Controllable Supramolecular Chiral Twisted Nanoribbons from Achiral

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Controllable supramolecular chiral twisted nanoribbons from achiral conjugated oligoaniline derivatives Chuanqiang Zhou, Yuanyuan Ren, Jie Han, Xiangxiang Gong, Zhixiang Wei, Ju Xie, and Rong Guo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12178 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Journal of the American Chemical Society

Controllable supramolecular chiral twisted nanoribbons from achiral conjugated oligoaniline derivatives Chuanqiang Zhou,† Yuanyuan Ren,†,‡ Jie Han,*,‡ Xiangxiang Gong,† Zhixiang Wei*,§Ju Xie‡ and Rong Guo‡ †Testing Center, Yangzhou University, Yangzhou, Jiangsu, 225002, P. R. China ‡School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu, 225002, P. R. China § Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing, 100190, P. R. China Supporting Information Placeholder light,7 temperature,8 solvent,9 pH,10 and rotary stirring.11 In these cases, chiral guests or asymmetric factors are always present, and the chirality of supramolecular nanostructures is usually determined by the molecular chirality of chiral guests or substituent groups.12 In fact, the supramolecular chirality of nanostructures is not only affected by the molecular chirality, but also is closely depended on the guests around them which might be chiral, asymmetric, or achiral. Supramolecular chiral systems can also be formed from achiral molecules which have shown the ability to induce or change their self-assembly to generate helical or twisted supramolecular systems. However, non-selective supramolecular self-assembly generally leads to equal numbers of left- and right-handed entities from achiral systems that do not show any optical activity under thermodynamic equilibrium conditions. In some specific cases, optical active helical or twisted nanostructures can be formed using achiral molecules through asymmetric induction (symmetry breaking).13 Symmetry breaking, leading to a specific handedness of biological structure, is one of the most fascinating phenomena in nature. Under external stimuli (i.e. solvent effect), the symmetry breaking in assembly could generate single chirality in the supramolecular structure via adjusting weak intermolecular interactions (such as hydrogen bonds, π-π interaction).14,15 Liu’s group has reported that formation of chiral supramolecular assembly from the symmetry breaking of achiral molecules via π-π interaction in cyclohexane.15a They have also found that achiral C3symmetric molecules can self-assemble into supramolecular nanostructures with macroscopic chirality in the organic solvent/water mixture.15b Studies on the symmetry breaking are of great importance to understand the origin of homochirality in nature as well as to construct chiral functional materials. Conjugated polymers are interesting materials due to their multiple applications in electrically and optically active materials and devices. Chiral conjugated polymers have tremendous potential for a wide variety of potentials in various fields such as asymmetric synthesis, chiral sensors and separation of enantiomers employed as chiral stationary phase and molecular imprinting.16 In the past decades, various helical or twisted conjugated polymer materials, including polyaniline,17,18 polythiophene19 and polypyr-

ABSTRACT:

The fabrication of supramolecular chiral nanostructures from achiral materials without the need of preexisting chirality is a major challenge associated with the origin of life. Herein, supramolecular chiral twisted nanoribbons of achiral oligoaniline derivatives were prepared via simply performing the chemical oxidation of aniline in alcohol/water mixed solvent. In particular, the supramolecular chirality of the twisted nanoribbons could be controlled by facilely tuning the alcohol content in the mixed solvent. A tetra-aniline derivative C24H20O3N4 was attested to be the major component of the obtained nanoribbons. The main driving forces for the assembly of the oligoaniline derivative into twisted nanoribbons might be the π-π stacking and hydrogen bonding interactions among the chains which could be modulated by the alcohol content in the mixed solvent. The single-handed twisted nanoribbons could be used to separate chiral phenylalanine from a racemic mixture. Thus, it is highly anticipated that the supramolecular chirality endows π-conjugated molecules with potential application in chiral recognition.

▇ INTERODUCTION Chirality is widely discovered as one of the key structural factors in nature to perform a series of complicated functions, especially in biological molecules such as proteins and DNA, in which the helix is one of the most simple and important states.1 Chirality of a system can be expressed at different levels, including chiral small molecules and chiral supramolecular nanostructures (for example helical conformation of macromolecules, and helical or twisted nanostructures).2 Chiral supramolecular nanostructures are attracting attention owing to their crucial roles in the fields of chiroptical switches,3 sensing or recognition,4 catalysis,5 and so on. Different from the chirality of small molecules that have asymmetric carbon atoms, supramolecular chirality of nanostructures is often achieved by supramolecular assembly in a controllable way through non-covalent interactions including hydrogen bonds, π-π stacking and electrostatic interactions.6 Because of the non-covalent interactions, some chiral supramolecular nanostructures could be influenced or controlled by external stimuli, such as

