Orientational ordering and dynamics in the columnar phase of a discotic liquid crystal studied by deuteron NMR spectroscopy

We report on a deuteron NMR study of quadrupolar splittings and spin-lattice relaxation times T-1Q and T-1Z as a function of temperature and at two different Larmor frequencies in the columnar phase of hexakis(n-hexyloxy)triphenylene (HAT6). The additive potential method is used to model the quadrupolar splittings, from which the potential of mean torque is parameterized, and the order parameter tensor for an "average" conformer is determined. The small-step rotational diffusion model is used to find the rotational diffusion constants D-parallel to and D-perpendicular to for the spinning and tumbling motions of the molecular core, It is found that D-perpendicular to is slightly larger than D-parallel to in contrast with the findings in calamitic liquid crystals. The decoupled model of Dong for correlated internal rotations in the end chains is used for the first lime in a discotic liquid crystal. Both jump constants for one- and three-bond motions are nearly independent of temperature, while the jump constant for two-bond motion is thermally activated, The relational speeds D-parallel to and D-perpendicular to are some two orders of magnitude slower than a typical charge hopping frequency between the aromatic cores of adjacent molecules in the columns, Thus, to a migrating charge, the "lattice" appears static with disorder being due to the instantaneous displacement of the cores with respect to each other. (C) 1998 American Institute of Physics. [S0021-9606(98)51210-X].