English

• Motivated by widespread biological switches such as protein tyrosine phosphatases (PTPs), protein tyrosine kinases (PTKs), and GTPases, the construction of artificial molecular switches (AMSs), displaying dynamic stimuli-responsive behaviors, has a long history of capturing the full attention of scientists [1-5]. From the characteristics of AMS, it is considered that molecular tweezers with specific on−off switching properties are excellent candidates for building AMSs [6]. During the last decades, various molecular tweezers [7-13] have been developed via non-covalent interactions such as π–π interactions, metal–ligand coordination, ionic interactions and hydrogen bonding, which have shown fascinating applications in molecular recognition, optical switch and complex structure construction. As well known, precise regulation the conformation of the molecular tweezer is the key point in realizing their dynamic on−off switching properties. In this respect, the 2,2′-bipyridine ligand, whose conformation was able to interconvert between its anti and syn pattern when it coordinated with suitable metal ions, is an ideal allosteric center for constructing dynamic molecular tweezer [14,15].

  Fullerenes are well-known carbon allotrope with defined numbers of Csp² atoms, which own plenty of potential applications in multiple research areas [16-19]. Molecular tweezers as fullerene receptors have been thoroughly investigated due to their potential applications in the field of material science [20,21], photosynthetic techniques [22-24], and biochemistry [25,26]. More importantly, the development of dynamic molecular tweezers with on−off fullerenes recognition ability will promote the research of fullerenes. Porphyrins and their derivatives have a wide range of applications and functions regarding molecular recognition [27,28], solar cells [29-31], radical-based materials [32,33], photochemical catalysis [34], and optoelectronic materials [35]. Attributing to the highly delocalized π–electron rich characteristic, porphyrins can bind electron–deficient fullerenes through π–π interactions in the solid state [36]. Notably, when introducing porphyrin units to jaw- or pocket-like structures, the binding affinity to fullerenes can be dramatically enhanced as the result of the chelate effect for bidentate complexation [37,38]. The donor-acceptor property of the porphyrin-fullerene combination had numerous superiorities in establishing photovoltaic solar cells for mimicry of natural photosynthesis.

  To develop porphyrins-based molecular tweezers as fullerenes receptors should take the following objectives into consideration: (1) high binding ability towards fullerenes, (2) distinct selectivity for either C₆₀ or C₇₀, and (3) dynamic performance with on−off switchable behavior. One of the possible approaches to endow the dynamic switchable property is introducing coordination units into molecular tweezers structures. Controlling the coordination and decoordination processes, their binding ability could be adjusted as the conformation fluctuated. To attain this objective, we anticipated that molecular tweezers with 2,2′-bipyridine ligand and porphyrin units could exhibit high affinity to fullerenes and dynamic switchable molecular recognition ability. When the 2,2′-bipyridine ligands coordinated with metal ions like Zn(Ⅱ) and resulted in syn conformation, molecular tweezers were in the active model with an advisable cavity for fullerenes binding. After the treatment with certain chemicals (specific anion like H₂PO₄⁻ could competitively bind with Zn(Ⅱ)) that lead to the decoordination process, molecular tweezers could be unfolded into anti−conformation with the binding ability losing. Molecular tweezers could re-recognize with fullerenes after H₂PO₄⁻ precipitated by Ca²⁺ [39]. The dynamic porphyrin-based molecular tweezer's reversible recognition of fullerenes capability was adjusted by successively introducing H₂PO₄⁻ and Ca²⁺ (Fig. 1).

  Figure 1

  

  Figure 1.  Structures of C-ZnPP, and T-ZnPP and illustration of the molecular tweezer on and off fullerene recognition.

  DownLoad: Full-Size Img PowerPoint

  As shown in Scheme 1, anti-conformation T-ZnPP was designed and synthesized in good yield through a Suzuki coupling reaction between 4,4′-dibromo-2,2′-bipyridine and biphenyl-substituted zinc porphyrin, which was further characterized by ¹H NMR, ¹³C NMR, and mass (HRMS) spectroscopy (Figs. S14-S16 in Supporting information). The molecular tweezer C-ZnPP, in syn-conformation, was easily prepared in situ by zinc acetate coordinating with T-ZnPP (Scheme 1). As for the complex C-ZnPP, downfield and upfield shifts of the signals corresponding to protons H³ (∆δ = 0.04 ppm), H⁵ (∆δ = −0.31 ppm), and H⁶ (∆δ = −0.49 ppm) of the 2,2′-bipyridine parts indicated the fully sufficient conversion from T-ZnPP to C-ZnPP (Fig. S19 in Supporting information).

