Redox switching mechanisms

Our goal is to develop a method for visualizing extremely complicated electroactive polymer film redox switching mechanisms. To exemplify the problem and our proposed solution, we discuss the mechanisms of two... 

Fig. 13.3. Dependence of redox switching mechanism on overpotential and the rates of...

Transfer of the methyl carbanion from methylcobalamin to Hg " " ion is non-enzymatic. Production of CH3B12 is enzyme dependent. The initial methylation of Hg " " proceeds at a rate 6000 times faster than the second methylation. In addition to electrophilic attack involving the displacement of a carbanion, two other reactions result in (To—C bond breakage are proposed. One involves displacement of a methyl radical from CH3B,2. The other is a redox switch mechanism which requires the presence of two different oxidation states of the metal. 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Reaction 3. This reaction has been described as a "Redox-Switch" mechanism. The metal ion in its lower oxidation state first forms an "outer sphere" complex with the corrin macrocycle, followed by a two electron transfer of the complexed metal ion to a two electron acceptor. Oxidation of the complexed metal ion facilitates carbanion transfer from the cobalt to the complexed metal. 

Scheme 1 Scheme-of-squares representation of the redox-switching mechanism of a polythionine film exposed to acetic acid solution. Th represents a monomeric thionine unit, A represents acetate, and X can be either a water or acetic acid molecule. (Reproduced from Ref [124] with permission from the American Chemical Society.)... 

Such a redox-switch mechanism results from the blocking of the associative process at the Cu state, imposed by the caUxarene funnel. All of this suggests that the embedment of a reactive redox metal ion in a funnel-like cavity can play a crucial role in catalysis, particularly for metallo-enzymes associating electrOTi transfer and ligand exchange. 

The redox-controlled mechanical switching in SAMs of disulfide-functionalized bistable TTF-DMN rotaxanes consisting of cyclophane 124+ and a dumbbell-shaped component containing TTF and DMN stations was also extensively investigated.49... 

Systems 13 and 14 represent prototypes of molecular switches of luminescence which are operated through a redox input the system consists of a luminescent unit (the light bulb of the everyday life) and of a metal-containing redox unit (the true switch) [8], The on/off situation is achieved when one of the two stable oxidation states of the metal quenches the nearby excited fluorophore and the other does not. In principle, redox switches of luminescence based on an electron transfer mechanism can be obtained by properly assembling a photoactive fragment and a metal-centered redox couple, possibly hosted by a cyclic framework. A further example based on the Cu VCu redox couple will be discussed in Section 5.4. 

This model of the switching process was confirmed by Kamitsos et al. Using Raman spectroscopy, they showed that a fraction of 10-15 mol% neutral TCNQ exists in the high conducting state immediately after switching [7]. A field-induced redox reaction was postulated by Potember [8] to describe the phase transition described above. Many publications deal with the switching mechanism but the details are still not yet fully understood. 

A rather neat example of the manipulation of film electroneutrality maintenance mechanism is provided by the redox behavior of poly(xylylviologen) (PXV) films, in which the nominal counterion is poly(styrenesulfonate) (PSS). In this case, the counterion is effectively immobile, and the question arises as to whether the PXV and PSS ion charges compensate each other or are separately compensated by (other) electrolyte ions. This will clearly have implications for the possible sources and sinks of ionic charge available to satisfy electroneutrality upon PXV redox switching, and the EQCM is ideally placed to make the distinction. It was found that... 

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