Perylene Bisimide Dye Assemblies as Archetype Functional

Review pubs.acs.org/CR

Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials Frank Würthner,* Chantu R. Saha-Möller, Benjamin Fimmel, Soichiro Ogi, Pawaret Leowanawat, and David Schmidt Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany 4.6. Coassembly of PBI with Other Dyes 5. Metallosupramolecular PBI Assemblies 5.1. Metallosupramolecular Polymers 5.2. Metallosupramolecular Macrocycles 5.3. Metallosupramolecular Polyhedra 5.4. Metallosupramolecular Coassemblies 6. Ionic Self-Assembly of PBI Dyes 6.1. Ionic Self-Assembly in Solution 6.2. Liquid-Crystalline Ionic PBIs 6.3. Self-Assembly of PBI−DNA Conjugates 7. Summary and Outlook Author Information Corresponding Author Notes Biography Acknowledgments References

CONTENTS 1. Introduction 1.1. Prologue 1.2. Parent Perylene Bisimide Chromophore 1.3. Core-Substituted Perylene Bisimides 1.4. Perylene Bisimides in the Solid State 1.5. Areas of Application 2. Linear, Dendritic, and Macrocyclic Covalent PBI Ensembles 2.1. Excitonic Coupling and Deactivation Processes of Photoexcited PBIs 2.2. Rigid PBI Dimers 2.3. Flexible PBI Ensembles 2.4. Cyclic PBI Ensembles 3. π-Stacked PBI Assemblies 3.1. Self-Assembly of Core-Unsubstituted PBIs in Solution 3.2. Organization of Core-Unsubstituted PBIs in the Bulk Solid State 3.3. Self-Assembly of Amphiphilic PBIs in Aqueous Media and Solid Bulk State 3.4. Self-Assembly of Core-Substituted PBIs 3.5. Self-Assembly of Dye Arrays Composed of Multiple PBIs 3.6. Self-Assembly of Multichromophoric PBI Conjugates Containing Other Dyes 4. Hydrogen-Bond Directed Self-Assembly 4.1. Self-Assembly Directed by Imide−Imide HBonding Interactions 4.2. Self-Assembly Directed by Side-Chain Amide−Amide H-Bonding Interactions 4.3. Self-Assembly Directed by Other H-Bonding Interactions 4.4. Coassembly Directed by Imide−Melamine HBonding Interactions 4.5. Coassembly Directed by Melamine−Cyanurate/Barbiturate H-Bonding Interactions © 2015 American Chemical Society

962 962 963 964 966 966 968

1. INTRODUCTION

969 971 974 976 979

1.1. Prologue

1013 1016 1017 1022 1024 1026 1029 1030 1033 1034 1037 1038 1038 1038 1039 1039 1039

When many of the currently largest chemical companies were founded about 150 years ago, textile dyes constituted their first products. Interestingly, while most of the utilized colorants colored the textiles as monomeric dyes, others such as indigo were of nanocrystalline character.1 For the latter, the coloristic properties are accordingly influenced strongly by dye−dye interactions as in organic pigment colorants.2 This division of colorants into two classes, dyes and pigments, prevails today also in all kinds of functional dye-based materials, which are based on either single dye molecules, for example, fluorescent labels,3 or bulk materials, for example, organic semiconductors.4 Because of the increasing importance of the latter for many high technology applications, there is a growing interest in understanding dye− dye interactions and how these interactions impact the properties of nano- or bulk solid-state materials such as absorption, solidstate fluorescence, exciton, and hole and electron transport, to mention just a few. Such understanding can be derived from investigations on supramolecular dye assemblies5 that constitute the intermediate state of matter between monomeric dyes and solid-state materials.

979 984 987 991 995 996 1000 1000 1003 1006

Special Issue: Frontiers in Macromolecular and Supramolecular Science

1008

Received: March 30, 2015 Published: August 13, 2015

1011 962

DOI: 10.1021/acs.chemrev.5b00188 Chem. Rev. 2016, 116, 962−1052

Chemical Reviews

Review

Figure 1. Molecular structure of the parent perylene-3,4:9,10-tetracarboxylic acid bisimide scaffold, typical cyclic voltammogram, UV/vis and fluorescence spectrum, and B3LYP/6-31++G** calculated HOMO and LUMO and S0 → S1 transition density.

oligomeric PBI scaffolds (section 2) that exhibit “supramolecular” interactions between their constituent PBI units that are related to those found in later discussed (sections 3−6) self-assembled architectures.

During the past decade, perylene bisimide dyes (PBI, derived from perylene-3,4:9,10-bis(dicarboximide); often also abbreviated as PDI from perylene-3,4:9,10-tetracarboxylic acid diimide) emerged as an archetype class of colorants for the elucidation of this transition from monomeric to bulk materials via the supramolecular state.6 The reason is given by an unmatched combination of favorable properties for fundamental studies (sections 1.2 and 1.3), the usefulness of these dyes for a broad variety of applications (section 1.5), and the simplicity to tailor these dyes into building blocks for their self-assembly into supramolecular architectures (sections 2−6). Accordingly, it is not surprising that reviews with different focus have been continuously published for this class of dyes. Among them there are reviews devoted to the synthesis and structure−property relationships of various PBI scaffolds,7,8 applications of PBIs in fluorescence spectroscopy,9 organic electronic and photovoltaic devices,10−12 and about the variety of hydrogen-bonded13 as well as metallosupramolecular14 architectures.6,15 Furthermore, in recent reviews, PBI-based supramolecular architectures in water and their interaction with DNA and RNA have been covered.16 The last comprehensive review on supramolecular architectures based on PBI dyes dates,6 however, back more than 10 years during which an impressive progress in the field has happened. Accordingly, the objective of this Review is to provide a comprehensive update on PBI-based supramolecular architectures by highlighting the most important developments during this period of time and to illustrate the relationship between structural and functional features of PBI assemblies. To keep this Review focused, we will neither include perylene monoimides nor core-enlarged systems such as terrylene or quaterrylene bisimides9 or other annulated scaffolds17 consisting of perylene bisimide subunits. Because the main emphasis of this Review is on noncovalent architectures formed by the self-assembly of PBI building blocks, we will furthermore only include those covalent

1.2. Parent Perylene Bisimide Chromophore

The reason for the suitability of PBIs for such a large and diverse range of applications is encoded in its molecular structure: a rigid polycyclic aromatic scaffold (perylene) substituted with two dicarboxylic acid imide groups at the 3,4- and 9,10-peri-positions (Figure 1). The strong conjugation between the electron-rich perylene core and the electron-withdrawing imide groups (acceptor−donor−acceptor scaffold) shifts the UV/vis absorption band of perylene from ∼440 to ∼525 nm, while the characteristic vibronic progressions of the aromatic scaffold remain almost unchanged by the double imidization. A most important feature of this chromophore is its mirror image fluorescence with fluorescence quantum yield of (very close to) unity.7 High fluorescence quantum yields >0.9 are indeed observed for perylene bisimides in all common solvents including aliphatic, aromatic, chlorinated, and dipolar solvents. While there was some debate about the origin of quenched fluorescence in protic solvents and in particular in water, recent studies for dendronized or with strongly charged side chain substituted PBIs (to prohibit aggregation) have demonstrated that also proton transfer is not a major quenching pathway and that even in water fluorescence quantum yields of 0.9 are possible for this chromophore.18,19 This important property is attributed to the rather rigid and planar scaffold and a very low-lying triplet level20 (Figure 2), making S1 → T1 intersystem crossing (ISC) a slow and accordingly unlikely process (quantum yield ISC < 0.01%20). Only recently the phosphorescence spectrum of PBI (λmax = 1160 nm; 1.1 eV) could be detected in glassy butyronitrile matrix containing methyl iodide at 77 K.21 If the spin−orbit coupling is, 963


Chemical Reviews

Review

favorable because reactions of short-lived photoexcited singlet states are less likely. The electron poor character is also exemplified by cyclic voltammetry studies.26,27 The very typical signature of all PBIs is two reversible redox couples that are located at around −1.0 and −1.2 V vs the ferrocene/ferrocenium redox couple in dichloromethane solvent.6 The oxidative wave is typically irreversible at >1.2 V and at the detection limit for common solvents. For this reason, in Figure 1 the cyclic voltammogram of a 1,6,7,12tetraphenoxy-PBI is illustrated whose reduction waves are almost identical to those of the parent PBI but whose oxidative wave is reversible at a lower potential of 0.9 V.6,28 Spectroelectrochemical studies have shown that indeed both reduced species (radical anion and dianion) as well as oxidized species are highly stable compounds in a protective environment such as nitrogen atmosphere.27 The high stability of PBI dianions29,30 has already been noted more than 80 years ago when water-soluble reduced species of PBIs were patented for the dyeing of cotton.31 However, in contrast to indigo and other vat dyes, textile dyeing never became an important application for PBI dyestuffs. Quantum chemical calculations reveal that the oxidation and reduction processes as well as the S0 → S1 transition can be interpreted in a most simple way to originate from an electron removal from the HOMO and/or an electron addition into the LUMO. Accordingly, the optical (an absorption maximum of 527 nm corresponds to 2.35 eV) and electrochemical (the difference between the oxidation and reduction waves is 2.19 eV)26 bandgaps are in excellent agreement, and the S0 → S1 transition can be attributed as a HOMO → LUMO transition that is polarized along the long molecular axis (Figure 1).

Figure 2. Simplified Jablonski diagram showing the energy levels of singlet and triplet states of the parent PBI chromophore and including the H- and J-coupled Frenkel states of a typical rotationally displaced πstacked dimer aggregate. On the left axis are shown UV/vis absorption spectra of PBI monomers (black) and rotationally displaced dimer aggregates (red).

however, enhanced in PBI transition metal complexes, PBI triplet states can be populated with high quantum yields up to 55%,22,23 and also significant phosphorescence quantum yields of up to 11%24 can be achieved. For the majority of applications, it is, however, advantageous that the large energy gap between S1 and T1 makes the ISC process negligible and allows for intense fluorescence with lifetimes of ∼4 ns and quantum yields of >95%.20,25 While also other fluorophores are known with similarly high fluorescence quantum yields, the high thermal, chemical, and photochemical stability of PBI makes this chromophore indeed outstanding for a multitude of applications, among which red shade color pigments2 and fluorescence emitters, even for single molecule spectroscopy,9 are the most relevant ones. The outstanding stability of this dye is attributed to the electronpoorness of the π-conjugated scaffold, which makes it very resistive to oxidative degradation and other decomposition pathways. The fact that intersystem crossing into longer lived triplet states is almost absent for this chromophore is also

1.3. Core-Substituted Perylene Bisimides

It is evident from the HOMO, LUMO, and transition density distributions that the perylene bisimide core is not electronically connected to any substituent attached to the imide nitrogen. Accordingly, the imide substituent has only negligible influence on the position of the lowest energy transition, which is

Figure 3. Impact of various core substituents on the redox potentials (black lines; vs ferrocene/ferrocenium redox couple) and the optical properties (colored boxes) of PBI dyes (N* = pyrrolidinyl; Alk = −CH2CH2Ph). With the exception of the electron-rich 1,7-pyrrolidinyl-substituted PBI whose HOMO is of different character than for the other PBIs (leading to a charge-transfer character of the S0 → S1 transition), there is a good match between the energy of the optical gap (ΔE = h × ν) and the electrochemical gap (ΔE = Eox − Ered). Notably, for PBIs bearing electron-withdrawing substituents, no oxidation processes are observed in cyclic voltammetry experiments in common electrolytes, and therefore no value for the oxidation potential is given. 964


Chemical Reviews

Review

substituents, partial dehalogenations are, however, a serious drawback for bay-tetrabrominated PBIs,51,52 while the issue of isomeric purity of 1,6 and 1,7 twofold brominated compounds makes the synthesis of twofold bay-substituted PBIs somewhat cumbersome (although chromatographic as well as crystallization methods for the separation of various 1,6- and 1,7substituted PBIs have been developed).51,53−55 As shown by Müllen and co-workers, 1,7-dibromo-PBIs are also the ideal precursors toward lateral core expansion of PBIs by annulation leading to coronene-, dibenzocoronene-, and dinaphthocoronene bisimides. This lateral extension leads to dyes with quite different optical properties, which will not be discussed here as the focus of this Review is on the intrinsic PBI π-scaffold. For readers interested in various kinds of longitudinal or lateral extensions of the PBI scaffolds, we refer to an excellent recent review of Müllen and co-workers on this subject.56 More recently, also highly selective methods for the functionalization of the perylene “headland” positions 2,5,8,11 became available, enabling the whole arsenal of arylation and alkylation,37,57,58 borylation,59,60 halogenation and cyanation,61 hydroxylation,59 and amination60 of these positions. It is interesting that the position of the absorption maximum is little influenced by the respective substituent (505 nm for tetrachlorinated,61 512 nm for tetraaminated,59 and 518 nm for tetracyanated61 derivatives), while the fluorescence quantum yields are strongly decreased by some of these substituents,59,61 rather different from those for the respective bay-substituted regioisomers. The latter fact is particularly surprising because bay substituents exert a pronounced distortion of the PBI scaffold (see below), whereas the PBI scaffold remains quite planar in the case of headland substitutions.57,62 Most recently, sequential functionalizations of bay and headland positions have been realized as well, providing the so far most electron-deficient 1,6,7,12-tetrachloro-2,5,8,11-tetracyano PBI derivatives with reduction potentials as high as −0.07 V vs ferrocene/ferrocenium (Figure 3). The high electron affinity of these dyes enabled the synthesis of the first ambient stable PBI dianions, which constitute interesting near infrared absorber materials.40 An important aspect arising from the substitution of the small hydrogen atoms by more bulky substituents in bay area is the concomitant sterical congestion. Indeed, already for the parent dye the sterical situation is not trivial. This becomes apparent from temperature-dependent UV/vis spectra, which reveal a broadening at low temperatures rather sharp vibronic progressions of the PBI S0 → S1 absorption band. The latter has been attributed by AM1 calculations to an equilibrium between two conformations with planar and twisted PBI scaffolds, respectively.63 A reinvestigation by more accurate quantum chemical methods revealed, however, that only one low-energy conformation is given for the parent chromophore, which is the planar one.64 However, the potential energy surface is very shallow and allows for the population of bay-distorted conformations whose amount is dependent on temperature. Further calculations show that as soon as one or more substituents are introduced in bay positions, the lowest energy conformation exhibits two rotationally displaced naphthalene imide subunits.15,64 Other contortions of the PBI scaffold have been observed occasionally as well in single crystals,65 but it appears that the PBI waistline, in simple view consisting of two naphthalene imide subunits connected by single sp2 hybridized carbon−carbon bonds (bond length of 1.46 Å), is most easily distorted by a propeller-type contortion to accommodate larger substituents in the bay area.66

independent of the respective alkyl or aryl substituents located between 524 and 527 nm in the UV/vis absorption spectra in dichloromethane.32 The solvent effect is also quite modest between 517 nm (aliphatic solvent) and 530 nm (chloroform) for alkyl-substituted PBIs32 and can be attributed to the transient polarization by the solvent molecules (related to the solvents’ refractive index). This closed character of the chromophoric unit is ideal with respect to the utilization of these chromophores in multichromophoric supramolecular architectures by means of covalent or noncovalent connections via the imide groups.6,15 With regard to the fluorescence properties, however, the imide substituents have a strong impact. Thus, it has been shown that already modestly electron-rich aromatic substituents such as alkylated and in particular alkoxylated phenyl groups quench the fluorescence in solvents of intermediate or high polarity by photoinduced electron transfer from the substituent to the electron-poor PBI scaffold.33,34 In contrast to the imide substituents, substituents at core positions have a pronounced influence on the redox potentials and absorption properties of these dyes (Figure 3). This opened the possibility to tune the dyes’ functional properties while maintaining the self-assembly properties encoded at the imide subunits. Such possibility for strong modulation of optical and electronic properties at identical (longitudinal) molecular dimensions is indeed only surpassed by the smaller homologues naphthalene diimides.11,35,36 Figure 3 provides an overview on the impact of electrondonating and electron-withdrawing substituents on the redox potentials (related to ionization energies and electron affinities as well as HOMO and LUMO levels) and absorption maxima. Notably, due to the strong fluorescence, the hue of these dyes is often different from expectations solely based on the absorption maxima. If we exclude the green 1,7-pyrrolidinyl-substituted PBI dye due to its distinct properties (the HOMO shown in Figure 1 is now the HOMO−1, while the new HOMO looks different and is mainly localized on the amino nitrogens; charge transfer character of S0 → S1 transition; weak fluorescence), all other PBI derivatives exhibit almost identical band gaps and absorption wavelengths concomitant with a substituent-dependent increase of both reduction and oxidation potentials from the bay-tetraaryloxy28 and headland-tetraalkyl-substituted37 PBIs via the parent PBI,26 tetra-halogenated PBIs,38 to the tetrachlorotetracyano-substituted PBIs.39,40 The latter are currently the most electron-poor PBIs with a reduction potential at −0.07 V vs ferrocene/ferrocenium for the CH2C3F7 derivative (this imide substituent leads to a decrease of the redox potential by about 0.13 V as compared to alkyl substituents).40 Historically, the first derivatizations of PBIs were accomplished by halogenations and subsequent nucleophilic exchange reactions.41 The fact that the latter are limited to a few nucleophiles such as aryloxy41 and thiol42 substituents for the easily accessible 1,6,7,12-“bay area” tetrachlorinated PBIs (although overchlorination is difficult to avoid for small-scale reactions with gaseous chlorine43,44 and even 1,2,5,6,7,8,11,12octachloro-PBIs can be obtained under harsh conditions)45 points at a radical-nucleophilic aromatic substitution (SRN1) mechanism,46 which has, however, never been elucidated in detail. Accordingly, the broader scope of substituents (alkyl, aryl, cyano, alkoxy, aryloxy, amine, etc.) became available via bromination and subsequent nucleophilic substitution or transition metal-catalyzed cross-coupling reactions.47−50 Because of sterical congestions by the large size of the bromine 965


Chemical Reviews

Review

While for up to two small oxygen or fluorine substituents in 1,7-positions, presumably supported by hydrogen bonds donated by the opposing CH-acidic 6,12-hydrogens, planarization of the PBI scaffold is energetically achievable by packing effects in the solid state,64 distortion is unavoidable for larger substituents such as chlorine67 or bromine or an increasing number of substituents such as four phenoxy groups68,69 (where the oxygens now not only sterically but also electrostatically repel each other). The relationship between the size of the bay substituents and the experimentally observed distortion could indeed be elucidated most simply without quantum chemical calculations by means of the apparent overlap parameter ∑r* (calculated from van der Waals radii) of the bay substituents (Figure 4).66 In this study, it could also be shown that the

properties (Figure 5).70−74 For 18 PBI compounds bearing different imide substituents (but no core modifications), including industrially relevant color pigments, these authors distinguished two main classes: pigments of red or maroon shade characterized by small longitudinal offsets (e.g., the brilliant red Pigment Red 178 bearing azobenzene substituents at the imide nitrogens and the maroon Pigment Red 179 bearing methyl substituents) and pigments of black shades characterized by pronounced longitudinal shifts (e.g., Pigment Black 31 bearing phenethyl substituents). In all cases, the PBI scaffold is planar, and no additional visible absorption bands are attributable to the imide substituents. Accordingly, the pronounced color changes have to be attributed to mutual interactions between the closely stacked PBI dyes (distances between neighboring π-scaffolds are between 3.34 and 3.55 Å)70−74 that arise from the different substituent-dependent packing arrangements.75 Recent quantum chemical studies on PBI−PBI interactions were able to rationalize these preferential packing patterns, revealing two energy minima on the ground-state potential energy surface (PES) for quite distinct longitudinal displacements (blue surface in Figure 5).76,77 Thus, for closely stacked dimers (calculated π−π-distance of 3.33 Å), one minimum is characterized by modest transversal (Y = 0.90 Å) and longitudinal (X = 1.40 Å) displacements and the other by a much larger longitudinal displacement (Y = 1.15 Å, X = 3.40 Å). Interestingly, however, a third and indeed energetically even lower minimum is found for a dimer stack without translational offsets (X,Y = 0 Å; R = 3.33 Å) but a rotational displacement of 29.4°. Such rotationally displaced PBI packing is most commonly found in onedimensional supramolecular stacks of these dyes28 but only rarely found in crystal structures (two structures are known with rotational offsets of 35.2° and 35.5°, respectively).78 For bay-substituted nonplanar PBIs, the packing patterns become obviously more diverse because simple dislocations as favored for the planar molecular building blocks are now not suitable to accomplish close packing anymore.38 In addition, crystals of bay-contorted PBIs typically consist of two atropoenantiomers, which often pack as M/P-dimeric units.65,67 In the majority of crystal structures, it has also been shown that upon bay substitution, the symmetry of the PBI scaffold is lost and the two bay areas are characterized by different dihedral angles. Only in rare cases like for tetra- and octachloro-PBIs bearing NH imide groups did a proper match between slipped π−π-stacking and hydrogen-bonded imide subunits provide well-defined twodimensional brickwork-type packing motifs with alternating chains of M- and P-atropo-enantiomeric PBIs (Figure 6). These molecules constituted the first examples for the crystal engineering of slip-stack packing arrangements in the PBI family, and the solid-state materials (single crystals and thin films) exhibited highly interesting charge and exciton transport, as well as waveguiding properties.45,79,80 It is interesting that slipped stack packing motifs could recently also be accomplished by bulky substituents in 2,5,8,11-headland positions.62,81

Figure 4. Conversion between propeller-like distorted PBI atropisomers and relationship between observed dihedral angles and the sterical demands of the bay substituents (for exact calculation of the apparent overlap parameters ∑r*, see ref 66). As an example of a tetra-baysubstituted PBI, the molecular structure of an octachloro-PBI45 in the single crystal is also shown.

activation barrier for interconversion, presumably via a planarized scaffold, becomes rather large for four bay substituents (up to 126 kJ/mol for tetrabromo-PBI), enabling the isolation of P- and M-atropoenantiomers on chiral columns.66

1.5. Areas of Application

For several decades, PBIs have been utilized for numerous fundamental studies and in commercial products as well. Besides the focus topic of this Review, that is, supramolecular materials systems, fundamental studies that rely on PBI dyes were in particular devoted to conventional and single molecule fluorescence spectroscopy,82−84 photoinduced energy,9,85−88 and electron transfer processes,89,90 and more recently to singlet fission62 and artificial photosynthesis.91,92 The traditional areas

1.4. Perylene Bisimides in the Solid State

The pronounced influence of the packing arrangements on the functional properties of PBIs becomes already apparent by the color of PBI pigments, which ranges from red to maroon and even black shades. It is a gratifying situation that researchers from industry published a significant number of crystal structures and empirical correlations between packing structures and coloristic 966


Chemical Reviews

Review

Figure 5. Packing arrangements found in three color pigments of red (PR 178), maroon (PR 179), and black (PB 31) shades and calculated potential energy surface (PES) of the ground state for PBI dimers.76 For the blue PES, only translational offsets were allowed, while for the green PES, also rotational displacements were considered.

