Vibrational Study of the Phase Transitions in Three Triphenylene Hexa n-Alkanoates

Abstract

Triphenylene hexa-n-alkanoates (C_mHAT) present thermotropic structural phase transitions in the solid state, before giving rise to columnar mesophases and to the isotropic state. A temperature-dependence study of the infrared and Raman spectra of C₈HAT, C₁₀HAT and C₁₂HAT provides a description of the molecular conformations in the ordered phases and shows that the disordering process begins in the crystalline state. It implies angular deformations about the aliphatic carbon skeleton in the disorderd solid phases, while a very important conformational disorder appears at the crystal-mesophase transition, together with a change in the position of the ester plane with respect to the aromatic core. Dynamics of the alkanoates chains thus play an important part in the mechanism of the phase transitions.