pubs.acs.org/Langmuir © 2009 American Chemical Society
Azobenzene-Based Light-Responsive Hydrogel System† Yan-Li Zhao and J. Fraser Stoddart* Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095, and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 Received December 30, 2008. Revised Manuscript Received February 13, 2009 A deoxycholic acid-modified β-cyclodextrin derivative (2) and an azobenzene-branched poly(acrylic acid) copolymer (3) were prepared, and the association and dissociation of 2 with the trans/cis-azobenzene units in 3 were characterized by UV/vis spectroscopy, induced circular dichroism, and 1H NMR spectroscopy. The experimental results indicate that the trans-azobenzene units are bound strongly within the cavities of 2 whereas the cis-azobenzene is not bound at all. A supramolecular inclusion complex (1), formed by 2 and 3, is accompanied by the formation of a hydrogel. The light-responsive gel-to-sol and sol-to-gel phase transitions of the hydrogel, induced by trans-cis photoisomerization of the azobenzene units, were investigated. In the hydrogel system, the trans-azobenzene units in 3 are included inside the hydrophobic cavity of 2. Upon photoirradiation with UV light of 355 nm, the hydrogel is converted efficiently to the sol phase because the trans-azobenzene units are converted photochemically to their cis configurations, whereupon the resulting cisazobenzene units dissociate from 2. The hydrogel can be recovered from the sol phase by photoirradiation with visible light of 450 nm. The swelling ratio for fresh hydrogel samples, which was found to be 8.7 ( 0.7, was measured for a number of gel-to-sol and sol-to-gel phase-transition cycles.
Introduction Stimuli-responsive supramolecular hydrogels, which can respond to external stimuli such as pH or redox changes and light, have attracted1 a great deal of attention recently from scientists because such hydrogel systems can serve as functional materials with potential applications in the areas of drug/gene delivery, photography, paints/coatings, sensors, and so forth. Because the formation of the supramolecular hydrogels is usually driven1 by specific noncovalent intermolecular forces such as hydrogen bonding, π-π stacking, hydrophobic, dipole-dipole, and van der Waals interactions, the rational design and precise preparation of the stimuli-responsive supramolecular hydrogels produced by the self-assembly processes are still a challenging research topic. Because activating processes by light are rapid and clean and allow for low invasiveness as far as biological systems are concerned, we have recently given particular attention to the preparation of *To whom correspondence should be addressed. E-mail: stoddart@ northwestern.edu. † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue.
(1) (a) Newkome, G. R.; Baker, G. R.; Arai, S.; Saunders, M. J.; Russo, P. S.; Theriot, K. J.; Moorefield, C. N.; Rogers, L. E.; Miller, J. E.; Lieux, T. R.; Murray, M. E.; Phillips, B.; Pascal, L. J. Am. Chem. Soc. 1990, 112, 8458–8465. (b) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (c) Petka, W. A.; Harden, J. L.; McGrath, K. P.; Wirtz, D.; Tirrell, D. A. Science 1998, 281, 389–392. (d) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263–2266. (e) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (f) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (g) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656. (h) Trewyn, B. G.; Slowing, I. I.; Giri, S.; Chen, H.-T.; Lin, V. S.-Y. Acc. Chem. Res. 2007, 40, 846–853. (i) Penard, A.L.; Gacoin, T.; Boilot, J.-P. Acc. Chem. Res. 2007, 40, 895–902. (j) Kang, D. J.; Bae, B.-S. Acc. Chem. Res. 2007, 40, 902–912. (k) Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. Rev. 2007, 36, 1857–1867. (l) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715.
