Two-Dimensional Self-Assembly of a Pair of Triangular Macrocycles

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Two-Dimensional Self-Assembly of a Pair of Triangular Macrocycles Studied by STM Jing Xu,† Tian Li,‡ Yanfang Geng,† Dahui Zhao,*,‡ Ke Deng,*,† Qingdao Zeng,*,† and Chen Wang*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), 11 ZhongguancunBeiyitiao, Beijing 100190, China ‡ Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: In this investigation, we reported the twodimensional (2D) self-assembly of a pair of triangular macrocycles (TMC1 and TMC2) at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. Although with the similar triangle-shaped phenyl backbones, TMC1 and TMC2 displayed different 2D nanopatterns. Control experiments with varying concentrations and temperatures have been carried out. Phase separations were recorded in the coassembly of TMC1 and TMC2. Scanning tunneling microscopy (STM) measurements, as well as density function theory (DFT) calculations, revealed the formation mechanism of the TMC1 and TMC2 nanoarrays. Moreover, minor ringopening phenomena of TMC2 were detected by STM, which demonstrates the advantages of STM in trace content analysis.

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INTRODUCTION Discotic liquid crystals (DLCs) are widely used in photovoltaic devices,1 organic light emitting diodes (OLED),2−5 molecular wires,6 and compensating thin films of LCD displays.7,8 Typical structures of DLCs often include a π-electron-rich aromatic core and attached flexible alkyl chains.8 The central aromatic parts can transport charge carriers while the side alkyl chains will contribute to the self-assembly process and improve the solubility into various solvents.8 Previously, substituted benzene, triphenylene, perylene, and coronene derivatives, porphyrin and phthalocyanine derivatives, and macrocycles with aromatic groups were extensively studied as central cores.8,9 DLCs are favorable organic semiconductive molecules due to their anisotropy, processability, self-assembled and selfhealing properties, especially in solution-processed systems.1,10−12 Since most electronic devices involve a conductive substrate, the behavior of DLC molecules on the twodimensional (2D) scale is of great importance. The orientation, conformation, packing patterns, and structural defects of the interfacial region between substrate and organic structures may affect the rate of carrier mobility and even change the way of carrier transportation.13−17 With resolution at atomic level, scanning tunneling microscopy (STM) has an absolute advantage in reflecting the position and distribution of molecules on 2D surfaces or interfaces through the variation of tunneling current.13,18,19 Compared with scanning electron microscopy (SEM), trans© 2015 American Chemical Society

mission electron microscopy (TEM), and X-ray diffraction (XRD), which often require vacuum condition, STM under ambient atmosphere performs better in detecting the selfassembly process at 2D liquid/solid interfaces without damaging the samples.13,19−21 Furthermore, since STM can record electron density of single molecules,13,18 it may also be utilized in analyzing molecules with trace amounts. New molecules, which are hard to detect by traditional methods due to their tiny amounts, may be studied by STM. Previously, STM has been employed to study the self-assembly behavior of phthalocyanine, porphyrin, pyrene, coronene, and their derivatives.14,19,22−26 Among various DLC molecules, shape-persistent macrocycles represent a unique group. They are viewed as promising building blocks in fabricating 2D nanostructures.9,27,28 In a previous report, Shen et al. reported a triangle-shaped macrocycle. In their work, the macrocycle molecules selfassembled into glassy state networks with many triangular nanopores via van der Waals force.27 In this Article, a pair of triangle-shaped macrocycles (TMC1 and TMC2) investigated by STM is reported for the first time. Their chemical structures are shown in Scheme 1. In TMC1, the triangle scaffold was constituted by 3,6-phenanthrylene bis(ethynylene) units Received: December 4, 2014 Revised: April 8, 2015 Published: April 8, 2015 9227

DOI: 10.1021/jp512079z J. Phys. Chem. C 2015, 119, 9227−9233

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The Journal of Physical Chemistry C Scheme 1. Chemical Structures of TMC1 (a) and TMC2 (b)

together with p-phenylene groups while, in TMC2 the pphenylene groups were replaced by 3,6-phenanthrylene units in the triangle skeleton. Meanwhile, TMC2 molecule has more alkyl chains than TMC1. The length of the side alkyl chains of TMC1 was also shorter than that of TMC2. In this research, we constructed the arrangements of TMC1 and TMC2 on the highly oriented pyrolytic graphite (HOPG) surface. We not only focused on the separate self-assembly behavior, but also investigated their coassembled properties. STM measurements and density functional theory (DFT) calculations were utilized to reveal the formation mechanism of two-dimensional nanopatterns.

