Chiroptical Vesicles and Disks That Originated ... - ACS Publications

May 12, 2015 - ABSTRACT: We report a chiral gel of vesicles and disklike micelles that originated from achiral molecules. The supramolecular chirality...
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Chiroptical Vesicles and Disks That Originated from Achiral Molecules Wenrong Zhao, Lei Feng, Lu Xu, Wenlong Xu, Xuan Sun, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: We report a chiral gel of vesicles and disklike micelles that originated from achiral molecules. The supramolecular chirality was obtained via regulating pH, in which a sol− gel−sol transition in a colloidal system consisting of a gelator, 4,4-di(2,3-dicarboxylphenoxyl)azobenzene (AzoNa4), and a zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO), happened. The supramolecular chirality was related to the state of aggregation, i.e., only the condensed gels show chiral sense and sols are chiral-silent. The coexistence of vesicles and disklike micelles was captured for the first time in supramolecular chiral hydrogels by cryo- and freeze-fracture transmission electron microscopy (cryo- and FF-TEM) observations. Ascribed to the photoisomerization of the azobenzene units, upon alternative UV/visible light irradiation, the gel chirality can be switched reversibly with the macroscopic changes between vesicles/disks and wormlike micelles. A pH- and light-dual-responsive chiroptical switch can be constructed, which may require understanding the regulating membrane permeability and reagent release of structural transformation through photoisomerization and also require understanding the origin of gelation-induced supramolecular chirality completely based on achiral molecules.

1. INTRODUCTION Chirality is no more than a description for the handedness of a single molecule, and it is also for nanostructures, such as supramolecular chirality or even macroscopic items. Especially for the supramolecular chirality describing the nonsymmetrical arrangements of molecules, chirality has fascinated the chemistry world of asymmetric catalysis, molecular recognition, and chiroptical responsiveness. In the supramolecular gel systems, a chirality switch could be acquired by three main strategies: (i) intrinsically chiral gelator, (ii) chiral/achiral hybrid gelator system, and (iii) totally achiral gelators.1 Among these, a chiroptical switch entirely composed of achiral molecules is the highest pursuit. It is only rarely reported and there is difficulty in characterizing the resulting products, except when the products are large enough to be captured by electron microscopy.2 Occasionally, achiral molecules are revealed to form nanostructures,3 which is important in understanding the self-assembly process, chirality amplification, and symmetry breaking. Liu and co-workers reported a chiroptical supramolecular switch exclusively formed from an achiral molecule at the air/water interface.3 The achiral 5-(octadecyloxy)-2-(2thiazolylazo)phenol (TARC18) can organize into a film with chirality, showing a reversible change when alternatively exposed to HCl gas and air. Among these exceptional works, the supramolecular chirality composed of achiral molecules via symmetry breaking is mainly reported in the crystalline state,4 as well as in solution5 and liquid crystals.6 Nevertheless, in gel systems, especially colloidal (soft material) systems, gel-induced supramolecular chirality composed of totally achiral molecules © XXXX American Chemical Society

via symmetry breaking remain unexploited to date. Zhang et al. reported the first chiral gel example based exclusively on the achiral molecules via mirror symmetry breaking, induced by strong directional interactions derived from the complex between the rigid bent bridging ligands and Ag+.7 We have reported that achiral azobenzene derivatives functionalized with four carboxylic acid groups, i.e., 4,4di(2,3-dicarboxyl phenoxyl)azobenzene (AzoNa4, Scheme 1 and Figure S1 in the Supporting Information), can selfassemble into chiral nanotubes induced by symmetric breaking8 and chiral wormlike micelles induced by the achiral molecules.9 Possessing an ingenious structure, the azobenzene gelator Scheme 1. Molecular Structures of the Achiral Gelator AzoNa4 (1) and C14DMAO (2)

Received: March 29, 2015 Revised: May 9, 2015

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switch in creating pH-responsive and photoresponsive supramolecular assemblies. Colloidal chiral gels containing vesicles and disks were constructed by introducing the achiral molecules AzoNa4 into a zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO) micellar solution, upon adding HCl. On the basis of the acid-induced gelation and morphology change, spatially fixed H-type aggregation of chromophores in the gel state and tilting chiral bilayers in a certain tilting direction field were suggested to produce the supramolecular chirality via Hbond. Regulating pH, the chirality can be tuned with a sol− gel−sol transition, on account of the hydrophilicity variation of Azo molecules (AzoH4 is much more hydrophobic than AzoNa4) resulting from strength change of the H-bond. The colloidal chirality is related to the state of aggregation, that is, only the condensed gels of vesicles and disks show a chiral sense but the sols are chiral-silent. Because of the H+dependent gelation, chiroptical switch responding to pH was well-displayed. On account of the isomerization of AzoH4 in response to light irradiation, the light-responsive supramolecular chirality was obtained with structural transformation between vesicles/disks and wormlike micelles, evidenced by distinct spectral and geometric properties.

