Competitive Influence of Hydrogen Bonding and van der Waals

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Competitive Influence of Hydrogen Bonding and van der Waals Interactions on Self-Assembled Monolayers of Stilbene-Based Carboxylic Acid Derivatives Ling-yan Liao,† Yi-bao Li,‡ Jing Xu,† Yan-fang Geng,*,† Jun-yong Zhang,§ Jing-li Xie,*,§ Qing-dao Zeng,*,† and Chen Wang*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China ‡ Key Laboratory of Organo-pharmaceutical Chemistry, Gannan Normal University, Ganzhou 341000, China § College of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing 314001, China S Supporting Information *

ABSTRACT: The molecule−molecule and molecule−substrate interactions play an important role during the formation of two-dimensional (2D) supramolecular nanostructure. In this paper, the self-assembled monolayers of four stilbene derivatives possessing different chemical structures at the liquid−solid interface were investigated by employing scanning tunneling microscopy (STM). Chemical structures that affect the 2D molecular self-assembly, such as number of alkoxyl chain with carboxylic acid end-group and length of alkoxyl chain, were elucidated in detail. Systematic investigation indicated that various self-assembly structures consequently formed on highly oriented pyrolytic graphite (HOPG) surface, via a combination of intermolecular hydrogen bonding and van der Waals interactions. It is proposed that hydrogen bonding and van der Waals interactions competitively control the morphology of the monolayer, and the selfassembled 2D nanostructure is determined by balance of these two interactions.

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electronegative atoms), and weak (∼1−5 kcal mol−1, e.g., CH··· O).19 The carboxylic acid species is the most frequently used building block to form hydrogen bonding motifs. The hydrogen atom in the hydroxyl group serves as the hydrogen bonding donor, while the oxygen atom in the carboxylic group acts as the hydrogen bonding acceptor.20 Two carboxylic acid groups can form two types of hydrogen bonding interaction. One is the cyclic dimer where two carboxylic acid groups interact with each other and form two equivalent hydrogen bonding.21 The other one is open catemer where two adjacent carboxylic acid groups form only one hydrogen bond. The role of hydrogen bonding in the process of 2D supramolecular organization has been perfectly illustrated by representative molecules 1,3,5benzenetricarboxylic acid (TMA)22−26 and 1,3,5-tris(10carboxydecyloxy)benzene (TCDB).27−30 The self-assembly of TMA is a pioneering example of porous self-assembled network by means of hydrogen-bonding interactions. The well-defined and robust 2D supramolecular networks of TMA or TMA with other components can been used as molecular template for complicated and functional systems. Template molecule TCDB with long alkoxyl chains terminated with carboxylic acid endgroup has been widely studied in depth in our previous reports, and we found that TCDB can form variable 2D supramolecular

INTRODUCTION The phenomenon of molecular self-assembly is ubiquitous in a variety of chemical, physical, and biological systems and has led to great advances in the field of nanoscience and nanotechnology.1,2 Among various molecular self-assemblies, twodimensional (2D) self-assembly of special building blocks in nanometer scale at surfaces or interfaces is of increasing interest because of their potential applications.3−7 Thus, controlling and tuning the 2D self-assembly structure on surface is a current attractive research field. The successful construction of 2D supramolecular pattern by a chemical design of molecular building blocks has been widely reported, and the assembled structure strongly depends on two kinds of interactions: molecule−molecule and molecule−substrate.8−13 The molecule−molecule interactions, which determine the molecule alignment by interaction between building blocks, are the key to control the self-assembly structure. Hydrogen bonding not only has been widely observed in various fields such as chemical, biological, and material14−17 but also is the most commonly encountered interaction to control the selfassembled structure owing to its relatively strong, highly selective and directional features.8,17,18 According to the strength, hydrogen bonding is often divided into three categories: very strong (∼15−40 kcal mol−1; typically involves a charged hydrogen bonding acceptor, e.g., COOH···COO−), moderate (∼5−15 kcal mol−1, most often observed for neutral hydrogen bonding donors and acceptors with N and O as © XXXX American Chemical Society

