UV light enhanced confined Fréedericksz transition in photoisomerizable nematic nanocomposite with photoactive molecules of azobenzene nematic liquid crystal

Based on experimental results, we consider an important feature of the electro-optical (EO) response of nematic liquid crystal (NLC) nanocomposite doped with a photoactive NLC. The basic nanocomposite was NLC with nanosilica fillers. The host compound of the investigated nanocomposite was the nematogen 4-n-heptyl cyanobiphenyl (7CB). This NLC was filled (3 wt.%) with hydrophilic silica nanoparticles (Aerosil 300) of size ∼ 7 nm. To obtain a photoresponsive nanofilled NLC, the photo-insensitive aerosil/7CB nanogel was doped (3 wt.%) with azobenzene NLC 4-(4′-ethoxyphenylazo)phenyl hexanoate (EPH). By illumination with UV light at the wavelength of 375 nm, a large reduction of the threshold voltage takes place for the voltage-dependent optical transmittance of thin (25 µm) optical films of the produced EPH-doped aerosil/7CB NLC. The photo-induced depression of the threshold of the LC reorientation upon AC electric field occurs due to trans-cis photoisomerization of EPH molecules that decorate the network of the aerosil/7CB nanogel. The origin and nature of this effect are discussed. Our explanation involves a model consisting of uniform distributed capacitances related to LC domains confined in the gel structure that are connected in series. Based on this model the term Confined Freedericksz Transition (CFT) is coined.