Article pubs.acs.org/Langmuir
Reversible Sol-to-Gel Transformation of Uracil Gelators: Specific Colorimetric and Fluorimetric Sensor for Fluoride Ions Ling-Bao Xing,† Bing Yang,† Xiao-Jun Wang,† Jiu-Ju Wang,† Bin Chen,† Qianhong Wu,‡ Hui-Xing Peng,† Li-Ping Zhang,† Chen-Ho Tung,† and Li-Zhu Wu*,† †
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry and University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Department of Mechanical Engineering, Villanova University, Villanova, Pennsylvania 19085, United States S Supporting Information *
ABSTRACT: A new type of anthracene organogelator based on uracil was obtained using organic aromatic solvents, cyclohexane, DMSO, ethanol, and ethyl acetate. It was further characterized by field-emission scanning electron microscopy and transmission electron microscopy. Specifically, the resulting organogels were demonstrated to be promising colorimetric and fluorescent sensors toward fluoride ions with high sensitivity and selectivity, accompanying the disruption of the gelators. Spectroscopic study and 1H NMR titration experiment revealed that the deprotonation of the hydrogen atom on the N position of uracil moiety by fluoride ions is responsible for the recognition events, evidenced by immediate transformation from the sol phase to the gel state upon adding a small amount of a proton solvent, methanol. The process is reversible, with zero loss in sensing activity and sol-to-gel transformation ability even after five runs.
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subunits.17−20 The receptor bears one or more hydrogenbonding donor sites for selective binding and sensing of specific anions. Recently, Ž inić and co-workers reported fluoride sensing using a gel of oxalamide-derived anthraquinone.21 As fluoride ions were present to deprotonate the amide group, the gel showed color change and gel-to-sol phase transition. Lee and co-workers reported fluoride sensing using a gel based on benzoxazole derivative.22 In the presence of fluoride ions, the translucent colorless gel was transformed to a solution with strong greenish fluorescence. Wei and co-workers designed a gelator bearing phenol O−H and acyl-hydrazone N−H groups. The organogel in N,N-dimethylformamide is very stable and contains two channels responsive to F−, AcO−, and H2PO4−.23 Very recently, Ju and co-workers designed a novel functionalized tweezer of uracil-appended glycyrrhetinic acid.24 The tweezer possesses excellent gelation ability and responds to fluoride ions and mercury ions by undergoing a gel-to-sol phase transition. Review of the literature indicates that the recognition of fluoride ions has been realized in organogelator systems,21−32
INTRODUCTION Gelation is an intriguing phenomenon characterized by the formation of supramolecular architectures in nano/micrometer dimensions, from molecules in aqueous or organic solvents.1,2 Self-assembled gelation is generally organized by intermolecular interactions, such as hydrogen bonding, π−π interactions, van der Waals forces of long alkyl chains, and electrostatic forces.3−5 As a soft matter, gelators have important applications in tissue engineering and drug delivery.6−8 As a light-sensitive matter, they are also widely applied in sensors, light harvesting, and photoresponsive systems.9−12 Recently, supramolecular structures composed of low molecular weight gelators (LMWGs) attracted much attention from the chemistry community. There are extensive reports on the gelation of small molecules coordinated with cations.13 On the other hand, considerable efforts have been made for the development of anion-responsive (e.g., fluoride ions) gels, which is of particular importance in revealing a number of biological processes, determining various disease states, and detecting environmental pollution.14−16 It is generally accepted that a synthetic anion sensor involves an optical-signaling chromophore covalent-linked to a neutral anion receptor, like urea, thiourea, amide, phenol, or pyrrole © XXXX American Chemical Society
Received: December 12, 2012 Revised: February 7, 2013
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Scheme 1a
a
(a) K2CO3, DMF; (b) 2-methyl-3-butyn-2-ol, [Pd(PPh3)4], CuI, Et3N, THF; (c) NaH, toluene; (d) [Pd(PPh3)4], CuI, Et3N, THF; (e) BuLi, I2, THF; (f) [Pd(PPh3)4], CuI, Et3N, THF.
