FULL PAPER DOI: 10.1002/asia.201402590
Self-Assembly of Pyridinium-Tailored Anthracene Amphiphiles into Supramolecular Hydrogels Peiyi Wang,[a] Jun Hu,[b] Song Yang,[a] Baoan Song,*[a] and Qian Wang*[b] Abstract: The mixing of a polyacid cross-linker with a pyridinium-functionalized anthracene amphiphile afforded a supramolecular hydrogel through a self-assembly process that was primarily driven by p-stacking and electrostatic interactions.
Introduction
polyanions could lead to the formation of supramolecular hydrogels based on electrostatic interactions between the negative charge of the polyanions and the positive charge on the surface of the nanofibers. Herein, we used a pyridinium-functionalized 2-anthracene (2-AP) species as the amphiphile (Figure 1 a),[6a] in which the hydrophobic aromatic ring and the hydrophilic pyridinium cation were used to afford p-stacking interactions and a positive charge, which could undergo electrostatic attractions, respectively. Disodium ethylenediaminetetraacete (EDTA·2 Na, 1; Figure 1 f), which has wide applications in industry, medicine, and cosmetics, owing to its chelating ability,[8] was chosen as a model polyacid to test its ability to cross-link these nanofibers, thereby resulting in a supramolecular hydrogel.
Supramolecular hydrogels, primarily formed through noncovalent interactions, have received great attention in recent years for their potential applications in drug delivery, tissue engineering, and cosmetics.[1] By combining different noncovalent interactions, such as hydrogen-bonding, dipole– dipole, charge-transfer (CT), hydrophobic, and electrostatic interactions, many supramolecular hydrogels have been engineered.[2] As a member of the p-conjugated acene family, anthracene is of considerable interest in the development of new gelators, not only owing to its rich electron density but also to its intrinsic planar anisotropy.[3] For example, Zhang and co-workers developed a supramolecular polymer gel based on an anthracene-derived bola-amphiphile driven by host-stabilized CT interactions and this gel exhibited strong sensitivity towards potassium cations.[4] Sako and Takaguchi fabricated a photo-responsive hydrogel based on an anthryl dendron that contained gluconamide residues.[5] Previously, we showed that a pyridinium-functionalized anthracene amphiphile could assemble into nanofibers, primarily promoted by p-stacking interactions, and that the fibers had a positive charge on its surface.[6] It is known that the entanglement of nanofibers can result in a 3D cavity to entrap solvent molecules, thus generating a gel.[7] Inspired by those reports, we wondered whether the introduction of
Results and Discussion As a general procedure, a mixture of 2-AP and 1 (2:1 molar ratio) in deionized water was heated at 55 8C until a clear solution was formed. Then, the mixture was cooled to room temperature and the “inverted test tube method” was used to determine whether a hydrogel had formed or not.[9] As shown in Figure 1 b, a colorless supramolecular hydrogel was formed and its microstructure was examined by TEM and AFM. A typical entangled 3D network of these fibers was observed (Figure 1 c–e) and its viscoelastic behavior was characterized by rheological measurements, in which the storage modulus (G’) and the loss modulus (G’’) were measured as functions of frequency and time sweep. As shown in Figure 2 a, G’ was about 20-times greater than G’’, which indicated elastic character of the hydrogel. The fact that both G’ and G’’ decreased dramatically after 10 Hz suggested that it was a medium-elastic gel.[10] The dynamic timesweep data showed that the G’ and G’’ values remained constant after the first 15 min gelling time, further indicating that it was a medium-strength physical gel (Figure 2 b).[11] Studies on the thermal stability as a function of stoichiometry were performed on hydrogels with different molar ratios of 2-AP and 1 (4:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4; 5.6 mm 2-
[a] P. Wang,+ Prof. S. Yang, Prof. B. Song State Key Lab Breeding Base of Green Pesticide and Agricultural Bioengineering Center for R&D of Fine Chemicals, Guizhou University Guiyang, 550025 (China) E-mail: [email protected] [b] Dr. J. Hu,+ Prof. Q. Wang Department of Chemistry and Biochemistry University of South Carolina Columbia, SC, 29208 (USA) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article, including MS and NMR spectra of 2-AP, 5, and TNF, is available on the WWW under http:// dx.doi.org/10.1002/asia.201402590.
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Keywords: amphiphiles · electrostatic interactions · gels · pi interactions · supramolecular chemistry
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Figure 1. a) Assembly process of the hydrogel. b) Photograph, c, d) TEM image, and e) AFM image of a hydrogel of 2-AP/1 (2:1 molar ratio, 5.6 mm 2AP, pH 4.3). f) Structures of the molecules used in this work. Scale bars are 500 nm for (c), 100 nm for (d), and 1 mm for (e).
