Divalent Pseudorotaxane with Polarized Plug–Socket and Padlock

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Divalent Pseudorotaxane with Polarized Plug−Socket and Padlock Functions Zhengliang Qian,† Xin Li,‡ Tao Yuan,† Xinghua Huang,† Qiaochun Wang,*,† Hans Ågren,‡ and He Tian*,† †

Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, China ‡ Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden S Supporting Information *

ABSTRACT: A dual-pore structured host composed of one 24-crown-8 and one 34-crown-10, and a “U”-shaped guest consisting of two different recognition units, one dibenzylammonium and one viologen, were synthesized and bound 1:1 into a divalent pseudorotaxane P1. P1 can mimic the inserting and pulling out functions of a polarized plug−socket system under solvent driven stimulus and can also realize the locking and unlocking actions of a padlock under pH stimulus.

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Although many smart molecular plug−socket systems have been reported,7 no polarized molecular plug−socket systems have been developed yet. It has been proved that a DB24C8 unit exclusively binds to DBA unit and a BPP34C10 moiety combines selectively with a viologen moiety.8 Therefore, the guest molecule G1 was hoped to combine unidirectionally with the host molecule H1, in the way that the DBA and viologen legs enter into the DB24C8 and BPP34C10 holes, respectively. G1 and H1 were synthesized (see Supporting Information), and their interactions were investigated. Viologens are known to thread aromatic crown ethers, and the formation of such pseudorataxane structures causes the formation of a chargetransfer (CT) state featured by the appearance of a broad and weak CT absorption band in the visible region.7b,c,9 Upon the addition of equivalent amount of H1 to G1 in CHCl3/CH3CN (2:1, v/v), a characteristic CT absorption at around 480 nm (Figure S7) was found, and the color of the solution changed from pale yellow to orange, showing strong interactions between the two molecules. To further understand the binding behaviors between H1 and G1, isothermal titration calorimetry (ITC) experiment was carried out. As illustrated in Figure S12, the association constant of G1+H1 in acetonitrile was obtained as 3.7 × 103 M−1. The inclusion ratio n was determined as 0.995, indicating a 1:1 binding mode rather than a 1:2 or others. 1 H NMR experiments were then carried out to verify the formation of the divalent pseudorotaxane P1. Model molecules 1, 2, 3, and 4 (Figure S1), which correspond to DBA, viologen unit, DB24C8, and BPP34C10, respectively, were used, and the NMR spectra of 1 + 2 + 3 indeed proves that the DB24C8 unit

o manipulate molecules to work in analogy to the machines in the macroscopic world and mimic their functions,1 various artificial molecular machines have been manufactured such as molecular shuttles,2 molecular rotary motors,3 molecular elevators,4 molecular muscles,5 and so on. A multifunctional molecular machine system means that the machine components have various motions in response to external stimuli and different kinds of functions can thus be achieved within a single machine molecule. Hence, it could be anticipated that multifunctional molecular machine systems would have as good applicative prospects as those of their macroscopic counterparts. To date, although many multiresponsive molecular machine systems have been set up,6 most of them can only implement a single function such as logic calculation,6e,f switching,6g,h and sensing.6i,j However, to the best of our knowledge, a molecular machine system that can realize imitating two macroscopic actions has not been reported yet. Herein we report a multiresponsive divalent pseudorotaxane P1 (Figure 1), which was assembled by a host molecule H1 and a guest molecule G1. H1 is composed of two directly linked crown ether rings, one dibenzo-24-crown-8 (DB24C8) and one bis(p-phenylene)-34-crown-10 (BPP34C10), and can act as a polarized socket and a padlock body. G1 consists of a dibenzylammonium (DBA) unit and a viologen moiety connected by a p-biphenyl linker and can play the roles of a polarized plug and a shackle of the padlock. As a result, P1 is a multifunctional molecular machine system that is capable of mimicking both actions of a polarized plug−socket system and a padlock when suitable stimuli are applied. A polarized plug−socket system guarantees that the neutral conductor and the neutral pole on the appliance will be connected and as a result effectively avoids electric shock. © XXXX American Chemical Society

