Reversible switching between positive and negative thermal expansion in a metal–organic framework DUT-49

Abstract

Three-dimensional architectures constructed via coordination of organic ligands to metal ions (broadly termed metal–organic frameworks, MOFs), are highly interesting for many demanding applications such as gas adsorption, molecular separation, heterogeneous catalysis, molecular sensing, etc. Being constructed from heterogeneous components, such framework solids show characteristic features from both the individual components and framework-specific features. One such interesting physicochemical property is thermal expansion, which arises from thermal vibration from the organic linker and metal ions. Herein, we show a very unique example of thermal responsiveness for the DUT-49 framework, a MOF well-known for its distinctive negative gas adsorption (NGA) properties. In the guest-free form, the framework shows another counter-intuitive phenomenon of negative thermal expansion (NTE), i.e. the lattice size increases with decrease of temperature. However, in the solvated state, it shows both NTE and positive thermal expansion (i.e. lattice size decreases with lowering of temperature, PTE) based on a specific temperature range. When the solvent exists in the liquid form inside the MOF pore, it retains the pristine NTE nature of the bare framework. But freezing of the solvent inside the pores induces the strain, which causes a structural transformation through in-plane bending of the linker and this squeezes the framework by ∼10% of the unit cell volume. This effect has been verified using 3 different solvents where the structural contraction occurs immediately at the freezing point of the individual solvent. Furthermore, studies on a series of DUT-49(M) frameworks with varying metals confirm the general applicability of this mechanism.