Ionic Liquid Crystals: Versatile Materials - Chemical Reviews (ACS

Review pubs.acs.org/CR

Ionic Liquid Crystals: Versatile Materials Karel Goossens,*,†,‡ Kathleen Lava,‡,§ Christopher W. Bielawski,†,∥ and Koen Binnemans‡ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 689-798, Republic of Korea Department of Chemistry, KU Leuven, Celestijnenlaan 200F, P.O. Box 2404, B-3001 Heverlee, Belgium § Department of Organic and Macromolecular Chemistry, Ghent University, Krijgslaan 281 S4, B-9000 Ghent, Belgium ∥ Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea ‡

S Supporting Information *

ABSTRACT: This Review covers the recent developments (2005−2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the “state-of-the-art” in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

CONTENTS 1. Introduction 2. Liquid Crystals (LCs): The “Fourth State of Matter” 3. Classification of Thermotropic Liquid Crystals and Their Mesophases 3.1. Importance of Molecular Structure and Driving Forces for Mesophase Formation 3.2. Calamitic Mesogens 3.3. Discotic Mesogens 3.4. Cubic Phases 3.5. Other Types of Liquid Crystals 4. Thermotropic Ionic Liquid Crystals (ILCs) 4.1. When Cations Get along with Anions 4.2. Liquid-Crystalline Ionic Liquids? 4.3. Mesophase Behavior of Thermotropic Ionic Liquid Crystals 4.3.1. Influence of Charges on the Mesophase Behavior 4.3.2. Theoretical Studies 4.3.3. Note: Thermal Stability and Hygroscopicity of Ionic Liquid Crystals 5. Imidazolium-Based Ionic Liquid Crystals 5.1. Imidazolium-Based Mesogens Having a Predominantly Amphiphilic Character 5.2. Taper-Shaped Imidazolium-Based Mesogens 5.3. Attachment of Mesogenic Groups To Influence the Mesophase Behavior 5.4. Mesogens in Which the Imidazolium Cation Makes Part of the Rigid Core © XXXX American Chemical Society

5.5. Dendrimeric Imidazolium-Based Ionic Liquid Crystals 6. Ammonium-Based Ionic Liquid Crystals 6.1. Ammonium-Based Mesogens Having a Predominantly Amphiphilic Character 6.2. Taper-Shaped Ammonium-Based Mesogens 6.3. Attachment of Mesogenic Groups 6.4. Dendrimeric Ammonium-Based Ionic Liquid Crystals 7. Phosphonium-Based Ionic Liquid Crystals 8. Pyridinium-Based Ionic Liquid Crystals 8.1. Pyridinium-Based Mesogens Having a Predominantly Amphiphilic Character 8.2. Taper-Shaped Pyridinium-Based Mesogens 8.3. Attachment of Mesogenic Groups 8.4. Mesogens in Which the Pyridinium Cation Makes Part of the Rigid Core 8.5. Pyridinium-Containing Ionic Liquid Crystals Whose Mesomorphism Is Anion-Driven 8.6. Dendrimeric Pyridinium-Based Ionic Liquid Crystals 9. 4,4′-Bipyridinium-Based Ionic Liquid Crystals 10. New Cationic Cores for Ionic Liquid Crystals 10.1. Ionic Liquid Crystals with Pyrrolidinium, Piperidinium, Piperazinium, and Morpholinium Cations

B B C C C E E I J J L N N R V V W AJ

BB BC BC BG BM BP BW CB CB CC CC CC CF CH CH CM

CM

AN Received: July 11, 2013

AT A

DOI: 10.1021/cr400334b Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 10.2. Ionic Liquid Crystals with ε-Caprolactam Moieties 10.3. Ionic Liquid Crystals with Amidinium Moieties 10.4. Ionic Liquid Crystals with Guanidinium Moieties 10.5. Ionic Liquid Crystals Based on Spiro− Merocyanine Isomerization 10.6. Ionic Liquid Crystals with Triazolium Cations 10.7. Ionic Liquid Crystals with Pyrazolium Moieties 10.8. Ionic Liquid Crystals with 1,10-Phenanthrolinium Cations 10.9. Ionic Liquid Crystals Containing Positively Charged Polycyclic Aromatic Hydrocarbons (PAHs) 11. Ionic Liquid Crystals Formed by “Ionic SelfAssembly” (ISA) 12. Applications of Ionic Liquid Crystals 12.1. Adaptive, Nanostructured, and Potentially Anisotropic Ion-Conductive Materials 12.2. Nonvolatile Electrolytes in Dye-Sensitized Solar Cells (DSSCs) 12.3. Organized Media for Organic and Inorganic Reactions, for Gas Adsorption, and for Structure Determination 12.4. Electrochromic Materials 12.5. Anisotropic Photoluminescent Soft Materials 12.6. Future Directions 13. Conclusions and Outlook Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

Review

molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds, the so-called metallomesogens, but some references to polymeric and/or lyotropic ILCs and particularly to ILs will also be described. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquidcrystalline materials consisting of organic cations and organic/ inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions. After a general introduction to liquid crystals and a thorough description of the behavior of thermotropic ILCs (including an overview of theoretical studies), recently reported ILCs will be discussed on the basis of their cation structure, with special attention to imidazolium-based ILCs. It is common practice to classify ionic mesogens according to their type of cation because the synthesis and substitution of the cationic part of ILCs is generally more straightforward than that of the anionic component and mesomorphism often originates from the cationic moiety. The sections of this Review have thus been conceived accordingly. Section 11 is devoted to ionic mesogens built up by “ionic self-assembly” (ISA). Important structure− property relationships for ILCs are highlighted throughout this Review. The last section covers current and potential future applications of ILCs. The phase transition temperatures for all thermotropic ionic mesogens that have been reported since 2005 can be found in the Supporting Information.

CO CO CO CT CT CU CU

CV CZ DH DH DK

DM DS

2. LIQUID CRYSTALS (LCs): THE “FOURTH STATE OF MATTER” The liquid-crystalline (LC) state of matter exists between the solid phase and the isotropic liquid phase, and is therefore defined as a mesophase (from ancient Greek word “mesos”, which means “intermediate”). It is sometimes referred to as the “fourth state of matter”.9−16 Materials that show LC properties are named mesomorphic compounds. LC behavior is also known as mesomorphism. Mesogenic groups are molecular fragments that can be introduced into a structure to promote LC behavior. Such groups often contain several aromatic rings. In a LC phase, the high order typical of crystalline solids is partially reduced, which gives the molecules some degree of mobility, and renders the material fluidic or plastic. In a recent review, Tschierske defined the LC state as a condensed matter state in which there is orientational and/or positional long-range order in at least one direction and no fixed position for individual molecules.17 Orientational order means that the molecules align themselves parallel to each other to minimize the excluded volume and to maximize attractive intermolecular interactions. This contrasts with an orientationally and positionally disordered liquid state. The optical, electric, magnetic, and mechanical properties (i.e., refractive index, dielectric permittivity, magnetic susceptibility, conductivity, elasticity, etc.) of LC materials generally depend on the direction in which the relevant quantities are measured. Moreover, the mobility of the molecules in the LC mesophase enables such assemblies to respond to different types of external stimuli (e.g., electric fields) and allows “self-healing” of defects.17−19 In the case of thermotropic liquid crystals, a LC phase is obtained by heating the solid mesomorphic sample. At the melting point (Tm), the thermal motion of the molecules has increased to such an extent that the material passes from the solid phase to the LC phase (this can happen below room temperature as well). A mesomorphic compound that exists in the glass state will enter the LC phase at the glass-transition temperature (Tg).

DU DZ EA EB EB EB EB EB EB EC EC EE

1. INTRODUCTION Ionic liquid crystals (ILCs) are fascinating liquid-crystalline materials that solely consist of cations and anions. Although the first ILCs were reported in 1938,1 the field has experienced a revival during the last two decades, partly due to the increasing interest in ionic liquids (ILs). Thermotropic ILCs can effectively combine the characteristics of liquid crystals (i.e., dynamic molecular order and self-assembling ability, anisotropic physical properties, etc.) with those of ionic liquids (ionic conductivity, “tuning” possibilities, and so forth). They also share structural and dynamic features with both nonionic thermotropic liquid crystals and phases of surfactant/water mixtures. An extensive review on ionic mesogens was published by Binnemans in 2005.2 Review articles that have appeared since then and that partially cover the field of ILCs include those of Axenov, Mansueto, and Laschat,3,4 Douce et al.,5 Chen and Eichhorn,6 Causin and Saielli,7 and Pal and Kumar.8 The present contribution was designed to give a comprehensive overview of the “state-of-theart” in the field. It covers the recent developments (2005−2015) in the design, synthesis, characterization, and application of thermotropic ILCs. The discussion herein is focused on low B


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molecular packing. On the other hand, microphase segregation is of prime importance in the case of amphiphilic mesogens (including most ionic liquid crystals), which do not necessarily have an anisometric shape. Because of the chemical bonding between the incompatible regions, segregation does not lead to macroscopic phase separation;18,19 instead, it results in the formation of different microscopic regions, which are separated by interfaces at a molecular scale.23 The relative size, shape, and volume fraction of the incompatible segments determine the interface curvature and strongly influence the mesophase morphology. Depending on the degree of chemical and/or structural difference and the relative dimensions of the constituent molecular blocks, nanosegregation can occur with the formation of lamellar, columnar, worm-like, or spheroidic aggregates, that organize into LC smectic (Sm), columnar (Col), and cubic (CubV and CubI) mesophases (Figure S1).23 Incorporation of perfluoroalkyl chains (which are more stretched and rigid than normal alkyl chains), oligo(ethylene oxide) chains, or organosiloxane chains (which are bulkier and more flexible than normal alkyl chains) into smectic, cubic, and discotic mesogens can increase the mesophase stability, due to a more efficient nanosegregation as compared to normal hydrocarbon chains.35−38 Still another factor that determines the type of mesophase, and that may compete with nanosegregation, is space filling.23,27,39 The whole space must be “reachable” and filled up by the mesomorphic molecules in their condensed matter state. The desire of rigid anisometric units to minimize the excluded volume (which is, in fact, a general organization principle of matter), for instance by aligning themselves parallel to one another, adds extra limitations to the structures that can be displayed by thermotropic LCs.27

On further heating, the thermotropic LC can form one or more other LC phases (in the case of polymorphism) or transform into an isotropic, clear liquid at the clearing point (or isotropization point) (Tc). When the mesophase is found on both heating and cooling the material, the phase is thermodynamically stable and is termed enantiotropic. When the mesophase only appears on cooling a material below its melting point, and is therefore metastable, the phase is termed monotropic. Another way to induce mesomorphism is through the addition of a solvent to the solid phase, which has a disruptive effect on the crystal lattice. This is termed lyotropic mesomorphism. Usually lyotropic LCs are amphiphilic. Compounds that exhibit both thermotropic and lyotropic mesomorphism are termed amphotropic liquid crystals.20,21

3. CLASSIFICATION OF THERMOTROPIC LIQUID CRYSTALS AND THEIR MESOPHASES 3.1. Importance of Molecular Structure and Driving Forces for Mesophase Formation

Many factors determine the type of mesophase formed by low molar mass thermotropic LCs, but molecular shape is an important parameter. A common classification scheme of LC phases is based on the specific anisometric shape of the constituent mesogenic molecules, although it should be emphasized that mesophase types should in principle be described and distinguished by their symmetry (which governs their physical properties) rather than by the shape of the individual components.22 Conventional LCs often have a rod-like shape (calamitic mesogens) or a disk-like shape (discotic mesogens). In both cases, the molecules can be described as cylinders with a high degree of structural anisotropy. The cylinders represent the average or effective shape of the mesogens, which are free to rotate about their primary molecular axis in fluidic LC phases. Moreover, both types of mesogens are able to form nematic phases, which show only orientational order (see below). On the other hand, it is possible that these compounds undergo molecular stacking, which leads to the formation of layers (“2D fluids”) or columns (“1D fluids”). In this way, smectic phases and columnar phases are formed, respectively (see below). Fluidic smectic phases and columnar phases show long-range (or quasi long-range) positional/translational order in at least one direction, besides orientational order (although the latter is not a prerequisite17). The formation of LC mesophases originates from the selforganization of the mesogenic molecules, which is driven by different kinds of intermolecular interactions (see Table S1). By increasing the temperature, the molecular mobility increases, and, as a consequence, the weak interactions dissipate while the stronger interactions remain. In this self-organization process, nanosegregation (or microsegregation), caused by contrasts within the mesomorphic molecules, often plays a primary role.18,23−34 Many mesogens have a molecular structure consisting of at least two structurally and/or chemically incompatible parts (or “blocks”). An example is the incompatibility between hydrophilic and hydrophobic moieties. Compatible regions interact with one another in the mesophase, whereas incompatible molecular segments segregate into distinct subspaces, thus causing a microphase segregation.29 This driving force does however not apply to nematic phases. These are stabilized by anisotropic dispersion forces that result from the anisometry of the constituent molecules, which facilitates dense

3.2. Calamitic Mesogens

Calamitic mesogens are rod-like molecules that have some rigidity in their central region. Terminal groups are attached to the central part: these can be flexible chains (largely responsible for the molecular mobility in the mesophase) and/or polar groups. The molecules should preferably possess a permanent dipole moment or undergo anisotropic polarization. An optically uniaxial nematic phase (N) is the least ordered (and therefore least viscous) mesophase exhibited by LCs (Figure 1).40 It is formed when mesogens align their primary molecular axis on average along a common direction defined by a vector, the director n. This results in long-range, 1D orientational order with full rotational symmetry around n, but without any long-range positional/translational ordering. The director is usually not constant throughout the bulk sample, but it fluctuates in space: for nonaligned LC samples, several so-called microdomains, each with a specific director orientation, exist. Nematic phase formation is favored by mesogens with a high aspect ratio. The complexity of the nematic phase structure can be increased through the introduction of chirality, yielding an optically active chiral nematic phase (or cholesteric phase) (N*). The local molecular ordering in this phase is identical to that of a simple nematic phase, but the director n is additionally continuously rotated along an axis perpendicular to it. In this way, a helical superstructure is formed. Smectic phases (or lamellar phases) (Sm) are characterized by a 1D periodic stacking of “layers” formed by the LC molecules or supermolecular aggregates; that is, unlike nematic phases they show positional order (translational symmetry) to a certain extent.10,15,41,42 The thickness of the layers is the so-called layer periodicity d. The smectic layers are not necessarily well-defined, C


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The smectic A phase (SmA) is the least ordered smectic modification. The molecules in the smectic layers have, on average, their long molecular axes parallel to the normal to the layer planes (they may also be slightly tilted in a random, noncorrelated way). The lateral distribution of the molecules within each layer is random, and they enjoy a considerable freedom for rotation about their long molecular axis, and even for translation within the smectic layer. The layers are free to slide past each other and can be relatively diffuse. The SmA phase is the most common phase for ionic liquid crystals.2−4 There exist some variations on the basic SmA structure, such as bilayer SmA2 phases, partial bilayer (interdigitated/intercalated) SmAd phases, modulated ribbon-like SmÃ phases, biaxial SmA phases (SmAb and SmAb2), and de Vries SmA phases.25 In a smectic C phase (SmC), the molecules are additionally tilted within the layers, along a preferred direction. In a normal SmC phase, molecules in adjacent layers have the same tilt direction (synclinic interlayer correlation); in the far more rare SmCA subtype, the tilt direction alternates from layer to layer (anticlinic). Several chiral SmC phases (SmC*) are known as well. SmC phases have not yet been found in amphiphilic LCs without rigid cores.23 In general, these tilted phases are rarely observed for ionic liquid crystals, as evidenced by the few examples in this work and previous reviews.2,3 A smectic B phase (SmB) is of lower entropy and higher viscosity than SmA and SmC phases. It resembles the SmA phase, but shows an additional ordering of the molecules within the smectic layers: the molecular centers of mass are arranged into a 2D close-packed hexagonal pattern. In true LC smectic B phases, the molecules are able to rotate about their long molecular axes, the layers are free to slide over one another, and diffusion between the layers occurs readily. The 2D hexagonal lattice is not correlated over a large number of layers, and interlayer shift distortions occur. Nevertheless, in “hexatic” smectic B phases, there exists bond orientational ordering: the layers cannot rotate relative to one another. This latter type of ordering is absent in “rotationally disordered” smectic B phases. Such true LC behavior does not always give an accurate picture. It is possible that the hexagonal close-packed arrangement within a given layer is extremely long-range, and that the 2D hexagonal lattice is correlated over a large number of layers. In this case, the undisturbed phase is structurally solid-like and not simply a LC phase; thus, it should be termed crystal smectic B phase or crystal B phase (see below). Rotation of the molecules about their long molecular axes is restricted: it occurs concerted; that is, the rotating molecules must share space (or it occurs in an oscillatory way).43 The smectic I phase (SmI) and the smectic F phase (SmF) are tilted versions of a SmB phase. These phases exhibit a pseudohexagonal packing of the molecules within the layers: in a plane at right angles to the tilt direction, the packing arrangement is of the hexagonal type. There also exist chiral variants of these phases (SmI* and SmF*, respectively). As mentioned above, some highly ordered smectic phases are no longer considered as genuine LC phases, but as soft or disordered crystal phases, or, more appropriately, as anisotropic plastic crystal phases. These phases are also termed crystal smectic phases and are denoted by a letter code that refers to their historical classification as a LC smectic phase but without the prefix “Sm”. They show long-range (intralayer and/or interlayer) positional order. However, the thermal rotational molecular motion is not completely frozen out, and the alkyl chains are not fully crystallized. The following phases are usually distinguished: crystal B (B), crystal J (J and J*), crystal G (G and

Figure 1. Some of the fluidic LC phases formed by conventional rod-like mesogens. The molecules are depicted as colored ellipsoids (Figure N* phase: Adapted with the authors’ permission from a figure by K. G. Yager, Brookhaven National Laboratory (NY), and C. J. Barrett, McGill University (Montreal, Canada)).

although this is usually the case for ionic liquid crystals as will be explained in section 4.3.2. Tschierske noted that it is often argued that the parallel organization of the rigid cores of rod-like molecules is the main reason for their liquid-crystallinity and stressed that in fact only the formation of nematic phases is strictly bound to the rigid anisometric shape of the individual molecules; the formation of smectic layers is strongly driven by nanosegregation.23,27 The most common smectic and crystal smectic phases, which will be introduced below, can be categorized into two groups: the orthogonal phases and the tilted phases (Table 1 and Figure S2). Table 1. General Structural Features of Achiral Smectic and Crystal Smectic Phases (Variations May Occur)

Phase

Range of orientational order (if present)

Range of positional order

SmAa SmCa SmBd SmId

short short long long

short short short short

orthogonalb tiltedc orthogonal tilted (to apex)

SmFd

long

short

tilted (to side)

Be Je

long long

long long

orthogonal tilted (to apex)

Ge

long

long

tilted (to side)

Ee Ke

long long

long long

He

long

long

Te

long

long

orthogonal tilted (to longer side of 2D lattice unit cell) tilted (to shorter side of 2D lattice unit cell) orthogonal

Orientation of long molecular axis

Distribution of molecular centers of mass random random hexagonal (pseudo) hexagonal (pseudo) hexagonal hexagonal (pseudo) hexagonal (pseudo) hexagonal orthorhombic monoclinic monoclinic square

a

Fluidic smectic phases. bOr randomly tilted. cUniform tilt direction in the smectic layers. dHexatic smectic phases. eCrystal smectic phases. D


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The stacking into 1D columns due to interdisk interactions can also be found in the most typical LC phases of discotic mesogens, the columnar (non-nematic) phases. In these phases, the columns are arranged in a 2D lattice with the column axes on average parallel to each other. While the nematic phases are often fluidic, the columnar non-nematic phases are usually waxy. Discotic mesogens with large aromatic cores showing π-stacked columnar arrays are of interest for organic electronics, primarily for applications as “one-dimensional” semiconductors that are processable as well as potentially defect-free.47,48,51−54 The different types of columnar phases are distinguished by the symmetry of the 2D lattice (i.e., hexagonal, rectangular, oblique, or square) and by their intracolumnar order (i.e., disordered columns, showing an irregular stacking of the disks in the columns, sometimes denoted by the subscript “d”; ordered columns, showing a regular, equidistant stacking of the cores, whereas flexible tails are still disordered, sometimes denoted by the subscript “o”; or tilted columns, in which the cores of the disks are tilted with respect to the column axis, sometimes denoted by the subscript “t”). It can be very difficult to determine the symmetry of a columnar mesophase by polarizing optical microscopy (POM) alone, and powder X-ray diffraction (PXRD) is usually necessary for mesophase identification. In the most common type of columnar phase, the hexagonal columnar phase (Colhex), the columns are arranged into a planar hexagonal pattern, according to a 2D p6mm lattice (or, exceptionally, a p3m1 symmetry). Three different lattice symmetries are common for a rectangular columnar phase (Colrec): c2mm, p2gg, and p2mg. Other symmetries such as p2mm have been found as well. In an oblique columnar phase (Colobl), the columns are arranged in a lattice with a p1 or p2 symmetry. Examples are relatively rare because strong core−core interactions are required.47 The square columnar phase (sometimes termed tetragonal columnar phase) (Colsqu) is characterized by a square 2D lattice of p4mm or p4gm symmetry. There also exist so-called lamello-columnar mesophases (LCol or ColL), in which disk-like molecules or molecular cores assemble in supramolecular microcolumns within smectic layers.

G*), crystal E (E), crystal K (K and K*), and crystal H (H and H*), where an asterisk denotes chiral modifications. Furthermore, another type of highly ordered smectic phase, the so-called crystal smectic T phase, can be found for certain types of ionic liquid crystals. This phase will be discussed in section 4.3.1. There exist nonanisotropic plastic crystal phases (so-called rotator phases) as well. Such plastic crystals possess full positional order like classical crystals (i.e., the individual molecules have fixed positions on the crystal lattice, and therefore they are not considered as LC phases17), but because the molecules are usually globular in shape and able to rotate very rapidly in all directions about their lattice points, there is no longrange orientational order. The crystals have weakly bonded molecules and hence deform easily. 3.3. Discotic Mesogens

Discotic mesogens have a disk-like molecular structure. In the most typical cases, the molecules consist of a more or less rigid flat core (usually aromatic), surrounded by at least three flexible chains that make up the “soft” region. The rigid core can also be bowl-like, cone-like, or pyramidal (it can even be a hollow ring). In many cases, individual molecules do not match these criteria, but they are capable of aggregating into units, which do fulfill them.42,44−49 The same nematic structure as in the case of calamitic mesogens, with orientational order but no positional/translational order, can be formed by discotic mesogens, when they align their principal, short molecular axes on average along a director n to produce the discotic nematic phase (ND) (Figure 2).50 It is also possible that the discotic mesogens pile up into 1D

3.4. Cubic Phases

Cubic phases (Cub) are mesophases of cubic symmetry, whose unit cell consists of several hundreds of molecules. These phases are very common for lyotropic LCs, where a cubic phase is possible between any pair of phases, but are relatively rare for thermotropic LCs.55−60 This Review shows that quite a few thermotropic ionic liquid crystals that form a cubic mesophase have been developed in recent years. Cubic LC phases show long-range positional order (translational symmetry) in three dimensions that is accompanied by rotational disorder as well as conformational mobility. Unambiguous identification of the symmetry of a cubic mesophase is only possible via small-angle X-ray scattering (SAXS) studies on aligned monodomain samples. Micellar cubic phases (CubI) consist of ordered 3D arrays of micelles. Multicontinuous cubic phases (CubV) consist of interpenetrating, infinite, periodic 3D molecular networks (the surfaces obtained by connecting all of the midway points between the networks are referred to as “infinite periodic minimal surfaces”). Thermotropic CubV mesophases of Ia3̅d, Im3̅m, Pn3̅m, and Pm3̅m symmetry, and thermotropic CubI mesophases of Pm3n̅ , Im3m ̅ , Pm3m ̅ and Fm3m ̅ symmetry have been found to date.17,55−65 Because of the high symmetry of cubic mesophases, and contrary to most other LC mesophases, the cubic phases are optically isotropic. Essentially no defect

Figure 2. Some of the LC phases formed by conventional disk-like mesogens.

columns, which tend to align parallel to each other but without long-range lateral positional correlations, to form the columnar nematic phase (or nematic columnar phase) (NC). The columns can be formed by the charge-transfer interaction between an electron donor and an electron acceptor, or via strong electrostatic interactions between cations and anions (see section 4.3.1). If the difference between the lengths or volumes of the side chains is large enough, the arrangement of the columns in a 2D lattice (like in the columnar non-nematic phases, see below) is disturbed. E


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Scheme 1. General Molecular Structure of the Organic Cations That Have Been Used To Obtain Low Molar Mass Thermotropic ILCsa

a

Nomenclature: (1) ammonium; (2) phosphonium; (3) pyrrolidinium; (4) piperidinium; (5) morpholinium; (6) piperazinium (including 1,4-bis(nalkyl)-1,4-diazoniabicyclo[2.2.2]octane (i.e., doubly quaternized DABCO) cations); (7) homopiperazinium; (8) 1,5,9-triazoniacyclododecane-1,5,9triium; (9) 1,4,8,11-tetraazoniacyclo-tetradecane-1,4,8,11-tetraium; (10) substituted ε-caprolactam; (11) guanidinium; (12) imidazolium; (13) triazolium; (14) pyrazolium; (15) dithiolium; (16) pyridinium; (17) pyrylium; (18) thiopyrylium; (19) benzimidazolium; (20) benzobis(imidazolium); (21) quinolinium; (22) isoquinolinium; (23) pyrimidinium; (24) viologen (4,4′-bipyridinium) (n = 0, 1); (25) 1,10phenanthrolinium; (26) vinamidinium; (27) bis(amidinium); (28) 2,2′,2″-(benzene-1,3,5-triyl)tris(imidazolinium); (29) protonated merocyanine; (30) fulleropyrrolidinium; (31) protonated cyclo[8]pyrrole; (32) 9-phenyl-benzo[1,2]quinolizino[3,4,5,6-fed]phenanthridinylium (PQP); (33) benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (BNAX); and (34) 4,8,12-trialkyl-4,8,12-triazatriangulenium (TATA) cations. F


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Scheme 2. Anions That Have Been Used To Obtain Low Molar Mass Thermotropic ILCs

G


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Scheme 2. continued

H


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Scheme 2. continued

morphologies.17−19,23−29,32−34,72−77 Metallomesogens are metal-containing liquid crystals.78−89 It can be challenging to achieve liquid-crystalline properties for bulky metal fragments with a high metal coordination number, which do not have an intrinsic anisometric shape such as in the case of metal complexes with a linear or square-planar geometry. There are different approaches to tackle this problem. One possibility is to use highly anisometric mesogenic ligands with a large number of (aromatic) rings. This often leads to rather high transition temperatures. A second possibility is to increase the number of diverging flexible chains attached to the central coordinating unit, and in that case the metal complexes usually exhibit columnar mesomorphism. Another strategy is a spatial decoupling (via a flexible spacer) of mesogenic, rigid aromatic cores from the bulky metal fragment. Such a system resembles a covalent “host−guest” system where the “guest” metal complex is dissolved in the LC “host”, but phase separation is prevented by covalently linking the metal complex to the mesogenic part(s). These general approaches can be adopted for inducing LC properties with other systems as well (e.g., C60 fullerenes90), and have also been widely used in the field of ionic liquid crystals as will become clear in the remainder of this Review.

texture is observed by POM. A cubic LC phase can be quite easily detected by POM when it is present between two optically anisotropic mesophases, but it is difficult to observe its formation when it appears on cooling an isotropic liquid (or, for example, a SmA phase in its homeotropic arrangement). In that case, a deformation of air bubbles in the melt is observed, as well as a sharp increase in the viscosity of the liquid. The cubic phase can also be recognized by its crystal-like shapedomains with straight edges and sharp cornerswhile growing or nucleating from another phase. Formation kinetics of cubic phases are slow, due to the rather substantial structural rearrangements that occur. 3.5. Other Types of Liquid Crystals

Polycatenar mesogens consist of an elongated linear rigid (mostly aromatic) core that is decorated with several flexible chains at the ends.23,66−68 The nomenclature of polycatenar systems is related to the number of terminal chains, so that, for example, those containing four chains are named tetracatenar. They usually show a rich mesomorphism. Bent-core mesogens, which are V-shaped or banana-shaped, have attracted interest because of their unique properties, such as the ability to form LC mesophases with superstructural chirality, while the constituent molecules are achiral.69−71 There exist many other LC materials whose molecular structure differs from a classical rod-like or disklike shape. Some of these exhibit complex mesophase I


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Scheme 3. Metal-Containing Anions That Have Been Used To Obtain Thermotropic ILCsa

Note: boron, arsenic and antimony are generally considered as “metalloids”, and therefore anions containing these elements are displayed in Scheme 2.

a

4. THERMOTROPIC IONIC LIQUID CRYSTALS (ILCs)

consists of compounds containing (positively charged) cations

4.1. When Cations Get along with Anions

and (negatively charged) anions. Because of their ionic character,

While most LCs are neutral organic compounds, there exist many ionic liquid crystals (ILCs) as well. This class of mesogens

the properties of ILCs may differ significantly from those of conventional LCs. J


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Figure 3. Different general molecular structures of ILCs (examples given for an imidazolium ring as the organic cation).

flexible spacer).103−105 The wedge-shaped type (d) compounds can form cylindrical supramolecular aggregates forming columnar and cubic mesophases (see below). The amphiphilic compounds that are combined with anisometric structural units bridge the gap between typical thermotropic and lyotropic LCs.23 While originally most research was focused on halide salts (with chloride, bromide, or iodide anions), other counterions were also investigated from the 1990s on. Schemes 2 and 3 give an overview of all of the anions that have been used up until now, to the best of our knowledge, to obtain thermotropic ILCs. Especially in the past fifteen years, the choice of anions was partially inspired by counterions typically used in ionic liquids (see section 4.2). Perfluorinated anionic species such as [BF4]−, [PF 6 ] − , [OTf] − (=[CF 3 SO 3 ] − ), and [NTf 2 ] − (=[N(SO2CF3)2]−) are commonly incorporated into ionic liquids instead of halide anions to decrease the melting point and viscosity, because of their larger size, the delocalization of the negative charge, and their weaker hydrogen-bonding ability (as well as conformational flexibility in the case of [NTf2]−).106−108 Furthermore, the hydrophilicity (and hygroscopicity) and thermal stability can be greatly affected by the anion choice. Different counterions have different shapes (spherical, globular, linear, etc.), structures (containing either no chains, one chain, or multiple chains, which can be linear or branched), and coordinating abilities, representing a great source of tunability. It should be noted that anion exchange reactions are not necessarily straightforward. Care should be taken that all anions have been exchanged, and often additional purification is necessary.109 Most of the anions that have been used in ILCs are singly charged. Commonly used anions that carry more than one negative charge are the tetrahalogenometalate(II) complexes, [MX4]2− (Scheme 3).2 Those ILCs consist of two cations and a doubly negatively charged, metal-containing [MX4]2− anion. In recent years, ILCs incorporating other metal-containing complex anions were reported as well: these include, for example, linear [Ag(CN)2]− and [Au(CN)2]− complexes;110−112 tetrahedral [ReO4]− and [AlCl4]− anions;113,114 octahedral [UO 2 Br 4 ] 2 − , 1 1 5 , 1 1 6 [La(NO 3 ) 6 ] 3 − , 1 1 7 [EuBr 6 ] 3 − , 1 1 8 [TbBr6]3−,119 and [DyBr6]3−120 complexes; highly luminescent inorganic [Mo6Xi8Xa6]2− clusters;121 a highly luminescent tetrakis(2-thenoyltrifluoroacetonato)europate(III) ([Eu(tta) 4 ] − ) anion with a distorted square-antiprismatic shape; 116,122 and many highly charged polyoxometalate ions123−140 (Scheme 3). We would like to describe the [MX4]2−, [M(CN)2]−, [ReO4]−, [AlCl4]−, [UO2Br4]2−, [La(NO3)6]3−, [LnBr6]3−, [Mo6Xi8Xa6]2−, [Eu(tta)4]−, and poly-

From the 1980s on, an increasing number of reports on several types of thermotropic ILCs appeared in the literature.2−4 In particular, ammonium salts and pyridinium salts, as well as metal carboxylate compounds, were investigated.20,80,91−93 Thermotropic mesomorphism of amphiphilic pyridinium salts had been noticed as early as 1938.1 Gradually other organic cations, such as substituted imidazolium, phosphonium, and viologen (1,1′disubstituted 4,4′-bipyridinium) cores, were also used to obtain mesomorphic salts. In the past few years, ILCs based on pyrrolidinium, piperidinium, morpholinium, piperazinium, εcaprolactam, guanidinium, triazolium, pyrazolium, benzobis(imidazolium), quinolinium, isoquinolinium, pyrimidinium, 1,10-phenanthrolinium, bis(amidinium), protonated merocyanine and fulleropyrrolidinium cations, besides large positively charged fused ring systems and protonated cyclo[8]pyrrole, have been reported as well (Scheme 1). Interest in especially the quaternary ammonium, pyridinium, imidazolium, phosphonium, and viologen ILCs arose from their relatively facile preparation by quaternization with alkyl halides. Because the synthesis and substitution of the cationic part of ILCs is generally more straightforward than that of the anionic part, it is common practice to classify the different types of ILCs according to the type of cation. The different sections of this Review have been conceived accordingly. Cases of “anion-induced” mesomorphism exist, and these will be discussed in particular in sections 8.5 and 10.9. There exist several review articles that (partially) cover the field of ILCs, of which the 2005 review by Binnemans is the most comprehensive.2−8,20,80,91,92,94−97 It should be noted that LC charge-transfer systems98 and hydrogen-bonded LCs99,100 show similarities to ILCs, but these compounds will not be discussed here unless relevant in the context of ionic mesogens. A further distinction can be made between (i) ILCs with cations carrying simple long alkyl chains (which make up the majority of all investigated compounds, mainly because of the ease of synthesis) (Figure 3a); (ii) ILCs in which the charged moiety (e.g., an imidazolium cation, which is a polarizable rigid group) is incorporated in the rigid part of the mesogen (Figure 3b); (iii) ILCs with conventional mesogenic groups attached via a flexible spacer to the organic cation (either terminally or laterally; Figure 3c); and (iv) wedge-shaped ILCs (Figure 3d). The thermotropic LC properties of compounds of type (a) are mainly due to their strong amphiphilic character, which also explains their lyotropic mesomorphism.101,102 Compounds of type (b) constitute examples of more classically shaped systems, where excluded volume effects play a primary role apart from amphiphilicity. Compounds of type (c) can be considered in a way as nonsymmetric LC dimers (two rigid moieties whose molecular motion is restricted due to their connection via a K


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oxometalate-containing mesomorphic compounds as “truly ionic” metallomesogens. Many of the “classical” metallomesogens are ionic compounds as well. Indeed, alkali metal, alkaline earth metal, d-block transition metal, lanthanide, and uranyl cations that are surrounded by neutral and/or mesogenic ligands in their first coordination sphere (i.e., the charged metal ion belongs to the rigid organic core of the mesogen) can also have small negative counterions in their second coordination sphere.79,85 However, these compounds, and also the wellknown LC metal carboxylate compounds, do not show a clear distinction between a mesogenic organic cation and a metalcontaining anion, and are outside the scope of this Review. A relatively recent trend in materials science is to make use of the so-called ionic self-assembly (ISA) concept to obtain functional nanostructured materials, such as ILCs.141−147 In essence, this strategy involves the combination of cations and anions (typically charged oligoelectrolytes and oppositely charged surfactants) to form highly organized, hierarchical superstructures with the attractive, noncovalent electrostatic interactions as the primary driving force. Secondary driving forces for self-organization can be hydrophobic interactions, π−π interactions, hydrogen bonding, etc. ISA is related to molecular tectonics, which refers to the assembly, via all kinds of noncovalent interactions, of complementary molecular building blocks (= “tectons”) into predesigned 1D, 2D, or 3D periodic architectures (this term is commonly used in the context of selfassembly in crystalline solid phases or on surfaces).148 The combination of oppositely charged, functional, structurally different, and preferably simple (or commercially available) oligoelectrolytic building blocks is a convenient way to obtain processable nanostructured materials with specific properties without the need for elaborate covalent chemistry. The practical realization of the ISA concept is not always easy because suitable reaction conditions have to be found for the actual ion exchange (stoichiometric precipitation of high-purity products). However, true ISA is accompanied by a cooperative binding mechanism, in the sense that the first bonds stimulate further binding of charged molecules toward a final self-assembled structure.2,141,142 Such cooperativity is not observed for traditional Coulombic salt formation. For polyelectrolyte molecules, an additional driving force for ISA “complexation” is the entropic gain in free energy through the release of small molecule counterions.149,150 ISA can be useful to transform water-soluble functional units (such as fluorophores) into compounds that are soluble in nonpolar volatile organic solvents. This can be convenient for processing, for instance, into thin films via spin-coating or dropcasting. The term “ISA” is mostly used in the context of polyelectrolytes, but has also been adopted for low molar mass compounds. In LC chemistry, the ISA approach (although not always recognized as such) proved to be useful, among many other examples, for the preparation of disk-like ionic mesogens, mesomorphic dye− surfactant complexes, LC luminescent 1,10-phenanthrolinium compounds, and LC cyclobis(paraquat-p-phenylene)s with “switchable” side chains;141,145,151−163 highly charged mesomorphic surfactant-encapsulated polyoxometalate complexes (SEP complexes, with either negatively or positively charged polyoxometalate cores);125−131,133−140 as well as LC complexes of negatively charged double-stranded DNA164−167 and of genetically engineered supercharged polypeptides168 with amphiphilic cations. These and other examples will be discussed in section 11. Liquid-crystallinity (fluidity) is usually induced by the incorporation of surfactant molecules.

Many ILCs can be purified by means of simple recrystallization, precipitation, or extraction. Typically, ionic mesogens are not soluble at room temperature in solvents of low polarity such as diethyl ether, toluene, ethyl acetate, n-pentane, n-hexane, and n-heptane, while they can be dissolved in the lower alcohols, acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, dichloromethane, and/or chloroform. Solubility is of course highly dependent on the specific molecular structure. In some cases, purification by means of column chromatography is required. It is advisable to postpone the quaternization or “ionization” of neutral precursors to (one of) the last synthesis step(s), because the purification of neutral molecules is more straightforward and does not suffer from potential ion exchange with other reagents. Purification on a silica gel column with gradient elution is possible, for example, using a methanol/dichloromethane mixture with a methanol content exceeding 5−10 vol % (see also ref 169 for a modified normal-phase chromatographic method). It can be helpful to deactivate the silica gel with for example triethylamine or an alkyltrichlorosilane, or to treat it with hydrogen chloride (for chloride salts). When using highly polar eluents, one should make sure to separate potentially dissolved silica from the purified samples by means of postcolumn filtration. In some cases, purification on an alumina column or even on a reversed-phase silica column is required. 4.2. Liquid-Crystalline Ionic Liquids?

Ionic liquid crystals are materials that may combine the characteristics of liquid crystals (anisotropy of physical properties) and some of the properties of ionic liquids (such as ionic conductivity). The tremendous increase in research activity on the latter during the past two decades has certainly contributed a lot to the growing interest in ionic mesogens. This is exemplified by the fact that two of the most highly cited publications about ionic liquid crystals170,171 are from groups that are well-known for their research on ionic liquids (not liquid crystals). Ionic liquids (ILs) are defined as solvents that entirely consist of ions and typically melt below 100 °C.172−175 Early research on ILs, as early as the 1970s and 1980s, focused on the electrochemical application as electrolytes. The so-called “first-generation” ILs were moisture-sensitive halogenoaluminate(III) salts.176 Further interest in ILs, particularly room-temperature ILs, developed from the fact that aprotic molten salts have an extremely low vapor pressure,177,178 which made them candidates for replacing volatile organic solvents in organic synthesis, and which allows their use in high-vacuum systems. Furthermore, many ILs show a broad liquidus range, high thermal stability, and/or a broad electrochemical window, and they are often nonflammable. The first air- and moisture-stable “second-generation” IL, 1-ethyl-3methylimidazolium tetrafluoroborate ([C 2 mim][BF 4 ]; [C2mim]+ = 1-ethyl-3-methylimidazolium), was reported in 1992.179 As from then, the halogenoaluminate(III) salts were largely replaced by ILs composed of an organic cation and an “inert” anion such as [BF4]−, [PF6]−, [OTf]−, or [NTf2]− (it should be noted, however, that [PF6]− and especially [BF4]− anions can slowly hydrolyze to HF in the presence of water180−183). ILs can be considered to be “designer solvents”, because of the fact that by combination of an appropriate cation and an appropriate anion, a material with specific desired properties (e.g., (im)miscibility with water and other solvents, dissolving ability, polarity, viscosity, density, etc.) can be prepared. This is why many molten salts may be categorized as “third-generation” functionalized or task-specific ILs, in which one or more functionalities have been added to their primary role L


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as a solvent.174,184−187 Imidazolium, pyrrolidinium, pyridinium, and quaternary ammonium and phosphonium cations have been most often used to obtain ILs. The conductivity of ILs is inversely proportional to their viscosity and is related to the ion mobility.188 ILs have been investigated as (i) (sometimes recyclable) reaction media for organic reactions, where in some cases a higher catalytic activity and/or a higher selectivity has been observed; (ii) solvents for immobilizing transition metal catalysts in biphasic catalysis; (iii) extraction solvents and gas processing media; (iv) electrolyte solution substituents for batteries, fuel cells, solar cells, and capacitors, because most ILs are intrinsically ion-conducting fluids;189−194 (v) electrolytic media for electrodeposition of reactive metals and semiconductors;194−199 (vi) solvents for luminescence studies;200,201 and (vii) as templating agents for the synthesis of inorganic nanostructures.202−206 They have already found application in the chemical industry.207 Because ILs are often composed of poorly coordinating ions, they have the potential to be highly polar but nevertheless noncoordinating solvents. The use of ILs in the preparation of inorganic materials is a rapidly emerging field. The nonvolatility and thermal stability of many ILs allow a nearly complete removal of water (and other nonionic solvents), and previously unknown inorganic materials could be synthesized by exploiting the IL as a “structure-directing” (templating) solvent.208−214 Protic as well as nonprotic ILs are also being investigated as amphiphile self-assembly media, including the formation of (“lyotropic”) LC mesophases.214−226 The melting points of ILs are mainly determined by the structural properties of the cation and the anion and the interaction strength between them. A low melting point can be obtained by reducing the Coulombic attractive forces between cations and anions (e.g., by a delocalization or shielding of the charges, or by increasing the interionic separation), by reducing the symmetry and increasing the volume of the ions, and by using ions with a high conformational flexibility.227,228 By doing so, the efficient packing of the ions in a stable crystal lattice is disturbed. Several models have been developed to predict the melting points of ILs; these are based on, for instance, quantitative structure−property relationships and/or quantum-mechanical calculations.229−232 An interesting example of melting point depression was given by Weiss and co-workers: they showed that replacement of the halide anion (spherical Cl− or Br−) in P,P,Ptris(n-alkyl)-P-(n-alkyl)phosphonium halide salts by a trihalide anion (linear [Br3]−, [ClBr2]−, or [BrCl2]−) resulted in a transition from room-temperature solids to room-temperature ILs.233 It is important to emphasize the distinction between the terms “ionic liquid” and “ionic liquid crystal”. The term “ionic liquid crystal” refers to an ionic liquid-crystalline material, that is, a material that consists of cations and anions and that shows at least one (either enantiotropic or monotropic) liquidcrystalline mesophase. It is a synonym of “ionic mesogen”. The term can be used irrespective of the material’s melting point (it does not need to be below 100 °C), viscosity (which can be very high), or potential use as a reaction medium (which has been explored only in a few cases, see section 12.3). As such, we recommend not to refer to a nonmesomorphic ionic liquid in its crystalline state (as opposed to a liquid-crystalline state) as “ionic liquid crystal”, to avoid any confusion. We would also avoid using the phrase “liquid-crystalline ionic liquid”. An ionic liquid can indeed show one or more liquid-crystalline phases at a lower temperature than its liquid state (this may be below room temperature), but one should always keep in mind that the liquid

and liquid-crystalline states are two different states of matter. It is important that both the ionic liquid community and the liquid crystal community are consistent in their terminology. Nonetheless, it is clear that the revived interest in ionic mesogens and intense research activity in that field during the past few years is largely due to the popularity of ionic liquids in various scientific areas. Researchers should however refrain from abusing this buzzword in the context of ionic mesogens. Throughout this Review, it will become clear that the knowledge that was accumulated in the field of ionic liquids has contributed to the development of novel, low-melting ionic mesogens and smart ion-conducting liquid-crystalline systems. It should be mentioned at this point that even the “isotropic liquid” state of many nonmesomorphic ILs displays some degree of nanoscale structuring, charge ordering, and local anisotropy (see also section 4.3.2). Typical ILs with sufficiently long alkyl chain substituents (already for propyl or butyl side chains in the case of imidazolium ILs) are characterized by some spatial mesoscopic structural heterogeneity in their liquid state, as shown by SAXS/WAXD and neutron scattering measurements as well as other experimental techniques, and by molecular dynamics (MD) simulations.225,234−279 Differences have been found between aromatic ILs and nonaromatic ILs.273 The heterogeneity is related to aggregation of (or network formation by) the polar headgroups (due to the charge-ordering effect resulting from the strong long-range Coulombic interactions, but cooperative hydrogen-bonding interactions between the cations and anions, which induce structural directionality, also play a role) and concomitant domain formation by the alkyl chains, albeit not to the same extent and not on the same length scale as in the case of ILCs. The latter contain, in their simplest form, longer alkyl chains (usually at least undecyl or dodecyl), which allow for a genuine, long-range correlated microphase segregation with long-range positional order/periodicity.262 Nevertheless, the medium-chain-length ILs constitute a natural transition from the close-packed cation−cation second-shell radial distribution observed for [C1mim]+ salts to the smectic layers with interdigitating alkyl chains exhibited by [Cnmim]+ (n > 11) ILCs.276 On the basis of SAXS and ionic conductivity measurements, Bradley et al. and De Roche et al. proposed that some short-range structural ordering, probably related to persisting ionic aggregates or clusters, is also still present in the isotropic liquid phase of the latter compounds.280,281 Abdallah et al. also reported on such persistent ordering in relation to phosphonium-based ILCs,282 and the phenomenon was confirmed by adiabatic scanning calorimetry (ASC) measurements on piperidinium- and morpholinium-based ILCs.283 A similar phenomenon has been proposed to rationalize the low enthalpy/entropy values associated with the SmA-to-isotropic liquid transition of many carbohydrate-based LCs, for which hydrogen-bonded aggregates are proposed to remain in the isotropic liquid phase.284,285 These nanoscale spatial heterogeneities are much less pronounced and only short-living for typical ILs with very short alkyl chains, such as [C2mim][NO3].276,286,287 Interestingly, a similar absence of liquid-state spatial heterogeneity was found both theoretically and experimentally for ILs with short to medium-sized ether-containing alkyl substituents, in contrast to their isoelectronic alkyl-substituted counterparts.273−275,288−290 This is because the ether-containing substituents are more polar than simple alkyl chains so that the polarity difference with the cationic core is lower; the ether-containing chains can even be involved in dipolar interactions and possibly (intra- and/or M


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intermolecular) hydrogen-bonding interactions with the cations, in competition with the anions.275,288,291−296 It raises the interesting question as to whether the analogues of [Cnmim]+ (n > 11) salts with long oligo(ethylene oxide) chains instead of simple alkyl substituents (i.e., [Rmim][X] with R = (CH2CH2O)m(CnH2n+1), m ≥ 4, n ≥ 1 or 0) also display LC properties, an issue which, to the best of our knowledge, has not yet been investigated. Several oligo(ethylene oxide)-containing imidazolium salts were found to be ILs (not always roomtemperature ILs), but potential mesomorphism was not investigated.294,297−300 The flexible oligo(ethylene oxide) chains can lower the melting point of salts as well as their viscosity,288,289,291,292,301−303 but the degree of amphiphilicity of such salts may not be high enough to induce thermotropic mesomorphism. This issue can be resolved by including a sufficiently long hydrophobic alkyl chain in the structure: Lin and co-workers demonstrated that [(H(OCH2CH2)m)Cnim][Cl] salts show SmA phases for m = 1 with n ≥ 10; m = 2 with n ≥ 12; and m = 3 with n ≥ 16.304,305

Figure 4. Stabilization of a columnar mesophase (and induction of a cubic mesophase) by incorporation of charged moieties.349

electrostatic interactions partially compensate the unfavorable influence of the bulky ferrocene core. Induction of columnar mesomorphism was found for a similar system.351 Even more compelling is recent work of Kaszynski and co-workers. They compared the thermal phase behavior of ionic mesogens with that of isosteric nonionic systems.352 The ionic systems contained a substituted pyridinium cation and a 1,10disubstituted [closo-1-CB9H8]− or 1,12-disubstituted [closo-1CB11H10]− anion; the nonionic mixtures consisted of two neutral rod-like molecules, one of which has a neutral {closo-1,10C2B8H8} or {closo-1,12-C2B10H10} fragment (Figure 5; see also

4.3. Mesophase Behavior of Thermotropic Ionic Liquid Crystals

4.3.1. Influence of Charges on the Mesophase Behavior. The presence of charges in ILCs has a profound influence on the mesophase behavior of these compounds. Purely electrostatic (Coulombic) interactions between ions constitute very strong noncovalent interactions (Table S1). Intra- and intermolecular associations based on ion-pairing control, for instance, the formation of host−guest systems, protein conformations, and the assembly of layered polyelectrolytes.306 Ionically cross-linked supramolecular networks were developed as well.307 Electrostatic interactions are isotropic forces, which means that they are not directional and predictable in the same manner as hydrogen bonds or metal−ligand bonds. [Remark: In contrast to Coulombic interactions, hydrogen bonds are short-range, directional, and selective interactions. In the strictest sense, they only occur between N, O, and F atoms, with the link being a shared H atom. However, a special class of hydrogen bonds are “ionic” or “charge-assisted” hydrogen bonds between ions and molecules, with bond strengths of 20−160 kJ mol−1.148,308,309 “Nonclassical” hydrogen bonds such as the C− H···X− and C−H···X interactions found in many ILs and ILCs can be considered to belong to this class.310−341] Introduction of charges is a very efficient way to increase the degree of amphiphilicity of LCs342,343 and to reinforce nanosegregation, which in turn can lead to an enhanced mesophase stability. A similar strategy has been adopted in the design of LC dendrimers.344,345 Smectic ordering in ILCs is stabilized to a large extent by nanosegregation, with orientational ordering typically being less important than in conventional smectic LCs (see also section 4.3.2).346,347 Ujiie et al. compared the stabilizing effect of the ionic “domains” or “sheets” on the smectic order of ILCs with the effect of the polymer backbone of a nonionic LC side-chain polymer.348 It was found that incorporation of imidazolium ion functionalities into the alkyl side chain termini of a triphenylene derivative resulted in stabilization of the columnar mesophase (as well as induction of a cubic mesophase) (Figure 4; see also section 5.3).349 Deschenaux et al. showed that electron transfer can be used in the ferrocene−ferrocenium redox system to induce mesomorphism: the neutral ferrocene compound is not LC, while the ferrocenium compound with a tosylate counterion shows a monotropic SmA phase.350 In this case, favorable

Figure 5. Stabilization of (lamellar) mesophases by creating favorable electrostatic interactions.352

section 8.5). The ionic compounds show lamellar mesophases and a clearing point that exceeds 200 °C. The neutral mixtures have a much lower melting point, but they only show a monotropic nematic phase with a clearing point that is about 181 °C lower as compared to the ionic systems. These phenomena are general for ILCs, as will be further discussed below. Another striking example of stabilization of mesophases due to electrostatic interactions can be found in the work of Piechocki et al. on LC bis[octa(alkyloxymethylene)phthalocyaninato]-lanthanide(III) complexes.353 For instance, [(C12H25OCH2)8Pc]2Lu forms a Colhex phase between 24 and 30 °C, whereas the chemically oxidized analogue [[(C12H25OCH2)8Pc]2Lu][SbCl6] shows the same phase between 13 and 118 °C. Even more N


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striking is the case of [(C8H17OCH2)8Pc]2Lu, which is not LC and melts at 25 °C, whereas [[(C8H17OCH2)8Pc]2Lu][SbCl6] forms a Colhex phase between 10 and 130 °C. Cho and coworkers could induce lamellar, cubic, and columnar mesomorphism in neutral, nonmesomorphic block molecules (both small molecules and codendrimers) via ion doping of the hydrophilic block that consisted of oligo- or poly(alkylene oxide) chains.354−357 The ion doping leads to enhanced nanosegregation and formation of LC mesophases. Cabezón et al., on the other hand, reported suppression of the mesomorphism of a triazolephthalocyanine after introduction of a charged group via quaternization of the triazole fragment with 1-bromododecane.358 This was explained by the fact that the ionic nature of the charged compound increases its melting point beyond the thermal decomposition temperature, although steric effects due to the bulky dodecyl substituent cannot be ruled out in this particular case. The viscosity of the mesophases shown by ionic mesogens is usually quite high, although few quantitative data are available.80,359−361 It is known that the viscosity of an IL can vary with electrostatic forces, molecular weight of the constituting ions, hydrogen bonding, van der Waals interactions, as well as geometry of cation and anion.362 For ILCs, the type of LC phase that is formed is an additional factor. Usually viscosity and melting point tend to increase with increasing charge valency of the constituting ions, due to increasingly stronger electrostatic interactions.363 In general, the strong ionic interactions tend to stabilize lamellar mesophases due to nanosegregation of nonionic parts and strongly bonded ionic parts, and the formation of “ion-rich” layers. The SmA phase is by far the most commonly observed phase for ILCs. Other smectic phases are less common, but many low molecular weight ILCs do display one or more ordered smectic phases (whether or not in combination with a highertemperature SmA phase). Bazuin suggested that the ordered LC mesophase that is shown by some low molecular weight ion pairs with small anions can disappear in favor of a disordered smectic phase after ionic complexation with an anionic polymer backbone.364 It is remarkable that tilted smectic phases, such as the SmC phase, are seldom observed for truly ionic mesogens,116,128−130,133,365−381 while these are common phases for neutral LCs.2,382 Some disk-like ionic mesogens, obtained via ISA and carrying “switchable” side chains, were shown to exhibit lamellar phases instead of expected columnar phases.153,155 Apart from electrostatics, there are also other factors that have to be taken into account to understand the mesophase behavior of amphiphilic salts. Steric interactions, for instance, are equally important. To form a stable thermotropic lamellar mesophase, the cross-sectional area of the polar part (i.e., the ionic headgroup) as projected onto the ion-rich planes should be balanced by a sufficiently large apolar part. This can be easily understood by considering the theory of Israelachvili about the self-assembly of amphiphiles with hydrocarbon chains in a fluid state (geometric packing parameter model).383,384 In this concept, packing constraints are considered by the critical parameter P = v/(a0lc), where a0 is the (polar) headgroup area, and v and lc are the volume and (extended) length of the alkyl chains, respectively. For a lamellar arrangement, P should roughly equal 1. It is useful to refer to previous studies by Weiss and co-workers on neat, monocationic quaternary ammonium and phosphonium salts carrying one, two, three, or four equivalent long n-alkyl chains.282,385 Figure 6 summarizes their models for the packing of these salts in their crystalline phases

Figure 6. Schematic representation of lamellar packing arrangements of monocationic quaternary ammonium and phosphonium halide salts with one to four equivalent long n-alkyl chains, as deduced from singlecrystal structures and PXRD studies (R = (CH2)xH where x ≥ 10; R′ = (CH2)yH where y ≤ 4 (typically y = 1); Y = nitrogen or phosphorus; A− = halide anion).282 Although models b and c for the packing of the threechain compounds in their LC phases cannot be ruled out, model a is preferred because of more favorable electrostatic interactions within the ionic sublayers. The concepts are also applicable to related salts with other cationic cores. Reprinted with permission from ref 282. Copyright 2000 American Chemical Society.

and smectic LC phases. These models are generally applicable to related amphiphilic thermotropic ILCs without an extended rigid core. The alkyl chain volume (i.e., number and length of alkyl chains) determines the placement of the long alkyl chains (i.e., either all on the same side of the ionic planes, or on opposite sides of the ionic planes), as well as the degree of interdigitation between alkyl chains of neighboring molecules. The different architectures can in principle only show a smectic mesophase if the ionic headgroups and the alkyl chains are able to project comparable cross-sectional areas onto the ionic sublayer planes. As such, the salts with three equivalent long n-alkyl chains require larger anions than the corresponding salts with only two long nalkyl chains to exhibit a smectic LC phase.282,386 It is important to emphasize that the anions of amphiphilic salts play a very important role in the packing of the cationic parts within crystalline and thermotropic LC phases, unlike in lyotropic mesophases. Interestingly, the preferred mode of organization in the smectic phase, which maximizes electrostatic interactions within the ionic planes, generally involves the formation of ionic bilayers (with the ionic headgroups interacting in a “head-tohead” arrangement), even in the case of the compounds with three long n-alkyl chains and one small methyl substituent, which form ionic monolayers in the crystalline solid state (at least as seen in the single-crystal structure of N,N,N-tris(n-octadecyl)-Nbenzylammonium bromide387) (Figure 6). True LC mesophases have not been found for symmetric tetrakis(n-alkyl)ammonium and -phosphonium halides.2 In sections 5.1 and 5.4, where relevant work by Douce and co-workers is discussed, we will further elaborate on the importance of a good balance between the cross-sectional areas of the polar and apolar parts of amphiphilic salts. The SmA phase of ionic mesogens is regularly seen as a socalled oily streak texture14 by POM, especially during heating runs. On cooling from the isotropic liquid phase into the SmA phase, spontaneous homeotropic alignment is common. This may be due to interactions between the cationic headgroups and the glass substrate surface (orientational adsorption to the O


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surface), and to subsequent ordering of the attached hydrophobic chains or mesogenic groups.2,348 Ujiie et al. considered ILCs to be “perpendicular alignment agents” and correlated their behavior with the role of surface-active agents such as alkylammonium halides.348 In 1972 already Proust et al. showed that a suitably oriented monolayer of N-(n-hexadecyl)-N,N,Ntrimethylammonium bromide (CTAB) on a suitable substrate could induce either homeotropic or planar orientation of a bulk nematic LC sample on top of it, via physicochemical coupling (“anchoring”388).389 Just as for conventional mesogens, the alignment of ILCs can in many cases be influenced by the specific sample preparation. For example, by inserting the sample between two large microscopy glass slides rather than between a large glass slide and a small coverslip (which exerts less pressure on the underlying sample), one can distort the homeotropic orientation. Naturally, surface treatment of the glass substrates (e.g., with covalently bonded silanes or strong acids or bases) can also have a tremendous impact on the ordering of ionic mesogens.390 Maeda and co-workers reported on homeotropic alignment of a columnar phase consisting of alternately stacked positively and negatively charged planar ions under the influence of an electric field.391 Macroscopic alignment (planar alignment in particular) of ILCs through, for example, simple shearing is often hampered by their high viscosity. More than two decades ago, the so-called crystal smectic T phase of 2D square symmetry (or tetragonal symmetry, and, hence, smectic “T”, although tetragonal refers in principle to 3D symmetry; Table 1) was observed for the first time during the study of the thermal behavior of N,N-bis(n-alkyl)-N,Ndimethylammonium bromides.392 [Remark: According to Przedmojski and Dynarowicz-Łat̨ ka, the T phase is only shown by the homologues with two n-hexadecyl or two n-octadecyl chains; the compounds with two n-decyl, two n-dodecyl, or two n-tetradecyl chains were said to exhibit crystal smectic E phases.393] Since then, several other ILCs have been shown to exhibit a T phase.115,361,365,394−401 We are not aware of any experimental reports on the crystal smectic T phase involving nonionic LCs, but there seems to be no reason to believe that it cannot be formed by neutral mesogens as well. Houssa et al. theoretically predicted the formation of T phases by calamitic mesogens (of a specific aspect ratio) with a strong terminal dipole (similar systems lacking the strong terminal dipole showed a smectic phase with hexagonal instead of square inplane ordering).402 In an earlier theoretical paper by Zannoni and co-workers, a tilted variant of the T phase shown by rod-like molecules with two terminal axial dipoles was mentioned.403 During Monte Carlo simulations, Józefowicz and Longa also observed local “frustration-free” square intralayer ordering for calamitics with a sufficiently strong off-center longitudinal dipole.404 In the highly ordered T phases shown by ILCs, there is a periodical square lateral arrangement of the cationic headgroups and anionic moieties inside the smectic ionic sublayers, while the alkyl chains in the well-segregated aliphatic sublayers are disordered (partial melting was suggested by temperature-dependent FT-IR spectroscopy394) (Figure 7). Formally, the phase can be seen as a genuine crystal phase exhibiting 3D positional order, at least within the ionic sublayers (see also below). In practice, it is soft and flowing (although quite viscous) thanks to the largely molten alkyl chains and to the fact that the smectic layers can slide over each other rather easily. With only a few exceptions,399−401 T phases have only been observed for salts that (i) contain cations which carry one, two, or more nondelocalized charges and whose (whether or not

Figure 7. Schematic representation of the structure of the crystal smectic T phases formed by [Cnmpyrr][Br] salts (n = 11−20). Left: Unit cell of the tetragonal lattice (the carbon and hydrogen atoms of the pyrrolidinium ring and the methyl group on the nitrogen atom of the pyrrolidinium ring are omitted for clarity). Right: Two ionic sublayers separated by the aliphatic continuum; d is the thickness of the smectic layers. Reprinted with permission from ref 115. Copyright 2009 Wiley.

heterocyclic) headgroups have an average sphere-like or cylinder-like shape under conditions of rotational molecular motion (e.g., N-(n-alkyl)-N-methylpyrrolidinium, N,N-bis(nalkyl)-N,N-dimethylammonium, etc.); and (ii) contain anions which either have a spherical shape (e.g., Br−, [BF4]−, and [PF6]−) or consist of a spherical negatively charged headgroup connected to a linear alkyl chain (as in the case of n-alkylsulfate anions). Slight deviations from these structural features result in a breakdown of the square intralayer order that would result from the strong, localized Coulombic interactions, and in a transition to a SmA phase. For example, 1,4-piperazinium n-alkylsulfates show T phases, whereas their less symmetric 1,5-homopiperazinium n-alkylsulfate counterparts form only SmA phases,396 and [C 18 mpyrr][SCN] ([C 18 mpyrr] + = N-methyl-N-(noctadecyl)pyrrolidinium), which contains a linear [SCN]− anion, exhibits only a SmA phase, while both a T phase and a SmA phase are found for [C18mpyrr][Br] (Figure S18),115 but [C18mim][Br] shows only a SmA phase (Figure 16a). The number of alkyl chains in the structure of the ionic mesogen also plays an important role: T phases were observed for [C 14 mpiperid][Br] ([C 14 mpiperid] + = N-methyl-N-(ntetradecyl)piperidinium; 1 chain), for [C 14 mpiperid][C12H25OSO3], and for [C14C14piperid][Br] ([C14C14piperid]+ = N,N-bis(n-tetradecyl)piperidinium) (both 2 chains), but not for [C14C14piperid][C12H25OSO3] (3 chains), which shows a Col h e x phase. 3 9 7 [C 1 4 C 1 4 mmpiperaz][C 1 2 H 2 5 OSO 3 ] 2 ([C14C14mmpiperaz]+ = N,N′-dimethyl-N,N′-bis(n-tetradecyl)piperazinium; 4 chains), on the other hand, shows a SmA phase.397 [C14C14mmpiperaz][Br]2 (2 chains) decomposes before melting, but its DABCO-based counterpart, 1,4-bis(ntetradecyl)-1,4-diazoniabicyclo[2.2.2]octane dibromide (2 chains), shows a T phase.395,397 The transition from a crystal smectic T phase to a smectic A phase is accompanied by the loss of long-range lateral order inside the ionic sublayers, as indicated by the disappearance of any sharp peaks in the PXRD diffractogram that would be related to an ordered arrangement of the cations and anions. For most SmA phases exhibited by ionic mesogens, no such signals are visible at all, despite the P


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groups within the smectic layers, whereas the ionic headgroups are locally arranged (within ionic sublayers) in a 2D square network. When the tilt angle of the rigid moieties with respect to the normal to the smectic layer planes (or, more precisely, the sublayers formed by the ionic headgroups) equals the “magic angle” of 54.7°, it is indeed mathematically possible that these moieties are arranged laterally in a hexagonal fashion, while the (cat)ionic parts to which they are connected via a flexible spacer are arranged laterally in a 2D square fashion.365 It should be noted that, despite the physical separation of the ionic sublayers in smectic phases of ILCs, interlayer correlations (indicated by hkl (l ≠ 0) reflections in PXRD) can exist due to long-range Coulombic interactions when the distance between the sublayers is not too large (i.e., when the alkyl chains forming the aliphatic sublayers are not too long). This can be the case for ordered smectic phases, wherein the ionic moieties are periodically distributed inside the ionic sublayers.394,408 Interlayer correlations have been observed, for example, for some T phases.115,394,401 Despite such correlations, these mesophases are usually still soft and flowing, although appreciably viscous. Indeed, the Coulombic interactions are stronger inside the polar layers than between them, and so the smectic layers can slide over each other rather easily. As far as the columnar phases are concerned, the hexagonal columnar phase Colhex is not at all unusual for ionic mesogens. In these phases, charge alternation is guaranteed along the columnar long axes, that is, parallel to the normal to the 2D hexagonal lattice. The ionic mesogens are stacked directly on top of each other within the columns (with the ionic parts in the column centers or at the peripheries). This is not possible in SmB phases, where ionic sublayers with 2D hexagonal ordering would be separated from one another by disordered alkyl chains. In this context, it is interesting to note that a so-called hexagonal channeled layer phase (ChLhex) has been observed for both Tshaped facial amphiphilic alkali metal carboxylates415 and supramolecular ionic dendritic complexes of a T-shaped facial amphiphilic carboxylic acid with a second-generation DAB poly(propylene imine) (PPI) dendrimer.416 This mesophase combines a layer structure with a hexagonal organization of columns. It is a viscous, optically uniaxial phase with a 3D lattice. The terphenyl cores of the carboxylic acids are organized in layers, which are separated from one another by aliphatic sublayers, whereas the “laterally attached” polar moieties are segregated into undulating columns, which are arranged in a 2D hexagonal lattice (Figure S3). The columns penetrate the layer structures perpendicularly, and, in this way, the polar groups are not separated from one another. For the dendritic complexes, it was found that increased protonation of the dendritic core (∼an increased charge density) gives rise to a transition from a “purely” columnar phase to a ChLhex phase.416 Interestingly, many examples of the relatively rare square columnar phase Colsqu involve ionic mesogens. Colsqu phases have been observed, for instance, for supramolecular ionic dendritic complexes of alkanoic acids or T-shaped facial amphiphilic carboxylic acids with first- to fifth-generation DAB PPI dendrimers;416,417 for ionic dendritic complexes of anionic lipids with cationic dendronized polymers;418,419 for an ionic complex of ethyl orange and N,N-bis(n-hexadecyl)-N,Ndimethylammonium;156 for ionic complexes of negatively charged DNA with positively charged PPI dendrimers;167 and for an ionic complex of a planar, π-conjugated, negatively charged (oligopyrrole receptor)-anion complex and a planar 4,8,12trialkyl-4,8,12-triazatriangulenium (TATA) cation.420

commonly high electron density associated with the anions (such as Br−, [NTf2]−, etc.). The tetragonal mesophase symmetry is rather unusual, as the most encountered mesophases that are characterized by a 2D ordering of the mesomorphic molecules have a hexagonal symmetry: this is true for the SmB, SmI, SmF, B, J, and G phases (Table 1), as well as the Colhex phase. However, ordering of singly charged ions on a square or rectangular 2D lattice, with cations and anions at the corners and centers, allows a strict alternation of positive and negative charges (in a manner similar to that found in the faces of the face-centered cubic crystals of alkali halides). When both the cations and the anions are relatively large and/or anisometric (and assuming that they do not form tight ion pairs or a zwitterion), a hexagonal 2D ordering of the ions seems to be less favorable. Indeed, for ILCs consisting of a singly charged cation and anion, only very few examples of a (Sm)B phase have been reported.405−411 In the case of the guanidinium n-alkylsulfonates 317-0/n (see section 10.4), hydrogen bonding plays a very important role in the formation of the (Sm)B phases.408 Bazuin et al. reported a SmB phase for N(n-hexadecyl)-4-cyanopyridinium iodide, but this phase has an unusual partial bilayer honeycomb structure that is associated with interactions between the polar cyano substituents and involves pairing of the iodide anions.411 It is striking that addition of even small amounts of the ionic compound 1-(n-heptyl)-4-(4pyridyl)pyridinium bromide to the neutral LC trans-4-(noctyl)cyclohexane-1-carboxylic acid, which forms a SmB phase and a nematic phase, causes the disappearance of both phases in favor of a SmA phase.412 This is not the case for mixtures of the same carboxylic acid and an anisometric neutral hydrogen-bond acceptor.413 Addition of a small amount of LiOTf to a neutral rod−coil mesogen leads to the disappearance of its SmB phase in favor of the SmA phase.414 It should be noted that it is not trivial to distinguish between (Sm)B and T phases. A phase type assignment based on POM observations is very difficult for ordered smectic phases, and even with PXRD results obtained on nonaligned samples, one should be very careful. The PXRD diffractograms of unaligned B and T phases are very much alike: in both cases, a rather sharp signal is observed in the medium- to wide-angle part, superposed on the broad signal due to the molten aliphatic chains; this signal is indexed as the (100) reflection from a 2D hexagonal lattice for B phases, and as the (110) reflection from a 2D square lattice for T phases.115,405 Additional medium- to wide-angle reflections might be observed for highly ordered (crystal) smectic B phases. For the T phases, at least one and nearly always two additional signals (the (200) and (210) reflections from a 2D square lattice) are found in practice in the medium- to wide-angle region.115,365,392,394−397,401 Depending on the exact molecular structure and the order developed in the mesophase, even more wide-angle reflections ((010), (111), (020), (112), etc.) are observed. Yet because these additional reflections are often rather weak (and sometimes only visible at high 2θ values), these signals might easily be overlooked. Tilted SmI and SmF phases, with a pseudohexagonal packing of the molecules, are virtually nonexistent for ILCs. To the best of our knowledge, there are only three reported examples: some ammonium bromide compounds with a pendant nitrostilbene mesogenic group;365 a tetrabromouranyl salt with two pyrrolidinium cations, each carrying a three-ring rod-like mesogenic group; 116 and two dicationic ILCs with a pentaphenylene mesogenic group.374 In the first two cases, there is a pseudohexagonal ordering of the tilted rigid mesogenic Q


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random hyperbranched poly(ethylene imine) (PEI) and fully methylated poly(ethylene imine) (PEIMe) polymers, were reported by Frey and co-workers455 and by Serrano, Marcos, Barberá, Sánchez, and co-workers.456−460 The anhydrous silver(I) stilbazole complexes with short alkyloxy chains reported by Bruce et al. exhibit a nematic phase, but these complexes are only formally ionic because the compounds form tightly associated ion pairs (a very low conductivity is measured in the mesophase).421,461−468 Similar tight ion pairing may explain the conservation of the ND phase when LiCl rather than LiOTf is added to the oligo(ethylene oxide)-substituted discotic mesogens reported by Kohmoto et al. (see above).427 Silver(I) complexes containing cholesteryl moieties,468 quaternized cholesteryl isonicotinates with tosylate counterions,469 axially chiral pyridinium salts with a 1,3-dioxane ring in the central core,369 cholesteryl-containing imidazolium tetrachloroaluminate(III) salts,114,470 and poly(siloxane)s containing both cholesteryl and imidazolium groups471 were shown to exhibit a chiral nematic phase. Nematic phases have also been observed for zwitterionic 1,2,4-triazine-4-oxides,472 zwitterionic LCs based on a sydnone core,473 and zwitterionic derivatives of anionic boron clusters.474−479 However, it can be expected that these compounds, in their pure form, do not show some of the interesting properties of “true” ILCs, such as high ionic conductivity. From the viewpoint of applications, nematic ILCs are potentially very interesting: the nematic phase is the least viscous mesophase, and typically the constituting molecules are easily aligned. Nematic ionic mesogens could be useful, for example, to increase the efficiency and selectivity of selected chemical reactions and electrochemical processes, and could be used as medium- to low-viscosity switchable LC electrolytes. 4.3.2. Theoretical Studies. In his 2005 review, Binnemans pointed out that theories that could explain the influence of the charge distribution on the mesophase type and mesophase stability of ILCs were lacking.2 Such theories would support the rational design of ionic mesogens that show uncommon mesophases. In 2010, Harnau and co-workers presented a theoretical approach based on density functional theory (DFT).480 They modeled the LC molecules as charged prolate hard ellipsoids of length L and width R (Figure 8). Concerning

Nematic phases are very uncommon for ILCs, as the orientationally ordered nematic phase is stabilized by weak, anisotropic dispersion forces, and not by strong, isotropic electrostatic forces that cause strong intermolecular interactions.2,292,421 In the case of type (a) compounds (Figure 3), excluded volume effects induced by a rigid anisometric molecular core, which constitute an important driving force for nematic phase formation in conventional LCs, can play no role. Even compounds of types (b) and (c) almost exclusively exhibit smectic (or columnar or cubic) phases, even when the mesogenic moiety used is a strong nematogen. We already mentioned that addition of small amounts of an anisometric pyridinium bromide salt to neutral carboxylic acid LCs causes the disappearance of the SmB and nematic phases, shown by the latter, in favor of a SmA phase.412,422 In sharp contrast, an equimolar mixture of one of those nonionic LCs and a neutral pyridine derivative displays both a SmA phase and a nematic phase.423 Furthermore, a 50:50 mixture of a stilbazolium bromide and a neutral carbazolyl nematogen only shows a smectic phase.424 A SmA phase is also induced by adding the same ionic compound to a nematogenic polymer of the carbazole mesogen.425 Similar observations were made for mixtures of salts 274-R9, 275-R9, and 279-R9 (see section 8.5) with a neutral nematogen.426 Kohmoto et al. demonstrated how a previously unknown phase transition from a discotic nematic (ND) phase to a non-nematic Colhex phase can be effected through the addition of an alkali metal triflate salt to a neutral discotic mesogen decorated with oligo(ethylene oxide) chains.427 This is not only due to the “straightening” of these chains by metal ion complexation; the stacking into columns also originates from the sequential electrostatic interactions between the ionic moieties in the direction of the columnar axes. Nevertheless, several examples of low molar mass thermotropic ILCs displaying a nematic phase, or a (not commonly observed) columnar nematic phase, can be found in the literature.121,122,134,156,369,428−445 [Remark: The quaternary ammonium halide salts based on tris(n-octadecyl)amine that were originally claimed to show a nematic phase432 were later reported to show a SmA phase.282,387] Veber and co-workers proposed that the formation of the NC phase by some tricatenar dithiolium salts can be rationalized by the fact that the columns in the lowertemperature Colhex and Colrec phases are stabilized by ionic interactions, so that the 2D hexagonal lattice can “melt” without complete destruction of the columns.2,433 Several neutral nematogenic calamitic LCs with a crown ether moiety and/or short oligo(ethylene oxide) chains were found to dissolve large amounts of LiBF4, NaOTf, or sodium picrate (with complexation of the alkali metal cations by the crown ether moiety if present) and still exhibit a nematic phase.446−450 On the other hand, mesomorphism was lost upon addition of small amounts of LiBF4 to some derivatives with a lower shape anisotropy.451 Yoshio et al. mentioned a short-range nematic phase for an equimolar mixture of an IL ([C2mim][BF4]) with a neutral, hydroxyl-terminated LC.215 Yousif et al., Al-Dujaili and coworkers, and Tong et al. reported nematic mesomorphism for some ionic polymers.452,453,913 Deschenaux and co-workers found a nematic phase for an oxidized ferrocene-containing sidechain LC poly(methacrylate) (the ferrocene groups are oxidized to ferrocenium moieties).454 Surprisingly, the reduced, neutral form of this polymer shows SmC and SmA phases but no nematic phase. Presumably the substantial size of the I3− counterions plays an important role in disrupting the layered organization in this case. Nematic ammonium salts of poly(propylene imine) (PPI) and poly(amidoamine) (PAMAM) dendrimers, and

