Dual-Responsive [2]Pseudorotaxane On the basis of a pH-Sensitive

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Dual-Responsive [2]Pseudorotaxane On the basis of a pH-Sensitive Pillar[5]arene and Its Application in the Fabrication of Metallosupramolecular Polypseudorotaxane Danyu Xia,*,† Liyun Wang,† Xiaoqing Lv,† Jianbin Chao,† Xuehong Wei,*,† and Pi Wang*,‡ †

Scientific Instrument Center, Shanxi University, Taiyuan 030006, P. R. China Ministry of Education Key Laboratory of Interface Science and Engineering in Advanced Materials, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, P.R. China

‡

S Supporting Information *

ABSTRACT: Metallosupramolecular polymer, an appealing polymeric material, plays important roles in many fields including catalysis, electrochemical devices, conducting materials and so on. As a class of metallosupramolecular polymers, metallosupramolecular polypseudorotaxane has attracted great attention not only because of its wide applications but also due to its facile synthesis which is by metal coordination between metal and macrocycle-based pseudorotaxane. The introducing of stimuli-responsive property into the metallosupramolecular polypseudorotaxane system will enrich their functionality. Herein, a triple stimuli-responsive metallosupramolecular polypseudorotaxane constructed by pillararene-based host− guest interaction and copper coordination. First, a new pHsensitive pillar[5]arene host (H) was synthesized. An azastilbenzene derivative, trans- 4,4′-vinylenedipyridine (trans-G) was chosen as the guest molecule to construct a [2]pseudorotaxane based on H and trans-G. The [2]pseudorotaxane displayed pH- and photo- dual stimuli-responsiveness. Then the [2]pseudorotaxane was used to construct a pH-, photo- and cyanide-triple stimuli-responsive metallosupramolecular polypseudorotaxane based on Cu(II) ion coordination.

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INTRODUCTION In recent years, the construction of metallosupramolecular polymeric architectures has gained commom interest in the field of polymer and materials science.1−5 The introducing of metal coordination to the fabrication of polymer opens avenues to new classes of functional materials with tailored mechanical or electronic properties, accompanied by the additional features imported by the metal centers, such as molecular magnetism, conductivity, redox, bioactivity, electrochromic properties, nonlinear optics, and sensitization.6 These intriguing properties endow metallosupramolecular polymers with wide application in diverse areas of research, for example, energy/information storage, conducting materials, electrochemical devices, biomedical field, self-healing materials, and so on.3,7−10 As a kind of appealing and special polymeric architectures, polypseudorotaxanes,11 in which repeated macrocyclic hosts or pseudorotaxane units are introduced into a long-chain polymeric backbone by noncovalent interactions, such as metal coodination, dynamic covalent bonds, hydrogen bonding,12,13 have attracted great attention due to their fascinating topologies and potential applications in materials science, nanotechnology and so on.14−16 Among these noncovalent interactions, metal coordination, which are strong, © XXXX American Chemical Society

directional, and highly versatile driving forces, play important roles in constructing various coordination geometries and supramolecular polymers with considerable stability and reversibility.17−19 The incoporation of metal coordination brings advanced and interesting features to polypseudorotaxanes, which will extend their application in various fields, such as catalysts, molecular wires and organic solar cell.6 Therefore, the preparation of polypseudorotaxanes by metal coodinaton is a viable method for the programmed organization of molecular components into advanced architectures with interesting functions.20 Moreover, the convinient construction of repeated macrocyclic hosts or pseudorotaxane is the foundation of effective way to obtain metallosupramolecular polypseudorotaxanes.14,21 Pillar[n]arenes,22,23 a new class of host macrocycles have become one of the most popular topics since 2008 owing to their wide applications in the field of self-assembly, catalysis, transmembrane channels, molecular machine, drug delivery systems, sensors, supramolecular polymers, gas adsorption, and Received: February 17, 2018 Revised: March 22, 2018

