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Fabrication of Chiral Langmuir-Schaefer Films of Achiral Amphiphilic Schiff Base Derivatives through an Interfacial Organization Peizhi Guo†,‡ and Minghua Liu*,† CAS Key Laboratory of Colloid, Interface and Chemical thermodynamics, Institute of Chemistry, and Graduate School of the Chinese Academy of Sciences, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received November 27, 2004. In Final Form: January 30, 2005 Supramolecular chirality in the Langmuir-Schaefer (LS) films of two achiral amphiphilic Schiff bases, 2-(2′-benzimidazolyliminomethyl)-4-octadecyloxyphenol (BSC18) and 2-(2′-benzthiazolyliminomethyl)-4octadecyloxyphenol (TSC18), was investigated. Both of these amphiphiles could form LS films from the water surface or coordinate with Ag(I) in the subphase to form Ag(I)-coordinated LS films. Although both of these amphiphiles were achiral, TSC18 formed a chiral LS film from the water surface, while BSC18 formed a chiral Ag(I)-coordinated LS film from the aqueous AgNO3 subphase. The supramolecular chirality in these LS films was suggested to be due to a cooperative stereoregular π-π stacking of the functional groups together with the long alkyl chains in a helical sense. The relationship between the chirality of the LS films and the molecular structures of TSC18 and BSC18 as well as their H-bond or coordination behaviors was discussed. The Schiff base films showed a reversible color change upon exposure to HCl and NH3 gas alternatively; however, the supramolecular chirality was irreversible during these processes.
Chirality and the chiroptical properties of supramolecular assemblies, which are formed through the noncovalent interactions such as hydrogen bond, electrostatic interaction, hydrophobic interaction, π-π stacking, and coordination, are of considerable importance in supramolecular chemistry and material sciences.1-11 Besides chiral molecules, achiral molecules can also contribute to the chirality of the supramolecular systems.4-8,12-18 Particularly, some systems composed of * To whom correspondence should be addressed. E-mail: liumh@ iccas.ac.cn. † Institute of Chemistry, The Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences, The Chinese Academy of Sciences. (1) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. Rev. 2000, 100, 1789-1816. (2) Green, M. M.; Park, J.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138-3154. (3) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376-379. (4) Hirschberg, J. H. K. K.; Brunsveld, L.; Ramzi, A.; Vekemans, J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407, 167-170. (5) Prins, L. J.; Jong, F. D.; Timmerman, P.; Reinhoudt, D. N. Nature 2000, 408, 181-184. (6) Prins, L. F.; Huskens, J.; Jong, F. D.; Timmerman, P.; Reinhoudt, D. N. Nature 1999, 398, 498-502. (7) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409-416. (8) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449451. (9) Yashima, E.; Goto, H.; Okamoto, Y. Macromolecules 1999, 32, 7942-7945. (10) Cheuk, K. K. L.; Lam, J. W. Y.; Lai, L.; Dong, Y.; Tang, B. Macromolecules 2003, 36, 9752-9762. (11) 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, 323-328. (12) Yuan, J.; Liu, M. J. Am. Chem. Soc. 2003, 125, 5051-5056. (13) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322-1323. (14) Zhai, X.; Zhang, L.; Liu, M. J. Phys. Chem. B 2004, 108, 71807185. (15) Zhang, L.; Lv, Q.; Liu, M. J. Phys. Chem. B 2003, 107, 25652569. (16) Ziegler, M.; Davis, A. V.; Johnson, D. W.; Raymond, K. N. Angew. Chem., Int. Ed. 2003, 42, 665-668. (17) Ribo, J. M.; Crusats, J.; Sagues, F.; Claret, J.; Rubires, R. Science 2001, 292, 2063-2066.
wholly achiral molecules could also show supramolecular chirality.12-18 Our group has been investigating the supramolecular chirality of the molecular assemblies from achiral molecules by employing the control of the molecular orientation and packing at an air/water surface and has succeeded in fabricating the chiral molecular assemblies from achiral molecules.12-15 We have found that the larger steric hindrance and the cooperative arrangement of the achiral molecules played an important role in forming the chiral assemblies.12 While the introduction of larger steric hindrances to the molecules could be easily realized, how to design the molecules which could be cooperatively arranged at the air/water interface still remained unclear. In this paper, we selected two achiral amphiphilic Schiff bases and studied the chirality of the deposited LangmuirSchaefer (LS) films through an interfacial fabrication to disclose the effect of the molecular structure on the supramolecular chirality of the organized molecular films. A deep insight into the relationship between the molecular structure and the chirality was gained. These two Schiff bases, 2-(2′-benzimidazolyliminomethyl)-4-octadecyloxyphenol (BSC18) and 2-(2′-benzthiazolyliminomethyl)-4octadecyloxyphenol (TSC18), as shown in Scheme 1, were selected on the basis of the following reasons. First, BSC18 has a similar structure to an amphiphilic long-chain naphtha[2,3]imidazole derivative, for which we have observed the supramolecular chirality of the assembled films with the Ag(I) ion.12 TSC18 has a similar structure to BSC18 with the only difference being replacing the NH group in the benzimidazole ring with the sulfur atom. Second, both of the compounds could form complexes with Ag(I) at the air/water interface.19 However, while BSC18 could coordinate through both the benzimidazole and imino as well as OH groups, TSC18 could only coordinate with Ag(I) through the imino and OH groups. We have found that such a slight difference in the molecular structure leads to the completely distinct supramolecular (18) Pawlik, A.; Kirstein, S. J. Phys. Chem. B 1997, 101, 5646-5651. (19) Liu, M.; Xu, G.; Liu, Y.; Chen, Q. Langmuir 2001, 17, 427-431.
