Binary Supramolecular Gel of Achiral Azobenzene with a Chaperone

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Binary Supramolecular Gel of Achiral Azobenzene with a Chaperone Gelator: Chirality Transfer, Tuned Morphology and Chiroptical Property Lukang Ji, Guanghui Ouyang, and Minghua Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02285 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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Binary Supramolecular Gel of Achiral Azobenzene with a Chaperone Gelator: Chirality Transfer, Tuned Morphology and Chiroptical Property Lukang Ji, †, ‡ Guanghui Ouyang,*,† and Minghua Liu,*,†,‡,§, ||

†

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid,

Interface and Chemical Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

§

Key Laboratory of Nano system and Hierarchical Fabrication, Chinese Academy of

Sciences, National Center for Nanoscience and Technology, Beijing 100190, China

||

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R.

China

KEYWORDS: Co-assembly, Chaperone gelator, Supramolecular gel, Azobenzene, Chirality

ABSTRACT

Binary supramolecular gels based on achiral azobenzene derivatives and a chiral chaperone gelator, long alkyl chain substituted L-histidine (abbreviated as LHC18) which

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could assist many non-gelling acids to form gels, were investigated in order to fabricate the chiroptical gel materials in a simple way. It was found that although the carboxylic acid terminated achiral azobenzene derivatives could not form gels in any solvents, when mixed with LHC18 they formed the co-gels and self-assembled into various morphologies ranging from nanotube, loosed nanotube to nanosheet, depending on the substituent groups on the azobenzene moiety.

The ether linkage and the number of the carboxylic acid groups

attached on the azobenzene moiety played important roles. Upon gel formation, the localized molecular chirality in LHC18 could be transferred to the azobenzene moiety. Combined with the trans-cis isomerization of the azobenzene, optically and chiroptically reversible gels were generated. It was found that the gel based on azobenzene with two carboxylic acid groups and ether linkages showed clear optical reversibility but less chiroptical reversibility, while the gel based on azobenzene with one carboxylic acid and an ether linkage showed both the optical and chiroptical reversibility. Thus, new insights into the relationship among the molecular structures of the azobenzene, self-assembled nanostructures in the gel and the optical as well as chiroptical reversibility were disclosed.

Introduction

Supramolecular gels from the self-assembly of small organic molecules in organic solvents (organogel) or water (hydrogel) have attracted extensive research interests over past decades due to their easy design, controlled nanostructure and the wide scope applications including optoelectronic devices, sensing system, catalysis, tissue engineering and biomaterials and so on.1-10 Among these applications, the reversible chiroptical materials, i.e. 2


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optical materials with reversible chirality, are currently attracting great interest,11-12 in which the gel materials can provide many opportunities. Azobenzene, as one of the most widely investigated photo-active building blocks, is a major candidate to form chiroptical materials.13-14 In order to fabricate chiroptical materials based on azobenzene, it is necessary to introduce the chiral component into the system. A most common strategy is to synthesize new gelators, in which the azobenzene moiety and chiral component are covalently linked together. Thus, various chiral gels based on the azobenzene-containing gelators are developed.15-20

Molecular self-assembly, in which molecules aggregate into nanostructures through non-covalent bonds, has provided an efficient alternative way of creating new structures and functions.21-28 Besides the single-component gels, two-component or binary supramolecular gels and multi-component gels have been developed using the supramolecular approach.29, 30 In binary supramolecular gels, three aspects of combination of small molecules can be considered. The first way is that both the components are gelators.31-33 The second approach is that one kind of gelator molecules are mixed with non-gelling functional molecules.34-37 The third way is that both the components cannot produce gel individually, when mixed together, they formed gels.38-40 In the binary gels, different properties can be separately endowed into each component. By combing them, two properties can be assembled and sometimes, new properties can be created. Recently, our group has developed a kind of “chaperone gelator”, LHC18, which can aid all kinds of L-amino acids to form supramolecular gels.41 In further investigation, we have found that this gelator is quite universal and can make many 3