Figure 8. Side view of two prolate ellipsoids of revolution with orientations ω1 and ω2, representing two LC molecules in the theoretical description of the phase behavior of ILCs by Harnau and co-workers.480 Black and white circles represent possible charges in the center of the particles and at the tail ends, with valencies zc and zt, respectively. Adapted with permission from ref 480. Copyright 2010 AIP Publishing LLC. R


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considered as a combination of 19 particles or “interaction sites” (Figure 9): the imidazolium ring (A), the N-methyl group (B),

the charge distribution, two general cases were investigated: fluids consisting of particles (i) with a single point charge located in the center (zc), or (ii) with two like point charges located at the tail ends (zt), each at a distance D from the particle center. The intermolecular pair potential was expressed as a sum of the contributions from excluded-volume interactions and from longrange interactions; the latter were considered in terms of the Gay−Berne pair potential and the screened Coulombic interaction (effect of small counterions). One of the most interesting findings is that nematic ordering should be stable for molecules with a single charge located in their center or with two like charges with D = 1.4R (in combination with a sufficiently anisometric shape); for D = R, on the other hand, the SmA phase (no other smectic phases were considered) is the only stable phase at high packing fractions. The stability of the nematic phase could be improved by an increase in strength of the Coulombic interactions (at least for particles with a single charge). Increasing the length L of particles with two like charges located at a fixed distance L/2 − D from the end of the particles had a stabilizing effect on the SmA phase. Suitable molecules should now be designed to verify these theoretical predictions. Furthermore, an extended theory that includes a quantitative treatment of the counterions (which can also show an anisotropic charge distribution) would be of interest. Shortly after the report of Harnau and co-workers, Ganzenmüller and Patey presented a molecular dynamics (MD) simulation study of an electroneutral mixture of oppositely charged oblate ellipsoids (height-to-width aspect ratio = 1/3, single point charge located in the center of each particle).481 Instead of the Gay−Berne potential, they used an anisotropic RE2-pair potential. For this system, they found, besides a high-temperature nematic phase, a “charge-ordered” SmA phase in which particles with charges of equal sign are (counterintuitively) packed in distinct sublayers. Interestingly, the same system without any charges (in which the particles only interact via a repulsive potential) exhibited only a nematic phase. Furthermore, the charged system evolved on cooling toward a crystalline solid of hexagonal columnar character, consisting of stacks of alternatingly charged disks, which are shifted parallel to each other, such that particles of opposite charge are also found laterally (i.e., the “charge layers” are destroyed). Saielli performed an interesting MD simulation of the thermotropic mesomorphic behavior of 1-hexadecyl-3-methylimidazolium nitrate ([C16mim][NO3]).347 The study was meant as an assessment of the performance of a coarse-grained force field that was originally developed and parametrized by Wang and Voth for the simulation of the isotropic liquid phase of imidazolium-based ionic liquids with one short alkyl chain.234,482−484 As an alternative to computationally costly fully atomistic models, coarse-grained force fields are useful to get a semiquantitative description of computationally demanding systems such as ILCs, which are usually quite large molecules. These materials are also more viscous (thus requiring simulations on long time scales) than simple liquids because they are more ordered and show strong electrostatic interactions, and the entire phase diagram is of interest in this case.485 It should be mentioned that an atomistic MD simulation of [C8mim][C8H17SO3] (3-8/8) at 400 K did show the incipient formation of layers related to its SmA phase.259 The smectic phases shown by three bis(n-alkyl)-tris(imidazolium) salts (50-X-20) were also studied via atomistic MD simulations.486 In the coarse-grained model potential used by Saielli, the [C16mim][NO3] molecules are not simplified to charged hard ellipsoids, but they are

Figure 9. Fully atomistic model (left) and coarse-grained model (right) of [C16mim][NO3]. Reproduced with permission from ref 347 (http:// dx.doi.org/10.1039/c2sm26376a). Copyright 2012 The Royal Society of Chemistry.

the N-methylene group (M1), the next three methylene groups of the alkyl chain (M2, M3, and M4), the remaining 11 methylene groups of the alkyl chain (C), the methyl end group (E), and the anion (D). The force field consisted of these nine different interaction sites, each characterized by a charge and each pair characterized by a tabulated short-range interaction potential resulting from the coarse-graining procedure. The MD simulation with periodic boundary conditions was performed as a function of the temperature, both upon heating and cooling. It encompassed 512 molecules of [C16mim][NO3] inside a box that was rationally constructed on the basis of the knowledge that amphiphilic ionic mesogens without an extended rigid core and with a single long alkyl chain tend to form an ionic bilayer smectic phase with a “head-to-head” arrangement of the organic cations (see section 4.3.1), and making use of the known density and smectic layer thickness found for related salts. Several temperature-dependent features could be extracted from the MD simulations, including (i) the enthalpy, total energy, and volume per ion pair; (ii) density profiles along the director orientation z (Figure 11); (iii) orientational order parameters and (iv) translational order parameters (Figure 10); and pair distribution functions (PDFs) of the distance vector r between two

Figure 10. Orientational order parameter (○) and translational order parameter (■) of [C16mim][NO3] as a function of the temperature, as obtained from MD simulations. Reproduced with permission from ref 347 (http://dx.doi.org/10.1039/c2sm26376a). Copyright 2012 The Royal Society of Chemistry. S


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interaction sites projected onto either (v) the z axis (rz, i.e., parallel to the director) or (vi) the xy plane (rxy, i.e., perpendicular to the director, in a layer plane). Whereas data values (i) clearly show a first-order type transition just above 500 K but not a second transition, data series (ii), (iii), and (iv) also point to a second transition above 560 K (Figures 10 and 11),

current model potential ([C16mim][NO3] actually shows a SmA phase between 53 °C (326 K) and 183 °C (456 K) (Figure 16a)487). Nonetheless the simulation revealed interesting structural features and dynamics of the ionic smectic mesophase, such as the highly disordered conformation of the long alkyl chains and the very low orientational order parameter of the phase. [Remark: The orientational order parameter, S, indicates the degree to which the constituent molecules are aligned along the director and is defined as S = 0.5·⟨3 cos2 θ − 1⟩, where θ is the angle between the primary molecular axis of an individual molecule and the director n.] Highly disordered alkyl chains and low orientational order parameters have experimentally been found for many ILCs, including, for example, [C12mim][Br], azobenzene-containing guanidinium-based ILCs, N-benzylsubstituted imidazolium ILCs, LC N-arylsubstituted imidazolium salts, and smectogenic 3,5-diaryl-1,2-dithiolium salts.489−492 Recently, Saielli et al. tried to give a more detailed description of the transitions between the different phases of [C16mim][NO3].493 Rather than focusing on the long-range, “macroscopic” orientational and translational order parameters (see above) that are commonly used to characterize LC structures, they monitored the evolution of the “heterogeneity order parameter” (HOP), ⟨h⟩,235 for several coarse-grained sites during the simulations. The latter is more appropriate for quantifying the “microscopic” structural changes on the nanometer scale. It measures the deviation of a given site distribution from a perfectly uniform distribution and can be seen as a measure for the degree of segregation at the nanoscale level. HOP values that exceed 15.75 indicate the appearance of microphase segregation. As previously discussed in section 4.2 in the context of ionic liquids, microphase segregation occurs to a certain extent even in the isotropic liquid phase of the modeled [C16mim][NO3] (Figure 12). Upon cooling to the smectic A

Figure 11. Left: Density profiles along the director orientation z at 450 K (top, in the crystal phase of [C16mim][NO3]), 505 K (middle, in the SmA phase), and 600 K (bottom, in the isotropic liquid phase). Black, A site; red, E site; blue, D site; green, C site (see Figure 9). In the crystal phase, the site A profile related to the imidazolium rings has a double peak in each layer, due to the ionic bilayer structure. In the SmA phase, the profiles for the cation site A and the anion site D completely overlap: the translational order along the z direction has become limited to an alternation of aliphatic and ionic sublayers. The profiles for E and C in the SmA phase suggest that melting of the alkyl chains in the aliphatic sublayers has occurred. Right: Snapshots of MD simulations of 512 molecules of [C16mim][NO3] at the same temperatures. Reproduced with permission from ref 347 (http://dx.doi.org/10.1039/ c2sm26376a) and ref 488 (http://dx.doi.org/10.1039/c3sm50375e). Copyright 2012 and 2013 The Royal Society of Chemistry.

and the intermediate phase can be recognized as the LC SmA mesophase. The three distinct phases shown by the ILC were further characterized by the calculation of PDFs. As would be expected, the PDFs of rz showed some degree of order both in the crystal phase and in the smectic phase but lacked any order in the isotropic liquid phase, whereas the PDFs of rxy in the smectic phase and in the isotropic liquid phase showed some degree of similarity and differed from the ones in the crystal phase. This is because the smectic A phase has translational order along z but has a liquid-like structure in the xy plane. The simulated density profiles along z and the PDFs of rz were found to reflect pretty well the smectic layer thickness that had previously been found experimentally for other [C16mim][X] systems. Saielli also gave suggestions on how to reparametrize the coarse-grained force field to achieve a better modeling accuracy. His study showed that the phase transition temperatures (particularly the crystalto-smectic transition) are poorly reproduced/predicted by the

Figure 12. Evolution of the heterogeneity order parameter (HOP) (which is a measure for microphase segregation) for selected coarsegrained sites of [C16mim][NO3] (see Figure 9) as a function of the temperature during MD simulations. Reprinted with permission from ref 493. Copyright 2015 American Chemical Society.

phase, however, the HOPs for sites A, D, C10, and E (see Figure 9; C10 is the tenth carbon atom in the hexadecyl chain) all increase. This is particularly true for the HOP for the aliphatic tail sites E. Interestingly, the latter value drops significantly upon cooling to the crystalline phase. These observations were explained by means of the model depicted in Figure 13. In the crystalline state, the methyl end groups of the aliphatic tails show a relatively large separation and poor aggregation, whereas the cationic and anionic parts are much more ordered because of the T


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Figure 13. Schematic representation of the structure of the “nanodomains” of the alkyl chains in a sample of [C16mim][NO3], in the (from left to right) isotropic liquid, SmA, and crystalline phase. In the crystalline phase, the alkyl chains are highly ordered in a hexagonal arrangement, resulting in a high HOP value for the C10 sites. The methyl end groups of the alkyl chains (depicted as gray spheres), however, show a relatively large separation and poor aggregation, yielding a much lower HOP value for the E sites. Reprinted with permission from ref 493. Copyright 2015 American Chemical Society.

strong electrostatic interactions between them. In the isotropic liquid state, on the other hand, there exist spherical aggregates of chains with the end groups approximately in the center, but there must be several nonsegregated tails as well to avoid voids. The intermediate smectic A phase that has a layered structure but at the same time shows considerable disorder within each hydrophobic sublayer allows the largest extent of close contacts between methyl end groups (and thus segregation of these groups), explaining the higher HOP values for the E sites that were found in this phase. The HOP values for the ionic sites A and D are also relatively high, but smaller than for E, because the ionic groups are predominantly arranged in ionic sublayers, but in the smectic phase they still retain a continuous polar network to a certain extent by having a small amount of charged groups connecting adjacent sublayers. Overall, the authors were able to demonstrate qualitatively the importance of long-range microphase segregation as the driving force for the formation and stabilization of the smectic phase. They further argued that typical ILCs (without a pronounced anisometric rigid core) are in essence mesomorphic systems that try to maximize the extent of microphase segregation in their structure upon cooling, which explains the small number of ionic nematogens found experimentally (see section 4.3.1). A nematic phase cannot show a relatively high HOP value for the aliphatic end groups: such a phase cannot have sphere-like aggregates as in the isotropic phase (because molecules in a nematic phase should be oriented on average) and at the same time it lacks the possibility of an arrangement in layers where the tail end groups can have a degree of aggregation larger than that in the isotropic liquid phase (because then it would be a smectic phase). Low orientational order of individual mesogens seems to distinguish “typical” ILCs such as [C16mim][NO3] (ionic amphiphiles without an extended rigid core, type (a) in Figure 3) from conventional neutral LCs that do contain an extended rigid core and whose smectic phases show a higher translational and orientational order than their nematic phase due to some degree of translational−rotational coupling.494 As mentioned above, the smectic layer structure of rather flexible ILCs is stabilized to a large extent by nanosegregation, whereas high orientational order also contributes to the layering in conventional nonionic smectogens. The relatively large size of typical ILC anions (such as Br−, I−, [BF4]−, [PF6]−, [OTf]−, [NTf2]−, etc.) as compared to typical ILC cations also contributes to the high disorder of the alkyl chains and low orientational ordering. Weiss and co-workers proposed to exploit amphotropic ammonium- and phosphonium-based ILCs with low order parameters as partially ordered solvents for structural studies of organic molecules using NMR spectroscopy (see also section 12.3).432,495−499

The model system [C16mim][NO3] was also used to investigate the self-diffusion mechanisms that occur in an amphiphilic, smectogenic ILC that lacks an anisometric core.488 The self-diffusion anisotropy, that is, the ratio between the parallel and perpendicular diffusion coefficients (relative to the director), in the ionic SmA phase was found to be about 0.5 for both the cations and the anions, which actually compares to the values previously found for nonionic smectogenic LCs. The anisotropy is typically much more pronounced in lamellar phases of lyotropic surfactant/water systems, where the layers are not only spatially separated but also chemically distinct: typically a hydrophobic layer alternates with a water/ionic layer, which is actually similar to the structure of the thermotropic SmA phase of [C16mim][NO3]. Coarse-grained MD simulations showed that two types of diffusion of the constituent molecules occur parallel to the director of the smectic phase of that ILC: (i) diffusion through a “parking-lot” mechanism where the cationic headgroup acts as a pivotal site to displace its alkyl chain between two adjacent hydrophobic sublayers, but itself remains in the same ionic sublayer (Figure S4(a); this pivot mechanism results in a reorientation of the molecules, and is known to occur for mixtures of hard rods and spheres as well); but also (ii) direct “permeation” of ions from one layer to the other, via pore defects that connect two ionic sublayers (Figure S4(b)). The latter type of defects, which provide channels for smooth diffusion through the hydrophobic sublayers by a local “isotropization” of the structure, has been reported before for lamellar phases formed by surfactant/water systems. As such, ILCs like [C16mim][NO3] seem to share structural and dynamic features with both nonionic thermotropic LCs and lamellar phases of surfactant/water mixtures. The less pronounced self-diffusion anisotropy in smectic phases of thermotropic ILCs as compared to the latter seems very relevant with respect to the search for systems that show highly anisotropic ionic conductivity under the influence of an electric field (see below). The MD simulations also showed that neither the cationic headgroups, nor the anions can still escape from the ionic sublayers in the lower-temperature crystalline phase of [C16mim][NO3]. Some of the aforementioned studies were extended to include all members of the homologous series [Cnmim][NO3], with n equal to an even number between 6 and 22.276 Through coarsegrained MD simulations, it was shown that the nonmesomorphic ionic liquids with intermediate alkyl chain lengths ([Cnmim][NO3] with n = 6, 8, 10, 12, 14) have nanoscale spatial heterogeneities (with aggregated aliphatic tail domains separated by a continuous polar ionic network; see also section 4.2), and that this structure gradually evolves to a LC-like structure with increasing length of the alkyl chain ([Cnmim][NO3] with n = 16, 18, 20, 22). The simulations were carried out at 400 K; it is not U


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obvious to which “real” phase of the ILs and ILCs this temperature corresponds to. Nevertheless, it is clear that for the intermediate-chain salts, the strong long-range electrostatic interactions dominate over the short-range dispersive van der Waals interactions between the aliphatic chains, which makes the ionic headgroups form a continuous polar network that forces the nonpolar chains to aggregate and form scattered aliphatic domains (Figure S5, left). Upon going from a tetradecyl chain to a hexadecyl chain, however, the attractive van der Waals interactions between the long alkyl chains have become sufficiently strong to compete with the ionic interactions, and they cause the aliphatic chains to assemble themselves into twodimensional sublayers to lower the total free energy (rather than aggregating into separated domains) (Figure S5, right). The “critical” chain length of n ≈ 15 was deduced from a strong decrease in the HOP value for the coarse-grained tail sites (i.e., the E sites in Figure 9) upon going from [C14mim][NO3] to [C16mim][NO3] (Figure S6). It corresponds reasonably well to the experimental finding that [Cnmim][NO3] salts are only LC from the tetradecyl-substituted homologue on (although [C13mim][NO3] has not been studied thus far; Figure 16a).487 Two-dimensional phase diagrams as a function of both temperature and chain length will be the logical next step to provide a more complete picture of the temperature-dependent structural changes shown by ILs and ILCs. In the future, hopefully other systems will be investigated as well, using more advanced models that also incorporate the role of hydrogen bonding, with the final goal of developing a set of “rules” to predict ionic mesomorphism and the temperature stability range of the mesophase in silico as a function of molecular structure. Three other reports on ILCs also contain MD simulations of the ionic mesophase to support experimental PXRD data.486,500,501 These results will be discussed in sections 5.1, 5.2, and 6.1, respectively. A relatively unexplored426,502 way to achieve a better understanding of the (lack of) mesomorphism shown by ionic compounds is the calculation and evaluation of partial atomic charges in the constituting molecules via wave function population analysis, to get an idea about the charge distribution. Among the different available methods, the natural population analysis (NPA) output from natural bond orbital (NBO) analysis503−506 has emerged as a widely used technique in the field of ionic liquids.109,507−510 NPA of the natural atomic orbitals originating from the NBO analysis is much less basis-setdependent than standard population analyses such as Mulliken analysis or Davidson−Roby population analysis, and usually leads to a chemically intuitive picture.109 Although data provided by population analysis should not be overinterpreted and it is important to consider relative values rather than absolute values (partial charges are in principle artificial), such results can help in the understanding of specific cation−anion interactions that influence the mesomorphic behavior.426,502 NPA data are related to the molecular and atomic orbitals and atomic centers. Bader’s analysis of “atoms in molecules” (AIM) directly starts from the molecular electron density.511,512 This method can provide information about the topography of the charge distribution within a molecule, and this can be conveniently visualized in three dimensions, but the technique has, to the best of our knowledge, not yet been applied to the study of ILCs. 4.3.3. Note: Thermal Stability and Hygroscopicity of Ionic Liquid Crystals. Several reports have shown that the thermal stability of typical organic salts is strongly determined by the type of anion.115,116,180,362,381,490,492,513−526 For example,

thermogravimetric analysis (TGA) under nitrogen indicated a 28% weight loss for [C18mpyrr][Br] at 250 °C, whereas [C18mpyrr][NTf2] is thermally stable up to 365 °C under the same conditions.115 At least two modes of thermal decomposition of quaternary ammonium salts are known: the reverse Menschutkin reaction type and the Hofmann elimination type.521−523,525,527−534 The nucleophilicity of the anion plays a major role. For example, decomposition of 1,3-bis(n-alkyl)imidazolium salts can occur via nucleophilic attack on an alkyl substituent by the anion at elevated temperatures, and as a consequence anions of low nucleophilicity produce thermally more stable compounds.362 Often thermal decomposition of ionic mesogens occurs in two steps: first the cationic heterocycle is dealkylated to yield a neutral molecule, after which the residue degrades upon further heating.115,528,535,536 For 1,3-bis(nalkyl)imidazolium tetrachloropalladate(II) salts, thermal decomposition was found to generate palladium(II) dicarbene and carbene-imidazole complexes.537 Besides giving information about thermal stability, TGA allows one to get an idea about the water content of the ionic compounds, which can have a dramatic effect on the mesophase behavior (see section 5.1).381,492 The water content can also be checked by elemental analysis, by NMR spectroscopy and by consideration of single-crystal structures (if these are available). Tschierske and co-workers described a coulometric Karl Fischer titration setup to determine the water content of amphiphilic LCs.538 Particularly halide salts are very hygroscopic, especially the chloride539 and bromide compounds. Anhydrous samples of [C12mim][Br] and [C14mim][Br], for example, are slowly converted into the corresponding monohydrates when exposed to air.540 Iodide anions are much weaker hydrogen-bond acceptors. It was shown by TGA that [C12mim][Br]·H2O and [C14mim][Br]·H2O start to lose their crystal water at about 50 °C (at a heating rate of 10 °C min−1).540

5. IMIDAZOLIUM-BASED IONIC LIQUID CRYSTALS As pointed out before, the study of imidazolium ILCs grew rapidly in the last 15 years as a result of the growing research in the field of ILs, and because imidazolium salts can serve as Nheterocyclic carbene precursors (which are excellent ligands for transition-metal-based catalysts). The imidazolium cation is a flat, pentagonal ring. In combination with weakly coordinating anions, the ring bonds exhibit a pronounced aromatic bond character with a delocalized π-electron system (i.e., the positive charge is delocalized over the ring system, Figure 14a). However, NBO analysis indicates that in combination with strongly coordinating anions such as the chloride ion (which is able to form a hydrogen bond with the H(2) proton (Figure S7)), the ring bonds resemble more an Arduengo carbene: C(4)···C(5) possesses double-bond character, while N(1)···C(5) and N(3)··· C(4) exhibit single-bond character (Figure 14b).541 In the

Figure 14. Anion-dependent changes in hybridization of the imidazolium ring: (a) with a weakly coordinating anion; and (b) with a strongly coordinating anion.541 (c) Comparison with the structure of an Arduengo carbene (complete proton abstraction). V


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lying in the plane of the imidazolium ring in front of the C(2)− H(2) bond, and an “on-top” configuration with the anion located above the imidazolium ring plane in close proximity to the C(2)−H(2) bond.541 Only the latter is energetically favored for imidazolium salts with weakly coordinating anions and for imidazolium compounds in which the most acidic H(2) proton is substituted by an alkyl substituent. Counterintuitively, the latter type of salts show higher melting points and a higher viscosity than their counterparts with a H(2) proton, despite the fact that they have a lower tendency to participate in hydrogen bonding. This was explained by a concomitant loss in entropy, and by electron density changes leading to changes in the interionic interactions and reduced configurational variations.318,508,546,547 Ludwig and co-workers showed that strong, localized, and directional hydrogen bonds between cations and anions destroy the charge symmetry and thus can “fluidize” ILs.330,334 In other words, such hydrogen bonds introduce “defects” into the Coulomb network of cations and anions, thus increasing the dynamics of the ions and reducing the melting point and viscosity (in contrast to neutral hydrogen-bonded molecular liquids). Replacing the imidazolium H(2) proton with a methyl group essentially corresponds to replacing a strong, localized, and highly directional hydrogen bond in favor of a nonlocalized (“smeared out”) electrostatic interaction, or in other words the interaction type is shifted from short-range hydrogen bonding to long-range Coulombic interactions. Further support for this theory comes from the fact that 1,2,3,4,5-pentamethylimidazolium bis(trifluoromethylsulfonyl)imide shows an even higher melting point (118 °C) than 1,2,3-trimethylimidazolium bis(trifluoromethylsulfonyl)imide (106 °C), which itself has a much higher melting point than 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (between 22 and 26 °C).332,333,335 The single-crystal structures of several 1,3-bis(n-dodecyl)imidazolium ([C12C12im]+) salts were elucidated.548 It was found that, depending on the geometry of the negatively charged counterion, the disubstituted imidazolium cation can show different structures. It has a rod-like shape when combined with (spherical) I−, (linear) [I3]−, and (tetrahedral) [B(CN)4]−; a Ushape in combination with (planar) [C(CN)3]− and (V-shaped) [N(CN)2]−; and a V-shape when combined with I−, (V-shaped) [I5]−, and (octahedral) [SbF6]−. Such differences in solid-state structure can be expected to have an influence on the mesomorphism. Remarkably, the only mesomorphic salt in the latter series, [C12C12im][I] (Figure 16b, Table S2), contains two crystallographically independent cationic structures in its crystalline solid state: one with a rod-like shape and the other one with a V-shape. Imidazolium salts generally display a higher thermal stability than their pyridinium analogues, which tend to undergo Ndealkylation reactions more easily upon prolonged heating, particularly in the presence of nucleophilic counterions.549

remainder of the text, imidazolium cations will be represented as in Figure 14a, irrespective of the type of anion. In 1H NMR spectra, the chemical shift of the most acidic H(2) proton is moved downfield (corresponding to a higher ppm value) when it participates in a hydrogen bond. The H(2) chemical shift increases with increasing hydrogen-bond acceptor ability of the anion. Infrared spectroscopy measurements can also give an idea about the strength of hydrogen bonding, through the band frequency of the C(2)−H(2) stretching vibration in the mid-infrared region; direct spectroscopic observation of the hydrogen bonds is possible via far-infrared spectroscopy.330,331,333,334,542,543 The H(4) and H(5) protons can participate in hydrogen bonding as well, albeit to a lesser extent.541 It should be noted here that, particularly in the field of imidazolium-based ILs, van der Waals close contacts observed in single-crystal structures have often been incorrectly termed “hydrogen bonds”, although hydrogen bonds are strictly speaking directional in nature and have a distance/angle criterion.308−311,322−324,328,334,337−339,544 Being also more electrostatic in nature, hydrogen bonds are thermodynamically stronger than van der Waals contacts. For imidazolium-based ILs with small and coordinating anions (such as Cl− anions), it has been shown by X-ray photoelectron spectroscopy (XPS) measurements and theoretical calculations that partial charge transfer between the anions and cations due to orbital mixing results in a smaller effective positive charge on the imidazolium ring (i.e., a noninteger value smaller than +1e).541 This charge transfer is greatly reduced in the case of larger, weakly coordinating anions such as [NTf2]−. Moreover, hydrogen bonding and charge transfer occur independently from each other, although both are more pronounced for small and more strongly coordinating anions, which carry a more localized negative excess charge. As outlined in section 4.3.2, the fractional charges on the different atoms of the imidazolium cation can be obtained by DFT calculations on isolated ion pairs, more specifically by NBO analysis (NPA).502,507,541 In 1-methyl3-(n-alkyl)imidazolium salts, the positive charge resides quasi entirely on the imidazolium ring atoms and on the CH3 and CH2 groups directly connected to the imidazolium nitrogen atoms (nearly equal distribution) (Figure 15).541

Figure 15. Natural population analysis (NPA) charges on the CH2 units and on the terminal CH3 group of the octyl chain in 1-methyl-3-(noctyl)imidazolium salts, as a function of position and anion type. Reprinted with permission from ref 541. Copyright 2010 Wiley.

5.1. Imidazolium-Based Mesogens Having a Predominantly Amphiphilic Character

The first report on LC properties of long-chain-substituted 1,3bis(n-alkyl)imidazolium salts appeared in 1996.550 Extending the length of the alkyl chain(s) linked to the cation of an IL, to increase the degree of amphiphilicity, is a general approach toward ILCs. Another, less explored, strategy is the combination of a typical IL cation with one or more short alkyl substituents, with an anion with one or more long alkyl chains. Figures 16 and 17 display phase diagrams for [Cnmim][X]·xH2O (X− = Cl−,

Besides electrostatic, hydrogen-bonding, and charge-transfer interactions, dispersive and inductive interactions between cations and anions were also found to play a considerable role in imidazolium-based ILs.545 Quantum-chemical calculations generally yield two lowenergy conformers for imidazolium-based ILs with an unsubstituted C(2) atom: an “in-plane” configuration with the anion W


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Figure 16. (a) Phase diagrams for [Cnmim][Cl]·xH2O (0 < x < 1),280 [Cnmim][Br]·xH2O (0 < x < 1),122,280 [Cnmim][NO3],487 [Cnmim][BF4],171,230 [Cnmim][PF6],170,230,281 and [Cnmim][OTf].280 Additional literature references can be found in the text. The values that are shown here for [Cnmim][BF4], which were reported by Holbrey et al.,171 slightly differ from those reported by Wang et al.551 The values that are shown here for [Cnmim][PF6], which were reported by Gordon et al.,170 slightly differ from those reported by Xu et al.519 At high temperatures (>ca. 200−250 °C), thermal decomposition can be expected for the halide salts.537,540 The [Cnmim][BF4] salts are thermally stable until at least 250 °C.171 (b) Phase diagrams for [CnCnim][Cl]·H2O,552 [CnCnim][Br]·H2O,552 [CnCnim][I],548,552 [CnCnim][BF4],359,553 [CnCnim][PF6],552,554 and [CnCnim][ClO4].359,553 For [C10C10im][I], slightly different transition temperatures were mentioned in another report.555 The [CnCnim][BF4] salts are thermally stable until 350 °C.553

Br−), [Cnmim][X] (X− = [NO3]−, [BF4]−, [PF6]−, [OTf]−), [CnCnim][X]·H2O (X− = Cl−, Br−), [CnCnim][X] (X− = I−, [BF4]−, [PF6]−, [ClO4]−), [CnHim][X] (X− = Cl−, [NO3]−, [BF 4 ] − ), [C n mim] 2 [MCl 4 ] (M = Co 2+ , Ni 2+ , Pd 2+ ), [CnCnim]2[CuCl4]·H2O, and [CnCnim]2[PdCl4] compounds ([Cnmim]+ = 1-methyl-3-(n-alkyl)imidazolium, with n indicating the number of carbon atoms in the alkyl chain; [CnCmim]+ = 1,3bis(n-alkyl)imidazolium, with n and m indicating the number of carbon atoms in both alkyl chains, respectively; [CnHim]+ = 1-

(n-alkyl)imidazolium) (refs 122, 170, 171, 230, 280, 281, 359, 487, 519, 537, 540, 548, 550−568). The neutral counterparts of salts [Cnmim][X], that is, the corresponding N-(n-alkyl)imidazoles, are not LC.552 The thermal phase behavior of other [Cnmim][X], [CnCmim][X], and [CnHim][X] salts that were not included in Figures 16 and 17 is listed in Table S2. The thermal transitions of all other compounds (and mixtures) discussed in the remainder of this Review can be found in the Supporting Information. The temperatures shown in Figure 16a for [Cnmim][Cl] and X


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Figure 17. Phase diagrams for [C nHim][Cl],556 [Cn Him][NO 3 ],556 [C n Him][BF4 ],556 [Cn mim] 2 [CoCl4 ],550 [C nmim] 2 [NiCl4 ],550 [Cnmim]2[PdCl4],557 [CnCnim]2[CuCl4]·H2O,537 and [CnCnim]2[PdCl4].537 The SmA phases shown by the [CnHim][BF4] salts may actually be only monotropic.556 [C16mim]2[PdCl4] and [C18mim]2[PdCl4] start to decompose above 190 °C,557 and the [CnCnim]2[CuCl4]·H2O and [CnCnim]2[PdCl4] complexes start to decompose close to their clearing points.537

[C22mim][N(SO2F)2], with a slightly smaller anion, even show an enantiotropic SmA phase.575 [C12C12im][NTf2] does not show any enantiotropic LC phases,553 although one report mentions the appearance of an unidentified mesophase during a second heating run (Table S2).563 Mudring and co-workers studied the thermal phase behavior of asymmetrically substituted, anhydrous 1,3-bis(n-alkyl)imidazolium bromide salts; the results can be found in Table S2. In the series [CnCmim][Br] with n + m = 13, only [C12mim][Br] and [C13Him][Br] are LC.561 [C10C3im][Br], [C9C4im][Br], [C8C5im][Br], and [C7C6im][Br] are (possibly supercooled) room-temperature ILs that vitrify upon cooling. In the series [C12Cmim][Br] with m = 0−13, all compounds show at least one mesophase, except for [C12C2im][Br] and [C12C6im][Br].562 Salt [C16C2im][Br], on the other hand, shows an enantiotropic SmA phase.568 Whereas it has been generally assumed that at least a dodecyl chain substituent is required to obtain a thermotropic mesomorphic [Cnmim][X] salt (see also below), Godinho et al. reported that a (probably metastable) room-temperature lamellar mesophase can be induced in crystalline [C8mim][OTs] and [C10mim][OTs] by simple shearing.567 Sheared films of these materials apparently have a morphology that resembles (wet) 2D liquid foams.576,577 The probable influence of water was not investigated. Moreover, both salts are also LC without shearing, but not at room temperature (Table S2). Nonmesomorphic imidazolium salts with short alkyl chains often exhibit a glass transition on cooling from the liquid state, rather than undergoing crystallization. For the longer-chain analogues, both glass formation and crystallization occur, depending on, for instance, the cooling rate. During subsequent heating, solid-state polymorphism is usually observed (i.e., transitions and rearrangements between several solid organizations: crystal-to-crystal transitions, and cold crystallization). From Figures 16 and 17, several conclusions can be drawn, which are generally valid for amphiphilic systems of this kind: (i) for the compounds to exhibit LC properties, the alkyl chain that

[Cnmim][Br] are the values that were reported by Bradley et al. (for the first DSC heating run).280 However, Getsis and Mudring reported other values for [C12mim][Br] and [C14mim][Br], both for anhydrous samples (that were kept under inert atmosphere for all thermal measurements) and monohydrate samples (see Table S2).540,561,562 Probably, the samples of Bradley et al. were partially hydrated (i.e., [Cnmim][Cl]·xH2O and [Cnmim][Br]· xH2O with 0 < x < 1). [C12mim][Br]·H2O and [C14mim][Br]· H2O show a higher clearing point and larger mesophase temperature range than their respective anhydrous counterparts, and the enthalpy changes that accompany the thermal transitions are also larger for the hydrated compounds. Hydrogen bonding between the halide anions and the water molecules stabilizes the smectic layers. This is supported by the single-crystal structures of several long-chain imidazolium, benzimidazolium, and ammonium salts,539,540,559,569−573 from which it is clear that cohesion in the solid state, and possibly, by extension, in the LC state, is not necessarily ensured by the electrostatic interactions alone.572 In this context, it is noteworthy that the Weiss group found that addition of at least 1 molar equivalent of water or MeOH to P,P,P-tris(n-alkyl)-P-methylphosphonium halides induces LC properties in some cases, and enhances the mesophase temperature range and lowers the melting point in some other cases.497−499,574 These general findings on the importance of hydrogen-bonding interactions for the induction and stabilization of ILC smectic phases will be further discussed in section 7. The transition temperatures in Figure 16b for [CnCnim][Cl] and [CnCnim][Br] are also those for the hydrated salts as they were obtained under ambient conditions, that is, [CnCnim][Cl]·H2O and [CnCnim][Br]·H2O, respectively.552 The thermal phase transitions of anhydrous [C10C10im][Br] and [C12C12im][Br] can be found in Table S2.558,562 1-Methyl-3(n-alkyl)imidazolium [NTf2]− salts are only mesomorphic if the attached alkyl chain is very long: [Cnmim][NTf2] is not LC for n = 12, 14, 16, or 18, but [C22mim][NTf2] shows a monotropic SmA phase (Table S2).280,575 [C18mim][N(SO2F)2] and Y


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breaking region that decreases the melting point by reducing the cation symmetry, and which has a maximal length of about six to seven carbon atoms; and (iii) the hydrophobic region that increases the melting point due to increasing van der Waals interactions.230 In this context, it is noteworthy that the latter effect can be counterbalanced again by introducing structural irregularities (“kinks”) in the long alkyl substituents, for instance, by using unsaturated alkyl chains with at least one (preferably cis) CC double bond (Figure 19).578−580 This disrupts the packing

is attached to the cationic core needs to have a certain length, and this minimum length is longer for salts with larger anions (and for anions that have a nonspherical shape); (ii) the longer the alkyl chain is, the higher the melting point and clearing point are (the latter corresponds to a higher mesophase stability); (iii) the clearing points, however, increase to a larger extent, so that the mesophase temperature range increases with the alkyl chain length as well; and (iv) for very long chain lengths, the clearing point does not significantly increase anymore. The observation that a minimum alkyl chain length is required to induce (smectic) mesomorphism is due to the fact that the amphiphilicity of the ionic compound needs to be high enough. This is necessary to achieve a balance between the strong electrostatic interactions (between the ionic headgroups) and van der Waals interactions (between the molten aliphatic chains), and to obtain long-range nanosegregation of the ionic headgroups and the hydrophobic moieties, which leads to the formation of ionic sublayers and hydrophobic sublayers in the smectic mesophase.276 Because a larger anion contributes to the polar part of the molecule, the apolar part should also be more substantial to preserve the amphiphilic properties. Moreover, as mentioned in section 4.3.1, the cross-sectional area of the polar part (as projected onto the ion-rich planes) should be balanced by a sufficiently large apolar part to form a stable thermotropic lamellar mesophase. This explains why the [Cnmim][NTf2] compounds only show a smectic mesophase for very long alkyl chains (n = 22, see Table S2); the volume of a [NTf2]− anion is quite large, and it has a quite extended shape.575 Possibly, [CnCnim][NTf2] salts with two alkyl chains do show thermotropic mesomorphism for shorter chain lengths, but no literature data are available except for [C12C12im][NTf2] (which possibly shows an unstable monotropic LC phase; see Table S2).553,563 The observation that the compounds with a longer alkyl chain display a higher melting point is due to the fact that a higher temperature is required to induce melting of the alkyl chains, which experience stronger attractive van der Waals interactions (compare the melting point of n-octane (−57 °C) with that of noctadecane (27−29 °C)).276 It should be noted, however, that this reasoning is only valid for salts with alkyl chain substituents that contain at least eight or more carbon atoms (Figure 18). Indeed, Seddon and Rothenberg divided long-chain 1-methyl-3(n-alkyl)imidazolium cations into three structural regions that have a different impact on the melting point: (i) the charge-rich region localized on the imidazolium ring; (ii) the symmetry-

Figure 19. Melting points of “lipidic” imidazolium salts with an unsaturated alkyl chain substituent, investigated by West, Davis, and coworkers (potential LC properties were not reported).578

of the ions in the solid state. In a similar way, cell membrane fluidity is regulated by the balance between saturated and unsaturated fatty acids in a process termed “homeoviscous adaptation”. The increase in clearing point, or mesophase stability, with increasing chain length is a consequence of the increasing degree of amphiphilicity, with a concomitant better defined sublayer alternation. One must note, however, that some upper limit should exist: if the alkyl chain length becomes too large, the amphiphilicity starts to diminish again; one can see this as a dilution of the ionic sublayers and a reduction of the correlations between them. This can already be seen in the phase behavior of the [Cnmim][Br](· xH2O) salts that is displayed in Figure 16a. In general, the highest clearing points are observed for the chloride and bromide salts. This may reflect the strength of (3D) hydrogen bonding between the imidazolium cations and the halide anions. Higher temperatures are required to disturb this hydrogen-bonding network, and thus to induce the collapse of the ionic sublayers in the LC phase, which results in the transition to an isotropic liquid. For instance, for the same cation, the mesophase stability often decreases in the order Cl− ≈ Br− > [BF4]− > [PF6]− > [OTf]−. In most cases, the smectic layer thickness is also dependent on the anion species, following a similar trend and reflecting the ability of the anions to build extended structures within the ionic sublayers.115,280,492,568,581 [Remark: In this context, it is interesting to note that NMR spectroscopy measurements at identical temperatures pointed to a considerably higher cation self-diffusion anisotropy and much lower cation translational diffusion coefficients in the SmA phase of [C12mim][Cl] as compared to [C12mim][BF4].582] To characterize the hydrogen-bond acceptor ability of halides in halogen-containing anions, the charge density ρ on each halide in the complex anion is often used (eq 1).170,550,583,584

z2 (1) n where z is the overall charge on the anion, and n is the number of halogen atoms in the anion. The strength of hydrogen bonding ρ=

Figure 18. Influence of the length (n) of the alkyl chain substituent in [Cnmim][X] salts (X− = [BF4]−, [PF6]−, [NTf2]−) on the melting point Tm. Reprinted with permission from ref 578. Copyright 2010 Wiley. Z


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between cations and anions is considerably lower for ρ < 1 (as in the case of, for example, [AlCl4]−) than for ρ = 1 (as in the case of, for example, Cl−, Br−, and [PdCl4]2−). A lower overall anion charge density can also contribute to a lower mesophase stability as a result of decreased electrostatic interactions. For more bulky anions, the molten alkyl chains are also more folded to compensate for the larger area occupied by the ionic headgroups in the ionic sublayers. This contributes to a decreased mesophase stability as well. The occurrence of extensive hydrogen bonding in ionic sublayers consisting of suitable polar moieties is probably also the reason for the mesophase stabilization by water molecules, which has been observed, for example, for P,P,P-tris(n-decyl)-Pmethylphosphonium and 1-methyl-3-(n-alkyl)imidazolium halide salts (see above and Figure S7(c)).385,540 A comparison of the phase behavior of the [Cnmim][Cl]· xH2O salts and the corresponding [Cnmim]2[MCl4] metal complexes (M = Co2+, Ni2+, Pd2+) shows that the melting points are comparable for the [Cnmim][Cl]·xH2O, [Cnmim]2[CoCl4], and [Cnmim]2[NiCl4] compounds, but much higher for the [Cnmim]2[PdCl4] compounds (Figure 17). Pd2+ is only slightly larger than Co2+ and Ni2+, so this observation emphasizes the importance of the coordination geometry around the metal cation: Co2+ and Ni2+ form tetrahedral complexes, whereas Pd2+ forms square-planar complexes. It is also remarkable that the minimum chain length to induce mesomorphism is higher for the Pd2+ complexes. This is probably a combined effect of size and shape. As far as the clearing points are concerned, one can see that for shorter chain lengths, these are much lower for the metal complexes than for the chloride salts, but that for longer chain lengths the metal complexes display much higher clearing points (the clearing points of [C16mim]2[PdCl4] and [C18mim]2[PdCl4] are probably much higher than the values in Figure 17, but these may not have been mentioned in the original paper because decomposition starts to set in from 190 °C on557). Apparently, if the degree of amphiphilicity of the metal complexes is sufficiently high, the mesophase formed is stabilized by the stronger electrostatic interactions with a doubly charged anion ([MCl4]2−) than with a singly charged anion (Cl−). The vast majority of simple amphiphilic ILCs contains one or multiple alkyl chains with an even number of carbon atoms, because the starting reagents are more readily available. However, it can be interesting to prepare homologues with an odd number of carbon atoms. In Figure 16a, one can observe a so-called “odd−even” effect for the [Cnmim][BF4] compounds: the homologues with a n-tridecyl chain or a n-pentadecyl chain have higher melting points than their nearest neighbors with an even number of carbon atoms in the alkyl chain. Wang et al. investigated the phase behavior of binary mixtures of the ILCs [C12mim][I] (1) and [C12mim][BF4] (2).584 [C12mim][I] has a melting point similar to that of [C12mim][BF4], but a higher clearing point and thus a larger mesophase stability range. On the other hand, imidazolium salts with fluorinated anions tend to display a higher ionic conductivity than the halide salts, partly because of reduced interactions between the cations and anions (see also above).519,581,585 Making mixtures is a possibility to combine the advantages of the separate components. For an increasing fraction x of [C12mim][I], mixtures [C12mim][(BF4)1−xIx] indeed show an increasingly broader mesophase stability range but a lower ionic conductivity. Other groups have investigated mixtures of ILCs as well (see below).2,162,170,292,426,439,537,549,553,555,586−588 Usually ionic mesogens containing the same cation were mixed, but mixtures

of ILCs with the same anion but different cations (e.g., [C16mim][PF6] and [C16pyr][PF6]170) have also been studied.170,292,555,588 Yamanaka et al. “diluted” [C12mim][I]/I2 with [C12mim][BF4] to vary the iodide concentration ([I−] + [I3−]), which allowed one to determine the contribution of so-called “exchange-reaction-based diffusion” to the total I3− diffusion (see section 12.2).565 Following Xu et al.581 (see below), Wang et al. proposed that the dominant charge carriers in the ionic sublayers of the smectic phase are the anions. The cations are obviously much larger than the anions (it should be noted that this is often not the case for nonmesomorphic ILs with short alkyl chains, for which the shape and size of both the cation and anion were found to have a profound influence on the diffusion and ionic conductivity106,589−591), and apart from interactions with the anions (electrostatic attraction, hydrogen bonding, etc.), they additionally experience van der Waals interactions with neighboring cations. MD simulations on [C16mim][NO3] also showed that the diffusion coefficient of the anions in the SmA phase is larger than that of the cations.488 The total conductivity is then very roughly approximated by eq 2:592 σ = n·q·μ ≈ nanion ·qanion ·μanion (2) where n is the number of dissociated charge carriers per unit volume; q is the charge carried by them; and μ is the mobility of the charge carriers. Because pure [C12mim][I] shows a larger smectic layer thickness than [C12mim][BF4] (see above), the number of ion-conductive layers per unit volume and thus nanion decreases with increasing x for [C12mim][(BF4)1−xIx]. On the other hand, μanion is smaller for I− than for [BF4]− because it interacts more strongly with the imidazolium cations. This explains the decrease in ionic conductivity with increasing x. Following Yamanaka et al.,565 the authors also made a ternary mixture of [C12mim][I], [C12mim][BF4], and I2 to form the ILC [C12mim][(BF4)0.40I0.45(I3)0.15]. Interestingly, both the melting and the clearing point of [C12mim][(BF4)0.40I0.45(I3)0.15] are lower than for [C12mim][(BF4)0.40I0.60] (Cr1 · 15 · Cr2 · 15 · SmA · 39 · I (°C) versus Cr1 · 13 · Cr2 · 17 · Cr3 · 21 · SmA · 57 · I (°C)). Recall that [C12C12im][I3] is not mesomorphic.548 The diffusion of iodide species in the LC ternary mixture was measured by the ultramicroelectrode technique, and compared to that in the IL [(C12mim)0.40(C11mim)0.60][(BF4)0.40I0.45(I3)0.15] (which is apparently not mesomorphic). Despite its higher viscosity, [C12mim][(BF4)0.40I0.45(I3)0.15] was found to show a higher I 3 − diffusion coefficient than [(C12mim)0.40(C11mim)0.60][(BF4)0.40I0.45(I3)0.15], indicating that the diffusion is mainly based on I−/I3− exchange reactions. This was explained by a higher collision frequency of iodide species inside the ionic sublayers in the SmA phase (see also section 12.2 regarding the potential application in dye-sensitized solar cells).

Judeinstein and co-workers performed 2H, 11B, 19F, and 7Li NMR spectroscopy experiments on pure [Cnmim][BF4] and [Cnvim][BF4] salts (n = 14, 16; [Cnvim]+ = 1-vinyl-3-(nalkyl)imidazolium, see below (28-BF4-n)), and their mixtures with LiBF4.560 These samples could be easily aligned in the strong magnetic field of the NMR spectrometer. Cifelli et al. did AA


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1

H and 19F NMR measurements on bulk [C12mim][Cl] and [C12mim][BF4].582 The results will be discussed in section 12.1. Zhang et al. managed to switch [C12mim][BF4] (2) isothermally and reversibly between a LC state (SmA) and an isotropic liquid state, by doping the ILC with a relatively small amount (1−5 wt %) of a photoisomerizable nonmesomorphic azobenzene derivative (4-methoxy-4′-methylazobenzene (MMAB)) and subsequently irradiating it with light of a suitable wavelength.593 UV light (365 nm) causes isomerization of MMAB to its bent-shaped cis state, and the disturbance in the local smectic LC alignment due to this nonlinear shape of the dopant destabilizes the entire bulk LC phase because of the cooperative motion of the mesogens.594 The LC state can restored upon irradiation with visible light to convert the azobenzene chromophores back to their trans states. The optical response times were rather long (seconds) but could be shortened by applying a higher irradiation power. Isothermal phase transitions could only be observed at temperatures between TSmA→I,trans (i.e., the clearing point under continuous irradiation with visible light) and TSmA→I,cis (i.e., the clearing point under continuous irradiation with UV light). An advantageous side effect of the addition of the rod-like azobenzene derivative appeared to be a broadening of the mesophase temperature range, maybe thanks to stabilizing cation−π interactions between the imidazolium and azobenzene moieties. On the other hand, addition of MMAB resulted in the loss of the spontaneous homeotropic alignment of [C12mim][BF4] between glass slides, which is a disadvantage if one would like to use the isothermal phase transition to switch between anisotropic and isotropic ion conduction. As is apparent from Scheme 2, the anions that have been employed to obtain mesomorphic salts are not at all limited to inorganic species. Anions with a long alkyl chain can also be used. The dodecylsulfate anion has been used very often in the past, for example, to prepare LC silver(I) stilbazole complexes421,461−463,468 or lanthanide complexes with Schiff’s base ligands.595,596 Alkylsulfate anions have also been used to obtain, for instance, LC N-(n-alkyl)pyridinium salts.597,598 Alkylsulfonate, alkyl- and alkyloxybenzenesulfonate, alkylphosphate, and alkylcarboxylate species are other examples of readily available anionic fragments that contain an alkyl chain. Seddon, Holbrey, Rebelo, and co-workers investigated the phase behavior of “catanionic” [Cnmim][CmH2m+1SO3] salts (3-n/m), and identified a SmA phase for [C8mim][C8H17SO3] (3-8/8).259 Biswas et al. studied some imidazolium salts with carboxylate counterions containing a long alkyl chain (4-n/m) (see also compounds 414n that will be discussed in section 11).599 Compound 4-4/13 forms a smectic phase between 15 and 21 °C, in contrast to 4-10/ 13, which is not LC. Campbell et al. observed a monotropic smectic phase for 1-(n-dodecyl)-3-methylimidazolium 2-hydroxybenzoate ([C12mim][salicylate], Table S2).600

homologue with only a methyl group on the imidazolium ring (n = 1) is LC. The phase diagram is quite interesting: the largest mesophase stability ranges were observed for the methyl and ndecyl homologues (n = 1 and n = 10, respectively), with the methyl homologue showing the highest clearing point. One might explain this by the fact that the incompatible hydrocarbon chains and fluorocarbon chains prefer to occupy distinct subspaces, and that this microphase segregation should only be efficient for a sufficiently long alkyl chain. For n = 1, one could imagine the direct alternation of ionic and fluorocarbon sublayers, but for larger n, three sublayers (ionic, hydrocarbon, and fluorocarbon) should be formed in the SmA phase. Surprisingly, however, PXRD measurements showed that the smectic layer spacing remains nearly constant for n = 2−8, indicating that the hydrocarbon and fluorocarbon chains are completely mixed inside one sublayer, with a thickness of about twice the length of the fluorocarbon chain. This emphasizes again the necessity for efficient charge alternation within the ionic sublayers, which cannot be achieved when the hydrocarbon and fluorocarbon chains point away from each other. One of the motives of the authors to prepare these salts was the finding from IL research that imidazolium ILs with [OTf]− anions generally show lower melting points and viscosity than their [CH3SO3]− counterparts. Despite the strongly electron-withdrawing perfluoroalkyl chain in the anion in the series 5-n, the electrostatic interactions still govern their mesophase structure. The authors also investigated the dynamic ionic conductivity parallel to the smectic layers of an aligned sample,215,216,602 and found that the ionic conductivity is higher in the SmA phase than in the isotropic liquid phase, despite the lower viscosity of the latter. They ascribed this to the higher ion density in the ionic sublayers of the LC phase, as compared to the isotropic phase, which constitutes a state of mixed polar ionic moieties and nonpolar (insulating) groups. The successive ionic sublayers act as ionconductive pathways. This property of ILCs will reappear several times throughout this Review.

Xu et al. prepared 1-methyl-3-(n-alkyl)imidazolium ILCs with a [F(HF)2]− anion (6-n), by mixing [Cnmim][Cl] salts with a large excess of anhydrous HF.581 The vacuum-stable HF composition x for [Cnmim][F(HF)x] at room temperature ranges from 2.0 to 2.3 ([F(HF)x]− room-temperature ILs with organic cations with a short alkyl chain contain a 7:3 ratio of [F(HF)2]− and [F(HF)3]−, corresponding to x = 2.3). For comparison purposes, the x value was adjusted to 2.0 for all investigated compounds. On heating above 45 °C, the [Cnmim][F(HF)2] salts slowly lose HF (which presents a serious risk), to fully decompose around 230 °C. Fortunately, the compounds show low melting temperatures, with 6−10 even being LC below room temperature. The [Cnmim][F(HF)2] salts have lower melting points than the corresponding [Cnmim][Cl](· xH2O) and [Cnmim][Br](· xH2O) compounds (Figure 16a); furthermore, [C10mim][Cl] and [C10mim][Br] are not LC. Homeotropically aligned SmA phases could be obtained by applying a pressure on the top glass slide of a conductivity measurement cell. Quite high ionic conductivity values were found. For 6-12, 6-14, and 6-16, the ionic conductivity parallel to the smectic layers (σ||) is at least 10 times higher than the ionic

The alkyl chain in alkylsulfonate anions can also be replaced by a perfluorinated chain. Mukai et al. reported on the mesomorphic behavior of [CnHim][C8F17SO3] salts (5-n) that were prepared by mixing the appropriate N-(n-alkyl)imidazole and C8F17SO3H in ethanol.601 These salts contain both a hydrocarbon part and a fluorocarbon part. All compounds show a SmA phase. Even the AB


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conductivity perpendicular to the smectic layers (σ⊥) (e.g., at 40 °C, in the SmA phase of 6-12: σ|| = 1.14 × 10−2 S cm−1, σ⊥ = 1.11 × 10−3 S cm−1; for practical use as electrolytes in devices, ionic conductivity values of 10−2 to 10−3 S cm−1 at room temperature are required). As expected, the anisotropy increases with increasing chain length (Figure 20). Interestingly, 6-10 does

Other fluorinated imidazolium ILCs, with iodide counterions, were reported by Abate et al. (9-x/m/n).606 Most of the studied compounds exhibit a SmA phase (which spontaneously aligns homeotropically between two glass slides), with high clearing points (around 200 °C) for 9-1/2/6, 9-1/3/6, and 9-6/2/8. The SmA order is frozen upon cooling to the solid state. Derivative 98/3/6 shows a SmA phase between 48 and 119 °C. After mixing with I2 in a 4:1 molar ratio, the melting point was lowered by 21 °C, but, interestingly, the clearing point remained largely unaffected. Dissolution of 0.65 M I2 in the ILC [C12mim][I] lowers the clearing point by ∼34 °C.564,565 As mentioned above, the aligned SmA phase provides 2D pathways for efficient I−/I3− exchange reactions and diffusion of the I−/I3− redox couple. The temperature stability range of the SmA phase of the (9-8/3/6)/I2 mixture makes it interesting for use as the electrolyte in a DSSC (see section 12.2). This was actually tested, but only at room temperature.

Figure 20. Temperature dependence of ionic conductivity for 6-n: σ|| for n = 10 (●), 12 (▲), 14 (■), 16 (◆); σ⊥ for n = 10 (○), 12 (△), 14 (□), 16 (◇). Reprinted with permission from ref 581. Copyright 2010 Wiley.

not exhibit any significant anisotropy in ion conduction, presumably because the smectic layers are not as well-defined as for the longer-chain analogues. The latter also display a higher σ|| than 6-10 thanks to the well-defined ion-conductive pathways. In contrast, the conductivity of (isotropic) imidazolium ILs decreases with increasing alkyl chain length because of the increase in viscosity.585 As noted above, the authors proposed that the dominant charge carrier in the ion-conductive layers is [F(HF)2]−. In a follow-up report, the influence of the HF content x in [C12mim][F(HF)x] (7-x; the possible anionic species are [FHF]−, [F(HF)2]−, and [F(HF)3]−) on its phase behavior was investigated.603 Both the melting point and the clearing point (as well as the hygroscopicity) were found to decrease with increasing x, but the clearing point was affected to a greater extent. The ionic conductivities σ|| and σ⊥ both increase as a function of x (as a result of a decreasing cation−anion interaction), with the anisotropy σ||/σ⊥ remaining nearly unaffected because the insulating aliphatic sublayer remains mostly unchanged. Semifluorinated analogues of 6-n were reported as well (8-n/x).604 The shorter-chain homologues (n < 6) are room-temperature ILs, but 8-6/2.0 and 8-8/2.0 are mesomorphic. Salt 6-8 which is the nonfluorinated counterpart of 8-6/2.0, is not LC; this suggests a positive influence of fluorous interaction between the side chains on mesophase formation. Introduction of fluorine atoms in the cation resulted in a higher density and viscosity, lower thermal stability, lower ionic conductivity, and lower electrochemical stability against reduction. Similar trends had been found before for 1-methyl-3[F(CF2)n(CH2)2]imidazolium [NTf2]− salts as compared to their nonfluorinated counterparts.605

Xu et al. synthesized 1-methyl-3-(n-alkyl)imidazolium ILCs with octahedral anions related to the [PF6]− anion: [AsF6]−, [SbF6]−, [NbF6]−, and [TaF6]− (10-Z-n).519 This allowed a systematic investigation of the influence of the anion size on the phase behavior. All of the [C16mim]+ and [C18mim]+ salts are mesomorphic, but the clearing point and mesophase temperature range decrease with increasing anion size. The [SbF6]−, [NbF6]−, and [TaF6]− salts have remarkably similar transition temperatures. Recall that [C12C12im][SbF6] is not mesomorphic.548 The smectic layer thickness measured by PXRD also decreases with increasing anion size, as a result of a higher degree of alkyl chain folding and/or more pronounced alkyl chain interdigitation (see also below). The [Cnmim][NbF6] salts gradually hydrolyze in air, even at room temperature. The authors managed to obtain single-crystal structures for all 10-Z18 salts (Z = P, As, Sb, Nb, Ta) at −100, 25, and 55 °C. At −100 °C, the octadecyl chain has a bent (“cis”) conformation in the immediate vicinity of the imidazolium ring, whereas it adopts an “all-trans” conformation at 55 °C, near the melting point. At room temperature both conformations coexist, in a ratio dependent on the anion size. All F atoms of the [ZF6]− anions are involved in short imidazolium C−H···F contacts.

AC


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N−H protons is even beneficial for LC properties ([C16mim][BF4], Cr · 50 · SmA · 182 · I (°C); [C16Him][BF4], Cr · ca. 97 · SmA · ca. 189 · I (°C) (probably monotropic); 13-BF4, Cr · 91 · SmA · 117 · I (°C)).171,513,556 In an attempt to obtain protonconductive LCs, the authors also investigated mixtures of 13-BF4 and 14 (90:10 and 50:50 molar ratios). Imidazole 14 contains both proton-donating and -accepting sites and allows proton transport through successive intermolecular hydrogen bonds, while 13-BF4 is meant to serve as a LC proton-conductive matrix (see also section 7 for other work of these groups on LC protonconductive systems). The 90:10 mixture shows a monotropic SmA phase, but the 50:50 mixture does not display LC properties. It should be noted that it is possible to obtain 2substituted imidazolium ILCs (i.e., without a H(2) proton) with large mesophase ranges. Li et al. reported that [C16dmim][Br] (17) shows a SmA phase between 90 and 232 °C,609 while [C16mim][Br]·xH2O clears out at 220 °C (Figure 16a). On the other hand, the latter compound already melts at 66 °C. Substitution of the H(2) proton by a methyl group appears to result in higher melting points. This was also observed for shortchain imidazolium ILs (see also above in the introduction to section 5).610 Fox and co-workers reported in 2003 slightly different transition temperatures for 17.610 They also observed mesomorphism for [C12dmim][Br] and [C14dmim][Br]. Li et al. investigated the lyotropic mesomorphism of binary mixtures of 17 with water, which occurs between 0 and 20 °C.609 For a concentration of 17 between 10 and 30 wt %, rod-like micelles are formed, whereas for a concentration between 40 and 75 wt %, the micelles self-assemble into larger structures, and a hexagonally ordered H1 phase is observed.

Tokarev et al. prepared [Cnmim][Ag(CN)2] and [Cnmim][Au(CN)2] salts (11-M(CN)2-n) from the corresponding [Cnmim][BF4] salts.111 These metal-containing ILCs show a SmA phase at low temperatures. Moreover, salts 11-Au(CN)2-n display gold-based phosphorescence (see also section 12.5). For salts 11-Ag(CN)2-n, only blueish luminescence originating from the imidazolium cations (around 432 nm), as displayed by [Cnmim][BF4], was observed.

Apart from their study on salts 5-n, Mukai et al. also investigated the ionic conductivity of [C12C2im][C12H25SO3] (12) and 12/LiBF4 (5 mol % LiBF4).607 Addition of LiBF4 slightly raises the clearing point of the SmA phase of 12: Li+ enhances electrostatic interactions inside the ionic sublayers because of its large surface charge density. For the 12/LiBF4 mixture, an additional metastable lamellar phase was observed during heating and cooling runs (the phase recrystallizes on heating, before the higher-temperature SmA phase is reached). The authors stated that in this phase the alkyl chains are crystallized, while the ionic sublayers are “fused”. The ionic conductivity is higher in the SmA phase than in the isotropic liquid phase for both 12 and 12/LiBF4. Addition of LiBF4 lowers the ionic conductivity parallel to the smectic layers, presumably due to the increased electrostatic interactions and concomitant reduced ion mobility. During cooling from the isotropic liquid phase, the samples spontaneously formed aligned monodomains in the LC phase and started to show anisotropic ion conduction. The ionic conductivity of 12/LiBF4 measured parallel to the smectic layers was 150 times higher than the ionic conductivity measured perpendicularly to the layers (at 51.5 °C, in the SmA phase: σ|| = 4.1 × 10−5 S cm−1, σ⊥ = 2.6 × 10−7 S cm−1). This system therefore shows a much higher ion conduction anisotropy but significantly lower ionic conductivity values than salt 6-12 (see above).

The same groups also reported on three 2-(n-heptadecyl)imidazolium salts (13-X) and a protonated imidazolium nalkylsulfonate salt (15).513 In the series 13-X, only 13-BF4 is LC: it shows a SmA phase over a rather narrow temperature range. The nonprotonated analogue, 2-(n-heptadecyl)-1H-imidazole (14), is not LC, emphasizing the importance of ionic charges to augment the amphiphilicity of the molecular structure. Salt 15 shows a SmA phase over a broad temperature range. These results suggest the importance of the presence of a proton in the 2-position of the imidazolium ring, which can stabilize the ionic sublayers in the smectic phase via hydrogen bonding with a suitable anion. Mukai et al. reported already in 2004 that replacement of the H(2) proton by a methyl group destabilizes the SmA phase formed by simple imidazolium n-dodecylsulfonate compounds (16-H/H, Cr · 90 · SmA · 177 · I (C°); 16CH3/H, Cr · 202 · I (C°); 16-H/CH3, Cr · (41 · SmA ·) 95 · I (C°)).608 It is not clear why the H(2) proton in particular is so important, because the N−H protons of the imidazolium ring are also able to form hydrogen bonds. It seems that the absence of

Lin and co-workers focused on variations on the simple 1,3bis(n-alkyl)imidazolium motif. They synthesized analogues of [CnCmim][Br] salts with a hydroxyl group on the second carbon atom of one of the alkyl chains (18-n and 19-n).558 These compounds can be prepared by the solventless reaction between a substituted epoxide and imidazole, and subsequent quaternization with an alkyl halide. The hydroxyl group can participate in hydrogen bonding. The same group had previously presented amide-functionalized imidazolium ILCs (20-X-n), in which the amide functionality is also involved in hydrogen bonding (these salts have been used as precursors for LC Ag(I) and Au(I) Nheterocyclic carbene complexes611).315 The crystal structure of 19-14 shows that the imidazolium cation adopts a “rod-like” geometry in the sense that the two alkyl chains are stretched outward along the imidazolium core plane and do not interdigitate. This is different from, for instance, 1,3-bis(nalkyl)benzimidazolium salts, in which the two alkyl chains are AD


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LCs, but zwitterionic imidazolium molten salts had been known for quite some time,614 and other LC examples (26-n, 27-n, 458, and 459) have been reported in the meantime.615,616 Adducts 24-X-n exhibit wider mesophase temperature ranges than 22-Xn. The single-crystal structures of 22-Br-10 and 23-10·(H2O)2 consist of planes of helical superstructures formed by C−H···Br−, O−H···Br−, C−H···O, and/or O−H···O hydrogen-bonding interactions. In the case of 22-Br-10, neighboring helical strands are linked by the bromide anions via C−H···Br− and O−H···Br− interactions. In the case of 23−10·(H2O)2, each imidazolium column is surrounded by three 1D water channels. The planes with the helices are separated from one another by aliphatic sublayers. Interestingly, 22-Br-10 does not dimerize via conventional CO···H−O hydrogen bonding between the carboxylic acid groups; instead, the carbonyl oxygen atoms form intermolecular hydrogen bonds with neighboring imidazolium C(5)−H(5) groups. In the series 22-X-n, changes in the type of anion affect the 1H NMR chemical shift of the imidazolium H(2) proton more than those of the H(4) and H(5) protons. This suggests that the anions preferentially locate near the H(2) protons. The H(2) δ values follow the order Cl− > Br− > [BF4]− ≈ [PF6]−. Mixing of 23-10 with LiBF4, LiClO4, LiNTf2, or CH3COOLi resulted in sticky soft materials showing a smectic phase at or just above room temperature. Addition of a lithium salt leads to an increase in smectic layer thickness, probably due to coordination of the carboxylate groups to the Li+ ions. For 2310/LiBF4 (3:1) and 23-10/LiClO4 (3:1), the ionic conductivity was found to increase with increasing temperature throughout the SmA phase, but to slightly decrease again in the isotropic liquid phase. The ester derivatives of 22-X-n were reported as well (25-X-n).617 These can form thermotropic mesophases, as well as lyotropic mesophases when mixed with chloroform. Moreover, salts 25-X-16 form LC gels in a variety of organic solvents. Trace amounts of water in chloroform play a crucial role for the development of the hydrogen-bonded network during gel formation. The zwitterionic compounds 26-n and 27-n, and their mixtures with LiClO4, show SmA phases that spontaneously align homeotropically on glass slides and display highly anisotropic transport of lithium ions.615 In both series, addition of ∼14 mol % of LiClO4 resulted in reduced melting points and increased clearing points. Zheng and co-workers investigated aqueous lyotropic LCs formed by an equimolar mixture of 26-12 and LiNTf2 (which forms a room-temperature IL) with 20−80 wt % of added water, and measured their temperature-dependent ionic conductivity in different mesophases (hexagonal columnar, lamellar, and bicontinuous cubic phases).618

more or less orthogonal to the benzimidazolium core plane (Ushaped conformation) as a result of benzene ring π−π interactions,559,612 and from [CnCnim]2[CuCl4]·H2O and [CnCnim]2[PdCl4] complexes, in which the cations also adopt a U-shaped conformation.537 A rod-like cation shape was also found in the crystal structures of [C12C12im][I], [C12C12im][I3], and [C12C12im][B(CN)4] (see above), but in those cases the ndodecyl chains interdigitate.548 The cations of 19-14 form ionic monolayers within the lamellar crystalline structure, in contrast to the usual “head-to-head” arrangement of imidazolium cations.537,559,569 The proton of the hydroxyl group forms a hydrogen bond with a bromide anion, whereas the oxygen atom is involved in hydrogen bonding with a neighboring imidazolium ring C−H proton. Infrared spectroscopy showed that the hydrogen bonds remain in the LC state, but that they are weakened due to the increased mobility of the molecules and less directional interactions. It is striking that the symmetrically substituted homologues (19-n) display much higher clearing points and much larger mesophase temperature ranges than the asymmetrically substituted analogues (18-n). Compounds (1912)−(19-18) also show much higher clearing points and larger mesophase temperature ranges than their [CnCnim][Br] counterparts, thanks to the additional hydrogen bonding with the hydroxyl groups. Furthermore, while anhydrous [C10C10im][Br] appears to be not LC, 19-10 is a room-temperature ILC with a large mesophase range. As mentioned in section 4.2, imidazolium salts with an N-(2-hydroxyethyl) or N-oligo(ethylene oxide) side chain and a relatively long hydrophobic alkyl chain on the other side of the imidazolium cation (21-X-0/n and 21-X-m/n (m = 1, 2), respectively) can show SmA phases as well.304,305 To exhibit mesomorphism, the degree of amphiphilicity of the salts needs to be sufficiently high: all members of the series 21-Cl-0/n (n = 10, 12, 14, 16, 18) are LC, whereas 21-Cl1/10 and 21-Cl-2/14 are nonmesomorphic, room-temperature ILs and LC properties only start to develop for a minimum alkyl chain length of n = 12 and n = 16 in the series 21-Cl-1/n and 21Cl-2/n, respectively. Elongation of the hydrophilic substituent lowers the thermal stability of the SmA phase: the clearing points of 21-Cl-0/18, 21-Cl-1/18, and 21-Cl-2/18 are ∼273, ∼228, and 142 °C, respectively. The addition of 0.5 equiv of LiCl to 21Cl-m/16 (m = 0, 1, 2) had only a minor effect on the melting points, but caused a considerable increase in clearing points (see also section 5.4292).

The same group presented carboxyl-functionalized imidazolium ILCs (22-X-n), which can be regarded as amphiphilic analogues of betainium salts.613 By deprotonation of the carboxylic acid group with NaOH, LC zwitterionic compounds (23-n) were obtained, and these could be combined with 22-X-n to form LC hydrogen-bonded dimers (24-X-n). The series 23-n represented the first examples of zwitterionic imidazolium-based

The Lin group also reported on the phase behavior of vinylfunctionalized imidazolium salts, [Cnvim][X] (28-X-n).568 AE


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[Remark: 28-I-n (X− = I−) and 28-BF4-n (X− = [BF4]−) are also mentioned in a more recent report from Wang et al.551] 28-Br16 and 28-I-16 display a slightly higher melting point and a higher clearing point than [C16C2im][Br] and [C16C2im][I] (Table S2), respectively. 1H NMR chemical shift values for the imidazolium and vinyl protons suggest that this is due to additional C−H···X− hydrogen-bonding interactions. On the other hand, the [Cnmim][Br] salts (n = 12, 14, 16, 18) display much higher clearing points than the corresponding vinylsubstituted salts 28-Br-n (n = 12, 14, 16, 18).

upfield after exchange of the bromide anion, in the following order: Br− > [CH3COO]− > I− > [BF4]− ≈ [SCN]− > [PF6]−. This order is exactly the same as that found for the mesophase stability, emphasizing the influence of the H(2) proton that was also observed by Mukai et al.513,608 and by Lin et al.613 Mixtures of 31-Br-9 and 31-X-9 (X− ≠ Br−) yielded 1H NMR spectra with a single set of signals: rapid anion exchange occurs on the NMR time scale. Other chiral imidazolium salts, with different anions, were reported more recently (32-X-n-R and 33-X-n-R).621 These were prepared starting from the amino acids glycine (for R = H), alanine (R = CH3), valine (R = i-Pr), leucine (R = i-Bu) and phenylalanine (R = Bn). The formation of LC mesophases appeared to be highly dependent on the steric hindrance caused by the R group. SmA phases were only observed for the glycinederived salts 32-OTf-n-H (n = 13, 15, 17) and 33-OTf-n-H (n = 15, 17), and for the alanine-derived salt 32-OTf-15-CH3, but not for the valine- and leucine-based compounds. Despite the bulkiness of the benzyl group, salts 32-Cl-n-Bn (n = 8, 9, 11) also show a SmA phase, and this was ascribed to favorable π−π stacking interactions.

The influence of a sulfur atom in the side chain of 1-methyl-3(n-alkyl)imidazolium salts was studied by Laschat and coworkers (29-X-n).619 Sulfur atoms have a “softer” character than oxygen and nitrogen atoms, due to their more delocalized dorbitals. In the case of the triflate salts, the presence of a sulfur atom appears to have a beneficial effect on the LC properties in the sense that compounds 29-OTf-n show lower melting points and larger mesophase widths than the related [Cnmim][OTf] salts.280 Compounds 29-OTf-16 and 29-OTf-18 also exhibit more stable SmA phases than 32-OTf-15-H and 32-OTf-17-H, respectively, where the latter contain a “harder” nitrogen atom in their alkyl side chain (see below). The bromide salts 29-Br-n are already LC for n = 8.

Nieuwenhuyzen et al. reported on nonhygroscopic ILs and ILCs containing the boron cluster anions [closo-B10Cl10]2−, [closo-B12Cl12]2−, [nido-C2B9H12]−, and [commo-3,3′-Co(1,2C2B9H11)2]−.362 These weakly coordinating anions exhibit delocalized charges, very weak nucleophilicity, and relative chemical inertness. Only [C 16 mim] 2 [B 10 Cl 10 ] (34-16), [C18mim]2[B10Cl10] (34-18), [C16mim]2[B12Cl12] (35-16), and [C18mim]2[B12Cl12] (35-18) are LC. The [B10Cl10]2− and [B12Cl12]2− salts are thermally more stable than the corresponding [C2B9H12]− and [Co(1,2-C2B9H11)2]− compounds (e.g., 3418 is thermally stable up to about 380 °C under nitrogen).

