A water-soluble pillar[6]arene: synthesis, host–guest chemistry, controllable self-assembly, and application in controlled release

Abstract

A new water-soluble pillar[6]arene was successfully prepared. It complexed with a sodium p-hydroxybenzoate derivative to form a supra-amphiphile. The controllable self-assembly and application in controlled release of this supra-amphiphile in water were investigated.

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Self-assembly of amphiphiles, a class of fascinating molecules bearing both hydrophilic and hydrophobic groups, is a natural phenomenon in biological systems.¹ Inspired by this, scientists have devoted considerable efforts to the design and fabrication of artificial supramolecular structures from self-assembly of amphiphiles for their wide application in chemistry, biology, and materials science.² Different from the irreversible covalent amphiphiles, supra-amphiphiles are amphiphiles that are formed on the basis of noncovalent interactions, which may include π–π interactions, hydrogen bonding, charge-transfer interactions, and electrostatic interactions.³ Because supra-amphiphiles are synthesized by multiple weak and therefore reversible interactions, they are responsive and tunable building blocks for soft matter. Until now, supra-amphiphiles based on macrocycles, such as crown ethers, cyclodextrins, calixarenes and cucurbiturils have gained great progress.⁴

Pillar[n]arenes, mainly including pillar[5]arenes^5,6 and pillar[6]arenes,⁷ are a new class of macrocycles, which consist of hydroquinone units linked by methylene (–CH₂–) bridges at their 2,5-positions. Their syntheses, conformational mobility, derivatization, host–guest complexation, self-assembly in different solvents and applications have been actively explored recently.^5–7 They have been described as “fascinating cyclophanes with a bright future”.^7g The properties and applications of pillar[5]arenes have been widely reported. However, there are only a few investigations on pillar[6]arenes, which may have some unique supramolecular properties and applications since they have larger cavities than pillar[5]arenes.^7c,f

Herein, we report the synthesis of a new water-soluble ionic liquid pillar[6]arene (WILP6) (Scheme 1). Its molecular recognition with sodium p-hydroxybenzoate (G1) was investigated. Such water-soluble recognition motif has not only high binding strength but also pH-responsiveness. Interestingly, the encapsulation of amphiphlic molecule G2 by WILP6 changed the self-assembly morphology from micelles based on G2 to vesicles based on WILP6⊃G2. Due to the pH-responsiveness of G2, the vesicular structure based on WILP6⊃G2 collapsed with a decrease of the solution pH. This pH-responsive aggregation behaviour was further used in the controlled release of calcein.

Scheme 1 Synthetic route to water-soluble ionic liquid pillar[6]arene WILP6, crystal structure of compound 1, and cartoon representation of WILP6 and G2.

WILP6 was synthesized in three steps as described in Scheme 1.^5k The X-ray single crystal structure of pillar[6]arene 1 confirmed its pillar structure. The complexation of WILP6 with G1 was first studied by ¹H NMR spectroscopy. When one equiv. of G1 was added into a D₂O solution of WILP6 (1.00 mM), the signals related to protons H_b and H_c on G1 shifted upfield significantly as a result of the host–guest complexation between WILP6 and G1 (spectra b and c in Fig. 1). The resonance peak related to protons H_a of WILP6 disappeared after complexation caused by broadening effects (spectra a and b in Fig. 1).^7d The reason is that protons H_a on WILP6 and –COO⁻ on G1 form hydrogen bonds.⁸

Fig. 1 Partial ¹H NMR spectra (400 MHz, D₂O, room temperature) of: (a) WILP6 (1.00 mM); (b) G1 (1.00 mM) and WILP6 (1.00 mM); (c) G1 (1.00 mM).

For the estimation of the association constant (K_a) of the complexation between WILP6 and G1, fluorescence titration of WILP6 with G1 was conducted at room temperature in water. As shown in Fig. S9,† the quenching of fluorescence intensity was found to be significant upon gradual addition of G1. A mole ratio plot based on the fluorescence titration experiments demonstrated that the complexation between WILP6 and G1 had a 1 : 1 stoichiometry (Fig. S10, ESI†). Additionally, the K_a value was calculated to be (3.21 ± 0.82) × 10⁶ M⁻¹ by using a non-linear curve-fitting method. We speculate that the formation of the complex WILP6⊃G1 was mainly driven by multiple electrostatic interactions, hydrophobic interactions, and π–π stacking interactions between the benzene rings on the pillar[6]arene host and G1 in water. The extremely high binding affinity of this host–guest system should be attributed to the cooperativity of these noncovalent interactions. We also used ITC method to investigate the host–guest property between WILP6 and G1. The ITC results demonstrated that the complexation between WILP6 and G1 had a 1 : 1 stoichiometry with an association constant about (1.63 ± 0.03) × 10⁶ M⁻¹ (Fig. S12, ESI†), which is close to the above mentioned value obtained from the fluorescence titration.

