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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.
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