Elaborate chemical design is of utmost importance in order to slow down the relaxation dynamics in single-molecule magnets (SMMs) and hence improve their potential applications. Much interest was devoted to the study of distinct relaxation processes related to the different crystal fields of crystallographically independent lanthanide ions. However, the assignment of the relaxation processes to specific metal sites remains a challenging task. To address this challenge, a new asymmetric Dy₂ SMM displaying a well-separated two-step relaxation process with the anisotropic centers in fine-tuned local environments was elaborately designed. For the first time a one-to-one relationship between the metal sites and the relaxation processes was evidenced. This work sheds light on complex multiple relaxation and may direct the rational design of lanthanide SMMs with enhanced magnetic properties.

The reaction of the rigid polydentate Schiff-base H₃L with different dysprosium carboxylates afforded two novel centrosymmetrical dinuclear Dy^III compounds, [Dy₂(HL)₂(CH₃COO)₂(DMF)₂] (1) and [Dy₂(HL)₂(PhCOO)₂(DMF)₂]·4DMF (2) with differing carboxylate groups. The two differing carboxylate groups affect the magnetic interaction and/or local anisotropy of the Dy ion, and therefore we have two compounds showing dissimilar magnetic behaviour. The acetato bridging groups with a syn-syn η¹:η¹:μ₂ mode in compound 1 lead to the disappearance of slow magnetic relaxation under zero dc field. Nevertheless, typical SMM behaviour is observed for compound 2 with η²-chelating benzoate ligands.

Efficient modulation of single-molecule magnet (SMM) behavior was realized by deliberate structural modification of the Dy₂ cores of [Dy₂(a′povh)₂(OAc)₂(DMF)₂] (1) and [Zn₂Dy₂(a′povh)₂(OAc)₆]⋅4 H₂O (2; H₂a′povh=N′-[amino(pyrimidin-2-yl)methylene]-o-vanilloyl hydrazine). Compound 1 having fourfold linkage between the two dysprosium ions shows high-performance SMM behavior with a thermal energy barrier of 322.1 K, whereas only slow relaxation is observed for compound 2 with only twofold connection between the dysprosium ions. This remarkable discrepancy is mainly because of strong axiality in 1 due to one pronounced covalent bond, as revealed by experimental and theoretical investigations. The significant antiferromagnetic interaction derived from bis(μ₂-O) and two acetate bridging groups was found to be crucial in leading to a nonmagnetic ground state in 1, by suppressing zero-field quantum tunneling of magnetization.

Three pairs of enantiopure chiral triangular Ln₃ clusters, [Ln₃L_{RRRRRR/SSSSSS}(μ₃-OH)₂(H₂O)₂(SCN)₄]⋅xCH₃OH⋅yH₂O (R-Dy₃, Ln=Dy, x=6, y=0; S-Dy₃, Ln=Dy, x=6, y=1; R-Ho₃, Ln=Ho, x=6, y=1; S-Ho₃, Ln=Ho, x=6, y=1; R-Er₃, Ln=Er, x=6, y=0; S-Er₃, Ln=Er, x=6, y=1), have been successfully synthesized by a rational enantioselective synthetic strategy. The core of triangular Ln₃ is bound in the central N₆O₃ of the macrocyclic ligand, and the coordination spheres of Ln ions are completed by four SCN⁻ anions and two H₂O molecules in axial positions of the macrocycle. The circular dichroism (CD) and vibrational circular dichroism (VCD) spectra of the enantiomers demonstrate that the chirality is successfully transferred from the ligands to the resulting Ln₃ clusters. Ac susceptibility measurements reveal that single-molecule magnet behavior occurs for both enantiopure clusters of R-Dy₃ and S-Dy₃. This work is one of the few examples of the successful design of a pair of triangular Dy₃ clusters showing simultaneously slow magnetic relaxation and optical activity, and this might open up new opportunities to develop novel multifunctional materials.