From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to

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Temperature-Triggered Self-Assembled Structural Transformation: from Pure Hydrogen-Bonding Quadrilateral Nano-Network to Trihexagonal Structures Siqi Zhang, Linxiu Cheng, Zhong-Liang Gong, Wubiao Duan, Bin Tu, Yu-Wu Zhong, and Qingdao Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00666 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Temperature-Triggered Transformation: Quadrilateral

from

Self-Assembled Pure

Nano-Network

Structural

Hydrogen-Bonding to

Trihexagonal

Structures Si-Qi Zhang, §,a,b Lin-Xiu Cheng,§,b,d Zhong-Liang Gong, c Wu-Biao Duan,*,a Bin Tu,*,b Yu-Wu Zhong*,c and Qing-Dao Zeng*,b,d aDepartment

of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, P. R.

China bCAS

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

Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, P. R. China cBeijing

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

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China dCenter

of Materials Science and Optoelectonics Engineering, University of Chinese Academy of

Sciences, Beijing 100049, P. R. China

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ABSTRACT

The adequate control over the structures of molecular building blocks acts as an important role in the fabrication of desired supramolecular nano-structures at interfaces. In this study, the formation of a pure hydrogen-bonding co-assembly supramolecular nano-network on a highly oriented pyrolytic graphite (HOPG) surface was demonstrated by means of scanning tunneling microscope (STM). Thermal annealing process was conducted to monitor the temperature triggered structural transformation of the self-assembled nano-network. On the base of the single molecule level resolution STM images, together with the density functional theory (DFT) calculations, the formation mechanisms of the formed nano-arrays were proposed. The results have great significance in regard to controlled construction of complex nano-structures on surface.

KEYWORDS: Scanning tunneling microscopy ‧ self-assembly ‧hydrogen-bonding ‧nano-network ‧ temperature triggered


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INTRODUCTION As a promising approach to build functional nanostructures and nano-devices, supramolecular self-assembly of organic molecules has attracted considerable attentions over decades.1-8 The progress in the design and fabrication of complex self-assembly supramolecular architectures on solid surface has been achieved by taking advantage of non-covalent interactions such as hydrogen bonding9-11, metal-ligand coordination12-14, van der Waals’ force15-18, etc19-21. In particular, hydrogen-bonded two-dimensional (2D) nano-networks on solid surface are of great significance to the field of nano-framework due to the relatively strong and highly directional interactions between the adsorbates.21-23 With higher selectivity and directionality25, hydrogen-bonding networks on solid surface are more predicable when designing the geometries and controlling the placement of functional groups of the building blocks. Notably, in most cases the pure hydrogen-bonded 2D nano-porous networks are formed by molecular self-assembly of single-component molecular building blocks with threefold or quartic symmetry, where these building blocks contain hydrogen-bond donor and acceptor moieties such as hydroxyl, carboxyl, amino groups, etc. For example, hexagonal and square nano-porous networks could be formed by self-assembly of 1,3,5-benzenetricarboxylic acid (TMA) and thymine functionalized porphyrin (tetra-TP), respectively.26,27 Although hydrogen-bonding is a frequently encountered non-covalent force that determines the structure of 2D supramolecular assembly, relatively few studies of the hydrogen-bonding directed multi-component 2D nano-networks has been carried out. Among these studies, De Feyter and co-workers28 investigated the co-assembly of an OPV-melamine moiety with a perylene bisimide derivative, a variety of complicated coassembly structures were formed through hydrogen-bonding. Bernasek and co-worker29 probed the co-adsorption mixture domain of 5-octadecyloxyisophthalic acid (5OIA) and terephthalic acid, where the terephthalic acid with two equivalent carboxylic acid groups was chosen as a linker between two 5OIA molecules. Moreover, Lackinger and co-workers used TMA and TPA as hydrogen-bond linker molecules to obtain the stable adsorption of TPT on surface30. These studies ACS Paragon Plus Environment

