Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges

Abstract
A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)₂(LL)](PF₆)₂ were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, E^o(Ru^III/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO₄/CH₃CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, −ΔG^°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed −ΔG^°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, H_ab, were independent of the applied potential, rate constants measured as a function of −ΔG^° and analyzed through Marcus–Gerischer theory provided estimates of H_ab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, H_ab decreased markedly with distance from 0.54 to 0.11 cm^–1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp³-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Citation
Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges Marzieh Heidari, Quentin Loague, Rachel E. Bangle, Elena Galoppini, and Gerald J. Meyer ACS Applied Materials & Interfaces 2022 14 (30), 35205-35214 DOI: 10.1021/acsami.2c07151