Develop Charge Transport Structures to Deliver Redox Equivalents to Catalysts
Structured assemblies need to be developed that promote organization of the active units (light-
harvesting, charge-conduction, catalytic) to optimize coupling between them for efficient fuel
production. Molecular linkages, such as molecular “wires,” need to be developed for efficient
charge conduction between catalytic sites and photoactive components embedded in the
assembly. For example, one class of such assemblies is 3-D mesoporous inert supports that allow
precise spatial arrangement of the active components in a predetermined way for optimum
coupling and protection from undesired chemistries. These supports must have structural
elements (walls, membranes) that allow separation of primary redox products on the nanometer
scale to prevent undesired cross-reactions and facilitate prompt escape of the products from the
fuel-forming sites. Catalytic sites should be separated in such a way that energy-rich products,
such as H 2 and O 2 , cannot recombine thermally. A few molecular catalytic components are
currently available for multi-electron H 2 O and CO 2 activation, but methods are lacking that allow
coupling of these components to electron/hole conducting moieties in 3-D frameworks.
Couple Single Photon Events to Accumulation of Multiple Redox Equivalents
In most cases, the absorption of light by a chromophore leads to the production of a single
electron-hole pair. However, fuel-forming reactions involve the formation of covalent bonds,
which are formed from electron pairs. Thus, an integrated solar fuels production system must
accumulate electrons from single-photon events and deliver them to the site of fuel molecule
formation. An excellent example of this function is the water oxidation catalyst of
photosynthesis, which can accumulate the oxidation equivalents needed to split water. There has
been very little research along these lines in molecule-based systems, and finding practical ways
to accumulate redox equivalents at a particular molecular site is a major scientific challenge.
Develop Control Elements that Modulate Energy and Charge Flow between Active
Components
Photosynthesis incorporates control elements that maximize photosynthetic performance under
low light conditions and protect the photosynthetic apparatus during times of very high light
intensity that could lead to photodamage. Integrated artificial photosynthetic systems for solar
fuel production will ultimately need similar built-in control elements. For example, in times of
excessively high light intensity, antennas could be decoupled from charge-separation centers,
and the excess light energy degraded to heat or emitted as fluorescence in order to prevent
photodamage. It is also necessary to create architectures that actively partition excitation energy
absorbed by an antenna among different reaction centers in order to maintain each reaction
center at maximum efficiency.
RELEVANCE AND POTENTIAL IMPACT
The design and development of light-harvesting, photoconversion, and catalytic modules capable
of self-ordering and self-assembling into an integrated functional unit will make it possible to