Roles of both amines and acid in supramolecular hydrogel formation of tetracarboxyl acid-appended calix[4]arene gelator

Abstract

A tetracarboxylic acid-appended calix[4]arene derivative 1 which were insoluble in water could form a supramolecular hydrogel upon addition of both amines and HCl. The added amines such as primary, secondary and tertiary amines efficiently acted as base to enhance the solubility of gelator. The stability of supramolecular gels is strongly dependent to the number of amino groups involved in the amines and the number of equivalents of amines added, relative to the equivalents of carboxylic acid groups in the calix[4]arene. In addition, the sequence of addition of both the amines and acid is critically important to form the supramolecular hydrogel from a gelator which, without any other additives, is insoluble in pure water. Furthermore, chiral gelator 1 formed a right-handed chiral arrangement of the amide chromophore in the presence of achiral amines.

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Introduction

Supramolecular gels have attracted considerable research interest due to their potential applications in diverse fields such as molecular electronics of light emission, light-energy conversion, catalysis, cosmetics and drug delivery systems.^1–6 For the self-assembly of nano- and micro-structures from low-weight molecules, the design of starting molecules as well as preparation method are important steps.^7–12 For the preparation of supramolecular gels, gelators with hydrophilic and hydrophobic parts within a single molecule are among the most widely used building blocks and have been the core topics of colloid chemistry.

Besides the traditional hydrogelators, which contains one head group as a hydrophilic part and one or two alkyl chains, new hydrogelators such as bola- and gemini amphiphiles have been developed.^13–17 To realize additional functions, more complex gelators have been designed which exhibit relatively low stability in water. Thus, the introduction of additional functional groups in macrocycle-based hydrogelators such as calixarenes, crown ether or pillarene, which are well-known as receptor molecules, has been limited due to poor solubility in water.^18–23 For example, the gelators possessing the calix[4]arene moiety as a core could not usually form gel in pure water, but could form gel in a mixed organic solvent and water. In particular, the 1,3-alternated calix[4]arene moiety has been useful as a core to form polymeric structures by self-assembly,²⁴ due to its simple synthetic method without the need for various functional groups. Although diverse structures of hydrogelators have been reported,^25–30 there have been few reports for simple preparation methods for hydrogels formation upon addition of both using amines and acids as additives to improve solubility in water.^31,32

In recently, there were several reports on pH responsive hydrogels.^33–37 Both acids and bases in the previously reported papers were used to reversibility in sol–gel transition, because gelators can form gel without both of acid and base. For example, dual-responsive supramolecular hydrogels reported by Chun Wang et al. has been prepared from PEG-grafted colypolymers and cyclodextrin. In particular, Ballester et al. have reported pH responsive hydrogel formation by the supramolecular system calix[4]pyrrole derivative in the presence of tetramethylammonium (TMA) cation. The gel formation of calix[4]pyrrole derivative is due to that the TMA acted as connector between the two shallow bowls formed by the pyrrole units belonging to adjacent calix[4]pyrrole molecules. In addition, Liu et al. have reported supramolecular binary hydrogels from tetra-proline modified calixarenes and amino acids. The gel formation of tetra-proline modified calixarene is due to by coulombic forces between gelator and amino acids, in which amino acids did not acted as base.

With this background in mind, we have synthesized tetracarboxylic acid-appended calix[4]arene derivative (1) and tested its ability to form gel by controlling its solubility upon addition of base, as amines, and acid. Compound 1 showed completely different manners of gel formation behaviour depending on the sequence of the addition of amines and acid. Furthermore, the number of amino residues within the amine molecule and the total equivalents of amines played an important role in supramolecular hydrogel formation (Scheme 1).

Scheme 1 Synthetic route of compound 1.

