A breathing MOF with nano-correlations

This work was was result of a collaboration with a number of universities and facilities but particularly the group of Prof. Dirk De Vos at KU Leuven university and with Bart Bueken and Frederik Vermoortele.

One of the remarkable features of MOFs is their flexibility - they can show all kinds of unusual behaviour, from negative linear compressibilities to The most distinctive behaviour is perhaps breathing - guest triggered transitions between very two structures with very different porosities.

In this paper we describe a breathing MOF that reversibly transforms between a crystalline and an amorphous phase on loss and reuptake of solvent molecules. From NMR spectroscopic measurements, we were able to deduce that the organic linker, 1,4-cyclohexanedicarboxylic acid, flips between two different shapes. Using a combination of X-ray pair distribution function data and structural modelling, we were able to show that this transformation happens in a locally correlated manner, leading to a contraction in the unit cell volume, but also destroying the long-range order in the material. The ‘closed’ form of ZrCDC only switches back to the ‘open’ form with specific solvents conditions, suggesting that it might be useful for selective separations.