Orthogonal Supramolecular Polymer Formation on Highly Oriented

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Orthogonal Supramolecular Polymer Formation on Highly Oriented Pyrolytic Graphite (HOPG) Surfaces Characterized by Scanning Probe Microscopy Yongxiang Gong,†,∇ Siqi Zhang,‡ Yanfang Geng,‡ Chunmei Niu,∇ Shouchun Yin,*,§ Qingdao Zeng,*,‡ and Min Li*,† †

CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ‡ CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), Beijing 100190, People’s Republic of China § College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou, 310036, People’s Republic of China ∇ College of Materials Science & Engineering, Heibei University of Science and Technology, Shijiazhuang, 050018, People’s Republic of China ABSTRACT: Formation of an orthogonal supramolecular polymer on a highly oriented pyrolytic graphite (HOPG) surface was demonstrated for the first time by means of scanning probe microscopy (SPM). Atomic force microscopy (AFM) was employed to characterize the variation of both the thickness and the topography of the film formed from (1) monomer 1, (2) monomer 1/Zn2+, and (3) monomer 1/Zn2+/ cross-linker 2, respectively. Scanning tunneling microscopy (STM) was used to monitor the self-assembly behavior of monomer 1 itself, as well as 1/Zn2+ ions binary system on graphite surface, further testifying for the formation of linear polymer via coordination interaction at the single molecule level. These results, given by the strong surface characterization tool of SPM, confirm the formation of the orthogonal polymer on the surface of graphite, which has great significance in regard to fabricating a complex superstructure on surfaces.

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INTRODUCTION Supramolecular polymers have been widely developed, in a great variety of applications, in the last decades.1−5 Selfassembled materials based on metal-containing polymers have a broad range of applications in the field of nanoscience and nanotechnology.6−12 Great efforts have been made to construct self-assembled supramolecular polymers with tunable properties. Crown ethers and their derivatives have attracted much attention, because of their applications in ionic conduction and unique properties of complexing with various alkali and inorganic cations. Specifically, the recognition between dibenzo-24-crown-8 (DB24C8) and dibenzylammonium salt was utilized in the construction of molecular machines and supramolecular gels, because of their pH and K+ responsiveness.13−17 Crown ether-based host−guest interactions and terpyridine-based metal ligand interactions have been widely used in the construction of supramolecular polymers.18−24 In our previous work, a low-molecular-weight monomer (1) was synthesized by linking DB24C8 with two terpyridine units, which can form a linear polymer in the presence of Zn(OTf)2 via coordination bond in solution. A supramolecular polymer © XXXX American Chemical Society

network then could be further obtained after adding a bisammonium cross-linker (2).20 It is well-known that the selfassembly of functional molecules at surfaces and interfaces is of profound significance in supramolecular chemistry.25−35 For example, self-assembled nanoporous networks at interfaces can act to immobilize and isolate functional objects at a molecular level, which is essential for the study of molecular functionality and, consequently, benefits fundamental research in nanoscience and nanotechnology.36−40 In the present work, we demonstrate, for the first time, this orthogonal supramolecular network can form on a highly oriented pyrolytic graphite (HOPG) surface by means of scanning probe spectroscopy (SPM). Both atomic force microscopy (AFM) and scanning tunneling microscopy (STM) were employed here to monitor the formation of the polymer network on HOPG at the singlemolecule level. This research provides direct evidence to the polymerization and the cross-linking of monomer and is of Received: August 10, 2015 Revised: October 11, 2015

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DOI: 10.1021/acs.langmuir.5b02883 Langmuir XXXX, XXX, XXX−XXX

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imaging, 1 was dissolved in toluene at the concentration below 0.1 mM, and the droplet was deposited onto freshly cleaved HOPG surfaces. For the metal coordination experiment, the Zn(OTf)2 acetonitrile/chloroform solution was deposited on the sample of 1. A small droplet of 1-phenyloctane was added to the surface to maintain the liquid/solid interface after evaporation of the sample solvents. The tip was mechanically formed from Pt/Ir (80:20) wire. The reproducibility of each image was checked by using different samples and tips. All the STM data was analyzed using the Nano Scope Image software.

great importance in fabricating a supramolecular polymer network on surfaces.