The gelation system used in this study is related to one previously used by us, in which the core component is a 1,3-alternated calix[4]arene with carboxylic acids groups (Fig. 1). Compound 1 itself does not self-assemble in pure H₂O, due to insolubility, but calix[4]arene derivatives possessing carboxylic acids groups could gelate in organic solvent and also in a mixture of organic solvents/water in the presence of metal ions. In this study, we investigated the gelation of 1 using various amines as a base and HCl as an acid to forms gel in pure H₂O. Surprisingly, upon addition of amines, compound 1 dissolved, and then addition of HCl solution to the solution of 1 and amine gave rise to immediate gelation. Instantaneous, in situ gelation is very rare. Most gelators require either heating or sonication in order to allow solubilization to occur prior to nanofiber assembly. This system is quite different from previously reported acid–base complexes.

Fig. 1 Chemical structures of 1 and amines derivative.

Results and discussion

The ability of 1 to form hydrogels was evaluated with a library of different amines. Compound 1 (10 mM) was mixed with amines (10–40 mM) and HCl (5–40 mM) in H₂O. For gelation testing, a constant weight of 1 was added to H₂O, which gave an insoluble suspension. When any of the amines were added, compound 1, dispersed in H₂O, was completely dissolved without heating or sonication. Then, when HCl was slowly added to the solution of 1 and amines, the mixture of 1-amine could be gelated in H₂O (Fig. 2a–c). For example, when diamines were used, 1 was complete dissolved upon addition of 2 equiv. of diamiopropane, diaminobutane and diaminopentane. On the other hand, with the addition of monoamines, such as aminopropane, aminobutane and aminopentane, gel formation of 1 required twice the molar amount of diamines. Furthermore, use of tert-amines, such as 1,4-diazabicyclo[2,2,2]octane (DABCO), N,N′-dimethylpiperazine and N,N,N′,N′-tetramethylethylenediamine (TMEA), required the same molar amount as needed with diamines. Thus, the gel formation process was to occur strongly dependent to the molar equivalents of amines added to the gelator. In addition, the basicity of the amines such as primary amines, secondary amines, and tert-amines was not a factor in gel formation. This gel formation is quite different from reported previously papers.^33–37

Fig. 2 These samples were prepared by two different addition sequences of amines and HCl; photographs of (a) 1 (4.0 wt%), (b) 1 + diaminobutane (2 equiv.), (c) 1 + diaminobutane (2 equiv.) + HCl (2 equiv.), (d) 1 (4.0 wt%), (e) 1 + HCl (2 equiv.) and (f) 1 + HCl (2 equiv.) + diaminobutane (2 equiv.) in H₂O.

In a converse experiment, HCl was first added to 1 dispersed of H₂O, which did not result in dissolution. Then, diaminobutane was added to 1 dispersed in HCl solution, and this mixture was also insoluble (Fig. 2d–f). Thus, the sequence for addition of amines is important to dissolve 1. The solubility of 1 was dramatically enhanced because, in the presence of amine, an acid proton was dissociated, leaving the carboxyl group as the anion. Then, upon addition of HCl to the 1-amine solution, molecules of 1 self-assembled with each other, and forming a supramolecular gel was formed. This approach will be important for the fabrication of soft supramolecular materials.

When 1 was mixed with the ammonium salt prepared from amines and HCl in water, 1 was insoluble. As we mentioned in the manuscript, 1 was soluble in the presence of base in water, and then, forming gel upon addition of HCl, due to the network structural formation by intermolecular hydrogen-bonding interaction.

To investigate the role of both amines and HCl in the gelation process, the –COOH and –COO⁻ vibrational bands of 1 were observed by FT IR spectroscopy (Fig. 3 and Tables S1–S3†). When compound 1 was partially dissolved in H₂O with 1.0 equiv. of diaminobutane, the IR spectrum of 1 showed the presence of 52% for –COOH and 48% for –COO⁻. On the other hand, the –COOH vibrational band of 1 was completely changed into the –COO⁻ vibrational band in the presence of more than 2.0 equiv. of amine (Fig. S1†). These results indicate that diaminobutane acted as a base to abstract a proton from the –COOH of 1. Furthermore, we observed the FT IR spectra of hydrogels at different concentrations of HCl in the presence of 2.0 equivalents of diaminobutane (Fig. 3). Ten percent of the –COO⁻ species of hydrogel remained after addition of 1.0 equiv. of HCl, which gave an unstable gel as shown in Fig. 3b. On the other hand, the –COO⁻ vibrational band in a stable hydrogel disappeared when 2.0 equiv. of HCl were added (Fig. 3d), indicating that the presence of the –COOH species of gelator 1 was important to form gel, due to the intermolecular hydrogen-bonding interaction.

