Adaptive Reorganization of 2D Molecular Nanoporous Network

5 Mar 2014 - Adaptive Reorganization of 2D Molecular Nanoporous Network Induced by Coadsorbed ... Surface Host–Guest Supramolecular Assemblies on ...
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Adaptive Reorganization of 2D Molecular Nanoporous Network Induced by Coadsorbed Guest Molecule Qing-Na Zheng,†,§ Lei Wang,‡ Yu-Wu Zhong,‡ Xuan-He Liu,†,§ Ting Chen,† Hui-Juan Yan,† Dong Wang,*,† Jian-Nian Yao,‡ and Li-Jun Wan*,† †

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 ‡ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: The ordered array of nanovoids in nanoporous networks, such as honeycomb, Kagome, and square, provides a molecular template for the accommodation of “guest molecules”. Compared with the commonly studied guest molecules featuring high symmetry evenly incorporated into the template, guest molecules featuring lower symmetry are rare to report. Herein, we report the formation of a distinct patterned superlattice of guest molecules by selective trapping of guest molecules into the honeycomb network of trimesic acid (TMA). Two distinct surface patterns have been achieved by the guest inclusion induced adaptive reconstruction of a 2D molecular nanoporous network. The honeycomb networks can synergetically tune the arrangement upon inclusion of the guest molecules with different core size but similar peripherals groups, resulting in a trihexagonal Kagome or triangular patterns.

1. INTRODUCTION Two-dimensional supramolecular self-assembly on solid surfaces is a prominent approach toward the bottom-up fabrication of supramolecular architectures.1−13 Controlling the arrangement of multicomponents in a designed manner is a challenge of surface molecular engineering. The so-called surface-confined nanoporous networks are of great interest due to the possibility of using them to immobilize functional guest molecules in a spatially ordered manner.14−26 So far, a rich library of patterned nanoporous networks, such as honeycomb, Kagome, and square, have been demonstrated on surfaces by tailoring supramolecular noncovalent interactions.27−46 The ordered array of nanovoids in nanoporous networks provides a molecular template for the accommodation of “guest molecules” through surface confinement interactions. The size and/or shape complement between the host cavity and the guest molecules is frequently proposed as a key factor to be responsible for the guest inclusion assembly.21,23,26,47−49 In most cases, the incorporation of the guest molecules into the host network does not bring any movement of the host matrix. © 2014 American Chemical Society

Therefore, by the so-called host−guest assembly method, the 2D pattern of the guest molecules is spatially defined by that of the nanovoids in the host nanoporous networks.16,18,19,21,23,49−51 In a few exceptional cases, however, the coassembly of guest molecules can introduce interesting reorganization of host assemblies. For example, De Feyter and co-workers demonstrated that the host structure can transform from a linear structure to a honeycomb network structure as a response to guest molecules.47 Blunt et al. used a guest coronene to induce the host tetracarboxylic acid species to fabricate Kagome structure.52 Beton et al. show that the trapping of fullerene in nanoporous networks can induce the formation of double-layer structure of host networks.53 Zhang et al. reported the structural flexible linear template in response to the size of trapped guest molecules.54 Gruber and co-workers clarified a system combining both host−guest behavior and Received: January 20, 2014 Revised: March 5, 2014 Published: March 5, 2014 3034

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templated polymorph assembly.55 The so-called templated assembly can result in a rich library of intricate pattern of functional molecules and deserves increased attention to understand the intermolecular interaction during the assembly process. Herein, we report the formation of distinct patterned superlattices of “guest molecules” by selectively trap into the honeycomb network of trimesic acid (TMA) in a coassembly process. The chemical structures of 1,3,6,8-tetrakis(1-butyl-1H1,2,3-triazol-4-yl)pyrene (TP)56 and 1,2,4,5-tetrakis(1-butyl1H-1,2,3-triazole-4-yl)benzene (TB) are shown in Figure 1.

