Figure 10 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)artificial RCs, have proven to be important tools to directly gauge the magnitude of the
intermolecular interactions responsible for a given rate of electron transfer and determine how
this rate depends on the details of molecular structure.
Integrating Artificial Photosynthetic Functions. An antenna, or light-harvesting molecular
array, increases the amount of solar energy absorbed without carrying out charge separation
itself. Following photoexcitation, a series of one or more energy transfer steps occurs; this series
of steps funnels the excitation energy to a site at which charge separation occurs. This process is
similar to what occurs in photosynthetic organisms, and it limits the need to produce large
amounts of the complex charge separation structures, while maintaining highly efficient light
collection. Covalently-linked arrays of light-harvesting chromophores that funnel energy to a
central site have been demonstrated, and they require significant synthetic efforts to produce
(Seth et al. 1996). By contrast, the ability to create self-assembling, robust, functional antenna
arrays is at an early stage of development. For example, new self-assembling antenna systems
produced from robust dyes used as industrial paint pigments hold significant promise as antenna
molecules (van der Boom et al. 2002). Several systems have been constructed that successfully
mimic the light-harvesting, energy-funneling, and charge-separation functions of the
photosynthetic RC (Liddell et al. 2004). These include systems in which self-assembly of a light-
harvesting antenna structure elicits co-assembly of an appropriate RC to carry out charge
separation (Rybtchinski et al. 2004).
Two of the most important photo-driven biological processes are the oxidation of water to O 2
and protons, and proton pumping across membranes. The protons that result from the photo-
oxidation of water can be used to produce H 2.
Photo-initiated, multi-step charge separation
(using a donor-acceptor triad contained within the
walls of a spherical nanoscale compartment made
from a lipid, i.e., a liposome) has been used to
pump protons to drive the synthesis of ATP, a
major energy-rich biological molecule (Figure 10)
(Steinberg-Yfrach et al. 1998). In addition, part of
the oxidative side of PSII has been modeled by
using a multi-step electron transfer cascade to
generate a potential sufficiently positive to
oxidize a Mn complex (Sun et al. 2001). These
examples illustrate the potential of artificial RC
components to carry out useful reactions to
produce fuels.
Photocatalysis and Photodriven Reactions
Photocatalysis is the process by which absorbed light is used to drive a chemical transformation
aided by a catalyst. The catalyst can either absorb the light itself or harness the light absorbed by
another molecule. Efficient solar fuel generation requires efficient (1) light absorption, (2) charge
separation, and (3) use of the separated charges in fuel-forming reactions (Figure 11). These