Fabrication of Single-Handed Nanocoils with Controlled Length via a

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Fabrication of Single-handed Nanocoils with Controlled Length via Living Supramolecular Self-assembly Ke Hu, Yin Liu, Wei Xiong, Yanjun Gong, Yanke Che, and Jincai Zhao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04923 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Chemistry of Materials

Fabrication of Single-handed Nanocoils with Controlled Length via Living Supramolecular Self-assembly Ke Hu,†,‡,§ Yin Liu,†,‡,§ Wei Xiong,†,‡ Yanjun Gong,†,‡ Yanke Che,*,†,‡ and Jincai Zhao†,‡ †Key

Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ‡University of Chinese Academy of Sciences, Beijing, 100049, China. ABSTRACT: We report the fabrication of single-handed nanocoils with controlled length from chiral perylene diimide (PDI) molecules via a living seeded self-assembly method. We demonstrate that the competition among -interactions, steric repulsion, and transfer of chirality causes the morphological transition from metastable microspheres and microsheets to stable nanocoils. Importantly, the complex assembly pathways allow the living seeded self-assembly to yield single-handed nanocoils with controlled length, which may have promising applications in optoelectronics, fluorescent sensors, and biological imaging fields. Fabrication of length-controlled one-dimensional (1D) πelectronic nanomaterials has attracted extensive attention because functions and properties of the resulting nanomaterial are associated with their length size distribution.1-6 However, it remains challenging to yield 1D nanomaterials with precise length and narrow distributions due to the spontaneous nucleation and growth in the conventional self-assembly process.7 Living supramolecular self-assembly has emerged as a particularly promising method to enable access the nanostructures with controlled dimensions,7-27 which has well demonstrated in the block copolymer (BCP) systems.8-18 Recently, this method has also been applied to fabricate 1D and 2D nanostructures of small molecules with precisely-controlled dimensions.7,19-33 However, despite the many advances in fabrication of 1D nanoribbons and nanotubes from small molecules, stable chiral nanocoils with controlled length, which might have potential applications in sensors and solenoids, have not been achieved. In this work, we report the fabrication of single-handed nanocoils with controlled length from chiral perylene diimide (PDI) 1 via a living supramolecular self-assembly (Figure 1). We reveal that 1 undergoes a morphological transition from kinetically-trapped metastable assemblies (i.e., microspheres and microsheets) to thermodynamically-stable nanocoils in the self-assembly process (Figure 1b). This morphological transition is driven by the competition among -interactions, steric repulsion, and transfer of chirality, which are encoded in the molecular structure of 1. Addition of nanocoil seeds to the solution of metastable microspheres leads to the uniform nanocoils whose lengths are dependent on the molar ratio of the seeds to the metastable microspheres. We previously reported that the competition between -interaction and steric repulsion gave rise to a top-down morphological transition from metastable microribbons to

Figure 1. (a) Molecular structures of chiral molecule (S)-1, (R)-1, (±)-1, and molecule 2. (b) Schematic representation of the complex morphological transition and seeded growth (living self-assembly) of 1.

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thermodynamically stable nanotubes.24 This result motivated us to explore if a further increasing in steric repulsion can facilitate the formation of nanocoiled structures rather than nanotubular structures. To this end, chiral molecule 1 was synthesized (see details in the Supporting Information), which bears two chiral sec-butoxy substituents at the 2,6-position of the benzyl moiety as one side chain and a dodecyl group as the other (Figure 1a). It is expected that the introduction of chiral substituents could induce the formation of singlehanded nanocoils via transfer of chirality.

time was shortened to 1 d. These uniform nanowires actually had the coiled structure, which were left-handed and had diameter of ca. 12 nm and a pitch length of ca. 130 nm as revealed by transmission electron microscopy (TEM) (Figure 2e) and atomic force microscopy (AFM) (Figure 2f). The nanocoiled morphology remained unchanged for a few weeks in solution, indicating the thermodynamically stable characteristic. Likewise, stepwise formation of right-handed nanocoils was observed in the self-assembly of (S)-1 (Figures 3 and S2). Mirror image circular dichroism (CD) spectra of

Figure 2. SEM images of assemblies from (R)-1 at 5 min (a), 30 min (b), 3 h (c), 24 h (d), and 7 d (e) after beginning the self-assembly process. Inset in (e): TEM image of a single nanocoil from (R)-1. (f) AFM image of nanocoils from (R)-1. Inset: diameters of nanocoils from (R)-1.

Figure 3. SEM images of assemblies from (S)-1 at 5 min (a), 30 min (b), 3 h (c), 24 h (d), and 7 d (e) after beginning the self-assembly process. Inset in (e): TEM image of a single nanocoil from (S)-1. (f) AFM image of nanocoils from (S)-1. Inset: diameters of nanocoils from (S)-1.

