Supramolecular Chirality from an Achiral Azobenzene Derivative

Lu Lin , Tianyu Wang , Zhou Lu , Minghua Liu , and Yuan Guo. The Journal of ... Penglei Chen, Xiaoguang Ma, Yiqun Zhang, Kaiming Hu, and Minghua Liu...
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10246

Langmuir 2006, 22, 10246-10250

Supramolecular Chirality from an Achiral Azobenzene Derivative through the Interfacial Assembly: Effect of the Configuration of Azobenzene Unit Yiqun Zhang, Penglei Chen,* and Minghua Liu* Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, China ReceiVed August 16, 2006. In Final Form: September 16, 2006 The Langmuir-Schaefer (LS) films of an achiral azobenzene derivative, 4-octyl-4′-(5-carboxypentamethyleneoxy) azobenzene (C8AzoC5), were fabricated and their optical activities were investigated. It was found that the LS film of the trans-C8AzoC5 showed strong Cotton effect, while that of cis-C8AzoC5 did not. The characterization of the LS films by UV-vis, Fourier transform infrared (FT-IR) spectra, and X-ray diffraction (XRD) revealed that this interesting phenomenon was due to the different packing of the azobenzene unit in the LS film. The planar conjugated trans-azobenzene favored ordered cooperative packing in a helical sense and produced the supramolecular chirality, while the cis-isomer did not due to the bulky twisted configuration.

Introduction Optically active molecular assemblies, which are generally constructed through the noncovalent interactions such as hydrogen bond, π-π stacking, electrostatic interaction, coordination, and hydrophobic interactions, have recently received much attention, ascribing to their significant importance in supramolecular chemistry, biochemistry, and material sciences.1,2 The intrinsically chiral building blocks are commonly believed to be responsible for the optical activities of certain supramolecular assemblies.1,2 The optically active supramolecular assemblies could also be constructed by using achiral building blocks, which could be induced to show chirality in a chiral matrix. In some particular cases, optically active supramolecular assemblies could be constructed exclusively from achiral building blocks, which resulted from the spontaneous symmetry breaking under an asymmetric circumstance. So far, several systems on the optically active supramolecular assemblies from wholly achiral molecules have been reported.3-6 For instance, some achiral dyes and achiral inorganic salts have been reported to form optically active supramolecular assemblies and crystals, respectively, under circularly polarized light irradiation or a directional stirring.4,5 * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. Tel.: 86-10-82612655. Fax: 8610-62569564. (1) For selected reviews, see: (a) Crego-Calama, M.; Reinhoudt, D. N.; Ten Cate, M. G. J. Top. Curr. Chem. 2005, 249, 285. (b) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. ReV. 2001, 101, 4039. (c) Cheng, R. P.; Gellman, S. H.; DeGrado, W. F. Chem. ReV. 2001, 101, 3219. (d) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893. (e) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. ReV. 2001, 101, 4071. (f) Feringa, B. L.; Van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. ReV. 2000, 100, 1789. (g) Nakano, T.; Okamoto, Y. Chem. ReV. 2001, 101, 4013. (h) Okamoto, Y.; Nakano, T. Chem. ReV. 1994, 94, 349. (2) For recent papers, see: (a) Gestel, J. V. J. Phys. Chem. B 2006, 110, 4365. (b) Renikuntla, B. R.; Armitage, B. A. Langmuir 2005, 21, 5362. (c) Parschau, M.; Romer, S.; Ernst, K.-H. J. Am. Chem. Soc. 2004, 126, 15398. (d) Li, B.; Cheuk, K. K. L.; Salhi, F.; Lam, J. W. Y.; Cha, J. A. K.; Xiao, X.; Bai, C.; Tang, B. Nano Lett. 2001, 1, 323. (e) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (f) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409. (g) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167. (h) Prins, L. F.; Huskens, J.; Jong, F. D.; Timmerman, P.; Reinhoudt, D. N. Nature 1999, 398, 498. (i) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; Mackintosh, F. C. Nature (London) 1999, 399, 566. (j) Verbiest, T.; Van Elshocht, S.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 1998, 282, 913.

