Combined Effects of the Polarity of Terminal Groups and the Direction of Ester Linkages on Crystalline and Mesophase Structures of Biphenyl Esters with a Cyano or a Methoxy Group

The crystal structures have been determined for 4′-octyloxybiphenyl-4-yl 4-cyanobenzoate (abbr. OCO-CN) and 4-methoxyphenyl 4′-octyloxybiphenyl-4-carboxylate (abbr. COO-OCH3) with the identical phase sequence of crystal 1 (CR1)-crystal 2 (CR2)-monolayered smectic A (SA1)-nematic (N)-isotropic. It is found that the combination of two factors, (1) opposite polarity of the terminal groups (electron-donating OCH3or -accepting CN) and (2) opposite direction of the central ester linkages, results in almost the same structures of CR1. They are composed of bilayers, in which the polar groups face each other. Methoxy groups are in van der Waals contact, while cyano groups have an unfavorable head-to-head arrangement. It is interpreted that this arrangement of CN groups is responsible for the large molecular rearrangements at the CR1–CR2 transition.