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.     

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.     

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.     

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.     

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.     

Steryl polyfluorobenzoate liquid crystals

Two series of steryl di- and tri-fluorobenzoate liquid crystals were synthesized and their phase transition behaviours studied by DSC and thermal polarizing microscopy. The results show that while para-fluorine substituents stabilize the mesogenicities, meta-fluorine substituents slightly depress the mesogenicities.     

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.     

Star-like discotic liquid crystals

The game of molecular engineering of liquid crystalline systems via a combination of molecular architecture and functionalization of mesogens has opened interesting perspectives in the field of liquid crystals. Here we report on the synthesis and characterization of star-like heptameric triphenylenes. The discotic subunits in these molecules have been arranged in a hexagonal fashion around a central discotic core, thus leading to a new type of molecular arrangement for discotic liquid crystals. These oligomers do not crystallize and combine a well-defined structure with a discotic polymer-like processing.     

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.     

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.     

Coumarin-based emissive liquid crystals

Calamitic luminophores are presented based on an elongated aryl-substituted coumarin core. The substitution pattern at terminal and lateral positions were systematically modified. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. The benzopyranones are miscible with acrylate-substituted reactive nematic mesogens. Uniaxial orientation of the mixed systems was achieved by surface-assisted alignment on polyimide orientation layers. The film anisotropy was stabilised by subsequent photocrosslinking. The crosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.     

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.     

Carbazole nematic liquid crystals

A number of calamitic 2,7-diary-N-alkyl-substituted carbazoles with an enantiotropic nematic phase have been prepared. Branching of the aliphatic chain attached to the nitrogen atom in the carbazole ring leads to significantly lower liquid crystal transition temperatures. These new materials show a lower ionisation potential than fluorene analogues and blue photoluminescence in solution and as thin solid films.     

Thermodilato-microscopy of liquid crystals

Liquid crystals exhibit successively several fluids and/or viscous phases and therefore microscopical methods play an important role for their characterization. No really efficient routine dilatometric method exist for rather small samples. By simple addition of a Michelson's interferometer objective and a photo-multiplier to the basic equipment for the investigation of liquid crystals — i.e. a polarizing microscope associated to a heating and cooling stage — dilatometric investigations as well as studies of phase transitions are getting accessible. In fact, when a droplet of liquid crystal, set up in a spherical stamp, is lightened through the interferometer with a quasi monochromatic source, equal thickness circular fringes are observable on the surface of the sample. A thermal expansion of the droplet leads to displacements of these fringes. The principle for the measurement of the volume changes consists by recording, versus the temperature, the variations of the light intensity of the central fringe. The volume of the sample, at given temperatures, is determined from photomicrographs of the whole interference feature. This volume is lower than 0.1 mm³. Experiments on the pure compound octyl-cyanobiphenyl show that the method is convenient for the detection of the phase transition, even if the transformation is weakly first order asS _A -N. Tested with a commercial ternary mixture the expansivity of phases and the transitions volumes changes can be deduced from experiments.

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Zusammenfassung Flüssige Kristalle besitzen mehrere flüssige und viskose Phasen, so dass zu ihrer Charakterisierung mikroskopische Methoden eine grosse Bedeutung besitzen. Zur Zeit besteht keine routinemÄssig einsetzbare dilatometrische Methode im Mikromassstab. Durch die Erweiterung eines Mikroskopes mit einem Interferometer nach Michelson und unter Einsatz eines Photoelektronenvervielfachers bei gleichzeitiger Verwendung eines Kühl- und Heiztisches werden sowohl dilatometrische Messungen als auch Untersuchungen von Phasenumwandlungen zugÄnglich.Es werden in der vorliegenden Arbeit Messungen an Oktyl-cyano-biphenyl und an einer ternÄren Mischung dargestellt und diskutiert.