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showed that for the construction of hydrogen-bonding multicomponent architectures on solid surface, the pre-design and organization of the building blocks is of great importance. While producing a variety of structures, hydrogen-bonding directed co-assembly also brings more precisely control over the network morphology. For this reason, it is meaningful and challenging to unravel the formation mechanisms of hydrogen-bonding co-assembly nano-patterns. In our previous report31, the self-assembly behaviors of a pair of linear molecules with amino functionalized triazine end groups were investigated. These series of derivatives possess rich hydrogen-bond donor and acceptor moieties at both end of their rigid linear phenylethynyl skeletons, where hydrogen-bonding could be formed for the self-assembly of closely-packed parallel linear structures under room temperature or trihexagonal porous structures after thermal annealing. This class of rigid molecules with different skeleton length may serve as basic building blocks to construct complex and diverse nano-structures through pure hydrogen-bonding stoichiometric molecular co-assembly. Particularly, as an important external stimulate factor, temperature is frequently utilized to control and modify the self-assembly arrangement32-34. On the basis of molecular modeling and theoretical calculations, we are able to interpret the temperaturetriggered structural transformation in the self-assembly and thereby gain a deeper understanding of the thermodynamic equilibrium process during the course. Here, we present our investigation of the construction of a pure hydrogen bonding co-assembly nano-network formed by two spatially matching rigid linear molecules (L1 and L2) at the solidliquid interface. Their chemical structures were shown in Scheme 1. First, the independent selfassembly behavior of each molecule was conducted, resulting in the formation of closely packing structures under room temperature and trihexagonal structures after thermal annealing. Interestingly, quadrilateral nano-patterns were formed in the binary system consists of molecule L1 and L2. We noticed that the spatial matching of the two components was crucial to the formation of the assembled nano-framework. Moreover, after thermal annealing, the quadrilateral nanoACS Paragon Plus Environment

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network was decomposed, and the two components were separated into different domains in their trihexagonal self-assembly structures. Their temperature triggered structural transformation behavior may provide potential applications in many fields such as sensing and switching. Scheme 1. Chemical structures of molecule L1 and L2.

EXPERIMENTAL SECTION STM Investigation. Molecule L1 and L2 were synthesized according to the established procedures30,35,36 Detailed synthetic protocols were provided in the supporting information (SI). Heptanoic acid (Aldrich, without further purification) was used as solvent in this work. Firstly, Molecule L1 and L2 were dissolved separately in heptanoic acid to reach the saturated concentration. The typical saturated concentrations are generally ~10-3 M. Then the solutions were diluted into 10% saturation. Furthermore, mixture solutions of L1 and L2 with different molar ratio were prepared by mixing up the 10% saturated solutions of L1 and L2 with a series of volume ratio (2: 1, 1: 1, and 1: 2). The samples were prepared by depositing a droplet (0.4 μL) of the above prepared solutions on the basal plane of freshly cleaved (using adhesive tape) HOPG (grade ZYB, NTMDT, Russia) substrates. As for the thermal annealing process, the samples were heated on a hotplate with certain temperatures. During the heating process, pure heptanoic acid was added continuously to keep the liquid/solid interface. After a fixed annealing time of 1 hour, the samples were taken away from the hotplate and cooled to room temperature. After the treatments, we performed the STM experiments using a Nanoscope Ⅲa scanning probe microscope system (Bruker, USA) under ambient conditions. All of STM images were recording in constant current ACS Paragon Plus Environment

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mode using mechanically cut Pt/Ir (80/20) tips. The STM images provided are raw data without any treatment except for the flattening procession. Detailed tunneling conditions are given in the figure captions. Computational Details. We performed theoretical calculations using density functional theory (DFT-D3) provided by DMol3 code37. We used the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work. The Perdew and Wang parameterization38 and Perdew-Burke-Ernzerh parametrization39 of the local exchange correlation energy was applied in the local spin density approximation (LSDA) to describe exchange and correlation. All-electron spin-unrestricted Kohn-Sham wave functions were expanded in a local atomic orbital basis. Numerical basis set was applied for the large system. The calculations equipped with the medium mesh and were all-electron ones. Self-consistent field procedure was done with a convergence criterion of 10-5 a.u. on the energy and electron density. Combined with the experimental data, we had optimized the unit cell parameters and the geometry of the adsorbates in the unit cell. When the energy and density convergence criterion reached the desired degree, we obtained the optimized parameters and the interaction energy between adsorbates. The model system showed the interactions between the adsorbates and HOPG substrate. In this investigation, the adsorption of adsorbates with π-conjugated benzene-ring on graphite is similar to that of graphene, which leads to performing calculations on infinite graphene monolayers using PBC. Graphene layers were divided by the normal direction of 35 Å in the superlattice. Graphene supercells were used, and the Brillouin zone was sampled by a 1 × 1 × 1 k-point mesh when adsorbates were modeled on graphene. The interaction energy (Einter) of adsorbates on graphite is Einter = Etot(adsorbates/graphene) − Etot(isolated

adsorbates in vacuum)

