Phase transition and dielectric studies of a three-ring homologous series with the -NCS terminal group

The synthesis route and phase properties for a new three-ring homologous se ries, 4-(trans,trans-4-alkylbicyclohexyl)benzene-isothiocyanates (nCCHBT) w ith n=2-7, are presented. The substances exhibit very broad range nematic p hases exceeding 200 K, and low viscosity with a low activation enthalpy. Th e dielectric properties of the nCCHBT materials are compared with those of their long known two-ring analogues (nCHBT). The static permittivity, epsil on(s parallel to) is considerably lower for the three-ring compounds. It is found that epsilon(s parallel to) decreases with n at temperatures as low as T-r = T/T-NI = 0.7. The relaxation time, tau(parallel to) shows the odd- even effect, the amplitude of which reverses on going from T-r=0.7 to 1. Th e activation enthalpy for molecular rotation about the short axis alternate s with n practically in the same manner for the corresponding members of bo th homologous series. It is argued that the present results are at odds wit h models assuming that the activation enthalpy consists of the viscosity an d nematic potential components.