A flash in the crystalline pan.

A flash in the crystalline pan

Sometimes it pays to work in the dark. About 17 years ago,chemist Linda M. Sweeting, then a graduate student, was preparing a chemical compound that she had been told was light-sensitive. Working alone and at night, she was astonished to see blue flashes of light as she scraped the freshly crystalized compound off a piece of filter paper into a bottle. Intrigued by her observation, Sweeting explored the subject further and discovered that the phenomenon, known as triboluminescence, was not well understood although it had been studied for centuries. Early reports describe the sparkling light given off when a chunk of crystalline sugar or rock salt is scraped with a knife or crushed in a mortar. Many children have seen similar flashes when rubbing together pieces of hard candy or while watching an open-mouthed companion crunching a wintergreen Life Saver in the dark.

Sweeting, now at Towson State University in Baltimore, hasrecently come up with an explanation for why one particular compound is triboluminescent. This result suggests a general theory of triboluminescence based on the way electrical charge is separated when crystalline compounds are fractured. Sweeting and Arnold L. Rheingold of the University of Delaware in Newark report their findings in the April 29 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

Sweeting and Rheingold studied the compound triethylammoniumtetrakis (benzoylmethanato) europate. This particular crystalline substance is known to produce a brilliant orange glow when it is crushed--bright enough to be seen in daylight. By accident, Sweeting discovered that this compound also crystalizes into a second form that is not triboluminescent. By studying the differences between the two crystal structures, Sweeting was able to deduce why one form gives off light and the other does not.

The triboluminescent form, when recrystalized from methanol,appears as irregular, yellow flakes, while the nontriboluminescent form, obtained by recrystalization from the solvent dichloromethane, shows up as yellow tablets. The latter crystals turn out to contain solvent molecules that are trapped within the structure during crystalization. The triboluminescent crystals contain no solvent molecules but show a degree of internal disorder.

Earlier research had indicated that electrical effects play animportant role in triboluminescence. As crystals are broken apart, positive and negative charges are separated so that pockets of like charge build up. Then, like miniature lightning bolts, the charges recombine, exciting nitrogen molecules in the air to produce a blue-green glow or causing the chemical compound itself to fluoresce. The problem is determining why the charges are separated in the first place, especially in substances in which molecules are symmetrically arranged.

Sweeting suggests that the disorder found in the fluorescenteuropate provides the local dissymmetry necessary for charge separation to occur when cracks appear. These crystal imperfections allow small pockets of like charge to accumulate. In nontriboluminescent crystals, the presence of dichloromethane changes the crystal structure just enough to reduce the amount of molecular disorder.

"We believe this discovery of the importance of disorderprovides a sufficient condition for the triboluminescence of other centrosymmetric crystals,' the researchers conclude, "and that the triboluminescence of other fluorescent materials is excited by light generated by a gas discharge.' Sweeting is now studying several other materials to check the theory and plans to investigate the role of impurities in producing triboluminescence. The theory may also apply to the generation of light when rock is fractured (SN: 6/14/86, p.373) and when wet clay dries (SN: 9/12/81, p.166).