Time-domain magnetic resonance studies of short-lived radical pairs in liquid solution

Magnetic resonance spectra of radical-ion pairs possessing lifetimes as short as 12 ns have been obtained using a new time-resolved optically detected magnetic resonance technique. Short-lived radical pairs are produced by a laser flash. The transient optical absorbance of the radical pairs or the triplet products resulting from their collapse is monitored as a function of time in the presence of high-power 9.1 GHz radiation as a magnetic field is swept. At resonance the microwaves induce transitions among the radical-pair energy levels that are observed as changes in the population of either the radical pair or the triplet products resulting from radical-pair collapse. These resonances can be used to obtain radical-pair structure and dynamics. Radical-ion pairs produced in the reaction-centre protein from the photosynthetic bacterium R. sphaeroides and radical-ion pairs resulting from the photoreduction of anthracene by N,N-diethylaniline in acetonitrile are discussed. All experiments are performed at ambient temperature in liquid solution.