Synergistic Coassembly of Two Structurally Different Molecular

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Synergistic co-assembly of two structurally different molecular gelators Jingyu Chen, Bing Yuan, Zhenyu Li, Bin Tang, Akhil Gupta, Sheshanath-Vishwanath Bhosale, and Jingliang Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03527 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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Synergistic co-assembly of two structurally different molecular gelators Jing-Yu Chen,† Bing Yuan,‡ Zhen-Yu Li,† Bin Tang,† Akhil Gupta, † Sheshanath-Vishwanath Bhosale§ and Jing-Liang Li*,† †

‡

Institute for Frontier Materials, Deakin University, Geelong, Australia.

Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou, 215006, P. R. China.

§

School of Applied Science, RMIT University, GPO Box 2476, Melbourne, Victoria 3001,

Australia

Keywords: molecular gel, organogel, fiber, self-assembly, NMR

ABSTRACT. Co-assembly of molecules can produce materials with improved properties and functionalities. To this end, achieving a molecular level understanding of the interactions governing the co-assembly is essential. In this work, two molecular gelators with significantly different structures and main intermolecular forces for assembly were co-assembled. The elastic moduli of the hybrid gels are more than one order of magnitude higher than those of the gels

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formed by the individual gelators, showing an obvious synergistic effect. The interactions between the gelators were investigated with confocal microscopy and both one dimensional and two dimensional nuclear magnetic resonance. It was found that the two gelators co-assemble to form fibers due to the nonspecific van der Waals interactions between their alkyl chains and the specific interactions between their functional groups. Switching from one gelator-dominated fiber network to the other gelator-dominated fiber network was achieved at a critical molar ratio of the gelators. The two gelators serve as additives of each other to tune the nucleation and growth of the fiber networks. The observations of this work are significant to the development of materials with improved properties by co-assembly of different molecules.

1. INTRODUCTION

Supramolecular assembly is the basis for the formation of many types of tissues in a human body. In a tissue, multiple types of molecules/components are involved in the assembly to provide complex functionalities, which inspired the design and co-assembly of molecules to achieve materials with new structures and improved properties.1 As a typical class of supramolecular materials, gels formed by low molecular weight gelators (i.e. molecular gels) have attracted special interests due to their many potential applications in food industry,2 pharmaceutics,3 chemical sensing and optoelectronics,4-6 etc. The hierarchical three dimensional fiber network and rheological properties are important for the applications of gels. Progress in fundamental understanding of molecular gel formation has led to the development of various approaches to tune the structure and rheological properties of molecular gels. These include manipulating the thermodynamics of gelator crystallization,7 using suitable additives,7 applying


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external stimuli such as ultrasound,8-9 apply volume confinement10 and structural design/modification of gelators,11 etc. Using dual or multiple gelators to achieve improved/synergistic properties can be another alternative, but has received limited attention. The assembly of a supramolecular gel is due to one or more types of non-covalent molecular interactions such as hydrogen bonding, π-π interaction and van der Waals force between the gelator molecules.12-14 The molecules of a gelator stack together to form a one or two dimensional structure (e.g. fiber or ribbon).12 In a fiber, the molecular stacking is generally governed by specific/strong interactions (such as hydrogen bonding and π-π interaction) between the major/core functional groups of the gelator molecules, while weak interactions such as hydrophobic or van der Waals’ interactions between the minor groups (e.g. alkyl chains) play a minor but essential role in assembly.15-16 In a gel with multiple gelators, depending on the molecular interactions between the different gelators, self-sorting or co-assembly of the individual gelators have been generally resulted.17-18 In case of self-sorting, individual assembly of the gelators (orthogonal assembly) occurs, leading to the formation of a network consisting of fibers of respective gelators. Co-assembly leads to the formation of fibers consisting of different gelators. While much efforts have been devoted to achieve self-sorting of gelators due to its potential applications such as excitation energy transfer and charge separation for organic electronics,19 co-assembly has received little attention. Co-assembly can lead to the formation of gels with new structures and possibly novel/synergistic properties. Understanding the molecular interactions between gelators will help the selection of suitable ones to fabricate co-assembled gels with desired properties. Despite the significant efforts that have been devoted to the design of multi-gelator systems, understanding the interactions between different gelators, in particular those with significant


