Insight into th - ACS Publications - American Chemical Society

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From a Two-Dimensional Supramolecular Network to OneDimensional Covalent Polymer at the Liquid/Solid Interface: Insight into the Role of the Stoichiometric Ratio of the Precursors Yanxia Yu,†,‡ Yali Zheng,† and Shengbin Lei*,†,‡ †

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, People’s Republic of China Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Science & Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Co-condensation reaction between 2,6-diaminoanthraquinone and aromatic aldehyde at the octanoic acid/ highly oriented pyrolitic graphite (HOPG) interface has been studied with scanning tunneling microscopy (STM). We found that the stoichiometric ratio of the precursors plays a vital role in the formation of the assembling structures. By controlling the molar ratio of the amine and aldehyde monomers, either an ordered, hydrogen-bond-stabilized two-dimensional supramolecular network or assembly of one-dimensional covalent polymers can be successfully constructed. The supramolecular network can also be transformed to covalent polymers by annealing the sample to 373 K.

1. INTRODUCTION In the past decade, numerous examples of surface-supported covalent nanostructures through the controlled bottom-up strategy have been reported,1−10 which have a wide range of potential applications, such as molecular electronics, nanofiltration, and so on.11−15 To gain well control on the formation and final topography of the surface-confined nanostructures at the atomic level, it is necessary to understand the affecting factors such as monomer concentration, reaction temperature, and precursor mobility, as well as reversibility of bond formation.16−27 For example, formation of the ordered nanoporous two-dimensional polymer (2DP) is facilitated by low monomer concentration at the liquid/solid interface,27 and is influenced by both reaction temperature and amount of monomers at the gas/solid interface, which highlights the significance of the amount and mobility of precursors on the surface.17−20 In addition, the stoichiometric proportion and preferential adsorption of building blocks are known to affect the formation of assembled nanostructures,28,29 but its effect on the on-surface reaction has not attracted much attention previously. Specifically, Chi and co-worker found that by adjusting the stoichiometric proportions of reactants, reaction products can be selected under ultrahigh vacuum condition.30 At the liquid/solid interface, such phenomena have also been studied in the synthesis of surface supported 2DPs.27 In this work, we report the surface-mediated structural conversion from a hydrogen-bond-stabilized two-dimensional supramolecular network of diamine and dialdehyde to an assembly of covalently connected one-dimensional polymers © 2016 American Chemical Society

(1DP) at the octanoic acid/highly oriented pyrolitic graphite (HOPG) interface. The compounds utilized in this study are 2,6-diaminoanthraquinone (DAAQ), terephthalaldehyde (TPA), and 2,5-dioctyloxy-terephthalaldehyde (OTPA). The structures of the precursors and some of the products are shown in Scheme 1. DAAQ was chosen because its structure facilitates intermolecular hydrogen bonding between the quinone and amino groups. The amino group can also react with the aldehyde group, which thus forms imine bond linked nanostructures on the surface. Our in situ STM characterization demonstrates that OTPA and DAAQ form H-bond-stabilized supramolecular networks at the liquid/solid interface, which can transform into one-dimensional imine polymers upon annealing or form directly by tuning the molar ratio of both precursors. The present finding highlights the importance of the molar ratio of monomers and the interplay of thermodynamics versus kinetics in the formation of surface confined architectures at the liquid/solid interface.

2. EXPERIMENTAL SECTION 2,5-Dioctyloxy-terephthalaldehyde (OTPA), terephthalaldehyde (TPA), 2,6-diaminoanthraquinone (DAAQ), octanoic acid, and dimethyl sulfoxide (DMSO) were purchased from J&K and used without further purification. For sample preparation at the octanoic acid/HOPG interface, the Received: November 16, 2016 Revised: December 27, 2016 Published: December 27, 2016 593

