Solvent Effect on Host-Guest Two-Dimensional Self-Assembly

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Solvent Effect on Host-Guest Two-Dimensional Self-Assembly Mediated by Halogen Bonding Juntian Wu, Jinxing Li, Meiqiu Dong, Kai Miao, Xinrui Miao, Yican Wu, and Wenli Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07194 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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The Journal of Physical Chemistry

Solvent Effect on Host-Guest Two-Dimensional Self-Assembly Mediated by Halogen Bonding Juntian Wu, Jinxing Li, Meiqiu Dong, Kai Miao, Xinrui Miao,* Yican Wu, Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China

Corresponding authors：：Xinrui Miao ([email protected]) Wenli Deng ([email protected])

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ABSTRACT: Host-guest self-assemblies of methyl 5-bromo-2-(hexadecyloxy)benzoate (host) and 1-bromohexadecane (guest) molecules were investigated at liquid/solid interface by scanning tunneling microscopy. Polar solvent (1-octanoic acid) and nonpolar solvents (1-phenyloctane, n-pentadecane, n-tetradecane, and n-decane) were selected to study the solvent effect on the self-assembly of host-guest system. It is observed that the host-guest system self-assembles into different morphologies at low concentrations due to the solvent adsorption. Only a linear structure can be obtained at high concentration in different kinds of solvents. In particular, the different self-assembled patterns obtained at low concentration will eventually transform to the same linear structure. Owing to the participation of guest molecules, the strong Br···O=C halogen bond is formed between host and guest molecules, further leading to the occurrence of physisorbed monolayers and structural transition. The density functional theory calculations are performed to reveal the underlying mechanisms of halogen bond formation.

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INTRODUCTION

Design and control the molecular ordering and self-assembled pattern of multicomponent two-dimensional (2D) supramolecular systems are of increasing interest due to their potential applications in fabricating molecular nanodevices.1-3 2D host-guest chemistry is a powerful method to control molecular ordering and morphology in multicomponent systems via molecular recognition.4-5 Therefore, it is an active field of research. The surface-confined host-guest structures often form in the following situations: (1) the guest molecules fit well with the caves formed by the host molecule in the size and shape;6-10 (2) there are noncovalent interactions between the host and the guest molecules. Until now, various 2D molecular host-guest structures mediated by non-covalent interactions have been constructed on highly oriented pyrolytic graphite (HOPG) surface and investigated by scanning tunneling microscopy (STM). These non-covalent interactions include van der Waals force,11 hydrogen bond,12-13 dipole-dipole interaction,14-15 and metal-organic interaction.16-18 However, host-guest assembly mediated by halogen bond has not been reported widely.19 Halogen bond, which can be denoted in general as R−X···Y (“R−X” and “Y” represent for the halogen bond donor and acceptor, respectively), is an attractive interaction between the positively charged cap (a so called σ-hole) along the axes of R−X covalent bond and a nucleophilic region of another molecule or the negative potential ring-like region of another halogen atom.20 Halogen bond, due to its high specificity and directionality in geometry, has been vastly investigated in areas of 3D crystal engineering and soft materials.21-25 Recently, the nature of halogen bond has been studied both experimentally and theoretically via the 2D molecular self-assembly at liquid/solid interface.26-32 Our group has demonstrated that different solution concentration could induce different patterns with different halogen bond densities in the self-assembly of thienophenanthrene derivatives.30 Moreover, we found that cooperation and competition of halogen bond with hydrogen bond and van der Waals interaction could significantly tune the self-assembly 3