8442 DOI: 10.1021/la804316u
light-responsive supramolecular hydrogels. Azobenzene and its derivatives have a unique light-induced trans-cis isomerization property.2,3 Irradiating azobenzene units with light causes them to isomerize from the more stable trans to the less stable cis configuration. Thus, they have been used as light-responsive units in many gel systems3 in which the sol-to-gel or gel-to-sol phase transition of the gel systems is controlled by light-induced trans-cis isomerization of the azobenzene units. For example, Shinkai and co-workers3a have prepared some cholesterol derivatives containing azobenzene units and have investigated their sol-gel phase transitions induced by light-responsive trans-cis isomerism of the azobenzene units. They found that the gel formed from the trans isomer was converted efficiently to the sol when the trans-to-cis isomerization of the azobenzene units was induced photochemically and that this phase-transition process can be repeated reversibly. A host-guest chemistry approach is advantageous for producing light-responsive hydrogels. Cyclodextrins (CDs) and their derivatives acting as hosts can form supramolecular inclusion complexes with a series of guests in which the guest molecules are included within the hydrophobic inner cavities (2) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915–1925. (3) (a) 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. (b) Kim, Y. S.; Sung, C. S. P. Macromolecules 1996, 29, 462–467. (c) Jung, J. H.; Ono, Y.; Shinkai, S. Angew. Chem., Int. Ed. 2000, 39, 1862–1865. (d) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Mater. 2003, 15, 2141–2145. (e) Moriyama, M.; Mizoshita, N.; Yokota, T.; Kishimoto, K.; Kato, T. Adv. Mater. 2003, 15, 1335–1338. (f) Koumura, N.; Kudo, M.; Tamaoki, N. Langmuir 2004, 20, 9897–9900. (g) Lee, C. T.Jr.; Smith, K. A.; Hatton, T. A. Macromolecules 2004, 37, 5397–5405. (h) Yagai, S.; Karatsu, T.; Kitamura, A. Langmuir 2005, 21, 11048–11052. (i) Moriyama, M.; Mizoshita, N.; Kato, T. Bull. Chem. Soc. Jpn. 2006, 79, 962–964. (j) Zhou, Y.; Xu, M.; Yi, T.; Xiao, S.; Zhou, Z.; Li, F.; Huang, C. Langmuir 2007, 23, 202–208.
Published on Web 03/13/2009
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Letter
of the CDs.4 Some research groups have reported5 that a few CD complexes can show gelation abilities under specific conditions. For instance, Harada and co-workers5c,5d,5f have reported that the sol-gel phase transition of the CD-complex-based hydrogels can be induced by light or by redox control. Previous investigations6 have demonstrated a high binding affinity in aqueous solution between β-CD and trans-azobenzene units and low, if any, binding between β-CD and cis-azobenzene units. These observations suggest that CD/azobenzene inclusion complexes might have the ability to produce light-responsive supramolecular hydrogels. Herein, we demonstrate (Scheme 1) that supramolecular inclusion complex 1 of the deoxycholic acid-modified β-CD derivative (deoxycholate-β-CD) 2, when mixed with azobenzene-branched poly(acrylic acid) copolymer 3, can form a hydrogel that shows the gel-sol phase-transition phenomenon induced by the light-responsive trans-cis isomerization of the azobenzene units. We show that the hydrogel, formed from the complex in which the trans-azobenzene units in copolymer 3 are included inside the hydrophobic cavity of β-CD derivative 2, is converted efficiently to the sol phase when the trans-azobenzene units are induced photochemically into their cis configurations and dissociate from β-CD derivative 2. Furthermore, we demonstrate that the hydrogel can be recovered from the sol phase under photoirradiation with visible light (i.e., the gel-to-sol and sol-to-gel phase transitions are reversible). In addition, the swelling ratios (q) for several gel-to-sol and sol-to-gel phase-transition cycles have been measured by irradiating the hydrogel samples with UV light (355 nm) and visible light (450 nm). Finally, we have employed UV/vis spectroscopy, circular dichroism, and 1H ROESY (rotating-frame Overhauser enhancement spectroscopy) NMR experiments to elucidate the mechanism of light-responsive gelation. (4) (a) Szejtli, J. Chem. Rev. 1998, 98, 1743–1754. (b) Nepogodiev, S. A.; Stoddart, J. F. Chem. Rev. 1998, 98, 1959–1976. (5) (a) Dreiss, C. A.; Cosgrove, T.; Newby, F. N.; Sabadini, E. Langmuir 2004, 20, 9124–9129. (b) Takashima, Y.; Nakayama, T.; Miyauchi, M.; Kawagushi, Y.; Yamaguchi, H.; Harada, A. Chem. Lett. 2004, 33, 890–891. (c) Tomatsu, I.; Hashidzume, A.; Harada, A. Macromol. Rapid Commun. 