A guest-responsive flexible material with molecular-scale pores

In a recent paper published in the journal Nature Chemistry the Brammer group, in collaboration with colleagues at Strathclyde University and Diamond Light Source, have reported novel flexible behaviour in a type of porous material known as a metal-organic framework (or MOF).

Research on MOFs is a very active area of materials chemistry internationally. These materials are assembled from metal ions linked by organic ligands into 2D or 3D frameworks with pores that have dimensions (typically 0.3-3 nm) suitable to contain molecular guests. MOFs are being intensely studied for applications in gas separation, sensing, catalysis, drug delivery and a variety of other applications that take advantage of their ability to reversibly trap molecules. Their modular design means that pore size, shape and chemical composition can be varied widely to enable tuning of behaviour for specific applications. It is estimated that some 20,000 MOFs have been reported in the past 20 years, but perhaps only 100 of these are flexible.

The MOF reported by the Brammer group, now known as SHF-61 (Sheffield Framework 61), is formed from indium metal ions linked by organic dicarboxylate ligands into a 3D network and has diamond-shaped pores of approximately 1 nm in diameter. The pores can open and close due to flexible metal-ligand bonding and this is found to occur in response to the intermolecular interactions between the molecular guests and the framework. Strongly-interacting guests cause the pores to close upon guest removal, whereas weakly interacting guests leave the pores upon removal. This allows SHF-61 to adopt both an open and a closed empty form, which is highly unusual. Each form has different adsorption capacities for CO2 and CH4 gas and different separation selectivities between the two gases, showing that the adsorption characteristics of the material can be tuned.

Equally remarkable is that the opening and closing of the pores occurs in a continuous rather than abrupt manner, which is atypical of the almost all of the flexible MOFs previously reported. Thus, any state in between the most open and closed forms can be accessed by partial removal of strongly interacting guests. These partially closed states exhibit pressure-gated adsorption of CO2 gas, which is not observed in the most open and closed forms.

Changes of the Sheffield MOF when the solvent system is changed.

The MOF is prepared as small crystals allowing its molecular structure to be studied by X-ray diffraction. Among the many experimental methods used to study the behaviour SHF-61, in situ X-ray diffraction carried out in the Chemistry Department X-ray laboratory in Sheffield and at the synchrotron X-ray facilities at Diamond Light Source, near Oxford has enabled accurate structural details to be obtained during the removal of guest molecules or adsorption of gases. The changes in the framework structure upon pore opening and closing is also large enough to be observed in macroscopic changes in crystal shape and dimensions.

The research on the MOF has been mostly carried out by Dr. Elliot Carrington, who was first a PhD student in the Brammer group associated with the E-Futures Doctoral Training Centre, and subsequently held an EPSRC Doctoral Prize Fellowship also in the Brammer group.