functionalized with four carboxylic acid groups in periphery acts favorably in triggering gel formation via noncovalent interaction. Herein, the intriguing gelator AzoNa4 once more induced a chiral gel with novel structures, composed of vesicles and disklike micelles that originated from achiral molecules with the assistance of a common nonionic surfactant C14DMAO. Besides, the vesicles and disks immobilize water via the special bilayers structures, which is important in understanding and widening the field of how gelator immobilizes the solvent. Gel-induced supramolecular chirality might be expected if the strong intermolecular H-bond remains dominant within the gel. In gel systems comprising amphiphilic molecules, surfactants can self-assemble into kinds of aggregates like micelles (globular, elliptic, and wormlike micelles) or bilayers (vesicles, disk, and lamellae) of different morphologies and sizes. Among them, long-range supermolecular chirality can be acquired more easily in cylindrical wormlike micelles2 but is rarely reported in chiral bilayer structures. 10−14 The chiral bilayer effect phenomenon15,16 has been extensively investigated by Fuhrhop et al. with potassium tartaric dodecylamide,15,16 in which the bilayer helices are formed in water with a thickness of 4.0 nm. Whitten and co-workers reported that azobenzene derivatives containing aromatic/dye chromophores formed bilayer aggregates in solution with circular dichroism (CD) signals for transazobenzenes and no induced CD signals for cis-azobenzenes upon alternative light irradiation.17 Although they were unable to calculate the possible structure of bilayers of azobenzene phospholipids,17 a chiral “pinwheel” unit aggregate structure comprising asymmetric trimers on the basis of “H”-type aggregate to account for the chiral nature by using Monte Carlo cooling methods was proposed. Yonemori and coworkers reported chiral bilayer membranes having CD when a hydrophobic azo dye was added.10 Kunitake and co-workers reported enhanced CD and phase separation of azobenzenecontaining chiral bilayers, in which the large CD enhancement of chiral bilayers arises from the interaction of spatially fixed chromophores in the gel state and the rigidity of chiral bilayers.11 Ou-Yang and Liu derived the tilt and surface shapeequilibrium equations for tilted chiral lipid bilayers in analogy with cholesteric crystals.12 Referring to bilayers structures, constructed by surfactants or cationic/anionic (catanionic) surfactant mixtures, vesicles are hollow aggregates with enclosed amphiphilic molecule bilayer membranes filled with and dispersed into a liquid solvent.18 Both the interior and exterior of the vesicles are filled and distributed with water. The bilayers of the vesicles made the water in the interior enclosed. The water in the exterior of the vesicles is also restricted by the densely packed vesicles and so flow is difficult. The bilayer structures of the vesicles play a very important role in immobilizing the solvent in the gel. Being spherical on average, vesicles can vary largely in shape and size and can have uni- or multilamellar or oligovesicular forms.19 Disks are high-energy structures with high curvature edges and possess a high rigidity to bending that hinders their spontaneous closure to vesicles.18 In our work, we report an example of supramolecular chiral gel of vesicles and disks based on achiral molecules. To the best of our knowledge, disk micelles are captured for the first time in supramolecular chiral hydrogels. AzoNa4, which contains two functional moieties, viz. the carboxylic groups, which are susceptible to H+, and the azobenzene group, which is sensitive to light irradiation, was employed. AzoNa4 can be used as a dual-responsive molecular

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Tetradecyldimethylamine oxide (C14DMAO) was purchased from the affiliate of Clariant Company (Germany) in China and was delivered as an aqueous solution. The C14DMAO solution was freeze-dried and recrystallized three times in acetone. The gelator, 4,4-di(2,3-dicarboxylphenoxyl)azobenzene (AzoNa4) was synthesized according to our earlier study;8,9 the structure and the photoisomerization are characterized by mass and 1H NMR measurements, respectively (Figure S1 in the Supporting Information). The deionized water was distilled three times using a UPH-IV ultrapure water purifier. The other reagents were products of analytical reagent (A.R.) grade. 2.2. Gel Formation and Adjustment with HCl. Typically, 0.3870 g of C14DMAO and 0.0630 g of AzoNa4 were dissolved in 8.0 mL of distilled H2O to form the stock solution of 187.50 mmol L−1 C14DMAO and 12.50 mmol L−1 AzoNa4 mixtures. HCl solution with an accurate concentration of 1.0 mol L−1 was prepared. HCl solution (0.1200 g; 1.0 mol L−1) and 0.0800 g of distilled H2O were mixed and then added dropwise into 0.8000 g of the stock solution to be 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4/120 mmol L−1 HCl. Upon the addition of the HCl, the gel was formed immediately, which can be confirmed by both naked eyes and the characterization. To illustrate the influence of the HCl concentration on the formation of the gel, controlled experiments were executed by changing the amount of the HCl solution. As an example, 0.1800 g of HCl solution (1.0 mol L−1) and 0.02000 g of distilled H2O were mixed and then added to 0.8000 g of the stock solution to be 150 mol L−1 C14DMAO/10 mol L−1 AzoNa4/180 mol L−1 HCl, which forms a sol. Accordingly, a series of sols/gels with different ratios, namely, solutions of 150 mol L−1 C14DMAO/10 mol L−1 AzoNa4/80 mol L−1 HCl, 150 mol L−1 C14DMAO/10 mol L−1 AzoNa4/50 mol L−1 HCl, and 150 mol L−1 C14DMAO/10 mol L−1 AzoNa4/20 mol L−1 HCl were fabricated, respectively, according to a phase diagram of the C14DMAO/AzoNa4 system (Figure S2 in the Supporting Information). As for comparison, solution without HCl was also prepared, i.e., 0.20 g of distilled water was added to 0.8000 g of the stock solution to form the solution of 150 mol L−1 C14DMAO/10 mol L−1 AzoNa4/0 mol L−1 HCl. KOH solution with an accurate concentration of 1.0 mol L−1 was prepared. KOH solution (0.2000 g, 1.0 mol L−1) was added dropwise into 1.0000 g of all the above gel systems. Upon the addition of the KOH, the gel was destroyed gradually and finally turned into waterlike fluid. 2.3. Rheological Measurements. Rheological measurements were carried out on a HAAKE RS6000 rheometer with a coaxial B

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Figure 1. (a) Photographs of samples formed by 10 mmol L−1 AzoNa4 /150 mmol L−1 C14DMAO with different cHCl: 0, 120, and 180 mmol L−1 (from left to right). (b) Viscosity measurements of samples formed by 10 mmol L−1 AzoNa4 /150 mmol L−1 C14DMAO with different cHCl: 0, 120, and 180 mmol L−1. cylinder sensor system (Z41 Ti) for lower-viscosity samples and a cone−plate system (C35/1° Ti L07116) for samples with high viscosity. 2.4. Dynamic Light Scattering Measurements. The solutions were determined by dynamic light scattering (DLS, using a Brookhaven BI-200SM instrument). A 200 mW green laser (λ = 532 nm) with variable intensity was used, and measurements were carried out with scattering angles of 90°. The intensity−intensity time correlation functions were analyzed by the CONTIN method. To ensure the stainless condition for use, the cells used for light scattering were rinsed with distilled acetone. 2.5. Freeze-Fracture Transmission Electron Microscopy Observations. Aggregate structures in the high-viscosity solution were determined by freeze-fracture transmission electron microscopy (FF-TEM) observations. A small amount of the solution was mounted onto a specimen holder. The sample was frozen by quickly plunging the specimen holder into liquid ethane that had been cooled with liquid nitrogen. Fracturing and replication were performed on a freezefracture apparatus (EM BAF 060, Leica, Germany) at a temperature of −150 °C. Pt/C was deposited at an angle of 45° to shadow the replicas, and C was deposited at an angle of 90° to consolidate the replicas. The replicas were transferred onto a copper grid and then observed by using a JEOL JEM-1400 TEM that was operated at 120 kV. 2.6. Cryogenic TEM Observations. Cryogenic (cryo)-TEM samples were prepared in a controlled environment vitrification system (CEVS) at 25 °C. A micropipette was used to load 5 μL of solution onto a TEM copper grid with carbon supporting film, which was blotted with two pieces of filter paper to form thin films suspended on the mesh holes. After waiting for ∼5 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by nitrogen) at −165 °C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 TEM (120 kV) at about −174 °C. The phase contrast was enhanced by under-focus. The images were recorded on a Gatan multiscan charge-coupled device (CCD) and processed with Digital Micrograph. The cryo-TEM observations were performed by Dr. J. Hao’s group. 2.7. Spectroscopic Measurements. UV−vis spectroscopy measurements before and after UV irradiation were carried out using a U-4100 UV−vis spectrophotometer. 1H NMR spectra were recorded on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Z-axis). Fourier transform infrared (FT-IR) spectra were obtained on a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). Circular dichroism (CD) measurements were performed on a JASCO J-810 spectropolarimeter, which was flushed with constant nitrogen flow during operation to purge the ozone generated by the light source of the instrument. The spectra were smoothed by using the noise-reducing option in the operating software of the instrument. Three scans were averaged per spectrum to improve the signal-tonoise ratio. Wavelength scans were recorded at 1 nm intervals from

700 to 200 nm. The solution was determined using a 0.1 mm pathlength quartz cuvette at 25 °C. To exclude the possible influence of linear dichroism, we measured the spectra of xerogels, which were cast gel sample on a quartz slide with solvents evaporated under vacuum. During the process of CD spectra measurement, the slides were held perpendicular to the light route and rotated within the film plane to exclude polarizationdependent reflections and avoid possible angle-dependent CD signals.

3. RESULTS AND DISCUSSION The responsive gelator, AzoNa4, combines two different functional moieties, the azobenzene moiety (azo group), which is sensitive to light irradiation, and the o-phthalic acid moiety (carboxylic group), which is sensitive to acidity. AzoNa4 can act as a dual-responsive switch for constructing photo- and acid-responsive molecular assemblies. By employing C14DMAO, a zwitterionic surfactant, to mix with AzoNa4 with varying pH, an assembly was constructed to test the hypothesis. 3.1. Gel Formation and pH-Responsiveness. At a fixed concentration of AzoNa4 , the C 14DMAO/AzoNa 4 selfassembled system can undergo a series of different phase regions by varying the amount of C14DMAO and acid concentration: solutions with precipitates, viscoelastic gel, biphasic solution (colorless phase/orange oily phase), and sols (small micelles) with precipitates, as shown in the phase diagram (Figure S2 in the Supporting Information). It is noted that the samples in the right part (150 mmol L−1 C14DMAO/ 10 mmol L−1 AzoNa4) of the phase diagram exhibit a sol−gel− sol transition by regulating the concentration of HCl, which reflects pH-responsiveness. The stock solution of C14DMAO and AzoNa4 mixtures keeps good fluidity, which is water-like sol state of micelles. AzoNa4 plays a role as hydrotropes that can strongly bind to the O− N(CH3)2 headgroup of C14DMAO noncovalently through hydrophobic effects and electrostatic attraction. As HCl solution was added to this stock solution, pH-sensitive groups, i.e., COO−, are composed within the system; charges will have a great impact on electrostatic attraction and H-bonding, which may influence the molecular stacking and the microstructure. For the purpose of investigating the gel microstructures, different amounts of HCl with cHCl = 0, 120, and 180 mmol L−1 (pH = 6.92, 2.88, and 1.44, respectively) were exerted on the stock solutions of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4, which are micelles. The typical concentration ratio was selected by the phase diagram in Figure S2 in the Supporting Information. C

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Figure 2. Dynamic laser light scattering of the samples 10 mmol L−1 AzoNa4/150 mmol L−1 C14DMAO/0 mmol L−1 HCl (a) and 10 mmol L−1 AzoNa4/150 mmol L−1 C14DMAO/180 mmol L−1 HCl (b). (c) Plots of the effective diffusion coefficient obtained from cumulants as a function of acid concentration (cHCl) at fixed 10 mmol L−1 AzoNa4/150 mmol L−1 C14DMAO. The scattering angle is 90 o.

Figure 3. (a) Stress sweep rheogram as a function of shear stress (τ); (b) oscillatory shear rheogram as a function of the frequency ( f). The sample solution contains cAzoNa4 = 10 mmol L−1, cC14DMAO = 150 mmol L−1, and cHCl = 120 mmol L−1.

structural transition from micelles to disks and vesicles, then deformed at high cHCl (Figure S5 in the Supporting Information), could be deduced from the DLS data, which is also confirmed by the rheological properties in Figure 1b. Although the viscosity decreased unexpectedly (cHCl: from 120 to 180 mmol L−1) with an excessive amount of acid, the viscosity of the gel (cHCl = 120 mmol L−1) decreased as expected as the pH increases with the addition of KOH (1000 mmol L−1), and the sample finally recovered into water-like liquid, indicating the deformation of vesicles and disks. Accordingly, a pH-responsively viscoelastic solution was prepared by introduction of the H+-sensitive gelator AzoNa4 into a zwitterionic surfactant, C14DMAO. To reveal the viscoelasticity of gels of disks and vesicles, rheological data are shown in Figure 3. The selected rheological properties of the gels of C14DMAO/AzoNa4 mixtures were measured by stress sweep mode and oscillatory sweep (dynamic shear) mode. The stress sweep data (Figure 3a) present elastic-dominant responsivity with linear viscoelastic region (yield stress 26.64 Pa), in which the storage modulus G′, called the elastic modulus, and the loss modulus G″, called the viscous modulus, are parallel. The oscillatory data (Figure 3b) are presented as plots of the elastic modulus G′ and viscous modulus G″ as a function of the angular frequency, showing a typical viscoelastic response versus the frequency. As captured by the FF-TEM images (Figure 4) and cryoTEM images (Figure 5), unilamellar vesicles and disks were observed instead of the global micelles at cHCl = 120 mmol L−1. The deformation of vesicles and disks brings about the viscosity decrease at high cHCl = 180 mmol L−1, which could be attributed to the microphase separation resulting from overhydrophobicity of AzoH4 compared with AzoNa4. The microstructure transformation from micelles to vesicles and

