Bilayer adsorption of porphyrin molecules substituted with carboxylic

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Bilayer adsorption of porphyrin molecules substituted with carboxylic acid atop the NN4A network: STM and DFT reveal Xiao-Yang Zhu, Siqi Zhang, Hongjun Xiao, Chao Li, Weiming Huang, Qiaojun Fang, XiaoKang Li, Min Zhang, Faliang Cheng, Bin Tu, Yanfang Geng, Jianxin Song, and Qingdao Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03507 • Publication Date (Web): 10 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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Bilayer adsorption of porphyrin molecules substituted with carboxylic acid atop the NN4A network: STM and DFT reveal XiaoyangZhu,1,# Siqi Zhang,1,# Hongjun Xiao,1,# Chao Li,2 Weiming Huang,2 Qiaojun Fang,1 Xiaokang Li,3 Min Zhang,4 Faliang Cheng,4 Bin Tu,1,* YanfangGeng,1,* Jianxin Song,2,* Qingdao Zeng1,4,5* 1

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

Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China. E-mail: [email protected], [email protected], [email protected] 2

Key Laboratory of Assembly Organic Functional Molecules, Hunan Normal University,

Changsha 410081, China. E-mail: [email protected] 3

Key Laboratory of Organo pharmaceutical Chemistry, Gannan Normal University, Ganzhou

34100, China 4

Guangdong Engineering and Technology Research Center for Advanced Nanomaterials, School

of Environment and Civil Engineering, Dongguan University of Technology, Dongguan 523808, P.R. China. 5

Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy

of Sciences, Beijing 100049, China. #

These authors contributed equally to this work.

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Abstract Bottom-up technology is a bridge connecting two-dimensional (2D) monolayer structure with three-dimensional (3D) bulk structure. From 2D to 3D, it helps us to understand the driving force of organization process to control the molecular arrangement in 3D phase. Here, we aimed at the fabrication of multilayer nanostructures on solid substrates. The bis(3,5-diacidic) diazobenzene (NN4A) was chosen as one molecule because of its photosensitive azo group and carboxylic group possessing hydrogen bonding effect, while porphyrin molecules comprised of different numbers and positions of carboxylic acid groups were used as the other component. It was found that the porphyrin molecules could adopt different adsorption configurations due to the influence of carboxylic groups, leading to different subsequent co-assemblies on solid surface. The NN4A/porphyrin systems underwent structural transformation when NN4A molecules adsorbed on the HOPG surface with pre-deposited porphyrin. This work displayed an efficient method on the construction of multilayer nanostructures in the molecular surface engineering and provided a new way to construct 3D structures based on the molecular design. Keywords: porphyrin, carboxylic acid, hydrogen bonding, bilayer, self-assembly

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Introduction Two-dimensional (2D) molecular self-assemblies in nanometer scale are paramount in material chemistry and have received considerable attention for directing functional nanostructures on the solid substrates. The bottom-up strategy offers a significant approach to fabricate unique nanostructures and complex functional nano-devices.1-4 This simple and basic nanotechnology uses the predefined molecules deposition on surface or interface, and then generates the complex organized hetero-structures and ordered nanostructures. The formation of defined diverse self-assembled nanostructures is still challenging because the molecular configuration is difficult to be determined. Since the self-assemblies are influenced not only by the molecular chemical structures but also by the interactions between molecule and substrate, the molecular configuration becomes much more complex and has drawn great interest.5 Therefore it is of great significance to understand the dynamic behaviors and mechanisms of the molecular arrangements in monolayers or multilayers. With supramolecular network as the template, various 2D and three-dimensional (3D) nanostructures have been successfully constructed.6-9 Recently, many co-assembled systems on the solid surface have been investigated by using scanning tunneling microscopy (STM) technique with the atomic resolution to verify the interactions and their influences on the nanostructures.10-15 For example, the length of the substituted chains affected the assembled structure of unsymmetrically substituted fluorenone derivatives.16 In addition, it has been proved that the external electric field also controlled the co-adsorption behavior for two components of supramolecular systems assembled on surface by changing molecular orientation induced by the field switching behavior.17 The assembled structures can also be tuned from monolayer to bilayer through accommodating guest molecules or electric field.18-19 In order to obtain expected 3D bulk structures, the multilayer structures have been achieved based on fundamental 2D monolayer self-assembled behavior. In comparison with the single-component bilayer structures, the well-defined 2D frameworks consisting of more than one material were determined by their molecular arrangements. The intermolecular bonding types and the driving forces for multicomponent assemblies seemed to be more valuable and needed to be further studied. Because porphyrins have exhibited thermal stability, broad absorption, and large extinction coefficient in the visible light region, they have been widely used to construct single-molecule junctions, molecular switch, electrocatalytic hydrogen evolution reaction and solar cell.20-23 For the advantages of porphyrin molecule, it is also an interesting material to construct the layered 3


