Photomechanically induced ferroelectricity in smectic liquid crystals

Abstract

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm⁻²) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P _s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P _s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm⁻²) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P _s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.