Electrochromism, reversible redox systems

Electrochromism is observed in reversible redox systems, which exhibit significant color changes in different oxidation states. Violenes, whose general structure is represented in Figure 1, are typical examples that exhibit electrochromism (1). 

The Donnelly Corporation have also devised a rear-view mirror using an hybrid system. A metal oxide electrochromic layer is used in conjunction with a non-elec-trochromic reversible redox complex in the contacting solution. ... 

The reaction of thioxanthone with various 3-thienyllithium compounds is the initial step in the synthesis of the diols such as 602 from which the bis(thioxanthylium) dication 603 is obtained. This species functions as a reversible redox pair with its reduction product, the hexaarylethane, creating an electrochromic system in which electron transfer brings about bond making and bond breaking. These oligomers 604 may be considered to be a new class of molecular wires (Scheme 238) <2004OL2523>. 

In the last few years McCleverty, Ward, and co-workers have reported the NIR electrochromic behavior of a series of mononuclear and dinuclear complexes containing the oxo-Mo(iv) v core unit [Mo(Tp )(0)Cl(OAr)], where Ar denotes a phenyl or naphthyl ring system [Tp = hydro-hydrotris(3,5-dimethylpyrazolyl)borate].184-189 Mononuclear complexes of this type undergo reversible MoIV/Mov and Mov/MoVI redox processes with all three oxidation states accessible at modest potentials. Whilst reduction to the MoIV state results in unremarkable changes in the electronic spectrum, oxidation to MoVI results in the appearance of a low-energy phenolate- (or naphtholate)-to-MoVI LMCT process.184,185... 

The instability of PANI above 0.8 V aside, the redox behavior of the first couple of polyaniline, leucoemeraldine to emeraldine, is stable and has been exploited in EC systems (9,13,24). Thin layer electrochromic devices have been reported by a number of groups, however despite the seemingly good results derived it would be desirable to address the pemigraniline (blue) form as well. If it were possible to access this state reversibly, then PANI or related materials could find utility in glare reduction applications and in smart windows since the band associated with this form best matches the solar spectrum (25). 

As will be seen in the sequel, an important factor neglected in the construction of many electrochromic devices is the provision of an adequate counter-electrode reaction which would permit reasonable faradaic reversibility of the electrochemical system. Frequently, the only available counter electrode reaction is redox of impurities such as O2, H2O and trace metal ions, leading to poorer cyclability. 

The voltammetric behavior of PB films is characterized by the presence of two prominent reversible peaks. This is shown in Figure 18. The nature of the redox processes associated with the electrochromic effect have been investigated using in situ Mossbauer spectroscopy by Itaya and coworkers/ in what may be regarded as a classical example of the use of this technique in the study of electrochemical systems. 

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