Figure 2. Dynamic frequency (a) and time sweep (b) of the storage modulus (G’) and the loss modulus (G’’) of a hydrogel of 2-AP/1 (2:1 molar ratio, 11 mm 2-AP, pH 4.3). 2 % strain was used for (a) and (b); a frequency of 1 Hz was used for (b).
AP, pH 4.3). As shown in the Supporting Information, Figure S1, the Tgel value reached a maximum at a 2-AP/1 molar ratio of 2:1, thus implying the electrostatic nature of this 2:1 stoichiometric complex. Under such conditions, the critical gel concentration (CGC) of the hydrogel was 3.7 mm (see the Supporting Information, Figure S2). UV/Vis, fluorescence, and IR spectroscopy were used to study the driving forces of the gelation process. As shown in the absorption spectra, a red shift (20 nm) was observed upon the addition of compound 1, which indicate the formation of “J”-type aggregates, owing to p-stacking of the anthracene rings (Figure 3 a).[12] However, it is still unclear whether compound 1 or NaBr induced the red-shift. The formation of “J”-type aggregates was further confirmed by the emission spectra, in which an excited-state dimer peak at
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Figure 3. a) UV/Vis and b) fluorescence spectra of 2-AP and 2-AP/1. Inset shows photographs of aqueous solutions of 2-AP (left) and 2-AP/ 1 (right) under 365 nm UV light. c) UV/Vis spectra of 2-AP/1 at different concentrations. d) Powder X-ray diffraction spectrum of the xerogel. e) TEM image of 2-AP/1. f) Statistical diameter of the fibers. The molar ratio is 2:1 for (a)–(f). The concentration of 2-AP is 0.2 mm for (a) and (b), 5.6 mm for (d), and 0.56 mm for (e). The scale bar is 50 nm for (e).
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471 nm was observed (Figure 3 b).[13] Compared to our previous work, in which CT interactions primarily promoted the formation of microtube assemblies of 2-AP in water,[6a] the involvement of electrostatic interactions from compound 1 led 2-AP to adapt a different stacking model. This result implied that even the same molecule could adopt different packing patterns depending on the assembly conditions. A comparison of the IR spectra of the freeze-dried xerogel and compound 1 showed that the stretching vibration of the carbonyl group in compound 1 changed from 1612 to 1629 cm 1 after the formation of the hydrogel (see the Supporting Information, Figure S3), thus implying that compound 1 served to cross-link the fibers together, promoted by electrostatic attractions between the pyridinium cations and the carbonate anions.[14] In addition, concentration-dependent UV/Vis spectra revealed a red shift on increasing the concentration of complex 2-AP/1 (Figure 3 c), caused by the formation of twisted fibers.[15] To determine the molecular packing pattern of the hydrogel, powder X-ray diffraction (XRD) of the hydrogel was performed. The sharp reflection peak in the XRD data revealed a semicrystalline nature of the hydrogel and a highly ordered layer-packing arrangement, with an inter-layer distance of about 3.45 nm (Figure 3 d). This distance was larger than the d value for the microtubes in our previous work (about 3.07 nm),[6a] thus confirming that “J”-type aggregates formed in the hydrogel, mainly driven by a combination of p-stacking and electrostatic interactions (Figure 1 a), while linear “head-to-tail” dimers formed for microtubes promoted primarily by the CT interactions.[6a] Moreover, the average fiber diameter was about 3.5 nm, as measured by TEM (Figure 3 e, f). Based on these results, it is clear that 2-AP assembled into nanofibers, likely driven by p-stacking of the anthracene rings, with the positive charge exposed on the outer surface. Compound 1 severed to cross-link the nanofibers together, thereby leading to 3D networks and a supramolecular hydrogel, promoted by electrostatic attractions between the negative charge of compound 1 and the positive charge on the surface of the nanofibers (Figure 1 a). Owing to the involvement of non-covalent p-stacking and electrostatic interactions in the gelling process, its responses to light, pH value, and electron-deficient molecules were investigated. A clear gel–sol transition was observed when the gel was irradiated under UV light at 365 nm (Figure 4 a, inset), owing to dimerization of the anthracene rings,[16] which resulted in destruction of the p-stacking interactions between the anthracene rings. The dissociation process was further confirmed by time-dependent absorption and emission spectroscopy. As shown in Figure 4 a, b, the classic absorption peaks belonging to anthracene group decreased over time, as well as the excimer peak of the anthracene ring at 471 nm. The hydrogel collapsed when the pH value was outside the range 3–5.5, but reformed after adjusting the pH value back again (see the Supporting Information, Figure S4), probably because at high pH value the dissociation of free carboxylic groups led to a decrease in hydrogen-bond for-
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Figure 4. Time-dependent a) UV/Vis and b) fluorescence spectra of 2AP/1 triggered by UV light at 365 nm (2:1 molar ratio, 0.2 mm 2-AP). Inset in (a) shows a photograph after irradiation. c) UV/Vis and d) fluorescence spectra of 2-AP/1, TNF, and 2-AP/1/TNF. The concentration of 2-AP was 0.2 mm. e) Photos of hydrogels before (left) and after (right) the addition of TNF (2:1:2 molar ratio, CGC of 2-AP is 5.6 mm, Tg = 33 8C). f) TEM image of 2-AP/1/TNF. Scale bar is 50 nm.
