English

• The studies on artificial molecular machines/devices and the control of their mechanical motions have attracted significant interest in the recent decades, which displayed promising potential in materials and biological applications [1-4]. To date, a variety of molecular machines/devices based on chemical structures, such as rotaxanes, catenanes and molecular knots, have been built [5-7], which underwent mechanical motions in response to external stimuli, including light [8, 9], temperature [10-12], pH [13], redox [14, 15] and chemical additives [16, 17]. On the other hand, chirality is a fundamental property of nature [18, 19], and the regulation of molecular chirality is playing more and more important roles in asymmetric catalysis [20], material science [21], biology and medicine science [22]. Chiral molecular devices often show circular dichroism (CD) spectral responses, which have significant advantages of allowing for distinctively determining mechanical motions on the basis of the sign of CD spectra over the intensity-based absorptive or emissive detection [23, 24]. Recently, bicyclic pillar[n]arene derivatives, in which a subring is fused in one hydroquinone unit, has attracted great attention [25, 26]. We defined these type of bicyclic compounds as molecular universal joints (MUJs) due to the subring's flexible rolling in/out property [27]. Such pillar[n]arene-based bicyclic structure has been demonstrated to undergo self-included/excluded conformational change, accompanying by the chirality switching of the pillar[n]arene core upon the variation of solvent, ion, and pH [28-31]. We have recently reported a series of MUJs showing chirality switching induced by stimuli, including temperature [32], pressure [33], redox [34] or light [35]. In these cases, the subring was often regarded as a guest for the pillar[n]arene and exerting external stimulus which resulted in the change of host-guest binding properties of the pillar[n]arene was mainly responsible for rolling in/out of the subring. Cooperative complexation of a guest by the pillar[n]arene cavity and the subring to regulate mechanical motions accompanying with chirality switching has been rarely reported, as simultaneously manipulating two active site in a single molecule to interact with a guest is challenging despite that such phenomena are widespread in biological systems [36]. Wen and coworkers reported that the two macrocyclic rings of crown ether-fused pillar[5]arene could individually complex guests, but first complexation showed a negative effect towards the second complexation [37]. Lee and coworkers reported a pillar[5]thiacrown whose planar chiral inversion was driven by a Hg²⁺ with rolling-out mechanical motion of a self-included subring upon complexation of Hg²⁺ [38]. Herein, we report a cation triggered rolling-in mechanical motion of a subring with significant chiroptical switching in crown ether-fused MUJs by synergisticly stepwise complexing the cation by two macro rings of the MUJs.

  The bicyclic compounds based on pillar[5]arene (Scheme 1a), fusing with 17-crown-5 (MUJ1) and alkyl ring (MUJ2) were synthesized according to our previous report [10]. A new MUJ (MUJ3) with increased alkyl chain length grafting on pillar[5]arene was synthesized to investigate the influence of alkyl chain length on the conformational flipping. Enantioseparation of enantiomeric pair of each MUJ, i.e., out-(S_p)/in-(R_p) and out-(R_p)/in-(S_p) pair, was achieved by preparative chiral-phase HPLC. Each enantiomer of the MUJs has two conformational isomers, which can interconvert with each other with accompanying chirality switching by the in-out equilibrium (Scheme 1b) [28, 29].

  Scheme 1

  

  Scheme 1.  (a) The chemical structures of the bicyclic MUJs (MUJ1–3); (b) schematic illustrations for the in-out equilibrium of the MUJ1 enantiomeric pair; (c) chemical structures of G1-G4.

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  The CD spectra of MUJ3 in different solvents were investigated (Fig. S5c in Supporting information). The out-(R_p)/in-(S_p)-MUJ3 showed a positive CD signal in DCM, MeCN, THF, n-hexane (n-H), CHCl₃ and ethyl acetate (EA), which is similar to that of MUJ1 (Fig. S5a in Supporting information) [32], demonstrating that MUJ3 also favored the out-configurations in these solvents. The binding of the solvent by the pillar[5]arene cavity [39] and solvation of the subring which stabilized the out-configuration should be responsible for the positive CD signal, as changing the solvent to cyclohexane (CH) and decalin (DECA) with large size, the CD sign of the MUJs inverted, indicating the inversion of planar chirality to the in-configurations, showing a unique character for MUJs that solvent could lead to a reversal of unimolecular chirality [40, 41].

