Luminescent cyclometalated iridium(III) polypyridine complexes appended with an alkyl chain or indole as biological probes

Abstract

A series of new luminescent cyclometalated iridium(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 (bpy-C18), 4-n-decylaminocarbonyl-4’-methyl-2,2’-bipyridine (bpy-C10), 4-ethylaminocarbonyl-4’-methyl-2,2’-bipyridine (bpy-C2); HN−C = 1-phenylpyrazole (Hppz), N−N = bpy-C18, bpy-C10, bpy-C2; HN−C = 2-phenylquinoline (Hpq), N−N = bpy-C18, bpy-C10, bpy-C2) bearing an alkyl pendant has 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) → π*(diimine)) excited state. The emissive states of the pq complexes are probably mixed with some triplet intraligand (3IL) (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 Po/w values are dependent on the cyclometalating and diimine ligands.Additionally, the cytotoxicity of these cyclometalated iridium(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. A series of luminescent cyclometalated iridium(III) polypyridine indole complexes [Ir(N−C)2(N−N)](PF6) (HN−C = Hppy, N−N = 4-(2-(indol-3-yl)ethyl)aminocarbonyl-4’-methyl-2,2’-bipyridine (bpy-ind), 4-(5-(2-(indol-3-yl)ethyl)aminocarbonyl-n-pentyl)aminocarbonyl-4’-methyl-2,2’-bipyridine (bpy-C6-ind); HN−C = 7,8-benzoquinoline (Hbzq), N−N = bpy-ind, bpy-C6-ind; HN−C = Hpq, N−N = bpy-ind, bpy-C6-ind) has been synthesized and characterized, and their electronic absorption, photophysical, and electrochemical properties and lipophilicity 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. On the basis of the photophysical and electrochemical data, the nature of the emissive states has been assigned. The interactions of these complexes with an indole-binding protein bovine serum albumin (BSA) have been studied by emission titrations. Additionally, the cytotoxicity of the complexes toward HeLa cells has been examined by the MTT assay, and the cellular uptake characteristics of the complexes have been investigated by flow cytometry and laser-scanning confocal microscopy.