In a series of investigations on the supramolecular chirality of the molecular assemblies, we have systematically demonstrated that some achiral compounds can be organized into chiral supramolecular assemblies at the air/water interface, where a spontaneous symmetry breaking occurs.6 A general understanding derived from these studies is that the achiral building blocks, bearing a relatively bulky headgroup and a hydrocarbon tail, are possible to form optically active supramolecular assemblies through the spontaneous symmetry breaking at the air/water interface. We suggested that the cooperative arrangement of the molecules in the assemblies, which were triggered by the overcrowded alignment of the adjacent bulky headgroups, caused such supramolecular chirality in the interfacially organized films. In this paper, we further extended our work to the effect of the molecular configuration on the supramolecular chirality in the interfacially assembled films. We have found that an achiral amphiphilic azobenzene derivative, 4-octyl-4′-(5-carboxypentamethyleneoxy) azobenzene (C8AzoC5, as shown in the insert of Figure 1), can be organized into the supramolecular assemblies showing supramolecular chirality through the interfacial assembly. While the Langmuir-Schaefer (LS) films of trans-C8AzoC5 showed strong Cotton effect, those of cis-isomer did not. Azobenzene is one of the well-known functional materials showing excellent properties. One of the most important proper(3) (a) Yamaguchi, T.; Kimura, T.; Matsuda, H.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 6350. (b) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Krblova, E.; Walba, D. M. Science 1997, 278, 1924. (c) Pawlik, A.; Kirstein, S.; De Rossi, U.; Daehne, S. J. Phys. Chem. B 1997, 101, 5646. (d) De Rossi, U.; Da¨hne, S.; Meskers, S. C. J.; Dekkers, H. P. J. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 760. (e) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Nature (London) 1994, 368, 440. (4) (a) Ribo´, J. P.; Crusats, J.; Sague´s, F.; Claret, J.; Rubires, R. Science 2001, 292, 2063. (b) Rubires, R.; Farrera, J.-A.; Ribo´, J. M. Chem.sEur. J. 2001, 7, 436. (5) (a) Saito, Y.; Hyuga, H. J. Phys. Soc. Jpn. 2005, 74, 535. (b) Viedma, C. Phys. ReV. Lett. 2005, 94, 065504. (c) Kondepudi, D. K.; Asakura, K. Acc. Chem. Res. 2001, 34, 946. (d) Kondepudi, D. K.; Bullock, K. L.; Digits, J. A.; Hall, J. K.; Miller, J. M. J. Am. Chem. Soc. 1993, 115, 10211. (e) Kondepudi, D. K.; Bullock, K. L.; Digits, J. A.; Hall, J. K.; Miller, J. M. J. Am. Chem. Soc. 1993, 115, 10211-10216. (f) McBride, J. M.; Carter, R. L. Angew. Chem., Int. Ed. Engl. 1991, 30, 293. (6) (a) Yuan, J.; Liu, M. J. Am. Chem. Soc. 2003, 125, 5051. (b) Zhang, L.; Lv, Q.; Liu, M. J. Phys. Chem. B 2003, 107, 2565. (c) Huang, X.; Liu, M. Chem. Commun. 2003, 66. (d) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322. (e) Guo, P.; Liu, M. Langmuir 2005, 21, 3410.

10.1021/la062427a CCC: $33.50 © 2006 American Chemical Society Published on Web 10/27/2006

Supramolecular Chirality from Achiral Azobenzene

Figure 1. Surface pressure-molecular area isotherms of the monolayers of trans-C8AzoC5 (a) and cis-C8AzoC5 (b) on a pure Milli-Q water surface. Insert: the molecular structure of C8AzoC5.

