Surprisingly large magnetic field effect in the electron-transfer reaction of 4,4′-bipyridine with triethylamine in acetonitrile

For a triplet born radical-ion pair in homogeneous non-viscous acetonitrile solvent, curiously large magnetic field effects (MFEs) of about 40–60%, and exceedingly high saturation fields (150–200 mT) are observed in the photoreaction of 4,4′-bipyridine with triethylamine. The field effect on the transient absorption spectrum shows the existence of a triplet excited complex (3ECT) around 330 nm. Complementary field effects on different intermediates and patterns of the MFE curves indicate the reformation of 3ECT from the radical-ion pair. To account for the observed MFEs, inter-radical hydrogen bonding mediated by water molecules is suggested.