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A simple structural hydrazide-based gelator as a fluoride ion colorimetric sensor† Binglian Bai,*a,b Jie Ma,b Jue Wei,b Jianxi Song,a Haitao Wanga and Min Li*a A 4-nitrobenzohydrazide derivative, N-(3,4,5-octyloxybenzoyl)-N’-(4’-nitrobenzoyl)hydrazine (C8), was synthesized. It could form stable gels in some of the tested organic solvents. The wide-angle X-ray diffraction analysis showed that the xerogels exhibited a layered structure. SEM images revealed that the molecules self-assembled into fibrous aggregates in the xerogels. FT-IR studies confirmed that the intermolecular hydrogen bonding between CvO and N–H groups was the major driving force for the formation of self-assembling gel processes. The gel is utilized for a ‘naked eye’ detection of fluoride ions,
Received 8th January 2014, Accepted 24th March 2014
through a reversible gel–sol transition, which is associated with a color change from colorless to red. An extended conjugated system formed through the phenyl group and a five-membered ring based on
DOI: 10.1039/c4ob00056k
intramolecular hydrogen bonding between the oxygen atom near the deprotonation nitrogen atom and
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the other NH, which is responsible for the dramatic color change upon addition of fluoride ions.
Low-molecular-weight organogels are the materials in which three-dimensional networks are formed due to the self-assembling of low-molecular-weight compounds through noncovalent interactions (such as hydrogen bonding, π–π stacking, and van der Waals interactions), and the networks can absorb a large amount of solvents therein.1 Organogels are nowadays intensively investigated for different actual and emerging applications such as drug delivery,2–4 the templating of inorganic and polymer materials,5,6 dynamic gels,7,8 biological applications,9 etc. Recently, some responsive gels whose gelation characteristics can be tuned by a variety of stimuli, such as thermal treating,10 changes in pH,11 and host–guest chemistry,12 have been developed for especially appealing potential applications. Anions are ubiquitous and play a crucial role in a wide range of chemical and biological processes. For example, fluoride is a useful chemical for dental caries and treatment of osteoporosis, but too much accumulation of fluoride can result in fluorosis. Therefore, design and development of sensing and recognition for different anions have grown into an area of great interest in supramolecular and biological chemistry in recent years.13 A variety of artificial neutral recep-
tors have been designed and tested for anion recognition and sensing in solution over the past few years. Such molecules should be able to behave as hydrogen-bond donors towards the envisaged anion, and the most frequently employed fragments are urea,14,15 amide16 and indole17 subunits. Because the gel–sol transition allows naked eye detection and the gels are more convenient to use, anion-responsive gels have attracted increasing attention and some reports on aniontuning organogels have been reported. For example, Jiang et al. studied an organogel based on cyano-substituted amide in which the gel state and color could be selectively controlled by F− stimulus.18 Yi et al. studied a switchable fluorescent organogel based on urea-substituted naphthalene derivatives. Although it could be translated into a solution by the addition of F−, no significant absorption spectral changes have been observed.19 And until now, the organogel systems which showed both reversible color change and sol–gel transition by anion stimuli are still limited.18,20–29 Herein, we report the synthesis, self-assembly and anionresponsive behavior of the compound N-(3,4,5-octyloxybenzoyl)-N′-(4′-nitrobenzoyl)hydrazine (C8, Scheme 1). C8 can form stable gels in organic solvents such as benzene and 1,2dichloroethane (Table 1). An FT-IR study confirmed that intermolecular hydrogen bonding was the major driving force in
a Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130012, PR China. E-mail: [email protected], [email protected]; Tel: +86-431-85168254 b College of Physics, Jilin University, Changchun 130012, PR China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ob00056k
Scheme 1
Introduction
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Molecular structure of C8.
