Cyclometalated Iridium(III) Polypyridine Complexes as Bioimaging Reagents, Sensors for Biothiols, and Photoactivatable Cytotoxic Agents


Student thesis: Doctoral Thesis

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Award date21 Nov 2018


A series of cyclometalated iridium(III) polypyridine complexes carrying a perylene bisimide (PBI) unit through a non-π-conjugated triethylene glycol (TEG) linker [Ir(N^C)2(bpy-TEG-PBI-OEG)](PF6) (HN^C = 2-(2,4-difluorophenyl) pyridine (Hdfppy), 2-phenylpyridine (Hppy), 4-methoxycarbonyl-2-phenylquinoline (Hpqe); bpy-TEG-PBI-OEG = N-(13-N-(4’-methyl-2,2’-bipyridin-4-ylmethoxycarbonyl) amino-4,7,10-trioxatridecanyl)-N’-(4,7,10,13-tetraoxapentadecyl)perylene-3,4:9,10-tetracarboxylic acid bisimide) were synthesized and characterized. Their PBI-free counterparts [Ir(N^C)2(bpy-TEG-NH2)](PF6) (HN^C = Hdfppy, Hppy, Hpqe; bpy-TEG-NH2 = 4-(N-(13-amino-4,7,10-trioxatridecanyl) aminocarbonyloxymethyl)-4’-methyl-2,2’-bipyridine) were also prepared. The photophysical and electrochemical properties of all the complexes were investigated. Upon photoexcitation, all PBI complexes displayed intense green emission with lifetimes in the nanosecond timescale in fluid solutions at 298 K, resembling the S1→S0 fluorescence of the PBI unit. In contrast, the PBI-free complexes exhibited green to red phosphorescence originating from longer-lived triplet metal-to-ligand charge transfer (3MLCT) (dπ(Ir) → π*(N^C and/or N^N)) emissive states. Photophysical studies suggested that the emission of the iridium(III) polypyridine moiety was quenched by the PBI unit via Förster resonance energy transfer (FRET) and triplet-triplet energy transfer (TTET). Notably, the quenching efficiencies of the PBI complexes were different and dependent on the emission energies of the iridium(III) polypyridines, while photoinduced electron transfer (PET) from the iridium(III) polypyridines to the PBI unit was thermodynamically favorable for all PBI complexes. Thus, weak phosphorescence was observed in one of the PBI complexes owing to incomplete quenching. The excited-state properties of the PBI complexes were examined. The presence of the iridium(III) metal centers facilitated intersystem crossing (ISC) and the population of long-lived 3PBI excited states that led to high 1O2 generation quantum yields. Upon strong aggregation in aqueous media, the fluorescence of the PBI complexes was quenched, while the 3MLCT phosphorescence was apparent. Cellular uptake experiments using live HeLa cells were performed. Although the aggregation process lowered the uptake efficiencies of the PBI complexes, confocal images of the cells treated with one of the PBI complexes showed red emission in the cytoplasm. Additionally, the PBI complexes exhibited photoinduced cytotoxicity owing to their strong 1O2-photosensitizing ability.

The synthesis and characterization of biothiol sensors derived from a class of cyclometalated iridium(III) polypyridine complexes containing a 2,4-dinitrophenyl ether moiety [Ir(pq)2(N^N)](PF6) (Hpq = 2-phenylquinoline; N^N = 4-(N-(4-(2,4-dinitrophenoxy) benzyloxy) carbonyl) aminomethyl-4’-methyl-2,2’-bipyridine (bpy-dinitro-1), 4-(2,4-dinitrophenoxy) methyl-4’-methyl-2,2’-bipyridine (bpy-dinitro-2), 4-(4-(2,4-dinitrophenoxy) phenyl)-2,2’-bipyridine (bpy-dinitro-3)) were reported. Due to the quenching effect of the dinitroaromatic moiety, these complexes were weakly emissive. The structural features of the weak emission bands suggest the involvement of triplet intraligand (3IL) (ππ*) (pq) character in the excited states of the complexes. Upon reaction with biothiols, however, the emission was significantly enhanced as a consequence of the departure of the quenching moiety. Among the three complexes, the complex with the shortest linker exhibited much smaller enhancement after the incubation with biothiols. The results may be due to greater steric hindrance between the dinitroaromatic moiety and biothiols. On the contrary, the complex with the longest linker showed the greatest emission enhancement and high selectivity toward biothiols including L-cysteine (L-Cys), glutathione (GSH), and Na2S over other reactive sulfur species (RSS), reactive oxygen species (ROS), and reactive nitrogen species (RNS). The results from a range of experiments demonstrated that this complex was noncytotoxic, showed facile cellular uptake, and can serve as a phosphorogenic intracellular sensor for biothiols including GSH and H2S.

Two novel photoactivatable mitochondria-targeting luminescent iridium(III) poly(ethylene glycol) (PEG) complexes modified with a nitrobenzyl-containing photolabile protecting group (PPG) [Ir(pq)2(N^N)](PF6) (N^N = 4-(N-(1-(5-methoxy-2-nitro-4-((1-(ω-methoxypoly (1-oxapropyl))-1H-[1,2,3]-triazol-4-yl)methoxy) phenyl)ethoxy) carbonyl) aminomethyl-4’-methyl-2,2’-bipyridine (bpy-NB-PEG5k) (number average molecular weight (Mn) = 5996.86, weight average molecular weight (Mw) = 6073.06, polydispersity index (PDI) = 1.013), bpy-NB-PEG10k (Mn = 10781.10, Mw = 10788.95, PDI = 1.001)) and a photoactivatable iridium(III) complexes containing a much shorter TEG pendant [Ir(pq)2(bpy-NB-TEG)](PF6) (bpy-NB-TEG = 4-(N-(1-(5-methoxy-2-nitro-4-((1-(8-hydroxy-3,6-dioxaoctyl)-1H-[1,2,3]-triazol-4-yl)methoxy) phenyl) ethoxy) carbonyl) aminomethyl-4’-methyl-2,2’-bipyridine) were synthesized and characterized. Additionally, the photoreleased product [Ir(pq)2(bpy-CH2NH2)](PF6) (bpy-CH2NH2 = 4-aminomethyl-4’-methyl-2,2’-bipyridine) was prepared. Upon irradiation, all the complexes were studied displayed intense and long-lived yellow emission with structural features in fluid solutions at 298 K, indicative of substantial 3IL (ππ*) (pq) character in their emissive states. Upon photoirradiation, the PPG-PEG pendant was detached from the iridium(III) polypyridine, as confirmed by HPLC analyses. All the complexes showed efficient cellular uptake toward HeLa cells and were localized in the mitochondria of the cells. The (photo) cytotoxicity of the PEG complexes was examined and compared to that of the PEG-free complex. Without irradiation, the PEG complexes were more biocompatible (IC50 = 36.2 – 65.9 μM) than the TEG complex (IC50 = 10.4 μM), indicative of successful attenuation of the cytotoxicity of the complexes through PEGylation. Importantly, the PEG complexes became remarkably cytotoxic (IC50 = 7.2 – 7.9 μM) upon irradiation for as short as 5 min. Control experiments demonstrated that the photoinduced cytotoxicity originated from the photorelease of the iridium(III) polypyridine, indicating that the attachment of the PPG-PEG pendant allowed oxygen-independent photocontrollable cytotoxicity.