Supramolecular Self-Assembly of Hexaphenylbenzene Derivatives

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Supramolecular Self-Assembly of Hexaphenylbenzene Derivatives with Different Symmetry and Number of Carboxylic Acid at Liquid/Solid Interfaces Lixin Cai, Lian-Cheng Wang, Shi-Zhao Kang, Yanfang Geng, Ke Deng, Qi-Yu Zheng, and Qingdao Zeng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06605 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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The Journal of Physical Chemistry

Supramolecular SelfAssembly of Hexaphenylbenzene Derivatives with Different Symmetry and Number of Carboxylic Acid at Liquid/Solid Interfaces Lixin Cai,†,§Liancheng Wang,ϕShizhao Kang,†Yanfang Geng,*§Ke Deng,*§Qiyu Zheng*ϕ and Qingdao Zeng*§ †

School of Chemical and Environmental Engineering, Ministry of Education, Shanghai Institute

of Technology, HaiQuan Road 100, Shanghai 201418, P. R. China §

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for

Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, P. R. China ϕ

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular

Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190, P. R. China

Corresponding Author *Qingdao Zeng, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China; Tel: 86-10-82545548; E-mail: [email protected] *Ke Deng, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China; Tel: 86-10-82545550; E-mail: [email protected]

ACS Paragon Plus Environment

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*Qiyu Zheng, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing 100190,China;Tel:86-10-62652811; E-mail: [email protected] *Yanfang Geng, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China; Tel: 86-10-82545691; E-mail: [email protected]


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ABSTRACT: Functional molecules, especially with carboxyl groups are crucial in building supramolecular structures. It is great important to study the effect of the symmetry, number of carboxyl groups on the self-assembly behavior of corresponding molecules. A series of hexaphenylbenzene (HPB) derivatives (HPB-1,3,5-3A, HPB-1,2,4-3A and HPB-1,4-2A) substituted with different number of carboxyl groups at different position have been synthesized and their self-assembled structures were investigated at both 1-phenyloctane/HOPG and heptanoic acid/HOPG interfaces by using scanning tunneling microscopy (STM) technique. The self-assembled mechanisms of these HPB-based compounds were further studied with the help of density functional theory (DFT) calculations. The results indicate that symmetry and number of carboxyl groups as well as solvent play a significant role on the tuning self-assemble process resulting in various structures.


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Introduction The prerequisite for applications of various molecular devices is constructing controllable surface nanostructure at the scale of molecule, especially for the fabrication of functional devices with multi-components.1-2 The self-assembly technique is one of the most important approach to obtaining supramolecular nanostructures with diverse dimensions.3 In general, weak noncovalent forces between molecules, such as hydrogen bond, π-π stacking, electrostatic interactions, metal-ligand coordination interactions and van der Waals forces,4-7 form the molecular architectures. Due to the properties of high selectivity, directivity and moderate strength, there have been a number of reports studying on molecular self-assembles based on hydrogen bond.8-10 The formed networks based on hydrogen bond with large cavities can be used to accommodate various guest molecules.11-12 There are several forms of hydrogen bonds such as =O⋅⋅⋅H−O−, =O⋅⋅⋅H−N−, −N−H⋅⋅⋅N− and so on.13 The carboxylic acid group −COOH is usually considered as a promising interaction site by forming hydrogen bond. With the help of scanning tunneling microscopy (STM) technique, the influence of hydrogen bonds on the self-assembled structures has been partially understood. The design and synthesis of functional molecules, especially that consisting of phenyl and carboxyl groups are of great interest and have received special attention because of their rigid structure and versatile chemistry.14-20 As previous reports, trimesic acid (TMA), with one central benzene ring and C3 symmetrical carboxyl groups, could form hexagonal network based on intermolecular hydrogen bonds.21-22Hexaphenylbenzene (HPB), with star-shaped polyphenylenes as the central core, has recently been designed as the building block of porous crystal.23-25 Furthermore, HPB-based compounds are highly symmetrical molecules that can exhibit specific arrangement substituted with interactive groups.26-30 When six peripheral benzenes are


