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, O