10.1021/la047089x CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005
LS Films of Amphiphilic Schiff Base Derivatives
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Scheme 1. Illustration of the Formation of the Chiral Molecular Assemblies (R ) -C18H37)
Figure 1. CD and UV-vis absorption spectra of the LS films of BSC18 (A) and TSC18 (B) transferred from the water subphase (a) and aqueous 1 mM AgNO3 subphase (b). The CD spectra of the BSC18 (A) and TSC18 (B) films, which were first deposited from the water surface and then immersed into a 10 mM AgNO3 solution for 90 min (c).
chirality of the assembled molecular films. These new results are helpful in understanding why achiral molecules could form chiral molecular assemblies and can provide further insight into the design of new achiral molecules, which could possibly form chiral molecular assemblies. The monolayer formation at the air/water interface, in situ coordination of both the Schiff bases with the Ag(I) ion in the monolayers, and the characterization of the Langmuir-Blodgett (LB) films of both ligands and metal complexes are reported in our previous paper.19 By comparison with the absorption spectra of the film with those of the solutions, we have confirmed that both of the Schiff bases formed H aggregates20 in the transferred multilayer films from the water surface. When we measured the circular dichroism (CD) spectra of the transferred films, it is interesting to find that the TSC18 film showed a strong Cotton effect in the corresponding absorption band of the film even though TSC18 is achiral. In contrast, no CD signal was detected for the BSC18 film from the water surface. However, the situation was just opposite for the Ag(I)-coordinated films; that is, the Ag(I)-coordinated BSC18 film showed a CD signal, while that of TSC18 did not. Figure 1 shows the CD and UV-vis absorption spectra of the multilayer films in various conditions.21 A strong bisignated CD signal with a crossover of 340 nm is observed for TSC18 film (Figure 1B,a), while no CD signals were detected for the BSC18 film from the water surface (Figure 1A,a). In the case of the TSC18 film, the split Cotton effect appeared around the absorption band of the film and can be regarded as an exciton couplet due to the interaction between the adjacent chromophores in the film. In the Ag(I)-coordinated BSC18 film (Figure 1A,b), a negative Cotton effect was observed at 350 nm, which is very close to the absorption band of the film, indicating that the Cotton effect was due to the chromophores without any coupling. No CD signal was observed for the Ag(I)coordinated TSC18 film (Figure 1B,b). In addition, we have further observed that the CD signals could be opposite in different deposition batches. (20) Mooney, W. F.; Brown, P. E.; Russel, J. C.; Costa, S. B.; Pederson, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984, 106, 5659-5667. (21) Monolayers of BSC18 and TSC18 were formed by spreading the chloroform solutions (6 × 10-4 M) onto a water surface or aqueous 1 mM AgNO3 subphase using a computer-controlled KSV-1100 film balance system. UV-vis absorption and CD spectral of the LS films (50 layers at 25 mN/m) were recorded by a JASCO UV-530 and JASCO J-810 CD spectrophotometer, respectively. The possible effect of linear dichroism of the film was removed by rotating the film plate around the incident light: Spitz, C.; Daehne, S.; Ouart, A.; Abraham, H. W. J. Phys. Chem. B 2000, 104, 8664-8669.
