Self-Assembly of Asymmetric Fan-Shaped Dendrimers with Different

Oct 8, 2008 - These results were confirmed by the electron density profile of G1 and G2 .... This result indicates that the hydrophilic core group of ...
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Langmuir 2008, 24, 12426-12430

Self-Assembly of Asymmetric Fan-Shaped Dendrimers with Different Generation Numbers at the Air-Water Interface Jisun Lee,† Kyungbae Kim,† Songyi Lee,† Hwan Kyu Kim,‡ and Daewon Sohn*,† Department of Chemistry, Hanyang UniVersity, Seoul 133-791, Korea, and Center for Smart Light-HarVesting Materials and Center for AdVanced PhotoVoltaic Materials and Department of AdVanced Materials Chemistry, Korea UniVersity, Jochiwon, ChungNam 339-700, Korea ReceiVed July 29, 2008. ReVised Manuscript ReceiVed September 9, 2008 Monolayer formation of two dendrimers containing a hydrophilic core group (COOH) and hydrophobic peripheral groups (anthracene and aryl ether tail groups), 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid (G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid (G2), were studied. To understand the mechanism of the self-assembly of these molecules, we measured the surface pressure-surface area (Π-A) isotherm and investigated the surface texture of Langmuir-Blodgett monolayers transferred onto hydrophilic silicon wafers. Both dendrimers form circular domains at the onset point of surface pressure as a result of the difference in hydrophobicity between the core group and the peripheral end group. The core group has a functional group at the end of dendrimer and can be anchored on the water surface. Upon further compression, monolayer of G1 shows a domain of molecules whereas a monolayer of G2 is aligned in the direction of compression at 10 mN/m. At higher surface pressure (20 mN/m), G1 molecules have several aggregates of domains, but G2 molecules maintain their ordering. These results were confirmed by the electron density profile of G1 and G2 monolayers transferred to silicon substrates, as measured by X-ray reflectivity.

Introduction Dendrimers, a new class of synthetic macromolecules, are built from connectors and branching units around a small molecule or a linear polymer core.1,2 Recently, synthetic dendrimers have attracted increasing attention as self-assembling building blocks for the construction of a variety of macromolecular systems with multifunctonalities because they are capable of organizing themselves into regularly ordered features with nanosized structures.3,4 It is well known that the molecular shape, generation number, and surface properties determine the dendrimers’ structure after self-assembly. Numerous studies have been performed on polyether-type dendrimers because of applications in molecular light harvesting5 and molecular encapsulation.6,7 Nevertheless, many low-generation dendrons are too small to exhibit the properties of dendrimers; however, they are frequently used as branched oligomeric building blocks and have a size relationship to dendrimers that is somewhat akin to that between oligomers and polymers. Low -generation dendrimers are known to be flexible, thus both the inner and the outer groups can come into contact with a solid surface and an air/water interface.8 In addition, the morphology and self-assembly of dendrimers at the air/water interface depend on the chemical structure and generation number.9 * Corresponding author. E-mail: [email protected]. † Hanyang University. ‡ Korea University.

(1) Fre´chet., J. M. J. Proc. Natl. Acad. Sci. U.S.A 2002, 99, 4762. (2) Zhao, M.; Helms, B.; Slonkina, E.; Friedle, S.; Lee, D.; DuBois, J.; Hedman, B.; Hodgson, K. O.; Frechet, J. M. J.; Lippard, S. J. J. Am. Chem. Soc. 2008, 130, 4352. (3) Zimmerman, S. C.; Quinn, J. R.; Burakowska, E.; Haag, R. Angew. Chem. Int. Ed. 2007, 46, 8164. (4) Gaveffotti, A. Acc. Chem. Res. 1994, 27, 309. (5) Jiang, D.-L.; Aida, T. Nature 1997, 388, 1681. (6) Arunkumar, E.; Forbes, C. C.; Smith, B. D. Eur. J. Org. Chem. 2005, 4051. (7) Tanaka, K.; Dai, S.; Kajiyama, T.; Aoi, K.; Okada, M. Langmuir 2003, 19, 1196. (8) Su, A.; Tan, S.; Thapa, P.; Flanders, B. N.; Ford, W. T. J. Phys. Chem. C 2007, 111, 4695.