The molecular recognition of WILP6 with G1 in water not only has high binding strength but also pH-responsiveness. ¹H NMR studies (Fig. S11, ESI†) provided convincing evidence for the pH-responsive complexation between WILP6 and G1. When the pH of the complex solution was adjusted to 4.0, the carboxylate group on G1 was protonated to the water-insoluble carboxylic acid group. The guest precipitated from the solution, resulting in the disappearance of the signals related to the protons on the guest (Fig. S11, spectrum c, ESI†). While the water-soluble host WILP6 still stayed in D₂O with their protons' resonances existed in the ¹H NMR spectrum. On the contrary, when the solution pH was returned to 7.4, the insoluble –COOH unit was deprotonated and G1 was soluble in water again. The peaks related to the protons on G1 shifted upfield and exhibited remarkable complexation-induced broadening effects again (Fig. S11, spectrum d, ESI†), indicating the reformation of WILP6⊃G1. These results demonstrated that the association and disassociation processes of complex WILP6⊃G1 were reversibly controlled by adjusting the solution pH.

After the establishment of the new WILP6/G1 recognition motif in water, we further applied it to construct a supra-amphiphile⁹ and utilize it in controllable self-assembly. G2 itself is an amphiphilic molecule that contains a long alkyl chain as the hydrophobic part and a benzoate unit as the hydrophilic part. The solubility of G2 in water was very poor, so the conductivity of the solution of G2 was close to pure water. However, when we added WILP6 into the solution of G2, WILP6 complexed with G2 and induced G2 to dissolve in water. The critical aggregation concentration (CAC) of WILP6⊃G2 was calculated to be (3.44 ± 0.21) × 10⁻⁴ M using concentration-dependent conductivity (Fig. S13, ESI†). As shown in Fig. S14,† the Tyndall effect could be observed when the concentration of WILP6⊃G2 was 5.00 × 10⁻⁴ M, indicating the formation of self-assembled aggregates for WP6⊃G2 at this concentration. However, due to the poor solubility of G2, the Tyndall effect could not be observed for free G2.

Transmission electron microscopy (TEM) experiments assisted in the visualization of the nano-structures of G2 and WILP6⊃G2. As shown in Fig. 2a, only a few solid spherical structures formed by G2 along with an average diameter about 8.0 nm were observed due to its poor solubility. The diameter is close to the length of two G2 molecules, confirming the formation of solid micelles. Upon addition of WILP6, a supra-amphiphile formed on the basis of the novel recognition motif, resulting in the significant changes in the aggregation structures. Vesicles with an average diameter about 85 nm were observed (Fig. 2b). It should be noted that the extended length of the WILP6⊃G2 complex is about 5.0 nm. The thickness of the hollow vesicles was calculated to be about 10 nm from the TEM image of WILP6⊃G2 (Fig. 2b, inset), which corresponds to the extended length of two WILP6⊃G2 complexes, suggesting that the vesicles have a bilayer wall. From these data, it was known that the packing structure of the complex in the membrane of the vesicles formed by G2 and WILP6 was in an antiparallel packing pattern. Interestingly, the intermediate state from micelles to vesicles was revealed through TEM as shown in Fig. S15.† The above demonstrated pH responsiveness of the complexation between WILP6 and G1 was used to control the aggregate nanostructure from WILP6⊃G2 by simply changing the solution pH. As shown in Fig. 2c, a precipitate with irregular structure appeared by adjusting the pH to 4.0. Moreover, when the pH was slightly higher than 7.0, vesicles rather than micelles formed in solution again with the same thickness of the membrane as shown in Fig. 2d, proving the pH-responsive self-assembly behaviour of WILP6⊃G2.

Fig. 2 TEM images of: (a) G2; (b) WILP6⊃G2; (c) WILP6⊃G2 when the solution pH is 4.0; (d) WILP6⊃G2 when the solution pH is 7.4. Insets are TEM images of the corresponding samples stained by osmium tetroxide.

