English

• 1.   Introduction

  Supramolecular polymers (SPs) are dynamic materials constructed from monomeric units via reversible non-covalent interactions.^[1-5] In recent years, many interesting SPs have been constructed by different non-covalent interactions, including multiple hydrogen bonds, ^[6-8] π-π interaction, ^[9] metal-ligand coordination, ^[10-12] halogen bond, ^[13] host-guest interaction, ^[14-21] or a combination of them. For example, Liu and co-workers^[22] reported a supramolecular ternary polymer fabricated by cucurbituril and cyclodextrin. Li, Zhao and co-workers^[23] reported a very interesting hydrogen bonding-driven supramolecular alternate block copolymers which could be tuned by ion-pair binding. Huang and co-workers^[24] reported a series of SPs formed by orthogonal self-assembly based on metal-ligand and host-guest interactions. Ureidopyrimidinone (UPy), which is a self-complementary quadruple hydrogen bonding unit reported by Meijer and co-workers, ^[25] is a widely used supramolecular polymerization building block on account of its high binding constant (K_dimer > 10⁷ L•mol^-1 in CHCl₃). Since then, an increasing number of functional SPs have been constructed from UPy.^[26-33] In recent years, our group^[34-36] has also been working on constructing various UPy-based supramolecular complexes.

  As mentioned above, π-π interaction is also an important non-covalent interaction besides hydrogen bonding in supramolecular chemistry. Interestingly, the UPy unit can not only form dimer via quadruple hydrogen bonding but also can undergo π-π stacking behavior.^[37] More recently, we^[38] reported a UPy-based supramolecular polymerization system, in which supramolecular polymerization of different monomers could be regulated by introducing π-π interaction to the monomer. In that work, the strength of the π-π interaction depends on the orientation and the size of the aromatic groups, which localized at the central of the spacer of the ditopic UPy monomer. Herein, we aim to take a step forward to investigate the impact of spacer length on the supramolecular polymerization of dioxyphenylene bridged ditopic UPys. To this end, three ditopic UPy derivatives M1~M3 were prepared, the only difference of which is the spacer length between two UPy units (Figure 1). By studying this interesting model, we would like to have an insight into the relationship between molecular structure and supramolecular polymerization, which is important to produce tailor-made SPs with functional properties.

  Figure 1

  

  Figure 1.  Chemical structures of M1~M3 and cartoon representation of their ring-opening supramolecular polymerization process

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  2.   Results and discussion

  The synthesis of M1~M3 is straightforward (Scheme 1). The preparation of M2 and M3 is according to our previous report.^[38-39] Coupling of the ditopic amine compound 1a with 1, 1'-carbonyldiimidazole (CDI) activated alkylsubstituted pyrimidinone 2 resulted in the targeted UPy monomer M1, which was fully characterized by ¹H NMR, ¹³C NMR and HR-MS.

  Scheme 1

  

  Scheme 1.  Synthesis of M1~M3

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  Ring-chain equilibrium is an important supramolecular polymerization mechanism, which shows that linear SP species are in equilibrium with their cyclic monomers or oligomers in one system.^[40] During the polymerization process, there exists a critical polymerization concentration (CPC), below which the cyclic species, for example, cyclic monomers or cyclic dimers, are predominant. But above CPC, the linear SPs are the main products in the system. Previously, we^{[39, 41-42]} have studied the ring-chain mechanism by incorporating functional group into the ditopic UPy monomers. In this study, the supramolecular polymerization of M1~M3 also follows ring-chain mechanism. Herein, M1 forms cyclic dimer while both M2 and M3 form cyclic monomers below CPC and SPs above CPC.

  The supramolecular polymerization behaviors of M1~M3 were firstly studied by concentration-dependent ¹H NMR. As shown from Figure 2, the characteristic amide proton signals located in the down field region (H₁, H₂ and H₃; between δ 9 and 14), indicating all the UPy units from M1~M3 exist in dimerization form via quadruple hydrogen bonding. Figure 2a shows the concentration-varied ¹H NMR spectra of M1. At low concentration (8 mmol•L^-1), two sets of peaks appeared, represent the cyclic dimer and the linear polymer respectively. Due to the short spacer of M1 (n＝1), cyclic dimers should be the smallest species below CPC. As the concentration increased, the peaks representing cyclic dimers gradually disappeared and only one set of peaks remained, which was corresponding to linear SPs. Moreover, the CPC of M1 was calculated based on H_1e by plotting the concentration of cyclic monomer versus the total monomer concentration. Therefore, a CPC of 5 mmol•L^-1 for M1 was calculated (Figure 3).

  Figure 2

  

  Figure 2.  ¹H NMR spectra (300 MHz, CDCl₃, 298 K) of M1 (a), M2 (b), and M3 (c) at different monomer concentrations

  The blue squares indicate cyclic species and the red dots stand for polymeric assemblies

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  Figure 3

  

  Figure 3.  CPC values of M1, M2, and M3 calculated from ¹H NMR

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  The ¹H NMR spectra of M2 showed a concise picture with only one set of peaks until the concentration increased to 192 mmol•L^-1 (Figure 2b), indicating that it's not easy for M2 to undergo a ring-opening polymerization process. With the increase of the concentration, a new set of peaks representing the linear SPs appeared and their intensity increased gradually. This is because the spacer in M2 is long enough to form a cyclic monomer, which generates a π-π interaction between the dimerized UPy plane and the dioxyphenylene group. The additional π-π interaction greatly stabilizes the quadruple hydrogen bonded cyclic monomer structure, which makes ring-opening of M2 much more difficult. During the ring-opening supramolecular polymerization process, H_2c and H_2e displayed obvious downfield shifts (from δ 11.27 to 11.87 for H_2c, from δ 6.39 to 6.69 for H_2e), reflecting the loss of shielding effect between the dioxyphenylene group and the dimerized UPy motif. Meanwhile, the signals of H_2a and H_2d moved a little to upfield upon ring-opening, suggesting that these protons are located at the edge of the phenylene ring in the cyclic structure. Notably, all proton signals in M2 became broad at high concentration, revealing the formation of large molecular weight SPs. A CPC value of 189 mmol•L^-1 for M2 was calculated based on H_2e (Figure 3).

  What will happen if we further increase the length of the spacer? To clarify this problem, the molecule M3 with n＝3 was designed and synthesized. ¹H NMR experiments showed that M3 is neither as difficult as M2 nor as easy as M1 for ring-opening polymerization. The ring-opening of M3 happened at around 64 mmol•L^-1 (Figure 2c). There is no doubt that M3 can form cyclic monomers below CPC due to its long spacer. Compared with M2, the distance between the dioxyphenylene group and the dimerized UPy motif in the cyclic form of M3 is longer, leading to a weaker π-π interaction between them and make the cyclic structure of M3 less stable than M2. However, the cyclic monomer structure of M3 is more stable than the cyclic dimer structure of M1 due to the additional π-π interaction, although it is weak. As a result, the CPC of M3 is much less than M2 but higher than M1 and was calculated to be 51 mmol•L^-1 (Figure 3).

  NOESY of M2 further evidenced its cyclic monomer structure below CPC. As shown from Figure 4, obvious NOE signals between UPy motif (H_2a, H_2b, and H_2c) and phenylene protons (H_2e) were observed. Additionally, correlation signals were also observed between 1-ethylpentyl group of the UPy and phenylene protons (H_2e). These evidences indicated that M2 formed a cyclic monomer structure via quadruple hydrogen bonding. The NOESY of M3 showed no similar correlation. It might be due to the longer distance between the dioxyphenylene group and the dimerized UPy motif in the cyclic form of M3.

