Photoinduced Anisotropy in a Polymer Doped with Azo Dyes in the Photostationary State Studied by Polarized FT-IR Spectroscopy

Abstract

Photoinduced optical anisotropy in poly(methyl methacrylate) (PMMA) films doped with Disperse Orange 3 (DO3, NO2–C6H4–N=N–C6H4–NH2) was investigated by polarized FT-IR spectroscopy. Symmetric (NOs2) and antisymmetric (NOas2) stretching modes of the NO2 group and the C–N stretching mode of C–NH2(C–N) in the infrared absorption spectra of DO3 were found to be useful in order to evaluate the degree of orientation of DO3 molecules in the polymer matrix. Infrared dichroism was observed in the NOs2, NOas2, and C–N bands under the stationary irradiation by visible light. The orientation factors, K(n)Zf (f = x, y, z, n = trans, cis), determined from the observed dichroism of each isomer in the photostationary state indicate that the p-NO2–C6H4 group is less mobile than the p-NH2–C6H4 group in PMMA during trans-cis-trans isomerization. This dynamic behavior of the DO3 molecules can be interpreted in terms of restriction of rotation within the free volume of PMMA. This study shows that polarized FT-IR spectroscopy is one of the most powerful methods available for the analysis of the physical mechanisms of photoinduced anisotropy.