but the selectivity of fluoride detection over other anions has yet to be improved. In the present work, we report a low molecular weight gelator 1 composed of an anthracene unit, a uracil unit, and long alkyl chains. The aromatic anthracene unit serves as a signaling chromophore, the long alkyl chains were selected for their gelation ability, and the amidic N−H in the uracil unit serves as the recognition site for its hydrogen-bonding ability (Scheme 1). The group of nitrogen-based molecules necessary for hydrogen bonding of complementary DNA and RNA strands enables nuclebases to assemble elegant macroscopic structures using hydrogen-bonding interactions.33,34 However, reports of gelators based on uracil, especially for recognition events, remain scarce. It is anticipated that with the cooperative multiple intermolecular interactions, including π−π interaction, hydrogen-bonding interaction, and van der Waals force in these components, compound 1 can self-assemble into long fibers in organic solvents and entangle further to form gels. This expectation was found indeed to be the case. Compound 1 is capable of forming organogelators in aromatic solvents, cyclohexane, DMSO, ethanol, and ethyl acetate. Significantly, the resulting organogels, upon the disruption of the gelators, present intriguing characteristics, making them promising colorimetric and fluorescent sensors in response to fluoride ions with high sensitivity and selectivity. Other anions, including Cl−, Br−, I−, ClO4−, AcO−, HSO4−, and H2PO4−, however, do not cause any alternation in either the color, fluorescence, or in the phase state of the organogelators. This unique feature of 1 allows for so-called “naked-eye” detection of fluoride ions without interference of other anions. The detailed results are presented below.
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26.14, 22.84, 14.27; EI-MS (m/z) calcd for C18H29IO 388.13, found 388.20 [M]+. Synthesis of 3. To the mixture of 9,10-dibromoanthracene (1 g, 3 mmol), CuI (5.7 mg, 0.03 mmol), and Pd(PPh3)4 (69.3 mg, 0.06 mmol) were added anhydrous THF (40 mL) and TEA (20 mL) under argon. While stirring, 2-methyl-3-butyn-2-ol (0.50 g, 6 mmol) was injected through a syringe. The reaction mixture was stirred at 75 °C overnight under argon atmosphere and was monitored by TLC. Upon completion, the solution was evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH2Cl2/EtOAc, 4/1) to give compound 3 (0.6 g, 17.7 mmol, 60%) as a yellow solid: 1H NMR (400 MHz, CDCl3) δ 8.64−8.45 (m, 4H), 7.67−7.57 (m, 4H), 1.85 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 132.96, 130.12, 128.19, 127.38, 127.02, 126.82, 124.22, 117.57, 106.33, 78.71, 66.34, 31.88; EI-MS (m/z) calcd for C19H15BrO 