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EXPERIMENTAL SECTION

Materials. Chloroform (CHCl3, analytical reagent (AR) grade) was purchased from Beijing Chemical Works and acetonitrile (CH3CN, 99.8%) was obtained from Tianjin Fuchen Chemical Reagents Factory. Zinc triflate (Zn(OTf)2) was purchased from Aldrich. All reagents were used without further purification. Monomer 1 and cross-linker 2 were synthesized according to the previous method,20 and their structures are shown in Scheme 1.

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RESULTS AND DISCUSSION To better investigate the formation of supramolecular polymers on the HOPG surface, it is necessary to monitor the 2D assembly behavior of monomer 1 first. Figure 1a shows the STM images of 1 at the 1-phenyloctane/HOPG interface. 1 could assemble into a large-scale regular pattern on the HOPG surface. The higher-resolution STM image inset in Figure 1b enables us to identify the location of the 1 molecules on the surface. The tentative molecular model is depicted in Figure 1c, on the basis of the STM observations, with the unit-cell parameters of a = 7.1 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 76° ± 1°. The angle (θ) formed between two neighboring 1 molecules, which are indicated by the red and blue rods in the figure, is measured to be 30°. STM could reach submolecule scale resolution; however, to detect and identify self-assembled objects on the top surface formed upon dropcasting, AFM is a better tool. To avoid the molar ratio mismatch between the three components (1, Zn2+, 2) that is due to the self-assembly of 1 itself on HOPG surface, we first deposited a certain amount of 1 molecules (10 μL, 0.32 μM on the 1 cm2 HOPG surface) for monolayer formation after accurately calculating the required number of 1 molecules to cover the hole surface based on the unit-cell parameters in Figure 1. Considering that 1 would likely adopt an orientation, instead of adsorbing in parallel with its first monolayer, because of the comparatively weak interaction between the substrate and the molecule, the concentration of 1 solution was increased by a factor of 5 for monolayer formation (that is, it increased from 0.32 μM to 1.6 μM). An AFM image of the assembled structure formed from 1 is shown in Figure 2. Island film-like features were observed on top of the first layer of 1, and the average thickness of the island film was measured to be 0.6 nm, based on the AFM observations. It is higher than the case when molecules are adsorbed when facing the graphite surface (∼0.3

Scheme 1. Chemical Structures of 1 and 2

Scanning Probe Microscopy. AFM experiments were carried out on samples prepared by drop-casting 10 μL of sample solutions (onecomponent, 1; dicomponent, 1 with Zn(OTf)2; tricomponent, 1 with Zn(OTf)2 and 2) at certain concentrations on HOPG surfaces. The system was then allowed to stand for 20 min, for absolute solvent evaporation before measuring. Details of the sample preparation are as follows. For the one-component scenario, 1 that had been dissolved in CH3Cl/CH3CN (v/v, 1:1) at different concentrations (1.6 and 0.32 μM) was dropcast on HOPG surfaces. For the bicomponent case, the mixture of 1 (1.6 μM in 1:1 CHCl3/CH3CN, v/v) and Zn(OTf)2 (32 μM in CH3CN), at a molar ratio of 1:1, was dropcast on HOPG surfaces. For the tricomponent case, the mixture of 1 (1.6 μM in 1:1 CHCl3/CH3CN, v/v), Zn(OTf)2 (32 μM in CH3CN) and 2 (10 μM, 1:1 CHCl3/CH3CN, v/v), at the molar ratio of 1:1:0.5, was dropcast on HOPG surfaces. AFM images were obtained with a Nanoscope V SPM system (Bruker, USA) at the air/solid interface in tapping mode with an E-type scanner. Image processing was performed with the NanoScope Analysis software (version 1.40). All images presented here were flattened using the software to remove image artifacts due to vertical scanner drift, image bow, etc. The STM experiments were performed on a Nanoscope IIIa system (Bruker, USA) under ambient conditions at the 1-phenyloctane/HOPG interface. Prior to STM

