Modulating Supramolecular Chirality in Alanine Derived Assemblies

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Interface-Rich Materials and Assemblies

Modulating Supramolecular Chirality in Alanine Derived Assemblies by Multiple External Stimuli Fang Wang, Minggao Qin, Ting Peng, Xianhui Tang, Auphedeous Yinme Dang-i, and Chuan-Liang Feng Langmuir, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Modulating Supramolecular Chirality in Alanine Derived Assemblies by Multiple External Stimuli Fang Wang,[a] Minggao Qin,[a] Ting Peng,[a] Xianhui Tang,[b] Auphedeous Yinme Dang-i,[a] and Chuanliang Feng*[a] [a]

State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, and

[b]

School of Chemistry and Chemical Technology, Shanghai Jiao Tong University 800

Dongchuan Road, Shanghai, 200240, China Corresponding author: Fax: +86 2154747651; E-mail: [email protected] Keywords: Gels, Chiral twists, Solvent effects, Self-assembly, Supramolecular chirality

Abstract: Having control over the supramolecular chirality through multi-external stimulators provides many possibilities in realizing functional chiral materials. Herein, the supramolecular chirality of nanotwists comprising PA centered with 1,4-phenyldicarboxamide bearing two L/Dhelicogenic alanine motifs and achiral COOH at each terminus of the alanine arms is modulated by solvent, temperature, and ultrasound. The modulations are mainly due to the hydrogen bonds among gelators and solvent-gelator interactions, resulting in changes of the molecular arrangement and subsequent self-assembled nanostructures. Typically, the gel of PA in ethyl acetate prepared by ultrasonication method exhibits thixotropic property due to the participation of ethyl acetate in the self-assembly process, resulting in relatively flexible and tolerant networks.

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This study provides a simplistic way to control the handedness of chiral nanostructures and a rational design of the self-assembly system with multi-stimuli-responsive supramolecular chirality.

1. Introduction Controlling the chirality of supramolecular systems is becoming more relevant owing to its increasing significance in chemistry and materials science.[1-16] Regulations of supramolecular chirality have been successfully achieved by employing external stimuli such as temperature,[1719]

solvents,[20-26] additives,[27-30] ultrasound[31] and so on.[32-37] Although various regulations

based on a single stimulus are largely reported, it still remains a big challenge to achieve controllable

supramolecular

chirality

upon

subjecting

to

multiple

external

stimuli.

Supramolecular chirality turned by multiple external stimuli has far-reaching implications in the field of supramolecular chemistry and extends to many possibilities in designing functional chiral materials.[38] To circumvent this issue, a rational design of the self-assembly system with multi-stimuli-responsive supramolecular chirality will be very necessary and it is important to gain a deep insight into the chiral assembly process and regulation of the supramolecular aggregation under multi-stimuli. Supramolecular gelators offer many opportunities for regulating supramolecular chirality due to their controllable self-assembly process through non-covalent interactions, such as hydrogen bonding, π-π stacking, and hydrophobic interactions.[39-44] Usually, the chirality of supramolecular nanostructures depends on the chirality of gelator molecules.[45] However, chiral nanostructures can also be modulated by external stimuli. Under the stimuli, solvent-solute interactions can be adjusted in some cases resulting to chirality modulation, since the stability of


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conformationally flexible molecules and their self-assembled assemblies can be deeply affected by solvent-solute interactions. Liu et al. reported that varying the water content in the water/organic mixture could change the morphology of the assembled structures obtained from a cationic amphiphile.[39] Faul showed that the supramolecular assembly of perylene derivatives was modulated through solute-solvent interactions and obtained opposite handed helical aggregates.[22] The amphiphilic discotic molecules which self-assembled into supramolecular fibers were found by Meijer and that the presence of water induced the formation of a triple helix.[26] Although solvent-gelator interactions are of great importance in supramolecular chirality, there are scarce studies that consider the influence of preparatory methods on solventgelator interactions. This can pave a way for fabricating smart materials displaying multi-stimuliresponsive supramolecular chirality.[36] Herein, new alanine-based organogelators (DPA and LPA, Scheme 1) were designed and their self-assembled supramolecular chirality can be regulated by temperature, ultrasound, and even polarity of solvents. The preparatory methods of heating-cooling and ultrasonication changed the solvent-gelator interactions and subsequent gelator-gelator interactions. Moreover, the gel in nonpolar solvents was stabilized by strong hydrogen bonds, while weak hydrogen bonds between amide groups were formed in organogels obtained from polar solvents. Therefore, the regulations of supramolecular chirality were mainly mediated by the intermolecular hydrogen bonding among gelators and the solvent-gelator interactions which can be easily influenced by multiple external stimuli and further change the packing mode of gelator molecules and chiral assemblies. Interestingly, the gels formed in ethyl acetate by using the ultrasonication method showed good thixotropic properties due to the participation of ethyl acetate in the self-assembly process, resulting in relatively flexible and tolerant networks. This study not only designs a simple self-