Fig. 3 IR spectra of 1 (a) without and with (b) 1.0 equiv., (c) 1.5 equiv., (d) 2.0 equiv. and (e) 4.0 equiv. of HCl in the presence of diaminobutane (2 equiv.).

In the case of aminobutane as the monoamine, the IR spectra of 1 showed distribution of the unionized and ionized forms to be 73% for –COOH and 27% for –COO⁻ in the presence of 1.0 equiv. of aminobutane (Fig. S2†), in which gelator 1 was insoluble. Compound 1 was complete dissolved upon addition of 4.0 equiv. of aminobutane in H₂O. With 4.0 equiv. of aminobutane, the –COOH vibrational band of 1 completely disappeared, accompanied by the appearance of the –COO⁻ vibrational band. The results are strongly suggestive of a 1 : 1 acid–base reaction being responsible for solubility. Furthermore, 1 could also gelate H₂O upon addition of 4.0 equiv. of aminobutane and 2.0 equiv. of HCl. According to the IR data, with addition of HCl, the spectrum of 1 changed from the –COO⁻ species of 1 into the corresponding –COOH species (Fig. S3†), clearly indicating that the re-protonation and generation of the –COOH group of 1 acted as a driving force to form gel, while the acid–base complex formation between gelator 1 and the amine was not responsible for gelation. We also observed gel formation of 1 upon addition of such tert-amine derivatives as DABCO, N,N′-dimethylpiperazine and TMEA (Fig. S4 and S5 and Table S3†). The tert-amine derivatives gave the same outcome as that obtained with diaminobutane.

To examine the relationship between the gel strength and the amount of added amines, we monitored the sol–gel transition temperatures (T_gel) of gels formed with different ratios of 1 to amines in the presence of 2.0 equivalent of HCl. The experiment was carried out with a fixed concentration of 1 and the amines were added at varying concentration over the range of 1 equiv. to 4 equiv. As shown in Fig. S6 and S7,† the sol–gel transition temperature (T_gel) by heating increased up to the addition of 4.0 equivalents of diaminobutane, and then, as additional amine was added, no further increase in thermal stability was observed. Furthermore, the T_gel values of gel upon addition of diaminobutane were higher than those obtained upon addition of monoaminobutane, which is attributed to the better solubility of 1 with the diamino compound.

We performed circular dichroism (CD) measurements to observe the effect of the achiral amines on the assembly of the chiral 1. The CD spectrum of gel (0.1 mmol) exhibited a negative signal for the first Cotton effect at 225 nm (Fig. S8†), indicative of a right-handed chiral arrangement of the amide chromophores. The CD intensities of gel were enhanced in the same direction by increasing concentrations of diaminobutane (1–4 equiv.). These findings suggested that increasing the amount of achiral amine induced chiral organization of 1 with the intermolecular hydrogen-bonding interaction between amide and amide groups.

To insight gain into the morphologies of gels formed by various amines, we observed the morphologies of xerogels prepared with diaminobutane, aminobutane and DABCO by FE-SEM (Fig. 4 and S9†). We observed two different morphologies of the xerogel formed with 1.0 equiv. of diaminobutane, which was in a partially gelled condition. The xerogel made with 1.0 equivalent of diaminobutane showed large sized particles with 2–5 μm diameters and a short connecting fiber structure of ca. 100 nm diameter in the nanoparticle. In contrast, the xerogel made with 4.0 equivalent of diaminobutane showed fiber structures connecting sphere to sphere, as for a pearl necklace. In addition, SEM images of the xerogels prepared with aminobutane or DABCO were the same as that obtained with diaminobutane. Thus, we propose a gel formation mechanism based on SEM observation as well as gel preparative method used in this study (Scheme 2). First, the acidic proton of the –COOH of gelator 1 in aqueous solution are dissociated upon addition of amine, simultaneously producing the ammonium derivative as the cationic species. In the first stage, the carboxyl anion of gelator 1 induces the spherical structures. Secondly, the addition of HCl re-protonates the acidic protons to the carboxylate anions of gelator 1 in aqueous solution, which generates a neutral molecule. In this stage, gelator 1 forms the intermolecular hydrogen-bonding interactions, which induce gel formation. In addition, the spherical particles are connected from sphere to sphere by the intermolecular hydrogen-bonding interactions.