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structural differences, on a molecular level, is essential for designing novel hybrid materials.20-21 However, most of the designed co-gelators are similar in both molecular structure and the molecular interactions governing their assembly.22 In this work, two molecular gelators, 2,3-di-ndecyloxyanthracene (DDOA) and N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) with different governing interactions, which are π-π stacking and hydrogen bonding, respectively, for gelation are used to construct a hybrid gel with tunable structure and synergistic viscoelasticity. GP-1 has been demonstrated to form gels in a variety of solvents and is used as an additive in cosmetics. The strong fluorescence of DDOA and non-fluorescence of GP-1 are ideal for observing hybrid fiber formation with a combination of confocal and bright field microscopy. In addition, for both gelators, van der Waals force is the secondary interaction for gelation. This will help understand the role of this interaction in co-assembly of gelators. One (1D) and two dimensional (2D) proton nuclear magnetic resonance (1H NMR) were used to understand the gelation and molecular arrangements of the two gelators in their individual and co-assembly. By combining with confocal/optical microscopy, the molecular packing of the gelators in the dual gelator system and the synergistic rheological behavior of the gel are elucidated.

2. MATERIALS AND METHODS 2.1 Materials Dimethyl sulfoxide (DMSO), deuterated DMSO and all the chemicals for the synthesis of 2,3di-n-decyloxyanthracene (DDOA) were purchased from Sigma-Aldrich Co. DMSO was dried with A4 molecular sieve before using. N-lauroyl-L-glutamic acid di-n-butylamide (GP-1) was purchased from Wako Pure Chemical Industries, Ltd., Japan. DDOA was synthesized following reported procedures.23 2.2 Methods


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Gel formation: Thin (300 µm) gel films of DDOA without and with GP-1 and films of GP-1 gels were formed in self-made glass cells. A Linkam heating and cooling stage (THMS600) was used to melt the gels at 90 oC and cooled to 25 oC at a cooling rate of 20 or 5 oC/min for gel reformation. In this manner, all the gel samples were formed under the same thermal conditions. The gel films were used for characterization with confocal microscopy. Confocal microscopic imaging: The DDOA fibers were imaged with a Leica confocal microscope. The excitation wavelength was 405 nm. Fluorescence spectra were obtained with a Hitachi fluorescence spectrometer (F4500). The excitation wavelength was 300 nm. UV-visible absorbance: Gels were melted at 90 oC and aliquots of the sols were dropped into cuvettes with a light path length of 2 mm for gel formation. The absorbance of the gels were examined with a UV-visible spectrometer (Varian Cary-3).

Plain DMSO was used as a

reference. Preparation of Xerogels and characterization with scanning electronic microscopy (SEM): Aerogels were obtained by removing the solvent DMSO in the gels using a supercritical fluid extraction system (SFE, Applied Separations). The SEM images of the xerogels were obtained using a Zeiss Supra 55VP FEG Scanning electron microscope at a voltage of 3.0 KV. The samples were coated with a 5 nm gold layer before examination. X-ray diffraction (XRD): The xgerogels were studied with XRD (PANalytical X’pert Pro) using a Cu Kα radiation source operated at 40 KV with 30 mM current. The samples were scanned with a step angle of 0.013o, and the curves were normalized using Origin. Rheological characterization: The rheological properties were characterized with an Advanced Rheological Expansion System (ARES-2, TA). Briefly, the sol-gel process was performed in-situ between two parallel plates with a gap of 0.5 mm. The samples were subjected to sinusoidal


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oscillation by moving both the upper (with a diameter 40 mm) and the lower plates. The amplitude of the oscillation was controlled to obtain a strain of 0.02% in the sample and the oscillation frequency was set at 0.1 Hz. The temperature ramp rate was 20 or 5 oC/min. Proton nuclear magnetic resonance (1H-NMR): Proton nuclear magnetic resonance (1HNMR): All 1H NMR experiments were carried out on a Bruker Avance III 500 MHz 5 mm broadband NMR spectrometer, operating at 500.13 MHz at 295 K unless stated otherwise. The spectra were processed with Origin and ACD/NMR processor academic edition. In all the experiments of variable temperature, unless further specified, the gels were heated to 90 oC, cooled to selected temperatures, and equilibrated for 5 min before testing. The 2D-COSY spectra were acquired by the standard COSY program of Bruker, with 1024 × 2048 data points. The 2D-NOESY spectra were acquired by Bruker program noesyetgp, using a mixing time of 700 ms to optimize the cross-peak signals. For both COSY and NOESY measurements, the temperature was set at 40 oC and the relaxation delay was fixed at 1.5 s, with 8 scans averaged.

