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

SUMMARY OF RESEARCH DIRECTION
The key role played by the protein in regulating and facilitating the primary energy and electron
transfer reactions of photosynthesis is best illustrated with reference to purple non-sulfur
photosynthetic bacteria. In this case, the same chemical entity, bacteriochlorophyll, is used in the
construction of both the light-harvesting complexes and the reaction centers. Whether a specific
bacteriochlorophyll molecule is destined to fulfill a light-harvesting function or participate in
electron transfer within the reaction center is solely controlled by the protein into which it is
assembled. For example, the antenna proteins modulate the spectroscopic properties of the
bacteriochlorophylls to increase the fraction of the solar spectrum absorbed. In addition, this
wavelength programming, coupled with hierarchical structural organization, creates an energy
funnel, which directs the captured energy downhill to the reaction centers. Within the reaction
center, the protein controls the directionality of the electron-transfer and charge-separation
processes, so that losses by wasteful deactivating recombination processes are prevented.
Although we are now beginning to realize in general terms what the protein achieves, we do not
understand how it achieves it.
The design of bio-inspired solar-energy systems is moving toward hierarchical supramolecular
structures and their integration into interfacial host architectures as means to achieve control of
light-initiated reaction sequences. This increase in structural complexity is also dynamic in
nature. Molecular motions intrinsic to the individual molecular components are altered in the
complex assembly; the resulting dynamics of the assembly most frequently dictate overall
function, often in ways that are difficult to predict using current theoretical and experimental
tools. Biology provides numerous examples of complex supramolecular structures with functions
that are unexpectedly sensitive to minimally perturbative single-site mutations, or ones that show
long-range cooperative effects. Molecular materials also show significant site-selective
conformational sensitivities. For example, the nature of the connection between conductive
molecules and metals dictates whether the molecule will behave as a molecular wire. From these
examples, it can be anticipated that a definitive resolution of mechanistic function within
complex bio-inspired supramolecular assemblies and the smart matrices in which they reside will
require the application of new in-situ structural probes.
A grand challenge is to resolve structural and electronic dynamics over the full time scale of
energy capture and conversion. At best, we currently have only a fragmentary understanding of
the dynamic structural features of complex molecular systems in their electronic ground states.
The complexities introduced by higher-order structures raise significant theoretical and
experimental challenges that must be addressed by (1) the development of new theoretical
concepts and predictive models for discovering structure-function relationships within biological,
molecular, and supramolecular systems; (2) the in-situ determination of supramolecular structure
and dynamics to resolve the dynamic interplay between supramolecular charge separation and
host environments that are relevant to solar-energy conversion; and (3) the integration of
theoretical and physical techniques to provide the knowledge necessary to achieve maximum
photoconversion system performance. These research directions will exploit new, emerging
methods for dynamic molecular structure determination, including multi-dimensional near- and
far-field optical, vibrational, and magnetic spectroscopies; pulsed X-ray, neutron, and electron
diffraction; and coherent scattering combined with multi-scale dynamic modeling.
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