Structural Characterization and Chemical Response of a Ag

Shear induced gelation in a copper(ii) metallogel: new aspects of ion-tunable rheology and gel-reformation by external chemical stimuli. Marc-Oliver M...
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Langmuir 2007, 23, 8217-8223

8217

Structural Characterization and Chemical Response of a Ag-Coordinated Supramolecular Gel Qingtao Liu, Yinglin Wang, Wen Li, and Lixin Wu* Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin UniVersity, Changchun 130012, People’s Republic of China

Langmuir 2007.23:8217-8223. Downloaded from pubs.acs.org by TULANE UNIV on 01/11/19. For personal use only.

ReceiVed February 8, 2007. In Final Form: April 6, 2007 Supramolecular gels exhibit potential applications in the areas of sensors, nanodevices, drug and catalyst carriers, and so on. To develop a novel organogel with a multiresponse, we designed a component molecule bearing a pyridyl group for metal coordination and an amide group for the formation of intermolecular hydrogen bonding. A complex building block with a symmetrical structure was selectively constructed by the coordination of a silver cation to the organic component. The coordination existing in the complex and the hydrogen bonding existing between complexes were examined by IR, Raman, and 1H NMR spectroscopy. The gel formation and phase transition were examined by viscosity and differential scanning calorimetry measurements. The selection of metal ions for the formation of a gel has proved to be crucial as only the complex of a binary coordinated metal ion, Ag+, was found to form a gel structure. From the band shift of the L1 solution with different amounts of silver ion, the binding ratio of silver ion to L1 was estimated to be 1:2 and the calculated stability constant was 3.6 × 108 M-2. On the basis of the analysis of X-ray diffraction and transmission electron microscopy results, we proposed a possible stacking structure of the complex in the fibrous aggregates. Of interest is that the organogel exhibits a 3-D network structure of a beltlike fiber composed of ordered lamellar arrangements of the coordinated complex and shows a rapid response to wide chemical stimulations such as anions I-, Br-, and Cl-, gases such as H2S and NH3, and a change of pH.

Introduction The organogel, as a kind of supramolecular soft material, has attracted considerable attention over recent years due to its smart response to the stimulation of the chemical and physical microenvironment such as light,1-3 temperature,4 pH,5,6 and ions.7 Such supramolecular gels exhibit potential applications in sensors,1,5 nanodevices,8,9 drug delivery and release,10,11 catalyst carriers,12 template synthesis,13 and so on. Especially, the exploitation of the function properties of the gels which are facile in preparation, reversible in stimulation, and rapid in response to the physical and chemical surroundings14 has proved to be significant and becomes one of the purposes of new gel systems. There are several approaches to realize supramolecular gels, * To whom correspondence should be addressed. Fax: +86-43185193421. E-mail: [email protected]. (1) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem. Soc. 2005, 127, 11134-11139. (2) Sugiyasu, K.; Fujita, N.; Takeuchi, M.; Yamada, S.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 895-899. (3) Miljanic´, S.; Frkanec, L.; Meic´, Z.; Zˇ inic´, M. Langmuir 2005, 21, 27542760. (4) Kawano, S.-I.; Fujita, N.; Shinkai, S. Chem.sEur. J. 2005, 11, 47354742. (5) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (6) Ahmed, S. A.; Sallenave, X.; Fages, F.; Mieden-Gundert, G.; Mu¨ller, W. M.; Mu¨ller, U.; Vo¨gtle, F.; Pozzo, J.-L. Langmuir 2002, 18, 7096-7101. (7) Kim, H.-J.; Lee, J.-H.; Lee, M. Angew. Chem., Int. Ed. 2005, 44, 58105814. (8) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922-13923. (9) (a) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179-183. (b) Enomoto, M.; Kishimura, A.; Aida, T. J. Am. Chem. Soc. 2001, 123, 5608-5609. (10) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Shinkai, S. Chem. Commun. 2005, 4149-4151. (11) Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 8932-8938. (12) Gao, P.; Zhan, C.; Liu, M. Langmuir 2006, 22, 775-779. (13) (a) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. J. Mater. Chem. 2001, 11, 2412-2419. (b) Llusar, M.; Monro´s, G.; Roux, C.; Pozzo, J. L.; Sanchez, C. J. Mater. Chem. 2003, 13, 2505-2514. (c) Kawano, S.-I.; Tamaru, S.-I.; Fujita, N.; Shinkai, S. Chem.sEur. J. 2004, 10, 343-351. (14) Ghoussoub, A.; Lehn, J.-M. Chem. Commun. 2005, 5763-5765.