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nanostructures.26-27 These innovative findings open up new insights in the 2D self-assembly based on intermolecular halogen bonding and provide valuable information about the strength of halogen bonding. However, these works are carried out in solutions containing monocomponent. Halogen bonds formed between multicomponent still remain ambiguous. Thus, it is highly desirable to develop a host-guest system that comprises of halogen bonding sites in both host and guest molecules. In our previous work, the competition of halogen bonding and hydrogen bonding induced 2D self-assembled nanostructures of 5-bromo-2-hexadecyloxy-benzoic acid (5-BHBA) at the liquid/solid interface has been investigated.27 We found that except for the hydrogen bonds between the carboxyl groups, the Br atom could form the halogen bond with the carboxyl group. The published work arouse us to explore whether the halogen bond will be formed between the Br atom and carbonyl group when the carboxyl group of 5-BHBA molecule is displaced by an ester group. In this work, we synthesize the methyl 5-bromo-2-(hexadecyloxy)benzoate (5-BHDB) molecule in order to investigate the 2D molecular self-assembly. 5-BHDB is an ideal molecule to study halogen bond because its ester group and Br atom can provide sufficient halogen bonding sites. The flexible alkyl chain can fit well with the graphite lattice and enhance the molecule-substrate van der Waals interactions. Unfortunately, 5-BHDB molecules cannot form the physisorbed monolayer on the HOPG surface in different solvents (1-phenyloctane, octanoic acid; n-pentadecane, n-tetradecane, and n-decane). In order to fabricate halogen-bonded nanostructures, 1-bromohexadecane (1-BH) is chosen as the guest molecule due to its rational side chain length and a halogen atom. We expect that, by the participation of guest molecule, the 2D self-assembly patterns can be constructed by host-guest halogen bonds. As expected, after 5-BHDB and 1-bromohexadecane are mixed by 2:1 mol ratio, a 2D host-guest self-assembly pattern is obtained. In addition, the self-assembled structures show solvent dependence. Density functional theory (DFT) calculations are employed to estimate the binding energy and stability of dimers, trimers, and tetramers. The results on host-guest system mediated by 4


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halogen bond will be significant in the understanding of halogen bond and provide a route to control 2D molecular ordering.

EXPERIMENTAL SECTION

The synthetic procedure of 5-BHDB molecule is described in Scheme S1. The solvents (1-phenyloctane, octanoic acid, n-pentadecane, n-tetradecane, and n-decane) and guest molecule (1-bromohexadecane) were purchased from TCI and used without further purification. The highly oriented pyrolytic graphite (HOPG, grade ZYB, Bruker, USA) substrate was freshly cleaved using adhesive tape to obtain an atomically flat surface. Pt/Ir wires (80/20) wires were mechanically cut prior to being used as STM tips. The samples for STM measurement were prepared by mixing 5-BHDB and 1-bromohexadecane and dried under vacuum to obtain the mixture powder with 2:1 mol ratio. X-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi (ThermoScientific) was carried out to confirm the composition of the mixture. Then the mixture powder was dissolved in 1-phenyloctane, 1-octanoic acid, n-pentadecane, n-tetradecane, and n-decane (concentration: 10-3 ~ 10-5 mol L-1), respectively. A droplet of solution (~ 2 µL) containing 5-BHDB/1-BH was deposited onto the HOPG surface to obtain the host-guest molecular assembly structures. All the samples were studied by STM with its tip immersed directly into the supernatant immediately. STM images were acquired in constant-current mode under ambient conditions, using a Nanoscope IIIa Multimode SPM (Bruker, USA). Detailed tunneling conditions were given in the corresponding figure captions. All of the STM measurements were repeated at less three times using different tips to check for reproducibility and avoid possible experimental artifacts. The structural models were built by Material Studio 7.0. The models of physisorbed monolayers were constructed by placing the molecules according to the intermolecular distance and angles obtained from the analysis of the high-resolution STM images. Theoretical calculations were performed using DFT provided 5



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by the Gaussian 09 software package. The geometry optimizations of dimers, trimers and tetramers were done through the hybrid M06-2X method and the split-valence polarized 6-31+g(d) basis set. The three-dimensional electrostatic potential map (ESP) was visualized with the VMD program.33

RESULTS

Top view

Side view

Figure 1. (a) Chemical structures of methyl 5-bromo-2-(hexadecyloxy)benzoate (5-BHDB) and 1-bromohexadecane (1-BH). (b) Top and side views of the calculated 3D electrostatic potential map (ESP) of 5-BHDB molecule. The map color scales from −5 kcal mol-1 (blue) to 5 kcal mol-1 (red).

