Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled

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Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled Naphthyl-containing L-Glutamide Xiufeng Wang, Fan Xie, Pengfei Duan, and Minghua Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02867 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled Naphthyl-containing L-Glutamide Xiufeng Wang,†‡ Fan Xie,‡ Pengfei Duan,§ and Minghua Liu*, ‡§

†College of Science, China University of Petroleum (East China), Qingdao 266580, Shandong, China ‡ CAS Key Laboratory of Colloid Interface and Chemical Thermodynamics, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China §Key Laboratory of Nanosystem and Hierarchical Fabrication, Chinese Academy of Sciences, National Center for Nanoscience and Technology, Beijing 100190, China

ABSTRACT

Supramolecular gelation provides an efficient way of fabricating functional soft materials with various nanostructures. Amphiphiles containing naphthyl group and dialkyl L-glutamide with a methylene spacer, 1NALG and 2NALG, have been designed and their self-assembly in various organic solvents were investigated. Both of these compounds formed organogels in organic solvents. In the case of the alcohol solvents, the initially formed organogel underwent gelprecipitate transformation, which process was monitored by the UV-Vis, CD spectra and SEM observation.

It was revealed that both the compounds formed the nanofiber structures in gel

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phases. Interestingly, in alcohol solvents, during the phase transition from the gel to precipitates, the nanofibers gradually transformed into a series of long coaxial solid nanotoruloid, a unique nanostructure that has never been observed in other self-assembly systems. In addition, during the gel formation, the nanofibers with supramolecular chirality or M-chirality were obtained. However, the coaxial nanotoruloid showed an inversed P-chirality. Comprehensive analysis based on various data and the gelator structure, substituent position, type of organic solvents, it was suggested that the synergistic interactions between the amide H-bond and π−π stacking of the naphthyl groups played important roles in the formation of the gels as well as the nanofiber, while the H-bonding ability of alcohol to the amide group can subtly regulate the gelator-gelator interactions and lead to the dynamic and hierarchical evolution of the unique nanostructures.

1. INTRODUCTION

Supramolecular self-assembly of low molecular weight gelators accompanied by the gel formation have been attracting considerable attention over last decades, due to their elegance in providing the well-defined nanostructure as well as wide application in tissue engineering, biomaterials, sensor, optoelectronic devices, nanomaterials and so on.1-7 During the selfassembly of small molecules, the non-covalent interactions such as hydrogen bonding, π-π stacking, van der Waals interaction, electrostatic interaction and coordination play an important role, which driving the small molecules to form the one-dimensional (1D) aggregate first, and further the 3D network with the vast majority of solvent molecules entrapped to let the system expressed as semisolid gelation macroscopically.

In the formation of the gels, not only the

interactions between the gelators, but also the gelator-solvents interactions play important roles, thus, a certain gelator can form the gels in several solvents, while it cannot in the other solvents.


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It is hardly to find the gelators that can form gels in vast kinds of solvents.8 Meanwhile, noncovalent interactions of gelator-gelator and gelator-solvents can be tuned subtly, thus providing the flexibility and responsiveness of supramolecular gel as a good platform for fabricating smart soft materials.9-13 On the other hand, the gel formation is not a static and immutable process but a dynamic one.14-19 While many gels can kept their structures for a long time at room temperature, several others can experience changes from gel to sol, gel to shrink gel20 or gel to precipitate phase transitions to form a thermodynamically more stable states.21,

22

Therefore, diverse

nanostructures and microstructure can emerge during the gel formation and/or subsequent changes. For instance, Raghavan et al observed the assembling pathway and structural changes in mixtures of a single-tailed diacetylenic surfactant and geraniol by light microscopy in real time, from nanoscale vesicle, to helical microribbons, to helical microtubules.23 Ma and Yi et al monitored a cholesterol-based assembly and investigated the reversible vesicle-tube-ribbon transition with intermediates such as fused vesicles and short, long tubes.24 Our group has found that a glycine-based lipid formed nanofiber at low temperature (-15°C), and then evolved into twist, dendritic twist nanostructures, and finally microbelt if exposed to room temperature.25 However, in comparison with the large amount researches on the static gel structure or functions, such dynamic process on the structural changes have been less investigated. Glutamic acid, as one of the proteinogenic amino acids, is a good building block for the selfassembly via derivation with many substituted groups or alkyl chains. Since the pioneer work on the self-assembled chiral nanostructures by Kunitake et al using the amphiphilic glutamic acid derivatives,26-28 a variety of self-assemblies have been developed. The dialkyl building block is quite efficient in forming the organogels. Several gelators containing amphiphilic L-or D-