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Table 1 Gelation properties of C8 (G: gel; S: solution; for gels the minimum gelation concentration is given)
Solvents
State
Solvents
State
N-Hexane Toluene Benzene Cyclohexane 1,2-Dichloroethane Chloroform
G G (1.67 wt%) G (1.82 wt%) G (0.45 wt%) G (1.30 wt%) S
Methanol Ethanol Acetonitrile THF DMF DMSO
G (0.73 wt%) G (2.65 wt%) G (3.78 wt%) S S S
the formation of self-assembling gels. Moreover, the C8 gel systems with dual-channel response exhibited reversible sol–gel transition and color change upon addition of anion (F−, AcO− and H2PO4−) stimuli and proton control. To the best of our knowledge, there is only one anion-responsive report based on hydrazide derivatives containing conventional anthraquinone chromophore till now.20
Experimental Synthesis of C8 The compound N-(3,4,5-octyloxybenzoyl)-N′-(4′-nitrobenzoyl)hydrazine (C8) was synthesized according to the route shown in the literature.30 3,4,5-Trioctyloxy-benzoylhydrazine (2.22 g, 4.26 mmol) and 4-nitrobenzoyl chloride (0.79 g, 4.26 mmol) were dissolved in tetrahydrofuran (100 mL), pyridine (2 mL) was added, and the resulting mixture was stirred at room temperature for 8 h. The reaction mixture was poured into an excess of ice water, and the precipitate was recrystallized from anhydrous alcohol for further 1H NMR, 13C NMR (Fig. S1†), MS and elemental analysis. 1 H NMR (500 MHz, DMSO-d6) ( ppm, from TMS): 10.9 (s, 1H), 10.6 (s, 1H), 8.47 (d, J = 7.95 Hz, 2H), 8.23 (d, J = 7.34 Hz, 2H), 7.31 (s, 2H), 4.10 (t, J = 5.90 Hz, 4H), 4.01 (t, J = 6.24 Hz, 2H), 1.83 (td, J = 13.13, 6.49 Hz, 4H), 1.72 (td, J = 8.05, 3.76 Hz, 2H), 1.53 (dd, J = 13.56, 6.75 Hz, 6H), 1.38 (dd, J = 25.40, 7.83 Hz, 24H), 0.94 (t, J = 6.89 Hz, 9H). 13 C NMR (500 MHz, DMSO-d6, Fig. S1†), ( ppm): 165.618, 164.872, 152.832, 149.908, 140.655, 138.649, 129.460, 127.457, 124.293, 106.251, 72.952, 68.907, 31.715, 30.256, 29.219, 26.084, 22.585, 14.409. Elem. Anal: Found: C 68.09, N 6.45, H 9.05%. Calcd for C38H59N3O7: C 68.13, N 6.27, H 8.88%. MALDI-TOF MS: m/z: calculated for: 669.4, found: 668.8.
S.P.A. FlashEA1112 apparatus. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. Mass spectra were obtained by MALDI-TOF mass spectrometry. The detailed gelation tests were the same as in our reported paper.31
Results and discussion Gelation properties of C8 The gelation abilities of C8 were examined for twelve different organic solvents, and the minimum gel concentrations are summarized in Table 1. As shown in Table 1, the gelator C8 could form stable gels in most of the tested solvents, but was easily dissolved in tetrahydrofuran (THF), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). While C8 is slightly soluble in N-hexane, in which it can gel in very low concentration, we cannot measure the minimum gel concentration because there are some insoluble solid samples in the organogels. In order to investigate the thermal behavior of the gels, we took the gels of C8 in cyclohexane as samples and studied the relationship between the gel–sol transition temperature (Tgel ) and the concentration of C8 (Fig. 1). The results showed that the Tgel increased as the gelator concentration increased at first and then remained almost unchanged at about 46 °C. In order to investigate the aggregation morphology of the organogels, xerogels C8 in cyclohexane and acetonitrile were prepared and subjected to field emission scanning electron microscopy (FE-SEM). The SEM image revealed that the gelator C8 could self-assemble into three-dimensional networks composed of threadlike fibrous aggregates, which are tens of micrometers in length and about hundreds of nanometers in width (Fig. 2). The formation of elongated fiber-like aggregates indicated that strong directional intermolecular interactions are responsible for the self-assembly of C8 molecules. To reveal the packing mode of the molecules in the gel phase, X-ray diffraction was measured (Fig. S2†). The XRD profile of the C8 xerogel from cyclohexane consists of a sharp strong diffraction (22.7 Å) and up to third-order diffractions,
Characterization NMR spectra were recorded with a Bruker Avance 500 MHz spectrometer using dimethyl sulfoxide-d as a solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00). Scanning electron microscopy (SEM) images were taken with a JSM-6700F apparatus. X-Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). Elemental analyses were carried out on a ThermoQuest Italia
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Fig. 1 Concentration-dependent melting temperature of C8 gels in cyclohexane.
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Fig. 2 Field emission scanning electron microscopic (FE-SEM) images of C8 xerogels derived from (a) cyclohexane (1.32 wt%), (b) acetonitrile (3.78 wt%).
suggesting a layered structure. The layer spacing is a little smaller than the calculated full extended molecular length (about 25 Å); thus C8 molecules have a little tilt angle in layers. To ascertain whether hydrogen bonding plays a role in the gelation process, the infrared spectrum of the C8 xerogel from cyclohexane was examined (Fig. S3†). The observation of hydrogen bonded N–H stretching bands at 3199 cm−1 (the absence of free N–H, a relatively sharp peak with frequency higher than 3400 cm−1) and intense absorption of bonded CvO stretching vibrations at 1625 cm−1 and 1676 cm−1 clearly indicates that the N–Hs of the hydrazide group are associated with the CvO groups via N–H⋯OvC intermolecular hydrogen bonding in the gelation process; such a hydrogen bonding interaction was considered to be the main driving force for the formation of the fibers.30 In addition, the absorption bands of the antisymmetric (νas) and symmetric (νs) CH2 stretching vibrational modes are observed at 2926 cm−1 (νas CH2) and 2856 cm−1 (νs CH2) in the gel, respectively, indicating that the alkyl chains are disordered in part, which is consistent with the XRD results. The intermolecular hydrogen bonding of the hydrazide group can also be confirmed by concentrationdependent 1H NMR as well as temperature-dependent FT-IR measurements.30 Anion responsive properties In order to investigate the interactions of the receptor C8 with different anions, characteristic UV-vis absorption changes were monitored upon the addition of TBA salts of F−, Cl−, Br−, I−, AcO− and H2PO4− to its DMSO solution. As can be seen in Fig. 3, in the presence of 20 equivalents (equiv.) of AcO− and H2PO4−, the maximum absorption at 268 nm decreased, and a new absorption at 450 nm developed. In contrast, apart from the disappearance of absorption at 268 nm, two new absorptions at 300 and 450 nm appeared in the presence of 20 equiv. of F−. No significant spectral change was observed in the presence of other halide anions, suggesting no bonding or very weak interactions between the receptor C8 and the other anions. Due to the appearance of a new absorption in the visible region, the addition of F−, AcO− or H2PO4− can cause a vivid color change, e.g. from colorless to red (Fig. S4†).