Chemical structures of host molecule (5-BHDB) and guest molecule (1-BH) are displayed in Figure 1a, 5-BHDB molecule has a methyl benzoate functionalized by one hexadecyloxy chain and one bromine atom at the 2 and 5 positions. From the top and side views of ESP map (Figure 1b), the particular potential distribution around the Br atom is clearly observed. The positive charged σ-hole at the tip of bromine atom and the negative potential part of oxygen atom in O=C bond demonstrate the capability of carbonyl group and Br atom to form intermolecular halogen bond. However, when the 5-BHDB was dissolved in different solvents, no self-assembly monolayer was formed on the graphite surface. This indicates that 5-BHDB molecules prefer to stay in the solution phase to minimize the free energy instead of lying on the HOPG 6


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surface. Then we prepared the 5-BHDB/1-bromohexadecane mix powder with the 2:1 mol ratio. The XPS Br3d spectra (Figure S1 and S2) confirm that the mixture powder comprises of two kinds of bromine atom ascribed to 5-BHDB and 1-BH the 2:1 mol ratio. Control experiments reveal that adding 1-BH molecules to the preloaded 5-BHDB solution on the HOPG with the same mol ratio cannot lead to the formation of self-assembly patterns. 1-BH is the liquid phase. When 1-BH molecules are added in the preloaded 5-BHDB solution on the surface, 1-BH molecules prefer to disperse in the solvent instead interact with the 5-BHDB molecules. Self-Assembly of Host-Guest System (5-BHDB/1-BH) at the 1-octanoic Acid/HOPG Interface. To investigate the halogen bonded self-assembled nanostructure between host and guest molecules, a droplet of 5-BHDB/1-BH solution in 1-octanoic acid (about 3.9 × 10-3 ~ 6.9 × 10-5 mol L-1) is deposited on the HOPG surface. The self-assembled pattern which is named as linear I structure emerges immediately and covers the whole scanning area (Figure 2a). The high-resolution STM image shows the molecular packing details (Figure 2b). It is clearly seen from the image that there are independent brighter dots in the trough. According to the size of the dots, each dot consists of three phenyl rings of 5-BHDB molecules. By counting the number of the conjugated core and the side chain, we find some 1-BH molecules marked by red dotted lines in the alkoxy part of the stripe. The length of brighter rods is measured to be 1.9 nm, in agreement with the length of 1-BH molecule. The corresponding molecular model is tentatively proposed as shown in Figure 2c and the co-assembled guest molecules are colored in yellow. The main forces to stabilize these three 5-BHDB molecules are the H···Br···O=C halogen bonds and the Br···Br bond marked as dashed lines in the inset of Figure 2c. Moreover, the ester group of one host molecule adopts different orientation to form the Br···O=C halogen bond with the bromine atom of the guest molecule. The unit cell parameters of the linear I structure are a = 7.1 ± 0.1 nm, b = 1.4 ± 0.1 nm, and α = 79 ± 1°.

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Figure 2. (a,b) Large-scale and high-resolution STM images of the self-assembled pattern at the 1-octanoic acid/HOPG interface. Tunneling parameters: Vbias = 610 mV and It = 490 pA. (c) Proposed model for the linear I pattern. The inset shows the intermolecular halogen bonds (Br···C=O, red; Br···Br, blue; H···Br, brown).

Self-Assembly of 5-BHDB/1-BH System at the 1-phenyloctane/HOPG Interface. 5-BHDB/1-BH could form the same linear I pattern at 1-phenyloctane/HOPG interface at high concentrations (Figure S3a). When 1-phenyloctane is used as the solvent at a relative low concentration (~ 4.7 × 10-4 mol L-1). A double-line structure emerges and dominates the whole surface (Figure 3a). After a close-up inspection, it can be noticed that the monolayer comprises of two double-line structures with subtle differences. These two structures in the STM image are denoted with domain I and II, respectively, and the domain boundaries are illustrated by blue dashed lines.

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Figure 3. (a) Large-scale STM image of the self-assembled pattern at the 1-phenyloctane/HOPG interface. Tunneling parameters: Vbias = 600 mV and It = 490 pA. (b) High-resolution STM image of domain II. Tunneling parameters: Vbias = 630 mV and It = 470 pA. (c) Proposed molecular model for the linear II pattern. Inset shows the intermolecular halogen bonds (Br···C=O, red; Br···Br, blue; H···Br, brown).

