Local Probing of Electronic Gaps in NH2-MIL-101(Cr) Metal–Organic Frameworks by Resonant Inelastic Soft X-ray Scattering

The electronic gap of metal–organic frameworks (MOFs), i.e., the energy difference between the highest occupied and lowest unoccupied electronic states, is crucial in many MOF applications. The magnitude and the orbital nature of the electronic gap determine the fundamental chemistry of the MOFs. Hence, local, atomic-level information about the electronic states is critical for MOFs with tailored properties or targeted functionalities; however, such information is not accessible through standard ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy and is usually derived from quantum chemical calculations. Here, we introduce resonant inelastic (soft) X-ray scattering (RIXS) as a direct experimental tool to locally probe electronic gaps in MOFs. Using NH2-MIL-101(Cr) as a test case, we compare our RIXS results with published UV-Vis-NIR spectra. The transitions observed in the UV-Vis-NIR data are also present in our RIXS data. By locally probing the chromium L2,3 and carbon, nitrogen, and oxygen K edges, we can identify specific local sites for each transition.