Spin-orbit coupling mechanism of singlet oxygen a1Δg quenching by solvent vibrations

Degenerate character of the O2(a1Δg) state and of the charge-transfer configurations (CTCs) from solvent to the oxygen open-shell orbitals explains the enhancement of spin-orbit coupling (SOC) which is necessary to overcome spin prohibition during singlet oxygen a1Δg quenching. The former mechanism of non-radiative transition O2(a1Δg) → O2(X3Σg-) based on electronic energy transfer to the solvent vibrational levels (e-v mechanism) is supplemented here by explicit analysis of SOC effects mediated by solvent and O2 vibrations. The SOC matrix element between one component of the initial electronic excited singlet a1Δg state and the final ground triplet X3Σg- state in the oxygen moiety is not equal to zero (as in free O2) in the collision complex with solvent molecule (M) when all possible CTCs of the type O2-…M+ are accounted for. Intermolecular configuration interaction between CTC and locally excited states obeys a simple symmetry selection rule which provides finally the SOC matrix element with a guarantee of large orbital rotation around the molecular oxygen axis creating a torque. The CTCs admixtures into the singlet and triplet wave functions in the collision complex O2…M ensure the SOC enhancement inside the O2 moiety and let the spin-prohibited singlet oxygen a1Δg quenching to become effectively allowed in terms of e-v mechanism. In the new model the solvent is not only a passive “sink” for the singlet oxygen excitation energy but serves as an active perturber of the oxygen open shell and finally - of the whole spin dynamics in the collision system.

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