English

• Supramolecular self-assembly behavior is widespread in nature and attracts intensive attention. This bottom-up self-assembly method is trying to be applied in the field of chemistry, biology etc. [1-5]. Self-assembly can be a promising approach of preparing functional nanomaterials with special nanostructures by rationally designing building blocks [6, 7]. The surface-confined nanostructures are related with the intermolecular interactions and molecule-substrate interactions [8, 9], so the relationship between them need to be further investigated. Scanning tunneling microscopy (STM) with atomic and molecular resolution can give details about molecular conformation and distribution, which is beneficial to the in-depth understanding and regulation of surface-supported self-assembly nanostructures [10-14].

  Oligothiophene derivatives show broad prospects for applications in electronic devices, such as organic photovoltaics [15-17], molecular electronics [18] and organic electroluminescence devices [19, 20] attributed to their outstanding and tuneable photoelectric properties. Earlier STM researches showed the two-dimensional assembly structures of the linear oligothiophene derivatives depended on their molecular structures, and van der Waals forces between the alkyl side chains of linear oligothiophene derivatives can be a dominant driving force in the construction of large-area ordered lamellar structure [21-23]. In general, the surface-confined self-assembly behavior is not the result of one single driving force, but the synergy of multiple forces [24, 25]. Functional groups such as carboxylic groups and halogen substituents can be easily introduced in the α-position of oligothiophene derivatives [26, 27]. The number of halogen substituents can influence the dipole moment of the linear oligothiophene derivatives. The formed dipole-dipole interaction and the van der Waals forces between the alkyl side chains together induced the oligothiophene derivatives to aggregate into different two-dimensional nanostructures [28]. For the linear oligothiophene derivatives modified with carboxyl groups at the α-position, the difference in the alkyl side chains modified on the thiophene unit and the length of the thiophene skeleton had an important impact on the intermolecular interactions and molecule-substrate interactions, which led to different assembly results [29]. Recently, linear oligothiophene derivatives consisting of electron-rich thiophene backbone and two terminal electron-deficient dicyanovinyls have been widely adopted to cooperate with the acceptors in photovoltaic devices for high power conversion efficiencies (PCEs) [30, 31]. As for linear dicyanovinyl-hexylthiophene (DCV6T) molecules without alkyl chains, the islands and chains assembly structures can be formed on the surface, since the two terminal dicyanovinyls of each DCV6T molecule interacted with the thiophene moieties or olefinic bonds of another two DCV6T molecules via C—H···N hydrogen bonds [32].

  In this work, a series of linear oligothiophene derivatives (DCV-nT-Hex, n = 3~11) modified with terminal dicyanovinyls and alkyl chains were adopted to further investigate the effect of the orientation of alkyl chains on their assembly behaviors by STM and DFT. The synergy of van der Waals interactions and hydrogen bonding on the nanostructures of DCV-nT-Hex (n = 3~11) was revealed here. The chemical structures of DCV-nT-Hex (n = 3~11) molecules were displayed in Scheme 1 and the difference between their chemical structures was the number of thiophene groups decorated by alkyl chains. Interestingly, linear oligothiophene derivatives in previous studies formed the lamellar structures [33-35], while DCV-3T-Hex in this work aggregated into special zigzag and flower structures and DCV-nT-Hex (n = 4~11) self-assembled into similar lamellar structures. Alkyl chains were found to distribute on one side of the DCV-3T-Hex but on two sides of other DCV-nT-Hex (n = 4~11) molecules, which affected intermolecular interactions and ultimately led to the difference in their assembled structures. Besides, the assembled structure's stability weakened with increasing the odd or even numbered thiophene groups, and overall, even-numbered ones were relatively more stable than odd-numbered ones.

  Scheme 1

  

  Scheme 1.  Chemical structures of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11).

