Synthesis and electroluminescent property of novel europium complexes with oxadiazole substituted 1,10-phenanthroline and 2,2′-bipyridine ligands

Abstract

Several 1,10-phenanthroline derivatives (PhoR), 2,2′-bipyridine derivatives (BpoR) and related europium complexes Eu(TTA)₃PhoR and Eu(TTA)₃BpoR were synthesized (HTTA = 2-thenoyltrifluoroacetone). Single crystal X-ray diffraction of Eu(TTA)₃Php (Php = 2-(pyridyl)-1,10-phenanthroline) shows that Php acts as a tridentate N⁁N⁁N ligand, leading to a high stability of the complex for vacuum evaporation. When an oxadiazole moiety is incorporated into the ligand, the corresponding europium complexes show improved carrier-transporting abilities as well as thermal stabilities under vapor deposition for electroluminescence (EL) applications. Experiments revealed that these complexes have high photoluminescence (PL) quantum yields due to suitable triplet energy levels (E_T) of the ligands, between 19 724 and 22 472 cm⁻¹, for the sensitization of Eu(III) (⁵D₀: 17 500 cm⁻¹). Utilizing Eu(TTA)₃PhoB (PhoB = 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-1,10-phenanthroline) as the dopant emitter in CBP (4,4′-N,N′-dicarbazolebiphenyl), EL devices with a structure of TPD (4,4′-bis[N-(p-tolyl)-N-phenylamino] biphenyl, 30 nm)/Eu(TTA)₃PhoB:CBP (7.5%, 20nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 20 nm)/Alq₃ (tris(8-hydroxyquinoline), 30 nm) exhibited a pure emission from europium ions. The highest efficiency obtained was 5.5 lm W⁻¹, 8.7 cd A⁻¹ and the maximum brightness achieved was 1086 cd m⁻². At a practical brightness of 100 cd m⁻², the efficiency remains above 2.0 cd A⁻¹.