Viologen salts, reduction

It proves possible to anchor catalysts of H2 evolution to the outer and inner surface of the vesicle membrane. These catalysts are finely dispersed (10-20 A in diameter) metal Pt or Pd particles formed via reduction of appropriate salts in vesicle suspension (see [15, 16] and refs, therein). Among the viologen-type electron carriers a promising one is p-bis (1,2,5-triphenyl-4-pyridil)benzene which possesses reduction potential low enough for water reduction at neutral pH. Recently, using this mediator we succeeded in H2 evolution conjugated with PET... 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

The reduction of tetrazolium salts by NADH is greatly accelerated by electron transfer agents (ETAs) such as phenazine methosulfate (PMS 233) or its derivatives.451-454 Other classes of ETAs such as quinones.455,456 ferricinium,457 phenothiazine,458 the viologens,459 acridiniums,460 and phe-nazinium or quinoxalinium salts461 as well as the enzyme diaphorase462 have been used. 

The redox system i (R = alkyl Iqx Viologenes was the first to be interpreted correctly (E. Weitz ). It is especially well suited for studying the effects of N-substituents because steric effects are virtually absent. In spite of the great importance of some of these quaternary salts as universal herbicides (R = CH3, paraquat only potentials Ej were known for a long period of time, since the reductions SEM/RED in aqueous medium are mostly irreversible In aprotic media, however, as in acetonitrile and DMF, E2 and Ei are ideally reversibleThis holds true for most of the investi ted substituents as can be seen from Table 1. 

Colorless viologens composed of diquatemary salts of 4,4 bipyridine(I), 2,2 -bipyridine, and 1,10-phenanthroline turn into a violet-blue radical cation(II) by one-electron reduction,5 and are further reduced to the yellowish quinonoid(III ) via the biradical (III) (Scheme 1). 

These chiral viologens were applied to the reduction of metmyoglobin, too [72]. In this reaction, the one-electron reduced viologen radical cations were photochemically produced by tris(2,2,-bipyridme)ruthenium(II) in the presence of disodium salt of ethylene diamine tetra acetic acid (Na2H2edta), and the cation radical reacts with metmyoglobin to reduce it. The (S,S) isomer more rapidly reacts with metmyoglobin than the (R,R) isomer. The reaction rate is analyzed with Michaelis-Menten mechanism, as shown in Scheme 29. 

Cysteamine was used to couple redox-active carboxylalkyl-4,4 -bipyridinium salts to the gold surface . The nonordered monolayer assembly was then transformed into a densely packed monolayer with 1-hexadecanethiol and cyclic voltammetry of the surface bound viologen was performed. The electron transfer rate constants to the bipyridinium sites depended on the alkyl chain length Abridging the redox site to the electrode. Electron transfer rate constants followed the Marcus theory. Cysteic-acid-active ester monolayers chemisorbed on gold were used to electrode-immobilize the protein glutathione reductase, then a bipyridinium carboxylic acid was condensed onto the enzyme in the presence of urea to wire the protein towards electrochemical reduction (Figure 6.26). 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

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