A mechanism was proposed to explain why the shape of G2 aggregates transformed from micelles to vesicles after its complexation with WILP6 (Fig. 3). The nano-assembled structure of the aggregates formed by amphiphiles is determined by the curvature of the membrane. When the amphiphilic guest G2 is dissolved in water, the hydrophobic part tends to aggregate while the hydrophilic part favors staying in water, generating micelles. Accompanied with the addition of WILP6, the hydrophilic head of G2 containing the water-soluble carboxylate group threaded into the cavity of WILP6 driven by electrostatic interactions, hydrophobic interactions and π–π stacking interactions, forming a supra-amphiphile. Due to the steric hindrance and the electrostatic repulsion generated upon the insertion of the WILP6 molecules, the formation of a vesicular structure with low curvature is obtained.⁹ When the solution pH was adjusted to 4.0, G2 precipitated from the solution, so the vesicles formed by WILP6⊃G2 collapsed. Therefore, self-assembly of this host–guest system can be controlled by simply changing the solution pH.

Fig. 3 The illustration of the aggregate transformation from micelles based on G2 to vesicles based on WILP6⊃G2.

The pH-responsive self-assembly of this host–guest system was further verified by dynamic light scattering (DLS). As shown in Fig. 4a, the main diameter distribution of the micelles formed from G2 alone is around 8.3 nm, which is in agreement with the corresponding TEM image shown in Fig. 2a. Upon addition of WILP6, the diameter of the aggregates formed by WILP6⊃G2 increased to 85 nm (Fig. 4b), also in accordance with the corresponding TEM image shown in Fig. 2b. By adjusting the solution pH to 4.0, the diameter of the aggregates became 2.4 nm, in agreement with the size of WILP6. What's more, there existed a few large-size aggregates, indicating the formation of a precipitate (Fig. 4c). On the other hand, by adjusting the pH of the solution to 7.4 again, the diameter of the aggregates changed to be about 100 nm due to the formation of the vesicles again (Fig. 4d). These DLS experiments confirmed the transformations of the micelle structure to the vesicular structure, then to the irregular structure, and lastly back to the vesicular structure, providing convincing proof to support the pH-responsive self-assembly phenomena.

Fig. 4 DLS results of: (a) G2 (5.00 × 10⁻⁴ M); (b) WILP6⊃G2 ([WILP6]₀ = [G2]₀ = 5.00 × 10⁻⁴ M), pH = 7.4; (c) WILP6⊃G2 when the solution pH is 4.0; (d) WILP6⊃G2 when the solution pH is resumed to 7.4.

The transformation between micelles formed by G2 alone and vesicles formed by the host–guest complex WILP6⊃G2 was then utilized for controlled release.¹⁰ The vesicles were expected to encapsulate hydrophilic guest molecules within their interior at neutral or weakly basic condition and release the guest molecules in response to a decrease in pH. With this in mind, water-soluble calcein as a hydrophilic fluorescent guest was used to investigate the pH-responsive release. As shown in Fig. S16,† at the pH of 7.4, almost no fluorescence intensity of WILP6⊃G2 could be observed corresponding to the characteristic absorbance of calcein, indicating that the calcein molecules were stably located in the vesicles. By adjusting the solution pH to acidity, the release of calcein molecules from the inside of vesicles was achieved accompanied by an increase in the fluorescence emission. This phenomenon could be explained by considering a pH-triggered vesicle to irregular solid–structure transition. The decrease of pH resulted in the collapse of the vesicles into irregular solid-structures with concomitant release of the encapsulated calcein molecules (Fig. 5).

Fig. 5 The illustration of pH-responsive release of calcein molecules from the vesicles based on WILP6⊃G2.

In summary, we have successfully synthesized the first water-soluble ionic liquid pillar[6]arene (WILP6), and investigated the complexation between WILP6 and sodium p-hydroxybenzoate G1 in water. WILP6 and G1 form a stable complex with an extremely high association constant of (3.21 ± 0.82) × 10⁶ M⁻¹ mainly driven by electrostatic interactions, hydrophobic interactions and π–π stacking interactions. The association and disassociation processes of this complex could be reversibly controlled by changing the solution pH. This novel recognition motif was further used to control the aggregation of a complex between WILP6 and an amphiphilic sodium p-hydroxybenzoate acid derivative (G2) in water. The transformations between solid micelles based on G2 and vesicles based on WILP6⊃G2 were realized by adjusting the solution pH due to the pH-responsiveness of G2. The controlled release of calcein dye molecules from the vesicles was achieved by the collapse of the vesicles upon changing the solution pH to acidity. The present highly efficient recognition motif in aqueous media will find applications in the successful fabrication of large supramolecular systems. Their great versatility and flexibility are very important for future applications.

We acknowledge the National Natural Science Foundation of China (21202145 and J1210042) and the China Postdoctoral Science Foundation (2013M541767).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Determination of association constant, UV-Vis spectra, TEM images, and other materials. CCDC 946657. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra46681g

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