Chemiluminescence

Chemiluminescence (also chemoluminescence) is the emission of light (

emits light by chemiluminescence.

Physical description

As in many chemical reactions, chemiluminescence starts with the combining of two compounds, say A and B, to give a product C. Unlike most chemical reactions, the product C converts to a further product, which is produced in an electronically excited state often indicated with an asterisk:

A + B → C

C → D*

D* then emits a photon (hν), to give the ground state of D:^[1] I

D* → D + hν

In theory, one

") is often low owing to side reactions.

For example, A could be

3-aminophthalate

.

Chemiluminescence differs from

.

Liquid-phase reactions

Chemiluminescence was first observed with lophine (triphenylimidazole).^[2] When in basic solution, this compound converts to the imidazolate, which reacts with oxygen to eventually give a dioxetane. Fragmentation of the dioxetane gives the excited state of an anionic diamide.^[3]

Chemiluminescence in aqueous system is mainly caused by redox reactions.^[4]

• oxidant,^[6]
  produces chemiluminescence. The luminol reaction is

${\displaystyle {\ce {{\underset {luminol}{C8H7N3O2}}+{\underset {hydrogen\ peroxide}{H2O2}}->3-APA[\lozenge ]->{3-APA}+light}}}$

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 (PO)₂ and HPO.^[7]
• Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air-quality testing. Ozone (O₃) is combined with nitric oxide (NO) to form nitrogen dioxide (NO₂) in an activated state [◊]:

${\displaystyle {\ce {NO{}+O3->NO2[\lozenge ]{}+O2}}}$

The activated NO₂[◊] luminesces broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier and associated electronics counts the photons that are proportional to the amount of NO present. To determine the amount of nitrogen dioxide, NO₂, in a sample (containing no NO) it must first be converted to nitric oxide, NO, by passing the sample through a converter before the above ozone activation reaction is applied. The ozone reaction produces a photon count proportional to NO that is proportional to NO₂ before it was converted to NO. In the case of a mixed sample that contains both NO and NO₂, the above reaction yields the amount of NO and NO₂ combined in the air sample, assuming that the sample is passed through the converter. If the mixed sample is not passed through the converter, the ozone reaction produces activated NO₂[◊] only in proportion to the NO in the sample. The NO₂ in the sample is not activated by the ozone reaction. Though unactivated NO₂ is present with the activated NO₂[◊], photons are emitted only by the activated species that is proportional to original NO. Final step: Subtract NO from (NO + NO₂) to yield NO₂^[8]

Infrared chemiluminescence

In chemical kinetics, infrared chemiluminiscence (IRCL) refers to the emission of infrared photons from vibrationally excited product molecules immediately after their formation. The intensities of infrared emission lines from vibrationally excited molecules are used to measure the populations of vibrational states of product molecules.^[9][10]

The observation of IRCL was developed as a kinetic technique by John Polanyi, who used it to study the attractive or repulsive nature of the potential energy surface for gas-phase reactions. In general the IRCL is much more intense for reactions with an attractive surface, indicating that this type of surface leads to energy deposition in vibrational excitation. In contrast reactions with a repulsive potential energy surface lead to little IRCL, indicating that the energy is primarily deposited as translational energy.^[11]

Enhanced chemiluminescence

Enhanced chemiluminescence (ECL) is a common technique for a variety of detection assays in biology. A

well below the detection limit for most assay systems.

Applications

• Gas analysis: for determining small amounts of impurities or poisons in air. Other compounds can also be determined by this method (ozone, N-oxides, S-compounds). A typical example is NO determination with detection limits down to 1 ppb. Highly specialised chemiluminescence detectors have been used recently to determine concentrations as well as fluxes of NO_x with detection limits as low as 5 ppt.^[14][15][16]
• Analysis of inorganic species in liquid phase
• Analysis of organic species: useful with enzymes, where the substrate is not directly involved in the chemiluminescence reaction, but the product is
• Detection and assay of biomolecules in systems such as ELISA and Western blots
• DNA sequencing using pyrosequencing
• Lighting objects. Chemiluminescence kites,^[17] emergency lighting, glow sticks^[18] (party decorations).
• Combustion analysis: Certain radical species (such as CH* and OH*) give off radiation at specific wavelengths. The heat release rate is calculated by measuring the amount of light radiated from a flame at those wavelengths.^[19]
• Children's toys.
• Glow sticks.

Biological applications

Chemiluminescence has been applied by

to solve crimes. In this case, they use luminol and hydrogen peroxide. The iron from the blood acts as a catalyst and reacts with the luminol and hydrogen peroxide to produce blue light for about 30 seconds. Because only a small amount of iron is required for chemiluminescence, trace amounts of blood are sufficient.

In biomedical research, the protein that gives

imaging allows scientists to test drugs in the pre-clinical stages cheaply. Another protein, aequorin, found in certain jellyfish, produces blue light in the presence of calcium. It can be used in molecular biology to assess calcium levels in cells. What these biological reactions have in common is their use of adenosine triphosphate (ATP) as an energy source. Though the structure of the molecules that produce luminescence is different for each species, they are given the generic name of luciferin. Firefly luciferin can be oxidized to produce an excited complex. Once it falls back down to a ground state a photon is released. It is very similar to the reaction with luminol.

${\displaystyle {\ce {Luciferin{}+O2{}+ATP->[{\text{Luciferase}}]Oxyluciferin{}+CO2{}+AMP{}+PPi{}+light}}}$

Many organisms have evolved to produce light in a range of colors. At the molecular level, the difference in color arises from the degree of conjugation of the molecule, when an electron drops down from the excited state to the ground state. Deep sea organisms have evolved to produce light to lure and catch prey, as camouflage, or to attract others. Some bacteria even use bioluminescence to communicate. The common colors for the light emitted by these animals are blue and green because they have shorter wavelengths than red and can transmit more easily in water.

In April 2020, researchers reported having

Chemiluminescence is different from

bioluminescence resonance energy transfer

(BRET), which increases the quantum yield of light emitted in these systems.

See also

• Eclox
• Electrochemiluminescence
• List of light sources
• Lyoluminescence
• Will-o'-the-wisp

References