338.03, found 338.03 [M]+. Synthesis of 4. To a toluene (30 mL) solution of 3 (0.6 g, 1.8 mmol) was added NaH (0.22 g, 9 mmol), and the mixture was stirred at 80 °C for over 20 min and monitored by TLC. Upon completion, the solution was evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (petroleum ether) to give compound 4 (0.47 g, 1.67 mmol, 93%) as a yellow solid: 1 H NMR (400 MHz, CDCl3) δ 8.68−8.61 (m, 2H), 8.59−8.54 (m, 2H), 7.70−7.56 (m, 4H), 4.06 (s, 1H); 13C NMR (101 MHz, CDCl3) δ 133.67, 130.26, 128.37, 127.57, 127.18, 125.03, 117.08, 89.46, 80.16; EI-MS (m/z) calcd for C16H9Br 279.99, found 279.98 [M]+. Synthesis of 5. To the mixture of compound 2 (0.56 g, 2 mmol), 4 (0.67 g, 2.4 mmol), CuI (3.8 mg, 0.02 mmol), and Pd(PPh3)4 (46.2 mg, 0.04 mmol) were added anhydrous THF (40 mL) and TEA (20 mL) under argon. The reaction mixture was stirred at 75 °C overnight under argon atmosphere and was monitored by TLC. Upon completion, the solution was evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (petroleum ether/EtOAc, 10/1) to give compound 5 (1.0 g, 1.85 mmol, 93%) as a red solid: 1H NMR (400 MHz, CDCl3) δ 8.70 (dd, J = 6.9, 2.5 Hz, 2H), 8.57 (dd, J = 7.1, 2.4 Hz, 2H), 7.69 (dd, J = 9.2, 2.3 Hz, 2H), 7.67−7.55 (m, 4H), 6.97 (d, J = 8.8 Hz, 2H), 4.03 (t, J = 6.6 Hz, 2H), 1.82 (m, 2H), 1.52−1.45 (m, 2H), 1.32 (m, 16H), 0.89 (t, J = 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 159.81, 133.25, 132.96, 130.42, 128.30, 127.51, 127.02, 126.73, 123.66, 118.96, 115.31, 114.90, 102.33, 84.83, 68.33, 32.08, 29.80, 29.75, 29.55, 29.51, 29.36, 26.18, 22.85, 14.27; EI-MS (m/z) calcd for C34H37BrO 540.20, found 540.20 [M]+. Synthesis of 6. To a THF solution (40 mL) of 5 (1.0 g, 1.85 mmol) was added BuLi (2.3 mL of a 1.6 M solution, 3.7 mmol) dropwise, and the resulting solution was stirred under argon at −78 °C. For 1.5 h, I2 (1.88 g, 7.4 mmol) was added and the reaction mixture was stirred for 2 h. The solution was allowed to warm to room temperature. The organic phase was diluted with CH2Cl2 (30 mL), washed with Na2SO3 (3 × 30 mL) and brine (3 × 30 mL), dried over
EXPERIMENTAL SECTION
Synthesis of 2. To a solution of 4-iodophenol (3 g, 13.6 mmol) in DMF (30 mL) were added 1-bromododecane (4.1 g, 16.5 mmol) and dry K2CO3 (11.3 g, 81.8 mmol), and the mixture was stirred at 70 °C overnight. Then the solution was poured into water (100 mL) and extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was washed with 1 M HCl (1 × 100 mL) and brine (3 × 150 mL), dried over Na2SO4, and evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (petroleum ether) to give compound 2 (5.1 g, 13.1 mmol, 96%) as a white solid: 1 H NMR (400 MHz, CDCl3) δ 7.61−7.48 (m, 2H), 6.73−6.60 (m, 2H), 3.91 (t, J = 6.6 Hz, 2H), 1.85−1.67 (m, 2H), 1.39−1.17 (m, 18H), 0.88 (t, J = 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 159.17, 138.28, 117.08, 82.52, 68.27, 32.07, 29.79, 29.73, 29.51, 29.29, B