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glutamides have been developed by Ihara’s group and ours.5, 29-32

In addition, if one wants to

fabricate the π-gels,33-37 it is easy to conjugate the aromatic functional groups to the L-glutamide. Previously, we have developed the naphthyl containing π-gelator via conjugating with the dialkyl L-glutamide,21,

22, 38

as shown in Figure 1.

It has been found that the naphthyl-

conjugated π-gelator formed nanotubes and can be used to detect the chiral amines.38

A

naphthylacryl conjugated L-glutamide in ethanol formed initially the nanofibers and then transformed into a family of coaxial hollow toruloid-like nanostructures due to a topochemical [2+2] cycloaddition in the organogel.21,

22

Such structural transition is unique and remains

largely serendipitous, which has never been found in other systems. In order to get a deep insight into the system, we designed new gelators with naphthylacetic acid attached to Lglutamide, as shown in Scheme 1. With this molecular design, the conjugation between the naphthyl group and the L-glutamide was broken by a methylene spacer.

We have found the

compounds formed organogels in various organic solvents also. Interestingly, a family of long coaxial nanotoruloid structures appeared unexpectedly in the ethanol organogel, which were generated gradually during the dynamic transformation of the gel to precipitates.

Various

spectroscopic measurements such as FT-IR, CD, XRD and the SEM observation were used to characterize the gel formation and transformation. A possible mechanism for the evolution of the coaxial nanotoruloid was proposed.


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Scheme 1. Chemical structures of the naphthyl containing gelator molecules. Self-assembly of the gelators 1NLG, 2NLG, 1-NA and 2-NA were reported previously.21,38 Gelators 1NALG and 2NALG with a red methylene spacer were newly synthesized and used in this work. 2. MATERIALS AND METHODS All the starting materials were obtained from commercial suppliers and used as received, including 1-Naphthylacetic acid (TCI, Shanghai, China), 2-Naphthylacetic acid (TCI, Shanghai China), 1-ethyl-3-(3-(dimethylamino) propyl) carbodiimide hydrochloride (GL Biochem Ltd., Shanghai, China) and 1-hydroxybenzotrizole (GL Biochem Ltd., Shanghai, China). Solvents were purified and dried according to standard methods. The synthetic route of the compounds is shown in Supporting Scheme S1. First, N,N’bisoctadecyl-L-glutamic diamide (LGAm) were synthesized according to the report.39 LGAm (1.3 g, 2 mmol) was dispersed in 60 mL dichloromethane. Then, 1-Naphthylacetic acid or 2Naphthylacetic acid (0.45 g, 2.4 mmol) was added into the above mixture and stirred at 0℃ for 30 min. After that, 1-ethyl-3-(3-(dimethylamino) propyl) carbodiimide hydrochloride (EDC HCl; 0.46g, 2.4 mmol) and 1-hydroxybenzotrizole (HOBt; 0.32 g, 2.4 mmol) were added to the


