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

Abstract

Three new homometallic lanthanide complexes with mixed carboxylate-modified rigid ligands, [Ln­(μ₃-OH)­(na)­(pyzc)]_<i>n</i> (na^– = 1-naphtholate, pyzc^– = 2-pyrazinecarboxylate, Ln = Dy (<b>1</b>), Yb (<b>2</b>), and Gd (<b>3</b>)), were solvothermally synthesized, and their structures and magnetic as well as photophysical properties were completely investigated. Complexes <b>1</b>–<b>3</b> 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 Dy^III and weak anisotropic Yb^III 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 Δ<i>E</i>/<i>k</i>_B = 39.6 K and pre-exponential factor τ_o = 1.52 × 10^–8 s for <b>1</b> and Δ<i>E</i>/<i>k</i>_B = 14.1 K and τ_o = 2.13 × 10^–7 s for <b>2</b>. By contrast, complex <b>3</b> with isotropic Gd^III ion and weak intracluster antiferromagnetic coupling shows a significant cryogenic magnetocaloric effect, with a maximum −Δ<i>S</i>_m 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 Dy^III- and Yb^III-based luminescence of <b>1</b> and <b>2</b> in the UV–visible region by an intramolecular energy transfer process. Thus, complexes <b>1</b>–<b>3</b>, 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