Self-assembled Carcerand-like Cage with Thermoregulated Selective

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Self-assembled Carcerand-like Cage with Thermoregulated Selective Binding Preference for Purification of High-Purity C60 and C70 Weidong Sun, Ying Wang, Lishuang Ma, Lu Zheng, Weihai Fang, Xuebo Chen, and Hua Jiang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02674 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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The Journal of Organic Chemistry

Self-assembled Carcerand-like Cage with Thermoregulated Selective Binding Preference for Purification of High-Purity C60 and C70 Weidong Sun, Ying Wang, Lishuang Ma, Lu Zheng, Weihai Fang, Xuebo Chen, and Hua Jiang* Key Laboratory of Theoretical and Computational Photochemistry and Key Laboratory of Radiopharmaceuticals, Ministry of Education; College of Chemistry, Beijing Normal University, Beijing 100875, China

Supporting Information Placeholder

ABSTRACT: Fullerene molecules have attracted considerable interest because of the unique curved aromatic π-conjugated systems. However, the complicated and costly technologies for purification of highly pure fullerenes hamper easy access to these attractive molecules and consequently limit most of fullerene applications. Here, we report the discovery of a carcerand-like cage acting as a standalone host for efficient separation and purification of C60 and C70 from fullerene soot. The cage, built through the self-assembly of metal coordination, is capable of quantitative encapsulating fullerenes C60 and C70. The fullerene complexes are highly stable at high temperatures due to the small crevices with precisely defined sizes, multiple favorable CH-π interactions and concave-convex aromatic interaction between fullerenes and corannulenes. Importantly, the carcerand-like cage shows a temperature-dependent selective binding preference for C60 over C70, which allows us to develop an efficient and green procedure for isolating C60 and C70 with high-purity and low mass loss from fullerene soot without the help of recrystallization or HPLC.

INTRODUCTION The search of synthetic hosts able to encapsulate fullerenes with high stability and selectivity in solution so as to achieve their purifications has attracted great attentions because fullerenes represent one of attractive molecules with the unique curved aromatic -conjugated surface in a wide range of fields from material science to pharmacy.1,2 Benefitting from the rapid developments of host-guest chemistry and self-assembly, a number of fullerene hosts3,4 including macrocycles,5-7 cages8-12 and MOFs13-15 has been developed. One of popular designing strategy is to covalently or noncovalently combine two planar or concave aromatic binding motifs in one host to enhance ~ interactions between fullerenes and host molecules so that high stability and

selectivity for fullerenes can be achieved. These planar or concave aromatic binding units included porphyrins,5,6,16,17 cycoltriveratrylenes18-21 and calixarene,22-29 corannulenes,30-37 extended tetrathiafulvalene,38-44 subphthalocyanine,45,46 and carbon nanorings,47-49 and so on,4,50 which are well-known to display favorable ~ interactions with fullerenes. Although significant advances on selective encapsulation of C60/C70 have been made,4,50-53 purification of C60/C70 mixtures using synthetic hosts still represents a great challenge because most of methods based on synthetic hosts for fullerene purification are mainly selective for C60 but less for C70.7,18,21,26,34,37,42,54,55 On the other hand, the releases of the encapsulated fullerenes usually require chemical or physical treatments, which generally either destroy hosts or block the binding cavity of hosts by a high-affinity

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secondary guest,5,56 consequently leading to obvious mass loss of both valuable hosts and fullerenes. So far, no stand-alone fullerene host has been reported for simultaneously separating and purifying fullerenes C60 and C70 from fullerene soot.4,50-53 Herein, we report the self-assembled carcerand-like cage COPY1-Pd with temperature-dependent selectivity for C60 over C70 and its application on highly efficient purification of C60 and C70 from fullerene soot. Specifically, the cage COPY1-Pd, constructed by the self-assembly of two corannulene units through five linear Pd coordination bonds, acts as a stand-alone host for quantitative encapsulating both C60 and C70 with remarkable thermodynamic stability in deuterated tetrachloroethane (TCEd2) at 130 oC. Moreover, COPY1-Pd displays unprecedented thermoregulated selective binding preference for C60 over C70, which allowed us to develop an efficient procedure for easy purification of high-purity (≥ 99.5%) C60 and C70 from fullerene soot. We also demonstrate that the mass extractions and releases of C60 and C70 are ≥96%, and the recovery of the ligand is up to 98.5% per turn due to the reversible self-assembly. To our best knowledge, it is the first time that a stand-alone self-assembled carcerand-like cage has been used to efficiently separate and purify C60 and C70 from fullerene soot based on a temperaturedependent binding preference for C60 over C70.

RESULTS AND DISCUSSION Design of the ligand and cage. Corannulene is considered as a fullerene fragment and an ideal candidate for designing hosts capable of associating with fullerenes through the favorable concave-convex aromatic interaction.32,34,35,37,57,58 We recently reported a Ag-direct self-assembled cage based on 1, 3, 5, 7, 9pentakis-pyridyl corannulene derivative that displayed a helical bias induced by chiral anions,59 but its cavity is too small to host large aromatic guests. We expected to increase the diameter of the ligand by inserting triple bonds between the corannulene core and pyridyl moieties, 1, 3, 5, 7, 9- pentakis-(3-pyridylethynyl) corannulene derivative (COPY1) was thus designed and synthesized (Figure 1, Scheme 1). We anticipated that the selfassembly of linear metal coordination between two pyridyl moieties and Pd(CH3CN)2Cl260,61 would generate a cage COPY1Pd with a large cavity for hosting fullerenes and five rigid coordination linkers in the periphery of the cage. The latter can provide five crevices with tunable size as entries for fullerenes when taking the rotation and geometry of coordinated PdCl2 into consideration. Given five crevices can influence each other through partition, they may function as ratchet-like portals that allow fullerenes to enter the cavity but block the escape of fullerenes from the cavity. The assembly/disassembly strategy based on metal association/dissociation is utilized to release guests and regenerate the cage, rendering the release of fullerenes and regeneration of COPY1-Pd easy and efficient. It is noteworthy that COPY1-Pd displays an inflexible conformation that originates from the inherent rigidity of triple bonds and the linear coordinated bonds of N-Pd-N. Though sufficient flexibility of hosts has nicely been illustrated to be favorable for maximizing host-guest interactions,16,38-44 provided that the cavity size and shape of COPY1-Pd matches the dimension of fullerenes, the rigid conformation of COPY1-Pd would be a favorable factor for the formation of stable fullerene complexes because of a high degree of preorganization.17,47-50,53,62

Figure 1. Self-Assembly of nanocage. (a) the synthesis of COPY1-Pd. i) Pd(CH3CN)2Cl2, 90oC, 2h in TCE-d2. (b) Cartoon illustrations of COPY1-Pd and its fullerene complexes.

Scheme 1. Synthesis of COPY1.a O

O

O

ⅰ

OH

O

O

O

OTs

2 N HO

N

ⅱ Br

N

ⅲ

Peg3O

N

ⅳ

Peg3O

Br

Peg3O 4

3

5

TMS

N

ⅴ Peg3O

R

Sn R

N

ⅶ

6

OPeg3 R Cl

R COPY1

Cl ⅵ

N R=

OPeg3

Cl Cl 7

Cl

aReagents

and conditions: (i) TsCl, TEA, 25 oC, 16 h, 90%; (ii) compound 2 (1.5equiv), K2CO3 (3 equiv), DMF, 70 oC, 16 h, 81.7%; (iii) trimethylsilylacetylene (3 equiv), CuI (0.1 equiv), Pd(PPh3)2Cl2 (0.1 equiv), TEA, 90 oC, 16 h, 88.9%; (iv) TBAF(1 equiv), THF, 2 h, 81%; (v) n-BuLi, n-Bu3SnCl, THF, -78 oC to room temperature,16 h, 81%; (vi) ICl, DCM, from -78oC to room temperature, 4 d, 57%; (vii) Pd(OAc)2, IPr.HCl, K2CO3, THF, reflux, 3d, 54%.

Fullerene encapsulations. The self-assembly of COPY1 and Pd(CH3CN)2Cl2 was observed to quantitatively generate the cage COPY1-Pd (Figure 1a) when a mixture COPY1 and Pd(CH3CN)2Cl2 (2.5 equiv) in TCE-d2 was heated at 90 oC for 2 h, which was confirmed by NMR spectroscopy and ESI-MS. The 1H NMR spectrum of COPY1-Pd shows four signals in the aromatic region assignable to the corannulene and pyridyl moieties (Figure 2b, Figure S1–10, SI). The upfield shift of the corannulene protons Ha (Δδ= -0.1 ppm) is indicative of aromatic shielding effects as a result of the formation of the cage. The slight downfield shifts of the signals for the pyridyl protons are attributed to the combined effects of aromatic shielding and the linear metal-ligand coordination. NMR diffusion-ordered spectroscopy (DOSY) experiments revealed the formation of a unique assembled architecture as the evidences that all aromatic proton signals of COPY1-Pd showed the same diffusion coefficient of 1.43 10-10 m2s–1, which is smaller than that (2.14


