Electron Transfer From Photoexcited Singlet and Triplet Bacteriopheophytin—ii. Theoretical

A previous paper showed that collision of the first excited singlet state of bacteriopheophytin (Bph*) and p ‐benzoquinone (Q) returns Bph* to the ground state; however, excited triplet (Bph + ) and quinone on collision produce the radical ions, (Bph + ) and (Q − ). This paer rationalizes these findings by first estimating the half cell potentials Bph + /Bph* and Bph + /Bph T , the energy for the various collision complexes, and the energy of the charge separated ions Bph + + Q− and then estimating the rates for conversion among these various states. Thus it is estimated that the complexes [Bph*Q] or [Bph T Q], live ˜5 ps before dissociating. This is long enough for electron transfer to occur, producing the singlet and triplet charge transfer complexes, [Bph + Q−] S or [Bph + Q−] T , either of which could separate to Bph + + Q− in ˜230ps. In the singlet case, quenching by reverse charge transfer [Bph + Q−] S →[Bph Q] occurs more rapidly than ion separation; however, the analogous triplet process, [Bph + Q−] T → [Bph Q], is spin forbidden, so that ion separation competes successfully with quenching. Spin scrambling, [Bph + Q−] S ↔ [Bph + Q−] T , is estimated to be slow, as this explanation requires. In the bacterial photosynthetic reaction center, the initial electron transfer from an excited singlet state of the bacteriochlorophyll dimer complex (BB)* to bacteriopheophytin, giving [(BB + )(Bph−)] S , successfully leads to ion separated species (i) because reverse charge transfer [(BB + )(Bph−)] S → [(BB)(Bph)] is slowed by a fairly large Franck‐Condon energy, Δ E ˜ lev, which is difficult to convert from electronic to vibrational degrees of freedom and (ii) because of the rapid subsequent electron transfer from (Bph + ) to another acceptor X .