Ag(I)-Coordinated Supramolecular Metallogels Based on Schiff Base

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Ag(I)-Coordinated Supramolecular Metallogels Based on Schiff Base Ligands: Structural Characterization and Reversible Thixotropic Property Min Xue,*,† Yanchao Lü,‡ Qingqing Sun,‡ Kaiqiang Liu,‡ Zhen Liu,† and Ping Sun† †

School of Chemical Engineering, Xi’an University, Xi’an 710065, P. R. China Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, No. 620, West Chang’an Avenue, Chang’an District, Xi’an 710119, P. R. China

‡

S Supporting Information *

ABSTRACT: Three Schiff base ligands containing two pyridine rings, S1, S2, and S3, show strong gelation abilities with AgNO3 in several pure or mixed solvents at room temperature. 1H NMR and Fourier transform infrared spectroscopy measurements not only demonstrated the coordination interaction between silver ion and nitrogen atom of the pyridine ring of the S2 ligand in S2-Ag metallogel, but also showed that hydrogen bonding contributes to the formation of the metallogels. In particular, S2-Ag metallogel shows a super smart and fully reversible thixotropic property, which has been rarely reported before in metallogels. X-ray diffraction (XRD) analysis revealed that S2-Ag metallogel in DMF takes a mixture of hexagonal and tetragonal packing modes. On the basis of the results of XRD and mass spectrometry analysis, a possible structure evolution process for the gel was proposed. And this model was further demonstrated by the results of polarizing microscopy and thermogravimetric analysis of S2-Ag metallogel.

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INTRODUCTION As a new challenging area of supramolecular chemistry and materials sciences,1−5 supramolecular gels based on low molecular mass gelators (LMMGs) have attracted considerable attention during the past two decades because of their unique properties and their numerous potential applications in sensors,6,7 nanodevices,8,9 drug delivery and release,10−12 catalysis carriers,13 template synthesis,14 self-cleaning membranes,15 self-healing materials,16 and so on. In forming supramolecular gels, the noncovalent intermolecular interactions are usually hydrogen bonding, van der Waals, π−π stacking, electrostatic, dipole−dipole, or a combination of some of these.17−19 However, it was only in the past decade that metal−ligand interactions have also been utilized to construct supramolecular gels, which was also called metallogels.20,21 And, metallogels have attracted a rapidly increasing interest because the diversity of metal−ligand coordination could readily and subtly regulate the self-assembly process and microstructures of gels and thereby influence the gel properties. Furthermore, incorporation of metal centers could endow gels with a great deal of fascinating properties, such as photo,22 catalysis,23 redox responsivity,5 sensing,24 gas adsorption,25 and so on. Accordingly, metallogels would possess multiple functions that could not be obtained in gels formed only by organic small molecule gelators. It is well-known that the structures and coordination capabilities of ligands could produce a dramatic influence on topologies and functional properties of metal complexes; © XXXX American Chemical Society

therefore the design and selection of ligands is vital in preparing functional metallogels. Ligands with diverse structural features have been used in metallogels, for example, pyridylbased compounds,26 porphyrin derivatives,27 azoles,28 carboxylic acids,23,29 and Schiff bases.30,31 In coordination chemistry, Schiff bases have attracted significant interest because of not only their ability to construct metal−organic molecular architectures, but also their potential applications in the areas of catalysis, magnetism, analgesia, anti-inflammation, antibiotics, antimicrobials, and especially anticancer activities.32 Consequently, it is envisaged that using Schiff bases as ligands would imbue metallogels with excellent properties. However, there have been very few reports on metallogels that utilize Schiff bases as organic ligands.30,31 In addition, in all Schiff base ligand-based metallogels reported, the structures of Schiff base ligands all include bigger blocks without coordination ability such as long chain alkyls and cholesterol. Schiff base ligands with simple structures have been largely used in crystal chemistry, yet which have been absolutely ignored in metallogels. Herein we designed and synthesized three simple-structure di-Schiff base ligands containing two pyridine rings, which could form metallogels with AgNO3, a very common silver salt, in different liquids through a coordination-driven selfReceived: July 8, 2015 Revised: September 25, 2015

