Dynamics of the Chiral Liquid Crystal 4?-Butyl-4-(S)-(2-methylbutoxy)azoxybenzene in the Isotropic, Cholesteric, and Solid Phases: A Fast Field-Cycling NMR Relaxometry Study

1H NMR relaxometry was applied to investigate dynamic
processes in the isotropic liquid, cholesteric, and crystalline phases of the
chiral mesogen 4?-butyl-4-(S)-(2-methylbutoxy)azoxybenzene (4ABO5*). To
this aim, 1H longitudinal relaxation rates were measured as a function of
temperature (between 257 and 319 K) and Larmor frequency (from 10 kHz
to 35 MHz by a fast field-cycling relaxometer and at 400 MHz by an NMR
spectrometer). The NMR relaxation dispersion (NMRD) curves so obtained
were analyzed in terms of models suitable for the description of dynamic
processes in the different phases, thus quantitatively determining values of
characteristic motional parameters. In particular, internal and overall
rotations/reorientations, molecular translational diffusion, and collective
motions contribute to relaxation in the isotropic and cholesteric phases,
whereas, in the crystalline phase, relaxation is mainly determined by internal motions and molecular reorientations. The results were discussed and compared with those previously obtained on the same compound by dielectric relaxation spectroscopy.