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Na2SO4, and then evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (petroleum ether/EtOAc, 10/1) to give compound 6 (0.94 g, 1.59 mmol, 86%) as a yellow solid: 1H NMR (400 MHz, CDCl3) δ 8.73−8.62 (m, 2H), 8.58−8.47 (m, 2H), 7.70 (d, J = 8.5 Hz, 2H), 7.67−7.55 (m, 4H), 6.97 (d, J = 8.6 Hz, 2H), 4.03 (t, J = 6.6 Hz, 2H), 1.88−1.78 (m, 2H), 1.52−1.45 (m, 2H), 1.28 (s, 16H), 0.88 (t, J = 6.6 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 159.80, 134.04, 133.61, 133.24, 132.72, 127.95, 127.68, 126.67, 120.32, 115.30, 114.87, 106.62, 102.76, 84.89, 68.30, 32.08, 29.80, 29.76, 29.55, 29.52, 29.36, 26.18, 22.85, 14.28; EI-MS (m/z) calcd for C34H37IO 588.19, found 588.19 [M]+. Synthesis of 7. This is described in the Supporting Information. Synthesis of 1. To the mixture of compound 6 (0.88 g, 1.49 mmol), 7 (0.54 g, 1.79 mmol), CuI (5.68 mg, 0.03 mmol), and Pd(PPh3)4 (17.3 mg, 0.015 mmol) were added anhydrous THF (40 mL) and TEA (20 mL) under argon. The reaction mixture was stirred at 75 °C overnight under argon atmosphere and was monitored by TLC. Upon completion, the solution was evaporated in vacuo to dryness. The crude product was purified by silica gel flash column chromatography (CH2Cl2/EtOAc, 10/3) to give compound 1 (1.09 g, 1.43 mmol, 96%) as a red solid: 1H NMR (400 MHz, CDCl3) δ 8.84− 8.62 (m, 2H), 8.44−8.33 (m, 2H), 7.80−7.60 (m, 6H), 6.99 (d, J = 8.1 Hz, 2H), 6.26 (s, 1H), 4.34−4.19 (m, 2H), 4.04 (t, J = 6.5 Hz, 2H), 2.00−1.78 (m, 4H), 1.58−1.06 (m, 36H), 0.87 (dt, J = 13.7, 6.8 Hz, 6H); 13C NMR (101 MHz, CDCl3) δ 160.21, 150.72, 138.99, 133.48, 132.95, 132.12, 131.65, 128.08, 127.94, 126.98, 126.02, 122.75, 114.99, 114.78, 113.61, 106.98, 105.12, 98.58, 92.19, 85.09, 68.39, 47.26, 46.49, 32.03, 29.70, 29.53, 29.47, 29.34, 29.21, 26.92, 26.16, 22.80, 14.23, 8.78; MALDI-TOF-MS (m/z) calcd for C52H64N2O3 764.50, found 764.50 [M]+.
Table 1. Gelation Experimental Results of Compound 1 in Various Solvents at Room Temperaturea,b solvent
gelation result
solvent
hexane cyclohexane dichloromethane chloroform acetone benzene chlorobenzene toluene o-xylene
I TG (9.6) P P I OG (12.4) P OG (6.8) OG (5.4)
m-xylene pyridine THF 1,4-dioxane ethyl acetate ethanol DMSO DMF acetonitrile
gelation result OG S S PG OG OG OG P I
(7.2)
(18.5) (14.4) (8.6)
a
TG, transparent gel; OG, opaque gel; PG, partial gel; P, precipitation; S, soluble; I, insoluble when heated. bThe critical gelation concentrations (CGC, mg mL−1) of gelators are shown in the parentheses.
