Terminal chains, chiral nematics

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. 

Similar structural modifications using chiral aliphatic chains (citronellyloxy and its hydrogenated analog) were performed, and these yielded complexes with a chiral nematic phase, though when both terminal chains were chiral, mesomorphism was suppressed. The phases typically occurred between 120 and 160 C. Siting the bulky rhenium fragment in one extremity of the molecule resulted in the destabilization of the mesomorphism, but not its suppression. 

Materials that generate the chiral nematic phase are very common and in many respects parallel those that give the nematic phase except that a chiral unit is present. Accordingly, it is the chiral unit that is of interest in this chapter. The chiral rmit is almost always found in the terminal chain because of the relative ease of synthesis (see Chapter 8). The inclusion of a chiral centre in the terminal chain invariably produces a sizeable steric effect (usually from a branched chain) which inevitably depresses the clearing point. So if... 

Using the same materials, chiral derivatives were prepared in which one or both of the terminal chains was replaced with (R)- or (5 )-citronellol where only one chiral chain was used, the other chain was butyloxy, octyloxy, or dodecyloxy (n, m = 4, 8, or 12). All complexes showed a chiral nematic phase (Figure 47). 

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... 

Many other structural possibilities for the terminal chains can have a great influence on melting point, transition temperatures and mesophase morphology. For example, branched chains are common, usually for the purpose of generating a chiral centre for chiral nematic or chiral smectic C liquid ciystals. The effect of the branch is to broaden the molecules and hence the transition temperatures are usually depressed significantly, often with the largest reduction in the smectic phase stability, but melting points are often not so much affected (compare compounds 29 and 30) [32]. 

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