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Photochromism is a reversible change of color, specifically a process that describes a change of color in the presence of ultraviolet (UV), visible, and infrared (IR) light. This phenomenon is commonly seen in transitional lenses, which are the kinds of eyeglass lenses that turn dark in outdoor sunlight and become clear in indoor light. A photochromic substance exhibits color change under the presence of certain kinds of light, for example, the UV sunlight that activates transitional lenses. The phenomenon occurs due to the absorption characteristics of molecular material in response to wavelength radiation. Different materials may respond with their own characteristic transmission spectra that transform in the presence of light variations.
An accurate understanding of the phenomenon was first discovered by German Jewish organic chemist Dr. Willi Marckwald (1864–1950), who also went by the name of Willy Markwald, in 1899 and labeled phototropy until the 1950s. He is also credited with the discovery of Radium F, an isotope of Pierre and Marie Curie's polonium, during his tenure at the University of Berlin. Although the photochromic phenomenon had been observed by others as early as 1867, Marckwald determined it factually in his study of the behavior of benzo-1-naphthyrodine and tetrachloro-1,2-keto-naphthalenone under light.
Simply put, a chemical compound exposed to light transforms into another chemical compound. In the absence of light, it transforms back to the original compound. These are labeled as forward and back reactions.
Color shifts can occur in organic and artificial compounds and also take place in nature. Reversibility is a key criterion in naming this process, although irreversible photochromism can occur if materials undergo a permanent color change with exposure to ultraviolet radiation. This, however, falls under the umbrella of photochemistry.
Numerous photochromic molecules are categorized into several classes; these may include spiropyrans, diarylethenes, and photochromic quinones, among others. Inorganic photochromics may include silver, silver chloride, and zinc halides. Silver chloride is the compound typically used in the manufacture of photochromic lenses.
Other applications of photochromism are found in supra-molar chemistry, to indicate molecular transitions by observing characteristic photochromic shifts. Three-dimensional optical data storage employs photochromism in order to create memory disks capable of holding a terabyte of data, or essentially 1,000 gigabytes. Many products use this alteration to create attractive features for toys, textiles, and cosmetics.
Observation of photochromic bands in certain portions of the light spectrum permits nondestructive monitoring of light-related processes and transitions. Nanotechnology relies on photochromism in the production of thin films. The effect can correlate with coloration responses on the surface area of a film, which may be used in any number of optical or material thin-film applications; for example, uses include production of semiconductors, filters, and other technical surface treatments.
Usually, photochromic systems are based on unimolecular reactions occurring between two states with notably different absorption spectra. The process is often a reversible shift of thermal radiation, or heat, as well as visible spectral light. Applying this phenomenon to consumer products as well as industrial technologies involves tying these natural molecular changes to desirable light transmissions and absorptions for a multitude of desirable effects. Energy band engineering of products and technologies is greatly enhanced by these color-sensitive modifications between light, materials, and elements.