dimensional fibrous network. TEM images (Figure 1d−f) also indicate that molecules of 1 are piled up to form cross-linking network structures. The interaction of 1 with anions (using their tetrabutylammonium salts as the sources) in the gel state was investigated in DMSO solution. When excess tetrabutylammonium fluoride was added to the hot DMSO solution of 1, the mixture was cooled to a clear solution at room temperature instead of forming a gel as it would form in its original state (Figures 2 and S1, Supporting Information). The transformation of gel-tosol is faster for compound 1 at high temperature than that at the low temperature. Under the same condition, addition of other anions (Cl−, Br−, I−, ClO4−, AcO−, HSO4−, H2PO4−) did not trigger the gel-to-sol transition (Figure S1, Supporting Information). More strikingly, the addition of fluoride ions caused solution color changes from salmon pink to dark red and fluorescence changes from yellow to green (Figure 2). Other anions have no obvious effect on the absorption and fluorescence spectra or the gel-to-sol transition (Figure S1, Supporting Information), indicating the high selectivity of gel 1 toward fluoride ions by naked-eye sensing. To examine the binding site of gel 1 and fluoride ions, we determined the spectral change at a low concentration (1 × 10−5 M) in DMSO. Compound 1 is strongly fluorescent in DMSO solution with a maximum at 530 nm. Upon addition of fluoride ions, however, 1 becomes weakly fluorescent and blueshifted to 498 nm (Figure S2, Supporting Information), and at the same time the UV−vis absorption is blue-shifted from 465 to 455 nm accompanied by enhancement at 320 nm and decrease at 340 nm (Figure 3a). After 20 equiv of fluoride ions were added to the DMSO solution, the fluorescence spectra and absorption spectra of compound 1 were saturated. The decreased fluorescence intensity and blue-shifted absorption are as a function of the concentration of fluoride ions. In the case of other anions, alterations in the absorption and fluorescence spectra were hardly observed (Figures 3b and S3, Supporting Information). Similar results were also observed in CH2Cl2 (Figures S4−S6, Supporting Information). Clearly, only fluoride ions could lead to the color and fluorescent changes of the solution (Figure S7, Supporting Information). The dramatic color changes may be interpreted by the fact that the hydrogen atom on the N position of uracil moiety was deprotonated by fluoride ions20 (Scheme 2), which subsequently modulated intramolecular charge transfer from the anthracene unit to the amide moiety of compound 1. This
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RESULTS AND DISCUSSION The synthesis of compound 1 started from 9,10-dibromoanthracence, which was reacted with 2-methyl-3-butyn-2-ol to produce compound 3. In the presence of toluene, compound 3 was deprotected with NaH to give compound 4 in a yield of 93%. Then compound 4 was reacted with 2 to afford compound 5 as a yellow solid, which was iodinated to give compound 6 in 86% yield. Compound 6 and 7, synthesized according to the literature35, were dissolved in THF and Et3N and catalyzed by CuI and Pd(PPh3)4 to give compound 1 as the product in 96% yield (the details are in the Supporting Information). All of the compounds were characterized by 1H and 13C NMR spectroscopy, as well as mass spectrometry. The gelation ability of compound 1 was examined in various solvents by means of the “stable to inversion of a test tube” method (for more details see the Supporting Information). The corresponding critical gelation concentrations (CGC) at room temperature were also measured. As shown in Table 1, compound 1 can gel aromatic solvents, such as benzene, toluene, o-xylene, and m-xylene, while it is insoluble in hexane, acetone, and acetonitrile, even after heating. These gels were found to be stable in closed tubes for at least 3 months at 25 °C. The better gelation for aromatic solvents relative to the hydrocarbon or halogenated hydrocarbons is probably due to the enhancement of π−π interactions in aromatic solvents.5 The morphology of xerogels obtained from different solvent (after evaporation of the solvent) was characterized by fieldemission scanning electron microscopy (FE-SEM). Most of the gels show an entanglement of fibrous aggregates. Figure 1a−c shows the typical FE-SEM images of the xerogels upon depositing the gels from cyclohexane, ethanol, and DMSO on silicon wafers under identical conditions. Note that the molecules in the gel phase were self-assembling into onedimensional fibers with a few micrometers long and 50−100 nm wide that further extended to form a large threeC
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Figure 1. FE-SEM and TEM images of xerogels of compound 1 prepared from (a, d) cyclohexane, (b, e) ethanol, and (c, f) DMSO solution, respectively.
Figure 2. Illustration of the gel-to-sol transformation, as well as the color and fluorescence changes, of compound 1 (10 mg/mL) in DMSO with the addition of fluoride ions (1 equiv) under ambient conditions.
