  Due to their favorable properties and high porosity, zeolitic imidazolate
frameworks (ZIFs) have recently received much limelight for key technologies
such as energy storage, optoelectronics, sensorics, and catalysis. Despite the
widespread interest in these materials, fundamental questions regarding the
zinc coordination environment remain poorly understood. By focusing on
zinc(II)2-methylimidazolate (ZIF-8) and its tetrahedrally coordinated analogs
with Br-, Cl-, and H-substitution in the 2-ring position, we aim to clarify how
variations in the local environment of Zn impact the charge distribution and
the electronic properties of these materials. Our results from
density-functional theory confirm the presence of a Zn coordinative bond with a
large polarization that is quantitatively affected by different substituents on
the organic ligand. Moreover, our findings suggest that the variations induced
by the functionalization in the Zn coordination have a negligible effect on the
electronic structure of the considered compounds. On the other hand, halogen
terminations of the ligands lead to distinct electronic contributions in the
vicinity of the frontier region which ultimately reduce the band-gap size by a
few hundred meV. Experimental results obtained from X-ray absorption
spectroscopy (Zn $K$-edge) confirm the trends predicted by theory and, together
with them, contribute to a better understanding of the structure-property
relationships that are needed to tailor ZIFs for target applications.
