Regulation of Supramolecular Chirality and Morphology of the LB Film

supramolecular chirality and surface morphologies of Lanmuir-Blodgett (LB) films were investigated. It was observed that, at a low molar ratio of the ...
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Langmuir 2006, 22, 4110-4115

Regulation of Supramolecular Chirality and Morphology of the LB Film of Achiral Barbituric Acid by Amphiphilic Matrix Molecules Xin Huang and Minghua Liu* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, CAS, 100080, Beijing, P. R. China ReceiVed NoVember 13, 2005. In Final Form: February 22, 2006 Previously, we have found that an achiral barbituric acid (BA) derivative, 5-(4-(N-methyl-N-hexadecylaminobenzylidene))-2,4,6-(1H,3H)-pyrimidinetrione (BAC16), could form molecular assemblies showing supramolecular chirality through the organization at the air/water interface. To acquire more knowledge of the formation mechanism of such supramolecular assemblies, some achiral molecules, such as stearic acid (SA), octadecylamine (ODA), and an analogue of BA without an alkyl chain, were mixed into the BAC16 system. The effects of these matrix molecules on the supramolecular chirality and surface morphologies of Lanmuir-Blodgett (LB) films were investigated. It was observed that, at a low molar ratio of the matrix molecules (below 10%), the chirality of the BAC16 assemblies could be maintained with only a reduction in the intensity. When the matrix fraction was increased, the supramolecular chirality of the mixed films disappeared. The addition of the matrix molecules can greatly change the surface morphologies of the mixed films. When SA was mixed with BAC16, the spiral nanofibers of BAC16 were changed to long nanofibers. When ODA was mixed, the hydrolytic cleavage reaction of BAC16 took place at the air/water interface and disordered spirals were obtained. When the analogous BA derivate without an alkyl chain was mixed, the phase-separating morphology was observed. These changes in the chirality and surface morphologies indicated firmly that the supramolecular chirality of BAC16 films were formed due to the cooperative arrangement of the molecules. A certain amount of matrix molecules will destroy the cooperative arrangement and thus the chirality.

Introduction Supramolecular chirality, which describes the chirality in the supramolecular level or the chirality of the supramolecular assemblies, has been attracting great interest in recent years.1-6 The research on the supramolecular chirality can not only help us deeply understand and mimic various important molecular processes via a precise molecular and supramolecular chirality recognitions in the biological systems but also expand our research versions on the functionality of supramolecular systems including the electronic, photomagnetic, and sensing properties.3 Supramolecular chirality can be achieved through various noncovalent interactions such as hydrogen bonding (H-bonding), electrostatic interaction, hydrophobic interaction, π-π stacking, and coordination.6-8 Interestingly, achiral molecules can also contribute to the supramolecular chirality through interaction with a chiral matrix.9-11 In a particular case, even complete * To whom correspondence should be addressed. E-mail: [email protected]. Tel: (+86)10-82612655. Fax: (+86)10-62569564. (1) Clines, D. The Physical Origin of Homochirality in Life; AIP Press: Woodbury, New York, 1996. (2) (a) Lehn, J. M. Angew. Chem., Int. Ed. 1990, 29, 1304. (b) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63. (c) Kawasaki, T.; Tokuhiro, M.; Kimizuka, N.; Kunitake, T. J. Am. Chem. Soc. 2001, 123, 6792. (3) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725. (b) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229. (4) (a) Bellacchio, E.; Lauceri, R.; Gurrieri, S.; Scolaro, L. M.; Romeo, A.; Purrello, R. J. Am. Chem. Soc. 1998, 120, 12353. (b) Lauceri, R.; Raudino, A.; Scolaro, L. M.; Micali, N.; Purrello, R. J. Am. Chem. Soc. 2002, 124, 894. (5) Prins, L. J.; Jong, F. D.; Timmerman, P.; Reinhoudt, D. N. Nature 2000, 408, 181. (6) (a) Verbiest, T.; Van elshocht, S.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T.; Persoons, A. Science 1998, 282, 913. (b) Boiadjiev E. S.; Lightener, D. A. Chirality 2000, 12, 204. (7) Constable, E. C. Angew. Chem., Int. Ed. Engl. 1991, 30, 1450. (8) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; Mackintosh, F. C. Nature 1999, 399, 566. (9) Wang, M.; Silva, G. L.; Armitage, B. A. J. Am. Chem. Soc. 2000, 122, 9977. (10) Chen, H.; Law, K.-Y.; Peristein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7257.