studied by STM with the tips immersed directly into the droplet. the Same experiments were also carried out for TMC2 molecules. The concentrations were denoted by C21, C22, and C23, while C21 = 2 × C22 = 4 × C23. Thermal Annealing Experiments. Three pieces of highly oriented pyrolytic graphite (HOPG, grade ZYB, NT-MDT, Russia) substrates were freshly cleaved using adhesive tape. A droplet of TMC1 solution with concentration C2 was added on each HOPG substrate. One sample was kept in the room temperature (20 °C) without further treatment. The second sample was heated to 45 °C and kept for 10 min. Then the sample was cooled to room temperature. The third sample was heated to 60 °C and kept for 10 min before cooled to room temperature. After the treatments, the samples were studied by STM with its tips immersed directly into the droplet. The samples of TMC2 molecules were treated for the same experiment procedures. Computational Details. Theoretical calculations were performed using DFT provided by DMol3 code.29 We used the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work. The Perdew and Wang parametrization30 of the local exchange correlation energy was applied in local spin density approximation (LSDA) to describe exchange and correlation. All-electron spinunrestricted Kohn−Sham wave functions were expanded in a local atomic orbital basis. For the large system, the numerical basis set was applied. All calculations were all-electron ones, and performed with the medium mesh. Self-consistent field procedure was done with a convergence criterion of 10−5 au 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, adsorbates have benzene-ring π-conjugated structures. Since adsorption of benzene on graphite and graphene should be very similar, we have performed our calculations on infinite graphene monolayers using PBC. In the superlattice, graphene layers were separated by 35 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 kpoint mesh. The interaction energy Einter of adsorbates with

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EXPERIMENTAL SECTION Sample Preparation. The solvent used in this work was 1phenyloctane (Aldrich) without further purification. A pair of triangular macrocycles TMC1 and TMC2 (chemical structures shown in Scheme 1) was synthesized according to the established procedures9 (SI1). They were dissolved separately with concentration less than 1.0 × 10−4 M. Three pieces of highly oriented pyrolytic graphite (HOPG, grade ZYB, NTMDT, Russia) substrates were freshly cleaved using adhesive tape. A droplet of solution containing TMC1 was deposited on one piece of HOPG substrate while a droplet of TMC2 was dropped on another piece of HOPG. Then a droplet of the hybrid solution of TMC1 and TMC2 with ratio 1:1 was added onto the third piece of HOPG. After the treatments, the samples were studied by STM with its tips immersed directly into the droplet. STM Measurement. All STM experiments were performed with a NanoscopeIIIa scanning probe microscope system (Bruker) in constant current mode under ambient conditions. STM tips were prepared by mechanically cutting of Pt/Ir wire (80/20). All the STM images provided are raw data and were calibrated by referring the underlying graphite lattice. Detailed tunneling conditions were given in the corresponding figure captions. Concentration Experiments. First, TMC1 molecules were dissolved with concentration less than 1.0 × 10−4 M. The concentration was denoted by C11. Then 2-fold dilutions of C11 solution were carried out for two sequent times to obtain solution C12 and C13. The concentration relations were C11 = 2 × C12 = 4 × C13. Three pieces of highly oriented pyrolytic graphite (HOPG, grade ZYB, NT-MDT, Russia) substrates were freshly cleaved using adhesive tape. A droplet of C11, C12, and C13 solution was deposited on separate HOPG substrate, respectively. After the treatments, the samples were 9228

DOI: 10.1021/jp512079z J. Phys. Chem. C 2015, 119, 9227−9233

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Figure 1. (a) Large-scale STM image of TMC1 molecular self-assembly at HOPG/1-phenyloctane interface, Iset = 299.1pA, Vbias = 599.1 mV. (b) High-resolution STM image of the self-assembled structures of TMC1 molecules, Iset = 349.1pA, Vbias = 599.1 mV. Two reverse molecular directions of TMC1 molecules are marked by the green and pink triangles. TMC1 molecules with the same orientation in the linear structure are demonstrated by the yellow arrows. The white line in (a) and (b) shows the staggered arrangements of TMC1 molecules with opposite orientations. (c) Suggested molecular model for (b). The red arrow shows TMC1 with the same orientations in the model. A unit cell was imposed on (b) and (c), respectively. Their parameters are shown in Table 1. Scale bar in (a) and (b) represents 5 nm.

graphite is given by Einter = Etot(adsorbates/graphene) − Etot(isolated adsorbates in vacuum) − Etot(graphene).