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substituted with carboxyl groups, the molecules can form supramolecular networks through hydrogen-bond pairs between carboxyl groups at the liquid/HOPGinterface.31 Therefore, the carboxylic acid group plays a crucial role on building new network structures. Although the self-assembled structures of benzene-based carboxylic acid derivatives have been reported by far, the assembly behavior of molecules directly connected with two or more carboxyl groups has rarely been reported. Additionally, the arrangement and amount of the phenyl group should also affect the self-assembled structure due to π-π stacking. In order to further clarify the influence of hydrogen bond on the self-assembled structure, it is meaningful to study the effect of number and position of carboxylic acid functional groups on the selfassembled structure. On the other hand, solvent could play a significant role on controlling and tuning the two dimensional (2D) self-assembly process as well as the final structures because the presence of solvent makes the supramolecular self-assembly more available and diverse.32-42 In this contribution, three kinds of HPB derivatives (HPB-1,3,5-3A, HPB-1,2,4-3A and HPB-1,4-2A) containing different number carboxyl groups at different position were designed and synthesized as shown in Scheme 1. HPB-1,3,5-3A molecule has three symmetrical carboxyl groups at 1, 3 and 5 position, HPB-1,2,4-3Acontains three asymmetrical carboxyl groups at 1, 2 and 4 position, while HPB-1,4-2A only includes two carboxyl groups at para-position. The selfassembled structures have been studied at both heptanoic acid (HA)/HOPG and 1-phenyloctane (PO)/HOPG interfaces by using STM technique. The influence of relative position of carboxyl groups on the molecular self-assembly as well as the solvent effect will be verified based on density functional theory (DFT) calculations.


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O O

OH

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O

OH

OH

O OH

HO

O O

OH

HO

HPB-1,3,5-3A

O

HO

HPB-1,2,4-3A

O

HPB-1,4-2A

OH O

PO

HA

Scheme 1. Chemical structures of HPB-based compounds HPB-1,3,5-3A, HPB-1,2,4-3A,HPB1,4-2A and solvents PO and HA. Experimental and theoretical methods Materials HPB-1,3,5-3A and HPB-1,2,4-3A were synthesized with a similar route according to the literature method.43HPB-1,3,5-3A. HPB-1,4-2A was synthesized as follows. Scheme 2 exhibits the detailed synthetic routes. The 1H-NMR and

13

C-NMR characterizations (Figure S1-S9)are

shown in the supporting information.


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COOMe

COOMe COOMe

COOMe

PhOH, AlCl3

hydrolysis

toluene

COOMe (a)

HPB-1,2,4-3A

COOMe (b) COOEt

Br

(HO)2B

COOEt

hydrolysis

HPB-1,4-2A

Suzuki coupling Br COOEt (d)

(c)

Scheme 2. Synthetic routes of HPB-1,2,4-3A and HPB-1,4-2A Synthesis of HPB-1,2,4-3A. Compound (a) was prepared according to the literature method.44 1.26 g (a) (1.44 mmol), 487 mg PhOH (5.18 mmol) and 1.92 g AlCl3 (14.38 mmol) were added into a 100 ml two-necked round bottle. The flask was degassed by three evacuationAr-backfilled cycles. 60 ml anhydrous toluene was added and the flask was again degassed by three evacuation-Ar-backfilled cycles. The reaction mixture was reacted at 30 °C for 40 hrs, and then poured into dilute hydrochloric acid solution. The aqueous solution was extracted with ethyl acetate. The organic phase was washed with water and brine, respectively, then dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by column chromatography to give 980 mg white solid (b) in 96% yield. Compound (b) was subsequently hydrolyzed with 30 equivalent NaOH in a THF/water 1:1 solution at reflux to afford HPB-1,2,4-3A in 97% yield. Synthesis ofHPB-1,4-2A. Compound (c) was prepared according to the previously reported method.451.08 g (c) (2 mmol) and 1.55 g 4-ethoxycarbonylphenylboronicacid (8 mmol) were


