Structural Modulation of Fluorescent Rhodamine-Based Dysprosium(III) Single-Molecule Magnets

Two rhodamine 6G-based mononuclear dysprosium complexes, [Dy(L^R)(L^A)₂](ClO₄)₃·Et₂O·1.5MeOH·0.5H₂O (1) and [Dy(L^R)(H₂O)₄(MeCN)](ClO₄)₃·2H₂O·MeCN (2) (L^R = salicylaldehyde rhodamine 6G hydrazone, L^A = 2-pyridylcarboxaldehyde benzoyl hydrazone), are synthesized, aiming at improving the magnetic behavior by modulating their coordination environment. Both complexes own one exclusive short Dy-O_phenoxy coordination bond as the predominant bond and exhibit single-molecule magnet behavior under zero dc field with the energy barrier (U_eff/k_B) of 90 K (1) and 320 K (2) and apparent hysteresis at 1.9 K. The ab initio calculations indicate that the short Dy-O_phenoxy bond determines the direction of magnetic anisotropic axis for 1 and 2. The quantum tunneling of magnetization (QTM) between the ground Kramers doublets (KDs) in 1 cannot be neglected, leading to an experimental U_eff/k_B much lower than the calculated energy of the first excited state (318.2 K). For 2, the stronger magnetic anisotropy and negligible QTM between the ground KDs guarantees that the energy barrier is close to the calculated energy of first KDs (320.8 K). On the other hand, the presence of ring-opened xanthene moiety makes complexes 1 and 2 in the solid state emit red light with emission bands of 645 and 658 nm, respectively.