Frustrated smectic phases in swallow-tailed compounds with perfluorinated chains

A group of 4-[4-(4-n-octyloxybenzoyloxy)benzoyloxy]benzylidenemalonic acid bis(perfluoralkyl) esters and 4-{4-[2,2-bis(n-octyloxycarbonyl)ethenyl]benzoyloxy}benzoic acid 4-perfluoroalkyloxyphenyl esters with varied lengths of the perfluorinated chains were investigated by X-ray measurements. The liquid crystalline behaviour (SmA, SmC, Col) and the structures of the phases are described. The substances show the influence of the steric interaction caused by the wedge shape of the molecules and the repulsion caused by chemically incompatible segments (fluorophobic effect). In the first group of compounds investigated the steric interaction clearly dominates the structure, so that an antiparallel packing of the molecules in SmA and SmC phases results. In the second group, the influence of the incompatibility is more and more enhanced and a low temperature phase with a columnar structure arises in which the incompatible parts are separated on a microscopic.