Figure 1. (a) Large-scale STM image (Iset = 916 pA, Vbias = 277 mV) and (b) high-resolution STM image (Iset = 498 pA, Vbias = 278 mV) of the selfassembled structures of 1 at the 1-phenyloctane/HOPG interface. (c) A suggested molecular model corresponding to the observed structure. B

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Figure 2. (a) AFM image of the film formed from 1 on the HOPG surface. Island film-like structures (outlined by dotted lines) are visible on the surface. Inset shows the chemical structure of 1. (b) Cross-section of the film along the white line shown in panel (a).

Figure 3. (a) AFM image shows the topography of the film formed from 1/Zn(OTf)2 at a molar ratio of 1:1 on the HOPG surface. (b) Small-scale topography image of the film. (c) Cross-section of both the film (h1) and rodlike feature (h2) along the line shown in panel (b). (d) Small-scale phase image of the film.

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Figure 4. (a) Large-scale STM image (Iset = 598 pA, Vbias = 300 mV) and (b) high-resolution STM image (Iset = 598 pA, Vbias = 300 mV) of the selfassembled structures after metal coordination treatment of the 1-covered HOPG surface. (c) A suggested molecular model corresponding to the observed structure.

Figure 5. (a) Large-scale and (b) small-scale topography AFM images of the film formed from 1/Zn(OTf)2/2 at a molar ratio of 1:1:0.5 on the HOPG surface (inset in panel (a) shows the cross-section of the film along the line shown in panel (a)). (c) Cross-section of the film along the line shown in (b).

the surface in an edge-on manner and was stabilized by the interactions (van de Waals, π−π interactions, etc.) among molecules. Importantly, all the “rods” are originated from the film instead of being in a separate domain. Thus, it is safe to conclude that the small rodlike structures are the second assembled layer formed from 1 on top of its first assembled monolayer shown in Figure 2, but assembled in a standing mode on surface. This has also been previously demonstrated in another system, where the porphyrin derivative standing on the first monolayer was monitored by means of AFM at the air/ graphite interface.9 The coordination reaction between 1 and Zn2+ was further demonstrated by STM at a submolecule scale resolution. Metal coordination experiments were performed via a deposition of 0.01 mL of Zn(OTf)2 on the precursor-covered HOPG samples, followed by maintaining it under ambient conditions for 30 min. The STM observation reveals that the surface morphology has significantly changed, in contrast with 1 itself, because the pyridine groups react with the introduced Zn ions, resulting in an extended self-assembly. Figure 4 shows a representative STM image of the 1/Zn2+ complex on HOPG. A metallo-supramolecular complex has been formed, in which each Zn2+ ion is bound to two 1 monomers. The images were very reproducible. The unit-cell parameters of the 2D structure of 1/Zn2+ were determined to be a = 7.4 ± 0.1 nm, b = 2.3 ± 0.1 nm, and α = 80° ± 1°.

nm). Therefore, the molecules on the top layer might orient on the surface at a certain angle and remain stabilized via the weak interactions among molecules, like π−π interaction, hydrogen bonding between tripyridine groups, van der Waals force, etc. Few triangular-shaped islands appear in the image, as indicated by the dashed line, indicating that the substrate−molecule interaction affects the formation direction of some island films, although on top of the first layer. On the basis of our previous work, 1 and Zn2+ would form a linear polymer via coordination bond in solution, which was characterized by NMR.20 Then, what will happen when 1 is mixed with Zn(OTf)2 on the HOPG surface? Figure 3 shows the surface topography after a 1/Zn2+ mixture was deposited on the first monolayer of 1 from CHCl3 and CH3CN at a molar ratio of 1:1, following solvent evaporation. It is clearly seen that the island film becomes consecutive when 1 is mixed with Zn(OTf)2, compared with the case of 1 itself (recall Figure 2). Interestingly, the film thickness of this film is determined to be 0.7 nm, which is slightly larger than that of 1 (0.6 nm). The fact that the formed film in this system is more consecutive indicates that a coordination reaction between 1 and Zn2+ occurs on the surface and forms a binary monolayer. Figure 3b is the higher-resolution image, which shows a closer look of the film. Some rodlike features were observed. The height of the rodlike structure is ∼0.5 nm, which matches well with the height of the molecule, suggesting 1 in the “rod” assembled on D

