following challenges must be met: (1) understanding repair and photoprotection mechanisms in
natural photosynthesis; (2) exploring assembly-disassembly strategies as a mode of self-repair in
artificial photosynthetic systems; (3) developing active repair molecules that specifically identify
and target defects in artificial photosynthetic systems and repairing them; and (4) exploring
redundancy and distributed function as a strategy to circumvent damage.
Photocatalysis and Photodriven Reactions
Significant scientific challenges remain in the effort to design and synthesize efficient, high-
turnover catalysts for the conversion of solar energy into energy-rich fuels. Important reactions
include the splitting of water to O 2 and H 2 and the reduction of CO 2 to CH 4. To carry out these
reactions, researchers will need to design and synthesize robust ligands, multi-metallic catalysts,
and tailored environments surrounding the catalysts. Guideposts for the development of new
systems will come, in part, from the new understanding acquired from bioenergetic enzymes
involved in fuel-producing reactions, especially the water-splitting reaction of PSII and the
H 2 -producing reaction of hydrogenases. A combination of techniques (ranging from x-ray
crystallography and magnetic resonance spectroscopies to genetic engineering) is needed to
elucidate the structure and dynamics of the intermediates of catalytic reactions occurring at redox
centers of key enzymes involved in solar energy conversion. This knowledge will provide the
blueprint necessary to formulate the precise molecular mechanisms of these biological processes
and apply them to catalyst design and synthesis.
Newly designed catalysts for fuel formation must rely on complex mechanisms that incorporate
multi-electron, atom, and proton-coupled electron-transfer reactions. The need to understand
bond-breaking and bond-making processes that accompany electron transfer during fuel-forming
transformations will require new synergistic theoretical treatments and mechanistic studies that
address these events. Mechanistic studies are therefore essential to the rational design of
advanced catalytic systems. Important targets include multi-metallic systems, with particular
emphasis on catalysts that use non-noble metals to replace costly and rare noble metals. It is also
critical to develop efficient catalysts for solar-driven production of fuels (H 2 , CH 3 OH, etc.) that
do not depend on consumption of a “sacrificial” molecule. For example, solar-driven water
splitting will produce H 2 and O 2 , both of which are consumed to regenerate H 2 O when the fuel is
used.
With guidance from theoretical calculations, researchers must develop an understanding of
catalytic mechanisms at a molecular level, whether the catalyst acts in solution, at interfaces, or
on surfaces. In addition, they must understand how catalysts interact with their surrounding
environment. This understanding should include detection of the intermediates and identification
of their sequence of formation, kinetics, and energetics. The development of molecular strategies
to compartmentalize mutually incompatible products, such as H 2 and O 2 , in the fuel formation
process is also important. New fuel-forming catalysts must also be integrated into the higher-
order assemblies required to construct practical photoelectrochemical devices. To achieve this
level of mechanistic understanding, a combination of structural and time-resolved techniques
must be employed, together with careful design of model systems that facilitate the application
of these techniques.