Cathodic Co-reactant Electrogenerated Chemiluminescence of Water-soluble Heteroleptic Iridium(III) Complexes Bearing N–Methyl(pyridyl)pyridinium Cyclometalating Ligands

Heteroleptic iridium(III) complexes containing N-methyl-(2′-pyridyl)pyridinium cyclometalating ligands were examined as luminophores for cathodic electrogenerated chemiluminescence (ECL) with peroxydisulfate co-reactant. These water-soluble iridium(III) complexes exhibit reduction potentials that are far less negative than those of the archetypal tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) luminophore. As observed with tris(2,2′-bipyrazine)ruthenium(II) ([Ru(bpz)3]2+), these shifts in potential are favourable for cathodic ECL because they lower the oxidative quenching of the excited state, and improve the stability of the reduced metal complex in aqueous solvent. The ECL intensities increased with repeated scanning or pulsing of the applied potential, due to the deposition of the less-soluble reduced species at the electrode surface. Iridium(III) complex ‘Ir4’, which contains cyclometalating ligands derived from the 1-methyl-4-(2ʹ-pyridyl)pyridinium cation, produces ECL with peroxydisulfate co-reactant in aqueous solution that is far more intense than that of [Ru(bpy)3]2+, and comparable to [Ru(bpz)3]2+, but in contrast to the red emissions of the conventional ruthenium(II) complexes, the iridium(III) species exhibits green luminescence, creating opportunities for novel spectrally and potentially resolved ECL systems.