Discotic Liquid Crystals - Chemical Reviews (ACS Publications)

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Discotic Liquid Crystals Tobias Wöhrle,† Iris Wurzbach,‡ Jochen Kirres,† Antonia Kostidou,† Nadia Kapernaum,‡ Juri Litterscheidt,† Johannes Christian Haenle,† Peter Staffeld,‡,§ Angelika Baro,† Frank Giesselmann,*,‡ and Sabine Laschat*,† †

Institut für Organische Chemie, and ‡Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany 4.10. Metallomesogens 4.10.1. Carboxylate Complexes 4.10.2. Pyridine, Bipyridine, Terpyridine Complexes 4.10.3. Pyrazolate Complexes 4.10.4. Imines, Salens, and Enamines 4.10.5. Miscellaneous 4.11. Ionic Liquid Crystals 4.12. Hydrogen-Bonded Liquid Crystals 5. Tabular Overview on Mesophases and Charge Mobilities 6. Summary and Outlook Author Information Corresponding Author Present Address Notes Biographies Acknowledgments Abbreviations References

CONTENTS 1. Introduction 2. Structures of Discotic Liquid Crystals 2.1. Basic Structures 2.1.1. Columnar Stacking 2.1.2. Nematic Phases 2.1.3. Columnar Phases 2.1.4. Lamellar Mesophase 2.2. Intracolumnar Helical Order 2.3. Ferroelectric Columnar Liquid Crystals 3. Charge Transport Properties 3.1. Discotic Liquid Crystals as Organic Semiconductors 3.2. Discotic Liquid Crystals as Ion Conductors 4. Classes of Compounds 4.1. Triphenylenes 4.2. Perylenes 4.3. Hexa-peri-hexabenzocoronenes, PAHs 4.4. Benzene Derivatives 4.4.1. Star-Shaped Benzene Derivatives 4.4.2. Miscellaneous Benzene Derivatives 4.5. Heteroaromatic Discotic Mesogens 4.5.1. Triazines 4.5.2. Azatriphenylenes, Dibenzo[a,c]phenacines, and Hexaazatrinaphthylenes 4.5.3. Carbazoles and Triindoles 4.5.4. Thiophenes 4.5.5. Oxazoles, Oxadiazoles, and Thiazoles 4.5.6. Miscellaneous Heterocycles 4.6. Porphyrins and Tetraazaporphyrins 4.7. Phthalocyanines 4.8. Shape-Persistent Macrocycles 4.9. Crown Ethers and Azacrowns © 2015 American Chemical Society

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1. INTRODUCTION Today where everybody is using a large variety of electronic devices such as notebook and tablet computers, smart phones, digital cameras, MP3 players, flat panel TVs, as well as embedded systems in household appliances, automobiles, and large industrial facilities, there is still an urgent need for downsizing such electronic equipment in order to save costs, space, weight, energy, and raw materials. While calamitic (rod-like) liquid crystals have already contributed a lot to this development, for example in large area flat planel displays,1,2 their corresponding discotic (disk-like) counterparts, besides some selected applications as optical compensator films for LC displays,3,4 gas sensors,5,6 and the like, are still in the proof-ofconcept device state. Nevertheless, much progress has been achieved up to now, and extensive research efforts worldwide have been devoted to the development of novel materials,7 the understanding of structure−property relationships in order to tailor compounds with desirable mesomorphic8 and other physical properties9 for the use of discotic liquid crystals as anisotropic organic semiconductors in organic field effect

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Special Issue: Frontiers in Macromolecular and Supramolecular Science Received: March 31, 2015 Published: October 20, 2015 1139

DOI: 10.1021/acs.chemrev.5b00190 Chem. Rev. 2016, 116, 1139−1241

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1D columns eventually leading to 2D columnar mesophases with 1D long-range orientational order and 2D translational order or alternatively into 3D cubic mesophases. However, with the occurrence of nonconventional mesogens, which are neither rod- nor disk-shaped, such as polyphilic mesogens, the significance of nanosegregation between aromatic cores and alkyl chains as driving force for mesophase formation became commonly accepted. Therefore, incompatible side chains (alkyl, oligoethylene glycol, perfluoroalkyl, etc.) can induce liquid crystalline phases.21−24 Based on our previous review from 200725 it is thus the aim of the current review to provide the reader with an up to date overview covering the literature from 2006 until December 2014. After this introductory chapter, a section dealing with the different phase types of discotic liquid crystals, recent discoveries of helical columnar ordering and ferroelectric columnar liquid crystals will be presented. A subsequent chapter will discuss the relevance of columnar order for electric or photoconductivity. Then the major part of this review will deal with the different compound classes. Where appropriate, historical remarks on the discovery, interesting synthetic strategies, mesomorphic and other physical properties, as well as applications will be discussed. The following general reviews on discotic liquid crystals by Bushby and Kawata,26 Kaafarani,27 Takezoe and Araoka,28 Kato et al.,29 Goodby et al.,30 Roy et al.,31 and Kumar32,33 are highly recommended. More focused reviews can be found in the specific chapters.

transistors (OFETs), organic light emitting diodes (OLEDs), and organic photovoltaic devices (OPVs).10,11 The discussion of discotic liquid crystals started in 1923, when Vorländer suggested the possibility of liquid crystalline phases with a packing behavior similar to “Voltas columns” while studying flat molecules such as triphenylene and perylene.12,13 Unfortunately, he was not able to observe any mesomorphism for these compounds, which have later been recognized as the archetypal core units of many discotics. The experimental breakthrough came in 1977, when Chandrasekhar published his findings on the mesomorphic properties of benzene-hexa-n-alkanoates, which were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD).14 This is today considered as the birth of discotic liquid crystals. Besides this first observation of thermotropic mesomorphism of a disk-shaped molecule, Chandrasekhar15 made a comment in the introduction of his seminal paper, which must be considered from retrospective as a statement of great foresight, when he wrote that “Mesophases composed of large plate-like molecules are known to occur at high temperatures during the carbonization of graphitizable substances, such as petroleum and coal tar pitches, but these are rather complex materials and certainly cannot be regarded as single-component liquid crystalline systems.”

2. STRUCTURES OF DISCOTIC LIQUID CRYSTALS 2.1. Basic Structures

Here the structures of basic discotic phases will briefly be presented. For a more detailed description of the structures and their characterization by X-ray diffraction and polarizing optical microscopy see our previous review.25 2.1.1. Columnar Stacking. The stacking of the disklike mesogens of discotic liquid crystals into 1D columns is fundamental for their most common phases. The columns are in the ideal case of infinite length and the molecules in a column exhibit only short-range positional order. The mesogens with their rigid, relatively flat cores self-organize into columns. This is caused by steric packing and π−π interactions between the cores.8 As a result of the high entropy and disorder of the flexible aliphatic chains on the periphery, the formation of a 3D crystal is hindered. The columnar stacking also leads to nanosegregation between the cores and the tails.

Figure 1. Model of the carbonaceous mesophase, a lamellar liquid crystal. Reprinted with permission from ref 17. Copyright 1977 Gordon and Breach Science Publishers, Ltd..

Thus, Brooks and Taylor, who already in 1965 described mesophases consisting of planar aromatic compounds of high molecular weight16 as well as Zimmer and White, who reported on disclinations in the carbonaceous mesophase and proposed a model with lamellar organization (Figure 1)17 can be considered as the grandfathers of graphene and polycyclic aromatic hydrocarbon derived columnar liquid crystals, which have received enormous interest over the last two decades due to their high electronic charge carrier mobility up to 1.1 cm2 V−1 s−1, which is comparable to amorphous silicon.18−20 However, not only potential applications of discotic liquid crystals have driven the research but also basic questions regarding the origin of self-assembly in such systems. According to the classical view, liquid crystalline phases appear as a result of strong interactions between π-conjugated cores which are counterbalanced by the thermal motion of the alkyl chains thus inhibiting crystallization into a 3D structure with true 3D longrange translational and orientational order. Thus, it was generally thought that shape-anisotropic rod-like molecules prefer the formation of nematic or smectic mesophases, whereas disk-shaped molecules favor the self-assembly into

Figure 2. Different types of columnar stacking: (a) ordered and (b) disordered. Reprinted with permission from ref 25. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Different types of stacking can be found in the columns (Figure 2): There are “disordered columns” with irregular stacking of the disks, “ordered columns” where the cores are 1140


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equidistant while the flexible alkyl-tails are still disordered as well as “tilted columns” with the cores of the disks being tilted with respect to the column axis. All these columns do not exhibit true 1D-translational order and can therefore be considered as 1D-fluids. 2.1.2. Nematic Phases. Nematic phases are the simplest liquid crystalline phases formed since they only have long-range orientational order (of molecules, columns, etc.) and no degree of long-range translational order. For discotic molecules different kinds of nematic phases are known (Figure 3). In a discotic nematic mesophase ND the nematic phase is built of single flat molecules which possess full translational and orientational freedom around their principal short axes. Their long axes however are on average oriented parallel to a general plane (Figure 3a). In contrast to the rather waxy columnar phases the nematic phases are fluid. There is also a chiral variant of the discotic nematic phase known, the cholesteric phase ND* (Figure 3b), which is formed by chiral discotic mesogens or after the addition of a chiral dopant to an achiral discotic nematic phase. It exhibits a twisted nematic structure where the director field is continuously twisted in the direction normal to the local directors n(r) (Figure 3b) with the pitch p of the helical structure.

one direction but also in two additional mutually perpendicular directions.37 These biaxial nematic phases are known for quite some time in lyotropic,38 polymeric,39 and more recently in mineral liquid crystals.40 In the case of discotic liquid crystals the formation of a biaxial nematic phase is expected in mixtures of rod- and disc-like mesogens.41 This mixing of rod- and disclike molecules was investigated by many groups using computer simulation (for examples see refs 42−44). In these simulations the occurrence of a biaxial nematic phase was shown. Nevertheless, to the best of our knowledge until now it was not possible to achieve this biaxial nematic phase experimentally from mixtures of rod- and disc-like particles. Artal et al. presented in 2001 a system where the mixing of rod- and disk-like molecules without demixing was achieved. But still no biaxial nematic phase could be observed.45 Therefore, it was stated by Bisoyi and Kumar in 2009 in their review on discotic nematic liquid crystals that “in some of the monomeric and dimeric compounds, optical biaxiality was speculated but later it was experimentally clarified that they do not possess any optical biaxiality”.36 This was also confirmed by Tschierske and Photinos in 2010 in their review on biaxial nematic phases41 and is to our knowledge still valid. 2.1.3. Columnar Phases. Discotic mesogens tend to form columns which are the building blocks of the columnar phases. In these phases the columns order in a 2D-lattice with the column axes being parallel to each other. Due to the arrangement of the columns in a 2D-lattice, columnar phases can be considered as 1D fluid (along the columns) and 2D crystalline (along the 2D lattice vectors).8 Depending on the crystal system hexagonal (Colh), rectangular (Colr), or oblique (Colob) phases are distinguished (Figure 4). The notation of these columnar liquid crystal phases developed in different ways: on the one hand there are the IUPAC recommendations,46 such as Colh, and on the other hand Levelut in her classic papers47,48 suggested the use of planar space groups, like P6/mmm. Nowadays the use of 2D plane groups is often found (see, e.g., ref 23 and Figure 4b). In this notation the Colh phase is described by the plane group p6mm. In the Colh phase the columns are arranged on a 2D-hexagonal lattice, the 6-fold symmetry of which requires nontilted columns (Figure 4a(i),b). The family of rectangular columnar mesophases Colr includes several symmetry groups (Figure 4a(ii−iv),b). The group of Levelut distinguished three different Colr mesophases with the planar space groups P21/a, P2/a, and C2/m, which correspond to the plane groups p2gg, p2mg, and c2mm, respectively (see Figure 4a(ii−iv)).47,48 Additionally a fourth columnar rectangular phase with the plane group p2mm is described by Tschierske (Figure 4b).23 In general, the disks in this kind of mesophases are tilted with respect to the column axis leading to an elliptic cross section of the column. As the cross section of the tilted columns is elliptical, the symmetries of the Colr phases differ from a proper hexagonal symmetry. Sometimes rectangular phases are also entitled as pseudohexagonal. In Colr phases stronger core−core interactions need to be present than in hexagonal phases as the tilt angle and tilt direction need to be correlated from one column to the next. This often leads to a change from the Colr to the Colh phase with increasing chain length. The organization of the columns in the Colob phase is found in Figures 4a (v) and 4b. The tilted columns are here also illustrated by their elliptic cross section. The space group of this phase according to Levelut is P1, whereas nowadays also the oblique plane group p2 is used. This

Figure 3. Different nematic phases of discotic liquid crystals: (a) discotic nematic ND, (b) cholesteric ND*, (c) columnar nematic Nc, and (d) lateral nematic NL phase. Redrawn from ref 36.