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XRD Measurement. Diffraction patterns were carried out on a Bruker D2 PHASER diffractometer with Cu Kα X-ray generated (λ = 1.5418 Å). The scan rate was 2°/min. The xerogel was prepared by freezing the gel in liquid nitrogen and then evaporated by a vacuum pump for 12−24 h. XPS Measurement. XPS data of samples were collected on a Kratos AXIS Ultra X-ray photoelectron spectroscopy. MS Measurement. HRMS data were recorded on a Bruker Apex IV FTMS with ESI source. TEM Measurement. A field transmission electron microscopy (FEI Tecnai G2 F20) was used for taking the TEM pictures. Polarizing Microscope. Polarizing microscope measurements were conducted on LEICA-DMLP and EC600. Thermal Analysis. The TG and DTA curves were obtained on a TA Q50 thermogravimetric analyzer in the flowing N2 with a temperature ramp rate of 10 °C min−1. Rheological Measurement. Rheological measurements were carried out with a stress-controlled rheometer (TA Instruments ARG2) equipped with steel-coated parallel-plate geometry (20 mm diameter). The gap distance was fixed at 1000 μm. A liquid-trapping device was placed above the plate, and measurement temperature was set at 25 °C. For rheological measurements, a stress sweep measurement at fixed frequency was first conducted, which provides information about the mechanical strength of the gel sample. And then, a time sweep was made to observe the recovery property of the gel. For this measurement, an oscillatory shear stress was applied to destroy the sample, and then, a small constant shear stress was applied, and the storage modulus G′ and the loss modulus G″ of the sample were monitored as functions of time. This process was repeated 10 times for the same sample.

assembling process. One of the obtained metallogels exhibited wonderful properties. This paper reports the details.

