April 16, 2007

Volume 85, Number 16

p. 12

Materials Engineering

Crystals' Shape Shifts In A Flash

Light induces crystals to bend without breaking

Bethany Halford

Shine a little ultraviolet light on the crystals in Masahiro Irie's lab at Japan's Kyushu University and something unusual happens: The crystals' shape changes—shrinking in one instance, going from a rectangle to a parallelogram in another, and bending but not breaking in a third (Nature 2007, 446, 778).

Shine visible light on the deformed crystals, and they'll shift back to their original shape.

The effects take place in a matter of microseconds—far faster than similar phenomena previously observed in liquid crystals and polymers, Irie says, suggesting that the crystals could be used as components in microscale devices.

According to Irie and coworkers, the shape shifting occurs as the result of an electrocyclic reaction that transforms the open-ring isomers of diarylethene chromophores into their corresponding closed-ring isomers.

Yale University chemistry professor J. Michael McBride calls the development "a breakthrough" in the area of device fabrication. "A long-cherished aim" of solid-state organic chemistry is "to discover new single-crystal to single-crystal transformations, with a smooth transition between the crystal lattices of the starting material and the product," he writes in a commentary that accompanies the paper.

Rod-shaped crystals, in one example, bent and straightened 80 times in response to light before losing the shape-shifting trait. Irie's group showed that one crystalline rod could behave like a light-activated slingshot, launching a gold microparticle that weighed 90 times more than the crystal itself over a distance of 30 µm.

"An improved understanding of this unexpected resilience is required, along with a method to assemble microscale components, if we are ever to make a useful chemicomechanical device from such materials," McBride adds. "But simply demonstrating this behavior suggests a possibility that previously seemed remote."

Chemical & Engineering News

ISSN 0009-2347

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