Effect of Aryl Substituents in Pyridyl-Imidazopyridine Ligands on the Aggregation-Induced Emission of Organoiridium(III) Complexes: Implications of 3MLCT and 3ILCT Excited States

In this study, three cyclometalated iridium(III) complexes, 1[PF6], 2[PF6], and 3[PF6], incorporating 3-aryl-substituted pyridyl imidazopyridine ligands, were synthesized and characterized using spectroscopic and single-crystal X-ray diffraction analysis. Photophysical studies revealed that the complexes 1[PF6] and 2[PF6] exhibited weak emission in acetonitrile but displayed pronounced aggregation-induced emission (AIE) in water-rich mixtures, whereas 3[PF6] remained nonemissive both in solution and in the aggregation state. Crystallographic analysis and DFT-optimized structure showed that the nearly parallel arrangement of the anthracenyl and 2-phenylpyridine moieties restricts intramolecular motion in 3[PF6] both in solution and aggregated state but does not show emission due to the population of a dark 3LC state. In contrast, the phenyl and naphthyl substituents in 1[PF6] and 2[PF6] undergo greater intramolecular motion in dilute solution, while their restriction upon aggregation promotes a strong emission enhancement. Additionally, biological response analysis of complexes 1[PF6] and 2[PF6] demonstrated low cytotoxicity toward HeLa cells and the complexes were successfully employed for live-cell imaging studies. This work provides new insights into an unusual fluorescence-quenching mechanism in cyclometalated iridium(III) complexes and offers valuable guidelines for the rational design of luminescent iridium-based materials.