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EXPERIMENTAL SECTION

Materials. Isophthalaldehyde, isonicotinyl hydrazine, nicotinic hydrazine, and 2-pyridine-carboxylic acid hydrazine were purchased from Aladdin Industrial Corporation and used as received. Other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. All solvents used in the syntheses were purified, dried, or freshly distilled as required. Preparation of S1, S2, and S3. Isonicotinyl hydrazine (3.1 g, 22.5 mmol) was dissolved in 50 mL of ethanol, and the mixture was heated and refluxed. To the system, 50 mL of ethanol solution of isophthalaldehyde (1.0 g, 7.5 mmol) was added slowly dropwise. After adding, the reaction mixture was refluxed for 10 h. After the reaction, there was a lot of white precipitate in the round flask. The mixture was filtered, and the resulting solid was washed with hot ethanol four times and dried in a vacuum to give a desired product, S1, in 61% yield as a white powder. The procedures used for the preparation of S2 and S3 are similar to that for S1. For S1: 1H NMR (DMSO-d6/Me4Si, 400 MHz): δ(ppm) 12.17 (s, 2H, NH), 8.81 (d, 4H, pyridine), 8.54 (s, 2H, CHN), 8.17 (s, 1H, benzene), 7.84 (m, 6H, benzene and pyridine), 7.61 (t, 1H, benzene). Elemental analysis, Calc. for C20H16N6O2: C, 64.51, H, 4.33, N, 22.57. Found: C, 64.30; H, 4.10; N, 22.03%. HRMS (ESI): m/e calcd for [(M +H)+]: 373.1413. Found: 373.1414. For S2: 1H NMR (DMSO-d6/Me4Si, 400 MHz): δ(ppm) 12.12 (s, 2H, NH), 9.09 (s, 2H, pyridine), 8.79 (d, 2H, pyridine), 8.52 (s, 2H, CHN), 8.29 (d, 2H, pyridine), 8.16 (s, 1H, benzene), 7.81 (d, 2H, benzene), 7.60 (m, 3H, pyridine and benzene). Elemental analysis, Calc. for C20H16N6O2: C, 64.51, H, 4.33, N, 22.57. Found: C, 64.08; H, 4.07; N, 22.76%. HRMS (ESI): m/e calcd for [(M+H)+]: 373.1413. Found: 373.1414. For S3: 1H NMR (DMSO-d6/Me4Si, 400 MHz): δ(ppm) 12.25 (s, 2H, NH), 8.74 (d, 2H, pyridine), 8.73 (s, 2H, CHN), 8.15 (d, 2H, benzene), 8.10 (s, 1H, benzene), 8.08 (t, 2H, pyridine), 7.76 (d, 2H, pyridine), 7.70 (t, 2H, pyridine), 7.58 (t, 1H, benzene). Elemental analysis, Calc. for C20H16N6O2: C, 64.51, H, 4.33, N, 22.57. Found: C, 64.33; H, 4.07; N, 22.88%. HRMS (ESI): m/e calcd for [(M+H)+]: 373.1413. Found: 373.1411. General Methods. Preparation of Gels. A typical gelation experiment procedure: a weighed amount of the selected Schiff base ligand was dissolved in measured volume of selected organic solvent. One equivalent of AgNO3 with respect to the ligand was dissolved in water. The silver solution was swiftly injected into the ligand solution, and the system was shaken in order to make the mixture homogeneous. After mixing, the system was left to stand. Finally the test tube was inversed to observe if the solution inside could still flow. Gelation was considered to have occurred when a homogeneous substance was obtained which exhibited no gravitational flow, and it was denoted “G”. For systems in which only solution was remained until the end of the tests, they were referred to as solutions (S). For systems in which the produced complex formed precipitation from solvents, these systems are denoted by “P (precipitation)”. A gel typically formed after several minutes upon mixing. SEM Measurement. A scanning electron microscopy spectrometer (Hitachi S-3400N II) was used for taking the SEM pictures. The accelerating voltage was 5 kV, and the emission was 128 μA. The gel was prepared in a sample tube and frozen by liquid nitrogen. The frozen specimen was evaporated by a vacuum pump for 12−24 h. Prior to examination, the xerogel was attached to a copper holder by using conductive adhesive tape, and then it was coated with a thin layer of gold. FT-IR Measurement. All FT-IR measurements were performed on a Thermo Scientific Nicolet iS50 infrared spectrometer in an attenuated total reflection (ATR) mode. The KBr pellets mixed with samples were measured on the transparent mode. 1 H NMR Measurement. 1H NMR data of samples were collected on Bruker AVANCF 400 MHz spectrometer.

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RESULTS AND DISCUSSION Formation of Metallogels with Schiff Bases As Ligands. Though there exists intermolecular hydrogen bonding sites in synthesized Schiff base ligands (Scheme 1), Scheme 1. Chemical Structures of Schiff Base Ligands S1−S3

the three ligands, S1−S3, cannot self-organize into organogels in tested solvents. In fact, three compounds do not take on typical structures of low-molecular-mass gelators, which possess some moieties with cross-linking capability. However, the three compounds are representative organic ligands, and N atoms of pyridine rings have excellent coordination abilities. Hence, considering the coordination property of three compounds, their abilities of forming metallogels with AgNO3, the commonest silver salt, were tested in different solvents, including pure and mixed solvents. The reason for choosing silver ion is because it possesses linear coordination mode; hence fibrous aggregates for gels may be obtained. The tested results are summarized in Table 1. Here, for increasing the solubility of AgNO3, we selected DMF, DMSO, and H2O with high polarity as test solvents. Examination of the table reveals that only one metallogel system was formed in three pure solvents. At room temperB

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variation in ligand structures could lead to dramatic changes in gelation abilities of ligands and properties of metallogels formed. Characterization of Coordination and Hydrogen Bonding. Generally speaking, 1H NMR measurements can provide rich information on the mechanism of self-assembly of gelator molecules in a gel state.33,34 In order to investigate driving forces for the formation of supramolecular gels, 1H NMR measurements of S2 ligand in DMF-d7 and S2-Ag metallogel in DMF-d7 were conducted. First, S2 (0.02 mmol, 7.45 mg) was dissolved in DMF-d7 (0.5 mL), and its 1H NMR was measured at 298 K. Then, after AgNO3 (0.02 mmol, 3.40 mg) was added to the above solution of S2, a transparent metallogel was formed and 1H NMR of this metallogel was measured at 298 K. According to the above experimental procedure, it is very clear that the concentration of S2 is just the same in solution and metallogel. The results are shown in Figure 2.

