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

Abstract

Homoleptic and heteroleptic cyclometalated blue iridium(III) complexes, namely, 1 and 2, incorporating 2 & PRIME;,6 & PRIME;-dimethoxy-5-trimethylsilyl-2,3 & PRIME;-bipyridine (OMe(2)py-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, pi(bpy)-pi*(bpy)) mixed with metal-to-ligand charge transfer (MLCT, Ir-d-pi*(bpy)), while for 2, it is attributed to the combination of ILCT/MLCT and additional ligand-to-ligand charge transfer (LLCT, pi(acac)- pi*(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.