Structural Motif Modulation in 2D Supramolecular Assemblies of

Article pubs.acs.org/JPCC

Structural Motif Modulation in 2D Supramolecular Assemblies of Molecular Dipolar Unit Tethered by Alkylene Spacer Xuan-Yun Wang,† Ti-Feng Jiao,‡ Zhu-Xia Zhang,† Ting Chen,† Ming-hua Liu,*,‡ Li-Jun Wan,† and Dong Wang*,† †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and ‡CAS Key Laboratory of Colloid, Interface and Chemical Themodynamics Institute of Chemistry, Chinese Academy of Sciences and Beijing National Laboratory for Molecular Sciences, Beijing 1000190, People’s Republic of China S Supporting Information *

ABSTRACT: The present work investigates the role of tethered alkylene spacer in the formation of 2D supramolecular assemblies of the gemini amphiphiles (Gn-Cn, n = 2, 4, 6, 8, 10) by scanning tunneling microscopy (STM). All GnCn molecules self-organize into the Type I lamellar structure with close-packed alkyl side chains, whereas Type II lamellar structure with interdigitated alkyl side chains is also observed for Gn-C6 and -C8. Two different dipole−dipole interaction modes, that is, collinear and antiparallel arrangement of Schiff base molecular dipole, are proposed to modulate the formation of two types of lamellar assemblies. The results highlight that the tethered alkyl chain length is far from being a passive part of the self-assembled system and plays a definitive role in the supramolecular engineering at the liquid−solid interface.

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INTRODUCTION Self-assembly, which embodies different components to form various packing arrangements in a permissive environment and is considered to be a promising supplement to nanofabrication, has historically come from studying supramolecular systems.1−5 During the self-assembly, individual components interactions via noncovalent forces such as van der Waals (vdWs) interactions, hydrogen bonds, π−π stacking, electrostatic, metal−organic coordination, and dipole−dipole intermolecular forces are always present as competing forces to dominate the supramolecular order.6−11 Among a variety of organic functional groups, alkane or alkyl is one of most important building units to control the self-assembly structures.12−15 The adsorption of assembled alkanes, which is considered essential to the stabilization and epitaxial growth,16 is closely related to the intermolecular interactions through two-dimensional (2D) crystallization, in addition to the adsorbate−substrate interactions.17,18 This important effect of stabilization of organized assemblies by alkyl substituents has been used as an effective approach to anchor functional molecules on surfaces.19−22 Dipole−dipole interaction is another ubiquitously existed noncovalent interaction in 2D supramolecular self-assemblies as long as polar chemical groups are incorporated into the molecules.23−28 It has been reported that the interfacial dipole moment created by charge-transfer between Schiff base group and the substrate can significantly modulate the self-assembly patterns.24 Zimmt and co-workers described the critical role of ether oxygen atoms and {CF2/CF2} dipolar interactions exert in driving the morphology and patterning of the anthracene moiety on surfaces.25−27 Although some useful knowledge has been gained by theoretical simulation,29−31 the understanding © 2013 American Chemical Society

of dipole−dipole interactions in 2D self-assemblies is generally limited, especially as compared to that about hydrogen bonding, for example. With the aim of tailored fabrication of functional nanostructures on surfaces, the simultaneous involvement of multiple competing noncovalent interactions is always encountered and may result in interesting packing motifs due to the delicate balance of intermolecular interaction and molecular− substrate interaction. In the present investigation, we focus on the assembly structures of Gemini molecules with two hydrophilic Schiff base units tethered by flexible alkylene spacers with different lengths. As compared to the welldocumented chain length effect of molecular periphery alkyl chain on self-assembly, there are few reports about the selfassembly behavior of flexible alkylene spacers tethered functional moieties.32,33 The molecular structure of Gemini amphiphiles34 (Gn-Cn, n = 2, 4, 6, 8, 10) is shown in Scheme 1. Gemini amphiphiles are Scheme 1. Chemical Structure of Gemini Amphiphiles with Schiff Base Units Linked by an Alkylene Spacer (Gn-Cn, n = 2, 4, 6, 8, 10)

