pubs.acs.org/Langmuir © 2010 American Chemical Society
Langmuir-Blodgett Films and Chiroptical Switch of an Azobenzene-Containing Dendron Regulated by the in Situ Host-Guest Reaction at the Air/Water Interface† Pengfei Duan, Long Qin, and Minghua Liu* Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Colloid and Interface Science, Institute of Chemistry, CAS, Beijing 100190, PR China Received September 30, 2010. Revised Manuscript Received November 19, 2010 An amphiphilic dendron containing an azobenzene ring at the focal point and the L-glutamate peripheral groups was designed. Its monolayer formation and host-guest reaction with cyclodextrins at the air/water interface and the properties of the transferred Langmuir-Blodgett (LB) films were investigated. The individual dendron, although without any long alkyl chains, could still form a stable monolayer at the air/water interface because of the good balance between hydrophilic and hydrophobic parts within the molecule. When cyclodextrin (CyD) was added to the subphase, a host-guest reaction occurred in situ at the air/water interface. The inclusion of the focal azobenzene moiety into the cavity of cyclodextrin decreased the packing of the aromatic ring and also led to the diminishment of the molecular area. Both the films formed at the surface of pure water and aqueous cyclodextrins were transferred onto solid substrates. Nanofiber structures were obtained for the film from the water surface as a result of the packing of the azobenzene groups, and circular domains were obtained for the film transferred from the aqueous CyD phases. The film transferred from the water surface showed an exciton couplet in the absorption band of azobenzene, whereas a negative Cotton effect was obtained for the film from CyD subphases. It was found that the supramolecular chirality in the LB film transferred from water was due to the transfer of the molecular chirality to the assemblies whereas that from the CyD subphase was due to the inclusion of azobenzene into the chiral cavity. Interestingly, the film from the water surface was photoinactive, whereas a reversible optical and chiroptical switch could be obtained for the film from the R-CyD subphase. The work provided a way to regulate the assembly and functions of organized molecular films by taking advantage of the interfacial host-guest reaction.
Introduction Dendrimers are a unique class of molecules with a spherical or hemispherical shape, uniform size, and a determined molecular weight.1 As well-defined building blocks, dendrimers have drawn intense attention as photoresponsive and electroresponsive materials.2 Dendrimers can be assembled not only in 3D solutions to form various organized nano/microstructures3 but also in a 2D air/ water interface to form stable organized molecular films.4 Several kinds of dendrimers with long alkyl chains and dendrimers containing peptides as their peripheral groups have been assembled into Langmuir monolayers and subsequent LB films.5 We have found that with the appropriate balance between the focal and peripheral groups, even without the long alkyl chain, a stable monolayer could be formed, which provided an easier modification for obtaining the organized molecular films.6 Besides the possible † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding author. E-mail:
[email protected]. Tel: þ86-10-82615803. Fax: þ86-10-62569564.
(1) (a) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem., Int. Ed. 1990, 29, 138. (b) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (2) (a) Vogtle, F.; Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym. Sci. 2000, 25, 987. (b) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171. (c) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692. (3) Zimmerman, S. C.; Zeng, F. W.; Reichert, D. E. C.; Kolotuchin, S. V. Science 1996, 271, 1095. (4) (a) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (b) Schenning, A.; Elissen-Roman, C.; Weener, J. W.; Baars, M.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (5) (a) Elliott, E. K.; Stine, K. J.; Gokel, G. W. J. Membr. Sci. 2008, 321, 43. (b) Nierengarten, J. F. New J. Chem. 2004, 28, 1177. (c) Cheng, C. X.; Jiao, T. F.; Tang, R. P.; Chen, E. Q.; Liu, M. H.; Xi, F. Macromolecules 2006, 39, 6327. (d) Costa, A. S.; Imae, T. Langmuir 2004, 20, 8865. (e) Sunde, M.; Kwan, A. H. Y.; Templeton, M. D.; Beever, R. E.; Mackay, J. P. Micron 2008, 39, 773. (f) Nierengarten, J. F.; Eckert, J. F.; Rio, Y.; Carreon, M. D.; Gallani, J. L.; Guillon, D. J. Am. Chem. Soc. 2001, 123, 9743. (6) Duan, P. F.; Liu, M. H. Phys. Chem. Chem. Phys. 2010, 12, 4383.
