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

This is the authors' abstract. We don't add key points for this paper.

The formation of singlet oxygen 1 O 2 provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C 5 H 8 –O 2 ) 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 1 O 2 is found to be a one-photon process. In the UV-C region (253–278 nm) the quantum yield of 1 O 2 is measured. This yield of 1 O 2 is governed mainly by photoexcitation of O 2 molecules to the Herzberg III ( 3 Δ u ) state via enhanced absorption by C 5 H 8 –O 2 collision complexes. So excited triplet O 2 gives rise to singlet oxygen because of triplet–triplet annihilation in the collisions with unexcited O 2 molecules. In the UV-B (308 nm) region the appearance of 1 O 2 is attributed to the excitation of a double spin–flip (DSF) transition in complex C 5 H 8 –O 2 . In the UV-A region (355 nm) besides DSF the O 2 -assisted T 1 ← S 0 excitation of isoprene to the triplet state takes place, which is a sensitizer of 1 O 2 formation. The contribution of the encounter complexes C 5 H 8 –O 2 to the production of singlet oxygen and to the lifetime of isoprene in the Earth’s troposphere are estimated.