We used electronic microscopy and optical characterization techniques to monitor the self-assembly of 1. Typically, the self-assembly was proceeded by injecting 0.3 mL chloroform solution of (R)-1 (4 mM) into 4.5 mL ethanol followed by quickly stirring and then aging at room temperature. At 5 min after beginning the self-assembly process, microspheres with . diameters ranging from 0.3 to 1 m were formed as revealed by scanning electron microscopy (SEM) (Figures 2a and S1). These microspheres then bound together and began to form microsheets at ca. 30 min (Figures 2b and S1). After 3 h, only microsheets were observed (Figures 2c and S1), indicating that all microspheres evolved into microsheets. Intriguingly, the resulting microsheets began to split into nanowire networks as time progressed (Figures 2d and S1). After 7 d, only entangled nanowires were observed and no microsheets were left (Figures 2e and S1). Notably, slight agitation (e.g., 200 rpm) was able to greatly accelerate the morphological transition from kinetically trapped metastable assemblies to thermodynamically stable nanocoils; the whole transition

(R)-1 and (S)-1 nanocoils, which are consistent with the absorption of nanocoils (Figure S3a and c), further confirmed that the molecular stacking of (R)-1 or (S)-1 adopts one helical sense within their individual nanocoils.34 To gain the molecular organization information of the assemblies, we performed X-ray diffraction (XRD) measurements of the metastable microspheres, microsheets, and the stable nanocoils obtained at different time points, respectively. No well-defined diffraction peak was observed on microspheres, indicative of the amorphous structure. In contrast, few welldefined diffraction peaks were observed on the microsheets (Figure S4), indicating that some extent of ordered molecular packing. Unfortunately, the XRD patterns are not sufficient to deduce a reliable molecular packing motif. Notably, because of the helical organization and small size intrinsic to nanocoils, no well-defined diffraction peak was observed on stable nanocoils. To further understand how the competition among interactions, steric repulsion, and transfer of chirality resulted


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Chemistry of Materials

Figure 4. SEM images of assemblies from ()-1 at 5 min (a), 30 min (b), 12 h (c), and 7 d (d) after beginning the selfassembly process.

Figure 5. (a) SEM image of (R)-1 nanocoil seeds obtained through ultrasonication. Inset: Length distribution of seeds (more than 100 nanocoils were measured and a polydispersity index (PDI) of 1.02 was obtained). (b-d) SEM images of the elongated nanocoils formed when the molar ratio of (R)-1 seeds to (R)-1 microspheres feeds was 1:1 (b), 1:5 (c), and 1:10 (d), respectively. Insets: length distribution of the formed nanocoils (more than 100 nanocoils were measured and PDI values of 1.04, 1.02, and 1.03 were obtained, respectively).

in the aforementioned morphological transition, we synthesized racemate ()-1 and molecule 2 that bears two 3methoxy substituents at the 2,6-position of the benzyl moiety and thereby has smaller steric repulsion than molecule 1 (Figure 1a). At the very beginning of the self-assembly, ()-1 formed microspheres followed by transformation into microsheets under identical self-assembly conditions, analogous to (S)-1 and (R)-1 (Figure 4a-b). Afterwards, the . metastable microsheets gradually became narrower and fewer and concomitantly small nanofibers appeared (Figure 4c). After 7 d, only entangled nanofibers were observed and no microsheets were left (Figure 4d). A magnified SEM image showed that entangled fibers actually consisted of small nanofibers with diameters of ca. 100 nm (Figure 4d). After a few weeks of suspension in solution, these nanofibers remained the same morphology, indicative of their stable characteristic. As expected, the CD signal of ()-1 nanofibers was silent, indicating that these nanofibers were optically inactive (Figure S3b). Herein, the formation of small nanofibers from ()-1 suggests that the morphological transition is dominantly driven by the competition of interactions and steric repulsion, while transfer of chirality can bias the fiber assemblies to form the coiled structure. The competition of -interactions and steric repulsion was supported by optical characterization. As shown in Figure S3c and d, the absorption and fluorescence spectra recorded at 5 min after beginning the self-assembly of (R)-1 corresponding to the metastable microspheres were similar to those of the molecular dissolved 1. This result suggests that the formation of metastable microspheres was dominantly driven by steric repulsion among bulky side chains of 1. At 3 h, a new absorption band between 535 and 625 nm, assignable to the

