OPTICAL-ACTIVITY AND LIGHT-SCATTERING IN THE ISOTROPIC AND SMECTIC A-ASTERISK PHASES OF A HIGHLY CHIRAL SMECTIC LIQUID-CRYSTAL

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4'-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral-racemic mixture of S-442-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twist-grain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic-smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.