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carboxylic acid-terminated molecules to form gels. Here, we selected several achiral azobenzene compounds containing carboxylic acid groups to co-assemble with our chaperone gelator LHC18, as shown in Figure 1. The three compounds contain the carboxylic acid groups covalently linked to azobenzene unit or with one or two ether linkages. The purposes of this strategy are as follows. First, we want to explore if our chaperone gelator can aid the carboxylic acid-functionalized azobenzene to self-assemble into gels, so that we do not need to synthesize azobenzene gelator using covalent bonds to develop new functional materials. Second, we want to find out whether the chirality can be transferred to the azobenzene moiety through such supramolecular process. Third, and most importantly, we want to know how the linkage and number of carboxylic acid groups on the azobenzene unit will influence the photo-responsive properties of azobenzene and the subsequent chiroptical properties to accumulate the knowledge of photo-responsive molecular design.

Result and Discussion Molecular design. The chaperone gelator, abbreviated as LHC18 in Figure 1, is covalently linked through a urea bond between L-histidine and a long alkyl chain. With such a molecular design, the compound possesses several non-covalent sites. For example, the urea group can form hydrogen bonds with water or between themselves. The imidazole group can form strong electrostatic and hydrogen bond interactions with carboxylic acid groups. The saturated alkyl tails have intense hydrophobic interactions. These non-covalent bonding sites make it easy for the molecule to self-assemble with carboxylic acid-terminated molecules like amino

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acids to form gels.41 Such property is expected to be applicable to the carboxylic acid-terminated azobenzenes. To study the influence of the position and number of carboxylic acid groups on the azobenzene unit, we designed three kinds of carboxylic acid group-containing azobenzenes: AZO-1, AZO-2, and AZO-3, as shown in Figure 1. AZO-1 has two alkoxy spaced carboxylic acid groups. AZO-2 has only one alkoxy spaced carboxylic acid group and AZO-3 has directly-attached carboxylic acid group. The difference in the number and linking mode of the carboxylic acid groups on the azobenzene unit may show different responsiveness to photo irradiation by these AZO compounds, and may further tune the chiroptical properties and morphologies of the supramolecular co-gels.

Figure 1. Molecular structures of LHC18, AZO-1, AZO-2 and AZO-3. When LHC18 and the three AZOs were mixed together respectively, supramolecular co-gels with various morphologies and different chiroptical reversibility were obtained.

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Co-assembly of LHC18 with azobenzene acid derivatives. Experimentally, 5 mg of LHC18 and a certain amount of azobenzene derivatives were added into dimethylformamide (DMF). The mixture was heated to form a transparent solution and then injected with boiling Milli-Q water and cooled naturally to room temperature to provide supramolecular gels. All the three azobenzene derivatives (AZO-1, AZO-2 and AZO-3, Figure 1) could form gels with LHC18 through the above method. When LHC18 and AZO-1 (molar ratio 2:1) were co-assembled in the mixed solution of 300 µL DMF and 600 µL H2O, an opaque orange gel was obtained which was confirmed by inverted test tube experiments. AZO-2, which has only one carboxylic acid group, when co-assembled with LHC18 at equal molar ratio, an opaque orange gel was also obtained, but the solution condition was changed to 100 µL DMF and 800

µL H2O. AZO-3, which has two carboxylic acid groups directly linked to the azobenzene, could co-assemble with LHC18 to form an opaque gel in the mixed solution of 100 µL DMF and 800 µL H2O.

We also tested other single or mixed solvents for these binary systems and

no gels could be obtained (Figure S1, Supporting Information), indicating that the DMF/H2O mixed solvent was the best system for the co-assembly of LHC18 and azobenzene acid derivatives.

In addition, water was added into a hot solution of DMF.

If the dispersion was

heated from the room temperature, no transparent solution could be obtained and gels were difficult to be obtained. (Figure S2).

We also tested the different ratio of the azobenzene

derivative to LHC18 and found that a stoichiometric ratio based on the carboxylic acid and LHC18 was the best for the gel formation (Figure S3).

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Characterization by SEM and TEM analysis. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the co-assembled gels were recorded. The co-assembled nanostructures of different two-component molecules are shown in Figure 2. Different AZO molecules lead to distinct co-assembly behaviours according to the structural differences of the components.

