Moieties chiral nematics

The complementary materials 52 and 53, based on a central scaffold made up of pentaerythritol and tris(hydroxymethyl)aminomethane units linked together, where one unit carries three cyanobiphenyl (CB) (smectic preferring), see Fig. 61, and the other three chiral phenyl benzoate (PB) (chiral nematic preferring) mesogenic moieties, see Fig. 62, or vice-versa were investigated [115], see Fig. 63. 

Other metallomesogens containing the optically active butadienetricarbonyliron(O) moiety, incorporated into promesogenic chiral nematic ligands derived from cholesteryl, have been reported. " Both the diastereoisomers exhibited a chiral nematic phase and a monotropic smectic phase, the chiral nematic phase being monotropic for the (-l-)-isomer (Cr 193 (S 93 N 131) I), and enantiotropic for the (—(-isomer (Cr 117 (S 99) N 133 I). 

Yelamaggad et al reported the more elaborate acetylide complexes 164 (n = 3, 4, 5, 7), all of which showed a chiral nematic phase due to the presence of the chiral cholesteryl moiety. For all complexes, decomposition occurred somewhere above 200 °C—the lowest melting point was seen for the complexes with the two heptamethylene spacers. 

The chiral nematic state, first observed in cholesteryl derivatives, was later detected in other chiral mesogens, and can also he induced hy adding small chiral molecules to a host nematic TCP. Chiral nematic LCPs have thus heen synthesized as side-chain polymers hy introducing a chiral unit in the tail moiety of mesogens or hy copolymerizing cholesterol-containing monomers with another potential mesogenic monomer. Examples [4] of these types are shown as structures (V) and (VI) ... 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

Vanin et al. studied order proflles of the perdeuterated laurate chain in chiral and achiral phases by means of deuterium NMR. The quadrupole splittings measured for the sequence of CD2 moieties are all quite similar for the nematic and chiral nematic phase the conclusion was that chiral distortions of the micellar shape must be very small. Their proposal for the mechanism behind the phase chirality is analogous to the one for polypeptides an asymmetric point charge distribution which in this case was speculated to be created by the chiral dopant on the micellar surface [19]. Radley and Tracey prepared samples from a racemic detergent and observed high fluidity, whereas those consisting of some enantiomeric excess showed a significantly increased viscosity [30]. But this is no proof for the association of micelles to necklaces. 

Despite the wide variety of chiral nematic liquid crystals (and of course low molar mass chiral liquid crystals in general), it is possible to sub-divide them into three class types, according to the relationship of the chiral moiety and the liquid-erystalline core [29]. Firstly, we consider type I here the chiral centre (or multiplieity of chiral centres) is situated in a terminal alkyl chain attached to the effective liquid-crystalline core. As will be seen later, it is entirely possible that the compound can have two or more chiral terminal groups associated with the molecule. Type I materials are probably the most frequently encountered chiral nematic liquid-crystalline compounds, often... 

Typical examples of chiral azobenzenes are (/f)-4-(2-methylbutyl)-4 -substituted azobenzenes (9-13) which appeared with the emergence of synthetic methods which allowed the preparation of chiral precursors, such as (-i-)-2-methylbutylbenzene [51-54]. In the case of compound 12, the presence of two chiral terminal moieties was found to increase the helical twisting power ()8) by a factor of two [51]. Both the compounds 9 and 10 show enantiotropic chiral nematic phases which have relatively low clearing points, unlike compounds 11 and 12 which are not liquid crystalline. 

The ability of the chiral 2-methylbutyl moiety to generate very short pitch chiral nematic phases was by now widely recognised. It was not therefore surprising that a number of research groups should attempt... 

Initial work revealed that chiral nematic phases were only obtained when the carboxylic acid moiety of RHPP was converted into either a simple alkyl ester or a cyano group. 

Compound 120 shows a short monotropic chiral nematic and smectic A phase, whilst 121 shows a short enantiotropic chiral nematic and a monotropic smectic A phase. From this it may be concluded that the lactic acid derived ( )-2-oxypropanoy-loxy moiety is not particularly suited to sustaining liquid crystal phases this may either be partly due to poor conjugation between the liquid crystalline cores or because of the non-linear nature of molecules. 

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