Understanding the Optical and Magnetic Properties of Ytterbium(III) Complexes

The absorption and emission spectra of three Yb³⁺ complexes possessing D₃, D₂, and C₂ symmetries were analyzed with the aid of ab initio calculations based on Complete Active Space (CAS) self-consistent field wave functions (CAS(13,7)). The absorption spectra present contributions from both cold and hot bands, involving thermally populated excited sublevels of the ²F_7/2 manifold. The high-resolution emission spectrum of the tris-picolinate complex [Yb(DPA)₃]^3- recorded at 77 K presents four components, while the complexes with macrocyclic ligands show both cold and hot emission bands, resulting in more than four components for the ²F_5/2 → ²F_7/2 transition. The combined information provided by the absorption and emission spectra allowed to identify most of the crystal field sublevels of the ²F_5/2 and ²F_7/2 states. The energies of these crystal field components are well-reproduced by the ab initio calculations, with deviations typically lower than 100 cm^-1. The crystal field splitting is very sensitive to subtle changes of the Yb³⁺ coordination environment. The magnetic anisotropy of [Yb(DPA)₃]^3- obtained with ab initio calculations was found to be extremely sensitive to changes in the twist angle of the upper and lower faces of the tricapped trigonal prismatic coordination polyhedron. Ab initio ligand field theory provides a straightforward chemical justification for the changes in magnetic anisotropy, which are responsible for the observed pseudocontact shifts in the NMR spectra.