English

• Supramolecular chemistry [1] has become an important field of research with the emergence of crown ether [2] as the first synthesized macrocyclic host, and has attracted more scientists for its broad applications in nanoscience [3], material science [4] and biology science [5], etc. Mechanically interlocked molecules (MIMs) [6] have been broadly studied in supramolecular chemistry and led to the emergence of special structures such as rotaxanes [7, 8], catenanes [9], molecular shuttles [10] and switches [11], which made them gradually applicable for the design of molecular machines [12], chemical probes [13] and drug delivery [14]. Among MIMs, rotaxanes have been intensively studied owing to the classical topological structure composed of a linear axle and a threaded macrocycle interconnected by non-covalent interactions such as cation-π interactions, metal-ligand interactions, C-H···π interactions, hydrogen-bonding interactions and so on. As a promising perspective of rotaxanes, pseudo[1]rotaxanes [15, 16], where the axle is linked and self-included to the macrocycle, are becoming of great interest in the field of MIMs. However, there have been only a few reported strategies for the design of bis-[1] rotaxanes due to the difficulties of their synthetic routes and characterizations.

  Pillar[5]arene [17], first reported in 2008 by Tomoki Ogoshi, with pillar-like structure consisting of five 1, 4-dimethoxybenzene linked by methylene bridges at their 2, 5-positions, is a popular macrocycle along with crown ether, cyclodextrin [18], cucurbituril [19], cyclophane and calixarene [20]. Because of their electron-rich cavity, symmetrical pillar-like shape and easy modification of the rims, pillar[n]arene has been widely used as an emerging host in the fields of chemistry [21-24], biology [25] and materials [26-32]. With the development of modified pillar[n]arenes including hydrophobic, hydrophilic and amphiphilic pillar[n]arenes, their applications have been extended to molecular machines [33], adsorption materials [34], catalysis [35], drug delivery [36] and so on. They tend to be attractive for the construction of bistable pseudo[1]rotaxanes, [1]rotaxanes and bis-[1]rotaxanes. For example, Tomoki Ogoshi et al. reported the synthesis of a mono-guest functionalized pillar[5]arene-based pseudo[1]rotaxane with different architectures (dethreaded structure in acetone and self-interlocked structure in CDCl₃) [37]. Wang et al. reported a pillar[5]arene-based interlocked pseudo[1]catenane and pseudo[1]rotaxane by a condensation reaction (one included diamino alkane and an acid-functionalized pillar[5]arene, and the other one included diisocyanate and amine-functionalized pillar[5]arene), which displayed different conformers responsive to solvents and guests [38]. Yang et al. reported a mono-functionalized pillar[5]arene bearing an imidazolium moiety that formed stable pseudo[1]rotaxane even at high concentration in chloroform [39]. Xue et al. reported an efficient method to fabricate pillar[5]arene-based[1]rotaxane with the contribution of C-H···π interactions and ionpair interactions [40].

  In 2017, our group designed an efficient strategy for the synthesis of pillar[5]arene-based pseudo[1]rotaxane by amide formation between a monoester-functionalized pillar[5]arene and dialkylamine [41]. Herein, we designed two different kinds of routes to fabricate the desired pillar[5]arene-based bis-[1]rotaxane, one route involves the construction of a stable pseudo[1] rotaxane followed by a click reaction at the terminal group of the axle leading to the desired bis-[1]rotaxane; the other route involved a linear bis-amino containing moiety which was threaded by an acid-functionalized pillararene from the two ending sides for the obtention of the targeted bis-[1]rotaxanes.

  The synthetic routes of pillar[5]arene-based mechanically selfinterlocked molecules (MSMs) are illustrated in Scheme 1. It showed two synthetic routes for the preparation of BR, route 1 and route 2. Route 1 involves the construction of a pseudo[3]rotaxane through host–guest interactions by interacting an electron-rich pillar[5]arene and a triazole-based guest in chloroform. The solution was stirred at room temperature for 12 h, then dicyclohexylcarbodiimide (DCC) and 4-dimethylaminepyridine (DMAP) were added to the solution, and the solution was stirred at room temperature for an additional 24 h. After purification using column chromatography, the expected BR was successfully obtained. Route 2 involves the formation of a self-included pseudo[1]rotaxane followed by the preparation of the final product by a click reaction between pseudo[1]rotaxane h and 1, 4-bis(2-propynyloxy)benzene. The compounds h and BR were fully characterized by ¹H NMR, ¹³C NMR, 2D NMRs including ¹H-¹³C HSQC NMR, ¹H-¹H COSY NMR, ¹H-¹H NOESY NMR and LC-ESI-MS.

  Scheme 1

  

  Scheme 1.  Synthetic procedure of compound BR via two different synthetic routes, route Ⅰ and Ⅱ.

