The Science of Glow Sticks
Light All of the forms of light work the same way: outside energy excites atoms which then release particles of light called photons. When an atom is excited the electrons go up in energy and energy levels. When they fall back down to their normal spot, they release energy in the form of light photons. A glow stick uses the same principles, but creates light through a chemical reaction called chemiluminescence.
Chemiluminescence is the emission of light through a chemical reaction. A significant property of this form of emission is that it produces no heat. Unlike light bulbs, in which a strip of filament is heated by electrical resistance to a flowing current until it glows, chemiluminescent light does not require electricity. In chemiluminescent reactions, one or more of the products of the reaction is produced at a high energy state, or more precisely, when the molecule is produced, one of its electrons is in a higher energy orbital than it would be in the ground state. The electron then falls to its ground state orbital, in the process releasing energy as electromagnetic radiation of a specific wavelength. If the wavelength is in the visible light spectrum, then the reaction produces light. This type of reaction is typified by the reaction that makes glow sticks glow.
The Chemistry of Glow Sticks
The glow stick contains two chemicals and a suitable fluorescent dye (sensitizer, or fluorophor). The chemicals in the plastic tube are a mixture of the dye and diphenyl oxalate. The chemical inside the glass vial is hydrogen peroxide. By mixing the peroxide with the phenyl oxalate ester, a chemical reaction takes place; the ester is oxidized, yielding two molecules of phenol and one molecule of peroxyacid ester. The peroxyacid decomposes spontaneously to carbon dioxide, releasing energy that excites the dye, which then deexcites by releasing a photon. The wavelength of the photon—the color of the emitted light—depends on the structure of the dye. There are three compounds that are necessary for glow sticks to produce light. The first two, bis(2,4,6-trichlorophenyl)oxalate (TCPO) and hydrogen peroxide, combine to produce a complex which reacts with the third chemical, a dye (generally an anthracene derivative) and the dye emits the light.
The hydrogen peroxide oxidizes the TCPO, to form trichlorophenol and an unstable peroxyacid ester. The unstable peroxyacid ester decomposes, resulting in phenol and 1,2-dioxetanedione. The dioxetanedione complexes with the dye molecule and releases light. In Mechanism (1) the dye does not decompose or become inactive after emmitting light while in Mechanism (2) it is inactive after emitting light.
The dye releases light of a color dictated by its molecular structure. A list of common dyes and their colors follows:
Blue - 9,10-diphenylanthracene Green - 9,10-bis(phenylethynyl)anthracene Yellow - 1-chloro-9,10-bis(phenylethynyl)anthracene Yellow - Rubrene Orange - 5,12-bis(phenylethynyl)-naphthacene Orange - Rhodamine 6G Red - Rhodamine B
Although red fluorophors such as Rhodamine B are available, red-emitting light sticks tend not to use them in the oxalate reaction. The red fluorophors are not very stable when stored with the other chemicals in the light sticks. Instead, a fluorescent red pigment is molded into the plastic tube that encases the light stick chemicals. The red-emitting pigment absorbs the light from the high yield (bright) yellow reaction and re-emits it as red. This results in a red light stick that is approximately twice as bright as it would have been had the light stick used the red fluorophor in the solution.
Brightness and duration By adjusting the concentrations of the two chemicals, manufacturers can produce glow sticks that either glow brightly for a short amount of time, or glow more dimly for a much longer amount of time. At maximum concentration (typically found in laboratory settings), mixing the chemicals results in a furious reaction, producing large amounts of light for only a few seconds.
Temperature Dependence As with most chemical reactions, the rate of reaction for the production of light from a glow sticks increases with increases in temperature. Analogously, if you place a glow stick in a cold environment then the chemical reaction rate will slow down. For every 10 degree (Fahrenheit) increase in temperature, the rate of reaction for light production doubles.
Ultraviolet Reaction The dyes used in glow sticks usually exhibit fluorescence when exposed to ultraviolet radiation. Therefore even a used glow stick will shine under a black light.