Three Isostructural One-Dimensional LnIII 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. [ABSTRACT FROM AUTHOR]