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 [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+ (RuP), where bpy is 2,2'-bipyridine. The small inner-sphere reorganizations, λi, for RuIII/IIP and TPA+/0 make them excellent probes of outer-sphere reorganization energy, λo, as λi ≪ λo such that λ = λi + λo ≈ λo. Consecutive layer-by-layer addition of ZrIV-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-)|RuIIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e-)|TPA+. The kinetics for ET from ITO to RuIIIP or TPA+ were quantified spectroscopically as a function of applied potential (Eapp) and hence driving force, -ΔG°. Marcus-Gerischer analysis of this data provided λ. Significantly, λo was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to Eapp. 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.