including tuning the interaction between molecules for singlecomponent materials such as gels based on hydrogen bonding15-17 and fabricating complex building blocks through hydrogen and coordination bonding first and then tuning the interaction between these building blocks for multicomponent materials.18 Consequently, the functionalization of supramolecular gels can be carried out by the rational design of building blocks, and thus, macroscopic characteristics can be controlled by adjusting the interaction and chemical structure of the molecular components. As the gel-sol phase transformation is always accompanied by a change of the supramolecular interaction, the additional components and the driving force can make the gels more sensitive to various stimulations. For example, a puny change in the anion size can induce the occurrence of a gel-sol phase transition of a coordinated polymer gel7 and a change in the molar ratio of the components can alter the existing state of the molecular aggregates in the gels.18 Among noncovalent interactions, coordination interaction19 is proved to be an effective path in creating a multiresponse organogel besides hydrogen bonding and π-π interactions as it is not only applied to control the chemical structure of the building blocks, but also used for the fabrication of responding sites because of its reversible binding and unbinding. To develop novel organogels with a multiresponse, a component molecule, L1 (Scheme 1), which possesses a coneshaped structure with a pyridyl head group connecting to a gallic acid with three alkyl chains through an amide group, was herein designed and prepared. We try to bridge L1 molecules through (15) Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 5620-5621. (16) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393-1397. (17) (a) Gao, P.; Zhan, C.; Liu, L.; Zhou, Y.; Liu, M. Chem. Commun. 2004, 1174-1175. (b) Zhan, C.; Gao, P.; Liu, M. Chem. Commun. 2005, 462-464. (18) (a) Hirst, A. R.; Smith, D. K. Chem.sEur. J. 2005, 11, 5496-5508. (b) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.; Wright, A. C. J. Am. Chem. Soc. 2003, 125, 9010-9011. (c) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurst, H. P. M. Chem.sEur. J. 2004, 10, 5901-5910. (19) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680-1682.

10.1021/la700364t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007

8218 Langmuir, Vol. 23, No. 15, 2007 Scheme 1. Schematic Representative of the Synthetic Path of Ligand L1 and the Chemical Structure of Its Ag-Coordinated Complex

metal ion coordination and hydrogen bond formation into a complex to generate a supramolecular gel. The expected linear complex is first considered by the coordination of L1 with a silver ion because the complex with a linear or planar symmetrical structure is favorable not only for the construction of a fibrous structure, which is necessary for a complex gel, but also for the multiresponse to different physical and chemical stimulations. Interestingly, the coordinated complex of L1 with a silver ion indeed exhibits an organogel property and shows a rapid response to wide chemical stimulations such as anions I-, Br-, and Cland gases H2S and NH3, as well as a change of pH. A structural characterization for the formation and phase transition of the complex gel is then investigated in detail. Experimental Section Materials and Samples. All analytical grade solvents were purified and dried before use. The precursor 3,4,5-tris(hexadecyloxy)benzoic acid (A) was synthesized according to the literature method,20 and column chromatography for the purification of the product was applied using silica gel. Analytical thin-layer chromatography was performed on commercial plates coated with silica gel F254. For the gelation experiment, the sample was prepared by adding 0.5 molar equiv of AgSO3CF3 dissolved in ethanol to a toluene solution of ligand L1 in a screw-capped test tube. The immediately formed complex gel was heated until it was dissolved, and then the solution was allowed to cool to room temperature. Thus, a homogeneous organogel was obtained after 1 h of standing. For the measurement of the critical gelation concentration, a suspension of the gelator in a given volume of solvent was heated until it was dissolved and then cooled to room temperature. By controlling the amount of the complex gelator in the solvent, a minimum gelation concentration was estimated during the cooling process of the hot solution. To measure the stability constant of the complex, an appropriate amount of AgSO3CF3 in ethanol was gradually added to a chloroform solution of ligand L1, giving a sample for UV-vis absorption measurements after sonication and standing for 10 min in ambient conditions. Here the reason that chloroform was used instead is to avoid the absorption disturbance of toluene. In the measurement of the gas response, H2S or NH3 gas was prepared by a simple chemistry reactor and was permitted to pass an airtight conical flask containing a test tube in which the complex gel was placed previously. Synthesis. A mixture of A (0.84 g, 1 mmol) and thionyl chloride (5 mL) was refluxed for 2 h. After the mixture was cooled to room temperature, the excess thionyl chloride was evaporated off under reduced pressure to give the intermediate B. To the mixture of the (20) Percec, V.; Mitchell, C. M.; Cho, W.-D.; Uchida, S.; Glodde, M.; Ungar, G.; Zeng, X.; Liu, Y.; Balagurusamy, V. S. K.; Heiney, P. A. J. Am. Chem. Soc. 2004, 126, 6078-6094.

