Photochromic windows

It also became evident before long, that the application of HABIs in photochromic windows would be a daunting challenge. We noted that the radicals were excellent hydrogen abstractors, and we could not develop media that lacked abstractable hydrogen. Additionally, the color of the generated free radicals was weak, compared to real dyes, with extinction coefficients under 10,000, compared to dyes, such as Crystal Violet, with over 140,000. 

The discovery of a photochromic oxidation product of triphenylimidazole by Hayashi and Maeda [1] stimulated wide interest in hexaarylbiimidazole dimers. At that time, an effort to develop photochromic windows and related products was underway at the DuPont Company under the direction of Dr. George R. Coraor, and Hayashi and Maeda s work was examined and duplicated. Fortunately, this interest attracted several synthetic chemists who were eager to develop related compounds with superior properties, what ever these might be. Obviously, for the proposed application, there was a need for a large number of reversals, different colors from the purple of the parent radical and different speeds of photolysis and recombination. 

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. 

Obvious applications of photochromism include glasses which darken according to the intensity of the surrounding light, and such glasses have now been available for some time both in the form of plate glass for windows and in the form of lenses for sunglasses. Their action is very slow, it takes indeed many minutes for the photostationary state to be established. This is due to the fact that the photochromic effect takes place at the molecular level... 

This experiment confirmed the fact that photochromic materials are beginning to find a broad range of applications that goes beyond their familiar use in sunglasses, windows, and other everyday devices. As researchers develop abetter understanding of the fundamental principles involved in the way photochromic materials operate, advanced applications in molecular switches, nanocomputers, and other nanometer-scale devices are likely to become much more common. 

In order to make these general procedures more concrete a design is described here of an optothermal active material that might be used as a photochromic cover of a solar collector with automatic feedback. It could also be used as an active window material for interior climate control. 

Photochromic materials can be applied to smart windows and sunglasses. In view of thermal control, photochromism in the infrared region... 

Research on photochromic compounds by itself is an important issue with great industrial impact from ophthalmic lens fabrication to the achievement of intelligent windows and displays. [l,2]Furthermore, the possibility of having photochromism in ionic liquids is an appealing objective due to the unique properties exhibited by these compounds. The research on this field is very recent and can be divided in two main categories i) photochromic compoimds dissolved in ionic liquids, ii) and more recently the paepjaration and study of intrinsically photochromic ionic liquids. 

A program to investigate various organic materials as photochromic components in automotive and architectural windows, initiated about 1958 at DuPont seriously examined hexaaryl-biimidazoles (HABIs), which had been reported in the literature [see Chapter 5]. These compounds were relatively easy to synthesize, by the route indicated below (Scheme 8.2). 

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