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assembled system with multi-stimuli-responsive supramolecular chirality, but also provides enlightenment to the design of functional soft materials.

Scheme 1. Molecular structures of PA (DPA or LPA enantiomers), and schematic illustration of nanostructures self-assembled from PA in dichloromethane (DCM), chloroform (CHCl3), benzene, toluene, acetonitrile (ACN) and ethyl acetate (EA) all prepared by the heating-cooling method, and in EA prepared by the ultrasonication method. M and P denote left- and righthanded chiral nanostructures, respectively. 2. Experimental Section 2.1 Materials Chemical reagents and solvents were purchased from Aladdin and used without further purification. 1H NMR and

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C NMR were obtained on a Bruker Advance III 400 Instrument

operating at 400 MHz. HRMS were recorded on a Water Q-Tof Mass Instrument. 2.2 Synthesis


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1,4-benzenedicarbonyl dichloride (2.6 g, 13.0 mmol) in dry dichloromethane (DCM, 20 mL) was added dropwise to a solution of D-alanine methyl ester hydrochloride (3.6 g, 26.1 mmol) and triethylamine (Et3N, 8.0 mL, 58.3 mmol) in dry DCM (100 mL). The resultant mixture was stirred at room temperature for 24 h and the solvents were removed under vacuum leaving a residue which was further dissolved in ethanol (100 mL). The solute substances obtained after filtration were collected and dried to give dimethyl ester of DPA (3.6 g, 10.7 mmol, 82%). For the hydrolysis, aqueous sodium hydroxide (10 ml, 2.0 M) was added to a cooled (25 °C) suspension of the dimethyl ester of DPA (2.1 g, 6.1 mmol) in methanol (20 mL). The mixture was gradually brought back to room temperature and stirred (about 24 hours) until a clear solution was obtained. The solution was then acidified with 3.0 M hydrogen chloride until pH value no more than 3.0 and gellike precipitate formed. The gel phase was filtered, washed with deionized water, and finally dried in the vacuum oven to give DPA (1.7 g, 5.5 mmol, 87%). 1H NMR (400 MHz, DMSO-d6, δ, ppm): δ = 1.41 (d, J = 7.4 Hz, 6H; CH3), 4.45 (m, 2H, CH), 7.97 (s, 4H, Ar-H), 8.82 (d, J = 7.2 Hz, 2H; CO-NH), 12.60 (s, 2H, COOH). 13C NMR (100 MHz, DMSO-d6, δ, ppm): δ = 174.56, 165.94, 136.71, 127.84, 48.70, 17.32. EI-MS (m/z) for C14H16O6N2 calcd. 308.1008; found 309.1094 [M+H]+, 331.0886 [M+Na]+. Similarly, LPA was obtained as a white solid (1.5 g, 82%). 1H NMR (400 MHz, DMSO-d6, δ, ppm): δ = 1.41 (d, J = 7.4 Hz, 6H; CH3), 4.45 (m, 2H, CH), 7.97 (s, 4H, Ar-H), 8.82 (d, J = 7.2 Hz, 2H, CO-NH), 12.60 (s, 2H, COOH).

