Self-Assembly of Four-Claw Discotic Mesogenic Molecules

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The Journal of Physical Chemistry

Self-Assembly of Four-Claw Discotic Mesogenic Molecules: Influence of Core on Chirality Yanfang Geng,a Shaoqing Chang,a Keqing Zhao,*,b Qingdao Zeng,*,a and Chen Wang*,a a

National Center for Nanoscience and Technology (NCNST), 11 ZhongguancunBeiyitiao,

Beijing 100190, China b

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu Sichuan

610066, China

Corresponding Authors *Keqing Zhao, College of Chemistry and Materials Science, Sichuan Normal University, Chengdu Sichuan 610066, China; Tel: 86-28-84764743; E-mail: [email protected] *Qingdao

Zeng, National Center for Nanoscience and Technology (NCNST), 11

ZhongguancunBeiyitiao,

Beijing

100190,

China;

Tel:

86-10-82545548;

E-mail:

[email protected] *Chen

Wang,

National

ZhongguancunBeiyitiao,

Center Beijing

for

Nanoscience

100190,

China;

and Tel:

Technology

(NCNST),

86-10-82545561;

11

E-mail:

[email protected]

ACS Paragon Plus Environment

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ABSTRACT: Morphology of self-assembled monolayers determined by molecular structure and involved interactions plays a crucial role in their properties. Herein, we report a study on twodimension (2D) self-assembly of two kinds of triphenylene (TP)-submitted discotic mesogenic species, which have similar four TP moieties with alkyne spacer but different rigid aromatic cores pyrene (Py) and carbazole (Cz). Two types of stable periodic long-ranged supramolecular networks on the highly oriented pyrolytic graphite (HOPG) surface have been visualized via high-resolution scanning tunneling microscopy (STM) technique. A comparative study reveals that how the aromatic core of molecular building blocks affects achiral and chiral arrangement of discotic molecule on surface. The asymmetry backbone results in molecular orientation and varied van der Waals forces between molecule-molecule and molecule-substrate. Our results demonstrate that design of functional molecules plays an important role in the construction of 2D supramolecular assembly possessing desirable structure for the specific applications.


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INTRODUCTION Molecular self-assembly provides a powerful means of constructing controlled functional surfaces

relying

on

molecular

noncovalent

interactions.1-3

These

surface-supported

nanostructures represent promising materials for various potential applications in molecular recognition, surface templating or nanopartterning. The self-assembly is a spotaneous organization process which takes advantage of intramolecular interaction during thin-film growth. The noncovalent interactions facilitate the formation of interesting feature occurring in flexible two-dimension (2D) supramolecular assemblies.4-5 The requirement of designing specific nanostructures and materials needs understanding of how to control and balance molecular assemblies into defined structure at the molecular level. Most of the early findings reveal that molecular orientation in the assemblies plays a signigicant role not only on symmetric or unsymmetric arrangement as well as their chemical/physical properties.6 In particular, chirality is a particular phenomenon encoutered at different low-dimensional levels.7-13 It has been reported many achiral molecules can form localized 2D chiral structures.14-16 These studies of chirality in individual moelcular building blocks or self-assembled molecular monolayer could provide deep understanding of how to creat chirality in three-dimension (3D). Discotic mesogenic molecules have been considered as a interesting material concerning intramoelcular interaction and molecule-surface forces.17 Moreover, as a class of functional materilas for optoelectronic, catalysis and life science applications, they have attracted much attention.18-21 These compounds generally consist of rigid and flat aromatic cores surrounded by two or more flexible side-chains, resulting in two or more symmetries. The well-known cores are mainly aromatic hydrocarbons, macrocyclic, heterocyclic, metallo-mesogens and saturated cores, which are mostly attached with flexible chains through various lingkages such as ether, ester,


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alkyne, etc.22 Discotic mesogenic molecules can assemble into 2D or 3D well-defined selforganized architures via non-covalent interactions such as van der Waals forces, dipolar interactions and hydrogen bonding, which play a significant role not only in the formation of assemblies but laso in the determination of their properties. Close-stacking assemblies formed by aromatic moieties have been widely used as efficient electron-transport pathway in the field of photovoltaics and field effect transistors.23-26 To date, a number of discotic mesogenic moelcules have been designed and synthesized towards the formation of functional supramolecular nanostructures. It is known that disc-shaped molecules can easily assemble into ordered aggregates in solutions. However, it is need to deposit the discotic mesogenic materials onto the semiconducting films or electrodes in the organic electronics. It has been well-established that the molecular orientation essentially controls its packing in solid state.27 On surface, the adsorption behavior depending on the chemical substitutents is of special interest. Therefore, it is necessary and also important to make certain how the disc-shaped molecules arranged on solid surfaces. In particular, the monolayer directly attahced on the surface serving as a template for overall morphology determines the subsequent growth. Notably the formation of mesogenic adlayers on surface is of great interest from the applicational points of view. Self-organization of discotic mesogenic molecules is ralated to building species as well as the corresponding complex interactions. The length of the flexible lateral alkoxylic chains surrounding the core affects the interactions between neighbouring molecules. Additionally, as an essential part determining the mesogenic structure, the influence of core structure on the interactions should also be clarified. The interactions between molecule-molecule and molecule-substrate determine the structure of molecules on the surface and further determine the architure of the discotic mesogenic molecules


