Surface groups hydroquinone

Yousaf and coworkers used an OEG-based surface bearing hydroquinone groups for electrochemical control of cell adhesion [186]. The hydroquinone groups were electrochemically converted into quinones, to which a cyclopentadienyl-modified RGD motif was then coupled via a Diels-Alder reaction. Although the hydroquinone-bearing surface was cell-repellent, fibroblasts attached to the areas... 

The absence of silver oxidation and/or reduction peaks is evidence for the electrochemical inactivity of the silver deposited on this carbon (in the form of metallic crystallites). The cyclic voltammogram recorded for the D—Ox carbon (Fig. 50, curve 2) exhibits two anodic peaks (fp., = +0.27 V, p,a = +0.77 V) due to the oxidation of adsorbed silver and surface hydroquinone-like groups, respectively. A single cathodic peak (Ep,) = +0.16 V) is due to the reduction of quinone-like surface groups according to Scheme 19. The large cathodic reduction wave confirms the presence of adsorbed silver cations and their reduction... 

The acid/base properties of carbons are of particular interest in the present context, since it will be seen that catalytic activity of carbons may often be related to amounts of oxygen adsorbed or to the acid/base characteristics of carbons. Thus, for example, the carbon catalysed rate of auto-oxidation of stannous chloride in acid was found to be maximal when the carbons were activated at 550 °C (high oxygen adsorption), but the carbon catalysed oxidation of hydroquinone to quinone was maximal for activation at 875 °C (low oxygen adsorption). It would obviously be of considerable interest to relate catalytic activity with specific surface groups, and such cases will be discussed later in this Chapter. However, the difficulty of analysing surface groups does make the correlation difficult to make. 

This is demonstrated in Fig. 5.50. The scheme depicted in Fig. 5.51 (Emeline and Serpone, 2002c) illustrates the possible steps during the photodegradation of 4-chlorophenol (4-ClPhOH). Benzoquinone (BQ) is formed from interaction of 4-chlorophenol with electrons localised on the surface, whereas ClCat (chlorocatechol) is formed by interaction of the initial substrate 4-ClPhOH with the OH radicals formed by hole trapping by surface OH groups hydroquinone (HQ) is formed both... 

The experimental results were well described by a kinetic model based on a redox mechanism of the Mars-van Krevelen type, where the quinone surface groups are reduced to hydroquinone by adsorbed ethylbenzene, and reoxidized back to quinone by oxygen [46,63,64], as shown in Figure 6.3. A recent... 

Several chemical groups of different stability on the carbon surfaces were recognized as carboxylics, phenols, lactones, carbonyls, quinones and hydroquinones. The TPD profiles were distinctly different, carbon for carbon, indicating differences in the stability of the carbon surface groups. The activated carbon, CAL, has the highest contents of surface oxygen complexes. 

OH radical formed from OH- and h+ rapidly adducts benzene to form a cyclohexadienyl radical, which is subsequently oxidized to a peroxy radical in the presence of 02. The peroxy radical transforms to the various intermediates including phenol, hydroquinone and to CO and C02. In the presence of H20, the surface hydroxyl groups consumed in the photoreaction are regenerated, leading to a successive catalytic cycle. Although the mechanism for the decomposition of the polymeric compounds is not well understood, it can be deduced that the OH radicals produced from the hydroxyl groups play a very significant role in the decomposition reaction. 

The electroactivity of the adsorbed states of this group of compounds is evidently associated with the presence on the adsorbed layer of a pendant functionality which is electroactive in the unadsorbed molecule and is attached to the surface in such a way that the pendant is virtually unperturbed structurally or electronically by the surface. In contrast, a molecule such as hydroquinone which is electroactive prior to adsorption but is strongly perturbed by adsorption (that is, by direct covalent attachment to the Pt surface) is not reversibly electroactive in the adsorbed state (Fig. 26). Accordingly, adsorbed DMBM is reversibly electroactive under almost all conditions, while adsorbed HQ is not reversibly electroactive under any conditions thus far studied. In between these two extremes is THBP, which is reversibly electroactive in one of its adsorbed states but not in the other two. 

The data also correlate with the electrochemical investigations of Epstein, et al., ( ), i.e., that quinone-hydroquinone groups are most likely present on LTI carbon surfaces. They further concluded that such groups can be readily converted electrochemically from one to the other. As previously noted, our data indicates that approximately 25% of the surface oxygen is quinone-like in nature. 

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... 

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