Photophysics of organic photostabilizers. Ab initio study of the excited-state deactivation mechanisms of 2-(2'-hydroxyphenyl)benzotriazole.

Excited-state reaction paths and the corresponding energy profiles of 2-(2'-hydroxyphenyl)benzotriazole (TIN-H) have been determined with the CC2 (simplified singles-and-doubles coupled-cluster) ab initio method. Hydrogen transfer along the intramolecular hydrogen bond, torsion of the aromatic rings and pyramidization of the central nitrogen atom are identified as the most relevant photochemical reaction coordinates. The keto-type planar S(1) state reached by barrierless intramolecular hydrogen transfer is found to be unstable with respect to torsion. The latter mode, together with a moderate pyramidization of the central nitrogen atom, provides barrierless access to a S(1)-S(0) conical intersection. Only the pi-type orbitals of the aromatic rings are involved in the open-shell structures. The S(1)-S(0) conical intersection, which occurs for perpendicular geometry of the aromatic rings, is a pure biradical. From the conical intersection, a barrierless reaction path steers the system back to the enol-type minimum of the S(0) potential-energy surface, thus closing the photocycle. This photophysical pathway accounts for the remarkable photostability of the molecule.