− Etot(graphene). The adsorption energy per

molecule is given by Eads = (Etot(adsorbates/graphene) − nE(per molecule of adsorbates) − Etot(graphene))/n. RESULTS AND DISCUSSION


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After deposited a droplet of heptanoic acid solution containing L1 molecules (10% saturated concentration) on freshly cleaved HOPG substrate, an area of molecule L1 adsorption on the surface of HOPG was recorded by STM. The large-scale STM image in figure 1a showed that the L1 molecules formed a closely packing pattern in the long range (denoted as L1-Packing). Each of the bight nano-rod in the STM image corresponded to one L1 molecule on the basis of the electron density of its linear π-conjugated backbone. To study the packing structure in detail, a highresolution STM image of L1-packing was shown in Figure 1b. The nano-rods in the STM image were closely packed with the unit cell parameters measured to be: a = 1.4 ± 0.1 nm, b = 1.4 ± 0.1 nm, α = 67° ± 1°. Interestingly, a trihexagonal porous self-assembly structure of L1 (denoted as L1Porous) appeared after 40°C annealing of the sample. The structural transition was irreversible, and no new structure formed when we further increased the temperature. Large scale and highresolution STM images of the L1-Porous pattern were shown in Figure 1d and Figure 1e, respectively. The measured unit cell was presented with: a = 2.3 ± 0.1 nm, b = 2.3 ± 0.1 nm, α = 60° ± 1°. Careful analysis of the two assembly structures of L1-Packing and L1-Porous revealed that the intermolecular hydrogen-bonds formed by the amino functional end groups of adjacent L1 molecules dominated the assembly process. The length of one bright rod that represented one L1 molecule was measured to be 1.1 ± 0.1 nm. Figure 1c and Figure 1f were the corresponding molecular models calculated by DFT method on the basis of STM observations.


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Figure 1. (a) A large-scale STM image of the self-assembled L1 molecules (L1-Packing pattern) at HOPG/Heptanoic acid interface under room temperature, Iset = 357.1 pA, Vbias = 719.9 mV. (b) A high-resolution STM image of the L1-Packing pattern, Iset = 357.1 pA, Vbias = 719.9 mV. (c) Corresponding molecular model for b. (d) A large-scale STM image of the self-assembled L1 molecules (L1-Porous pattern) at HOPG/Heptanoic acid interface after 40°C annealing for 1 hour, Iset = 329.6 pA, Vbias = 770.0 mV. (e) A high-resolution STM image of the L1-Porous pattern, Iset = 329.6 pA, Vbias = 770.0 mV. (f) Corresponding molecular model for e. Unit cells were imposed on the STM images and their corresponding molecular models. Their measured and calculated parameters were all shown in Table 1. The self-assembly processes of L2 were also investigated at HOPG/Heptanoic acid interface. The L2 molecule has the same amino functionalized triazine end groups and similar linear backbone as L1 molecule. The self-assembly behavior of L2 was also similar as L1 molecule that it tended to form closely packed linear structure under room temperature (denoted as L2-Packing) and transformed into trihexagonal porous pattern (denoted as L2-Porous) after annealing at desired temperature. Figure 2a displayed a large-scale STM image of the L2-Packing self-assembly pattern. In the high-resolution STM image presented in Figure 2b, the bright slender nano-rods that


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represented L2 molecules were packed into linear arrangements. A large-scale of L2-Porous pattern was observed when the sample was annealed at 80 °C (Figure 2c). Detailed STM image in Figure 2d showed that the network structure was constructed via hydrogen-bonds between the amino groups at the terminal of L2 molecules. As discussed in our previous report31, L2-Porous pattern started to appear at HOPG/Octanoic acid interface when the sample was annealed at 60 °C. However, the complete conversion was not easy to realize even the sample was heated up to 100 °C. According to the results of experiments, the self-assembly behavior of L2 was not significantly affected by the different solvents used. The length of one bright rod that represented one L2 molecule was measured to be 2.2 ± 0.1 nm. The measured unit cell for L2-Packing was presented with: a = 4.5 ± 0.1 nm, b = 1.8 ± 0.1 nm, α = 70° ± 1°, and for L2-Porous was presented with: a = 3.0 ± 0.1 nm, b = 3.0 ± 0.1 nm, α= 60° ± 1°.

