Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants

With few exceptions, triplet excited states of organic molecules, ³M₁, are quenched by ground state molecular oxygen, O₂(X³Σ_g^-), with rate constants k_q greater than ∼10⁹ 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, O₂(a¹Δ_g). In the interaction between ³M₁ and O₂(X³Σ_g^-), the magnitudes of both k_q and the yield of the O₂(a¹Δ_g) depend appreciably on mixing with the M-O₂ 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 × 10⁸ M^-1 s^-1) but have quantum yields for the photosensitized production of O₂(a¹Δ_g) that approach unity. Because these molecules possess high oxidation potentials (∼3.5 V vs SCE), we suggest that charge transfer character in the ³M₁-O₂(X³Σ_g^-) encounter complex is reduced, thereby lowering k_q while maintaining high O₂(a¹Δ_g) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O₂(X³Σ_g^-).