Fig. 4 SEM images of (a) 1 + diaminobutane (1 equiv.) + HCl (2 equiv.), (b) hydrogel 1 + diaminobutane (4 equiv.) + HCl (2 equiv.), (c) 1 + aminobutane (1 equiv.) + HCl (1 equiv.), (d) hydrogel 1 + aminobutane (2 equiv.) + HCl (2 equiv.), (e) 1 + DABCO (1 equiv.) + HCl (2 equiv.) and (f) hydrogel 1 with DADCO (2 equiv.) and HCl (2 equiv.). The samples (a, c and e) gave precipitations. In contrast, the samples (b, d and f) are formed gels.

Scheme 2 Systematic mechanism for dissociation and re-protonation of 1 upon addition of ammine and HCl to form gel.

The storage and loss modulus (G′ and G′′ respectively) were obtained from rheology to offer a means for assessing the behavior of the hydrogel with different amines under mechanical stress.^38,39 We examined the dynamic strain sweep in order to find the appropriate conditions for the dynamic frequency sweep of gels obtained with 4.0 equiv. of amines. Fig. S10† illustrates that G′ and G′′ were almost constant with the increase of frequency from 0.1 to 100 rad s⁻¹. G′ and G′′ of gels obtained with diaminobutane and DABCO were ca. 10-fold larger than that for the gel obtained with aminobutane. We attribute this observation to the complete dissociation of the COOH protons of 1 in the presence of the diaminobutane or DABCO, but this outcome does not depend on the basicity of the amines. Upon addition of amines, G′ is ca. 20 times larger than G′′ over the total range (0.1–100 rad s⁻¹), indicating that the gel is moderately tolerant to external force. Furthermore, the storage modulus (G′) and the loss modulus (G′′) showed a weak dependence, from 0.1 to 1.0% of strain (with G′ dominating G′′) (Fig. S10†; A-b, B-b, C-b). The observation of these met criteria indicates the sample exists as a gel. We also measured time-dependent oscillation for the gelation processes of gels with 4.0 equiv. of amines (Fig. S10†; A-c, B-c, C-c). Rapid increases in G′ and G′′ were both observed at the initial stage of the time sweep. A long, slow approach to a final pseudo-equilibrium plateau was then found to occur. The final value of G′ as the end of the experiment was almost one order of magnitude higher than that of G′′.

Experimental

Characterization

¹H and ¹³C NMR spectra were measured on a Bruker DRX 300 apparatus. IR spectra were obtained for KBr pellets, in the range 400–4000 cm⁻¹, with a shimadzu FT-IR 8400S instrument, and Mass spectra were obtained by a JEOL JMS-700 mass spectrometer. All fluorescence spectra were recorded in RF-5301PC spectrophotometer. The optical absorption spectra of the samples were obtained at 278–77 K using a UV-vis spectrophotometer (Thermo Evolution 600).

Electron microscopy

A field emission scanning electron microscope (Philips XL30 S FEG) was also used to observe the samples in which an accelerating voltage of 5–15 kV with an emission current of 10 μA was used. Prior to scanning electron microscopy (SEM) visualization, the hydrogel was also freeze-dried under vacuum at −35 °C.

Measurement of rheological properties

Hydrogels were analyzed for their rheological properties using a controlled stress rheometer (AR-2000ex, TA Instruments Ltd., New Castle, DE, USA). Throughout the experiments, a cone-type geometry of 40 mm diameter was used. By using a frequency of 0.1 rad s⁻¹, the dynamic oscillatory behavior was examined. In addition, the response was observed upon increased amplitude oscillations with up to 100% apparent strain on shear and having frequency sweeps at 25 °C (from to 100 rad s⁻¹, respectively). Finally, the unidirectional shear was examined at 25 °C with a shear-rate regime between 10⁻¹ and 10³ s⁻¹ using transient measurements in the mechanical spectroscopy routines.