Received: September 6, 2014 Revised: October 27, 2014

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shown in TCDB case.28 Scanning tunneling microscopy (STM), a preferred technique for investigating structures and properties of the self-assembled monolayer on surface due to the submolecular resolution, was used to investigate the 2D self-assembled structures. This contribution mainly focused on the influence of the molecule−molecule and molecule− substrate interactions on the 2D supramolecular structures. The competitive influence of hydrogen bonding and van der Waals on the formation process of 2D self-assembly at the liquid−solid interface was explored in detail. It is proposed that the role of hydrogen bonding and van der Waals interactions might be switched by varying the number and length of alkoxyl chain terminated with carboxylic acid group and consequently different 2D assembly structures formed.

networks according to the accommodated guest molecules. It was demonstrated that hydrogen bonding, served as a switching, is responsible for the flexible networks for specific guest.27,28 The underlying substrate is another factor in control of the self-assembly structure due to the molecule−substrate interaction. In particular, the highly oriented pyrolytic graphite (HOPG) is one of the most widely used substrate for 2D selfassembly because of its low cost, ease of handling, defined surface, chemical stability, and so on. The molecule−HOPG interactions mainly include van der Waals interactions and electrostatic interactions.21 The van der Waals interaction is general between molecular monolayer and HOPG substrate for self-assembly in two dimensions. The strength of van der Waals interaction between molecule and HOPG substrate strongly depends on the chemical structure of the adsorbed molecule such as functional groups and the substituted alkyl chain. It has been found that long alkyl chains can more easily adsorb to the surface via van der Waals interactions.31 Other than the alkane−substrate interaction, the long alkyl chain also can form interdigitated alignment to modify the distance between molecules. It is important to investigate the influence of length and number of substituted alkyl chain on the strength of van der Waals interactions for well-defined 2D self-assembly structures. In self-assembly systems, a variety of interactions usually coexist for control of the molecular alignment; therefore, it is necessary to consider the competitive effect of various interactions on the molecular arrangement. Both molecule− molecule and molecule−substrate interactions simultaneously determine the overall adsorption structure. Jayakannan reported the role of π-stacking and van der Waals interactions on the molecular self-assembly depending on the chemical structure of molecules.32 van Esch et al. researched the balancing of hydrogen bonding and van der Waals interactions in specific systems.33 Steven and co-workers demonstrated that intermolecular hydrogen bonding and molecule−substrate van der Waals interactions control the 2D self-assembled pattern.20 Although competitive influence of various interactions on selfassembled structure has been studied, the effect of the number of acids and length of alkoxyl chain on the molecular alignment has not been systematically studied. Herein, we design molecular chemical structure, control its self-assembled pattern, and bring insight into the correlation between chemical structure and the well-defined molecular self-assembly. Stilbene functional group was selected because its derivatives have been widely studied due to their important role in the field of chemistry and materials nanotechnology.34,35 Four stilbenebased terminal carboxylic acid compounds with different number and length of substituted alkoxyl chains were designed and synthesized as shown in Scheme 1. Alkoxyl chain, not alkyl chain, was used based on the consideration of its tenability as

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EXPERIMENTAL SECTION Sample Preparation. All solvents for STM experiments were purchased from Tokyo Chemical Industry (TCI) and used without further purification. All of the studied samples were dissolved by n-heptanoic acid, and the concentrations of all the solutions for STM investigation were less than 1.0 × 10−4 mol/L. A droplet (∼0.4 μL) of solution containing compound 1 (or 2−4) was cast onto a freshly cleaved highly oriented pyrolytic graphite (HOPG, grade ZYB, NTMDT, Russia). After 20 min, the sample was measured by the STM technique. STM Measurement. The STM measurements were performed under atmospheric conditions. All the STM images were acquired on a Nano III scanning probe microscope system (Bruker) operating in constant current mode with mechanically cut Pt/Ir (80/20) tips. All images presented here were raw data without any treatment except for the flattening procession. The drift was calibrated using the underlying graphite lattice as a reference. Molecular models were constructed using a HyperChem software package.