Received: March 13, 2013 Revised: June 11, 2013 Published: July 25, 2013 16392

dx.doi.org/10.1021/jp402563d | J. Phys. Chem. C 2013, 117, 16392−16396

The Journal of Physical Chemistry C

Article

a family of amphiphiles that consist of two hydrophilic headgroups and two alkyl chains linked by a rigid or flexible spacer unit,35,36 and they have attracted considerable interest in colloidal chemistry and liquid/gas interface assembly. Highresolution scanning tunneling microscopy (STM) reveals that the Gn-Cn molecules are able to form lamellar supramolecular structures on highly oriented pyrolytic graphite (HOPG) surface, depending on the length of alkylene spacers. Intriguingly, it is found that a lamellar structure with interdigitated alkyl chains is observed only when the tethered alkylene chain is 6 or 8 carbons, whereas the other lamellar structure with fully close-packing alkyl chains is formed for all of the Gn molecules with alkylene spacer of 2−10. The unexpected spacer length effect is ascribed to the change of structural motifs with different dipole−dipole interaction mode, which is determined by the arrangement of the tethered alkylene spacer. The present study provides a case example to demonstrate that not only terminal alkyl chains but also tethered alkylene spacers are far from passivity and can modulate the structural motifs of self-assemblies. We anticipate that the molecules with alkylene spacer tethered functional groups may provide a new class of building unit for the design and tailored assembly of 2D supramolecular architectures on surfaces.

Figure 1. Large-scale STM images of Gn-C2 (a) and Gn-C4 (c) molecules at the solid−liquid interface. High-resolution STM images of Gn-C2 (b) and Gn-C4 (d). Molecular models are drawn in both high-resolution STM images.

Gn-C2, as shown in Figure 1c and d, except that the ordered domain is larger and with fewer defects for Gn-C4. The packing parameters are measured to be a = 6.7 ± 0.1 nm, b = 0.8 ± 0.1 nm, α = 44 ± 2°, and β1 is measured to be 44 ± 2°. As expected, the unit cell stays the same along the b direction, whereas it elongates ca. 0.3 nm at a direction due to the increased length of the alkylene spacer. With the same preparation procedure, the assembly of GnC6 is obtained at the HOPG/phenyloctane interface. Figure 2a

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EXPERIMENTAL SECTION Gemini-type Schiff base amphiphiles (Gn-Cn, n = 2, 4, 6, 8, 10) were synthesized as described in the literature.34 1-Phenyloctane (TCI, HPLC grade) was used as solvent without further purification. Prior to imaging, a drop of pure 10−4 M singlecomponent solution (∼2 μL) was deposited onto a freshly cleaved surface of HOPG (Grade-ZYH, Digital Instruments). All experiments were carried out at the liquid−solid interface using a Nanoscope IIIa scanning probe microscope (Bruker, U.S.) operating in constant-current mode at ambient condition. Mechanically cut Pt/Ir wires (90/10) were employed as STM tips. The presented STM images were not subjected to any processing other than routine plane correction. The dipole moment of Schiff base unit was calculated by Gaussian 09, revision A.02 at b3lyp/6-31g* level.37

Figure 2. (a) STM image showing Gn-C6 physisorbed at the solid/ liquid surface, including Domain I and Domain II. (b) Domain I, close-packing structure (Type I); (c) Domain II, interdigitated structure (Type II). Molecular models are overlaid in both (b) and (c).

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presents the large-scale STM image of assembly. Two types of lamellar structures with a wide and narrow dark trough between bright stripes are disclosed. The domains with wider interlamellar space are confirmed to be Type I structure by zoom-in high-resolution STM images (Figure 2b). The other lamella structure is named Type II (interdigitated structure). Both types of the individual domains are labeled in Figure 2a. Initially, two different types of lamellar structures can coexist at the solid/liquid interface. The structural transformation from Type I to Type II is observed at local area in the assembly with elapsed time (see the Supporting Information). The highresolution STM image of Type I domain in Figure 2b reveals structural details of the Gn-C6 lamellar structure. The intersection angle between the lamella direction and alkylene spacer (β1) is 43 ± 2°. The unit cell constants are measured to be a = 7.1 ± 0.1 nm, b = 0.8 ± 0.1 nm, and α = 46 ± 2°. The high-resolution STM image of Type II domain is shown in Figure 2c. The Gn-C6 molecules adapt a zigzag conformation with interdigitated side alkyl chains. The aromatic moieties can be discerned evidently as paired rows of bright spots separated by the alkylene chains with dark contrast. The most-notable feature of Type II is that the side alkyl chains of neighboring

RESULTS AND DISCUSSION Figure 1a displays an image of a monolayer of Gn-C2 on HOPG observed a few minutes after the addition of solution. A densely packed lamellar structure is observed to cover large areas. The structural details are revealed by a high-resolution STM image shown in Figure 1b. The image is submolecularly resolved, which enables us to identify the aromatic groups as well as the octadecyl chains. A close-packing structure with parallel bright stripes and dark troughs is spontaneously formed at the liquid/solid interface. The bright stripes can be ascribed to the Schiff base units of Gn-C2 molecules, whereas the dark troughs are attributed to the close-packed alkyl side chains and ethylene spacers. The peripheral alkyl chains are arranged in a close-packing manner, which drives the formation of lamellar structure. This assembly structure is named Type I featuring the close-packed alkyl chains. To guide the eye, the molecular models have been schematically overlaid in the STM image. The intersection angle between the lamella direction and alkylene spacer (β1) is 44 ± 2°. A unit cell with a = 6.4 ± 0.1 nm, b = 0.8 ± 0.1 nm, and α = 47 ± 2° is superimposed on the image. The Gn-C4 molecules are packed in the same way as 16393



