Deuterium N.M.R. relaxation and spectral densities in the discotic mesophase of hexahexyloxytriphenylene

Abstract

Longitudinal T _1Z and quadrupolar T _1Q , and transverse T ₂ relaxation times of deuterium in all the hydrogen sites of deuterated hexahexyloxytriphenylene (THE6), were measured over the whole stability range of the discotic mesophase. Two isotopic species were used for the measurements, i.e. THE6 deuterated in the aromatic sites, ar d THE6 perdeuterated in the aliphatic chains. Several experimental techniques were used in order to obtain the relaxation times, including the Jeener-Broekaert, selective inversion, and quadrupole echo pulse sequences. Measurements were mainly performed on samples in which the liquid crystalline domains were distributed in a plane with the directors perpendicular to the field direction. Experiments were also made on a single domain sample with the director parallel to the field direction. The experimental results were used to obtain several spectral densities J_p (pω) for both the parallel and perpendicular orientations, of all the deuterium sites in the THE6 molecule. The results are discussed in the light of existing theories.