Light-Driven Water Oxidation with the Ir-blue Catalyst and the Ru(bpy)₃ ²⁺/S₂O₈ ^2- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency

Light-driven water oxidation is achieved with the Ru(bpy)₃ ²⁺/S₂O₈ ^2- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an Ir^IV,IV ₂(pyalc)₂ μ-oxo-dimer [pyalc = 2-(2′-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)₃ ³⁺, 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 1 to Ru(bpy)₃ ³⁺, leading to the Ir(IV) monomer I₁ (k₁ ∼10⁸ M^-1 s^-1). The competent Ir-blue catalyst is then obtained from I₁ upon photooxidative loss of the Cp∗ ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)₃ ²⁺/S₂O₈ ^2- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)₃ ³⁺ [with k_Ir-blue = (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,V ₂ 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.