X. Shen et al., ORIENTATIONAL ORDERING AND DYNAMICS IN THE COLUMNAR PHASE OF A DISCOTIC LIQUID-CRYSTAL STUDIED BY DEUTERON NMR-SPECTROSCOPY, The Journal of chemical physics, 108(10), 1998, pp. 4324-4332
We report on a deuteron NMR study of quadrupolar splittings and spin-l
attice 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 ''avera
ge'' conformer is determined. The small-step rotational diffusion mode
l is used to find the rotational diffusion constants D-parallel to and
D-perpendicular to for the spinning and tumbling motions of the molec
ular core, It is found that D-perpendicular to is slightly larger than
D-parallel to in contrast with the findings in calamitic liquid cryst
als. 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 indep
endent of temperature, while the jump constant for two-bond motion is
thermally activated, The relational speeds D-parallel to and D-perpend
icular to are some two orders of magnitude slower than a typical charg
e hopping frequency between the aromatic cores of adjacent molecules i
n the columns, Thus, to a migrating charge, the ''lattice'' appears st
atic with disorder being due to the instantaneous displacement of the
cores with respect to each other. (C) 1998 American Institute of Physi
cs. [S0021-9606(98)51210-X].