Liquid-crystalline compounds with lateral aromatic branches

Abstract

We present the synthesis of 2-substituted hydroquinone-bisbenzoates which have large 2-substituents containing aromatic and other ring systems. Contrary to the general accepted opinion these large lateral substituents which cause remarkable deviations from the rod-like shape of the molecules do not prevent the liquid-crystalline properties, the compounds are nematic and smectic. The influence of different chemical groups on the liquid-crystalline properties is investigated systematically. The compounds tend to exhibit the glassy nematic state above room temperature. This property may be used for the construction of thermo-electrooptic devices.     

Liquid-crystalline ionomers

Liquid-crystalline (LC) ionomers combine the properties of ionomers (cluster formation of the ionic groups) with LC phases. They can be prepared with LC main-chain and LC side-chain polymers.     

Liquid-crystalline terpyridines

5,5"-Disubstitution of the terpyridine core leads to the first inherently liquid-crystalline terpyridines. Mesophases characteristic of bent-core and calamitic systems may be obtained depending on the core structure employed.     

Liquid-crystalline silsesquioxanes

Liquid-crystalline silsesquioxanes were synthesized by combining mesogens with spherical silsesquioxanes. Most of the synthesized compounds exhibit smectic phases. Lateral connection of a mesogen results in a nematic phase. All phases are optical uniaxial positive, i.e. calamitic structures are favoured. LC-silsesquioxanes represent a new group of liquid-crystalline side-chain polymers with backbones ranging from centrosymmetric cage like structures of siloxanes or heterosiloxanes to linear ladder structures. They can be considered as the link between the linear side-chain polymers and surface treated silica or quartz.     

Liquid-crystalline physical gels

Liquid-crystalline (LC) physical gels are a new class of dynamically functional materials consisting of liquid crystals and fibrous aggregates of molecules that are called "gelators". Liquid-crystalline physical gels, which are macroscopically soft solids, exhibit induced or enhanced electro-optical, photochemical, electronic properties due to the combination of two components that form phase-separated structures. In this tutorial review, we describe the materials design and structure-property relationships of the LC physical gels. The introduction of self-assembled fibers into nematic liquid crystals leads to faster responses in twisted nematic (TN) mode and high contrast switching in light scattering mode. Furthermore, the LC physical gels can be exploited as a new type of materials for electro-optical memory. This function is achieved by the control of reversible aggregation processes of gelators under electric fields in nematic liquid crystals. Electronic properties such as hole mobilities are improved by the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. The incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.     

Liquid-crystalline main-chain elastomers

A main-chain elastomer was synthesized for the first time by a cross-linking hydrosilylation of a cyclosiloxane with a new main-chain liquid crystalline semiflexible polyether. Imposing a uniaxial mechanical field to the unstretched nematic polydomain sample results in a macroscopic alignment of the director. For this process the sample has to be stretched by a factor of about 5, which considerably exceeds the effect observed for nematic side-chain elastomers.     

Liquid-crystalline fork-like dendrons

ABSTRACT

Dendritic molecules having several rigid-rod moieties can be applied to induce liquid crystallinity for a variety of non-mesomorphic functional molecules such as metal complexes, nanoparticles, fullerenes and π-conjugated molecules when these dendritic molecules are covalently bonded to those non-mesomorphic molecules. These complex molecules are called supermolecular liquid crystals. Due to the cooperation of several mesogenic moieties, these dendritic molecules exhibit very stable liquid-crystalline (LC) phases. We have used fork-shaped LC dendrons having two or three rigid-rod moieties to induce liquid crystallinity for functional molecules such as interlocked molecules and π-conjugated molecules. In these fork-like molecules, the rigid-rod cores are attached to the 3,4,5-position of the phenyl moieties through flexible spacer, and these molecules are bonded to functional molecules through the 1-position. They basically form smectic LC phases, which induce the layered arrangement of functional moieties. Here we report on a new family of fork-like mesogens containing a hydrogen bonding moiety or an ionic group. They are designed to build supramolecular materials.     

