In chemistry, chromism is a process that induces a reversible change 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.

• 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.
• electrochromism is induced by the gain and loss of electrons. This phenomenon occurs in compounds with redox active sites, such as metal ions or organic radicals.
• solvatochromism depends on the polarity of the solvent. Most solvatochromic compounds are metal complexes.

Chromic phenomena

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:

• Reversible color change
• The absorption and reflection of light
• The absorption of energy followed by the emission of light
• The absorption of light and energy transfer (or conversion)
• The manipulation of light.

Color changing phenomena

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. They are:

• Photochromism - color change caused by light.
• Thermochromism - color change caused by heat.
• Electrochromism - color change caused by an electrical current.
• Solvatochromism - color change caused by solvent polarity.
• Ionochromism - color change caused by ions.
• Halochromism - color change caused by a change in pH.
• Tribochromism - color change caused by mechanical friction.
• Piezochromism - color change caused by mechanical pressure.

Dyes

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.

Dyes are also made using the properties of chromic substances: Photochromic dyes and Thermochromic dyes

Luminescence

The absorption of energy followed by the emission of light is often described by the term luminescence. The list of these phenomena is long, and many are widely used in consumer products and other important outlets. Cathodoluminescence is used in cathode ray tubes, photoluminescence in fluorescent lighting and plasma display panels, phosphorescence in safety signs and low energy lighting, fluorescence in pigments, inks, optical brighteners, safety clothing, and biological and medicinal analysis, chemoluminescence and bioluminescence in analysis, diagnostics and sensors, and electroluminescence in the burgeoning areas of light-emitting diodes (LEDs), displays and panel lighting.

Light transfer

Absorption of light and energy transfer (or conversion) involves colored molecules that can transfer electromagnetic energy, usually from a laser light source, to other molecules in another form of energy, such as thermal or electrical. These laser addressable colorants are used in optical data storage, organic photoconductors, in photomedicine (such as photodynamic therapy of cancer), and in solar energy utilization.

Light manipulation

Materials may be used to manipulate light via a variety of mechanisms. For instance, a change of orientation of molecules as in liquid crystal displays, by interference and diffraction as in holography, and by modifying the movement of light through materials by electrical means, as in opto-electronics, or by optical means in photonics.