Quenching of electronically excited states by molecular oxygen in fluid solution

Rate constants, k(T)(O2), for quenching by oxygen of the triplet states of anthracene, biphenyl and naphthalene derivatives in acetonitrile and the efficiencies of formation thereby of singlet oxygen, f(Delta)(T), are discussed in the light of the new values obtained for biphenyl derivatives. In the case of anthracene derivatives little variation in k(T)(O2) or f(Delta)(T) is observed but for biphenyl an naphthalene derivatives the rate constants k(T)(O2) decrease as their oxidation potential rises whilst f(Delta)(T) varies in the opposite direction. In the case of biphenyl derivatives f(Delta)(T) rises from 0.31 to 0.84 as k(T)(O2) decreases from 12.5 to 0.88 x 10(9) dm(3) mol(-1) s(-1) on going from 4,4'-dimethoxybiphenyl to 4-cyanobiphenyl. The mechanism of quenching via singlet and triplet channels is discussed and the importance of the energy of the charge transfer state involving electron transfer to oxygen reative to the energy of the locally excited triplet state is established as important in determining k(T)(O2) and f(Delta)(T). These results indicate that charge transfer assisted quenching occurs via both singlet and triplet complexes with only partial (up to 13.5%) charge transfer character.