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on surface. Thus, a clearer understanding of the relationship between molecular structural features and suparmolecular patterns could lead to the development of functional surfaces. Recently, we found that discotic molecules with different units as the core and triphenylene units surrrounded outside can form mesomorphic ordered architectures with excellent semiconduting properties.28 In this contribution, we reported self-assembly properties of two discotic mesogenic molecules Py-TP4 and Cz-TP4 (as shown in Scheme 1). Scanning tunneling microscopy (STM), a visualization tool with outstanding real space imaing capabilities was used for probing the self-assembled molecular monolayers which adsorbed on a typically highly oriented pyrolytic graphite (HOPG) substrate. Starting with studies of molecular self-assembly on surface, this work describes the influence of the cores on the self-assembled structures of these two discotic mesogenic molecules and demonstrates that the symmetry of core is important to govern the molecule-molecule and molecule-substrate interactions, resulting in achiral or chiral surface nanostructures.

R

R

R

R

R

R

R

Py-TP4

N

R

Cz-TP4 C5H11O

OC5H11

C5H11O

OC5H11

O

R=

OC5H11

O

Scheme 1. The chemical structures of discotic mesogenic molecules Py-TP4 and Cz-TP4.


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EXPERIMENTAL SECTION Materials. Molecules Py-TP4 and Cz-TP4 were synthesized through Sonogashira coupling reaction as our previous studies and the experimental details will be addressed elsewhere.29 The solvent 1-phenyloctane (HPLC grade) was purchased from Tokyo Chemical Industry (TCI) company and used directly without any purification. The freshly cleaned highly oriented pyrolytic graphite (HOPG) (grade ZYB, Russia) is used as substrate. STM Measurements. The monolayers of Py-TP4 and Cz-TP4 compounds were fabricated by depositing their solution with concentration less than 10-4 M onto the freshly slaved HOPG surface, respectively. STM were performed at the 1-phenyloctane/solid interface under ambient condition via Nanoscope Multimode SPM (Bruker Nano Inc.). The tip (Pt:Ir = 80:20) was cutted mechanically. All the images were recorded under constant current mode. The obtained STM images were corrected through HOPG crystal in order to eliminate the thermal drift error during scanning.

RESULTS AND DISCUSSION Two discotic mesogenic molecules Py-TP4 and Cz-TP4 were designed and synthesized. They consist of different rigid aromatic cores (pyrene and carbozle) and similar four substituted terminal triphenylene moieties. Pyrene moiety is one of the most frequently encountered discotic liquid-crystal cores.30 Carbazole as an aromatic group with tricyclic structure has not been widely used in discotic mesogenic molecules. Triphenylene-based molecules have been paid great attention among numerous mesogenic molecules.22 The fourfold terminal triphenylenes, referred to as four claws, are able to stabilize the moelcules on surfaces. Flexible alkyl chains were inserted between core and claw to allow conformational adaptation of compound to the


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local enviroment. Based on the molecular structure, Py-TP4 molecule is C2 symmetric, while CzTP4 has only one symmetric axis as marked by using dotted lines ignoring the dynamic structures and the conformation of claw-part substituted to the central Py or Cz unit. The selforganizations of Py-TP4 and Cz-TP4 on the HOPG surface were firstly performed in order to clarify the molecular packing of the first monolayer on the solid surface. STM technique has been proved to probe such monolayers adsobed on the electric solid surfaces. From the results the molecular conformation adsorbed on the surface was determined and the possible reason was analysized. Self-organization of Py-TP4 on Surface. After adsorbed at the 1-phenyloctane/HOPG interface, the 2D assembly of Py-TP4 is investigated by STM technique under ambient condition. As shown in Figure 1, Py-TP4 can spontaneously form large-area and long-ranged periodic assemblies on HOPG surface. The formation of such large domain at room temperature suggests a low diffusion barrier of Py-TP4 molecule on HOPG surface. Because of similar electron cloud density of pyrene and triphenylene moieties, they appear bright in the packing pattern.31 From the molecular shape we assign each rectangle to pyrene and triangle to triphenylene. Besides these bright spots, the lower lines drawn in white can be attributed to the long alkyl chain linkages. High-resolution STM image as shown in Figure 1b can clearly reveals the exact arrangement of Py-TP4 moelcules. The moieties marked by red arrows are rectangle, and the length (L) and width (W) of the rectangle are measured to be 1.6 nm and 0.8 nm, respectively. Therefore, the marked blue rectangle should correspond to the pyrene central unit. The distance (d) between two adjactant pyrenes is estimated to 4.2 nm. Around the rectangle pyrene unit, there are six triangular bright spots assigned to triphenylene groups as shown with capitals A, B, C, D, E and


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F. It is proposed that one central rectangle and four triangle units among of them should make up one Py-TP4 molecule. And then, the distances between these triangle spots were measured. We found that the distances of both AD and CF are 4.4 nm, which are longer than that of BE estimated to be 3.9 nm. In addition, the distance between A (or D, C, F) and pyrene is 1.0 nm, which is consistent with the length of alkyl chain linkage between pyrene and triphenylene of Py-TP4 molecule. Therefore, four triangle spots (A, C, D, F) and one rectangle spot constitute one Py-TP4 molecule. A rhombic unit cell with pyrene group in every corner is described as shown in Figure 1b. The measured unit parameters are as follows: a = 4.2 ± 0.1 nm, b = 6.2 ± 0.1 nm, and α = 76 ± 1°.

Figure 1. (a) Large scale (scale bar = 10 nm) and (b) high resolution (scale bar = 4 nm) STM image of Py-TP4 assemblies at 1-phenyloctane/HOPG interface (Iset = 0.340 nA, Vbias = 0.635 V); A unit cell is indicated and superimposed in the image b; (c) A suggested schematic model for the observed area in b, in which the red triangle and the green rectangle presents the TP and Py group in Py-TP4, respectively.

After carefully inspecting the STM image, the schematic molecular model of Py-TP4 arrangement has been proposed as depicted in Figure 1c. Although the lateral resolution of STM


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technique is not precise enough, the corresponding molecular packing clarifies the molecular features. Within the molecular self-assembled structure, Py-TP4 might interact with each other through the van der Waals forces between the interdigitated and parallel arranged terminal alkoxyl chains and the stacking forces between the TP moieties from adjacent lines. In addition, there are strong π-π stacking between Py-TP4 and HOPG substrate. The balance of these forces finally enhances the stability of the self-organized morphology. More interestingly, the packing of Py-TP4 on solid surface is different from that of previously reported analogue Py-TP2 with two substituted triphenylene groups. In the case of Py-TP2, pyrene and alkyl chain linkage form depression herringbone arrangement, whereas the triphenylene groups assemble into lines of bright beads.29 The similar brightness of pyrene and triphenylene in Py-TP4 molecule indicates they completely adsobed on surface, which might be at the origin of the increased van der Waals force of the molecule to the substrate. Therefore, the number of the claw affects the balance between molecule-molecule and molecule-substrate interactions which control the self-assembly of molecules. Self-organization of Cz-TP4 on Surface. In contrast, Cz-TP4 with carbazole as the core represents different arrangement from Py-TP4. As shown in Figure 2, Cz-TP4 molecules selfassembled into staggered arrangement with clear molecular units. One can identify two types of features on HOPG surface in every row separated with white arrows. When one molecule arranges with specific direction, the neighboring molecule in the same row switches. Figure 2b highlights the molecular packing features based on the large scales of Figure 2a. As shown in Scheme 1, each Cz-TP4 molecule consists of five aromatic groups. In the high-resolution STM image, five bright spots well-distributed in the white rectangle make up one Cz-TP4 molecule. The central smaller triangle-shaped spot can be identified to carbazole unit, while the larger four


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triangular spots can be assigned to the triphenylene units of Cz-TP4. Therefore, the Cz-TP4 molecules adsorbed on the surface and arranged linear close-packing due to the strong π-π stacking between Cz-TP4 molecules and substrate as well as the van der Waals forces between partial terminal alkoxyl chains.32 From another point of view, this result indicates that the molecule-substrate interactions are main forces dominating the self-assembly strcuture when the substituted side chains are enough short.33 It is interesting to note that four triphenylene groups show two types of packing on HOPG surface. At one side, the longer distance (L1) between two triphenylenes is measured to be 3.2 nm, while at the other side the shorter distance (L2) is measured to be 2.4 nm. This phenomenon suggests that the peripheral triphenylene groups might rotate relative to the central carbazole unit. On the basis of the above analysized result, it can be easily identified that there are two types of conformations in Cz-TP4 assembly. Figure 2b shows that four triphenylene groups appear as an clockwise pinwheel, and thus form (P)-chiral assembly as marked with light blue arrow.13 For the molecule marked with red arrow, four triphenylene groups anticlockwise rotate with respect to the carbazole core and show (M)-chiral assembly. Two kinds of conformations might result from the C1 symmetry of four triphenylenes due to the structure of carbazole core.

