Investigation of physical triplet quenching by electron donors

Absolute radical yields, as determined by flash spectroscopy, are reported for the reaction of the thionine triplet 3TH+ and its protonated form 3TH22+ with a variety of electron donors in methanolic solution. The yields are extrapolated to infinite donor concentrations. Two methods of obtaining absolute yields are described. The different yields with different donors are due to partial physical quenching of the triplets in the electron transfer reaction. The results are discussed in terms of different models, capable of explaining bimolecular intersystem crossing processes. A triplet exciplex as an intermediate in the electron transfer reaction is favored.