Polymer Chemistry

Polymer Chemistry

Published on 02 August 2016. Downloaded by Zhejiang University on 21/09/2016 08:02:57.

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Cite this: Polym. Chem., 2016, 7, 5221 Received 28th June 2016, Accepted 2nd August 2016 DOI: 10.1039/c6py01123c www.rsc.org/polymers

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Construction of a neutral linear supramolecular polymer via orthogonal donor–acceptor interactions and pillar[5]arene-based molecular recognition† Panpan Chen, Julfikar Hassan Mondal, Yujuan Zhou, Huangtianzhi Zhu and Bingbing Shi*

A neutral linear supramolecular polymer was efficiently constructed by an orthogonal combination of donor–acceptor interactions and pillar[5]arene-based molecular recognition.

Supramolecular polymers,1 which can be defined as polymeric arrays of low molecular weight monomeric units that are brought together by noncovalent interactions instead of conventional covalent polymerization, have attracted significant attention from scientists in recent years. Due to the relatively low activation energy required to break weak noncovalent interactions,2 chemists have achieved predictable control over various noncovalent interactions to develop a plethora of intricate supramolecular polymers. On account of the dynamic and reversible nature of noncovalent interactions, these materials not only show traditional polymeric properties, but also present stimuli-responsive and unprecedented macroscopic properties, endowing their potential applications as advanced materials.3 In addition, supramolecular assemblies constructed by low molecular weight monomers via orthogonal noncovalent interactions have enjoyed great superiority in recent research owing to their resultant unique properties.4 Taking the advantages of orthogonal self-assembly, we can perfectly unify the themes of multiple noncovalent interactions. Thus, supramolecular polymers with novel topological structures can be efficiently constructed due to the synergy of multiple noncovalent bonds. Among the numerous noncovalent interactions, π–π stacking, which exists between two electronically complementary components, is a valuable methodology for the construction of supramolecular assemblies. As a class of π-stacks, donor– acceptor (D–A) complexes arise from a combination of electron donors (D) and electron acceptors (A),5 and the integration of

Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China. E-mail: [email protected]; Fax: +86 571-8795-3189 † Electronic supplementary information (ESI) available: Synthetic procedures, characterization and other materials. See DOI: 10.1039/c6py01123c

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donor and acceptor chromophores in a mixed array is a convenient approach to construct supramolecular assemblies.6 Pillararenes,7 a new class of supramolecular hosts, present a hydrophobic core sandwiched between two functionalizable rims. The unique structures and easy functionalization of pillararenes provide a useful platform for the construction of various interesting supramolecular systems, which have captured more and more attention from scientists in recent years.8 Until now, there are lots of good examples of supramolecular polymers constructed by pillararene-based molecular recognition.9 However, supramolecular polymers constructed in an orthogonal fashion by unifying the themes of donor– acceptor interactions and pillararene-based molecular recognition are still rare. Additionally, the neutral supramolecular polymers derived from pillararene-based molecular recognition are also seldom reported. Thus, it is necessary to conduct some relevant research to promote the development of pillararene-based supramolecular polymers. N. S. Saleesh Kumar and co-workers have demonstrated that a preferentially π-stacking pair between pyrene and naphthalene diimide (NDI) rationalized by frontier orbital congruence presents a high association constant, leading to alternate π-stacking even in an excellent solvent.10,11 Furthermore, it is worth noting that the neutral guest bearing a cyano site and a triazole site at its ends (TAPN) is an effective guest for pillar[5]arene, with an association constant (Ka) of 104 M−1 in the case of an alkylated derivative.8a,12 Enlightened by these, we designed and synthesized a linear pillar[5]arene-based supramolecular polymer with structural novelty constructed by a neutral guest possessing two triazole binding sites for pillar [5]arene cavities and the donor–acceptor interactions between pyrene and NDI through hierarchical orthogonal strategies. Benefiting from the strong donor–acceptor interactions and the high association constant of the host–guest recognition, we can effectively obtain the monomeric unit 12·NDI·2. The component 12·NDI (Scheme 1) may be thought of as an unusual homoditopic AA-type monomer, and a neutral guest 2 was used as a BB-type monomer. As a result, a linear supra-

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Scheme 1 Chemical structures of 1, NDI and 2. Cartoon representation of the formation of the linear supramolecular polymer.

