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.

High-efficiency near-infrared organic light-emitting devices based on an iridium complex with negligible efficiency roll-off

“…The oxidation process Fig. 36 On the other hand, complex 1 has a double reversible reduction process (Fig. The PL spectra were recorded using a red PMT (excited at 465 nm, corrected to 850 nm corresponding to the instrument limit) and an infrared-sensitive InGaAs photodiode (inset, excited at 400 nm), respectively.…”

DFT and Time-dependant DFT Investigation of eLectronic Structure, Phosphorescence and Electroluminescence Properties of Iridium (III) Quinoxaline Complexes

“…The main structural parameters are summarized in Table 1 together with the X-ray crystal structures data of 2. 7 The three complexes show a pseudo-octahedral coordination around the metal centers with the two N atoms residing at the trans location, and two carbon atoms at the cis location. Table 1 shows that the calculated bond lengths and bond angles are in good agreement with available crystal structural data, 7 and by changing ancillary ligands, three complexes show different bond lengths variation trend.…”

Red-Orange Emissive Cyclometalated Neutral Iridium(III) Complexes and Hydridoiridium(III) Complex Based on 2-Phenylquinoxaline : Structure, Photophysics and Reactivity of Acetylacetone Towards Cyclometalated Iridium Dimer

“…The bite angles at iridium metal are 79.87(11) and 80.34 (11) o for the cyclometalating ligands and 87.22 (8) o for acetylacetone ligand. In the iridium hydride complex, (pqx)IrH(acac) 2 (4), there are two discrete molecules in an unit cell which are related to mirror images each other as shown in Figure 2.…”

“…7 The reported complexes emitted from red-orange to deep red phosphorescence with high emission quantum yields depending on the substituent of cyclometalated ligands, and we suggested them as good triplet emitters for OLEDs applications. As an extension of our study on emissive iridium(III) complexes, herein, we synthesize cyclometalated neutral iridium(III) complexes using two 2-phenylquinoxaline-based main ligands and one acetylacetone ancillary ligand since the nitrogen-containing heterocycles showed orange to red phosphorescent emissions 8 and the ancillary ligand played an important role in tuning the emission color finely. 7,9 The emissive iridium(III) complexes were prepared via a reaction of acac with a dimeric iridium complex [(C^N) 2 Ir(μ-Cl)] 2 where C^N were 2-phenylquinoxaline-based ligands.…”