Two-Photon Excitation of Neat Aerated Solvents with Visible Light Produces Singlet Oxygen

Aeration of common liquid solvents (e.g., cyclohexane, benzene, or water) gives rise to a unique absorption band associated with a transition from a ground state oxygen-solvent collision complex to an oxygen-solvent charge-transfer (CT) state. Although the absorption band is solvent-dependent, the energies involved invariably correspond to a one-photon transition in UV. We now show that this CT state can also be produced in a nonlinear two-photon process using visible light. Moreover, the CT state decays to produce an appreciable amount of singlet oxygen, O₂(a¹Δ_g). As such, we demonstrate a convenient method to generate O₂(a¹Δ_g) and oxygen-based radicals with high spatial resolution and localization control under conditions in which a photosensitizer is not added as a solute. We also provide a reasonable explanation for the undesired photoinduced oxidative degradation often observed in Raman and nonlinear optical microscopy experiments and in other laser-based applications where the solute itself does not absorb light. Finally, we provide evidence for a comparatively long-lived photoproduced species in liquid water, possibly the triplet state of water itself, that sensitizes O₂(a¹Δ_g) production.