2005, 26, 825–829. (d) Tomatsu, I.; Hashidzume, A.; Harada, A. Macromolecules 2005, 38, 5223–5227. (e) Isobe, Y.; Sudo, A.; Endo, T. Macromolecules 2006, 39, 7783–7785. (f) Tomatsu, I.; Hashidzume, A.; Harada, A. Macromol. Rapid Commun. 2006, 27, 238–241. (g) Kretschmann, O.; Choi, S. W.; Miyauchi, M.; Tomatsu, I.; Harada, A.; Ritter, H. Angew. Chem., Int. Ed. 2006, 45, 4361–4365. (h) Deng, W.; Yamagushi, H.; Takashima, Y.; Harada, A. Angew. Chem., Int. Ed. 2007, 46, 5144–5147. (i) Ogoshi, T.; Takashima, Y.; Yamagushi, H.; Harada, A. J. Am. Chem. Soc. 2007, 129, 4878–4879. (j) Ma, X.; Wang, Q.; Qu, D.; Xu, Y.; Ji, F.; Tian, H. Adv. Funct. Mater. 2007, 17, 829–837. (k) Zhu, L.; Ma, X.; Ji, F.; Wang, Q.; Tian, H. Chem.-Eur. J. 2007, 13, 9216–9222. (l) Deng, W.; Yamagushi, H.; Takashima, Y.; Harada, A. Chem.-Asian J. 2008, 3, 687–695. (6) (a) Bortolus, P.; Monti, S. J. Phys. Chem. 1987, 91, 5046–5050. (b) Yoshida, N.; Seiyama, A.; Fujimoto, M. J. Phys. Chem. 1990, 94, 4254–4259. (c) Sanchez, A. M.; de Rossi, R. H. J. Org. Chem. 1996, 61, 3446–3454. (d) Murakami, H.; Kawabuchi, A.; Kotoo, K.; Kunitake, M.; Nakashima, N. J. Am. Chem. Soc. 1997, 119, 7605–7606. (e) Anderson, S.; Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. Engl. 1997, 36, 1310–1313. (f) Takei, M.; Yui, H.; Hirose, Y.; Sawada, T. J. Phys. Chem. A 2001, 105, 11395–11399. (g) Stanier, C. A.; Alderman, S. J.; Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. 2002, 41, 1769–1772. (h) Liu, Y.; Zhao, Y.-L.; Chen, Y.; Guo, D.-S. Org. Biomol. Chem. 2005, 3, 584–591. (i) Murakami, H.; Kawabuchi, A.; Matsumoto, R; Ido, T.; Nakashima, N. J. Am. Chem. Soc. 2005, 127, 15891–15899. (j) Tomatsu, I; Hashidzume, A.; Harada, A. J. Am. Chem. Soc. 2006, 128, 2226–2227. (k) Inoue, Y.; Kuad, P.; Okumura, Y.; Takashima, Y.; Yamaguchi, H.; Harada, A. J. Am. Chem. Soc. 2007, 129, 6396–6397. (l) Pouliquen, G.; Amiel, C.; Tribet, C. J. Phys. Chem. B 2007, 111, 5587–5595. (m) Jog, P. V.; Gin, M. S. Org. Lett. 2008, 10, 3693–3696.
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Results and Discussion Deoxycholate-β-CD 2 was prepared (Scheme S1 in Supporting Information) by the condensation of mono(6-triethylenetetramino-6-deoxy)-β-CD (5) and deoxycholic acid (6) in 56% yield. Azobenzene-branched poly(acrylic acid) copolymer 3 was synthesized (Scheme S2 in Supporting Information) from poly(acrylic acid) (9) and 2-(4-phenylazophenoxy) ethanol (10) in 86% yield. The content of the azobenzene unit in copolymer 3 is ca. 8%, calculated in the 1H NMR spectrum of copolymer 3 from the ratio between the azobenzene units’ protons and the methine units’ or methylene units’ protons in the main chain of the copolymer. On account of the inclusion complexation5 of the β-CD cavity with trans-azobenzene units, hydrogel 1 was prepared from deoxycholateβ-CD 2 and copolymer 3 in aqueous solution (pH 9), where the molar ratio of deoxycholate-β-CD (24 10-3 mol L-1) and the azobenzene units (16 10-3 mol L-1) in copolymer 3 was ca. 1.5:1. Under the same conditions, neither deoxycholate-β-CD 2 nor copolymer 3 forms a hydrogel. Also, copolymer 3 does not exhibit the gelation phenomenon in the presence of β-CD. For a quantitative assessment of the association of deoxycholate-β-CD (2) with the azobenzene unit in aqueous solution, titration experiments were performed using UV/vis spectroscopy in phosphoric acid-disodium hydrogen phosphate buffer solution (pH 9) at 25 °C to give a complex formation constant (KS) for a model system of deoxycholate-βCD (2) with trans-2-(4-phenylazophenoxy)ethanol (trans-10). In the UV/vis spectra, the UV absorption intensity of trans-2(4-phenylazophenoxy)ethanol (trans-10) (3.0 10-5 mol L-1) around 355 nm gradually decreases upon the addition