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mixture. The above mixture was stirred for five days at room temperature. Then the solvent was removed by rotary evaporation and little yellow solid was obtained. The crude product was dissolved in 15 mL of THF and poured into a 400 mL saturated aqueous solution of NaHCO3. After filtration, the product was purified by recrystallization in THF/ethanol (2:1 volume ratio) mixed solvents twice to give target product as white solid with yield of 72.2 % (1NALG) and 67.9 % (2NALG). 1NALG, 1H-NMR (CDCl3, 400Hz): δ(ppm) 0.86-0.89 (t, 6H, CH3), 1.23-1.45 (m, 64H, CH2), 1.76-1.95 (m, 2H, CH2), 2.08-2.19 (m, 2H, CH2), 3.03-3.08 (t, 2H, CH2), 3.13-3.20 (t, 2H, CH2), 4.02 (s, 2H, CH2), 4.29-4.32 (q, 1H, CH), 5.74 (s, 1H, NH), 6.50 (s, 1H, NH), 6.87-6.89 (d, 1H (naphthene) ), 7.45-7.54 (m, 4H, 3H (naphthene)+1H (NH) ), 7.80-7.82 (d, 1H (naphthene) ), 7.86-7.88 (d, 1H (naphthene) ), 7.98-8.00 (d, 1H (naphthene) ). MALDI-TOF C53H91N3O3, 818.3; found M: m/z = 818.9, [M+ Na]+: m/z = 841.0, [M + K]+: m/z = 856.9. Elemental analysis calcd. C, 77.79; H, 11.21; N, 5.13. found: C, 77.98; H, 11.08; N, 5.20. [ߙ]ଶହ ஽ = -5.2 (c = 0.33, CHCl3). 2NALG, 1H-NMR (CDCl3, 400Hz): δ(ppm) 0.86-0.89 (t, 6H, CH3), 1.23-1.25 (m, 60H, CH2), 1.36-1.45 (m, 4H, CH2), 1.84-2.04 (m, 2H, CH2), 2.12-2.39 (m, 2H, CH2), 3.13-3.16 (t, 4H, CH2), 3.71 (s, 2H, CH2), 4.34-4.38 (q, 1H, CH), 5.93 (s, 1H, NH), 6.79 (s, 1H, NH), 7.19-7.20 (d, 1H (naphthene) ), 7.39-7.42 (d, 1H (naphthene) ), 7.45-7.47 (m, 2H (naphthene) ), 7.74(s, 1H, NH), 7.79-7.81 (t, 3H (naphthene) ). MALDI-TOF C53H91N3O3, 818.3; found [M+ Na]+: m/z = 841.0, [M + K]+: m/z = 857.0. Elemental analysis calcd. C, 77.79; H, 11.21; N, 5.13. found: C, 78.03; H, 11.15; N, 5.30. [ߙ]ଶହ ஽ = -8.0 (c = 0.11, CHCl3).


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Scanning electron microscopy (SEM) images were performed on Hitachi S-4800 with accelerating voltage 10 kV, and Transmission electron microscopy (TEM) were captured by JEM-1011 with accelerating voltage 200 kV. UV/Vis spectra and CD spectra were recorded with a Hitachi U-3900 and Jasco J-810 spectrophotometers, respectively. Fourier transforminfrared (FTIR) results were measured by Bruker Tensor-27 spectrophotometer with a wavenumber resolution of 4 cm-1 at room temperature in the range of 400-4000 cm-1. X-ray diffraction (XRD) data was obtained on an X'Pert PRO MPD with Cu Kα radiation. 3. RESULTS AND DISCUSSION 3.1. Supramolecular gelation of the compounds in various organic solvents Compounds 1NALG and 2NALG displayed little solubility in water and organic solvents at a room temperature. However, they can be dissolved into transparent solution in many organic solvents upon heating. When these solutions were cooled down to room temperature, both of the compounds could form the organogels, as confirmed by the method of inverting the vials upside down. The gelation property of 1NALG and 2NALG in various organic solvents was showed in Table S1. Transparent organogels were obtained in nonpolar solvents such as toluene, hexane, and cyclohexane, while opaque organogels formed in some polar solvents. Depending on the solvents, most of the gels were stable. However, the gels formed in ethanol experienced a gradual change from the opaque gel to precipitates. To gain an insight into such difference and see if the nanostructures were changed, the scanning electron microscopy (SEM) of the xerogels from various organic solvents were measured, as shown in Figure 1.


