  A new mechanism for the primary photoinduced charge separation in
photosynthesis is proposed. It involves as real intermediate between the
excited special pair state P* and the primary charge separated state P+HL- a
trip-trip-singlet PTBLT, which consists of a triplet on the dimer P and a
further triplet on the monomer BL. 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+BL-, is reinterpreted as a transient excitation of this trip-trip
singlet.