ties of the azobenzene is its trans-cis-isomerization and related photochromism.7-11 A series of chiral molecular assemblies and/ or chiral polymers from azobenzene derivatives have been built up with azobenzene units bearing intrinsic chirality and several chiroptical devices have been proposed.12-17 There are also interesting reports that achiral azobenzene derivatives could be photoinduced into chiral assemblies by circularly and/or elliptically polarized beam irradiation, which creates chiral electromagnetic environments.18-20 However, there is no report on the fabrication of optically active molecular assemblies from achiral azobenzene derivates without the introduction of external chiral factors. Here, combining the advantage of the interfacial assembly (7) (a) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (b) Matsumoto, M.; Terrettaz, S.; Tachibana, H. AdV. Colloid Interface Sci. 2000, 87, 147. (8) Yamamura, M.; Kano, N.; Kawashima, T. Inorg. Chem. 2006, 45, 6497. (9) (a) Dugave, C.; Demange, L. Chem. ReV. 2003, 103, 2475. (b) Bossi, M. L.; Murgida, D. H.; Aramendia, P. F. J. Phys. Chem. B 2006, 110, 13804. (c) Yuan, W.; Jiang, G.; Wang, J.; Wang, G.; Song, Y.; Jiang, L. Macromolecules 2006, 39, 1300. (10) (a) Muraoka, T.; Kinbara, K.; Aida, T. Nature (London) 2006, 440, 512. (b) Crecca, C. R.; Roitberg, A. E. J. Phys. Chem. A 2006, 110, 8188. (c) Li, Y.; Deng, Y.; Tong, X.; Wang, X. Macromolecules 2006, 39, 1108. (11) (a) Chanishvili, A.; Chilaya, G.; Petriashvili, G.; Collings, P. J. Phys. ReV. E 2005, 71, 051705. (b) Kumar, S. K.; Hong, J.-D.; Lim, C.-K.; Park, S.-Y. Macromolecules 2006, 39, 3217. (c) Liu, M.; Asanuma, H.; Komiyama, M. J. Am. Chem. Soc. 2006, 128, 1009. (12) (a) Muraoka, T.; Kinbara, K.; Kobayashi, Y.; Aida, T. J. Am. Chem. Soc. 2003, 125, 5612. (b) Kurihara, S.; Nomiyama S.; Nonaka, T. Chem. Mater. 2001, 13, 1992. (c) Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Chem. Mater. 2001, 13, 1998. (13) (a) Li, Q.; Li, L.; Kim, J.; Park, H.-S.; Williams, J. Chem. Mater. 2005, 17, 6018. (b) Carreo`o, M. C.; Garcı´a, I.; Ribagorda, M.; Merino, E.; Pieraccini, S.; Spada, G. P. Org. Lett. 2005, 7, 2869. (c) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (14) (a) Tie, C.; Gallucci, J. C.; Parquette, J. R. J. Am. Chem. Soc. 2006, 128, 1162. (b) Iftime, G.; Natansohn, A.; Rochon, P. Macromolecules 2002, 35, 365. (c) Mruk, R.; Zentel, R. Macromolecules 2002, 35, 185. (d) Kurihara, S.; Nomiyama, S.; Nonaka, T. Chem. Mater. 2000, 12, 9. (15) (a) Khan, A.; Hecht, S. Chem.sEur. J. 2006, 12, 4764. (b) Khan, A.; Kaiser, C.; Hecht, S. Angew. Chem., Int. Ed. 2006, 45, 1878. (16) (a) Mitsuoka, T.; Sato, H.; Yoshida, J.; Yamagishi, A.; Einaga, Y. Chem. Mater. 2006, 18, 3442. (b) Angiolini, L.; Benelli, T.; Giorgini, L.; Salatelli, E. Macromolecules 2006, 39, 3731. (17) (a) Tamaoki, N.; Wada, M. J. Am. Chem. Soc. 2006, 128, 6284. (b) Angiolini, L.; Benelli, T.; Giorgini, L.; Salatelli, E. Polymer 2006, 47, 1875. (18) (a) Wu, Y.; Natansohn, A.; Rochon, P. Macromolecules 2004, 37, 6801. (b) Nikolova, L.; Nedelchev, L.; Todorov, T.; Petrova, Tz.; Tomova, N.; Dragostinova, V.; Ramanujam, P. S.; Hvilsted, S. Appl. Phys. Lett. 2000, 77, 657. (c) Nikolova, L.; Todorov, T.; Ivanov, M.; Andruzzi, F.; Hvilsted, S.; Ramanujam, P. S. Opt. Mater. 1997, 8, 255. (19) (a) Ivanov, M.; Naydenova, I.; Todorov, T.; Nikolova, L.; Petrova, T.; Tomova, N.; Dragostinova, V. J. Mod. Opt. 2000, 47, 861. (b) Naydenova, I.; Nikolova, L.; Ramanujam, P. S.; Hvilsted, S. J. Opt. A: Pure Appl. Opt. 1999, 1, 438. (20) (a) Fukuda, T.; Kim, J. Y.; Sumimura, H.; Itoh, M.; Yatagai, T. Mol. Cryst. Liq. Cryst. 2006, 446, 61. (b) Tejedor, R. M.; Millaruelo, M.; Oriol, L.; Serrano, J.; Alcala´, R.; Rodrı´guez, F. J.; Villacampa, B. J. Mater. Chem. 2006, 16, 1674. (c) Choi, S. W.; Ha, N. Y.; Shiromo, K.; Rao, N. V. S.; Paul, M. K.; Toyooka, T.; Nishimura, S.; Wu, J. W.; Park, B.; Takanishi, Y.; Ishikawa, K.; Takezoe, H. Phys. ReV. E 2006, 73, 021702.

