Quinone sensitized photooxidation of aromatic hydrocarbons ampersand arylmethanols: Influence of substituents on sequential electron/proton transfer

The photochemistry of a strong quinone oxidant, chloranil (Q), with several electron donors (naphthalene, 1-, 2-methylnaphthalenes, 1,2-, 1,3-, 1,4-, 1,5-, 1,8-, 2,3-, 2,6-dimethylnaphthalenes and acenaphthene) in benzene has been investigated by laser flash photolysis. Combination of Q with high concentrations of the donors leads to formation of ground state electron-donor-acceptor complexes, irradiation of which at long wavelengths (e.g. 532 nm) failed to exhibit any excited state chemistry. In contrast, absorption of 355 nm laser light by ground state Q resulted in the formation of triplet quinone (3Q) that was efficiently quenched by the donors (kq = ca. 5 x 109 M-1 s-1). A study of photoreduction of the inorganic photosensitizer, uranyl ion (UO22+), has been carried out for comparison with the organic photosensitizer, chloranil. Aromatic hydrocarbons with uranyl ion did not produce transients in pulsed laser experiments. The high rate constants for the quenching of the acceptor excited states by these donors compared to those for alcohol donors are interpreted in terms of two distinctive H-atom and electron transfer pathways for reduction