Photochromism is a reversible color change caused by exposure to UV, visible, and IR light. It is commonly seen in transition lenses. The phenomenon was discovered by Dr. Willi Marckwald in 1899. Chemical compounds exposed to light turn into another compound and revert in the absence of light. Numerous photochromic molecules are classified into different classes. Other applications include supramalar chemistry, 3D optical data storage, and nanotechnology. The process is often a reversible displacement of thermal radiation and visible spectral light.
Photochromism is a reversible color change, specifically a process that describes a change in color in the presence of ultraviolet (UV), visible and infrared (IR) light. This phenomenon is commonly seen in transition lenses, which are the types of eyeglass lenses that darken in outdoor sunlight and lighten in indoor light. A photochromic substance exhibits a color change in the presence of certain types of light, such as UV sunlight which activates transition lenses. The phenomenon occurs due to the absorption characteristics of the molecular material in response to wavelength radiation. Different materials can respond with their own characteristic transmission spectra that transform in the presence of changes in light.
An accurate understanding of the phenomenon was first discovered by the German Jewish organic chemist Dr. Willi Marckwald (1864–1950), who also went by the name Willy Markwald, in 1899 and called phototropy until the 1950s. He is also credited with the discovery of Radium F, an isotope of polonium by Pierre and Marie Curie, during his tenure at the University of Berlin. Although the photochromic phenomenon had been observed by others as early as 1867, Marckwald actually determined it in his study of the behavior of benzo-1-naphthyrodine and tetrachloro-1,2-keto-naphthalonene in light.
Simply put, a chemical compound exposed to light turns into another chemical compound. In the absence of light, it transforms back into the original compound. These are labeled forward and reverse reactions.
Color changes can occur in organic and man-made compounds and also occur 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 within the realm of photochemistry.
Numerous photochromic molecules are classified into different classes; these can include spiropyrans, diarylethene, and photochromic quinones, among others. Inorganic photochromics can include silver, silver chloride and zinc halides. Silver chloride is the compound typically used in the production of photochromic lenses.
Other applications of photochromism are found in supramalar chemistry, to indicate molecular transitions by observing characteristic photochromic changes. Three-dimensional optical data storage uses photochromism to create memory disks capable of holding one terabyte of data, or essentially 1,000 gigabytes. Many products use this alteration to create attractive features for toys, fabrics and cosmetics.
The observation of photochromic bands in certain portions of the light spectrum allows non-destructive monitoring of light-related processes and transitions. Nanotechnology relies on photochromism in the production of thin films. The effect can be correlated with coloration responses over the surface area of a film, which can be used in any number of optical or thin-film applications; for example, uses include semiconductor manufacturing, filters, and other technical surface treatments.
Photochromatic systems are usually based on single-molecule reactions that occur between two states with significantly different absorption spectra. The process is often a reversible displacement of thermal radiation, or heat, as well as visible spectral light. Applying this phenomenon to consumer products and industrial technologies involves linking 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 changes between light, materials and elements.
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