OPTICAL-ACTIVITY STUDIES IN THE PRETRANSITIONAL ISOTROPIC REGION OF STRONGLY CHIRAL PROPIOLATE ESTER LIQUID-CRYSTALS

Optical rotation and circular dichroism measurements performed in the pretransitional isotropic region above the chiral smectic phases TGBA , TGBC, S-C*, and S-A* exhibited by 1-alkylalkyl 4'-(4 ''-n-alkoxyphe nylpropioloyloxy)biphenyl are reported. These results revealed a high degree of increasing chiral ordering on cooling in the isotropic phase , particularly in the temperature region where a broad DSC peak appear s. However, the behaviour of the pretransitional optical activity was unlike that commonly observed for chiral nematic or blue phase compoun ds and did not follow a simple Landau-de Gennes temperature dependence . Characteristic features of the pretransitional optical activity may aid in understanding the local structure of chiral smectic phases. Ele ctric fields did not seem to influence the results significantly, but strong electric field-induced optical rotation behaviour was demonstra ted for a mixture constituted of the laterally fluorinated propiolate ester component and a nematic with a large positive dielectric anisotr opy. These field-induced results could be fitted to expressions from t he existing theory of pretransitional optical activity. Circular dichr oism studies at low temperatures were performed for two compounds in d ilute solution and were found to exhibit very large changes in their e xtinction coefficient at wavelengths in the ultraviolet region, which may indicate strong chiral interactions even in dilute isotropic solut ions of these materials.