Biomimetic solar energy conversion

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Biomimetic Solar Energy Conversion Systems Design Issues... 

Three key issues must be addressed in the development of effective biomimetic solar energy conversion systems. First, the molecular system should possess a large optical absorption cross-section in the desired spectral region. Second, the system should possess appropriate characteristics to insure formation of a sufficiently long-lived, low-lying state which can initiate the primary ET efficiently. And third, the system should be able to effect the ET process irreversibility, that is electron-hole recombination should be substantially inhibited. 

The first issue can be addressed in two ways a primary ET species which has a large optical absorption cross-section can be chosen or arrays of molecules with large optical absorption cross-sections can be used as "antennas" that will efficiently collect and transport the electronic excitation energy to the primary ET species, in direct analogy to photosynthetic systems. While in the latter case it should be possible to develop systems with more efficient solar photon collection, the number of primary ET species will have to be reduced due to the spatial limitations, which will also reduce the potential electric current that can be produced by the system. Thus, questions related to the detailed molecular architecture of biomimetic solar energy conversion devices will have to address this issue, and it is quite likely that a number of compromises will have to be made before optimal design characteristics are obtained. 

Thus, it appears that several systems have been developed which hold promise as prototypes of biomimetic solar energy conversion devices. And in conjunction with the advances being made in experimental and theoretical methods for investigating molecular excited-state processes, prospects for the development of practical biomimetic devices are now substantially better than they were only a decade ago. 

Self-assembled and spontaneously adsorbed monolayers offer a facile means of controlling the chemical composition and physical structure of a surface. As discussed later in Chapter 5, applications of these monolayers include modeling election transfer reactions, biomimetic membranes, nano-scale photonic devices, solar energy conversion, catalysis, chemical sensing and nano-scale lithography. 

Another area of increasing emphasis is the elucidation of chemical bonding rearrangements either initiated by or accompanying ET for example, coupled proton- (or other ion ) electron transfer cpet) [20, 22] and dissociative ET [80]. Such a focus, of course, lies at the heart of much current research in solar-energy conversion. An especially exciting recent development is the construction of a functioning biomimetic photon-driven proton pump [81]. 

Production of H2 fuel from water via solar energy is of exceedingly high interest.115 Catalysis may involve H2 complexes at least as intermediates, and H2 complexes had previously been implicated in solar energy conversion schemes based on photoreduction of water.116 Industrially important water-gas shift and related H2-producing reactions undoubtedly proceed via transient H2 complexes.117 Biomimetic H2 production, particularly solar driven (via photocatalysis), is also a challenge and may take a cue from models of the active site of hydrogenase coupled with models of nature s photosystems.91-93 Here, the formation of H-H bonds from protons and... 

Fig. 2 A biomimetic system for solar-energy conversion. A molecular triad embedded in the bilayer membrane allows photoexcited electrons to be transported across the membrane to acceptor species in the interior. This charge transport is accompanied by the transmembrane flow of hydrogen ions, leading to a decrease in pH inside the compartment. This proton-motive force is then used to drive ATP synthesis by the membrane protein ATP synthase, as it is in photosynthesis.

Solar energy conversion into chemical fuels is one of the holy grails of the 21st century. Significant research efforts are currently underway toward understanding natural photosynthesis and artificial biomimetic systems. Photocatalytic cells absorb solar energy and use it to drive catalytic water oxidation at photoanodes ... 

Sivakumar R, Thomas J, Yoon M (2012) Polyoxometalate-based molecular/nano composites advances in environmental remediation by photocatalysis and biomimetic approaches to solar energy conversion. J Photochem Photobiol C 13 277-298... 

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