Two Types of Lanthanide Coordination Polymers Based on a Homophthalate Ligand Exhibiting Luminescence and a Significant Magnetocaloric Effect

Two types of lanthanide coordination polymers based on homophthalic acid (H₂HPA), one-dimensional {[Ln­(HPA)­(NO₃)­(H₂O)₂]­·H₂O}_<i>n</i> (Ln = Eu (<b>1</b>); Gd (<b>2</b>); Tb (<b>3</b>); Dy (<b>4</b>)) and two-dimensional {[Ln₂(HPA)₃­(H₂O)₂]­·H₂O}_<i>n</i> (Ln = Eu (<b>5</b>); Gd (<b>6</b>); Tb (<b>7</b>); Dy (<b>8</b>)), were solvothermally synthesized. Compounds <b>1</b>–<b>4</b> show a one-dimensional (1D) linear chain structure consisting of a HPA^2– linker and [Ln₂] cluster unit. Compounds <b>4</b>–<b>8</b> are crystallographically isostructural and contain 1D lanthanide-carboxylate building units [Ln₄(CO₂)₁₀]_<i>n</i> built from the adjacent Ln³⁺ ions and carboxyl groups of the H₂HPA ligands, which can further give two-dimensional layer structures via the link of HPA^2–. The magnetic studies reveal that complexes <b>2</b> and <b>6</b> with isotropic Gd³⁺ ion exhibit significant cryogenic magnetocaloric effects with a maximum −Δ<i>S</i>_m value of 35.58 and 35.41 J kg^–1 K^–1 at 2 K and 7 T, respectively. In addition, the solid-state photophysical properties of <b>1</b>/<b>5</b> and <b>3</b>/<b>7</b> respectively display strong characteristic Eu³⁺ and Tb³⁺ photoluminescent emission in the visible region, suggesting that Eu- and Tb-based luminescences are sensitized by the effective energy transfer from the ligand to the metal centers