On-Surface Synthesis of Highly Ordered Covalent Sierpiński Triangle

Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

pubs.acs.org/JACS

On-Surface Synthesis of Highly Ordered Covalent Sierpiń ski Triangle Fractals Yiping Mo,†,§ Ting Chen,† Jingxin Dai,‡ Kai Wu,*,‡ and Dong Wang*,†,∥

Downloaded via GUILFORD COLG on July 17, 2019 at 08:52:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemical and College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

to the intermolecular interactions, the reported covalent STs suffer from the formation of many other disordered motifs. The unwanted aggregates prevent covalent STs growing larger, and the second generation fractal is the largest covalent STs obtained in the previous studies. Herein, we report the synthesis of large-scale highly ordered covalent STs by the onsurface dehydration reaction of 1,3-benzenediboronic acid (1,3-BDBA) (Scheme 1) on highly oriented pyrolytic graphite

ABSTRACT: The Sierpiński triangle (ST) is a wellknown fractal structure. Synthesis of stable molecular STs with robust covalent linkages is attractive but challenging. Here, we demonstrate the formation of a series of highquality covalent STs via the on-surface dehydration reaction of 1,3-benzenediboronic acid with the presence of water as an equilibrium regulator at ambient atmosphere. Extended molecular fractals up to third generation are obtained, as disclosed by scanning tunnel microscope. The covalent STs show intriguing bright and dark contrasts irrespective of the fractal generations, which is related with epitaxial relationship of fractal structure to the underlying graphite lattice, as supported by theoretical simulations.

Scheme 1. Schematic Diagram of the Fabrication of Covalent ST Fractals from 1,3-BDBA

F

ractals, which display a self-similar nature at every scale,1 are of great importance in mathematics, aesthetics, science and technology and exist widely in nature, such as snowflakes, trees, and coastlines. To explore the formation process of such complicated yet fascinating structures, great efforts have been devoted to design and fabricate molecular fractals in the last two decades. 2−12 The Sierpiń s ki triangle (ST) is a representative fractal structure and can be constructed from an equilateral triangle by recursively removing its smaller central triangle. The molecular STs with different building blocks have been predicted to possess interesting electronic, magnetic, optical, and mechanical properties.13,14 Shang et al. achieved the first self-assembled STs based on synergistic halogen and hydrogen bonds on Ag(111).15 Subsequently, other types of intermolecular interactions, including coordination interaction and hydrogen bond, are also proved feasible for constructing ST fractals with high quality.16−22 The stability of molecular STs is vital to their further applications, and thus fabrication of STs with robust covalent bonds is of great interest. The covalent STs bonded with imine or thioether linkages have been demonstrated via on-surface reaction in an ultrahigh vacuum environment recently.23,24 However, owing to the relatively poor reversibility compared © XXXX American Chemical Society

Received: May 5, 2019 Published: July 10, 2019 A

DOI: 10.1021/jacs.9b04815 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society (HOPG) under an ambient atmosphere. By introducing water as the thermodynamic equilibrium regulator, a whole series of covalent STs up to third generation are successfully prepared. Intriguing, the covalent STs show an intriguing bright and dark contrast on HOPG, which is ascribed to the epitaxial orientation of STs with graphite lattice as supported by theoretical calculations. Figure 1a depicts a typical large-scale scanning tunneling microscopy (STM) image acquired by on-surface dehydration

Figure 2. Whole family of the observed covalent ST fractals. (a) High-resolution STM image revealing details. (b−e) The structural models (the unreacted boric acid groups in the vertices are omitted for clarity) and the corresponding STM images of zero, first, second, and third generation covalent STs, respectively. Imaging conditions: Vbias = 630 mV, It = 495 pA.

