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