Bioinspired Design of an Uncharged Ambipolar Helical Scaffold To Achieve Efficient Solid-State Proton Conduction.

Abstract

This work demonstrates highly efficient solid-state proton conduction in helical organic scaffolds inspired by the biomolecule gramicidin A. The scaffold, 1, derived from a pyridine-2,6-dicarboxamide (PDC) residue adopts a helical conformation that is stabilized by a network of strong bifurcated intramolecular H-bonds between the polar residues that align the inner (concave) face of the molecule, while the aromatic units in 1 are oriented outwards. As a result, the helix attains an ambipolar nature just like gramicidin A. Two different solid forms of 1 could be isolated: a yellow solid from high-polarity solvents and an orange solid from low-polarity solvents. Single-crystal X-ray diffraction (SCXRD) studies showed that in the former, molecules of 1 are stacked in a homochiral fashion, while in the latter heterochiral stacks of 1 were present. The yellow form exhibited an almost ∼300-fold higher conductivity (of up to 0.12 mS cm-1 at 95 °C and 95 % relative humidity) than the orange form as a result of closer intermolecular proximity and lower activation energy of 0.098 eV, thus indicating a Grotthus mechanism of proton transport. This study establishes the key role of bioinspired design and controlled stereo-organization of such discrete uncharged organic molecules in achieving efficient solid-state proton conduction.