2D Nanomaterials

Isomeric Routes to Schiff-Base Single-layered Covalent Organic Frameworks Xuan-He Liu, Yi-Ping Mo, Jie-Yu Yue, Qing-Na Zheng, Hui-Juan Yan, Dong Wang,* and Li-Jun Wan*

With

graphene-like topology and designable functional moieties, single-layered covalent organic frameworks (sCOFs) have attracted enormous interest for both fundamental research and application prospects. As the growth of sCOFs involves the assembly and reaction of precursors in a spatial defined manner, it is of great importance to understand the kinetics of sCOFs formation. Although several large families of sCOFs and bulk COF materials based on different coupling reactions have been reported, the synthesis of isomeric sCOFs by exchanging the coupling reaction moieties on precursors has been barely explored. Herein, a series of isomeric sCOFs based on Schiff-base reaction is designed to understand the effect of monomer structure on the growth kinetics of sCOFs. The distinctly different local packing motifs in the mixed assemblies for the two isomeric routes closely resemble to those in the assemblies of monomers, which affect the structural evolution process for highly ordered imine-linked sCOFs. In addition, surface diffusion of monomers and the molecule-substrate interaction, which is tunable by reaction temperature, also play an important role in structural evolutions. This study highlights the important roles of monomer structure and reaction temperature in the design and synthesis of covalent bond connected functional nanoporous networks.

1. Introduction Two-dimensional structure-robust porous nanostructures with covalent linkages, what we called single-layered covalent organic frameworks (sCOFs), have recently attracted

X.-H. Liu, Y.-P. Mo, J.-Y. Yue, Q.-N. Zheng, H.-J. Yan, Prof. D. Wang, Prof. L.-J. Wan CAS Key laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing 100190, P. R. China E-mail: [email protected]; wanli[email protected] X.-H. Liu, Y.-P. Mo, J.-Y. Yue, Q.-N. Zheng University of Chinese Academy of Sciences Beijing 100049, P. R. China DOI: 10.1002/smll.201400899 small 2014, DOI: 10.1002/smll.201400899

enormous interest due to their higher chemical stability and wide range of potential applications, such as nanofiltration, molecular electronics sensors and so on.[1–11] The bottomup strategy proves to be an effective synthetic strategy to construct sCOFs.[12–21] Recently, the strategies of thermodynamic equilibrium control, kinetic growth process control, and surface-catalyzed polymerization have been developed to construct large-scale high-quality sCOFs.[22–27] In the construction of high-quality sCOFs, plenty of influencing parameters including concentration, substrate, temperature, the chemical structure of the precursors et al. should be controlled. As an interesting comparison, there already has been a ton of extensive research in the effect of the nonexclusive list of these factors on non-covalent nanostructures of molecular self-assembly.[28–33] Structural phase transitions of non-covalent nanostructures from one initial phase into a transition, followed by the final one by spontaneity or induced by environmental conditions have been subjected to a host of observations.[31–35] Nevertheless, studies

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Scheme 1. Molecular structures of monomers: 1, 1, 3, 5-tris (4-formylphenyl) benzene; 2, 1, 3, 5-tris (4-aminophenyl) benzene; 3, [1,1′-biphenyl]-4,4′-diamine dihydrochloride; 4, [1,1′-biphenyl]-4,4′dicarboxaldehyde and the chemical structures of the expected isomeric sCOFs.

in kinetic or thermodynamic control over 2D sCOFs are still rare so far, which are necessary to high-quality sCOFs synthesis. For a binary linear linkage based COF, the isomeric networks with the same diameter can be targeted by exchanging the reactive groups in two monomers with different symmetry. So far, there is no previous report about the effect of isomeric routes to the formation process of sCOFs or bulk COF materials. In the present work, we aim at exploring the general kinetic growth process of sCOFs by comparing the structural evolution process of the isomeric sCOF structures via exchanging terminal reactive groups on C3 and C2 building blocks. Schiff-base chemistry is chosen as the linkage reaction for sCOFs synthesis due to merits of easily accessible building blocks and mild reaction conditions.[36–45] Diverse building blocks we choose are illustrated in Scheme 1. Molecules 1 and 2 have C3 symmetry, whereas molecules 3 and 4 have C2 symmetry. Two C3 building blocks have the same aromatic backbone but carry different reaction groups. Homologue hexagonal networks of 2D sCOFs are expected by integrating the C3 building blocks (1, 2) with corresponding C2 (3, 4) blocks (shown in Scheme 1). The sCOFs fabricated from building blocks 1 and 3 would have the isomeric framework structures compared to those from building blocks 2 and 4. The condensation reactions are performed by heating C3 and C2 blocks on the highly oriented pyrolytic graphite (HOPG) surfaces at designated temperatures in a closed system. In addition, the effect of surface diffusion of monomers and the molecule-substrate interaction has also been investigated by varying length of monomer backbone with conformation rigid entities under the guidance of reticular design principle. The effect of reaction temperature over the structural evolution of isomeric sCOFs formation has been extensively interrogated. The present work provides insight into the growth kinetics of sCOFs and sheds

