Oxygen uptake upon photolysis of 3-benzoylpyridine and 3,3′-dipyridylketone in air-saturated aqueous solution in the presence of formate, ascorbic acid, alcohols and amines

Abstract

The effects of oxygen in the photolysis of 3-benzoylpyridine (3-By) and 3,3′-dipyridylketone (3,3′-Dy) in aqueous solution were studied in the presence of appropriate donors: formate, ascorbic acid, amines, e.g. triethylamine and alcohols, e.g. 2-propanol. The quantum yield of oxygen uptake ( Φ − O 2 ) increases with the donor concentration, approaching Φ − O 2 = 0.6 , e.g. for ascorbic acid or triethylamine (>1 mM) and 2-propanol (>0.1 M). Quenching of the triplet state by the donor and subsequent reaction of both acceptor and donor radicals with oxygen leads to formation of hydrogen peroxide. The radicals of the mono- and dipyridylketones terminate into an “iso”benzpinacol-type product. This reaction is minor and major in the presence and absence of air, respectively. For N-methylphthalimide, a related heterocyclic sensitizer, the mechanism of oxygen uptake is the same: oxidation of the donor and conversion of oxygen via the hydroperoxyl radical into H 2O 2, but the Φ − O 2 values are smaller. The relevant properties of donors, the radicals involved and the pH and concentration dependences are discussed.