Chromism

In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances. It is known that there are many natural compounds that have chromism, and many artificial compounds with specific chromism have been synthesized to date.

Chromism is classified by what kind of stimuli are used. The major kinds of chromism are as follows.

Chromic phenomena are those phenomena in which color is produced when light interacts with materials in a variety of ways. These can be categorized under the following five headings:

Phenomena which involve the change in color of a chemical compound take their name from the type of external influence, either chemical or physical, which is involved. Many of these phenomena are reversible. The following list includes all the classic chromisms plus others of increasing interest in newer outlets.

Commercial applications of color change materials are very common and include in ophthalmics, fashion/cosmetics and optical memory and optical switches, thermochromics in paints, plastics and textiles and architecture, electrochromics in car mirrors and smart windows, and in biological probes.

Classical dyes and pigments produce color by the absorption and reflection of light; these are the materials that make a major impact on the color of our daily lives. In 2000, world production of organic dyes was 800,000 tonnes and of organic pigments, 250,000 tonnes. There is also a very large production of inorganic pigments. Organic dyes are used mainly to color textile fibers, paper, hair, leather, while pigments are used largely in inks, paints and plastics.

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