Electron transfer reactions are commonplace in both chemical and biochemical transformations. These reactions generally involve the formation of transient radical ion pairs. Non-adiabatic electron transfer results in formation of a correlated radical ion pair1. Only when the radical ions have separated to distances of the order of 10 Å are the internal magnetic fields in the radical ions able to perturb this correlation. Thus radical pairs initially born in a singlet state will develop triplet character as a function of time, and vice versa. Because the ensuing chemistry of the radicals is dependent on their spin state, external perturbations of the radical pair spin state may permit some control over the course of the chemistry. Microwave radiation can provide a strong internal magnetic field perturbation on a radical pair such as the one that results from photoin-duced charge separation in bacterial reaction centres. We show here how this perturbation can be used to control the lifetime of the initially formed radical pair.
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