coupling in multi-cofactor-containing assemblies has used 2-D transient optical spectroscopy to
map the time evolution of coupled electrons in light-harvesting proteins (see Figure 42). The
crystal structures of these light-harvesting proteins show impressively large arrays of cofactors.
The complexities of these arrays and their protein hosts prevent a definitive determination of
structure-based function and elucidation of underlying design principles. Two-dimensional
transient optical and related coherent spectroscopies offer new approaches for achieving
breakthroughs in understanding the design and function of multi-cofactor arrays. These
spectroscopies are well-suited for extension to in-situ analysis of cofactor arrays within
specialized micro-environments.
Building upon these coherent optical techniques are emerging analogous X-ray spectroscopic
techniques for deciphering electronic structure at metal centers and finer, higher-resolution
length scales. Pioneering examples include inelastic X-ray scattering techniques that have
imaged spatial and temporal electric-field-induced electron density disturbances associated with
charge and electric-field perturbations in water with 40-attosecond (10-18 s) time resolution
(Abbamonte et al. 2004). These measurements allowed mapping of electronic disturbances
calculated to be produced by an oscillating molecular dipole and diffusing ion fields. These
studies suggest unprecedented opportunities to map the dynamic electronic responses of solar-
fuel-producing materials.
Multi-scale Theoretical/Computational Approaches. The complex nature of supramolecular
assemblies associated with a variety of host architectures and the anticipated explosion in
experimental detail concerning light-initiated electronic and nuclear dynamics raise significant
theoretical challenges. New, multi-scale theoretical/computational methods are critically needed
to account for the complexities of excited-state energetics applied across multiple spatial length
scales relevant to supramolecular structures within complex host architectures, and on the range
of time scales encompassing solar-energy capture, conversion, and storage. New theoretical
methods are essential for establishing predictive methods to accelerate the design of efficient
systems for solar fuels production.
Figure 42 Dynamically resolved electronic coupling in the FMO protein using 2-D pulsed
spectroscopy (Source: Brixner et al. 2005)