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technique and the photoinduced configuration control of the azobenzene derivative, we have successfully fabricated the optical active supramolecular assemblies from achiral azobenzene derivative. Furthermore, the results revealed that even with the same molecule, the configuration of the employed building blocks could contribute significantly to the molecular packing in the film and thus affect the supramolecular chirality of the formed molecular assemblies. The interfacial properties of the azobenzene derivative were investigated by the π-A isotherm and the transferred LS films were characterized by UV-Vis, Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD), and circular dichroism (CD) spectra. Experimental Section Materials. 4-Octyl-4′-(5-carboxypentamethyleneoxy) azobenzene (C8AzoC5, insert of Figure 1) was purchased from Dojindo Laboratories and used without further purification. Milli-Q water (18.2 MΩ cm) was used in all the cases. Procedures. The compound was dissolved in chloroform solution as a trans-form. To obtain cis-C8AzoC5 in solution, the chloroform solution of trans-C8AzoC5 (1 × 10-4 M) was irradiated with 365 nm UV light for 10 min to achieve the photoisomerization from trans-C8AzoC5 to cis-C8AzoC5. The solutions of trans-C8AzoC5 and cis-C8AzoC5 in chloroform were spread on a water surface and their interfacial properties as well as their transferred films were investigated. After the evaporation of the solvent, surface pressurearea (π-A) isotherms were recorded with a compression speed of 8 cm2/min. The monolayers were transferred onto solid supports at a surface pressure of 20 mN/m by a horizontal lifting technique to get the LS multilayer films. Immediately, the LS films were subjected to UV-Vis, CD spectra, FT-IR spectra, and XRD measurements. In general, a certain multilayer was fabricated from the same monolayer at different places. Quartz slides were used as the substrates for UV-vis, CD, and XRD investigations, while single-face polished silicon (111) wafers were used for the FT-IR measurement. All the experiments and measurements were operated under darkness to avoid possible photoisomerization. Apparatus and Measurements. The surface pressure-area (πA) isotherms were recorded on a KSV 1100 minitrough (KSV instruments 1100, Helsinki, Finland) at 20 °C. JASCO UV-530 and JASCO J-810 CD spectropolarimeters were used for the UV-vis and CD spectra measurements, respectively. In the measurement of the CD spectra, the samples were placed perpendicular to the light path and rotated within the film plane to avoid the polarizationdependent reflections and eliminate the possible angle dependence of the CD signal. The angle dependence behavior of all 36 CD spectra which were measured in a step of 10° around the optical axis has been achieved as described elsewhere.6 XRD of the LS multilayers was performed on a Rigaku X-ray diffractometer (D/Max-RB) with Cu KR radiation.

Results and Discussions Surface Pressure-Molecular Area (π-A) Isotherms. Solutions of the compound in chloroform were spread onto a pure Milli-Q water surface at 20 °C and their compression isotherms were recorded as shown in Figure 1. Reproducible π-A isotherms were obtained. In the case of trans-C8AzoC5, the limiting molecular area, which was obtained by extrapolating from the steepest linear part of the isotherms to zero surface pressure, was 0.28 nm2 per molecule. This value was close to the cross-sectional area perpendicular to the long axis of trans-azobenzene chromophore (0.25 nm2),21,22 suggesting that nearly all the C8AzoC5 molecules were in the trans-form configuration.21,23,24 For cisC8AzoC5, the onset area of the spreading film was 0.85 nm2 per (21) Yim, K. S.; Fuller, G. G. Phys. ReV. E 2003, 67, 041601, 1. (22) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378.

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Zhang et al.

