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‐difluorooct‐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 λ = 510 nm caused a sign inversion of P S , from –0.88 to +0.42 nC cm –2 at T  –  T C  = –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 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.