Orientation of alkyl chains and hindered rotation of carbonyl groups in the smectic-C*phase of antiferroelectric liquid crystals studied by polarized Fourier transform infrared spectroscopy

Abstract

Several absorbance peaks in electrically unwound, uniform smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) were studied as a function of polarization direction of incident ir radiation by using homogeneously and homeotropically aligned cells of four antiferroelectric liquid crystals. Contrary to the ‘‘zigzag model,’’ the average stretching direction of phenyl rings and the average orientation of two alkyl chains are almost parallel to the long molecular axis. Carbonyl rotation in the chiral and core parts is confirmed to be hindered. The most probable orientation of each individual chiral carbonyl group is not in the plane containing the two-fold axis of the ${\mathit{C}}_2$ point group to which Sm-${\mathit{C}}^{\mathrm{*}}$ belongs; the spontaneous polarization appears along the ${\mathit{C}}_2$ axis perpendicularly to the tilting plane because of head-and-tail equivalence. A possible orientational distribution, mirror symmetric with respect to the tilting plane, is suggested in connection with the hindered rotation of the core carbonyl group. The electric-field-induced phase transition between Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ and Sm-${\mathit{C}}^{\mathrm{*}}$ was also investigated by an asynchronous time-resolved method; all the molecular segments reorient simultaneously in the microsecond time scale.