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RESULTS AND DISCUSSION After a droplet of solution containing TMC1 was deposited on freshly cleaved HOPG surface, the sample was detected by STM. An area of TMC1 molecules adsorbed on the surface was recorded. One TMC1 molecule appeared as a bright triangular macrocycle due to the electron density of the aromatic rings on the edges and vertexes of the backbone. The length of the edge was measured to be 2.0 nm. Since the acetylene linkers on the edges increased the rigidity of TMC1 molecule, the molecules exhibit good planarity in the self-assembly nanostructure. The large-scale STM image of TMC1 is shown in Figure 1a. We can notice that TMC1 molecules take turns to self-assemble into regularly linear structure in the long-range. Detailed assembly structures are shown in Figure 1b. Two kinds of molecular directions of triangles were observed in the self-assembly. One direction is denoted by the green-color triangle in Figure 1b, while the inverse direction was marked by the pink-color triangle. The two adjacent triangles in the linear structure were in opposite orientations to reduce the steric hindrance. Careful analysis of this rectangular network structure revealed that both the phenyl backbones of the two adjacent TMC1 molecules in the linear structure and the side alkyl chains of TMC1 triangles attracted each other via van der Waals interaction. Especially, the conjugated cores on the edges strengthened such interaction. Figure 1c shows the corresponding molecular model calculated by DFT method on the basis of STM observations. The measured unit cell was also superimposed on the molecular model with a1 = 3.85 ± 0.1 nm, b1 = 3.19 ± 0.1 nm, α = 88° ± 2°. We also investigated the self-assembly process of TMC2 at HOPG/1-phenyloctane interface. Figure 2a presents a large scale STM image of the self-assembled structure of TMC2. Owing to the 3,6-phenanthrylene units in the triangle skeleton, the TMC2 molecule appeared as a larger triangle with the measured length of the edge 2.7 nm, more than that of TMC1. Therefore, TMC2 was with the enlarged triangular-shaped cavity, and exhibited even better planarity than TMC1. Obviously, TMC2 molecular arrangements appeared disordered in the large scale. A higher resolution STM image is shown in Figure 2b, which shows the detailed assembled structure. In Figure 2b, we could recognize two kinds of TMC2 self-assembly structures in the local area, which were marked by domain A1 and domain A2. The two patterns were denoted by

Figure 2. (a) Large-scale STM image of TMC2 molecular selfassembly at HOPG/1-phenyloctane interface, Iset = 299.1pA, Vbias = 599.1 mV. (b) High-resolution STM image of the self-assembly structures of TMC2 molecules, Iset = 349.1pA, Vbias = 599.1 mV. Two self-assembly structures are marked by domains A1 and A2. (c) Proposed molecular model for domain A1 (TMC2@A1) in (b). (d) Proposed molecular model for domain A2 (TMC2@A2) in (b). The scale bar in (a) and (b) is 5 nm. Unit cells were imposed on TMC2@ A1 and TMC2@A2, together with their molecular models. Their parameters are shown in Table 1.

TMC2@A1 and TMC2@A2, respectively. In domain A1, TMC2 molecules closely packed with the reverse directions of triangles to assembly into linear structure just like TMC1, but the long-range order was obviously interrupted. In domain A2, the packing arrangements of triangles in TMC2@A2 were even looser with a clear distance between two neighboring molecules. According to the STM image, the corresponding molecular models are proposed in Figure 2c and d. The measured unit cell for TMC2@A1 is presented with W1 = 5.63 ± 0.1 nm, L1 = 4.86 ± 0.1 nm, and γ1 = 79° ± 2°; and for TMC2@A2 is presented with W2= 7.86 ± 0.1 nm, L2 = 4.65 ± 0.1 nm, and γ2 = 83° ± 2°. 9229