achiral molecules can lead to the molecular assemblies showing supramolecular chirality through the cooperative stereo arrangement or spontaneous symmetry breaking.12,13 For example, some achiral porphyrin derivatives and cyanine dyes were reported to show chirality under stirring, where the chirality followed the direction of the stirring.14 Some inorganic salt such as NaClO3 could spontaneously form the chiral crystals without any chiral factor.15 It was also found that some banana-shaped achiral molecules could form chiral liquid crystals.16 In polymer systems, such a phenomenon was more thoroughly investigated.17 The air/water interface provides an important platform where the molecular orientation and packing can be controlled. In these two-dimensional planes, subtle chiral discrimination could be observed in the monolayers.18 Our group has been interested in the formation of the supramolecular chirality from the achiral amphiphilic molecules through the organization at the air/water interface.19 For example, we have found that 2-heptadecylnaphth(11) (a) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449. (b) Petty, J. T.; Zheng, J.; Hud, N. V.; Dickson, R. M. J. Am. Chem. Soc. 2004, 126, 5207. (12) Ezuhara, T.; Endo, K.; Aoyama, Y. J. Am. Chem. Soc. 1999, 121, 3279. (13) DeRossi, U.; Da¨hne, S.; Meskers, S. C. J.; Dekkers, H. P. J. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 760. (14) Ribo´, J. M.; Crusats, J.; Sague´s, F.; Claret, J.; Rubires, R. Science 2001, 292, 2063. (15) Kondepudi, D. K.; Kaufman, R. J.; Singh, N. Science 1990, 250, 975. (16) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Krblova, E.; Walba, D. M. Science 1997, 278, 1924. (17) (a) Ishikawa, M.; Maeda, K.; Yashima, E. J. Am. Chem. Soc. 2002, 124, 7448. (b) 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. (18) (a) Nandi, N.; Vollhardt, D.; Brezesinski, G. J. Phys. Chem. B 2004, 108, 327. (b) Fix, M.; Lauter, R.; Lobbe, C.; Brezesinski, G.; Galla, H. J. Langmuir 2000, 16, 8937. (c) Zhai, X. H.; Brezesinski, G.; Moehwald, H.; Li, J. B. J. Phys. Chem. B 2004, 108, 1347. (19) (a)Yuan, J.; Liu, M. H. J. Am. Chem. Soc. 2003, 125, 5051. (b) Zhang, L.; Lu, Q.; Liu, M. J. Phys. Chem. B. 2003, 107, 2565. (c)Guo, P. Liu, M. Langmuir 2005, 21, 3410. (d) Huang, X.; Liu, M. Chem. Commun. 2003, 66. (e)Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Am. Chem. Soc. 2004, 126, 1322. (f) Huang, X.; Li, C.; Jiang, S.; Wang, X.; Zhang, B.; Liu, M. J. Colloid Interface Sci. 2005, 285, 680.

10.1021/la0530586 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/25/2006

LB Film of Achiral Barbituric Acid

Langmuir, Vol. 22, No. 9, 2006 4111 Scheme 1. Structures of the Molecules Used in the Work

