Rotational dynamics of a discotic liquid crystal studied by deuteron spin relaxation

Abstract

We report on measurements of deuteron quadrupolar splitting and spin-lattice relaxation times T_1Q and T_1Z in the columnar phase of a ring-deuteriated hexaoctyloxyrufigallol at 46 MHz as a function of temperature. To describe small-step rotations of these molecules within each column in the columnar phase, a space-fixed frame is used to diagonalize the molecular diffusion tensor. The principal diffusion constants in this so called 'anisotropic viscosity' model D_alpha and D_beta are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities at seven temperatures in a minimization procedure was carried out. We found that D_alpha > D_beta, which is consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column.