Role of Singlet Excited States of Molecular Oxygen in the Quenching of Organic Triplet States

Abstract

The quenching of triplet‐state molecules by molecular oxygen is examined theoretically. Two different quenching processes are considered: namely, (i) quenching by transfer of electronic excitation energy from the triplet state of the donor molecule to oxygen, resulting in a ground‐state donor molecule and an electronically excited singlet‐state oxygen molecule; and (ii) quenching by enhancement of intersystem crossing in the donor molecule, resulting in a vibrationally excited donor molecule in its ground electronic state and a ground‐state molecule. The relative importance of these two quenching processes is determined almost entirely by Franck—Condon factors, rather than by electronic factors. From our analysis we conclude that quenching by transfer of electronic excitation energy from the triplet‐state molecule to oxygen is usually 100–1000 times faster than quenching by enhanced intersystem crossing. Exchange interactions appear to be less important in promoting the radiationless transitions than do interactions involving charge‐transfer virtual states. The absolute value of the quenching rate calculated for the energy transfer process satisfactorily accounts for the experimental observations on the oxygen quenching of triplet‐state molecules in solution. These theoretical results give strong support to the previous suggestion that quenching of triplet‐state molecules by oxygen leads to the production of electronically excited singlet‐state (¹Σ_g⁺ or ¹Δ_g) oxygen molecules.