Table 1. Gelation Properties of Compound S1, S2, and S3 with AgNO3 solvents

S1 + AgNO3

S2 + AgNO3

S3 + AgNO3

DMF DMSO H2O DMF:H2O = 1:1 DMSO:H2O = 1:1

P S P G G

TG S P P P

P S P G G

ature, after AgNO3 solid (0.04 mmol) was added to a DMF solution of S2 (0.04 mmol, 1.0 mL), and the resulted mixture was sufficiently shaken with by hand for 1 min, immediately, a totally transparent and colorless metallogel (Figure 1a) was obtained through formation of S2-Ag complexes. The S2-Ag metallogel in DMF is very stable over several mouths and show no visible sign of weakening. Moreover, the metallogel exhibits fully reversible thixotropic property. However, after 1 week on the open bench the gel takes on a slight yellow coloration (Figure 1b) that after further 1 week turns into deeper yellow (Figure 1c). And then, the color of the gel remains invariant. The color change of the gels results from reduction of silver ions.26 In order to estimate the gelling ability of S2-Ag for DMF, the minimum amount needed to gel 1.0 mL of DMF (critical gelation concentration, CGC) was tested. The result shows the CGC of S2-Ag metallogel for DMF was 0.54 wt %, indicating that S2-Ag is a very efficient gelator for gelling this liquid. Further examining Table 1 reveals that both S1-Ag and S3Ag can form opaque metallogels in the mixed solvents of DMF/H2O (v:v = 1:1) and DMSO/H2O (v:v = 1:1), while S2Ag can gel none of the two mixed solvents and precipitates have only been obtained. Interestingly, in four metallogels formed in mixed solvents, the gel of S3-Ag in DMSO/H2O (v:v = 1:1) did not shown any coloration after several weeks on the open bench (Figure 1g−i). However, after 1 week colors of the other three metallogels deepened, and both of the gels of S1-Ag and S3-Ag in DMF/H2O (v:v = 1:1) were almost black after a further 1 week (Figure 1d−f and Figure S1). These observations indicated that the rate of silver reduction in metallogels depends on not only structures of ligands but also different solvents. The gelation test results have demonstrated that the three Schiff base ligands, S1−S3, exhibited different behaviors when they were used to construct metallogels in different solvents through coordination with silver ions. However, the three ligands differ only in relative positions of the nitrogen atom in two pyridine rings, which have clearly indicated that a slight

Figure 2. Partial 1H NMR spectra of ligand S2 in DMF-d7 (solution, a) and S2-Ag metallogel in DMF-d7 (metallogel, b).

It is clear that the coordination of silver ion to ligand S2 can be supported by 1H NMR spectroscopy. With reference to Figure 2, in the solution of S2 in DMF-d7, the chemical shifts of four protons of the pyridyl group appear at 9.19, 8.78, 8.37, and 7.60 ppm, respectively. However, in the metallogel of S2-Ag in DMF-d7, the chemical shifts of the four protons significantly shifted to downfield, and the data are 9.26, 8.87, 8.46, and 7.68 ppm, respectively, indicating the coordination interaction between silver ion and nitrogen atom of the pyridine ring in S2-Ag metallogel. At the same time, further reference to Figure 2, it can be seen that the chemical shift of the amide protons moved strikingly downfield (from 12.14 ppm in the solution to