Figure 6. Brickwork-type packing of 1,2,5,6,7,8,11,12-octachloro-PBI directed by hydrogen bonds (blue) and π−π-stacking of alternating M- (gray πsurfaces) and P- (white π-surfaces) PBI atropo-enantiomers. 967


Chemical Reviews

Review

the limited exciton diffusion116 (∼10 nm) in noncrystalline organic solids. In this regard, fullerenes are the safe choice material. They will provide a working device with publishable efficiencies between 1% and 10% for properly designed donor components (i.e., dyes and conjugated polymers with strong absorption and matching redox properties), while PBIs are the challenging materials where subtle packing and phase separation issues make the difference between a decent device and complete failure. The latter issue may be the reason why PBIs constitute the underdogs in the field, so far not being attractive for a larger community. However, if molecular properties are tailored (which opens a much wider space for PBIs as shown in Figure 3) and packing arrangements are properly controlled, and function− packing relationships are understood, PBIs should offer the potential to surpass fullerene-based devices for both PHJ and BHJ thin films. To acquire such an understanding on elementary processes in perylene bisimide-based dye assemblies is one of the main driving forces for investigations on self-assembled PBI materials that are covered in this Review. Most recent improvements of PBI-acceptor-based BHJ solar cells propel them into promising materials with efficiencies of up to 4% in 2013114,117,118 and up to 6% in 2014.119,120 Because of the many parameters that influence the performance of an organic solar cell, it is difficult to derive structure−property relationships for the available successful and not successful PBI-based materials. However, it is awakening that the mentioned progress came from covalently tethered PBI dimers and trimers whose supramolecular organization and phase separation from the psemiconducting polymer is obviously more favorable than that for simple mono-PBIs.

of commercial applications of PBIs are as color pigments and as fluorescent dyes, which rely, besides of the optical properties, on the excellent (photo)stability of these chromophores. Furthermore, PBIs have been investigated for a broad variety of optical and optoelectronical applications including dye lasers,93−95 optical power limiters,96 fluorescent solar light collectors,41,97 xerographic photoreceptors,98 optical sensors,99−102 and as probes for biomacromolecules (DNA, RNA, proteins).103,104 Furthermore, the anhydride of PBI, that is, perylene tetracarboxylic acid dianhydride (commonly known as PTCDA), had been investigated widely by solid state and surface physicists as an archetype electron-deficient aromatic scaffold.105 The latter aspect prepared the ground for significant recent activities on PBI-based organic semiconductors. The highly successful utilization of several PBI derivatives in ambient stable organic transistor devices with n-channel mobility >1 cm2 V−1 s−1 has been highlighted in several recent review articles10−12 and will accordingly not be expatiated here. The favorable outcome of these studies was that suitably packed PBIs with electron affinities of parent PBI (X,Y,Z = H; in Figure 3) may already provide ambient stable operation38 and that a significant number of PBIs with electron-withdrawing substituents are highly suitable as well. The latter appear in particular to be advantageous for noncrystalline materials such as polymeric semiconductors.106 Highly crystalline thin films of PBI semiconductors were shown to also afford organic spintronic devices such as spin valves.107 Taking into consideration the excellent electron transport capabilities of PBIs and their intense absorption of visible light, in particular for the whole visible range by black materials, it appears rather surprising that the utilization of PBIs in organic solar cells108 has strongly lacked behind fullerenes.109,110 This holds in particular true if we consider that the electron affinity of coreunsubstituted PBIs is almost identical to that of C60 fullerene, and accordingly from the electronic point of view these two materials should be combinable with the same p-type semiconductors, providing the same open circuit voltage (Voc), while the short circuit current (Isc) should take advantage of additional photons absorbed by the much better absorber PBI as compared to fullerene. Indeed, the very first organic solar cell with a power conversion efficiency >1% was reported for a vacuum deposited two-layer thin film structure (nowadays called planar heterojunction, PHJ) consisting of electron-donating copper phthalocyanine (CuPc) and electron-accepting perylene tetracarboxylic bisbenzimidazole (PTCBI), a PBI derivative.111 The intriguing idea behind this combination of materials, one being a hole conductor and absorber of red light (CuPc), the other being the electron conductor and absorber of blue light (PBI derivative), continues to attract scientists now for almost three decades.81,112−114 However, as compared to fullerene based n-type semiconductors, PBIs are the much more challenging materials. They exhibit lower solubility and are accordingly less easily applicable for solution-processing as required in combination with polymeric semiconductors.109 Next, their material properties are strongly influenced by the respective supramolecular organization (as already exemplified by the coloristic properties; see Figure 5). Most importantly, fullerenes developed as the favored counterparts to both small molecule110 and polymeric109 p-type semiconductors because their unique globular shape is highly suitable to direct phase separation from planar electron donor π-scaffolds, which is a prerequisite to accomplish the bulk heterojunction (BHJ) morphology115 that is the so far most successful approach to overcome the problem encountered by

2. LINEAR, DENDRITIC, AND MACROCYCLIC COVALENT PBI ENSEMBLES This Review primarily concerns noncovalent self-assemblies of PBI dyes as functional materials. Because the functional properties of such supramolecular materials are decisively influenced by the electronic interactions of constituent dye building blocks, a comprehensive understanding of those interactions is of vital importance for the development of novel materials. Such understanding can be favorably acquired by exploring the optical and redox properties of covalently linked small PBI oligomers where dye−dye interactions are confined to highly defined small systems and particular geometries. Indeed, in the past years, numerous covalently linked PBI oligomers have been elaborated, and valuable insights into the dye−dye interactions have been acquired, which are highly instructive for the understanding of larger PBI self-assemblies that are discussed in the subsequent sections. Thus, we will present in this section prominent examples of recently reported covalent PBI scaffolds starting with rigid PBI dimers, in which dye−dye interactions are predetermined by well-defined arrangement of both dye units, and continue with more flexible PBI oligomers that exhibit conformational dynamics leading to intramolecular assembling (folding) and finally attend macrocyclic PBI arrays. Prior to presenting examples of covalent PBI scaffolds, in the following we will first discuss some fundamental photophysical processes that are of avail for the understanding of functional properties of PBI assemblies such as optical properties, excitation energy transfer, charge carrier and exciton mobility. 968


Chemical Reviews

Review

Figure 7. UV/vis absorption (solid lines) and fluorescence spectra (dashed lines) of (a) an H-aggregating PBI and (b) a J-aggregating PBI in methylcyclohexane. (a) Monomer absorption (black, solid line; denoted with M) and emission spectra (black, dashed line) of the PBI at a low concentration of 2 × 10−7 M and hypsochromically shifted absorption (blue, solid line; denoted with H) and bathochromically shifted, broad excimer emission spectra (blue, dashed line) upon aggregation at a higher concentration of 1 × 10−3 M at 25 °C. (b) Monomer absorption (black, solid line; at 90 °C; denoted with M) and emission spectra (black, dashed line; at 50 °C) of the PBI, and bathochromically shifted J-aggregate absorption (red, solid line; denoted as J) and emission spectra (red, dashed line) at 15 °C. For (b), the concentrations of the absorption measurements are 6 × 10−7 M and of the fluorescence experiments 2 × 10−7 M. Adapted with permission from refs 123 and 124. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 8. Schematic energy level diagram for the excitonic coupling of PBI dimers with coplanar transition dipole moments (μeg, depicted as double arrows) tilted toward the interconnecting axis by the slip angle θ. The two borderline cases, i.e., in-line (θ = 0°) and sandwich-like arrangement (θ = 90°), are shown in the gray boxes on the left and right side of the diagram. Additionally, for these two alignments, the resulting overall oscillator strengths (small dashed arrows) as well as the oscillator strengths (f, small solid arrows) of the two individual PBI units are indicated. For further explanation on this model, see the text.

2.1. Excitonic Coupling and Deactivation Processes of Photoexcited PBIs

drastically changed. Absorption bands of PBI assemblies are shifted either hypsochromically (to shorter wavelengths) with concomitant band broadening and decrease in intensity or bathochromically (to longer wavelengths) with significant band sharpening and increase of absorption coefficient with regard to that of the respective monomers as shown in Figure 7 for two PBI dyes123,124 as illustrative examples. Self-assemblies with hypsochromic shifts of absorption bands are commonly termed as H-aggregates and those with bathochromic shifts as Jaggregates (this term was initially introduced presumably in the

As discussed in the introductory section 1, most PBIs show strong absorption in the visible spectral region with absorption maxima between 510 and 570 nm depending on core substituents and very high fluorescence quantum yields of >90% (see Figure 3; exceptions are the “green” PBIs bearing amino substituents at bay positions50,121,122). Upon selfassembly of these PBI building blocks, their absorption and fluorescence properties, and thus the spectral features, are 969


Chemical Reviews

Review

Figure 9. Simplified schematic energy level diagram for different excited states of a hypothetical PBI dimer (PBI−PBI) with possible radiative (solid arrows) or radiationless (dashed arrows) relaxation and excitation processes (A, absorption; F, fluorescence; SF, singlet fission; ISC, intersystem crossing; P, phosphorescence; CS, charge separation; CR, charge recombination). The respective higher vibrational levels of each state are not shown for clarity.

1960s in photographic literature on cyanine dyes,125 and “J” denotes Jelley,126 one of the inventors of such dye aggregates).127 Both types of PBI assemblies exhibit in general lower fluorescence quantum yields as compared to those of their respective monomers. While such fluorescence quenching due to aggregation is well-explained for H-aggregates by exciton coupling theory (see below), the situation for J-aggregates is more subtle (here exciton coupling theory predicts a strongly emissive aggregate) and probably involves quenching by trap sites128,129 and processes such as singlet fission.62 The above-mentioned optical properties of PBI self-assemblies can be explained on the basis of Kasha’s exciton coupling theory,130−132 which deals with the interaction of transition dipole moments of chromophores with respect to their geometrical arrangements upon photoexcitation. In the following, we will discuss Kasha’s model in a qualitative manner based on a dimer system to explain the spectral changes observed for Hand J-type PBI assemblies (Figure 8). According to Kasha’s model, the vertical excitation from S0 to S1 can lead to different higher electronic states depending on the spatial arrangement of the dye units in a composite molecule. The prerequisites for the most simple situation depicted in Figure 8 are the coplanar, parallel, and equidistant orientation of the participating transition dipole moments (depicted as black, solid double arrows in the schematic illustration for such a dimeric system in Figure 8). In a dimer dye aggregate, the energies of the ground state as well as the excited state are reduced relative to that of monomer units due to the energy gain achieved by van der Waals forces. Equalizing the ground-state energy levels of the monomers and the formed dimer results in an energetically more stable excited dimer situation because the more polarizable excited molecule shows an increased van der Waals interaction (ΔEvdW). As a consequence of excitonic coupling, however, the excited singlet state of such dimers splits into two excitonic states (also called Frenkel excitons). The magnitude of this splitting (Δε) depends on the magnitude of the dyes transition dipole moments (μeg), the distance between the chromophores, and their mutual arrangement (Figure 8). While for the general case of not coplanar and/or not parallel dye units both excitonic states are (partially) allowed, the more simple and highly illustrative situation depicted in Figure 8 arises for coplanar, parallel chromophores. Here, two borderline cases

in which the transition dipole moments are either in-line (with a slip angle of θ = 0°, J-aggregate) or stacked on top of each other (θ = 90°, H-aggregate) are shown on the left and right sides, representatively, while the alignment with no energetically splitting of both states is indicated by the middle dashed line at the so-called magic angle (θ = 54.7°). At this value, both sigmoidal curves have an intersection point, and the two states are equal in energy. It is noteworthy to point out the fact that the spectra of such kind of aggregates are not distinguished from those of their monomers. The transitions from the S0 state to the different S1 energy levels on the solid sigmoidal curve are strongly allowed (high oscillator strengths), whereas the ones to the energy levels on the dashed line are strictly forbidden (no oscillator strength). Thus, for PBI−PBI arrangements with θ values smaller than 54.7°, the transition is energetically decreased with respect to the monomers’ transition, whereas larger θ values result in an energy increase. Consequently, the aggregates’ absorption maxima are shifted bathochromically to lower energies or hypsochromically to higher energies, respectively. In this simplistic picture, only longitudinal shifts along the dyes’ transition dipole moments are considered. However, other geometrical arrangements, that is, the transversal displacement as well as a rotational offset to each other, also influence the excitonic coupling and the resulting spectral behavior.81 For rotationally displaced PBI dimers and also larger oligomers, the excitonic coupling has been elucidated by quantum dynamical calculations by Engel and co-workers.133−135 These calculations also took into account the pronounced vibronic progressions of the PBI chromophores, which make the analysis of the excitonic coupling more cumbersome than for dyes with less strong vibronic coupling, for example, cyanine dyes. Upon photoexcitation of a chromophore from the electronic ground state (S0) to higher energy levels (Sn, n ≥ 1), a very fast relaxation to the first excited state (S1) occurs, which according to Kasha’s rule136 serves as a starting point for further relaxation processes from S1 → S0. The main deactivation channel for excited PBI monomers is the radiative process to the ground state (fluorescence), while the intersystem crossing (ISC) to the triplet state (T1) is a relatively slow process as the energy level of this state is very low-lying. Furthermore, PBIs are very rigid; therefore, also the radiationless internal conversion to the ground state is slow. As a result, the fluorescence quantum yields of PBIs 970


Chemical Reviews

Review

Chart 1. Covalent PBI Dimers with In-Plane Arrangements

2.2. Rigid PBI Dimers

are in general very high. However, for an ensemble of coupled chromophores, several additional relaxation processes become feasible, which may lead to either radiative or nonradiative decays. For instance, structural rearrangements among aggregated PBIs may lead to excimer states, 1(PBI−PBI)*, with enhanced lifetimes (up to 20 ns) and markedly displaced fluorescence, and modest to high quantum yields as observed for the H-type aggregate shown in Figure 7. Furthermore, other relaxation pathways, which direct the system to its ground state, may become more prominent upon PBI aggregation. For a hypothetical PBI dimer (PBI−PBI), the possible radiationless and radiative decays are schematically illustrated in Figure 9. Another relaxation channel than the S1 → S0 or excimer fluorescence is the ISC to triplet states, resulting in a singlettriplet state (3PBI−PBI). While the S1 → T1 process is very slow for the PBI monomers due to the large energy gap,20 this process may be accelerated in PBI aggregates, for example, via an intermediate PBI+−PBI− charge transfer137 or excimer state. Spin-allowed singlet (exciton) fission is also a possible deactivation pathway, which has, however, so far only been observed in solid materials.62 In this process, one excited state (1PBI−PBI) converts to two excited triplet states (3PBI−3PBI). According to these studies, this pathway is slightly endergonic and might therefore only occur in the solid state where it becomes favored by entropy. Finally, a most competitive nonradiative process (neither fluorescence nor phosphorescence) results via charge transfer states (even more if PBIs bear electron-rich substituents), so that after charge separation (PBI+−PBI−) the subsequent charge recombination takes place.122,137

Covalently tethered PBI dimers are well suited to derive structure−property relationships with regard to electronic communication between the constituent chromophores depending on their center-to-center distance and spatial alignment. Thus, through space electronic coupling can be studied with such PBI dimers if appropriate linkers are used that bring the monomer units into a predictable position to each other. Indeed, in the past years, quite a few PBI dimers that are tethered by rigid linkers have been reported. These PBI dimer systems can be grouped into two classes. The first one contains molecules in which the two PBI dyes are connected by rigid π-conjugated spacer units (Chart 1), while the second one exhibits spacer units that preorganize the two PBI chromophores into different kinds of π−π-contacts (Chart 2). One of the first examples of such PBI dimers, in which the monomeric units are directly linked by a N−N single bond between the imide nitrogen atoms (2-1a), was reported by Langhals and Jona.138 This dimer showed a bathochromically shifted (ca. 10 nm) lowest energy absorption maximum at 535 nm in chloroform with significant band narrowing and increase in absorption coefficient (ε = 241 800 M−1 cm−1)138 as compared to a related monomeric PBI (ε = 85 700 M−1 cm−1).139 These spectral properties are reminiscent of J-aggregates pointing at intramolecular exciton coupling conferred by linear arrangement of the chromophore units. Linear PBI dimers with larger distances between the monomeric units were achieved by connecting the monomers with mono-, di-, and tri-paraphenylene bridges (PBIs 2-1b−d).140−142 Adams and coworkers have studied this homologous series of PBI dimers 21a−d in a comparative manner to explore their photoinduced intramolecular electron transfer properties by steady-state and 971


Chemical Reviews

Review

Chart 2. Covalent PBI Dimers with Close PBI−PBI Distances

time-resolved fluorescence spectroscopy as well as femtosecond transient absorption spectroscopy, and single-molecule spectroscopy.141−143 These studies showed that the rate constants for photoinduced electron transfer in these dimers are strongly dependent on the length of the oligomeric spacer, the chromophore geometry, and the solvent polarity (dielectric constant). Single-molecule spectroscopic studies showed that fluorescence blinking behavior of these PBI dimers is dependent on their bridge length and chromophore alignment, which affect the electronic coupling and thus the through bridge electron transfer. Very recently, this series of PBI dimers has also been investigated by single-molecule spectroscopy in condensed state on thin films by Basché and co-workers.144 These authors have shown that the electronic coupling and static disorder control the quantum-mechanical coherence between the electronically excited PBI units in these dimers. These coherences were clearly found to influence fluorescence lifetimes as well as the emission spectra.144 Further interesting photophysical studies of in-line arranged PBI ensembles, in particular with trimer analogues similar to 2-1a, were performed by Matile, 145 Hulst, Hoogenboom, and Garcia-Parajó,146−149 Müllen, De Schryver, and Hofkens,150 Debije and Schenning,151,152 Herz,153 and their co-workers. It has been convincingly shown that the degree of

PBI dye alignment in a nematic liquid-crystalline host could be appreciably improved by attaching PBI units through rigid N−N linkage as in PBI dimer 2-1a and its trimer analogue.152 As the focus of our Review is on supramolecular systems, we will obviate here further discussion on these photophysical studies and refer the interested readers to the cited original works. The wedge-shaped PBI dimer 2-2 with a meta-phenylene bridge, a regioisomer of linear dimer 2-1b, was reported to exhibit a slightly more bathochromically shifted absorption maximum as compared to that of its linear para-phenylene bridged isomer.140 A similar PBI dimer and its trimer homologue were subjected to photophysical investigations at their ensemble and single-molecule levels by Osuka and Kim.154 These larger PBI ensembles were shown to be weakly coupled systems and to exhibit incoherent Förster-type energy hopping. More conformational dynamics are present in PBI dimers 23155 and 2-4a−c156 in which the monomeric chromophores are tethered at a bay position either with a diethynylene bridge (2-3 and 2-4a) or with phenylene ethynylene linkers of different lengths (2-4b,c). The dimers 2-3 and 2-4a with shorter intramolecular distances showed markedly blue-shifted absorption bands in the excitation spectra as compared to that of a monomeric reference PBI bearing the respective bay substituent, 972