Figure 2. STM topography image of TMA in octanoic acid. (a) STM image of honeycomb structure of TMA. Tunneling conditions: Vbias = 700 mV, It = 450 pA. The inset at the upper right corner shows largescale STM image of the TMA network. (b) Model of TMA honeycomb structure.

molecules, as have been reported previously. Contrary to the well-documented host−guest assembly with the rigid host matrix, herein we found that the matrix may flexbily reorganize itself upon guest inclusion. 2.2. TMA/TP Kagome Coassembly. After obtaining the honeycomb network of TMA, a drop of guest molecules TP in solution was added to the network. The Kagome architecture is almost always observed within a few minutes after deposition of TP molecules, without showing any significant structural evolution during this process. Such a structure can be consistently observed at various concentrations from 10−4 to 10−6 M and wide molar ratio (TMA:TP range from 2000 to 100; the stoichiometric excess of TMA has to be employed, which is ascribed to the strong adsorption affinity of TP to HOPG). Figure 3a is a typical large-scale STM image acquired by adding a droplet of TP solution to TMA honeycomb

Figure 1. Chemical structures of (a) TP, (b) TB, and (c) TMA.

TP consists of a central pyrene backbone and four 1,2,3-triazole groups on the corners. The triazole unit is decorated with four butyl chains providing enhanced solubility as well as improved propensity to physisorb on highly oriented pyrolitic graphite (HOPG) at the solid−liquid interface.57 The TB molecule is structurally similar to TP except for a benzene ring as backbone. The assembly process starts from a hexagonal honeycomb network of TMA on HOPG.58−60 Typically, guest molecules with size smaller than nanovoids can fill in the nanovoids evenly, such as coronene in TMA networks. In sharp contrast, we found that TP and TB can be trapped into TMA network selectively and results in the superlattice of guest molecules in network. In particular, TP molecules can form a trihexagonal or Kagome superlattice, whereas TB molecules form a triangular tiling. Structural analysis from high resolution scanning tunneling microscopy (STM) images and molecular mechanics (MM) simulation of the assemblies reveals that the inclusion of the guest molecules results in the concomitant adjustment of the TMA network. Guest-inclusion-induced reorganization of surface molecular networks not only enriches the breadth of surface supramolecular chemistry but also provides a feasible route for the design of nanoarchitecture with high complexity.

2. RESULTS AND DISCUSSION 2.1. TMA Template Matrix. After cleavage of the HOPG substrate, the honeycomb structure, formed by self-assembly of TMA molecules at liquid−solid interface, is fabricated on the surface. The high resolution STM image of honeycomb structure of TMA is shown in Figure 2a. TMA molecules occupy the corner positions and enclose a hexagonal pore. Each of the three carboxylic groups per molecule forms two hydrogen bonds with its neighbors. The honeycomb-like structure, or so-called “chicken-wire” structure, exhibits a periodic arrangement of cavities of 1.1 nm in diameter and a nearest-neighbor distance of 1.7 nm. The inset in Figure 2a depicts the large scale, defect-free honeycomb structure. The typical domain size can reach 200 × 200 nm2. The honeycomb structure of TMA polymorph on graphite has been observed both in ultrahigh vacuum (UHV) and at the liquid−solid interface. The large-scale and defect-free TMA honeycomb network serves as a templete for the ordering of guest

Figure 3. Kagome structure formed through coassembly of TP/TMA. (a) Large-scale STM image of the Kagome structure. The main symmetry axes of the underlying HOPG lattice are indicated by white arrows in the top left corner of the STM image. Tunneling conditions: Vbias = 561 mV, It = 550 pA. (b) High-resolution STM image of the Kagome structure. Tunneling conditions: Vbias = 770 mV, It = 320 pA. The small red triangle indicates basic unit of Kagome network formed by three TP molecules. Kagome lattice made by connecting the centroids of the TP molecules trapped in the honeycomb TMA supramolecular structure is indicated by white lines. The gray triangles indicate the TMA molecules with explicit orientation. The yellow ellipses indicate the TP molecules with explicit orientation. (c) Tentatively proposed structural model for the Kagome structure. TP molecules and TMA molecules are colored yellow and gray, respectively. (d) Illustration for hydrogen bonds. Possible hydrogen bonds are depicted by cyan dashed line. Color code: C = gray; H = white; O = red; N = blue. 3035