Figure 2. SEM images of co-assembly of LHC18 with azobenzene derivatives: a) LHC18/AZO-1 (the inserted figure is TEM image); b) LHC18/AZO-1 under UV 365 nm irradiation for 2 hours; c) LHC18/AZO-2 (the inserted figure is TEM image); d) LHC18/AZO-2 under UV 365 nm irradiation for 2 hours; e) LHC18/AZO-3 = 2/1; f) LHC18/AZO-3 under UV 365 nm irradiation for 2 hours. When LHC18 and AZO-1 were co-assembled at a molar ratio equal to 2/1, an opaque orange gel was obtained. Uniform loosed nanotube structures were formed, which were visualised by the SEM, and further confirmed by TEM observations (Figure 2a, the inserted

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figure is a TEM image). The inner diameter of the loosed nanotube is about 20 nm, and the outer diameter is about 100 nm, according to the TEM analysis results. It should be noted that AZO-1 itself formed precipitates without ordered nanostructures in the mixed solution, as shown in Figure S4 (supporting information). The SEM image of the co-assembly of LHC18 with AZO-2 (at a ratio of 1/1) showed a nanotube structure, whose length was much shorter than that of the above loosely nanotube (Figure 2c). The inner diameter of these nanotubes are about 50 nm, which are bigger than that of loosely nanotubes formed in AZO-1. The structure of the co-assembly of LHC18 with AZO-3 (at a ratio of 2/1) was an obvious plate structure (Figure 2e), different from the nanotube structure of the co-assembly of LHC18 with AZO-1 and AZO-2. The SEM and TEM data demonstrated that the morphology of the co-assembled gels could be well tuned by changing the structure of non-gelling azobenzenes. Morphological changes triggered by UV irradiation. The gels contain photo-active azobenzene derivatives and their changes upon the alternative UV/Vis irradiation were investigated. The gels formed by LHC18/AZO-2 could collapse into sol under UV irradiation, while the gels of LHC18/AZO-1 and LHC18/AZO-3 remained as the gels. The morphological changes of the nanostructures in three gels triggered by UV irradiation are shown in Figure 2. For the LHC18/AZO-1 gel after exposure to 365-nm UV irradiation for 2 hours, partial of the nanotubes were found to break into ribbon (Figure 2b). These nanostructures cannot be recovered by visible light irradiation, even though the UV-Vis spectrum could be recovered (Figure 5a). For the LHC18/AZO-2 gel, partial of the nanotubes collapsed into plates after

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365-nm UV irradiation for 2 hours (Figure 2d) and also could not be recovered by visible light, as shown in Figure S6. For the LHC18/AZO-3 gel, the plate nanostructure did not change after 365-nm UV irradiation (Figure 2e-f). The rheological data for these gels before and after UV irradiation were measured as shown in supporting information (Figure S5). It was found that all the storage modulus (G’) were an order of magnitude larger than the loss modulus (G’’), indicating that these gels were more like elastic materials rather than viscous materials. Upon UV irradiation, LHC18/AZO-1 and LHC18/AZO-3 gels did not collapsed into sol but both the G’ and G’’ value decreased (Figure S5a-b). LHC18/AZO-2 gel (Figure S5c) could collapse into sol under UV irradiation. These results demonstrated that introduction of a spacer group between the carboxylic acid and azobenzene unit, for example alkoxy group, changed the molecular packing of the azobenzene and further caused the different morphological changes in the assemblies.17 FTIR Spectroscopy

To gain further insight into the molecular packing and non-covalent

interactions within the co-assemblies, FTIR spectra were recorded for the different binary xerogels, as shown in Figure 3a. These FT-IR spectra showed some common characteristics. First, strong vibrations are observed at 3375 and 1637-1633 cm-1, which could be assigned to the N-H vibrations of amide groups, indicating the H-bond in these assemblies. Second, all the FT-IR spectra of the co-assemblies showed strong vibrations at around 1620 cm-1, which demonstrated a strong and ordered hydrogen bonding between the urea groups.42-44 Third, except LHC18, all the FT-IR spectra of the co-assemblies showed wide absorption bands at around 1950 or 2550 cm-1, which could be attributed to the formation of the imidazolium

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carboxylate salts (Figure 3b).45, 46 Finally, all the samples of the asymmetric and symmetric stretching vibrations of CH2 appeared at around 2920 cm-1 and 2850 cm-1, respectively, indicating that the alkyl chains were closely and orderly packed in a zigzag way. These results indicated that non-covalent bonds like hydrophobic interactions, hydrogen bonding between urea groups, and the electrostatic interaction between carboxylic acid and imidazole groups exist in the co-assemblies regardless of the components and ratio.