The columnar nematic mesophase Nc is formed out of columns as building blocks (Figure 3c). An example is the case of an electron donor doped with an electron acceptor.34,35 Here ordered columns are built because of charge-transfer interactions. The formation of a 2D-lattice is prevented by the use of molecules with strongly differing lengths of their side chains. The columns now arrange parallel to each other in a columnar nematic mesophase and therefore exhibit positional short-range and orientational long-range order. Besides these two nematic phases also the lateral nematic phase NL was observed.36 The lateral nematic phase is built of aggregates formed by multiple discotic mesogens. These supramolecular aggregates then organize into a nematic phase (Figure 3d). The biaxial nematic phase is an optically biaxial nematic phase where long-range orientational order is not only found in 1141


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Figure 5. Lamellar mesophase DL. This structure was proposed by Sakashita et al.52 Copyright 1988 Gordon and Breach Science Publishers S. A.

Figure 6. Hexagonal columnar phase of highly concentrated solution of B-DNA: (a) Polygonal texture (b) X-ray diffraction pattern of an aligned fiber sample (fiber axis vertical) showing an X-shaped pattern characteristic of a helical structure. Reprinted with permission from ref 68. Copyright 1989 Nature Publishing Group.

Figure 4. (a) Classical indexing of the different columnar mesophases after Levelut et al., where the elliptical cross section indicates a tilted column in top view.47,48 2D-lattice of (i) hexagonal, (ii−iv) rectangular, and (v) oblique columnar phase. In parentheses the point-group symmetry according to the “International System” is given. (b) Modern classification of columnar phases according to the relevant plane groups after Tschierske.23 Here the Colh phase with the plane group p6mm is equivalent to the space group P6/mmm, the rectangular phases c2mm, p2gg, and p2mg are equivalent to C2/m, P21/a, and P2/a, respectively. Reprinted with permission from ref 25. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; reprinted with permission from ref 23. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

of soaps49) and also more recently found in thermotropic polyphilic molecules which combine more than two incompatible units. Ungar et al. presented a broad variety of columnar phases in their review.50 They describe different kinds of columnar phases, like hexagonal, rectangular, and even a square columnar phases in, e.g., T-shaped bolaamphiphiles. The symmetry of these square columnar phases are described by the plane groups p4mm or p4gm (Figure 4b). An interesting example of columnar analogue phases in lyotropic liquid crystals is found for the system 5-[4-(5-n-heptylpyrimidine-2-yl)phenoxy]pentane-1,2-diol in water.51 This system shows several complex phases which have in common with columnar phases that they are 2D long-range ordered. It exhibits in dependence on the water content a hexagonal, a p2 and a pseudo c2mm phase

phase needs strong core−core interaction and is therefore quite rare. Columnar phases are not exclusively formed by discotic mesogens. They are also long known as a very common structure in lyotropic liquid crystals (the so-called middle phase 1142


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Figure 7. C3-discotic mesogens adopt a propeller like conformation and achieve optimum stacking by forming a left- or a right-handed helical column. The rotation angles between consecutive disks range between 13° (1b) and 16° (1a). In the case of chiral alkyl tails (1a) one helical twist sense is preferred, while for nonchiral R (1b,c) both helical senses are found in equal amounts. Reprinted with permission from ref 74. Copyright 2011 The Royal Society of Chemistry.

structure of DNA.66,67 Later Livolant et al. showed that the highly concentrated phase of DNA is indeed a hexagonal columnar liquid crystal phase of B-DNA double helices68 which, in terms of lyotropic discotic liquid crystals, can be seen as a helically twisted stack of planar (discotic) complementary base pairs, linked along the stacking sequence by two desoxyribose-phosphate polymer backbones. Figure 6 shows the typical polygonal texture and the X-ray fiber diffraction pattern of the hexagonal columnar phase of B-DNA. Since in the lyotropic liquid crystalline state the phase of the helices is decorrelated from column to column, the inner part of the X-ray diffraction pattern (Figure 6b) mainly reflects the form factor of a helical structure69,70 which is signified by the appearance of periodic layer lines with diffuse scattering peaks, the intensity of which in the n-th layer line is related to the square of the nth order Bessel function. This gives rise to the famous X-shaped diffraction pattern, characteristic of a helical structure. A careful analysis of such patterns (see, e.g., ref 71) can confirm the presence of a helical structure and reveals its symmetry and pitch. In particular, for a mesogen with n-fold rotational symmetry the helical pitch length is n times the helical repeat unit, which is obtained from the meridional distance of the layer lines in the diffraction pattern. Early observations of helical intracolumnar order in neat thermotropic discotics were reported by Malthête et al. in 198272 and by Fontes et al. in 1988.73 In the latter case it was shown that the disordered columnar hexagonal phase of 2,3,6,7,10,11-hexakis(hexylthio)triphenylene (3, Figure 18) transforms on cooling into an ordered phase (denoted as H) with helically twisted columns. It was proposed that the helical stacking originates from the competition between the attractive π-stacking interactions between the aromatic cores of consecutive mesogenic molecules and the steric repulsion between their flexible alkyl tails. The interaction energy is optimized if the mesogens adopt a propeller like conformation and stack on each other with a relative rotation of about 45°, either clockwise or counterclockwise, “so that once the initial

which are lyotropic analogues to the Colh, the Colob, and a Colr phase, respectively. 2.1.4. Lamellar Mesophase. In a lamellar mesophase DL the mesogens are ordered in layers like in calamitic smectic phases. The structure of the lamellar mesophase is still not completely understood. Figure 5 illustrates a possible arrangement proposed by Sakashita et al.52 Recently lamellar discotic phases were observed for triphenylene silanes by Mansueto et al.53 and in dimeric discotic mesogens by Prasad et al.54 and Ong et al.55 2.2. Intracolumnar Helical Order

Unique macroscopic chirality effects in liquid crystals such as helicity and ferroelectricity have been widely investigated in the case of rod-like calamitic mesogens since many years.56−58 In chiral nematic (cholesteric) or chiral smectic C (SmC*) phases of rod-like molecules the helical structures (as well as the spontaneous electric polarization of the SmC* layers) originate from the molecular chirality of the mesogens. In the more recently discovered case of bent-core (“banana-shaped”) mesogens, however, spontaneous achiral symmetry breaking results from steric interactions and close chiral packing of inherently nonchiral but bent-shaped mesogens.59 This particular case of supramolecular chirality gives rise to a plethora of chirality-related phenomena such as the spontaneous formation of chiral domains,60 polar electro-optic switching,61 electric-field-driven deracemization,62 and helical nanofilament phases.63,64 In comparison to their calamitic counterparts, the understanding of chirality effects and helical structures in discotic liquid crystals is not so far developed, even though indications of an intracolumnar helical structure in nonchiral hexa-alkoxy triphenylenes were found by Levelut et al.65 as early as in 1979, only two years after the discovery of discotic liquid crystals by Chandrasekhar et al.14 However, the first observation of a columnar phase with intracolumnar helical order was probably in 1952 the famous “Photo 51” by Raymond Gosling and Rosalind Franklin, which was key to unravel the double helix 1143


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Figure 9. Temperature dependence of (a) core−core-spacing and (b) associated correlation length ξ in the Colh and H phases of hexakis(hexylthio)triphenylene (3). Reprinted with permission from ref 76. Copyright 2003 Taylor and Francis, Inc.

Figure 8. Circular dichroism spectra of 1a in methylcyclohexane (10−5 M) recorded while cooling from 90 to 0 °C (arrows indicate decreasing temperature), indicating the growth of homochiral helical columns. (b) Results of quantum chemical RI-DFT/SVP calculations, showing the minimum energy configuration of a twisted dimer stack and the dependence of the dimer energy on the twist angle φ. Reprinted with permission from ref 74. Copyright 2011 The Royal Society of Chemistry.

length drastically increases from about ∼20 Å in the Colh phase to values of ∼300 Å in the H phase (Figure 9). Both effects of helical intracolumnar ordering, a decreasing core−core spacing dcc and an increasing intracolumnar correlation length ξ, improve the charge carrier mobility along the column (see Figure 18 in chapter 3). Other important factors influencing the charge carrier mobility are the reorganization energy λ and the transfer integral t (see chapter 3). While λ essentially depends on the size of the conjugated π-electron system of the mesogen, t is related to the π-orbital overlap between consecutive molecules of the same column and thus essentially depends on their twist angle φ (cf. Figure 8) and, again, on the core−core distance.77 The formation of helical intracolumnar order with appropriate values of dcc, t, and ξ has thus the potential to enhance all critical key factors (except λ) for high charge carrier mobility. Therefore, the understanding, which core structures and peripheral groups enable the formation of a certain helical order and which do not is crucial for the rational design of new discotic materials with enhanced charge carrier mobility.77−81 It is thus not surprising that the investigation of helical intracolumnar order has attained increasing interest over the last years. Vera et al. comprehensively reviewed the subject in 2009 and summarized the role of helical π-stacking, hydrogen bonding and metal-ion assisted columnar twisting in the formation of helical columnar phases.82 This review covers a wide range of materials from triphenylenes, hexabenzocoronenes, perylene bisimides, dendron-like fluorenones, and helicenes over various H-bonded mesogenic complexes to discotic metallomesogens. More recent developments in new materials classes such as the star-shaped mesogens are addressed in the subsequent sections of this review.

clockwise-counterclockwise symmetry is broken the helical order can extend to large distances”.73 This mechanism of helical intracolumnar ordering is illustrated for the more recent case of bipyridine diaminederived discotics with C3-symmetry (“C3-discotics”) in Figure 7.74 Quantum chemical calculations on the core structure of 1 (without the peripheric gallic moieties) confirm a propeller like equilibrium conformation with the three bipyridinyl substituents twisted out of the aromatic core plane and, subsequently, a twisted stacking of the dimer (Figure 8b). In the case of the chiral C3-mesogen 1a one helical twist sense is preferred. Circular dichroism (CD) spectroscopy confirms the isodesmic growth of homochirally twisted columns in methycyclohexane solution (Figure 8a). Van Gestel et al. found in mixed octane solutions of the (R)- and (S)enantiomers of 1a a strongly sigmoid dependence of the CD intensity on the enantiomeric excess and thus a pronounced “majority-rules” effect, i.e., “a slight excess of one enantiomer leads to a strong bias toward the helical sense preferred by the enantiomer that is present in majority”.75 In comparison to disordered columns, the reduced steric hindrance in helically modulated columns allows closer intracolumnar packing and longer correlation lengths of the aromatic cores. High-resolution X-ray measurements by Prasad et al.76 revealed that in the first-order transition from the disordered Colh phase into the helically ordered phase (H) of hexakis(hexylthio)triphenylene 3 the core−core spacing abruptly decreases from 3.61 to 3.59 Å, while the core−core correlation

2.3. Ferroelectric Columnar Liquid Crystals

Pyroelectric crystals have nonzero electric polarization even in the absence of an electric field. This polarization is thus 1144


