Luminescent lanthanide complexes supported by ditopic Schiff-base/calix[4]arene macrocycles: synthesis, structure, and luminescence properties of [Ln2(H2L2)(H2O)2] (Ln = La, Eu, Tb, Yb)†
Abstract
The ditopic macrocycle H8L2 comprising two 3,3′-dihydroxy-4,4′-bis(iminomethyl)-biphenyl and two calix[4]arene units was synthesized and its ability to bind lanthanide ions has been studied. H8L2 reacts readily with Ln(III) nitrate salts in the presence of NEt3 to support dinuclear neutral complexes of composition [Ln2(H2L2)(H2O)2] (Ln = La3+ (1), Eu3+ (2), Tb3+ (3), and Yb3+ (4)). The crystal structure of the ditopic calix[4]arene ligand reveals a centrosymmetric molecule with an overall “S-shape” structure and a cone-conformation. The molecular structure of the Yb complex 4 is also “S-shape” indicative of some degree of preorganization imposed by the ligand constraints. The Yb3+ ions are seven-coordinated (distorted monocapped trigonal prismatic coordination environment (NO6 donor set)). DFT calculations for the La complex 1 (at the r2SCAN-3c level of computation) implies an isostructural series of compounds. The ligand and complexes exhibit strong absorptions (ε > 104 M−1 cm−1) in the 250–440 nm range attributed to ligand-based π–π* and n–π* transitions. The bis(iminomethyl)biphenyl linkers were found to sensitize EuIII and TbIII emission (λex = 370 nm and 405 nm) in the solid state at 295 K. A material containing statistically distributed TbIII and EuIII ions reveals upon 370 nm excitation only Eu3+ emission lines indicative of intermolecular energy transfer processes between the Ln3+ ions.
- This article is part of the themed collection: Rare Earth Materials