Chemical Reviews

Review

revealing an H-type excitonic coupling between the constituent monomers.155,156 The prominent hypsochromism observed for 2-4a (λmax = 534 nm) was attributed to intramolecular interactions through the conjugated linker, which completely disappeared for 2-4b and 2-4c with larger distances between the chromophore units. Thus, for the latter examples, no communication between PBI moieties occurred, which resulted in absorption spectra similar to that of the respective monomeric PBIs.156 In addition to the pronounced blue-shift of the absorption spectra, the fluorescence quantum yields were drastically reduced for the H-coupled PBI dimers 2-3 and 2-4a, whereas dimers 2-4b,c showed higher values.155,156 In contrast to the PBI dimers discussed in the previous section, the covalent PBI ensembles shown in Chart 2 enforce considerable π−π-interactions as the constituent chromophores are in close vicinity to each other with predefined alignments imposed by the respective rigid backbones. In the atropoenantiomeric PBI dimers 2-5a and 2-5b (one enantiomer of each is shown) with axially chiral biphenylene (2-5a)157 or binaphthylene (2-5b)158 tether, the chromophore units are aligned in an almost perpendicular fashion as revealed by quantum chemical calculations.157,158 The center-to-center distances of PBI units in these dimers are much shorter as compared to those of the dimers with in-line arrangement (Chart 1), which results in more pronounced changes of the absorption spectra of these chiral dimers, in particular, a decreased 0−0 absorption band with an additional shoulder at the higher-energy flank (H-coupling). In recent years, another axially chiral binaphthyl-linked PBI dimer with slightly shorter branched alkyl chains, (C6H13)2, than in 2-5b, (C7H15)2, at the imide positions has been investigated very elaborately with regard to their circularly polarized luminescence properties by the groups of Kawai159−163 and Cohen.164 Further PBI dimers with close distances between PBI scaffolds were achieved by employing xanthene derivatives as rigid backbones (2-6a−d) by Wasielewski and co-workers.122,165 In dimer 2-6a containing core-unsubstituted PBIs and linear alkyl imide substituents, both chromophore units are stacked cofacially mimicking an almost perfect H-dimer.165 Only a small deviation occurred when more bulky branched alkyl imide substituents were used as in 2-6b, whereas sterically constrained bay-substituents, as in 2-6c,d, led to contortion of the PBI scaffold. The ratio of the 0−1 versus 0−0 transitions (A0−1/A0−0) of 2-6a is greater than for 2-6b,165 corroborating a more pronounced H-type coupling for 2-6a. As a consequence, a 10fold lower fluorescence quantum yield (2%) for 2-6a, as compared to that for 2-6b (19%), was observed with each excimer-like, broad emission band shifted to lower energies. Intramolecular electron transfer properties of such xanthenetethered PBI dimers have also been studied by femtosecondtransient absorption spectroscopy. Interestingly, Janssen and coworkers discussed the pronounced decrease of the fluorescence quantum yield in a molecule similar to 2-6b (differing only in the alkyl imide substituents) as a consequence of triplet state formation.137 For 2-6c bearing two pyrrolidinyle substituents on each PBI moiety, the electron transfer kinetics showed subpicosecond charge separation into PBI+−PBI− with subsequent recombination.122 Notably, such charge transfer process was not observed for the related linear, dipyrrolidinyl-substituted PBI dimer (structurally similar to 2-1a) as the distance between PBI centers in the linear dimer is more than 3 times larger than that in cofacially arranged dimer 2-6c, making charge separation energetically less favorable.122

More insight into the slip angle dependency for the energy splitting of excitonic states (compare Figure 8) was gained by shifting one of the two chromophores in the longitudinal direction that could be achieved by introducing linear fragments with varied lengths to the xanthene tether. As accomplished by the groups of Wasielewski166 and Li,167 phenylene, phenylene ethynylene, or para-diphenylene spacer allowed the adjustment of the interchromophoric distances and their angular relationship in dimers 2-7a−d. For the parent systems 2-6a−d (without an additional spacer), the cofacial alignment (θ ≈ 90°) revealed center-to-center distances (R, see the inset in the bottom left corner of Chart 2) of approximately 3.5−4.0 Å, which could be extended up to 9 Å with concomitant decrease of the angles to 23° (2-7b). With decreasing θ values from 44° (2-7a)166 to 41° (2-7c),167 32° (2-7d),167 and 23° (2-7b),166 the energy splitting and the population of the excitonic states became more pronounced. However, the biggest impact on the photophysical properties comes from the center-to-center distance, even if the distances between π-surfaces (Rππ, compare the schematic graph in Chart 2) for all systems discussed here are close to the optimal values for π−π-stacking. For 2-7b having the largest longitudinal shift of 7.9 Å (with θ = 23°), the most favored deactivation pathway is not anymore the evolution of an excimer, but the formation of the triplet state.166 This deactivation mode could be reached either by rapid singlet exciton fission (S1 → 2 T1), by conventional spin−orbit induced ISC, or by spin−orbit charge transfer ISC. For the latter process, proper energetics for the charge-separated state are required. In PBI dimers 2-7a,b, the xanthene spacer can indeed serve as electron donor to yield the xanthene+−PBI− intermediate, which enables subsequent deactivation to the triplet state.166 Hence, for larger distances, the excimer formation becomes less important and other relaxation pathways arise. A similar series for PBI dimers bearing triptycene linkers with (2-8a) or without (2-8b) an additional phenylene fragment was introduced by the Wasielewski group as well. Photophysical studies of these dimers provided further insight into the distance dependency.168 For the PBI dimers with calix[4]arene backbone 2-9a−c, Würthner and co-workers could demonstrate the solvent-driven equilibria between nonstacked and π-stacked conformations.169 As mentioned before, the extent of stacking is strongly dependent on the structural features of the chromophores and the surrounding environment. Thus, the more flat PBI units bearing hydrogens at the bay-positions of the perylene core in 29a exhibited a better stacking ability, which resulted in folded states of closely stacked PBIs in low polarity media, or unfolded conformations in solvents with higher polarity. This example nicely demonstrates that stacking/unstacking processes can be controlled by external conditions. However, gradual distortion of the perylene core by introducing more bulky substituents (2-9b, two pyrrolidinyle groups in 1,7-positions; 2-9c, four para-tertbutylphenoxy substituents in the bay area) led to a less or even completely vanished stacking ability. It is to note, however, that the electron donor character as well as the conformational flexibility of the macrocyclic calix[4]arene moiety complicate the control and precise evaluation of PBI−PBI interactions by ensemble measurements in these dimers. Therefore, for the PBI dimer 2-9c, single-molecule spectroscopic studies have been performed recently. The results of these studies suggest that the photo-blinking of 2-9c mainly occurs due to the presence of three different levels of intensity, attributable to dimer, monomer, and intermediate level.170,171 Moreover, photophysical properties of multichromophoric PBI-calix[4]arene arrays containing up to 973


Chemical Reviews

Review

five PBI chromophores have been investigated with the results that these arrays exhibit sequential fluorescence-resonance energy transfer (FRET) processes (Figure 10).88,172

fold into a predictable and thus defined way, the conformational diversity can be reduced to a minimum set of possible structures as masterly exemplified by proteins. In favorable cases, these structures are almost congruent, that is, exhibit very similar alignments of the monomeric PBI units. To achieve some dynamic character for these systems, however, external stimuli are needed, which drive the systems to different states. Consequently, in one of the resultant states, the chromophores are prone to interact with each other due to their close vicinity, and in other states no electronic communication can occur due to larger distances between the chromophores. Thus, the excitonic coupling can be switched on or off by the surrounding environment. In the approaches toward such flexible PBI foldamers by Würthner and co-workers, an alternating ortho− meta phenylene ethynylene scaffold (Chart 3) was used to allow solvent-triggered coiling from the unfolded states to the folded states.173,174 The PBI ensembles depicted in Chart 3 exhibit solventdependent folding and unfolding processes, leading to distinctive spectroscopic features of the folded and unfolded states.173,174 While methylcyclohexane (MCH) being the solvent of choice for the folding of octamer 2-12, this solvent was not suitable for folding of the smaller foldamers 2-10 and 2-11 due to a complex interplay between intra- and intermolecular aggregation driven by strong solvophobic effects. However, with the medium polar solvent THF, the folding by intramolecular aggregation of these smaller ensembles could be achieved. In Figure 11, the UV/vis absorption spectra of 2-10 in CHCl3, THF, and mixtures thereof are shown.174 The major differences in the spectra of foldamer 210 in the good solvent (CHCl3), yielding the opened or unfolded state, and in the bad solvent (THF), enforcing stacking events to folded species, were observed for the most red-shifted band representing the 0−0 vibronic transition between S0 and S1.174

Figure 10. PBI-calix[4]arene array consisting of five PBI moieties (X = Y = 4-tert-butylphenoxy; Z = pyrrolidinyl) exhibiting sequential FRET processes by excitation of the middle PBI yielding emission of light from the outer PBIs with λ = 739 nm.88

2.3. Flexible PBI Ensembles

Covalent PBI systems highlighted so far possess stiff tethers, which in most cases enforce one of the possible PBI−PBI arrangements controlled by the linker unit. In this section, we will present PBI ensembles that are tethered by flexible bridges to allow for different arrangements of the constituent chromophores by folding and unfolding processes. It is to note that so far much less examples of flexible, covalently linked PBI systems were reported as compared to the rigid ensembles discussed in the previous section. This might be due to the possibility that for allowing flexibility the complete loss of conformational control could result. However, by prudent design allowing the system to Chart 3. PBI Foldamers Based on Phenylene Ethynylene Scaffold

974


Chemical Reviews

Review

constitutes the exclusive deactivation channel for the unfolded, excited systems, the formation of excimeric state by π−π-stacking seems to disfavor charge separation and to switch the excited molecule into another deactivation pathway (compare Figure 9).175 Li and co-workers have synthesized a broad series of homologous PBI oligomers containing up to 10 coreunsubstituted PBI chromophores that are tethered by flexible tetra(ethylene glycol) linkers (2-13a−i in Chart 4) and investigated their folding versus self-assembling properties.176−179 1H NMR and UV/vis spectroscopic studies, for example, in chloroform, revealed that the folding and selfassembly of these flexible PBI oligomers are strongly dependent on the concentration. Thus, at low concentrations ( 0.9) (Figure 17). In addition, the fluorescence lifetime increased from monomeric to aggregate state due to the reduced transition probability of an S1 to S0 transition commonly observed in Htype aggregates. In-depth theoretical studies showed that a rotational displacement of ∼30° is the energy minimum for πstacked PBIs in the ground state (in the absence of specific additional influences from substituents), while an almost sandwich-type (angle close to 0°) is preferred if one PBI is in the excited state.76 Accordingly, upon optical excitation, a change in aggregate structure leads to an energetic relaxation and a bathochromically displaced excimer-type fluorescence band. The

Chart 7. PBI Macrocycles with More than Two PBI Chromophores

980


Chemical Reviews

Review

Figure 16. (a) Simplified molecular structure of PBI macrocycle 2-26a (X = Y = 4-tert-butylphenoxy) with single PBI chromophores denoted with A, B, and C. (b) Representative fluorescence intensity trajectories (FITs) of 2-26a exhibiting the three distinct levels indicative for A, B, or C. FITs are plotted as the integrated intensity of each defocused image measured at a bin time with 1 s. (c) Left images are experimental and right images are the corresponding simulated ones revealing specific orientations for individual chromophores A, B, and C in 2-26a.87

Chart 8. Structures of Core-Unsubstituted PBIs

aggregation and solvent polarity was observed for 3-1a and 3-1d as shown in the plot of ΔGo values against solvent polarity parameter ET(30) (Figure 18). Different contributions of intermolecular forces were found to operate on π−π-stacks of these core-unsubstituted PBIs. In the

details of this relaxation process were recently elucidated by timeresolved spectroscopy209 and quantum dynamic calculations.210 The dependency of self-assembly properties of PBIs on the solvent polarity has been extensively studied by Würthner and co-workers.32 A biphasic behavior of Gibbs free energy (ΔGo) of 981


Chemical Reviews

Review

Figure 17. (a) Concentration-dependent UV/vis absorption spectra of 3-1a (2.0 × 10−7 to 1.0 × 10−3 M) in MCH. Arrows indicate spectral changes upon increasing concentration. Blue line: UV/vis spectrum of a spin-coated film of PBI 3-1a after annealing at 150 °C for 3 h. (b) Normalized concentration-dependent fluorescence spectra (excitation at 469 nm) of 3-1a (2.1 × 10−7 to 2.1 × 10−4 M) in MCH solution and of a spin-coated thin film (blue line). Inset: Color photographs of MCH solutions of 3-1a with increasing concentration (left to right) and a thin film of 3-1a. Adapted with permission from ref 123. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

from electron-rich trialkyloxyphenyl groups to the electrondeficient perylene core. Incorporation of chiral side chains in imide substituents as in PBI 3-1c provided helical supramolecular assemblies. Interestingly, no defined isodichroic points were detected from temperature-dependent CD spectroscopic studies. Therefore, it was proposed that at low concentration or high temperature lefthanded dimer (M-dimer) is preferably formed. Upon cooling or increasing concentration, this left-handed dimer (M-dimer) then transforms into a right-handed one (P-dimer) and further elongates to form an extended right-handed helical stack, which is more preferable at lower temperature.211 Extension of alkoxyphenyl groups with one methylenic unit spacer in PBI 3-2 showed aggregation behavior similar to that of 3-1b that does not have an additional spacer. However, the binding constant of 3-2 was 3 orders of magnitude lower than that of 3-1b. This methylene spacer decreases the rigidity of the molecule, which lowers the binding constant of the aggregates.212 The effect of long alkoxy terminal substituents on self-assembly was elucidated by varying the number of these substituents (R1) at the phenyl ring in PBIs 3-3a−c containing no, one, and three dodecyl groups, respectively.213 Combining the properties of PBI building block with pentadecyl phenol (PDP) or cardanol, an unsaturated analogue of PDP, generates PBI derivatives 3-4a and 3-5, which also exhibited highly fluorescent aggregates in solution with a quantum yield ∼0.5 in MCH.214 Furthermore, the morphology of the supramolecular architectures was studied by different microscopic techniques, including SEM, TEM, and AFM. These studies revealed that both PBIs 3-4a and 3-5 bearing saturated and unsaturated PDP chains, respectively, selfassembled into cylindrical bundles of sickle-shaped nanorods. The nanorods formed from derivative 3-5 are shorter than those obtained from derivative 3-4a.214 When the substituents at the imide positions are simple alkyl chains as in PBI 3-6a and 3-6b, the self-assembly of these derivatives resulted in 1D nanobelt crystals under optimized conditions.215,216 The limited solubility of these molecules, however, prohibits detailed thermodynamic studies on the aggregation process, which probably follows a kinetic pathway similar to other crystallization processes.217 However, when the imide substituents are branched swallow tails as in 3-7b, solubility becomes high, aggregation constants becomes low,32 and 0D nanoparticles were obtained due to the steric hindrance

Figure 18. (a) Plot of the standard Gibbs free energy ΔG° for isodesmic aggregation vs solvent polarity parameter ET(30) for 3-1a (●) and 3-1d (▲) and (b) model for the columnar PBI aggregates. Adapted with permission from ref 32. Copyright 2012 The Royal Society of Chemistry.

nonpolar solvent regime, the aggregation constants decrease from low polarity aliphatic solvents (n-hexane and MCH) to intermediate polarity solvent (toluene, THF). This can be explained in terms of reduced electrostatic interaction between these highly quadrupolar molecules in higher polarity solvents. On the other hand, the increase of the aggregation constants in very dipolar (e.g., acetone, MeCN) and protic solvents (e.g., MeOH) is imparted by dispersion interactions, which become stronger as the solvent polarity increases. As for the special case of halogenated solvents such as chlorinated CCl4 and CHCl3, the high polarizability of these solvents provides the best solvation of PBI units, leading to reduced binding strength of aggregates. In the case of water, the pronounced hydrophobic effect causes tremendous values for the aggregation constants, which are not easy to determine because dissociation is difficult to accomplish in the required concentration range for UV/vis spectroscopy. Self-assembly of a large number of core-unsubstituted symmetric PBIs (Chart 8) with various imide substituents has been studied in the past years. The self-assembly behavior of core-unsubstituted PBI 3-1b bearing trialkoxyphenyl imide substituents, along with derivatives containing bay substituents, in solution and in the bulk state was studied by Würthner and coworkers already in 2001.28 The self-assembly process of this PBI can be well described by the isodesmic model. It is to note that fluorescence was observed neither for monomers nor for the aggregates of this PBI due to photoinduced electron transfer 982


Chemical Reviews

Review

Chart 9. Structures of Core-Unsubstituted Liquid-Crystalline PBIs

that interrupts one-dimensional growth of the π−π-stack.215 These findings emphasize the important effect of imide substituents on the morphology of π−π-stacked assemblies of core-unsubstituted PBIs. It is interesting to note that a comparative study with a core-unsubstituted PBI and its phenylethyl headland-substituted derivative revealed that such ortho-substitution can alter both the self-assembly and the dichroic properties of PBIs.218 George and co-workers have reported that PBI functionalized with dipicolylethylenediamine-zinc (DPA-Zn) receptors at the imide positions self-assembled into helical supramolecular polymers by homotropic allosteric control upon binding with adenosine triphosphate (ATP) or heterotropic allosteric control

upon subsequent binding with adenosine diphosphate (ADP) or pyrophosphate (P2O74−).219 They have also reported that at the imide positions, cholesterol- and dihydrocholesterol-functionalized PBIs with dipolar carbonate linkers self-assemble into Htype one-dimensional aggregates in MCH through a nucleation− elongation mechanism. Molecular dynamics simulation and bulk dielectric measurements revealed that dipole−dipole interaction between the carbonate linkers along the π−π-stacking direction is the origin of the cooperativity in the supramolecular polymerization process.220 Würthner and co-workers have further demonstrated that unsymmetric PBIs with bulky rigid and flexible substituents at imide positions can be utilized to limit the growth of the PBI self983


Chemical Reviews

Review

the PBI molecules in solution. Charge transport properties of PBI 3-1a in the solid state were investigated by using PR-TRMC, and one-dimensional TRMC mobility of 0.42 cm2 V−1 s−1 was observed for the crystalline phase, which is among the highest values for PBI dyes. The conductivity in the liquid crystal phase was more than 1 order of magnitude lower (0.023 cm2 V−1 s−1) than that for the crystal phase, which is probably due to relaxed polaronic dimer trap sites (similar to the excimers) within the column.123 In contrast to PBI 3-1a and 3-1b, which showed a disordered columnar hexagonal mesophase, PBI 3-1c containing chiral side chains exhibited a highly ordered columnar hexagonal (Colho) mesophase.211 Consequently, improved charge carrier mobilities were obtained for the chiral PBI 3-1c as compared to those for achiral PBI 3-1a.211 Marder and co-workers have reported the charge carrier mobilities of PBI 3-1b and 3-2 in liquid-crystalline phases at room temperature by steady-state space-charge limited current (SCLC) technique and observed the values of 0.77 cm2 V−1 s−1 (for PBI 3-1b) and 1.3 cm2 V−1 s−1 (for PBI 3-2).227 These values were found to be higher than those obtained from PR-TRMC technique (0.0078 cm2 V−1 s−1 for PBI 3-1b and 0.011 cm2 V−1 s−1 for PBI 3-2), which might be due to different sample preparation and/or differences in the respective methodologies for charge carrier mobility determination (different methods measure on different length scales and show different impact by trap states). It is interesting to note that, related to previously investigated coronene bisimides by Müllen and co-workers,48,228 Marder’s group has also investigated a series of liquid-crystalline coronene bisimides, which can be considered as core-extended PBI derivatives, and reported an exceptionally high SCLC charge-carrier mobility of 6.7 cm2 V−1 s−1 for a N,N′bis(pentadecylfluorooctyl)-substituted derivative.229 Thermotropic properties of PBIs containing swallow-tail substituents with varied length of alkyl (3-7a,c) (see Chart 8) or oligoethylene oxide (3-11a,b) (see Chart 9) chains were studied in detail by Müllen, Thelakkat, and their co-workers.230,231 The first study by Müllen’s group elucidated PBI 3-7a in comparison to various longitudinally and laterally π-extended PBI derivatives.230 The two interesting outcomes of this study were that 3-7a did not form a liquid-crystalline mesophase (but still exhibits columnar organization) while the core-extended quaterrylene bisimides and coronene bisimides formed such mesophases, and that these molecules showed quite different orientations on a substrate. Thus, liquid-crystalline coronene bisimide arranged face-on, leading to a homeotropic phase, while all other dyes arranged in edge-on fassion. It is noteworthy that face-on orientations are preferred for applications in organic photovoltaics, while edge-on orientations are preferred for organic transistors. For the structurally related oligoethylene glycol derivative 3-11b a broad mesophase is found, which raised the interest in this compound. Two studies by Sebastiani, Spiess, and their co-workers applied solid-state NMR techniques to elucidate the packing arrangements and dynamics of these molecules in their liquid-crystalline phase.232,233 In their more comprehensive study on a larger number of alkyl and oligoethylene glycol swallow-tail PBI derivatives, Thelakkat and co-workers aimed at structure−property relationships. From these studies, it can be concluded that only oligoethylene oxide chains, which have higher conformational flexibility, can better stabilize the liquid-crystalline mesophases. Accordingly, the PBIs with oligoethylene oxide chains 3-11a and 3-11b exhibited hexagonal columnar liquid crystal mesophases in a broad temperature range, while the alkyl chain derivatives showed a

assembly. PBI containing 2,5-di-tert-butylphenyl and benzyl substituents as in 3-8 showed a very unique discrete dimer in the CHCl3/MCH mixture. This observation can be attributed to the interplay between conformationally locked 2,5-di-tert-butylphenyl group and the benzyl group rotation, which allows for only one perylene facet to be accessible for the π−π-interaction. Once the binding of the other molecules is formed (dimerization process), the bulky substituents will block further aggregation leading to the discrete stabilized dimers.221 The excimer formation process and exciton dynamics in this defined dimer were studied by timeresolved fluorescence and transient absorption spectroscopy by Kim and co-workers.222 3.2. Organization of Core-Unsubstituted PBIs in the Bulk Solid State

