Article pubs.acs.org/JPCC
Two-Dimensional Supramolecular Self-Assembly of Stilbene Derivatives with Ester Groups: Molecular Symmetry and Alkoxy Substitution Effect Ling-yan Liao,§,† Xue-mei Zhang,§,† Fang-yun Hu,† Shuai Wang,† Shan-Dong Xu,*,‡ Qing-dao Zeng,*,† and Chen Wang*,† †
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China ‡ College of Science, Beijing Forestry University, Beijing 100083, China S Supporting Information *
ABSTRACT: In this paper, four stilbene derivatives substituted with ester groups were synthesized and their two-dimensional (2D) self-assembled behaviors were investigated by scanning tunneling microscopy (STM) on the highly oriented pyrolytic graphite (HOPG) surface. The four target molecules are different in the symmetrical properties and the length of substituted alkoxy chains which extremely influence the self-assembly of these molecules on graphite surface. As a result, we obtained four kinds of nanostructures including chiral supramolecular networks and linear lamellae, as probed at a single molecule level.
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INTRODUCTION
clipped motifs. STM observations showed that this minimal clip can form stable linear lamellae on HOPG and Au(111).20 Here, such a single stilbene unit has been chosen to also build up new nanostructures. Importantly, we designed and synthesized four stilbene derivatives with terminal ester groups (Scheme 1, also see the Supporting Information), and these molecules are different in the molecular symmetry or the length of substituted alkoxy chains. In nature, molecular symmetry is widely applied in the phase behavior of molecules, self-assembly on surfaces,21−23 and electronic properties in the case of conjugated polymers.24 Notably, the symmetry of peripheral substitution has an important effect on the self-assembly.25,26 On the other hand, alkyl substitution has been thought of as one of the most effective molecular design strategies to generate a rich variety of physical and chemical properties, such as solubility, phase transitions, and optoelectric behavior. This method is commonly practiced in preparing discotic liquid crystal species. Also, the substituted alkyl chains can take effect27−31 on the assembling structures of the target molecules. Studies have shown that it influenced the surface mobility of adsorbed molecules on a surface.28,32 Moreover, previous work indicated that 21,23-dihydro-5,10,15,20-tetrakis(4-(ethyl 11-phenoxyundecanoate) porphyrin (ester alkyl-TTPP) could form an
Self-assembly has been demonstrated as one of the most promising bottom-up routes toward atomically precise fabrication of functional systems.1 Under such a principle, scientists have developed many new functional materials in various fields,2−4 among which two-dimensional (2D) supramolecular self-assembly at surfaces or interfaces have been widely studied during the past decade, utilizing the scanning tunnelling microscopy (STM) technique.5−9 STM is a powerful technique, by which assembly behaviors and structures of the adsorbed molecules can be probed with atomic scale resolution. In general, 2D supramolecular self-assembly was influenced by the molecule−molecule and molecule−substrate interactions. Although the 2D crystal structures are often dominated by the properties of the target molecule,10 several external factors, such as heating, light stimuli, solvent, and electric potential, have also been identified as important triggers to regulate the assembling structures.6,11−18 In 2007, Charra’s group proposed a functional “clip” with two stilbene units as the π-conjugated bridge, which could form a lamellae structure that strictly preserved the Groszek geometry through the interdigitation of the substituted alkyl chains.19 In a later work, Charra and colleagues optimized the molecule and designed another clip just using one single stilbene unit as π-conjugated bridge. The novel clip presents many advantages, such as fewer rotational degrees of freedom, enhanced clipping energy, and optimized surface density of © 2014 American Chemical Society
Received: December 31, 2013 Revised: March 24, 2014 Published: March 25, 2014 7989
dx.doi.org/10.1021/jp412793w | J. Phys. Chem. C 2014, 118, 7989−7995
The Journal of Physical Chemistry C
Article
Scheme 1. Chemical Structures of Stilbene Derivatives (Compounds 2, 4, 5, 6)
graphite (HOPG) surface, a well-ordered Kagomé network with two types of cavities (marked A and B in Figure 1b) has
alternating single/double layer pattern on the graphite surface, which should be attributed to the introduction of ester groups into the aliphatic chains.33 Other factors such as the concentration of solvent also play an important role. Some studies have reported the concentration-dependent formation of surface-confined 2D monolayers on HOPG.34−37 Therefore, in our present work, the molecular symmetry and the alkyl substitution effect on the 2D self-assembly of the stilbene derivatives with terminal ester groups have been investigated by STM. As displayed in Scheme 1, compounds 2 and 4 possess a 2-fold rotational axis, respectively, therefore having the same symmetry (C2 symmetry). The symmetry of compounds 5 and 6 is different from that of compounds 2 and 4, and they are asymmetric molecules. Meanwhile, the length of alkoxy chains for compounds 2 and 5 is equal, which is shorter than that of compounds 4 and 6. As a result, four kinds of nanostructures were obtained, including supramolecular chiral network and different lamella organizations.
