Electrochromic systems

Another electrochromic system based on mesoporous EISA-produced films was realized by Jheong et al.,74 who fabricated Pluronic-templated Ti02... 

Several other organic systems have been stndied as potential electrochromes because of their redox behaviour. These include carbazoles, methoxybiphenyls, fluorenones, benzoquinones, naphthaquinones and anthraqninones, tetracyanoquinodimethane, tetrathiafnlvalene and pyrazolines. ° Of particnlar interest are the 1,4-phenylenedi-amines, which form highly colonred species on oxidation. These, known as Wurster s salts, exemplified by Wnrster s Bine (1.97), are anodically colouring and this type of material has found nse in composite electrochromic systems for car rearview mirrors (see 1.5.4.1). 

The main application areas for electrochromic systems are in electrically switchable rear-view car mirrors for anti-dazzle, in glazing units for temperature and light control and in visnal displays. The main advantage offered by electrochromic systems in the first two of these applications is the fact that the cells can be made very large, i.e. one cell can comprise a whole glazing unit. 

The desirable properties of an electrochromic system for use in both rear-view mirrors or glazing units are ... 

Catenane 404+ was also incorporated into a solid-state device that could be used for random access memory (RAM) storage.42 In addition, this compound could be employed for the construction of electrochromic systems, because its various redox states are characterized by different colors.41,43... 

Metal-oxide-based electrochromic systems are especially interesting for the development of electrochromic windows because they mostly switch from a transparent state to a dark colored state [38,39]. In addition, their relatively slow response times are acceptable for this kind of application, possibly even preferable from an aesthetic point of view. Again, W03 has seen the most use in the development of actual devices. Several different deposition techniques have been applied. For example, a prototype electrochromic window based on W03 with reasonable dimensions (0.7 X 1 m) has been assembled that reduces light transmission by a factor of 4 in its colored state [28]. 

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>. 

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. ... 

M. Green (1996) Chemistry Industry, p. 643 - The promise of electrochromic systems . 

Many of the techniques used to study electrochemical kinetics involve perturbation and measurement of electrical variables such as voltage and current. However, an electrochemical system in some initial steady state condition can also be perturbed by a suitable periodic non-electrical stimulus, and the monitored response may also be non-electrical examples are periodic modulation of mass (electrochemical quartz crystal microbalance [97] and of optical transmission (electrochromic systems) [98-100], In general, the relationship between input and response is described by the transfer function, G, which contains information about the system under study [101], and analysis can be performed either in the time or frequency domain. In the latter case, the frequency dependent transfer function G o)) is defined as... 

In automotive industry [2], rearview mirrors are a good application for electrochromic systems. Transport sunroofs also have been manufactured, in particular in Japan. The durability—characterized by the number of oxidation/reduction cycles which can be applied to the material without excessive degradation is in agreement with the automobile mean durability. 

Ishigaki Y, Suzuki T, Nishida J et al (2011) Hysteietic tricolor electrochromic systems based on the dynamic redox properties of imsymmetrically substituted dihydrophenanthrenes and biphenyl-2,2 -diyl dications efficient precursor synthesis by a flow micTOTeactor method. Materials 4 1906-1926... 

Titanium(ll) oxide is manufactured by heating Ti02 and Ti in vacuo. It is a black solid and a metallic conductor which adopts an NaCl-type structure with one-sixth of both anion and cation sites unoccupied. The oxide is a non-stoichiometric compound with a composition typically in the range TiOo 82 TiOi 23. A commercial use of TiO is in electrochromic systems (see Box 23.4). Conducting properties of the first row metal(II) oxides are compared in Section 28.3. 

Leventis, N. and Y.C. Chung. 1992. New complementary electrochromic system based on poly-pyrrole-Prussian Blue composite, a benzylviologen polymer, and polyfvinylpyrrohdone)/ potassium sulfate aqueous electrolyte. Chem Mater 4 1415-1422. 

Finally, cation selection is application dependent. It has already been emphasized that there are relatively few alternatives to lithium for high energy density power sources, but clearly a calcium sensor requires a calcium-based electrolyte. Certain applications are of course dictated by performance rather than species an electrochromic system requires an optical response that may be satisfied by more than one choice of system. Even here, however, mechanical factors and long-term stability to exposure may override considerations of the conductivity contributions of individual species. The specific requirements for particular applications are discussed in detail in later chapters. 

Figure 8.12 illustrates the cyclic voltammetry and the transmittance of the window. The feasibility of the electrochromic system is confirmed by the excellent peak definition and low peak separation, as well as by the large transmittance variation upon cycling. One problem associated with the use of lithiated nickel oxide counter-electrodes remains the low diffusion of the Li" ions (calculated to be of the order of 10 cm s [24]), which may lead to long response times. However, this problem may be alleviated by optimizing the morphology of the electrode. 

These precipitation reactions have been previously reported with other hydrophobic viologens, like the commercially available heptylviologen [14], and form the basis for the proposed use of viologens as active components in electrochromic systems. 

Optical memory (open-circuit memory). Optical memory is defined as the time that an electrochromic material maintains its absorption state once the applied voltage is removed. Solution-based electrochromic systems will bleach (lose their absorptive state) more quickly than their solid-state counterparts [24]. In the solution case, the mobile redox-active species, which are dissolved in an electrolyte, can diffuse to both electrodes when the circuit is open. Therefore, there is no open-circuit memory in these devices and power must be supplied continuously to maintain coloration. 

The secondary electrode redox reaction is chosen to be a system where there is little perceptible visible color change or as an electrochromic system where the color change is complementary to that of the color change at the primary electrochromic electrode. 

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