Deciphering crystal field splitting of lanthanide ions using magnetism, luminescence and ab-initio calculations: A friendly discussion between van Vleck and Boltzmann
Résumé
This chapter highlights the potential of magneto-optical correlation as a powerful tool to probe the unique properties of lanthanide-based molecular complexes. This emerging approach synergistically combines high-resolution luminescence spectroscopy, magnetic measurements, and ab initio calculations, performed on well-characterized single-crystal or polycrystalline samples. The exceptional properties of these systems stem from the coordination geometry around the lanthanide ion, which governs the crystal-field (CF) splitting of ground and excited (2S+1)ΓJ multiplets into discrete CF states. In lanthanide complexes, the ligand(crystal)-field splitting is modest—typically below 1000 cm–1—so all CF states are thermally populated at room temperature, following Boltzmann statistics. To access single-state populations essential for observing single-molecule magnet (SMM) behavior or sharp luminescence spectra free from hot bands, measurements must be carried out at cryogenic temperatures (typically<10 K). Under these conditions, magneto-optical correlations become highly informative, enabling deep insight into low-symmetry coordination environments, often encountered in organometallic complexes. This methodology also facilitates the characterization of divalent lanthanide species and the acquisition of “pure” chiroptical data, offering new perspectives for understanding structure–property relationships in f-element chemistry.