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 luminescent dyes for cell imaging in aqueous solution. Their different stain ability in cell imaging was fairly well supported by the experimental data based on the measurement of oil/water partition coefficients and encapsulation/release with liposomes.