The magnetic field influence on bridge-assisted electron transfer

The influence of an external magnetic field on the rate of electron transfer is studied theoretically for the case of a donor-acceptor complex with paramagnetic ions as bridging sites. We demonstrate for a bridge with a single and with two paramagnetic ions that the applied magnetic field causes a blocking effect of the electron transfer. Each paramagnetic ion staying in its electronic ground state is assumed to reduce its spin from if the transferred electron is coupled to it. Such a spin reduction determines the magnetic field dependence of the transfer rate in a specific manner. The magnetic field dependence is derived at low temperatures in utilizing the Wigner 6j-symbols method. For the case of two paramagnetic ions per bridge the exchange interaction between them has been additionally included into the calculations. It is responsible for the step-like dependence of the rate constant on the magnetic field, as well as for the creation of a rather narrow field-strength region where the rate constant drops to zero.