Fluorescence lifetime measurements

By using a conventional laser system, the emission lifetimes were measured upon generation with an excitation source (420 nm output of a Spectra-Physics Quanta-Ray Q-switched GCR-150-10 pulsed Nd/YAG laser). The signals for the luminescence decay were obtained using a Hamamatsu R928 PMT, and the data was recorded on a Tektronix model TDS-620A (500 MHz, 2 GS s⁻¹) digital oscilloscope from which it was exponentially fit for analysis.

Preparation of gels

Compound 1 (8 mg, 4.0 wt%) added to H₂O (0.1 mL). Amines (1–4 equiv.) were added to disperse 1 of H₂O. Then, HCl (2 equiv.) was added to 1 + amine solution. The sample immediately was maintained at room temperature to form the gel.

Synthesis of compound 6

p-tert-Butyl phenol (100 g, 0.67 mol) and NaOH (1.20 g, 0.03 mmol) was dissolved in 37% formaldehyde (67.31 g, 67.31 mol). The reaction mixture was refluxed at 120 °C for 12 h. After the solution was cooled to room temperature, H₂O was removed in vacuo, and then, diphenyl ether (400 mL) and toluene (50 mL) were added. The reaction mixture was refluxed at 230 °C, again. The color of the reaction mixture changed to dark brown. Then, the crude product was recrystallized from ethyl acetate (200 mL) and was washed with acetic acid (100 mL) to give white crystalline solid in 56.79% yield. mp 343 °C; IR (KBr pellet) 3176, 2958, 2866, 1603, 1482, 1361, 1242, 1200, 1042, 871, 816, 783; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 10.36 (s, 4H), 7.07 (s, 8H), 3.52 (s, 4H), 1.23 (s, 36H); ¹³C NMR (75 MHz DMSO-d₆) δ ppm 147.62, 143.63, 126.96, 125.74, 34.52, 33.1, 31.2; ESI-MS: calculated for C₄₄H₅₆O₄ 648.42, found 648.40 [M + H]⁺; anal. calcd for C₄₄H₅₆O₄: C, 81.44; H, 8.70. found: C, 81.75; H, 8.59.

Synthesis of compound 5

A suspension of AlCl₃ (24 g, 180 mmol) and toluene (50 mL) was stirred in a 2 L two-necked roundbottom flask. The contents of the flask were poured into a suspension of compound 2 (20 g, 30.8 mmol), CH₂Cl₂ (500 mL). After the reaction mixture was stirred for 0.5 h, CH₂Cl₂ (100 mL) and 10% aqueous HCl (400 mL) solution were added to the reaction mixture in an ice bath. Finally, the reaction mixture was extracted with CH₂Cl₂ (3 × 200 mL), washed twice with water, and dried over anhydrous MgSO₄, and the solvent was removed in vacuo. The crude product was recrystallized from CH₂Cl₂/ethyl ether (1 : 30, v/v) to give a beige crystalline solid in 66.5% yield (8.7 g). mp 315 °C; IR (KBr pellet) 3160, 2935, 2870, 1594, 1465, 1448, 1244, 752; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 9.76 (br, 4H), 7.16 (d, 8H), 6.66 (t, 4H), 3.89 (s, 4H); ¹³C NMR (75 MHz DMSO-d₆) δ ppm 149.8, 129.2, 129.0, 121.7, 31.1; ESI-MS: calculated for C₂₈H₂₄O₄ 424.17, found 425.16 [M + H]⁺; anal. calcd for C₂₈H₂₄O₄: C, 79.22; H, 5.70. Found: C, 79.31; H, 5.65.