With the addition of HCl (Figure 1a), the stock solution exhibited remarkable viscosity changes from easily flowing micelle solution at cHCl = 0 mmol L−1 to highly viscous gel at cHCl = 120 mmol L−1, at which the sample is almost “frozen”, tough enough to support its own weight. At cHCl = 180 mmol L−1, the sample turned into easily flowing solution again, which is enough to provide deeply supporting evidence of its pHresponsiveness. The steady shear measurements in Figure 1b indicate that the sol−gel−sol viscosity changes from water-like state to gel-like state and then to sol, eventually along with microstructure transformation as pH varies. Dynamic laser light scattering (DLS) measurements clearly demonstrate the existence of small micelles with a hydrodynamic radius (Rh) of 4.9 nm of 150 mmol L−1 C14DMAO solution (Figure S3 in the Supporting Information) and spherical micelles with Rh of 7 nm in a mixture of 150 mmol L−1 C14DMAO and 10 mmol L−1 AzoNa4 in water (Figure 2a), in which Rh was obtained by CONTIN analysis.19,20 It is noted that at cHCl = 120 mmol L−1, while after acidizing the above system, plots of the effective diffusion coefficient obtained from cumulants as a function of pH are presented. With the concentration of HCl increasing, the effective diffusion coefficient also varies (Figure 2c), which indicates a structural transition, in correspondence with the coexistence of disks and vesicles captured by FF-TEM and cryo-TEM. SEM or atomic force microscopy (AFM) pictures for the xerogel samples are also provided (Figure S4 in the Supporting Information). Further acidification brings about both a drastic change in the DLS data (Figure 2b) and a viscosity decrease (Figure 1). The DLS peak turned out to be a single-distributed one of 3.6 nm again, implying the deformation of gel structures at high cHCl due to the microphase separation resulting from overhydrophobicity of AzoH4 compared with AzoNa4. A clear D

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and hydrophobic effects, which can account for the high stability of the disks. To determine the stability of the vesicles, the radii of 765 vesicles in the cryo-TEM images were calculated to obtain a size distribution histogram (Figure 6). We can see that most

Figure 4. FF-TEM micrographs for the sample: cAzoNa4 = 10 mmol L−1, cC14DMAO = 150 mmol L−1, and cHCl = 120 mmol L−1. The arrows show disklike micelles.

Figure 6. Vesicle size distribution histogram through the method of statistic analysis. The average radius is 17.50 ± 10.00 nm.

radii are in the range 5.1−67.4 nm, and the average radius is about 17.50 ± 10.00 nm. On the basis of this, we applied a curvilinear equation to fit the size distribution histogram (indicated by the black curve). The effective bending constant, K = 0.3kBT, can be obtained through the comparison between the curvilinear equation and the theory equation, eq 1, giving the vesicle size distribution as a function of R0 and K:

Figure 5. Cryo-TEM micrographs for the sample: cAzoNa4 = 10 mmol L−1, cC14DMAO = 150 mmol L−1, and cHCl = 120 mmol L−1. The arrows show disklike micelles.

disks can be well-understood by the critical packing parameter p.21 Global micelles are usually obtained when p is small (p ≤ 1/3), and bilayer structures such as vesicles or disks are obtained at 1/2 < p < 1. Salt addition can screen the charge repulsion of surfactant headgroups and reduce the headgroup size, which contributes to the formation of microstructures of low curvature, such as vesicles and disk micelles. When adding AzoNa4 to the solution of C14DMAO with high cHCl at 25 °C, intermolecular H-bonded AzoH4 efficiently reduced the headgroup size and the vesicles and disks formed, which is perfectly consistent with the critical packing parameter. The FF-TEM images in Figure 4 display nearly multidispersed unilamellar vesicles with diameters ranging from 14.1 to 101.7 nm, which is slightly narrower than the ones ranging from 10.2 to 134.8 nm measured by cryo-TEM (Figure 5). Meanwhile, corresponding well with the DLS conclusion, small amounts of disk micelles with some slight bending and with diameters ranging from 22.4 to 42.5 nm could be clearly distinguished, which is a time-dependent and concentration-dependent intermediate aggregate structure formed during the transition from globular micelles to vesicles.22 According to a previous report,18 only when the disks grow in size above a critical dimension could the disks could enclose into vesicles through thermal fluctuation.18 As a transient formation in most cases, the disks can enclose into unilamellar vesicles and finally grow toward an equilibrium distribution over a time scale varying from hours to months for different systems.18 In our system, the disks compared with the other systems are stable and can be kept at least for one month according to the cryo-TEM measurement time. On the basis of the fact of the coexistence of disks and multidispersed unilamellar vesicles, we conclude that the system has not reached the critical dimension, at which the vesicle radius distribution is nearly monodispersed.18 In our system, spatially fixed chromophores in the disk bilayers greatly increased the rigidity of the disks via intermolecular H-bond

