Singlet oxygen formation efficiencies following quenching of excited singlet and triplet states of aromatic hydrocarbons by molecular oxygen

Abstract

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k S O 2 and k T O 2 , respectively, of 11 aromatic hydrocarbons in cyclohexane are reported. Measured values of k S O 2 are in the range 0.44 to 2.5×10 10 dm 3 mol −1 s −1 and are therefore in many cases close to k d, the diffusion controlled value of 2.8×10 10 dm 3 mol −1 s −1. The fraction of excited singlet states quenched by oxygen which result in triplet states, f T O 2 , was measured for all the compounds and found to be unity. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f Δ S and f Δ T, respectively, were also measured. Values of f Δ S were shown to be 0, within experimental error, for all compounds except perylene for which f Δ S=0.28±0.05. Values of k T O 2 vary from 0.46 to 2.32×10 9 dm 3 mol −1 s −1. Thus, k T O 2 / k d was found to be less than one-ninth for all the compounds studied. Three different methods were used to measure f Δ T values, which were found to range from 0.57 to 1. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways. Quenching of both excited singlet and triplet states, with and without energy transfer, are shown to be ‘charge-transfer assisted’ and mechanisms accounting for these results are discussed.