Andrzej L. Sobolewski,^*† Wolfgang Domcke,^‡ Christof Hättig^§
Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland, Department of Chemistry, Technical University of Munich, D-85747 Garching, Germany, and Institute of Nanotechnology, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany

J. Phys. Chem. A 110, 6301-6306 (2006).
Received: December 23, 2005; In Final Form: March 17, 2006

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 S₁ 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 S₁-S₀ conical intersection. Only the π-type orbitals of the aromatic rings are involved in the open-shell structures. The S₁-S₀ 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 S₀ potential-energy surface, thus closing the photocycle. This photophysical pathway accounts for the remarkable photostability of the molecule.

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