Redox quinone/hydroquinone

Kinetic Parameters of Quinone, Hydroquinone Redox Couple at a Platinum Interface0... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

D. Note that the oxidation of hydroquinone near 0.3 V is not observed until the second positive-going sweep is made. Also note the rather large separation in peak potentials of the quinone-hydroquinone redox couple. 

The quinone-hydroquinone redox couple built into complexes 124 and 125 fulfils the requirements for the design of a bistable electro-photoswitch both the oxidized and reduced forms are isolable and stable the reduced form 125 is luminescent, whereas the oxidized form 124 is quenched the electrochemical interconversion of the two species is reversible [8.256]. 

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to thoroughly studied carbonaceous materials (glassy carbon and alike) in their electrode behavior. Redox reactions proceed in a quasi-reversible mode on these films [75], On the contrary, no oxidation or reduction current peaks were observed on diamondlike carbon electrodes in Ce3+/ 41, Fe(CN)63 4. and quinone/hydroquinone redox systems the measured current did not exceed the background current (see below, Section 6.5). We conventionally took the rather wide-gap DLC as a model material of the intercrystallite boundaries in the polycrystalline diamond. Note that the intercrystallite boundaries cannot consist of the conducting graphite-like carbon because undoped polycrystalline diamond films possess excellent dielectric characteristics. 

On the contrary, no oxidation or reduction current peaks were observed on the sp3-carbon-comprising wide-gap DLC (Eg 1.7 eV) electrodes in Ce3+/4+, Fe(CN)63 /4. and quinone/hydroquinone redox systems, as already mentioned in Section 6.3. Thus, we conclude that DLC is electrochemically inactive in itself. It gains electrochemical activity upon introducing a significant (ca. 10 %) admixture of platinum to the film bulk. Figure 34 shows the dependence of the Fe(CN)63 reduc-... 

Thiophenols and thiophenol derivatives chemisorbed on well-defined electrode surfaces have also been studied by HREELS. The cyclic voltammetric peaks for the quinone/hydroquinone redox reaction of the 2,5-dihydroxythiophenol immobilized on the Pt surface was much broader than for the unadsorbed species the broadening vanished when a methylene group was placed between the—SH group and the phenyl ring. These results indicated strong substrate mediated adsorbate-adsorbate interactions. Such... 

Two cases are distinguishable according to Fig. 3. The conventional one (a) holds for all molecular systems, which will be treated in Section 2, e.g., quinones or disulfides. The redox partner reacts in the dissolved state this means at low concentrations for electrodes of the second kind. The reactant is positioned in the outer Helmholtz plane, or even in the inner Helmholtz plane, as shown for adsorbed molecules. One example is the quinone/hydroquinone redox reaction according to Eq. (13) ... 

Prominent examples are the redox pairs o- or p-quinones/hydroquinones, the corresponding quinoneimines, the diimines and the azobenzenes and disulfides [68]. V. Stackelberg [69] has pointed out that the exclusive formation or cleavage of 0-H, N-H, S-H, or S-S bonds is a necessary precondition for reversible organic redox partners. This can be clearly recognized in the case of the quinone/hydroquinone redox reaction (cf. Eq. (13)). Only O -H bonds are formed or cleaved. In contrast, in the case of the acetone/isopropanol redox system, 0-H and C-H bonds participate. 

Thus, in analogy with the monomer quinone/hydroquinone redox couple, two electrons per monomer unit are assumed to be transferred finally. Examples for lithium metal negative electrodes in combination with PANI are reported in [508-510]. Details of the preparation of PANI positive electrodes are given in the patent literature (e.g., [358, 511-513]). 

More properly, the above remark refers to the initial step of this reaction. Studies performed using platinum and Sn02 electrodes indicate that the quinone/hydroquinone redox reaction involves two distinct, consecutive charge transfer steps. Also the hydroquinone oxidation at the Ti02 photoanode follows presumably a two-step mechanism. 

Conjugated conducting polymers have been found in some cases to present by themselves an intrinsic electrocatalytic activity. Among the few examples, polypyrrole has been shown to be an interesting catalyst for the quinone-hydroquinone redox system [34,35] or for the electro-reduction of dioxygen [36]. 

A quinone/hydroquinone redox couple is used to test the functioning of platinum electrode. Thereductionreactionofthiscoupleisquinone + 2H+ + 2e = hydroquinone ° = 0.699V. Assume the ratio of the activities to be [quinone]/hydroquinone] = 1. The electrodes are tested in buffer solutions with pH of 7 or 4 containing the redox couple at 25°C. 

C.2 for further discussion of electron-mediated reductions) (Schwarzenbach, et al., 1990 Tratnyek and Macalady, 1989). Quinoid-type compounds are thought to be constituents of natural organic matter (Thurman, 1985 see Chapter l.B.3c). It has been hypothesized that some free radicals in humic substances are quinone-hydroquinone redox couples (Tollin et al., 1963 Steelink and Tollin, 1967). 

As an example, the surface excess of the quinon-hydroquinone redox components on mercury is shown in Figure 4.7. 

Bhat MA (2012) Mechanistic, kinetic and electroanalyUcal aspects of quinone-hydroquinone redox system in N-alkylimidazohum based room temperature ionic liquids. Electrochim Acta 81 275-282. doi 10.1016/j.electacta.2012.07.059... 

The electrochemical processes involved in the quinone-hydroquinone redox couple have recently been examined by Bagotzky et In acid solutions the rate determining step was found to be ... 

Among polymer coated electrodes which have been the object of active investigations during this last decade, particular attention has been paid to the conductive polypyrrole films, obtained by electrooxidation of pyrrole in acetonitrile. Such electrodes have been used to study the electrochemical behaviours of the quinone-hydroquinone redox couple " and tetrathiafulvalene , The controlled release of ferrocyanide from polypyrrole by reduction of the polymer has been demonstrated . As an application, electroinactive anions can be determined using a polypyrrole modified electrochemical detector in flow-injection analysis. Pyrrole can be polymerized from aqueous solutions. Enlarging the modification field, the polymerization step may be preceded by a chemical reaction between pyrrole and another substrate . 

Fig. 4.4. Dependence of the faradaic resistance measured at the equilibrium redox potential for polycrystaUine diamond electrodes on their resistivity for (1) Fe(CN)63 / and (2) quinone/hydroquinone redox systems [13].

The PBPA electrodes possess an ion-sensitive surface, and the conductivity of the film depends on the degree of protonation or deprotonation as a function of pH.4 This electrochemical mechanism of PBPA films when interacting with protons of the solution is analogous to the quinone/hydroquinone redox system. ... 

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