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added into a Schlenk tube. 115.6 mg Pd(PPh3)4 (0.1 mmol), 1.38 g K2CO3 (10 mmol) and 30 ml dioxane were added under argon. After three freeze-pump-thaw cycles, the mixture was reacted at 85 °C overnight. A white solid product (d) was obtained by column chromatography with a PE/dichloromethane eluent. Yield: 975 mg, 72%. Compound (d) was subsequently hydrolyzed with 20 equivalent NaOH in a THF/water 1:1 solution at reflux to afford HPB-1,4-2A in 98% yield. STM measurements These three molecules were dissolved in solvent PO and HA with concentration less than 1 mM, respectively. Highly oriented pyrolytic graphite (HOPG, grade ZYB) purchased from Agilent (USA) was used as substrate. All the samples were prepared by depositing a droplet (5 µL) of the corresponding solution onto the freshly cleaved HOPG surface. STM measurements were performed by using an Agilent scanning tunneling microscope (Nanoscope IIIa, USA). The tips were mechanically formed from a Pt/Ir wire (80/20, diameter 0.25 mm). All STM images were recorded in constant current mode at room temperature. The specific tunneling conditions were presented in the corresponding figure captions. Computational details The theoretical calculation was performed by using density functional theory (DFT) provided by the DMol3code.46We used the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work. The Perdew and Wang Parameterization of the local exchange correlation energy are applied in the local spin density approximation (LSDA) to describe exchange and correlation.47We expanded the all-electron spin-unrestricted Kohn-Sham wave functions in a local atomic orbital basis. In such double-numerical basis set polarization


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was described. All calculations were all-electron ones, and performed with the Extra-Fine mesh. Self-consistent field procedure was done with a convergence criterion of 10-5a.u. on the energy and electron density. Combined with the experimental data, we have optimized the unit cell parameters and the geometry of the adsorbates in the unit cell. When the energy and density convergence criterion are reached, we could obtain the optimized parameters and the interaction energy between adsorbates. To evaluate the interaction between the adsorbates and HOPG, we design the model system. In our work, since adsorption of the adsorbates on graphite and graphene can be considered as very similar, we have performed our calculations on infinite graphene monolayers using PBC. In the superlattice, graphene layers were separated by 40 Å in the normal direction and represented by orthorhombic unit cells containing two carbon atoms. When modeling the adsorbates on graphene, we used graphene supercells and sampled the Brillouin zone by a 1 × 1 × 1 k-point mesh. The interaction energy Einter of adsorbates with graphite is given by Einter = Etot(adsorbates/graphene) − Etot(isolated adsorbates in vacuum) − Etot(graphene). Results and discussion


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Figure 1.(a) Large-scale STM image of HPB-1,3,5-3A monolayers at the PO/HOPG interface (Vbias = 594.5 mV, Iset = 289.9 pA, Scale bar = 10 nm). (b) The statistical percentage for yellow pentagon, blue hexagon, green heptagon and red octagon structures formed by HPB-1,3,5-3A in STM image (a). (c) High-resolution STM image of HPB-1,3,5-3A self-assembled structure (Vbias = 688.9 mV, Iset = 299.1 pA, Scale bar = 2 nm). (d) Suggested molecular model for the structure marked with blue hexagons. After a droplet of dilute PO solution containing HPB-1,3,5-3A molecules was deposited onto the HOPG surface, large-scale complex network with multiple cavities were recorded as shown in Figure 1a. With careful inspection, there are mainly four types of cavities, including pentagon,


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hexagon, heptagon, and octagon structure. Further statistical analysis has been performed for these structures (Figure1b). Obviously, the main structures are the hexagonal structures with an approximate percentage of 60%. The pentagonal and heptagonal structures account for about 20 percent of the network, respectively. The amount of octagonal structure is the least, and only less than 1%. The detailed structures with high-resolution are shown in Figure 1c. The pentagonal, hexagonal, heptagonal and octagonal structures are marked with orange pentagon, blue hexagon, green heptagon and red octagon, respectively. Evidently, each bright hollow circle on the vertex corresponds to one HPB-1,3,5-3A molecule. The circle center presents low-contrast feature, possibly due to the fact that the peripheral benzene rings tend to rotate by certain angle from the central plane to decrease the steric hindrance, and accordingly the peripheral benzene rings are higher than the central benzene ring. The measured side lengths of these cavities are 1.8 ± 0.1 nm, indicating that HPB-1,3,5-3A molecules directly connect with each other through hydrogenbond pairs. Each HPB-1,3,5-3A molecule can interact with three neighbouringHPB-1,3,5-3A molecules by forming three pairs of hydrogen bonds. The hydrogen bonding between a HPB1,3,5-3A dimer is sufficiently strong (about -33.085 kcal mol-1) to connect the two molecules tightly. Every six HPB-1,3,5-3A molecules form a hexagonal network via intermolecular hydrogen-bond pairs. The unit cell is superimposed on the STM image with a = b = 3.2 ± 0.1 nm and α = 60 ± 1º. An optimized molecular model by DFT method corresponding to the hexagonal structure marked by the blue hexagon is shown in Figure1d.


