Photoinduced anchoring transitions in a nematic doped with azo dyes

Recently, we reported on a light-induced anchoring transition of an azobenzene nematic from planar to homeotropic alignment. In the proposed model of the transition, the changes in shape of the liquid crystal molecules and of their net dipole moment, due to the photoisomerization, were considered to play a vital role in the occurrence of the transition. In order to assess the validity of this model, a study of the anchoring behaviour of nematic guest-host liquid crystal mixtures containing two photochromic dyes, 3,3'- and 4,4'-substituted azobenzenes, was carried out. The dyes have very similar molecular structures to that of the azobenzene nematic previously studied, and their molecules, having a linear shape in the trans-form, maintained this shape after photo-isomerization in the case of the 3,3'-azo dye, and changed it to bent in the case of the 4,4'-azo dye. The dyes possessed similar net dipole moments that increased substantially after photoisomerization, resulting in a preferential adsorption of their cis-isomers on the solid substrate. However, only the mixture containing the 4,4'-azo dye exhibited an anchoring transition from planar to homeotropic alignment upon illumination with unpolarized UV light, a behaviour in excellent agreement with the prediction of the model for the light-induced anchoring transition. An anchoring transition from random planar to uniform planar alignment was found to take place in the mixtures when linearly polarized UV light was used, requiring, however, a different exposure time for the two dyes.