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nano/microstructures formed by the dendrimers, the functionalization is expected. Generally, the dendrimer molecules can be modified through the peripheral group and focal points to regulate their self-assembly and functionalization. In this article, we have investigated the assembly and function of an L-glutamate-based dendron with azobenzene at the focal point. We designed this molecule for two reasons. Because azobenzene is apt to show a strong π-π interaction, its reversible photochromic property in the organized molecular films is frequently inhibited when using simple azobenzene-containing amphiphiles.7 We introduce azobenzene into the focal point to allow it to be protected by the peripheral group and hope to realize the reversible photoresponse. Another reason is to functionalize the L-glutamate-based dendron film with a chiroptical switch. The chiroptical switch described the reversible changes in the chirality, which can be used to increase the recording density in data storage.8 Although the optically active aromatics could be synthesized and designed as chiroptical switches, the substitution on the aromatic ring with the chiral group is an easier way. However, in this case, it is necessary to transfer the chirality of the substituted group to the aromatic rings. Although such a chiroptical switch has been widely investigated in the solution and gel system,9 fewer have (7) Matsumoto, M.; Terrettaz, S.; Tachibana, H. Adv. Colloid Interface Sci. 2000, 87, 147. (8) (a) Feringa, B. L. Acc. Chem. Res. 2001, 34, 504. (b) Sato, O. Acc. Chem. Res. 2003, 36, 692. (c) Guo, P. Z.; Liu, M. H.; Zhao, X. S. Prog. Chem. 2008, 20, 644. (9) (a) Kim, M. J.; Yoo, S. J.; Kim, D. Y. Adv. Funct. Mater. 2006, 16, 2089. (b) Li, Y. G.; Wang, T. Y.; Liu, M. H. Soft Matter 2007, 3, 1312. (c) Pijper, D.; Feringa, B. L. Soft Matter 2008, 4, 1349. (d) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38, 3419. (e) Pu, L. Chem. Rev. 2004, 104, 1687. (f) Maheut, G.; Castaings, A.; Pecaut, J.; Daku, L. M. L.; Pescitelli, G.; Di Bari, L.; Marchon, J. C. J. Am. Chem. Soc. 2006, 128, 6347. (g) Soloshonok, V. A.; Ueki, H.; Moore, J. L.; Ellis, T. K. J. Am. Chem. Soc. 2007, 129, 3512.
Published on Web 12/15/2010
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Article Scheme 1. Molecular Structures of the Molecules Used in This Work and Their Abbreviations
been reported for the molecular films.10 However, the latter is sometimes more important considering the real applications. Unfortunately, our designed azobenzene-containing dendron did not show reversible trans-cis isomerization as expected because of strong π-π stacking of the azobenzene groups in the films. However, we have found that if we spread the azobenzene-containing dendron on the subphase containing cyclodextrin, a well-known host molecule with a chiral, hydrophobic cavity, then a host-guest reaction between azobenzene and cyclodextrin occurred in situ at the air/water interface. Owing to this host-guest reaction, we obtained a molecular film showing reversible photochromic properties. Previously, the reversible photoisomerization of azobenzene derivatives incorporated into cyclodextrins through a host-guest reaction has been reported either in solution or LB films, which were focused on the amphiphiles.11-13 Our work showed that the hostguest reaction could also occur for the azobenzene-containing dendrimers in situ at the air/water interface. Moreover, such a host-guest reaction cannot only change the packing of the dendrimers on the surface but also can make the film photoactive. Under alternative UV/vis irradiation, an