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

Abstract

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 possible mechanisms based on charge-transfer (CT) or double spin-flip (DSF) transitions. The results obtained in this work for complexes with X = ethylene C₂H₄, 1,3-butadiene C₄H₆, deuterated methyl iodide CD₃I, benzene C₆H₆ and water H₂O and for those investigated previously indicate the DSF mechanism as a source of singlet oxygen. The formation of O₂(¹Δ_g) is observed only when the energy of exciting quantum is sufficient for DSF transition. Universally detected low vibrational excitation of O₂(¹Δ_g) arising in the photodissociation of van der Waals complexes X–O₂ indicates the DSF mechanism as its source. For complex of ethylene C₂H₄–O₂ab initio calculations of vertical energy ΔE_vert for DSF and CT transitions have been carried out. The positive results of singlet oxygen formation from C₂H₄–O₂ can be explained by the DSF but not by the CT mechanism.