Basic Research Needs for Solar Energy Utilization

Energy Transfer, 21 ps

Electron Transfer, 7 ps

Energy Transfer, 21 ps

Electron Transfer, 7 ps

Figure 44 The red

perylenebis(imide) dyes function

both as antenna molecules and as

self-assembly points, which bring the

two green perylenebis(imide) dyes

close enough to form a dimer within

which ultrafast charge separation

occurs. (Source: Rybtchinski et al.

2004)

largely unknown are the fundamental concepts of how to
prepare individual light-harvesting complexes, reaction
centers, or catalysts that can readily engage in specific
intermolecular interactions promoting their assembly into
ordered supramolecular structures with the ability to
function as complete artificial photosynthetic systems. It is
a major scientific challenge to develop small, functional
building blocks, having a minimum number of covalent
linkages, which also have the appropriate molecular
recognition sites that facilitate self-assembly into
complete, functional artificial photosynthetic assemblies.

The construction of an integrated artificial photosynthetic
system will be achieved through hierarchical organization
of molecular and supramolecular constructs. The synthesis
of molecular building blocks allows very fine control over
the chemistry and physics of energy and electron transfer.
This is required for the first steps of solar energy
conversion because the excited states and initially formed, charge-separated states are
thermodynamically and kinetically prone to reactions that convert the stored solar energy to
useless heat. Self-assembly, or supramolecular organization, on the other hand, provides a facile
mechanism for assembling large numbers of molecules into structures that can bridge length
scales from nanometers to macroscopic dimensions. It can also lead to synergistic and emergent
properties that are not intrinsic to the building blocks themselves. For example, the ability of the
light-harvesting antenna illustrated in Figure 44 to form ordered aggregates elicits self-assembly
of a reaction center at which charge is separated following photoexcitation.

Visible light-driven water-splitting or CO 2 reduction with high efficiency is currently achieved
only in the presence of sacrificial reagents. The conversion of H 2 O to H 2 or O 2 , or of CO 2 to a
liquid fuel like methanol requires two or more visible quanta. The reasons for the inefficiency in
the absence of sacrificial donors or acceptors involve adverse processes that occur upon
absorption of a photon: spontaneous back-reaction, trapping of excitation energy or migrating
charge by defects or impurities (semiconductors), unwanted chemical reactions due to lack of
materials robustness, and lack of separation of intermediates and products. The design of new
self-assembled photocatalysts that eliminate the need for sacrificial reagents is imperative for
achieving efficient solar fuel production. The challenge is to develop assemblies that afford
coupling of the active components for efficient solar to fuel conversion without the need of
sacrificial reagents. These assemblies currently do not exist.

Understanding the Relationships between Electronic Communication and the
Molecular Interactions Responsible for Self-assembly

Hierarchical structures that provide directional organization on different length scales can be
prepared by molecular self-assembly. While individual interactions between covalent molecules,
such as a single hydrogen bond, are generally too weak to maintain the structure of a
supramolecular assembly, several such interactions with proper design can lead to robust