Luminescence tuning of imidazole-based lanthanide(iii) complexes [Ln = Sm, Eu, Gd, Tb, Dy]

Abstract

To tune the lanthanide luminescence in related molecular structures, we synthesized and characterized a series of lanthanide complexes with imidazole-based ligands: two tripodal ligands, tris{[2-{(1-methylimidazol-2-yl)methylidene}amino]ethyl}amine (Me₃L), and tris{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H₃L), and the dipodal ligand bis{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H₂L). The general formulas are [Ln(Me₃L)(H₂O)₂](NO₃)₃·3H₂O (Ln = 3+ lanthanide ion: Sm (1), Eu (2), Gd (3), Tb (4), and Dy (5)), [Ln(H₃L)(NO₃)](NO₃)₂·MeOH (Ln³⁺ = Sm (6), Eu (7), Gd (8), Tb (9), and Dy (10)), and [Ln(H₂L)(NO₃)₂(MeOH)](NO₃)·MeOH (Ln³⁺ = Sm (11), Eu (12), Gd (13), Tb (14), and Dy (15)). Each lanthanide ion is 9-coordinate in the complexes with the Me₃L and H₃L ligands and 10-coordinate in the complexes with the H₂L ligand, in which counter anion and solvent molecules are also coordinated. The complexes show a screw arrangement of ligands around the lanthanide ions, and their enantiomorphs form racemate crystals. Luminescence studies have been carried out on the solid and solution-state samples. The triplet energy levels of Me₃L, H₃L, and H₂L are 21 000, 22 700, and 23 000 cm⁻¹, respectively, which were determined from the phosphorescence spectra of their Gd³⁺ complexes. The Me₃L ligand is an effective sensitizer for Sm³⁺ and Eu³⁺ ions. Efficient luminescence of Sm³⁺, Eu³⁺, Tb³⁺, and Dy³⁺ ions was observed in complexes with the H₃L and H₂L ligands. Ligand modification by changing imidazole groups alters their triplet energy, and results in different sensitizing ability towards lanthanide ions.