Application of the collision-complex model to the photophysical processes of singlet oxygen in liquids

In this paper we propose a collision-complex model for the quenching of singlet oxygen by solvents. Using this model, it is possible to explain the diffusion-controlled quenching rate constants observed for quenchers such as carotenes and the group additivity rules observed for quenchers such as ordinary organic molecules by Hurst, Schuster and Rodgers. We also present a theoretical treatment of the spectral shift of singlet oxygen in various solvents. It is applied to interpret the experimental data reported by Bromberg and Foote. It is shown that the agreement between experiment and theory is reasonable. The emission intensity and radiative rate constant of singlet oxygen in liquids are discussed from the viewpoint of the collision-complex model. Both of these properties are shown to be enhanced by solvents and the addition of other quenchers. We also report the experimental data of the quenching of singlet oxygen by 4-amino-TEMPO in methylene chloride solution. The data are analyzed by the collision-complex model. We show that, from the analysis of the experimental decay curves, both relative radiative and non-radiative rate constants can be determined.