Synthesis, transition temperatures, some physical properties and the influence of linkages, outboard dipoles and double bonds on smectic C formation in cyclohexylphenylpyrimidines

Abstract

A trans-1,4-disubstituted cyclohexane ring has been introduced into known two-ring phenylpyrimidines to produce a wide variety of new three-ring Cyclohexylphenylpyrimidines. The length and type of the terminal chains and linking units have been varied systematically. The effect of introducing a carbon–carbon double bond of defined configuration into various positions of both terminal chains has also been investigated. The influence of lateral dipoles (i.e. oxygen and carboxyl groups) in different positions (central and terminal) in the molecular core of a model system on the smectic C (S_c) transition temperature has been studied and related in a simple empirical way to standard theories for S_c phase formation. Isolated, non-conjugated outboard dipoles (i.e. in cyclohexyl ethers and esters) have been found to destabilise the S_c and nematic (N) phases. Conjugated outboard dipoles (i.e. in phenyl ethers and esters) lead to substantial increases in the S_c transition temperature and usually to a widening of the S_c temperature range. Most of the new Cyclohexylphenylpyrimidines exhibit a variety of smectic phases as well as S_c and N phases. Several homologous series of the most interesting Cyclohexylphenylpyrimidines incorporating oxygen atoms or carboxy groups and/or a carbon–carbon double bond were synthesized and found to exhibit a relatively wide-range S_c phase at elevated temperatures. In admixture with a chiral smectic C (S_c* base mixture, some of the new three-ring cyclohexylphenylpyrimidines can induce a substantial increase in the S_c* and N transition temperatures without increasing the viscosity (and thus response times) excessively.