CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 43–46
Articles Different Self-Assembly Behavior of Amphiphilic Molecules under Diverse Precipitating Conditions Shuping Pang,† Fangfang Jian,*,†,⊥ Zongwei Xuan,‡ and Jing Wang† The Laboratory of New Materials and Functional Compounds, Qingdao UniVersity of Science and Technology, Qingdao 266042, P. R. China, Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing UniVersity of Science and Technology, Nanjing 210094, P. R. China, and Microscale Science Institute, Weifang UniVersity, Weifang, 261061, P. R. China ReceiVed May 13, 2007; ReVised Manuscript ReceiVed July 1, 2008
ABSTRACT: We present investigations on the microcosmic self-assembly process of new synthesized amphiphilic DMAPNPP molecules. During the temperature decrease and solvent evaporation of the amphiphilic molecules’ saturated solution, the amphiphilic molecules first assembled into thinner belt-like arms extending from a core. And then the curled belts not only served as the directing template but also were straightened during the subsequent assembly process. This conclusion well illuminates why the dimensions can be controlled by adjusting the amphiphilic molecules’ deposition temperature and rate from their saturated solution. It opens a new avenue for conveniently adjusting the self-assembly dimension of similar amphiphilic molecules. Introduction In addition to the development of nanomaterials from inorganic compounds and polymers, organic nanomaterials based on low-molecular-weight compounds have attracted increasing attention because their electronic and optical properties are fundamentally different from those of the inorganic type.1-4 Among the traditional methods of preparing organic nanostructures, self-assembly as a powerful tool for constructing various types of nano/microstructures shows ever increasing importance in chemistry, material science, life science, and nanotechnology.5,6 It has generated a wide variety of objects with nanoscale or micrometer-scale morphologies, assembled mainly through noncovalent interactions such as van der Waals, hydrogen bonding, hydrophilic/hydrophobic, electrostatic, donor and acceptor, and metal-ligand coordination networks.7-9 Amphiphilic molecules have many advantages in the assembly of nano/microstructures because of the properties of hydrophilic/ hydrophobic groups. The middle parts of amphiphilic molecules can also influence the final self-assembly morphologies.10-12 At present, the strategy of self-assembly is an interesting alternative for the bottom-up design of organic electronic devices. Aida et al. prepared discrete nanotubes of an amphiphilic hexa-peri-hexabenzocoronene.13 Yan et al. prepared macroscopic tubes from designed amphiphilic multiarm co* Corresponding author. E-mail:
[email protected]. † Qingdao University of Science and Technology. ‡ Nanjing University of Science and Technology. ⊥ Weifang University.
polymers. The tube assembly process is mainly directed by the force of hydrophilic/hydrophobic groups.14 Self-assembly of amphiphilic dendron-rod molecules in selected solutions have been investigated for a long time. Tsukruk et al. observed compressed monolayers composed of interdigitated layering and circular planer surface structure in LB layers,15 and they subsequently reported a novel mechanism of self-assembly of that molecule into star shaped aggregates featuring self-propelled mechanistic motion.16 But to date, only a few attempts have been made to study how the structures of amphiphilic molecules assembled from smaller to larger and why the dimensions can be controlled by adjusting the precipitation rate of their saturated solution. We now synthesize a new simple amphiphilic molecule and study the assembly behavior during the temperature decrease and the solvent evaporation. Experimental Section Synthesis of DMAPNPP. The synthesis of (E)-3-(4-(dimethylamino)phenyl)-1-(4-nitrophenyl)prop-2-en-1-one (DMAPNPP) is a typical chalcone reaction, and the synthetic process of similar molecules has been reported elsewhere.17 The detailed steps are described as follows: First, 3.3 g (0.02 mol) of 1-(4-nitrophenyl)ethanone and 20 mL of 10% NaOH aqueous solution were added to 40 mL ethanol in turn with stirring to form a uniform solution. Subsequently, 2.9 g (0.02 mol) of 4-(dimethylamino)benzaldehyde was added slowly into the above solution. Then the mixture was stirred for 2 h at room temperature until a red precipitate formed. The red precipitate was collected and further purified by recrystallization from acetone at room temperature. The yield of the DMAPNPP is estimated to be higher than 80%, and the melting point is about 215-216 °C.
10.1021/cg070436r CCC: $40.75 2009 American Chemical Society Published on Web 11/24/2008
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Scheme 1. Synthesis of DMAPNPP
Figure 3. SEM image of larger DMAPNPP belts that grow along thinner ones (a). Planeform and side elevation of the junction of larger-thinner structures (b). SEM image of the step morphologies of the DMAPNPP belt (c). Scale bar in a and b is 5 µm; scale bar in d is 1 µm. and atom force microscopy (AFM, Veeco Group) were adopted to characterize the morphologies of the products.