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Fig. 3 UV-vis absorption spectra of C8 (8 × 10−5 mol L−1) in the presence of 20 equiv. of various anions in DMSO at room temperature.
The interaction between C8 and fluoride was investigated in detail through absorbance spectra titration experiments. Fig. 4 shows the complete family of spectra during the titration of a 1 × 10−4 mol L−1 solution of C8 in DMSO with [Bu4N]F. Addition of low equiv. F− (Fig. 4a) caused the band at 268 nm to decrease gradually, and meanwhile a new very weak band at 450 nm appeared. The color change is almost invisible. The clear isosbestic point at 306 nm indicates that a single component is produced in response to the interaction between C8 and F−.17,27 Addition of high equiv. F− (Fig. 4b) caused the absorption at 268 nm to be shifted to 300 nm, and that at 450 nm increased and leveled off in the case of more than 25 equiv. of F− added. The color turns from colorless to pale orange. Significant changes in absorbance spectra titration experiments begin not after 2 equiv. addition (significant changes in 1H NMR titration experiments begin after 2 equiv. addition, Fig. 5), but after the addition of 10 equiv. or more of fluoride, which is very similar to the urea derivative result of Fabbrizzi et al.15 This may be due to the water in the solvent or [Bu4N]F·3H2O. The interaction between the receptor C8 and the fluoride was further investigated by 1H NMR titration experiments in DMSO-d6 (Fig. 5). The amide NH proton signals at 10.9 and 10.6 ppm disappeared, and a weak broad signal at 10.7 ppm simultaneously appeared with increasing fluoride concentration from 0 to 1 equiv. The appearance of the weak broad band at 10.7 ppm indicated that the N–H⋯F− hydrogen bond formed,20 which is consistent with the result of the isosbestic point appearing in absorbance spectra titration experiments at low equiv. F− (Fig. 4a). When 2 equiv. of fluoride was added, the weak broad signal at 10.7 ppm disappeared, and a narrow peak at 10.4 ppm simultaneously appeared; meanwhile three new signals emerged at 16.0, 16.2 and 16.5 ppm with a 1 : 2 : 1 integration ratio, which was ascribed to the [HF2]− dimer18,20 and also suggested that a hydrazide N–H group might undergo a deprotonation process. Ha and Hc protons undergo an upfield shift in both the 0–1 and 1–2 equiv. ranges, while Hb
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Fig. 4 Absorbance spectral changes of C8 (1 × 10−4 mol L−1) upon the addition of (a) 0, 1, 2, 3, 5 equiv. fluoride in DMSO; (b) 0–30 (5 equiv. interval) equiv. fluoride in DMSO.
Fig. 5
Plots of 1H NMR spectra of C8 (1 × 10−2 mol L−1) upon the addition of fluoride in DMSO-d6.
protons are insensitive to the addition. In any case, no change occurred after the addition of the second equiv. Based on the analyses of UV-vis absorption and 1H NMR spectra, it can be concluded that there are two stepwise equilibria in this responsive process. As can be envisioned, the antihydrazide isomer of C8 is the most stable conformation, which isomerized to the syn-hydrazide upon the addition of F−, which may be similar to the changes of thiourea protons promoted by F−,32 because the proton was abstracted slightly from the hydrazide subunit and resulted in the formation of a 1 : 1 supramolecular complex [C8·F]−. The following equilibrium (i) and Scheme 2 are given to describe the above changes: ½C8 þ F ! ½C8F
ðiÞ
½C8F þ F ! ½C8 þ ½HF2
ðiiÞ
On the addition of the second fluoride ion, the base becomes very strong, and F− exhibits a large affinity toward H+, which induces the deprotonation of the [C8·F]− complex to form the very stable [HF2]− dimer, as illustrated by eqn (ii). Meanwhile the charge-transfer might result in a fivemembered ring based on intramolecular hydrogen bonding
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between the oxygen atom near the deprotonation nitrogen atom and the other NH could thus be formed (Scheme 2), which results in NH proton signal at 10.4 ppm unchanged, the absorption at 268 nm shift to 300 nm and the absorption at 450 nm developed. The intramolecular hydrogen bonding can also be confirmed by concentration-dependent 1H NMR experiments (Fig. S5†). Thus, the extended conjugated system was formed through the adjacent phenyl group and a five-membered ring, which is responsible for the dramatic color change upon the addition of fluoride ions. In addition, the increase of the shielding effect induces a moderate upfield of the Ha and Hc protons.14 The Hb protons do not shift all the while, maybe due to the result of the electron-withdrawing effect of the nitro group as well as the increasing shielding effect of the F− titration process. When the receptor C8 was titrated with AcO−, different spectroscopic patterns were obtained. Fig. 6 displays the family of UV-visible spectra recorded in the course of the titration of an 8 × 10−5 mol L−1 solution of C8 in DMSO. Addition of AcO− caused the band at 268 nm to decrease gradually, and meanwhile a new band at 450 nm developed. The clear isosbestic point at 288 nm indicates that a stable hydrogen-bond complex [C8·AcO]− is produced in response to the interaction
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Scheme 2
Organic & Biomolecular Chemistry
The proposed reaction mechanism of the host C8 with fluoride.
