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Tuesday, 12 February 2008 11:46

Everything you need to know about glowsticks-from the history, down to the chemical reaction.
WHAT'S THAT STUFF?

January 18,1999
Volume 77, Number 3
CENEAR 77 3 p.65
ISSN 0009-2347
LIGHT STICKS ~by Elizabeth Wilson

As a child, I loved nothing so much as to see an object glowing in the dark. Back then, before such items were commonplace, I scoured toy stores in search of elusive phosphorescent balls, paints, or rubber bugs. I quickly learned to eschew the common "fluorescent" toys that required black lights--it was long-lasting, unaided glow I was after.

When I was a teenager, another wonderful glowing species began to appear at concerts and outdoor festivals: necklaces and ropes filled with mysterious liquids that shone brilliant greens, yellows, and blues. The glow lasted only a few hours, and unlike phosphorescent toys, once their light faded, they were irreversibly spent. My friends and I put them in the freezer, trying to extend their luminescence just a little longer.

And, of course, there were the light sticks, available at drug stores: short, plastic, tapered rods that contained a bizarre, brilliant green liquid. It was great fun to bend the plastic stick slightly, and hear the crunch of a small, liquid-filled glass capsule breaking within. As the two ingredients blended, an eerie bright green glow spread throughout the stick.

No longer a curiosity, nowadays chemiluminescent products are fabricated in every color and shape imaginable. But despite my fascination and chemiluminescence's ubiquity, I had never learned the chemistry that produced the glow and made the different colors. The shine of the green light stick and blue necklace was still largely a mystery.

Of course, as long as there have been fireflies glowing in the dark, humans have been fascinated by "cold" light. The firefly's glow mechanism, which hinges on the oxidation of firefly luciferin, is incredibly efficient--80 out of 100 reacting molecules go on to produce a photon of light.

In the early 1960s, when scientists took the first steps toward developing their own version of a firefly, they knew what was required: a molecule that radiates light when it's excited and an energy source to excite the molecule. There are numerous possible energy sources, such as light, heat, and electricity. In chemiluminescence, that source is a chemical reaction.

But the reaction has to generate a huge bundle of available energy, and transfer it as a package instantaneously to the fluorescent molecule without radiating any as heat. Known examples of such perfectly orchestrated reactions were few.

Also in the early 1960s, Edwin A. Chandross, a young chemist at Bell Labs in Murray Hill, N.J., was searching for a general way to explain chemiluminescence. Peroxides, with their potential to liberate large amounts of energy during some chemical reactions, seemed to be likely participants.

After a number of experiments, he found to his great excitement that oxalyl chloride mixed with hydrogen peroxide and a fluorescent dye produced chemical light. The efficiency was only about 0.1\, but it was the foundation from which sprang modern chemiluminescence. Chandross, unaware of the powerful potential of his discovery, never patented it.

At about the same time, chemist Michael M. Rauhut was manager of exploratory research at American Cyanamid in Stamford, Conn. He and his colleagues corresponded with Chandross about his oxalyl chloride chemistry, then went to work on the reaction--studying it and looking for avenues that would produce chemical light intense enough to be of practical use.

Rauhut and his colleague Laszlo J. Bollyky developed a series of oxalate esters. Ultimately, Rauhut designed a phenyl oxalate ester that, when mixed with hydrogen peroxide and a dye, gave a quantum yield of 5\--not as efficient as a firefly, but still brilliantly useful. They dubbed it Cyalume, and it became the trademark name for American Cyanamid's chemical light products.

"It was a great project," Rauhut, who is now retired, recalls fondly. "There were a lot of surprises."

The mechanism that he and other researchers have proposed for the process still stands as the best candidate: The oxalate ester and H2O2 react with the help of a salicylate catalyst to form a peroxyacid ester and phenol. The peroxyacid ester decomposes to form more phenol, and most important, a highly energetic intermediate, presumed to be a four-membered ring dimer of CO2. As the cyclic dimer decomposes into two CO2 molecules, it gives up its energy to a waiting dye molecule, which then fluoresces.

