Modified Pyridine-2,6-dicarboxylate Acid Ligands for Sensitization of Near-Infrared Luminescence from Lanthanide Ions (Ln3+ = Pr3+, Nd3+, Gd3+, Dy3+, Er3+)

A detailed study of the ability of pyridine-2,6-dicarboxylic acid (1) and its 4-mono- and 3,4,5-tri-substituted analogues to sensitize emission from Pr³⁺, Nd³⁺, Gd³⁺, Dy³⁺ and Er³⁺ is presented. Sensitization of Ln³⁺ emission was demonstrated via the ligands in all complexes, excluding Gd³⁺, with emission covering the spectral range from 500 nm to 1850 nm obtained with variation of the Ln³⁺ ion. From the study of the ligand-based photoluminescence obtained from Gd³⁺-complexes, and the relative ligand and Ln³⁺ emission obtained from the other complexes, the singlet and triplet state energies of complexes of (1) are estimated to be at 3.1 eV and 2.6 eV respectively whilst for the 3,5-dibromo-substituted complexes (4) they are at 2.9 eV and 2.3 eV. Hypersensitivity of the Er^{3+ 4}I_15/2 → ²H_11/2 and ⁴I_15/2 → ⁴G_11/2 intra-atomic transitions is also observed in the 4-chloro-substituted (3) complex. Enhanced sensitization of Nd³⁺ (ca. 5-fold) and Er³⁺ (ca. 2-fold) near-infrared emission is demonstrated for complexes of (3) and (4) respectively in comparison with those of (1).