K.S. Kisel, G. Linti, G.L. Starova, V.V. Sizov, A.S. Melnikov, A.P. Pushkarev, M.N. Bochkarev, E.V. Grachova, S.P. Tunik
“Syntheses, Structures, and Photophysical Properties of Eu and Lu Diketonates with a Neutral Polydentate Imidazolylmethanamine Ligand”
Eur. J. Inorg. Chem., 2015
The Schiff base NNO ligand 1-(furan-2-yl)-N-[(2-methyl-1H-imidazol-4-yl)methylene]methanamine was synthesized and structurally characterized by XRD crystallography, mass spectrometry, and NMR spectroscopy. Quantum-chemical calculations revealed conformational flexibility of the ligand backbone to give two different conformations with nearly equal ground-state energies. The orientation of two nitrogen atoms and the oxygen atom in one conformation is a good fit for the NNO tridentate coordination mode, whereas the other would allow the NN coordination mode only. Two lanthanide complexes [Ln(tta)3(NNO)] (Ln = EuIII and LuIII; tta = thenoyltrifluoroacetone) were prepared and studied spectroscopically. The structures of the complexes were optimized by the DFT approach. The NNO ligand in the Eu complex displays tridentate NNO coordination, whereas the ligand is only NN-coordinated in the Lu complex. The Eu complex shows bright red metal-centered phosphorescence under excitation into the ligand (π–π*) absorption bands with a quantum yield of ca. 80 % and a lifetime of 580 μs. A mechanism for energy transfer between the ligands and metal centers was suggested and confirmed by DFT and time-dependent DFT (TDDFT) studies. An organic light-emitting diode (OLED) device based on the Eu complex incorporated into a poly(9-vinylcarbazole) (PVK) matrix was prepared. A study of the characteristics of the device revealed electrochromism of the system owing to variations in the efficacy of metal-centered and matrix emission at different strengths of applied electric field.