Thioxanthone Dioxide Triplet States Have Low Oxygen Quenching Rate Constants

• Published in: The journal of physical chemistry. B 2024-01, Vol.128 (1), p.244-249
• Main Authors: Jockusch, Steffen, Kazancioglu, Elif Ozcelik, Karaca, Nurcan, Arsu, Nergis, Landgraf, Stephan, Ogilby, Peter R.
• Format: Article
• Language: English
• Subjects: B: Liquids; Chemical and Dynamical Processes in Solution
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.3c06077

With few exceptions, triplet excited states of organic molecules, 3M1, are quenched by ground state molecular oxygen, O2(X3Σg –), 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(a1Δg). In the interaction between 3M1 and O2(X3Σg –), the magnitudes of both k q and the yield of the O2(a1Δg) 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(a1Δg) 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(X3Σg –) encounter complex is reduced, thereby lowering k q while maintaining high O2(a1Δg) yields. These results provide important experimental support for existing models for the quenching of organic molecule excited states by O2(X3Σg –).