The Chemiluminescent Tutorial
The History of Chemiluminescent
Chemiluminescent (Chemical light) has been known to mankind ever since man first realized that the cold light of the firefly was different from the hot light of the campfire. While hot light, or incandescence, has been understood for many years, we have begun to understand chemical light only recently.
Chemical light, or chemiluminescence, converts the energy released in a chemical reaction directly to light with out the involvement of heat or flame. The biochemical reaction of the firefly is the most efficient example we know. In principle, it is possible for each molecule of a chemiluminescent reactant to produce one photon of light. The firefly approaches this theoretical limit by producing 88 photons for each 100 molecules for a quantum yield of 88%.
Until recently, chemists have been much less efficient. More than 50 chemiluminescent reactions have been discovered, but quantum yields are no better than 0.1%. While such reactions make interesting demonstrations, they produce far to little light for practical use. Modern chemical research, however, has discovered a great deal about the chemiluminescent process, and it has become possible in recent years to invent new reactions having efficiencies as high as 23%. It is reasonable to expect that further research will produce reactions rivaling the firefly.
Incandescence, as in a candle, involves the conversion of chemical energy to heat, followed by conversion of some of the heat energy to light. Chemical light or chemiluminescence differs in that chemical energy is converted directly to light without the involvement of heat as an intermediate energy form. Conversion of chemical energy to heat in chemical reactions is commonly observed and well understood. Light is just as legitimate a form of energy as heat, but conversion of chemical enregy to light is a rare phenomenon.
THE TECHNICAL STUFF
We now recognize that chemiluminescence requires a combination of two special kinds of chemistry. The first is called fluorescence. In ordinary fluorescence, a molecule absorbs light to become an electronic excited state. After a lifetime - as short as 1 billionth of a second - the energetic exited state releases its energy as light.
Chemiluminescence includes this fluorescent process, except that the necessary excited state is produced as a product of chemical reaction rather than by light absorption. The second and more unusual kind of chemistry required is, of course, the chemical reaction that produces the excited state. This is called the excitation process and is the real key to chemiluminescence. We now know that certain decomposition reaction of organic peroxides can produce excited products efficiently, and we believe we understand why.
An excitation reaction must be capable of generating at least 40 to 70 kilocalories/mole of energy, the energy range of visible light. This is a substantial amount of energy in chemical terms, and only highly energetic molecules are capable of meeting the requirement. Not only must the energy be available, but it must be provided essentially instantaneously in a single chemical step.
In additioin to substantial instantaneous energy release and the formation of a fluorescent product, other more subtle requirements must be met which involve the distribution of energy released from a reaction between light emitting (or electronic) excited states and heat emitting (or vibrational) excited states.
Since all of these requirements must be met together in an efficient chemiluminescent reaction, and since none of the requirements are commonly met even individually, it is understandable that efficient chemiluminescence is rare.
The first step is essentially conventional chemistry producing the key intermediate (K1). The second step is the critical excitation process where the chemical energy of K1 is converted and transfered to electronic excitation energy in a separate fluorescent chemical molecule (fluorescer). The third step is conventional fluorescent emission.
The critical feature in the process of course, is the structure of the key intermediate. Its efficiency is believed to result in part from its high energy content, its ability to release its energy instantaneously through a concerted peroxide decomposition reaction, the quantum mechanical reluctance of a small molecule like carbon dioxide to accept a large amount of chemical energy as heat, and the inability of carbon dioxide itself to become electronically excited by the available energy.
Since the key intermediate does not have a favorable pathway by which it an get rid of its unwanted energy, it has an appreciable lifetime. On the other hand, because of its energy content, it is glad to have an opportunity to decompose when it encounters a fluorescer with the ability to accept its energy. The fluorescer thus acts as a catalyst for the decomposition of the key intermediate, and this catalyst is an important factor in the efficiency of this chemical reaction.
Because the fluorescer is separate from the energy producing components of the reaction, it can be varied without changing the basic chemistry. Since the color of the light depends on the fluorescer selected, peroxyoxalate chemiluminescence can be formulated in any color desired.
The lightstick is a tube containing the chemical reactants separated by a capsule. Activation is accomplished by bending the lighstick to break the capsule. The result is a bright, long lasting light.
Chemical light is finding many important uses. It is especially useful because it is cold and cannot cause fire or explosion. Chemical light is an example of the value of chemical research in providing novel and useful products to make life safer and more fun!*The information and statements herein are believed to be reliable but assumes no legal responsibility.