range from 0.7 to 6.0 nm, corresponding to the family of covalent STs from ST-0 to ST-3. Under the optimized reaction condition, the occupancies (in term of the number of molecules) of zero, first, second, third generation covalent STs and fractal fragments are 9.6%, 32.7%, 43.5%, 12.0%, and 2.2%, respectively, by statistical analysis (Figure S3). The effect of the reaction conditions on the fractal size distribution is shown in Figures S4−S7. It is hard to obtain the ST fractal with high generation due to its unique growth process. Thermodynamically, the formation of STs is related to the conformation of the elementary motif. For a 120° tick-shaped monomer such as 1,3-BDBA, there are two types of node conformation after the dehydration reaction, i.e., heterotactic and homotactic. STs are exclusively composed of heterotactic nodes at every vertex. The formation of the homotactic node acts as a defect and is detrimental for further ST growth. As the two kinds of nodes are energetically equivalent, theoretical simulations show that the formation of the fractal aggregates is mainly due to the entropic stabilization of the heterotactic nodes (the heterotactic nodes have a larger number of in-plane orientations compared to the homotactic nodes).7 Since the covalent bond has relatively poor reversibility compared to the supramolecular interactions, the growth of covalent STs under ultrahigh vacuum turn out to form many other disordered motifs coexisting with the target product.23,24 The presence of homotactic nodes is the main reason for the unwanted aggregates and prevents covalent STs growing larger. To improve this situation, we introduce a small amount of water into our reaction system as a chemical equilibrium regulator. Previously, this method has been demonstrated to improve significantly the quality of on-surface synthesis of single-layered covalent organic frameworks based on dehydration reactions.25,26 The releasing water vapor during the heating process can increase the reversibility of the boroxine ring formation reaction by regulating the thermodynamic equilibrium.27,28 Herein, the similar mechanism works for growth of STs. A monomer can form both homotactic and heterotactic nodes after reaction. However, the heterotactic nodes are entropically favorable, and this delicate free energy bias results in the growth of STs. In addition, the defects due to the

Figure 1. (a) Large-scale STM image of hierarchical covalent ST fractals obtained by on-surface dehydration reaction of 1,3-BDBA. (b) Close-up view of the selected area in panel a showing the existence of two types of contrasts in formed products. (c) Delineated image of panel b. The blue and gray symbols represent the covalent STs with bright and dark contrasts, respectively. Imaging conditions: Vbias = 630 mV, It = 495 pA.

reaction of 1,3-BDBA. The reaction was carried out in a closed reactor with the presence of CuSO4·5H2O power as a chemical equilibrium regulation agent (using water saturated silica gel affords similar results; see SI). After heating at 150 °C for 3 h, the HOPG surface was covered with a series of highly ordered equilateral triangular structures, or ST fractals. Statistical analysis indicates that the surface coverage of fractal layer is 83%. Figure 2a presents the high-resolution STM image revealing the self-similar nature of covalent STs with different generations. They are denoted here as covalent ST-n, where n represents the order of the ST. In this study, the third generation covalent ST is the largest ST observed and other STs with n equal to 0, 1, 2, and 3 are all observed. The structural models and the corresponding STM images of the successive generations of STs are shown in Figure 2b,c. A few fragment structures, such as incomplete covalent ST-1 fractal (marked by white dashed triangle in Figure 2a), are also disclosed by STM. The elementary motif of covalent STs is the heterotactic polymerization product of three 1,3-BDBA monomers (pink triangles in Scheme 1), or ST-0. The difference between the heterotactic and homotactic nodes is displayed in Figure S1. Then, the growth of the higher generation STs is an iterative procedure of the elementary motifs with the formed boroxine serving as the linker. Although the boroxine linker is sensitive to moisture, the prepared fractal patterns can remain on HOPG with high quality after exposing to air for 24 h (Figure S2). For arbitrary covalent ST-n, the needed precursor numbers are An = (3n+1 + 3)/2. The side lengths of the acquired STs B

