Chemiluminescence, is the emission of electromagnetic radiation, particularly in the visible and near infrared, that can accompany a chemical reaction. Particular type of luminescence that accompanies some chemical reactions, such as the slow oxidation of phosphorus and many organic substances in decomposition. When the phenomenon occurs in living organisms, it is more properly called bioluminescence.

The light emission from a molecular chemical species, produced by a reaction in an excited state, loses in the form of radiation the excess energy decaying in the fundamental energy state (lower energy). Often the emission falls in the visible part of the spectrum, but examples of chemiluminescence in the infrared and ultraviolet are also known. A particular type of chemiluminescence is bioluminescence, in which the emission is caused by an enzymatic reaction that occurs in a living organism (typical examples are those of fireflies and some jellyfish).

Chemiluminescence is used in analytical chemistry for the qualitative and quantitative determination of various substances. Application examples are the measurement of nitrogen monoxide (NO) concentration in the atmosphere and the luminol assay, used among other things for the detection of blood traces. In the first case, the reaction between NO and ozone (O3) is used, which produces NO2 in an excited state. In the luminol assay we use the fact that this compound (5-amino-2,3-dihydro-1,4-phthalazindione) is oxidized with hydrogen peroxide in alkaline solution producing the 3-aminophthalate ion (chemiluminescent with bluish light) only in the presence of certain substances, including iron, whose presence can be traced back to blood containing iron bound to hemoglobin.

Chemiluminescence in the infrared is exploited in research of chemical dynamics, because it allows to observe the distribution of vibrational energies in the products of a reaction and to deduce from this some aspects of the microscopic mechanism with which the reaction occurs. Chemiluminescent reactions can also occur under the presence of electrical stimuli (e.g., at the interface between an electrode and a solution), in which case we speak of electrochemiluminescence or electrogenerated (or electroinduced) chemiluminescence.

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