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Figure 1. SEM images of 1NALG assemblies formed in (A) DMSO, (B) ethyl acetate, (C) ethanol, (D) TEM image from ethanol, and 2NALG in (E) DMSO, (F) ethyl acetate, (G) ethanol, (H)TEM image from ethanol. Essentially, one-dimensional nanostructures were formed in all the organic solvents. However, depending on the solvents, there are slightly different. In the DMSO gels of 1NALG and 2NALG, long nanofibers were formed and these fibers further rolled into chiral fiber bundles (Figure 1 A and E). The nanofiber was also obtained from 1NALG and 2NALG ethyl acetate gels, as shown in Figure 1 B and F. In nonpolar solvent such as hexane in Figure S1, smaller and thinner nanofibers were obtained, indicating that polarity of solvents impact on the self-assembly to a certain extent. The interesting morphology is observed in the ethanol gels. For 1NALG, when the hot solution of ethanol was cooled down to room temperature naturally, the initially gel showed as nanofiber structures. However, after more than 4 hours where precipitates were formed, a series of coaxial nanotoruloid structures were formed. These nanotoruloids are formed by different diameter toruloid-like or disk block piled one above another, as shown in Figure 1C and D. For 2NALG a similar change occurred. However, it was a mixture of the short derived nanotoruloid structures and nanofibers. The structures were further confirmed by the TEM observation in Figure 1D and 1H.

The TEM image shows all the 1NALG nanotoruloid

structures are the solid nature, no matter the length and width the assemblies owned. Most of these nanotoruloid structures are 80 to 350 nanometers in width, and can extend tens of micrometers. For 2NALG, the mixture of short nanotoruloid structures, nanofibers are observed. Both the TEM nanostructures of 1NALG and 2NALG are in accordance with the SEM ones. 3.2. Dynamic evolution of nanotoruloid in ethanol upon ageing


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Figure 2. SEM images of 1NALG gel in ethanol with different ageing time, (A) 1 minute, (B) 5 minutes, (C) 30 minutes, (D) 2 hour, (E) 4 hours, (F) 8 hours. In order to further clarify the phase change from the gel to precipitates in alcohol gel, the dynamic evolution of nanostructures was tracked with different time intervals by the SEM, as shown in Figure 2. When hot ethanol solution of 1NALG were kept at ambient condition, white aggregates suffused everywhere quickly within one minute and the self-assembled behavior cannot find significant difference with other polar solvents. At one minute, entangled nanofiber structures were formed, as shown in Figure 2A. When the gel was keeping at the room temperature for 5 minutes, short nanotoruloid structures came into being (Figure 2B). More and more nanotoruloid structures appeared and the length increased with time (Figure 2C to E). At last, the assemblies were completely dominated by long nanotoruloid structures (Figure 2F) after 4 hours. The precipitates were observed simultaneously. It was obvious that the dynamically formed gel collapsed gradually into the precipitate, during which the gel-trapped solvents was expelled. When the gel was aged for 4 hours or more, equilibrium reached. It is supposed that


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original kinetic-controlled assembly gradually changed into thermodynamic assembly with the growth of nanotoruloid structures. Furthermore, since every building block of one nanotoruloid structure share the same symmetry axis and stack along the axis, we can infer the nanotoruloid structures were generated from nanofibers. It should be noted that although the concentration of the gelation and the temperature could affect the formation speed of the precipitates, the final nanostructures were similar. There was no significant change in the size of the nanotoiud.

Figure 3. SEM images of 2NALG gel in ethanol with different ageing time, (A) 1 minute, (B) 30 minutes, (C) 4 hours, (D) 24 hours. Figure 3 showed the dynamic changes of the morphologies of the 2NALG gels, which had a similar tendency to that of 1NALG.

However, the structural details are some different.

Generally, a short nanotoruloid was formed. In addition, the transform from the nanofiber to the toruloid structure was incomplete. There are always mixtures of diverse nanotoruloid fragment and nanofibers. Prolonged ageing can cause the formation of nanohelix, nano-donuts, which is much similar to our previously reported photo-irradiated 2-NA gels structures.22


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3.3 Effect of alcohol solvents Since NALG in ethanol displayed unusual phenomenon, we paid more attention on the alcohol solvents. Several common alcohols such as methanol, n-propanol and n-butanol were selected to further investigate the solvent effect.