  Figure 4

  

  Figure 4.  Partial NOESY NMR spectrum (400 MHz, CDCl₃, 298 K) of M2 (64 mmol•L^-1)

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  Viscosity measurements is a powerful tool to study supramolecular polymerization. To further investigate the ring-opening supramolecular polymerization process of M1~M3, viscosity measurements were carried out in CHCl₃ solution. Double logarithmic plots of specific viscosity versus molecule concentration are shown in Figure 5. At the beginning of the concentration increasing, a slope of ca. 1 was observed for all curves, which is characteristic for non-interacting assemblies with constant size. This suggests the predominance of cyclic species below CPC. Upon increasing the concentration, M1 quickly shows the turning point at 7 mmol•L^-1 (CPC) and a moderate rise in the viscosity (k'_M1＝1.93). The CPC value is close to the ¹H NMR result (5 mmol•L^-1). By contrast, M2 showed a turning point at a relatively high concentration and a very steep slope was observed (k'_M2＝8.70). The CPC value of M2 measured by viscosity (191 mmol•L^-1) is also in accordance with ¹H NMR result (189 mmol•L^-1). Different from M1 and M2, M3 showed a moderate CPC value of 55 mmol•L^-1 and a moderate turning slope (k'_M3＝5.71), which is also in agreement with the results from concentration-varied ¹H NMR of M3. Subsequently, the viscosity measurements again show the big impact of spacer length on the ring-opening supramolecular polymerization.

  Figure 5

  

  Figure 5.  Specific viscosity of chloroform solutions of M1 (a), M2 (b), and M3 (c) against the concentration (298 K)

  Values on the curves indicate the slope

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  Considering that the dioxyphenylene moiety is an electron rich group that can form mechanically interlocked molecules with electron deficient cyclophane, we envisioned what differences would happen in the host-guest complexation of M1~M3 with blue-box^[43-44] (also named as CBPQT⁴⁺). Notably, we have demonstrated that the ethylpentyl UPy motif could thread into the blue-box.^[39] The host-guest complexation of M1~M3 with blue-box in mixed CHCl₃/CH₃CN solvent was investigated. Interestingly, M3+CBPQT⁴⁺ exhibited red color (Figure 6), indicating the existence of host-guest complexation and formation of a hydrogen bonded ^[2]catenane, which was verified in our previous report.^[39] However, neither M1 nor M2 showed any evidence of complexation with CBPQT⁴⁺, resulting in a colorless solution with white precipitate after several days even heating to 50 ℃. The reason for M1 should be due to the short spacer, which has a very poor binding ability with CBPQT⁴⁺.^[45] By contrast, the reason for M2 is because it forms a more stable cyclic monomer in dilute solution, which prevents its threading behavior with the blue-box. Compared with M2, the cyclic monomer of M3 is less stable and can undergo threading after ringopening. The colorless solutions of M1+CBPQT⁴⁺ and M2+CBPQT⁴⁺ also suggests that both M1 and M2 could not form relatively strong ^[2]pseudorotaxane with blue-box in CHCl₃. Then the threading behavior in dimethyl sulfoxide (DMSO) was studied, which can totally break quadruple hydrogen bonding. According to ¹H NMR, no ^[2]pseudorotaxane was found in M1+CBPQT⁴⁺ while very few ^[2]pseudorotaxanes were formed in M2+CBPQT⁴⁺. By contrast, considerable amounts of ^[2]pseudorotaxane were formed in M3+CBPQT⁴⁺. These phenomena further indicate that the longer the ethylene glycol chain, the greater the ability to thread the ring of blue-box.

  Figure 6

  

  Figure 6.  Pictures of mixture of M1, M2 and M3 with CBPQT⁴⁺ in mixed CHCl₃/CH₃CN solvent (V:V＝1:1) (left) and chemical structure of ^[2]catenane formed from M3+CBPQT⁴⁺ (right)

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  3.   Conclusions

  In conclusion, we have described a ring-chain equilibrium supramolecular polymerization system based on quadruple hydrogen bonding, in which the spacer length of the monomers has a big influence on the ring-opening process. A combination of techniques including concentration-dependent ¹H NMR, NOESY and viscosity measurement were employed to study this system. Moreover, the host-guest complexation between the monomers with the π-electron deficient macrocycle "blue-box" were also investigated. From this interesting model, new insight into the relationship between molecular structure and supramolecular polymerization is discovered, which is important to create tailor-made supramolecular polymeric materials.

