Chiral liquid crystal

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

Almost all the smectic phases, in which the molecules are arranged in layers and are tilted with respect to the layers, have counterpart chiral phases. The most important one of this class is the chiral smectic C phase — Sc phase. In these chiral liquid crystal phases, the molecules are tilted at a constant angle with respect to the layer normal but the tilt azimuthal rotates uniformly along the chiral axis and forms a helical structure. 

The local symmetry group of the Sc phase is a C2 group and thus the Sc phase has helical electricity. The spontaneous polarization, Ps, is perpendicular to the layer normal and molecular axis. Due to its helical structure Ps changes its direction uniformly, evolving along the helical axis so that the Sc phase does not show a measurable ferroelectricity except in the unwinding of its helical structure. The Sc phase is one of the very important liquid crystal phases that has a prospective application in fast response display. The detailed structure of the Sc phase will be shown in Chapter 6. 

Once the helical structure of the Sc phase is unwound, ferroelectricity is displayed (see Chapter 6 for the details). In recent years, many experimental studies have revealed that some liquid crystal compounds show new types of smectic phases with complex tilt and dipole order, such as the anti-ferroelectric smectic C phase, Sca phase, and the ferrielectric smectic C phase, Sc7 phase. For instance, in the Sca phase, the spontaneous polarization Ps is opposite for successive layers. It was found experimentally that the chiral So phase is in fact similar to the anti-ferroelectric Sca phase. 

Screw Dislocations Grain Boundaries Molecules LayeK 

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Chiral liquid crystals Chiral recognition Chiral separation Chiral separations Chiral shift reagents... 

The compounds crystallise in noncentrosymmetric space groups namely PI, P2i, C2, and P2i2i2i (but with priority of P2i) due to the chirality of the molecules. Most of the compounds have a tilted layer structure in the crystalline state. The tilt angle of the long molecular axes with respect to the layer normal in the crystal phase of the compounds is also presented in Table 18. Some compounds show larger tilt angles in the crystalline state than in the smectic phase. In the following only the crystal structures of some selected chiral liquid crystals will be discussed. 

A very different model of tubules with tilt variations was developed by Selinger et al.132,186 Instead of thermal fluctuations, these authors consider the possibility of systematic modulations in the molecular tilt direction. The concept of systematic modulations in tubules is motivated by modulated structures in chiral liquid crystals. Bulk chiral liquid crystals form cholesteric phases, with a helical twist in the molecular director, and thin films of chiral smectic-C liquid crystals form striped phases, with periodic arrays of defect lines.176 To determine whether tubules can form analogous structures, these authors generalize the free-energy of Eq. (5) to consider the expression... 

Chiral Liquid Crystals from Achiral Molecules Banana Phases... 

CHIRAL LIQUID CRYSTALS FROM ACHIRAL MOLECULES ... 

DR Hall, Use of Stereoscopic Systems Using Chiral Liquid Crystals, U.S. Patent 5,699,184, 1997. 

Xu Z, Gao C. Graphene chiral liquid crystals and macroscopic assembled fibers. Nat Commun. 

Chiral liquid crystals - [LIQUID CRYSTALLINE MATERIALS] (Vol 15)... 

A good deal in synthesis effort has been devoted to chiral liquid crystals, especially those w ith chiral smectic C phases. The chiral smectic C phase is ferroelectric. w hich gives it properties quite useful lor applications. Perhaps the most important properly of these phases is that a lateral dipnle can produce a spontaneous polarization... 

Scheme 5 Complexation of chiral liquid crystals 8 and 10 with alkaline metal salts...

De Feyter S, De Schryver FC (2003) Two-dimensional supramolecular self-assembly probed by scanning tunneling microscopy. Chem Soc Rev 32 393 Ernst K-H (2006) Supramolecular surface chirahty. Top Curr Chem 265 209 Perez-Garcia L, Amabrlino DB (2007) Spontaneous resolution, whence and wither from enantiomorphic solids to chiral liquid crystals, monolayers and macro- and supramolecular polymers and assemblies. Chem Soc Rev 36 941... 

