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Precise quantum yields for generation of singlet molecular oxygen, O2(1Δg), have been measured for tetrakis(4-sulfonatophenyl)porphyrin (TSPP), closely-related water-soluble porphyrin derivatives, and tris(2,2‘-bipyridyl)ruthenium(II) (bpy3Ru2+) in O2-saturated water. Under the experimental conditions, TSPP is present in an aggregated state, but the measured quantum yield for formation of O2(1Δg), ΦΔ = 0.51, remains on the same order as that of the corresponding monomer in methanol, ΦΔ = 0.70. Comparison with the quantum yield for formation of the triplet state indicates that quenching of the triplet by O2 gives O2(1Δg) with an efficiency of ca. 80%. There is a small but significant increase in ΦΔ when D2O is used in place of H2O. For the other porphyrin derivatives, the ability of the aggregate to sensitize formation of O2(1Δg) appears to depend on the total electronic charge resident on the molecule, decreasing with decreasing negative charge. A strong solvent dependence noted for O2(1Δg) production with (bpy3Ru2+) is explained in terms of competition between triplet energy transfer and photoinduced electron (or partial charge) transfer. Similar behavior could be responsible for the relatively low ΦΔ value observed for TSPP.

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Illumination of polyoxotungstates generates a short-lived excited state which decays to form the reactive intermediate wO that is responsible for subsequent transformation of organic substrates. It is shown by laser flash photolysis that wO is formed with a quantum yield of 0.6 and that it survives for ca. 55 ns in deoxygenated acetonitrile. In the presence of a substrate such as propan-2-ol wO reacts to produce the one-electron-reduced species, as characterized by transient spectroscopy and as confirmed by pulse radiolysis. Oxygen intercepts the initially-formed radicals so that this methodology may be used to follow the course of the reaction. Under continuous photolysis, propan-2-ol is converted into acetone and hydrogen peroxide but with adamantane as substrate the corresponding hydroperoxides are formed. Quantum yields of oxygen consumption have been measured for a variety of other substrates and range from ca. 0.05 for pure acetonitrile to ca. 0.5 for propan-2-ol. In the absence of other substrates, reaction of organic counterions may be observed. All these observations are consistent with hydrogen-atom abstraction from an organic substrate by wO, followed by rapid deprotonation of • wOH and/or trapping by O 2 . A concise reaction scheme involving coupling between oxyradicals is proposed that adequately describes the overall chemistry.

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