From 1D Polymers to 2D Polymers: Preparation of Free-Standing

From 1D Polymers to 2D Polymers: Preparation of Free-Standing SingleMonomer-Thick Two-Dimensional Conjugated Polymers in Water Na Zhang,† Taisheng Wang,† Xing Wu,† Chen Jiang,† Taiming Zhang,‡ Bangkun Jin,† Hengxing Ji,‡ Wei Bai,*,§ and Ruke Bai*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, and ‡Department of Materials Science and Engineering & CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, 96 Jin Zhai Road, Hefei, Anhui 230026, P.R. China § Department of Chemistry, University of Massachusetts Amherst, 300 Massachusetts Avenue, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Recently, investigation on two-dimensional (2D) organic polymers has made great progress, and conjugated 2D polymers already play a dynamic role in both academic and practical applications. However, a convenient, noninterfacial approach to obtain single-layer 2D polymers in solution, especially in aqueous media, remains challenging. Herein, we present a facile, highly efficient, and versatile “1D to 2D” strategy for preparation of freestanding single-monomer-thick conjugated 2D polymers in water without any aid. The 2D structure was achieved by taking advantage of the side-by-side self-assembly of a rigid amphiphilic 1D polymer and following topochemical photopolymerization in water. The spontaneous formation of single-layer polymer sheets was driven by synergetic association of the hydrophobic interactions, π−π stacking interactions, and electrostatic repulsion. Both the supramolecular sheets and the covalent sheets were confirmed by spectroscopic analyses and electron microscope techniques. Moreover, in comparison of the supramolecular 2D polymer, the covalent 2D polymer sheets exhibited not only higher mechanical strength but also higher conductivity, which can be ascribed to the conjugated network within the covalent 2D polymer sheets. KEYWORDS: 2D polymers, single-monomer-thick, 1D to 2D, topochemical photopolymerization, conductivity

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conductivity, high thermal conductivity, and outstanding mechanical properties, graphene has become one of the most well-known examples of conjugated 2D polymers that exhibit great potential in many areas.10 However, owing to the lack of efficient and convenient preparation methods, the systematic study of the 2D polymers is far behind those of onedimensional (1D) linear polymers and three-dimensional (3D) polymer networks, which severely hindered the exploration of the 2D polymeric materials.

ecently, two-dimensional (2D) materials have attracted much attention because of their exceptional properties and potential applications.1−4 Generally, synthetic organic 2D material is defined as a periodic network of repeating organic units, arranged in two orthogonal directions within isolated sheet-like structures. The connectivity between building blocks within each 2D organic layer can be both noncovalent5−7 and covalent,8 and each has its own merits. It will be meaningful to synthesize 2D supramolecular polymers together with 2D covalent polymers, in turn, by rational control of various noncovalent interactions. The archetypal example of the 2D polymers is graphene, in which every sp2-hybridized carbon atom serves as a repeat unit with planar recurrence, resulting in a sheet-like macromolecule.9 Due to its high specific surface area, excellent charge carrier mobility, superior electrical © 2017 American Chemical Society

Received: May 4, 2017 Accepted: June 22, 2017 Published: June 22, 2017 7223

DOI: 10.1021/acsnano.7b03109 ACS Nano 2017, 11, 7223−7229

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Figure 1. Schematic illustration of the formation pathway of single-monomer-thick two-dimensional polymers.

single-layer 2D covalent polymers (2D-CP) were prepared by photopolymerization of the diacetylene groups within the single-layer 2D-SP. The results demonstrated that the approach from 1D polymer to 2D polymer was very convenient and highly effective for preparation of free-standing single-layer 2D polymers. Additionally, the exceptional properties of these 2D polymers also revealed the potential of our strategy in fabrication of practical materials.