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nanostructures. Some studies have revealed that a porphyrin dimer covalently linked two metalporphyrins, Cu-porphyrin and Mn-porphyrin, could rearrange by exchanging the molecule between the bilayer.24 In addition, the azobenzene derivative bis(3,5-diacidic) diazobenzene (NN4A) is a famous nanotemplate because of its photosensitive azo group and four carboxylic acid groups and can form porous networks though hydrogen bonding. In our previous work, we have reported that the formed well-ordered 2D porous network by NN4A could serve as molecular template for trapping guest species.25-26 These results create a way to develop new 2D networked materials based on these systems. The important structural feature of the building layered structures is still challenging. A very important reason for choosing these two kinds of molecules is that they all contain different carboxylic groups and may produce unexpected assembled structures. Based on the previous studies taking porphyrin or NN4A as template, a problem about whether these two molecules could co-assemble has emerged. Therefore, we planned to configure the 2D frameworks based on these two molecular systems. Herein, we investigate the site selectivity in the binary system composing of NN4A and four porphyrin compounds containing different numbers and positions of carboxylic acid groups, as shown in Scheme 1. To investigate the co-assembled process, a series of the experiments were carried out by STM at the liquid/solid interfaces. Density functional theory (DFT) calculations were also performed to reveal the formation mechanism of the nanoscale patterns. Our results showed that the porphyrin molecules adopt different adsorption configurations on top of NN4A cavities due to the influence of tertiary butyl and carboxylic groups, and the co-assembled systems underwent configuration adjustments when different numbers of carboxylic groups adsorb on the cavity area of the bottom layer.

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OH

OH

O

OH

OH

OH

O

O

OH

O

O

O

O

NH

NH

N

N

HN

N

OH

N

NH N

HN

N HN

OH O

O

O OH

o-2A

1A OH

OH

p-2A

OH

O

O

O OH

O NH N

N

N N

HN

O

OH

HO

O O

HO O

NN4A

O OH

OH

O

OH

OH

3A

Scheme 1. Chemical structures of porphyrin carboxylic compounds and NN4A molecule.

Experimental Section Sample preparation The azobenzene NN4A and porphyrin molecules were synthesized by the reported literatures.27-29. First, the porphyrin molecules and NN4A molecule were dissolved in heptanoic acid with a concentration at the magnitude of 10−4 mol/L. In order to ensure a large-scale structure of the underlying layer, two droplets of the corresponding solution (0.4μL) were firstly deposited on the highly oriented pyrolytic graphite (HOPG, grade ZYB, NTMDT, Russia) substrate, which was freshly cleaved using adhesive tape. And then, a droplet (0.2μL) solution of the other component was deposited onto the prepared layer. After a few minutes, the sample was detected by STM. There are two kinds of dropping order for preparing the co-assembly of NN4A and porphyrin molecules. One is that the NN4A solution was firstly deposited, the other is that the porphyrin solution was dropped firstly. STM investigation The STM experiments were operated with a Nanoscope IIIA system (Veeco Inc. USA) in constant current mode under ambient conditions. STM tips were prepared by mechanical cutting of a Pt/Ir wire (80/20). All the STM images provided were raw data and were calibrated by referring the underlying graphite lattice. Detailed tunneling conditions are given in the corresponding figure captions. Computational details The theoretical calculations were performed using Density functional theory (DFT) provided by DMol3 code to reveal the interactions involved between the molecular5


