Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants.

With few exceptions, triplet excited states of organic molecules, 3 M 1 , are quenched by ground state molecular oxygen, O 2 (X 3 Σ g - ), with rate constants k q greater than ∼10 9 M -1 s -1 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 2 (a 1 Δ g ). In the interaction between 3 M 1 and O 2 (X 3 Σ g - ), the magnitudes of both k q and the yield of the O 2 (a 1 Δ g ) depend appreciably on mixing with the M-O 2 charge-transfer state. Here, we report that triplet states of several thioxanthen-9-one-10,10-dioxide derivatives have unusually low k q values (as low as ∼1 × 10 8 M -1 s -1 ) but have quantum yields for the photosensitized production of O 2 (a 1 Δ g ) that approach unity. Because these molecules possess high oxidation potentials (∼3.5 V vs SCE), we suggest that charge transfer character in the 3 M 1 -O 2 (X 3 Σ g - ) encounter complex is reduced, thereby lowering k q while maintaining high O 2 (a 1 Δ g ) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O 2 (X 3 Σ g - ).