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On the basis of detailed structural analysis from STM image, a structural model for the Kagome structure of TP is proposed in Figure 3c. Possible hydrogen bonds formed on Kagome lattices are presented in Figure 3d by dashed lines. The TMA hexamer inside the hexagon pore of Kagome structure are stabilized by 12 hydrogen bonds between carboxyl groups in neighboring TMA molecules. There are three hydrogen bonds existing between the trapped TMA molecule and the shrunk TMA hexamer and three hydrogen bonds existing between hydroxyl groups of TMA molecules trapped in the triangular nanopores of Kagome network and nitrogen in triazole groups of TP as indicated in the inset image in Figure 3d, which is helpful to stabilize the enlarged TMA hexamers. 2.3. TMA/TB Triangular Coassembly. To understand the unexpected selective immobilization of TP molecules in nanopores of TMA honeycomb network, we choose another guest molecule TB with the same substituent triazole groups but a benzene ring as backbone to fill in the TMA network. Figure 4a is a typical STM image of the self-assembly adlayers

network preformed on HOPG surface. We note that pure guest TP molecules in THF solution self-assemble into a close packed pattern (see Supporting Information). A well-ordered Kagome network can be clearly seen. It is worth mentioning that the Kagome structure is formed by the “guest molecules”. The typical domain size can reach 150 × 150 nm2. The different contrast is due to the electronic density of the pyrene backbone is higher than that of aromatic cores of TMA; the big bright spots are ascribed to TP molecules and small dark spots to TMA molecules. By comparing with the atomic images of underlying HOPG substrate, it is found that the lattice vectors of Kagome structure is along the primary vector of HOPG substrate, as indicating by arrows in Figure 3a (see Supporting Information for composite image of Kagome assembly and substrate). More details of the structure are shown in the high resolution STM image of Figure 3b. Each TP molecule appears as ellipse shape. The major and short axis of the ellipse are 1.3 and 0.9 nm, respectively, which are in accord with the length (1.19 nm) and width (0.98 nm) of TP molecule. TP molecules are supposed to adsorb on the substrate in a lying manner. The orientation of TP is clearly defined by the shape and direction of the TMA matrix: the major axis of TP is point to the vertex of the hexagonal hole of TMA network and the triazole groups in the four peripheral sides interact with the TMA dimer namely the side of the TMA hexamer. Close inspection reveals that the TP molecules have an explicit orientation enclose a hexagon in the Kagome network, as highlighted by yellow ellipses in Figure 3b, indicating that the intermolecular interactions between TP and TMA molecules restrict the flexibility of TP. Every three neighboring TP molecules constitute a symmetric triangle, as clarified by red triangle in Figure 3b. The six triangles enclose a regular hexagon in a vertex-shaping configuration, as illustrated by white line in the image. The small dark spots in STM image are ascribed to TMA molecules. Each TMA molecule can be resolved as a small triangle shape feature at the optimal imaging condition, as highlighted by the gray triangle in Figure 3b. By comparing with honeycomb network shown in Figure 2, the TP molecules are trapped alternately in the pores of TMA network. One can see that there are also some small spots trapped in the TMA hexamers alternately. These small spots are ascribed to the excess TMA molecules in the liquid environment. In comparison, please note that the trapping of excess TMA trapped in the monocomponent TMA network is barely observed as illustrated in Figure 2. As revealed in Figure 2, each TMA molecule is bounded to three adjacent molecules via 2fold hydrogen bonds between carboxylic functionalities in the honeycomb network. The TMA molecules take a vertex-tovertex manner to form a uniform hexagonal pore. With the inclusion of TP molecules, the pores of TMA network adaptive adjusting. The trapping process of TP molecules on the boundary of Kagome and pre-existing TMA network domains (see Supporting Information Figure S5) indicates that the TMA network adjusted itself to load TP molecules. The TMA pores that are filled in TP molecules are stretched a little, with a diameter of 2.5 nm. These results indicate that the flexibility of TMA network leads to adjust itself to incorporating the TP molecules. As a complement, the incorporation of TP molecules can also contribute to incorporate excess TMA in the changed TMA network. The parameters of the unit cell outlined in Figure 3b are a = b = 4.0 ± 0.2 nm and γ = 60 ± 2°.

