Article pubs.acs.org/Langmuir
Growth Mechanisms of 2D Organic Assemblies Generated from Dialkylated Melaminium Derivatives: The Length Difference of the Two Alkyl Chains That Matters Jun Xu, Guanglu Wu, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing100084, P. R. China S Supporting Information *
ABSTRACT: This research is aimed to understand the growth mechanisms for self-assembly of dialkylated melamine derivatives. The dialkylated melamine derivatives with different alkyl chains (Mela-m-n) are able to self-assemble with hydrochloric acid in dichloromethane to form 2D organic assemblies, exhibiting similar lamellar structures as Mela-n·HCl with identical alkyl chains. The most interesting finding is that the growth mechanism of Mela-n·HCl with identical alkyl chains is revealed to be layer growth, while Mela-m-n·HCl with asymmetric alkyl chains adopts a spiral growth mechanism. The asymmetric alkyl chains in Mela-m-n may lead to the formation of dislocation, which is responsible for the spiral growth mechanism.
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INTRODUCTION Two-dimensional (2D) micro/nanostructures have attracted considerable attention due to their unique properties and promising applications.1−4 Self-assembly of small organic molecules in solution is an effective bottom-up strategy to fabricate 2D micro/nanostructures.5−17 A lot of work has been done to fabricate 2D organic assemblies with various size, shape, and thickness, but the good control of parameters such as lateral size, shape, thickness, and monodispersity of 2D assemblies is still a great challenge.18 To this end, a better understanding of the growth mechanisms of 2D organic assemblies is crucial, which may provide guidelines for the fabrication of 2D materials in a rational way. Two most commonly noted crystal growth mechanisms are layer growth and spiral growth in terms of the surface structure of crystals.19−21 For layer growth, growth units attach to the kink sites and move along the step on the same plane, and then the step advances to the edge of the crystal to form one layer. For spiral growth, due to the perpetuating step created by dislocation, growth units attach to the step and generate a spiral pattern around the dislocation. Atomic force microscopy (AFM) is widely used as a powerful approach to monitor the crystal growth process.22−27 The above-mentioned theoretical models and observation technique of crystal growth may provide some inspiration to reveal growth mechanisms of 2D organic assemblies. Herein, we are aimed to study the growth mechanisms of 2D organic assemblies generated by dialkylated melaminium derivatives. For this purpose, we synthesized two types of dialkylated melamine derivatives, Mela-n and Mela-m-n, as shown in Scheme 1. Mela-n (n = 4, 8, 12) have two identical © 2013 American Chemical Society
Scheme 1. Chemical Structures of Dialkylated Melamine Derivatives with Identical Alkyl Chains (a) and Asymmetric Alkyl Chains (b)
alkyl chains, Mela-m-n (m = 4, n = 8; m = 4, n = 12; m = 8, n = 12) have two alkyl chains of different length, and n and m refers to the carbon number of each alkyl chain. Mela-n and Mela-m-n protonated by hydrochloric acid (HCl) can both form 2D organic assemblies in dichloromethane. Wide-angle X-ray diffraction (WAXD) showed that the d-spacing in the lamellar structure of Mela-m-n·HCl was equivalent to that of Mela-(m + n)/2·HCl, which suggested the “interdigitation” of alkyl chains within Mela-m-n·HCl assemblies. Although the protonated Mela-m-n exhibits the same driving forces and the same lamellar molecular arrangements as previously reported protonated Mela-n, the length difference of alkyl chains does influence the growth mechanism of the 2D assemblies. By taking advantage of AFM, the growth mechanism of Mela-n·HCl is revealed to be layer growth, while the growth mechanism of Mela-m-n·HCl is spiral growth. The asymmetric alkyl chains of Mela-m-n may Received: July 11, 2013 Revised: July 29, 2013 Published: August 2, 2013 10959
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Scheme 2. Synthetic Route of Mela-m-n
Figure 1. TEM images of 2D rectangular assemblies formed by (a) Mela-8-12·HCl, (b) Mela-4-12·HCl, and (c) Mela-4-8·HCl. N-Butyl-N′-dodecyl-1,3,5-triazine-2,4,6-triamine (Mela-4-12). This compound was synthesized by adopting a procedure similar to that used for Mela-8-12. White powder; yield: 96%. 1H NMR (400 MHz, CDCl3, ppm): δ 4.95−4.50 (br, 4H), 3.44−3.22 (m, 4H), 1.59− 1.45 (m, 4H), 1.41−1.18 (m, 20H), 0.93 (t, 7.3 Hz, 3H), 0.88 (t, 6.8 Hz, 3H). ESI-MS: 351.4 ([M + H]+), calcd 351.3. N-Butyl-N′-octyl-1,3,5-triazine-2,4,6-triamine (Mela-4-8). This compound was synthesized by adopting a procedure similar to that used for Mela-8−12. White powder; yield: 77%. 1H NMR (400 MHz, CDCl3, ppm): δ 5.10−4.65 (br, 4H), 3.44−3.24 (m, 4H), 1.60−1.46 (m, 4H), 1.43−1.18 (m, 12H), 0.93 (t, 7.3 Hz, 3H), 0.88 (t, 6.9 Hz, 3H). ESI-MS: 295.2 ([M + H]+), calcd 295.3. Preparation Procedures of 2D Assemblies. In a typical preparation, 5.0 mM of Mela-m-n was dissolved in 5.0 mL of dichloromethane and then 1.5 μL of 3.5 M HCl was added. After sonication for 2 min and standing for several hours at room temperature, turbid suspensions were obtained, indicating the formation of 2D assemblies. Characterization. 1H NMR spectra were recorded on JEOL JNMECA 400 (400M) apparatuses at 25 °C. ESI-MS spectra were obtained on an Esquire-LC 00136 mass spectrometer. Transmission electron microscopy (TEM) experiments were performed at 80 kV with a HITACHI H-7650B transmission electron microscope. The samples were prepared by dripping 5 μL of 1.0 mM Mela-m-n·HCl suspensions of assemblies on carbon-coated copper grids and drying in air. AFM images were obtained on a commercial MultiMode 8 atomic force microscope with ScanAsyst mode on glass substrate, using silicon cantilevers (ScanAsyst-Air from Bruker). The sample was prepared by dripping 10 μL of 1.0 mM Mela-n·HCl and Mela-m-n·HCl assembly suspensions on glass substrate and drying in air. WAXD data were collected at room temperature on a Rigaku D/ max 2500 diffractometer with Cu Kα radiation. The samples were prepared by successively dripping 15 μL of 1.0 mM Mela-m-n·HCl suspensions of assemblies on glass substrates and drying in air dozens of times.