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Figure 2.(a), (b) and (c) Structure models for the self-assembled pattern 6-6-6, 7-5-6 and 8-5-6. It is worth pointing out that the pentagon, heptagon, and octagon rings, however, are irregularly distributed in the hexagonal network. In Figure 1c, we can notice that a hexagonal ring is connected together with a pentagonal ring and a heptagonal ring with a common vertex (denoted as “7-5-6” circle), while a hexagonal ring, a pentagonal ring and an octagonal ring together form “8-5-6” circle. It has been rarely observed that such complex structures coexisted in the previous reported self-assembled structures. To compare with 7-5-6 and 8-5-6 structure, we selected the similar structure from the hexagonal network in which three hexagonal rings were connected with a common vertex and denoted it as "6-6-6" circle. The optimized molecular model by DFT method for these three kinds of structures denoted as 6-6-6, 7-5-6 and 8-5-6 are shown in Figure 2a, Figure 2b and Figure2c. Table 1. Total energies and energies per area for the observed three kinds of structures in HPB1,3,5-3A self-assembly

6-6-6 7-5-6 8-5-6

The interaction between adsorbates (kcal mol-1) -686.688 -677.554 -694.280

The interaction between adsorbates and substrate (kcal mol-1) -570.544 -575.470 -617.641

Total energy (kcal mol-1)

Total area Å2

-1257.232 -1253.024 -1311.921

2520.460 2562.025 2950.364

Energy per area (kcal mol1 -2 Å ) -0.499 -0.489 -0.445


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It is interesting to investigate why these three kinds of structures could coexist in the network. To answer this question, DFT calculations have been performed, and the related interaction energies were presented in Table 1. The total energies were calculated based on the interactions between the molecules as well as the interactions between the molecules and the substrate. As we know, more negative energy per area indicates that the assembled structure is more stable. The energy per area of 6-6-6 system is the lowest (-0.499 kcal mol-1 Å-2), which is less than that of 7-5-6 system (-0.489 kcal mol-1 Å-2) and 8-5-6 system (-0.445 kcal mol-1 Å-2). It means that the 6-6-6 system is more stable than 7-5-6 and 8-5-6 systems. Considering that the discrepancy between the energy per area of 6-6-6 system and 7-5-6/8-5-6 system is small, we could observe 7-5-6/8-5-6 system in the network. Consequently, due to the energetically favorable stability, the amount of the hexagonal ring is much more than that of other three rings formed by HPB-1,3,5-3A molecule on HOPG surface.


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Figure 3. (a) Large-scale STM image of HPB-1,2,4-3A monolayers at the PO/HOPG interface (Vbias = 653.1 mV, Iset = 275.5 pA, Scale bar = 10 nm). (b) High-resolution STM image corresponding to image (a) (Vbias = 688.9 mV, Iset = 299.1 pA, Scale bar = 2 nm). (c), (d), (e) and (f) Molecular models of domain I, II, III and IV.


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HPB-1,2,4-3A molecule has three asymmetrically substituted carboxyl groups at the 1, 2 and 4 positions, which is different with the 1,3,5-positions in HPB-1,3,5-3A molecule. Large-scale STM image as shown in Figure 3a reveals different self-assembled structure formed by HPB1,2,4-3A molecule at the PO/HOPG interface. The bright hollow circles should be ascribed to the HPB-1,2,4-3A molecules. In Figure 3b, there are mainly two kinds of distances (d1 and d2) between two adjacent HPB-1,2,4-3A molecules, in which d1 is measured with 1.8 ± 0.1 nm and d2 is 1.2 ± 0.1 nm. Similar to HPB-1,3,5-3A, d1 corresponds to the distance between two adjacent HPB-1,2,4-3A molecules with hydrogen bond pairs. The shorter distance d2 might be attributed to the close interdigitation of two HPB-1,2,4-3A molecules. From STM image in Figure 3b, four local ordered patterns (pattern I, II, III and IV) can be observed. In domain I, each HPB-1,2,4-3A molecule is located in the center of a hexagonal ring with the distance d1. In domain II, molecules with the distance d1 form two adjacent rows. In domain III, two HPB1,2,4-3A molecules interdigitate with each other with the distance d2, and then such pairs are arranged into rows with the distance d1 between pairs. In domain IV, pairs of molecules with the distance d2 only form into one row. Optimized models of these three kinds of patterns are shown in Figure 3c, Figure 3d, Figure 3e and Figure 3f.