  In view of the strong binding ability of crown ether with alkali metal ions [42], G1–4 (Scheme 1c) were employed to investigate the stimuli-responsibility of MUJs toward cations and, therefore to regulate the mechanical behavior and the planar chirality of MUJs. Out-(R_p)/in-(S_p)-MUJ1 showed a positive CD signal (CD_ex) at the extremum (308 nm) in DCM, demonstrating an out conformation. Interestingly, upon adding guest G1, the intensive positive CD signal at ca. 308 nm decreased sharply firstly and then inverted to a strong negative CD signal when adding 1.0~1.5 equiv. of G1 (Fig. 1). The CD spectral changes were directly proportional to the amount of G1 and reached a plateau at [G1]/[MUJ1] = 1.0 (Fig. S6 in Supporting information). The anisotropy (g) factor varied as much as 0.014 (Fig. S7 in Supporting information). In addition, the UV–vis absorption at ca. 300 nm of out-(R_p)/in-(S_p)-MUJ1 decreased upon adding G1, and the changes also stopped when the amount of G1 exceeded 1.0 equiv. (Fig. 1). These results suggested a strong 1:1 host-guest complexation between MUJ1 and G1 in DCM, and the inversion of the CD signal suggested a conformational flipping from out-(R_p)-MUJ1 to the in-(S_p)-MUJ1, for which the binding of sodium ion by the fused 17-crown-5 should be responsible, as changing the side ring to an alkyl ring induced only decrease of CD signal but no inversion even in the presence of an excess amount of G1 (Fig. S8 in Supporting information), demonstrating a significant chiral reversal induced by cation complexation.

  Figure 1

  

  Figure 1.  CD (top) and UV–vis (bottom) spectra of out-(R_p)/in-(S_p)-MUJ1 (24 µmol/L) with titrations of G1 in DCM.

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  The solvent effect on the G1-induced CD changes was further investigated (Figs. S9–S17 in Supporting information). The CD signal of out-(R_p)/in-(S_p)-MUJ1 in CHCl₃ also showed inversion which was similar with that in DCM. However, in the polar solvents, including MeOH, MeCN, THF and EA, almost no CD changes for both MUJ1 and MUJ2 could be observed. We ascribe this to the strong solvation of G1 with the polar solvents, which prevent the complexation of G1 by the MUJs.

  For the MUJ3, an analog decorated with bulky octyl group on the portal of pillar[5]arene, CD inversion was also observed for out-(R_p)/in-(S_p)-MUJ3 upon adding guest G1 (Fig. S19 in Supporting information), and absorption and CD changes also reached a plateau when equal proportion of G1 was added (Figs. S20 and S21 in Supporting information). The varied g factor was similar (Fig. S22 in Supporting information). These phenomena indicated that the alkyl substitutions on the portal of pillar[5]arene exert little effect on the chirality switching of MUJs triggered by G1. Instead, the subring of crown ether played a pivotal role.

  The rolling of the side ring into the cavity of pillar[5]arene induced by G1 complexation was verified by the NMR titration. As can be seen in Fig. 2, increasing the concentration of G1 led to broadening and upfield shift of all proton signals of the subring, with the protons a and a' showing the largest upfield shift of −0.76 ppm shifts when 1.0 equiv. of G1 was added (Fig. 2b). This is in good consistent with the fact that the rolling-in subring will suffer from the shielding effect of benzene rings when was located in the cavity of the pillar[5]arene, and the protons a and a' should insert mostly deep into the cavity, thus, suffer from the strongest shielding effect. The broadening and shifting of the proton signals demonstrated a fast equilibrium of the threading/unthreading of the side ring in the pillar[5]arene cavity. The aromatic protons of the pillar[5]arene, however, showed unsynchronous shifting, with three shifting upfield and the other two showing significant downfield shift (Fig. 2c). This was a little bit unexpected as the inclusion of just a crown ether subring will only result in upfield shifting. Considering the strong binding ability of crown ether with Na⁺, cation-π interaction was primarily responsible for downfield shifting of the aromatic protons. This deduction was validated by the NMR titration of C1 with G1. As can be seen in Fig. 3, both the aromatic and methylene protons of C1 showed significant downfield shifting upon the complexation of G1. The protons of C1 showed extremely broadening effects upon adding G1, demonstrating a fast equilibrium of complexation/decomplexation of the sodium ion. More significantly, the proton signals recovered clear and stopped changing after more than 1.0 equiv. of G1 were added, indicating again a strong 1:1 complexation between C1 and G1 in CD₂Cl₂ [43, 44].