Figure 2. (a) A large-scale STM image of the self-assemled L2 molecules (L2-Packing pattern) at HOPG/Heptanoic acid interface, Iset = 335.7 pA, Vbias = 614.0 mV. (b) A high-resolution STM image of the L2-Packing pattern, Iset = 335.7 pA, Vbias = 614.0 mV. (c) A large-scale STM image of the self-assembled L2 molecules (L2-Porous pattern) at HOPG/Heptanoic acid interface after 80°C annealing for 1 hour, Iset = 204.5 pA, Vbias = 681.2 mV. (d) A high-resolution STM image of the L2Porous pattern, Iset = 204.5 pA, Vbias = 681.2 mV. Some L2 molecules were marked by white lines


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in b and d to clarify the assembled structures. Unit cells were imposed on the high-resolution STM images. Their measured and calculated parameters were shown in Table 1. This class of rigid linear molecules with strongly amino functionalized triazine end groups showed their ability to form stable nano-patterns that were directed by hydrogen-bonds. With same hydrogen bond acceptor and donor moieties at the terminal, two rigid linear molecules with different skeleton length have the possibility to form stable complicated co-assembly structures. However, it should be noted that the stoichiometric co-assembly of two or more components is a challenging task because there lays the competition between the formation of mixture domain and the formation of one-component domains. The controlled construction of a well-defined ordering co-assembly depends on many factors, such as the structural and spatial match of each component. The highly selectivity and directionality of hydrogen-bond can also be a double-edged sword in constructing composite supramolecular structures because it increases the difficulty of structural matching. After several tries with this series of molecules, we were only able to obtain stoichiometric co-assembly of molecule L1 and L2. The proper direction and relatively strong strength of the hydrogen bonds between L1 and L2 molecules were the critical factors that derived the formation of the quadrilateral network. Co-assembly results in Figure 3a-c showed that the inclusion of L1 molecules would cause the rearrangement of L2-Packing structure, a quadrilateral nano-network that consists of both L1 and L2 molecules (denoted as L1-L2) was constructed at the HOPG/Heptanoic acid interface. As the STM results shown, the most beneficial condition to build L1-L2 structure was under molar ratio of L1: L2 = 1: 1.


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Figure 3. Co-assembly structures of the L1 and L2 binary system at HOPG/Heptanoic acid interface under molar ratio of (a) L1 : L2 = 1 : 2, Iset = 375.4 pA, Vbias = 715.0 mV. (b) L1 : L2= 1 : 1, Iset = 512.7 pA, Vbias = 612.5 mV. (c) L1 : L2 = 2 : 1, Iset = 453.5 pA, Vbias = 770.0 mV. As shown in the high-resolution STM image (Figure 4a), we could ascribe the bright nanorods to the conjugated backbones of the linear L1 and L2 molecules. The shorter and taller nanorods could be assigned to molecule L1 and L2, respectively. Evidently, the length of the two bright rods were measured to be 1.1 ± 0.1 nm and 2.2 ± 0.1 nm, which coincided with the length of molecule L1 and L2 in their single self-assembly STM images, indicated that the quadrilateral nano-network was formed via formation of hydrogen-bonds between the adjacent L1 and L2 molecules. As we had expected, pure hydrogen-bonding co-assembly nano-network was constructed. The spatial matching of the two building blocks was crucial to the formation of the coassembled nano-network. A closer inspection revealed that there were some dim stripes inside the parallelograms, which could possibly be assigned to the heptanoic acid solvent molecules. Figure 4b was the corresponding molecular model calculated by a DFT method on the basis of STM observations. As shown in the optimized molecular model, the heptanoic acid molecules also aided the co-assembly stability through hydrogen bonding with L2 molecules. A measured unit cell was superimposed on the STM image and the corresponding molecular model with a = 3.1 ± 0.1 nm, b = 3.0 ± 0.1 nm, α= 55° ± 1°.