Liu et al. intermediate B and 4-aminopyridine (0.2 g, 2 mmol) in THF (70 mL) under an ice bath was added triethylamine (1.0 g, 6 mmol) dropwise with continuous stirring at 0 °C. The mixture was stirred at this temperature for 10 h and then poured into ice-water. The aqueous solution was extracted with three portions of chloroform (40 mL). The combined organic phase was dried over anhydrous MgSO4 and then filtered and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel with a 3/1 mixture of chloroform/ethyl acetate as the eluent to give 0.45 g (48.4% yield) of a white solid, 3,4,5-tris(hexadecyloxy)-N(pyridin-4-yl)benzamide (L1). 1H NMR (500 MHz, CDCl3, TMS): 0.88 ppm (t, J ) 7 Hz, 9H), 1.26 ppm (s, 72H), 1.48 ppm (m, 6H), 1.79 ppm (m, 6H), 4.04 (m, 6H), 7.41 ppm (s, 2H), 8.31 ppm (s, 2H), 8.73 ppm (d, J ) 5.5 Hz, 2H), 10.8 ppm (s, 1H). Anal. Calcd for C60H106N2O4: C, 78.37; H, 11.62; N, 3.05. Found: C, 78.20; H, 11.77; N, 3.14. MALDI-TOF MS: m/z 920.7 (M+), 665.9 (M OC16H33+). Mp: 76.6 °C. Instruments. FT-IR spectra were obtained on a Bruker IFS66V spectrometer equipped with a DGTS detector (32 scans), recording with a resolution of 4 cm-1. 1H NMR spectra (TMS) were obtained on a Bruker UltraShield 500 MHz spectrometer. The small-angle X-ray diffraction (XRD) of the thick casting film on a silicon wafer was characterized on a Rigaku X-ray diffractometer (D/max rA, Cu KR radiation of wavelength 1.542 Å). MALDI-TOF spectra were recorded on a Kratos-Shimadzu AXIMA-CFR mass spectrometer. Scanning electron microscopy (SEM) measurement was carried out with a JEOL FESEM 6700F electron microscope operating at an accelerating voltage of 25 kV. Atomic force microscopy (AFM) measurement was carried out on a commercial instrument (Digital Instrument, Nanoscope III, and Dimension 3000), operating in tapping mode at room temperature in air. Transmission electron microscopy (TEM) images were obtained on a Hitachi H 8100 transmission electron microscope with an accelerating voltage of 200 kV without staining. Differential scanning calorimetry (DSC) curves were obtained on a Netzsch DSC 204 instrument. The FT-IR spectra were obtained on a Bruker IFS66V FT-IR spectrometer by using a CaF2 cell filled with a CHCl3 solution of the complex. The Raman spectra were recorded with a Raman spectrometer (Renishaw model 1000). The 514.5 nm radiation from a 200 mW air-cooled argon ion laser was used as an excitation light source. UV-vis spectra were obtained on a Shimadzu UV3100PC spectrometer. The fibers of the coordinated gel were studied using a polarized optical microscope from Leica DMLP, Germany.

