Reversible redox processes

The present technique enables light-induced redox reaction UV light-induced oxidative dissolution and visible light-induced reductive deposition of silver nanoparticles. Reversible control of the particle size is therefore possible in principle. The reversible redox process can be applied to surface patterning and a photoelectrochemical actuator, besides the multicolor photochromism. 

In general we work in an analyte solution without stirring, as in polarography, unless mentioned otherwise. The simplest situation is that where a reversible redox process such as ox + ne red takes place at an inert electrode such as Pt, Pd, Ir or Eh (and sometimes Au or Ag) and where both ox and red... 

Moore et al. later reported [98] the design and synthesis of triarylamine based dendrimers. These fluorescent macromolecules exhibited reversible redox processes and their potential use in electro-optic film applications was envisioned. 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Experimentally, this means that for a given (reversible) redox process one records a series of cyclic voltammograms at different scan rates... 

Just zinc complexes bearing redox-active ligands sometimes display apparently reversible redox processes. Really, in these cases the electron transfer processes are centred on the ligand. This is, for example, the case of [Zn(papm)Cl2] (papm = 2-(phenylazo)pyrimidine), the (distorted) trigonal bipyramidal molecular structure of which is illustrated in Figure 134.194... 

Metal-sulfur clusters probably constitute the best known class of clusters due to their considerable importance and common occurrence in the biological world, where they fulfil the role of electron carriers (see Chapter 12). This function is related to their high capacity to undergo a cascade of reversible redox processes. As an illustrative and introductive example, Figure 1 shows the redox ability of [Fe4(/r3-S)40/-C5H5)4].la... 

Figure 12 shows that also this derivative undergoes a rich series of reversible redox processes (3 + /2 + / + /0). 

Although [Rh4(CO)i2] is isoelectronic and isostructural with [Co4(CO)i2] (mean Rh-Rh distance 2.78 A), it does not display any chemically reversible redox processes. 

Since such correlations belong to a series of treatments which are commonly identified as Linear Free Energy Relationships (LFER), and as only the standard potential is an electrochemical quantity directly linked with free energy (AG° = -n F AE°), one can make use of these mathematical treatments only in cases of electrochemically reversible redox processes (or in the limit of quasireversibility). Only in these cases does the measured redox potential have thermodynamic significance. 

As a compromise between the above two approaches, the third approach adopts nonactive (inert) materials as working electrodes with neat electrolyte solutions and is the most widely used voltammetry technique for the characterization of electrolytes for batteries, capacitors, and fuel cells. Its advantage is the absence of the reversible redox processes and passivations that occur with active electrode materials, and therefore, a well-defined onset or threshold current can usually be determined. However, there is still a certain arbitrariness involved in this approach in the definition of onset of decomposition, and disparities often occur for a given electrolyte system when reported by different authors Therefore, caution should be taken when electrochemical stability data from different sources are compared. 

It is evident that the square wave charge-potential curves corresponding to surface-bound molecules behave in a similar way to the normalized current-potential ones observed for a soluble solution reversible redox process in SWV when an ultramicroelectrode is used (i.e., when steady-state conditions are attained), providing the analogous role played by 2sw (surface-bound species) and (soluble solution species), and also 2f (Eq- (7.93)) and the steady-state diffusion-limited current (7 css), see Sect. 2.7. This analogy can be made because the normalized converted charge in a surface reversible electrode process is proportional to the difference between the initial surface concentration (I ) and that... 

Resulting cyclic voltammogram for a Nernstian reversible redox process. 

Figure 1.15. Typical cyclic voltammogram for a reversible redox process [19]. (From Wang J., Analytical electrochemistry. 2006 Wiley-VCH. Reproduced with permission.)...

The reversibility of electrochemical reactions is determined by the reaction rates for a reversible reaction, k° > 0.3 v /2 cm/s, for a quasi-reversible reaction, 0.3 v172 > k° > 2 x 10 V12 cm/s, while for an irreversible reaction, k° < 2 x KTV° cm/s. Figure 1.16 shows the cyclic voltammograms for irreversible and quasi-reversible redox processes. 

Electrochemical investigations have shown that Mo( OA r)(S 2C2R 2)21 complexes exhibit two reversible redox processes, at (versus SCE) E /2 = —1.95 and 0.10 V (R = Me Ar = C6H3-2,6-/-Pr2) and -1.74 and 0.30 V (R = Ph Ar = C6H3-2,6-/-Pr2), that are attributed to the Mo(IV/III) and Mo(V/IV) couples, respectively. The very negative potential that is required to produce the Mo(III) state of these systems clearly suggests that this oxidation state is unlikely to be accessible to corresponding molybdenum centers of the Mo MPT enzymes. 

The electrochemistry of the polymeric and isomorphous cobalt(II) and nickel(II) methylsquarates was also studied by Iwuoha et al. In aqueous solutions, they found evidence that both the nickel(II) methylsquarate and its cobalt analog were dissociated without any reversible redox processes occurring for the metal ions. However, the cyclic and Osteryoung square wave voltammograms, obtained using a Pt electrode for solutions of these complexes in dimethylformamide and dimethylsulfoxide, contained signals attributable to both ligand-based and metal-based redox processes 142). 

According to cyclovoltammetric investigations some of the derivatives possess even quasi-reversible oxidation waves although no extended n systems are present. This is illustrated in Fig. 7.4 for [l.l.l.ljpagodadiene. A first quasi-reversible redox process can be detected at 1/2 = 0.66 V versus Ag/AgCl resembling the formation of a persistent radical cation. 

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