A soft-bent dimer composite exhibiting twist-bend nematic phase: Photo-driven effects and an optical memory device

Abstract

We report the influence of photoisomerization on a guest-host composite comprising a soft-bent dimer exhibiting the presently sought-after twist-bend nematic (NTB) phase, doped with small amounts of a calamitic photoactive azobenzene-based dimer that assumes a bent shape when photo-driven. Trans-cis photoisomerization of the latter compound leads to a substantial decrease in the nematic-isotropic and nematic-NTB transition temperatures. The results bring out the puzzling feature that the thermal stability of the NTB phase, having a helical structure, despite the molecules being achiral, is influenced more than the regular nematic. Differential influence is also seen in the Frank elastic constants of the regular nematic: while the splay elastic constant is significantly diminished its bend counterpart is hardly affected. Molecular conformational aspects of the entities are proposed to explain these features. Through a judicious combination of this photo-driven effect and an AC electric field, an attractive type of optical memory device is demonstrated.