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Supramolecular chirality of the hydrogen-bonded complex Langmuir–Blodgett film of achiral barbituric acid and melamine
Journal of Colloid and Interface Science 285 (2005) 680–685 www.elsevier.com/locate/jcis
Supramolecular chirality of the hydrogen-bonded complex Langmuir–Blodgett film of achiral barbituric acid and melamine Xin Huang a , Chao Li b , Siguang Jiang a , Xuesong Wang b , Baowen Zhang b , Minghua Liu a,∗ a CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, CAS, 100080, Beijing, People’s Republic of China b Technical Institute of Physics and Chemistry, CAS, 100101, Beijing, People’s Republic of China
Received 8 July 2004; accepted 1 December 2004 Available online 4 February 2005
Abstract Complex monolayers of barbituric acid and melamine were formed by spreading a chloroform solution of amphiphilic barbituric acid on the subphase of melamine solution. It was confirmed that the complex monolayer was formed through in situ complementary hydrogen bonding at the air–water interface. It was interesting to find that the complex LB films showed supramolecular chirality although both of the molecules were achiral, as verified by the circular dichroism spectral measurements. It was suggested that the π –π stacking of the neighboring barbituric acid and melamine group in a helical sense resulted in the chirality of the molecular assemblies. Due to the directionality of the hydrogen bonding, the BA-M film could form regular aligned nanofibers on the AFM images. Increasing the subphase temperature will lead to the decrease of CD intensity and the change of the morphologies. We suggested that the strength of the hydrogen bonding resulted in the difference. 2005 Elsevier Inc. All rights reserved. Keywords: Barbituric acid; Circular dichroism; Hydrogen bonding; LB film; Supramolecular chirality
1. Introduction Chirality plays an important role both in life and material sciences [1,2]. Besides the persisting research interest in chiral molecules, many efforts have been devoted to the chirality of supramolecular assemblies in recent years [3–5]. In a supramolecular level, the chirality of the whole system can be formed not only by the chiral molecules but also by the chiral building block in the system [6,7]. One extreme case is that achiral molecules could assemble into chiral supramolecular assemblies in an aggregate in solution [8,9], in crystals [10], in the Langmuir films [11], and in supramolecular assemblies [12–14]. This case is very important because it may relate to the origin of the chirality and even the origin of life. Recently, our group has focused on the supramolecular chirality of the interfacial organized molecular films from * Corresponding author. Fax: +86-10-62569564.
E-mail address: [email protected] (M. Liu). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.12.025
achiral amphiphiles and obtained some interesting results [15–20]. We have found that some organized molecular films of achiral compounds could show chirality if the molecules were rationally designed and assembled through noncovalent interactions, which involved J-aggregation, electrostatic interaction, coordination, and hydrogen bonding. The hydrogen bond is one of the most fascinating noncovalent interactions favored by nature [21,22]. In proteins, the hydrogen bonding between macromolecules cause them to fold into specific shapes such as α-helix or β-sheet, which is vital in realizing the physiological and biochemical functions. Similarly, it is of significant importance in DNA. Hydrogen bonding between the base pairs determine the link of the complementary strands and enable the replication. People are interested in the hydrogen bond for the same reasons and use it to investigate molecular recognition, self-assembly, biomimicking, and supramolecular chirality [23–27]. The complementary hydrogen bonds between melamine or 2,6-diaminopyrimidines and cyclic imides or barbituric acid are one of the most investigated hydrogen
X. Huang et al. / Journal of Colloid and Interface Science 285 (2005) 680–685
bonding systems [28–35]. They can form a 1:1 complex through H-bonding, which is like that in DNA, and various structures such as linear tapes, crinkled tapes, circular structures, and helical tapes have been proposed in this system [36–38]. Previously we reported that 5-(4-(N -methyl-N -hexadecylaminobenzylidene))-2,4,6-(1H,3H)-pyrimidinetrione (BA) could form a chiral supramolecular assembly through hydrogen bonding on the water surface although the component molecule was achiral. Furthermore, spiral nanoarchitectures were observed for the LB film due to the directionality of the H-bond [15]. Since BA could easily form complementary hydrogen bonding with melamine at the air–water interface, as a continuing work, we investigated the supramolecular chirality of achiral barbituric acid when melamine was added in the subphase in this paper. Although this kind of supramolecular assembly has been studied both in solution and in 2D organized molecular films, the supramolecular chirality of the LB films from achiral amphiphilic barbituric acid and melamine have not been reported. In this research we found that BA could easily form a 1:1 complex with melamine at the air/water interface. In the complex LB films, BA could form H-aggregates and show supramolecular chirality. Interesting morphologies were observed on their AFM images. While aligned nanofibers were observed for the film transferred at low temperature, distorted spiral structures were formed for the films transferred at higher temperature.