Results and Discussion Formation of the Complex Gel. Though there exists intermolecular hydrogen bonding in its solution, pure L1 cannot self-organize into a gel state due to the disadvantage of the structure and weak strength of the hydrogen bonding for the fibrous aggregation and cross-linking or entanglement of the fibers. Considering the coordination of the pyridyl head group of L1 for metal ions, the multicoordinated L1 with an ion will provide a more symmetrical structure and increase the strength of the hydrogen bonding by increasing the hydrogen-bonding sites in the singly coordinated complex. To obtain fibrous aggregates for gels, linear or planar coordination of metal ions in the complex is necessary, and thus, silver becomes the first metal ion to be considered. The reason we employed AgSO3CF3 rather than AgNO3, a rather more common silver salt, is due to its good solubility in organic solvents such as ethanol, acetone, and so forth. When an ethanol solution of AgSO3CF3 is added to a toluene solution of L1 at a molar ratio of 1/2, a complex gel forms quickly in the mixed solvent with a toluene/ethanol ratio of 10/1 (v/v). In contrast to the gradual increase of the viscosity with the L1 concentration in a pure L1 solution, the viscosity of the complex reveals a sharp enhancement near the critical gel concentration (CGC) in the complex solution. This property is quite similar to the behavior of the polymer gel and

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band shift versus the molar ratio, implying a 1/2 ratio of Ag+ to the ligand L1 in the complex, which just corresponds to the well-known coordination number of Ag+. In addition, with this stoichiometry, we can calculate the stability constant of the complex on the basis of the following reaction equation:22 +

+

[Ag ] + 2[L1] f [Ag(L1)2 ]

Ksc )

[Ag(L1)2+] [Ag+][L1]2

)

[Ag(L1)2+] ([Ag+]0 - [Ag(L1)2+])([L1]0 - 2[Ag(L1)2+])2

(1)

where [Ag+], [L1], and [Ag(L1)2+] are the concentrations of silver ion, ligand L1, and complex at equilibrium, respectively, and [Ag+]0 and [L1]0 denote the general concentration of added silver ion and ligand L1. To obtain the ratio of bound L1 to the total L1, we define

θ)

Figure 1. (a) UV-vis absorption spectra of an L1 chloroform/ ethanol (10/1) solution (fixed at 4.7 × 10-4 M) with different amounts of silver ions (0, 2.9 × 10-5, 5.9 × 10-5, 7.8 × 10-5, 1.2 × 10-4, 1.6 × 10-4, 2.4 × 10-4, 4.7 × 10-4, 9.4 × 10-4, and 1.4 × 10-3 M). (b) Band shift of the Ag complex corresponding to pure L1 versus the concentration ratio of silver ion to L1.

implies the presence of long and entangled supramolecular fibrils in solution.21 The difference between pure L1 and its silver complex also reflects the solubility in the mixed solvent as the saturated concentration of pure L1 in the same solvent exceeds 20 mg/mL, whereas the CGC of the coordinated complex can only reach 2 mg/mL. DSC measurement demonstrates that the gel-sol transition temperature (Tgel) relies on the concentration of the complex gel. The Tgel appears at 38.6 °C under the CGC, while it increases to 42 °C under a concentration of 20 mg/mL. The complex shows a good gelation property in various mixed solvents such as mixtures of chloroform with acetone, ethanol, and other polar solvents. UV-vis absorption spectra have been employed to determine the effect of the stoichiometry of the two components on the formation of the complex gel. The concentration of L1 was fixed at 4.7 × 10-4 M, and considerable shifts of the absorption band of L1 to long wavelength have been observed by altering the added amount of silver ion to 1.4 × 10-3 M (Figure 1a), indicative of the formation of the metal complex. As the value of the band shift is correlated to the concentration of Ag ion, we can estimate the stoichiometry of the Ag ion to the ligand L1 from the plot of the band shift versus the molar ratio of Ag to L1 in the given region of the concentration, as shown in Figure 1b. The turning point appears at a molar ratio of ca. 0.55 by elongating two linearly increasing regions of the (21) (a) Guan, Y.; Yu, S.-H.; Antonietti, M.; Bo¨ttcher, C.; Faul, C. F. J. Chem.s Eur. J. 2005, 11, 1305-1311. (b) Wu¨rthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42, 3247-3250.

