A pillar[5]arene/imidazolium [2]rotaxane: solvent- and thermo-driven molecular motions and supramolecular gel formation

Abstract

Based on the pillar[5]arene/imidazolium recognition motif, a [2]rotaxane was effectively prepared. Solvent/temperature triggered molecular motions of the pillar[5]arene ring on the imidazolium axle were successfully realized. By comparison of proton NMR spectra of the [2]rotaxane in different solvents, we found that if we increased the solvent polarity, the pillar[5]arene ring gradually moved away from the imidazolium part. In DMSO, we also could adjust the binding site of the pillar[5]arene ring by changing the temperature. Furthermore, in DMSO, the [2]rotaxane self-assembled to form a supramolecular gel, which showed multiple stimuli-responsiveness.

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Introduction

Molecular motions are quite common in biosystems and closely related to macroscopic motions.¹ Inspired by the dynamic and reversible nature of natural supramolecular assemblies, scientists have applied artificial supramolecular architectures to mimic and control molecular motions.² Rotaxanes, considered as a typical type of mechanically interlocked structures, are crucial precursors for the fabrication of advanced supramolecular architectures³ and have been widely used as building blocks to fabricate molecular shuttles and switches which show molecular motions induced by certain stimuli.⁴ Though many well-known molecular recognition motifs,⁵ such as crown ether/ammonium salt recognition motifs, have been applied to investigate rotaxane-based molecular motions, the introduction of new molecular recognition motifs to the realm of rotaxanes will no doubt expand the applications of rotaxanes and show some unique stimuli-responsiveness.⁶

Pillar[5]arenes,⁷ a new kind of macrocyclic host, were first reported in 2008 and have achieved quick development over the past five years.⁸ Though pillar[5]arenes have become among the most promising candidates to prepare supramolecular assemblies, such as supramolecular polymers,^8b micelles^8f and nano fibers,^8e their application in molecular motions still remains limited.^6c,7g,8h In our previous work, we found that a pillar[5]arene can complex an imidazolium salt with a high binding constant and form a [2]pseudorotaxane.⁹ Considering the rigid and pillar-like structures and host–guest properties of pillar[5]arenes, we wonder whether it is feasible to utilize this pillar[5]arene/imidazolium recognition motif to investigate pillar[5]arene-based reversible molecular motions. Here we designed and synthesized a pillar[5]arene-based [2]rotaxane and studied the temperature/solvent-controllable molecular motions of the pillar[5]arene ring on the imidazolium axle.

Results and discussion

As shown in Scheme 1, DEP5 (1,4-diethoxypillar[5]arene) and the semi-blocked rod-like component 1 were mixed together for two hours at low temperature (in an ice-salt bath), then 1-isocyanato-3,5-dimethylbenzene (2), which acted as the stopper, and dibutyltindilaurate (DBTDL) were added.¹⁰ This reaction gave [2]rotaxane 4 in quite a high yield, 92%. No dumbbell-shaped component 3 was observed in this reaction as monitored by TLC.

Scheme 1 Synthetic route to [2]rotaxane 4 and the corresponding dumbbell-shaped component 3 and their cartoon representations.

Considering the high binding capacity between pillar[5]arenes and imidazolium salts,⁹ we estimated that the imidazolium part and the adjacent methylene groups would be located in the cavity of the pillar[5]arene ring. Indeed, the ¹H NMR spectrum and 2D NOESY spectrum of [2]rotaxane 4 in chloroform-d confirmed our deduction (Fig. 1, S7 and S12†). In chloroform-d, we found that due to the shielding effect, the signals of protons on the imidazolium part and adjacent methylene protons H_m, H_b, H_c and H_d shifted upfield obviously (Fig. 1). For example, the signals of protons H_m shifted from 10.77 ppm to 8.11 ppm and the signal of protons H_b and H_c showed chemical shifts below even 0 ppm. These changes were quite in accordance with previously reported pillar[5]arene-based threaded structures with alkyl chains,^7h,8b,g,9 which meant that the imidazolium part and adjacent methylene groups were covered by the aromatic cavity of the pillar[5]arene ring. The NOESY spectrum of 4 in chloroform showed cross-signals between the aromatic protons of the pillar[5]arene ring and the benzyl protons H_p, indicating the formation of the host–guest inclusion complex (Fig. S12†).