  DownLoad: Full-Size Img PowerPoint

  With the deposition of DCV-3T-Hex solution on HOPG, two types of self-assembled structures, zigzag and flower structures, were observed in the large-area HOPG surface with the aid of STM (Fig. S1a in Supporting information). As seen from Fig. 1a and Fig. S1b (Supporting information), DCV-3T-Hex molecules showed as short stick-liked bright spots with the measured length of 1.6 ± 0.1 nm. Every two DCV-3T-Hex molecules interacted with a head-to-tail manner to form a dimer as the building block (marked as two blue sticks), and further aggregated parallel into rows. While the neighboring rows were arranged with a certain angle (about 120°, marked as white dashed line intersection angle), the zigzag structure was formed. The spacing between two adjacent rows was about 0.8 ± 0.1 nm, which was slightly larger than the length of DCV-3T-Hex's alkyl chains. Thus, the alkyl chains of DCV-3T-Hex molecules were possibly to distribute in the slightly darker gap between the two rows and adsorbed on HOPG surface to enhance the stability of the self-assembly system. The measured lattice parameters in Fig. 1a were: a = 5.9 ± 0.1 nm, b = 1.4 ± 0.1 nm, α = 91° ± 1° (Table 1). Based on the experimental data (Table 1), DFT calculations were performed and the optimized assembly model for DCV-3T-Hex's self-assembly structure was shown in Fig. 1b. Considering the rotatability of the single bond, we performed theoretical calculations for different isomers of each DCV-nT-Hex (n = 3~11) in this work, and adopted the most energetically favorable structures in Fig. S6 (Supporting information) for the assemblies. For example, we have demonstrated nine isomers of DCV-3T-Hex in Fig. S4 (Supporting information) and chosen the optimal configuration in the subsequent assembly. It is noticed that in every building block two molecules interacted through C—H···N hydrogen bonding between the terminal dicyanoethylene moieties (marked with red circle). The building block further interacted with another one through C—H···N hydrogen bonding both between the terminal dicyanoethylene moieties (marked with red circle) and between the terminal dicyanovinyl and the central thiophene moiety (marked with black circle). This resulted in two orientations of DCV-3T-Hex molecules adsorbing on HOPG, and a certain angle (about 120°) was formed between DCV-3T-Hex rows. Additionally, DCV-3T-Hex building blocks connected with others in the same row via the van der Waals interactions between their alkyl chains (marked with purple rectangle in Fig. 1b).

  Figure 1

  

  Figure 1.  High-resolution of DCV-3T-Hex's assembled structures at the 1-phenyloctane/HOPG interface: (a) zigzag structure, I_set = 329.6 pA, V_bias = 748.6 mV; (c) nanoporous structure, I_set = 376.0 pA, V_bias = 763.6 mV. (b, d) The calculated molecular models for zigzag structure and porous structure respectively.

  DownLoad: Full-Size Img PowerPoint

  Table 1

  

  Table 1.  Experimental (Expt.) and calculated (Calcd.) lattice parameters of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11) self-assembled structures.

  DownLoad: CSV

  Another flower porous self-assembly structure of DCV-3T-Hex that stably co-existed with the zigzag structure can be observed in Fig. S1a, and more details can be acquired from the high-resolution STM image in Fig. 1c. Two DCV-3T-Hex arranged in parallel head to tail can be regarded as a dimer (marked as two red/blue sticks), and three DCV-3T-Hex dimers constituted a triangular cavity, which were highlighted by blue lines in Fig. 1c. And the cyano end of each DCV-3T-Hex dimer was located near the thiophene moiety of other adjacent dimer. Among the three DCV-3T-Hex dimers, two of them respectively formed inverted triangular cavities with two adjacent dimers indicated by red lines in Fig. 1c, while the rest one was adjacent to another two DCV-3T-Hex molecules both marked by white lines. The lattice parameters a, b and α of the flower structure (Table 1) were measured to be 4.1 ± 0.1 nm, 5.4 ± 0.1 nm and 72° ± 1° respectively. As indicated by the proposed molecular model (Fig. 1d), the three DCV-3T-Hex dimers constituting the triangular cavity formed C—H···N hydrogen bonding through the terminal dicyanovinyl and the central thiophene moiety (marked with red circle and black circle), and their alkyl chains were distributed in the triangular cavity (marked with purple circle). The two DCV-3T-Hex molecules in a dimer interacted with each other through the C—H···N hydrogen bonding between the terminal dicyanoethylene moieties (marked with red circle). As for the DCV-3T-Hex molecules identified by white lines, their terminal dicyanoethylenes and the central thiophene moiety respectively formed C—H···N hydrogen bonds with the central thiophene moiety and the terminal dicyanoethylenes of other DCV-3T-Hex molecules in adjacent dimers (marked with orange circle).

