A novel low-molecular-mass gelator with a redox active ferrocenyl group: Tuning gel formation by oxidation

Journal of Colloid and Interface Science 318 (2008) 397–404 www.elsevier.com/locate/jcis

A novel low-molecular-mass gelator with a redox active ferrocenyl group: Tuning gel formation by oxidation Jing Liu, Junlin Yan, Xuanwei Yuan, Kaiqiang Liu, Junxia Peng, Yu Fang ∗ Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, People’s Republic of China Received 9 July 2007; accepted 3 October 2007 Available online 19 November 2007

Abstract A novel low-molecular-mass gelator containing a redox-active ferrocenyl group, cholesteryl glycinate ferrocenoylamide (CGF), was intentionally designed and prepared. It was demonstrated that the gelator gels 13 out of the 45 solvents tested. Scanning electron microscopy (SEM) measurements revealed that the gelator self-assembled into different supramolecular network structures in different gels. Chemical oxidation of the ferrocenyl residue resulted in phase transition of the gel from gel state to solution state. FTIR and 1 H NMR spectroscopy studies revealed that hydrogen bonding between the gelator molecules in the gel was one of the main driving forces for the formation of the gels. © 2007 Elsevier Inc. All rights reserved. Keywords: Gelator; Ferrocene; Stimuli-responsive gels; Cholesterol

1. Introduction Low-molecular-mass organic gelators (LMOGs) and their thermally reversible organogels have experienced an enormous increase in interest during the past decade because of their numerous potential applications in template synthesis of nanostructured and functional materials, controlled release, capture of pollutants in the environment, and so on [1–12]. These organogels are materials that consist of, generally speaking, an organic liquid and a small amount of a LMOG. The gelator molecules self-assemble into crystalline fibers, tapes, strands, or other aggregates with high length-to-width ratios. These elongated objects link with each other at “junction zones” to form three-dimensional networks that immobilize the solvent by capillary forces and surface tensions [1,2]. Clearly, the gels formed in this way are supramolecular gels. Unlike chemical gels, one of the advantages of the supramolecular gels is their reversibility and sharp sol–gel phase transition as a result of thermal stimulus. Such unique behaviors originate from the noncovalent nature of the interaction between the molecules, * Corresponding author. Fax: +86 29 85310097.

E-mail address: [email protected] (Y. Fang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.10.005

consisting of the three-dimensional networks in the gels. The nature of the noncovalent interaction could be hydrogen bonding, π–π interaction, van der Waals force, coordination, electrostatic interaction, or a combination of some of these. The organogels from LMOGs not only are thermoreversible; some of them are even responsible to stimuli such light [13–16], sound [17], pH [15,18], host–guest interaction [19–21], charge transfer [22], complexation [22], oxidation/reduction [23], and even a combination of these [24,25]. To the best of our knowledge, light- and pH-responsive organogels are most commonly encountered in the supramolecular gels reported. In contrast, oxidation–reduction-responsive gels are very limited. Shinkai and co-workers [23] reported the first redoxresponsive organogel from a LMOG that contains a redoxactive Cu(I)/Cu(II) centre. The gel from the LMOG exhibits reversible chromatic and sol–gel phase-transition phenomena when the oxidation state of the Cu(I)/Cu(II) center is altered by chemical oxidation or reduction. Besides, they also synthesized a series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl moieties at the a position. It was found that a sol–gel phase transition can be implemented by addition of oxidizing and reducing reagents [26]. Zhu and his colleagues [24] designed and synthesized an electroactive LMOG containing a tetrathiafulvalene (TTF) group. The gel

