Pathways Following Electron Injection: Medium Effects
and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst
Assembly on TiO2
Posted on 2018-06-08 - 10:30
Interfacial dynamics following photoexcitation
of the water oxidation
assembly [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]4+, −[RuaII–RubII–OH2]4+, on nanocrystalline
TiO2 electrodes, starting from either −[RuaII–RubII–OH2]4+ or −[RuaII–RubIII–OH2]5+, have been
investigated. Transient absorption measurements for TiO2–[RuaII–RubII–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–[RuaII–RubIII–OH2]5+, in a variety
of media, generates −[RuaIII–RubIII–OH2]6+. The injected
electron randomly migrates through the surface oxide structure reducing
an unreacted −[RuaII–RubIII–OH2]5+ assembly to −[RuaII–RubII–OH2]4+. In a parallel reaction, −[RuaIII–RubIII–OH2]6+ formed by electron injection undergoes proton
loss giving −[RuaII–RubIVO]4+ with possible conversion to
−[RuaII–RubII–OH2]4+ by an electrolyte-mediated reaction.
In the following slow step, re-equilibration on the surface occurs
either by reaction with added FeIII/II or by cross-surface
electron transfer between spatially separated −[RuaII–RubIVO]4+ and −[RuaII–RubII–OH2]4+ assemblies to give −[RuaII–RubIII–OH2]5+ with a half-time of t1/2 ∼ 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.
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Brennaman, M. Kyle; Gish, Melissa K.; Alibabaei, Leila; Norris, Michael R.; Binstead, Robert A.; Nayak, Animesh; et al. (2018). Pathways Following Electron Injection: Medium Effects
and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst
Assembly on TiO2. ACS Publications. Collection. https://doi.org/10.1021/acs.jpcc.8b04837