The Laschat group also built further on their original work from 2004, in which they studied an imidazolium ILC with a chiral (R)-citronellyl chain (30).620 Interestingly, 30 only shows a monotropic SmA phase, whereas 18-14, for instance, shows an enantiotropic SmA phase from room temperature to 81 °C, emphasizing again the influence of the hydroxyl group. When comparing the phase behavior of [C14C14im][Br]·H2O (Figure 16b), 18-14, 19-14, and 30, it appears that the most stable mesophases are obtained when both long N-(n-alkyl) substituents on the central imidazolium ring have the same size. Neither the analogues of 30 with a shorter (CH3, n-C4H9, nC6H13, n-C12H25) or longer (n-C18H37) alkyl chain, nor the analogue with two citronellyl substituents, are LC.620 The LC properties could however be improved by extending the chiral substituent instead of the nonchiral alkyl chain: salts 31-Br-n show enantiotropic SmA phases at or below room temperature and over a broad temperature range.549 An “odd−even” effect was found. A branched N-(n-alkyl) substituent lowers the melting point and destabilizes the mesophase: bromide salts 31Br-n show lower melting and clearing points than their nonbranched [Cnmim][Br](· xH2O) counterparts (Figure 16a). The effect of other counterions was also investigated. The mesophase stability decreases in the order Br− > [CH3COO]− > I− > [BF4]− > [SCN]− > [PF6]−. The stereogenic center was found to act only as a “stopper” for alkyl chain interdigitation; no superstructural chirality was induced. 1H NMR spectra of salts 31-X-9 showed that the chemical shift of the most acidic proton H(2) of the imidazolium ring had moved

Other imidazolium-based, metal-containing ILCs were presented by Mudring and co-workers.118−120,622 They synthesized [C12mim]3[DyBr6] (36-DyBr6) and [C12mim]3[TbBr6] (36-TbBr6) (as well as [C12mpyrr]3[TbBr6]), [C12mim]4[EuBr6]Br (37), and [Cnmim]2[Mo6Cl14] (38-n; see Figure 66 for the structure of [Mo6Cl14]2−). Due to the low nucleophilicity of [Mo6Cl14]2−, salts 38-n are thermally stable up to about 350 °C under an inert gas. Single crystals of the solvates [C12mim]3[DyBr6]·(CH3CN)2, [C12mim]3[TbBr6]·(CH3CN)2, and [C12mim]4[EuBr6]Br·CH3CN were obtained from cold acetonitrile solutions, but the acetonitrile solvent molecules are AF


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based on such anions have not yet been reported. Even more highly charged monolanthanide complex anions include [Ln(NCS)7(H2O)]4−,624 [Ln(NCS)8]5−,624,627 and several lanthanide-containing polyoxometalate anions (e.g., [EuW10O36]9− and [Eu(BW11O39)2]15−; see section 11). Lanthanide-containing ILs based on [Ln(dcnm)6]3− (dcnm− = dicyanonitrosomethanide) anions have been reported.628,629 Compounds 39-n in which the imidazolium cations carry a short alkyl chain (i.e., ethyl, n-butyl, i-butyl, n-hexyl, n-octyl) are low-melting nonmesomorphic salts, but the homologues with long alkyl chains (n = 12, 14, 16, 18) show a SmA phase, even at room temperature in the case of 39-12. The stability of the mesophase increases to a remarkable extent with increasing alkyl chain length. No hydrogen-bonding network was detected in the single-crystal structure of 39-1. It was proposed that this lack of hydrogen bonds contributes to the low melting points of the imidazolium hexanitratolanthanate(III) compounds. All of these salts are thermally stable up to 300 °C under nitrogen, in contrast to some triazolium and tetrazolium counterparts.630

lost upon warming to room temperature, as indicated by IR and Raman spectroscopy. The “double salt” [C12mim]4[EuBr6]Br can be structurally understood as a solvate of the ILCs [C12mim][Br] and [C12mim]3[EuBr6] ([C12mim]3[EuBr6]· [C12mim][Br]). The [LnBr6]3−complex metal anions are quite bulky and symmetric, but sufficient structural anisotropy and amphiphilicity to induce mesomorphism is provided by the three imidazolium cations per complex anion. The [Mo6Cl14]2− cluster complex anion is also quite large, with a diameter of about 1 nm. It is surprising that only two long-chain-substituted imidazolium cations are sufficient to induce mesomorphism. Other LC complexes with [M6Xi8Xa6]n− anions (i.e., combinations of 389(397-n) with 405 or 406; Figure 66) will be discussed in section 11. The DSC thermogram of 36-DyBr6 showed multiple peaks, which was ascribed to SmA−SmA polymorphism (although this was not supported by PXRD measurements).120 SmA−SmA polymorphism had been reported before by Ikeda and coworkers for some long-chain tetraalkylammonium tetrachlorometalates.623 They proposed that several slightly different SmA phases can be formed because the alkyl chains melt rather independently from the ionic headgroups that are concentrated in the ionic sublayers, and thus can impose different space requirements depending on their specific conformation in a certain temperature range. The DSC thermograms of 36-TbBr6 and 37 displayed only a glass transition or melting peak, and a clearing peak. Melting occurs at low temperatures, despite the highly charged metal complex anions (low melting points were also found for [C4mim]x−3[Ln(NCS)x(H2O)y] ILs (x = 6−8, y = 0−2, x + y < 10)624). The luminescence properties of 36-DyBr6, 36-TbBr6, 37, and 38-n will be discussed in section 12.5. Magnetism measurements were performed on 36-DyBr6, because Dy3+ is a strongly paramagnetic ion (effective magnetic moment of Dy3+ in 36-DyBr6: μeff. = 9.6 μB at room temperature). Its large magnetic anisotropy should in principle allow one to align the mesophase in an external magnetic field,625 although this was not shown by the authors.

The blue-colored benzimidazolium salt [C12C12Bim]2[Co(NCS)4] (40) shows a SmA phase upon heating.612 The analogous copper(II) salts [CnCnBim]2[CuCl4] (n = 8, 10, 12) were found to be nonmesomorphic.631 Surprisingly, cobalt(II) complex 40 could be prepared by mixing [C12C12Bim][Br] (not [C12C12Bim][SCN]) and Co(NCS)2 in ethanol and subsequent filtration. Intermolecular short C−H···S contacts were observed in its single-crystal structure. This compound is one of the few examples of 1,3-bis(n-alkyl)benzimidazolium ILCs.515,559,632,633

Douce and co-workers reported on a new type of imidazoliumbased ILCs, in which the cationic core is substituted with a 4-(nalkyloxy)benzyl group (41-X-n).110,492,514 All mesomorphic compounds show a SmA phase. For an identical anion and alkyl chain length, salts 41-X-n show higher melting points (except for the bromide salts) and a much higher mesophase stability than the 1-methyl-3-(n-alkyl)imidazolium compounds (Figure 16a). For solutions of 41-X-12 (X− ≠ [M(CN)2]−) in acetonitrile, an irreversible reduction process was observed at ca. −1.7 V versus a SCE reference electrode by cyclic voltammetry. This corresponds to reduction of the cationic core. Salts 41Ag(CN)2-12 and 41-Au(CN)2-12 were used for the shapeselective electrosynthesis of silver and gold nanoparticles, respectively (see section 12.3).110 41-Br-12 forms aggregates in concentrated CDCl3 solutions, as seen by 1H DOSY NMR. Interestingly, the authors determined the molecular volume Vmol of 41-Br-12 as a function of temperature by means of dilatometry.492 From these values, the molecular volumes of the other compounds could be estimated by considering the methylene and anion partial volumes (from dilatometric measurements on liquid alkanes: VCH2(T) = (26.5616 + 0.02023T) Å3, with T in °C634). Vmol allows one to estimate

Tao and co-workers reported on water-free ionic lanthanidomesogens that are related to 36-LnBr 6 but contain a hexanitratolanthanate(III) anion ([La(NO3)6]3−) rather than a hexabromo complex (39-n).117 Coordination of the central lanthanide(III) ion to six negatively charged, bidentate nitrato ligands (i.e., total coordination number = 12) has the advantage that its coordination sphere is completely filled with strongly coordinating O-donor ligands, thus avoiding coordination of water, which has a detrimental influence on the luminescence properties of photoactive lanthanide ions due to vibrational quenching. Moreover, as in the case of [LnBr6]3−, the 3-fold negative charge of the [Ln(NO3)6]3− ion allows its combination with three “mesogenic” cations. This would also be possible with highly luminescent [Ln(dpa)3]3− (H2dpa = 2,6-pyridinedicarboxylic acid) complexes that contain three tridentate dpa2− (i.e., pyridine-2,6-dicarboxylate or dipicolinate) ligands,626 but ILCs AG


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the so-called “molecular (cross-sectional) area” AM in the smectic mesophase, via the expression AM = NVmol/d, in which N is the number of “superposed” cationic headgroup−anion combinations within the ionic sublayers, and d is the smectic layer thickness.65,115,116,122,492 More precisely, AM is the projection area of the cation−anion assembly within the smectic plane. The molecular packing in the SmA phases shown by amphiphilic ionic mesogens of the type described thus far, is in nearly all cases one in which the cationic headgroups and anions of two different molecules interact in a “head-to-head” arrangement (i.e., the ionic sublayers are in fact bilayers), with the alkyl chains homogeneously distributed on either side of these ionic sublayers. For such arrangement, N equals 2. It was found that while AM obviously varies with the size (and shape) of the anion, the thickness d0 of the ionic sublayers at a given temperature is relatively independent of the anion choice. The difference between the projection area of the cationic headgroup-anion assembly within the smectic plane, and the cross-sectional area of the interdigitating alkyl chains, needs to be compensated by a strong folding of the alkyl chains, the effect being larger for larger anions and higher temperatures (see also below in section 5.4, Figure 28). AM increases with temperature, indicating a decreasing “gathering” (or in other words, “relaxation”) of the ions in the ionic sublayer. For a given anion, the temperature dependence of AM is larger for longer chain lengths. Interestingly, when plotting the (extrapolated) AMT isotherms as a function of chain length, it is seen that these isotherms cross each other on the lower side of the X axis. The intersection of a horizontal line through the AM value of this point, with the curve connecting the AMmax values close to isotropization, corresponds roughly to the alkyl chain part that is incorporated in what the authors call the diffuse interface between the ionic and aliphatic sublayers, so more or less to the minimal chain length required for (smectic) liquid-crystallinity. Indeed, the alkyl chains need to jut out of the “interface layer” to form the aliphatic sublayer. The authors deduced that on average seven methylene groups are inside the interface layer (this value might be related to the “symmetrybreaking region” that was defined by Seddon and Rothenberg for 1-methyl-3-(n-alkyl)imidazolium cations (see above)230). The thermal behavior of the homologues with a n-hexyl or a n-heptyl chain was not reported as a further support. While significant alkyl chain folding has been often proposed to describe the molecular packing in mesophases formed by ILCs (see also section 4.3.2),65 Douce and co-workers presented additional evidence: PXRD measurements with a 2D detector on an aligned monodomain sample of 41-PF6-12 revealed a good alignment of the smectic layers (narrow first- and possibly higher-order spots on the meridian), yet the diffuse band in the wide-angle region corresponding to the lateral packing of the molecules did not lie on the equator but was spread over the whole azimuthal range. This was also observed for guanidinium-based ILCs.491

imidazolium moieties.63,349,371,400,486,515,516,520,632,633,636−639 The I− and [BF4]− bis- and tris(imidazolium) salts are LC, showing an enantiotropic SmA phase. The [NTf2]− compound with three imidazolium groups shows a monotropic SmA phase. Although the trisubstituted compounds seem to have a disk-like shape at first sight, considerable conformational freedom of the imidazolium moieties due to the methylene linker to the central benzene ring (as well as steric hindrance by the methyl groups) prohibits the formation of a columnar mesophase. In fact, the imidazolium rings are supposed to be oriented perpendicularly to the plane of the central ring, with the alkyl chains extending away from this plane. It should be noted that, for a particular anion, the cation structure does not affect the melting point very much, while the clearing point is much higher for the tris(imidazolium) salts. This supports the hypothesis that the clearing process involves mainly the collapse of the sublayers formed by the ionic moieties, which interact with each other via electrostatic interactions and potentially hydrogen bonding.537 The authors investigated supercooling of the SmA phase shown by 42-BF4, and the glass transition from the mesophase to a metastable, solid lamellar phase that structurally resembles the LC phase but has a higher degree of order. The degree of order of the metastable phase is influenced by the rate of cooling from the SmA phase. In contrast to the SmA phase, the layers in the low-temperature phase contract with decreasing temperature.

Casal-Dujat et al. prepared similar imidazolium-based compounds, including macrocyclic derivatives (protophanes 44-n and cyclophanes 45-X-n).520 The “closed” macrocyclic compounds (45-Br-n) display a higher melting and clearing point than their “open” counterparts (44-n). The mesophase stabilization probably results from the lower conformational flexibility of the cyclophanes. Salt 45-NTf2-18 shows a thermotropic cubic phase, but this phase was not characterized in detail. In the case of 45-Br-18, the authors observed that the chemical shift of the H(2) protons of the imidazolium rings and the benzene rings moved upfield in DMSO-d6 as compared to CDCl3, as a result of decreased interactions between the imidazolium moieties and the bromide anions.

Pleixats and co-workers presented ILCs based on mesitylenecontaining bis- and tris(imidazolium) salts (42-X and 43-X).635 These belong to the few examples of low molar mass thermotropic ILCs wherein the cationic core contains multiple

The thermotropic mesophase behavior of other, symmetric gemini imidazolium salts (46-n, 47-n, 48-X-m/n/n (x = n), and 49-X-m/n) was investigated by Gin and co-workers,637 and by AH


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Mudring and co-workers.400 [Remark: “Gemini surfactants” (or “dimeric surfactants”) are amphiphiles consisting (in sequence) of a hydrophobic tail, a polar group, a spacer, another polar group, and another hydrophobic tail. When the two polar groups are not the same, the term “heterogemini” is used.] The alkynebridged bis(imidazolium) bromide salts 47-n show a lower thermal stability than the other compounds, but 46-12, 48-Br-6/ 12/12, and 49-Br-1/12 are thermally more stable than [C12mim][Br]. It had been shown before for analogous, nonmesomorphic ILs that the dicationic salts are thermally more stable than their monocationic counterparts.640 For 48-Xm/n/n, the length of the flexible spacer between the imidazolium cations was found to have the most significant effect: only the salts with a relatively short spacer in combination with sufficiently long hydrophobic tails show a smectic phase. [Remark: A polymerizable derivative of 48-Br-6/n/n has been used in combination with glycerol as a bicontinuous cubic lyotropic LC precursor for the preparation of nanoporous thin-film membranes.641] On the other hand, most gemini salts with an oligo(ethylene oxide) spacer (49-X-m/n with m = 1, 2, 3) are mesomorphic, possibly thanks to more efficient nanosegregation (similar gemini quaternary ammonium salts had been reported before642). Replacement of the bromide anions by [BF4]− anions resulted in a lowering of transition temperatures and narrowing of the mesophase temperature range. When comparing [C12mim][X] and [C14mim][X] (X− = Br−, [BF4]−) with 48X-6/12/12 and 48-X-6/14/14 (X− = Br−, [BF4]−), it appears that both melting and clearing points are higher for the gemini salts. In a series of asymmetric bis(imidazolium) salts (48-X-6/ n/1 (m = 6, x = 1)), compounds 48-Br-6/16/1, 48-BF4-6/18/1, and 48-BF4-6/20/1 were found to show a thermotropic bicontinuous cubic phase, most probably of Ia3̅d symmetry.638 The latter salt also shows a lyotropic bicontinuous cubic phase upon addition of water, glycerol, or the IL [C2mim][BF4]. Apart from the bis(imidazolium) salts, symmetric bis(n-alkyl)-tris(imidazolium) compounds 50-X-n were studied as well.486,639 The bromide salts show optically uniaxial smectic phases for n ≥ 16. As compared to the series 48-Br-6/n/n, longer alkyl chains are necessary to compensate for the extended linear tris(imidazolium) core. The [BF4]− salts show higher clearing points in this case and are already LC for n ≥ 12. It was found that very long alkyl chain lengths produce, counterintuitively, more ordered phases (i.e., SmX). In general, the smectic phases of salts 50-X-n are highly viscous and show slow formation kinetics, as expected for a highly charged system. MD simulations of 50-X20 showed that the bulkier [NTf2]− anions result in less dense ionic sublayers and a higher extent of interdigitation between alkyl chains originating from neighboring ionic regions.

investigated the thermotropic and lyotropic LC behavior. The tris(imidazolium) chloride salts with n ≥ 10 show a SmA phase at room temperature, whereas, interestingly, the corresponding tris(benzimidazolium) salts show a columnar mesophase. The phase behavior of the compounds with four cations is not entirely clear, although the tetrakis(imidazolium) chloride salts seem to show either a columnar or a SmA phase. All corresponding [NTf2]− compounds are liquids at room temperature.

Atypical imidazolium-based ILCs with a chiral camphorsulfonamide part were reported by Hesemann and co-workers (52-Xn).643 Mesomorphism was only observed when both the imidazolium cation and the anion contained a long alkyl chain.

Wasserscheid, Meyer, and co-workers reported strong nonNewtonian viscosity behavior for the SmA phases shown by both [CnCnim][BF4] and [CnCnim][ClO4] (n = 12, 16).359,553 This means that the viscosity depends on the shear rate applied to the sample. While in the isotropic liquid phase the viscosity is relatively low and independent of shear rate, it steeply increases and starts to show non-Newtonian behavior when cooling to the underlying SmA phase (Figure 21). Similar observations had been reported earlier by Seddon et al. for [Cnmim][X] ILs and ILCs.644 Burrell et al. observed non-Newtonian viscous shear thinning in several imidazolium-, ammonium-, and pyrrolidinium-based ILs (however, upon application of a sufficiently high temperature the liquids adopted the characteristics of a Newtonian fluid again).265 Other rheological and tribological studies on imidazolium-based ILs (including [C12mim][PF6]) and ILCs ([C12mim][Cl] and [C12mim][Br]) were performed by Amann et al., who also found strong non-Newtonian viscosity behavior and viscoelastic properties in the LC phase (they also studied the orientation of the mesomorphic molecules under shear stress and its influence on the viscosity).360 Mudring and

Al-Mohammed et al. recently reported on tris(imidazolium), tris(benzimidazolium), tetrakis(imidazolium), and tetrakis(benzimidazolium) salts with a tetrahedral core (51-R-Yn-n), as a new class of biodegradable surfactants.632,633 They also AI


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Figure 21. Viscosity versus shear rate for [C12C12im][BF4], which shows a SmA phase between 50 and 69 °C. Reproduced with permission from ref 359 (http://dx.doi.org/10.1039/b914939b). Copyright 2009 The Royal Society of Chemistry. Figure 22. Alignment of the supramolecular columns in the Colhex phase of 53-BF4-12, parallel to a supporting glass slide with gold electrodes, by mechanical shearing: creation of 1D ion-conductive channels, formed by the ionic cores (depicted in pink) of the columns.18,390,647 Reprinted with permission from ref 18. Copyright 2006 Wiley.

co-workers reported that 46-12, 47-12, 48-Br-6/12/12, and 49X-1/12 show non-Newtonian viscosity behavior as well.400 This apparently general property of ILCs could be advantageous for the use as working fluids in mechanical applications. Wu and coworkers studied the rheological properties of more complex, laterally substituted ILCs, as well as [C18mim][PF6] and [C18dmim][PF6] (see section 6.3).361 They also observed nonNewtonian viscosity behavior for these salts below their clearing point. Maximo et al. found non-Newtonian behavior for mixtures of ethanolamine and diethanolamine with oleic acid.645 Mann and co-workers, and Clark, Herrmann, and co-workers, observed the same for LC complexes of positively and negatively charged polypeptides with oppositely charged surfactant ions, in their respective LC states.168,646

cylinder structure. Macroscopic alignment of the supramolecular columns parallel to a supporting glass slide could be achieved by shearing of a polydomain sample between two microscopy glass slides (i.e., rubbing treatment of the glass slides was not necessary), and results in 1D ion-conductive channels formed by the ionic cores of the columns (Figure 22).647 For example, 53BF4-8 shows the following conductivities in its oriented Colhex phase at 130 °C: σ|| = ionic conductivity parallel to the long axes of the columns = 4.8 × 10−4 S cm−1, and σ⊥ ≈ σ||/10. The authors tentatively attributed the relatively low anisotropy σ||/σ⊥ to the possibility that the aligned columnar structures do not completely extend from one electrode to the other, due to the fluidity in the mesophase and liquid crystal defects. Nevertheless, the ionic conductivity is anisotropic in the columnar mesophase, but not in the isotropic liquid phase. The anion is supposed to be

5.2. Taper-Shaped Imidazolium-Based Mesogens

In principle, the taper-shaped mesomorphic imidazolium salts that will be considered in this section also have a predominantly amphiphilic character. The reason why we prefer to group these compounds in a separate section is the fact that they generally show a phase behavior different from that of the salts described above: in addition to smectic phases, columnar and cubic phases are very common. The phase behavior is highly dependent on the length of the alkyl chains that are attached to the imidazolium core in the “apex” of the wedge shape. In the wider context of LC research, the mesomorphism of (neutral) taper-shaped amphiphiles, and the importance of nanosegregation for it to be observed, has been thoroughly investigated for many years.17,18,23,24,27 Kato and co-workers were the first to prepare taper-shaped imidazolium-based mesogens (53-X-n, 54-X).217,647−649 Most of the compounds with three sufficiently long alkyl chains show a Colhex phase. The Colhex phase is formed as a result of nanosegregation of the ionic cores and the benzyl moieties, on the one hand, and the hydrophobic alkyl chains, on the other hand (Figure 22). It should be mentioned that, although one can conceptually regard the columnar structures as stacked slices that contain a certain number of self-assembling molecules, in reality there are no discrete slices for such taper-shaped amphiphilic compounds.23 Besides the molten aliphatic chains, the aromatic parts that make up the center of the columns are quite mobile. No discrete π−π stacking distance has been observed by PXRD in any of the columnar phases formed by simple taper-shaped amphiphiles. It should also be mentioned that the single-crystal structure of the chloride salt 53-Cl-12·H2O consists of stacked bilayers, not of supramolecular cylinders;571 apparently, the thermal motion of the molten alkyl chains in the mesophase causes the transition from a lamellar to a phase-segregated

the dominant charge carrier in the ion-conductive channels; the large cations are less mobile. The σ|| values at 110 °C increase in the order [PF6]− > [BF4]− > [OTf]−, while the anisotropy σ||/σ⊥ decreases with an increase in anion size (e.g., for 53-BF4-16, σ||/ σ⊥ = 1.1 × 102 at 110 °C; for 53-PF6-16, σ||/σ⊥ = 8.1 × 101 at 110 °C; and for 53-OTf-16, σ||/σ⊥ = 1.5 × 101 at 110 °C).648 Because the conductivity depends on the anion type, the predominant conduction mechanism is supposed to be diffusion and/or hopping of anions through the columnar structures (see also section 6.2650,790 and section 7). Besides the planar alignment, the columns could also be oriented perpendicularly to an ITO glass slide, after surface treatment of the slide with (3aminopropyl)triethoxysilane to make it hydrophilic.390 The authors demonstrated that both types of alignment in the LC phase could be fixed by in situ photopolymerization of a similar AJ


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monotropic for 53-OTf-12. Interestingly, the intercolumnar distances in the Colhex phases of the [OTf]− salts are smaller than those in the Colhex phases of the [BF4]− and [PF6]− salts. The authors attributed this to intrusion of the [OTf]− anions into the hydrophobic regions, due to their weaker Coulombic interactions with the imidazolium cations. Addition of LiBF4 (25 mol %) to 53-BF4-12 slightly raises the clearing point (from 183 to 193 °C); the metal salt appears to stabilize the ionic columnar cores in the mesophase (this was also observed for the smectic phase of a mixture of LiBF4 and [C12C2im][C12H25SO3]607 (see above)). Furthermore, both σ|| and σ||/σ⊥ increased after addition of LiBF4. Mixtures of LC zwitterionic analogues of 53-X-12 (458and 459) with LiNTf2 and propylene carbonate were investigated as lithium-ion-conducting materials (see section 12.1).616 In an attempt to increase the ionic conductivity by tapershaped imidazolium-based mesogens, the Kato group decided to adopt another strategy: instead of preparing compounds with a covalent bond between the ionic and hydrophobic parts, they investigated mixtures of neutral, taper-shaped dihydroxy amphiphiles (NDA1-n, consisting of an ionophilic and an ionophobic part, see Figure S8(a) for their structure) and simple aprotic imidazolium ILs (57-X) with short alkyl chains, in a “noncovalent approach”.217 In such mixtures, the mobility of the ionic parts should be enhanced. Compounds NDA1-8 and NDA1-12 were found to be miscible with 57-Br up to a molar fraction of 0.50 for 57-Br. This is because the dihydroxy

compound having two acrylate groups at the periphery (55), leading to transparent 1D ion-conductive polymer films.390 The groups of Kato and Ohno had reported before on 2D ionconductive polymer films.651−653 It should be noted that 1D ion conduction through aligned Colhex phases fixed by polymerization had already been achieved for nonionic crown etherbased taper-shaped LCs in 2000.654 Nonionic LC diblock copolymer films with perpendicularly oriented poly(ethylene oxide) cylindrical domains were reported to show anisotropic 1D ion conduction as well.655 In general, the concept of stabilization of columnar LC phases by cross-linking polymerization had been explored before, both for thermotropic and for lyotropic (ionic and nonionic) LCs.654,656−669 More recently, Feng et al. could prepare polymer membranes with vertically aligned 1 nm pores through photo-cross-linking of the magnetically aligned thermotropic and lyotropic Colhex phases shown by a polymerizable, taper-shaped, amphiphilic sodium carboxylate compound.670 The solid polymer films of 55 showed a higher anisotropy of ionic conductivity the aligned LC monomers prior to cross-linking, presumably thanks to fixation and decreased thermal fluctuation of the oriented columns due to polymerization (e.g., for the film made from 55 with “vertical” column orientation, σ||/σ⊥ ≈ 2.6 × 102 at 50 °C and 1.4 × 103 at 150 °C; for the film with “parallel” column orientation, σ||/σ⊥ ≈ 1.2 × 102 at 150 °C; these values are 10−100 times higher than for unpolymerized 55). The anisotropy also remained until higher temperatures because it did not vanish due to transformation into an isotropic liquid. Compound 56 is not LC (indicating the importance of dipole−dipole interactions between the ester groups to stabilize the columnar packing), nor the analogues of 55 and 56 with three bulky acrylate groups instead of two. For all compounds in the series 53-X-n that contain the same anion, the melting point increases with increasing chain length, whereas the clearing point first increases but then decreases again when the alkyl chains become too long. Introduction of a methyl group at the 2-position of the imidazolium ring (54-X) has no severe detrimental impact on the mesomorphism; [OTf]− salt 54-OTf even shows an enantiotropic Colhex phase, while this phase is only

compounds can form stable hydrogen bonds with the imidazolium cations and particularly with the bromide anions (confirmed by 1H NMR and FT-IR spectroscopy).215,216,217 The [BF4]− salt 57-BF4, on the other hand, is not miscible with the

Figure 23. Top: Relationship between the nanosegregated structure of the mesophase and the ionic conductivity for an NDA2/58-Br (90:10) mixture (see Figure S8(a) for the structure of neutral amphiphile NDA2). Bottom: A 45:55 mixture of NDA2 and 58-Br forms a “normal-type” bicontinuous cubic phase (CubV1), in contrast to the 90:10 mixture (top) that forms an “inverted-type” bicontinuous cubic phase (CubV2). Reproduced with permission from ref 221 (http://dx.doi.org/10.1039/c2sc00981a). Copyright 2012 The Royal Society of Chemistry. AK


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70%. This system thus offers remarkable tunability: it is possible to switch between the two CubV phase types just by changing the relative amounts of the neutral amphiphile and the noncovalently bonded, nonvolatile IL. It should be noted that most of the reported bicontinuous cubic phases shown by thermotropic ILCs (see below) are “inverted-type” phases, in which the continuum between the interconnected ionic nanochannels is formed by aliphatic chains. To the best of our knowledge, the only exceptions are the CubV phases shown by 89-(1-Me-imid)-X-n and 89-(1,2-Me-imid)-X-n (see section 5.3);63 those formed by binary mixtures of N-(n-alkyl)-N,N,N-trimethylammonium chlorides and some transition metal chlorides (see section 6.1);675 and those shown by (240-n/1)/HNTf2 (1:1) with 2−3 wt % of water (see section 7; these systems can be considered as lyotropic LCs).676 Cho and co-workers presented ion-doped LC block codendrimers that can show either a CubV1 phase or a CubV2 phase, depending on their (covalent) substitution pattern.355 Remarkably, only a columnar phase and a smectic phase were observed for mixtures of NDA2 and 57-Br, the only difference with the NDA2/58-Br system being the lack of an ether group in 57-Br. Furthermore, NDA3/58-Br mixtures form a Colhex phase, whereas NDA3/57-Br mixtures are not LC, and the isotropization temperatures of NDA2/58-Br mixtures are slightly higher than those of NDA2/57-Br mixtures. This emphasizes the importance of the ether oxygen atom to reinforce the hydrogenbonding interactions between the amphiphile and IL components. Mixtures of NDA2 with ILs with an amino acid anion (57Gly (X− = [Gly]− = deprotonated form of glycine), 57-Ala (X− = [Ala]−), and 57-Leu (X− = [Leu]−)) show a rich mesophase behavior, including Colhex, CubV, and smectic phases.223 The amino acid anions have a high hydrogen-bonding capability. Recently, the “noncovalent approach” was combined with the concept of fixation of 1D ion-conducting channels through in situ photopolymerization, which was discussed above for 55. For this purpose, IL 57-Br was mixed with a neutral dihydroxy amphiphile that contains three polymerizable 1,3-dienyl groups (NDA5, see Figure S8(a)).224 These mixtures show a Colsqu mesophase for mole fractions of 57-Br between 20% and 50% (Figure S8(c)). At low temperatures, the nanostructured polymer film obtained from an equimolar mixture of NDA5 and 57-Br, in which the IL is confined inside the 1D channels, shows a 12 times higher ionic conductivity parallel to the columnar axes than a polymer film made from 55. It is interesting to compare the anisotropies of ionic conductivity of columnar ILCs with those of electric conductivity of columnar π-conjugated LCs. The anisotropies of electric conductivity of triphenylene677 and tricycloquinazoline678 derivatives were found to be in the range of 102−103. Bruce and co-workers reported on dimeric analogues of compounds 53-X-n (59-X-m/n).671 Like 53-X-n, these symmetric dimers show Colhex phases, but over broader temperature ranges than their asymmetric counterparts, particularly for short spacers and long alkyloxy chains. Moreover, the thermal stability of the dimeric compounds is slightly higher. The Colhex phase of 59-BF4-4/12 extends over more than 250 °C, from below room temperature to 256 °C. Because the chloride salts 59-Cl-m/n underwent decomposition at their clearing point, no characteristic POM textures could be obtained on cooling from the isotropic liquid. However, POM contact experiments with the analogous [BF4]− salts 59-BF4-m/n (which clear well below their decomposition temperature) pointed to Colhex phases (the samples were continuously miscible within their mutual

dihydroxy compounds and does not form a homogeneous LC phase, probably because of weaker interactions with [BF4]− and because of the slightly larger size of this anion. Thus, the structure of the Colhex phase appears to be quite sensitive to the anion type, because the same group had previously found good miscibility between [C2mim][BF4] and rod-like hydroxyl-terminated amphiphiles; these mixtures showed smectic phases.215,216 Mixtures of NDA1-8 and 57-Br (100:0−50:50) show a Colhex phase, whereas mixtures of NDA1-12 and 57-Br show a micellar cubic phase (100:0−80:20) or a Colhex phase (70:30−50:50). One can consider these systems as “lyotropic” LCs, with the nonmesomorphic IL acting as the solvent. In comparison to classical aqueous lyotropic LC systems, ILs offer additional functionality and the advantage of nonvolatility. For an aligned sample of the Colhex phase exhibited by an equimolar mixture of NDA1-8 and 57-Br (obtained by mechanical shearing), a σ|| value of 3.9 × 10−3 S cm−1 was found at 50 °C. For comparison, the aligned Colhex phase displayed by “covalent-type” 53-Br-8 shows a σ|| value of only 5.3 × 10−6 S cm−1 at 50 °C. The anisotropy values σ||/σ⊥, however, are similar for both the “noncovalent-type” and the “covalent-type” materials. Later on, the “noncovalent approach” was extended to amphiphilic diethanolamines (NDA2, NDA3, and NDA4; see Figure S8(a)) instead of amphiphiles NDA1-n.221 Just as in the case of NDA1-n and 57-X, the diethanolamines are only miscible with the bromide salts 57-Br and 58-Br, but not with 58-BF4, despite the ability of the ether oxygen atom to act as a hydrogenbond acceptor. By mixing NDA2 and 58-Br in different molar ratios, it was possible to obtain Colhex, bicontinuous cubic, as well as smectic phases (Figure S8(b)). In contrast to the micellar cubic phases shown by NDA1-12/57-Br (100:0−80:20), the bicontinuous cubic phases contain 3D interconnected ionic nanochannel networks, which function as efficient transportation pathways for ions even in a nonaligned (polydomain) sample (Figure 23; see also sections 5.3, 6.2, and 7). For smectic and columnar LC materials, macroscopic alignment of mesophase domains to avoid discontinuities is required to achieve high conductivity. The advantage of a bicontinuous cubic mesophase structure for ion conduction had already been reported by the same group for taper-shaped ammonium-based mesogens 161X-m/n and 164 (see section 6.2).672−674 For the NDA2/58-Br system, small increases in IL content were found to result in significantly higher ionic conductivity values (e.g., at 55 °C, in the cubic phase: σ = 7.1 × 10−6 S cm−1 for NDA2/58-Br (90:10); σ = 4.0 × 10−5 S cm−1 for NDA2/58-Br (80:20); and σ = 1.7 × 10−4 S cm−1 for NDA2/58-Br (70:30)), due to a higher amount of mobile ions. For NDA2/58-Br ratios between 90:10 and 70:30, an “inverted-type” CubV2 phase is formed, whereas a “normaltype” CubV1 phase is observed for ratios between 45:55 and 40:60 (i.e., mixtures with a higher mole fraction of the hydrophilic IL) (Figure 23). [The terms “inverted-type” and “normal-type” stem from their use in the context of lyotropic bicontinuous cubic LCs in an aqueous environment, to differentiate between phases in which the central parts of the interconnected nanochannels are formed by the hydrophilic moieties or hydrophobic moieties, respectively (see also Figure S1). In general, “normal-type” refers to phase structures where the stronger attractive forces (electrostatic interactions and/or hydrogen bonding, versus weaker van der Waals interactions) are located in the continuum outside the supramolecular aggregates, whereas “inverted-type” refers to phase structures where the stronger attractive forces are located inside the aggregates.] Liquid-crystallinity is lost if the mole fraction of 58-Br exceeds AL


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mesophase ranges). Karl Fischer titration measurements and CHN elemental analysis indicated that the chloride salts are, as expected, very hygroscopic.

More recently, a taper-shaped imidazolium salt was described that contains three rod-like azobenzene moieties connected to the termini of the alkyl chains (60).441 In contrast to the previously discussed taper-shaped mesogens, this salt exhibits a SmA phase and a nematic phase. It spontaneously aligns homeotropically on glass and quartz substrates upon cooling from the isotropic liquid state, so that the ion-conductive pathways in the SmA phase are on average parallel to the substrate surface (Figure S9). Photoisomerization of the azobenzene chromophores to their bent-shaped cis states under the influence of nonpolarized 365 nm light causes a transition to a viscous, disordered liquid state. Subsequent illumination of this photostationary state with linearly polarized 540 nm light triggers the recovery of the trans isomers, and during this process the chromophores will also reorient and align themselves parallel to the linear polarizer. After annealing, this results in a SmA phase whose ion-transporting pathways are now perpendicular to the substrate surface (Figure S9). This is a very elegant way of switching the macroscopic orientation of 2D ionconducting sheets in a thin film, in a noninvasive and reversible way. It should be noted that photoinduced reorientation of azobenzene-containing ILCs has been used before to align LC domains and achieve optical anisotropy, but not in the context of anisotropic ion conduction.156,157,456−458,679,680 Furthermore, anisotropic ion transport was not only observed in the photoaligned SmA phase of 60, but also in the nematic phase upon heating. During the SmA-to-N transition, the anisotropy changes, because the ion conduction along the long molecular axis of 60 becomes more efficient than that perpendicular to this direction (which corresponds to the original direction of the ionic sublayers in the SmA phase, Figure 24).

Figure 24. Schematic representation of the anisotropic ion transport in the photoaligned SmA phase of 60 (right), and the “inverse” anisotropic ion conduction in its nematic phase upon heating (left). Reprinted with permission from ref 441. Copyright 2014 American Chemical Society.

Figure 25. Schematic representation of the LC mesophases formed by 61-n, 62-n, and 63-n. Reprinted with permission from ref 375 (http:// dx.doi.org/10.1016/j.tetlet.2010.01.045). Copyright 2010 Elsevier.

to form hydrogen-bonded dimers (e.g., 4-(n-octyloxy)benzoic acid, Cr · 101 · SmC · 108 · N · 148 · I (°C); 61-8, Cr · 90 · SmC · 117 · I (°C)). Similar work bridging the field of ILCs and hydrogen-bonded LCs had been performed before by Kresse and co-workers, who also constructed binary phase diagrams.412,422,681,682 Kohmoto et al. published interesting work on ILCs formed by noncovalent interactions with imidazolium cores: they mixed a nonmesomorphic pyridine-substituted imidazolium chloride salt with a 4-(n-alkyloxy)-, 3,4-bis(n-alkyloxy)-, or 3,4,5-tris(nalkyloxy)benzoic acid in a 1:1 molar ratio.375 The pyridine moiety and the carboxylic acid can interact via a hydrogen bond/ acid−base interaction. Depending on the alkyl chain number, a SmC (61-n), Colrec (62-8 and 62-12), or monotropic micellar cubic phase (63-n) was obtained, albeit over rather narrow temperature ranges (Figure 25; it should be noted that salts 61-n do not have a taper shape). As mentioned before, the SmC phase is seldomly observed for ILCs. It is remarkable that the transition temperatures are hardly influenced by the alkyl chain length. Compound 62-16 does not form a columnar LC phase because the alkyl chain volume is too large. The phase behavior differs from that of the pure substituted benzoic acids, which are known

Douce and co-workers completed the research on ILCs with a substituted benzyl group attached to the imidazolium core (see above, 41-X-n and 53-X-n), by synthesizing the variants with two alkyloxy chains (64-X-n).500 These compounds show the most AM


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anisometric structure. Moreover, π-conjugated fragments can add electronic functions to the ILC. In this section, we will consider those systems that consist of a (cat)ionic core, with one or more mesogenic groups attached to it (Scheme 4, Scheme 5). It should be noted that many potential structures have been patented.685 The first examples of mesomorphic imidazolium salts with mesogenic groups were described by Koide and co-workers: they prepared salts consisting of a rod-shaped 4-(n-butyloxy)-4′biphenyl moiety connected via a flexible alkyl spacer (C8 or C12) to a 1-methylimidazolium fragment (65-X-n).686 Both the Br− and the [BF4]− salts exhibit a crystal smectic E phase and a higher-temperature SmA phase. A longer spacer resulted in slightly higher transition temperatures. In a follow-up report, the same group presented an analogous compound with a 1vinylimidazolium instead of a 1-methylimidazolium moiety (80).687 This salt shows only a SmA phase. Photopolymerization of 80 in its mesophase (at 155 °C) yielded a polymer, which exhibits flow between room temperature and about 200 °C. A highly ordered smectic phase, presumably a crystal smectic E phase, was proposed on the basis of POM and PXRD observations.687 Kumar and Pal attached two rod-shaped cyanobiphenyl groups to a central imidazolium core (84-Br-m/n).370 They observed a monotropic SmC phase in the case of 84-Br-9/9 and 84-Br-4/9, and, surprisingly, an enantiotropic SmC phase in the case of 84-Br-6/9. The phase assignment was based on the observed Schlieren texture with four-brush point defects. The achievement of a tilted SmC phase instead of an orthogonal SmA (or E) phase demonstrates the power of the concept of attaching anisometric mesogenic groups to simple ionic moieties: SmC phases have never been found in amphiphilic LCs without rigid cores.23 Goossens et al. connected cyanobiphenyl and cholesteryl mesogenic groups to imidazolium cores via a flexible C11 spacer, and combined the resulting cations with different anions (66-X, 67-X, 84-X-11/11, and 85-X), including a bulky but highly luminescent tetrakis(2-thenoyltrifluoroacetonato)europate(III) complex anion ([Eu(tta)4]−, Figure 26).122 The Br− and [NTf2]− salts with a cholesteryl group display wide mesophase temperature ranges, with 67-Br and 67-NTf2 showing a SmA phase already at room temperature. The cholesteryl group appeared to be a stronger mesogenic promoter for these systems, as the Br− and [NTf2]− cyanobiphenyl-containing salts only show monotropic mesophases. Nevertheless, attachment of two cyanobiphenyl groups to one imidazolium cation (84-Br-11/11 and 84NTf2-11/11) yielded rare examples of ILCs showing a weakly ordered nematic phase. The nematic phase was unequivocally recognized by its Schlieren texture with two- and four-brush disclinations and by its very characteristic thread-like texture. A possible explanation for the appearance of a nematic phase might be an effective “shielding” of the strong electrostatic interactions (which tend to stabilize lamellar arrangements) by hydrogen bonding between the anions and the imidazolium rings, as seen in the single-crystal structure of 66-Br (Figure 27). Such theory was also used to explain the nematic phase formation by anhydrous silver(I) stilbazole complexes with short alkyloxy chains, which were found to form tightly associated (“uncharged”) ion pairs (see section 4.3.1). The number of cyanobiphenyl groups should nevertheless play an important role as well, because no nematic phase was observed for 66-Br and 66-NTf2. It is surprising that Kumar and Pal observed a SmC phase instead of a nematic phase for 84-Br-9/9.370 It is known

interesting phase behavior: depending on the chain length, SmA, bicontinuous cubic (Ia3̅d) or Colhex phases were observed (64-X6: SmA; 64-X-12: CubV + Colhex; 64-X-16 and 64-X-18: Colhex). Such evolution from smectic mesomorphism for one-chain compounds (41-X-n) to columnar mesomorphism for threechain compounds (53-X-n), with the intermediate two-chain compounds (64-X-n) showing smectic, cubic, as well as columnar phases depending on the chain length, has been commonly observed for (nonionic) amphiphilic compounds in the past.23,24 Nanosegregation has been shown to play a crucial role in the mesomorphism. The occurrence of columnar mesomorphism with increasing chain length/number is a result of the increasing curvature at the ionic/aromatic-aliphatic interface. As was done for 41-Br-12, the molecular volume Vmol of 64-Br-12 was measured as a function of temperature by dilatometry. The Cr-to-Cub transition is accompanied by an abrupt increase of Vmol, corresponding to the melting of the alkyl chains. Above this transition, Vmol increases quasi-linearly with temperature. Because of increasing chain mobility and conformational disorder (increasing number of gauche conformers), the columnar cross-sectional area S in the Colhex phases decreases with increasing temperature, the effect being greater for longer alkyl chain lengths. This is also the reason why the smectic layer thickness d in the SmA phase decreases as a function of temperature. Interestingly, the AM value at the isotropization temperature is equal for 64-Cl-6 (T(SmA-to-I) = 180 °C) and 64-Br-6 (T(SmA-to-I) = 165 °C): this indicates that the mechanism of breakdown of the ionic sublayers is the same for both compounds, and that it is governed by the temperaturedependent molecular cross-sectional area (i.e., there is no balance anymore between the projection area of the cationic headgroupanion assembly, and the cross-sectional area of the interdigitating alkyl chains). Molecular dynamics (MD) simulations showed that in the Colhex phases the molecules are not confined into slices that are stacked on top of each other, but instead densely aggregated around the central columnar axes (see also the comment in the beginning of this section). It was proposed that these columnar structures are preserved upon cooling to the CubV phases, in which two intertwined but unconnected 3D continuous networks of short column segments exist. Molecular flexibility is important in this aspect. The transformation may occur through regular column undulations and interconnections, which increase substantially with decreasing temperature.500 The authors also investigated the possibility of using these imidazolium salts as vectors for short interfering RNA (siRNA) transfection (in formulations with a neutral colipid).

5.3. Attachment of Mesogenic Groups To Influence the Mesophase Behavior

As mentioned in section 3.5, the attachment of “mesogenic” or “mesophase-inducing” molecular fragments to other moieties is a powerful strategy to induce LC behavior. The concept has been successful to produce, for instance, LC C60 fullerenes90,683 and LC octasilsesquioxanes.684 The role of the mesogenic groups usually lies in imparting the molecular assembly with the shape anisotropy that is required for liquid-crystallinity. Therefore, classical mesogenic moieties often have a rod-like or otherwise AN


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Scheme 4. Imidazolium-Based ILCs That Contain at Least One Rod-like Mesogenic Group That Is Connected to the Cationic Core via a Flexible Spacer114,122,370,371,378,636,686−695

AO


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Scheme 4. continued

that the spacer length in LC dimers and oligomers can have a great influence on the phase behavior,103−105 as also seen in the series 84-Br-m/n. Apart from the induction of a nematic phase, another achievement thanks to the use of mesogenic promoters was the creation of a LC luminescent [Eu(tta)4]− salt, 85Eu(tta)4. The large isometric metal fragment can be counterbalanced by the two large cholesteryl groups, which allows the formation of a smectic mesophase. This phase could be vitrified on fast cooling. On the contrary, [C18mim][Eu(tta)4] and 66Eu(tta)4, 67-Eu(tta)4, and 84-Eu(tta)4-11/11 are not LC. In their paper, Goossens et al. also showed that careful fitting and modeling of the small-angle X-ray diffraction patterns to determine the electron density profile of the smectic phases can reveal the detailed molecular arrangement in the mesophases.122 Each sublayer corresponds to a different level in the electron density profile. Because the anions of ILCs often are/contain

Figure 26. Left: Structure of the tetrakis(2-thenoyltrifluoroacetonato)europate(III) complex anion ([Eu(tta)4]−, Htta = 2-thenoyltrifluoroacetone). The imidazolium salts containing this anion show an intense red photoluminescence. Right: Single-crystal structure of [C18mim][Eu(tta)4] (which is not LC), showing the globular shape of the metal complex anion, and hydrogen bonding between the oxygen atoms of the β-diketonate groups and the imidazolium rings. Reprinted with permission from ref 122. Copyright 2008 American Chemical Society.

Figure 27. Hydrogen bonding in the single-crystal structure of 66-Br, resulting in the formation of trans-dimeric units. Reprinted with permission from ref 122. Copyright 2008 American Chemical Society. AP


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heavy, electron-rich atoms (e.g., Br−; I−; sulfur in [OTf]− and [NTf2]−; etc.), their separation in the ionic sublayers of a smectic phase can dominate the diffractogram at higher angles, apart from the diffuse signal that corresponds to the lateral short-range order of the molten aliphatic chains. [Remark: Although anions of ILCs are commonly very electron-rich species, PXRD signals related to them are often not (clearly) observed because the anions can be spatially disordered in the mesophase.489,696] Including the WAXD data in the fit of the diffraction pattern therefore reveals the detailed structure of the ionic sublayers as well. The conclusions could be supported by an analysis based on molecular volumes and cross-sectional areas. It was found that there is no interdigitation of the cholesteryl groups in the mesophase shown by 67-Br, in contrast to the phase shown by 67-NTf2, which contains larger and more elongated [NTf2]− anions and thus ionic clusters with a larger cross-sectional area. It was also found that the stacking disorder in the case of 67-Br is mainly due to disorder in the packing of the branched tails of the cholesteryl groups, while in the case of 67-NTf2 it originates from disorder in the ionic sublayers. More recently, Meng and co-workers reported on LC imidazolium salts that are structurally similar to 67-X (68-X-x/ n).114,470 Whereas the chloride salts only show a SmA phase, a chiral nematic phase was found for the [AlCl4]− compounds, which contain a more voluminous anion. The length of the short linkage between the imidazolium cation and the cholesteryl group (i.e., the value of n) has a larger influence on the phase transition temperatures than the nature of the small alkyl substituent on the other side of the imidazolium ring (i.e., methyl, isopropyl, or n-butyl in the investigated series). In general, both the melting point and the clearing point decrease with increasing n. Laschat and co-workers attached substituted 5-phenylpyrimidine groups to an imidazolium core via a flexible spacer (69X-x/m/n).689 They envisioned that the introduction of two electronegative nitrogen atoms in the biphenyl-type mesogenic group would increase the dipole moment, polarizability, and dielectric anisotropy of the compounds as compared to, for example, 65-X-n, and that it would result in higher clearing points. This seems not to be the case: 69-Br-1/9/6 shows a lower clearing point and a smaller mesophase stability range than the similar compound 65-Br-8. However, by careful tuning of several structural parameters, such as the spacer length and the length of the alkyl chain connected to the pyrimidine ring, the authors succeeded in obtaining ILCs with high clearing points and a broad mesophase temperature range. The highest clearing point was found for the salt with a short spacer (C4) and a long pyrimidine alkyl chain (C14) (69-Br-1/4/14). In contrast to the neutral bromosubstituted precursors, no odd−even effect of the spacer length on the phase behavior was found. A n-butyl instead of a methyl substituent on the imidazolium ring consistently resulted in a decrease of the clearing point and the mesophase stability range, or even in complete loss of mesomorphism. Furthermore, it should be noted that neutral analogues with a hydroxyl or cyano group instead of the 1-methylimidazolium moiety are not LC, indicating the importance of the electrostatic interactions for mesophase formation. A lower melting point could be achieved by attaching the 5-phenylpyrimidine fragment to the imidazolium cation via the meta position of the phenyl ring rather than the para position, thus reducing the molecular symmetry (compare 69-Br-1/9/12 and 70).378 The phase behavior of 2-phenylpyrimidine analogues (71-x/m/n and 72-x/ m) was also investigated.378 Other structural variations in this

follow-up paper included the direct attachment of an additional phenyl ring to the imidazolium core (86-m/n and 87-m), and replacement of the 5-phenylpyrimidine moiety by a pyrimidine moiety that was connected to the cationic core via an ester group at its 5-position (73-X-x/m and 88-X-m).378 In the series 71-1/ m/n, a SmC phase in addition to a SmA phase was observed for the compounds with the longest spacer and terminal alkyl group (71-1/8/10 and 71-1/8/12), whereas this phase is absent for 69Br-1/8/12. This can be attributed to the more planar conformation of 2-phenylpyrimidine groups as opposed to the twisted structure of 5-phenylpyrimidine fragments.697 No SmC phase was found for the “bent” analogue 72-1/8, however. Elongation of the rigid cationic core resulted in higher transition temperatures, but compounds 86-m/n and 87-m are interesting because they can be regarded as rare examples of LC dimers consisting of a classical neutral mesogenic group, on the one hand, and a charged rigid mesogenic moiety, on the other hand. Near-ambient melting points could be achieved with compounds 73-X-x/m, particularly with the triflate salts. It is striking that, once again, and despite all structural variations, the vast majority of the ILCs in these contributions show a SmA phase. Zhang et al. investigated LC salts with a rod-shaped pnitroazobenzene moiety as mesogenic group, both with only one and with two imidazolium cores (74-X-1/n-NO2 (Z = NO2), 81n, and 75-X-1/n-O (Z = O)).371,690,691 In the series 75-X-1/n-O, only 75-Br-1/10-O is mesomorphic. Interestingly, this salt shows a SmC phase instead of a SmA phase. Because 75-BF4-1/ 10-O is not LC, extensive hydrogen bonding between the imidazolium cations and the bromide anions is probably important for stabilization of the mesophase. A longer flexible alkyl spacer appears to promote mesomorphism, inducing lower melting points and higher clearing points. A very short C3 spacer does not provide sufficient decoupling between the cationic core and the mesogenic group, and no mesomorphism was observed for 74-X-1/3-NO2. The neutral precursors, with a terminal bromo substituent instead of a 1-methylimidazolium or 1vinylimidazolium bromide moiety, are not LC. Mudring and coworkers reported on related structures (74-Br-x/n-CH3 (X− = Br−, Z = CH3) and 75-Br-x/0-CH2 (X− = Br−, Z = CH2)).692 Enantiotropic SmA phases were found for 74-Br-10/2-CH3 and 74-Br-12/2-CH3 (despite the very short C2 spacer), and for 75Br-12/0-CH2 and 75-Br-14/0-CH2. Significant differences in solid-state molecular conformation and packing were observed in the crystal structures of 74-Br-x/2-CH3 (x = 8, 10, 12) (Ushaped and L-shaped conformations), 74-Br-x/6-CH3 (x = 12, 16) (elongated zigzag structures with parallel arrangement), and 75-Br-6/0-CH2 (S-shaped conformation). Kato and co-workers synthesized imidazolium salts 76-X tethered with a relatively electrochemically stable, phenylcyclohexyl-based mesogenic group.693 These show a SmA phase at low temperatures. Mixtures of 76-OTf (X− = [OTf]−) with the ILs [C2mim][X] (X− = [OTf]−, [BF4]−, [NTf2]−) were also investigated: all of these still form the SmA phase up to high concentrations of 70−90 mol % of the IL (even for two different anions), albeit with reduced clearing points. As seen for the cubic phases of NDA2/58-Br mixtures (see section 5.2), addition of an IL lowers the viscosity and benefits the ionic conductivity (e.g., at 41 °C: σ|| = 2.6 × 10−5 S cm−1 for 76-OTf, σ|| = 2.9 × 10−4 S cm−1 for 76-OTf/[C2mim][OTf] (60:40)). This had previously been demonstrated for smectogenic mixtures of ILC [C18mim][BF4] and IL [C2mim][BF4].602 The latter are LC up to a concentration of 60 mol % of IL and show even higher ionic conductivities (σ|| > 10−2 S cm−1 above 70 °C), but the SmA phase appears at higher AQ


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authors observed a very rare example of a thermotropic cubic mesophase of Pn3̅m symmetry. They also derived from PXRD data that the compounds form π-stacked triphenylene columns not only in their Colhex phases, but also in the bicontinuous cubic phases. This may explain the semiconducting properties of the latter. A quite high intrinsic charge carrier mobility of 3.5 × 10−4 cm2 V−1 s−1 was found for the cubic phase of 89-(1-Me-imid)BF4-10 by laser flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements.699 This maximum transient conductivity is comparable to that of the Colhex phase of 89-(1,2-Me-imid)-BF4-10 and even higher than that of a crystalline sample of the nonionic analogue. The fast carrier mobility is due to electronic (electron/hole) conduction, because a possible contribution of slower ionic transport (mobility ≈ 10−5−10−6 cm2 V−1 s−1) is excluded due to a high resonant frequency (∼9.1 GHz) of the microwave employed.700 Interestingly, the lifetimes of the generated charge carriers were much higher in the ionic compounds than in the neutral analogue. It is rather unexpected that such fast charge transport can occur in a highly ionic medium, because localized charges can act as traps for holes and electrons and prevent carrier drifting. These significant results demonstrate that ILCs can also be used as organic semiconductors. Moreover, bicontinuous cubic phases allow for “alignment-free” conduction through their 3D interconnected networks, in contrast to Colhex phases where conduction is hindered by domain barriers in samples that are not macroscopically aligned (see also sections 5.2, 6.2, and 7). Such alignment of conducting columns in a device can be challenging, and therefore the existence of ion- and electron/ hole-conducting assemblies in a bicontinuous cubic phase is very promising (see also section 5.4).

temperatures than in the case of 76-OTf/[C2mim][OTf]. Interestingly, 76-OTf/[C2mim][OTf] (80:20) shows a considerably higher ionic conductivity than an 80:20 mixture of a neutral dihydroxysubstituted analogue of 76-OTf with the IL [C2mim][BF4].216 Addition of a lithium salt (LiOTf) to 76OTf/[C2mim][OTf] lowers the crystallization temperature but also results in lower ionic conductivities due to Li+−anion associations. The Kato group also broadened the applicability of the concept of attaching mesogenic groups to ionic cores, by synthesizing electroactive imidazolium salts containing functional π-conjugated oligothiophene moieties that can transport electronic charges (77, 78, and 79).694,695 They exploited not only the anisometric shape of these rod-like (or lath-like) mesogenic groups and their nanosegregation from the ionic headgroups, but also their electronic/redox properties, and combined these elegantly with the ion mobility in the nanosegregated SmA phase. Because of the presence of both πconjugated and ionic parts in the molecules, the compounds display efficient, reversible electrochromism in the bulk LC state without the need for an additional electrolyte. These results will be discussed in section 12.4. Two polymerizable LC imidazolium salts with a mesogenic group (82 and 83) were reported by Jazkewitsch and Ritter.636 Compound 82 shows a mesophase between 102 and 129 °C. Salt 83 is mesomorphic, in sharp contrast to compound 75-Br-1/6O. Both 82 and 83 were polymerized by free radical polymerization in solution. The polymer obtained from 82 was amorphous; the one obtained from 83 showed a LC phase above 127 °C. LC imidazolium salts 125-n and 126-n, which contain a tethered propylcyclohexylphenyl or cyanobiphenyl mesogenic group, respectively,445 will be discussed in section 5.4. After an initial communication by Fukushima, Aida, and coworkers about a disk-like triphenylene derivative with six imidazolium ion functionalities in its periphery (see also section 4.3.1 and Figure 4),349 the same groups published a follow-up report on the phase behavior of homologous compounds with different spacer lengths, different substitution patterns of the cationic headgroups, and other anions (89-(1-Me-imid)-X-n and 89-(1,2-Me-imid)-X-n).63 It should be noted that already in 1986 Keller-Griffith et al. investigated the lyotropic LC properties of similar compounds with peripheral N,N,Ntrimethylammonium or pyridinium groups.698 Most of the new discotic compounds are low-melting (LC at or near room temperature) and show both a bicontinuous cubic phase (rare for discotic mesogens) and a higher-temperature Colhex phase. Recall that a nonionic analogue (in which the imidazolium groups are replaced by hydrogen atoms) only shows a columnar phase (Figure 4), emphasizing the importance of the ionic pendants for the formation of the bicontinuous cubic structure. A longer spacer resulted in higher transition temperatures, and in particular higher clearing points with a stabilization of the cubic phases versus destabilization of the columnar phases. Salt 89-(1Me-imid)-BF4-14 thus shows a bicontinuous cubic phase over a very wide temperature range, from room temperature to over 200 °C. Larger counterions ([BF4]− → [PF6]− → [NTf2]−) require longer spacers to form the cubic phase. This is because larger ionic pendants sterically disturb close packing of the disklike triphenylene cores. Interestingly, incorporation of a 1,2dimethylimidazolium instead of a 1-methylimidazolium part appeared to promote columnar phase formation, at the expense of the cubic mesophase. For salt 89-(1,2-Me-imid)-BF4-14, the

Salts 89-(1,2-Me-imid)-BF4-10 and 89-(1-Me-imid)-BF4-14 have been used as dispersion media for orienting unfunctionalized (subμm long) single-walled carbon nanotubes (SWCNTs).701 SWCNT contents of 5 wt % and even higher could easily be attained without the formation of phase-separated SWCNT aggregates and with conservation of the LC properties. Only the clearing point was slightly decreased upon doping with the nanotubes. PXRD indicated that 89-(1-Me-imid)-BF4-14 accommodates the SWCNTs in its cubic lattice, whereas 89(1,2-Me-imid)-BF4-10 prefers to maintain its original 2D hexagonal lattice without incorporation of the nanotubes. The ionic parts of the ILCs are a prerequisite for good dispersion with high loading levels, which could not be attained with a nonionic LC triphenylene derivative (it had been previously shown that SWCNTs can be mixed with imidazolium-based ILs and that the mixtures form physical gels thanks to cation−π and interionic interactions702−705). 89-(1,2-Me-imid)-BF4-10/SWCNT mixtures showed spontaneous homeotropic orientation of the LC columns between two glass slides, with the nanotubes oriented randomly in the sample. Despite the fact that they are not incorporated into the hexagonal lattice and their random orientation, the nanotubes appeared to be essential for the homeotropic orientation. Shearing of the LC hybrid material allowed to align both the LC columns and the nanotubes parallel to the glass slides along the shear direction, as shown by POM, AR


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polarized absorption spectroscopy, and measurements of the conductivity anisotropy. Short annealing of a sheared sample at 150 °C led to a homeotropic orientation of the LC columns with preservation of the horizontal orientation of the nanotubes, whereas longer annealing also restored the random orientation of the nanotubes. Cui and Zhu synthesized ILCs consisting of a single imidazolium cation linked to a disk-like triphenylene moiety (90-n).706 Interestingly, the phase behavior of the nonionic precursors (i.e., with a bromine atom instead of the 1methylimidazolium bromide fragment) opposes that of the ionic compounds: large asymmetry results in the loss of LC properties for the neutral structures, whereas a sufficiently long spacer is required to induce mesomorphism in the case of the imidazolium salts. This decoupling allows segregation of the ionic moieties in a distinct subspace. A relatively rare Colobl phase and a highly ordered lamello-columnar Colrec-type phase were observed for 90-7 and 90-10, respectively, in contrast to a Colhex phase for the neutral precursor with n = 4. A double-layer Colrec phase (of unusual pm symmetry) and a single-layer Colrec phase (of p2mg symmetry) may coexist in a sample of 90-10. However, annealing at 95 °C eliminated the single-layer Colrec phase in the sample, indicating that it concerns a metastable, kinetically formed structure (which should be the case indeed, because otherwise the Gibbs phase rule would be violated). The lamellar structure of the Colrec phase clearly results from the strong ionic interactions. Cui and Zhu also prepared complexes of imidazolium salts 90-n with negatively charged double-stranded DNA (456-n).164,165 These results will be discussed in section 11. Kumar and Pal reported on similar asymmetric triphenylenebased imidazolium LCs, with an ether instead of an ester linkage (91-X).707 Remarkably, 91-Br shows a columnar mesophase, in contrast to compound 90-4 which is not mesomorphic. The neutral precursor with a terminal imidazole fragment instead of a 1-methylimidazolium halide moiety is not LC, indicating again the importance of the electrostatic interactions. Exposure of the hygroscopic salts 91-X to atmospheric moisture resulted in

slightly higher clearing points, as was also observed for the smectogenic [C12mim][Br] and [C14mim][Br] salts.540 Thin films of 91-Br (as well as of a similar pyridinium bromide salt) at air−water and air−solid interfaces were also investigated.708 An imidazolium salt related to 91-Br, with an N-vinyl group instead of an N-methyl group, a C6 spacer, and n-butyloxy instead of npentyloxy chains, exhibits a monotropic columnar phase.709 The same authors prepared an imidazolium LC with two triphenylene mesogenic groups, which shows a Colrec phase over a very narrow temperature range (92).370 Interestingly, an asymmetric analogue with both a calamitic cyanobiphenyl and a disk-like triphenylene group (93) did not show mesomorphic properties. Kumar and Gupta examined some symmetric gemini imidazolium salts tethered with triphenylene groups (97-m/n,

Scheme 6).710 As in the series 48-Br-m/n/x, only the compounds with rather short spacers are LC, and the mesophase temperature range and clearing point decrease with an increasing combined spacer length (m + n). Cı̂rcu and co-workers synthesized other symmetric gemini imidazolium bromides, tethered with two or four cyanobiphenyl

Scheme 5. Imidazolium-Based ILCs That Contain at Least One Disk-like Mesogenic Group That Is Connected to the Cationic Core via a Flexible Spacer370,706,707

AS


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Scheme 6. LC Symmetric Gemini Imidazolium Salts Tethered with Mesogenic Groups443,710

combination with a relatively short rigid core does not provide a sufficiently anisometric molecular shape. This limitation can be overcome by the introduction of strongly interacting charged groups. Introduction of an additional n-decyloxy chain on the “nonionic side” of the molecules resulted in a lower melting point (compare 99-Br and 100, and 102-Br-10/10 and 103-Br-10) and in the appearance of an additional, lower-temperature LC phase (neither smectic nor hexagonal columnar in nature) in the case of 100. In the series 102-X-n/10, N-alkylation with n-decyl (102-X-10/10) instead of a methyl group (102-X-1/10) lowered both the melting point and the clearing point when comparing salts with the same anion. Salt 102-Br-12/12, which had previously been reported by Pedro et al.,715 shows a phase behavior similar to that of 102-Br-10/10, except for the appearance of a plastic crystal phase just below the SmA phase. Interestingly, 99-C12H25OSO3 does not form a LC phase, whereas 102-C12H25OSO3-10/10 does. The compounds are luminescent in solution. Müllen and co-workers reported on the columnar selfassembly of amphiphilic hexa-peri-hexabenzocoronenes (108X) into fibrous aggregates.716 They did not observe any selfassembly into fluidic, thermotropic LC columnar phases, though. On the other hand, the analogue of 108-Cl with terminal chloro substituents instead of methylimidazolium chloride groups exhibits a Colhex phase above 190 °C.

groups, or with two cholesteryl groups (94, 95, and 96, Scheme 6).443 The three salts exhibit thermotropic SmA phases, and 95 also shows a nematic phase between 79 and 81 °C. Interestingly, the latter compound can be seen as a dimeric analogue of bromide salt 84-Br-11/11, which also shows a nematic phase (see above).122 Silver(I) N-heterocyclic carbene complexes prepared from 94, 95, and 96 only show SmA phases. Cholesteryl-containing salt 96 could be used as a chiral dopant to induce a N* phase in the common LCs 5CB and 5OCB. To the best of our knowledge, there are only two reports on ILCs containing one or more bent-shaped mesogenic groups. The ionic LC dendrimers with banana-shaped carboxylate anions that were reported by Vergara et al.711 will be discussed in section 6.4. He et al. synthesized a LC imidazolium salt tethered with a bent-shaped, rigid, π-conjugated group (98-n, n = 8).712 This compound shows a thermotropic Colhex phase with an interesting molecular packing (Figure S10). Its homologues with two n-hexyl or two n-hexadecyl substituents melt directly without an intermediate LC mesophase. Gallardo and co-workers attached photoluminescent 1,3,4oxadiazole moieties to imidazolium cations (99-X, 100, and 101), and compared their thermal behavior and photophysical properties in solution with those of analogous salts in which cationic pyridinium moieties are directly connected to the oxadiazole fragments (102-X-n/10, 103-X-n, 104, and 105) [remark: these pyridinium-based systems are discussed here and not in section 8 because of their relation to the imidazoliumbased salts 99-X, 100, and 101] and with those of neutral derivatives (106-n and 107-n) (Scheme 7).536 LC 1,3,4oxadiazolylpyridinium salts similar to 102-X-n/m had already been investigated by Haristoy and Tsiourvas.713,714 Gallardo et al. observed mesomorphism only for the monocationic salts, and only SmA phases were found (except for 100, see below). The SmA phases have a monolayer or bilayer structure depending on the exact molecular structure of the cation and the anion type. The dicationic pyridinium-based compounds (104 and 105) decompose before melting. The melting point of bis(imidazolium) salt 101 is only slightly higher than that of its neutral counterpart 106-10. The neutral compounds (106-n and 107-n) are not LC, possibly because their bent shape in

5.4. Mesogens in Which the Imidazolium Cation Makes Part of the Rigid Core

Up until now, with the exception of the pyrimidine-substituted N-arylimidazolium salts reported by Starkulla et al. (86-m/n, 87AT


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Scheme 7. LCs Containing 1,3,4-Oxadiazole Moieties536,a

a

See sections 6.4 and 8.4 for other examples of oxadiazole-containing ILCs.

m, and 88-X-m),378 no examples were given of imidazoliumbased ILCs in which the cationic core is directly connected to other rigid moieties. Such ionic mesogens were named “rigidcore” ILCs,292 but this term can be misleading because, for instance, the ILCs described in the previous section also contain rigid mesogenic moieties. Even the mesomorphic salts reviewed in section 5.1 can be termed “rigid-core” ILCs, because the imidazolium moiety itself is a rigid (albeit not anisometric) molecular fragment. As mentioned above, the mesogens described in this section constitute examples of more classically shaped systems where excluded volume effects play a primary role. An additional advantage of direct attachment of aromatic groups to the imidazolium core (via the imidazolium nitrogen atoms and/or carbon atoms), and thus the possibility of electronic communication between those aromatic groups and the core, is that π-system-based electronic effects can be used to tune the properties. This is particularly the case when the aromatic moieties carry specific substituents (electron-withdrawing or electron-donating, etc.).579,717 Furthermore, the

potentially high birefringence of this type of ILCs can be advantageous for optical applications. Calamitic ionic mesogens with an elongated rigid charged core are potentially easier to align in classical LC cells with planar alignment layers than the compounds discussed in sections 5.1 and 5.3. It should be noted again that many potential structures have been patented.685,718 The first report on this type of ILCs was published by Douce and co-workers. They prepared salts consisting of an imidazolium moiety with a substituted aryl ring directly connected to both nitrogen atoms, by condensation of 2 equiv of a substituted aniline with glyoxal and subsequent ring closure (109-n).719,720 The melting point is nearly unaffected by the alkyl chain length, in sharp contrast to the clearing point. In a later report, analogous salts with only one substituted aryl ring (110X) were described.490 [Remark: Some LC imidazolium salts that are structurally related to 110-X (339-(a−d) and 340-(a−e)) will be discussed in section 10.4, together with similar guanidinium salts.721] These were prepared via a solventless modified Ullmann reaction. Compound 110-OTf shows a lower AU


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of charge carriers in the nonaligned SmA phase of 109-10 to be about 10−4 cm2 V−1 s−1,720 or 10−3 cm2 V−1 s−1 for a further purified sample;723 no slower mobility component was found. The observed response or carrier mobility is too fast for ionic transport (mobility ≈ 10−5−10−6 cm2 V−1 s−1), and was explained by an electronic charge hopping process, presumably along the phenyl groups but electrostatically affected by the ionic moieties.723 As mentioned in section 5.3, it is surprising that such fast electronic charge hopping can occur in a highly ionic medium, because of the possibility that the ionic moieties act as traps and prevent carrier drifting (in this context, it should be mentioned that room-temperature ILs could be observed by scanning electron microscopy without charging of the liquid724). It was proposed that the fast charge transport might be due to the ordering of the ionic parts in a layered structure with segregated sublayers. The carrier mobility decreased when the applied electric field was increased, in contrast to neutral amorphous organic semiconductors.

melting point but also a much lower mesophase stability than salt 109-12. On the other hand, it is mesomorphic, in contrast to the related compound 41-OTf-12. The conformationally more flexible salts 41-X-12 show slightly lower melting points and clearing points in comparison to 110-X. In the single-crystal structure of 110-I, the n-dodecyl chains, which are segregated from the cationic headgroups and iodide anions, are highly tilted to compensate for the lattice area imposed by the organization within the ionic sublayers, and the polar sublayers are nearly flat. The huge tilting of 71° with respect to the layer normal was not observed in other single-crystal structures of ILCs with simple alkyl chain substituents (for example, it amounts to only 57° in the crystal phase of [C12mim][PF6]),120,170,281,514,540,558,561,562,569,570,719,722 and is a direct consequence of the extension of the rigid cationic core. The tilting was not observed in the crystal structure of 109-12, which contains a longer rigid core but two long alkyl chains instead of one.719 The authors proposed that in the series 110-X, at the transition to the SmA phase, both undulation of the ionic sublayers (resulting in a lateral “shrinkage” and a longitudinal extension) and strong folding of the nontilted molten aliphatic chains allow area matching between headgroup parts and aliphatic parts.490 The alkyl chains are no longer tilted relative to the layer normal to minimize the aromatic−aliphatic interface (incompatibility between flexible alkyl chains and rigid aromatic cores).437 The degree of undulation is highest at the Cr-to-SmA transition and decreases again with increasing temperature (indicated by the decrease of the thickness of the ionic sublayers), with a concomitant lateral “broadening” or “relaxation” of the ionic sublayers (Figure 28). This is possible

Kouwer and Swager built further on the idea of attaching a rigid group directly to one of the nitrogen atoms of an imidazolium cation, and performed an extensive structure− property relationship study (111-X, 112-X, 113-X, 114-X, and 115-X; Scheme 8).292 They investigated the influence of (i) the size and nature (branching, chirality, etc.) of the chains attached to the “free” imidazolium nitrogen atom and to the aromatic group; (ii) the size of the rigid core; (iii) replacement of the imidazolium H(2) atom by a bulkier group; and (iv) the anion. The compounds were prepared via Ullmann-type amination reactions and subsequent quaternization reactions with iodide compounds. Most of the salts display a SmA phase (with an unidentified SmX phase at lower temperatures in the case of 114X-a (see the Supporting Information for an explanation of structure codes)), with either an interdigitated (SmAd) or a bilayer (SmA2) organization. The neutral analogue of compounds 115-X (without an R substituent and without a counterion) is also LC. Some interesting observations are listed below. (i) In some cases (111-I-a, 112-BF4-a, 113-I-d, 113-BF4-a, 113-ClO4, 114-X-c, 115-BF4-a), crystallization could be completely suppressed: the SmA phase could be vitrified to a low-melting glass (gSmA) on cooling, thus extending the mesophase window to room temperature. No cold crystallization and melting were observed in a subsequent heating run. The supercooled state appeared to be stable for months. (ii) The compounds with the shortest rigid core (an imidazolium ring and only one phenyl ring) are either not LC (111-BF4-a) or exhibit a lower clearing point (111-I-(a-c) and 111-BF4-(b-c)) than their analogues with a longer rigid core, because of a lower shape anisotropy.