mation and an increase in intermolecular electrostatic repulsion between dissociated carboxylic groups.[17] Conversely, although enough hydrogen bonds could form at lower pH value, electrostatic attraction between the dissociated carboxylic groups and the pyridinium cations were weakened. This result was further confirmed by comparison of the IR spectra for 2-AP/1 at different pH values. As shown in the Supporting Information, Figure S3, the wavenumber shifts of the carbonyl group in compound 1 changed on increasing or decreasing the pH value, thus indicating a change in the equilibrium between the free and dissociated carboxylic groups, which were the donors in hydrogen bonds and electrostatic attractions, respectively. Apparently, such an equilibrium played a critical role in the gelation process. The gelation process was further confirmed by gelation studies of control molecules under the same condition as 2AP/1. As shown in Figure 1 f, compound 2, which contained both free and dissociated carboxylic groups at pH 3–5.5, could form a hydrogel with 2-AP, whereas molecules 3 and 4 could not. At the same time, when control molecule 5 replaced 2-AP to study its gelation with compound 1, no hydrogel was observed, thus suggesting that the existence of the aromatic plane was also needed for the formation of a supramolecular hydrogel. This result may be due to effective packing of the pyridium rings, which could better assist the formation of fibrous structures, as we reported previously.[6b]
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However, the gelation behavior of 2-AP/1 was not clearly affected upon the addition of trinitrofluorenone (TNF), which was strongly electron-deficient and widely used to form CT interactions with all sorts of electron-rich moieties.[18] As shown in Figure 4 e, the color changed from colorless to red on the introduction of TNF to the 2-AP/1 gel system, which revealed the formation of CT interactions between TNF and the anthracene groups.[19] This result was further confirmed by a new CT peak at 550 nm in the absorption spectrum and by a quenching effect in the emission spectrum (Figure 4 c,d). Rheology studies showed that the gel was a medium-strength physical gel because the G’ and G’’ values remained constant after the first 18 min gelling time (see the Supporting Information, Figure S5 a). The fact that G’ was around 10-times higher than G’’ (see the Supporting Information, Figure S5 b), thus revealing that it had a weaker elastic character than 2-AP/1. TEM images indicated that the 2-AP/1/TNF system formed long nanofibrous structures (Figure 4 f). These results implied that this supramolecular hydrogel had the potential to carry electron-deficient guest molecules.
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Discover diffractometer (l = 0.15406 nm). The Bragg peaks were extracted from the XRD data and the layer thickness (d) was obtained according to the Bragg equation, d = l/2sinq; for the sample preparation, the hydrogel was placed onto a silicon plate and dried in air before the tests. Synthesis of 1-(11-(Anthracen-2-ylmethoxy)-11-oxoundecyl)pyridinium Bromide (2-AP) The synthesis of 2-AP was performed according to the procedure described in Ref. [6a]. M.p. 145–147 8C; 1H NMR (300 MHz, [D6]DMSO): d = 9.11 (d, J = 6.9 Hz, 2 H; pyridinium-H), 8.60 (t, J = 7.2 Hz, 1 H; pyridinium-H), 8.57 (s, 1 H; anthracene-H), 8.56 (s, 1 H; anthracene-H), 8.15 (dd, J = 7.2, 6.9 Hz, 2 H; pyridinium-H), 8.07 (m, 4 H; anthracene-H), 7.50 (m, 3 H; anthracene-H), 5.28 (s, 2 H; OCH2), 4.57 (t, J = 7.5 Hz, 2 H; NCH2), 2.39 (t, J = 7.2 Hz, 2 H; CH2CO), 1.84 (m, 2 H; CH2), 1.54 (m, 2 H; CH2), 1.19 ppm (m, 12 H; CH2); 13C NMR (75 MHz, [D6]DMSO): d = 173.33 (C=O), 145.91, 145.16, 128.92 (5 C; pyridinium-C), 133.86, 131.88, 131.81, 131.18, 131.07, 128.51, 128.46, 127.06, 126.59, 126.42, 126.19, 126.16, 126.02 (14 C, anthracene-C), 65.95, 61.12, 33.95, 31.15, 29.16, 29.11, 29.05, 28.85, 28.76, 25.78, 24.93 ppm; MS (ESI+): m/z: 454. Synthesis of 1-[11-(Benzyloxy)-11-oxoundecyl]pyridinium