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RESULTS AND DISCUSSION

Molecule Synthesis. The synthetic routes of four target molecules are summarized in Scheme S1 (see Supporting Information). Precursor compounds H1−H4 were synthesized according to our previous work.31 Self-Assembly at the Interface. As shown in Scheme 1, compounds 1 and 2 have four substituted alkoxyl chains with different length, terminated with carboxylic acid groups, while compounds 3 and 4 have only three alkoxyl chains with different length, also terminated with carboxylic acid groups. The self-assembly structures for these four molecules formed at the heptanoic acid/HOPG interface were investigated by STM, in order to study the effect of the number and length of the substituted alkoxyl chain terminated with carboxylic acid on the self-assembled structure. Self-Assembly of Compound 1. After a droplet of heptanoic acid solution containing compound 1 was deposited onto HOPG substrate surface, two types of well-ordered and periodic linear patterns with bright and dark stripes formed as observed by STM, namely, 1-I and 1-II (as shown in Figure 1a). In type 1-I, a close-packed linear pattern was formed as displayed in Figure 1b. The length (L1) and width (W1) of the bright rod are measured to be 1.2 ± 0.1 nm and 0.6 ± 0.1 nm, respectively, which is in accordance with the size of the stilbene part with high electron density in STM images.36 The width (L2) of the dark area is estimated to be 0.7 ± 0.1 nm, which

Scheme 1. Chemical Structures of Compounds 1−4

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indicated from the molecular model. In the case of 1-I, van der Waals interaction between alkoxyl chains as well as molecule and HOPG plays the major role, while hydrogen bonding between molecules and van der Waals interactions between molecules and HOPG control the molecular alignment in the case of 1-II. It is well-known that the self-assembly is a simultaneous and rapid process. The adsorbates on surface have the tendency of preferentially arranging in stable state. Therefore, these two conditions 1-I and 1-II are all stable, suggesting the hydrogen bonding and van der Waals interactions that controlled the self-assembly have similar influence on the molecular alignment. Self-Assembly of Compound 2. By contrast, compound 2 has longer alkoxyl chain than compound 1. After depositing a droplet heptanoic acid solution containing compound 2 onto HOPG surface, only one type of linear alignment form as shown in Figure 2a. In Figure 2b, the length (L5) and width

Figure 1. (a) Large-area STM image of two assembling structures for compound 1 at heptanoic acid/HOPG interface, Iset = 329.6 pA, Vbias = 700.1 mV, scale bar = 15 nm. (b) High-resolution STM image of the white rectangle area in (a), Iset = 329.6 pA, Vbias = 677.8 mV, scale bar = 5 nm. (c) Suggested molecular model for the case of 1-I. (d) Highresolution STM image of the red rectangle area in the image (a), Iset = 350.5 pA, Vbias = 534.3 mV, scale bar = 5 nm. (e) Proposed molecular model for the case of 1-II. Figure 2. (a) Large-scale STM image for the self-assembled structure of compound 2 at heptanoic acid/HOPG interface, Iset = 296.0 pA, Vbias = 679.0 mV, scale bar = 15 nm. (b) High-resolution STM image of the white rectangle area in (a). Iset = 296.0 pA, Vbias = 679.0 mV, scale bar = 5 nm. (c) Molecular model of the assembly structure for (b).

agrees with the length of substituted alkoxyl chain, O(CH2)3COOH. Thus, the alkoxyl chains are proposed to form interdigitated alignment in the dark domain. As presented tentative molecular model in Figure 1c, the unit cell parameters are determined to be a = 1.3 ± 0.1 nm, b = 1.9 ± 0.1 nm, and α = 72 ± 2°. The close-packed linear structure at the heptanoic acid/HOPG interface might result from van der Waals interactions between the interdigitated alkoxyl chains as well as the molecules and HOPG. Compared with type 1-I structure, type 1-II shows a loose linear alignment as displayed in Figure 1d. The length and width of the bright spot are determined to be L3 = 1.3 ± 0.1 nm and W2 = 0.7 ± 0.1 nm, which are coincident with the size of the stilbene part. The width (L4) of the dark stripe is measured to be 1.2 ± 0.1 nm, which is approximately twice the length of alkoxyl chain, indicating that the alkoxyl chains might align in parallel. Two alkoxyl chains coming from two aligned compound 1 connected with each other through hydrogen bonding between two terminal carboxylic acid groups (COOH···COOH), as indicated by the black circle in Figure 1e. The nonlinear array of alkoxyl chains reveals that two terminal COOH groups formed cyclic dimer hydrogen bonding as shown in the packed model in Figure 1e. The unit cell parameters are measured to be a = 1.8 ± 0.1 nm, b = 2.4 ± 0.1 nm, and α = 86 ± 2°. For these two kinds of alignments in the case of compound 1, different interactions control the self-assembled structures as

