Molecular Chirality Due to Twisted Conformation in a Bent-Shaped Liquid Crystal Studied by Polarized Ft-Ir Spectroscopy

Microscopic polarized fourier-transform infrared (FT-IR) measurements were performed in the antiferroelectric B2 phase of a bent-shaped liquid crystal molecule. In homogeneously chiral domains, the absorbance maximum appeared when the transmission axis of a polarizer was parallel to the layer normal for the phenyl ring stretching and C-O-C asymmetric stretching and that for the C = O stretching was almost perpendicular to that for C-O-C in the absence of a field. When a dc field was applied, the maximum for C-O-C rotated by about 35°, which is almost the same as the apparent tilt angle of the molecule. In contrast, the axis of maximum absorbance for C = O was neither parallel nor perpendicular to that of C-O-C under a field. It indicates that the two ester groups are twisted with respect to the central phenyl ring and are not perpendicular to the molecular long axis. Moreover, one of the twist conformations is chosen and the rotation around the molecular long axis is strongly hindered.