3. RESULTS AND DISCUSSION 3.1 Microstructure and rheological properties GP-1 forms a self-supporting gel in DMSO when its concentration is 100 mM and above. The minimal gelation concentration of DDOA is 2 mM. At concentrations below 100 mM, aggregates of GP-1 can be identified after a long time (several days), but gels cannot be formed (Fig. S1a, supporting information). In mixed gels, the concentration of DDOA was fixed at 2 mM and different concentrations of GP-1, which are 20, 50, 100 and 150 mM, were used, thus resulting in DDOA to GP-1 molar ratios of 1:10, 1:25, 1:50 and 1:75 respectively. A photo of the


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DDOA gel and the hybrid gels are given in Figure S1b. With an increase in GP-1 concentration, the gel becomes more opaque due to the increase in fiber mass. Figure 1a-e are the confocal microscopic images of 2 mM DDOA gel (a) and the mixed gels with different DDOA/GP-1 molar ratios (b-e). Since GP-1 is not fluorescent, the optical transmission microscopic image of 150 mM GP-1 gel is given in Figure 1 f. In the hybrid gels with different DDOA to GP-1 ratios, all the fibers are fluorescent.

Figure 1. Confocal and optical microscopic images of plain DDOA and GP-1 gels and their hybrid gels. (a) Confocal fluorescence image of DDOA (2 mM) gel, (b-e) hybrid gels at different DDOA to GP-1 molar ratios, and (f) Transmission optical images of 150 mM GP-1 gels .The scale bars represent 100 µm. A combination of fluorescence and bright field images indicates that the two gelators coassembled in the fibers (Fig. S2, supporting information). UV-visible spectra show that in the presence of GP-1, the characteristic absorption of DDOA at 250 nm is red-shifted to 260 nm (Fig. S3, supporting information), indicating that the GP-1 affects the π-π stacking of DDOA


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molecules. A red shift indicates the π-π stacking turns to be more off-face.24 In DMSO, DDOA crystallize into spherulitic fiber networks with a few fiber branches growing from the centers of the spherulites. The crystallization of DDOA, GP-1 and their mixtures at the various ratios takes place non-isothermally, which means their crystallization happened before a hot solution is cooled to a certain temperature. Non-isothermal crystallization widely happens in molecular gels when the gelators have a strong gelling capacity in solvents or when the gelators are highly supersaturated.25-26 It was observed from real-time monitoring of the gelation process that the crystallization of DDOA was significantly retarded by GP-1. For example, at a cooling rate of 20 o

C/min, when GP-1 was not present, the crystallization of DDOA (2 mM) takes place at 36 oC.

This temperature was reduced to 28 oC, when GP-1 was 20 mM (ratio 1:10). Consequently, DDOA formed dense spherulites with numerous branches springing out from the nucleation centers (Fig. 1b). This indicates that GP-1 enhances the mismatch nucleation of DDOA. Mismatch nucleation governs branching, including that from a nucleation center, of fibers in a molecular gel.7 It has been observed in our previous work that even in a solution state in DMSO, a large fraction of DDOA molecules does not exist as monomers, but as small aggregates due to strong π-π interactions.27 The attachment of GP-1 molecules on DDOA molecules and aggregates hinders the nucleation of DDOA by retarding their diffusion and further aggregation (Fig. 2a). The interaction between GP-1 and DDOA could be through the non-specific interactions between their alkyl chains or some specific interactions, which have been proven later from the two dimensional NMR spectra of their mixtures. In addition, nucleation preferably takes place on substrates (e.g. dust particles or air bubbles). Due to its significantly higher concentration, GP-1 molecules strongly compete with DDOA molecules for substrates to hinder


