Zwitterionic iridium complexes: synthesis, luminescent properties, and their application in cell imaging.

A series of phosphorescent zwitterionic iridium(III) complexes, with 4-carboxy-2, 2′-bipyridine-4′-carboxylate (Hdcbpy) as ancillary ligand, Ir(C∧N)2(Hdcbpy) (C∧N = 1-phenylpyrazole (ppz), 1-phenyl-pyridine (ppy), 2-(4′,6′-difluoro-phenyl)pyridine (dfppy), 1-phenyl-isoquinoline (piq), dibenzo[f,h]quinoxaline (dbq)), were prepared and characterized. Their photophysical properties were studied, and intense luminescence emissions were observed based on metal-to-ligand-charge-transfer (3MLCT), ligand-to-ligand charge-transfer (3LLCT), ligand-centered transitions (3LC, i.e., 3π → π*), or intraligand-charge-transfer (3ILCT) excited states, which were confirmed by theoretical calculations. The quantum yield of Ir(dfppy)2(Hdcbpy) is as high as 0.106 in aqueous solution. With Hdcbpy as a hydrophilic part, their amphiphilic structures as further confirmed by X-ray single crystal data endow them with different solubilities in phosphate buffer solution (PBS, pH 7.0). The compounds were successfully applied as lumines...