DOI: 10.1021/jacs.9b04815 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society homotactic nodes can be self-corrected and eliminated during continuous reversible reaction process. The formation of a third ST requires 27 (3N, N = generation of ST) reactions to be proceeded in series, and the conformation of monomers at each step needs to be correct. The formation of a series of high-quality covalent STs with an order up to 3 indicates that the chemical equilibrium control strategy is suitable to achieve an entropy-driven structure by dynamic covalent chemistry. Growing of covalent ST fractals on Au(111) has also been investigated. We can hardly acquire fractal products by directly annealing 1,3-BDBA on Au(111). The improved result with low generation fractal structures (mainly ST-0) was obtained by annealing in a humid atmosphere (Figure S8). Kinetically, the growth of STs follows the limited kinetic growth. The growth of STs is only possible by the reaction or interaction of atoms or molecules at the three accessible vertices since the sides of STs are saturated. The growth of high generation STs requires that the new-coming monomer diffuses to the right position and attaches to the existing ST vertices before it generates a new nuceli. In other words, the growth of STs requires each precursor exists as a monomer before landing and attaching to the existing nuceli. In contrast, for a typical 2D periodic structure, the nucleation sites scale with the perimeters of the domain and the grow process can sustain beyond the critical nucleation step. In our experiment, the monomers are vaporized and then land on the surface to form covalent STs. This method can minimize the nucleation process and is helpful for growth of STs with high generation. In contrast, if the monomers were loaded on HOPG by dropcasting and then subject to a similar growth condition, the occupation of fractal STs is reduced and the disordered structures were also obtained (Figure S5), presumably due to the unwanted fast nucleation process under high surface dosage of monomers. Intriguingly, the formed fractals consist of two types of contrasts as indicated in Figure 1b. The cross-sectional analysis results preclude the possibility of a contrast difference caused by the formation of a double layer (Figure S9). To visualize the position relations of the bright and dark covalent STs, the delineated image is displayed in Figure 1c. The blue and gray triangles represent the covalent STs with bright and dark contrasts, respectively. These two kinds of triangles differ by 60, 180, or 300°, and it is impossible to interconvert them only by translation operation. This phenomenon is observed throughout the whole adlayer irrespective of the fractal generations. The STs are perfectly orientated into a 3-fold symmetry, indicating a strong orientation effect from the HOPG substrate. The neighboring STs intergrowing by sharing a growing facet is frequently observed, mainly due to the high coverage and epitaxial growth of STs on HOPG (see SI for details). To understand the contrast difference in fractals, the adsorption sites of molecules on the HOPG surface were investigated. By comparing the calculated adsorption energy of the hollow, bridge, and top sites (Table S1), it was determined the top site is preferred for the monomers. There are two types of top sites for STs. We chose ST-0 as a model, and the two different top sites are displayed in Figure 3a,b, respectively. Here, Top-A is defined as the stacking mode in which boron atoms of the boroxine locate just above the carbon atoms of the substrate, while the oxygen atom is just above the carbon atoms in Top-B. The calculated adsorption energy of two kinds of top sites based on covalent ST-0 is −84.448 and −84.950

Figure 3. Theoretical simulation results for two kinds of top sites. (a) The optimized model of Top-A based on covalent ST-0. (b) The optimized model of Top-B based on covalent ST-0. (c) Top views of the simulated STM results for panel a. (d) Top views of the simulated STM results for panel b. Only the first HOPG layer is shown for clarity.

kJ/mol, respectively. Such a low energy difference is well-below the thermo-fluctuation at room temperature, and two kinds of top sites can coexist. These two top sites cannot be interchanged by translation operation, but can be overlapped by rotating 60, 180, or 300°, which is consistent with the characteristic of bright and dark triangles discussed above. Therefore, we propose that two types of contrasts in STM images may be related to the two different adsorption sites. According to this hypothesis, we simulated the STM images of two top sites based on covalent ST-0. The theoretical results (Figure 3c,d) indicate that the ST adsorbing in the Top-B site presents higher electronic state density than the one in the Top-A site. Therefore, the bright covalent STs in STM imaging should be from the STs adsorbing in Top-B sites while the covalent STs present in dark contrast are related to Top-A sites. The occupancies of bright and dark fractals are 48.6% and 51.4% by statistical analysis, which is consistent with the small energy difference between the two types of top sites. It should be pointed out that the size of boroxine is almost the same as a benzene ring. As a result, every boroxine in high generation covalent STs is always located in the same sites (Figure S10), and therefore the contrast difference is persistent for the adlayer irrespective of the fractal progression. In summary, we have successfully prepared a whole series of high-quality covalent STs under an ambient atmosphere from the 1,3-BDBA precursors. By introducing a small amount of water to improve the reversibility of the reaction, the formation of large-scale highly ordered covalent fractals up to the third gerneration is achieved. Intriguingly, an unprecedented contrast difference in fractals is observed by STM, which is ascribed to the different adsorption sites of STs on the HOPG surface. This study provides new insights on the growth of robust molecular fractals from on-surface reaction. The synthesis of entropy driven covalent fractal structures based C