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the interactions of DDOA with the substrate. As a consequence of these two effects of GP-1, the nucleation and crystallization of DDOA was inhibited. Increasing the molar ratio of DDOA to GP-1 to 1:25, the spherulitic structure disappeared and fibrillar structures formed. Further increasing the ratio to 1:50 and 1:75, densely branched spherulitic fiber networks appeared. These spherulitic networks are similar to the pure GP-1 fiber network (Fig. 1f), indicating the fiber networks at these high GP-1 concentrations are governed by the crystallization of GP-1. Real time monitoring of the fiber network formation shows that at the molar ratio of 1:25, the fiber growth consists of two stages. When a hot solution of the mixture was cooled to 25.3 oC, fiber nucleation and growth occurred until reaching a static state (i.e. fibers growth stopped) (Fig. S4-a, supporting information). At the second stage, new fibers grew from the surface of existing fibers (Fig. S4-b). This phenomenon was not observed for gels at other DDOA to GP-1 ratios. At the first stage, assembly of DDOA may dominate the fiber formation since GP-1 is harder to crystallize at this low concentration (50 mM). The DDOAdominated fibers formed during the first stage served as templates to lower the Gibbs free energy for GP-1 nucleation, promoting the second-stage fiber nucleation and growth. In other words, GP-1 may only act as an additive to tune the growth of DDOA fibers at the initial stage. The fiber formed at the later stage should consist mainly of GP-1. As observed from confocal fluorescence microscopy, all the fibers formed at this DDOA/GP-1 molar ratio are fluorescent, indicating that DDOA molecules participated in the formation of GP-1 fibers at the second stage. This molar ratio (i.e. 1:25) is a critical point that switches a DDOA-dominated fiber network to one dominated by GP-1. A comparison of Fig. 1e and 1f, which have the same GP-1 concentration, shows that the spherulite networks formed in the hybrid gel are significantly smaller. This indicates that the


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nucleation of GP-1 was promoted by DDOA. The DDOA aggregates can serve as nucleation centers to promote the nucleation of GP-1 (Fig. 2c), primarily due to molecular interactions which will be discussed later.

Figure 2. Schematic illustration of retarded nucleation of DDOA due to (a) interaction of GP-1 with DDOA aggregates and (b) competitive adsorption of GP-1 molecules on substrates (e.g. a dust particle or an air bubble), and (c) illustration of promoted assembly/nucleation of GP-1 by DDOA aggregates. The molecular structures of DDOA and GP-1 are shown.

The SEM images of the pure and hybrid aerogels are shown in Figure 3. The diameters of the individual DDOA fibers are in the range of 100 to 200 nm. Fibers of DDOA formed in the absence of GP-1 are elongated and entangled (Fig. 3a). In the presence of GP-1 (1:10), much


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thinner (20 to 85 nm) fibers formed, which is consistent with the confocal images. At a higher molar ratio of 1:25, more intensive fiber fusion and entanglements occurred which is consistent with the fusion of fibers in microscopic images (Fig. 1c). Elongated fibers that are less entangled are obvious for hybrid gels at higher GP-1 ratios and the pure GP-1 gel (Fig. 3d-f). Spherulitic structures are also obvious for these gels (the insets of Fig. 3d-f). XRD of the aerogels of DDOA, GP-1 and their hybrid gels are given in Figure S5 (supporting information). Compared to the spectra of the pure GP-1 and DDOA aerogels, the main peaks of the hybrid aerogels shifted to smaller angles, indicating co-assembly of the two gelators and larger lattice spacing (looser molecular packing) of the hybrid fibers.

Figure 3. SEM images of gels. The scale bars represent 1 µm, that of the inset in (b) is 200 nm, and those of the insets in (d), (e) and (f) represent 10 µm.

Storage modulus, a measure of elasticity, is an important rheological property of a gel. Figure 4 shows the evolution of storage moduli of DDOA (2 mM), GP-1 (150 mM) and the hybrid gels


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during sol-gel transition. The pure DDOA gel has a modulus of 1300 Pa and that of GP-1 gel is 2500 Pa. The modulus of DDOA gel was improved to about 4700 Pa when 20 mM GP-1 (ratio 1:10) was present. GP-1 at this low concentration cannot form a gel by itself. Therefore, the increase in storage modulus could be due to the change in the structure of fiber networks (dominated by DDOA). GP-1 molecules which participated in DDOA fiber formation can also affect the modulus due to interactions, in particular the hydrogen bonding between GP-1 molecules and interactions between GP-1 and DDOA molecules, which will be discussed in more detail later.