2

2

R /R 0 ⎧ ⎡ −8πK ⎛ R 0 ⎞ 2 ⎤⎫ ⎜ ⎟ ⎥⎬ CN = ⎨ CM exp⎢ 1− ⎪ R ⎠ ⎦⎪ ⎣ kBT ⎝ ⎩ ⎭ ⎪

⎪

(1)

where CM (=XM/M) and CN are the molar or number fractions of vesicles of size M and N, R0 is the radius of the minimumenergy vesicle, and K is an effective bending constant. The histogram (10.2−134.8 nm in diameter) could be wellexplained by the Helfrich fluctuation theory, namely, when K ≈ kBT, vesicles are stabilized by Helfrich fluctuation with a broad size distribution.23,24 The equilibrium size distribution of the population of vesicles is determined by a subtle competition between the entropy of mixing and the curvature elasticity of the bilayers. The regulation of the surfactant/cosurfactant ratio and determination of the appropriate chain length for cosurfactant25 should be considered for the preparation of unilamellar vesicle gels. Recently, a couple of studies have been reported involving H+-induced gelation, in an effort to design pH-responsive gel/ fluid.26,27 It was reported that this happened at the presence of a hydrotrope, which can protonate C14DMAO noncovalently. The electrostatic repulsion of C14DMAO headgroups could be shielded, making the hydrocarbon chain packing tighter, and large aggregates such as vesicles formed.27 We suggest that the AzoNa4 can be considered as a hydrotrope that possesses an amphiphilic structure with an aromatic group and four ionized carboxylic acid (or salt) moieties. On account of the excellent hydrophilicity, AzoNa4 can interact with C14DMAO headgroups only in the surface of mixed micelles and compensate the charges via electrostatic attraction and hydrophobic effects, which made the mixed spherical micelles containing two components more neutral and stable. Upon protonation, AzoNa4 molecules were transformed to be AzoH4 and the hydrophobicity of Azo4− ions increased greatly as a result of E

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Figure 7. (a) UV−vis spectra of 150 mmol L−1 C14DMAO and 10 mmol L−1 AzoNa4 solutions with different HCl concentrations: 0, 120, and 180 mmol L−1; all samples for UV−vis characterization were diluted 167 times before UV−vis measurements. (b) CD spectra of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4 with 0, 120, and 180 mmol L−1 HCl.

It is noted that the gelator AzoNa4 is an achiral molecule.8,9 The solution of 10 mmol L−1 AzoNa4 and 150 mmol L−1 C14DMAO in water gives no CD signals, as shown in Figure 7b. CD signal changes are observed upon increasing cHCl to 120 mmol L−1. Positive (Figure 7b) or negative Cotton effects (CEs) appeared at ∼360 nm at cHCl = 120 mmol L−1 but disappeared at cHCl = 180 mmol L−1. The CEs appeared at the corresponding absorption range of AzoNa4 in the UV−vis spectra; we propose that these CD signals were brought about by the chromophores in the solution. At cHCl = 120 mmol L−1, all the AzoNa4 are protoned to be AzoH4 and nearly all of the C14DMAO are coupled to be dimers by an acid−base equilibrium; vesicles and disks were formed, and a strong positive CD signal was obtained. With the increase of HCl, at cHCl = 180 mmol L−1, no CD signal was produced and there were remarkable viscosity decreases. The azobenzene derivative chirality might be a combination process of linear polarization and circular polarization of incident light.31 To exclude the possible influence of linear dichroism, we measured the spectra of xerogels, which were cast gel sample on a quartz slide with solvents evaporated under vacuum. During the process of CD spectra measurements, the slide was held perpendicular to the light route and rotated within the film plane to exclude polarization-dependent reflections and avoid possible angledependent CD signals. As shown in Figure S6 in the Supporting Information, the obtained xerogel CD spectra almost keep the same peak shape unrelated to the rotation angles, which affirms the authenticity of the gel in CD spectra. The supramolecular chirality is definitely generated by gelation. If a stoichiometric amount of OH− was added to the acidified gel sample, the CD signal can turn to silent reversibly (Figure S7 in the Supporting Information), which asserted clearly the pH-responsive conclusion of the supramolecular chirality to the contrary. In our previous work, we reported that supramolecular chirality could be gained from achiral gelator, AzoNa4, resulting from a cooperative stereoregular arrangements.8,9 In the present work, the supramolecular chirality is obtained via regulating pH exclusively by achiral molecules in soft matter systems, which is rarely reported and characterized.2 The supramolecular chirality is connected with the state of aggregation, that is, only the condensed gels show a chiral sense, with sols being chiral-silent. 3.3. Mechanism of the Chiral Gel Formation of Vesicles and Disks. As we reported previously, the tetraalkylammonium ion of cetyltrimethylammonium bromide (CTAB) has no hydrogen-bond donor or acceptor sites.9 The

aggregation of AzoH4 on account of H-bond formation. Because of the loss of hydrophilicity, AzoH4 escaped from the bulk water and inserted into the hydrophobic hydrocarbon chains of C14DMAO molecules. In acid solution, C14DMAO can couple into dimers, whose aminoxides are in an acid−base equilibrium.28 Bearing all this in mind, the synergism of AzoH4 and C14DMAO played an important role in forming bilayers of disks and vesicles. Nevertheless, excessive amounts of acid can overcompensate the charges and make the AzoH4 overhydrophobic or even cause microphase separation, breaking the elegant charge balance and destroying the synergism. The effect of acid concentration (or pH) in regulating the state of aggregates has been well-illustrated in our earlier work8,9 and was evidenced once more in this discussion. Intriguingly, gelation-induced supramolecular chirality is also accomplished in this work, which exhibits the ordered molecular stacking evoked by the H-bond. 3.2. Supramolecular Chirality. Except as a gelator to drive the gel formation, the gelator AzoNa4 can arrange nonsymmetrically via H-bond and lead to supramolecular chirality.8,9 As shown in Figure 7, by regulating the cHCl, the supramolecular chirality gel composed of vesicle and disk bilayers can be gained. For bilayer systems, Kunitake and co-workers reported enhanced CD of azobenzene-containing chiral bilayers, in which the large CD enhancement of chiral bilayers arises from the interaction of spatially fixed chromophores in the gel state and the rigidity of the chiral bilayer.11 In our system, chiral bilayers remained within the gel, and the supramolecular chirality was gained via the interaction of spatially fixed chromophores in chiral bilayers. For this reason, a pH-chiral switch can be built, demonstrated by UV−vis and CD spectra in Figure 7. The solution of 150 mmol L−1 C14DMAO and 10 mmol L−1 AzoNa4 in water displays a strong absorption band at 360 nm (Figure 7a), which is the typical π−π* transition of trans-azobenzene derivatives.29 With the increase of cHCl, one can note a notable reduction of absorption intensity and blueshift of absorption peak at 362 nm, indicating the compact packing between the AzoH4 molecules via H-bond. Azobenzene derivatives are known to form a variety of aggregates such as J/ H aggregate,30 which produces dramatic shifts in the absorption maximum of UV−vis spectra. The small blue-shift with an increase of cHCl, i.e., 362−355 nm in Figure 7a, in UV−vis spectra should be assignable to the tightly packed H-aggregate arrangement of the H-bonded AzoH4 molecules within the vesicle and disk bilayers, as a result of the synergism of the stronger H-bond and hydrophobic effects. F

