Factors Affecting the Rate of Decay of the First Excited Singlet State of Molecular Oxygen O₂(aΔg) in Supercritical Fluid Carbon Dioxide

The photosensitized production and subsequent decay of singlet oxygen in supercritical fluid carbon dioxide has been studied as a function of pressure and temperature. The rate of decay is shown to be a function of the fluid density, demonstrating a quenching effect by the solvent. The observed rate constant for decay is also found to depend on the oxygen concentration, as a result of quenching of singlet oxygen, O2(a1Δg), by ground state oxygen. The rate constant for quenching by oxygen is found to be 1.9 × 104 dm3 mol-1 s-1, at 41 °C and at a pressure of 150 kg cm-2, which is compared with previous findings in conventional solvents. The lifetime of singlet oxygen in supercritical fluid carbon dioxide, under these conditions, extrapolated to the value in the absence of ground state molecular oxygen, was found to be 5.1 ms (at 150 kg/cm2 and 41 °C). The factors determining the lifetime of O2(a1Δg), in supercritical fluid carbon dioxide are discussed.