Orientation of a photo-sensitive polymeric monolayer studied by second-harmonic generation

We determine the orientational distribution functions of azo-side-chain polymeric monolayers before and after the photo-irradiation by optical surface second-harmonic generation (SHG). The polymeric monolayer, composed of azo-dye-labeled polyamic acid (long alkyl) amine salt, is prepared by the Langmuir-Blodgett technique. Oblique incident surface SHG measurement from the monolayer provides the nonlinear-optical tensor components related to the orientational distribution functions of the azo-side-chain. The analysis of experimental results shows that the point symmetry of the azo-pendent unit in the monolayer changes from C_∞ν to C_1ν by the photo-isomerization process, with the azobenzene conformation retained as trans-isomer.