easily cause the formation of dislocation, which is responsible for the spiral growth.
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EXPERIMENTAL SECTION
Materials. Cyanuric chloride (Sigma-Aldrich, 99%), 1-dodecylamine (Aladdin, 98%), 1-octylamine (Aladdin, 99%), n-butylamine (Aladdin, 99%), hydrochloric acid (Beijing Chemical Works, AR), and dichloromethane (Aladdin, Super Dry, H2O ≤ 0.004%) were used as received. Synthesis. The synthetic route of Mela-m-n is shown in Scheme 2. 2-Amine-4,6-dichloro-1,3,5-triazine. The synthesis was performed according to a standard procedure.28 White powder; yield: 67%. ESIMS: 165.3, 167.2 ([M + H]+), calcd 165.0, 167.0. 6-Chloro-N-dodecyl-1,3,5-triazine-2,4-diamine. The synthesis was performed according to the literature.29 2-Amino-4,6-dichloro-1,3,5triazine (0.37 g, 2.2 mmol) and 1-dodecanamine (0.50 g, 2.7 mmol) were mixed in 30 mL of water. Sodium bicarbonate (0.22 g, 2.6 mmol) was added, and the mixture was stirred at 25 °C for 5 h. The crude product was then extracted with chloroform; the organic layer was dried over Na2SO4, and the solvent was removed in vacuum. The crude product was purified by column chromatography (20:1 dichloromethane/methanol) to afford a white powder (0.41 g, 59%). ESI-MS: 314.2, 316.2 ([M + H]+), calcd 314.2, 316.2. 6-Chloro-N-octyl-1,3,5-triazine-2,4-diamine. This compound was synthesized by adopting a procedure similar to that used for 6-chloroN-dodecyl-1,3,5-triazine-2,4-diamine. White powder; yield: 22%. ESIMS: 258.2, 260.1 ([M + H]+), calcd 258.2, 260.2. N-Dodecyl-N′-octyl-1,3,5-triazine-2,4,6-triamine (Mela-8-12). The synthesis was performed according to a standard procedure.5 6Chloro-N-dodecyl-1,3,5-triazine-2,4-diamine (0.21 g, 0.67 mmol) and 1-octylamine (0.22 mL, 1.3 mmol) were mixed in 10 mL of water. After stirring for 0.5 h, sodium hydroxide (0.032 g, 0.80 mmol) in 10 mL of water was added, and the mixture was refluxed at 110 °C for 20 h. After cooling, the crude product was extracted with chloroform, the organic layer was dried over Na2SO4, and the solvent was removed in vacuum. The crude product was purified by column chromatography (20:1 dichloromethane/methanol) to afford a white powder (0.26 g, 95%). 1H NMR (400 MHz, CDCl3, ppm): δ 4.95−4.50 (br, 4H), 3.43−3.22 (m, 4H), 1.59−1.46 (m, 4H), 1.39−1.18 (m, 28H), 0.88 (t, 6.8 Hz, 6H). ESI-MS: 407.4 ([M + H]+), calcd 407.4.
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RESULTS AND DISCUSSION The self-assembly of Mela-m-n·HCl, having two asymmetric alkyl chains, forms 2D rectangular assemblies in dichloro10960
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Figure 2. (a) The lamellar structures of Mela-m-n·HCl assemblies confirmed by WAXD measurements. (b) The linear relationship between lamellar d-spacings of Mela-m-n·HCl assemblies and average carbon number [(m + n)/2] of alkyl chains, which has almost the same slope as for Mela-n·HCl.