2. Materials and methods 2.1. Materials 5-(4-(N -Methyl-N -hexadecylaminobenzylidene))-2,4,6(1H,3H)-pyrimidinetrione (BA) was synthesized according to the literature and has been confirmed by NMR and elemental analysis. Melamine (M) was purchased from Acros Organics. Measurements of surface pressure–area (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). After the chloroform solution of BA (5×10−4 M) spread on the subphase of melamine solution (10−4 M) for 30 min, the π –A isotherms were measured with a compressing speed of 12 cm2 /min. The monolayer was transferred onto quartz and CaF2 plates for UV– vis, circular dichroism (CD), and FT-IR spectra measurements, respectively. JASCO UV-530, J-810 CD, and FT/IR660 plus spectrophotometers were used for UV–vis, CD, and FT-IR spectrum measurement, respectively. In the process of CD spectrum measurement, the multilayer film was 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 BA monolayer compressed at a certain
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Fig. 1. π –A isotherms of BA on the subphase of melamine solution (10−4 M) at different temperature.
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.
3. Results and discussion 3.1. Surface pressure–area (π –A) isotherms Fig. 1 shows the surface pressure–area (π –A) isotherms of BA monolayers spread on the aqueous solution of melamine (10−4 M) at various temperatures. The isotherms show inflection points and with the increase of the subphase temperature the inflection point become obvious. Comparing the isotherms with previous report, the inflection points and plateau regions on the melamine were not obvious as BA on the pure water surface. It has been reported that there existed strong hydrogen bonding between melamine and barbituric acid [28–38]. Present isotherms clearly indicated that such hydrogen bonding has been formed in accordance with previous studies. The complementary H-bonding could be illustrated as Fig. 2, where BA and melamine form 1:1 alternate hydrogen bonding linear tape. The complex monolayers formed at the air/water interface could be deposited onto solid substrate and the properties of the LB films were characterized by the UV–vis, CD spectra, and AFM. 3.2. UV–vis and CD spectra Fig. 3 shows the UV–vis and CD spectra of BA-M LB films deposited at different temperature of the subphase. In the UV–vis spectra, a peak band is observed at 460 nm with a shoulder at 485 nm. The peak band at 460 nm can be assigned to the H-aggregation of BA and the shoulder at 485 nm is ascribed to the intermolecular charge trans-
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Fig. 2. Linear tape of BA-M formed by complementary hydrogen bonding at the air–water interface.
Fig. 3. UV–vis and CD spectra of BA-M LB films at different subphase temperature.