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thus allow the appearance of a spontaneous electric polarization along the polar C2 axis, transverse to the column axis (Figure 10). The first and, to the best of our knowledge, so far only example of this C2-type ferroelectric columnar liquid crystal (FCLC) phase was reported by Bock and Helfrich in 199586 for a disc-shaped dibenzopyrene with chiral and strongly polar carboxylic side chains (Figure 11a). At zero electric field the material has a helical columnar ground state where the normal to the molecular disks precesses around the column axis (Figure 11b, E = 0). On application of an electric field the helical structure is unwound into states with opposite directions of (now uniform) tilt and a (spontaneous) polarization normal to the tilt plane, that is the plane defined by the column axis and the disks normal. The directions of tilt and Ps are reversed on reversing the field direction. In the so-called low-field phase they found a tilt angle of 24.5 deg and a spontaneous polarization of 60 nC cm−2. At an electric field strength higher than 10 V μm−1 the transformation into a high-field phase was observed, the tilt and polarization of which increases to 37 deg and 180 nC cm−2, respectively. Another option to achieve liquid crystal phases with polarpoint group symmetry is the close packing of molecules with pronounced “polar” shape such as bent core mesogens (Figure 10). Here, the long-range dipole order follows from the minimization of free volume, which requires a noncentrosymmetric packing of the (nonchiral) molecules. The analog to the polar bent-core smectics are columnar phases of conical or bowl shaped mesogens (Figure 10), which, according to their C∞v symmetry, can possess a spontaneous polarization along the C∞ column axis. There have been several promising attempts to find this kind of FCLC phase,88,89 but in these cases the seemingly “spontaneous” polarization was found to relax after poling the samples in an electric field. The first clear-cut case of an FCLC with stable Ps along the columns (Figure 12) was reported by Miyajima et al. in 2012.90 Fan-shaped dendrons (Figure 12a) assemble into supramolecular conical mesogens with strong cyano-dipoles at the tip of the cones. The conical assemblies stack on top of each other to form polar columns, which in turn arrange on a 2D hexagonal lattice (Figure 12b). Since all dipoles add up along the column axis, each column has a net polarization, which enables a very efficient control of the column alignment by an electric field (Figure 12c). Finally, the reversal of the electric field direction reverses the spontaneous polarization of the columns, such that a typical ferroelectric hysteresis loop is observed (Figure 12d). Even though the spontaneous polarization of this phase (∼5 μC cm−2) is higher than in any smectic FLC reported so far (150 °C, but also improved the charge carrier mobility to 1.4 cm2 V−1 s−1. Presumably the ethynyl linker increases the supramolecular order in the discotic mesophase and simultaneously reduces the intracolumnar stacking distances.104,377 Garcı ́a-Frutos et al. synthesized highly ordered π-extended discotic liquid-crystalline triindoles 174 via Scholl reaction of the corresponding triindoles 173 with lateral o-terphenyl units (Scheme 75).378 Surprisingly, even those derivatives 173b and 174a,b displayed Col phases at ambient temperature, which carried just a methyl or methoxy group at the periphery. Cyclic voltammetry studies revealed easy oxidation to stable radical cations and higher cationic charged species. It should be pointed out that for related o-terphenyl-substituted triindoles with NH or N−CH3 units short intracolumnar stacking distances of 3.30 Å were found due to optimum packing via twisting.379 As a consequence high charge carrier mobilities up to 2.8 cm2 V−1 s−1 were obtained for these materials. Ye et al. reported somewhat smaller values up to 0.69 cm2 V−1 s−1 for triindoles with peripheral diazatriphenylene units.380 4.5.4. Thiophenes. The first discotic thiophenes were discovered by the Swager group in 1996,381 who found that the thiophene unit was a necessary extension of the ligand in Cu bis-β-diketonato complexes in order to obtain columnar mesomorphism. It then took until 2002, when Eichhorn et al. reported bent-shaped di-, tri-, and tetracatenar liquid crystals carrying a thiophene core.382 Since then many groups have contributed to the progress in this area. H-bonded benzotristhiophenes 175 studied by Demenev et al. (Scheme 76) form Colh phases that ranged from 172c > 172a matches the order of intracolumnar disk−disk spacings (172b < 172c < 172a) as determined by XRD. More recent Scheme 77

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77).383 Since this columnar phase is not composed out of diskshaped molecules or aggregates, it is outside of the scope of this review and related to structures of bent-core molecules. This exciting new field in liquid crystal research is comprehensively treated in recent reviews.384,385 Substitution of the thiophene moiety by a phenyl group led to monotropic Colr phases. When the alkyl side chains were replaced by oligosiloxanes tethered via alkyl spacers Colob phases were found. With a linear thiophene derivative 177 exhibiting a Colh mesophase Li et al. successfully demonstrated the tailor-made design of LC chromophores (Scheme 77).386 Compound 177 possessed a strong thin-film absorption in the NIR region (λmax = 890 nm) and a very low band gap of 1.06 eV as deduced from cyclic voltammetry studies. Such donor−acceptor−donor compounds

are highly attractive for tandem solar cells and NIR LC displays. When thiophenes are directly fused to tetracene diimide, an ambipolar semiconductur with DL phases could be obtained.387 Elgueta et al. developed polycatenar thiadiazole diamides 178 with Colh phases (Scheme 78).388 The mesomorphic properties not only depended on length and number of peripheral alkoxy chains but also on the nature of the central connecting ring. Thus, phase widths decreased in the order phenyl > cyclohexyl > thiophene, while linear optical properties remained similar to quantum yields of 8−13%. Walba reported an electric-field-responsive hybrid composed of hexa-peri-hexabenzocoronene 179 with lateral tetrathiophene groups (Scheme 79).389 Compound 179 self-assembled into a Colh mesophase, which could be homeotropically aligned by

Scheme 78

Scheme 79

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application of an electric field due to its low frequency dielectric anisotropy Δε. In a series of papers Tschierske presented in cooperation with other groups their concept of X-shaped bolaamphiphiles 180−182 carrying fluorescent oligothiophene cores with two glycerol end groups and two lateral alkyl side chains (Scheme 80).390−393 While 5,5′-(phenylethynyl)-2,2′-bithiophene derivatives 181 showed Colh phases, square honeycomb phases Colsqu were observed for bi- and tetrathiophenes 180 and 182 (R = C10H21). In addition, the complex phase behavior was found to be dependent on the side-chain volume.393 Such bolaamphiphiles provide an example of a dynamic self-assembled system where, depending on the molecular mobility, the transition between two periodic structures with different symmetry either leads to an increase of complexity, or to a chaotic regime with reduced order. A truncated version 183 of the bolaamphiphiles is shown in Scheme 80.394 In this case, only the drawn 183 formed a Colh phase, whereas derivatives with shorter chain lengths exhibited SmA phases. A related system where one-half of the molecule was replaced by a flexible obligoethylene glycol side chain with terminal carboxylate was published by Jing et al.395 The temperature range of the Colh phase could be increased below room temperature upon replacement of the carboxylic acid with a lithium carboxylate. 4.5.5. Oxazoles, Oxadiazoles, and Thiazoles. Although thiadiazoles and oxadiazoles as liquid crystalline photoconductors were already explored in 1993 by Closs et al., the study involved only heterocycles with SmA and N phases.396 It took until 2001, when Kim et al. reported the first star-shaped discotic nematic liquid crystal containing 1,3,5-triethynylbenzene and oxadiazole-base rigid arms.397 The progress since then has been summarized by Han.398 Triazoles and pyrazoles as building blocks of columnar liquid crystals were first reported in 1997 by the groups of Torres and Serrano, respectively, which used them as ligands for metallomesogens.399,400 Tang et al. developed polycatenar LCs 184 with 1,3substituted benzene ring interconnecting two 1,3,4-oxadiazoles (Scheme 81).401 All compounds exhibited Colh phases and displayed in solution a strong blue emission (λmax = 425 nm) with large Stokes shifts of 116 nm and high quantum yields up to 73%. A stacking model was proposed, in which two molecules form a dimer. Recently, related compounds 185 (Scheme 81) with 1,4benzene ring connecting the oxadiazoles were published.402,403 In contrast to the 1,3-benzene series Colh phases were only observed with minimum chain length of C10, thus revising previous reports.404,405 Westphal et al. further extended their concept by attaching mesogenic amide groups in meta-position to obtain nonsymmetrical 1,3,5-trisubstituted derivatives 186 (Scheme 81).406 Compound 186b formed a Colh phase as a result of assumed dimer formation. 1,4-Benzene-linked bent-shaped bis-1,3,4-oxadiazoles 187 with terminal stilbene moieties (Scheme 82) were reported by Sivadas et al.407 Derivative 187 was able to assemble into Colob and Colh mesophases and formed organogels in n-decane. Rheological measurements revealed the thixotropic nature of these gels. Bent-rod mesogens with regioisomeric 1,2,4oxadiazole carrying a hydrogen-bond donor (OH, NH2) at the central 1,3,5-trisubstituted benzene were reported by Gallardo et al.408 The columnar mesophase was stabilized by H-bonded dimer formation. If 1,2,4-oxadiazoles are directly

Scheme 80

Scheme 81

Scheme 82

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compared to 190 (23%), making the former a promising candidate for emissive LC displays, polarized organic lasers, and anisotropic OLEDs. Oxadiazoles with central azobenzene core were developed by Westphal et al. and compared with the corresponding azobenzenes with four lateral amides instead of the oxadiazoles.412 The former possessed broader Colh phases and stronger absorption than the latter. Photoisomerization was studied in solution. Tetra- and hexacatenar 1,3,4-oxa- and 1,3,4-thiadiazoles with a central heterocyclic unit studied by He et al. showed Col phases, although no exact assignment was made.413 Bisthiazoles and bisoxazoles anellated via central benzene or biphenyl exhibited Colh and Colr phases.414 By extending these mesogenic cores with quinoxaline terphenyl moieties, Lin et al. obtained polycatenar propeller-shaped compounds 192 with broader Col phases for the 1,3,4-oxadiazoles 192a as compared to the 1,3,4-thiadiazoles heterocycle 192b (Scheme 84).415 This difference between oxygen and sulfur heterocycle was even more pronounced for truncated derivatives.416,417 Lai compared polarization effects in isoxazoles and 1,3,4oxadiazoles and found, that a rather minimalistic approach led to isoxazoles with hydroxymethyl and dialkyloxyphenyl group attached to the heterocycle resulting in Colh phases due to H-bonded tetramer formation. Similar observations were made by Parra et al. for columnar LCs based on amino-1,3,4thiadiazole derivatives.418 Star-shaped oxadiazole derivatives are among the liquid crystalline compounds studied in detail. For example, Varghese et al. developed highly luminescent octupolar 1,3,4-oxadiazoles 195, which were synthesized by condensation of the tetrazolylstilbene 193 and 1,3,5-trimesic acid chloride 194 (Scheme 85).419 Hierarchical self-assembly into nanofibers at higher concentrations, observed by TEM in n-decane solutions, was caused by J-aggregates, whereas at lower concentrations spheres were formed due to H-aggregates. XRD measurements indicated a Colh geometry in the fibers. Upon replacement of the stilbene units in these 1,3,4-oxadiazoles by tolane groups, blue emitting liquid crystals and gels were obtained.420 In a similar fashion luminescent columnar tris(N-salicylideneaniline)s with pendant 1,3,4-oxadiazoles were prepared by Yelamaggad et al.421 Three groups independently reported star-shaped 1,3,4oxadiazoles 196 and 198 and the sulfur analogues 197 (Scheme 86).340,422,423 As mentioned above, the Colh phase stability was influenced by the heterocycle decreasing from oxadiazole to thiadiazole. The same tendency was observed for the photoluminescence quantum yields with 25−31% for sulfur and 43−48% for oxygen heterocycles despite the larger band gaps of the latter.422 By utilizing branched side chains, Colh phases at ambient temperature were obtained.424 Barberá et al. reported 1,3,5-benzene trisamides with pendant 1,3,4-oxadiazoles, which displayed broad Colh phases enforced by intradisk hydrogenbonding resulting in supramolecular liquid crystals.425 Thus, each disk is composed of 2 or 3 molecules depending on the number of peripheral side chains. 3,5-Diphenyl-1H-pyrazoles 199 with four chiral side chains self-assembling into helical Colh phases were discovered by Beltrán et al. (Scheme 87).426 Crystallization was inhibited for several weeks for these room temperature mesogens. However, introduction of a fifths or sixth side chain (chiral or achiral)

Scheme 83. Synthesis of Derivatives 190 and 191

Scheme 84

Scheme 85

connected, ColL and Colh phases were observed according to Qu and Li and Zhang et al.409,410 The impact of H-bonding on mesophase stabilization was also demonstrated by Seo et al. for hydroquinone-tethered bis1,3,4-oxadiazoles 190, 191, which were obtained from gallic acid hydrazide 189 and acid chloride 188 (Scheme 83).411 Catalytic hydrogenation of dibenzyl ether 190 gave compound 191 with free hydroquinone moiety. Both compounds not only differ in their mesophase stability but also in their fluorescence properties. 191 showed J-type stacking in the mesophases, resulting in highly enhanced fluorescence emission and a larger quantum yield (34%) as 1182


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Scheme 86

Scheme 90. Boroxines 204 and Their Mesophase Stabilities upon Second Heating (below)437

Scheme 87

prevented any mesomorphism. The packing was rationalized by H-bonded dimer formation. 4.5.6. Miscellaneous Heterocycles. In a series of papers the Kaszynski group studied derivatives of 6-oxoverdazyl radicals 200 (Scheme 88).427 Monotropic Colr phases were observed for thioethersubstituted derivative 200. SQUID measurements revealed that 200 is paramagnetic in all phases. Furthermore, hole mobilities up to 1.25 × 10−4 cm2 V−1 s−1 were detected via time-of-flight method. In contrast, the corresponding ether derivative 201 possessed enantiotropic Colh phases and similar charge carrier mobilities as compared to the thioether 201, although charge photogeneration was inefficient.428 Furthermore, compound 201 underwent a reversible color change at the melting transition from red (λmax = 552 nm for Colh) to green (λmax = 630 nm in the isotropic phase), which is very rare for radicals and indicates strong π−π*-absorption. In addition unsymmetrical 6-oxoverdazyls were studied, complemented by HOMO/LUMO calculations and cyclic voltammetry studies.429,430 A discotic 2,4,7,9-tetraaryl-6H-dibenzo[c,e][1,2]thiazine, which could be transformed into a persistent radical via oxidation, was also reported.431 The group of Ohta reported anion-responsive mesogenic boron difluoride-pyrromethene analogues (BODIPYs) 202 (Scheme 89).432−434 Compound 202a with NH moieties showed Colh phases presumably due to H-bonded dimer formation, whereas the corresponding N-methylated derivative 202b yielded Colr phases. Anion-binding properties of these BODIPYs were examined by UV/vis titration, giving association constants Ka of 140, 890, 4200, and 18 000 M−1 for bromide, chloride, dihydrogen phosphate, and acetate, respectively. Sánchez et al. investigated the influence of symmetry on the mesomorphism of polycatenar BODIPYs.435 Furthermore, fluores-

