Molecular motion and orientational order in a nematic liquid crystal. An electron resonance investigation

Abstract

The electron resonance spectrum of the spin probe vanadyl acetylacetonate dissolved in the nematic mesophase of 4,4′-dimethoxyazoxybenzene has been studied as a function of temperature. The line shape is observed to deviate from a true lorentzian probably as a result of thermal fluctuations in the director orientation. An approximate expression for the true line shape in the presence of such fluctuations was, therefore, derived and employed to analyse the spectra to yield the line positions and, more importantly, their widths. A standard treatment of these line positions gives P₂, the component of the solute ordering matrix along the V—O bond. The dominant spin relaxation process contributing to the linewidths comes from modulation of the g and vanadium hyperfine tensors by the random molecular reorientation; theoretical expressions for the linewidth coefficients are obtained for vanadyl acetylacetonate in a liquid crystal with due allowance for the anisotropic environment and the large hyperfine interaction. The linewidth coefficients are found to depend on a new solute order parameter, P₄, for the V—O bond and analysis of the B and C coefficients gives P₄ as a function of P₂; the observed dependence is found to support a statistical mechanical model of liquid crystal solutions based on the molecular field approximation. The rotational correlation time is also obtained from the linewidth coefficients and found to decrease with decreasing temperature. This unusual behaviour is in qualitative agreement with the diffusion model for molecular reorientation in liquid crystals. Not all of the A linewidth coefficient can be accounted for with the assumed relaxation process, and the large excess linewidth is attributed to a spin-rotation relaxation mechanism. However, attempts to predict this excess linewidth using relations adopted from normal fluids are found to be in serious error and the spin-rotational correlation time appears to be much longer in a liquid crystal than in an isotropic phase under comparable conditions. Finally the experiments also provide the temperature dependence of the root-mean-square fluctuation in the director and this is found to be in good agreement with theory.