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C NMR (100 MHz, DMSO-d6, δ, ppm): δ =

174.56, 165.94, 136.71, 127.84, 48.70, 17.32. EI-MS (m/z) for C14H16O6N2 calcd. 308.1008; found 309.1090 [M+H]+, 331.0882 [M+Na]+. 2.3 Gel preparation


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The gel preparation followed two different methods, including heating-cooling and ultrasonication. In the heating-cooling method, the gelator and each of the solvent such as ethyl acetate, dichloromethane, chloroform and acetonitrile were put in a septum-capped 5 mL glass vial and heated to just below the boiling point of the solvent. All samples were then left to stand for 2 h before carrying out any spectroscopic investigations. In the ultrasonication method, the suspension of the compound was prepared by subjecting the sample to sonication for 20 min in a water-bath sonicator and was equilibrated at room temperature for 2 h. 2.4 Instruments and methods SEM characterization was determined using an FEI QUANTA 250 microscope. Samples were prepared by depositing dilute solutions (approximately 0.05 wt%) of gel on silicon slice, dried and sprayed with a thin gold layer. FT-IR spectra of xerogels and powders were taken using Bruck EQUINOX55 instrument. The KBr disk technique was used for the solid-state measurement. The samples were scanned between the wavelengths of 4000 and 400 cm-1 at an interval of 1.9285 cm-1. CD spectra were measured using a JASCO J815 CD spectrometer with a bandwidth of 1.0 nm. CD spectra of gels were recorded in the UV region (230-600 nm) using a 0.1 mm quartz cuvette at room temperature. VCD spectra were measured at BioTools, Inc., Jupiter, FL, using a ChiralIR-2X Fourier transform VCD (FT-VCD) spectrometer equipped with an MCT detector and the DualPEM option for enhanced VCD baseline stability. The XRD patterns were obtained from xerogels and recorded on a D8 Advance instrument from Bruker Company. Crystals of DPA and LPA suitable for X-ray diffraction were obtained by the heating-cooling method in aqueous solution with low critical gelation concentration (CGC) of 2 and 5 mg mL-


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1

, respectively. Single-crystal data were collected on a Bruker SMART Apex II CCD-

based X-ray diffractometer with Mo-Kαradiation (λ = 0.71073 Å) at 173 K. 3 Results and discussion Alanine derivative gelators LPA and DPA were synthesized with a high yield through a conventional liquid-phase reaction (Scheme S1). Chemical structures and purities of these new compounds were fully characterized and confirmed by 1H and

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C NMR and HRMS

(Figures S1-S6). The gelation ability of PA (DPA or LPA) in various organic solvents was investigated. It was found that PA was soluble in dimethyl sulfoxide, tetrahydrofuran, methanol, 1,4-dioxane and pyridine, but precipitated in toluene and benzene with the heating-cooling method (Figure S7). However, PA formed gels with the heating-cooling method in ethyl acetate (EA), dichloromethane (DCM), chloroform (CHCl3), and acetonitrile (ACN) with low critical gelation concentration (CGC) of 3, 20, 20, and 7 mg mL-1, respectively. In particular, PA could also form gels in EA by using the ultrasonication method with the CGC of 10 mg mL-1 and the formed gels exhibited thixotropic behavior. Namely, the gel was destroyed after shaking by hand and reformed when rested without any interference (Figures 1 and S8). Moreover, the gels were stable under sonication and responsive to temperature.


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Figure 1. Reversible gel-sol transitions of DPA gels prepared by the ultrasonication method from EA triggered by temperature and shear stress. The gels were stable under sonication. Scanning electron microscopy (SEM) was employed in the characterization of the morphologies of the gels obtained in various solvents. All the self-assembled gels were organized into single-handed twisted nanoribbons with the twist pitches in a few microns and diameters in hundreds of nanometers (Figures 2, S9, S10 and S11). The gel of DPA in EA prepared by the heating-cooling method (abbreviated as DPA/EA-H) exhibited righthanded twisted ribbons (Figures 2a and S9a). However, the gel of DPA in EA prepared by the ultrasonication method (abbreviated as DPA/EA-S) showed left-handed twisted ribbons (Figures 2c and S9c), which had opposite chirality of DPA/EA-H gel. Lefthanded twisted ribbons were observed for DPA/DCM, DPA/CHCl3, DPA/benzene and DPA/toluene (Figures 2e, g, S9g and S10a, c). Unexpected right-handed twisted ribbons were achieved for DPA/ACN (Figures 2i and S9i). By contrast, right-handed twisted ribbons were formed for LPA/EA-S, LPA/DCM, LPA/CHCl3, LPA/benzene and LPA/toluene (Figures 2d, f, h, S9d, h and S10b, d). Left-handed twisted ribbons were obtained for LPA/EA-H and LPA/ACN (Figures 2b, j and S9b, j). Moreover, except the chiral nanotwists achieved for PA/benzene and PA/toluene, there were irregular higherorder structures formed by the intertwining of thin fibers (Figures S9e, f and S10e, f). These results suggest that nanotwists of opposite handedness could be obtained from the same chiral molecule and the chiral inversion was triggered by changing the solvent or preparatory method.