  4.   Experimental section

  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 1a, ^[46] 2, ^[47] M2, ^[38] and M3^[39] were prepared according to literature procedure. All yields were given as isolated yields. NMR spectra were recorded on a Bruker AVANCE Ⅲ 300 MHz or a Bruker AVANCE Ⅲ 400 MHz spectrometer with internal standard tetramethylsilane (TMS) and solvent signals as internal references, where CDCl₃ and CD₃CN were dried using neutral aluminum oxide. NOESY experiments were performed on a Bruker AVANCE Ⅲ 400 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. Viscosity measurements were carried out with Ubbelohde micro viscometers (Shanghai Liangjing Glass Instrument Factory, 0.40 mm inner diameter) at 298 K in chloroform.

  4.2   Synthesis of compound M1

  Imidazolide 2 (0.68 g, 2.24 mmol) and 1a (0.20 g, 1.01 mmol) were dissolved in dry CHCl₃ (30 mL) and this solution was stirred for 12 h under nitrogen at room temperature. To the reaction mixture CHCl₃ (20 mL) was added and the organic layer was washed with 1 mol/L HCl (50 mL), saturated NaHCO₃ (50 mL), brine (50 mL), and then dried over anhydrous MgSO₄ and concentrated under reduced pressure. The resulting residue was subjected to column chromatography over silica gel (CHCl₃/MeOH, V:V＝100:1) to afford compound M1 as a colorless viscous solid (0.50 g, 0.75 mmol, 74%). m.p. 263~265 ℃; ¹H NMR (300 MHz, CDCl₃) δ: 13.13 (s, 2H, NH), 12.04 (s, 2H, NH), 10.50 (s, 2H, NH), 6.85 (s, 4H, ArH), 5.82 (s, 2H, alkylidene-H), 4.06 (t, J＝5.7 Hz, 4H, OCH₂), 3.73~3.58 (m, 4H, OCH₂), 2.36~2.23 [m, 2H, CH(CH₂)₂], 1.73~1.50 (m, 8H, CH₂), 1.35~1.21 (m, 8H, CH₂), 0.92~0.85 (m, 12H, CH₃); ¹³C NMR (75 MHz, CDCl₃) δ: 173.1, 157.1, 155.4, 154.7, 153.1, 115.7, 106.4, 67.0, 45.4, 39.4, 32.9, 29.3, 26.6, 22.5, 13.9, 11.7; ESI-MS m/z: 667.25 ([M+H]⁺), 665.20 ([M－H]^-); HR-ESI-MS calcd for C₃₄H₅₁N₈O₆ [M+H]⁺ 667.3926, found 667.3922.

  Supporting Information   Characterization spectra of compound M1 and ¹H-¹H NOESY of M2. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn.

  
  Dedicated to the 40th anniversary of Chinese Journal of Organic Chemistry.
  
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• 

  Figure 1  Chemical structures of M1~M3 and cartoon representation of their ring-opening supramolecular polymerization process

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  Scheme 1  Synthesis of M1~M3

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  Figure 2  ¹H NMR spectra (300 MHz, CDCl₃, 298 K) of M1 (a), M2 (b), and M3 (c) at different monomer concentrations

  The blue squares indicate cyclic species and the red dots stand for polymeric assemblies

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  Figure 3  CPC values of M1, M2, and M3 calculated from ¹H NMR

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  Figure 4  Partial NOESY NMR spectrum (400 MHz, CDCl₃, 298 K) of M2 (64 mmol•L^-1)

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  Figure 5  Specific viscosity of chloroform solutions of M1 (a), M2 (b), and M3 (c) against the concentration (298 K)

  Values on the curves indicate the slope

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  Figure 6  Pictures of mixture of M1, M2 and M3 with CBPQT⁴⁺ in mixed CHCl₃/CH₃CN solvent (V:V＝1:1) (left) and chemical structure of ^[2]catenane formed from M3+CBPQT⁴⁺ (right)

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•