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, kSO2and kTO2, respectively, of 11 aromatic hydrocarbons in cyclohexane are reported. Measured values of kSO2are in the range 0.44 to 2.5×1010dm3mol-1s-1and are therefore in many cases close to kd, the diffusion controlled value of 2.8×1010dm3mol-1s-1. The fraction of excited singlet states quenched by oxygen which result in triplet states, fTO2, 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, fΔSand fΔT, respectively, were also measured. Values of fΔSwere shown to be 0, within experimental error, for all compounds except perylene for which fΔS=0.28±0.05. Values of kTO2vary from 0.46 to 2.32×109dm3mol-1s-1. Thus, kTO2/kdwas found to be less than one-ninth for all the compounds studied. Three different methods were used to measure fΔTvalues, 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. © 2001 Elsevier Science B.V.