DOI: 10.1021/acs.jpcc.6b11553 J. Phys. Chem. C 2017, 121, 593−599

Article

The Journal of Physical Chemistry C

Scheme 1. Schematic Illustration of the Formation of 1DPOTPA−DAAQ and a H-Bond-Stabilized Supramolecular Assembly of OTPA and DAAQ on HOPG Surface

monomers OTPA, TPA, and DAAQ were first dissolved in DMSO with concentrations of 1.4 × 10−2, 4.1 × 10−2, and 2.3 × 10−2 mol/L, respectively. The solutions then were diluted to the desired concentration with octanoic acid. The selected monomers were mixed with a desired molar ratio, a droplet (∼5 μL) of the mixed solution was directly deposited onto a freshly cleaved HOPG surface, and then it was characterized by scanning tunneling microscopy (STM). Detailed molar ratio, concentration, and imaging conditions are given in the figure captions. For the sample prepared at the gas/solid interface, the drop-cast samples were positioned in a preheated vacuum oven at 373 K for ∼30 min. The samples were taken out of the oven and cooled to room temperature, and then characterized by STM. STM measurements were performed under room temperature with a constant current mode (Agilent 5100, U.S.). Mechanically cut Pt/Ir (80/20) tips were used.

3. RESULTS AND DISCUSSION We first investigated the self-assembling behavior of the precursors at the liquid/solid interface. By applying a droplet of a 3.8 × 10−4 mol/L solution of DAAQ in octanoic acid on the freshly cleaved HOPG surface, the DAAQ molecules adsorb with their molecular plane parallel to the substrate and form a well-ordered structure that consists of lamellar architectures (Figure 1a). The dimensions of the unit cell are a = 0.9 ± 0.1 nm, b = 7.2 ± 0.1 nm, and β = (120 ± 2)°. The self-assembly is stabilized by intermolecular N−H···O hydrogen bonds between amino and aldehyde groups and van der Waals interactions (the structural model is shown in Figure 1b). At the lower right corner of Figure 1a, ordered self-assembly of octanoic acid was also observed. OTPA self-assembles at the octanoic acid/ HOPG interface at relatively high concentration into an ordered crystalline monolayer that consists of lamellar architectures and coadsorption of octanoic acid (Figure 1c), which have been obtained in our previous work.31 However, ordered self-assembly of TPA was not observed at the octanoic acid/HOPG interface, independent of the concentration of TPA.

Figure 1. High-resolution STM images of DAAQ (a, 3.8 × 10−4 mol/ L) and OTPA (c, 2.3 × 10−3 mol/L) at the octanoic acid/HOPG interface. The panels of (b) and (d) show the corresponding model of the self-assembly island, respectively. Imaging conditions: (a) Iset = 30 pA, Vbias = 0.44 V; (b) Iset = 22 pA, Vbias = 0.68 V. All scale bars = 3 nm.

Figure 2a shows a representative STM image obtained several minutes after applying a drop of mixture solution of DAAQ and OTPA with the molar ratio of 1:1 on the HOPG surface. A well-defined two-dimensional network with apparent square lattice was revealed, covering the entire surface. From the high-resolution image (Figure 2b), individual molecules can be clearly distinguished, in which the stripe-like species are attributed to rows of DAAQ molecules connected by N−H···O hydrogen bonds between the amino and quinone, while the clover-shaped objects between the rows are attributed to the OTPA molecules. Thus, the 2D network is attributed to a twocomponent supramolecular assembly of DAAQ and OTPA, 594


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Figure 2. Large (a, scale bar = 5 nm) and high-resolution (b, scale bar = 2 nm) STM images of the monolayer formed several minutes after a mixture of DAAQ and OTPA in octanoic acid (the molar ratio of DAAQ and OTPA is 1:1, concentration of DAAQ: 1.4 × 10−4 mol/L) is drop-casted onto the surface of HOPG. (c) The structural model of the self-assembly. Imaging conditions: (a) Iset = 65 pA, Vbias = 0.44 V; (b) Iset = 86 pA, Vbias = 0.35 V.

Figure 3. STM images of the monolayer formed immediately (a) and 75 min (b) after a mixture of DAAQ and OTPA in octanoic acid (the molar ratio of DAAQ and OTPA is 1:1, concentration of DAAQ: 1.4 × 10−4 mol/L) is drop casted onto the surface of HOPG. (d) STM image of the sample (b) after annealing at 373 K for 30 min. (c) STM image of the sample annealed at 373 K for 30 min immediately after deposition. Imaging conditions: (a) Iset = 65pA, Vbias = 0.44 V; (b) Iset = 67 pA, Vbias = 0.30 V; (c) Iset = 26pA, Vbias = 0.60 V; (d) Iset = 19 pA, Vbias = 0.66 V. All scale bars = 5 nm.

