Magneto-OpticalSpectroscopy Across Three OxidationStates of a Europium Terpyridine Complex

Abstract We report the magneto-optical characterization of a homoleptic europium terpyridine complex 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 D3h local symmetry in solution, higher than suggested by the solid-state structure, through the splitting pattern of its 7F0→5D2 transition. The Eu(II) complex, by contrast, exhibited broad MCD features spanning the visible region arising from 4f7[8S7/2]→4f6[7FJ]×ψ1 transitions (ψ = 5d or Lπ*, J = 0–6). We show theoretically and experimentally that when individual components of this J progression are unresolved, MCD and magnetic linear dichroism (MLD) spectra display characteristic “pseudo-A1” and “pseudo-A2” lineshapes, respectively. Variable-temperature variable-field measurements of both techniques reveal a near-zero Eu(II) zero-field splitting by monitoring MCD C0 and MLD G0 signal intensities. The formally Eu(0) complex was serendipitously isolated and its properties are consistent with a redox noninnocent [EuII(mtpy)(mtpy•–)2] formulation. Together, these results establish MCD and MLD spectroscopy as information-rich tools for probing f-block electronic structure across multiple oxidation states and transition types.