Theoretical design of bioinspired macromolecular electrets based on anthranilamide derivatives

Abstract

Polypeptide helices possess considerable intrinsic dipole moments oriented along their axes. While for proline helices the dipoles originate solely from the ordered orientation of the amide bonds, for 3(10-) and alpha-helices the polarization resultant from the formation of hydrogen-bond network further increases the magnitude of the macromolecular dipoles. The enormous electric-field gradients, generated by the dipoles of alpha-helices (which amount to about 5 D per residue with 0.15 nm residue increments along the helix), play a crucial role in the selectivity and the transport properties of ion channels. The demonstration of dipole-induced rectification of vectorial charge transfer mediated by alpha-helices has opened a range of possibilities for applications of these macromolecules in molecular and biomolecular electronics. These biopolymers, however, possess relatively large bandgaps. As an alternative, we examined a series of synthetic macromolecules, aromatic oligo-ortho-amides, which form extended structures with amide bonds in ordered orientation, supported by a hydrogen-bond network. Unlike their biomolecular counterparts, the extended pi-conjugation of these macromolecules will produce bandgaps significantly smaller than the polypeptide bandgaps. Using ab initio density functional theory calculations, we modeled anthranilamide derivatives that are representative oligo-ortho-amide conjugates. Our calculations, indeed, showed intrinsic dipole moments oriented along the polymer axes and increasing with the increase in the length of the oligomers. Each anthranilamide residue contributed about 3 D to the vectorial macromolecular dipole. When we added electron donating (diethylamine) and electron withdrawing (nitro and trifluoromethyl) groups for n- and p-doping, respectively, we observed that: (1) proper positioning of the electron donating and withdrawing groups further polarized the aromatic residues, increasing the intrinsic dipole to about 4.5 D per residue; and (2) extension of the pi-conjugation over some of the doping groups narrowed the band gaps with as much as 1 eV. The investigated bioinspired systems offer alternatives for the development of broad range of organic electronic materials with nonlinear properties.

Building blocks for bioinspired electrets: molecular-level approach to materials for energy and electronics

“…3b, Table 1). This finding is consistent with our theoretical predictions that placing dialkylamine at the 5 th position of anthranilamides elevates the energy levels of their HOMOs with about 1 eV [48].…”

“…The predicted dipole moment of Ant is about 4.7 D (Table 1), which is consistent with the contributions of 1.9 D from each of the two amides and of 0.9 D from the polarization due to the hydrogen bonding (Fig. 1) [48]. Adding methyl to the 5 th position, such as in Met, slightly enhances the dipole (Table 1), which is consistent with the 5-methyl-induced polarization of the aromatic ring co-directionally with the total dipole from the amide and hydrogen bonds.…”

“…In agreement with the contribution from the amide and hydrogen bonds [48], the magnitudes of the dipole moments of the Aa residues exceed 4 D. The chemical derivatization with the three listed electrondonating groups at the two positions of the Aa aromatic ring (i.e. the 4 th and 5 th ) allow for adjusting the reduction potentials of the Aa oxidation over the range of about 1 V. This range is quite significant: adjusting the HOMO energy levels over 1 eV provides an incomparable means for modulating the hole-transfer pathways along sequences composed of such Aa residues.…”

“…To address some of the challenges with polypeptide biological and biomimetic structures, we have undertaken a bioinspired approach to designing molecular electrets in the search of properties that are beyond what natural systems can offer [45][46][47][48]. Composed of anthranilamide (Aa) residues, the bioinspired electrets possess ordered amide and hydrogen bonds that, similar to the ordered peptide bonds in protein helices, generate an axial electric dipole (Fig.…”

“…Our focus, conversely, is on extended aromatic structures, such as Aa oligomers ( Fig. 1) [46,48,58], which are not truly foldamers. While the common theme as electrets with extended conformations and large intrinsic dipoles is conserved in the Aa structures, alterations of the side chains (R 1 and R 2 , Fig.…”