Figure 4. Coassembly structure formed in the control experiment. (a) Large-scale STM image of the supramolecular coassembly architecture. The main symmetry axes of the underlying HOPG lattice are indicated by white arrows in the top left corner of the STM image. Tunneling conditions: Vbias = 913 mV, It = 406 pA. (b) High-resolution STM image of the supramolecular coassembly architecture. Tunneling conditions: Vbias = 913 mV, It = 406 pA. The gray triangles indicate the TMA molecules. The yellow spheres indicate the TB molecules. The small pink sphere indicates the TMA molecule filled in the hexagonal hole. (c) Tentatively proposed structural model for the supramolecular coassembly architecture. TB molecules are colored yellow. Two kinds of TMA molecules, i.e. the TMA molecules incorporated in the holes of TMA honeycomb and the TMA honeycomb network, are colored pink and gray, respectively. (d) Illustration for hydrogen bonds. Possible hydrogen bonds are depicted by a cyan dashed line.

obtained by adding a drop of TB solution to the preformed TMA network. A high-ordered 2D coassembly with a hexagonal motif can be seen in the STM image. The large-scale and defect-free domain can reach 300 × 300 nm2. A high-resolution STM image with detailed internal structural information is shown in Figure 4b. A honeycomb-like network, as outlined by gray triangles, resembling the TMA network is still discernible after careful comparison. The big bright spots are observed to be trapped alternately in the honeycomb networks as highlighted by using yellow spheres in Figure 4c and are ascribed to coabsorbed TB molecules. The less pronounced contrast of TB molecule, relative to that of TP molecules, can 3036

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Figure 5. Schematic diagram for the guest inclusion induced nanoporous network reconstruction. (a) TP molecule structure with a rectangle shape. The arrows depict the a- and b-axis of the TP molecules. (b) Trapping of a TP molecule in the hexagonal void of TMA honeycomb network results in the distortion of nanovoid. The TP molecule interacts with TMA network at four shoulder sites (highlighted by red color) and then enlarges the TMA hexagonal due to the size mismatch. The other two sides of the network are highlighted by the blue color. (c) TP molecule inclusion induced Kagome pattern. The 2D tiling of surface with the distorted hexagon shown in panel b results in the formation of Kagome lattice of TP molecules. (d) TB molecule structure with a rectangle shape. (e) Trapping of a TB molecule in the hexagonal void of TMA honeycomb network. (f) The coassembly pattern of TB/TMA system. Along with the inclusion of the TB molecules, the neighboring TMA hexagonal holes are able to incorporate a TMA molecule as depicted with pink hexagon. Please note the change of the hexagonal lattice is not drawn to scale.

2.4. Adaptive Coassembly. The selective trapping of these two kinds of triazole molecules in the TMA honeycomb network highlights the importance of size compatibility between guest molecules and nanovoids of network for coassembly. Figure 5a shows the length and width of the TP molecules, which features a rectangle backbone with four triazole peripherals, as represented by the superimposed symbols. The TP molecule has a rectangle shape and has mirror symmetry along the a-axis of the molecule. Figure 5b shows schematic diagram of trapping of TP into a TMA hexagon. The black honeycomb network indicates the TMA network. The honeycomb network has a diameter of ∼1 nm, which is too small to incorporate a TP molecule. The affinity of triazole group to carboxylic acid group results in the alignment of the a-axis of TP molecules with the diagonal direction (a′) of TMA honeycomb network. Furthermore, the four interacting sites between the TP molecules and network enlarge the four sides of the TMA hexamer and thus expand the TMA holes (illustrated in Figure 5b) to a new motif in order to load a TP molecule. The synergistic recognition and reorganization between the flexible 6-fold symmetrical TMA network and the 2-fold symmetrical TP molecule bring about the Kagome tiling. Figure 5d shows TB molecule with a smaller size. With benzene unit as its center backbone, TB features a smaller dimension and has a shorter a-axis. Upon loading into TMA network, as schematically shown in Figure 5e, TB molecule prefers aligning the longer b-axis with the a′-axis of honeycomb network, also driven by the interaction between triazole and carboxylic acid groups. In addition, with the compatibility of the TB and TMA hole, the TMA hexagonal holes keep fixed with the inclusion of TB molecule. Then the hexagonal sides shared

