Molecular dynamics has been studied by broadband dielectric relaxation spectroscopy in the $\mathrm{Sm}-A,$ $\mathrm{Sm}-B,$ and $\mathrm{Sm}-E$ phases (Sm denotes smectic) of a homologous series of nonchiral stilbenes. An assignment of modes is presented based on their dependence on temperature and molecular length, and, as far as they obey the Arrhenius law, their activation energy has been determined. In general, reorientations of entire molecules around their short axis are active, whereas reorientations of entire molecules around their long axis are locked out in the $\mathrm{Sm}-E$ phase of shorter homologs, yet intramolecular reorientations of polar sites have been established. Strong evidence is presented for an interdependence of reorientations of entire molecules around the short and long axes within the biaxial $\mathrm{Sm}-E$ phase of longer homologs.

• Received 15 July 1999

DOI:https://doi.org/10.1103/PhysRevE.61.3926