Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants

With few exceptions, triplet excited states of organic molecules, M, are quenched by ground state molecular oxygen, O(XΣ), with rate constants k greater than ∼10 M s in fluid solutions. If the energy of the triplet state is above 94 kJ/mol, then such quenching can result in the sensitized production of singlet oxygen, O(aΔ). In the interaction between M and O(XΣ), the magnitudes of both k and the yield of the O(aΔ) depend appreciably on mixing with the M-O charge-transfer state. Here, we report that triplet states of several thioxanthen-9-one-10,10-dioxide derivatives have unusually low k values (as low as ∼1 × 10 M s) but have quantum yields for the photosensitized production of O(aΔ) that approach unity. Because these molecules possess high oxidation potentials (∼3.5 V vs SCE), we suggest that charge transfer character in the M-O(XΣ) encounter complex is reduced, thereby lowering k while maintaining high O(aΔ) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O(XΣ).