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role20 have been synthesized through the asymmetric polymerization, which can be proceeded if chiral monomers, dopants or templates are involved, or if some external chiral influence is present. For instance, Guo et al. have synthesized the optical active polyaniline nanospheres induced by protein through the chiral template-assisted polymerization route.17c Wei’s group has reported the preparation of polyaniline helical nanofibers,18a,b and superhelical microstructures,18c by a polymerization process in the presence of chiral dopant acids. In these preparations, chiral dopants or templates are usually required for inducing the chirality of the objective materials. Although the supramolecular chirality of achiral amphiphilic molecules have been achieved in a confined air/water interface,15c the supramolecular chirality of achiral conjugated polymers or oligomers achieved in the bulk solution of an achiral reaction system has not been reported. In this work, we report the interesting finding that supramolecular chiral twisted nanoribbons can be generated by the chemical oxidation of aniline in the alcohol/water mixture without any chiral molecules. Specially, the twisted handedness and chiral sense of the nanoribbons could be facilely adjusted by alcohol content in the mixture. As characterized by matrix-assisted laser desorption ionization time of flight mass spectrum (MALDI-TOF MS), high-performance liquid chromatography (HPLC)-MS, Fourier transform infrared (FTIR), 1H NMR, and UV-vis techniques, the main chemical component of the obtained nanoribbons was found to be a tetra-aniline derivative. Based on our experimental results, the driving forces for the assembly of tetra-aniline derivative into twisted nanoribbons were explored, and a possible formation mechanism of the twisted nanoribbons was deduced.

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character, whereas a few twisted nanoribbons are found at 30 % ipropanol (Figure S1). But, if the i-propanol content is larger than 45 %, almost no precipitates can be found since only soluble oligomers are generated in the reaction medium. In fact, the twisted nanoribbons are merely obtained with the i-propanol content in the range of 35 %-45 %. Thus, it is inferred that the moderate ipropanol content of the reaction medium should be a prerequisite for the successful fabrication of twisted nanoribbons. (a)

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Figure 1 (a, b, f-h) FESEM and (i-k) TEM images of the twisted nanoribbons synthesized at 40 % i-propanol of the mixed solvent. (c-e) Schematic diagrams show the torsional directions of twisted nanoribbons.

▇ RESULTS AND DISCUSSION

Further experiments find that i-propanol content of the reaction medium varying in the range of 35 %-45 % can be employed to tune twisted orientation of the nanoribbons. At 35 % i-propanol, almost all the products are left-handed nanoribbons with 80-150 nm in a width and 200 nm in an average pitch (Figures 2a and 2b). With 40 % i-propanol, the left-handed nanoribbons are slightly less than right-handed nanoribbons (Figure 1b). At 45 % ipropanol (Figures 2c and 2d), the major morphology of the product is the right-handed nanoribbons with 90 nm in a mean width and 150-180 nm in the pitch. For illustrating the moderation of ipropanol content to the configuration of twisted nanoribbons, Figure 2e gives a percent histogram of single-handed nanoribbons as a function of i-propanol content. The statistical results are obtained by observing more than 200 twisted nanoribbons that are randomly selected in different regions of FESEM samples. Thus, it is suggested that the twisted handedness of nanoribbons can be facilely adjusted by tuning i-propanol content in a suitable range. According to the above morphology observations, these welldefined twisted nanoribbons may be originated from the supramolecular chirality of products since no chiral reagents are used in the synthesis. In order to attest the supramolecular chirality presented in the products, these twisted nanoribbons dissolved in organic solvent N,N-dimethylformamide (DMF) and dispersed in water were measured by the circular dichroism (CD) technique. As for DMF solutions containing single molecules, flat curves with no Cotton effect are observed in the CD spectrum (Figure S2), indicating that the products are chiral silent in the molecule level and do not contain any chiral carbon atom. However, it is surprisingly found that the twisted nanoribbons dispersed in water show obvious optical activity, as seen from the UV-vis and CD spectra (Figure 2f). CD spectrum of the product obtained at 35 % i-propanol displays a negative Cotton effect at about 201 nm,

Controllable fabrication of twisted nanoribbons. Conventionally, oligoaniline was synthesized by the chemical oxidation of aniline using water as the reaction medium and ammonium peroxydisulfate as the oxidant, leading to achiral nanoarchitectures.21 When alcohol/water mixture is used instead of pure water as a reaction medium, supramolecular chiral twisted nanoribbons could be fabricated. Herein, i-propanol is firstly chosen as the typical alcohol for example to form the mixed solvent with water for the preparation. Figure 1 shows the typical field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) images of twisted nanoribbons synthesized by the chemical oxidation of aniline in i-propanol/water mixed solution using ammonium peroxydisulfate as the oxidant. Figure 1a exhibits that the product is composed of uniform and dense 1D nanostructures with several microns in length. It is observed from a magnified FESEM image (Figure 1b) that a majority of nanostructures are twisted nanoribbons with 50-150 nm in width. Due to the absence of chirality induction, left-handed and righthanded twisted nanoribbons even torsional heterojunctions emerge in the product. Their schematic diagrams are given in Figures 1c-e for visualizing the torsional conformations, and the corresponding FESEM and TEM images are shown in Figures 1fh and Figures 1i-k, respectively. The thread pitch of the twisted nanoribbons with about 20 nm in thickness is observed to be approximately 200 nm. Above observations indicate that the twisted nanostructures can indeed be fabricated in the bulk solution without any chiral factor. The i-propanol content in the mixed solvent (which is denoted by the volume fraction) was found to be able to affect the nanostructures of the obtained product. At a low i-propanol content (20 %), the product contains only nanorods without any chiral


