Photoinduced electron transfer, photosynthetic

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

In the study of the ultrafast dynamics of photosynthetic bacterial reaction centers, we are concerned with the photoinduced electron transfer [72]... 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Sc heme 1 Multistep photoinduced electron transfers in a natural photosynthetic system. 

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... 

Significant progress in the development of such artificial photosynthetic systems, particularly aimed at the photolysis of water, has been reported in recent years. Several approaches to resolve the problems involved in controlling the photoinduced electron transfer process as well as the development of catalysts for multi-electron fixation processes will be discussed in this paper. 

In making rotaxanes usable as parts of molecular devices and with the purpose of studying long range election transfer processes within large molecular systems of well controlled geometries, the introduction of photoactive and electroactive compounds has been a valuable development. Photoinduced electron transfer between porphyrin species has a particular relevance to the primary events occurring in bacterial photosynthetic reaction center complexes, and so is a well studied phenomenon. 

Photoinduced Electron Transfer in Electron Donor-Acceptor Linked Molecules Mimicking the Photosynthetic Reaction Center... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Cowan, J.A., Sanders, J.K.M., Beddard, G.S. and Harrison, R.J. 1987. Modelling the photosynthetic reaction centre photoinduced electron transfer in a pyromellitimide-bridged special pair Porphyrin Dimer, J. Chem. Soc., Chem. Commun., 55-58. 

J. L. Sessler, B. Wang, A. Harriman, Long-Range Photoinduced Electron Transfer in an Associated But Noncovalently Linked Photosynthetic Model System , J. Am. Chem. Soc., 115, 10418 (1993)... 

Photoinduced electron transfer reactions have been studied in the dyads 44(6), 45(9), and 46(11). The 6-bond porphyrin-quinone dyad 44(6) serves as a model for aspects of ET in the photosynthetic reaction center, since congeners of these chromophores are present in the center as cofactors (Figure 37). Photoinduced ET in... 

The majority of the research on the photochemistry of porphyrins linked to other moieties has been in the area of photoinduced electron transfer, and the systems studied are all in some sense mimics of the photosynthetic process described above. The simplest way to prepare a system in which porphyrin excited states can act as electron donors or acceptors is to mix a porphyrin with an electron acceptor or donor in a suitable solvent. Experiments of this type have been done for years, and a good deal about porphyrin photophysics and photochemistry has been learned from them. Although these systems are easy to construct, they have serious problems for the study of photoinduced electron transfer. In solution, donor-acceptor separation and relative orientation cannot be controlled. As indicated above, electron transfer is a sensitive function of these variables. In addition, because electron transfer requires electronic orbital overlap, the donor and acceptor must collide in order for transfer to occur. As this happens via diffusion, electron transfer rates and yields are often affected or controlled by diffusion. As mentioned above, porphyrin excited singlet states typically have lifetimes of a few nanoseconds. Therefore, efficient photoinduced electron transfer must occur on a time scale shorter that this. This is difficult or impossible to achieve via diffusion. Thus, photoinduced electron transfer between freely diffusing partners is confined mainly to electron transfer from excited triplet states, which have the required long lifetimes (on the micro to the millisecond time scale). 

Photoinduced intramolecular electron transfer reactions in triad assemblies have been extensively examined with regard to mimicking the photosynthetic center. Novel bridging ligands L25a and L25b were prepared, in which the naphthalene bis(dicarboximide) unit is linked by two bidentate (2-pyridyl)benzimidazole (Ll).55 Under photoirradiation, photoinduced electron transfer from the Ru(bpy)2 moiety to the diimide site takes place. 

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