  Scheme 1

  

  Scheme 1.  Synthetic routes of T-ZnPP and C-ZnPP.

  DownLoad: Full-Size Img PowerPoint

  With the molecular tweezer (C-ZnPP) in hand, a series of UV−vis and fluorescence titration experiments were performed to verify the recognition capability of C-ZnPP to the fullerenes. Compound C-ZnPP exhibited an intense Soret band at 427 nm and two less intense Q-bands in the 500−700 nm region. As shown in Fig. 2a, upon adding C₆₀ with progressive concentrations (0−70 equiv.) to 0.5 × 10⁻⁶ mol/L solutions of C-ZnPP in toluene at 25 ℃, the Soret band at 426 nm withstood a slight bathochromic shift accompany with an obvious isosbestic point (431 nm). Small bathochromic shifts of the Soret bands of porphyrins were commonly observed on complexation of fullerenes, which here demonstrated typical π−π interaction between C-ZnPP and C₆₀, lowering the energy of the porphyrin π to π* transition [40]. Moreover, intensities of Q-bands were gradually increased due to the charge transfer interactions between porphyrin units and fullerenes [41]. The Job's plot analysis revealed the stoichiometry of C-ZnPP and C₆₀ was 1:1 (Fig. S20a in Supporting information). The binding constant (K_a) for the formation of C-ZnPP⊃C₆₀ complex was 1.05 × 10⁴ L/mol, which was evaluated (1:1 mode) spectrophotometrically by UV–vis titration data (Table S1 in Supporting information) [42]. The similar UV–vis titration experiments were conducted for C-ZnPP and C₇₀. As shown in Fig. 2c, the Soret band at 427 nm underwent barely shifts and a little hypochromicity with the addition of C₇₀ into the toluene solution of C-ZnPP, which suggested the binding ability was not as strong as that of C-ZnPP with C₆₀. The binding stoichiometry of C-ZnPP and C₇₀ was 1:1 (Fig. S20b in Supporting information) according to the corresponding Job's plot analysis (Fig. S20b), and the K_a value for the formation of complex C-ZnPP⊃C₇₀ was 2.38 × 10³ L/mol calculated from the UV–vis titration results using a 1:1 model (Table S1). Thus, the molecular tweezer C-ZnPP exhibited reasonable recognition capability towards to fullerenes (C₆₀ and C₇₀) and visible binding selectivity to C₆₀. The fluorescence titration experiments were also accomplished to estimate the binding ability of C-ZnPP to fullerenes. It is generally reported that porphyrin fluorescence is quenched when fullerenes approach, as a result of photoinduced electron transfer between ¹S of the porphyrins and fullerenes [43]. Upon addition of C₆₀ and C₇₀ to the toluene solution of C-ZnPP respectively, conspicuous quenching effect occurred when exciting at 427 nm as a result of complex C-ZnPP⊃C₆₀ and C-ZnPP⊃C₇₀ formation (Figs. 2b and d). Notably, the fluorescence measurements showed a larger quenching of the excited state of C-ZnPP by C₆₀ than C₇₀, which also reveals the binding selectivity that was consistent with the UV–vis titration results as mentioned above.

  Figure 2

  

  Figure 2.  UV–vis titration spectra of a toluene solution of C-ZnPP (0.5 × 10⁻⁶ L/mol) with addition of 0−70 equiv. of C₆₀ (a) and C₇₀ (c). Fluorescence titration spectra (λ_ex = 427 nm) of a toluene solution of C-ZnPP (0.5 × 10⁻⁶ L/mol) with addition of 0−70 equiv. of C₆₀ (b) and C₇₀ (d).