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Figure 8. (a) 1H NMR spectra of 150 mmol L−1 C14DMAO and 10 mmol L−1 AzoNa4 mixture with different cHCl, 180, 120, and 0 mmol L−1, and 1H NMR spectra of AzoNa4 and C14DMAO; (b) FT-IR spectra of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4 with different cHCl: 180, 120, and 0 mmol L−1.

intermolecular H-bonds are formed between AzoH4’s, with the influence of CTAB on the formation of O−H−O intermolecular H-bonds of AzoH4 and CTAB distributed around AzoH4’s via electrostatic interactions to arrange in a tight packing.9 The mixture of CTAB and AzoNa4 forms wormlike micelles at suitable pH values. Herein, the H-bond evoked by the increasing concentration of HCl leads to the gel formation of vesicles and disks that possess the supramolecular chirality. The gels of vesicles and disks are formed from the close packing of C14DMAO hydrocarbon chain bilayers, which were dominated by H-bonded AzoH4. The supramolecular chirality is brought about from the chiral tilting bilayer that originated from the H-type aggregation packing of the H-bonded AzoH4. In acid condition, C14DMAO molecules are coupled to be dimers via H···O−H hydrogen bond, whose aminoxides are in an acid− base equilibrium.28 The steric hindrance of the headgroups of C14DMAO and CTAB are different, which will definitely result in the different headgroup areas of aggregation types according to the critical packing parameter p (p = v/a0l). Considering the similarities and differences between CTAB and C14DMAO, it is interesting that vesicles/disks are formed when the CTAB is changed to C14DMAO. As is well-documented,26 we employed 1 H NMR and FT-IR31 to study the association between hydrotropes (AzoNa4 or AzoH4) and zwitterionic C14DMAO for confirming the presence of H-bonds. Figure 8a shows the 1H NMR chemical shifts of the relative components in the AzoNa4 (or AzoH4)/C14DMAO aqueous solution with the addition of HCl. An apparent upfield shift (ppm: from 7.84 to 7.79) appears in Figure 8a for the protons on the benzene ring of AzoNa4 with the addition of C14DMAO. In the absence of C14DMAO, the protons on the benzene ring of AzoNa4 experienced a polar environment exposed to the bulk water; however, with the addition of C14DMAO, the hydrophobic moiety of AzoNa4 intercalated into the hydrocarbon chains of zwitterionic C14DMAO via hydrophobic effects. That is to say, the benzene ring protons experience a microenvironment change from a polar to an apolar one, resulting in the NMR signals shifting upfield. The NMR results of the benzene protons adjacent to carboxylic groups also reveal the H-bond between AzoH4 and C14DMAO molecules with the increase of acid. The electron cloud densities at the protons involved in hydrogen bonds are decreased, and consequently their NMR signals are shifted to lower magnetic fields.32 The magnitude of the chemical shift is a symbol of the strength of the H-bond. As shown in Figure 8a, the signal peak for the

benzene protons adjacent to carboxylic groups undergoes a downfield shift (ppm: 7.79, 7.82, and 7.85) gradually as cHCl increases, and it was ascribed to the influence of hydrogen bond formed by AzoH4 carboxylic groups because the proton electron densities are decreased on account of being involved in hydrogen bonds. On the contrary, the protons of −(CH3)2 of C14DMAO (Figure 8a) show quite an opposite change with an upfield shift (ppm from 3.11 to 3.07) with the addition of AzoNa4 and a downfield shift (ppm: 3.07, 3.40, and 3.44) gradually as the acid concentration increases because of the Hbond formation. According to this analysis, it can be firmly believed that the H-bonds between AzoH4 molecules were gradually strengthened with cHCl increase, considering the fact that the magnitude of the chemical shift relates to the strength of the hydrogen bond.32 H-bonds between AzoH4 molecules can account for the fact that AzoH4 molecules are overhydrophobic and even cause microphase separation from the bulk solution at high acid concentration (cHCl = 180 mmol L−1). As shown in Figure 8b, FT-IR data also confirmed the formation of H-bonds with a variation of pH. Upon addition of HCl (cHCl = 120 mmol L−1), the asymmetric stretching vibrations (1568.22 cm−1) of CO (AzoNa4) shifted to 1711.47 cm−1, indicating the intermolecular H-bond. At the higher acid concentration, e.g., cHCl = 180 mmol L−1, this vibration band shifts from 1711.47 to 1724.35 cm−1, correlative of the H-bonded strength (Badger−Bauer relation).32 That is, C14DMAO molecules are distributed around H-bonded AzoH4 molecules via cooperative interaction of electrostatic interactions and H-bonds, which agrees well with 1H NMR results. According to the results of 1H NMR and FT-IR, we concluded that the asymmetric H-bonded AzoH4 molecules act as the smallest aggregate unit that plays a chiral-inducting role in the extended asymmetric bilayers in vesicles and disks. As we have mentioned earlier, with the increase of HCl, the microstructure transformation from micelles to vesicles and disks can be well-understood by the critical packing parameter p, forming bilayer structures such as vesicles or disks at 1/2 < p < 1.21 With the strengthening of H-bonds, the hydrophobicity of AzoH4 molecules gradually increases. Only at cHCl = 120 mmol L −1 HCl, the intermolecular H-bonds of AzoH 4 molecules formed, which have been demonstrated by the NMR and IR data. The hydrophobic AzoH4 molecules can be incorporated into the C14DMAO bilayers elegantly. It could shield the electrostatic repulsion between C14DMAO headgroups and facilitate the tight packing of hydrophobic chains, resulting in the gels of vesicles and disks being stable by G

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Langmuir Helfrich fluctuation theory.23,24 AzoH4 molecules are inclined to tilt with a certain tilting angle in request of the intermolecular H-bonds8,9 and form H-type aggregate within the vesicle/disk asymmetric bilayers with chiral sense, as illustrated in Scheme 2.

