The synthesis and characterisation of iridium(III) bis(2-(2,4-difluorophenyl)pyridinato-N, C2')-2(4-carboxylphenyl)imidazo[4,5-f][1,10]phenanthroline perchlorate, [Ir(dfpp)(2)(picCOOH)](+) and its octaarginine conjugate [Ir(dfpp)(2)(picCONH-Arg(8))](9+) are reported. Both complex and conjugate exhibit intense and long-lived luminescence, which is O-2 and pH sensitive. Conjugation to the polyarginine peptide renders the complex very water soluble. The uptake of the parent iridium(III) complex and conjugate are compared in two mammalian cell lines; SP2 myeloma and Chinese hamster ovary (CHO). Both complexes internalise into the cytoplasm, however dye uptake rate and distribution vary with peptide conjugation and with cell identity. Whereas transmembrane transport is thought to have been facilitated by the dimethyl sulfoxide (DMSO) used as co-solvent (0.05% v/v) for the parent complex, the octaarginine, the dye-conjugate (iridium-R-8) is membrane permeable in water only. Both complexes exhibit high cytotoxicity, evident through blebbing and vacuole formation within living cells, indicative of apoptosis, within 30 min of exposure to the probe. The IC50 recorded for the cells in the dark was independent, in the case of the parent complex, of the identity of the cell, with IC50 of 84.8 mu M and 88 mu M respectively for SP2 and CHO cells. The IC50 approximately doubled for the polyarginine conjugate and displayed a significant dependence on cell type with IC50 of 35 mu M and 54.1 mu M respectively for SP2 and CHO cells. These IC50 values were recorded in the dark. However under irradiation cell death is considerably faster. Evidence from imaging suggests that the conjugate penetrates the nucleus whereas the parent does not, indicating that nuclear penetration may play a role in cytotoxicity. (C) 2012 Elsevier Inc. All rights reserved.