(Invited) Why Do Dipole Effects on Charge Transfer Matter?

Abstract

Charge transfer (CT) and charge transport (CTr) are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible (Phys. Chem. Chem. Phys. 2020, 22, 21583-21629). In natural and manmade sensing systems, for example, well synchronized CT processes drive the conversion of mechanical, optical, chemical and thermal signals into electric outputs, which is essential for interfacing with processing modules. Concurrently, electric dipoles are everywhere and therefore, understanding how they affect CT and CTr is of principal importance for attaining and controlling the functionality of a wide range of materials and devices. Discussions of the idea about dipole effects on CT date back to the mid 20th century and reported experimental evidence from the 1990s and 2000s demonstrated their key importance. The dipole-generated localized electric fields induce Stark effects and modulate the electronic properties of the CT moieties in their vicinity. The notion for such effects focusses on dipole-induced changes in the reduction potentials of the donor and the acceptor, 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 (Angew. Chem. 2018, 57, 12365-12369). 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. The same localized field effects in non-polar medium completely shut down the electron transfer against the dipole. While in the vicinity of dipoles, the electric fields can reach GV/m, they fall of rapidly with distance. In our work, we incorporate the dipoles in the electron donors. This geometry provides the closest possible placement of the dipole to the CT moieties. Most importantly, it has also allowed us to discover for the first time dipole effects on donor-acceptor electronic coupling that dominate and oppose the established notion about effects on the FC contributions to the CT kinetics. These discoveries change the ways of thinking about dipole effects on CT and CTr, and set unprecedented paradigms for the development of electronic materials and devices.