Viologen, reduced electron transfer

For added viologens, photochemical electron transfer from inner pools containing EDTA or K2C204 and [Ru(bipy)3]2+ or [ZnTMPyP]4+ to methylene blue, [Fe(CN)fi]3, [ Ru(bipy)2(H20) 20]4+ or [SiMo,2042]8 was observed and in the absence of an electron acceptor in the bulk, viologen cation radical was formed. Again, tunnelling of an electron from a reduced viologen on the inner surface to one on the other surface is believed to occur. Quantum yields up to 0.1 have been observed.340... 

Indeed, the (200-fs) laser excitation of the EDA complexes of various benz-pinacols with methyl viologen (MV2+) confirms the formation of all the transient species in equation (59). A careful kinetic analysis of the decay rates of pinacol cation radical and reduced methyl viologen leads to the conclusion that the ultrafast C—C bond cleavage (kc c = 1010 to 1011 s- ) of the various pinacol cation radicals competes effectively with the back electron transfer in the reactive ion pair. 

One of the most popular photo-promoted electron transfer systems is the trisbipyridylruthenium(II)-methyl viologen (MV++) system which, on excitation, produces Ru and reduced methyl viologen (MV+) ... 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

In the same study, redox polymers (223) were prepared that contained pendant viologens (Scheme 108). An active reducing agent was obtained by chemical reduction with dithionite or zinc, electrochemically, or by exposure to light. Utilization of the reduced poly(viologen) (224) as an electron transfer mediator was demonstrated by addition of a catalytic amount of the polymer to a mixture of zinc powder, ethyl benzoylformate (225) and water-acetonitrile (1 5). A quantitative yield of ethyl mandelate (226) was obtained after two days at room temperature (Scheme 109). Without the polymer, no reaction was observed after a month. 

In the presence of colloidal Pt, the decay of the reduced viologen was enhanced, presumably because of electron transfer to Pt and this decay became extremely rapid when the solution was acidified with 0.5 mol dm-3 H2S04. However, when EDTA was added to the system, hydrogen production was not observed, as EDTA is not an efficient electron donor (to [Ru(bipy)3]3+) at this pH. Nevertheless, hydrogen was evolved on irradiation (in the presence of EDTA) at neutral pH, although much more slowly than in a system containing MV2+, and in fact PV+ accumulated. This points to proton reduction as the rate determining step and the effect is ascribed to a shift... 

These systems have been used in attempts to allow charge separation in electron transfer events. For example, with microemuisions of toluene containing dodecylammonium propanoate with EDTA and [Ru(bipy)3]3 + in the trapped water pools, it is possible to observe reduction of bis(hexadecyl)viologen ((C]near surface region (4 — 0.013). Interestingly, neither MV2+ nor diheptylviologen are reduced in a similar system made up of dodecane, CTAB and hexanol.334... 

Photoinduced electron transfer rates can be considerably reduced when the counterion X- is changed from chloride to bromide. Charge transfer between the cationic part of a molecule and the bromide ion may be responsible to the reduction of photoinduced electron-transfer rates. Such a counterion effect on the photoinduced electron transfer and the reverse process has been demonstrated for examples of porphyrin-viologen-linked compounds (Mitsui et al. 1989). 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

The direct evidence for the fact that transmembrane electron transfer is provided by the migration of the reduced carrier across the membrane from one aqueous phase to another was obtained also for such water-soluble carriers as methyl-viologen and methylene blue [77, 179]. The corresponding rate constants are 5.3 x 10 2 and 9 x 10 3 s-1. 

Excitation of the complexes leads to photoinduced electron transfer from the excited ruthenium polypyridyl site to the viologen acceptor. The Ru2+ site is restored through electron transfer from the TEOA or back-electron transfer from the bipyridine, while the viologen is oxidized by the electrode, thus generating the photocurrent. As illustrated in Figure 5.51, this mechanism is supported by experiments in which the electron acceptor 4ZV (see Figure 5.50) reduced the... 

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