Lanthanide complexes assembled from two flexible amide-type tripodal ligands: terminal groups effect on photoluminescence behavior

Abstract

To explore the effect of terminal groups of tripodal ligands on the photoluminescence behaviors of the complexes, lanthanide (Eu(III), Tb(III)) nitrate complexes with two flexible amide-type tripodal ligands, 2,2′,2′′-nitrilotris-(N-phenylmethyl)-acetamide (L^I) and 2,2′,2′′-nitrilotris-(N-naphthalenemethyl)-acetamide (L^II) were synthesized and characterized. The general formulas of the complexes are [EuL^I₂(C₃H₆O)]·(NO₃)₃·(HCCl₃)·(H₂O)₄ (1), TbL^I₂(NO₃)₃·2H₂O (2), EuL^II(NO₃)₃ (3), and TbL^II(NO₃)₃ (4). Among them, 1, 3, and 4 were characterized by single-crystal X-ray diffraction. Complex 1 demonstrates a 1 : 2 (ML₂) capsule type stoichiometry, and the complexes 3 and 4 confirm 1 : 1 (ML) type coordination structures. What is more, the triplet energy levels of L^I and L^II are 24 331 and 19 802 cm⁻¹, which were determined from the phosphorescence spectra of the Gd(III) complexes. Ligand modification by changing the terminal groups alters their triplet energy, and results in a different sensitizing ability towards lanthanide ions. The density functional theory (DFT) calculations of energy levels including HOMO, LUMO, singlet, and triplet energies tuned by the different terminal groups are also discussed in detail, and the trends are almost consistent with the experimental conclusions.