Photoaddition and photoreduction of chloranil with arenes via singlet and triplet excited complexes. Effects of irradiation wavelength and radical-ion pair spin multiplicity

TLDR: The photochemistry of chloranil (Q) in the presence of the arenes hexamethylbenzene, 1-methyl-, 1-3-dimethyl-, 1,4-dimmethyl-, and 2,3dimethylnaphthalene, and acenaphthene has been investigated as discussed by the authors.

Abstract: 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, (QH/sub 2/), and in one case, an identifiable oxidation product, acenaphthylene. The quantum yield for these transformations was shown to have a complex dependence on the concentration of the arene and irradiation wavelength. On irradiation at 366 nm under conditions in which free chloranil absorbs, the preferred product was the reduced species, QH/sub 2/. 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/sup ./ and DH/sup ./) by a sequential electron-transfer, proton-transfer mechanism. Irradiation of QDH/sub 2/ complexes appears alternatively to lead to singlet radical ion pairs (Q/sup .-/, DH/sub 2//sup .+/) which evolve into singlet radical pairs (QH/sup ./, DH/sup ./) and final products inmore » competition with back reactions which return radical pairs to ground-state reactants.« less