Electrochromism methyl viologen

The divalent cation methyl viologen is highly colored in reduced form and is used as an electrochromic in solution cells (Eig. 11). 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

For instance, inclusion of l,T-dimethyl-4,4 -dipyridyl cation-radical (the cation-radical of methyl viologen) into a cavity of sulfocalixarenes prevents the disadvantageous n dimerization. This is important for more effective use of viologens as components of electrochromic displays and electric condnctors (Gno et al. 2007, references therein). 

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... 

The properties of benzothiazolinone azine redox systems have been studied with a view to their practical applications Shelepin and co-workers studied a mixed system involving 3-ethylbenzothiazolin-2-one azine and methyl viologen as an electrochromic system activated by optically transparent electrodes Sharp has described perchlorate-sensitive electrodes also involving 3-ethylbenzothiazolin-2-one azine solid-state electrical properties of salts of the 3-methylbenzothiazolin-2-one azine radical-cation have been reported. ... 

Five-colour polyelectrochromicity, by application of an outer Nafion layer (with subsequent electrostatic incorporation of the methyl viologen system) to an inner layer of PB, is possible (Figure 19). The transparent/purple viologen dication/radical cation electrochromicity operates in the potential region where the PB is in its (reduced) transparent state the bilayer electrode system thus exhibits yellow/green/ blue/transparent/purple colours. 

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