Received: December 12, 2017

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DOI: 10.1021/acs.orglett.7b03867 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Figure 1. (a) Chemical structures of H1, G1. (b) Schematic diagram of multiresponsive divalent pseudorotaxane P1.

strong NOE signals between the viologen protons Hc‑f on G1 and the crown ether protons, as well as between Hh, Hi in DBA and the crown ether protons, again prove the interactions between G1 and H1. MALDI-TOF mass and dynamic light scattering (DLS) measurements were also carried out to identify P1. The MALDI-TOF MS of 1:1 mixture (Figure S9) exhibited strong signals corresponding to H1+, [H1+Na]+, [H1+K]+, [P1− PF6]+, [P1−2PF6−H]+, and [P1−3PF6−Viologen]+, and no other peaks appeared in the range m/z 2000−20000. The DLS result of the 1:1 mixture showed that the size distribution is less than 3 nm (Figure S10). The above results indicate that only P1, rather than polymer or oligomers, forms when H1 and G1 are 1:1 mixed. To gain insight into host−guest interactions among P1, we optimized the structure using the general Amber force field (GAFF)10 and the GROMACS program package,11 as shown in Figure 3. The interaction energy between the host and guest

was bound exclusively to the DBA unit, accompanied by the unaffected proton signals of 2 and the downfield-shifted of Hh and Hi on DBA. The NMR result of 1 + 2 + 4 also confirms that BPP34C10 combines selectively with the viologen moiety because the NMR signals of 1 remain unchanged, while the chemical shifts of the viologen protons Hc, Hd, He, and Hf move upfield. The NMR spectra of 1 + 3 + 4 and 2 + 3 + 4 also indicate that only the DBA/DB24C8 and the viologen/ BPP34C10 complexes are formed, respectively, which further confirm the selective bindings. Then H1 and G1 were mixed in an equivalent ratio and the NMR is illustrated in Figure 2. The

Figure 3. (a) Side view and (b) top view of optimized structure of divalent pseudorotaxane P1.

Figure 2. Partial 1H NMR spectra (400 MHz, CDCl3/CD3CN (2:1, v/v), 6 mM, 298 K) of (a) G1, (b) equivalent G1+H1, and (c) H1. ∗ = peaks of solvent.

molecule is calculated as −674 kJ/mol, and the contributions from electrostatic and van der Waals interactions are −426 kJ/ mol and −248 kJ/mol, respectively. A π−π stacked structure is identified in the optimized host−guest complex, in accordance with the dominant van der Waals interaction. In addition, electrostatic interaction from the Viologen/BPP34C10 and DBA/DB24C8 pairs further stabilize the pseudorotaxane P1 system. The assembling of G1 and H1 into P1 is quite efficient in CHCl3/CH3CN = 2:1(v/v). Replacing the solvent by DMSO breaks the van der Waals and electrostatic interactions between H1 and G1, and P1 disintegrates into G1 and H1. This was

signals of Hh and Hi on the DBA unit undergo downfield shifts (both Δδ = 0.47 ppm), and those of Hc, Hd, He, and Hf upfield shift by values of 0.06, 0.36, 0.36, and 0.13 ppm, respectively. These results are in accordance with the above NMR models, indicating that H1 combines with G1 in equal stoichiometry, forming the divalent pseudorotaxane P1, where the DBA arm locates in the DB24C8 hole and the viologen branch is encapsulated by BPP34C10. The 2D NOESY NMR spectrum of equimolar mixtures of G1 and H1 in CHCl3/CH3CN (2:1, v/v) solution was also obtained. As illustrated in Figure S6, the B

DOI: 10.1021/acs.orglett.7b03867 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters confirmed by the 1H NMR measurements, as can be seen in Figure 4. When the solvent was removed and replaced by

Figure 5. Partial 1H NMR spectra (400 MHz, CDCl3/CD3CN (2/1, v/v), 6.0 mM, 298 K) of (a) G1+TEA (equivalent), (b) P1 + TEA (equivalent), (c) P1, and (d) P1 + TEA (equivalent) + TFA (equivalent). ∗ = solvent peaks; # = TEA peaks. Figure 4. Partial 1H NMR spectra (400 MHz, DMSO-d6, 6.0 mM, 298 K) of (a) G1, (b) equivalent mixtures of G1 and H1, and (c) H1. ∗ = peaks of solvent and H2O.