Figure 4 shows the SEM images of the

precipitates formed various alcohols, which were all transferred from the gel after 4 hours. Precipitates were always obtained upon ageing in all these alcohols and each expressed the similar transformation as to that of ethanol. In addition, long nanotoruloid structures appear in the corresponding morphologies for every 1NALG precipitates, and mixed nanostructures for 2NALG. However, the formation of the nanotoruloid was incomplete when the higher alcohol used after 4 hours, and nanofiber structure still existed in SEM images. If transform completely, the longest time is needed in n-butanol, then n-propanol and ethanol, and the shortest one is methanol. This phenomenon is much similar to our previous vicinal effect of the NA cases.22 Since alcohol can accept and donate a proton easily,40, 41 so the hydrogen bond formation or dissociation between alcohol and NALG molecule become more flexible. In the progress of dynamic assembly, alcohol adjusted the intermolecular interaction to approach thermodynamic equilibrium.


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Figure 4. SEM images of 1NALG precipitate in methanol (A), n-propanol (B) and n-butanol (C), 2NALG precipitate in methanol (D), n-propanol (E) and n-butanol (F). All the scale bar are 1000 nm. 3.4 Spectral characterization of the gels and nanostructures Figure 5 shows the UV−vis spectra of the gels in ethanol at different assembling time. When 1NALG is dissolved in hot ethanol, the monomer showed the peak maximum at 281 nm, which


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can be assigned to the π–π* transition of naphthyl ring. Upon precipitation, the peak maximum shifted to 289 nm, which is 8 nm red-shift in comparison with the solution. This suggested that J type π–π stacking between naphthalene rings was formed. When 2NALG dissolved in hot ethanol, the monomer peak appeared at 277 nm, and the aggregates was at 278 nm, showed in Figure S2, only a slight change occurred, indicated 2NALG arranged less order than 1NALG.

Figure 5. Ageing time-dependent UV−vis spectra (A) and CD spectra (B) of 1NALG in ethanol, from initial hot solution (a) , to 5 minutes (b), 30 minutes (c) , 1.5 hours (d), and 8 hours (e). Since the compounds have chiral centers in the L-glutamide moiety, CD spectra will be very effective in characterizing the interactions of the chromophores.42-44 Figure 5B shows the timedependent CD spectra of 1NALG in ethanol. No CD signal was detected in the hot solution of 1NALG. When forming organogel, CD signal appeared, which means the supramolecular chirality of the system was produced during the self-assembly. A negative Cotton effect was observed at 242 nm and a positive band at 223 nm with a crossover at 233 nm, which ascribed to the π–π* transitions of secondary amide. In the progress of morphology transition, a negative CD signal was observed at 286 nm in 5 minutes, which corresponds to the π–π* transition of


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aromatic rings, and this signal red-shifted to 289 nm at 30 minutes, 291 nm at 1.5 hours. These results were in good agreement with the UV spectra. Meanwhile, the intensity of CD signal in 30 min is stronger than that of 5 min, suggesting the supramolecular chirality increase with the formation of self-assemblies. With more and more nanofiber-nanotoruloid morphological transition, the CD signal decrease slightly in 1.5 h. Interestingly, in the final precipitate, a negative CD signal appeared at 216 nm, and a positive one at 273 nm, suggesting the different arrangement of aromatic rings. The CD spectra confirmed that the chirality can transfer from amino acid to naphthyl ring via self-assembly, and aggregates with different topology probably possess the opposite supramolecular chirality. 3.5 FT-IR spectra and X-ray diffraction patterns of the xerogels from various solvents

Figure 6. FT-IR spectra of dried 1NALG (A) and 2NALG (B) self-assemblies from different solvents: (a) toluene, (b) DMSO, (c) ethyl acetate, (d) acetonitrile, (e) cyclohexane, (f) ethanol. The self-assembly is a result of the interactions between gelator and gelator, gelator and solvent molecule, which determine the properties of the assembly ultimately.45,46 The organic