2

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10-10 m2s–1) of free COPY1 (Figure S26–33, SI). ESI-MS also confirmed an M5L2 stoichiometry according to two prominent peaks at m/z 1974.8085 and 1304.2122 for [COPY1-Pd-nCl]n+ (n=2 and 3, Figure S12, SI).

3). Moreover, the diffusion coefficients are calculated to be 1.24 and 1.50 10-10 m2s–1 for C60⊂COPY1-Pd and C70⊂COPY1-Pd through DOSY experiments, respectively (Figure S26–33, SI), comparable with that of the free host, hinting at that encapsulation placed little effect on the dimension of the cage. This phenomenon is presumably attributed to the highly rigid conformation of the cage. Encapsulating C60 or C70 within the cavity were also confirmed by 13C NMR experiments. The signal of the encapsulated C60 appeared at 141.8 ppm that is obviously different from the signal of free C60 observed at 143.0 ppm (Figure 2h and i). Five 13C NMR peaks of C70 were also shifted toward higher field with Δδ= 1.0~-3.1ppm under the same conditions (Figure 2e and f), indicative of strong aromatic shielding effects as a result of encapsulations and consistent with the previously reported fullerene encapsulations.63 No free fullerene was detected by 13C NMR experiments in each case when an equimolar mixture of COPY1-Pd and C60 or C70 was heated at 90°C (Figure 2e-i), which further confirmed quantitative encapsulation.

Figure 2. (a-d) Partial 1H NMR (0.5 mM in TCE-d2, 600 MHz, 298 K) spectra of a) COPY1, b) COPY1-Pd, c) C60⊂COPY1-Pd and d) C70⊂COPY1-Pd. (e-i) Partial 13C NMR (TCE-d2, 125 MHz, 298 K) spectra of e) C70, f) C70⊂COPY1-Pd, g) COPY1-Pd, h) C60⊂COPY1Pd and i) C60.

The investigation on host-guest interactions revealed that COPY1-Pd quantitatively encapsulated fullerenes. The progressive addition of fullerenes C60 or C70 (0~1equiv. for each fullerene) to a TCE-d2 solution of COPY1-Pd resulted in the gradual disappearance of the signals of COPY1-Pd with the concomitant appearance of one set of new signals assignable to host-guest complexes C60⊂COPY1-Pd or C70⊂COPY1-Pd at 90°C within 2 h (Figure 2c-d, Figure S13–19, SI). Upon addition of 1 equiv. fullerenes C60 or C70, the signals of COPY1-Pd completely disappeared, hinting at quantitative formation of 1:1 host-guest complexes. Specifically, in the case of C60⊂COPY1-Pd, the protons Hc of pyridyl moieties located in the inside cavity obviously shifted upfield with Δδ= -0.26 ppm, while the protons Hd of pyridyl moieties located in the outer cavity shifted downfield with Δδ= 0.16 ppm. Unexpectedly, the proton Ha of corannulene remained constant. However, the 1H NMR spectrum of C70⊂COPY1-Pd revealed a completely different scenario in which proton Ha showed an upfield shift with Δδ= -0.15 ppm, whereas proton Hc slightly shifted downfield with Δδ= 0.08 ppm. The upfield shift of the corannulene protons Ha (Δδ= -0.1 ppm) is indicative of aromatic shielding effects as a result of the formation of the cage. The slight downfield shifts of the signals for the pyridyl protons are attributed to the combined effects of aromatic shielding and the linear metal-ligand coordination. However, the signals of the protons Ha, Hc and Hd split into two after the nanocage formation, suggesting the existence of two isomers of the nanocage, consistent with our previous observations on silver mediated corannulene based cage.59 (Figure