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Figure 3. Self-assembly structures of TMC1 molecules under concentration of (a) C11, (b) C12, and (c) C13. Iset = 299.1 pA, Vbias = 599.1 mV, C11 > C12 > C13. Each scale bar in the STM images represents 10 nm.

assembly under varying temperatures. Both the loose and the closely packing pattern could still be observed under different temperatures. When the temperature was increased, the percentage of the close pattern increased in our observations of the experiments. We suggested that, with the higher temperature, the system would be easy to achieve more stable state, and the loose pattern of TMC2 tends to transform into the more stable packing pattern. At last, we have observed the arrangement of TMC1 and TMC2 combination at HOPG/1-phenyloctane interface. In Figure 7, STM results show that TMC1 and TMC2 molecules phase separated into different domains, which are marked by domains B1 and B2. In domain B1, TMC1 displayed the similar arrangements as the previous observation with TMC1 alone. In domain B2, TMC2 also appeared disordered in large scale, and the lattice parameters were between that of TMC2@A1 and TMC2@A2. To further investigate the different formation mechanism of TMC1 and TMC2 assembly behavior, we performed the DFT calculations based on the above observed phenomena. The calculated lattice parameters for 2D networks are summarized in Table 1. The calculated parameters agreed well with the experimental data. In the surface assembly system, the interaction between adsorbates and substrate plays an important role. Therefore, we present the total energy (including the interaction energy between adsorbates and the interaction energy between adsorbates and substrate) in Table 2. Furthermore, a reasonable way to compare the thermodynamic stability of the different arrays should be the total energy per unit area. Then we also present the total energy per unit area of the system in Table 2. From Table 2, we notice that the total energy per unit area of TMC1 assembled structure is with the most thermodynamic stability (−0.39 kcal mol−1 Å−2). It indicated that the orderly assembly of TMC1 is the energetically favorable structure.

According to previous report, concentration variations tend to have effects on the arrangements of molecules with long alkyl chains.31 We have carried out concentration experiments for TMC1 and TMC2. Figure 3 shows the STM results of TMC1 self-assembly under varying concentrations. Evidently, the packing patterns of TMC1 still exhibited in long-range linear structures under different concentrations. Through careful inspection of the images in Figure 3, we can see that there were slight changes under different concentrations. The packing pattern was tighter with higher concentration, and slightly looser with lower concentration. However, the changes of parameters of the unit cells were within the margin of error (i.e., ±0.1 nm). Figure 4 shows the STM results of TMC2 self-

Figure 4. Self-assembly structures of TMC2 molecules under concentration of (a) C21, (b) C22, and (c) C23. Iset = 299.1 pA, Vbias = 599.1 mV, C21 > C22 > C23. Each scale bar in the STM images represents 10 nm.

assembly under varying concentrations. The loose pattern and the closely packing pattern could still be observed under different concentrations. Furthermore, we have performed the experiments under varying temperatures. Figure 5 shows the self-assembly structures of TMC1 molecules under different temperatures. The packing patterns still exhibited in long-range linear structures. Figure 6 shows the STM results of TMC2 self-

Figure 5. Self-assembly structures of TMC1 molecules under temperature of (a) 20 °C, (b) 45 °C, and (c) 60 °C. Iset = 299.1 pA, Vbias = 599.1 mV. Each scale bar in the STM images represents 10 nm. 9230

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Figure 6. Self-assembly structures of TMC2 molecules under temperature of (a) 20 °C, (b) 45 °C, and (c) 60 °C. Iset = 299.1 pA, Vbias = 599.1 mV. Each scale bar in the STM images represents 10 nm.