of varying amounts of deoxycholate-β-CD (2) ((0-84.0) 10-5 mol L-1). Using the nonlinear least-squares curve-fitting method,7 we calculated that the complex formation constant for the 1:1 host-guest combination of deoxycholate-β-CD (2) with trans-2-(4-phenylazophenoxy)ethanol (trans-10) is 970 ( 50 M-1. However, the spectral change of cis-2-(4-phenylazophenoxy)ethanol (cis-10) in the presence of deoxycholate-β-CD (2) is too weak to be determined under the same conditions.5b These experimental results indicate that the inclusion complexation of deoxycholate-β-CD (2) with trans-2-(4-phenylazophenoxy)ethanol (trans-10) is more favorable than that with cis-10, a result that provides experimental support for constructing a light-responsive hydrogel system controlled by the trans-cis photoisomerization of the azobenzene units. It is important to investigate the geometrical change that occurs in deoxycholate-β-CD (2) in the absence and in the presence of the trans-azobenzene unit. Thus, we carried out experiments that can reveal (i) the relative geometries of deoxycholate-β-CD (2) and the azobenzene unit in the hydrogel system and (ii) the gel-sol phase-transition mechanism of hydrogel 1 induced by light-responsive trans-cis isomerization of the azobenzene unit. We began by performing a 1H ROESY NMR experiment8 on deoxycholate-β-CD (2) (4.8 10-4 mol L-1) in D2O at 25 °C. The ROESY spectrum of 2 displays identifiable NOE (nuclear Overhauser effect) cross peaks between the H-3 and H-5 protons on the inside of the β-CD torus and the key protons of the deoxycholate unit, a result that suggests that the deoxycholate unit in 2 is either (7) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703–2707. :: (8) Schneider, H.-J.; Hacker, F.; Rudiger, V.; Ikeda, H. Chem. Rev. 1998, 98, 1755–1786.
DOI: 10.1021/la804316u
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Letter Scheme 1. Supramolecular Inclusion Complex 1 Formed from Deoxycholate-β-CD Derivative 2 and Azobenzene-Branched Poly(acrylic acid) Copolymer 3a
a Complex 1 can form a hydrogel in aqueous solution, which has a gel-sol phase transition induced by light-responsive trans-cis isomerization of the azobenzene units.
self-included in the hydrophobic cavity of the β-CD ring to form the intramolecular inclusion complex or included in another β-CD ring to form the intermolecular inclusion complex. The fact that the azobenzene unit of copolymer 3 can be included in the cavity of deoxycholate-β-CD (2) is demonstrated by another 1H ROESY experiment. To simplify such a complicated system, we pursued the 1H ROESY experiment using the complex of deoxycholate-β-CD (2) (4.8 10-4 mol L-1) with trans-2-(4-phenylazophenoxy)ethanol (trans-10) (4.8 10-4 mol L-1) in D2O at 25 °C. NOE cross peaks between (i) the H-R protons of the azobenzene unit and the H-3 protons on the inside of the β-CD ring (peak A), (ii) the H-β protons and the H-3/5 protons (peak B), and (iii) the H-γ protons and the H-5 protons (peak C) are observed (Figure 1) in the 1H ROESY spectrum. However, there are no NOE cross peaks found between the H-3 and H-5 protons on the β-CD ring and the protons of the deoxycholate unit in 2. These observations indicate that (i) the trans-azobenzene unit is included in the hydrophobic cavity from the secondary face of the β-CD ring as illustrated in Scheme 1 and (ii) the deoxycholate unit is excluded from the hydrophobic cavity. It is well known3a,9 that the steroid units can participate in 1D stacking aggregation, driven by intermolecular van der Waals and/or hydrogen bonding interactions. We have already demonstrated that the deoxycholate unit in 2 is included intramolecularly or intermolecularly inside the cavity of the β-CD ring. This particular inclusion geometry may well prevent the deoxycholate unit in 2 from undergoing intermolecular association to form the hydrogels under the current experimental conditions. Because the inclusion complexation (9) (a) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17, 5825–5833. (b) Kawano, S.-i.; Fujita, N.; Shinkai, S. Chem. Commun. 2003, 1352–1353. (c) Liu, Y.; Zhao, Y.-L.; Zhang, H.-Y. Langmuir 2006, 22, 3434–3438.
