Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

A novel core–shell structured columnar liquid crystal composed of a donor‐acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self‐assembles in solution into helical J‐aggregates guided by π–π interactions and hydrogen bonds which organize into a liquid‐crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs’ (J‐type) transition dipole moment parallel and the bithiophene side arms’ (H‐type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self‐assembled core–shell structure enables the implementation of the dye in two‐contact photoconductivity devices giving rise to a 20‐fold increased photoresponse compared to a reference dye without bithiophene donor moieties.     

Self‐Sorting Supramolecular Polymerization: Helical and Lamellar Aggregates of Tetra‐Bay‐Acyloxy Perylene Bisimide

A new perylene bisimide (PBI), with a fluorescence quantum yield up to unity, self‐assembles into two polymorphic supramolecular polymers. This PBI bears four solubilizing acyloxy substituents at the bay positions and is unsubstituted at the imide position, thereby allowing hydrogen‐bond‐directed self‐assembly in nonpolar solvents. The formation of the polymorphs is controlled by the cooling rate of hot monomer solutions. They show distinctive absorption profiles and morphologies and can be isolated in different polymorphic liquid‐crystalline states. The interchromophoric arrangement causing the spectral features was elucidated, revealing the formation of columnar and lamellar phases, which are formed by either homo‐ or heterochiral self‐assembly, respectively, of the atropoenantiomeric PBIs. Kinetic studies reveal a narcissistic self‐sorting process upon fast cooling, and that the transformation into the heterochiral (racemic) sheetlike self‐assemblies proceeds by dissociation via the monomeric state.     

Diketopyrrolopyrrole Columnar Liquid‐Crystalline Assembly Directed by Quadruple Hydrogen Bonds

A diketopyrrolopyrrole (DPP) dye self‐assembles via a unique hydrogen‐bonding motif into an unprecedented columnar liquid‐crystalline (LC) structure. X‐ray and polarized FTIR experiments reveal that the DPPs organize into a one‐dimensional assembly with the chromophores oriented parallel to the columnar axis. This columnar structure is composed of two π–π‐stacked DPP dimers with mirror‐image configurations that stack alternately through quadruple hydrogen bonding by 90° rotation. This exotic packing is dictated by the complementarity between H‐bonds and the steric demands of the wedge‐shaped groups attached at the core. This novel LC supramolecular material opens a new avenue of research on DPP dye assemblies with photofunctional properties tailored by H‐bonding networks.     

Plank‐Shaped Column‐Forming Mesogens with Substituents on One Side Only

Prolonged glyoxylation of pyrenyl‐1‐glyoxylic acid ethyl ester leads to a mixture of isomers with polar pyrenylene‐1,8‐diglyoxylic acid as the main product, whereas the centrosymmetric 1,6‐isomer is obtained in good yield from the corresponding dibromopyrene. Perkin condensations followed by Pd‐catalyzed cyclizations lead to isomeric dinaphthopyrene‐tetracarboxdiimides that self‐assemble into columnar liquid crystals of hexagonal and rectangular symmetry, of which the rectangular mesophases have unusually elongated unit cells. The cisoid diimides with both alkylimide substituents on the same side of the oblong arene system show a much greater tendency to self‐assemble into fluid stacks of disks than their centrosymmetric isomers. With racemically branched alkyl substituents, uniform vertical surface alignment of the columns in the high‐temperature hexagonal mesophase is resilient to cycling through the lower‐temperature rectangular and crystalline phases.     

Nanopatterned Superlattices in Self‐Assembled C2‐Symmetric Oligodimethylsiloxane‐Based Benzene‐1,3,5‐Tricarboxamides

The synthesis of C₃‐ and C₂‐symmetric benzene‐1,3,5‐tricarboxamides (BTAs) containing well‐defined oligodimethylsiloxane (oDMS) and/or alkyl side chains has been carried out. The influence of the bulkiness of the oDMS chains in the aggregation behavior of dilute solutions of the oDMS‐BTAs in methylcyclohexane was studied by temperature‐dependent UV spectroscopy. The formation of hierarchically self‐assembled aggregates was observed at different BTA concentrations, the tendency of aggregation increases by shortening or removing oDMS chains. Chiral BTAs were investigated with circular dichroism (CD) spectroscopy, showing a stronger tendency to aggregate than the achiral ones. Majority rules experiments show a linear behavior consistent with the existence of a high mismatch penalty energy. The most efficient oDMS‐BTAs organogelators have the ability to form stable organogels at 5 mg mL^?1 (0.75 wt %) in hexane. Solid‐state characterization techniques indicate the formation of an intermolecular threefold hydrogen bonding between adjacent molecules forming thermotropic liquid crystals, exhibiting a hexagonal columnar organization from room temperature to above 150 °C. A decrease of the clearing temperatures was observed when increasing the number and length of the oligodimethylsiloxane chains. In addition to the three‐fold hydrogen bonding that leads to columnar liquid crystalline phase, segregation between the oDMS and aliphatic chains takes place in the BTA functionalized with two alkyl and one oDMS chain leading to a superlattice within the hexagonal structure with potential applications in lithography.     

Advances in Metal‐Free Heterocycle‐Based Columnar Liquid Crystals

Liquid crystals are ordered soft materials formed by self‐organized molecules and can potentially be used as new functional materials for electron‐, ion‐ or molecular‐transport; optical; and bio‐active materials. In particular, the columnar liquid crystals are promising candidates used in various optical and electronic devices. For this purpose, design and synthesis of unconventional materials are essential. In this review, we have summarized several approaches for the synthesis of columnar liquid crystals composed of various heterocyclic systems. We also outline their liquid crystalline and other relevant properties, and their suitability for applications in diverse fields.     

Guest⊂Guest⊂Host Multicomponent Molecular Crystals: Entrapment of Guest⊂Guest in Honeycomb Networks Formed by Self‐Assembly of 1,3,5‐Tri(4‐hydroxyaryl)benzenes

Sterically‐engineered rigid trigonal molecular modules based on 1,3,5‐tri(4‐hydroxyphenyl)benzenes H1 and H2 undergo O‐H???O hydrogen‐bonded self‐assembly into eight‐fold catenated hexagonal (6,3) and two‐fold interpenetrated undulated square (4,4) networks, respectively. In the presence of [18]crown‐6 as a guest, the triphenol H1 is found to self‐assemble into a honeycomb network with hexagonal voids created between three triphenol building blocks. The guest [18]crown‐6 molecules are found to be nicely nested in hexagonal enclosures. The empty spaces within the crowns can be further filled with neutral (MeOH/water, MeOH/MeNO₂) or ionic guest species such as KI/KAcAc to furnish novel multicomponent assemblies, that is, guest ? guest ? host, that typify Russian dolls. In contrast, triphenol H2 is found to yield analogous multicomponent molecular crystals in which the guest crown–K⁺ acts as a spacers in the hydrogen‐bonded self‐assembly that leads to distorted chicken wire networks.     

Dendronized supramolecular polymers self‐assembled from dendritic ionic liquids

The synthesis, structural, and retrostructural analysis of a library of self‐assembling dendrons containing triethyl and tripropyl ammonium, pyridinium and 3‐methylimidazolium chloride, tetrafluoroborate, and hexafluorophosphate at their apex are reported. These dendritic ionic liquids self‐assemble into supramolecular columns or spheres which self‐organize into 2D hexagonal or rectangular and 3D cubic or tetragonal liquid crystalline and crystalline lattices. Structural analysis by X‐ray diffraction experiments demonstrated the self‐assembly of supramolecular dendrimers containing columnar and spherical nanoscale ionic liquid reactors segregated in their core. Both in the supramolecular columns and spheres the noncovalent interactions mediated by the ionic liquid provide a supramolecular polymer and therefore, these assemblies represent a new class of dendronized supramolecular polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4165–4193, 2009     

Synthesis and self‐assembly of rod‐coil molecules with n‐shaped rod building block

The rod‐coil molecules with n‐shaped rod building block, consisting of an anthracene unit and two biphenyl groups linked together with acetylenyl bonds at the 1,8‐position of anthracene as a rigid rod segment, and the alkyl or alkyloxy chains with various length (i.e., methoxy‐ ( 1 ), octyl‐ ( 2 ), hexadecyl‐ ( 3 )) at the 10‐position of anthracene and poly(ethylene oxide) with the number of repeating units of 7 connected with biphenyl as coil segments were synthesized. The molecular structures were characterized by ¹H NMR and MALDI‐TOF mass spectroscopy. The self‐assembling behavior of new type of molecules 1–3 was investigated by means of DSC, POM, and SAXS at the bulk state. These molecules with a n‐shaped rod building block segment self‐assemble into supramolecular structures through the combination of π–π stacking of rigid rod building blocks and microphase separation of the rod and coil blocks. SAXS studies reveal that molecules 1 and 2 show hexagonal columnar and rectangular columnar structures in the liquid crystalline phase, respectively; meanwhile, molecules 1–3 self‐organize into lamellar structures in the crystalline state. In addition, self‐assembling studies of molecules 1–3 by DLS and TEM indicated that these molecules self‐assemble into elongated nanofibers in aqueous medium. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1415–1422, 2010     

Dithienophosphole‐Based Phosphinamides with Intriguing Self‐Assembly Behavior

A new, highly adaptable type of phosphinamide‐based hydrogen bonding is representatively demonstrated in π‐conjugated phosphole materials. The rotational flexibility of these intermolecular P=O?H?N hydrogen bonds is demonstrated by X‐ray crystallography and variable‐concentration NMR spectroscopy. In addition to crystalline compounds, phosphinamide hydrogen bonding was successfully introduced into the self‐assembly of soft crystals, liquid crystals, and organogels, thus highlighting the high general value of this type of interaction for the formation of organic soft materials.     

Self‐Assembly of Imidazolium‐Based Rodlike Ionic Liquid Crystals: Transition from Lamellar to Micellar Organization

By using aryl‐amination chemistry, a series of rodlike 1‐phenyl‐1H‐imidazole‐based liquid crystals (LCs) and related imidazolium‐based ionic liquid crystals (ILCs) has been prepared. The number and length of the C‐terminal chains (at the noncharged end of the rodlike core) and the length of the N‐terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self‐assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X‐ray diffraction. For the single‐chain imidazole derivatives nematic phases (N) and bilayer SmA₂ phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C‐terminal chains and in this series it leads to the phase sequence SmA–columnar (Col)–micellar cubic (Cub_I/Pm3n). Elongation of the N‐terminal chain gives the reversed sequence. Short N‐terminal chains prefer an end‐to‐end packing of the mesogens in which these chains are separated from the C‐terminal chains. Elongation of the N‐terminal chain leads to a mixing of N‐ and C‐terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA₂) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end‐to‐end packing leads to core–shell aggregates. In this case, elongation of the N‐terminal chains distorts core–shell formation and removes Cub_I and Col phases in favor of single‐layer SmA phases. Hence, by tailoring the length of the N‐terminal chain, a crossover from taper‐shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self‐assembly in ILCs.     

Toward Ultralow‐Bandgap Liquid Crystalline Semiconductors: Use of Triply Fused Metalloporphyrin Trimer–Pentamer as Extra‐large π‐Extended Mesogenic Motifs

In contrast with their dimeric homologue, triply fused zinc porphyrin trimer–pentamer, as extra‐large π‐extended mesogens, assemble into columnar liquid crystals (LCs) when combined with 3,4,5‐tri(dodecyloxy)phenyl side groups ( 3 P_Zn – 5 P_Zn , Figure 1 ). Their LC mesophases develop over a wide temperature range, namely, 41–280 °C (on heating) for 5 P_Zn , and all adopt an oblique columnar geometry, typically seen in columnar LC materials involving strong mesogenic interactions. These LC materials are characterized by their wide light‐absorption windows from the entire visible region up to a near infrared (NIR) region. Such ultralow‐bandgap LC materials are chemically stable and serve as hole transporters, in which 5 P_Zn gives the largest charge carrier mobility (2.4×10^?2 cm V^?1 s^?1) among the series. Despite a big dimensional difference, they coassemble without phase separation, in which the resultant LC materials display essentially no deterioration of the intrinsic conducting properties.     

Development of Structural Complexity by Liquid‐Crystal Self‐assembly

Since the discovery of the liquid‐crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self‐assembly of liquid crystals. The focus is on new exciting soft‐matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry‐breaking. Comparisons are made with developments in related fields, such as self‐assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.     

Ferroelectrically Switching Helical Columnar Assembly Comprising Cisoid Conformers of a 1,2,3‐Triazole‐based Liquid Crystal

The 1,2,3‐triazole molecule, which is a product of click chemistry, possesses a high dipole moment and can be a useful polar motif for ferroelectric columnar liquid crystal (LC) materials—though it has not been used to date. Herein, we report the helical assembly and ferroelectric switching properties of a columnar liquid crystal comprising a naphthalene core and 1,2,3‐triazolyl linkages. The molecule assembles into a double‐stranded helical columnar LC structure (Col_hel). The X‐ray simulations of cisoid and transoid columnar models suggest that the helical assembly comprises cisoid conformers with a non‐zero dipole moment. The helical columns in the Col_hel phase are aligned homeotropically under an electric field. The ferroelectric switching of the axial polarization can be observed in the temperature range of 105–115 °C in the Col_hel phase, wherein the triazolyl hydrogen bonding along the column axis is weakened. The ferroelectric switching event is attributed to the rotation of the polar triazolyl units in response to the electric field.     

Supramolecular Columnar Liquid Crystals with Tapered‐Shape Simple Pyrazoles Obtained by Efficient Henry/Michael Reactions

A straightforward synthesis of mesogenic pyrazoles starting from benzaldehydes by a combination of efficient Henry and Michael reactions led to novel supramolecular liquid crystals. The mesogens are fluorescent 3,5‐dimethyl‐4‐(di or trialkoxyphenyl)pyrazoles and, in spite of the tapered shape of these molecules and their structural simplicity (only one phenyl ring), columnar liquid‐crystal phases were formed that are stable at room temperature. The self‐assembled structure was studied by XRD and the columnar cross section contains two molecules on average with an antiparallel arrangement of pyrazoles interacting through hydrogen bonds. In contrast, the single‐crystal structure of a trimethoxy analog did not show hydrogen‐bonded pyrazoles but chains of head‐to‐tail arranged molecules.     

Exploiting Host–Guest Interactions for the Synthesis of a Rod–Rod Block Copolymer with Crystalline and Liquid‐Crystalline Blocks

By making use of the host–guest interactions between the host molecule tris‐o‐phenylenedioxycyclotriphosphazene (TPP) and the rod–coil block copolymer (BCP) poly(ethylene oxide)‐block‐poly(octyl 4′‐octyloxy‐2‐vinylbiphenyl‐4‐carboxylate) (PEO‐b‐PVBP), the supramolecular rod–rod block copolymer P(EO@TPP)‐b‐PVBP was constructed. It consists of a crystalline segment P(EO@TPP) with a hexagonal crystalline structure and a columnar nematic liquid‐crystalline segment (PVBP). As the PVBP segments arrange themselves as columnar nematic phases, the crystalline structure of the inclusion complex P(EO@TPP), which has a smaller diameter, is destroyed. The self‐assembled nanostructure is thus clearly affected by the interplay between the two blocks. On the basis of wide‐ and small‐angle X‐ray scattering analysis, we conclude that the supramolecular rod–rod BCP can self‐assemble into a cylinder‐in‐cylinder double hexagonal structure.     

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self‐assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction provided insight into the unique self‐assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p‐alkoxyphenyl units led to bilayer‐type smectic mesophases, wedge‐shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.     

4‐Aryl‐3,5‐bis(arylethynyl)aryl‐4H‐1,2,4‐triazoles: Multitasking Skeleton as a Self‐Assembling Unit

The synthesis of a series of 4‐aryl‐3,5‐bis(arylethynyl)aryl‐4H‐1,2,4‐triazoles derivatives is reported and the influence exerted by peripheral substitution on the morphology of the aggregates generated from these 1,2,4‐triazoles is investigated by SEM imaging. The presence of paraffinic side chains results in long fibrillar supramolecular structures, but unsubstituted triazoles self‐assemble into thinner ribbons and needle‐like aggregates. The crystals obtained from methoxy‐substituted triazoles have been utilised to elaborate a model that helps to justify aggregation of the investigated 1,2,4‐triazoles, in which the operation of arrays of C?H???π non‐covalent interactions plays a significant role. The results presented herein demonstrate the ability of simple molecules to behave as multitasking scaffolds with different properties, depending on peripheral substitution. Thus, although 1,2,4‐triazoles without long paraffinic side chains exhibit optical waveguiding behaviour, triazoles endowed with peripheral paraffinic side chains exhibit hexagonal columnar mesomorphism.     

Dual Photo‐functionalized Amphiphile for Photo‐reversible Liquid Crystal Alignments

Without the conventional polymer‐based liquid crystal (LC) alignment process, a newly synthesized dual photo‐functionalized amphiphile (abbreviated as ADMA₁) was successfully applied as a robust photo‐reversible LC alignment layer by self‐assembly and photo‐polymerization. The LC alignment layer constructed by directly adding dual photo‐functionalized amphiphiles into LC media significantly cuts the manufacturing cost as well as opens new doors for the fabrication of novel electro‐optical devices.     

Helical arrangement of a hydrogen‐bonded columnar liquid crystal induced by a centered triphenylene derivative bearing chiral side‐chains

A hydrogen‐bonded helical columnar liquid crystal was synthesized, in which the helical structure is induced by a centered triphenylene derivative bearing chiral side‐chains. The triphenylene derivative, 2,6,10‐tris(carboxymethoxy)‐3,7,11‐tris((S)‐(‐)‐2‐methyl‐1‐butanoxy)triphenylene ( TPC4(S) ), and a dendric amphiphile, 3,5‐bis‐(3,4‐bis‐dodecyloxy‐benzyloxy)‐N‐pyridine‐4‐yl‐benzamide ( DenC12 ), were mixed in a 1:3 ratio to obtain a complex, TPC4(S)‐DenC12 . Analyses by ¹H‐NMR spectroscopy, diffusion ordered spectroscopy (DOSY), CD spectroscopy, infrared (IR) spectroscopy, polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X‐ray diffractometry revealed that TPC4(S)‐DenC12 self‐assembles to form helical columnar stacks in solution and a helical columnar liquid crystal in bulk. The hydrogen bonding between TPC4(S) and DenC12 is essential for the helical columnar organization, and the preference for a one‐handed helical conformation is likely derived from the steric interaction between the chiral side‐chains and the dendric amphiphiles in the packing of the hydrogen‐bonded columnar assemblies. Copyright © 2008 John Wiley & Sons, Ltd.