Toward a Two-Dimensional Supramolecular Organic Framework with High Degree of Internal Order via Amphiphilic Modification

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Jan 2022Toward a Two-Dimensional Supramolecular Organic Framework with High Degree of Internal Order via Amphiphilic Modification Zhi-Jian Yin†, Shu-Yan Jiang†, Na Liu, Qiao-Yan Qi, Zong-Quan Wu, Tian-Guang Zhan and Xin Zhao Zhi-Jian Yin† Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Province, Hefei 230009 †Z.-J. Yin and S.-Y. Jiang contributed equally to this work.Google Scholar More articles by this author , Shu-Yan Jiang† Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 †Z.-J. Yin and S.-Y. Jiang contributed equally to this work.Google Scholar More articles by this author , Na Liu Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Province, Hefei 230009 Google Scholar More articles by this author , Qiao-Yan Qi Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 Google Scholar More articles by this author , Zong-Quan Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Province, Hefei 230009 Google Scholar More articles by this author , Tian-Guang Zhan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004 Google Scholar More articles by this author and Xin Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000602 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Solution-phase self-assembly of two-dimensional (2D) networks with a high degree of internal order and long-range periodicity is a great challenge. Herein, we report a rational design to improve 2D self-assembly in water through amphiphilic modification of the building block. An amphiphilic tritopic molecule ( 1) is designed and synthesized by introducing three hydrophilic oligo(ethylene glycol) moieties and three hydrophobic hexyl chains. The assembly of 1 and cucurbit[8]uril (CB[8]) leads to the formation of a Janus 2D supramolecular organic framework (SOF), which further creates unique bilayer supramolecular networks and exhibits an unprecedentedly high degree of internal order and long-range periodicity. In contrast, the assembly of a nonamphiphilic analog ( 2) with CB[8] only generates a 2D SOF with a lower degree of internal order, suggesting that the inherent amphiphilicity of 1 plays a crucial role in improving its 2D self-assembly in aqueous phase. Download figure Download PowerPoint Introduction Over the past decade, one of the most astounding achievements in chemistry and materials science is the emergence and rapid development of two-dimensional (2D) nanomaterials.1–5 Such fascinating materials have demonstrated appealing unprecedented properties and versatile applications attributed to their unique structural peculiarities of ultrathin thicknesses, enormously high surface areas, and well-defined 2D extended skeletons. So far, while significant progress in structural diversity and functional exploration has been made for 2D materials, extensive efforts have also been devoted to developing various methods to efficiently produce such a burgeoning class of 2D polymeric structures. In this context, two strategies are dominantly used to fabricate 2D materials. One refers to the top-down strategy as represented by the production of graphene from the exfoliation of graphite.6 In addition, some recent examples have revealed that few- and single-layered metal–organic frameworks (MOFs)7,8 or covalent organic frameworks (COFs)9–11 could be obtained through the exfoliation of their layered bulky parent materials. Very recently, Loh and co-workers12 have developed a supramolecular approach to facilitate self-exfoliation of bulky 2D COFs into COF sheets with controllable thickness. In contrast to the top-down strategy, the other strategy is the more straightforward bottom-up construction of 2D materials via the direct polymerization of small organic building blocks at interfaces,13,14 in solid state,15,16 or even in solution.17,18 To produce structurally well-defined 2D materials, not only the aspect ratio but also the degree of long-range internal order must be taken into consideration. In fact, the degree of in-plane long-range internal order is a crucial factor that plays a pivotal role in determining the properties, functions, and applications of 2D materials. However, owing to the lack of sufficient and broadly applicable construction strategies, it remains a grand challenge to obtain 2D polymeric structures possessing a 2D skeleton with a high degree of long-range internal order. Although 2D polymers with high degree of internal order could be produced by using templates to direct and confine the 2D growth of building blocks, they often strongly adhere to templates and thus cannot exist independently.19 Self-assembly, with decades of development, has been proven