The group went searching for fluorescing dyes to make different colors. For example, the common green in most light sticks comes from 9,10-bis(phenylethynyl)anthracene, and 9,10-diphenylanthracene gives blue. "We invented a beautiful yellow," Rauhut remembers.

American Cyanamid eventually sold its chemical light division in 1993 to Springfield, Mass.-based Omniglow, a manufacturer of chemiluminescent products.

E. Earl Cranor, head of Omniglow's R&D, continues to develop new commercial uses for chemical light. His latest project is a light stick that works at below-freezing temperatures. He's also always seeking greater efficiencies and better colors. Reds and blues are typically the most difficult to produce, Cranor says. Purple, made from a combination of three dyes, is the most intractable color of all. "Green and yellow," he notes, "are a piece of cake."

There are those who would claim that all this cold scientific knowledge strips away romance. Now that the secrets of chemiluminescence are revealed in stark detail, has the magic disappeared? Hardly. Standing in the dark, green light stick in hand, for me the thrill of the glow remains undimmed.

A glowstick is a very simple construct, consisting of a glass vial containing chemicals inside a plastic stick containing more chemicals. There's usually a fluorescent dye in the mix for color. The rate of reaction, and therefore the intensity of the light emitted is affected by temperature. A heated glowstick will produce intense light, but will wear out quickly. Put the glowstick in the freezer and it will last longer, but will not glow as brightly.

Simply bending the plastic outer stick breaks the glass vial inside, and the chemicals are allowed to mix. The two chemical components interact, releasing energy, and the action of the energy on the fluorescent dye turns it into light. The chemicals involved are typically hydrogen peroxide, which is also used as a helper in hair coloring, phenyl oxalate ester and a fluorescent dye. The hydrogen peroxide is called the activator, and is kept in the glass vial. The phenyl oxalate ester and fluorescent dye fill most of the plastic outer tube. Here's what happens when they mix:

- Hydrogen peroxide oxidizes the ester, producing phenol and an unstable peroxyacid ester, which decomposes, resulting in more phenol, and a cyclic peroxy compound.

- The cyclic peroxy compound decomposes, and carbon dioxide is formed, releasing energy into the fluorescent dye.

- This excites the electrons in the dye, and they jump up and down like excited children, releasing energy in the form of light.

Chemical reactions that result in the emittance of light is termed "chemiluminescence."

The commercial lightstick contains dilute hydrogen peroxide in a phthalic ester solvent contained in a thin glass ampoule, which is surrounded by a solution containing a phenyl oxalate ester and the fluorescent dye 9,10-bis(phenylethynyl)anthracene. When the ampoule is broken, the hydrogen peroxide and oxalate ester react. During the course of the reaction, an intermediate is produced which transfers energy to the dye molecule. Visible light is emitted when the excited dye molecule returns to the ground state.

What is Chemiluminescence?

Chemiluminescent products evolved from original U.S. Government research that began in 1963. For over two decades, federal agencies including the Office of Naval Research, NASA and the Federal Highway Administration were instrumental in developing practical lighting systems that did not generate heat or require electricity to operate. The result was chemiluminescence - a process that has been captured in SNAPLIGHT from Omniglow - the world's largest manufacturer of safety lightstick technology.

How does it work?

The science is basic. Chemiluminescence begins with two liquid chemicals: oxalate (which includes special fluorescers) and an activator. One liquid is placed in a sealed glass vial then floated in a plastic tube containing the other liquid. The tube is sealed. When the light is needed, the user simply bends the plastic tube, breaking the glass vial within. The two liquids then interact to produce glow.

The duration and intensity of the glow varies inversely: the longer the duration of light, the lower the light intensity; the greater the intensity of light, the shorter its duration. For example, upon activation, a 12-hour, 6" green lightstick produces approximately 300 LUX (average luminance) while a 5-minute, 6" Ultra Orange lightstick emits 2000 LUX.

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