[2,3]imidazole, an amphiphile with a larger headgroup, could show chirality when coordinating with Ag(I) in the air/aqueous AgNO3 interface.19a An achiral amphiphilic barbituric acid derivative, 5-(4-(N-methyl-N-hexadecylaminobenzylidene))-2,4 6-(1H,3H)-pyrimidinetrione (BAC16), could form a chiral LB film through interfacial H-bonding.19e Furthermore, due to the directionality of the H-bond, spiral nanoarchitectures were observed in the transferred LB films.19e In all these cases, it has been suggested that a stereo-cooperative arrangement of the functional amphiphiles plays a key role in the formation of the supramolecular chirality. To further verify if such stereocooperative arrangement of the molecules is really related to the chirality, in this paper, we investigated the effect of the matrix molecules, which can possibly destroy the arrangement and further give a regulation over the miscibility of two kinds of molecules.20-22 To exclude the possible chiral effect caused by the chiral molecules, we selected three achiral molecules as matrixes, stearic acid (SA), octadecylamine (ODA), and 5-(4-(N,N-diphenylaminobenzylidene))-2,4,6-(1H,3H)-pyrimidinetrione (DPBA), as shown in Scheme 1. The choice of these three compounds is based on the following considerations. The SA and ODA are two of the most-investigated amphiphiles which show elegant interfacial behaviors. One is acidic and the other is alkaline. They can serve as good candidates to investigate the interaction between the hydrophobic alkyl chains, as well as the headgroups. These two compounds were usually used as a matrix to obtain a stable monolayer. DPBA, an analogue to BAC16 without an alkyl chain, was used to exclude the effect of the alkyl-chain interaction of BAC16 and DPBA in forming the chiral supermolecules. Since the simple barbituric acid derivative was watersoluble, we added the phenyl group in the molecule in order to help the film formation. We have found that the addition of each kind of these compounds will significantly affect the interfacial properties, the supramolecular chirality, and the surface morphologies. Furthermore, dependent on the molecular structure and molar ratio of the matrix molecules, a regulation on the supramolecular chirality and the surface morphologies of the films was observed. The interfacial behaviors of the mixed monolayers, supramolecular chirality, and surface morphologies of the transferred LB films were investigated by the surface pressure-area (π-A) isotherm, UV-vis and CD spectroscopy, and AFM measurements, respectively. (20) O’Driscoll, B. M. D.; Gentle, I. R. Langmuir 2002, 18, 6391. (21) Ekelund, K.; Sparr, E.; Engblom, J.; Wennerstrom, H.; Engstrom, S. Langmuir 1999, 15, 6946. (22) Matsumoto, M.; Tanaka, K.; Azumi, R.; Kondo, Y.; Yoshino, N. Langmuir 2003, 19, 2802.

Materials and Procedures Materials. The two derivatives of barbituric acid, BAC16 and DPBA, were synthesized according to the literature 23 and confirmed by 1H NMR and elemental analysis. High-grade SA and ODA were purchased from Aldrich and crystallized from methanol. Measurements of π-A isotherms and the deposition of multilayer films were carried out using a computer-controlled KSV-1100 film Balance system (KSV instruments, Helsinki, Finland). A mixed solution of BAC16 with difference amphiphiles in chloroform was spread on the water surface, and the π-A isotherms were recorded after 20 min for evaporation of the solvent with a compressing speed of 12 cm2/min. The monolayers were transferred onto quartz and CaF2 plates for UV-vis, circular dichroism (CD), and FT-IR spectra measurements, respectively. The JASCO UV-530, J-810 CD, and FT/IR-660 plus spectrophotometers were employed during the UVvis, CD, and FT-IR spectra measurements, respectively. In the process of measuring CD spectra, the multilayer films were placed perpendicular to the optical axis and rotated within the film plane in order to avoid the polarization-dependent reflections and eliminate the possible angle dependence of the CD signal. One layer of monolayer compressed at a certain surface pressure was deposited onto a freshly cleaved mica surface, and the AFM images of the transferred films were recorded on a Digital Instrument Nanoscope IIIa multimode system (Santa Barbara, CA) with a silicon cantilever using the tapping mode.

Results 1. Mixed Film of BAC16 and SA. Figure 1A shows the π-A isotherms for the mixed-monolayer films of BAC16/SA with different molar ratios at the air/water interface at 15 °C. The isotherm of BAC16 shows a distinct inflection point and plateau region at about 5 mN/m. When SA was mixed into BAC16, a clear change of the π-A isotherms was observed. The isotherm could be significantly different at various molar ratios of SA to BAC16. When the molar ratio of SA/BAC16 changed into 7:10, the inflection point and plateau region disappeared. The interactions between components in the binary mixed monolayers can be studied from the view of miscibility. For the ideal miscibility, the mean molecular area, Am, versus the molar ratio of two compounds fit the additive rule.24

Am ) A1X1 + A2X2

(1)

Where Am is the mean and ideal areas per molecule of the mixed monolayer at a given surface pressure, X1 and X2 imply the mole fractions of components 1 and 2, respectively, and A1 and A2 are the areas per molecule of each pure monolayer at the same surface pressure. (23) Bohanon, T. M.; Caruso, P.-L.; Denzinger, S.; Fink, R.; Moebius, D.; Paulus, W.; Preece, J. A.; Ringsdorf, H.; Schollmeyer, D. Langmuir 1999, 15, 174. (24) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interface; Wiley: New York, 1966, 22.

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Figure 1. (A) Surface pressure-area (π-A) isotherms of BAC16/SA mixed monolayers at different mixed ratios on the pure water surface at 15 °C. (B) The plot of the mean molecular area of the mixed film as a function of the molar ratio of SA at different surface pressures.

