Fabrication of chiral Langmuir-Schaefer films from achiral TPPS and amphiphiles through the adsorption at the air/water interface

Complex monolayer formation between three amphiphiles: cetyltrimethylammonium bromide (CTAB), octadecylamine (ODA), dioctadecyldimethylammonium bromide (DOAB), and tetrakis(4-sulfonatophenyl)-porphine (TPPS) at the air/water interface were investigated. It was found that TPPS could form complex monolayers with the amphiphiles at the air/water interface through the electrostatic interaction at pH = 3.1. The complex monolayer could be deposited onto solid substrates using a Langmuir-Schaefer (LS) transfer method to form well-defined LS films. Although TPPS existed as a monomer in aqueous solution at pH 3.1, it could form a J-aggregate in the complex monolayers with the amphiphiles. It is interesting to note that the LS films showed strong split Cotton effect in the circular dichroism (CD) spectra although TPPS and the amphiphiles are all achiral. It was found that only if the chirality of the LS film was related to the J-aggregate formation of TPPS in the LS film, i.e., only when the TPPS formed J-aggregate, can a CD signal be detected. Further investigation through AFM measurements revealed that the nanothread formed in the LS films was responsible for the chirality of the LS film.