Compounds chiral nematics

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

The simplest mesophase is the nematic phase. It is very fluid and involves highly disordered molecules having only short-range positional order, but with the molecules preferentially aligned on average in a particular direction (the director). If the constituent compound is racemic then it is possible to form a phase from the enantiomerically pure compound which is a chiral nematic phase. 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

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). 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

Just because a molecule is long, narrow, and meets the requirement of geometric anisotropy docs not ensure that it will have a liquid crystal phase. The particular phase structure that occurs in a compound, i.e.. smectic, nematic, or chiral nematic, not only dc >cnds on the molecular shape hut is intimately connected with the strength and position of the polar or polarizable groups within the molecule, the overall polarizability of the molecule, and the presence ol chiral centers. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Li and co-workers investigated derivatives of 43c,d [62]. Instead of the terminal heptyl chains, chiral (S)-2-methylbutyl or dodecyl chains were used. For the methylbutane side chain, a chiral smectic and a chiral nematic phase were observed and the clearing points were higher than in 43c,d due to the shorter chain while they were lower for the longer dodecyl side chains. It was also observed that the trans-compounds possess higher clearing temperatures compared to their c/.v-derivatives. 

Crown ether binaphthyl derivatives 128-131 (Scheme 71) were synthesized and investigated by Akagi [139], Compounds 128-131 were used to induce chiral nematic phases (N ) in liquid crystals. It was found that the helical twisting power increased with decreasing ring size. Helical polyacetylenes were synthesized in the N phases. It was found that the interdistance between the fibril bundles of the helical polyacetylene was equal to the half-helical pitch of the N liquid... 

One compound may exist in more than one liquid crystal form. The structure of cholestryl myristate changes from solid to smectic A at 71 °C, to chiral nematic at 79 °C, and finally to an isotropic liquid at 85 °C. Figure 16.10 shows the transitions in p-methoxybenzoic acid. Note that the order increases with increased pressure and lower temperature. Liquid crystal phases are more densely packed than the... 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

The pitch of the helix for compound 42 was found to be approximately 0.2-0.3 xm, thus the material selectively reflects visible light over a wide temperature range. Moreover, the pitch is relatively temperature insensitive thus the material can be used in large area non-absorbing polarizers, or in optical notch filters or reflectors. In addition, in the glassy state the helical macrostructure of the chiral nematic phases is retained, thus similar applications are possible. 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

Fig. 56 a fingerprint texture for an uncovered droplet of compound 47, and b the Grandjean texture of the chiral nematic phase... 

In contrast, compound 53 exhibits only one enantiotropic transition by DSC, which occurs between a chiral nematic phase and the isotropic liquid, in... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

Four chiral homologous complexes were also prepared. None of the palladium complexes showed mesomorphic properties, whereas a monotropic chiral discotic nematic phase was observed for the platinum complex (Table 18). The absence of mesomorphism for the dinuclear palladium complexes may be due to the type of chiral chain used, which differed from that used for the platinum system. All of the complexes form charge-transfer complexes with TNF. A Colh phase was induced for the two halo-bridged palladium complexes and for the platinum complex, as was observed for their non-chiral analogs. Flowever, the chiral nematic phase of the platinum compound was suppressed. At low TNF content, a chiral Nq phase was stabilized for the thiocyanato-bridged compound along with a non-chiral No phase at higher concentration. 

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