Ground vs. excited state electron transfer: Adsorbed monolayers and trimers in solution

Abstract

Transient emission spectroscopy has been used to probe the rate of photoinduced electron transfer between metal centres within a novel trimeric complex {[Os(bpy) 2 (bpe) 2 ][Os(bpy) 2 Cl] 2 } 4+ , where bpy is 2,2 0 -bipyridyl and bpe is trans-1,2-bis-(4pyridyl)ethylene. Transient emission experiments on the trimer, and on [Os(bpy) 2 (bpe) 2 ] 2+ in which the [Os(bpy) 2 Cl] + quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 AE 0.1 Â 10 8 s À1 . Investigations into the driving forces for oxidation and reduction of the electronically excited state within the trimer indicate that quenching of the [Os(bpy) 2 (bpe) 2 ] 2+* centre within the trimer involves electron transfer from the [bpe Os(bpy) 2 Cl] + centres to the electronically excited state with a driving force of À0.3 eV. Monolayers of the complex, [Os(bpy) 2 bpe pyridine] 2+ , have been formed by spontaneous adsorption onto platinum microelectrodes and used to probe the dynamics of electron transfer across the trans-1,2bis-(4-pyridyl)ethylene bridge in the ground state. These monolayers are stable and exhibit well defined voltammetric responses for the Os 2+/3+ redox reaction. Cyclic voltammograms recorded at high scan rates can be accurately modelled according to a nonadiabatic electron transfer model based on the Marcus theory using a standard heterogeneous electron transfer rate constant, k , of 3.1 AE 0.2 Â 10 4 s À1 and a reorganization energy of 0.4 AE 0.1 eV. This rate constant is a factor of approximately two orders of magnitude smaller than that found for photoinduced electron transfer across the same bpe bridge for identical driving forces. This significant difference is interpreted in terms of both the nature of the orbitals involved in electrochemically and optically driven electron transfer, as well as the strength of electronic coupling between two molecular components as opposed to a molecular component and a metal electrode.

Electrochemistry of redox-active self-assembled monolayers

“…A significant amount of work is directed towards developing new molecular junctions such as peptides [452–454]. Electrochemical SAM studies have allowed the direct comparison of photoinduced electron transfer (transient emission spectroscopy) to ground state electron transfer [155,455]. …”

Mathematical modelling and numerical simulation of adsorption processes at microdisk electrodes

“…However, a significant part of these do not contain a rigorous quantitative treatment of the results or the treatment is based on certain simplifying assumptions. The simplified analytical treatment of the results has been performed, for example, in a series of experimental reports on adsorbed monolayers by Forster et al [8][9][10]. In this case the authors assumed the total electrode coverage to be constant during the timescale of the experiment.…”

“…All the published approaches for the simulation of adsorptive electrochemical processes lack presentation of the general results due to their complexity. The corresponding mathematical models, even for the planar or spherical diffusion conditions considered in the literature [8][9][10][11][12][13][14][15][16][17][18][19][20], have too many independent parameters for the results to be easily represented in a suitable form. If one wanted to employ microelectrodes for studying adsorptive kinetics, the number of variables would increase even more.…”

Ruthenium Molecular Wires with Conjugated Bridging Ligands: Onset of Band Formation in Linear Inorganic Conjugated Oligomers

“…Transition-metal complexes incorporating π-conjugated bridging ligands are extremely attractive as potential linear molecular wires because of high stability, the accessibility of multiple stable redox states, and the ability to synthetically tailor both the ligand itself as well as utilizing a wide range of metal ions to form complexes. Bimetallic and trimetallic complexes, particularly those of cobalt, ruthenium, and osmium, have previously been studied via electrochemistry. ,,− In the limit of zero communication between metal centers, only one oxidation peak will be observed (as a two-electron process), so if multiple oxidation peaks are observed, the separation between the peaks indicates the degree of communicationalthough the increase in electronic repulsion, as well as statistical factors, needs to be taken into account . Tridentate bridging ligands, such as tetra-2-pyridyl-1,4-pyrazine (tppz) (Figure ), are particularly attractive since a linear one-dimensional geometry is enforced when bound to octahedral transition-metal ions.…”