Understanding and Controlling the Dielectric Response of Metal-Organic Frameworks

Metal-organic framework (MOF) materials have recently been shown to have promising electronic and dielectric properties. This study involves investigating a diverse range of MOFs to rationalise how the different building blocks that form the structure can affect the electronic properties and dielectric response. The analysis, based on quantum mechanical calculations, includes the contribution from the metals involved, the organic linkers and the symmetry and topology of the framework and makes suggestions for future work on low-kappa dielectric MOFs. The results confirm that the band gap is primarily due to the electronic levels of the organic linkers and that tuning the band gap can be easily achieved either by linker functionalisation or by increasing the aromaticity. The relevance of simple structure-property relationships for different families of isoreticular MOFs through the use of Hammett sigma constants is also highlighted. It is also shown that the polarisability of the framework can be tuned comparably to the band gap. However, the expected low static dielectric constant is less influenced by the composition of the MOF and can be modified by acting on the crystal structure. Indeed, it is shown that it can be directly linked to the framework porosity.