difference between the primary donor in PSII and those in PSI and purple bacteria is that the
oxidized PSII donor, P680+, is one of the most powerful oxidants known in biology; it provides
the oxidizing power to split water into O 2 and H+ in the Mn-containing, oxygen-evolving
complex (OEC) within the protein. Understanding the overall coupling between the
photoinduced electron transfer process within PSII and the functioning of the OEC is essential
because efficient water splitting, coupled to catalytic reduction of H+ to H 2 , is one of the most
important routes to clean solar fuels (Krauss 2003). The recently determined structure of PSII
has provided insights into the organization of the OEC, serves as a framework for describing the
water splitting chemistry of PSII, and therefore is of major importance for designing artificial
catalytic systems for reproducing this chemistry.
Bio-inspired Approaches to Photochemical Energy Conversion
The construction of artificial photosynthetic systems for practical solar fuels production must
incorporate both molecular-level and supra-molecular organization to collect light energy,
separate charge, and use charge transport structures to deliver the oxidizing and reducing
equivalents to catalytic sites where water oxidation and CO 2 reduction will occur. Thus, a
principal target of artificial photosynthetic energy conversion is the environmentally sound
production of H 2 directly from water and CH 4 from CO 2. While some progress has been made on
each aspect of this complex problem, researchers have not yet developed components that are
both efficient and robust and have not yet integrated the existing functional components into a
working system for solar fuels production. The design and development of light-harvesting,
photoconversion, and catalytic modules capable of self-ordering and self-assembling into an
integrated functional unit will make it possible to realize an efficient artificial photosynthetic
system for solar fuels production. It is also imperative to develop systems that will either be
defect-tolerant or can execute self-repair strategies to ensure long service lifetimes.
The main focus of current research is the design and synthesis of molecular systems consisting of
electron donors and acceptors that mimic the charge separation function of photosynthetic
proteins. Researchers have prepared synthetic systems to study the dependencies of electron
transfer rate constants on donor-acceptor distance and orientation, the free energy of the reaction,
and electronic interaction. The most useful and informative systems are those in which there are
structural constraints to control both the distance and the orientation between the electron donors
and acceptors. Along with ease of synthesis and stability, bio-inspired systems for photochemical
solar energy conversion must have components with intense electronic absorptions that cover the
solar spectrum. As is the case in photosynthetic RC proteins, multi-component donor-acceptor
arrays that carry out multi-step charge separation reactions are most useful for producing long-
lived charge-separated states. Most bio-inspired systems employ light absorbers
(i.e., chromophores that absorb broad regions of the solar spectrum) as do the natural
chlorophylls. These same chromophores also readily engage in rapid electron transfer reactions
leading to stored charges. Unambiguous identification of both the short- and long-lived
intermediates produced by photoinitiated electron transfer is critical to determining the
mechanisms by which charge separation and storage occur in these bio-inspired systems. This
information is generally obtained using time-resolved optical and electron paramagnetic
resonance spectroscopy (Levanon et al. 1998).