C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts
2022; Chinese Chemical Society; Volume: 5; Issue: 4 Linguagem: Inglês
10.31635/ccschem.022.202202019
ISSN2096-5745
AutoresJialin Xie, Xia Li, Zhenglin Du, Yandie Liu, Kelong Zhu,
Tópico(s)Boron and Carbon Nanomaterials Research
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE25 May 2022C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts Jialin Xie†, Xia Li†, Zhenglin Du, Yandie Liu and Kelong Zhu Jialin Xie† School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Xia Li† School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Zhenglin Du School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Yandie Liu School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 and Kelong Zhu *Corresponding author: E-mail Address: [email protected] School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 https://doi.org/10.31635/ccschem.022.202202019 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Hydrogen-bonded capsules have been widely employed as supramolecular hosts for organic molecular guests. Encapsulation of fullerenes by capsules is relatively scarce, especially those that utilize sulfur atoms as hydrogen-bond acceptors. Herein, we describe, in both solution and solid state, a bowl-shaped nanobelt [8]cyclophenoxathiin 1a and its tetra-methylated derivative 1b that can form C–H⋯S hydrogen-bonded capsules induced by complexation with suitable fullerenes. 1a strongly encapsulates C60, C70, or 6,6-phenyl-C61-butyric acid methyl ester (PC61BM) to form a 2∶1 ternary complex featuring 16 equatorial (sp2)C–H⋯S hydrogen bonds. A pseudorotaxane structure was further obtained for the complex of 1a with PC61BM. Conversely, a 1∶1 inclusion complex was observed for binding C60 or PC61BM with 1b indicating the reduced tendency to form capsules by introducing methyl groups into the belt. Surprisingly, the capsule-like structure was retained for the 1:2 complex of C70 with 1b as observed by the presence of multiple (sp3)C–H⋯S hydrogen bonds. The strong binding affinity and tailorable complexation mode enable further applications of nanobelts in fullerene chemistry. Download figure Download PowerPoint Introduction Fullerenes, as stable 0D carbon allotropes, have emerged as a set of well-known star molecules, not only due to their intriguing fully π-conjugated spherical surfaces,1 but also their wide applications in energy storage and conversion materials, superconductors, and biomedicines.2–4 To date, the molecular-recognition based host–guest chemistry of fullerenes through non-covalent interactions is gradually turning into one of the landmarks in supramolecular chemistry because it opens up new opportunities in fullerenes purification and functionalization, which can lead to the improvement of photoelectric generating efficiency.5 Compared with covalent derivation of fullerenes, a supramolecular modification method is highly desirable since it could maintain the superior electronic properties of these balloon cages to the greatest extent possible. In this vein, design of macrocyclic receptors with strong binding affinity and tailorable complexation selectivity has emerged as the focal point of fullerene host–guest chemistry. Against this backdrop, various sophisticated artificial macrocycles6,7 with large lipophilic cavities have been developed based on calixarenes,8–10 cyclotriveratrylenes,11,12 π-extended-tetrathiafulvalenes,13,14 porphyrins,15–20 and calixpyrroles.21,22 Recently, this field has entered a new stage with the emergence of hooped or belt-shaped macrocycles.23–28 Due to their large π-conjugated and curved surfaces, polycyclic aromatic hydrocarbon nanohoops have proven to be particularly effective in binding fullerenes.29–33 Despite these reports, there are very few examples of double-stranded nanobelts being used as fullerene receptors.34–38 In particular, nanobelts capable of efficiently binding a variety of fullerenes by forming dimeric capsules remain inadequately explored. Hydrogen-bonded39 dimeric supramolecular capsules have been known for calixarenes,40,41 resorcin[4]arene,42,43 