For further clarification of these self-assembled structures, high-resolution STM images of two domains were obtained. The high-resolution image of domain I (Figure S4) reveals that this domain consists of the linear I structure. Figure 3b is the high-resolution STM image of domain II termed as linear II structure. It is clear to conclude that this pattern is composed of trimers and tetramers alternately by measuring the size of the bright dots. Two bright dots marked by blue dashed circles could be observed between the stripes. The diameter of each dot is measured to be 0.45 nm, which is in accord with the diameter of benzene ring in 1-phenyloctane molecule. This indicates that 1-phenyloctane molecules co-adsorb at the interval between alkoxyl chains. Carefully observation suggests that all the alkoxyl chains of 5-BHDB molecules adsorb on the surface and one 1-BH molecule co-assembles in every trimer or tetramer. A tentative molecular model is proposed in Figure 3c. The inset in Figure 3c displays the possible intermolecular interactions. The trimer in domain II adopts the same configuration with that in domain I. In each tetramer, one guest molecule forms the Br···O=C halogen bond with the host molecule and the other guest molecule forms the triangular C−Br···O···Br−C bonds with the neighboring host molecule. The 9



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H···Br···O=C halogen bond and the Br···Br type-I bonds are formed to stabilize this structure. After sequential scanning, the linear II pattern still exists in a large-scale area. We deduce that the Br···O=C halogen bonds are strong enough to stabilize the formation of the linear II structure. The unit cell parameters are a = 7.8 ± 0.2 nm, b = 3.8 ± 0.1 nm, and α = 64 ± 1°. Self-Assembly of 5-BHDB/1-BH System at the n-pentadecane or n-tetradecane /HOPG Interface. The formation of 5-BHDB monolayer is investigated at the n-pentadecane /HOPG interface under ambient condition to evaluate the effect of solvent on the formation of 2D host-guest networks. Similarly, 5-BHDB/1-BH could form the same linear I pattern at high concentrations (Figure S3b, S3c). After a droplet of the mixed solution with a low concentration (~5.6 × 10-4 mol L-1) was deposited on the HOPG surface, the mix self-assembled patterns covering the whole scanning area emerged immediately. Figure 4a shows the coexistence of wave-like and double-line patterns which are labelled as domain I and II. After continuous scanning, the domain size of the double-line structure gradually increases at the same scanning area (Figure 4b). The whole surface is exclusively covered with the double-line structure after the sample is scanned for 3 hours. This transition indicates that the double-line structure is more energetically preferred. STM images of energetic favored double-line structure (domain II) are displayed in Figure S5, which are same with the linear I structure discussed above.

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Figure 4. Large-scale STM images of 5-BHDB/1-BH adlayer showing the scanning domain size of domain II grows at the same scanning area. Scan parameters: Vbias = 600 mV and It = 490 pA.

Figure 5a is the large-scale STM image showing the wave-like pattern. The high-resolution image (Figure 5b) reveal that each bright dot consists of two molecules. The conjugated cores of neighboring dimers in each line adopt different configurations. The alkoxy chains in adjacent lamellae are interdigitated to enhance the molecule-molecule van der Waals interactions. It is noteworthy that there are two parallel brighter rods (marked by blue dashed circles) between two dimers in the same line. Each rod is measured to be 1.6 nm and is ascribed to be the coadsorbed n-pentadecane. The tentative molecular model and detailed intermolecular interaction are proposed in Figure 5c. The conjugate cores of two dimers adopt the same bonding mode. In each dimer, two 5-BHDB molecules are antiparallel to form a pair of H···Br···O=C halogen bonds labelled by red and brown dashed lines. Based on the STM images, two n-pentadecane molecules are paralleled to the alkoxy chains and coadsorb in the gap between two dimers which are indicated by blue color to distinguish solvent molecules from solute molecules. Because the length of n-pentadecane molecule is a little shorter than that of the alkoxy chain of 5-BHDB, the pentadecane molecules are fully matching the alkoxy chains to maximize the molecule-solvent van der Waals interaction. Thus, this wave-like structure is driven by the intermolecular H···Br···O=C halogen bonds, and the molecule-solvent van der Waals interactions. The unit cell parameters are a = 4.4 ± 0.1 nm, b = 3.1 ± 0.3 nm, and α = 86 ± 1°.

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Figure 5. (a,b) Large-scale and high-resolution STM images showing the wave-like structure of the co-assembled adlayer for 5-BHDB/1-BH at the n-pentadecane/HOPG interface. Concentration: ~ 5.2 × 10-4 mol L-1. Scanning parameters: Vbias = 580 mV and It = 460 pA. (c) Structure model of the wave-like structure. Inset shows the intermolecular halogen bonds (Br···C=O, red; H···Br, brown).