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molecular and the formation mechanism. We used the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work.30 For a large system, the numerical basis set was applied. All calculations were all-electron ones and performed with a medium mesh. A self-consistent field procedure was carried out with a convergence criterion of 10−5 au on the energy and electron density. Combined with the experimental data, we have just optimized the unit cell parameters. When the energy and density convergence criterion are reached, then we could obtain the optimized parameters and the interaction energies. So all the models proposed in this work were optimized DFT models. To evaluate the interactions between the adsorbates and HOPG, we have performed our calculations on infinite graphene monolayers using PBCs because adsorption of the adsorbates on graphite and graphene can be considered as very similar. In the super lattice, the 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 using 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 Self-assembly of porphyrin molecules Four kinds of porphyrin molecules (1A, o-2A, p-2A, 3A) respectively substituted with one, two, two and three carboxylic acid groups were introduced in this study, whose chemical structures were shown in Scheme 1. The difference between o-2A and p-2A molecule was the position of substituted carboxylic acid groups. Their self-assembled nanostructures at the heptanoic acid/HOPG interface were investigated by STM after depositing a droplet of solution onto the HOPG surface. Their large-scale self-assembled structures are shown in Figure S1. 1A molecules formed a small-scale ordered structure as shown in the low-resolution STM image observed after several scanning, while o-2A and 3A self-assembled into ordered structures, which can be clearly detected by STM scanning. Unfortunately, no assemblies for p-2A molecules were observed under the same experimental conditions. In the case of 1A, two kinds of structures in region I and II were accidentally found as shown in Figure S1, and the blow-up images in Figure 1(a, c). According to the shape and size of 6


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porphyrin molecule, the bright spot containing two lobes are generally assigned to one porphyrin molecule. Herein, the distance between two adjacent lobes is estimated to be 1.0 ± 0.1 nm in region I, while 1.7 ± 0.1 nm in region II. Therefore, the red rectangles are assigned to 1A molecules, respectively. In other words, 1A molecules are parallel to the solid surface in region I, but in region II the 1A molecules are perpendicular to the HOPG surface. From the proposed molecular models displayed in Figure 1(b, d), it can be seen that the structure I is possibly dominated by the intermolecular interaction between carboxylic group and tertiary butyl as well as the molecule-substrate interaction, and the =O···H hydrogen bonding comes from the O atom in carboxylic acid group and the H atom in the side chain. In the structure II, 1A molecules seemed to align in order connecting with one =O···H hydrogen bonding between carboxylic groups of diagonally arranged 1A molecules. Due to the nonplanar structure of 1A, these interactions might be so weak that only small region thermodynamically unstable structures were recorded. The high-resolution STM images of o-2A are shown in Figure 1(e). The bright protrusions composed of two lobes are similar with the STM images of other porphyrin molecules previously observed. In addition, the measured length and width of the bright quadrilateral are determined respectively to be 1.2 ± 0.1 nm and 1.2 ± 0.1 nm, which are in accordance with the size of the central porphyrin skeleton. The most scanned region is covered by structure I, while there is still a small region exhibiting structure II. In the structure I, o-2A molecules formed chessboard structure, and the quadrilateral structure is arranged in an approximate angular diagonal way in the structure II. Obviously, the relative orientation of o-2A molecules in region II structure is different from that in region I, which is preliminarily deduced by the driving forces of o-2A molecules. In addition, there is a clear boundary marked with red line, at both sides of that the orientations of molecular stripe are different. The unit cell of these two structures superimposed on STM images were determined to be a =1.9 ± 0.1 nm, b= 1.8 ± 0.1 nm, α = 97 ± 2° and a = 2.1 ± 0.1 nm, b =2.1± 0.1 nm, α =51 ± 2°, respectively. On the basis of the unit cell parameters, the molecular shape and size, possible intermolecular interaction, and the molecular arrangements are optimized and proposed. As far as we know, carboxylic groups preferentially formed hydrogen bonding in the process of self-assembly. Therefore, the adjacent o-2A molecular chains could be connected by hydrogen bonding. However, the proposed model as shown in Figure S2 is difficult to match with STM image. As shown in Figure 1(f), it is proposed that the neighboring o-2A molecules interact with 7