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Figure 4. (a) A high-resolution STM image of the L1-L2 co-assembly pattern at HOPG/Heptanoic acid interface under room temperature, Iset = 512.7 pA, Vbias = 612.5 mV. (b) Corresponding molecular model for a. Unit cells were imposed on the STM image and the molecular model. Their parameters were all shown in Table 1. Furthermore, we have studied the co-assembly behavior of L1 and L2 molecules after annealing at different temperatures. After having observed stable binary L1-L2 network at room temperature, we performed thermal annealing process of the same sample under 60 °C. Figure 5a was the STM result of L1 and L2 binary system at HOPG/Heptanoic acid interface after 60 °C annealing for 1 hour. The STM image was divided into two domains (A1 and A2) that exhibited different nano-patterns. Through careful inspection we could see that the nano-pattern in domain A1 was L1-Porous structure and in domain A2 was L2-Packing structure. The result indicated that after 60 °C annealing of the binary system, the co-assembly network of L1-L2 was broken down, L1 and L2 molecules had isolated from each other to form self-assembly structures. At this point L1 molecules had completed their assembly structural transition while L2 molecules still took the form of closely packing. Additionally, from the large scale STM image shown in Figure S1, we could see some L2 molecules on the edge of the L2-Packing domain started to rotate. The result was quite conceivable because the phase transition temperature of L1 was about 30 - 40 °C and for L2 was about 60 - 80 °C. We repeated the annealing experiment at 80 °C, the STM image in Figure 5b showed that with the higher annealing temperature of 80 °C, the assembly structure of L2 molecules could also transform into trihexagonal porous structure (see in domain B2) as L1 molecules (see in domain B1), despite L2-Packing domains still occupied a large proportion (see in domain B3 and the large scale STM image shown in Figure S2).


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Figure 5. (a) STM image of L1 and L2 binary system at HOPG/Heptanoic acid interface after 60 °C annealing for 1 hour, Iset = 366.2 pA, Vbias = 770.0 mV. Two self-assembly structures were marked by domains A1 and A2 (b) STM image of L1 and L2 binary system at HOPG/Heptanoic acid interface after 80 °C annealing for 1 hour, Iset = 468.6 pA, Vbias = 541.4 mV. The selfassembled structures were marked by domain B1, B2, and B3. In order to further explain the mechanism during the formation of the above observed selfassembled and co-assembled nano-patterns, DFT calculations were performed based on the experiment results. The calculated lattice parameters for the different nano-patterns were summarized in Table 1. Obviously, the calculated lattice parameters agreed well with the corresponding experimental values. Furthermore, we presented the total energy and total energy per unit area for the assembled nano-patterns in Table 2. The total energy includes the interactions between adsorbates and the interactions between adsorbates and substrate. Therefore, thermodynamic stability of the different nano-patterns could be compared by the energy per unit area.


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Table 1. Experimental (Expt.) and calculated (Cal.) lattice parameters for the nano-patterns. Unit cell parameters

L1-Packing

L1-Porous

L2-Packing

L2-Porous

L1-L2

a (nm)

b (nm)

α (°)

Expt.

1.4 ± 0.1

1.4 ± 0.1

67 ± 1

Cal.

1.39

1.36

67.0

Expt.

2.3 ± 0.1

2.3 ± 0.1

60 ± 1

Cal.

2.18

2.18

60.0

Expt.

4.5 ± 0.1

1.8 ± 0.1

70 ± 1

Cal.

4.40

1.80

70.0

Expt.

3.0 ± 0.1

3.0 ± 0.1

60 ± 1

Cal.

3.26

3.26

60.0

Expt.

3.1 ± 0.1

3.0 ± 0.1

55 ± 1

Cal.

2.96

30.4

55.0

From Table 2, for the L1 molecule, from the value of the total energy per unit area we noticed that L1-Porous assembled structure was with the better thermodynamic stability (-0.204 kcal mol-1 Å-2) than L1-Packing (-0.138 kcal mol-1 Å-2). It indicated that the L1-Porous structure was the energetically favorable structure. The same principle also applied to L2 molecule, the total energy per unit area of L2-Packing and L2-Porous pattern were -0.087 kcal mol-1 Å-2 and -0.163 kcal mol-1 Å-2, respectively. It is clear that the assembly of L2-Packing would be with less stable than that of L2-Porous structure. With reference to the literature40,41 and according to our experiment results, we concluded that the self-assembly process of L1 and L2 molecules under room temperature is likely kinetically controlled. The trihexagonal porous patterns of L1 and L2 molecule were not the kinetic preferred assembly structure. However, after the thermal annealing process, the less thermodynamic stable closely-packed patterns can be irreversible transit to the more thermodynamic stable porous patterns that remained stable upon cooling to room temperature.


