An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

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Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

Transition metal liquid crystals: advanced materials within the reach of the coordination chemist

Phase transitions in liquid crystals

An overview over the current state of research in the field of biaxial nematic liquid crystalline materials is given. After a short introduction, providing some general aspects and summarizing the classical approaches, the main part focuses on recent developments of new concepts for designing biaxial nematics. First, the major theoretical concepts are outlined, including the classification to different symmetries, the importance of cooperativity and cluster formation for the development of biaxial order and the conditions for the establishment of field induced and spontaneous biaxiality in nematic phases. These new concepts also require the re-evaluation of the tools used for the identification of phase biaxiality, which are discussed briefly. In the second part, recent progress in the design of potential biaxial nematic materials, especially focussing on bent-core molecules with nematic phases, is reported and, finally, comparisons with phase biaxiality as observed in smectic liquid crystals are made.

Biaxial nematic phases

Biaxial nematic liquid crystals: fact or fiction?

High pressure, Xray and miscibility studies on binary mixtures of terminally substituted cyano and nitro compounds have led to the observation of two kinds of A-A transitions, viz., Ad-Ad and Ad-A1...

A New Kind of A-A Transition: Studies on Binary Mixtures of Terminally Substituted Cyano and Nitro Compounds

Miscibility, high pressure, Xray and dielectric studies on binary mixtures of terminally nitro and cyano substituted compounds are presented. The results show the existence of the Ad-Ad transition which occurs from a partially bilayer phase with a temperature-dependent layer spacing to another with a temperature-independent layer spacing.

Ad-Ad Transition in a Binary Liquid Crystal System

We have measured the static dielectric constants e∥ and e⊥ as well as the low-frequency dispersion of epar for mixtures of two eutectic compositions exhibiting a reentrant nematic phase over a range of concentration. The dielectric properties of a nematic mixture, whose concentration is close to the critical concentration ( C cr) at which the smectic phase appears, are found to be markedly different from those for a concentration far away from ( C cr). This can be understood as being due to an increase in the smectic like ordering in the nematic phase as the critical concentration is approached.

Effect of Smectic Ordering on the Dielectric Properties of Reentrant Nematic Mixtures

It is shown that by tuning the experimentally observable critical region, singularities of the phase boundaries can be observed in the immediate vicinity of the reentrant-nematic\char21{}smectic-C\char21{}smectic-A multicritical point. The exponents evaluated from these singularities are found to be identical to the universal exponents associated with the nematic\char21{}smectic-A\char21{}smectic-C multicritical point.

Universality of the reentrant-nematic-smectic-C-smectic-A and nematic-smectic-A-smectic-C multicritical points.

It is demonstrated experimentally that there can be a reversal of the sign of the dielectric anisotropy (Δϵ) with decreasing temperature in a nematic. The results are explainable in terms of the Maier-Meier mean field theory and indicate that the occurrence of the smectic phase and the consequent dipole correlations within the smectic layers is not a necessary condition for the sign reversal of Δϵ.

B. R. Ratna

Papers

A New Kind of A-A Transition: Studies on Binary Mixtures of Terminally Substituted Cyano and Nitro Compounds

Ad-Ad Transition in a Binary Liquid Crystal System

Effect of Smectic Ordering on the Dielectric Properties of Reentrant Nematic Mixtures

Universality of the reentrant-nematic-smectic-C-smectic-A and nematic-smectic-A-smectic-C multicritical points.

Near Neighbour Correlations and the Dielectric Properties of Liquid Crystals