cavitands,44–47 calix[4]pyrroles,48,49 cyclodextrins,50–52 and so on.53–58 So far, various experiments have shown that there are two main mechanisms for the formation of these capsules: (1) two receptors form a capsule spontaneously without guest (or solvent) occupying the cavity, and (2) host–guest complexation induced encapsulation processes in the presence of a suitable guest molecule.59 In most cases, nitrogen and oxygen atoms are employed as hydrogen bond acceptors (X–H⋯Y, Y = N or O), whereas very few examples have been reported on the use of sulfur atoms, which are much less electronegative and usually not favorable for hydrogen bonding.60,61 In 2011, Rebek and co-workers62 reported a unique capsule assembled by dimerization of a thiourea-derived cavitand with 16 N–H⋯S hydrogen bonds in the presence of a small guest molecule (Figure 1a). To date, very few examples of hydrogen-bonded dimeric capsules are known for encapsulating fullerenes.42,43,50,53,63 A representative work was reported by de Mendoza and co-workers demonstrating that ureidopyrimidinone-derived cyclotriveratrylene can form a hydrogen-bonded capsule to preferentially bind C70 obeying the first mechanism.53 Figure 1 | (a) N–H⋯S hydrogen-bonded (active hydrogen as donor) capsule in the presence of small molecule; (b) An example of aromatic C–H⋯S hydrogen-bonded capsule induced by complexing fullerene C60 with two nanobelts. (c) Structures of a tetra-methylated nanobelt and fullerene guests discussed in this study. Download figure Download PowerPoint Recently, we developed a bowl-shaped heteroatom (S, O)-bridged nanobelt 1a, namely [8]cyclophenoxathiin (Figure 1b). With a π-electron-rich concave aromatic cavity and preorganized hydrogen bond donors (sp2-C–H) and acceptors (S atoms) on its upper rim, 1a efficiently forms a dimeric capsule when C60 is encapsulated.63 The strong binding affinity and its unique (sp2)C–H⋯S hydrogen-bonded capsule structure have promptly raised two questions: (1) how is the host–guest chemistry and binding mechanism of 1a towards fullerenes other than C60; and (2) could such a dimeric encapsulation mode be tunable and controllable by structure engineering?64 To address these questions, we have first targeted the complexation of 1a with ellipsoidal C70 and a functionalized C60 (6,6-phenyl-C61-butyric acid methyl ester, PC61BM) for comparison with that of C60. Moreover, to probe the role of side groups in dimerization, we have replaced four (sp2)C–H hydrogen atoms with methyl groups at the upper rim of 1a to afford a deeper belt container 1b (Figure 1c).64,65 By comparison of its fullerene binding behavior with 1a, we would like to gain more insights into the mechanism of hydrogen-bonded-capsule formation utilizing nanobelts. Herein, we report the synthesis and host–guest complexation study of the heteroatom (S, O)-bridged nanobelt 1a with C70 and PC61BM and contrast the host–guest behavior of 1a with its tetra-methylated counterpart 1b. Experimental Methods All reagents were purchased from commercial suppliers and used without further purification unless otherwise noted. Solvents were either used as purchased or degassed and dried under a Vigor VSGS-5 Solvent Purification System (Vigor Gas Purification Technologies (Suzhou) Co., Ltd., Suzhou, Jiangsu, China). Detailed synthetic procedures are listed in the Supporting Information Scheme S1. The compounds were characterized by 1H NMR, 13C NMR, 2D NMR, and mass spectroscopy ( Supporting Information Figures S1–S2 and S27–S42). NMR spectra and 1H NMR titrations experiments were recorded on a JEOL 400YH instrument (JEOL Co., Ltd., Akishima, Tokyo, Japan). The UV–vis titrations experiments were conducted on a Shimadzu UV-2600 spectrophotometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). X-ray diffraction analysis was conducted on a Bruker D8 VENTURE PHOTON III diffractometer using Ga Kα or Mo Kα radiation (Bruker AXS GmbH - Karlsruhe, Germany). The crystal data are summarized in the Supporting Information Figures S20–S26 and Tables S4–S6. Synthesis of [8]cyclophenoxathiin 1b Under a N2 atmosphere, a 100 mL dry