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During the experiment, we also noticed that with shorter skeleton length, the temperature triggered transformation for molecule L1 was more convenient and faster, it could happen at a relatively lower temperature (40°C) and complete in about 15 minutes. As for the molecule L2 with longer skeleton, it is more difficult to move and reassemble on the HOPG surface to form the porous pattern. As a result, higher annealing temperature (80°C) and longer annealing time (>30 minutes) were required to get a large area of L2-Porous structure. In this study, we fixed the annealing time at 1 hour for every system to achieve a better transform rate. However, L2 molecules cannot achieve complete transformation even at 100°C. The experiment results and theoretical calculations suggested that the kinetics factor was occupying the predominant position in the L2 molecular self-assembly. To better understand the co-assembly process of L1 and L2 molecules in theoretical aspect, we carried out DFT calculation of total energy for L1, L2 and heptanoic acid molecules multicomponent system on HOPG substrate. The total energies per unit area for L1-L2 structure was 0.180 kcal mol-1 Å-2, which was lower than L1-Packing (-0.138 kcal mol-1 Å-2) and L2-Packing structures (-0.087 kcal mol-1 Å-2). The calculation results revealed that L1-L2 structure was more stable than L1-Packing and L2-Packing structure, which explained why the L1-L2 structure could be constructed at room temperature from the theoretic aspect. During the thermal annealing process, the L1-L2 structure was broken down and decomposed into separated one-component domains of L1 and L2, because the L1-Porous structure (-0.204 kcal mol-1 Å-2) was a thermodynamic favored state compared to the L1-L2 network (-0.180 kcal mol-1 Å-2). As for the spare L2 molecules after the L1-Porous pattern formed, they will mainly take the form of L2-Packing pattern (-0.087 kcal mol-1 Å-2) at the lower annealing temperature (60°C) and partly transformed into L2-Porous pattern (-0.163 kcal mol-1 Å-2) at the higher annealing temperature (80°C) in the L1 and L2 binary system. All the calculation results agreed well with our STM observations.


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Table 2. The total energy and energy per unit area for adsorbates on the HOPG surface. The total energy includes the interactions between adsorbates and the interactions between adsorbates and substrate. Here, the more negative energy means the system is more stable. Interactions between molecules (kcal mol-1)

Interactions between molecules and substrate (kcal mol-1)

Total energy (kcal mol-1)

Energy per unit area (kcal mol-1 Å-2)

L1-Packing

-6.903

-15.021

-21.924

-0.138

L1-Porous

-43.926

-40.883

-83.809

-0.204

L2-Packing

-2.950

-69.342

-72.292

-0.087

L2-Porous

-58.684

-91.045

-149.729

-0.163

L1-L2

-68.418

-63.683

-132.101

-0.180

CONCLUSIONS In summary, the supramolecular self-assembly and co-assembly of two rigid linear molecules (L1 and L2) with different skeleton lengths were studied at HOPG/Heptanoic acid surface by STM in combination with DFT calculations. The results suggested that they were able to self-assembling into defined temperature depended nano-structures profiting from the strongly amino functional end groups, with the hydrogen bond between the end amino groups being the most contributing factor. A pure hydrogen bonding co-assembly nano-network was constructed on HOPG by the two spatially matching molecules. Based on the analysis of the STM results and the DFT theoretical calculations, we have got one step closer to the adequate control and tuning of molecular structures to construct desired supramolecular structures.


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ASSOCIATED CONTENT Supporting Information Synthesis of molecules L1 and L2. Large scale STM images of L1 and L2 binary system after annealing at 60 °C and 80 °C, respectively.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]; [email protected]; [email protected]. Author Contributions § These

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2016YFA0200700) and the National Natural Science Foundation of China (Nos. 21773041, 21472029, 21601194).


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Table of Contents

This work describes temperature triggered transformation of separated self-assembly of L1 and L2 molecules and their co-assembly structures.


From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to

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