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lamellar pack in an interdigitated mode, which is in accordance with the narrower interlamellar spacing, as described earlier. At the same time, due to the interdigitation of alkyl chains, the periodicity along the lamellar direction (b) increases to ca. 1.3 nm, as compared to ca. 0.8 nm for Type I close-packing structure. The intersection angle between the lamella direction and methylene spacers (β2) is 38 ± 2°. The constants of unit cell are measured to be a = 3.2 ± 0.1 nm, b = 1.3 ± 0.1 nm, and α = 85 ± 2°. The Gn-C6 molecules adapt a zigzag conformation in Type II structure, as outlined by the overlaid structural models in Figure 2c. Figure 3a is a large-scale STM image acquired on Type I structure of Gn-C8 monolayer on HOPG surface. The domain

Figure 4. (a) Large-scale STM image of Gn-C10 molecules at the solid−liquid interface. (b) High-resolution STM image of (a). Structural model is overlaid in (b).

resolution STM image shown in Figure 4b. The aromatic rings are organized into rows that are separated by alkyl chains, which are packed in a tail-to-tail arrangement. The intersection angle between the lamella direction and the side alkyl chains (γ1) is 46 ± 2°, and the angle between the lamella direction and alkylene spacers (β1) is 44 ± 2°, indicating each Gn-C10 molecule shows a zigzag appearance. The “zigzag” molecular models of Gn-C10 have been superimposed in Figure 4b. The distance of the aromatic moieties within an individual Gn-C10 molecule is 1.4 ± 0.1 nm, consistent with the tethered length of 10 methylene units. The unit cell parameters are determined as a = 5.4 ± 0.1 nm, b = 0.8 ± 0.1 nm, and α = 85 ± 2°. As has been well-demonstrated, the close-packing mode and the interdigitation mode of alkyl side chains are both typical assembly modes for functional molecules equipped with peripheral alkyl chains.14,38,39 Therefore, the formation of Type I or Type II assemblies must be originated from the intermolecular interactions between aromatic moieties of Gn molecules. The backbone moiety of the surfactant Gn is composed of two rigid Schiff base groups connected by a flexible alkylene spacer. The dipole−dipole interaction between Schiff base manifests itself by the formation of two different structural motifs. Figure 5a shows the theoretical simulation

Figure 3. Large-scale (a) and high-resolution (b) STM images of GnC8 Type I structure on HOPG. Large-scale (c) and high-resolution STM images of Type II structure. Structural models are overlaid in (b) and (d).

size with regular molecular rows extends to more than 100 nm. Figure 3b presents a high-resolution STM image showing the details of the self-assembly. Each bright spot corresponds to an aromatic ring of Gn-C8 molecule, and the molecules in a lamellar unit form a close-packed assembly. As resolved from the STM image, the alkyl side chains and the aromatic rings of each molecule adapt a straight linear configuration. A unit cell is outlined in Figure 3b. The lattice constants were determined to be a = 7.4 ± 0.1 nm, b = 0.8 ± 0.1 nm, and α = 46 ± 2°. The intersection angle between the lamella direction and alkylene spacer (β1) is 45 ± 2°. Gn-C8 can also form Type II pattern, similar to Gn-C6. Figure 3c shows a large-scale STM image of the Gn-C8 monolayer with the interdigitated structure (Type II). The distance between two neighboring bright spots along the lamella axis (b) is measured to be 1.3 ± 0.1 nm. The distance between two rows of aromatic rings (ΔL) is 1.2 nm, matching well with the molecular model of aromatic rings including tethered octylene spacers. A unit cell is overlaid with the measured unit cell parameters of a = 3.6 ± 0.1 nm, b = 1.3 ± 0.1 nm, and α = 70 ± 2°. The intersection angle between the lamella direction and alkylene spacer (β2) is 37 ± 2°. Figure 4a represents a large-scale STM image after applying a drop of a solution of Gn-C10 in 1-phenyloctane to a graphite surface. Type I lamellar structure is spontaneously formed at the liquid/solid interface. The bright stripes can be ascribed to the Schiff base units of Gn-C10 molecules, whereas the dark troughs are attributed to the close-packed side chains and alkylene spacers. The structural details are revealed by a high-

Figure 5. Schematic diagram illustrating the molecular models in the Type I and Type II structures. (a) The dipolar direction of Schiff base backbone. (b) Collinearly arranged Schiff base dipole, and (c) antiparallel arranged Schiff base dipole. Schematic diagram for the molecular assembly model without side chains for Gn-C6 in the Type I structure (d) and Type II structure (e).

