Photoaddition and Photoreduction of Chloranil with Arenes via Singlet and Triplet Excited Complexes: Effects of Irradiation Wavelength and Radical Ion Pair Spin Multiplicity

Article abstract of DOI:10.1021/ja00214a043

The photochemistry of chloranil (Q) in the presence of the arenes hexamethylbenzene, 1-methyl-, 1,3-dimethyl-, 1,4-dimethyl-, and 2,3-dimethylnaphthalene, and acenaphthene has been investigated.Irradiation in a nonpolar solvent provided ether adducts resulting from formal addition of the quinone to the arene side chain, the hydroquinone, (QH₂), and in one case, an identifiable oxidation product, acenapthylene.The quantum yield for these transformations was shown to have a complex dependence on the concentration of the arene and the irradiation wavelength.On irradiation at 366 nm under conditions in which free chloranil absorbs, the preferred product was the reduced species, QH₂.On irradiation at 436 nm under conditions in which the ground-state charge-transfer (CT) complex of Q and the arene chiefly absorbs, the preferred product was the ether adduct.Excitation of complexes at 546 nm resulted in a sharply reduced quantum yield.Flash photolysis studies show that the quenching of Q triplets by the arenes leads to free radicals (QH^. and DH^.) by a sequential electron-transfer, proton-transfer mechanism.Irradiation of Q/DH₂ complexes appears alternatively to lead to singlet radical ion pairs (Q^.-, DH₂^.+) which evolve into singlet radical pairs (QH^., DH^.) and final products in competition with back reactions which return radical pairs to ground-state reactants.The wavelength dependence of product quantum yield for excitation of Q/DH₂ complexes is discussed in terms of the facilitation of proton transfer within radical ion pairs due to the ability of side-chain C-H bonds of the arenes to act as acceptor modes for deposition of excess vibrational energy in charge transfer excited states.