Figure 2. (A) UV-vis (bottom) and CD (top) spectra of trans-C8AzoC5 in chloroform solution (a and a′, respectively), 25-layer LS films (b and b′, respectively) deposited at 20 mN/m, respectively; (B) the corresponding spectra of those of cis-C8AzoC5.

molecule, while the limiting area was 0.53 nm2 per molecule, as indicated in Figure 1. The value was much larger than that of the trans-C8AzoC5. It is well-known that the trans-azobenzene unit is roughly a planar molecule, whereas the cis-form species has a twisted molecular skeleton with a dihedral angel of 53.3° between its two phenyl groups.25 The larger onset and limiting molecular areas deduced from cis-C8AzoC5 compared to those of its trans-isomer could thus be ascribed to the bending of the cis-azobenzene unit, which was more bulky than that of the trans-form unit.21,23-25 As reported in the literature, the nearly pure cis-form of a azobenzene analogue, 4-octyl-4′-(3-carboxytrimethyleneoxy) azobenzene (cis-C8AzoC3), gave an onset and limiting molecular area of 0.62 and 0.5 nm2 on pure Milli-Q water at 20 °C, respectively. The experimental results for the monolayer of cis-C8AzoC5 were close to these reported ones, indicating that nearly pure cis-C8AzoC5 monolayer was obtained under the present experimental conditions.21,23 At the same time, it was noted that the monolayer of trans-C8AzoC5 had a higher collapsed surface pressure (ca. 37 mN/m) than that of cisC8AzoC5 (ca. 26 mN/m), suggesting the intermolecular interactions were stronger in the case of trans-C8AzoC5.21 UV-Vis Spectra and Optically Activities of the LS Films of trans-C8AzoC5 and cis-C8AzoC5. The as-prepared monolayers were transferred onto solid substrates at 20 mN/m by a horizontal lifting method under darkness. Figure 2 shows the UV-Vis and the CD spectra of both of the isomers in chloroform solutions and the LS films. The UV-vis spectrum of transC8AzoC5 exhibits a strong absorption and a weak peak at 352 and 444 nm, respectively, in solution. The two bands can be ascribed to the π-π*, which originated from the long axis transition moment of the trans-form azobenzene, and n-π* transitions of the azobenzene chromophore, respectively. The absorption of its 25-layer LS film exhibits a broadened band centered around 337 nm, which is blue-shifted by 15 nm with respect to that of the π-π* transition in chloroform solution. In addition, an absorption peak ascribed to the π-π* transition, whose transition moment is parallel to the short axis of the transform azobenzene, is observed at around 248 nm in the LS film.22,24,26-28 These data suggested the trans-C8AzoC5 molecules were arranged in an H-aggregate in the LS film.22,24 The cisC8AzoC5 species was obtained by irradiating with 365 nm light on the trans-form isomer in chloroform solution, which showed an increased peak at 444 nm and a new peak at 310 nm, (23) Crusats, J.; Albalat, R.; Claret, J.; Igne´s-Mullol, J.; Sague´s, F. Langmuir 2004, 20, 8668. (24) Sato, T.; Ozaki, Y.; Iriyama, K. Langmuir 1994, 10, 2363. (25) Kurita, N.; Tanaka, S.; Itoh, S. J. Phys. Chem. A 2000, 104, 8114. (26) Seki, T.; Kukuchi, T. Langmuir 2002, 18, 5462. (27) Manna, A.; Chen, P.-L.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20. (28) Seki, T.; Kukuchi, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1998, 71, 2807.

accompanied by the disappearance of the absorption at 352 nm. The spectrum is in accordance with those reported in the literature for cis-C8AzoC5 species in solution.22,24,26,27 Such solution was used to fabricate the multilayer LS films of cis-form isomer. And thus transferred film shows a broadened band centered around 333 nm, which is red-shifted by 23 nm with respect to that of the absorption peak at 310 nm in chloroform solution, whereas the peak at 444 nm maintains its position except for the peak broadening. These data indicated that we surely obtained cisC8AzoC5 LS films in which the cis-C8AzoC5 molecules were arranged in a J-like aggregate. When the LS films of trans-C8AzoC5 and cis-C8AzoC5 samples were subjected to CD spectral measurements,6 it was interesting to find that the trans-C8AzoC5 LS film displayed an exciton-type Cotton effect (CE) at 320 and 360 nm with a crossover at 337 nm. Since the solution did not show any CD signals and the compound was achiral, the present CD signal in the LS film could be ascribed to the nonsymmetrical arrangement of the trans-form azobenzene chromophores in the LS films. These results indicated that the achiral trans-C8AzoC5 molecules could form optically active supramolecular assemblies through the organization at the air/water interface. In addition, we have found that the films fabricated in different batches could show CD signals with different signs in most of the cases. We have fabricated 23 batches of the LS films of trans-C8AzoC5 and found that 10 films showed positive (43.5%) CD signals and 13 films showed negative (56.5%) signals around 320 nm. This phenomenon was similar to the results we previously reported and indicated that the macroscopic chirality of the LS film was indeterminate and that the observed macroscopic supramolecular chirality was indeed not from any chiral impurities but from the chiral supramolecular assemblies that arranged in helical structures.6 More surprisingly, no CD signals were detected either in the solution or in the LS film of cis-C8AzoC5. This indicated that the configuration of azobenzene played an important role in forming the supramolecular chirality in the films, which will be further discussed in a later section. To confirm that the supramolecular chirality in the LS film was really from the stacking of trans-C8AzoC5 molecules, we have measured the angle dependence of the CD spectra of the LS films, which were operated by measuring the CD spectra in a step of 10° rotating around the optical axis. The angle dependence of the CD amplitude was determined by the difference between the maximum value at 360 nm and minimum value at 320 nm, and the corresponding angle dependence of the background was determined by the difference between the values at the upper wavelength edge at 500 nm and lower edge at 200 nm, which were shown as filled squares and circles, respectively, in Figure 3. In the case of angle dependence of the background, the observed differences between the values at the upper

