Near-Infrared-Emitting Iridium(III) Complexes as Phosphorescent Dyes for Live Cell Imaging

The three near-infrared-emitting cationic iridium(III) complexes [Ir(pbq- g ) 2 (N ∧ N)] + PF 6 – (pbq- g = phenylbenzo[ g ]quinoline; N ∧ N = bipyridine ( 1 ), 1,10-phenanthroline ( 2 ), 4,7-diphenyl-1,10-phenanthroline ( 3 )) have been demonstrated as phosphorescent dyes in live cell imaging. These complexes with different ancillary ligands show similar near-infrared (NIR) emission with λ max,peak at 698 nm and λ max,shoulder at 760 nm in CH 2 Cl 2 solutions, with a moderate quantum yield of around 3%. However, these complexes behave quite differently as NIR dyes for live cell imaging. Complexes 1 and 2 exhibit exclusive staining in the cytoplasm with good cell membrane permeability under excitation at 488 nm, while 3 gives almost no cell uptake, as further determined by flow cytometry. Although the lipophilicities of these complexes follow the order 1 < 2 < 3, their cytotoxicities are in the reverse order. The exceptionally low cytotoxicity of 3 could be attributed to its poor solubility in aqueous buffer and thus substantially low exposure dose. This comparative study suggested that the ancillary ligands could fine-tune the amphiphilicity and cytotoxicity of the cyclometalated iridium(III) complexes and thus might play a key role in the design of NIR-emitting iridium(III) complexes for practical applications in bioimaging.