The intrinsic photoferroelectric effect in the smectic C* phase of a chiral azobenzene

The ferroelectric properties in the liquid-crystalline smectic C* phase of the chiral mesogenic azobenzene (S)-4-(4-decyloxy-phenylazo) benzoic acid 4-(1-methylhexyloxy)-3-nitro-phenyl ester ('W470') were studied and found to be strongly dependent on the intensity of incident light. The light-induced trans to cis isomerization of the azobenzene chromophore which is an intrinsic constituent of this SmC* mesogen leads to a pronounced photoferroelectric effect since the ferroelectric SmC* ordering is considerably disturbed by the presence of bent-shaped cis-isomer molecules: At increasing light intensity a strong decrease of both SmC* order parameters, spontaneous electric polarization P-s and director tilt angle theta, is observed. It is further seen that even the nature of the ferroelectric transition is affected by the photoisomerization which leads to a crossover from the nearly tricritrical transition observed in the absence of light to a typical second-order transition. This result is interpreted theoretically taking into account a coupling between the director tilt and the biaxial order parameter which is strongly affected by the light-induced generation of the bent-shaped cis-isomers. The dynamics of the intrinsic photoferroelectric effect in W470 was studied by time-resolved measurements of spontaneous polarisation and director tilt.