Figure 3. FT-IR spectra of LHC18 and the co-assemblies of LHC18 with AZO-1, AZO-2, AZO-3. a) in a range from 4000 to 800 cm-1, b) expanded FT-IR spectra in the region from 2700-1900 cm-1. X-ray Diffraction (XRD) patterns. In order to further evaluate the nanostructures of the different samples, the xerogels or the air-dried suspensions were cast on quartz plates and their X-ray diffractions (XRD) were measured. The XRD patterns of all the samples are shown in Figure 4. For pure LHC18 sample, two obvious peaks are observed at 2θ values of 2.51 and 4.98, which correspond to the d values of 3.51 and 1.77 nm, respectively, suggesting a bilayer stacking of LHC18. Upon co-assembling with the azobenzene derivatives, the XRD changed obviously. In the case of LHC18/AZO-1, diffraction peaks are observed in 2θ values 10


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of 1.78, 3.61, 5.37, 7.03 and 9.05, which is corresponding to a layer distance of 5.02, 2.45, 1.64, 1.25 and 0.97 nm, respectively, indicating a well-defined lamellar structure with a d-spacing of 5.02 nm (Figure 4 and Figure S7a). A shoulder peak at 3.18 nm, which is deviated from the bilayer structure of LHC18/AZO-1, might be due to the packing of LHC18, which partially existed in the mixtures. For the XRD of the LHC18/AZO-3 co-assembly, three clear peaks are observed at 2.47, 4.99 and 9.93, which correspond to a d value of 3.55, 1.78 and 0.88 nm, respectively. This pattern also indicated that the co-assembly formed a lamellar structure. For the LHC18/AZO-2 assembly, three peaks are observed at 2θ values of 3.18, 6.50 and 9.70, which correspond to a d value of 2.78, 1.36 and 0.91 nm, respectively, suggesting also a lamellar structure with a layer distance of 2.78 nm. These XRD patterns indicated that in all the gels, the complex between AZOs and LHC18 formed layers structures and the detailed molecular packing will be shown in the discussion section.

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Figure 4. XRD patterns of LHC18 and the co-assemblies of LHC18 with AZO-1, AZO-2, AZO-3. Optical and Chiroptical properties. Azobenzene is well-known for its reversible isomerization upon alternative UV/Vis irradiations. In order to see if such properties can be kept in the gels, the UV-Vis spectra of the gels under alternative UV/Vis irradiations were investigated, as shown in Fig. 5a, c and e for LHC18/AZO-1, AZO-2 and AZO-3 gel, respectively. For the LHC18/AZO-1 co-assembly, before 365-nm UV light irradiation, the spectrum was dominated by the 349-nm absorption which was ascribed to the π–π* absorption band of the trans-azobenzene moiety. As UV irradiation proceeded for one hour, the 349-nm absorption band decreased with concomitant increase of the π–π* and n–π* bands of the cis-isomer at around 310 nm and 440 nm, respectively. After visible light irradiation for one hour, the 349-nm absorption band of the trans-azobenzene again appeared, and the 310-nm and 440-nm signals of the cis-isomer disappeared. The UV-Vis spectrum result strongly demonstrated the azobenzene trans-cis transition triggered by UV or visible light. Meanwhile, such a conformational change could be reversed by light.

For the

LHC18/AZO-2 co-assembly, similar change could be observed and the spectra showed an obvious reversible change by 365-nm UV irradiation and visible light. For LHC18/AZO-3 assembly, in which the carboxylic acid group was directly linked with the azobenzene moiety, the spectra were different from those of AZO-1 and AZO-2 containing assemblies.