Fig. 1 ¹H NMR spectra (400 MHz, chloroform-d, 25 °C) of 2.00 mM DEP5 (a), 2.00 mM [2]rotaxane 4 (b), and 2.00 mM dumbbell-shaped component 3 (c).

In order to confirm this mechanically interlocked structure, a polar solvent, DMSO-d₆, was used for proton NMR investigations. In DMSO-d₆, we observed that due to the shielding effect, the signals of protons on the alkyl chains displayed upfield chemical shifts, which indicated the formation of a mechanically interlocked structure (Fig. 2). From ¹H NMR spectra in DMSO-d₆, we found that the signals of protons H_g, H_h, H_i and H_j moved upfield and all had chemical shifts below 0 ppm, which meant these protons were located in the cavity of the pillar[5]arene ring. For example, H_g,j and H_h,i shifted from 1.10 ppm to −0.12 and −0.41 ppm, respectively. The NOESY spectrum of 4 in DMSO-d₆ showed obvious cross-signals between methylene protons H₁ and H₃ and phenyl protons H₄ of the pillar[5]arene ring and the signal of methylene protons H_k of the alkyl chain (Fig. S13†). Compared with the ¹H NMR spectrum of [2]rotaxane 4 in chloroform-d (Fig. 1), we found that in DMSO-d₆, the pillar[5]arene ring was on average located on the methylene groups close to the carbamic stopper instead of the imidazolium unit. A possible reason for this was that DMSO-d₆ could destroy the interactions between the pillar[5]arene ring and the imidazolium moiety and showed a strong solvophobic effect for the alkyl chain close to the stopper. From the difference between the binding sites, we thought that the addition of polar solvent molecules would not only destroy the hydrogen bonds between the pillar[5]arene ring and the imidazolium guest unit but also drag the imidazolium unit out of the pillar[5]arene cavity because the imidazolium guest unit is solvophilic when its counterion is the bromide anion. This result is similar to the findings of Gibson et al. with crown ether–polyurethane systems^11a–e and cyclobis(p-xylyleneparaquat)–polyurethanes.^11f Hence we thought that we could realize molecular motions through changing the solvent polarity.^2a,d,6c,8h,9

Fig. 2 ¹H NMR spectra (400 MHz, DMSO-d₆, 25 °C) of 2.00 mM DEP5 (a), 2.00 mM [2]rotaxane 4 (b), and 2.00 mM dumbbell-shaped component 3 (c).

We changed the volume ratio of chloroform-d/DMSO-d₆ to adjust the solvent polarity and control the molecular motion of the pillararene ring on the imidazolium axle. In Fig. 3, we found that as the solvent polarity increased due to the stepwise addition of DMSO-d₆ to chloroform-d, the signals of the protons H_b, H_c and H_d, which were close to the imidazolium unit, were all moved downfield. On the contrary, the signals of the protons near to the carbamic stopper, such as H_g, H_h, H_i and H_j, shifted upfield. For example, the signal of proton H_b moved downfield from −1.27 ppm (chloroform-d) to −0.58 ppm (chloroform-d/DMSO-d₆ = 1 : 3), −0.14 ppm (chloroform-d/DMSO-d₆ = 1 : 5), 0.09 ppm (chloroform-d/DMSO-d₆ = 1 : 7), 0.19 ppm (chloroform-d/DMSO-d₆ = 1 : 9) and 0.38 ppm (DMSO-d₆). On the other hand, the signal of proton H_i moved upfield from 1.27 ppm (choroform-d) to 1.06 ppm (chloroform-d/DMSO-d₆ = 1 : 2), 0.49 ppm (chloroform-d/DMSO-d₆ = 1 : 3), 0.24 ppm (chloroform-d/DMSO-d₆ = 1 : 5), −0.26 ppm (chloroform-d/DMSO-d₆ = 1 : 9) and −0.53 ppm (DMSO-d₆). Similar chemical shift changes were also observed for protons H_a and H_k (Fig. S14†). From these observations, it was demonstrated that the imidazolium unit and the adjacent methylene groups gradually moved out of the cavity of the pillar[5]arene ring; meanwhile the pillar[5]arene ring moved onto the methylene groups close to the carbamic stopper. Upon changing the solvent polarity, the pillar[5]arene ring was located at different places on the axle on average. These observations indicated that as the solvent polarity changed, [2]rotaxane 4 showed the corresponding molecular motions and acted as a molecular shuttle.^2a,d,6c,8h

