SENSITIZED PHOTOOXYGENATION: CHANGE FROM TYPE I (RADICAL) TO TYPE II (SINGLET OXYGEN) MECHANISM

Abstract— As we have shown in previous papers, thionine-sensitized photooxygenation reactions follow a Type I (radical) mechanism. We now demonstrate that, by an appropriate choice of the acceptor and its concentration (solvent: pyridine) or by working in a rigid matrix (ethyl cellulose), the reaction can be switched to a Type II (singlet oxygen) mechanism. The system studied in the present investigation, thionine and 9,10-dimethylanthracene, represents to a certain extent an intermediate type. Photooxygenation at low DMA-concentration occurs according to a Type II mechanism as verified by the method of competitive photooxygenation, while in oxygen-free solutions, the sensitizer is partially photoreduced by the acceptor, which is typical for Type I systems. Whereas the photooxygenation of allylthiourea (ATU) with thionine as sensitizer takes place via radicals at high ATU concentrations, a change to the singlet oxygen mechanism could be observed at low ATU concentrations in pyridine solution.