Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O2–O2 and N2–O2 Encounter Complexes

Singlet Oxygen Generation via UV-A, -B, and -C Photoexcitation of Isoprene-Oxygen (C5H8-O2) Encounter Complexes in the Gas Phase

The formation of singlet oxygen ¹O₂ provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C₅H₈-O₂) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 μm. The photogeneration of ¹O₂ is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of ¹O₂ is measured. This yield of ¹O₂ is governed mainly by photoexcitation of O₂ molecules to the Herzberg III (³Δ_u) state via enhanced absorption by C₅H₈-O₂ collision complexes. So excited triplet O₂ gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O₂ molecules. In the UV-B (308 nm) region the appearance of ¹O₂ is attributed to the excitation of a double spin-flip (DSF) transition in complex C₅H₈-O₂. In the UV-A region (355 nm) besides DSF the O₂-assisted T₁ ← S₀ excitation of isoprene to the triplet state takes place, which is a sensitizer of ¹O₂ formation. The contribution of the encounter complexes C₅H₈-O₂ to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.

Singlet oxygen photogeneration from X–O2 van der Waals complexes: double spin-flip vs. charge-transfer mechanism

The channel of singlet oxygen O₂(¹Δ_g) photogeneration from van der Waals complexes of oxygen X–O₂ has been investigated to discriminate between two mechanisms based on charge-transfer or double spin-flip transitions.