Synthesis of compound 4

Compound 3 (5.00 g, 11.78 mmol) and Cs₂CO₃ (38.4 g, 11.78 mmol) were suspended in dry acetone (250 mL) and added to the solution of ethyl 2-bromoacetate (11.81 g, 70.68 mmol) in dry acetone (25 mL). The reaction mixture was refluxed for an additional 4 h. After cooling to room temperature, the salt was filtered, and the solvent (acetone) was removed in vacuo. To the resulting pale yellow oil, 10% aqueous HCl (100 mL) solution and CH₂Cl₂ (100 mL) were added, and the organic layer was separated, washed twice with water, and dried over anhydrous MgSO₄, and the solvent was removed in vacuo. The crude product was recrystallized from CH₂Cl₂/n-hexane (1 : 30, v/v) to give a white crystalline solid in 45.5% yield (4.12 g). mp 118 °C; IR (KBr pellet) 3062, 2980, 2938, 1758, 1453, 1180, 1095, 1060, 769; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 7.07 (d, 8H), 6.65 (t, 4H), 4.17 (q, 8H), 3.96 (s, 8H), 3.79 (s, 8H), 1.23 (t, 12H); ¹³C NMR (75 MHz DMSO-d₆) δ ppm 169.9, 156.2, 133.8, 130.6, 122.7, 69.8, 60.7, 35.7, 14.5; ESI-MS: calculated for C₄₄H₄₈O₁₂ 768.31, found 791.25 [M + Na]⁺; anal. calcd for C₄₄H₄₈O₁₂: C, 68.74; H, 6.29. Found: C, 68.68; H, 6.34.

Synthesis of compound 3

A solution of compound 4 (2 g, 2.6 mmol) in the mixture of THF (40 mL) and EtOH (40 mL) was heated to reflux temperature. The reaction mixture was then added to aqueous KOH (1 mL, 26 mmol). After refluxing for 4 h, the organic solvents were removed in vacuo, and water (10 mL) was added. The remaining aqueous solution was acidified to pH 1 by addition of 6 N HCl. The resulting precipitate was filtered and washed with water. The precipitation was dried under vacuum to give compound 3 as a white solid in 79.7% yield (1.36 g). mp 303 °C; IR (KBr pellet) 3448, 3412, 3015, 2930, 1759, 1732, 1458, 1356, 1322, 1195, 1057, 767; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 12.48 (br, 4H), 7.12 (d, 8H), 6.69 (t, 4H), 4.12 (s, 8H), 3.83 (s, 8H); ¹³C NMR (75 MHz DMSO-d₆) δ ppm 169.5, 154.4, 134.7, 124.1, 122.7, 72.5, 34.1; ESI-MS: calculated for C₃₆H₃₂O₁₂ 656.16, found 695.25 [M + K]⁺, 679.50 [M + Na]⁺; anal. calcd for C₃₆H₃₂O₁₂: C, 65.85; H, 4.91. Found: C, 66.21; H, 4.97.

Synthesis of compound 2

To a suspension of compound 3 (0.2 g, 0.30 mmol) in toluene (10 mL) was added SOCl₂ (0.265 mL, 3.65 mmol). The reaction mixture was refluxed for 5 h and was cooled to room temperature. The solvent and unreacted SOCl₂ were removed to give 1,3-alternate calix[4]arene tetra-acid chloride. The 1,3-alternate calix[4]arene tetra-acid without purification was dissolved in CH₂Cl₂ (10 mL). Then, the solution of D-alanine methyl ester hydrochloride (0.179 g, 1.28 mmol) in CH₂Cl₂ (5 mL) and TEA (0.5 mL) was added and stirred for 12 h. After filtration of the reaction mixture, the solvent was removed in vacuo. The crude product was dissolved in CH₂Cl₂, washed twice with water, and dried over anhydrous MgSO₄, and the solvent was removed in vacuo. The crude product was recrystallized in CH₂Cl₂/ethyl ether (1 : 30, v/v) to give a withe solid in 63.3% yield (0.19 g). mp 243 °C; IR (KBr pellet) 3405, 2949, 1746, 1683, 1527, 1450, 1345, 1192, 1159, 1094, 1047, 916, 849, 776; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 7.15 (d, J = 7.43 Hz, 1H), 7.04 (t, J = 6.24, 6.24 Hz, 8H), 6.70 (t, J = 7.46, 7.46 Hz, 4H), 4.35 (dd, J = 14.32, 7.17 Hz, 4H), 3.95 (q, J = 14.15, 14.15, 14.12 Hz, 8H), 3.74 (d, J = 5.55 Hz, 8H), 3.70 (s, 9H), 1.32 (d, J = 7.27 Hz, 9H); ¹³C NMR (75 MHz, DMSO-d₆) δ ppm 172.9, 168.6, 154.5, 134.2, 124.7, 123.9, 67.6, 51.9, 51.8, 32.4, 17.3; ESI-MS: calculated for C₅₂H₆₀N₄O₁₆ 996.40, found 997.33 [M + H]⁺, 1019.50 [M + Na]⁺; anal. calcd for C₅₂H₆₀N₄O₁₆: C, 62.64; H, 6.07; N, 5.62. Found: C, 62.67; H, 6.02; N, 5.65.

