Spectroscopic and photochemical properties of aromatic thioketones: xanthione

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 (¹A₁ » ¹A₁) 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 CS 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.