Electrochromic behavior

Some materials change color corresponding to the applied electrochemical potential, a phenomenon called electrochromism. Electrochromic materials have been known for a long time. Common to all electrochromic materials is the fact that a redox process is triggered electrochemically, which changes the material s optical absorption. 

The glass industry has exploited the electrochromic effect in the development of multilayer glasses that darken electrically ( smart windows ). However, until now these windows have had little success because of harsh lifetime requirements, their complicated and cost-intensive production process, and their high price that appears to be prohibitive for a major market breakthrough. 

More recently, electrochromic displays especially based on FEDOT have attained attraction in the field of printed electronics. Here the main idea is to develop a paperlike display by printing techniques. 

Inorganic oxides as well as electroactive polymers are known for their electrochromic behavior. Tungsten trioxide - has been utilized in electrochromic windows. Among electroactive polymers polyaniline - and polythiophenes have been studied most extensively as electrochromic solids. 

Layer structure of a viologen electrochromic cell ( TCO Transparent-conducting oxide [e.g.. Indium Tin Oxide].) 

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Metallopolymer films have also been prepared by oxidative polymerization of complexes of the type [M(phen)2(4,4 -bipy)2]2+ (M = Fe, Ru, or Os phen= 1,10-phenanthroline, 4,4 -bipy = 4,4 -bipyridine).23 Such films are both oxidatively and reductively electrochromic reversible film-based reduction at potentials below —IV lead to dark purple films,23 the color and potential region being consistent with the viologen dication/radical cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L13)3]2+.24 Electropolymerized films prepared from the complexes [Ru(L16)-(bipy)2][PF6]22 and [Ru(L17)3][PF6]226,27 exhibit reversible orange/transparent electrochromic behavior associated with the Run/Ruin interconversion. 

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 2 Electrochromic behavior of [ Mo(Tp )(0)Cl 2( -OC6H4-C6H4-C6H40)]w+ in the oxidation states Mov-Mov (n = 0), Mov—MoVI (n= 1), MoVI-Mo (n = 2). Spectra were measured at 243 K in CH2C12.185...

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...

Sonochemical methods have been used by the Cordoba de Torresi group to prepare Ni(OH)2, Co(OH)2 and mixed Ni/Co hydroxide NPs [33, 34]. For the sonochemical synthesis, the appropriate metal nitrate was mixed with ammonium hydroxide solutions and then sonochemically irradiated for various times. This produced about 5-nm diameter metal-hydroxide NPs that were then immobilized at ITO surfaces using a LbL approach with poly(allylamine) hydrochloride (PAH). In one study, the electrochromic behavior of the LbL deposits was compared with that of bulk deposits... 

Examples for electrochromic behavior upon electrochemical oxidation can be found among group VIII metal oxides. Thin films of transparent hydrated iridium oxide turn blue-black, whereas nickel oxide switches from pale green to brown-black, possibly due to the absorbance of Ni3+ centers [26]. The systems are much less thoroughly investigated and a detailed mechanistic explanation is not known. However, proton extraction and anion insertion have been suggested. 

Several such polymers have shown electrochromic behavior, among them poly(n-vinylcarbazole) [73] which switches from colorless in the neutral state to green in the doped state (Scheme 10) and poly(Ar-phenyl-2-(2 -thienyl)-5-(5"-vinyl-2"-thienyl)pyrrole) [74], which changes from yellow to reddish brown upon oxidation (Scheme 11). A study of the electrochromic properties of blends consist-... 

The demonstration that MLCT and LLCT bands can, in principle, be treated with the same theoretical model has a tremendous impact on practical aspects and, therefore, the search and the development of electrochromic compounds in the area as well. Especially, the inclusion of organic ligands with their own redox properties along with their interaction with the metal cores and their mediation of the interactions between metals opens up a virtually limitless supply of possible combinations with potential electrochromic behavior. 

Other mixed-valence metal complexes have been published that display electrochromic behavior, among them bipyridyl ruthenium complexes linked through dioxolene containing bridging ligands, such as 3,3, 4,4 -tetrahydroxybi-phenyl, 3,3",4,4"-tetrahydroxy-p-terphenyl, and 9-phenyl-2,3,7-trihydoxy-6-flu-orone [115,116] (Chart 3). Many of the redox processes that lead to frequently... 

Ionization of the corresponding diols with HBF4 or oxidation of 9,9,10,10-tetra-aryldihydrophenanthrenes results in the formation of the 2,2 -bis(triarylmethylium) dications 187 402 These were characterized by UV spectroscopy and found to show tricolor electrochromic behavior in reversible reduction-oxidation cycles. 

Chromotropism in the presence of an electric field, electrochromism, has been observed for just one polysilane, the copolymer (CF3CH2CH2SiMe) -co-(ftPrSiMe)m, n m = 45 55.89 The UV band for this polymer is intensified by 50% and shifted from 294 to 299 nm, in an electric field of 10s v/m. The changes are reversible when the field is removed. Other polysilanes with polar side groups might also show electrochromic behavior, but none have been studied. 

Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

The electrical and electrochromic properties of PANi depend not only on its oxidation state but also on its protonation state, and hence the pH value of the electrolyte used. As shown in Figure 1 PANi exhibits electrochromic behavior. Electrochromic behavior of PANi is shown in Figure 9. As demonstrated, small change in the pH of the solution and/or potential could create color and conductivity changes. PANi is green in the oxidized state and is transparent yellow in the reduced states. It has violet color in very acidic and yellow brownish in very basic media [14-16]. 

The electrochromic behavior of BPEB was explored in the Ti02/ [ Ru dcbpy)2Cl 2(BPEB)] system. As previously shown, this system exhibits a good performance in photoelectrochemical cells, and therefore, the same arrangement can be employed for investigating the electrochromic behavior. In Fig. 59, one can see the CVs in connection with the measurements of the absorption... 

Nickel(II) oxide crystallizes in the NaCl structure. Thin amorphous films of it exhibit electrochromic behavior and are antiferromagnetic at Tn = 247 °C. Nickel(II) oxide films with smooth surfaces and columnar structures of preferred (100) orientation on MgO(lll), Si(lll), soda lime glass, fused silica and stainless steel can be obtained by and... 

Redox series of metal-polypyridines still await their practical exploration. The existence of multistep, reversible, sequential reduction processes, each step occurring at a defined potential and being localized at a specific molecular site, is very promising for possible applications in molecular electronics. This would require to organize the active complexes in films, polymers or supermolecules. Up to now, only the electrochromic behavior of some [Ru(N,N)a] + complexes has been explored with potential applications in electrochromic glasses, displays and redox sensors [206, 262, 264]. 

R. O. Lezna, Preparation, photoelectro-chemical characterization, and photo-electrochromic behavior of metal exacyanoferrate-titanium dioxide composite films. Electrochim Acta 2000, 45, 3403-3411. 

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