Figure 2. UV-vis and CD spectra of mixed BAC16/SA films at various molar ratios: (a) 10:1, (b) 10:4, (c) 10:7, and (d) 10:10. The LB films were transferred at 30 mN/m. The layer numbers of the films were 20, 30, 40, and 40 for a, b, c, and d, respectively.

Figure 1B shows the plot of the mean molecular area Am(ovs) of the mixed film BAC16/SA at a certain surface pressure as a function of the percentage molar ratio of SA at different surface pressures. The mean molecular areas are larger than the ideal ones at a lower molar ratio of SA in the mixed monolayer. In other molar ratios, the situations are different. At a surface pressure of 4 mN/m, which is below the phase-transition region, the mean molecular areas are always smaller than the ideal ones, which is a negative deviation from the ideal, indicating the attracting interaction between the two components. At a surface pressure higher than the phase-transition region, the mean molecular areas nearly fit the ideal curves, except at a higher molar ratio of SA, in which the mean molecular areas showed a positive deviation from the ideal one. These results indicated that at a lower surface pressure the attracting interaction between the headgroups of BAC16 and SA predominated in the mixed monolayer. At a higher surface pressure, both of the components showed good miscibility or little repulsion. To further clarify the effect of SA on the aggregation of BAC16, both UV-vis and CD spectra of the transferred LB films were measured, as shown in Figure 2. In the UV-vis spectra, the maximum absorbance of BAC16 film is observed at 457 nm, which can be assigned to the H-aggregation of BAC16. Upon mixing with SA, this band did not show obvious change. This indicated that SA did not affect the H-aggregate of BAC16 molecules. Previously, we have found that BAC16 LB film showed the Cotton effect at 445 nm, although the BAC16 itself was achiral.19e After SA was mixed with BAC16, the CD spectra of the mixed films showed some changes. When the molar fraction of BAC16/SA was 10:1 (Figure 2a), the CD intensity of the mixed film was almost the same as that of BAC16 film alone. When the BAC16/SA ratio was decreased to 10:4, the CD intensity decreased greatly. No CD signal could be detected when the SA fraction was larger than 50%, although the absorbance of the film did not change. This indicated that addition of SA would not alter the aggregation of BAC16. However, they greatly

Figure 3. AFM images of one-layer mixed films of BAC16 and SA. BAC16/SA ) (a) 10:1 at 15 mN/m, (b) 10:4, (c) 10:7, and (d) 10:10 at 8mN/m (bar ) 1 µm).

affected the cooperative arrangement of BAC16 molecules, particularly when large amount of SA was mixed into the BAC16 monolayers. It should be noted that both quartz and CaF2 substrates were used to transfer the films. In addition, both vertical and horizontal methods were employed to transfer the films. The same results of UV-vis and CD spectra were obtained. This suggested that both the aggregation and supramolecular chirality were already formed in the monolayer. AFM measurement can provide a direct visualization of the possible phase separation in the LB films. BAC16 could form spiral nanofibers on the water surface. SA could form flat and uniform film on the air/water interface. When they were mixed in the monolayers, great changes in the surface morphologies were observed. Figure 3 shows the AFM images of monolayer mixed films of BAC16/SA at molar ratio of 10:1, 10:4, 10:7, and 10:10. When BAC16 was mixed with SA, most of the spiral nanofibers became straight, although some spiral still existed. 2. BAC16/DPBA Mixture Film. Figure 4A shows the π-A isotherms for BAC16/DPBA mixtures on a water surface with different molar ratios at 15 °C. The isotherm of DPBA shows an increase of the surface pressure at a molecular area of 0.35 nm2/molecule and a phase transition at about 13 mN/m. When mixed with BAC16, the transition region was found to be dependent on the molar ratio of the two components. When BAC16 is in excess, the transition region appeared at around those of the BAC16, and vice versa. Figure 4B shows the plot of the mean molecular area, Am, of the mixed film BAC16/ DPBA as a function of the mole ratio of DPBA at different surface pressures. At any surface pressure, the mean molecular areas showed a positive deviation from the ideal miscibility, indicating that a strong repulsion occurred between the two

LB Film of Achiral Barbituric Acid

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Figure 4. (A) Surface pressure-area (π-A) isotherms of BAC16/DPBA mixed monolayers at different mixed ratios on the pure water surface at 15 °C. (B) The plot of the mean molecular area of the mixed film as a function of the molar ratio of DPBA at different surface pressures.