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formation process can be tuned via oxidation/reduction of the TTF group chemically or electrochemically. In addition, some electron-conductive organogels have been obtained by doping some electrolytes, such as organic salts [27,28], into the gel systems. It is to be noted, however, that doping is not an effective way to introduce a property such as redox-controllable phase transition into a system. Ferrocene and its derivatives are of considerable interest in various areas such as asymmetric catalysis, nonlinear optics, and electrochemistry [29]. Although they are often photochemically inert, ferrocene and ferrocenyl derivatives may undergo chemical modification in the presence of light, or may be used as excited-state quenchers or photosensitizers, that is, as catalysts of photochemical reactions. They therefore find interesting applications in photography and as photoresists. Due to the quasi-reversible oxidation of Fe(II), new applications are emerging, in which advantage is taken of the presence of ferrocene acting as a redox center; this gives optically and electrochemically active sensors. Besides, ferrocenyl derivatives find numerous applications in organized media, whatever the complexity of the media. Owing to their hydrophobic character, they are mainly encountered in lipid phases or incorporated into membranes and films. For instance, a ferrocenyl group linked to a hydrophobic alkyl chain, to the head-group of an amphiphile [30], or to polypeptides [31] has been incorporated into artificial membranes, with the aim that the ferrocenyl redox state controls the permeability of the membranes. This systems aim at mimicking certain biological processes occurring at synapses, where potential variation induces the release of transmitter metabolites. As redox-active molecules, ferrocene and its derivatives have also been widely used to construct smart supramolecular structures [32–34]. Although organometallic derivatives are important LMOGs [35], to the best of our knowledge, there has been no report of LMOGs containing ferrocenyl residue. As a continuation of our work on studies of LMOGs [36,37], a new LMOG with a ferrocenyl group as its redox-active center has been designed and synthesized. The gelator consists of three parts: a cholesteryl moiety, a ferrocenyl group, and an amino acid residue functioning as a linker (CGF; cf. Scheme S1 in the supporting material). The main ideas behind this design are as follows: (1) as is well known, cholesteryl units could aggregate in solution through van der Waals interaction [38]; (2) amino acid residues have a strong tendency to form hydrogen bonds [39]. Furthermore, introduction of amino acid residue into the structure of LMOGs must increase the solubility of them in polar solvents, which should be favorable for the gelation of solvents of greater polarity: (3) a ferrocenyl residue is intentionally introduced into the structure. This is because the oxidation of its central ion, Fe(II), and the reduction of its oxidized state could be realized by simple addition of oxidizing and reducing reagents, respectively, or by electrochemical methods. It is expected that a change in the oxidation state of the central ion of the ferrocenyl residue may result in a dramatic change in the gelling ability of the gelator containing it. On the bases of this discussion, cholesteryl glycinate ferrocenoylamide (CGF; cf. Scheme S1) was designed and syn-

thesized, and its gelling behavior in various solvents has been studied. The details are presented in this report. 2. Experimental 2.1. Synthesis of CGF The synthesis of CGF is schematically shown in Scheme S1. The details are described below. 2.1.1. Step (a) Triethylamine (247 µL, 1.78 mmol) was added to a suspension of hydrochloric salts of cholesteryl glycinate [36] (0.8521 g, 1.78 mmol) in 50 mL benzene. The mixture was stirred and refluxed for 5 h; after that the formed precipitate was filtered off, the resulting solution was evaporated to dryness, and the residue was dried in vacuum to give cholesteryl glycinate (primary amine) as a white or yellowish solid (0.62 g, 78%). 2.1.2. Step (b) Cholesteryl glycinate (1.3311 g, 3 mmol) and triethylamine (417 µL, 3 mmol) were dissolved in 30 mL dichloromethane at room temperature, and then 30 mL of dichloromethane solution of ferrocenoyl chloride [40] (0.7452 g, 3 mmol) was added dropwise. The reaction mixture was stirred for 4 h at room temperature. After the reaction, the mixture was filtered and the filtrate was washed with 0.01 mol/L hydrochloric acid (100 mL × 3) and pure water (100 mL × 3), respectively. The organic layer was evaporated to dryness and the residue was recrystallized from a THF/water mixture three times to give the desired product as a yellow solid (1.38 g, 70%). 1 H NMR, δH (300 MHz; CDCl3 ; Me4 Si): 6.13 (1H, b, NH), 5.40 (1H, s, alkenyl), 4.75 (3H, b, 2H for ferrocenyl and 1H for oxycyclohexyl), 4.41–4.31 (7H, d, ferrocenyl), 4.15 (2H, s, CH2 ), 2.36– 2.38 (2H, d, CH2 ), and 0.68–1.88 (42H, m, cholesteryl protons). FT-IR, υmax /cm−1 : 3348 (NH), 2942 (CH), 1745 (C=O, –O), 1629 (C=O, –NH), 1540 (NH, bending), and 1259 (–C–O). Elemental analysis, Found: C, 73.44; H, 8.48; N, 2.17%. Calc’d. for C40 H57 O3 NFe: C, 73.28, H, 8.70, N, 2.14%. 2.2. Gelation test A known weight of potential gelator and a measured aliquot of liquid were placed into a sealed test tube and the system was heated in an oil bath until the solid was dissolved; then the solution was cooled slowly to room temperature in air, and finally the test tube was inverted to look at whether the solution inside could still flow. A positive test was obtained if the flow test was negative. When a gel was formed at this stage, it was denoted by “G.” In some cases, a solution and a solidlike gel may coexist within a system. This kind of system has been referred to as “partial gels” (PG). Systems in which only solution remained until the end of the tests were referred to as solution (S). When the gelator of a system appeared as a precipitate or crystals, the system was denoted as “R.” A system in which the potential gelator could not be dissolved even at the boiling point of the solvent was called an insoluble system (I).