molecular polymer was successfully prepared in the A2B2 form as suggested in Scheme 1. In order to explore the interaction between 1 and NDI, UV-Vis, fluorescence and 1H NMR spectroscopy techniques have been performed in solution. The UV-Vis spectrum of 12·NDI (1.0 × 10−5 M) in CHCl3 showed that the characteristic absorption of pyrene presented a significant red shift, indicating the formation of a π-stacked complex (Fig. S12†).10a Fluorescence spectra of 12·NDI (1.0 × 10−6 M) was recorded upon excitation at 355 nm in CHCl3 solution, fluorescence intensity of 1 obviously decreased at λ = 387, 409 and 432 nm upon addition of NDI (Fig. S13†). This feature was attributed to a quenching of the emission due to a photo-induced intermolecular charge transfer taking place between interacting pyrene and NDI moieties. Such fluorescence intensity changes clearly suggest the occurrence of π–π intermolecular interactions between 1 and NDI even in dilute solutions.13 Furthermore, 1H NMR spectroscopy provided additional evidence of aromatic stacking interactions in the donor–acceptor complex. As shown in Fig. S14,† after the complexation of 1 with NDI, the peaks related to the protons Hα on NDI and protons H1–9 on 1 showed upfield shifts, which is characteristic of π-stacked moieties.14 Taken together, UV-Vis, fluorescence and 1H NMR spectroscopy of 12·NDI provide clear evidence of the interactions between the closely packed π-electron donors ( pyrene) and acceptors (NDI), confirming the existence of the interaction between 1 and NDI in solution. The binding behavior between per-methylated pillar[5] arene (MeP5A) and TAPN has been reported by Li and coworkers and the association constant (Ka) can be as high as (1.2 ± 0.2) × 104 M −1 in chloroform.12a Based on this, we investigated the molecular recognition behavior between MeP5A derivative hosts 1 and 2 in chloroform. 1H NMR spectroscopy experiments clearly indicated the formation of the [3]pseudorotaxane-type complex 12·2 (Fig. S15†). Upon the addition of 1 to the solution of 2, the peaks related to the protons Ha–d on 2 exhibited substantial upfield shifts (Δδ = −2.09, −2.87, −3.48 and −1.56 ppm for Ha, Hb, Hc and Hd, respectively) compared to the free axle as a consequence of inclusion-induced shielding effects, which indicated the formation of a threaded

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structure between 1 and 2. Additionally, the signals of protons H10 and H11 on 1 also exhibited slight chemical shift changes in the presence of 2 due to the formation of host–guest inclusion complexes. The assignment and correlation of the protons were further validated by a 2D NOESY NMR spectrum of 12·2; correlations were observed between methylene protons (Ha–d) of 2 with the aromatic protons H10 and methyl protons H11 of 1 (Fig. S16 and S17†), suggesting that the TAPN group was deeply encapsulated in the cavity of the pillararene moiety. The supramolecular polymer formation was characterized by various techniques including 1H NMR, two-dimensional diffusion-ordered 1H NMR spectroscopy (DOSY), specific viscosity and scanning electron microscopy (SEM). 1H NMR spectra of 12·NDI·2 were recorded over a concentration range of 5.0 mM to 200 mM (Fig. 1). As expected, the proton NMR spectra of 12·NDI·2 are concentration dependent. As the concentration increased, the signals of protons Hα of NDI and protons H1–9 of 1 underwent substantial upfield shifts. Moreover, all the signals became broad at a high concentration, which confirmed the formation of high molecular weight aggregates (Fig. S18 and S19†).15 Two-dimensional diffusion-ordered 1H NMR spectroscopy (DOSY) experiments were performed to investigate the aggregation of 12·NDI·2 during linear supramolecular polymerization. The measured weight-average diffusion coefficient (D) of 12·NDI·2 in CDCl3 decreased gradually from 3.98 × 10−10 m2 s−1 to 8.71 × 10−11 m2 s−1 upon increasing the concentrations of 12·NDI·2 from 5.0 mM to 200 mM, which implied that the increase of average aggregation size led to the transition from supramolecular oligomer species to supramolecular polymers (Fig. S20†). From previous studies,4c we know that a significant decrease in the diffusion coefficient resulted from a high polymerization degree value. Thus, the current measurements clearly indicated the formation of an extended high-molecularweight polymeric structure.

