The balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by ππ* triplet states

Abstract

A charge transfer (CT) channel and a non-CT deactivation channel, both leading to formation of O₂(¹Σ⁺_g), O₂(¹Δ_g) and O₂(³Σ⁻_g), compete in the quenching of triplet states by O₂. Recent studies by our group demonstrated that these channels are described by rather simple and general quantitative relations. In the present paper we use the detailed kinetic data on the quenching by O₂ of ππ* triplet sensitizers of three homologous aromatic series in CCl₄ to derive a parameter, which describes the balance between CT and non-CT deactivation. This quantity, p_CT, is the relative contribution of CT mediated deactivation and is easily calculated for a sensitizer of known triplet energy from its quenching rate constant. The parameter p_CT quantitatively describes the balance between both deactivation channels without requiring any knowledge of oxidation potentials. It is shown how the variation of p_CT influences the efficiencies and the rate constants of O₂(¹Σ⁺_g), O₂(¹Δ_g) and O₂(³Σ⁻_g) formation in the quenching process.