The

LHC18/AZO-3 gel showed no obvious spectral change, except that the signal at 370 nm was weakened.

Upon visible light irradiation, the absorption at 370 nm can also recover to its

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original but with fewer changes in the longer wavelength region. This indicated that the packing of the trans-azobenzene in AZO-3 is closer than that in the other two derivatives and the trans-cis isomerization is largely supressed due to the packing.

The ether linkage

between the carboxylic acid group and azobenzene moiety could make the packing loose and favour for the trans-cis isomerization of azobenzene.17 Since LHC18 molecule contains a chiral center, circular dichroism (CD) investigation may provide further information on the molecular packing and assembly structures. Although all the aqueous solutions of the three individual azobenzene acid derivatives were CD-silent due to their achiral nature, the co-gels with LHC18 showed obvious CD band in the azobenzene moiety, as shown in Figure 5b, 5d and 5f. To gain further insight into the self-assembly information of the binary gels, circular dichroism (CD) studies were performed. For the LHC18 and AZOs co-assemblies, clear Cotton effects are observed, which are different for the three azobenzenes. For LHC18/AZO-1, a positive and negative Cotton effect is observed at 354 nm and 422 nm with a crossover at 401 nm. For LHC18/AZO-2, a negative and positive Cotton effect is observed at 318 nm and 376 nm with a crossover at 339 nm, LHC18/AZO-3, a positive and negative Cotton effect is observed at 324 nm and 373 nm with a crossover at 332 nm. These CD activities observed in the assemblies demonstrated that upon cooperative self-assembly, LHC18 transferred its chirality to AZOs. It is interesting to note that the assemblies of LHC18/AZO-1 and LHC18/AZO-3 showed an opposite Cotton effect to that of LHC18/AZO-2.

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Figure 5. UV-Vis spectra of co-assembly of LHC18 with azobenzene acid derivatives (a) LHC18/AZO-1 = 2/1; (c) LHC18/AZO-2 = 1/1, (e) LHC18/AZO-3 = 2/1; Circular Dichroism of different co-assemblies under 365-nm UV irradiation and visible light irradiation (b) LHC18/AZO-1 = 2/1; (d) LHC18/AZO-2 = 1/1; (f) LHC18 and AZO3 = 2/1; Plot of g) CD of LHC18/AZO-2 = 1/1 after repeated photo irradiation cycles: UV (365 nm) half hour; then visible light half hour, repeated 5 cycles.

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On the other hand, the gel samples also showed different changes in CD spectra upon photo-irradiation. Under 365-nm UV irradiation for 2 hours, only slight changes in the intensities are found in the CD spectra of LHC18/AZO-1 and LHC18/AZO-3. However, a significant change in the Cotton effect is observed in the LHC18/AZO-2 gel. The exciton couplet changed into a negative valley at 360 nm with great decrease in intensity. Upon photo irradiation with visible light, the CD spectrum almost recovered to its original. Interestingly, this process could be repeated at least for five cycles, and the CD signals were not weaken. For the LHC18 and AZO-3 co-assembly, it only had weak CD signals and no obvious absorbance change in the CD spectrum under 365-nm UV irradiation. Discussion Based on the above experimental results, a possible mechanism for the co-assembly of the chaperone gelator LHC18 with different non-gelator azobenzene acid derivatives is proposed, as shown in Figure 6. It is evident from the above results that the LHC18 amphiphile could form gels with AZO-1, AZO-2 and AZO-3 in a proper ratio. Multiple non-covalent interactions were involved in the process. As shown in Fig. 6c, compounds AZO-1 and AZO-3 formed 1: 2 complexes with LHC18 like gemini amphiphiles, while AZO-2 formed a conventional amphiphile (Fig. 6b). The linkage between LHC18 and AZOs are due to the electrostatic interaction and H-bond, as shown in Fig. 6e. Based on the XRD patterns, all these complex amphiphiles formed bilayer structures but their packings are slightly different. For LHC18/AZO-1 and LHC18/AZO-3, the packing are very similar but their layer distance are