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EXPERIMENTAL SECTION
1. STM Investigation. 1.1. Sample Preparation. All solvents for STM experiments were purchased from Acros Company and used without further purification. The sample syntheses for STM investigations are described in Supporting Information. All of the studied samples were dissolved in 1phenyloctane, and the concentration of all the studied solutions for STM investigation was less than 1.0 × 10−4 mol L−1. A small droplet of solution containing compound 2 (or 4−6) was deposited onto freshly cleaved highly oriented pyrolytic graphite (HOPG, grade ZYB, NTMDT, Russia). A few minutes later, the sample was characterized by STM. 1.2. STM Measurement. The STM investigation was performed with a Nanoscope IIIa scanning probe microscope system (Bruker, USA) under atmospheric conditions. All STM images presented were acquired in constant current mode using a mechanically formed Pt/Ir (80/20) tip. All images presented here were flattened to correct the tilting effect of the substrate, and the thermal drift was corrected using the underlying graphite lattice as a reference. Molecular models were constructed using a HyperChem software package.
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Figure 1. (a) Large-scale STM image for the self-assembled structure of compound 2 at 1-phenyloctane/HOPG interface, Iset = 289.9 pA, Vbias = 620.1 mV, scale bar = 15 nm. (b) High-resolution STM image of the black rectangle area in part a. Iset = 289.9 pA, Vbias = 620.1 mV, scale bar = 5 nm. (c, d) Molecular models of the assembly structures for domain I and domain II in part b, respectively.
been formed as observed by STM. The comparatively larger Atype cavity with 6-fold symmetry is composed of six molecules of compound 2, while the triangular B-type cavity is formed by three molecules of compound 2. Each A-type cavity is surrounded by six B-type cavities. The inner widths for cavities A and B are 12.9 and 8.2 Å, respectively. Similar to the structure constructed by the diimine molecules in our previous work,38 the two-dimensional (2D) flower-shaped architecture of compound 2 also obviously displays the surface chirality. As shown in Figure 1b, each flower in domain I shows a clockwise direction which is opposite to the flowers in domain II. To better illustrate these chiral structures, here we define the clockwise direction as an R-configuration and the counterclockwise direction as an S-configuration. The distances (L1 for R-configuration and L2 for S-configuration) between two bright rods are both determined as 0.9 ± 0.1 nm as shown in Figure 1b, which agrees with the length of interdigitated alkoxy chains. The unit cell parameters for the R- and S-configuration are measured to be a = a′ = 2.3 ± 0.1 nm, b = b′ = 2.8 ± 0.1 nm, α
RESULTS AND DISCUSSION
1. Self-Assembly Structures of Compounds 2 and 4 on HOPG Surface. Compounds 2 and 4 have similar chemical structures with the same molecular symmetry, except for the length of the substituted alkoxy chains. The surface assembly structures for these two molecules have been investigated on freshly cleaved HOPG surfaces by STM, separately. After depositing a droplet of 1-octylbenzene solution containing compound 2 onto a highly oriented pyrolitic 7990
dx.doi.org/10.1021/jp412793w | J. Phys. Chem. C 2014, 118, 7989−7995
The Journal of Physical Chemistry C
Article