Experiments are performed in n-tetradecane at low concentrations to investigate the 2D host-guest molecular ordering of the 5-BHDB/1-BH mixture. Similar to the molecular packing of 5-BHDB at the n-pentadecane/HOPG interface, the wave-like structure is obtained at the front period of scanning and gradually transforms to the linear I structure. STM images of the wave-like structure and the linear I structure are obtained (Figure S6). After carefully inspection and comparison, these two molecular patterns are similar with those observed in n-pentadecane. Self-Assembly of 5-BHDB/1-BH System at the n-decane /HOPG Interface.

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Figure 6. (a,b) Large-scale and high-resolution STM images of 5-BHDB/1-BH physisorbed monolayer in n-decane on the HOPG surface. Concentration: ~ 3.7 × 10-4 mol L-1. Scanning parameters: Vbias = 600 mV and It = 570 pA. (c) Host-guest structural model of the linear III pattern. (d,e) Large-scale and high-resolution STM images of 5-BHDB/1-BH displaying the linear I structure in n-decane. Concentration: ~ 3.7 × 10-4 mol L-1. Scanning parameters: Vbias = 580 mV and It = 560 pA. (f) Host-guest structural model of the linear I pattern. Insets show the intermolecular bonds (Br···Br, blue; Br···C=O, red; H···Br, brown).

In order to further evaluate the formation of the wave-like pattern resulting from the coadsorption of alkyl chains instead of the 1-HB molecules, the physisorbed monolayer of 5-BHDB/1-BH at the n-decane/HOPG interface was further investigated at high (Figure S3d) and low (Figure 6) concentrations, respectively. A new uniform linear structure (linear III) covering the whole surface is observed (Figure 6a). The detailed self-assembly pattern of 5-BHDB is revealed in the high-resolution STM image (Figure 6b). Four 5-BHDB moieties form a tetramer as the basic unit of the molecular assembly. The side chains in neighboring lamellae are adsorbed by a tail-to-tail fashion. The dark gap between neighboring lamellae is measured to be 0.7 nm which is wider than that in the generally densely-packed structure. A careful 13



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observation reveals that there are dislocated bright rods (labelled by red dashed lines) coadsorbed which are measured to be the guest molecules. On the basis of the chain length and the intermolecular noncovalent binding mode, a molecular model is proposed (Figure 6c). The inset shows the 5-BHDB molecules form cyclic Br···Br type-II halogen bonds (blue dashed lines) in each tetramer. 1-BH molecules occupy the space between the alkoxy chains of 5-BHDB and form a “bridge” with another 1-BH molecule in adjacent lamellae via Br···Br type-I bond (blue dashed line). These intricate noncovalent interactions co-stabilize the linear structure. We speculate that n-decane molecule is too short to form strong enough van der Waals forces and co-assemble on the HOPG interface. The unit cell parameters of the linear III pattern are a = 4.7 ± 0.2 nm, b = 0.8 ± 0.1 nm, and α = 84 ± 1°. After a period of scanning, the linear I structure (Figure 6d-f) emerges and displaces the linear III structure. Finally, this linear I structure covers the whole area. The result indicates that the linear I pattern is energetic favorable than uniform linear III structure. To further demonstrate our speculation that linear I structure is thermodynamically favored, control experiments are carried out at different temperatures. The STM samples are prepared at 10°C and 35°C, respectively. STM images (Figure S7) exhibit that the structural transition from the linear III pattern to the linear I pattern is faster at higher temperature.

DISCUSSION

For 5-BHDB, two functional groups (ester group and Br atom) are capable of halogen bond formation and effectively involved in the formation of each self-assembly structure (Figure 7). Among these structures, the wave-like and the linear III structures only exist at a relatively low concentration and will evolve to the linear I structure, which indicates the linear I structure is more energetically preferred. As pointed out previously, solution of pure 5-BHDB and mechanically mix 5-BHDB solution with 1-BH molecule cannot lead to the formation of physisorbed monolayers. This result reveals that the halogen bonds between host 14


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and guest molecules play a significant role to induce the self-assembly process.

Figure 7. Optimized structures of 5-BHDB complexes observed from STM images. (a-d) trimer I, tetramer I, tetramer II and dimer I, respectively. Dashed lines represent the noncovalent interactions (Br···Br, blue; Br···C=O, red; H···Br, yellow).

Solvent Effect on Host-Guest Assembled Patterns. Except for intermolecular halogen bonds, solvent plays a dominant role in the formation of self-assembly polymorphs. The formation of linear II, wave-like and linear III structures further confirms the solvent effect. The dissolution of mixed host-guest powder in different solvents can temporarily break the host-guest intermolecular interactions. The cooperative and competitive of noncovalent interactions between solvent and host-guest molecules induce the emergency of self-assembly patterns at the initial assembly stage. Especially, when the STM measurements are carried out at low concentration, molecular dynamic stable structures are observed firstly because the number of solvent molecule is much larger than that of 1-BH molecule. However, with the scanning time going, more guest molecules gradually take part in the assembly process and replace the solvent molecules. This structural transition indicates that the halogen bonds formed between host and guest molecules are stronger and more energetically favored than the van der Waals interaction and other interactions between solvent and host

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molecules. Specifically in the linear II structure, solvent molecule (1-phenyloctane) and guest molecule (1-BH) are coadsorbed. Host-guest molecules form halogen bonds to stabilize this structure. Solvent molecules fill the vacancy between side chains to elevate the pattern density. This three-component structure is formed due to the low affinity of 1-phenyloctane and graphite lattices. When n-pentadecane and n-tetradecane are used as solvents, the wave-like structures which are comprised of 5-BHDB and solvent molecules are observed. The capability of n-pentadecane and n-tetradecane molecules to co-assemble with host molecules reveals that the molecule-solvent van der Waals force are strong enough to stabilize the wave-like structure. Moreover, in the linear III pattern, the host-guest co-assembled structure is observed without the participation of n-decane. The carbon chain of n-decane is so short that it cannot provide enough van der Waals forces. Although the host-guest structure is formed, the intermolecular interactions are different with that in the linear I structure. In the linear III pattern, host molecules form cyclic type-II halogen bonds while guest molecules form type-I bonds. The results demonstrate that the C−Br···Br halogen bond could be more stable in the nonpolar solvent. DFT Calculations. Density functional theory (DFT) calculations are performed at M06-2X/6-31G(d) level to determine the binding energy and optimal geometry for complexes, which allows us to illustrate whether these halogen bonds are sufficient to stabilize these structures and gain insight into the role of halogen bonding in the formation of 2D monolayers. For the sake of simplicity, the hexadecyl groups are replaced by methoxy groups in the 5-BHDB molecules. Table 1 and Figure 7 list the DFT results for the representative 5-BHDB complexes in the polymorphic configurations observed in the physisorbed monolayers. By comparing the distance between Br atoms and oxygen or hydrogen atoms with the sum of van der Waals radii of each atom, the Br···Br and Br···O=C halogen bonds and the H···Br hydrogen bonds are further verified among these building blocks. In trimer I which is the building block of the linear I structure, a carbonyl group and two Br atoms constitute a triangular halogen-bonded motif. The triangular 16


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binding model is the crucial motif to induce the formation of trimer I and stabilize the monolayer. The binding energy is calculated to be −18.23 kJ mol-1. For the linear II pattern, trimer I and tetramer I are the building blocks. Tetramer I is formed based on the Br···O=C halogen bonds and the H···Br hydrogen bonds. The binding energy is calculated to be −18.29 kJ mol-1. In dimer I, a pair of equivalent H···Br···O=C halogen bonds are observed and the binding energy is −9.81 kJ mol-1, which is higher than that of trimer I. Two pairs of homogeneous Br···Br halogen bonds are formed in tetramer II. These four bonds constitute a rectangular motif, which is the main driving force to form tetramer II. The binding energy of tetramer II is calculated to be −10.54 kJ mol-1. The structural transform to the linear I pattern occurs due to the lower binding energy of trimer I compared with the binding energies of dimer I and tetramer II, as well as the participation of the guest molecule. The formation of the Br···O=C halogen bond between 5-BHDB and 1-BH molecules play another important role.