Table 2. The calculated total energies and energies per area for the observed four structures of HPB-1,2,4-3A as shown in Figure 3

I II III

The interaction between adsorbates (kcal mol-1)

The interaction between adsorbates and substrate (kcal mol-1)

-86.754 -71.658 -143.805

-79.989 -63.722 -124.52

Total energy (kcal mol-1) -166.743 -135.38 -268.325

Energy per area (kcal mol-1 Å-2) -0.298 -0.298 -0.300


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IV

-70.032

-67.256

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-137.288

-0.302

For the molecular models, we define the direction along the para-substituted (1, 4) carboxylic acid as x, the direction along the only substituted carboxylic acid as y. In all four patterns, along x direction, HPB-1,2,4-3Amolecules connect with each other by hydrogen bond pairs and form molecular wire. If the molecular conformations along y direction in the same wire are alternately arranged, then the hydrogen bond pairs are alternately formed, and HPB-1,2,4-3A molecules can assemble into structure I (Figure 3c). Otherwise, with the same molecular conformation along y direction, two adjacent wires connect with each other by hydrogen bond pairs resulting in structure II. Notably, only hydrogen bonding interactions appear in structure I and II. However, in structure III and IV, two adjacent wires with the distance d2interdigitate with each other by ππ stacking between the unsubstituted phenyls (marked by the blue circle), in which the carboxyl groups form hydrogen bond trimers (marked by the red circle). In structure III, half of the carboxyl groups at ortho-position along y direction participate in hydrogen bond trimers (Figure3e). While in structure IV, all the carboxyl groups at ortho-position participate in hydrogen bond trimers (Figure3f). The π-π stacking interaction is about -11.255 kcal mol-1. It should be noted that the hydrogen bond trimer plays an important role on stabilizing the structure III and IV, which even contributes about -50.399 kcal mol-1 to the system. Although we have not observed the expanded network of the four patterns, we still present the related interactions and energies in Table 2. The energies per area of these structures are nearly equal. Thus, we could observe the four structures dispersedly coexist in Figure3b. Obviously, such structural diversity is caused by the directional uncertainty of the substituted carboxylic acid group at ortho-position along y direction. If we remove such uncertainty, the


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structure is stable or not? And then, HPB-1,4-2A molecule contains two carboxylic acids groups at the para-position of the peripheral phenyls was designed. After dropping a droplet of HPB1,4-2A PO solution, however, no assembled structure can be obtained. This phenomenon not only indicates that the substituted carboxyl groups at ortho-position are important on stabilizing of the structure, but also proves the importance of molecular design. Considering the effect of solvent,32, 48,49the other solvent HA was selected to be used to dissolve the molecule in order to study the self-assembled structure of HPB-1,4-2A molecule.

Figure 4.(a) Large-scale STM image of HPB-1,4-2A monolayers at the HA/HOPG interface (Vbias = 688.9 mV, Iset = 299.1 pA, Scale bar = 10 nm). (b) High-resolution STM image of HPB1,4-2A assembled structure (Vbias = 688.9 mV, Iset = 299.1 pA, Scale bar = 2 nm). (c) The suggested structure model of HPB-1,4-2A self-assembly on HOPG surface.