Figure 4. Evolution of storage moduli of gels. Time zero is when the hot solutions started to be cooled.

When the GP-1 concentration was increased to 50 mM (1:25) and 100 mM (1:50), the storage modulus G’ was enhanced to 26 and 27 KPa, respectively, which are more than 20 and 10 times those of the plain DDOA and GP-1 gel, respectively. This indicates a synergistic effect on the storage modulus that was brought about by mixing the two gelators. However, a further increase of GP-1 to 150 mM (1:75), the modulus (c.a. 13 Kpa) of the gel was compromised, though it is still ten times higher than those of the individual gels. It has been demonstrated that the storage


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modulus of a gel is not only affected by fiber mass, the topology of fiber network also has an important role in determining G’.28-29 The fiber network formation in a gel is nucleation and growth process. The entire fiber network of a gel generally consists of a collection of individual fiber networks, each originating from a nucleation center. If the individual fiber networks are compact enough due to significant fiber branching, they will form domains that do not interact with each other or with very weak interactions. The boundary area between the domains is mechanically weak. In this case, the elastic modulus of the gel is weak.7, 28 As shown in Figure 1e, clear domain networks formed when the molar ratio of DDOA to GP-1 is 1:75, which interprets the lower G’ of this gel compared to those with other molar ratios. At the molar ratio of 1:25 the fiber networks are non-spherulitic (non-domain) so that fibers are interconnecting. At the molar ratio of 1:50, although spherulites can be identified, boundary between the neighboring spherulites is not clear. Therefore, the G’ of the gels formed at these two DDOA/GP-1 ratios are higher. The higher fiber mass at the ratio of 1:50 may contribute to the slightly higher G’ of this gel, compared to the one with the ratio of 1:25. In a gel with a single gelator, a network with small domains is generally weaker than one with bigger domains since a network with smaller domains has a larger fraction of domain boundaries, which are mechanically weak. Reducing the boundary area by converting smaller domains/spherulites to bigger ones or by changing a domain network to a network with interconnecting fibers is effective to enhance the G’ of a gel.7 Although the G’ of the gel formed at the molar ratio of 1:75 has smaller domains (Fig. 1e) compared to the plain GP-1 gel (Fig. 1f), it is still higher than (> 6 times) the G’ of the plain GP1, which could be partly due to the larger fiber mass. However, the mass of fibers in Fig. 1e is only 1.5% (contributed by 2 mM DDOA) higher than that for Fig. 1f formed by pure GP-1. The


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enhancement in G’ should also be contributed by some specific interactions that strengthen the network. Suitable additives such as surfactants and polymers have been demonstrated to affect the nucleation and growth of gelator fibers.26-27,

30-33

However, the enhancement in G’, although

observed for some additives, is generally less significant (< 50%) than the enhancement observed (more than one order of magnitude) in this work. A co-gelator can be considered as a special additive, which with a proper structure takes a more active role in participating fiber formation and strengthening the fiber network. The G’ of plain DDOA gel as a function of DDOA concentration was also examined. The G’ reached 17 KPa when the concentration of DDOA was increased 24 mM, which is still far below the value of the hybrid gels at the molar ratios of 1:25 and 1:50. Further enhancement of DDOA concentration was not possible as became highly insoluble at high high concentrations. Therefore, using a suitable co-gelator can be an efficient approach to improve the storage modulus of a gel. This is particularly significant for a gel formed by a precious gelator. The cost for producing such a gel with a desired modulus by increasing the concentration of a gelator can be significantly higher than simply using a cheaper co-gelator. Cooling rate is an important parameter that affect the nucleation and growth of fibers, which can either positively or negatively affect the rheological properties of a gel, depending on the structure of a gel. We examined the rheological property of the pure and hybrid gels at a lower cooling rate of 5 oC/min. For a pure GP-1 gel, this low cooling rate significantly promotes the rheological properties of the gel. At 150 mM, the G’ of GP-1 gel is so high that it is over the limit of the rheometer. Therefore, we tested the G’ of a gel at a lower GP-1 concentration of 100 mM. At this concentration, the G’ of the gel is 4000 Pa, which is more than an order of magnitude


