Dinuclear iridium complexes with non-conjugated ditopic bis-terdentate cyclometallating ligands and their electroluminescence properties

Abstract

Two dinuclear iridium(III) complexes (3, 4) with a new bridged cyclometallating bis-tripodal phosphite ligand and two correspongding mononuclear complexes (1, 2) have been synthesized. The effects of different substituents and adding iridium atoms on the photophysical properties of these iridium complexes are studied. These dinuclear complexes have excellent thermal stability with decomposition temperatures exceeding 440 °C, which are more than 70 °C higher than that of mononuclear complexes. The dinuclear complexes possess higher quantum yields and lower nonradiative decay rates than the mononuclear complexes due to the increased heavy atom effect and molecular rigidity. The steric protective tertiary butyl groups on 2, 2′-bipyridine unit also improve the quantum yields and reduce the nonradiative decay rates of iridium complexes. The dinuclear complexes exhibit higher oxidation potential (0.579–0.594 V) than the mononuclear complexes (0.558–0.560 V). The organic light-emitting diodes based on complex 2 shows a turn-on voltage of 3.8 V, high maximum brightness of 9550 cd m−2 with moderate external quantum efficiency (EQE) of 5.1%. The device based on complex 4 shows lower turn-on voltage of 3.2 V, higher maximum brightness of 9639 cd m−2, but with lower EQE of 2.2% because of the serious concentration quenching.