Historically favoured for their incredible host-guest properties, metal–organic frameworks (MOFs) are increasingly found to exhibit an range of extremely unusual and useful mechanical properties. Some shrink instead of expand when heated (NTE: negative thermal expansion). Others expand instead of shrink under increasing pressure (NLC: negative linear compresssibility). And yet others are capable of switching between crystalline and amorphous forms under changes in either pressure or temperature. These bizarre properties find potential application in the development of nanoscale artificial muscles and shock-resistent materials, in data storage and in high-precision pressure sensors.
Whereas the rules governing host-guest behaviour in MOFs are reasonably well established — we know how to control framework geometry and influence pore size — there are no ‘recipes’ for controlling mechanical behaviour.
By studying the pressure- and temperature-dependent behaviour of on particular MOF — namely silver(I) 2-methylimidazolate — we have found tantalising evidence that such rules might exist more generally. And we have made a start on working out what the rules might actually be.
Our approach is to think of MOFs in terms of smaller structural motifs each with their own predefined mechanical response. Understanding how these components (which we term mechanical building units, or XBUs) respond to external stimuli such as changes in temperature or pressure, means that we can start to understand and predict the mechanical behaviour of the MOF as a whole. So some XBUs favour NTE; others favour NLC. Some show extreme responses; others are affected less strongly by a given stimulus. It is almost as if we have access to a whole new Lego set: the properties of the materials we make are limited only by the XBUs we choose and the way in which we bring them together..