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2.3. Gel-to-sol transition temperature (Tgel ) measurement

2.8. UV measurements

Temperatures of solution-to-gel or gel-to-solution transition (Tgel ) were measured using a conventional falling ball method [41]. In the test, a small glass ball (diameter ∼3 mm) was placed on top of the gel in the test tube (10 mm). The tube was slowly heated (1 ◦ C/min) in a thermostated water bath until the ball fell from the surface of the gel to the bottom of the tube. The two specific temperatures corresponding to the starting point of the falling process and the ending of the process were recorded. The average of the two temperatures was taken as the Tgel of the system.

UV spectra were measured with a Perkin–Elmer Lambda 950 UV/vis spectrophotometer at room temperature.

2.4. SEM observation SEM pictures of the xerogel were taken on a Quanta 200 scanning electron microscopy spectrometer (Philips-FEI). The accelerating voltage was 15 kV and the emission was 10 mA. The xerogel was prepared by freezing the gel in liquid nitrogen and then freeze-dried. 2.5. AFM observation AFM images were obtained on a SPM-9500J3 atomic force microscope in constant force mode. The sample was prepared by spreading a heated mixture of gelator and ethyl acetate (2.0% w/v) onto freshly cleaved mica, then it was cooled in air to room temperature, and finally the mica was placed in the AFM chamber for imaging. 2.6. Light microscopy observation Microscopic pictures were taken on a LEICA-DMLP microscope. CGF was dissolved in ethyl acetate at high temperature, then the solution was introduced into a flat quartz cell, and finally the cell was sealed and used for observation. 2.7. Cyclic voltammetric measurements Cyclic voltammetric measurements were carried out on an electrochemical analyzer system, Bas100B. The cyclic voltammogram of the ethyl acetate (dried over P2 O5 before use) solution of CGF containing n-Bu4 NPF6 (0.05 mol/L) as the supporting electrolyte was measured with platinum as the working and counter electrodes and a saturated calomel electrode as the reference electrode, with a scanning rate of 50 mV/s. For recording the corresponding cyclic voltammogram of the gel of CGF (20 mg in 1.0 mL of ethyl acetate (dried before use), ∼30 mmol/L), a small amount of the gel was carefully put on the platinum electrode, which was left in the open air for several minutes. This modified platinum electrode (as working electrode), together with a platinum wire as the counter electrode and a saturated calomel electrode as the reference electrode, was put into the ethyl acetate solution containing n-Bu4 NPF6 (0.05 mol/L). The scanning rate was 50 mV/s.

2.9. FTIR measurements The solution and gel sample were measured in attenuated total reflectance (ATR) mode by a Bruker EQUINX55 infrared spectrometer. The gel sample for measurement was coated on a KI slice as a smooth gel film and then freeze-dried. 2.10. 1 H NMR measurements 1H

NMR spectra were recorded with a Bruker AV 300 (300-MHz) NMR spectrometer. 2.11. X-ray diffraction (XRD) measurements The measurements were conducted on a Japan D/Max2550VB+/PC diffractometer. The xerogel was prepared by loading the fresh gel directly onto a rectangular glass sample holder and then freeze-dried. The XRD patterns were obtained using CuKα radiation with an incident wavelength of 0.1541 nm. The scan rate was 3◦ /min. 3. Results and discussion 3.1. Gelation behaviors The gelation ability of CGF was tested for 45 different solvents with 2.5% (w/v) as a standard concentration. The results are summarized in Table 1. Examination of the table reveals that CGF can gel 13 out of the 45 organic solvents tested. In addition to the ether system (for its evaporability), the gels formed from other solvents are so stable that there was no liquid separated from the gel after two months kept in a closed bottle. By reference to the data shown in Table 1, it can be also seen that CGF can gel most of the alcohols tested, but the gelation Table 1 Gelation properties of CGF (2.5%, w/v) in various solvents Solvent

