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Cholesterol-Appended Aromatic Imine Organogelators: A Case Study of Gelation-Driven Component Selection† Gui-Tao Wang, Jian-Bin Lin, Xi-Kui Jiang, and Zhan-Ting Li* State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China Received December 19, 2008. Revised Manuscript Received February 4, 2009 This letter describes a novel approach for developing organogelators through the formation of reversible imine bonds from two molecular components and the enriching behavior of the gelating imines. Cholesterol-appended aniline 1 and 4-substituted benzaldehydes 2a-d did not gelate any solvents. Their condensation products, imines 3a-d, however, could gelate alcohols because of the enhanced stacking interaction of the imine unit. For a further component selectivity test, the reactions of the mixture of 1, 2b-d, and cholesterol-free aniline 7 (1:1:1) in different solvents were performed. The resulting imines were reduced to the corresponding amines and analyzed with 1H NMR. It was revealed that, for the reactions resulting in no formation of the gel phase, imines 8a-c formed from 2b-d and 7 were obtained as the major product (64-76%) and all of the reactions that led to the formation of the gel phase gave rise to 3b-d as the major product (55-61%).
Introduction Self-assembly provides new possibilities in both biological and materials science because it enables the rapid formation of complicated, functionalized architectures.1 One family of self-assembled architectures is organogels in which the organic liquid is immobilized by the continuous 3D entangled network of fibers formed by gelators.2,3 In the past decade, the development of new organogels has received considerable † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. E-mail:
[email protected]. Phone: 008621-54925122. Fax: 0086-21-64166128.
(1) aLehn, J.-M. Supramolecular Chemistry: Concepts and PerspectivesWiley-VCH: Weinheim, Germany, 1995.bSteed, J. W.; Atwood, J. L. Supramolecular Chemistry; John Wiley: New York, 2000.cPelesko, J. A. Self Assembly: The Science of Things That Put Themselves Together; Chapman Hall/CRC: Boca Raton, FL, 2007. (2) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3160. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (c) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (d) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (e) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (f) Sada, K.; Takeuchi, M.; Fujita, N.; Numata, M.; Shinkai, S. Chem. Soc. Rev. 2007, 36, 415–435. (g) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656. (h) Yang, Z.; Xu, B. J. Mater. Chem. 2007, 17, 2385–2393. (i) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715. (j) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37, 109–122. (3) Weiss, R. G.; Terech, P., Eds.; Molecular Gels: Materials with SelfAssembled Fibrillar Networks; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2005. (4) For representative examples, see (a) Jung, H; Kobayashi, H; Masuda, M; Shimizu, T; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785–8789. (b) De Loos, M.; Van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613–616. (c) Maitra, U.; Mukhopadhyay, S.; Sarkar, A.; Rao, P.; Indi, S. S. Angew. Chem., Int. Ed. 2001, 40, 2281–2283. (d) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550–6551. (e) Zhan, C.; Wang, J.; Yuan, J.; Gong, H.; Liu, Y.; Liu, M. Langmuir 2003, 19, 9440–9445. (f) Chow, H.-F.; Zhang, J. Chem.-Eur. J. 2005, 11, 5817–5831. (g) Li, Y.; Wang, T.; Liu, M. Soft Matter 2007, 3, 1312–1317. (h) Wang, C.; Zhang, D.; Xiang, J.; Zhu, D. Langmuir 2007, 23, 9195–9200. (i) Cardolaccia, T.; Li, Y.; Schanze, K. S. J. Am. Chem. :: Soc. 2008, 130, 2535–2545. (j) Li, X.-Q.; Zhang, X.; Ghosh, S.; Wurthner, F. Chem. -Eur. J. 2008, 14, 8074–8078. (k) Yang, X.; Lu, R.; Xue, P.; Li, B.; Xu, D.; Xu, T.; Zhao, Y. Langmuir 2008, 24, 13730–13735. (l) Cai, W.; Wang G.-T.; Xu, Y.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2008, 130 6936–6937.