Results and Discussion Figure 1. Time-lapse optical micrographs of growth video after (a) 20, (b) 30, (c) 40 s in acetone solution in a glass vessel. A higher resolution optical micrograph of the belts obtained after 30 s (d). Scale bar in a, b, c are 5 µm, d is 1 µm.
Figure 2. Time-lapse optical micrographs of self-assembly process of DMAPNPP belt along thinner and curled belts. Scale bar in a, b, c, and d is 10 µm. Self-Assembly of DMAPNPP. 0.3 g of DMAPNPP was added to 100 mL of acetone and heated slowly until the DMAPNPP was absolutely dissolved (about 50 °C). Red floccules precipitated from the saturated DMAPNPP solution as the temperature decreased to room temperature (17 °C) and then the solution evaporated. The assembly behavior was revealed in the course of direct in situ monitoring of its growth with optical microscopy (OM, angel, AQ-2010B, China). Fieldemission scanning electron microscopy (FE-SEM, JSM 6700F, Japan)
The reason for selecting an amphiphilic chalcone molecule as the self-assembly monomer is that chalcones are easily synthesized and widely used in optical materials, food technology, and medical therapy. The synthetic process of DMAPNPP is outlined in Scheme 1. N,N-Dimethyl and nitryl can be considered as hydrophilic18,19 and hydrophobic20 groups, respectively. The structure of the target compound was identified by elemental analysis, 1H NMR, and FT-IR spectroscopy (see Supporting Information). The as-synthesized compound is soluble in acetone, tetrahydrofuran, ethyl acetate, n-butanol, methanol, etc. In this experiment, acetone was adopted for its good solvating ability and quick evaporation rate. The solubility of DMAPNPP was estimated according to our approximate experimental statistical data: 1 g of DMAPNPP can be dissolved in 1 L of acetone (about 17 °C), and nearly 3 g of DMAPNPP can be absolutely dissolved when the temperature was raised to 50 °C. In our experiment, 5 mL of saturated DMAPNPP solution (50 °C) was transferred to a transparent glass vessel which had also been maintained at 50 °C in advance. Then the selfassembly process was quickly monitored with OM at the room temperature. Figure 1a-c are time-lapse optical micrographs from growth video of the structural transformation of DMAPNPP after 20 s, 30 s, 40 s in acetone solution, respectively. It is estimated that the temperature of the DMAPNPP solution dropped to room temperature in only a few seconds. It is difficult to capture the initial morphologies of the star shaped structure. The self-assembly model of many arms stretching from a core is indicated. After the initial formation of the star shaped nuclei, the growth of the star shaped structure was observed with the optical micrographs. The length of the arms changed quickly and the width changed slowly during the self-assembly process as shown in Figure 1a-c. An astonishing discovery is shown in Figure 1d. Besides the thick and straight belts, there are some thinner and curly ones, which are soft enough to form spring
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Figure 4. AFM image of final belt-like structure (a). High resolution AFM image of the edge of the DMAPNPP belt (b), and its corresponding 3D image (c). High resolution AFM image of the top area (d). The line across the section (line in d) shows a domain height of 1.25 nm (e).
Figure 5. SEM images of the belt-like structure obtained with acetone evaporating naturally at a constant temperature of 50 °C (a), with the temperature decrease and acetone evaporation at room temperature (b), and with the temperature decreasing sharply to 17 °C (c).
