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

Abstract

Doping the novel chiral photochromic dye (R,R)-6,6′-bis(1-methylheptyloxy) thioindigo into the S_c liquid crystal host MX6120 at a concentration of 7.2 mol % gives rise to a ferroelectric S∗_c phase with a reduced polarization (P ₀) of +14.4 nCcm⁻². Results of time-resolved experiments demonstrate that the dye undergoes trans→cis photoisomerization in the S_c phase, although the efficiency of photoisomerization is significantly lower than in the nematic and isotropic liquid phases. A reversible modulation of P _s on the order of 10 per cent is observed 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 photostationary state (+5.6 nC cm⁻²) at T – Tc = −10°C. Control experiments using mixtures of the racemic and optically active dye in a S∗_c host suggest that P _s photomodulation does not arise from a photomechanical effect. Consideration of the Boulder model for the molecular origins of P _s provides a rationale for the observed P _s photomodulation based on changes in the conformational equilibria of the 1-methylheptyloxy side-chains in the S_c binding site going from the trans form to the cis form of the thioindigo dopant.