Over the past decade, much effort has been devoted to design and synthesize 2D polymers, and several strategies have been reviewed.11−13 So far, three major methods have been reported to obtain monolayer or few-layer 2D polymers: (a) “top-down” exfoliation of layered covalent organic frameworks (COFs)14−16 and 3D-layered single crystals, (b) “bottom-up” synthesis of monolayer 2D polymers on heterophase interfaces, and (c) noninterfacial synthesis of individual free-standing nanosheet in solution. Recently, the King and Schlüter groups synthesized 2D polymers through the photopolymerization of rigid anthracene-based monomers in single crystals.17−22 However, the possibility of causing defects in the process of exfoliation obviously limited their applications. For the interface-assisted “bottom-up” synthesis approach, monomers were allowed to polymerize within confined 2D interfaces.23−30 By the aid of phase interface with a certain size, the size of the 2D polymers could be finely tuned. The main limitation of approach (b) is that the 2D polymers can only be formed at the interface. In order to obtain a high reaction yield, the phase interface needs to be continuously regenerated, which is timeconsuming. Moreover, in the case of using a solid substrate, it may also become a problem when transfer of the 2D polymer from one interface to other environments is needed. It is noteworthy that, though preparation of 2D polymers in solution has been reported, only very few obtained monolayer 2D polymers directly.31−33 What’s more, instead of organic solvents or reagents containing harsh chemicals, preparation of a conductive 2D polymer in environment-friendly solvent such as water is still rarely reported. Here, we report a facile, highly efficient, and versatile strategy for preparation of free-standing single-monomer-thick 2D polymers in water without any aid. The monolayer 2D polymer was conveniently obtained in water by self-assembly and photopolymerization of an amphiphilic rigid 1D polymer. The strategy included three steps (Figure 1): (1) an amphiphilic rigid conjugated 1D polymer (A-1DP) was designed and synthesized; (2) single-layer 2D supramolecular polymers (2DSP) were formed by self-assembly of the A-1DP in water; (3)

RESULTS AND DISCUSSION In our design, the structure of A-1DP was crucial for the formation of the single-layer 2D-SP and 2D-CP. We hypothesized that the synergetic effect of rigidity, π−π stacking, and amphiphilicity of the A-1DP could facilitate the precise side-by-side self-assembly of linear polymers to form supramolecular 2D sheets in water, so the diphenylanthracene units and carboxylic groups were incorporated in the main chain and side chain, respectively, to introduce these associative interactions. Moreover, the diacetylene units as polymerizable groups in the main chain of the A-1DP could be polymerized to form a 2D network under UV light in the second step. In the meantime, we took advantage of the electrostatic repulsion among the carboxylic anions to keep each individual sheet separated from one another in the process of self-assembly and photopolymerization to avoid any chemical or physical exfoliation. Figure S1 shows the design and synthesis route of the amphiphilic conjugated A-1DP. To examine the effect of molecular weight of A-1DP on its self-assembly behavior into the 2D supramolecular sheets, three A-1DPs with different degrees of polymerization (DP) were synthesized. All the nonamphiphilic 1D polymers (N-1DP) have good solubility in chloroform and emit yellow green fluorescence in solution. The structure and the molecular weight of the polymers were characterized by NMR spectroscopy (Figures S24 and S25) and MALDI-TOF-MS (Figure S2), respectively. Then, the amphiphilic 1D A-1DP polymers were obtained by hydrolysis of the ester groups with tetramethylguanidine (TMG) as a base, 7224


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Figure 2. Transmission electron microscope images of single-layer 2D-SP (a) and 2D-CP (b). Fluorescence microscope image (c), HRTEM image (d) of the 2D-SP in water with a zoomed-in image on the top-right corner and an electron diffraction pattern (e) of 2D-SP on a carbon support film.

