A Dual-Characteristic Bidentate Ligand for a Ternary Mononuclear Europium(III) Molecular Complex - Synthesis, Photophysical, Electrochemical, and Theoretical Study

The new bipolar ligand 4-{1-(9,9-diethyl-9H-fluoren-2-yl)-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl}-N,N-diphenylbenzenamine (Phen-Fl-TPA) and its -diketonate Eu-III complex have been designed, synthesized, characterized, and their photophysical and electrochemical properties have been investigated. The UV/Vis absorption and photoluminescence (PL) emission spectra of the Eu complex and Phen-Fl-TPA in solution as well as in the solid state and as thin films were recorded. The PL study indicated that the Eu complex emits narrow-band red emission with an appropriate International Commission on Illumination (CIE) color gamut. Moreover, it also confirmed the efficient energy transfer from the ligand to the Eu3+ ion. Time-dependent DFT (TD-DFT) calculations were performed for the ligand to determine the exact positions of the excited singlet and triplet energy levels. The Eu-III complex shows a dominant pathway involving energy transfer between the ligand triplet level and the excited (D-5(0)) level of the Eu-III ion. In addition, the theoretically calculated UV/Vis spectrum of the ligand is similar to the experimental one. The Judd-Ofelt theory was applied to the emissive properties of the Eu-III complex, and the lifetime was found to be 0.66 ms. The ground-state optimized ligand structure was a good match with the single-crystal structure. The photoluminescence quantum yield (PLQY) of Eu(TTA)(3)Phen-Fl-TPA (TTA = thenoyltrifluoroacetone) in solution was 34.1%, and it possess a high thermal decomposition temperature (317 degrees C), as determined by differential scanning calorimetry/thermogravimetric analysis (DSC-TGA). Electrochemical analysis revealed highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for the Eu-III complex of 5.6 and 2.8 eV, respectively.