The Influence of Lateral Fluoro-Substituents on TGB Phases in Chiral Propiolates

Abstract

The transition temperatures and associated enthalpies of chiral and racemic 1-methylheptyl 4′-[(2- or 3-fluoro-4-alkoxyphenyl)propioloyl-oxy]biphenyl-4-carboxylates are reported along with phase diagrams for binary mixtures of (R)- and (S)-isomers. The position of the fluoro substituent was found to influence the formation of twist grain boundary phases both in the enantiomers and in binary mixtures of enantiomers. DSC studies revealed that the enantiomeric 3-fluoro compounds possessed a diffuse liquid-liquid transition above the clearing point, which did not arise for the racemates or for the 2-fluoro analogues. Circular dichroism and optical rotation studies carried out over the temperature range of the diffuse liquid phase, confirm that a form of cybotactic, chiral organisation in the isotropic liquid is responsible for this phenomenon.