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quite different. Due to the existence of the ether linkage, the layer distance of LHC18/AZO-1 is longer than that of LHC18/AZO-3. In the case of LHC18/AZO-3, the layer distance is only a little larger than that of LHC18, due to the compact packing of the azobenzene between two LHC18 molecules. In the case of LHC18/AZO-2, the layer distance is even smaller than that of LHC18. This suggested that the complex is rather inclined both for the alkyl chain and azobenzene moiety. Furthermore, the azobenzene moiety packed differently for the AZO-1 and AZO-3 in comparison with AZO-2. The former two is suggested to be nearly perpendicular to the alkyl chain or bilayer direction, while the latter is parallel to the layer direction. This is clearly reflected in the CD spectra. We have observed just opposite Cotton effect in LHC18/AZO-2 gel with respect to those in LHC18/AZO-1 or LHC18/AZO-3. Since the supramolecular chirality of the gel was due to the transfer of the chirality from the localized chiral centre on LHC18 to the chromophore, the direction of the transition moment is vitally important. Since the orientation and the transition moment of the azobenzene moiety in the gels of LHC18/AZO-2 is vertical each other to those in LHC18/AZO-1 or LHC18/AZO-3, as illustrated in Fig. 6i, their Cotton effect are opposite. The bilayer structure served as the basic unit and further stacked each other and strengthened by the hydrogen bond between the urea groups and the hydrophobic interaction between the long alkyl chains. They stacked into multi-bilayer structures and formed the nanotube or nanosheet structures due to the packing differences. Ultimately, the nanostructure woven into three-dimensional (3D) networks, which could trap solvent molecules, thereby leading to the formation of gels.

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Figure 6. Illustration on the molecular packing in the binary gels. (a) Cartoon showing molecular structure and the chaperone gelator; (b) Cartoon representing the LHC18/AZO-2; (c) Cartoon representing the LHC18/AZO-1 or LHC18/AZO-3, (d) Interaction between the headgroup of the azo-compound and LHC18; (e) Non-covalent bond between AZO3 and LHC18; (f-h) molecular packing of LHC18/AZO-1, AZO-3, and AZO-2, respectively; (i) Transition moment of the azo-compounds to the chiral center localized on the LHC18. The nanostructures are sensitive to the light irradiation. For the azobenzene compounds with ether linkage, they showed better reversibility even in the gel. However, in the case of AZO-1 and AZO-3, the two head groups are complexed with the LHC18, making the azobenzene moiety lacking of free space. Thus, their reversibility is worse. And this is even worse for AZO-3, whose carboxylic acid group was directly linked to the azobenzene. This could be verified from the XRD pattern related to the packing of the azobenzene, a shown in 17


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supporting information Fig. S6, a loose packing distance of 0.40 or 0.39 nm was observed for AZO-1 and AZO-2, while a tight packing of 0.36 nm was observed for AZO-3. Thus, we observed the clear optical reversibility in the LHC18/AZO-1 and AZO-2 but less with AZO-3. For the chiroptical activity, since the azobenzene moiety are tightly clipped by two LHC18 in LHC18/AZO-1 and LHC18/AZO-3, it did not show good reversibility. In the case of LHC18/AZO-2, with good optical reversibility and relative free azobenzene moiety, it showed the best chiroptical reversibility in the three AZOs. The above results indicated that for the self-assembly of azobenzene based chiroptical materials, the number and position of the terminal chiral groups attached to the azobenzene moiety had a critical influence on the co-assembly structure and chiroptical properties. In order to achieve a well-reversed chiroptical property, the spacer of the covalent junction of the azobenzene with the chiral functional group is of utmost importance. An ether spacer is preferred. In addition, the azobenzene should not be confined to a limited space, such as in the case of a bola form.

Conclusions In summary, a simple supramolecular approach has been proposed to achieve morphological change and the reversible chiroptical property in a binary system consisting of histidine acid-based amphiphiles and azobenzene acid derivatives. It was suggested that the intense electrostatic interaction and the H-bond between the carboxylic acid groups and imidazole group, the hydrogen bonding between urea groups, and the hydrophobic

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interactions between alkyl chains were responsible for the self-assembled nanostructures. We found that all the carboxylic acid terminated azobenzene derivatives could form gels with LHC18. However, depending on the linkage and the number of linkages, the nanostructures of the self-assemblies and their chiroptical properties are completely different. For azobenzene based amphiphile, a flexible spacer like alkoxy is benefit to obtain efficiently reversible chiroptical properties.

