Orientational bias of carbonyl groups in the chiral smectic C phase

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Abstract
The IR absorbance of carbonyl groups in an electrically unwound SC* phase was studied as a function of polarizer rotation angle, temperature and the sign of the applied electric field by using homogeneously aligned cells of several ferroelectric liquid crystals. The various absorbance profiles under the electric field were analyzed quantitatively, using new general formulas derived for polarized IR absorption. The rotational motion of all the carbonyl groups is confirmed to be biased, and there are no significant differences correlated with their position in the molecule. The various C˭O absorbance profiles are described in terms of rotational bias parameters, and by the polar angle of the transition moment with respect to the molecular long axis. The calculated orientational distribution function around the molecular axis makes it clear that although the most probable orientation of the C˭O is not normal to the tilt plane, the spontaneous polarization nevertheless appears along the C2 symmetry axis due to the head-and-tail equivalence of the molecules in the SC* phase.