Self-assembly of PBIs in bulk solid state has been widely investigated in terms of liquid crystallinity. Prominent examples of liquid-crystalline PBIs are shown in Chart 9. Liquid-crystalline (LC) properties of PBIs were first reported in 1997 by Gregg and co-workers using PBIs 3-9a−e that contain oligooxyethylene substituents at the imide positions.223 Most of these PBIs exhibited LC properties over a wide temperature range. PBI 3-9d spin-coated from THF solution as thin films crystallized into ribbon-like structures, and the film color changed from red to black during the self-assembly process. The black phase that has a higher phase order exhibited higher fluorescence quantum yield and narrower emission band than the less ordered red phase, which could be transformed into the black phase by thermal or solvent vapor annealing. The highly branched chains of 3-9e were found to inhibit the packing of PBI dyes with the absence of LC phase formation.224 Another investigation of liquid-crystalline PBIs was performed by the group of Sudhölter with PBIs 3-10a,b and 3-6a containing linear alkyl chains C7, C18, and C12, respectively, in the imide positions.225 According to DSC and X-ray diffraction analysis, the liquid-crystalline mesophases of these compounds displayed highly ordered mesophases, presumably of smectic nature, with the interdigitation of the alkyl chains. In general, several phase transitions are observed upon heating. The lower temperature mesophase has a higher degree of interdigitation among the alkyl chains than the higher temperature one, leading to tighter packing in the former case. Because of the intermolecular dipole−dipole interactions of carbonyl groups at the imide position, rotational displacement of PBI units was also observed within the π−π-stacks, confirming the spectroscopic results from solution aggregates. Charge carrier mobilities of 3-10b were determined by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique, revealing a value of 0.1 and 0.2 cm2 V−1 s−1 for the liquid crystal and crystalline phases, respectively. These relatively high values indicate a close packing of PBI units in both crystal and mesophases. Würthner reported the thermotropic properties of trialkoxyphenyl- and trialkylphenyl-substituted PBIs 3-1a and 3-1b.28,123 These compounds feature hexagonal columnar mesophases over a broad temperature range, similar to traditional discotic LC molecules such as triphenylenes, hexabenzocoronenes, or phthalocyanines.226 On first glance, this is an unexpected result for rectangular-shaped and highly quadrupolar PBIs. However, due to the rotational offset among π-stacked PBIs, a discoid “supramolecular” unit becomes possible leading to hexagonal columnar mesophases. This structural model for bulk material fits nicely with the results obtained by the authors from concentration-dependent absorption spectroscopic studies of 984


Chemical Reviews

Review

Figure 19. (a) Molecular structure of swallow-tail N-substituted PBIs. Optical textures under crossed polarizers showing (b) focal conic textures of hexagonal columnar phase of 3-7a in the mesophase at 118 °C, (c) dendritic growth aggregates of hexagonal columnar phase of 3-11a in the mesophase at 144 °C, and (d) fan-shaped focal conic textures of 3-11b at 142 °C.231

Figure 20. Molecular models of the aromatic core region of dendronized PBIs 3-1b, 3-2, 3-14, 3-15i, and 3-16 (a), and top (b) and side (c,d) views of the supramolecular structures obtained from the analysis and simulation of the WAXS fiber patterns. The stacking features are indicated for the corresponding dimer for 3-2 (m = 1) and tetramer for PBIs with m = 0, 2, 3, and 4. Color code used in (a)−(d): O atoms, red; H atoms, white; N atoms, blue; C atoms of the PBI, green; C atoms of the dendron phenyl, orange and light blue; all other C atoms, gray.235

narrow monotropic mesophase (for 3-7a) or no mesophase at all (for 3-7c) (Figure 19).231 Recently, the charge carrier mobilities of a series of PBIs 312a−h containing ester groups at the imide nitrogens have been investigated by Grozema and co-workers. The PR-TRMC studies of powders of these PBIs revealed mobilities values in the range of 0.03 cm2 V−1 s−1 (for 3-12a) to 0.22 cm2 V−1 s−1 (for 3-12h).234 It was observed that the ester group position on chains of the imide substituents affected the tight packing of polymethylene chains, leading to the differences in melting point and solubility of these PBIs (3-12a−h). Percec and co-workers have explored in great detail the supramolecular organization of dendronized PBIs in the solid state by employing a very broad series of compounds with the

general structure (3,4,5)nG1-m-PBI (shown in Chart 9), in which the length of the spacer (m) between the imide position and first generation dendron (3,4,5-trialkoxyphenyl) as well as chain length (Cn) of the peripheral alkyl group were systematically varied. The results on the thermotropic properties of these dendronized PBIs were disclosed in a series of three papers.235−237 In the first paper of this series,235 the authors studied the effect of the spacer group on the self-assembly properties using PBIs 3-1b, 3-2, 3-14, 3-15i, and 3-16 in which the length of spacer group was varied from 0 to 4 methylenic units, respectively, while the dendron moiety 3,4,5-tridodecyloxylphenyl remained unchanged. Investigation by a combination of different techniques such as DSC, X-ray diffraction, solid-state NMR, and molecular modeling revealed that these dendronized 985


Chemical Reviews

Review

Chart 10. Structures of Amphiphilic Core-Unsubstituted PBIs

temperature were observed,235 in a subsequent paper, this research group has studied the temperature-dependent selfassembly processes by using a library of dendronized PBIs that contain a spacer with three methylenic units and varied chain length of alkoxy substituents (n = 4−14, PBI 3-15a−k).236 It was found that for PBI 3-15i−k with larger alkyl chains (n = 12−14) the 3D orthorhombic phases from low temperature remain kinetically controlled, while for PBI 3-15e−h with shorter chain lengths (n = 8−11) the 3D phases start to become thermodynamically controlled. However, for PBIs with n = 8 and 9, the 3D orthorhombic columnar arrays transform from thermodynamic products to 3D monoclinic arrays through a kinetically controlled process.236 Similar kinetic phenomena were observed for dendronized PBI systems with one methylenic unit spacer (m = 1) and varied chain length of alkyl groups (n = 6−12; PBI 3-13a−f, 3-2) except that the supramolecular columns are constructed from dimer

PBIs self-organized in the solid state into helical columns with complex intracolumnar structures. At high temperature, these PBIs self-organized into a 2D hexagonal columnar phase. However, at low temperature, compounds with m = 0, 2, 3, and 4 methylenic units (3-1b, 3-14, 3-15i, and 3-16) selfassembled into 3D orthorhombic, while compound with one methylenic unit (3-2) formed 3D monoclinic columnar phase. The former compounds formed tetramers as a basic repeat unit for column stacking to maximize the π−π-interactions, while the latter formed a dimer repeat unit due to the short methylenic spacer (m = 1), which prevents the accommodation of the tetramer. In Figure 20 is illustrated the effect of spacer groups on the supramolecular structures of dendronized PBIs. Because for the PBI 3-15i (with m = 3, n = 12) a thermodynamically controlled process into a 2D hexagonal columnar phase at high temperature and a 3D orthorhombic columnar phase by kinetically controlled process at low 986


Chemical Reviews

Review

obtained from 3-10c are the result of smaller steric hindrance of disiloxane moiety than that of trisiloxane in 3-10d. As a consequence, TOF electron mobility of 3-10c (0.1 cm2 V−1 s−1) is 2 orders of magnitude higher than that for 3-10d (0.001 cm2 V−1 s−1).239,240 The PBI in this series containing bulky cyclotetrasiloxane moieties (3-10e) also exhibited a rectangular disordered columnar phase with an electron mobility of 0.01 cm2 V−1 s−1 laying between those of 3-10d and 3-10c.241 The electron transport properties of liquid-crystalline oligosiloxane PBIs were found to follow a hopping transport mechanism.242

repeat units instead of tetramer repeat units as in the case of three methylenic unit spacer PBIs. The self-organization of helical columns of dendronized PBI was transformed from a kinetically controlled process for 3-13f (n = 11) and 3-2 (n = 12), to a thermodynamic one for PBI 3-13c−e (n = 8−10). For PBI 313a,b (n = 6 and 7), no 2D liquid-crystalline phase was observed.237 Asha and co-workers have reported for the PBI system 3-3a (see Chart 8) containing six methylene units spacer and terminal benzoyl group (without any side chain) only transitions between different crystalline phases without the presence of any mesophases.213 However, incorporation of dodecyloxy substituent in the phenyl ring as in PBI 3-3b resulted in the formation of smectic LC phase as detected and analyzed by DSC and X-ray diffraction. Interestingly, by increasing the number of dodecyloxy substituents from one to three in PBI 3-3c, a columnar liquid-crystalline phase was obtained.213 This research group has also studied the thermotropic properties of PBIs 3-4a and 3-5 (Chart 8) bearing pentadecyl phenol (PDP) and cardanol spacer, respectively, and 3,4,5-tridodecyloxybenzoyl terminal groups. Columnar hexagonal mesophases were observed for both PBIs that were retained at room temperature (25 °C). X-ray diffraction analysis revealed strong π−π-stacking interaction in liquid-crystalline phase of PDP containing PBI 34a, while this interaction was absent in the LC phase of cardanol spacer compound 3-5. This was explained as a consequence of cis double bonds in the spacer side chain of PBI 3-5, which is apparently not favorable for π−π-stacking.214 They have also investigated the effect of chain length of terminal alkoxyl substituents by employing the series of PBIs 3-4a−i containing a PDP spacer and varied alkyl chain length from C4 to C12 (Chart 9).238 The thermotropic properties such as phase transition enthalpies of this series exhibited an odd−even effect due to the inefficient packing of an odd number of carbon atoms in the alkyloxy chains in the solid state. No liquid-crystalline phases were observed for compounds with short alkyloxy chain 3-f−i (n = 7−4). With longer chains as in 3-4a−e (n = 12−8), thermotropic LC phases were obtained with different periodicity such as rectangular columnar phase for 3-4b−e (n = 11−8) and hexagonal columnar phase for 3-4a (n = 12). The space charge limited current (SCLC) measurements were performed to demonstrate charge mobility dependency on the different liquidcrystalline mesophases. The hexagonal columnar phase of PBI 34a showed a mobility of 10−3 cm2 V−1 s−1, which is 1 order of magnitude higher than that for the crystalline phase of PBI 3-4f− i (10−4 cm2 V−1 s−1). The least mobility in this series was observed for the rectangular columnar phase of PBI 3-4b−e with the charge mobility of 10−5 cm2 V−1 s−1 (2 orders of magnitude less than the hexagonal columnar phase). These findings emphasize the importance of packing order for the charge transport properties in bulk.238 Modification of PBI side chains using oligosiloxane can induce phase segregation between the flexible oligosiloxane units and rigid PBI cores. Funahashi’s laboratory reported liquid-crystalline properties and their electron mobilities of PBIs 3-10c and 3-10d containing oligosiloxane moieties. The oligosiloxane groups not only improve the solubility in nonpolar solvent, but also decrease the crystallinity of the PBI leading to low isotropic temperature and a wide range of the liquid crystal phases. 3-10d with trisiloxane units exhibited a hexagonal disordered columnar phase, whereas 3-10c with disiloxane units showed a hexagonal ordered columnar phase at high temperature and a rectangular ordered columnar phase at low temperature. The ordered phases

3.3. Self-Assembly of Amphiphilic PBIs in Aqueous Media and Solid Bulk State

By attaching water-soluble first and second generation Newkome-typed dendron to PBIs, highly water-soluble amphiphilic PBIs 3-17a,b (Chart 10) could be achieved by Hirsch and coworkers.243 Self-assembly studies in aqueous media revealed that symmetric PBI 3-17a with first generation dendron exhibited higher aggregation tendency than 3-17b with second generation dendron because of less steric bulkiness and less hydrophilicity of the former, which was also reflected by the low fluorescent intensity even at low concentrations. The analogue unsymmetric PBI amphiphile 3-18a bearing a first generation dendron is not water-soluble, while PBI 3-18b containing a second generation dendron showed water solubility combined with pronounced aggregation properties. TEM images revealed that symmetrical PBIs 3-17a,b formed small irregular shaped aggregates, while unsymmetric PBI 3-18b self-assembled into globular micelles. The same group also reported the aggregation studies of PBI 319 containing ethylenediaminetetraacetic acid (EDTA) in aqueous and organic conditions based on absorption/fluorescence spectroscopy and zeta potential measurement.244 The complexation of metal cations by EDTA moieties revealed that bi- and trivalent metal cations triggered the fluorescence quenching with the formation of excimer bands for the latter case. Haag and Würthner and their co-workers reported that dendronization of PBIs 3-20a−d with first, second, third, and fourth generation polyglycerol dendrons improved the water solubility of PBIs and demonstrated that their aggregation was governed by a strong dendritic effect.18 Third and fourth generation dendrons in 3-20c and 3-20d were indeed able to suppress the π−π-stacking of PBI, while lower generation dendronized PBIs 3-20a and 3-20b still exhibited aggregation behavior in water. Accordingly, the fluorescence quantum yields increased with increasing generation of polyglycerol dendrons. These results imply that the excited-state proton transfer mechanism is not the reason for fluorescence quenching of PBIs in water as generally assumed, because almost 100% quantum yield could be obtained for 3-20d when its aggregation was fully inhibited by a large dendritic shell. In contrast to the introduction of large dendrons at imide positions, Nau, Scherman, and co-workers have demonstrated a supramolecular approach to prevent PBI aggregation in aqueous media. Curcurbit[8]uril (CB[8]) has been shown to form a strong host−guest inclusion complex with dicationic PBI leading to the deaggregation of the PBI units and enhanced fluorescence intensity with a quantum yield almost equal to unity (0.90 ± 0.10).245 The reversible stimuli-responsiveness of the complexes to competitive guests and electrochemical reduction might allow applications in fluorescence sensors. The same group also reported the self-assembly of tetracationic cyclobis(4,4′-(1,4phenylene)bispyridine-p-phenylene)tetrakis(chloride) (ExBox) 987


Chemical Reviews

Review

Chart 11. Structures of Unsymmetrical Core-Unsubstituted Amphiphilic PBIs

and dicationic PBIs supramolecular complexes.246 These complexes showed efficient energy transfer from ExBox (donor) to PBIs (acceptor) and underwent further complexation with two neutral cucurbit[7]uril (CB[7]) at the outer quaternary ammonium moieties to form 3-polypseudorotaxane. Combining the water solubility and host−guest properties of cyclodextrin, functionalization of the PBI at the imide positions with β-cyclodextrin as in symmetric PBI 3-21a provided PBI aggregates that are useful as fluorescence sensors.247 According to UV/vis and fluorescence studies, cofacial H-type aggregates were formed from 3-21a in aqueous media. Solid-state investigations of 3-21a showed strong fluorescence, which can be used for probing organic vapors. Thus, the membrane of 321a aggregates embedded in poly(vinylidene fluoride) (PVDF) was able to detect organic vapors such as amines and organic solvents. This observation was explained by the uptake of organic vapors by cyclodextrin receptors, which can not only interrupt the π−π-stacking of PBI stacks but also promote photoinduced electron transfer from electron-rich amines to electron-poor PBIs leading to fluorescence quenching. Moreover, it was reported that PBI analogue 3-21b bearing β-cyclodextrin with amino (NH) group spacer exhibited pH-dependent aggregation behavior due to the ionization of the amino moieties. According to concentration- and temperature-dependent UV/vis studies, the aggregation constant of protonated form of PBI 3-21b was almost 1 order of magnitude lower than that of neutral form as a result of electrostatic repulsion. The enhanced fluorescence

intensity of protonated form of PBI 3-21b was observed due to the inhibition of intramolecular electron transfer process and weakening of π−π-stacking interaction caused by the electrostatic repulsion. In a further report, it was shown that the morphology of the assemblies of unsymmetric PBI 3-22 bearing one cyclodextrin receptor can be controlled by the change of solvent polarity.248 The stronger π−π-interaction of 3-22 aggregates as compared to that of 3-21 reduced the nonselective fluorescence quenching resulting from disrupted π−π-stacking after binding of organic vapor. As a result, 3-22 aggregates exhibited higher selectivity and sensitivity toward organic amine vapor than aggregates of 3-21a. Core-unsubstituted amphiphilic PBIs containing hydrophilic and hydrophobic imide substituents of different size and shape were synthesized, and their self-assembly in water was investigated by the Würthner group.249 The molecular shape of these amphiphilic PBIs (wedge or dumbbell-shaped) allows the formation of different morphologies in water, which could be rationalized by Israelachvili’s critical packing parameters. The wedge-shaped amphiphilic PBIs 3-23a,b (Chart 11), which assemble into aggregates with a high degree of curvature, formed spherical micelles, while dumbbell-shaped amphiphilic PBI 324b formed rod aggregates with columnar structures similar to those formed by PBIs 3-1a,b in organic solvents. Mixing of dumbbell-shaped unsymmetric amphiphilic PBI 324a with wedge-shaped PBI 3-23b reduced the curvature of interface, leading to the co-self-assembly of these differently 988


Chemical Reviews

Review

nanocapsules with interesting photofunctional properties.250 Encapsulation of bispyrene-based energy donors with pHdependent folding properties into the PBI acceptor nanocapsules generated unique optical properties to these functional vesicles. The bispyrene donor was carefully selected to show different conformations depending on the pH value. The unstacked conformer is favored in acidic conditions due to the electrostatic repulsion of protonated amine linker, while at high pH (basic conditions), the stacked conformation prevails due to π−πinteractions between the pyrenes. Because the former conformer shows the characteristic blue emission of pyrene monomer while the latter exhibits green excimer emission, the overlap integral for Förster resonance energy transfer (FRET) to the green light absorbing PBI aggregate vesicle membrane gradually increases with increasing pH as proved by time-resolved fluorescence spectroscopy. Accordingly, FRET from bispyrene donor inside the vesicles to the bilayer vesicle membrane can produce fluorescence color changes for pH gradient with high sensitivity, visible to the naked eye (Figure 22). Moreover, white fluorescence could also be obtained at pH = 9, which offered a new system for producing white-light emissive nanoparticles. A related more simplistic system composed of biscarbazoles embedded in PBI 3-23a micellar nanoparticles with white light emission could be created as well.251 Micellar aggregates of amphiphilic PBI dyes in water (buffer) have further been investigated for PBI bearing Jeffamine T-403 as reported by Montalti’s group.252 These low cytotoxicity nanoparticles were reported to be used as fluorogenic and phototunable agents for multicolor imaging of biological cells by exploiting a disaggregation-induced emission mechanism (DIE).253

Figure 21. Formation of micelles assembled from wedge-shaped PBI 323a (top), bilayer vesicles assembled from the co-self-assembly of PBI 323a and dumbbell-shaped unsymmetric PBI 3-24a (middle), and rod aggregates assembled from dumbbell-shaped symmetric PBI 3-24b (bottom).249

shaped PBIs and formation of spherical vesicles. The size of these vesicles can be controlled by changing the ratio of these two amphiphilic PBIs. Thus, average diameters of 94 nm (for [323a]/[3-24a] = 8/1) and 133 nm (for [3-23a]/[3-24a] = 4/1) were observed. The formation of nanostructures with different morphologies is illustrated in Figure 21. The resulting photo-cross-linking-stabilized vesicles still retained their spherical shape and could be utilized as

Figure 22. (a) The donor-loaded PBI vesicles with pH-tunable energy transfer. The inner and outer layers of the vesicle consist of PBI acceptor molecules. Their hydrophilic chains (blue) are exposed to water, with the hydrophobic part (yellow) packed together and stabilized by polymerized double bonds (green). (b) Resulting fluorescence color changes upon increasing pH. Adapted with permission from ref 250. Copyright 2009 Nature Publishing Group. 989


