Enhancement of Terbium(III)-Centered Luminescence by Tuning the Triplet Energy Level of Substituted Pyridylamino-4-R-Phenoxo Tripodal Ligands

A series of luminescent phenoxo-bridged dinuclear Tb^III complexes with tripodal ligands, 2,2′-[[(2-pyridinylmethyl)imino]di(methylene)]-bis(4-R-phenol), where R = CH₃ (L^CH3) (I), Cl (L^Cl) (II), CH₃O (L^CH3O) (III), COOCH₃ (L^COOCH3) (IV), were prepared to probe the effect of para-substitution on the phenol ring of the ligand on the Tb^III luminescence. For these Tb^III complexes a complete suppression of the ligand-centered fluorescence is observed, which demonstrates an efficient ligand-to-metal energy transfer. Complex IV was found to be the one that shows the greater intensity of the emission at room temperature. The obtained quantum yields follow the trend IV > II ≫ I > III. The quantum yield for II and IV is approximately five times greater than those obtained for I and III, indicating that the L^Cl and L^COOCH3 are better sensitizers of the Tb^III ions. These results were rationalized in terms of the variation of the energy gap between the triplet level (T₁) of the ligand and the emissive ⁵D₄ level of Tb^III, due to the electron-acceptor or electron-donor properties of the substituents. The τ_av values are in the millisecond range for all the studied complexes and resulted independent of temperature. The Commission International d'Eclairage coordinates (CIE) for all complexes are in the green color region, being insensitive to the variation of temperature. Moreover, the color purity (CP) is ca. 90% for all complexes, being ca. 100% for IV. Thus, the introduction of electron-acceptor substituents on the ligand permitted us to improve the luminescent properties of the Tb^III complexes.