Ligand field tuning of d-orbital energies in MOF clusters

Abstract

Linker functionalization is a common route used to affect the electronic and catalytic properties of metal-organic frameworks. By either pre- or post-synthetically installing linkages with differing linker moieties the band gap, workfunction, and exciton lifetimes have been shown to be affected. One overlooked aspect of linker functionalization, however, has been the impact on the metal d-orbital energies to which they are bound. The ligand field differences should result in substantial changes in d¬-splitting. In this study we use DFT to study the energetics of d-orbital energy tuning as a function of linker chemistry. We offer a general descriptor, linker pKa, as a tool to predict the resultant d-splitting in MOFs. Our calculations reveal that simple functionalizations can affect the d-energies by up to 2 eV and illustrate the significance of this band modularity using four archetypal MOFs: UiO-66, MIL-125, ZIF-8, and MOF-5. Together, we show that linker functionalization dramatically affects d-energies in MOF clusters and highlight that linker functionalization is a useful route for fine-tuning band edges centered on the metals, rather than linkers themselves.