J. Robert Huber

Spectroscopic and photochemical properties of aromatic thioketones: xanthione

Abstract Some aspects of the absorption, emission and photochemistry of xanthione (XS), an aromatic thioketone with a C 2ν molecular geometry, have been investigated in solution at room temperature and 77 K. Contrary to the observation in similar aromatic ketones, the first Π, Π* transion ( 1 A 1 » 1 A 1 ) of thioketones has charge-transfer character as indicated by the results of a CNDO/2-CI calculation. The short lived emission observed at room temperature and at 77 K (τ p (77 K) = 43 μs; Φ p (77 K) = 0.11) is polarized parallel to the Cue5fbS axis and is assigned as phosphorescence of the lowest 3 ( n , Π*) state. The exceptionally effective spin-orbit coupling of aromatic thioketones (about two orders of magnitude larger than in the ketone analogs) indicates that a different intersystem crossing mechanism is operative in aromatic thioketones from that of ketones. The photo-oxidation of XS to xanthone proceeds via the lowest n , Π* triplet state of XS. The photodecomposition is most efficient in oxygen-free alcoholic solutions and involves the excited XS molecule and the solvent.

Triplet state deactivation and photochemistry of aromatic thioketones in solution: xanthione

Abstract The decay of triplet xanthione has been studied by means of the phosphorescence decay kinetics and the photodecomposition quantum yields φ x measured in a variety of solvents at room temperature. Both the triplet decay and φ x are strongly dependent not only on the solvent but also on the xanthione concentration. Based on these results a mechanism for the photochemical transformation of xanthione is proposed which features a primary photochemical step involving hydrogen abstraction from the solvent and the formation of a solvent-caged radical pair. Following these steps there is a competition between the recombination of the caged radicals and the reaction of the thioketyl radical with a xanthione ground state molecule. The reaction scheme is supported by quantitative calculation of the yields.