Figure 28. Schematic representation of the Cr → SmA melting process, which involves undulation of the ionic sublayers (depicted in red) and the start of alkyl chain folding.490,539 In the SmA phase, the degree of undulation decreases with increasing temperature.

because the conformationally molten alkyl chains can fold increasingly more easily. A similar model had been proposed before and is generally applicable.492,558 Furthermore, the authors stated that the stability of the SmA phase is not determined by the undulation degree of the ionic sublayers, but by the degree of folding of the alkyl chains and therefore the thickness of the aliphatic sublayers; isotropization in the series 110-X was indeed found to occur at similar values for AMmax and therefore similar minimal aliphatic sublayer thicknesses dchainsmin. By deprotonation of the acidic H(2) proton of 109-10, its palladium(II) carbene complex could be prepared, consisting of a central Pd(II) ion surrounded by two carbene ligands in a cis coordination geometry, and by two iodide ligands.720 The phase behavior of this complex was not reported, but it could be used as a catalyst for a Suzuki−Miyaura cross-coupling. Time-of-flight (TOF) photoconductivity measurements (with pulsed laser illumination of the positive electrode) revealed the drift mobility AV


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Scheme 8. Imidazolium-Based ILCs Investigated by Kouwer and Swager292,725

(iii) Replacement of the imidazolium H(2) atom by a “lateral”, sterically undemanding methyl group resulted in a lower melting point or glass transition, and a similar clearing point for the I− salts (114-I-(a-b)), but either a lower or higher clearing point for the [BF4]− salts (114-BF4-(a-b)). The introduction of an ethyl group (114-X-(c-d)) steadily resulted in mesophase destabilization. Suppression of crystallization by a bulkier group also allowed for the formation of another smectic mesophase in some cases (114-I-a and 114-BF4-d). The salts with an even larger lateral n-pentyl or phenyl group (114-X-(e-g)) are not LC. (iv) Interestingly, attachment of a triethylene glycol monomethyl ether chain to the imidazolium core resulted in a very low melting point or glass transition and in either the loss of LC properties (112-I-d and 112-BF4-c) or a reduced clearing point (113-I-d and 113-BF4-d). This was attributed to dipolar interactions and possibly hydrogen-bonding interactions between the polar tails and the imidazolium cations (supported by molecular modeling and by the observation of the large extent of interdigitation of the oligo(ethylene oxide) chains by PXRD; see also section 4.2). The clearing point of 113-BF4-d could be increased by >100 °C by addition of 1 equivalent of LiBF4 (see also above305,427,607,647); probably the Li+ ions compete with the imidazolium groups in complexing the polar oligo(ethylene oxide) chains, allowing for a more rod-shaped conformation of the mesogens. (v) The compounds exhibiting a bilayer SmA2 phase show a more pronounced temperature dependence of the smectic layer thickness than the salts exhibiting a SmAd phase. (vi) When comparing the phase behavior of, for instance, 112BF4-a and 113-BF4-a, it is clear that the position of the alkyl chains with respect to the rigid core is important. This is because the core is not symmetric since the cationic charge is not in the center. Possibly, the lower melting point of 112-BF4-a can be explained by the fact that its long alkyl tails cannot pack as efficiently in the crystalline phase as in the case of 113-BF4-a,

because of the counterions, which will stay close to the cationic part. The lower mesophase stability of 112-BF4-a might be due to a less efficient packing of the ionic moieties, which can be disturbed by the molten aliphatic chains. The difference between 112-BF4-a, which shows a SmA phase between −8 and 121 °C, and 112-C12H25SO3-a, which is not LC at all, is remarkable. The difference in phase behavior is much smaller between 113-BF4-b and 113-C12H25SO3. The authors also investigated mixtures of the reported ILCs containing the same anion. POM contact samples revealed complete miscibility of structurally similar mesogens. Compounds 113-I-a and 112-I-b, 113-BF4-a and 112-BF4-a, and 114-BF4-a and 112-BF4-b were found to be immiscible (or only miscible in the isotropic liquid phase). On the other hand, 115-Ia and 111-I-a, having a different core structure, were miscible; the 50:50 mixture shows a SmA phase and displays a lower viscosity than the individual components. The work of Kouwer and Swager inspired Strassner and co-workers to prepare “tunable” aryl-alkyl ILs.579,717 More recently, Kouwer and co-workers investigated an additional structural parameter to tune the phase behavior of this class of ILCs. They prepared analogues of 113-BF4-b, but instead of attaching an n-dodecyl chain to the imidazolium core they incorporated a cis unsaturated fatty acid tail (116-X, Scheme 8) (see also the work of West, Davis, and co-workers in relation to imidazolium-based ILs that was discussed in section 5.1578).725 They also further investigated the influence of the lateral substituent R2 (on the C(2) atom of the imidazolium ring), and prepared salts with [NTf2]− instead of [BF4]− anions. The combined effect of the “kink” in the oleyl tail to hinder close packing726 and the [NTf2]− anion caused a very significant decrease in transition temperatures for 116-NTf2-c as compared to its counterpart 116-BF4-f with a linear n-octadecyl chain and a [BF4]− counterion: the former shows a SmA phase between 17 and 55 °C, whereas the latter exhibits a SmA phase between 59 AW


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Scheme 9. Imidazolium-Based ILCs Investigated by Cheng, Tschierske, and Co-workers437,731

and 233 °C. The SmA phase of salt 116-NTf2-e even exists only between −2 and 25 °C; the clearing point of its [BF4]− counterpart 116-BF4-e is about 100 °C higher. Importantly, at lower temperatures the viscosity of the SmA phases shown by the [NTf2]− salts is lower than that of the mesophases exhibited by the [BF4]− compounds, although no quantitative data were provided. As mentioned in section 4.1, it is widely known that ILs with [NTf2]− anions display relatively low viscosities at room temperature as compared to many other molten salts.106 [This applies to an even greater extent to [N(CN)2]−-based ILs,727,728 but to the best of our knowledge only one ILC with this anion has been reported up until now.729] This is a good example of how the knowledge that was accumulated in the field of ILs can be used advantageously in the design of ILCs. Finetuning the phase behavior and transition temperatures and lowering the viscosity of ionic mesogens might prove useful in the optimized design of anisotropic solvents (see section 12.3) and (bio)sensors. The authors also calculated the energy barriers for rotation between the imidazolium and biphenyl groups in compounds 116-X. For R2 = H, the energetic penalty of the flat conformation is relatively small, which allows for intermolecular ion−dipole and π−π interactions. Yet already for the relatively small methyl group, a significant barrier was predicted. It was proposed that the reduction in conformational flexibility, in combination with the known steric effects described for classical mesogens (which actually constitute the dominating factor),68,730 can explain the important impact of the lateral substituents R2 on the clearing points. Related compounds were investigated by Cheng, Tschierske, and co-workers (117-m/n, 118-m/n, 119-m/n, 120-m/n, 121, 122-m/n, 123-n, 124-m/n; Scheme 9).437,731 They focused on the influence of the number and length of the alkyl chains attached to the rigid core, as well as on the effect of the central linkage group (ester function, versus relatively less polar ether group versus relatively more polar amide group), and observed

that (i) increasing the number and length of the alkyl chains attached to the noncharged end of the rod-like core (= Cterminal chain(s)) results in the phase sequence SmA → Colhex → CubI(Pm3̅n) for the ester- and ether-based salts (Figure 29), as a result of a gradually increasing aromatic−aliphatic interfacial

Figure 29. Evolution of the mesophase morphology as a function of the length of the “N-terminal” chain, and the length and number of “Cterminal” chains, in the series 117-m/n, 118-m/n, 119-m/n, 120-m/n, 121, 122-m/n, 123-n, and 124-m/n.437,731 There are 5−6 molecules per “slice” in a column of the Colhex phase, and 80−87 molecules in each spheroidic micelle of the CubI(Pm3̅n) phase. The micellar cubic phase is only formed by molecules with a very high interface curvature, provided by three long “C-terminal” chains and one very short “N-terminal” chain. Reproduced with permission from ref 731 (http://dx.doi.org/10.1039/ c2sm06854k). Copyright 2012 The Royal Society of Chemistry. AX


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curvature (SmA for f V,C < 0.5, Colhex for f V,C ≈ 0.6−0.72, and CubI for f V,C > 0.72, with f V,C being the volume fraction of the Cterminal chains); (ii) increasing the length of the alkyl chain attached to the “free” imidazolium nitrogen atom (= N-terminal chain) results in the reverse sequence for these compounds (Figure 29); and (iii) the amide-based compounds shows higher melting points and only forms SmA and Colhex phases (Figure 29); the single-chain amide salts have such high melting points that decomposition occurs before a potential mesophase can develop. The authors concluded that for short N-terminal chains, a “head-to-head” packing is preferred, in which the N-terminal and C-terminal chains are located in different subspaces. Elongation of the N-terminal chain (i.e., evolution from a taper-shaped structure toward a polycatenar structure) leads to mixing of Nand C-terminal chains, with a concomitant intercalation of the rigid cores. In the series 117-m/n, this corresponds to a transition from a “bilayer” SmA2 phase (Figure 30a) to a monolayer SmA

Figure 31. Models showing the molecular organization (a) in the SmA phase shown by 119-8/12 (antiparallel organization in ribbons), and (b) in the Colhex phase shown by 119-3/12 (rosette-like organization; the empty space at the periphery is filled up by the “C-terminal” chains from neighboring columns437,735). Adapted with permission from ref 437. Copyright 2010 Wiley.

tures) in the homologous series 119-3/16, 119-4/16, and 1195/16. This is because the space available for the N-terminal chains in the middle of the spheroidic micelles (CubI) or columnar structures (Colhex) is very limited (Figure 31b), and thus only slight elongation of these chains disturbs the packing of the ionic moieties and causes the micelles or columns to burst. This reasoning can also explain the unexpected CubI−Colhex phase sequence observed for 122-4/16, because a micellar cubic phase should be expected to form at a higher temperature than a columnar phase. However, thermal expansion of the N-terminal chain has a stronger effect on mesophase morphology than thermal expansion of the C-terminal chains. The ether-based compounds generally show lower melting points but also reduces mesophase stability as compared to the ester-based compounds, which was explained by the higher flexibility and lower polarity of the ether linkage. Interestingly, the neutral precursors for the ester-based and amide-based salts (without an N-terminal chain and without a counterion) with two and three C-terminal chains (C12, C14, or C16) are not LC; the precursors with a single Cterminal chain show nematic and/or SmA2 phases, with lower transition temperatures than the ionic compounds in the case of the ester-containing structures but with a higher mesophase stability in the case of the amide-containing species. These compounds lack the strong electrostatic interactions and increased nanosegregation that are typical for the ionic structures. Similar neutral compounds with three C-terminal chains but with a free N−H group were reported to show Colhex and CubI phases thanks to the mesophase-stabilizing effect of hydrogen bonding between the N−H protons and the “free” imidazole nitrogen atoms.732−734 Such intermolecular hydrogen bonding can also occur in the neutral amide-containing compounds, which explains their relatively high mesophase stability. As far as their ionic counterparts are concerned, it is believed that polar interactions between the amide groups and the imidazolium cations (Figure 32) cause a reduction of the interfacial curvature and an increased intracolumnar aggregate rigidity, and thus the absence of a micellar cubic phase in favor of a columnar phase and eventually a smectic phase (compare, for example, 119-4/16, 122-4/16, and 124-4/16). The antiparallel core packing, in which the short N-terminal chains are forced to mix with the long C-terminal chains, can also explain the unexpected reduced mesophase stability of 124-m/n as compared to 119-m/n and 122-m/n.

Figure 30. Schematic representations of (a) the “bilayer” SmA2 phases shown by compounds 117-m/n with a short “N-terminal” chain (parallel side-by-side packing of the aromatic cores within the sublayers), and (b) the monolayer SmA phases shown by compounds 117-m/n with a long “N-terminal” chain (antiparallel side-by-side packing of the aromatic cores, mixing of “N-terminal” and “C-terminal” chains). Adapted with permission from ref 437. Copyright 2010 Wiley.

phase, which is favored because of higher entropy and direct dipole compensation between adjacent molecules (Figure 30b) (it should be noted that 117-4/10 also exhibits a nematic phase, but only over 0.5 °C). A similar dependence of the nature of the SmA phase on the length of the N-substituent was also found by Laschat and co-workers for pyrimidine-substituted imidazoliumbased ILCs.378 Columnar and cubic (Pm3̅n) phases can only be formed when three C-terminal chains create a curved interface with the rigid core (as in the series 119-m/n), and if the segregated “head-to-head” arrangement is achieved (i.e., for very short N-terminal chains) (taper shape), as it allows the formation of “core−shell” aggregates composed of hydrophobic cores that contain the short N-terminal chains and which are enclosed by shells of the ionic, aromatic, and aliphatic parts (Figure 31b). [Remark: The term “vesicular LC phases” has been used for such mesophases.17] A slightly longer N-terminal chain (m > 4) completely distorts the aggregate formation (because there is a lack of space inside to accommodate the N-terminal chains) and results in a lamellar SmA phase being formed again (Figure 31a). This SmA phase has a lower clearing point than for the series 117-m/n because of the layer distortion that is a consequence of the increased alkyl chain volume. It is striking that a difference of only one methylene group in the N-terminal chain causes a transition from a cubic, over a columnar, to a lamellar organization (corresponding to very different interface curvaAY


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monolayer and an ionic bilayer. In both ionic sublayers, the ions are arranged according to a tetragonal lattice (hence “T′”). The ionic monolayer contains parallel rigid cores, whereas the ionic bilayer is formed by slight translation of one rigid core relative to its neighbors along the director. It is not entirely clear what is the driving force for this remarkable structure, nor do the authors give an explanation for the difference in alkyl chain density between layers A−B and layer C in Figure S11(b). A comparison of different anions (e.g., substitution of Br− by linear [SCN]−115) would be of interest.

Figure 32. Proposed model for the molecular organization in a columnar slice of the Colhex phase shown by 124-2/16, indicating the possibility of polar interactions between the amide groups and the imidazolium cations, which favor intercalation of the aromatic cores.

Akagi and co-workers elegantly combined the strategies of mesogenic group attachment and incorporation of a cation into a rigid anisometric core to obtain ILCs 125-n and 126-n.445 These contain either two propylcyclohexylphenyl moieties, or one such group and one tethered cyanobiphenyl fragment. In the latter case, an enantiotropic nematic phase (with cybotactic character) was found for 126-6 and 126-12. Salt 125-12 shows smectic mesomorphism, but 125-6 is not LC. The nematogen 126-12 was found to be miscible with custom-made, axially chiral tetrasubstituted binaphthyl derivatives carrying the same types of mesogenic groups ((R)-BNDx and (S)-BNDx (x = 1−3); see Supporting Information). Small amounts of these neutral chiral dopants could induce a chiral nematic phase (N*) in the ionic nematogen (recall that salt 96 was used as a chiral dopant to induce a N* phase in the neutral nematogens 5CB and 5OCB443). Mixtures 126-12/(R)-BND1 and 126-12/(S)BND1 were shown to have a right-handed helical sense and a left-handed helical sense, respectively. The ionic conductivity in the chiral nematic phase of 126-12/(R)-BND1 is slightly lower than that in the nematic phase of pure 126-12 at the same temperature.

Other examples of ILCs in which an imidazolium cation makes part of the anisometric rigid core, but that have a flexible linear alkyl substituent on only one side (128-n/m, 129-n, and 130-Xn/m, with m = 1, 2 or 3), were presented by Chen et al.736 Salts 129-n and 130-X-n/m contain fluorinated moieties. In general, selective fluorination of mesogenic molecular fragments has attracted a lot of attention as a tool to tune mesomorphic and physicochemical properties.38,737 The small size of the fluorine atom enables its incorporation into various organic LC materials without disturbing mesomorphism. However, the fluoro substituent is larger than hydrogen and introduces a dipole moment due to its very high electronegativity. When comparing the phase behavior of the series 128-n/m and 130-Br-n/m, it appears that the introduction of a fluorinated pyrrolidine-based or homopiperidine-based ring benefits the mesomorphic properties. Also, the use of a rigid 3,4-difluoropyrrole substituent proved very successful to stabilize the SmA phase, because compounds 129-n display much higher clearing points than their 128-n/m and 130-Br-n/m counterparts. Interestingly, the clearing points of 130-Br-n/1 could also be increased significantly via replacement of the bromide anion by a [BF4]− anion. Several of the neutral imidazole-containing precursors to these salts, which do not carry a flexible alkyl chain, are also LC, and they show a more ordered crystal smectic E phase.

Wu et al. investigated the phase behavior of salts related to 115-X and 117-m/n, but with a biphenyl ester moiety instead of a C-terminal alkyl chain (127-n).381 The C12, C14, and C16 derivatives were proposed to show an enantiotropic SmC2 phase, and the C18 derivative shows an enantiotropic SmA2 phase. The authors described a novel, highly ordered mesophase at room temperature for the long-chain derivatives (n ≥ 12) upon cooling from the isotropic liquid state, which was designated as “crystal smectic T′ phase”. This supercooled state was found to be stable for several days. Two modifications were proposed: orthogonal T′ and tilted T′, with transitional phases designated as Sm(A-T′)2 and Sm(C-T′)2, respectively (Figure S11). It is remarkable that these molecules with a long rigid core and only one flexible alkyl chain form a flowing, albeit presumably highly viscous, phase at room temperature. The T′ phase has a peculiar structure, which was described as “sandwichlike layered” (Figure S11). Whereas the higher-temperature SmA2 and SmC2 phases contain ionic monolayers, each smectic layer in the lower-temperature T′ phase is composed of an ionic

Shreeve and co-workers reported on dicationic imidazoliumbased ILs and ILCs bearing fluoro substituents on the central benzene ring and/or peripheral benzene rings (131-X-R1/ R2).516 Lateral fluoro substitution of aromatic systems increases the transverse dipole moment of the LC molecules, and is widely used to obtain materials with a negative dielectric anisotropy Δε (Δε = ε|| − ε⊥, where ε|| is the dielectric constant parallel to the director, and ε⊥ is the dielectric constant perpendicular to the director). Four of the investigated compounds, with n-dodecyl chains (131-X-R1/R2), are LC. Interestingly, 131-NTf2-H/F does not show a regular SmA phase, but a modulated ribbon-like AZ


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SmÃ phase. In a SmÃ phase, the smectic layers are collapsed into ribbons (Figure S12), and because these ribbons can be organized into a rectangular or an oblique 2D lattice, the phase can alternatively be described as a columnar phase. It is not clear why 131-Br-H/F and 131-NTf2-H/F behave differently. The Laschat group published the synthesis of related, phasmidic compounds, but these are not LC.738

ILCs whose cation is a positively charged receptor−anion complex between a π-conjugated bis(imidazolium) receptor and a chloride anion ([375][Cl], [376-Z][Cl], 361-a-H·Cl−/375, and 361-a-H·Cl−/376-Z) will be discussed in section 10.9.739 Bielawski and co-workers were the first ones to investigate benzobis(imidazolium) (BBI) ILCs (133-n and 134).515 These structures contain a conjugated rigid core formed by two imidazolium rings fused together via a central benzene ring,740−742 and they can be highly luminescent both in solution and in condensed phases.743,744 The group investigated different substitution patterns of the BBI core to check the influence on the phase behavior and the electronic/spectroscopic properties. For several non-LC derivatives, low glass transition temperatures could be achieved by reducing the molecular symmetry (with different substituents on the two imidazolium rings, among which substituents that disrupt π−π interactions) and by using noncoordinating counterions such as [CH3OSO3]−. A cubic LC mesophase, probably bicontinuous in nature, was observed for 134, which moreover exhibits an emission maximum in the visible region of the electromagnetic spectrum. Salt 133-12 shows an unidentified ordered smectic phase. The single-crystal structure of 133-12 shows ionic sublayers and aliphatic sublayers with interdigitated alkyl chains, with the two n-dodecyl chains attached to one imidazolium group extending toward opposite sides.

Other dicationic imidazolium-based ILCs with a rigid-rod structure (132-X-Yn-n) were reported by Schmitzer and coworkers.399,401 The single-crystal structures of [OTf]− salt 132OTf-Y1-12 and [NTf2]− salt 132-NTf2-Y1-12 (Figure 33) provide interesting information about the molecular structure and the cation−anion interactions in the solid state. It turns out that the two imidazolium cations of the dicationic core are in a trans conformation relative to one another, giving rise to an “S” shape. The authors suggested that, in the case of salts 132-OTfY1-n, this leads to the formation of a SmC phase upon melting. The single-crystal structures also show that the [NTf2]− anions are involved in more short contacts with the dicationic core than the [OTf]− anions; this could explain the more ordered smectic phases found for 132-NTf2-Y1-n. This is a rare case in which [NTf2]− salts show another, more ordered mesophase than their [OTf]− counterparts. Generally, if there is any difference in phase behavior, the [NTf2]− compounds tend to be not LC while the [OTf]− salts do form mesophases (as in the case of simple [Cnmim][X] compounds280), or they show a less stable mesophase due to the sterically more demanding [NTf2]− anions. 45-OTf-18 and 45-NTf2-18 present another example of different phase types observed for [OTf]− and [NTf2]− salts. The ionic conductivity of 132-OTf-Y1-n and 132-NTf2-Y1-n was measured in the mesophase and in the isotropic liquid state. For a given anion and temperature, a higher conductivity was found for a longer alkyl chain length. One of the derivatives with a naphthalene core (132-NTf2-Y2-12) was tested as an organized solvent for intramolecular Diels−Alder reactions (see section 12.3).401

Figure 33. Single-crystal structure of (a) 132-NTf2-Y1-12 and (b) 132-OTf-Y1-12. Reproduced with permission from ref 399 (http://dx.doi.org/10. 1039/c2sm26213d). Copyright 2012 The Royal Society of Chemistry. BA


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Figure 34. (a) Schematic representation of the structures of the SmA phases formed by 135-10 (left) and 136-10 (right), which are governed by nanosegregation, space-filling, and strong electrostatic interactions. The smectic layer thickness is indicated by d. When compared to its solid-state single-crystal structure, melting of 136-10 involves softening of the alkyl chains to result in an interdigitated arrangement in which the tilted cationic headgroups and iodide anions are counterbalanced by three alkyl chains (because the SmA phase is an optically uniaxial phase, there is no long-range correlation in the direction of the tilt of the rigid cores). (b) Schematic representation of one column slice in the Colhex phase formed by 136-18 (as mentioned in section 5.2, the concept of discrete column slices is a simplification for taper-shaped amphiphilic molecules). The outer, solid circle indicates the imaginary edge of the column; the aliphatic chains of adjacent columns interdigitate. Reprinted with permission from ref 65. Copyright 2011 Wiley.

Goossens et al. prepared the first T-shaped, 2-arylsubstituted imidazolium ILCs (135-n and 136-n).65 In order to attach a substituted aryl moiety to the imidazolium cation in its 2position, the cationic core was constructed starting from ethylenediamine. The compounds with one long alkyloxy chain (135-n) display only SmA phases, whereas the salts containing two alkyloxy chains (136-n) exhibit SmA, multicontinuous cubic and/or Colhex phases depending on the chain length and the resulting molecular shape and curvature at the ionic/aromaticaliphatic interface. None of the neutral imidazole precursors are LC. The SmA phases shown by 135-10 and 136-10 have a similar smectic layer thickness, and both contain interdigitated, folded alkyl chains (Figure 34a). Intuitively and on the basis of their single-crystal structures one would have expected an arrangement with interdigitated aliphatic chains in the case of the singlechain compound, in contrast to an arrangement with noninterdigitated chains in the case of the double-chain salt. The imidazolium headgroups and the iodide counterions were found to adopt a peculiar orientation in the central part of the columns of the Colhex phases (Figure 34b). This arrangement differs from the Colhex phases reported by the groups of Kato,647,648 Douce,500 Percec,517 and Tschierske437 for N-alkyl/arylsubstituted imidazolium-based mesogens. The proposed models for the SmA and Colhex phases (Figure 34) were supported by a packing constraint analysis based on Israelachvili’s theory383,384 (see also section 5.1). There were no indications for the creation of “supramolecular” polycatenar structures via “head-to-head” dimer formation between two molecules of 136-n mediated by intermolecular short C−H···I− contacts. The latter interactions (which can be observed in the single-crystal structures) are indeed even weaker than nonclassical C−H···Cl− and C−H···Br− interactions.310 The enantiotropic cubic phase shown by 136-15 probably represents the first example of a multicontinuous

thermotropic cubic mesophase of Pm3m ̅ symmetry. As noted by Douce and co-workers,500 it can be assumed that the columnar structure of the higher-temperature Colhex phase is preserved during the Colhex-to-Cub phase transition on cooling (i.e., the minicolumns forming the channel networks in the multicontinuous cubic phase reflect the structure of the columns in the Colhex phase). The “rigid-core” structure of 135-n and 136-n can be a versatile scaffold to synthesize more elaborate molecular architectures by using appropriately substituted benzaldehyde compounds as starting materials.

5.5. Dendrimeric Imidazolium-Based Ionic Liquid Crystals

Only two examples of imidazolium-containing dendrimeric ILCs have been reported up until now. Kato and co-workers presented two LC imidazolium salts with a fan-shaped L-glutamic acid moiety bearing bis(n-alkyloxy)phenyl groups (137-X);745 these compounds can be considered to be first-generation dendrons. The second-generation variants of 137-X with a folic acid derivative at the focal point of the dendrons instead of an imidazolium cation were shown to exhibit supramolecular chiral mesomorphic behavior in the presence of NaOTf.746 Compounds 137-X contain a more voluminous lipophilic part (four long alkyl chains per molecule, thanks to the branched architecture) than, for example, the taper-shaped imidazolium salts 53-X-n (three long alkyl chains). This is the reason for the appearance of a micellar cubic phase for 137-Br, besides a lowertemperature Colhex phase. 137-NTf2 only shows a Colhex phase, BB


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but in a wider temperature range than 53-NTf2-16 and 53-NTf218. Wiesner and co-workers proposed to use charge-transporting, amphiphilic LC materials that exhibit both a Colhex phase and an “inverted-type” micellar cubic phase as supramolecular “on−off switches” for conduction (Figure 35).747 Indeed, in the

cubic (CubI(Pm3̅n)) or tetragonal (Tet(P42/mnm), which can be seen as an intermediate structure between cubic Pm3n̅ and cubic Im3̅m phases749) mesophases, respectively. Four compounds (140-(144-Cl), 140-(145), 142-(144-PF6), and 143(144-Cl)) form cylinders at low temperatures (resulting in a Colhex phase) and spheres at higher temperatures (resulting in a CubI(Pm3̅n) or a Tet(P42/mnm) phase). As a result of the electrostatic interactions between the ionic moieties and the nanosegregation of the ionic, aromatic, and aliphatic parts, the supramolecular columns and spheres have a hydrophilic, charged interior. The authors proposed to use these as nanosized “IL reactors”, to perform reactions in a confined environment. In general, the clearing points appear to decrease with an increasing size of the cationic headgroup, and with an increasing anion size.

6. AMMONIUM-BASED IONIC LIQUID CRYSTALS Ammonium salts with long alkyl chains are well-known cationic surfactants, which can show lyotropic LC behavior at high concentrations in aqueous solution. A typical example is N-(nhexadecyl)-N,N,N-trimethylammonium bromide (CTAB, = [N 16,1,1,1 ][Br]; [N n,m,x,y ] + = N,N,N,N-tetrakis(n-alkyl)ammonium, with n, m, x, and y indicating the number of carbon atoms in each alkyl chain). In some cases, these compounds also exhibit thermotropic LC behavior. The n-alkylammonium halide salts ([Nn,0,0,0][X]) and N,N-bis(n-alkyl)-N,N-dimethylammonium halide salts ([Nn,m,1,1][X]) are some of the simplest examples of thermotropic ILCs.2 An important difference with the imidazolium-based ILCs is that the positive charge is much more localized. Simple ammonium salts do not absorb near-UV light, in contrast to aromatic systems. Furthermore, it is known that ILs with a quaternary ammonium cation (as well as ILs with a pyrrolidinium cation) have a wider electrochemical window than the corresponding imidazolium ILs (without a substituent at the C(2) carbon atom).750,751 In the past few years, significant advancements were made in the synthesis and characterization of ammonium-based LC dendrimers (see section 6.4).

Figure 35. Top: Ionic conductivities of 137-Br (○ = σ|| = ionic conductivity parallel to the long axes of the columns in the Colhex phase; □ = σ⊥ = ionic conductivity perpendicular to the long axes of the columns in the Colhex phase) and 137-NTf2 (● = σ||; ■ = σ⊥) as a function of temperature. The anisotropy in ion conduction disappears in the cubic and isotropic liquid phases. Bottom: Schematic representation of an ideal “on−off switch” for ion conduction, based on the temperature-dependent structural change from a columnar to a micellar cubic mesophase. Reprinted with permission from ref 745. Copyright 2008 The Chemical Society of Japan.

“inverted-type” micellar cubic phase, the ionic moieties are preferentially confined inside the hydrophilic micelles that are embedded in a hydrophobic matrix (see Figure S1). This prevents fast long-range ion transport in the latter phase. The Kato group also observed such a conductivity drop (from 8.4 × 10−5 to 9.6 × 10−6 S cm−1) at the (aligned Colhex)-to-CubI transition for 137-Br (Figure 35; a similar decrease in proton conductivity was found for the micellar cubic phases shown by amphiphilic N-(2,3-dihydroxypropyl)benzamides748). Because the Colhex phase of 137-Br was aligned by mechanical shearing, the conductivity drop was only observed upon heating from the Colhex to the CubI phase; in the subsequent cooling run, a polydomain Colhex phase was formed again. FT-IR measurements suggested that the bromide anions in 137-Br form hydrogen bonds with the N−H proton of the glutamic acid moiety, thus preventing intermolecular hydrogen bonding between amide groups, while the [NTf2]− anions in 137-NTf2 are nearly not involved in hydrogen bonding. This, as well as the weaker ionic interactions between the [NTf2]− anions and the cations, might be an explanation for the higher conductivity of 137-NTf2 as compared to 137-Br. Percec and co-workers investigated the self-assembly of dendritic building blocks with a methylimidazolium, pyridinium, triethylammonium, or tris(n-propyl)ammonium group at their apex ((138-R)−(143-R), Scheme 10).517 The dendrons were found to self-assemble into supramolecular columns or spheres, which self-organize into 2D Colhex or Colrec mesophases, and 3D

6.1. Ammonium-Based Mesogens Having a Predominantly Amphiphilic Character

Wang and Marques described the thermotropic LC behavior of gemini surfactants with quaternary ammonium groups and a C2 spacer (148-Br-0/n).587 The lyotropic and thermotropic phase behavior of similar compounds had been investigated before.2,642,752−754 In these reports, the melting point was found to increase with increasing length of the hydrophobic tails, and to decrease with increasing spacer length. Compounds 148Br-0/n show a disordered smectic phase, but over a narrow temperature range. This is possibly due to a mismatch in crosssectional area between the polar and nonpolar moieties, which was proposed on the basis of surface pressure−area (π−A) isotherm measurements on condensed monolayers in a Langmuir trough. Mixtures of 1 equiv of 148-Br-0/n with 2 equiv of [Na][C12H25OSO3] (the Na+ and Br− counterions were not removed) also show smectic phases, but the phase behavior is very complex. Manet et al. studied the thermal phase behavior of salts 148-X-0/14 with a broad variety of anions (X− = I−, Cl−, F−, BC


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Scheme 10. Dendrimeric ILCs Investigated by Percec and Co-workers517

[NO3]−, [CF3COO]−, [H2PO4]−, [CxH2x+1COO]− (x = 0−2, 5, 7, 9, 11, 15)).755

12 exist as highly ordered (in fact quasi-solid) crystal smectic phases (probably crystal smectic B and crystal smectic T, respectively). In contrast, the nonmesomorphic “monomer” (i.e., [N12,1,1,1][Br]: Cr1 · 72 · Cr2 · 98 · Cr3 · 166 · I/dec (°C)) and the LC “dimer” (148-Br-0/12) are fully crystalline at room temperature with well-developed 3D structural ordering, probably because of less stringent geometric constraints imposed by the smaller ionic headgroups and lower number of alkyl chains. The melting point in the series of oligomeric cationic surfactants increases with the degree of oligomerization (despite the increasingly lower degree of structural order in the solid/ quasi-solid state), due to increasingly stronger electrostatic interactions between the constituting ions. The “monomer”, “dimer”, “trimer”, and “tetramer” show lyotropic mesomorphism upon addition of a small amount of water. Tomašić, Mihelj, and co-workers extended the research on these types of ILCs to ndodecylsubstituted salts with [C 1 2 H 2 5 OSO 3 ] − , [(1pentylheptyl)PhSO3]−, or [cholate]− counterions (148-X-m/ 12 with m = 0−2). They compared the thermal phase behavior of these compounds to that of their analogues with a [N12,1,1,1]+,

Higher oligomeric analogues of 148-Br-0/12 (i.e., 148-Br-1/ 12 and 148-Br-2/12) were investigated by Jurašin et al.398 The “trimer” (m = 1) is also mesomorphic, but extensive thermal decomposition already occurs in the vicinity of its melting point so that its SmA phase is only transient. The “tetramer” (m = 2) completely decomposes before melting. At all temperatures, lamellar phases with interdigitated n-dodecyl chains were found. At room temperature, compounds 148-Br-1/12 and 148-Br-2/ BD


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[N12,12,1,1]+, or [N12,12,12,1]+ cation.756−758 Many of these salts exhibit one or more thermotropic smectic mesophases; [N12,12,1,1][(1-pentylheptyl)PhSO3], on the other hand, shows a Colhex phase. Zhou and Zhao reported on LC heterogemini bromide salts containing a quaternary ammonium cation and a hydroxyl group (149-Br-m/n).759 The hydroxyl group is close to the positively charged nitrogen atom, and can participate in hydrogen bonds with the bromide anion (and probably with other hydroxyl groups as well).760 This causes a bending of the alkyloxy chain near the hydroxyl group, so that the alkyloxy chain and the alkyl chain are parallel in a “cis” configuration relative to the polar headgroups. In situ variable-temperature IR spectroscopy indicated the loss of water from these hygroscopic compounds above 50 °C, and showed that the hydrogen bonds persist until the mesophase temperature range, although they are severely weakened. This persistence of intermolecular hydrogen bonding in the SmA phase may seem surprising at first, because the intralayer ordering should only extend over a few molecules in this disordered phase. However, it was also experimentally observed for, for instance, N-(n-alkyl)-N,N-bis(2-hydroxyethyl)N-methylammonium bromides showing T and SmA phases,394 for SmA-forming guanidinium 4-(n-alkyl)benzene- and nalkylsulfonates (see section 10.4), 408,761 and for many imidazolium-based ILCs (see section 5). In comparison to the “homogemini” salts with two quaternary ammonium groups such as 148-Br-0/n, replacement of one of those groups by a hydroxyl moiety weakens the electrostatic interactions between the polar headgroups in the crystal lattice, and results in lower melting points. These findings are in good agreement with the idea of “molecular lubrication” that was proposed by Weiss and coworkers in relation to phosphonium-based ILCs.385,497−499,574 This concept will be dealt with more extensively in section 7. The compounds with the highest overall length (m + n) of the hydrophobic chains (149-Br-16/8, 149-Br-14/10, 149-Br-12/ 12, and 149-Br-10/14) show the highest clearing points. No crystal smectic T phases were observed, in contrast to the N,Nbis(n-alkyl)-N,N-dimethylammonium bromide salts,392 which lack a hydroxyl group; this is probably due to steric factors.

hydrogen-bonding interactions in the ionic sublayers when replacing the halide anions by [BF4]− anions, which is also reflected in the lower clearing points found for the [BF4]− salts. The same group also studied two-chain, gemini-type surfactants composed of two tetraalkylammonium groups and a spacer unit with a central hydroxyl group (150-n).763 Once again, the central hydroxyl group is involved in hydrogen bonding with the halide anions (which are themselves hydrogenbonded to water molecules), as revealed in single-crystal structures of 150-12, 150-14, and 150-16. Surprisingly, the two identical alkyl chains, which are parallel in a “cis” configuration relative to the polar headgroups, adopt an asymmetric chain packing mode. Therefore, the dicationic part does not have a perfect U shape. In contrast, the single-crystal structure of 148-Br-0/12 (with a hydroxyl-free spacer) showed that its two alkyl chains extend on each side of the spacer plane.764 Enhanced hydrogen-bonding interactions between neighboring molecules of 150-n lead to a herringbone-like packing in the solid state (when viewed down the direction of the alkyl chains). Furthermore, salts 150-n display a lower melting point than their counterparts lacking a hydroxyl group.765 Remarkably, these compounds show a SmB phase below their higher-temperature SmA phase, in contrast to 148-Br-0/n and the less symmetric salts 149-Br-m/n. This should be related to the hydroxyl-substituted spacer and maybe even to the solid-state herringbone structure. Upon cooling the SmA phase at a rate of 2 °C per minute, the POM texture is maintained down to room temperature, indicating slow formation of the more ordered SmB phase. The SmB phases were further investigated by means of rheological measurements. The elastic modulus G′ is nearly independent of frequency because of the formation of an elastic network; this indicates that the SmB phase responds like a viscoelastic solid to small deformations. The phase behaves as a plastic material under high oscillatory shear. In the series 150-n, higher G′ values were found for increasing alkyl chain length n, presumably because of the increasing extent of chain interdigitation. In general, the rheological properties of the SmB phases of ILCs 150-n appeared to be consistent with those of other small-molecule smectic phases but with viscosities that are at least 2 orders of magnitude larger, possibly due to the extensive hydrogen bonding exhibited by these compounds.

Wei et al. synthesized very similar, SmA-forming ammonium compounds (149-X-m/1).762 The cations of these salts can be regarded as choline cations (choline = N-(2-hydroxyethyl)N,N,N-trimethylammonium) with a long-chain substituent. Hydrogen bonding between the hydroxyl groups and the bromide anions again resulted in a bent configuration of the alkyloxy chain relative to the polar headgroups, as well as in a reduction of the electrostatic and hydrogen-bonding interactions between the cationic headgroups and the bromide anions. Overall, this leads to lower melting points as compared to the simple N-(n-alkyl)-N,N,N-trimethylammonium bromide salts. These largely decompose before melting.2 The SmA phases were further investigated by means of rheological measurements. The elastic modulus G′ can be an indicator of the structural strength. Higher G′ values were found at 130 °C for 149-Br-m/1 (m = 14, 16) than for 149-Br-12/1, and for the Br− salts as compared to the [BF4]− salts. The latter is probably due to a decrease in

Kunz and co-workers investigated the thermotropic LC properties of choline-based salts with a long-chain alkanoate counterion (151-n).766 These compounds were already known to exhibit lyotropic LC behavior in aqueous solutions, and to show very low Krafft points.767,768 All of the (anhydrous) salts show a SmA phase, but also a lower-temperature highly viscous phase (M), which might be some sort of highly ordered and biaxial surfactant gel/rotator (Lβ) phase with partially molten but mainly all-trans alkyl chains. This was supported by 1H NMR spin−spin relaxation time (T2,eff.) measurements. The value of T2,eff. is sensitive to local reorientational dynamics and reflects the “average” degree of molecular order: the lower is the value, the higher is the order. An intermediate T2,eff. value was found for the M phase. The thermal behavior of the choline soaps is in line with the trend seen for the long-chain alkali metal alkanoate series, for BE


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spectroscopy (presence of CO stretching vibration around 1600 cm−1 (COO−), and absence of CO stretching vibration around 1670−1700 cm−1 (COOH) in the 1:1 complexes). Most 1:1 complexes show a SmA phase. Hydrogen-bonding motifs in the direction of the molecular short axes tend to destabilize mesophases due to obstruction of rotational fluctuations along the molecular long axes, but in this case they stabilized the nanosegregated structure of the SmA phases. This was supported by the fact that a 1:1 mixture of 4-(n-dodecyloxy)benzoic acid and (n-C18H37)NH2 does not show LC properties.

which the phase behavior simplifies with increasing size of the metal ion, with a lamellar molecular arrangement becoming the main mode of organization. However, thanks to the relatively large and asymmetric choline cation, compounds 151-n melt at much lower temperatures than the alkali soaps. Maximo et al. determined the full phase diagrams for mixtures of ethanolamine and diethanolamine with oleic acid, C17H33COOH, and with stearic acid, C17H35COOH, respectively (152-m and 153-m represent the equimolar mixtures).645 They also did extensive rheological measurements on these samples. Tolentino et al. reported on the thermal phase behavior of long-chain alkanoylcholine iodide salts (154-n).769 Apart from a stable and a metastable crystalline phase at lower temperatures, these compounds too exhibit an intermediate, ordered lamellar mesophase before forming a SmA phase above 160 °C. The transition to the intermediate phase was studied by multiple techniques, including 1H−13C CP-MAS solid-state NMR spectroscopy. Recall that the related imidazolium bromide salts 25-Br-n only display SmA phases, at lower temperatures.617

The branched bis(2-ethylhexyl) sulfosuccinate anion ([DOSS]− or “AOT”, Scheme 2) has often been used as anionic surfactant moiety to obtain (low-melting) thermotropic ILCs. Following an earlier report,771 Ungar et al. showed that the anhydrous sodium salt of this anion (159) is also mesomorphic, as are its anhydrous n-alkylammonium salts (160-n).501 All of these compounds show a Colhex phase (at subambient temperatures in the case of 160-n). Based on electron density reconstruction from experimental PXRD patterns and on molecular modeling and molecular dynamics (MD) simulations, detailed mesophase packing models were proposed. The relatively highly ordered structure of the Colhex phase shown by 159 entails three ion pairs in a columnar cross-section and is locally trigonal, but probably hexagonal over longer distances as a result of space- and time-averaging of local trigonal domains. The [DOSS]− anions are closely packed and ordered into an interdigitated assembly mode that allows efficient interlocking of the protruding ethyl branches (Figure 36a). The columnar

Chiral ammonium-based ILCs were prepared by Rao et al. through protonation of n-alkylamines with D- or L-camphorsulfonic acid, respectively (155-n and 156-n).410 These salts are somewhat related to compounds 52-X-n and show a smectic phase with intralayer ordering, tentatively assigned as SmB. In case of 155-12, 155-14, 156-12, and 156-14, it may concern a more highly ordered smectic phase. The phase transition temperatures of 155-n are very similar to those of their 156-n counterparts.

Lu et al. reported on other chiral ILCs, with a [Dy(NCS)8]5− anion (157-n).627 These salts form smectic phases for a sufficiently long alkyl chain. They are also luminescent in acetonitrile solution and in their solid and LC states, and possess magnetic properties thanks to the [Dy(NCS)8]5− anion.

Figure 36. Snapshots of MD simulations of (a) 159 and (b) 160-14 in their Colhex phase. These simulations were supported by reconstructed electron density maps. Reprinted with permission from ref 501. Copyright 2009 American Chemical Society.

phases formed by 160-n contain only two ion pairs in a columnar cross-section and are less ordered (Figure 36b), particularly for

Kohmoto et al. investigated the thermal behavior of ionic complexes of linear n-alkylamines with LC benzoic acid derivatives with lateral hydrogen-bonding moieties (158-m/ n).770 The ionic nature of the interaction between the amine groups and the carboxylic acid groups was confirmed by FT-IR

the longest chain lengths, for which dense packing can only be achieved through introduction of high conformational disorder in the radially extending alkyl chains. BF


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investigated the analogous ammonium-based salts (161-BF4-2/ n).672 Surprisingly, 161-BF4-2/10 and 161-BF4-2/12 show a bicontinuous cubic phase of Ia3̅d symmetry, with an additional higher-temperature Colhex phase in the case of 161-BF4-2/12. No cubic phases were found for the fully analogous imidazolium salts 53-BF4-n. The different phase behavior must be due to the differently shaped and sized cationic headgroups (see below for a more detailed explanation), and possibly also due to the difference in charge delocalization, with the positive charge being more localized in the case of the ammonium salts. In a follow-up report it was investigated how the anion, and the length of the alkyl groups directly attached to the cationic core, affect the mesophase behavior (compounds 161-X-m/n).674 The phosphonium analogues of 161-BF4-m/n (162-m/n) were also synthesized. Synchrotron SAXS experiments on the bicontinuous cubic phases of 161-BF4-2/10 and 161-PF6-2/10 allowed one to reconstruct 3D electron density maps of these phases in the bulk. Subtraction of the electron density map of 161-BF4-2/ 10 from that of 161-PF6-2/10 even revealed the positions of the [PF6]− anions ([PF6]− has a higher electron density than [BF4]−), confirming that they are located in the center of the 3D interconnected nanochannel networks of the bicontinuous cubic Ia3̅d phase. The different phase behavior of 161-BF4-2/n, 161PF6-2/n, and 161-OTf-2/n (n = 10, 12, 14) once again emphasizes the important influence of the anions. None of the [OTf]− salts is mesomorphic. The isotropization temperatures decrease with increasing anion size. In the series 161-BF4-3/n, only the salt with three n-tetradecyl chains is mesomorphic (Colhex). The three n-propyl groups are probably too bulky for efficient formation of the columnar phase. In contrast to compounds 161-BF4-2/n, trimethylammonium salts 161-BF41/n only show columnar phases, with a significantly higher clearing point. The higher mesophase stability might be due to stronger electrostatic interactions between the smaller cationic moiety and the anion, as well as a better match between the geometry of the cationic headgroup and the space available to it in the Colhex phase. The absence of cubic mesomorphism in this series is surprising. The authors explained this phase behavior by predicting (semiquantitatively) the “ideal” shape of a molecule that would fit perfectly into a given mesophase. Their geometrybased scheme was based on previous work by Ungar et al. in relation to LC wedge-shaped dendritic molecules.749 The rate of increase in volume of the branched cylinder network in a bicontinuous cubic phase (CubV(Ia3̅d)), dV/dr, was calculated as a function of increasing radius r of the cylinders (Figure S13). To have the ideal shape to fit into such bicontinuous cubic phase, the actual molecular cross-sectional area profile, A(r), should match the calculated dV/dr profile as closely as possible to achieve maximal and uniform space-filling. As it turns out, close to r = 0, the dV/dr profile for the CubV(Ia3̅d) phase is slightly higher than the profile calculated for the Colhex phase. This explains why 161-BF4-2/12, which has a slightly larger cationic headgroup, forms a CubV(Ia3d̅ ) phase and 161-BF4-1/12 does not; the extra cross-sectional area near the apex of the molecule is provided by the three extra methylene groups. Clearly the size and shape of the narrow end of the taper-shaped molecules are crucial in determining the type of mesophase assembly, as had already been found before (see, for example, section 5.4 for the phase behavior of 119-3/16, 119-4/16, and 119-5/16).437,772 The proposed scheme can also explain why the imidazoliumbased taper-shaped mesogens 53-BF4-n do not form cubic phases but columnar phases. A LC zwitterionic analogue of 161-

Martin et al. observed a rich thermotropic mesophase behavior for binary mixtures of N-(n-alkyl)-N,N,N-trimethylammonium chlorides and some transition metal chlorides (ZnCl2, CdCl2, CuCl 2 ).675 The detailed phase diagrams underline the importance of the ratio (surfactant cation)/metal (Figure 37).

Figure 37. Phase diagram for mixtures of N-(n-dodecyl)-N,N,Ntrimethylammonium chloride and ZnCl2. Crystalline phases are indicated by solid vertical lines; Sm = crystal smectic phase; SmA = smectic A phase; Cub = bicontinuous cubic phase (Ia3d̅ ); Colhex = hexagonal columnar phase; Cub′ = micellar cubic phase. Reprinted in part with permission from ref 675. Copyright 2006 Macmillan Publishers Ltd.

Prior to this report, only stoichiometric [MCl4]2− salts with longchain surfactant cations had been investigated (corresponding to 33.3 mol % MCl2 in a phase diagram such as the one shown in Figure 37).2 Martin et al. found that much lower amounts of the tetraalkylammonium surfactant are sufficient to template intermediate-range order in amorphous melts of transition metal chlorides. This can result in LC materials with a very high metal content (up to 80 mol % in the case of zinc(II)). The authors coined the term “metallotropic” (by analogy with the well-established expressions “thermotropic” and “lyotropic”) to describe mesophases formed by the reaction of an inorganic network former with a structure-directing agent. Depending on the exact composition, the low-melting metal chloride networks may consist of discrete molecular anions or oligomeric anionic networks. For example, for zinc(II) discrete tetrahedral [ZnCl4]2− anions are formed at the 33.3 mol % composition, but edge-shared tetrahedral dimers [(ZnCl3)2]2− are found at the 50 mol % composition. The authors stated that the amphiphilic character of the inorganic/organic hybrid and the tunability of charge density of the inorganic anions (compare for example [ZnCl4]2− and [(ZnCl3)2]2−) are responsible for the observed variety of homogeneous LC phases. The surfactant plays a double role: it lowers the melting temperature of the metal halide salt, and subsequently structures the melt (which would not be possible above the melting points of pure metal halides because of thermal decomposition). The authors presented preliminary data which indicate that tuning the structure of the melt of metalrich (>50 mol %) paramagnetic copper(II) networks also changed the magnetic communication. 6.2. Taper-Shaped Ammonium-Based Mesogens

After their initial reports on taper-shaped imidazolium-based mesogens (see section 5.2),390,647,648 Kato and co-workers BG


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X-2/12 (241) will be discussed in section 7 in the context of proton-conducting ILC materials.773

The phosphonium-based [BF4]− salts 162-1/n show a phase behavior similar to that of their ammonium-based counterparts 161-BF4-1/n. [Remark: These phosphonium-based systems are discussed here and not in section 7 because of their relation to the ammonium-based salts.] They form only a Colhex phase, but with lower melting and clearing points than the ammonium salts. The lower clearing points can be explained by the weaker electrostatic interactions between the larger phosphonium-centered cations and the [BF4]− anions (this was also demonstrated by 1H NMR spectroscopy).774,775 The same similarity in phase behavior was found for the series 162-2/n and 161-BF4-2/n, except for 162-2/ 12: this compound only forms a Colhex phase, whereas both a bicontinuous cubic and a Colhex phase were observed for the corresponding ammonium salt 161-BF4-2/12. The tris(npropyl)onium-based salts 162-3/n and 161-BF4-3/n differ in their phase behavior. In the case of the phosphonium salts, a Colhex phase is already formed for the n-decyloxy- and ndodecyloxy-substituted derivatives (162-3/10 and 162-3/12), whereas the corresponding ammonium salts (161-BF4-3/10 and 161-BF4-3/12) are not LC. Taking the isotropization enthalpy values into consideration, the authors attributed this to stronger van der Waals interactions between the phosphonium cation moieties in comparison to the ammonium structures. Salts 161-BF4-2/10 and 162-2/10 were also mixed with different amounts of LiBF4. An increasing amount of LiBF4 induced the formation of a higher-temperature Colhex phase, but the bicontinuous cubic phase was maintained in all investigated compositions. This compatibility of the ILC CubV phase with lithium cations is very promising in the search for efficient lithium ion conductors for batteries (see section 12.1). Investigation of the temperature dependence of the ionic conductivity revealed that, while the conductivity generally increases with increasing temperature, it is higher in the bicontinuous cubic phase of 161-BF4-2/12 than in its unaligned Colhex phase (Figure 38).672 This was attributed to the formation of 3D interconnected ion-conductive channels formed by the ionic cores of the interpenetrating infinite networks in the Ia3̅d cubic phase (see also the discussion on NDA2/58-Br mixtures in section 5.2). No macroscopic alignment is required to facilitate ion transport, in contrast to the columnar phases reported before (see also sections 5.2, 5.3, and 7). Structural defects in the bicontinuous cubic phase are less detrimental for ion migration than in other phases. This is because the 3D branched channels form alternative pathways for the diffusing ions, thus efficiently bypassing defects where some of the channels do not continue. The ionic conductivity shown by 161-BF4-2/12 in its unaligned cubic phase at 48 °C is still about one magnitude higher than the ionic conductivity parallel to the columns in the aligned Colhex phase shown by the related imidazolium salt 53-BF4-12 at 48 °C.647,672 It is also remarkable that in the case of 161-BF4-2/10, ion conduction takes place faster in the cubic phase than in the higher-temperature disordered isotropic liquid phase, where the ion mobility is expected to be higher (Figure 38; see also

Figure 38. Top: Ionic conductivities of 161-BF4-2/10 (red) and 161BF4-2/12 (blue) as a function of temperature. Bottom: Schematic representation of the nanoscale ion-conductive channels formed in the bicontinuous cubic phase. Reprinted with permission from ref 672. Copyright 2007 American Chemical Society.

below650).672 Phosphonium salt 162-2/10 shows a similar behavior.674 It should be mentioned that the situation is completely the opposite for micellar cubic phases, which act as insulators for ion conduction. This was observed for LC imidazolium salts having an L-glutamic acid moiety (see section 5.5).745 An important finding is the fact that phosphonium salt 162-2/10 shows a significantly higher ionic conductivity than its ammonium counterpart 161-BF4-2/10 at the same temperature (e.g., at 48 °C: 162-2/10, σ = 2.3 × 10−4 S cm−1 versus 161-BF42/10, about 30 times lower), despite the similar self-assembled cubic mesophase structure.674 This was attributed to the weaker electrostatic interactions between the phosphonium cationic headgroups and the [BF4]− anions (see above). It is known that phosphonium-based ILs have a lower viscosity and a higher ionic conductivity than their ammonium-based analogues, due to the lower extent of ion association by phosphonium salts and the higher flexibility of the bond angles and dihedral angles in phosphonium cations.774−776 A mixture of 162-2/10 and LiBF4 in a 4:1 molar ratio showed a temperature dependence of the ionic conductivity similar to that of pure 162-2/10. Following the same strategy that was applied to the tapershaped imidazolium salts forming Colhex phases,390 the Kato group synthesized analogues of salts 161-BF4-2/n with polymerizable groups at two chain ends (163 and 164), to fix the mesophase structure by in situ photopolymerization.673 Addition of two acrylate groups (163) resulted in the loss of mesomorphism, but the salt with two 1,3-dienyl groups (164) shows a bicontinuous Ia3̅d cubic phase at and below room temperature. Polymerizable 1,3-dienyl moieties seem to be very attractive for incorporation in known LC materials, because 1,3-dieneterminated alkyl chains resemble normal alkyl chains in polarity and size.777 Salt 164 was also mixed with different amounts of LiBF4. Again, an increasing amount of LiBF4 induced the formation of a higher-temperature Colhex phase. PhotopolymeBH


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temperatures and despite the highly ordered 3D structure of cubic mesophases (a similar difference in diffusion coefficient between the anions and cations was found during MD simulations on the smectogenic ILC [C16mim][NO3]488). When the transition to the isotropic liquid was approached on heating, the diffusion of the cations gradually speeded up, and became comparable to that of the anions. This was ascribed to the formation of ion pairs/clusters in the isotropic phase (Figure 39). The authors proposed that ion pairing decreases the fraction

rization of 164/LiBF4 (4:1) yielded lithium ion-conductive, solid, free-standing, flexible, thermally stable (no phase transitions up to 120 °C), and transparent polymer films. Polymerization was conducted in the cubic phase, in the unaligned Colhex phase (the Colhex phase of the 164/LiBF4 mixture could not be aligned, in contrast to 55390), and in the isotropic liquid state, to achieve polymer films with a different nanostructure. At a particular temperature, the ionic conductivity of the “cubic” film was higher than that of the “isotropic” film, which was in turn higher than that of the “columnar” film (e.g., at 90 °C: σ = 3.1 × 10−4 S cm−1 versus 7.8 × 10−5 S cm−1 versus 8.8 × 10−6 S cm−1, respectively). Moreover, the “cubic” film showed an ionic conductivity that was comparable to that of unpolymerized 164/LiBF4 (4:1) in its (mobile) isotropic liquid state. It should be noted that fixation of a bicontinuous cubic mesophase structure by cross-linking had been reported before for lyotropic ILCs,656,658,663,667,668,778−784 but the “all-solid” polymer electrolytes presented by Kato et al. require no organic solvent/liquid electrolyte, which might be of interest for safety reasons and ease of fabrication. Compound 164 photopolymerized in its bicontinuous cubic phase has also been used to construct nanostructured membranes that show selective permeation of ions.785 These membranes can potentially be used for water purification (desalination) because of their combination of good water permeability and salt-rejection performance. Thanks to the “frozen” bicontinuous cubic phase structure, the 50−100 nm thick polymerized LC film that acts as the separating layer in the composite membrane has self-organized uniform pores with a diameter of about 0.60 nm (as estimated by positron-annihilation lifetime spectroscopy (PALS)). On a support of 7 cm diameter, no micrometer-scale defect pores could be found. NaCl and MgCl2 were rejected with an efficiency of more than 60%, whereas rejection efficiencies of an amorphous film made by photopolymerization of 164 in its isotropic liquid state were below 14%. The flux of the aqueous feed solutions through the “CubV membrane” was reasonable, although not as high as for a commercial, cross-linked aromatic polyamide nanofiltration membrane. In contrast to the latter and in contrast to the amorphous membrane, however, the “CubV membrane” rejected divalent [SO4]2− ions to a much lower extent than monovalent Cl− ions ([SO4]2−/Cl− selectivity of about 3 for Na+ and Mg2+ salts). This rather unique anion selectivity is not well understood yet, but was tentatively attributed to specific interactions of the ions with the pore walls during their passage through the 3D interconnected ionconductive nanochannels.

Figure 39. Schematic representation of the proposed ion self-diffusion phenomena in (a) the isotropic liquid phase, and (b) the bicontinuous cubic LC phase of 161-BF4-2/10. Reproduced with permission from ref 650 (http://dx.doi.org/10.1039/b915931b). Copyright 2010 The Royal Society of Chemistry.

of ions available to participate in conduction, while cluster formation may limit the path length for ionic transport, and that these two effects might explain the lower ionic conductivity found in the isotropic liquid phase compared to the bicontinuous cubic phase (see above672). Similar experiments on the roomtemperature IL N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium tetrafluoroborate showed that the (smaller) ammonium cations and the [BF4]− anions also move at a similar speed in the liquid phase, but with diffusion coefficients that are about 2 orders of magnitude higher than in the isotropic liquid phase of 161-BF4-2/10. Other groups had already proposed that correlated ion motions and ion pairing occur in ILs, and that the conductivity calculated from the Nernst−Einstein equation (which only takes “free” charged species into account) in combination with diffusion data is generally higher than the actual, measured conductivity.591,786 The effective concentration of ions available to participate in conduction, cion,eff. (eq 3), was introduced as a parameter to estimate the electrostatic interactions and “ionicity” in ILs.787

Furó and co-workers performed variable-temperature pulsedfield-gradient spin−echo NMR measurements on neat samples of 161-BF4-2/10, to obtain distinct self-diffusion coefficients for the cations (via 1H NMR) and anions (via 19F NMR) in the CubV(Ia3̅d) phase.650 They found that the [BF4]− anions move about twice as fast as the cations in the cubic phase. This rather small (albeit significant) difference is somewhat surprising, given the large size difference between the two ionic species. It indicates the dynamic nature of the LC phase, even at low

⎛ Λ impedance ⎞ c ion,eff. = ⎜ ⎟ ·c ion ⎝ ΛNMR ⎠

(3) −1

where Λimpedance is molar ionic conductivity [S cm mol ] measured by the electrochemical impedance method; ΛNMR is molar ionic conductivity [S cm2 mol−1] estimated by pulsedfield-gradient NMR and the Nernst−Einstein equation; and cion is molar concentration = ρ/M (with ρ = density [g cm−3], and M 2

BI


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Scheme 11. Ionic Complexes of Star-Shaped Tris(2-aminoethyl)amine (165) and Benzoic Acid Derivatives, Which Were Investigated by Kohmoto, Kishikawa, and Co-workers735,791−793

= molecular mass [g mol−1]). Another popular approach to estimate “ionicity” is consideration of Walden plots, that is, plots of log(Λ) versus log(η−1) (with η = viscosity [poise]).178,788,789 The Walden plot of a “high ionicity” IL (with low extent of cation-anion association) should be close to a straight line passing through the origin. MacFarlane et al. noted that the concept of “ionicity” does not necessarily describe the chemical availability of individual ions.789 Similar pulsed-field-gradient NMR measurements (1H NMR and 19F NMR) were performed on the magnetically aligned Colhex phase of 53-PF6-8, to investigate the anisotropic ion diffusion.790 The macroscopic orientation of the Colhex phase in the magnetic field of the spectrometer could be checked by 2H NMR spectroscopy (for this purpose, a small amount (100 °C) partial formation of amide bonds between the −NH3+ and −COO− groups can occur. Furthermore, the internal NR3 tertiary amino groups can also become protonated.

After their initial report on the symmetric gemini imidazolium salts 97-m/n,710 Kumar and Gupta investigated their ammonium-based analogues with an ethylene spacer between the two positively charged nitrogen atoms and with [BF4]− counterions (190-m/n).824 These show Colhex phases.

Mizoshita and Seki synthesized bolaamphiphiles with flexible trisiloxane spacers and a central biphenyl (191) or azobenzene (192) core.825 Compound 191 shows only lyotropic mesomorphism in the presence of water. Azobenzene derivative 192 exhibits both thermotropic and lyotropic LC properties, probably due to stronger core−core interactions between the azobenzene groups, as indicated by UV−vis spectroscopy. UV irradiation of 192 in its thermotropic LC state leads to an isothermal LC-to-isotropic phase transition as a result of reversible cis−trans isomerization of the azobenzene moieties.

After a comprehensive study of the mesophase behavior of ammonium salts of PPI and PAMAM dendrimers of different generations with simple long-chain alkanoate counterions (PPIGx/(199)2(x+1) (x = 1−3), PPI-Gx/(202)2(x+1) (x = 1−5), and PPIGx/(204)2(x+1) (x = 1−5); PAMAM-Gx/(204)2(x+2) (x = 0− 5)),2,417 the Zaragoza group investigated the same systems with mono-, di-, and trisubstituted benzoate counterions (PPI-Gx/ (206)2(x+1), PPI-Gx/(207)2(x+1), and PPI-Gx/(208)2(x+1) (x = 1− 5)).826 Compounds PPI-Gx/(206)2(x+1) and PPI-Gx/(207)2(x+1) exhibit a SmA phase, while a Colrec or Colhex phase was found for the series PPI-Gx/(208)2(x+1) with trisubstituted benzoate anions because of a larger alkyl chain volume. A Colrec phase was only observed for the first generation, PPI-G1/(208)4. In the

6.4. Dendrimeric Ammonium-Based Ionic Liquid Crystals

As already mentioned in section 5.5, ammonium-containing dendritic building blocks that self-assemble into supramolecular columns were investigated by Percec and co-workers (138-(146X), 138-(147), 139-(146-X), 139-(147), 140-(146-X), and BP


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Scheme 13. Carboxylic Acid Derivatives That Were Used by Serrano, Marcos, Barberá, Ros, Sánchez, and Co-workers, and by Sijbesma and Co-workers, To Make Ionic Complexes with PPI and PAMAM Dendrimers of Different Generations (193, 194, 195, 196a)417,456−460,711,826,828−835

BQ


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Scheme 13. continued

a

See also Table S3.

Figure 45. Left: Phase diagram for PPI-G3/(200)y(205)16−y.830 SmAH = SmA phase with dimensions similar to those for PPI-G3/(200)16(205)0; SmAF = SmA phase with dimensions similar to those for PPI-G3/(200)0(205)16; SmA+ = frustrated SmA phase. Right: Phase diagram for PPI-Gx/ (200)0.75·2(x+1))(205)0.25·2(x+1)) (x = 1−5).832

The concept of fluorination was extended by reacting PPIG3 with mixtures of CH3(CH2)9COOH and CF3(CF2)7(CH2)2COOH to prepare ionic codendrimers (PPI-G3/ (200)y(205)16−y).830 Importantly, because of this synthesis method, not every individual molecule will have exactly the same composition. The isotropization temperature increased with increasing degree of fluorination (Figure 45). Furthermore, SmA phases with a different layer thickness were found for PPIG3/(200)16(205)0 and PPI-G3/(200)0(205)16, denoted as SmAH and SmAF, respectively. This is because the semifluorinated chains are more stretched and rigid than the hydrocarbon chains, and it was already observed in the previous report.828 It could be calculated that, as soon as the contribution of a particular type of chain (hydrocarbon or fluorocarbon) to the total cross-sectional area of the chains exceeds 70%, a single SmAH phase or SmAF phase is formed, respectively. Because fluorocarbon chains have a larger cross-sectional area than hydrocarbon chains, a single SmAF phase was already observed for PPI-G3/(200)6(205)10. However, for contents between 30% and 70% of the respective chain types, that is, for PPI-G3/ (200) 12 (205) 4 , PPI-G3/(200) 10 (205) 6 , and PPI-G3/ (200)8(205)8, a peculiar “intermediate” frustrated SmA phase (designated as SmA+) was suggested (Figure 46). Importantly, this phase exhibits a typical SmA POM texture, but shows only a diffuse small-angle X-ray diffraction signal because it contains no long-range lamellar arrangement. Whereas in the SmAH and

series PPI-Gx/(206)2(x+1), there was a partial protonation of the internal NR3 groups of the PPI scaffold. Interestingly, the analogues of PPI-Gx/(207)2(x+1) (x = 1−4) with covalent amide bonds instead of ionic −NH3+−OOC− interactions show Colhex phases above 60 °C instead of SmA phases above 22−45 °C.827 The difference was ascribed to the formation of intermolecular hydrogen bonds in the covalently bonded complexes. This is again an interesting example of the decisive influence electrostatic interactions can have on the phase behavior. Subsequently, PPI and PAMAM dendrimers were functionalized with a semifluorinated carboxylic acid (PPI-Gx/(205)2(x+1) (x = 1−5) and PAMAM-Gx/(205)2(x+2) (x = 0−4)).828 SmA phases were observed, except in the case of PPI-G5/(205)64, which shows a Colrec phase. PAMAM-G4/(205)64 did not show any columnar mesomorphism due to the larger volume of the PAMAM structure. This larger volume prevents steric congestion of the peripheral NH2 groups at the base surfaces of the “cylinder” formed by elongated dendrimers in the SmA phase, which would cause the transition to a columnar phase. Such transitions are possible because of the flexibility of the dendrimer structures. The mesophase stability of the dendrimers with a fluorinated shell was increased as compared to the counterparts with hydrocarbon chains (compare PPI-G3/ (205)16 and PPI-G3/(200)16), as a result of enhanced nanosegregation (fluorophobic effect). Oriented films and fibers could be made with these ionic dendrimers. BR


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G3/(200)y(205)16−y),830 only SmA phases were found. However, for PPI-G3/(206)12(209)4 and PPI-G3/(206)8(209)8, an additional lower-temperature modulated ribbon-like SmÃ phase was observed. It is not a coincidence that only for these compositions the contributions of the hydrocarbon and fluorocarbon parts to the total cross-sectional area of the carboxylate moieties are between 30% and 70%, so that there is no majority group that can control the supramolecular organization by its own. The sharp small-angle X-ray diffraction signals of the SmÃ phase do not fit a simple lamellar structure, but rather a 2D centered rectangular lattice (c2mm symmetry), with lattice parameter a equal to roughly twice the layer spacing d in the higher-temperature SmA phase. The proposed molecular packing in the SmÃ phase is depicted in Figure 47. Its formation

Figure 46. (a and b) Schematic representation of the structure of the SmAH phase formed by PPI-G3/(200)14(205)2. (c and d) Schematic representation of the structure of the frustrated SmA+ phase formed by PPI-G3/(200)10(205)6. (a) and (c) represent top views, while (b) and (d) are side views. Semifluorinated chains are shown in red. Reprinted with permission from ref 830. Copyright 2010 Wiley.

