Electron-Transfer Photochemistry of Thianthrene. Nucleophile-Assisted Photooxidation to Sulfoxide

Article abstract of DOI:10.1021/jo00060a016

The photochemistry of thianthrene (1) in the presence of a variety of electron acceptors in acetonitrile/water has been investigated.Diffusion-limited rates of fluorescence quenching were observed on excitation of 1 (with fumaronitrile and with methyl phthalate).Alternatively, addition of 1 led to quenching of the fluorescence emission of ketones, biacetyl, and fluorenone and to the quenching of the excited triplet states of quinones such as 2,5-dichloro-p-benzoquinone (5) and 9,10-anthraquinone (3).Steady irradiation of 1 with selected acceptors results in photooxidation yielding the sulfoxide (2) and the expected photoreduction products.The electron-transfer mechanisms were further studied by laser flash photolysis.The quenching of ketone triplets by 1 resulted in formation of the radical cation of 1 (λ_max 540 nm).The decay of this species and its dependence on the concentration of water present were determined; the bimolecular rate constant for trapping of 1^.+ by water in acetonitrile-water solutions is 4.9x10⁴ M^-1s^-1 under conditions in which 5 acts as a sensitizer.The nucleophilic trapping of the 1 radical cation by imidazole (k=3.6x10⁷ M^-1s^-1) was also investigated.The mechanism of photoinduced two-electron oxidation of 1 and the role played by nucleophiles in facilitating the reaction are discussed in detail.