be ascribed to the smaller conjugated core in TB molecule. In addition, the leftover nanovoids are occupied with less bright spherical features, presumably the coabsorbed TMA molecules. These coabsorbed TMA are highlighted by small pink sphere overlaid on Figure 4b. We also note that trapping of TMA molecules into TMA network has been barely observed for the monocomponent assembly of TMA on HOPG. We propose that, unlike the larger TP molecule which can enlarge the TMA network, the relative smaller size TB molecules can be trapped into TMA networks fitly because of the comparable sizes of TMA nanopore and TB molecule. The parameters of the unit cell outlined in Figure 4b are a = b = 3.0 ± 0.1 nm and γ = 60 ± 1°. The distance between two TB molecules is 3.0 nm, which is √3 times the diameter of TMA honeycomb network. On the basis of the above analysis, a structural model for the coassembled TB/TMA adlayers is proposed in Figure 4c. Pink TMA indicated the TMA molecules trapped in the holes of the honeycomb network, and the gray TMA indicated the TMA honeycomb network. Possible hydrogen bonds formed on coassembly pattern are presented in Figure 4d by dashed lines. The TMA hexamer surrounding the TB molecules is stabilized by 12 hydrogen bonds between carboxyl groups in neighboring TMA molecules, and this bonding scheme is similar to the honeycomb structure of TMA network. TMA molecules immobilized in the interspaces of the hexagonal motif of honeycomb network form another three hydrogen bonds with hydroxyl groups in the TMA hexagons. The results of control experiment indicate that the formation of unique selective heterogeneous coassembly architecture is related to the interactions between triazole groups and carboxylic acid groups. 3037

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with the incorporated guest molecule TMA hexamer in three directions are enough to incorporate other TMA molecules excess on solid−liquid interface, as represented by pink distorted hexamer in Figure 5f. In the two different systems, the size compatibility is the decisive factor to influence the template assemblies. The larger TP molecules enlarged the network anisotropically with directional hydrogen bond to the TMA network; on the other hand, the smaller size TB molecule incorporated into TMA holes fitly. Besides that, we also analyze the energies for the two systems, i.e. TP/TMA Kagome structure and TB/TMA triangle structure (see Supporting Information Tables S1 and S2), using MM calculation. For the TP Kagome structure, with 3 TP molecules and 1 TMA molecule trapped in a unit cell, the MM simulation generates 4.02 kcal mol−1 nm−2 gain of adsorption energy per area. For TB triangle structure with 1 TB molecule and 2 TMA molecules trapped in the TMA network, the adsorption energy gains 2.92 kcal mol−1 nm−2. The simulation results suggest that the synergic reorganization of network upon guest inclusion is energetically favored.

Article

ASSOCIATED CONTENT

S Supporting Information *

STM image of pure TP and TB molecules and the orientation of the main symmetry axes of the heterogeneous coassembly structures as well as the trapping process of the guest molecules on the boundary of the Kagome structure and bare TMA network. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Authors

*Tel +86 10 62558934, e-mail [email protected] (D.W.). *Tel +86 10 62558934; e-mail [email protected] (L.-J.W.). Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is supported by National Key Project on Basic Research (Grants 2011CB808700 and 2011CB932300), National Natural Science Foundation of China (Grants 91023013, 21127901, 21121063, 91227104, and 21233010), Beijing Municipal Education Commission (20118000101), and the Chinese Academy of Sciences.

3. CONCLUSION In summary, we have found an intriguing reorganization process of TMA network upon incorporation of low symmetrical guest molecules. We find that the TP molecules can be selectively trapped in the pores of the TMA network to form Kagome structure, whereas selectively trapping of TB molecules results in a triangular superlattice. We discuss the synergistic effect between the flexible network and the low symmetrical guest molecules. The adaptive reorganization of the TMA network templated by guest inclusion is driven by the interactions between the functional groups of TMA network and peripheral functional groups of guest molecules as well as the symmetry of the involved components. The difference between the two systems is due to the size compatibility of the network and guest species. The results presented here offer an in-depth understanding of complex multicomponent selfassembly, which is helpful in guiding the bottom-up fabrication of functional nanostructures with well-defined patterns.



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