Proton-Coupled Electron Transfer Model Systems

The design of artificial photosynthetic systems is an important area of nanoscience. Significant progress has been achieved in recent years through elaborate supramolecular assemblies that mimic various aspects of biological systems that achieve charge separation. A strategy that nature uses extensively is proton-coupled electron transfer (PCET). We are studying a PCET reaction that is of great importance in biological and biomimetic charge transfer nanostructures,

In the above scheme, an electron is transferred from a donor molecule, DH, to an electron acceptor, A. Following one electron oxidation of DH, a proton is transferred along the hydrogen bond to a separate proton acceptor, B. Since oxidation of DH provides the driving force for its deprotonation, the scheme emphasizes the presumed sequential character of electron transfer (ET) and proton transfer (PT). Since D· is generally a weaker oxidant than DH·+, the rate of back electron transfer (BET) in the final complex can be lower than in the middle complex. Perhaps the finest example of this strategy is found in the oxygen-evolving complex of Photosystem II. However, it is difficult to fully characterize the effect of solvent, temperature, thermodynamic driving force and other factors that influence the rates of the relevant charge transfer reactions in a system as complex as Photosystem II. We study simple model systems that display PCET behavior which are highly amenable to detailed comparison with current theories.

In order to measure the intrinsic charge transfer rates by ultrafast laser spectroscopy, A and DH must be pre-organized to eliminate the need for diffusion. We study covalently linked dyads and a prototype system in which the donor and acceptor are in virtual contact due to a high concentration of donor molecules. In our experiments an electronically excited viologen functions as the electron acceptor. We have recently shown that the high excited-state reduction potential of methyl viologen can remove an electron from the non-bonding electron pair of an oxygen atom in a hydroxy group [1]. To our knowledge this is the first example of ultrafast intermolecular electron transfer in a hydrogen-bonding solvent. Upon one-electron oxidation, many hydroxy-containing compounds such as linear alkanols become strong acids and deprotonate rapidly. This makes these good model systems for studying proton-coupled electron transfer.

References

1. Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B., "Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile," J. Phys. Chem. A 2001, 105, 5768-5777.