Article,
Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants
Affiliations
- [1] Bowling Green State University [NORA names: United States; America, North; OECD];
- [2] Yildiz Technical University [NORA names: Turkey; Asia, Middle East; OECD];
- [3] Graz University of Technology [NORA names: Austria; Europe, EU; OECD];
- [4] Aarhus University [NORA names: AU Aarhus University; University; Denmark; Europe, EU; Nordic; OECD]
Abstract
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).