isomerization occurred in the organized molecular films of dendron/CyD. In addition, because of the inclusion of azobenzene into the cavity of CyD, chirality was induced. More interestingly, the chirality of the dendron/CyD film was a reversible response to photoirradiation, and a chiroptical switch was realized. We have further revealed that R-CyD is more efficient than β-CyD in realizing a switchable chiroptical film. Therefore, through the in situ host-guest reaction, we can functionalize the azobenzene-containing dendron and obtain a chiroptical switch, whereas this was impossible without such a host-guest reaction. (10) (a) Guo, P. Z.; Zhang, L.; Liu, M. H. Adv. Mater. 2006, 18, 177. (b) Zhang, G. C.; Liu, M. H. J. Mater. Chem. 2009, 19, 1471. (c) Kickova, A.; Donovalova, J.; Kasak, P.; Putala, M. New J. Chem. 2010, 34, 1109. (d) Liao, B.; Liu, R. G.; Huang, Y. Polym. J. 2007, 39, 1071. (11) (a) Matsumoto, M.; Tanaka, M.; Azumi, R.; Tachibana, H.; Nakamura, T.; Kawabata, Y.; Miyasaka, T.; Tagaki, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1992, 210, 803. (b) Matsuzawa, Y.; Noguchi, S.; Sakai, H.; Abe, M.; Matsumoto, M. Thin Solid Films 2006, 510, 292. (c) Li, Y. G.; Liu, M. H. J. Colloid Interface Sci. 2007, 306, 386. (d) Rivera, E.; Carreon-Castro, M. D. P.; Rodriguez, L.; Cedillo, G.; Fomine, S.; Morales-Saavedra, O. G. Dyes Pigments 2007, 74, 396. (e) Yoshida, N.; Yamaguchi, H.; Higashi, M. J. Chem. Soc., Perkin Trans. 2 1994, 2507. (12) (a) Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds. Inclusion Compounds; Academic Press: NewYork, 1985; Vols. 1-3.(b) Vogtle, F., Weber, E., Eds. Host-Guest Complex Chemistry; Springer-Verlag: Berlin, 1985.(c) Lehn, J. M. Angew. Chem., Int. Ed. 1988, 27, 89. (d) Liao, X. J.; Chen, G. S.; Liu, X. X.; Chen, W. X.; Chen, F.; Jiang, M. Angew. Chem., Int. Ed. 2010, 49, 4409. (e) Nalluri, S. K. M.; Ravoo, B. J. Angew. Chem., Int. Ed. 2010, 49, 5371. (13) (a) Lednev, I. K.; Petty, M. C. Adv. Mater. 1996, 8, 615. (b) Shankar, B. V.; Patnaik, A. J. Colloid Interface Sci. 2006, 302, 259. (c) Degen, P.; Optenhostert, T.; Rehage, H.; Verhaelen, C.; Lange, M.; Polkowska, J.; Klarner, F. G. Langmuir 2007, 23, 11611. (d) Azov, V. A.; Beeby, A.; Cacciarini, M.; Cheetham, A. G.; Diederich, F.; Frei, M.; Gimzewski, J. K.; Gramlich, V.; Hecht, B.; Jaun, B.; Latychevskaia, T.; Lieb, A.; Lill, Y.; Marotti, F.; Schlegel, A.; Schlittler, R. R.; Skinner, P. J.; Seiler, P.; Yamakoshi, Y. Adv. Funct. Mater. 2006, 16, 147. (e) Xu, W. H.; Wang, Y. H.; Xiao, Y. X.; Liu, F.; Lu, G. Y. Langmuir 2009, 25, 3646.
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Experimental Section Materials. N-(4-Phenyldiazenylcarboxyl)-1,5-bis(L-glutamic acid diethyl ester)-L-glutamic diamide (AzoGE), as shown in Scheme 1, was synthesized by the EDC coupling method.6,14 First, a peptide dendron (Boc-GE) was obtained by reacting the Boc-protected L-glutamic acid with diethyl esters of L-glutamic acid containing a free amino group using EDC as a condensation agent. Second, the Boc protection was deleted by reacting Boc-GE with trifluoroacetic acid (TFA). Boc-GE (0.62 g, 1 mmol) was dissolved in 45 mL of dry CH2Cl2. Then 5 mL of TFA was added to the flask, and the mixture was stirred at room temperature for 3 h. After the reaction, the solvent was removed by rotary evaporation. The obtained oily compound, CF3COO-NH3þ-GE, could be used without further purification. Third, CF3COO-NH3þ-GE was dissolved in dry CH2Cl2 (40 mL), and TEA (3 mL) was added to the solution. The mixture was stirred at 0 °C for 30 min. Then 4-(phenyldiazenyl) benzoic acid (0.23 g, 1 mmol) was added to the above solution, and the resulting