Chemical Reviews

Review

Figure 23. TEM images of 3-24b aggregates prepared from water, (a) [3-24b] = 0.25 mg/mL, (b) 0.76 mg/mL, and (c) 1.0 mg/mL. (d) TEM images of the fusion and fission of two, three, or more nanorods; arrows indicate the segmented nanostructures, [3-24b] = 0.25 mg/mL; scale bar, 10 nm. (e) Schematic illustration for the hierarchical self-assembly from nanorods to nanoribbons by fusion and fission. Reproduced with permission from ref 254. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 24. (a) Molecular structure of 3-25. (b) CPK-models of open conformation and closed conformation of 3-25. The tetrahedral conformation (middle) shows the allocation of hydrophobic (red) and hydrophilic parts (blue). (c) Cryo-TEM image of 3-25 showing a dense population of fibrous aggregates and small platelets (white arrowheads) prepared in the water/THF mixture (70:30, v/v); scale bar, 50 nm. (d) Perspective (left) and top view (right) of a proposed cylindrical micelle arrangement of the closed conformers of the fibrous aggregates that are observed in the binary water/THF mixtures. Dimensions of the model correspond to the observed fibrous aggregates. The open space in the core volume of the cylindrical micelle is thought to be filled with THF.256

promote a hierarchical growth by fusion−fission process to merge PBI monomers side-to-side. By this means, the hydrophobic PBI cores are enclosed by hydrophilic ethylene oxide chains resulting in the nanoribbon structure (Figure 23) that resembles the packing of some related PBIs in liquid-crystalline phases (Figure 20).235 Interestingly, the fluorescence intensity

More recently, the mechanism for the growth of PBI nanorods resulting from the self-assembly of dumbbell-shaped amphiphilic PBI (3-24b) in water was elucidated by direct visualization with transmission electron microscopy (TEM), cryogenic scanning electron microscopy (cryo-SEM), and AFM.254 It was observed that the strong cohesive forces between PBIs in water can 990


Chemical Reviews

Review

Chart 12. Structures of Core-Substituted PBIs with Tridodecylphenyl Imide Substituents

temperature (Tiso), which offers the possibility for tailoring the mesophase range. The same group has also investigated the thermotropic properties of unsymmetrical PBIs (3-27a−c) containing one swallow-tail and one linear chain imide substituent.257 An important effect of oligoethylene oxide chains has been reported: While PBI 3-27a bearing alkyl swallow-tail and linear alkyl chain at imide positions is crystalline, replacing the alkyl swallow-tail by oligoethylene oxide swallow-tail as in 3-27b and 3-27c resulted in the formation of mesophases with hexagonal columnar and lamellar-columnar packing, respectively. Upon annealing, the electron mobilities for 3-27b (6 × 10−3 cm2 V−1 s−1) and 3-27c (7 × 10−3 cm2 V−1 s−1) were improved by 2 orders of magnitude as compared to that of their pristine state (∼10−5 cm2 V−1 s−1), and the mobility values of 3-27b and 3-27c are higher than that of 3-27 due to trap generating in domain boundaries of crystalline 3-27a. In a related study, Funahashi and co-workers reported that PBI 3-27d with linear oligoethylene oxide and swallow-tail pentamethyldisiloxane imide substituents formed an ordered lamellar mesophase, instead of columnar phase as observed for 327b due to the nanosegregation of ethylene oxide and disiloxane moieties.258 A similarly high electron mobility as observed for the related molecules by Thelakkat exceeding 1 × 10−3 cm2 V−1 s−1 was measured for 3-27d by time-of-flight (TOF) technique. Moreover, the ability of the oligoethylene oxide chain of 3-27d to complex lithium triflate, while still retaining the packing order, efficient electron transport, and ionic activity at room temperature, provides the opportunity for new stimuli-responsive materials. Application of perfluoroalkyl groups was shown as an alternative strategy to promote nanosegregation. By attaching the semifluorinated dendron to one of the imide positions in 328, an unprecedented π−π-stacking of PBI units parallel to the long axis of the supramolecular column could be achieved.259

was enhanced in nanoribbon (ΦF = 6.8%) as compared to that of nanorod (ΦF = 1.7%). This aggregation enhanced emission can be attributed to the disfavored excimer formation pathway (involving structural rearrangement) of these tightly packed PBIs in the nanoribbon morphology. Co-self-assembly of planar PBI 3-24b and the corresponding core twisted PBI derivative containg four tert-butylphenoxy substituents at 1,6,7,12 bay positions in water has recently been reported to form supramolecular block copolymer nanowires with alternating sequence of their self-assembled blocks in a kinetically controlled self-assembly pathway.255 Very recently, Hirsch and co-workers have investigated the aggregation behavior of a unique amphiphilic, covalent PBI dimer bridged with a calix[4]arene moiety containing two Newkome-type dendrons (PBI 3-25 in Figure 24) in water and THF/water mixtures by cryo-TEM, UV/vis, and fluorescence spectroscopy.256 Different supramolecular assemblies of H-type PBI aggregates were observed including ribbon and tube-like structures in water and long fibrous aggregates in THF/water mixture followed by the formation of extended lamellae and nanocrystalline platelets and eventually microcrystalline precipitates. On the basis of molecular dynamics simulation, the open and closed conformations of the molecular structures are responsible for the differences in optical behavior and supramolecular assemblies in pure water and THF/water mixture systems (Figure 24). Thermotropic behaviors of unsymmetrically N-substituted PBI derivatives have been systematically studied by Thelakkat and co-workers.230 Introducing the swallow-tail chains with different polarity and flexibility has been shown to change the thermotropic properties of PBIs. As confirmed by OPM, DSC, and XRD, compounds 3-26a−d (Chart 11) containing both alkyl and oligoethylene oxide swallow-tail substituents exhibited thermotropic hexagonal columnar mesophase, while symmetrical alkyl swallow-tail PBI 3-7c showed only the crystalline phase. This difference was explained in terms of higher conformational flexibility imparted by the C−O bonds of oligoethylene oxide swallow-tails that suppressed the crystallization of the PBIs (3-26a−d) in contrast to linear alkyl chain substituted PBI 3-7c. Similarly, an increase of the length of oligoethylene oxide unit resulted in a lower isotropization

3.4. Self-Assembly of Core-Substituted PBIs

In contrast to substitution at the imide position, substitution at the bay area of PBI core imposes steric constraint that leads to distortion of the PBI core. As a result, the aggregation constants of core-twisted PBIs are 2−3 orders of magnitude lower than 991


Chemical Reviews

Review

those of core-unsubstituted ones.260 Moreover, the arrangement of substituents at bay positions directing toward the same side of the PBI plane creates the sterically more hindered π-surface on one face that inhibits further aggregation. The other side of the πsurface, which is more accessible, can then stack with another dye to form a stable dimer. The core twisting angle also has a strong effect on the supramolecular arrangements of these selfassemblies and their electronic as well as liquid-crystalline properties. Thus, core-disubstituted PBIs 3-29c−e (Chart 12) possessing relatively small twist angles (15°−25°) show stronger excitonic coupling and hypsochromic shift of absorption maxima as compared to those of core-tetrasubstituted PBIs 3-29a and 329b with larger twist angles (27°−36°). These results were explained by the distance between the dyes in the dimer, that is, larger twist angles that prevent close contact of PBI cores.260 The substituent-dependent core-distortion also has a pronounced impact on the mutual organization of these dyes in the solid state. Thus, the tetrachloro-substituted strongly twisted PBI 3-29a exhibits a rectangular columnar mesophase, while for disubstituted PBIs 3-29c,d with smaller twist angles hexagonal mesophase was observed (Figure 25).260 These

LC phase like its core-unsubstituted analogue 3-1b. However, the charge carrier lifetime of the core-twisted PBI is markedly greater than that for the core-unsubstituted one.261 A very recent comparative study by Percec and co-workers with a series of dendronized tetrachlorinated PBIs 3-30a−c and their nonchlorinated analogues (3-1b, 3-2, 3-14) has shown that molecularly less ordered, core-twisted PBIs self-organize into ordered 3D crystalline phases via a thermodynamically controlled process, while nonchlorinated PBI derivatives selfassemble into 2D periodic arrays and 3D crystalline phases via kinetic control.262 The atropo-enantiomeric nature of core-twisted PBIs66,263 allows for a variety of possible π−π-contacts in aggregates and in the solid state; that is, a homochiral contact will have a different geometry and energy than a heterochiral contact. To addresss this issue, Würthner and co-workers introduced diethylene glycole bridges to connect both substituents at 1,7 bay positions as in strapped PBI 3-31. By this means, the conformation of twisted PBI was prevented from interconversion between P- and M-atropo-enantiomers, and both enantiomers could be resolved by chiral HPLC. These chiral PBIs were subjected to selfassembly studies for pure enantiomers and various mixtures of enantiomers, and it was found that the self-recognition process leading to homodimers, that is, self-sorting, is more preferable than the self-discrimination resulting in heterodimers.264 These results could be rationalized by the better overlap of the π-surface in homodimer than in heterodimer. Moreover, by using chiral PBIs with varied lengths of the bridging unit, it was demonstrated that the fidelity of self-sorting is dependent on the rigidity of the core-twisted conformation.265 The core chirality also effects the aggregation behavior of these PBIs in the condensed state. The racemic mixture of PBI 3-31 exhibited soft columnar crystalline phase, while enantiopure PBIs (P)-3-31 and (M)-3-31 selfassembled into a lamellar liquid-crystalline mesophase. On the basis of the experimental data and force field calculations of the condensed state packing, it was proposed that nanosegregation and interaction between the bridging units are responsible for the observed differences in the condensed state properties of racemic and homochiral PBIs.266 The effect of bay substitution on the morphology of nanoaggregates was also investigated with core-disubstituted PBIs 3-32a−c containing dodecyloxy, thiododecyloxy, or both groups, respectively (Chart 13).267 PBI 3-32a containing two dodecyloxy groups self-assembled into long, flexible nanowires, while thiododecyloxy derivative 3-32b formed spherical nanoparticles and the mixed system 3-32c formed nanorods. This diversity in morphology of self-assembly was rationalized in terms of different degrees of core twisting depending on the substituents and different tilting of dodecyloxy and thiododecyloxy groups with regard to the core.267 Once the PBI core was disubstituted with more bulky, propeller-shaped tetraphenylethane (TPE) at bay positions as in regioisomeric PBI 3-33a and 3-33b, their self-assemblies exhibited slipped aggregates due to the large degree of twisting. The concomitant J-type coupling was evidenced by bathochromic shift of absorption bands,268 and similar to previously discussed PBI 3-29a these aggregates of 333a showed a highly red-shifted fluorescence band with enhanced quantum yield of ΦF = 29.7% as compared to its monomeric state (ΦF < 0.1%). Similar fluorescence properties were observed for the regioisomer 3-33b.268 This observation has been explained by the restricted rotation of the TPE groups in aggregates, in contrast to freely rotated TPE in monomer

Figure 25. Schematic representation of the proposed packing patterns and the orientation of the transition dipole moments in columnar mesophases of the core-twisted PBI dyes: (a) longitudinally displaced Jtype π−π-stacking of 3-29a, and (b) rotationally displaced cofacial π−πstacking of 3-29c,d. Reproduced with permission from ref 260. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

differences in packing also influence the functional properties; that is, longitudinally slipped stack of tetrachloro PBI 3-29a afforded highly fluorescent J-aggregates.260 These results suggest that a twisted PBI core may direct the preferential π−π-stacking mode from usual rotational displacement (leading to highly ordered columnar mesophase) toward longitudinal displacement (leading to rectangular columnar mesophase). Charge carrier mobility studies of structurally closely related PBI 3-30a revealed that this PBI exhibited similarly good charge carrier mobilities in 992


Chemical Reviews

Review

Chart 13. Structures of 1,7 and 1,6 Core-Disubstituted PBIs Bearing Alkyl Imide Substituents

Figure 26. Cryo-TEM images of a solution of PBI 3-34c complex (M = Co2+) in water/THF (19:1 v/v) after (a) 5 h and (b) 2 days. Scale bars: 50 nm for main graphic and 10 nm for the inset in (b).272 Reproduced with permission from ref 272. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

structures in both solution and dry state as revealed by cryo-SEM, cryo-TEM, and AFM experiments.270 Similarly, the strong π−πinteraction of PBI building block in PBI-peptide conjugate 334b, where the peptide moiety was connected through a terpyridine-Pt complex, was reported to undergo stepwise transformation of various organic nanostructures in aqueous media under kinetically controlled self-assembly.271 The peptide ligands were attached to PBI core for the purpose of providing additional interaction modes such as hydrogen bonding, hydrophobic, and hydrophilic interactions for the self-assembly process. Inversion of 0−1 and 0−0 vibronic bands with

solution, which is known as a quenching mechanism of excited states.269 Various PBIs functionalized with polyethylene glycole at bay positions have been investigated for their self-assembly in aqueous media by taking advantage of well-defined and strong noncovalent π−π-interaction among PBI building blocks. Rybtchinski and co-workers reported a series of studies based on such amphiphilic core-substituted PBI structures. Coresubstituted amphiphilic PBI 3-34a (Chart 13) was employed to control the self-assembly of polyphenyl acetylene (PPA) in aqueous media to form stable two-dimensional porous network 993


Chemical Reviews

Review

Chart 14. Structures of Multichromophoric PBI Arrays

observed by cryo-TEM (Figure 26). The wall width of only about 2 nm suggests that the molecules form monolayer membranes whose curvature into spirals and toroids (with diameters of around 25 nm) is driven by water molecules that act as bridging ligands between two adjacent Zn2+ centers.272,273 Recently, it was discovered by the same group that the incorporation of perfluorooctyl chain into PBI amphiphiles as in PBI 3-34d has a tremendous influence on the self-assembly behavior of such amphiphiles in water as compared to nonfluorinated analogue.274 At room temperature, it was found that the association constant of PBI 3-34d was 2 orders of magnitude larger than that of nonfluorinated analogue as a result of a larger hydrophobic surface area of the fluorocarbon group. However, increasing the THF:water ratio can lower the aggregation strength and even the aggregation mechanism and morphologies of PBI 3-34d. According to cryo-TEM, CD, and molecular calculations, tube-like fibers resulting from isodesmic process were observed for 3-34d aggregates in the solvent mixtures containing low THF content, while the formation of

concomitant band broadening upon aggregation suggests that PBI 3-34b self-assembled by face-to-face π-stacking in THF/ water mixture. CD and cryo-TEM measurements were performed to monitor the self-assembly pathways of this PBI− peptide conjugate. Upon increasing the THF content in the solution or allowing longer monitoring time, PBI 3-34b gradually changed its morphologies to more ordered straight nanofibers as a result of the structural equilibration to more organized thermodynamically stable morphologies. More diverse structures with similar assembly behavior were also observed for the PBI analogue containing phenylalanine instead of alanine due to its stronger hydrophobicity and higher conformational rigidity. The Rybtchinski group has further utilized terpyridine-metal complex containing PBI scaffolds 3-34c to study the impact of various metal ions (Zn2+, Co2+, and Ni2+) on the morphology of the dye assembly in water/THF (19:1 v/v). Depending on the preferential coordination geometry of the respective metal ion, different morphologies are formed. In particular, for Zn2+ and Co2+ ions, intriguing spiral and toroidal morphologies could be 994


Chemical Reviews

Review

Figure 27. Schematic illustration of membrane fabrication and recycling process. Molecular structure of 3-36, schematic depiction of supramolecular fibers, 3D network, and membrane. Recycling of the membrane is achieved by disaggregation or physical removal of the supramolecular layer from the support, followed by purification, and subsequent reassembly in aqueous solution. Adapted with permission from ref 282. Copyright 2013 PLOS.

way switching, which are of interest for controlled supramolecular polymerization.279 Amphiphilic bis-PBI 3-36 containing bipyridine spacer exhibits supramolecular fibers that can form three-dimensional networks of supramolecular gels in water/THF mixtures.280 The supramolecular gel was reported to be responsive toward chemical redox reagents and temperature, suitable for reversible conversion between gel-state and fluid solution. The robust fiber assemblies of PBI 3-36 could also be utilized to prepare supramolecular membranes for size-selective separation of nanoparticles281 and immobilization/biocatalytic utilization of proteins.282 This supramolecular membrane has advantages over the conventional ones as it can be recycled multiple times by disassembly/assembly process (Figure 27). Recently, tris-PBI 337 was designed to produce crystalline 2D nanostructures in aqueous media.283 This highly ordered 2D array exhibited excellent solar light absorption and fast exciton transfer, thus being promising for light harvesting applications. Hexasubstituted benzene scaffold bearing six amphiphilic PBIs has been also demonstrated to form supramolecular polymer in aqueous medium as a result of pairwise directional hydrophobic/ π-stacking interactions.284 The supramolecular polymerization showed enthalpically and entropically driven self-assembly according to isodesmic mechanism. These assemblies revealed exciton confinement and localized emission, which are uncommon for extended π-stacked systems.284 In another study, Yagai and co-workers reported on the selfassembly of bis-PBIs 3-38a−f (n = 5−10), which were connected through alkane linkers to control self-organized structures in both solution and solid state.285 An odd/even effect of the linker units was observed due to the different molecular packing efficiencies, which influence the thermal as well as electronic properties and charge-carrier transportation characteristics of these bis-PBIs. The difference in self-organized morphologies of PBI 3-38c (n = 7) and PBI 3-38d (n = 8) could be visualized by

helical columnar stacks from cooperative self-assembly mechanism was identified in the solution with a high THF ratio. Such cooperativity effect could not be found in nonfluorinated PBI derivatives. The change in aggregation mechanism between fluorinated and nonfluorinated analogues was rationalized in terms of the conformational rigidity of the fluorocarbon substituent. 3.5. Self-Assembly of Dye Arrays Composed of Multiple PBIs

PBI building blocks can be connected with different spacers at bay positions, leading to interesting modulations of absorption and self-assembly properties275 as well as an exciting prospect for organic photovoltaics.276 This approach has been utilized by Rybtchinski and co-workers to strengthen the hydrophobic interaction for π−π-stacking in aqueous media.277 Amphiphilic bis-PBI 3-35 (Chart 14) bearing ethynyl spacer was reported to self-assemble into one-dimensional fibers in water/THF mixtures. The self-assembly and disassembly process of the fibers could be triggered reversibly by the redox properties of the PBI unit. Under reducing conditions, fiber assemblies of neutral 3-35 transformed into spherical micelles of the respective anionic radical species due to their enhanced water solubility and electrostatic repulsion. Upon exposure to air, anionic PBI units are reoxidized to their neutral states and the fiber assembly is restored. A longer PEG chain (PEG 44) analogue of 3-35 has been recently reported to show improved solubility in neat water.278 As evidenced by UV/vis spectroscopy and cryo-TEM, two supramolecular polymer isomers of H-aggregates and Jaggregates were observed leading to the differences in electronic and photophysical properties. These isomeric aggregates are very stable in neat water but can be interconverted in the presence of organic solvents. According to UV/vis spectroscopy, the kinetic mechanistic study of the H- and J-aggregates transformation revealed the nucleation/growth mechanism with sharp nucleation-to-growth transition and temperature-dependent off-path995


Chemical Reviews

Review

field-emission scanning electron microscopy (FE-SEM) (Figure 28).