fer. But the peak of H-aggregation of BA alone in LB film was at 450 nm. The UV–vis spectra clearly indicated the complex was formed in the organized molecular films. The supramolecular assemblies of BA-M are formed by directional six H-bonding, π –π stacking, and hydrophobic chain interactions. The hydrogen bonding network in the BA-M LB film was verified in the FT-IR spectra. The N–H stretching bands of BA at 3194 and 3060 cm−1 disappeared and new bands appeared at 3395 and 3220 cm−1 in the BA-M LB film. The carbonyl stretches of BA are observed at 1724, 1692, and 1660 cm−1 in KBr pellet. Upon assembling with melamine, the carbonyl vibrations changed to new positions at 1747, 1733, 1718, 1699, 1683, and 1670 cm−1 . These changes confirmed that the hydrogen bonds have been formed between BA and melamine [14]. Previously, we have found that BA film could show chirality in the LB film, although the BA itself was achiral. It was attributed to the overcrowded and cooperative stacking of the neighboring aromatic rings in a helical sense. In order to study the effect of melamine, we measured the CD spectra of BA-M films. Fig. 3 shows the CD spectra of BA-M films deposited at different temperatures. Two features are observed for the CD spectra. First, similarly to the case of BA LB films, the complex film also showed a strong split Cotton effect around 445 nm. This indicates that there existed an exciton coupling between the BA molecules. Second, the sign of the CD signals could be completely opposite in dif-
ferent deposition batches, as shown in Fig. 3. We suggested that the supramolecular chirality of BA-M film resulted from symmetry breaking on the air–water interface, whose direction was random [8]. These phenomena are essentially the same as we previously observed in the LB film [15–20]. However, BA-M films show two distinct different features from the BA films took from the water surface. It was observed that the CD intensity of BA-M film was almost double that of the BA films although the absorption intensity in the UV–vis spectra were almost equal. It means that BA-M could form a more conjugated structure than BA assemblies. Second, it is interesting to note that there exists another Cotton effect at 212 nm, which is not shown in the BA-LB film. The absorption band at 212 nm was assigned to the localized absorption of melamine [28]. It is the exciton interaction among the melamine aligned in the linear complementary hydrogen-bond networks. It is well known that when achiral molecules interacted with chiral compounds or put into a chiral environment, induced CD could be observed in the absorption band of achiral molecules [39,40]. In the present case, achiral melamine and BA formed chiral supramolecular assemblies. And more importantly, achiral melamine showed chirality upon interaction with the chiral assemblies of BA. That is to say, the chirality of achiral molecules could be formed by intermolecular interactions in a chiral matrix or environment no matter what the chiral environment is composed of chiral molecules or not. Although lots of researches have been reported on the chirality of achiral molecular assemblies induced by chiral molecules, it has not been reported that the chirality could form by the chiral assemblies which composed of achiral molecules. It presents a new way to give rise to the chirality of achiral molecules. Furthermore, we have observed that although the sign of BA signals appeared by chance, the sign of melamine always followed the chirality of BA. That is, when BA showed a positive CD, melamine showed positive CD, and vice versa, as shown in Fig. 3. The strength of hydrogen bonding is related to the temperature. In the above, we have found that temperature of the subphase could affect the monolayer behavior, as shown in Fig. 1. The intensity of CD decreased with the increase of the temperature. We suggest that at the higher temperature, the strength of hydrogen bonding is weakened and the
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Fig. 4. Angle dependence of the CD amplitude (triangles) and the background (squares) of the CD spectra while the BA-M LB film was rotated in steps of 10◦ within the sample plane.
supramolecular structures were less orderly. It will be further discussed in the following section of AFM measurements. 3.3. Confirmation of the CD spectra measurements In order to distinguish the intrinsic chirality from the possible parasitic artifacts, the angle dependence of the CD amplitude had been made [41]. As the LB plate was perpendicular to the light beam, the birefringence contribution was avoided. The artifacts could be canceled through averaging the CD spectra measured at the different rotating angles. We measured 36 CD spectra in a 10◦ step and the CD intensities at different angles were plotted against the rotating angle, as shown in Fig. 4. The background was determined by the difference between the values at 600 and 300 nm. It is clearly shown that both the CD intensities of the sample and the background can be approximated by a cosine (2β) function. The cosine function of the background situated around zero while that of the sample was positively shifted about 50 millidegrees. This is a clear indication that intrinsic chirality really existed in the LB film of BA-M. 3.4. AFM of the BA-M LB films It has been reported that supermolecules of barbituric acid and melamine in solution can form nanorods, nanofibers and helical structures [14,28,29,34]. We have reported that spiral architectures could be formed by BA itself on the water surface. The bulky groups of the BA were expected to change the supramolecular structures when melamine rings insert between the BA headgroups through complementary hydrogen bonding. We transferred one layer of the BA-M monolayer onto a freshly cleaved mica surface and measured their AFM pictures. Fig. 5 shows the AFM images for the films deposited at different subphase temperatures. At 20 ◦ C the regularly aligned fibers with width of 7 nm are observed. These morphologies are greatly different from spiral
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structure of BA alone. For the directionality of the hydrogen bonding, spiral structures were formed when BA was on the water surface. The insertion of melamine between the BA increased the rigidity of the linear tapes, and the complex molecular assemblies are difficult to wind into a spiral. However, due to the directionality of the complementary hydrogen bonding, the nanofibers orderly lined. It suggests that the different mode of hydrogen bonding will change the surface morphologies significantly. Temperature of the subphase greatly affected the surface morphologies. It could be observed that the surface morphologies changed significantly by the increase of the temperature. For the film deposited at 20 ◦ C, the nanofibers were very regular. At 30 ◦ C, nanofibers distorted and spirals were observed while all the lines disappeared at 40 ◦ C. It is well known that hydrogen bonding is greatly affected by the temperatures. With increasing the temperature of the subphase the hydrogen bonding could be weakened. Furthermore, at higher temperature, the aromatic rings could be free to rotate and the H-bonded BA-M complex will lost some rigidity and become flexible to some extent. At the same time, a little part of BA-M complex will disperse. Therefore, we could observe the decrease of the CD intensity and the ordered nanofibers. 3.5. Model for the chiral supermolecules The chirality of the supramolecular assemblies of BA and melamine can be explained by a helical model, as shown in Scheme 1. The BA and melamine assembled and formed linear tapes by complementary hydrogen bonding at the air– water interface. The air–water surface was limited in 2D and the long alkyl chains sterically interact. Due to the steric hindrance of the bulky group of BA-M, the neighboring headgroups were overcrowded and deviated from a certain plane. The symmetry breaking occurred at the air–water interface. If these headgroups were cooperative and twisted to some extent, a semihelical linear tape could form. A certain amount of the helical linear tapes aggregated and caused chiral nanofiber in which BA molecules assembled into Haggregates. So the LB films of BA-M could show CD signals. The direction of the starting aggregate was determined by chance so we could obtain opposite CD sign in different fabrication batches [41].
4. Conclusion Amphiphilic barbituric acid can form complex monolayers with melamine through in situ complementary hydrogen bonding. We found that the organized BA molecules assembled into H-aggregates. The LB films showed supramolecular chirality although both of the barbituric acid and melamine were achiral. It was suggested that the chirality was caused by the overcrowded stacking of BA and melamine in a helical sense. In addition, both the BA and melamine chromophore showed CD signals. The spiral
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Fig. 5. AFM images of one-layer BA-M LB films deposited at 20 mN/m at different temperatures: (a) 20 ◦ C; (b) 30 ◦ C; (c) 40 ◦ C.
Scheme 1. The helical linear tape of BA-M: side view (left) and top view (right).
X. Huang et al. / Journal of Colloid and Interface Science 285 (2005) 680–685
structures were formed by BA alone on the pure water surface but regular aligned nanofibers appeared on the AFM images of BA-M. We suggested that this difference was due to the change of the mode of hydrogen bonding. Temperature would affect the CD intensity and morphologies because at higher temperature the hydrogen bonding could be weakened.
Acknowledgments We thank Dr. Zhuqing Zhang for the helical model. This work was supported by the National Science Foundation of China (No. 20025312 and 90306002, 20273078) and the Major State Basic Research Development Program (G2000078103, 2002CCA03100) and the fund from the Chinese Academy of Sciences.
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Supramolecular chirality of the hydrogen-bonded complex Langmuir–Blodgett film of achiral barbituric acid and melamine Xin Huang a , Chao Li b , Siguang Jiang a , Xuesong Wang b , Baowen Zhang b , Minghua Liu a,∗ a CAS Key Laboratory of Colloid and Interface Science, Institute of Chemistry, CAS, 100080, Beijing, People’s Republic of China b Technical Institute of Physics and Chemistry, CAS, 100101, Beijing, People’s Republic of China
Received 8 July 2004; accepted 1 December 2004 Available online 4 February 2005
Abstract Complex monolayers of barbituric acid and melamine were formed by spreading a chloroform solution of amphiphilic barbituric acid on the subphase of melamine solution. It was confirmed that the complex monolayer was formed through in situ complementary hydrogen bonding at the air–water interface. It was interesting to find that the complex LB films showed supramolecular chirality although both of the molecules were achiral, as verified by the circular dichroism spectral measurements. It was suggested that the π –π stacking of the neighboring barbituric acid and melamine group in a helical sense resulted in the chirality of the molecular assemblies. Due to the directionality of the hydrogen bonding, the BA-M film could form regular aligned nanofibers on the AFM images. Increasing the subphase temperature will lead to the decrease of CD intensity and the change of the morphologies. We suggested that the strength of the hydrogen bonding resulted in the difference. 2005 Elsevier Inc. All rights reserved. Keywords: Barbituric acid; Circular dichroism; Hydrogen bonding; LB film; Supramolecular chirality
1. Introduction Chirality plays an important role both in life and material sciences [1,2]. Besides the persisting research interest in chiral molecules, many efforts have been devoted to the chirality of supramolecular assemblies in recent years [3–5]. In a supramolecular level, the chirality of the whole system can be formed not only by the chiral molecules but also by the chiral building block in the system [6,7]. One extreme case is that achiral molecules could assemble into chiral supramolecular assemblies in an aggregate in solution [8,9], in crystals [10], in the Langmuir films [11], and in supramolecular assemblies [12–14]. This case is very important because it may relate to the origin of the chirality and even the origin of life. Recently, our group has focused on the supramolecular chirality of the interfacial organized molecular films from * Corresponding author. Fax: +86-10-62569564.
E-mail address: [email protected] (M. Liu). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.12.025
achiral amphiphiles and obtained some interesting results [15–20]. We have found that some organized molecular films of achiral compounds could show chirality if the molecules were rationally designed and assembled through noncovalent interactions, which involved J-aggregation, electrostatic interaction, coordination, and hydrogen bonding. The hydrogen bond is one of the most fascinating noncovalent interactions favored by nature [21,22]. In proteins, the hydrogen bonding between macromolecules cause them to fold into specific shapes such as α-helix or β-sheet, which is vital in realizing the physiological and biochemical functions. Similarly, it is of significant importance in DNA. Hydrogen bonding between the base pairs determine the link of the complementary strands and enable the replication. People are interested in the hydrogen bond for the same reasons and use it to investigate molecular recognition, self-assembly, biomimicking, and supramolecular chirality [23–27]. The complementary hydrogen bonds between melamine or 2,6-diaminopyrimidines and cyclic imides or barbituric acid are one of the most investigated hydrogen
X. Huang et al. / Journal of Colloid and Interface Science 285 (2005) 680–685
bonding systems [28–35]. They can form a 1:1 complex through H-bonding, which is like that in DNA, and various structures such as linear tapes, crinkled tapes, circular structures, and helical tapes have been proposed in this system [36–38]. Previously we reported that 5-(4-(N -methyl-N -hexadecylaminobenzylidene))-2,4,6-(1H,3H)-pyrimidinetrione (BA) could form a chiral supramolecular assembly through hydrogen bonding on the water surface although the component molecule was achiral. Furthermore, spiral nanoarchitectures were observed for the LB film due to the directionality of the H-bond [15]. Since BA could easily form complementary hydrogen bonding with melamine at the air–water interface, as a continuing work, we investigated the supramolecular chirality of achiral barbituric acid when melamine was added in the subphase in this paper. Although this kind of supramolecular assembly has been studied both in solution and in 2D organized molecular films, the supramolecular chirality of the LB films from achiral amphiphilic barbituric acid and melamine have not been reported. In this research we found that BA could easily form a 1:1 complex with melamine at the air/water interface. In the complex LB films, BA could form H-aggregates and show supramolecular chirality. Interesting morphologies were observed on their AFM images. While aligned nanofibers were observed for the film transferred at low temperature, distorted spiral structures were formed for the films transferred at higher temperature.
2. Materials and methods 2.1. Materials 5-(4-(N -Methyl-N -hexadecylaminobenzylidene))-2,4,6(1H,3H)-pyrimidinetrione (BA) was synthesized according to the literature and has been confirmed by NMR and elemental analysis. Melamine (M) was purchased from Acros Organics. Measurements of surface pressure–area (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). After the chloroform solution of BA (5×10−4 M) spread on the subphase of melamine solution (10−4 M) for 30 min, the π –A isotherms were measured with a compressing speed of 12 cm2 /min. The monolayer was transferred onto quartz and CaF2 plates for UV– vis, circular dichroism (CD), and FT-IR spectra measurements, respectively. JASCO UV-530, J-810 CD, and FT/IR660 plus spectrophotometers were used for UV–vis, CD, and FT-IR spectrum measurement, respectively. In the process of CD spectrum measurement, the multilayer film was 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 BA monolayer compressed at a certain
681
Fig. 1. π –A isotherms of BA on the subphase of melamine solution (10−4 M) at different temperature.
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.
3. Results and discussion 3.1. Surface pressure–area (π –A) isotherms Fig. 1 shows the surface pressure–area (π –A) isotherms of BA monolayers spread on the aqueous solution of melamine (10−4 M) at various temperatures. The isotherms show inflection points and with the increase of the subphase temperature the inflection point become obvious. Comparing the isotherms with previous report, the inflection points and plateau regions on the melamine were not obvious as BA on the pure water surface. It has been reported that there existed strong hydrogen bonding between melamine and barbituric acid [28–38]. Present isotherms clearly indicated that such hydrogen bonding has been formed in accordance with previous studies. The complementary H-bonding could be illustrated as Fig. 2, where BA and melamine form 1:1 alternate hydrogen bonding linear tape. The complex monolayers formed at the air/water interface could be deposited onto solid substrate and the properties of the LB films were characterized by the UV–vis, CD spectra, and AFM. 3.2. UV–vis and CD spectra Fig. 3 shows the UV–vis and CD spectra of BA-M LB films deposited at different temperature of the subphase. In the UV–vis spectra, a peak band is observed at 460 nm with a shoulder at 485 nm. The peak band at 460 nm can be assigned to the H-aggregation of BA and the shoulder at 485 nm is ascribed to the intermolecular charge trans-
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Fig. 2. Linear tape of BA-M formed by complementary hydrogen bonding at the air–water interface.
Fig. 3. UV–vis and CD spectra of BA-M LB films at different subphase temperature.
fer. But the peak of H-aggregation of BA alone in LB film was at 450 nm. The UV–vis spectra clearly indicated the complex was formed in the organized molecular films. The supramolecular assemblies of BA-M are formed by directional six H-bonding, π –π stacking, and hydrophobic chain interactions. The hydrogen bonding network in the BA-M LB film was verified in the FT-IR spectra. The N–H stretching bands of BA at 3194 and 3060 cm−1 disappeared and new bands appeared at 3395 and 3220 cm−1 in the BA-M LB film. The carbonyl stretches of BA are observed at 1724, 1692, and 1660 cm−1 in KBr pellet. Upon assembling with melamine, the carbonyl vibrations changed to new positions at 1747, 1733, 1718, 1699, 1683, and 1670 cm−1 . These changes confirmed that the hydrogen bonds have been formed between BA and melamine [14]. Previously, we have found that BA film could show chirality in the LB film, although the BA itself was achiral. It was attributed to the overcrowded and cooperative stacking of the neighboring aromatic rings in a helical sense. In order to study the effect of melamine, we measured the CD spectra of BA-M films. Fig. 3 shows the CD spectra of BA-M films deposited at different temperatures. Two features are observed for the CD spectra. First, similarly to the case of BA LB films, the complex film also showed a strong split Cotton effect around 445 nm. This indicates that there existed an exciton coupling between the BA molecules. Second, the sign of the CD signals could be completely opposite in dif-
ferent deposition batches, as shown in Fig. 3. We suggested that the supramolecular chirality of BA-M film resulted from symmetry breaking on the air–water interface, whose direction was random [8]. These phenomena are essentially the same as we previously observed in the LB film [15–20]. However, BA-M films show two distinct different features from the BA films took from the water surface. It was observed that the CD intensity of BA-M film was almost double that of the BA films although the absorption intensity in the UV–vis spectra were almost equal. It means that BA-M could form a more conjugated structure than BA assemblies. Second, it is interesting to note that there exists another Cotton effect at 212 nm, which is not shown in the BA-LB film. The absorption band at 212 nm was assigned to the localized absorption of melamine [28]. It is the exciton interaction among the melamine aligned in the linear complementary hydrogen-bond networks. It is well known that when achiral molecules interacted with chiral compounds or put into a chiral environment, induced CD could be observed in the absorption band of achiral molecules [39,40]. In the present case, achiral melamine and BA formed chiral supramolecular assemblies. And more importantly, achiral melamine showed chirality upon interaction with the chiral assemblies of BA. That is to say, the chirality of achiral molecules could be formed by intermolecular interactions in a chiral matrix or environment no matter what the chiral environment is composed of chiral molecules or not. Although lots of researches have been reported on the chirality of achiral molecular assemblies induced by chiral molecules, it has not been reported that the chirality could form by the chiral assemblies which composed of achiral molecules. It presents a new way to give rise to the chirality of achiral molecules. Furthermore, we have observed that although the sign of BA signals appeared by chance, the sign of melamine always followed the chirality of BA. That is, when BA showed a positive CD, melamine showed positive CD, and vice versa, as shown in Fig. 3. The strength of hydrogen bonding is related to the temperature. In the above, we have found that temperature of the subphase could affect the monolayer behavior, as shown in Fig. 1. The intensity of CD decreased with the increase of the temperature. We suggest that at the higher temperature, the strength of hydrogen bonding is weakened and the
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Fig. 4. Angle dependence of the CD amplitude (triangles) and the background (squares) of the CD spectra while the BA-M LB film was rotated in steps of 10◦ within the sample plane.
supramolecular structures were less orderly. It will be further discussed in the following section of AFM measurements. 3.3. Confirmation of the CD spectra measurements In order to distinguish the intrinsic chirality from the possible parasitic artifacts, the angle dependence of the CD amplitude had been made [41]. As the LB plate was perpendicular to the light beam, the birefringence contribution was avoided. The artifacts could be canceled through averaging the CD spectra measured at the different rotating angles. We measured 36 CD spectra in a 10◦ step and the CD intensities at different angles were plotted against the rotating angle, as shown in Fig. 4. The background was determined by the difference between the values at 600 and 300 nm. It is clearly shown that both the CD intensities of the sample and the background can be approximated by a cosine (2β) function. The cosine function of the background situated around zero while that of the sample was positively shifted about 50 millidegrees. This is a clear indication that intrinsic chirality really existed in the LB film of BA-M. 3.4. AFM of the BA-M LB films It has been reported that supermolecules of barbituric acid and melamine in solution can form nanorods, nanofibers and helical structures [14,28,29,34]. We have reported that spiral architectures could be formed by BA itself on the water surface. The bulky groups of the BA were expected to change the supramolecular structures when melamine rings insert between the BA headgroups through complementary hydrogen bonding. We transferred one layer of the BA-M monolayer onto a freshly cleaved mica surface and measured their AFM pictures. Fig. 5 shows the AFM images for the films deposited at different subphase temperatures. At 20 ◦ C the regularly aligned fibers with width of 7 nm are observed. These morphologies are greatly different from spiral
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structure of BA alone. For the directionality of the hydrogen bonding, spiral structures were formed when BA was on the water surface. The insertion of melamine between the BA increased the rigidity of the linear tapes, and the complex molecular assemblies are difficult to wind into a spiral. However, due to the directionality of the complementary hydrogen bonding, the nanofibers orderly lined. It suggests that the different mode of hydrogen bonding will change the surface morphologies significantly. Temperature of the subphase greatly affected the surface morphologies. It could be observed that the surface morphologies changed significantly by the increase of the temperature. For the film deposited at 20 ◦ C, the nanofibers were very regular. At 30 ◦ C, nanofibers distorted and spirals were observed while all the lines disappeared at 40 ◦ C. It is well known that hydrogen bonding is greatly affected by the temperatures. With increasing the temperature of the subphase the hydrogen bonding could be weakened. Furthermore, at higher temperature, the aromatic rings could be free to rotate and the H-bonded BA-M complex will lost some rigidity and become flexible to some extent. At the same time, a little part of BA-M complex will disperse. Therefore, we could observe the decrease of the CD intensity and the ordered nanofibers. 3.5. Model for the chiral supermolecules The chirality of the supramolecular assemblies of BA and melamine can be explained by a helical model, as shown in Scheme 1. The BA and melamine assembled and formed linear tapes by complementary hydrogen bonding at the air– water interface. The air–water surface was limited in 2D and the long alkyl chains sterically interact. Due to the steric hindrance of the bulky group of BA-M, the neighboring headgroups were overcrowded and deviated from a certain plane. The symmetry breaking occurred at the air–water interface. If these headgroups were cooperative and twisted to some extent, a semihelical linear tape could form. A certain amount of the helical linear tapes aggregated and caused chiral nanofiber in which BA molecules assembled into Haggregates. So the LB films of BA-M could show CD signals. The direction of the starting aggregate was determined by chance so we could obtain opposite CD sign in different fabrication batches [41].
4. Conclusion Amphiphilic barbituric acid can form complex monolayers with melamine through in situ complementary hydrogen bonding. We found that the organized BA molecules assembled into H-aggregates. The LB films showed supramolecular chirality although both of the barbituric acid and melamine were achiral. It was suggested that the chirality was caused by the overcrowded stacking of BA and melamine in a helical sense. In addition, both the BA and melamine chromophore showed CD signals. The spiral
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X. Huang et al. / Journal of Colloid and Interface Science 285 (2005) 680–685
Fig. 5. AFM images of one-layer BA-M LB films deposited at 20 mN/m at different temperatures: (a) 20 ◦ C; (b) 30 ◦ C; (c) 40 ◦ C.
Scheme 1. The helical linear tape of BA-M: side view (left) and top view (right).
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structures were formed by BA alone on the pure water surface but regular aligned nanofibers appeared on the AFM images of BA-M. We suggested that this difference was due to the change of the mode of hydrogen bonding. Temperature would affect the CD intensity and morphologies because at higher temperature the hydrogen bonding could be weakened.
Acknowledgments We thank Dr. Zhuqing Zhang for the helical model. This work was supported by the National Science Foundation of China (No. 20025312 and 90306002, 20273078) and the Major State Basic Research Development Program (G2000078103, 2002CCA03100) and the fund from the Chinese Academy of Sciences.
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