confirmed by fluorescence microscopy (FM) images (Figure 2c). Owing to the instability of the single layer against a strong electron beam, multilayered sheets of A-1DP were measured by high-resolution electron microscopy. The high-resolution TEM (HRTEM) image (Figure 2d) suggested the 1D polymer selfassembled into lamellar structure with a repeat distance of 2.8 Å, which indicated that each amphiphilic A-1DP was facing one another owing to the presence of inter-macromolecular π−π stacking interactions. This could be further confirmed by selected area electron diffraction (SAED) patterns (Figure 2e) of 2D-SP with diffused diffraction rings, which displayed a typical polycrystalline structure. The strongest diffraction spot at 3.7 nm−1 (0.27 nm) was attributed to the π−π stacking between 1D polymer chains. This value was smaller than the stacking distance calculated from X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) spectra (Figure S9) (0.31 nm). This was because, from an energetic standpoint, the anthracene rings of 1D polymer chains were not perpendicular to the surface of the matrix when transferring the 2D-SP onto it. They may be prone to incline like falling dominoes. In this situation, the measured stacking distance from HRTEM and SAED between 1D polymer chains was actually a projected distance of adjacent anthracenes to the matrix, the value of which would be smaller than the typical π−π stacking distance. The as-prepared 2D-SP sheets dispersed in water were transferred onto a SiO2/Si wafer and then investigated by tapping-mode atomic force microscopy (AFM) (Figure 3). The results further supported our previous observation that the layered structure was formed with lateral sizes of up to several micrometers (Figure 3c). Cross-section analysis indicated that the sheets were very flat and uniform. The height of the sheets was measured to be ∼4.2 ± 0.1 nm (statistically average height based on 30 different sites, Figure S8). Although the height measured by AFM was larger than the theoretical thickness of the predicted supramolecular monolayer (∼3.5 nm, Figure 3a,b, calculated by semiempirical method at the PM3 level), this value still strongly suggested the formation of single-layer sheets, and the difference may due to an adhered water layer or surface roughness.32 Previous studies36−38 have demonstrated that the 1,4polymerization of diacetylenes could be proceed via thermal

and they were characterized by Fourier transform infrared (FTIR) spectra (Figure S3). In comparison of FT-IR spectra of A1DP and N-1DP (Figure S3), the disappearance of the ester carbonyl band at 1725 cm−1 and the appearance of the carboxylic acid ion at 1558 and 1399 cm−1 demonstrated the occurrence of ester hydrolysis. Unfortunately, except for water, the A-1DP showed extremely poor solubility in common polar solvents including DMSO, methanol, acetonitrile, THF, and DMF. What’s more, A-1DP in water displayed severely broadened peaks due to a strong stacking tendency among them.34 So, we tried to use the high-temperature 1H NMR. As shown in Figure S25, the resonance peaks between 6.0 and 9.0 ppm, which were assigned to the hydrogen atoms of the aromatic rings, did not show an obvious reinforcement but asymptotically shifted to downfield with increasing temperature. The δ of Hd, He, and Hf exhibited the same phenomenon. The low δ values observed for Ha, Hb, and Hc at room temperature arised because π-stacked polymer chains enhanced the ring-current effect.35 A higher δ value indicated that the π−π interaction was weakened at 80 °C, which unfortunately still could not be destroyed. However, we think the corresponding MALDI-TOF-MS spectra of A-1DP (Figure S2d−f) could demonstrate their structures well. The self-assembly of the amphiphilic A-1DP was performed in water. The transmission electron microscopy (TEM) results (Figure S2) indicated that the formation of 2D-SP was strongly related to the molecular weight of the A-1DP. For example, a stable and uniform 2D-SP sheet was successfully obtained by self-assembly of the amphiphilic A-1DP with higher DP (Mn = 5840 and Mn = 8750), whereas the amphiphilic A-1DP with Mn = 3700 formed irregular aggregates, instead of sheets because of weak synergetic interaction among the oligomers with lower molecular weight. TEM images of N-1DP with higher DP in dioxane and XRD of N-1DP in powder form (Figure S13) showed that nonamphiphilic N-1DP formed a multilayer lamellar structure, which indicates the significance of electrostatic repulsion to obtain monolayer 2D polymers. The TEM images (Figures 2a and S6) and cryogenic TEM (cryo-TEM) (Figure S9) images showed that the size of these sheets was about several dozen micrometers with some wrinkles. Moreover, the formation of the sheets was also 7225


