DYNAMICS OF LIQUID-CRYSTALS BY MEANS OF H-2-NMR - A COMPARISON BETWEEN 4,4'-BIS(HEXYLOXY)AZOXYBENZENE AND THE DERIVATIVE PD(II) COMPLEX AZPAC

In the present work we report a molecular dynamics investigation of th e two mesogens 4,4'bis-(hexyloxy)azoxybenzene (HL) and its Pd(II) acet ylacetonate derivative Azpac in their nematic phases. Deuterium Zeeman and quadrupolar spin-lattice relaxation times have been measured at 4 6.04 MHz on two isotopomers of Azpac (Azpac-d(4) and Azpac-d(26), deut eriated on the aromatic core and on the alkoxy chains, respectively) a nd on HL-d(4), an isotopomer of HL partially deuteriated on the aromat ic core, by means of the Wimperis pulse sequence. The spectral densiti es obtained from the measured relaxation times are discussed in terms of internal and overall molecular motions. A small step rotational dif fusion model for the overall molecular motions, superimposed on a free rotational model for internal motions, allowed diffusion coefficients for molecular spinning and tumbling and for phenyl ring rotations to be derived for HL; the same models were used in describing the dynamic s of Azpac.