Reversible redox behavior

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Certain lead(II) complexes were shown to display reversible redox behavior on the CV scale. Examples are Pb(II) complexes of 2(o-hydroxyphenyliminomethyl)-pyrrole and 2(o-hydroxyphenyliminomethyl)-thiophene. Their stability constants were also determined by polarography140. Anodic exchange reactions of extracted metal chelates were carried out with 1-pyrrolidinecarbodithioic acid in isobutyl methyl ketone141. 

Figure 3.22 (a) Cyclic voltammogram of myoglobin covalently attached to a CNT forest in PBS solution under nitrogen atmosphere. The reversible redox behavior of the iron redox center is observed, (b) and (c) electrocatalytic response of Myoglobin/CNT forest electrode to oxygen and peroxide... 

Electrochemical and structural studies of oxovanadium complexes with Schiff-base ligands attract particular attention because of their reversible redox behavior, which allows possible applications to electrocatalysts. VO(salen) and its oxidized product VvO(salen)Cl04 crystallize readily and their x-ray structures have been solved [108,109],... 

An ideal photosensitizer must satisfy several stringent requirements (Balzani et. al., 1986) 1) stability towards thermal and photochemical decomposition reactions 2) sufficiently intense absorption bands in a suitable spectral region 3) high efficiency of population of the reactive excited state 4) long lifetime in the reactive excited state 5) suitable ground state and excited state potentials 6) reversible redox behavior 7) good kinetic factors for outer sphere electron transfer reactions. 

The treatment of hexaphenylstannane with 3 equiv of lithium in THF at room temperature selectively leads the formation of the pentaphenylstannole anion <2007EJI1297>. The reversible redox behavior between tetraphenyl-stannole dianion and octaphenylbistannole 1,2-dianion was investigated by H NMR spectroscopy <2006JA4934>. 

As regards the electrooxidation of the corresponding monomers, they have less anodic Epa values than N-phenylpyrrole even with the nitro substituent and the reactions remain irreversible. The substituents influence the oxidation of these monomers much more than was observed with pentaphenylpyrrole. For example, substitution of a p-methoxy group in the N-phenyl of die latter produces a 20 mV cathodic shift in the Epa value (10). The dimethylaminophenyl and nitrophenyl groups show reversible redox behavior and appear to behave independent of the pyrrole moiety in these derivatives. 

In conclusion, it should be noted that the recently reported [Pd33Ni9(CO)4i-(PPh3)6] " and Hi2Pd2gPti3(CO)27(PMe3)(PPh3)i2 "" carbonyl-substituted bimetallic clusters contain bulk metal atoms (one and four, respectively) but do not have reversible redox behavior. 

For example, Mirkin has demonstrated that Fc-ODN films attached through a gold-thiol linkage display reversible redox behavior [7]. In addition, the surface coverage of a DNA probe can be controlled using alkylthiol diluents, as shown in Fig. 7.2. The surface coverage of a ss-DNA capture strand has a dramatic effect on the hybridization efficiency since sufficient space between the capture... 

Various molecules are capable to bind to the DNA duplex or to single-stranded DNA. The application of DNA binding molecules for the detection of base-pair mismatches is discussed below. For instance, Millan et al. demonstrated sequence-selective electrochemical DNA sensing using hybridization indicators [9]. In this detection scheme, DNA capture strands were covalently immobilized on a glassy carbon 14 electrode and [Co(bpy)3] + and [Co(phen)3] + served as hybridization indicators that display reversible redox behavior. Presumably, electrostatic interactions with the negatively charged... 

Dihydro-l,2,4-benzotriazinyls 12 (R = Aik, Ar, HAr = Ar, HAr) (Figure 4), first reported in 1968 (1968TL2701), are air, moisture, and thermally stable and show fuUy reversible redox behavior. They can be readily reduced by mild reductants such as ascorbic acid in DMSO to the... 

Abstract. The evolution of lariat ethers from relatively simple, substituted crown ethers into electrochem-ically sensitive ligands is presented. Although nitrogen-pivot lariats were observed to be better binders than the corresponding parent crowns and to retain considerable flexibility after complexation, overall stability constants were not favorable for cation transport applications. This led to the syntheses of nitrobenzene- and anthraquinone-substituted systems capable of reversible redox behavior and drastically enhanced cation binding abilities when reduced. Application of these in enhanced cation transport processes has been demonstrated. 

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