Light-induced twisting, untwisting, and retwisting of aromatic polyamides: an interplay between the induced chirality and the co-facial π-stacking interactions

Abstract

The ability of proteins to undergo conformational changes in response to varying environmental conditions has inspired chemists to devise smart materials that can achieve comparable functions. Oligopeptides, which are simplified versions of proteins, have demonstrated the ability to undergo conformational changes in response to stimuli, transitioning between two ordered structures: helix and sheet. In contrast, such conformational transitions in non-peptidic synthetic polymers are generally limited to order-disorder transitions, specifically shifting between helix and coil states. This report presents a novel approach in which we designed a periodically functionalized aromatic polyamide that exhibits the conformational dynamicity between two order structures (helix ↔ pleated-sheet ↔ helix). The enantiopure pendants of this aromatic polyamide induce a helical structure into the achiral polyamide backbones. At the same time, incorporating the guest molecule enhances the co-facial π-stacking and mediates a conformational transition from a helix to a pleated sheet-like structure. Subsequently, we employed photoresponsive merocyanine as the planar guest molecule, which served as a reversible conformational switch for this aromatic polyamide. The planer merocyanine induces the host–guest complex with this polymer and transforms the helical structure of polyamides into a pleated sheet-like structure. When exposed to visible light, the planar merocyanine changes into a non-planar spyropyran, which breaks apart the host–guest complex and effectively restores the helical structure of aromatic polyamides. Therefore, we present an intriguing demonstration of the twisting, untwisting, and retwisting of aromatic polyamides by balancing two key interactions, such as co-facial π-stacking along the aromatic polyamide backbone and the helical induction from the grafted enantiopure residues.