Photoswitching of ferroelectric liquid crystals using unsymmetrical chiral thioindigo dopants: structural effects on polarization photomodulation

Abstract

A series of unsymmetrical chiral thioindigo dopants were synthesized and doped in the SmC liquid crystal host (±)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB) to give ferroelectric SmC* liquid crystals. The unsymmetrical design of these new dopants resulted in improved compatibility with PhB, as shown by ²H NMR spectroscopy. One unsymmetrical thioindigo derivative, (R)-6-heptyloxy-5′-nitro-6′-(2-octyloxy)thioindigo, forms an enantiotropic SmA* liquid crystal phase (Cr 123 SmA* 129 I). Irradiation of the doped SmC* liquid crystal phases at 510 nm causes an increase in spontaneous polarization (P_S) as a result of trans–cis photoisomerization of the thioindigo dopants. A correlation between the polarization power of the thioindigo dopants in the trans form and the calculated transverse dipole moment µ_⊥ of the thioindigo cores is consistent with the assumption that the thioindigo core contributes to the induced polarization by virtue of steric coupling to the chiral 2-octyloxy side-chain. A correlation between the degree of polarization photomodulation and the calculated change in transverse dipole moment Δµ_⊥ of the thioindigo core upon trans–cis photoisomerization is also consistent with the contribution of the thioindigo core to the induced polarization. This study suggests that the design of any chiral thioindigo dopants capable of high P_S photomodulation must include a symmetrical thioindigo core with zero transverse dipole moment in the trans form in order to maximize Δµ_⊥.