Pr. Ogilby et J. Sanetra, MAGNETIC-FIELD EFFECTS ON EXCITED-STATE OXYGEN ORGANIC-MOLECULE INTERACTIONS, Journal of physical chemistry, 97(18), 1993, pp. 4689-4694
The effects of an applied magnetic field (<22 kG) on (1) the lifetime
of singlet oxygen (1DELTAgO2) and (2) the quenching of triplet chrysen
e 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) encounte
r pair. Under certain conditions, the 1DELTAgO2 and 3chrysene deactiva
tion 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 lyi
ng 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 ch
aracter and are most pronounced (1) in solvents where the charge-trans
fer state is more stable, (2) when M and O2 are held in close proximit
y for a longer period of time, and (3) when the rate of singlet-triple
t spin evolution is much slower than dissociation of the excited-state
M-O2 encounter pair to regenerate solvated reactants. Furthermore, th
e observation of a solvent hydrogen/deuterium magnetic isotope effect
on the 1DELTAgO2 lifetime is consistent with a mechanism in which hype
rfine interactions influence the rate of electron spin evolution. Wher
e magnetic field effects were observed, the data indicate that singlet
-state-triplet-state mixing becomes less probable at higher field stre
ngths.