The Langmuir-Blodgett (LB) technique allows for the fabrication of ordered monolayers and organized molecular assemblies with well-defined molecular orientation10-12 on a desired substrate. Such monolayers with nanosized features and supramolecular ordering have been applied to nanofabrication, molecular-electronic devices, and display devices.13-15 Langmuir-Blodgett films of dendrimers at the air-water interface demonstrate that these molecules are able to arrange themselves in monolayers. In this work, we investigate the morphology of LB monolayers transferred onto a solid substrate and show the mechanism of self-assembly of asymmetric fan-shaped dendrimers ([G1-An]CO2H and [G2-An]-CO2H) with CO2H core groups and aryl ether tail groups at the air-water interface at various surface pressures using Π-A isotherms,16 atomic force microscopy (AFM), and X-ray reflectivity.17

Experimental Section Materials. The fan-shaped dendrimers used in this study are depicted in Figure 1. We synthesized amphiphilic 4-{10-[4-(3,5bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid ([G1-An](9) Jung, H.-T.; Kim, S. O.; Ko, Y. K.; Yoon, D. K.; Hudson, S. D.; Percec, V.; Holerca, M. N.; Cho, W.-D.; Mosier, P. F. Macromolecules 2002, 35, 3717. (10) Guo, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630. (11) Youm, S.-G.; Paeng, K.; Choi, Y.-W.; Park, S.; Sohn, D.; Seo, Y.-S.; Satija, S. K.; Kim, B. G.; Kim, S.; Park, S. Y. Langmuir 2005, 21, 5647. (12) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (13) Miyahara, T.; Kurihara, K. J. Am. Chem. Soc. 2004, 126, 5684. (14) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. J. Am. Chem. Soc. 2004, 126, 3708. (15) Henderson, P.; Beyer, D.; Jonas, U.; Karthaus, O.; Ringsdorf, H.; Heiney, P. A.; Maliszewskyj, N. C.; Ghosh, S. S.; Mindyuk, O. Y.; Josefowicz, J. Y. J. Am. Chem. Soc. 2000, 119, 4740. (16) (a) Yoon, D. K.; Jung, H.-T. Langmuir 2003, 19, 1154. (b) Pao, W.-J.; Stetzer, M. R.; Heiney, P. A.; Cho, W.-D.; Percec, V. J. Phys. Chem. B 2001, 105, 2170. (c) Guodong, S.; Miodrag, M.; Qun, H.; Roger, M. L. Colloids Surf., A 2000, 171, 185. (17) Seo, Y.-S.; Kim, K.-S.; Shin, K.; White, H.; Rafailovich, M.; Sokolov, J.; Lin, B.; Kim, H. J.; Zhang, C.; Balogh, L. Langmuir 2002, 18, 5927.

10.1021/la802438n CCC: $40.75  2008 American Chemical Society Published on Web 10/08/2008

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Figure 2. Surface pressure-surface area isotherms of [G1-An]-CO2H and [G2-An]-CO2H at the air/water interface at 25°C: (-) [G1-An]CO2H and (---) [G2-An]-CO2H. Figure 1. Structures and conformations of the novel fan-shaped dendrimers. (a) 4-{10-[4-(3,5-Bis-benzyloxy)-phenyl]-anthracen-9-yl}benzoic acid ([G1-An]-CO2H) and (b) 4-(10-{4-[3,5-bis-(3,5-bisbenzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid ([G2-An]CO2H). (a-1, b-1) Molecular structures. (a-2,3, b-2,3,4) Ball-and-stick models of G1 and G2, respectively.

CO2H: G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]phenyl}-anthracen-9-yl)-benzoic acid ([G2-An]-CO2H: G2) as reported in a previous paper.18 The nomenclature G1 indicates a molecule with one layer of monomer units (first generation), and G2 indicates a molecule with two layers (second generation). G1 and G2 are different from conventional amphiphilic molecules that contain a hydrophilic head group and a hydrophobic tail because the strength of the hydrophilicity of the core group (COOH) is relatively weaker than that of conventional organic amphiphiles. The anthracene part of these compounds also increases the rigidity of the molecule. Figure 1a-2 shows the side view of a G1 molecule, and Figure 1a-3 shows the top view. Figure 1b-2 shows the side view, and Figure 1b-3,b-4 shows the top view of a G2 molecule. G2 has the ability to rotate the carbon atoms indicated by arrows in Figure 1b-1. When such a carbon atom rotates, the molecule makes a compact structure as shown in Figure 1b-4. Isotherm Measurements. The surface pressure of the monolayer was measured as a function of area (Π-A) at 298.0 ( 0.5 K on a KSV3000 equipped with a Wilhelmy balance. Two computercontrolled systemmetrically movable, hydrophobic Teflon barriers were used to regulate the surface area. [G1-An]-CO2H and [G2An]-CO2H solutions (0.2-0.5 mg/mL in CHCl3) were spread on the Milli-Q (resistivity ∼18 M cm) water subphase, and compression of the monolayer began 30 min after spreading in order to allow the CHCl3 solvent to evaporate completely. The compression rate was 5 mm/min. Each surface pressure versus surface area isotherm was measured at least three times. Topological Studies. Before observation, all specimens were prepared using a Langmuir-Blodgett vertical dipping technique. After monolayer formation, it was transferred onto a quartz slide or silicon wafer. Substrates were cleaned by immersion in acetone and then ethanol, followed by rinsing with Milli-Q water; they were then sonicated in acetone for 20 min. After being cleaned, the silicon wafers were treated for 20 min in an ozone cleaner to ensure a hydrophilic surface. Surface topological measurements were performed under ambient conditions with an atomic force microscope (XE-100 microscope, Park Systems Corp.) using 910M-NCHR silicon cantilevers (noncontact mode, 42 N m-1 force constant, and 330 kHz resonance frequency). Synchrotron X-ray scattering measurements were performed at beam line 10C1 XRD(II) at Pohang Light Source in Korea with an X-ray wavelength of 1.54 Å, a beam size of 0.5 mm (H) × 0.3 mm (18) Baek, N. S.; Kim, Y. H.; Roh, S. G.; Kwak, B. K.; Kim, H. K. AdV. Funct. Mater. 2006, 16, 1873.

(V), and an energy resolution of DE/E ) 2 × 10-4. Symmetric θ ≈ 2θ scans were used to analyze the texture, ordering, and nanostructures using a 2θ range of 0-7°, a 0.02° step size, and a 2 s data collection time for each data point. Reflectivity profiles were obtained as a function of the scattering wave vector transfer, q ) (4π/λ) sin(θ), where λ is the wavelength and θ is the reflected angle, which equals the incident angle.

Results and Discussion Figure 2 shows Π-A isotherms of [G1-An]-CO2H (G1) molecules and [G2-An]-CO2H (G2) molecules at the air/water interface. In the case of G1, the surface pressure-surface area isotherm shows a steep increase. G1 experiences a direct transition from the gaseous phase to the solid phase, and molecules are compressed without complicated transitions. The onset area of ∼50 Å2 indicated in Figure 2a is comparable to 51 Å2, which is calculated from the top-view area of G1 shown in Figure 1a-3. This result indicates that the hydrophilic core group of G1 is anchored in the water whereas the hydrophobic aryl ether groups are oriented away from the water during compression. Furthermore, the size of the projected area is the same even though ether linkages can rotate. In contrast to G1, the surface pressure-surface area isotherm for G2 shows different transitions during the compression. Specifically, G2 has two transition points consisting of phase changes from the gaseous phase to the condensed phase and to the solid phase. The first onset area of 71 Å2, indicated by Figure 2b, is comparable to 74 Å2, calculated from the top-view area shown in Figure 1b-3. In this area, G2 has a fully extended conformation of tail groups. A monolayer of G2 can be compressed to 13.5 mN/m. At 13.5 mN/m, the G2 monolayer collapses and reaches a plateau indicated in Figure 2c. The flexibility of the second-generation part of G2 makes the π-π stacking of benzene rings in G2 possible. Therefore, we can suppose that, for higher compression, G2 molecules have π-π stacking of the outside tail groups fold completely as allowed by free rotation, so the intermolecular interaction of G2 molecules is possible only among the peripheral groups. After point (d) in Figure 2, the surface pressure rises sharply. The surface pressure versus surface area isotherm of G2 shows a similar tendency to that of G1 when the monolayer is extremely compressed. G1 molecules experience domain aggregation whereas G2 molecules have broken monolayers. G2 molecules maintain their ordering in the monolayer. This is represented in Figure 8d, and total mechanism of self-assembly of two dendrimers will be explained in detail by X-ray data.

