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

BIO-INSPIRED MOLECULAR ASSEMBLIES FOR INTEGRATING
PHOTON-TO-FUELS PATHWAYS
Molecular systems that mimic the photoconversion steps of photosynthesis have been synthesized
using complex and costly sequences of chemical reactions. Yet, modular systems that avoid these
difficulties by self-assembling into complete artificial photosynthetic systems remain largely
unknown. 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 realize an efficient artificial photosynthetic system for solar fuels production.
EXECUTIVE SUMMARY
A scientific grand challenge is making bio-inspired, molecular assemblies that integrate light
absorption, photo-induced charge separation, and catalytic water oxidation/fuel formation into a
single fully functional 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 reaction centers. 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 fuel production occur. The self-organization of
molecular structures using a variety of nanoscale motifs must be implemented to make these
processes highly efficient. 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
to give rise to ordered structures. Achieving the goal of producing a functional integrated
artificial photosynthetic system for efficient solar fuels production requires: (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; and (3) understanding and controlling the reactivity of
hybrid molecular materials on many length scales.
SUMMARY OF RESEARCH DIRECTION
Innovative Architectures for Coupling Light-harvesting, Photoredox, and Catalytic
Components
Research into the design and synthesis of molecular systems comprised of chromophores,
electron donors, and acceptors, which mimic both the light-harvesting and the charge separation
functions of photosynthetic proteins, has clearly demonstrated that covalent systems can perform
these functions. In addition, catalysts for fuel-forming reactions are also based largely on
covalently linked molecules, even though they are less well developed. However, what remain
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