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each other by carboxylic groups through pairs of hydrogen bonding. Along the other direction, the stability of o-2A stripes mainly stems from the interaction between molecule and substrate. For the pattern II as shown in Figure 1(g), each carboxylic group and tertiary butyl of neighboring o-2A molecules connected by hydrogen bonding. Because of the non-planar structure of tertiary butyl, the formed hydrogen bonding is much weaker than that between carboxylic groups. Therefore, it can be considered that structure II is an unstable intermediate state, which is the main reason for the presence of large-scale assembled structure I. On the other hand, the existence of this unstable intermediate state makes the assembly of o-2A molecules form discrete structure I. Although o-2A and p-2A have similar chemical structures except for the position of carboxylic acid substituents, the self-assembled structures of p-2A molecule could not be observed at the same condition. It is expected that the para-substituted carboxylic groups of two adjacent p-2A molecules could be linked by hydrogen bonding to form molecular chains, as shown in Figure S2(c). There may be three possible modes of hydrogen bonding between p-2A molecules. We speculated that the existence of indeterminate connections between adjacent p-2A molecules may be the reason for chaotic structure, which is difficult to be observed. Therefore, the hydrogen bonding between the molecules plays an important role on the 2D self-assembled networks for porphyrin molecules on the HOPG surface. The high-resolution STM image of 2D self-assembled nanostructure of 3A is shown in Figure 1h. From the STM image, it can be seen that 3A molecule formed ordered stripes, which is similar with that of o-2A molecule. 3A showed two-lobe protrusions similar to 1A and o-2A molecules, indicating that the increment of carboxylic group did not obviously affect the configurations of porphyrin molecules on HOPG surface. The unit cell parameters were determined to be a =1.8 ± 0.1 nm, b= 1.8 ± 0.1 nm, α = 94 ± 2°. The molecular model is proposed as shown in Figure 1i. The hydrogen bonding formed between the carboxylic groups plays a significant role in stabilizing the assembled structure. In addition, the non-planar structure of phenyl at meta position with respect to porphyrin core leads to a twisted configuration of carboxylic acid, making a carboxylic group dangle without participating hydrogen bonding.

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(I) (b)

(a)

(II)

(c)

b α a

b b

1.0 nm 2 nm

(d)

1.7 nm

α a

2 nm

(f)

(e)

(I)

ba a

I

II

b a a 4 nm

(g)

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ba a

2 nm

Figure 1. STM image of the assembled structure of 1A at heptanoic acid/HOPG interface with two kinds of regions I (a) and II (c) (Iset = 204.5 pA, Vbias = -819.1 mV); (b) and (d) Proposed molecular models for the structures in (I) and (II); (e) High-resolution STM image of the assembled structure of o-2A at heptanoic acid/HOPG interface (Iset = 299.1 pA, Vbias = 313.4 mV); (f) and (g) Proposed molecular models for the monolayer structure of I and II in (e); (h) High-resolution STM image of 3A at heptanoic acid/HOPG interface (Iset = 296.0 pA, Vbias = 698.9 mV); (i) Proposed molecular model for the monolayer structure in (h).

Bilayer formation of NN4A/porphyrin After NN4A molecular solution is deposited onto the HOPG surface, a 2D well-ordered network formed at the liquid/solid interface, in which two types of cavities have different size and symmetry.25 The STM image and schematic model of NN4A network are shown in Figure S3. Two adjacent carboxylic groups form a pair of hydrogen bonding, which confers great stability to the 2D network. The unit cell (a = 2.6 ± 0.1 nm, b = 2.6 ± 0.1 nm, and α = 60 ± 2°) is superimposed on the molecular model. Two cavities show different inner diameters of 1.2 ± 0.1 nm and 0.9 ± 0.1 nm, which are formed by six symmetry benzene rings and three NN4A 9


