Liquid crystal chirality

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

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

The extraordinarily strong chiral properties of [nfhelicenes provide an impetus for the development of synthetic approaches to nonracemic [nfhelicenes for applications as organic materials. From this point of view, asymmetric syntheses of functionalized long [n]helicenes (n > 7), and also [n]helicene-like molecules and polymers with novel electronic structures and material properties, are important. The properties of helicenes related to materials are relatively unexplored, compared with the more synthetically accessible n-conjugated molecules and polymers. Notably, redox states of helicenes are practically unknown [33, 34]. Assembly of helicenes on surfaces, their uses as liquid crystals, chiral sensors, ligands or additives for asymmetric synthesis and helicene-biomolecule interactions are in the exploratory stages [35-43],... 

Some chiral ILs have been designed and synthesized. They have already been applied in different fields snch as asymmetric synthesis, stereoselective polymerization, chiral chromatography, liquid crystals, chiral resolution, and NMR shift reagents [20,106, 107]. Chiral solvents have been reported in asymmetric syntheses. In the BayUs-HUlman reaction of benzaldehyde and methyl acrylate in the presence of bases, chiral ILs demonstrate their ability in the transfer of chirality, even if the enantiomeric excesses (ee) are stiU moderate. The presence of an alcoholic functional group on the Af-alkyl-fV methylephedrinium is primordial and acts as a fixing point of the chiral IL on the reactants. It is assumed that the OH is connected... 

It should be noted that cholesteric liquid crystals (chiral nematics) having point group symmetry Dqo are also periodic with flie pitch considerably exceeding a molecular size. The preferable direction of the local molecular orientatiOTi, i.e. the director oriented along the Coo axis, rotates additionally through subsequent infinitesimal angles in the direction perpendicular to that axis. Hence a helical structure forms with a screw axis and continuous translation group. 

Figure 14.4 Examples of nematic liquid crystals, chiral dopants, and monomers.

Another type of LCD uses a chiral smectic C Uquid crystal instead of a nematic liquid crystal. Chiral smectic C liquid crystals are ferroelectric, spontaneously developing an electric polarisation parallel to the smectic layers. In an tmdistorted chiral smectic C liquid crystal, the polarisation is at 90° to the normal to the layers and rotates around the normal as the director rotates aroimd a cone centred on the normal to the layers. However, if the chiral smectic C liquid crystal is placed between properly prepared pieces of glass separated by only several micrometres, it is possible to establish a texture in which the director is parallel to the glass surfaces and uniform throughout the hquid crystal. In this texture, the smectic planes are perpendicular to the glass surfaces and the... 

Figure 17.7 Schematic showing the liquid crystal chiral nematic phase.

The second living system feature we select is that they are chiral. We take this to mean broken mirror symmetry. In liquid crystals, chirality is expressed as a macroscopic helix with a pitch, />(,. In cholesteric liquid crystals, 1500 A < Pq < oo, and qo, its wavevector (wavenumber qo = 2it/po), is perpendicular to n, the direction of orientational order. The existence of this intrinsic equilibrium length has several implications of which we mention two particularly insightful ones [1], [2]. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

The raw materials from which di-D-fructose dianhydrides can be obtained in appreciable yield are readily available from comparatively inexpensive agricultural feedstocks. Thus, these compounds are attractive as chiral-starting materials for chemical synthesis. Their stability to acid and heat, and their relative rigidity, because of the conformational constraints covered here, are also features that might be exploited during syntheses.119 A series of variously substituted di-D-fructose dianhydrides has been prepared,119 starting from 6,6 -dideoxy-6,6 -di-halosucroses. The properties of these and other derivatives of di-D-fructose dianhydrides are summarized in Tables XIV-XX. Two of these derivatives, 48 and 56, exhibit thermotropic liquid-crystal properties.119... 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

In the operation of ferroelectric liquid crystal devices, the applied electric field couples directly to the spontaneous polarisation Ps and response times depend on the magnitude E Ps. Depending on the electronic structure (magnitude and direction of the dipole moment as well as position and polarity of the chiral species) and ordering of the molecules P can vary over several orders of magnitude (3 to 1.2 x 10 ), giving response times in the range 1-100 ps. 

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

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. 

Mixtures of a nematic liquid crystal (LC or LC ) with small quantities of gold nanoparticles coated with alkylthiolates (<5 wt%) including an alkylthiolate functionalized with a chiral group have been studied (Figure 8.29) [72]. All mixtures show nematic mesophases with transition temperatures and phase stability very similar to those oftheliquid crystal precursors LC or LC. The introduction ofachiral center into the mixtures (mixtures of Au ) produce chiral nematic mesophases. A similar result is obtained in mixtures of Au and LC doped with the chiral dopant (s)-Naproxen. 

Omenat, A., Serrano, J.L., Sierra, T., Amabilino, D.B., Minguet, M., Ramos, E. and Veciana, J. (1999) Chiral linear isocyanide palladium(II) and gold(I) complexes as ferroelectric liquid crystals. Journal of Materials Chemistry, 9, 2301-2305. 

Burnham, K. S. Schuster, G. B. A search for chiral photochromic optical triggers for liquid crystals photoracemization of l,l -binaphthylpyran through a transient biaryl quinone methide intermediate. J. Am. Chem. Soc. 1998, 120, 12619-12625. 

The synthesis of technologically interesting ROMP materials using (233) includes the preparation of molecular wires,701 liquid-crystal polymers,702,703 chiral supports for catalysis,704 redox-active macromolecules,705 photochromic materials706 and embedded clusters of CdSe.707 Polymers... 

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

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