Scheme 88

Scheme 89

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Scheme 91

Scheme 92

central BODIPY core as reported by Florian et al.436 However, even in this case the fluorescence is quenched in the Colr mesophase. Tris(3,4,5-alkoxyphenyl)boroxines 204 were studied by Wöhrle et al. (Scheme 90).437 Compounds 204 showed broad Colh mesophases with strong homeotropic alignment on glass surfaces. Reducing the number of peripheral side chains resulted in loss of mesomorphism. Jiménez et al. developed six-armed cyclotriphosphazenes 205 with dendron-type gallic esters (Scheme 91).438 With increasing number of peripheral side chains mesomorphism changed from Colh to micellar Cub phase (Figure 35). This was rationalized by the interface curvature as a major parameter to determine the mesophase morphology. Thus, the change from Colh to micellar Cub is due to an increase in the aromatic−aliphatic interface curvature by increasing the effective volume of the peripheral tails. The resulting overcrowding renders a columnar packing difficult and leading to a 3D cubic arrangement of segmented columns. More recently, Cheng et al. reported related six-armed iminecontaining cyclotriphosphazenes with Colh phases at room temperature for the whole series.439 Phasmidic indigo derivatives were investigated by Porada and Blunk.440 The textures of the Col mesophases beared striking similarities with the B7 and B2 phase of bent-core mesogens. 4.6. Porphyrins and Tetraazaporphyrins

In 1980 the first discotic phase was described by Goodby et al. for uro-porphyrin I octa-n-decyl ester; however, the phase width was just 0.1 K.441 It then took until 1987, when Gregg et al. reported porphyrin octaesters as new discotic liquid crystals.442 Since then a huge variety of porphyrin derivatives have been explored and progress has been summarized previously.443,444 In 2009 Shearman et al. reported the phase transition behavior of β-octaalkyl porphyrins 206 and 207 (Scheme 92).445 Whereas the free macrocycles 206 were nonmesomorphic, the corresponding Zn complexes 207 displayed Colr phases. Sengupta et al. grafted a second-generation dendritic unit to a Zn chlorophyll derivative and the resulting hybrid 208 selfassembled into a Colob phase (Scheme 93).446 Semiconducting behavior of 208 was investigated by the PRTRMC method, and charge carrier mobilities of ∼10−2 cm2 V−1 s−1 were observed. Such organized columnar superstructures constructed from semisynthetic Zn chlorins can be considered as mimics of the tubular organization of the bacteriochlorophyll dyes in the light harvesting chlorosomal antennae of green sulfur bacteria. A comparison of aryl-substituted Zn porphyrin complexes 209 with Zn porphycene complexes 210 was reported by Step̨ ień et al. (Scheme 94).447 Whereas the Zn porphycene 210

Figure 35. Schematic representation of the proposed self-assemblies of (a) 205a and 205b in the Colh phase and (b) 205c in the micellar cubic phase. Reprinted with permission from ref 438. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

cence spectrra revealed that the emission decreased with increasing temperature and almost disappeared in the isotropic liquid. Electronic communication could be achieved by grafting two peripheral oligo(p-phenyleneethynylene) units to the 1184


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Scheme 93. Proposed Self-Organization of Molecules 208 within Column Stratuma

a Column diameter obtained by AFM and XRD are given. Reprinted with permission from ref 446. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 94

The flattening of octaaryl-substituted porphyrins 211 to porphyrins 212 with peripherally fused phenanthrene units via Scholl reaction was reported by Myśliwiec et al. (Scheme 94).448 The sterically highly congested π-systems displayed thermotropic liquid crystallinity and self-assembled into Colr mesophases. Li et al. and Zhou et al. developed tetraphenylporphyrins 213a with peripheral gallic acid amides and their correponding Zn complexes 213b with broad Colr phases (Scheme 95).449,450 Probably H-bonding of the amide moieties contributes to the

and its corresponding tetracyanoquinodimethane (TCNQ) adduct showed lamello-columnar (Lcol) and Colh phases, respectively, the Zn porphyrin 209b formed a Colr phase. Similar clearing points were detected for both macrocycles 209b and 210b. The degree of charge transfer in the TCNQ adduct was investigated by electronic and vibrational spectroscopy in solution and thin film. DSC revealed an improved thermal stability of the TCNQ adduct as compared to the neat complex 210b. 1185


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Scheme 95

Scheme 97

Scheme 98

Scheme 96

215 and their Zn complexes 216, the corresponding Cu and Ni complexes were nonfluorescent. In order to obtain materials for organic photovoltaics, tetraphenyl porphyrins 214 with peripheral gallic esters were synthesized (Scheme 95), which displayed broad Colho and Colh phases and spontaneous homeotropic alignment.452,453 Compound 214a was incorporated as electron donor into a bulk-heterojunction solar cell fabricated from PCBM (electron acceptor), poly(3,4-ethylenedioxythiophene) (PEDOT), and ITO. The power conversion efficiency could be improved from 0.22% to 0.71% by postannealing. For the related porphyrin 214b with fluorous ponytails (Scheme 95) a significant decrease of the melting point was observed.454 Triply fused metalloporphyrin dimers 217 with peripheral alkoxy and oligoethylenglycol side chains (Scheme 96) were described by Sakurai et al.455 Columnar mesomorphism was found, but the assignment was inconclusive. However, the structurally related dimers 217d covered completely by peripheral fluorous pony-tails (Scheme 96) and dimers with partially fluorinated side chains showed Colr and Colob phases already at ambient temperature. Time-of-flight measurements of 217d revealed electron mobilities of 4.3 × 10−3 cm2 V−1 s−1. The design concept could be further extended to triply fused pentamers 218 showing a phase width of almost 300 K (Scheme 96).456

stability of the liquid crystalline phase. However, amides are not always sufficient to promote mesomorphism as was published by Miao and Zhu, who tethered tetraphenylporphyrin via amide spacers to two pentaalkoxytriphenylenes.170 For such triads no mesomorphism could be detected. Fatty acid meta-octaesters 215 of tetraphenylporphyrins were reported by Wu et al. (Scheme 95).451 Temperature ranges of the Colh phase were considerably increased by complexation of Zn2+ and other metal ions. In contrast to the neat compounds 1186


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More exotic porphyrins were published by Sessler, prepared a cyclo[8]pyrrole, which was nonmesomorphic, but showed broad Colh phases upon complexation of TNF, trinitrobenzene, trinitrotoluene, or picric acid.461 21,23-Dithiaporphyrins with Colr and Colh phases, exhibiting electrochromic properties, were described by Bromby et al.462 Thin films of these materials generated by spin-casting showed an increase of the conductivity by 4 orders of magnitude upon electrochemical p-doping, making them interesting candidates for organic photovoltaics.

Scheme 99

4.7. Phthalocyanines

The first discotic Cu phthalocyanines and their metal-free counterparts were described in 1982 and 1983, respectively, by the research group of Guillon.463,464 Since then a huge amount of work has been devoted to the development of novel materials as well as their applications in semiconductors and gas sensors.465−467 Crown ether-substituted phthalocyanines 223 were accessible by condensation of dicyanobenzene 222 with hydroquinone (Scheme 98).468 The corresponding template-assisted condensation with metal acetates provided the Zn(II)- and Ni(II)-phthalocyanine complexes 223a and 223b, respectively. Interestingly, Logacheva et al. found that the peripheral crown resulted in the formation of ND phase with decreasing phase width in the order 223 > 223a > 223b. Other 223−metal complexes such as Cd(II), Co(II), and Cu(II) were not liquid crystalline.469 Several studies focused on structure−property relationships of phthalocyanines. For example, Tate et al. investigated the influence of iso-alkyl vs n-alkyl side chains on the mesomorphic properties.470 The synthesis of compounds 228 commenced with bis-alkylation of thiophene 224, followed by oxidation to the sulfone 225. The latter was treated with fumaronitrile to provide phthalonitrile 227 (Scheme 99). Alternatively, 227 was obtained from 3,6-dihydroxyphthalonitrile 226 by Ni-catalyzed cross-coupling of the corresponding triflate. Steric assistance of the α-substituents in the cyclooligomerization resulted in a direct access to the metal-free phthalocyanines 228, which displayed Colh phases. A hole mobility of 0.14 cm2 V−1 s−1 in the Colh phase of 228b at 185 °C was measured by the time-of-flight method. This value is higher than the charge carrier mobilities of the corresponding n-alkyl-substituted phthalocyanines. Cammidge et al. compared phthalocyanine 229 and its Cu complex 229a with the corresponding macrocycles 230−232 (and their Cu complexes 230a−232a) carrying a reduced number of nitrogen atoms in the ring (Scheme 100).471 Compounds 229−232 were obtained from 3,6-dihexylphthalonitrile and MeMgBr, which provided a whole range of the different ring types. A rather unusual structural motif can be considered as the “sleeping beauty” for nonconventional discotic mesogens. Already in 1911 Vorländer reported the liquid crystalline properties of sodium diphenylacetate.472 However, the paper was almost forgotten, since Ohta et al. picked up the story in 2006 and performed temperaturedependent XRD studies, which clearly revealed that sodium diphenylacetate exhibits a Colh mesophase.473 Since then structurally related compounds were dubbed as “flying-seed-like liquid crystals”. Later this group attached such flying-seed-like units to phthalocyanines (Scheme 101).474 Compounds 233 showed columnar (Colh and Colr) mesomorphism from room temperature to extremely high

Scheme 100

Lee et al. studied Co complexes of symmetrical octa(thioalkyl)azaporphyrins 219 with Colh phases (Scheme 97) and found magnetic uniaxial alignment of the columnar superstructure over a centimeter length scale via SANS measurements.116,457 Searching for push−pull systems, which are suitable for NLO applications, Belviso et al. envisaged nonsymmetrical aryl-substituted octa(thioalkyl)azaporphyrins 220 (Scheme 97).458,459 A series of derivatives 220 were prepared which assembled into Colh phases. It was proposed that the permanent dipole induced by the nonsymmetric shape of the macrocycle contributes to the stabilization of ordered liquid crystalline phases (e.g., for bromo derivative 220 (R1 = Br)). Quantum chemical calculations of the HOMOs and LUMOs provided insight into the degree of charge transfer interaction and explained the experimentally observed absorption spectra. Kayal et al. reported symmetrical octa(thioalkyl)azaprophyrins 221 with terminal azide and acetylene groups, respectively (Scheme 97), which could be cross-linked via click reaction (CuAAC) as Langmuir films or thermally activated azide−alkyne cycloaddition (AAC) in the Colh phase.460 Surprisingly, the mesophase is only little affected by the cross-linking. 1187