founded in the 2D NOESY NMR experiment of the P1+ TEA (Figure S15). These results indicate that the molecular padlock P1 changes into the unlocked state by adding TEA. Then trifluoroacetic acid (TFA) was added to the unlocked P1, and the resulting spectrum was illustrated in Figure 5d. The chemical shifts of Hh and Hi at 4.7 ppm reappear, and the 1H NMR spectra are consistent with those of P1, indicating that the DBA unit inserts again into DB24C8 and the molecular padlock restores to the locked state. It should be noted that the padlock function can be repeated (Figure S17). In summary, a host molecule H1 with two different crown ether holes and a “U”-shaped guest molecule G1 with two different binding sites were synthesized. H1 bound 1:1 with G1 to generate a divalent pseudorotaxane P1, which was confirmed by ITC, 1H NMR, 2D NOESY NMR, MALDI-TOF, UV−vis spectroscopy, fluorescence spectroscopy, and DLS experiments. P1 can fulfill the inserting/pulling out actions of a polarity plug−socket system and can also execute switching behavior like a padlock under the stimulation of acid/base. Such a divalent pseudorotaxne offers a constructive strategy to obtain a complex multifunctional molecular machine system by using simple molecular components.

DMSO-d6, the proton signals became those of free G1 and H1, indicating that the disassembling of P1 in DMSO was quantitative. Further NMR experiments indicate that P1 completely disintegrates into G1 and H1 when the ratio of DMSO versus CDCl3/CD3CN is up to 25:75 (Figure S16). The formation of the CT interactions between a viologen and an aromatic crown ether is also known to cause the quenching of the fluorescence of the aromatic crown ether, through the energy conversion from the aromatic ether excited state to the viologen/crown CT excited state.7b,c,9a We then carried out the fluorescence measurements, as shown in Figures S13 and S14. In agreement with these reported results, the additions of model viologen 2 or G1 to H1 solution were both found to quench the fluorescence of H1 in CHCl3/CH3CN (2:1), the dissociation of the plugged state in DMSO stopped the charge transfer interactions, and the fluorescence of H1 recovered. These results indicate that pseudorotaxane P1 can serve as a molecular polarized plug−socket. Besides the changing of solvents, the assembly and disassembly between DBA and DB24C8 could also be driven by pH.12 Therefore, it is also possible for P1 to implement a padlock function swing the DBA shackle arm away from the G1 body and leave to the viologen arm still secured in the BPP34C10 hole by adding a base, and reverting to the original state by addition of an acid. The pH driven padlock actions of P1 were monitored by 1H NMR, as shown in Figure 5. After the addition of equivalent of trimethylamine (TEA) to G1, the chemical shifts of Hh and Hi both moved upfield from 4.3 to 3.8 ppm because of the deprotonation of the ammonium group. Then one equivalent of TEA was added to P1. It can be seen that, on one hand, the Hh and Hi proton signals disappear and the integral value at around 3.8 ppm increased by 2, which is identical with the situation of G1+TEA, suggesting that the DBA leg of G1 was pulled out of from the DB24C8 hole; on the other hand, the chemical shifts of the viologen protons Hc, Hd, He, and Hf were unchanged when compared with P1, indicating that the viologen moiety was still wrapped by BPP34C10, which was further confirmed by the interrelated signals of the viologen protons with the alkyl crown protons

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03867. Details of experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qiaochun Wang: 0000-0002-2852-2463 Hans Ågren: 0000-0002-1763-9383 He Tian: 0000-0003-3547-7485 C

DOI: 10.1021/acs.orglett.7b03867 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC/China (21572063, 21372076), the Science Fund for Creative Research Groups (21421004), the Programme of Introducing Talents of Discipline to Universities (B16017), and the Fundamental Research Funds for the Central Universities (222201717003). We are grateful for the help from Ting Yin and Prof. Xinlong Ni in the group of Prof. Zhu Tao (Guizhou University, China) for carrying out the ITC measurement.

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DOI: 10.1021/acs.orglett.7b03867 Org. Lett. XXXX, XXX, XXX−XXX