Synthesis, Characterization, and Photophysical Properties of Visible-Emitting Lanthanide Isophthalate-Based Coordination Polymers

Abstract

Four homonuclear lanthanide-based coordination polymers were synthesized under hydrothermal conditions and obtained as microcrystalline powders. The resulting compounds, with the general formula {[Ln₂(ip)₃(H₂O)₂]·H₂O}_n (H₂ip = isophthalic acid; Ln = Sm (Ia), Eu (Ib), Tb (Ic), Dy (Id)), are isostructural and exhibit extended polymeric architectures driven by the coordination versatility of the isophthalate ligands. Powder X-ray diffraction (PXRD) confirmed their phase purity and structural consistency, while Fourier-transform infrared spectroscopy (FTIR) evidenced the coordination through the carboxylate groups. Thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) revealed high thermal stability, with framework decomposition occurring only above 516°C. Photoluminescence investigations revealed characteristic lanthanide-centered visible emissions. The emission lifetimes were measured as 0.60699 ms for (Ia), 0.39287 ms for (Ib), 0.79544 ms for (Ic), and 0.69231 ms for (Id), and the quantum yields were also determined to evaluate their photophysical performance. CIE 1931 coordinates spanning orange (Sm³⁺: 0.34, 0.31), pink-red (Eu³⁺: 0.34, 0.27), green (Tb³⁺: 0.31, 0.52), to cyan (Dy³⁺: 0.35, 0.37), reflecting efficient ligand-to-metal energy transfer. These results highlight isophthalate ligands as versatile platforms for robust lanthanide coordination polymers with full-spectrum visible tunability, underscoring their potential for luminescent devices, optical sensors, and photonic materials.