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‐9 H ‐fluoren‐2‐yl)‐1 H ‐imidazo[4,5‐ f ][1,10]phenanthrolin‐2‐yl}‐ N , N ‐diphenylbenzenamine (Phen‐Fl‐TPA) and its β‐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 Eu 3+ 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 ( 5 D 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 °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.