Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO₂

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO₃H₂)₂bpy)₂Ru^II(bpy-bimpy)­Ru^II(tpy)­(OH₂)]⁴⁺, −[Ru_a^II–Ru_b^II–OH₂]⁴⁺, on nanocrystalline TiO₂ electrodes, starting from either −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ or −[Ru_a^II–Ru_b^III–OH₂]⁵⁺, have been investigated. Transient absorption measurements for TiO₂–[Ru_a^II–Ru_b^II–OH₂]⁴⁺ in 0.1 M HPF₆ or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO₂–[Ru_a^II–Ru_b^III–OH₂]⁵⁺, in a variety of media, generates −[Ru_a^III–Ru_b^III–OH₂]⁶⁺. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ assembly to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺. In a parallel reaction, −[Ru_a^III–Ru_b^III–OH₂]⁶⁺ formed by electron injection undergoes proton loss giving −[Ru_a^II–Ru_b^IVO]⁴⁺ with possible conversion to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ by an electrolyte-mediated reaction. In the following slow step, re-equilibration on the surface occurs either by reaction with added Fe^III/II or by cross-surface electron transfer between spatially separated −[Ru_a^II–Ru_b^IVO]⁴⁺ and −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ assemblies to give −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ with a half-time of t_1/2 ∼ 68 μs. These results and analyses show that the transient surface behavior of the assembly and cross-surface reactions play important roles in producing and storing redox equivalents on the surface that are used for water oxidation.