Langmuir 2007, 23, 12791-12794
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Optical Switching of Self-Assembly and Disassembly of Noncovalently Connected Amphiphiles Jiong Zou,† Fenggang Tao,† and Ming Jiang*,‡ Department of Chemistry and Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, and Department of Macromolecular Science, Fudan UniVersity, Shanghai, 200433, China ReceiVed September 11, 2007. In Final Form: October 29, 2007 A hydrophobic compound, which we name 3C18-Azo, containing an azo head and three 18 C alkyl chains has the capacity to form an amphiphile by capping it with a cyclodextrin (CD) by inclusion complexation. The amphiphilic compound self-assembles into vesicles in water. Optical switching of the assembly and disassembly is realized by alternating visible and UV irradiation, which causes the isomerization of the azo groups, thus affecting their complexation with the CDs.
Now, most biological cell membranes have the structure of bilayer vesicles, which can be mimicked by the self-assembly of synthetic amphiphiles.1 In addition to the “classical building blocks” for vesicles such as surfactants and phospholipids, amphiphilic block copolymers have attracted great attention because their diverse structures could meet different requirements in fabricating vesicles.2 In recent years, amphiphilic dendrimers,3a,b calixarenes,3c-e cyclodextrins,3f,g and hyperbranch polymers3h,i have joined the family of building blocks for making the vesicles. Very recently, Tew et al.4 reported that discotic macrocycles based on ortho-phenlene ethynylene are able to self-assemble into vesicles, which clearly demonstrates the importance of the molecular architecture of small molecules in self-assembly. It is interesting that in all of the above building blocks the hydrophobic and hydrophilic parts are connected by covalent bonds. In other words, the two opposite parts cannot be separated without destroying the bonds. In this work, we report a new type of amphiphile in which the hydrophobic and hydrophilic parts are connected by supramolecular interactions, specifically, by molecular recognition between azobenzene groups and cyclodextrins.5 Because this interaction largely depends on the isomers * Corresponding author. E-mail:
[email protected]. Fax: 86-2165640293. Tel: 86-21-65643919. † Department of Chemistry. ‡ Key Laboratory of Molecular Engineering of Polymers, Ministry of Education and Department of Macromolecular Science. (1) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (2) (a) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (b) Lim, S. P.; Eisenberg, A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 923. (c) Terreau, O.; Soo, P. L.; Duxin, N.; Eisenberg, A. In Macromolecular Self-Assembly; Jiang, M., Eisenberg, A., Liu, G., Zhang, X., Eds.; Science Press: Beijing, 2006; Chapter 1, in Chinese. (3) (a) Rico-Lattes, I.; Blanzat, M.; Franceschi-Messant, S.; Perez, E.; Lattes, A. C. R. Chim. 2005, 8, 807. (b) Yang, M.; Wang, W.; Yuan, F.; Zhang, Z.; Li, J.; Liang, F.; He, B.; Minch, B.; Wegner, G. J. Am. Chem. Soc. 2005, 127, 15107. (c) Lee, M.; Lee, S. J.; Jiang, L. H. J. Am. Chem. Soc. 2004, 126, 12724. (d) Houmadi, S.; Coquiere, D.; Legrand, L.; Fare, M. C.; Goldmann, M.; Reinaud, O.; Remita, S. Langmuir 2007, 23, 4849. (e) Kellermann, M.; Bauer, W.; Hirsch, A.; Schade, B.; Ludwig, K.; Bottcher, C. Angew. Chem, Int. Ed. 2004, 43, 2959. (f) Donohue, R.; Mazzaglia, A.; Ravoo, B. J.; Darcy, R. Chem. Commun. 2002, 2864. (g) Falvey, P.; Lim, C. W.; Darcy, R.; Ravoo, B. J. Chem.sEur. J. 2005, 11, 1171. (h) Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2004, 43, 4896. (i) Zhou, Y.; Yan, D. Angew. Chem., Int. Ed. 2005, 44, 3223. (4) Seo, S. H.; Chang, J. Y.; Tew, G. N. Angew. Chem., Int. Ed. 2006, 45, 7526. (5) (a) Nepogodiev, S. A.; Stoddart, J. F. Chem. ReV. 1998, 98, 1959. (b) Tomatsu, I.; Hashidzume, A.; Harada, A. J. Am. Chem. Soc. 2006, 128, 2226. (c) Ipsita, A.; Banerjee; Yu, L. T.; Matsui, H. J. Am. Chem. Soc. 2003, 125, 9542. (d) Harada, A. Acc. Chem. Res. 2001, 34, 456. (e) Murakami, H.; Kawabuchi, A.; Matsumoto, R.; Ido, T.; Nakashima, N. J. Am. Chem. Soc. 2005, 127, 15891. (f) Liu, Y.; Zhao, Y. L.; Zhang, H. Y.; Fan, Z.; Wen, G. D.; Ding, F. J. Phys. Chem. B 2004, 108, 8836. (g) Wang, Y. P.; Ma, N.; Wang, Z. Q.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823.
