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)]+PF6– (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 λmax,peak at 698 nm and λmax,shoulder at 760 nm in CH2Cl2 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.