A Superelastochromic Crystal

Elastic fluorescent crystals reversibly change color when bent or deformed. This material may lead to the development of durable mechanical sensors that require little or no external power. (Source: Inst. Industrial Science, U. Tokyo)

Researchers at the Institute of Industrial Science, part of The University of Tokyo, and Yokohama City University have introduced novel color-changing organic crystals that spon­taneously return to their original shape and hue after being stressed, a property they call super­elasto­chromism. These materials can be used to make sensors for shear forces to monitor locations susceptible to damage.

The ability to visua­lize forces can be very useful in many industries, parti­cularly heavy manufacturing and shipping. For example, a color-changing material that shows where beams are being stressed would be great for con­struction companies. However, such devices often work once and have to be replaced after being stretched. Materials that bounce back after being stretched or squeezed, like a rubber ball, are elastic. But even these objects can suffer a permanent change of shape when stressed too much, in a plastic deformation. Now, a team has intro­duced a new organic material that changes the color of its emitted fluores­cence from green to red under mechanical stress, and bounces right back to its original configuration when this stress is removed.

“We called this property super­elasto­chromism because the color changes are due to completely reversible changes to the arrangements of molecules in the material,” says Toshiki Mutai. Based on 7-chloro-2-(2?-hydroxy­phenyl)imidazo[1,2-a]­pyridine (7Cl), the crystals consist of molecules that can exist in two distinct confi­gurations. In both states, a hydrogen atom is covalently bonded to an oxygen atom, and only weakly attracted to a nearby nitrogen atom.

When the material is squeezed or bent, the molecular arrange­ment switches to the other confi­guration. This mechanically controlled phase transition alters the wave­lengths of light the molecule will emit as fluores­cence when excited by an external UV light source. The change is clearly apparent to the unaided eye as a shift in color from neon green to reddish orange.

“Chromatic changes in sensors are highly desirable, because they are easily seen and inter­preted by people,” says Satoshi Takamizawa. “If more precise measure­ments are needed, spectro­scopy can be used to quantify the amount of stress.” This work can help led to a wide range of smart material sensors. For example, one could be used to determine the time when mechanical stress is applied or removed. (Source: U. Tokyo)

Reference: T. Mutai et al.: A superelastochromic crystal, Nat. Commun. 11, 1824 (2020); DOI: 10.1038/s41467-020-15663-5

Link: Dept. of Materials and Environmental Science, Institute of Industrial Science, University of Tokyo, Tokyo, Japan

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