Synthesis, characterization, and properties of luminescent organoiridium(III) polypyridine complexes appended with an alkyl chain and their interactions with lipid bilayers, surfactants, and living cells

A series of new luminescent organoiridium(III) polypyridine complexes [Ir(N-C)₂(N-N)](PF₆) (HN-C = 2-phenylpyridine, Hppy, N-N = 4-n-octadecylaminocarbonyl-4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C₁₈H₃₇) (1a), 4-n-decylaminocarbonyl- 4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C₁₀H₂₁) (1b), 4-ethylaminocarbonyl-4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C₂H₅) (1c); HN-C = 1-phenylpyrazole, Hppz, N-N = Me-bpy-CONH-C₁₈H₃₇ (2a), Me-bpy-CONH-C ₁₀H₂₁, (2b), Me-bpy-CONH-C₂H₅ (2c); HN-C = 2-phenylquinoline, Hpq, N-N = Me-bpy-CONH-C₁₈H₃₇ (3a), Me-bpy-CONH-C₁₀H₂₁ (3b), Me-bpy-CONH-C ₂H₅ (3c)) bearing an alkyl pendant have been synthesized and characterized. The photophysical and electrochemical properties of these complexes have been investigated. Upon irradiation, all the complexes exhibited intense and long-lived luminescence in homogeneous fluid solutions at 298 K and in alcohol glass at 77 K. The emission has been assigned to a triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Ir) →*(diimine)) excited state. The emissive states of the pq complexes 3a-c are probably mixed with some triplet intraligand (³IL) (π → *) (pq) character. All the complexes have been incorporated into phospholipid vesicles composed of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and the resulting liposomes have been examined by cryogenic transmission electron microscopy (cryo-TEM) and luminescence spectroscopy. Also, the emission properties of the complexes in aqueous solutions containing the surfactants sodium dodecylsulfate (SDS), Triton X-100 (TX), and cetyltrimethylammonium bromide (CTAB) have been studied. The lipophilicity of the complexes has been determined by reversed-phase HPLC, and the log P_o/w values were dependent on the cyclometalating and diimine ligands. Additionally, the cytotoxicity of these organoiridium(III) complexes toward the human cervix epithelioid carcinoma (HeLa) cell line has been evaluated by the 3-(4,5-dimethyl-2-thiazolyl)-2,5- diphenyltetrazolium bromide (MTT) assay. Furthermore, the cellular uptake of all the complexes by HeLa cells has been examined by flow cytometry and laser-scanning confocal microscopy. © 2008 American Chemical Society.