methane. As indicated by TEM in Figure 1, in the case of Mela8-12·HCl, the 2D rectangular assemblies are microsheets, and the lateral size can cover a range of a few micrometers, and the thickness of the microsheets is about several hundred nanometers. For Mela-4-12·HCl and Mela-4-8·HCl, they also form similar 2D microsheets with a little difference in aspect ratio, which may be caused by the difficulty in accurately controlling the molar ratio of melamine derivatives and HCl. WAXD was used to get more structure information of the above-mentioned 2D assemblies. As demonstrated by WAXD in Figure 2a, a series of periodic diffraction peaks is clearly observed, indicating highly ordered lamellar structures in the 2D assemblies formed by Mela-8−12·HCl. The d-spacing of the lamellar structures is calculated to be 3.15 nm. Similarly, Mela4-12·HCl and Mela-4-8·HCl formed 2D assemblies of ordered lamellar structures as well, whose d-spacings are 2.63 and 2.21 nm, respectively. The self-assembling behavior and structure of Mela-n·HCl have been fully explored in our previous work.17 Mela-n·HCl self-assembled in dichloromethane to form 2D microsheets through counterion bridged multiple hydrogen bonds as well as electrostatic interactions and π−π stacking. WAXD results indicated the existence of lamellar structures in Mela-n·HCl assemblies, and the lamellar d-spacings increase linearly with the carbon number of the alkyl chain. As shown in Figure 2b, interestingly, the lamellar d-spacings of Mela-m-n·HCl also change linearly with the average carbon number [(m + n)/2] of two alkyl chains, in spite of their length difference. Moreover, the slope of Mela-m-n·HCl is almost the same as that of Mela-n· HCl, and the d-spacing in the lamellar structure of Mela-m-n· HCl can be equivalent to the one of Mela-(m + n)/2·HCl. From these results, we can deduce that the molecular arrangements of Mela-m-n·HCl within their 2D assemblies should be the same as Mela-n·HCl, whereas the alkyl chains of the former should probably adopt an “interdigitated” structure. The plausible molecular arrangements of Mela-8-12·HCl are shown in Figure 3, which is also supported by WAXD in Figure 2a. Therefore, the melaminium moieties can serve as versatile structure-directing groups for the fabrication of 2D assemblies, regardless of the variation of the appending alkyl chains. Although Mela-m-n·HCl and Mela-n·HCl have the same lamellar molecular arrangements, the different symmetry of alkyl chains leads to different growth mechanisms of obtained
Figure 3. The plausible molecular arrangement of Mela-8-12·HCl in the 2D assemblies. The driving forces are counterion bridging and π−π stacking, while the alkyl chains adopt interdigitated structures.
2D assemblies. The growth mechanism of Mela-m-n·HCl is revealed to be spiral growth by AFM. In a typical preparation, the protonated melamine derivatives were obtained by mixing an equal molar amount of HCl with Mela-m-n in dichloromethane. After sonication for 2 min and standing for various times, the solution was dripped and dried on glass substrate for AFM measurements. Figure 4a−d shows the AFM images of Mela-8-12·HCl assemblies at each stage. From these images, we can see the enlargement of lateral size and the increase of layer number as time progressed during the self-assembling process. It is noteworthy that the spiral growth patterns can be observed clearly in each image. Figure 4e,f shows typical spiral patterns 10961
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Figure 5. The AFM images of Mela-12·HCl assemblies after sonication for 2 min (a) and standing for 1 h (b).
For Mela-8·HCl and Mela-4·HCl, similar flat-surface structures can also be observed by AFM (Figure S2, Supporting Information), instead of spiral patterns. Therefore, the growth mechanism of Mela-n·HCl is determined to be layer growth. The difference of the growth mechanisms for these two types of dialkylated melaminium derivatives may result from their difference in molecular symmetry. Mela-m-n molecules have two alkyl chains with different length, which adopt “interdigitated” structures in the Mela-m-n·HCl assemblies, as shown in Figure 3. During the self-assembling process, although an “interdigitated” structure of the alkyl chains is preferred, there is a possibility that the alkyl chains are met in a “noninterdigitation” mode. It means that in Mela-m-n molecules, the short and long alkyl chain may not fully “interdigitated”. This kind of mismatch may cause the formation of screw dislocations, leading to spiral growth. However, for Mela-n· HCl, the identical alkyl chains have no chance to mismatch and form dislocations; therefore, a layer growth mechanism is adopted.
Figure 4. AFM images of Mela-8-12·HCl assemblies after sonication for 2 min (a) and standing for 1 h (b), 4 h (c), and 7 h (d). The spiral growth patterns of Mela-8-12·HCl assemblies after standing for 1 h with a layer thickness of 2.8 nm were observed by AFM: (e) peak force error and (f) height. The peak force error image, which can differentiate the boundaries in plane clearly, is provided here to help to observe the spiral pattern. Meanwhile, the height image and section analysis can quantitatively demonstrate the spiral growth.
