Generation, characterization, and deprotonation of phenol radical cations

A variety of methoxy- and methyl-substituted phenol radical cations have been generated and characterized by laser flash photolysis in solution under ambient conditions. The radical cations were generated by either photosensitized electron transfer using 1,4-dicyanonaphthalene with biphenyl as a cosensitizer in acetonitrile or by direct excitation of the phenol at 266 nm. The phenol radical cations have absorption maxims between 410 and 460 nm, with the exception of the 3,5-dimethoxyphenol which absorbs at 580 nm. The assignment of the observed transients to phenol radical cations is based on their spectral similarity to matrix spectra for the same species and to the corresponding methoxybenzene radical cations, as well as their characteristic reactivity. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only observed transient in the photosensitized electron transfer. For those phenols for which authentic phenoxyl spectra are not available, the identity of the radical was confirmed by its generation by hydrogen abstraction by the tert-butoxyl radical. Although several of the phenols can be photoionized in either 1:1 aqueous ethanol or in acetonitrile, this is a less general route for the formation and characterization of the phenol radical cations. The rate constants for deprotonation of the phenol radical cations by water were measured and fall within the range (0.6-6) x 10(8) M-1 s(-1); the 2-methoxyphenol radical cation is more reactive than the 4-methoxy, consistent with a recent estimate of the pK(a) for these species.