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attention, and advances in this field have opened access to a range of functional nanostructured materials that have potential applications in separation technologies, medicine, biomaterials, electronics, and photonics and sensors.4 In most cases, organogels are generated on the basis of single-molecular gelators. Many supramolecular organogels have also been developed from two components that are organized through a self-complementary network of noncovalent or electrostatic interactions.5-7 In 2005, Lehn et al. utilized dynamic combinatorial chemistry (DCC)8 to develop a class of guanosine hydrazide-based supramolecular hydrogels.9 (5) (a) Hirst, A. R.; Smith, D. K. Chem.-Eur. J. 2005, 11, 5496–5508. (b) Smith, D. K. Adv. Mater. 2006, 18, 2773–2778. (c) Diaz, D. D.; Rajagopal, K.; Strable, E.; Schneider, J.; Finn, M. G. J. Am. Chem. Soc. 2006, 128, 6056– 6057. (d) Qiu, Y.; Chen, P.; Guo, P.; Li, Y.; Liu, M. Adv. Mater. 2008, 20, 2908–2913. (e) Liu, Q.; Wang, Y.; Li, W.; Wu, L. Langmuir 2007, 23, 8217– 8223. (f) Tu, T.; Assenmacher, W.; Peterlik, H.; Weisbarth, R.; Nieger, M.; Doetz, K. H.. Angew. Chem., Int. Ed. 2007, 46, 6368–6371. (g) Chen, K.; Tang, L.; Xia, Y.; Wang, Y. Langmuir 2008, 24, 13838–13841. (h) Seo, J.; Chung, J. W.; Jo, E.-H.; Park, S. Y. Chem. Commun. 2008, 2794–2796. (i) Zhang, S.; Yang, S.; Lan, J.; Yang, S.; You, J. Chem. Commun. 2008, 6170– 6172. (j) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5746–5749. (k) George, M.; Funkhouser, G. P.; Weiss, R. G. Langmuir 2008, 24, 3537–3544. (6) (a) Maeda, H.; Haketa, Y.; Nakanishi, T. J. Am. Chem. Soc. 2007, 129, 13661–13674. (b) Zhou, Y.; Xu, M.; Yi, T.; Xiao, S.; Zhou, Z.; Li, F.; Huang, C. Langmuir 2007, 23, 202–208. (c) Shu, T.; Wu, J.; Lu, M.; Chen, L.; Yi, T.; Li, F.; Huang, C. J. Mater. Chem. 2008, 18, 886–893. (d) Seo, J.; Chung, J. W.; Jo, E.-H.; Park, S. Y. Chem. Commun. 2008, 2794–2796. (e) Zhao, Y.-L.; Aprahamian, I.; Trabolsi, A.; Erina, N.; Stoddart, J. F. J. Am. Chem. Soc. 2008, 130, 6348–6350. (f) Cai, W.; Wang, G.-T.; Du, P.; Wang, R.-X.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2008, 130, 13450–13459. (7) (a) Ayabe, M.; Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744–2747. (b) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Chem. Mater. 2006, 18, 3795–3800. (c) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715. (8) (a) Lehn, J.-M. Chem.-Eur. J. 1999, 5, 2455–2463. (b) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 899–952. (c) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.; Sanders, J. K. M.; Otto, S. Chem. Rev. 2006, 106, 3652–3711. (d) Lehn, J.-M. Chem. Soc. Rev. 2007, 36, 151–160. (e) Meyer, C. D.; Joiner, C. S.; Stoddart, J. F. Chem. Soc. Rev. 2007, 36, 1705–1723. (f) Ludlow, R. F.; Otto, S. Chem. Soc. Rev. 2008, 37, 101–108. (9) (a) Sreenivasachary, N.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5938–5943. (b) Buhler, E.; Sreenivasachary, N.; Candau, S.-J.; Lehn, J.-M. J. Am. Chem. Soc. 2007, 129, 10058–10059.
Published on Web 03/13/2009
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Letter
Because this approach was based on the formation of the reversible hydrazone bonds, both the structural and environmental factors that affected gelation could be readily investigated. As a result, a unique gelation-driven amplification of a gelating constituent from a multiconstituent system has been revealed.9,10 Such amplification or selectivity bears implications on practical applications in component separation or enrichment and therefore is worthy of further investigation. Cholesterol is one of the most versatile units utilized to design functional gelators for the immobilization of organic fluids.11 Since the first report of cholesteryl 4-(2-anthry1oxy)butyrate as a gelator,12 a large number of cholesterol-based gelators have been developed. Most typically, the cholesterol moiety is connected to an aromatic or other rigid unit by a flexible linker to produce the so-called ALS structures (i.e., molecules with aromatic (A), linking (L), and steroidal (S) groups).13 Our interest in dynamic covalent synthesis has prompted us to develop new gelators with DCC.14 By using the ALS motif, we have designed a simple system in which the self-assembly of the organogels can be realized through the formation of an imine from a cholesterol-appended aniline and discrete 4-substituted benzaldehydes.15 The reversible feature of the imine bond has led to the amplification of the gelating imine over another ungelating imine through gelation. This letter reports the detailed results.