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CONCLUSIONS In conclusion, we have prepared 2D organic microsheets from dialkylated melaminium derivatives with identical and different alkyl chains. The molecular arrangements of Mela-m-n·HCl are demonstrated to be the same as for Mela-n·HCl. More importantly, the growth mechanism of Mela-n·HCl is revealed by AFM to be layer growth, while Mela-m-n·HCl adopts a spiral growth mechanism because of its asymmetric alkyl chains. This work not only reveals two types of growth mechanisms of 2D organic assemblies based on the different structures of building blocks but also supplies inspiration for the fabrication of ultrathin 2D assemblies and spiral structures from small organic molecules.
with quadrilateral shape after standing for 1 h, and the layer thickness is measured to be 2.8 nm by cross-sectional analysis of height, which is consistent with the d-spacing indicated by WAXD in Figure 2a. In addition, it holds true for Mela-4-12· HCl and Mela-4-8·HCl assemblies that similar spiral growth patterns are observed by AFM (Figure S1, Supporting Information). Therefore, the growth mechanism of Mela-m-n· HCl assemblies is determined to be spiral growth. Contrary to Mela-m-n·HCl, the growth mechanism of Melan·HCl assemblies is revealed to be layer growth. Figure 5 shows the AFM images of Mela-12·HCl assemblies after sonication for 2 min and standing for 1 h. We can clearly see the increase of the thickness and the enlargement of the lateral size, but no spiral growth patterns can be observed. When increasing the incubation time from 2 min to 1 h, the height of the 2D assemblies increases from less than 10 nm to more than 30 nm. The thickness of each layer within the 2D assemblies can be measured at the edge of the flat sheet and is determined to be 3.2 nm, which is close to its d-spacing measured by WAXD. It is worthy to note that ultrathin or even single-layer-thick 2D assemblies may be obtained by controlling the incubating time.
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ASSOCIATED CONTENT
S Supporting Information *
Spiral growth patterns of Mela-4-12·HCl and Mela-4-8·HCl, layer growth patterns of Mela-8·HCl and Mela-4·HCl, and 1H NMR spectra of Mela-m-n. This material is available free of charge via the Internet at http://pubs.acs.org. 10962
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Interactions: From Nanosheets to Nanofibers. Langmuir 2012, 28, 10697−10702. (17) Xu, J.; Wu, G.; Wang, Z.; Zhang, X. Generation of 2D Organic Microsheets from Protonated Melamine Derivatives: Suppression of the Self Assembly of a Particular Dimension by Introduction of Alkyl Chains. Chem. Sci. 2012, 3, 3227−3230. (18) Govindaraju, T.; Avinash, M. B. Two-Dimensional Nanoarchitectonics: Organic and Hybrid Materials. Nanoscale 2012, 4, 6102−6117. (19) De Yoreo, J. J.; Vekilov, P. G. Principles of Crystal Nucleation and Growth. Rev. Mineral. Geochem. 2003, 54, 57−93. (20) Dhanaraj, G.; Byrappa, K.; Prasad, V.; Dudley, M. Springer Handbook of Crystal Growth; Springer: New York, 2010; Vol. 1, pp 4− 6. (21) Cubillas, P.; Anderson, M. W. Zeolites and Catalysis: Synthesis, Reactions and Applications; Wiley-VCH: New York, 2010; Chapter 1, pp 6−11. (22) Penn, R. L.; Banfield, J. F. Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals. Science 1998, 281, 969−971. (23) Pina, C. M.; Becker, U.; Risthaus, P.; Bosbach, D.; Putnis, A. Molecular-Scale Mechanisms of Crystal Growth in Barite. Nature 1998, 395, 483−486. (24) Malkin, A. J.; Kuznetsov, Y. G.; McPherson, A. In Situ Atomic Force Microscopy Studies of Surface Morphology, Growth Kinetics, Defect Structure and Dissolution in Macromolecular Crystallization. J. Cryst. Growth 1999, 196, 471−488. (25) Davis, K. J.; Dove, P. M.; De Yoreo, J. J. The Role of Mg2+ as an Impurity in Calcite Growth. Science 2000, 290, 1134−1137. (26) Shöaê è, M.; Agger, J. R.; Anderson, M. W.; Attfield, M. P. Crystal Form, Defects and Growth of the Metal Organic Framework HKUST-1 Revealed by Atomic Force Microscopy. CrystEngComm 2008, 10, 646−648. (27) Cubillas, P.; Holden, M. A.; Anderson, M. W. Crystal Growth Studies on Microporous Zincophosphate-Faujasite Using Atomic Force Microscopy. Cryst. Growth Des. 2011, 11, 3163−3171. (28) Thurston, J. T.; Dudley, J. R.; Kaiser, D. W.; Hechenbleikner, I.; Schaefer, F. C.; Holm-Hansen, D. Cyanuric Chloride Derivatives. I. Aminochloro-s-triazines. J. Am. Chem. Soc. 1951, 73, 2981−2983. (29) Baliani, A.; Peal, V.; Gros, L.; Brun, R.; Kaiser, M.; Barrett, M. P.; Gilbert, I. H. Novel Functionalized Melamine-based Nitroheterocycles: Synthesis and Activity against Trypanosomatid Parasites. Org. Biomol. Chem. 2009, 7, 1154−1166.
AUTHOR INFORMATION
Corresponding Author
*Fax: (+86) 10-6277-1149. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2013CB834502), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21121004), and the NSFC− DFG joint grant (TRR61).