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Figure 1. A,B) High-resolution STM image and molecular model of molecule 1 adlayer on HOPG. C,D) High-resolution STM image and molecular model of molecule 1 adlayer on HOPG. Imaging conditions: A) Vbias = 800 mV, It = 200 pA; B) Vbias = 700 mV, It = 500 pA.

important insight on the design of sCOFs and bulk COF materials with large diameters.

2. Results and Discussion 2.1. Adlayers of Monomers on HOPG We first survey the self-assembly behavior of monomers on HOPG. Figure 1A shows a typical STM image of the selfassembled adlayers of molecule 1 on HOPG prepared at ambient temperature. Ordered adlayers with domain size about 20 nm of densely packed structure can be observed in the image, and larger domain size over 100 nm can be obtained after annealing at 120 °C. The dimensions of the unit cell outlined in Figure 1A are a = b = 1.3 ± 0.2 nm and α = 60 ± 2°. Each molecule 1 is typically observed as a trefoil feature under optimized imaging conditions. The structural model depicting the molecular arrangement on the basis of the STM images is proposed in Figure 1B. The high-resolution STM image of the self-assembled adlayers of molecule 2 is displayed in Figure 1C. The ordered patterning of molecules 2 can only be obtained after annealing at 120 °C. Each molecule 2 appears as a trefoil shape, similar to molecule 1. Different from closing packing of molecule 1, molecules 2 form an open network structure of molecular trimer as basic building units. The structural model depicting the molecular arrangement is shown in Figure 1D. The parameters of the unit cell shown in Figure 1C are a = b = 1.8 ± 0.2 nm and α = 60 ± 2°. Selfassembly of molecules 3 and 4 were also studied. However,

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small 2014, DOI: 10.1002/smll.201400899

Isomeric Routes to Schiff-Base Single-layered Covalent Organic Frameworks

Figure 2. A) Scheme diagram of general co-condensation reaction of 1 with 3 at 120 °C. Condensation of trigonal precursor 1 and linear precursor 3 can form Macrocycle A. B, C) Large-scale and high resolution STM images showing the assembly structures of Macrocycle A with Oligomer A, and intact molecules 1. White lines indicate graphite symmetry axes. Imaging conditions: Vbias = 700 mV, It = 500 pA.

Figure 3. A) Scheme diagram of formation of sCOFs-IC2 at 130 °C. Condensation of trigonal precursor 1 and linear precursor 3 can form sCOFs-IC2. B) Large-scale STM image of sCOFs-IC2 with the inset depicting the corresponding FFT spectrum of the STM image. C) High resolution STM image of sCOFs-IC2. Imaging conditions: Vbias = 700 mV, It = 500 pA.

ordered self-assembled monolayers of molecules 3, 4 were not observed on HOPG.

size of 4.62 nm by DFT calculation (Figure S3, Supporting Information), which evidences that ring structures are Macrocycles A formed through a [3 + 3] diamine-trialdehyde condensation with molecules 1 at each vertex of a regular hexagon. The general co-condensation reaction of 1 with 3 at 120 °C is outlined in Figure 2A. Interestingly, we find that the structural motif in the adlayer (Figure 2) is similar to the selfassembled packing of molecule 1 (Figure 1a), indicating that the close packing of terminal molecule 1 plays an important role in guiding the multi-walled porous structures. By choosing higher reaction temperature of 130 °C, the networks of sCOFs-IC2 were constructed. The general cocondensation reaction of 1 with 3 at 130 °C is outlined in Figure 3A. Figure 3B displays the typical large scale STM image exhibiting well-defined sCOFs-IC2 with high-quality honeycomb network structures and the two-dimensional FFT of images in the inset of Figures 3B show well-defined 6-fold symmetry. Figures 3C displays the high-resolution STM image of sCOFs-IC2. The lattice parameter is measured to be 4.8 ± 0.2 nm, in agreement with the expected size of 4.62 nm (shown in Figure S4, Supporting Information) by DFT calculation, confirming the covalent bond formation. In addition, the frames of sCOFs show nearly uniform contrast as reported in our previous work,[26] further supporting covalent bond-linked networks formation. We note that the structural transformation from multi-walled porous structures formed at lower temperature to sCOFs-IC2 by heating at 130 °C or even higher is not observed. The multi-walled porous structures are stable at room temperature, and their molecular orderings cannot be changed to sCOFs-IC2 by increasing the temperature. This may indicate that both the multi-walled porous structures

2.2. Structural Evolution for sCOFs from Monomer 1+3 Co-condensation of 1 with 3 was performed by heating HOPG preloaded with the two building blocks in a closed system at 120 °C. After cooling to ambient condition, multiwalled porous structures are observed by STM. Close-examination of STM image reveals that the structure is lack of perfect orderliness, which is also supported by fast Fourier transform (FFT) (shown in Figure S1, Supporting Information). Figure 2B shows the representative large-scale STM image. The adlayer with domain size over 100 nm can be clearly resolved. The structural details of the porous structure are illustrated in the high-resolution STM image shown in Figure 2C. The pattern structure of the assembly is composed of ring structures surrounded by bone-like structures and trefoil structures. On the basis of the result in Figure 1, trefoil structures are presumably ascribed to the intact molecules 1. In addition, we propose that bone-like structures are the imine-linked Oligomers A formed by coupling of molecule 1 and molecules 3 at a stoichiometric ratio of 2:1 based on the feature of STM imaging and the length agreement between experimental value of 3.3 ± 0.2 nm and the expected size of 3.30 nm by DFT calculation (Figure S2, Supporting Information). The perfect hexagon ring structure can be ascribed to the incomplete reaction between molecules 1 and molecules 3 at a stoichiometric ratio of 1:2 with the leftover aldehyde group at six corners of the ring. The ring diameter of 4.8 ± 0.2 nm measured by STM agrees well with the expected small 2014, DOI: 10.1002/smll.201400899

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Figure 4. A) Scheme diagram of general co-condensation reaction of 2 with 4 at 110 °C. Condensation of trigonal precursor 2 and linear precursor 4 can form Macrocycle A′. B,C) Large-scale and high resolution STM images showing the assembly structures of Macrocycles A′, Oligomers A′, and intact molecules 2. White lines indicate graphite symmetry axes. Imaging conditions: Vbias = 700 mV, It = 500 pA.

and sCOFs-IC2 are thermodynamically stable. The closing packing of unreacted precursors and oligomers in the multiwalled porous structures presumably renders higher energy barrier to overcome for the effective diffusion of precursors or oligomers, which is very important for fully developed sCOF formation. The failure of transformation suggests the multi-walled porous structures might not exist as the intermediate state in sCOFs-IC2 formation at higher temperature.

3.3 ± 0.2 nm agrees well with the expected size of 3.53 nm by DFT calculation (shown in Figure S2, Supporting Information). As shown in phase III, ring structures are referred as Macrocycles A’ formed through a [3 + 3] triamine-dialdehyde condensation with molecules 2 at each vertex of a regular hexagon. The experimental ring diameter of 4.8 ± 0.2 nm measured by STM agrees well with the expected size of 4.62 nm (shown in Figure S3, Supporting Information) by DFT calculation. Different from multi-walled regular porous structures, the ring structures can self-assemble into ordered domains shown in phase III. The arrangement of these phases is all guided by packing pattern of terminal unreacted amino groups on molecule 1, namely 3 amino groups forming a trimer, which is similar to the assembly motifs for some C3 symmetric molecules via non-covalent interactions.[46,47] It is worth mentioning that small flakes of sCOFs-IC2′ already can be observed even at 110 °C shown in phase IV, indicating that it is much easier to overcome the van der Waals forces of terminal amino groups for structural transformation to sCOF-IC2’. After heating the mixed adlayers on HOPG obtained at 110 °C to higher temperature 120 °C, sCOFs-IC2′ was constructed as expected. The general co-condensation reaction of 2 with 4 is outlined in Figure 5A. Figure 5B and C displays the typical large scale and high resolution STM images exhibiting well-defined high-quality sCOFs-IC2′. FFT of the image in the inset of Figure 5B shows well-defined sixfold symmetry and the lattice parameters are measured to be 4.8 ± 0.2 nm, in agreement with the expected size of 4.62 nm (shown in Figure S4, Supporting Information) by DFT calculation, confirming the covalent formation of sCOFs-IC2′. The