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Octaalkoxycarbonyl phthalocyanines 235 with swallow-tailed side chains (Scheme 102) were described by the Geerts group.476 In comparison with the corresponding peripherally n-alkyl substitued phthalocyanines derivative 235a with branched chains showed Colr phases with lower clearing transitions. Quantum chemical calculations predicted a strong decrease in energy of the HOMO and LUMO levels, as required for n-type conductors. This assumption was supported by electrochemical studies. Besides their beneficial effect on the mesomorphic properties, branched alkyl chains prevented self-aggregation in solution according to NMR studies. Thus, 235a seemed to be well suited for solution processing via zone-casting. In order to get a detailed insight into the alignment of discotic liquid crystals, phase diagrams of blends of miscible phthalocyanines 235a/236 with swallow-tailed alkyl chains were investigated.477 Colr − Colh transitions were characterized by a continuous reversible distortion of the Colr/Colh lattices. Furthermore, homeotropic alignment was adopted by mixed samples displaying Colh phases, whereas planar alignment was preferred for Colr samples. This result provides important information for the design of novel materials suitable for devices. More recently, Sergeyev et al. reported the transition temperature engineering of octaalkoxycarbonyl phthalocyanines by esterification of octaacid 234 with binary mixtures of alcohols (Scheme 102).478 A linear dependence of the average number of side chain carbon atoms and the clearing point of the Colr phase was found. The important role of the substrate in mesophase formation was reported by Chattopadhyay et al., who proved the presence of substrate-induced phases for thin films of tetraalkoxyphthalocyanine 236 (Scheme 103). On a solid support the liquid crystalline Colr phase converts into a thermodynamically more stable plastic crystalline Coltet phase due to the geometrical effect of the rigid and impenetrable flat surface.479 In a further paper the authors studied the effect of solid interfaces and demonstrated that the control of columnar alignment of phthalocyanine 236 is determined by the confinement induced by solid substrates rather than by the nature of those substrates (e.g., glass, ITO, PCBM, gold, and PEDOT:PSS).480 That means, if a phthalocyanine film is spincoated on a substrate, whatever its surface energy or roughness, a planar alignment is always observed. This was explained by the fact that forces acting at the liquid crystal−air interface favor planar alignment, and they overcome the face-on anchoring of discotic mesogens acting at the liquid crystal− substrate interface. In addition, planar alignment was found for films deposited on one substrate, upon cooling from the isotropic liquid, developing from the air interface. This is due to so-called surface freezing, which induces the preferential orientation, already in the isotropic phase, of the alkyl chains of the phthalocyanine toward air. On the other hand, if the phthalocyanine film is confined between two identical substrates, whatever their surface energy or roughness, homeotropic alignment was detected in most cases. A lithographic alignment of phthalocyanine-derived liquid crystals as a time−temperature integrating framework, which is a crucial element for device performance, was reported by Cavallini et al.481 McKeown et al. studied binary systems of phthalocyanine 236 with swallow-tailed alkoxy substituents and (Z)-alkenylsubstituted perylene bisimide 237 in various molar ratios regarding structure and morphology (Scheme 104).482 The

Scheme 101

Scheme 102

Scheme 103

isotropization temperatures. Mesomorphism was assumed to originate from the thermal fluctuations due to free rotation of the bulky propeller-shaped substituents. Efimova et al. reported related phthalocyanines with terminal tert-butyl groups showing lyotropic behavior in chloroform or toluene.475 1188


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Scheme 104

phthalocyanine components with respect to the axis of the column (Scheme 105b). The synthesis of electron-deficient phthalocyanines 245 bearing eight alkylsulfonyl groups started from the dichlorophthalonitrile 242, which was converted in the dithioether 243. (Scheme 106).485 Condensation of 243 to the phthalocyanine 244 was performed, which was further oxidized with MCPBA to the target compound 245. The electron-deficient mesogens assembled into Colr mesophases. Quantum chemical calculations predicted a decrease in the energy of HOMO and LUMO levels induced by the electron-withdrawing sulfone groups. The conclusion was supported by cyclic voltammetry studies. The influence of halides on the mesomorphic properties of thioether-substituted phthalocyanines 246 was studied by Ahmida et al. (Scheme 107).486 All compounds showed Colh phases as long as four thioethers were present. For octaalkylthio substituted derivatives like 246f Colr phases were observed together with a pronounced odd−even effect.302 In addition, the halide atoms seem to suppress crystallization. Based on DFT calculations and cyclic voltammograms HOMO/LUMO levels were little affected by the change from octa-alkylthio to tetra-alkylthio substitution or introduction of Cl, Br, or I. A remarkable decrease in LUMO energy was only observed for the fluorinated 246a. More recent work on partly chlorinated alkoxy-substituted phthalocyanines by Usol’tseva et al. pointed along the same lines and revealed also lyotropic mesomorphism, although no details were provided.487 De la Escosura et al.488 investigated the self-organization of nonmesogenic phthalocyanine−[60]fullerene dyads 248 in

miscibility between the differently shaped mesogens depended strongly on the composition. For blends containing at least 60 mol % of disk-like 236, the two components are fully miscible, and the homogeneous blends form a Colh mesophase with enhanced thermal stability as compared to pure 236. In blends containing less than 40 mol % of 236 phase separation is observed, the Colh phases are replaced by LCol phases and no homeotropic alignment could be achieved. In order to achieve large-area homeotropic alignment, which is crucial for device applications such as thin-film organic solar cells or ultrahigh density memory devices, Kajitani et al.483 successfully utilized helically folded o-phenylene octamers as surface modifiers for octakis(dodecyloxy)phthalocyanine. The o-phenylene octamers could be spin-coated to the surface. A series of phthalocyanines 238−241 containing eight phenoxy substituents with varying number of peripheral alkoxy chains were synthesized by Ichihara et al. (Scheme 105).484 The phase transition behavior depended considerably on the number of alkyl chains. Thus, the C8 chain substituted 238Cu complexes displayed only Colh phases. The Cu complexes of 239 and 240, however, exhibited various Colr mesophases together with a cubic mesophase [Cub(Pn3̅m)] over a temperature range of 90 °C in the case of 240. McKeown et al. reported that the phthalocyanine is a rather robust discotic mesogen tolerating even the presence of out-of-plane alkyl substituents such as derivative 241 (Scheme 105).482 Broad Colh phases were detected with isotropization temperatures up to 350 °C. Single-crystal X-ray diffraction of 241a (C6) revealed how the columnar assembly accommodates the out-of-plane alkyl chains by tilting the macrocyclic plane of the 1189


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Scheme 105. (a) Structure of Derivatives 238−241 and (b) XRD Structure of 241aa

a

The tilted arrangement of 241a relative to the axis of the columnar stack that facilitates the accommodation of the out-of-plane alkyl chains. Reprinted with permission from ref 482. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 106

Scheme 107

liquid crystalline phthalocyanine hosts 247. The blends displayed indeed Colh phases. In the proposed structure of the mesophase formed by the 247/248 blends the bulky substituents are easily accommodated within the columns (Figure 36). Double-decker phthalocyanine complexes are highly desirable materials, because the metal contributes additional features to the overall properties of the complex such as luminescence, or magnetism. Nekelson et al. prepared a homologous series of double-decker cerium phthalocyanine complexes 249 and 1190


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Figure 36. Schematic representation of the most propable structure of the mesophases formed by 247/248 blends. Reprinted with permission from ref 488. Copyright 2008 American Chemical Society.

studied their mesomorphic properties (Scheme 108).489,490 Complexes 249 displayed Colho and Colhd phases. TOF measurements revealed a relatively low hole mobility of 7 × 10−3 cm2 V−1 s−1 for 249 (n = 18). This value may be explained by the absence of free radicals, which can delocalize within the columns. The Tb complex 250 prepared by Gonidec et al. (Scheme 108) behaved like a liquid crystalline single-molecule magnet.491 A reversible change of the magnetic properties was achieved by simple heating and cooling cycles.

Scheme 108

4.8. Shape-Persistent Macrocycles

While macrocycles such as crown ethers or azamacrocycles owe their flexible structure to the presence of ethylene glycol or amino tethers between rigid aromatic subunits, shape-persistent macrocycles consist of aryl or heteroaryl moieties at the edge positions, which are linked by rigid alkyne or oligo-yne units. Usually macrocycles with triangular or hexagonal shape are known. The first example of a discotic shape-persistent macrocycle was described in 1994 by Fischer et al.492 When discussing this class of compounds, two different systems have to be considered, i.e., those with peripheral alkyl or alkoxy side chains in the usual fashion and systems with intraannular side chains, which fill the “empty” interior space of the macrocycle. Thus, in the latter case, mesophase formation is due to an inverse type of nanosegregation much like inverse micellar lyotropic systems. The triangular compound 251 reported by Seo et al. represents the smallest member of shape-persistent macrocycles with extraannular side chains (Scheme 109).493 A strong influence of the side chains on mesomorphism was evident. The short branched pentyl chain in 251a led to a Colr phase, the linear hydrophilic triethylene glycol in 251b produced a Colh phase and a mixture of both side chains in the unsymmetrical 251c resulted in a ND phase. Large triangular and hexagonal macrocycles 252 and 253 (Scheme 110) have been developed by the groups of Zhao494 and Höger495 utilizing Sonogashira-Hagihara and oxidative Glaser couplings, respectively, for the assembly of the macrocycles. Depending on the phenylene spacers and aryl edge units Colrd, Colho, and ND mesophases were detected for cyclic trimers 252.494 In contrast, the liquid crystalline hexagonal macrocycle 253a behaved more traditional and self-assembled into Colh and Colr phases. However, the formation of empty helical nanochannels was observed by

Scheme 109

Figure 37. Molecular models for the SAMs of 253a corresponding to oblique and pseudohexagonal lattices, respectively (a and c). STM image of 253a at the phenyloctane/HOPG interface (image size 100 × 100 nm2, Vbias = −0.990 V, It = 6 pA). Reprinted with permission from ref 495. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. 1191


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Scheme 110

gel permeation chromatography. Regarding mesomorphic properties, macrocycle 258 with template self-assembled into a Colr phase, whereas the corresponding “empty” analogue displayed two ND phases. Surprisingly, little change of the phase transition temperatures was observed. The pronounced tendency of shape-persistent macrocycles to form ND phases can also be seen in compound 259a with two flexible intraannular chains developed by Li et al. (Scheme 112). Derivative 259b with six intraannular flexible chains, however, was only crystalline.500 A macrocyclic compound having both intra- and extraannular chains was reported by Shimura et al.501 The combination of intraannular diethylene glycol moieties and glutamate-tethered bis(dialkoxybenzyl) units enforced nanosegregation resulting in Colh phases already at ambient temperature, which are further stabilized via hydrogen bonding. The condensation of o-formylchromone 260 and o-phenylenediamine 261 to chiral tetra(dihydrocitronellyl)-substituted dibenzotetraaza[14]annulene 262 was described by Grolik et al. (Scheme 113).502,503 Small-angle X-ray diffractometry revealed lamellar Colr phases. The results from temperature-variable ECD spectroscopy were interpreted in terms of a helical

XRD and STM even for 253a, which also formed highly ordered self-assembled monolayers (SAMs) at the HOPG/ phenyloctane interface (Figure 37).495 Photoconductivity studies by Luo et al. on triangular compounds related to 252 afforded materials with nAphotocurrents and on/off ratios of 103 suggesting their use as photoswitches.496 Although details of the columnar mesomorphism were not disclosed, Kato et al. reported tetraalkoxyphenanthrene-fused dehydro[n]annulenes, i.e., shape-persistent macrocycles with phenanthrenes sitting at the edges of the triangle (or square).497 Due to the antiaromaticity of [12]annulenes no fluorescence could be detected, in contrast to the corresponding luminescent [18]annulenes. HOMO/ LUMO gaps of 1.94 and 2.33 V were found for [12]- and [18]annulenes, respectively. Recently, Höger’s group studied the effect of intraannular templates on shape-persistent macrocycles.498,499 Key step in the synthesis of 258 was a 4-fold Sonogashira−Hagihara coupling of alkyne precursor 255 and tetraiodide 254 (Scheme 111).498 The template-assisted cross coupling proceeded relatively clean with less tendency toward oligomerization, as stated by 1192


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Scheme 111

Scheme 113

Scheme 114

Scheme 115

Scheme 112

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organization of the molecules in the mesophase caused by the chiral substituents attached to the macrocycle. This packing model was further supported by molecular dynamics (MD) simulations. An unprecendented observation regarding Schiff basederived macrocycles 263 with crown-ether-like interior was reported by Hui et al. (Scheme 114).504 Neat compounds 263 were nonmesomorphic; however, they showed nematic lyotropic mesophases in various solvents (CHCl3, toluene or chlorobenzene) when complexed to metal salts such as LiBF4 or NH4BF4. Presumably the mesogens were formed by ioninduced columnar assemblies of a Schiff base macrocycle− cation complex. Photoresponsive LCs based on macrocyclic azobenzenes 264, 265 were developed by Norikane et al. (Scheme 115).505 The self-assembling can be switched by light. Dimer 264 behaved as a rod-like molecule exhibiting Sm phases, while the disk-like trimer 265 formed Col phases. Upon photoirradiation isothermal phase transitions were observed, which were intensively studied for 264.506 Pecinovsky et al. followed a similar concept by tethering the azobenzene units via mesogenic dipyridino-platinum-diphosphane complexes resulting in photochromic metallomesogens 266 (Scheme 115), which displayed both thermotropic and lyotropic (in dioxane, diglyme, and ethylene glycol) Colh phases and whose shape could be adjusted by irradiation behaving as responsive nanopores.507 The combination of polar substituents and arylsulfonamide units in macrocyclic trisallylamines 267 (Scheme 116)508,509 turned out to be beneficial for obtaining supramolecular columnar liquid crystals via dipole···dipole or H-bonding interactions. Cyclo-3,6-trisphenanthrylene revealed a columnar assembly in which the molecules self-organized by π-stacking. A record mesophase width ranging from 148 to 500 °C was reported by Pisula et al.198 Related cyclohexa-m-phenylenes did not show the same degree of macroscopic order of the columnar structures. Cone-shaped discotic liquid crystals are particularly attractive, because their directionality might allow ferroelectricity and thus switching in an electric field (see also chapter 2.3.).510 Inspired by naturally occurring peptidic macrocycles isolated from the marine organism Lissoclinum, which exist in robust, bowl-shaped conformation with a large dipole moment,511,512 Sato et al. developed peptidic macrocycles 268, which selfassembled into Colh phases already at ambient temperature (Scheme 117).513 Presumably intracolumnar H-bonding contributes to a major extent to the mesophase stability. When an electric field was applied to the LC phase, homeotropic alignment over large domains was observed. It should be pointed out that the unidirectional orientation persisted even in the absence of an electric field as long as the materials were kept in their mesophase temperature range.