Fig. 3 ¹H NMR spectra (400 MHz, 25 °C) of [2]rotaxane 4 in solvents with different polarities: (a) chloroform-d; (b) 1 : 1 chloroform-d/DMSO-d₆; (c) 1 : 2 chloroform-d/DMSO-d₆; (d) 1 : 3 chloroform-d/DMSO-d₆; (e) 1 : 5 chloroform-d/DMSO-d₆; (f) 1 : 7 chloroform-d/DMSO-d₆; (g) 1 : 9 chloroform-d/DMSO-d₆; (h) DMSO-d₆.

In nature and biological systems, relative molecular motions are very common and mostly induced by different stimuli, such as temperature, concentrations or electrical signals.⁵ Inspired by the molecular motions induced by the solvent polarity, here we further realized the rotaxane-based molecular motions in DMSO-d₆ by changing the temperature. By applying temperature-dependent ¹H NMR, we observed these molecular motions (Fig. 4 and 5). As the temperature increased, the signals of the methylene protons H_g, H_h, H_i, H_j, and H_k, which were close to the carbamic stopper, gradually moved downfield. For example, due to the deshielding effect, the chemical shift of proton H_h shifted from −0.44 ppm at 25 °C to −0.20 ppm at 35 °C, 0.35 ppm at 65 °C, and 0.82 ppm at 105 °C. In contrast, the signals of methylene protons H_a, H_b, H_c and H_d, which are close to the imidazolium unit, all showed upfield movements and the signals of protons H_c and H_d appeared at chemical shifts below 0 ppm at about 65 °C. With the further increase of temperature, these signals (H_b, H_c and H_d) shifted more and more upfield. For example, the signal of proton H_c shifted from −0.17 at 65 °C to −0.63 at 105 °C. In Fig. 4, we also observed that as the temperature increased, the signal of proton H_a moved upfield, along with the gradual downfield movement of the signal of proton H_k (Fig. 4 and 5). From these ¹H NMR results, it was demonstrated that at low temperature, due to the high polarity of DMSO-d₆, the pillar[5]arene ring was on the methylene groups close to the carbamic stopper, which caused a shielding effect of the alkyl chain near the carbamic stopper. With the increase of temperature, the pillararene cavity gradually slid to the imidazolium unit and made the signals of the protons on the imidazolium unit and adjacent methylene groups moved upfield. By comparing the ¹H NMR spectra of [2]rotaxane 4 at 25 °C and 115 °C, it was indicated that heating and cooling effectively controlled the binding sites and drove the shuttle-like molecular motions. A possible reason for these observations was that the relative stabilities of the pillar[5]arene/imidazolium complexation and the pillar[5]arene/alkyl chain binding were quite different and had a temperature dependence, which meant that at different temperatures the pillar[5]arene ring favored moving to different binding sites to form a more stable complex.^2c From these results, we think that [2]rotaxane 4 was proven to be a good candidate for stimuli-controlled molecular shuttles and would have great potential application in investigating molecular motions.^{2b,c,6c,8g,10c}

Fig. 4 Temperature-dependent chemical shifts of H_a and H_k (400 MHz, DMSO-d₆).