mixture was stirred at 0 °C for another 30 min. Then 1-ethyl-3-(3-dimethyllaminopropyl) carbodiimide hydrochloride (EDC 3 HCl, 0.23 g, 1.2 mmol) and 1-hydroxybenzotrizole (HOBt, 0.16 g, 1.2 mmol) were added to the mixture. The obtained mixture was stirred at 0 °C for 3 h, the ice bath was removed, and the mixture was stirred for another 72 h at room temperature. The solution was washed with a saturated sodium chloride (3 30 mL) and water, and the organic phase was dried over magnesium sulfate. The solvent was removed by rotary evaporation, and the crude product was obtained. After purification by silica column chromatography (2/1 CH2Cl2/ethyl acetate, Rf = 0.3), the target product was obtained as a fine solid, mp 131.2 °C (0.43 g, 59%). 1H NMR (400 MHz, CDCl3): δ 8.07-8.09 (d, J = 8.0, 1H), 7.93-7.95 (m, 6H), 7.52-7.55 (m, 4H), 7.04 (s, 1H), 4.69-4.72 (m, 2H), 4.57 (s, 1H), 4.08-4.31 (m, 8H), 2.01-2.48 (m, 12H), 1.68 (s, 2H), 1.20-1.32 (m, 12H). MALDI-TOF-MS Calcd. for C36H47N5O11, 725.8. Found: 748.8 (M þ Na)þ. Anal. Calcd. for C36H47N5O11 (%): C, 59.57; H, 6.53; N, 9.65. Found: C, 59.35; H, 6.96; N, 9.15. Procedure. Monolayers were formed by spreading a chloroform solution of the compound (4.2 10-4 M) on the surface of pure Millipore (18.2 MΩ cm) water, and the π-A isotherm was recorded by compressing the film at a rate of 5 mm2/min after allowing the chloroform to be evaporated for 20 min. The R-CyD and β-CyD subphases were prepared by dissolving the corresponding cyclodextrins in pure Millipore (18.2 MΩ cm) water, and the concentrations were adjusted to 1 10-3 mol/L. For the AFM measurement, one layer of a floating film was deposited on freshly cleaved mica by a vertical-dipping technique with an upstroke speed of 2 mm/min at the selected surface pressure. For spectral measurements, multilayer films were fabricated. Although the vertical dipping method could be applied to fabricate the multilayers, the horizontal lifting method was more effective in fabricating the (14) Duan, P. F.; Liu, M. H. Langmuir 2009, 25, 8706.
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Figure 1. Surface pressure-area isotherms of the spreading films
of AzoGE on (a) a pure water subphase, (b) a β-CyD subphase, and (c) a R-CyD subphase at 20 °C. The concentrations of both R-CyD and β-CyD subphases are 1 10-3 mol/L.
multilayer films. Therefore, the multilayer films were mainly transferred by a horizontal lifting method at the corresponding surface pressure for UV-vis and CD spectral measurements. Photoreaction in the transferred films was performed with a 25 W UV lamp (365 nm) and a 40 W visible light lamp, which was hung over the film at a distance of 20 cm. Apparatus and Measurements. 1H NMR spectra were obtained on an ARX400 (Bruker) NMR spectrometer in CDCl3 with TMS as an internal standard. MALDI-TOF mass spectra were determined with a BIFLEXIII. Elemental analysis was carried out with a Flash EA Carlo-Erba-1106 Thermo-Quest. The surface pressure π-A isotherms were recorded on a KSV 1100 minitrough (KSV Instruments 1100, Helsinki, Finland) at 20 °C. AFM images of the monolayers were recorded on a Digital Instruments Nanoscope IIIa multimode system (Santa Barbara, CA) with a silicon cantilever in tapping mode. All of the AFM images are shown in height mode without any image processing except flattening. UV-vis and CD spectra were obtained using Jasco UV-550 and Jasco J-810 spectrophotometers, respectively. When recording the CD spectra, all of the films were placed perpendicular to the light path and rotated within the film plane to avoid polarization-dependent reflections and eliminate the possible angle dependence of the CD signals.