Figure 1. (1) Metallogels of S2-Ag in DMF (a) fresh, (b) after 1 week on the bench, (c) after 2 weeks on the bench; (2) metallogels of S1-Ag in the mixed solvents of DMF/H2O (v:v = 1:1) (d) fresh, (e) after 1 week on the bench, (f) after 2 weeks on the bench; (3) metallogels of S3-Ag in the mixed solvents of DMSO/H2O (v:v = 1:1) (g) fresh, (h) after 1 week on the bench, (i) after 2 weeks on the bench. C

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enhancement of the hydrogen bonding strength. The similar shifts can be seen in the FT-IR spectra of S1-Ag metallogel (Figure S2). To further investigate the chemical state of silver in metallogels, XPS was carried out on the metallogels and ligands. Compared with the XPS spectrum of ligand S2 (Figure S3), the XPS spectrum of S2-Ag metallogel showed clear peaks of Ag 3d (Figure 5a), and the binding energy of the Ag 3d peaks was 373.53 and 367.53 eV (Figure 5b). Meanwhile, the X-ray induced Auger electron spectroscopy (XAES) was used to measure Ag MNN in metallogels, and the results showed the kinetic energy of Ag MNN in S2-Ag metallogel appeared at 350.50 eV (Figure 5c), instead of about 358 eV of Ag simple substance, suggesting that silver element exists in monovalent form.35 On the basis of the results of 1H NMR, FT-IR, and XPS study, coordination interaction and intermolecular hydrogen bonding are two of the driving forces for the formation of S2Ag metallogel and silver element exists in monovalent form in metallogels. Aggregation Structure of Metallogels. Images of molecular aggregates can be observed on transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and they provide a visual insight into the morphologies of the self-aggregation of gelator molecules. Some representative TEM and SEM images of the xerogels are shown in Figure 6. As shown in Figure 6a, the TEM images of S2-Ag metallogel exhibited granular aggregates formed by short fibers. And, in the SEM images of S2-Ag metallogel (Figure 6b), the irregular granular aggregates were more distinct and the diameters of the aggregates were ca. 8−10 μm. The S3-Ag metallogel formed from a mixture of DMF/H2O (v:v = 1:1) showed a short rod structure and the width of the rod is ca. 0.2−0.5 μm (Figure 6c). However, The S3-Ag metallogel formed from a mixture of DMSO/H2O (v:v = 1:1) exhibited a very long belt structure (Figure 6d), and the width of the belt is 0.5 μm, which is almost uniform. And, the SEM images of S1-Ag metallogels formed from a mixture of DMF/ H2O (v:v = 1:1) or DMSO/H2O (v:v = 1:1) showed fibrous structures (Figure S4). The TEM images of S1-Ag and S2-Ag metallogels showed different fibrous structures (Figure S5). Upon the basis of comparisons of the microstructures of different metallogels, it is safe to conclude that the morphology of a metallogel depends not only on the structure of a ligand but also on the nature of the liquid gelled by it. In addition, in order to elucidate the detailed packing mode of molecules of metallogels, the xerogels of S2-Ag metallogel in DMF and S3-Ag metallogel in DMSO/H2O (v:v = 1:1) were studied by employing a XRD technique. Figure 7 shows the XRD trace of S2-Ag metallogel in DMF. As is shown in the figure, the XRD pattern is characterized by a group of reflection peaks, and all peaks can be classified into two sets. The d values of one set are 1.11, 0.90, 0.57, 0.49, 0.40, and 0.34 nm, respectively, and they are in the ratio of 1:1/√2:1/2:1√5:1/ √8:1/3, which is in good agreement with the (100), (110), (200), (210), (220), and (300) reflections of a tetragonal packing. At the same time, the d values of another set are 1.11, 0.69, 0.57, 0.42, 0.34, and 0.32 nm, respectively, corresponding to the ratio of 1:1/√3:1/2:1/√7:1/3:1/√12, which is in good agreement with the (100), (110), (200), (210), (300), and (220) reflections of a hexagonal packing. The findings indicate that S2-Ag metallogel takes on mixed packing models of tetragonal and hexagonal, and two packing models possess the

