Redox potential oligomers

As a simplification, it was assumed that the interactions between charged oligomeric segments were negligible. These redox data correlate well with potential values obtained by extrapolation from quantum ma hanical calculations and redox potential measurements on oligomers of defined chain length. As expected,... 

Table IV lists the redox potentials of conjugated ferrocene oligomers (mainly dimers with a single bridge). Potential values are denoted against different reference electrodes as given in the references. The values can be primarily compared using the relationship mentioned in the footnote of the table, although care should be taken with some errors derived from junction potentials which depend on experimental conditions. There have been several reports on the quantitative estimation of structural factors affecting internuclear electron delocalization.

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

Moreover, almost in all the early steps, the redox potential of the clusters, which decreases with the nuclearity, is quite negative. Therefore the growth process undergoes another competition with a spontaneous corrosion by the solvent and the radiolytic protons, corrosion which may even prevent the formation of clusters, as mostly in the case of nonnoble metals. Monomeric atoms and oligomers of these elements are so fragile to reverse oxidation by the medium that H2 is evolved and the zerovalent metal is not formed [11]. For that reason, it is preferable in these systems to scavenge the protons by adding a base to the solution and to favor the coalescence by a reduction faster than the oxidation [53]. 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

Stability means that clusters do not undergo coalescence nor corrosion by the medium, at least in the absence of oxygen. The quite negative value of ii°(MVM ) and the dependence of the cluster redox potential on the nuclearity have crucial consequences in the formation of early nuclei, their possible corrosion or their growth. As an example, the faster the coalescence, the lower is the probability of corrosion of the small clusters by the medium. The property of stability offers the means to apply to these clusters a larger amount of suitable characterization techniques than to transient oligomers. 

Similarly, when PtClg ions are irradiated in the presence of PA, which prevents coalescence, most of the Pt atoms are found via STM imaging in the form of very small platinum oligomers of 3-7 atoms only [92]. In the presence of PP, PtCle irradiated solutions present a UV band with a maximum at 215 nm [93]. About 20% of the initial signal is lost by oxidation when the sample is exposed to air, the rest being stable. This result again confirms that the redox potential of the smallest clusters is markedly shifted to the negative values. 

Figure 1. Radiolytic reduction of metal ions. After their formation, the atoms coalesce and also become associated with excess metal ions. The clusters can be stabilized by ligands, polymers or supports. The cluster redox potential increases with the nuclearity n. The smallest oligomers can undergo corrosion by...

The couple S/S is selected with a specific and intense optical absorption of S or S , so that the electron-transfer reaction can be observed directly. In the early stages of atom coalescence, the redox potentials of the atom and of the smallest clusters are generally far below that of the donor and the transfer from S to the oligomer does not occur. The ion reduction is caused exclusively by solvated electrons and alcohol radicals (Eqs. 2, 8, and 9). The nucleation and coalescence dynamics are thus the same as in the absence of (Eqs. 10 and 11). Beyond a certain critical time, tc, that is large enough to enable the growth of clusters and the increase of their potential above the threshold imposed by the electron donor S , electron transfer from this monitor to the supercritical clusters is allowed (Eq. 32) and detected by the absorbance decay of S (Fig. 6). For n > ny. 

Oxidation of atoms and small oligomers in the presence of oxygen is often so fast that the corrosion is over before the formation of clusters. Even large clusters such as Ag ,cN-, Au ,cn, or Cu cr, which are stable after their radiolytic formation under anaerobic conditions, are readily oxidized when exposed to air, suggesting that their redox potential is lower than E° 02/02 ) = -0.33 Vnhe-... 

An alternative model uses the Crank-Nicholson method to generate a voltammogram that consists of a layer with a series of microscopic formal potentials, most situated at O.OV and the rest equally spaced 50 mV apart. This also yields a voltanmiogram (Fig. 6.16) similar to the experimental one (Fig. 6.14). The basis for this is the fact that different oligomers of different chain length possess a range of redox potentials. Thus at least qualitatively, two models may account for the electrochemical behavior of a conducting polymer coated on an electrode. 

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