2.3. Structural Evolution for sCOFs from Monomer 2+4 To obtain more detailed insight into the kinetic process of the structural evolution in formation of sCOFs, we designed sCOFs-IC2′ with the exact same dimension as sCOFs-IC2 but different connection mode of imine bond in the frameworks by choosing monomers equipped with 3 amine groups and 2 aldehyde groups respectively, namely molecule 2 and 4. Interestingly, we observe the slightly different temperature dependent structural evolution for sCOFs growth. Several phases are observed when heating HOPG pre-loaded by 2 and 4 at 110 °C and the co-condensation reaction of 2 with 4 is shown in Figure 4A. Figure 4B displays a typical large-scale STM image with all the phases present in the same image. More detailed structural information can be obtained from the high resolution STM image in Figure 4C. In phase I, molecules 2 have the same arrangement shown in Figure 1B. Within phase II, bone-like structures arrange in order, which is perceived as Oligomer A′ formed by molecule 2 and two molecules 4 at t a stoichiometric ratio of 2:1, similar to the formation of Oligomer A. The experimental length

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Figure 5. A) Scheme diagram of formation of sCOFs-IC2′ at 120 °C. Condensation of trigonal precursor 2 and linear precursor 4 can form sCOFs-IC2′. B) Large-scale STM image of sCOFs-IC2′ with the inset depicting the corresponding FFT spectrum of the STM image. C) High resolution STM image of sCOFs-IC2’. Imaging conditions: Vbias = 700 mV, It = 500 pA.

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small 2014, DOI: 10.1002/smll.201400899

Isomeric Routes to Schiff-Base Single-layered Covalent Organic Frameworks

sCOFs domain size can reach more than 300 mm × 300 nm (shown in Figure S5, Supporting Information). It is worth mentioning that sCOFs-IC2′ can also be obtained by heating HOPG preloaded by monomers, molecule 2 and 4 at 120 °C. In contrast to the failure of structural evolution from the formed multi-walled structures to sCOFs-IC2, conversion of structural phase to sCOFs-IC2′ occurs by increasing heating temperature. The network packing of terminal amine groups lower the energy threshold for the structural evolution. The phenomenon of initial metastable structure formation and structural evolution to the most stable one is within Ostwald’s rule, which is applied to crystallizations of most chemical compounds.

2.4. Structural Evolution for sCOFs with Larger Diameter We have further designed sCOFs with larger lattice parameter by employing C2 monomers with terphenyl backbone to further explore the general kinetic rules of structural evolution. The fully developed sCOFs with extended order are not obtained by combination of monomers. Instead, the adlayers of macrocylces are formed at the temperature range from 150 °C up to 200 °C. For example, the multi-walled structures with large diameters are observed by co-condensation of 1 with [1,1′:4′,1′′]terphenyl-4,4′′-diamine(shown in Figure S6, Supporting Information). The ordered ring structures with domain size over 100 nm are obtained by co-condensation of 2 with [1,1′:4′,1′′]terphenyl-4,4′′-dicarbaldehyde (shown in Figure S7, Supporting Information). Poor mobility of large oligomers and macrocylces on surfaces impede the further formation of sCOFs with large diameters, which can be attributed to the increases substrate-molecule interaction with the molecular weight of adsorbates.