SmAF phases incompatibility effects are suppressed and the different chain types can mix homogeneously over the dendrimer surface, these incompatibility effects dominate the dendrimer structure and chain packing in the SmA+ phase, where compatible chains try to segregate in nanodomains (Figure 46). In this way, the regular layer structure found in SmAH and SmAF is disrupted into domains too small to be detected by PXRD. Interestingly, the analogues of PPI-G3/(200)12(205)4 and PPI-G3/(200)8(205)8 with covalent amide bonds show a single SmA phase with a typical long-range layer structure. These compounds cannot form the frustrated SmA phase, because the covalent bonds force the chains to be in fixed positions on the dendrimer surface (despite their incompatibility). This indicates that the reversibility of the noncovalent electrostatic interactions is crucial for the ionic dendrimers to adopt the thermodynamically most favorable dendrimer and phase structure. PPI-G3/ (200)12(205)4 presents an interesting case, because a mixture of the SmAH and SmAF phase is formed at room temperature. The authors discussed their observations in terms of an enthalpy− entropy competition, where entropic factors dominate in case of the presence of a majority group (which can obscure the effect of the minority group that in principle would like to form separate domains), but enthalpic factors dominate for similar quantities of both groups. In a follow-up report, the influence of the PPI dendrimer generation was investigated (PPI-Gx/ (200)0.75·2̂(x+1)(205)0.25·2̂(x+1) (x = 1−5); Figure 45).832 A frustrated SmA+ phase was only observed for PPI-G2/ (200)6(205)2 and PPI-G3/(200)12(205)4, whereas PPI-G1/ (200)3(205)1 and PPI-G4/(200)24(205)8 show a SmAH phase. The origin of this difference should be further elucidated. Just like PPI-G5/(200)0(205)64, PPI-G5/(200)48(205)16 exhibits a Colrec phase; apparently a small fraction (25 mol %) of the more voluminous fluorocarbon chains is sufficient to induce the transition from a lamellar to a columnar phase, because a SmA phase was observed for PPI-G5/(200)64(205)0. PPI-derived ionic codendrimers with aromatic benzoate derivatives carrying either a nonfluorinated chain or a semifluorinated chain were prepared as well (PPI-G3/ (206)y(209)16−y).831 Just like for the alkanoate analogues (PPI-

Figure 47. (a) Schematic representation of the structure of the SmA phase formed by PPI-G3/(206)0(209)16. (b) Schematic representation of the proposed structure of the modulated ribbon-like SmÃ phase formed by PPI-G3/(206)8(209)8, showing the undulation in a direction perpendicular to the layer normal. Semifluorinated moieties are shown in red, perhydrogenated moieties in blue, and the PPI dendrimer matrix in black. d is the smectic layer thickness of the SmA phase; a and b are the lattice parameters of the SmÃ phase. Reprinted with permission from ref 831. Copyright 2010 American Chemical Society.

was ascribed to the chemical incompatibility of the hydrocarbon and fluorocarbon chains, and to the mismatch between their respective space-filling and cross-sectional area requirements, which causes the molecules to adopt the shape of a conical frustum. The strain generated by this particular shape is relieved by the layers breaking into ribbons arranged parallel to the layer planes and organized in a rectangular lattice. It is interesting to note that the SmÃ phase can apparently evolve to a regular, lessordered SmA phase at higher temperatures. None of the LC ionic dendrimers from the Zaragoza group discussed thus far contain rigid, anisometric mesogenic parts. As pointed out by Donnio et al., 344 the supramolecular organizations displayed by these amphiphilic dendrimers resemble those of lyotropic mesophases, with the (noncharged) dendritic core playing the role of the polar solvent. The concept of functionalization of the PPI and PAMAM dendrimer outer surfaces by means of electrostatic interactions was also used to decorate the dendrimer structures with rod-like mesogenic groups. Whereas covalent linkage of (pro)mesogenic units to dendrimers to induce mesomorphism is well-known,344 noncovalent linking is less common. By using a p-cyanoazobenzenefunctionalized carboxylic acid, photoresponsive nematic ionic dendromesogens (as well as nematic ionic random hyperbranched PEI and PEIMe polymers) could be obtained (PPIG3/(210)16 and PAMAM-G2/(210)16),456 as had already been shown by Frey and co-workers for PEIMe.455 Analogous PPIderived ionic codendrimers with a branched aliphatic carboxylic acid were presented in two follow-up reports (PPI-G3/ (210)y(212-R1/R1)16−y).457,458 Some of the latter compounds BS


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(y ≤ 4) show a higher solubility in common organic solvents, which is beneficial for thin-film processability. Increasingly large fractions of the aliphatic carboxylic acid 212-R1/R1 resulted in the gradual appearance and stabilization of a SmA phase, at the expense of the nematic phase. Using 488 nm linearly polarized light, stable birefringence could be induced in thin films of the azobenzene-containing compounds (the optical anisotropy is generated due to the reorientation of the azobenzene chromophores in response to the incoming linearly polarized light, through trans−cis−trans photoisomerization cycles). Birefringence kept increasing for some time after switching off the excitation light; this well-known phenomenon of further enhancement of the photoalignment is associated with cooperative reorientation of the azobenzene chromophores in the LC state. Subsequent linear dichroism measurements resulted in a maximal dichroic ratio of 4.3 and a maximal value of 0.67 for the in-plane order parameter. Higher dichroic ratios were found for the compounds with higher azobenzene contents. Replacement of the p-cyanoazobenzene moieties by pcyanobiphenyl groups resulted in the disappearance of the nematic phase in favor of a smectic phase (PPI-G3/(211)16 and PAMAM-G2/(211)16).459 This phase was designated as SmC, although homeotropic areas can be seen in the POM image. Interestingly, the nematic phase is preserved for random hyperbranched PEI and PEIMe polymers carrying the pcyanobiphenyl groups. This can probably be ascribed to the lower degree of structural order in the polymers than in the PPI and PAMAM dendrimers. This structural order facilitates nanosegregation and thus the formation of smectic phases. The first report on ILCs containing typical bent-core mesogenic groups also appeared in relation to dendrimeric ammonium salts. Vergara et al. combined bent-shaped (mostly nonmesomorphic) carboxylic acids 217-m/n and 218-m/n with PPI dendrimers of different generations (PPI-Gx/(217-m/ n)2̂(x+1) (x = 1−2, 5) and PPI-Gx/(218-m/n)2̂(x+1) (x = 1−3, 5)) and with a random hyperbranched PEI polymer.711 It could be concluded that the bent-core structure strongly determines the supramolecular arrangement of the ionic macromolecules, whereas the size and branching regularity of the dendritic core are less important. The length of the flexible spacer (m) and the number of aromatic rings in each bent-core unit (217-m/n versus 218-m/n) are parameters that influence the transition temperatures. The length of the terminal alkyloxy chain (n) determines whether the macromolecules self-organize in a rectangular columnar arrangement (Colrec, for n = 8) or in a lamellar arrangement (SmCP, for n = 14). In the polar SmCP or B2 “banana” phase, the bent-core units are tilted within their separate sublayers, and four possible states can in principle be distinguished depending on the relative tilt direction and the polar order of the molecules in adjacent layers (two ferroelectric states and two antiferroelectric states).71 The switching response of the compounds in their mesophase under the influence of an electric field was however not investigated. The molecular order in the mesophases could be frozen into a glassy state at room temperature. Interestingly, aligned fibers could be drawn from the materials in their (viscous) mesophases. Other types of mesogenic groups with a nonlinear shape were also combined with PPI and PAMAM dendritic cores. Hernández-Ainsa et al. investigated the phase behavior of ionic complexes of PPI or PAMAM with carboxylic acids containing luminescent 1,3,4-oxadiazole-based or 1,2,4-oxadiazole-based rigid groups (PPI-Gx/(215-n)2(x+1) (x = 1−5), PPI-Gx/(216n)2 (x+1) (x = 3, 5), and PAMAM-G4/(215-n)64).834 All

compounds show a SmA phase, at the expense of the nematic phase shown by some of the carboxylic acid precursors. PPI-G5/ (215-4)64 and PPI-G5/(215-10)64 additionally show a lowertemperature Colhex phase. In general, the 1,2,4-oxadiazole-based materials exhibit lower melting points and broader mesophase ranges than the dendrimers containing 1,3,4-oxadiazole groups, but also a lower thermal stability. The analogue of PPI-G3/(21510)16 with covalently connected mesogenic groups (via amide bonds) shows a slightly smaller mesophase temperature range and higher transition temperatures than its ionic counterpart, because of the reduced mobility of the different molecular components. For the 1,3,4-oxadiazole compounds with a long spacer (n = 10), PXRD data pointed to full interdigitation of the oxadiazole moieties (in a head-to-tail fashion) of molecules in adjacent layers in the SmA phases, whereas only partial interdigitation (only of the pentyl chains, not of the aromatic moieties) was found for their counterparts with a short spacer (n = 4). In contrast, full interdigitation occurs for all 1,2,4oxadiazole-based compounds as a result of more efficient intermolecular side-by-side interactions. The different supramolecular arrangements did not have a significant influence on the photophysical properties (absorption and emission) of casted films of the compounds (see also section 12.5). More recently, the ionic LC dendrimers PPI-Gx/(212-Y1/ Y2)2(x+1) (x = 1−5; Y1 = R2, and Y2 = R3 or R4) and PPI-Gx/(213Y1/Y2)2(x+1) (x = 1−5; Y1 = R2, and Y2 = R3 or R4) were reported.460 In this case, the dendritic PPI core is surrounded by luminescent and electroactive carbazole-containing bifunctional dendrons. The materials based on PPI-G1, PPI-G2, PPI-G3, or PPI-G4 display either a nematic phase (in the series with 212-R2/ R3 or 213-R2/R3 dendrons) or a SmA phase (in the series with 212-R2/R4 or 213-R2/R4 dendrons), except for PPI-G4/(213R2/R3)32, which shows a Colrec phase. The PPI-G5 materials with first-generation bifunctional dendrons (PPI-G5/(212-R2/R3)64 and PPI-G5/(212-R2/R4)64) also exhibit a Colrec phase, whereas a Colhex phase was found for their counterparts with secondgeneration dendrons (PPI-G5/(213-R2/R3)64 and PPI-G5/ (213-R2/R4)64). Finally, the Serrano group reinvestigated the ammonium salts of PAMAM dendrimers of different generations with aliphatic alkanoate counterions,417 but this time with a higher (carboxylic acid)/dendrimer ratio and with carboxylic acids of different lengths (PAMAM-G2/(197) 30 , PAMAM-G2/(198) 3 0 , PAMAM-G2/(199)30, PAMAM-Gx/(201)2̂(x+3)−2 (x = 0−2), PAMAM-Gx/(202) 2̂ ( x+3)−2 (x = 0−4), PAMAM-Gx/ (203)2̂(x+3)−2 (x = 0−2), and PAMAM-Gx/(204)2̂(x+3)−2 (x = 0−2)).833 Because of the higher acid-to-dendrimer ratio, not only the outer primary amino groups but also the inner tertiary amino groups were protonated, as indicated by 1H and 13C NMR spectroscopy. As a result, the dendritic core is also charged in these compounds. The ionic complexes with the shortest-chain carboxylic acids (i.e., hexanoic acid and octanoic acid) are not LC. For the other compounds, both the glass transition temperature and the melting point increase with increasing length of the carboxylate anions. The clearing point is rather independent of both the dendrimer generation and the anion chain length. PAMAM-G4/(202)126 shows a Colrec phase below its SmA phase; the columnar-to-smectic transition was assigned to the loss of correlation along the columnar axes upon increasing the temperature (Figure 48). The effect of varying the (carboxylic acid)/dendrimer ratio was investigated in detail for the PAMAM-G2/(202)y system, which forms a SmA phase for y ≥ 8.835 The highest clearing point was found for y = 16, BT


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structures, successive generations can be easily prepared in one or only a few steps from commercially available precursors in a thermodynamically controlled self-assembly process. This allows extensive variation of the peripheral functionalization of the dendrimers. Naturally, they also show the advantages of dendrimeric compounds in general as compared to, for example, linear polymers; these include monodispersity and a better defined molecular architecture. Furthermore, the reversibility of the electrostatic interactions allows the ionic complexes to adapt their molecular structure, as well as their supramolecular arrangement in the mesophase, to specific conditions. This has been shown by the appearance of an uncommon frustrated SmA phase and a modulated ribbon-like SmÃ phase for some PPIderived ionic codendrimers. Interestingly, the analogues of PPIGx/(199)2(x+1) (x = 1−5) and PPI-Gx/(202)2(x+1) (x = 1−5) with covalent amide bonds instead of ionic −NH3 +−OOC− interactions are not mesomorphic.417 Sijbesma and co-workers also exploited the concept by reacting a roughly disk-shaped carboxylic acid (instead of a simple long-chain alkanoic acid) with PPI-G2 (PPI-G2/(214)n (n = 0, 2, 4−8, 10, 12, 15)).829 The compounds with a (carboxylic acid)/dendrimer ratio between 4:1 and 8:1 exhibit a peculiar Colobl phase with a well-defined superlattice of column-shaped dendrimer domains (Figure 49). Pure 214 shows a Colhex phase, consisting of rather short columns of hydrogen-bonded molecules (π−π stacking is less important for such a small aromatic core), but addition of PPI-G2 and subsequent interaction between the amino groups of PPI-G2 and the carboxylic acid groups results in the transition to the Colobl phase (Figure 49). The structure of the oblique superlattice could be covalently stabilized via amidation at elevated temperature (annealing of PPI-G2/(214)8 at 120 °C). In “dendrimerenriched” mixtures (acid/PPI ratios 7:1 to 4:1), the lattice swells up (in the direction along the (100) planes, Figure 49) because of the incorporation of additional dendrimer nanodomains. “Diskenriched” mixtures (acid/PPI ratios 12:1 to 15:1) show lamellar phases with a columnar structure closely related to the oblique superlattice. In these phases, no or less correlation exists between the dendrimer nanodomains. The formation of the unusual Colobl structure instead of a regular Colhex phase was ascribed to

Figure 48. Colrec-to-SmA transition shown by PAMAM-G4/(202)126. This transition is probably caused by the loss of correlation along the columnar axes upon increasing the temperature, which leads to the disruption of the 2D rectangular positional ordering so that a lamellar structure is formed. Reproduced with permission from ref 833 (http:// dx.doi.org/10.1039/c0sm01074j). Copyright 2011 The Royal Society of Chemistry.

corresponding to full protonation of the PAMAM outer primary amino groups but no protonation of the inner tertiary amino functions. The compounds with y > 16 all show similar transition temperatures. PXRD studies showed that the gradual addition of carboxylic acids makes the dendrimer core change its shape: the core expands to accommodate all carboxylic acids (resulting in a decreased smectic layer thickness), and in this way the inner amino groups become accessible for protonation. The authors exploited the amphiphilic character of these ionic complexes not only to achieve bulk mesomorphic properties, but also to obtain self-assembled structures in water, such as “onion-like” nanospheres. The successful formation of stable nanoobjects in water can depend strongly on the hydrophilic/hydrophobic composition balance, indicated by Hc = (molar mass aliphatic chains/ molar mass dendrimer) × 100. Interestingly, the nanoobjects were found to be pH-sensitive (they are disrupted in acidic and basic media), and to be capable of hosting both hydrophilic and hydrophobic compounds thanks to their amphiphilic nature. In summary, the supramolecular ionic dendrimers and codendrimers described above (which can also be considered as oligoelectrolyte−surfactant complexes) constitute an interesting class of ILCs. Unlike many covalently bound dendrimer

Figure 49. (a) Columns of hydrogen-bonded molecules in the Colhex phase of 214. Green spheres represent the COOH groups. Addition of PPI-G2 induces the transformation to a Colobl phase. (b) Schematic representation of the structure of the Colobl phase formed by PPI-G2/(214)8. Gray circles correspond to PPI-G2 domains, and white circles to acid-modified disk columns. The black parallelogram indicates the unit cell of the Colobl phase; the gray lines indicate the rectangular grid on which the PXRD analysis was based. The apparent stoichiometry of the superlattice is 1:5, but one has to realize that PPI-G2 is a flexible molecule, which extends over more than one column slice of the stacked carboxylic acid molecules. Reprinted with permission from ref 829. Copyright 2008 American Chemical Society. BU


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Scheme 14. Ammonium-Based Dendrons and Dendrimers with n-Alkylsulfate Counterions, Investigated by Mezzenga, Schlüter, and Co-workers837

chains are confined within the column centers (discrete domains) instead of forming a continuum between the different columns (see section 5.2 for more information about “normaltype” versus “inverted-type”). As noted by the authors, this “frustrated” configuration is in conflict with the intrinsic curvature imposed by the dendron/dendrimer architecture (“concave” with respect to the focal point, versus “convex” with respect to the focal point in the “normal-type” Colhex phase), but is favored by an imbalance of conformational entropy between short alkyl chains and longer dendron segments. These remarkable observations were supported by self-consistent field theory (SCFT) simulations (Figure 50). A similar phenomenon had been noticed before for columnar mesophases shown by (cationic dendronized polymer)/(anionic surfactant) supramolecular ionic complexes.839 The latter polymeric compounds were thoroughly investigated,418,419,840,841 but are outside the scope of this Review. Van de Coevering et al. reported on positively charged dendrimers in which the polycationic core is surrounded by a shell of n-dodecyl chains (225−226, Scheme 15).842 These compounds could be used as hosts for a predefined number of anionic guests such as methyl orange. The dendrimers, as well as their assemblies with methyl orange, are presumably LC, but their phase behavior was not thoroughly investigated. Effective shielding of the ionic moieties by the aliphatic periphery renders these molecules soluble in organic solvents as apolar as n-hexane.

the asymmetry imposed by the restricted mobility of the acidmodified side chain in 214 due to its electrostatic interaction with the dendrimer domains. Indeed, asymmetry inside the columns due to tilting of the molecules (which is usually the case in Colrec and Colobl phases) is not to be expected for this system, given the directional hydrogen bonds of equal length between the amide groups of stacked molecules of 214. The same group also investigated mixtures of 214 with branched PEI, linear PEI, and poly(ferrocenyl(3-aminopropyl)methylsilane), for which similar phenomena were observed.836 Mezzenga, Schlüter, and co-workers synthesized covalently bound dendrons and dendrimers with terminal ammonium groups, and combined these with n-alkylsulfate counterions to form LC materials (219-n, 220-n, 221-n, and 222-n, Scheme 14).837 [Remark: Nguyen et al. combined ionic hyperbranched PAMAM polymers with [C12H25OSO3]− counterions to form high molecular weight LCs showing Colrec and lamellar mesophases (see also section 12.3).838] Besides SAXS measurements, they also performed TEM experiments on the mesophases. TEM is a “direct space” technique, whereas SAXS is a “reciprocal space” technique. The aromatic rings in the dendron/dendrimer parts of the supramolecular ionic complexes can be selectively stained for TEM analysis. The TEM pictures showed that the Colhex phases displayed by the complexes with an alkyl chain volume fraction smaller than about 0.7 are “normal-type” Colhex phases, in the sense that the pendant alkyl BV


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162-m/n were discussed).774−776 The different behavior can be attributed to the differences between nitrogen and phosphorus, which both belong to group VA in the periodic table, but only in the case of phosphorus can the 3d orbitals participate in the bonding. Because of the larger size of the phosphorus atom, the electrostatic interactions of phosphonium cationic headgroups with anions are weaker than those of ammonium cations (in other words, phosphonium salts are characterized by a lower degree of cation−anion association; see also section 6.2). Continuing their earlier work on phosphonium-based ILs and ILCs,2,282,385,387,843,844 Weiss and co-workers synthesized a variety of quaternary phosphonium salts that resembled their previously reported compounds with one short alkyl substituent and three equivalent long n-alkyl chains attached to the positively charged phosphorus atom (227-X-n, 228-X-n, 229-n, 230-n, 231-n, and 232-X-n; Scheme 16).497,574 [Remark: Analogues of P,P,P-tris(n-alkyl)-P-methylphosphonium salts 227-X-n with an ethyl, n-propyl, n-butyl, n-pentyl, n-dodecyl, n-octadecyl, or benzyl substituent or a hydrogen atom instead of the methyl group had already been reported prior to Binnemans’ 2005 review2,282 and will not be discussed again here.] The combined results of their studies nicely show how subtle balances between entropic disorder caused by chain melting (and subsequent disruption of the ionic planes via “molecular tumbling”), the strength of the electrostatic interactions versus attractive van der Waals forces, as well as space-filling factors and steric interactions form the key to mesophase formation. The P,P,P-tris(n-alkyl)-Pmethylphosphonium compounds (= [Pn,m,x,1][X]; [Pn,m,x,y]+ = P,P,P,P-tetrakis(n-alkyl)phosphonium, with n, m, x, and y indicating the number of carbon atoms in each alkyl chain) have a remarkable stucture in the sense that they contain a high amount of alkyl chain volume relative to a rather small ionic headgroup (i.e., the phosphorus center and the anion) (see also section 4.3.1, Figure 6). At the time these represented the first LC amphiphiles containing three equivalent long alkyl chains and a

Figure 50. (a) Electron density profiles for compounds 221-n for a gradually increasing alkyl chain volume fraction x, calculated by SCFT simulations: (1) x = 0.3 (“normal-type” Colhex phase); (2) x = 0.5 (lamellar mesophase); (3) x = 0.7 (“inverted-type” Colhex phase). Dendrons/dendrimers and alkyl tails are depicted in blue and red, respectively. (b) Progressive changes in the dendron/(alkyl tails) interfacial curvature in the series 220-n, upon increasing the alkyl chain volume fraction from about 0.4 to about 0.7. Adapted with permission from ref 837 (http://dx.doi.org/10.1039/b814972k). Copyright 2009 The Royal Society of Chemistry.

7. PHOSPHONIUM-BASED IONIC LIQUID CRYSTALS Phosphonium salts resemble ammonium salts in many aspects, but their thermal behavior is much less studied, primarily because fewer precursors (trialkylphosphines) are commercially available. Nevertheless, phosphonium-based ILCs can present some advantages as compared to their ammonium counterparts, such as potentially wider mesophase ranges, higher clearing points, and a higher chemical and thermal stability.2,282,386,776 Moreover, phosphonium-based ILs show a lower viscosity and a higher ionic conductivity than the corresponding ammonium salts (see also section 6.2 where the different properties of 161-BF4-m/n and

Scheme 15. Ammonium-Based Structures Studied by Klein Gebbink, van Koten, and Co-workers842

BW


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Scheme 16. Phosphonium-Based ILCs Investigated by Weiss and Co-workers496−499,574

single-atom cationic headgroup.282 Relatively short chains (such as n-decyl chains) and small anions (e.g., F−, Cl−, and Br−) do not favor thermotropic liquid-crystallinity because the energy of the system is in that case controlled by the relatively strong P+−X− interactions of the ionic headgroups that are closely packed together. More voluminous anions with a more delocalized charge (e.g., [BF4]−, [PF6]−, [NO3]−, and [ClO4]−) tend to attenuate such electrostatic interactions so that the order within the ionic planes is reduced and a certain extent of “fluidity” is introduced into the system. Only lamellar phases (SmA2 phases; see, e.g., Figure 6) are found for these “three-chain” salts, in contrast to, for example, [C14C14piperid][C12H25OSO3] (311C12H25OSO3). However, if the anion becomes too large, as in case of the [NTf2]− salts, the LC packing is destroyed because the anions can no longer be accommodated in the layered mesophase structure. Homologues possessing longer alkyl chains tend to maintain the layered structure more effectively, so that, for example, mesomorphism is induced in the series of bromide salts for [P14,14,14,1][Br] (227-Br-14) and [P18,18,18,1][Br] (227Br-18), in contrast to [P10,10,10,1][Br] (227-Br-10), which is (in its anhydrous, pure form) not LC but forms a soft solid phase before melting. Replacement of the methyl substituent by a −CH2COOH or −CH2CONH2 group (228-X-n and 229-n) does not have a positive effect on the mesomorphic properties, with only 228-Br-14 being LC. The zwitterionic compounds 230-n and 231-n do not show thermotropic LC behavior either. Although many P,P,P-tris(n-alkyl)-P-methylphosphonium halide salts do not display thermotropic mesomorphism, they turn into lyotropic LCs upon addition of at least 1 molar equivalent of water or a monofunctional alcohol (i.e., methanol, ethanol, n-propanol, i-propanol, n-butanol, sec-butanol, tertbutanol, n-pentanol, or n-hexanol), or upon addition of at least 0.5 molar equivalent of the difunctional alcohol ethylene glycol (Figure S14).385,496−499 In other words, at least one molar equivalent of hydroxyl groups is necessary to effect complete (isothermal) conversion of the solid salts to a LC. This inspired the Weiss group to replace the methyl substituent by a hydroxyl group covalently linked to the positively charged phosphorus atom (232-X-n).497,574 [Remark: This had been done before for quaternary ammonium salts with one long alkyl chain.760] The hydroxyl groups, whether they originate from added solvent

molecules (water or alcohols) or are covalently attached, attenuate strong electrostatic interactions within the ionic headgroup region and create more fluidity. This is because the anions are moved farther away from the P+ ion as a result of hydrogen bonding between the −OH moieties and the anions (in the case of a covalently attached hydroxyl group, it can concern intra- and/or intermolecular hydrogen bonding). In addition, the oxygen atoms of the −OH groups were proposed to interact electrostatically with the P+ ions, thus “decreasing the specificity and increasing the spatial diversity of the electrostatic interactions”.497 Moreover, the effective headgroup area is increased. As a result, the long alkyl chains can adopt additional gauche bends. This so-called “molecular lubrication” induces LC properties for some phosphonium halides that are nonmesomorphic in their neat form, and enhances the mesophase temperature range and lowers the melting point for other salts that are already LC without the presence of hydroxyl groups.497 For example, neat chloride salts 227-Cl-n with n = 10, 14, or 18 are not LC, whereas SmA2 phases are found for their hydroxylsubstituted counterparts 232-Cl-n. It is also interesting to see that 232-Br-10 is LC, while 228-Br-10 and 229-10 are not, suggesting that the −COOH and −CONH2 groups are less effective than −OH in inducing a LC phase. This is probably because the specific shape and size of the hydrogen-bond donor substituents are also important as they disturb the order in the general lamellar structure. The greater freedom of motion at and near the ionic sublayers as a result of “molecular lubrication” is evident from the lower transition temperatures of 232-Br-14 and 228-Br-14 as compared to 227-Br-14. Addition of water or an alcohol to zwitterionic compounds 230-n and 231-n was not effective to induce mesomorphism. We already highlighted the power of this “lubrication” concept in section 6.1, in relation to ammonium-based ILCs. The hydroxyl-containing ammonium bromide salts 149-Br-m/1 that were reported by Wei et al., for example, form SmA phases over a broad range of temperatures,762 whereas their nonfunctionalized counterparts, the N(n-alkyl)-N,N,N-trimethylammonium bromides, decompose before melting.2 The amphotropic LC behavior of several of the previously discussed phosphonium salts in the presence of different solutes (such as acetonitrile or DMSO, and the alcohols, which allow for BX


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“lubrication” as seen above) was extensively studied using quadrupolar 2H NMR spectroscopy, besides the usual techniques of POM, DSC, and PXRD.496−499 For oriented deuterated molecules in an aligned mesophase, splitting of the 2H NMR peaks occurs because of quadrupolar interactions, whereas single sharp signals appear in the isotropic liquid state (because the quadrupolar interactions are orientationally averaged). Hence, the transition to an anisotropic LC state can be monitored via NMR spectroscopy. The quadrupolar splittings of added solutes were used to calculate the order parameters of the smectic mesophases, which could surprisingly easily be aligned in the strong magnetic fields of the NMR spectrometer. Low order parameters of about 10−2 were found, depending on the type of anion of the ionic mesogen and the concentration and nature of the added solute. As will be further explained in section 12.3, such amphotropic LCs that can be oriented and that possess low order parameters (∼10−2−10−3) can be used as partially ordered NMR solvents for structure studies of solute molecules of interest, such as small organic molecules but eventually also medium-sized biomolecules.495−499 It should be noted that Judeinstein and coworkers also monitored the local anisotropy in oriented ionic LC media (i.e., mixtures of neutral calamitic mesogens containing a crown ether moiety and/or oligo(ethylene oxide) chains with Li+ and Na+ salts, as well as pure [Cnmim][BF4] and [Cnvim][BF4] salts and their mixtures with LiBF4) via 2H, 7Li, 11B, 13C, and 19F NMR spectroscopy (see section 12.1).448,450,560 The taper-shaped phosphonium-based mesogens 162-m/n that were reported by Kato and co-workers674 have already been discussed in section 6.2. An interesting development in the field of phosphonium-based functional materials is the use of π-conjugated phospholium structures. Phospholes are the phosphorus analogues of pyrroles, thiophenes, and furans. The phosphorus center can be considered as an electronic “switch” for the conjugated moieties through appropriate functionalization, efficiently influencing photophysical properties (fluorescence quantum yield, band gap, charge transfer processes, etc.) and redox properties. Baumgartner and co-workers were the first to report on LC phospholium salts (they used the term “self-assembling phosphole-lipids”; 234-X, 235-X, 236-X, and 237).845,846 They incorporated a substituted phospholium core into several π-conjugated systems, and were able to obtain fluorescent, stimuli-responsive LC materials. A key property of these systems is their ability to undergo concentration- and temperature-dependent intramolecular conformational changes that are related to the flexible 3,4,5-tris(n-alkyloxy)benzyl group that is attached to the phosphorus center and that lends the compounds their amphiphilic character. They can switch between a “closed” form (i.e., benzyl ring close to the conjugated phosphole backbone, occurring for “low” concentrations (10−4 M) or temperatures) and an “open” form (i.e., benzyl ring further away, occurring for “high” concentrations (10−2 M) or temperatures) (Figure 51). These changes are triggered by intermolecular interactions that result from external stimuli, such as the appearance of intermolecular π−π stacking between conjugated backbones at high concentrations and concomitant “pushing away” of the benzyl substituent. The conformational changes about the phosphorus center could be monitored via variableconcentration/temperature 1H and 31P NMR spectroscopy. The two conformations were also separately found in two singlecrystal polymorphs of a model compound without long alkyloxy chains. In the condensed state, salts 234-Br, 234-BF4, and 235Br with relatively small counterions (Br− and [BF4]−) show two

Figure 51. Proposed conformational changes of the “phosphole-lipids” in solution. The conformation on the right-hand side is assumed to occur in the mesophases. Reprinted with permission from ref 845. Copyright 2011 American Chemical Society.

types of unidentified ordered smectic phases, whereas compounds 234-OTf and 234-B(C6H5)4 with larger anions ([OTf]− and [B(C6H5)4]−) only show a highly ordered crystal smectic phase. The latter was also the case for salts 236-X with an extended conjugated core (it is not entirely clear whether it does not concern a crystalline phase). As expected, a too long extension of the conjugated backbone, as for 237, is detrimental for LC properties. Interestingly, NMR experiments indicated a stronger cation−anion interaction for 234-B(C6H5)4 than for 234-Br, probably resulting from additional π−π and/or CH−π interactions. The photophysical properties of the phospholium salts are discussed in section 12.5. The lower tendency of phosphorus to undergo nucleophilic substitution as compared to nitrogen is emphasized by the fact that the trivalent phosphole center could not be quaternized with n-dodecyl bromide under similar conditions as for the tris(n-dodecyloxy)benzyl bromide.

Kato, Ohno, and co-workers prepared LC mixtures of the mesomorphic phosphonium salts 238-Li-n (A+ = Li+) and the nonmesomorphic, hydrophilic ammonium salt 239.847 Compound 239 is an IL that is miscible with water at room temperature, despite its typically hydrophobic [NTf2]− anion. Salts 238-Li-n and 239 form macroscopically homogeneous mixtures, thanks to the compatibility of the hydrophilic ion pair −SO3−/Li+ in 238-Li-n with the hydrophilic cation of 239. On the other hand, the incompatibility of the hydrophobic ion pair −P(CnH2n+1)3+/[NTf2]− in 238-Li-n (this ion pair resembles a hydrophobic low-melting IL environment) and the hydrophilic cation of 239 induces the formation of nanosegregated hydrophobic and hydrophilic domains, which leads to the formation of a LC phase. Depending on the molar fraction of 239 in the 238-Li-n/239 mixtures, a cubic or Colhex phase is formed (Figure S15). These systems can be considered as “lyotropic” BY


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zwitterionic systems incorporating two preferential ion pairs is that selective ion conduction can be achieved. Indeed, in classical nonmesomorphic ILs, the migration of component ions (e.g., under the influence of an electric field) lowers the so-called transference number of target carrier ions, such as Li+ (important for lithium-ion batteries) or H+ (important for fuel cells). Zwitterions, on the other hand, are not expected to migrate under an applied potential gradient because of the covalent tethering of positively charged and negatively charged moieties and their net neutrality.614 As mentioned above, addition of LiNTf2 or HNTf2 drastically lowers the melting point of the zwitterions, and at the same time a liquid system is formed in which selective cation transport (of either Li+ or H+) can occur under the influence of an applied voltage. Furthermore, selforganization of the system in combination with water (thanks to nanosegregation) leads to the formation of water networks consisting of flexible and successive hydrogen bonds (between the water molecules and the sulfonic acid groups). The latter structures contribute to the gel formation process and allow anisotropic ion (proton) conduction via the fast hopping or bond-exchange (Grotthuss) mechanism857,858 rather than the slower, diffusion-controlled transport (vehicle) mechanism.859 Liquid (nonmesomorphic, isotropic) imidazolium-based analogues of 238-Li-n do exhibit selective Li+ transport, but their high viscosity and the absence of pathways for fast ion conduction result in low ionic conductivity values (e.g., only about 10−6 S cm−1 at room temperature).849 In contrast, a LC sample of 238-H-6 containing 20 wt % water could be macroscopically aligned by shearing, and the ionic conductivity of the aligned sample was found to be 7.9 × 10−2 S cm−1 at 25 °C. The conductivity of water-free, nonmesomorphic 238-H-6 is about 2000 times lower. From the activation energy values for ionic conductivity, it was inferred that conduction in water-free 238-H-6 primarily takes place via the vehicle mechanism, whereas the process in 238-H-6 containing 20 wt % water is dominated by the hopping mechanism. The ionic conductivity is mainly related to proton conduction, given the quasiimmobilization of the zwitterions in their self-assembled structures. Pulsed-field-gradient NMR measurements (19F NMR) could give more information about the diffusion ability of the [NTf2]− anions.560,650,790 Interestingly, on cooling a watercontaining sample below 0 °C, no exothermic DSC signal corresponding to water crystallization was observed because of the confinement of the water molecules within the hydrogenbonding network in the center of the columns. Further progress was made when Ichikawa et al. reported on smectogenic zwitterionic pyridinium compounds (240-n/1; these can be seen as pyridinium analogues of 26-n and 27-n) that, when mixed with HNTf2, form bicontinuous cubic phases, besides columnar and smectic phases.676 [Remark: These pyridinium-based systems are discussed here and not in section 8 because of their relation to the previously discussed phosphonium systems.] Karl Fischer titration indicated that the mixtures contain 2−3 wt % water, due to their hygroscopic nature and the strong hydration ability of the sulfonic acid group (note that the water content is much lower than in the 238-H-n/ water systems, however). The advantage of the bicontinuous cubic phases shown by these systems as compared to the columnar phases formed by 238-H-n/water and the columnar and micellar cubic phases exhibited by 238-Li-n/239 mixtures is that they contain 3D continuous structural features that can function as efficient ion transportation pathways, even in a nonaligned (polydomain) sample. This concept was already

ILCs in which the IL 239 plays the role of solvent. Interestingly, pure 238-Li-6 and 238-Li-8 also form a cubic phase, even at room temperature. Recall that compounds 231-n are not LC (see above). The purely zwitterionic precursors for 238-Li-n (without Li+ and [NTf2]−) are also nonmesomorphic solids with a melting point above 130 °C. The decrease in crystallinity upon addition of LiNTf2 was attributed to the formation of the preferential ion pairs −SO3−/Li+ and −P(CnH2n+1)3+/[NTf2]−, as confirmed by IR measurements. These preferential interactions of the “soft” organic cations with the [NTf2]− anions, on the one hand, and of the sulfonate moieties with Li+, on the other hand, are in accordance with Pearson’s “hard and soft acids and bases” (HSAB) principle.848 Yoshizawa and Ohno had previously observed homogeneous mixing of imidazolium-, phosphonium-, and pyridinium-based zwitterions (with a sulfonate anionic moiety) with LiNTf2 and HNTf2, and they had found that the mixtures were room-temperature liquids, whereas the pure zwitterions were solids with melting points above 100 °C.614,849−852 [Yoshizawa et al. also found that attachment of an oligo(ethylene oxide) moiety to an imidazolium-based zwitterion can decrease its crystallinity.301] The thermal stability of the nanosegregated mesophase of the 238-Li-n/239 mixtures decreases with an increasing fraction of 239 (Figure S15). For 238-Li-6/239 mixtures, a molar fraction of 239 exceeding about 0.3 results in an isotropic liquid phase lacking long-range nanosegregation. For 238-Li-8/239 mixtures, even an equimolar mixture still forms a Colhex phase. The Colhex phase of 238-Li-8/ 239 (0.63:0.37) could be macroscopically aligned by simple shearing, and its anisotropic ionic conductivity could be measured. The transition from a micellar cubic phase toward a Colhex phase with an increasing amount of 239 can be explained by a volume fraction increase of the hydrophilic inner components of the micellar structures in the cubic phase, resulting in a decrease of the interfacial curvature between hydrophilic and hydrophobic domains. This work of the Kato and Ohno groups is in fact a new strategy to combine different ILs into a single “nanobiphasic” matrix with conservation of their original properties, avoiding the macroscopic phase separation that occurs when adding together two immiscible ILs (such as [P8,8,8,8][NTf2] and 239).853

Analogues of 238-Li-n with a proton H+ instead of Li+ (238H-n (A+ = H+)) were investigated as aqueous lyotropic LCs that can serve as anisotropic proton-conductive materials (see also section 5.1: mixtures of 13-BF4 and 14).854 In contrast to the 238-Li-n/239 mixtures, water plays the role of solvent in these systems instead of the IL 239. Depending on the water content, mixtures of 238-H-6 with water form either a nonmesomorphic liquid (0, 10, or 50 wt % water) or a mesomorphic gel showing a Colhex phase (20 wt % water) or an unidentified mesophase (30 or 40 wt % water). Remarkably, no cubic phases are formed unlike in the case of 238-Li-n(/239). Compounds 238-H-n resemble the most well-known proton-conductive organic material, the ionomer Nafion,855,856 in the sulfonic acid structural element. The reason for turning toward self-organizing BZ


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Figure 52. (a) Schematic representation of the nanostructure of the bicontinuous cubic phase formed by (240-12/1)/HNTf2 (1:1) containing 2.7 wt % water. (b) Temperature dependence of ionic conductivity for four (240-14/1)/HNTf2 (1:1) mixtures that contain different amounts of water (2.6, 6.6, 8.5, or 9.4 wt % water). Reprinted with permission from ref 676. Copyright 2012 American Chemical Society.

highlighted in sections 5.2,221 5.3,63 and 6.2.672−674 The difference with the previously discussed systems, however, is that the small amount of water present in (240-12/1)/HNTf2 (1:1) and (240-14/1)/HNTf2 (1:1) occupies the so-called “gyroid minimal surface” (infinite periodic minimal surface) in the bicontinuous cubic phase of Ia3̅d. This means that the water is located in the middle of the 3D periodic continuous hydrophilic sheath domain that surrounds the hydrophobic channel networks composed of the alkyl chains (Figure 52a). The phase can thus be considered as a “normal-type” CubV1 phase, because the 3D interwoven channel networks are formed by the hydrophobic alkyl chains (see Figure S1). This is because each molecule of 240-12/1 contains only a single alkyl chain, so that the aliphatic volume fraction is not dominant, particularly after addition of HNTf2; for the systems with a longer alkyl chain length ((240-16/1)/HNTf2 (1:1) and (240-18/1)/HNTf2 (1:1)), the cubic phase is destabilized and gradually replaced by a SmA phase. The ionophilic gyroid minimal surface formed by the extremely thin, hydrogen-bonded water nanosheet (only about 5 Å thick) is a macroscopically continuous feature that allows for “alignment-free” proton conduction. The hydrogenbonded water network is surrounded by a hydrophobic IL-like layer formed by the pyridinium−[NTf2]− ion pairs (preferential ion pairs, see above) (Figure 52a). The ionic conductivity of the system (240-14/1)/HNTf2 (1:1) was found to increase (and the activation energy to decrease) with increasing water content (Figure 52b). Just as for 238-H-6, ion conduction under waterpoor conditions probably occurs via the vehicle mechanism, whereas the hopping mechanism becomes possible as soon as a successive hydrogen-bonded water network is established on the gyroid minimal surface. The water content of (240-14/1)/ HNTf2 (1:1) should not exceed 10 wt %, because then a columnar mesophase is formed instead of a cubic phase a a result of the increased volume fraction of the hydrophilic moieties. In a follow-up paper, compounds 240-n/2 and mixtures of 240-n/m (m = 1, 2) with lithium salts (LiCl, LiBF4, LiOTf, and LiNTf2) and with HNTf2 were investigated.860 The formation of pyridinium−[NTf2]− ion pairs was confirmed via Raman spectroscopy.

More recently, an anhydrous proton-conducting system was reported, which is based on mixtures of a LC zwitterionic analogue of 161-X-2/12 (241) with relatively nonvolatile benzenesulfonic acid (PhSO3H) as a source of mobile protons.773 [Remark: This ammonium-based system is discussed here and not in section 6 because of its relevance in the context of ILC proton conductors.] In its pure form, the taper-shaped sulfobetaine 241 shows a Colhex phase from room temperature to 200 °C, but when mixed with the sulfonic acid PhSO3H an intermediate bicontinuous cubic phase is formed, with 3D interconnected nanochannels for ion conduction. No such phase is formed by mixtures with phenylphosphonic acid or CF3SO3H. Interestingly, pure benzenesulfonic acid is hygroscopic, but the ionic 241/PhSO3H complexes are not. Thanks to the absence of water, the ionic conductivity does not drop above 100 °C, making this system potentially useful as proton conductor component in a medium- to high-temperature proton exchange membrane fuel cell. Standard Nafion membranes show very high proton conductivities when hydrated (i.e., when mobile water molecules are present in hydrophilic nanochannels), but ∼90 °C they will dehydrate and gradually lose proton conductivity.856 A maximum ionic conductivity of about 10−4 S cm−1 (mainly attributable to proton conduction) was achieved with 241/ PhSO3H (70:30) at 130 °C. Other LC anhydrous proton conductors, such as 4-(n-octadecyloxy)phenylsulfonic acid (Cr · 63 · SmA · 83 · I (°C)) and a sulfonated side-chain LC polymer (g · 145 · SmA · 215 · I (°C)), were proposed previously.861,862 Chen et al. described anhydrous proton transport by facially amphiphilic polymers that show supramolecular assemblies in which the proton-conducting functionalities (either benzotriazole or imidazole groups) are organized in nanoscale ionconducting channels.863 Zheng and co-workers investigated the proton-conducting properties of lyotropic LCs formed by (i) mixtures of water with 26-n (n = 12, 14, 16) and PhSO3H, CF3SO3H, or CH3SO3H;864 and (ii) mixtures of water with a vinyl-substituted analogue of 26-n and (C12H25)PhSO3H (the latter LC structures were also fixed through photopolymerization).865 Mixtures of an imidazolium-based analogue of 241 (458) with LiNTf2 and propylene carbonate were investigated as anhydrous lithium-ion-conducting materials (see section 12.1).616 CA


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alkylation of the nitrogen atom or protonation with hydrogen chloride. Ionic interactions are thus a prerequisite for mesophase formation, but the extent of their influence depends on the type of cation and anion. For example, the transition temperatures in the series of bromide salts 245-Br-m/n (m = n) are not dependent on the chain length, indicating that Coulombic interactions as well as dipolar interactions and hydrogen bonding are dominating the system. The same conclusion can be drawn for salts 245-Cl-0/n, all forming SmA phases. For the compounds with the more voluminous [BF4]− and [PF6]− anions, hydrophobic interactions become more important, and melting and clearing temperatures increase with elongation of the alkyl chain. Also, for the salts with the shortest chain length (245-BF4-10/10 and 245-PF6-10/10), no mesophase was observed. Introduction of a hydroxyl function in the 4-position surprisingly destabilizes the liquid-crystallinity; no mesophase could be observed for the n-dodecyl and n-tetradecyl homologues. Salts 245-Br-m/n (m = n) form SmA phases with noninterdigitated monolayers, whereas for all of the other LC compounds partially interdigitated bilayers were proposed.

8. PYRIDINIUM-BASED IONIC LIQUID CRYSTALS 8.1. Pyridinium-Based Mesogens Having a Predominantly Amphiphilic Character

Pyridinium-based ILCs were discovered as early as 1938 by Knight and Shaw.1 Although the type of mesophase formed by N-(n-alkyl)pyridinium chlorides (= [Cnpyr][Cl]; [Cnpyr]+ = N(n-alkyl)pyridinium, with n indicating the number of carbon atoms in the alkyl chain) was never thoroughly investigated, a SmA phase was found for [C16pyr][Cl],93 and it was assumed that the homologues with other alkyl chain lengths also exhibit such an orthogonal lamellar phase. Surprisingly, not only a SmA phase but also a cubic phase and a monotropic columnar phase were observed for the n-dodecyl derivative, [C12pyr][Cl], by Ujiie and Mori.866 The columnar phase was also observed by Mihelj and Tomašić.867 Replacement of the chloride anion by a polymeric poly(acrylate) counteranion results in the disappearance of the SmA phase and the formation of enantiotropic Colhex and cubic phases.866 The polymeric counteranion enhances the thermal stability of the cubic phase with respect to low molar mass [C12pyr][Cl]. The thermal phase behavior of [C12pyr]+ salts with [(C12H25)PhSO3]− (242), cholate, and picrate anions has also been reported a few years ago.867 Compound 242 shows an ordered smectic phase at room temperature, which transforms into a SmA phase upon heating.

Following earlier work on amide-functionalized imidazolium ILCs (see also section 5.1),315 Su and Lee reported on LC nicotinamide-based salts with tetrahedral metal-containing anions (246-M-n).869 In the crystalline solid state, the cation of 246-Cu-12 has an L-shaped structure because the first two carbon atoms in the dodecyl chain adopt a gauche conformation. Each cation interacts with four different [CuCl4]2− anions, via two classical N−H···Cl hydrogen bonds involving the amide group, two weak nonclassical Caromatic−H···Cl hydrogen bonds, one weak nonclassical Cα−H···Cl hydrogen bond, and an anion−π interaction (bond distance of 3.379 Å). Remarkably, the oxygen atom of the amide group is not involved in N−H···OC hydrogen bonding; it only forms weak nonclassical Cγ−H···O and CCH3−H···O hydrogen bonds. The authors proposed that in the higher-temperature mesophase, however, the packing is dominated by strong N−H···Cl and N−H···OC hydrogen bonds at the expense of the nonclassical C−H···Cl interactions. The mesophase type was assigned as SmC, but the shown POM texture is consistent with a SmA phase.

In 2004, Laschat and co-workers synthesized a chiral pyridinium bromide salt (243) by reacting (R)-citronellyl bromide with pyridine. This compound is a room-temperature IL.620 They succeeded in obtaining mesomorphic chiral pyridinium salts by extending the chiral substituent (244-X-n), in exactly the same way as for imidazolium salts 31-X-n (see section 5.1).549 Compounds 244-X-n display enantiotropic SmA phases over a broad temperature range, but the mesophase range is smaller than for the imidazolium counterparts. Just as for the latter compounds, the stereogenic center merely acts as a “stopper” for alkyl chain interdigitation, and no superstructural chirality was induced.

Ren and co-workers also synthesized a pyridinium salt with a metal-containing anion. Instead of the typical combination of the organic cation with a tetrahalogenometalate anion ([MX4]2−), they exchanged the bromide anion of 4-amino-N-(n-hexyl)pyridinium bromide with bis(maleonitriledithiolato)nickelate(III) ([Ni(mnt)2]−) (247).870 This work was inspired by some of the very first examples of metallomesogens, the bis(1,2dithiolato) complexes of nickel and platinum reported by Giroud-Godquin and Mueller-Westerhoff.871,872 Because of the square-planar coordination of the [mnt]2− ligands to the

Instead of using a long N-alkyl chain, mesomorphic pyridinium salts can also be obtained by the attachment of a long alkyl substituent in the 4-position of the pyridinium ring. Lin and coworkers synthesized three types of pyridinium salts: N-(n-alkyl)4-(n-alkyloxy)pyridinium compounds 245-X-m/n (m = n), N(n-alkyl)-4-hydroxypyridinium chlorides 245-Cl-m/0 (X− = Cl−, n = 0) and 4-(n-alkyloxy)pyridinium hydrochlorides 245-Cl-0/n (X− = Cl−, m = 0).868 Whereas the neutral precursors are not LC, mesomorphism is induced in the ionic compounds after either CB


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nickel(III) cation, [Ni(mnt)2]− is a planar anion with an anisometric, lath-like shape, which makes it suitable for incorporation in LC systems. The authors suggested that this might be the reason for the observed mesomorphism, despite the short n-hexyl substituent on the pyridinium cation.

previously described the LC behavior of the neutral bromosubstituted precursors, which show SmI/SmF and SmC phases.879 All pyridinium salts suffered from thermal decomposition above 250 °C, whereas the neutral terphenylene bromide precursors proved to be stable up to 316 °C. A rich smectic polymorphism was observed for the ionic compounds, including tilted ordered smectic phases and SmC phases. It was found that π−π-type interactions play an important role, which is not unexpected for systems containing extended aromatic cores. The occurrence of such strong π−π stacking in addition to the electrostatic interactions resulted in complete segregation of the different molecular components into ionic, aliphatic, and aromatic sublayers. Polymerized 4-vinylpyridinium analogues of 250-m/ n are also LC.373

The pyridinium-based zwitterions 240-n/m that were developed by Kato, Ohno, and co-workers676,860 have already been discussed in section 7. 8.2. Taper-Shaped Pyridinium-Based Mesogens

Beginn and co-workers reported on amphiphilic taper-shaped mesogens with a sulfonate group at the apex of the anionic moiety and with a protonated pyridinium counterion (180 and 181-n; see section 6.2).804−807 Both lamellar and columnar phases were found. Cı̂rcu and co-workers studied taper-shaped pyridinium salts 248-X.873 Remarkably, only the bromide salt shows columnar mesomorphism.

8.4. Mesogens in Which the Pyridinium Cation Makes Part of the Rigid Core

In continuation of the work of Bazuin and co-workers,880 the thermal behavior of four series of pyridinium-based compounds with variations in chain length, substituents, substitution patern, and anion was studied by Ster et al. (251-X-n, 252-X-n, 253-X-n, and 254-X-n-R).881 All LC compounds showed SmA phases and/or an unidentified mesophase M on the basis of their optical textures and confirmed by X-ray diffraction measurements on aligned samples. For the N-(n-alkyl)pyridinium derivatives 251X-n, 252-X-n, and 253-X-n, the clearing temperature, and thus the mesophase stability, increases with increasing alkyl chain length, whereas the melting temperature remains more or less constant. Such a trend was not observed for the N-(nalkyl)stilbazolium halides 254-X-n-R, although it must be noted that these compounds are partially decomposing at the clearing point so that no unambiguous conclusion can be drawn. Enlargement of the aromatic system via a CC double bond results in an increase of the transition temperatures. It was found that the type of substituent in the 4′-position of the stilbazolium moiety plays an important role in mesophase formation; the mesophase stability generally decreases in the order −OCH3 > −OH > −H. Absorption spectroscopy measurements on pyridinium salts (251-X-n)-(253-X-n) showed that the Lambert−Beer law is not followed for iodide salts due to ion pairing between the iodide anion as the electron donor and the pyridinium ring as electron acceptor. For the more electrophilic bromide anion, a a linear correlation was found in the Lambert− Beer plot.

8.3. Attachment of Mesogenic Groups

The LC behavior of triphenylene-based pyridinium salts with variations in chain length, spacer length, and the type of anion was previously investigated by Kumar and Pal.874 These compounds show an unidentified columnar phase, similarly to their imidazolium analogues (however, for one of the imidazolium salts, the mesophase was characterized as a Colrec phase; see section 5.3). In a later report, Langmuir−Blodgett (LB) films of compound 249 were obtained.708 These monolayers show a phase transition from the expanded phase, where the molecules are ordered face-on, to the condensed phase, in which the molecules are arranged edge-on. In general, pyridinium-based LB films proved to be more stable than the imidazolium-based ones. Suresh and Nayak also investigated films of complexes of 249 with negatively charged doublestranded DNA.875−878

Santos-Martell et al. prepared mesomorphic pyridinium bromides consisting of an ionic headgroup, a spacer, an aromatic terphenylene unit, and an aliphatic part (250-m/n).372 They had CC


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To gain further insight into the influence of the shape of the rigid cationic core on the mesophase behavior of ILCs, the Binnemans group investigated in detail homologous series of both N-(n-alkyl)quinolinium (255-X-n, with n up to 22 (ndocosyl chain)) and N-(n-alkyl)isoquinolinium salts (256-X-n, with n up to 22).882 For identical anions and alkyl chain lengths, both series show SmA phases over a similar temperature range. The thermal behavior of these ILCs is atypical in the sense that these compounds only exhibit liquid-crystallinity when their alkyl chain is very long. Exchange of the halide anion resulted in lower clearing temperatures and smaller mesophase temperature ranges in the order Br− > [BF4]− > [PF6]− (Figure S16(a), (b)). For the dodecylsulfate salts, in which both the cation and the anion bear a long alkyl chain, SmA phases were already observed for shorter alkyl chain substituents on the cations (Figure S16(c)). This was to be expected, because in these systems the anion is contributing to the amphiphilicity of the molecules as well. However, a further increase in the aliphatic content of the counterion, as in the case of 255-DOSS-4 (X− = [DOSS]−, see Scheme 2), leads to the loss of LC properties. Shortening of the aliphatic chain from n-butyl to methyl restores mesomorphism for the [DOSS]− salts 255-DOSS-1 and 256DOSS-1 (both LC at room temperature) and the [DHSS]− salt 255-DHSS-1 (monotropic SmA phase). The dicyclohexyl sulfosuccinate salts 255-DcHSS-1 and 256-DcHSS-1 are not mesomorphic, emphasizing the importance of the structure of the counterion.

out over the pyridinium and 1,2,4-oxadiazole rings, that is, 4substituted pyridinium salts 261-X-Z1/Z2, do not show LC behavior, presumably due to weaker electrostatic interactions. For 3-substituted pyridinium iodides 260-I-Z1/Z2, a SmA phase was observed, despite the decrease in structural anisotropy by the introduction of a methyl group in that position. Strong electrostatic interactions between a more localized charge in the pyridinium ring and the anion seem to be required for mesophase formation. This is also indicated by the fact that the [OTf]− salts, where the negative charge is more delocalized, do not form LC phases. PXRD showed that a bilayer is formed in the mesophase, in which the bulky fluorinated chains are packed endto-end. The salts are luminescent in solution. The iodide derivatives also show reversible thermochromism thanks to formation/destruction of a charge-transfer complex between the electron-accepting pyridinium ring and the electron-donating halide anion.

Peculiar chiral ILCs with a bent shape, based on pyridinium and with a 1,3-dioxane ring in their central core, were described by Baudoux et al. (257, 258-X-R, and 259-n).369 Chiral ILCs are of interest as they can be used as stereospecific, ordered reaction media or asymmetric catalysts. The compounds show, apart from smectic phases (including SmC phases), also nematic and chiral nematic phases. The salts containing a bromo substituent (258X-Br) or phenyl substituent (258-X-Ph) are ILs at room temperature, just like the salts incorporating voluminous anions like [PF6]− or [NTf2]−. Transition temperatures were found to be similar for the racemic and enantiopure compounds.

As a side note we would like to mention the work of Yelamaggad et al., which is slightly related to the oxadiazolecontaining ILCs that were discussed above and in section 5.3. They synthesized polar LCs based on a mesoionic sydnone core (262-(a-m) and 263-n).473,883 [Remark: “Mesoionic” or mesomeric-ionic compounds are dipolar heterocyclic compounds in which both the positive and the negative charges are delocalized. No completely uncharged mesomeric structure exists for such compounds. Only one canonical form is shown here.] A monotropic nematic phase could be observed for 262-b and 262-d. Compounds 262-(d−i) were found to pack in an antiparallel way in a partial bilayer SmAd phase to account for the minimization of dipolar energy, in a manner reminiscent of the packing of 4′-(n-alkyl)-4-cyanobiphenyls. Although the authors envisioned a high dielectric anisotropy Δε, a moderate Δε value of only 5.4 was found for 262-d. The work of Kozhevnikov et al. on LC zwitterionic 1,2,4triazine-4-oxides (264-n, 265-n/m, and 266) is also related to the field of pyridinium-based ILCs in which the charged moiety makes part of the rigid core.472 These compounds show a rich polymorphism, exhibiting SmC, chiral SmC*, SmA, and nematic phases. Extension of the aromatic system resulted in stabilization of the SmC phase. A nematic phase was observed if the aliphatic part of the molecule did not become too large (265-4/4, 265-4/ 8, 265-8/2, and 265-8/4) because longer chains aggregate more

Lo Celso et al. reported on novel pyridinium salts synthesized from perfluoroalkylated 1,2,4-oxadiazolylpyridines (260-X-Z1/ Z2 and 261-X-Z1/Z2).535 Recall that the 1,3,4-oxadiazolylpyridinium salts studied by Gallardo and co-workers and by Pedro et al. (102-X-n/m, 103-X-n, 104, and 105)536,715 have already been discussed in section 5.3. Lo Celso et al. found that localization of the positive charge plays a major role in mesophase formation, besides the choice of the anion. Salts in which the charge is spread CD


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of the three arms of the tripodal molecules due to sterics and electrostatic repulsion between the different cationic cores, these molecules adopt a disk-like (or bowl-like) shape and tend to assemble in columnar phases (Figure 53). For 268-Br-Y1-a,

Figure 53. Schematic representation of the formation of columnar phases by C3-symmetric tripodal ILCs. Reprinted with permission from ref 886. Copyright 2012 American Chemical Society.

easily giving rise to smectic layers. Compounds 264-n display higher transition temperatures and narrower mesophase ranges than their neutral 1,2,4-triazine counterparts, while very similar transition temperatures were found for 265-n/m and their 1,2,4triazine analogues. Neutral 5-cyano-1,2,4-triazines could readily be synthesized from zwitterions 265-n/m. For these compounds, the smectic and even nematic phases were destabilized due to the lateral cyano group.

containing the smallest anion within the series, a micellar cubic phase was also found. As the anion size decreases, the pyridinium moieties are forced to pack more closely so that electrostatic interactions are maximized. Through variation of the cationic moieties (pyridinium vs pyrimidinium vs quinolinium) and of the substitution pattern of the outer phenyl rings (different numbers of n-dodecyloxy chains or, more electron-donating, bis(n-dodecyl)amine moieties), both the phase behavior and the spectroscopic properties could be modulated. This resulted in a series of luminescent ILCs covering the visible region from bluegreen to red (see also section 12.5). The photoluminescence arises from intramolecular charge transfer in the donor−acceptor (push−pull) π-conjugated chromophores (where the cations act as electron-accepting units), and could be observed both in solution and in the solid and LC states. The attachment of three instead of two alkyloxy chains consistently resulted in a melting point depression to or below room temperature, as well as in a slight stabilization of the mesophase. Interestingly, the cationic core structure, the anion type, and the peripheral substitution pattern all appeared to have an influence on the type of columnar phase that is formed, that is, Colhex or Colrec.

Other fluorinated pyridinium-based ionic mesogens besides 260-X-Z1/Z2 and 261-X-Z1/Z2 were presented by Tao et al. (267-n/m, with m = 0 or 1; see also 130-X-n/m in section 5.4).884 No attempt was made to compare the thermal properties of these salts that contain a terminal fluorinated pyrrolidine ring with those of analogous salts without fluorine atoms. Compounds 267-n/1 are not LC because the seven-membered 3,3,4,4,5,5,6,6-octafluoroazepane ring has a twisted L-shaped conformation that is sterically too demanding in this case (in contrast to the LC salts 130-Br-n/3).

Photoluminescent C3-symmetric tripodal ILCs based on pyridinium cations but also on pyrimidinium and quinolinium fragments (268-X-Yn-(a−c) (see the Supporting Information for an explanation of structure codes)) were investigated by Kato and co-workers.885,886 Thanks to the limited rotational flexibility

LC behavior could be induced in nonmesomorphic β-diketone pyridinium chloride salts 270-n by the preparation of their tetrachlorozincate salts (269-n).887 Chloride precursors 270-n CE


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makes these clusters among the least nucleophilic anions available. Such systems in which the cation and anion are efficiently dissociated are of interest for use as electrolytes in batteries. Other ILCs with a boron-cluster-containing anion (3416, 34-18, 35-16, and 35-18) have been discussed in section 5.1.362 The term “anion-driven mesogenicity” refers to the idea that the rigid, anisometric anion functions as a mesogen, in contrast to traditional imidazolium, ammonium, or pyridinium halide ILCs in which the anion is merely present for charge compensation. In section 10.9, other examples of this concept will be discussed; those structures contain anisometric receptor− anion complexes as the anionic moieties. Synthetic methods have been developed to connect the boron clusters on two sides to alkyl chains and ring systems, and this allows one to construct rod-like anionic structures with an alkyl chain on both termini, which are difficult to obtain otherwise. These anions also have a much higher degree of rigidity than, for example, simple alkylsulfate, alkylsulfonate, and alkanoate anions that have often been used to induce mesomorphism in combination with cations lacking one or more long alkyl chains (see Scheme 2). The precursors for the mesomorphic pyridinium salts 273-R1, 273-R2, 273-R4, 273-R15, 274-R1, 274-R2, 274-R3, 274-R4, 274R14, and 274-R15 with a tetramethylammonium cation ([N(CH3)4]+) or triethylammonium cation ([NH(C2H5)3]+) (rather than the pyridinium cation) are not mesomorphic, indicating that the LC behavior is still heavily dependent on the cation type as well. The analogue of 274-R13 with a cesium cation is not LC either. LC compounds could however be obtained by the combination of an N-(n-hexadecyl)-N,N,N-trimethylammonium cation and a sufficiently anisometric anion (277-R13 and 278-R13). It should be noted that 4-(n-heptyloxy)-N-(nbutyl)pyridinium bromide is a nonmesomorphic solid that melts at 34 °C. It appears that good mesomorphic properties are only obtained for compounds that contain at least three rigid moieties (ring structures + the boron cluster) in their mesogenic core in addition to the cationic pyridinium ring structure. The [closo-1-CB9H8]−-containing salts carrying an ester group (274R) generally have higher clearing points than their counterparts with an azo linkage (273-R). The salts comprising a 1,12disubstituted [closo-1-CB11H10]− cluster (series 275-R) generally show higher melting points and narrower SmA temperature ranges than their counterparts with a 1,10-disubstituted [closo-1CB9H8]− cluster (series 274-R), but they are easier to prepare. Compounds 274-R8 and 275-R8, on the one have, and 274-R9 and 275-R9 on the other hand, display a remarkably different phase behavior, despite the small variation in the structure of the mesogenic anion. The former two salts are not LC at all, whereas the latter two compounds exhibit a SmA phase and, interestingly, an enantiotropic nematic phase (albeit within a rather narrow temperature range). In contrast to the trend observed for the SmA phases, the nematic phase of 275-R9 is slightly more stable than that of 274-R9. The authors also investigated binary mixtures of 274-R8 or 275-R8 with 275-R9 and observed ideal miscibility. Crystallization of the components was significantly suppressed in the mixtures. Salt 274-R11 showed complete miscibility with a classical neutral smectogen, but only partial miscibility with a neutral mesogen that contains a substituted {closo-1,12-C2B10H10} core. A further structural parameter that was varied is the connecting group between the boron cluster and its terminal alkyl chain.426 Surprisingly, the replacement of the methylene connecting group of the n-hexyl chain of 274-R9 by an oxygen atom has a dramatic effect on the phase behavior: salt 279-R9 is not LC. This is in sharp contrast to the neutral

show only crystal-to-crystal transitions, but metallomesogens 269-n form crystal smectic and SmA phases over a broad temperature range. It appears that the packing efficiency of the tetrachlorozincate salts is reduced so that solid phases are destabilized as compared to their chloride precursors. The metal salts are unusual in the sense that the β-diketone moiety is typically deprotonated to act as a ligand for metal coordination. The type of crystal smectic phase shown by 269-n was not elucidated, but the authors proposed a structural model where the molecules are tilted with respect to the smectic planes (Figure 54). Single-crystal X-ray diffraction measurements on the

Figure 54. Schematic representation of the proposed molecular arrangement in the smectic phases of 269-16. Reprinted with permission from ref 887 (http://dx.doi.org/10.1016/j.inoche.2008.12. 016). Copyright 2009 Elsevier.

chloride and [ZnCl4]− salts proved that hydrogen bonding, improved by crystal water molecules, plays a prominent role in the lamellar organization of the solid phase. In two follow-up reports, Mayoral et al. investigated the phase behavior and photophysical properties of the metal-free β-diketone pyridine and pyridinium ligands, and their allyl-palladium(II) complexes (271-n and 272-n).376,377 In solution there exists an equilibrium between 271-n and 272-n. The different coordination modes in the crystalline state were established by IR and 13C−15N solidstate NMR spectroscopy. The ligands and complexes 271-n and 272-n could be used as fluorescent chemosensors for Zn2+ and Cu2+. It was shown for 272-14 that the luminescence is not quenched in the LC state.

8.5. Pyridinium-Containing Ionic Liquid Crystals Whose Mesomorphism Is Anion-Driven

Kaszynski and co-workers coined the term “anion-driven mesogenicity” for ILCs based on anisometric, weakly coordinating 1,10-disubstituted [closo-1-CB9H8]− and 1,12-disubstituted [closo-1-CB11H10]− clusters (273-R, 274-R, 275-R, 276-R-n, 277-R, 278-R, 279-R, 280-R, and 281-R-n; Scheme 17).352,439,440,888−890 The negative charge of closo-monocarbaborate anions is fully delocalized over their cage structure, which CF


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Scheme 17. ILCs Based on 1,10-Disubstituted [closo-1-CB9H8]− and 1,12-Disubstituted [closo-1-CB11H10]− Clusters, Investigated by Kaszynski and Co-workers352,426,439,440,888−890

a substantial negative charge, and this is thought to cause a less favorable alignment of the anion and the pyridinium cation. These results nicely illustrate the importance of charge distribution in relation to the phase behavior of ionic mesogens. The study of binary mixtures of 275-R9 with 274-R9 and 279-R9, respectively, confirmed the destabilization of the nematic and particularly the smectic phase by 279-R9. Removal of the ester

analogues of 274-R9 and 279-R9 in which the boron cluster is replaced by a phenyl ring: in that case, the alkyloxy-substituted derivative shows a more stable nematic phase than its alkylsubstituted counterpart. DFT modeling suggested that this is due to the difference in charge distribution between 274-R9 and 279R9 rather than conformational differences. The connecting oxygen atom in 279-R9 has an unusually high basicity and carries CG


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Scheme 18. Zwitterions Based on Boron Clusters, Investigated by Kaszynski and Co-workers474−476,889,891,a

a

More examples can be found in other references.477−479,892,893

from slightly negative (Δε = −0.59) to a positive value, depending on the concentration (Δε ≈ 0.57 for 1 mol %).891 Nematogenic zwitterionic LCs were obtained by linking a ring system to the [closo-1-CB9H8]− cluster via an ester linkage (284R and 285-n-R).474,475 The 4-pentylbicyclo[2.2.2]octane group in particular appeared to be very successful in generating mesogens with a relatively high clearing point and broad nematic phase range. Comparison of the longitudinal molecular electric dipole moments of 284-R (μ|| = 13.2−20.2 D) and their nonionic counterparts (in which the pyridinium ring is replaced by a phenyl ring and which contain the neutral {closo-1,10-C2B8H8} cluster; μ|| = 1.3−8.1 D) proves that the dipole moment is effectively enhanced by the introduction of two opposite charges. For 284-R5, an extremely high dielectric anisotropy (Δε = 113, with μ|| = 20.2 D) was measured; for its nonionic analogue, a Δε value of only 15 (with μ|| = 8.1 D) was found.474 For the sulfonium salts 285-n-R, a quick interconversion between the cis and trans isomer was observed. The presence of such fluxional behavior enhances the solubility in a nematic host, but lowers order parameters and destabilizes the nematic phase. More recently, a broad range of other zwitterions based on boron clusters were reported, and their use as high Δε additives to nematic hosts or as LC nonlinear optical chromophores was evaluated.477−479,892,893

linkage between the boron cluster and the ring system (R group) leads to a slight destabilization of the mesophases for the series 276-R-6 as compared to 275-R. UV−vis absorption spectra of 274-R3 and 273-R15 in acetonitrile are the sum of the spectra of the cationic and anionic parts, and reveal full transparency above 280 nm in the case of 274-R3. There were no indications for charge transfer between the boron cluster in 274-R3 and the pyridinium cation. No charge separation and transfer was achieved with thin films of 273-R15 and 274-R15 inside electrooptical cells. Mesogens based on boron cages are unique systems in the sense that the charge of the boron cluster can be changed by varying the number of carbon atoms in the cage. The Kaszynski group could thus elegantly show the influence of electrostatic interactions on the thermal behavior of LCs, by comparing ILCs 280-R2 and 281-R2-11 with isosteric and isoelectronic, but nonionic, systems.352 The latter were prepared by mixing two neutral rod-like molecules, one of which with a neutral carborane fragment ({closo-1,10-C2B8H8} or {closo-1,12-C2B10H10}, respectively) rather than a monocarbaborate cluster ([closo-1CB9H8]− or [closo-1-CB11H10]−, respectively) (see also section 4.3.1, Figure 5). Salts 280-R2 and 281-R2-11 both show lamellar E and SmA phases with a clearing point that exceeds 200 °C. The neutral mixtures have a much lower melting point and show a nematic mesophase, but this phase is only monotropic and the clearing points are 181 °C lower than those of the ionic systems. DFT calculations showed that this difference corresponds to a difference in association enthalpy of 7 kcal mol−1 in appropriate dielectric media. The Coulombic interactions in the ion pairs cause a tighter molecular arrangement as compared to the nonionic binary mixtures. In search for systems with high dielectric anisotropy Δε, zwitterionic derivatives of closo-monocarbaborates were prepared by the attachment of a pyridinium, quinuclidinium, or sulfonium heterocycle to the [closo-1-CB9H8]− cluster (282, 283, 284-R, and 285-n-R; Scheme 18).474−476,889,891 High dielectric anisotropy, and thus high dipole moments, are desirable for lowering the threshold “switching” voltage for the reorientation of LC molecules in an external electric field (for instance, in liquid crystal displays (LCDs)). The compounds without an ester function (282 and 283) do not show mesomorphic behavior, but they are soluble in a nematic host. Upon mixing, the dielectric anisotropy of the investigated nematic host altered

8.6. Dendrimeric Pyridinium-Based Ionic Liquid Crystals

Pyridinium-containing dendritic building blocks that selfassemble into supramolecular columns or spheres were reported by Percec and co-workers (138-(145), 139-(145), and 140(145); see section 5.5).517 The supramolecular cylinders and spheres self-organize into a 2D Colhex phase and a 3D cubic (CubI(Pm3̅n)) mesophase, respectively.

9. 4,4′-BIPYRIDINIUM-BASED IONIC LIQUID CRYSTALS Viologens or 4,4′-bipyridinium salts have received a great deal of interest as redox-active, electrochromic, photochromic, and thermochromic materials.894−896 They can be reduced in two steps, and this process is reversible (Figure 55). In the period before 2005, their LC behavior had been comparatively less explored.2 Mesomorphic viologen salts were found to be semiconducting in the LC state.897−899 Bhowmik and co-workers investigated the mesophase behavior of symmetric viologens with [NTf2]− CH


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Interestingly, the temperature at which the SmX-to-SmA transition takes place is hardly affected by the length of the outer alkyl substituents. It is probably dependent on the length of the linker between the dications, but this effect has not yet been investigated. Derivatives with short outer substituents (ethyl groups, i.e., n = 2) and various linker lengths are not mesomorphic.903

Protonation of 4,4′-bipyridine using a benzenesulfonic acid derivative with a branched alkyl chain resulted in a monotropic mesophase for the 4,4′-bipyridinium salt 289-Y1.904 The type of mesophase was not identified, but a Schlieren texture was observed. None of the 2,2′-bipyridinium salts (289-Y2 and 289Y3) are thermotropic LCs, but lyotropic mesophases are formed in toluene, methanol, etc.

Figure 55. Structural changes in viologen salts by electrochemical reduction.

counterions (286-NTf2-m/n with m = n).899,900 These [NTf2]− salts are thermally much more stable than their halide counterparts, and show smectic phases at low temperatures. They present some of the most successful examples of halide anion exchange with [NTf2]− to lower the melting point, without the loss of mesomorphism. As discussed in section 5, the [Cnmim][NTf2] salts have melting points at or near room temperature, but are not LC, except for n = 22 (Table S2).280,575 The mesophases shown by 286-NTf2-m/n for m = n = 4−8 were originally identified as SmA phases, but more recent studies suggest that these are actually highly ordered lamellar phases with substantial intralayer ordering (denoted as SmX).588,901

Asaftei et al. also synthesized viologen-based ILCs in which the 4,4′-bipyridinium units do not carry a long alkyl chain (290 and 291-Yn).905 Mesomorphism was induced by combination of the cations with 3,4,5-tris(n-dodecyloxy)benzenesulfonate ([(C12H25O)3PhSO3]−) anions. The ionic complexes were obtained under phase-transfer conditions by mixing equimolar solutions of the viologen halide in water and of Cs[(C12H25O)3PhSO3] in chloroform at room temperature. A 1:1 molar ratio of the reagents yielded stoichiometric complexes 290-[(C12H25O)3PhSO3]2, 291-Y1-[(C12H25O)3PhSO3]3, and 291-Y2-[(C12H25O)3PhSO3]6 after separation and evaporation of the organic phase, whereas, for example, a 1:3 ratio of 291-Y1Br3 and Cs([(C12H25O)3PhSO3] resulted in a 1:5.2 complex (Figure S17). The very small difference in transition temperatures between 291-Y1 and 291-Y2 is remarkable, because a higher number of charges (for a similar molecular shape) generally leads to a substantially higher melting point. The much higher alkyl chain content of the latter compound probably explains the observed phase behavior. The exact mesophase types were not determined, but the redox activity of the viologen salts was investigated (see section 12.4). Earlier on, similar redox-active ILCs based on 4,4′bipyridinium (292-n) had already been investigated by Kato and co-workers.906 They quaternized 4,4′-bipyridine on both sides with tris(n-alkyloxy)-substituted benzyl bromide to obtain mesomorphic salts 292-n, which show a Colhex or Colrec phase depending on the chain length. Probably two to three molecules aggregate into a disk-shaped system. Elongation of the alkyl chains results in a more elliptical assembly, promoting Colrec phases.

Causin and Saielli studied asymmetric variants of the compounds reported by Bhowmik et al. (286-NTf2-m/n with m ≠ n), to investigate the effect of increasing the system’s entropy on the LC behavior.588 Strongly asymmetric salts, bearing one short alkyl chain, do not form a mesophase. For the other compounds highly ordered lamellar mesophases were observed, even at room temperature. Binary mixtures of these compounds show similar phase behavior, but with a lower melting and/or clearing point. The SmX phases of 286-NTf2-14/14 and 286NTf2-7/10 were investigated by 1H−13C CP-MAS and 19F solidstate NMR spectroscopy.901,903 The combined solid-state NMR and PXRD results suggest that the alkyl chains are almost entirely molten in the SmX phase, but that there still exists a high degree of order inside the ionic sublayers. Lyotropic LC behavior in benzene was also observed. Widening of the 4,4′-bipyridinium core by four methyl groups, as in compounds 287-n, appears to be detrimental for mesophase formation.902

A study of symmetric viologen dimers with long alkyl chains (288-n) revealed that the linker between the 4,4′-bipyridinium cores enables the formation of a SmA phase in addition to the highly ordered (and highly viscous) SmX phase, presumably because it makes the central polycationic region more flexible.901 CI


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Also, X-ray diffraction showed only a slight decrease of the smectic layer spacing d in contrast to the discontinuous expansion in the heating run pointing to the lack of a SmA-toSmF/SmI phase transition upon cooling. The molecules adopt a single layered organization going from tilted to orthogonal as the lateral n-butyloxy groups are melting. The polymeric compound 295 exhibits similar phase behavior; a SmC phase is followed by a higher-temperature SmA phase. Interestingly, this transition is smeared out over 40 °C, implying an extremely slow melting process.

In search for more functional self-organizing LC compounds, Stoddart, Kato, and co-workers investigated a bistable [2]rotaxane with a mesogenic periphery (296).908 [2]Rotaxanes consist of a dumbbell-shaped core of which the rod-like part is encircled by a macrocycle. The core of compound 296 contains two recognition sites for the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring (which contains two 4,4′-bipyridinium units): an electron-donating tetrathiafulvalene (TTF) unit and a 1,5dioxynaphthalene (DNP) unit. It can act as a molecular switch because the TTF part in its neutral state is more likely to be associated with the tetracationic CBPQT4+ macrocycle, whereas in its oxidized state electrostatic repulsion causes the positively charged CBPQT4+ to move away from the oxidized TTF recognition site and to associate with the DNP part. The dendrons that are attached to both ends of the central axle via azide−alkyne “click” chemistry not only serve as mesogenic units but also as stoppers for the macrocycle. The cyclohexylbiphenyl group was designed to be electrochemically inactive. Despite the presence of the bulky macrocycle, the rotaxane shows a monolayer SmA phase at room temperature and up to 150 °C. The neutral dumbbell-shaped core without the CBPQT4+ macrocycle also exhibits a SmA phase between 7 and 146 °C. Electrochemical switching, with reversible shuttling of the CBPQT4+ ring between the two recognition sites, was initially only demonstrated in solution,908 but in a follow-up paper switching in the mesophase itself (LC film of 296) was reported.909 The redox-driven mechanical movement of the CBPQT4+ ring is accompanied by a color change (electrochromism, see section 12.4), and apparently does not affect the mesomorphic behavior because no change in texture was observed upon oxidation. Simple cholesteryl groups have also been attached to the same [2]rotaxane core as in 296, but no thermotropic mesomorphism was reported for these compounds.910 The CBPQT4+ ion appeared to be mesomorphic itself if dialkylsulfosuccinate anions were used as counterions.162 Although compound 297-DOSS is not LC, its mixtures with sodium bis(2-ethylhexyl) sulfosuccinate ([Na][DOSS]) show a lamello-columnar (LCol) or Colhex phase, depending on the molar ratio. A higher aliphatic volume fraction in the anion, as in 297DDSS, leads to the formation of a Colrec phase. Upon

Kohmoto et al. recently reported on “extended” analogues of 292-n, that is, 4,4′-(9,10-anthracenediyl)bipyridinium salts 293X-n.907 Despite the fact that the pyridinium rings are twisted with respect to the anthracene fluorophore, as seen in the crystal structures of methoxysubstituted (non-LC) homologues (n = 1), the Br− and [BF4]− salts form enantiotropic Colrec phases for n = 8 and n = 12. The corresponding [PF6]− salts show monotropic mesophases. The fluorescence spectra of 293-PF6-n (n = 8, 12) in their vitrified LC state (obtained by rapid cooling) are redshifted with respect to the spectra obtained in their crystalline solid state before heating. This seems to imply that the central aromatic system becomes more planar when forming the LC phase.

Dı ́az-Cuadros et al. synthesized luminescent LC molecules and a low molecular weight polymer based on an aromatic pentaphenylene unit and 4,4′-bipyridinium (294-n and 295).374 For the charged compounds 294-n, a SmF/SmI and a SmA phase were successively observed on heating, whereas their nonionic precursors show a nematic phase. Upon cooling of 294-10, the molecules had the tendency to align homeotropically so that a black image was seen using POM, even at room temperature. CJ


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complexation with TTF (to form a pseudorotaxane), an LCol phase was again observed (Figure 56). Figure 57. Single-crystal structures of methyl-substituted analogues of 298-crown-c and 298-crown-d (top), and 299-crown-a (bottom), showing “shielding” of the charges of the pyridinium moieties (depicted in blue) by the interlocked neutral crown ether macrocycle (depicted in red). The triflate anions are depicted in yellow. A similar conformation of the [2]rotaxane components can be expected in the mesophase. Reprinted with permission from ref 363. Copyright 2013 American Chemical Society.

compared to the dumbbell species, because only highly viscous soft crystal phases (whether or not with a simple lamellar structure) were found for the latter, whereas the former show a genuine SmA phase for sufficiently long alkyl chains. These results elegantly show that for low molecular weight rigid-rod species, the potentially beneficial effect of an ionic core on LC properties (e.g., thanks to increased nanosegregation) can be undone by a too high number of “naked” charges and consequently too strong intermolecular electrostatic interactions. Compounds 298-(a-e) are potentially LC, but a hypothetical mesophase will only exist at temperatures exceeding the thermal decomposition point. The authors found that a reduction in charge from +4 (298-crown-c and 298-crown-d) to +2 (299-crown-a) also results in lower transition temperatures and a less viscous SmA phase (the diffraction patterns of the SmA phases of 298-crown-(c-d) still show a reflection related to a repeating distance of about 15.0−15.5 Å, probably corresponding to partial in-plane lateral ordering of the triflate anions). Moreover, 299-crown-a is easier to prepare and purify. The stability of the mesophase shown by 299-crown-a could be increased by replacing the dodecyl chains by organosiloxane chains that are more powerful promoters of nanosegregation (299-crown-b). Although such organosiloxane chains often induce SmC phases, the cross-sectional area occupied by the [2]rotaxane core is probably sufficiently large to easily accommodate the four bulky organosiloxane chains in a nontilted SmA fashion. Also, the phase behavior of 299-crown-b shows once again how strongly the SmA phase is promoted by ionic mesogens as opposed to tilted phases. Complexation with the interlocked macrocycle and the resulting improved delocalization of the positive charges on the axle not only improved the mesomorphic properties to a great extent, but also increased the thermal stability, with decomposition only starting above 250 °C for rotaxane 299-crown-a and above 190 °C for rotaxanes 298-

Figure 56. Formation of the LCol phase upon complexation of 297DDSS with TTF. Reproduced with permission from ref 162 (http://dx. doi.org/10.1039/b900140a). Copyright 2009 The Royal Society of Chemistry.

Other LC [2]rotaxanes were reported by Loeb and Eichhorn.363 They did not use a charged macrocycle (like CBPQT4+) but a charged core (axle) consisting of two 4,4′-bipyridinium groups linked by an ethylene chain, or two 4-phenylpyridinium groups linked by the same spacer; the charged core was decorated with four alkyl chains in a tetracatenar fashion. The influence of “shielding” of the charges of the pyridinium moieties by an interlocked neutral dibenzo[24]-crown-8 ether macrocycle could be investigated by comparing the “naked” charged dumbbell species (298-(a−e) and 299-a) with the corresponding [2]rotaxanes (dumbbells including the macrocycle, 298crown-(a−e), and 299-crown-(a-b); Figure 57). Remarkably, the [2]rotaxanes show superior mesomorphic properties as CK


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crown-(a−e) as opposed to 170 °C for the dumbbells. Weight losses below 100 °C and (irreversible) differences between the phase behavior in the first and second heating runs of pristine samples were attributed to the loss of solvent molecules. Siloxane-substituted rotaxane 299-crown-b, which shows the most desirable mesomorphic properties, shows an unexpectedly low thermal degradation temperature, with decomposition of the organosiloxane chains already starting at 175 °C. Although this temperature is still well above the clearing point of 299-crown-b and thus does not affect its practical applicability to a great extent, the proposed relatively facile thermal decomposition of the organosiloxane substituents in ionic compounds (despite the shielding of the charges in 299-crown-b) should be kept in mind when designing future ILCs that would show a SmC phase. It should be noted that the number of reported ionic mesogens containing siloxane moieties is very limited,825,911−914 and except for the compounds synthesized by Mizoshita and Seki (191 and 192), all of the examples encompass high molecular weight poly(siloxane) species. Ikker et al. observed slow degradation of the poly(siloxane) backbone only above 400 °C by TGA.911

anion. However, it has an anisometric shape, and in the singlecrystal structure of 300 the long axes of the cations and anions are aligned parallel to each other. This arrangement can explain the observation of a nematic phase, which was proposed by the authors on the basis of POM textures. Two redox processes were observed in 300 by cyclic voltammetry on acetonitrile solutions: a reversible process at ca. −0.76 V versus Ag+/Ag related to the [4,4′-bipyridinium]2+/[4,4′-bipyridinium]•+ redox couple, and an irreversible oxidation process at ca. 0.53 V versus Ag+/Ag related to [Zn(mnt)2]2−→ [Zn(mnt)2]•−. It would be of interest to investigate whether a nematic phase is also formed by a salt analogous to 300, but with a flat [M(mnt)2]2− anion instead of the tetrahedral [Zn(mnt)2]2−. Such a planar [M(mnt)2]2− anion could also be combined with two 4-amino-N-alkylpyridinium cations,870 to create an elongated rigid structure by hydrogen bonding between the cyano groups of the maleonitriledithiolato ligands and the hydrogen atoms of the amino groups.