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REFERENCES
(1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Rao, C. N.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: the New Two-Dimensional Nanomaterial. Angew. Chem., Int. Ed. Engl. 2009, 48, 7752−7777. (3) Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132−145. (4) Osada, M.; Sasaki, T. Two-Dimensional Dielectric Nanosheets: Novel Nanoelectronics from Nanocrystal Building Blocks. Adv. Mater. 2012, 24, 210−228. (5) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake, T. Supramolecular Membranes. Spontaneous Assembly of Aqueous Bilayer Membrane via Formation of Hydrogen Bonded Pairs of Melamine and Cyanuric Acid Derivatives. J. Am. Chem. Soc. 1998, 120, 4094−4104. (6) Song, B.; Wang, Z.; Chen, S.; Zhang, X.; Fu, Y.; Smet, M.; Dehaen, W. The Introduction of π−π Stacking Moieties for Fabricating Stable Micellar Structure: Formation and Dynamics of Disklike Micelles. Angew. Chem., Int. Ed. 2005, 44, 4731−4735. (7) Yip, H. L.; Ma, H.; Jen, A. K.; Dong, J.; Parviz, B. A. TwoDimensional Self-Assembly of 1-Pyrylphosphonic Acid: Transfer of Stacks on Structured Surface. J. Am. Chem. Soc. 2006, 128, 5672−5679. (8) Davis, R.; Berger, R.; Zentel, R. Two-Dimensional Aggregation of Organogelators Induced by Biaxial Hydrogen-Bonding Gives Supramolecular Nanosheets. Adv. Mater. 2007, 19, 3878−3881. (9) Zhong, C.; Wang, J.; Wu, N.; Wu, G.; Zavalij, P. Y.; Shi, X. Anion Bridged Nanosheet from Self-Assembled G-Quadruplexes. Chem. Commun. 2007, 3148−3150. (10) Chen, Y.; Zhu, B.; Zhang, F.; Han, Y.; Bo, Z. Hierarchical Supramolecular Self-Assembly of Nanotubes and Layered Sheets. Angew. Chem., Int. Ed. 2008, 47, 6015−6018. (11) Lee, E.; Kim, J. K.; Lee, M. Reversible Scrolling of TwoDimensional Sheets from the Self-Assembly of Laterally Grafted Amphiphilic Rods. Angew. Chem., Int. Ed. 2009, 48, 3657−3660. (12) An, Q.; Chen, Q.; Zhu, W.; Li, Y.; Tao, C. A.; Yang, H.; Li, Z.; Wan, L.; Tian, H.; Li, G. A Facile Method for Preparing OneMolecule-Thick Free-Standing Organic Nanosheets with a Regular Square Shape. Chem. Commun. 2010, 46, 725−727. (13) Nam, K. T.; Shelby, S. A.; Choi, P. H.; Marciel, A. B.; Chen, R.; Tan, L.; Chu, T. K.; Mesch, R. A.; Lee, B. C.; Connolly, M. D.; Kisielowski, C.; Zuckermann, R. N. Free-Floating Ultrathin TwoDimensional Crystals from Sequence-Specific Peptoid Polymers. Nat. Mater. 2010, 9, 454−460. (14) Han, M.; Hirade, T. Anisotropic Two-Dimensional Sheets Assembled from Rod-Shaped Metal Complexes. Chem. Commun. 2012, 48, 100−102. (15) Wu, G.; Verwilst, P.; Xu, J.; Xu, H.; Wang, R.; Smet, M.; Dehaen, W.; Faul, C. F.; Wang, Z.; Zhang, X. Bolaamphiphiles Bearing Bipyridine as Mesogenic Core: Rational Exploitation of Molecular Architectures for Controlled Self-Assembly. Langmuir 2012, 28, 5023−5230. (16) Liu, K.; Yao, Y.; Liu, Y.; Wang, C.; Li, Z.; Zhang, X. SelfAssembly of Supra-Amphiphiles Based on Dual Charge-Transfer 10963
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Growth Mechanisms of 2D Organic Assemblies Generated from Dialkylated Melaminium Derivatives: The Length Difference of the Two Alkyl Chains That Matters Jun Xu, Guanglu Wu, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing100084, P. R. China S Supporting Information *
ABSTRACT: This research is aimed to understand the growth mechanisms for self-assembly of dialkylated melamine derivatives. The dialkylated melamine derivatives with different alkyl chains (Mela-m-n) are able to self-assemble with hydrochloric acid in dichloromethane to form 2D organic assemblies, exhibiting similar lamellar structures as Mela-n·HCl with identical alkyl chains. The most interesting finding is that the growth mechanism of Mela-n·HCl with identical alkyl chains is revealed to be layer growth, while Mela-m-n·HCl with asymmetric alkyl chains adopts a spiral growth mechanism. The asymmetric alkyl chains in Mela-m-n may lead to the formation of dislocation, which is responsible for the spiral growth mechanism.