Scheme 116

Scheme 117

Scheme 118

crown ethers were pioneered by Wright et al., who developed in 1993 columnar phthalocyanines with anellated crown ethers as gas sensors.518 In 1994 Johansson et al. reported the selfassembly of taper-shaped crowns into columnar mesophases.519 Since then tremendous progress has been made both regarding basic research as well as applications520,521 such as ion-selective sensing based on selective reflection of polymer-stabilized cholesteric liquid crystalline membranes.522,523 Guo et al. reported novel pentaalkoxytriphenylene LCs with tethered benzo[15]crown-5 unit which showed columnar mesophases (Scheme 118).524 However, upon complexation mesomorphism was completely lost. Based on Lattermann’s initial report on C3-symmetrical tris(dialkoxybenzoate) azacrown, Mori et al. developed related

4.9. Crown Ethers and Azacrowns

Early reports on the interactions of crown ethers and their metal complexes with liquid crystals dates back to the work by Haller et al. in 1973 dealing with their use as conductive dopants for nematic liquid crystals.514 However, incorporation of azacrowns into mesogenic units forming columnar mesophases was discovered independently by Lehn et al.,515 Mertesdorf and Ringsdorf,516 and Lattermann.517 Columnar 1194


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and a minimum chain length of 14 carbon atoms. Rather unexpectedly, a bicontinuous cubic phase with Pn3m space group was detected for troponoid 271a with C12 chains. Thus, troponoids 271 represent the first azamacrocyclic liquid crystals with cubic mesophase. A monotropic Colh phase of dibenzo[18]crown-6 272 with 4 lateral gallic esters formed only upon rapid cooling was reported by Zhang et al. (Scheme 120).526 They doped the fluorophoric 1,1-diethynyl-2,3,4,5-tetraphenylsilole into 272 and observed a significant increase of the fluorescence intensity response upon mechanical stimuli, thus generating a mechanoresponsive material. XRD measurements of dibenzo[24]crown-8 273a with larger dendron substituents in the periphery revealed a reversible phase transition between Colh and body-centered Cub phase (Scheme 121, Figure 38).527

Scheme 119

Scheme 120

Figure 38. Illustration of the LC structures and reversible Colh-toBCC phase transition of 273. Disk represents the dibenzo crown ether core. Colh: p6mm symmetry, BCC: body-centered cubic-like phase. Reprinted with permission from ref 527. Copyright 2014 The Royal Society of Chemistry.

The related dibenzo[30]crown-10 derivative bearing dodecyloxygallic acid substituents undergoes gel formation in n-dodecane solutions following a proposed mechanism closely related to living polymerization.528 Dibenzo[m]crowns[n] with lateral o-terphenyl and triphenylene units, respectively, were systematically studied by Kaller et al. with the focus on structure−property relationships (Scheme 122). While variation of alkyl chain lengths had only a minor influence on the phase behavior,529 increasing the πsystem by conversion of the o-terphenyl to the correponding triphenylene improved phase stability and temperature range, especially for [12]crown-4 derivatives 274 (Scheme 122 c).530 However, flatter is not always better, as demonstrated in particular for salt complexes of [18]crown-6 derivatives,531 where in some cases columnar mesomorphism disappeared. The studies revealed the important role of the counterion in mesophase stabilization. Soft anions such as iodide or thiocyanate increase the mesophase widths significantly, hard anions such as fluoride, however, had only little effect (Scheme 122d). The mesomorphic properties correlated well with the anion-dependent downfield shifts of the 1H and 13C NMR crown ether signals in solution, which are presumably due to contact ion pairs in case of soft anions (resulting in large downfield shifts). Hard anions form solvent-separated ion pairs and thus almost no shifts relative to the uncomplexed crown ether were detected. Besides the counterion also the ring size and symmetry of the central crown core affects the mesophase stability, with the smaller rings and less symmetrical systems yielding more stable discotics (Scheme 122a,b).530,532 It should be noted that the trends regarding flexibility and geometry are

Scheme 121

N,N,N-tris(alkoxybenzoyl)- and N,N,N-tris(alkoxytroponyl)substituted azacrowns 270b,c and 271 (Scheme 119).525 Despite their similar shape, different mesomorphic properties were observed depending on the side chains. Compound 270a with 6 peripheral alkoxy substituent with C10 chain length showed a Col mesophase. In contrast, a SmA phase was observed for the derivative 270c with 3 alkoxy chains 1195


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Scheme 122

Scheme 123

Scheme 124

somewhat different, when it comes to photoconductivity and applications in OFETs. While smaller crown sizes are beneficial for high charge carrier mobility, unsymmetrical crowns reduce the mobility.533 For example, the smallest and most rigid [12]crown-4 with lateral triphenylene units showed hole mobilities of 1 × 10−4 cm2 V−1 s−1 in solid and 5 × 10−6 cm2 V−1 s−1 in the Colh phase, whereas the values dropped down by 2 orders of magnitude for the more flexible, unsymmetrical [15]crown-5 (n = 1, m = 0). More recently, sterically biased crowns were synthesized from the tetrabromo[15]crown-5 278 and boronic acids via a 4fold Suzuki cross coupling providing the o-terphenyls 279 and 280. Despite the severe steric hindrance, the Scholl reaction of 279 and 280350 proceeded in yields of 61−83% to give the triphenylenes 281 and 282 (Scheme 123).534 Most surprisingly, the steric interaction of the two alkoxysubstituents in the bay region of the triphenylene 281 has less impact on the mesomorphism as compared to the alkoxysubstituents in the 2″-position. Thus, derivatives 281 formed a Colh phase, whereas only 282a with the shortest chain length was liquid crystalline displaying a Colr phase. 1196


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Scheme 125

al.537,538 and Beginn et al.539 In contrast, the corresponding [15]crown-5 285b with one lateral o-terphenyl unit and ester side chains was nonmesomorphic, but complexation with NaI (287) initiated a Colr phase formation at ambient temperature, which transformed into a Colh phase at elevated temperatures.540 This time, tetramers of 287 self-assembled into supramolecular disks. Surprisingly, the [18]crown-6 derivative with two lateral o-terphenyl units and peripheral gallic ester side chains showed the opposite behavior, i.e., Colh phases in the uncomplexed state, but no liquid crystallinity in the KI complexes.541 Coco et al. and Espinet et al. reported ortho-palladated complexes derived from dibenzo[18]crown-6. However, although a columnar arrangement was expected, both the free crown ether and its KClO4 complex seemed to prefer arrangement in a smectic order.542,543

In an alternative route to discotic crown ethers the tetraalkoxy-dihydroxytriphenylenes 283 were prepared by Li et al. in a separate step and reacted in a 2-fold Williamson etherification with intermediate 284 (Scheme 124) to the desired products 275.535 By a related approach the corresponding C3-symmetrical crown was also available. The dichotomy between polarity and flexibility of the side chain and polarity and flexibility of the central crown core can be seen in studies published by Steinke et al. For example, [15]crown-5 derivative 285a with only one lateral o-terphenyl unit and alkoxy side chains showed a smectic phase resulting from an antiparallel orientation of the wedge-shaped mesogens (Scheme 125).536 Upon complexation with NaI (286) the phase type changed to Colh with three molecules forming a disc according to XRD measurements. This supramolecular selfassembly is in good agreement with earlier work by Percec et 1197


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Scheme 126

Scheme 128

Scheme 129

Scheme 127

4.10.1. Carboxylate Complexes. Since the early reports by Heintz and others, carboxylate complexes are still studied. For example, Hachisuga et al. explored the concept of flyingseed-like liquid crystals, i.e., structural anisotropic molecules with sterically bulky, propeller-like appendages instead of long alkyl chains toward Tb(III) carboxylate complexes 289, which indeed showed Colr phases (Scheme 127).551 Chaia et al. reported ionic coordination polymers based on mixed-valent diruthenium tetracarboxylates 290 forming Colh and Lam phases depending on the number of alkoxy chains on the benzoate unit (Scheme 128).552 Based on the single crystal Xray structure analysis of a short chain homologue of 290b (R1 = OEt) a packing model of the mesophase was proposed, in which the molecules are tilted inside the column and the backbone of the Ru − Ru − Cl − Ru − Ru − Cl strand is oriented in a zigzag conformation. 4.10.2. Pyridine, Bipyridine, Terpyridine Complexes. Luminescent liquid crystals are highly desirable materials for OLEDs. In particular metallomesogens are very attractive for this purpose. Therefore, Kozhevnikov et al. prepared phosphorescent terdentate Pt(II) pyridine complexes 291 with stimulus-dependent emission (Scheme 129).553 Thus, while a pure film of 291b displayed only excimer-like red emission (λem = 660 nm), heating to 110 °C followed by cooling to room temperature shifted the emission color to yellow (λem = 530 nm), indicating simultaneous emission from monomer and excimer. Upon mechanical rubbing, the red emission of the excimer returns. A further heat−cool cycle reestablished monomer emission. Thus, emission was under tribological control. It should be noted that the ligands were nonmesomorphic, while the complex possessed Colh mesophases.

Aiming at luminescent metal complexes Ziessel et al. grafted gallic acids to 1,3-diamino-phenylalkyne-benzo[15]crown-5 (Scheme 126).544 In order to rationalize the observed Colr phases of compounds 288, nanosegregation into three distinct zones, i.e., aliphatic part, the tolane unit with the gallic amides, and the crown moiety, was proposed, which was further stabilized by H-bonding. Hou et al. presented redox-active tetrathiafulvalene-thiacrown ether porphyrazine triads showing a Colob phase.545 In solution, a high tendency toward complexation of Ag+ and Hg2+ was observed, resulting in dimerization of the triad. 4.10. Metallomesogens

The origins of both metallomesogens and ionic liquid crystals can be traced back to 1855, when Heintz discovered two melting points for Mg(tetradecanoate)2.546 At that time liquid crystals were not even known. More than 50 years later Vorlä n der described the “double melting” of Na(tetradecanoate) as evidence for an enantiotropic mesophase. He also studied the mesomorphism of other fatty acid-derived metal carboxylates.472 However, it took until 1992, when the first true mesomorphic coordination compounds, i.e., dinuclear Cu triketonato complexes and Ni, Co, and Zn octakis(alkylthio)tetraazaporphyrine complexes, were described by Lai et al.547 and Lelj et al.548 Since then tremendous progress has been achieved in the development of novel materials.549,550 1198