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indicative of superhelical n-π* transition,22 corresponding to the left-handed twisted nanoribbons (Figures 2a and 2b). A positive Cotton effect at about 202 nm is found in the CD spectrum of the twisted nanoribbons prepared at 45 % i-propanol, which is related to the right-handed twisted nanoribbons (Figures 2c and 2d). Without any asymmetric carbon atom in the products, the optical activity should be originated from the helical arrangement of molecule chains in nanoribbons, proving the supramolecular chirality of the twisted nanoribbons. As expected, more right-handed twisted nanoribbons prepared at 40 % i-propanol (Figure 1b) exhibit a relatively weak positive peak at about 202 nm in the CD spectrum. Since left- and right-handed enantiomers express opposite Cotton effects at a near wavelength, the CD scan provides only a weak signal in intensity because of the counteraction of their optical rotation. It should be pointed out that the left- and righthanded nanoribbons are not exactly enantiomer, and their CD spectra are not exact mirror images, where the handedness selection of nanoribbons might be affected by the kinetics of the molecular association.22b,c From these results, a relationship between the handedness of twisted nanoribbons and the CD signal can be established, that is, the left-handed nanoribbons show a negative Cotton effect at 201 nm, whereas the right-handed nanoribbons exhibit a positive Cotton effect at 202 nm. Therefore, it is concluded that the sense of supramolecular chirality of the twisted nanoribbons can be successfully inverted by controlling the ipropanol content.

long chain alcohols, such as butanol, they cannot form miscible solution with water due to their low solubility in water, thus this fabrication cannot be achieved. In n-propanol/water or ethanol/water mixed medium, twisted nanoribbons could also be successful prepared, and the supramolecular chiral transformation of twisted nanoribbons from left-handed to right-handed could also be adjusted through controlling the alcohol content (Figure S3 and Figure S4). However, in methanol/water mixed medium, there were no twisted nanoribbons, and the supramolecular chirality was not observed in the CD spectra (Figure S5). Obviously, the alcohol species could affect the formation of supramolecular chiral nanostructures. In fact, the solvent-induced supramolecular chirality has been reported in other aqueous co-solvent mixtures.15a, 15b, 23 During the formation process of twisted nanoribbons, the alcohol used is believed to participate or interfere in the self-assembly of molecules.

(b)

Figure 3 (a) MALDI-TOF MS of twisted nanoribbons synthesized at 40 % i-propanol. (b) MS of the maximum HPLC component of twisted nanoribbons synthesized at 40 % i-propanol with the retention time of 10.23-10.29 min. (c) FTIR and (d) 1H NMR spectra of the twisted nanoribbons synthesized at 40 % i-propanol. Inset in (b) gives the chemical structure of the major molecule with Mw=412 m/z. 80 60 40

(f)30

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Chemical structures of twisted nanoribbons. In order to infer the origin of the supramolecular chirality, the chemical structures of the product should firstly investigated via characterizing by various techniques. The MALDI-TOF MS (Figure 3a) displays low-molecular weight peaks with a maximum signal at 412 m/z, basically consistent with gel permeation chromatography (GPC) result where only one peak is observed in the range of 300-1000 (Figure S6). For further determining the chemical structures with different molecular weights, HPLC technique is employed to separate these molecules (Figure S7). The maximum HPLC peak with the retention time of 10.23-10.29 min is analyzed by MS spectrometer equipped onto HPLC (Figure 3b) to be C24H20O3N4 with Mw=412 m/z, which is reasonably corresponded to the molecular structure of a tetra-aniline derivative (inset in Figure 3b).24 MS spectra of other components with different retention times can be assigned to other aniline oligomers (Figure S8). UV-vis spectrum of the product dissolved in N-methylpyrrolidone (Figure S9) proves also the oligomer character of the product.

UV-vis absorbance (a.u.)

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Figure 2 (a, c) FESEM and (b, d) TEM images of twisted nanoribbons synthesized at different i-propanol contents: (a, b) 35 %, (c, d) 45 %. (e) Proportion histograms of left- and right-handed nanoribbons synthesized at different i-propanol contents. (f) UVvis and CD spectra of twisted nanoribbons dispersed in aqueous solution synthesized at different i-propanol contents. To inspect the applicability of this fabrication, other alcohols were also used to replace i-propanol as co-solvents to form mixed mediums with water for the chemical oxidation of aniline. As for



