MODULATING THE SPONTANEOUS POLARIZATION OF A FERROELECTRIC LIQUID-CRYSTAL VIA THE PHOTOISOMERIZATION OF A CHIRAL THIOINDIGO DOPANT - (R,R)-6,6'-BIS(1-METHYLHEPTYLOXY)THIOINDIGO

Doping the novel chiral photochromic dye (R,R)-6,6'-bis(1-methylheptyl oxy)thioindigo into the S-C liquid crystal host MX6120 at a concentrat ion of 7.2 mol % gives rise to a ferroelectric S-C phase with a reduc ed polarization (P-0) of +14.4 nC cm(-2). Results of time-resolved exp eriments demonstrate that the dye undergoes trans --> cis photoisomeri zation in the S-C phase, although the efficiency of photoisomerization is significantly lower than in the nematic and isotropic liquid phase s. A reversible modulation of P-S on the order of 10 per cent is obser ved for the doped S-C phase upon photoisomerization of the dye from a thermally relaxed trans form (+6.2 nC cm(-2)) to a cis-enriched photo stationary state (+5.6 nC cm(-2)) at T-T-c = -10 degrees C. Control ex periments using mixtures of the racemic and optically active dye in a SF host suggest that P-S photomodulation does not arise from a photome chanical effect. Consideration of the Boulder model for the molecular origins of P-S provides a rationale for the observed P-S photomodulati on based on changes in the conformational equilibria of the 1-methylhe ptyloxy side-chains in the S-C binding site going from the trans form to the cis form of the thioindigo dopant.