[L1]0 - [L1] [L1]0

)

2[Ag(L1)2+] [L1]0

(2)

It can also be expressed as θ ) (λobsd - λL1)/(λAg(L1)2 - λL1) ) ∆λobs/∆λAg(L1)2. Here, λobsd is the wavelength of the absorption band of the complex solution, λL1 is the wavelength of the absorption band of L1, λAg(L1)2 is the wavelength of the absorption band of the complex in which all L1 molecules have coordinated with a silver ion, ∆λobsd denotes the band shift of the complex solution corresponding to L1, ∆λAg(L1)2 is the band shift of the complex corresponding to L1 when all L1 molecules have coordinated with a silver ion. When M is applied as the ratio of silver ion and L1 in the solution, we get

M ) [Ag+]0/[L1]0

(3)

Combining eqs 1-3, we obtain

Ksc )

θ (2M - θ)(1 - θ)2[L1]02

(4)

From the plot of Figure 1b, ∆λAg(L1)2 is calculated to be ca. 7.0 nm. Thus, when the stoichiometry of silver ion to L1 is 1/2, ∆λobsd is calculated to be ca. 5.5 nm, and thus, θ is estimated to be ca. 0.79. Therefore, the stability constant Ksc obtained is about 3.6 × 108 M-2, indicative of quite stable coordination of the complex. Characterization of Coordination and Hydrogen Bonding. Because the pyridyl ring breathing vibration is sensitive to the chemical binding of the N atom of the pyridyl group, Raman spectroscopy was used to detect the coordination between silver ion and L1 in the complex. The pyridyl ring breathing mode of free L1 at 999 cm-1 is found to wholly shift to 1024 cm-1 after the addition of AgSO3CF3 (Figure 2a, top). The band shift can be regarded as a sign of the pyridyl group in its combined state, implying complete coordination of silver ion.23 The coordination of silver ion to the ligand L1 can also be supported by 1H NMR spectroscopy (Figure 2b, top). With the addition of Ag ion to the solution of L1, the chemical shifts of the amide protons and R-protons of the pyridyl group of L1 shift to high field, and that of the β-protons of the pyridyl group shifts to low field. All the (22) (a) Polyakov, N. E.; Khan, V. K.; Taraban, M. B.; Leshina, T. V.; Salakhutdinov, N. F.; Tolstikov, G. A. J. Phys. Chem. B 2005, 109, 2452624530. (b) Resendiz, M. J. E.; Noveron, J. C.; Disteldorf, H.; Fischer, S.; Stang, P. J. Org. Lett. 2004, 6, 651-653. (23) Hou, X.; Wu, L.; Xu, M.; Qin, L.; Wang, C.; Zhang, X.; Shen, J. Colloids Surf., A 2002, 198-200, 135-140.

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Figure 3. FT-IR spectra of the (a) Ag-coordinated complex in the xerogel state and (b) Ag-coordinated complex in chloroform solution.

Figure 2. (Top) Raman spectra of (a) the Ag-coordinated complex gel and (b) ligand L1. (Bottom) 1H NMR spectra of (a) the Agcoordinated complex gel and (b) ligand L1.

