Determining Optical Band Gaps of MOFs

Assigning optical band gaps to MOFs is paramount for understanding their optical, electronic, and reactivity properties, but literature reports have produced a wide range of values for even the same MOF. Despite the molecular nature of MOF electronic structures, experimental assignments employ Tauc analysis, a method applied to semiconductors. Here, we report optical band gaps of common MOFs and demonstrate that Gaussian fitting is more appropriate for assigning accurate gap energies. We further support this claim with DFT simulation, providing a reliable method for estimating optical band gaps from ground-state hybrid-GGA. MOFs that require Tauc analysis exhibit overlapping optical transitions uncommon for typical carboxylate-based MOFs and more akin to narrow-gap semiconductors. Taken together, these results provide a simple roadmap for assigning MOF optical band gaps.