Increasing Cage Escape Yields for Halide Oxidation in Aqueous Solutions Using Ir(III) Photosensitizers with π-Extended Aromatic Ligands

A series of five Ir(III) photosensitizers featuring π-extended aromatic ligands is reported for iodide, bromide, and chloride photo-oxidation in acetonitrile/water (50:50) mixtures. All photosensitizers exhibited excited-state reactivity toward iodide, bromide, and chloride, with quenching rate constants in the (1.2-2.1) 10¹⁰ M⁻¹s⁻¹, (0.8-4.8) 10⁹ M⁻¹s⁻¹, and (0.049-1.3) 10⁸ M⁻¹s⁻¹ range, respectively. Nanosecond transient absorption spectroscopy confirmed electron transfer from the halides to the excited photosensitizers, with cage escape yields (ΦCE) that were significantly higher in acetonitrile than in acetonitrile/water mixtures. In acetonitrile, up to unitary cage escape yields were obtained for iodide, bromide, and chloride oxidation. In water-enriched conditions, those yields decreased drastically, but the introduction of π-extended aromatic ligands allowed to maintain appreciable cage escape yields (ΦCE = 0.02-0.32).