Electrochromic mirrors

In general, the term electrochromism is used to describe the change in light absorption as a result of an electrochemical reaction. Recent interest in electrochromism stems from its potential applications in numerous devices, such as flat screen displays, antidazzle mirrors, smart windows, and others. 

It s quite common when driving at night to be dazzled by the lights of the vehicle behind as they reflect from the driver s new-view or door mirror. We can prevent the dazzle by forming a layer of coloured material over the reflecting surface within an electrochromic mirror. Such mirrors are sometimes called smart mirrors or electronic anti-dazzle mirrors . 

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... 

Electrochromic mirrors are now a common feature in expensive cars. 

We can now explain how an electrochromic car mirror operates. The mirror is constructed with II in its colourless form, so the mirror functions in a normal way. The driver activates the mirror when the anti-dazzle state of the mirror is required, and the coloured form of methylene blue (MB+) is generated oxidatively according to Equation (7.24). Coloured MB+ blocks out the dazzling reflection at the mirror by absorbing about 70 per cent of the light. After our vehicle has been overtaken and we require the mirror to function normally again, we reduce MB+ back to colourless MB0 via the reverse of Equation (7.24), and return the mirror to its colourless state. These two situations are depicted in Figure 7.6. 

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...

The reasoning above helps explain why MB0 and MB+ have different colours. To summarize, we say that the colours in an electrochromic mirror change following oxidation or reduction because different orbitals are occupied before and after the electrode reaction. 

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

Electrochromism Electrical current Windows, mirrors, displays... 

The materials that change colour on passing a charge are called electrochromes, and these can be classified into three groups. In the first type the colouring species remain in solution in the second type the reactants are in solution but the coloured product is a solid the third type are those where all the materials are solids, e.g. in films. The first type is used in car, anti-dazzle, rear-view mirrors, the second type in larger mirrors for commercial vehicles and the third type in smart windows (see section 1.5.4.2). 

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 ... 

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 electrochromic, self-dimming mirror market is expanding rapidly, comprising around 6 M of the estimated 150 M total for mirrors in cars and trucks in 2000. The market is set to grow to around 15 M by 2010, with additional growth in self-dim-mable sunroofs and sun visors. 

The detection limit for TLV has been improved substantially by using differential pulse and square-wave voltammetry (Chap. 5). For example, detection limits in the 10 8 M range and below have been demonstrated in thin-layer cells requiring less than 100 /xL of sample [61,62]. One practical application of twin-electrode thin-layer cells is in the automatic electrochromic rearview mirror for automobiles. A cell with optically transparent electrodes is placed in front of a mirrored surface. At night, electrolysis in the cell to generate colored material can rapidly reduce glare from following vehicles. 

In Fig. 4, we have included a portion of the spectral region above 400 nm, where the electrochromic shifts of chlorophyll spectra, possibly ofP680 itself, induced by the presence of positive charges on the various S-states are very prominent. While this electrochromic shift is absent in the S2->S3 transition, those appearing in the spectra for the 8,- 83 and Sq- S, transitions are interestingly almost mirror images of each other. 

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. 

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