Scheme 1
Results and Discussion Design and Synthesis. It was reported that cholesterolattached 1,2-diphenylethene16 or 1,2-diphenyldiazene17 derivatives gelate several organic solvents. It was envisioned that imine derivatives of the similar frameworks might also gelate organic solvents. Compounds 1 and 2ad, which were readily expected to form corresponding imines 3a-d, were chosen to test this possibility. The synthesis route for 1 is provided in Scheme 1. Thus, compound 4 was first coupled with 5 in the presence of EDCI in chloroform to afford 6 in 44% yield. This intermediate was then treated with trifluoroacetic acid in dichloromethane to give 1 quantitatively.
(10) For examples of dynamic chiral selection and amplification of organogels, see (a) De Jong, J. J. D.; Tiemersma-Wegman, T. D.; Van Esch, J. H.; Feringa, B. L. J. Am. Chem. Soc. 2005, 127, 13804–13805. (b) de Jong, J. J. D.; van Rijn, P.; Tiemersma-Wegeman, T. D.; Lucas, L. N.; Browne, W. R.; Kellogg, R. M.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Tetrahedron 2008, 64, 8324–8335. (11) (a) Abdallah, D. J.; Weiss, R. G. J. Braz. Chem. Soc. 2000, 11, 209– 218. (b) Mallia, V. A.; Tamaoki, N. Chem. Soc. Rev. 2004, 33, 76–84. (c) Jung, :: J. H.; Shinkai, S. Top. Curr. Chem. 2005, 248, 223–260. (d) Zinic, M.; Vogtle, F.; Fages, F. Top. Curr. Chem. 2005, 256, 39–76. (e) Llusar, M.; Sanchez, C. Chem. Mater. 2008, 20, 782–820. (12) Lin, Y.-C.; Weiss, R. G. Macromolecules 1987, 20, 414–417. (13) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489–497. (14) (a) Lin, J.-B.; Xu, X.-N.; Jiang, X.-K.; Li, Z.-T. J. Org. Chem. 2008, 73, 9403–9410. (b) Lin, J.-B.; Wu, J.; Jiang, X.-K.; Li, Z.-T. Chin. J. Chem. 2009, 27, 117–122. (15) An example of Schiff base-based organogels has been described, see: Xue, P.; Lu, R.; Chen, G.; Zhang, Y.; Nomoto, H.; Takafuji, M.; Ihara, H. Chem.-Eur. J. 2007, 13, 8231–8239. (16) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241–2245. (17) (a) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715–1718. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664–6676.
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Gelation Behaviors. The capacity of compound 1 to form organogels was first investigated in hydrocarbons and small alcohols. The results showed that 1 did not possess such a capacity. The compound was insoluble in hydrocarbons but soluble in hot alcohols. The SEM images of its samples, obtained by evaporating its solution in alcohols, showed only sheet crystals. Adding 1 equiv of p-substituted aldehydes 2a-d to the solution of 1 (0.1 M) in n-butanol caused all of the samples to change to the gel state at room temperature (Table 1). The lowest gelation concentrations for the 1:1 mixtures of 1 and 2b-d were estimated to be approximately 25, 11, and 14 mM, respectively. The concentrations of 3b-d in these mixture systems were evaluated by the 1H NMR method (vide infra) to be ca. 21, 11, and 12 nm, respectively. To investigate whether the unreacted 1 and 2b-d in the mixtures affected the gelating capacity of 3b-d, the lowest gelation concentrations of pure 3b-d were also determined, which were ca. 22, 11, and 13 mM, respectively. These values were identical or comparable, indicating that the unreacted 1 and 2b-d in the mixture systems did not promote the gelating capacity of 3b-d. The value of the mixture of 1 and 2a was not available because of DOI: 10.1021/la804188z
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Letter Table 1. Gelation Results of Organic Solvents by Mixtures of 1 and 2a-da
n-C4H9OH n-C5H11OH n-C8H17OH n-nonane decahydronaphthalene hexafluorobenzene nitrobenzene
1
1 + 2a
1 + 2b
1 + 2c
1 + 2d
P P P P P P S
G G G
G G G P P Gb S
G G G P P Gb S
G G G P P Gb S
P I
a [1] = [2] = 50 mM; G = gel, S = solution, I = insoluble, and P = precipitation. b Sonication in 50 °C for 30 s with cooling to room temperature.
Figure 2. AFM height images of the (a) 1 + 2a, (b) 1 + 2b, and (c) 1 + 2c gels and (d) AFM phase image of the 1 + 2c gel in n-BuOH (1:1, 10 mM). The white bars are 0.5 μm each.
