Vaentine Vullev

Abstract:

The importance of charge transfer (CT) and charge transport (CTr) for supporting life on Earth and for making our modern ways of living possible cannot be overstated.¹ Concurrently, electric dipoles are everywhere and understanding how they affect CT and CTr is of principal importance for addressing medical challenges and controlling the functionality of a wide range of materials and devices.^2,3 Discussions of the idea about dipole effects on CT date back to the mid 20th century.⁴ Reported experimental evidence from the 1990s and 2000s demonstrated the importance of dipole effects on CT.^5-7 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. 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.^8,9 Polar media, indeed, stabilize charged states and in general, enhances the rates of CT. Polar media, however, screen the field permeation and damps the dipole effect on the electron donor and acceptor. 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, such as acetonitrile.⁸ 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 kinetics and thermodynamics on medium polarity is, the interfacial nature of electrode processes presents challenges for characterizing this dependence.^10,11 While the polarity that solvated species experience in the bulk of a electrolyte solution is readily attainable, it is still challenging to determine the polarity of the microenvironment at the electrode surfaces where the redox processes occur.¹² An increase in electrolyte concertation in organic solvents increases their polarity, which is opposite for aqueous solutions.^11,12 At the electrode surfaces, however, the increase rigidity of the double layer, especially under applied voltage, decreases the oreitnational polarization, which principally contributed to the dielectric constant of solvents with large dipoles.¹² That is, the polarity that CT species experience at electrode surfaces is smaller than that in the bulk of the solution. Global-fit analysis on the dependence of measured reduction potentials on the polarity of the solvent and the electrolyte concentrations offers a means for estimating the effective polarity that redox species experience at electrode surfaces.¹² Such 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.