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Figure 2. SEM images of DPA assemblies formed in a) EA-H (prepared by heatingcooling), c) EA-S (prepared by ultrasonication), e) benzene, g) DCM, i) ACN; and SEM images of LPA assemblies formed in b) EA-H, d) EA-S, f) benzene, h) DCM, j) ACN.


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The chirality of the formed nanostructures of PA assemblies in different solvents was investigated by circular dichroism (CD) spectroscopy (Figures 3 and S12). For DPA/EAS, DPA/DCM, and DPA/CHCl3 gels, the CD spectrum showed a positive Cotton effect at 258 nm, 265 nm and 260 nm, respectively. In the case of DPA/benzene suspension, a positive Cotton effect at 260 nm and a negative band at 307 nm were observed. Similarly, a positive Cotton effect at 261 nm and a negative band at 306 nm were obtained for DPA/toluene. These were in accordance with the UV absorption of the gels or suspensions (Figure S12), indicating that the chirality was transferred to the central aryl group according to our previous reports.[29,30] Whereas for DPA/ACN gels, a negative Cotton effect was observed at 266 nm. The inverted CD signals of gels in different solvents imply the reversed supramolecular chirality. The CD spectra of DPA and LPA in different organic solvents were almost mirror images of each other. However, the CD signals of PA/EA-H gels were too weak to be detected. A chiral transition into the opposite optically active gel was achieved by exchanging the solvent, which correlated well with the chiral microscopic structures observed in the SEM images. Furthermore, the shape of the CD spectra changed drastically from one solvent to other. The differences found in these CD spectra are likely due to the dissimilar aggregation mode for PA derivatives in the different solvents.


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Figure 3. CD spectra of PA assemblies formed in a) EA-S prepared by ultrasonication and EA-H prepared by heating-cooling, b) benzene, c) DCM, and d) ACN. The optical activity and the chirality of these assemblies were also investigated by vibrational circular dichroism (VCD) spectroscopy (Figures 4 and S13). DPA/EA-S, DPA/DCM, DPA/CHCl3, DPA/benzene, and DPA/toluene exhibited a negative VCD signal of C=O stretching band at 1651 cm-1, whereas the VCD signal of the band switched to a positive signal for DPA/EA-H and DPA/ACN. The VCD patterns imply the inversion of the chirality from DPA/EA-S, DPA/DCM, DPA/CHCl3, DPA/benzene, and DPA/toluene to DPA/EA-H and DPA/ACN at room temperature. By contrast, the C=O stretching band of LPA in EA-S, DCM, CHCl3, benzene, and toluene gave a positive signal, and that in EA-H and ACN showed an opposite negative signal. These observations reveal that the supramolecular chirality of the nanostructures is turned by multiple external stimuli of temperature, ultrasound and solvent and influenced by the stacking of the molecules through intermolecular hydrogen bonding during the selfassembling process.


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Figure 4. VCD spectra of PA assemblies formed in EA prepared by heating-cooling (EAH) and ultrasonication (EA-S), benzene, DCM, and ACN. To understand the organization of PA in different solvents, FT-IR spectrum was recorded (Figures 5 and S14). The PA assemblies in nonpolar solvents such as DCM, CHCl3, benzene and toluene showed a characteristic peak for a hydrogen bonded amide: 1633 cm-1 and 1544 cm-1 for amide I and amide II, respectively.[46] The amide II band of PA/EA-S gels at 1544 cm-1 became weak, indicating a slightly weaker hydrogen-bond


