Photophysical and Electrochemical Properties of Phenanthroline‐Based Bis‐cyclometallated Iridium Complexes in Aqueous and Organic Media

Abstract

AbstractThe 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)2(N∩N)]+/– 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 SO3– 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 (> 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)2BPS]– and [Ir(ppy)2phen]+ was irreversible. Results from spectroelectrochemical experiments indicate that electrolysis produces significant changes to the UV region of the spectrum associated with the ppy moieties. Significantly, this demonstrates that the highest occupied molecular orbital (HOMO) is not entirely metal based but is also associated with the cyclometallating ligand. The variation in emission maxima between the iridium complexes was small in organic and aqueous media, but the colour of the emission depends on solvent polarity and strongly on temperature. Most significantly, the quantum yields for these species are also strongly influenced by the solvent: high quantum yields, ranging from 14.0 to 28.6 %, are observed in CH2Cl2. However, in aqueous media the water‐soluble complexes, [Ir(ppy)2BPS]– and [Ir(ppy)2phen]+, exhibited significantly diminished quantum yields of 2.5 and 2.6 %, respectively. Theoretical calculations confirm the spectroscopic assignments and that the HOMO is significantly delocalized over the metal and cyclometallating ligands. The relationship between the electrochemical, spectroscopic and theoretical results is discussed.