Bromide (5) The synthesis of 5 was performed according to the procedure described in Ref. [6a]. M.p. 72–74 8C; 1H NMR (400 MHz, CDCl3): d = 9.47 (d, J = 6.8 Hz, 2 H; pyridinium-H), 8.48 (t, J = 7.6 Hz, 1 H; pyridinium-H), 8.11 (dd, J = 7.6, 6.8 Hz, 2 H; pyridinium-H), 7.27 (m, 5 H; phenyl-H), 5.04 (s, 2 H; OCH2), 4.93 (t, J = 7.2 Hz, 2 H; NCH2), 2.28 (t, J = 7.6 Hz, 2 H; CH2CO), 1.98 (m, 2 H; CH2), 1.53 (m, 2 H; CH2), 1.23 ppm (m, 12 H; CH2); 13C NMR (100 MHz, CDCl3): d = 173.79 (C=O), 145.28, 145.23, 128.63 (5 C, pyridinium-C), 136.17, 128.59, 128.25, 128.17 (6 C, phenyl-C), 66.12, 62.12, 34.35, 32.07, 29.28, 29.19, 29.08, 29.06, 26.07, 24.95 ppm; MS (ESI+): m/z: 354.
Conclusions We have developed a supramolecular hydrogel based on a pyridinium-functionalized anthracene and a polyanion. In the hydrogel, the pyridinium-functionalized anthracenes assembled into nanofibers driven by p-stacking interactions and the polyanions were used as a cross-linker to combine the fibers together, thereby generating a supramolecular hydrogel primarily promoted by electrostatic interactions between the negative charge of the polyacids and the positive charge on the surface of the nanofibers. Our results demonstrated that the structure of the supramolecular assembly of a simple amphiphilic molecule (like 2-AP in our case) could be readily manipulated by varying the assembly conditions. In addition, such amphiphile-based supramolecular hydrogels showed multi-response properties, which could find potential applications in drug delivery, tissue engineering, and cosmetics.
Synthesis of 2,4,7-Trinitrofluorenone (TNF) The synthesis of TNF was performed according to the procedure described in Ref. [18c]. M.p. 170–172 8C; 1H NMR (300 MHz, [D6]DMSO): d = 8.99 (d, J = 2.1 Hz, 1 H; 3-H), 8.61 (m, 2 H; 6-H, 7-H), 8.43 (d, J = 2.1 Hz, 1 H; 1-H), 8.19 ppm (d, J = 8.4 Hz, 1 H; 5-H); 13C NMR (75 MHz, [D6]DMSO): d = 186.42, 149.67, 148.90, 144.80, 143.35, 138.86, 137.86, 136.17, 130.78, 128.01, 125.99, 122.20, 119.00 ppm; MS (EI+): m/z: 315.
Acknowledgements The research was supported by the US NSF (CHE-1307319), the University of South Carolina, and the China Scholarship Council (200001). B.S. is grateful for financial support from the International Science & Technology Cooperation Program of China (2010DFB60840), the National Key Program for Basic Research (2010CB126105), and the National Nature Science Foundation of China (21132003).
Experimental Section NMR spectra were recorded on Varian Mercury 300/400 spectrometers in CDCl3 and [D6]DMSO. Mass spectra were recorded on Micromass QTOF and Finningan TSQ spectrometers in positive-ion mode. UV/Vis spectra were recorded on Agilent Technologies 95–03 spectrometers. Fluorescence spectra were recorded on a Varian Cary Eclipse spectrophotometer. IR spectra were recorded on a PerkinElmer Spectrum 100; for the sample preparation, the hydrogel was freeze-dried before the tests. TEM measurements were performed on a Hitachi H8000 electron microscope operating at an acceleration voltage of 120 kV; for the sample preparation, a small amount of the hydrogel was placed onto a copper grid and negatively stained with a uranyl acetate solution before the tests. AFM was performed in air on a NanoScope IIIA MultiMode AFM (Veeco); for the sample preparation, a small amount of the hydrogel was placed onto a silicon plate and dried in air before the tests. Powder X-ray diffraction spectroscopy was performed on a BrukerD8
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Received: June 1, 2014 Revised: June 14, 2014 Published online: August 5, 2014
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Self-Assembly of Pyridinium-Tailored Anthracene Amphiphiles into Supramolecular Hydrogels Peiyi Wang,[a] Jun Hu,[b] Song Yang,[a] Baoan Song,*[a] and Qian Wang*[b] Abstract: The mixing of a polyacid cross-linker with a pyridinium-functionalized anthracene amphiphile afforded a supramolecular hydrogel through a self-assembly process that was primarily driven by p-stacking and electrostatic interactions.
