Exploratory Study of the Intermolecular Reactivity of Excited Diphenylmethyl Radicals

Article abstract of DOI:10.1021/ja00301a005

A series of intermolecular reactions of excited diphenylmethyl radicals has been examined with laser flash photolysis techniques.The radicals, Ph2CH(.), were generated by 308-nm photodecomposition of 1,1,3,3-tetraphenylacetone or diphenyldiazomethane.In the latter system the radicals result from reaction of diphenylcarbene with hydrogen-donor solvents.The radicals were then excited with the 337-nm pulses from a nitrogen laser, leading to the formation of the readily detectable excited state of the radical, Ph2CH(.) (λ_max 355 nm).The excited radical is an excellent electron donor, reacting with methyl viologen with k_q=1.3*10¹⁰ M^-1s^-1 in wet acetonitrile and leading to the formation of methyl viologen radical cation.Ph2CH(.) is readily quenched by amines, but no evidence for full electron transfer could be obtained, e.g., for triethylamine k_q = 5.2*10⁸ M^-1 s^-1 in cyclohexane.The excited radical is 14 times more reactive toward oxygen than its ground state, but the process seems to involve a different mechanism, probably leading to singlet oxygen generation.Halogenated substrated react readily with Ph2CH(.); for example, for CCl4 k_q = 1.6*10⁸ M^-1 s^-1 and leads to Ph2CHCl in a process that is presumed to involve charge transfer.Excited diphenylmethyl is not a good hydrogen abstractor in spite of the fact that its hydrogen abstraction reactions would be more exothermic than those of phenyl radicals.