Results Surface Pressure-Area Isotherms of the Spreading Films. Figure 1 shows the surface pressure-area (π-A) isotherms of AzoGE spread on a water surface and the subphase containing cyclodextrins. Although AzoGE has no substituted long alkyl chains, it still showed surface activity on the surface of pure water with an isotherm showing a surface pressure of as high as 44 mN/m. The onset of surface pressure appeared over a molecular area of 1.25 nm2. Upon compression, an inflection point was observed at 5 mN/m in the isotherm. After the inflection point, the surface pressure decreased slightly and increased again at around 0.3 nm2/molecule. On the basis of the CPK models of AzoGE, if we assume that peripheral L-glutamate formed an expanded phase on the water surface, then a molecular area of 1.2 nm2 can be estimated. Thus, we suggested that the molecular area of 1.25 nm2 could be assumed to be an average of the molecular dimensions. This indicated that AzoGE formed monolayers on the water surface upon spreading. However, the aromatic ring with a vertical arrangement can be estimated to be around 0.3 nm2, corresponding to the molecular area of the condensed regions. These results indicated that the AzoGE dendron, although without the alkyl chains, can still form stable 1328 DOI: 10.1021/la103934g
Langmuir films on a water surface. During this film formation, it is suggested that the balance between the hydrophobic aromatic ring and the hydrophilic dendritic part is important. When the subphase was changed to cyclodextrins, although AzoGE could also be spread, its molecular areas decreased significantly. The film formation of AzoGE on R-CyD subphase was very poor, the molecular area of the condensed region become less than 0.1 nm2, and the surface pressure decreased to as low as 14 mN/m. Compared with the R-CyD subphase, AzoGE on the β-CyD subphase showed relatively good surface activity with a molecular area of the condensed regions of around 0.15 nm2 and a surface pressure of up to 30 mN/m. These results clearly suggested that both R-CyD and β-CyD could interact with AzoGE at the air/water interface. Because both R-CyD and β-CyD have a hydrophobic cavity and azobenzene is also hydrophobic, it can be suggested that the focal azobenzene group might be incorporated into their cavities. Because of such a host-guest reaction, AzoGE become more hydrophilic and small molecular areas were obtained in comparison to that on the pure water surface. The smaller molecular areas of AzoGE on the CyD subphase implied the formation of 3D multilayer films on the subphase. From the even smaller area of the film on R-CyD, it seemed that the host-guest reaction between R-CyD and AzoGE might be more effective than that between β-CyD and AzoGE at the air/water interface. Morphological Investigation of the Transferred OneLayer LB Films. To further disclose the differences in these systems, we have transferred one-layer monolayer films at different surface pressures onto mica and have investigated their morphologies by AFM measurement. We have divided the isotherms into three pressure regions of film deposition. Region I is for the monolayer at a surface pressure of ca. 0 mN/m. Region II corresponds to the middle part of the isotherm where the surface pressure is between 3 and 5 mN/m. Region III represents the region above the second transition. Figure 2 shows the AFM images of the AzoGE monolayers deposited at pressures in the three regions on various subphases. The AzoGE LB film deposited in region I on a pure water surface exhibited a spiral nanofiber pattern, as shown in Figure 2a. By further compressing the monolayer in region II to 5 mN/m, the spiral nanofibers were packed densely and some creases were observed. At a surface pressure above the inflection point, well-defined straight nanofibers were observed, as shown in Figure 2c. The average height of the nanofibers is about 1.7 nm, which corresponds to the monolayer thickness. This implies that the azobenzene ring in AzoGE is vertically oriented in the transferred film. These results are in agreement with the deductions from the isotherms, confirming the monolayer formation of AzoGE, and further disclosed that the dendron formed ordered or aligned nanostructures. When the subphase was changed to cyclodextrin, great changes in the surface morphologies were observed. Figure 2d-f shows AFM images of the AzoGE monolayer on R-CyD subphase. Disklike domains are observed even for the films deposited in region I. It is interesting to observe that above some domains there are other higher domains at the center with an average height of 1.8 nm with respect to the first floor, as shown in the 3-D images in Figure 2d0 -f0 . The heights of the domains increased with the compression surface pressure. At a compression pressure of 10 mN/m, the domains grew larger and packed tightly and the height of the domains increased up to 10 nm or more. A similar phenomenon was observed in the AzoGE/β-CyD system. As shown in Figure 2g-i and g0 -i0 , disklike domains were observed in regions I and II but in region II the domains packed tightly. Langmuir 2011, 27(4), 1326–1331
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Figure 2. AFM images of one-layer films transferred at different surface pressures for various subphases. Pure water: (a) 0, (b) 5, and (c) 15 mN/m; R-CyD subphase: (d) 0, (e) 3, and (f) 10 mN/m; β-CyD subphase: (g) 0, (h) 5, and (i) 10 mN/m. (d0 -i0 ) Three-dimensional images of d-i by a side-view method.
