Perturbed and Activated Decay: The Lifetime of Singlet Oxygen in Liquid Organic Solvents.

Singlet oxygen, O(aΔ), the lowest excited electronic state of molecular oxygen, plays an important role in a range of chemical and biological processes. In liquid solvents, the reactions of singlet oxygen with a solute kinetically compete with solvent-mediated deactivation that yields the ground electronic state of oxygen, O(XΣ). In this regard, the key parameter is the solvent-mediated lifetime of singlet oxygen, which embodies fundamental physical principles ranging from intermolecular interactions that perturb the forbidden O(aΔ) → O(XΣ) transition to the transfer of oxygen's excitation energy into the vibrational modes of a solvent molecule M. Extensive research performed by the global community on this oxygen-related issue over the past ∼50 years reflects its significance. Unfortunately, a satisfactory quantitative understanding of this unique solvent effect has remained elusive thus far. In temperature-dependent studies, we have quantified the singlet oxygen lifetime in common aromatic and aliphatic organic solvents, including partially deuterated molecules that exploit the H/D solvent isotope effect on the lifetime. We now account for experimental data, including previously intractable data, using a model that exploits both weak and strong coupling in the M-O complex to accommodate the roles that M plays to (1) induce the forbidden O(aΔ) → O(XΣ) transition and (2) accept the excitation energy of O(aΔ). As such, our approach brings us appreciably closer to an accurate and predictive solution for the long-standing oxygen-dependent problem that, in turn, should be relevant for a host of other molecular systems.