12.25 ppm in the metallogel), suggesting that the amide groups participated in the aggregation process, an indicator of intermolecular hydrogen bonding formation. Such intermolecular hydrogen bonding formation also can be revealed by temperature-dependent 1H NMR studies of the metallogel (Figure 3). With reference to Figure 3, it is clear that the signal

Figure 3. Partial 1H NMR spectra of S2-Ag metallogel in DMF-d7 at different temperatures.

of the amide proton shifts strikingly to upfield along with increasing the temperature of the system from 298 to 328 K. At 298 K, the amide proton appears at 12.25 ppm. However, the corresponding signal shifts to 12.13 ppm when the temperature reached 328 K. The results demonstrate again that the amide proton had taken part in intermolecular hydrogen bonding formation. FT-IR spectroscopy was also used to detect the coordination between silver ion and S2 in metallogel.29,33,34 From the vibration spectra, we observed that the characteristic CN stretching bands of pyridine ring in ligand S2 appeared at 1561 and 1482 cm−1. However, in S2-Ag metallogel, the bands shifted to 1546 and 1474 cm−1, respectively (Figure 4). The shift to low wavenumbers of these bands suggests the lone pair of electrons of a nitrogen atom is donated to silver ion causing a decrease of bond order of the pyridine ring. Furthermore, the stretching vibrations of the amide CO shift from 1682 cm−1 in ligand S2 to 1664 in S2-Ag gel (Figure 4), suggesting the

Figure 4. FT-IR spectra of ligand S2 (solid, a) and S2-Ag metallogel (xerogel, b). D

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Figure 5. XPS spectra of S2-Ag metallogel: (a) wide, (b) Ag 3d; and (c) Ag MNN.

gelation has been referred quite often as an incompleted crystallization37,38 and the crystal is anisotropic. Therefore, fresh S2-Ag metallogel and its xerogel were characterized by polarizing microscopy and obtained images were shown in Figure 8 and Figure S6, respectively. According the two images,

Figure 6. (a) TEM image of S2-Ag metallogel in DMF; (b) SEM images of S2-Ag metallogel in DMF; (c) S3-Ag metallogel in DMF/ H2O (v:v = 1:1); (d) S3-Ag metallogel in DMSO/H2O (v:v = 1:1).

Figure 8. Polarizing microscope images of the fresh S2-Ag metallogel in DMF (0.04 mmol/mL).

it can be seen that both fresh S2-Ag metallogel and its xerogel exhibited clear polarized phenomena, which illustrated that there was, to a certain extent, crystallization in S2-Ag metallogel. Unlike S2-Ag metallogel in DMF, the XRD pattern of S3-Ag metallogel in DMSO/H2O (v:v = 1:1) is characterized by three reflection peaks (Figure S7), and the corresponding spacings (d) are 1.18, 0.55, and 0.31 nm, respectively, and they are in the ratio of 1:1/2:1/3, indicating that the gel takes a layered structure, and the interlayer distance is 1.18 nm. For obtaining further information on the gelation mechanism at the molecular lever, electrospray ionization (ESI) mass spectrometry was performed on metallogels. Figure 9 showed the MS data of S2-Ag metallogel measured using methanol as the solvent. It can be seen that there were three main peaks in the figure. The three peaks appeared at 481.0393, 853.1718, and 959.0677 (m/z), respectively, which can be assigned to [AgS2], [Ag(S2)2], and [Ag2(S2)2-H+], respectively. In the MS data of S2-Ag metallogel, there is no bigger peak than 959.0677 (m/z), which indicated that, in S2-Ag metallogel, S2 molecules and Ag+ have not formed linear coordination polymer. According to the data of MS and XRD, S2 molecules and Ag+ should form cyclic dimer (Figure 10) in the metallogel. At the same time, from the MS data of S3-Ag metallogels (Figure S8), it can be seen that, besides three peaks as same as that of S2-Ag metallogel, one new peak appeared at 1064.9665 (m/z), which can be assigned to [Ag3(S3)2-2H+], indicating that S3

