Crystalline porous organic frameworks are typically hard and inflexible, with a certain degree of rigidity being necessary to retain the framework structure. However, a rigid crystal structure means guest molecule uptake will remain limited by the size of the voids within the framework. A new, novel chemical design developed by researchers at Dartmouth College improves the balance between strength and flexibility in porous organic frameworks by allowing carbon-based crystals to expand or contract based on interactions with a specific guest molecule.
The new “stretchy” crystal design relies on the incorporation of “soft joints” made from bisulfate anion clusters. These joints are kept in place through interactions with other molecules within the framework, but upon exposure to a certain chemical, they are disrupted and repel each other, causing the framework to expand. The molecular crosslinks within the framework prevent the crystal from expanding too far, making the reaction reversible when the disrupting chemical is removed.
The researchers designed their flexible crystal to expand when exposed to phenol, and found that the crystal stretched to more than twice its original length in under 20 minutes when placed in a phenol solution. This transformation allowed the phenol guest molecule to occupy the additional space, and when the phenol was washed out, the crystal contracted and regained its original shape in less than 10 minutes. This research was published in the journal Chem.
“PIcture a diamond that behaves like a rubber band,” said Chenfeng Ke, who led the research. “... Seeing the crystal expand and contract to this extent is remarkable.”
This new porous organic framework design could allow for the creation of new nanofilters that can selectively trap certain chemicals, such as impurities in water. The research team is looking forward to creating similar crystals using their novel design to target different chemicals.