Dynamic properties of discotic liquid crystals with triphenylene and benzene cores, studied by broadband dielectric spectroscopy

Abstract

Broadband dielectric spectroscopy in the frequency range from 10⁻⁴ to 10⁹Hz was employed to study the molecular dynamics of various triphenylene and benzene derivatives in their isotropic and liquid crystalline phases. For all derivatives, two relaxation processes were detected in their columnar mesophases. The β-process shows an Arrhenius-like temperature dependence with activation energies between 37 and 40 kJ mol⁻¹. This process is assigned to a librational motion of the ester groups in the side chains. The β-process follows a Williams-Landel-Ferry temperature dependence and is correlated to the glass transition. This process is assigned to the hindered rotation of the molecular discs around the columnar axis. In the isotropic phase, the dielectric properties of the benzene derivatives are essentially different from the triphenylenes, most probably because of the difference in capability of conformational rearrangement.