Impacts of ancillary ligand coordination modes on red-emitting cyclometalated iridium complexes

Abstract

Cyclometalated iridium complexes are widely used in optoelectronic technologies, but creating efficient red emitters remains challenging. Prior investigations have demonstrated the efficiency of electron-rich salicylaldimine and 2-picolinamide ligands in promoting red to deep red luminescence. This work introduces a series of ten new red or deep-red emitting heteroleptic bis-cyclometalated iridium(III) complexes supported by eight different ancillary ligands, including some from the salicylaldimine and 2-picolinamide families. Our study reveals that the effectiveness of salicylaldimine ligands at supporting efficient red phosphorescence is dependent on the cyclometalating ligand they are paired with. A more significant finding is that the 2-picolinamide ligands can adopt three different coordination modes. Firstly, the N-propyl-substituted 2-picolinamide proligand can bind to the iridium center in its neutral, protonated form through the pyridyl N and amide O atoms, forming five-membered metallacycles. Furthermore, N-aryl-substituted ligands can coordinate with the iridium center in either N,N′ or N(pyridyl),O modes, yielding two structurally distinct isomers. Notably, the change in coordination mode minimally influences emission wavelength while significantly modulating the photoluminescence quantum yield. This study advances our comprehension of how ligand coordination impacts cyclometalated iridium complexes, offering invaluable insights into the design of high-performance red phosphors for potential optoelectronic applications.