Substrate Orientation Effect in the On-Surface Synthesis of

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Substrate Orientation Effect in the On-Surface Synthesis of Tetrathiafulvalene-Integrated Single-Layer Covalent Organic Frameworks Wei-long Dong,†,‡ Lin Wang,†,‡ Hui-min Ding,§ Lu Zhao,† Dong Wang,*,† Cheng Wang,*,§ and Li-Jun Wan*,† †

Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, PR China University of CAS, Beijing, PR China § Key Laboratory of Biomedical Polymers (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China ‡

S Supporting Information *

ABSTRACT: The on-surface reactions of tetrathiafulvalene equipped with four benzaldehyde groups (4ATTF) and ditopic diamine molecules are investigated. 4ATTF tends to form large-scale-ordered rhombus structures when reacted with pphenylenediamine (PPDA). A longer ditopic diamine molecule, 1,1′-biphenyl-4,4′-diamine dihydrochloride (BPDA), causes the domain size of the regular rhombus structure to decrease and triangular and irregular rhombus structures to appear upon reaction with 4ATTF. However, in the rhombus structures formed by different-length ditopic diamine molecules, the single-layer covalent organic frameworks on the graphite surface preferentially orient in alignment with the underlying HOPG substrate lattice.

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INTRODUCTION When matter is decreased from three dimensions (3D) to two dimensions (2D), materials may exhibit unique dimensionalityrelated properties.1 Graphene, for example, is one of the most intensively investigated 2D materials with single-atom layer thickness and features distinct electronic properties compared to 3D graphite.2 Single-layered covalent organic frameworks (sCOF), composed of periodically arranged functional organic building blocks linked by covalent bonds, can be regarded as single-layer sheets of bulk COF materials and an organic analogue to graphene. 3−6 Theoretical calculations have predicted that the electronic properties of 2D materials with conjugated backbones can be greatly tuned by the periodicity and topology of the materials.7 Therefore, it is highly desirable to synthesize sCOF with designable and tunable structures and thereafter interrogate their intrinsic properties.8−11 In recent years, on-surface synthesis has increasingly been recognized as a powerful “bottom-up” strategy for fabricating ordered 2D sCOFs.12 For example, under optimal synthesis conditions, highly ordered sCOF structures based on on-surface dynamical covalent chemistry can achieve single domain sizes of several © XXXX American Chemical Society

hundred nanometers, which is 1 order of magnitude larger than the typical domain sizes for bulk COFs synthesized in the solution phase.13−15 During the on-surface synthesis process, the atomically flat substrate is believed to act as a template to promote the organization of precursors and therefore facilitates the formation of highly ordered sCOFs.16,17 Previously, high-symmetry precursors such as triangular and cross-shaped molecules have been employed to fabricate hexagonal honeycomb sCOFs18,19 and square sCOFs.20−24 From the viewpoint of reticular design, combining a tetradentate precursor with a linear precursor could generate a rich library of sCOFs with different topology, including a rhombus structure (I), a square structure (II), and a triangular structure (III) (Scheme 1a). In fact, these topological structures have been observed for bulk COF materials by using tetradentate building blocks such as pyrene,25 tetrathiafulvalene (TTF),26 and tetraphenyl-ethylene.27 It is of great curiosity to Received: July 2, 2015 Revised: October 11, 2015

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Scheme 1. (a) Topological Design of sCOFs Using Tetradentate Building Blocks and Linear Ditopic Building Blocks and (b) Chemical Structures of Molecules Used in the Present Work

study the topology diversity of sCOFs formed from tetradentate precursors and compare the results with that of solution-phase synthesized bulk COFs.28 TTF and its derivatives, as excellent electron donors, have been widely studied as organic conductors and superconductors.29 Recently, both the rhombus structure (I) and square structure (II) have been reported for the bulk COF formed from TTF equipped with four benzaldehyde groups (4ATTF) and p-phenylenediamine (PPDA).26,30,31 Here, we use 4ATTF and linear ditopic diamine molecules (PPDA and 1,1′-biphenyl-4,4′-diamine dihydrochloride, BPDA) to build sCOFs on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunnelling microscopy (STM) results reveal that the rhombus structure is dominant for both sCOFs. The longer diamine precursor results in the decreased domain size of the ordered rhombus structure and increased coverage of triangular motifs.

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RESULTS AND DISCUSSION Figure 1a shows a large-scale STM image of an sCOF formed by 4ATTF and PPDA on HOPG after heating to 150 °C for 3 h in a closed autoclave. (A detailed experimental procedure for sCOF synthesis is provided in the Supporting Information.) Ordered rhombus bright dot arrays with a typical domain size of about 100 nm can be routinely obtained. In the large-scale STM image, several point defects due to missing 4ATTF molecules are visible. A high-resolution STM image reveals more information about its structure (Figure 1b). Bright rectangular spots are attributed to electron-rich TTF cores. A rhombus structure is formed by linking neighboring 4ATTF molecules with PPDA molecules. Linking molecule PPDA has a weaker contrast compared to 4ATTF. In the rhombus structures every 4ATTF is arranged in the same orientation, which is similar to that in the bulk COF formed under solvothermal reactions.31 The measured lattice parameters of the formed structure are a = b = 2.5 ± 0.2 nm, α = 65 ± 2°, which agree well with expected parameters for the imine-linked 4ATTF sCOF and are supported by theoretical calculation (Figure S1). The two pairs of redox peaks of TTF in sCOF on HOPG formed by 4ATTF and PPDA show negative shifting compared to monomer 4ATTF (Figure S2). During the formation of 4ATTF-integrated sCOF, the HOPG substrate affects the orientation of sCOF. Figure 2a shows a composite STM image obtained by suddenly changing the scanning scale from 100 to 10 nm during image collection. The lower part of the STM image shows rhombus sCOF, and

Figure 1. (a) STM image showing ordered rhombus 4ATTF sCOF. (b) High-resolution STM image of sCOF showing details. A model of the rhombus structure is overlaid and drawn to scale. (c) The schematic diagram of sCOF formation. The condensation of tetradentate precursor 4ATTF and linear precursor PPDA can form an ordered rhombus structure. Imaging conditions: (a) Vbias = 644 mV, It = 523 pA; (b) Vbias = 630 mV, It = 503 pA.

the upper part is the atomic image of the graphite substrate. The black line in the image represents the 6-fold-symmetry lattice direction of HOPG, and the white line represents the direction of the long diagonal axis of the rhombus structure (the direction of the 4ATTF long axis). Comparing the orientation of TTF with the graphite lattice, we find that a 15° difference between the two orientations exists. Figure 2b is a schematic showing the measured angle between the substrate lattice orientation (the black arrow) and the TTF arrangement direction in rhombus networks (the white line). The relationship between orientations of different domains was further analyzed. Figure 2c is a large-scale STM image containing eight domains of 4ATTF sCOF outlined by different black dot lines. The white lines are used to represent the directions of each 4ATTF sCOF area. The relative orientation of different domains is represented by an acute angle between diagonal lines (white lines) of the domains. Statistical analysis of the relative angles between different B

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Figure 3. (a, d) STM images of ordered and disordered sCOF obtained from 4ATTF and BPDA. (a) The white line in every black dotted circle represents the orientation of each small sCOF area. (b, e, g) Zoomed-in STM images of the rhombus structure, irregular rhombus structure, and triangular structure, respectively. (c, f, h) Sketches of the three different structures marked by black circles in (b), (e), and (g). Imaging conditions: (a) Vbias = 700 mV, It = 500 pA; (d) Vbias = 700 mV, It = 500 pA.

Figure 2. (a) STM image containing 4ATTF sCOF (lower part, 100 nm) and atomic image of the substrate (upper part, 10 nm). (b) Schematic of the measured angle between the substrate lattice orientation and the TTF arrangement direction in rhombus networks. (c) STM image with eight areas of networks distinguished by black dotted lines (scale: 100 nm × 100 nm). White lines represent the directions of the 4ATTF sCOFs. (d) Distribution of the 132 angle values. The 132 angle values distribute very well within 4 different values (0, 30, 60, and 90°). Imaging conditions: (a) (up) Vbias = 33.4 mV, It = 564 pA; (down) Vbias = 726 mV, It = 564 pA; (c) Vbias = 600 mV, It = 536 pA.

