Photoswitching of Ferroelectric Liquid Crystals Using Unsymmetrical Chiral Thioindigo Dopants: Photoinduced Inversion of the Sign of Spontaneous Polarization

Authors


  • We are grateful to the Natural Sciences and Engineering Research Council of Canada, to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the Canada Foundation for Innovation and to the Ontario Challenge Fund for support of this work.

Abstract

A new unsymmetrical chiral thioindigo dopant 6-[(R,R)-2,3-difluorooct-1-yloxy]-5′-nitro-6′-[(R)-2-octyloxy]thioindigo (4) designed to photoinvert the sign of spontaneous polarization (PS) 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 PS, whereas the (R,R)-2,3-difluorooctyloxy side-chain is decoupled from the core and induces a larger negative PS. 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 λ = 510 nm caused a sign inversion of PS, from –0.88 to +0.42 nC cm–2 at T – TC = –5 °C, 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 thioindigo core upon transcis photoisomerization. The PS 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 transcis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation.

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