The electrochemical and photophysical properties of biscyclometallated iridium(III) complexes containing phenanthroline-based ligands have been investigated and compared in organic and aqueous media. Complexes having general formula [Ir(ppy) +/-were synthesized, where ppy is the cyclometallating ligand, 2-phenylpyridine, and N _ N represents one of the following phenanthroline based ligands: 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline (dpp) and 4,7-diphenyl-1,10-phenanthrolinedisulfonate (BPS). In the case of [Ir(ppy) 2 (BPS)] -, the SO 3 -group located on the phenyl ring of BPS dramatically increases the aqueous solubility of the complex compared to the dpp complex, which is only soluble in organic media. The chloride salt of the underivatized phen complex, though not as soluble as the BPS chelated complex, is sufficiently soluble (Ͼ 1 mM) for sensing and other applications in aqueous media. The electrochemical response depends markedly on the medium; all the complexes showed reversible behaviour in organic media, whereas the oxidative electrochemistry of [Ir(ppy) 2

Chemical reactions between certain bis-cyclometalated iridium complexes, cerium(IV) and organic reducing agents in aqueous solution produce an emission of light which in some cases is more intense than that from analogous reactions with conventional ruthenium-based reagents, thus providing a new avenue for chemically-initiated luminescence detection.