Introduction
polyanions could lead to the formation of supramolecular hydrogels based on electrostatic interactions between the negative charge of the polyanions and the positive charge on the surface of the nanofibers. Herein, we used a pyridinium-functionalized 2-anthracene (2-AP) species as the amphiphile (Figure 1 a),[6a] in which the hydrophobic aromatic ring and the hydrophilic pyridinium cation were used to afford p-stacking interactions and a positive charge, which could undergo electrostatic attractions, respectively. Disodium ethylenediaminetetraacete (EDTA·2 Na, 1; Figure 1 f), which has wide applications in industry, medicine, and cosmetics, owing to its chelating ability,[8] was chosen as a model polyacid to test its ability to cross-link these nanofibers, thereby resulting in a supramolecular hydrogel.
Supramolecular hydrogels, primarily formed through noncovalent interactions, have received great attention in recent years for their potential applications in drug delivery, tissue engineering, and cosmetics.[1] By combining different noncovalent interactions, such as hydrogen-bonding, dipole– dipole, charge-transfer (CT), hydrophobic, and electrostatic interactions, many supramolecular hydrogels have been engineered.[2] As a member of the p-conjugated acene family, anthracene is of considerable interest in the development of new gelators, not only owing to its rich electron density but also to its intrinsic planar anisotropy.[3] For example, Zhang and co-workers developed a supramolecular polymer gel based on an anthracene-derived bola-amphiphile driven by host-stabilized CT interactions and this gel exhibited strong sensitivity towards potassium cations.[4] Sako and Takaguchi fabricated a photo-responsive hydrogel based on an anthryl dendron that contained gluconamide residues.[5] Previously, we showed that a pyridinium-functionalized anthracene amphiphile could assemble into nanofibers, primarily promoted by p-stacking interactions, and that the fibers had a positive charge on its surface.[6] It is known that the entanglement of nanofibers can result in a 3D cavity to entrap solvent molecules, thus generating a gel.[7] Inspired by those reports, we wondered whether the introduction of
Results and Discussion As a general procedure, a mixture of 2-AP and 1 (2:1 molar ratio) in deionized water was heated at 55 8C until a clear solution was formed. Then, the mixture was cooled to room temperature and the “inverted test tube method” was used to determine whether a hydrogel had formed or not.[9] As shown in Figure 1 b, a colorless supramolecular hydrogel was formed and its microstructure was examined by TEM and AFM. A typical entangled 3D network of these fibers was observed (Figure 1 c–e) and its viscoelastic behavior was characterized by rheological measurements, in which the storage modulus (G’) and the loss modulus (G’’) were measured as functions of frequency and time sweep. As shown in Figure 2 a, G’ was about 20-times greater than G’’, which indicated elastic character of the hydrogel. The fact that both G’ and G’’ decreased dramatically after 10 Hz suggested that it was a medium-elastic gel.[10] The dynamic timesweep data showed that the G’ and G’’ values remained constant after the first 15 min gelling time, further indicating that it was a medium-strength physical gel (Figure 2 b).[11] Studies on the thermal stability as a function of stoichiometry were performed on hydrogels with different molar ratios of 2-AP and 1 (4:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4; 5.6 mm 2-
[a] P. Wang,+ Prof. S. Yang, Prof. B. Song State Key Lab Breeding Base of Green Pesticide and Agricultural Bioengineering Center for R&D of Fine Chemicals, Guizhou University Guiyang, 550025 (China) E-mail: [email protected] [b] Dr. J. Hu,+ Prof. Q. Wang Department of Chemistry and Biochemistry University of South Carolina Columbia, SC, 29208 (USA) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article, including MS and NMR spectra of 2-AP, 5, and TNF, is available on the WWW under http:// dx.doi.org/10.1002/asia.201402590.
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Keywords: amphiphiles · electrostatic interactions · gels · pi interactions · supramolecular chemistry
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Figure 1. a) Assembly process of the hydrogel. b) Photograph, c, d) TEM image, and e) AFM image of a hydrogel of 2-AP/1 (2:1 molar ratio, 5.6 mm 2AP, pH 4.3). f) Structures of the molecules used in this work. Scale bars are 500 nm for (c), 100 nm for (d), and 1 mm for (e).