  Figure 2

  

  Figure 2.  (a) Schematic illustrations for the in-out equilibrium of the MUJ1 induced by the binding of Na⁺; partial ¹H NMR spectra of MUJ1 (10 mmol/L) with titrations of G1 at upfield (b) and downfield (c) regimes, respectively.

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  Figure 3

  

  Figure 3.  (a) Schematic illustrations for complexing equilibrium of C1 with G1; partial ¹H NMR (400 MHz, CD₂Cl₂, r.t.) spectra of C1 (17 mmol/L) with titrations of G1 at (b) downfield and (c) upfield regimes, respectively.

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  The sodium ion triggered rolling-in motions with chirality switching of out-(R_p)/in-(S_p)-MUJ1 was a little bit unexpected. Previously, a pillar[5]thiacrown derivatives showed a rolling-out motion of the subring when complexing with mercury ion [38], for which the expansion of the thiacrown unit upon complexation of Hg²⁺ was responsible. Therefore, the preference of the in-conformation of the present Na⁺⊂MUJ1 to the out-conformer was further investigated. NMR titration of DEP5 with G1 was carried out in CD₂Cl₂, and the proton signals of methylene at both the waist and on the portal of DEP5 showed significant upfield shifting and extremely broadening effects (Fig. S27 in Supporting information), demonstrating that DEP5 exhibited host-guest complexing with G1, cation-π and electrostatic attraction between the electron-rich cavity with the electron-defect Na⁺ should be the main driving force. The binding constant was determined to be (5.4 ± 0.55) ×10³ L/mol (Fig. S28 in Supporting information). Theoretical simulation of the complex Na⁺⊂out-(R_p)/in-(S_p)-MUJ1 also showed that the included crown ether subring coordinated with sodium ion could be stabilized by the synergetic cation-π interaction between sodium ion and the aromatic units of pillar[5]arene as well as the strong C—H···π interactions of the positive glycol ether subring with the electron-rich cavity of P[5] (Fig. 4) [44-46]. Therefore, agile rolling in of the subring was realized upon adding G1 and chirality switching of MUJs was achieved firstly by synergistically complexation of G1 by the two macrocyclic hosts in MUJ1.

  Figure 4

  

  Figure 4.  Top (a) and front (b) view of the optimized structures of Na⁺⊂out-(R_p)/in-(S_p)-MUJ1 by DFT at the B3LYP/6–31G(d) level with Gaussian 09 W.

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  The counter anion could significantly affect the chiral inversion of the MUJ1, as can be seen in Fig. S32 (Supporting information), adding excessive G2 induced CD inversion but the negative signal was much weaker than that by adding equivalent G1, which was reasonable as the ion-pairing strength will affect the host-guest binding affinities. Other alkali metal ions, including K⁺ (G3) and Rb⁺ (G4) were also employed to activate the chirality switching of out-(R)/in-(S)-MUJ1 (Figs. S33 and S34 in Supporting information), it was found that little changes in the CD spectra were observed even by adding excessive G3 and G4, demonstrating the good selectivity of MUJ1 with Na⁺ over other alkali metal ions.

