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-<i>o</i>-aminophenolato)-bis(8-quinolinolato)-bis-gallium(III) [Ga<sub>2</sub>(saph)<sub>2</sub>q<sub>2</sub>], has been synthesized and structurally characterized by single-crystal X-ray crystallography. Crystal data for C<sub>44</sub>H<sub>30</sub>Ga<sub>2</sub>N<sub>4</sub>O<sub>6</sub> are as follows:  space group, triclinic, <i>P</i>1̄; <i>a</i> = 11.357(3) Å, <i>b</i> = 12.945(3) Å, <i>c</i> = 12.947(3) Å, <i>α</i> = 103.461(15)°, <i>β</i> = 100.070(7)°, <i>γ</i> = 96.107(18)°, <i>Z</i> = 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 Ga<sub>2</sub>(saph)<sub>2</sub>q<sub>2</sub> can readily form a stable amorphous glass with a high glass transition temperature (<i>T</i><sub>g</sub> = 204 °C), which is 27 °C higher than that of tris(8-hydroxyquinolinolate)aluminum (Alq<sub>3</sub>). In addition, a high photoluminescence efficiency (φ<sub>PL</sub>) 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 Ga<sub>2</sub>(saph)<sub>2</sub>q<sub>2</sub> devices with 4-(dicyanomethylene)-2-<i>tert</i>-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4<i>H</i>-pyran (DCJTB) as the dopant exhibited excellent red emission centered at 628 nm with improved durability, compared with the case of Alq<sub>3</sub> as the host. These distinguishing properties of Ga<sub>2</sub>(saph)<sub>2</sub>q<sub>2</sub> make it a good candidate as a novel electron-transporting and emitting material for OLEDs.