LINEAR DICHROISM AND TRANS-] CIS PHOTO-ISOMERIZATION STUDIES OF AZOBENZENE MOLECULES IN ORIENTED POLYETHYLENE MATRIX

U.v.-vis linear dichroism spectra of trans-and cis-azobenzene in stretched poly(ethylene) (PE) films are reported. Trans --> cis photo-isomerization kinetics were measured at 337 nm using polarized light with the electric vector either parallel or perpendicular to the stretching direction of the matrix. Three types of samples were examined, viz. unoriented, kept at constant elongation stretched to (800%) and relaxed (stretched to 600%). Analysis of the kinetic data reveals that the photo-isomerization is purely exponential in time up to 83% conversion, with nearly the same rate constants in both directions. It is faster in the stretched than in the unstretched sample. This suggest that trans --> cis isomerization of azobenzene in nonpolar solid solution occurs according to an inversion mechanism. Cis-azobenzene molecules obtained from phototransformation of the trans species in stretched PE attain globular geometry with the phenyl rings twisted perpendicular to the azo plane.