Reorientation mechanism of azobenzenes within the trans → cis photoisomerization

We have recently shown, using waveguide spectroscopy, that irradiation with linearly polarized UV light (360 nm) induces a strong anisotropy in an azobenzene-containing polyglutamate film. Further irradiation with linearly polarized blue light (450 nm) suggested that the cis state aligns perpendicular to the initial UV light polarization. It this paper we can confirm unambiguously these results by using polarized UV-vis spectroscopy, since each isomer (e.g., trans and cis) may be assigned to a characteristic absorption band within the UV-vis spectrum. We also present a theoretical model that describes the photoisomerization-induced reorientation of azobenzenes in a viscous environment. The equations of this phenomenological theory are solved by using the formalism of Legendre polynomials, and analytical solutions are derived. These solutions fully explain the experimental findings and give a good physical insight into the reorientation phenomenon that occurs within the photoisomerization reaction. © 1995 American Chemical Society.