In FTIR (Figure 3c), two bands at 1573 and 1506 cm-1 correspond to C=C stretching vibration in quinonoid and benzenoid rings, respectively. The bands at 1416, 859 and 838 cm-1 are assigned to characteristic peaks of phenazine units.24 A shoulder at 1635 cm-1 is due to N-H scissoring vibrations of aromatic amines in the benzoquinoneimine ring. The sharp band at 1445 cm-1 is attributed to the skeletal C=C stretching vibration of the substituted aromatic ring. The bands at 1347 and 1289 cm-1 correspond to the C-N stretching vibrations of benzoquinoneimine ring.24,25 Two peaks at 756 and 693 cm-1 are related to the C-H out-of-plane bending and out-out-plane ring deformations of a monosubstituted phenylene ring, respectively. FTIR bands are well attested the presence of the main component (tetra-aniline derivatives). In the 1H NMR data (Figure 3d), the typical benzenoid protons are identified by these signals centered at 7.48, 7.26 and 7.00 ppm. The 1H signals at 7.15, 6.68, 6.03 and 5.93 ppm are associated with protons directly bonded to different aromatic rings.26, 27 The chemical shift at 6.43 ppm is attributed to the protons in amino group, while the peaks at 5.35 ppm corresponds to the protons of the benzenoid ring in phenazine segment.26a Three signals at 8.93, 8.58 and 8.00 ppm may be due to the π-π interactions among moleucles.14a,15 The chemical shifts at 7.42, 7.31 and 6.25 ppm are related to the benzenoid protons of phenol ring.26b The peaks centered at 6.16 and 5.60 ppm are assigned to the benzenoid protons of substituted benzoquinoneimine ring.27 1H NMR results affirm further the tetra-aniline derivative C24H20O3N4 as a main component. As a typical character of conjugated molecules, the reversible acid/base doping/dedoping property for the obtained product was verified by UV-vis measurement. As shown in Figure 4, two obvious absorption bands at 295 nm and 425 nm are the typical characteristic of the oxygen-containing oligoaniline.24, 25b When the product is doped by sulfuric acid, the absorption maximum at 537 nm corresponds to the n-π* transition. If the doped product is dedoped with ammonia, its spectrum exhibits similar absorption peaks to those of the original ones. Results indicate that the prepared oligoaniline derivative can be doped and dedoped, maintaining the unique characteristic of conducting polymer.28

Moreover, the FTIR spectra of the products with different reaction stages (Figure S12) attest that van der Waals forces between the neighboring oligomer skeletons and intra- and intermolecular hydrogen bonding interactions increase with the reaction time. Above experimental results confirm that the main intermolecular interactions occurred in the twisted nanoribbons include π-π stacking and hydrogen bonding interactions.

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Figure 5 (a) Theoretical optimized structure of the tetra-aniline derivative C24H20O3N4. (b) XRD pattern of the twisted nanoribbons synthesized at 40 % i-propanol. (c, d) Theoretical optimized conformation of the C24H20O3N4 predominated by (c) hydrogen bonding interactions with the total binding energy of -251.71 kJ/mol, or (d) conjugated π-π stacking interactions with the total binding energy of -276.73kJ/mol. For exploring the formation mechanism of twisted nanoribbons, theoretical computation was performed to study the molecular conformation and self-assembly behavior of the tetra-aniline derivatives. The density functional theory (DFT) calculation by using Gaussian 09 was frequently used to optimize the molecular structure in supramolecular self-assembly systems and could obtain reliable results.15b, 30 Based on the relative reports, Gaussian 09 package should be creditable software and DFT calculation could provide a suitable computation methodology for optimizing the molecular structure. Considering the achiral character of the tetra-aniline derivative C24H20O3N4, we optimized its molecular structure at B3LYP/6-31G(d,p) level with Gaussian 09 program.15b As shown in Figure 5a, the optimized horizontal length of the tetra-aniline derivative is 1.66 nm, and the vertical distances are 0.49 and 0.46 nm, respectively, marking the benzoquinoneimine ring as the datum plane. In the XRD pattern (Figure 5b), the d-spacing of 1.61, 0.50 and 0.47 nm should correspond to the theoretical molecular horizontal length and vertical distances, respectively. A diffraction peak centered at 23.88° corresponds to the d-spacing of 0.37 nm as a typical π-π stacking distance.17a As found, the vertical distance of the tetra-aniline derivative (0.49 and 0.46 nm) is larger than 0.37 nm, suggesting that the molecules should assume a certain conformation, and their misalignment might happen during the self-assembly process with a staggered angle to fit into such space.15 Meanwhile, four reflection peaks corresponding to d-spacing of 2.86, 1.41, 0.75 and 0.37 nm verify the lamellar structures in the helices. Based on the above results, a possible formation mechanism of twisted nanoribbons can be deduced. As the oligoaniline derivative C24H20O3N4 is an achiral molecule, the chirality origin of

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800

Wavelength (nm)

Figure 4 UV-vis spectra of the obtained product synthesized at 40 % i-propanol with doping/dedoping treatments. Formation mechanism of twisted nanoribbons. To probe the intermolecular interactions in the twisted nanoribbons, the offspring generated at different reaction times were separated from the reaction system for FESEM, 1H NMR and FTIR characterizations. FESEM results (Figure S10) suggest that the twisted nanoribbons are formed at the reaction time of 90-120 min. The 1H NMR data (Figure S11) indicate that with the increasing reaction time these aromatic rings of oligomers in product are gradually involved into the π-π interactions during the self-assembly.15,29


conformers of conjugated molecules,30a, 33 and that the strong hydrogen bonding interactions are beneficial to the right-handed conformers.33,34 To validate the occurrence of the symmetry breaking in the system, time-dependent density functional theory (TDDFT) calculations with the B3LYP and the 6-31G(d,p) basis set were conducted to calculate the CD spectra of the dimer assembly of the “S” and “V” shaped molecules (Figure S14). The results approve indeed that the co-assembly of two “S” shaped molecules prefers the left-handed chiral assemblies with negative Cotton effect, while the right-handed congeries with positive Cotton effect are prior for “V” shaped molecules. Once the formation rate for one-handed dimer assemblies as minor chiral species is slightly larger than another in the beginning, such difference would be amplified in the next association to generate excess and lager single-handed supramolecular structures with an evident optical activity.15c, 31, 33

supramolecular chiral twisted nanoribbons may be ascribed to an extrinsic induction occurred in the studied system. However, the supramolecular chiral twisted nanoribbons of achiral oligoaniline derivatives were synthesized without rotary stirring, and the twisted nanoribbons were composed of undetectable amounts of chiral impurities (Figure S8). Although the extrinsic induction effect of chirality origin is not fully understood, the minute difference of the beginning chiral conformations at different i-propanol contents is amplified by the following assembly processes, finally resulting in the generation of chiral twisted nanoribbons. From the molecular structure point of view, the obtained tetra-aniline derivative is asymmetry along its long axis (Figure 5a), which would cause the asymmetry arrangement when the molecules assemble along their long axis in the π-π stacking system.18c,d Theoretical computation and XRD results suggest that the tetra-aniline derivative should assume a certain conformation during the assembly processes, which may be crucial for the formation of supramolecular chiral twisted nanoribbons.15c, 30a Obviously, the conformation of the molecular chain is closely related to the intermolecular interactions occurred in the assembly.15, 30a, 31 Although it is difficult to calculate the accurate geometry structures of this molecule under multiple intermolecular interactions, the conformations of the tetra-aniline derivatives should depend severely on the predominated interaction (π-π stacking or hydrogen bonding forces). For instance, if the conjugated π-π stacking force is the major interaction, this molecule would assume the “S” shaped conformation (Figure 5c), while its conformation might similar to the “V” shape (Figure 5d) when the hydrogen bonding interaction is predominated. HN

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To validate the relationship between the major molecular interactions and the chiral conformation, it is necessary to characterize the products containing single-handed twisted nanoribbons. As shown in Figure 2, the product synthesized at 35 % i-propanol is mainly composed of left-handed nanoribbons, while the righthanded nanoribbons are the dominating morphology at 45 % ipropanol. Thus, the product mainly containing left-handed nanoribbons can be collected in the reaction system at 35 % i-propanol, and the product obtained at 45 % i-propanol is deemed as righthanded nanoribbons. Both twisted nanoribbons were characterized by MALDI-TOF MS, FTIR, 1H NMR and XRD techniques. As found from these characterizations, although their chemical components are basically the same (Table S1, Figure S15-17), the intermolecular interactions seem to be different in strength for left- and right-handed nanoribbons (Figure 7). The left-handed nanoribbons obtained at 35 % i-propanol show stronger XRD diffraction peak at 23.88° (d=0.37 nm) than that of the righthanded nanoribbons obtained at 45 % i-propanol (Figure 7a), meaning stronger π-π stacking interaction in left-handed nanoribbons. In addition, the right-handed nanoribbons appear a stronger IR bands at 3235 cm-1 compared to that of the left-handed nanoribbons (Figure 7b), indicative of the dominating hydrogen bonding interaction in the right-handed nanoribbons. The experimental results provide reliable proofs that the π-π stacking forces are the major intermolecular interactions in the left-handed nanoribbons, whereas that the right-handed nanoribbons are mainly driven by hydrogen bonding interactions. As known, with the low ipropanol content (35 %), the relative high water content causes high polarity and hydrophilicity of the solvent,35 which make the hydrophobic interactions of hydrophobic moieties (benzene/quinone rings of tetramer) much stronger,36 thus resulting in enhanced π-π stacking interactions.34 As the i-propanol content

predominated by hydrogen bonding

Ⅰ the misalignment of tetramers with certain conformation leads to twisted conformer

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Figure 7 (a) XRD patterns and (b) FTIR spectra of twisted nanoribbons obtained at 35 % and 45 % i-propanol.

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Transmittance

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Top view

Figure 6 Schematic illustration of the possible formation processes of left- and right-handed helix conformer by the tetra-aniline derivatives. Conformations (I) and (II) are the same to Figure 5c and d. Due to steric hindrance and electrostatic repulsion (Figure S13), the molecule with the “S” shaped conformation would selfassemble with each other and adjust their tilted orientation to form left-handed conformer, and the right-handed conformer should be generated by the misalignment of “V” shaped molecules (Figure 6).30a, 32 In fact, it has been reported that the system dominated by π-π stacking interactions favors the generation of the left-handed



increases from 35 % to 45 %, the polarity of the solvent is decreased,35 and therefore the hydrophobic backbone of tetra-aniline derivative will extend to expose more hydrophilic groups (such as -OH, -NH- or =O) on its surface,37 which favors the formation of the intermolecular hydrogen bonds.33 Therefore, it is reasonably to conclude that the major intermolecular interactions occurred during the assembly which are controlled by i-propanol content, do indeed adjust the handedness of the twisted nanoribbons. In our proposed formation mechanism, the product is deemed as the tetra-aniline derivative C24H20O3N4. In fact, the obtained product contains also other species besides this material (Figure S8). For guaranteeing the credibility of the proposed formation mechanism, the special material C24H20O3N4 as a major component in the products is purified according to its solubility property. Compared with other species (Figure S8), this tetra-aniline derivative contains two hydroxyl groups and thus possesses relative good solubility in the alcohol solvent. When the product is dispersed into methanol, the tetra-aniline derivative can dissolve but other species are insoluble, thus, other species can be removed by filtration. On the other hand, due to hydrophobic phenyl segments, the objective material is difficult to dissolve in water-rich mixture. Then water is added to the above alcohol solution to deposit a precipitation from the mixed solution which is further separated by centrifugation (see Experimental Section for details). Using the dissolving-filtrating-depositing-separating method, the product has been carefully purified for characterization (Table S1, Figure S18), and the relatively pure tetra-aniline derivative C24H20O3N4 is obtained. (a)

the tetra-aniline derivatives can indeed self-assemble into twisted nanostructures in i-propanol/water mixture. Chiral recognition application of single-handed nanoribbons. To explore the potential of the twisted nanoribbons of the tetra-aniline derivatives for chiral recognition, the helically distinct forms of twisted nanoribbons were separately incubated with racemic mixtures of D- and L-phenylalanine. The 1:1 mixtures of the D- and L-phenylalanine have no observable CD signal (Figure 9), due to the offset of opposite Cotton effects. The left- or righthanded nanoribbons were then added into this racemic mixture for incubation, and the mixture was filtered to obtain filtrate for CD measurement. The CD spectrum of the filtrate incubated by lefthanded nanoribbons shows positive Cotton effect centered at 216.8 nm coming from D-phenylalanine, which suggests that the left-handed nanoribbons can capture and remove L-phenylalanine from the solution. Similarly, right-handed nanoribbons can bind and separate the D-phenylalanine from the solution. The separation efficiency of single-handed nanoribbons for the 1:1 mixtures of the D- and L-phenylalanine can be evaluated based on the linear change of UV-vis and CD intensities with the concentration in a certain range (Figure S19). The enantiomeric excess (ee (%)) of left-handed nanoribbons is 58.62%, and the ee (%) of righthanded nanoribbons is about 25.00%. This result manifests that the single-handed nanoribbons of tetra-aniline derivative are indeed valuable for chiral sensing applications. The similar observations have also been reported in other researches.13, 38, 39 The binding driving forces of tetra-aniline derivatives to phenylalanine may originate from the hydrophobic force and hydrogen bond between two function groups (-COO- and –NH2) of amino acid and the responding groups on oligoaniline.38, 39 Chiral spatial conformations of tetra-aniline derivative would affect the contacting area and binding interactions with phenylalanine.38a, 39 When the molecular geometry of one phenylalanine was matched to that of tetra-aniline derivative, they would be chelated and bound with each other,4 or else, the phenylalanine might be released, leaving the another enantiomer in solution, similar to those of the other chiral materials.18b, 38, 39

(b)

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Figure 8 FESEM images of the purified tetra-aniline derivatives self-assembling into twisted nanofibers when they are transferred from good solvent THF to poor solvent i-propanol/water mixture at different i-propanol content: (a) 35 %, (b) 45 %. Inserts in (a) and (b) give the corresponding CD spectra of these self-assembled twisted nanofibers dispersed in water.

1:1 mixture of L-/D-phenylalanine incubated by left-handed ribbons incubated by right-handed ribbons

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Interestingly, the purified tetra-aniline derivatives are able to self-assemble into supramolecular chiral twisted nanofibers (Figures 8a and 8b) when they are transferred from good solvent THF to poor solvent i-propanol/water mixture. The material is soluble and exists in the molecule state in good solvent THF. When ipropanol/water mixture as a poor solvent was slowly added into the above THF solution, the tetra-aniline derivatives could progressively self-assemble into supramolecular aggregates. During the assembly, the π-π stacking and hydrogen bonding interactions will drive the molecules to assume certain conformations and to align with a staggered angle, finally resulting in the twisted nanostructures. At 35 % i-propanol, left-handed twisted nanofibers are seen in the products (Figure 8a), and the right-handed twisted nanofibers appear in the products at 45 % i-propanol (Figure 8b). CD measurements indicate that these twisted nanofibers dispersed in water have obvious optical activity (inserts in Figures 8a and 8b). These observations attest from the molecular level that

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Figure 9 CD spectra of 1:1 mixture of L-/D-phenylalanine and filtrates incubated by left-handed and right-handed nanoribbons of tetra-aniline derivatives. ▇ CONCLUSIONS In summary, an achiral reaction system was reported for the first time to controllably fabricate the supramolecular chiral twisted nanoribbons of conjugated molecules. The alcohol content in ipropanol/water reaction medium varying in the range of 35 %-45 % has been employed to tune the twisted orientation and supra-


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molecular chirality of the nanoribbons. The tetra-aniline derivative C24H20O3N4 was proven to be the major component of the obtained products. It has been deduced that the solvent surrounding can control the assembly of tetra-aniline derivatives through adjusting the intermolecular interactions, and further determine the handedness of the twisted nanoribbons. The helically distinct forms of the twisted nanoribbons could be used for chiral recognition, showing the prospective of sensing application. This system is one of very few examples where achiral conjugated molecules synthesized in achiral solution self-assemble into nanostructures with supramolecular chirality. This work will provide a facile approach in the fabrication of supramolecular chiral nanostructures of conjugated molecules in an achiral system, and will be instructive for further understanding of the homochirality in nature.

tively. The FTIR spectra (670-IR + 610-IR, Varian Co., USA) were recorded in the range of 400-4000 cm-1. The UV-vis spectra (Cary 5000, Varian Co., Japan) of the samples were measured in the range between 200-800 nm. 1H NMR spectra of samples dissolved in d6-acetone were collected at 296 K on a Bruker AVANCE 600 spectrometer. The samples were taken to the MALDI-TOF MS analysis using MALDI-TOF/TOF mass spectrometer (AB 5800, SCIEX, USA) on the linear model. The MALDI-TOF MS sample was prepared by adding the matrix [2,5dihydroxybenzoic acid (DHB)] into about 2 µl of the dilute sample solution in dimethylformamide dropped on the stainless-steel sample holder, and the solvent was evaporated on an electrical heater. The concentration of DHB in the solvent was about 10 mg/mL. CD analysis was performed on a Jasco J-810 CD spectropolarimeter (Jasco Inc., Japan) over a wavelength range 190-350 nm with a resolution of 1 nm. Spectra were recorded three times using a scanning speed of 100 nm min-1 with a bandwidth of 1 nm. The resonance of the methyl group of the solvent was used as reference for shift tabulated values. XRD analysis was performed on a Rigaku D/Max-2500 X-ray diffractometer (Japan) with Cu Kα radiation (λ=1.5406 Å), which was operated at a voltage of 40 kV and a current of 200 mA. Samples were cast on glass substrates and vacuum-dried for XRD measurements. Elemental composition was determined with a CHNS/O Analyzer (Vario EL cube, Germany Elementar Co.). The molecular weights distribution of as-synthesized products dissolved in THF were measured with GPC (Agilent1100, Agilent Co.). Before analysis, the sample was filtered through a 0.45 µm syringe filter, and THF was used as the eluent with the flow rate of 1 mL/min at 25 oC. Calibration was accomplished with monodisperse polystyrene standards. A maXis ultra-high resolution QTOF-MS (Bruker Daltonik, Bremen, Germany) combined with a Dionex U3000 HPLC system (Dionex Softron, Germering, Germany) was used to analyze the products. Data acquiring was executed by using Compass 1.3 SR3 software (Bruker Daltonik, Bremen, Germany). Chromatographic separation was achieved on an Agilent Eclipse plus C18 column (2.1×150 mm, 5 µm; USA) with a guard column, and column temperature was set at 30 °C. The system was run in gradient elution with mobile phase of methanol and water contained 0.1% (v/v) formic acid as shown in Table S2. Mobile phase was delivered at a flow rate of 0.3 mL/min. The mass spectrometer was operated at electrospray positive ionization mode (ESI+). The parameters were as follows: Capillary voltage was set at 4.5 kV, and end plate offset voltage was 0.5 kV. Nitrogen was used as dry gas, flow rate and temperature of which were 6 L/min and 180 °C, respectively. Pressure of nebulizer gas (N2) was 1.5 bar. Detector voltage of the TOF analyzer was 2.9 kV. Accurate mass was corrected by using sodium trifluoroacetate calibration solutions. Data were acquired in line spectra, and the ratio of mass to charge (m/z) was from 50 to 1000.

▇ ASSOCIATED CONTENT

Supporting Information Additional characterization data. The Supporting Information is available free of charge on the ACS Publications website. ▇ EXPERIMENTAL SECTION Materials: Aniline monomer (Shanghai Chemical Co.) was distilled under reduced pressure, ammonium peroxydisulfate, methanol, ethanol, n-propanol, i-propanol, THF, D-phenylalanine, L-phenylalanine and other chemicals of analytical reagent grade were commercially obtained from Shanghai Chemical Co. and used as received. Preparation of twisted nanoribbons: In a typical synthesis, aniline (0.80 mmol) was dissolved in 18.0 mL alcohol/water mixed solution with shaking for 3 min to obtain a uniform solution. After that, 2.0 mL the aqueous solution containing 0.80 mmol APS was added to the above mixture in one portion, and the resulting solution was shaken for 5 seconds to ensure complete mixing and then the reaction solution was allowed to proceed without agitation for 12 h at room temperature. Finally, the product was washed with deionized water until the filtrate became colorless and then dried in a vacuum at 60 °C for 24 h. Purification and self-assembly of the main component. In a purification experiment, 0.2 g of the obtained twisted nanoribbons was firstly added into 10 mL of methanol, and then the resulting solution was shaken for 10 seconds at room temperature to completely dissolve the soluble component (the objective material). After 2 h without agitation, the solution was filtrated to remove insoluble species. When 20 mL water as a poor solvent was added to the above filtrate, precipitation (the objective tetramer) was deposited from the water-rich system and then was separated by centrifugation. Repeating the above dissolving-filtratingdepositing-separating method for three times, the relatively pure tetra-aniline derivatives were obtained. For the self-assembly experiment, the purified tetramer (0.02 g) was dissolved in 2 mL THF to form a solution at room temperature, which then was slowly introduced into 8 mL ipropanol/water mixture. The helical nanostructures could be formed and deposited from the solution.

Separation of L and D-phenylalanine enantiomers: For separation L- and D-phenylalanine, the single-handed nanoribbons (5.0 mg) were added to 20.0 mL aqueous solution containing Land D-phenylalanine (1:1 v/v, total concentration is 0.5 mM). The mixture was stirred for several hours at room temperature and finally filtered. The solid filter residue containing any bound amino acid was separated from the solution by filtration, and the remaining filtrate was monitored by CD spectrum. For evaluating ee (%) value, the concentrations of L- and D-phenylalanine was inferred based on the UV-vis and CD intensities at the characteristic absorption bands (206 and 216.8 nm, respectively) which were

Instruments and Characterization: The morphology of asprepared samples were examined by FESEM (S-4800, Hitachi Co., Japan) and TEM (Tecnai-2 Philip Apparatus Co., USA), respec-


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linearly changed with the concentration in a certain range (Figure S19). ▇ AUTHOR INFORMATION

Corresponding Author * [email protected]; * [email protected]

Notes The authors declare no competing financial interests. ▇ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21673202, 91427302 and 21104093), Topnotch Academic Programs Project of Jiangsu Higher Education In stitutions (TAPP), Qing Lan Project and the Priority Academic Program Development of Jiangsu Higher Education Institutions. ▇ REFERENCES (1) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism Principles and Applications, 2nd ed.; Wiley-VCH: New York, 2000. (2) (a) Liu, M.; Zhang, L.; Wang, T. Chem. Rev. 2015, 115, 7304-7397. (b) Aida, T.; Meijer, W. W.; Stupp, S. I. Science 2012, 335, 813-817. (3) (a) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (b) Faul, C. F. J. Acc. Chem. Res. 2014, 47, 3428-3438. (c) Echue, G.; Lloyd-Jones, G. C.; Faul, C. F. J. Chem. Eur. J. 2015, 21, 5118-5128. (d) Ahmed, R.; Patra, S. K.; Hamley, I. W.; Manners, E.; Faul, C. F. J. J. Am. Chem. Soc. 2013, 135, 2055-2058. (4) (a) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. Rev. 2001, 101, 4039-4070. (b) Edwards, W.; Smith, D. K. J. Am. Chem. Soc. 2014, 136, 1116-1124. (c) Schaefer, J. L.; Moganty, S. S.; Yanga, D. A.; Archer, L. A. J. Mater. Chem. 2011, 21, 10094-10101. (5) (a) Rodríguez-Llansola, F.; Escuder, B.; Miravet, J. F. J. Am. Chem. Soc. 2009, 131, 11478-11484. (b) Sato, I.; Kadowaki, K.; Urabe, H.; Ono, Y.; Shinkai, S.; Jung, J. H.; Soaia, K. Tetrahedron Lett. 2003, 44, 721724. (6) (a) Rowan, A. E.; Nolte, R. J. M. Angew Chem. Int. Ed. 1998, 37, 63-68. (b) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. R.; Meijer, E. W. Nature 2000, 407, 167-170. (7) Li, Q.; Green, L.; Venkataraman, N.; Shiyanovskaya, I.; Khan, A.; Urbas, A.; Doane, J. W. J. Am. Chem. Soc. 2007, 129, 12908-12909. (8) (a) Brizard, A.; Aimé, C.; Labrot, T.; Huc, I.; Berthier, D.; Artzner, F.; Desbat, B.; Oda, R. J. Am. Chem. Soc. 2007, 129, 3754-3762. (b) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336-3343. (9) (a) Johnson, R. S.; Yamazaki, T.; Kovalenko, A.; Fenniri, H. J. Am. Chem. Soc. 2007, 129, 5735-5743. (b) Sakurai, S. I.; Okoshi, K.; Kumaki, J.; Yashima, E. J. Am. Chem. Soc. 2006, 128, 5650-5651. (c) Goto, H.; Okamoto, Y.; Yashima, F. Macromolecules 2002, 35, 4590-4601. (10) Lin, R.; Zhang, H.; Li, S. H.; Chen, L. Q.; Zhang, W. G.; Wen, T. B.; Zhang, H.; Xia, H. P. Chem. Eur. J. 2011, 17, 2420-2427. (11) (a) Ribó J. M.; Crusats, J.; Sagués, F.; Claret, J.; Rubires, R. Science 2001, 292, 2063-2066. (b) Amabilino, D. B. Nat. Mater. 2007, 6, 924-925. (12) (a) Ahmed, S.; Mondal, J. H.; Behera, N.; Das, D. Langmuir 2013, 29, 14274-14283. (b) Duan, P.; Cao, H.; Zhang, L.; Liu, M. Soft Matter 2014, 10, 5428-5448. (c) Lara, C.; Reynolds, N. P.; Berryman, J. T.; Xu, A.; Zhang, A.; Mezzenga, R. J. Am. Chem. Soc. 2014, 137, 47324739. (13) (a) Zhang, S.; Yang, S.; Lan, J.; Yang, S.; You, J. Chem. Commun. 2008, 6170-6172. (b) Kimura, M.; Hatanaka, T.; Nomoto, H.; Takizawa, J.; Fukawa, T.; Tatewakio, Y.; Shirai, H. Chem. Mater. 2010, 22, 5732-5738. (14) (a) Chen, P.; Ma, X.; Duan, P.; Liu, M. ChemPhysChem 2006, 7, 2419-2423. (b) Ribó, J. M.; Crusats, J.; Sagues, F.; Claret, J.; Rubires, R. Science 2001, 292, 2063-2066. (c) Micali, N.; Engel-kamp, H.; Ven Rhee,


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Table of Contents Controllable supramolecular chiral twisted nanoribbons from achiral conjugated oligoaniline derivatives 60

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Supramolecular chiral twisted nanoribbons of achiral oligoaniline derivatives were prepared via simply performing the chemical oxidation of aniline in alcohol/water mixed solution, which showed potential application for chiral recognition.

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