Effect of the terminal branching structure of some liquid-crystalline biphenyl carboxylates on the stability of the antiferroelectric phase

Three different series of smectic liquid-crystalline biphenyl carboxylates have been prepared and their smectic properties investigated. The 'lateral' alkyl chain at the branching carbon was varied from methyl to hexyl for the homologous series of 4-(1-alkylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylates (series A). A smectic phase which has the same structure as the antiferroelectric phase (S-CA*) or 'anticlinic' phase (S-CA') was observed for all the members of the series except the pentyl homologue. The smectic layer thicknesses for the homologous series were found to increase as ascending the 'lateral' alkyl chain, indicating that the 'lateral' alkyl group of the series is in fact directed along the molecular long axis to some extent, which can occur if the molecule has a 'bent' structure. A 'swallow-tailed' compound, 4-(1-propylbutyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate, and the related 'ring-tailed' compounds, 4-(1-cycloalkyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylates, were also prepared (series B). The swallow-tailed compound showed an S-CA' phase, whereas the related ring-tailed compounds exhibited an ordinary S-C phase, indicating that steric interactions via the terminal alkyl chains stabilize the S-CA' phase. The biphenyl carboxylates possessing a 2-alkyloctyl group at the branching chiral centre, i.e., 4-(2-methylalkyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylates (series C), were found to exhibit no S-CA* phase.