  Polar solvent methanol and competitive host 15-crown-5 ether (C2) easily disassembled the complexation between MUJ1 and G1 and resulted in the rolling-out of the subring. The CD spectra of Na⁺⊂out-(R_p)/in-(S_p)-MUJ1 recovered to that of free out-(R_p)/in-(S_p)-MUJ1 after about 3% MeOH was added (Fig. S36 in Supporting information), this is mainly due to the solvation of sodium ion by methanol, which weakens the inclusion of sodium ion by crown ether/pillar[5]arene. Also, adding excessive C2 to competitively binding with Na⁺ recovered the CD signal (Fig. S37 in Supporting information). The CD signs of out-(R_p)/in-(S_p)-MUJ1 were inverted by adding G1 and C1 alternatively for several cycles without obvious fatigue (Fig. 5 and Fig. S38 in Supporting information). These results indicated that the agile rolling in /out of the subring and chirality switching of MUJs could be reversibly manipulated, which will be beneficial for the construction of supramolecular stimuli-responsive systems based on molecular machines [47].

  Figure 5

  

  Figure 5.  CD signal changes of out-(R_p)/in-(S_p)-MUJ1 (20 µmol/L, 307 nm, dichloromethane) with additions of G1 and C2 alternately.

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  In summary, three MUJs based on pillar[5]arene derivatives fusing with 17-crown-5 ether (MUJ1, MUJ3) or aliphatic subring (MUJ2) were synthesized, planar-chiral enantiomers of MUJs were isolated, and the absolute configuration was determined by circular dichroism. Stimuli-responsibility of MUJs toward sodium ion was investigated, and the crown ether fused out-(R_p)-MUJ1 and out-(R_p)-MUJ3 recognize Na⁺ to trigger the chiral inversion to in-(S_p)-MUJ1 and in-(S_p)-MUJ3, respectively. The intensive positive CD signal inverted to a strong negative CD signal upon adding 1.0 equiv. of G1, with varied anisotropy factor of as much as 0.014, while the aliphatic subring fused MUJ2 showed only CD decrease but no inversion. NMR titrations and theoretical calculation indicated that the cation-π and C—H⋅⋅⋅π interactions synergistically stabilized the in-conformer of MUJs, thus led to the agile rolling-in motion of the subring upon complexing with G1. Other alkali metal ions like K⁺ and Rb⁺ did not induce the chirality inversion, showing good selectivity of MUJ1 toward Na⁺. The rolling in/out motion of the subring and the chirality switching were reversibly manipulated by adding G1 and C2 (or methanol) alternatively. This work opened a new window for manipulating supramolecular equilibria by synergetic complexing of cations and will help understanding the mechanical motion of natural or artificial molecular machines.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We acknowledge the support of this work by the National Natural Science Foundation of China (Nos. 22171194, 21971169, 92056116, 21871194), the Science & Technology Department of Sichuan Province (Nos. 2022YFH0095, 2021ZYD0052). Compound characterization was obtained with the support of the Comprehensive Training Platform of Specialized Laboratory, College of Chemistry, Sichuan University, and Prof. Peng Wu of Analytical & Testing centre, Sichuan University.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.072.

  
  
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  Scheme 1  (a) The chemical structures of the bicyclic MUJs (MUJ1–3); (b) schematic illustrations for the in-out equilibrium of the MUJ1 enantiomeric pair; (c) chemical structures of G1-G4.

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  Figure 1  CD (top) and UV–vis (bottom) spectra of out-(R_p)/in-(S_p)-MUJ1 (24 µmol/L) with titrations of G1 in DCM.

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  Figure 2  (a) Schematic illustrations for the in-out equilibrium of the MUJ1 induced by the binding of Na⁺; partial ¹H NMR spectra of MUJ1 (10 mmol/L) with titrations of G1 at upfield (b) and downfield (c) regimes, respectively.

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  Figure 3  (a) Schematic illustrations for complexing equilibrium of C1 with G1; partial ¹H NMR (400 MHz, CD₂Cl₂, r.t.) spectra of C1 (17 mmol/L) with titrations of G1 at (b) downfield and (c) upfield regimes, respectively.

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  Figure 4  Top (a) and front (b) view of the optimized structures of Na⁺⊂out-(R_p)/in-(S_p)-MUJ1 by DFT at the B3LYP/6–31G(d) level with Gaussian 09 W.

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  Figure 5  CD signal changes of out-(R_p)/in-(S_p)-MUJ1 (20 µmol/L, 307 nm, dichloromethane) with additions of G1 and C2 alternately.

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