= α′ = 85 ± 2°. On the basis of these phenomena, a pair of corresponding molecular models for the chiral patterns in domain I and domain II have been proposed in Figure 1c,d, respectively. The self-assembly behavior changes dramatically when inserting six methylene units in each alkoxy chain of compound 2, which corresponds to the structure of compound 4. As shown in Figure 2a, compound 4 forms a 2D periodic linear
interaction between alkyl chains of compound 4 can be enhanced. When the van der Waals interaction becomes dominant in the assembling process, compound 4 shows entirely a different assembled structure from that of compound 2. It can therefore be concluded that the terminal ester group in each alkoxy chain plays an important role in determining the surface chirality of compound 2 or 4. Without the ester groups, Bléger et al. reported that the four-alkoxy-derived stilbene molecule formed an achiral linear structure on graphite through the van der Waals interactions between the interdigitated alkyl chains.20 Introducing the ester groups into the aliphatic chains leads to the distortion of the alkyl chains of compound 2 and 4, and results in the rotation of molecules to reach the minimum Gibbs free energy on the HOPG surface.33 2. Assembly Structures of Compounds 5 and 6 on HOPG Surface. Different from compounds 2 and 4, compounds 5 and 6 are asymmetrical molecules. When a droplet of 1-phenyloctane solution containing compound 5 or 6 was deposited onto a graphite surface, the self-assembly process occurred. As shown in Figure 3a, compound 5 formed a
Figure 2. (a) Large-scale STM image for the self-assembled structure of compound 4 at 1-phenyloctane/HOPG interface, Iset = 289.9 pA, Vbias = 594.5 mV, scale bar =20 nm. (b) High-resolution STM image of the white rectangle area in part a, Iset = 357.8 pA, Vbias = 777.6 mV, scale bar = 5 nm. (c) High-resolution STM image of the red rectangle area in part a, Iset = 293.0 pA, Vbias = 821.2 mV, scale bar = 5 nm. (d, e) Proposed molecular models for parts b and c, respectively.
pattern with bright and dark stripes at the 1-octylbenzene/ HOPG interface. The linear pattern also displays the surface chirality, which corresponds to the report.38 The fine chiral structure can be resolved from Figure 2b,c. From Figure 2b we can clearly see that the stilbene directs clockwise, while the stilbene in Figure 2c turns counterclockwise. Because of the high electron density of aromatic rings,39 each bright feature in Figure 2b,c is attributed to a stilbene unit, whereas the dark area should be ascribed to the interdigitated alkoxy chains. It can also be proved by the length (L3 and L3′) of the bright rod (1.3 ± 0.1 nm) and the distances (L4 and L4′) between two bright rods (1.5 ± 0.1 nm). The alkoxy chains (marked by the blue lines) extend on each side of the stilbene unit and make two angles (β and β′) of 168 ± 2° as displayed in Figure 2b. The packed models corresponding to the STM images (Figure 2b,c) are shown in Figure 2d,e, respectively. The unit cell parameters are measured to be a = a′ = 2.2 ± 0.1 nm, b = b′ = 2.5 ± 0.1 nm, α = α′ = 75 ± 2°. The monolayer structure of compound 2 is not the same as that reported;20 maybe there is weak CO···HC hydrogen bonding between the ester group and methylene group, and the interaction of hydrogen bonds is stronger than the van der Waals interaction, so we get the nanoporous network. With the increase of the alkyl chain length, the strength of van der Waals
Figure 3. (a) Large-scale STM image for the self-assembled structure of compound 5 at 1-phenyloctane/HOPG interface, Iset = 289.9 pA, Vbias = 570.7 mV, scale bar = 15 nm. (b) High-resolution STM image of compound 5, Iset = 289.9 pA, Vbias = −321.2 mV, scale bar = 5 nm. (c) Proposed molecular model for the compound 5 structure.
