Back electron transfer zinc porphyrins

FIG. 4 (a) Latimer diagram of the water soluble zinc tetra-Af-methyl-4-pyridium porphyrin (ZnTMPyP ). (Reprinted with permission from Ref. 47.) (b) Schematic representation of a photosynthetic process based on porphyrin sensitized water-organic interface. Dotted line corresponds to the back electron-transfer process. (Reprinted from Ref. 51 with permission from Elsevier Science.)... 

In a later research, Sereno and co-workers found efficient energy transfer from zinc porphyrin to metal-free porphyrin in the dyad (90) even in the adsorbed states. The fact that the dimer is less effective in comparison with metal free porphyrin monomer in the generation of photocurrent is explained as that the metallized porphyrin enhances the back electron-transfer process [96],... 

The zinc(II) porphyrin of 1 is particularly suitable for electron transfer studies because it is a photoactivated electron donor with easily monitored spectral properties. Forward and back electron transfer rate constants for la ( f = 5.0 x lO ... 

The presence of p-CD leads to stabilization of the photoproducts formed by electron transfer from zinc(II)meso-tetrakis[l-(3-sulfonatopropyl)-4-pyridino]porphyrin (120) to anthraquinone-2-sulfonate (121, AQS ). This is due to the ability of CD to prevent ground-state association between the sensitizer and 121 by complexation of the latter. Moreover, the interaction of the reduced form AQSH " (formed in presence of cysteine) with the cavity lowers the back-electron transfer rate by one order of magnitude, further contributing to the accumulation of the radical anion [327]. 

These reactions were further studied at miCTO-ITIES by Quinn et al. who found the reactions to be reversible [204]. In the early days, most electron-transfer reactions were considered heterogeneous, but, as discussed, the locus of the ET step may occur in one of the adjacent phases. More recently, Sugihara et al. showed that the oxidation of ascorbic acid and chlorogenic acid in water by a zinc porphyrin (5,10,15,20-tetraphenylporphirinato zinc(II)) in nitrobenzene occurs on the organic side of the interface, accompanied by a back proton transfer reaction [205]. According to Osakai et al., one case of truly heterogeneous ET reactions was observed with a cadmium tetraphenylporphyrin in nitrobenzene and ferricyanide in water [206]. 

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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