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

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The synthesis route and phase properties for a new three-ring homologous series, 4-(trans,trans-4-alkylbicyclohexyl)benzene-isothiocyanates (nCCHBT) with n=2-7, are presented. The substances exhibit very broad range nematic phases exceeding 200 K, and low viscosity with a low activation enthalpy. The dielectric properties of the nCCHBT materials are compared with those of their long known two-ring analogues (nCHBT). The static permittivity, epsilons is considerably lower for the three-ring compounds. It is found that epsilons decreases with n at temperatures as low as Tr=T/TNI=0.7. The relaxation time, tau shows the odd-even effect, the amplitude of which reverses on going from Tr=0.7 to 1. The activation enthalpy for molecular rotation about the short axis alternates with n practically in the same manner for the corresponding members of both homologous series. It is argued that the present results are at odds with models assuming that the activation enthalpy consists of the viscosity and nematic potential components.