Chromogenic Materials, Photochromic

AbstractPhotochromism has been defined as a reversible change in the color, or darkening of a material caused by absorption more generally of ultraviolet or visible light. The change in color is due to a modification of the compound structure and implies a change in the absorption spectrum in the visible range (400–750 nm). The reversion of the system to its original state can be driven either by thermal or photochemical energy (photobleaching), or both, depending on the chemical systems. Photochromic systems are naturally divided into two broad categories : inorganic and organic.Within the inorganic category, the most well‐studied and commercially used systems are those containing silver halide crystallites dispersed throughout a glass matrix. The mechanism of the silver halide photochromism is the reversible formation of silver metal. Both the photochemical formation of silver and the thermal reformation of silver halide are catalyzed by copper ions. The silver halide glass systems have been used commercially in photochromic eyewear.Since 1990, the organic photochromic systems have been a dramatic development in the area of variable optical transmission materials and especially for ophthalmic applications. Indeed polymer glasses have also been greatly developed and are compatible with organic pigments. The most common families are those involving electrocyclic reactions and within this class thermoreversible indolinospirooxazines and naphthopyrans have received the most attention.Indolinospirooxazines were the first photochromic compound used in the preparation of commercially acceptable organic photochromic ophthalmic lenses.Moreover, the area of optical memories and switches has been also a prodigious development and the families of photoreversible fulgides and diaryl (or heteroaryl) ethenes have been extensively studied and used for this type of applications.Finally, many potential applications are possible with photochromic pigments using not only the change of color, but the modifications of physico‐chemical properties under the effect of light irradiation.