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strength. However, a shoulder peak at 1651 cm-1 was observed and the amide II band at 1544 cm-1 almost disappeared in PA/EA-H and PA/ACN assemblies, which means that the amide hydrogen-bond strength in PA/EA-H and PA/ACN gels is much weaker than in PA/EA-S gel. Due to the different ability of the solvents to accept hydrogen bonds which can disrupt hydrogen bond interactions to some extent,[47] these different amide hydrogen bonding interactions can be ascribed to different interactions between PA and the solvents. Furthermore, the interaction between PA and solvent EA can also be influenced by preparatory methods and subsequently change the hydrogen bonds between amide groups. The stretching vibration bands of C=O from carboxyl groups of PA in various organic solvents at 1699 cm-1 were observed, indicating that the hydrogen bonds were formed between carboxylic acid units. It can be concluded that during the gelation process, although amide and carboxylic acid groups should act cooperatively, their relative dominance should differ depending on the solvent. The gel formation in nonpolar solvents is driven by hydrogen bonds between the amide and carboxylic acid units while hydrogen bonds between the carboxylic acid units are the main driving force in PA/EA-H and PA/ACN gels because of the much weak hydrogen bonds between amide groups. On the other hand, the hydrogen bonds in PA/EA-S are different from the above circumstances.


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Figure 5. FT-IR spectra of PA assemblies that were obtained in EA prepared by heatingcooling (EA-H) and ultrasonication (EA-S), benzene, DCM, and ACN. To get a deep insight into the underlying mechanism of the chiral inversion and better understand the molecular arrangement of assemblies formed by using different preparatory methods, single crystals of DPA and LPA were grown in aqueous solution at room temperature (Figures S15 and S16). As shown in Figure 6a, there is oxygen of two water molecules in each unit cell of DPA single crystal. Detailed crystallographic data are summarized in Table S1. The crystal structure and packing diagram of DPA suggest that the main driving forces for the self-assembly are six types of intermolecular hydrogen bond interactions according to the structural parameters (Figures 6b, S17 and Table S2). The detailed information on these multiple hydrogen bonds is as follows: one of the oxygen atom of the water molecules formed two hydrogen bonds with one hydrogen atom from the amide group of an adjacent DPA molecule (N-H···O-H, bond length: 2.20 Å, Table S2) and with another hydrogen atom from the carboxylic group of the other neighboring DPA molecule (CO-O-H···O-H, bond length: 1.79 Å). The oxygen atom of


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the other water molecule formed a hydrogen bond with one hydrogen atom from the carboxylic group of the neighboring DPA molecule (CO-O-H···O-H, bond length: 1.76 Å). Intermolecular hydrogen bond formed between one hydrogen atom of the amide group and an oxygen atom of the carboxylic group (N-H···OH-C=O, bond length: 2.30 Å). In addition, C-H···O=C-OH formed intermolecular hydrogen bond, which came from a hydrogen of the methine and the C=O of the carboxylic group (H···O = 2.42 Å). CH···O=C-NH could also form an intermolecular hydrogen bond to further increase the stability of self-assembly, which came from a hydrogen of the benzene ring and the C=O of the amide group (H···O = 2.44 Å). These six types of multiple intermolecular hydrogen bonds are the main driving forces to stabilize the single-crystal framework of DPA (Figures 6b and S17). Similarly, the packing diagram of LPA was also mainly stabilized by six kinds of intermolecular hydrogen bonds (N-H···O-H, bond length: 2.29 Å; CO-O-H···O-H, bond length: 1.73 Å; CO-O-H···O-H, bond length: 1.59 Å; NH···OH-C=O, bond length: 2.33 Å, C-H···O=C-OH, bond length: 2.47 Å, C-H···O=CNH, bond length: 2.47 Å, Figures 6a', b' and S18, and Tables S3 and S4). Moreover, four other kinds of intermolecular hydrogen bonds formed between the hydrogen atoms of the two water molecules and the two oxygen atoms of the carboxylic groups (O=C-OH) of LPA molecule (HO-C=O···H-O-H, bond length: 1.96 Å and 1.89 Å) as well as two oxygen atoms of the amide groups of the same LPA molecule (HN-C=O···H-O-H, bond length: 1.94 Å and 1.85 Å, Table S4).


