Phosphorescence imaging of living cells with amino acid-functionalized tris(2-phenylpyridine)iridium(III) complexes

A series of nine luminescent cyclometalated octahedral iridium(III) tris(2-phenylpyridine) complexes has been synthesized, functionalized with three different amino acids (glycine, alanine, and lysine), on one, two, or all three of the phenylpyridine ligands. All starting complexes and final compounds have been fully analyzed by one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy, and photophysical data have been obtained for all the mono-, bis-, and tri- substituted iridium(III) complexes. Cellular uptake and localization have been studied with flow cytometry and confocal microscopy, respectively. Confocal experiments demonstrate that all nine substituted iridium(III) complexes show variable uptake in the tumor cells. The monosubstituted iridium(III) complexes give the highest cellular uptake, and the series substituted with lysines shows the highest toxicity. This systematic study of amino acid-functionalized Ir(ppy)(3) complexes provides guidelines for further functionalization and possible implementation of luminescent iridium complexes, for example, in (automated) peptide synthesis or biomarker specific targeting.