Handedness switch of synthesized helical polyaniline nanofibers featuring the use of a single enantiomeric acid and aniline oligomers
Graphical abstract
Polymerization of aniline assisted by an aniline oligomer having two amino end groups (2AO) in the presence of a single enantiomeric acid ((S)CSA) produced the helical PANI nanofibers with the opposite helical sense due to the concentration changes of (S)CSA in the polymerization system.
Introduction
Helicity is widely expressed in biological molecules such as DNA and proteins, owing to the homochirality of their components (D-sugars and l-amino acids). Helix sense control of artificial polymer has been an important issue in the fields of chemistry, biology, and pharmacology [[1], [2], [3]]. Yashima developed helicity induction of optically inactive polymer, in which one-handed helical conformation was induced into the mainchain by noncovalent bonding of chiral small molecules with the polymer [4]. Since this discovery, the helicity inductions of polymers with various binding sites have already been investigated.
Optically active, nanostructured conducting polymers, such as polyaniline (PANI) nanofibers, are of great interest because of their wide applications in electrically and optically active materials and devices [[5], [6], [7], [8]]. Optically active PANI and its derivatives are usually prepared using chemical and electrochemical methods. They can be obtained by ionic interaction with a chiral dopant, and show induced circular dichroism (ICD) in circular dichroism (CD) spectra. For instance, one preparation method involves co-dissolving PANI powder and a chiral acid in common solvents or polymerizing aniline in the presence of a chiral acid [[9], [10], [11], [12], [13], [14], [15]]. However, most PANI materials synthesized from the above methods suffer from low chirality. Oligomer-assisted polymerization results in more efficient preparation of chiral PANI nanofibers. The aniline oligomers or PANI nanofibers are utilized as seeds to catalyze and accelerate the polymerization of aniline in the presence of a chiral acid and produce PANI nanofibers possessing a high degree of one-handed helicity [[16], [17], [18]]. It is suspected that the chirality of the oligomer seed in the early stage of the polymerization is important and decisive for controlling the helicity of the PANI nanofibers. However, it is still unclear that how the oligomer seeds control the chirality or helical sense of the resulting PANI fibers, there are no reports regarding the contribution of the oligomer's chirality to the control of the chirality of the resulting PANI nanofibers.
In addition, there is always great interest in helical inversion of macromolecules because they provide new opportunities for their potential applications [[19], [20], [21], [22], [23], [24]]. It is very difficult to realize the helical inversion of PANI because it is a rigid polymer, and only in a few cases has this helical inversion been achieved by external stimuli, such as solvent, temperature, and pH value or by copolymerization [[25], [26], [27]]. So far, there are no reports regarding the handedness switch of synthesized helical polyaniline nanofibers featuring the use of a single enantiomeric acid and aniline oligomers.
Herein, we find a handedness switch of the synthesized helical PANI nanofibers triggered by polymerization temperature and concentration changes of a single enantiomeric acid in an oligomer assisted polymerization of AN. In order to investigate mechanism of the helical sense control of PANI nanofibers, a batch of oligomer-assisted polymerizations and oligomer oxidation tests are carried out in the presence of a single enantiomeric acid ((1S)-(+)-10-camphor sulfonic acid (CSA)). The influences of the oligomer structure, concentration of (S)CSA, temperature and media on the chirality of the oligoaniline oxides and the helical sense of the resulting PANI nanofibers prepared with the assistance of the oligomers are systematically investigated.
Section snippets
Materials
Aniline, (1S)-(+)-10-camphor sulfonic acid (CSA), 1-4-phenylenediamine, N-phenyl-p-phenylenediamine, and analytical grade pure ammonium persulfate (APS) were purchased from Aladdin Chemical Reagent Corporation. Aniline was used after distillation; other reagents and solvents used in the experiments were all of analytical grade purity.
Measurements
The UV–Vis and CD spectra of polyaniline dispersed in distilled water were recorded on a UV–Vis spectrophotometer (UV-6100) and a spectropolarimeter (Jasco 815),
Induced chirality of aniline oligomers
To investigate the effect of the chirality of the AN oligomer on the resulting PANI nanofibers’ chirality, two achiral AN oligomers, 2AO having two amino end groups and 1AO having a single amino end group, were separately mixed with a chiral acid ((S)CSA). However, neither mixture 2AO/(S)CSA or 1AO/(S)CSA exhibited a detectable CD signal, which indicates that the helical structure could not be induced on the oligomers without further treatment. The failure of chiral induction on the oligomers
Conclusions
Polymerizations of aniline assisted by aniline oligomers having two (2AO) and single (1AO) amino end groups in the presence of (S)CSA were carried out. By merely changing the concentration of (S)CSA, the polymerization assisted by oligomer 2AO successfully produced helical PANI nanofibers with the opposite helical sense. (S)CSA content-dependent sign inversion and pattern change in the CD signal were also observed in the oxidization of 2AO in the presence of different quantities of (S)CSA. The (
Acknowledgements
This work is supported by the National Natural Science of China (21574033), the National Natural Science of Heilongjiang Province, China (B2018003), and the Fundamental Research Funds for the Central Universities (HEUCF181006 and HEUCFQ1417).
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