Beneduci et al. investigated the mesophase behavior of calamitic thienoviologens (i.e., 4,4′-(2,2′-bithiophene-5,5′-diyl)bis[N-(n-alkyl)pyridinium] salts, which are π-extended viologens) substituted with moderately long alkyl chains (301-Xn).920,921 Just as in the case of series 286-X-m/n, halide counterions are not suitable to induce LC properties (except in the case of 301-I-12), but [NTf2]− anions are. The authors described a peculiar “bimesomorphic behavior” for the series 301-NTf2-n. Molecules of 301-NTf2-9 form more or less diskshaped π-stacked dimers that stack into columns to form ordered Colrec phases. The mesophase behavior evolves to a lamellar SmA phase for 301-NTf2-11 and 301-NTf2-12 (in the first heating run of pristine samples a columnar phase is however observed). Incipient lamellar mesomorphism is observed for 301-NTf2-10, under the form of a lamello-columnar phase LCol (which should be considered as an ordered SmA analogue in which the molecules are ordered into columns within the layers). The LC thienoviologens are intrinsically switchable electroactive fluorophores: they combine a redox-active extended viologen architecture with a highly luminescent π-conjugated core.921 Recently, the bulk photophysical, electrochemical, and spectroelectrochemical properties of 301-NTf2-9 and 301-NTf2-11 were reported, and it was demonstrated that these electronacceptor materials show both electrochromic and electrofluorochromic responses in the bulk LC state.922,923 These results will be discussed in section 12.5.

Future challenges in the research on LC rotaxanes908,909,915−919 are macroscopic alignment of the mesomorphic samples and molecular switching in the mesophase (as already demonstrated by Stoddart, Kato, and co-workers for a low molecular weight sample;909 see above). The ultimate goal is to achieve self-organizing molecular switches that show a directed macroscopic response (and perform “work”) thanks to amplification resulting from cooperative motion in the mesophase. In continuation of their work on ionic compounds with a metal-containing anion with maleonitriledithiolato ligands (see section 8.1, 247), Ren and co-workers prepared a 1,1′-bis(ndecyl)-4,4′-bipyridinium salt with the bis(maleonitriledithiolato)zincate(II) ([Zn(mnt)2 ] 2− ) anion (300).438 This is a 2-fold negatively charged anion, which can form a 1:1 complex with the 4,4′-bipyridinium cation. In contrast to [Ni(mnt)2]−, the [mnt]2− ligands are tetrahedrally coordinated to the zinc(II) center in [Zn(mnt)2]2−, so that it is not a flat CL


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10. NEW CATIONIC CORES FOR IONIC LIQUID CRYSTALS During the past decade, several new ILCs with other cationic cores than those that were discussed in the previous sections have been developed. These positively charged cores are mainly quaternized N-heterocyclic ring systems, but other structural motifs have been reported as well.

metal-containing pyrrolidinium salt, [C12mpyrr]3[TbBr6] (304), which shows a phase behavior similar to that of [C12mim]3[TbBr6] (36-TbBr6).119 The clearing point of the pyrrolidinium salt is about 60 °C higher than that of the imidazolium compound, as is the case for the corresponding bromide salts (the difference in clearing point between [Cnmpyrr][Br] and [Cnmim][Br] salts fades out with increasing alkyl chain length). It is not clear whether 304 forms a SmA phase or an ordered smectic phase.

10.1. Ionic Liquid Crystals with Pyrrolidinium, Piperidinium, Piperazinium, and Morpholinium Cations

Goossens et al. synthesized a series of LC pyrrolidinium salts with variation of the alkyl chain length and the anion (302-X-n and 303-n).115 The pyrrolidinium cation is structurally different from the imidazolium cation: both are five-membered Nheterocyclic ring systems, but pyrrolidinium is nonplanar (with the nitrogen atom residing above the plane of the four ring carbon atoms) and nonaromatic, lacking any charge delocalization.924 The absence of conjugation has the advantage that no inconvenient background fluorescence is measured in the study of metal-centered luminescence of ionic pyrrolidinium-based metallomesogens. The pyrrolidinium cation also displays a higher electrochemical stability than imidazolium.925 Moreover, it will not be involved in hydrogen bonding due to the lack of any acidic hydrogen atom, in contrast to the imidazolium cation. Short C−H···Cl− contacts in the single-crystal structure of [C3mpyrr][Cl] were termed (weak) hydrogen bonds, though.926 Very weak C−H···I− interactions were also found in crystals of [C1mpyrr][I] at 123 K927 but not for [C3mpyrr][I].928 Anderson et al. observed weak hydrogen bonds in the crystal structure of a hydrated dicationic pyrrolidinium IL.640 Dealing with a point charge localized on the nitrogen atom rather than a positive charge spread over the ionic core greatly affects the intra- and intermolecular electrostatic interactions and, as a consequence, mesophase formation. A rich mesomorphism was observed for the N-(n-alkyl)-N-methylpyrrolidinium salts, including highly ordered smectic phases (the relatively uncommon crystal smectic T phase (see section 4.3.1) and the crystal smectic E phase) and SmA phases. In contrast, the analogous 1-methyl-3-(n-alkyl)imidazolium salts only show SmA phases, emphasizing the importance of charge delocalization. A minimum chain length of 11 and 14 carbon atoms is required to induce thermotropic mesomorphism for the N-(n-alkyl)-N-methylpyrrolidinium bromides 302-Br-n (Figure S18) and bis[N-(n-alkyl)-Nmethylpyrrolidinium] tetrabromouranyl salts 303-n, respectively. [Remark: [C10mpyrr][Br] was also considered to be LC by Getsis and Mudring.929] It must be noted that no “odd−even” effect was observed (in contrast to the [Cnmim][BF4] salts; see Figure 16a). Upon elongation of the alkyl chain length, the melting and clearing temperatures increase. The influence of the anion appeared to be more pronounced. The mesophase stability decreases with increasing anion size, the [NTf2]− salt 302-NTf218 and the [Eu(tta)4]− compound 302-Eu(tta)4-18 being nonmesomorphic. For compound 302-SCN-18, with a linear [SCN]− anion, only a SmA phase was observed, whereas salts containing a spherical anion (i.e., Br−, [BF4]−, and [PF6]−) show T phases and, for long alkyl chains, an additional SmA phase. These results point to the incompatibility of nonspherical anions with tetragonal ordering inside the ionic sublayers, as was already discussed in section 4.3.1. For tetrabromouranyl salts 303-n, no photoluminescence could be observed in the condensed state, probably due to autoquenching. However, solutions in the IL [C4mpyrr][NTf2] showed green luminescence. As already mentioned in section 5.1, Getsis and Mudring also prepared a

Xu et al. extended their work on ILCs with the [F(HF)2]− anion by synthesizing the N-(n-alkyl)-N-methylpyrrolidinium analogues of imidazolium salts 6-n (305-n).518 As for [SCN]− salt 302-SCN-18, a SmA phase rather than an ordered smectic phase was found for the tetradecyl, hexadecyl, and octadecyl compounds. The bent-shaped [F(HF)2 ]− anion is not compatible with a crystal smectic T structure. Apparently, a longer alkyl chain is required to induce mesomorphism than in the case of the corresponding imidazolium salts, for which the decyl derivative (6-10) is already LC. The [Cnmpyrr][F(HF)2] salts have lower melting and clearing points than the corresponding [Cnmpyrr][Br] compounds, but their melting points are higher than those of the [Cnmim][F(HF)2] salts. Their ionic conductivity anisotropy is about 10, just like for the imidazolium counterparts.

Related to their work on imidazolium ILCs with pendant mesogenic groups, Goossens et al. attached a mesogenic group to an ionic pyrrolidinium core via an alkyl spacer (306-X, 307-X-n/ m, and 308-X-n/m).116 Apart from crystal smectic E phases and SmA phases, which were also observed for imidazolium salts with a cyanobiphenyl functionality, these pyrrolidinium salts exhibit tilted smectic phases, which is quite unusual for ILCs that are not built up by a rigid ionic core. The cyanobiphenyl-containing salts 306-Br and 306-NTf2 only show a monotropic SmA phase, just like their imidazolium analogues.122 None of the luminescent [Eu(tta)4]− salts seemed to be mesomorphic. In general, bulkier anions destabilize the mesophase as Coulombic forces are attenuated due to charge delocalization and the size of the anion. This effect becomes less pronounced as the mesogenic group becomes larger (308-X-n/m versus 307-X-n/m). This can be easily understood in terms of space-filling considering that the molecular area of the ionic headgroups has to be counterbalanced by the mesogenic moieties. The structural organization of the reported compounds into smectic layers is considerably different from the pyrrolidinium salts lacking a mesogenic unit. No T phases were observed. Instead, the bromide, [NTf2]−, and CM


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[UO2Br4]2− salts display a rich mesophase behavior, including highly ordered smectic (G, J, H, or K), crystal smectic E, SmF/ SmI, SmC, and/or SmA phases, depending on the molecular structure. On the basis of detailed PXRD investigations, the SmA phases could be described by the de Vries “diffuse cone” model.41 Indeed, it was found that the mesogenic groups needed to be strongly tilted with respect to the smectic layer normal, to compensate for the cross-sectional area imposed by the ionic headgroup moieties (Figure 58). On the other hand, POM

three n-eicosyloxy chains on the lateral phenyl group (309) and its neutral precursor (with only one methyl group on the pyrrolidine nitrogen atom). The use of extremely long aliphatic chains is probably necessary to compensate for the bulky fullerene component. The thermal behavior did not change much upon quaternization. However, for ionic compound 309, a deionization back-reaction to its neutral precursor was observed at elevated temperatures (or even at room temperature, albeit at a much slower rate). This was ascribed to the low nucleophilicity of the pyrrolidine ring nitrogen atom resulting from the presence of the electron-deficient C60 moiety. Differences in electronic structure between 309 and its neutral analogue were observed in UV−vis spectra and electrochemical measurements. For both compounds the formation of flower-like microparticles consisting of flakes, as well as nanowire formation was observed. Figure 58. Structural model for the de Vries type SmA phase exhibited by compound 307-Br-4/10 (bromide anions are represented by gray spheres). Reprinted with permission from ref 116. Copyright 2009 American Chemical Society.

observations confirmed the optical uniaxiality of the phases, which indicated the lack of long-range correlation in the tilt direction. Although the SmA phases were originally designated as “de Vries type” SmA phases, the terms “de Vries SmA phase” or “de Vries character” should strictly speaking only be used for SmA phases (or materials) that undergo a “de Vries phase transition” to a lower-temperature SmC phase.41 Such transition is characterized by a very minor layer shrinkage (≤1%−2%). The layer shrinkage observed by PXRD is much smaller than would be expected from the optical tilt angle. This phenomenon could be explained by de Vries via his “diffuse cone” model, that is, lack of correlation in the tilt direction within the smectic layers (the “noncorrelation” model that proposes lack of correlation between the smectic layers is generally discarded nowadays).41,930−933 During the transition, the azimuthal distribution of the molecular tilt directions, which is degenerate in the SmA phase, becomes biased (ordered) toward a certain direction in the azimuthal plane in which the macroscopic tilt of the SmC director n then appears. The interested reader is referred to the work of (among others) Giesselmann, Lagerwall, Lemieux, Clark, and Walba for more information about de Vries phase transitions and de Vries materials.41,934−953 Functionalized pyrrolidine-based LCs incorporating a fullerene moiety were synthesized by Li et al.954 A highly ordered smectic phase was observed for both the ionic derivative bearing

A rich mesomorphism was also observed for ILCs based on a piperidinium (310-X-n and 311-X), piperazinium (312-X), or morpholinium (313-X) core that were synthesized by Lava et al.397 The compounds show highly ordered smectic phases (crystal smectic E and T phases), SmA phases, and Colhex phases. In general, short alkyl chains and voluminous spherical anions destabilize the mesophase. A crystal smectic E or T phase was proposed for the ordered smectic phases designated as “SmX”, on the basis of their optical textures and by analogy with the pyrrolidinium salts mentioned above. The “X” phases are plastic crystal phases. Interestingly, for compounds 311-C12H25OSO3, 313-DOSS, 313-DcHSS, and 313-DHSS, a Colhex phase was observed, even at room temperature in case of the salts with a branched anion. PXRD data suggested that about three molecules are present in one columnar slice of about 5 Å thickness. Apparently, the formation of the disks is governed by space filling requirements; that is, a sufficient amount of alkyl groups must be present in either the anion or the cation for the disk to be formed, as is shown in Figure 59. Piperidinium- and morpholinium-based ILCs 310-BF4-14, 311-BF4, 311-PF6, and 313-BF4 were further investigated by high-resolution adiabatic CN


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phase of 314-I on a glass slide could be vitrified to a glassy state. A neutral piperidine analogue of 314-X showed only a monotropic SmA phase, which did not show homeotropic alignment. Ionic polymers resulting from radical polymerization of a methacrylate monomer obtained from 314-I also showed an enantiotropic SmA phase over a broad temperature range.

10.2. Ionic Liquid Crystals with ε-Caprolactam Moieties

Deng and co-workers investigated ILCs based on N-(n-alkyl)-εcaprolactam cations (315-X-n·H2O).956 These could be obtained as monohydrates in a single synthesis step by reacting ε-caprolactam with an alkyl-substituted tosylate or mesylate. Surprisingly, in the series 315-CH3SO3-n·H2O, the homologue with the longest alkyl chain (C18) shows a monotropic SmA phase, whereas the (n-hexadecyl)-substituted compound exhibits an enantiotropic SmA phase. The ε-caprolactam [OTs]− ILCs possess two interesting properties: (i) they show high specific heat capacities (Cp), sensible heat storage densities (Es), and latent heat storage densities (El), which can render them useful as thermal storage media; and (ii) they appear to show lower acute toxicity values than, for instance, the common IL [C4mim][BF4]. These ILCs were also investigated as organized reaction media for the hydroformylation of 1-octene (see section 12.3).566

Figure 59. Molecular model for the packing of 313-DHSS in the Colhex phase. Reprinted with permission from ref 397. Copyright 2009 American Chemical Society.

scanning calorimetry (ASC).283 All phase transitions, that is, Cr → Cr, Cr → T, Cr → SmX, T → I, SmX → I, SmX → SmA, as well as SmA → I, displayed a nonzero latent heat and were thus found to be first-order. For the particular case of crystal-to-highly ordered smectic phase transitions, pretransitional effects were observed in the low-temperature wing. Such effects may be ascribed to partial loss of orientational order due to premelting of the alkyl chains in the crystalline phase. The latent heat associated with the SmA → I transition of 313-BF4 (0.8 J g−1) was found to be much smaller than the value of about 13.5 J g−1 previously determined for nonionic long-chain cyanobiphenyl mesogens, which also form a bilayer SmA phase with interdigitated alkyl chains and whose SmA → I transition is strongly first-order.955 In addition, the SmA → I transition of 313-BF4 is also much broader than what is commonly observed for a long-chain cyanobiphenyl LC. These observations suggest that even in the isotropic liquid state a substantial amount of ordering remains. This aspect has already been discussed in section 4.2 in relation to ILs and ILCs. ILC phase transitions appear to be generally broader than those of conventional nonionic LCs, which may indicate that many of these phase transformations are gradual processes, with important pretransitional changes.

10.3. Ionic Liquid Crystals with Amidinium Moieties

Hosseini and co-workers realized molecular self-assembly directed by charge-assisted hydrogen bonding for bis(amidinium) dications combined with chloride or dicyanometalate anions (316-X).112 Compound 316-Cl shows a Colrec phase, while a Colhex phase was found for 316-Ag(CN)2. The lipophilic 3,4,5-tris(n-dodecyloxy)benzene unit appeared to be crucial for mesophase formation; analogues with four n-propyl, nhexyl, or n-dodecyl chains directly connected to the central core do not show LC behavior. PXRD measurements indicated that the Colhex phase of 316-Ag(CN)2 consists of columnar slices made up by one molecule. Within each column, the slices are interconnected by charge-assisted hydrogen bonds between the N−H groups of the amidinium cations and the cyano groups of the linear [Ag(CN)2]− anions (Figure 60). Interestingly, 316Ag(CN)2 displays blue phosphorescence, presumably originating from short silver−silver interactions imposed by the molecular self-assembly.957 The approach adopted by the Hosseini group can be termed “molecular tectonics” (see section 4.1).

Ujiie et al. attached an azobenzene-based mesogenic group to a 4-(2-hydroxyethyl)-N-methylpiperidinium moiety (314-X).348 Both the bromide and the iodide salt show an enantiotropic SmA phase. Salt 314-Br shows a lower melting point and a higher clearing point than imidazolium salt 74-Br-1/6-NO2. The SmA phase of 314-Br crystallized upon cooling, whereas the spontaneous uniform homeotropic orientation of the SmA

10.4. Ionic Liquid Crystals with Guanidinium Moieties

Like amidinium, guanidinium is a highly stable cation due to the possibility of charge delocalization, and is as such an excellent candidate for the design of LC molecules. Smectic phases were observed at high temperatures for guanidinium 4-(n-alkyl)benzenesulfonates 317-1/n (m = 1) with n > 6 and for CO


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Figure 60. Proposed molecular packing in the Colhex phase formed by 316-Ag(CN)2 (a 1D ladder-type network is formed). Reproduced with permission from ref 112 (http://dx.doi.org/10.1039/c0cc03970e). Copyright 2011 The Royal Society of Chemistry.

Figure 62. Polymorphism observed for the guanidinium n-alkylsulfonates 317-0/n. Reprinted with permission from ref 408. Copyright 2005 American Chemical Society.

and the ordered smectic phases lies in the melting of the alkyl chains. It is therefore not surprising that the hexagonal structure that is observed in the crystalline state is preserved in the ordered smectic phases. Up to four different ordered smectic phases were distinguished by X-ray diffraction (Figure 62; “SmX” and “SmX’” are monotropic phases). For the long-chain homologue 317-0/ 18, two SmB-type phases were identified, in which the ionic layers can glide freely past each other. The authors suggested “SmY” to be a “hexatic” SmB phase and “SmY’” to be a “rotationally disordered” SmB phase (see section 3.2). In the shorter-chain homologues 317-0/10 and 317-0/12, Coulombic forces between the smectic layers are comparatively stronger because the ionic cores are less separated. Consequently, the molecules adopt well-defined positions within the smectic layers, and in this case “SmY” corresponds to a crystal smectic B type phase in which the alkyl chains are molten but the ionic sublayers are “locked” in well-defined positions according to a 3D crystal lattice (monoclinic or triclinic). For intermediate chain lengths (317-0/14 and 317-0/16), the X-ray diffraction patterns pointed to a gradual transition from a crystal smectic B to a SmB-type phase. The SmA structure of the guanidinium n-alkylsulfonates is similar to that of compounds 317-1/n described above. Variabletemperature IR spectroscopy showed that the hydrogen bonding that was observed in the crystal structure is preserved in the LC state. Even in the SmA phase, about 75% of the hydrogen bonds continue to exist, even though the distance over which intralayer ordering occurs decreases from “infinity” in the crystal, over ∼200−300 Å in the ordered smectic phases to ∼20 Å in the SmA phase.

guanidinium 4′-(n-alkyl)biphenyl-4-sulfonates 317-2/n (m = 2) with n > 7, in which the negative charge is delocalized as well.958 Mathevet et al. proposed a fully interdigitated head-to-tail arrangement of the molecules in such a way that the ionic layer is well segregated from the apolar alkylbenzene moieties.959 The systems without aromatic rings, in which the negative charge is delocalized only over the oxygen atoms of the sulfonate moiety (317-0/n, m = 0), show highly ordered smectic phases in addition to the less ordered SmA phase.408 A schematic representation of the bilayered honeycomb structure of the crystalline state of 317-0/2, which is governed by hydrogen bonding, is shown in Figure 61. Figure 62 depicts the phase

Figure 61. Honeycomb structure of the crystalline state of guanidinium ethanesulfonate (317-0/2): (a) packing of the molecules viewed along the b axis; and (b) network of hydrogen bonds as viewed along the c axis. Carbon atoms are depicted in green, sulfur atoms in yellow, oxygen atoms in red, nitrogen atoms in blue, and hydrogen atoms in white. Reprinted with permission from ref 408. Copyright 2005 American Chemical Society.

Kim and co-workers described guanidinium salts with a tris(nalkyloxy)benzene unit covalently linked to the cation (318X).960 Different phases were obtained by altering the anion. This shows again that “anion-directed self-assembly” is an attractive approach because no complicated synthetic pathways are needed to modify the thermal behavior. Aggregates of four molecules were found to self-assemble into columnar phases of either hexagonal or rectangular symmetry depending on the shape of the anionspherical or linear, respectively. For the small chloride anion, a micellar cubic phase of Pm3̅n symmetry was also observed at higher temperatures.

behavior of the guanidinium n-alkylsulfonates with longer alkyl chains. Interestingly, two types of phase transitions could be distinguished (Figure S19): a set of transitions whose enthalpy change is independent of the alkyl chain length and which are related to a structural rearrangement of the ionic cores (SmY → SmA and SmX → SmY), and another set of transitions whose enthalpy change depends linearly on the alkyl chain length and which are related to melting of the alkyl chains (Cr → SmX and Cr → SmY). The only difference between the crystalline solid CP


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to that of the corresponding iodide salts 320-I-n/m, in which the cation and anion have very different shapes (more or less rod-like and spherical, respectively). The sulfonate and iodide salts could be conveniently prepared by reaction of 320-Cl-n/0 with a base and with the appropriate methyl sulfonate or with methyl iodide, respectively, eliminating the need for further anion exchange. Both 321-n/m and 320-I-n/1 exclusively show SmA phases (which become homeotropically aligned on cooling from the isotropic liquid phase), but in the series 321-n/m mesomorphism was already observed for the homologue with two n-nonyl chains. Among the sulfonimide salts, only one derivative (322-H/CH3) is LC. For the shorter alkyl chain lengths, the sulfonate salts are superior to the iodide compounds in the sense that they display mesomorphism with a lower melting point and a higher clearing point. The relatively bulky and spherical iodide anions disrupt the smectic bilayer structure in the case of the short-chain iodide compounds. However, 321-16/16 and 320-I16/1 have similar phase transition temperatures, and starting from an n-octadecyl substituent, the iodide salts show a much broader mesophase temperature range than the sulfonate salts thanks to a much higher clearing point. Too long alkyl chains in salts 321-n/m tend to decrease the clearing point, probably as a result of a diminishing amphiphilicity (see section 5.1). Thanks to a less efficient crystal packing, lower melting points could be obtained for the “asymmetric” salts 321-12/m (m ≠ 12) and 321-n/12 (n ≠ 12) as compared to 321-12/12. A similar phase behavior has been reported for N-(n-alkyl)pyridinium nalkylsulfates.598 The clearing point of guanidinium salts 321-n/ m seems to depend on the value of n + m: among the derivatives that were investigated, the highest clearing point is obtained for n + m ≈ 32. For the N-(n-alkyl)pyridinium n-alkylsulfates, the optimal n + m value was found to be 28.598 The smectic layer spacing of the “asymmetric” guanidinium salts is determined by the longer alkyl chain in an ion pair and is independent of its location (attached either to the cation or to the anion), indicating mixing of the congruently shaped cations and anions within the smectic layers. Single-crystal structures could be obtained for 321-12/14 and 320-I-12/1. In the crystalline solid state, the cationic guanidinium moieties and the anionic sulfonate groups are located next to each other, but the aryl rings of the cation and anion are not parallel to each other and are not involved in π−π stacking. “Bent” analogues of 320-X-n/m and 325-n (323-X-n/m and 326-n, respectively) show lower melting points and lower clearing points than their linear analogues.965 All derivatives show a SmA phase. For salts 324-R1/R2-n/m, which contain substituted benzenesulfonate anions, the mesophase behavior is highly dependent on the exact structure of the anion (number of n-alkyloxy chains), and the difference between alkyl chain lengths in the cation and anion. Depending on these parameters, a SmA, Colrec, or Colhex phase is formed by these compounds. Models for the molecular packing in the different mesophases were proposed on the basis of PXRD data. The influence of the substitution pattern of both the guanidinium cation and the anion was investigated in even more detail by considering the series 327-R, 328-(a−c)/(d−g), 329-R1/R2, and 330-(a−c)/(d−g), as well as the related imidazolium salts 339-(a−d) and 340-(a−e).721 The compounds that contain only one n-alkyloxy chain in either their cation or their anion and up to three n-alkyloxy chains in total display SmA phases. The derivatives that contain in total four to five n-alkyloxy chains within one ion pair show Colhex phases, irrespective of the structure of the headgroups. With a total of six

In recent years, many other LC guanidinium salts in which the cationic core is covalently connected to a substituted aromatic ring or ring system have been reported by Laschat and coworkers (Scheme 19; see the Supporting Information for an explanation of structure codes).491,502,721,961−967 They prepared both “acyclic” guanidinium compounds (such as 320-X-n/m) and “cyclic” guanidinium salts (such as 329-R1/R2). In the first instance, guanidinium salts with one or two phenyl rings directly attached to the acyclic ionic moiety (319-X-(a-k)-m and 320-Xn/m) were studied extensively.502,961 Structure−property relationships were investigated both experimentally and theoretically by varying a number of factors, including the length of the aromatic moiety attached to the ionic core, the counterion, the terminal alkyl chain length, the type of terminal alkyl chain, and the possibility of hydrogen bonding, which can be modulated via N-alkylation. The thermotropic phase behavior is very similar to that of the imidazolium analogues.292 All mesomorphic salts display SmA phases that, upon cooling, usually vitrify into a glassy state. In a second heating cycle, LC behavior can be observed over a wide temperature range. For 4-(n-alkyloxy)biphenyl salts 319-X-(a-k)-m and 4-(n-alkyloxy)phenyl salts 320-X-n/m, a minimum chain length of 8 or 11 carbon atoms, respectively, is required to induce mesomorphism. As expected, an additional phenyl ring stabilizes the smectic phase thanks to an increased shape anisotropy. Clearing temperatures decrease with the anionic radius in the order Cl− > Br− > I− > [BF4]− > [SCN]− > [PF6]− > [B(C6H5)4]− (the tetraphenylborate salts are actually not mesomorphic). The anion exchange could be easily monitored by 1H NMR spectroscopy for compounds that possess the N−H proton. This proton was deshielded according to Cl− > Br− > [SCN]− > I− > [BF4]− > [PF6]−, independent of the alkyl chain length n (this was also found for the acidic H(2) proton of imidazolium salts; see section 5.1). This implies that the anion must be situated close to the N−H group and not directly above the central carbon atom. However, NBO analysis showed that the central carbon atom of the guanidinium moiety carries the highest positive partial charge independent of the substituents, and no indications for charge delocalization in the aryl system were found (i.e., the guanidinium moiety behaves like an isolated cation). As such, hydrogen bonding rather than electrostatic interactions seem to play a crucial role in the positioning of the anion, and this was confirmed by the singlecrystal structures of 319-Cl-f-0 (R = n-C12H25) and 319-I-f-0 (R = n-C12H25). In the series 319-I-(d,f,g,h,i)-m, the substitution of the N−H proton with an ethyl or n-propyl group (m = 2 or 3, respectively) reduces the mesophase stability due to a reduction in shape anisotropy. The Laschat group also prepared derivatives of compounds 320-X-n/m with 4-(n-alkyloxy)benzenesulfonate anions (321n/m) and with 4-(n-dodecyloxy)benzenesulfonimide anions (322-R1/R2).962,963 The cation and anion of these salts have a similar molecular shape (the authors use the term “congruently shaped anions” or “complementary ion pairs”; the anions of 321n/m and 322-R1/CF3 can be regarded as derivatives of [OTf]− and [NTf2]−, respectively, in which a CF3 group is replaced by a 4-(n-alkyloxy)phenyl group). The phase behavior was compared CQ


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Scheme 19. Guanidinium-Based ILCs Investigated by Laschat and Co-workers491,502,721,961−967

CR


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n-alkyloxy chains, however, the symmetry of the arylsulfonate anion and the type and symmetry of the cationic headgroup become relevant, and a Colhex phase, a micellar cubic phase, a plastic phase, or a lack of mesomorphism is observed (see the Supporting Information for the detailed thermal phase behavior of all compounds that were investigated).

increasing alkyl chain length. Anion exchange for spherical [BF4]− or [PF6]− (331-BF4 and 331-PF6, respectively) appeared to have a much less negative effect on the mesophase stability. Ring closure in the cationic headgroup region to form a cyclic imidazolinium-like structure did not have much influence on the phase behavior, although a slightly higher clearing point could be achieved for the [OTf]− salts (compare series 331-X-(c−f) and 333-X-(e−h), and series 332-X-n and 334-X-n, respectively). “Bent” guanidinium salts 332-X-n and 334-X-n show lower clearing points than their linear counterparts. The latter can achieve a tighter molecular packing in their mesophase. In early work of the Laschat group on LC guanidinium salts, a disk-like pentakis(n-alkyloxy)triphenylene moiety was attached to the guanidinium unit (338-n).967 Two types of Colrec phases were characterized, one of common C2/m symmetry and another one of uncommon P2m symmetry (Figure S20). The columns are stacks of disks formed by four guanidinium salt molecules. The tilt of the columns, being inversely related to the cross-sectional area of the ellipsoids, decreases with increasing temperature and alkyl chain length. Analogues bearing long alkyl chains cannot be accommodated in the Colrec phase of C2/m symmetry in which the unit cell comprises two columns. In contrast, the unit cell in the Colrec phase of P2m symmetry is constituted of only one column, leaving enough space for long or highly disordered (i.e., at higher temperatures) alkyl chains. For compound 338-6, a third type of Colrec phase was characterized at 65 °C, probably of P2/a symmetry. More recently, rod-like mesogenic units were used as well to obtain LC guanidinium salts (335-R, 336-X-R-n, and 337n).491,966 None of the salts with a cyanobiphenyl substituent (335-CN and 336-Cl-CN-10) is LC. Out of the 12 derivatives with a 4-(n-decyloxy)biphenyl substituent (335-OC10H21 and 336-X-OC10H21-n), only four show an enantiotropic SmA phase, and four show a monotropic SmA phase. The spacer needs to consist of at least six carbon atoms to obtain mesomorphic properties. Both the melting point and the clearing point of 336Cl-OC10H21-10 are lower than those of its linear analogue 335OC10H21. The chloride salts show poor thermal stability, but this could be improved by anion exchange. All of the chloride salts with a photoresponsive azobenzene unit (337-n) show a SmA phase with a high translational order parameter but a rather low orientational order parameter. We already mentioned in section 4.3.2 that low orientational order seems to be typical for classical amphiphilic ILCs without an extended rigid core, but this property seems to be general and also applicable to ionic

Further research efforts focused on a series of mesomorphic guanidinium salts bearing a phenyl (n-alkyloxy)benzoate moiety (331-X-(a−f), 332-X-n, 333-X-(a−h), and 334-X-n).964 The effect of the number of alkyl chains, their length, and the type of anion, as well as the symmetry and detailed structure of the rigid part on the mesophase behavior was investigated. The linear “acyclic” and “cyclic” chloride salts with a single long alkyl chain (331-Cl-a and 333-Cl-a) show a SmA phase, whereas no mesophase was observed for the corresponding triflate salts (331-OTf-a and 333-OTf-a). Attachment of a second long alkyl chain to 331-Cl-a and 333-Cl-a (to form 331-Cl-b and 333-Cl-b, respectively) resulted in the formation of a Colhex phase rather than a SmA phase, but, surprisingly, a SmA phase was induced for the triflate analogues 331-OTf-b and 333-OTf-b. Such evolution from columnar to lamellar mesomorphism through simple exchange of a halide anion for an [OTf]− ion is rather exceptional. All other reported compounds contain three long alkyl chains, and for the mesomorphic homologues a Colhex phase was found. Exchange of the chloride anion for a nonspherical triflate anion was detrimental for mesophase formation in case of the shorter alkyl chains or resulted in a significantly lower clearing point (i.e., a decreased mesophase stability), with the effect becoming less pronounced for CS


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Scheme 20. Isomerization Process of Spiropyran-Based Compound 341 upon Addition of Varying Amounts of p-Toluenesulfonic Acid968

10.5. Ionic Liquid Crystals Based on Spiro−Merocyanine Isomerization

mesogens that do contain an anisometric moiety (which is however decoupled from the ionic headgroups). Overall, nanosegregation clearly dominates the structure of mesophases shown by ILCs, unless the ionic parts are sufficiently “diluted” by neutral molecular fragments (see, for example, the formation of a nematic phase by compound 60441). The cis-to-trans reisomerization of the azobenzene-containing guanidinium salts was monitored in the ILC [C12mim][Br] or in water as an anisotropic and isotropic host, respectively, and it was found that the activation energies are unexpectedly very similar in both types of host. This was attributed to a size mismatch between the guanidinium salts and [C12mim][Br], but also to the low orientational order in the SmA phase of the latter.

Mesomorphic behavior was induced in spiropyran-based compounds with a fan-shaped tris(n-alkyloxy)benzene group by Kato and co-workers.968,969 Spiropyran is known to isomerize to the (zwitter)ionic merocyanine form either chemically, by a temperature change, or by light irradiation (Scheme 20). By mixing a sulfonic acid ((343-n)−(348-X), Scheme 21) and a nonmesomorphic spiropyran compound (341 or 342-n) in equimolar quantities, the acid-induced formation of a Colhex phase was achieved. The formation of the protonated merocyanine form was proven by means of UV−vis spectroscopy. The mixtures with a sulfonic acid carrying a long alkyl chain (343-12, 345, and 347) were not LC, nor the mixtures with a benzoic acid or phenol derivative (349 and 350), which did not induce spiropyran−merocyanine isomerization (due to their weaker acidities). The LC properties of the merocyanine derivatives can be attributed to the planarity of the system allowing a close packing of the molecules, in contrast to the nonplanar spiro form. Additionally, nanosegregation of the long apolar alkyl chains and the hydrogen-bonded and ionic parts can be considered as one of the driving forces. Interestingly, mesophase stability was greatly enhanced by the introduction of an imidazolium-based sulfonic acid (348-X), with which the clearing temperatures could be increased by more than 100 °C as compared to the imidazolium-free analogues ((343-n)−347).

Scheme 21. Acidic Compounds That Were Tested by Kato and Co-workers for the Induction of a LC Phase in Mixtures with 341 or 342-n968,969

10.6. Ionic Liquid Crystals with Triazolium Cations

As mentioned before, the H(2) proton of an imidazolium cation is very much involved in hydrogen-bonding interactions with anions. Because of its acidity, imidazolium salts will be deprotonated in basic conditions, thereby forming a reactive carbene species. Mudring and co-workers studied the thermal phase behavior of 1,3-bis(n-alkyl)-1,2,3-triazolium salts (351-Xn/m), which lack this proton, to compare it with the LC properties of analogous 1,3-bis(n-alkyl)imidazolium compounds.729,970,971 They prepared both symmetric and asymmetric derivatives and investigated the influence of the anion. Nonmesomorphic 1,2,3-triazolium ILs had been reported before.972 In general, the 1,3-dialkyltriazolium salts show higher melting points than their anhydrous 1,3-dialkylimidazolium counterparts, but usually also a higher clearing point. In some instances, a triazolium salt is LC but its imidazolium analogue is not. For example, 351-PF6-1/12, 351-SbF6-12/12, 351-Br-2/ 12, and 351-Br-6/12 show smectic phases, but [C12mim][PF6], [C12C12im][SbF6], [C12C2im][Br], and [C12C6im][Br], respecCT


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tively, do not.170,548,562 Compound 351-N(CN)2-12/12 is, to the best of our knowledge, the first example of a LC [N(CN)2]− salt; [C12C12im][N(CN)2] is not mesomorphic.548 In other instances, the triazolium salt shows both an ordered SmX phase and a SmA phase, whereas only a SmA phase is found for the corresponding imidazolium salt. This is the case for 351-X-1/12 (X− = Cl−, Br−) and 351-X-12/12 (X− = I−, [BF4]−) versus [C12mim][X] (X− = Cl−, Br−) and [C12C12im][X] (X− = I−, [BF4]−), respectively.280,359,548 Interestingly, while [C12C12im][NTf2] does not show any enantiotropic LC phase (Table S2),553,563 351-NTf2-12/12 seems to show an enantiotropic mesophase just above room temperature. The single-crystal structure of 351-Br-12/12 shows that the triazolium cations adopt a U-shaped conformation. In the crystal packing of the nonmesomorphic salt 351-I3-12/12, on the other hand, the ndodecyl chains are seen to stretch outward into opposite directions along the triazolium core plane, parallel to the linear triiodide anions.729 This was observed previously for the nonmesomorphic imidazolium salt [C12C12im][I3] and for the imidazolium ILCs [C12C12im][I] and 19-14.548,558

The Maeda group reported on protonated 3,5-dipyrrolylpyrazole derivatives (354-R3 and 355) that form planar [2+2]-type ion-pair complexes with bridging [CF3COO]− anions through hydrogen bonding (Figure S21).64,974 Compounds 354-R3 (R3 = H or F) show thermotropic columnar and cubic mesomorphism, and form supramolecular gels, as well as 2D patterns on a HOPG substrate. The hole mobility of 354-R3 was measured in the solid state at room temperature by FP-TRMC, and it is higher than that of their nonprotonated precursors. The mobility in the mesophases could not be measured because the ionic complexes are susceptible to partial loss of trifluoroacetic acid upon heating.

10.8. Ionic Liquid Crystals with 1,10-Phenanthrolinium Cations

10.7. Ionic Liquid Crystals with Pyrazolium Moieties

Lafuente, Giménez, and co-workers prepared luminescent boomerang-shaped pyrazolo[1,2-a]-4-pyrazolium derivatives (352-R).409,973 These could be synthesized by a phase-transfer reaction between the neutral pyrazole precursors with 1,3dichloropropane in the presence of K2CO3. Lamellar mesophases (SmB and SmA) were observed for 352-H (R = H), whereas 352-OC10H21 (R = OC10H21) displays a Colhex phase with intracolumnar order. Stable Langmuir and Langmuir− Blodgett films could be obtained for both compounds, which allowed the study of their supramolecular organization at the air−liquid interface. The architecture of the films under compression could be correlated to the specific LC structures shown by the separate compounds. For example, salt 352OC10H21 has a tendency to organize into supramolecular columns both in the bulk LC state and in the films.

1,10-Phenanthroline is well-known for its ability to form stable complexes with many metal ions and has been, as such, incorporated into a variety of LC compounds.975−977 However, quaternization of the nitrogen atoms opens the way toward a novel type of ILCs based on 1,10-phenanthroline, as was demonstrated by Binnemans and co-workers (356-X-R1/R2 and 357-X).161 Traditionally one or more alkyl chains attached to the cationic core provide the fluidity necessary for the formation of LC phases. In this case, however, flexibility is ensured by alkyl chains present in the anions (see previous sections for other examples, such as 317-0/n). For the dodecylsulfate salts 356C12H25OSO3-R1/H, highly ordered crystal smectic E phases were observed, whereas the dioctyl sulfosuccinate salts 356DOSS-R1/H and 357-DOSS display less ordered SmA phases. The larger cross-sectional area occupied by the branched dioctyl sulfosuccinate anions in comparison to the dodecylsulfate anions does not allow strict 2D intralayer ordering needed for an E phase. An increase of the planarity of the system, as for compounds 357-X, results in a decrease of mesophase stability or no mesophase formation at all. The degree of planarity also influences the fluorescence properties: a blue-shift occurs with increasing deviation from planarity.

Sánchez et al. investigated the thermal phase behavior of a large number of asymmetrically shaped pyrazolium-based ILCs (353-X-n).113 All of the mesomorphic salts exhibit a SmA phase. Interestingly, all [SbF6]− and [ReO4]− homologues are LC, whereas none of the [OTf]− or [OTs]− salts is. Single-crystal structures suggested that this is due to the CF3 and p-(CH3)(C6H4) groups of those anions, respectively, that extend laterally from the hydrogen-bonded dimeric structure formed by each pair of ionic headgroups and disrupt the formation of stable smectic layers. The neutral pyrazole precursors are not LC either. CU


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Columnar so-called “charge-by-charge assemblies” (consisting of alternately stacked positively and negatively charged disk-like species, Figure 63a985) of a quite unique planar, π-conjugated, negatively charged receptor−anion complex (361-a-H·Cl− (R1 = R2 = n-C16H33; R3 = H)) and planar 4,8,12-trialkyl-4,8,12triazatriangulenium (TATA) cations (369-n) were designed by the group of Maeda (Scheme 22).391,986,987 A xerogel obtained from n-octane and composed of 361-a-H·Cl− and 369-3 (after mixing the neutral dipyrrolyldiketone-BF2 anion receptor 361-aH with [369-n][Cl]) shows a Colhex phase, whereas the longchain analogue (361-a-H·Cl− in combination with 369-16) exhibits a lamello-columnar (LCol) structure. In these systems, both the anisometric cation and the anisometric anion, which mainly interact via electrostatic and π−π interactions, play a prominent role in the self-organization process. The neutral anion receptor 361-a-H was also reported to be LC,988 so for this particular system charges are not required to induce mesomorphism. The Colhex phase of 361-a-H·Cl−/369-3 could be aligned by shearing, and it could even be switched by application of an electric field. In a follow-up paper, the influence of substitution of the two pyrrole rings in 361-a-H was investigated.990 Pyrrole β-methyl and β-fluorine substituents (361-a-CH3·Cl−/369-3 and 361-a-F·Cl−/369-3, respectively) appeared to interfere with the formation of stable “charge-bycharge” stacking columns, and to induce a higher contribution of “charge-segregated assemblies” (Figure 63a) in the Colhex phase. In the latter type of assemblies, stacking of the cationic planes (369-3) occurs instead of the alternate stacking of positively and negatively charged species; the receptor−anion complexes (361a-CH3·Cl− and 361-a-F·Cl−), whose β-substituents prevent efficient stacking with the TATA species, are probably located in the grooves of these cationic stacks (Figure 63b). This finding was inferred from a specific WAXD signal and was partly supported by single-crystal structures of nonmesomorphic analogues of 361-a-CH3·Cl−/369-3 and 361-a-F·Cl−/369-3 without n-alkyloxy substituents (Figure 64). Given the proposed helical arrangement in the Colhex phases, one could think of chiral mesophases as well, although this was not mentioned by the authors. It was noted that even for 361-a-H·Cl−/369-3, there is a contribution of “charge-segregated assemblies”, but to a lesser extent. The effect of the introduction of fluorine substituents is remarkable, because they are only slightly larger than hydrogen atoms; this suggests some electronic effect apart from the steric change (possibly stacking interactions between the receptor− anion complexes are facilitated thanks to inductive electron withdrawal by the fluorine substituents). “Charge-segregated assemblies” comprising stacked planar cations are probably also formed by compounds 358-C 12 H 25 SO 3 -H and 358C12H25PhSO3-H,978 360-(a-b),984 and 377-(a-b)998 (see below for the latter), but in these materials the location of the nonplanar/nondiscotic anions is much less defined, and the properties are mainly determined by the cationic species (see also section 11 for the columnar phase structure of 411). Time-offlight (TOF) photoconductivity measurements shed light on the influence of the “charge-segregated” nature of the Colhex phases of 361-a-CH3·Cl−/369-3 and 361-a-F·Cl−/369-3 on the rather high charge carrier drift mobility (10−2−10−3 cm2 V−1 s−1, without special purification procedures, and for samples that were not macroscopically aligned). It was suggested that the higher potentials resulting from the charged stacks are beneficial for fast, long-range charge transport. The “charge-segregated” LC mesophase architecture seems very interesting to achieve ambipolar charge mobility (i.e., transport of both electrons and

10.9. Ionic Liquid Crystals Containing Positively Charged Polycyclic Aromatic Hydrocarbons (PAHs)

Müllen and co-workers investigated several positively charged polycyclic aromatic hydrocarbons (PAHs), which have the potential of forming 1D channels for charge transport, as components for ILCs. PAHs 358-C12H25SO3-H and 358C12H25PhSO3-H, obtained by photochemical cyclodehydrogenation (photobis-cyclization) of a tetraphenylsubstituted pyridinium cation to form a 9-phenyl-benzo[1,2]quinolizino[3,4,5,6-fed]phenanthridinylium (PQP) cation, show columnar phases.978 The [BF4]−, benzenesulfonate, and [C8H17SO3]− salts are not mesomorphic. This means that a certain amount of fluidity, in the form of a sufficiently long alkyl chain, is needed to induce mesomorphism. Campagna, Lainé, and co-workers investigated the influence of pericondensation of branched pyridinium cations on their electrochemical, electronic, and photophysical properties; only the latter two properties are significantly affected by the cyclization.979 Besides the PQP core, the same groups also presented other “expanded pyridinium” compounds.979−981 Surprisingly, attachment of an n-tetradecyl chain to the aromatic PQP core (358-CnH2n+1SO3-C14H29 and 359) gave rise to lamellar solid-state structures rather than columnar phases.982,983 In this case, not the π−π stacking interactions, but the segregation of the incompatible parts (i.e., the ionic aromatic core and the hydrophobic aliphatic chains) dominates the self-assembly process. X-ray diffraction studies of compounds 358-CnH2n+1SO3-C14H29 showed that the layer spacing d is increasing upon elongation of the anionic alkyl chain from n = 6 to n = 12. Apart from the supramolecular organization in the bulk, these compounds form nanostructured aggregates in methanolic solution. The nanobelts thus obtained are more wrinkled, and thus less structured, for compound 358C12H25SO3-C14H29 than for compound 359.

The Müllen group also investigated the LC self-assembly of benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (BNAX) bromide substituted with more than one long alkyl chain (360-(a-b)).984 Both 360-a (R1 = H, R2 = n-C12H25) and 360-b (R1 = R2 = n-C12H25) exhibit a Colhex phase over a very broad temperature range. Three molecules self-assemble into a disk-like structure, which further organizes into a Colhex phase. The analogue with only one n-dodecyl chain is not mesomorphic, but forms aggregates in methanol.

CV


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Figure 63. (a) Schematic representation of “charge-by-charge assemblies” (left) and “charge-segregated assemblies” (right), and one of many possible intermediate states (middle). Reprinted in part with permission from ref 990. Copyright 2012 Wiley. (b) Schematic representation of the structure of the Colhex phases formed by 361-a-CH3·Cl−/369-3 and 361-a-F·Cl−/369-3. The stacked TATA cations (369-3) are depicted as blue disks, whereas the receptor−anion complexes (361-a-R3·Cl−) are represented as purple-green discotic-like structures. Reprinted in part with permission from ref 990. Copyright 2012 Wiley. (c) Schematic representation of the structure of the Colhex phase formed by 361-a-H·Cl−/[N4,4,4,4]+. The tetraalkylammonium cations are depicted as blue tetrahedrons. Reprinted in part with permission from ref 991. Copyright 2012 Wiley.

The Maeda group also investigated other β-substitution patterns for the π-conjugated receptor−anion complexes (362· Cl−, 363·Cl−, and 364·Cl−).420 β-Aryl and β-(n-alkyloxy) substituents appeared to be unsuitable for the induction of mesomorphism in combination with a planar TATA cation, probably because of less efficient stacking of the different planar components. A Colsqu phase was however found for β-benzofused 362·Cl− in combination with 369-3, in contrast to the Colhex phase previously observed for 361-a-H·Cl−/369-3 (see above). In the Colsqu phase, the anions are located in the interior of the supramolecular “charge-by-charge” columns consisting of tetrameric assemblies of 362·Cl−/369-3 complexes. The neutral anion receptors are LC too (362, lamellar structure during first heating, Colhex phase afterward, based on N−H···F hydrogenbonded dimers; 363, Colhex during first heating, Colrec afterward; 364, lamellar structure). 363 and 364 form J aggregates in the condensed state, whereas H aggregates were found for 362 in the solid state. Modification of the boron center was explored as well as a way to influence the phase behavior of the receptor−anion complexes. One boron-modified derivative of 361-a-H·Cl−/ 369-3, complex 365·Cl−/369-3, was found to show two slightly different Colrec phases consisting of “charge-by-charge” columns.992 The central catechol moiety thus induced a transition from a hexagonal (361-a-H·Cl−/369-3) toward a rectangular (365·Cl−/369-3) mesophase symmetry. Derivative 366·Cl−/ 369-3 is not LC, presumably because of the high steric demands imposed by the two phenyl groups attached to the boron center. It shows crystal-like phases with a columnar structure. The boron substituents resulted in moderate changes in the absorption and emission wavelengths of the neutral anion receptors in dichloromethane solution, and the catechol-boron substitution significantly decreased the fluorescence quantum yield from 0.98 for an analogue of 361-a-H with methoxy groups to only 0.02 for a similar analogue of 365. Compound 365 is also LC: it shows a Colhex phase just like 361-a-H. It also forms a supramolecular noctane xerogel, just like its ionic complex 365·Cl−/369-3. Suitable boron substituents may be used in the future to obtain “charge-segregated assemblies” instead of “charge-by-charge” stacking structures. Another report discusses the mesophase behavior of the planar receptor−anion complex 361-a-H·Cl− in combination with

holes), because the segregated cationic and anionic stacks can be considered as electron acceptor moieties and electron donor moieties, respectively, and these can act as stable (and, thanks to the LC nature, potentially “self-repairing”, boundary-free) pathways for charge transport (provided the charged nature of the system does not pose a problem). This concept is related to work by Hayashi et al., who reported on neutral columnar LCs in which an electron-rich phthalocyanine moiety and an electronaccepting fullerene moiety are covalently linked; these materials form segregated donor−acceptor columns in the LC state and were found to exhibit highly efficient ambipolar charge transport.999 More recently, formation of a LC columnar mesophase with a high degree of “charge-segregation” was also observed for ion pairs of 361-a-H·Cl− and a planar, cationic terpyridine platinum(II) complex (374).996 Attractive PtII···PtII interactions help to stabilize the segregated stacks of cationic moieties. Pyrrole β-substituents do not seem to be necessary in this case. Replacement of the peripheral alkyl chains in 361-a-H·Cl−/ 369-3 and 361-b-H·Cl−/369-3 (for the latter: R1 = R2 = nC12H25, R3 = H) by semifluorinated chains (giving 361-c-H·Cl−/ 369-3, with R1 = R2 = (CH2)6(CF2)5CF3 and R3 = H) resulted in a stabilization of the Colhex phase to higher temperatures.997 The anion-free receptor 361-c-H is also LC, showing an ordered and disordered Colhex phase at different temperatures, respectively. Derivatives with chiral peripheral chains (361-(d−f)-H·Cl−/ 369-3, where R1 and/or R2 = (S)-3,7-dimethyloctyl) do not show organized LC mesophases (probably because the branched chains are too bulky for an efficient packing), but their chiroptical properties in solution were investigated.995 It was found that, at low temperatures, the ion-free solution-state assemblies of 361(d−f)-H show opposite signs of both circular dichroism and circularly polarized luminescence as compared to solution-state assemblies of the ionic complexes 361-(d−f)-H·Cl−/369-3. The asymmetrically substituted, corannulene-fused anion receptor 368 shows a higher intrinsic charge carrier mobility in its mesophase than 367, which lacks the corannulene moiety and thus any electron-conductive pathways along those stacked bowlshaped π-conjugated units.994 Both receptors show a LC phase with a lamellar structure, and 368 also shows a Colrec and Colhex phase. Ion pair 367·Cl−/369-3 is also LC, but 368·Cl−/369-3 is not. CW


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Scheme 22. ILCs Based on Receptor−Anion Complexes, Investigated by Maeda and Co-workers64,391,420,739,985−997

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361-a-H·Cl−/[N12,12,1,1]+ (Cr · 36 · Colhex · 70 · I (°C)) and 361a-H·Cl−/[N8,8,8,1]+ (Cr · 35 · I · (°C)), despite the same number of carbon atoms in the countercations. The Colhex phases consist of hexagonally ordered columns in which each column slice is built up by about four pairs of 361-a-H·Cl− and [Nn,m,x,y]+, and consecutive column slices can be considered to be alternately stacked “charge-by-charge assemblies” (Figure 63c). The Maeda group also published related work on LC receptor−anion complexes consisting of a planar dipyrrolyldiketone BF2 compound and a tris(n-alkyloxy)benzoate anion, with a [N4,4,4,4]+ counterion (370-n-R).989 These compounds exhibit extremely ordered lamellar structures, but the exact mesophase structure is not clear. Compounds related to the series 361-a-H·Cl−/[Nn,m,x,y]+ and 370-n-R were reported as well (371-R1/R2, 372-R1/R2, and 373R1/R2).993 Just like in the case of 370-n-R, the anion receptor moiety does not contain any long alkyl chains in these ionic complexes. As previously observed for 361-a-F·Cl−/369-3, the compounds with pyrrole β-fluorine substituents (371-F/R2, 372-F/R2, and 373-F/R2) show columnar phases with a contribution from “charge-segregated assemblies”. Predominantly “charge-by-charge” stacking was observed for the derivatives without fluorine substituents (371-H/R2, 372-H/ R2, and 373-H/R2). Interestingly, FP-TRMC measurements revealed that the “charge-segregated assemblies” of 371-F/H show a substantially higher charge carrier mobility (0.22 cm2 V−1 s−1) than the “charge-by-charge” stacks of 372-H/Ph (0.05 cm2 V−1 s−1). This was attributed to the smaller stacking distances in the former and consequently more favorable overlap of πconjugated moieties. TOF measurements should point out whether the Colhex phase of 371-F/H also exhibits ambipolar charge transport, as was observed for 361-a-F·Cl−/369-3 (see above).990

Figure 64. Single-crystal structures of (a) the analogue of 361-a-H·Cl−/ 369-3 without n-hexadecyloxy chains, (b) the analogue of 361-a-CH3· Cl−/369-3 without n-hexadecyloxy chains, and (c) the analogue of 361a-F·Cl−/369-3 without n-hexadecyloxy chains. Reprinted with permission from ref 990. Copyright 2012 Wiley.

bulky, nonplanar tetraalkylammonium countercations [Nn,m,x,y]+ carrying at least one long alkyl chain.991 Non-mesomorphic analogues of complexes 361-a-H·Cl−/[N4,4,4,4]+, 361-a-CH3· Cl−/[N3,3,3,3]+, and 361-a-F·Cl−/[N4,4,4,4]+ (without -OC16H33 chains) were previously found to show “charge-by-charge assemblies” in the solid state.1000,1001 The nonhygroscopic, low-melting complexes 361-a-H·Cl−/[Nn,m,x,y]+ show a Colhex phase (of slightly lower stability than that shown by 361-a-H· Cl−/369-3), except for the compounds with a very bulky tetraalkylammonium cation carrying three or four long (≥C8) alkyl chains, even though these alkyl chains are still much shorter than those in 361-a-H. The LC derivatives are yet other examples of rather uncommon “anion-driven mesogenicity” (see also section 8.5).888 One should note the different phase behavior of CY


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All ionic structures from the Maeda group that were discussed thus far contained anionic receptor−anion complexes. Cationic π-conjugated receptor−anion complexes (375 and 376-Z) have also been used to prepare ILCs.739 In the chloride salts of those singly positively charged species ([375][Cl] and [376-Z][Cl]), one of the chloride ions is complexed by the pyrimidine or phenylene bridge and the imidazolium moieties of the dicationic bis(imidazolium)-based receptor molecules, whereas the other chloride ion is still “free” and acts as the counterion of the complex between the dicationic receptor and the first chloride ion. [375][Cl] and [376-Z][Cl] form a Colhex phase and lamellar LC phases, respectively. The “free” chloride anion could subsequently be captured by pyrrole-based, neutral anion receptor 361-a-H, to form LC complexes 361-a-H·Cl−/375 and 361-a-H·Cl−/376-Z. These form an unidentified mesophase and Colhex phases, respectively. The successful combination of the planar cationic receptor−chloride complexes and planar anionic receptor−chloride complexes to form organized “chargeby-charge” mesophases partially relies on their similar core geometry (i.e., a central six-membered ring, and two neighboring five-membered rings in each of the receptors). Simonsen et al. reported on the columnar self-assembly of planar carbenium salts with two n-decyl chains (377-(a-b)) in Langmuir−Blodgett films.998 They did not investigate the thermotropic phase behavior of these compounds, but it can be expected that analogues with a higher number of long alkyl chains exhibit a columnar LC phase. A related compound with four n-decyl chains (377-c) could be macroscopically aligned in spin-cast thin films on PTFE substrates.1002

that have been specifically designed in the context of ISA. Most of these are collected in Schemes 23 and 24. All examples concern ionic complexes of multivalent, “functional” anions or cations with oppositely charged surfactant or “mesophase-inducing” molecules. The only exceptions are salts 381-(399-n), which contain the monovalent dye ethyl orange as the anionic component. However, these complexes were reported specifically in the context of ISA by Faul and co-workers,156 and are therefore also treated here. We would like to emphasize that many compounds that have been discussed in previous sections could also be termed “ISA compounds”, in the sense that a charged functional component is converted into a LC material by combining it with a suitable, oppositely charged building block. This is, for example, the case for the [Eu(tta)4]−,122 [UO2Br4]2−,115,116 [ZnCl4]2−,869,887 [CuCl4]2−,869 [Ag(CN)2]− and [Au(CN)2]−,110−112 [Co(NCS)4]2−,612 [Mo6Cl14]2−,622 [LnBr6]3− (Ln = Eu3+, Tb3+, Dy 3+ ), 118−120 [La(NO 3 ) 6 ] 3− , 117 [Dy(NCS) 8 ] 5− , 627 [Zn(mnt)2]2−,438 and [Ni(mnt)2]−870 metal complexes; redox-active ionic 4,4′-bipyridinium complexes with sulfonate surfactant molecules;162,904,905 luminescent LC 1,10-phenanthrolinium compounds with a central propylene or ethylene bridge and with sulfate and sulfonate counterions;161 and the thermotropic ionic LC receptor−anion complexes of the Maeda group.391,420,985,989−993 A possible cooperative binding mechanism, which should be typical for ISA (see section 4.1; particularly in cases involving polyelectrolytes), was however not investigated in these reports, but one could argue that this can also be said about many reports focused specifically on socalled “ISA materials”. Therefore, the treatment of the latter compounds in a separate section can appear somewhat arbitrary. Despite the usefulness of the ISA approach in the area of LCs, one must be very cautious not to overuse the term “ionic selfassembly” as a synonym for “LC salt formation”. It should also be noted that ISA goes beyond the field of LCs, because LC mesophase formation is only one of several possible modes of self-assembly. In many papers dealing with ISA compounds, it is not entirely clear whether the materials are genuinely LC, or, for example, just ordered in the solid state but not soft and flowing. A full characterization of mesomorphic compounds should include at least POM, DSC, and (small-angle and wide-angle) PXRD investigations. We will start this discussion with the mesomorphic azobenzene-containing dyes 381-(399-n) that were mentioned above. Faul and co-workers did an extensive study on the

Sessler and co-workers synthesized supramolecular liquid crystals based on cyclo[8]pyrrole (378, 379, and 380 in combination with electron-deficient 2,4,7-trinitrofluorenone (TNF), 1,3,5-trinitrobenzene (TNB), or 2,4,6-trinitrophenol (TNP)).1003 These donor−acceptor systems contain hydrogenbonded sulfate anions in the center of the aromatic expanded porphyrin framework. The importance of the charged nature of the oligopyrrole cores for the LC behavior is not clear; the sulfate anion is included because of the specific synthetic methodology toward cyclo[8]pyrroles. Pure 378, 379, and 380 are not mesomorphic due to an inefficient face-centered interaction between the electron-rich cyclo[8]pyrrole cores. The lack of liquid-crystallinity for 378/TNB as compared to 378/TNF emphasizes the importance of the choice of the electron-deficient partner. The possibility of inducing liquid-crystallinity in thin films of 379 and 380 upon exposure to vapors of 2,4,6trinitrotoluene (TNT) makes the cyclo[8]pyrroles of interest as potential explosive-sensing materials.

11. IONIC LIQUID CRYSTALS FORMED BY “IONIC SELF-ASSEMBLY” (ISA) The concept of “ionic self-assembly” (ISA) was introduced in section 4.1. Here, we will discuss recently reported LC materials CZ


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Scheme 23. Components of LC ISA Compounds with Functional Anions, That Have Been Reported since 2005 (See Scheme 24 for LC ISA compounds with polyoxometalate anions.)121,156−160,442,444,1004−1010,a

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a

Superscript “i” denotes inner ligands; superscript “a” denotes apical ligands (Figure 66).

photoalignment of these ionic complexes.156,157 Combination of ethyl orange and quaternary ammonium salts resulted in the formation of a Colrec phase followed by a bilayer SmA2 phase and a low-ordered SmA phase at high temperatures for 381-(39912). In the Colrec phase, the ionic core is surrounded by the hydrophobic chains of the ammonium cations and the azobenzene moieties. The latter are not randomly distributed within the aliphatic continuum, but rather associate to form stacked aggregates (Figure 65). The transition of the columnar phase to the SmA-type phase is accompanied by stretched-tobent-shaped conformational changes of the alkyl chains. Dissociation of the aggregated azobenzene units takes place at the SmA2-to-SmA transition. For 381-(399-16), a Colsqu phase and columnar nematic phase were found. Upon irradiation of thin films of these complexes with linearly polarized light of an Ar+ laser (488 nm), photoalignment occurred yielding high dichroic ratios up to 50 (note that the dichroic ratio of LC polymers typically varies between 2 and 10). The reorientation process was investigated in detail.157 In the LC state, small domains are formed by molecules that are aligned in the same direction. During irradiation with polarized light, macroscopic alignment of these domains takes place due to photoinduced, cooperative reorientation of the azobenzene chromophores (see also the discussions above about ILCs 60, PPI-G3/(210)y(212R1/R1)16−y, and PAMAM-G2/(210)16). According to the authors, such a cooperative process is in contrast to the localized alignment process observed for polymers. The dichroic ratio in LC polymers is restricted by the internal order, whereas for low molar mass LCs the size of the domains is the determining factor. Ziessel’s group combined a negatively charged luminescent anthracene core (382) with ammonium (399-n and 400-n) and imidazolium cations (403-n).159 While the ammonium salts 382-

Figure 65. Schematic representation of (a) the azobenzene complex 381-(399-12); (b) its molecular packing within the SmA2 phase; and (c) its molecular packing within the Colrec phase. Reprinted with permission from ref 157 (http://dx.doi.org/10.1103/PhysRevE.75.031703). Copyright 2007 American Physical Society.

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of benzene-1,2,3,4,5,6-hexacarboxylate with [N12,12,1,1]+ cations.155 For 410 in a PS matrix, moderate dichroic ratios (∼1.48, for Mw(PS) = 115 000 g mol−1) could be obtained while maintaining the oblique columnar ordering of the mesophase. The alignment quality was highly dependent on the molecular weight of the polymer: low-molecular-weight PS resulted merely in a dilution of the ionic complex, and high-molecular-weight PS was not suitable due to its high viscosity. The use of chiral, enantiomerically pure phosphate counterions in combination with the perylenediimide dication (411) produced a helical supramolecular structure.1012 CD spectra supported the formation of aggregates with their transition dipoles oriented in a (left-handed) helical fashion. The authors proposed a helical self-assembly of the molecules in the oblique columnar LC state as shown in Figure S22.

(399-n)2 and 382-(400-n)2 do not show mesomorphic behavior, all imidazolium salts 382-(403-n)2 form Colhex mesophases. The shortest-chain homologue 382-(403-8)2 exhibits a somewhat peculiar Colhex-to-Cr transition at 73 °C, indicating a metastable mesophase. The number of complexes within one supramolecular disk was estimated to be two. The authors proposed a molecular model in which the anthracene moieties are aggregated within the core of the column and are surrounded by the surfactants. The strong emission from the anthracene moiety is maintained in the LC phase, although broadening and a red-shift of the emission band can be seen in comparison to the luminescence spectra recorded in solution, which confirms the formation of aggregates (J aggregates). Other rigid luminescent dyes were also functionalized with sulfonate groups at their periphery (383 and 384). Mesomorphic materials were obtained by combination of the resulting 4,4difluoro-1,3,5,7,8-pentamethyl-2,6-disulfonato-4-bora-3a,4adiaza-s-indacene ([sulfoBODIPY] 2− ) and tetrakis(4sulfonatophenyl)porphyrin ([TPPS]4−) anions with ammonium and imidazolium cations (383-(400-n)2, 383-(403-n)2, 383(404)2, and 384-(400-n)4).158,160 In most cases, Colhex phases are formed at room temperature by the piling up of disks composed of either two or three [sulfoBODIPY]2− complexes or one [TPPS]4− complex. Especially the [sulfoBODIPY]2− compounds show interesting photophysical properties (see section 12.5). Again, a broadening of the emission band was observed in the solid state and LC state, attributed to the aggregation of [sulfoBODIPY]2− dianions. ISA complexes of the blue-absorbing [sulfoBODIPY]2− dianions with red-absorbing cationic distyryl-BODIPY moieties were reported as well (383-(407-n)2).1009 These 1:2 complexes also show a Colhex phase (in which one column slice is made up of a dimer of 383-(407-n)2 complexes), and the mesomorphic order can be frozen into a glassy state. The melting point was found to decrease with increasing length of the alkyl chains on the cationic parts. The oppositely charged BODIPY dyes 383 and 407-n also associate in THF solution. Because of the close proximity of the differently substituted BODIPY chromophores in the ISA complexes, these materials display efficient intracomplex electronic energy transfer from the red donor dyes 383 to the blue acceptor dyes 407-n, both in solution and in their Colhex mesophase (see also section 12.5). Luminescent tris(8-hydroxyquinoline-5-sulfonato)aluminum(III) complexes 385-(399-n)3 and 385-(400-n)3 were investigated by the same group.1004 The former form roomtemperature SmA phases, while the latter form room-temperature Colhex phases. Their photophysical properties were measured in solution as well as in thin films (see section 12.5). Faul and co-workers used chiral lysine-based surfactant cations and a perylenediimide dianion to prepare luminescent LC ionic complexes 387-(408)2 and 387-409.1007 Discotic lamellar ordering was proposed for both complexes on the basis of PXRD measurements. A perylenediimide dication, on the other hand, could be turned into LCs forming oblique columnar phases by combining it with phosphate anions containing two branched alkyl chains (410 and 411).1011,1012 ILC 410 could be macroscopically aligned within a poly(styrene) (PS) matrix. Alignment of ionic mesogens can be challenging because of their high charge density and high viscosity, although spontaneous alignment occurs in some cases. We mentioned before that simple ILCs exhibiting a SmA phase often spontaneously align homeotropically when cooled from the isotropic melt. Zakrevskyy et al. observed spontaneous alignment of a complex

By using similar phosphate anions, doped oligoaniline surfactant complexes (412 and 413) were obtained.1013 Interestingly, these complexes exhibit Colrec mesophases, whereas the polymer analogues order into highly ordered hexagonal lamellar assemblies. This can be ascribed to the inhibition of π−π interactions in the polymer due to its coil-like conformation. An intermediate situation is exhibited by the octamer, which shows lamellar and hexagonal structures. The small ionic complexes show a conductivity of 3 × 10−3 S cm−1 at room temperature.

Davis et al. proposed lamellar phases for calix[4]resorcinarenesurfactant complexes ((386-R)-(399-16)4 and (386-C11H23)(399-12)4) on the basis of PXRD measurements.1005 The ISA complexes show strong adsorption of ammonia, p-nitrophenol, and aromatic dyes. As such, these materials have the potential to act as chemochromic sensors. Faul et al. observed a change in tripeptide binding properties of the acid-rich diketopiperazine receptor 388 (containing two azobenzene chromophores) upon complexation with ammonium surfactants 399-n and 401-n (Figure S23).1006 This receptor is known to mark selectively arginine-rich peptides in aqueous solution, but addition of the ammonium surfactant led to a decoloring of the peptide beads. A new binding preference was found between the ISA receptors 388-(399-16)6 and 388(401-16)6 and histidine-rich polypeptides in chloroform. The DC


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= Br−, [NTf2]−) that have been discussed previously.122 Because the cluster anions 391, 389, and 390 have very similar dimensions but a different charge, the influence of these relative volume fractions on the phase behavior of their ISA complexes with the same cations could be probed directly. Only a SmA phase is observed for 389-(406)3 and 390-(406)4, but 391(406)2 shows a SmA phase and a nematic phase. Interestingly, 391-(405)2 and 392-(405)2 are not LC.121,442 The phase behavior of the series (397-n)-(406)2 is only marginally influenced by the length of the apical ligands in 397-n. Clustomesogens have also been obtained through covalent and supramolecular approaches.1014−1019 Another ILC with a central inorganic (but metal-free) cluster was developed by Tanaka et al.1010 They combined an octaanionic, polyhedral oligomeric silsesquioxane (POSS) core with eight [Cnmim]+ cations (398-(402-n)8). For n ≤ 16, roomtemperature ILs were obtained, but ionic complex 398-(40218)8 shows a smectic phase from 45 °C until at least 100 °C. The inorganic cluster apparently stabilizes the mesophase structure: 414-18, which mimics one of the “arms” of 398-(402-18)8, also shows a smectic phase but clears out at 83 °C. Complexes 398(402-n)8 also show lower melting points and higher decomposition temperatures than the corresponding salts 414-n.

complexed receptors exhibit a lamellar organization. Apart from the (001) and (002) reflections in the SAXS pattern, several other peaks were observed, which should be attributed to 2D intralayer ordering of the molecules. Molard and co-workers investigated the thermotropic mesomorphism of so-called “clustomesogens”, that is, hybrid LCs containing rigid transition metal clusters, in which the intrinsic properties of the latter (such as luminescence and magnetism) are retained.1014 In general, to obtain metal-containing thermotropic LC systems, metal atoms or ions are usually combined with mesogenic organic ligands, resulting in either neutral or charged metallomesogens. Nonmesomorphic metal complexes can also be mixed with a LC host. Thermotropic LCs containing metal complexes with merely inorganic ligands have only been achieved via the latter doping procedure, or by relying on electrostatic interactions to combine inorganic metal complex anions with mesogenic cations to obtain hybrid LC materials (previously discussed examples include 36-LnBr6, 303-n, 39-n, etc.). For their first report on ionic metallomesogens with [M6Xi8Xa6]n− anions (Figure 66), synthesized via the latter

Saramago and co-workers reinvestigated ionic complexes of the active pharmaceuticals lidocaine and ibuprofen with [DOSS]− anions and [N10,10,1,1]+ cations, respectively.1020 These had been previously reported as “third-generation” room-temperature viscous ILs, but the nature of a so-called “liquid−liquid transition” was not clear.1021 Temperaturedependent surface tension and contact-angle measurements revealed a peculiar surface behavior for these compounds, and the authors tentatively attributed this to the presence of an ordered mesophase below the isotropic liquid phase, although POM observations did not give conclusive evidence for a LC phase. The ISA process has been applied with great success to form mesomorphic surfactant-encapsulated polyoxometalate (SEP) complexes (Scheme 24). Multivalent polyoxometalate anions (Figure 67) are of great interest because of their potential luminescent, photochromic, electrochromic, redox, magnetic, catalytic, etc., properties. Their solubility in water and/or organic solvents enables combination with mesogenic counterions, to improve their processability and to reinforce their functionality. The groups of Wu and Faul in particular have been very productive in the field of self-organizing SEP complexes,2,144,147,1022−1024 and Floquet, Terazzi, and co-workers and Wang and co-workers have recently joined their efforts (see below).136−138,1025−1029 Wu and co-workers observed thermotropic LC behavior for europium(III)- and terbium(III)-based polyoxometalates complexed with azobenzene-functionalized ammonium surfactants (428-(448)9, 431-(448)11, and 436-(448)13).126,127 Highly ordered smectic phases were observed, as the elliptical shape of the polyoxometalates favors layered structures. The transition temperatures for the first two transitions (Cr−SmX1 and SmX1− SmX2) are comparable for all three complexes. The phase transitions could be ascribed to conformational changes in the

Figure 66. General structure of [M6Xi8Xa6]n− anions containing an octahedral metal cluster (M = Mo, W, Re; Xi = inner ligand = chalcogen or halogen; Xa = apical ligand = halogen or CN; 2 ≤ n ≤ 4), shown here for M = Mo and n = 2. Reproduced with permission from ref 121 (http://dx.doi.org/10.1039/c5tc00632e). Copyright 2015 The Royal Society of Chemistry.

procedure, Molard et al. prepared [Re6Se8(CN)6]3− and [Re6Se8(CN)6]4− clusters surrounded by ammonium cations tethered with two cyanobiphenyl mesogenic groups (389-(405)3 and 390-(405)4).1008 Apart from lyotropic mesomorphism in chloroform, these compounds show a monotropic thermotropic mesophase, which is very unstable in the case of 390-(405)4. No nematic phase was observed, despite the fact that precursor [405][Br] shows a monotropic nematic mesophase.430 A related salt with three cyanobiphenyl groups, [406][Br], was found to form a nematic phase as well.442 Its cation was successfully combined with molybdenum clusters 391-(397-n) (all [Mo6Xi8Xa6]2− anions; see also the related [Cnmim]2[Mo6Cl14] salts (38-n) that were reported by Mudring and co-workers622) to induce nematic mesomorphism.121,442,444 Ionic complexes 392-(406)2, 393-(406)2, 394-(406)2, 395-(406)2, 396-(406)2, and (397-n)-(406)2 (n = 1−3) even show exclusively a nematic phase below their clearing point (i.e., no additional smectic phase is formed), which makes these highly luminescent materials of potential interest for use in optoelectronic devices (see section 12.5). The authors note that the nematic phases are quite viscous, however. The mesophases could be vitrified upon cooling. These results indicate the importance of a suitable balance between the volume fractions of the organic and inorganic/ionic parts of the hybrids to achieve a nematic phase. This is also obvious from the differences in phase behavior between 66-X and 84-X-11/11 (X− DD


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SmC-to-SmA and SmA-to-I transitions occur at very similar temperatures, since these phase transitions are influenced by the nature of the polyoxometalate. Replacement of the polyoxometalate resulted in a different phase behavior for SEP clusters 415-(449-8)3, 417-(449-8)3, and 420-(449-8)5. Complexes 415-(449-8)3 and 437-(449-8)15, with a planar and elliptical cluster, respectively, form SmA and SmC phases. The phase behavior of complexes 417-(449-8)3 and 420-(449-8)5, which contain a spherical cluster, appears to be dependent on the charge density of the polyoxometalate. As 417-(449-8)3 needs less surfactant molecules for charge compensation, the sizes of the inorganic cluster and the organic shell are comparable. In this case, the formation of a SmB phase is promoted. For complex 420-(449-8)5, SmC and SmA phases are observed (as well as an unidentified nonlamellar mesophase), because the sizes of the inorganic core and the organic coating are no longer comparable. A similar reasoning could be applied to compounds 416(446)3, 417-(446)3, 418-(446)4, and 420-(446)5, some of which show thermotropic LC behavior without the need for rigid mesogenic groups.131,1022 SEP complexes 417-(446)3 and 420(446)5 show a SmB phase. Because ammonium cation 446 contains only one long alkyl chain, the sizes of the inorganic cluster and the amphiphiles in 420-(446)5 are believed to be comparable, as opposed to the larger disubstituted amphiphiles used in SEP complex 420-(449-8)5 described above. The smectic layers are built up by polar sublayers, consisting of the polyoxometalates, the ammonium headgroups, and the tris(ethylene oxide) chains, and aliphatic sublayers. In the proposed model, the tris(ethylene oxide) chains are fully embedded in the ionic layer so that the lateral area of the hydrophilic domains is increased. To compensate for this cross-sectional enlargement, the aliphatic chains adopt a highly folded conformation. This could explain the small layer spacings found by PXRD measurements. Complexes 416-(446)3 and 418-(446)4 are not LC due to a subtle interplay between the presence of strong electrostatic interactions and packing efficiency. For example, the high packing efficiency and relatively strong electrostatic interactions in 418-(446)4 make that this compound decomposes before melting. Polyoxometalates 417 and 418 were also encapsulated with ammonium salt 450 to produce thermotropic LCs.133 Hydrogen bonding between the benzoic acid end groups of the ammonium cations was suggested to be one of the driving forces for mesophase formation; the dimer formed by two benzoic acid moieties can be regarded as a mesogenic unit (it should be noted that 450[Br] itself shows a SmC phase besides a SmA phase). From this perspective, larger mesogenic entities could be obtained by interaction of the SEP complexes with hydrogenbond-accepting pyridine moieties (453, 454-(a−d)). Depending on the type of pyridine derivative, SmA (418-(450/454-a (1:1))4 and 418-(450/454-c (1:1))4), SmC (418-(450/454-d (1:1))4) or SmB (418-(450/454-b (1:1))4 and 417-(450/454b (1:1))3) phases were observed. The mesophase stability increases with increasing alkyl chain length and number of alkyl chains in the pyridine component. The charge of the polyoxometalate anion has a major influence on the transition temperatures as well: these are higher for the compounds containing the more negatively charged polyoxometalate 418, as exemplified by the phase behavior of 417-(450/454-b (1:1))3 and 418-(450/454-b (1:1))4, respectively. Similar results were found for europium(III)-containing SEP clusters 428-(450)9, 431-(450)11, and 435-(450)13 without added pyridine derivatives.130 The use of larger, more asymmetric polyoxometalates

Figure 67. Sketches of the molecular structures of some polyoxometalate anions that have been used as building blocks for LC surfactant-encapsulated polyoxometalate (SEP) complexes. From left to right, top to bottom: [ZnH 6 Mo 6 O 24 ] 3− , [BW 12 O 40 ] 5− , [Na 2 [P 2 W 15 O 56 ]] 10− , [Eu(SiW 11 O 39 ) 2 ] 13− , [Mo 132 O 312 S 60 (SO 4 ) 23 (H 2 O) 86 ] 58− , [(Mo 4 O 4 S 4 (OH) 2 (H 2 O) 3 ) 2 (P 8 W 48 O 184 )] 36− , and [EuP5W30O110]12−.

aliphatic chains of the surfactants. FT-IR measurements confirmed the increasing accumulation of gauche conformers in the alkyl chains. Absorption bands originating from the azobenzene units also showed changes with temperature, but no clear conclusions could be drawn about their orientation. Interestingly, 436-(399-18)13 does not show mesomorphic behavior, indicating the importance of the rigid azobenzene moieties. Clearing temperatures increase with increasing polyoxometalate charge: 428-(448)9 < 431-(448)11 < 436(448)13. This implies that isotropization is dependent on the strength of the electrostatic interactions, which are the strongest for 436-(448)13. Lanthanide-free Keggin-type polyoxometalates 417 and 418 were also combined with 448 (417-(448)3 and 418(448)4).129 A spectroscopic study of the complexes showed that the SEP complexes synthesized from their H 3PW12O40, Na3PW12O40, and H4SiW12O40 precursors in acidic solution resulted in 50% protonation of the azobenzene groups for 417(448)3 and 25% for 418-(448)4. However, if 418-(448)4 was prepared starting from Na4SiW12O40, no protonation was observed, the reason for this still being unclear. It must be noted that in all complexes the cationic ammonium headgroups, rather than the protonated azobenzene groups, are binding to the polyoxometalate cluster. SmB phases were observed for protonated and deprotonated 417-(448)3 complexes regardless of the precursor. Protonation did have a marked effect on the phase behavior of 418-(448)4, though. SmC and SmA phases were observed for the protonated derivative, whereas the deprotonated complex shows a SmB phase. The Wu group also studied the influence of the structure of SEP complexes 415-(449-8)3, 417-(449-8)3, 420-(449-8)5, and 437-(449-n)15 on their thermal behavior.128 For the series 437(449-n)15 (n = 6, 8, 10, 12), a similar phase behavior was observed for all homologues (SmC and SmA phases) (Figure S24). Melting temperatures decrease gradually with increasing alkyl chain length, indicating that the melting process is governed by melting of the hydrophobic chains. At the same time, the DE


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the europium(III) spectrum is influenced by the type of surfactant. For the complexes with ferrocenyl cation surfactants, no photoluminescence was observed. Electrochromism, which could be observed for the other complexes, was also absent in complexes 433-(447-n)12. This was ascribed to a charge-transfer interaction between the polyoxometalate and the ferrocenyl group. Previously, SEP complexes containing [Eu(SiW9Mo2O39)2]13− (434) had been investigated by the same group.124 It was proposed that these complexes are formally in a thermotropic LC state at room temperature because the alkyl chains are not crystallized while the polyoxometalate anions form an organized structure as seen from SAXS measurements. However, the authors also mentioned that the materials are solids, and no characteristic textures nor “double-melting behavior” could be detected by POM or DSC. The materials are probably in an organized glassy state at room temperature. As compared to the precursor polyoxometalate salt K13[Eu(SiW9Mo2O39)2], fluorescence quantum yields increased upon complexation with ammonium surfactants 399-n and 401-n. The ferrocenyl-functionalized SEP complexes 434-(447-n)13, however, showed a drop in quantum yield. Floquet et al. reported on the thermotropic mesomorphism of the surfactant-encapsulated “keplerate” [Mo132O372(CH3COO)30(H2O)72]42− (440-(39918)36(NH4)6·75H2O).136 The latter cluster, which is actually a hollow sphere of about 3 nm diameter (“nanocapsule”), is more negatively charged than any of the polyoxometalates discussed above. It is capable of encapsulating substrates into its cavity (with the acetate moieties acting as internal ligands). Upon heating above 150 °C, water is lost and rehydration does not occur upon cooling. The SEP complex shows a lamellar mesophase over a very broad temperature range (from room temperature to >220 °C). Analysis of PXRD data led to the conclusion that the layers in the mesophase are composed of oblate clusters, that is, clusters with a nonuniform distribution of 399-18 cations around the rigid spherical polyoxometalate (Figure S25). The clusters are locally arranged in a hexagonal pattern. More recently, two related surfactant-encapsulated, sulfurated “keplerate” complexes ((441-23)-(399-18)44(NH4)14 and (441-30)-(399-18)56(NH4)16·15H2O) were described.137 These even more highly charged systems, with a higher number of 399-18 counterions, also form lamellar mesophases at room temperature. These are slightly less organized than the mesophase shown by 440-(399-18)36(NH4)6·75H2O; the clusters are not ordered within the smectic layers. The same groups also described LC SEP complexes based on the cyclic polyoxothiometalate 439, with “two-chain” ammonium counterions (399-18) or “single-chain” imidazolium counterions (445n, n = 12, 14, 16, 18, 20).138 They found high-viscosity SmA-type phases with highly ordered alkyl chains for these complexes, whereas similar systems with long-chain [N16,1,1,1]+ and [Cndmim]+ counterions are not LC. The interaction between the polar headgroups of the cationic surfactants and the negatively charged surface of the polyoxometalate anion was nicely demonstrated via multinuclear solid-state NMR spectroscopy. Wang and co-workers also used 399-18 as cationic surfactant to induce thermotropic LC properties for several types of polyoxometalates, including the Waugh-type polyoxoanion 423, the Silverton-type species 426, and the photosensitive polyoxometalate 421 (421-(399-18)5·28H2O, 423-(399-18)6· 16H2O, 424-(399-18)6·16H2O, 426-(399-18)8·9H2O, 427(399-18)8·8H2O, 432-(399-18)12·16H2O, and 438-(399-

seems to disturb the ordering within the layered packing so that less ordered smectic phases were observed as compared to 417(450)3 (SmB) and 418-(450)4 (SmX). A spectroscopic study revealed that the local environment of Eu3+ is strongly affected by the formation of LC phases as the transition from the amorphous powder to the LC state is accompanied by an increase in fluorescence quantum yield.

