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.

• 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.
• 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.
• cathodochromism is induced by electron beam irradiation.

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:

• Stimulated (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. The following list includes all the classic chromisms plus others of increasing interest in newer outlets.

• Photochromism - color change caused by light.
• Thermochromism - color change caused by heat.
• Electrochromism - color change caused by an electrical current.
• Gasochromism- color change caused by a gas - hydrogen/oxygen redox.
• Solvatochromism - color change caused by solvent polarity.
• Vapochromism - color change caused by vapour of an organic compound due to chemical polarity/polarisation.
• Ionochromism - color change caused by ions.
• Halochromism - color change caused by a change in pH.
• Mechanochromism - color change caused by mechanical actions.
• Tribochromism - color change caused by mechanical friction.
• Piezochromism - color change caused by mechanical pressure.
• Cathodochromism - color change caused by electron beam irradiation.
• Radiochromism - color change caused by ionising radiation.
• Magnetochromism - color change caused by magnetic field.
• Biochromism - color change caused by interfacing with biological entity.
• Chronochromism - color change indirectly as a result of the passage of time.
• Aggregachromism - color change on dimerisation/aggregation of chromophores.
• Crystallochromism - color change due to changes in crystal structure of a chromophore.

Commercial applications of color change materials are very common and include photochromics 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 solvatochromics in biological probes.

Dyes and pigments

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 exact term used is based on the energy source responsible for the luminescence as in color-change phenomena.

• Electrical - electroluminescence Galvanoluminescence Sonoluminescence.
• Photons (light) - Photoluminescence Fluorescence Phosphorescence Biofluorescence.
• Chemical - Chemiluminescence Bioluminescence Electrochemiluminescence.
• Thermal - Thermoluminescence Pyroluminescence Candololuminescence.
• Electron Beam - Cathodoluminescence Anodoluminescence Radioluminescence.
• Mechanical - Triboluminescence Fractoluminescence Mechanoluminescence Crystalloluminescence Lyoluminescence Elasticoluminescence.

Many of these phenomena 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 and diagnostics, chemoluminescence and bioluminescence in analysis, diagnostics and sensors, and electroluminescence in the burgeoning areas of light-emitting diodes (LEDs/OLEDs), displays and panel lighting. Important new developments are taking place in the areas of quantum dots and metallic nanoparticles.

Light and energy 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, photodiagnosis and photoinsecticides). The absorption of natural sunlight solar energy by chromophores is exploited in solar cells for the production of electrical energy by inorganic photovoltaics and dye sensitized solar cell (DSSC) and also in the production of useful chemicals via artificial photosynthesis.

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 lustre pigments and holography, and by modifying the movement of light through materials by electrical means as in organic lasers, or in conjunction with light, opto-electronics, or by purely optical means photonics for instance by using photonic crystals made by colloidal synthesis and other methods.

References

1. Bamfield Peter and Hutchings Michael G, Chromic Phenomena; technological applications of colour chemistry, Royal Society of Chemistry, Cambridge, 2010. ISBN 978-1-84755-868-8.