Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
Basic Research Needs for Solar Energy Utilization - Office of ...
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NEW SCIENTIFIC OPPORTUNITIES<br />
Making a Smart Matrix. A bio-inspired smart matrix must be able to promote (1) the<br />
conduction <strong>of</strong> holes and electrons over required distances and at moderate redox potentials,<br />
without significant losses; (2) the accumulation and storage <strong>of</strong> numbers <strong>of</strong> charges to enable<br />
chemical catalysis; (3) growth <strong>of</strong> the assembly in an ordered way from the nanoscale to the<br />
macroscale; and (4) compartmentalization <strong>of</strong> redox components and incompatible products, such<br />
as hydrogen and oxygen. In addition, a matrix that mediates a specific process (light harvesting,<br />
charge separation, chemical catalysis, etc.) must be compatible with integration into functional<br />
solar energy conversion systems.<br />
Charge Transport in Dynamically Constrained Environments. When a charge separation<br />
reaction occurs in solution, it is well known that the newly <strong>for</strong>med charges interact with solvent<br />
dipoles in their immediate vicinity, leading to a reorganization <strong>of</strong> the overall orientation <strong>of</strong> the<br />
solvent molecules relative to the charged intermediates. This change in solvent orientation<br />
requires an energy penalty that may be reasonably large in polar media, which results in an<br />
overall slowing <strong>of</strong> the electron transfer rate. The protein in photosynthetic reaction centers<br />
provides an environment that dynamically adjusts to minimize the energy penalty as the chargeseparation<br />
process occurs. Specific motions <strong>of</strong><br />
individual amino acids may be critical in<br />
gating electron flow within the protein. In<br />
addition, the overall electrostatic environment<br />
<strong>of</strong> the protein provides a spatially tailored<br />
potential that promotes directional electron<br />
flow. This concept is illustrated schematically<br />
in Figure 39. It is important to understand<br />
which protein motions are responsible <strong>for</strong> this<br />
optimization, how to control this process, and<br />
how to adapt this process <strong>for</strong> use in bioinspired<br />
artificial photosynthetic systems.<br />
This is a challenging problem that requires<br />
new techniques to probe molecular structure<br />
at the ultrafast time scales characteristic <strong>of</strong><br />
these electron-transfer events.<br />
129<br />
hν<br />
energy charge<br />
energy<br />
hν<br />
charge<br />
Figure 39 A smart matrix. Left: The present<br />
situation <strong>for</strong> transporting energy or charge with<br />
covalently linked, synthetically tuned<br />
chromophores. Right: Self-assembled<br />
chromophores whose properties are tailored by<br />
the smart matrix.<br />
Proton-coupled Electron Transfer and Multiple Electron Transfers. In most biological<br />
redox processes, single electron-transfer events are followed by proton transfers that diminish the<br />
overall energy penalty paid by accumulating several negative charges in one location. Typically,<br />
the acid-base properties <strong>of</strong> the amino acids that are in the vicinity <strong>of</strong> the reduced species are<br />
involved in this process. It is <strong>of</strong>ten very difficult to discern the molecular details <strong>of</strong> protoncoupled<br />
electron-transfer processes, because the proton movements occur over short distances<br />
and x-ray structural probes are generally not capable <strong>of</strong> determining the positions <strong>of</strong> the protons.<br />
A major challenge is to find new ways to understand proton-coupled electron transfer and the<br />
structural requirements <strong>for</strong> minimizing the energetic requirements <strong>for</strong> such processes. New time-<br />
and spatially resolved probes <strong>for</strong> determining the mechanisms <strong>of</strong> these reactions are critical <strong>for</strong>