Photophysical properties of a new series of water soluble iridium bisterpyridine complexes functionalised at the 4′ position

Abstract

Four new hetero- and homo-leptic iridium(III) bisterpyridine complexes have been prepared which incorporate aniline (tpy-φ-NH₂), benzoic acid (tpy-φ-COOH), and benzyl alcohol (tpy-φ-CH₂OH) substituents at the 4′ positions of the tpy ligands (tpy = 2,2′:6′,2′′-terpyridine, φ = phenylene). The electrochemical behaviour and ground and excited state spectroscopic properties of the complexes are reported, and the X-ray crystal structures of a homoleptic benzyl alcohol [Ir(tpy-φ-CH₂OH)₂](PF₆)₃, homoleptic aniline [Ir(tpy-φ-NH₂)₂](PF₆)₃, and heteroleptic benzyl alcohol/aniline substituted complex [Ir(tpy-φ-CH₂OH)(tpy-φ-NH₂)](PF₆)₃ have been solved. Complexes with aniline substituents were found to display absorption bands at around 430 nm corresponding to intraligand charge transfer (ILCT) that are sensitive to changes in solvent and pH. Strong emission in the visible region involving the ILCT state is observed in two of the complexes (Φ_e = 0.7% and 2.6%) in acetonitrile. In the heteroleptic aniline/benzyl alcohol complex the Stokes shift is shown to be linearly related to solvent polarisability according to the Lippert equation, but only for solvents with weak hydrogen bonding interactions. Additionally, in water, emission from the ILCT state is quenched and only weak ligand centred (LC) emission is observed. The long lifetimes and quantum yields of these complexes make them interesting candidates for probes in sensing applications, especially [Ir(tpy-φ-CH₂OH)(tpy-φ-NH₂)₂](PF₆)₃ due to its unusual sensitivity to the solvent environment.