Kinetic study of the radiationless deactivation of singlet oxygen (1Δg) in liquid solution under high pressure

Abstract

The mechanism of the radiationless deactivation of singlet oxygen, ¹O₂(¹Δ_g), in several solvents at pressures up to 400 MPa was investigated. The lifetime decreased significantly with increasing pressure and the activation volume, ΔV_D^≠, for the bimolecular quenching constant, k_D, was determined to be in the range from − 12.0 to − 7.0 cm³ mol⁻¹. It was found for a series of n-alkanes (C₅–C₈ and C₁₂) that k_D increases linearly with increasing number of methylene groups at pressures up to 400 MPa (additivity relationships). The activation volume separated by the additivity relationships was determined to be from − 14.4 to − 12.5 cm³ mol⁻¹ for CH₃ and from − 9.2 to − 7.3 cm³ mol⁻¹ for CH₂, depending on the solvents examined. The lifetime of ¹O₂ was also measured in methylcyclohexane at temperatures from − 37 to 84°C and 0.1 MPa, and the activation energy for k_D was found to be positive (1.6 kJ mol⁻¹). These results are interpreted successfully by assuming a kinetic model in which encounter and collision complexes formed between ¹O₂ and solvent are involved. It was also shown that the kinetic model can be applied to systems with charge transfer interactions between ¹O₂ and solvent.