  DownLoad: Full-Size Img PowerPoint

  To gain further insight into the mechanism of the molecular tweezer C-ZnPP specifically binding with fullerenes, similar UV−vis and fluorescence titration experiments for T-ZnPP (anti-conformation) and control molecule 2HPP (no zinc in porphyrin units) were also performed. As for T-ZnPP, with the separate addition of fullerenes (0−50 equiv. of C₆₀ or C₇₀), the Soret band at 427 nm sustained no shifts and a slight lowering in intensity (Figs. S21a and c in Supporting information). Meanwhile, there was only a modest decrease in fluorescence emission intensity, which implied the fullerenes binding ability of T-ZnPP was relatively weak (Figs. S21b and d in Supporting information). Therefore, the UV–vis and fluorescence titration results affirmed that the syn-conformational structure of C-ZnPP played crucial roles in fullerenes recognition. C-ZnPP, which possesses the tweezer-like bis-porphyrin-based structure could tightly bind with fullerenes due to the appropriate cavity size between two porphyrin clefts for accepting fullerenes, while anti-conformational T-ZnPP does not. What is more, upon the separate addition of fullerenes (0−50 equiv. of C₆₀ or C₇₀) to a toluene solution of 2HPP, there was no obvious change in the UV–vis and fluorescence titration data (Fig. S22 in Supporting information). It is well known that metallo-porphyrins have a larger π−conjugated systems with 18 electrons, which are excellent components for binding with π−acceptor fullerenes [44]. 2HPP, without metal insertion, basically has extremely weak fullerenes recognition capability.

  Diffusion ordered spectroscopy (DOSY) experiments were also conducted to prove the formation of supramolecular complexes between C-ZnPP and fullerenes (C₆₀ or C₇₀). With the addition of 1 equiv. of C₆₀ to a 1.5 × 10⁻³ mol/L toluene solution of C-ZnPP, signals of C-ZnPP⊃C₆₀ complex were detected with a diffusion coefficient of 2.15 × 10⁻⁶ cm²/s, which was same as signals (D = 2.14 × 10⁻⁶ cm²/s) of free molecular tweezer C-ZnPP within error (Fig. S23 in Supporting information). It was proved that C-ZnPP can bind with C₆₀ to form a 1:1 complex and maintain its cavity size [8,45]. Furthermore, the diffusion coefficient of C-ZnPP⊃C₇₀ (D = 1.86 × 10⁻⁶ cm²/s) was smaller than C-ZnPP⊃C₆₀ complex, which could be believed that C-ZnPP were expanded due to its insufficient cavity for C₇₀. The diffusion coefficient data of C-ZnPP⊃C₆₀ and C-ZnPP⊃C₇₀ complexes demonstrated that the cavity size of C-ZnPP was more suited for C₆₀ than C₇₀, explaining why it had superior selectivity towards C₆₀ as determined by the binding constant, UV–vis, and fluorescence titration results previously.

  Thereafter, a series of ion-controlled coordination/decoordination experiments were carried out to investigate the switchable fullerene recognition ability of molecular tweezer C-ZnPP. As mentioned above, the significant fluorescence quenching phenomenon occurred when excessive C₆₀ added into the toluene solution of C-ZnPP. As shown in Fig. 3a, following the introduction of 0−5 equiv. H₂PO₄⁻ (tetrabutylammonium salt) to a toluene solution of C-ZnPP/70.0 equiv. C₆₀ mixture, it was found that the quenched fluorescence emission of C-ZnPP by C₆₀ was reasonably restored. It is inferred that Zn cation, which coordinated with the 2,2′-bipyridine components of C-ZnPP, was gradually competitively replaced by H₂PO₄⁻, and C-ZnPP was transformed to T-ZnPP bearing anti-conformation accompany with the binding ability to C₆₀ restraining. Therefore, the fluorescence emission intensity of the mixture solution was recovered. When 5 equiv. of H₂PO₄⁻ was added, the emission intensity was able to return to its original state and kept the stated steady as up to 10 equiv. of H₂PO₄⁻ was introduced (Fig. S24 in Supporting information). Moreover, the C₆₀ recognition on−off behaviors of C-ZnPP were further monitored by ¹H NMR spectroscopy in toluene-d₈ (Fig. 4a). Upfield shifts (∆δ = 0.051 ppm) of the signals corresponding to the β protons of porphyrin units of C-ZnPP⊃C₆₀ complex by virtue of shielding effect which compared to free C-ZnPP indicated that π–π stacking interactions between C-ZnPP and C₆₀. When adding H₂PO₄⁻ to the C-ZnPP⊃C₆₀ complex solution, it was found that signals for β protons suffered downfield shifts (∆δ = 0.046 ppm) and came back to the same position as the C-ZnPP itself, within error, due to the structural transformations of the molecular tweezer from syn to anti conformation. ¹³C NMR spectroscopy is also very practical for disclosing the details of fullerenes complexation, since C₆₀ has a unique signal. Obvious upfield shifts (∆δ = 0.15 ppm) of the signals ascribing to C₆₀ belonging to C-ZnPP⊃C₆₀ complex were found compared to free C₆₀ because of ring current effects of porphyrins (Fig. 4b). Thus, the upfield shift should be the evidence for effective complexation between C-ZnPP and C₆₀ [24,37]. Subsequently, upon the addition of H₂PO₄⁻ to C-ZnPP⊃C₆₀ complex solution, signals of C₆₀ moved to downfield and returned to the free C₆₀ state due to interactions between C-ZnPP and C₆₀ diminishing. In general, ¹H and ¹³C NMR spectroscopy results were consistent with the switchable ion-controlled fullerene recognition process as assumed and in line with the conclusion obtained by UV−vis and fluorescence spectroscopy experiments.

