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 (MLCT)-M-3 and (3)pipi(*) 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 voltarnmetry 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 CAN 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/M-2 at a current density of 67.4 mA/cm(2) and the maximum brightness of 7750 cd/m(2) was achieved at the applied voltage of 21 V and with CIE coordinates of (0.64, 0.31).