Efficient and Long-Time Stable Red Iridium(III) Complexes for Organic Light-Emitting Diodes Based on Quinoxaline Ligands

Abstract

We report the design and characterization of three heteroleptic orange-red phosphorescent iridium(III) complexes bearing two 2-(4-fluorophenyl)-3-methyl-quinoxaline (fpmqx) cyclometalated ligands combined with three different ancillary ligands, triazolylpyridine (trz), picolinate (pic), and acetylacetonate (acac). All of these complexes emit an orange to red color in the spectral range of 605-628 nm in dichloromethane. Strong spin-orbit coupling of the iridium atom allows the formally forbidden mixing of singlet and triplet states. Because of the structureless phosphorescent line shapes and low Stokes shifts between triplet metal-to-ligand charge-transfer ((3)MLCT) absorption and phosphorescent emission, we propose that emission originates predominantly from the (3)MLCT state with a lesser admixture of totally ligand-based (3)(pi-pi*) states. The influence of 5d-electron densities of the iridium center on highest occupied molecular orbitals leads to high emission quantum yields in toluene (Phi(p) = 0.39-0.42) and to short triplet lifetimes. Cyclovoltammetry measurements show reversible oxidation peaks from 0.74 to 0.92 V and reversible reduction waves with potentials ranging from -1.58 to -2.05 V versus Cp(2)Fe/Cp(2)Fe(+). All complexes have been applied in simple test devices and also in stable, long-living devices to evaluate their electroluminescent device performances, for which we especially report the influence of the chosen ancillary ligands on emission colors, efficiencies, and device lifetimes. We obtained narrowband emission ranging from 613 to 630 nm with a full width at half-maximum of 64-71 nm, and a maximum in power efficiency of eta(p) = 14.6 lm/W at a current density of J = 0.01 mA/cm(2) for [(fpmqx)(2)Ir(pic)]. The operating lifetimes of [(fpmqx)(2)Ir(trz)] in both neat and mixed matrixes were longer than that of the established stable tris(1-phenylisoquinolinato)iridium(III) [Ir(piq)(3)]. From the lifetime measurements, it becomes clear that the stability is strongly correlated to the type of ancillary ligand. An extrapolated lifetime of 58 000 h with an initial brightness of 1000 cd/m(2), together with a very low voltage increase of 0.2 V over a time period of 1000 h (starting voltage of 4.1 V), was achieved. Such a high device lifetime is attributed to the chemical stability of all materials toward both charge carriers and excitons.