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DOI: 10.1021/acs.macromol.8b00354 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Chemical Structures and Cartoon Presentation of Compound H, A-H, trans-G, A-trans-G and cis-G and (b) Cartoon Representation of the Dual-Responsive [2]Pseudorotaxane Based on H⊃trans-G and the Fabrication of the Triple Stimuli-Responsive Metallosupramolecular Polypseudorotaxane

so on.24−32 Because of their special structure, electron-donating cavities, and easy functionalization, pillararenes have displayed interesting host−guest recognition properties with various guest molecules, providing a useful and appealing platform for the construction of pseudorotanxane.33−36 However, the construction of metallosupramolecular polypseudorotaxanes by pillararene-based pseudorotaxane and metal coordination only has a few examples. For instance, Wang et al. reported a linear supramolecular polyrotaxane by a pseudorotaxane which was based on a dimethoxypillar[5]arene host and an amidefunctionalized pyridine derivative guest coordinated with Pd(II).37 More importantly, the introducing of stimuliresponsive property into the metallosupramolecular polypseudorotaxane system will extend their applications in the field of sensors, adaptive coatings, biomedical materials and so on.38,39 In the previously reported work, the stimuli-responsiveness of metallosupramolecular polypseudorotaxanes was either bringing in by the host molecules, or the guest molecules, or the inherent heat-responsive or concentration-dependent property of supramolecular polymer itself. To gain the diversity of the responsiveness of the metallosupramolecular polypseudorotaxane system, it is necessary to take full adavantage of the responsive property of host and guest molecules, even the metal coordination. Herein, we designed and synthesized a new triple stimuli-responsive metallosupramolecular polypseudorotaxane constructed by a dual-responsive pillararene-based [2]pseudorotaxane and cyanide-responsive copper(II) coordination. First, a new class of pH-sensitive pillar[5]arenes, a morpholine group per-substituted pillar[5]arene (H) was

synthesized. Then H was used to construct a dual-responsive [2]pseudorotaxane with a UV light-sensitive azastilbenzene derivative, trans-4,4′-vinylenedipyridine (trans-G). Finally, the triple stimuli-responsive metallosupramolecular polypseudotaxane was fabricated using Cu(II) coordination with the [2]pseudorotaxane because of copper’s high natural abundance, low cost, and strong coordination with pyridine groups.21,32,40 (Scheme 1).

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RESULTS AND DISCUSSION In order to obtain the pillararene-based pseudorotaxane, a new pH-sensitive pillar[5]arene was synthesized. As shown in Scheme S1, compound a was prepared according to published procedures.41 Then compound H was obtained by the reaction of a and morpholine, and its single crystals were successfully obtained by slowly evaporation of its saturated acetonitrile solutions and shown in Figure 1. In addition, the pH-sensitive property of H was studied. As shown in Figure S4, upon addition of trifluoroacetic acid (TFA) to the chloroform solution of H, the peak related to the proton H1 shifted upfield, the peaks related to the protons H2, H3, H4, H5, and H6 shifted downfield and became broad because the morpholine groups on H were protonated to become compound A-H. After further addition of triethylamine (TEA), the peaks related to the protons on H went back to their original state because of the deprotonation. These phenomena showed the pH-sensitive property of H. Next, the formation of the [2]pseudorotaxane was investigated. Trans-4,4′-vinylenedipyridine (trans-G), a comB

DOI: 10.1021/acs.macromol.8b00354 Macromolecules XXXX, XXX, XXX−XXX

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As shown in Figure S6, NOE correlation signals were observed between proton Ha of trans-G and protons H2, H4, H5 and H6 of H (Figure S6, parts A, B, C, and D), between proton Hb of trans-G and protons H2 and H3 of H (Figure S6, parts E and F). To determine the stoichiometry and association constant between H and trans-G, 1H NMR titration experiments were done with solutions which had a constant concentration of trans-G (1.00 mM) and different concentrations of H (Figure S7). By a mole ratio plot, a 1:1 stoichiometry was obtained for this system (Figure S8). By a nonlinear curve-fitting method, the association constant between the H and trans-G was calculated to be (2.30 ± 0.46) × 102 M−1 (Figure S9). UV−vis absorption spectroscopy was also used to monitor the host− guest complexation between H and trans-G. As shown in Figure S10, trans-G exhibited a maximum absorption band at around 287 nm, while after adding H to the solution, the maximum absorption band moved to around 292 nm and the intensity increased, indicating the complexation between H and trans-G. These phenomena provided convincing evidence for the formation of the [2]pseudorotaxane based on H and transG. The [2]pseudorotaxane based on H and trans-G displayed stimuli-responsive property in chloroform. First, its pHresponsive property was investigated by 1H NMR spectroscopy. As shown in the 1H NMR spectra (Figure 3, parts a and b),