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solvent was solidified in the formation of supramolecular gels, and different solvents can govern the process of gelation and impact the assembly on its morphology.47-49 The unusual nanotoruloid structures were obtained in ethanol, while nanofibers in other solvents. The FT-IR spectra of various solvents are measured to provide this detailed information on the intermolecular interactions. Figure 6 shows the FT-IR spectra of the xerogels from toluene, DMSO, ethyl acetate, acetonitrile, cyclohexane and ethanol. For the xerogels from non-alcohol solvents, similar FT-IR spectra were obtained. The stretching N-H vibration was observed at 3290 cm-1 for all the non-alcohol gels, indicating the strong H-bonding formation for the samples. In the case of ethanol gel, significant changes are observed. Vibration band split into three bands at 3329, 3304, and 3277 cm-1, which suggest that the three amide groups of 1NALG show differences in the vibrations. Compared to other solvents, the amide I band shift to a higher wavenumber in ethanol, from 1636 cm-1 to 1649 cm-1, and amide II simultaneously shift to a lower wavenumber, from 1562, 1539 cm-1 to 1556, 1537 cm-1. These changes of the vibration bands indicated that the hydrogen-bonding interactions between 1NALG molecules in ethanol were weaker than those of the other solvents.50 In addition, strong asymmetrical and symmetrical CH2 stretching band of alkyl chains are at 2918 and 2850 cm-1 respectively for all the samples, indicating that the alkyl chains packed in an all-trans conformation. Meanwhile, the CH2 scissoring vibrations move to a higher wavenumber in ethanol, suggesting the alkyl chain packed looser than the other systems. Moreover, sharper peaks appeared from ethanol than that from other solvents, probably owing to the high homogeneity of the self-assembled structures obtained from ethanol. For 2NALG, similar changes in FT-IR are observed. An interesting feature is similar that the N-H vibration split into 3330 and 3285 cm-1 in ethanol precipitates.


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X-ray diffraction is also employed to characterize the structures of the gels. Well-defined diffraction pattern was only observed in the precipitates of 1NALG in Figure S3. The two θ values appeared at 2.58 and 5.18 degree, respectively. On the basis of Bragg’s equation, the layer distance was estimated as 3.42 nm and 1.70 nm, respectively, which suggested the lamellar structure in the precipitates with a layer distance of 3.41 nm. The value of the layer distance is larger than one single molecular length and less than the twice, which indicated the formation of bilayer structure with alkyl chains interdigitated. For 2NALG, the XRD pattern displayed two broaden diffraction peaks, also formed a lamellar structure with a d-spacing of 3.48 nm. 3.6 Discussion Based on the results on the self-assembled nanostructures of our previously reported naphthyl analogues (Scheme 1) and the above results, the morphological changes during the transformation of the gel to precipitates in the 1NALG and 2NALG can be possibly illustrated in Figure 7. These compounds formed multilayers structures with the bilayer as the basic unit, as shown in Fig.7A. Such multi-bilayer structures are stabilized by three amide H-bonds and the π−π stacking as well as the hydrophobic interactions between the alkyl chains. The multilayers can be nanofiber or nanotube depending on the substitutions. When the naphthyl group is directly conjugated to the L-glutamide, as in the case of 1NLG and 2NLG, it formed nanotube, otherwise it formed nanofiber. These compounds have three amide groups, which could form intermolecular H-bond upon self-assembly. During the formation of these structures, both the Hbonds between three amide groups and the π-π stacking between the naphthyl groups behaved synchronously. In addition, in the conjugated gelators, the intensities of the three H-bonds are similar and they showed the same vibrations in the FT-IR. For the gelators in the present work, however, since the naphthyl group is not conjugated with the amide, three H-bonds may different.


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When the solvents are inert such as non-alcohol, the formed nanofiber structures are stable. However, when alcohol was involved, the initial packing was affected by the alcohol since alcohol molecules have strong interactions with the amide groups.22

Such interaction can

destroy the synchronous amide H-bond and the π-π stacking in the gels. As a consequence, the gels collapsed into the precipitates.


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Figure 7. (A) Illustration of 1NALG molecules assembled in ethanol via hydrophobic interaction, π-π stacking and multiple hydrogen-bonding interactions.