2.5. Discussion It is very interesting to compare the structural evolution process of the isomeric sCOF structure by exchanging terminal reactive groups on C3 and C2 building blocks to explore the general kinetic rules. The leftover terminal reactive groups on C3 building blocks play important roles in the mixed assembly. The close packing of C3 molecule 1, namely aldehyde, leads to the multi-walled porous structures. Similarly, the self-assembled ring structures derived from C3 molecule 2 are guided by the patterning of un-reacted amine groups. The packing motif formed by terminal aldehyde groups is much denser than that by terminal amine. Furthermore, the local packing motifs in the mixed assemblies affect the structural evolution process for highly ordered sCOFs. With further increasing of phenyl spacer in C2 building blocks, however, no expected sCOFs structures are obtained even at higher reaction temperature, which can be attributed to the increases substrate-molecule interaction with the molecular weight of adsorbates. Hence, we propose that the formation of well-ordered sCOFs on surfaces is a result of delicate balance of reaction thermodynamic of covalent bond formation, surface diffusion of monomers, the intermolecular interaction small 2014, DOI: 10.1002/smll.201400899

between monomers and oligomers, and the molecule-substrate interaction. The reaction temperature is found to be a critical factor to control the structural evolution of sCOFs formation. By increasing heating temperature, the mixed assemblies with un-reacted monomers, oligomers or partially developed sCOFs-IC2′ rings observed at lower reaction temperature can transform directly into sCOFs-IC2′, while structural transition from multiwall porous structure with closing packing of terminal aldehyde groups to well-ordered sCOFs-IC2 is not accomplished. We propose the close-packed multi-walled porous structure is stabilized by relatively stronger van der Waals interaction, and thus less prone to be disassembled to form sCOFs-IC2. Not surprisingly, the fully developed highly ordered sCOFs-IC2 and IC2′ are observed at higher temperature, as the enhanced thermo movement can rupture the weak intermolecular interactions to certain extent and active the diffusion of oligomers.

3. Conclusion In summary, we have designed a series of homologue iminebond linked isomeric sCOFs and explored the general kinetic rules of their structural evolutions. Two isomeric routes show distinctly different structural evolution process in sCOFs synthesis process. By varying the reaction temperature, structural evolution from mixed assemblies to sCOFs has been precisely observed. Different porous structures composed by isomeric macrocyles were observed at lower temperature. The local packing motifs in the mixed assemblies closely resemble to those of the assemblies of monomers. By varying the backbone length of precursors with C2 symmetry, the macrocycle structures instead of sCOFs formation are obtained due to poor mobility of the molecular weight of adsorbates. The present work could be conducive for profoundly understanding the growth kinetics of sCOFs and highlight the importance of monomer structure for the sCOF design and synthesis.

4. Experimental Section 1,3,5-tris(4-formylphenyl)benzene (1) and [1,1′:4′,1′′] quaterphenyl-4,4′′-dicarbaldehyde (6) was synthesized according to the reported procedures. Their 1H NMR and 13C NMR spectra agree well with those reported previously.[48–50] 1,3,5-tris(4-aminophenyl)benzene (2) and [1,1′:4′,1′′]quaterphenyl-4,4′′-diamine (4) were purchased from J&K. [1,1′-biphenyl]-4,4′-diamine dihydrochloride (3) was acquired from Sigma and [1,1′-biphenyl]-4,4′dicarboxaldehyde (5) was bought from Acros. All the chemicals used in this study, unless otherwise specified, were used without further purification. In a typical synthesis procedure, a droplet (≈ 5 µL) of tetrahydrofuran (THF) solution containing two building blocks (∼10−5m) was deposited on freshly cleaved HOPG surfaces and allow them to dry. Then, the treated HOPG and CuSO4·5H2O power (≈1.1 g) were transferred into a 100 mL Teflon-sealed autoclave (Figure S8, Supporting Information). After autoclave was sealed and subject to the designated temperature, different structures are expected to form without further processing. Typical

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reaction time was 3 h. STM images were recorded using the Nanoscope IIIa SPM (Bruker Nano) with mechanically cut Pt/Ir wires (90/10) under ambient conditions. All of the images were performed in constant-current mode and are shown without further processing. FFT spectrums of the STM images were performed by Nanoscope software.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Key Project on Basic Research (Grants 2011CB808700, 2011CB932300, and 2009CB930400), National Natural Science Foundation of China (91023013, 21121063, 20905069, 21073204), and the Chinese Academy of Sciences.

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Received: April 2, 2014 Revised: June 25, 2014 Published online: small 2014, DOI: 10.1002/smll.201400899

Isomeric routes to Schiff-base single-layered covalent organic frameworks.

With graphene-like topology and designable functional moieties, single-layered covalent organic frameworks (sCOFs) have attracted enormous interest fo...
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