Figure 2. (a) Large scale (scale bar = 10 nm) and (b) high resolution (scale bar = 2.5 nm) STM


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image of the Cz-TP4 assemblies at 1-phenyloctane/HOPG interface (Iset = 0.263 nA, Vbias = 0.554 V). The green arrow and the red arrow marked the (P)- and (M)- conformational assembly on surface, respectively.

In addition, as shown in Figure 2b, it needs to be pointed out that the conformations of molecule A marked with red dashed trapezoid and molecule B marked with blue dashed trapezoid are opposite, however, molecules B and C marked with green dashed trapezoid exhibit the same conformation. Molecule C rotates a slight angle relative to molecule B. A large-scale STM image was carefully analyzed as shown in Figure 3. Two farther triphenylene groups at one side of central carbazole unit are connected by white arrow, while two closer triphenylene groups at the other side are connected by blue arrow. The arrangement of alternate white or blue arrow indicates that two adjacent Cz-TP4 molecules display the same conformation. In contrast, adjacent two white or blue arrows suggests two neighbouring Cz-TP4 molecules exhibit different conformation. Thus, the prochiral Cz-TP4 molecules self-assemble into chiral domains on HOPG surface. Furthermore, one Cz-TP4 molecule rotates relative to the adjacent Cz-TP4 molecule with the same conformation. The molecular model is drawn schematically in Figure 3b. Three Cz-TP4 molecules as marked by red rectangle were picked out and shown in Figure 3c. The angle is estimated to be α = 8 ± 1o. Therefore, the asymmetry distribution of triphenylenes might lead to molecular rotation on surface and bear surface chiral due to the packing constraints.34


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Figure 3. (a) Large scale (scale bar = 10 nm) STM image of the Cz-TP4 assemblies with arrow indicating the molecular conformation and rotation. (b) A suggested molecular model for the observed area (scale bar = 2 nm). (c) Schematic illumination of molecular rotation on surface.

After discussing the packing and conformation of Cz-TP4 on HOPG surface, we now address the role of molecular chemical structure on the surface structure.35 Previous study has shown that discotic mesogenic moelculs physisorb on HOPG with the triphenylene groups orienting toward in order to facilitate van der Waals force.29 This demonstrates that the number of the triphenylene substitutents determines the morphology of the self-assembled monolayer at the liquid/HOPG interface. Comparing Py-TP4 with Cz-TP4, it can be concluded that the chemical structure of the core controls the morphology of the formed monolayer. We can obtain that discotic molecules functionalized by pyrene show an intermolecular ordering. The unsymmetric substitutents allow molecule to rotate on surface, hence two different cores affect the monolayer pattern on surface.36 It is possible that the C-C bond linking the triphenylene to the atomatic core should have a free rotation. From the point of view of interaction, molecule/substrate interactions might determine the formation of homogeneous arrangement in the case of Py-TP4. However, molecule-molecule interactions might lead to a chiral monolayer which is spotaneously formed


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for Cz-TP4.37 The possible influence of the core suggests that the intermolecular interactions played a crucial role in the expression of the localized chiral structure at the monolayer level. The interactions between the resulted unsymmetric terminal alkoxyl chains might also play an important role in the process of self-assembly.15, 38 From the above description, it appears that formation of chiral structures from achiral molecules is a result of interfacial confinement in which the effect of core is especially well expressed. Consequently, it is particularly useful to understand the interactions between molecule-molecule and molecule-substrate in order to obtain specific molecular architecture with functional properties.39

CONCLUSIONS We obtained two new discotic mesogenic molecules and investigated their supramolecular structures by using STM techniques. The results reveal that their assembly behaviors on the surface are closely related to their composition as well as the inermolecular interaction. For PyTP4 molecule, pyrene and triphenylene displayed the same height and arranged periodically together on HOPG surface. In contrast, Cz-TP4 with carbazole and triphenylene assembled into long shaped periodic structure on the HOPG surface. The asymmetry of carbazole and triphenylene makes Cz-TP4 discotic molecules rotate on the surface and produce surface chiral due to the repulsive interactions. These observation will provide an important guidance in the design and production of functional discotic mesogenic molecules.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (Nos. 2011CB932303, 2013CB934203) and the National Natural Science Foundation of China (Nos. 51173031, 91127043, 51203030, 21472029, 51443004, 51273133).

Supporting Information Available: Synthesis and characterization of Py-TP4 and Cz-TP4. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents Graphic


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Self-Assembly of Four-Claw Discotic Mesogenic Molecules

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