Light-Driven Water Oxidation with the <b>Ir-blue</b> Catalyst and the Ru(bpy)₃²⁺/S₂O₈^2– Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency

Abstract

Light-driven water oxidation is achieved with the Ru(bpy)₃²⁺/S₂O₈^2- cycle employing the highly active <b>Ir-blue</b> water oxidation catalyst, namely, an Ir^IV,IV₂(pyalc)₂ μ-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. <b>Ir-blue</b> is readily formed by stepwise oxidation of the monomeric Ir(III) precursor <b>1</b> by the photogenerated Ru(bpy)₃³⁺, with a quantum yield ϕ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from <b>1</b> to Ru(bpy)₃³⁺, leading to the Ir(IV) monomer <b>I</b>_<b>1</b> (<i>k</i>₁ ∼ 10⁸ M^-1 s^-1). The competent <b>Ir-blue</b> catalyst is then obtained from <b>I</b>_<b>1</b> upon photooxidative loss of the Cp* ligand and dimerization. The <b>Ir-blue</b> catalyst is active in the Ru(bpy)₃²⁺/S₂O₈^2- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)₃³⁺ [with <i>k</i>_{<b>Ir-blue</b>} = (3.00 ± 0.02) × 10⁸ M^-1 s^-1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a Ir^V,V₂ steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of ϕ(O₂) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.