Electron Transfer Reorganization Energies in the Electrode-Electrolyte Double Layer

The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been determined experimentally. Conductive indium-tin-oxide (ITO, In₂O₃:Sn) mesoporous films were functionalized with 4-[N,N-di(p-tolyl)-amino]benzylphosphonic acid (TPA) and/or [Ru^II(bpy)₂(4,4'-(PO₃H₂)₂-bpy)]²⁺ (RuP), where bpy is 2,2'-bipyridine. The small inner-sphere reorganizations, λ_i, for Ru^III/IIP and TPA^+/0 make them excellent probes of outer-sphere reorganization energy, λ_o, as λ_i ≪ λ_o such that λ = λ_i + λ_o ≈ λ_o. Consecutive layer-by-layer addition of Zr^IV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO(e^-)|Ru^IIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e^-)|TPA⁺. The kinetics for ET from ITO to Ru^IIIP or TPA⁺ were quantified spectroscopically as a function of applied potential (E_app) and hence driving force, -ΔG°. Marcus-Gerischer analysis of this data provided λ. Significantly, λ_o was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to E_app. In agreement with dielectric continuum theory, λ increased to values expected in CH₃CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.