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Proceedings of a Workshop: Potential Applications of Concentrated Solar Energy
under some circumstances (high temperature, adsorbed on surfaces, etc.) than is currently thought to be the case. Concentrated solar photons may have considerable utility in materials processing applications in which chemistry occurs at a surface or interface. Such opportunities should be sought.
There appears to be a need for a shared research facility to which the photochemical community could bring or submit samples for exposure to real solar concentrator conditions. While much useful and productive work could be (and should be) accomplished in laboratory scale solar concentrators, the existence of an accessible user facility would allow any photochemist to try a ''dumb'' experiment, and it would also serve to link the photochemical community more tightly to the solar energy community.
Models for Charge Separation in Photosynthesis
Solar photosynthesis is accomplished on an enormous scale each day. Photosynthetic reaction centers in green plants and in photosynthetic bacteria employ low energy photons in the solar spectrum to drive endothermic reactions for the production of high energy metabolic products. The driving force for the chemistry comes from the initial separation of charges (electrons and holes) along a charge transport chain initiated by light absorption in porphyrin pigments. During very rapid initial charge transfer events, electrons are driven away from the porphyrin site down an energy gradient to become trapped on a quinone acceptor some 20 Å away within about 200 ps. At this point in the process, the charge separation is effectively complete. Subsequent, slower events transport the electron further away from the initial site of excitation and initiate the chemical parts of the electron transport process. The cationic hole left behind on the once-excited porphyrin is also used in the redox chain of events.
Chemists have had considerable success building small molecule models for the initial events of the photosynthetic process. The current state of the modelers' art is provided by the latest linked carotenoid-porphyrinquinone "pentad" system devised by Gust, Moore, and coworkers, which can absorb a red photon and produce a charge separated ionic state containing 1 eV of potential energy, in which the initial electron-hole pair is separated by nearly 80 Å and which will live for about 10 µs before collapsing back to its ground state.1 This is a remarkable achievement which gives chemists confidence that one could design molecules capable of storing considerable electrochemical redox potential, and that one could use the redox sites to initiate energy storing chemical synthesis.
Excellent progress is being made to understand the factors which control the reactivity of such model systems. Energetics (molecular spectroscopy, electrochemistry, and reorganizational energies associated with various redox processes), structural factors (distances among chromophores or redox pairs, their relative orientations, and the flexibility of the linkages connecting them), and parameters relating to the medium in which the reaction occurs (dielectric constant, index of refraction, and viscosity) all are considerations which bear on the outcome of the photoinitiated event. They are all related by the theoretical framework of the Marcus treatment of electron transfer processes.
Much research remains to be done to develop molecular devices or membrane based organized assemblies which can truly mimic the photosynthetic process, but the goal is achievable. Research to these ends should continue. It is a highly interdisciplinary effort requiring the combined talents of synthetic chemists, spectroscopists, electrochemists, photophysicists and others. The research is synthesis limited. Opportunities for application of concentrated solar photons in this area are not apparent.
See for example Gust, D., and Moore, T.A. 1989. Science. 224:35.