English

• 1.   Introduction

  In biological systems, many functional biomolecules self-assemble into 3D architectures via multiple non-co- valent interactions. For example, DNA employs hydrogen bonding and π-π stacking interactions to form stable double helix structures.^[1~3] Supramolecular polymers, in which monomeric building blocks are usually hold together by non-covalent interactions, not only show polymer-like properties, but also exhibit many interesting properties, such as stimuli-responsiveness, self-healing and degradability.^[4~17] Inspired by biological systems, chemists have been trying to construct supramolecular polymers by employing two or multiple types of non-covalent interactions, especially in an orthogonal way.^[18~21] For instance, Liu and co-workers developed a supramolecular ternary polymer based on two kinds of host-guest interactions by employing two macrocyclic molecules cucurbit[8]uril and cyclodextrin and a ditopic guest molecule.^[22] Huang and co-workers prepared a quadruple-responsive supramolecular gel formed by crown ether based host-guest interaction and Pd- based metal-ligand coordination.^[23] Recently, Yin, Stang, and co-workers reported a fluorescent metallacage-cored supramolecular network gel, which assembled by orthogonal host-guest interaction and metal coordination.^[24] Besides these examples, various supramolecular complexes fabricated by multiple orthogonal non-covalent interactions have been reported in recent years.^[25~30] As we know, short spacer may endow supramolecular polymers with special properties. However, in the previous reports, functional groups in the monomer are usually linked together by long alkyl chain.^[31~33] In this paper, the construction of supramolecular polymers was reported by employing two different monomers containing short spacers.

  UPy-based quadruple hydrogen bonding interaction, first disclosed by Meijer and co-workers, has gained much attention in the area of supramolecular materials on accounts of its strong dimerization constant and synthetic simplicity.^{[34, 35]} Since then, a large number of functional supramolecular polymers have been prepared based on UPy.^[36~46] Dialkylammonium salts could be efficiently complexed with benzo-21-crown-7 (B21C7), which was developed by Huang and co-workers.^[47~49] B21C7 has been used to construct self-sorting systems and supramolecular polymers.^[50~53] We have been endeavored to study various non-covalent interactions in supramolecular chemis- try.^[54~59] Herein, we report a linear supramolecular polymer constructed by orthogonal quadruple hydrogen bonding and host-guest interactions, which was realized by employing two types of monomers, UPy-functionalized B21C7 (H) and homoditopic secondary ammonium salt molecule (G) (Figure 1). Firstly, H forms a dimer via quadruple hydrogen bonding in a mixed CHCl₃/CH₃CN (V:V=1:1) solvent. Secondly, a linear supramolecular polymer could be obtained upon the addition of G.

  Figure 1

  

  Figure 1.  Cartoon representation of the construction of linear supramolecular polymer from two different monomers by orthogonal self-assembly

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  2.   Results and discussion

  The monomers G and H were well-designed and successfully synthesized. The UPy motif in H was directly connected to the benzene group of B21C7. The UPy unit endows the monomer with the ability to self-dimerize via quadruple hydrogen bonds, while B21C7 provides a macrocycle to further complex with G via host-guest interaction. The synthetic process of G and H is shown in Scheme 1. Starting from fluorenone, compound 1 is prepared and further converted to compound 2 in toluene solution by reacting with 4-formylphenylboronic acid through Suzuki reaction. Finally, compound 2 reacts with n-butylamine to afford dialkylammonium G. Hexa(ethyleneglycol)ditto- sylate condenses with 4-nitrobenzene-1, 2-diol to produce crown ether 3, which can be further reduced to compound 4. Coupling 4 with 1, 1'-carbonyldiimidazole activated pyrimidinone M results in the host molecule H. G and H were fully characterized by ¹H NMR, ¹³C NMR and HR-MS. ¹H-¹H COSY NMR was employed to assist the assignment of protons in compound H. Moreover, a host-guest complex was evidenced by HR-MS.

