Charge separation in photosynthesis via a spin exchange coupling mechanism

A new mechanism for the primary photoinduced charge separation in photosynthesis is proposed. It involves as a real intermediate between the excited special pair state P* and the primary charge separated state P+ H-L(-) a trip-trip-singlet B-T B-L(T), which consists of a triplet on the dimer P and a further triplet on the monomer B-L Both combine to a singlet. The electron transfer is caused by spin exchange couplings. The transient spectrum of the short lived intermediate, formerly taken as evidence for the charge transfer state P+ B-L(-) is reinterpreted as a transient excitation of this trip-trip singlet.