Figure 5. UV-vis and CD spectra of BAC16/DPBA mixed films (a) 10:1, (b) 10:4, (c) 10:7, (d) 10:10, and (e) DPBA in CHCl3 solution. The layer numbers of the films were 20, 30, 30, and 30 for a, b, c, and d, respectively.

components in the mixed monolayers. In addition, we observed that DPBA only appeared at a relative small area, while BAC16 showed the surface pressure at larger molecular areas. This suggested that DPBA did not form a true monolayer but a multilayer. Similar to the case of BAC16/SA, we have measured the UVvis, CD, and AFM spectra of transferred BAC16/DPBA films. Figure 5 shows the UV-vis and CD spectra of BAC16/DPBA mixture films which were transferred at 30 mN/m at various molar ratios in comparison with the UV-vis spectrum of DPBA in chloroform solution. DPBA in the chloroform solution showed a maximum absorbance at 485 nm, which is the same as that of BAC16 in the chloroform solution. This band could be ascribed to the intramolecular charge transfer. The absorption maximum of DPBA film (485 nm) did not shift, although that of the BAC16 film (450 nm) showed a blue-shift in comparison with the absorption maximum in chloroform solution. This implied that DPBA molecules, in contrast with the BAC16, did not form a H-aggregate in the film. In the mixed films, the absorption maximum showed the absorption band between that of the pure BAC16 and DPBA films. On the other hand, the CD signals could be detected for the mixed film with a small amount of DPBA, as shown in Figure 5. When the ratio of BAC16/DPBA was 10:1, the CD spectrum showed a positive and negative Cotton effect with a crossover at 435 nm. The CD intensity was almost the same as that for the BAC16 LB film. With an increase in the ratio of DPBA, the CD intensity rapidly decreased to zero. This indicated that the supramolecular chirality was destroyed by DPBA in the mixed films. Figure 6 shows the AFM images of one-layer mixed films of BAC16/DPBA at 10:1, 10:4, 10:7, and 10:10 at 15 mN/m. When the BAC16/DPBA was 10:1 (Figure 6a), orderly aligned spiral nanofibers were observed, which were very similar to those of BAC16 alone on the pure water surface. When the molar ratio of BAC16/DPBA was decreased to 10:4 (Figure 6b), the morphologies changed significantly, although some spiral

Figure 6. AFM images of one-layer mixed films of BAC16 and DPBA. BAC16/DPBA ) (a) 10:1, (b) 10:4, (c)10:7, and (d)10:10 at 15 mN/m (bar ) 500 nm).

structures could still be investigated. Some dotted domains were observed in the AFM images. These dotted aggregates were higher than the spirals, suggesting that they were squeezed from the BAC16 spirals. When the ratio of BAC16/DPBA was further decreased to 10:7 (Figure 6c), these dotted aggregates increased and randomly distributed over the flat films. Spiral domains disappeared completely, and a clear sublayer was shown in the AFM images (Figure 6d). 3. BAC16/ODA Mixture. Figure 7A shows the π-A isotherms for pure BAC16, pure ODA, and a BAC16/ODA mixture on the pure water surface at 15 °C. The isotherm of ODA was the same as reported before. With an increase in the ODA molar ratio, the phase transition of BAC16 disappeared gradually. Figure 7B shows the plot of the mean molecular area, Am, of the mixed film BAC16/ODA as a function of the percentage mole ratio of ODA at different surface pressure. The mean molecular areas showed a positive deviation from the ideal miscibility. Figure 8 shows the UV-vis and CD spectra of 30-layer mixed films of BAC16 and ODA, which were transferred at 30 mN/m at different molar ratios. Compared with the case of SA and DPBA, great changes of the UV-vis spectra were observed. When the molar ratio of BAC16/ODA was 10:1 (Figure 8a), the mixed film showed a maximum absorbance band at 470 nm, which could be assigned to the H-aggregation of BAC16. When the molar ratio of BAC16/ODA was 10:4 (Figure 8b), the maximum peak at 470 nm decreased and two new bands appeared at 400 and 340 nm. When the BAC16/ODA ratio was further increased to 10:7 (Figure 8c), the bands appeared at 465, 400, 335, 267, and 238 nm. When the BAC16/ODA ratio was 10:10 (Figure 8d), the band at 470 nm was very weak and the maximum absorbance appeared at 267 nm. It has been reported that the

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Figure 7. (A) Surface pressure-area (π-A) isotherms of BAC16/ODA mixed monolayers at different mixed ratios on the pure water surface at 15 °C. (B) The plot of the mean molecular area of the mixed film as a function of the molar ratio of ODA at different surface pressures.

