Iridium(III) Complexes with Orthometalated Quinoxaline Ligands: Subtle Tuning of Emission to the Saturated Red Color

Abstract

Rational design and syntheses of four iridium complexes (1−4) bearing two substituted quinoxalines and an additional 5-(2-pyridyl) pyrazolate or triazolate as the third coordinating ligand are reported. Single-crystal X-ray diffraction studies of 1 reveal a distorted octahedral geometry, in which two dpqx ligands adopt an eclipse configuration, for which the quinoxaline N atoms and the C atoms of orthometalated phenyl groups are located at the mutual trans- and cis-positions, respectively. The lowest absorption band for all complexes consists of a mixture of heavy-atom Ir(III)-enhanced ³MLCT and ³ππ* transitions, and the phosphorescent peak wavelength can be fine-tuned to cover the spectral range of 622−649 nm with high quantum efficiencies. The cyclic voltammetry was measured, showing a reversible, metal-centered oxidation with potentials at 0.76−1.03 V, as well as two reversible reduction waves with potentials ranging from −1.61 to −2.06 V, attributed to the sequential addition of two electrons to the more electron-accepting heterocyclic portion of two distinctive cyclometalated C∧N ligands. Complex 1 was used as the representative example to fabricate the red-emitting PLEDs by blending it into a PVK−PBD polymer mixture. The devices exhibited the characteristic emission profile of 1 with peak maxima located at 640 nm. The maximum external quantum efficiency was 3.15% ph/el with a brightness of 1751 cd/m² at a current density of 67.4 mA/cm², and the maximum brightness of 7750 cd/m² was achieved at the applied voltage of 21 V and with CIE coordinates of (0.64, 0.31).