  Scheme 1

  

  Scheme 1.  Synthetic routes for G and H

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  The formation of linear supramolecular polymers was first investigated by concentration-dependent ¹H NMR, which was recorded in CDCl₃/CD₃CN (V:V=1:1) at concentrations in the range of 8~128 mmol/L (Figure 2). The UPy N—H peaks in mixed CDCl₃/CD₃CN solvents showed large downfield shifts (between δ 9 and 13.5), giving direct evidence for the dimerization. Upon the addition of G, these peaks still showed large downfield shifts, indicating the orthogonal properties of quadruple hydrogen bonding and host-guest interactions in this system. The benzyl protons (δ 4.21) and two of the aryl protons (δ 6.48) in G and protons in crown ether of H (δ 4.20~3.50) showed obvious down field shifts, indicating the strong host-guest complexation between G and H. Moreover, all the proton peaks became broad as the concentration increases, offering evidence for the generation of high-molecular-weight assemblies driven by orthogonal quadruple hydrogen bonding and host-guest interactions.

  Figure 2

  

  Figure 2.  Partial ¹H NMR spectra (300 MHz, CDCl₃/CD₃CN, V:V=1:1, 298 K) of H; mixtures of H and 0.50 equiv. G at different H concentrations: 8, 16, 32, 64 and 128 mmol/L; and G

  The blue dots indicate solvent peaks

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  To further study the supramolecular polymers driven by orthogonal self-assembly, viscosity experiments were performed in mixed CDCl₃/CD₃CN solvents (V:V=1:1) by employing a micro-Ubbelohde viscometer. A double logarithmic plot of specific viscosity versus concentration was depicted in Figure 3, which displays a critical polymer- rization concentration (CPC) at about 51 g/L of H concentration. A slope of 1.01 was found in the initial stage as the concentration increases, which is a feature of cyclic species with constant size.^[35] In this study, we speculated that a cyclic oligomer composing two H dimers and two G molecules could be formed below CPC. Of course, besides this smallest one, larger cyclic oligomer composing more H dimers and G may also exist. The slope was turned to 2.45 when the concentration was above CPC, suggesting the transition of assemblies from cyclic oligomers to supra- molecular copolymers with increasing size. The slope value is similar to the value obtained for supramolecular polymers constructed from homoditopic building blocks comprised of bis-B21C7-based AA monomer and bis(dialkylammonium salt)-based BB monomer, indicating that the quadruple hydrogen bonding in UPy dimers has little affection on the properties of such types of supramolecular polymers.^[60]

  Figure 3

  

  Figure 3.  Specific viscosity of H with 0.5 equiv. of G vs. the H concentration in CDCl₃/CD₃CN (V:V=1:1) solutions (298 K)

  Inset: a concentrated solution of G and H

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  Further direct evidence for the formation of supramolecular copolymers was obtained by scanning electron microscope (SEM). Rod-like fiber could be drawn from a concentrated solution of G and H (molar ratio G/H=1/2). Such fibers could be only formed by entanglement of linearly connected large aggregates. By contrast, no fibers could be pulled out from the single concentrated solution of the quadruply hydrogen bonded H dimer or the solution of individual G molecule. Since the rigid spacer in G is a polycyclic aromatic hydro-carbon group, we envisioned whether the supramolecular polymers possess fluorescent property. However, the fluorescence test showed that the supramolecular polymer had no fluorescence. This might be due to that the two dialkylammonium groups at both ends quench the fluorescence.

  Due to the strong binding property of B21C7 group and K⁺, the reversibility of the obtained supramolecular polymers upon addition/removal of a competitive K⁺ was investigated by viscosity measurement (Figure 4). When 1 equiv. of KPF₆ was added to a solution of H (78 g/L) with 0.5 equiv. of G in mixed CHCl₃/CH₃CN (V:V=1:1) solution, the K⁺ was placed into the B21C7 ring of H, pushing the dialkylammoniun salt to slide out of the cavity of B21C7 owing to its stronger binding affinity to K⁺. At this time, the dramatic decrease of specific viscosity was observed due to the disassembly of the linear supramolecular polymer. After 1 equiv. of benzo-18-crown-6 (B18C6) was subsequently added to the above solution, the specific viscosity was increased largely, indicating that the supramolecular polymer was reformed due to the stronger complexation of K⁺ with B18C6. This process was repeated for one more time and similar result was observed, indicating that the quadruple hydrogen bonding interaction was orthogonal to the host-guest interaction.