Fig. 1 1H NMR spectra (400 MHz, 298 K) of 12·NDI·2 in CDCl3 at various concentrations: (A) 5.00 mM; (B) 10.0 mM; (C) 25.0 mM; (D) 50.0 mM; (E) 62.5 mM; (F) 76.5 mM; (G) 100 mM; (H) 140 mM; (I) 200 mM.


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To further investigate the supramolecular aggregates, viscosity measurements were carried out in CHCl3. As presented in Fig. 2, the linear supramolecular polymers assembled from the monomers exhibited a viscosity transition that was characterized by a change in the slope in the double logarithmic plots of specific viscosity versus concentration. In the low concentration range, the slope of the curve was 1.48, indicating a linear relationship between specific viscosity and concentration, which demonstrated the presence of cyclic oligomers. When the concentration exceeded the critical polymerization concentration (CPC; approximately 67.0 mM), a sharp increase in the viscosity was observed (slope = 1.72, at 298 K), indicating that the monomers self-assembled into supramolecular polymers of increasing sizes. Then the maximum possible polymerization degrees, n, is easily derived as being related to the equilibrium constant Ka and the initial concentration of 12·NDI·2 (Table S1†).16 As the concentration increases, the calculated size of aggregates increases to large values. Scanning electron microscopy (SEM) studies showed that rod-like fibers could be easily drawn from a highly concentrated solution of 12·NDI·2, reflecting a significant chain extension to generate the desired high-molecular-weight polymeric

Fig. 2 Specific viscosity of a 2 : 1 : 1 molar mixture of 1, NDI and 2 in CHCl3 at 298 K versus the concentration of 12·NDI·2.

Fig. 3 SEM images of rod-like fibers drawn from a high concentration solution of 12·NDI·2 in CHCl3.


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aggregates (Fig. 3). All these images confirmed the formation of a linear supramolecular polymer driven by orthogonal donor–acceptor interactions and pillar[5]arene-based molecular recognition. In addition, the macroscopic properties of the high concentration supramolecular polymer were also investigated. Glue-like viscous liquids were obtained from a concentrated solution of 12·NDI·2 by dissolving the monomers in CHCl3 at 55 °C followed by cooling to room temperature. Upon increasing the concentration of the monomers, the high concentration supramolecular polymers finally formed at a phasetransition temperature of approximately 41–43 °C for 12·NDI·2 at a concentration of 200 mM.9f,17 Moreover, it is interesting to find that the high concentration supramolecular polymers showed reversible glue–sol phase transitions upon heating and cooling (Fig. 4). In the process of molecular recognition and supramolecular self-assembly, elevated temperatures always decrease the stabilities of host–guest systems due to the accompanying more unfavorable entropy term (TΔS°) governing their complexation free energies.18 On the other hand, π–π stacking between two electronically complementary components is also temperature-dependent.19 It raises the question as to which interaction was mainly changed when heating the high concentration supramolecular polymers. To address the issue, temperature-dependent 1H NMR measurements of 12·NDI·2 at the concentration of 150 mM in CDCl3 were performed. Briefly, both signals of protons Hα of NDI and protons H2–9 of 1 exhibited remarkable downfield shifts at elevated temperatures, while no obvious changes occur for the peaks related to the protons Ha–d on 2 (Fig. S21–S23†). Hence, we can draw the conclusion that donor–acceptor interactions between 1 and NDI are more susceptible to temperature variation, leading to reversible glue–sol phase transitions upon heating and cooling the high concentration supramolecular polymers. In summary, we have successfully prepared a neutral linear supramolecular polymer using donor–acceptor interactions and pillar[5]arene-based molecular recognition in a hierarchical orthogonal fashion. Through 1H NMR, DOSY and specific viscosity, we found that the monomer concentration exerts a significant impact on the formation of the supramolecular polymer. Moreover, rod-like fibers were drawn from a highconcentration solution of the monomers, which provided direct evidence for the formation of a linear supramolecular

Fig. 4 The glue–sol transitions of the supramolecular polymer triggered by temperature stimulus.

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polymer. Interestingly, the supramolecular polymer shows reversible glue–sol phase transitions upon heating and cooling. Considering the favorable properties induced by donor–acceptor interactions and host–guest molecular recognition, this pillararene-based supramolecular polymer may be of high importance for developing supramolecular materials with more complex structures and functions in the future.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities.

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