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More recently, this group reported octahedral Ir(III) 2phenylpyridine complexes 292 with Colh phases (Scheme 130).554,555 The use of tetracatenar ligand gave complex 292b, which was nonmesomorphic, but a very bright orange emitter (λem = 586 nm) with high fluorescence quantum yield of 50%.554 When axially twisted diketonates were employed as bridging ligands, axially chiral dinuclear Ir complexes were obtained.555 Halfdiscoid cycloplatinated square planar metallomesogens were studied by Venkatesan et al. (Scheme 131).556 For example, Pt complex 293 showed a Colh phase. By decreasing the number of alkyl chains attached to the cyclometalating ligand and the 1,3-diketonate units, a transition from columnar to lamellar organization was observed. Shi et al. connected a tetraalkyloxytriphenylene to the 2-phenylpyridine moiety of structurally related Pt complexes,557 and Ghedini et al. studied cyclopalladated complexes derived from nile red and a mesogenic diketonato ligand.558 They observed broad Colr phases from 35 to 173 °C and a red emission with strongly solvent dependent quantum efficiencies (cyclohexane: λem = 640 nm, ϕ = 0.6%; CH2Cl2: λem = 660 nm, ϕ = 23%). Furthermore, the authors investigated octahedral Ru(II) bipyridine complexes, where one bipyridine ligand carries two gallic esters and found both Colh and Colr phases for these complexes.559 Chang et al. reported square planar Pt(II) bipyridine catecholato complexes, which not only showed broad Colh phases, but also reversible redox behavior according to cyclic voltammetry studies.560 Cardinaels et al. developed imidazo[4,5-f ]-1,10-phenanthrolines 294, which turned out to be rather versatile ligands for lanthanidomesogens (Scheme 132).561 Surprisingly, ligand 294b was not liquid crystalline, whereas the analoguous ligand 294a with two alkyl side chains displayed Colh and SmX phases. In contrast, the corresponding 294bLa(III) complex showed Cub mesomorphism, whereas the uranyl complex of 294b yielded a Colh phase. The influence of the lanthanide contraction was visible from the phase behavior, thus decreasing ionic radii of the lanthanide ion resulted in decreasing Cub phase stability. Escande et al. connected 12 peripheral dodecyloxy chains to a central tridentate aromatic binding core to obtain dodecacatenar pyridine ligand 295 with room temperature mesomorphism. Its lanthanide complexes [Ln(295) (NO3)3] exhibited extremely broad Colh phases (Scheme 133).562 Square planar C2-symmetrical Pd(II) and Cu(II) metallomesogens based on phenacylpyridines showed enantiotropic room temperature Colh phases.563 4.10.3. Pyrazolate Complexes. Lintang et al. reported triangular trinuclear Au(I) pyrazolate complexes 296 (Scheme 134),564 which self-assembled into columns through Au(I)− Au(I) metallophilic interactions, showing a DL phase. Upon heating the luminescence emission (λem = 693 nm) decreased. The hydrophilic parts of the side chains were used for polymerization with tetrabutoxysilane to form fixed nanochannels. It was also found that these nanoscopic channels of mesoporous silica not only protected included 1D assemblies of the metallomesogen against thermal disruption but also improved their self-recovery from heat-induced structural damage, thus increasing the long-term stability of such materials. Barberá et al. developed a Ag(I) pyrazolate coordination polymer (Figure 39), which exhibits Colh phases and broad luminescence emission in the region λem = 350−500 nm.565

Scheme 130

Scheme 131

Scheme 132

Scheme 133

1199


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Scheme 134

Figure 40. Schematic illustration of the packing mode for 297 and 298 in the columnar phase. Reprinted with permission from ref 566. Copyright 2007 American Chemical Society.

Scheme 135

Figure 39. Schematic representation of a 2:1 helix. Reprinted with permission from ref 565. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

An interesting packing mode in the Col phase was described by Cavero et al. for Zn(II) pyrazolate complexes 297 and 298 (Figure 40; 297 Col 103 I, 298 Col 110 I), which depended on the presence of methylene-bridged bispyrazoles or two isolated pyrazoles.566 Compounds 297 and 298 displayed strong emissions at 412 and 437 nm, respectively, with large Stokes shifts. Columnar Ni(II) complexes derived from unsymmetrical pyrazoles were developed by Chou et al.567 H-bonded dimerization led to Colh phases already for the ligands. Cuerva et al. prepared polycatenar pyrazole ligands 299 with ColL phases for C14 and C16 side chains, which were converted either to the Pd(II) (pyrazolate)2 (300) or the Pd(II)Cl2(pyrazole) complexes (301) (Scheme 135).568 Thermal studies showed that both complexes 300 and 301 display discotic mesophases, namely Colh phases (300) and ColL phases (301). Based on X-ray crystal structure analyses of the Pd(II) (pyrazolate)2 complex packing models for the mesophases of 300 and 301 were proposed (Figure 41). Presumably the lamellar arrangement is caused by dimer formation.

Also related Pt(II) (pyrazolate)2 complexes behaved as discotic liquid crystals and exhibited Colh mesophases.569 However, the mesomorphic behavior strongly depended on the alkyl chain length. It was reasoned when core−core interactions predominate over van der Waals interactions (i.e., for complexes with C4−C10 chains), a columnar arrangement is favored. These compounds could be used as fluorescent probes for the detection of Hg2+, Zn2+, Cd2+, and Pd2+. Upon titration with the respective metal salt a chelation enhancement of fluorescence quenching was observed. Hexacatenar Pt complexes similar to 300 were described by Liao et al.570 A rather unexpected observation was made by Pucci et al., who reported that “tail-free” Pd complexes 302 with 3,5-disubstituted 2-(2′pyridyl)pyrrol ligands yielded Colh phases despite the absence of terminal long alkyl chain (Scheme 136).571 Presumably the trifluoromethyl groups at the diketonato ligand contribute to phase segregation in addition to stabilizing C−H···F hydrogen bonds. 1200


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Scheme 138. Structure of Complex 304a

Figure 41. Proposed schematic model representing the (a) columnar hexagonal and (b) columnar lamellar packing in the mesophase for compounds 300 and 301, respecitively. Chloride was omitted for clarity. Reprinted with permission from ref 568. Copyright 2014 The Royal Society of Chemistry.

Scheme 136

a Below: the “bow-tie”-like dimeric species viewed along the columnar axis (i.e. cross-section of column). Reprinted with permission from ref 574. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 137

Scheme 139

4.10.4. Imines, Salens, and Enamines. A variety of imine and salen ligands have been utilized to prepare metallomesogens. Godbert et al. reported an ortho-metalated Pd(II) complex 303 with Colh phases (Scheme 137).572 According to cyclic voltammetry complex 303 has a very low HOMO energy (−6.1 eV) and a low band gap of 1.2 eV. When the ester moieties were replaced with ether units, photoconduction ranging from UV/vis to NIR (300−850 nm) was observed. Recently, Ghedini’s group showed that the azo moiety in similar Pd complexes could be substituted by mesogenic nile red dyes, resulting in metallomesogens with high photoconductivity and amphoteric redox behavior.573 Imine ligands are also suitable for dinuclear lanthanide complexes 304, as was reported by Binnemans et al. (Scheme 138).574 In order to explain the observed Colr phases a “bowtie”-like dimeric packing was proposed (Scheme 138). However, even when a dinuclear coordination mode is possible, this does not necessarily lead to dinuclear metal-

lomesogens, as was published by Morale et al. for tetrahedral Zn(II) and Mn(II) imine complexes.575 Different types of mono- and binuclear imine complexes were studied by Szydlowska et al. For example, binuclear Ni(II) and Cu(II) oxamide complexes 305 showed Colr and Colh phases (Scheme 139).576 For complex 305c a re-entrant Colr phase was observed, i.e. upon inreasing the temperature the rare phase sequence Colr− Colh−Colr was found, presumably due to a monotonical change of the 2D lattice parameters with temperature (Figure 42). According to ESR spectroscopy and voltammetric studies the dicopper complexes behaved as a coupled Cu−Cu system with paramagnetic properties at high and diamagnetic properties at low temperatures. 1201


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Scheme 140. Complexes 306 (Left) and Structure of Ferromagnetic Columnar Phse with Spin−Spin Interactions Imposed by Chelating of Metal Center by Atoms of Neighboring Complexes (Right)a

a

Reprinted with permission from ref 577. Copyright 2008 The Royal Society of Chemistry.

Scheme 141. Structure of Complex 307 and Its Molecular Organization in Discoid Shell

In order to obtain 1D magnets, mononuclear Ni(II) and Cu(II) complexes of barbituric acid derivatives 306 with Colh phases were prepared (Scheme 140).577 SQUID experiments revealed that the ferromagnetic state is observed for Ni(II) complexes of 306 at low temperatures. It was proposed that the spin interactions leading to ferromagnetism are enhanced by bridging the metal complexes along the columns due to chelation of the metal ions by atoms of neighboring molecules (Scheme 140, right). For structurally related enaminoketone complexes with fluorinated side chains Krzyczkowska et al. found that the number of fluorinated side chains determines whether SmA or Col h phases are observed.578 It was rationalized that the preferred formation of the Colh phase with increased degree of fluorination is due to the stiffer structure of the perfluorinated chains compared to hydrocarbon chains, broadening the molecular core. A reentrant isotropic phase was found below the columnar phase because of the competing tendency to form columnar and lamellar structures over a specific temperature range. In addition, dipole−dipole interactions and Cs symmetry favors the formation of Colh phases, as was published by Krówczyński et al. for a series of similar enamino complexes.579 Instead of

Figure 42. Proposed structure of the columnar phases of 305c: (a) supposed molecular conformation in the lower Colr phase and (b−d) molecular arrangement in subsequent phases. Reprinted with permission from ref 576. Copyright 2008 The Royal Society of Chemistry. 1202


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Metallomacrocycles 311 composed of four carbazoles linked via salen bridges were recently reported by Kawano et al. (Scheme 143).586 Both the free ligand and the tetranuclear Ni4 and Cu4 complexes displayed Colr mesophases with an increased phase stability for the metal complexes and an exceptionally broad phase widths for the Ni4 complex. 4.10.5. Miscellaneous. Trimetallic nitride template endohedral metallofullerenes are highly useful materials for bulk heterojunction solar cells. For device preparation and processing, the attachment of functional groups which improve the solubility are desirable. To achieve this goal, Toth et al. developed a liquid-crystalline derivative of Y3N@C80 312 (Scheme 144), which showed a Colh phase, while its “empty” precursor self-assembled into a Colr phase (Scheme 144 below).587 The most striking difference, however, was the observed luminescence efficiency. While the “empty” precursor was only weakly fluorescent, a very strong emission was detected for 312. The oligo(phenylene)ethynylidene units act as a 100% efficient light-harvesting antennae to sensitize a bright and long-lived fullerene core emission.587 Complex triazine-gold thiolate supramolecular aggregates 313 were reported by two research groups (Scheme 145).588,589 While the neat Au complexes were not liquid crystalline, the hybrid compounds 313 displayed Colh phases. XRD results were rationalized by the packing pattern in Scheme 145 (below). Hydrogen-bonded triazine adducts were also used to attach Fe, Cr, Mo, and W carbonyl complexes.590

Scheme 142

4.11. Ionic Liquid Crystals

As already mentioned in the chapter on metallomesogens, the history of metallomesogens coincides with those of ionic liquid crystals (ILCs). Thus, the Mg(tetradecanoate)2 described by Heintz in 1855 was the first ILC composed of an organic anion and an inorganic cation.546 The inverse combination, i.e., organic cation combined with an inorganic anion was first reported by Knight and Shaw in 1938, who studied the mesomorphic properties of N-alkylpyridinium chlorides and other pyridinium salts.591 Since then a huge variety of different ILCs has been reported, which has been summarized in several focused reviews.520,592−594 Although pyridinium salts belong to the archetypal ILCs, they are mostly forming SmA phases. Colh and Cub phases occur less frequently, mostly in combination with viologens, i.e., 4,4′-bipyridinium salts. Tanabe et al. developed star-shaped tripodal pyridinium, pyrimidinium and quinolinium salts 314− 316 via 3-fold Suzuki coupling of 1,3,5-trimethylbromobenzene (Scheme 146).595 While pyrimidinium and quinolinium ILCs 315 and 316 displayed Colr and Colh phases, additionally micellar Cub phases were formed by pyridinium salts 314 via nano-

the 1,2-phenylenediamine unit in the chelate ligand also diaminophenazines and -quinoxalines could be used for such Cu(II) and Ni(II) metallomesogens.578 Quinoxaline-salicylaldimine conjugates were also suitable ligands for planar Cu(II), Pd(II) complexes showing Col h phases, whereas the correponding tetrahedral Zn(II) complexes were nonmesomorphic.580 Scissor-shaped tetradentate enaminoketones yielded H-bonded Pd(II) complexes with Colh phases.581 When bidentate benzoxazoles with attached o-phenols were converted to the Zn(II) complexes, Colh phases and strong luminescence (λem = 471 nm) were observed.582 A board-shaped dinuclear salicylimine Zn(II) complex 307 was developed by Bhattacharjee et al.583 Despite the nondiscoid structure of the complex, a Colr phase was detected resulting from dimer formation (Scheme 141). Complex 307 exhibited blue emission both in solution and in the liquid crystalline phase mainly due to a ligand-based π−π* transition. Debnath et al. investigated Ni(II) bis(dithiolene) complexes with pendant gallic esters (309a) and amides (309b), respectively (Scheme 142).584 While ester 309a formed Colr and Colh phases, the corresponding amide 309b displayed Colh and broad Cub phases. It should be pointed out that this was the first example of a Cub phase in a metallomesogen. Due to hydrogen bonding the Colh phase of the amides was stabilized down to room temperature. The related Au complex 310 (Scheme 142) showed columnar mesomorphism, but EPR studies revealed that the complex is not a neutral closed-shell species, but a well delocalized radical, which forms dark green organogels in dodecane solution.585

Figure 43. Schematic representation of the self-assembled columnar structure of salts 314−316. PF6 anions are omitted for clarity. Reprinted with permission from ref 595. Copyright 2012 American Chemical Society. 1203


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Scheme 143. Metallomacrocycles 311a

a

Reprinted with permission from ref 586. Copyright 2015 American Chemical Society.

