Luminescent Cyclometalated Iridium(III) Polypyridine Di-2-picolylamine Complexes: Synthesis, Photophysics, Electrochemistry, Cation Binding, Cellular Internalization, and Cytotoxic Activity

A series of luminescent cyclometalated iridium (III) polypyridine complexes containing a di-2-picolylamine (DPA) moiety [Ir(N boolean AND C)(2)(phen-DPA)](PF6) (phen-DPA = 5-(di-2-picolylamino)-1,10-phenanthroline) (HN boolean AND C = 2-phenylpyridine, Hppy (1a), 2-(4-methylphenyl)pyridine, Hmppy (2a), 2-phenylquinoline, Hpq (3a), 4-(2-pyridyl)benzaldehyde, Hpba (4a)) and their DPA-free counterparts [Ir(N boolean AND C)(2)(phen-DMA)](PF6) (phen-DMA = 5-(dimethylamino)-1,10-phenanthroline) (HN boolean AND C = Hppy (1b), Hmppy (2b), Hpq (3b), Hpba (4b)) have been synthesized and characterized, and their photophysical and electrochemical properties investigated. Photoexcitation of the complexes in fluid solutions at 298 K and in alcohol glass at 77 K resulted in intense and long-lived luminescence. The emission of the complexes has been assigned to a triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (d pi(Ir)->pi*(N boolean AND N)) or triplet intraligand ((IL)-I-3) (pi ->pi*) (N boolean AND C) excited state and with substantial mixing of triplet amine-to-ligand charge-transfer ((NLCT)-N-3) (n ->pi*) (N boolean AND N) character, depending on the identity of the cyclometalating and diimine ligands. Electrochemical measurements revealed an irreversible amine oxidation wave at ca. +1.1 to +1.2 V vs saturated calomel electrode, a quasi-reversible iridium (IV/III) couple at ca. +1.2 to +1.6 V, and a reversible diimine reduction couple at ca. -1.4 to -1.5 V. The cation-binding properties of these complexes have been studied by emission spectroscopy. Upon binding of zinc ion, the iridium(III) DPA complexes displayed 1.2- to 5.4-fold emission enhancement, and the K-d values determined were on the order of 10(-5) M. Job's plot analysis confirmed that the binding stoichiometry was 1:1. Additionally, selectivity studies showed that the iridium(III) DPA complexes were more sensitive toward zinc ion among various transition metal ions examined. Furthermore, the cytotoxicity of these complexes toward human cervix epithelioid carcinoma cells have been studied by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide assay and their cellular-uptake properties by inductively coupled plasma mass spectrometry and laser-scanning confocal microscopy.