University of Delaware
Office of Public Relations
UpDate - Vol. 16, No. 32, May 22

            Structural anomalies cause materials to 'flash'
     
     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 UD and
Towson State University (TSU) report in this month's
Chemistry of Materials.
     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 Arnold L. Rheingold and his
students in the Department of Chemistry and Biochemistry,
who co-authored the journal article with Linda M. Sweeting
of TSU.
     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," Rheingold
explained. 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," said Sweeting. "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 said. While sugar emits light
mainly in the ultraviolet range, she added, 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 said. "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 said. "Just prior to fracture, a plastic
deformation changes the material from centrosymmetric to
noncentrosymmetric," he added. It's also likely, Sweeting
said, 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.
     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
triboluminescent 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 TSU 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 explained. 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.)
                                          -Ginger Pinholster