Photo-orientation of liquid crystals due to light-induced desorption and adsorption of dye molecules on an aligning surface - art. no. 051709

We show that adsorption of dye molecules control the light-induced alignmen t of dye-doped nematic liquid crystal (LC) on a nonphotosensitive polymer s urface. The dependencies of light-induced twist structures on exposure, the rmal baking, thickness, and aging before irradiation of the LC cells allowe d us to propose the following mechanism for the alignment. Before irradiati on, the "dark"-adsorbed layer on the tested surface is formed from dye mole cules predominantly aligned along the initial direction of the director. Ir radiation of the cell with linearly polarized light produces an additional layer with different orientational ordering of dye molecules. The final eas y axis is determined by the competition of "dark" and light-induced contrib utions to anchoring and is aligned between the "dark" easy axes and polariz ation of the light. For quantitative interpretation, we apply the tensor mo del of anchoring and assume that the photoalignment in the mesophase is a c umulative effect of the light-induced anchoring on the background of the al ready existing anisotropic "dark" dye layer.