However, as time passed, the 2D network transforms into a more compact and disordered structure as shown in Figure 3b. This less ordered structure is attributed to an assembly of imine oligomers and intact DAAQ precursors, and the gradual structure transformation is attributed to the proceeding of on-surface reaction between amine and aldehyde monomers (Figure S2). Determined from the interchain distances and close pack characteristics, the octyloxy side chains on the OTPA units are supposed to be floated in the solution rather than adsorbed on the surface. Unfortunately, at room temperature, the on-surface reaction does not proceed further at this concentration and molar ratio of monomers, and no ordered assembly of 1D imine polymer as revealed with high molar ratio of DAAQ, which will be discussed in the following section, was revealed even after being layed aside for a long time. However, after annealing the sample at higher temperature (373 K), distinctively different structures were revealed (Figures 3d and S1a). Now the surface was covered by both loosely and compactly packed domains, corresponding to assembling domains of 1DPOTPA−DAAQ with the octyloxy side chain floated in the solution and adsorbed on the surface, respectively. The latter structure is identical to that obtained with a higher molar ratio of DAAQ at room temperature, the

instead of assembly of 1D imine polymer from the on-surface condensation from both monomers. The lattice is stabilized via the N−H···O hydrogen bonds between the amino and quinone or aldehyde groups. The bright stripes are attributed to supramolecular assembly of intact DAAQ monomers stabilized by H-bonds between amino and quinone groups, while each OTPA molecule interacts with two DAAQ monomers from two adjacent rows (the structural model is shown in Figure 2c). Noteworthy, in most cases, each alternative DAAQ in the same row interacts with one out of two of its amino groups with OTPA on the opposite side of the row. However, as a kind of linear defect (as highlighted with the black arrow in Figure 2a and b), on one side the OTPAs can shift for a certain distance, and in this case each alternative DAAQ in the same row interacts with both of its amino groups with OTPA on the opposite side of the row. A unit cell is superimposed on the STM image in Figure 2b with a = 1.7 ± 0.1 nm, b = 1.9 ± 0.1 nm, and α = 89 ± 1°. As can be seen, each unit cell is composed of two DAAQ monomers and one OTPA. The ratio of OTPA to DAAQ is 1:2 on the surface, different from the molar ratio of 1:1 in solution, which indicates that the composition of the supramolecular network is determined by the packing mode rather than the composition in solution. 595


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Figure 4. Real-space imaging of the transition of the ordered supramolecular network into disordered structure at the octanoic acid/HOPG interface (the molar ratio of DAAQ and OTPA is 1:1, concentration of DAAQ: 1.4 × 10−4 mol/L). Imaging conditions: (a−c) Iset = 92 pA, Vbias = 0.35 V. All scale bars = 10 nm.

Figure 5. Representative STM images of monolayers formed from mixture solutions of DAAQ and OTPA with different molar ratios: (a−c) 1:2 (concentratiosn of DAAQ, 8.9 × 10−5 mol/L; OTPA, 1.8 × 10−4 mol/L), (d,e) 3:1 (concentrations of DAAQ, 2.5 × 10−4 mol/L; OTPA, 8.3 × 10−5 mol/L). A tentative molecular model of 1DPOTPA−DAAQ is shown in panel (f). Imaging conditions: (a) Iset = 26 pA, Vbias = 0.60 V; (b) Iset = 24 pA, Vbias = 0.86 V; (c) Iset = 28pA, Vbias = 0.66 V; (d) Iset = 30 pA, Vbias = 0.68 V; (e) Iset = 26 pA, Vbias = 0.60 V. For (a), (c), and (d), scale bar = 10 nm; for (b) and (e), scale bar = 3 nm.

details of which will be discussed later. More interestingly, if the sample was immediately heated at 373 K after the solution containing DAAQ and OTPA with the molar ratio of 1:1 was applied on the HOPG surface, it results in long-range ordered 1DPOTPA−DAAQ assembly with most of the octyloxy side chains adsorbed on the surface (Figures 3c and S1b). Therefore, we can conclude that the thermal history of the sample has an important impact on the final topology of the assembly; this might arise from that the compact and disordered assembly of the oligomers formed under room temperature may hinder the diffusion of the monomers on the surface during the annealing process, thus resulting in the formation of compact domains of 1DP with short chain length. Interestingly, the dynamic process of the transformation from the 2D supramolecular network to the disordered structure was also recorded in situ by high-resolution imaging. Sequential STM images recorded at the liquid/solid interface with a 1:1 molar ratio of the DAAQ and OTPA are shown in Figure 4. These consecutive images clearly show that the surface coverage of the ordered 2D network decreases over time, and gradually transforms into the disordered structures. In Figure 4a, the green arrow marks the point of a fuzzy region, which, after 8 min, disappeared from the surface (Figure 4b).

