Heptyl viologen

MV /MV " (HV is heptyl viologen and MV is methyl viologen). The specific effects of iodide on the electrochemical behavior of the layer-type compounds were compared, and the characteristics of several PEC cells were described. The interface energies for n-MoSe2 in contact with various redox couples were given as in Fig. 5.9. 

Worked Example 7.5. The one-electron reduction of the heptyl viologen dication (HV +) (to form the radical cation HV+ ) occurs at a platinum electrode with an area of 1.50 cm. Calculate the value of kd, given that the intercept on a Tafel plot of log / (as y ) against tj (as x ) is —2.2. 

The adsorbed state of heptyl viologen on the Au(lll) electrode surface was... 

Cyclic voltammetry and in situ IRAS have been utilized in the studies on adsorption of heptyl viologen (HV) at mercury electrode [147]. A set of very sharp cathodic and anodic peaks formed at potentials more positive than the... 

Despite some small spectral differences, the similarities have been sufficient to confirm the slow step in the electrochemistry of immobilized cobalt porphyrin mediators (113) and to identify the intermediates involved in a tetrathiafulvalene polymer coated electrode (7). A polyxylylviologen -polystyrenesulfonate co-polymer coated electrode, on the other hand, showed no changes in the position of the peaks in the absorption spectra upon immobilization (111). Presumably this indicated an absence of interactions between neighboring viologen moieties. Similar spectral results have been obtained using photoacoustic spectroscopy (PAS). Heptyl viologen adsorbed on Pt exhibited an unshifted spectrum which correlated with the electrochemical results (115). 

Electrochromic materials are of three basic types [i]. In a given -> electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states, an example being l,l -di-methyl-4,4 -bipyridylium ( methyl viologen ), which, on reduction, switches from the colorless di-cation to the blue radical cation. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. An example here is l,l -di-heptyl-4,4 -bipyridylium ( heptyl viologen ). In type III materials, such as -> tungsten oxide, - Prussian blue, and electroactive conjugated polymers, both... 

Examples include the radical anion of tetracyanoethylene [327] and the radical cation derived from heptyl viologen [328]. 

As shown in this symposium, interest in chemical modification of electrode surfaces has been extended in many directions, including the study of light-assisted redox reactions, and the use of modified electrodes in electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (A,5). 

In the case of the n-heptyl viologen deposition, nucleation rates of the first molecular layers of this molecule control the deposition rates of subsequent layers. The nucleation reaction follows the instantaneous nucleation model — and is found to be highly sensitive to the chemical and physical nature of the electrode surface prior to deposition. RF-plasma of ion-beam etched surfaces generally show greatly enhanced nucleation and bulk deposition rates. 

More recently we have extrapolated these surface activation experiments to the examination of the reduction of n-heptyl viologen dication to its insoluble cation radical... 

Linear sweep voltammetry delineates several regions in the current/potential relationships for the n-heptyl viologen... 

Figure 6. Current vs. time, and transmittance-decrease vs. time plots for n-heptyl-viologen reduction obtained on a clean (top) and ion-beam modified (bottom) ITO, MPOTE. Current increases with (tinted in the nucleation region, and then decreases, while the absorbance changes linearly. Rates of nucleation are enhanced on the modified surface. [n-HV ] = 10 3M, 0.1 M KHP. Potential steps are indicated with each plot.

Further studies of the n-heptyl viologen reaction on modified surfaces are reported elsewhere (25). It is clear that the interaction of ion-beams or plasmas with the electrode surface can be a powerful modification tool, complementary to chemical modification procedures for application to either photoelectrochemical or electrochromic reactions. 

The high sensitivity of SEIRAS (see above) allows measurements in real time during a slow electrode potential scan [303-305] for particularly fast acquisition, step-scan interferometers may be used [306]. A series of time-resolved SEIRA spectra recorded during reduction of heptyl viologen to HV + at a silver electrode in an aqueous solution of 0.3 M KBr is displayed in Fig. 5.60 [274]. 

The use of a mercury drop bottom electrode placed on an underlying Nation fihn in ER measurement was demonstrated for the reaction of heptyl viologen incorporated in a Nafion film (Fig. 2.13) [48], A mercury drop was placed on a Nation 117 tihn of thickness 0.175 mm. The transparent nature of the Nafion as weU as the high conductivity for cations through its film made it actually possible to measure the ER spectrum of the redox reaction of heptyl viologen with a perpendicular incidence of the hght to the mercury electrode surface through the cell bottom window and the Nafion fihn. 

Fig. 2.13 (a) Schematic view of the cell used to measure the normal incidence ER spectrum at the bottom of a mercury drop electrode placed on an underlying Nafion film, (b) Solid line ER spectrum (real part) for heptyl viologen at a mercury electrode on Nafion film equilibrated with 1 mM heptyl viologen+ 1 M KBr (Ejc=-0.5 V vs Ag/... 

AgCI-sat d KCI, AEac=71 mV./=8 Hz) at a normal incidence. Dotted line Absorption spectrum of reduced form of heptyl viologen in an aqueous solution containing excess of sodium hydrosulfite. 

Fig. 15 Time-resolved infrared spectra of the reduction of heptyl viologen, HV +, at a silver electrode during a potential step from —0.2 to —0.55 V versus Ag/AgCl. The spectra were taken with a 100-ps acquisition time, but only the spectra of every 1-ms interval are shown, for clarity, see text for details. (From the work of Osawa and...

The physical classification dilTerentiates three types of electrochromic materials. Type I electrochromic materials are always in solution. MetaUic ions belong to this class. Type II electrochromic materials are colourless and in solution at one state and coloured and solid at the other state. Heptyl viologen is type II. Type in electrochromic materials are always solid. Most electrochromic materials are type HI, including conducting polymers or metal oxides [59,60]. 

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