Figure 1. SEM images of the n-butanol gels formed from the mixtures of (a) 1 + 2a, (b) 1 + 2b, (c) 1 + 2c, and (d) 1 + 2d (1:1, 50 mM) on mica upon evaporation. the instability of the imine product at the reduced concentration. SEM images of the samples on mica, after evaporation of the solvent, revealed the formation of long fibers on the order of millimeters (Figure 1a-d). The latter three gels were less transparent, which is consistent with their larger fiber thickness. No assembled structures were observed from the SEM images of 2a-d under the same conditions. Therefore, it is reasonable to propose that these fibrous structures were generated by the correspondent imine products (i.e., 3a-d). AFM images of the above n-butanol gels also showed fibrous structures of varying width and length, further supporting the formation of the assembled structures (Figure 2). The phase image of the sample obtained from 1 and 2c also exhibited helicity for some long fibrils (Figure 2d), reflecting the chiral feature of the cholesterol moiety. Under the same conditions, the mixture samples of 2a-d with cholesterol-free aniline 7 in n-butanol did not form the gel phase, indicating that the cholesterol moiety of 1 was indispensable in enhancing the gelating capacity of the aromatic imine unit. The gelation of other solvents by the above imine samples of 1 and 2a-d was also investigated. It was found that these samples could gelate n-pentanol, n-octanol, and hexafluorobenzene (for the systems of 2b-d) but not alkanes, nitrobenzene, or smaller alcohols (Table 1). 8416 DOI: 10.1021/la804188z
To gain deeper insight into the assembly behavior of the above imine derivatives, we further prepared and separated the pure samples of 3a-d from the corresponding reactions of 1 with 2a-d (Scheme 1). As expected, all four compounds gelated n-butanol. Their SEM images were also very similar to those obtained for the above mixture samples of 1 and 2a-d. For comparison, we also tried to prepare the cholesterol-free imine derivatives from the reactions of 2a-d and 7. Pure imine product was not obtained from the reaction of 2a and 7 possibly because of the instability of the product. However, 8a-c could be readily obtained from another three reactions. These three compounds were found to be incapable of gelating any organic solvents, once again indicating that it was the cooperative interaction of the cholesterol and imine units in 3a-d that led to the formation of the fibrils and consequently the gelation of the solvent, as shown in Figure 3. A similar assembly model has been proposed for other cholesterol-appended aromatic structures.10 Component Selection. The different self-assembly behavior of the above two series of imines made it possible to investigate their expression in the gel and solution phases in a mixture system. As a case study, we chose the simplest threeLangmuir 2009, 25(15), 8414–8418
Letter
component systems, which consisted of the same molar amounts of 1, 2b-d, and 7 to study the effect of gelation on the formation of the correspondent imine derivatives in the solution and gel phases. Because it is difficult to determine the amount of the imine derivatives in the gel state
Figure 3. Unidirectional self-assembly of the cholesterol-imine gelators that are driven by the cooperative cholesterol-cholesterol and aromatic-aromatic stacking interactions. The gray, green, and red segments represent the cholesterol, the amide, and alkoxyl-bearing benzenes, respectively. Table 2. Conditions and Results of the Component Selection Experiments entry
aldehyde
solvent
conc (mM)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
2b 2b 2b 2b 2b 2b 2c 2c 2c 2c 2c 2c 2d 2d 2d 2d 2d 2d
n-BuOH n-BuOH n-C5H11OH n-C8H17OH CHCl3 CH2Cl2 n-BuOH n-BuOH n-C5H11OH n-C8H17OH CHCl3 CH2Cl2 n-BuOH n-BuOH n-C5H11OH n-C8H17OH CHCl3 CH2Cl2
50 3 50 50 50 50 50 3 50 50 50 50 50 3 50 50 50 50
state G S pG pG S S G S G G S S P S P P S S
fraction (%)a 61 26 43 50 36 32 56 29 55 59 30 29 26 28 24 25 31 24
a [9]/([9] + [10]; G = gel,pG means that the solvent was partially gelated, S = solution, and P = precipitate.
