ANISOTROPIC REORIENTATION IN 4-N-ALKYLOXYBENZYLIDENE-4'-N-ALKYLANILINES BY DEUTERIUM NMR RELAXATION

In this paper the relevant dynamical parameters necessary for describing molecular reorientation of a single molecule in the mesophases of 4-n-alkyloxybenzylidene-4-n-alkylaniline (nO.m) liquid crystals are examined. The spectral densities of motion J1(0) and J2(20) were determined at 15.3 MHz by the Jeener-Broekaert method in partially deuterated 1O.4 and 5O.7 molecules. These results are discussed in the framework of rotational diffusion processes in anisotropic potentials. In particular, the anisotropic viscosity model of Freed as modified by Vold and Vold [J. Chem. Phys. 88, 1443 (1988)] is used to determine the spinning (D) and tumbling (D) diffusion constants of the molecule. In this model, molecules also rotationally diffuse around the nematic director to give another rotational diffusion constant D±. It was reported by Raman and infrared band-shape analysis that in several liquid crystals including 2O.4, the spinning motion of the molecules slows down in the nematic phase as the orientational order decreases. Observation of slowing down in the spinning motion of 5O.7 molecules based on the above model is reported. Molecular reorientation of 1O.4 molecules is found to be slow on the deuterium NMR time scale, thus causing frequency dependences in the spectral densities of motion. © 1990 The American Physical Society.