Structure, Photophysical and Electrochemical Properties, Biomolecular Interactions, and Intracellular Uptake of Luminescent Cyclometalated Iridium(III) Dipyridoquinoxaline Complexes

A series of luminescent cyclometalated iridium(III) dipyridoquinoxaline complexes [Ir(N boolean AND C)(2)(N boolean AND N)](PF6) (HN boolean AND C= 1-phenylpyrazole, Hppz, N boolean AND N = dipyrido[3,2-f:2',3'-h]quinoxaline, dpq (1a), 2-(n-butylamido)dipyrido-[3,2-f.2',3'-h]quinoxaline, dpqa (1b); HN boolean AND C = 7,8-benzoquinoline, Hbzq, N boolean AND N = dpq (2a), dpqa (2b); HN boolean AND C = 2-phenylquinoline, Hpq, N boolean AND N = dpq (3a), dpqa (3b)) has been synthesized and characterized. Cyclic voltammetric studies revealed a reversible or quasi-reversible iridium(IV/III) oxidation couple at about +1.13 to +1.32 V and a reversible diimine reduction couple at about -1.10 to -1.29 V versus SCE. Upon photoexcitation, all the complexes displayed intense and long-lived green to orange triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (d pi(lr) -> pi*(dpq or dpqa)) emission in aprotic organic solvents at room temperature and in low-temperature glass. In aqueous solution, these complexes were only weakly emissive or even non-emissive. The lipophilicity of all the complexes has been determined by reversed-phase HPLC. The cytotoxicity of these iridium(III) complexes toward the human cervix epithelioid carcinoma (HeLa) and Madin-Darby canine kidney (MDCK) cell lines has been evaluated by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cellular uptake of the complexes by MDCK cells has been examined by laser-scanning confocal microscopy. Most importantly, apparent nucleolar staining was observed after the cells were treated by the complexes. The interactions of these complexes with proteins, DNA, and RNA have also been studied by emission titrations and SDS-PAGE gel staining. The results revealed that the complexes bound to the hydrophobic pockets of proteins, intercalated into the base-pairs of double-stranded DNA, but did not appear to interact with RNA.