Deuteron NMR study of dynamics and order in the columnar phase of a fluorinated discotic liquid crystal

Measurements have been made of deuteron quadrupolar splitting and spin-latt ice relaxation times T-IQ and T-IZ in the columnar phase of a ring deuterat ed 1-fluoro-2,3,6,7,10,11-hexahexyloxytriphenylene (F-HAT6) as a function o f temperature and at two different Larmor frequencies. The disc-like molecu les are stacked, with only short range positional order, into columns which are arranged on a 2-dimensional hexagonal lattice. To describe small step reorientations of these molecules, a rotational diffusion model which uses a space-fixed frame to diagonalize the molecular diffusion tensor is adopte d. The principal diffusion constants in this so called 'anisotropic viscosi ty model' D-alpha and D-beta are for rotations of a molecule around and per pendicular to the columnar axis, respectively. A global target analysis of the spectral densities J(1)(omega(0)) and J(2)(2 omega(0)) at seven tempera tures and two Larmor frequencies in a single minimization procedure was car ried out. It was found that D-alpha > D-beta, which is consistent with the picture that the motion towards or away from the local director (i.e., the columnar axis) tends to disrupt the packing of molecules within the column. Furthermore, the activation energies E-a(alpha) and E-a(beta) for these mo tions were found to be 67.7 kJ mol(-1) and 56.3 kJ mol(-1). That these valu es are higher in comparison with those for HAT6 is interesting, and believe d to be a result of a strong lateral dipole in F-HAT6.