Mixed Tb/Dy coordination ladders based on tetra(carboxymethyl)thiacalix[4]arene: a new avenue towards luminescent molecular nanomagnets

Abstract

The macrocyclic ligand calix[4]arene (L1) and its sulphur-containing analogue thia[4]calixarene (L2) are promising precursors for functional molecular materials as they offer rational functionalization with various organic groups. Here, we present the first example of lanthanide-based coordination polymers built from the macrocyclic thiacalix[4]arene backbone bearing four carboxylic moieties, namely, ligand H₄L3. The combination of H₄L3 with the Tb³⁺ and Dy³⁺ cations led to the formation of 1D ladder-type coordination polymers with the formula [Ln^IIIHL3DMF₃]·(DMF) (where DMF = dimethylformamide and Ln = Tb or Dy, denoted as HL3–Tb and HL3–Dy), which resulted from the coordination of the lanthanide cations with the partially deprotonated ligand HL3³⁻ that behaved as a T-shape connector. The coordination sphere around the metal was completed by the coordinated DMF solvent molecules. By combining both Tb³⁺ and Dy³⁺ cations, isostructural heterobimetallic solid solutions HL3–Tb_1−xDy_x were also prepared. HL3–Tb and HL3–Dy showed visible light photoluminescence originating from the f–f electronic transitions of pale green emissive Tb³⁺ and pale yellow emissive Dy³⁺ with efficient sensitization by the functionalized thia[4]calixarene ligand HL3. In the HL3–Tb_1−xDy_x solid solutions, the Tb/Dy ratio governed both the emission colour as well as the emission quantum yield, which reached even 28% at room temperature for HL3–Tb. Moreover, HL3–Dy exhibited a slow magnetic relaxation effect related to the magnetic anisotropy of the dodecahedral Dy³⁺ complexes, which were well isolated in the crystal lattice by expanded organic spacers.