In this seminar, two topics in which ultrafast optical spectroscopy has been used to probe fundamental aspects of charge separation will be discussed: (1) electron transfer in the bacterial photosynthetic reaction center and (2) ground-state hole transfer tetrapyrrole arrays.

The primary photochemical steps in the energy conversion process in photosynthesis occur in pigment-protein complexes called reaction centers (RCs).  In the bacterial RC, the membrane-spanning multi-step charge separation process utilizes only one of two parallel electron-transport chains (the so-called A branch, or A side).  Bi-directional electron transfer has been achieved in mutants in which electron transfer from the excited bacteriochlorophyll dimer (P*) to the B-side bacterioheopheophytin (HB) has a yield up to about 30%, with faster electron transfer to the A-side accounting for most of the remaining decay of P*.  Such results have been applied to tailoring a mutant lacking the A-side bacteriopheophytin (HA) that gives a 70% yield of electron transfer to HB.  In a related mutant that also lacks HA, electron transfer has been switched back to the A-side and an electron has been trapped on the earliest intermediate, a bacteriochlorophyll molecule dented BB, for about 0.5 ns; in the wild-type RC, this intermediate state achieves only a small (~15%) population from P* and lives for only ~0.5 ps.  The switching between the A- versus B-side pathways is achieved by manipulating the amino acid residues near the cofactors and thereby the energetics of charge separation on the two branches.

Efficient solar-energy conversion, such as in molecular solar cells, requires that holes generated after excited-state electron-transfer or electron-injection can move efficiently away from the anode, thereby preventing charge-recombination.  Thus, understanding hole mobility in prototypical light-harvesting and charge-separation systems is of fundamental interest.  Hole transfer is often studied in molecular arrays following photoinduced electron transfer, and thus occurs in the presence of a Coulomb field.  The studies also typically involve transfer between different pigments, and thus down a free-energy gradient.  The rate constants for ground-state hole transfer in the absence of a Coulomb potential and in the absence of a free-energy gradient (i.e., between nominally identical sites and thus lacking a simple spectral signature) are normally difficult to measure.  In arrays of covalently linked porphyrins, only upper limits of about 50 ns for the times of such processes have been determined by EPR spectroscopy.  We have developed systems in which ground state hole transfer between redox equivalent sites (e.g. two zinc porphyrins) and in the absence of a Coulomb potential.  In the arrays studied, there is only a small effect of linker length on the rate of this process, and the same is true for hole transfer between non-equivalent sites (e.g. zinc and free base porphyrins).  Part of the explanation may be a balancing of the effects of linker length and energetics (MO energies).  Implications of such results for artificial solar-energy conversion systems based on molecular arrays will be discussed.