The self-assembly of the more electron-rich green dipyrrolidinyl-PBI core connected with four red diphenoxy PBI units (339) was investigated by Wasielewski and co-workers already a decade ago.286 It was revealed that this compound formed πstacked dimers with H-type coupling of the PBIs in toluene according to UV/vis spectroscopy and X-ray scattering experiments. This dimer exhibited energy transfer from outer PBI units to the central PBI core followed by ultrafast charge separation at the central PBI units. Therefore, the PBI outer units harvest and funnel the light energy to the central PBI dimer unit. 3.6. Self-Assembly of Multichromophoric PBI Conjugates Containing Other Dyes

Self-assembly provides an efficient and cost-effective approach toward large and highly ordered architectures in comparison to conventional covalent synthesis. The attractiveness of this approach has been recognized in particular for the generation of highly ordered segregated arrays of donor and acceptor units for photovoltaic devices and light-harvesting systems.276 Thus, PBIs 3-40a (Chart 15) were prepared by Janssen, Meijer, and coworkers to incorporate oligo(p-phenylenevinylene) (OPV4) donors and PBI acceptor into the same molecule for the purpose to create donor−acceptor supramolecular networks for potential optoelectronic applications.287 According to UV/vis studies of thin films of 3-40a, the blue shift of OPV4 moieties and the red shift of PBI units suggested π-stacking of alike subunits. Liquidcrystalline mesophases were also observed for these compounds.287 The impact of intermolecular orientation on photoinduced charge transfer kinetics of PBI-OPV assemblies was further studied by Janssen and co-workers.288 PBI 3-40b and 3-40c formed J-type aggregates in apolar solvent as revealed by absorption spectroscopy and molecular mechanic calculations. The aggregates of 3-40b and 3-40c exhibited faster charge separation rates than their monomeric states. This can be explained by the short intermolecular distance between OPV and PBI that facilitates the intermolecular charge separation process

Figure 28. (a) Photograph of a gel of 3-38c (c = 5 × 10−3 M in 1,2dichlorobenzene) under UV irradiation (λ = 365 nm). (b) Fluorescence optical-microscopy image of the pristine gel. (c) FE-SEM images of the freeze-dried gel. Inset: A magnified image of individual fibers. (d) FESEM and (e) TEM images of flower-shaped micro-objects formed by drop-casting a solution of 3-38d. (f) A magnified image of flower-shaped microstructure. Adapted with permission from ref 285. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chart 15. Structures of Triads Consisting of Electron-Poor PBIs and Various Electron-Rich Donor Moieties

996


Chemical Reviews

Review

lived charge separation and charge migration through the selfassemble structure were observed implying the segregated ZnTBTPP and PBI charge conduits. Self-assembled multichromophoric phthalocyanine−PBI systems including π-stacked heptamer of zinc phthalocyanine tetrakis(perylene bisimide)294 and aggregates of phthalocyanine (Pc)-PBI8 octad295 have also been investigated in photophysical studies. It was shown that within the self-assembled structures ultrafast energy transfer occurs from peripheral PBIs to the central phthalocyanine cores294 rather than a charge separation and the excitons are more stabilized in the aggregated forms than the monomeric state.295 Recently, X-shaped PBIs 3-44a and 3-44b containing two PBI acceptor units and two ZnTPnP (zinc tripentylporphyrin) donors, respectively, were synthesized by the same group, and the impact of molecular arrangement on charge transport properties of their self-assemblies was studied.296 According to X-ray scattering measurements, PBI 3-44a self-assembled in toluene into π-stacked aggregates consisting of 5 ± 1 monomeric units at a concentration of 1.1 × 10−5 M, while PBI 3-44b remained as a monomeric species due to the steric hindrance among the phenyl groups. UV/vis spectroscopic studies suggested that the arrangement of PBIs within 3-44a aggregates might be of crisscrossed stacking instead of segregated stacking between ZnTPnP and PBI units due to the absence of enhanced (0−1) vibronic band relative to (0−0) band usually found for Htype PBI aggregates. This view was also supported by the observation of shorter charge separation and recombination lifetime by the intermolecular process in PBI 3-44a (compare Figure 29) in contrast to that in the monomeric PBI 3-44b, whose charge separation occurred intramolecularly through a multistep sequence within the same molecule. A similar observation of supramolecular ordering was reported by Müllen and co-workers for hexabenzocoronene (HBC)/PBI dyads 3-45a−c.297 The combination of UV/vis and fluorescence spectroscopy, STM, and X-ray scattering investigation revealed that these dyads self-assembled into two-dimensional ordered structures in both solution and bulk solid state. The 6-fold starshaped dyad 3-45a exhibited segregated columnar stacks of HBC and PBI subunits, while linear dyads 3-45b and 3-45c bearing only two or one PBI chromophore formed interdigitating stacks consisting of alternating HBC and PBI subunits. Because of the large intermolecular distance within the columnar stacks of 345a, photoinduced electron transfer between HBC and PBI moieties could not be detected. As for the interdigitating stacks of 3-45b and 3-45c, direct overlap of HBC and PBI allowed the formation of charge transfer state, but no charge transport properties could be observed due to the lack of charge transport channels. Multichromophoric system 3-46 containing a Ru(II) complex fragment and axial PBI ligands has been reported to self-assemble into supramolecular nanofibers in acetonitrile/water system as evidenced from UV/vis absorption spectroscopy, AFM, and cryo-SEM.298 The Ru(II) complex embedded in the nanofibers was utilized as a water oxidation catalyst, and an improved catalytic stability was observed as compared to the monomeric reference complex (Figure 31). This can be explained by the stabilization of such aggregates toward dissociation of the axial ligands leading to the commonly observed decomposition of these catalysts after a few hundreds of turnovers. Monodisperse conjugated oligomers containing oligo(fluorene-alt-bithiophene)s and PBI were reported to exhibit smectic liquid-crystalline mesophases, and their vapor annealed

Figure 29. Schematic representation showing π-stacking of OPV-PBIOPV to give J-type (left) and H-type (right) aggregates.288

as compared to the intramolecular one. In contrast to Jaggregates of the former PBIs, H-aggregates resulting from 3-40a exhibited minor difference in rates of electron transfer between aggregates and molecularly dissolved state. This is a result of the longer intermolecular distance in the H-type aggregates (Figure 29) where close contact between OPV subunit (red) and PBIs (blue) is missing. The self-assembly of donor−acceptor−donor triads 3-41 (Chart 15) in MCH was investigated by Wasielewski and coworkers.289 This triad formed helical hexamers with the πstacking of the PBI units in MCH as determined by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements. Upon photoexcitation, electron hopping through the PBI πstack was detected proving that charge transport between PBI acceptors is fast enough to compete with the charge recombination process. A 3-fold symmetric PBI 3-42 (Chart 16) containing diethylaniline (DEA) donors at the imide position of PBI was synthesized by Wasielewski and co-workers.290 On the basis of UV/vis spectroscopy, vapor pressure osmometry (VPO), and dynamic light scattering (DLS) experiments, this tris(DEA-PBI) showed the formation of π-stacked dimer with H-type coupling in toluene, methyltetrahydrofuran, and CH2Cl2. According to femtosecond transient absorption spectroscopy, the photoinduced electron transfer of the dimer showed a significant difference in transient spectral features as compared to the monomeric state. However, not much change of the charge separation and charge recombination time constants could be observed for the dimer and monomer. Wasielewski and co-workers have shown by UV/vis spectroscopic studies that multichromophoric system 3-43a containing four PBI acceptors and a central zinc tetraphenylporphyrin donor (ZnTPP) self-assembled by face-to-face stacking in both solution and the solid state to form nanoparticles with Htype coupling among the PBI units.291 A model for the stacking arrangement was proposed with close PBI−PBI distance of ∼3.5 Å and large ZnTPP−ZnTPP distances of ∼7 Å. The interlocked stacked assemblies revealed quantitative charge separation and charge transport properties within each segregated stack of donors and acceptors. These properties are desirable for artificial photosynthetic systems. Similar to 3-43a assemblies, segregated stack aggregates were observed for 3-43b assemblies. However, according to SAXS and GPC studies, monodisperse aggregates of five stacked molecules (pentamer) could be generated from 343b in toluene instead of polydisperse aggregates in 3-43a292 (Figure 30). Moreover, different charge recombination mechanisms for 3-43a and 3-43b assemblies were observed as a result of different electronic properties of the monomeric building blocks. Photoinduced charge separation and transport were recently reported in dodecamers self-assembled from multichromophoric system bearing four PBI acceptors and a central zinc tetrabenzotetraphenylporphyrin (ZnTBTPP) donor.293 Long997


Chemical Reviews

Review

Chart 16. Structures of Multichromophoric PBI Conjugates Containing Other Dyes

thin films revealed the highly ordered lamellar nanostructures with alternating donor−acceptor blocks as evidenced from TEM and selected area electron diffraction (SEAD).299 Accordingly, efficient charge transport carriers and high-performance single molecular photovoltaic devices of power conversion efficiency

(PCE) up to 1.5% were observed for these materials. Recently, a systematic self-assembly study of conjugates between PBIs and conductive oligomers was performed by Méry and co-workers with a series of acceptor−donor dyads 3-47 (AD) and triads 3-48 (ADA) and 3-49 (DAD).300 It was found that both 3-47 (AD) 998


Chemical Reviews

Review

Figure 30. Top view (a) and side view (b) of the suggested structure of 4-43b, (ZnTPP-PDI4)5 in toluene according to SAXS/WAXS data.292

Figure 31. Schematic illustration of catalytic water oxidation by multichromophoric Ru(II) catalyst 3-46 embedded in PBI nanofibers. Ceric(IV) ammonium nitrate (CAN) was used as a sacrificing oxidant. Reproduced with permission from ref 298. Copyright 2015 The Royal Society of Chemistry.

those of the polymer blend device.303 The donor block of the copolymer can be replaced by more electron-rich bis(4methoxyphenyl)phenylamine (DMTPA) (3-50b) and N,N′bis(4-methoxyphenyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (DMTPD) (3-50c) to enhance driving force at the D−A interface and improve power conversion efficiency (η) to 4-fold (η = 0.26%) and 5-fold (η = 0.32%) higher, respectively, as compared to the TPA analogue (η = 0.065%).304 Recently, Thelakkat, Thurn-Albrecht, and co-workers have revealed the synthesis of donor−acceptor poly(3-hexylthiophene)-block-poly PBI acrylate (P3HT-b-PPerAcr) (3-51) and observed the microphase separation in melt and in subsequent confined crystallization without morphology change.305 These crystalline liquid-crystalline, donor−acceptor diblock copolymers exhibited lamella and cylindrical phase separated structures depending on molecular weight and volume ratio between the two blocks analogous to amorphous coil−coil systems (Figure 32). The charge transport properties of different morphologies in thin film were also investigated for these copolymers.306

and 3-48 (ADA) (Chart 17) self-assembled into 3D ordered lamellae structures with a different degree of interdigitation. However, 3-49 (DAD) revealed only amorphous structures with no π-stacking among PBI units due to the large sectional area of the donor blocks, which prevents stacking of PBI central unit. On the basis of a combination of low dose electron diffraction (ED), high-resolution TEM, and grazing incident-X-ray diffraction (GIXD) data, the zipper-like molecular packing was observed for the lamellar structures. Different alignment methods for controlling nanostructure orientation were performed such as high-temperature rubbing to produce edge-on oriented lamellae or epitaxy on oriented PTFE to form flat-on lying lamellae.301 Thelakkat and co-workers have reported the synthesis and photovoltaic studies of donor−acceptor diblock copolymers (350a) bearing bisphenyl-4-vinylphenylamine (TPA) and PBI acrylate moieties as hole and electron transport moieties, respectively.302,303 According to TEM and AFM, the morphologies of the diblock copolymers were investigated and revealed nanostructure assemblies of separating interfaces leading to photovoltaic properties of short circuit current (JSC), open circuit voltage (VOC), and power conversion efficiency (η) superior to 999


Chemical Reviews

Review

Chart 17. Structures of Donor−Acceptor Dyads and Donor−Acceptor Diblock Copolymers

Figure 32. TEM image of 3-51 at increasing PBI volume fraction from 44% (a) to 61% (b) in bulk exhibiting a lamellar and cylindrical morphology, respectively.305

4.1. Self-Assembly Directed by Imide−Imide H-Bonding Interactions

4. HYDROGEN-BOND DIRECTED SELF-ASSEMBLY Functional groups that enable hydrogen-bonding interactions are ubiquitously used in supramolecular chemistry. While an individual hydrogen bond is too weak to overcome the entropic costs that are demanded to form supramolecular assemblies, particularly in dilute solution, hydrogen bondings accompanied by π−π-stacking between aromatic cores combine strength and directionality to accomplish well-defined organization of πconjugated systems, for example, perylene bisimides (PBIs), into nano- and mesoscopic assemblies. Such supramolecular design of highly ordered assemblies is of significance for improving photonic and electronic properties. Thus, in this section, we will focus on the self-assembly of PBI dyes supported by hydrogen-bonding interactions.

In this section, we will highlight examples of PBI assemblies directed by imide−imide hydrogen-bonding interactions that are derived from the molecular structure of PBI itself. Würthner and co-workers designed N,N′-unsubstituted PBI 4-1a (Chart 18) bearing four trialkoxyphenyl wedges at the 1,6,7,12 bay positions.124 The sterical crowding of the bulky substituents at bay area affords an about 25° rotational twist between the two naphthalene imide subunits. This distortion and the bulkiness of the solubilizing alkyl side chains prevent the dyes from forming sandwich-type columnar π stacks and neat crystallization in solution. Instead, the combination of (decreased) π−πinteractions among slipped PBI stacks and intermolecular hydrogen-bonding interaction among imide groups leads to a 1000


Chemical Reviews

Review

Chart 18. Structures of N,N′-Unsubstituted and N-Monoarylated PBIs

Figure 33. (a) Concept for self-assembly of core-substituted perylene bisimides directed by imide−imide hydrogen-bonding interactions. The solubilizing groups at bay positions are simplified as X. (b) Schematic representation of a one-dimensional packing of PBI 4-1a in a strongly slipped arrangement. Red twisted blocks represent the perylene bisimide core, gray cones with a blue apex represent the bay substituents, and green lines represent hydrogen bonds.310

1001


Chemical Reviews

Review

aggregate of the “green” PBIs, that is, 1,7-diamino-substituted PBIs,309 which were originally introduced by Wasielewski and coworkers as synthetic counterparts of natural chlorophylls due to their green color.122 Similar to 4-1a, the 1,7-diamino PBI 4-1b self-assembles into J-aggregate fibers through slipped π-stacking and hydrogen-bonding interactions. These aggregates exhibit a remarkably red-shifted J-type absorption band beyond 800 nm and a second higher energy band at around 450 nm. The dye stacking mode and spectral features of these aggregates indeed resemble those of natural light-harvesting bacteriochlorophyll dye aggregates. Given the inherent thermal and photochemical stability of PBIs, self-assemblies of PBIs with absorption bands in the NIR region are promising materials for NIR light absorption in organic solar cells and NIR photosensitization. PBI 4-1c containing 12 alkyl side chains with chirality centers self-assembled into J-aggregate nanofibers, which are CD active.310 Thus, the molecular chirality of the side chains is transmitted to the self-assembled PBI π-core. The CD spectra of these J-aggregates showed bisignated signals in the S0−S2 transition range (around 430 nm), while monosignated signals were observed in the S0−S1 transition range (660−500 nm). These results corroborate the proposed aggregate model (Figure 33b), in which only the transition dipole moments of S0−S2 transition are perpendicular to the hydrogen-bonded selfassembly direction and thus interact with each other due to a helical displacement. The J-aggregate formation of chiral PBI 41c in MCH was investigated regarding mechanistic aspects, revealing that this self-assembly process proceeds according to a nucleation−elongation mechanism.310 CD studies revealed that chiral PBI 4-1c and achiral PBI 4-1d self-assembled into chiral heteroaggregates with a nonlinearity of chiral amplification, indicating that the sergeants-and-soldiers principle311 applies to the co-self-assembly process of these two PBIs.310 A superb organogelator was obtained based on N,N′unsubstituted PBI 4-1e bearing four ortho-methylphenoxy substituents at bay positions without additional long alkyl chains or any other solubilizing groups.312 In contrast to the appreciable solubility of 4-1e in chloroform (ca. ∼5 mg mL−1), a reference PBI derivative that contains four phenoxy substituents, that is, lacking ortho-methyl groups, shows very poor solubility (only around 1 μg mL−1) in chloroform. Therefore, the methyl groups at the ortho-position of phenoxy substituents in 4-1e play an important role in the fixation of the contorted conformation of PBI core and the associated possibility to direct one-dimensional fiber growth via specific intermolecular contacts to neighbor molecules. The self-assembly of 4-1e into highly fluorescent onedimensional J-aggregate follows again a nucleation−elongation growth mechanism by cooperative hydrogen bonding and π−πinteractions between PBI monomers, leading to gelation of toluene at extremely low critical gelation concentrations (CGCs) of as low as 0.05 wt %. This alkyl-chainless PBI gelator forms highly condensed “brick-wall” layers with a uniaxial molecular stacking directed by hydrogen bonds in the solid state, and thus appears to be particularly promising for application in photonic and electronic devices. Formation of a unique example of homochiral J-aggregate fibers was achieved by using the core-chiral perylene bisimide 4-2 that contains NH groups in the imide positions and a biphenoxy bridge at one of the bay areas.313 The sterically demanding substituents at the bay area rigidify the chiral π-core and thus enable the isolation of pure atropisomeric M- and P-enantiomer at ambient conditions. While the activation enthalpy value of ΔG⧧333 K = 99.8 kJ mol−1 is already substantial for monomeric

head-to-tail alignment of the dyes in helical double string nanofibers (Figure 33). This supramolecular design with slipped dye arrangement afforded a unique J-aggregate with outstanding fluorescence properties. The J-aggregate formation of PBI 4-1a in nonpolar solvent methylcyclohexane (MCH) is characterized by a narrow and intensive absorption band (J-band) that is 89 nm bathochromically shifted with respect to the monomer band (see Figure 7b). Such pronounced bathochromic shifts cannot be rationalized solely in terms of hydrogen-bonded perylene bisimide chains, but can arise only from closely stacked π-systems. According to Kasha’s exciton model,132 this spectral feature further implies that the monomer units are stacked in a slipped fashion. Atomic force microscopy (AFM) images of PBI 4-1a aggregates on silicon wafers revealed a network of helical strands whose height (2.0 ± 0.2 nm) and width (8.4 ± 2.6 nm) are in agreement with the expected dimensions of a “double string cable-like” structure displayed in Figure 33b.124 The fluorescence spectrum of 4-1a J-aggregate has a mirrorimage relationship to the absorption spectrum with a small Stokes shift of 12 nm and a fluorescence quantum yield of 0.96 ± 0.03 in MCH (see Figure 7b). Polarization fluorescence measurements of the highly fluorescent J-aggregates revealed a strong increase in the fluorescence anisotropy value from 0.032 (monomers at 80 °C in MCH) to 0.158 (aggregates at 20 °C in MCH) for the whole S0−S1 band. This result indicates the formation of an extended aggregate with a rather parallel alignment of the S0−S1 transition dipole moments of the monomeric units with respect to the long axis of the aggregate. Whereas the bathochromic shift and the band narrowing for the J-aggregate provide a clear indication of a strong coupling between the aggregated chromophores, the decrease in the fluorescence lifetime from 6.8 ns (monomer in dilute MCH solutions) to 2.6 ns (J-aggregate at higher concentration) indicates a substantial exciton delocalization. However, temperature-dependent absorption and fluorescence spectroscopic studies on exciton dynamics reveal PBI 4-1a J-aggregates have a very large level of structural disorder even at low temperature.307 The structural disorder was related to different orientations of the four aryloxy substituents at the bay area, varying torsional angles between the two naphthalene imide subunits, and cis- and transoid hydrogen-bonding alignments. Single molecule spectroscopic studies on the highly fluorescent Jaggregate confirmed strong fluctuations of fluorescence intensity (so-called fluorescence blinking) in individual J-aggregates of 41a, which could be related to one-dimensional exciton transport over distances up to 70 nm in these nanomaterials.308 An elaborate investigation of more than 200 individual fibers by super resolution microscopy further disclosed examples where excitons got trapped in even more red-shifted blinking fluorescent states.129 On the basis of a theoretical analysis of the exciton transport in such fibers, it was proposed that the trap is a Frenkel exciton state formed much below the main exciton band edge due to an environmentally induced heavy-tailed Lévy disorder. Water-induced reorganization of individual onedimensional J-aggregates of 4-1a was observed by fluorescence microscopy as well. 128 Time-dependent changes of the fluorescence spectra and lifetimes upon exposure to water vapor suggest an initial coordination of water molecules at defect sites, leading to the formation of H-type dimer units that act as exciton traps. The supramolecular design based on the imide−imide hydrogen-bonding interaction has been extended to the 1002


Chemical Reviews

Review

two PBI molecules with equilibrium constant Kh leads to an unfavorable species because the subsequent π−π-stacking of these dimers with equilibrium constant Ki is energetically more favorable. While such a process typically leads to cooperative growth of the supramolecular polymer, here upon polymerization increased steric crowding results in a diminished aggregation tendency via Ki = K/i with increasing polymer length i. This leads to the formation of small self-assembled objects instead of elongated fibers. Calculations of the degree of aggregation as well as DPn and DPw as a function of the dimensionless concentration xtot = Kctot capture the experimental results: a critical point is obtained, whereas DPn and DPw are low, even at very high values of xtot. These observations serve as a warning that typical hallmarks of cooperative growth of selfassembled nanostructures do not per se imply the formation of large objects.