Lamellar and hexagonal columnar assemblies were observed for complexes 418-(451)4 and 420-(451)5, and 425-(451)7 and 429-(451)10, respectively.132 These contain ammonium cations with a dendritic shape. It is not entirely clear whether the assemblies are genuine LCs. The charge of the polyoxometalate anion and the choice of the dendritic surfactant are determining factors for the type of assembly that is formed, because these parameters directly affect the shape of the complexes: rod-like for 418-(451)4 and 420-(451)5, and disk-like for 425-(451)7 and 429-(451)10. The strong propensity of polarizable, rigid polyoxoanions for self-organizing in the bulk phase was further demonstrated by the Wu group for hybrids of nonmesogenic (isometrically shaped) tetrakis(n-alkyl)ammonium cations (443-n) and the sphere-like Keggin-type polyoxometalates 417, 418, 419, and 420 (417(443-8) 3 , 418-(443-n) 4 , 419-(443-8) 4 , and 420-(443− 8)5).135,140 Ionic complexes 418-(443-8)4, 418-(443-10)4, 419-(443-8)4, and 420-(443-8)5 show viscous lamellar or pseudolamellar mesophases thanks to nanosegregation and fluidization by the n-octyl or n-decyl chains, but complexes 417-(443-8)3, 418-(443-4)4, and 418-(443-6)4 are not LC. Interestingly, SEP complexes showing a rather viscous nematic phase at low temperatures were obtained by combining 420, 422, and 430 with an ammonium cation with a laterally attached mesogenic group (452).134 This is despite the fact that the bromide salt of 452, in which the ammonium headgroups are much less restricted in their motion, had previously been shown to exhibit a SmA phase (see section 6.3).436 Complex 430(452)10, with the highest number of surfactant cations, shows the highest clearing point in this series. The broadening and highfield shift of the NMR signals for the aromatic protons in 420(452)5 as compared to [452][Br] suggest that the aromatic rings are in close proximity to the metal cluster thanks to the lateral architecture. Such differences were not observed for systems in which the mesogenic groups are attached in an “end-on” fashion (see above). The magnetic properties of 422 and 430 were preserved in their SEP complexes. The photophysical and electrochemical properties of hybrids of the Preyssler-type polyoxometalate [EuP5W30O110]12− (433) and substituted ammonium-based (399-n, 401-n, 442, 443-4, 447-n) or viologen-based (444) surfactants were examined by Faul and co-workers.125 No unambiguous identification of the mesophases was possible on the basis of POM or PXRD. The authors proposed a lamellar ordering for complexes with N-(nalkyl)-substituted ammonium-based or viologen-based amphiphiles (399-n, 401-n, 442−444), and a hexagonal structure for the complexes with ferrocene-containing ammonium groups (447-n). UV−vis spectra of these SEP complexes showed that DF


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Scheme 24. Components of LC ISA Compounds with Polyoxometalate Anions, That Have Been Reported since 2005124−138,140,1025−1029

Its analogue with five pseudospherical [N7,7,7,7]+ cations is not LC. To end this section, we would like to mention that ionic biomacromolecules have also been used as building blocks for the creation of thermotropic (i.e., solvent-free) ionic LC assemblies. As mentioned in section 5.3, Cui and Zhu prepared complexes of negatively charged double-stranded DNA with imidazolium salts

18)16·36H2O).1025−1029 All of these SEP complexes show SmA and/or slightly more ordered smectic phases, with clearing points between about 100 and 150 °C. Their lamellar structure at room temperature could be visualized by TEM. Surface photovoltage spectroscopy showed that a photocurrent can be generated in 421-(399-18)5·28H2O upon illumination with simulated sunlight.1029 This compound has the character of a p-type material. DG


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with N,N-bis(n-alkyl)-N,N-dimethylammonium cations.168 These quasi-water-free materials exhibit smectic phases (in which they show non-Newtonian fluid behavior; see also the end of section 5.1), and above their clearing point they exist in an isotropic liquid state (with Newtonian behavior). In contrast, solvent-free solid ELPs that lack surfactant cations with flexible alkyl chains undergo thermal degradation upon heating rather than melting, due to the existence of very large persistent structures in the dry solid state. The ionic complexes show a very high elasticity in their nanosegregated LC state. The authors demonstrated that they can be elastically stretched without the need for cross-linking. The elastic modulus and viscosity could be controlled simply by adjusting the length of the polypeptide or the alkyl chain length of the countercations. In addition, green fluorescent protein (GFP) was fused to the ELPs, and it was shown that this model protein did not unfold upon complexation with the surfactant cations and that it did not disturb the mesophase formation. As such, these remarkable materials could be used as a general platform for the study of enzymes in the absence of water.

90-n (456-n), as well as with imidazolium compounds containing a rod-like cyanobiphenyl group (455) or a cube-like POSS group (457), respectively.164−166 Such nucleic acid complexes (lipoplexes, in formulations with a colipid) might be useful for nonviral gene delivery applications. Gene transfection efficiency is determined by multiple parameters, including the structure of the cationic moieties and neutral colipid(s), surface charge density (or cationic to helper lipid ratio), and so forth, but also the mesophase morphology of the lipoplexes.1030 The transition from a rod-like over a disk-like to a cube-like substituent on the imidazolium cation in the series 455−457 results in an increase of the cross-sectional area of the hydrophobic tail with respect to the hydrophilic cationic headgroup, and thus the development of curvature. For an identical spacer length, this induced the transition from a thermotropic SmA-type lamello-columnar phase for 455 (with parallel aligned columns formed by the DNA molecules, intercalated between bilayers of cyanobiphenyl moieties), over a double lamello-columnar phase for 456-10 (with both the DNA and the triphenylene moieties forming columns), to an inverted hexagonal columnar phase for 457 (with lamellar POSS crystals between the DNA columns). Figure S26 displays the proposed structures of the Colobl phase shown by 456-4, and the lamello-columnar phase shown by 456-7 and 456-10. The mesophase structures were visualized by TEM. FT-IR spectroscopy and PXRD suggested a 1:1 complexation of negatively charged phosphate groups and positively charged imidazolium parts. However, the DNA double helices could only be partly complexed as they rapidly precipitated from the reaction mixture (phosphate/imidazolium ratios ranging from 1.4 to 2.0 in the series 456-n). The triphenylene moieties in 456-n were found to stack on top of each other (π−π stacking); the face-to-face interdisk spacing of 3.5 Å is very similar to the period of hydrogen-bonded base pairs in DNA (∼3.4 Å). Interestingly, the columnar stacking of the triphenylene parts destroyed the helical conformation of the double-stranded DNA, as evidenced by CD spectroscopy. Replacement of the semirigid DNA molecules in 456-4 by flexible single-stranded total RNA resulted in the loss of columnar stacking of the triphenylene groups: a lamellar morphology with a random “discotic-nematic-type” packing of the triphenylene parts between neighboring RNA layers was observed. It should also be mentioned that Evans et al. reported in 2003 on the mesophase behavior of ionic complexes of DNA with positively charged PPI dendrimers.167

12. APPLICATIONS OF IONIC LIQUID CRYSTALS 12.1. Adaptive, Nanostructured, and Potentially Anisotropic Ion-Conductive Materials

Liquid crystal displays (LCDs) are the best known application of LCs.1031−1033 LCDs rely on the light-modulating properties of LCs and the possibility to switch LC molecules by external electric fields, which are based on their optical and dielectric anisotropy, respectively. It is unlikely that ILCs will be used in LCDs for switching purposes, because the strong electric fields in the LC cells in these devices can lead to charge separation or even to oxidation or reduction of the ILC components. However, ILCs are very promising candidates to design adaptive, nanostructured, anisotropic (“low-dimensional”) ion-conductive materials for molecular electronics, batteries, fuel cells, and capacitors.1034 In such materials, the constituting ions act as mobile charge carriers,1035,1036 and the ionic conductivity depends on the direction in which it is measured because of the anisotropic structural organization. The long alkyl chains that are typically present in mesomorphic molecules can act as insulating layers for the ion-conductive channels (similar “insulation” is provided by the alkyl chains in neutral, electronconductive LCs with large conjugated cores that form columnar mesophases, in which the aligned columnar stacks can be seen as “molecular wires”47,48,51−53). Depending on the type of mesophase, nanostructured materials for one-dimensional or for two-dimensional long-range ion conduction can be obtained, in contrast to electronic conduction via π−π-stacked structures, which can only happen efficiently in one direction.18,33 LCs showing a columnar mesophase can be used to create 1D ion conductors, with ion conduction taking place in the direction of the columnar axes (see also section 5.2, including Figure 22) (refs 49, 217, 221, 224, 390, 647−649, 655, 674, 745, 747, 847, 968, 1034, and 1037−1039). Appropriately designed smectic LCs can be considered as 2D ion conductors, with ion conduction occurring within the smectic ionic sublayers (for genuine ILCs consisting only of cations and anions) or the smectic oligo(ethylene oxide) sublayers (for LCs that contain oligo(ethylene oxide) moieties complexed with metal salts (usually lithium salts)) (refs 215, 216, 414, 441, 560, 581, 602, 607, 651−653, 686, 687, 747, 1034, and 1040−1048). [Remark: This situation can be different for smectic ILCs under nanoconfinement; see

Mann and co-workers reported the first example of a solventfree protein showing a stable liquid-crystalline and liquid state.646 For this purpose, they successfully combined cationized ferritin with an anionic polymeric surfactant. More recently, Clark, Herrmann, and co-workers designed thermotropic LC complexes of genetically engineered, highly charged elastin-like polypeptides (ELPs, with 9, 18, 36, 72, or 144 negative charges) DH


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below.] The field of organic, nanostructured liquid-crystalline electrolytes has recently been reviewed.1049 Strategies to obtain high ionic conductivity values in smectic,581,602,693 columnar,217,221,224,674 and bicontinuous cubic221,674 phases of ILCs, by an optimal choice of the cation and/or anion, by addition of an IL, and/or via a “noncovalent approach”, have been discussed in sections 5.1, 5.2, 5.3, and 6.2. [Remark: Recall that for practical use as electrolytes in devices, ionic conductivity values of at least 10−2−10−3 S cm−1 at room temperature are required. It should be noted that organic ionic plastic crystals are currently receiving a great deal of interest as highly conductive solid-state electrolytes,1050−1052 but these materials are outside the scope of this Review.] Phosphoniumbased ILCs 162-m/n were found to have a higher ionic conductivity than their ammonium-based counterparts 161BF4-m/n.674 This was explained by a weaker electrostatic interaction between the phosphonium headgroups and the [BF 4 ] − counterions, in accordance with the fact that phosphonium-based ILs display lower viscosities and higher ionic conductivities than their ammonium-based analogues (see section 6.2). These results are valuable in the design of future ionconducting materials. In a similar way, a rational choice of the anion is important. Higher dynamic ionic conductivities can be obtained with anions with a delocalized negative charge (such as [NTf2]−, [F(HF)2]−, etc.), which interact less strongly with the cationic parts.581,616,693,745 Clearly the knowledge that has accumulated in the field of ILs189,238,787,1036,1053−1057 can be used advantageously to optimize the ionic conductivity of new ILC materials. The advantages of using zwitterionic amphiphiles in combination with lithium salts or sources of mobile protons (e.g., organic acids) to achieve fast and selective lithium transport or proton transport, respectively, have been highlighted at the end of section 7. In mixtures of amphiphilic zwitterions with lithium salts, the latter “dissociate” to form preferential ion pairs with the oppositely charged moieties of the zwitterions.613,615,847,853,860 Soberats et al. recently studied LC mixtures of two imidazolium-based zwitterionic mesogens, containing different charge-delocalized anionic parts (458 and 459), with LiNTf2.616 [Remark: 458 is very similar to 241, which was studied as a component of proton-conducting materials (see section 7), and is also the taper-shaped analogue of smectogens 26-n and 27-n.] They also added propylene carbonate (PC) as a polar additive. Solutions of lithium salts in PC are typical liquid organic electrolytes, because carbonate derivatives have high dielectric constants for the dissociation of lithium salts and good electrochemical stability.1058 The authors showed via 7Li NMR and IR measurements that the lithium ions interact both with the sulfonate (in the case of 458) or dicyanoethenolate (in the case of 459) anionic moieties, and (through ion−dipole interactions) with the PC molecules. Addition of the latter results in significantly enhanced ionic conductivity, with a maximum of about 10−4 S cm−1 for a nonaligned Colhex sample of 458/LiNTf2 (50:50) + 10 wt % propylene carbonate at 100 °C (as compared to σ < 10−8 S cm−1 for pure 458 at 100 °C, and σ ≈ 10−5 S cm−1 for 458/LiNTf2 (50:50) at 100 °C). Pure 458 and 459 both form only a Colhex phase, but in the case of 459 the addition of at least 50 mol % LiNTf2 or the addition of 20 mol % LiNTf2 and 5−10 wt % PC causes sufficient changes in the relative volume fractions of the polar and apolar parts to induce the formation of a bicontinuous cubic phase (with 3D lithium ion transport). Earlier work on ion-conductive LCs was directed to the design of materials with high ionic conductivity, but no high anisotropy for ion conduction because of the difficulties in obtaining

oriented LC domains.686 Indeed, macroscopically aligned samples with large LC monodomains are required to get highly anisotropic ion conduction over reasonable distances in smectic and columnar phases, because the boundaries between randomly oriented domains are detrimental for the anisotropic ion conduction process. Ideally, the ordered molecular arrangements in the LC monodomains are stabilized to maintain the anisotropic ionic conductivity over a longer period of time. This can be achieved by in situ photopolymerization of aligned LC molecules with groups, as already discussed in sections 5.2 and 6.2. Cross-linking of suitable polymerizable LC monomers in their mesophase can yield nanostructured and thermally and mechanically stable membrane materials with permanent pathways for ion transport. Thermal polymerization has to be avoided, because the thermal agitation of the molecules can destroy the ordered mesophase. Photopolymerization can be performed at a suitable temperature within the mesophase stability range. In the period before 2005, Hubbard et al., as well as Kato and co-workers, had already reported on nanostructured LC polymers showing anisotropic ion conduction in two dimensions.2,18,1059 Hubbard et al. used a magnetic field (field strength 7.1 T) to align LC poly(ethylene oxide) derivatives doped with lithium triflate.1059 The conductivity in the direction parallel to the smectic layers was found to be 5 times higher than that in the direction perpendicular to these layers. The compounds from the Kato group were based on polymerizable smectogenic monomers containing either an ionic moiety651 or an oligo(ethylene oxide) moiety.652,653 In the case of the polymerized ionic monomers, the polymer films with a monodomain structure show in the SmA phase at 150 °C an ionic conductivity that is about 40 times higher than that of polymer films with a polydomain structure at the same temperature. This shows that the boundaries between randomly oriented domains disturb the ion conduction. Significant advancements were made in the past few years, with the preparation of ionic 1D ion-conductive49,224,390 (see section 5.2) and 3D lithium-ion-conductive673,782,1060 (see section 6.2) polymer films. Such nanostructured polymer-based electrolytes could be very interesting for use in lithium-ion batteries,1058,1061−1063 for which the following technological requirements need to be met: (i) high and selective lithium-ion conductivity (preferably above 10−3 S cm−1 at room temperature); (ii) chemical, electrochemical, and thermal stability (and preferably nonflammability); (iii) mechanical stability;1060,1064 (iv) processability; (v) interfacial compatibility between the electrolyte and the electrode materials; and (vi) no risk of electrolyte leakage or evaporation, for durability and safety.1060,1064,1065 Electrolyte leakage is not an issue if no liquid electrolyte is needed to obtain ion conduction, which is, for example, the case for the polymer films prepared from 164/LiBF4 (4:1).673 However, the ionic conductivity shown by the latter materials is presently still too low for practical applications (see above). Gin and co-workers presented an interesting, highly lithium-ion-conducting material DI


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tion in response to incoming linearly polarized light (Figure S9). In this way, a thin film could be made in which the 2D ionconducting “sheets” are oriented perpendicular to the substrate surface, which is the most useful orientation for practical devices. Interestingly, the anisotropic ionic conductivity shown by 60 was found to reverse during the transition from its SmA phase to its nematic phase (Figure 24). An important development in the past few years is the successful preparation of ILCs that show a bicontinuous cubic mesophase at low temperatures (with, in the case of mixtures of NDA2 and 58-Br, even the possibility of switching between an “inverted-type” phase and a “normal-type” phase;221 see section 5.2). As stated throughout the text, bicontinuous cubic phases of ILCs allow for “alignment-free”, fast ion conduction through their 3D interconnected, continuous ionic nanochannel networks, in contrast to, for example, Colhex phases where conduction is hindered by domain barriers in samples that are not macroscopically aligned (and in contrast to micellar cubic phases with segregated ionic domains, naturally). Such alignment in a device can be challenging. The bicontinuous cubic phase structure of suitable ILCs was found to be compatible with added lithium salts,673,674 and it could be “frozen” by cross-linking (see above) to achieve highly lithium-ion-conductive polymer films673 (as well as ion-selective membranes785). The cubic mesophase structure (either bicontinuous or micellar) could also be obtained via a “noncovalent approach” (i.e., by mixing neutral amphiphiles with nonmesomorphic ILs).217,221,223,860 The induction of cubic phases by addition of lithium salts or organic acids to amphiphilic zwitterions has been discussed above. As mentioned in section 6.2 (including Figure 39), the translational self-diffusion coefficients for the quaternary ammonium cations and [BF4]− anions of 161-BF4-2/10 were investigated by variable-temperature pulsed-field-gradient spin− echo NMR measurements, to elucidate the process of ion conduction/ion self-diffusion in the CubV(Ia3̅d) phase.650 Similar experiments were performed on the magnetically aligned Colhex phase of 53-PF6-8.790 Cifelli et al. did variable-temperature 1 H and 19F NMR measurements on bulk [C12mim][Cl] and [C12mim][BF4] in their SmA phase.582 They found a cation selfdiffusion anisotropy of ∼4.2 for the Cl− salt and of ∼1.8 for the [BF4]− salt. These values are considerably higher than for the Colhex phase of 53-PF6-8, for which hardly any cation selfdiffusion anisotropy was found.790 Furthermore, the anion selfdiffusion anisotropy in the SmA phase of [C12mim][BF4] was found to be ∼1.2,582 as compared to a value of ∼5 for the Colhex phase of 53-PF6-8.790 Judeinstein and co-workers did variabletemperature 2H (natural abundance), 11B and 7Li quadrupolar nuclei NMR, as well as 11B, 19F, and 7Li pulsed-field-gradient NMR measurements on the SmA phases formed by pure [Cnmim][BF4] and [Cnvim][BF4] salts (n = 14, 16) and their mixtures with LiBF4.560 They could show that these phases are easily aligned homogeneously in the magnetic field of the NMR spectrometer (field strengths of 9.4 and 14.1 T were used) upon cooling from the isotropic liquid state, even with fast cooling. The ionic slabs become arranged parallel to the magnetic field. It should be noted that macroscopic alignment of classical neutral smectogens can often only be achieved by slow cooling, preferably through an intermediate nematic phase. As mentioned before (see, for example, section 4.3.2), the orientational order in the aligned ILC mesophases is mainly determined by the longrange ionic sublayers, whereas that part of the flexible alkyl chain that is relatively far from the imidazolium cation is much more disordered. This was also evident from differences in line widths

that was prepared by cross-linking of a polymerizable lyotropic ILC (460).782 Instead of just using water as the solvent to obtain a lyotropic bicontinuous cubic mesophase, they used a solution of LiClO4 (0.245 M) in propylene carbonate (PC). The crosslinked material contained 3D interconnected nanopores filled with about 15 wt % of the liquid electrolyte. Even with such low amounts of electrolyte (the gelled poly(ethylene oxide) electrolytes currently used in lithium-ion batteries contain 40− 80 wt % of liquid electrolyte with a much higher lithium salt concentration (1 M)782,1060), the free-standing, flexible, and robust films showed an ionic conductivity of about 10−4−10−3 S cm−1 at and far below room temperature, thanks to the liquid-like mobility inside their channel nanostructure. The covalently bonded network, in combination with the relatively low amount of liquid electrolyte, minimizes the risk of leakage. Similarly, mixtures of amphiphilic neutral LCs with short-chain ILs (such as mixtures of NDA1-n with 57-X; see section 5.2) can also be regarded as nonaqueous lyotropic LCs, with the IL being the “functional solvent”.214−226,860,1060,1066 It should be noted that the polymers discussed above show a higher ionic conductivity at low temperatures than, for instance, nongelled, solvent-free poly(ethylene oxide)s doped with lithium salts, which are some of the most studied components for lithium-ion batteries. This is because in the latter case the ions are transported by large-scale segmental motion of the polymer backbone, and consequently the ion mobility becomes severely restricted near and below the glass transition temperature; ionic conductivity values of less than 10−4 S cm−1 around ambient temperature are usually obtained,782,1060,1067 with the exception of some LC polymers.653,1068

The use of high molar mass thermotropic LC ionic polymers/ ionomers for membrane applications (including charge and solvent transport) has been reviewed.1069 It is worth pointing out here that LC mixtures of LiNTf2 with a neutral small-molecule mesogen, consisting of an electrochemically stable phenylcyclohexyl-based mesogenic group covalently bonded to a cyclic carbonate moiety (NCM, see Figure S8(a) and also section 12.2), have successfully been used as nonvolatile electrolytes in “real-world” lithium-ion batteries.1070 Apparently the NCM/LiNTf2 mixtures are electrochemically more stable than mixtures of LiOTf with an analogue of NCM that comprises a tetraethylene glycol-based ion-conductive part.414 The mixtures are easily processable and show nanostructured smectic phases over a broad temperature range. The 2D lithium ion transport within the smectic sublayers, composed of the carbonate groups, Li+, and [NTf2]−, is sufficiently high for practical use. Previously reported Colhex-forming mixtures of LiOTf with a fan-shaped neutral mesogen that also contained a cyclic carbonate moiety did not show sufficiently high conductivity values to be used in a real device.1039 The macroscopic photocontrol over the orientation of the 2D ion-transporting pathways that are formed by taper-shaped imidazolium salt 60 in its SmA phase has been discussed in section 5.2.441 This noninvasive and reversible switching could be achieved by making use of the trans-to-cis photoisomerization of the azobenzene substituents and their subsequent reorientaDJ


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bicontinuous cubic phases.63 Compounds 77-79 and 301NTf2-n are other ILCs that combine ionic and electronic functions (the former salts contain hole-transporting (p-type) πconjugated moieties, whereas the latter salts are electrontransporting (n-type) materials), and were successfully applied in electrochromic devices (see section 12.4).694,695,920−923 It should be mentioned that nonionic, discotic, crown ethersubstituted phthalocyanine mesogens capable of both ionic and electronic conduction (via the crown ether moieties and the conjugated cores, respectively) have been reported by Nolte and co-workers1072 (the authors coined the term “molecular ionoelectronics”1073). However, the latter materials do not show high ionic conductivity because the crown ether rings are not well aligned inside the columnar stacks in the mesophase, which results in distortion or even complete absence of extended channel structures.1074 The LC “charge-segregated assemblies” reported by Maeda and co-workers (361-a-CH3·Cl−/369-3, 361-a-F·Cl−/369-3, 371-F/R2, 372-F/R2, 373-F/R2, and 374) seem promising as materials that can show ambipolar charge transport (i.e., of both electrons and holes).990,993,996

and quadrupolar splitting between individual 2H doublets in the NMR spectra. The long-range anisotropic conduction of mobile Li+ and [BF4]− ions in the aligned smectic phases was also confirmed. Uchida and co-workers studied the anisotropic ionic conductivity of commercially available porous aluminum oxide membranes (∼60 μm thick, with cylindrical pores of about 100 nm diameter running parallel to the membrane normal) that were filled with the ILC [C16mim][PF6].1071 Prior to the addition of the ILC, the membranes were treated with poly(vinyl alcohol) (PVA) or with [N16,1,1,1][Br] (CTAB), or they remained untreated. A PVA coating induces homogeneous alignment of the ILC molecules parallel to the pore walls, whereas homeotropic alignment is achieved with CTAB treatment. PXRD measurements showed that, locally, the SmA layer structure of bulk [C16mim][PF6] is preserved inside the pores. The ionic conductivity along the membrane normal in the mesophase temperature range was found to be higher for the CTAB-treated composite membrane than for the PVA-treated sample, thanks to the homeotropic alignment and formation of concentric tubular aggregates (Figure 68). This work illustrates

12.2. Nonvolatile Electrolytes in Dye-Sensitized Solar Cells (DSSCs)

ILCs can improve the efficiency of dye-sensitized solar cells (DSSCs) (Grätzel cells). In a typical DSSC, a thin mesoporous TiO2 layer is placed on a transparent electrode and the high surface area of the TiO2 is covered with a monolayer of a photosensitive charge-transfer dye, typically a ruthenium complex.1075,1076 Photoexcitation of the dye by incident solar light causes the injection of an electron into the conduction band of TiO2, leaving the photosensitizer dye in its oxidized state. An electric current is generated by the subsequent migration of the electron to the ITO photoelectrode and further on via an electric circuitry to a counter-electrode. The function of the electrolyte in the DSSC is the transfer of the charged components of a redox system from the counter-electrode to the oxidized dye molecule, thus regenerating the ground state of the dye. The most commonly used redox system is the I−/I3− couple, which is dissolved in an organic solvent such as acetonitrile. Evaporation and potential leakage of the organic solvent(s) seriously limits the lifetime of a DSSC. Therefore, nonvolatile and conducting ILs (as well as the gels formed by mixtures of ILs and smallmolecule gelators or polymers) have been proposed as alternatives for the organic solvents in DSSCs.192,193,1077−1080 Unfortunately, the efficiency of DSSCs using ILs is lower than that of DSSCs using organic solvents, because the high viscosity of the ILs hampers the diffusion of I− and I3−. Rather than embarking upon the challenging task of substantially lowering the viscosity of the ILs to enhance the physical diffusion of ions, their conductivity can also be increased by providing optimal pathways for fast charge transport (see also section 12.1). It was previously reported that the effective charge transport at high concentration of the I−/I3− redox couple (with comparable [I−] and [I3−]) in ILs may be attributed to a significant extent to the exchange reaction I− + I3− → I3− + I−.1081−1083 Larger polyiodide species (i.e., I2n+1− with n ≥ 2) can also be involved in this process, as confirmed by Raman spectroscopic measurements. This means that Grotthuss-type857 electron-exchange conduction or polyiodide bond-exchange conduction could take place, in such a way that charge transport occurs via electron exchange or bond exchange between rather closely packed I− and I3− ions instead of via long-range effective mass transfer. [Remark: In recent years, it has been proposed that the role of a Grotthuss-like

Figure 68. 1D anisotropic ion conduction in porous aluminum oxide membranes, filled with the smectogenic ILC [C16mim][PF6] after surface treatment of the pore walls with CTAB to induce “homeotropic alignment”. A PVA coating induces “homogeneous alignment”. The cylindrical pores have a diameter of about 100 nm. Adapted with permission from ref 1071 (http://dx.doi.org/10.1039/c5tc00314h). Copyright 2015 The Royal Society of Chemistry.

that, under ILC nanoconfinement, it is possible to create a system for anisotropic ion transport that is not derived from the intrinsic symmetry of the LC phase: typically 2D ion conduction rather than 1D ion transport is achieved with bulk smectogenic ILCs. The Kato and Ohno groups were also successful in preparing efficient proton-conducting ILC-based materials (see the end of section 7 for a full discussion). They presented phosphonium/ sulfonate-based, zwitterionic, aqueous lyotropic LCs that are capable of anisotropic and selective proton conduction.854 Shortly thereafter, they proposed proton-conducting zwitterionic pyridinium and ammonium systems that form bicontinuous cubic phases (besides smectic and columnar phases) and that require much less or even no water for smooth proton conduction, even at elevated temperatures.676,773 Much is to be expected from organic semiconducting ILC systems that show both ionic and electronic conductivity in their mesophase(s). Although it is surprising that fast electron or hole transport can occur in a highly ionic medium, compounds 109-n are examples of ionic smectogens that show both types of conduction.720,723 Salts 89-(1-Me-imid)-BF4-n and 89-(1,2Me-imid)-BF4-n are examples of discotic mesogens that are capable of both ionic and electronic conduction in their DK


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the electrolyte itself, other components of the ILC-based DSSC have been optimized as well. It was found that amphiphilic ruthenium dyes containing long tridecyl chains result in better device performance than their counterparts lacking alkyl chains, when the ILC [C12mim][I]/I2 is used as the electrolyte.1087 This phenomenon was not observed for a DSSC based on nonmesomorphic [C11mim][I]/I2. This indicates that interfacial charge transfer between the electrolyte and the dye at the photoelectrode differs between the ILC-based and IL-based DSSCs, respectively. The smectic organization of the ILC electrolyte and the resulting ordered ionic pathways are possibly stabilized at the interface with the photoelectrode thanks to interactions between the amphiphilic dye and the ionic mesogen. It should be noted that long-range (“monodomain-like”) LC order is not necessarily achieved inside the mesopores of the TiO2 electrode. Nonetheless, even the creation of optimal pathways for fast charge transport in separate LC domains already shows rather significant positive effects on the DSSC performance. In section 12.1, we already discussed the work of Uchida et al. on [C16mim][PF6] confined in porous inorganic membranes.1071 In section 12.3, we will see that Meyer, Libuda, Wasserscheid, and co-workers demonstrated the preservation of the mesomorphism of ILCs under confinement in mesoporous materials.1088,1089 These results are very relevant in the further development of ILC-based DSSCs. As mentioned in section 5.1, Abate et al. used a mixture of the fluorinated imidazolium iodide salt 9-8/3/6 and I2 (4:1 molar ratio), which shows a SmA phase between 27 and 119 °C, in a DSSC, but they did not test it at different temperatures.606 Pan et al. compared the performance of DSSCs with the ILC [C12C2im][I] and the IL [C10C2im][I] as the electrolyte, respectively.1090 Kim and co-workers proposed I2-free DSSCs containing substituted imidazolium iodide salts that are apparently LC at elevated temperatures, but they only tested the DSSC performance at room temperature.1091 Thorsmølle et al. investigated the phase diagram of [C3mim][I]/I2 as a function of the concentration of I2.1092 They found that above a threshold I2 concentration of 3.9 M, larger polyiodide species (I5−, I7−, etc.) start to dominate the physical properties, and mention that a nematic phase is formed between about −13 and 7 °C as a result of orientational ordering of these polyiodide chains. However, this conclusion was drawn from terahertz time-domain spectroscopic measurements and was not supported by any POM or PXRD results. Costa et al. tried to confirm the beneficial effect of a smectic LC phase on the performance of DSSCs.555 They investigated “[C10C10im][I3]” and “[C12C12im][I3]” as LC electrolytes. Although their paper creates some confusion about which compositions have actually been studied, it seems that measurements were performed on [C10C10im][I]/I2 (20:1 molar ratio) and [C12C12im][I]/I2 (20:1 molar ratio) (the I2 content in these systems is smaller than in the [C12mim][I]/ I2(0.65M) system proposed by Yamanaka et al.564,565). The selection criterium for the alkyl chain lengths was to have compounds with a LC mesophase between 50 and 80 °C, which are relevant outdoor operation temperatures. [C12C12im][I] exhibits a SmA phase between 40 and 89 °C.1093 The singlecrystal structures of both [C10C10im][I3]555 and [C12C12im][I3]548 reveal highly ordered pathways for I3− diffusion. A solar cell based on the ILC [C12C12im][I]/I2 (20:1) as a “singlecomponent” electrolyte showed an excellent stability of more than 1000 h under outdoor operation temperature conditions (65 °C) and 1 sun illumination, as well as good conversion

mechanism in systems with a high concentration of I2, such as the ones based on ILs, may have been overestimated by neglecting the possible impact of ion association, i.e., reduced cation−anion pair formation with increasing size of the anion and concomitant delocalization of its single negative charge (compare, e.g., I− and I3−).1076,1084] Such fast exchange-reaction-based charge transport, as opposed to physical diffusion, could not be observed in the molecular, nonionic liquids poly(ethylene glycol) dimethyl ether (Mn ≈ 500) and acetonitrile.1083 Yamanaka et al. reasoned that, to enhance the short circuit photocurrent density (JSC) of DSSCs using ILs with a high concentration of I−/I3−, the aforementioned exchange reaction1082,1083 needed to be promoted. They proposed to use ILCs as electrolyte constituents, as a new strategy to enhance the conductivity of IL electrolytes in DSSCs.564,565 The self-assembled, lowdimensional structures formed by ILCs, reminiscent of those found in the solid state but maintaining liquid-like molecular dynamics, were expected to promote the exchange reaction by a local increase of the I− and I3− concentrations. A similar concept was used in the design of all-solid-state DSSCs based on solid imidazolium salts carrying a carboxylic acid, hydroxyl, or ester group, which form ordered ionic channels in the solid state.1085,1086 Yamanaka et al. tested the ILC [C12mim][I] (Cr · 27 · SmA · 80 · I (°C)) with 0.65 M iodine dissolved in it ([C12mim][I]/I2(0.65 M): Cr · 21 · SmA · 45−46 · I (°C)) as electrolyte in a DSSC.564,565 [C12mim][I] was chosen because this compound contains the required I− anions and because it shows the lowest melting point and viscosity within the series of LC [Cnmim][I] salts (n ≥ 12). The DSSC showed a higher JSC value and a higher light-to-electricity (incident photon-tocharge) conversion efficiency at 40 °C than that based on the nonmesomorphic IL electrolyte [C11mim][I] (Cr · 50 · I (°C)) with 0.65 M iodine dissolved in it ([C11mim][I]/I2(0.65 M): Cr · 37 · I (°C)), although the viscosity of [C12mim][I]/I2 is about 2.5 times higher than that of [C11mim][I]/I2 and physical diffusion is thus slower in the former medium. The contribution of exchange-reaction-based charge transport to the total I3− diffusion was determined by measuring the apparent diffusion coefficient as a function of the iodide concentration ([I−] + [I3−]), through mixing of [C12mim][I]/I2 or [C11mim][I]/I2 with [C12mim][BF4].565 This contribution was found to be higher for [C12mim][I]/I2 than for [C11mim][I]/I2. It was also shown that the ionic conductivity in the direction parallel to the SmA layer planes of the ILC is higher than that in the direction perpendicular to the layer planes (see also section 12.1),564,565 and that [C12mim][I]/I2 contains a higher amount of polyiodide species (i.e., I2n+1− with n ≥ 2) relative to I3− than [C11mim][I]/ I2, once again due to local concentration of the iodide species in the 2D ionic sublayers of the LC phase.565 Moreover, because the redox couple is largely confined within these ionic sublayers, the I−, I3−, and I2n+1− anions experience a strong ionic strength field and can collide with each other more easily than in a molecular, nonionic liquid due to the “kinetic salt effect”. While this also applies to nonmesomorphic ILs, the iodide species are in that case not confined into ordered pathways, and this reduces the collision frequency.565,1083 A quasi-solid-state DSSC obtained by gelation of the [C12mim][I]/I2 ILC electrolyte (leading to a LC physical gel) showed an even higher efficiency than the device based on nongelated [C12mim][I]/I2, possibly because of stabilization of the LC order in the gel state.565 It had previously been reported that gelation of IL electrolytes yields DSSCs with improved thermal stability, because it suppresses sublimation of I2 and concomitant decrease of the I3− concentration.1078 Besides DL


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regeneration) by relying on the plastic nature of ILC electrolytes, could lead to even more performant and stable quasi-solid-state DSSCs in the years to come. Several device aspects can still be optimized. The [SeCN]−/[SeCN]3− pseudohalide redox couple has been proposed as an alternative to the I−/I3− system a few years ago,1094,1095 but ILCs containing the [SeCN]− anion have not yet been reported. More recently, the Kato group also contributed to this field. They proposed a two-component LC electrolyte consisting of the neutral carbonate-based mesogen NCM (see Figure S8(a) and also section 12.1) and the IL [C3mim][I] (adopting the “noncovalent approach” that was discussed in section 5.2).1066 Mixtures of NCM and 30−80 mol % of [C3mim][I]/I2 (4:1 molar ratio) show a SmA phase over a very broad temperature range, from below 0 °C to ∼110 °C. This range extends to higher temperatures than any of the (imidazolium salt)/I2 combinations that were mentioned above, except for (9-8/3/6)/I2 (4:1 molar ratio). It was demonstrated that DSSCs with these electrolytes show high open-circuit voltages and improved power conversion efficiencies with increasing temperature. The solar cells still convert light to electricity at temperatures up to ∼120 °C. In contrast, a DSSC containing only [C3mim][I]/I2 suffers from a large drop in power conversion efficiency at higher temperatures. The authors proposed that the good DSSC performance is due to partial suppression of electron recombination reactions at the photoelectrode/electrolyte interface, possibly because of the selforganization of the NCM-([C3mim][I]/I2) electrolyte on the TiO2-dye surface (the carbonate mesogens may form an insulating barrier for I3− and I− species on the TiO2 electrode surface). Measurements of the electron lifetime τ as a function of temperature showed that the DSSCs based on NCM-([C3mim][I]/I2) show a longer lifetime than those based on [C3mim][I]/ I2 alone, and that the lifetime does not decrease as strongly with increasing temperature as for the latter.

efficiencies of 1.5% and 3.9% under 1 and 0.25 sun illumination conditions, respectively. Importantly, the photoelectrochemical measurements were carried out as a function of temperature (this is actually what distinguishes the work of Costa et al. from the pioneering studies of Yamanaka et al., who compared an IL electrolyte and an ILC electrolyte at 40 °C564,565), and these efficiency values were only achieved above the crystal-to-LC phase transition (Figure 69). Upon heating the [C12C12im][I]/

Figure 69. Efficiency η versus temperature of DSSCs containing a standard solvent-based electrolyte (see main text for details) and different ILC-based electrolytes (1, [C10C10im][I]/I2 (20:1 molar ratio); 2, [C12C12im][I]/I2 (20:1 molar ratio)) at 1 sun illumination upon heating (solid line) and cooling (dashed line).555 The LC mesophase temperature ranges of the ILC electrolytes are indicated with dashed−dotted lines (during heating runs) and dotted lines (during cooling runs). Reprinted with permission from ref 555. Copyright 2013 Wiley.

12.3. Organized Media for Organic and Inorganic Reactions, for Gas Adsorption, and for Structure Determination

LCs can be regarded as anisotropic solvents. As such, they can be used as (partially) organized reaction media, at least if their viscosity is sufficiently low, the mesophase stability range is sufficiently wide, and the LC is available in sufficient quantities. Because of the orientation and confinement of reagent molecules dissolved in the LC host (“template effect”), a different chemo-, regio-, stereo-, and/or enantioselectivity may be achieved as compared to conventional solvents, and/or aligned reaction products can be obtained (as in the case of polymerization reactions). As previously discussed in the context of ion conduction, self-organized LC nanostructures can also be stabilized by in situ polymerization of the constituents.666,667 This strategy afforded, for example, organic analogues of zeolites, containing precisely defined nanosized pores and channels, and even specific reactive catalytic sites within the channels of these nanostructured polymer networks.657,659,663,1096−1098 Most of the investigations of organic reactions in LC solvents involved neutral LCs.1099−1103 Very few studies have considered the use of thermotropic ILCs as solvents for organic reactions, despite potential advantages such as the stabilization of transient polar, ionic, or radical intermediates involved in a process and the possibility of combined use as solvents and electrolytes. Lin and co-workers demonstrated the influence of the LC 1(n-alkyl)imidazolium salts [CnHim][X], (with X− = Cl−, [BF4]−, [NO3]−) on the stereoselectivity of a Diels−Alder reaction.556 A preference for the exo over the endo product was found for the

I2-based device from room temperature to 40 °C (i.e., within the temperature range of the crystalline solid phase of the electrolyte), the efficiency increased linearly. Upon further heating, however, the efficiency increased more rapidly between 40 and 90 °C, that is, within the temperature range of the smectic mesophase. The heating/cooling process could be repeated without changes in the efficiency-versus-temperature profile. In sharp contrast, the performance of a reference device based on a standard solvent-based electrolyte (i.e., a solution of 0.6 M [C4mim][I], 0.03 M I2 ([C4mim][I]/I2: 20:1 molar ratio), 0.1 M guanidinium thiocyanate, and 0.5 M 4-(tert-butyl)pyridine in acetonitrile/valeronitrile (85:15)) decreased linearly with increasing temperature due to solvent evaporation and leakage, particularly at temperatures exceeding the boiling point of acetonitrile (82 °C) (Figure 69). Interestingly, the DSSC performance is not significantly affected by the LC-to-isotropic liquid phase transition. The dye regeneration and hole transport in the latter phase were however not discussed. Increased mesophase ionic conductivities, in combination with improved penetration of the electrolyte into the mesoporous TiO2 electrode to maximize physical contact at the TiO2−dye− electrolyte interfaces (which is important for efficient dye DM


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cycloaddition of cyclopentadiene with diethyl maleate. The exo/ endo ratio in the ILC (54:46) was much higher than that in ethanol (12:88). More recently, Schmitzer and co-workers demonstrated a higher selectivity for the intramolecular Diels− Alder product of ester-tethered 1,3,9-decatriene substrates than for their intermolecular Diels−Alder product, when the ILC 132NTf2-Y2-12 was used as the reaction medium rather than the IL [C4mim][NTf2], even under high concentration conditions.401 This was ascribed to the highly organized environment provided by the crystal smectic T phase of the former, which ensures a certain isolation of the reactant molecules. Interestingly, the selectivity in the T phase of 132-NTf2-Y2-12 was found to be much higher than in its solid state. Because the reaction products can easily be removed by extraction with diethyl ether, the ILC could be reused several times as an organized reaction medium without any significant loss of reactivity and selectivity. The hydroformylation of 1-octene yielding nonanal and 2methyloctanal was carried out in a series of 1-methyl-3-(nalkyl)imidazolium ILCs (with [BF4]− and [CH3SO3]− anions) and a series of N-(n-alkyl)-ε-caprolactam-based ILCs (315-X-n· H2O, with [OTs]− and [CH3SO3]− anions).566 The lamellar mesophase was found to have a strong influence on the regioselectivity and the turnover frequency of the reaction product. Variation of the alkyl chain length of the caprolactambased cation from C12H25 to C18H37 caused the nonanal/2methyloctanal ratios to vary from 1.7 to 2.9, whereas the turnover frequency decreased from 148 to 122 mol mol−1 h−1. The imidazolium ILCs favored the formation of the unbranched nonanal over the branched product 2-methyloctanal. A nonanal/ 2-methyloctanal ratio of 5.2 was observed for [C16mim][BF4] with triphenylphosphine as additive. Incorporation of a metal in the LC (e.g., Pd(II)) might result in additional catalytic properties. In this respect, it is noteworthy that Lee et al. observed complete miscibility of the mesomorphic [CnCnim][Cl] and [CnCnim]2[MCl4] salts (M = Pd2+, Cu2+), with the mixtures also displaying SmA phases. 537 The [CnCnim]2[MCl4] salts can act as precursors for catalytically active carbene complexes. Meyer, Libuda, Steinrück, Wasserscheid, and co-workers initiated a research program dedicated to the study of ILCs under confinement and to the development of ILC-based “supported IL phase” (SILP) materials.1104−1106 In SILP materials, a very thin film of IL (typically only a few nm thick) is confined onto the surfaces of a highly porous solid by physisorption, tethering, or covalent anchoring. The nonvolatile IL acts as a support-modifying, functional coating. Other active compounds such as catalysts can be dissolved in the supported IL thin film. This is an elegant way of combining the advantages of heterogeneous and homogeneous catalysis: the SILP material appears macroscopically as a dry solid that can be processed as a heterogeneous catalyst, but microscopically the dissolved catalyst complex acts as a well-defined homogeneous catalyst in a liquidlike environment. Moreover, the amount of required IL solvent, which is often expensive, is greatly minimized, as are mass transfer problems that can be an issue in high-viscosity ILs. The use of an ILC instead of an IL (to form a “SILCP” or “ILC-SILP” material) could have the added value of an organized reaction environment. In general, it would also yield solid materials with temperature-swichable properties. In a first report, Sobota et al. investigated the phase behavior of a spin-coated film (about 10 μm thick) of the ILC [C12C12im][BF4] by means of a surface vibrational spectroscopy method, temperature-dependent infrared reflection absorption spectroscopy (IRAS).1107 Their goal

was to demonstrate the applicability of this technique to the study of future SILCP materials. Typical techniques to investigate the phase behavior of LCs, such as POM and DSC, are not easily applicable to porous hybrid materials with only small loadings of mesomorphic material. Sobota et al. showed that temperature-dependent IRAS can be used to probe the phase transitions and ordering of an ILC film, under ultrahighvacuum (UHV) conditions (which is not a problem due to the nonvolatility of ionic mesogens). The transition from the isotropic liquid state to the SmA phase on cooling is accompanied by subtle IR spectral band shifts and intensity changes related to both the cation and the anion. The frequencies of the alkyl CH2 symmetric (νs(CH2)) and antisymmetric (νas(CH2)) stretching vibrations and scissoring modes provide information on the temperature-dependent conformational and structural changes related to the alkyl chains. It is possible to distinguish a trans-zigzag conformation of the alkyl chains from a conformation with gauche defects. Whereas a large number of gauche defects is observed throughout the liquid state, a sudden decrease of the concentration of these defects occurs at the transition to the SmA phase (accompanied by a red-shift of the νs(CH2) and νas(CH2) bands). With decreasing temperature, the number of gauche defects decreases further, indicating that the degree of ordering in the mesophase is temperature-dependent. A second abrupt change occurs at the transition to the crystalline solid state, in which the alkyl chains adopt an all-trans conformation (although previous work of Hardacre and Seddon suggested that the alkyl chains remain at least partially disordered after crystallizing from the mesophase (see also below)1108). Importantly, the transition temperatures that were extracted from the evolution of the νs(CH2) and νas(CH2) peak positions upon cooling were in fairly good agreement with the results previously obtained by POM and DSC.359 The authors did not investigate spectral changes related to the imidazolium core or the [BF4]− anion. These could give information about changes in interionic interactions. In principle, IRAS can equally well be used to study the structure of the mesophase under “reaction conditions” (i.e., with dissolved catalyst and reactants), but such experiments were not reported. Following the demonstration of the applicability of IRAS, the same groups made further progress by preparing genuine SILCP materials.1088 Kohler et al. immobilized [C18mim][OTf] onto porous silica supports (powders and glasses) with pore sizes ranging from 11 to 50 nm. The materials were investigated by means of POM, DSC, and temperature-dependent diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The latter technique was validated as a suitable method to monitor the phase transitions and ordering of the immobilized ILC thin layers (nanometers thick as opposed to the ca. 10 μm thick films studied in the first report), in a way similar to IRAS (see above)1107 but allowing the study of “3D” porous powder samples rather than only films on planar supports.1109 It should be mentioned however that the mesophase-to-isotropic liquid transition could not be identified very clearly by DRIFTS, whereas it was easy to detect by means of IRAS. Nonetheless, the authors were able to show that for calcined silica-100 with a pore size of 11 nm and filled with the ILC to achieve a pore filling α ≥ ∼0.8, the phase behavior of the neat ILC (Cr · 64 · SmA · 107 · I (°C)280) was reproduced quite well under confinement in the mesopores (see also the work of Uchida et al. that was discussed in section 12.11071). On the other hand, for the same material with a pore filling α ≈ 0.6, liquid-crystallinity seemed to be lost: only a single thermal event around 37 °C was detected, that is, DN


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Figure 70. Schematic representation of the SILCP materials, consisting of a mixture of the ILC [C12C12im][BF4] and a structurally similar nickel(II) Nheterocyclic carbene (Ni−NHC) complex immobilized onto a porous support. Reproduced with permission from ref 1089 (http://dx.doi.org/10. 1039/c1jm13416g). Copyright 2012 The Royal Society of Chemistry.

reproduced again. In fact, in 50 nm pores, a pore filling α > ∼0.4 is sufficient to reproduce the phase transitions of the neat ILC, opening the way to the use of such SILCP materials for, for instance, continuous gas-phase catalysis purposes. For such applications, a better understanding of the detailed structure of the ILC−vacuum (gas phase) interface would be desirable, because reactants would have to pass this interface. AR-XPS could be very helpful in this respect, because it has successfully been applied to elucidate the chemical composition of the nearsurface region in various ILs.296,1110,1115−1120 As was already mentioned in section 12.2, the results of Kohler et al. are also very relevant in relation to dye-sensitized solar cells (DSSCs) based on ILC electrolytes,555,564,565 because the TiO2 photoelectrodes within the DSSCs are mesoporous materials. In relation to the “contact layer” concept, it should be noted that in 2001 Hardacre, Seddon, and co-workers had already reported on molecular layering and local order in very thin films of selected ILs (not liquid at room temperature) and ILCs ([C n mim][PF 6 ] (n = 12, 18), [C 18 mim][BF 4 ], and [Cnmim]2[PdCl4] (n = 4, 12)) that were spin-coated on Si(111).1108 Their X-ray reflectivity measurements supported the formation of a multilayer stack of interdigitated bilayers in a well-structured film (only about 15 nm thick) of [C18mim][PF6], with the ionic headgroups located at both the silicon−salt and the salt−air interfaces and with the alkyl chains oriented toward the bulk. Such ordered films could only be obtained after an annealing step (heating to the isotropic liquid state followed by cooling to the LC phase or to a crystalline state). The alkyl chains appeared to remain disordered after crystallizing from the mesophase upon cooling. The authors mentioned that they were not able to include in their structural model of the thin film a modified interfacial structure that differs from the structure in the bulk. They noted that this could be due to the limited signal-tonoise ratio of the X-ray reflectivity data and/or the fact that the material structure at the interface with the substrate surface is only slightly different from the bulk. Having demonstrated that the mesomorphism and temperature-induced reorganization of ILCs can be preserved under confinement in a mesoporous support material, the Meyer, Libuda, and Wasserscheid groups took a next step toward SILCP

about 30 °C below the Cr-to-SmA transition of the neat ILC. This led the authors to define a so-called “contact layer”, with properties and structure differing from those of the bulk, of about 2 nm thickness. This “contact layer” was proposed to represent a solid phase that has a different, probably less ordered structure than the crystalline solid phase, because the νs(CH2) and νas(CH2) peak maxima were located at slightly higher wavenumbers for the lower-temperature phase at pore filling 0.6 than for the crystal phase at pore filling ≥0.8. The results are in line with previous work on supported IL films, ILs under confinement, and supported neutral LCs. Using angle-resolved X-ray photoelectron spectroscopy (AR-XPS), Cremer et al. found charging effects (pointing to an immobile, nonconductive IL) for the first two ion-pair layers in an ultrathin film of the IL [C2mim][NTf2] that was deposited on glass via physical vapor deposition, and they attributed this to interactions with the surface.1110 The very first ion-pair layer was found to consist of a “bilayer”, with the [C2mim]+ cations in contact with the surface and the [NTf2]− anions at the vacuum side. Only from a thickness of three ion-pair layers on [C2mim][NTf2] behaved like a liquid again. Other reports have shown that the melting point and other physical properties of ILs are affected under confinement. For example, when the room-temperature IL [C4mim][PF6] is confined into multiwalled carbon nanotubes, it is converted into a high-melting-point crystal.1111,1112 Similar effects were observed for [C4mim][BF4] confined in IL− (aluminum hydroxide) hybrids.1113 It is well-known that neutral LC molecules that are located close to a support surface are subject to different interactions as compared to the bulk, which greatly influences the phase behavior. For example, Rayss et al. reported that a silica support modifies the LC properties of p-(nbutyl)-p′-(n-hexanoyl)azobenzene up to a distance of about 2 nm from the surface.1114 In the second part of their study, Kohler et al. compared the silica-100α([C18mim][OTf])∼0.6 material with Fractosil-500α([C18mim][OTf])∼0.6 with a pore size of 43 nm, and with porous glassα([C18mim][OTf])∼0.6 with a pore size of 50 nm. They were able to show that for larger pores at similar relative pore filling, the influence of the “contact layer” is minimized (because a higher number of successive ILC layers are formed on the support) and the phase behavior of the neat ILC can be DO


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stabilizing π−π aromatic stacking interactions between the imidazolium rings. The latter type of interactions cannot play a role for [N16,1,1,1][Cl]. A similar “hydrogen bond-co-π−π” stacking mechanism had previously been proposed by Zhou et al. in relation to the formation of mesoporous silica with wormhole-like pores under acidic conditions using the shortchain IL [C4mim][BF4].1125 These authors based themselves on IR spectroscopic data. Tanaka et al. suggested that the stacking interactions can be facilitated by the delocalization of charge in the imidazolium cations, which reduces their mutual repulsion. The compact “self-assembled” packing of [C16mim][Cl] inside the rigid mesopores to form ionic organic−inorganic interfaces with the silica surface, which is apparently absent for [N16,1,1,1][Cl], was proposed to explain the difference in CO2 uptake between im-MSM and amm-MSM (Figure 71). The CO2

materials that could be useful for catalysis purposes. They incorporated suitable catalyst precursors in the ILC phase.1089 Two nickel(II) N-heterocyclic carbene complexes ([NiCl2(C12mim)2] and [NiCl2(C12C12im)2], where C12mim = 1-(n-dodecyl)-3-methylimidazolin-2-ylidene and C12C12im = 1,3-bis(n-dodecyl)imidazolin-2-ylidene) were prepared and mixed with the ILC [C12C12im][BF4] prior to immobilization onto the solid support (Figure 70). These complexes are catalyst precursors for olefin dimerization reactions. The creation of an organized LC reaction environment around the catalytic centers may increase the reaction selectivity, for example, toward linear dimers at the expense of branched dimers. Thanks to their structural similarity to [C12C12im][BF4], the complexes showed good miscibility with the ILC, and, importantly, mesomorphism in the bulk was maintained for a 10 wt % mixture of each complex in the ionic mesogen. It was furthermore shown by POM, DSC, IRAS, and DRIFTS that the smectic mesomorphism was preserved after immobilization of the complex/ILC mixture onto Pt(111) (planar film of about 10 μm thickness) and porous silica-100 (pore size of 11 nm; pore filling α = 1.0). Actual olefin dimerization experiments with these SILCP materials were not reported. Related to the previous work is a report by Tanaka et al. that demonstrates anomalous CO2 adsorption at low pressure by mesoporous silica functionalized with the ILC [C16mim][Cl] inside its pores.1121 The ionic mesogen was not adsorbed onto the mesoporous material in a postsynthesis functionalization step, but was used during the synthesis of the monodisperse silica microspheres under basic conditions as a templating agent and was not removed by calcination afterward. [Remark: Ionic surfactants can act as a template for silica frameworks, by directing the electrostatic assembly of the 3D silica structures through S+I− charge matching between the cationic headgroups S+ and the framework-forming anionic silicate precursors I−.1122] The CO2 adsorption by the [C16mim][Cl]-containing mesoporous silica microspheres (hereafter called im-MSM) was compared to that of a similar [N16,1,1,1][Cl]-containing material (hereafter called amm-MSM). In contrast to [C16mim][Cl], [N16,1,1,1][Cl] does not show a thermotropic LC mesophase.2,623 The pore structures of im-MSM and amm-MSM were very similar. The [N16,1,1,1][Cl] template could easily be removed from amm-MSM through extraction with organic solvents, but this proved to be much more difficult for [C16mim][Cl] (as previously reported for [C16mim][BF4]1123). This was interpreted as a stronger interaction of the imidazolium salt with the inner silica surface as compared to the quaternary ammonium salt. Unlike for neat [C16mim][Cl], the 13C NMR resonances related to the imidazolium ring carbon atoms and the carbon atoms right next to the nitrogen atoms could not be detected anymore by 13C solid-state NMR spectroscopy for im-MSM, which suggests reduced conformational and diffusional freedom of the cationic headgroup of [C16mim]+ inside the mesopores. Such difference was not seen between [N16,1,1,1][Cl] and ammMSM, but also not between [C16mim][Cl] and [C16mim][Cl]templated silica prepared under acidic instead of basic reaction conditions (the latter is in agreement with work of Zhou and Antonietti, who mentioned that [C16mim][Cl] can easily be removed from silica prepared under acidic conditions through simple solvent extraction1124). Because the two templates only differ in the structure of their cationic headgroup, the authors proposed that the [C16mim]+ cations, together with the Cl− anions, are aligned along the silica surface through electrostatic interactions involving the silica polar surface and additional,

Figure 71. CO2 adsorption isotherms of [C16mim][Cl]-templated mesoporous silica microspheres (im-MSM; ○), [N16,1,1,1][Cl]templated mesoporous silica microspheres (amm-MSM; □), and neat [C16mim][Cl] (●) (measured at 70 °C). The CO2 adsorption isotherm of im-MSM could be fitted according to a dual-site-mode sorption model (red curve): C = CH + CL = kHP + [(bPCL*)/(1 + bP)], where CH = Henry-type sorption, CL = Langmuir-type sorption, kH = Henry’s solubility coefficient, P = applied gas pressure, b = Langmuir affinity constant, and CL* = Langmuir saturation constant. The isotherm was deconvoluted into a linear and nonlinear contribution (blue line and blue curve, respectively). Reprinted with permission from ref 1121. Copyright 2011 American Chemical Society.

adsorption isotherm of im-MSM could be fitted with a dual-sitemode sorption model, involving contributions from Henry-type sorption (i.e., random mixing with weak interactions) and Langmuir-type sorption (i.e., sorption in specific domains with strong interactions). The latter type of sorption, not seen for amm-MSM (Figure 71), is expected to dominate at wellstructured ionic headgroup/silica interfaces where CO2 molecules can occupy well-defined adsorption sites. Remarkably, the CO2 adsorption capacity was hardly affected by exchange of the Cl− anions in [C16mim][Cl] for [BF4]− or [NTf2]−, whereas the solubility of CO2 in pure [C16mim][Cl], [C16mim][BF4], and [C16mim][NTf2] differs markedly. Although the observations by Tanaka et al. seem to originate from differences in specific cation−silica interactions between imidazolium and quaternary ammonium cations, rather than genuine LC self-assembly, it is striking that [C16mim][Cl] actually shows a thermotropic LC mesophase,280 whereas [N16,1,1,1][Cl] does not.623 On the other hand, the peculiar CO2 adsorption behavior was also observed for microspheres prepared with [C16mim][NTf2], although this is not a thermotropic LC280 and although it was suggested that the “hydrogen bond-co-π−π” stacking mechanism is not effective for [NTf2]− ILs.1125 It was not investigated whether the mesomorphic properties of neat [C16mim][Cl] and [C16mim]DP


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into gases (i.e., HF, alkenes, haloalkanes, CF3H, SO2, etc.) upon decomposition above 350 °C, thus facilitating the exfoliation process. The decreasing viscosity of the ILC with increasing temperature helps to achieve a homogeneous composite. It is, however, not entirely clear what is the advantage of using 286NTf2-12/12 rather than, for example, its shorter-chain analogues (which are either ILs or ILCs with clearing points below 150 °C). The same group also reported on carbon composite electrodes made up of graphite and either 286-NTf2-4/4 or 286-NTf2-12/ 12 as a conductive “binder” that also affects the electrode surface morphology.1127 It is remarkable that mixtures of graphite and 286-NTf2-4/4 or 286-NTf2-12/12 (70/30 w/w) still display liquid-crystalline properties, with only slightly different transition temperatures as compared to the neat ILCs. Moreover, the mixtures can be aligned on a glass slide by simple rubbing. In the composite electrodes, the ILC binder lowers the background current, improves the electrochemical performance, and imparts stability to the electrodes, for reasons that remain unclear. Interestingly, for the graphite/286-NTf2-4/4 electrode, these improved properties were only obtained after heating the electrode, because the graphite/286-NTf2-4/4 mixture is not LC around room temperature in contrast to the mixture with 286-NTf2-12/12. The performance of composite electrodes with nonmesomorphic analogues of 286-NTf2-4/4 or 286NTf2-12/12 with Br−, [BF4]−, or [PF6]− anions was inferior to that of the composite electrodes containing the ILC binders. The use of ILCs as organized media is not limited to organic synthesis. Taubert used an ILC to synthesize inorganic copper(I) chloride nanoplatelets.1128 The ionic mesogen N-(n-dodecyl)pyridinium tetrachlorocuprate(II) ([C12pyr]2[CuCl4]) was mixed with the amphiphilic reducing agent 6-O-palmitoyl ascorbic acid, and on heating the Cu(II) ions are reduced to Cu(I) by the ascorbic acid. The Cu(I) ions react with the chloride ions in the medium to form CuCl. The CuCl platelets that were formed in the mesophase at 85 °C were relatively large and interconnected, whereas smaller platelets without permanent junction were formed in the isotropic liquid phase. The fact that CuCl platelets are formed in both phases indicates that some ordering of the ILC must persist in the isotropic phase (see also above). The platelets were in fact too thick to be the product of direct phase replication or templating. It should be noted that a platelet morphology is uncommon for CuCl, though. In followup studies, the formation of the CuCl particles was studied in more detail by thermal analysis methods, EPR spectroscopy, and high-temperature XRD.1129,1130 Unfortunately, the two-component system [C12pyr]2[CuCl4]/6-O-palmitoyl ascorbic acid is rather unstable: optical microscopy showed demixing of the two components after a few hours at room temperature.206 The methodology was also used for the synthesis of CuBr and CuI, but it seemed difficult to generalize it to other metals and to nonhalide compounds (oxides or sulfides). N-(n-Dodecyl)pyridinium tetrachloroaurate(III) ([C12pyr][AuCl4]) is a layered compound, but it does not show mesomorphic behavior: at 50− 55 °C it melts directly to a yellow liquid, and the melting process is accompanied by thermal reduction of Au(III) to micrometersized spheres of Au(0). Gold platelets with a nanometer thickness and a length of up to 15 μm were formed by photoreduction of Au(III) within the layered, crystalline gold complex.1131 The morphology of the gold particles was found to be a function of the distance between the UV-lamp and the sample, but this effect can be attributed to local heating and melting of the sample at short lamp−sample distances.

