Synthesis, crystal structure and luminescence of novel Eu3+, Sm3+ and Gd3+ complexes of 1,3,5- and 1,2,4-triazines

New Eu3+, Sm3+ and Gd3+ complexes of triazine ligands 2,4,6-tri(2-pyridyl)-1,3,5-triazine, 3,5,6-tri(2-pyridyl)-1,2, 4-triazine and 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine were synthesized. The structure of five of them was established by single crystal X-ray crystallography. Various aspects of the triazine ligands such as steric interactions, deviation from the planarity and coordination mode define the structure of the complexes. The 1H NMR spectra of the solutions of Eu3+ and Sm3+ molecules showed broad paramagnetic shifted peaks assigned to the protons of the ligated triazines. The powder EPR spectra of the Gd3+ complexes exhibited a broad peak centered at g = 2. All Eu3+ and Sm3+ complexes display characteristic lanthanide emission upon UV excitation. The enhancement of the luminescence is attributed to the effective energy transfer from the triazine ligand to the lanthanide ion. 1,2,4-Triazines show to be better sensitizers for Eu3+ and Sm 3+ than the 1,3,5-triazine ligand. Co-coordination of the triazines with β-diketonate ligands further enhances the luminescence of the lanthanides. This enhancement was attributed to the synergistic effect of the two ligands and/or the decrease of the vibrational quenching processes