Iridium(III) Complexes of Bifunctional 2-(2-Pyridyl)imidazole Ligands: Electrochemiluminescent Emitters in Aqueous Media

A series of electrochemiluminescent (ECL) iridium(III) complexes with the general formula [Ir(C^∧N)₂(pim)]⁺ (where C^∧N = cyclometalating ligands 2-phenylpyridinato (ppy) or 2-(2,4-difluorophenyl)pyridinato (dFppy), and pim = 2-(2-pyridyl)imidazole) have been synthesized. In each case, the 2-(2-pyridyl)imidazole ancillary ligand has been modified to facilitate bioconjugation and ECL label development. All complexes exhibit blue-shifted optical and electro-generated phosphorescence relative to the archetypal complex [Ir(ppy)₂(bpy)]⁺ (bpy = 2,2'-bipyridine). The emission energies for the complexes were unperturbed by functionalization of the imidazole unit of the pim ligand, whereas the emission energy was significantly blue-shifted when the pyridyl group was modified with an electron-donating oxyethanol unit. Cyclic voltammetric studies provide results consistent with fluorine substituents on the cyclometalating ligands, or an oxyethanol substituent on the neutral pim ligand, widening the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of these complexes. Most of the complexes have high photoluminescence quantum yields (Φ_PL) in acetonitrile (up to 0.91), and some have higher ECL efficiencies than [Ru(bpy)₃]²⁺ in both acetonitrile (up to 177%) and ProCell buffer (up to 202%). Theoretical studies provide additional insights into the photophysical and electrochemical properties of this series of compounds.