Dielectric properties of liquid crystalline isothiocyanato-tolane derivatives with fluorine atom at various lateral positions

Dielectric studies were performed for several isothiocyanato-tolane compounds having the F atom attached to the benzene rings at different lateral positions and the alkoxy or alkyl chain at the p-position. They form the nematic phase in broad temperature intervals. The static as well as dynamic properties of the compounds in the nematic and isotropic phases were studied. Tensor permittivity components: epsilon(parallel to), epsilon(perpendicular to), Delta epsilon, as functions of temperature were measured. The complex dielectric permittivity, epsilon*(f) = epsilon'(f)-i epsilon"(f), was measured in the frequency range of 1 kHz - 3 GHz from which the relaxation times tau(parallel to) and tau(perpendicular to) were calculated. The order parameter S(T) was determined from the dielectric data. It was found that the close vicinity of the F atom and the alkoxy group results in blocking of the internal rotation of the wing around the O-phenyl bond.