To further investigate the influence of the molar ratio of the monomers on the supramolecular assembling and Schiff-base coupling reaction at the liquid/solid interface, we performed systematic experiments on the assembling and reaction of monomers with different molar ratio. When the molar ratio of DAAQ and OTPA is 1:2, the ordered self-assembled supramolecular networks are formed, which are identical to the structures that are shown in Figure 2 (as shown in Figure 5a). Occasionally, imine oligomers with a length of ∼3.5 nm were also observed. Considering the size and the request for hydrogen bonding, which means the oligomer should terminate with aldehyde group, the oligomer is supposed to have consisted of three OTPAs and two DAAQs, as indicated by the red panel in Figure 5b. However, after about 92 min, the network also transforms into disordered structures (Figure 5c), which is similar to that with a molar ratio of 1:1. Upon further decreasing the ratio of DAAQ to OTPA to 1:3, STM reveals that the 2D supramolecular networks coexist with the disordered structures at the interface (Figure S3). More interestingly, assembly of covalent one-dimensional imine polymer 1DPOTPA−DAAQ is successfully formed, extending over the entire surface upon increasing the DAAQ to OTPA ratio to 3:1, as shown in Figure 5d. Closer inspection (Figure 596


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Figure 6. Large-scale (a, scale bar = 15 nm) and high-resolution (b, c, scale bar = 3 nm) STM images of the monolayer formed after a mixture of TPA and DAAQ in octanoic acid (the molar ratio of DAAQ and TPA is 1:1, concentration of DAAQ: 2.4 × 10−4 mol/L) is drop-casted onto the HOPG surface. The lower panels of (b,c) show the structural models of 1DPTPA−DAAQ. Imaging conditions: (a) Iset = 30 pA, Vbias = 0.60 V; (b) Iset = 30 pA, Vbias = 0.60 V; (c) Iset = 30 pA, Vbias = 0.60 V.

as well. Because the speed for forming supramolecular assembly is much faster than forming a covalent bond, the bicomponent supramolecular network is kinetically favored. Although this bicomponent supramolecular network can gradually transform into disordered assembly composed of oligomer and intact monomers due to on-surface condensation, the disordered assembly limits the diffusion of monomers on the surface, and induces a barrier for further on-surface reaction; even annealing to higher temperature (373 K) cannot provide enough energy to overcome this barrier. When the molar ratio of DAAQ and OTPA is 3:1 (concentrations of DAAQ, 2.5 × 10−4 mol/L; OTPA, 8.3 × 10−5 mol/L), both monomers do not have enough concentration to form a stable assembly, and thus provide enough space for the on-surface diffusion and condensation between both monomers to form a 1D polymer. Finally, we designed an experiment to clarify the role of octyloxy side chains on OTPA in forming the bicomponent 2D supramolecular network. To do so, the octyloxy side chain of the OTPA is eliminated (OTPA is replaced by TPA). In this case, upon applying the mixture of the DAAQ and TPA in octanoic acid with the molar ratio of 1:1 on the HOPG surface, crystalline assembling domains of 1DPDAAQ−TPA were directly formed, and no sign of bicomponent supramolecular assembly was observed. The domain is in the range of 20−30 nm in lateral size, and orients along the three symmetry-related orientations of the graphite substrate, as illustrated in Figure 6a. Interestingly, it is quite clear from the high-resolution STM images that both cis- and trans-conformations of the imine linkage (with respect to the DAAQ moiety) are present in the polymer (Figure 6b and c). Although from an energetic aspect of view, 1DPTPA−DAAQ with all imine linkages adapting an alltrans conformation will be most favorable, cis-conformation could present in the 1D polymer due to kinetic factors. Direct proof for the existence of cis-conformation of the imine linkage has been presented in 2DP on the HOPG surface.20 In this case, we have, as well, observed the existence of cisconformation of the imine linkage in 1DPTPA−DAAQ, as indicated by the black arrow in Figure 6b. Figure 6c displays the highresolution STM image of 1DP TPA−DAAQ with all-trans conformation. The repeating period along the 1DPTPA−DAAQ chain measures 1.8 ± 0.2 nm, fully consistent with that expected from a covalent imine polymer and identical to that in 1DPOTPA−DAAQ. The interchain distance measures 0.9 ± 0.2 nm, suggesting that the interchain interaction is dominated by van der Waals interactions. This experiment proves the importance of the octyloxyl side chains of the aldehyde for the formation of the supramolecular network on the surface.

5e) clearly reveals that larger dimmer spots and smaller brighter spots are aligned alternatively along the backbone of 1DPOTPA−DAAQ; these spots are assigned to the anthraquinone group from DAAQ and phenyl group from OTPA, respectively. Along the polymer backbone of 1DPOTPA−DAAQ, the repeating period measures 1.8 ± 0.1 nm, in good agreement with those expected from the chemical structure, confirming the covalent bond formation. The interchain distance averages ∼1.8 nm, consistent with that expected with the octyloxy side chains fully extended and interdigitated on the surface. It is also noteworthy that the 1DPOTPA−DAAQ is relatively stable, so that we did not observe any decomposition of the structure during the scanning, which is rather different from the above-mentioned supramolecular structures, and further confirm the covalent bond formation. At some domain boundaries, lamellae of DAAQ assembly can be identified, as marked by the red arrow in Figure 5d. In this case, the terminal DAAQ moiety of the 1DPOTPA−DAAQ chains composes part of the lamellae. The above experiments have clearly demonstrated the importance of molar ratio of the building blocks in determining the assembling and reaction of a dynamic covalent library at the liquid/solid interface. The total balance of self-assembling/ Schiff base condensation can be shifted by tuning the molar ratio of precursors. Apparently, in the current system, the bicomponent supramolecular network is a kinetically favored structure, which is evidenced by the observation of its gradual transformation into thermodynamically more stable disordered assemblies. To rationalize the above observation of the dependence of assembly and reaction on the molar ratio of precursors at the liquid/solid interface, some primary issues must be taken into account. These include the competitive adsorption of the precursors and reaction products, including both polymer and oligomer, and the assembling properties of these compounds as well. Both precursors, especially DAAQ, show relatively weak adsorption ability, only form assembly at the interface with high concentration. Even with high concentration, DAAQ only forms assembly of submonolayer coverage (Figure 1a). The 1H NMR spectroscopy revealed that the reaction between DAAQ+OTPA in solution is neglectable, as nearly all precursors exist as intact monomers in solution (Figure S4). When brought into contact with the graphite surface, competitive adsorption of these species as well as surface condensation may happen at the same time. Because OTPA has a stronger tendency to form ordered assembly, thus with higher OTPA concentration it turns to form assembly on the surface, and due to the strong intermolecular H-bond between OTPA and DAAQ, it drags DAAQ into the assembly 597


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4. CONCLUSIONS By keeping the total mass concentration constant, we have performed an STM study on the influence of molar ratio of precursors on the assembling and reaction of Schiff base coupling at the liquid/solid interface. Either bicomponent supramolecular assembly or 1DP via on-surface condensation was obtained depending on the molar ratio of the building blocks. The octyloxy side chain of the OTPA plays a key role in the formation of the H-bond-stabilized supramolecular network of DAAQ and OTPA. The results prove clearly that the selfassembly of DAAQ and OTPA hampers the on-surface condensation reaction, and the effect of thermal history observed in this work indicates the important role of mobility of the building blocks. The selective pattern formation is a result of the competitive adsorption and the assembling properties of both the monomers and the products. This study demonstrates that the adsorption ability, assembling property, and stoichiometric composition of the monomers have significant effects on the final structure of a dynamic covalent library at the liquid/solid interface.

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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.jpcc.6b11553. Extra STM images and 1H NMR spectra of bicomponent mixtures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shengbin Lei: 0000-0002-2483-5286 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21572157, 21373070). ABBREVIATIONS HOPG, highly oriented pyrolitic graphite; STM, scanning tunneling microscopy; 2DP, two-dimensional polymer; 1DP, one-dimensional polymers; DAAQ, 2,6-diaminoanthraquinone; TPA, terephthalaldehyde; OTPA, 2,5-dioctyloxy-terephthalaldehyde; DMSO, dimethyl sulfoxide

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REFERENCES

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