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 complexes with tripodal ligands, 2,2'-[[(2-pyridinylmethyl)imino]di(methylene)]-bis(4-R-phenol), where R = CH (L) (), Cl (L) (), CHO (L) (), COOCH (L) (), were prepared to probe the effect of -substitution on the phenol ring of the ligand on the Tb luminescence. For these Tb complexes a complete suppression of the ligand-centered fluorescence is observed, which demonstrates an efficient ligand-to-metal energy transfer. Complex was found to be the one that shows the greater intensity of the emission at room temperature. The obtained quantum yields follow the trend > ≫ > . The quantum yield for and is approximately five times greater than those obtained for and , indicating that the L and L are better sensitizers of the Tb ions. These results were rationalized in terms of the variation of the energy gap between the triplet level (T) of the ligand and the emissive D level of Tb, due to the electron-acceptor or electron-donor properties of the substituents. The τ 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 . Thus, the introduction of electron-acceptor substituents on the ligand permitted us to improve the luminescent properties of the Tb complexes.