Molecular orientational motion in discotic liquid crystals

Abstract

The spectral densities of motion for the aromatic and chain deuterons of the discotic mesogen hexahexyoxytriphenylene (THE6) have been reported in the literature for a frequency of 46 MHz. Most spectral densities J_p (pω₀, 90°) have been obtained from samples consisting of a planar distribution of domains in which the directors were perpendicular to the magnetic field Limited data J_p (pω₀) have also been available from single-domain samples with the director aligned parallel to the magnetic field. We have applied the small-step rotational diffusion model of Nordio et al. to the data from the aromatic deuterons of THE6-ard in its uniaxial columnar D_ho phase, to describe the spinning (D _∥, rotational diffusion constant about the planar normal to the disc) and the tumbling (D?, rotational diffusion constant of the planar normal) motions of the molecular core. Although this model has been successfully used for rod-like nematic liquid crystals, its use has not been attempted for discotic liquid crystals. The model seems to indicate that molecular reorientation has slowed down in the D_ho phase, giving frequency dependence to the spectral densities. This can be explained by the high orientational order of the molecules. We are able to account for the four spectral densities J ₁ (ω₀), J ₁ (ω₀, 90°), J ₂ (2ω₀) and J ₂(2ω₀, 90°) with a calculated ratio D∥/D? of about 1. This is quite different from that of rodlike liquid crystals.