Highly luminescent Eu(III) complexes with 2,4,6-tri(2-pyridyl)-1,3,5-triazine ligand: Synthesis, structural characterization, and photoluminescence studies

Two new Eu(III) complexes featuring 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) and a rigid Lewis base ligand 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz), [Eu (hfac)₂ (H₂ O) (EtOH) (tptz)] [CF₃ CO₂^-] (1) and Eu(hfac)₃(tptz) (2), were synthesized. Their structures were established by single crystal X-ray diffraction. The europium ion in each of these complexes is nona-coordinate with six oxygen and three nitrogen atoms forming a coordination polyhedron best describable as a monocapped square antiprism. The difference in the composition and structure between these two complexes is caused by simply reversing the order of ligand (hfac and tptz) addition during the complex synthesis, and is rationalized in terms of the structural and electronic properties of the ligands and the overall steric bulk of the coordination sphere. Both complexes display characteristic Eu(III)-originated red emission upon UV excitation. The high quantum yields observed, 52% (1) and 60% (2), are rationalized in terms of the strong absorptions of both hfac and tptz ligands near the excitation wavelength (295 nm), an interpretation consistent with the well-established mechanism of ligand-mediated energy transfer for lanthanide-based light emission.