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INTRODUCTION Two-dimensional (2D) micro/nanostructures have attracted considerable attention due to their unique properties and promising applications.1−4 Self-assembly of small organic molecules in solution is an effective bottom-up strategy to fabricate 2D micro/nanostructures.5−17 A lot of work has been done to fabricate 2D organic assemblies with various size, shape, and thickness, but the good control of parameters such as lateral size, shape, thickness, and monodispersity of 2D assemblies is still a great challenge.18 To this end, a better understanding of the growth mechanisms of 2D organic assemblies is crucial, which may provide guidelines for the fabrication of 2D materials in a rational way. Two most commonly noted crystal growth mechanisms are layer growth and spiral growth in terms of the surface structure of crystals.19−21 For layer growth, growth units attach to the kink sites and move along the step on the same plane, and then the step advances to the edge of the crystal to form one layer. For spiral growth, due to the perpetuating step created by dislocation, growth units attach to the step and generate a spiral pattern around the dislocation. Atomic force microscopy (AFM) is widely used as a powerful approach to monitor the crystal growth process.22−27 The above-mentioned theoretical models and observation technique of crystal growth may provide some inspiration to reveal growth mechanisms of 2D organic assemblies. Herein, we are aimed to study the growth mechanisms of 2D organic assemblies generated by dialkylated melaminium derivatives. For this purpose, we synthesized two types of dialkylated melamine derivatives, Mela-n and Mela-m-n, as shown in Scheme 1. Mela-n (n = 4, 8, 12) have two identical © 2013 American Chemical Society
Scheme 1. Chemical Structures of Dialkylated Melamine Derivatives with Identical Alkyl Chains (a) and Asymmetric Alkyl Chains (b)
alkyl chains, Mela-m-n (m = 4, n = 8; m = 4, n = 12; m = 8, n = 12) have two alkyl chains of different length, and n and m refers to the carbon number of each alkyl chain. Mela-n and Mela-m-n protonated by hydrochloric acid (HCl) can both form 2D organic assemblies in dichloromethane. Wide-angle X-ray diffraction (WAXD) showed that the d-spacing in the lamellar structure of Mela-m-n·HCl was equivalent to that of Mela-(m + n)/2·HCl, which suggested the “interdigitation” of alkyl chains within Mela-m-n·HCl assemblies. Although the protonated Mela-m-n exhibits the same driving forces and the same lamellar molecular arrangements as previously reported protonated Mela-n, the length difference of alkyl chains does influence the growth mechanism of the 2D assemblies. By taking advantage of AFM, the growth mechanism of Mela-n·HCl is revealed to be layer growth, while the growth mechanism of Mela-m-n·HCl is spiral growth. The asymmetric alkyl chains of Mela-m-n may Received: July 11, 2013 Revised: July 29, 2013 Published: August 2, 2013 10959
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Scheme 2. Synthetic Route of Mela-m-n
Figure 1. TEM images of 2D rectangular assemblies formed by (a) Mela-8-12·HCl, (b) Mela-4-12·HCl, and (c) Mela-4-8·HCl. N-Butyl-N′-dodecyl-1,3,5-triazine-2,4,6-triamine (Mela-4-12). This compound was synthesized by adopting a procedure similar to that used for Mela-8-12. White powder; yield: 96%. 1H NMR (400 MHz, CDCl3, ppm): δ 4.95−4.50 (br, 4H), 3.44−3.22 (m, 4H), 1.59− 1.45 (m, 4H), 1.41−1.18 (m, 20H), 0.93 (t, 7.3 Hz, 3H), 0.88 (t, 6.8 Hz, 3H). ESI-MS: 351.4 ([M + H]+), calcd 351.3. N-Butyl-N′-octyl-1,3,5-triazine-2,4,6-triamine (Mela-4-8). This compound was synthesized by adopting a procedure similar to that used for Mela-8−12. White powder; yield: 77%. 1H NMR (400 MHz, CDCl3, ppm): δ 5.10−4.65 (br, 4H), 3.44−3.24 (m, 4H), 1.60−1.46 (m, 4H), 1.43−1.18 (m, 12H), 0.93 (t, 7.3 Hz, 3H), 0.88 (t, 6.9 Hz, 3H). ESI-MS: 295.2 ([M + H]+), calcd 295.3. Preparation Procedures of 2D Assemblies. In a typical preparation, 5.0 mM of Mela-m-n was dissolved in 5.0 mL of dichloromethane and then 1.5 μL of 3.5 M HCl was added. After sonication for 2 min and standing for several hours at room temperature, turbid suspensions were obtained, indicating the formation of 2D assemblies. Characterization. 1H NMR spectra were recorded on JEOL JNMECA 400 (400M) apparatuses at 25 °C. ESI-MS spectra were obtained on an Esquire-LC 00136 mass spectrometer. Transmission electron microscopy (TEM) experiments were performed at 80 kV with a HITACHI H-7650B transmission electron microscope. The samples were prepared by dripping 5 μL of 1.0 mM Mela-m-n·HCl suspensions of assemblies on carbon-coated copper grids and drying in air. AFM images were obtained on a commercial MultiMode 8 atomic force microscope with ScanAsyst mode on glass substrate, using silicon cantilevers (ScanAsyst-Air from Bruker). The sample was prepared by dripping 10 μL of 1.0 mM Mela-n·HCl and Mela-m-n·HCl assembly suspensions on glass substrate and drying in air. WAXD data were collected at room temperature on a Rigaku D/ max 2500 diffractometer with Cu Kα radiation. The samples were prepared by successively dripping 15 μL of 1.0 mM Mela-m-n·HCl suspensions of assemblies on glass substrates and drying in air dozens of times.
easily cause the formation of dislocation, which is responsible for the spiral growth.
