Broad band dielectric relaxation spectroscopy of molecular reorientation in smectic liquid crystalline phases (SmA, SmB, and CrE)

Broad band dielectric measurements reveal that the reorientation of non-chi ral rod-shaped low molecular mass liquid crystals is active around their mo lecular long axis and a short axis, in the smectic A and hexatic smectic B phase, respectively, as well as in the soft crystalline E phase of two isom eric stilbene compounds possessing an equal molecular length of their all-t l ans-conformations. One ('generalized') Arrhenius equation describes the t emperature dependence of the reorientation around a molecular short axis fo r each of these phases of both compounds. A change of the activation energy related to the reorientation around a molecular short axis is accompanied by a slowing down of the reorientation around the molecular long axis in th e soft crystalline E phase in one of these compounds, compelling evidence f or a coupling of both reorientations. This result is discussed with respect to the biaxiality of the soft crystalline E phase.