Figure 1. Ball−stick views of the crystal structure of H. Hydrogen atoms and solvent molecules were omitted for clarity. Color code: C, red; O, green; N, blue.

mercial available compound, was chosen as the guest because of its special electron-deficient structure, UV light-/pH-responsive property and pyridine group which can be used as metal ligands. To the beginning, 1H NMR spectroscopy was performed to investigate the host−guest interactions between H and trans-G. Compared with free H and trans-G (Figure 2,

Figure 2. Partial 1H NMR spectra (400 MHz, CDCl3, room temperature): (a) H (5.00 mM); (b) trans-G (5.00 mM) and H (5.00 mM); (c) trans-G (5.00 mM).

Figure 3. Partial 1H NMR spectra (400 MHz, CDCl3, room temperature): (a) trans-G (5.00 mM); (b) after addition of 2.0 equimolar trifluoroacetyl acid to sample a; (c) H⊃trans-G (5.00 mM); (d) after addition of 12.0 equimolartrifluoroacetyl acid to sample c; (e) after addition of 10.0 equimolar trifluoroacetyl acid to H (5.00 mM); (f) after further addition of 12.0 equimolar triethylamine to sample b.

parts a and c), remarkable chemical shift changes of the signals for the protons on trans-G appeared in the presence of equimolar H (Figure 2b). The peaks related to Ha and Hb shifted upfield and became broad, and the peak related to Hc disappeared, indicating that these protons are located within the cavity of H and shielded by the electron-rich cyclic structure because of the complexation between H and trans-G. Additionally, protons signals for H2 on H also slightly shifted downfield and became broad. The broad pehnomenoum of these peaks was due to complexation dynamics.42 In addition, varied-temperature 1H NMR experiments of equimolar trans-G and H were performed. As shown in Figure S5, the complexation induced broadening effect of the peak related to the proton Hc on trans-G in higher temperatures were less remarkable than those in lower temperatures. It can be seen that the peak related to Hc appeared in higher temperature and shifted upfield from 7.22 to 5.25 ppm, confirming that trans-G threaded into the cavity of H. Moreover, a 2D NOESY NMR experiment was also performed to confirm the 1H NMR results.

when the solution of trans-G was added with TFA, the signals related to all the protons on trans-G shifted downfield, indicating that the pyridine groups on trans-G were protonated to become compound A-trans-G. Meanwhile, when the solution of the [2]pseudorotaxane was added with TFA, both H and trans-G changed into protonated products, A-trans-G and A-H, respectively. As shown in Figure 3, parts c and d, the peaks related to the protons Ha, Hb, and H2−6 shifted downfield, H1 shifted upfield, and Hc appeared. There were no chemical shift changes or broadening effects of the signals for the protons on A-trans-G and A-H compared to free Atrans-G and A-H (Figure 3, parts b and e), indicating that the complexation between the H and trans-G was destroyed by TFA. When the solution was added with TEA, H and trans-G formed again. The peaks corresponding to the protons on C

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Macromolecules trans-G and H turned back to the original state (Figure 3f), indicating the recovery of the [2]pseudorotaxane. These data provided evidence for the pH-responsive property of the [2]pseudorotaxane. Then the photoresponsive property of the [2]pseudorotaxane was studied by 1H NMR spectroscopy and UV−vis absorption spectroscopy. To the beginning, the photoresponsive ability of trans-G was investigated via 1H NMR spectroscopy and UV−vis absorption spectroscopy. As shown in Figure 4, parts a and b, upon irradiation by UV light

After construction and invesitgation of the external stimuliresponsive property of the [2]pseudorotaxane, it was used to fabricate a metallosupramolecular polypseudorotaxane linked by Cu(II). First, 1H NMR spectroscopy was used to study the coodination process. As shown in Figure 5, parts a and b, upon

Figure 5. Partial 1H NMR spectra (400 MHz, CDCl3, room temperature): (a) trans-G (5.00 mM); (b) after addition of equimolar Cu(II) to sample a; (c) H⊃trans-G (5.00 mM); (d) after addition of equimolar Cu(II) to sample c.