round-bottom flask was charged with compound C (139 mg, 0.1 mmol) and 20 mL trifluoromethanesulfonic acid. The reaction mixture was stirred at 80 °C for 60 h and cooled to room temperature, then slowly poured into 80 mL 1/1 (v/v) pyridine/ice water and stirred at 105 °C for another 15 h. After the reaction was complete, the solvent was removed under vacuum, acidified with aqueous HCl (1 M), and extracted with three portions of CH2Cl2. The organic layers were combined and dried over anhydrous Na2SO4, filtered, and concentrated in vacuum. The crude product was purified by column chromatography on silica gel with (CH2Cl2/cyclohexane = 1/10) as eluent to give the product 1b (53 mg, 42% yield) as a white solid. 1H NMR (400 MHz, CDCl3, δ): 7.05 (s, 4H), 6.82 (s, 4H), 2.85 (t, J = 7.2 Hz, 8H), 2.28 (s, 12H), 1.46–1.38 (m, 16H), 0.97–0.92 (m, 12H). 13C NMR (100 MHz, CDCl3, δ): 154.7, 151.5, 130.3, 125.9, 121.3, 120.0, 119.3, 108.7, 32.4, 23.3, 22.8, 17.3, 14.0. HRMS (m/z): [M]+ calcd for C68H56O8S8, 1256.1735; found, 1256.1735. Results and Discussion Synthesis and characterization The synthesis of nanobelt 1a has been reported previously.63 The deeper belt container 1b was successfully constructed by similar procedures using a cyclization and subsequent bridging strategy (Scheme 1). Starting from 1,3-dimethoxy-5-methylbenzene, the key monomer diphenolic A was obtained in five steps with an overall yield of 25% (See Supporting Information). The following [2+2] Ullmann ether cyclization reaction of A with the dibromo connector B generated the oxo-stranded macrocyclic intermediate C with 36% yield. Finally, the sulfur bridges were installed by an intramolecular Pummerer-like reaction and afforded the double-stranded 1b with a yield of 42%. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis showed an ion peak at m/z of 1256.1735, corresponding to M+•, confirming its correct formula as predicted ( Supporting Information Figure S2). The 1H NMR spectrum of 1b is relatively simple, as predicted by its high C4v symmetry, with two aromatic resonance peaks, Ha and Hc, centered at 7.05 and 6.82 ppm, respectively (Figure 2a). The peak assignment was achieved by the assistance of 2D NMR analysis ( Supporting Information Figures S41–S43). Single crystals of 1b were obtained from slow evaporation of a toluene solution. Single-crystal X-ray diffraction (SCXRD) analysis unambiguously revealed a bowl-shaped belt structure of 1b with four methyl groups distributed at the upper rim (Figure 2b and Supporting Information Figure S20).a 1b adopts an elliptical conformation with an average short and long axis of 11.1 and 13.3 Å, respectively. Such a de-symmetrized structure in solid state is most likely dictated by molecular packing. Furthermore, unlike that of 1a, 1b does not form a dimeric capsule in the solid state when the macrocyclic cavity is filled with solvents indicating its retarded ability to form hydrogen-bonded capsules as we designed (Figure 2c). Scheme 1 | Synthesis of deeper belt-container 1b. Conditions: (i) Cs2CO3, CuI, N,N-dimethylglycine, N,N-dimethylacetamide, 150 °C for 48 h; (ii) CF3SO3H, 80 °C for 60 h, then pyridine/H2O, 105 °C for 15 h. Download figure Download PowerPoint Host–guest chemistry of nanobelts with C60 In our previous report, we investigated the complexation of 1a with C60 and found that 1a can efficiently bind C60 to form a unique (sp2)C–H⋯S hydrogen-bonded capsule structure with a strong binding affinity (Ka = 3.6 × 109 M−2) in o-dichlorobenzene (o-DCB).63 Accordingly, with 1b in hand, we first looked into its host–guest chemistry towards C60. MALDI-TOF-MS analysis of a 2∶1 mixture of 1b and C60 in chloroform only exhibited an intense m/z peak at 1977.1769 corresponding to 1∶1 complex C60@ 1b ( Supporting Information Figure S4). The binding stoichiometry was further supported by 1H NMR spectroscopic titration in dichloromethane (Figure 3a and Supporting Information Figure S3). When 1b was subjected to 0.5 equiv of C60, two sets of broadened proton signals associated with an exchange rate slower than the NMR time scale were observed. Upon addition of 1.0 equiv