Fig. 7 (a) Organogel formed from a solution of C8 in acetonitrile (3.86 wt%); (b) 5 s after addition of solid TBAF ([Bu4N]F·3H2O, 3 equiv.); (c) 10 s; (d) 50 s; (e) immediately after addition of 200 μL methanol to (d).
Fig. 6 Absorbance spectra changes of C8 (8 × 10−5 mol L−1) upon the addition of 0–22 equiv. AcO− in DMSO.
between C8 and AcO−.17,27,33 (The results of absorbance spectra titration experiments of H2PO4− were similar to those of AcO−, Fig. S6†.) The formation of a hydrogen-bond complex of 1 : 1 stoichiometry was also confirmed by 1H NMR titration experiments (Fig. S7†). Even on excess addition of AcO−, only very little upfield shift of Ha and Hc was observed, indicating a weak hydrogen-bond interaction. The above behaviour should be ascribed to the much lower basicity and hydrogenbond acceptor properties of AcO− compared to F−. In particular, AcO− is unable to form a stable [H(AcO)2]− complex as found for the [HF2]− dimer. Fluoride-responsive gels In order to study the potential application in sensing materials, we also checked the fluoride-responsive properties of gel C8. On careful addition of solid [Bu4N]F·3H2O onto the top of the acetonitrile (MeCN) gel, a thin layer of red solution immediately appeared at the upper part. As time passed, the
3482 | Org. Biomol. Chem., 2014, 12, 3478–3483
white gel gradually vanished and more and more red-colored solution appeared until all the gel transferred into solution, and the whole process took place in no more than one minute (Fig. 7). The presence of fluoride not only changed the color of the system but also disrupted the gel to a solution through slow diffusion of fluoride. The results indicated that C8 gel is sensitive to fluoride, with naked eye sensing by gel–sol transition and obvious color changes. But other halide anions could not lead to gel decomposition (Fig. S8†). We could presume a mechanism of F anion induced gel–sol transition and color change: upon the addition of the F− (more than 2 equiv.), the hydrazide NH group perhaps underwent a deprotonation process (Scheme 2). It destroyed the intermolecular hydrogen bonding between CvO and N–H groups which is the main driving force that supported gel formation, and subsequently caused the gel–sol transition. Meanwhile, the color change of the gel was mainly attributed to the extended conjugated system formed through the adjacent phenyl group and a five-membered ring. Other halide anions with weaker basicity could not lead to the gel decomposition and color change. To further certify the mechanism of deprotonation of the hydrazide group resulting in the gel–sol conversion and color change, methanol as a competing agent was added into the
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red solution formed by the F− disintegrated gel; the red color disappeared, while a new organogel formed again (Fig. 7). However, no change emerged during the titration of a 1 × 10−4 mol L−1 solution of C8 in ethanol with [Bu4N]F until 100 equiv. because there is a strong interaction between the fluoride and the protic solvent. When THF solution of [Bu4N]F was carefully added onto the top of the ethanol gel, a thin layer of red solution immediately appeared at the upper part (Fig. S9†). The C8 gel showed a similar response to AcO− and H2PO4− (Fig. S10†).
Conclusions A gelator C8 based on nitro-substituted hydrazide derivatives was synthesized, and it was demonstrated that it could form stable gels in some of the tested solvents. The C8 organogel can be utilized for an efficient ‘naked eye’ detection of F−, through a reversible gel–sol transition, which is associated with a color change from colorless to red. A possible mechanism for the F− responsive process was proposed. There are two stepwise equilibria: (i) C8 interacts with F− to give the [C8·F]− complex, and (ii) the [C8·F]− complex interacts with a second F− and deprotonation takes place, with formation of [HF2]− and the fivemembered ring based on intramolecular hydrogen bonding. The results might provide a strategy to design a novel gelator for constructing stimulus responsive soft materials and provide the basis for the development of non-fluid systems for sensing anions with the naked eye.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21072076, 51103057, and 51073071), the Natural Science Foundation of Jilin Province (201215009), and Project 985-Automotive Engineering of Jilin University.