to be an alternative, powerful bottom-up method in the fabrication of numerous supramolecular architectures and soft nanomaterials with diverse topology ranging from zero-dimensional (0D),20 one-dimensional (1D),21–23 and three-dimensional (3D),24–26 to the hierarchical self-assembled systems.27,28 In terms of 2D nanomaterials, self-assembly has also demonstrated its extraordinary utility in the construction of structurally well-defined 2D assemblies based on rationally designed building blocks driven by various noncovalent interactions including host–guest interaction,29–32 metal coordination,4,33–35 hydrogen bonding,36 π–π stacking,37–39 ionic interaction,40,41 as well as Pt⋯Pt42,43 and cation–π interactions.44 Among these, the cucurbit[8]uril (CB[8])-mediated shape-directed assembly has been proven to be a sufficient strategy capable of directing a 2D extension of building blocks, from which monolayer 2D supramolecular organic frameworks (SOFs) have been constructed in solution phase.45–51 However, despite these significant progresses, the development of self-assembled 2D structures toward more innovative applications has been discouraged by the annoying problem of rarely producing such nanomaterials with long-range internal order. Although this challenge can be partly addressed by performing self-assembly at interfaces,30,33–35 the interface-confinement effect will be lost when a self-assembly process occurs in solution phase. The main difficulty encountered is how to efficiently restrict the assembly process only in orthogonal directions within a 2D skeleton, which is a challenging task for the current strategies because of the intrinsic rotation of the building blocks and the dynamic nature of noncovalent connections between monomers. Therefore, to overcome this obstacle, it is crucial to develop new strategies to introduce an efficient confinement effect on solution-phase 2D self-assembly. Inspired by nature’s elegant use of amphiphilicity and the successful construction of numerous well-defined self-assembled architectures and functional systems,52–55 as well as 2D supramolecular assemblies56–60 based on artificial amphiphilic organic building blocks, herein, we report an amphiphilic modification approach to improve 2D self-assembly in water, which enables the construction of a 2D SOF with an unprecedentedly high degree of internal order and long-range periodicity (Scheme 1). In contrast to the previously reported 2D SOFs that all exist as monolayers with homogeneous surfaces,29–31,45–51 the as-formed SOF has a unique Janus feature and can exist as bilayers (Scheme 1). Furthermore, the crucial role that amphiphilicity plays in the construction of the highly ordered 2D SOF has been revealed by a comparative study between the amphiphilic building block and its nonamphiphilic analog. Scheme 1 | Structures of the building blocks and CB[8], and cartoon representation of the formation of the 2D SOF assembled from 1 and CB[8]. Although the benzene units that bear the hydrophilic and hydrophobic chains can rotate, amphiphilicity could result in self-sorting distribution of the chains at both sides of the SOF upon self-assembly in water. Download figure Download PowerPoint Results and Discussion The design principle for the building block was based on the approach of shape-directed self-assembly and the driving force was the CB[8]-based host–guest chemistry.61–63 In contrast to the previous designing strategies,29–31,45–51 in this work, three hydrophilic oligo(ethylene glycol) moieties and three hydrophobic hexyl chains were attached to the arms of a C3-symmetric building block ( 1 in Scheme 1). Such modification made the building block amphiphilic, which, as we envisioned, should facilitate structural preorganization during the self-assembly process. For comparison, a nonamphiphilic building block ( 2) bearing six oligo(ethylene glycol) moieties was also prepared. The formation of honeycomb-like 2D SOFs was expected by mixing 1 or 2 with CB[8] in water in a 2:3 molar ratio (Scheme 1). 1H NMR titration experiments were initially carried out to investigate the binding behavior between CB[8] and the building blocks. 