Figure 2. Dynamic frequency (a) and time sweep (b) of the storage modulus (G’) and the loss modulus (G’’) of a hydrogel of 2-AP/1 (2:1 molar ratio, 11 mm 2-AP, pH 4.3). 2 % strain was used for (a) and (b); a frequency of 1 Hz was used for (b).
AP, pH 4.3). As shown in the Supporting Information, Figure S1, the Tgel value reached a maximum at a 2-AP/1 molar ratio of 2:1, thus implying the electrostatic nature of this 2:1 stoichiometric complex. Under such conditions, the critical gel concentration (CGC) of the hydrogel was 3.7 mm (see the Supporting Information, Figure S2). UV/Vis, fluorescence, and IR spectroscopy were used to study the driving forces of the gelation process. As shown in the absorption spectra, a red shift (20 nm) was observed upon the addition of compound 1, which indicate the formation of “J”-type aggregates, owing to p-stacking of the anthracene rings (Figure 3 a).[12] However, it is still unclear whether compound 1 or NaBr induced the red-shift. The formation of “J”-type aggregates was further confirmed by the emission spectra, in which an excited-state dimer peak at
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Figure 3. a) UV/Vis and b) fluorescence spectra of 2-AP and 2-AP/1. Inset shows photographs of aqueous solutions of 2-AP (left) and 2-AP/ 1 (right) under 365 nm UV light. c) UV/Vis spectra of 2-AP/1 at different concentrations. d) Powder X-ray diffraction spectrum of the xerogel. e) TEM image of 2-AP/1. f) Statistical diameter of the fibers. The molar ratio is 2:1 for (a)–(f). The concentration of 2-AP is 0.2 mm for (a) and (b), 5.6 mm for (d), and 0.56 mm for (e). The scale bar is 50 nm for (e).
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471 nm was observed (Figure 3 b).[13] Compared to our previous work, in which CT interactions primarily promoted the formation of microtube assemblies of 2-AP in water,[6a] the involvement of electrostatic interactions from compound 1 led 2-AP to adapt a different stacking model. This result implied that even the same molecule could adopt different packing patterns depending on the assembly conditions. A comparison of the IR spectra of the freeze-dried xerogel and compound 1 showed that the stretching vibration of the carbonyl group in compound 1 changed from 1612 to 1629 cm 1 after the formation of the hydrogel (see the Supporting Information, Figure S3), thus implying that compound 1 served to cross-link the fibers together, promoted by electrostatic attractions between the pyridinium cations and the carbonate anions.[14] In addition, concentration-dependent UV/Vis spectra revealed a red shift on increasing the concentration of complex 2-AP/1 (Figure 3 c), caused by the formation of twisted fibers.[15] To determine the molecular packing pattern of the hydrogel, powder X-ray diffraction (XRD) of the hydrogel was performed. The sharp reflection peak in the XRD data revealed a semicrystalline nature of the hydrogel and a highly ordered layer-packing arrangement, with an inter-layer distance of about 3.45 nm (Figure 3 d). This distance was larger than the d value for the microtubes in our previous work (about 3.07 nm),[6a] thus confirming that “J”-type aggregates formed in the hydrogel, mainly driven by a combination of p-stacking and electrostatic interactions (Figure 1 a), while linear “head-to-tail” dimers formed for microtubes promoted primarily by the CT interactions.[6a] Moreover, the average fiber diameter was about 3.5 nm, as measured by TEM (Figure 3 e, f). Based on these results, it is clear that 2-AP assembled into nanofibers, likely driven by p-stacking of the anthracene rings, with the positive charge exposed on the outer surface. Compound 1 severed to cross-link the nanofibers together, thereby leading to 3D networks and a supramolecular hydrogel, promoted by electrostatic attractions between the negative charge of compound 1 and the positive charge on the surface of the nanofibers (Figure 1 a). Owing to the involvement of non-covalent p-stacking and electrostatic interactions in the gelling process, its responses to light, pH value, and electron-deficient molecules were investigated. A clear gel–sol transition was observed when the gel was irradiated under UV light at 365 nm (Figure 4 a, inset), owing to dimerization of the anthracene rings,[16] which resulted in destruction of the p-stacking interactions between the anthracene rings. The dissociation process was further confirmed by time-dependent absorption and emission spectroscopy. As shown in Figure 4 a, b, the classic absorption peaks belonging to anthracene group decreased over time, as well as the excimer peak of the anthracene ring at 471 nm. The hydrogel collapsed when the pH value was outside the range 3–5.5, but reformed after adjusting the pH value back again (see the Supporting Information, Figure S4), probably because at high pH value the dissociation of free carboxylic groups led to a decrease in hydrogen-bond for-
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Figure 4. Time-dependent a) UV/Vis and b) fluorescence spectra of 2AP/1 triggered by UV light at 365 nm (2:1 molar ratio, 0.2 mm 2-AP). Inset in (a) shows a photograph after irradiation. c) UV/Vis and d) fluorescence spectra of 2-AP/1, TNF, and 2-AP/1/TNF. The concentration of 2-AP was 0.2 mm. e) Photos of hydrogels before (left) and after (right) the addition of TNF (2:1:2 molar ratio, CGC of 2-AP is 5.6 mm, Tg = 33 8C). f) TEM image of 2-AP/1/TNF. Scale bar is 50 nm.