These phenomena were essentially similar to the cases of our previous results.12-15 However, these results gave us additional and new insight into how the supramolecular chirality of the assembled LS films was formed from achiral molecules. First, we have found that for 2-alkyl substituted benzimidazole derivatives, only those with larger aromatic rings such as naphtha[2,3]imidazole could form chiral LB films through an in situ coordination with the Ag(I) ion.12 Thus, 2-alkylbenzimidazole derivatives, even those with methyl or dimethyl substituted groups in the aromatic ring of the benzimidazole, did not show chirality. However, in the present case, chirality was obtained by replacing the 2 substitutent with a larger Schiff base group in the benzimidazole ring. This means that enlarging the group substituted in the 2 position of the benzimidazole ring is also effective in assembling the chiral molecular films. This added a further clue in designing the achiral molecules to form chiral molecular films. Second, TSC18, having different structure from BSC18 only in the S atom, showed chirality in the organized molecular film from the water surface, while BSC18 did not. This can be explained in Scheme 1. It is well-known that benzimidazole could easily form intermolecular hydrogen bonding between the neighboring benzimidazole rings.19 It was clear that both TSC18 and BSC18 formed H aggregates20 in the LS films. In forming the H aggregate, the aromatic ring should stack face to face with the aromatic ring, as shown in Scheme 1B, while the H bond caused the ring aligned in a linear way, as shown in Scheme 1A. For TSC18, only the π-π stacking of the aromatic rings occurred, and we could obtain chirality for the film from the water surface, as shown in Scheme 1B,D. For the BSC18 film from water, as a result of the competitive processes between the H bond and π-π stacking, a cooperative stacking of the aromatic ring was disrupted and no chirality could be observed for the film from the water surface. When coordinating with Ag(I), the situation changed. In the case of BSC18, the coordination in the benzimidazole part could help the group to align in one direction, while the coordination at the Schiff base part can increase the steric hindrance between the adjacent molecules. Therefore, we could observe the supramolecular chirality of the Ag(I)-coordinated BSC18 film (Scheme 1C). In the case of TSC18, because the headgroup was not linearly tied, the coordination caused a disordered arrangement of the functional groups. Therefore, the Ag(I)-coordinated TSC18 film did not show chirality. All these results indicated that the supramolecular chirality of the LS films could be caused by a stereoregular arrangement of the molecules.22,23 This is completely (22) Yashima, E.; Maeda, K.; Nishimura, T. Chem.sEur. J. 2004, 10, 42-51.
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different from the molecular chirality, which was caused by the asymmetric arrangement of the atoms. Because the intermolecular force is more easily destroyed than the covalent bond, we have further investigated whether the supramolecular chirality of the LS films could be destroyed by changing the intermolecular interactions though an external stimuli. The simplest way for the TSC18 system is that we can check this by reacting the TSC18 film with Ag(I) because the Ag(I)-coordinated TSC18 film did not show chirality. This was successfully done, as shown in Figure 1c. When the TSC18 film was immersed into an aqueous solution of AgNO3, the film changed its color rapidly and a distinct change in the UV-vis spectra was observed. As expected the CD signals decreased rapidly with the progress of the reaction (Figure 1c). However, the CD signal did not vanish even after reacting longer. This was because the coordination reaction could not be completed in the LS films due to the steric hindrances. The opposite case was observed for BSC18. The second way to confirm the supramolecular chirality was performed by reacting the film with HCl gas. We have previously observed that the Schiff base could show reversible color change upon alternate exposure to HCl and NH3 gas.24 A similar situation occurred in the present case. For the TSC18 film, reversible color changes were observed when the film was exposed in HCl gas and returned in the air, as shown in the UV-vis spectra (Figure 2A). The as-prepared films were very stable, and the CD signals did not change for more than several weeks in the air. However, no CD signal was detected for the film exposed to HCl gas. Although a diminished CD spectrum was still detected when the film was exposed to HCl gas for the first time and then kept in the air (Figure 2c), continuing exposing of the film to HCl gas caused the complete disappearance of the CD signal and the CD signal can no longer return even when kept in air for a long time. The acidichromism of the Schiff bases is basically limited as a molecular behavior, while the supramolecular chirality reflects the changes of the assemblies. During the film exposure alternately to HCl and NH3 gases, while the chemical change in the Schiff base was reversible, the (23) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860-1866. (24) Liu, Y.; Liu, M. Thin Solid Films 2002, 415, 248-252.
Guo and Liu
Figure 2. UV-vis absorption (A) and CD (B) spectra of the LS films of TSC18 transferred from the water subphase (a), after exposure to HCl vapor (b), and then deposited in air for 15 min or exposed for NH3 gas for 1 min (c).
change in the whole molecular packing was irreversible. Therefore, we could observe the reversible changes in color, but we could not detect the reversible change in chirality of the supramolecular system. In conclusion, two achiral amphiphilic Schiff bases, which contain heteroaromatic benzimidazole and benzthiazole rings, could form chiral LS films from water or the subphase containing AgNO3. TSC18, which has no ability to form an intermolecular H bond, formed a chiral LS film at the air/water interface, while BSC18 could only form a chiral LS film through the coordination with the Ag(I) ion. The supramolecular chirality in these LS films was formed through a cooperative π-π stacking of the functional groups together with the long alkyl chains in a helical sense. These Schiff base films could show a reversible color change upon alternate exposure to HCl and NH3 but not the supramolecular chirality. It is suggested that the color change was mainly a molecular behavior, while the chirality of the LS films was a supramolecular behavior in which molecular packing played an important role. The results provided an important insight into the design of chiral organized molecular films from achiral amphiphiles. Acknowledgment. This work was supported by the Outstanding Youth Fund (No. 20025312), the National Natural Science Foundation of China (Nos. 20273078 and 90306002), the Major State Basic Research Development Program (Nos. 2002CCA03100 and G2000078103), and the Fund of the Chinese Academy of Sciences. LA047089X
