The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.

https://www.mdpi.com/2073-8994/6/2/444/pdf

Chiral Liquid Crystals: Structures, Phases, Effects

Chirality in liquid crystal systems is a complex and sometimes difficult concept to understand and appreciate. In this article we review some of the reduced symmetry aspects of calamitic liquid crystal phases with reference to point symmetry, space symmetry, and helicity. We utilize these concepts in the discussion of new results obtained on Twist Grain Boundary phases, inversions in chiral dependent properties such as the helicity in cholesteric and smectic C* phases and the spontaneous polarization in ferroelectric liquid crystals, and antiferroelectric behaviour in low molar mass systems. Our observations suggest that many novel effects found in chiral liquid crystals are a result of conflicts between the effects of reduced symmetry and the desire to form normal liquid-crystalline structures, i.e., a form of frustration exists.

Chirality and Frustration in Ordered Fluids

A novel approach to designing liquid crystal dimers with high chirality is described. The synthesis, and liquid crystal properties of several new optically active unsymmetrical fluorescent dimers belonging to four homologous series are presented. These new oligomesogens possess pro-mesogenic cholesterol as a chiral moiety covalently linked to a fluorescent bent aromatic (chalcone) core through a trimethylene (4-oxybutanoyl)/tetramethylene (5-oxypentanoyl)/pentamethylene (6-oxyhexanoyl)/heptamethylene (8-oxyoctanoyl) central spacer giving rise to four series of chiral dimers. In each series, the chalcone entity is substituted with ethoxy, n-butyloxy to n-dodecyloxy tails, with the aim of learning the relation between the molecular structure and thermotropic behavior. The molecular structures have been characterized by routine characterization techniques. These compounds are fluorescent, as evidenced by recording the emission spectra for two different excitation maxima for some representative compounds. The...

Frustrated Liquid Crystals: Synthesis and Mesomorphic Behavior of Unsymmetrical Dimers Possessing Chiral and Fluorescent Entities

A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4′-(4″-alkyloxy-3″-fluorobenzoyloxy)tolanes (nFBTFO1M7). For the first time, the helical SA∗ phase or TGBA phase is found in all of the derivatives from heptyloxy to octadecyloxy. The SA-SA∗-N∗ phase sequence is observed in several compounds with short chains and the SC∗-SA-SA∗-N∗ phase sequence is obtained with the decyloxy derivative. The TGBA phase has filament or cholesteric textures. The helical pitch of the TGBA phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGBA phase shows that it is incommensurate with the fully elongated molecular length.

New TGBA series exhibiting a SC∗SASA∗N∗ phase sequence

We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in the NVT ensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parameter c describing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constant c, we obtained a phase region showing characteristics of the recently discovered helical smectic A∗ phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small sy...

Computer simulation of chiral liquid crystal phases. I. The polymorphism of the chiral Gay-Berne fluid

Based on a mean field analysis of the Chen-Lubensky model we extend an earlier discussion regarding the effect of chirality on the region of the phase diagram where the Cholesteric, Smectic-A, and Smectic- C* phases meet (i.e., the N* AC* point). We show that, on the Sm-C* side of the phase diagram, a second twist grain boundary (or chiral smectic) phase occurs. This phase, the “TGB C ”, is a highly dislocated version of Smectic-C. According to the deGennes analogy, this phase corresponds to Abri- kosov's flux lattice when the Ginzburg parameter K is negative. We also show that an Ising like transition between the TGB A and TGB C occurs and that this phase boundary, together with the Sm-A/TGB A Sm-C/TGBc, and the x-y like Sm-A/Sm-C* phase boundaries, meet at a multicritical point.

S. R. Renn

Papers

Existence of a Sm-C Grain Boundary Phase at the Chiral MAC Point