Molecular Conformation and Orientation in Some Smectic C Phases

X-ray diffraction measurements are reported on two types of smectic C phases. First, an example of a compound showing very small variation in layer spacing through the smectic A and C phases was studied. It is concluded that the results are consistent with the normal second-order behaviour in which the tilt angle in the smectic C phase is zero at the A to C transition, but in this case only reaches about 10° at lower temperatures because of the small temperature range of the C phase. Second, a homologous series of 4-alkyloxycinnamic acids was studied and some measurements were also made on 4-hexadecyloxybenzoic acid. The cinnamic acid results have been analysed to give information about the gross average molecular conformation in the smectic C phase. This conformation is similar to that determined earlier for the 4-alkyloxybenzoic acids in both crystal and smectic C phases. Direct measurements of the tilt angle from the symmetry of the diffraction patterns show that the appropriate long axis of the molecule is the vector joining the two ends of the markedly non-linear molecule. The implications of these results for the fast molecular rotations known to occur in smectic C phases are briefly discussed.