Computational Study on the Light-Induced Oxidation of Iridium-Aqua Complex to Iridium-Oxo Complex over WO₃(001)...

An iridium aqua complex [Ir^III(η⁵-C₅Me₅){bpy(COOH)₂}(H₂O)]²⁺ under visible light irradiation has been experimentally reported to form an iridium-oxo (Ir-oxo) complex [Ir^V(η⁵-C₅Me₅){bpy(COOH)₂}(O)]²⁺, which oxidizes H₂O to O₂. However, the mechanism for the formation of this Ir-oxo complex remains unclear, due to the difficulties in observing the unstable Ir-oxo complex and computing light-induced systems having different numbers of electrons. In this study, we perform density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to investigate more in detail our previously proposed deprotonation and light-induced oxidation reactions composing the formation of the Ir-oxo complex. In particular, we discuss effects of light irradiation and WO₃ support on the formation of the Ir-oxo complex. We suggest two distinct mechanisms, that is, direct and indirect for the light-induced oxidation. In the direct mechanism electrons are directly transferred from the occupied π* orbitals of Ir^III-OH or Ir^IV=O^• to the conduction band of the WO₃ surface, whereas in the indirect mechanism electrons are first excited from the valence band to the conduction band of the WO₃ surface due to the UV light, and then the resultant electron hole oxidizes the Ir complex. In the direct mechanism, in particular, we found that the lowest energy of the anode's conduction band determines the adsorption wavelength of the light irradiation, enabling us to predict alternative semiconductor anodes for more efficient formation of the Ir-oxo complex.