dyes in solution, the racemization process of 4-2 is entirely prohibited for the aggregates. Self-recognition of enantiopure PBI (M-4-2 or P-4-2) afforded well-defined one-dimensional helical nanowires with J-type excitonic coupling. Interestingly, for the racemic PBI 4-2, an enantiomer preferentially recognizes its mirror image, which results in the formation of particle-like aggregates. Both homochiral and racemic aggregates form through a cooperative nucleation−elongation process with similar equilibrium constants for the elongation, in which dimers are considered as nuclei. However, the nucleation equilibrium constant for the racemate is about 20 times larger than that of the enantiopure compounds, indicating that heterodimer of M-4-2 and P-4-2 is thermodynamically more stable than homodimer, presumably due to preferential formation of π-stacked dimers of opposing chirality. The fluorescence efficiency of the helical nanowire of enantiopure M-4-2 is 47 ± 3%, which is 4 times higher than that of the nanoparticle of racemate (12 ± 3%).313 These results clearly demonstrate the importance of the enantiopure building blocks for the construction of highly ordered chiral nanostructures that ensures their enhanced luminescence efficiency. Meijer and co-workers investigated the self-assembly of Nmonoarylated PBI 4-3 that showed characteristics of a cooperative growth mechanism, but unexpectedly yielded objects of small size due to anticooperativity possibly caused by increased steric hindrance upon stack growth.314 Combined UV/vis and IR observations strongly suggest that PBI 4-3 selfassembles into H-type columnar aggregates consisting of cofacially stacked, hydrogen-bonded dimers as schematically depicted in Figure 34. While temperature-dependent absorption changes suggested a cooperative growth mechanism, small-angle X-ray scattering (SAXS) studies confirmed only small objects with the radius of gyration (Rg) of 1.9 nm in MCH solution. For these at first glance contradictory results, the authors offered a plausible explanation. Accordingly, the initial formation of a hydrogen-bonded dimer via weak 2-fold hydrogen bonding of

4.2. Self-Assembly Directed by Side-Chain Amide−Amide H-Bonding Interactions

Examples described in this section are categorized in terms of amide-functionalized PBIs at the imide positions (Chart 19). In contrast to imide−imide hydrogen bondings, the network of amide−amide interactions is rather oriented to the stacking direction of PBI cores, often supporting the PBI-inherent preference for the formation of columnar π-stacks of rotationally displaced dyes. Shinkai and co-workers have designed a visible-light-harvesting organogelator system by utilizing one-dimensional supramolecular fibers of cholesterol-based PBIs 4-4a−d (Chart 19).315 The gelation ability of cholesterol groups enables a 1D alignment of PBIs in a H-aggregate manner with helically oriented transition dipole moments. The substituents at the bay positions were modified so that PBI derivatives absorb a wider range of light energy. In binary PBI gels, energy is transferred from selectively excited 4-4a to 4-4b, 4-4c, and 4-4d with efficiencies of 68%, 53%, and 34%, respectively. The organogel-based lightharvesting system was extended further to the 4-4a/4-4b/4-4c ternary system and 4-4a/4-4b/4-4c/4-4d quaternary system, in which the excited energy is collected by 4-4c and 4-4d, respectively. In a subsequent study, these authors showed that PBI 4-4a and amide group bearing oligothiophene gelators self-sort and assemble to form p-type and n-type fibers with p−nheterojunction points (Figure 35).316 The self-sorting process was well supported by the overlapping of both absorption and CD spectra of the mixed gels with those of the sum of two spectra of 4-4a gel and oligothiophene gel. Moreover, the dissociation temperatures of the gelators in the self-sorting gel are identical to those of individual gels, indicating that aggregation and dissociation processes are independent of each other. The πstacked assemblies entangled at p−n-heterojunction points were applied in photovoltaic devices that showed photoelectrical conversion by visible-light irradiation.316 Recently, on the basis of a combination of UV/vis spectroscopy, AFM, XRD, and Kelvin-probe force microscopy (KPFM), Ajayaghosh and co-workers demonstrated another example of mixing n-type semiconductor PBI 3-6a with p-type π-gelator trithienylenevinylene derivative (TTV). In this case, a 1:1 molar ratio of the two compounds in n-decane afforded coaxially aligned p−n heterojunctions of supramolecular fibers as a result of self-sorting and self-assembly process.317 According to the flash photolysis time-resolved microwave conductivity (FPTRMC) experiments, these PBI 3-6a/TTV assemblies exhibited

Figure 34. (a) Schematic representation of the self-assembly of PBI 4-3 into short stacks of hydrogen-bonded dimers. (b) Schematic illustration of the attenuated equilibrium model for supramolecular polymerization of 4-3 with dimerization as the unfavorable nucleation step.314 1003


Chemical Reviews

Review

Chart 19. Structures of Amide-Functionalized PBIs

the absorption maximum and appearance of a pronounced shoulder at longer wavelength. These spectral features are very characteristic of H-type PBI aggregates and indicative of a close face-to-face stacking of rotationally displaced chromophores. AFM studies confirmed the formation of well-defined fibrous structures upon spin-coating of diluted gel solutions. These fibers adopt both left-handed (M) and right-handed (P) helicities that can be explained by the hydrogen-bond directed π−π-stacking (Figure 36) where the desired π−π-stacking distances of ∼3.5 Å and the desired hydrogen-bonding distances of ∼2 Å (leading to ∼5 Å distance between amide nitrogen and carbonyl carbon atoms) are realized for ∼30° rotationally displaced PBIs (Figure 36a). As gel fibers arise upon a strong preference for anisotropic unidirectional growth, long-range chiral order can originate from nucleation of a small chiral supramolecular unit, for example, hydrogen-bonded dimeric or oligomeric π-stacks of M or P helicity. On both highly oriented pyrolytic graphite (HOPG) and mica surfaces, the mean height of the helical fibers was 3.1 ± 0.3 nm with the width of 8.0 ± 2.0 nm, helical pitch of 15.0 ± 2.0 nm, and several micrometers in length. Remarkably, well-defined macroscopic fibers can be observed at room temperature after slow cooling from the isotropic liquid at a temperature of 228 °C under the polarized optical microscope (POM). They adopt a uniform diameter of 3.8 ± 0.2 mm and can extend to beyond 1 mm in length. Such well-organized fibers and bundles, which are composed of extended π-stacks of the electron-poor dye 4-5a, provide efficient pathways for mobile n-type charge carriers. In fact, the sum of the isotropic electron and hole mobility in the solid state was determined to be ∑μTRMC = 0.052 cm2 V−1 s−1 at room temperature by using the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique.318

Figure 35. (a) Chemical structures of oligothiophene gelatores. (b) Schemetic representation of self-sorting organogel formation of PBI 44a (red) and oligothiophene derivatives (blue), yielding p−nheterojunction points.316

an almost 12-folded enhancement in photoconductivity as compared to single component TTV assemblies due to the charge separation in the coaxial arrangement. On the basis of a related concept, Würthner and co-workers designed an organogelator (4-5a) with a core-unsubstituted PBI containing benzamide group with peripheral long alkyl chains in the imide position. This PBI exhibits pronounced gelation capabilities leading to interpenetrating networks of PBI stacks in a broad variety of solvents, including electron-rich aromatic solvents such as thiophene.318 Upon cooling a solution of 4-5a in toluene, aggregation took place and the absorption coefficients decreased drastically with a concomitant blue-shift (ca. 30 nm) of 1004


Chemical Reviews

Review

Figure 36. (a) Concept for self-assembly of 4-5a supported by sidechain amide−amide hydrogen-bonding interactions. The dodecyl groups are simplified as R. Right part depicts schematic representation of face-to-face stacks of rotationally displaced PBI cores. (b) Toluene organogel formed at 1.5 mM. (c) AFM height image of an aggregate sample spin-coated from diluted gel solution of 4-5a in toluene onto mica; scale bar, 200 nm.318

CD spectroscopic studies on dilute solutions of achiral 4-5a in chiral solvent (R)- or (S)-limonene revealed that the utilization of chiral solvents was effective in preferential population of single-handed helical fibers with an enantiomeric excess close to 100%.319 In contrast to the diluted system, however, AFM studies confirmed less pronounced helical bias with an enantiomeric excess of 20% for organogel fibers of 4-5a obtained from concentrated solution in chiral solvent.320 This discrepancy could be explained by the fact that the gelation process of this PBI through a fast nucleation and growth process into extended fibers leads to kinetic rather than thermodynamic self-assembly products. Only recently was the mechanism of supramolecular polymerization for such an organogelation process disclosed for the first time by temperature-dependent UV/vis spectroscopic experiments.321 Thermodynamic and kinetic analyses at different cooling and heating rates in low-polarity solvents revealed that the helical fibers are formed through a cooperative nucleation− growth process, in which the monomeric 4-5a is initially kinetically trapped in an inactive conformation. The unique kinetics was confirmed as thermal hysteresis in a cycle of assembly and disassembly processes. The out-of-equilibrium monomeric state observed in the hysteresis loop was rationalized by the formation of intramolecular hydrogen bonds between the imide carbonyl oxygens of the PBI and the amide hydrogens of the benzamide units (Figure 37). Under appropriate temperature, solvent, and concentration conditions, a spontaneous polymerization was retarded during a significant lag time of 1 h. On the basis of the slow kinetics in the nucleated polymerization, seeded polymerization could be demonstrated by addition of preassembled nanofiber seeds to the kinetically trapped monomers that subsequently attached to the termini of the seeds in a living supramolecular polymerization process. Replacement of the dodecyl groups in core-unsubstituted PBI 4-5a with chiral alkyl chains resulted in chiral PBI 4-5b that showed the expected homochiral aggregate fibers with righthanded (P) helicity in solution and in the organogel state.322 Unexpectedly, however, the self-assembly process was accom-

Figure 37. Schemetic representation of seeded supramolecular polymerization of PBI 4-5a based on the cooperative nucleation− growth supramolecular polymerization accompanied by thermal hysteresis. The bottom part depicts chemical equilibrium between a non-hydrogen-bonded and an intramolecular hydrogen-bonded conformation of 4-5a, where the dodecyl groups are simplified as R.321

panied by a strongly bathochromically shifted absorption band above 600 nm that is characteristic for a J-aggregate, leading to a dark green to almost black color depending on the concentration. To explain such “black” aggregate and pronounced bathochromic shift, the PBI cores have to be stacked on top of each other with a substantial displacement in the longitudinal direction (see Figure 5). The ability to form defined extended supramolecular networks in various kinds of organic media suggested unique possibilities for these materials as photosensory systems or as n-type semiconductors in organic bulk heterojunction solar cells. In a subsequent project with Thelakkat and co-workers, an innovative and simple concept for donor− acceptor heterojunction was provided by utilizing the selfassembly of 4-5b in the presence of an amorphous hole conducting polymer.323 The organogel acceptor molecule builds nanostructures in the presence of the donor polymer, resulting in a large area of donor−acceptor interface suitable for charge separation and charge transport. Depending on the nature of the peripheral alkyl substituents, the π−π-stacking mode of the chromophores could be altered from commonly observed H-type for 4-5a to the rather uncommon J-type for 4-5b for this class of benzamide-PBI organogelators.319 A broader variation of linear and branched alkyl chains suggested that the increased spatial demand of branched chains directs the transition from the more compact cofacially stacked H-aggregate to the enlarged column of slipped stack J-aggregated chromophores. Co-assembly studies with Hand J-aggregating chromophores 4-5a and 4-5b revealed the formation of solely H-type π-stacks at a lower mole fraction of the J-aggregating chromophore. The values of the relative amount of 1005


Chemical Reviews

Review

newly derived aggregation model, the observed concentration and temperature dependences could be described quantitatively.328 A substantial series of PBIs 4-6a−c having amide linkage and end-capped by phenyl, monododecyloxyphenyl, or tridodecyloxyphenyl units were synthesized by Asha and co-workers.213 These amide-functionalized PBIs self-assemble into H-type aggregates, regardless of their end-capping, in organic solvents like tetrahydrofuran (THF), toluene, and dichloromethane. Scanning electron micrographs (SEMs) of thin drop-cast samples that were annealed in toluene showed the formation of supramolecular rods of several micrometers in length. PBI 46a does not show any mesogenic properties, while PBI 4-6b exhibits a liquid crystal window of 103 °C between two transitions at 146 and 249 °C as confirmed by DSC studies. The liquid crystal window was shifted to lower temperature between 34 and 132 °C upon increasing the number of dodecyloxy substituents from one to three in PBI 4-6c. Furthermore, Kawai and co-workers have reported that bichromophoric PBI molecules, in which two PBI units are connected through a homochiral cyclohexane diamide core, selfassemble into elongated helical fibers in MCH directed by intermolecular amide−amide hydrogen bonding and π-stacking interactions. For these helical assemblies, enhanced circularly polarized luminescence (CPL) was observed both in solution and in the solid state.329 Interestingly, the supramolecular chirality of such assemblies could be tuned by external stimuli, in particular temperature and ultrasonication.330

4-5b in the mixed H-aggregate were plotted as a function of feed composition of 4-5b, from which the maximum incorporation of 4-5b in the mixed stack was estimated to be 44%. This ratio implies that 4-5b is located between two 4-5a units in the mixed stack to reduce the steric hindrance among two adjacently stacked 4-5b units. The core tetra-aryloxy-substituted PBI organogelator 4-5c showed an unprecedented sharp J-type absorption band and favorable emission properties in lyotropic mesophases and bulk gel phases.324 Femtosecond pump−probe spectroscopy was applied to investigate the electronically excited states of this PBI to provide the ingredients for the description of excitons in the aggregates and their annihilation processes.325 The dynamics and mobility of excitons in J-aggregates of PBI 4-5c were investigated by transient absorption spectroscopy with a time resolution of 50 fs.326 A diffusion constant of 1.29 nm2/ps was deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. Such a large diffusion length recommends these J-aggregates for lightharvesting systems where trapping as encountered in PBI Haggregates is undesired. The size-dependent exciton dynamics of one-dimensional aggregates of 4-5c was further studied by ultrafast transient absorption spectroscopy and kinetic Monte Carlo simulations as a function of the excitation density and the temperature in the range of 25−90 °C.327 For low temperatures, the aggregates can be treated as infinite chains, and the dynamics is dominated by diffusion-driven exciton−exciton annihilation. With increasing temperature, the aggregates dissociate into small fragments consisting of very few monomers. This scenario was supported by the time-dependent anisotropy deduced from polarization-dependent experiments. Interestingly, closer inspection of PBI 4-5c aggregation in MCH revealed the presence of a competing aggregation pathway, that is, biphasic behavior (Figure 38).328 Thus, at

4.3. Self-Assembly Directed by Other H-Bonding Interactions

Besides imide−imide and amide−amide hydrogen-bonding interactions (see previous sections), other types of hydrogen bondings were successfully applied to self-assemble the PBIs shown in Chart 20. Li, Zhu, and co-workers reported on the selfassembly of core-disubstituted bis-urea PBI derivatives 4-7a,b.331 Proton NMR and fluorescence spectra of these PBIs confirmed that strong hydrogen-bonding interactions between neighboring urea groups take place in CHCl3. The TEM images indicated the formation of well-defined nanoscale rods with uniform diameter distribution by self-assembly of hydrogen-bonding and π−πstacking interactions of PBI cores. Photocurrent measurements showed that the self-assembled films of these bis-urea derivatives produce steady and rapid anodic photocurrent responses. Würthner and co-workers synthesized the urea-containing tetraphenoxy-substituted PBI 4-8, and studied its gelation ability.332 This functional dye forms fluorescent organogels in toluene and tetrachloromethane by hydrogen-bonding and π−πstacking interactions. AFM and confocal laser scanning microscopy revealed the formation of fibrous aggregates. Another series of related molecules depicted in Chart 20 has been investigated by various groups. Thus, Zhu and co-workers demonstrated hydrogen-bonding-driven self-assembly of coretetrasubstituted PBI 4-9a containing chiral amino acid residues into supramolecular helices in CCl4,333 while Zang and coworkers have achieved one-dimensional assemblies of PBI 4-9b with propanoic acid attached at imide nitrogen atoms in water through adjustment of solution pH (Figure 39).334 Addition of hydrochloric acid to a mixture of monomeric and dimeric 4-9b in pH 9.0 solution at a concentration of 4.4 mM resulted in the formation of a gel whose constituent nanofibers with a length of 3−5 μm and a width of 20−30 nm were visualized by TEM. Malik and co-workers reported that coassembly of the donor-

Figure 38. Schematic illustration of self-assembly behavior of 4-5c in methylcyclohexane (MCH). Left bottom: Calculated structure of the compact sandwich-type dimer species with H-type excitonic character. Right bottom: Calculated structure of the dimer acting as nucleus for the formation of J-type aggregates.328

intermediate concentrations and temperatures, a compact sandwich-type dimer species with H-type excitonic character and aggregation-induced quenching behavior dominates. Again, due to the congested cofacial structure, this aggregate cannot grow into extended aggregates. Growth into larger aggregates is, however, possible from a less favored slipped dimer nucleus, leading to aggregates with J-type excitonic coupling and enhanced fluorescence as experimentally observed at high concentrations (and/or lower temperatures). On the basis of a 1006


Chemical Reviews

Review

Chart 20. Structures of PBIs Bearing Diverse Functional Groups for Hydrogen Bonding

tuned by changing the ratio of donor and acceptor, and 98% efficiency was achieved at a 4:1 ratio of 4-10a and 4-10b. Faul, Wei, and co-workers synthesized the sugar-based amphiphilic PBI 4-11a and studied its aggregate morphologies in different solvents.336 CD, UV/vis absorption, and TEM studies confirmed that 4-11a formed optically active left-handed helical nanowires in a chloroform/n-octane solution, while nanostructures with a right-handed helical arrangement were obtained from a THF/water solution. They also synthesized the symmetrical sugar-based PBI derivative 4-11b and studied in detail its self-assembly in water/DMF solvent mixtures with changing volume ratios.337 These investigations revealed that 411b self-assembled into planar ribbons in 20/80 and 40/60 water/DMF (v/v) mixtures with opposite chirality, while lefthanded helical nanowires were formed in 60/40 and 80/20 water/DMF (v/v) mixtures. The supramolecular helicity could also be adjusted by replacing the 1-hexylheptyl group in 4-11a with alkyl chains of different lengths338 and by introducing chlorine substituents at bay positions in 4-11b.339 Furthermore, a

Figure 39. Molecular configuration of PBI 4-9b and schematic diagram showing the concerted intermolecular π−π-stacking and hydrogenbonding interactions that form the nanofibril structure. Adapted with permission from ref 334. Copyright The Royal Society of Chemistry 2013.

and acceptor-type PBIs 4-10a and 4-10b in water at lower pH provided aggregates with bright yellow fluorescence.335 The light-harvesting efficiency of the coassembled system could be 1007


Chemical Reviews

Review

Figure 40. Hierarchical self-organization of 4-12 from nanofibrils to microfibers with detailed structural control on all length scales.343

4.4. Coassembly Directed by Imide−Melamine H-Bonding Interactions

solvent-dependent chiroptical inversion was reported for selfassembly systems of a PBI derivative bearing six D-mannoses in different DMSO/water mixtures,340 whereas a PBI derivative functionalized with two D-lactoses exhibited only right-handed supramolecular stacking in different volume ratios of DMSO/ water.341 Noteworthy is that temperature-dependent transformation of supramolecular nanostructures from twisted ribbons to microplates of a core-tetrachlorinated PBI containing glucose units in imide positions was achieved by modulation of the interaction of glucose moieties.342 In a very comprehensive study, Frauenrath and co-workers demonstrated the self-assembly of oligopeptide-substituted PBI 4-12 into well-ordered nanofibrils (Figure 40).343 The individual nanofibrils were investigated by spectroscopic and imaging methods, and the preparation of hierarchically structured microfibers of aligned nanofibrils allowed for a comprehensive structural characterization on all length scales with molecular level precision. The subtle relationship between molecular structure and supramolecular arrangement of the chromophores was investigated in self-assembled nanowires prepared from PBIs with different lengths of oligopeptide−polymer side chains.344 A “two-fold” odd−even effect was observed in circular dichroism spectra of these derivatives, depending on both the number of Lalanine units in the oligopeptide segments and the length of the alkylene spacer between PBI and oligopeptide substituents. The observation of both left- and right-handed helical aggregates as well as assemblies with no global handedness is likely the result of a complex interplay between the “universal” translation of molecular chirality into supramolecular helicity and the molecules’ inherent propensity for well-defined one-dimensional aggregation into β-sheet-like superstructures in the presence of a central chromophore.

