Cyclometalated Anionic Benzimidazole-Based Luminescent Iridium Complexes with Tunable Organelle-Specific Cell Imaging

Herein, we present an example of a neutral [ 1 ] and a cationic [ 2 ] heteroleptic cyclometalated iridium complex incorporating benzimidazole-based ligands, where the N ∧ N benzimidazole ligand coordinates in its anionic form. The formation and molecular integrity of the complexes were confirmed by HRMS and multinuclear NMR spectroscopy, while single-crystal X-ray crystallography verified the neutral nature of [ 1 ] and the anionic coordination of the N ∧ N ligand. Complex [ 2 ] is water-soluble, whereas [ 1 ] shows reduced solubility in most solvents. Both complexes are luminescent, exhibiting better emission quantum yields and lifetimes (∼1.0 μs) in water than other solvents. Density functional theory (DFT) studies show that the HOMO is mainly localized on the iridium center and the N ∧ N benzimidazole ligand, while the LUMO is primarily localized on the C ∧ N benzimidazole ligand with minimal iridium contribution. Spin density analysis indicates that the emission originates from 3 MLLCT states. Both complexes show moderate cytotoxicity (IC 50 ∼ 15 μM) in HeLa and B16F10 cancer cells and CHO normal cells. Additionally, the cellular imaging of the complexes in HeLa cells revealed that complex [ 1 ] has specific localization in the endoplasmic reticulum (ER), whereas complex [ 2 ] has specific localization in mitochondria, demonstrating their potential as organelle-specific phosphorescent probes for bioimaging.