Electrochromic, electrochromism redox

Electrochromic redox reactions of vapour-deposited thin films of tetrap yridotetraazaporphyrinatozinc(II). J. Porphyrins Phthalocyanines, 4,112-122. 

Oxidative Eiectropoiymerization of Poiypyridyi Compiexes. Oxidative electropolymerization of suitably substituted [M (bipy)3l + complexes offers an alternative approach to the preparation of electrochromic redox active polymer films. Oxidative eiectropoiymerization has been described for iron(II) and ruthenium(II) complexes containing amino-substituted (31) and pendant aniline-substituted (32) 2,2 -bipyridyl ligands, and amino- and hydroxy-substituted 2,2 6, 2"-terpyridinyl ligands (33) [ligand structures (4) and (5)]. Analysis of IR spectra suggests that the eiectropoiymerization of [Ru(L )2l + [L = (4)], via the pendant aminophenyl substituent, proceeds by a reaction mechanism similar to that of aniline (33). The resulting metallopolymer film reversibly switches from purple to pale pink on oxidation of Fe(II) to Fe(III). For polymeric films formed from [Ru(L )2l + [L = (5)], via polymerization of the pendant hydroxyphenyl group, the color switch is from brown to dark yellow (see Electropolymerization). 

Photoactive films are formed by dispersing them in different glassy solids and studied for diode applications [70]. These compounds dispersed in polystyrene [227], poly A -vinyl carbazole or polyimides [141,142] show stable electrochromic redox properties in the absence of redox couples in an electrolyte. The electrochemical activity is found to depend on the type of the polymers, the amount of phthalocyanine, the layer thickness and the conditions of film preparation. Photovoltaic cells using Schottky junctions have been... 

The electrochemical polymerization of Ti-electron-rich aromatics, such as aniline, pyrrole and thiophene, to obtain electrically conducting polymers is well-known. Some reports describe the polymerization of amino-, pyrrolyl- and hydroxy-substituted tetraphenylporphyrins and suitable substituted phthalocyanines (for reviews see [230,231]) (anodic electropolymerization of 2,9,16,23-tetraaminophthalocyanine (M = Co(II), Ni(II)) [231,232] and 2,9,16,23-tetra(l-pyrrolylalkyleneoxy)phthalocyanines (M = 2H, Zn(II), Co(II) [232])) under formation of polymers 53 and 54 shown as idealized structures. Depending on the reaction conditions the film thicknesses are between around 50 nm and several pm. The films remain electroactive at the electrochemical potential so that oxidation or reduction current envelope grows with each successive potential cycle. Electrochromism, redox mediation and electrocatalysis of the electrically conducting films are summarized in [230,231]. 

Polynuclear transition metal cyanides such as the well-known Prussian blue and its analogues with osmium and ruthenium have been intensely studied Prussian blue films on electrodes are formed as microcrystalline materials by the electrochemical reduction of FeFe(CN)g in aqueous solutionThey show two reversible redox reactions, and due to the intense color of the single oxidation states, they appear to be candidates for electrochromic displays Ion exchange properties in the reduced state are limited to certain ions having similar ionic radii. Thus, the reversible... 

The reader may be aware that the terms electrochromism and electrochromic are also used in a large segment of literature to mean dyes whose absorption spectra are changed through redox reactions induced by electrochemical means. This subject is not included in this chapter. 

The electrochromism of the phthalocyanine ring-based redox processes of vacuum-sublimed thin films of [Lu(Pc)2] was first reported in 1970,32 and since that time this complex has received most attention, although many other (mainly lanthanide) metallophthalocyanines have been investigated for their electrochromic properties.1 Lu(Pc)2 has been studied extensively by Collins and Schiffrin33,34 and by... 

The structures and redox properties of these complexes have been extensively reviewed 170,171 of interest here is the presence of an intense NIR transition in the neutral and mono-anionic forms, but not the dianionic forms, i.e., the complexes are polyelectrochromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. A large number of substituted dithiolene ligands has been prepared and used to prepare complexes of Ni, Pd, and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1,400 nm and extinction coefficients up to ca. 40,000 dm3 mol-1 cm-1 (see refs. 170,171 for an extensive listing). 

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... 

Figure 3 Ligand-centered redox interconversions of [ Ru(bipy)2 3(/i-L22)]n+ ( = 3-6) (potentials vs. SCE), and the resulting electrochromic behavior. Spectra were measured at 243 K in MeCN.192...

As in the case of [3]radialenes, the individual redox stages of [4]radialenes may have different colors. Based on these electrochromic properties, the application of 77 as a component in liquid crystal display devices was patented85. 

These mirrors are electrochromic if they contain a substance that changes colour according to its redox state. For example, methylene blue, MB+ (II), is a chromophore because it has an intense blue colour. II is a popular choice of electrochromic material for such mirrors it is blue when fully oxidized, but it becomes colourless when reduced according to... 

Stabilized Carbocations as Redox Active Chromophores Preparation of Electrochromic Materials Using Stabilized Carbocations... 

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). 

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Bhandari, S. Deepa, M. Srivastava, A. K. Joshi, A. G. Kant, R., Poly (3, 4-Ethylene-dioxythiophene)- multiwalled carbon nanotube composite films structure-directed amplified electrochromic response and improved redox activity./. Phys. Chem. B2009,113, 9416-9428. 

In some contexts, an indicator (dye) such as this is said to be electrochromic, that is, the potential determines the colour (the name comes from the Greek word chromos, meaning colour ). A few representative redox indicator systems are shown in Table 4.1. 

Electrochromism The electrochemical generation of colour in accompaniment with a redox reaction, e.g. as displayed by a redox indicator. 

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