Involvement of light absorbing transients upon photoreduction of 4-benzoylpyridine and 4,4′-dipyridylketone via donor and acceptor radicals

The photochemical properties of 4-benzoylpyridine and 4,4′-dipyridylketone were studied in argon-saturated aqueous solution in the presence of donors, such as formate, amines, e.g. triethylamine, and several alcohols. Quenching of the triplet state by the donor and subsequent reaction of both acceptor and donor radicals leads to specific quinoid intermediates: light absorbing transients (LATs). The radicals of the donor and 4-benzoylpyridine are suggested to react to LATs with maximum at 370 nm which disproportionate on the ms-s time scale forming phenyl-4-pyridylmethanol as stable species. The LATs of 4,4′-dipyridylketone absorb up to 500 nm, have similar decay rate constants (kL) and eventually form 4-dipyridylmethanol. The properties of donor and acceptor radicals involved, the relevant effects of donor concentration and the solvent, pH and temperature dependences of kL are discussed. The disproportionation kinetics support a cross-coupling reaction mechanism for formation of LATs.