  Figure 3

  

  Figure 3.  Fluorescence spectra (λ_ex = 427 nm) of the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L) and 70 equiv. of C₆₀ in toluene with the addition of 0−5 equiv. of H₂PO₄⁻ (a), and the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L), 70 equiv. of C₆₀, and 5 equiv. of H₂PO₄⁻ with the addition of 0−5 equiv. of Ca²⁺ (b). (c) Fluorescence spectra (λ_ex = 427 nm) of the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L) and 70 equiv. of C₆₀ with the sequentially cyclical addition of 5 equiv. of H₂PO₄⁻ and Ca²⁺. (d) Reversible cycling of the fluorescence emission intensity at 576 nm of four consecutive association-dissociation processed by successively adding H₂PO₄⁻ and Ca²⁺.

  DownLoad: Full-Size Img PowerPoint

  Figure 4

  

  Figure 4.  (a) Stacked ¹H NMR spectra (400 MHz, toluene-d₈, 298 K) of a 1 × 10⁻³ mol/L solution of C-ZnPP, C-ZnPP with 1 equiv. of C₆₀, and 1:1 mixture of C-ZnPP and C₆₀ with 1 equiv. of H₂PO₄⁻. (b) Stacked ¹³C NMR spectra (100 MHz, toluene-d₈, 298 K) of a 1 × 10⁻³ mol/L solution of C-ZnPP with 1 equiv. of C₆₀, C₆₀, and 1:1 mixture of C-ZnPP and C₆₀ with 1 equiv. of H₂PO₄⁻.

  DownLoad: Full-Size Img PowerPoint

  In light of the relatively poor solubility of Ca(H₂PO₄)₂ in toluene, Ca²⁺ was employed to capture H₂PO₄⁻ and reproduce the syn conformational C-ZnPP with the C₆₀ binding ability recovery. The fluorescence titration experiments showed that upon 0−5 equiv. of Ca(ClO₄)₂ were progressively added to a toluene solution of C-ZnPP/70 equiv. C₆₀/5 equiv. H₂PO₄⁻ mixture, a distinct quenching effect took place, which suggested the regenerate of C-ZnPP (Fig. 3b). It is worth to mention that H₂PO₄⁻ or Ca²⁺ have no interactions with the zinc ions from the porphyrin units based on the UV–vis and fluorescence spectra of 2HPP, T-ZnPP, and T-ZnPP with separate addition of H₂PO₄⁻ or Ca²⁺ (Fig. S25 in Supporting information). With the addition of H₂PO₄⁻ or Ca²⁺ to T-ZnPP solution, it exhibited characteristic two Q-bands of metallo-porphyrins in the 500−700 nm region, whereas porphyrins, like 2HPP, exhibited four Q-bands (Fig. S25a). What's more, when introducing H₂PO₄⁻ or Ca²⁺ to T-ZnPP solution, the emission λ_max of around 597 nm was still the same (Fig. S25b). Generally, the whole dissociation−association of C₆₀ by C-ZnPP process controlled through alternately adding H₂PO₄⁻ and Ca²⁺ was identified to be effectively repeatable with the excellent reversibility (Figs. 3c and d).