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molecules, respectively. The head-to-head packing of hydrogen bonding makes the network stable enough, which would be interesting to the selectivity of the molecular networks. And then, the porphyrin solution is deposited followed by STM scanning. Unfortunately, few bright spots appeared, and large-areas are covered by NN4A grids, as shown in Figure 2(a). Then we tried to change the addition order of these two molecules. When 1A molecular solution was firstly added to the clean HOPG surface followed by depositing NN4A solution, a surprised assembled pattern was obtained as shown in Figure 2(b). In previous studies, we found that the addition of 4,4’-di(pyridine)-ethene (DPE) can break the firstly prepared NN4A grid structure and form a new cavity structure, which is mainly controlled by intermolecular hydrogen bonding -O-H···N=C-.31-32 Under UV light irradiation for 30 min, DPE molecules underwent desorption, resulting in another new structure dominated by hydrogen bonding -O-H···O=C- between carboxylic acid groups. In this work, the addition of 1A solution onto the NN4A network has no effect on the mesh of NN4A, indicating that the interaction between 1A and NN4A is weak, and then we tried to change the addition order of these two molecules. In contrast, the first deposition of 1A followed by the drop of NN4A solution does not lead to the formation of monolayer, but a bilayer structure. We also mixed the two molecules in solution and then dropped onto the HOPG, but unfortunately the co-assembly of the NN4A molecules was mainly obtained. This may be due to the delicate balance of interactions between NN4A molecules and that between NN4A and 1A. Based on this observation, the porphyrin solution is firstly deposited followed by deposition of NN4A solution for other three systems including NN4A/o-2A, NN4A/p-2A and NN4A/3A.

Figure 2. Large scale STM image of 1A/NN4A (a) and NN4A/1A (b) co-assembled networks at heptanoic acid/HOPG interface (Iset = 546.3pA, Vbias = 539.9 mV).

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Figure S4 shows large-scale STM images of these four systems including NN4A/1A, NN4A/o-2A, NN4A/p-2A and NN4A/3A. The high-resolution STM images of these coassembled systems are described in Figure 3. It can be clearly seen that large bright spots with diameters about 1 nm appear on the cavities of NN4A network. Compared with pure NN4A grid structure, these bright spots only appear in the hexagonal cavities. For the NN4A/1A system as shown in Figure 3a, the unit cell parameters are measured to be a = 2.6 ± 0.1 nm, b = 2.7 ± 0.1 nm, and α = 45 ± 2°. In comparison with the cell parameters of NN4A, the arrangement of 1A is different from the distribution of hexagonal cavities in NN4A. However, the cell parameters of pristine NN4A near the co-assembled region were not changed, indicating that the variation of cell parameters of NN4A/1A system is caused by the adsorption of 1A on the top of NN4A cavity. In the proposed model as shown in Figure 3b, 1A does not exactly adsorb in the center of the cavity induced by the interaction between 1A and NN4A owing to the asymmetric substituted carboxylic groups. In combination with DFT calculations, the total interaction energy for NN4A/1A system is calculated to be -161.353 kcal·mol-1, which is slightly larger than that (154.481 kcal·mol-1) of NN4A. Therefore, the inclusion of 1A molecules did induce the great enhancement of systematic stability, resulting in small region of co-assembled structure. Bilayer structures were also observed for NN4A/o-2A, NN4A/p-2A and NN4A/3A, as shown in Figure 3(c, e, g). The corresponding proposed models are displayed in Figure 3(d, f, h). The unit cell superimposed on STM images are determined for NN4A/o-2A (a = 2.6 ± 0.1 nm, b = 2.8 ± 0.1 nm and α = 53 ± 2°), NN4A/p-2A (a = 2.5 ± 0.1 nm, b = 2.8 ± 0.1 nm and α = 57 ± 2°), and NN4A/3A (a = 2.8 ± 0.1 nm, b = 3.2 ± 0.1 nm and α = 50 ± 2°). These different parameters reveal that the arrangements of porphyrin in the co-assembled bilayer structure strongly depend on the different numbers and positions of the carboxylic acid groups. Similar with the case of 1A, the other three molecules o-2A, p-2A and 3A did not locate the center of hexagonal cavity of NN4A network, but adsorb atop of the NN4A monolayer leading to the formation of 3D bilayer structures. In addition to the co-assembled structures, there are still some regions only containing o-2A or 3A structure. It should be noted that no single assembled structure of p-2A was found in the NN4A/p-2A co-assembled system. This is in accordance with the discovery of self-assembly of p-2A molecule above, which once again proves that p-2A molecules cannot form stable selfassemblies on HOPG surface. The backbones of porphyrin molecules appear as much brighter spots in the image than that of NN4A, which might be attributed to the position and higher electron density relative to NN4A 11