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further change could be detected, suggesting that the photopolymerization was complete. On the other hand, a notable change was also observed in the photoluminescence spectra (Figure 4c), where the emission peak at 488 nm gradually diminished, concomitantly with the appearance of an emission peak near 442 nm. In addition, a change in photoluminescence from green to blue was also observed (Figure 4d). These results indicated that the π-stacking among the diphenylanthracene units was weakened during the photopolymerization, which may be related to the change of chemical bonds in the main chains. After irradiation, a vibration band at 2157 cm−1 appeared in the infrared spectrum (Figure S4), which could be attributed to the conversion of diacetylene moieties into the alternating structure of the alkene and alkyne.39 The band at 1630 cm−1 became broader due to the overlap of the double bonds from the aromatic rings and the newly formed olefin units.39 Raman spectroscopy was used to monitor the reaction of butadiyne under UV irradiation, and the results obtained are presented in Figure 5a. 2D-SP possessed intense bands at 2212 and 1519 cm−1 associated with the unreacted butadiyne moieties and phenyl rings. After the irradiation of solution, the dramatically weakened band at 2212 cm−1 was observed as well as the appearance of two new bands at 2135 and 1470 cm−1 associated with new triple and double bonds of 2D-CP. According to the literature,40,41 the reaction of anthracene with acetylene or with anthracene could not occur when irradiated with UV light at 254 nm. So, these results of the spectroscopic analyses demonstrated that the photopolymerization of the diacetylene units occurred after the 2D-SP was irradiated and the 2D-CP was produced. The comparison of the TEM images of the single-layer 2DSP (Figures 2a and S6) and 2D-CP (Figures 2b and S7) indicated that the morphologies of these sheets had no obvious change after irradiation. The HRTEM, XRD, and SAED results of 2D-CP have been summarized in Supporting Information (Figure S11). Before and after UV polymerization, the WXRD patterns almost showed no changes. A broad diffraction band appeared at 5.5° (1.6 nm) in SXRD, which represented the distance between newly formed alkynyl. This was further confirmed by HR-TEM of 2D-CP (Figure S11c). However, from the diffraction rings of SAED (Figure S11d), the crystallinity of conjugated 2D-CP was not enhanced. To further examine mechanical stability of the single-layer 2D-SP and 2D-CP, they were then transferred onto the Quantifoil TEM grids with holes (Figure 5c,d).42 In a series of control experiments, the TEM images showed that the 2D-CP covered all the grids with occasional ruptures on the edge (Figure 5d), but in the case of the 2D-SP, none of the holes were spanned. The results showed that the covalent-linked 2D-CP have mechanical stability higher than that of the 2D-SP constructed with noncovalent interactions. In order to obtain quantitative determination of the in-plane elastic modulus value (E2D) of the covalent 2D-CP, AFM indentation experiments were performed. The samples were prepared by transferring 2D-CP onto Quantifoil TEM grids with holes (1.0 μm diameter). A typical force curve is shown in Figure 5b. By fitting such curves to a simplified continuum mechanics model,42,43 the in-plane elastic modulus of E2D = 89.0 ± 9 N/m was obtained. The inplane elastic modulus of 2D-CP was lower compared to that of naturally occurring 2D sheets. It was ca. 4 times smaller than that of graphene (342 N/m)43 and ca. 2 times smaller than that of MoS2 (180 N/m).44 The reason could be attributed to structural defects in the single-layered sheet.

Figure 3. Front view (a) and side view (b) of simulated monomer conformation giving hcalc ≈ 3.55 nm. AFM image and height profiles (c) of single-layer 2D-SP.

or photoinitiation when neighboring diacetylene moieties were prealigned with a minimum repeat distance of 4.9 Å. Therefore, it was possible to convert the 2D-SP into the 2D-CP by photopolymerization of diacetylene units. The photopolymerization was investigated by UV/vis absorption spectroscopy and fluorescence emission spectroscopy (Figure S5). When being irradiated with UV light (254 nm), the absorption bands of the 2D-SP gradually decreased at 400 nm (Figure 4a), accompanying an apparent color change from yellowish to colorless (Figure 4b). After 6 h of irradiation, no

