Preparation and fluorescence properties of ternary complexes of Eu(BA) ₃PIP and Eu(BA)₃phen

Rare earth complex materials have been widely used in cathodoluminescent display phosphor screens, lasers and lamps because their photoluminescence (PL) exhibits high quantum efficiencies and very sharp spectral bands. The luminescence process of the rare earth complex is as follows; the electrons in excited single state of the ligand move to the triplet state by intrasystem relaxation process, followed by the energy transfers from the triplet state of the ligand to the nearest resonance Ievel of the rare earth ion. In the lumines-cence process, the single state and triplet state can only be used. Theoretically, the internal quantum efficiency can reach 100% , which is four tinies than that of other fluorescent materials. When europium ion chelates with some organic ligands, such as benzoic acid (BA) , the emissions of rare earth Eu + ion exhibits highly pure red color and strong luminescence intensity. The intensity of sensitized luminescence of central lanthanide ions in a rare earth organic chelate depends on three factors. Firstly, the suitability of the energy gap between the excited triplet energy Ievel of the ligands and the lowest excited energy Ievel of rare earth ions. Secondly, the rigidity and planarity of the structure of the chelate molecule. Thirdly, the existence of a suitable secondary ligand which may increase the rigidity and the stability of the chelate molecule. In this paper, 1,10-phenanth-roline (phen) derivative, 2-phenyl-imidazo [4,5-f] 1, 10-phenanthroline (PIP) were synthesized. With benzoic acid(BA) as the first ligand and PIP as the secondary ligand, a new ternary organic europium com-plex Eu( BA)₃PIP was prepared. In the same condition, with the same first ligand and phen as the secondary ligand, Eu(BA)₃phen was also prepared. The structures and compositions of the ligand PIP, Eu(BA)₃PIP and Eu(BA)₃phen were confirmed by elemental analysis, IR spectra, TG analyses, the fluorescence proper-ties of the two complexes were also studied. Elemental analysis demonstrates that the compositions of the temary complexes are PIP, Eu(BA)₃PIP and Eu(BA)₃phen. IR spectra indicate that rare earth Eu + ion is coordi-nated with six oxygen atoms of three benzoic acid ligands and two nitrogen atoms of the second ligand. The emission spectra dcmonstrate that two kinds of temary organic europium complex exhibit characteristic fluorescence of rare earth Eu³⁺ and the second ligand PIP has larger conjugation system, and under UV excitation, energy absorbed by the ligands in the complex can be transfed to die rare earth Eu + ion, so that it can emit characteristic luminescence of Eu³⁺ , and the luminescence intensity of Eu(BA) ₃PIP is obviously higher than that of Eu(BA)₃phen. The luminescence of complex was greatly sensitized by appropriate secondary ligand. TG analyses show that the thermal decomposition temperature of Eu(BA) ₃PIP are higher than that of Eu(BA)₃phen. Thus the secondary ligand of 2-phenyl-imidazo [4,5-f] 1, 10-phenanthroline (PIP) obviously enhanced the fluorescent intensity of Eu³⁺ and the thermal stability of europium complex. So, the new ternary organic europium complex of Eu(BA)₃PIP is an excellent red-emitting material.