of C60, signals for free 1b disappeared, only leaving a set of sharp and slight up-field shifted signals, proving the 1∶1 stoichiometric binding. The up-field shifting of belt protons a and c could be attributed to the shielding effect from π–π interaction with C60. The binding affinity was further quantified by UV–vis absorption spectroscopic titration (Figure 3b and Supporting Information Figure S5), wherein an obvious broad charge-transfer band centered around 520 nm appeared and developed upon increasing the amounts of 1b. By fitting the titration data at 520 nm, the association constant Ka was calculated to be ca. 3.7 × 105 M−1 (Table 1).66 Single crystals of complex C60@ 1b were grown by diffusing n-hexane into an equimolar mixture of C60 and 1b in o-DCB. The X-ray crystal structure revealed that C60 is bound in the cavity of 1b with nearly half of the sphere wrapped by the panels of the belt (Figure 3c and Supporting Information Figure S21), confirming the 1:1 binding stoichiometry observed in solution. Compared with the elliptical conformation observed for the toluene bound structure (Figure 2b), the belt 1b adopts a round shape in the C60 complexed structure to maximize the concave–convex π–π interactions (centroid-to-centroid average distance = 3.80 Å, dash lines, Figure 3c). In addition, the average carbon–carbon distance between the methyl carbon atom and its nearest C60 carbon atom is approximately 3.04 Å, which is consistent with the existence of C–H⋯π interactions. Thus, the introduction of methyl groups on the upper rim successfully suppresses the formation of a dimeric capsule of 1b upon complexation of C60. Figure 2 | (a) 1H NMR spectra comparison of 1a (top) with 1b (bottom). (b) Single-crystal X-ray structure of 1b with two toluene molecules complexed. (c) Molecular packing of 1b in its single-crystal form. NMR peak assignment was referenced to Figure 1b and Scheme 1. *, CDCl3. Download figure Download PowerPoint Figure 3 | (a) Partial 1H NMR spectra of 1b with different amounts of C60 (400 MHz, CDCl3/CS2 = 4/1, 298 K); (b) UV–vis absorption spectra of C60 upon titrating with 1b from 0 to 3 equiv. Inset: fitting curve at λ = 520 nm; (c) X-ray crystal structure of C60@1b. The π–π and C–H⋯π interactions are highlighted in red and purple, respectively. Download figure Download PowerPoint Table 1 | Summary of Association Constantsa Host K, αb C60 C70 PC61BM 1a K1 = 1.3 × 104 M−1c K1 = 1.7 × 105 M−1c K1 = 8.0 × 105 M−1d K2 = 2.8 × 105 M−1 K2 = 3.5 × 106 M−1 K2 = 1.5 × 104 M−1 Ka = 3.6 × 109 M−2 Ka = 5.9 × 1011 M−2 Ka = 1.2 × 1010 M−2 α = 88 α = 82 α = 0.075 1b Ka = 3.7 × 105 M−1e K1 = 2.3 × 105 M−1d K2 = 5.8 × 103 M−1 Ka = 1.3 × 109 M−2 Ka = 1.2 × 106 M−1e α = 0.101 aThe association constants K were obtained by UV–vis titration spectroscopy. bThe cooperativity factor is defined as α = 4K2/K1. cMeasured in o-dichlorobenzene. dMeasured in 1,1,2,2-tetrachloroethane. eDichloromethane. Host–guest chemistry of nanobelts with C70 After gaining insights into binding C60 with nanobelts, we turned our attention to the larger fullerene C70. 1H NMR spectroscopic titration was first carried out on the complexation of 1a with C70 (Figure 4a and Supporting Information Figure S6). After adding 0.25 equiv of C70 to 1a in CDCl3, two sets of signals, corresponding to free host 1a and the ternary complex C70@ 1a2, instantly appeared in the 1H NMR spectrum. This binding behavior is very similar to that previously reported for 1a with C60 indicating a possibly identical complexation with a stoichiometric ratio of 2∶1.63 Besides this, the upper rim protons (a and b) shifted downfield 0.24 and 0.25 ppm, respectively, while the lower rim proton c only shifted 0.08 ppm. Due to the ellipsoidal geometry of C70, the fullerene has an extended shielding belt in the equatorial position, while a strong deshielding effect was observed for the two apical pentagons of C70.67 This allows us to reasonably speculate on the existence of C–H⋯S hydrogen bonds between the two belts and the formation of a hydrogen-bonded capsule complex. This was further supported by MALDI-TOF-MS analysis