Result Solvent

Result Solvent

Result

Methanol Ethanol n-Propanol n-Butanol n-Pentanol n-Hexanol n-Heptanol n-Octanol n-Nonanol n-Decanol Glycol Glycerol 1,2-Propanediol 1,3-Propanediol 1,3-Butanediol

I R R R G G G* G G G I I G R G

G G S R S R S S R R R G PG PG G

S S S R PG R PG PG R S S S S G S

1,5-Pentanediol Ethyl acetate Butyl acetate Diethyl oxalate Formic acid Acetic acid Caproic acid Benzene Toluene Xylene Acetone Ether CCl4 Triethylamine Ethylenediamine

1,3-Diaminopropane 1,4-Diaminobutane DMF Hexane Heptane Decane Cyclohexane DMSO Acetonitrile Chloroform THF Pyridine Nitrobenzene Nitromethane UDMH

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process is very slow. Several hours or even one night is needed for the effective gelation of the solvents. For the ethyl acetate system, however, the gel can be formed within several minutes. The lowest gelation concentration (LGC) test demonstrated that ethyl acetate is one of the most easily gelled solvents by CGF, and the value of LGC for this solvent is lower than 1% (w/v).

That is, one gelator molecule could gel statistically at least 670 acetic acid molecules. Thus, the ethyl acetate/CGF system was selected as a sample and has been studied in detail in order to obtain a better understanding of the formation process of the gels. 3.2. Gel stability studies Tgel , the critical temperature of a gel system at which the system experiences a gel-to-solution or solution-to-gel phase transition, is an important parameter to denote gel stability. Fig. 1 shows the plot of Tgel against the concentration of CGF in ethyl acetate. As expected, the value increases with concentration to ca. 2.6% (w/v) and then is virtually independent of concentration (a plateau region, Tgel = 341.5 K), indicating that a stable, complete network structure has been formed in the gel at this stage. 3.3. Morphology studies

Fig. 1. Plot of Tgel against the concentration of CGF in ethyl acetate.

To obtain visual insight into the morphologies of the aggregation mode, the morphologies of the xerogels of the ethyl acetate/CGF and 1.5-pentanediol/CGF gel systems were studied using a scanning electron microscopy (SEM) technique,

Fig. 2. SEM images of the xerogel of CGF/ethyl acetate (a, b) and CGF/1,5-pentanediol (c, d) (2.0%, w/v). Bar = 500, 2, 10, and 2 µm for (a), (b), (c), and (d), respectively.

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Fig. 3. X-ray diffraction patterns of the CGF powder (A) and the freeze-dried gel (B) of CGF/ethyl acetate (2.0% w/v).

and the results are displayed in Fig. 2. By reference to the images, it can be seen that the microstructures of the xerogel of ethyl acetate/CGF are very different from those of 1.5-pentanediol/CGF. The former adopt solid prism-like structures with high length-to-width ratios. The width of the prism is about 2 µm (cf. Fig. 2b), and the length could be several hundred micrometers (cf. Fig. 2a). Further examining the picture shown in Fig. 2a, it can be seen that all the rods assembled into starlike structures, in which the ends of all the component rods of a “star” got together and formed the center of the “star,” but the other ends kept away from the center. Unlike SEM, atomic force microscopy (AFM) observation can be made under ambient conditions and thus minimizes damage to the samples. In the present study, AFM images of a fresh ethyl acetate gel of CGF were obtained (cf. Fig. S2 in the supporting material), which is consistent with the results from SEM observation. Further evidence supporting the SEM observations has been obtained by monitoring the gel formation process with a light microscope, and the results are shown in Fig. S3 in the supporting material. In contrast, the gelator molecules of the 1,5-pentanediol/CGF gel formed flower-like structures (cf. Fig. 2c). On further examination of the structures, it is clear that they are composed of slices (cf. Fig. 2d), which should be assemblies of CGF molecules. The XRD patterns of the xerogel of CGF/ethyl acetate and CGF powder are shown in Fig. 3. It can be seen that the diffraction pattern of the powder sample is characterized by four obvious reflection peaks, corresponding to d values of 3.92, 1.98, 1.33, and 1.09 nm, respectively, in the region of 2θ values lower than 8.5 (Fig. 3a). The ratio of the d values is 1:1/2:1/3:1/4, suggesting a perfect lamellar organization. Further examination of the XRD pattern of the xerogel reveals that a similar structure is adopted by the aggregates of the gelators in the gel state, even though peak 2 did not appear in its XRD pattern. It is interesting to note that the interlayer distance of the lamellar structure, the first d value, just equals the length of the quasi dicholesterol structure (3.86 nm; see Fig. 3), indicating that the advanced structures of the aggregates of the gelator in the gel state might

