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 A 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 ap...