Figure 45 Hybrid light-driven proton
pump using charge separation within a
donor-acceptor triad and ATP synthase
incorporated into a liposome (Source:
Steinberg-Yfrach et al. 1998)structures. Supramolecular organization can lead to a diversity of structures, some of which are
highly symmetric with repeating patterns. Such motifs are useful for the construction of antenna
units and molecular conductors. However, repeating patterns will not by themselves lead to
functional devices. The reason for this is that the supramolecular assemblies required for an
artificial photosynthetic system must not be simply structural, but also functional. They must
provide pathways for migration of light excitation energy among antenna chromophores, and
from antennas to reaction centers. They must also incorporate charge-conduction devices, or
molecular “wires” that can efficiently move electrons and holes between reaction centers and
catalytic sites. Discovering the principles governing excitation and charge migration within
supramolecular assemblies is a major scientific challenge. The supramolecular assembly must
not only facilitate and correlate directional flow of energy and charge within the integrated solar
conversion device, but also provide an environment that preserves the functions of the individual
components (antennas, reaction centers, catalysts) and protects them from damage.
The overall organization of the integrated devices must also provide mechanisms for transport of
oxidizing and reducing equivalents across phase boundaries. This includes functionally
interfacing molecules with traditional materials such as conductors and semiconductors so that
charge transport across the boundaries is highly
efficient. Phase boundaries provided by lipid bilayers,
micelles, nanoparticles, polymers, and similar systems
offer mechanisms for separating molecular redox
equivalents, fuel molecules, and oxidizing agents. This
can prevent charge recombination and destruction of
fuels by oxidizing materials. Finally, the
supramolecular organization must also provide an
environment that separates the final products of the
fuel production process (e.g., H 2 and O 2 ), and allows
for their extraction and transport. A recent example of
a hybrid system that carries out proton pumping using
both artificial and natural building blocks incorporated
into a liposome is shown in Figure 45.
Understanding and Controlling the Reactivity of Hybrid Molecular Materials on
Many Length Scales
Supramolecular structures can cover many length scales, which allows integration of individual
molecules into nano- or meso-scale structures that can carry out the entire solar fuels production
process. Biological systems employ a hierarchical organization to carry out many functions,
including those of photosynthesis. Chemical processes such as microphase separation in block
copolymers, template directed sol-gel synthesis of porous materials, layer-by-layer synthesis,
nanoscale imprinting and patterning, and particle self-assembly have opened the door to a huge
variety of hierarchical structures that are organized on several length scales. The challenge is to
map these new synthetic techniques onto the demands of artificial photosynthesis in order to
better control light-harvesting, charge separation, traffic control of holes and electrons, catalytic
reactions, and permanent separation of the photo-generated fuel and oxidant. Tasks include the
development of novel methods for compartmentalizing oxidizing or reducing sites by