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Lighting Up The Lab: Structural Anomalies Make Many Materials Flash Under Pressure, New Study Shows

University of Delaware

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* Call for journal article or cartoon on triboluminescence.

Just as wintergreen candy sparkles when crunched in a darkened room, many different crystalline materials can change shape and flash under pressure--if they lack symmetry or contain structural anomalies, researchers from Towson State University and the University of Delaware report in this month's Chemistry of Materials, scheduled for release May 15.

Triboluminescence--the phenomenon that prompts certain materials to emit light when fractured or deformed--has traditionally been associated with structures that lack a center of symmetry, according to Linda M. Sweeting of Towson State, Arnold L. Rheingold of UD and their students.

Sugar and other "noncentrosymmetric" materials may be more likely to give off light because "breaking the crystal along a plane tends to leave one surface with a positive charge, and one surface with a negative charge," explains UD's Rheingold, a professor of chemistry and biochemistry. By contrast, centrosymmetric materials are built like the letter X, so that charged fragments are symmetrically arranged around the center.

"Triboluminescent materials are usually noncentrosymmetric, meaning that their structure looks more like an arrow than an X," says Sweeting, a professor of chemistry at Towson State. "But, it seems that even X-structured or centrosymmetric substances can give off light if they contain an impurity."

This may explain why wintergreen candy containing various ingredients is more intensely triboluminescent than cane sugar (sucrose), Sweeting notes. While sugar emits light mainly in the ultra-violet range, she says, the presence of methyl salicylate in wintergreen candy shifts its emissions into the visible range.

"There is simply no way to talk about this work without resorting to puns," Rheingold says. "The study really did shed light on the nature of solid-state materials. It told us a great deal about what happens when materials are mechanically deformed."

How could impurities prompt materials to sparkle? They seem to make centrosymmetric substances change shape; that is, some X-shaped structures can be transformed into arrows, Rheingold says. "Just prior to fracture, a plastic deformation changes the material from centrosymmetric to noncentrosymmetric," he adds. It's also likely, Sweeting says, that "impurities are arranged in the crystal so as to make the X-shape into something less symmetric, by putting a cap on the X, for example."

English philosopher Francis Bacon (1561-1626) was among the first to describe triboluminescence in 1605, when he reported that chopping large blocks of cane sugar at night created "a very vivid but exceeding short-lived splendour." In 1922, researcher H. Longchambon published the first triboluminescence spectrum, demonstrating that sucrose emits a light pattern identical to excited dinitrogen, which forms lightning. Longchambon noted that noncentrosymmetric crystals are more likely to give off light, but he couldn't explain why. Since then, other researchers have identified puzzling exceptions: centrosymmetric materials capable of triboluminescence.

So, Sweeting and Rheingold set out to learn exactly what makes materials emit light when smashed. First, Sweeting and her undergraduate students synthesized 12 esters--organic compounds resulting from a reaction of alcohol with 9-anthracencecarboxylic acid. Then, they compared the triboluminscent activity of the acid and its esters with each material's crystal structure and purity, determined by Rheingold and his students. Triboluminescence was assessed objectively, by measuring the wavelength of light. It also was judged subjectively--by crushing materials in a darkened laboratory where students graded the resulting light shows.

Materials synthesized at Towson State were characterized in UD's state-of-the-art X-Ray Crystallography Laboratory. Bombarded by a powerful laser beam, different crystalline materials emit a characteristic pattern of X-ray diffraction, Rheingold explains. A computer then detects the scattered X-rays, generating a color-coded 'map' of the molecular architecture of each sample. (To understand the process, Rheingold suggests thinking of the mirrored ball hanging above a dance floor, which reflects spots of light onto surrounding walls. The spots may reveal the exact structure of the mirrored ball, just as diffracted X-rays can be analyzed to determine the structure of a material.)


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