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)](PF 6 ) (HN−C = 2-phenylpyridine, Hppy, N−N = 4- n -octadecylaminocarbonyl-4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C 18 H 37 ) ( 1a ), 4- n -decylaminocarbonyl-4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C 10 H 21 ) ( 1b ), 4-ethylaminocarbonyl-4′-methyl-2,2′-bipyridine (Me-bpy-CONH-C 2 H 5 ) ( 1c ); HN−C = 1-phenylpyrazole, Hppz, N−N = Me-bpy-CONH-C 18 H 37 ( 2a ), Me-bpy-CONH-C 10 H 21 ( 2b ), Me-bpy-CONH-C 2 H 5 ( 2c ); HN−C = 2-phenylquinoline, Hpq, N−N = Me-bpy-CONH-C 18 H 37 ( 3a ), Me-bpy-CONH-C 10 H 21 ( 3b ), Me-bpy-CONH-C 2 H 5 ( 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 ( 3 MLCT) (dπ(Ir) → π*(diimine)) excited state. The emissive states of the pq complexes 3a − c are probably mixed with some triplet intraligand ( 3 IL) (π → 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.