dd excitations in CPO-27-Ni metal-organic framework: comparison between resonant inelastic X-ray scattering and UV-vis spectroscopy

We identify the dd excitations in the metal-organic framework CPO-27-Ni by coupling resonant inelastic X-ray scattering (RIXS) and UV-vis spectroscopy, and we show that the element selectivity of RIXS is crucial to observing the full dd multiplet structure, which is not visible in UV-vis. The combination of calculations using crystal-field multiplet theory and density functional theory can reproduce the RIXS spectral features, crucially improving interpretation of the experimental data. We obtain the crystal-field splitting and magnitude of the electron-electron interactions and correct previously reported values. RIXS instruments at synchrotron radiation sources are accessible to all researchers, and the technique can be applied to a broad range of systems.