Solvent-dependent singlet oxygen lifetimes: temperature effects implicate tunneling and charge-transfer interactions

Abstract

The effect of solvent on the lifetime of singlet oxygen, O₂(a¹Δ_g), particularly the pronounced H/D solvent isotope effect, has drawn the attention of chemists for almost 50 years. The currently accepted model for this phenomenon is built on a foundation in which the electronic excitation energy of O₂(a¹Δ_g) is transferred to vibrational modes in a solvent molecule, with oxygen returning to its ground electronic state, O₂(X³Σ_g⁻). This model of electronic-to-vibrational (e-to-v) energy transfer specifically focusses on the solvent as a “sink” for the excitation energy of O₂(a¹Δ_g). On the basis of temperature-dependent changes in the solvent-mediated O₂(a¹Δ_g) lifetime, we demonstrate that this energy-sink-based model has limitations and needs to be re-formulated. We now show that the effect of solvent on the O₂(a¹Δ_g) lifetime is more reasonably interpreted by considering an activation barrier that reflects the extent to which a solvent molecule perturbs the forbidden O₂(a¹Δ_g) → O₂(X³Σ_g⁻) transition. For a given solvent molecule, this barrier reflects contributions from (a) the oxygen-solvent charge transfer state that mediates nonradiative coupling between the O₂(a¹Δ_g) and O₂(X³Σ_g⁻) states, and (b) vibrations of specific bonds in the solvent molecule. The latter establishes connectivity to the desirable features of the energy-sink-based model. Moreover, temperature-dependent H/D solvent isotope effects imply that tunneling through this barrier plays a role in the mechanism for O₂(a¹Δ_g) deactivation, even at room temperature. Although we focus on a long-standing problem involving O₂(a¹Δ_g), our results and interpretation touch fundamental issues of interest to chemists at large.