  Figure 4

  

  Figure 4.  Specific viscosity of a solution of supramolecular polymer constructed from H and G upon stepwise addition of equal equiv. of K⁺ ion or B18C6

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  3.   Conclusions

  In conclusion, we have designed and successfully synthesized two different monomers comprised of B21C7, UPy, and dialkylammonium units. We further demonstrated that linear supramolecular copolymers could be fabricated from such two monomers by orthogonal quadruple hydrogen bonding and host-guest interactions. By employing combination of techniques, such as concentration-dep- endent ¹H NMR, ¹H-¹H COSY, SEM, and viscosity measurements, the supramolecular polymers were fully characterized. Moreover, such supramolecular polymers displayed stimuli-responsive capability, and their reversibility could be orthogonally switched by addition or removal of K⁺ without disruption of quadruple hydrogen bonding. Our future work will focus on developing fluorescent supramolecular copolymers driven by orthogonal self-assembly.

  4.   Experimental

  4.1   Materials and instruments

  The commercially available reagents and solvents were either employed as purchased or dried according to procedures described in the literature. Compounds 1, ^[61] 4, ^[62] and M^[63] were prepared according to literature procedure. All yields were given as isolated yields. NMR spectra were recorded on a Bruker DPX 300 MHz spectrometer with internal standard tetramethylsilane (TMS) and solvent signals as internal references, where CDCl₃ and CD₃CN were dried using neutral aluminium oxide. ¹H-¹H COSY experiments were performed on a Bruker AVANCE Ⅲ 300 MHz spectrometer. Low-resolution electrospray ionization mass spectra (LR-ESI-MS) were obtained on LCMS2020. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were recorded on an Agilent Technologies 6540 UHD Accurate-Mass. Scanning electron microscopy (SEM) image were recorded on a JSM-6360- LA. Viscosity measurements were carried out with a Ubbelohde micro viscometer (Shanghai Liangjing Glass Instrument Factory, 0.40 mm inner diameter) at 298 K in chloroform/acetonitrile (V:V=1:1).

  4.2   Synthesis of 9-(dibromomethylene)-9H-fluorene (1)

  Fluorenone (1.80 g, 10.0 mmol), carbon tetrabromide (6.63 g, 20.0 mmol), and triphenyl phosphine (10.49 g, 40.0 mmol) were combined in 50 mL of anhydrous dichloromethane. The reaction was run at 40 ℃ for 24 h. Column chromatography of the crude product on silica gel with hexanes as the eluent gave yellow solid, followed by recrystallization from hexane to provide 1 (1.40 g, 42%) as yellow crystals. m.p. 134~136 ℃ (lit.^[64] m.p. 128~130 ℃); ¹H NMR (300 MHz, CDCl₃) δ: 8.61 (d, J=8.0 Hz, 2H), 7.68 (d, J=7.4 Hz, 2H), 7.42 (td, J=7.4, 0.9 Hz, 2H), 7.31 (td, J=8.0, 1.2 Hz, 2H).