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  Fig. 1 represents the ¹H NMR spectrum of BR in CDCl₃ with a few protons below 0 ppm. From the ¹H-¹H COSY NMR spectrum of compound BR, we can emphasize that these protons were attributed to the protons H_l-p of BR with upfield shifts due to the anisotropic interactions of the corresponding -CH₂- with the electronic cloud of the aromatic cavity (Δδ = -1.20 to -3.27), indicating that BR might be self-included at this point. Due to the deshielding effect caused by the self-inclusion, the signals of protons H_h-i displayed a downfield shift (Δδ = +0.13 and +0.53). The analysis and results of the 2D NOESY NMR spectrum of BR (Fig. 2) showed unequivocal correlation peaks between H_l-p (the signals of alkyl chain protons) with the aromatic protons (H_b) and methylene protons (H_g) of BR. The similar correlation peaks were also observed in the NOESY spectrum of h (Fig. S17 in Supporting information). Thus, we concluded to a self-included structure for BR. Mass spectrometry (MALDI-TOF-MS) also confirmed of the right compound. In the mass spectrum of BR (Fig. S21 in Supporting infornation), a base peak at m/z 2181.60 (100%) corresponding to [M+2Na-H]⁺ was observed. The comparative NMR and mass studies led us to conclude to a self-interlocked structure for BR.

  Figure 1

  

  Figure 1.  ¹H NMR spectrum (600 MHz, 298 K, CDCl₃) of compound BR.

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  Figure 2

  

  Figure 2.  Partial 2D NOESY NMR spectrum of compound BR.

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  Since pillar[5]arene has highly symmetrical and rigid pillar architecture, we anticipated the potential self-assembly of BR in some solvents. BR was dissolved in various solvents and sonicated for 30 min, then the morphology of the aggregates were investigated by scanning electronic microscopy (SEM), dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results of SEM showed that BR could assemble into polydisperse nanoparticles in MeOH at 6.2 ×10^-4 mol/L (Fig. 3a). However, the size controllable morphologies in other solvents were not found (Fig. S26 in Supporting information). The TEM studies showed that the nanospheres were formed in methanol and displayed a dark core surrounded by a light corona and accompanied by a small percentage of coalescence. Furthermore, the dynamic light scattering (DLS) result showed that the spherical nanostructures in methanol have an average diameter of 700- 800 nm. All these results demonstrated that BR could selfassemble into polydisperse spherical nanostructures in methanol with punctual coalescence. We hypothesized that the solvent played an important role in the assembly process where the solubility and the polarity of the solvent play a pivotal role. Xi and co-workers reported that higher-polarity solvents lead to stronger interactions, giving rise to more favorable aggregations [42, 43]. Herein, when BR was dissolved in various solvents (CH₂Cl₂, DMF, CH₃OH and DMSO with increasing polarity), polydisperse and organized nanoparticles only existed in CH₃OH, due to the poor solubility in DMF and DMSO, and the lower polarity of CH₂Cl₂. We also suspected that the O–H···O interactions between methanol and the carbonyl group (C = O) of pillar[5]arene were favorable to the formation of spherical structures. On the other hand, as pillar[5]arenes have pillar-like structure, each 1, 4-dimethoxybenzene may interact with the neighboring 1, 4-dimethoxybenzene of the adjacent pillar[5]arene by π···π stacking interactions which lead to a self-assembled structure in some specific solvents. The synthesized mechanism of aggregation in MeOH is illustrated in Fig. 3c. These studies indicated that the solvent effect, the network of hydrogen bonding and π···π stacking synergistically control the self-assembly process and final morphologies.

  Figure 3

  

  Figure 3.  Self-assembly of BR into nanoparticles in methanol. (a) SEM data of BR aggregates (6.2 ×10^-4 mol/L). (b) TEM images of BR self-assemblies. (c) DLS data of BR aggregates. (d) Schematic illustration of the self-assembly mechanism of BR in methanol.

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  In conclusion, we developed two different strategies for the synthesis of a novel self-interlocked pillar[5]arene-based bis-[1]rotaxane. Meanwhile, the results of TEM, SEM and DLS measurements showed that BR tend to assemble into a spherical morphology with a dark core, where the driving forces of the self-assembly are the intermolecular non-covalent interactions and the solvation. Our current efforts are focused on the synthesis of novel MSMs and the extension of their potential applications of this new nanomaterial in the near future.

  Acknowledgment

  We thank NMAC, College of Chemistry at Jilin University for financial support.

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.10.014.

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• 

  Scheme 1  Synthetic procedure of compound BR via two different synthetic routes, route Ⅰ and Ⅱ.

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  Figure 1  ¹H NMR spectrum (600 MHz, 298 K, CDCl₃) of compound BR.

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  Figure 2  Partial 2D NOESY NMR spectrum of compound BR.

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  Figure 3  Self-assembly of BR into nanoparticles in methanol. (a) SEM data of BR aggregates (6.2 ×10^-4 mol/L). (b) TEM images of BR self-assemblies. (c) DLS data of BR aggregates. (d) Schematic illustration of the self-assembly mechanism of BR in methanol.

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