Helical Polyaniline Nanofibers Induced by Chiral Dopants by a Polymerization Process

Chirality is widely expressed in organic materials, perhaps most notably in biological molecules such as DNA and proteins, owing to the homochirality of their components (D-sugars and L-amino acids). Chiral materials have shown potential applications in materials science, chemical and biological sensors, catalysis, pharmaceutics, and enantioselective separation. Chirality can be expressed at different levels, from chiral small molecules to helical conformation of macromolecules, and even to helical nanostructures and supramolecular structures. Weak interand intramolecular interactions, including hydrogen bonding, solvophobic interaction, p–p interactions, and chiral templates, play an important role in forming helical architectures. Conducting polymers are interesting materials because of their multiple applications in electrically and optically active materials and devices. A helical conformation of conducting polymers such as helical polyaniline (PANI) can be obtained by ionic interaction with a chiral dopant, and shows induced circular dichroism (ICD) in circular dichroism (CD) spectra. On the other hand, nanostructured conducting polymers have been produced by template and “template-free” methods. In the template-free method, nanotubes or sub-micrometer tubes have been realized by using micelles as a “soft template”, while nanofibers have been obtained as a result of the intrinsic nanofibrillar morphology of PANI by preventing their overgrowth. However, the synthesis of enantiomerically pure helical nanostructures of conducting polymers still remains a scientific challenge. In this Communication, we report that conducting PANI nanostructures, including sub-micrometer tubes and helical nanofibers, can be produced by means of a polymerization process in the presence of Dor L-camphorsulfonic acid (CSA) as a dopant. CSA has attracted great attention because of the ability to induce the helical conformation of PANI using its enantiomers, which can be realized by either postprocessing of PANI or in situ polymerization of aniline in the presence of chiral CSA as a dopant. However, there are only a few reports relating to the synthesis of helical nanostructures of PANI. In order to controllably synthesize PANI nanostructures, the effect of the molar ratio of D-CSA to aniline (represented by [D-CSA]/[An]) was studied first. When [D-CSA]/[An] is 1/2 or 1 at a fixed aniline concentration of 0.025 M, dendritic PANI tubes with diameters of 120–550 nm were obtained (Fig. 1a and b). When [D-CSA]/[An] was higher than 1, such as 5 or 80, nanofibers with diameters of 30–80 nm were obtained with a large yield (Figs. 1c,d). Sub-micrometer tubes and nanofibers were formed through different mechanisms. At a low ratio of D-CSA to An, aniline monomer is in excess in the system. Since D-CSA has a surfactant-like property, monomer-filled micelles could form in the reaction system. The polymerization on the micelle “soft-template” had a tendency to form one-dimensional (1D) nanostructures due to the rigid polymer chain of PANI. As a result, sub-micrometer hollow tubes were produced. In contrast, at high molar ratio of D-CSA to aniline, the excess D-CSA acted as a protecting agent to prevent the aggregation of the nuclei of PANI in the polymerization process. Therefore, nanofibers were obtained due to the intrinsic nanofibrillar morphology of PANI. Although the formation mechanisms of sub-micrometer tubes and nanofibers are different, the rigid polymer chain of PANI played a key role in forming both kinds of 1D nanostructures. If a soft template existed and the polymerization occurred on their surface, hollow nanoor sub-micrometer tubes were obtained; whereas if there was no soft template, the efficient prevention of the aggregation of PANI nuclei produced nanofibers. Subsequently, nanostructured PANI was characterized by UV-vis and CD spectrometry. In the UV-vis spectra (Fig. 1e), absorptions at ca. 420 nm and 780 nm were observed due to the p–p* transition of the polarons, indicating that PANI is in the doped state. When the ratio is higher than 1, a free-carrier tail above 800 nm was observed, indicating that the PANI chains are in an extended conformation. Interestingly, negative and positive ICD peaks at 420 nm and 780 nm, respectively, are observed in CD spectra of PANI nanofibers if the ratio of D-CSA to An is higher than 20 (Fig. 1f). Since D-CSA has only a positive peak at 290 nm, the negative peak at 420 nm and the positive peak at 780 nm can be ascribed to the p–p* transition of the polarons in chiral PANI, which is C O M M U N IC A IO N