Photoinduced oxygen uptake for benzoylpyridines and dipyridylketones in aqueous solution

Abstract

The photochemical properties of 4-benzoylpyridine (4-By) and 4,4′-dipyridylketone (4,4′-Dy) were studied in air-saturated aqueous solution in the presence of formate, ascorbic acid, amines and alcohols as appropriate donors. Upon UV irradiation of these systems oxygen is consumed. The quantum yield of oxygen uptake increases with the donor concentration, approaching Φ − O 2 = 0.2 – 0.8 , e.g. for 4-By and 4,4′-Dy in the presence of ascorbic acid or triethylamine (0.3–30 mM), 0.03–1 M formate and 0.1–10 M 2-propanol. Quenching of the triplet state by the donor and subsequent reaction of both acceptor and donor radicals with oxygen was found. The operating mechanism is oxidation of the donor and conversion of oxygen via the hydroperoxyl radical into hydrogen peroxide. The relevant properties of donors, the radicals involved and the concentration dependencies are discussed. For 2-By and 2,2′-Dy oxygen is also converted into H 2O 2. The mechanism of the 2-pyridyl isomers in the presence of oxygen, however, differs from that of the n = 3,4 isomers in the respect that the photoreaction is due to ring closure and dehydrogenation. The photoinduced oxygen-uptake with 2-By and 2,2′-Dy therefore takes place in the absence of a donor and Φ − O 2 = 0.3 – 0.4 .