Photoinduced Electron Transfer Membranes

Energy transfer processes are intimately involved in the primary photoprocesses of photosynthesis. Considering that photosynthesis is the origin of all biological energy, it is easy to understand the importance of these processes. The mechanism of biological photosynthesis has been mimicked by scientists in ingenious devices. These will be described in this section after a brief description of plant photosynthesis. 

There have been several attempts to produce highly efficient artificial electron transport membranes modeled after the electron transfer processes in the sequence of oxidation/reduction reactions of photosynthesis in the chlorophyll pigments. Various self-assembly systems have been investigated, such as, for example, liquid membranes, bilayer membranes, micelles, LB-films (named after Langmuir and Blodgett). 

The formation of monolayer films at the air-water interface upon introduction of surface-active molecules in water has been known for a long time. Pioneering work by Langmuir and Blodgett has demonstrated that these monolayer films can be transferred intact onto solid 

To illustrate the kind of systems that have been achieved since that time, we present two examples. The first one uses two cyanine dyes carrying long alkyl chains, the oxacyanine dye (23) and the thiocyanine dye (24). The benzoxazole ring in compound (23) has an absorption band in the UV and it fluoresces in the blue. The benzothiazole ring in compound (24) 

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Lymar, S.V., Parmon, V.N., andZamarev, K.I. Photoinduced Electron Transfer Across Membranes. 159, 1-66 (1991). 

Bilayer membranes and vesicles provide not only charged surfaces but also two phases, separating the reaction sites and products. It was first demonstrated that photoinduced electron transfer occurs from EDTA in the inner water phase of vesicles incorporated with surfactant Ru(bpy)2+ to MV2+ in the outer water phase22 (Eq. (14)). 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

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