results agree well with our STM observations. By carefully comparing the chemical structures of TMC1 and TMC2, we suggested that such different assembly stability possibly due to their different side groups. For TMC1, with the conjugated cores on the backbone, it is easy to closely pack with opposite orientations in the linear order. While for TMC2 molecules, with the longer alkyl chains on the vertexes and two more alkyl chains on each backbone, it should overcome more steric hindrance to be closely packed. Considering the flexibility of alkyl chains, TMC2 could be packed closely or loosely. That is why TMC2 only appeared in ordered assembly in local area, and disordered in the large scale. Moreover, because the closely packed pattern TMC2@A1 is more stable than the loosely packed pattern TMC2@A2, we could observe that the percentage of the close pattern increased in our observations of the experiments when we increased the temperature. It should be noted that TMC2@A1 and TMC2@A2 only represent two typical patterns in the self-assembly of TMC2. In our experiment process, the lattice parameters may vary under different conditions due to the flexibility of alkyl chains. The discrepancy of energy of per unit area between TMC2@ A1 and TMC2@A2 is fairly comparable. Therefore, we could observe the coexistence of both the loosely and closely packing patterns of TMC2 in our experiments. In addition, through carefully inspecting the STM image, minor open-ring phenomena of TMC2 molecules are observed in Figure 7. They are highlighted by the yellow circles. Highresolution STM images for such phenomena are shown in Figure 8. In Figure 8a, there is a clover-shaped molecule, while in Figure 8c a cross-shaped molecule is observed. Their chemical structures are shown in Figure 8b and d, respectively. These macrocyclic molecules were formed during the synthetic process. Since the content of such molecules is minor, their structures are difficult to be isolated from the more abundant analogues and identified by regular analysis methods. Their existence was proved for the first time through STM method. With better resolution and higher sensitivity in locating a single molecule on 2D surfaces or interfaces, STM also demonstrates an absolute advantage in trace detection. Especially, when such

Figure 7. Large-scale STM image for the coassembly structure of TMC1 and TMC2 at HOPG/1-phenyloctane interface, Iset = 349.1pA, Vbias = 599.1 mV. The scale bar is 10 nm. Domain B1 is constituted by TMC1 molecules, while domain B2 is composed of TMC2 molecules. Open-ring molecules of TMC2 in the coassembly are highlighted by the yellow circles.

Table 1. Experimental (exptl) and Calculated (calcd) Lattice Parameters for TMC1, TMC2@A1, and TMC2@A2 unit cell parameters a (nm) TMC1

TMC2@A1 TMC2@A2

exptl calcd exptl calcd exptl calcd

3.85 ± 0.1 3.90 W (nm) 5.63 ± 0.1 5.60 7.86 ± 0.1 7.90

b (nm)

α (deg)

3.19 ± 0.1 3.20 L (nm)

88 ± 2 88 γ (deg)

4.86 ± 0.1 4.95 4.65 ± 0.1 4.70

79 ± 2 79 83 ± 2 82

While for TMC2, the total energy per unit area of TMC2@A1 and TMC2@A2 packing pattern is −0.37 and −0.26 kcal mol−1 Å−2, respectively. Obviously, the assembly behavior of TMC2 would be with less stable than that of TMC1. The calculation

Table 2. Total Energy and Energy per Unit Area for TMC1, TMC2@A1, and TMC2@A2a

TMC1 TMC2@A1 TMC2@A2

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)

−87.39 −54.91 −26.95

−394.68 −948.62 −938.71

−482.07 −1003.53 −965.66

−0.39 −0.37 −0.26

a

The total energy includes the interaction between molecules and the interaction between molecules and HOPG. Here, the more negative energy means the system is more stable. 9231

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AUTHOR INFORMATION

Corresponding Authors

*Tel: 86-10-62753973. Fax: 86-10-62753973. E-mail: dhzhao@ pku.edu.cn. *Tel: 86-10-82545550. Fax: 86-10-62656765. E-mail: kdeng@ nanoctr.cn. *Tel: 86-10-82545548. Fax: 86-10-62656765. E-mail: zengqd@ nanoctr.cn. *Tel: 86-10-82545561. Fax: 86-10-62656765. E-mail: wangch@ nanoctr.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2011CB932303, 2013CB934203, and 2012CB933001) and the Chinese Academy of Sciences (No. YZ201318). The National Natural Science Foundation of China (Nos. 51173031, 91127043, 21472029, 21174004, 21222403, and 51203030) is also gratefully acknowledged.

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Figure 8. (a) Clover-shaped molecule observed in TMC2 selfassembly. The clover-shaped molecule is highlighted by the yellow circle. Iset = 349.1pA, Vbias = 649.1 mV. (b) Chemical structure of the clover-shaped molecule in (a). (c) Cross-shaped molecule observed in TMC2 self-assembly. The cross-shaped molecule is highlighted by the yellow circle. Iset = 349.1pA, Vbias = 649.1 mV. (d) Chemical structure of the cross-shaped molecule in (c). The scale bar in (a) and (c) is 5 nm.