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Figure 1. 1H ROESY NMR spectrum (500 MHz) of deoxycholateβ-CD (2) (4.8 10-4 mol L-1) with trans-2-(4-phenylazophenoxy) ethanol (10) (4.8 10-4 mol L-1) in D2O at 25 °C with a mixing time of 250 ms. of the trans-azobenzene unit in the cavity of deoxycholate-βCD (2) orients the deoxycholate unit away from the cavity, the free deoxycholate units in complex 1 can interact with each other through intermolecular interactions, thus favoring the formation of the hydrogel. To monitor the photoirradiation process of the azobenzene units in complex 1, induced circular dichroism (ICD) and relative UV/vis spectroscopy experiments (Figure 2) of complex 1 containing deoxycholate-β-CD (2) (2.8 10-5 mol L-1) and copolymer 3 (2.8 10-5 mol L-1 based on the azobenzene unit) were carried out in phosphoric acid/disodium hydrogen phosphate buffer solution (pH 9) at 25 °C. The time required Langmuir 2009, 25(15), 8442–8446
Letter
Figure 2. (a) ICD and (b) UV/vis spectra of complex 1 containing deoxycholate-β-CD (2) (2.8 10-5 mol L-1) and copolymer 3 (2.8 10-5 mol L-1 based on the azobenzene unit) in phosphoric acid/ disodium hydrogen phosphate buffer solution (pH 9) upon photoirradiation at 355 nm at 25 °C. to scan one UV/vis or CD spectrum is about 1 min. As expected, no ICD signal was observed for deoxycholate-βCD (2) or copolymer 3. The ICD spectrum of complex 1, however, reveals a negative Cotton effect peak (black curve in Figure 2a) around 350 nm with a Δε value of -2.14 dm-3 mol-1 cm-1. With a knowledge of previous investigations carried out by Harata, Kajt ar, and Nau10 on the ICD properties of CD complexes, we can deduce that the trans-azobenzene unit in copolymer 3 is encircled by the deoxycholate-β-CD ring in complex 1. Upon photoirradiation with UV light of 355 nm, the absorption peak of complex 1 at around 350 nm decreases remarkably with the photoirradiation time, and the absorption peak around 440 nm increases concomitantly. Because the absorption peaks around 355 and 440 nm are assigned to π-π* and n-π* transitions of the azobenzene unit, respectively, the absorption change of complex 1 induced by UV photoirradiation indicates that the photoisomerization of the azobenzene unit from its trans to cis configurations is taking place. In the ICD spectra, the ICD intensity of complex 1 around 355 nm decreases gradually toward the baseline upon photoirradiation, a phenomenon that indicates that the cis-azobenzene unit is dissociating away from the deoxycholate-β-CD ring. When irradiated with visible light of 450 nm, the UV/vis and ICD spectra of complex 1 can be recovered in their original forms, a result that reveals the photoisomerization of the azobenzene unit from the cis to trans configuration. The UV/vis and ICD experimental results provide direct evidence for the association and dissociation between the azobenzene unit and deoxycholate-β-CD in complex 1 upon photoirradiation. Complex 1, containing deoxycholate-β-CD (2) (24 10-3 mol L-1) and copolymer 3 (16 10-3 mol L-1 accounted for by the azobenzene unit), forms a transparent, red hydrogel. A photograph of the hydrogel is shown in Figure S1a of Supporting Information. In the system, the trans-azobenzene units in copolymer 3 are included inside the cavity of (10) Achiral entities located in chiral environments will produce ICD signals in the corresponding transition bands. An empirical rule for the ICD behavior of cyclodextrin complexes with achiral chromophoric guests has been proposed as follows: if the transition moment of the guest chromophore is parallel to the axis of symmetry of the cyclodextrin, then the sign of the ICD signal for that transition will be positive, whereas if the moment axis is aligned perpendicularly to the principal axis, then the sign of ICD will be negative. See (a) Harata, K.; Uedaira, H. Bull. Chem. Soc. Jpn. 1975, 48, 375–378. (b) Kajtar, M.; Horvath-Toro, C.; Kuthi, E.; Szejtli, J. Acta Chim. Acad. Sci. Hung. 1982, 110, 327–355. (c) Zhang, X.; Nau, W. M. Angew. Chem., Int. Ed. 2000, 39, 544–547.