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Figure 3. AFM images of a [G1-An]-CO2H monolayer transferred onto a hydrophilic silicon wafer measured at different pressures: (a) 0.2 and (b) 10 mN/m at 25°C.

Figure 4. AFM images of a [G2-An]-CO2H monolayer transferred onto a hydrophilic silicon wafer at different pressures: (a) 0.2 and (b) 10 mN/m at 25°C.

On the basis of the above discussion, it is clear that the core groups of G1 and G2 molecules come together and make a circular domain on the water surface from the beginning of the onset area but each undergoes a different conformational transition. From the Π-A isotherm, the onset area of G1 (Π ≈ 1 mN/m) is significantly smaller than that of G2 that lies on the water. Because of the difference in hydrophobicity between core group and peripheral groups, a relatively hydrophilic core group remains in contact with the water subphase. With respect to conformational transitions, G2 dendrimers are more flexible than G1 because they possess more oxygen groups that can undergo free rotation. Thus, G2 molecules can change their conformation more easily than G1 molecules. Consequently, with increasing surface pressure, the structure of G2 molecules can be changed by the rotation of the linkages highlighted in Figure 1b-1, resulting in the more closed structure as shown in Figure 1b-4. Instead of interaction between the anthracene parts of the G2 dendrimers, we suggest a hydrophobic interaction between tail groups of G2

dendrimers. It is difficult to induce π-π stacking or a hydrophobic interaction between anthracene and phenyl groups of di-metasubstituted anthracene (dihedral angle 68°) because of high steric hindrance. To visualize and examine the self-assembly of G1 and G2 at the air-water interface, we used X-ray reflectivity and an atomic force microscope (AFM). Langmuir-Blodgett films of G1 and G2 are deposited on silicon substrates at different surface pressures. Figures 3 and 4 provide AFM images at 0.2 and 10 mN/m for G1 and G2, respectively. At low pressure (Π ≈ 0.2 mN/m), G1 and G2 dendrimers form a monolayer that consists of small domains. The size of the domain is about 20-25 nm with a height of about 0.7 nm, as shown in Figure 3a. On the water surface, circular domains appear spontaneously around the onset area of G1 and G2. This indicates that the difference in hydrophobicity between the core group and peripheral end groups induces domains of fan-shaped dendrimers to aggregate spontaneously at low pressure. The roughness of a monolayer

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Figure 5. X-ray reflectivity profiles of a [G1-An]-CO2H monolayer transferred onto a hydrophilic silicon wafer at 25°C (symbols) and the corresponding fits (lines). Each solid line is the best fit for each data set. (a) X-ray reflectivity profiles (curves are shifted in the y direction for clarity) and (b) electron density profiles.

Figure 6. X-ray reflectivity profiles of a [G2-An]-CO2H monolayer transferred onto a hydrophilic silicon wafer at 25°C (symbols) and the corresponding fits (lines). Each solid line is the best fit for each data set. (a) X-ray reflectivity profiles (curves are shifted in the y direction for clarity) and (b) electron density profiles.

of G1 is larger than that of G2, indicating that G1 can form aggregated structures more easily than G2. G1 and G2 show different morphologies at 10 mN/m. In the case of G1 (Figure 3b), domains of G1 organize into large-scale aggregates, whereas G2 forms close-packed ordering, as shown in Figure 4b, as a result of intermolecular hydrophobic interactions induced by peripheral end groups of G2. X-ray reflectivity studies were performed to prove transition changes of G1 and G2 molecules. Figures 5a and 6a show the X-ray reflectivity profiles of the G1 and G2 monolayers transferred onto a hydrophilic silicon wafer at different surface pressures. The thickness and roughness were then measured by X-ray reflectivity. Figures 5b and 6b provide individual and total layer electron density profiles at different surface pressures. The electron density of the molecule, Fel, is related to the refractive index of the molecule, n ) 1 - d where d ) λ2Fel ro/2π with the classical electron radius ro. The thickness of the silicon oxide layer was almost constant whereas the G1 and G2 layer thicknesses increased. Figure 7 shows the roughness of the films as a function of surface pressure. In the case of G1 films in Figure 7a, the thickness of the G1 film increases from 5.14 Å at 0.1 mN/m to 27.7 Å at 5 mN/m with increasing surface pressure. This is in accordance with the AFM observation that several circular domains of G1 were made spontaneously at low pressure and formed aggregates at higher compression. Whereas the thickness is constant, the roughness of G1 films decreases at a surface pressure of 20 mN/m. The