  To further explore the effect of molecular conjugation length on molecular self-assembly, we selected DCV-nT-Hex molecules with different amounts of thiophenes from 4 to 11. For DCV-nT-Hex (n = 4~11) molecules, all of them self-assembled into the similar lamellar structures in large domains (Figs. S2 and S3 in Supporting information). However, we found that the assemblies of DCV-nT-Hex molecules with odd and even numbers of thiophenes were different in some details.

  Self-assembly behaviors of DCV-nT-Hex molecules with even-numbered thiophenes were investigated in Fig. 2 by means of STM and DFT calculations. For example, DCV-4T-Hex molecules possessed a linear conjugated structure like that of the DCV-3T-Hex molecule, except that one thiophene units modified with the alkyl chain were added in the middle of its skeleton. DCV-4T-Hex molecules generated a lamellar structure on 1-phenyloctane/HOPG interface. The stubby rod-like bright spots with the measured length of 2.0 ± 0.1 nm correspond to the DCV-4T-Hex molecules were obtained and observed in Fig. 2a and Fig. S2a. We found that ends of each DCV-4T-Hex molecule were completely flush with that of another two DCV-4T-Hex molecules in the same row. The experimental values of the lattice parameters marked in Fig. 2a were: a = 2.2 ± 0.1 nm, b = 1.5 ± 0.1 nm, α = 118° ± 1° (Table 1). The optimized molecular assembly model in Fig. 2b showed that along axis a, in the same row, DCV-4T-Hex molecules interacted with the neighboring one head-to-tail through C—H···N hydrogen bonding between the terminal dicyanoethylene moieties (marked with red circle). And with the alkyl chains alternately arranged on two sides, DCV-4T-Hex molecules interacted with neighboring one in adjacent rows through van der Waals interactions between alkyl chains (marked with purple rectangles). In accordance to Figs. 2c, e and g, the lattice parameter a (Table 1) and the length of rod-like bright spots representing DCV molecules in the STM images gradually increased with the increment of molecular thiophene moiety. Moreover, DCV-nT-Hex (n = 6, 8, 10) molecules' assembled structures which depicted in Figs. 2c, e and g were very similar to DCV-4T-Hex. The optimized molecular models in Figs. 2d, f and h showed that each DCV-nT-Hex (n = 6, 8, 10) molecule also formed van der Waals forces with others in the adjacent rows and C—H···N hydrogen bonds with the neighboring ones in a row, which indicated that the interactions in their structures were the same with that in DCV-4T-Hex's structure.

  Figure 2

  

  Figure 2.  High-resolution STM image of the self-assembly structures of DCV-nT-Hex (n = 4, 6, 8, 10) molecules with even-numbered thiophenes at the 1-phenyloctane/HOPG interface: (a) DCV-4T-Hex, imaging conditions: I_set = 189.2 pA, V_bias = 1006 mV. (c) DCV-6T-Hex, imaging conditions: I_set = 299.1 pA, V_bias = 699.8 mV. (e) DCV-8T-Hex, imaging condition: I_set = 369.3 pA, V_bias = 673.8 mV. (g) DCV-10T-Hex, imaging conditions: I_set = 368.5 pA, V_bias = 557.4 mV. (b, d, f, h) The calculated molecular models for the DCV-4T-Hex's, DCV-6T-Hex's, DCV-8T-Hex's, and DCV-10T-Hex's structures respectively.

