Abstract

Fourteen compounds based on 2,6-disubstituted naphthalenes or related 1-benzothiophene moieties with butylsulfanyl and cyano or isothiocyanato terminal groups have been synthesised. The transition temperatures of the compounds and their refractive indices have been determined and the derived values for the optical anisotropies, polarisabilities and order parameters have been calculated. With one exception (which also shows a smectic A phase), all the compounds with naphthyl and phenyl groups are solely nematogenic; for these compounds the naphthyl unit gives an average increase in T_N–I value and melting point of 72 and 20 °C respectively compared to the values for the compounds with a phenyl in place of the naphthyl unit. The incorporation of a 2,5-thiophene unit in place of phenyl lowers T_N–I and when it is part of a 1-benzothiophene unit the depression is even greater. The naphthalene compounds increase the values of optical anisotropy by approximately 0.04, compared to the phenyl systems, mainly because they increase the refractive index of the e-ray (n_∥ values); compound 12 has an exceptionally high optical anisotropy (Δn value) of 0.54.