Highly-efficient solution-processed deep-red organic light-emitting diodes based on heteroleptic Ir(III) complexes with effective heterocyclic Schiff base as ancillary ligand

Abstract

Abstract Three new deep-red heteroleptic phosphorescent iridium(III) complexes Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3), comprising cyclometalated ligand 1-pheynylisoquionoloine(piq) and heterocyclic Schiff base ancillary ligands 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol(L1), 3-methyl-1-phenyl-4-(phenylimino)methyl-1H-pyrazol-5-ol (L2), and 4-(4-methoxyphenyl)imino-methyl-3-methyl-1-phenyl-1H-pyrazol-5-ol (L3) have been designed, synthesized, and characterized. All the compounds emit deep red emission with λmax values in the spectral range of 602–620 nm, high quantum yield 0.44 to 0.52 and short excited state lifetime τ (0.51–0.55 μs) due to dominant strong field ligands, resulting an efficient triplet metal-ligand charge transfer (3MLCT) excited state. Time dependent density functional theory (TD-DFT) calculations and electrochemical measurements of the compounds strongly support their genuine deep red phosphorescent emission. The combination of ancillary and cyclometalated ligands significantly influence the molecular orbitals of Ir(III) complex, leading to clearly distinct electron density distributions of the LUMO and HOMO. The compounds show good thermal stability and quantum yield, these characteristics making them an ideal candidate to exploit in phosphorescent organic light emitting diodes (PhOLEDs). Highly-efficient PhOLEDs were developed by using Ir(piq)2(L1), Ir(piq)2(L2), and Ir(piq)2(L3) in solution process as deep red emitters and device composed of Ir(piq)2(L3) exhibited an excellent external quantum efficiency of 14.9% and current efficiency of 10.8 cd/A with the stable CIE coordinates of (0.67,0.33).