Bioinspired Molecular Electrets: Bringing Proteomic Approaches to Charge-Transfer Systems
Authors: Vullev Valentine Ivanov 
Published: February 2020

The ability to control charge transfer at molecular and nanometer scales represents the ultimate level of electronic mastery, and its impacts on electronic, energy and other applications cannot be overstated. As electrostatic analogues of magnets, electrets possess ordered electric dipoles that present key paradigms for directing transduction of electrons and holes. We undertake a bioinspired approach in the design of molecular electrets based on anthranilamides.[J. Phys. Chem. Lett.
2011, 2, 503–508] Similar to protein helices, the anthranilamides possess intrinsic dipoles originating from ordered amide and hydrogen bonds, i.e., they are composed of non-native anthranilic residues.[J. Org. Chem.
2013, 78, 1994-2004; Pure Appl. Chem.
2015, 87, 779-792] Unlike proteins, however, the bioinspired molecular electrets have extended pi-conjugation along their backbones providing a means for efficient charge transfer. The anthranilic molecular electrets are polypeptides of non-native amino acids. This feature offers unexplored routes for bringing principles of proteomics into the de novo designs of electronic molecular systems and materials. In analogy, less than a couple of dozen transcribable native alpha-amino acids, each with a different single side chain, are the building blocks of proteins with countless structural and functional characteristics yielding the amazing diversity of life on our planet. Therefore, a similar set of Aa residues with different electronic properties should prove most essential for the design of molecular electrets with a wide range of charge-transfer properties. Unlike the native amino acids, each of the anthranic residues has two side chains that we selectively alter. Electrochemical and spectroelectrochemical studies revealed that the electronic properties of an anthranilic residue depend not only on the type of a substituent, but also on its exact position [J. Phys. Chem. Lett.
2016, 7, 758-764]. This regio-dependence of properties offers a larger diversity in anthranilamide residues than what native-type amino acids can offer, demonstrating once again the clear advantage of bioinspired over biomimetic or biomediated approaches. The bioinspired molecular electrets, indeed, rectify the kinetics of charge separation, i.e., they act as molecular diodes.[J. Am. Chem. Soc.
2014, 136, 12966-12973] The rates of electron transfer along the electret dipoles are faster than the rates against the dipoles. For processes with small driving forces, the modulation of the energies of the charge-transfer states accounts for the observed rectification trends. Conversely, for processes with a large graving force, such as charge-recombination, it is the asymmetry in the electronic coupling between the electret residues and the axillary photosensitizer. These effects become more pronounced as the length of the anthranilic oligomers increases. Another feature that is affected by the electret length is the kinetic isotope effect on charge-transfer kinetics. Replacing all amide protons with a deuterium significantly slows down charge separation and charge recombination. While the amide protons are not on the charge-transfer pathways, most likely, their modes of vibration couple with charge-transfer transition-state modes affecting its kinetics. Currently, we have developed more than 15 non-native anthranilic residues. Controlling the sequence of these residues in the molecular electrets allows for introducing barriers, wells, and rectifying junctions on the charge-transfer pathways. These findings and developments provide unexplored paradigms for energy science and organic electronics.