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Figure 6. Single-crystal structures of a) DPA and a') LPA obtained from aqueous solution and corresponding packing structures of b) DPA and b') LPA stabilized by intermolecular hydrogen bonds (purple dashed lines) in the single-crystal state. Furthermore, the self-assembly mechanism of PA in EA with different preparatory methods was investigated in both xerogel and single-crystal states by using wide-angle Xray diffraction (WAXD). Since DPF and LPF are enantiomers, the crystal structure and packing diagram of LPF are almost the same as that of DPF even though the hydrogen bonds between gelators and waters are a little difference (Figure S19). Analogous WAXD patterns were obtained from single-crystal and xerogel states of PA/EA-S, although the intensity of signals in xerogel was lower than that of the single crystal (Figure 7a). Therefore, a similar stacking mode was assumed for PA/EA-S in xerogel and singlecrystal states,[37,48] which was mainly driven by intermolecular hydrogen bond


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interactions. Moreover, due to the participation of EA in the self-assembly process, the networks of different noncovalent interactions within PA/EA-S gels are relatively flexible and tolerant, leading to desirable thixotropic properties. Well-defined WAXD patterns of PA/EA-H xerogel display d-spacing of 2.85, 1.41, 0.93, 0.70, 0.56, and 0.46 nm, calculated from Bragg’s equation, which correspond to ratios of 1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively, indicating a lamellar structure with a d-spacing of 2.85 nm[49] (Figure 7a). This d-spacing is comparable with twice the molecular length of PA (1.42 nm), indicating a bilayer structure of PA (Figure 7b), which served as the basic unit and stacked further into nanostructures. Due to the interaction between EA and amide group of PA, the hydrogen bonds among the carboxylic acid units were the main driving force in the process of assembly of PA/EA-H as confirmed by FT-IR spectra, which led to the bilayer stacking. These two kinds of stacking modes induced by different solvent-gelator interactions lead to the inversion of the supramolecular chirality of PA/EA gels prepared by the ultrasonication method and heating-cooling method.


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Figure 7. (a) WXRD patterns of assemblies of PA/EA-S, PA/EA-H, and DPA crystal. (b) Illustration of PA/EA-H packing as a bilayer structure with hydrogen bonding (purple dashed lines) of carboxylic moiety (gray C, dark grey H, red O, blue N). 4 Conclusions In summary, the supramolecular chirality in alanine derivative PA systems has been regulated by applying multiple external stimuli such as solvent, temperature and ultrasound. Moreover, the gels of PA in ethyl acetate prepared via the ultrasonication method can show thixotropic properties. All these ascribe to the noncovalent interactions among gelators including intermolecular hydrogen bonding among the amide and carboxylic acid groups and solventgelator interactions, which can be modulated by multiple stimuli. This system is a rare example where multiple external stimuli have been shown to turn the chirality of nanostructures. The


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control of chiral properties of the self-assembled nanostructures helps in the development of functional soft materials such as chiroptical switches. ASSOCIATED CONTENT Supporting Information: X-ray crystallographic data, supplemental figures, synthesis and characterization of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the NSFC (51573092), the Innovation Program of Shanghai Municipal Education Commission (201701070002E00061), and Program for Professors of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning. REFERENCES (1) Liu, G. F.; Zhang, D.; Feng, C. L. Control of Three-Dimensional Cell Adhesion by the Chirality of Nanofibers in Hydrogels. Angew. Chem., Int. Ed. 2014, 53, 7789-7793. (2) Liang, G.; Yang, Z.; Zhang, R.; Li, L.; Fan, Y.; Kuang, Y.; Gao, Y.; Wang, T.; Lu, W. W.; Xu, B. Supramolecular Hydrogel of a D-Amino Acid Dipeptide for Controlled Drug Release in Vivo. Langmuir 2009, 25, 8419-8422. (3) Thornton, P. D.; Mart, R. J.; Ulijn, R. V. Enzyme-Responsive Polymer Hydrogel Particles for Controlled Release. Adv. Mater. 2007, 19, 1252-1256.


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Table of contents


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Modulating Supramolecular Chirality in Alanine Derived Assemblies

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