Quenching and Reduction of Photoexcited Benzophenone by Thioethers and Mercaptans

Reactions of thioethers (sulfides) with excited triplet benzophenones have been studied (1) by retardation by the sulfides of photoreduction by isoborneol, (2) by quenching by naphthalene of photoreduction by the sul-fides, and (3) by quenching of phosphorescence of the ketone by the sulfides. Quenching rate constants, fe_r, are in the range 10⁷-10⁹ M ^_1 sec^-1. They are highest for aliphatic and lowest for aromatic sulfides, and values are de-creased by a branching and by electronegative substituents, and higher in acetonitrile than in benzene. Benzophenone is photoreduced by sulfides containing a H. Quantum yields are low, τ~ 0.05-0.2, and increase with decreasing values of kir,. Quenching of phosphorescence of benzophenone by mercaptans shows values of k_q in the range 10⁷-10⁹ M-^l sec^-1, highest for aromatic, lowest for aliphatic thiols, decreased by electron-attracting substituents. Reversible hydrogen abstraction is not important in reactions of sulfides, while probably dominant in reactions of thiols. Quenching and photoreduction by sulfides may proceed via a common charge transfer complex, in which a full unit of charge separation is not developed. Contributions of charge transfer, hydrogen transfer, and polarizability in quenching and reduction of excited carbonyl compounds by alcohols, ethers, amines, sulfides, and mercaptans are discussed.