Control of radical pair lifetimes by microwave irradiation. Application to photosynthetic reaction centres

Radicals produced by illumination or ionizing radiation are often produced in pairs, which quickly decay by recombination or by diffusion and subsequent reactions. For maximizing the yield of products, and for facilitating the study of reaction pathways, it is desirable to minimize the probability of radical pair recombination. We present a way of controlling the radical pair lifetime through the application of a pulse of resonant microwaves in the presence of a magnetic field. Herewith, two radical pair triplet states are coherently populated, from which the pair cannot recombine directly to the singlet ground state because of spin conservation. We illustrate the method with a photosynthetic photochemical reaction, where we have achieved an increase in the radical pair lifetime of up to two orders of magnitude.