mation and an increase in intermolecular electrostatic repulsion between dissociated carboxylic groups.[17] Conversely, although enough hydrogen bonds could form at lower pH value, electrostatic attraction between the dissociated carboxylic groups and the pyridinium cations were weakened. This result was further confirmed by comparison of the IR spectra for 2-AP/1 at different pH values. As shown in the Supporting Information, Figure S3, the wavenumber shifts of the carbonyl group in compound 1 changed on increasing or decreasing the pH value, thus indicating a change in the equilibrium between the free and dissociated carboxylic groups, which were the donors in hydrogen bonds and electrostatic attractions, respectively. Apparently, such an equilibrium played a critical role in the gelation process. The gelation process was further confirmed by gelation studies of control molecules under the same condition as 2AP/1. As shown in Figure 1 f, compound 2, which contained both free and dissociated carboxylic groups at pH 3–5.5, could form a hydrogel with 2-AP, whereas molecules 3 and 4 could not. At the same time, when control molecule 5 replaced 2-AP to study its gelation with compound 1, no hydrogel was observed, thus suggesting that the existence of the aromatic plane was also needed for the formation of a supramolecular hydrogel. This result may be due to effective packing of the pyridium rings, which could better assist the formation of fibrous structures, as we reported previously.[6b]
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However, the gelation behavior of 2-AP/1 was not clearly affected upon the addition of trinitrofluorenone (TNF), which was strongly electron-deficient and widely used to form CT interactions with all sorts of electron-rich moieties.[18] As shown in Figure 4 e, the color changed from colorless to red on the introduction of TNF to the 2-AP/1 gel system, which revealed the formation of CT interactions between TNF and the anthracene groups.[19] This result was further confirmed by a new CT peak at 550 nm in the absorption spectrum and by a quenching effect in the emission spectrum (Figure 4 c,d). Rheology studies showed that the gel was a medium-strength physical gel because the G’ and G’’ values remained constant after the first 18 min gelling time (see the Supporting Information, Figure S5 a). The fact that G’ was around 10-times higher than G’’ (see the Supporting Information, Figure S5 b), thus revealing that it had a weaker elastic character than 2-AP/1. TEM images indicated that the 2-AP/1/TNF system formed long nanofibrous structures (Figure 4 f). These results implied that this supramolecular hydrogel had the potential to carry electron-deficient guest molecules.
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Discover diffractometer (l = 0.15406 nm). The Bragg peaks were extracted from the XRD data and the layer thickness (d) was obtained according to the Bragg equation, d = l/2sinq; for the sample preparation, the hydrogel was placed onto a silicon plate and dried in air before the tests. Synthesis of 1-(11-(Anthracen-2-ylmethoxy)-11-oxoundecyl)pyridinium Bromide (2-AP) The synthesis of 2-AP was performed according to the procedure described in Ref. [6a]. M.p. 145–147 8C; 1H NMR (300 MHz, [D6]DMSO): d = 9.11 (d, J = 6.9 Hz, 2 H; pyridinium-H), 8.60 (t, J = 7.2 Hz, 1 H; pyridinium-H), 8.57 (s, 1 H; anthracene-H), 8.56 (s, 1 H; anthracene-H), 8.15 (dd, J = 7.2, 6.9 Hz, 2 H; pyridinium-H), 8.07 (m, 4 H; anthracene-H), 7.50 (m, 3 H; anthracene-H), 5.28 (s, 2 H; OCH2), 4.57 (t, J = 7.5 Hz, 2 H; NCH2), 2.39 (t, J = 7.2 Hz, 2 H; CH2CO), 1.84 (m, 2 H; CH2), 1.54 (m, 2 H; CH2), 1.19 ppm (m, 12 H; CH2); 13C NMR (75 MHz, [D6]DMSO): d = 173.33 (C=O), 145.91, 145.16, 128.92 (5 C; pyridinium-C), 133.86, 131.88, 131.81, 131.18, 131.07, 128.51, 128.46, 127.06, 126.59, 