Table 1. Calculated Building Block, Geometry Parameters and Interaction Energies in Different Complexes for the Studied System Trimer I

Tetramer I

Dimer I

Tetramer II

Host-guest

Guest-guest

Br···Br

Br···O=C

Br···Br

2.94

2.91

177

154

−4.96

−0.07

Building Block Bond

Br···O=C

Br···Br

H···Br

2.88 Distance (Å)

3.19

3.48

2.67

157

∆E (kJ mol-1)

114

−18.23

3.36 3.04

2.69 3.72

2.80

100

115 166

163

H···Br

118

155 105

Br···O=C

2.67

2.71 2.97

165

(°)

H···Br

3.31

2.97

Angle

Br···O=C

161

111 93

114

−18.29

−9.81 17


−10.54


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To further demonstrate the importance of guest molecule to induce the structural transition, tentative models which reflect the noncovalent interaction between host and guest molecules are proposed based on the linear I and linear III patterns (Figure 8). The Br···O=C halogen bond is verified between host and guest molecules. The geometry parameters are measured to be 2.94 Å, 177°. The binding energy of this halogen bond is calculated to be −4.96 kJ mol-1. It is well-established in studies of Br···O=C halogen bond that the ideal C−X−O angle is 180°.34 This value derives from the location of the electropositive region of the halogen (σ-hole) directly opposite the C−X bond. Only the C−Br−O (O=C) angle formed between host and guest molecule is close to 180° (177°), indicating that the halogen bond is stronger than other heterogeneous halogen bonds due to the intermolecular steric hindrance. As for possible Br···Br type I bond in the linear III pattern, the binding energy is only −0.07 kJ mol-1. Such low energy indicates that the Br···Br bond is not the driving force. The interchain van der Waals forces between host and guest molecules are formed, however, that are not strong enough to sustain the linear III pattern. We conclude that the formation of the Br···O=C halogen bond leads to the decline of binding energy in the linear I structure and induces the formation of 5-BHDB/1-BH adlayer. The relatively low binding energies of trimer I and host-guest binding motif induce the structural transformation.

Figure 8. (a) Intermolecular binding model of host and guest molecules which is extracted form structural model of the linear I structure. (b) Optimized host-guest binding model. The side chains are simplified by methyl group. (c) Optimized binding model for guest molecules.

CONCLUSION

In summary, we have accomplished STM experiments of 5-BHDB/1-BH mixture at the liquid/graphite 18


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interface. 5-BHDB acts as the host molecule and 1-BH acts as the guest molecule. At the 1-octanic acid/HOPG interface, the energetically favored linear I structure was observed. The linear I structure formed based on the intermolecular halogen bonds and the host- guest halogen bond. We found that 5-BHDB and 1-BH could form the linear structure with trimers (linear I) in different solvents under high solution concentrations. Under low concentrations, another kind of double-line host-guest nanostructure composed of trimers and tetramers (linear II) formed at the 1-phenyloctane/HOPG interface. The solvent molecule (1-phenyloctane) coadsorbed to fill the gap between side chains in the linear II pattern. In n-pentadecane and n-tetradecane, the wave-like structures were observed resulting from the solvent coadsorption. The molecule-solvent van der Waals interactions and the intermolecular halogen bonds dominate the formation of coadsorbed patterns. The linear III pattern formed when n-decane was employed to investigate the solvent effect of this host-guest system. This pattern was driven by the Br···Br type-I bond and the Br···Br type-II halogen bond. Prolonging the scanning time, the linear I pattern emerged and gradually replaced the linear II, linear III and wave-like structures. This finding reveals that the host-guest linear I structure is the most thermodynamically stable than other structures due to the formation of Br···O=C halogen bond between host and guest molecules. In combination with DFT calculations, it was concluded that halogen bonds induced the formation of each structure, and the presence of relatively stronger host- guest halogen bonds played a crucial role to stabilize these nanostructures and induced the structural transition. We believe that the systematic study in host-guest system based on halogen bond not only provides strategies in designing and controlling multicomponent self-assembled patterns at liquid/solid interface, but also constitutes a vital step toward technological applications in fabricating electronic nanodevices. ASSOCIATED CONTENT Supplementary Information Available: Molecular synthesis, XPS spectra, and additional STM images. This material is available free of charge via the Internet at http://pubs.acs.org. 19



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

Acknowledgements Financial supports from the National Natural Science Foundation of China (51373055and 21573077), the Natural Science Foundation of Guangdong Province (2018A030313452) and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged.

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GRAPHIC ABSTRACT

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Solvent Effect on Host-Guest Two-Dimensional Self-Assembly

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