Supramolecular Chirality from Achiral Azobenzene

Figure 3. Angle dependence of the CD amplitude (9) and the background (b) of the CD spectra of the LS film of trans-C8AzoC5 while the sample turned about the optical axis in a step of 10° within the sample plane.

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Figure 6. Top: Photoisomerization of the 15-layer LS films of trans-C8AzoC5 by irradiating ((a) before the irradiation; (b) after the irradiation) the sample with 365 nm light for 1 h. Bottom: Photoisomerization of the 15-layer LS films of cis-C8AzoC5 by irradiating ((a) before the irradiation; (b) after the irradiation) the sample with 440 nm light for 1 h. Scheme 1. Illustration of the Formation of Optically Active and Inactive Supramolecular Assemblies from trans-C8AzoC5 (Top) and cis-C8AzoC5 (Bottom), Respectively

Figure 4. FT-IR spectra of trans-C8AzoC5 (a) and cis-C8AzoC5 (b) in 25-layer LS films.

Figure 5. XRD pattern of 25-layer LS films of trans-C8AzoC5 (a) and cis-C8AzoC5 (b).

wavelength and lower edges fluctuated around 0 mdeg, whereas the CD amplitude, exhibiting as a cosine function and fluctuating around 128 mdeg, was positively shifted about 128 mdeg compared to that of the background. These results apparently indicated that the fabricated films indeed had genuine intrinsic chirality. FT-IR and XRD Patterns of the LS Films. Figures 4 and 5 show the FT-IR and XRD of the LS films. Both the trans- and cis-isomer displayed a strong vibration band at 1703 cm-1, which could be assigned to the carbonyl stretching mode of a carboxylic acid group with a hydrogen bond, suggesting the formation of hydrogen-bond-linked supramolecular assemblies, which were arranged in a sideways manner (Scheme 1).24,26 Strong vibration bands corresponding to the CH2 asymmetric and symmetric stretching modes were observed at 2925 and 2855 cm-1, respectively, indicating at least one of the hydrocarbon chains (the alkyl chain C5 that bonded to the carboxylic group or the alkyl tail C8 that bonded to the phenyl group) was arranged in gauche configurations.24 The vibrational peak exhibited by the LS film of trans-C8AzoC5 at 1250 cm-1, attributed to asymmetric stretching mode of phenyl-O-C group, shifted by 11 cm-1 to