These morphologies indicated that in the cyclodextrin subphase, AzoGE was no longer the monolayer and a multilayer film was formed. When the film was compressed into region III, the aggregation of the domains became more obvious and welldefined nanofibers, which are similar to the AzoGE fiber on the water surface, appeared. This may result from the weak interaction between β-CyD and the azobenzene group. When compressed to high surface pressure, β-CyD might drop out of the AzoGE complex, and AzoGE could reassemble into ordered or aligned nanofibers. In addition, such a phenomenon is more obvious in the case of AzoGE/β-CyD than in AzoGE/R-CyD. These results suggested that the interaction between AzoGE and R-CyD was stronger than for the AzoGE/β-CyD system. Further support came from the FT-IR spectra of the complex film, as shown in Supporting Information Figure S3. The transferred multilayer films from the R-CyD subphase at 10 mN/m showed a Langmuir 2011, 27(4), 1326–1331
typical cyclodextrin band with wide vibrational bands at around 3291 and 1020 cm-1. However, the film fabricated from β-CyD showed a spectrum similar to that of pure AzoGE on a water surface. This indicated that the azobenzene group could be squeezed out of the β-CyD cavity at a higher surface pressure. Supramolecular Chirality of Transferred LS Films. Because the AzoGE compound has a chiral center in L-glutamic acid, we have measured the CD spectra of the LS films obtained from various subphases. Commonly, accurate knowledge concerning the concentration and cuvette path length of the sample is required for the estimation of CD spectra in terms of the types and amounts of the chiral structures. For a thin solid film, the definitions of concentration and cuvette path length required for the quantification and analysis of CD data, however, are not clear. It has recently been suggested that a good estimation of the number of chiral structures could be obtained from the CD DOI: 10.1021/la103934g
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Figure 3. g-factor spectra of AzoGE obtained from various subphases: on (a) a pure water subphase, (b) a R-CyD subphase, and (c) a β-CyD subphase.
spectra for thin solid films by analyzing the wavelength dependence of the Kuhn’s g factor, which is also known as the dissymmetry or anisotropy factor and is defined as the ratio of the CD and UV/vis absorbance signals of a sample.15 To make a general comparison of the different optical activity of AzoGE LS films obtained from different subphases, the g-factor spectra of the LS films transferred from different subphases were investigated. Figure 3 compares the g-factor spectra for the LB films deposited from various subphases. For the LB film deposited from pure water, as shown in Figure 3a, an intensive negative CD splitting band intersected the θ = 0° line at 305 nm and a minimum negative band at 330 nm and a maximum positive band at 288 nm. However, we did not detect any CD signal for the compound in solutions. This suggests that the molecular chirality of the L-glutamic acid moiety transferred to the LB film upon forming organized molecular films. When the subphase changed to cyclodextrins, the CD signal changed greatly, from a splitting band to a solely negative band. According to Harata’s and Kodaka’s rule,16 the induced circular dichroism (ICD) of a chromophore located inside the cyclodextrin cavity will always be positive when its electric transition dipole is parallel to the principal axis of cyclodextrin and the ICD is postulated to be negative if the alignment inside the cavity is perpendicular to the principal axis of the host. In the case of azobenzene derivatives, the dipole of the n-π* transition of the azobenzene group is parallel to the principal axis of cyclodextrin and the dipole of the π-π* transition of the azobenzene group is perpendicular to the principal axis of cyclodextrin. Thus, the negative CD signal of AzoGE/R-CyD can be ascribed to the ICD of the azobenzene groups in the cavity of R-CyD. This also indicated that AzoGE could form a stable Langmuir film with R-CyD through hostguest interaction on an air/water interface. However, the supramolecular chirality in the AzoGE LB film from water and from those of the cyclodextrin subphase was different. The supramolecular chirality of the AzoGE LB film from water was determined by chiral transfer through the assembly whereas that from the cyclodextrin subphase was determined by the host-guest reaction. It is the chiral cavity of cyclodextrin that induced the chirality of the azobenzene moiety. In addition, we have observed that the CD intensity of AzoGE/R-CyD is stronger than that of AzoGE/β-CyD, which suggests a stronger host-guest interaction of AzoGE/R-CyD. (15) (a) McPhie, P. Anal. Biochem. 2001, 293, 109. (b) McPhie, P. Biopolymers 2004, 75, 140. (16) (a) Allenmark, S. Chirality 2003, 15, 409. (b) Kodaka, M. J. Am. Chem. Soc. 1993, 115, 3702. (c) Kodaka, M. J. Phys. Chem. 1991, 95, 2110.