observing an intense peak at m/z of 3243.231, corresponding to the C70@ 1a2 complex ( Supporting Information Figure S7). The absence of signals for 1:1 complex in either the NMR or MS analysis implies an immediate equilibration between both complexes, and possibly a positive cooperative binding with preference of forming the 2:1 complex.63 Conversely, both signals of 1:1 (m/z = 2098.1757) and 2:1 (m/z = 3355.3485) complex were observed on MALDI-TOF-MS analysis of a mixture of 1b and C70 ( Supporting Information Figure S10). Slow exchange between these two complexes was clearly evidenced by 1H NMR titration of 1b with C70 (Figure 4b and Supporting Information Figure S9). With a 1b/C70 mole ratio of 1∶1, two sets of resonance signals appeared with the major species ascribed to the C70@ 1b complex ( A labels in Figure 4b). Upon increasing the ratio to 2∶1, resonance signals for C70@ 1b almost disappeared while the other set of signals corresponding to the complex C70@ 1b2 ( B labels in Figure 4b) became predominant. Interestingly, the methyl proton Meb underwent an abnormal down-field shift, from 2.16 to 2.43 ppm, during the formation of the 2∶1 complex. This observation has led us to speculate that the 2∶1 complex of 1b and C70 could still form a hydrogen-bonded capsule structure with the assistance of unusual (sp3)C–H⋯S hydrogen bonds between the methyl protons and the sulfur atoms in the upper rim of the belt. Figure 4 | (a) Partial 1H NMR spectra of 1a with different amounts of C70 (400 MHz, CDCl3, 298 K). (b) Partial 1H NMR spectra of 1b with different amounts of C70 (400 MHz, 1,1,2,2-tetrachloroethane-d2, 298 K). (c) UV–vis absorption spectra of C70 upon titrating with 1a from 0 to 6 equiv. Inset: fitting curve at λ = 382 nm. (d) UV–vis absorption spectra of C70 upon titrating with 1b from 0 to 8 equiv. Inset: fitting curve at λ = 380 nm. A, 1∶1 complex; B, 2∶1 complex. Download figure Download PowerPoint The binding strength of 1a and 1b towards C70 was further probed by UV–vis absorption titration ( Supporting Information Figures S8 and S11). As shown in Figure 4c, with gradual increase of 1a/C70 ratio in o-DCB, two well-defined isosbestic points at 432 and 502 nm were observed, which means that the absorbing species had been converted. The absorption intensity of C70 at 382 nm noticeably decreased and gradually shifted to 390 nm upon raising the 1a/C70 ratio to 2∶1, further supporting the 2:1 binding stoichiometry. The association constants were estimated to be K1 = 1.7 × 105 M−1 and K2 = 3.5 × 106 M−1 (Table 1) by fitting the titration curve at 382 nm. The cooperativity factor (α) for this complexation was calculated to be 82 (α = 4K2/K1) which is quite rare for positive allosteric cooperativity of a rigid macrocycle through weak interactions.68 A similar UV–vis titration experiment was conducted on 1b and C70 in 1,1,2,2-tetrachloroethane (Figure 4d). The same trend of changing absorption intensity of C70 is observed upon gradually titrating 1b to C70. Based on the titration data, the first and second Ka values were calculated to be 2.3 × 105 M−1 and 5.8 × 103 M−1, respectively (Table 1). The negative cooperativity (α = 0.101) for 1b to complex C70 is presumably due to the steric hindrance of the methyl groups. Gratifyingly, both solid-state structures of C70@ 1a2 and C70@ 1b2 were unambiguously determined by X-ray crystallography analysis (Figure 5), which mirrored the dimeric capsules observed in solution. In the crystalline state of the C70@ 1a2 complex (Figure 5a and Supporting Information Figure S22), C70 goes deep into the cavity, almost close to the plane defined by the lower rim, to maximize the face-to-face interactions with panels of the belts. The centroid distances between the phenyl planes of 1a and the nearest pentagon or hexagon of C70 are 3.52–3.75 Å. One belt rotates ca. 22.5° relative to the other to reach a complementary state which facilitates the formation of a cyclic seam of 16 (sp2)C–H⋯S hydrogen bonds (averaged dS ···H = 3.11 Å, Supporting Information Table S1) on the equatorial window. By stitching two belts together with 16 hydrogen