Light-Driven Linear Helical Supramolecular Polymer Formed by Molecular-Recognition-Directed Self-Assembly of Bis(p-sulfonatocalix[4]arene) and Pseudorotaxane

A light-driven, linear, chiral supramolecular polymer was constructed in water by host-guest molecular recognition between bis(p-sulfonatocalix[4]arene) and the alpha-cyclodextrin-based pseudo[3]rotaxane containing axially chiral 1,1'-binaphthyl and photoresponsive azobenzene moieties. The successful supramolecular polymerization by non-covalent host-guest molecular recognition was confirmed by H-1 NMR spectroscopy and dynamic light scattering (DLS) measurements, and its photoresponsive behavior was investigated by UV-vis absorption spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The chirality of this supramolecular polymer was confirmed by circular dichroism spectroscopy. The dramatic morphology change of this chiral polymer driven by light was observed in SEM, AFM and TEM images. More interestingly, dynamically self-assembled, light-driven, single-helical linear supramolecular polymer molecules with lengths of hundreds of nanometers to micrometers in water were directly observed in their native state using cryo-TEM measurements. The observation of considerably lengthy individual supramolecular polymer molecules indicates that the molecular self-assembly in water by non-covalent host-guest molecular recognition is sufficiently strong to form the supramolecular polymer. Moreover, preliminary molecular modeling was performed to substantiate this interesting photoresponsive supramolecular structure.