DOI: 10.1021/jacs.9b04815 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Multitopic Terpyridine Ligands. J. Am. Chem. Soc. 2014, 136, 6664− 6671. (11) Jiang, Z.; Li, Y.; Wang, M.; Liu, D.; Yuan, J.; Chen, M.; Wang, J.; Newkome, G. R.; Sun, W.; Li, X.; Wang, P. Constructing HighGeneration Sierpiński Triangles by Molecular Puzzling. Angew. Chem., Int. Ed. 2017, 56, 11450−11455. (12) Wang, L.; Liu, R.; Gu, J.; Song, B.; Wang, H.; Jiang, X.; Zhang, K.; Han, X.; Hao, X. Q.; Bai, S.; Wang, M.; Li, X.; Xu, B.; Li, X. SelfAssembly of Supramolecular Fractals from Generation 1 to 5. J. Am. Chem. Soc. 2018, 140, 14087−14096. (13) Wang, A.; Zhao, M. Intrinsic Half-Metallicity in Fractal Carbon Nitride Honeycomb Lattices. Phys. Chem. Chem. Phys. 2015, 17, 21837−21844. (14) van Veen, E.; Yuan, S.; Katsnelson, M. I.; Polini, M.; Tomadin, A. Quantum Transport in Sierpiński Carpets. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 115428−115432. (15) Shang, J.; Wang, Y.; Chen, M.; Dai, J.; Zhou, X.; Kuttner, J.; Hilt, G.; Shao, X.; Gottfried, J. M.; Wu, K. Assembling Molecular Sierpiński Triangle Fractals. Nat. Chem. 2015, 7, 389−393. (16) Li, N.; Zhang, X.; Gu, G. C.; Wang, H.; Nieckarz, D.; Szabelski, P.; He, Y.; Wang, Y.; Lü, J. T.; Tang, H.; Peng, L. M.; Hou, S. M.; Wu, K.; Wang, Y. F. Sierpiński-Triangle Fractal Crystals with the C3v Point Group. Chin. Chem. Lett. 2015, 26, 1198−1202. (17) Sun, Q.; Cai, L.; Ma, H.; Yuan, C.; Xu, W. On-Surface Construction of a Metal-Organic Sierpiński Triangle. Chem. Commun. 2015, 51, 14164−14166. (18) Zhang, X.; Li, N.; Gu, G. C.; Wang, H.; Nieckarz, D.; Szabelski, P.; He, Y.; Wang, Y.; Xie, C.; Shen, Z.; et al. Controlling Molecular Growth between Fractals and Crystals on Surfaces. ACS Nano 2015, 9, 11909−11915. (19) Zhang, X.; Li, N.; Liu, L.; Gu, G.; Li, C.; Tang, H.; Peng, L.; Hou, S.; Wang, Y. Robust Sierpiński Triangle Fractals on SymmetryMismatched Ag(100). Chem. Commun. 2016, 52, 10578−10581. (20) Cai, L.; Sun, Q.; Bao, M.; Ma, H.; Yuan, C.; Xu, W. Competition between Hydrogen Bonds and Coordination Bonds Steered by the Surface Molecular Coverage. ACS Nano 2017, 11, 3727−3732. (21) Li, N.; Gu, G. G.; Zhang, X.; Song, D. L.; Zhang, Y. J.; Teo, B. K.; Peng, L. M.; Hou, S. M.; Wang, Y. F. Packing Fractal Sierpiński Triangles into One-Dimensional Crystals via a Templating Method. Chem. Commun. 2017, 53, 3469−3472. (22) Li, C.; Zhang, X.; Li, N.; Wang, Y. W.; Yang, J. J.; Gu, G. C.; Zhang, Y. J.; Hou, S. M.; Peng, L. M.; Wu, K.; Nieckarz, D.; Szabelski, P.; Tang, H.; Wang, Y. F. Construction of Sierpiński Triangles up to the Fifth Order. J. Am. Chem. Soc. 2017, 139, 13749−13753. (23) Gu, G. C.; Li, N.; Liu, L. W.; Zhang, X.; Wu, Q. M.; Nieckarz, D.; Szabelski, P.; Peng, L. M.; Teo, B. K.; Hou, S. M.; Wang, Y. F. Growth of Covalently Bonded Sierpiński Triangles up to the Second Generation. RSC Adv. 2016, 6, 66548−66552. (24) Rastgoo-Lahrood, A.; Martsinovich, N.; Lischka, M.; Eichhorn, J.; Szabelski, P.; Nieckarz, D.; Strunskus, T.; Das, K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. From Au-Thiolate Chains to Thioether Sierpiński Triangles: The Versatile Surface Chemistry of 1,3,5-Tris(4mercaptophenyl)benzene on Au(111). ACS Nano 2016, 10, 10901− 10911. (25) Guan, C. Z.; Wang, D.; Wan, L. J. Construction and Repair of Highly Ordered 2D Covalent Networks by Chemical Equilibrium Regulation. Chem. Commun. 2012, 48, 2943−2945. (26) Liu, X. H.; Guan, C. Z.; Ding, S. Y.; Wang, W.; Yan, H. J.; Wang, D.; Wan, L. J. On-Surface Synthesis of Single-Layered TwoDimensional Covalent Organic Frameworks via Solid-Vapor Interface Reactions. J. Am. Chem. Soc. 2013, 135, 10470−10474. (27) Spitzer, S.; Rastgoo-Lahrood, A.; Macknapp, K.; Ritter, V.; Sotier, S.; Heckl, W. M.; Lackinger, M. Solvent-Free On-Surface Synthesis of Boroxine COF Monolayers. Chem. Commun. 2017, 53, 5147−5150. (28) Dienstmaier, J. F.; Medina, D. D.; Dogru, M.; Knochel, P.; Bein, T.; Heckl, W. M.; Lackinger, M. Isoreticular Two-Dimensional

