Glow Stick - Chemistry

Chemistry

The glow stick contains two chemicals and a suitable fluorescent dye (sensitizer, or fluorophor). The chemicals inside the plastic tube are a mixture of the dye and diphenyl oxalate. The chemical in the glass vial is hydrogen peroxide. By mixing the peroxide with the phenyl oxalate ester, a chemical reaction takes place, yielding two molecules of phenol and one molecule of peroxyacid ester (1,2-dioxetanedione). The peroxyacid decomposes spontaneously to carbon dioxide, releasing energy that excites the dye, which then relaxes by releasing a photon. The wavelength of the photon—the color of the emitted light—depends on the structure of the dye. The reason the reaction is so slow and releases only light, not heat, is that the reverse 2+2 photocycloaddition of 1,2-dioxetanedione is a forbidden transition (it violates Woodward–Hoffmann rules) and cannot proceed through a regular thermal mechanism.

By adjusting the concentrations of the two chemicals, manufacturers can produce glow sticks that either glow brightly for a short amount of time or more dimly for an extended length of time. This also allows design of glow sticks that perform satisfactorily in hot or cold climates, by compensating for the temperature dependence of reaction. At maximum concentration (typically only found in laboratory settings), mixing the chemicals results in a furious reaction, producing large amounts of light for only a few seconds. Heating a glow stick also causes the reaction to proceed faster and the glow stick to glow more brightly for a brief period. Cooling a glow stick slows the reaction a small amount and causes it to last longer, but the light is dimmer. This can be demonstrated by refrigerating or freezing an active glow stick; when it warms up again, it will resume glowing. The dyes used in glow sticks usually exhibit fluorescence when exposed to ultraviolet radiation—even a spent glow stick may therefore shine under a black light.

After activation, the glow sticks gradually shift their emission spectral distribution somewhat towards red. The light intensity is high just after activation, then exponentially decays. Leveling of this initial high output is possible by refrigerating the glow stick before activation.

A combination of two fluorophores can be used, with one in the solution and another incorporated to the walls of the container. This is advantageous when the second fluorophore would degrade in solution or be attacked by the chemicals. The emission spectrum of the first fluorophore and the absorption spectrum of the second one have to largely overlap, and the first one has to emit at shorter wavelength than the second one. A downconversion from ultraviolet to visible is possible, as is conversion between visible wavelengths (e.g., green to orange) or visible to near-infrared. The shift can be as much as 200 nm, but usually the range is about 20-100 nm longer than the absorption spectrum. Glow sticks using this approach tend to have colored containers, due to the dye embedded in the plastic. Infrared glow sticks may appear dark-red to black, as the dyes absorb the visible light produced inside the container and reemit near-infrared.