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A simple structural hydrazide-based gelator as a fluoride ion colorimetric sensor† Binglian Bai,*a,b Jie Ma,b Jue Wei,b Jianxi Song,a Haitao Wanga and Min Li*a A 4-nitrobenzohydrazide derivative, N-(3,4,5-octyloxybenzoyl)-N’-(4’-nitrobenzoyl)hydrazine (C8), was synthesized. It could form stable gels in some of the tested organic solvents. The wide-angle X-ray diffraction analysis showed that the xerogels exhibited a layered structure. SEM images revealed that the molecules self-assembled into fibrous aggregates in the xerogels. FT-IR studies confirmed that the intermolecular hydrogen bonding between CvO and N–H groups was the major driving force for the formation of self-assembling gel processes. The gel is utilized for a ‘naked eye’ detection of fluoride ions,
Received 8th January 2014, Accepted 24th March 2014
through a reversible gel–sol transition, which is associated with a color change from colorless to red. An extended conjugated system formed through the phenyl group and a five-membered ring based on
DOI: 10.1039/c4ob00056k
intramolecular hydrogen bonding between the oxygen atom near the deprotonation nitrogen atom and
www.rsc.org/obc
the other NH, which is responsible for the dramatic color change upon addition of fluoride ions.
Low-molecular-weight organogels are the materials in which three-dimensional networks are formed due to the self-assembling of low-molecular-weight compounds through noncovalent interactions (such as hydrogen bonding, π–π stacking, and van der Waals interactions), and the networks can absorb a large amount of solvents therein.1 Organogels are nowadays intensively investigated for different actual and emerging applications such as drug delivery,2–4 the templating of inorganic and polymer materials,5,6 dynamic gels,7,8 biological applications,9 etc. Recently, some responsive gels whose gelation characteristics can be tuned by a variety of stimuli, such as thermal treating,10 changes in pH,11 and host–guest chemistry,12 have been developed for especially appealing potential applications. Anions are ubiquitous and play a crucial role in a wide range of chemical and biological processes. For example, fluoride is a useful chemical for dental caries and treatment of osteoporosis, but too much accumulation of fluoride can result in fluorosis. Therefore, design and development of sensing and recognition for different anions have grown into an area of great interest in supramolecular and biological chemistry in recent years.13 A variety of artificial neutral recep-
tors have been designed and tested for anion recognition and sensing in solution over the past few years. Such molecules should be able to behave as hydrogen-bond donors towards the envisaged anion, and the most frequently employed fragments are urea,14,15 amide16 and indole17 subunits. Because the gel–sol transition allows naked eye detection and the gels are more convenient to use, anion-responsive gels have attracted increasing attention and some reports on aniontuning organogels have been reported. For example, Jiang et al. studied an organogel based on cyano-substituted amide in which the gel state and color could be selectively controlled by F− stimulus.18 Yi et al. studied a switchable fluorescent organogel based on urea-substituted naphthalene derivatives. Although it could be translated into a solution by the addition of F−, no significant absorption spectral changes have been observed.19 And until now, the organogel systems which showed both reversible color change and sol–gel transition by anion stimuli are still limited.18,20–29 Herein, we report the synthesis, self-assembly and anionresponsive behavior of the compound N-(3,4,5-octyloxybenzoyl)-N′-(4′-nitrobenzoyl)hydrazine (C8, Scheme 1). C8 can form stable gels in organic solvents such as benzene and 1,2dichloroethane (Table 1). An FT-IR study confirmed that intermolecular hydrogen bonding was the major driving force in
a Key Laboratory for Automobile Materials (JLU), Ministry of Education, College of Materials Science and Engineering, Jilin University, Changchun 130012, PR China. E-mail: [email protected], [email protected]; Tel: +86-431-85168254 b College of Physics, Jilin University, Changchun 130012, PR China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ob00056k
Scheme 1
Introduction
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Molecular structure of C8.
