Synthesis of New Europium Complexes and Their Application in Electroluminescent Devices

Several substituted phenanthrolines (L = pyrazino[2,3‐f][1,10]phenanthroline (PyPhen), 2‐methylpyrazino[2,3‐f][1,10]phenanthroline (MPP), dipyrido[3,2‐a:2′,3′‐c]phenazine (DPPz), 11‐methyldipyrido[3,2‐a:2′,3′‐c]phenazine (MDPz), 11,12‐dimethyldipyrido[3,2‐a:2′,3′‐c]phenazine (DDPz), and benzo[i]dipyrido[3,2‐a:2,3‐c]phenazine (BDPz)) were successfully prepared and europium complexes Eu(TTA)₃L (Eu‐L) based on these ligands were synthesized from EuCl₃, 2‐thenoyltrifluoroacetone (TTA) and L in good yields. Irradiation at the absorption band between 320–390 nm of all these europium complexes, except Eu‐BDPz, in solution or in the solid state leads to the emission of a sharp red band at ～ 612 nm, a characteristic Eu³⁺ emission due to the transition ⁵D₀ → ⁷F₂. No emission from the ligands was found. The result indicates that complete energy transfer from the ligand to the center Eu³⁺ ion occurs for these europium complexes. In contrast, the photoluminescence spectrum of Eu‐BDPz exhibits a strong emission at around 550 nm from the coordinated BDPz ligand and a weak emission at 612 nm from the central europium ion. Incomplete energy transfer from the ligand to the central Eu³⁺ ion was observed for the first time. Several electroluminescent devices ( A – I ) using Eu‐PyPhen, Eu‐MPP, Eu‐DPPz, and Eu‐DDPz as dopant emitters with the device configuration: TPD or NPB (50 nm)/Eu:CBP (1.7–7 %, 30 nm)/BCP (20–30 nm)/Alq (25–35 nm) (where TPD: 4,4′‐bis[N‐(p‐tolyl)‐N‐phenylamino]biphenyl; NPB: 4,4′‐bis[1‐naphthylphenylamino]biphenyl; CBP: 4,4′‐N,N′‐dicarbazole biphenyl; BCP: 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline; Alq: tris[8‐hydroxyquinoline]aluminum) were fabricated. Some of these devices emit saturated red light and are the only europium complex‐based devices that show a brightness of more than 1000 cd m^–2.