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higher than the value obtained at the cooling rate of 20 oC/min (300 Pa). In general, a higher cooling rate promotes nucleation, which has a similar effect as a lower temperature, both creating a higher driving force for nucleation. Figure S6 (supporting information) compares the confocal/microscopic images of fiber networks of pure and hybrid gels. Enhanced nucleation at a higher cooling rate occurred to both the pure and hybrid gels, as evidenced by the small fiber networks formed at the higher cooling rate. GP-1 forms domain networks in a number of solvent including the DMSO used in this work. Since the boundary area between the domains is mechanically weak, a lower nucleation rate can lead to the formation of bigger domains/spherulites so that the boundary area can be reduced. Therefore the G’ of the gel can be improved at a lower cooling rate. In contrast, for a hybrid gel at a DDOA to GP-1 molar ratio of 1:50, the G’ of the gel is reduced when the cooling rate is reduced to 5 oC/min. The corresponding image in Figure 6S shows that at this low cooling rate, isolated spherulitic fiber networks are formed. The isolated domain network formed at 5 oC/min has an inferior G’ compared to the interconnecting network formed at the higher cooling rate of 20 oC/min. However, it is still higher than the G’ of the pure GP-1 and DDOA gels, indicating the formation of a hybrid gel. The network of 1:50 gel formed at 5 oC/min is similar to the domain network formed at 20 oC/min but with a DDOA to GP-1 molar ratio of 1:75 (Fig. 1e), which is dominated by GP-1. This indicates that reducing cooling rate, GP-1 becomes to dominate at a lower molar ratio, that is, the crystallization of GP-1 is promoted relatively. The results indicate that by manipulating cooling rate, the crystallization of the components of a hybrid gel can be selectively promoted/inhibited, so that the network structure and rheological properties of a gel can be tuned.


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3.2 Variable temperature 1H NMR for sol-gel transition To study in detail the influence of co-gelator on the gelation process, gelation of GP-1 at 10 mM and 100 mM was investigated, with and without DDOA (2 mM), using variable temperature 1

H NMR (VT NMR). In all the spectra in Figure 5, the peaks of –NH- groups of GP-1 at 7.67

ppm and 7.78 ppm shift down field with a chemical shift of 0.2 ppm. This indicates that these groups form H-bonds as the solutions are cooled down, which leads to de-shielding of these –NH protons. In the hybrid systems (Figs.5c and d) when the temperature decreases to 35 oC, the DDOA peaks merge together. This indicates that the movement of DDOA molecules are inhibited, a result that signs gelation.34 While it is noticeable that in the spectra a-c, the splitting of –NH- peaks of GP-1 becomes sharper when the sample was cooled down to 22 oC (the gels were held at each temperature for five minutes). It is because when the temperature decreases, before gelation occurs, the molecules start to assemble resulting in hindrance of movement, making the signals sharper. When gelation starts,

the signals decay until they are fully

vanished.34 However, when the concentration of GP-1 is high enough (100 mM) and with the presence of DDOA, the coalescence of peak splitting started at 35 oC (Fig. 5d). This proves the gelation of GP-1 was promoted by DDOA. A comparison of Figure 4a and 4d shows the presence of DDOA did not have an obvious effect on the peak positions of GP-1, indicating the packing of GP-1 molecules remains unchanged. DDOA molecules could adsorb on the surface of GP-1 stacks/fibrils.


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Figure 5. One dimensional variable temperature 1H NMR of a) 20 mM GP-1, b) a mixture of DDOA and GP-1 at a molar ratio of 1/10, c) 100 mM GP-1 and d) a mixture of DDOA and GP-1 at a molar ratio of 1/50.

Due to the high concentration of GP-1, the peaks of DDOA in the

hybrid systems (b and d) are too small, so they are zoomed in (7.4 -7.2 ppm) for clarity.

