Synthesis, Crystal Structure, and Luminescent Properties of a Binuclear Gallium Complex with Mixed Ligands

By introducing tridentate Schiff base ligands, a binuclear gallium complex with mixed ligands, bis(salicylidene-o-aminophenolato)-bis(8-quinolinolato)-bis-gallium(III) [Ga₂(saph)₂q₂], has been synthesized and structurally characterized by single-crystal X-ray crystallography. Crystal data for C₄₄H₃₀Ga₂N₄O₆ are as follows:? space group, triclinic, P1?; a = 11.357(3) ?, b = 12.945(3) ?, c = 12.947(3) ?, α = 103.461(15)°, β = 100.070(7)°, γ = 96.107(18)°, Z = 2. This complex was identified as a dimeric complex of hexacoordinated gallium with strong intermolecular and intramolecular π?π stacking interactions between the pyridyl/pyridyl rings. The thermal analysis showed that Ga₂(saph)₂q₂ can readily form a stable amorphous glass with a high glass transition temperature (T_g = 204 °C), which is 27 °C higher than that of tris(8-hydroxyquinolinolate)aluminum (Alq₃). In addition, a high photoluminescence efficiency (φ_PL) of 0.318 in DMF has been demonstrated, although the central gallium atom can result in heavy-atom quenching. Organic light-emitting diodes (OLEDs) based on this complex displayed a turn-on voltage as low as 2.5 V and a high efficiency. Even at a low doping concentration of 1%, the doped Ga₂(saph)₂q₂ devices with 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as the dopant exhibited excellent red emission centered at 628 nm with improved durability, compared with the case of Alq₃ as the host. These distinguishing properties of Ga₂(saph)₂q₂ make it a good candidate as a novel electron-transporting and emitting material for OLEDs.