The gelator 1NALG

formed the basic bilayer units in the assemblies. (B) Three kinds of amides in 1NALG, named as amide α, β, γ for the sake of easy discussion (C) The possible packing mode of the naphthyl ring in 1NALG gel and precipitate, the adopted opposite packing lead to the reversed supramolecular chirality. The explanation is supported from the following facts.

First, the FT-IR spectra of the

nanotoruloid showed an obvious difference from those of the nanofibers. In the case of 1NALG, the N-H vibration splits into three bands, which can be designated as the H-bonds between amide α, amide β and amide γ, respectively, as indicated in Fig.7B. In addition, an 8 nm red-shift was also observed for the precipitates, showing the J-like packing of the naphthyl groups. In the case of 2NALG, the N-H vibrations split into two peaks due to the less steric hindrance in comparison with that of 1NALG.

Second, as we reported before, the gelators with the naphthyl group

directly conjugated with the amide α (gelator 1NLG and 2 NLG in Scheme 1) formed nanotube structures and showed no changes in ethanol. This means that the synchronous of three H-bonds were kept. For the gelator that naphthyl group is conjugated with the amide a through an additional C=C double bond (1-NA, 2-NA in Scheme 1), the compounds formed nanofiber also. However, upon [2+ 2] reaction, the conjugation was destroyed and lead to the formation of nanotoruloid structures. In addition, in the case of 1-NA, where topological [2+2] reaction could not occur, no change could be observed since the H-bonds and π−π stacking took actions simultaneously due to the conjugation. In 1-NALG with a methylene spacer, no matter what the steric effect, the compound could still change into nanotoruloid upon elongated aging.


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It should be noted that due to the breaking of synchronous of the three H-bonds between the amide groups in the ethanol gel of 1NALG and 2NALG, the π−π packing mode of the naphthyl groups can be different, as illustrated in Fig.7C. M-chirality was obtained from gel, while in precipitate turned into P-chirality. Such differences are reflected in the CD spectra, supramolecular chirality inversion was displayed in gel and precipitate. It is clear that the supramolecular chirality was due to the transfer from the chiral center to the assemblies. However, it seemed that the presence or absence of the synergistic H-bonds caused different packing of the naphthyl groups, which determined the signals of the CD spectra,51 as illustrated in Figure 7C.

CONCLUSION We have designed two gelator molecules in which the naphthyl groups are covalently connected to the dioctadecyl L-glutamide via the N-terminal with a methylene spacer.

Both of the

compounds form organogels in polar or non-polar solvents. While the gels formed in the nonalcohol solvents are stable, those formed in alcohol undergo transformed from the gel to precipitate. Through SEM and TEM observations, it was revealed that during such gel collapse process, the initially formed entangled nanofibers changed into solid coaxial nanotoruloid structures, a unique nanostructure that has not been reported in other gel systems. Based on the comprehensive data related to the gelator structures, gelator-gelator interactions, it was suggested that synergistic intermolecular interactions among three H-bonds and π−π stacking caused the fiber formation.

When alcohol was involved, the gelator-alcohol interaction destroyed the

synchronous that lead to the gel collapse. During such changes, not only the three amides split


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into different intensities but also the naphthyl groups changed their packing, leading the gel from M-chirality to P-chirality in the precipitates. The current results present a simple approach to regulate the hierarchical self-assembly morphologies via the gelator-solvent interactions and provide new insight into the dynamic and thermodynamic sides of the supramolecular gels. ASSOCIATED CONTENT Supporting Information. Gelation property of 1NALG and 2NALG in various organic solvents, SEM images of organogel from hexane, UV-vis spectra of 2NALG gel in ethanol with different ageing time, and the XRD pattern of precipitates from ethanol. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Key Basic Research Program (2013CB834504), National Natural Science Foundation of China (21321063, 91427302, 21603276), and Fund of the Chinese Academy of Sciences (XDB12020200), Outstanding Young Scientists Award Fund of Shandong Province, China (BS2014CL028), and Fundamental Research Funds for the Central Universities, China (15CX02052A, 15CX05027A).


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Table of Contents Graphic and Synopsis Here


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Dynamic Evolution of Coaxial Nanotoruloid in the Self-Assembled

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