roughness of the G1 LB films increases from 1.95 Å at low surface pressure to 3.31 Å at medium surface pressure and 2.45 Å at high surface pressure. This is due to the rigid tail groups of G1 dendrimers at high pressure (Π ≈ 20 mN/m), which are arranged to make a relatively flat surface. For G2, the thickness of the films increases from 13.5 Å at 0.2 mN/m to 37.6 Å at 10 mN/m (Figure 7b). After point (d) of the Π-A isotherm in Figure 2, the thickness of the G2 monolayer film at a surface pressure of 20 mN/m suddenly increases to 45.5 Å, which is greater than the size of the G2 molecules standing on the substrate. This indicates that the G2 molecules slide up to the neighboring molecules as the surface pressure increases. In contrast with G1 films, the roughness of G2 films decreases with increasing surface pressure. In the case of G2 LB films, the roughness decreases from 5.68 Π-A at low pressure to 1.59 Å at high pressure. This indicates that G2 molecules move individually and align in the direction of the surface pressure increase, so the surface of G2 films is smoother than the surface of G1 films at high pressure. As a result, the roughness change shows a tendency to correspond to the roughness of AFM images. In addition, the thickness change of the data is equal to the thickness change of films from ellipsometry measurements. A gradual increase in the thickness of G1 and G2 films means that the molecules stand up. The different aspect of roughness for LB films is due to the difference in self-assembly behavior. After G1 molecules stand up, several molecules aggregate as the monolayer is compressed. However, G2 molecules align because of the π-π stacking of tail groups. The roughness of G1 films

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Figure 8. Schematic model of the molecular organization of the fanshaped dendrimer at the air-water interface. (a) In the gas phase, there is a weak interaction between the molecules and the water subphase. (b) At low surface pressure, there is the formation of small planar domains with centered core groups (-CO2H). (c) At high surface pressure, the molecules self-organize into a planar alignment of fan-shaped dendrimers.

Figure 7. Summarized results of the thickness (solid line) and the roughness (dash line) of G1 and G2 films from Figures 5 and 6: (a) [G1-An]-CO2H and (b) [G2-An]-CO2H.

increases, but G2 films transferred onto Si wafers at high pressure are slightly smoother than G2 LB films at low pressure because G2 molecules order in a close-packed arrangement.

Conclusions In this study, we determined the mechanism of self-assembly of fan-shaped dendrimers ([G1-An]-CO2H and [G2-An]-CO2H) at the air-water interface as investigated by X-ray reflectivity, AFM, and Π-A isotherm measurements. Figure 8 provides a

schematic of the whole self-assembly mechanism of G1 and G2 molecules. From the Π-A isotherm and morphology studies, we found that fan-shaped dendrimers (G1, G2) showed a different conformation at high surface pressure (Π ≈ 20 mN/m) owing to the flexibility of G2 molecules due to increased generation number. G1 and G2 molecules formed oblate domains at low surface pressure whereas G1 molecules were easily aggregated at high surface pressure. However, G2 molecules were ordered in a close-packed arrangement at high surface pressure. From X-ray reflectivity profiles, a comparison of the thickness and roughness of films, we found that the hydrophilic core groups (-CO2H) of G1 and G2 molecules were anchored on the water subphase and that the hydrophobic tail groups (aryl ether groups) were located far away from the water. The conformational transition is due to the rotational degree of freedom of G2. Acknowledgment. This work was supported by the KOSEF Nano Project Research Fund and the Research Fund of Hanyang University (HYU-2008-T). D.S. thanks Dr. Jaeyoung Choi at PAL (Phohang Accelerator Lab) for use of 10C1 beam line. LA802438N