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Fig. 4. The cyclic voltammograms of the solutions ((A) 1; (B) 2; (C) 3 mmol/L) and the gel ((D) 30 mmol/L) of CGF in ethyl acetate.

be based on this fundamental structure. Clearly, the key point of this hypothesis is the aggregation of the ferrocenyl units, which has been confirmed by the results from electrochemical studies. 3.4. Electrochemical studies and sensitivity to (NH4 )2 Ce(NO3 )6 The patterns of the cyclic voltammograms of the solutions and the gel of CGF in ethyl acetate are depicted in Fig. 4. Reference to Fig. 4 reveals that the oxidation potential (E1/2 ) of CGF depends on its concentration. For the systems studied, the values are 0.87, 0.92, and 0.99 V (vs SCE), respectively, corresponding to concentrations of 1, 2, and 3 mmol/L. The value reaches 1.08 V (vs SCE) in the gel state (20 mg in 1.0 mL of ethyl acetate, ∼30 mmol/L) of CGF in ethyl acetate. The increase in the oxidation potential may be caused by the aggregation of the ferrocenyl residues of the gelator molecules. This is because aggregation must be accompanied by electronic communication between neighboring ferrocenyl residues and results in a positive shift of the oxidation potential of the residues [42,43]. As a reducing agent, ferrocenyl group is easily oxidized both electronically and chemically, and for the present gel system, the oxidation of the ferrocenyl residues may result in a gelto-solution transition. To look at the sensitivity of the gel to oxidant, an equal amount (in moles) of (NH4 )2 Ce(NO3 )6 dissolved in methanol was carefully placed above the gel mentioned in Fig. 4. It was observed that the gel was gradually changed into a suspension within 30 min, and the color of the system changed from yellow to dark green (cf. Fig. 5a). As a control, the test was repeated using NH4 NO3 or Ce(NO3 )3 instead of (NH4 )2 Ce(NO3 )6 , and no change was observed during the experimental time scale. As another control, the solubility of the oxidized form of the gelator in methanol was tested, and it was found that it almost does not dissolve in the solvent. Therefore, it is concluded that both solubilization and salt effects play little role in the phase transition process. No doubt the nature of the phase transition is dis-aggregation of the gelator molecules,

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Fig. 5. (a) Tuning the gel formation by the oxidation of the ferrocenyl group; (b) UV–vis spectra of the CGF/ethyl acetate solution in the absence (A) and presence (B) of (NH4 )2 Ce(NO3 )6 and that of Ce(NO3 )3 (C) and NH4 NO3 (D).

which should be the result of breakage of the H-bonds induced by intermolecular electrostatic repulsion between the oxidized ferrocenyl residues in the gelator. The statement was further supported by the results shown in Fig. 5b, which shows the UV–vis spectra of the ethyl acetate solutions of CGF (5 mmol/L) in the absence and presence of (NH4 )2 Ce(NO3 )6 and their control systems, the ethyl acetate solutions of Ce(NO3 )3 and NH4 NO3 . By reference to the figure, it can be seen that the profiles, particularly the positions of the absorption bands (440 nm, 635 nm), of the two systems are very different from each other, strong evidence for the successful conversion of CGF to its oxidized form. It is to be noted, however, that introduction of the methanol solution of SnCl2 , a typical reducing agent, into the dark green suspension made the color of the system change to yellow, an indication of reduction of the ferrocenyl residues, but this solution cannot be gelled even though heating and cooling treatment were repeated many times. The irreversibility has been proved to be caused by the addition of methanol. Unlike ethyl acetate, methanol is not a good solvent for the gelator, and therefore introduction of it must break the balance of aggregation and dis-aggregation of the gelator molecules in the system, which is the basis for the formation of the gel network structure. Also, we have tried many times to break the gel by electrochemical methods; regrettably; we failed, because the gel is not a conductor or even a semiconductor. However, we have observed the appearance of a dark green solution on the electrode surface, which should be a sign of the phase transition of the CGF/ethyl acetate gel due to oxidation of the ferrocenyl residues. 3.5. Spectroscopic studies FT-IR measurements can provide useful information for confirming the formation of hydrogen bonds [44,45]. Accordingly, the FTIR spectra of CGF/ethyl acetate gel and CGF in CH2 Cl2 (10 mmol/L) were recorded and are shown in Fig. 6. CH2 Cl2 was chosen because it is a good solvent for CGF, and a real

