Three Isostructural One-Dimensional Ln^III Chains with Distorted Cubane Motifs Showing Dual Fluorescence and Slow Magnetic Relaxation/Magnetocaloric Effect

Three new homometallic lanthanide complexes with mixed carboxylate-modified rigid ligands, [Ln­(μ3-OH)­(na)­(pyzc)]n (na– = 1-naphtholate, pyzc– = 2-pyrazinecarboxylate, Ln = Dy (1), Yb (2), and Gd (3)), were solvothermally synthesized, and their structures and magnetic as well as photophysical properties were completely investigated. Complexes 1–3 are crystallographically isostructural, exhibiting linear chains with four bidentate bridging μ-COO– moieties encapsulated cubic {Ln4(μ3-OH)4}8+ clusters repeatedly extended by 4-fold chelating-bridging-pyzc– connectors. Magnetically, the former two complexes with highly anisotropic DyIII and weak anisotropic YbIII ions in the distorted NO7 triangular dodecahedron coordination environment display field-induced slow relaxation of magnetization. Fitting the dynamic magnetic data to the Arrhenius law gives energy barrier ΔE/kB = 39.6 K and pre-exponential factor τo = 1.52 × 10–8 s for 1 and ΔE/kB = 14.1 K and τo = 2.13 × 10–7 s for 2. By contrast, complex 3 with isotropic GdIII ion and weak intracluster antiferromagnetic coupling shows a significant cryogenic magnetocaloric effect, with a maximum −ΔSm value of 30.0 J kg–1 K–1 at 2.5 K and 70 kOe. Additionally, the chromophoric na– and pyzc– ligands can serve as antenna groups, selectively sensitizing the DyIII- and YbIII-based luminescence of 1 and 2 in the UV–visible region by an intramolecular energy transfer process. Thus, complexes 1–3, incorporating field-induced slow magnetic magnetization and interesting luminescence together, can be used as composite magneto-optical materials. More importantly, these interesting results further demonstrate that the mixed-ligand system with rigid carboxylate-functionalized chromophores can be excellent candidates for the preparations of new bifunctional magneto-optical materials.