Unusual odd-even effects depending on the monomer chain length in nematic liquid crystals made of rod-like dimers

We report physical studies on the nematic phase exhibited by five members of a homologous series of symmetric dimers which have cyanobiphenyl end groups and, a disulphide bridging group in the spacer. All of them have rod-like conformations in the nematic phase, but unusually, the nematic-isotropic transition temperature, the birefringence and the dielectric anisotropy exhibit "odd-even" effects depending on the number of carbon atoms in the alkyl chain of a monomeric moiety. Further, the lowest "even" homologue exhibits a nematic-nematic transition. We argue that these phenomena arise from the relatively facile rotational fluctuations of the two monomeric moieties around the central S-S bond, whose orientation with respect to the long molecular axis changes between the "even" and "odd" members.