sCOF domains formed by 4ATTF and BPDA was analyzed as described above. The result indicates that 4ATTF in the network structure again shows an angle of 15° relative to the HOPG lattice. Figure 3d shows an area mainly composed of a disordered structure. At the domain boundary of the regular rhombus structure, an irregular rhombus structure, in which one of the four 4ATTFs in a unit cell is perpendicular to the other three, making a torsional quadrilateral cell, can be clearly seen, as magnified in Figure 3e. Obviously, such an irregular arrangement of the 4ATTF core results in the distortion of the network and affects the formation of large domains for the sCOF. Figure 3g shows another zoomed-in STM image with a triangular structural motif. Such a motif is expected for a tetradentate precursor such as 4ATTF, as shown in Scheme 1a and Figure 3h. We note that triangular-motif-based bulk COF is not obtained when using 4ATTF as the monomer. At the same time, those irregular structural motifs shown in Figure 3e,g cannot form large-scale ordered structures from on-surface synthesis. Further variation of the reaction temperature and molar ratio between the two monomers did not result in an improvement of the domain size in the ordered area. The on-surface synthesis of sCOF using 4ATTF and two linear ditopic diamine molecules with different lengths exhibits a discrepancy in results. Reaction with the shorter molecule, PPDA, tends to form ordered rhombus structures. On the other hand, employing longer linear ditopic diamine molecule BPDA decreases the rigidity of the backbone and makes the angle of the rhombus structure more flexible, resulting in a decreased domain size of the ordered rhombus structure and the formation of disordered sCOF on the substrate surface. In the ordered rhombus structures, the 4ATTF monomer

domains in more than 35 STM images reveals a narrow distribution around 4 values, i.e., 0 ± 1.8, 30 ± 4.4, 60 ± 3.4, and 90 ± 3.4°. Such a result is consistent with the registry measurement of the 4ATTF to HOPG substrate shown in Figure 2a,b, considering the 6-fold symmetry of the HOPG substrate. Obviously, an orientation control effect has been imposed by the substrate−monomer interaction during sCOF growth. To study the influence of the length of the linear diamine on the synthesis of sCOF, BPDA (Scheme 1b) was utilized to build sCOF with 4ATTF. In contrast to the reaction between PPDA and 4ATTF, BPDA and 4ATTF mainly formed a disordered sCOF layer containing both a rhombus structure (Figure 3a) and triangular and irregular rhombus structures (Figure 3d). Figure 3a shows an STM image containing several small rhombus areas marked by black dotted lines. A zoomed-in STM image of the small-area rhombus structure shows 4ATTF and BPDA more clearly, although 4ATTF is much brighter than the ditopic diamine molecule (Figure 3b). Measured lattice parameters of this rhombus structure are a = b = 2.9 ± 0.2 nm. Interestingly, the intersection angle α varies over a relatively large range of 56−68°, which is presumably related to the flexibility of the longer diamine bridging molecule and also accounts for the relatively small domain size of the sCOF. In addition, some node spots with exceptional contrast are outlined by red circles in Figure 3b and e, which may be related to the formation of a bilayer structure. The schematic model for the rhombus sCOF by 4ATTF and BPDA is shown in Figure 3c. The relative orientation of the ordered rhombus C

