Direct Detection of Singlet Molecular Oxygen by Electron Paramagnetic Resonance and Development of a New Quantitative Method for the Concentration Measurement

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The lowest electronically excited singlet state of oxygen molecule, O_2(^1Δ_g), has been a subject of study for many years, with particular interest being taken in many photochemical and biological problems. O_2(^1Δ_g) is an active intermediate in the photooxygenation reactions. It is important to measure the concentration of O_2(^1Δ_g) for most applications requiring O_2(^1Δ_g) as a reactive intermediate. Since the ground-state oxygen molecule, O_2(^3Σ_g^-), is recommended as a quantitative standard, the most reliable technique for determining O_2(^1Δ_g) concentration has been electron paramagnetic resonance(EPR) spectroscopy. Oxygen in the ^1Δ_g state can be detected using the EPR technique, because O_2(^1Δ_g) is paramagnetic owing to its orbital angular momentum.O_2(^1Δ_g) molecules were generated from O_2(^3Σ_g^-) by gas-phase energy transfer from the excited triplet states of the sensitizer molecules. Naphthalene, 1-fluoronaphthalene, octafluoronaphthalene, and benzophenone were u … More sed as the sensitizer. The EPR spectra were measured at room temperature by a JEOL JES-FA200 X-band spectrometer equipped with a homemade 100 kHz power amplifier and a Varian V-4535 large sample access cylindrical cavity. The excitations were carried out using a Canrad-Hanovia 1 kW Xe-Hg arc lamp.The characteristic nearly symmetrical four-line EPR spectrum of O_2(^1Δ_g) was observed at about 1 T during excitation. The EPR signals of O_2(^3Σ_g^-) decreased immediately with rise of O_2(^1Δ_g) signals on excitation. After shutting off the exciting light, the O_2(^3Σ_g^-) signals rose and the O_2(^1Δ_g) signals disappeared. These results show that the loss of O_2(^3Σ_g^-) on excitation is negligible compared to the amount of produced O_2(^1Δ_g). We can safely assume that the amount of O_2 in some metastable state other than ^1Δ_g is negligible. Therefore we estimated the concentration of O_2(^1Δ_g) directly. Under our experimental conditions, the steady-state O_2(^1Δ_g) concentration of 40% was obtained by the photosensitization with octafluoronaphthalene at low pressures, 【approximately equal】0.3 Torr. Less