[BF4] were preserved under confinement in the mesoporous silica microspheres. As mentioned in section 6.2, Saigo and co-workers used ionic complexes of 177 and 178 with the amphiphilic 2-aminoalcohol derivatives 172u−w as chiral reaction media for the enantioselective photodimerization of the anthracenecarboxylic acid components.800,801 Upon irradiation of the LC complexes with visible light in their mesophase, the (structurally asymmetric) anthracene moieties photodimerized regio-, diastereo-, and enantioselectively, thanks to their ordered and constrained microenvironment and their defined relative orientation (Figure S27). For example, irradiation of 177/172v (1:1) at 45 °C resulted primarily in the formation of the antiHH anthracene dimer (HH = head-to-head), in 34% yield and with up to 81% enantiomeric excess (out of four possible configurational isomers), despite the higher thermodynamic stability of the head-to-tail (HT) isomers.800 Contrary to conventional LC reaction media, the reactive substrates were not randomly doped in the partially ordered medium, but constituted an integral part of it (via noncovalent interactions) and were thus efficiently preorganized. Interestingly, the synHH/antiHH diastereoselectivity in the photoreaction of 177 could be controlled by the choice of the 2-aminoalcohol and the mesophase (177/172v (1:1) at 35 °C in its M phase, synHH/antiHH 26:72; 177/172w (1:1) at 105 °C in its SmA phase, synHH/antiHH 61:37).801 On the other hand, the absolute configuration of the preferentially formed enantiomer of the antiHH product was determined by that of the carbon atom carrying the hydroxyl group in the chiral 2aminoalcohol unit. The dimeric products could be recovered by treatment of the reaction medium with an excess of trimethylsilyldiazomethane to esterify the carboxyl groups, followed by chromatographic separation. The fact that the same amphiphilic component, either 172v or 172w, could be used with two differently shaped substrates, 177 and 178, with in both cases acceptable to high reaction yields and very high HH/ HT regioselectivities, points to the advantages of these “flexible” LC systems as compared to crystalline-phase reactions. The latter proceed only under meticulously engineered conditions, in which crystal packings exactly meet topochemically stipulated demands; moreover, severe restriction on molecular motions strongly reduces the probability of reaction.800,801 It was found that the photoirradiated product of 177/172w (1:1) in its SmX phase also constituted a thermotropic LC, with slightly changed lattice parameters (increased layer thickness as a result of lateral contraction after photodimerization). This shows that the “flexible” LC reaction medium can adapt itself to a certain degree, and explains why very high HH/HT selectivities can be obtained even at high conversions. It should be noted that high HH/HT selectivities were also obtained upon photoirradiation in the isotropic liquid phases of the ionic complexes. This indicates once again that the order of the LC phase is partially preserved after clearing out into the isotropic liquid phase (see earlier sections). The strong electrostatic interactions in ILCs are probably important in this respect. Safavi et al. reported on a facile, large-scale, one-pot synthesis of large-area, unoxidized multilayer graphene nanosheets from a composite of graphite and the ILC 286-NTf2-12/12 (30/70 w/ w).1126 Thermal treatment of the composite at 700 °C under an argon flow resulted in the separation of graphene sheets from the graphite. The 4,4′-bipyridinium cations probably intercalate between graphene layers inside graphite (interacting via π−π stacking, as was also found for mixtures of imidazolium ILs and carbon nanotubes702), and the salts are completely converted DQ


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Douce and co-workers also used metal-containing ILCs as both inorganic material precursor, electrochemical reaction solvent, and nanostructured template for the resulting inorganic particles. They prepared gold and silver nanoparticles by reduction of the ILC precursors 41-Ag(CN)2-12 and 41Au(CN)2-12, containing dicyanoargentate(I) ([Ag(CN)2]−) and dicyanoaurate(I) ([Au(CN)2]−) anions, respectively.110 The compounds were sandwiched between two ITO-coated glass slides separated by a 100 μm spacer. The ILC functioned as electrolyte. The electrodeposition was conducted potentiostatically. Major differences were observed between the gold nanoparticles obtained from the isotropic liquid state (nanospheres of 20−30 nm diameter that aggregated to larger aggregates) and from the LC state (leaf-like forms interlocked in rosettes). For the silver nanoparticles, polydisperse spheres were obtained from the isotropic liquid and hexagonal platelets from the LC state, although the silver electrodeposition turned out to be more difficult to control than the gold electrodeposition. The advantage of this electrochemical method is that a single compound can be used. Another elegant method for the size-selective synthesis of (isotropic) gold nanoparticles was demonstrated by Lauth-de Viguerie, Marty, and co-workers. They exploited the interactions between the ionic regions within ionic LC hyperbranched PAMAM polymers, on the one hand, and added HAuCl4 (as the ionic metal source), on the other hand, to form homogeneous hybrid LC materials.838 Subjecting these to a flow of hydrogen gas resulted in the in situ reduction of Au(III) to relatively monodisperse gold nanoparticles. The nanoparticles that were obtained in the LC state (∼1.9−2.8 nm depending on the molar mass of the hyperbranched polymer) were found to be smaller than the ones prepared in the isotropic liquid state (∼3.3−4.3 nm). Interestingly, liquid-crystallinity was retained after formation of the nanoparticles, but at the expense of the highly organized columnar phase in favor of the lamellar phase of the hyperbranched ILCs. Larionova et al. investigated the size-controlled synthesis, as well as the long-range organization (in a 2D array) of magnetic cyano-bridged coordination polymer nanoparticles Mn1.5[Cr(CN)6] in the ILC [C12mim][BF4].1132 These particles could be prepared by mixing a [C12mim][BF4] solution of [Mn(H2O)6][NO3]2 with a [C12mim][BF4] solution of [C12mim]3[Cr(CN)6] at 60 °C (i.e., in the isotropic liquid state of [C12mim][BF4]). On cooling to room temperature, the mixture solidified to an opaque yellow (plastic) solid, which melted again to a transparent yellow solution at 60 °C. TEM indicated the organization of spherical nanoparticles (∼4.0 nm diameter) into parallel and equally spaced layers, reminiscent of a smectic ordering (Figure 72). Indeed, the Mn1.5[Cr(CN)6]/[C12mim][BF4] system itself is LC between room temperature and 51 °C (the phase behavior of [C12mim]3[Cr(CN)6] was not reported). The ILC acts as a structuring solvent and stabilizing agent for the nanoparticles. In contrast, if the IL [C10mim][BF4] was used rather than the ILC [C12mim][BF4], the nanoparticles were just randomly dispersed throughout the IL (however, with some short-range ordering). The Mn1.5[Cr(CN)6]/[C10mim][BF4] system is a liquid at room temperature. The authors proposed that local ordering in the isotropic liquid phase (see earlier sections) is of importance during the actual formation of the coordination polymer nanoparticles at 60 °C, in the case of both [C12mim][BF4] and [C10mim][BF4] (upper part of Figure 72). Because of the differences in particle size and interparticle separation in both systems, the magnetic properties of the

Figure 72. Proposed mechanism for the growth and organization of coordination polymer nanoparticles within [C12mim][BF4] containing 0.02 wt % of water. The complex at the bottom left (yellow, black, and green spheres) is [C12mim]3[Cr(CN)6]; the complex at the top right (blue sphere) is [Mn(H2O)6][NO3]2. Reprinted with permission from ref 1132. Copyright 2009 American Chemical Society.

nanoparticles were also different. In the ILC system, there exist relatively strong magnetostatic interparticle interactions that produce a collective nanocluster-glass-like behavior. More recently, Mudring and co-workers observed spontaneous decomposition (in the absence of H2) of Ni(COD)2 (COD = 1,5-cyclooctadiene) upon addition to several dodecylsubstituted ILs and ILCs (i.e., [C12mim][NTf2], [C12C12im][NTf2], the analogue of [C12C12im][NTf2] with a methyl group at the imidazolium C(2) atom, 48-NTf2-6/12/12, and 49-NTf21/12).563 This resulted in the formation of Ni(0) nanoparticles. The ether-containing salt 49-NTf2-1/12 in particular provided small nanoparticles with a very narrow size distribution (3.4 ± 0.2 nm). More importantly, regularly interspaced Ni(0) nanoparticle arrays were observed by TEM when [C12C12im][NTf2] or 49NTf2-1/12 was used as the catalytic reaction medium and stabilizer. The authors proposed that this is a consequence of the order imparted by the long alkyl chains, but it is not clear whether liquid-crystallinity plays a role. In continuation of their previous work on N-heterocyclic carbene metal complexes,96 the Lin group reacted LC long-chain 1,3-bis(n-alkyl)imidazolium and 1,3-bis(n-alkyl)benzimidazolium halide salts with Ag2O in dichloromethane to prepare (nonmesomorphic) Ag(I) N-heterocyclic carbenes of different structures.1133 When excess molar ratios of Ag2O were used, the formation of silver nanoparticles was observed along with the Ag(I)−NHC complexes. The pure Ag(I)−NHC complexes could also be used as “single-source” precursors for silver nanoparticles by reduction with NaBH4 in a biphasic system. Their mixtures with the corresponding LC 1,3-bis(nalkyl)imidazolium chloride salts are also LC, but the formation of silver nanoparticles in the mesophase was not tried. Wang et al. prepared blue- and green-emitting, stable zinc oxide nanocrystals by action of LiOH on the zinc(II) salts of 1(n-alkyl)-3-(carboxymethyl)imidazolium chlorides.1134 The latter were prepared by reacting 22-Cl-1 or 22-Cl-16 with DR


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nium-based ILCs with three equivalent long n-alkyl chains as partially ordered, nonpolymeric solvents for 3D structural studies of organic solute molecules via NMR spectroscopy.432,495−499 These ionic mesogens can show smectic phases that may easily be oriented in the strong magnetic field of an NMR spectrometer and that are characterized by low overall order parameters (∼10−2−10−3, as measured by 2H NMR spectroscopy on added selectively deuterated solutes as well as on the neat partially deuterated ILCs). Conventional, nonionic thermotropic LCs (typically rod-like nematic LCs) have been used before as media for NMR studies. Detailed information about the molecular geometry and orientation of solute molecules can be derived from residual dipolar or quadrupolar couplings (in 1H and 2H NMR spectroscopy, respectively).1152 However, because the order parameters of these LCs are usually quite large, the 1H NMR spectra of molecules dissolved in them become exceedingly complicated, as the number of interacting nuclei increases as a result of “strong coupling” (i.e., a large number of lines originating from residual dipolar couplings appear in the spectra).496 Media with relatively low order parameters that can be oriented in magnetic fields, that are able to retain their LC behavior up to high concentrations of a (nonmesogenic) solute (which makes amphotropic LC properties of interest), and in which the dissolved molecules are only “weakly coupled” are therefore useful alternatives because they can yield simpler, wellresolved spectra. As such, the ILCs studied by the Weiss group present a nonaqueous alternative to typically used aqueous and weakly aligning lyotropic LCs for the NMR determination of structural parameters of solutes of interestnot only small organic molecules (such as acetonitrile,496,499 alcohols,497−499 DMSO,495,499 CH3I,495 cis,cis-mucononitrile,495 etc.), but eventually also medium-sized biomolecules. The order parameters of these ILCs can be tuned through the type of anion, the concentration (and nature) of the added solute, and the temperature. The coexistence of oriented and isotropic phases in certain temperature ranges for some of the investigated ILC systems allows for the calculation of both anisotropic and isotropic parameters under identical sample conditions.499

Zn(OH)2. The IL/ILC precursor molecules 22-Cl-n function merely as stabilizers for the ZnO nanoparticles. It should be noted that lyotropic ILCs have also been successfully used as soft, nanostructured templates for the synthesis of anisometric gold nanoparticles. Firestone and coworkers exploited LC gels formed by the nonmesomorphic IL [C10mim][Cl] in an aqueous solution1135−1138 of HAuCl4 for this purpose.1139 Photochemical reduction of the [AuCl4]− ions in the constrained aqueous domains of the ionogel resulted in gold nanoparticles with a variety of sizes and morphologies, including trigonal prismatic nanorods. Replacement of [C10mim][Cl] by [C10vim][Cl] allowed the stabilization of the gold-containing self-assembled ionogel by photopolymerization, and this also prevented leaching of the metal nanoparticles.1140 Such a nanostructured composite exhibits both ionic conductivity (through the polymerized IL) and electronic conductivity (through the encapsulated columns of tightly packed gold nanoparticles) if it contains a sufficiently large amount of gold (Figure 73).1141 Furthermore, the plasmonic

Figure 73. Left: Schematic representation of the lyotropic Colhex phase shown by a LC ionogel formed by [C10vim][Cl] in an aqueous solution of HAuCl4. UV irradiation of the ionogel leads to reduction of the [AuCl4]− ions inside the hydrophilic column centers, yielding anisotropic gold nanoparticles (shown in red), and to polymerization of the imidazolium cations (shown in gray). Middle: Topographic AFM image showing the internal structure of alternating polymer layers (= “i”) and hydrophilic channels (incompletely) filled with gold nanoparticles (= “ii”). Right: Schematic representation of the AFM image. Adapted with permission from ref 1141 (http://dx.doi.org/10.1039/ b921444p, http://dx.doi.org/10.1039/b910059h). Copyright 2009 The Royal Society of Chemistry.

12.4. Electrochromic Materials

Redox-active ILCs can be electrochromic: they may show different colors depending on the applied potential.1153 In an efficient electrochromic device, fast formation of an electric double layer at the electrode surface is essential to avoid electrode polarization. Fast electronic charge transfer from the electrode to suitable π-conjugated systems is also required. Generally, electrochromism based on π-conjugated materials is achieved in thin films deposited on electrodes that are dipped in electrolyte solutions. The Kato group demonstrated electrochromism for ILCs in their bulk LC state, without the need for an additional electrolyte.694,695 This was initially shown for a nanostructured smectogenic ILC with a phenylterthiophene moiety covalently linked to an imidazolium group via a flexible alkyl spacer (77).694 The ionic headgroups act as ion-conductive part, and the π-conjugated phenylterthiophene groups allow for electronic charge transport. Nanosegregation of the latter from the ionic moieties leads to the formation of layered LC structures consisting of alternating 2D pathways for electronic charges and ionic species, respectively. The mechanism with p-type semiconducting moieties as in 77 is as follows: when they are heated between two ITO-coated glass slides the ionic mesogens form their nanostructured LC phase, and spontaneous alignment can occur; upon application of an appropriate voltage to the LC film,

spectrum of the gold nanoparticles could be tuned by selection of the solvent: in the dried composite there exist strong particle− particle interactions arising from aggregated 1D gold clusters or chains of gold nanoparticles, and these result in plasmon resonances located in the near-IR region; upon swelling in alcohol, however, the composite becomes more disordered, which is accompanied by a shift in the surface plasmon resonance to 527 nm, consistent with isolated, noninteracting particles. Naturally, thermally stable ILCs can be suitable reaction media and structure-directing (templating) agents1142−1144 for the ionothermal and hydrothermal synthesis of various inorganic materials, such as mesoporous silica (see also above), zeolites, etc.1125,1143−1145 Thanks to their anisotropic nature, ILCs can be advantageous when compared to ILs.208−214,1121,1124,1145−1150 Because this application generally makes use of lyotropic (I)LCs or the lyotropic properties of thermotropic (I)LCs, it will not be further discussed here. Encouraged by good results with the IL [C4pyr][NTf2], Vieira and Falvey proposed to use ILCs as organized media to increase the rate of solvent-facilitated photoinduced electron transfer (PET) processes.1151 As was already mentioned in section 7, Weiss and co-workers investigated the use of amphotropic ammonium- and phosphoDS


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poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) on the cathode surface allows coupling of the fast anodic oxidation of 77 and 78 with the fast cathodic reduction of PEDOT-PSS. This results in a fully reversible electrochromic response, lower driving voltages, and shorter response times. Overall, rather high ionic conductivities of the pure LC materials are required for a stable electrochromic response in the absence of an additional electrolyte. The following ionic conductivity values were found for the aligned SmA phases of 77−79: 77, σ|| = 3.9 × 10−4 S cm−1 at 120 °C; 78, σ|| = 1.2 × 10−4 S cm−1 at 130 °C; 79, σ|| = 4.7 × 10−4 S cm−1 at 160 °C. Other examples of electrochromically responsive ILCs are ionic mesogens incorporating redox-active viologen units (290 and 292-n), which can show a color change from yellow to blue during electrochemical reduction to the radical cation state (Figure 55).905,906 Application of a negative voltage over a film of 290 sandwiched between two ITO-coated glass slides resulted in such color change, and in a change in the POM texture (the latter observation suggests a change in molecular arrangement).905 Moreover, the same color change was observed for 290 upon heating above 75 °C, without the need for an external voltage; in this case, electron transfer occurs from the [(C12H25O)3PhSO3]− anions to the viologen dications. Pairing of the formed radical monocations (i.e., pimerization), which is accompanied by a color change to violet, also occurred to a certain extent. The dication state could be restored upon oxidation in air. No reduction cathode peak current, nor a (voltage- or temperatureinduced) color change were observed for ILC 291-Y2, though. This was ascribed to the higher content of nonconductive alkyl chains and lower ion mobility in the latter compound. Salts 292-n display reversible electrochromism in dichloromethane solution (with added [N4,4,4,4][ClO4]), going from light-yellow to darkblue upon reduction from the dication state to the radical cation state.906 Stoddart, Kato, and co-workers demonstrated electrochromic switching of the LC bistable [2]rotaxane 296 in solution908 but also in its condensed SmA phase.909 Within 10 s after application of a 1.6 V voltage, the whole LC film changes color from greenish brown to reddish purple as a result of redox-driven shuttling of the positively charged CBPQT4+ macrocycle from the oxidized tetrathiafulvalene (TTF) to the 1,5-dioxynaphthalene (DNP) recognition site of the rotaxane (Figure 76). This oxidation process and mechanical movement can occur without any changes in the mesomorphism, as suggested by the lack of any significant changes in the birefringent POM texture of the LC

holes are injected from the anode into the phenylterthiophene parts, and the holes are transported between them, leading to oxidation of the π-conjugated moieties and a concomitant color change; at the same time, a sufficiently high ionic conductivity should allow the mobile counterions to migrate rapidly toward the anode under influence of the electric field, to form an electric double layer and prevent electrode polarization that otherwise limits the electronic charge injection (in electrochromic systems based on neutral organic semiconductors, this polarization is prevented by the mobile ions in the electrolyte solution) (Figure 74).694 Application of a positive voltage to a LC layer of 77

Figure 74. Electrochromism in the bulk LC state of 77. The neutral phenylterthiophene moieties are depicted by red cylinders, while they are displayed as green cylinders in their oxidized state. The imidazolium headgroups and [OTf]− anions are depicted as light blue and dark blue spheres, respectively. Reprinted with permission from ref 694. Copyright 2008 American Chemical Society.

indeed resulted in a color change from pale yellow to dark blue (Figure 75; the color change is attributed to the formation of

Figure 75. Electrochromic response of a film of 77 (in its SmA phase at 120 °C) at an applied voltage of 2 V (left) and 0 V (right). In this LC cell, the cathode is coated with a thin PEDOT-PSS layer to lower the driving voltages (see text). Reprinted with permission from ref 695. Copyright 2010 American Chemical Society.

radical cations). The original color was restored when the voltage was decreased to −1 V. Importantly, no color changes were observed with a film of a neutral analogue that also contains the mesogenic phenylterthiophene group, but lacks an ionic headgroup at the end of the dodecyl chain. Electrochromism was also observed for imidazolium ILCs with a tetrafluorophenylterthiophene (78) or terthienylphenylcyanoethylene (79) moiety.695 The latter compound was found to exhibit reversible anodic oxidation and cathodic reduction (thanks to the electron-withdrawing cyano group, which stabilizes the reduced anionic state) in its SmA phase at 160 °C, while the cathodic reduction of 77 and 78 is irreversible and requires a higher driving voltage.695 However, deposition of a thin, electron-accepting PEDOT-PSS layer (PEDOT-PSS =

Figure 76. Electrochemical switching of 296 in its LC mesophase (LC film on an electrode surface, in an electrochromic cell with a polymer electrolyte). Left: Redox-driven mechanical movement (shuttling) of the CBPQT4+ ring. Right: Color changes of the LC film of 296 upon changes in the applied voltage. Reproduced in part with permission from ref 909 (http://dx.doi.org/10.1039/b922088g). Copyright 2010 The Royal Society of Chemistry. DT


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film. The original greenish brown color is restored within 40−50 s after application of a negative potential. Cyclic voltammograms of thin films of 1:1 complexes of 297-X with TTF show two consecutive one-electron oxidations of the TTF moieties, as well as reduction of the CBPQT4+ macrocycles.162 The spectroelectrochemical response of these thin films was studied as well: partial oxidation induces a color change from green to blue, whereas application of a negative voltage results in a color change to violet as a result of pimerization of partially reduced CBPQT4+ units. Electrochromism was also observed for some ISA complexes of [EuP5W30O110]12− with cationic surfactants (433-(cation)12).125 The photophysical and spectroelectrochemical properties of LC thienoviologens 301-NTf2-9 and 301-NTf2-11, which show both electrochromic and electrofluorochromic responses in their bulk LC states,922,923 will be discussed in the next section.

temperature to be useful for detection of phase transitions.1161 Intense NIR emission was observed by doping 1 mol % of the βdiketonate complexes [Ln(tta)3(phen)] (Ln = Nd3+, Er3+, Yb3+; tta− = 2-thenoyltrifluoroacetonate, phen = 1,10-phenanthroline) in the ILC [C12mim][Cl] host.1162 Luminescent ILCs could also be obtained by doping the lanthanide bromides EuBr2, SmBr3, TbBr3, and DyBr3 in the ILC [C12mim][Br].1163 The doping concentrations of about 1 mol % did not appreciably influence the mesophase behavior of [C12mim][Br]. However, it was found that the mesophase of [C12mim][Br] is stabilized by addition of larger amounts of the lanthanide bromides, to such an extent that the samples remain LC to temperatures below room temperature. [C12mim][Br] itself shows blueish-white emission upon excitation with UV light, originating from the imidazolium cation. The samples doped with EuBr2 showed blue emission (due to 4f−5d transitions); SmBr3-doped samples red emission; TbBr3-doped samples green emission; and DyBr3-doped samples orange emission (emission by SmBr3, TbBr3, and DyBr3 is due to 4f−4f transitions). As for the [C12mim][Cl]/EuX3 mixtures, the emission color of the DyBr3-doped samples could be tuned from blueish-white to orange-yellowish, depending on the excitation wavelength: excitation with 366 nm light resulted in blueishwhite luminescence from the imidazolium cations, whereas excitation with 254 nm light led to orange-yellowish emission from the dysprosium(III) complex.120 The imidazolium cations were found to act as sensitizers, which can activate Dy(III) via energy transfer. Likewise, the emission color of the TbBr3-doped samples could be tuned from blueish-white to green.119 If the [C12mim]+ imidazolium cation was replaced by the nonaromatic [C12mpyrr]+ pyrrolidinium cation (which does not act as a sensitizer), the emission became less intense, but the luminescence decay times became somewhat longer. The possibility to tune the luminescence color from blueish-white to green by a change of the excitation wavelength is lost by doping TbBr3 in [C12mpyrr][Br]. [C12mim]4[EuBr6]Br (37) is a room-temperature ILC, and it shows a strong red photoluminescence when cooled to 77 K.118 In contrast, the mesomorphic tetrakis(2-thenoyltrifluoroacetonato)europate(III) imidazolium salt 85-Eu(tta)4 that was reported by Goossens et al. displays an intense red photoluminescence of high color purity at room temperature.122 The SmA phase shown by 85-Eu(tta)4 can be vitrified on fast cooling. No major differences in the luminescence properties were observed between crystalline and vitrified samples, indicating that the lanthanide site symmetry (first coordination sphere) does not change significantly when heating the metal complex across different phase transitions. Intense red photoluminescence was also observed for homogeneous LC mixtures of (nonmesomorphic) 308-Eu(tta)4-4/10 (e.g., 1 mol %) with 308-Br-4/10.116 The first coordination sphere of the [Eu(tta)4]− anion remains largely intact in the presence of the bromide salt. The luminescence decay times were found to be shorter for the mixtures than for pure 308-Eu(tta)4-4/10; this may be caused by hydrogen bonding between the bromide anions and the 2thenoyltrifluoroacetonate ligands, leading to more vibrational deactivation pathways for the excited states. As described in section 11, several LC nanocomposite ISA materials have been prepared by combining negatively charged polyoxometalate complexes with cationic surfactants (SEP complexes).144 Luminescent ionic lanthanidomesogens were obtained by using lanthanide-containing polyoxometalates: [EuW10O36]9− (428-(cation+)9),127,130 [Eu(PW11O39)2]11− (431-(cation + ) 11 ), 127,130 [EuP 5 W 30 O 110 ] 12− (433-(cati-

12.5. Anisotropic Photoluminescent Soft Materials

Photoluminescent and electroluminescent LCs are of interest for use in emissive LCDs and other light-emitting devices such as organic light-emitting diodes (OLEDs).86 An interesting property of luminescent mesomorphic materials that contain suitable chromophores and that can be macroscopically aligned in their LC state without any defects is their ability to generate linearly polarized emission.1154−1156 Provided these systems have a sufficiently high brightness and stability, they can be used in a LCD as a source of polarized light and make some of the expensive and energy-inefficient polarization filters and color filters redundant. The high charge-carrier mobility shown by many ILCs should be beneficial for electroluminescence properties, although this field remains almost entirely open to exploration. Ionic lanthanidomesogens or lanthanide-doped ILCs have been studied as interesting types of luminescent soft materials.86,1157,1158 The emission observed for these materials is metal-centered between energy levels within the 4f shell of the trivalent lanthanide ion; thus the emission is originating from intraconfigurational f−f transitions. An advantage of luminescence by trivalent lanthanide ions is that their luminescence spectra consist of narrow emission lines of high color purity.1159,1160 The relative intensity of the emission lines and their fine structure is only marginally influenced by the nature of the ligand in the first coordination sphere, with the exception of the so-called “hypersensitive transitions”. The excited states have a long lifetime at room temperature, in the order of microseconds or even milliseconds. The emission wavelength can be tuned by a proper choice of the lanthanide ion: red emission by Eu3+, green emission by Tb3+, blue emission by Tm3+, orange emission by Sm3+, and NIR emission by Nd3+, Er3+, and Yb3+. Bünzli and coworkers dissolved different Eu(III) salts in the smectogenic ILC [C12mim][Cl].487 They observed that concentrations of Eu(III) salts as high as 10 mol % do not appreciably alter the LC behavior of the host matrix. Interestingly, the emission color of the Eu(III)-containing LC mixture could be tuned from blue (emission color of the host imidazolium matrix) to red (emission color of Eu3+), depending on the excitation wavelength and the counterion of Eu(III) (Cl−, [NO3]−, [ClO4]−, [OTf]−). In an attempt to detect the melting point and the clearing point of [C12mim][Cl] by monitoring the spectroscopic and photophysical properties of Eu(NO3)3 doped into the ILC, Bünzli and co-workers concluded that the variations of the luminescence intensities and lifetimes are too smooth as a function of DU


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on+)12),125 [Eu(SiW9Mo2O39)2]13− (434-(cation+)13),124 and [Eu(SiW11O39)2]13− (435-(cation+)13).130 The photoluminescence properties of the LC complexes of [Eu(BW11O39)2]15− (437-(cation+)15)128 and [Tb(SiW11O39)2]13− (436-(cation+)13)126,127 were not reported. The luminescence of films of the decatungsteuropate ([EuW10O36]9−) anion in combination with [N16,1,1,1][Br] (CTAB), n-octadecylamine (ODA), or 4-(nhexadecyl)aniline (HDA) was found to be sensitive to acidic (HCl) and basic (NH3) gases.1164 Interestingly, polarized luminescence was observed for self-organized films of [N18,18,1,1]9[EuW10O36]·9H2O that were prepared by solvent casting.1165 Depending on the position of the polarizers before and after the sample, the transitions in the luminescence spectra showed different intensities. Only the total luminescence intensity was found to change and not the relative intensity of the different transitions. A nanocomposite material consisting of [EuW10O36]9− in combination with a mixture of [N12,12,1,1]+ and N-(n-dodecyl)-N,N-dimethyl-N-[10-(4-(2-(pyridin-4-yl)vinyl)phenoxy)decan-1]ammonium ([N10(O-(C6H4)-CH=CH-(C5H4N)),12,1,1]+) operated as a luminescent logic gate, with dual output logic function (INHIBIT and NOR).1166 Although most of the work on the lanthanidecontaining polyoxometalates has been performed on europium(III)-containing samples, some samarium(III)-containing materials have been described as well. Examples include [N18,18,1,1]9[SmW10O36](· xH2O) and [N12,1,1,1]9[SmW10O36](· xH2O).1167,1168 The potential thermotropic mesomorphism of these compounds was not investigated. Besides lanthanidomesogens, other metal-containing luminescent ILCs have been reported as well. Imidazolium salts 38-n, which contain the [Mo6Cl14]2− cluster complex anion, show bright red luminescence centered at about 730 nm.622 As in the case of the imidazolium salts with [LnBr6]3− anions, the imidazolium cations were found to act as sensitizers that can transfer energy to the metal-containing anion. The related rhenium- and molybdenum-based clustomesogens 390-(405)4, 390-(406)4, 391-(406)2, 392-(406)2, 393-(406)2, 394-(406)2, 395-(406)2, 396-(406)2, and (397-n)-(406)2 (n = 2−3) (all with [M6Xi8Xa6]n− anions, see Figure 66), which were reported by Molard, Cordier, and co-workers, also show bright red-NIR long-lived luminescence between 600 and 800−900 nm in their c o n d e ns e d st a t e s (m e s o p h a s e o r v i t r ified m es ophase).121,442,444,1008 Their phosphorescence originates from the metal clusters; it is not of the ligand-to-metal charge transfer (LMCT) type. They show large Stokes shifts, and can in principle be excited with commercial blue LEDs, which is interesting with respect to applications (see also below). Curves of luminescence intensity as a function of temperature were recorded for a few of the compounds and showed a change in slope around their clearing point. The oxidized, dark green complexes 389-(405)3 and 389-(406)3, with paramagnetic [Re6Se8(CN)6]3− anions, are nonluminescent. Together with the well-known lanthanidomesogens that contain Nd3+, Er3+, or Yb3+ ions,88,1169 and the ISA complexes 383-(407-n)2 (see below), the luminescent [M6Xi8Xa6]n− ILCs constitute a great part of the reported intrinsically LC materials that show luminescence in the near-infrared region of the electromagnetic spectrum.1014 The advantage of the nonlanthanide-based compounds is that they do not have to rely on the “antenna effect”1159 for efficient NIR luminescence. For some clustomesogens, an enhancement in luminescence properties (such as a higher quantum yield) as compared to precursors with alkali

metal cations was tentatively ascribed to an antenna effect from the cyanobiphenyl units to the metallic clusters,121 similar to the effect of the imidazolium moieties in 38-n, which was mentioned above. This shows that the organic countercations can influence the spectroscopic properties. As discussed in section 11, Faul and co-workers also found this for the europium-containing SEP complexes 433-(cation+)12, whose luminescence properties even completely disappeared when a ferrocene-containing cationic surfactant (447-n) was used (due to charge transfer between the polyoxometalate anions and the ferrocenyl groups).125 The precursors of the LC complexes mentioned above, with inorganic countercations, are ceramic-like solids that are difficult to integrate in a functional device. The clustomesogens, on the other hand, are more easily processable. Nematogens 394(406)2, (397-2)-(406)2, and (397-3)-(406)2 were used to construct electroswitchable luminescent cells.121,444 Because the nematic phase of these ionic complexes is quite viscous, they still had to be mixed with commercially available nematic LC matrixes (E44 and E7, respectively) to achieve sufficiently fast responses. With mixtures containing quite high concentrations of complexes (397-2)-(406)2 and (397-3)-(406)2 (up to 20 wt %), orientational switching of the LCs upon application of a voltage on a LC cell (5 μm thick) with planar alignment layers was still possible. Moreover, application of an AC voltage resulted in a clear modulation of the integrated photoluminescence signal (by 45−50%) that was higher than the birefringence of the LC hosts, despite the fact that the metallic clusters are isotropic emitters. The original luminescence intensity was restored upon removal of the electric field. This phenomenon is not yet fully understood, but the authors proposed that the ability of the cyanobiphenyl units to act as anisotropic sensitizers can give at least a partial explanation (scattering effects may be important as well). Some emission lifetime measurements were performed at elevated temperature to mimic the working conditions in a LCD, where the backlight can generate temperatures up to 50−85 °C.121 As mentioned in section 5.1, the gold-containing ILCs [Cnmim][Au(CN)2] (11-Au(CN)2-n) display gold-based phosphorescence, for which the emission maximum depends on the physical state of the compounds: the emission band at 412 nm in the crystalline solid state at room temperature is red-shifted toward 458 nm upon heating to the mesophase.111 The authors suggested that a different disposition of the [Au(CN)2]− complex anions in the solid and LC phases results in different aurophilic intermolecular interactions (larger number of gold− gold interactions and/or shorter gold−gold distances in the LC state). On the other hand, for the analogous silver-containing salts 11-Ag(CN)2-n, only blueish luminescence originating from the imidazolium cations (around 432 nm), as also displayed by [Cnmim][BF4], was observed, presumably due to a lack of short silver−silver interactions. Nevertheless, Hosseini and co-workers observed blue phosphorescence by the silver-containing bis(amidinium) ILC 316-Ag(CN)2.112 This emission presumably originates from short silver−silver interactions specifically imposed by the molecular self-assembly (Figure 60).957 Douce and co-workers did not investigate possible luminescent properties of 41-Au(CN)2-12 and 41-Ag(CN)2-12.110 For tetrabromouranyl salts 303-n, no photoluminescence could be observed in the condensed state, probably due to autoquenching. However, solutions in the IL [C4mpyrr][NTf2] showed green luminescence.115 Ren and co-workers reported on the nonmetal-centered luminescence by a 1,1′-bis(n-decyl)-4,4′-bipyridinium salt with DV


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homogeneous blue anthracene fluorescence. The anthracenecontaining ILCs 293-X-n that were reported by Kohmoto et al. are also fluorescent, although emission spectra were only measured for the vitrified mesophases of 293-PF6-n (n = 8, 12) that were obtained after rapid cooling to room temperature.907 These are red-shifted relative to the fluorescence spectra of the same compounds in their solid state before heating. This suggests increased interaction between the anthracene moieties when the LC phase is formed, due to partial planarization of the central aromatic system. The fluorescence of both [PF6]− salts in solution is dependent on the solvent polarity. Crystals of the nonmesomorphic salts 293-Cl-1 and 293-PF6-1 were shown to exhibit piezoluminescence and vapoluminescence. As outlined in section 11, Ziessel and co-workers also presented quite unique ISA complexes that contain both a cationic and an anionic luminescent BODIPY part (383-(407n)2).1009 Because of the close proximity of the differently substituted BODIPY chromophores in the ISA complexes, these materials display efficient intracomplex electronic energy transfer, both in solution and in their Colhex mesophase. The blue-absorbing anionic [sulfoBODIPY]2− (383) acts as the donor moiety, and the red-absorbing cationic distyryl-BODIPY moiety (407-n) as acceptor. Thus, upon illumination of 383(407-12)2 with 500 nm light to excite predominantly the [sulfoBODIPY]2− (383) part, strong emission centered at 670 nm originating from 407-12 was observed (quantum yield in THF solution = 0.40), but nearly no luminescence from [sulfoBODIPY]2− (which is expected around 520 nm). Similar results were obtained for a nonmesomorphic covalently bonded dyad comprising neutral analogues of 383 and 407-12, respectively. However, when 407-12 was mixed with the neutral equivalent of 383 (without the sulfonate groups) in THF, no electron energy transfer was observed. An annealed film of 40712 and this neutral analogue also showed much less efficient energy transfer. This emphasizes the importance of the electrostatic interactions to form the functional ISA complex. Complete dissociation of 383-(407-12)2 in THF solution by addition of a large excess of [N(C4H9)4]+ restored emission from the donor as well. The first (distyryl-BODIPY)-ammonium ion was readily replaced by [N(C4H9)4]+, but this had only a minor effect on the fluorescence characteristics; replacement of the second ammonium cation required a much larger excess of [N(C4H9)4]+ because of the greater preference of 383 for 407-12 than for [N(C 4 H 9) 4] + (presumably thanks to ancillary interactions that do not occur with [N(C4H9)4]+). The fluorescence spectrum of 383-(407-12)2 in its Colhex phase is broadened and red-shifted with respect to its spectrum in THF, and extends to the NIR region (850 nm). Photoluminescence was also observed for the metal-free tripodal salts 268-X-Yn-(a−c), both in solution and in their solid and LC states.885,886 The light emission originates from the intramolecular charge transfer (ICT) excited state of the independent donor−acceptor (push−pull) π-conjugated chromophore in each of the three arms, and, at least for the series 268X-Y1-a,885 is almost independent of the nature of the counterions (Br−, [BF4]−, [PF6]−). The ICT character of the emission was confirmed by DFT calculations and by the study of substituent and solvent effects. By variation of the cationic acceptor structure (pyridinium versus more electron-deficient pyrimidinium versus π-expanded quinolinium) and the donor substituents (alkyloxy chains versus dialkylamino groups), a series of luminescent ILCs covering the visible region from blue-green to red could be obtained.886 In the series 268-PF6-(Y1-Y3)-b, the absorption and

the bis(maleonitriledithiolato)zincate(II) ([Zn(mnt)2]2−) anion (300).438 Ziessel, Camerel, and co-workers synthesized SmA- and Colhex-forming photoluminescent tris(8-hydroxyquinoline-5sulfonato)aluminum(III) complexes (385-(cation+)3).1004 Their work nicely illustrates the use of ISA to transform watersoluble fluorophores into compounds that are soluble in nonpolar volatile organic solvents. A water-soluble derivative of the well-known electroluminescent compound tris(8hydroxyquinoline)aluminum(III) (Alq3), tris(8-hydroxyquinoline-5-sulfonato)aluminum(III), was combined with amphiphilic quaternary ammonium cations. Homogeneous thin films of the resulting ISA compounds (385-(cation+)3) could be prepared by spin-coating solutions of the complexes in toluene (or via simple dropcasting). The thin films showed a higher photoluminescence quantum yield and longer excited-state lifetimes as compared to the toluene solutions. Also, the fluorescence quantum yield of thin films and toluene solutions of 385-(399-16)3 was found to be higher than that of standard Alq3. Aligned films were obtained by spin-coating on rubbed substrates coated with poly(Nvinylcarbazole) (PVK) and poly(3,4-ethylenedioxythiophene) (PEDOT). However, no polarized luminescence was observed for the surface-treated samples, because of unspecific orientation of the pseudospherical fluorophores. Electroluminescent properties were not investigated. The same group presented several metal-free luminescent ILCs that were prepared via ISA, following work by Faul and coworkers.152 The fluorescent materials were formed by combining a negatively charged luminophore with an oppositely charged (nonluminescent) surfactant. 4,4-Difluoro-4-bora-3a,4a-diaza-sindacene dyes (better known as BODIPY dyes) are popular fluorophores, due to their high photostability, large molar absorptivities, narrow emission band, and very high emission quantum yields.1170 BODIPY dyes are also relatively easy to modify, and such chemical modifications allow one to tune the emission properties. Fluorescent LC materials were obtained by combining an anionic BODIPY disulfonate ([sulfoBODIPY]2−) with 3,4,5-tris(n-alkyloxy)benzyl-functionalized (“Kato-type”) imidazolium cations (383-(403-n)2).160 The strong fluorescence of the BODIPY group is maintained in the mesophase. Fluorescence measurements confirmed the presence of J aggregates, indicating that the BODIPY units are not isolated from one another in the solid state and in the mesophase (this was also supported by PXRD measurements). The aggregate formation also results in a strong red-shift of the emission bands, as compared to the solution spectra. Ammonium salts of amido derivatives of 3,4,5-tris(n-alkyloxy)benzoic acid combined with [sulfoBODIPY]2− (383-(400-n)2) or with a sulfonated porphyrin derivative ([TPPS]4−) (384-(400-n)4) also gave compounds that were luminescent in the mesophase.158 Interestingly, observation of the samples without polarizers by fluorescence microscopy in combination with a heating stage revealed the presence of domains with different fluorescence colors. These differences in color were attributed to different orientations of the molecules in different domains of LC thin films. Finally, mesogenic imidazolium cations combined with an anionic anthracene derivative (382-(403-n)2) also yielded luminescent LC materials.159 Just as in the case of the BODIPY materials, the strong luminescence of anthracene was maintained in the mesophase. The fluorescence spectra confirmed the presence of J aggregates, indicating intermolecular interactions between the anthracene moieties in the mesophase. No domains with different emission colors were observed: thin films showed DW


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the work of the Kato group694,695 that was discussed in section 12.4). Both the electrochromic and the electrofluorochromic responses in the bulk were found to be reversible. Different fluorescence intensity contrast ratios could be obtained by varying the applied voltage (Figure 77). The switching times are

emission maxima shift to higher wavelengths in the order pyridinium < pyrimidinium < quinolinium. Surprisingly, the salts with six alkyloxy chains show a higher (i.e., bathochromically shifted) absorption maximum than their counterparts with nine of such donor substituents. All quinolinium derivatives (268PF6-Y3-(a−c)) show a much higher photoluminescence quantum yield in their condensed state at room temperature (annealed film) than in chloroform solution. This was attributed to aggregation-induced emission enhancement, the origin of which is not yet fully understood (probably aggregation-induced restriction of molecular motion and/or planarization of the chromophores play a role). 886 The aggregation-induced emission of THF solutions of 268-PF6-Y3-b upon addition of water as a “poor” solvent was also reported.1171 The pyridinium derivatives (268-PF6-Y1-(a−c)), on the other hand, show a higher quantum yield in solution than in their condensed state, probably because the less sterically demanding pyridinium-based chromophores can more easily assemble in a close-packed structure in the solid or LC state as compared to the bulkier quinolinium moieties. For as-spin-coated films, two fluorescence decay components were found, the shortest one of which became predominant after annealing. Annealing causes a closer packing and interaction of the LC molecules. Vibrational deactivation pathways are presumably suppressed in the as-spin-coated films because of the restriction of intramolecular vibrational and rotational motions. The authors did not report on aligned monodomain samples. The 4-(n-alkyloxy)substituted pyridinium salt 248-Br is also luminescent in solution and in its solid and LC states.873 In the latter states, it shows blueish-green emission originating from aggregation. Beneduci et al. demonstrated that the LC thienoviologens 301-NTf2-9 and 301-NTf2-11 show both electrochromic and electrofluorochromic responses in their bulk LC states.922,923 [Remark: More recently, the electro- and spectroelectrochemical properties of 301-NTf2-12 dissolved in propylene carbonate were reported in detail as well.921] These are the first examples of LCs showing electrofluorochromism in the mesophase. The latter property involves a change in photoluminescence features (i.e., fluorescence color and/or intensity) in response to electrochemical stimuli (redox processes). Thienoviologens 301-NTf2-9 and 301-NTf2-11 are intrinsically switchable electroactive fluorophores: they combine a redox-active and electrochromic extended viologen architecture with a highly luminescent π-conjugated core (it should be noted that the neutral thienoviologen precursor, 5,5′-bis(4-pyridyl)-2,2′-bithiophene, does not show any appreciable fluorescence). Even in the bulk LC state, their fluorescence quantum yield is quite high (up to 0.68), despite the rather high temperatures at which the measurements were performed (>100 °C). The emission in the LC state is heavily red-shifted (by 100−140 nm) with respect to the emission in solution or as a film in the isotropic liquid state, due to aggregation. When a suitable reductive voltage is applied on a micrometer-thick film of 301-NTf2-9 or 301-NTf2-11 in their LC state between two electrodes, direct reduction of the viologen-based fluorophore to its radical cation results in a color change from red to almost black (= electrochromism), but also in a roughly 10-fold increase of the fluorescence intensity and a slight fluorescence red-shift (= electrofluorochromism). This is what happens: after easy electron injection at the cathode, electrons move rapidly through the film via a hopping mechanism to reduce the thienoviologen dications; at the same time, electrode polarization is prevented thanks to the efficient migration of the [NTf2]− counterions toward the anode (see also

Figure 77. Electrochemical fluorescence switching (monitored at 630 nm, with excitation at 450 nm) of 301-NTf2-9 and 301-NTf2-11 in their LC mesophase. (a) Fluorescence switching of 301-NTf2-9 in its Colrec,o,1 phase, upon application of −0.7 V (cyan), −1.0 V (magenta), −1.5 V (blue), and −2.0 V (black) voltages for 10 s. (b) Fluorescence switching of 301-NTf2-11 in its SmA phase, upon switching between −1.0 and +1.0 V for a step duration of 5 s. Reprinted in part with permission from ref 922. Copyright 2014 Macmillan Publishers Ltd.

relatively short (in the order of a few seconds) thanks to the efficient electron transport in the nanosegregated mesophase via π−π intermolecular interactions, and the relatively high ionic conductivity of the pure ILCs.923 High voltages resulted in full reduction of the dications to their neutral state, which is essentially nonemissive. Application of a negative voltage on the isotropic liquid state (rather than the LC state) of 301-NTf2-11 caused a substantial red-shift of the fluorescence, resulting from field-induced molecular ordering and formation of a (more densely packed) SmA phase. This process was found to be reversible and was accompanied by the large increase in fluorescence intensity mentioned above. This is a highly interesting observation: not only does it show that besides the fluorescence intensity also its color can be modulated, but it also demonstrates directly how heavily a change in the total charge of a mesogen affects its phase behavior under the influence of an electric field. The authors also noted that reduction of the dications in their LC state did not alter the textures of the mesophases. More recently, 301-NTf2-9 was incorporated in a polymer gel based on poly(vinyl formal), N-methyl-2-pyrrolidone, and ferrocene, to construct a singlelayer electrofluorochromic device that operates efficiently at room temperature.1172 DX


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green fluorescence could be recovered, and this process could be repeated. PXRD showed that 237 is in a more amorphous state after mechanical grinding. The LC receptor−anion complexes that were designed by Maeda and co-workers (361-a-H·Cl−/369-n, 361-a-CH3·Cl−/ 369-3, 361-a-F·Cl−/369-3, 361-a-H·Cl−/[Nn,m,x,y]+, 365·Cl−/ 369-3, 366·Cl−/369-3 (not LC), 370-n-R, 371-R1/R2, 372-R1/ R2, and 373-R1/R2) are also fluorescent, both in solution and in the solid, LC, and/or gel state.391,989−993 Condensed-state emission from 361-a-H·Cl−/369-3 occurs at higher wavelengths than in the case of 361-a-H·Cl−/369-16, which was explained by a less efficient “charge-by-charge” stacking in the presence of long alkyl chains in the TATA cation.391 The solid-state fluorescence of the methyl- and fluorine-substituted analogues of 361-a-H· Cl−/369-3, that is, 361-a-CH3·Cl−/369-3 and 361-a-F·Cl−/ 369-3, respectively, is hypsochromically shifted as compared to 361-a-H·Cl−/369-3.990 The solid-state luminescence of salts 361-a-H·Cl−/[Nn,m,x,y]+ is blue-shifted with respect to the emission from the neutral anion receptor 361-a-H; the hypsochromic shift is larger for increased number and length of alkyl chains in the [Nn,m,x,y]+ cation, which could again be explained by a less efficient stacking in the presence of long alkyl chains.991 Substitution of the fluorine substituents on the boron atom by a catechol moiety or by two phenyl groups resulted in moderate changes in the absorption and emission wavelengths of dichloromethane solutions of the neutral anion receptors 365 and 366, respectively, as compared to 361-a-H (bathochromic shifts for the former, hypsochromic shifts for the latter).992 The catechol-boron substitution significantly decreased the fluorescence quantum yield from 0.98 for 361-a-H to only 0.02 for 365 (0.93 for 366). n-Octane gels of 365, 366, and 365·Cl−/3693, and annealed samples of 365, 366, 365·Cl−/369-3, and 366· Cl−/369-3 are fluorescent as well. The diphenyl-boronsubstituted ionic complex 366·Cl−/369-3 shows less efficient stacking of the π-conjugated moieties because of steric hindrance. Complexes 370-16-R display fluorescence in their solid state at room temperature, which is again blue-shifted with respect to the emission from the neutral anion receptors (i.e., without the carboxylate anion and the [N(C4H9)4]+ cation).989 This also applies to the ionic complexes 371-R1/R2, 372-R1/R2, and 373-R1/R2.993 The photophysical properties of imidazolium- and pyridinium-based salts with a 1,3,4-oxadiazole moiety (99-X, 100, 101, 102-X-n/10, 103-X-n, 104, and 105; 105 contains two oxadiazole fragments) and their neutral derivatives (106-n and 107-n) were studied in dichloromethane solution (or methanol in the case of 105).536 Similar spectroscopic properties were found for the imidazolium, bispyridinium, and neutral compounds (99-X, 100, 101, 104, 105, 106-n, and 107-n), on the one hand, and the other pyridinium salts (102-X-n/10 and 103-X-n), on the other hand. The imidazolium cations are effectively decoupled from the chromophore and as such do not exert any substantial influence on the photophysical behavior in solution, which explains the similarity to the neutral compounds and the lack of a dependence of the absorption and emission bands on the counterion. The asymmetric pyridinium compounds 102-X-n/10 and 103-X-n, however, exhibit a more complex photophysical behavior as a result of their donor− acceptor architecture and the possibility of intramolecular charge transfer from the alkyloxy substituents to the pyridinium units, which are part of the chromophore. The absorption bands of these compounds are slightly sensitive to the counterion. The absorption maximum shifts bathochromically in the order

The absorption and emission maxima of π-conjugated phospholium salts 234-X, 235-X, 236-X, and 237 are red-shifted by increasing the conjugation length.845 Remarkably, the emission is blue-shifted with respect to their counterparts with a methyl group instead of a 3,4,5-tris(n-dodecyloxy)benzyl group, probably due to the higher electron density on the phosphonium center imparted by the donating nature of the benzyl group. Furthermore, the photoluminescence quantum yields in solution (dichloromethane) of fused-ring compounds 234-X, 235-X, and 236-X are much lower than those of the methyl derivatives. Fortunately, the quantum yields of 234-X and 236-X are higher in their condensed (solid or LC) state at room temperature. For example, the quantum yield of 234-Br equals 0.005 in solution and 0.11 in the solid state. As in the case of quinolinium salts 268-PF6-Y3-(a−c), this was attributed to aggregation-induced enhanced emission. In solution, photoinduced electron transfer (PET) occurs from the electrondonating 3,4,5-tris(n-dodecyloxy)benzyl groups to the phospholium chromophores, which is probably a nonradiative decay channel. In the condensed state, the intramolecular rotation of the flexible benzyl group is restricted, and in addition PET is suppressed as a result of conformational changes that probably resemble those in solution accompanying an increase in concentration (see section 7 and Figure 51). The conformational changes significantly affect the phosphole-typical σ*−π* interaction, which strongly influences the photophysical behavior. Suppression of PET apparently does not occur for salts 235-X, which are nearly nonluminescent both in solution and in the solid state. On the other hand, the PET process is probably not active in the case of 237 which shows a relatively high quantum yield both in solution and in the solid state. Theoretical calculations pointed to a beneficial intramolecular charge transfer within the conjugated backbone of 237 (with the outer phenyl rings acting as weak donors, and the phosphonium center acting as an acceptor) that competes with PET. The photophysical properties of nonmesomorphic analogues of 234X, 235-X, and 236-X with an electron-donating or electronwithdrawing benzyl substituent were investigated in detail.1173 The shift between the emission wavelength λem in solution and in the condensed state decreases with increasing size of the anion. Larger anions reduce interactions between the π-conjugated phospholium chromophores. The lower solid-state quantum yields and higher λem shifts of 236-X as compared to 234-X were ascribed to more efficient electronic communication between the more extended and exposed planar chromophores of 236-X in the solid state. Because of twisting of the outer phenyl rings, such interactions are less efficient for 237, which exhibits nearly identical emission wavelengths in solution and in the solid state. The emission properties of 234-X, 235-X, and 236-X are also dependent on temperature. The emission intensity of 234-X at −60 °C in dichloromethane is about 30 times higher than at 25 °C thanks to restricted intramolecular rotation at lower temperatures. Temperature changes did not have much impact on the photophysical properties of 237 in solution. This salt, however, shows a mechanical photoluminescence response in its solid state, in contrast to 234-Br, 234-BF4, 235-X, and 236-X that have completely fused ring systems. The fluorescence color of a solid powder of 237 changes from green to yellow after grinding it (mechanochromic photoluminescence). This process was attributed to excimer formation by 237 made possible by chromophore planarization upon grinding (even in the presence of the bulky phosphonium center). After thermal annealing of this metastable state at 80 °C (i.e., still in the solid state), the DY


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[C12H25OSO3]− < [NO3]− < Br−/I− < [BF4]− < [ClO4]−. For both the imidazolium- and the pyridinium-based salts, the counterion was found to have a significant effect on the quantum yield, in the order [ClO4]− > [BF4]− > [C12H25OSO3]− > [NO3]− > Br−/I−. The halide ions in particular cause quenching of the fluorescence, and their effect is additive (compare the quantum yield of 106-10 (0.906) with that of 99-Br (0.804) and 101 (0.729)). It should also be noted that quaternization of the neutral pyridine precursor of 102-Br-10/10 causes a considerable decrease in the fluorescence quantum yield, from 0.736 to 0.055. Pedro et al. performed a detailed photophysical study on the 1,3,4-oxadiazole-containing LC salt 102-Br-12/12.715 The oxadiazolylpyridinium salts reported by Lo Celso et al. (260-XZ1/Z2 and 261-X-Z1/Z2) are luminescent in acetonitrile solution, in contrast to their neutral (nonmethylated) precursors.535 The 4-substituted pyridinium derivatives show slightly red-shifted absorption and emission maxima as compared to the 3-substituted pyridinium compounds, indicating a higher degree of conjugation and charge delocalization in the former. The iodide salts also show reversible thermochromism thanks to formation/destruction of a colored charge-transfer complex between the electron-accepting pyridinium ring and the electrondonating halide anion. The ionic dendritic PPI and PAMAM complexes with 1,3,4-oxadiazole- or 1,2,4-oxadiazole-containing mesogenic groups that were synthesized by Hernández-Ainsa et al. (PPI-Gx/(215-n)2(x+1) (x = 1−5), PPI-Gx/(216-n)2(x+1) (x = 3, 5), and PAMAM-G4/(215-n)64) are luminescent both in chloroform solution and as casted films.834 In solution the emission maxima are located between 364 and 379 nm, and in the solid state they are located between 384 and 415 nm. Apart from the type of heterocycle, the other structural parameters and the supramolecular arrangements do not have a significant influence on the photophysical properties. Emission bands appear broader in the solid state, and the Stokes shifts are also remarkably higher in a film than in solution. The ionic LC PPI dendrimers with peripheral carbazole-containing bifunctional dendrons (PPI-Gx/(212-Y1/Y2)2(x+1) (x = 1−5) and PPI-Gx/ (213-Y1/Y2)2(x+1) (x = 1−5)) are also luminescent in solution and as casted LC films.460 The luminescence intensity is related to the number of carbazole units present in the molecule. When comparing the condensed-state emission spectra to the solutionstate spectra, the dendrimers with peripheral 212-R2/R3 or 213R2/R3 dendrons show a more significant bathochromic shift than their counterparts with 212-R2/R4 or 213-R2/R4 dendrons. This is because the aromatic mesogenic group in the former type of dendrons can interact more strongly with the carbazole moieties than the cholesteryl groups. The metal-free LC benzobis(imidazolium) salt 134 that was synthesized by Bielawski and co-workers is luminescent, with an emission maximum at 451 nm in methanol (fluorescence quantum yield = 0.72).515 Salt 133-12 shows an emission maximum at 332 nm in methanol (fluorescence quantum yield = 0.85).515 Nonmesomorphic IL analogues of 133-n and 134 are not only fluorescent in solution, but also in their condensed solid and liquid phases.515 Counterion-dependent red-edge effects were detected for dilute solutions of nonmesomorphic BBI salts in methanol,743 but this was not investigated for the LC compounds. The LC pyrazolo[1,2-a]-4-pyrazolium derivatives reported by Lafuente, Giménez, and co-workers (352-R) show blueish luminescence in chloroform solution and under the form of a Langmuir−Blodgett film transferred onto a solid substrate.409,973 Emission from the latter state is slightly blue-shifted as compared

to the solution state. The hypsochromic shift is more pronounced for multilayers than for monolayers, due to a relatively higher chromophore rigidity in the former. An interesting perspective article discusses the design of functional ILs with luminescent (and/or magneto-active) ions.1174 These ideas could also be applied in the field of ILCs. 12.6. Future Directions

The applications discussed in the previous sections clearly illustrate the advantages of combining an ionic molecular structure (giving rise to, for example, ion-conductive behavior and increased nanosegregation) with LC properties. Other reported applications of ILCs, such as the preparation of nanofiltration membranes, etc., were discussed in earlier sections of this Review. Another development that might be envisioned is the design of alignable ionic mesogens with nonlinear optical (NLO) properties. The charged moieties can be exploited in a donor−acceptor-type system that can induce such properties, as has been demonstrated already for nonmesomorphic NLOactive ILs.1175 Self-organizing stimuli-responsive molecular switches that show a directed macroscopic response and perform “work” are of tremendous academic and commercial interest. The LC state is an ideal environment for the controlled operation of these switches and for the amplification of their response via cooperative motion in the mesophase. LC rotaxanes908,909,915−919 are promising candidates for such compounds. Because many rotaxane architectures comprise charged components, the knowledge about ionic mesogens that has accumulated in the past years can be used advantageously to design future smart materials. Furthermore, mesogens that can be reversibly switched between a neutral state and a state with charged moieties could be useful as sensor materials. The presence/absence of charges will have a profound influence on the phase behavior, as well as on, for example, the surface properties and optical properties of the material. The interesting results that have been obtained with metal electrodeposition and nanoparticle formation (see section 12.3) urge for further research in this area. Similar experiments should be carried out in a variety of mesophases and particularly in aligned monodomain samples on suitable substrates. One can also think of templated electropolymerization processes and other electrochemical reactions (without the need for an added electrolyte) in aligned ILC environments (or electropolymerization of the ILC monomers themselves1176). The fascinating opportunities offered by the 3D channel structure of bicontinuous cubic mesophases could be further exploited. Work by the Kato group regarding nanofiltration membranes that were prepared from a cross-linked thermotropic ILC bicontinuous cubic phase, and which allow selective permeation of ions for water treatment,785 shows that exciting applications outside the field of molecular electronics are within reach for these materials. Ionic mesogens can also play a useful role as additives to existing thermotropic or lyotropic LC systems, in applications where their ionic nature is important.1177 They may be applied as nonvolatile phase-change (energy-storage) materials as well, as has already been shown for [C16mim][Br] and [C16dmim][Br].1178 We already mentioned in section 10.2 that ILCs based on N-(n-alkyl)-ε-caprolactam cations have been proposed as thermal storage media.956 A largely unexplored field of research is the investigation of ILCs as biocompatible organized media. This can range from DZ


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formation of highly thermally stable materials. Further investigations should clarify the exact role of the size, shape, and charge distribution of relatively large anions with a delocalized negative charge (such as the boron cluster anions, [NTf2]−, etc.) in forming LC materials with desired properties such as low transition temperatures, low viscosity, high ionic conductivity, and high thermal stability. In addition to metalcontaining anions, metal-centered or metal-containing cationic cores could also be explored for the design of new ILCs, including heterobimetallic compounds (where metal-containing cations and anions are combined).199,996,1197,1198 Further advancements can also be expected in the field of “ionic selfassembly” (ISA), because there still exist many functional ionic building blocks, which could be conveniently combined with mesogenic counterions to obtain easily processable materials. One can think for example of some charged ruthenium dyes that are typically used in dye-sensitized solar cells. Zwitterionic molecular structures should not be neglected either, particularly in the fascinating search for conductive nematogenic ILCs and materials for electrooptical applications. Unless there is the option to work under glovebox conditions, hygroscopic compounds should be avoided, because water molecules can have a profound influence on the mesophase behavior of ILCs. Apart from thermotropic LC systems, further progress may be expected in the investigation of lyotropic LC systems made up of ionic or nonionic amphiphiles, on the one hand, and nonmesomorphic ILs acting as the solvent or additive, on the other hand.215−218,221,223−225,602,638,693,847,853,860,1066,1199 ILs can be considered as functional solvents, and offer the important advantage of nonvolatility. Many more possibilities also lie in the mixing of different ILCs or of ILCs with conventional neutral LCs, to achieve particular desired properties. This strategy has already found widespread use in the field of ILs1200,1201 and in the field of neutral LCs (e.g., for display applications). Notwithstanding the recent publication of some DFT-based theoretical studies and computer simulations (see section 4.3.2),276,347,480,481,486,488,493,501,502 more theoretical work about the phase behavior and bulk properties of ILCs would be warmly welcomed, because it remains challenging to rationally design ionic mesogens with desired properties. In particular, the subtle balance between Coulombic interactions, hydrogen bonds, and dispersion forces needs to be better understood. The development of a set of “rules” to predict ionic mesomorphism and the temperature stability range of the mesophase, as well as important material properties such as viscosity and ionic conductivity (possibly in combination with electronic conductivity, as was already found for some ILCs63,723,990,993), could ultimately lead to a de novo, in silico design of tailor-made ionic mesogens. The calculation and evaluation of partial atomic charges via population analysis,426,502 possibly supplemented with a Bader analysis (see section 4.3.2), for new as well as previously reported ILCs would be valuable in this respect. The theoretical analysis results for isolated cations and anions should be compared to those for the cation−anion pairs (both for isolated pairs and for the bulk material). The work of Meyer, Libuda, Wasserscheid, and co-workers as well as Uchida and co-workers related to the development of “supported ILC phase” (SILCP) materials (see sections 12.3 and 12.1) can be an impetus for additional theoretical and experimental research focused on interactions of ionic mesogens with different substrates and their properties under confinement. The surface properties of ILCs are not yet completely understood, while these are of primordial importance in their practical application,

their use as anisotropic solvents for biochemical reactions (involving, for example, enzymes) and biomimetic processes to the development of ILC-based, nontoxic, label-free optical biosensors for biomedical (diagnostic) purposes.1179 It is an advantage that many biocompatibility and biodegradability studies have already been performed for nonmesomorphic ILs.109,291,1180−1186 ILs have already been used for biocatalysis purposes, and it has been found that they can successfully accommodate or even accelerate reactions.1187−1191 The potential of charged groups that are capable of strong, yet reversible noncovalent interactions, has also been recognized in the field of responsive polymeric materials.1192 Azzaroni et al. described a system in which compression of a polyelectrolyte film with interspersed pH-sensitive dye molecules resulted in a color change.1193 This was ascribed to a shift in the equilibrium between the neutral and anionic forms of the dye toward the charged form, in response to the increase in ionic strength as a result of the compression. Similarly, one might envision an adaptive system in which neutral, nonmesomorphic molecules are ionized under mechanical stress, yielding charged molecules that are capable of forming a LC phase.

13. CONCLUSIONS AND OUTLOOK Many of the earlier examples of ILCs were discovered by an extension of the alkyl chain of 1-methyl-3-(n-alkyl)imidazolium, N-(n-alkyl)pyridinium, or quaternary ammonium and phosphonium ILs to very long chain lengths. Several of these ILCs were therefore discovered as a consequence of a systematic variation of the structure of ILs, rather than by a proper LC design. Since the review by Binnemans,2 the field of ILCs has developed more toward the field of LCs than that of ILs. This is evident from the more complex molecular architectures of recently introduced ionic mesogens, for instance, salts where mesogenic groups are covalently attached to the cation via a flexible alkyl chain. In this Review, we have tried to emphasize important structure− property relationships for ILCs that have been elucidated thanks to the intensive research in the field. It can be predicted that ionic mesogens with even more complex geometries, and more complex phase structures, will be developed in the next years. One can think of bent-core and chiral ILCs, or discotic mesogens with both an ionic core and an ionic periphery. Smectic C phases and nematic phases, and their chiral variants, have rarely been obtained for ILCs to date. Complex structures could be achieved by the interplay between different orthogonal interaction motifs (i.e., hierarchical architectures combining (long-range, nondirectional) electrostatic interactions with (short-range, directional) hydrogen bonding and halogen bonding, or structures containing both hydrocarbon, fluorocarbon, oligo(ethylene oxide), and ionic parts). Simple ILCs based on known structures in which the alkyl chains are replaced by perfluoroalkyl chains or organosiloxane chains have also remained largely unexplored up until now. Thanks to the higher flexibility of organosiloxane chains as compared to alkyl chains, these have the potential to yield lower transition temperatures and viscosity (which is important for alignment purposes, to achieve short response times in response to external stimuli, and to obtain high ionic conductivity) while still preserving a sufficient degree of amphiphilicity.1194−1196 In the vast majority of ILCs, the mesomorphism is still introduced via the cation. There is still room for development of ILCs with mesogenic anions (“anion-driven mesogenicity”) that are not based on carboxylate or sulfonate groups.439,440,888,989,991 It has been shown, both for ILs and for ILCs, that the use of inorganic cluster anions such as [B12Cl12]2− can lead to the EA


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Additional tables, additional figures, and thermal transitions of all compounds discussed in the main text (PDF)

for example, for the macroscopic alignment into large-area monodomain samples. Ideally, the charges present in nanostructured ILCs should not only be exploited to induce mesomorphic behavior or an increased mesophase stability, but also to achieve specific functional properties such as ionic conductivity (see sections 12.1 and 12.2); electrochromism and electrofluorochromism (see section 12.4); luminescence, including switchable emission under the influence of an electric field (see section 12.5); nonlinear optical properties (see section 12.6); a high dielectric anisotropy (see section 8.5); the ability to stabilize charged intermediates in chemical reactions or to catalyze specific reaction steps carried out in the anisotropic ionic mesogen (see section 12.3); the ability to disperse and orient carbon materials such as carbon nanotubes via, for example, cation−π interactions (see section 5.3); and so on. ILCs also offer the exciting possibility of processing via advanced yet low-cost techniques such as microcontact printing and layer-by-layer deposition. Recent studies have yielded a better understanding of the role of the constituting ions of ILCs, and of added ions (for example, in the form of lithium salts or dissociated organic acids), in the ion conduction process. The development of ion-conducting bicontinuous cubic LC phases that do not require macroscopic alignment as well as LC materials whose anisotropic iontransporting pathways can be reoriented under the influence of light have also been important steps forward in the search for efficient, processable, and self-repairing ion conductors that can be used in energy devices (see sections 5.2, 5.3, 6.2, 7, and 12.1). Pioneering work of Gin and co-workers in the field of lyotropic LCs641,663,668,778−783,1098,1202 has inspired several groups to “freeze” the LC order induced by reversible electrostatic interactions (as well as other noncovalent forces) by means of in situ polymerization of the mobile molecules in the mesophase, potentially followed by removal of one of the noncovalently bound components. This has proven most helpful in the preparation of advanced materials such as anisotropic solid-state ion conductors (see section 12.1) and nanoporous solid-state hosts (see sections 6.2 and 12.3). Such ideally defect-free, nanochanneled, robust polymeric materials with well-defined pores and functionality have promising applications as catalysts, nanocomposites, separation membranes, proton-conducting membranes for fuel cells, and so forth. All of these opportunities, in combination with the possibility to influence the mesomorphic behavior, phase transition temperatures, and other properties through the quasi-independent choice of the cationic and anionic constituents (“tunability”), reveal the true versatility of ILCs. The development of commercial applications of ILCs is still lacking far behind the design of new mesomorphic structures. This is not unexpected, and it is a natural process. However, industrial applications of ILCs could give a real boost to this research field. This has previously been observed for LCs and ILs. Notwithstanding these limitations, the research field of ILCs is flourishing and still growing and diversifying, and we enthusiastically believe that it has a very bright future.

AUTHOR INFORMATION Corresponding Author

*Tel.: +82-52-217-5753. Fax: +82-52-217-5759. E-mail: karel. [email protected]. Notes

The authors declare no competing financial interest. Biographies Karel Goossens received a M.Sc. degree in Chemistry from the University of Leuven (KU Leuven, Belgium). After doctoral research under the supervision of Prof. Dr. Koen Binnemans, he obtained a Ph.D. degree with a thesis entitled “Ionic liquid crystals based on imidazolium and pyrrolidinium cores”. He then pursued postdoctoral research with Prof. Dr. Johan Hofkens at the University of Leuven as a fellow of the Research Foundation Flanders (FWO-Vlaanderen), studying polymer properties and dynamics, polymerization processes, and giant dendrimeric luminophores by means of “single-molecule” spectroscopy and microscopy. In 2013 he started working in the group of Prof. Dr. Christopher W. Bielawski, first at The University of Texas at Austin (U.S.), as Francqui Foundation fellow of the Belgian American Educational Foundation (BAEF), and since December 2014 at the IBS Center for Multidimensional Carbon Materials located at the Ulsan National Institute of Science and Technology (UNIST, South Korea). Kathleen Lava graduated in 2007 as a M.Sc. in Chemistry at the University of Leuven (KU Leuven). In 2012, she successfully completed her Ph.D. thesis entitled “Ionic liquid crystals based on novel heterocyclic cores” in the Binnemans group, as fellow of the Agency for Innovation by Science and Technology in Flanders (IWT). She presently performs postdoctoral research in the team of Prof. Dr. Richard Hoogenboom at Ghent University, focusing on functional poly(2-oxazoline)s for (bio)materials applications. Christopher W. Bielawski received a B.S. degree in Chemistry from the University of Illinois at Urbana−Champaign in 1997 and a Ph.D. degree from the California Institute of Technology in 2003. Following a postdoctoral appointment (also at Caltech), he assumed an independent position at The University of Texas at Austin in 2004, where he directed synthetic efforts in a broad range of macromolecular and materials chemistry projects. Recently, he moved his research program to the Ulsan National Institute of Science and Technology (UNIST) in South Korea and is currently participating in a new initiative focused on the preparation of novel carbon materials as well as their applications. Koen Binnemans obtained his M.Sc. (1992) and Ph.D. (1996) degrees in Chemistry at the University of Leuven (KU Leuven). In the period 1999−2005, he was a postdoctoral fellow of the Research Foundation Flanders. He did postdoctoral work with Prof. Dr. Jacques Lucas (Rennes, France) and Prof. Dr. Duncan W. Bruce (Exeter, United Kingdom). In 2000, he received the first ERES Junior Award (ERES: European Rare-Earth and Actinide Society). From 2002 until 2005 he was (part-time) associate professor, from 2005 until 2010 professor, and presently he is full professor of Chemistry at the University of Leuven. He has published over 330 papers in international scientific journals. He made significant contributions to the development of ionic liquid crystals and wrote in 2005 the first major review on this topic. His current research interests are the use of ionic liquids for solvent extraction and battery applications, the separation of rare earths, and the recycling of critical materials.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/cr400334b. EB


Chemical Reviews

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[Cnpyr]+

ACKNOWLEDGMENTS K.G. acknowledges the Research Foundation Flanders (FWOVlaanderen) and the Belgian American Educational Foundation (BAEF) for postdoctoral fellowships, and would like to thank Dr. Bertrand Donnio (IPCMS, Strasbourg, France) and Prof. Dr. Duncan W. Bruce (University of York, UK) for helpful discussions. K.G. is very grateful to Mrs. Marianna Grenadier (The University of Texas at Austin) for her contribution to the graphical abstract. K.G. would also like to thank Kiliane De Vuyst for her endless patience and support. K.G. and C.W.B. were supported in part by the Korean Institute for Basic Science (IBSR019-D1). ABBREVIATIONS 1D 2D 3D 5CB 5OCB λ σ|| σ⊥ [CnCmBim]+

[CnCmim]+

[CnCmmmpiperaz]+

[CnCmpiperid]+

[CnCmpyrr]+

[Cndmim]+

[CnHim]+ [Cnmim]+ [Cnmpiperid]+ [Cnmpyrr]+

N-(n-alkyl)pyridinium (n indicates the number of carbon atoms in the alkyl chain) [Cnvim]+ 1-vinyl-3-(n-alkyl)imidazolium (n indicates the number of carbon atoms in the alkyl chain) [DcHSS]− dicyclohexyl sulfosuccinate [DDSS]− bis(2-butyloctyl) sulfosuccinate (didodecyl sulfosuccinate) [DHSS]− bis(n-hexyl) sulfosuccinate [DOSS]− bis(2-ethylhexyl) sulfosuccinate (dioctyl sulfosuccinate) [Eu(tta)4]− tetrakis(2-thenoyltrifluoroacetonato)europate(III) [Nn,m,x,y]+ N,N,N,N-tetrakis(n-alkyl)ammonium (n, m, x, and y indicate the number of carbon atoms in each alkyl chain) [NTf2]− bis(trifluoromethylsulfonyl)imide (bis(triflyl)imide) (Tf = SO2CF3) [OTf]− trifluoromethylsulfonate (triflate) (Tf = SO2CF3) [OTs]− p-toluenesulfonate (tosylate) (Ts = SO2(C6H4)CH3) [Pn,m,x,y]+ P,P,P,P-tetrakis(n-alkyl)phosphonium (n, m, x, and y indicate the number of carbon atoms in each alkyl chain) [sulfoBODIPY]2− 4,4-difluoro-1,3,5,7,8-pentamethyl2,6-disulfonato-4-bora-3a,4a-diazas-indacene AM molecular area Ar aryl group AR-XPS angle-resolved X-ray photoelectron spectroscopy ASC adiabatic scanning calorimetry a.u. arbitrary units B crystal smectic B phase BBI benzobis(imidazolium) Bn benzyl CD circular dichroism ChLhex hexagonal channeled layer phase Chol cholesteryl Col, Col1, Col2 unidentified columnar phase Colhex hexagonal columnar phase Colhex,1 “normal-type” hexagonal columnar phase Colhex,2 “inverted-type” hexagonal columnar phase ColL lamello-columnar phase Colobl oblique columnar phase Colrec rectangular columnar phase Colrec,d disordered rectangular columnar phase Colrec,o ordered rectangular columnar phase Colsqu square columnar phase CP-MAS cross-polarization magic-angle spinning Cr, Cr1, Cr2, Cr3, Cr4, Cr5 crystalline (or semicrystalline) phase Cub cubic phase CubI micellar cubic phase CubI1 “normal-type” micellar cubic phase CubI2 “inverted-type” micellar cubic phase

one-dimensional(ly) two-dimensional(ly) three-dimensional(ly) 4′-(n-pentyl)-4-cyanobiphenyl 4′-(n-pentyloxy)-4-cyanobiphenyl wavelength ionic conductivity parallel to smectic layers or columnar stacks (in S cm−1) ionic conductivity perpendicular to smectic layers or columnar stacks (in S cm−1) 1,3-bis(n-alkyl)benzimidazolium (n and m indicate the number of carbon atoms in both alkyl chains, respectively) 1,3-bis(n-alkyl)imidazolium (n and m indicate the number of carbon atoms in both alkyl chains, respectively) N,N′-bis(n-alkyl)-N,N′-dimethylpiperazinium (n and m indicate the number of carbon atoms in both alkyl chains, respectively) N,N-bis(n-alkyl)piperidinium (n and m indicate the number of carbon atoms in both alkyl chains, respectively) N,N-bis(n-alkyl)pyrrolidinium (n and m indicate the number of carbon atoms in both alkyl chains, respectively) 1,2-dimethyl-3-(n-alkyl)imidazolium (n indicates the number of carbon atoms in the alkyl chain) 1-(n-alkyl)imidazolium (n indicates the number of carbon atoms in the alkyl chain) 1-methyl-3-(n-alkyl)imidazolium (n indicates the number of carbon atoms in the alkyl chain) N-(n-alkyl)-N-methylpiperidinium (n indicates the number of carbon atoms in the alkyl chain) N-(n-alkyl)-N-methylpyrrolidinium (n indicates the number of carbon atoms in the alkyl chain) EC


Chemical Reviews CubV CubV1 CubV2 d DAB DABCO DAE dcnm− dec DFT DMSO DOSY DRIFTS DSC DSSC E EPR FP-TRMC FT-IR g G G* gIso gSmA H H* H2dpa HOP HOPG Htta I i-Bu IL ILC i-Pr IR IRAS ISA ITO J J* K K* LC LCol LED Ln M, M1, M2, M3 MD MeOH MMAB n N N* NBO

Review

NC N C* ND ND* NHC NIR NMR NPA PAH PAMAM PC PEDOT-PSS

bicontinuous cubic phase “normal-type” bicontinuous cubic phase “inverted-type” bicontinuous cubic phase smectic layer thickness 1,4-diaminobutane 1,4-diazabicyclo[2.2.2]octane 1,2-diaminoethane dicyanonitrosomethanide (thermal) decomposition density functional theory dimethyl sulfoxide diffusion ordered spectroscopy diffuse reflectance infrared Fouriertransform spectroscopy differential scanning calorimetry dye-sensitized solar cell crystal smectic E phase electron paramagnetic resonance flash-photolysis time-resolved microwave conductivity Fourier-transform infrared glass crystal smectic G phase chiral crystal smectic G phase vitrified isotropic liquid phase vitrified SmA phase crystal smectic H phase chiral crystal smectic H phase 2,6-pyridinedicarboxylic acid (dpa2− = pyridine-2,6-dicarboxylate or dipicolinate) heterogeneity order parameter highly oriented pyrolytic graphite 2-thenoyltrifluoroacetone (tta− = 2thenoyltrifluoroacetonate) isotropic liquid phase isobutyl ionic liquid ionic liquid crystal isopropyl infrared infrared reflection absorption spectroscopy ionic self-assembly indium tin oxide crystal smectic J phase chiral crystal smectic J phase crystal smectic K phase chiral crystal smectic K phase liquid crystal/liquid-crystalline lamello-columnar phase light-emitting diode lanthanide(III) ion unidentified mesophase molecular dynamics methanol 4-methoxy-4′-methylazobenzene director of the liquid crystal nematic phase chiral nematic phase natural bond orbital (analysis)

PEI PEIMe Ph POM PPI PR-TRMC PTFE PXRD rt SAXS SEP SILP SILCP SmA SmA* SmÃ SmA2 SmAd SmB SmC SmC* SmC2 SmCP SmF SmF* SmI SmI* SmX, SmX1, SmX2, SmX3 SWCNT T, T1, T2 Tc Tdec. TEM Tet Tg TGA THF Tm TOF TTF UV vis Vmol WAXD ED

columnar nematic phase chiral columnar nematic phase discotic nematic phase chiral discotic nematic phase N-heterocyclic carbene near-infrared nuclear magnetic resonance natural population analysis polycyclic aromatic hydrocarbon poly(amidoamine) propylene carbonate poly(3,4-ethylenedioxythiophene)poly(4-styrenesulfonate) poly(ethylene imine) fully methylated poly(ethylene imine) phenyl group polarizing optical microscopy poly(propylene imine) pulse-radiolysis time-resolved microwave conductivity poly(tetrafluoroethylene) powder X-ray diffraction room temperature small-angle X-ray scattering surfactant-encapsulated polyoxometalate (cluster/complex) supported ionic liquid phase supported ionic liquid crystal phase smectic A phase chiral smectic A phase modulated (ribbon-like) SmA phase bilayer SmA phase partial bilayer (interdigitated/intercalated) SmA phase smectic B phase smectic C phase chiral smectic C phase bilayer SmC phase polar SmC phase or B2 “banana” phase smectic F phase chiral smectic F phase smectic I phase chiral smectic I phase unidentified smectic phase single-walled carbon nanotube crystal smectic T phase clearing temperature decomposition temperature transmission electron microscopy P42/mnm tetragonal phase glass transition temperature thermogravimetric analysis tetrahydrofuran melting temperature time-of-flight tetrathiafulvalene ultraviolet visible region of the electromagnetic spectrum molecular volume wide-angle X-ray diffraction DOI: 10.1021/cr400334b Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews wt % X, X1, X2, X3, X4, X5 XL XPS XRD

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weight percent unidentified crystal smectic phase or soft crystal phase unidentified soft crystal phase with a simple lamellar structure X-ray photoelectron spectroscopy X-ray diffraction

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