Visible photooxidation of dibenzothiophenes sensitized by 2-(4-methoxyphenyl)-4, 6-diphenylpyrylium: an electron transfer mechanism without involvement of superoxide

We report here on a new electron-transfer mechanism for visible-light photooxidation of sulfides in which no superoxide ion is involved. Visible-light irradiation of 2-(4-methoxyphenyl)-4, 6-diphenylpyrylium tetrafluoroborate (MOPDPP(+)BF(4)(-)) in an O(2)-saturated acetonitrile solution containing dibenzothiophene (DBT) results in nearly 100% conversion to oxygenated products, DBT sulfoxide and sulfone. The photooxidation of DBT is initiated by a photoinduced electron-transfer process, where the excited MOPDPP(+) traps an electron from the ground-state DBT to form MOPDPP(*) and DBT radical cation. Such a mechanism is consistent with the studies of laser flash photolysis, electron spin resonance, and fluorescence quenching of the irradiated system. The photogenerated DBT radical cation undergoes a coupling reaction with O(2) to produce the intermediate responsible for the formation of the oxygenated products. The presence of O(2) has no effect on the decay kinetics of the transient absorption of MOPDPP(*), indicating that no redox reaction occurs between MOPDPP(*) and O(2), and thus no superoxide ion (O(2)(*-)) is formed. Moreover, the ESR signal of MOPDPP(*) was significantly enhanced in the presence of O(2), consistent with the assumption that the photogenerated DBT radical cation couples with O(2) to form the oxygen-adduct, which is subject to further reactions (Scheme 3) leading to the final oxygenated products. Similar results have been obtained when using 10-methylacridine hexafluorophosphate (AcrH(+)PF(6)(-), which has similar reduction potential in the ground state as MOPDPP(+)) as the sensitizer. This finding provides a possibility for the photooxidation of sulfides with dioxygen utilizing visible light (solar energy) and is also of significance in clarification of the reaction mechanism.