MAGNETIC-FIELD EFFECTS ON EXCITED-STATE OXYGEN ORGANIC-MOLECULE INTERACTIONS

The effects of an applied magnetic field (<22 kG) on (1) the lifetime of singlet oxygen (1DELTAgO2) and (2) the quenching of triplet chrysene by ground-state oxygen (3SIGMAg-O2) were examined in liquid solvents and solid organic polymers. Each process involves a ''spin-forbidden'' transition between states of an oxygen-organic molecule (M) encounter pair. Under certain conditions, the 1DELTAgO2 and 3chrysene deactivation rates decreased with an increase in the magnetic field strength. The data are consistent with a model in which the M-O2 charge-transfer state (M.+O2.-) imparts geminate radical ion character into lower lying states of the M-O2 encounter pair through configuration interaction. Magnetic field effects appear to derive from changes in the rate of singlet-triplet spin evolution in M-O2 states with radical ion pair character and are most pronounced (1) in solvents where the charge-transfer state is more stable, (2) when M and O2 are held in close proximity for a longer period of time, and (3) when the rate of singlet-triplet spin evolution is much slower than dissociation of the excited-state M-O2 encounter pair to regenerate solvated reactants. Furthermore, the observation of a solvent hydrogen/deuterium magnetic isotope effect on the 1DELTAgO2 lifetime is consistent with a mechanism in which hyperfine interactions influence the rate of electron spin evolution. Where magnetic field effects were observed, the data indicate that singlet-state-triplet-state mixing becomes less probable at higher field strengths.