Fig. 6. FT-IR spectra of CGF/ethyl acetate gel ((A) 2.0% w/v) and CGF/ CH2 Cl2 solution ((B) 10 mmol/L).

solution can be prepared and used as a control for the gel system during the FTIR measurement. In this case, three typical bands of CGF corresponding to the stretching vibrations of NH and C=O groups and the bending vibration of NH appeared at 3330, 1638, and 1538 cm−1 , respectively. Upon gelation, the three bands shifted to 3270, 1629, and 1540 cm−1 , respectively (cf. Fig. 6), direct evidence for the formation of intermolecular hydrogen bonds, which should be one of the driving forces for the formation of network structures in the gel [24]. The formation of intermolecular hydrogen bonds of the gelator molecules was also supported by 1 H NMR measurements of the gel of CGF/ethyl acetate (1.5%, w/v) at different temperatures (cf. Fig. 7). By reference to the figure, it can be seen that the signal of the NH group shifted from 7.15 to 6.98 ppm with increasing temperature of the system, indicating an increase in the strength of the N–H bond, direct evidence of the disentanglement of NH from a bonding state.

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to the formation of the gel networks through hydrogen bond formation. A similar result was reported during the studies of the formation process of the gel of tetrathiafulvalene derivatives in CHCl3 by Zhu’s group [24]. 4. Conclusions

Fig. 7. The 1 H NMR spectra of the gel of CGF (1.5% (w/v) in ethyl acetate; DMSO as the deuterated solvent) at different temperatures: (A) 298, (B) 310, (C) 320, (D) 328, and (E) 335 K.

A novel low-molecular-mass gelator containing a redox active ferrocenyl group, CGF, was intentionally designed and prepared. Its gelation properties in various solvents were studied. It was demonstrated that the gelator gels 13 out of the 45 solvents tested. SEM measurements revealed that CGF self-assembled into different supramolecular structures in different gels. Chemical oxidation of the ferrocenyl residue resulted in phase transition of the gel from the gel state to the solution state, which has been explained by considering that the positively charged ferrocenyl group would impair the intermolecular hydrogen bonding between the gelator molecules in the gel, which has been proved to be one of the main driving forces for the formation of the gels. On the basis of this study, new electrochemically or chemically controllable sol–gel phase transition events are currently under investigation. Acknowledgments Financial support from the Natural Science Foundation of China (No. 20674048) and the Ministry of Education of China (Nos. 20040718001, 306015) is gratefully acknowledged. Supporting material The online version of this article contains additional supporting material. Please visit DOI:10.1016/j.jcis.2007.10.005. References

Fig. 8. The 1 H NMR spectra of CGF in CDCl3 at different concentrations: (A) 20, (B) 30, (C) 40, (D) 50, and (E) 60 mg/mL.

The result from the concentration-dependent test of the 1 H NMR signal of the N–H bond of the gelator is also in support of the involvement of the group in the formation of gel network structures. In this case, CDCl3 was chosen as a solvent, since it has a strong ability to dissolve the gelator. The concentration of the gelator was increased from 20 to 60 mg/mL during the test, and correspondingly, the signal of NH shifted from 6.14 to 6.22 ppm (cf. Fig. 8). As expected, with a gradual increase in the temperature of the solution of highest gelator concentration, the signal shifted from 6.22 to 6.15 ppm (cf. Fig. S1 in the supporting material). In summary, FTIR and 1 H NMR measurements have proved that the carbonyl amide group, –NH, of the gelator contributed

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