bilayers (Scheme 2). The membrane elasticity concerning Helfrich theory and a linear term with D∞ symmetry supposed by Ou-Yang may account for the tilting direction field of bilayers and therefore the chirality. Our research results may give confirmative evidence experimentally. A transition scheme of disks and vesicles is shown in Scheme 2. 3.4. pH- and Light-Responsive Supramolecular Chiral Switches. The surfactant gels of vesicles and disks accompanied by the supramolecular chirality can be switched on and off reversibly by inputting of H+ and OH−. For pHresponsiveness, upon incorporating a typical photochromic unit of the achiral gelator, AzoH4 molecules, the samples are anticipated to be sensitive to light.33 AzoH4 molecules show clear-cut spectral and geometric properties, which can be tuned by light-irradiation of different lengths. It can generate remarkable changes of chromophore stacking modes and, hence, the switch of supramolecular chirality. In fact, kinds of chiroptical switches in response to one single stimulus have been vastly studied; a smart approach of achieving the multiresponsive switch of supramolecular chirality has been synchronously accomplished in this work. In the following, the responsiveness of the gel sample to UV/visible light irradiations will be demonstrated. All the AzoNa4 solutions comprising different amounts of H+ experience trans−cis isomerization irradiated with UV- and visible-light alternatively because of the AzoNa4 molecules. The absorbance spectra of the viscous solution of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4/120 mmol L−1 were obtained and are shown in Figure 9a at different UV irradiation times. Before light irradiation, a sharp absorption band ascribed to the typical π−π* transition of trans-azobenzene derivatives (trans-AzoNa4) was observed at 360 nm. UV irradiation on the solution resulted in the notable decrease in intensity of the absorption peak, indicating the photoisomerization of transand cis-AzoNa4. A photostationary state can be found,27 i.e., a constant ratio of trans-/cis-AzoNa4 is reached within 120 min of UV irradiation. The light-triggered transition between transand cis-AzoNa4 was also reversible, as shown in Figure 9b. If the UV-irradiated solution was subsequently irradiated by visible light for 120 min, the intensity of the absorption peak increased back and indicated the reappearance of trans-AzoNa4, which gives evidence for the reversible trans- and cis-photoisomerization of AzoNa4. It should be noted that, in Figure 9b, the intensity of the absorption peak at 360 nm after visible light irradiation was slightly lower than that of the primary state without light irradiation, indicating the incompleteness of

Scheme 2. Transition Scheme of Disks and Vesicles: Bilayers with Symmetric Breaking; Chain Length: C14DMAO (1.93 nm) and AzoNa4 (2.2 nm); (a) Micelles of C14DMAO Solution; (b) Mixed Micelles of C14DMAO and AzoNa4 Mixture Solution; (C) Disks and Vesicles of C14DMAO/ AzoH4 Gels; (d) Wormlike Micelles Transformed from the Gel of Disks and Vesicles with UV Irradiation

As for chiral bilayers, although unable to calculate the possible structure of bilayers, Whitten and co-workers proposed a chiral pinwheel unit aggregate structure on the basis of H-type aggregate to account for the chiral nature by using Monte Carlo cooling methods.17 With their approach following Helfrich and Prost in dealing with the membrane elasticity, Ou-Yang and Liu derived surface shape-equilibrium equations for tilted chiral lipid bilayers. 12 By introducing a linear term with D ∞ symmetry, the twisted-strip solution and the tilt field of spherical vesicles are obtained.12 For spherical vesicles they found more than two singular points, whose calculation indicates a decreasing sequence of the elastic energies associated with vesicle, twisted-strip, and wound-ribbon solution. The result could explain the experimental transition from vesicular dispersion to other structures. In the case of our system, the H-bonded AzoH4 molecules8,9 aggregating in Htype within vesicles and disks are proposed as the smallest chiral units, which induced the extended asymmetric tilting

Figure 9. (a) UV−vis absorption spectra of the viscous solution of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4/ 120 mmol L−1 at different UVlight irradiation times; (b) UV−vis absorption spectra for the viscous solution of 150 mmol L−1 C14DMAO/10 mmol L−1 AzoNa4/ 120 mmol L−1 under different conditions. H

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Langmuir isomerization of cis-AzoNa4, which turns back to its transisomer within a certain time.34 The supramolecular chirality of the system can also be tuned reversibly. It is accompanied by morphology changes (Figure 10) upon alternative exposure to UV and visible light, as

system. As is shown by the cryo-TEM micrographs in Figure 10, upon UV irradiation for 60 min, the disks disappeared and the integrity of the spherical vesicle began to be lost. The spherical vesicle membranes tended to deform and reemerged into larger irregular bilayer aggregates as shown in Figure 10b. With the longer (120 min) UV irradiation, the larger irregular bilayer aggregates finally transformed into wormlike micelles with a small amount of vesicles (Figure 10c). Both the oscillatory sweep (dynamic shear) mode and Cole−Cole plots of the dynamic rheological results for the 120 min of UV irradiation system are in accordance with Maxwell’s mechanical model at low shear frequency (Figure S9 in the Supporting Information), which demonstrates the existence of wormlike micelles. In an aqueous solution, C14DMAO can be easily protonated by the weak acid,27 AzoH4, to form a cationic−anionic (catanionic) surfactant mixture system. This combination should cause a decrease of the a value and an increase of packing parameter p. According to the theory of critical packing parameter, the vesicle/disk bilayer structure can be formed. Because the trans-azobenzene structure has a better ability to reduce the area of the headgroups than the cis-structures because of its lower steric hindrance, trans-AzoNa4 molecules are favorable in the formation of vesicles and disks. UV irradiation can increase the amount of cis-AzoNa4, and the a value increased largely. A transition from the vesicles and disks to wormlike micelles could be regulated by light irradiation. Kim et al. also reported similar conformational chirality optically induced by the irradiation of photochromic molecules.31

Figure 10. Cryo-TEM micrographs for the sample: cAzoNa4 = 10 mmol L−1, cC14DMAO = 150 mmol L−1, and cHCl = 120 mmol L−1, irradiated with 365 nm light at different times: (a) 0 min, (b) 60 min, and (c) 120 min.

