Reentrant phenomena in cyano substituted biphenyl esters containing flexible spacers

Abstract

A number of 4′-substituted phenyl 4-(4′-cyano-4-oxybiphenyl)butanoates have been synthesized to study the effects of the -O-(CH₂)₃COO- inter-ring linkage on mesomorphic behaviour. Transition temperatures have been determined and are compared with those of the phenyl cyanobiphenylmethanoate analogues. In general, the butanoates exhibit lower melting points but in all cases, a lower nematic-isotropic transition is observed. An interesting feature of some members of the butanoate series is the presence of several reentrant mesophases. A unique phenomenon observed while studying the behaviour of one of the butanoates was the appearance of smectic filaments from the isotropic liquid on cooling. These filaments, through an unusual process, eventually form the typical smectic A texture. Formation of an S_A phase from the isotropic liquid or the nematic phase in this fashion has not been reported previously.