  DownLoad: Full-Size Img PowerPoint

  Similarly, high-resolution self-assembly nanopatterns of DCV-nT-Hex molecules with odd-numbered thiophenes were depicted in Fig. 3. Overall, their assembled arrays were lamellar structures as those of DCV-nT-Hex molecules with even-numbered thiophenes. However, there were different in some details. Taking DCV-5T-Hex as an example, a lamellar-structure assembly was observed in Fig. 3a and Fig. S3a, where the rod-like bright spots with the measured length of 2.4 ± 0.1 nm corresponded to the DCV-5T-Hex molecules. Since the distance between rows was about 0.8 ± 0.1 nm, the alkyl chains of neighboring DCV-5T-Hex molecules were speculated to distribute in the slightly darker gap between rows. Along the row direction, i.e., a-axis, every two DCV-5T-Hex molecules formed into dimers as building blocks. Dimers further interacted head-to-tail in the same row, while in the neighboring row parallel and interlaced along b-axis. Finally, they aggregated into the double-row lamellar structures. The experimental values of the lattice parameters marked in Fig. 3a were: a = 5.2 ± 0.1 nm, b = 2.8 ± 0.1 nm, α = 92° ± 1° (Table 1). DFT calculations have been performed and the optimized assembly structure was shown in Fig. 3b. DCV-5T-Hex molecules formed dimers through the C—H···N hydrogen bonding between the terminal dicyanoethylene moieties (marked with red circle). Careful calculations show that the most energetically favorable configuration of DCV-5T-Hex (Fig. S5 in Supporting information), with the alkyl chains distributed on two sides of the backbone, could form optimal straight-line pattern, matching well with the rod-like bright spots in Fig. 3a. Van der Waals interactions existed between the interdigitated alkyl chains of the DCV-5T-Hex molecules arranged along b-axis (marked with purple circle and rectangle), and C—H···N hydrogen bonds were formed by the dicyanovinyls of DCV-5T-Hex molecules arranged along a-axis. In the same way, high-resolution self-assembled nanostructures and calculated molecular models of DCV-nT-Hex (n = 7, 9, 11) molecules were shown in the Figs. 3c-h, the combination of intermolecular hydrogen bonding and van der Waals interactions contributed to the generation of DCV-nT-Hex's (n = 7, 9, 11) double-row lamellar structures.

  Figure 3

  

  Figure 3.  High-resolution STM image of the self-assembly structures of DCV-nT-Hex (n = 5, 7, 9, 11) molecules with odd-numbered thiophenes at the 1-phenyloctane/HOPG interface: (a) DCV-5T-Hex, imaging conditions: I_set = 299.1 pA, V_bias = 699.8 mV. (c) DCV-7T-Hex, imaging conditions: I_set = 363.2 pA, V_bias = 629.9 mV; (e) DCV-9T-Hex, imaging condition: I_set = 434.5 pA, V_bias = 614.1 mV; (g) DCV-11T-Hex, imaging conditions: I_set = 402.8 pA, V_bias = 694.0 mV. (b, d, f, h) The calculated molecular models for the DCV-5T-Hex's, DCV-7T-Hex's, DCV-9T-Hex's, and DCV-11T-Hex's structures, respectively.

  DownLoad: Full-Size Img PowerPoint

  The calculated parameters for the nanopatterns of DCV-nT-Hex (n = 3~11) are summarized in Table 1, which agree well with the experimental values. Moreover, the total energies including the energies of adsorbate-adsorbate interaction and adsorbate-substrate interaction are further calculated and presented in Table 2. Generally, the total energy per unit area could be employed to estimate the thermodynamic stability of the assemblies. The more negative the total energy per unit area, the more stable the system is. As shown in Table 2, DCV-3T-Hex zigzag or flower nanostructure are with the lowest total energy per unit area (−0.423 kcal mol⁻¹ Å⁻², and −0.411 kcal mol⁻¹ Å⁻², respectively), indicating that the two self-assembled structures are the most favorable comparing with other DCV-nT-Hex's (n = 4~11) structures. And the two structures are with the comparable stability, resulting in the stable co-existence on HOPG. Furthermore, the total energies per unit area increase gradually with the odd or even number increasing, meaning that the stabilities of the self-assembled structures are weakened with the extension of the molecular backbone. In addition, it is worth mentioning that structures of even-numbered molecules are overall slightly more stable than those of odd-numbered ones.

