Basic Research Needs for Solar Energy Utilization - Office of ...

Efficient Photo-initiated Charge Separation and Storage. The dependence of the rates of
electron transfer reactions within covalently linked donor-acceptor molecules on the free energy
of the reaction and the electronic interaction between the donor and the acceptor are described
well by theory (Marcus 1956). Both theory and experiment show that there is an optimal free
energy for achieving the maximum electron transfer rate, and therefore the maximum efficiency,
for this process. Moreover, a key prediction of theory is that the rate of an electron transfer
reaction will slow when the free energy of the reaction becomes very large. The key to observing
this so-called “inverted region” in donor-acceptor molecules is maintaining a fixed distance
between the donor and the acceptor as the structure of the donor and/or the acceptor is changed
to modify the free energy (Miller et al. 1984; Wasielewski et al. 1985). The use of large free
energies for charge recombination to slow these energy-wasting reactions is critical to achieving
the long charge separation times essential for driving catalysts for fuel formation.
Another important prediction of electron transfer theory is that the rates (and efficiencies) of
electron transfer generally decrease exponentially as a function of distance. Experiments have
confirmed this exponential distance dependence and have shown that the steepness of this
dependence reflects the molecular structure of the molecules linking the electron donor to the
acceptor. Rates of electron transfer reactions generally decrease by about a factor of 30 for every
1 nm of distance (Paddon-Row et al. 1988).
The various electron donors and acceptors used in bio-inspired artificial photosynthetic systems
need not be covalently linked to one another. In fact, natural photosynthetic systems use the
surrounding protein to position the chlorophyll electron donors and suitable acceptors close to
one another. The nature of non-covalent interactions among electron donors and acceptors, such
as those found in molecules ranging from DNA to the bacterial photosynthetic RC, is an
important area of investigation. Non-covalent assemblies may be constructed through a variety
of weak chemical interactions between molecules, e.g., hydrogen bonding, coordination bonding,
π-π stacking, formation of donor-acceptor charge transfer complexes, and electrostatic
interactions. For example, it has been shown that photogenerated positive charges can move
within DNA by means of non-covalent interactions between the stacked base pairs (Lewis et al.
1997).
The importance of using a cascade of thermal electron transfer steps following the initial
photoinduced charge separation, as evidenced by natural photosynthesis, has been demonstrated
in numerous systems. Studies on the optimization of the free energy changes, distances, and
orientations between the various donors and acceptors have allowed researchers to determine
strategies for the development of novel molecular structures to tailor the charge separation and
storage characteristics to specific applications. For example, efficient performance in the solid
state requires (1) the use of specialized donor and/or acceptor molecules, such as C60, that
undergo minimal structural changes following electron transfer, or (2) the incorporation of highpotential
donors and acceptors to overcome the inability of the solvent to change its structure in
the solid state. In these systems, photoinduced charge separation, followed by 1–3 thermal
electron transfer steps, leads to overall charge separation efficiencies of about 80% that persist
for times approaching seconds (Gust et al. 2001; Wasielewski 1992). Ultrafast laser techniques
that measure events down to 20 fs (20 quadrillionths of a second), as well as time-resolved
measurements of the magnetic properties of charged intermediates produced within these
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