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.
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