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Chiroptical Switch of AzoGE/r-CyD LS Films. Because the azobenzene moiety is photoactive, we have investigated the photoresponsive property of the LB films. The LB film of AzoGE/R-CyD was irradiated with UV 365 nm and visible light to measure the photoisomerization. Figure 4 shows a change in the absorption and CD spectra of the AzoGE/R-CyD hybrid LS film upon alternate irradiation with UV (365 nm) and visible (>400 nm) light. AzoGE LS films obtained from the pure water subphase showed poor isomerization in the UV-vis spectra and nearly no chiroptical responsiveness to poor isomerization whereas AzoGE LS films obtained from β-CyD films showed poor chiroptical responsiveness to UV irradiation (Supporting Information). As for AzoGE/R-CyD LS films, though the degree of isomerization is poor, an obvious chiroptical switch could be proposed by alternate irradiation with UV (365 nm) and visible (>400 nm) light. When the film was irradiated with UV (365 nm) light, the negative CD signal increased. When the film was irradiated with visible light, the negative CD signal decreased and recovered. Figure 4C shows, as an example, a plot of the intensity change of the CD signal for the AzoGE/R-CyD LS film as a function of the irradiation cycles. More than five cycles are realized without obvious diminishment of the CD intensity. Here, a reversible chiroptical switch using a solid film is presented. Actually, the AzoGE/β-CyD system has poor responsiveness to alternative irradiation although a chiroptical switch property was observed. In addition, the reversibility and repeatability of the switch is poor. After three cycles, the difference in CD intensity disappeared (Supporting Information). Pure AzoGE LS films showed no chiroptical responsiveness to alternative irradiation, which suggested that the π-π stacking between the azobenzene rings is too strong to inhibit the trans-cis isomerization of azobenzene in the film.
Discussion Dendrimers are widely investigated as the building blocks of organized molecular films. However, in those cases, a covalently linked long alkyl chain is usually necessary. In the present case, no long alkyl chain is present in the dendron. Therefore, the formation of the spreading films of AzoGE at the air/water interface is suggested to be due to three important factors: the balance between the hydrophobic azobenzene rings and the peripheral hydrophilic L-glutamate groups, π-π stacking between the azobenzene rings, and the hydrogen bond between the L-glutamate groups. Upon spreading at the air/water interface, the azobenzene group could be packed cooperatively because of the strong π-π interaction. Such cooperative interaction caused the dendron to stay on the water surface. This was confirmed by the larger molecular area of AzoGE on the water surface. In such a monolayer, the azobenzene group was vertical to the water surface and formed the monolayer as shown in Scheme 2a. Because of the strong π-π interaction, we observed the nanofiber structures in the transferred monolayer films. In the beginning, the packed azobenzene formed a localized fiber domain and a spiral structure was observed. Upon compression, the fibers were compressed together to form the aligned fiber structures. In addition, because of the packing of the aromatic groups, the chirality in the glutamic moiety can be transferred to the chromophore. Therefore, we detected CD spectra for the LB films. Because these azobenzene ring packed tightly, we obtained the split Cotton effect. However, such packing is unfavorable for the photoreaction of azobenzene, and such a film failed to show a reversible change upon alternative UV/vis irradiation. When AzoGE was spread on the subphase containing cyclodextrin, a host-guest reaction could occur at the air/water interface, as shown in Scheme 2b,c. During such a reaction, the hydrophobic Langmuir 2011, 27(4), 1326–1331
Duan et al.
Article
Figure 4. (A) UV-vis and (B) CD spectra of AzoGE/R-CyD complex films (45 layers) irradiated with UV and visible light. (C) CD intensity at 312 nm as a function of the irradiation cycle: alternate irradiation with UV (365 nm) and visible (>400 nm) light. Scheme 2. Schematic Illustration of the Interfacial Assembly Methods of AzoGE on Various Subphasesa
switch based on azobenzene.9a,17 Here, using the host-guest interaction, a chiroptical switch can be proposed on the basis of AzoGE/R-CyD LS solid films. In the AzoGE/β-CyD system, most AzoGE molecules reassembled into nanofibers at high surface pressure and the azobenzene groups were squeezed out by a relatively weak interaction. Thus, the chiroptical activity in the AzoGE/β-CyD system was poor.