(W3) of the bright rod are measured to be 1.2 ± 0.1 nm and 0.6 ± 0.1 nm, respectively, which agree with one single stilbene moiety. The width (L6) of the dark area is measured to be 1.4 ± 0.1 nm, which corresponds to the length of alkoxyl chain, O(CH2)9COOH. It is suggested that the alkoxyl chains formed interdigitated alignment in the dark stripes, which is similar to 1-I of compound 1. The molecular model corresponding to the STM image in Figure 2b is shown in Figure 2c. The unit cell parameters are determined to be a = 2.2 ± 0.1 nm, b = 2.3 ± 0.1 nm, and α = 72 ± 2°. Additionally, the assembled structure also resembles our previously reported stilbene molecule with terminal ester groups.31 The uniform and well-ordered 2D supramolecular pattern is stable by van der Waals interactions between alkoxyl chains. Therefore, the terminal groups can not affect the molecular alignment when the length of alkoxyl chain increases to long enough. It is suggested that van der Waals interactions between alkoxyl chain mainly control the self-assembly process in the case of sufficiently long chain length existing. In other words, increasing the length of alkoxyl chain, van der Waals C

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respectively, which correspond to the stilbene part. As shown in Figure 3d, the distance (L12), measured to be 2.6 ± 0.1 nm, is twice the length of stilbene unit. Thus, the two molecules form a dimer indicated by the red rectangle. A corresponding molecular model for the image (Figure 3d) has been proposed in Figures 3e. The unit cell parameters are determined to be a = 3.2 ± 0.1 nm, b = 8.2 ± 0.1 nm, and α = 55 ± 2°. Based on above STM analysis, it can be concluded that van der Waals interactions between interdigitated alkoxyl chain mainly controlled the 2D supramolecular organization in the case of A, while open catemer hydrogen bonding (as marked with black rectangle in Figure 3e) between terminal carboxylic acids and van der Waals interactions simultaneously determined the molecular self-assembly in the case of B. As our previous study on the effect of external field on the molecular alignment on surface,37 compound 3 shows electrical response, which could be attributed to the interaction of the molecule with the electric filed. Compared with compound 1, compound 3 has one fewer alkoxyl chain, which suggests the molecule−molecule and molecule−HOPG van der Waals interactions could become weak. As a result, self-assembled structure of compound 3 has tendency of being affected by external condition. Self-Assembly of Compound 4. The self-assembled process of compound 4 dramatically changes when prolonging the alkoxyl chain in comparison with compound 3. After a droplet of heptanoic acid solution containing compound 4 was deposited onto HOPG surface, periodic lamella architecture is formed at the heptanoic acid/HOPG interface as displayed in Figure 4a. As displayed in Figure 4b, the length of the bright

interactions between alkoxyl chains increase and become stronger than the hydrogen bonding between terminal carboxylic acid groups. So it can be concluded that hydrogen bonding and van der Waals interactions completely control the self-assembled structure by changing the length of the alkoxyl chain. Self-Assembly of Compound 3. In the case of compound 3, it was found that the self-assembly of compound 3 interestingly displayed an obvious tunneling voltage effect. When the bias voltage changed (Vbias > 0 or Vbias < 0), the compound 3 molecules assembled into different 2D structures on HOPG surface as shown in Figure 3a. When positive bias voltage (Vbias

Figure 3. (a) Large-area STM image for the self-assembled structure of compound 3 at heptanoic acid/HOPG interface with tip bias changed from negative (−560.0 mV, 308.2 pA) to positive (770.0 mV, 306.2 pA) during the STM scan, as indicated by the blue arrows, scale bar = 15 nm. (b) High-resolution STM image for domain A in (a), Iset = 306.2 pA, Vbias = 629.0 mV, scale bar = 5 nm. (c) Suggested molecular model for (b). (d) High-resolution STM image for domain B in (a), Iset = 308.2 pA, Vbias = −524.3 mV, scale bar = 5 nm. (e) Proposed molecular model for (d). Figure 4. (a) Large-scale STM image for the self-assembled structure of compound 4 at heptanoic acid/HOPG interface, Iset = 259.4 pA, Vbias = 579.8 mV, scale bar = 20 nm. (b) High-resolution STM image of the white rectangle area in (a). Iset = 259.4 pA, Vbias = 579.8 mV, scale bar = 5 nm. (c) Molecular model of the assembly structure for (b).

