Electron transfer processes involving organic molecules are relevant to many biological systems, such as photosynthesis. As a result electron transfer is being studied in detail, both theoretically and experimentally. One of the key issues in the field is the mechanism of long-range electron transfer, that is how the electronic coupling of donor and acceptor sites is mediated by the intervening material between the donor and acceptor. However, the electronic coupling interaction between a donor and acceptor is still one of the least understood aspects of charge transfer reactions. In most intramolecular electron transfer reactions, this coupling interaction is presumed to occur primarily through the bonds of the spacer that tethers the donor and acceptor. Since this is an important source of coupling, it is useful to understand how this interaction occurs, including the magnitude of the effect, and to understand the reasons for the behavior.

In present work, we are investigating the dependence that coupling has on spacer structure by addressing the question of how the coupling is dependent on the bond angles of the spacer and by determining the magnitude of this effect. Both electron transfer and hole transfer rates were measured for a series of compounds and compared with results from computations being carried out on the supercomputers available at NERSC. With these results, we can test the limits of our current computational methods and use our computational results to understand the coupling mechanism and explain the observed effect. The calculation of accurate magnitudes of the couplings requires a high level of theory. Our compuations are giving new insights into the controlling factors of electron tranfer. An example is given in the attached figure, which shows a breakdown of electron transfer pathways in two of the compounds that have been studied experimentally and theoretically. The calculations indicate that the trans arrangement of the bonds provides for superior transmission of electronic coupling due to constructive interference between principal pathways, while in the cis or gauche arrangements destructive interference leads to poorer electronic coupling.

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