Symmetric and non-symmetric chiral liquid crystal dimers

Abstract

The synthesis and characterisation of three sets of non-symmetric liquid crystal dimers consisting of a cholesteryl-based unit and either 4-methoxybiphenyl, 4-cyanobiphenyl or (S)-2-methylbutyl 4′oxybiphenyl-4-carboxylate are described. The transitional properties of these non-symmetric dimers are compared to those of the corresponding parent symmetric dimers. The symmetric dimers exhibit only chiral nematic or nematic phase behaviour. By contrast, members of the non-symmetric dimer series containing either 4-cyanobiphenyl or (S)-2-methylbutyl 4′-oxybiphenyl-4-carboxylate groups exhibit smectic behaviour. The former series show an interdigitated smectic A phase while for the latter the structure of the smectic A phase depends on the length of the flexible spacer. Specifically, for short spacer lengths a monolayer arrangement is seen while for long spacers an intercalated smectic A phase is formed. For an intermediate spacer length, the small-angle X-ray scattering pattern for the intercalated smectic A phase reveals short-range incommensurate structural fluctuations associated with the monolayer smectic A phase. The driving force for the formation of the intercalated phase may be an electrostatic interaction between the electron rich carbonyl linking group attached to the cholesteryl unit and the electron deficient aromatic unit while the monolayer arrangement may be stabilised by the van der Waals interactions between the cholesteryl unit and the alkyl chains. Blue phases are observed only for a small number of these non-symmetric dimers and these all contain an odd-membered spacer. This is in accord with the rather general observation that blue phases are observed for odd-membered non-symmetric dimers and not their even-membered counterparts.