Bi-nuclear luminescent europium(III) molecular complexes for white light emitting diodes: Experimental and theoretical study

Two newly designed and synthesized ancillary ligands with N1 functionalization (phenyl and fluorene) and its influence on photophysical properties of their corresponding binuclear europium complexes were investigated systematically. Photoluminescence (PL) emission spectral study was shown dominant electric dipole transition of Eum ion (D-5(0) -> F-7(2)) and it is clearly indicated that the Eu-III ion occupies in the non-centrosymmetric site. The calculated parameter via Judd-Ofelt shown ligands are the best sensitizer for Eu-III metal ion. In addition, there is no emission in the region of 400-550 nm (belongs to ligand), indicates that the efficient energy transfer from ligand to central Eu-III ion in the complex. The energy transfer mechanism was proposed by the help of DFT and TD-DFT calculations. The obtained highest QY (59.5%, thin film) of the complex, Eu-2(TTA)(6)(L2) indicating efficient energy sensitization from fluorene-based ligand than that of phenyl based Eu-complex. The calculated CIE color coordinates are well matched with NTSC standard values. The binuclear Eu-complexes were combined with InGaN near UV LED, obtained pure red emission (forward bias 20 mA) with CIE color coordinate value x = 0.65, y = 0.34 and x = 0.66, y = 0.33 for Eu-2(TTA)(6)(L1) and Eu-2(TTA)(6)(L2), respectively.