Synthesis of compound 1

A solution of compound 2 (2 g, 2.0 mmol) in the mixture of MeOH (40 mL) was heated to reflux temperature. The reaction mixture was then added to aqueous NaOH (1 mL, 26 mmol). After refluxing for 4 h, the organic solvents were removed in vacuo, and water (10 mL) was added. The remaining aqueous solution was acidified to pH 1 by addition of 6 N HCl. The resulting precipitate was filtered and washed with water. The precipitation was dried under vacuum to give compound 1 as a white solid in 65% yield (1.23 g). mp 235 °C; IR (KBr pellet) 3399, 2935, 1738, 1660, 1537, 1454, 1249, 1191, 1159, 1093, 1049, 916, 848, 778; ¹H NMR (300 MHz, DMSO-d₆) δ ppm 12.78 (m, 4H), 7.20 (d, J = 6.84 Hz, 4H), 7.03 (d, J = 6.68 Hz, 4H), 6.88 (d, J = 7.51 Hz, 4H), 6.71 (t, J = 7.46, 7.46 Hz, 4H), 4.39 (m, 4H), 4.10 (q, J = 14.26, 14.26, 14.24 Hz, 8H), 3.66 (d, J = 5.23 Hz, 8H), 1.38 (t, J = 7.03, 7.03 Hz, 9H); ¹³C NMR (75 MHz DMSO-d₆) δ ppm 174.7, 168.6, 154.5, 134.2, 123.9, 124.7, 67.6, 51.3, 32.4, 17.0; ESI-MS: calculated for C₄₈H₅₂O₁₆ 940.34, found 940.71 [M + H]⁺; anal. calcd for C₄₈H₅₂O₁₆: C, 61.27; H, 5.57. Found: C, 61.21; H, 5.52.

Conclusions

In conclusion, we have demonstrated the formation of a supramolecular hydrogel from calix[4]arene-based derivatives possessing tetracarboxylic acid groups which were insoluble in water. We have also presented a mechanism to describe the gel formation. The calix[4]arene-based derivative possessing tetracarboxylic acid formed the supramolecular hydrogel upon addition of both amines and HCl. The added amines efficiently acted as base to enhance the solubility of gelator. The stability of supramolecular gels is strongly dependent to the number of amino groups involved in the amines and the number of equivalents of amines added, relative to the equivalents of carboxylic acid groups in the calix[4]arene. In addition, the sequence of addition of both the amines and acid is critically important to form the supramolecular hydrogel from a gelator which, without any other additives, is insoluble in pure water. Furthermore, chiral gelator 1 formed a right-handed chiral arrangement of the amide chromophore in the presence of achiral amines. We suggest that this approach may have applications in the development of evolvable, stimulus-responsive hydrogels.

Acknowledgements

This work was supported by grants from the NRF (2012R1A2A2A01002547 and 2012R1A4A1027750), Ministry of Education, Science and Technology and Environmental-Fusion Project (191-091-004), Korea. In addition, this work was partially supported by a grant from the Next-Generation BioGreen 21 Program (SSAC, grant#: PJ009041022014), Rural Development Administration, Korea.

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Footnote

† Electronic supplementary information (ESI) available: The synthesis recipe and experimental details, IR data, Photographs of another amine gels, SEM images, CD spetra, DSC data, Rheological data. See DOI: 10.1039/c5ra00685f

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