the higher wavenumber 1261 cm-1 in the case of the LS film of cis-rich C8AzoC5. However, previously published literature,24 which focused on the single-layer LS films of cis-C8AzoC5, did not present this phenomenon. This suggested that there existed interactions between the adjacent layers in the multilayer LS film of cis-C8AzoC5 in the present case. The XRD pattern of the LS film of trans-C8AzoC5 exhibits a diffraction peak at 5.19° (Figure 5). A layer thickness of 3.4 nm, which is close to the length of the CPK mode of the transC8AzoC5 molecule, can be estimated according to Bragg’s equation. This indicated that the trans-species were aligned almost perpendicularly to the substrate surface and the LS film had a good layered structure. Accompanied by the information deduced from the FT-IR, it could be sure that the alkyl chain C5 and C8 were arranged in gauche and trans zigzag configurations, respectively, since the gauche of the C5 alkyl chain would bring only a little bit of change to the length of the molecule. No diffraction peak was observed for the film of cis-rich species, suggesting that the molecules in the film were in a disordered arrangement. Photoisomerization of the LS Films. The photoisomerization of the LS films of these two species has also been examined, as shown in Figure 6. The UV-vis spectrum of trans-C8AzoC5 remained almost unchanged upon irradiation with 365 nm light. This suggested that no photoisomerization occurred in its LS film, which could be due to the close packing of the azobenzene units in the film. For the LS film of cis-C8AaoC5, the photoisomerization occurred as verified by the decreasing of the absorption at 444 nm and the shifting of the absorption from 333 to 341 nm upon irradiation with 440 nm light. Such photoisomerization was due to the fact that the cis-C8AzoC5 units in the LS film were loosely packed, which could provide free volume for the isomerization reaction. However, different from the LS films directly fabricated from the trans-form C8AzoC5, the resulted trans-form C8AzoC5 film, which was photoinduced from the cis-C8AzoC5 LS film, did not show any CD signals.

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This indicated that the organization at the air/water interface played an important role in obtaining the optical active molecular assemblies. A Possible Explanation for the Optical Activities of the LS Films. Based on these data, a possible mechanism can be proposed to explain our interesting phenomenon (Scheme 1). In forming the Langmuir films and the subsequent LS films, the packing states of these two isomers could be different due to their different spatial configurations. The trans-azobenzene unit has a planar molecular configuration, whereas the cis-isomer has a bending shaped molecular skeleton with a dihedral angel of 53.3° between its two phenyl groups.25 When these two forms of the C8AzoC5 molecule were arranged through the air/water interface, the transazobenzene packed in an H-aggregate, while the cis-isomer formed a J-like aggregate. It seems that the planar configuration of trans-azobenzene favors the cooperative packing in a helical sense due to the strong intermolecular interactions and the steric hindrance of the adjacent molecules, as in the case of our previously reported systems.6 As a result, a certain enantiomer (dextrorotatory or levorotatory) was predominantly formed, and we thus obtained the supramolecular assemblies showing macroscopic chirality. In the case of cis-C8AzoC5, owing to the complicated intermolecular steric hindrance and the disordered loose arrangement of the molecules, no cooperative arrangement could be obtained. Therefore, we could not observe the macroscopic supramolecular chirality from the cis-C8AzoC5 LS films. On the other hand, due to the dense packing of the molecules, no photoisomerization was observed for the LS films of transform species. In the case of cis-form isomers, photoisomerization could occur due to the free volume in the film. However, the packing status of thus-generated trans-C8AzoC5 molecules was different from that in the LS films directly fabricated from transC8AzoC5 monolayer because in LS film it did not provide virtual mobility for molecules to pack densely. Thus, no macroscopic chirality could be detected.

Zhang et al.

Finally, it should be noted that the Langmuir films of C8AzoC5 have been studied before.22,26,29-32 However, all of that research was concerned with the photochromism and was not conscious of the supramolecular chirality in the films. Here, we presented a new aspect of this organized molecular film.

Conclusions We have investigated the interfacial assembly of azobenzene amphiphiles with trans- and cis-configurations. We have demonstrated that trans-C8AzoC5, bearing a planar configuration in the transferred film, exhibited distinct optical activities in the LS films, although the molecule itself was achiral, whereas cisC8AzoC5, which had a more bulky configuration than its transform isomer, showed no optical activities in the LS films. The supramolecular chirality in the formed organized molecular films was suggested to be due to the cooperative stacking of the functional molecules. It seems that while the planar configuration favors the cooperative stacking, the bulky twisted configuration disfavors such arrangement. The present investigation provides not only a facile way for constructing optical active azobenzene supramolecular assemblies from wholly achiral azobenzene building blocks but also deep insight into how the different molecular configurations of the same compound could affect the supramolecular chirality. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20533050, 20403023, and 90306002), National Basic Research Program (No. 2007CB808005), and the Fund of the Chinese Academy of Sciences. LA062427A (29) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (30) Enomoto, T.; Hagiwara, H.; Tryk, D. A.; Liu, Z.-F.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. B 1997, 101, 7422. (31) Kawai, T. J. Phys. Chem. B 1999, 103, 5517. (32) Pedrosa, J.-M.; Romero, M. T. M.; Camacho, L. J. Phys. Chem. B 2002, 106, 2583.