Modeling the Effect of Solvents on Nonradiative Singlet Oxygen Deactivation: Going beyond Weak Coupling in Intermolecular Electronic-to-Vibrational Energy Transfer.

For almost 50 years, attempts have been made to account for the pronounced solvent effect on the lifetime of singlet molecular oxygen, O2(a1Δg). This process is dominated by the O2(a1Δg) → O2(X3Σg-) nonradiative transition. Given the comparatively low O2(a1Δg) 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(a1Δg) lifetime. However, recent experiments showing a pronounced temperature effect on the O2(a1Δg) 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(a1Δg) 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(a1Δg) → O2(X3Σg-) 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.