Figure 7. X-ray diffraction (XRD) profile of S2-Ag metallogel in DMF (0.04 mmol/mL).

same minimum repeat unit.36,37 Moreover, 1.11 nm is the basic distance which is close to the sum of the general Ag−N bond length (0.23 nm) and half of the length of S2 (1.84 nm) calculated from molecular dynamics simulation. As is known to all, the precise self-assembly process leads to aggregates in which molecules are highly organized. Indeed, E

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Figure 11. TG-DTA curve of S2-Ag xerogel.

Figure 9. ESI-MS data of S2-Ag metallogel in DMF.

First, the linear regime of the system was determined. For S2-Ag metallogel, its storage modulus G′, associated with the energy storage, and loss modulus G″, associated with the loss of energy, were measured as functions of shear stress at a constant frequency of 1 Hz at 25 °C. Figure 12a shows the result of this measurement. From the figure, it can be seen that G′ is much larger than G″ before the shear stress reaches a value of about 56.0 Pa, indicating elastic character of S2-Ag metallogel. When the shear strain was above 56.0 Pa, decreases of both G′ and G″ were observed, which can be attributed to a partial breakup of the gel that begins to flow. The critical shear stress value of around 56.0 Pa, known as the yield stress value of the gel, representing the upper limit of the linear regime. The concentration of a gelator also can influence the rheological property of a gel system, so the linear regime of different concentrations of S2-Ag metallogel in DMF were measured, and the results are collectively presented in Figure 12b (to be clear, only the values of G′ are shown). The figure shows that both of the value of G′ and the yield stress of the gels increased along with increasing the gelator concentration. When the concentration of S2-Ag increased from 0.02 mmol/ mL to 0.04 mmol/mL, G′ roughly increased from 218 to 695 Pa, and the corresponding yield stress changed from 15.8 to 56.0 Pa. The above results indicated that both the stability and the elastic property of the gel are very dependent upon the concentration of the gelator in the system. In order to examine the thixotropic property of S2-Ag metallogel, the destruction and recovery of the system were studied repeatedly. First, a constant stress of 100 Pa exceeding the yield stress value was applied on S2-Ag metallogel for 2 min at a shearing frequency of 1 Hz. And then, a low shear stress of 0.1 Pa was applied to monitored the evolution of the elastic modulus of the metallogel as a function of time. The result is shown in Figure 13a. It is can be seen that, on stress of 100 Pa, S2-Ag metallogel was been destroyed and G″ > G′, indicating the breakup of the gel networks. After the destroyed shear stress was removed, the sample recovered its elastic property almost at once as that shown by the fact that the value of G′ is larger than that of G″. The result demonstrated well that S2-Ag metallogel possessed pronounced thixotropic property. For investigating the reversibility of the thixotropic property, the destruction and recovery of the same S2-Ag metallogel was reexamined for 10 circles. In one circle, there were two continuous processes (1) deformation: the gel was sheared at

Figure 10. Schematic representation of the multistage self-assembly of S2-Ag metallogel in DMF.

molecules and Ag+ may form linear coordination polymer in S3-Ag metallogels. On the basis of the results obtained via 1H NMR, FT-IR, XPS, SEM, XRD, and MS measurements, a plausible gel formation process of S2-Ag metallogel in DMF is proposed and illustrated in Figure 10. In this model, two S2 molecules were linked via the coordination interaction between silver ion and nitrogen atom of the pyridine ring, which formed the basic unit of self-assembly process. And then, two or three basic units were assembled together via intermolecular hydrogen bonding, which further assembled to tetragonal or hexagonal packing modes. According this model, the mole ratio of S2 to Ag was 1:1, so it could be forecasted that the residual weight of thermogravimetric analysis (TG) of S2-Ag xerogel should be 23.73% ascribed to Ag2O. In order to verify this forecast, TGDTA of S2-Ag xerogel was conducted, and the result revealed that its residual weight is 21.59% (Figure 11), which was in good agreement with the forecast in consideration of solvent molecules in the xerogel. The TG-DTA data of other metallogels also showed alike results (Figures S9 and S10). Rheological Studies. Interestingly, S2-Ag metallogel in DMF shows pronounced thixotropic property, which has been rarely reported before in metallogels.8,39−42 In order to further investigate this thixotropic prpperty, the rheological properties of S2-Ag metallogel in DMF were studied in detail. F