Conclusions
a (a) On the surface of a pure water subphase, AzoGE assembles into well-defined nanofibers. (b) On the surface of R-CyD, AzoGE and R-CyD form an inclusion complex that will aggregate by further compression. (c) On the surface of β-CyD, AzoGE and β-CyD form inclusion complexes, but further compression will destroy the inclusion complexes and partial AzoGE molecules could reassemble into nanofibers. (d) The LB film from AzoGE/R-CyD could be proposed to be a chiroptical switch according to the active responsiveness of azobenzene to alternate irradiation with UV and visible light.
azobenzene was pulled into the cavity of CyD and the isomers shifted to smaller molecular areas. In addition, we observed many disklike structures in the AFM images. The host-guest reaction between AzoGE and cyclodexrin is related to the size of cyclodextrin. It seemed that azobenzene interacted with R-CyD more strongly than with β-CyD. Therefore, we obtained a smaller molecular area. Interestingly, upon compression, the azobenzene group can be squeezed out of the β-CyD cavity because we observed the nanofibers in the case of the AzoGE monolayer on the β-CyD subphase at a higher surface pressure. These results were further supported by the FT-IR data. The host-guest interaction between AzoGE and CyD caused several changes. First, because the cavity of CyD is chiral, the induced chirality of azobenzene was obtained. It should be noted that such induced chirality was different from supramolecular chirality in the case of the film from the pure water surface. Only a negative CD signal was detected. Second, because azobenzene was included in the cavity, it has some spaces. Therefore, it showed a reversible change upon irradiation. During such reversible isomerization of azobenzene, it stayed in the cavity. Therefore, the CD signal alternately changed the intensity, as shown in Scheme 2d. By changing the CD intensity, we could obtain a chiroptical switch. Several papers have reported the chiroptical
Langmuir 2011, 27(4), 1326–1331
The interfacial assembly of a photoactive azobenzene-containing dendron and the host-guest interaction with cyclodextrins has been investigated. It has been proven that even without long alkyl chains, an appropriate balance between the hydrophobic azobenzene group and the hydrophilic peripheral group can lead to the successful formation of a Langmuir film on the water surface. Through the host-guest interaction, the photoactive dendron and cyclodextrins could form inclusion complexes at the air/water interface. The assemblies on both the water surface and the CyD subphase showed supramolecular chirality. However, the supramolecular chirality of the AzoGE dendron assembly is determined by the molecular chirality in pure AzoGE LB films, which was transferred to the assembly by the packing of azobenzene. In the AzoGE/cyclodextrin system, supramolecular chirality is induced by the chiral cavity of cyclodextrins. Because of the strong packing of the azobenzene in the films fabricated from the water surface, the film became photoinactive. However, because of the strong host-guest interaction between AzoGE and R-CyD, the film became photoactive. Under alternative UV/vis irradiation, the inclusion film showed reversible changes in both the absorption and chirality, forming an optical as well as a chiroptical switch. Acknowledgment. This work was supported by the National Natural Science Foundation of China (nos. 21021003 and 50673095), the Basic Research Development Program (2007CB808005 and 2009CB930802), and the Fund of the Chinese Academy of Sciences. Supporting Information Available: UV-vis, FT-IR, and CD spectra of AzoGE LS films. AFM images of AzoGE onelayer LB films. This material is available free of charge via the Internet at http://pubs.acs.org. (17) (a) van Delden, R. A.; Mecca, T.; Rosini, C.; Feringa, B. L. Chem.;Eur. J. 2004, 10, 61. (b) Martinez-Ponce, G.; Solano, C.; Rodriguez-Gonzalez, R. J.; LariosLopez, L.; Navarro-Rodriguez, D.; Nikolova, L. J. Opt. A: Pure Appl. Opt. 2008, 10, 115006. (c) Zou, G.; Jiang, H.; Zhang, Q. J.; Kohn, H.; Manaka, T.; Iwamoto, M. J. Mater. Chem. 2010, 20, 285.
DOI: 10.1021/la103934g
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