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, <b>1[PF</b>_<b>6</b><b>], 2[PF</b>_<b>6</b><b>]</b>, and <b>3[PF</b>_<b>6</b><b>]</b>, 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 <b>1[PF</b>_<b>6</b><b>]</b> and <b>2[PF</b>_<b>6</b><b>]</b> exhibited weak emission in acetonitrile but displayed pronounced aggregation-induced emission (AIE) in water-rich mixtures, whereas <b>3[PF</b>_<b>6</b><b>]</b> 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 <b>3[PF</b>_<b>6</b><b>]</b> 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 <b>1[PF</b>_<b>6</b><b>]</b> and <b>2[PF</b>_<b>6</b><b>]</b> undergo greater intramolecular motion in dilute solution, while their restriction upon aggregation promotes a strong emission enhancement.