The starting point for the research summarized in section 4 was given by Würthner’s report on the supramolecular coassembly of mono- and ditopic PBIs with complementary melamine receptors in 1999 (Chart 21).345,346 In this initial study, the complexation process of monotopic PBI 4-13a with the complementary melamine 4-15 was first confirmed by monitoring the chemical shift of imide proton signal by NMR titration experiments. These studies showed that the binding constant and the free binding enthalpy of the resulting complex increase drastically upon changing the medium polar solvent chloroform to less polar solvents tetrachloromethane and methylcyclohexane and accordingly established the nowadays ubiquitously used conditions for hydrogen-bond driven selfassembly of π-conjugated molecules. Further titration experiments for complexation between N,N′-unsubstituted ditopic PBI 4-13b and monotopic melamine 4-15 revealed the independence of the two imide receptor sites of PBI. On the basis of the binding studies, supramolecular polymerization of the ditopic PBI 4-13b and ditopic melamine 4-16a was explored. UV/vis dilution experiments indicated that significant interactions of the πconjugated PBI cores take place only for the assemblies of 4-13b and 4-16a, and not for those of 4-13b and monotopic melamine 4-15.345 These observations point at a cooperative process, in which the imide−melamine hydrogen-bonded polymeric structures hierarchically aggregate to an extended three-dimensional supramolecular network through secondary π−π-interaction (Figure 41). Light-scattering experiments provided evidence for the existence of large-sized aggregates in solution, in which the size of the aggregates drops below the experimental resolution of the DSL technique upon addition of monotopic 1008


Chemical Reviews

Review

Chart 21. Structures of PBIs and Melamine Receptors

to (S,S)-4-17a, the assemblies of (S,S)-4-17b with PBI 4-13b exhibit a negative CD couplet, which is less intense with respect to that of (S,S)-4-17a, or its R,R enantiomer, by 1 order of magnitude, indicating the formation of less-ordered M-helical “frustrated” stacks for (S,S)-4-17b·4-13b.347 Wasielewski and co-workers reported that PBI 4-18 bearing a monotopic melamine at one imide position self-assembles in solution as well as in the solid state into large-diameter cylindrical nanostructures through the synergistic effects of π−π-stacking, microsegregation, and hydrogen bonding.348 UV/vis absorption, 1 H NMR titration, and small-angle X-ray scattering (SAXS) studies revealed that the solution-phase assemblies consist of 12 monomers arranged in either a face-to-face stacked pair of hydrogen-bonded hexagonal arrays or a two-turn helix (Figure 42). These cyclic arrays grow to lengths of about 1 μm and form bundles of cylindrical structures in the solid phase as probed by X-ray diffraction, TEM, and SEM. The solution-phase photophysics of the dodecamer were probed by UV/vis, time-resolved fluorescence, and femtosecond transient absorption spectroscopies, revealing the formation of an excimer-like state, followed

melamine 4-15 as chain-stoppers. Further self-organization took place with increasing concentration during the evaporation process, leading to a highly intertwined network of strands with a diameter ranging from about 20 to 500 nm as explored by TEM, SEM, and confocal fluorescence microscopy (Figure 41b).346 Moreover, it was shown that chiroptical properties of PBI chromophores can be induced by hydrogen-bond directed selfassembly with amino acid-derivatized chiral melamines 417a,b.347 Investigation of MCH solutions of a 1:1 mixture of alanine-derivatized melamine (S,S)-4-17a and ditopic PBI 4-13b by DLS revealed the presence of large aggregates (>200 nm, with an irregular shape), whereas for phenylalanine-derivatized melamine (S,S)-4-17b no particles larger than the detection limit of 2 nm could be observed. An exciton-coupled CD effect was observed for the S0−S1 absorption band of the PBI dye with a negative sign for the (S,S) enantiomer 4-17a and a positive sign for the (R,R) enantiomer 4-17a. The negative sign of the CD couplet observed for (S,S)-4-17a·4-13b and the positive sign for (R,R)-4-17a·4-13b coassemblies imply an arrangement of the PBI molecules in M- and P-helical stacks, respectively. Similarly 1009


Chemical Reviews

Review

Figure 41. (a) Concept for hierarchically organized functional superstructures based on orthogonal intermolecular forces as given with imide− melamine hydrogen bonding, π−π-interactions, and alkyl chain interactions. (b) Confocal fluorescence micrograph (λex = 543 nm) of mesoscopic superstructure of coassemblies of 4-13b and 4-16a on silanized glass.346

The presence of propylthio chains in the pores inhibits the entrapment of C60 molecules presumably due to a steric hindering effect imposed by these groups on the C60−surface interaction. Remarkably, despite the pigment characters of PBIs without solubilizing groups, Buck and co-workers accomplished a solution-based assembly of honeycomb arrangement by adsorption of a mixture of PBI 4-19a and melamine 4-16b in dimethylformamide on gold surface.353 The solution-based fabrication strategy allows subsequent processing using selfassembled monolayers (SAMs) of thiols, which offer a broad scope for surface modification such as an underpotential deposition (UPD) of copper. 354 De Feyter, Schenning, Würthner, and co-workers have explored the two-dimensional self-assembly of functional bicomponent hydrogen-bonding dye systems based on ditopic PBIs 4-13b and monotopic melamine derivative 4-20.355 STM studies revealed a well-defined 4-13b·420 pattern on HOPG that is different from those of the individual components, providing some design rules to immobilize multicomponent systems at the liquid−solid interface. Fasel and co-workers achieved bimolecular wires by deposition of PBI 4-19a and 1,4-bis(2,4-diamino-1,3,5-triazine)-benzene 4-21 through complementary multitopic hydrogen bondings.356 Double- and single-row hydrogen-bonding wires with a longrange growth could be obtained by the choice of deposition concentration and an appropriate template surface such as Au(11,12,12). Silly and co-workers achieved arrays of single

by an ultrafast electron-transfer from the electron-rich tridodecyloxybenzene group of melamine moiety to electronpoor PBI.348 Self-assembly on surfaces is a “bottom-up” approach that allows a simple and rapid creation of ordered structures with nanometer precision over an extended length scale. Champness, Beton, and co-workers achieved two-dimensional bimolecular assemblies with an open honeycomb network by adsorbing 2fold symmetric PBI 4-19a with the 3-fold symmetric melamine 416b on a silver-terminated silicon surface in an ultrahigh-vacuum environment (Figure 43).349,350 The honeycomb network is stabilized by multiple imide−melamine hydrogen bondings, in which the well-defined pores are large enough to localize and stabilize heptameric C60 clusters with a compact hexagonal arrangement formed by subliming C60 onto the network. The bimolecular honeycomb arrays stabilized by hydrogen bonding can also be formed on metallic surfaces such as gold with relatively minor changes in preparation conditions.351 They have also demonstrated the deposition of melamine 4-16b onto Ag− Si surface covered by a monolayer of dibrominated PBI 4-19b, followed by annealing at around 60 °C, resulting in a bimolecular honeycomb arrangement.352 4-16b·4-19b network stabilizes C60 clusters with geometries similar to that observed in the 4-16b·419a network. Deposition of a monolayer of propylthiosubstituted PBI 4-19c on Ag−Si surface resulted in the formation of a hexagonal arrangement in addition to a close-packed array. The unimolecular 4-19c hexagonal array could be converted to a bimolecular 4-16b·4-19c hexagonal phase by addition of 4-16b. 1010


Chemical Reviews

Review

Figure 42. Aggregation process of melamine-functionalized PBI 4-18 in solution and in the solid state. The right part depicts the formation of the twoturn helix structure followed by creation of an extended helix.348

linker affords extended supramolecular polymers of 4-23b·4-25a, which hierarchically organize into nanoscopic fibers. These results demonstrate the possibility to create distinct supramolecular structures based on melamine-linked functional dyes,361 particularly PBIs, by just changing the number of carbon atoms at the linker moieties. A transformation of H-aggregates of core-unsubstituted PBI 424 to J-aggregates in solution as well as in solid state was demonstrated by hydrogen-bond-directed complexation of the melamine unit in 4-24 with cyanuric acid derivative 4-25a at different stoichiometric ratios.360 In nonpolar MCH, 4-24 selfassembles into H-aggregates, which was locked as H-type dimers upon addition of 0.5 equiv of ditopic cyanuric acid 4-25a. Remarkably, further addition of 4-25a to the solution of Haggregate PBI dimers resulted in the formation of PBI J-aggregate with a pronounced red-shifted absorption band at 620 nm (Figure 44a,b), which is similar to previously discussed “black” PBI organogelator 4-5b.322 The transition between J- and H-type stacking modes was reversibly controlled even in the solid state upon thermal and mechanical stimuli as visualized by the change between green and red colors (Figure 44c,d). Moreover, welldefined columnar assemblies were obtained by complexation of cyanurate-anchored PBI 4-26 with melamine derivative 4-27.362 The columnar assemblies could be visualized by TEM and AFM as well-defined fibrillar nanostructures. Upon heating to 200 °C, the hexagonal columnar morphology of these coassemblies was changed to a highly ordered lamellar structure in the bulk state as

metallofullerene Lu@C82 molecules by using the empty pores of PBI−melamine supramolecular network on Au(111) surface.357 4.5. Coassembly Directed by Melamine−Cyanurate/Barbiturate H-Bonding Interactions

Yagai and co-workers have extensively studied the complexation behavior of melamine-anchored PBIs with complementary hydrogen-bonding cyanurate components.13,358−361 For this purpose, they synthesized a broad variety of melamine-anchored PBIs (4-22, 4-23a,b, 4-24) shown in Chart 22. The UV/vis absorption spectrum of PBI 4-22 in MCH at a concentration of 10 μM revealed characteristic spectral features for excitonically coupled PBIs with a face-to-face stacking arrangement. The 1:1 complexation of PBI 4-22 with cyanurate 4-25a resulted in disruption of the π−π-stacked aggregates of 4-22 and formation of hydrogen-bonded supramolecular polymers even at lower concentrations (10 μM). Upon increasing the concentration of 1:1 mixture of 4-22 and 4-25a, the hydrogen-bonded supramolecular polymers hierarchically self-organized by a π−πstacking interaction of PBI chromophores, affording ribbonlike or ropelike aggregates in aliphatic solvents.358 They have also investigated the effect of linker length in melamine-functionalized PBIs 4-23a,b on coaggregation with cyanurate 4-25a.359 DLS and AFM studies upon addition of 1 equiv of cyanurate component revealed that PBI 4-23a featuring ethylene linker generates a discrete dimer supported by two hydrogen-bonded cyanurate molecules, while PBI 4-23b containing a trimethylene 1011


Chemical Reviews

Review

Figure 43. (a) Structural motif of a melamine (4-16b)−PBI (4-19a) network created by multiple complementary hydrogen bondings. (b) STM image of a 4-16b·4-19a network on a silver-terminated silicon surface. Inset: High-resolution view of the Ag/Si(111)-√3 × √3R30° substrate surface; scale bar, 3 nm. (c) STM image of large-area network; scale bar, 20 nm. Adapted with permission from ref 349. Copyright 2003 Nature Publishing Group.

revealed by DSC, POM, X-ray diffraction (XRD), AFM, IR, and UV/vis studies. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient absorption studies confirmed a remarkable change in the optoelectronic properties upon the exothermic transition from the hexagonal columnar to lamellar structures. Self-assembly of PBI dimer 4-28 (Chart 22) bearing a melamine linker was studied in Langmuir and Langmuir− Blodgett (LB) films by Chen, Li, and co-workers.363 Surface pressure−area isotherm measurements and the spectroscopic studies indicated that in Langmuir or the multilayer LB films, this PBI dimer adopted a face-to-face configuration and edge-on

orientation. In the presence of the barbituric acid 4-29 in water, coassemblies of PBI 4-28 and 4-29 were formed by melamine− barbiturate hydrogen bondings (Figure 45). TEM images of the LB films deposited from the barbituric acid solution revealed uniform nanowire morphology, while the X-ray diffraction studies indicate that the molecules in the solid film packed with high order. The strong excimer emission in LB films suggests enforced face-to-face configuration for the PBI units in LB films as compared to that in solution. The swallow-tail containing PBI dimer 4-30a with a monotopic melamine bridge was synthesized, and its selfassembly with tritopic cyanuric acid 4-25b was studied by Yagai 1012


Chemical Reviews

Review

Chart 22. Structures of Melamine/Cyanurate-Linked PBIs and Corresponding Receptors

and co-workers.364 UV/vis absorption, fluorescence, and CD titration experiments in MCH and TEM studies confirmed the formation of a 3:1 complex between 4-30a and 4-25b (Figure 46) that subsequently stack to form helical columnar assemblies. The swallow-tail containing PBI dimer 4-30b (Chart 22) with a ditopic melamine bridge showed a remarkable increase in the solubility in organic solvents upon mixing with the complementary hydrogen-bonding barbiturate 4-31 or cyanurate 425a.365 The film obtained upon slow cooling from the isotropic melts of 1:1 mixtures of 4-30b·4-31 and 4-30b·4-25a displayed powder X-ray diffraction (PXRD) patterns assignable to multilamellar structures, suggesting the formation of linear tapelike hydrogen-bonded strands. The axis of the semiconductive PBI stacks in thin films is parallel to the substrate; thus the lamellar architectures act as electron transporting layers in organic field effect transistors.

4.6. Coassembly of PBI with Other Dyes

The unique optical and redox properties of PBIs make them very suited for various kinds of mixtures with other dyes for lightharvesting and energy transfer as well as light-induced charge transfer studies or even functional materials for electronics, photonics, or photovoltaics. Accordingly, in the past decade, quite a few highly interesting studies were reported on the coassembly of PBI dyes with different melamine-anchored functional dyes (Chart 23). Schenning, Meijer, Würthner, and co-workers pioneered this idea with a series of studies on the coassembly of melamine-linked oligo(p-phenylenevinylene)s (OPV) 4-32a−c and mono- and ditopic PBIs. In the first study, OPV 4-32a self-assembly with 4-13b was investigated in nonpolar solvent MCH by UV/vis, fluorescence, and CD titration experiments.366 These studies revealed the formation of imide−melamine hydrogen-bonded OPV−PBI−OPV donor− acceptor−donor complexes that further stack into J-type 1013


Chemical Reviews

Review

Figure 44. (a) Transformation of H-aggregate dimer to J-aggregate nanostructure based on complexation of 4-24 with 4-25a. (b) Temperaturedependent UV/vis absorption spectra and the corresponding solution colors of the 1:1 mixture of 4-24 with 4-25a. (c,d) Color change of thin film upon (c) thermal and (d) mechanical stimuli. Adapted with permission from ref 361. Copyright 2014 The Chemical Society of Japan.

13b.367 These well-defined coaggregated dyes exhibit photoinduced electron transfer on subpicosecond time scale. Furthermore, it was reported that donor−acceptor dyad complexes were created by hydrogen-bond driven coassembly of OPV 4-32b and monotopic tetrachloro-PBI 4-14a into πstacks in apolar solvents.368 Field-effect transistors constructed with these dyad complexes showed ambipolar transport of holes and electrons that were related to transport through either OPV or PBI stacks, respectively. Coassembly of OPV 4-32b with complementary ditopic PBI 4-13b and monotopic PBIs 4-14a,b, respectively, was investigated by absorption, CD, photoluminescence, and femtosecond transient absorption spectroscopy.369 These studies revealed formation of hydrogen-bonded OPV−PBI arrays with J-type PBI stacking. In these stacks, an ultrafast photoinduced charge separation occurs via an intermolecular pathway (compare Figure 29). The subsequent charge recombination was shown to strongly depend on small structural differences within the J-type arrangements as shown by comparison of stacked supramolecular dimers, trimers, and covalently linked OPV−PBI dyads. A coupled oscillator model was applied to analyze absorption and circular dichroism spectra of these arrays and to identify intermolecular arrangements that are consistent with the experimental spectra and the chargetransfer kinetics.369 Coassembly of melamine-bridged OPV dimer 4-33 with ditopic PBI 4-13b was investigated in great detail by De Feyter and co-workers.370 The authors revealed that the coassembly of these complementary system leads to the formation of linear and cyclic hydrogen-bonded structures with different morphologies

Figure 45. Schematic arrangement of PBI 4-28 on the surface of 1 mM barbituric acid 4-29 solution.363

aggregates with a helical screw sense. AFM studies confirmed that these J-aggregates have helical rod-like morphology with a length of several micrometers and a width of 7 nm. Moreover, for these helical coassemblies, photoinduced electron transfer from electron-rich OPV to electron-poor PBI, followed by charge separation, was observed. Following this concept, supramolecular p−n heterojunctions composed of OPV−PBI−OPV arrays were achieved by hierarchical coself-organization of melamine-functionalized electron-donor OPV derivatives 4-32a−c (Chart 23) with increasing number of OPV units and electron-acceptor PBI 41014


Chemical Reviews

Review

Figure 46. Proposed structure of 3:1 hydrogen-bonded complex between 4-30a and 4-25b.364

tween the two dyes in the coassembly give rise to intramolecular electron transfer upon selective photoexcitation of PBI dye, affording a several nanosecond lived radical ion pair species, while photoexcitation of Pc dye is only followed by an intersystem crossing. Hydrogen-bonded supramolecular triads of electron-rich triaryl amine triazine derivatives 4-37,38 with PBI 4-13b were synthesized and characterized by Lin and co-workers.374 FTIR, NMR, UV/vis, and fluorescence spectroscopic studies confirmed the formation of 2:1 triads through multiple hydrogen bondings between 4-37,38 and 4-13b. Braunschweig and co-workers investigated the coassembly of diamidopyridine-linked diketopyrrolopyrrole (DPP) electron donors 4-39 and 1,7-substituted PBI electron acceptors 4-19d into a heteroaggregate by variabletemperature UV/vis absorption and CD spectroscopy.375 A new assembly model for the formation of heteroaggregates that arise from both H-bonding and π−π-stacking was developed and applied to fit the changes in absorption with temperature, providing the corresponding thermodynamic parameters for each interaction. Furthermore, electronic and optical properties of the donor−acceptor−donor superstructure composed of DPP 4-39 and PBI 4-19e were explored by fluorescence and transient absorption spectroscopy.376 These results confirmed that fluorescence quenching arises with the formation of heterosuperstructures as a result of photoinduced charge separation, which is not possible between the disaggregated individual components alone. Coassembly of melamine-anchored photochromic diarylethenes (DAEs) 4-40 (Chart 23) and PBI 4-13b was explored

as visualized by scanning tunneling microscopy (STM). In a related project, Yagai, Würthner, and co-workers studied the coassembly of PBI 4-13b and azobenzene-functionalized ditopic melamine 4-34.371 AFM studies revealed the formation of illdefined nanostructures for a 1:1 mixture of 4-13b and 4-34 in MCH. However, for a 1:2 mixture, well-defined fibrous nanostructures with a helically coiled architecture were observed. Furthermore, it was shown that the self-assembly process was accompanied by the formation of J-aggregated PBIs with a substantial equilibration time of several hours, which became faster upon increasing the amount of melamine 4-34 against PBI 4-13b. UV/vis titration studies under thermodynamic control revealed that a 1:1.5 ratio of 4-13b and 4-34 is required for the formation of the J-aggregate state. The unconventional stoichiometric ratio suggests that additional 4-34 units are inserted into alternate triple hydrogen-bonded arrays of 4-13b and 4-34 by forming melamine−melamine double hydrogen bonds. In another interesting study, Guldi, Torres, and co-workers applied imide−melamine hydrogen-bonding interactions to create supramolecular donor−acceptor−donor arrays of melamine-anchored zinc phthalocyanine (ZnPc) 4-35 and PBI 413b.372 Photoexcitation of the PBI component afforded transfer of singlet excited-state energy to the energetically lower lying triplet excited state of ZnPc through an intersystem crossing. It was also reported by the same groups that melamine-linked phthalocyanine (Pc) 4-36 coassembles with PBI 4-13b through two triple hydrogen bonds in chloroform, leading to an electron donor−acceptor−donor array.373 Electronic interactions be1015


Chemical Reviews

Review

Chart 23. Structures of Melamine-Functionalized Dyes Used in Coassembly with PBIs

by Yagai and co-workers.377 AFM, UV/vis spectroscopy, and molecular modeling studies revealed that DAEs 4-40 and PBI 413b coassembled in toluene to form well-defined helical nanofibers featuring J-type dimers of PBI dyes. The cooperative stacking of PBI dyes could be used to stabilize the parallel conformer of DAE through complementary hydrogen bonds. Upon irradiating the coassembly solution in toluene with UV and visible light in turn, a reversible morphology change between nanofibers and nanoparticles was observed.377 Narayan, Asha, and co-workers reported on the coassembly of pyridine-linked PBI 4-41 with OPV 4-42 functionalized with a hydroxyl unit.378 NMR and FTIR spectroscopic studies confirmed hydrogen-bonding interaction between pyridine and hydroxyl units to form a 1:1 donor−acceptor complex, which was further subjected to photoinduced polymerization of methacrylamide unit in 4-42 in the presence of photoinitiator 2,2-

diethoxyacetophenone (Figure 47). Lamellar structures with an average thickness of 18 Å were observed in TEM images of the thin films on copper grids for the polymerized complex. As compared to the pristine acceptor PBI 4-41 device, higher photocurrent response was confirmed in the donor−acceptor system.

5. METALLOSUPRAMOLECULAR PBI ASSEMBLIES In contrast to other noncovalent forces like hydrogen bonding and ionic interactions, ligand coordination to metal ions can easily surpass the inherent strength of π−π-interactions among PBI dyes, particularly in dipolar aprotic and halogenated solvents. Therefore, metal−ligand coordination with its high directionality and adequate thermodynamic stability is of particular interest for the creation of defined nanoscale PBI architectures. The binding energies of commonly employed coordinative bonds (40−120 kJ 1016


Chemical Reviews

Review

Figure 47. Structure of 1:1 complex of PBI 4-41 and OPV 4-42, and photo polymerization of methacrylamide unit in 4-42.378

Figure 48. Step-growth polymerization of conventional nylon-6.6 (above) and a typical metallosupramolecular polymer (below).

mol−1) are in the intermediate range between the strong organic covalent ones (∼400 kJ mol−1) and other noncovalent interactions (

Perylene Bisimide Dye Assemblies as Archetype Functional

Recommend Documents