Magneto-Optical Spectroscopy Across Three Oxidation States of a Europium Terpyridine Complex

The multiconfigurational nature of lanthanide ions yields notoriously complex electronic structures, and europium is a useful exemplar of this complexity. Engineering of future europium-based molecules and materials—well known for their luminescence in the +3 oxidation state and magnetism in the +2 oxidation state—rests on a precise understanding of their structure–property relationships. Magneto-optical spectroscopies are well suited to probe these relationships, and we demonstrate their utility here through the characterization of a series of homoleptic europium terpyridine complexes in +3, +2, and formally 0 oxidation states. Magnetic circular dichroism (MCD) analysis of the f–f transitions of the Eu(III) complex revealed an effective D 3h local symmetry in solution, higher than suggested by the solid-state structure, through the splitting pattern of its 7 F 0 → 5 D 2 transition. The Eu(II) complex, by contrast, exhibited broad MCD features spanning the visible region arising from 4f 7 → 4f 6 ⊗ ψ 1 transitions (ψ = 5d or Lπ*). We show theoretically and experimentally that when individual components of this progression are unresolved, MCD and magnetic linear dichroism (MLD) spectra display characteristic “pseudo- A 1 ” and “pseudo- A 2 ” lineshapes, respectively. Variable-temperature variable-field measurements of both techniques reveal a near-zero Eu(II) zero-field splitting and confirm the first observation of G 0 MLD intensity for an f-block molecular complex. The formally Eu(0) complex was serendipitously isolated and its properties are consistent with a redox non-innocent [Eu II (mtpy)(mtpy •– ) 2 ] formulation. Together, these results establish MCD and MLD spectroscopies as information-rich tools for probing f-block electronic structure across multiple oxidation states and transition types.