New chemical design makes hard crystals stretchable

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Researchers have discovered a new way to make crystals stretchable, a modification that could allow them to act as very efficient nanofilters.

“Imagine a diamond behaving like a rubber band,” explains assistant chemistry professor Chenfeng Ke. His research team has designed a new type of porous carbon-based crystals that can stretch more than twice their length.

Known to chemists as porous organic frameworks, these materials are generally hard. They are built from a scaffolding of lightweight organic molecules like carbon, oxygen, and nitrogen. Additional molecular crosslinks are chemically stitched together to strengthen the structure. Their structures resemble open nets full of voids, or pores, which can house a variety of molecules as guests. This allows them to act as filters capable of removing certain pollutants from air and water, or of separating and storing commercially important chemicals. The size of the pores usually determines which molecules can be absorbed and stored.

By fine-tuning the design of molecular building blocks, researchers have now allowed specific chemicals to swell the crystal. It’s as if some molecules have a key that can unlock a lot of additional space that they can now occupy, explains Jayanta Samanta, associate researcher at the Ke Functional Materials Group.

In an article published in Chemistry, the researchers describe how they incorporate this functionality by adding what they call “soft joints” into the crystal scaffolding. Joints are made up of ions that repel each other but are put in place by interactions with other molecules in the scaffold. However, when they encounter the right chemical, they are easily confused and move away from each other. This causes the crystal to expand, but only to the extent that its crosslinking agents allow.

Samanta, the lead author of the article, describes the crystals as tiny, hard needles, about half a millimeter in length. He remembers the watershed moment when he first put one in a solution of phenol, an organic compound widely used in household cleaners. It stretched to twice its length in under 20 minutes, he says. When the phenol was washed away, it returned to its original form in half that time.

“To see the crystal expand and contract at this point is remarkable,” says Ke, who is particularly struck by the speed of the expansion. This physical response to specific chemicals in their environment can be used for interesting applications, he says. Ke is eager to implement the new design by creating similar crystals capable of absorbing impurities from water.

– This press release was originally published on the Dartmouth College website


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