Letter pubs.acs.org/macroletters
Supramolecular Polymerization Controlled by Reversible Conformational Modulation Jiang-Fei Xu, Zehuan Huang, Linghui Chen, Bo Qin, Qiao Song, Zhiqiang Wang,* and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: We report a new method for fabricating supramolecular polymers with controlled structure and molecular weight through reversible conformational modulation. To this end, the crown-ether-based “taco complex” was introduced. We prepared a monomer containing a bis(m-phenylene)-32-crown-10 in the core, which can supramolecularly polymerize efficiently in solution. When the conformation of the crown ether core was folded into a taco complex, the linear supramolecular polymerization could be significantly depressed, thus decreasing the molecular weight of the supramolecular polymer. In addition, extracting the depolymerizing agent with aqueous solution of cucurbit[7]uril could disassociate the taco complex and regenerate the supramolecular polymer with molecular weight as high as before. It is anticipated that this study can provide a facile and general methodology for controllable supramolecular polymerization.
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modulation of the rigidity of ditopic monomer through selfsorting.7 Herein, this communication is aimed to introduce a new method of controllable supramolecular polymerization through reversible conformational modulation. Effective control over the supramolecular polymerization process can be realized by precisely tuning the equilibrium between linear polymerization and cyclization of the building blocks. As shown in Scheme 1, a new kind of monomer was designed that contains one crown ether bis(m-phenylene)-32-crown-10 (BMP32C10) in the middle flanked by two quadruple hydrogen bonding units ureidopyrimidinone (UPy) as end groups. The di-UPyfunctionalized crown ether is noted as DUC. It has been well investigated by Gibson’s group and Huang’s group that the crown ether BMP32C10 adapts a “chair conformation” in solutions and solid states. However, upon complexation with a paraquat guest, the conformation of BMP32C10 is a folded one. The host−guest complex thus formed, in which the guest molecule is enveloped within the folded host, is called “taco complex”.8 Benefitting from the significant structural change from the crown ether to its taco complex, we envisioned that in the absence of guest molecules the bulky BMP32C10 moiety as
upramolecular polymers are polymeric arrays of monomeric units that are held together by highly directional and reversible noncovalent interactions.1 Due to the dynamic nature of noncovalent interactions, supramolecular polymers display fascinating properties such as reversibility, adaptiveness, selfhealing, and stimuli-responsiveness, which are different from their covalent counterparts. These attributes have motivated chemists to construct a variety of supramolecular polymers and materials.2 Although significant advances have taken place in the fabrication of supramolecular polymers in recent decades, the control over supramolecular polymerization is still a big challenge.3 The monomers of supramolecular polymers are mainly polymerized through three major mechanisms, including isodesmic, ring−chain, and cooperative supramolecular polymerization.1b In most examples of supramolecular polymers, there exist equilibria between linear chains and their cyclic counterparts during the polymerization process of ditopic monomers. Such processes are classified as ring−chain supramolecular polymerization. To promote linear polymerization, the cyclization of monomers should be prevented. Following this principle, a series of methods to realize effective supramolecular polymerization in dilute solutions have been reported by our research group, including the design of unique ABBA-type monomers,4 the utilization of either a rigid and bulky linker5 or very long and flexible linker,6 and the © XXXX American Chemical Society
Received: November 19, 2015 Accepted: December 3, 2015
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DOI: 10.1021/acsmacrolett.5b00831 ACS Macro Lett. 2015, 4, 1410−1414
Letter
ACS Macro Letters
Scheme 1. Supramolecular Polymerization Controlled by Reversible Conformational Modulation: (a) Structures of DUC and PV and the Formation of a “Taco Complex”, (b) Structures of Competitive Host CPT and CB[7], and (c) The Controllable Supramolecular Polymerization by Tuning the Ring−Chain Equilibrium
a linker would facilitate the linear supramolecular polymerization of the homoditopic monomer DUC in solutions. With the addition of a guest, such as a viologen derivative (PV), the formation of a taco complex could shift the ring−chain equilibrium to favor the formation of cyclized species. Thus, the ring−chain equilibrium in the supramolecular polymerization process and molecular weight of the supramolecular polymer could be precisely controlled by tuning the molar ratio of guest PV and monomer DUC. The host−guest complexation between DUC and PV was studied by isothermal titration calorimetry (ITC) experiment and UV−vis spectroscopy. As indicated by ITC, the binding stoichiometry between DUC and PV was confirmed to be 1:1 (Figure S4, Supporting Information), and the binding constant was measured to be 7.2 × 103 M−1 in a mixed solvent of CHCl3 and CH3CN (2/1, v/v). Both solutions of DUC and PV were colorless. Upon mixing the two solutions, a color of light yellow appeared immediately. The UV−vis spectrum of the solution of DUC and PV revealed an absorption band around 420 nm (Figure S5), suggesting the formation of a charge-transfer complex between the electron-rich aromatic rings of the crown ether and electron-poor pyridinium rings of viologen. The above results confirm that the host DUC forms a taco complex with the guest PV in solution.8 The self-assembly of DUC in solution was studied by NMR spectroscopy. As shown in Figure 1, the 1H NMR spectra of DUC in CDCl3/CD3CN (2/1, v/v) showed a downfield shift (between 10.0 and 13.5 ppm) for NH signals in UPy, suggesting that there exists a strong binding ability of UPy to form quadruple hydrogen-bonded dimers.9 At low concentrations (10 mM), only one set of signals for these protons appeared and gradually broadened, which indicated that linear supramolecular polymer species became predominant with increasing concentration.10 The self-assembly of DUC in solution was further studied by viscosity measurements which can provide more information about the process of supramolecular polymerization. As shown in Figure 2a, the double-logarithmic plot of specific viscosity versus concentration was linear with a slope of 1.1 at concentrations