directly, we chose to reduce the imines with excess sodium borohydride (10 equiv) for the correspondent amines (i.e., 9a-c and 10a-c) and then estimated their relative percentage yields on the basis of the integrative strength of the signals of the benzyl methylene hydrogen atoms of the mixture samples by 1H NMR spectroscopy. The relative percentages of amines 9a-c and 10a-c in the same system should reflect the relative amount of the two imines in the gels because the reduction reactions have been revealed to occur nearly quantitatively when excess sodium borohydride was used. For the quantitative measurement, we also obtained pure samples of 9a-c and 10a-c. All of these amines were found to be unable to gelate any organic solvent. The component selectivity experiments were carried out systematically in n-butanol, n-pentanol, n-octanol, chloroform, and dichloromethane for the three-component systems of 1, 2b-d, and 7. The results are listed in Table 2, and the representative 1H NMR spectra are provided in Figure 4. It was already established that both 3 and 8 could not gelate chloroform or dichloromethane. Therefore, the reactions that generated these imines in these two solvents did not cause the new gel phase. It can be found that, for the reactions carried out in these solvents, amines 9a-c were all generated as the minor product, with a relative percentage of 24-36% (Table 2, entries 5, 6, 11, 12, 17, and 18; Figure 4). Considering the possible experimental errors, these values are quite comparable, reasonably reflecting the fact that the electron-rich butoxyl group of 7 was stronger than the amide group of 1 in promoting the condensation reaction for forming the corresponding imines. In contrast, from the reaction systems of 2b and 2c in alcohols that led to the formation of the gel phase, 9a and 9b could be generated as the major products, with relative percentages of 55-61% (Table 2, entries 1, 7, 9, and 10). When these reactions were performed at the reduced concentration that could not lead to the formation of the gel phase, their relative percentages were reduced (Table 2, entries 2 and 8), which were close to those obtained for the reactions in chloroform or dichloromethane. The observations clearly showed that the cholesterol-appended imines could be enriched in the gel phase possibly because of their large stacking tendency in the gel phase. It is reasonable to propose that, in the early stage, the condensation reactions were kinetically controlled and imines 10 formed more favorably. Therefore, the above enrichment of 9a and 9b suggested that partial 10a and 10b
Figure 4. Partial 1H NMR spectra of the samples, obtained from the reduction of the three-component (1:1:1) system, in CDCl3 {(a) 1 + 2b + 7, (b) 1 + 2c + 7, and (c) 1 + 2d + 7}, highlighting the gelation-caused enrichment of the cholesterol-appended imines (b, signal of the benzyl methylene hydrogen atoms of 9a-c; O, signal of the benzyl methylene hydrogen atoms of 10a-c). Langmuir 2009, 25(15), 8414–8418
DOI: 10.1021/la804188z
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were converted to 9a and 9b, respectively, as a result of the reversible feature of the imine bond through the gelation process. The reaction systems of 1, 2b, and 7 in n-pentanol and noctanol led to partial gelation of the media (Table 2, entries 3 and 4). As a result, the relative percentages of 9a became lower than that observed in n-butanol but still were notably higher than that observed in chloroform and dichloromethane. The reaction systems of 1, 2d, and 7 in all of the alcohols did not result in the gelation of the media (Table 2, entries 13, 15, and 16). Instead, imine 3d precipitated during the reactions. The result was different from that observed for the two-component system of 1 and 2d, which gelated the solvent through the formation of 3d. In agreement with this result, the relative percentages of 9c formed from these threecomponent reactions were very similar to those observed from the identical reactions performed in chloroform or dichloromethane, once again reflecting the intrinsic reactivity of 1 and 7 in the solution phase. This result also further indicated that the above gelation-caused enrichment of imines 3a-c was driven by the increase in their stability in the gel state and could not be simply attributed to the phase separation.
Conclusions In this study, we describe a new approach to developing organogels through the formation of a reversible covalent bond (here, an imine bond). The approach opens up the
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possibility of generating the gel phases by simply combining two structurally simpler components. The approach may also be used to develop hydrogels if the hydrophilicity and hydrophobicity of rationally designed molecular components can be suitably balanced. A further application of this approach is the quick establishment of new ideal molecular frameworks and their appended groups in developing gelators for a specific liquid by simultaneously screening different molecular components of the similar structures. As the next step, we will design new backbones to systematically explore the enriching behavior of the gelating product from other competing products. For this purpose, we may use other reversible covalent bonds. In particularly, we will focus on discrete chiral components that condense to generate diastereomeric products. We hope that progress in this direction may lead to the development of new separation methods and techniques. Acknowledgment. We thank the National Science Foundation of China (nos. 20732007, 20621062, 20672137, and 20872167), the National Basic Research Program (2007CB808001), and the Chinese Academy of Sciences (KJCX2-YW-H13) for financial support. Supporting Information Available: Experimental details and characterization, additional SEM images, and 1H NMR spectra of pure samples and mixtures. This material is available free of charge via the Internet at http://pubs. acs.org.
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