large area of well-ordered lamellae at the 1-phenyloctane/ HOPG interface. The molecules of compound 5 are closepacked, and every two neighboring molecules form a dimer to make up a lamella. The length of the bright rod (L5) is measured to be 1.3 ± 0.1 nm, which corresponds to the stilbene part of the target molecule, similar to those of compound 4. Besides this, as displayed in Figure 3b, the distances between two bright rods along the lamellae (L6) and the row (L7) are both determined to be 0.9 ± 0.1 nm, which are coincident with the total length of the alkoxy chain. Therefore, the three alkyl chains of each molecule 5 are distributed along the lamellae or the row, as presented in the suggested molecular model in Figure 3c. Along the lamellae marked by the blue arrow, the alkyl chains are interdigitated, so compound 5 is stable by the 7991
dx.doi.org/10.1021/jp412793w | J. Phys. Chem. C 2014, 118, 7989−7995
The Journal of Physical Chemistry C
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The 6-I type structure is a close-packed lamellae (Figure 4b). The size of each bright spot is measured to be L6I = 1.2 ± 0.1 nm, W6I = 0.6 ± 0.1 nm, which corresponds to be one single stilbene motif. Importantly, the orientation of the two adjacent stilbene units is opposite to each other, forming an A−B−A alternating pattern. For the A pattern, the stilbene directs clockwise, while the stilbene in the B pattern turns counterclockwise. For domain 6-II, the compound 6 molecules assembled into a uniform and loose architecture (Figure 4d). Each bright spot should be attributed to the stilbene part. Molecules in the two neighboring rows form a dimer as displayed in the suggested model in Figure 4e. However, the size of every bright spot in the 6-III type structure is determined to be L6III = 1.2 ± 0.1 nm, W6III = 1.3 ± 0.1 nm, and the width is twice that in 6-I, indicating that each spot is composed of two compound 6 molecules. The two neighboring bright spots form a dimer marked by the purple rectangle. Also, the distance (L8) between the two dimers along the black arrow (Figure 4f) is determined as 1.5 ± 0.1 nm, which is a little longer than the alkoxy chain length of compound 6. On the basis of these phenomena, we proposed a molecular model shown in Figure 4g. Interestingly, the self-assembly of compound 6 showed an obvious concentration effect. At a high concentration (>9.2 × 10−3 M), the 6-I type structure is the main assembly (Figure 5a). With a lower concentration, the 6-I structure will be gradually transformed into 6-II and 6-III, which is shown in Figure 5b,c, respectively. In other words, the π−π stacking interaction dominated the supramolecular self-assembly of compound 6 at the high concentration, whereas the van der Waals interactions between the alkyl chains determined the
van der Waals interactions. The unit cell parameters are measured to be a = 1.7 ± 0.1 nm, b = 3.4 ± 0.1 nm, and α = 80 ± 2°. For compound 6, prolonging the alkoxy chain of compound 5, it can form three types of self-assembling structures (6-I, 6II, and 6-III) on HOPG surface, as shown in Figure 4a. Although these structures were close-packed lamellae with dimers, the organization of the whole molecule is different on surface in the three domains, depending on the pattern of stilbene and alkoxy chains.
Figure 4. (a) Large-area STM image of all three assembling structures for compound 6 (5.7 × 10−3 mol L−1) at 1-phenyloctane/HOPG interface, Iset = 271.6 pA, Vbias = 613.4 mV, scale bar = 20 nm. (b) High-resolution STM image for domain 6-I in part a, Iset = 298.8 pA, Vbias = 533.8 mV, scale bar = 5 nm, a = 2.3 ± 0.1 nm, b = 4.7 ± 0.1 nm, and α = 90 ± 2°. (c) Suggested molecular model for part b. (d) Highresolution STM image for domain 6-II in part a, Iset = 296 pA, Vbias = 423.6 mV, scale bar = 5 nm, a = 1.6 ± 0.1 nm, b = 4.1 ± 0.1 nm, and α = 89 ± 2°. (e) Proposed molecular model for part d. (f) Highresolution STM image for domain 6-III in part a, Iset = 296 pA, Vbias = 286.4 mV, scale bar = 5 nm, a = 2.5 ± 0.1 nm, b = 3.5 ± 0.1 nm, and α = 92 ± 2°. (g) Suggested molecular model for part f.
Figure 5. Series of large-area STM images obtained at the 1phenyloctane/graphite interface of monolayers formed at four compound 6 concentrations: (a) >9.2 × 10−3 mol L−1 (Iset = 299.1 pA, Vbias = 506 mV, scale bar = 20 nm), (b) 3.6 × 10−3 mol L−1 ∼ 9.2 × 10−3 mol L−1 (Iset = 271.6 pA, Vbias = 613.4 mV, scale bar = 20 nm), (c) 1.7 × 10−3 mol L−1 ∼ 3.6 × 10−3 mol L−1 (Iset = 296 pA, Vbias = 499.9 mV, scale bar = 20 nm), (d)
