UC Riverside

Accounting for the best features of biological electrets and biomolecular charge transfer (CT) systems, we design bioinspired molecular electrets based on anthranilamide (Aa) motifs. Similar to protein helices, ordered amide bonds generate a macromolecular dipole. The hydrogen-bonding network not only supports the extended Aa conformation, but also provides a polarization that enhances the total dipole of these molecular electrets. Unlike the protein helices, the aromatic Aa moieties and the extended π-conjugation along the Aa backbone provide pathways for efficient long-range CT. These structures illustrate the unexplored potentials of bioinspired approaches to design and development of electronic and energy-conversion systems. Despite all the advances in solid phase peptide synthesis, none of these synthetic protocols are applicable for making Aa oligomers. First, the anthranilic residues are considerably less reactive than aliphatic amino acids. The carbonyls at the ortho-position decrease the nucleophilicity of the free amines. Similarly, the protected ortho-amines decrease the electrophilicity of the carbonyl carbons of the activated carboxylates. Second and most important, activation of carboxyl groups at the ortho position to amides or protected amines leads to the formation of stable cyclic structures that cannot react with the aromatic free amines on the oligomer termini and suppress any further coupling all together. Introducing each of the Aa residues as its 2-nitrobenzoic acid analogue addresses both issues. The strongly electron-withdrawing nitro group increases the electrophilicity of the carbonyl carbon. In addition, the nitro group does not react with the neighboring activated carboxylates to form stable structures that terminate the coupling step. Therefore, I spent my early research synthesizing Aa oligomers from their C- to N-termini via a sequence of various amide coupling and nitro-group-reduction steps. For long-range CT in organic materials, it is important to attain a hopping (or incoherent) mechanism, for which the kinetics exhibits negligible distance dependence beyond about 1 nm. In order to prevent oxidative degradation of electrets mediating such hole hopping, it is crucial for the comprising Aa residues to form stable radical cations, Aa•+. For attaining such stability, we have determined that: (1) the spin density distribution (SDD) of Aa•+ should not extend over its C-terminal amide; and (2) the reduction potentials for oxidizing Aa, should not be too large, i.e., EAa•+|Aa < 1.5 V vs. SCE, to prevent the inherent oxidative degradation of the amides. The latter places a limit on how oxidizing the transferred holes can be. Hole hopping along moieties with as positive EAa•+|Aa as possible ensures the potency of the holes for attaining large open-circuit voltages and for driving chemical transformations. Therefore, a significant amount of my studies involved placing alkoxy side chains on Aa residues brings the reduction potentials of their radical cations, Aa•+, to the limit of 1.5 V vs. SCE, such ether conjugates present a key paradigm in the pursuit of organic derivatives that can transduce strongly oxidizing charge carriers. Overall, the most significant contributions to my doctoral research are: (1) developing a variety of nitro reduction and amide coupling methods to suit both solution-phase and solid-phase peptide synthesis; (2) designing alkoxy anthranilamides to positively shift the reduction potential and still be chemical reversible; and (3) designing the ether synthesis protocols that selectively lead to etherification, without esterification via solvent selection and microwave chemistry.