  In conclusion, we designed and synthesized a novel molecular tweezer based on the 2,2′-bipyridine-bridged zinc-porphyrin in syn-conformation, which has the appropriate cavity to adequately bind the fullerene forming 1:1 supramolecular complex. It exhibited distinct binding selectivity towards C₆₀ (K = 1.05 × 10⁴ L/mol) over C₇₀ (K = 2.38 × 10³ L/mol). Furthermore, the fullerene recognition capacity of the molecular tweezer was able to be switched by introducing H₂PO₄⁻ and Ca²⁺ successively. Upon the addition of H₂PO₄⁻, the molecular tweezer could undergo a syn−anti conformational conversion with the binding ability vanishing. After adding Ca²⁺ to competitively capture H₂PO₄⁻, the molecular tweezer regenerated and the fullerene binding ability was fully recovered. More importantly, the molecular tweezer underwent conformational conversion between its syn- and anti-form accompany with the association-dissociation process reversibly and repeatably, which were demonstrated by ¹H NMR, ¹³C NMR, UV−vis, and fluorescence spectroscopy analyses. This dynamically switchable molecular tweezer with on−off fullerene recognition ability is expected to be used in the field of fullerene separation and donor−acceptor photovoltaic solar cells.

  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 financial support of the National Natural Science Foundation of China (No. 21801060), Natural Science Foundation of Hebei Province (Nos. B2019205172, 226Z1501G), and China Postdoctoral Science Foundation (No. 2020TQ0087). We also gratefully acknowledge the support from the National Demonstration Center for Experimental Chemistry Education of Hebei Normal University.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109162.

  
  
• 1. [1]

     R.A. Bissell, E. Córdova, A.E. Kaifer, J.F. Stoddart, Nature 369 (1994) 133–137. doi: 10.1038/369133a0

  2. [2]

     L. Zhang, H.X. Wang, S. Li, M. Liu, Chem. Soc. Rev. 49 (2020) 9095–9120. doi: 10.1039/d0cs00191k

  3. [3]

     F. Gao, X. Yu, L. Liu, et al., Chin. Chem. Lett. 34 (2023) 107558. doi: 10.1016/j.cclet.2022.05.072

  4. [4]

     C. Wang, Y.M. Zhang, H. Li, et al., Chin. Chem. Lett. 33 (2022) 2447–2450. doi: 10.1016/j.cclet.2021.09.106

  5. [5]

     A. Blanco-Gómez, P. Cortón, L. Barravecchia, et al., Chem. Soc. Rev. 49 (2020) 3834–3862. doi: 10.1039/d0cs00109k

  6. [6]

     B.L. Feringa, Angew. Chem. Int. Ed. 56 (2017) 11060–11078. doi: 10.1002/anie.201702979

  7. [7]

     F.G. Klärner, T. Schrader, Acc. Chem. Res. 46 (2013) 967–978. doi: 10.1021/ar300061c

  8. [8]

     M. Takeda, S. Hiroto, H. Yokoi, et al., J. Am. Chem. Soc. 140 (2018) 6336–6342. doi: 10.1021/jacs.8b02327

  9. [9]

     D.C. Yang, M. Li, C.F. Chen, Chem. Commun. 53 (2017) 9336–9339. doi: 10.1039/C7CC03519E

  10. [10]

      G. Yin, J. Huang, D. Liu, et al., Chin. Chem. Lett. 34 (2023) 107290. doi: 10.1016/j.cclet.2022.03.013

  11. [11]

      D. Jia, H. Zhong, S. Jiang, R. Yao, F. Wang, Chin. Chem. Lett. 33 (2022) 4900–4903. doi: 10.1016/j.cclet.2022.02.081

  12. [12]

      A. Jozeliu̅naitė, T. Javorskis, V. Vaitkevičius, V. Klimavičius, E. Orentas, J. Am. Chem. Soc. 144 (2022) 8231–8241. doi: 10.1021/jacs.2c01455

  13. [13]