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As presented in Figure 4a, a large-scale supramolecular structure on HOPG surface can be observed. The detailed assembly structure revealed by high-resolution STM image is shown in Figure 4b. The diameter (d) of the bright circle is measured to be around 1.7 ± 0.1 nm, which presumably corresponds to the backbone of HPB-1,4-2A molecule. As shown in Scheme 1, one HPB-1,4-2A molecule only contains two carboxyl groups at the para-position. If two adjacent HPB-1,4-2A molecules connect with each other by forming hydrogen bond pair, the dark stripe between two HPB-1,4-2A molecules in Figure 4b should only accommodate a parallel hydrogen bond pair along one direction. However, the length (l) between two HPB-1,4-2A molecules is almost the same and estimated to be 2.9 ± 0.1 nm, which is much larger than the length of a dimer with hydrogen bond pair calculated above. Therefore, there should be other molecules to participate in the self-assembly of HPB-1,4-2A molecules on the surface. After carefully analyzing the experimental process, we suggest that the solvent HA take part in the assembly. DFT calculations have been performed to investigate the self-assembled structure of HPB1,4-2A molecules with the presence of HA. As shown in Figure 4c, a proposed structure model of the self-assembled HPB-1,4-2A monolayer is presented. Clearly, two adjacent HPB-1,4-2A molecules did not directly interact with each other through the carboxyl groups, but loosely arranged by bridging of HA molecule. Each carboxylic acid group of HPB-1,4-2A molecule interacts with a HA molecule by forming hydrogen bond pair. The hydrogen bonding interaction between HPB-1,4-2A and HA (-45.194 kcal mol-1) is more stable than that between HPB-1,4-2A dimer (-34.254 kcal mol-1), which leads to the preferable binding between HPB-1,4-2A and HA. Particularly, with the matching size, HA dimers locate side by side with the HA bridging with the carboxylic acid group of HPB-1,4-2A in the cavity, and strongly enhance the stability of coassembly of HPB-1,4-2A and HA molecules. When there is no HA dimer adsorbed on


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HOPGsurface, some defects can be detected in the large-scale STM image as shown in Figure 4a. This result not only further confirms the importance of HA solvent, but also explains why no assembled structure can be observed when PO is used as solvent for HPB-1,4-2Amolecules. Consequently, HPB-1,4-2A molecules formed a large-scale network structure with rhombic cavities at HA/HOPG interface. Based on the suggested molecular model, a unit cell with the parameters of a = 2.8 ± 0.1 nm, b = 4.8 ± 0.1 nm, α = 90 ± 1° was marked on the STM image (Figure 4b). Undoubtedly, solvent dramatically changes the self-assembled behavior of HPB-1,4-2A molecules in terms of the interaction sites. In order to further study the influence of solvent on the network structure, it is necessary to discuss solvent effect on the assembly of other molecules (HPB-1,2,4-3A and HPB-1,3,5-3A). As mentioned above, there are three peripheral phenyls substituted with carboxyl groups in HPB-1,2,4-3A and HPB-1,3,5-3A. After these two molecules were dissolved into HA solvent, and the surface of HOPG was scanned upon depositing the corresponding solution onto the surface.


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Figure 5.(a) Large-scale STM image of HPB-1,2,4-3A monolayers at HA/HOPG interface (Vbias = 688.9 mV,Iset = 299.1 pA, Scale bar = 10 nm). (b) High-resolution STM image of HPB-1,2,43A self-assembled structure (Vbias = 688.9 mV, Iset = 299.1 pA, Scale bar = 2 nm). (c) Suggested molecular model for HPB-1,2,4-3A architecture in STM image (b). For HPB-1,2,4-3A molecule, a large-scale STM image (Figure 5a) reveals the well-ordered lamellar self-assembled structure. Figure5b shows a high-resolution STM image, in which bowknot-shaped dimers consisting of two adjacent bright circles arrange into rows. Similar with HPB-1,4-2A molecule, the bright circles can be assigned to HPB-1,2,4-3A molecules with the diameter of 1.7 ± 0.1 nm. Because there are three asymmetrical carboxyl groups in HPB-1,2,43A molecule, it is very necessary to know the distances between one molecule and other molecules around it. As shown in Figure 5b, the molecule marked with red circle was selected as


