(Invited) Organic Molecular Electrets: Multifaceted Complexity of Dipole Effects on Charge Transfer

Abstract

Electric dipoles are everywhere. While they provide potentially important paradigms for controlling charge transfer (CT), dipole effects on CT still remain largely unexplored. CT is among the most fundamental processes responsible for sustaining life on earth and for ensuring the functionality of a range of energy and electronic devices that are an intricate part of everyday life. Therefore, we pioneer the concept of bioinspired molecular electrets that, in addition to possessing large macrodipoles, also can mediate long-range CT. (Electrets are systems with ordered electric dipoles, i.e., they are the electrostatic analogies of magnets.) Our electret designs are based on polypeptides composed of non-native aromatic beta-amino acids. We observe that even a single electret residue induces rectification of CT that surpasses what is reported for similar systems utilizing polypeptide helices composed of native amino acids. While dipoles generate enormous fields around them, the strength of these localized fields significantly decays with distance. Therefore, we incorporate the dipoles within the participants in the CT steps. It allows us to observe unprecedentedly large dipole effects on the CT kinetics.5 A decrease in solvent polarity increases the rates of CT. It appears counterintuitive, and it should be the other way around: polar media stabilizes the formed charged CT states and thus increases the rates of photoinduced CT in the Marcus normal region. Concurrently, polar media screens the dipole-generated fields and suppresses their effects on CT. For non-polar solvents, changing the dipole orientation can alter the reduction potentials of the photosensitizers with hundreds of millivolts. Therefore, a careful balance between the two opposing polarity effects allows us to demonstrate CT in lipophilic media, such as toluene. For non-polar solvent, we observe enhancement of electron-transfer (ET) rates along the dipole, while ET against the dipole is completely suppressed. By modifying the semi-classical CT theory, we quantify such experimentally observed dipole effects for the first time. In addition to affecting the reduction potentials and thus the Franck-Condon aspects of the CT kinetics, the molecular electrets reveal that dipoles can also drastically change the CT rates by altering the donor-acceptor electronic coupling. This discovery contradicts the accepted notion about dipole effects on CT, where the focus is on the Franck-Condon nuclear aspects of the kinetics and the electronic contributions are considered insignificant. Our findings not only lead to rethinking the fundamentals of dipole effects on CT, but also open new avenues for development of unprecedented paradigms for organic electronics and energy materials.