Liquid-crystalline phthalocyanines revisited

A Commentary on the paper “Homologous series of liquid-crystalline metal free and copper octa-n-alkoxyphthalocyanines„, by J. F. van der Pol, E. Neeleman, J. W. Zwikker, R. J. M. Nolte, W. Drenth, J. Aerts, R. Visser and S. J. Picken. First published in Liquid Crystals, 6, 577-592 (1989).     

Liquid-crystalline quinoline derivatives

A number of novel liquid crystals have been synthesized. 2-Phenylquinolines with alkyl or alkoxyl substituents in positions 6 and 4' show a texture typical of nematic mesophases and are characterized by a greater thermal stability than Schiff bases bearing the same substituents. The possibility of lowering the melting temperatures by means of eutectic mixtures is discussed.     

Liquid-crystalline fullerene-oligophenylenevinylene conjugates

Functionalization of C60-oligophenylenevinylene derivatives with a cyanobiphenyl-terminated dendromesogen leads to new donor-acceptor systems with liquid-crystalline properties.     

Liquid-crystalline phthalocyanines revisited

A Commentary on the paper ”Homologous series of liquid‐crystalline metal free and copper octa‐n‐alkoxyphthalocyanines?, by J. F. van der Pol, E. Neeleman, J. W. Zwikker, R. J. M. Nolte, W. Drenth, J. Aerts, R. Visser and S. J. Picken. First published in Liquid Crystals, 6, 577‐592 (1989).     

Liquid-crystalline glucamine derivatives

The aminoalditol glucamine was transformed into N-alkylated derivatives and also into ammonium salts of various organic acids. Studies of the liquid-crystalline properties revealed a series of novel smectic and discotic mesophases. Rules generally applicable to the occurrence of the various phases are presented.     

Liquid-crystalline symmetric triblock copolymers

Symmetric fully liquid-crystalline triblock copolymers of various structures containing optically active mesogenic groups are for the first time synthesized via pseudoliving radical reversible addition-fragmentation chain-transfer polymerization. Their phase behavior and physicochemical and optical properties are studied. It is shown that, depending on composition, at low temperatures block copolymers can form at temperatures phase-separated structures caused by microsegregation of blocks of different chemical natures and that, with an increase in temperature, these structures can mix to form a cholesteric mesophase characterized by a helical supramolecular structure. A model illustrating the molecular packing of block copolymers with a phase-separated lamellar structure is advanced. The effect of the molecular structure of the block copolymers on their optical properties is discussed.     

Liquid-crystalline 1,4-disubstituted cycloheptanes

Abstract

A new series of liquid-crystalline alicyclic compounds, the 1,4-disubstituted cycloheptane derivatives, have been synthesized. The physicochemical and electrooptical characteristics of the new mesogens have been compared with those of analogues in the cyclopentane and cyclohexane series, including cyclohexanes having a side methyl group.     

Liquid-crystalline 1,4-disubstituted cycloheptanes

A new series of liquid-crystalline alicyclic compounds, the 1,4-disubstituted cycloheptane derivatives, have been synthesized. The physicochemical and electrooptical characteristics of the new mesogens have been compared with those of analogues in the cyclopentane and cyclohexane series, including cyclohexanes having a side methyl group.     

Liquid-crystalline polymers for fibres

Recent advances in the development of liquid-crystalline polymers for fibres are reviewed. The discussion of lyotropic systems concentrates upon poly(p-phenylene terephthalamide) and its copolymer with 3,4′-oxydiphenylene terephalamide units. The discussion of thermotropic systems identifies specific problems restricting the development of fibres and suggests some possible means of improvement.     

Liquid-crystalline ordering in polymer brushes

The mean-field theory of liquid crystalline (LC) ordering is developed for a polymer brush immersed in a solvent. Additional attraction between neighbouring parallel mesogenic segments of the grafted chains is taken into account. It is shown that LC ordering in this brush is connected with the loss of solubility and occurs as a discrete first-order phase transition which is similar or even identical to the transition in polymer solution.