Electro electrochromism

Figure 29. (a) Evolution of the absorption spectra of an electro-chromic polypyrrole as a function of the oxidation potential obtained during voltammetry between -900 and 400 mV from a 2.5 M LiCI04 aqueous solution. The voltammetry was performed at a scan rate of 20 mV s 1. (From Ref. 161). (b) Evolution of the absorption spectra of an electrochromic polypyrrole as a function of the reduction potential obtained during voltammetry between 400 and -900 mV from a 2.5 M LiCI04 aqueous solution. The voltammetry was performed at a scan rate of 20 mV s 1. (From Ref. 161). 

It is important, however, to realize that whilst many types of chemical species exhibit electro-chromism, only those with favorable electrochromic performance parameters1 are potentially useful in commercial applications. Thus, most applications require electrochromic materials with a high contrast ratio, coloration efficiency, cycle life, and write-erase efficiency.1 Some performance parameters are application dependent displays need low response times, whereas smart windows can tolerate response times of up to several minutes. 

Figure 7.6 Mirrors (a) an ordinary car driver s mirror reflects the lights of a following car, which can dazzle the driver (b) in an electrochromic mirror, a layer of optically absorbing chemical is electro-generated in front of the reflector layer, thereby decreasing the scope for dazzle. The width of the arrows indicates the relative light intensity...

Chrom means colour and electro implies an electrochemical process, so electrochromic means colour change or generation of a new optical absorption band caused by an electron-transfer reaction. 

Electro-photoswitching devices possess a dual mode operation resulting from different electrochemical properties in the two photo-interconvertible states. They may display both photochromism and electrochromism. They allow the photomodulation of electrochemical properties as was shown to occur in photochromic dihydroazulene derivatives [8.247]. Separate photochemical and electrochemical interconversion has been realized in systems containing a photoactive azo [8.248, 8.249] or dihydroazulene [8.250] unit appended with an anthraquinone moiety. 

Metal-containing polymers, in which the metal is coordinated to the polymer chain, are interesting from the standpoint that the conjugated backbone can help stabilize redox activity, affording a continuum of accessible states. The very nature of these materials suggests an array of potential uses (e.g., electro-catalysis, electrochromic displays, and molecular recognition). There are many ex-... 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Chromic properties (- electrochromism, electro-chemically induced chemiluminescence). 

Electrochromism of a liquid solution [lm (-a) a),0,0))] electro-optical absorption measurements (EOAM)... 

All books, reviews, and entries on CPs describe the potential applications. Chandrasekhar and others ° have reviewed in comprehensive fashion the applications of CPs, including batteries sensors electro-optic and optical devices microwave- and conductivity-based technologies electrochromic devices electrochemomechanical and chemomechanical devices corrosion protection semiconductor, lithography, and electrically related applications— photovoltaics, heterojunction, and photoelectrochemical cells capacitors electrolytic and electroless metal plating CP-based molecular electronic devices catalysis and delivery of drugs and chemicals membranes and LEDs. 

A further electro-optical effect which is studied for applications in display is electrochromism. Electrochromism is characterized as a reversible colour change induced in some organic and inorganic materials by an applied electric field or current. According to Chang [263], the physical mechanisms responsible for these phenomena are electronic and electrochemical in nature. A schematic electrochromic reaction can be formulated as follows ... 

Electrochromism is the reversible change in optical properties of a material caused by redox reactions. The redox reactions can be initiated when the material is placed on the surface of an electrode. When the electro-chromic material is capable of showing several colors, it is addressed as polyelectrochromic. 

Electrochromism is a phenomenon displayed by some materials reversibly changing colors. Various materials can be used to construct electrochromic devices, such as transition metal oxides, liquid crystals, photonic crystals, and polymers (Booth and Casey, 2009 Nicoletta et al., 2005 Arsenault et al., 2007 Gamier et al., 1983). Here, we will focus on the electrochromic materials based on polymers. There are several mechanisms to explain the color changes of polymer electrochromic materials like electro-induced oxidation-reduction and electrothermal chromatic transition and so on. 

Conjugated polymers, discovered in the late 1970s, have attracted a variety of attentions because of their unique properties, such as electrical conductivity and color versatility. The conjugated polymers with different colors can be used as ideal electrochromic materials, which have potential applications in sensors, mirrors, displayers, and textiles. Most of their reversible electrochromic behaviors are caused by the electro-induced oxidation-reduction, that is, the reversible change of a chemical species between two redox states under a certain voltage (Niklasson and Granqvist, 2007 Beaujuge et al., 2010). 

Besides electricity-induced oxidation-reduction and heating mechanisms, there are other electrochromic mechanisms that give rise to the electrochromic behavior, such as electro-introduced conformational changes of conjugated polymers, reorientation of liquid crystals and change of lattice constant in opals. 

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