on dynamic covalent chemistry could further broaden the possibility to design complicated hierarchical molecular nanostructures with robust covalent chemistry.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04815. Experimental details and Figures S1−S10 (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Ting Chen: 0000-0002-6616-0512 Kai Wu: 0000-0002-5016-0251 Dong Wang: 0000-0002-1649-942X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 21725306, 21433011, and 91527303), the National Key R&D Program of China (2017YFA0204702), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100). The Supercomputing Environment of Chinese Academy of Sciences is acknowledged for providing the computation resources.

■

REFERENCES

(1) Mandelbrot, B. B. The Fractal Geometry of Nature; W. H. Freeman: San Francisco, 1982. (2) Sugiura, K.-i.; Tanaka, H.; Matsumoto, T.; Kawai, T.; Sakata, Y. A Mandala-Patterned Bandanna-Shaped Porphyrin Oligomer, C1244H1350N84Ni20O88, Having a Unique Size and Geometry. Chem. Lett. 1999, 28, 1193−1194. (3) Rothemund, P. W. K.; Papadakis, N.; Winfree, E. Algorithmic Self-Assembly of DNA Sierpinski Triangles. PLoS Biol. 2004, 2, e424. (4) Newkome, G. R.; Wang, P. S.; Moorefield, C. N.; Cho, T. J.; Mohapatra, P. P.; Li, S. N.; Hwang, S. H.; Lukoyanova, O.; Echegoyen, L.; Palagallo, J. A.; Iancu, V.; Hla, S. W. Nanoassembly of a Fractal Polymer: A Molecular Sierpinski Hexagonal Gasket. Science 2006, 312, 1782−1785. (5) Fujibayashi, K.; Hariadi, R.; Park, S. H.; Winfree, E.; Murata, S. Toward Reliable Algorithmic Self-Assembly of DNA Tiles: A FixedWidth Cellular Automaton Pattern. Nano Lett. 2008, 8, 1791−1797. (6) Newkome, G. R.; Shreiner, C. Dendrimers derived from 1→3 branching motifs. Chem. Rev. 2010, 110, 6338−6442. (7) Nieckarz, D.; Szabelski, P. Simulation of The Self-Assembly of Simple Molecular Bricks into Sierpinski Triangles. Chem. Commun. 2014, 50, 6843−6845. (8) Sarkar, R.; Guo, K.; Moorefield, C. N.; Saunders, M. J.; Wesdemiotis, C.; Newkome, G. R. One-Step Multicomponent SelfAssembly of a First-Generation Sierpiński Triangle: From Fractal Design to Chemical Reality. Angew. Chem., Int. Ed. 2014, 53, 12182− 12185. (9) Nieckarz, D.; Szabelski, P. Chiral and Fractal: From Simple Design Rules to Complex Supramolecular Constructs. Chem. Commun. 2016, 52, 11642−11645. (10) Wang, M.; Wang, C.; Hao, X. Q.; Liu, J.; Li, X.; Xu, C.; Lopez, A.; Sun, L.; Song, M. P.; Yang, H. B.; Li, X. Hexagon Wreaths: SelfAssembly of Discrete Supramolecular Fractal Architectures Using D

DOI: 10.1021/jacs.9b04815 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society Covalent Organic Frameworks Synthesized by On-Surface Condensation of Diboronic Acids. ACS Nano 2012, 6, 7234−7242.

E

DOI: 10.1021/jacs.9b04815 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX