Light-driven electron transfer

Gratzel, M. (1982) Artificial photosynthesis, light-driven electron transfer processes in organized molecular assemblies and colloidal semiconductors. Pure. Appl. Chem, 54, 2369-82. 

Light-driven electron transfer in plant chloroplasts during photosynthesis is accomplished by multienzyme systems in the thylakoid membrane. Our current picture of photosynthetic mechanisms is a composite, drawn from studies of plant chloroplasts and a variety of bacteria and algae. Determination of the molecular structures of bacterial photosynthetic complexes (by x-ray crystallography) has given us a much improved understanding of the molecular events in photosynthesis in general. 

Herrero C, Lassalle-Kaiser B, Leibl W, Rutherford AW, Aukauloo A. Artificial systems related to light driven electron transfer processes in PSII. Coord Chem Rev 2008 252 456-68. 

Light driven electron transfer is central to many important processes. Photosynthesis and colour photography both involve light stimulated... 

The theoretical understanding of electron-transfer processes in molecular systems is still lagging behind experiment. This is understandable in view of the intrinsic complexity of these systems. It appears that in order to design an efficient light-driven electron-transfer system, the complete assembly has to be considered. Electron transfer can be very rapid, but so is recombination. For the systems considered in this chapter to be useful in practical applications, one needs to consider the fate of both the electron and hole formed in the initial step. The multi-bridged systems that have recently been constructed may be the first step in that direction. 

Light-driven electron transfers between Chla and hydroquinones [Cho (30), White (200)] as well as between Chla and lumiflavin [Tu (184), 185)] have also been described and interpreted as simple models for some primary steps in photosynthesis [Wang (193)]. Through the use of j8 carotene as a quencher the rate constant for semiquinone radical formation in such a system was found to be 8.8 X 105 1/mol sec at —30° [Cho (30j. A descriptive theory of the relationships between the energy levels in different chlorophylls and the electron-transfer processes between them and other organic electron carriers has also been developed [Nelson (134)]. 

Gardner JM, Beyler M, Kamahl M, Tschierlei S, Ott S, Hammarstrom L (2012) Light-driven electron transfer between a photosensitizer and a proton-reducing catalyst Co-adsorbed to NiO. J Am Chem Soc 134(47) 19322-19325. doi 10.1021/ja3082268... 

Fig.15 Light driven electron transfer process via the intermediacy of a light absorption sensitizer (LAS).

Photosynthesis in photosynthetic bacteria involves light driven electron transfer across a bilayer lipid membrane which converts the light energy into chemical potential. After transmembrane charge separation, the chemical potential is in the form of reducing equivalents on the cytoplasmic side of the membrane, oxidizing equivalents on the periplasmic side and a membrane potential of perhaps 180 mV which is negative on the cytoplasmic side. From the crystal structure of the reaction center of Rb. sphaeroides (Yeates et (1988)) it is possible to construct an illustration of the supramolecular structure which accomplishes this process (Fig. 1). 

Other examples of electron transfer reactions in surfactant assemblies are those between pyrene and dimethylaniline in micelles, between viologen derivative and zinc porphyrin as an electron relay, and between chlorophyll a and methylviologen in microemulsions the photoinduced reduction of duroquinone by zinc porphyrin in micellar solution the photoinduced redox reaction of proflavine in aqueous and micellar solutions retardation of back reactions in micellar systems light-driven electron transfer from tetrathiafulvalene to porphyrin and tris a, a -bipyridine)... 

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