Bright luminescence in lanthanide coordination polymers with tetrafluoroterephthalate as a bridging ligand

Abstract

Ten new coordination polymers of the general compositions ²_∞[Ln^III(tfBDC)(NO₃)(DMF)₂]·DMF with Ln^III = Eu³⁺ (1), Gd³⁺ (2), Tb³⁺ (3), Ho³⁺ (4), Tm³⁺ (5), ²_∞[Ln^III(tfBDC)(CH₃COO)(FA)₃]·3FA with Ln^III = Sm³⁺ (6), Eu³⁺ (7) and ²_∞[Ln^III(tfBDC)(NO₃)(DMSO)₂] with Ln^III = Ho³⁺ (8), Er³⁺ (9) and Tm³⁺ (10) were synthesized and structurally characterized by X-ray single crystal diffraction (tfBDC²⁻ = 2,3,5,6-tetrafluoroterephthalate, DMF = N,N′-dimethylformamide, FA = formamide, DMSO = dimethyl sulfoxide). 1–5 crystallize in the monoclinic space group C2/c with Z = 8, 6 and 7 in P with Z = 2 and 8–10 in Pbca with Z = 8. All crystal structures contain binuclear lanthanide nodes that are connected by 2,3,5,6-tetrafluoroterephthalates (tfBDC²⁻) to form two-dimensional polymeric structural units. Despite this common structural feature the coordination within these binuclear units is quite different in detail, e.g. CN = 9 for 1–7 and CN = 8 for 8–10. The emission spectra of the europium (1, 7) and terbium (3) compounds reveal bright red and green emission in the visible region. The resulting high quantum yields of 53% (1) and 67% (3) at room temperature show that the replacement of organic ligands with C–H groups by perfluorinated ligands leads to compounds with intense emission, as vibrational quenching is reduced. On the other hand, the influence of the coordinating solvent and additional ligands cannot be neglected, as the replacement of DMF by FA and NO₃⁻ by CH₃COO⁻ in 7 leads to a reduced quantum yield of only 10%. Thermoanalytical investigations show that all compounds are stable up to 100–150 °C, before a stepwise release of solvent molecules starts followed by a decomposition of the coordination polymer.