Liquid-Crystalline Properties of Thioesters

An extension of a new method for forming thioesters with mesomorphic properties has been described, whereby the treatment of aryl tert-butylthioethers with long-chain acid chlorides in the presence of bismuth triflate afforded simple derivatives in good yields. This method in the case of 1-cyanoazulenes was, however, complicated by a competitive Friedel-Crafts-type acylation side reaction at the 3-position. Long-chain derivatives of cyanobiphenyl attached through a thioester linkage exhibited mesophases comparable with their ester analogues. The use of shorter chains to decorate the cyanobiphenyl moiety did not produce mesophasic behaviour, unlike their ester analogues. The cyanobiphenyl derivatives showing mesophasic behaviour were studied by small-angle X-ray diffraction, showing alignment for molecules possessing a smectic A phase. It was found that the layer spacing in the smectic A phase was similar to 1.5 times the length of the molecule, owing to antiparallel pairing arising from pi-stacking between neighbouring units. Calculations suggest these derivatives should exhibit large dielectric anisotropy; however, instability made an exact value difficult to determine. For potential use in a liquid crystal display (LCD), the thioester holds some promise, although this may be limited by the relative stability of the thioester functionality.