(1) Schmidt Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics. Science 2001, 293, 1119−1122. (2) Seguy, I.; Jolinat, P.; Destruel, P.; Farenc, J.; Mamy, R.; Bock, H.; Ip, J.; Nguyen, T. P. Red Organic Light Emitting Device Made from Triphenylene Hexaester and Perylene Tetraester. J. Appl. Phys. 2001, 89, 5442−5448. (3) Seguy, I.; Jolinat, P.; Destruel, P.; Mamy, R.; Allouchi, H.; Courseille, C.; Cotrait, M.; Bock, H. Crystal and Electronic Structure of a Fluorescent Columnar Liquid Crystalline Electron Transport Material. ChemPhysChem 2001, 2, 448−452. (4) Seguy, I.; Mamy, R.; Destruel, P.; Jolinat, P.; Bock, H. Photoemission Study of the ITO/Triphenylene/Perylene/Al Interfaces. Appl. Surf. Sci. 2001, 174, 310−315. (5) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated, HighMobility Polymer Field-Effect Transistors Driving Polymer LightEmitting Diodes. Synth. Met. 1999, 102, 857−860. (6) Steinhart, M.; Zimmermann, S.; Goring, P.; Schaper, A. K.; Gosele, U.; Weder, C.; Wendorff, J. H. Liquid Crystalline Nanowires in Porous Alumina: Geometric Confinement versus Influence of Pore Walls. Nano Lett. 2005, 5, 995−995. (7) Brown, P. J.; Sirringhaus, H.; Friend, R. H. Electro-Optical Characterisation of Field Effect Devices with Regioregular PolyHexylthiophene Active Layers. Synth. Met. 1999, 101, 557−560. (8) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Liquid-Crystals of Disc-Like Molecules. Pramana 1977, 9, 471−480. (9) Li, T.; Yue, K.; Yan, Q.; Huang, H.; Wu, H.; Zhu, N.; Zhao, D. Triangular Arylene Ethynylene Macrocycles: Syntheses, Optical, and Thermotropic Liquid Crystalline Properties. Soft Matter 2012, 8, 2405−2415. (10) Kreouzis, T.; Donovan, K. J.; Boden, N.; Bushby, R. J.; Lozman, O. R.; Liu, Q. Temperature-Independent Hole Mobility in Discotic Liquid Crystals. J. Chem. Phys. 2001, 114, 1797−1802. (11) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Haegele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; et al. Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics. Angew. Chem., Int. Ed. 2007, 46, 4832−4887. (12) Kreouzis, T.; Scott, K.; Donovan, K. J.; Boden, N.; Bushby, R. J.; Lozman, O. R.; Liu, Q. Enhanced Electronic Transport Properties in Complementary Binary Discotic Liquid Crystal Systems. Chem. Phys. 2000, 262, 489−497.

molecule has good planarity and can participate into the selfassembly of stable 2D nanostructure, it might be analyzed and investigated by STM.

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CONCLUSION In summary, we have investigated a pair of triangle-shaped macrocycles using STM at HOPG/1-phenyloctane interface. STM images showed that TMC1 molecules presented a regular linear structure while the arrangements of TMC2 molecules were lack of long-range order. Control experiments with varying concentrations and temperatures have been carried out. Phase separations were recorded in the coassembly of TMC1 and TMC2. TMC1 molecules maintained its regular linear order and TMC2 molecules arranged randomly in its domain. In addition, minor ring-opening phenomena of TMC2 were detected by STM. New molecules with minor amount were detected by STM. Furthermore, DFT calculations were utilized to reveal the formation mechanism of the TMC1 and TMC2 molecular nanoarrays. High-resolution STM images, as well as DFT calculations, provide a new perspective to study the 2D nanostructures at submolecular level. The method also demonstrates the advantages in analyzing molecules with trace amount. The new sights may enhance the understanding of DLC molecule self-assembly at interfaces and improve further applications in nanodevices.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Characterization data for macrocycle TMC2. This material is available free of charge via the Internet at http://pubs.acs.org. 9232

DOI: 10.1021/jp512079z J. Phys. Chem. C 2015, 119, 9227−9233

Article

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DOI: 10.1021/jp512079z J. Phys. Chem. C 2015, 119, 9227−9233