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

Bio-inspired Approaches to Photochemical Energy Conversion
A major scientific challenge is the preparation of bio-inspired, molecular assemblies that
integrate light absorption, photoinduced charge separation, and catalytic water oxidation/fuel
formation into a single unit. These integrated assemblies must take full advantage of both
molecular and supramolecular organization to collect light energy and transfer the resulting
excitation to artificial RCs. These centers must separate charge and inject electrons and holes
into charge transport structures that deliver the oxidizing and reducing equivalents to catalytic
sites where water oxidation and CO2 reduction occur. It is critical to understand how excitation
energy flow from the antenna to a RC depends on molecular structure. In addition, charge
transport structures for delivery of redox equivalents to catalysts must be developed. By analogy
to natural photosynthesis, it is important to provide control elements, or “throttles,” to optimize
energy and charge flow within an artificial photosynthetic system as it responds to varying light
intensities and spectral distributions. One of the most difficult tasks critical to achieving system
integration is coupling single-photon events to the accumulation of multiple-redox equivalents
necessary to drive multi-electron, fuel-forming chemistry within a catalyst.
The assembly of complex photoconversion systems with synergistic functionality depends on a
variety of weak, intermolecular interactions, rather than strong, individual covalent chemical
bonds. A critical step toward fully functional photoconversion systems is the ability to create
increasingly larger arrays of interactive molecules. Covalent synthesis of near-macromolecular
arrays becomes highly inefficient and costly, thus requiring that practical photoconversion
systems be prepared using self-assembly to achieve ordered architectures from properly
functionalized building blocks. Self-assembly is based on a variety of weak interactions — such
as hydrogen bonding, electrostatic, metal-ligand, and π-π interactions — that give rise to ordered
structures. Achieving the goal of producing a functional, integrated artificial photosynthetic
system for efficient solar fuels production requires the following: (1) developing innovative
architectures for coupling light-harvesting, photoredox, and catalytic components;
(2) understanding the relationships between electronic communication and the molecular
interactions responsible for self-assembly; (3) understanding and controlling the reactivity of
hybrid molecular assemblies on many length scales; and (4) applying new synthetic discoveries
in nanoscale materials (e.g., shape and pore control, nano- and microphase separation) to
organize functional parts of an integrated artificial photosynthetic system for efficient fuel
formation.
Biological systems, such as photosynthesis, have built-in repair mechanisms that can restore
useful function following damage to the system. This contrasts strongly with the lack of such
mechanisms the complex molecules used to develop artificial photosynthetic systems for solar
fuels production. The development of active repair and photoprotection strategies for artificial
photosynthetic systems is a major scientific challenge that is critical to the long-term efficient
performance of these systems. The usual strategy used by photosynthetic organisms to repair
photochemical damage is to degrade the pigment-protein complex and replace it with a newly
synthesized complex. Photosynthetic systems are continuously subjected to photochemical
damage, especially when the incident light intensity is high. The major scientific challenge lies in
understanding the photoprotection and repair mechanisms in natural systems and exploiting these
findings to engineer robust artificial systems. To ensure that complex, artificial, photosynthetic
systems designed for solar fuels production maintain their efficiency over long lifetimes, the
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