SENSITIZED PHOTOOXYGENATION ACCORDING TO TYPE I MECHANISM (RADICAL MECHANISM) — PART III. EXPERIMENTS WITH CONTINUOUS ILLUMINATION

Abstract— The photochemical reaction in the system thionine (sensitizer), allylthiourea (ATU, acceptor), and oxygen was studied with continuous illumination. In oxygen-free aqueous solution thionine is photoreduced to leucothionine. With oxygen, however, a photooxygenation of the acceptor takes place. At the same time the quantum yield of the bleaching reaction of thionine decreases markedly in comparison with that of the oxygen-free solution. At about 100 sec after the beginning of illumination, the overall quantum yield of the bleaching reaction diminishes further because the leucothionine formed during the reaction now becomes transformed into thionine. The quantum yields do not change significantly over the range of oxygen concentrations studied (initial concentration 1 × 10^-5 to 5 × 10^-5 M ). In addition they are independent of the light intensity. The influence of the pH and the acceptor concentration were also investigated.
The sensitizer is not only bleached reductively, but is also partly destroyed by oxidation. The results are in agreement with the reaction scheme elucidated by flash photolysis measurements.
In accordance with this reaction scheme, the primary reaction (a) of the reductive bleaching of the sensitizer, (b) of the photooxygenation of the acceptor and (c) of the oxidative destruction of the sensitizer, is identical in all cases. This process is the redox reaction between the sensitizer triplet and the acceptor, where a semithionine and an ATU-radical are formed. The reaction represents an example of a Type I photooxygenation according to the notation of Gollnick.