Hierarchical chirality observed from chiral supramolecular assembling of racemic and enantiopure helicene derivatives on silica nanohelix surfaces

Abstract

Cast films of racemic helicene derivatives adsorbed onto the surface of nanometric silica helices with controlled handedness exhibited distinct CD signals, whereas no CD signal was observed in the absence of silica nanohelices. These CD signals originate from the helical supramolecular assemblies formed by the racemic mixture of helicenes, with no evidence of enantiospecific adsorption. Interestingly, when enantiomerically pure forms of these helicenes were drop-cast onto the silica helices, different CD spectra were observed depending on the combination of the helicenes’ handedness with that of the silica nanohelices. In all cases, the sign of the CD signals was determined by the handedness of the silica helices, with positive signals for right-handed helices and negative signals for left-handed helices. Furthermore, the wavelength of the CD signal maxima is dictated by the combination of the handednesses both of the helicene and silica helix, and shifts from 250 to 340 nm when they have matching handednesses.