Mechanism of the excited singlet and triplet states quenching by molecular oxygen in acetonitrile

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k(q)(S) and k(q)(T), respectively, of some aromatic hydrocarbons in acetonitrile are reported. The fraction of excited singlet states quenched by oxygen which result in triplet states, f(T)(S), are in the range of 0.25-0.85. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f(Delta)(S), and f(Delta)(T) respectively, were also measured. Values of f(Delta)(S) were shown to be 0.28 +/- 0.05 for 1,2;5,6-dibenzanthracene, pyrene, fluoranthene, 1, 11-benzoperlyene and perlyene while values of f(Delta)(T) cover the range of 0.25-1.0. 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. Factors governing the production of singlet oxygen with different efficiencies during quenching of both excited singlet and triplet states by oxygen are discussed. (c) 2004 Elsevier B.V. All rights reserved.