Figure 1. DLS size distributions of sample A. (a) Self-assembled vesicles. (b) UV-irradiated sample a forming irregular aggregates. (c) Visible-light-irradiated sample b forming vesicles.
of the azobenzene groups, which in turn are governed by light irradiation, the formation of such noncovalently connected amphiphiles and their further self-assembly in water are lightswitchable. This work is a substantial extension of our work on noncovalently connected polymeric micelles (NCCMs) in which the core and shell are joined by hydrogen bonding or inclusion complexation.6 However, in our previous work on NCCMs, usually only spherical micelles rather than vesicles were obtained. Both R- and β-cyclodextrins (CDs) and their derivatives can form inclusion complexes with azo compounds in the trans form via host-guest recognition. However, the UV irradiation-induced isomerization of the azo compounds from the trans form to the cis form would exclude the azo compounds from the CD cavities.5 This photoswitching behavior has been used in designing advanced materials,5b,c “molecular machines”,5d-f and causing the vesicles to disassemble.5g It seems that such a photoresponsive procedure in constructing micelles and vesicles has not been explored so far. In this work, we designed and synthesized azo-containing compound 3C18-Azo, (E)-4-(phenyldiazenyl)phenyl 3,4,5-tris(6) (a) Chen, D. Y.; Jiang, M. Acc. Chem. Res. 2005, 38, 494. (b) Jiang, M. In Macromolecular Self-Assembly; Jiang, M., Eisenberg, A., Liu, G., Zhang, X., Eds.; Science Press: Beijing, 2006; Chapter 4, in Chinese. (c) Duan, H. W.; Chen, D. Y.; Jiang, M.; Gan, W. J.; Li, S. J.; Wang, M.; Gong, J. J. Am. Chem Soc. 2001, 123, 12097. (d) Zhang, Y. W.; Jiang, M.; Zhao, J. X.; Ren, X. W.; Chen, D. Y.; Zhang, G. Z. AdV. Funct. Mater. 2004, 14, 695.
