Imparting chiroptical property to achiral azobenzene derivative via incorporation into chiral-controlled helical nanofibers

Chiral transcription from chiral to achiral organic molecules is a fundamental research area in supramolecular chemistry, organic chemistry, and nanoscience. In this study, we demonstrated that chiral transcription of an achiral azobenzene derivative (Azo) can be achieved by embedding it within helically controlled nanofibers. Helical nanofibers were produced via supramolecular assembly of water-insoluble D-12-hydroxystearic acid (D-HSA) as a chiral source and a water-soluble long-chain amidoamine derivative (C18AA) in water. Azo-incorporated helical nanofibers exhibited a circular dichroism (CD) peak at 350 nm, which was assigned to the π−π* transition band of trans-azobenzene chromophore, suggesting a chiral arrangement of Azo molecules in the nanofibers. Because the nanofibers are dispersed in water and Azo is not soluble in water, the selection of an organic solvent to dissolve Azo is important for the incorporation of Azo molecules. When water-immiscible toluene was used as the solvent, Azo incorporation did not occur, because contact between the nanofibers and Azo was inhibited; however, when water-miscible methanol was used, incorporation was achieved. The incorporation of Azo gradually occurred in the pre-assembled C18AA + D-HSA nanofibers, but co-assembly did not occur during the formation of the C18AA + D-HSA nanofibers. We also showed that Azo-containing nanofibers can undergo a reversible thermal phase transition between gel-to-sol states, switching the CD signal of Azo on and off. Furthermore, trans–cis-photoisomerization of Azo embedded in the nanofibers eliminated the CD peak of the trans-isomer, and no new CD peak corresponding to the cis-isomer appeared.