Water-stable LnIII-based coordination polymers displaying slow magnetic relaxation and luminescence sensing properties

Abstract

Six new two-dimensional isostructural lanthanide coordination polymers (Ln^III-CPs), [Ln₂(L)₃(H₂O)₃]·DMF (Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5) and Ho (6), H₂L = 3-(3,5-dicarboxylphenoxy)pyridine), have been fabricated successfully under solvothermal conditions. In these compounds, Ln^III ions are linked by the mixed (μ-O_carboxylate)(COO)₂ triple bridges to give dinuclear motifs. The motifs are connected through single carboxylate bridges to give alternating chains, and the chains are interlinked by the L ligands to produce 2D layers. Their structures with accessible Lewis-base sites and coordinated water molecules could easily recognize various analytes. The dc magnetic susceptibility studies for Gd-CP (3) and Ho-CP (6) reveal the occurrence of probable antiferromagnetic interactions, but for Dy-CP (5) show ferromagnetic interactions. Further investigations suggest that Dy-CP (5) displays slow magnetic relaxation behavior with an energy barrier (ΔU_eff) and a pre-exponential factor (τ₀) of 25.47 K and 1.25 × 10⁻⁶ s at 1200 Oe dc field, respectively. The luminescence explorations suggest that Eu-CP (2) and Tb-CP (4) can exhibit high quenching efficiency and low detection limit for sensing Fe³⁺, MnO₄⁻, Cr^VI-anions (CrO₄²⁻, Cr₂O₇²⁻) and 2,4,6-trinitrophenol (TNP) in water systems. Moreover, in the process of detecting analytes, Eu-CP (2) and Tb-CP (4) not only exhibit excellent water stability and strong luminescence, but also show remarkable recyclability and regeneration. Especially, Eu/Tb-CP can detect Fe³⁺, MnO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻ simultaneously, which is rarely reported in the past.