Photochemistry of Diphenylketyl Radicals: Spectroscopy, Kinetics, and Mechanisms

Article abstract of DOI:10.1021/ja00210a035

The photochemistry of the diphenylketyl radical has been examined in nonpolar solutions.Transient studies using two-laser techniques yield an excited-state lifetime of 3.9 ns in toluene at room temperature, while for diphenylketyl-O-d the lifetime is 8.7 ns.Dye laser irradiation (515 nm) in the kethyl's visible absorption band leads to efficient photobleaching with Φ_bleach=0.27 +/- 0.06 for the parent radical and 0.39 and 0.26 for the 4-methyl and 4-chloro derivatives, respectively.The photobleaching reaction involves the cleavage of the O-H ketyl bond to yield benzophenone and hydrogen atoms; in cyclohexane the latter abstract hydrogen from the solvent to produce molecular hydrogen, which was characterized by Raman spectroscopy.In accordance with this mechanism, two-laser experiments produce lower yields of photoreduction products than the one-laser experiments in which the ketyls are not photobleached.When the ketyl radicals are generated by reaction of tert-butoxy radicals with benzhydrol, dye laser irradiation leads to a large increase in the yield of benzophenone (now a product), although the mechanism here is somewhat more complex due to the quenching of excited ketyl radicals by di-tert-butyl peroxide (k_q=1.9E9 M^-1s^-1).Detailed studies of the fluorescence, isotope effects, temperature effects, and products are also included.