Figure 4. Partial 1H NMR spectra (400 MHz, CDCl3, room temperature): (a) trans-G (5.00 mM); (b) trans-G (5.00 mM) after irradiation with UV light at 365 nm; (c) H⊃trans-G (5.00 mM); (d) H⊃trans-G (5.00 mM) after irradiation with UV light at 365 nm.

addition of Cu(II) to the solution of trans-G, the peaks corresponding to the protons Ha and Hb of trans-G shifted downfield with broadening, indicating the coodination between Cu(II) and trans-G. When the solution of H⊃trans-G was added with Cu(II), the peaks corresponding to the complexed protons Ha and Hb also shifted downfield and became broader (Figure 5, parts c and d), suggesting the formation of the metallosupramolecular polypseudorotaxane. The result was confirmed by 2D DOSY NMR spectroscopy. As shown in Figure 6, compared with H⊃trans-G solution, the diffusion

at 365 nm, the signals for protons Ha, Hb, and Hc of trans-G weakened accompanied by the enhancement of the protons Ha*, Hb*, and Hc* of cis-G, indicating the photoisomerization from the trans state to the cis state and reaching a photostationary state,33 which corresponds to a trans−cis conversion of 65%. Meanwhile, the photoinduced trans−cis isomerization behavior of trans-G was investigated by UV−vis absorption spectroscopy. The absorption spectrum of trans-G exhibited one absorption band at 287 nm, corresponding to the trans state absorption of azastilbenzene derivatives.43,44 Upon gradually irradiation with UV light at 365 nm, the absorption band of trans-G at 287 nm decreased dramatically, accompanied by enhancement of the band around 246 nm, indicating the photoisomerization from the trans state to cis state. One isosbestic point was observed at around 270 nm (Figure S11a).43 Then the photoresponsive behavior of the [2]pseudorotaxane based on H and trans-G was further studied. As shown in Figure 4, parts c and d, the peaks related to Ha, Hb, and Hc on trans-G became weaker and the signals for protons Ha*, Hb*, and Hc* on free cis-G were clearly observed after irradiating a solution of H⊃trans-G with UV light at 365 nm for 2.0 h, suggesting that the host−guest interactions between H and trans-G were destroyed. The reason is that the size of cis-G is larger than the cavity of H.33,45 Therefore, these cis-G molecules could not thread the cavity of H. UV−vis absorption spectroscopy data confirmed the results. As shown in Figure S11b, the intensity of the maximum absorption band around 292 nm ascribed to the [2]pseudorotaxane gradually decreased upon irradiation with UV light. These results suggested that the photoresponsive property of the [2]pseudorotaxane was achieved.

Figure 6. Diffusion coefficient D (600 MHz, CDCl3, room temperature) of H⊃trans-G (5.00 mM) and the sample after addition of equimolar Cu(II).

coefficient decreased upon addition of Cu(II), indicating the formation of large aggregates. Meanwhile, the intensity of the maxmiun absorption band around 287 and 292 nm, corresponding to trans-G and H⊃trans-G, respectively, increased remarkably after adding Cu(II), proving further evidence for the formation of the metallosupramolecular polypseudorotaxane (Figure S12). In addition, scanning electron microscopy (SEM) experiments were performed to investigate the formation of metallosupramolecular polypseudorotaxane in choloform with high molecular weight and a high degree of linear chain D

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Macromolecules extension. As shown in Figure 7, rodlike and ringlike fibers with the diameter about 2 μm were observed obtained from a high concentration solution of the mixtures of H⊃trans-G and Cu(II).

