PHOTOMECHANICALLY INDUCED FERROELECTRICITY IN SMECTIC LIQUID-CRYSTALS

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (approximately 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 (approximately 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 (greater-than-or-equal-to 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 (approximately 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.