  4.3   Synthesis of 4, 4'-((9H-fluoren-9-ylidene)meth- ylene)dibenzaldehyde (2)

  4-Formylphenylboronic acid (1.30 g, 9.0 mmol) and 1 (1.00 g, 3.0 mmol) were dissolved in 30 mL of toluene, and then 2 mol/L aqueous K₂CO₃ solution (4.5 mL) and aliquat 336 (0.36 g, 0.9 mmol) were added. The mixture was stirred for 0.5 h under N₂ atmosphere at room temperature. Then Pd(PPh₃)₄ (0.34 g, 0.3 mmol) catalyst was added and the reaction mixture was stirred at 80 ℃ for 16 h. After cooling to room temperature, the product was concentrated and purified by silica gel column chromatography with dichloromethane/n-hexane (V:V=2:1) to provide 2 (1.11 g, 96%) as yellow solid. m.p. 202~203 ℃; ¹H NMR (300 MHz, CDCl₃) δ: 10.09 (s, 2H), 7.96 (d, J=8.3 Hz, 4H), 7.69 (d, J=7.5 Hz, 2H), 7.57 (d, J=8.1 Hz, 4H), 7.28 (dt, J=7.4, 0.9 Hz, 2H), 6.99~6.88 (m, 2H), 6.59 (d, J=7.9 Hz, 2H); ¹³C NMR (75 MHz, CDCl₃) δ: 191.75, 148.28, 141.27, 140.98, 137.77, 136.38, 136.03, 130.60, 130.42, 128.75, 126.83, 125.00, 119.66. HR-ESI-MS calcd for C₂₈H₁₉O₂ [M+H]⁺ 387.1385, found 387.1394.

  4.4   Synthesis of N, N'-((((9H-fluoren-9-ylidene)me- thylene)bis(4, 1-phenylene))bis(methylene))bisbutyla-15mmonium bis(hexafluorophosphate) (G)

  Compound 2 (0.93 g, 2.4 mmol) and n-butylamine (0.70 g, 9.6 mmol) were dissolved in methanol (100 mL) and stirred at 70 ℃ under N₂ atmosphere overnight. After the reaction mixture was cooled to ambient temperature, NaBH₄ (0.36 g, 9.6 mmol) was added to the solution in small portions and the mixture was stirred at room temperature for another 12 h. Water was added to quench the remaining NaBH₄ and 2.4 mol/L HCl was added to acidify the amine. The solvent was removed to give a white solid which was dissolved in deionized water/methanol (500 mL, V:V=5:1). A saturated aqueous solution of NH₄PF₆ was added to afford white precipitate which was filtered off and washed with deionized water to afford G (1.2 g, 63%) as white solid. m.p. 257~259 ℃; ¹H NMR (300 MHz, CD₃CN) δ: 7.82 (d, J=7.5 Hz, 2H), 7.66~7.49 (m, 8H), 7.34 (td, J=7.5, 0.9 Hz, 2H), 7.02~6.93 (m, 2H), 6.48 (d, J=7.9 Hz, 2H), 4.26 (s, 4H), 3.17~3.04 (m, 4H), 1.79~1.63 (m, 4H), 1.51~1.40 (m, 4H), 0.99 (t, J=7.3 Hz, 6H); ¹³C NMR (75 MHz, CD₃CN) δ: 144.45, 143.72, 141.10, 138.53, 135.30, 131.64, 131.36, 129.96, 129.02, 127.25, 125.31, 120.29, 51.82, 48.45, 28.14, 19.90, 13.31; ESI-MS calcd for [M－PF₆]⁺ 647.30, found 647.10; HR-ESI-MS (C₃₆H₄₂F₁₂N₂P₂) calcd for [M－PF₆]⁺ 647. 2984, found 647.2979.

  4.5   Synthesis of 3-nitrobenzo-21-crown ether (3)

  A mixture of hexa(ethyleneglycol)ditosylate (1.00 g, 1.7 mmol), 4-nitrobenzene-1, 2-diol (0.26 g, 1.7 mmol), K₂CO₃ (0.47 g, 2.5 mmol), and KPF₆ (0.71 g, 5.1 mmol) in 25 mL of CH₃CN was stirred and refluxed for 12 h under nitrogen gas protection. After cooling, the mixture was filtered and CH₃CN was removed with a rotary evaporator, and then CH₂Cl₂ was added. After washing with water (20 mL×3) and brine (30 mL×3), the organic phase was dried with Na₂SO₄ and then concentrated. The product was used in the next step without further purification.

