Inhibited interlayer electron transfer in metal ion linked multilayers on mesoporous metal oxide films

Molecular Orientation and Energy Transfer Dynamics of a Metal Oxide Bound Self-Assembled Trilayer

Self-assembly of molecular multilayers via metal ion linkages has become an important strategy for interfacial engineering of metalloid and metal oxide (MOx) substrates, with applications in numerous areas, including energy harvesting, catalysis, and chemical sensing. An important aspect for the rational design of these multilayers is knowledge of the molecular structure-function relationships. For example, in a multilayer composed of different chromophores in each layer, the molecular orientation of each layer, both relative to the adjacent layers and the substrate, influences the efficiency of vectorial energy and electron transfer. Here, we describe an approach using UV-vis attenuated total reflection (ATR) spectroscopy to determine the mean dipole tilt angle of chromophores in each layer in a metal ion-linked trilayer self-assembled on indium-tin oxide. To our knowledge, this is the first report demonstrating the measurement of the orientation of three different chromophores in a single assembly. The ATR approach allows the adsorption of each layer to be monitored in real-time, and any changes in the orientation of an underlying layer arising from the adsorption of an overlying layer can be detected. We also performed transient absorption spectroscopy to monitor interlayer energy transfer dynamics in order to relate structure to function. We found that near unity efficiency, sub-nanosecond energy transfer between the third and second layer was primarily dictated by the distance between the chromophores. Thus, in this case, the orientation had minimal impact at such proximity.

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

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.