1 exhibits poor solubility in D2O. Its amphiphilic feature results in a high aggregation tendency, which prohibits the acquisition of a high-resolution 1H NMR spectrum (Figure 1, left). Upon incremental addition of CB[8] into a D2O solution of 1, its original peaks decreased gradually while a new set of signals appeared. The original peaks of 1 totally disappeared after 1.5 equiv of CB[8] was added, suggesting a 2:3 stoichiometry for 1 and CB[8], which was further confirmed by the Job’s plot ( Supporting Information Figure S1). This observation is also consistent with the theoretical expectation for an extended 2D framework assembled from 1 and CB[8]. Notably, it was found that the resolutions of the spectra were not improved upon the addition of CB[8] during the 1H NMR titration, which could be attributed to the formation of polymeric structures and further aggregation of the polymers at the high concentration of the 1H NMR titration experiment. The dynamic light scattering (DLS) study further revealed the formation of polymeric structures assembled from 1 and CB[8] (2:3) in aqueous solutions ( Supporting Information Figure S2). In addition, it was found that the hydrodynamic diameters (Dh) of the assembled species increased as the concentration of 1 and CB[8] increased, which indicated the formation of larger sized entities at higher concentrations. Isothermal titration calorimetry (ITC) study gave rise to an apparent association constant (Ka) of 1.35 × 107 M−2 ( Supporting Information Figure S3). Such a high binding constant should ensure highly stable assemblies in solution. In the case of nonamphiphilic 2, a high-resolution 1H NMR spectrum was obtained in D2O because of its highly hydrophilic feature and thus good solubility in water (Figure 1, right). The investigations of 1H NMR titration, DLS experiment, and Job’s plot indicated that 2 exhibited similar self-assembly behavior as that of 1 in the presence of CB[8] ( Supporting Information Figures S4 and S5), suggesting the generation of a polymeric framework structure through the assembly of 2 and CB[8] in a 2:3 stoichiometry. Figure 1 | Left: 1H NMR spectra (500 MHz) in D2O at 25 °C of (a) 1 (1.0 mM), (b) 1 + CB[8] (0.375 equiv), (c) 1 + CB[8] (0.75 equiv), (d) 1 + CB[8] (1.125 equiv), (e) 1 + CB[8] (1.5 equiv), and (f) 1 + CB[8] (1.875 equiv). Right: 1H NMR spectra (500 MHz) in D2O at 25 °C of (a) 2 (1.0 mM), (b) 2 + CB[8] (0.375 equiv), (c) 2 + CB[8] (0.75 equiv), (d) 2 + CB[8] (1.125 equiv), (e) 2 + CB[8] (1.5 equiv), and (f) 2 + CB[8] (1.875 equiv). Download figure Download PowerPoint To get the detailed structural information of the polymeric structures assembled from the building blocks and CB[8], the as-formed entities were investigated by microscopic techniques. They were initially characterized by transmission electron microscopy (TEM) at various magnifications. As revealed in Figure 2a, sheet-like objects are observed for the material assembled from 1 and CB[8] (2∶3) in water, indicating formation of 2D structures. The sizes of such nanosheets were estimated to range from approximately one-hundred to several-hundred nanometers. In addition, the enlarged TEM image clearly revealed tiny but orderly arranged pores of such 2D assembly (Figure 2b), which indicated its porous network feature. Moreover, the fine structure of the pores was further disclosed by a TEM image at a higher resolution, from which a honeycomb-like structure consisting of uniform pores was clearly observed (Figure 2c). This result strongly confirmed the formation of a 2D SOF with long-range translational order. To the best of our knowledge, it is the highest degree of internal order ever reported for 2D SOFs fabricated in solution phase. The high-resolution TEM (HR-TEM) image suggested that the quality of the as-formed 2D SOF could even be comparable in terms of the degree of internal order and periodicity with the 2D polymers prepared from photoinduced 2D polymerization in layered crystals.14 Notably, the 2D fast Fourier transform (FFT) of a selected area (as marked by the square in Figure 2c) in the TEM image gave rise to distinct hexagonal diffraction spots (the white spots in the inset image of Figure 2c), suggesting long-range order of the 2D SOF with hexagonal pores. The pore aperture (including the skeleton) was calculated to be 3.32 nm from the Fourier transform pattern, which is close to the theoretically predicted value (around 3.6 nm) estimated for the size of the hexagonal macrocycle self-assembled from 1 and CB[8] based on a computational model ( Supporting Information Figure S6). It should be noted that the 2D sheets were unstable under the high electron doses used for the HR-TEM investigation, and they were found to be decomposed after the image was taken. Figure 2 | (a–c) TEM images at various magnifications and (d) tapping-mode AFM image of the SOF fabricated from 1 (0.005 mM) and CB[8] (0.0075 mM) in water. (e) The corresponding cross-section analysis of the AFM image. Note: the white circles in the fast Fourier transform profile (inset of c) could be attributed to the diffractions of the carbon film on the TEM grid, or to the carbonaceous materials from the e-beam-induced sample decomposition. Download figure Download PowerPoint The morphology of the as-formed SOF from 1 and CB[8] was further investigated by atomic force microscopy (AFM). A flat and uniform sheet