Tunable Dyes for Smart Devices

The building blocks of rationally designed chemicals are simple elements: carbon, hydrogen, oxygen and so on. These elements can be combined in myriad ways to accom­plish a variety of chemicals with different charac­teristics. Even the same chemical can be treated differently with pressure or heat to show drastically different properties. A simpler version is to think of how water can be boiled to cook pasta or frozen to become ice. Now, researchers are working to better control how the chemicals respond to treatment, as well as how to reverse the chemicals back to their original state with little to no interference. Such control would allow scientists to prepare the sensing systems of environ­mental stimuli, as well as conti­nuously repeat the sensing.

Illustration: A new two-component dye shows self-recovering mechanochromic luminescence that exhibits a high-contrast emission color change between violet and orange. (Source: Yokohama Nat. U.)

A team of researchers at Yokohama National Univer­sity has achieved such results with a specific compound that can emit light and has potential appli­cations in the next generation of smart devices such as wearable devices and anti-counter­feiting paintings. The compound is a derivative of thiophene, which is a dye with mechano­chromic lumines­cence properties – it changes color under physical change. It starts emitting a violet glow under the irra­diation of UV light, but as it is exposed to mechanical stimuli, such as grinding, the violet glow shifts slightly to blue. Another external inter­vention can make the compound heal and become violet again.

“Mechano­chromically lumines­cent (MCL) dyes have recently attracted considerable interest on account of their potential applications,” said Suguru Ito, associate professor in the Depart­ment of Chemistry and Life Science in the Graduate School of Engineering Science at Yokohama National Univ­ersity. “However, it is still very difficult to rationally design MCL dyes with desired charac­teristics.” Now, the researchers disco­vered that by adding another chemical (DMQA), the dye changed to orange under mechanical stimuli. The dye did not need more external stimuli to revert back to violet either.

“We combined two kinds of rational design guidelines for tuning the lumines­cent properties, resulting in the desired and unprece­dented charac­teristics of high-contrast, self-recovering dyes,” Ito said. The first rational design guideline is that the recovery behavior of the dye can be attri­buted to the length of the alkyl group in the compound – a longer chain of carbon atoms with hydrogens in the dye allows the dye to recrys­tallize and heal in time. The second is that by mixing with DMQA, the color range between the original state and ground state differ greatly.

“The next step is to establish a rational design guideline to control the dye’s respon­siveness to mechanical stimuli,” Ito said. “My ultimate goal is to develop an inno­vative pressure-sensing system by rationally creating a material that can change its emission color in stages in response to mechanical stimuli of different inten­sity.” With such control, Ito could use mechanical stimuli to precisely induce a specific and intended response. A little pressure could shift the violet glow to blue, a little more pressure pushes the glow closer to red. A system with such ability would allow for stepwise changes and reco­veries by the stimulus, which could be highly bene­ficial in the next generation of smart materials, according to Ito. (Source: Yokohama Nat. U.)

Reference: M. Ikeya et al.: Tunable mechanochromic luminescence of 2-alkyl-4-(pyren-1-yl)thiophenes: controlling the self-recovering properties and the range of chromism, Chem. Commun. 55, 12296 (2019); DOI: 10.1039/C9CC06406K

Link: Dept. of Chemistry and Life Science, Graduate School of Engineering Science, Yokohama National University, Yokohama, Japan

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