Type II Chiral Nematic Liquid Crystals

The first examples of this kind of material belong to a homologous series of bis-azo-methines (113-114) which have the general structure represented in Table 9 [101]. The materials were prepared by esterification of (/ )-3-methyladipic acid with 4-hy-droxybenzaldehyde to the his-[4-formyl-phenyl] (/ )-3-methyladipate. This was then condensed with the appropriate 4-alkoxy-aniline to give the desired i is-azomethine dimer. In this homologous series, only the early members (ethoxy, butoxy and hexy-loxy, compounds 113 to 115) showed chiral nematic phases the transition temperatures of these materials are given in Table 9. 

2 Novel Highly Twisting Phenyl and 2-Pyrimidinylphenyl Esters of (/ )-3-Methyladipic Acid 

A more recent continuation of this theme using (i )-3-methyladipic acid, was the systematic study of dimeric or twin structures which are capable of forming highly twisted helical structures [102-105]. These examples, differ from the previously mentioned Z7/5-azomethines in that they employ 2-pyrimidinylpheny] (116 and 117) and phenyl core units (118) they were made simply by esterification of (/ )-3-methyl-adipic acid with the appropriately substituted phenol using dicyclohexylcarbodiimide (DCC) and the catalyst 4-iV,iV-dimethyl-aminopyridine (DMAP). The structures and phase transitions of members of these two classes are given in structures 116 to 118. 

It has been suggested that the marked difference in pitch lengths between the twins and monomeric materials is due in part to the further restriction of rotational motion about the chiral centre caused by the presence of a second bulky core moiety, result- 

Other attempts have been made to employ the (/ )-2-oxypropanoyloxy moiety as a chiral spacer between identical [106] and dissimilar [71, 107] liquid-crystalline core systems, typical structures are given by compounds 120 and 121. 

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