with lateral size <1 μm was clearly observed in the AFM image, again confirming the formation of 2D structures (Figures 2d and 2e). The height of the sheet was measured to be 5.1 nm, which matched well with the thickness of 5.3 nm theoretically estimated by a computational model of two stacked amphiphilic build block 1 ( Supporting Information Figure S8). As previous studies on self-assembled monolayers have revealed that flexible chains could stand up on solid surfaces,64 it is reasonable for this amphiphilic 2D SOF to adopt a bilayer arrangement in which the hydrophobic alkyl chains are buried inside, while the hydrophilic oligo(ethylene glycol) moieties point to the aqueous surroundings, as illustrated in Scheme 1. Such an assembly tendency observed for the obtained 2D SOF is a quite common phenomenon of amphiphiles, for example, the formation of bilayer membranes from phospholipids. Another key evidence for the formation of 2D SOF with highly ordered internal structures and long-range periodicity from the self-assembly of 1 and CB[8] was provided by a solution-phase synchrotron small-angle X-ray scattering (SAXS) experiment (Figure 3a). As shown in the SAXS profile, three sharp peaks, which are assignable to (100), (110), and (200) facets, respectively, are observed for the assembled material formed in the solution of 1 and CB[8] (2∶3) in water. The d spacing corresponding to the (100) peak was calculated to be 3.42 nm, which agreed well with the expected size of the hexagonal unit of the 2D SOF (3.59 nm), for which a computational model has been established ( Supporting Information Figure S6). The value was also consistent with the result obtained from the TEM study above. The experimental SAXS pattern was compared with the theoretically simulated one, which matched each other very well ( Supporting Information Figure S7). The presence of these sharp peaks provided a strong argument that the as-formed SOF possessed high internal order and long-range periodicity. Furthermore, it is notable that the formation of the bilayered structure is likely to increase structural rigidity of the 2D SOF, which is also a beneficial factor to increase the intensity of the SAXS signals. Figure 3 | Synchrotron SAXS profiles of the solutions of the SOFs assembled from (a) 1 (1.0 mM) and CB[8] (1.5 mM), and (b) 2 (1.0 mM) and CB[8] (1.5 mM) in water. (c) Comparison between the SAXS curves of the two SOFs in log–log coordinate. Download figure Download PowerPoint For comparison, the self-assembled material from nonamphiphilic building block 2 and CB[8] was also investigated with the same techniques used for characterizing the 2D SOF fabricated from 1 and CB[8] under the analogous conditions. Concretely, the TEM image revealed that 2 and CB[8] were capable of self-assembling into similar, smaller-in-size, sheet-like structures (Figure 4a), indicating the formation of a 2D SOF. However, no fine structure was observed from its HR-TEM image (Figure 4b), and the 2D FFT profile just exhibited weak diffraction points as displayed in the inset of Figure 4b. The AFM investigation further revealed flat and uniform nanosheets with a thickness of 3.8 nm, suggesting that the assembly of 2 and CB[8] gave rise to a monolayered 2D SOF (Figures 4c and 4d and Supporting Information Figure S9). Notably, no sharp diffraction peaks but only a broad (100) peak was observed in the solution-phase synchrotron SAXS pattern for the 2D SOF solution of 2 and CB[8] (Figure 3b). All these experimental results suggested that the self-assembly of nonamphiphilic 2 and CB[8] could also generate a 2D SOF, but its internal order and long-range periodicity were not as high as those of the 2D SOF assembled from amphiphilic 1 and CB[8]. Figure 4 | (a and b) TEM images at various magnifications and (c) tapping-mode AFM image of the SOF fabricated from 2 (0.005 mM) and CB[8] (0.0075 mM) in water. (d) The corresponding cross-section analysis of the AFM image. Download figure Download PowerPoint To acquire more information from the SAXS data, the SAXS patterns of the two SOFs were compared in a log–log coordinate (Figure 3c). The comparison showed that the two curves were almost parallel, suggesting that the two SOFs had a similar structure. This result was consistent with the fact that building blocks 1 and 2 share the same skeleton. As a result, they should give rise to similar framework structures after they coassembled with CB[8]. Moreover, the higher intensity and more abundant peaks in the SAXS pattern of the SOF formed from 1 and CB[8] indicated that it had higher crystallinity as well as larger crystalline domains. The larger supramolecular architectures enabled by 1 were also supported by the DLS result, from which the Dh value of assemblies formed from 1 and CB[8] was