Three-level hierarchical self-assembly of azobenzene conjugated phenylalanines into superhelical nanostructures with light-switchable helicity

Abstract

Superhelices are ubiquitous in biological systems, but they still remain a great challenge to be realized in artificial systems. In addition, dynamic regulation of the hierarchical superhelical morphology is far from being achieved. Here, photosensitive azobenzene conjugated phenylalanines were rationally designed as building blocks to construct superhelical nanostructures with controllable handedness and switchable morphology. In a mixed solvent of tetrahydrofuran and water, the as-designed phenylalanine derivatives could self-assemble into a series of hierarchical superhelices, including sea urchin-like superhelical spheres, dendritic superhelical nanofibers, superhelical nanofibers, double superhelices and triple superhelices. These superhelices were highly dependent on the solvent ratio of good solvent to bad solvent and the concentration of the phenylalanine derivatives. Furthermore, these chiral superstructures could be reversibly switched into achiral nanorods upon ultraviolet (UV) or visible light irradiation. The successful construction of superhelices and the reversible chiral–achiral switch system will open a new way for the fabrication of chiroptical materials.