Magnetic field effect on the photooxidation efficiency of ferrocene

The cage escape efficiency νce of radicals in the electron transfer quenching of triplet methylene blue by ferrocene in acetonitrile was found to be sensitive towards an external magnetic field. The field dependence of the effect corresponding to a monotonic decrease of νce was determined in a field range from 0.0 to 3.2T, where it reaches a value of −20.7%. The effect is analyzed in terms of a spin chemical model wherein the effective rate of geminate reverse electron transfer, regenerating the singlet ground state of the reactants, becomes magnetic field dependent due to magnetic mixing of non-uniformly reactive spin sublevels by the anisotropic Zeeman interaction. From the analysis, the absolute values of the rate constants of spin-allowed reverse electron transfer (605 ns−1), cage escape (15 ns−1, and electron spin relaxation in the ferricenium cation (154 ns−1) could be determined.