Electrochemical Properties of Residues for Bioinspired Molecular Electrets

Controlling charge transfer at nanometer scale is crucial for electronic materials and energy conversion. Because local electric fields provide a means for “steering” charge carriers, molecular electrets present an important paradigm for energy science and molecular electronics. An “electret,” or specifically, a “dipole-polarization electret,” is an electrostatic analogue of a magnet, i.e., an electret contains co-directionally ordered permanent electric dipoles. While it is paramount for the electrets to be dielectrics (free charge carriers will screen the fields generated by the intrinsic dipoles and suppress their effects), adjusting their reduction and oxidation properties provides a means for attaining electron or hole transduction through them. 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] Unlike their biological counterparts, however, the bioinspired molecular electrets have extended π-conjugation along their backbones providing a means for efficient charge transfer. Following principles of proteomics, the bioinspired electrets provide venues for attaining an immensely broad diversity of electronic and photonic functionalities by altering the sequences of the non-native anthranilic residues. In biological systems, 20 native amino acids (arranges in various sequences) dictate the countless functionalities of proteins. The single side chain of a native amino acid determines the structural and functional features it introduces to the biomolecules. The anthranilic residues have two side chains that we can alter. Electrochemical and spectroelectrochemical studies revealed that the electronic properties of an anthranilic residue depend not only on the type of a side-chain substituents, but also on its exact position. This regio-dependence of properties offers a much larger diversity in the anthranilic resideus than what native-type amino acids with single side chains provide, demonstrating 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. (The direction of the dipoles is defined from their negative to their positive poles.) For processes with small driving forces, the modulation of the energies of the charge-transfer states accounts for the observed rectification trends. Conversely, the same explanation could not account for the observed rectification of charge recombination. Regioselectivity of electron trasnfer proved to prevail the charge-recombination kinetics. These new mechanistic findings provide unexplored paradigms and venues for energy science and organic electronics.