Exploiting a New Strategy to Prepare Water-Soluble Heteroleptic Iridium(III) Complexes to Control Electrochemiluminescence Reaction Pathways in Aqueous Solution.

The direct derivatization of heteroleptic iridium(III) complexes with sulfonate groups removes the limitations of prior strategies for the preparation of water-soluble analogues, in which complexes were prepared from a narrow range of ligands with suitably polar or charged functional groups. Phenylpyridine, phenylpyrazole, and phenylbenzothiazole ligands were selectively sulfonated opposite to their cyclometalated carbon within iridium(III) complexes containing bipyridine or N-heterocyclic carbene ancillary ligands. Altering reaction conditions enabled additional sulfonation of the benzothiazole fragments. Informed by the electrochemical and photophysical properties of the parent complexes in organic solvents, we adopted this strategy to design six novel luminophores that proceeded through three different sets of coreactant electrochemiluminescence (ECL) reaction pathways under aqueous conditions. The intensity of the indirect coreactant ECL of one of the iridium(III) luminophores was enhanced by over an order of magnitude by introducing a redox mediator, extending this promising analytical approach beyond the conventional [Ru(bpy)