Ligand-field symmetry and magneto-optical correlations in a luminescent Dy(III) single-molecule magnet

Abstract

The establishment of ligand field (LF) symmetry around lanthanoid (Ln(III)) centres is of paramount importance to understand factors that govern magnetization relaxation in single-molecule magnets (SMMs). We investigated a luminescent mononuclear Dy(III) compound [Dy(BA)₄] (pip) (1), where pip is a piperidinium cation and BA is a benzoylacetonate ligand. The stoichiometric compound exhibits zero-field SMM characteristics, quantum tunneling of magnetization (QTM) dominates magnetization relaxation. In the diluted version of the compound (1@Y), the QTM is mitigated, Raman and Orbach processes are involved in the relaxation. The effective energy barrier (Ueff = 53.61 cm-1; HDC = 0 Oe) estimated for magnetization relaxation is comparable with the LF splitting (ΔE = 57.2 cm-1) between the ground and first excited Kramers doublets (KDs) determined from the 4F9/2→6H15/2 Dy(III)-based transition recorded at 2.4 K. By analysing the emission spectrum of the isostructural Eu(III) analogue as spectroscopic probe for site symmetry, the LF symmetry around the Dy(III) centre is assigned as C₂ᵥ. The close proximity (178 cm-1) of the triplet state of the ligand and 4F9/2 state of the Dy(III) facilitates back energy transfer, thereby rendering 1 emissive only at cryogenic temperatures. The predictive accuracy of the complete active space spin-orbit configuration interaction (CASOCI) method is benchmarked against the experimental emission profile. Overall, we propose a strategy to assign effective LF symmetry around Dy(III), establish magneto-optical correlations, and provide a comprehensive analysis of emission process of 1.