10.1021/la702815h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/16/2007
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Figure 2. TEM micrographs of sample A. (a) Self-assembled vesicles. (b) UV-irradiated (30 min) sample a forming irregular aggregates. (c) Visible-light-irradiated (30 min) sample b forming vesicles. Scheme 1. Synthesis of Hydrophobic (E)-4-(Phenyldiazenyl)phenyl 3,4,5-tris(octadecyloxy)benzoate
Scheme 2. Schematic Illustration of the Vesicular Structure
(octadecyloxy)benzoate, via the procedure shown in Scheme 1. This hydrophobic compound has an azo head and three tails of 18-carbon alkyl chains. The four-step synthesis gave a total yield of 38% of 3C18-Azo. The structure was confirmed by 1H NMR, 13C NMR, FTIR, and MALDI-TOF measurements (Supporting Information). The azo head in 3C18-Azo, as expected, shows perfect reversible isomerization in THF solutions by repeated alternating irradiation of UV (365 nm) and visible light (434 nm). This process was monitored by UV/vis spectroscopy, and the results are shown in Supporting Information (Figure S1a,b). 3C18-Azo can be easily dissolved in weakly polar organic solvents such as chloroform and THF, whereas the CDs can be dissolved in water and polar organic solvent such as DMF. We screened most of the common organic solvents and did not find
one that dissolves both. We used the following procedure to convert the hydrophobic 3C18-Azo into an amphiphile by capping it with a CD and then realizing its further self-assembly in water. Typically, 0.1 mL of 3C18-Azo in THF solution at low concentration (0.1 mg/mL) was slowly injected into 2 mL of R-CD in water (1 mg/mL) solution over 5 min at 10 °C. Bluish opalescence appeared immediately, which indicated the formation of assembled particles (samples A). β-CD was also used as the host, and the same result was obtained (sample B). Stable particles could be prepared by changing both the concentrations of 3C18Azo in THF and R-CD (or β-CD) in water over quite broad ranges provided the host was present in large excess. Dynamic light scattering (DLS) was employed to measure the size and size distributions of the particles (curve a, Figure 1). Sample A,
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Figure 3. UV/vis spectra of A for increasing durations of UV irradiation (λ ) 365 nm) of 0, 5, 10, 15, 20, 30, 40, 50, 60, 80,100, 120, 140, 200, 300, and 600 s (shown from the top down). The visible region is shown enlarged in the inset for UV irradiation times of 5, 30, 50, 300, and 600 s (from the bottom up).
Figure 4. From the bottom up, UV/vis spectra of UV-irradiated sample A for increasing durations of visible-light irradiation (λ ) 434 nm) of 5, 10, 15, 20, 25, 35, 45, 55, 65, 85, 105, 125, 145, 180, 240, 300, 420, and 720 s.
which is composed of R-CD and 3C18-Azo, has an average diameter of 241 nm and a polydispersity index (PDI) of 0.24 estimated from the relative line width, µ2/〈Γ〉2. Five samples (A1-A5) were prepared under the same conditions as for A, and DLS measurements were made. The results show fine reproducibility of the size and PDI (Table S1). The morphology of the particles was examined by transmission electron microscopy (TEM) observations. As an example, Figure 2a shows that the particles are spherical with diameters ranging from 150 to 200 nm, which is slightly smaller than that found by DLS. This seems reasonable because TEM and DLS showed solid and swollen vesicles, respectively. More importantly, all of the particles show a strong contrast between the center and the periphery, which is typical of vesicular structure. This is obviously caused by the self-assembly of the amphiphilic supramolecules of 3C18-Azo capped with R-CD. Although the vesicles have a large size polydispersity, they have a uniform thickness of around 10 nm. Because Chem3D gave a simple estimate of 5 nm for the molecular size of 3C18-Azo, the bilayer model of the membrene shown in Scheme 2 seems acceptable. In this model, the hydrophobic “tails” are directed inward, and the hydrophilic CD heads are facing the water, thereby enclosing an aqueous interior (Scheme 2). The vesicles formed by the supramolecules of 3C18-Azo/R-CD are quite stable, with no change in size observed by DLS over a period of a few months.