morphologies for minimizing the surface energy. It is interesting that the small belts attach to the big arms stretching from the center of the star shaped structure. In order to study the selfassembly behavior, another two similar chalcone molecules were successfully synthesized (Scheme S1, Supporting Information). A similar amphiphilic compound without nitryl was also assembled into one-dimensional belt-like structures, but when the nitryl was substituted with methoxyl, the compound with two hydrophilic groups on the opposite sides only formed irregular powders. So it is proved that the hydrophilic/ hydrophobic property plays an important role in the selfassembly process of one-dimensional belt-like structures, and the molecular self-assembly model of the 1D belt structure is shown in Figure S-3 in the Supporting Information. In order to investigate the novel self-assembly behavior, how the above morphologies (small belts linked with larger ones) formed was revealed in the course of direct in situ monitoring of its growth with optical microscopy. Figure 2 shows the timelapse optical micrographs of a short period of the self-assembly process. At the beginning, the thinner belt swayed in the acetone solution. A supersaturated area on the right side of the curving belt was formed when it swayed from left to right. Then the DMAPNPP molecules quickly assembled along the thinner belt at the place with the largest curvature. The growing rate along the thinner belt was much larger than in the vertical direction because of the different noncovalent interaction forces in the two directions.16,21 The self-assembly process occurred quickly, and it took about 20 s from Figure 2a to 2c. In addition, as Figure 2b and 2c show, the assembly rates in the two opposite directions along the thinner belt were the same. When the assembly route was blocked by another belt, the self-assembly
was stopped (Figure 2d), which is different from any other organic molecular assembly behavior having rigid structure properties.16 The detailed information of small belts linked with larger ones is shown in Figure 3a. The thickness and width of the thinner belt is about 500 nm and 2 µm, respectively. The larger belt is generally larger than the thinner one in both thickness and width (Figure 3a,b). It is clear that the larger DMAPNPP belt can grow along one broad face of the thinner one, but not with the thinner belt as an axis, to form a cable-like structure (Figure 3b). So the growth of the belt from thinner to larger is attributed to the layer by layer assembly behavior. The dimension of the original thinner belts did not change during the following assembly process as shown in Figure 2. Because the layers had different dimensions in thickness and width, steplike surface morphologies were formed as shown in Figure 3c. The detailed surface information of the final DMAPNPP belt is shown in Figure 4. The surface of the belt looks not as perfect as that in the single crystal belts of inorganic materials. Layered structure is very clear on the sides of the belt in Figure 4b, which may attribute to the layer by layer assembly behavior. There are about seven obvious layers in the high range of 1.2 µm as shown in the corresponding 3D image (Figure 4c). Another cross-section topography of the AFM image reveals that the surface is comparatively smooth in the middle part of the belt surface. The line cross-section shows step structure with a domain height distance of about 1.25 nm, which is about the thickness of a monolayer of DMAPNPP.15 On the basis of the above analysis, it is considered that the width of the belts significantly depends on the precipitation process. Figure 3a-c are SEM images of the samples which were obtained under different precipitation processes. Figure 5a is the SEM image of the irregular belt-like structure obtained with acetone evaporating naturally at a constant temperature of 50 °C. So the DMAPNPP tends to form a huge irregular belt structure with the width of 20 µm and coarse surface. Figure 5b is the SEM image of a regular DMAPNPP belt with a width of 1-4 µm. The rectangular cross-section of the belt-like structure can be seen clearly. The self-assembly proceeded during the temperature decrease and acetone evaporation observed at room temperature as described in the Experimental Section. When the temperature of 100 mL of saturated DMAPNPP solution decreases from 50 to 17 °C, 0.2 g of DMAPNPP precipitates rapidly. Another 0.1 g of DMAPNPP slowly precipitates from acetone during the acetone evaporation. The above estimation is logical, assuming that the evaporating loss during the temperature decrease can be ignored because this moment is too short compared with the entire acetone evapora-
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tion time. Quick precipitation leads to a thinner belt as discussed in Figure 3. The assembly behavior of DMAPNPP also has been studied with the different temperature rates. Figure 5c is the SEM image of thinner DMAPNPP belt with the width and thickness of about 200 and 30 nm, respectively. The thinner belt is prepared as follows: The saturated DMAPNPP solution was quickly transferred into a large volume of cold distilled water and then a red precipitate was quickly formed. Subsequently, the mixture was filtered and dried at room temperature for 10 h. So, we can conclude that the precipitation temperature and rate play an essential role in the self-assembly process. Low solvent evaporation temperature and a quick temperature decrease rate lead to thinner belts. The quality of the DMAPNPP structures can be adjusted by selecting proper solvent evaporation temperature. And the dimensions of the self-assembled belts also can be tailored by controlling the precipitation rate of the saturated DMAPNPP solution. Conclusions In summary, we have observed an astonishing self-assembly behavior from DMAPNPP amphiphilic molecules during two different linked processes of temperature decrease and solvent evaporation. The star shaped aggregates constructed with thinner and curled belts were first self-assembled during a sharp temperature decrease. Then the smaller belts served as template for the subsequent assembly during the acetone evaporation. The growth model from thinner belts to big ones is a nucleationgrowth process with the thinner belts as the directing template. Belt growth from thinner to larger is attributed to the layer by layer assembly of DMAPNPP molecules. Thus, the dimension of the DMAPNPP structures can be adjusted by selecting proper evaporation temperature of the solvent and precipitation rate of the saturated DMAPNPP solution. Research on the control of the assembly behavior of such amphiphilic molecules might be useful in bottom-up device fabrication. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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