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. Examples of the major kinds of chromism are as follows.
thermochromism is chromism that is induced by heat, that is, a change of temperature. This is the most common chromism of all.
photochromism is induced by light irradiation. This phenomenon is based on the isomerization between two different molecular structures, light-induced formation of color centers in crystals, precipitation of metal particles in a glass, or other mechanisms.
Chromic phenomena are those phenomena in which color is produced when light interacts with materials, often called chromic materials in a variety of ways. These can be categorized under the following five headings:
Those phenomena which involve the change in color of a chemical compound under an external stimulus fall under the generic term of chromisms. They take their individual names from the type of the external influence, which can be either chemical or physical, that is involved. Many of these phenomena are reversible. The following list includes all the classic chromisms plus many others of increasing interest in newer outlets.
Photochromism – color change caused by light.
Thermochromism – color change caused by temperature.
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 and the volume has grown at a steady rate throughout the early years of this century. In 2019 the value of the organic dyes/pigments market is forecast to be $19.5bn. Their value is exceeded by the 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, plastic and cosmetics. Both are used in the growth area of the digital printing of textiles, paper and other surfaces. Dyes are also made using the properties of chromic substances: Examples being Photochromic dyes and Thermochromic dyes
Luminescence
The absorption of energy followed by the emission of light is often described by the term luminescence. The exact term used is based on the energy source responsible for the luminescence as in color-change phenomena.
Materials may be used to control and manipulate light via a variety of mechanisms to produce useful effects involving colour. For instance, a change of orientation of molecules to produce a visual effect as in liquid crystal displays. Other materials operate by producing a physical effect, by interference and diffraction as in lustre pigments and opticallv variable pigments, colloidal photonic crystals and in holography. Increasingly inspiration is coming from Nature, in the form of bioinspired structural colours. Molecular materials are also used to increase the intensity of light by modifying its movement through the materials by electrical means, so increasing its intensity as in organic lasers, or in modifying the transmission of light through materials, as in opto-electronics, or by purely by all optical means as in optical limiters.