shifts reach their largest values at a stoichiometry of silver ion to L1 of 1/2 (Figure 2, bottom). Therefore, on the basis of the estimated stoichiometry and linear coordination geometry, one Ag+ must coordinate with two pyridyl groups, forming a complex with a molecular ratio of 1/2. To confirm the special hydrogen bonding existing in the complex gel, we compared the FT-IR spectra of the complex in the gel state and in its chloroform solution.24 From the vibration spectra, we observe that the N-H stretching and amide I bands of the complex shift from 3423 and 1693 cm-1 in chloroform solution to 3338 and 1687 cm-1 in its gel state, respectively (Figure 3). The shift to low wavenumbers of these absorption bands suggests the enhancement of the hydrogen-bonding strength. Stronger hydrogen bonding between amide groups in the gel state makes the complex self-assemble into ordered aggregates and becomes one of the main driving forces for the gel formation.24,25 In contrast to these results of the silver complex with L1, when silver ion coordinates with (E)-4-(4-(3,4,5-tris(hexadecyloxy)benzyloxy)styryl)pyridine, a ligand with a structure similar to that of L1 but without an amide group, the formed complex cannot self-organize into gels under the same conditions (see Figure S1 in the Supporting Information). In addition, by comparing the antisymmetric and symmetric stretching vibrations of CH2 alkyl chains, we can clearly see ordered aggregation of the complex in the gel state on the basis of the bands at 2917 and 2850 cm-1 and disordered aggregation in chloroform solution (without addition of ethanol) on the basis of the band positions at 2927 and 2855 cm-1. Therefore, we believe that the van der Waals interaction between alkyl chains helps to form the complex (24) Kim, C.; Kim, K. T.; Chang, Y.; Song, H. H.; Cho, T. -Y.; Jeon, H.-J. J. Am. Chem. Soc. 2001, 123, 5586-5587. (25) Chow, H. F.; Zhang, J. Chem.sEur. J. 2005, 11, 5817-5831.

Figure 4. Photographs of coordinated complexes of (a) Cu(CH3COO)2 (molar ratio 1/4, 6.6 mg/mL), (b) Co(CH3COO)2 (molar ratio 1/6, 10 mg/mL), and (c) NiCl2 (molar radio 1/6, 10 mg/mL) with ligand L1 in toluene/ethanol of 10/1 (v/v) molar ratio and the corresponding X-ray diffraction patterns of (d) the sample in (a) and (e) the sample in (b) in the bulk solid.

gel, especially when a little ethanol has been added to the complex solution because the addition of ethanol enhances the hydrophobic interaction between the alkyl chains of L1. Other metal ions, such as Co2+ and Cu2+, also can coordinate with L1 to form complexes, as confirmed by Raman and FT-IR spectra (Figures S2 and S3 in the Supporting Information), but interestingly, the gelation of the L1 complex exhibits selectivity for metal ions. Although the FT-IR spectra display similar ordered stacking of alkyl chains to the Ag complex, the amide I bands of Co2+ and Cu2+ complexes appear at 1693 and 1691 cm-1, indicative of weaker hydrogen bonding than that of the Ag+ complex. As shown in Figure 4, the complexes of other metal cations such as Co2+, Cu2+, and Ni2+ with L1 cannot generate gel states (Figure 4). Considering the smaller shifts of the amide I bands in the IR spectra by the comparison of these metal complexes with the Ag complex, it can be affirmed that the strength of the hydrogen bonding of these two metal complexes seems weaker. The difference in structure between these complexes of transition-metal ions and Ag ion was further examined by XRD. Both the spectra of Co2+ and Cu2+ complexes show only one diffraction pattern, implying the layered structure is quite disordered in their solid states, and the layer spacing

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Figure 6. XRD pattern of the coordinated xerogel and (inset) polarized optical photograph of the Ag-coordinated complex gel (magnification 300×).

Figure 5. SEM image of the xerogel (a) and AFM image of fibers in the sol (b), which is obtained by diluting the coordinated gel into a sol.

estimated from each single diffraction peak is much smaller than the simulated length of two L1 molecules. Considering these data and the coordination geometry, we propose that the aggregated structures of Cu and Co complexes are different from that of the Ag complex gel, as discussed in the following part, so that they cannot form a gel state under the same conditions. The main reason should be probably that the coordination of these metal ions with L1 is normally nonlinear and the coordination geometry of the complexes is disadvantageous for linear growth of hydrogen bonding and fibrous aggregation even if these complexes possess a layered aggregation structure. Therefore, we believe that both the linear coordination structure of the complex and the intermolecular hydrogen-bonding pattern as well as van der Waals interaction between alkyl chains are crucial for the gelation because the complex structure with double hydrogen bondings compared with the single bonding of L1 is favorable for stabilizing the architecture and orientated and entangled growth of fibrous aggregation. Aggregation Structure of the Complex Gel. As shown in Figure 5a, the complex gel can be demonstrated to be composed of a beltlike fibrous structure. The beltlike fibers 50-300 nm in width and several tens of micrometers in length weave into 3-D network, while only nublike aggregates are observed in the solution of pure L1. The AFM image also illustrates a long and beltlike structure of the complex gel, and when the complex gel is diluted into a sol, the belt fibers become straight and monodispersed, which is similar to the morphology of the crystalline state (Figure 5b). The polarized optical microscopic photograph exhibits strong birefringence of the gel state, indicating ordered arrangement of the silver complex in the fibrous aggregate. In addition, XRD of coordinated xerogel reveals that the beltlike fiber possesses a lamellar structure of the complex with a layer spacing of 6.3 nm (Figure 6), which is in good