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deoxycholate-β-CD (2), as demonstrated already. Because the azobenzene units in copolymer 3 can undergo reversible photoisomerization from trans to cis and from cis to trans upon photoirradiation with UV (355 nm) and visible light (450 nm), respectively, we investigated the light-responsive gel-to-sol and sol-to-gel phase-transition behavior of complex 1 induced by photoisomerization of the azobenzene units. When the hydrogel was irradiated with UV light (355 nm) for 5 min, it was converted gradually into its sol phase (Figure S1b in Supporting Information). Under these conditions, the trans-azobenzene unit is isomerized to its cis configuration, whereupon the cis-azobenzene unit dissociates from the cavity of deoxycholate-β-CD (2). The hydrogel can be recovered from its sol phase via irradiation with visible light (450 nm), demonstrating that the gel-to-sol and sol-to-gel phase transitions are reversible. Thus, the association and dissociation of the azobenzene unit with deoxycholate-β-CD (2) can be controlled by trans-cis photoisomerization of the azobenzene unit, leading to light-responsive gel-to-sol and sol-to-gel phase transitions of complex 1. To investigate quantitatively the light-responsive phase-transition properties,11 the swelling ratios (q) for several gel-to-sol and sol-to-gel phase-transition cycles were measured (experimental details and Table S1 in Supporting Information) by irradiating the hydrogel samples with UV light (355 nm) and visible light (450 nm). The swelling ratio (qtrans0) for fresh hydrogel samples was found to be 8.7 ( 0.7. Upon photoirradiation with UV (355 nm), a decrease in the swelling ratio (qcis0 = 0.4 ( 0.1) was observed as the trans-azobenzene unit was isomerized to its cis configuration. The hydrogel was converted completely into its sol phase after the second cycle of photoirradiation with UV light (355 nm). According to expectations, the hydrogel was recovered (qtrans1 = 5.6 ( 0.4 and qtrans2 = 4.3 ( 0.4) under photoirradiation with visible light (450 nm), where the cis-azobenzene unit is isomerized back to its trans configuration. These results all support a light-responsive phase-transition mechanism for the system illustrated in Scheme 1.
Conclusions A supramolecular inclusion complex, formed by deoxycholate-β-CD and azobenzene-branched poly(acrylic acid) copolymer, has been investigated and found to undergo reversible gelation in response to the trans-cis photoisomerization of the azobenzene unit. In the hydrogel system, the trans-azobenzene units in the copolymer are included inside the hydrophobic cavity of the β-cyclodextrin derivative. Upon photoirradiation with UV light of 355 nm, the hydrogel is converted efficiently into the sol phase because the trans-azobenzene units are photochemically converted into their cis configurations and the resulting cis-azobenzene units dissociate away from the β-cyclodextrin rings. The hydrogel can be recovered from the sol phase via photoirradiation with visible light of 450 nm. The light-responsive gel-to-sol and solto-gel phase transitions, induced by trans-cis photoisomerization of the azobenzene units, are reversible. The reversible sol-gel phase transition of the supramolecular complex can be controlled under these mild conditions, a fact that (11) (a) Rosen, O.; Piculell, L.; Hourdet, D. Langmuir 1998, 14, 777–782. (b) Kang, M.-S.; Gupta, V. K. J. Phys. Chem. B 2002, 106, 4127–4132. (c) Tsutsui, H.; Moriyama, M.; Nakayama, D.; Ishii, R.; Akashi, R. Macromolecules 2006, 39, 2291–2297. (d) Du, J.-Z.; Sun, T.-M.; Weng, S.-Q.; Chen, X.-S.; Wang, J. Biomacromolecules 2007, 8, 3375–3381.
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suggests that this gel material could have a promising role to play in bioengineering applications requiring light-controlled encapsulation or release of molecular and cellular species. Acknowledgment. This research was supported by the Microelectronics Advanced Research Corporation
8446 DOI: 10.1021/la804316u
(MARCO) and its Focus Center Research Program (FCRP) and the Center on Functional Engineered NanoArchitectonics (FENA). Supporting Information Available: Synthesis and characterization details. This material is available free of charge via the Internet at http://pubs.acs.org.
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