Hierarchical co-assembly of chiral lipid nanotubes with an azobenzene derivative: optical and chiroptical switching

Two enantiomeric glutamic acid-based bolaamphiphiles are found to form hydrogels in aqueous solution and self-assemble into chiral nanotubes with opposite helicity. An azobenzene derivative, 4-(phenylazo) benzoic acid sodium salt (Azo), was further used to co-assembly with the bolaamphiphiles or nanotube to realize the functions. The azo-containing hierarchical nanotube structures were obtained via different assembly methods. One route is to assemble the Azo with the preformed chiral nanotubes. The other is to mix Azo with the bolaamphiphiles in solution at elevated temperature and then cooled to form the co-gel. Helical tubular structures and supramolecular chirality were observed in the both cases. However, depending on the assembly method, different functions were obtained. The co-assembly of Azo with the preformed chiral nanotubes showed strong exciton-type Cotton effect. Unfortunately, such supramolecular chirality was irreversible to the alternative UV/Vis irradiation although the absorption of the system is reversible due to cis-trans isomerization of Azo. However, if Azo was co-assembled with the bolaamphiphile, starting from the aqueous solution, the resulting Azo-containing lipid nanotubes showed reversible changes both in the UV-vis and CD spectra upon alternative UV/Vis irradiation, indicating the simultaneous formation of an optical and chiroptical switch. These results opens a feasible way to regulate the function of the lipid nanotube systems.