result of dipolar moment of Schiff base subunit. The lateral dipolar moment of Schiff base backbone is calculated to be μ = 1.23 D. Generally, the molecular dipoles tend to arrange in collinear or antiparallel modes to maximize the dipole−dipole interactions, as is schematically shown in Figure 5b and c, respectively. Taking the Gn-C6 molecules as an example, according to the experiments, both Type I and Type II 16394



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CONCLUSION The self-assembly of gemini Gn-Cn molecules can form two types of lamellar structures with close-packed and interdigitated alkyl chains on HOPG surface. The structural dependence of the self-assembly on tethered alkylene spacer is disclosed by STM experiments. With the increase of tethered alkylene spacer length, the arrangement of Type I lamellar structure exists for all of the Gn molecules all of the time. On the other hand, when the tethered alkylene spacer is 6 or 8 carbon atoms, both types of the structure can be obtained on the surface. The detailed structural analysis points out two different dipole− dipole interaction modes presenting two types of lamellar assemblies. In particular, the antiparallel arrangement of molecular dipole of Schiff base units can tuned by the length of the tethered alkylene spacer, therefore resulting in the observation of Type II structures only for Gn-C6 and Gn-C8. The reported results highlight that the tethered alkylene spacer can modulate the structural motifs of self-assemblies. We anticipate that the molecules with alkylene spacer tethered functional groups may provide an orthogonal approach for the design and tailored assembly of 2D supramolecular architectures on surfaces.

structures are observed. In the Type I structure, the Schiff base dipoles of neighboring Gn molecules align collinearly along the lamellar direction, as shown in Figure 5d. This arrangement is synergetically stabilized by the close-packing of alkyl chains (not shown in Figure 5d; see the Supporting Information). By contrast, the Schiff base backbone appears as a pair by adaption antiparallel arrangement in the Type II structure (Figure 5c and e). The paired arrangement of Schiff base dipole is in concert with the interdigitation of alkyl chains, which on one hand provides 2D crystallization energy of close-packed alkyl chains, and on other hand separates neighboring molecular dipole pairs to avoid potential dipole repulsions. We found experimentally that all Gn molecules can form Type I assembly with similar arrangement of Schiff base backbone. This result is naturally consistent with the structural scheme in Figure 5d, because changing the length of alkylene spacer does not alter the intermolecular dipole−dipole interaction along the lamellar direction. One intriguing observation of the self-assemblies of Gn series molecules is that Type II structure is only present for the alkylene spacer with C6 and C8. This alkylene spacer length-dependent structure motif modulation can be understood by considering the geometry restriction imposed by interdigitation of alkyl chains. The formation of the Type II structure is schematically illustrated in Scheme 2. We assume all Gn series molecules can

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ASSOCIATED CONTENT

* Supporting Information S

Full description of structural models (Gn-Cn, n = 2, 4, 6, 8, 10). STM images of time-dependent structural evolution of the GnC6 adlayer consisting of two structures. Large-scale STM image of the Gn-C8 adlayer. A table of structural parameters of Gn-Cn assemblies. This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 2. Schematic Diagram for the Formation of the Interdigitated Arrangement with Schiff Base Units Linked by an Alkylene Spacer (Gn-Cn, n = 2, 4, 6, 8, 10)a

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82615803 (M.-h.L.); +86-10-82616935 (D.W.). E-mail: [email protected] (M.-h.L.); [email protected] (D.W.). Notes

The authors declare no competing financial interest.

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a

The lattice vector in the b direction is constrained by the preservation of the interdigitated arrangement of side alkyl chains (not shown).

ACKNOWLEDGMENTS This work is supported by the National Key Project on Basic Research (Grants 2011CB808700 and 2011CB932300), the National Natural Science Foundation of China (Grants 91023013, 21121063, 21233010, 21003131), and the Chinese Academy of Sciences.

form similar Type II structures with interdigitated alkyl chains. The requirement imposes a specific restriction of unit cell length in the b direction to be 1.3 nm, as was experimentally observed for the assemblies of Gn-C6 and Gn-C8. At the same time, due to the conformation being rigid of molecules in 2D assemblies, the location and direction of the Schiff base dipole, as well as the orientation (β2) of alkylene spacer relative to the b axis are all fixed. This hypothesis is consistent with the high similarity between STM images of different molecules. It can be seen from Scheme 2 that the distance between the Schiff base units (one is in direction b, pink arrowhead) is close to form an antiparallel dipole pair when the alkylene spacer contains 6 and 8 carbons. When the two Schiff base units are in the range of dipole−dipole interactions, the Schiff base units choose to form the paired arrangement. As such an arrangement is only possible for the interdigitated arrangement of alkyl chains, Type II structure is only observed for alkylene lengths of C6 and C8.

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