Rey, A.D. Theory of linear viscoelasticity of chiral liquid crystals. Rheol. Acta 1996, 35 (5), 400-409. 

A. E. E. Wacbtler, Synthesis and Application of Chiral Liquid Crystals in Chirality in Industry II (A. N. Collins, G. N. Sheldrake, J. Crosby eds.), Jobn Wiley and Sons Ltd., 1997, 264-285. 

As alluded to earlier, chiral modifications are possible for some of the phases namely the N, Sc, Sp and Si. These may be generated either by doping a non-chiral liquid crystal with a chiral additive, or by resolving a racemic material exhibiting one or more of these phases. 

In this chapter we will review the recent advances of supramolecular photochirogenesis in various confined media, excluding micelles, chiral solvents, liquid crystals, metal complexes, polymer matrices, clays, and crystals. Micelles are a typical supramolecular assembly with an internal hydrophobic core which shows a unique boundary effect, e.g., enhanced radical recombination of geminate radical pairs produced by ketone photolysis [26], but essentially no asymmetric photoreaction has hitherto been reported in micelles. Photochemical asymmetric induction in chiral solvents [27,28] and chiral liquid crystals [29,30] have been known... 

Chirality is also an important aspect of liquid crystals. The introduction of chiral moieties into the chiral smectic phases induces functions such as ferroelectricity and antiferroelectricity. A few of the unconventional chiral liquid crystals are described in Chapter 1. The blue phase is one of the exotic chiral liquid crystalline phases. In Chapter 3, Kikuchi introduces the basic aspects and recent progress in research of the blue phase. Recently, the materials exhibiting the blue phases have attracted attention because significant photonic and electro-optic functions are expected from the materials. 

Oxidative photocyclization of l-(2-naphthyl)-2-(3-phenanthryl)ethylene has been carried out in a chiral liquid crystal and an optically active helicene obtained. A synthetically useful conversion of 2-()S-arylvinyl)pyrazines to azaphenanthrenes has also been described using the same oxidative procedure. The position at which ring closure occurs is dependent on the structme of the aryl group. 

Figure 3.5.3 Chiral liquid crystals without a chromophore may reflect colors because of light-scattering effects in helices of 400-700 nm pitches. Models of the cholesteric phase and the Schadt-Helfrich cell for liquid crystal displays are given. Two perpendicular polarization filters let light pass only if its direction of polarization has been rotated by the liquid crystals. If the hquid crystals are destroyed by an electric field, no light is transmitted, because the crossed polarizers quench it.

Chirality in liquid crystalline systems has become an increasingly important area for both technological and fundamental reasons [129] and a wide range of chiral liquid crystal dimers have been reported. These have included both symmetric [50, 74, 130-138] and non-symmetric dimers [74, 139-144] and the chiral centre has been placed either in the terminal chains or in the spacer. 

Chiral solvation agents, chiral solvents and chiral liquid crystals. The differentiation of two enantiomers by NMR is possible by studying them in the presence of a chiral solvent or in chiral liquid crystals. The molecule-solvent or molecule-liquid crystal interaction is sufhciently energetic for the diastereomeric association to have a lifetime longer than the NMR timescale. 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

Figure 3.1. Induced CD (curves 1-5) and UV (curve 6) spectra of substrate N-acetylaminocinnamic acid included into chiral liquid crystal matrix of cholesteryl tridecanoate in 1-butanol at 54 C.

Pavlov V.A., Spitsyna N.l. and Klabunovskii E.I. (1983) Enantioselective hydrogenation in the presence of cholesteryltridecanoate as chiral liquid crystal matrix, Izv. Acad. Sci. SSSR. Ser. Khim. (russ.) 1653-1656 Chem. Abstr. 1983, 99,194048). 

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