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molecule. The porphyrin molecules exhibit approximatively ring-shaped features, which are different from the generally observed two-lobe structure. The ring-shaped structure may come from the central part of porphyrin, therefore the substitutions at the meta position are non-planar adsorption. As shown in Table 1, the total energy of NN4A/o-2A, NN4A/p-2A and NN4A/3A systems are similar with that of NN4A/1A system, which are all slightly higher than that of NN4A. The interaction between porphyrins and HOPG substrate is so weak that can be neglected. The interactions between NN4A and porphyrin provide the dominant contribution to the adsorption energy and thus these co-assembled molecular arrays have the nearly equal stability. (b)

(a) b

(c)

(d)

a

a

a

ba

2 nm

4 nm

(h)

(g)

(f)

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ba a b a a 2 nm

2 nm

Figure 3. (a) High-resolution STM image and model (b) of the NN4A/1A co-assembled at heptanoic acid/HOPG interface (Iset = 546.3pA, Vbias = 539.9 mV); (c) High-resolution STM image and model (d) of NN4A/o-2A coassembled networks at heptanoic acid/HOPG interface (Iset = 296.0pA, Vbias = 698.9 mV); (e) High-resolution STM image and model (f) of NN4A/p-2A co-assembled networks at heptanoic acid/HOPG interface (Iset = 299.1pA, Vbias = 699.8 mV); (g) High-resolution STM image and model (h) of NN4A/3A co-assembled networks at heptanoic acid/HOPG interface (Iset = 296.1pA, Vbias = 698.8 mV).

Molecular configurations of upper molecules Generally, the self-assembled structure results from the balance of a variety of interactions, including the molecule-molecule and molecule-substrate interactions, whether there is only one component or multi components. For the purpose of further understanding the self-assembled mechanisms based on the observed phenomena, we performed DFT calculations to investigate the arrangement of the co-assembled nanostructures on basis of the related interactions. In the DFT calculations, the interactions were not only between the adsorbates but also between the adsorbates and the substrate, so the total energy included the interaction energy between the adsorbates and the interaction energy between the adsorbates and the substrate. We provided the 12


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total energies and total energy per unit area for observed self-assemblies. In the single component systems, the interactions between the porphyrin molecules are smaller than that (111.194 kcal mol-1) of NN4A molecule because of the pairs of hydrogen bonding −O−H···O=C−. That is why NN4A can form a wide-range, stable grid while porphyrin molecules only form small-range ordered structure. The total energy for 3A system is larger than that for o-2A and p2A system, therefore, the 3A network is much stable than o-2A and p-2A structure. All the total energy of these systems is slightly larger than that of NN4A, indicating the hydrogen bonding plays an important role in these binary systems. The more negative energy per unit area means that the NN4A/3A system is more stable than the other systems. Table 1. Interactions between adsorbates, interactions between adsorbates and substrate, total energies and total energy per unit area for these observed self-assemblies.

Interactions between adsorbates (kcal·mol-1) NN4A 1A-I 1A-II o-2A-1 o-2A-2 3A 1A/NN4A o-2A/NN4A p-2A/NN4A 3A/NN4A

-111.194 -7.530 -10.668 -18.135 -5.312 -36.646 -117.344 -118.746 -118.126 -120.261

Interactions between adsorbates and substrate (kcal·mol-1) -43.287 -11.114 -19.258 -10.303 -9.891 -12.342 -44.009 -44.617 -44.327 -45.448

Total energy (kcal·mol-1)

Total energy per unit area (kcal·mol-1·Å-2)

-154.481 -18.644 -29.926 -28.438 -15.203 -48.988 -161.353 -163.362 -162.453 -165.709

-0.285 -0.040 -0.047 -0.081 -0.041 -0.145 -0.298 -0.302 -0.300 -0.306

As shown in Figure 4, the underlying NN4A molecules are highlighted with yellow color in order to distinguish the upper porphyrin molecules. From the schematic models, it has been demonstrated that these four porphyrin molecules have different configurations and positions relative to the NN4A cavity. It is worthy to note that 1A and 3A have better flatness than o-2A and p-2A molecules, which deviate from the center of NN4A hexagon cavity. That is why the unit cell parameters of these binary systems are different from those of NN4A system. In addition, it is easy to realize that the hydrogen bonding interactions between porphyrin molecules and NN4A molecules dominate the molecular configurations and adsorption sites, which are calculated to be -6.328 kcal·mol-1 (NN4A/1A), -11.825 kcal·mol-1 (NN4A/o-2A), -11.144 kcal·mol-1 (NN4A/p-2A), -14.348 kcal·mol-1 (NN4A/3A), respectively. These small interactions 13