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EXPERIMENTAL SECTION
Materials. Cyanuric chloride (Sigma-Aldrich, 99%), 1-dodecylamine (Aladdin, 98%), 1-octylamine (Aladdin, 99%), n-butylamine (Aladdin, 99%), hydrochloric acid (Beijing Chemical Works, AR), and dichloromethane (Aladdin, Super Dry, H2O ≤ 0.004%) were used as received. Synthesis. The synthetic route of Mela-m-n is shown in Scheme 2. 2-Amine-4,6-dichloro-1,3,5-triazine. The synthesis was performed according to a standard procedure.28 White powder; yield: 67%. ESIMS: 165.3, 167.2 ([M + H]+), calcd 165.0, 167.0. 6-Chloro-N-dodecyl-1,3,5-triazine-2,4-diamine. The synthesis was performed according to the literature.29 2-Amino-4,6-dichloro-1,3,5triazine (0.37 g, 2.2 mmol) and 1-dodecanamine (0.50 g, 2.7 mmol) were mixed in 30 mL of water. Sodium bicarbonate (0.22 g, 2.6 mmol) was added, and the mixture was stirred at 25 °C for 5 h. The crude product was then extracted with chloroform; the organic layer was dried over Na2SO4, and the solvent was removed in vacuum. The crude product was purified by column chromatography (20:1 dichloromethane/methanol) to afford a white powder (0.41 g, 59%). ESI-MS: 314.2, 316.2 ([M + H]+), calcd 314.2, 316.2. 6-Chloro-N-octyl-1,3,5-triazine-2,4-diamine. This compound was synthesized by adopting a procedure similar to that used for 6-chloroN-dodecyl-1,3,5-triazine-2,4-diamine. White powder; yield: 22%. ESIMS: 258.2, 260.1 ([M + H]+), calcd 258.2, 260.2. N-Dodecyl-N′-octyl-1,3,5-triazine-2,4,6-triamine (Mela-8-12). The synthesis was performed according to a standard procedure.5 6Chloro-N-dodecyl-1,3,5-triazine-2,4-diamine (0.21 g, 0.67 mmol) and 1-octylamine (0.22 mL, 1.3 mmol) were mixed in 10 mL of water. After stirring for 0.5 h, sodium hydroxide (0.032 g, 0.80 mmol) in 10 mL of water was added, and the mixture was refluxed at 110 °C for 20 h. After cooling, the crude product was extracted with chloroform, the organic layer was dried over Na2SO4, and the solvent was removed in vacuum. The crude product was purified by column chromatography (20:1 dichloromethane/methanol) to afford a white powder (0.26 g, 95%). 1H NMR (400 MHz, CDCl3, ppm): δ 4.95−4.50 (br, 4H), 3.43−3.22 (m, 4H), 1.59−1.46 (m, 4H), 1.39−1.18 (m, 28H), 0.88 (t, 6.8 Hz, 6H). ESI-MS: 407.4 ([M + H]+), calcd 407.4.
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RESULTS AND DISCUSSION The self-assembly of Mela-m-n·HCl, having two asymmetric alkyl chains, forms 2D rectangular assemblies in dichloro10960
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Figure 2. (a) The lamellar structures of Mela-m-n·HCl assemblies confirmed by WAXD measurements. (b) The linear relationship between lamellar d-spacings of Mela-m-n·HCl assemblies and average carbon number [(m + n)/2] of alkyl chains, which has almost the same slope as for Mela-n·HCl.
methane. As indicated by TEM in Figure 1, in the case of Mela8-12·HCl, the 2D rectangular assemblies are microsheets, and the lateral size can cover a range of a few micrometers, and the thickness of the microsheets is about several hundred nanometers. For Mela-4-12·HCl and Mela-4-8·HCl, they also form similar 2D microsheets with a little difference in aspect ratio, which may be caused by the difficulty in accurately controlling the molar ratio of melamine derivatives and HCl. WAXD was used to get more structure information of the above-mentioned 2D assemblies. As demonstrated by WAXD in Figure 2a, a series of periodic diffraction peaks is clearly observed, indicating highly ordered lamellar structures in the 2D assemblies formed by Mela-8−12·HCl. The d-spacing of the lamellar structures is calculated to be 3.15 nm. Similarly, Mela4-12·HCl and Mela-4-8·HCl formed 2D assemblies of ordered lamellar structures as well, whose d-spacings are 2.63 and 2.21 nm, respectively. The self-assembling behavior and structure of Mela-n·HCl have been fully explored in our previous work.17 Mela-n·HCl self-assembled in dichloromethane to form 2D microsheets through counterion bridged multiple hydrogen bonds as well as electrostatic interactions and π−π stacking. WAXD results indicated the existence of lamellar structures in Mela-n·HCl assemblies, and the lamellar d-spacings increase linearly with the carbon number of the alkyl chain. As shown in Figure 2b, interestingly, the lamellar d-spacings of Mela-m-n·HCl also change linearly with the average carbon number [(m + n)/2] of two alkyl chains, in spite of their length difference. Moreover, the slope of Mela-m-n·HCl is almost the same as that of Mela-n· HCl, and the d-spacing in the lamellar structure of Mela-m-n· HCl can be equivalent to the one of Mela-(m + n)/2·HCl. From these results, we can deduce that the molecular arrangements of Mela-m-n·HCl within their 2D assemblies should be the same as Mela-n·HCl, whereas the alkyl chains of the former should probably adopt an “interdigitated” structure. The plausible molecular arrangements of Mela-8-12·HCl are shown in Figure 3, which is also supported by WAXD in Figure 2a. Therefore, the melaminium moieties can serve as versatile structure-directing groups for the fabrication of 2D assemblies, regardless of the variation of the appending alkyl chains. Although Mela-m-n·HCl and Mela-n·HCl have the same lamellar molecular arrangements, the different symmetry of alkyl chains leads to different growth mechanisms of obtained
Figure 3. The plausible molecular arrangement of Mela-8-12·HCl in the 2D assemblies. The driving forces are counterion bridging and π−π stacking, while the alkyl chains adopt interdigitated structures.
