Managing Charge Transfer: Molecular Dipoles, Electrochemical Potentials, and the Elephant in the Room

MANAGING CHARGE TRANSFER: MOLECULAR DIPOLES, ELECTROCHEMICAL POTENTIALS, AND THE ELEPHANT IN THE ROOM

Prof. Val Vullev
University of California, Riverside

Charge transfer (CT) and charge transport (CTr) are crucial for supporting life on Earth and for making our modern ways of living possible. The ubiquitous nature of electric dipoles warrants the demand for deep understanding their multifaceted effects on charge transduction. Dipoles affect not only CT and CTr, but also molecular conformations and (enzymatic) catalysis. Discussions about dipole effects on CT date back to the mid 20th century. Reported experimental evidence since the 1990s have confirmed the importance of dipole effects on CT. The dipole-generated localized electric fields modulate the electronic properties of the CT moieties. The notion for such effects focusses on dipole-induced changes in the reduction potentials of the acceptor and the oxidized donor, affecting the CT driving forces and thus, the Franck-Condon (FC) contributions to the CT kinetics.

Contrary to the common belief, can dipoles affect other aspects of the CT kinetics, such as the donor-acceptor electronic coupling? Our work with bioinspired molecular electrets demonstrates that they can. (Possessing ordered electric dipoles, electrets are the electrostatic analogues of magnets.) We recently demonstrated that to harness such effects, which are inherently enormous, (1) the dipoles should be placed as closed as possible to the electron donor and acceptor, and (2) the media polarity should be lowered. Polar media, indeed, stabilize charged states and in general, enhances the rates of charge separation (CS). Polar media, however, screen the field permeation and damp the dipole effects on CS. Using hydrocarbons as a medium, results in electron transfer rates along the dipole that are six times larger than the rates for the same system when in polar solvents. The same localized field effects in non-polar medium completely shut down the electron transfer against the dipole. As important as the dependence of CT on medium polarity is, the interfacial nature of electrode processes presents challenges for characterizing this dependence. At electrode surfaces, the increased rigidity of the double layer, especially under applied potential, decreases the orientational polarization, which principally contributes to the dielectric constant of solvents with large dipoles. As our analysis reveals, the polarity that CT species experience at electrode surfaces is quite smaller than that in the bulk of the solution. Along with the liquid-junction potentials, this polarity effect indices considerable shifts in the measured electrochemical potentials. Taking these effects under consideration, our electrochemical analysis improves not only the characterization of CT thermodynamics, but also the quantification of the dipole effects on CT, which has key implications for electronics, photonics and energy science and engineering.