  4.6   Synthesis of 3-aminobenzo-21-crown ether (4)

  Compound 3 (0.78 g, 1.9 mmol) and 10% Pd/C (10 mg) were mixed in 20 mL of degassed ethanol under argon atmosphere and degassed hydrazine monohydrate (0.95 g, 19.0 mmol) was added to the mixture which was heated at 80 ℃ in the dark for 1 h. The mixture was filtered quickly through Celite while hot, the filtrate was evaporated under reduced pressure and the residue was purified by silica gel flash column chromatography (PE/EA, V:V=3:1, 5‰ Et₃N) to provide 4^[62] (0.37 g, 51%) as yellow oil. ¹H NMR (300 MHz, CDCl₃) δ: 6.70 (d, J=8.4 Hz, 1H), 6.25 (d, J=2.6 Hz, 1H), 6.18 (dd, J=8.4, 2.6 Hz, 1H), 4.09~4.02 (m, 4H), 3.88~3.82 (m, 4H), 3.76~3.63 (m, 16H).

  4.7   Synthesis of ureidopyrimidinone derived benzo- 21-crown ether (H)

  Imidazolide M (81.00 mg, 0.27 mmol) and 4 (0.10 g, 0.27 mmol) were dissolved in 15 mL of dry CHCl₃ and this solution was stirred for 12 h under nitrogen at room temperature. To the reaction mixture 50 mL of CHCl₃ was added and the organic layer was washed with 1 mol/L HCl (30 mL), saturated NaHCO₃ (30 mL), brine (30 mL) and dried over Na₂SO₄. After the solvent was removed, the resulting residue was subjected to column chromatography CH₂Cl₂/MeOH (V:V=100:1) to give H (0.12 g, 73%) as yellowish solid. m.p. 163~164 ℃; ¹H NMR (300 MHz, CDCl₃) δ: 13.11 (s, 1H), 12.25 (s, 1H), 12.08 (s, 1H), 7.31 (d, J=2.4 Hz, 1H), 7.16 (dd, J=8.7, 2.3 Hz, 1H), 6.86 (d, J=8.7 Hz, 1H), 5.90 (s, 1H), 4.24~4.12 (m, 4H), 3.97~3.88 (m, 4H), 3.82~3.64 (m, 16H), 2.38~2.29 (m, 1H), 1.73~1.52 (m, 4H), 1.31~1.20 (m, 4H), 0.94~0.84 (m, 6H); ¹³C NMR (75 MHz, CDCl₃) δ: 173.05, 155.76, 154.81, 154.75, 149.15, 145.58, 132.00, 115.12, 113.86, 108.20, 106.54, 71.17, 71.13, 71.08, 71.05 (2C), 70.98, 70.58 (2C), 69.86, 69.78, 69.74, 69.22, 45.48, 32.95, 29.36, 26.67, 22.50, 13.91, 11.74; ESI-MS m/z: 629.10 [M+Na]⁺; HR-ESI-MS calcd for C₃₀H₄₇N₄O₉ [M+H]⁺ 607.3343, found 607.3338.

  Supporting Information  ¹H NMR, ¹³C NMR, and HR-MS spectra of compounds 1, 2, 4, H and G; ¹H-¹H COSY of H; SEM micrograph of H and G complex; HRMS of complexes H and G. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  
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  Figure 1  Cartoon representation of the construction of linear supramolecular polymer from two different monomers by orthogonal self-assembly

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  Scheme 1  Synthetic routes for G and H

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  Figure 2  Partial ¹H NMR spectra (300 MHz, CDCl₃/CD₃CN, V:V=1:1, 298 K) of H; mixtures of H and 0.50 equiv. G at different H concentrations: 8, 16, 32, 64 and 128 mmol/L; and G

  The blue dots indicate solvent peaks

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  Figure 3  Specific viscosity of H with 0.5 equiv. of G vs. the H concentration in CDCl₃/CD₃CN (V:V=1:1) solutions (298 K)

  Inset: a concentrated solution of G and H

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  Figure 4  Specific viscosity of a solution of supramolecular polymer constructed from H and G upon stepwise addition of equal equiv. of K⁺ ion or B18C6

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