  Table 2

  

  Table 2.  Theoretical calculated total energy and total energy per unit area for self-assembled structures of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11) molecules. Total energy value included molecule-molecule interactions and molecule-HOPG substrate interactions.

  DownLoad: CSV

  As revealed by the experimental results, only the lamellar structure was observed in the self-assembly systems of DCV-nT-Hex (n = 4~11) despite the number of the thiophene moiety was increased in the molecular skeleton. In the lamellar structure, van der Waals forces existed among alkyl chains of DCV-nT-Hex (n = 4~11) molecules, and C—H···N hydrogen bonds were formed by the terminal dicyanovinyls. However, in addition to the above-mentioned interactions, there was another C—H···N hydrogen bonding between the terminal dicyanovinyl and the central thiophene moiety, hence two structures (zigzag structure and flower structure) of DCV-3T-Hex molecules were formed and different from DCV-nT-Hex (n = 4~11) molecules. This can be explained by the orientation of the alkyl chains on the thiophene moieties. For the optimal configuration of DCV-3T-Hex (Figs. S4 and S6), two alkyl chains located on one side of the molecular backbone, which exposed all thiophene moieties to the terminal dicyanovinyls of the other DCV-3T-Hex molecules and facilitated the formation of hydrogen bonds between them. However, the alkyl chains of DCV-nT-Hex (n = 4~11) molecules were distributed on both sides of the molecular backbone (Fig. S6), which hindered the hydrogen bonds between the terminal dicyanovinyls and the central thiophene moieties without alkyl chains. Therefore, hydrogen bonds were just formed by the terminal dicyanovinyls of molecules, which induced DCV-nT-Hex (n = 4~11) molecules to arranged in the head-to-tail manner in each row, and finally led to the formation of lamellar structures of DCV-nT-Hex (n = 4~11) molecules.

  With regards to DCV-nT-Hex (n = 4~11) molecules, DCV-nT-Hex molecules with even and odd numbers of thiophenes also differed in some details. It is worthwhile to note that in the DCV-nT-Hex's (n = 4, 6, 8, 10) assemblies, single molecules as building blocks assembled into single-row lamellar structures, while in DCV-nT-Hex's (n = 5, 7, 9, 11) assemblies, dimers as building blocks assembled into double-row lamellar structures. DFT results show that for DCV-nT-Hex molecules, the energetically favorable isomers are with the terminal dicyanovinyls on the same side as the neighboring thiophene moiety (Fig. S6). It means that for the even-numbered DCV-nT-Hex, terminal dicyanovinyls tend to be on opposite sides of the backbone; while for the odd-numbered DCV-nT-Hex's, terminal dicyanovinyls tend to be on same sides of the backbone. Therefore, even-numbered DCV-nT-Hex molecules were easy to form head-to-tail single-row lamellar structures via C—H···N hydrogen bonding. However, for odd-numbered DCV-nT-Hex molecules, they preferred to form dimers head-to-tail. And then the dimers interacted head-to-tail in the same row, but in the neighboring row dimers parallel and interlaced along the b-axis to aggregate into the double-row lamellar structures. Besides, the center of the odd-numbered DCV-nT-Hex's backbone is a thiophene without the alkyl chain differing from the even-numbered DCV-nT-Hex. In this case, the alkyl chains were distributed on the two sides of the odd-numbered DCV-nT-Hex's backbone unevenly. This also explained why self-assembled nanostructures of DCV-nT-Hex molecules with even numbers (n = 4, 6, 8, 10) were overall more stable than those with odd numbers (n = 5, 7, 9, 11).

