Electron transfer via a midway molecule as seen in primary processes in photosynthesis; A new process describable as superexchange or sequential in mutually opposite limits

Electron transfer (ET) mediated by a midway molecule should be described as a process new in itself, not in terms of direct ET between two molecules as a two-step sequential or a unistep superexchange mechanism. They describe only mutually opposite limits (not working in parallel as often assumed) that the reorganization time of phonons at the midway state is respectively much shorter or longer than the lifetime of an electron therein, when this state has energies comparable to the donor and acceptor states. We can obtain the rate constant of the ET by perturbational expansion in electronic coupling.