2D assemblies. The growth mechanism of Mela-m-n·HCl is revealed to be spiral growth by AFM. In a typical preparation, the protonated melamine derivatives were obtained by mixing an equal molar amount of HCl with Mela-m-n in dichloromethane. After sonication for 2 min and standing for various times, the solution was dripped and dried on glass substrate for AFM measurements. Figure 4a−d shows the AFM images of Mela-8-12·HCl assemblies at each stage. From these images, we can see the enlargement of lateral size and the increase of layer number as time progressed during the self-assembling process. It is noteworthy that the spiral growth patterns can be observed clearly in each image. Figure 4e,f shows typical spiral patterns 10961
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Figure 5. The AFM images of Mela-12·HCl assemblies after sonication for 2 min (a) and standing for 1 h (b).
For Mela-8·HCl and Mela-4·HCl, similar flat-surface structures can also be observed by AFM (Figure S2, Supporting Information), instead of spiral patterns. Therefore, the growth mechanism of Mela-n·HCl is determined to be layer growth. The difference of the growth mechanisms for these two types of dialkylated melaminium derivatives may result from their difference in molecular symmetry. Mela-m-n molecules have two alkyl chains with different length, which adopt “interdigitated” structures in the Mela-m-n·HCl assemblies, as shown in Figure 3. During the self-assembling process, although an “interdigitated” structure of the alkyl chains is preferred, there is a possibility that the alkyl chains are met in a “noninterdigitation” mode. It means that in Mela-m-n molecules, the short and long alkyl chain may not fully “interdigitated”. This kind of mismatch may cause the formation of screw dislocations, leading to spiral growth. However, for Mela-n· HCl, the identical alkyl chains have no chance to mismatch and form dislocations; therefore, a layer growth mechanism is adopted.
Figure 4. AFM images of Mela-8-12·HCl assemblies after sonication for 2 min (a) and standing for 1 h (b), 4 h (c), and 7 h (d). The spiral growth patterns of Mela-8-12·HCl assemblies after standing for 1 h with a layer thickness of 2.8 nm were observed by AFM: (e) peak force error and (f) height. The peak force error image, which can differentiate the boundaries in plane clearly, is provided here to help to observe the spiral pattern. Meanwhile, the height image and section analysis can quantitatively demonstrate the spiral growth.
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CONCLUSIONS In conclusion, we have prepared 2D organic microsheets from dialkylated melaminium derivatives with identical and different alkyl chains. The molecular arrangements of Mela-m-n·HCl are demonstrated to be the same as for Mela-n·HCl. More importantly, the growth mechanism of Mela-n·HCl is revealed by AFM to be layer growth, while Mela-m-n·HCl adopts a spiral growth mechanism because of its asymmetric alkyl chains. This work not only reveals two types of growth mechanisms of 2D organic assemblies based on the different structures of building blocks but also supplies inspiration for the fabrication of ultrathin 2D assemblies and spiral structures from small organic molecules.
with quadrilateral shape after standing for 1 h, and the layer thickness is measured to be 2.8 nm by cross-sectional analysis of height, which is consistent with the d-spacing indicated by WAXD in Figure 2a. In addition, it holds true for Mela-4-12· HCl and Mela-4-8·HCl assemblies that similar spiral growth patterns are observed by AFM (Figure S1, Supporting Information). Therefore, the growth mechanism of Mela-m-n· HCl assemblies is determined to be spiral growth. Contrary to Mela-m-n·HCl, the growth mechanism of Melan·HCl assemblies is revealed to be layer growth. Figure 5 shows the AFM images of Mela-12·HCl assemblies after sonication for 2 min and standing for 1 h. We can clearly see the increase of the thickness and the enlargement of the lateral size, but no spiral growth patterns can be observed. When increasing the incubation time from 2 min to 1 h, the height of the 2D assemblies increases from less than 10 nm to more than 30 nm. The thickness of each layer within the 2D assemblies can be measured at the edge of the flat sheet and is determined to be 3.2 nm, which is close to its d-spacing measured by WAXD. It is worthy to note that ultrathin or even single-layer-thick 2D assemblies may be obtained by controlling the incubating time.
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ASSOCIATED CONTENT
S Supporting Information *
Spiral growth patterns of Mela-4-12·HCl and Mela-4-8·HCl, layer growth patterns of Mela-8·HCl and Mela-4·HCl, and 1H NMR spectra of Mela-m-n. This material is available free of charge via the Internet at http://pubs.acs.org. 10962
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AUTHOR INFORMATION
Corresponding Author
*Fax: (+86) 10-6277-1149. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2013CB834502), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21121004), and the NSFC− DFG joint grant (TRR61).
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