Redox viologens

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

MV /MV " (HV is heptyl viologen and MV is methyl viologen). The specific effects of iodide on the electrochemical behavior of the layer-type compounds were compared, and the characteristics of several PEC cells were described. The interface energies for n-MoSe2 in contact with various redox couples were given as in Fig. 5.9. 

Figure 19. Scheme describing the redox switch, which is based on a viologen redox center incorporated within the nanoclusters ligand shell. For simplification the counterelectrode is not shown. (Adapted with permission from Ref [34], 2000, Nature Publishing Group.)... 

Mayhew, S.G. 1978. The redox potential of dithionite and SO,- from equilibrium reactions with flavodoxins, methyl viologen and hydrogen plus hydrogenase. European Journal of Biochemistry 85 535-547. 

The second-generation 02" biosensors are mainly based on the electron transfer of SOD shuttled by surface-confined or solution-phase mediators, as shown in Scheme 2(b). In 1995, Ohsaka et al. found that methyl viologen could efficiently shuttle the electron transfer between SOD and the glassy carbon electrode and proposed that such a protocol could be useful for developing 02 biosensors [125], Recently, Endo et al. reported an 02, biosensor based on mediated electrochemistry of SOD [148], In that case, ferrocene-carboxaldehyde was used as the mediator for the redox process of SOD. The as-developed 02 biosensor showed a high sensitivity, reproducibility, and durability. A good linearity was obtained in the range of 0 100 pM. In the flow cell system, tissue-derived 02 was measured. 

It has been reported that the electrical properties of single molecules incorporating redox groups (e.g. viologens [114, 119, 120, 123, 124], oligophenylene ethynylenes [122, 123], porphyrins [111, 126], oligo-anilines and thiophenes [116, 127], metal transition complexes [118,128-132], carotenes [133], ferrocenes [134,135],perylene tetracarboxylic bisimide [93, 136, 137] and redox-active proteins [138-143]), can be switched electrochemically. Such experiments, typically performed by STM on redox-active molecules tethered via Au-S bonds between a gold substrate and a tip under potential control, allow the possibility to examine directly the correlation between redox state and the conductance of individual molecules. 

Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

The purple colonies of T. roseopersicina are lifted on a filter paper, transferred onto a stack of filter papers soaked with oxidised redox dye (benzyl viologen) under air. Following heat treatment, the cells containing heat stable, active enzyme turn blue under hydrogen atmosphere,those containing defected hydrogenase remain purple [Fodor et al., 2001],... 

The redox electrochemistry of thin polymer films is a particularly useful field of application for the quartz microbalance. As an example, we review experiments on poly(xylylviologen) films [15]. The viologen groups can be reversibly reduced in two discrete one-electron steps. 

Scheme IV, A poly(I)-based microelectrochemical transistor that turns on when VG is moved from VG (ie +0.4 V vs. Ag+/Ag) where polythiophene is reduced and insulating to Vq4 (ie +0.7 V vs. Ag+/Ag) where polythiophene is oxidized and conducting. This transistor also turns on to a smaller extent at E0/ (V2+/+), Vq1 = -0.63 V vs. Ag+/Ag. At VG significantly (>0.2 V) more negative (Vq2 < -0.8 V vs. Ag+/Ag) or positive (+0.4 V > Vq > -0.4 V vs. Ag+/Ag) of E° (V2+/+) only the reduced or oxidized form of viologen redox centers is present, respectively, and this device is...

Electrochemical oxidation of X produces a polymer film with polythiophene as the backbone and viologen centers as pendant redox groups. The electrochemical properties of the polymer are the combination of polythiophene and viologen. Using viologen subunits as the internal standard (one per repeat unit of the polymer), the "doping level" of the oxidized polythiophene backbone at its maximum conductivity can be measured and is about 25%. The charge transport via the pendant V2+/+ of poly(l) has been studied by... 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

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