Competitive Decay Pathways of the Radical Ions Formed by Photoinduced Electron Transfer between Quinones and 4,4’-Dimethoxydiphenylmetane in Acetonitrile

The reactivity of the cation radical of (4-MeOC6H4)(2)CH2 photosensitized by 1,4-benzoquinone (BQ), 2,5-dichloro-1,4-benzoquinone (Cl(2)BQ), and tetrachloro-1,4-benzoquinone (chloranil, CA) was investigated in acetonitrile. The main photoreaction products obtained by steady-state irradiation were identified to be: (4-MeOC6H4)(2)-CHOC6H4OH, sensitized by BQ; (4-MeOC6H4)(2)CHCI, sensitized by Cl(2)BQ; (4-MeOC6H4)(2)CHOH, sensitized by CA. The mechanism of their formation was investigated by nanosecond laser flash photolysis that allowed transient species (radical ions, neutral radicals, and ions) to be detected and characterized in terms of absorption spectra, formation quantum yields, and decay rate constants. For all systems, the interaction between the triplet quinone (Q) and (4-MeOC6H4)(2)CH2 produced the corresponding radical ions (quantum yield phi greater than or equal to 0.72) which mainly decay by back electron transfer processes. Less efficient reaction routes for the radical ions (Q)is approximately equal to and (4-MeOC6H4)(2)CH2.+ were also: i) the proton-transfer process with the formation of the radical (4-MeOC6H4)(2)CH. by use of Cl(2)BQ; ii) the hydrogen-transfer process with the formation of the cation (4-MeOC6H4)2CH(+) in the case of CA. Instead, BQ sensitized a much higher yield of BQH(.) and (4-MeOC6H4)(2)CH., mainly by the direct interaction of triplet BQ with (4-MeOC6H4)(2)CH2 It was also shown that the presence of salts decreases significantly the rate of the back electron transfer process and enhances the quantum yields of formation of the neutral radicals and ions when Cl(2)BQ and CA are used, respectively The behavior of (BQ)is approximately equal to, (Cl(2)BQ)is approximately equal to, and (CA)is approximately equal to appears to be mainly determined by the Mulliken charges on the oxygen atom obtained from quantum mechanical calculations with the model B3LYP/6-311G(d,p). Spin densities seem to be much less important.