formed microsheets, appeared along with the decreasing of the molecular absorption (Figure S3c), indicative of the participation of -interactions of (R)-1 molecules. At 7 d, this red-shifted absorption became more intense, indicating that the -interactions outcompeted steric repulsion in the resulting nanocoils. Similarly, the fluorescence spectra reflected the enhancing -interactions where the new redshifted emission (centered at ca. 630 nm) relative to the monomer emission (centered at 575 nm) became prominent after 3 h self-assembly and more intense as time progressed (Figure S3d). To further verify the critical role of steric repulsion in the morphological transition, we monitored the time-dependent self-assembly of 2. As shown in Figure S5, no morphological transition was observed during the selfassembly process where the nanofibers were exclusively formed. Obviously, the dominant -interactions among molecules 2 favored the quick nucleation of 2 and growth into stable nanofibers in the whole self-assembly process. The above observations allow us to conclude that the proper interaction competition between -interactions and steric repulsion is required to create the minima in the energy landscape toward metastable assemblies. Given that the formation of metastable aggregates can retard spontaneous nucleation and thereby give rise to living supramolecular self-assembly,7 we explored the living selfassembly of 1 by adding nanocoil seeds into the prefabricated stock solution of 1 microspheres (See details in the Supporting Information). Prior to the experiments, we prepared nanocoil seeds of 1 with lengths of 0.6-1.0 μm (Figure 5a) by ultrasonicating the prefabricated nanocoils at 70 ℃ for 2 h. As shown in Figure 5, with molar ratios of (R)-1 nanocoil seeds to (R)-1 microspheres at 1:1, 1:5, and 1:10, the



resulting nanocoils exhibited about 2, 3, and 6 times the length of the seeds, respectively, indicating the living growth of (R)-1 on the ends of (R)-1 nanocoil seeds. A closer examination of the elongated nanocoils by SEM revealed that the helical sense of the newly grown nanocoils was identical to that of the seed (Figure S6). Notably, the complete growth of (R)-1 on the seeds was much faster than the spontaneous self-assembly process that required 7 d. The growth processes with molar ratios of 1 seeds to the microspheres feeds at 1:1, 1:5, and 1:10 only took 1 hour, 6 hour and 1 day, respectively, consistent with the autocatalytic characteristics of a living assembly.7, 25 Likewise, living seeded growth of (S)-1 on the ends of (S)-1 seeds in the same sense was observed (Figure S7). Therefore, mirror single-handed nanocoils with controlled length can be fabricated via living seeded selfassembly of chiral 1. To explored the possibility of heteroseeding that may lead to enhanced complexities and functionalities,8-18, 24, 27 we added (R)-1 nanocoils seeds in 0.5 mL chloroform/ethanol (volume ratio, 1:15) mixture into (S)1 microspheres, (R)-1 microspheres, and (±)-1 microspheres in 2.5 mL chloroform/ethanol (volume ratio, 1:15) mixture, respectively, at molar ratios of 1:1, 1:3, and 1:5. The corresponding time-dependent CD spectra were then monitored and recorded. As shown in Figure S8a-c, the CD signals of seeds began to weaken when (R)-1 seeds were added into (S)-1 metastable microspheres. Particularly, when the molar ratio of the metastable microspheres to the seeds was more than 1, the negative CD signals continued to weaken and then reverted to a positive Cotton effect. These results indicate that the majority rule eventually took control in this hetero-seeding self-assembly, resulting in a positive Cotton effect. Notably, the hetero-seeding self-assembly was finished in 24 h, indicative of the seeded growth of (S)-1 monomers originating from immediate disassembly of the metastable microspheres. SEM imaging also revealed the elongation of the nanocoil seeds. On the other hand, when (R)-1 seeds were added into (R)-1 and (±)-1 metastable microspheres, the same negative cotton effect continued to increase until metastable microspheres were consumed and no CD Cotton effect reversal was observed (Figure S8d-i), indicating that the chiral seeds initiated the epitaxial growth of (±)-1 or (R)-1 monomer in the same helical sense, resulting in the chiral amplification phenomenon. In conclusion, we report the fabrication of single-handed nanocoils with controlled length from chiral PDI molecule 1 via a living seeded self-assembly method. We demonstrate that the competition among -interactions, steric repulsion, and transfer of chirality causes the morphological transition from metastable microspheres and microsheets into thermodynamically stable nanocoils. Importantly, the complex assembly pathways of 1 allow the living seeded self-assembly to yield single-handed nanocoils with controlled length, which may have promising applications in optoelectronics, fluorescent sensors, and biological imaging.

ASSOCIATED CONTENT Supporting Information

Synthetic and self-assembling procedure, structural and property characterizations, SEM images, and optical spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Yanke Che: 0000-0002-9671-3704

Author Contributions §K. H. and Y. L. contributed equally to this work.

Notes

Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by the NSFC (Nos. 21577147, 21590811, and 21677148) and the “Key Research Program of Frontier Sciences” (No. QYZDY-SSW-SLH028) of the Chinese Academy of Sciences.

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Fabrication of Single-Handed Nanocoils with Controlled Length via a

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