agreement with the simulated length of the coordinated complex along its long axis (6.4 nm). It should be mentioned that a separated diffraction peak appeared at 2θ ) 6.2° corresponding to a 1.4 nm layer spacing (whose higher orders of diffraction were not found in the mean time), which can be primarily attributed to the length of the short axis of the complex due to the similarity of the experimental result with that of the simulation (1.3 nm). Considering that the symmetric and antisymmetric stretching bands of CH2 appear at 2850 and 2917 cm-1, respectively, and its scissor band appears at 1467 cm-1, the alkyl chains of the complex in the gel state are concluded to exist in an all-trans zigzag conformation and ordered hexagonal arrangement.26 Therefore, we suggest that the Ag-coordinated complex arranges perpendicularly to the direction of the beltlike fibers. To understand this hierarchical self-assembly structure, a model is proposed in Figure 7. Two L1 ligands are linearly connected by a binary coordinated silver ion symmetrically, and the complex further self-assembles into a primary fibril structure through hydrogen bonding between amide groups on both sides and further assembles into beltlike fibers as the secondary structure of the gel through van der Waals interaction between the outside alkyl chains. The beltlike fibers finally weave into a 3-D network and fix the solvents to form a gel through surface tension. Chemical Response of the Complex Gel. The organogel based on the Ag-coordinated complex shows a quick response to some anions, leading to macroscopic gel-sol transformation through destroying the coordination of silver ion with L1, symmetrical hydrogen bonds, and further the fibrous aggregate structure. As an example, when the proper amount of the ethanol solution of potassium iodide was added to the complex solution, the gel was seen to dissolve and transform into a sol, while a pale yellow precipitate was observed (Figure 8a,b), which could be clearly assigned to the silver iodide. From the TEM images, the intertwisted fibers in the gel state are found to be dissociated and become amorphous aggregates by the addition of an inorganic salt (Figure 8d,e). If we add excess silver ion to the fluid solution again after the silver iodide precipitate has been filtrated off, the gel re-forms. Such a gel-sol transformation can be repeated several times by adding potassium iodide and silver trifluo(26) Weers, J. G.; Sheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Sheuing, D. R., Ed.; American Chemical Society: Washington, DC, 1991; p 91.

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Figure 7. Hierarchical structure model of the Ag-coordinated complex gel: (a) 1-D molecular stacking along the orientation of the hydrogen bonds as the primary structure of the gel, (b) 1-D aggregation of beltlike fibers through van der Waals interaction as the secondary structure of gel, and (c) 3-D network weaved by beltlike fibers as the tertiary structure of gel.

Figure 8. Photographs of the response property of the Ag-coordinated complex gel before (a) and after the addition of potassium iodide (b) and H2S gas (c). (d)-(f) are the TEM images corresponding to the samples in (a)-(c), respectively.

romethanesulfonate to the gel and the sol solution alternately. Other anions that can react with silver ion, forming a precipitate, such as Br- and Cl-, can also induce the transformation of the gel to a sol. Obviously, the formation of the precipitate accelerates the dissociation of the silver complex in the gel state because the precipitation reduces the concentration of silver ion in the complex solution, which leads to a shift of the equilibrium toward the dissociation of the complex, as shown in eq 5.