demonstrated by CD data in Figure S8 in the Supporting Information. For the representative example (cAzoNa4/cC14DMAO/ cHCl, 10/150/120 mmol L−1) in Figure S8a in the Supporting Information, it is a gel phase consisting of vesicles and disklike micelles. The positive CE at 360 nm drastically shifts to chiralsilent when irradiated with 365 nm light for 60 min. The CD signal was recovered almost without significant degradation when the gel was exposed to 420 nm light for 60 min. Whitten and co-workers have also reported similar results on the bilayer aggregates formed by one kind of azobenzene derivative containing aromatic/dye chromophores that induced CD signals for trans-azobenzenes and did not induce CD signals for cis-azobenzenes upon alternative light irradiation.17 The photoinduced supramolecular chirality switch is totally reversible. As shown in Figure S8 in the Supporting Information, the reversible CD intensity at 360 nm could be accomplished at several irradiation cycles. A photoresponsive chiroptical switch can be achieved. The reverse CEs switched by photoirradiation might be ascribed to the photoinduced trans- and cis-isomerization of AzoH4 molecules, according to the UV−vis spectra (Figure 9). UV irradiation, which leads to the formation of the cis-isomer, can frequently result in chiralsilent behavior for the gel. The CD signal intensity of chiral gels involves the trans−cis state of azobenzene derivatives and gelation state. It can be strengthened while gelator molecules are assembling and weakened as gelator molecules are disassembling.1 The merely trans-isomer-generated CD signal indicates that supramolecular chirality can be induced only in the trans- rather than the cis-states. The helical twisting power (HTP, β) in the trans-state is higher than that in its cis-state, which is more frequently observed in chiral azobenzene derivatives.35 The chiral-silent behavior incited with UV-light irradiation is obviously caused by the molecule rearrangement via the trans−cis geometric isomerization of AzoH4. The HTP value decreases in cis-AzoH4, and the supramolecular chirality is regulated to almost silent as the geometry and chirality of the central unit are changed. Our results may give another corroborative evidence experimentally for the isomerization mechanism of azobenzene units, which has been discussed for decades and is still controversial. Because the supramolecular chirality originates from the specific packing of the bilayer arrangements, subtle molecularscale changes have remarkable impacts on the macroscopic properties. We have reported the photoinduced phase transition from multilameller vesicles to wormlike micelles via photoisomerization,27 and similar results are obtained in this

4. CONCLUSIONS In conclusion, a chiral gel of H-bonded vesicles and disks by achiral molecules in water is obtained by regulating HCl concentration in the colloidal system, which is rarely reported and characterized. The disklike micelles are captured for the first time in supramolecular chiral hydrogel systems. The samples can produce sol−gel−sol transition via regulating pH. Only the condensed gels show a chiral sense with sols being chiral-silent. The gel chirality can be switched reversibly with fantastic macroscopic changes upon light irradiation. The gel materials comprise a pH- and light-dual-responsive chiroptical switch.

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ASSOCIATED CONTENT

S Supporting Information *

Mass and 1H NMR of AzoNa4, phase diagram of the C14DMAO/AzoNa4 system, dynamic light scattering (DLS) data of 150 mmol L−1 C14DMAO solution, LD data, silent CD signal via adding KOH, CD signal in response to light irradiation, oscillatory shear rheogram, and Cole−Cole plots for irradiated sample. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b01147.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-531-88363532. Fax: +86531-88564750. Notes

The authors declare no competing financial interest. I

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(22) Silvander, M.; Karlsson, G.; Edwards, K. Vesicle solubilization by alkyl sulfate surfactants: A cryo-TEM study of the vesicle to micelle transition. J. Colloid Interface Sci. 1996, 197, 104−113. (23) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. The origins of stability of spontaneous vesicles. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 1353−1357. (24) Xu, W.; Wang, X.; Zhong, Z.; Song, A.; Hao, J. Influence of counterions on lauric acid vesicles and theoretical consideration of vesicle stability. J. Phys. Chem. B 2013, 117, 242−251. (25) Wang, D.; Wei, G.; Dong, R.; Hao, J. Multiresponsive viscoelastic vesicle gels of nonionic C12EO4 and anionic azoNa. Chem.Eur. J. 2013, 19, 8253−8260. (26) Lin, Y.; Han, X.; Huang, J.; Fu, H.; Yu, C. A facile route to design pH-responsive viscoelastic wormlike micelles: Smart use of hydrotropes. J. Colloid Interface Sci. 2009, 330, 449−455. (27) Wang, D.; Dong, R.; Long, P.; Hao, J. Photo-induced phase transition from multilamellar vesicles to wormlike micelles. Soft Matter 2011, 7, 10713−10719. (28) Hoffmann, H.; Oetter, G.; Schwandner, B. The aggregation behaviour of tetradecyldimethylaminoxide. Prog. Colloid Polym. Sci. 1987, 73, 95−106. (29) Beharry, A. A.; Woolley, G. A. Azobenzene photoswitches for biomolecules. Chem. Soc. Rev. 2011, 40, 4422−4437. (30) Saito, K. H-aggregate formation in squarylium Langmuir− Blodgett films. J. Phys. Chem. B 2001, 105, 4235−4238. (31) Kim, M. J.; Shin, B. G.; Kim, J. J.; Kim, D. Y. Photoinduced supramolecular chirality in amorphous azobenzene polymer films. J. Am. Chem. Soc. 2002, 124, 3504−3505. (32) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Noncovalent Synthesis Using Hydrogen Bonding. Angew. Chem., Int. Ed. 2001, 40, 2382−2426. (33) Lv, K.; Qin, L.; Wang, X.; Zhang, L.; Liu, M. A chiroptical switch based on supramolecular chirality transfer through alkyl chain entanglement and dynamic covalent bonding. Phys. Chem. Chem. Phys. 2013, 15, 20197−20202. (34) Tong, X.; Wang, G.; Soldera, A.; Zhao, Y. How Can Azobenzene Block Copolymer Vesicles Be Dissociated and Reformed by Light? J. Phys. Chem. B 2005, 109, 20281−20287. (35) Wang, Y.; Urbas, A.; Li, Q. Reversible vsible-light tuning of selforganized helical superstructures enabled by unprecedented lightdriven axially chiral molecular switches. J. Am. Chem. Soc. 2012, 134, 3342−3345.

ACKNOWLEDGMENTS This work is financially supported by the NSFC (21420102006 and 21273134).

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