Theoretical design of bioinspired macromolecular electrets based on anthranilamide derivatives

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.