The scientists based the material on copper atoms and the organic free radical polychlorinated triphenylmethyl (PTM) functionalized with three carboxylic groups (TC). They dubbed the resulting structure - Cu3(PTMTC)2(py)6(CH3CH2OH)2(H2O) - MOROF-1: MOROF stands for metal-organic radical open-framework.
“The MOROF-1 material showed hexagonal pores with a nanometric size (2.8-3.1 nm) formed by the piling up of honeycomb layers,” Jaume Veciana told nanotechweb.org. “In addition, we found that the material, when filled with solvent, was crystalline and ordered magnetically below 4 K.”
Whilst making MOROF-1, the scientists used solutions containing ethanol and water. The resulting crystals contained guest molecules of ethanol and water, which were hydrogen-bonded to the inner walls of the pores and inside other narrow pore channels. When the MOROF-1 crystals were removed from the solution, the ethanol and water molecules left the structure and it became amorphous, reducing the crystal volume by 25-35%. Re-immersing the crystals in ethanol reversed the process.
“The most exciting result was the solvent-induced ‘shrinking-breathing’ process that involved large volume changes and strongly influenced the magnetic properties,” said Veciana. “This ‘magnetic sponge-like’ behaviour was completely unexpected.”
At low temperatures, filled MOROF-1 samples showed a much higher magnetic response than samples that did not contain ethanol and water molecules. MOROF-1 also proved selective - it only regenerated its framework and crystallinity after exposure to ethanol and methanol.
“We are currently studying new materials similar to MOROF-1 but that have had their nanopore inner walls modified with the purpose of changing their selectivity towards the inclusion of different kinds of molecules,” added Veciana. “We are also trying to include different functional molecules in order to achieve multifunctional molecular magnetic materials.”
The researchers reported their work in Nature Materials.