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-C10H21 (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-C2H5 (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 ((MLCT)-M-3) (d pi(Ir) --> pi*(diimine)) excited state. The emissive states of the pq complexes 3a-c are probably mixed with some triplet intraligand ((IL)-I-3) (pi --> pi*) (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.