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a reference. The distance (d1) between the centers of the molecule marked red circle and the molecule marked with green circle was measured to be 1.8 ±0.1 nm. The distances (d2) between the molecule marked with red circle and neighboring molecule marked with blue circle was estimated to be 2.1 ± 0.1 nm. Therefore, it can be inferred that the molecule marked with red circle connects with the molecule marked with green circle through intermolecular hydrogen bond pair. The solvent might participate in the assembly process and lie in the dark stripe between the red and blue circles. The fine interacted structure will be clarified through DFT calculations in the following. The self-assembled structure model based on the DFT calculations is presented in Figure 5c. Every two HPB-1,2,4-3A molecules interact with each other via hydrogen bond between one of the three carboxyl groups and form a bowknot-shaped dimer. The solvent molecules really take part in the assembly and interact with other two carboxyl groups of HPB-1,2,4-3A molecule through forming hydrogen-bond pairs. Although HA dimer could not participate in the cavity due to the narrow gap between two rows, the alkyl chains of HA molecules bridging with HPB1,2,4-3A arrange in parallel, and thus the intermolecular van der Waals interactions further enhance the stability of the large-scale self-assembled structure, leading to more close-packed structure of HPB-1,2,4-3A than that of HPB-1,4-2A. Apparently, with the participation of solvent molecules, HPB-1,4-2A molecule and HPB-1,2,4-3A molecule all can form large-scale network structure on HOPG surface. However, no ordered structure can be obtained when HA solution of HPB-1,3,5-3A was dropped onto the HOPG surface, which is different with the case of HPB-6a.31 In previous reports, HPB-6a can form stable network at octanoic acid/HOPG interface. It is speculated that HA solvent interacts with HPB-1,3,5-3A molecules and disturbs the formation of regular


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network ofHPB-1,3,5-3A. Up to now, it can be deduced that three HPB-based molecules HPB1,3,5-3A, HPB-1,2,4-3A and HPB-1,4-2A can self-assemble into different network structures depending on the symmetry and number of carboxyl groups substituted in the molecular backbone. HPB-1,3,5-3A with C3 symmetry tends to form hexagonal network assembly, asymmetrical HPB-1,2,4-3A and HPB-1,4-2A with C2 symmetry tend to form linear assemblies. Furthermore, the solvent plays a great crucial role on the self-assembly of molecules. Table 3.Experimental (Expt.) and calculated (Cal.) unit cell parameters for the observed networks of HPB-1,3,5-3A, HPB-1,2,4-3A, and HPB-1,4-2A as shown in Figure 1d, Figure 4c and Figure 5c Unit cell parameters

PO

HPB-1,3,5-3A

HA

HPB-1,2,4-3A

HA

HPB-1,4-2A

a (nm)

b (nm)

α (o)

Expt.

3.1±0.1

3.1±0.1

60±1.0

Cal.

3.11

3.11

60.00

Expt.

2.2±0.1

3.6±0.1

100±1.0

Cal.

2.20

3.60

100.00

Expt.

2.8±0.1

4.8±0.1

90±1.0

Cal.

2.95

5.11

90.00

Table 4.The results of DFT calculations on the self-assembled structures of HPB-1,3,5-3A, HPB-1,2,4-3A and HPB-1,4-2A, respectively The interaction

The interaction

Total

between

between adsorbates

energy

adsorbates (kcal

and substrate (kcal

(kcal mol-

-1

-1

mol )

mol )

1

)

Energy per unit area (kcal mol-1 Å-2)

