Helicenes at Air/Water Interface: Spreading Film and Metal Ion Induced a Helical Ring Nanostructure

Two enantiomeric hydrohelicenes containing a hydroxyl group and a pi-conjugated nonplanar structure are assembled at the air/water interface. These molecules are found to form spreading films with well-defined surface pressure-area isotherms. Upon transferring the spreading film onto the mica surface, porous nanostructures are observed. The spreading film can be transferred onto solid substrates by the Langmuir-Schaefer (LS) method and the transferred LS films display optical activity as revealed by the circular dichroism (CD) spectra. The P- and M-hydrohelicene enantiomers showed mirrored CD spectra, suggesting that the chirality of the LS films was controlled by molecular chirality. When these molecules are spread on the aqueous solution containing metal ions such as Ag+, Cu2+, and Zn2+, a clear twisted ring nanostructure, which is similar to the Mobius strip, is observed. It is suggested that the interaction between the hydroxyl groups of helicenes and metal ions induced such a ring nanostructure.