found always larger than that of assemblies formed from 2 and CB[8] under the same concentration ( Supporting Information Figure S10). The comparison between the SOFs fabricated from 1 and 2 with CB[8], respectively, clearly indicated that amphiphilicity has played a crucial role in improving the accuracy of 2D self-assembly. Although the amphiphilic effect has rarely been applied for the construction of a 2D framework in water, it has been successfully used to fabricate ultrathin nanosheets56–60 and artificial membranes.65 In those cases, amphiphilicity endowed the building blocks with structural preorganization, which restricted the freedom of molecular motion and thus highly ordered aggregates could be produced. We believe that the significant improvement of 2D network formation through the coassembly of 1 and CB[8] should also benefit from a similar effect. Therefore, TEM images of a solution of 1 in water were also recorded, which showed the formation of well-ordered sphere-like aggregates (Figure 5a), suggesting that 1 had a preorganized structure in water. Accordingly, once CB[8] was introduced into the aqueous solution of 1, such structural preorganization should greatly facilitate the 2D assembly of 1 and CB[8]. Moreover, the advantage of the amphiphilic feature should also come into effect in the 2D supramolecular polymerization process by decreasing the degree of freedom of the building blocks, which was also favored for the formation of highly ordered supramolecular architectures and bilayer structures. In the case of nonamphiphilic 2, it only carries hydrophilic chains and can be well solvated by water molecules. In this context, the molecules of 2 exist freely in water, which decreases the controllability over 2D self-assembly. This assumption was supported by TEM study, which indicated that no well-defined structures formed from 2 in water (Figure 5b). Figure 5 | TEM images of the samples prepared by evaporating the solutions of (a) 1 (0.005 mM) and (b) 2 (0.005 mM) in water on TEM grids under ambient environment. Download figure Download PowerPoint Conclusion An effective approach capable of improving 2D self-assembly in water has been developed based on the amphiphilic modification of the building block, from which a 2D SOF with an unprecedentedly high degree of internal order and long-range periodicity has been successfully fabricated. With this advantage, the nanoscale framework structure of a water-soluble 2D SOF with exceptional long-range internal order has been visualized with TEM for the first time. It also endows the SOF with Janus feature that results in the formation of unique SOF bilayers. This work demonstrates that amphiphilicity provides excellent control over the internal order of 2D supramolecular networks, which is conducive to the accurate construction of more 2D SOFs featuring a high degree of internal order, long-range periodicity, large surface areas, and the nanoscale porosities. This kind of 2D SOF can be a versatile platform via outer-surface functionalization for the fabrication of advanced functional materials, for example, ultrahigh sensitive sensing, selective separation, and highly efficient catalysis. Furthermore, in contrast to common amphiphiles which usually are linear, the SOF assembled from 1 and CB[8] is planar and thus can be defined as a 2D amphiphile. This unique type of amphiphilic structure might exhibit interesting properties different from linear amphiphiles, which deserve to be further exploited for practical applications. Supporting Information Supporting Information is available and includes the procedures for the preparation of the building blocks and SOFs, Job’s plots, DLS and ITC profiles, computational models, and NMR spectra of the new compounds (Figures S11–S24). Conflict of Interest There is no conflict of interest to report. Acknowledgments This research was supported financially by the National Science Fund for Distinguished Young Scholars of China (no. 21725404), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. and the Science of Zhejiang (no. The of Chemistry, Chinese Academy of for on the SAXS The also Shanghai Synchrotron for the synchrotron X-ray scattering 1. Wu Zhao in Two-Dimensional Google Scholar 2D Google Scholar Wu A of Google Scholar Structures and Google Scholar Zhao Liu Organic in Synthetic 2D Organic Google Scholar Jiang in Google Scholar of a Google Scholar Liu Framework as for Molecular Google Scholar Organic from Organic via Google Scholar of Organic into as for Google Scholar of Organic into Google Scholar Zhao Loh the of Organic by in Google Scholar Liu of Two-Dimensional Organic via Google Scholar of a Two-Dimensional Organic through Chemistry at the Google Scholar

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