By designing a novel molecule called a pyrazinacene derivative, the team developed a crystal that shifts its color from greenish-blue to red-violet when exposed to naphthalene.
Imagine a crystal that changes color just by coming into contact with a pollutant, almost instantly revealing its presence. Scientists at Japan’s Shibaura Institute of Technology have developed a crystal that can achieve this.
By designing a novel molecule called a pyrazinacene derivative, the team developed a crystal that shifts its color from greenish-blue to red-violet when exposed to naphthalene, a well-known air and water pollutant. What makes this even more remarkable is that the change is reversible, meaning the crystal can return to its original color and be reused. This research could lead to the development of next-generation chemical sensors that help protect human health and the environment.“This crystal is capable of reacting with naphthalene to produce a reversible color shift. Such color-changing crystals can have various potential applications in materials science,” the SIT team notes.A crystal with dual charge transfer capabilityTo understand what makes this pyrazinacene crystal special, it is important to know a bit about the charge transfer process that actually makes the crystal color change.This can happen within a single molecule or between two different molecules . While ICT is often used in dyes and OLEDs, CT plays a major role in solar cells and semiconductors. However, combining both types of charge transfer in one material has been challenging. This is “because it requires precise control over the molecular design and intermolecular interactions. Additionally, the hybrid system must be composed of material that remains stable under such rapid transfer conditions,” The SIT team explained.To overcome this problem, the study authors designed a new molecule called compound one, a type of pyrazinacene with electron-donating groups and electron-accepting groups , all connected through an electron-deficient ring-shaped core. This structure allowed electrons to move within the molecule , giving it a greenish-blue color. When this compound was mixed with naphthalene, the two molecules co-crystallized in a perfect 1:1 ratio. This close pairing changed how the electrons moved. Instead of staying within compound one, some electrons moved from naphthalene to the new crystal. This shift in electron flow caused the crystal to change color dramatically, from greenish-blue to red-violet, achieving intermolecular charge transfer . “Our molecule’s design achieves competition between intramolecular and intermolecular charge transfer. This allows our molecule to act as a sensor that can, through a simple color change, identify even trace amounts of naphthalene—an environmentally regulated substance—in freshwater and seawater,” Kazushi Nakada, lead researcher and a graduate student at SIT, saidA selective and reusable pollutant detectorWhen the study authors tested the crystal to detect compounds like octafluoronaphthalene, which is very similar to naphthalene, surprisingly, the co-crystal didn’t form, and no color change occurred.This indicates that the crystal structure is highly selective for naphthalene. Further lab analysis and X-ray diffraction confirmed this selective co-crystallization. However, heating the crystal to 180°C drove the naphthalene out, restoring the crystal to its original greenish-blue color, proving the process is reversible and reusable. The study authors suggest that their crystal can be used to detect naphthalene in rivers, oceans, or the air, without using expensive electronic sensors. It could even be useful in lab tests, industrial settings, or environmental monitoring tools—offering a simple and reusable way to spot this harmful pollutant. Hopefully, further research will lead to the creation of more such innovative crystals capable of detecting various other pollutants.The study is published in Chemistry: A European Journal.
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