Figure 3. (a) UV−vis absorption of compound 1 upon addition of tetrabutylammonium fluoride (0−20 equiv), and (b) fluorescent changes of compound 1 upon addition of tetrabutylammonium anions (20 equiv) in DMSO at room temperature; [1] = 1.0 × 10−5 M, excitation was at 450 nm.
in DMSO-d6 was also carried out. As shown in Figure 4, the amide N−H proton signal (11.7 ppm) on the uracil unit of compound 1 disappeared completely with the addition of excess fluoride ions, and a new weak broad signal appeared at 16.6 ppm, the typical signal of HF 2−. Evidently, the deprotonation of N−H of uracil in compound 1 took place. As a consequence, the proton signal resonance of the uracil unit shifted upfield. More interestingly, the organogels of compound 1 could be re-formed with the aid of CH3OH. For example, when fluoride ions were added to the organogels of 1 in DMSO, the gel-to-sol
speculation was confirmed by the identical spectral changes of compound 1 when tetrabutylammonium hydroxide was used to replace fluoride ions (Figure S8, Supporting Information), suggestive that the deprotonation occurs. As a result, the absorption and fluorescence were blue-shifted, and at the same time the negatively charged compound would decrease the hydrogen-bonding interactions of the amide moiety, leading to the disruption of gelators in the presence of fluoride ions or tetrabutylammonium hydroxide. Furthermore, the 1 H NMR titration experiment of compound 1 before and after addition of excess fluoride ions D
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Scheme 2. Representative Deprotonation of Compound 1 by Fluoride Ions
Figure 5. The reversibility of gel-to-sol transition processes of 1 (10 mg/mL) in DMSO monitored at 630 nm in the presence of tetrabutylammonium fluoride (13 mM) and CH3OH (0.6 M).
transformation was clearly observed. Meanwhile, the solution color changed from salmon pink to dark red and fluorescence changed from yellow to green (Figure 2). When a small amount of CH3OH (0.6 M) was introduced into the system, the organogels were regenerated immediately (Figure 2). The influence of methanol is attributed to the reprotonation of compound 1. The equilibrium of protonation and deprotonation depends on not only the basicity but also the concentrations of methanol and fluoride, respectively. The higher concentration of methanol solution turned back into a gel in spite of the presence of fluoride. Furthermore, the better solvation of fluoride in a polar protic solvent makes it a less efficient competitor for the gelator amide NH groups. Taking the fluorescent maximum at 630 nm for gel state (Figure S9, Supporting Information), we inferred that the whole process is reversible with no loss in sensing activity and sol-to-gel transformation ability even after five runs (Figure 5).
formation of organogelators in aromatic solvents, cyclohexane, DMSO, ethanol, and ethyl acetate. Of particular significance, the resulting organogels can detect fluoride ions with high sensitivity and selectivity, accompanying with the disruption of the gelators. The addition of fluoride ions in DMSO of 1, for example, results in either absorption color change from vivid salmon pink to dark red or fluorescence change from yellow to green, respectively. A small amount of CH3OH can regenerate the organogels immediately. The whole process is reversible, with no loss in sensing activity and sol-to-gel transformation ability even after five runs. Other anions, including Cl−, Br−, I−, ClO4−, AcO−, HSO4−, and H2PO4−, caused no alternation either in the color, fluorescence, or the phase state of the organogelators. The experimental study presented herein could have important potentials for the fabrication of smarter uracil supramolecular systems for anion sensing applications.
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CONCLUSION In summary, we have developed a specific colorimetric and fluorimetric sensor 1 for detecting fluoride ions. Benefiting from the unique structure of uracil, this compound enables the
ASSOCIATED CONTENT
S Supporting Information *
Synthesis of 7, UV−vis absorption and fluorescence spectra, and color changes of 1 after addition of various anions. This
Figure 4. Partial 1H NMR spectra of 1 (8 mM) in DMSO-d6 upon addition of excess fluoride ions. Inset: the new typical signal of HF2−. E
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the Ministry of Science and Technology of China (2013CB834505 and 2009CB22008), the National Natural Science Foundation of China (91027041 and 20973189), and the Bureau for Basic Research of Chinese Academy of Sciences.
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REFERENCES
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