      S. Yuan, Y. Zhou, T. Gao, et al., Chin. Chem. Lett. 35 (2024) 108404. doi: 10.1016/j.cclet.2023

  14. [14]

      G. Bastien, P.I. Dron, M. Vincent, et al., Org. Lett. 18 (2016) 5856–5859. doi: 10.1021/acs.orglett.6b02915

  15. [15]

      A. Sacristán-Martín, H. Barbero, S. Ferrero, et al., Chem. Commun. 57 (2021) 11013–11016. doi: 10.1039/d1cc03451k

  16. [16]

      C. Thilgen, F. Diederich, Chem. Rev. 106 (2006) 5049–5135. doi: 10.1021/cr0505371

  17. [17]

      Q. Tang, S. Maji, B. Jiang, et al., ACS Nano 13 (2019) 14005–14012. doi: 10.1021/acsnano.9b05938

  18. [18]

      Y. Wang, M. Zhong, J. Li, et al., Chin. Chem. Lett. 33 (2022) 1074–1076. doi: 10.1016/j.cclet.2021.05.060

  19. [19]

      X. Dong, Z. Xin, D. He, et al., Chin. Chem. Lett. 34 (2023) 107459. doi: 10.1016/j.cclet.2022.04.057

  20. [20]

      J. Crassous, Chem. Soc. Rev. 38 (2009) 830–845. doi: 10.1039/b806203j

  21. [21]

      V. Valderrey, G. Aragay, P. Ballester, Coord. Chem. Rev. 258-259 (2014) 137–156. doi: 10.1016/j.ccr.2013.08.040

  22. [22]

      J. Song, N. Aratani, H. Shinokubo, A. Osuka, J. Am. Chem. Soc. 132 (2010) 16356–16357. doi: 10.1021/ja107814s

  23. [23]

      H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 14 (2004) 525–536. doi: 10.1002/adfm.200305172

  24. [24]

      R.M.K. Calderon, J. Valero, B. Grimm, J. de Mendoza, D.M. Guldi, J. Am. Chem. Soc. 136 (2014) 11436–11443. doi: 10.1021/ja5052236

  25. [25]

      J.R. Moffitt, Y.R. Chemla, S.B. Smith, C. Bustamante, Annu. Rev. Biochem. 77 (2008) 205–228. doi: 10.1146/annurev.biochem.77.043007.090225

  26. [26]

      A. Turchanin, A. Tinazli, M. El-Desawy, et al., Adv. Mater. 20 (2008) 471–477. doi: 10.1002/adma.200702189

  27. [27]

      Y. Rao, L. Xu, M. Zhou, et al., Angew. Chem. Int. Ed. 61 (2022) e202206899. doi: 10.1002/anie.202206899

  28. [28]

      I. Beletskaya, V.S. Tyurin, A.Y. Tsivadze, R. Guilard, C. Stern, Chem. Rev. 109 (2009) 1659–1713. doi: 10.1021/cr800247a

  29. [29]

      J. Zou, Y. Wang, G. Baryshnikov, et al., ACS Appl. Mater. Interfaces 14 (2022) 33274–33284. doi: 10.1021/acsami.2c07950

  30. [30]

      J. Luo, Y. Wang, S. Shi, et al., J. Mater. Chem. C 11 (2023) 5450–5460. doi: 10.1039/d2tc05167b

  31. [31]

      T. Zhang, X. Qian, P.F. Zhang, Y.Z. Zhu, J.Y. Zheng, Chem. Commun. 51 (2015) 3782–3785. doi: 10.1039/C4CC09640A

  32. [32]

      H. Gao, F. Wu, Y. Zhao, et al., J. Am. Chem. Soc. 144 (2022) 3458–3467. doi: 10.1021/jacs.1c11716

  33. [33]

      J. Xu, L. Zhu, H. Gao, et al., Angew. Chem. Int. Ed. 60 (2021) 11702–11706. doi: 10.1002/anie.202016674

  34. [34]

      A. Maldotti, R. Amadelli, C. Bartocci, et al., Coord. Chem. Rev. 125 (1993) 143–154. doi: 10.1016/0010-8545(93)85014-U

  35. [35]

