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Abstract

We used Ni K-edge resonant-valence-to-core X-ray emission spectroscopy (RVtC-XES, also referred to as direct RIXS), an element-selective bulk-sensitive synchrotron-based technique, to investigate the electronic structure of the CPO-27-Ni metal–organic framework (MOF) upon molecular adsorption of significant molecular probes: $H_{2}O$, CO, $H_{2}S$, and NO. We compare RVtC-XES with UV–vis spectroscopy, and we show that the element selectivity of RVtC-XES is of strategic significance to observe the full set of d–d excitations in $Ni^{2+}$, which are partially overshadowed by the low-energy π–π* transitions of the Ni ligands in standard diffuse-reflectance UV–vis experiments. Our combined RVtC-XES/UV–vis approach provides access to the whole set of d–d excitations, allowing us a complete discussion of the changes undergone by the electronic configuration of the $Ni^{2+}$ sites hosted within the MOF upon molecular adsorption. The experimental data have been interpreted by multiplet ligand-field theory calculations based on Wannier orbitals. This study represents a step further in understanding the ability of the CPO-27-Ni MOFs in molecular sorption and separation applications.

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