bonds, a perfect encapsulated structure was eventually formed. Figure 5 | (a) X-ray crystal structure of C70@1a2 with hemispherical (left) and fully encapsulated (right) views. (b) X-ray crystal structure of C70@1b2. The π–π, C–H⋯π interactions, and C–H···S hydrogen bonds are highlighted in red, purple, and blue, respectively. Download figure Download PowerPoint In the case of the C70@ 1b2 complex (Figure 5b and Supporting Information Figure S23), a well-defined 2:1 capsule was also observed confirming the inferred structure based on the NMR study. Although the assembled dimeric capsule of C70@ 1b2 is similar to that of C70@ 1a2, substantially different structural characteristics were observed. To form a capsule smoothly, C70 is slightly inclined away from the bottom of the cavity, exposing a larger volume outside the cavity to fit the second belt. Additionally, the relative rotation angle of the two belts is larger, ca. 35°, to reduce the steric congestion. This structural orientation results in the closest distances between interior panels and C70 (3.73–4.10 Å) being relatively larger than that of C70@ 1a2. These features further explain that the association constants for C70@ 1b2 are much smaller than those of C70@ 1a2 (Table 1). In particular, the average distance between the methyl group protons and the sulfur atoms is 3.19 Å ( Supporting Information Table S2), within the range of distance for weak C–H⋯S hydrogen bonds.61 This observation is consistent with the down-field chemical shift of the methyl group protons upon formation of the 2:1 complex in the NMR study (Figure 4b). Therefore bowl-shape 1b can self-tune the distance between two belts to fit C70 in their cavities, thereby reducing the obstacles of methyl groups and still forming a capsule-like complexed structure. Host–guest chemistry of nanobelts with PC61BM Fullerene C60 derivative PC61BM is widely used in material science.69 It has an inherent conjugated carbon sphere almost identical to C60 but with a larger substituent (tail) attached to the spherical surface. Since PC61BM is asymmetric, there are two complex structures likely for forming a 1:1 complex with either bowl-shaped 1a or 1b, that is, tail-through-lower-rim or tail-through-upper-rim (Scheme 2). Accordingly, both a symmetrical clam-like structure and an asymmetrical capsule-like structure are most possible for a fully encapsulated 2:1 complex (Scheme 2). With these considerations, we proceeded to explore whether 1a and 1b can form hydrogen-bonded capsules in the presence of PC61BM. 1H NMR titration of 1a with PC61BM turns out to be complicated but accountable ( Supporting Information Figure S12). Addition of 0.5 equiv of PC61BM to 1a resulted in two new sets of resonances with disappearance of signals for the free 1a (Figure 6a). This led us to infer that both 1:1 and 2:1 complexes were possibly formed in this mixture. The predominant species ( B) exhibited six singlets with equal intensity that could be attributed to an asymmetrical capsule complex PC61[email protected] 1a2 with two distinct hemispherical chemical environments upon binding the PC61BM. These aromatic protons from the upper rim obviously moved downfield implying the formation of a hydrogen-bonded capsule. The minor species ( A) with three singlets is most likely the 1:1 complex PC61[email protected] 1a with a preferred orientation of the tail of PC61BM. When one equivalent of PC61BM was introduced, the spectrum became simpler with species ( A) as the main product while only a trace amount was observed for ( B), indicating a nearly 1:1 complex formed (Figure 6a). Moreover, the PC61BM orientation preference in the cavity of 1a was further determined to be tail-through-upper-rim according to the 2D nuclear Overhauser effect spectroscopy (NOESY) experiment ( Supporting Information Figure S14). In contrast, 1H NMR titration of 1b with PC61BM straightforwardly displayed a 1:1 slow exchange binding process throughout the titration (Figure 6b and Supporting Information Figure S16). This clearly means that 1b forms exclusively a 1:1 complex with PC61BM ( Supporting Information