C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts

Abstract

Open 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 the methyl proton an from to ppm, the formation of the 2∶1 complex. This has us to speculate that the 2∶1 complex of 1b and C70 could form a hydrogen-bonded capsule structure with the assistance of (sp3)C–H⋯S hydrogen bonds between the methyl protons and the sulfur atoms in the upper rim of the belt. Figure | (a) Partial 1H NMR spectra of 1a with different amounts of C70 (400 MHz, CDCl3, 298 (b) Partial 1H NMR spectra of 1b with different amounts of C70 (400 MHz, 298 (c) UV–vis absorption spectra of C70 upon titrating with 1a from 0 to equiv. Inset: fitting curve at λ = UV–vis absorption spectra of C70 upon titrating with 1b from 0 to equiv. Inset: fitting curve at λ = 1∶1 2∶1 complex. Download figure Download PowerPoint The binding of 1a and 1b towards C70 was further by UV–vis absorption titration ( Supporting Information Figures and shown in Figure with of ratio in two at and nm were which that the species been The absorption of C70 at nm and gradually shifted to nm upon the ratio to 2∶1, further the 2:1 binding The association constants were to be K1 = 1.7 × 105 M−1 and K2 = 3.5 × 106 M−1 (Table by fitting the titration curve at The cooperativity factor for this complexation was calculated to be 82 = which is for positive cooperativity of a through A similar UV–vis titration was conducted on 1b and C70 in (Figure The of absorption of C70 is observed upon gradually titrating 1b to C70. on the titration the first and Ka were calculated to be 2.3 × 105 M−1 and 5.8 × 103 respectively (Table 1). The cooperativity = for 1b to complex C70 is due to the of the methyl both of C70@ 1a2 and C70@ 1b2 were unambiguously by X-ray analysis (Figure which the dimeric capsules observed in solution. In the state of the C70@ 1a2 complex (Figure and Supporting Information Figure C70 into the cavity, almost to the defined by the lower rim, to maximize the interactions with panels of the The between the of 1a and the nearest or of C70 are belt ca. to the other to a state which the formation of a of 16 (sp2)C–H⋯S hydrogen bonds = Å, Supporting Information Table on the equatorial By two belts with 16 hydrogen a structure was Figure | (a) X-ray crystal structure of with and fully (b) X-ray crystal structure of The C–H⋯π and hydrogen bonds are highlighted in purple, and respectively. Download figure Download PowerPoint In the of the C70@ 1b2 complex (Figure and Supporting Information Figure a 2:1 capsule was also observed confirming the structure based on the NMR study. the assembled dimeric capsule of C70@ 1b2 is similar to that of C70@ 1a2, different were observed. To form a capsule C70 is from the of the cavity, a larger the cavity to the belt. the of the two belts is ca. to the This in the between panels and C70 being relatively larger than that of C70@ further that the association constants for C70@ 1b2 are much than those of C70@ 1a2 (Table 1). In particular, the average distance between the methyl protons and the sulfur atoms is ( Supporting Information Table the of distance for C–H⋯S hydrogen This is consistent with the of the methyl protons upon formation of the 2:1 complex in the NMR study (Figure 4b). 1b can the distance between two belts to C70 in their the of methyl groups and forming a capsule-like complexed Host–guest chemistry of nanobelts with PC61BM C60 derivative PC61BM is widely used in has an carbon sphere almost identical to C60 but with a larger to the spherical PC61BM is there are two complex likely for forming a 1:1 complex with either bowl-shaped 1a or that or (Scheme Accordingly, both a structure and an capsule-like structure are most for a fully 2:1 complex (Scheme With these we to 1a and 1b can form hydrogen-bonded capsules in the presence of PC61BM. 1H NMR titration of 1a with PC61BM out to be but ( Supporting Information Figure of 0.5 equiv of PC61BM to 1a in two new sets of with of signals for the free 1a (Figure This us to that both 1:1 and 2:1 complexes were possibly in this The species ( exhibited with that could be attributed to an capsule complex protected] 1a2 with two upon binding the PC61BM. aromatic protons from the upper rim downfield the formation of a hydrogen-bonded The species ( with three is most likely the 1:1 complex protected] 1a with a of the of PC61BM. When one of PC61BM was the spectrum became with species ( as the main product while only a was observed for ( indicating a nearly 1:1 complex (Figure Moreover, the PC61BM preference in the cavity of 1a was further to be to the 2D effect spectroscopy ( Supporting Information Figure In 1H NMR titration of 1b with PC61BM a 1:1 slow exchange binding the titration (Figure and Supporting Information Figure This clearly that 1b forms a 1:1 complex with PC61BM ( Supporting Information Figure that of protected] 1a, the complexed species protected] 1b exhibited only one set of three that PC61BM has a in the belt cavity in solution as evidenced by the 2D ( Supporting Information Figure Scheme 2 | of 1:1 and 2:1 complex between bowl-shaped 1a and PC61BM. Download figure Download PowerPoint for binding PC61BM with nanobelts was from UV–vis absorption titration experiments and and Supporting Information Figures and The titration curve of binding PC61BM with 1a in a spectroscopic in the absorption band of PC61BM at when the ratio of is to the absorption band has an and then to This to the 2:1 stoichiometric binding of 1a and PC61BM. on this titration the association constant K1 × 105 is that of K2 × 104 a cooperativity factor α of 0.075 and indicating the 1:1 complex is more This unique binding was further evidenced by the absence of for protected] 1a2 in MALDI-TOF-MS which only showed a peak for protected] 1a at m/z of ( Supporting Information Figure The UV–vis absorption titration of 1b to PC61BM in dichloromethane an association constant of 1.2 × 106 M−1 with a 1:1 binding (Figure and Supporting Information Figure Figure | (a) Partial 1H NMR spectra of 1a with different amounts of PC61BM (400 MHz, CDCl3, 298 (b) Partial 1H NMR spectra of 1b with different amounts of PC61BM (400 MHz, CDCl3, 298 (c) UV–vis absorption spectra of PC61BM upon titrating with 1a from 0 to 3.5 equiv. Inset: fitting at λ UV–vis absorption spectra of PC61BM upon titrating with 1b from 0 to equiv. Inset: fitting at λ 1:1 for 2:1 *, PC61BM. Download figure Download PowerPoint Finally, of PC61BM bound nanobelts were successfully obtained to their binding of mole ratio of is used for the crystal these obtained crystals are to be the 2:1 complex. analysis complex protected] 1a2 a hydrogen-bonded capsule structure with a of unique (Figure and Supporting Information Figure a pseudorotaxane structure was by the of PC61BM through the lower rim of one belt. The was the forming a structure that PC61BM Furthermore, a very encapsulation was achieved by the π–π interactions between the panels of both belts and the carbon sphere of PC61BM with an distance of similar to those dimeric capsules observed for C60@ 1a2 and C70@ 1a2, one 1a belt ca. to the other one to form 16 C–H⋯S hydrogen bonds = Å, Supporting Information Table which can two belts into a the the structure of protected] 1b fully its 1:1 inclusion complex with a by the through the upper rim of 1b (Figure and Supporting Information Figures This is consistent with the from NMR The π–π interaction (centroid-to-centroid average distance = between PC61BM and 1b is the main to In addition, the C–H⋯π interactions between the methyl of 1b and the PC61BM sphere further this complex. Figure | (a) X-ray crystal structure of with and fully

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