Modeling the Effect of Solvents on Nonradiative Singlet Oxygen Deactivation: Going beyond Weak Coupling in...

For almost 50 years, attempts have been made to account for the pronounced solvent effect on the lifetime of singlet molecular oxygen, O₂(a¹Δ_g). This process is dominated by the O₂(a¹Δ_g) → O₂(X³Σ_g^-) nonradiative transition. Given the comparatively low O₂(a¹Δ_g) excitation energy of ∼7880 cm^-1, existing models have been built upon a foundation of electronic-to-vibrational (e-to-v) energy transfer in which C-H and O-H stretching modes in the solvent act as the dominant energy sink. The latter accounts for large H/D solvent isotope effects on the O₂(a¹Δ_g) lifetime. However, recent experiments showing a pronounced temperature effect on the O₂(a¹Δ_g) lifetime in some solvents reveal limitations in these models. We have developed a general and computationally tenable model that accounts for both temperature and H/D solvent isotope effects on the O₂(a¹Δ_g) lifetime. A key feature of our approach is the need to strike a balance in the oxygen-solvent interaction between weak and strong coupling. In the weak coupling limit, the O₂(a¹Δ_g) → O₂(X³Σ_g^-) transition probability is determined by the overlap of vibrational wave functions, and this is the main component defining the H/D isotope effects. In the strong coupling limit, the transition probability is determined by an activated process and thus accounts for the observed temperature dependence. In addition to resolving a long-standing oxygen-dependent problem, our model may provide useful insights into a wide range of bimolecular interactions that involve e-to-v energy transfer.