chemiluminescence

chemiluminescence, emission of electromagnetic radiation during the course of chemical reactions. Such radiation, whether ultraviolet, visible, or infrared, is most commonly generated by oxidation. The radiation can usually be ascribed to the transfer of the oxidation energy to an electronically excited unstable intermediate molecule that forms as a result of oxidation. This molecule then loses the excitation energy, which is emitted as light of the proper wavelength.

Types

Depending on which excited-state molecule emits radiation, chemiluminescence can be of direct or indirect type. When the high-energy intermediate produced by the oxidation of a chemiluminescent molecule directly gives off the excitation energy (by emitting a photon) to return to a lower-energy state (ground state), the phenomenon is called direct chemiluminescence. For example, if the oxidation reaction involving the chemiluminescent molecule X and the oxidizing agent Y gives the high-energy intermediate Z* along with other products, Z* returns to its ground state, Z, by emitting the excitation energy as light. $X + Y \to Z* + \mathrm{other} \mathrm{products} \to Z + \mathrm{light}$. The principle of direct chemiluminescence is at work when the forensic chemical luminol emits a blue light upon oxidation by hydrogen peroxide in the presence of the iron in blood, which acts as a catalyst.

On the contrary, the transfer of the excitation energy from the high-energy intermediate (Z*) to a fluorophore (a molecule capable of absorbing and emitting light) and the subsequent emission of radiation from this excited-state fluorophore (fluorophore*) results in indirect or sensitized chemiluminescence. $X + Y \to Z*+ \left[\mathrm{fluorophore}\right] \to Z + \left[fluorophore\right]*\to Z + \left[\mathrm{fluorophore}\right] + \mathrm{light}$. An example of indirect chemiluminescence is the emission of light from a glow stick—breaking of a plastic or glass tube inside the stick releases hydrogen peroxide (oxidizing agent), which reacts with an oxalic acid ester inside the stick to produce a high-energy intermediate that then transfers the excitation energy to a fluorescent dye. Emission of light from this excited dye as it returns to its ground state causes the stick to glow.

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photochemical reaction: Chemiluminescence

Examples and uses

A large number of substances (for example, formaldehyde, paraldehyde, acrolein, lophine, glucose, lecithin, and cholesterol) luminesce if slowly oxidized in alcoholic alkaline solution. Another group of chemiluminescences is connected with the oxidation of sulfur compounds. Chemiluminescence of these and other compounds helps to detect toxins, impurities, and other chemicals of interest in target liquids and gases. It also forms the basis of multiple assays used in biomedicine to detect immune molecules, cellular activities, disease markers, and biomolecules such as adenosine triphosphate (ATP) and calcium. The widespread luminescence of such living organisms as fireflies and bacteria is based on the oxidation of the light-emitting organic compound luciferin in the presence of an enzyme, luciferase, and is a special type of chemiluminescence called bioluminescence. Bioluminescence is seen in a wide range of protists and animals, from bacteria and fungi to insects, marine invertebrates, and fish.