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?

Optical studies have been carried out on a nematogenic copper complex, which incorporates the features of both rod-like and disk-like molecules. Conoscopic figures are presented demonstrating (i) the occurrence of a biaxial nematic phase in the pure complex, (ii) the uniaxial-biaxial (Na-Nb) transition in binary mixtures, and (iii) the temperature variation of the biaxiality near this transition. The I-Na-Nb phase diagram has been studied for the binary system.

A thermotropic biaxial nematic liquid crystal

The static dielectric constants of the following compounds have been measured as functions of temperature in their nematic and isotropic phases: (i) 4′-n-alkyl-4-cyanobiphyenyls, (ii) trans-p-n-alkoxy-α-methyl [pacute]-cyanophenyl cinnamates and (iii) CBOOA. All these exhibit a very large positive dielectric anisotropy because of the C≡N end group. For all the compounds the average dielectric constant ϵ evaluated from the measured values of ϵ ∥ and ϵ∥ is, throughout the nematic phase, less than the extrapolated isotropic value. Further, ϵ decreases with decreasing temperature, the effect being greter in compounds whose nematic phase is preceded by smectic phase. These facts are in conformity wiht the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. For the two homologous series (i) and (ii), the dielectric anistropy does not show an alternation with increasing chain length, but decreases continuously.

Dielectric Studies on Liquid Crystals of Strong Positive Dielectric Anisotropy

The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantse
 ‖ ande
 ⊥ as well as the anisotropy Δe decrease with increasing alkyl chain length. The experimental value of
 
$$\bar \in = \frac{1}{3}$$

 (e‖+2e⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.

Dielectric properties of 4′- n -alkyl-4-cyanobiphenyls in their nematic phases

Evidence of a first-order smectic-A--smectic-${C}^{\mathrm{*}}$ (SmA-Sm${C}^{\mathrm{*}}$) transition is presented. Detailed x-ray studies on 4-(3-methyl-2-chloropentanoyloxy)4'-heptyloxybiphenyl show that the SmA-Sm${C}^{\mathrm{*}}$ transition in this material is characterized by a discontinuous jump in the layer spacing (and hence in the tilt angle) and by a narrow two-phase region in which the density modulations due to the smectic-A and the smectic-${C}^{\mathrm{*}}$ phases coexist. Addition of a second compound drives the SmA-Sm${C}^{\mathrm{*}}$ transition towards second order.

Evidence of a first-order smectic-A-smectic-C transition and its approach to tricritical behavior.

We report a biaxial nematic liquid crystal in a simple thermotropic system.

B. R. Ratna

Papers

A thermotropic biaxial nematic liquid crystal

Dielectric Studies on Liquid Crystals of Strong Positive Dielectric Anisotropy

Dielectric properties of 4′- n -alkyl-4-cyanobiphenyls in their nematic phases

Evidence of a first-order smectic-A-smectic-C transition and its approach to tricritical behavior.

A biaxial nematic liquid crystal