126.42, 126.19, 126.16, 126.02 (14 C, anthracene-C), 65.95, 61.12, 33.95, 31.15, 29.16, 29.11, 29.05, 28.85, 28.76, 25.78, 24.93 ppm; MS (ESI+): m/z: 454. Synthesis of 1-[11-(Benzyloxy)-11-oxoundecyl]pyridinium Bromide (5) The synthesis of 5 was performed according to the procedure described in Ref. [6a]. M.p. 72–74 8C; 1H NMR (400 MHz, CDCl3): d = 9.47 (d, J = 6.8 Hz, 2 H; pyridinium-H), 8.48 (t, J = 7.6 Hz, 1 H; pyridinium-H), 8.11 (dd, J = 7.6, 6.8 Hz, 2 H; pyridinium-H), 7.27 (m, 5 H; phenyl-H), 5.04 (s, 2 H; OCH2), 4.93 (t, J = 7.2 Hz, 2 H; NCH2), 2.28 (t, J = 7.6 Hz, 2 H; CH2CO), 1.98 (m, 2 H; CH2), 1.53 (m, 2 H; CH2), 1.23 ppm (m, 12 H; CH2); 13C NMR (100 MHz, CDCl3): d = 173.79 (C=O), 145.28, 145.23, 128.63 (5 C, pyridinium-C), 136.17, 128.59, 128.25, 128.17 (6 C, phenyl-C), 66.12, 62.12, 34.35, 32.07, 29.28, 29.19, 29.08, 29.06, 26.07, 24.95 ppm; MS (ESI+): m/z: 354.
Conclusions We have developed a supramolecular hydrogel based on a pyridinium-functionalized anthracene and a polyanion. In the hydrogel, the pyridinium-functionalized anthracenes assembled into nanofibers driven by p-stacking interactions and the polyanions were used as a cross-linker to combine the fibers together, thereby generating a supramolecular hydrogel primarily promoted by electrostatic interactions between the negative charge of the polyacids and the positive charge on the surface of the nanofibers. Our results demonstrated that the structure of the supramolecular assembly of a simple amphiphilic molecule (like 2-AP in our case) could be readily manipulated by varying the assembly conditions. In addition, such amphiphile-based supramolecular hydrogels showed multi-response properties, which could find potential applications in drug delivery, tissue engineering, and cosmetics.
Synthesis of 2,4,7-Trinitrofluorenone (TNF) The synthesis of TNF was performed according to the procedure described in Ref. [18c]. M.p. 170–172 8C; 1H NMR (300 MHz, [D6]DMSO): d = 8.99 (d, J = 2.1 Hz, 1 H; 3-H), 8.61 (m, 2 H; 6-H, 7-H), 8.43 (d, J = 2.1 Hz, 1 H; 1-H), 8.19 ppm (d, J = 8.4 Hz, 1 H; 5-H); 13C NMR (75 MHz, [D6]DMSO): d = 186.42, 149.67, 148.90, 144.80, 143.35, 138.86, 137.86, 136.17, 130.78, 128.01, 125.99, 122.20, 119.00 ppm; MS (EI+): m/z: 315.
Acknowledgements The research was supported by the US NSF (CHE-1307319), the University of South Carolina, and the China Scholarship Council (200001). B.S. is grateful for financial support from the International Science & Technology Cooperation Program of China (2010DFB60840), the National Key Program for Basic Research (2010CB126105), and the National Nature Science Foundation of China (21132003).
Experimental Section NMR spectra were recorded on Varian Mercury 300/400 spectrometers in CDCl3 and [D6]DMSO. Mass spectra were recorded on Micromass QTOF and Finningan TSQ spectrometers in positive-ion mode. UV/Vis spectra were recorded on Agilent Technologies 95–03 spectrometers. Fluorescence spectra were recorded on a Varian Cary Eclipse spectrophotometer. IR spectra were recorded on a PerkinElmer Spectrum 100; for the sample preparation, the hydrogel was freeze-dried before the tests. TEM measurements were performed on a Hitachi H8000 electron microscope operating at an acceleration voltage of 120 kV; for the sample preparation, a small amount of the hydrogel was placed onto a copper grid and negatively stained with a uranyl acetate solution before the tests. AFM was performed in air on a NanoScope IIIA MultiMode AFM (Veeco); for the sample preparation, a small amount of the hydrogel was placed onto a silicon plate and dried in air before the tests. Powder X-ray diffraction spectroscopy was performed on a BrukerD8
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Received: June 1, 2014 Revised: June 14, 2014 Published online: August 5, 2014
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