Photosynthesis photoinduced charge separation

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

Such a photoinduced charge separation can proceed effectively provided an electric field (potential gradient) has been established at the position where the primary photoexcitation takes place. In general, a potential gradient can be produced at the interface between two different substances (or different phases). For example, a very thin (ca. 50 A) lipid membrane separating two aqueous solutions inside the chloroplasts of green plants is believed to play the essential role in the process of photosynthesis, which is the cheapest and perhaps the most successful solar conversion system available. 

We now report that we have observed intramolecular photoinduced charge separation similar to that in the primary steps of photosynthesis both with respect to the distances involved and to the transfer times... 

Artificial Photosynthesis, Fig. 2 Schematic state energy diagram for photoinduced charge separation in a photosensitizer-acceptor pair. Excitation to P-A results in electron transfer with a driving force —AG that is determined by the reduction potentials for the P /P and A/A couples, the excited-state energy bs, and... 

Moore, T. A., Gust, D., Moore, A. L., Seta, P., Bienvenue, E., and Bensasson, R. V. (1987) Transmembrane charge transfer in model systems for photosynthesis, in V. Balzani (ed.), Photoinduced Charge Separation and Energy Migration in Supramolecular Species, D. Reidel Press, Boston, pp. 283-297. 

Photoinduced electron transfer, using the presently described molecular systems, remains an interesting and promising topic—particularly in relation to charge separation and ultimately artificial photosynthesis. Nevertheless, a new area has recently emerged, which is that of multicomponent molecular sets undergoing controlled motions, under the action of an external signal. There is no doubt that this... 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

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