      A. Wang, J. Ye, M.G. Humphrey, C. Zhang, Adv. Mater. 30 (2018) 1705704. doi: 10.1002/adma.201705704

  36. [36]

      P.D.W. Boyd, M.C. Hodgson, C.E.F. Rickard, et al., J. Am. Chem. Soc. 121 (1999) 10487–10495. doi: 10.1021/ja992165h

  37. [37]

      D. Sun, F.S. Tham, C.A. Reed, et al., J. Am. Chem. Soc. 122 (2000) 10704–10705. doi: 10.1021/ja002214m

  38. [38]

      D. Sun, F.S. Tham, C.A. Reed, L. Chaker, P.D.W. Boyd, J. Am. Chem. Soc. 124 (2002) 6604–6612. doi: 10.1021/ja017555u

  39. [39]

      X. Fang, Y.Z. Zhu, J.Y. Zheng, J. Org. Chem. 79 (2014) 1184–1191. doi: 10.1021/jo4026176

  40. [40]

      P.D.W. Boyd, Acc. Chem. Res. 38 (2005) 235–242. doi: 10.1021/ar040168f

  41. [41]

      N. Armaroli, G. Marconi, L. Echegoyen, J.P. Bourgeois, F. Diederich, Chem. Eur. J. 6 (2000) 1629–1645. doi: 10.1002/(sici)1521-3765(20000502)6:9<1629::aid-chem1629>3.0.co;2-z

  42. [42]

      P. Thordarson, Chem. Soc. Rev. 40 (2011) 1305–1323. doi: 10.1039/C0CS00062K

  43. [43]

      D.M. Guldi, C. Luo, M. Prato, E. Dietel, A. Hirsch, Chem. Commun. 5 (2000) 373–374.

  44. [44]

      K. Tashiro, T. Aida, Chem. Soc. Rev. 36 (2007) 189–197. doi: 10.1039/B614883M

  45. [45]

      S. Kawano, T. Fukushima, K. Tanaka, Angew. Chem. Int. Ed. 57 (2018) 14827–14831. doi: 10.1002/anie.201809167

• 

  Figure 1  Structures of C-ZnPP, and T-ZnPP and illustration of the molecular tweezer on and off fullerene recognition.

  下载: 全尺寸图片 幻灯片
  

  Scheme 1  Synthetic routes of T-ZnPP and C-ZnPP.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  UV–vis titration spectra of a toluene solution of C-ZnPP (0.5 × 10⁻⁶ L/mol) with addition of 0−70 equiv. of C₆₀ (a) and C₇₀ (c). Fluorescence titration spectra (λ_ex = 427 nm) of a toluene solution of C-ZnPP (0.5 × 10⁻⁶ L/mol) with addition of 0−70 equiv. of C₆₀ (b) and C₇₀ (d).

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Fluorescence spectra (λ_ex = 427 nm) of the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L) and 70 equiv. of C₆₀ in toluene with the addition of 0−5 equiv. of H₂PO₄⁻ (a), and the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L), 70 equiv. of C₆₀, and 5 equiv. of H₂PO₄⁻ with the addition of 0−5 equiv. of Ca²⁺ (b). (c) Fluorescence spectra (λ_ex = 427 nm) of the mixture of C-ZnPP (0.5 × 10⁻⁶ mol/L) and 70 equiv. of C₆₀ with the sequentially cyclical addition of 5 equiv. of H₂PO₄⁻ and Ca²⁺. (d) Reversible cycling of the fluorescence emission intensity at 576 nm of four consecutive association-dissociation processed by successively adding H₂PO₄⁻ and Ca²⁺.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Stacked ¹H NMR spectra (400 MHz, toluene-d₈, 298 K) of a 1 × 10⁻³ mol/L solution of C-ZnPP, C-ZnPP with 1 equiv. of C₆₀, and 1:1 mixture of C-ZnPP and C₆₀ with 1 equiv. of H₂PO₄⁻. (b) Stacked ¹³C NMR spectra (100 MHz, toluene-d₈, 298 K) of a 1 × 10⁻³ mol/L solution of C-ZnPP with 1 equiv. of C₆₀, C₆₀, and 1:1 mixture of C-ZnPP and C₆₀ with 1 equiv. of H₂PO₄⁻.

  下载: 全尺寸图片 幻灯片
•