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(3) Clair, S.; Abel, M.; Porte, L. Growth of boronic acid based twodimensional covalent networks on a metal surface under ultrahigh vacuum. Chem. Commun. 2014, 50 (68), 9627−9635. (4) El Garah, M.; MacLeod, J. M.; Rosei, F. Covalently bonded networks through surface-confined polymerization. Surf. Sci. 2013, 613 (0), 6−14. (5) Xu, L.; Zhou, X.; Yu, Y.; Tian, W. Q.; Ma, J.; Lei, S. SurfaceConfined Crystalline Two-Dimensional Covalent Organic Frameworks via on-Surface Schiff-Base Coupling. ACS Nano 2013, 7 (9), 8066−8073. (6) Dienstmaier, J. F.; Medina, D. D.; Dogru, M.; Knochel, P.; Bein, T.; Heckl, W. M.; Lackinger, M. Isoreticular Two-Dimensional Covalent Organic Frameworks Synthesized by On-Surface Condensation of Diboronic Acids. ACS Nano 2012, 6 (8), 7234−7242. (7) Adjizian, J.-J.; Briddon, P.; Humbert, B.; Duvail, J.-L.; Wagner, P.; Adda, C.; Ewels, C. Dirac Cones in two-dimensional conjugated polymer networks. Nat. Commun. 2014, 5, 5842. (8) Ding, S.-Y.; Wang, W. Covalent organic frameworks (COFs): from design to applications. Chem. Soc. Rev. 2013, 42 (2), 548−568. (9) Perepichka, D. F.; Rosei, F. Extending Polymer Conjugation into the Second Dimension. Science 2009, 323 (5911), 216−217. (10) Colson, J. W.; Dichtel, W. R. Rationally synthesized twodimensional polymers. Nat. Chem. 2013, 5 (6), 453−465. (11) Feng, X.; Ding, X.; Jiang, D. Covalent organic frameworks. Chem. Soc. Rev. 2012, 41 (18), 6010−6022. (12) Liu, X.-H.; Guan, C.-Z.; Wang, D.; Wan, L.-J. Graphene-Like Single-Layered Covalent Organic Frameworks: Synthesis Strategies and Application Prospects. Adv. Mater. 2014, 26 (40), 6912−6920. (13) Colson, J. W.; Woll, A. R.; Mukherjee, A.; Levendorf, M. P.; Spitler, E. L.; Shields, V. B.; Spencer, M. G.; Park, J.; Dichtel, W. R. Oriented 2D Covalent Organic Framework Thin Films on SingleLayer Graphene. Science 2011, 332 (6026), 228−231. (14) 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 (24), 2943−2945. (15) 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 (28), 10470−10474. (16) Gutzler, R.; Walch, H.; Eder, G.; Kloft, S.; Heckl, W. M.; Lackinger, M. Surface mediated synthesis of 2D covalent organic frameworks: 1,3,5-tris(4-bromophenyl)benzene on graphite(001), Cu(111), and Ag(110). Chem. Commun. 2009, 29 (29), 4456−4458. (17) Marele, A. C.; Mas-Balleste, R.; Terracciano, L.; RodriguezFernandez, J.; Berlanga, I.; Alexandre, S. S.; Otero, R.; Gallego, J. M.; Zamora, F.; Gomez-Rodriguez, J. M. Formation of a surface covalent organic framework based on polyester condensation. Chem. Commun. 2012, 48 (54), 6779−6781. (18) Zwaneveld, N. A. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. Organized Formation of 2D Extended Covalent Organic Frameworks at Surfaces. J. Am. Chem. Soc. 2008, 130 (21), 6678−6679. (19) Dalapati, S.; Addicoat, M.; Jin, S.; Sakurai, T.; Gao, J.; Xu, H.; Irle, S.; Seki, S.; Jiang, D. Rational design of crystalline supermicroporous covalent organic frameworks with triangular topologies. Nat. Commun. 2015, 6, 7786. (20) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nano-architectures by covalent assembly of molecular building blocks. Nat. Nanotechnol. 2007, 2 (11), 687−691. (21) Tanoue, R.; Higuchi, R.; Enoki, N.; Miyasato, Y.; Uemura, S.; Kimizuka, N.; Stieg, A. Z.; Gimzewski, J. K.; Kunitake, M. Thermodynamically Controlled Self-Assembly of Covalent Nanoarchitectures in Aqueous Solution. ACS Nano 2011, 5 (5), 3923− 3929. (22) Huang, N.; Krishna, R.; Jiang, D. Tailor-Made Pore Surface Engineering in Covalent Organic Frameworks: Systematic Functionalization for Performance Screening. J. Am. Chem. Soc. 2015, 137 (22), 7079−7082.

maintains a well-defined orientation on HOPG, indicating the important role of substrate−molecule interaction during the growth of well-ordered sCOF. Preliminary molecular mechanics (MM) simulation indicates that the stacking energy of 4ATTF (45.86 kcal/mol) on HOPG is much higher than those of BPDA (9.98 kcal/mol) and PPDA (−0.85 kcal/mol). This simulation shows that the adsorption of 4ATTF plays a dominant role in the orientation of the sCOFs on HOPG. The increase in disordered structure for larger sCOFs manifests an important challenge for the conformation control for sCOF synthesis.

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CONCLUSIONS We have fabricated TTF-integrated sCOFs with tetradentate molecule 4ATTF. Regular rhombus structure sCOF has been observed when 4ATTF reacted with PPDA. When the reaction was performed with the longer ditopic diamine BPDA to enlarge the size of the sCOF, the domain size of the regular structure decreased, together with the appearance of triangular and irregular rhombus structures. The ordered rhombus sCOF preferentially orients with the HOPG substrate lattice direction. The result highlights the important role of substrate−monomer interactions for the on-surface synthesis of sCOF structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02412. Synthesis procedures and additional characterization results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86(0)1062558934. *E-mail: [email protected]. *E-mail: [email protected]. Fax: +86(0)1062558934. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Project on Basic Research (grant 2011CB808701), the National Natural Science Foundation of China (grants 21433011, 21127901, 21233010, and 21373236), and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDB12020100).

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ABBREVIATIONS STM, scanning tunnelling microscopy; sCOF, single-layered covalent organic frameworks; TTF, tetrathiafulvalene; 4ATTF, tetrathiafulvalene tetraaldehyde; PPDA, p-phenylenediamine; BPDA, 1,1′-biphenyl-4,4′-diamine dihydrochloride; HOPG, highly oriented pyrolytic graphite

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b02412 Langmuir XXXX, XXX, XXX−XXX