Mesomorphic phase transition behaviour of photopolymerisable liquid crystalline triphenylene ether compounds

This report discusses the preparation and the unusual mesomorphism of three homologues of photopolymerisable triphenylene ether compounds. These homologues showed ‘a cold crystallisation’ on heating differential scanning calorimetry curves, and this phenomenon is attenuated with increase methylene units, spacers between the triphenylene core and the terminal photopolymerisable acrylate group. Classical textures of hexagonal columnar (Col_h) mesophase were observed by polarising optical microscopy for all three photopolymerisable mesophase compounds described here. Also described is the mesomorphism of their intermediates, hydroxylalkoxytriphenylenes. In some cases, the discotic columnar hexagonal mesophases were confirmed by wide angle X-ray scattering techniques.     

From colloids in liquid crystals to colloidal liquid crystals

ABSTRACT

Liquid crystals in combination with nanoparticles are a fascinating topic of research, because of the wealth of aspects and questions to study. These range from simple effects of nanoparticles on phase transitions and phase diagrams, to the tuning of physical properties, adding of novel functionalities, all the way to the formation of spontaneous order by nanoparticles themselves and the possibilities that templating has for future materials design and applications. This article intends to provide a flavour of the multiplicity, variety and diversity that these thermotropic and lyotropic systems have to offer in the area of materials development, which we believe will become increasingly important, especially for switchable non-display applications and nanotechnology. It is not intended to provide a conclusive overview, which would be a presumptuous attempt considering the limited space available, but rather to place our own work into a wider context and to point out some more recent developments and trends in liquid crystal – nanoparticle dispersions.     

Amphotropic liquid crystals

The liquid crystalline state is a fundamental organization of matter, which combines order and mobility on a molecular, supramolecular and macroscopic level. In many cases the molecules can show both thermotropic and lyotropic liquid-crystalline (LC) phases, which is described as amphotropic behavior. Block-copolymers, polyhydroxy amphiphiles, disc-like, rod-like, polycatenar and banana-shaped LC molecules are discussed with respect to their amphotropic behavior with specific and non-specific solvents. The interactions of salts with polyether chains, leading to halotropic mesophases, and the interaction of aromatic electron acceptor molecules with electron-rich aromatic molecular parts are discussed in relation to lyotropic mesomorphism induced by classical solvent molecules. Polyphilic amphotropic materials showing more complex mesophase morphology and amphiphiles showing a hierarchical order of different levels of order are pointed out as future directions.     

Mineral liquid crystals

The contemporary state of studying mineral liquid crystals has been analyzed. Such crystals are lyotropic aqueous or water–organic colloidal solutions, the dispersed phases of which are represented by nano- and microsized crystalline particles. The methods of production, structure, and physicochemical properties of these systems, as well as the influence of electric and magnetic fields on them, have been discussed in detail.     

Mineral liquid crystals

This review describes recent developments in the field of liquid–crystalline suspensions of mineral nanoparticles. New families of chemical compounds have been investigated in the last few years. The most common mesophases (nematic, lamellar and columnar) have now been discovered in dispersions of disc-like and rod-like nanoparticles. New research thrusts presently focus on more subtle thermodynamic effects such as those of polydispersity and gravity. The specific physical properties brought by the mineral building blocks to the liquid–crystalline phases are now being examined. Mesomorphic ordering of the nanoparticles is increasingly used in materials science for templating and for preparing composites.     

Germanium liquid crystals

Liquid-crystalline compounds containing germanium atoms were synthesised and assessed for liquid-crystalline properties. These new compounds generally possess smectic C phases, and many also possess nematic, smectic A and higher order smectic phases. The germanium-containing liquid crystals were incorporated into smectic C mixtures. These mixtures tend to exhibit little change in smectic C*?layer thickness over temperature. This characteristic is associated with de Vries smectic A materials, but measurements show that, although they have high smectic C stability, the materials' smectic cone angles are small. Measurement of smectic cone angle versus temperature of an exemplar material and its analogues containing carbon and silicon in place of the germanium, all show small cone angles which fall smoothly and extrapolate to zero as the smectic C*?to smectic A transition is approached. These measurements largely explain the observed small layer changes and establish that the materials are not first-order de Vries materials. They must be located elsewhere along the de Vries-orthogonal continuum of smectic A phases.     

Fluoroalkylated discotic liquid crystals

The concept of mesophase stabilization by partial fluorination of side chains has been extended to discotic systems. The mesophase structure is essentially unchanged, whereas the temperature range of the mesophase is strongly affected by the fluorinated side chains. Triphenylene substituted with only one partially fluorinated side chain exhibits a decrease of the clearing temperature, whereas for symmetrically substituted systems a broad range mesophase has been observed. This behaviour can be attributed to a segregation of the incompatible parts of the molecule giving rise to a stabilized columnar structure for the symmetrical substitution pattern.     

Supramolecular hydrogen-bonded liquid crystals

The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes that may exhibit thermotropic liquid crystalline character. Depending on the nature, number and position of the groups able to form hydrogen bonds, a diversity of supramolecular structures, both dimeric and polymeric, have been obtained, affording in turn various liquid crystalline phases. The structure and stability of these hydrogen-bonded supramolecular complexes and their relation to the observed liquid crystalline phases are the main topics of this review.     

Steryl polyfluorobenzoate liquid crystals

Two series of steryl di- and tri-fluorobenzoate liquid crystal materials were synthesized and their phase transition behaviours studied by DSC and thermal polarizing microscopy. The results show that para-fluorine substituents stabilize the mesogenicity whilst meta-fluorines depress it to a weak degree.     

Cholesterol-based hydrogen-bonded liquid crystals

Hydrogen bonding is a powerful tool for assembling molecules and building new liquid crystalline structures. In this study, non-symmetric dimesogens were prepared by intermolecular hydrogen bonding between rationally designed H-bond donor (3-cholesteryloxycarbonylpentanoic acid) and acceptor (4-(pyridine-4-ylmethyleneimino)phenyl 4-alkoxybenzoate) moieties. Their liquid crystalline properties were investigated by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. Cholesteric and smectic phases were observed. As for the covalently linked dimesogens, several types of smectic periodicities occur for these H-bonded cholesteryl compounds depending on the molecular parameters.     

Potential infrared liquid crystals

Principles for selecting potential infrared nematic liquid crystals are described. Optical and electro-optic properties of two cyanotolanes, 4-n-alkyl-4'-cyanotolanes and three commercial liquid-crystal mixtures were chosen for evaluation. Their applications for modulating infrared radiation are addressed.     

Bromine-terminated azobenzene liquid crystals

A series of liquid crystals (LCs) of bromine-terminated azobenzene were synthesised and characterised. The LCs were composed of an azobenzene core, an alkyl chain and a flexible spacer with a bromine atom at a remote position. Mesomorphic properties were dependent on both the alkyl chain length and the relative position of the bromine atom at the end of the spacer group. The nematic phase was favoured over the smectic A phase for alkyl chains with one and seven carbon atoms. However, the SmA mesophase was dominant for compounds with 10-carbon alkyl chains. The remote bromine atom in the spacer group favoured SmA for homologous with n-decyl chains and the nematic phase for n-heptyl and methyl groups. Molecular modelling showed that the azobenzene LCs tended to adopt the all trans-conformation in the gas phase as the number of carbon atoms increased. For short spacer groups, bent conformations contributed to the level population proportion of conformers. For the non-LC 5a, the gauche conformation became the most stable with a torsional angle of –68.9°. X-ray experiments showed a monolamellar SmA mesophase in an antiparallel arrangement. Absorption maxima at 360 and 440 nm were assigned to π–π* and n–π* transitions, respectively.     

Thermotropic diacetylenic liquid crystals

Further studies are reported on polar diacetylenes with conjugated side chains which show liquid-crystalline behaviour. Results obtained using differential scanning calorimetry, thermogravimetry, and hot stage microscopy are reported. One group of materials forms a smectic liquid crystal phase and all polymerize from a nematic liquid crystal phase.     

TTF-based bent-core liquid crystals

The synthesis and characterization of bent-core liquid crystals which incorporate TTF groups is reported; different bent-core mesophases are induced depending on the molecular structure and properties derived from their compact packing have been studied.     

Gel dispersed liquid crystals

Microemulsion based gels (MBG) can be used as carrier materials for dispersed thermotropic liquid crystals (LC). The viscosity of the carrier material can be specifically changed by varying the gelatin content. The LC droplet sizes and their distribution in the MBG system are influenced by both the ratio between AOT surfactant and alcohol co-surfactant and the length of the carbon chain of the co-surfactant. LC droplets without or with only a small amount of alcohol co-surfactant have the same droplet size and show a radial structure.     

Polycatenar oligophenylene liquid crystals

We report the synthesis of oligophenylene polycatenar liquid crystals incorporating 1,4‐disubstituted phenyl rings joined by a direct carbon carbon bond and some pyrimidine analogues. The nature of the linkages does appear to affect the mesomorphism significantly. The ratio of the aromatic core to the aliphatic chains is varied systematically by changing the number of 1,4‐disubstituted phenyl rings and the length of the aliphatic chains. This strongly influences the transition temperatures of the mesophases. Some of the compounds are columnar over an extended temperature range of more than 200°C with melting points below room temperature. We suggest that a combination of the poor overlap of the conjugated electron system of the molecular cores making up the columnar structure and the high concentration of aliphatic chains leads to a low charge‐carrier mobility.     

