Theoretical and experimental study of the quenching of singlet oxygen by solvent

Abstract

The intermolecular electronic—vibrational energy transfer theory based on dipole—dipole interaction has been extended to treat the quenching of singlet ( 1Δ g) oxygen by solvent. According to this treatment, the off-resonance factor which relates to the singlet oxygen emission bandwidth in the solvent is included in the rate constant expression and has a definite value; the additivity relationship descovered by Schuster and Rodgers can also be explained. The lifetimes of singlet oxygen in n-chloroalkane, n-bromoalkane and n-iodoalkane solvents were determined by measuring the time-resolved decay of singlet oxygen luminescence at 1.27 μm. The excitation source is a laser light which has a pulse width of 15 ns with a maximum power of 7 mJ at 592 nm. A good linear relationship between the bimolecular rate constants and the number of methylene groups in the carbon skeleton of the solvent is found. The bimolecular rate constants contributed by methyl and methylene groups are 250 M −1 s −1 and 800 M −1 s −1 respectively. The finding that the methylene group is a better quencher than the methyl group eliminates the possibility that the highest frequency fundamental vibrational mode plays a more important role than the overtone or the combination mode. The significant increase in the quenching rate constants in iodoalkanes is probably due to an effect other than the heavy atom effect.