3.3 Two dimensional NMR for the interactions between gelators To study the interactions between DDOA and GP-1, and the arrangement of their molecules in fibers, 2D-NMR was performed on pure DDOA and GP-1, as well as their mixture (1:10) in DMSO. COSY experiments were performed to assign the peaks of GP-1 and DDOA. The results are shown in the supporting document (Fig. S7, supporting information).


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Peak crossing indicates close localization of protons in two molecules.35 NOESY experiments were carried out at 40 oC which was chosen based on results of 1D VT NMR. This temperature is just above the gelation point of the samples. Below this temperature, some of the signals starts to decay. If the temperature is too high, the assembly is greatly disturbed by the high mobility of the molecules. In the NOESY spectrum of pure DDOA (2 mM solution in DMSO), strong peak crossing in its aromatic and aliphatic regions were observed (Fig. 6a), indicating the existence of both strong interactions between the aromatic parts and between the aliphatic parts of DDOA molecules. The interactions should be attributable to π-π and van der Waals interactions, respectively. In the aromatic region, three groups of cross-peaks were observed. The D5D8-D6D7 and D5D8-D9D10 crossing is attributed to the close spatial distance within a molecule of DDOA. In the aliphatic region, only the NOESY cross-peaks between Db and its neighbouring methyl groups can be seen. No cross-peaks of D1D4-D5D8 or D1D4-D6D7 were observed, which suggests that all the molecules of DDOA stack in parallel (Fig. 7a). NOESY spectrum was obtained for GP-1 solution at 100 mM (Fig. 6b) since it is the critical concentration of gel formation. In this spectrum, peak crossing of proton G3, G27 to the proton along the chain (CH2-), and overlap of G10 with G16 and G18 were observed. However, it is impossible to identify whether the cross peaks are from the protons of same molecule or from another molecule when they assembles. In the spectra of variable temperature, the peaks of G3, G10, and G27 move noticeably downfield when the sample is cooled. It is a typical evidence of the formation of H-bonds between the -NH- and -C=O groups.

This indicates that GP1

molecules assemble in parallel with all the molecules aligned in the same direction by the Hbonds and the interactions among the alkyl chains. In the aliphatic region of the GP1 spectra, most of the cross-peaks can be attributed to the neighboring protons along the alkyl chain.


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Interestingly, interaction between G28, G4 and G16, -CH3, -CH2- were observed, although they are not spatially close along the aliphatic chain of GP-1. This may suggest that in the assembly the hydrophilic head of one GP1 molecule can come close to the tail of another molecule. This could be due to the van der Waals’ interactions between the short butyl alkyl chains on one GP-1 molecule and the long alkyl tail of another molecules. In the 1D NMR spectra of the mixture, all the peaks of the aliphatic chains of GP1 and DDOA overlap, so information of their interactions could not be obtained. However, cross-peaks of G8D1D4, G3/G27-D1/D4, and G3/G27-D6/D7 were observed in the 2D spectra (Figs. 6c and d), which suggests the aromatic part of DDOA molecules interact with the amide region of GP-1. The interaction could be through the formation of hydrogen bonds between the amide groups of GP-1 and the benzene rings of DDOA.36 However, the peak shift of DDOA is not demonstrated in the VT NMR spectra, meaning the π-π

stacking of DDOA was not affected and GP-1

molecules may only attach to the surface of the stacks. It is also possible for hydrogen bonds to form between the amide groups of GP-1 and the oxygen on DDOA.


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Figure 6. NOESY spectra of a) 2 mM DDOA, b) 100 mM GP-1, c) a mixture of DDOA and GP1 (ratio 1:10). d) Zoomed in down field area of the spectra in c). 3.4 Co-assembly mode of the two gelators


