Stochastic molecular motions in the nematic, smectic A and solid phases of 4,4’-di-heptyl-azoxybenzene as seen by quasi-elastic neutron scattering and 13C Cross-Polarization Magic-Angle-Spinning NMR

Molecular rotational dynamics in p,p(')-di-n-heptyl-azoxybenzene was studied by means of quasielastic neutron scattering (QENS) and C-13 cross-polarization magic-angle-spinning (CPMAS) NMR. Fast reorientation of the hydrogen nuclei was observed by QENS in the two liquid crystalline (LC) phases nematic and smectic A, as well as in the crystalline phase. The latter could not be restricted to the -CH3 rotations alone, and a clear indication was found of some other reorientation motions persisting in the crystal. Two Lorentz-type components convoluted with the resolution function gave an excellent fit to the QENS spectra in both LC phases. The narrow (slow) component was attributed to the reorientation of the whole molecule around the long axis. The corresponding characteristic time of similar to 130 ps agreed well with the values obtained in recent dielectric relaxation and H-2 NMR studies. The full width at half maximum of the broader (fast) component shows a quadratic Q dependence (Q is the momentum transfer). Hence the corresponding motions could be described by a stretched exponential correlation function and were interpreted as various "crankshaft-type" motions within the alkyl tails. The C-13 CPMAS experiments fully corroborated the QENS results, sometimes considered ambiguous in complex systems.