Two Luminescent Iridium Complexes with Phosphorous Ligands and Their Photophysical Comparison in Solution, Solid and Electrospun Fibers: Decreased Aggregation-Caused Emission Quenching by Steric Hindrance

In this paper, we prepared two phosphorescent Ir complexes with ligands of 2-phenyl pyridine (ppy), and two phosphorous ligands with large steric hindrance, hoping to allow enough time for the transformation of the highly phosphorescent (3)MLLCT (metal-to-ligand-ligand-charge-transfer) excited state. Their large steric hindrance minimized the π-π interaction between complex molecules, so that the aggregation-induced phosphorescence emission (AIPE) influence could be minimized. Their single crystals indicated that they took a distorted octahedral coordination mode. Photophysical comparison between these Ir complexes in solution, in the solid state and in electrospun fibers was performed to confirm the realization of limited aggregation-caused quenching (ACQ). The potential surface crossing and energy transfer from (3)ML(BPE)CT/(3)ML(BPE)L(ppy)CT to (3)ML(ppy)CT in these Ir complexes were revealed by density functional theory calculation and temperature-dependent emission. It was confirmed that these two phosphorous ligands offered large steric hindrance, which decreased the ACQ effect, allowing the efficient emissive decay of the (3)ML(ppy)CT excited state. This is one of the several luminescent Ir complexes with a high emission yield (Φ = 0.27) and long emission lifetime (0.43 μs) in the solid state.