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, P1̄;
a = 11.357(3) Å, b = 12.945(3) Å, c = 12.947(3) Å, 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 (Tg = 204 °C),
which is 27 °C higher than that of tris(8-hydroxyquinolinolate)aluminum (Alq3). In addition, a high photoluminescence
efficiency (φPL) 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.