Figure 8. UV-vis and CD spectra of BAC16/ODA mixtures (a) 10:1, (b) 10:4, (c) 10:7, and (d) 10:10.

barbituric acid headgroups were cleaved by the hydrolysis of a CdC double bond which linked the headgroup to the hydrophobic tail (retro-Knoevenagel reaction) with the presence of 2,4,6triaminopyrimidine in the subphase.25,26 This indicated that the 2,4,6-triaminopyrimidine inserted into the barbituric acid by H-bonding. Here ODA has a similar free amine group. Such a kind of spectral change might be due to the hydrolysis of BAC16 and the cleavage of the CdC double bond by ODA. The reaction mechanism will be discussed later. In the CD spectra, the CD signal changed greatly. When the BAC16/ODA ratio was 10:1, the CD signal decreased to become very weak. No CD signal was detected for the other films. From the UV-vis and CD spectra, we can see the ODA affect the H-aggregation and chirality greatly. The AFM images show different changes compared to the other two mixture systems. As BAC16/ODA was 10:1 (Figure 9a), the spiral nanofibers were not consecutive as BAC16 alone, although a spiral structure could be obtained. If the BAC16/ODA ratio was 10:4 (Figure 9b), the AFM images show broken fibers, and no spiral nanostructure was obtained. The spiral structure was broken completely. Increasing the ODA ratio (BAC16/ODA was 10:7, Figure 9c), fewer nanofibers could be observed. The fibers disappeared completely and a flat film was obtained when the BAC16/ODA was 10:10. (Figure 9d).

Discussion It has been shown from the above that the interactions between the BAC16 and matrix molecules are different depending on their structures. Using different kinds of matrix, both the chirality and surface morphologies of the films could be regulated. Such effects of the matrix were explained in Scheme 2. When BAC16 was spread on the water surface, the BAC16 molecules could cooperatively arrange in a helical sense to show chirality (Scheme (25) Bohanon, T. M.; Denzinger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.; Weck M. Angew. Chem., Int. Ed. Engl. 1995, 34, 58. (26) Yang, W.; Chai, X.; Chi, L.; Liu, X.; Cao, Y.; Lu, R.; Jiang, Y.; Tang, X.; Fuchs, H.; Li, T. Chem. Eur. J. 1999, 5, 1144.

Figure 9. AFM images of one-layer mixed films of BAC16 and ODA. BAC16/ODA ) (a) 10:1, (b) 10:4, (c) 10:7, and (d) 10:10 at 20 mN/m (bar ) 500 nm). Scheme 2. Cartoon Illustration on the Arrangement of the BAC16 and Other Molecules in the Mixed Films.a

a (a) BAC16 molecules; (b) BAC16 mixed with a small amount of SA; (c) BAC16 with a larger amount of SA; (d) BAC16 mixed with a small amount of DPBA; (e) BAC16 with a larger amount of DPBA.

2a). Due to the directionality of the H-bond between BAC16 headgroups and the larger steric hindrance, the molecules could arrange in a helical way to form the spiral nanoarchitectures. It is suggested that the larger headgroup in comparison with the small hydrophobic chain played an important role in forming such spiral structures. When a small amount of matrix molecules was mixed into the BAC16 monolayers, these molecules could only destroy a part of the arrangement of the BAC16 assemblies, and thus, the supramolecular chirality essentially remained (Scheme 2b and d). However, when a large amount of matrix molecules was added, these molecules destroyed the cooperative arrangement of BAC16 and the chirality disappeared (Scheme 2c and e). This verified our deduction that the supramolecular

LB Film of Achiral Barbituric Acid

Figure 10. FTIR spectra of BAC16 (a) and mixed films of BAC16/ ODA (b) 10:1, (c) 10:4, (d) 10:7, and (e) 10:10 on the CaF2 plate.