Fig. 5 Partial variable-temperature ¹H NMR of [2]rotaxane 4 (400 MHz, DMSO-d₆): (a) 115 °C; (b) 105 °C; (c) 95 °C; (d) 85 °C; (e) 75 °C; (f) 65 °C; (g) 55 °C; (h) 45 °C; (i) 35 °C; (j) 25 °C.

Surprisingly, we found that [2]rotaxane 4 formed supramolecular gels in DMSO or DMSO/water (5 : 1), while the dumbbell-shaped component 3 could not form supramolecular gels under the same conditions. Interestingly, under normal conditions these supramolecular gels were quite stable for at least half a year. Xerogels prepared by freeze-drying a gel of 4 in DMSO were examined by scanning electron microscopy (SEM). SEM pictures revealed that [2]rotaxane 4 aggregated into fibers with diameters of 200 nm and lengths of several hundred μm (Fig. 6b–e). These SEM images showed that the amphiphilic gelator 4 (the imidazolium unit was solvophilic when its counterion was the bromide anion while the alkyl chain was solvophobic in DMSO) self-assembled at the nanoscale via intermolecular interactions to produce fibrils, and these fibrils further physically cross-linked and interwove together to form a dense net-like structure. By entrapping solvent molecules, supramolecular gels formed.¹²

Fig. 6 Stimuli-responsive reversible sol–gel transitions ((a), AgBr was removed) and SEM images of freeze-dried supramolecular gels (b–e).

Considering the fact that reversibility and responsiveness are important features of supramolecular gels,¹² we investigated stimuli-triggered reversible gel–sol transitions of the supramolecular gel 4/DMSO (Fig. 6a). After the addition of CF₃COOAg (silver trifluoroacetate) to the supramolecular gel (Fig. 6a), we found that the gel gradually collapsed and finally became a transparent solution (AgBr was removed), while upon adding a little excess TBABr (tetrabutylammonium bromide) to the solution, we observed the reformation of the supramolecular gel. A presumed reason was that after ion exchange (from the bromide anion to the CF₃COO anion), gelator 4 was amphiphilic, which led to the disappearance of the driving forces of gelation. After the addition of TBABr, the counterion was changed back to the bromide anion, leading to the reformation of the supramolecular gel. We also achieved a reversible sol–gel transition upon sequential addition of CF₃SO₃Ag and TBABr (Fig. 6a).¹²

As expected, we found that this supramolecular gel was quite sensitive to temperature. The transparent gel gradually became solution phase when temperature increased and recovered at room temperature. A quick reversible gel–sol transition was also achieved by shaking or resting. We also observed that this supramolecular gel gradually collapsed when treated with ultrasonic waves and reformed after standing.¹¹

Conclusions

In conclusion, we prepared a pillar[5]arene-based [2]rotaxane. This [2]rotaxane showed solvent- and thermo-driven molecular motions. By adjusting the solvent polarity or changing the temperature, the pillar[5]arene ring can be on average located at different parts of the dumbbell-shaped component. These changes were monitored by ¹H NMR. Molecular motions and multiple binding sites in the [2]rotaxane will no doubt endow it with the ability to generate supramolecular architectures with dynamic functions and properties. This [2]rotaxane further self-assembled into supramolecular gels under appropriate conditions. The supramolecular gels showed reversible gel–sol phase transitions upon heating/cooling, shaking/resting, or the addition of different anions. The work presented here demonstrates that the pillar[5]arene/imidazolium complex can be used as a building block to construct multiple supramolecular aggregates, from mechanically interlocked structures to supramolecular gels under different conditions.

Acknowledgements

This work was supported by the National Basic Research Program (2013CB834502), the National Natural Science Foundation of China (91027006 and 21125417), the Fundamental Research Funds for the Central Universities (2012QNA3013) and an Alexander von Humboldt Fellowship. We thank Prof. Markus Antonietti from the Max Planck Institute of Colloids and Interfaces for his kind assistance.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Compound characterization, full synthetic details and other materials. See DOI: 10.1039/c3sc52481g

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