Effect of ancillary ligand on the photoluminescent and electroluminescent properties of blue Ir(III) complexes bearing main bipyridine ligand

Homoleptic- and heteroeptic-Ir(III) complexes based on bipyridine ligand for the direct comparison of photoluminescent and electroluminescent characteristics were developed. In virtue of adjusting the intermolecular interactions between adjacent molecules, both higher efficiency and more rapid radiative decay in excited state were achieved in hereroleptic complex, as compared to homoleptic counterpart. As the results, heteroleptic complex has one of the highest performance among the reported blue PhOLEDs with bipolar host (mCPPO1, 9-(3-(9H-Carbazol-9-yl)phenyl)-3-(diphenyl phosphoryl)-9H-carbazole), making it an outstanding blue triplet emitter in PHOLEDs with 26.7% of external quantum efficiency. [Display omitted] •Homoleptic and heteroleptic cyclometalated blue Ir(III) complexes, 1 and 2, developed.•Structures, photophysical properties, and electroluminescent characteristics compared.•1- and 2-based PhOLEDs investigated.•2-based PhOLED exhibited a high EQE of 26.7% Homoleptic and heteroleptic cyclometalated blue iridium(III) complexes, namely, 1 and 2, incorporating 2′,6′-dimethoxy-5-trimethylsilyl-2,3′-bipyridine (OMe2py-TMSpy) as the main ligand and acetylacetonate (acac) as the ancillary ligand, are developed for the comparison of their structure, photophysical properties, and electroluminescent characteristics. Both 1 and 2 exhibit distorted octahedral geometries around the iridium center, and the aspect ratios are 1.0 for 1 and 0.76 for 2, respectively. The interatomic contacts involving hydrogen bond interactions (H···N/O) in 1 are greater than those in 2, based on Hirshfeld surface analysis. TD-DFT calculations reveal that the electronic transition for 1 may be attributed to intra-ligand charge transfer (ILCT, πbpy−π*bpy) mixed with metal-to-ligand charge transfer (MLCT, Ird−π*bpy), while for 2, it is attributed to the combination of ILCT/MLCT and additional ligand-to-ligand charge transfer (LLCT, πacac−π*bpy). Owing to the introduction of the ancillary ligand, the emission of complex 2 is red-shifted by only 6 nm compared to complex 1. However, the photoluminescent quantum efficiency of 2 is higher than that of 1 owing to the high radiative decay rate. Furthermore, phosphorescent organic light-emitting diodes (PhOLEDs) based on 2 achieve a high external quantum efficiency of 26.7%, which is one of the highest performances observed among reported blue PhOLEDs.