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This is different from the vesicles3g prepared by amphiphilic cyclodextrins with chemically connected hydrophobic tails, which tended to precipitate on standing. We took full advantage of the noncovalent connection between 3C18-Azo and CDs to realize reversible self-assembly and disassembly. As shown in Figure 3, UV irradiation at 365 nm of the vesicles of sample A composed of 3C18-Azo and R-CD at room temperature caused a substantial change in the UV/vis spectra. The absorption band at around 303 nm decreased gradually as the irradiation went on. At the same time, the band at around 430 nm increased slightly (inset, Figure 3). Because the absorption bands at 303 and 430 nm are ascribed to π-π* of the trans form and n-π* of the cis form of azo, respectively, this spectral variation clearly shows the isomerization of the azo groups from the trans form to the cis form, and sample B showed a similar variation in the UV/vis spectra (Figure S2a). However, this variation caused by the isomerization of azo groups in the presence of R-CD and β-CD is much less pronounced than that observed for 3C18-Azo without CDs (Figure S1). This difference can be attributed to the fact that the absorptions observed in the solutions of 3C18-Azo and R-CD or β-CD contain contributions of both the azo groups and CD molecules. DLS measurements were carried out just after A had been irradiated by UV for 10 min. As shown in Figure 1 (curve b), the size distribution of sample A substantially changed, leading to a bimodal curve; accordingly, the polydispersity index increased from 0.23 to 0.49. A remarkable change in the morphology of the vesicles caused by UV irradiation was clearly brought out by TEM observations. As shown in Figure 2b, after 30 min of UV irradaton, the vesicular structure completely disappeared, and particles with a large size polydispersity formed. Some of the particles had irregular shapes, and the size reached around 400 nm. This morphological change is obviously caused by the isomerization of the azo groups. The guest molecules in the cis form left the R-CD cavities and caused 3C18-Azo to become hydrophobic again; therefore, the vesicles disassembled. Because guest molecules with long alkyl chains are strongly hydrophobic, they aggregated. Because the concentration of 3C18-Azo was very low, the formed aggregate particles did not grow into a macroscopic precipitate. In fact, in a control experiment, a dilute solution of 3C18-Azo in THF was added to pure water, leading to irregular particles with sizes as large as 400-500 nm (Figure S3), very similar to that shown in Figure 2b. In other words, after UV irradiation of A, the free R-CD molecules no longer affect the aggregation of 3C18-Azo. Afterward, UV-irradiated samples A and B were subjected to visible-light irradiation at 434 nm at room temperature. Figure 4 shows the results of monitoring A by UV/vis spectroscopy: here the absorption band around 303 nm increased remarkably, and the band around 430 nm decreased slightly. These results indicate the reverse isomerization of the cis form to trans. For sample B, similar variations in the spectra with visible irradiation were observed (Figure S2b). The TEM observations clearly show that visible-light irradiation results in the irregular aggregates reverting to the vesicles. As shown in Figure 2c, every sphere shows a clear contrast between a dark periphery and a gray central part, typical of hollow spheres. The thickness of the vesicular membrane is rather uniform at about 10 nm. This morphological change caused by visible-light irradiation was supported by DLS measurements. As shown in Figure 1, after visible-light irradiation, the bimodal distribution of the irregular aggregates (curve b) changed back, showing a distribution (curve c) very similar to that of the original vesicles (curve a). The recovered A vesicles had an average
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diameter of 260 nm and a PDI of 0.29, similar to the original values of 241 nm and 0.23, respectively. Clearly, we have realized an optical switching cycle between the vesicles and irregular particles. This reversibility of the morphological transition can be attributed to the reversible photoisomerization of the azo group in 3C18-Azo and the different inclusion complexation abilities between the cis and trans forms with cyclodextrins. For sample B composed of 3C18-Azo and β-CD, we observed the same morphological changes (i.e., UV transformed the vesicles to irregular particles, and then visible-light irradiation changed them back). The results are shown in Figure S4. Although the azo/ R-CD complex has an obviously greater stability constant5g than the azo/β-CD complex7 (2.8 × 104 M-1 vs 1.7 × 103 M-1), they show similar assembly and disassembly behavior with 3C18Azo. Here we report two levels of supramolecular assembly: hostguest recognition between CDs and azobenzene-containing (7) Lahav, M.; Ranjit, K. T.; Katz, E.; Willner, I. Chem. Commun. 1997, 259.
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hydrophobic molecules leading to noncovalently connected amphiphiles and subsequent organization of the amphiphiles to bilayer vesicles. The vesicles can undergo disassembly and assembly reversibly by light irradiation because of the photoisomerization of the azo group. Acknowledgment. This work is supported by the National Natural Science Foundation of China (NNSFC no. 50333010) Supporting Information Available: Synthesis of hydrophobic azobenzene derivative 4, as confirmed by 1H NMR, 13C NMR, FTIR, and MALDI-TOF spectroscopy. Reversible isomerization of the azo head in 3C18-Azo in THF solutions by repeated alternating irradiation of UV and visible light. Dh and PDI of sample A and of those prepared under the same conditions. TEM micrographs of (1) the aggregates formed by injecting a THF solution of 3C18-Azo into water and (2) sample B. This material is available free of charge via the Internet at http://pubs.acs.org. LA702815H