Ag(L1)2+ + X- f AgXV + 2L1

(5)

In addition, the complex gel is sensitive to the change in pH as the transformation from the gel to a sol shows a tight dependence at pH 2 and 11 under the addition of acid or base solution, respectively, when nitric acid and sodium hydroxide are used to adjust the chemical environment of the gel. Under the low pH, the coordinated pyridyl group of the ligand L1 can be protonized, resulting in the decomposition of the coordinated complex, which destroys the gel aggregation. Under the high pH, the silver ion in the complex will react with OH-, forming silver oxide and leading to the dissociation of the complex gel. This can be clearly affirmed by the formation of a black precipitate

in the sol solution. These processes can be described from reaction eqs 6 and 7, respectively.

Ag(L1)2+ + 2H+ f Ag+ + 2H(L1)+

(6)

2Ag(L1)2+ + 2OH- f Ag2OV + 4L1 + H2O

(7)

Notably, the coordinated gel also responds to certain gases such as H2S and NH3 rapidly. The gel surface immediately changes from colorless to brown when it is exposed to an atmosphere of H2S. In several minutes of exposure, the coordinated gel completely transforms into a brown sol, and the sol can transform into a gel again if we add Ag ion to the solution after the precipitate has been filtered off. The change from a gel to a sol can be evidently confirmed by TEM measurement. In the beginning stage of the complex gel being exposed to the H2S gas, we can see that the long and entangled beltlike fibers rapidly break into short and unentangled fibrils. Because the short beltlike fibers cannot keep the 3-D network structure which is crucial for the gel, the gel-sol transformation takes place, as demonstrated in Figure 8c,f. Substantially, the gel-sol transformation is evidently derived from the reaction of the gas with silver ion, which relieves

Ag-Coordinated Supramolecular Gel Chemical Response

Langmuir, Vol. 23, No. 15, 2007 8223

the coordination of silver ion with L1 and thus destroys the gel structure as expressed in eq 8. This can be confirmed by the dark precipitate of silver sulfide after a longer time of exposure as well. If Ag ion is supplied again into the sol solution, the complex gel re-forms for a reason similar to that which occurred in the repeated transformation by adding other reactive anions with silver cation. NH3 gas can also make the Ag complex gel disassociate into a sol through its coordination to silver ion as described in eq 9.

2Ag(L1)2+ + S2- f Ag2SV + 2L1

(8)

Ag(L1)2+ + 2NH3 f Ag(NH3)2+ + 2L1

(9)

Although the stability constant of the silver complex is a little bit larger than that of Ag(NH3)2+, 1.67 × 107 M-2, the addition of a high concentration of NH3 still can catch silver ion in the complex gel, resulting in the transformation of the gel to a sol.

Conclusions By the combination of a metal ion and an organic component bearing a pyridyl head group and an amide, we successfully prepare a silver-coordinated complex, which can be used as the building block of an organogel. Of interest is that the binary coordination of Ag ion is critical for the gelation of the complex as we find that other metal complexes with polyhedron coordination cannot form gels. In addition, both the coordination structure of the complex and the symmetrical hydrogen bonding between complexes as well as the van der Waals interaction of

hydrophobic alkyl chains are confirmed to be crucial for the gelation. By the investigation of the structure of the complex gel, we find that the complex aggregates into a layered structure and further self-organizes into a beltlike fibril network, which is enhanced by intermolecular hydrogen bonding. In contrast to the case in which the dissociation of a single-component organogel is mainly derived from the driving force of the unbinding of hydrogen bonding, the repeated and rapid response of the supramolecular gel to some anions, gases, and pH is completely attributed to the release of silver ion through its combination with the chemical surroundings, which makes this kind of gel applicable for fabricating smart soft materials. Acknowledgment. We acknowledge financial support from the National Basic Research Program (Grant 2007CB808003), National Natural Science Foundation of China (Grants 20574030 and 20473032), PCSIRT of the Ministry of Education of China (Grant IRT0422), 111 Project (Grant B06009), and Open Project of the State Key Laboratory of Polymer Physics and Chemistry of the Chinese Academy of Sciences. We thank Professor F. Schue from the University of Montpelier for helpful discussion on the occasion of his visiting us. Supporting Information Available: Details of the DSC curves, photographs of the sol-gel phase transformation, and TEM and SEM images of the coordinated gel and ligand L1 solution. This information is available free of charge via the Internet at http://pubs.acs.org. LA700364T