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A gel fiber can be considered as a bundle of fibrils. Each fibril is one stack of gelator molecules. On the basis of the 1H NMR results, it can be deduced that at a low DDOA to GP-1 ratio, DDOA molecules stack into columns/fibrils which are held together by π-π interactions between the anthracene part and the van der Waals interactions between their alkyl chains to form a fiber (Fig. 7a). GP-1 molecules should attach to the surface of DDOA fibrils through two types of interactions, one is the van der Waals interactions between the alkyl chains and the other is the interactions between the aromatic parts of DDOA and the amide parts of GP-1. Both types of interactions would result in the attachment of GP-1 molecules on surface of DDOA fiber or incorporation inside DDOA fibers. The intercalation of GP-1 molecules inside DDOA fibers or adsorption on surface of fibers strengthens the individual DDOA fibrils (stacks) inside a fiber and the interactions between fibers that are close enough, due to formation of hydrogen bonds between the hydrophilic groups of GP-1. When the molar ratio of GP-1 to DDOA is high enough, GP-1 fibers dominate and DDOA molecules interact with GP-1 molecules in fibers in a similar manner. The incorporation of DDOA molecules in GP-1 fibers or their adsorption on the surface of GP-1 fibers also strengthens the fibers and their interactions due to π-π stacking of the aromatic parts of DDOA (Fig. 7b). Therefore, the hybrid gels show synergistic storage moduli that are significantly higher than the gels formed by the individual gelators.


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Figure 7. Schematic illustration of molecular interaction between GP-1 and DDOA in (a) DDOA dominated fibers and (b) GP-1 dominated fibers. Molecules of one gelator serve as an additive to enter into or be adsorbed on the fibers of the other gelator. The interactions between the additive molecules and those between the different gelators strengthen the fiber network by enhancing the interactions between the fibrils of a fiber and between fibers.

Fluorescence measurements demonstrated that DDOA has a strong emission peak at 390 nm, which was reduced in a gel state. This peak disappeared from the spectra at high GP-1 concentrations (1: 50 and 1:75) (Fig. S8, supporting information). Simultaneously, the emission of DDOA at 437 nm was enhanced in the gel state particularly at high GP-1 concentrations. Although the mechanism is not clear, this observation may indicate more restricted


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motion/vibration of DDOA molecules in the hybrid gels. The solubility of GP-1 in DMSO at room temperature (c.a. 22 oC) is about 18 mM. When the concentration of GP-1 is 100 mM, about 82 mM of GP-1 is in gel fibers. Due to the small molar ratio of DDOA to GP-1, DDOA molecules should be well dispersed on/inside gel fibers. The highly restricted movement of DDOA molecules should thus be due to their nano-confinement in the inter-fibrillar space and their interactions with GP-1 molecules.

4. CONCLUSIONS Synergistic assembly of two molecular gelators DDOA and GP-1 produced gels with significantly improved storage modulus, compared to gels formed by the assembly of the individual gelators. Variable temperature 1H NMR spectra demonstrated that the molecular packings of DDOA and GP-1 are not affected. This indicates self-sorting for the dominant gelator (depending on the molar ratio) still exists, with the other gelator as an additive to tune the fiber nucleation and growth. As suggested by the 2D NMR results, molecules of one gelator adsorb on the surface of the fibrils of the other gelator through specific interactions most probably between the amide groups of GP-1 and the benzene rings (and oxygen) of DDOA. Van der Waals’ interactions between the alkyl chains of two gelators could also contribute to their coassembly although the interactions could not be detected due to the similarities between the alkyl chains of the two gelators. The neighboring gelator molecules adsorbed on the surface of fibrils/fibers could interact with each other to strengthen the fiber network, which improves the storage modulus of the gel. The molecular level understanding achieved in this work will help in designing of molecular gelators for preparing soft materials with improved properties such as the storage modulus and energy transfer from one gelator to another.


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ASSOCIATED CONTENT Supporting Information Photos of GP-1 gels, confocal, bright field and merged images of hybrid gels, fiber networks formed at the two stages during DDOAA-GP-1 (molar ratio of 1:25) gel formation, NMR peak assignment of DDOA and GP-1, and fluorescence spectra of DDOA sol, gel and hybrid gels. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Notes The authors declare no competing ﬁnancial interest

ACKNOWLEDGMENT This work was supported by the Australian Research Council (ARC) through a Future Fellowship Grant (FT130100057) and an Industrial Transformation Research Hub in Future Fibres (IH140100018). The ARC is also acknowledged for funding Deakin University’s Magnetic Resonance Facility through a LIEF grant (LE110100141). The authors would like to thank Mrs Shan Du for help with the XRD characterization.

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Table of Contents Graphic

Co-assembly of two molecular gelators enhance the interfibril/interfiber interactions to bring about synergistic effects on the storage modulus of the gels


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Synergistic Coassembly of Two Structurally Different Molecular

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