Heterocyclic polycatenar liquid crystals

We report the synthesis of polycatenar liquid crystals incorporating 2,5‐disubstituted 1 O’Neill, M. and Kelly, S. M. 2003. Adv. Mater., 15: 1135[Crossref], [Web of Science ®] , [Google Scholar] 3 Catry, C., van der Auweraer, M., de Schryver, F. C., Bengs, H., Häussling, K., Karthaus, O. and Ringsdorf, H. 1993. Makromol. Chem., 194: 2985[Crossref] , [Google Scholar] 4 Adam, D., Schumacher, P., Simmerer, J., Häussling, K., Siemensmeier, K., Etzbach, K. H., Ringsdorf, H. and Haarer, D. 1994. Nature., 371: 141[Crossref], [Web of Science ®] , [Google Scholar]‐oxadiazole and 1 O’Neill, M. and Kelly, S. M. 2003. Adv. Mater., 15: 1135[Crossref], [Web of Science ®] , [Google Scholar] 3 Catry, C., van der Auweraer, M., de Schryver, F. C., Bengs, H., Häussling, K., Karthaus, O. and Ringsdorf, H. 1993. Makromol. Chem., 194: 2985[Crossref] , [Google Scholar] 4 Adam, D., Schumacher, P., Simmerer, J., Häussling, K., Siemensmeier, K., Etzbach, K. H., Ringsdorf, H. and Haarer, D. 1994. Nature., 371: 141[Crossref], [Web of Science ®] , [Google Scholar]‐thiadiazole rings joined by a combination of carbon–carbon single and double bonds (–CH = CH–). The ratio of the aromatic core to the aliphatic chains was varied systematically by changing the number of the aliphatic chains, from two to six, and their length, from short to very long, i.e. from methoxy to hexadecyloxy. The shape anisotropy of the core was varied by exchanging the oxygen atom in the 1 O’Neill, M. and Kelly, S. M. 2003. Adv. Mater., 15: 1135[Crossref], [Web of Science ®] , [Google Scholar] 3 Catry, C., van der Auweraer, M., de Schryver, F. C., Bengs, H., Häussling, K., Karthaus, O. and Ringsdorf, H. 1993. Makromol. Chem., 194: 2985[Crossref] , [Google Scholar] 4 Adam, D., Schumacher, P., Simmerer, J., Häussling, K., Siemensmeier, K., Etzbach, K. H., Ringsdorf, H. and Haarer, D. 1994. Nature., 371: 141[Crossref], [Web of Science ®] , [Google Scholar]‐oxadiazole for a sulfur atom to form the corresponding 1 O’Neill, M. and Kelly, S. M. 2003. Adv. Mater., 15: 1135[Crossref], [Web of Science ®] , [Google Scholar] 3 Catry, C., van der Auweraer, M., de Schryver, F. C., Bengs, H., Häussling, K., Karthaus, O. and Ringsdorf, H. 1993. Makromol. Chem., 194: 2985[Crossref] , [Google Scholar] 4 Adam, D., Schumacher, P., Simmerer, J., Häussling, K., Siemensmeier, K., Etzbach, K. H., Ringsdorf, H. and Haarer, D. 1994. Nature., 371: 141[Crossref], [Web of Science ®] , [Google Scholar]‐thiadiazole ring with a smaller deviation from coaxiality of the bonds in the 2,5‐positions. The shape anisotropy of the core was increased by the presence of an additional phenylenevinylene unit in a series of tetracatenar oxadiazoles. We report the synthesis, physical properties and polymerization of a polycatenar reactive mesogen in a columnar phase to form a polycatenar polymer network.     

Polycatenar oligophenylene liquid crystals

We report the synthesis of oligophenylene polycatenar liquid crystals incorporating 1,4-disubstituted phenyl rings joined by a direct carbon carbon bond and some pyrimidine analogues. The nature of the linkages does appear to affect the mesomorphism significantly. The ratio of the aromatic core to the aliphatic chains is varied systematically by changing the number of 1,4-disubstituted phenyl rings and the length of the aliphatic chains. This strongly influences the transition temperatures of the mesophases. Some of the compounds are columnar over an extended temperature range of more than 200°C with melting points below room temperature. We suggest that a combination of the poor overlap of the conjugated electron system of the molecular cores making up the columnar structure and the high concentration of aliphatic chains leads to a low charge-carrier mobility.