Figure S17). Like that of PC61[email protected] 1a, the complexed species PC61[email protected] 1b exhibited only one set of three singlets, implying that PC61BM has a preferred orientation in the belt cavity in solution as evidenced by the 2D NOESY experiment ( Supporting Information Figure S18). Scheme 2 | Proposed structures of 1:1 and 2:1 complex between bowl-shaped 1a and PC61BM. Download figure Download PowerPoint Further evidence for binding PC61BM with nanobelts was gained from UV–vis absorption titration experiments (Figures 6c and 6d and Supporting Information Figures S15 and S19). The titration curve of binding PC61BM with 1a in 1,1,2,2-tetrachloroethane reveals a singular spectroscopic change phenomenon in the absorption band of PC61BM at 431 nm. Namely, when the ratio of 1a/PC61BM is close to 1.2, the absorption band has an instant redshift (6 nm) and then continues to increase. This feature points directly to the 2:1 stoichiometric binding model of 1a and PC61BM. Based on this titration experiment, the association constant K1 (8.0 × 105 M−1) is 53 times that of K2 (1.5 × 104 M−1), giving a cooperativity factor α of 0.075 and indicating the 1:1 complex is more favored. This unique binding motif was further evidenced by the absence of signal for PC61[email protected] 1a2 in MALDI-TOF-MS analysis, which only showed a signal peak for PC61[email protected] 1a at m/z of 2112.2131 ( Supporting Information Figure S13). The UV–vis absorption titration of 1b to PC61BM in dichloromethane provided an association constant of 1.2 × 106 M−1 with a 1:1 binding model (Figure 6d and Supporting Information Figure S19). Figure 6 | (a) Partial 1H NMR spectra of 1a with different amounts of PC61BM (400 MHz, CDCl3, 298 K). (b) Partial 1H NMR spectra of 1b with different amounts of PC61BM (400 MHz, CDCl3, 298 K). (c) UV–vis absorption spectra of PC61BM upon titrating with 1a from 0 to 3.5 equiv. Inset: fitting curves at λ =518 nm. (d) UV–vis absorption spectra of PC61BM upon titrating with 1b from 0 to 4.0 equiv. Inset: fitting curves at λ =495 nm. A, 1:1 complex; B, stands for 2:1 complex; *, PC61BM. Download figure Download PowerPoint Finally, solid-state structures of PC61BM bound nanobelts were successfully obtained to elucidate their binding mechanisms. Interestingly, regardless of what mole ratio of 1a/PC61BM is used for the crystal growth, these obtained single crystals are constantly determined to be the 2:1 complex. SCXRD analysis confirms complex PC61[email protected] 1a2 a hydrogen-bonded capsule structure with a bunch of unique characteristics (Figure 7a and Supporting Information Figure S24). First, a pseudorotaxane structure was formed by threading the substituent of PC61BM through the lower rim of one belt. The substituent was interlaced among the butyl alkyl chains forming a gear structure that hindered PC61BM rotation.70 Furthermore, a very rare full encapsulation was achieved by the π–π interactions between the panels of both belts and the carbon sphere of PC61BM with an averaged centroid-to-centroid distance of 3.90 Å. Most importantly, similar to those dimeric capsules observed for C60@ 1a2 and C70@ 1a2, one 1a belt rotated ca. 22.5° relative to the other one to form 16 C–H⋯S hydrogen bonds (averaged dS ···H = 3.01 Å, Supporting Information Table S3), which can stitch two belts into a perfect capsule. On the contrary, the solid-state structure of PC61[email protected] 1b fully confirms its 1:1 inclusion complex with a preferred orientation by pointing the substituent through the upper rim of 1b (Figure 7b and Supporting Information Figures S25–S26). This observation is consistent with the conclusions drawn from NMR experiments. The π–π interaction (centroid-to-centroid average distance = 3.75 Å) between PC61BM and 1b is the main driven forces to complexation. In addition, the C–H⋯π interactions between the methyl group of 1b and the PC61BM sphere (2.97–3.36 Å) further stabilize this complex. Figure 7 | (a) X-ray crystal structure of PC61[email protected]1a2 with hemispherical (left) and fully encapsu
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