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completely. This indicated that the CdC double bond in BAC16 was cleaved and the cleaved barbituric acid was dissolved in the subphase. Therefore, in comparison with the intensity of the CH2 vibrations, those vibration bands decreased significantly when reacting with ODA. Such a mechanism was supported by the disappearance of CdC vibration at 1539 cm-1 after the reaction. Ringsdorf et al. studied the hydrolytic reaction of barbituric acid lipid with 2,4,6-triaminopyrimidine (TAP). They proposed that TAP inserted into the self-organized monolayer of barbituric acid by H-bonding and led to the cleavage of CdC double band.24 Similar mechanism can be applied to our systems. When the mixture of BAC16 and ODA were spread on the air/ water interface, the amino groups of ODA inserted into the headgroups of barbituric acid by H-bonding. The water molecules were trapped in the hydrophobic cleft and attacked the polarized double bonds. Due to the ordered molecular arrangement of both the BAC16 and ODA at the air/water interface, such a reaction proceeded rapidly.

Conclusion chirality of BAC16 was really due to the cooperative arrangement of the BAC16 molecules in a helical sense. On the other hand, due to the insertion of the matrix molecules into the BAC16 monolayers, the surface morphologies changed significantly. In the case of SA, the interaction between SA and BAC16 is essentially based on the alkyl chains. From the miscibility study, the alkyl chain showed a near-ideal miscibility, which decreases the free volume of the hydrophobic region of BAC16. As a result, the spiral structure changed into a nanofiber structure (Scheme 2c). In the case of DPBA, the self-association of BAC16 or DPBA was predominant so that phase separation was observed at a higher molar ratio of DPBA. When the DPBA was present in a small amount, it essentially strengthened the interaction between the headgroups. Therefore, we observed a clearer spiral structure. When the amount of DPBA increased, these molecules aggregated strongly to form a multilayer. These multilayered DPBA molecules blocked the aggregation of BAC16 and destroyed the chirality of the whole system. Furthermore, these molecules will be squeezed out from the BAC16 layer and form dotted domains in the AFM pictures. When BAC16 was mixed with ODA, the situation was completely different. A reaction between BAC16 and ODA occurred. It was observed that reaction proceeded rapidly at the air/water interface due to the ordered arrangement of the functional groups, while it was very slow in the chloroform solution. The progress of the reaction was verified by measuring the FT-IR spectra of the transferred films. Figure 10 shows the FTIR of BAC16 and mixed films at different ratios. The BAC16 LB films show that the N-H stretching band of barbituric acid appeared at 3052 and 3187 cm-1 (Figure 10a). It was weakened and then disappeared when increasing the ODA molar ratio. The carbonyl vibrations were observed at 1745, 1733, 1718, 1699, and 1683 cm-1 (Figure 10a).24,25 As the ODA ratio increased, the band ascribed to the CdO vibration disappeared. At a molar ratio of 1:1, only the band at 1683 cm-1 was left. Other carbonyl vibrations disappeared

Achiral BAC16 could form molecular assemblies showing both supramolecular chirality and spiral structures through the organization at the air/water interface. Three kinds of matrix molecules, SA, ODA, and a barbituric acid analogue (DPBA), could form mixed monolayers with BAC16 and regulate both the chirality and surface morphologies of the mixed films. BAC16 and SA showed a predominant attracting interaction between the headgroups at a lower surface pressure and good miscibility at a higher surface pressure. The other two matrix molecules showed a positive deviation from the ideal miscibility, indicating that the two components have a strong repulsion in the mixed monolayers. When small amount of matrix molecules (less than 10%) were mixed with BAC16, although the surface morphologies were changed, the supramolecular chirality still remained with a reduction in intensity. Depending on the used matrix molecules, surface morphologies changed differently. Addition of DPBA strengthened the interaction between the headgroups, and the spiral structures were enhanced. Mixing of SA led the spiral structure of BAC16 to nanofibers. Introduction of ODA to the mixed film caused the cleavage of BAC16 molecule in the monolayer where the reaction was accelerated due to the ordered arrangement of the functional groups. When larger amount of matrix molecules were mixed in the BAC16, the supramolecular chirality was completely destroyed. This indicated that the chirality of the BAC16 film was due to a stereo-cooperative stacking of the molecules and introduction of the matrix molecules can destroy such an arrangement of this chirality. The results provided an easy method to verify and regulate the supramolecular chirality of the assemblies from achiral molecules. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20533050 and 90306002), the Basic Research Development Program (2005CCA06600), and the Fund of the Chinese Academy of Sciences. LA0530586