Fabrication Of Chiral Nanostructures Of Aniline Polymer In Achiral Systems And Their Application In Chiral Separation

As one of the most intensively investigated functional synthetic materials,polyaniline(PANI)with chiral nanostaictures shows potential applications in chiral sensing,chiral catalysis,enantioselective separation and chiral electronic devices.In general,the synthesis of chiral PANI requires a chiral dopant or template to induce the chirality of the synthetic material.However,the construction of PANI chiral nanostructures in an achiral system is still a challenge.Oligoaniline(OANI)is a novel functional conjugated molecule with the conjugated chain structure and electrical activity similar to PANI.The intennolecular force and assembly process of OANI nanostructures with short molecular chain can be adjusted easily.Although studies on OANI chiral nanostructures are rarely reported,there are many studies on their assembly into multi-level micro/nanostructures.In this paper,we intend to control the assembly process of OANI molecule by adjusting the composition of the reaction system and the molecular interaction,so as to realize the construction of OANI chiral nanostructures.Moreover,reactive OANI can be further polymerized to PANI under suitable conditions.Once the OANI chiral nanostructure can be realized,it is expected that the PANI chiral nanostructure would be realized by in-situ polymerization in the presence of OANI as a sacrificial template.In this paper,the supramolecular chiral twisted nanoribbon formed by the achiral oligoaniline(OANI)was prepared simply via the chelical oxidation of aniline in an achiral alcohol/water mixed solvent.The supramolecular chirality could be controlled by facilely adjusting the alcohol content in the mixed solvent.The benzoquinone-containing tetra-aniline derivative C24H20O3N4 was proved to be the major component of the obtained nanoribbons by a series of characterization techniques.Based on the experimental data and the theoretical computation of molecular structure,the formation mechanism of helical nanoribbons was proposed:the tetramer molecules with a specific conformation were packed asymmetrically under the action of molecular interaction during the assembly process,which may be the reason for the formation of supramolecular chiral nanostructures assemblied by the achiral molecules in the achiral system.The main driving force for assembly of the OANI into twisted nanoribbons may be ?-? stacking and hydrogen bonding interactions among the chains,which could be modulated by the alcohol content in the mixed solvent.Based on the above research,the commonly used hydrochloric acid(HCl)with low concentration is introduced into alcohol/water mixed solvent.The PANI hollow nanotwists as complex chiral nanostructures have been prepared via chemical oxidation of aniline in an achiral HCl/alcohol/water mixed solvent.Unlike OANI twisted nanoribbons synthesized in isopropanol/water systems,the proper concentration of inorganic acid ensures the production of PANI polymers and the construction of hollow structures,resulting in PANI hollow nanotwists with higher conductivity,larger specific surface area and wider pH range stability.Based on their identical morphology and chemical structure,it is believed that the formation mechanism of OANI twisted nanoribbons obtained at the early reaction stage in HCl/isopropanol/water mixed solvent should be coincident with that of the products prepared in isopropanol/water mixed solvent.Therefore the chirality of the PANI hollow nanotwists can be controlled by adjusting the isopropanol content.Finally,the application of chiral nanostructures of OANI/PANI in chiral separation was studied.Chiral nanomaterials have high efficiency,low cost and low pollution for the separation of racemic mixtures.The single-handed nanoribbons of OANI obtained in the isopropanol/water system are effective for separating chiral amino acids(phenylalanine,alanine,tryptophan,glutamic acid)from the racemic mixture.Molecular interaction(hydrogen bonding and hydrophobic interaction)provides the driving force for chiral separation.Asymmetric molecular conformation is the key to chiral separation.Compared to OANI twisted nanoribbons,PANI hollow nanotwists exhibited superior enantioselective separation efficiency for all amino acids tested due to strong interactions between PANI and amino acids,as well as a fine chiral matching of the spatial conformations of PANI chains and amino acid molecules.The single-handed PANI hollow nanotwists can be used to separate chiral amino acid for multiple cycles,showing separation stability of amino acid enantiomer.Therefore,the chiral nanostructures of aniline polymers prepared in this paper have broad application prospects in enantioselective separation.