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indicate that the adsorption modes are unstable, which further proves the small region of bilayer assembled structure. As a similar bilayer system, two HPB derivatives which were modified with six carboxyl groups have been investigated with different sizes of stable networks through the hydrogen bonding between carboxyl groups, with the introduction of coronene as the guest specie, it exclusively formed a heterogeneous bilayer structure on top of the networks, and the results showed that the electronic interaction and van de waals interactions might contribute to the bilayer stability. 33 It is worth noting that the hydrogen bonding interaction also played the main role on the molecular configurations in our results. By optimizing the chemical structures of porphyrins, more complex nano-architectures and molecular nanodevices can be observed.34 Furthermore, this bilayer system can be considered as a vertical heterojunction in surface molecular engineering, which opens perspectives for the manipulation of surface molecular nanoarchitectures and helps developing high-performance organic molecular devices prepared by related materials. (a)

(b)

(c)

(d)

Figure 4. The top and side views of molecular configurations of porphyrin molecules on top of the NN4A networks.

Conclusions In conclusion, a series of porphyrin-based molecules substituted different numbers and positions of carboxylic acid groups have been designed. The formation of the coassembled hetero bilayer has been systematically studied through STM technique in combination with theory calculations. The results indicated that the various different 2D supramolecular nanostructures could be formed through changing the substituents of molecules. In addition to the intermolecular interaction, the hydrogen bonding between porphyrin molecules and NN4A molecules dominated the molecular configuration and adsorption site of the 2D packing patterns. Our bottom-up approach can also provide a good platform to clearly understand the molecules in stabilized networks. Although it is 14


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difficult to determine the effect of a double-layer structure, this work displays an efficient method on fabricating complex self-assembled networks in surface molecular engineering, and it will provide the new way to construct the 3D structures from a 2D surface in supramolecular chemistry and interfacial science. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21472029, 21773041, 21772036 and 51463002), the National Basic Research Program of China (No.2016YFA0200700). References (1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. About Supramolecular Assemblies of Pi-Conjugated Systems. Chem. Rev. 2005, 105, 1491-1546. (2) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. Solvent Controlled Self-Assembly at the Liquid-Solid Interface Revealed by Stm. J. Am. Chem. Soc. 2006, 128, 317-325. (3) Tahara, K.; Okuhata, S.; Adisoejoso, J.; Lei, S. B.; Fujita, T.; De Feyter, S.; Tobe, Y. 2d Networks of Rhombic-Shaped Fused Dehydrobenzo[12]Annulenes: Structural Variations under Concentration Control. J. Am. Chem. Soc. 2009, 131, 17583-17590. (4) Liao, L. Y.; Zhang, X. M.; Hu, F. Y.; Wang, S.; Xu, S. D.; Zeng, Q. D.; Wang, C. TwoDimensional Supramolecular Self-Assembly of Stilbene Derivatives with Ester Groups: Molecular Symmetry and Alkoxy Substitution Effect. J. Phys. Chem. C 2014, 118, 79897995. (5) Fernandez, L.; Thussing, S.; Manz, A.; Sundermeyer, J.; Witte, G.; Jakob, P. The Discrete Nature of Inhomogeneity: The Initial Stages and Local Configurations of Tiopc During Bilayer Growth on Ag(111). Phys. Chem. Chem. Phys. 2017, 19, 2495-2502. (6) Chen, T.; Pan, G. B.; Wettach, H.; Fritzsche, M.; Hoger, S.; Wan, L. J.; Yang, H. B.; Northrop, B. H.; Stang, P. J. 2d Assembly of Metallacycles on Hopg by Shape-Persistent Macrocycle Templates. J. Am. Chem. Soc. 2010, 132, 1328-1333. 15


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OH

OH

OH

O

O

N N O

O NH N

OH

O

OH

N HN

HO

HO O

OH O

O

O OH

OH

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Bilayer adsorption of porphyrin molecules substituted with carboxylic

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