Photoswitching of ferroelectric liquid crystals using unsymmetrical chiral thioindigo dopants: Photoinduced inversion of the sign of spontaneous polarization

A new unsymmetrical chiral thioindigo dopant 6-[(R,R)-2,3-difluorooet-1-yloxy]-5'-nitro-6'-[(R)-2-octyloxy]thioindigo (4) designed to photoinvert the sign of spontaneous polarization (P-S) in a ferroelectric chiral smectic C (SmC*) liquid crystal was prepared using a synthetic approach previously developed in our laboratory. In this new "ambidextrous" design, the (R)-2-octyloxy side-chain is sterically coupled to the thioindigo core and induces a positive P-S, whereas the (R,R)-2,3-difluorooctyloxy side-chain is decoupled from the core and induces a larger negative P-S. In the trans form, this dopant induces a negative polarization in the SmC host (+)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB). Irradiation of a 1 mol-% mixture of 4 in PhB at lambda = 510 nm caused a sign inversion of P-S, from -0.88 to +0.42 nC cm(-2) at T-T-C = -5 degreesC, which is consistent with an increase in the polarization power of the coupled 2-octyloxy/thioindigo unit over that of the 2,3-difluorooctyloxy unit, due to the increase in transverse dipole moment of the thioincligo core upon trans-cis photoisomerization. The P-S sign inversion was confirmed by a surface-stabilized ferroelectric liquid crystal photoswitching experiment. Spectroscopic measurements on films of the doped liquid crystal mixtures showed that trans-cis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation.