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Supramolecular Chirality Transfer through Alkyl Chain Entanglement and a Chiroptical Switch Based on Dynamic Covalent Bonding
Published on 01 October 2013. Downloaded by Washington University in St. Louis on 02/10/2013 19:39:32.
Kai Lv, Long Qin, Xiufeng Wang, Li Zhang*, and Minghua Liu* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Chirality transfer is an interesting phenomenon in nature, which represents an important step to understand the evolution of chiral bias and the amplification of the chirality. In this paper, we reported the chirality transfer via the entanglements of the alkyl chains between chiral gelator molecules and achiral amphiphilic Schiff base. We have found that although an achiral Schiff base amphiphile could not form organogels in any kind of organic solvents, it formed co-organogels when mixed with a chiral gelator molecule. Interestingly, the chirality in the gelator molecules was transferred into the Schiff base chromophore in the mixed co-gels and there was a maximum mixing ratio for the chirality transfer. Furthermore, the supramolecular chirality was also produced based on a dynamic covalent chemistry of imine between the aldehyde and amine reaction. Such covalent bond of imine was formed reversibly depending on the pH variation. When covalent bond was formed the chirality transfer occurred, while it was destroyed, the transfer stopped. Thus, a supramolecular chiroptical switch is obtained based on supramolecular chirality transfer and dynamic covalent chemistry.
Introduction 20
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The induction, transfer, amplification and memory of chirality are intriguing issues in exploring the origin of homochiral structures in nature1-3 and for possible application in the various fields such as enantioselective synthesis, sensing, drug delivery and molecular device applications.4-9 The process of chirality transfer can be realized either in a synthetic process or by virtue of supramolecular chemistry approach.10-15 In the supramolecular way, the chirality transfer can be realized from a chiral matrix or template to achiral molecules through various noncovalent interactions such as π-π stacking, hydrogen bonding, electrostatic and hydrophobic interactions.16-23 These noncovalent interactions should be appropriate to preserve the initial chiral matrix and to induce the chiral information transferring. In many cases, the non-covalent interaction is easy to be broken and the process could be reversible, which provides many opportunity for developing chirality-involved stimuli-responsive properties or new applications.24-30 For example, the formation and breakage of the interactions between chiral matrix and achiral dopants make it easy to realize the switch of supramolecular chirality between “on” and “off” state. Thus, this strategy will be convenient to fabricate chiroptical switch.31-36 In the chiral organogel systems, apart from thermal responsiveness, other external stimuli including light irradiation and redox have been used to trigger such on-off state of supramolecular chirality.37-39 While the individual gelators possessing noncovalent interactions such as hydrogen bonding, electrostatic interactions, hydrophobic interactions or metal-ligand coordination are widely investigated, the mixing of the two components will also help to fabricate the This journal is © The Royal Society of Chemistry [year]
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chiral nanostructures. In these cases, the H-bond and electrostatic pairing of the two components are generally necessary to fabricate the chiral structures. The interchain or hydrophobic interaction is possible to form the chiral nanostructure although it is scarcely reported.23 Previously, we have found that when achiral porphyrin mixed with a chiral gelator, the chirality can be transferred to the porphyrin assemblies. However, there still remain important issues that how such transfer occurred and if such alkyl chain entanglements could be extended to other systems. In addition, during such chiral transfer through the alkyl chain entanglement, how about the effect of the mixed ratio of alkyl chains belong to chiral gelator and achiral molecules on the transfer capacity? In this paper, we developed a supramolecular chiral organogel system formed by a chiral gelator and an achiral Schiff base, whose supramolecular chirality was produced through the transfer via the alkyl chain entanglements. Furthermore, we have sought out the optimum mixing ratio of gelator to the achiral molecules and realized a chiroptical switch based on a dynamic covalent bond of Schiff base. Schiff base compounds contain an imine bond formed by the reversible condensation of amine derivatives and aldehydes. The formation and cleavage of the imine bond have been extensively explored in the self-assembly systems and a large number of novel nanostructures as well as responsive properties appeared through such dynamic imine chemistry.40-44 In this paper, we have extended such dynamic covalent bond into the organogel system. We have found that although the amphiphilic achiral Schiff base 2((octadecylimino)methyl)phenol (OMP) could not form organogels in any organic solvents, it formed co-organogels [journal], [year], [vol], 00–00 | 1
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when mixed with LBG (N, N’-bis(octadecyl)-L-Boc-glutamic diamide), which is a universal gelator found in our previous study.23,45 Upon gel formation, supramolecular chirality appeared in the Schiff base chromophore, which presumably come from the chirality transfer through interchain interactions. Further, once the imine bond was destroyed, the chirality transfer would be stopped. The imine bond can be subsequently recovered under alkaline conditions and the chirality reappeared. Thus, a chiroptical switch can be realized through the formation and cleavage of the covalent imine bond, during which the organogels were not broken.
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Scheme 1. Molecular structure of the non-gelator Schiff base OMP (A) and the gelator molecule BG (B).
Experimental section
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Materials The common chemical reagents were used as purchased from commercial chemical company without further purification. The synthesis method of the N,N’-bis(octadecyl)-L(D)-Boc-glutamic diamide (LBG and DBG) was reported previously by our group.23 Gel formation The gelator (BG, 5mg) and OMP (different molar fraction to BG) were mixed with 0.5mL ethanol in a capped test tube and the mixture was heated until the solid was completely dissolved. The hot solution was naturally cooled at room temperature and then the gel formed in a few minutes through the confirmation with the inversed test tube. The co-organogels containing metal ions were prepared by introducing the Cu(Ac)2 with 0.5 equivalent of OMP into BG ethanol system. The mixture was heated until a uniform transparent solution was obtained. After cooling to the room temperature, the co-gels were subsequently formed. During the investigation on the responsiveness of the organogels to acid or base, equimolar HCl or triethylamine was added into the OMP doped BG/ethanol gels alternatively. The xerogels obtained from ethanol were used for SEM measurements, and the organogels in ethanol were directly used for UV-vis and CD spectral measurements. Characterization 1 H NMR spectra were recorded on a Bruker AV400 spectrometer. Matrix-assisted laser desorption ionization time-of flight mass spectrometry (MALDI-TOF MS) were recorded on a BIFLEIII instrument. Elemental analysis was performed on a Carlo-Erba1106 instrument. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 FE-SEM microscope. The fully aging gels were cast onto single-crystal silica plates (Pt coated), and the trapped solvents in gels were evaporated at ambient conditions first, then vacuum-dried for 12 hours for SEM measurements. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 FTIR spectrometer at room temperature. The KBr pellets made from the vacuum-dried samples were used for FT-IR spectra measurements. UV-Vis spectra were measured on a Hitachi U-3900 UV-VIS spectrophotometer. The CD spectra were performed on a JASCO815 spectrometer. The 0.1mm quartz cells were used for UV-Vis and CD measurements.
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Results and discussion Gel formation and chirality transfer
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The Schiff base OMP is synthesized from the octadecylamine (C18H37NH2) and salicyaldehyde, and this compound can be dissolved in many organic solvents but cannot form organogels in any solvents. The LBG has been reported to be a universal gelator, which can drive various solvents and materials to form co-organogels.23,45 Scheme 1 shows the molecular structure of the Schiff base OMP and the gelator molecule BG. When OMP was mixed with the gelator LBG, it could form organogels. Different amount of OMP was doped into LBG ethanol organogels when the LBG concentration was kept at 10 mg/mL (Fig. 1A). The samples were heated to produce a homogeneous solution and then cooled to room temperature, in which the gels formation was confirmed by inverted test tube, as shown in Fig. 1A. The doped amount of OMP (molar fraction) in the co-organogels could be as high to 68% (the critical amount of OMP to destroy gels is 70%), indicating that the LBG possessed strong combined capacity with OMP. With the increase of OMP, the co-gels gradually turned faint yellow. In order to further investigate the gel structures, the organogels with various mix ratios of OMP and LBG were measured with the SEM, as shown in Fig. 1B and Fig. S1. The twist structures are observed for all the samples. In addition, in a large area, there were no other structures. This indicated that OMP and LBG molecules were miscible between 5-68% without any phase separation. Furthermore, the co-gels of LBG/OMP exhibited as left-handedness. If we use DBG to perform the same gel formation, similar results were obtained. The obtained twist of OMP/DBG, however, was right-handed. This indicated that upon formed the co-gel, the chirality of BG could be transferred to the co-gel, which followed the chirality of the gelator molecules. In order to further characterize the gel properties, both UV-Vis and CD spectra were measured, as shown in Fig. 2. When OMP was mixed with LBG to form the co-organogels, an obvious CD signal was seen in Fig. 2A. A strong Cotton effect appeared at 260 nm and a weak Cotton effect existed at 320 nm, which is the characterized adsorption band of imine, indicating that the chirality of LBG was transferred to Schiff base chromophore through the hydrophobic interaction between alkyl chains of LBG and OMP. In order to further confirm such chirality transfer, we used the gelator with opposite chirality, DBG, to perform the same spectral measurements. The mirror CD spectrum of LBG/OMP and DBG/OMP organogels were obtained, as shown in Figure 2A, implying that during the chirality transfer, the supramolecular chirality followed the chirality of the gelators. The CD spectra of LBG/OMP co-gels with different molar fraction were monitored, and the Cotton effects at 260 nm were observed in all samples. Although the absorption increased This journal is © The Royal Society of Chemistry [year]
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Figure 1. (A) The photo images of OMP/LBG ethanol organogels with different molar fraction of OMP. (B) SEM images of OMP/LBG xerogels at OMP molar fraction at a) 30%, b) 50%, c) 24%, d) OMP/DBG ( the OMP molar fracrion is 24%). The scale bar is 1μm.
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Figure 2. (A) CD (up) and UV-vis (down) spectra of LBG/OMP and DBG/OMP ethanol organogels , the OMP moalr fraction is 24% (B) The variation of G-Value at 260nm as t molar fraction of OMP in OMP/LBG organogels. 25
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Figure 3. CD spectra (G Value) of the LBG/OMP gels in various solvents. a) cyclohexane, b) ethanol, c), methanol d) ethanol+5% H2O, e) ethanol+10% H2O.
As shown above, the hydrophobic interaction plays an important role in the chirality transfer. Therefore, the factors that affect the hydrophobic interactions, especially solvents, will be expected to influence the packing of the alkyl chains. We have investigated the co-gel formation of OMP with LBG or DBG in various organic solvents and compared their CD spectra. In order to compare the effect of the solvent, we also used the G value as a function of wavelength to evaluate such differences. Fig. 3 screens the G values of OMP/LBG organogels in the organic solvents varying from nonpolar cyclohexane, hexane, to polar ethanol and methanol and so on. It is found that with increasing the solvent polarity, the positive Cotton effect at around 260 and 320 nm become more obvious. For the gels from non-polar solvents, the Cotton effect was very weak. For example, the cyclohexane gel showed signals fluctuation along zero, suggesting that the chirality was almost not being transferred from LBG chiral matrix. The most significant Cotton effect was observed in the ethanol and methanol gels. Furthermore, when increasing the polarity of the solvent by adding small amount of water into ethanol, the CD intensity increased accordingly, as can be seen in Fig. 3. This suggested that the hydrophobic interaction between the alkyl chains of LBG and OMP in non-polar cyclohexane or hexane is too weak to perform the chirality transfer, while polar solvents favor the chiral transfer through enhancing the hydrophobic interactions. It should be noted although the addition of water into ethanol could enhance the chirality transfer, when the water amount was above 10%, the chiral matrix of LBG would be broken and no gel was formed. As a consequence, the CD signals of OMP disappeared. Chiroptical switch based on a dynamic covalent bonding From the data above, it seems that the chirality was transferred through the alkyl chain entanglement. To further verify this we have performed the following experiments, we mixed LBG with salicyaldehyde, LBG with octadecylamine. The CD signals were silent for both C18H37NH2-doped and salicylaldehyde-doped LBG organogels. For co-gels containing salicylaldehyde, which showed obvious adsorption band at 260 nm (Fig. 4) due to the π-π transition, the CD was also silent, indicating that the chirality of LBG was not transferred to salicylaldehyde. Since OMP is synthesized from the octadecylamine (C18H37NH2) and salicyaldehyde, we then mixed LBG/octadeyclamine gel with Journal Name, [year], [vol], 00–00 | 3
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linearly with the OMP ratio, the CD spectra did not. In order to compare the CD signals or the chirality transfer ability, the Gvalue, which is the ratio of CD signal to the UV-Vis absorbance, at 260 nm function as molar fraction of OMP was described, as shown in Fig. 2B. An interesting phenomenon was observed. The G-value of the co-gel system increased firstly with the amount of OMP, and reached a maximum at about the molar fraction 24%. After that the G-value of OMP/LBG gels decreased with increasing OMP. These results indicated that upon mixing gelator and achiral molecules, there is a maximum ratio for the chirality transfer. In the present case, the optimal amount of OMP in LBG gels is around 24%, and we chose the system containing OMP molar fraction 24% as the gelation in the following work.
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salicyaldehyde. After a heating and cooling cycle, the organogels with faint yellow color was obtained. Meanwhile, the CD signals was observed at 260nm, which is similar to that of organogels of OMP/LBG. This result indicated that octadecylamine and salicylaldehyde formed Schiff base by heating and imine formation is crucial in the chirality transfer from LBG assemblies to Schiff base moiety through the hydrophobic interaction.
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Figure 4. (A) CD (up) and UV-vis spectra (bottom) of a) LBG/ C18H37NH2, b) LBG/salicyaldehyde,, c) LBG/ C18H37NH2/ salicyaldehyde gels after a heat to cool cycle.
It is known that the imine bond could be reversibly switched by acid and base.44 When the pH is tuned down to slightly acidic, the imine bond can be hydrolyzed. Subsequent addition of base can cause the reformation of imine bond. So we investigated the LBG(DBG)/OMP organogels with alternative addition of hydrochloric acid and triethylamine. As shown in the Fig. 5A, when the acid was added into the gel system, the CD signals disappeared as the result of the broken of imine bond. However, when triethylamine was subsequently added into the gel system, the CD spectrum recovered to the initial state. The obvious Cotton effect was observed at 260nm and a weak CD signal followed at 320nm, implying that the chirality transfer route was connected again as the consequence of reformation of imine bond. This reversible CD signal on-off upon acid/base alternative treatment can be repeated several times, showing that a acid-base responsive chiroptical switch can be realized based on the dynamic covalent interactions. It is well-known that the Schiff base can coordinate with many transition metal ions, so it is interesting to explore whether the chiroptical switch can be still realized when the Schiff base was coordinated with metal ions. The Cu2+ ion was selected owing to its ability to obviously enhance the CD signals of OMP/LBG gels, Fig. 5B shows the CD spectra of organogels in ethanol with the addition of Cu2+. The addition of Cu2+ ions caused the remarkable increase of CD signals. Moreover, the new positive bands at 375 nm was observed, which is ascribed to the charge transfer band from ligand to Cu2+.46,47 When acid was introduced into the gels, the chromophore moiety were apart from the alkyl chains, the CD signals (Fig. 5B) in 320 nm and 375 nm disappeared completely, while the CD band at 260 nm decreased remarkably, indicating that the interaction between the OMP molecules and Cu2+ ions was broken. Then we added the triethylamine into the system, the 4 | Journal Name, [year], [vol], 00–00
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gels can return to the initial situation at the appearance sight. And the CD spectra recovered to the initial state system. In View Article Online addition, when DBG was used, similar chiroptical switch can be DOI: 10.1039/C3CP53620C obtained but the CD signals were just opposite (Fig. 5B).
Figure 5. CD spectra of the LBG(DBG)/OMP (A) and LBG(DBG)/OMP/Cu2+ gels with the acid-base response. (The letters in picture A represent different gels: L-a LBG/OMP; L-b LBG/OMP/Acid; Lc LBG/OMP/Acid/Base; D-a DBG/OMP; D-b DBG/OMP/Acid; D-c DBG/OMP/Acid/Base. The letters in picture B represent different gels: La LBG/OMP/Cu2+; L-b LBG/OMP/Cu2+/Acid; L-c LBG/OMP/Cu2+/Acid/Base; D-a DBG/OMP/Cu2+; D-b DBG/OMP/Cu2+/Acid; D-c DBG/OMP/Cu2+/Acid/Base.)
The dynamic formation of imine covalent bond upon treatment of acid and base was further confirmed by FT-IR spectra, as shown in Fig. 6. The OMP showed strong vibrations at 1635, 1580, and 1470cm-1, which can be ascribed to the vibrations of C=N band, aromatic rings skeleton, scissor vibration of the CH2 group, respectively.48
Figure 6. FT-IR spectra of a) OMP, b) OMP+Cu2+, c) OMP+Cu2++Acid, d) OMP+Cu2++Acid+Base.
When Cu2+ ions were added, the C=N band shifted from 1635cmto 1623cm-1 and the vibration of the aromatic rings shifted from l580cm-1 to 1540cm-1, suggesting that the OH and the imine groups have coordinated with the Cu2+. When acid was added to the system, the vibration band of C=N band has disappeared, indicating that the imine group was destroyed by the acid, and the coordination with the Cu2+ ions was also destroyed, as characterized by the red shift of aromatic rings skeleton vibration. The new broad band at about 3130cm-1 can be ascribed to the vibration of N-H group, which also indicated the hydrolyzing of Schiff base. When base was subsequently added to the system, 1
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Scheme 2. (a) A Schiff base based on a dynamic covalent bond. (b) The gelator BG molecules self-assembled into gels through hydrophobic interactions and H-bonding in ethanol. When the Schiff base OMP molecules were added into the system, the OMP molecules can be inserted into the alkyl chains of BG molecules. Based on the hydrophobic interaction, a supramolecular assembly with twist structure can be formed, and the supramolecular chirality can be transferred from BG molecules to Schiff base moiety. The supramolecular chirality showed “on” and “off” state through the alternate treatment of acid and base.
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In conclusion, an achiral Schiff base with long alkyl chain was found to form organogel with an amphiphilic glutamide gelator although the Schiff base could not form organogels itself. The supramolecular chirality of amphiphilic Schiff base was induced by the chiral gelator via the interchain interactions, the chirality of which followed the gelator chirality. On account of the dynamic covalent chemistry of imine in amphiphilic Schiff base, the salicyaldehyde moiety can be connected or departed with long alkyl chain upon treatment by base or acid. During such process, the chirality transfer path was opened or closed alternately, leading to the emergence or disappearance of Schiff base supramolecular chirality. A acid-base driven chiroptical switch could be realized based on both chirality transfer and dynamic covalent chemistry.
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The authors are grateful to National Natural Science Foundation of China (Nos. 91027042, 21021003 and 21227802), Basic Research Development Program (2013CB834504), and the Fund of the Chinese Academy of Sciences.
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Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. Email:[email protected], [email protected] † Electronic Supplementary Information (ESI) available: Synthesis, characterization of the compond OMP, and SEM images of OMP/LBG(DBG) xerogels at different OMP molar fraction. See DOI: 10.1039/b000000x/
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the vibration band of the imine group was detected again at 1623cm-1, similar to the system before acid was added. These FT-IR spectral changes indicated that acid and base influenced the imine group and consequently on the coordination between Cu2+ ions and Schiff base. Based on the above results, it is obvious that the cooperative interaction between OMP and LBG or DBG plays an important role in realizing the gel formation, chirality transfer and the chiroptical switch. The process can be illustrated in Scheme 2. LBG or DBG could easily form organogels and self-assemble into fiber or twist structures through the H-bond between three amide groups and the hydrophobic interactions. When OMP was added into BG matrix, due to the good affinity of the alkyl chains, they may entangle each other to form a chiral twist structures. With this interaction, the chirality was transferred from LBG or DBG to whole assemblies including the OMP moiety. Therefore, we observed the Cotton effect for the Schiff base moiety. In this case, the chirality of the whole system followed the chirality of the gelator molecules. If we only mixed the salicylaldehyde, no CD spectra were detected, indicating that the interaction between the alkyl chain of OMP and LBG is very important. It should be noted that during such chirality transfer, the ratio of the gelator molecules and the achiral molecules is also important. In the mixed ratio range when organogels were formed, it revealed that both OMP and LBG or DBG formed only one kind of nanostructures and the absorption was linearly increased with the increment of OMP, however, the chirality signal showed a maximum at around 24%. This means that there existed a maximum combination of the two components for the chirality transfer. On the other hand, the amphiphilic Schiff base has a dynamic imine covalent, which could be reversibly switched by acid/base reaction. OMP molecules can be disconnected into two building blocks under acid condition. When acid was added into the gel system, the Schiff base was hydrolyzed although the gel was maintained. Then the route of chirality transfer through the alkyl chain was cut off. Therefore, we cannot detect the CD spectra for the acid regulated gels, as shown in Scheme 2b. The imine bond can be easily reformed by base. By subsequently addition of triethylamine into the system, the Schiff base formed again and the supramolecular chirality of the gel was recovered. Thus, a chiroptical switch based on dynamic imine covalent can be realized by the alternate addition of acid and base.
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R. Fasel, M. Parschau, K.-H. Ernst, Angew. Chem., Int. Ed., 2003, 42, 5178–5181. R. Purrello, A. Raudino, L. M. Scolaro, A. Loisi, E. Bellacchio, R. Lauceri, J. Phys. Chem. B, 2000, 104, 10900–10908. A. R. A. Palmans, E. W. Meijer, Angew. Chem., Int. Ed., 2007, 46, 8948–8968. N. Ousaka, Y. Takeyama, E. Yashima, Chem. Sci., 2012, 3, 466-469. T. Tu, W. Fang, X. Bao, X. Li, K. H. Doetz, Angew. Chem., Int. Ed. 2011, 50, 6601-6605. M. Siczek, P. J. Chmielewski, Angew. Chem., Int. Ed., 2007, 46, 7432-7436. L. You, J. S. Berman, E. V. Anslyn, Nat. Chem., 2011, 3, 943-948. W. Zhang, W. Jin, T. Fukushina, N. Ishii, T. Aida, J. Am. Chem. Soc., 2013, 135, 114-117. S. Shinoda, Chem. Soc. Rev., 2013, 42, 1825-1835. B. L. Feringa, R. A. van Delden, Angew. Chem., Int. Ed., 1999, 38, 3419-3438. L. Pu, Chem. Rev., 2004, 104, 1687-1716. R. M. Hazen, D. S. Sholl, Nature Mater., 2003, 2, 367-374. M. Suzuki, K. Hanabusa, Chem. Soc. Rev., 2009, 38, 967-975. B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geertsema, Chem. Rev., 2000, 100, 1789-1816. D. K. Smith, Chem. Soc. Rev., 2009, 38, 684-694. J. J. D. de Jong, T. D. Tiemersma-Wegman, J. H. van Esch, B. L. Feringa, J. Am. Chem. Soc., 2005, 127, 13804-13805. A. Ajayaghosh, R. Varghese, S. J. George, C. Vijayakumar, Angew. Chem., Int. Ed., 2006, 45, 1141-1144. A. R. Hirst, D. K. Smith, in Low Molecular Mass Gelators: Design, Self-Assembly, Function, ed. F. Fages, 2005, 256, 237-273. A. R. Hirst, D. K. Smith, M. C. Feiters, H. P. M. Geurts, Chem. -Eur. J., 2004, 10, 5901-5910. S. Bhattacharya, S. K. Samanta, Langmuir, 2009, 25, 8378-8381. M. George, R. G. Weiss, Acc. Chem. Res., 2006, 39, 489-497. S. Bhattacharya, S. K. Samanta, Chem. -Eur. J., 2012, 18, 1663216641. Y. G. Li, T. Y. Wang, M. H. Liu, Soft Matter, 2007, 3, 1312-1317. D. D. Diaz, D. Kuhbeck, R. J. Koopmans, Chem. Soc. Rev., 2011, 40, 427-448. T. Muraoka, C.-Y. Koh, H. Cui, S. I. Stupp, Angew. Chem., Int. Ed., 2009, 48, 5946–5949. D. J. Abdallah, S. A. Sirchio, R. G. Weiss, Langmuir, 2000, 16, 7558-7561. G. O. Lloyd, J. W. Steed, Nat. Chem., 2009, 1, 437-442. H. Maeda, Chem. -Eur. J., 2008, 14, 11274-11282. F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680-1682. Y. Kubo, W. Yoshizumi, T. Minami, Chem. Lett., 2008, 37, 12381239. H. Cao, J. Jiang, X. F. Zhu, P. F. Duan, M. H. Liu, Soft Matter, 2011, 7, 4654-4660. N. Yan, G. He, H. Zhang, L. Ding, Y. Fang, Langmuir, 2010, 26, 5909-5917. Y. L. Si, G. C. Yang, Z. M. Su, J. Mater. Chem. C., 2013, 1, 13991406. J. Crassous, Chem. Soc. Rev., 2009, 38, 830–845. Y. F. Qiu, P. L. Chen, P. Z. Guo, Y. G. Li, M. H. Liu, Adv. Mater., 2008, 20, 2908-2913. X. W. Zhu, M. H. Liu, Soft Matter, 2011, 7, 11447-11452. J. Jiang, T. Y. Wang, M. H. Liu, Chem. Comm., 2010, 46, 7178-7180.
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38 P. F. Duan, Y. G. Li, L. C. Li, J. G. Deng, M. H. Liu, J. Phys. Chem. B, 2011, 115, 3322-3329. View Article Online 39 W. G. Miao, L. Zhang, X. F. Wang, H. Cao,DOI: Q. 10.1039/C3CP53620C X. Jin, M. H. Liu, Chem. -Eur. J., 2013, 19, 3029-3036. 40 J. F. Folmer-Andersen, J. Lehn, J. Am. Chem. Soc., 2011, 133, 10966-10973. 41 H. Wu, B. Ni, C. Wang, F. Zhai, Y. Ma, Soft Matter, 2012, 8, 54865492. 42 M. E. Belowich, J. F. Stoddart, Chem. Soc. Rev., 2012, 41, 20032024. 43 L. Tauk, A. P. Schröder, G. Decher, N. Giuseppone, Nat. Chem., 2009, 1, 649-656. 44 Y. Liu, Z. T. Li, Aust. J. Chem., 2013, 66, 9–22. 45 P. F. Duan, Y. G. Li, J. Jiang, T. Y. Wang, M. H. Liu, Sci China Chem, 2011, 54, 1051-1063. 46 S. Dalapati, S. Jana, M. A. Alam, N. Guchhait, Sens. Actuators B: Chem., 2011, 160, 1106-1111. 47 M. X. Liu, T. B. Wei, Q. Lin, Y. M. Zhang, Spectrochim. Acta A, 2011, 79, 1837-1842. 48 T. F. Jiao, G. C. Zhang, M. H. Liu, J. Phys. Chem. B, 2007, 111, 3090-3097.
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Supramolecular Chirality Transfer through Alkyl Chain Entanglement and a Chiroptical Switch Based on Dynamic Covalent Bonding
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Kai Lv, Long Qin, Xiufeng Wang, Li Zhang*, and Minghua Liu* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Chirality transfer is an interesting phenomenon in nature, which represents an important step to understand the evolution of chiral bias and the amplification of the chirality. In this paper, we reported the chirality transfer via the entanglements of the alkyl chains between chiral gelator molecules and achiral amphiphilic Schiff base. We have found that although an achiral Schiff base amphiphile could not form organogels in any kind of organic solvents, it formed co-organogels when mixed with a chiral gelator molecule. Interestingly, the chirality in the gelator molecules was transferred into the Schiff base chromophore in the mixed co-gels and there was a maximum mixing ratio for the chirality transfer. Furthermore, the supramolecular chirality was also produced based on a dynamic covalent chemistry of imine between the aldehyde and amine reaction. Such covalent bond of imine was formed reversibly depending on the pH variation. When covalent bond was formed the chirality transfer occurred, while it was destroyed, the transfer stopped. Thus, a supramolecular chiroptical switch is obtained based on supramolecular chirality transfer and dynamic covalent chemistry.
Introduction 20
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The induction, transfer, amplification and memory of chirality are intriguing issues in exploring the origin of homochiral structures in nature1-3 and for possible application in the various fields such as enantioselective synthesis, sensing, drug delivery and molecular device applications.4-9 The process of chirality transfer can be realized either in a synthetic process or by virtue of supramolecular chemistry approach.10-15 In the supramolecular way, the chirality transfer can be realized from a chiral matrix or template to achiral molecules through various noncovalent interactions such as π-π stacking, hydrogen bonding, electrostatic and hydrophobic interactions.16-23 These noncovalent interactions should be appropriate to preserve the initial chiral matrix and to induce the chiral information transferring. In many cases, the non-covalent interaction is easy to be broken and the process could be reversible, which provides many opportunity for developing chirality-involved stimuli-responsive properties or new applications.24-30 For example, the formation and breakage of the interactions between chiral matrix and achiral dopants make it easy to realize the switch of supramolecular chirality between “on” and “off” state. Thus, this strategy will be convenient to fabricate chiroptical switch.31-36 In the chiral organogel systems, apart from thermal responsiveness, other external stimuli including light irradiation and redox have been used to trigger such on-off state of supramolecular chirality.37-39 While the individual gelators possessing noncovalent interactions such as hydrogen bonding, electrostatic interactions, hydrophobic interactions or metal-ligand coordination are widely investigated, the mixing of the two components will also help to fabricate the This journal is © The Royal Society of Chemistry [year]
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chiral nanostructures. In these cases, the H-bond and electrostatic pairing of the two components are generally necessary to fabricate the chiral structures. The interchain or hydrophobic interaction is possible to form the chiral nanostructure although it is scarcely reported.23 Previously, we have found that when achiral porphyrin mixed with a chiral gelator, the chirality can be transferred to the porphyrin assemblies. However, there still remain important issues that how such transfer occurred and if such alkyl chain entanglements could be extended to other systems. In addition, during such chiral transfer through the alkyl chain entanglement, how about the effect of the mixed ratio of alkyl chains belong to chiral gelator and achiral molecules on the transfer capacity? In this paper, we developed a supramolecular chiral organogel system formed by a chiral gelator and an achiral Schiff base, whose supramolecular chirality was produced through the transfer via the alkyl chain entanglements. Furthermore, we have sought out the optimum mixing ratio of gelator to the achiral molecules and realized a chiroptical switch based on a dynamic covalent bond of Schiff base. Schiff base compounds contain an imine bond formed by the reversible condensation of amine derivatives and aldehydes. The formation and cleavage of the imine bond have been extensively explored in the self-assembly systems and a large number of novel nanostructures as well as responsive properties appeared through such dynamic imine chemistry.40-44 In this paper, we have extended such dynamic covalent bond into the organogel system. We have found that although the amphiphilic achiral Schiff base 2((octadecylimino)methyl)phenol (OMP) could not form organogels in any organic solvents, it formed co-organogels [journal], [year], [vol], 00–00 | 1
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when mixed with LBG (N, N’-bis(octadecyl)-L-Boc-glutamic diamide), which is a universal gelator found in our previous study.23,45 Upon gel formation, supramolecular chirality appeared in the Schiff base chromophore, which presumably come from the chirality transfer through interchain interactions. Further, once the imine bond was destroyed, the chirality transfer would be stopped. The imine bond can be subsequently recovered under alkaline conditions and the chirality reappeared. Thus, a chiroptical switch can be realized through the formation and cleavage of the covalent imine bond, during which the organogels were not broken.
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Scheme 1. Molecular structure of the non-gelator Schiff base OMP (A) and the gelator molecule BG (B).
Experimental section
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Materials The common chemical reagents were used as purchased from commercial chemical company without further purification. The synthesis method of the N,N’-bis(octadecyl)-L(D)-Boc-glutamic diamide (LBG and DBG) was reported previously by our group.23 Gel formation The gelator (BG, 5mg) and OMP (different molar fraction to BG) were mixed with 0.5mL ethanol in a capped test tube and the mixture was heated until the solid was completely dissolved. The hot solution was naturally cooled at room temperature and then the gel formed in a few minutes through the confirmation with the inversed test tube. The co-organogels containing metal ions were prepared by introducing the Cu(Ac)2 with 0.5 equivalent of OMP into BG ethanol system. The mixture was heated until a uniform transparent solution was obtained. After cooling to the room temperature, the co-gels were subsequently formed. During the investigation on the responsiveness of the organogels to acid or base, equimolar HCl or triethylamine was added into the OMP doped BG/ethanol gels alternatively. The xerogels obtained from ethanol were used for SEM measurements, and the organogels in ethanol were directly used for UV-vis and CD spectral measurements. Characterization 1 H NMR spectra were recorded on a Bruker AV400 spectrometer. Matrix-assisted laser desorption ionization time-of flight mass spectrometry (MALDI-TOF MS) were recorded on a BIFLEIII instrument. Elemental analysis was performed on a Carlo-Erba1106 instrument. Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 FE-SEM microscope. The fully aging gels were cast onto single-crystal silica plates (Pt coated), and the trapped solvents in gels were evaporated at ambient conditions first, then vacuum-dried for 12 hours for SEM measurements. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Tensor 27 FTIR spectrometer at room temperature. The KBr pellets made from the vacuum-dried samples were used for FT-IR spectra measurements. UV-Vis spectra were measured on a Hitachi U-3900 UV-VIS spectrophotometer. The CD spectra were performed on a JASCO815 spectrometer. The 0.1mm quartz cells were used for UV-Vis and CD measurements.
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Results and discussion Gel formation and chirality transfer
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The Schiff base OMP is synthesized from the octadecylamine (C18H37NH2) and salicyaldehyde, and this compound can be dissolved in many organic solvents but cannot form organogels in any solvents. The LBG has been reported to be a universal gelator, which can drive various solvents and materials to form co-organogels.23,45 Scheme 1 shows the molecular structure of the Schiff base OMP and the gelator molecule BG. When OMP was mixed with the gelator LBG, it could form organogels. Different amount of OMP was doped into LBG ethanol organogels when the LBG concentration was kept at 10 mg/mL (Fig. 1A). The samples were heated to produce a homogeneous solution and then cooled to room temperature, in which the gels formation was confirmed by inverted test tube, as shown in Fig. 1A. The doped amount of OMP (molar fraction) in the co-organogels could be as high to 68% (the critical amount of OMP to destroy gels is 70%), indicating that the LBG possessed strong combined capacity with OMP. With the increase of OMP, the co-gels gradually turned faint yellow. In order to further investigate the gel structures, the organogels with various mix ratios of OMP and LBG were measured with the SEM, as shown in Fig. 1B and Fig. S1. The twist structures are observed for all the samples. In addition, in a large area, there were no other structures. This indicated that OMP and LBG molecules were miscible between 5-68% without any phase separation. Furthermore, the co-gels of LBG/OMP exhibited as left-handedness. If we use DBG to perform the same gel formation, similar results were obtained. The obtained twist of OMP/DBG, however, was right-handed. This indicated that upon formed the co-gel, the chirality of BG could be transferred to the co-gel, which followed the chirality of the gelator molecules. In order to further characterize the gel properties, both UV-Vis and CD spectra were measured, as shown in Fig. 2. When OMP was mixed with LBG to form the co-organogels, an obvious CD signal was seen in Fig. 2A. A strong Cotton effect appeared at 260 nm and a weak Cotton effect existed at 320 nm, which is the characterized adsorption band of imine, indicating that the chirality of LBG was transferred to Schiff base chromophore through the hydrophobic interaction between alkyl chains of LBG and OMP. In order to further confirm such chirality transfer, we used the gelator with opposite chirality, DBG, to perform the same spectral measurements. The mirror CD spectrum of LBG/OMP and DBG/OMP organogels were obtained, as shown in Figure 2A, implying that during the chirality transfer, the supramolecular chirality followed the chirality of the gelators. The CD spectra of LBG/OMP co-gels with different molar fraction were monitored, and the Cotton effects at 260 nm were observed in all samples. Although the absorption increased This journal is © The Royal Society of Chemistry [year]
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Figure 1. (A) The photo images of OMP/LBG ethanol organogels with different molar fraction of OMP. (B) SEM images of OMP/LBG xerogels at OMP molar fraction at a) 30%, b) 50%, c) 24%, d) OMP/DBG ( the OMP molar fracrion is 24%). The scale bar is 1μm.
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Figure 2. (A) CD (up) and UV-vis (down) spectra of LBG/OMP and DBG/OMP ethanol organogels , the OMP moalr fraction is 24% (B) The variation of G-Value at 260nm as t molar fraction of OMP in OMP/LBG organogels. 25
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Figure 3. CD spectra (G Value) of the LBG/OMP gels in various solvents. a) cyclohexane, b) ethanol, c), methanol d) ethanol+5% H2O, e) ethanol+10% H2O.
As shown above, the hydrophobic interaction plays an important role in the chirality transfer. Therefore, the factors that affect the hydrophobic interactions, especially solvents, will be expected to influence the packing of the alkyl chains. We have investigated the co-gel formation of OMP with LBG or DBG in various organic solvents and compared their CD spectra. In order to compare the effect of the solvent, we also used the G value as a function of wavelength to evaluate such differences. Fig. 3 screens the G values of OMP/LBG organogels in the organic solvents varying from nonpolar cyclohexane, hexane, to polar ethanol and methanol and so on. It is found that with increasing the solvent polarity, the positive Cotton effect at around 260 and 320 nm become more obvious. For the gels from non-polar solvents, the Cotton effect was very weak. For example, the cyclohexane gel showed signals fluctuation along zero, suggesting that the chirality was almost not being transferred from LBG chiral matrix. The most significant Cotton effect was observed in the ethanol and methanol gels. Furthermore, when increasing the polarity of the solvent by adding small amount of water into ethanol, the CD intensity increased accordingly, as can be seen in Fig. 3. This suggested that the hydrophobic interaction between the alkyl chains of LBG and OMP in non-polar cyclohexane or hexane is too weak to perform the chirality transfer, while polar solvents favor the chiral transfer through enhancing the hydrophobic interactions. It should be noted although the addition of water into ethanol could enhance the chirality transfer, when the water amount was above 10%, the chiral matrix of LBG would be broken and no gel was formed. As a consequence, the CD signals of OMP disappeared. Chiroptical switch based on a dynamic covalent bonding From the data above, it seems that the chirality was transferred through the alkyl chain entanglement. To further verify this we have performed the following experiments, we mixed LBG with salicyaldehyde, LBG with octadecylamine. The CD signals were silent for both C18H37NH2-doped and salicylaldehyde-doped LBG organogels. For co-gels containing salicylaldehyde, which showed obvious adsorption band at 260 nm (Fig. 4) due to the π-π transition, the CD was also silent, indicating that the chirality of LBG was not transferred to salicylaldehyde. Since OMP is synthesized from the octadecylamine (C18H37NH2) and salicyaldehyde, we then mixed LBG/octadeyclamine gel with Journal Name, [year], [vol], 00–00 | 3
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linearly with the OMP ratio, the CD spectra did not. In order to compare the CD signals or the chirality transfer ability, the Gvalue, which is the ratio of CD signal to the UV-Vis absorbance, at 260 nm function as molar fraction of OMP was described, as shown in Fig. 2B. An interesting phenomenon was observed. The G-value of the co-gel system increased firstly with the amount of OMP, and reached a maximum at about the molar fraction 24%. After that the G-value of OMP/LBG gels decreased with increasing OMP. These results indicated that upon mixing gelator and achiral molecules, there is a maximum ratio for the chirality transfer. In the present case, the optimal amount of OMP in LBG gels is around 24%, and we chose the system containing OMP molar fraction 24% as the gelation in the following work.
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salicyaldehyde. After a heating and cooling cycle, the organogels with faint yellow color was obtained. Meanwhile, the CD signals was observed at 260nm, which is similar to that of organogels of OMP/LBG. This result indicated that octadecylamine and salicylaldehyde formed Schiff base by heating and imine formation is crucial in the chirality transfer from LBG assemblies to Schiff base moiety through the hydrophobic interaction.
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Figure 4. (A) CD (up) and UV-vis spectra (bottom) of a) LBG/ C18H37NH2, b) LBG/salicyaldehyde,, c) LBG/ C18H37NH2/ salicyaldehyde gels after a heat to cool cycle.
It is known that the imine bond could be reversibly switched by acid and base.44 When the pH is tuned down to slightly acidic, the imine bond can be hydrolyzed. Subsequent addition of base can cause the reformation of imine bond. So we investigated the LBG(DBG)/OMP organogels with alternative addition of hydrochloric acid and triethylamine. As shown in the Fig. 5A, when the acid was added into the gel system, the CD signals disappeared as the result of the broken of imine bond. However, when triethylamine was subsequently added into the gel system, the CD spectrum recovered to the initial state. The obvious Cotton effect was observed at 260nm and a weak CD signal followed at 320nm, implying that the chirality transfer route was connected again as the consequence of reformation of imine bond. This reversible CD signal on-off upon acid/base alternative treatment can be repeated several times, showing that a acid-base responsive chiroptical switch can be realized based on the dynamic covalent interactions. It is well-known that the Schiff base can coordinate with many transition metal ions, so it is interesting to explore whether the chiroptical switch can be still realized when the Schiff base was coordinated with metal ions. The Cu2+ ion was selected owing to its ability to obviously enhance the CD signals of OMP/LBG gels, Fig. 5B shows the CD spectra of organogels in ethanol with the addition of Cu2+. The addition of Cu2+ ions caused the remarkable increase of CD signals. Moreover, the new positive bands at 375 nm was observed, which is ascribed to the charge transfer band from ligand to Cu2+.46,47 When acid was introduced into the gels, the chromophore moiety were apart from the alkyl chains, the CD signals (Fig. 5B) in 320 nm and 375 nm disappeared completely, while the CD band at 260 nm decreased remarkably, indicating that the interaction between the OMP molecules and Cu2+ ions was broken. Then we added the triethylamine into the system, the 4 | Journal Name, [year], [vol], 00–00
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gels can return to the initial situation at the appearance sight. And the CD spectra recovered to the initial state system. In View Article Online addition, when DBG was used, similar chiroptical switch can be DOI: 10.1039/C3CP53620C obtained but the CD signals were just opposite (Fig. 5B).
Figure 5. CD spectra of the LBG(DBG)/OMP (A) and LBG(DBG)/OMP/Cu2+ gels with the acid-base response. (The letters in picture A represent different gels: L-a LBG/OMP; L-b LBG/OMP/Acid; Lc LBG/OMP/Acid/Base; D-a DBG/OMP; D-b DBG/OMP/Acid; D-c DBG/OMP/Acid/Base. The letters in picture B represent different gels: La LBG/OMP/Cu2+; L-b LBG/OMP/Cu2+/Acid; L-c LBG/OMP/Cu2+/Acid/Base; D-a DBG/OMP/Cu2+; D-b DBG/OMP/Cu2+/Acid; D-c DBG/OMP/Cu2+/Acid/Base.)
The dynamic formation of imine covalent bond upon treatment of acid and base was further confirmed by FT-IR spectra, as shown in Fig. 6. The OMP showed strong vibrations at 1635, 1580, and 1470cm-1, which can be ascribed to the vibrations of C=N band, aromatic rings skeleton, scissor vibration of the CH2 group, respectively.48
Figure 6. FT-IR spectra of a) OMP, b) OMP+Cu2+, c) OMP+Cu2++Acid, d) OMP+Cu2++Acid+Base.
When Cu2+ ions were added, the C=N band shifted from 1635cmto 1623cm-1 and the vibration of the aromatic rings shifted from l580cm-1 to 1540cm-1, suggesting that the OH and the imine groups have coordinated with the Cu2+. When acid was added to the system, the vibration band of C=N band has disappeared, indicating that the imine group was destroyed by the acid, and the coordination with the Cu2+ ions was also destroyed, as characterized by the red shift of aromatic rings skeleton vibration. The new broad band at about 3130cm-1 can be ascribed to the vibration of N-H group, which also indicated the hydrolyzing of Schiff base. When base was subsequently added to the system, 1
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Scheme 2. (a) A Schiff base based on a dynamic covalent bond. (b) The gelator BG molecules self-assembled into gels through hydrophobic interactions and H-bonding in ethanol. When the Schiff base OMP molecules were added into the system, the OMP molecules can be inserted into the alkyl chains of BG molecules. Based on the hydrophobic interaction, a supramolecular assembly with twist structure can be formed, and the supramolecular chirality can be transferred from BG molecules to Schiff base moiety. The supramolecular chirality showed “on” and “off” state through the alternate treatment of acid and base.
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In conclusion, an achiral Schiff base with long alkyl chain was found to form organogel with an amphiphilic glutamide gelator although the Schiff base could not form organogels itself. The supramolecular chirality of amphiphilic Schiff base was induced by the chiral gelator via the interchain interactions, the chirality of which followed the gelator chirality. On account of the dynamic covalent chemistry of imine in amphiphilic Schiff base, the salicyaldehyde moiety can be connected or departed with long alkyl chain upon treatment by base or acid. During such process, the chirality transfer path was opened or closed alternately, leading to the emergence or disappearance of Schiff base supramolecular chirality. A acid-base driven chiroptical switch could be realized based on both chirality transfer and dynamic covalent chemistry.
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The authors are grateful to National Natural Science Foundation of China (Nos. 91027042, 21021003 and 21227802), Basic Research Development Program (2013CB834504), and the Fund of the Chinese Academy of Sciences.
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Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. Email:[email protected], [email protected] † Electronic Supplementary Information (ESI) available: Synthesis, characterization of the compond OMP, and SEM images of OMP/LBG(DBG) xerogels at different OMP molar fraction. See DOI: 10.1039/b000000x/
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the vibration band of the imine group was detected again at 1623cm-1, similar to the system before acid was added. These FT-IR spectral changes indicated that acid and base influenced the imine group and consequently on the coordination between Cu2+ ions and Schiff base. Based on the above results, it is obvious that the cooperative interaction between OMP and LBG or DBG plays an important role in realizing the gel formation, chirality transfer and the chiroptical switch. The process can be illustrated in Scheme 2. LBG or DBG could easily form organogels and self-assemble into fiber or twist structures through the H-bond between three amide groups and the hydrophobic interactions. When OMP was added into BG matrix, due to the good affinity of the alkyl chains, they may entangle each other to form a chiral twist structures. With this interaction, the chirality was transferred from LBG or DBG to whole assemblies including the OMP moiety. Therefore, we observed the Cotton effect for the Schiff base moiety. In this case, the chirality of the whole system followed the chirality of the gelator molecules. If we only mixed the salicylaldehyde, no CD spectra were detected, indicating that the interaction between the alkyl chain of OMP and LBG is very important. It should be noted that during such chirality transfer, the ratio of the gelator molecules and the achiral molecules is also important. In the mixed ratio range when organogels were formed, it revealed that both OMP and LBG or DBG formed only one kind of nanostructures and the absorption was linearly increased with the increment of OMP, however, the chirality signal showed a maximum at around 24%. This means that there existed a maximum combination of the two components for the chirality transfer. On the other hand, the amphiphilic Schiff base has a dynamic imine covalent, which could be reversibly switched by acid/base reaction. OMP molecules can be disconnected into two building blocks under acid condition. When acid was added into the gel system, the Schiff base was hydrolyzed although the gel was maintained. Then the route of chirality transfer through the alkyl chain was cut off. Therefore, we cannot detect the CD spectra for the acid regulated gels, as shown in Scheme 2b. The imine bond can be easily reformed by base. By subsequently addition of triethylamine into the system, the Schiff base formed again and the supramolecular chirality of the gel was recovered. Thus, a chiroptical switch based on dynamic imine covalent can be realized by the alternate addition of acid and base.
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R. Fasel, M. Parschau, K.-H. Ernst, Angew. Chem., Int. Ed., 2003, 42, 5178–5181. R. Purrello, A. Raudino, L. M. Scolaro, A. Loisi, E. Bellacchio, R. Lauceri, J. Phys. Chem. B, 2000, 104, 10900–10908. A. R. A. Palmans, E. W. Meijer, Angew. Chem., Int. Ed., 2007, 46, 8948–8968. N. Ousaka, Y. Takeyama, E. Yashima, Chem. Sci., 2012, 3, 466-469. T. Tu, W. Fang, X. Bao, X. Li, K. H. Doetz, Angew. Chem., Int. Ed. 2011, 50, 6601-6605. M. Siczek, P. J. Chmielewski, Angew. Chem., Int. Ed., 2007, 46, 7432-7436. L. You, J. S. Berman, E. V. Anslyn, Nat. Chem., 2011, 3, 943-948. W. Zhang, W. Jin, T. Fukushina, N. Ishii, T. Aida, J. Am. Chem. Soc., 2013, 135, 114-117. S. Shinoda, Chem. Soc. Rev., 2013, 42, 1825-1835. B. L. Feringa, R. A. van Delden, Angew. Chem., Int. Ed., 1999, 38, 3419-3438. L. Pu, Chem. Rev., 2004, 104, 1687-1716. R. M. Hazen, D. S. Sholl, Nature Mater., 2003, 2, 367-374. M. Suzuki, K. Hanabusa, Chem. Soc. Rev., 2009, 38, 967-975. B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geertsema, Chem. Rev., 2000, 100, 1789-1816. D. K. Smith, Chem. Soc. Rev., 2009, 38, 684-694. J. J. D. de Jong, T. D. Tiemersma-Wegman, J. H. van Esch, B. L. Feringa, J. Am. Chem. Soc., 2005, 127, 13804-13805. A. Ajayaghosh, R. Varghese, S. J. George, C. Vijayakumar, Angew. Chem., Int. Ed., 2006, 45, 1141-1144. A. R. Hirst, D. K. Smith, in Low Molecular Mass Gelators: Design, Self-Assembly, Function, ed. F. Fages, 2005, 256, 237-273. A. R. Hirst, D. K. Smith, M. C. Feiters, H. P. M. Geurts, Chem. -Eur. J., 2004, 10, 5901-5910. S. Bhattacharya, S. K. Samanta, Langmuir, 2009, 25, 8378-8381. M. George, R. G. Weiss, Acc. Chem. Res., 2006, 39, 489-497. S. Bhattacharya, S. K. Samanta, Chem. -Eur. J., 2012, 18, 1663216641. Y. G. Li, T. Y. Wang, M. H. Liu, Soft Matter, 2007, 3, 1312-1317. D. D. Diaz, D. Kuhbeck, R. J. Koopmans, Chem. Soc. Rev., 2011, 40, 427-448. T. Muraoka, C.-Y. Koh, H. Cui, S. I. Stupp, Angew. Chem., Int. Ed., 2009, 48, 5946–5949. D. J. Abdallah, S. A. Sirchio, R. G. Weiss, Langmuir, 2000, 16, 7558-7561. G. O. Lloyd, J. W. Steed, Nat. Chem., 2009, 1, 437-442. H. Maeda, Chem. -Eur. J., 2008, 14, 11274-11282. F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680-1682. Y. Kubo, W. Yoshizumi, T. Minami, Chem. Lett., 2008, 37, 12381239. H. Cao, J. Jiang, X. F. Zhu, P. F. Duan, M. H. Liu, Soft Matter, 2011, 7, 4654-4660. N. Yan, G. He, H. Zhang, L. Ding, Y. Fang, Langmuir, 2010, 26, 5909-5917. Y. L. Si, G. C. Yang, Z. M. Su, J. Mater. Chem. C., 2013, 1, 13991406. J. Crassous, Chem. Soc. Rev., 2009, 38, 830–845. Y. F. Qiu, P. L. Chen, P. Z. Guo, Y. G. Li, M. H. Liu, Adv. Mater., 2008, 20, 2908-2913. X. W. Zhu, M. H. Liu, Soft Matter, 2011, 7, 11447-11452. J. Jiang, T. Y. Wang, M. H. Liu, Chem. Comm., 2010, 46, 7178-7180.
6 | Journal Name, [year], [vol], 00–00
65
70
75
80
38 P. F. Duan, Y. G. Li, L. C. Li, J. G. Deng, M. H. Liu, J. Phys. Chem. B, 2011, 115, 3322-3329. View Article Online 39 W. G. Miao, L. Zhang, X. F. Wang, H. Cao,DOI: Q. 10.1039/C3CP53620C X. Jin, M. H. Liu, Chem. -Eur. J., 2013, 19, 3029-3036. 40 J. F. Folmer-Andersen, J. Lehn, J. Am. Chem. Soc., 2011, 133, 10966-10973. 41 H. Wu, B. Ni, C. Wang, F. Zhai, Y. Ma, Soft Matter, 2012, 8, 54865492. 42 M. E. Belowich, J. F. Stoddart, Chem. Soc. Rev., 2012, 41, 20032024. 43 L. Tauk, A. P. Schröder, G. Decher, N. Giuseppone, Nat. Chem., 2009, 1, 649-656. 44 Y. Liu, Z. T. Li, Aust. J. Chem., 2013, 66, 9–22. 45 P. F. Duan, Y. G. Li, J. Jiang, T. Y. Wang, M. H. Liu, Sci China Chem, 2011, 54, 1051-1063. 46 S. Dalapati, S. Jana, M. A. Alam, N. Guchhait, Sens. Actuators B: Chem., 2011, 160, 1106-1111. 47 M. X. Liu, T. B. Wei, Q. Lin, Y. M. Zhang, Spectrochim. Acta A, 2011, 79, 1837-1842. 48 T. F. Jiao, G. C. Zhang, M. H. Liu, J. Phys. Chem. B, 2007, 111, 3090-3097.
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