Effect of conformations of trans- and cis-azobenzenes on photoinduced anchoring transitions in a nematic liquid crystal

Two azobenzenes with different positional substituents, 3,3'-dihexyloxy-2,2'-dimethylazobenzene (3,3'-azo) and 4,4'-dihexyloxy azobenzene (4,4'-azo), were doped onto a nematic liquid crystal (LC) to form photoresponsive guest-host LC mixtures. The photoinduced anchoring transition of the LC, triggered by trans-cis photoisomerization of the two dyes subjected to linearly polarized UV light was found to be different. Whereas the photoisomerization of 3,3'-azo only induced a transition from random planar (RP) to uniform planar (UP) (in-plane) alignment, that of 4,4'-azo additionally induced a transition from uniform planar to homeotropic (H) (out-of-plane) alignment of the LC at a longer exposure time. The anchoring behavior of the two azobenzenes is discussed in terms of the conformations of their trans- and cis-isomers and the interactions of the isomers with the solid surface of the substrate.