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

Rate constants for quenching by molecular oxygen of excited singlet and tri plet states k(s)(O2) and k(T)(O2), respectively, of 11 aromatic hydrocarbon s in cyclohexane are reported. Measured values of k(s)(O2) are in the range 0.44 to 2.5 x 10(10) dm(3) mol(-1) s(-1) and are therefore in many cases c lose to k(d), the diffusion controlled value of 2.8 x 10(10) dm(3) mol(-1) s(-1). The fraction of excited singlet states quenched by oxygen which resu lt in triplet states, f(T)(O2), was measured for all the compounds and foun d to be unity. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f(Delta)(S) and f(Delt a)(T), respectively, were also measured. Values of f(Delta)(S), were shown to be 0, within experimental error, for all compounds except perylene for w hich f(Delta)(S) = 0.28 +/- 0.05. Values of k(T)(O2) vary from 0.46 to 2.32 x 10(9) dm(3) mol(-1) s(-1). Thus, k(T)(O2)/k(d) was found to be less than one-ninth for all the compounds studied. Three different methods were used to measure f(Delta)(T) values, which were found to range from 0.57 to 1. C ombination of the total quenching rate constants with the fractional effici encies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways. Quenching of both excited singlet and tr iplet states, with and without energy transfer, are shown to be 'charge-tra nsfer assisted' and mechanisms accounting for these results are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.