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Table 1 Gelation properties of C8 (G: gel; S: solution; for gels the minimum gelation concentration is given)
Solvents
State
Solvents
State
N-Hexane Toluene Benzene Cyclohexane 1,2-Dichloroethane Chloroform
G G (1.67 wt%) G (1.82 wt%) G (0.45 wt%) G (1.30 wt%) S
Methanol Ethanol Acetonitrile THF DMF DMSO
G (0.73 wt%) G (2.65 wt%) G (3.78 wt%) S S S
the formation of self-assembling gels. Moreover, the C8 gel systems with dual-channel response exhibited reversible sol–gel transition and color change upon addition of anion (F−, AcO− and H2PO4−) stimuli and proton control. To the best of our knowledge, there is only one anion-responsive report based on hydrazide derivatives containing conventional anthraquinone chromophore till now.20
Experimental Synthesis of C8 The compound N-(3,4,5-octyloxybenzoyl)-N′-(4′-nitrobenzoyl)hydrazine (C8) was synthesized according to the route shown in the literature.30 3,4,5-Trioctyloxy-benzoylhydrazine (2.22 g, 4.26 mmol) and 4-nitrobenzoyl chloride (0.79 g, 4.26 mmol) were dissolved in tetrahydrofuran (100 mL), pyridine (2 mL) was added, and the resulting mixture was stirred at room temperature for 8 h. The reaction mixture was poured into an excess of ice water, and the precipitate was recrystallized from anhydrous alcohol for further 1H NMR, 13C NMR (Fig. S1†), MS and elemental analysis. 1 H NMR (500 MHz, DMSO-d6) ( ppm, from TMS): 10.9 (s, 1H), 10.6 (s, 1H), 8.47 (d, J = 7.95 Hz, 2H), 8.23 (d, J = 7.34 Hz, 2H), 7.31 (s, 2H), 4.10 (t, J = 5.90 Hz, 4H), 4.01 (t, J = 6.24 Hz, 2H), 1.83 (td, J = 13.13, 6.49 Hz, 4H), 1.72 (td, J = 8.05, 3.76 Hz, 2H), 1.53 (dd, J = 13.56, 6.75 Hz, 6H), 1.38 (dd, J = 25.40, 7.83 Hz, 24H), 0.94 (t, J = 6.89 Hz, 9H). 13 C NMR (500 MHz, DMSO-d6, Fig. S1†), ( ppm): 165.618, 164.872, 152.832, 149.908, 140.655, 138.649, 129.460, 127.457, 124.293, 106.251, 72.952, 68.907, 31.715, 30.256, 29.219, 26.084, 22.585, 14.409. Elem. Anal: Found: C 68.09, N 6.45, H 9.05%. Calcd for C38H59N3O7: C 68.13, N 6.27, H 8.88%. MALDI-TOF MS: m/z: calculated for: 669.4, found: 668.8.
S.P.A. FlashEA1112 apparatus. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer. Mass spectra were obtained by MALDI-TOF mass spectrometry. The detailed gelation tests were the same as in our reported paper.31
Results and discussion Gelation properties of C8 The gelation abilities of C8 were examined for twelve different organic solvents, and the minimum gel concentrations are summarized in Table 1. As shown in Table 1, the gelator C8 could form stable gels in most of the tested solvents, but was easily dissolved in tetrahydrofuran (THF), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). While C8 is slightly soluble in N-hexane, in which it can gel in very low concentration, we cannot measure the minimum gel concentration because there are some insoluble solid samples in the organogels. In order to investigate the thermal behavior of the gels, we took the gels of C8 in cyclohexane as samples and studied the relationship between the gel–sol transition temperature (Tgel ) and the concentration of C8 (Fig. 1). The results showed that the Tgel increased as the gelator concentration increased at first and then remained almost unchanged at about 46 °C. In order to investigate the aggregation morphology of the organogels, xerogels C8 in cyclohexane and acetonitrile were prepared and subjected to field emission scanning electron microscopy (FE-SEM). The SEM image revealed that the gelator C8 could self-assemble into three-dimensional networks composed of threadlike fibrous aggregates, which are tens of micrometers in length and about hundreds of nanometers in width (Fig. 2). The formation of elongated fiber-like aggregates indicated that strong directional intermolecular interactions are responsible for the self-assembly of C8 molecules. To reveal the packing mode of the molecules in the gel phase, X-ray diffraction was measured (Fig. S2†). The XRD profile of the C8 xerogel from cyclohexane consists of a sharp strong diffraction (22.7 Å) and up to third-order diffractions,
Characterization NMR spectra were recorded with a Bruker Avance 500 MHz spectrometer using dimethyl sulfoxide-d as a solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00). Scanning electron microscopy (SEM) images were taken with a JSM-6700F apparatus. X-Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). Elemental analyses were carried out on a ThermoQuest Italia
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Fig. 1 Concentration-dependent melting temperature of C8 gels in cyclohexane.
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Fig. 2 Field emission scanning electron microscopic (FE-SEM) images of C8 xerogels derived from (a) cyclohexane (1.32 wt%), (b) acetonitrile (3.78 wt%).
suggesting a layered structure. The layer spacing is a little smaller than the calculated full extended molecular length (about 25 Å); thus C8 molecules have a little tilt angle in layers. To ascertain whether hydrogen bonding plays a role in the gelation process, the infrared spectrum of the C8 xerogel from cyclohexane was examined (Fig. S3†). The observation of hydrogen bonded N–H stretching bands at 3199 cm−1 (the absence of free N–H, a relatively sharp peak with frequency higher than 3400 cm−1) and intense absorption of bonded CvO stretching vibrations at 1625 cm−1 and 1676 cm−1 clearly indicates that the N–Hs of the hydrazide group are associated with the CvO groups via N–H⋯OvC intermolecular hydrogen bonding in the gelation process; such a hydrogen bonding interaction was considered to be the main driving force for the formation of the fibers.30 In addition, the absorption bands of the antisymmetric (νas) and symmetric (νs) CH2 stretching vibrational modes are observed at 2926 cm−1 (νas CH2) and 2856 cm−1 (νs CH2) in the gel, respectively, indicating that the alkyl chains are disordered in part, which is consistent with the XRD results. The intermolecular hydrogen bonding of the hydrazide group can also be confirmed by concentrationdependent 1H NMR as well as temperature-dependent FT-IR measurements.30 Anion responsive properties In order to investigate the interactions of the receptor C8 with different anions, characteristic UV-vis absorption changes were monitored upon the addition of TBA salts of F−, Cl−, Br−, I−, AcO− and H2PO4− to its DMSO solution. As can be seen in Fig. 3, in the presence of 20 equivalents (equiv.) of AcO− and H2PO4−, the maximum absorption at 268 nm decreased, and a new absorption at 450 nm developed. In contrast, apart from the disappearance of absorption at 268 nm, two new absorptions at 300 and 450 nm appeared in the presence of 20 equiv. of F−. No significant spectral change was observed in the presence of other halide anions, suggesting no bonding or very weak interactions between the receptor C8 and the other anions. Due to the appearance of a new absorption in the visible region, the addition of F−, AcO− or H2PO4− can cause a vivid color change, e.g. from colorless to red (Fig. S4†).
