Thermotropic chirality enhancement of nanoparticles constructed from foldamer/bis(amino acid) complexes

Abstract

Thermo-responsive chiral nanomaterials have garnered much attention owing to their prospective applications in drug delivery, smart surfaces, chiroptical switches, etc. However, as the system temperature is increased, the nanostructures tend to collapse, leading to decreased chirality. Therefore, it remains a formidable challenge to endow chiral nanomaterials with thermotropic chirality enhancement. Herein, chiral nanoparticles have been constructed by mixing a poly(m-phenylene diethynylene) derivative (Poly-1) bearing aza-18-crown-6 pendants with an adequate amount of L-homocystine perchlorate salt (L-Hcy). The L-Hcy guest acts as an intermolecular cross-linker to bind with two aza-18-crown-6 pendants in different Poly-1 chains via three N⁺–H⋯O hydrogen bonds. In this case, Poly-1 features an extended preferred-handed helix, accompanied by a partial helix-coil transition. Upon heating to 50 °C, two pendant aza-18-crown-6 rings in the same Poly-1 chain can be occupied by one L-Hcy guest, leading to the enforced π–π stacking interaction in the main chain. As a result, Poly-1 folds into a contracted preferred-handed helix, so that enhanced chirality is achieved. After further increasing the temperature, L-Hcy is isolated because hydrogen bonds are destroyed completely. Finally, Poly-1 possesses an equal mixture of the extended left-handed and right-handed helices. This work not only provides a novel strategy to regulate folded helices by changing the host–guest binding modes during the heating process, but opens a way for developing stimuli-responsive chiral nanomaterials.