segregation of ionic and nonionic parts (Figure 43). All compounds 314−316 were strongly luminescent both in solution as well as annealed films with quantum efficiencies up to 41.4% in solution and 12.6% in thin films, respectively. Multicolor photoluminescence emissions were successfully achieved by simple tuning of electron-donating and−accepting moieties covering the visible region from blue-green to red. DFT calculations yielded low band gaps of 2.39 eV (for 314c), 2.30 eV (for 315c), and 2.00 eV (for 316c). Zhu et al. reported wedge-shaped azobenzenesulfonates 318 and compared them with the corresponding p-amidobenzenesulfonates 317 (Scheme 147).596 Whereas the latter derivatives showed Col phases for both Na+ and pyridinium salts as well as the free acid, the columnar mesomorphism of the azobenzene pyridinium sulfonate 318 was lost upon cation exchange with Na+. When the azobenzene pyridinium sulfonate was combined with pyridinium ions on a polymeric support, the observed phase type depended on the ratio sulfonic acid/pyridine unit.597 For ratios 0.8−1 Col phases were obtained, while ratios 25 71 86 79 14 64 291 209 85 82 74 69 183 164 142 121 101 73 45

42 45 59 67

50 50 69 80

161

Tc [°C] 93 206 198 122 120 105 78 67 68 157 87 130 147

compd

chain

residual

>290 272 256 241 224 187 190 190 190 171 156 77 93 123 105 89 104 69 66 104 90 144 113

3 4a 4b 5a 5b 6a 6b 6c 6d 7 8a 8b 9a 9b 10 12 13a 13b 13c 15a 15b 15c 15d 16 17 18 19a 19b 19c 19d 20a 20b 20c 20d 21a 21b 25 26a 26b 26c 27b 27c 29a 29b 29c 29d 30a 30b

93 83 117 121 106 125 153 168

31b 31c 33a 33b 34a 34b 35 36a

SC6H13 C14H29 C14H29 CH2CH2CH2CH(CH3)2 CH2CH(CH2CH3)2 CH2CH2CH(CH3)2 CH2CH(CH2CH3)2 (CH2)5CCH (CH2)6CCH OC5H11 OC9H19 OC9H19 OC6H13 OC6H13 C6H13 C12H25 n=2 n=3 n=4 C6H13 C6H13 C6H14 C6H15 OC6H13 n=1 n=2 C8H17 C10H21 C12H25 C14H29 C8H17 C10H21 C12H25 C14H29 OC5H11 OC5H11 C2H5 C8H17/C8H17 C8H17/C4H9 C8H17/C8H19 C8H17/C4H9 C8H17/C8H19 TP C6H13/ruffigalol C6H13 TP C6H13/ruffigalol C7H15 TP C6H13/ruffigalol C6H13 TP C5H11/ruffigalol C7H15 TP C5H11/ruffigalol H TP C5H11/ruffigalol COCH3 C6H13 C6H13 C5H11 C5H11 C5H11 C5H11 C6H13 C4H9

100

36b

C4H9

69 160 224 147 76 136 127 157 159 296dec

cis azobenzene trans azobenzene

X = NO2, X1 = H, X2 = NO2 X = H, X1 = NO2, X2 = NO2 H C6H13 thioureas peripheral thiophene oligoethylene glycol tails oligoethylene glycol tails oligoethylene glycol tails benzene anisole 2-furan 3-furan peripheral thiophene

X X X X X X X X

= = = = = = = =

N N N N CH CH CH CH

methacrylates n=5 n=5 n=9 n=5 n=9 n=7 n=7 n=9 n=9 n = 12 n = 12 n n n n n n

= = = = = =

4 5 3 (alkyne) 1 (alkyne) 3 1

C4H9 CH3

1214

class/type

section

triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene triphenylene/homodimer triphenylene/homodimer triphenylene/homodimer triphenylene/heterodimer triphenylene/heterodimer triphenylene/heterotrimer triphenylene/heterotrimer triphenylene/heterotrimer triphenylene/heterotrimer triphenylene/heteropentamer triphenylene/heteropentamer

3 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1

triphenylene/homodimer triphenylene/homodimer triphenylene triphenylene triphenylene triphenylene/star-shaped trimer bis-triazolyl-tethered triphenylene triphenylene/trimer with triazole unit triphenylene/trimer with triazole unit

4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1


Chemical Reviews

Review

Table 2. continued phase Colh Colo Colho, Colhd

Tm [°C]

Tc [°C]

compd

chain

residual

38

111

37

C6H13

152

213 195

38 39a

C12H25 C5H11

n=6

Colho

11

96

39b

C5H11

n = 10

Colro

38

166

39c

C12H25

n=6

Colob

27

71

39d

C12H25

n = 10

Col Col Colh Colh Colh Colh Colh Col Colh Colh Colh Colh Colh Colh

42 35 15 34 53 78

40a 40b 41b 41c 41d 42a 43 46 47 48 52 53 55a 55b

C5H11 C5H11 C6H13 C8H17 C10H21 C6H13 C5H11 OC4H9

n=6 n = 10

77 >RT >RT >−30 65 78 78

100 128 107 96 83 188 69 137 148 91 131 120 146 154

Colh Colh Colh

62 31 50

68 248 251

62 65a 65b

120

172 305 215 195 349 110 200 302 151 247 280dec 104 150 86 131 500 500 150

Colh Colhp, Colhd Colhp, Colhd Colhd Colho Colro Colr1, Colr2 Colh Colh Colh Colh Colh Colh Colr Colr Colp1, Colp2, Colho Colp, Colho Colh (Colh) (Colh) (Colh) (Colh) Colh Colh Colh Colh Colh Colh Colh Colr Colr Colh

20 29 >0

312 219 101

−100 112 71 66 40 59 145 −100 −100 74 −70 −150 38 −100 −100 −100

200 200 200 >500 >500 232 >200 >200 >250

67 68 69a 69b 70 71 72 73 74 75 76 77a 77b 78 79 81b 81c 82 86a 86b 86c 86d 88 89a 89b 89c 90a/b 91 99c 100 101 102

branched side chains branched side chains branched side chains branched side chains

(OCH2CH2)3OCH3 CH2(OCH2CH2)2OCH3 R2 = CH2(OCH2CH2)2OCH3 C4H8 R1 = C16H33 R1 = C16H33 C12H25 C12H25 R1 = C12H25 R1 = C12H26 C12H25 O(CH2)4C8F17

CO2Et CO2Pr

R1 = R2 R1 = C7H15 oligosiloxane chains R = CH2CH(C8H17)C10H21 R = CH2CH(C8H17)C6H13 R = R1 R = C15H31 R = C15H29 branched side chains R1 = R2 = Cl branched side chains branched side chains CO2Me pentaalkyloxytriphenylene dihydrocitronellyl

R1 = CH(C6H13)C8H17 R1 = CH(C6H13)C8H17 C12H25 C5H10 C12H25 C12H25 C12H25 C12H25 C12H26 (OCH2CH2)3OCH3 (OCH2CH2)3OCH3 (OCH2CH2)3OCH3

R = CH2CH(C6H13)C8H17 branched side chains acrylate units X=I X = Br X = Cl X=F OMe branched side chains C12H26 dodecylbenzene branched side chains branched side chains

C12H25 thiophene dendrons thiophene dendrons diketopyrrolopyrrole 1215

class/type silox-triphenylene star-shaped oligomer triphenylene/porphyrin hybrid triphenylene/Cu-phthalocyanine hybrid triphenylene/Cu-phthalocyanine hybrid triphenylene/Cu-phthalocyanine hybrid triphenylene/Cu-phthalocyanine hybrid calixarene-triphenylene triad calixarene-triphenylene triad ester-substituted triphenylene ester-substituted triphenylene ester-substituted triphenylene ester-substituted triphenylene triphenylene triphenylene perylene perylene perylene imido diester perylene perylene bisimide perylene bisimide perylene bisimide dibenzocoronene tetracarboxdiimide dibenzocoronene tetracarboxdiimide perylene imide benzimidazole perylene diester benzimidazole perylene bisimide perylene bisimide perylene bisimide perylene bisimide perylene bisimide perylene bisimide perylene bisimide perylene coronene perylene perylene peropyrenequinone peropyrenequinone HBC HBC HBC HBC HBC HBC HBC HBC HBC HBC HBC triangle-shaped discotic graphene triangle-shaped discotic graphene hexathienocoronene HBC with thiophene dendron HBC with thiophene dendron HBC with diketopyrrolopyrrole

section 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3


Chemical Reviews

Review

Table 2. continued phase Colr, Colh Colho Colh Colh Colh Colh Colh Colh Colh Col Colhd Colrd, SmA Colx, CoIr Colr Colr Colr Colr Colr Colr Colr Colr Colr Colh Colh Colh Colx, Colr Colh Colh Colh Col Colh Colh Colr Colr Colh Cub Colh Colh Colh Col Colh Colh Colr, N Colh Colh Colh Col Colh Colr Colr Colr Colh Colh Colh Colh Colh Colob, Colr, Colh Colh Colr Col

Tm [°C] 93 200 165 137 113 89 46 −18

−39 −50 −21 86 −43 −39 −12 −7 18 −38 70 109 24 25 48 8 28 23 70 64 68 95 92 42 86 55 57 173 122 88 107 69 123 15 78 62 47 73 109 29 58 161 42 31 126 15

Tc [°C]

compd

216 254 382 380 370 357 337 287 239 78 65 65 182 118 99 105 172 178 183 174 161 146 58 95 135 225 107 114 125 80 194 183 112 78 126 112 208 167 215 130 111 263 209 260 242 80 >263dec 75 82 71 68 87 126 100 98 221 192 110 276 174

104 105 106a 106b 106c 106d 106e 106f 106g 107 108a 108b 109 110a 110b 110c 111a 111b 111c 111d 111e 111f 114 115 116 117 119a 119b 120a 121a 124a 124b 130a 130b 131 131 138b 138c 142 148d 148e 150a 150b 151a 151b 152 157 158 161a 161c 161d 162 165 166a 166b 167a 167b 168 169a 169b

chain C11H23 C6H13 C4H9 C5H11 C6H13 C7H15 C8H17 C10H21 C12H25 C12H25 (CH2)4(CF2)8F OC6H13 C9H19 C10H21 C11H23 C8H17 C9H19 C10H21 C11H23 C12H25 C14H29 C10H21 C10H21 C10H21 C16H33 C12H25 C8H17 C12H25 C12H25 C10H21 C10H21 C12H25 dihydrocitronellyl C10H21 C12H25 C12H25 citronellyl C10H21 C10H21 C10H21 C6H13 C10H21 C6H13 C10H21 C10H21 C12H25 C12H25 C10H21 C14H29 C16H33 C8H17 C8H17 C12H25 C16H33 C12H25 C16H33 C16H33 C12H25 2-hexyldecyl

residual

terminal gallic ester terminal gallic ester terminal gallic ester terminal gallic ester terminal gallic ester terminal gallic ester terminal gallic ester branched side chains

n=4 X=H X=H X=H X = OMe X = OMe X = OMe X = OMe X = OMe X = OMe

2x C10H21 3x C10H21

X = Et X = Et

1216

class/type dibenzochrysene PAH truxene truxene truxene truxene truxene truxene truxene polyalkynylbenzenes star-shaped-mesogen star-shaped-mesogen benzenetrisamide wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene wedge-shaped 1,2-diaminobenzene tetraphenylene tetraphenylene tetraphenylene spiro-fluorene triazine triazine triazine triazine triazine triazine triazine triazine triazine triazine triazine triazine hexaazatriphenylene azatriphenylene azatriphenylene azatriphenylene azatriphenylene azatriphenylene azatriphenylene azatriphenylene azatriphenylene carbazole carbazole carbazole carbazole carbazole carbazole carbazole carbazole carbazole carbazole carbazole quaterrylene quaterrylene

section 4.3 4.3 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.1 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.3.2 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.1 4.4.2 4.4.2 4.4.2 4.4.2 4.4.2 4.4.2 4.4.2 4.4.2 4.4.2 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3 4.4.3


Chemical Reviews

Review

Table 2. continued phase Colh Colh Colh Colh Colh Colr Colh Colh Colh Colh Colh Colh Colh Colh Colh Colh Colh (Colh) Colh Colh Colob, Colh Colh Colh Colh Colh Colh Cub (Colr) Colh Colh Colh Colh Colh Colh Colh Colh Cub Col Colob Lcol Colh Colh Colh Colh Colob Colob Colob Colh Colh Colh ND ND ND Colh Colh Colr Colr, Cub Colh

Tm [°C]

Tc [°C]

compd

9 85 76 39

Discotic Liquid Crystals - Chemical Reviews (ACS Publications)

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