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Figure 6. Topography AFM image of the film formed from (a) 1, (b) 1/Zn(OTf)2, and (c) 1/Zn(OTf)2/2 at a concentration of 16 μM on the HOPG surface. (d) Cross-section of the film along the line shown in panel (a). (e) Cross-section of the film along the lines shown in panel (b).

Also, the film covers almost the entire surface, as shown in Figure 6c, implying the formation of an orthogonal supramolecular network on the surface. However, the near-intact film makes it difficult to measure the film thickness. The experiments at higher concentration further demonstrate the formation of the orthogonal supramolecular polymer on the HOPG surface.

Depositing the three-component mixture of 1, Zn(OTf)2, and 2 at the molar ratio of 1:1:0.5 onto the HOPG surface resulted in a more consecutive film, as shown in Figure 5. The film coverage becomes much larger than that of the binary system (Figure 3a). The film thickness was measured to be 0.9 nm, which is slightly higher than that in the binary system, implying the occurrence of the cross-link process in which the formation of the orthogonal polymer network would increase the size in the z-axis direction. Unfortunately, it is a big challenge for STM to obtain the image of such structure, because of the significant steric effect and poor tunnelling situation. Some rodlike features could still be observed in this system, mainly located between neighboring domains, like “bridges”. Different from the smooth surface of the 1/ Zn2+ binary film (Figure 3b), the film surface for three components system looks much rougher, as seen in Figure 5b. To detect the coordination reaction of this system at higher concentration, instead of the case of monolayer polymers formation, additional experiments were performed and showed similar observations for these three systems (Figure 6). Island films were formed from 1 at a concentration of 16 μM, which is similar to the case of 1.6 μM but with larger film thickness (1.2 nm; see Figure 6a). The film thickness decreased to 0.8 nm when mixing with Zn(OTf)2 at a molar ratio of 1:1. This indicates that a more regular superstructure formed, which could make the thickness smaller. In addition, the film becomes more concecutive, as shown in Figure 6b, suggesting the occurrence of a coordination reaction between the pyridine group and Zn2+ to form a linear polymer structure. Some bright dots appeared on the surface, which could be attributed to the aggregatation of extra salts. After combining with 2, the film becomes rougher again, which is the same as the phenomenon observed in the case of lower concentration (recall Figure 5b).

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CONCLUSIONS

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AUTHOR INFORMATION

We demonstrate, for the first time, that the orthogonal supramolecular network can form on highly oriented pyrolytic graphite (HOPG) surfaces, which was characterized by means of scanning probe microscopy (SPM). Atomic force microscopy (AFM) was employed to monitor the variations of the surface morphology and the thickness of the single-molecule layer of the polymer network on HOPG. Scanning tunneling microscopy (STM) images reveal the assembly structure of the host molecule, as well as the linear polymers formed through metal−ligand interaction at the submolecule resolution level. This research provides us direct evidence to the polymerization as well as the cross-linking of host molecule and is of great significance in fabricating supramolecular polymers on surfaces.

Corresponding Authors

*E-mail: [email protected] (S. Yin). *E-mail: [email protected] (Q. Zeng). *E-mail: [email protected] (M. Li). Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2011CB933101 and 2011CB932303) and the National Natural Science Foundation of China (Grant Nos. 21303208, 51173031, 91127032, 21174035, and 21574034). The project was also sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. Y31Z55), the Start-Up Funding from the Institute of High Energy Physics of the Chinese Academy of Sciences (No. 2011IHEPYJRC504), and Zhejiang Provincial Natural Science Foundation of China (No. LY16B040006).

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