Figure 4. UV/vis absorption (a) and fluorescence emission (c) spectra of a solution of 2D-SP without irradiation (red line) and after irradiation (blue line) at 254 nm for 6 h. Corresponding visible light (b) and fluorescent image (d) of the solutions. 7226


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Figure 5. Raman spectra of 2D-SP (blue line) and 2D-CP (black line) (a). An example of a loading curve recorded at the center of the freestanding 2D-CP (b); the inset is a schematic representation of the geometry and principle of the nanoindentation method; TEM images of single-layer 2D-SP (c) and 2D-CP (d) on a carbon film with holes. The hole size of the Quantifoil TEM grid is 1.0 μm.

Synthesis of monomers and polymers, 1H NMR, 13C NMR, FT-IR, PXRD, TGA, MALDI-TOF-MS, AFM images, TEM images, and the electrical conductivity data of 2D sheets (PDF)

Although the electrical property of the monolayer 2D polymer sheet has not yet been measured due to the lack of suitable equipment, electrical conductivity of the multilayer 2D polymer sheet was examined, and the results are shown in Figure S12. We directly compared the electrical conductivities of 2DPs before and after in situ photopolymerization. The electrical conductivities of 2D-SP and in situ polymerized 2DCP were measured to be σs = 1.35 × 10−7 S/cm and σc = 6.56 × 10−7 S/cm, respectively. Obviously, the electrical conductivity of 2D-CP was higher than that of 2D-SP, and this may be due to the formation of the covalently conjugated network within the single-layer 2D-CP sheet.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hengxing Ji: 0000-0003-2851-9878 Ruke Bai: 0000-0002-1310-316X

CONCLUSION In conclusion, amphiphilic conjugated A-1DPs containing diphenylanthracene and diacetylene units in the main chain were designed and synthesized. Based on the “1D to 2D” approach, we have successfully developed a facile, efficient, and versatile strategy for preparation of single-monomer-thick 2D polymers in water by synergetic self-assembly of the 1D polymers and photopolymerization within the 2D supramolecular polymer sheet. By this method, the free-standing single-monomer-thick 2D-SP and 2D-CP were prepared in water without any aid. The structure and the morphology of these sheets were characterized by electron microscope techniques and spectroscopic analyses. Interestingly, compared with 2D-SP, the 2D-CP exhibited better mechanical stability as well as higher conductivity. Therefore, our work provided a powerful strategy for preparation of not only free-standing 2D supramolecular polymers but also 2D covalent polymers, especially conjugated 2D polymers with several nanometer thickness as conductive or semiconductive materials, which are very useful in many fields.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for financial support from the National Natural Science Foundation of China (No. 21674101) and kind assistance of Professor Hengxing Ji for electrical conductivity tests. REFERENCES (1) Mas-Ballesté, R.; Gomez-Navarro, C.; Gomez-Herrero, J.; Zamora, F. 2D Materials: to Graphene and beyond. Nanoscale 2011, 3, 20−30. (2) Liu, X. H.; Guan, C. Z.; Wang, D.; Wan, L. J. Graphene-like Single-Layered Covalent Organic Frameworks: Synthesis Strategies and Application Prospects. Adv. Mater. 2014, 26, 6912−6920. (3) Mendoza-Sánchez, B.; Gogotsi, Y. Synthesis of Two-Dimensional Materials for Capacitive Energy Storage. Adv. Mater. 2016, 28, 6104− 6135. (4) Zheng, Z. K.; Grünker, R.; Feng, X. L. Synthetic TwoDimensional Materials: a New Paradigm of Membranes for Ultimate Separation. Adv. Mater. 2016, 28, 6529−6545. (5) Dong, R. H.; Pfeffermann, M.; Liang, H. W.; Zheng, Z. K.; Zhu, X.; Zhang, J.; Feng, X. L. Large-Area, Free-Standing, Two-Dimensional Supramolecular Polymer Single-Layer Sheets for Highly Efficient Electrocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2015, 54, 12058−12063.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03109. 7227


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