Figure 7. SEM image of gold-coated fibers drawn from a high concentration solution of mixtures of H⊃trans-G and Cu(II) in the molar ratio of 1:1 in choloroform.

Figure 8. Partial 1H NMR spectra (400 MHz, CDCl3, room temperature): (a) trans-G-based Cu(II) coordinated polymer; (b) sample after addition of enough tetrabutylammonium cyanide to sample a; (c) H⊃trans-G-based metallosupramolecular polypseudorotaxane; (d) sample after addition of enough tetrabutylammonium cyanide to sample c.

Furthermore, the stimuli-responsive property of the metallosupramolecular polypseudorotaxane was investigated by 1H NMR spectroscopy. As the [2]pseudorotaxane based on H and trans-G was pH- and photoresponsive, it is obvious that the dual-responsiveness will endow the polypseudorotaxane with the stimuli-responsive property. First, the pH-responsive property was studied. As shown in Figure S13, upon addition of TFA to the solution of trans-G-based Cu(II) coordinated polymer, trans-G turned into protonated product A-trans-G, the signals for the protons Ha, Hb and Hc on the polymer shifted downfield, indicating that the metal coordination was destroyed (Figure S13b), after addition of TEA, the signals for these protons turned back, suggesting that the polymer formed again (Figure S13c). Analogously, the metallosupramolecular polypseudorotaxane was destroyed upon addition of TFA and recovered after addition of TEA, indicating its pH-responsive property (Figure S13d−f). Second, the photoresponsive property was monitored. As shown in Figure S14, after irradiation by UV light at 365 nm for 2.0 h, the signals for the protons Ha, Hb, and Hc of trans-G-based Cu(II) coordinated polymer weakened with enhancement of the protons Ha*, Hb*, and Hc* of cis-G, it can be seen from Figure S14, parts b and c that cis-G also coordinated with Cu(II). Therefore, it is reasonable that the metallosupramolecular polypseudorotaxane became cis-G-based coordinated polymer after irradiation by UV light at 365 nm (Figure S14d−f), which was in consistent with the photoresponsive property of the [2]pseudorotaxane based on H and trans-G. Moreover, as the cyanide is known to react with copper ions to form very stable Cu(CN)2 species,46 the metallosupramolecular polypseudorotaxane could be destroyed by cyanide, which was studied by 1 H NMR spectroscopy. As shown in Figure 8, upon addition of CN− to the solution of the trans-G-based Cu(II) coordinated polymer, the peaks corresponding to the protons Ha and Hb at the coordinated state went back to its original state as free trans-G. Similar phenomenon can be observed in the metallosupramolecular polypseudorotaxane system (Figure 8, parts c and d). As a result, the metallosupramolecular polypseudorotaxane system was triple stimuli-responsive.

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CONCLUSIONS In conclusion, a [2]pseudorotaxane based on the host−guest interactions between a new pH-sensitive pillar[5]arene (H) and a photosensitive guest (trans-G) was synthesized. The [2]pseudorotaxane was then used to fabricate a pH-, photoand cyanide-triple stimuli-responsive metallosupramolecular polypseudorotaxane by Cu(II) coordination. This is the first pH-sensitive pillararene-based pH-, photo- and cyanide-triple stimuli-responsive metallosupramolecular polypseudorotaxane, which can be of high potential in the application of sensors, drug develivery systems, adaptive coatings, and so on. Therefore, this work expanded pillararene-based host−guest recognition system and enriched the field of metallosupramolecular polymer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00354. Synthesis, characterizations, 1H NMR data, 2D NOESY NMR data, UV−vis data, and other material (PDF) Cif crystallographic file for H (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.X.) E-mail: [email protected]. *(X.W.) E-mail: [email protected]. *(P.W.) E-mail: [email protected]. ORCID

Danyu Xia: 0000-0001-6575-6448 Xuehong Wei: 0000-0002-9490-7265 Pi Wang: 0000-0002-8803-7953 Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation for Young Scientists of China (21704073) for financial support.

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DOI: 10.1021/acs.macromol.8b00354 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00354 Macromolecules XXXX, XXX, XXX−XXX