Experimental Section Materials Preparation. All the starting materials and solvents were obtained from commercial

suppliers

and

used

as

received.

(E)-4-(phenyldiazenyl)phenol,

(E)-4,4'-(diazene-1,2-diyl)diphenol, methyl 2-bromoacetate were purchased from Acros and AZO-3 was purchased from TCI. The experimental details for the synthesis of AZO-1 and AZO-2 were provided in the Supporting Information. Gels Formation in Mixed Solvents. A typical procedure for the gels formation in mixed solution was as follows: 5.0 mg of LHC18 and 1.8 mg AZO-1 were mixed in 300 µL DMF solution, heat to form a transparent solution, and then injected 600 µL boiled Milli-Q water, the mix solution was cooled to room temperature naturally and an orange gel was obtained after 30 min. The produces for gel preparation between AZO-3 and LHC18 were the same as mentioned above, but for AZO-2, 100 µL DMF and 800 µL Milli-Q water were used instead. Gelation was confirmed by tube inversion method.

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Spectral measurements. UV−Vis spectra were recorded in quartz cuvettes (light path 0.1 mm) on a JASCO UV-550 spectrometer. Circular Dichroism (CD) spectra were recorded in quartz curettes (light path 0.1 mm) on a JASCO J-810 spectrophotometer. The samples were prepared and cast on quartz plates. The irradiation was performed on the 0.1 mm cell with UV (30 W at 365nm) and visible light (40 W compact fluorescent lamp, >400nm), respectively. Fourier transform infrared (FT-IR). FT-IR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer at room temperature. The KBr pellets made from the vacuum-dried samples were used for FT-IR spectra measurements. Scanning Electron Microscopy (SEM). Samples were cast onto single-crystal silica plates, the solvent was evaporated under the ambient conditions, and then vacuum-dried. The sample surface was coated with a thin layer of Pt to increase the contrast. SEM images were recorded on a Hitachi S-4800 FE-SEM instrument with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM). TEM images were obtained on a JEM-1011 electron microscope at an accelerating voltage of 100 kV. The TEM samples were prepared by casting a small amount of sample on carbon-coated copper grids (300 mesh) and dried under strong vacuum. X-ray Diffraction (XRD). XRD analysis was performed on a Rigaku D/Max-2500 X-ray diffract meter (Japan) with Cu Kα radiation (λ = 1.5406 Å), which was operated at a voltage of 40 kV and a current of 200 mA. Samples were cast on glass substrates and vacuum-dried for XRD measurements.

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1

H NMR Spectra. 1H NMR (400 MHz) spectra were recorded on a Bruker Avance 400

spectrometer with TMS as internal standard at 298 K. Mass Spectra. Mass spectral data were obtained by using a BIFLEIII matrix-assisted laser desorption/ionization time of fight mass spectrometry (MALDI-TOF MS) instrument. Rheology Study. Mechanical properties were measured on a strain-controlled rheometer (Discoevry-DHR-1, TA Instruments) using a cone-plate geometry (40 mm diameter). The dynamic moduli of the gels were conducted as a function of frequency between 0.01 and 100 Hz. The experiments were performed at 25±0.05°C and the temperature was controlled with an integrated electrical heater.

AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected] ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the Basic Research Development Program (2013CB834504), the National Natural Science Foundation of China (Nos. 91427302, 21602223), “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12020200). REFERENCES

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Table of Content Entry

Binary Supramolecular Gel of Achiral Azobenzene with a Chaperone Gelator: Chirality Transfer, Tuned Morphology and Chiroptical Property Lukang Ji,†, ‡ Guanghui Ouyang,*,† and Minghua Liu,*,†,‡,§, ||

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Binary Supramolecular Gel of Achiral Azobenzene with a Chaperone

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