Some Particular Viologens

One of the most important properties of viologens is their ability to reversibly accept one electron, giving stable and colored radical cations. New viologens have been created, where the two pyridine moieties are separated by an unsaturated chain or by an aromatic or heteroaromatic ring, such as compounds 21-24.25 

While dications 21 and 22 only underwent a one-stage two-electron reduction without formation of the radical cations, the reduction of the thiophene and furan derivatives proceeded in two steps, in the first of which the radical cations 23+ and 24+ were formed. The stability of these radicals was proved by the fact that the EPR spectrum of 23+, consisting of a strong line (g = 2.0038) with rather complicated hyperfine structure, persisted with undiminished intensity for weeks at room temperature. 

Upon reduction in the region where the cation radical PTV+ was formed, a new signal with hyperfine structure, due to the reduced viologen moiety, could be seen superimposed on the previous one.26 The presence of the hyperfine structure indicated the high flexibility of the alkyl-chain-bound viologen units. 

The proposed mechanism for the formation of these carbon-centered radicals produced in the hydrocarbon interior of an organic aggregate by photoinitiated radical conversion from AV+ involves hydrogen abstraction from a location of the 

When the AV2+ dication was photoreduced in DODAC micelles in the presence of variable amounts of a-tocopherol (vE), the nature of the observed radicals depended on the concentration of the latter compound.36 AV+ was observed in the absence of vE, and its signal increased slightly with the amount of tocopherol. When the vE concentration exceeded ca. 9%, a second radical was also observed, i.e the neutral radical originating from proton abstraction from the tocopherol cation formed in the electron-transfer process, this latter species becoming the only observed radical for tocopherol concentrations in the range of 17 to 23%. The disappearance of the radical AV+ is attributed to its reduction to AV by tocopherol with formation of more vE+  

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This enzyme is of wide occurrence in bacteria where it is concerned with the reduction of nitrate and CO2 as well as sulphur. Methods for its estimation depend on measuring some activity of hydrogenase by (a) dye reduction (benzyl viologen or methylene blue), (b) isotopic exchange and (c) evolution of molecular hydrogen. Interpretation of quantitative results is difficult due to the complex relationship between the enzyme cell structure and the particular method selected. ... 

Low-potential electron-transfer mediators such as viologens can substitute natural cofactors (particularly NADH) in some enzymatic reactions [184], The electrochemical reduction of viologens has been studied extensively [185] and they and other reductive electron mediators have been utilized to drive enzyme-catalyzed reactions [186], For instance, the electrochemical reduction of NAD(P)+ to NAD(P)H with a current efficiency of more than 97 % was achieved using alcohol dehydrogenase in the presence of acetophenone as an electron mediator [187], The addition of acetone or acetaldehyde as a substrate to the above bioelectrocatalytic system allowed the reduction of the substrate to the corresponding alcohol at alcohol dehydrogenase accompanied by the oxidation of the resulting NAD(P)H. 

Table 11 shows various reactions, where electrons are transferred from pyrene anion to various entities such as carbon dioxide and methyl viologen. The reactions with some of these solutes, in particular CC are very slow in homogeneous solution in micelles. Nevertheless, the long lifetime of the pyrene anion in micellar systems allows sufficient time for complete electron transfer to the added solute to produce reduced methyl viologen,... 

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