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Figure 12. (a) The evolution of G′ and G″ as functions of the applied shear stress, where the system was S2-Ag metallogel in DMF (0.04 mmol/ mL); (b) evolution of G′ as a function of the applied shear stress at different concentrations of S2-Ag metallogel in DMF.

Figure 13. (a) Evolution of G′ and G″ as functions of recovery time, where the system was S2-Ag metallogel in DMF (0.04 mmol/mL); (b) reversibility of the thioxtropic property of S2-Ag metallogel in DMF (0.04 mmol/mL) examined via alternative stress and time sweeps. Cx refers to the xth time circle, and x = 1−10.

actions between silver ions and the pyridine nitrogen atoms of ligands and hydrogen bondings contributed to the formation of the metallogels. In particular, the rheological studies showed that S2-Ag metallogel sample possesses a super smart and fully reversible thixotropic property, which has been rarely reported before in metallogels. XRD analysis revealed that S2-Ag metallogel takes a mixture of hexagonal and tetragonal packing modes; however S3-Ag metallogel in DMSO/H2O (v:v = 1:1) takes a layered structure. On the basis of the results of XRD and mass spectrometry (MS) analysis, a plausible gel formation process of S2-Ag metallogel in DMF is proposed. And this model was further demonstrated by the results of polarizing microscope and thermogravimetric analysis of S2-Ag xerogel. The findings demonstrated in the present work suggest that Schiff base ligands can be used not only to construct metalorganic molecular architectures in coordination chemistry, but also to fabricate metallogels with excellent properties.

a continuous stress from 1 to 100 Pa at a frequency of 1.0 Hz and at 25 °C; (2) recovery: a low shear stress of 0.1 Pa was applied on the destroyed gel at same frequency and at same temperature, and the recoveries of storage modulus G′ and the loss modulus G″ of the system were monitored as a function of time. The results are collectively presented in Figure 13b. It is very clear that, for the same gel sample, the reversible switch of the rheology by alternating the thixotropic could be repeated efficiently for at least 10 cycles. This indicated that this gel had an excellent fatigue resistance. No doubt, this property is valuable for some specific applications such as injection molding and drug delivery, etc.

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CONCLUSION In conclusion, we have investigated the gelling behaviors of three similar Schiff base ligands with AgNO3 in different solvents, including pure and mixed solvents. It is revealed that three ligands exhibited different gelling abilities with AgNO3 only because of the different positions of the nitrogen atom in pyridine rings, which indicated that a slight variation in ligand structures might lead to dramatic changes in gelling behaviors of ligands and properties of resulted metallogels. 1H NMR and FT-IR measurements demonstrated that coordination inter-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00952. G

DOI: 10.1021/acs.cgd.5b00952 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Article

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Photographs of S1-Ag and S3-Ag metallogels; FT-IR spectra of S1 system; XPS data of metallogels and ligands; SEM images of S1-Ag metallogels; TEM images of metallogels; polarizing microscope images of S2-Ag xerogel; XRD of S3-Ag metallogel; ESI-MS data of S3-Ag metallogels; TG-DTA curve of S1-Ag and S3-Ag xerogels (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-29-88258553; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (Grant Nos. 21403166, 21473110, 21303135), and Science and Technology Program of Xi’an (CXY1443WL14) and Key Disciplines of Analytical Chemistry in Shaanxi Province for financial support.

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DOI: 10.1021/acs.cgd.5b00952 Cryst. Growth Des. XXXX, XXX, XXX−XXX