Kinetics of the photoferroelectric effect in chiral smectic-C liquid crystals studied by time-resolved measurements of spontaneous electric polarization and director tilt angle.

The origin of the photoferroelectric effect in liquid crystals, where the spontaneous polarization of a chiral ferroelectric smectic-C* (SmC*) host phase is changed by the E,Z-photoisomerization of azobenzene dopant molecules, was investigated by kinetic studies on the molecular isomerization and the subsequent changes in the SmC* order parameters, the director tilt angle, and the spontaneous electric polarization. The photoresponse of a liquid-crystal mixture consisting of 5 mol% 4,4'-bis-[(2-methyl)butyloxy]azobenzene dissolved in the SmC* host phase FLC 6430 was studied at low UV-light intensities (lambda = 366 nm, 15 microW cm-2) using an electrooptical technique that measured the desired parameters with a time resolution of about 1 s. The time-resolved measurements of optical absorption, tilt angle, and spontaneous polarization during the thermal reisomerization after ending the sample irradiation showed that the photoinduced changes in the spontaneous polarization simultaneously followed the molecular isomerization with the same rate constant and activation energy, while the director tilt angle remained basically unchanged. Minor changes in the tilt are explained by the local heating of the sample due to the optical absorption. Since the photoinduced change in polarization was observed at constant tilt, we conclude that in the limit of low UV intensity the photoferroelectric effect originates from a photo-induced change of the bilinear coupling coefficient between the polarization and the tilt. In the molecular theory of chiral SmC* liquid crystals, the coupling coefficient is related to the bias of molecular rotations. This bias may be considerably disturbed by the formation of the bent Z-isomers during the photoisomerization.