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Fig. 3 UV-vis absorption spectra of C8 (8 × 10−5 mol L−1) in the presence of 20 equiv. of various anions in DMSO at room temperature.
The interaction between C8 and fluoride was investigated in detail through absorbance spectra titration experiments. Fig. 4 shows the complete family of spectra during the titration of a 1 × 10−4 mol L−1 solution of C8 in DMSO with [Bu4N]F. Addition of low equiv. F− (Fig. 4a) caused the band at 268 nm to decrease gradually, and meanwhile a new very weak band at 450 nm appeared. The color change is almost invisible. The clear isosbestic point at 306 nm indicates that a single component is produced in response to the interaction between C8 and F−.17,27 Addition of high equiv. F− (Fig. 4b) caused the absorption at 268 nm to be shifted to 300 nm, and that at 450 nm increased and leveled off in the case of more than 25 equiv. of F− added. The color turns from colorless to pale orange. Significant changes in absorbance spectra titration experiments begin not after 2 equiv. addition (significant changes in 1H NMR titration experiments begin after 2 equiv. addition, Fig. 5), but after the addition of 10 equiv. or more of fluoride, which is very similar to the urea derivative result of Fabbrizzi et al.15 This may be due to the water in the solvent or [Bu4N]F·3H2O. The interaction between the receptor C8 and the fluoride was further investigated by 1H NMR titration experiments in DMSO-d6 (Fig. 5). The amide NH proton signals at 10.9 and 10.6 ppm disappeared, and a weak broad signal at 10.7 ppm simultaneously appeared with increasing fluoride concentration from 0 to 1 equiv. The appearance of the weak broad band at 10.7 ppm indicated that the N–H⋯F− hydrogen bond formed,20 which is consistent with the result of the isosbestic point appearing in absorbance spectra titration experiments at low equiv. F− (Fig. 4a). When 2 equiv. of fluoride was added, the weak broad signal at 10.7 ppm disappeared, and a narrow peak at 10.4 ppm simultaneously appeared; meanwhile three new signals emerged at 16.0, 16.2 and 16.5 ppm with a 1 : 2 : 1 integration ratio, which was ascribed to the [HF2]− dimer18,20 and also suggested that a hydrazide N–H group might undergo a deprotonation process. Ha and Hc protons undergo an upfield shift in both the 0–1 and 1–2 equiv. ranges, while Hb
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Fig. 4 Absorbance spectral changes of C8 (1 × 10−4 mol L−1) upon the addition of (a) 0, 1, 2, 3, 5 equiv. fluoride in DMSO; (b) 0–30 (5 equiv. interval) equiv. fluoride in DMSO.
Fig. 5
Plots of 1H NMR spectra of C8 (1 × 10−2 mol L−1) upon the addition of fluoride in DMSO-d6.
protons are insensitive to the addition. In any case, no change occurred after the addition of the second equiv. Based on the analyses of UV-vis absorption and 1H NMR spectra, it can be concluded that there are two stepwise equilibria in this responsive process. As can be envisioned, the antihydrazide isomer of C8 is the most stable conformation, which isomerized to the syn-hydrazide upon the addition of F−, which may be similar to the changes of thiourea protons promoted by F−,32 because the proton was abstracted slightly from the hydrazide subunit and resulted in the formation of a 1 : 1 supramolecular complex [C8·F]−. The following equilibrium (i) and Scheme 2 are given to describe the above changes: ½C8 þ F ! ½C8F
ðiÞ
½C8F þ F ! ½C8 þ ½HF2
ðiiÞ
On the addition of the second fluoride ion, the base becomes very strong, and F− exhibits a large affinity toward H+, which induces the deprotonation of the [C8·F]− complex to form the very stable [HF2]− dimer, as illustrated by eqn (ii). Meanwhile the charge-transfer might result in a fivemembered ring based on intramolecular hydrogen bonding
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between the oxygen atom near the deprotonation nitrogen atom and the other NH could thus be formed (Scheme 2), which results in NH proton signal at 10.4 ppm unchanged, the absorption at 268 nm shift to 300 nm and the absorption at 450 nm developed. The intramolecular hydrogen bonding can also be confirmed by concentration-dependent 1H NMR experiments (Fig. S5†). Thus, the extended conjugated system was formed through the adjacent phenyl group and a five-membered ring, which is responsible for the dramatic color change upon the addition of fluoride ions. In addition, the increase of the shielding effect induces a moderate upfield of the Ha and Hc protons.14 The Hb protons do not shift all the while, maybe due to the result of the electron-withdrawing effect of the nitro group as well as the increasing shielding effect of the F− titration process. When the receptor C8 was titrated with AcO−, different spectroscopic patterns were obtained. Fig. 6 displays the family of UV-visible spectra recorded in the course of the titration of an 8 × 10−5 mol L−1 solution of C8 in DMSO. Addition of AcO− caused the band at 268 nm to decrease gradually, and meanwhile a new band at 450 nm developed. The clear isosbestic point at 288 nm indicates that a stable hydrogen-bond complex [C8·AcO]− is produced in response to the interaction
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Scheme 2
Organic & Biomolecular Chemistry
The proposed reaction mechanism of the host C8 with fluoride.
