Three dinuclear [Ln₂H₂OL¹ ₂(acac)₂]·solvent (1, Ln = Gd, solvent = 2CH₂Cl₂; 2, Ln = Tb, no solvent; 3, Ln = Er, solvent = (C₂H₅)₂O), and two tetranuclear lanthanide clusters [Ln₄(μ₃-OH)₂L² ₂(acac)₆]·2(solvent) (4, Ln = Tb, solvent = CH₃OH; 5, Ln = Dy, solvent = CH₃CN) were characterized in terms of structure, fluorescence and magnetism. The dinuclear lanthanide complexes were constructed by a rigid salen-type ligand H₂L¹ = N,N′-bis(salicylidene)-o-phenylenediamine and β-diketonate (acac = acetylacetonate) ligands, while the tetranuclear clusters were formed from the flexible ligand H₂L² = N,N′-bis(salicylidene)-1,2-ethanediamine. Crystal structure analysis indicates that the rigid ligand favors the double-decker sandwich structure (Ln₂L¹ ₂), in which the two lanthanide ions have different coordination numbers and geometry, while the more flexible ligand (H₂L²) leads to planar tetranuclear clusters. The relationship between their respective magnetic anisotropy and ligand-field geometries and their fluorescence properties was investigated. The Dy and Tb-containing clusters exhibit typical visible fluorescence properties, and single-molecule magnet behavior is seen in complex 5.

The significantly improvement of single-molecule magnetic performance was achieved by removing or replacing the lattice solvents in dinuclear Dy₂ SMMs.

A series of lanthanide binuclear complexes with formula [Dy₂(DMOP)₂(NTFA)₄] (1), [Dy₂(DMOP)₂(NTFA)₄(CH₃OH)₂·2H₂O] (1a), [Dy₂(DMOP)₂(BTFA)₄] (2), [Dy₂(DMOP)₂(TTA)₄] (3), [Y₂(DMOP)₂(TTA)₄] (3Y), [Tb₂(DMOP)₂(NTFA)₄] (4), [Tb₂(DMOP)₂(NTFA)₄(CH₃OH)₂·2H₂O] (4a), [Er₂(DMOP)₂(NTFA)₄] (5), [Gd₂(DMOP)₂(NTFA)₄(CH₃OH)₂·2H₂O] (6), and [Gd₂(DMOP)₂(TTA)₄] (7) (DMOP = 2,6-dimethoxyphen, NTFA = 2-naphtoyltrifluoroacetone, BTFA = benzoyltrifluoroacetone, TTA = thenoyltrifluoroacetone) have been structurally and magnetically characterized. They possess nearly the same dinuclear Ln2 cores bridged by double μ₂-phenoxyl oxygen atoms and differ in the peripheral coordinated β-diketonate ligands or coordinated neutral MeOH molecule. The Dy(III)-based derivatives show significant zero-field single-molecule magnet behavior. It is noteworthy that the single-molecule magnet (SMM) performance of 1 was dramatically improved after transforming to 1a with a nearly double energy barrier, which is mostly due to the enhancement of local symmetry around Dy(III) ions in 1a by the introduction of a CH₃OH molecule. In addition, the nature of intramolecular Dy–Dy interaction changed from antiferromagnetic to ferromagnetic coupling. The detailed ab initio calculation reveals the alternate domination of the intramolecular exchange coupling and dipole–dipole interactions in 1 and 1a, respectively. Furthermore, improved SMM performance with a hysteresis temperature of 3 K was observed in diluted samples of 3. In the present work, the joint contribution of intramolecular coupling and local symmetry to improve their dynamic magnetic relaxation was unambiguously affirmed.