Flexibility of crystal surfaces shown in real-time

Real-time imaging has shown the flexibility of crystal surfaces upon the introduction of a guest molecule.

Researchers at Kyoto University’s Institute for Integrated Cell-Material Sciences (iCeMS) observed flexible and dynamic changes that occur on the surfaces of porous coordination polymer (PCP) crystals after the addition of a biphenyl solution.

Nobuhiko Hosono of iCeMS said: “We were surprised to find that the porous coordination polymer surface is exceptionally flexible and constantly fluctuating in solution, on the contrary to the common perception that the crystal is hard and immobile.”

The team at iCeMS used atomic force microscopy to observe the changes that happened to the surface of a single-crystalline porous coordination polymer, made of zinc cluster and two types of ligands. Researchers added a biphenyl solution to the crystal in gradually increasing concentrations and the metal-organic lattices changed from a tetragonal to a rhombic shape.

Decreasing concentrations of the biphenyl solution resulted in a rapid return to the lattices’ tetragonal shapes, while further studies showed that the surface changes barely affected the rest of the crystal’s structure. The team said that the findings have implications for investigations into materials that can be used for storing and sensing molecules.

PCPs can change structure when other molecules are introduced without losing their crystallinity, making them suited for the development of devices that can selectively adsorb gas molecules and filter or store them, such as storing hydrogen for energy.

Researchers are aware some crystalline structures change when exposed to certain molecules, but hadn’t been able to observe these changes in real-time. The University of Illinois previously demonstrated structural transformations of a PCP after the introduction of azobenzene, including drastic changes in gas adsorption properties.

The iCeMS team’s findings have been published in Nature Chemistry.