  In conclusion, zigzag and flower structures can be formed by the hydrogen bonding and van der Waals interactions between DCV-3T-Hex molecules on HOPG. As the number of the molecular thiophene moieties modified with alkyl chains increased, just the lamellar structures can be obtained in DCV-nT-Hex's (n = 4~11) self-assembly systems. Because of different orientation of alkyl chains in DCV-nT-Hex (n = 4~11) molecules, the hydrogen bonding between the terminal dicyanovinyls and the central thiophene unit was hindered. Thus, the hydrogen bonding between the terminal dicyanovinyls and the van der Waals forces between the alkyl chains resulted in the lamellar structures of DCV-nT-Hex (n = 4~11), which differed from DCV-3T-Hex's structures. Apart from DCV-3T-Hex's structures' energetically advantageous, DCV-nT-Hex molecules (n = 4, 6, 8, 10) with even-numbered thiophene moieties were overall more stable than DCV-nT-Hex molecules (n = 5, 7, 9, 11) with odd-numbered thiophene moieties, and the stability of the self-assembled structure was weakened with the extension of the molecular backbone, individually. This work revealed the impact of the orientation of the molecular alkyl chains on the intermolecular interactions that induced the formation of self-assembly structures, and may guide the molecular design of linear oligothiophene.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Basic Research Program of China (No. 2017YFA0205000), the National Natural Science Foundation of China (No. 21972031) and the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB36000000).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.082.

  
  
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• 

  Scheme 1  Chemical structures of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11).

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  Figure 1  High-resolution of DCV-3T-Hex's assembled structures at the 1-phenyloctane/HOPG interface: (a) zigzag structure, I_set = 329.6 pA, V_bias = 748.6 mV; (c) nanoporous structure, I_set = 376.0 pA, V_bias = 763.6 mV. (b, d) The calculated molecular models for zigzag structure and porous structure respectively.

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  Figure 2  High-resolution STM image of the self-assembly structures of DCV-nT-Hex (n = 4, 6, 8, 10) molecules with even-numbered thiophenes at the 1-phenyloctane/HOPG interface: (a) DCV-4T-Hex, imaging conditions: I_set = 189.2 pA, V_bias = 1006 mV. (c) DCV-6T-Hex, imaging conditions: I_set = 299.1 pA, V_bias = 699.8 mV. (e) DCV-8T-Hex, imaging condition: I_set = 369.3 pA, V_bias = 673.8 mV. (g) DCV-10T-Hex, imaging conditions: I_set = 368.5 pA, V_bias = 557.4 mV. (b, d, f, h) The calculated molecular models for the DCV-4T-Hex's, DCV-6T-Hex's, DCV-8T-Hex's, and DCV-10T-Hex's structures respectively.

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  Figure 3  High-resolution STM image of the self-assembly structures of DCV-nT-Hex (n = 5, 7, 9, 11) molecules with odd-numbered thiophenes at the 1-phenyloctane/HOPG interface: (a) DCV-5T-Hex, imaging conditions: I_set = 299.1 pA, V_bias = 699.8 mV. (c) DCV-7T-Hex, imaging conditions: I_set = 363.2 pA, V_bias = 629.9 mV; (e) DCV-9T-Hex, imaging condition: I_set = 434.5 pA, V_bias = 614.1 mV; (g) DCV-11T-Hex, imaging conditions: I_set = 402.8 pA, V_bias = 694.0 mV. (b, d, f, h) The calculated molecular models for the DCV-5T-Hex's, DCV-7T-Hex's, DCV-9T-Hex's, and DCV-11T-Hex's structures, respectively.

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  Table 1.  Experimental (Expt.) and calculated (Calcd.) lattice parameters of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11) self-assembled structures.

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  Table 2.  Theoretical calculated total energy and total energy per unit area for self-assembled structures of DCV-nT-Hex (n = 3, 4, 5, 6, 7, 8, 9, 10, 11) molecules. Total energy value included molecule-molecule interactions and molecule-HOPG substrate interactions.

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