Optical anisotropy in films of photoaddressable polymers

The photoinduced anisotropy in polymers and copolymers with a mesogenic and a nonmesogenic azobenzene moiety in the side chain were investigated. The results of the light-induced optical anisotropy study in thin films obtaine d by means of polarized W-vis and IR absorption spectroscopy are presented. Starting from in-plane optically isotropic samples, irradiation with polar ized laser at 514 nm induces significant and long-term stable optical aniso tropy. The build-up of the optical anisotropy has been followed by transien t absorption experiments with time resolution in the range of seconds. The coupling of photoinduced alignment between the two different azobenzenes co uld be proven by irradiation in the absorption band of the nonmesogenic chr omophore. In- and out-of-plane refractive indices were determined using att enuated total reflection waveguide spectroscopy. High values of the in-plan e light-induced birefringence of up to 0.23 depending on the relative conte nt of the mesogenic and the nonmesogenic azobenzene moiety in polymer were found. The pronounced dependence of the birefringence on probe wavelength s uggests a resonant enhancement of refractive index in the visible region. R efractive indices as a function of wavelength were evaluated from the measu red polarized absorption spectra using the Kramers-Kronig relationship. Com parison with the results from waveguide spectroscopy showed that the anisot ropy and dispersion of the refractive index below the absorption could almo st entirely be explained by the reorientation of the two side-chain groups of the copolymer.