ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET AND TRIPLET BACTERIOPHEOPHYTIN—II. THEORETICAL

Abstract

Abstract— 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+/BphT, 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 [BphTQ], 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, ΔE˜ 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.