Fig. 7 (a) Organogel formed from a solution of C8 in acetonitrile (3.86 wt%); (b) 5 s after addition of solid TBAF ([Bu4N]F·3H2O, 3 equiv.); (c) 10 s; (d) 50 s; (e) immediately after addition of 200 μL methanol to (d).
Fig. 6 Absorbance spectra changes of C8 (8 × 10−5 mol L−1) upon the addition of 0–22 equiv. AcO− in DMSO.
between C8 and AcO−.17,27,33 (The results of absorbance spectra titration experiments of H2PO4− were similar to those of AcO−, Fig. S6†.) The formation of a hydrogen-bond complex of 1 : 1 stoichiometry was also confirmed by 1H NMR titration experiments (Fig. S7†). Even on excess addition of AcO−, only very little upfield shift of Ha and Hc was observed, indicating a weak hydrogen-bond interaction. The above behaviour should be ascribed to the much lower basicity and hydrogenbond acceptor properties of AcO− compared to F−. In particular, AcO− is unable to form a stable [H(AcO)2]− complex as found for the [HF2]− dimer. Fluoride-responsive gels In order to study the potential application in sensing materials, we also checked the fluoride-responsive properties of gel C8. On careful addition of solid [Bu4N]F·3H2O onto the top of the acetonitrile (MeCN) gel, a thin layer of red solution immediately appeared at the upper part. As time passed, the
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white gel gradually vanished and more and more red-colored solution appeared until all the gel transferred into solution, and the whole process took place in no more than one minute (Fig. 7). The presence of fluoride not only changed the color of the system but also disrupted the gel to a solution through slow diffusion of fluoride. The results indicated that C8 gel is sensitive to fluoride, with naked eye sensing by gel–sol transition and obvious color changes. But other halide anions could not lead to gel decomposition (Fig. S8†). We could presume a mechanism of F anion induced gel–sol transition and color change: upon the addition of the F− (more than 2 equiv.), the hydrazide NH group perhaps underwent a deprotonation process (Scheme 2). It destroyed the intermolecular hydrogen bonding between CvO and N–H groups which is the main driving force that supported gel formation, and subsequently caused the gel–sol transition. Meanwhile, the color change of the gel was mainly attributed to the extended conjugated system formed through the adjacent phenyl group and a five-membered ring. Other halide anions with weaker basicity could not lead to the gel decomposition and color change. To further certify the mechanism of deprotonation of the hydrazide group resulting in the gel–sol conversion and color change, methanol as a competing agent was added into the
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red solution formed by the F− disintegrated gel; the red color disappeared, while a new organogel formed again (Fig. 7). However, no change emerged during the titration of a 1 × 10−4 mol L−1 solution of C8 in ethanol with [Bu4N]F until 100 equiv. because there is a strong interaction between the fluoride and the protic solvent. When THF solution of [Bu4N]F was carefully added onto the top of the ethanol gel, a thin layer of red solution immediately appeared at the upper part (Fig. S9†). The C8 gel showed a similar response to AcO− and H2PO4− (Fig. S10†).
Conclusions A gelator C8 based on nitro-substituted hydrazide derivatives was synthesized, and it was demonstrated that it could form stable gels in some of the tested solvents. The C8 organogel can be utilized for an efficient ‘naked eye’ detection of F−, through a reversible gel–sol transition, which is associated with a color change from colorless to red. A possible mechanism for the F− responsive process was proposed. There are two stepwise equilibria: (i) C8 interacts with F− to give the [C8·F]− complex, and (ii) the [C8·F]− complex interacts with a second F− and deprotonation takes place, with formation of [HF2]− and the fivemembered ring based on intramolecular hydrogen bonding. The results might provide a strategy to design a novel gelator for constructing stimulus responsive soft materials and provide the basis for the development of non-fluid systems for sensing anions with the naked eye.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21072076, 51103057, and 51073071), the Natural Science Foundation of Jilin Province (201215009), and Project 985-Automotive Engineering of Jilin University.
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