HPB-1,3,5-3A

PO

-87.087

-90.827

-177.914

-0.212

HPB-1,2,4-3A

HA

-163.022

-110.689

-273.711

-0.351

HPB-1,4-2A

HA

-259.876

-172.704

-432.58

-0.287


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To further evaluate the stabilities of these self-assembled structures, DFT calculations have been performed. The calculated lattice parameters for these structures are summarized in Table 3, which agree well with the experimental data. The interactions between molecules, the interactions between molecules and substrate, the total energy and the energy per unit area are all listed in Table 4. Especially, the energy per unit area presents a reasonable way to compare the thermodynamic stability of the different arrays with different unit cells. The negative values of energy per unit area for these three molecules indicate that these three molecules can assemble stable structure on HOPG surface. We notice that both the energies per unit area of HPB-1,2,43A/HA and HPB-1,4-2A/HA are less than that of HPB-1,3,5-3A/PO. Evidently, the participation of HA strengthens the stability of the system and makes the system more stable. It is notable that the energy per unit area of HPB-1,2,4-3A/HA is -0.351 kcal mol-1 Å-2, which is the lowest value among these three systems, indicating that the pattern is the most energetically favorable. It is interesting to investigate different epitaxy behaviors for the three HPB assemblies. Generally, the epitaxy is mainly dependent on the interaction between adsorbates and substrate. In HPB-1,3,5-3A network, the strong interaction totally comes from HPB. In HPB-1,4-2A and HPB-1,2,4-3A network, although HA participated in the assemblies, 80% and 50% contribution to the interactions come from HPB, respectively. Furthermore, most of HA directly interacted with the carboxyl groups on the HPB backbone with the chain along the direction of the carboxyl group. Therefore, we believe the crystal orientation mainly results from the strong π-π interaction between HPB and HOPG, especially the central benzene ring of HPB and HOPG. Furthermore, we have inspected the direction of overlayer relative to the substrate in epitaxy behavior for the three different networks. DFT calculations have been carried out to investigate the crystal orientations of the assemblies relative to HOPG substrate, in which we cited the high


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symmetry direction (the zigzag direction) of HOPG substrate as the reference. Our DFT calculations show that in the most stable adsorbed configuration, the crystal orientation of the HPB-1,3,5-3A assembly relative to the zigzag direction of HOPG substrate is 0° (Figure S10). For HPB-1,4-2A, the crystal orientation of the molecular assembly relative to the zigzag direction of HOPG substrate in the most stable adsorbed configuration is 90° (Figure S11). It should be noted that HPB-1,2,4-3A contains three asymmetrical carboxyl groups and the molecules form into rows along a direction in the network. Figure S12 shows that in the most stable adsorbed configurations, the crystal orientations of the molecular assemblies relative to the zigzag direction of HOPG substrate are -30°, 30° and 90°. They are energetically equivalent. It is why we could observe the expected domain orientations for epitaxial adsorption in Figure 5a. Conclusions In summary, we have demonstrated that these HPB-based molecules HPB-1,3,5-3A, HPB1,2,4-3A and HPB-1,4-2A can self-assemble into various hydrogen-bonded networks on HOPG surface depending on the site and number of carboxyl groups. For HPB-1,3,5-3A molecule, only the structure with multiple cavities formed at PO/HOPG interface can be observed by STM. HPB-1,2,4-3A molecule formed irregular structure at the PO/HOPG interface and regular largescale lamellar structure at HA/HOPG interface. HPB-1,4-2A molecules only formed wellordered two-dimensional self-assembly structures with the participation of the HA molecules in the process of molecular self-assembly. Different epitaxy behaviors for the three HPB assemblies have been discussed. ASSOCIATED CONTENT


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Supporting Information.1H-NMRspectra,13C-NMR spectra and the most stable adsorbed configurations on HOPG by DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2016YFA0200700, 2013CB934200, 2012CB933001, 2013CB834504) and the National Natural Science Foundation of China (No. 21472029). REFERENCES (1) Zhang, D. W.; Zhao, X.; Hou, J. L.; Li, Z. T. Aromatic Amide Foldamers: Structures, Properties, and Functions. Chem. Rev.2012, 112, 5271-5316. (2) Gale, P. A.; Quesada, R. Anion Coordination and Anion-TemplatedAssembly: Highlights from 2002 to 2004. Coordin. Chem. Rev.2006, 250, 3219-3244. (3) Wurthner, F.; You, C. C.; Saha-Moller, C. R. MetallosupramolecularSquares: from Structure to Function. Chem. Soc. Rev.2004, 33, 133-146. (4) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in Self-Assembly. Small2009, 5, 1600-1630. (5) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. NoncovalentSynthesis using Hydrogen Bonding. Angew. Chem., Int. Ed.2001, 40, 2382-2426. (6) Claessens, C. G.; Stoddart, J. F. Pi-Pi Interactions in Self-Assembly. J. Phys. Org. Chem.1997, 10, 254-272.


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The self-assembly behaviors of hexaphenylbenzene-based carboxylic acid derivatives depend on the symmetry and number of substituted carboxyl groups as well as the solvent.


32

Supramolecular Self-Assembly of Hexaphenylbenzene Derivatives

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