> 0) was applied, a well-ordered and close-packed lamella pattern formed (A for short) as presented in Figure 3a. The length of the bright rod (L7) is measured to be 1.2 ± 0.1 nm, which corresponds to the stilbene unit. Every two neighboring molecules form a dimer as marked by the white rectangle. Besides, both the lamella distance (L8) and the dimer distance (L9) are measured to be 0.7 ± 0.1 nm, which agrees with the total length of the alkoxyl chain, O(CH2)3COOH. The unit cell parameters are measured to be a = 1.7 ± 0.1 nm, b = 2.7 ± 0.1 nm, and α = 78 ± 2°. When negative voltage was applied (Vbias < 0), the assembled structure changes dramatically. Uniform and periodic monolayer architecture (B for short) was formed as shown in Figure 3d. The lengths of the bright rods (L10 and L11) are determined to be 1.2 ± 0.1 nm and 1.3 ± 0.1 nm,

rod (L13) is determined to be 1.2 ± 0.1 nm, which is assigned to the stilbene part. Every two neighboring molecules form a dimer indicated by the red rectangle in Figure 4a,b, and lamella structure is made up. The distance (L14) between two bright rods along the row is measured to be 1.3 ± 0.1 nm, which can be assigned to the length of the substituted alkoxyl chain, O(CH2)9COOH. Therefore, the dark area along the a-direction indicated by the black arrow in Figure 4b is constructed by the D

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*Tel 86-573-83643264; Fax 86-10-62656765; e-mail jlxie@ mail.zjxu.edu.cn (J.X.).

interdigitated alkoxyl chains. In Figure 4b, careful inspection of the high-resolution image reveals that two alkoxyl chains of compound 4 are distributed perpendicular to the a-direction, and a hydrogen bonding between terminal carboxylic acid group and a benzene ring of a neighboring stilbene derivative is indicated by the black circle in Figure 4c. The monolayer structure is finally stabilized by hydrogen bonding and van der Waals interactions. The packed model corresponding to the STM image (Figure 4b) is shown in Figure 4c. The unit cell parameters are measured to be a = 1.9 ± 0.1 nm, b = 4.5 ± 0.1 nm, and α = 64 ± 2°. Compared with compound 2, it can be concluded that the intermolecular hydrogen bonding and van der Waals interactions were tuned by changing the number of alkoxyl chain. On the basis of the presented STM investigations, the 2D self-assembled structures of compounds 3 and 4 are obviously different from those of compounds 1 and 2, which suggests that the number of alkoxyl chains terminated with carboxylic acid groups exerts great influence on the molecular alignment on HOPG surface. That is to say, the balance of various controlled interactions was tuned by altering the chemical structure. As the above analysis, hydrogen bonding and van der Waals interactions mainly determine the self-assembly structure. Therefore, it can be concluded that the number and length of substituted alkoxyl chain with carboxylic acid terminal group could directly affect the strength of hydrogen bonding and van der Waals interactions and ultimately induced the dominant interaction switch and new balance between hydrogen bonding and van der Waals interactions emerge. This contribution focusing on the interaction balance could offer important help for constructing designed and desirable 2D supramolecular selfassemblies.

Author Contributions

L.L. and Y.L. contributed to this work equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2011CB932303 and 2013CB934203). The National Natural Science Foundation of China (Nos. 51173031, 91127043, 21472029, 21371078, 21303024, 21365003, and 51203030) is also gratefully acknowledged.

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CONCLUSION In conclusion, four stilbene derivatives (compounds 1−4) with substituted alkoxyl chain terminated with carboxylic acid groups have been successfully synthesized. STM investigations of their 2D self-assembly monolayers on HOPG surface were systematically performed, and various different 2D supramolecular organizations were obtained. The experimental results present that the intermolecular hydrogen bonding and van der Waals interactions can be tuned by changing the length or number of the alkoxyl chains terminated with carboxylic acid group. The competitive influence of hydrogen bonding and van der Waals interactions on the molecule−molecule and molecule− substrate interactions consequently affects 2D self-assembled structures. It is concluded that a perfect balance of hydrogen bonding and van der Waals interactions is necessary for constructing required 2D monolayer structures on the surface.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of compounds 1−4, Scheme S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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*Tel 86-10-82545548; Fax 86-10-62656765; e-mail zengqd@ nanoctr.cn (Q.Z.). *Tel 86-10-82545561; Fax 86-10-62656765; e-mail wangch@ nanoctr.cn (C.W.). *Tel 86-10-82545691; Fax 86-573-83646048; e-mail gengyf@ nanoctr.cn (Y.G.). E

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