Synthesis, Properties, and live-cell imaging studies of luminescent cyclometalated iridium(111) polypyridine complexes containing two or three biotin pendants

Three luminescent cyclometalated iridium(Ill) bis-biotin complexes [lr(N-C)₂(N-N)](PF₆) (HN-C = 2-(4-(N-(6-(biotinamido) hexyl)amlnomethyl)phenyl)pyridine, HppyC6B, N∧N = 2,2′-bipyridine, bpy (1); HN-C = 2-phenylpyridine, Hppy, N∧N = 4,4′-bis((2-(biotinamido) ethyl)amlnocarbonyl)-2,2′-bipyridine, bpyC2B2 (2); HN∧C = Hppy, N-N = 4,4′-bis((2-((6-(biotinamido)hexanoyl)amino)ethyl)aminocarbonyl)-2, 2′-bipyridine, bpyC2C6B2 (3)) and one tris-biotin complex [lr(ppyC6B) ₂(bpyC6B)](PF₆) (bpyC6B = 4-((6-(biotinamido)hexyl) aminocarbonyl)-4′-methyl-2,2′-bipyridlne) (4) have been synthesized and characterized. The biotin-free complex [lr(ppy)₂(bpyC4)](PF ₆) (bpyC4 = 4,4′-bis(n-butylaminocarbonyl)-2,2′- bipyridine) (5) has also been prepared for comparison studies. Upon photoexcitation, all the complexes displayed intense and long-lived greenish-yellow to red triplet metal-to-ligand charge-transfer ( ³MLCT) (dπ (Ir) →π^z.ast(N6N)) emission in fluid solutions at room temperature and in low-temperature glass. Cyclic voltammetric studies revealed iridium(IV/lll) oxidation at about +1.21 to + 1.29 V and diimine-based reductions at about -1.07 to -1.39 V versus SCE. The interactions of the bis-biotin and tris-biotin complexes with avidin have been studied by 4′-hydroxyazobenzene-2-carboxylic acid (HABA) assays, emission titrations, and dissociation assays. The possibility of these complexes as cross-linkers for avidin has been examined by microscopy studies using avidin-conjugated green fluororescent microspheres and size-exclusion HPLC analysis. Utilization of these luminescent iridium(111) biotin complexes in signal amplification has been demonstrated using avidin-coated nonfluorescent microspheres and complex 3 as an example. Additionally, the lipophilicity of all the complexes has been determined by reversed-phase HPLC. The cytotoxicity of these iridium(111) complexes toward the human cervix epithelioid carcinoma (HeLa) cell line has been evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assays. Furthermore, the cellular uptake of the complexes has been examined by ICP-MS, laser-scanning confocal microscopy, and flow cytometry. © 2009 American Chemical Society.