Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate ◊, [A]{ } + [B] -> { }[\lozenge ]{ } -> { }[products]{ } + light
For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:
\underset{ luminol}{ C8H7N3O2} + \underset{ hydrogen\ peroxide}{ H2O2} -> 3-APA[\lozenge] -> { 3-APA} + light
where:
3-APA is 3-aminophthalate
3-APA[◊] is the vibronic excited state fluorescing as it decays to a lower energy level.
General description
The decay of this excited state [◊] to a lower energy level causes light emission. In theory, one photon of light should be given off for each molecule of reactant. This is equivalent to the Avogadro number of photons per mole of reactant. In actual practice, non-enzymatic reactions seldom exceed 1% QC, quantum efficiency.
In a chemical reaction, reactants collide to form a transition state, the enthalpic maximum in a reaction coordinate diagram, which proceeds to the product. Normally, reactants form products of lesser chemical energy. The difference in energy between reactants and products, represented as , is turned into heat, physically realized as excitations in the vibrational state of the normal modes of the product. Since vibrational energy is generally much greater than the thermal agitation, it rapidly disperses in the solvent through molecular rotation. This is how exothermic reactions make their solutions hotter. In a chemiluminescent reaction, the direct product of the reaction is an excited electronic state. This state then decays into an electronic ground state and emits light through either an allowed transition (analogous to fluorescence) or a forbidden transition (analogous to phosphorescence), depending partly on the spin state of the electronic excited state formed.
Chemiluminescence differs from fluorescence or phosphorescence in that the electronic excited state is the product of a chemical reaction rather than of the absorption of a photon. It is the antithesis of a photochemical reaction, in which light is used to drive an endothermic chemical reaction. Here, light is generated from a chemically exothermic reaction. The chemiluminescence might be also induced by an electrochemical stimulus, in this case is called electrochemiluminescence.
The first chemiluminescent compound to be discovered was 2,4,5-triphenylimidazole (lophine), which was reported, in 1877, to emit light when mixed with potassium hydroxide in aqueous ethanol in the presence of air. A standard example of chemiluminescence in the laboratory setting is the luminol test. Here, blood is indicated by luminescence upon contact with iron in hemoglobin. When chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits light by chemiluminescence.
Liquid-phase reactions
Chemiluminescence in aqueous system is mainly caused by redox reactions.
Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper, or an auxiliary oxidant, produces chemiluminescence. The luminol reaction is
\underset{ luminol}{ C8H7N3O2} + \underset{ hydrogen\ peroxide}{ H2O2} -> 3-APA[\lozenge] -> { 3-APA} + light
Gas-phase reactions
One of the oldest known chemiluminescent reactions is that of elemental white phosphorus oxidizing in moist air, producing a green glow. This is a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing excited states of and HPO. Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air-quality testing. Ozone () is combined with nitric oxide (NO) to form nitrogen dioxide () in an activated state [◊]: