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

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 tunnelling of magnetization (QTM) dominates magnetization relaxation. In the diluted version of the compound (1@Y), the QTM is mitigated, and Raman and Orbach processes are involved in the relaxation. The effective energy barrier (U_eff = 53.61 cm^-1; H_DC = 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 ⁴F_9/2 → ⁶H_15/2 Dy(III)-based transition recorded at 2.4 K. By analysing the emission spectrum of the isostructural Eu(III) analogue as a spectroscopic probe for site symmetry, the LF symmetry around the Dy(III) centre is assigned as C_2v. The close proximity (178 cm^-1) of the triplet state of the ligand and the ⁴F_9/2 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 the emission process of 1.