Blue heteroleptic iridium(iii) complexes for OLEDs: simultaneous optimization of color purity and efficiency

Abstract

High-performance blue phosphorescent iridium(III) complexes having good efficiency and color purity simultaneously are still a huge challenge. Two blue phosphorescent iridium(III) complexes, (dfdmappy)₂Ir(phim) (Ir1) and (dfdmapypy)₂Ir(phim) (Ir2), were developed by employing 2-(2,4-difluorophenyl)-N,N-dimethylpyridin-4-amine (dfdmappy) and 2′,6′-difluoro-N,N-dimethyl-[2,3′-bipyridin]-4-amine (dfdmapypy) as cyclometalating ligands and N-heterocyclic carbene (phim) as an ancillary ligand. The introduction of a strong electron-donating dimethylamino (dma) group onto the N-coordinating pyridine ring of the main ligands and the adoption of high field-strength phim effectively raise the cyclometalating ligand-centered emitting triplet states (i.e.³LC). Meanwhile, the phim ligand with strong σ-donating electron character destabilizes the non-radiative d–d*state as well. These strategies make both complexes achieve charge transfer (CT) state dominated emission, i.e., metal-to-ligand/ligand-to-ligand charge transfer (³MLCT/³LLCT) dominated emission from the T₁ state and resulted in Ir1 and Ir2 single-peak blue phosphorescence with high photoluminescence quantum yields (PLQYs) of 91% and 60%, respectively, and shorter excited-state lifetimes of 1.10 and 3.33 μs, respectively. Quantum chemical calculations verified the CT-dominated feature and lower root-mean-square displacement/deviation (RMSD) value of Ir1versus more metal centered d–d*transition and larger RMSD value of Ir2, well accounting for the higher PLQY and superior color purity of Ir1. Pure blue organic light-emitting diodes (OLEDs) of Ir1 exhibit a maximum external quantum efficiency (EQE) of 28% with Commission Internationale de I’Eclairage (CIE) coordinates of (0.16, 0.21), which is one of the best performances for blue phosphorescent OLEDs reported so far.