ESI-MS also confirmed 1:1 host-guest complexes on the basis of two prominent peaks at m/z 2335.3043 and 1544.2196 for [C60⊂COPY1-Pd-nCl]n+ (n=2 and 3, Figure S18, SI), and at m/z 2395.3061 and 1584.2137 for [C70⊂COPY1-Pd-nCl]n+ (n=2 and 3, Figure S24, SI). Additional evidence in support of encapsulating C60 or C70 within the cavity comes from the obvious colour change. The solutions of C60⊂COPY1-Pd and C70⊂COPY1-Pd appear purple and deep purple, respectively (Figure S25a, SI). The UV/Vis spectra further revealed that C60⊂COPY1-Pd and C70⊂COPY1-Pd displayed broadened absorption bands of the fullerenes between 450 and 650 nm (Figure S25b, SI). Interestingly, no encapsulation of higher fullerenes was observed presumably due to small cavity of COPY1-Pd for favourable encapsulating higher fullerenes. Having established encapsulation, we next investigated the stability of fullerene complexes. All signals of protons in C60⊂COPY1-Pd or C70⊂COPY1-Pd stayed constant and no dissociated species was detected after TCE-d2 solutions of complexes were heated at 130°C for 3 days (Figure S34, SI), demonstrating the high stability and reminiscent of carcerands that cannot release their encapsulated small guests.64,65 This unique behaviour renders C60⊂COPY1-Pd and C70⊂COPY1-Pd easy to be handled during their purification without bothering their dissociation, and makes them stand out from most of reported fullerene complexes that were formed readily at room temperature but dissociated at elevated temperature.


3


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in C70⊂COPY1-Pd may be attributed to C70’s ellipsoidal shape that potentially affects the partition of the crevices. Additionally, multiple CH- interactions between fullerenes and those pyridine CH protons, located in the inside cavity of the cage were revealed by the computational studies. The distances of CH- interactions are about 2.9 Å in both complexes (Figure S80, SI), which perfectly match the favourable distance of CH- interactions around 2.9 Å in the reported fullerene complexes.67 These observations suggest that the highly co-operative selfassembly provide not only small crevices with precisely defined sizes but also multiple favourable CH- interactions in C60/70⊂COPY1-Pd complexes that account for high stability of fullerene complexes.

Figure 3. Optimized structures of COPY1-Pd (a), C60⊂COPY1-Pd (b), and C70⊂COPY1-Pd (c) using the Gaussian 09 program package at the B3LYP/LANL2DZ/6-31G*/PCM level of theory. In each column, Top: Side view in stick; Middle: Side view in CPK; Bottom: Top view in CPK. The methyl group was used in the calculation instead of 2-(2-(2-methoxyethoxy)ethoxy) ethoxy group for simplicity. The fullerenes are labelled in golden. The height of the cage h is defined as the distance between two corannulene units (dash lines in red). The radius of the cage r is defined as the distances between the centroid of the cage and Pd atoms (dash lines in pink). The size of the crevice d is defined as the distance between two neighbouring chloride atoms in a crevice (dash lines in blue). The dihedral angle is defined as the angle between the planes of two pyridine rings in an individual coordination unit.

Density functional theory (DFT) calculations were performed to determine the energy-minimized structures of COPY1-Pd, C60⊂COPY1-Pd and C70⊂COPY1-Pd (SI). The computational modeling studies reveal that COPY1-Pd shows a cylinder-like structure with a height of 12.9 Å and a radius of 8.2 Å (Figure 3a), which well matches the sizes of C60/70 (~ 1nm in diameter).66 The heights of fullerene complexes are found to be 14.3 Å, which is slightly larger than that of free COPY1-Pd, but the radii of the occupied cages are decreased slightly to 7.7~7.8 Å from 8.2 Å in free COPY1-Pd (Figure 3), demonstrating that the structures of the occupied cages were stretched slightly. Five coordination linkers in both fullerene complexes displayed a helical conformation, leading to the significant change on all dihedral angles between the planes of two pyridine rings in each individual coordination unit (Figure 3, Figure S78 and Table S2, SI). Thanks to its rigid conformation, it would be difficult for COPY1-Pd itself to adjust the distance between two corannulene units, but it seems feasible to adjust the dihedral angles so as to achieve a better conformation for hosting fullerenes. Modelling studies also displayed that the optimizing sizes of five crevices in the unoccupied COPY1-Pd are 6.8 Å (Figure 3a), which were slightly smaller than the diameter (~1 nm) of fullerenes, suggesting that there exist potential barriers for fullerenes to freely enter and exit the cavity in support of our experimental observations that encapsulation occurred at elevated temperatures (vide infra). The crevice sizes were found to be 5.6 Å in C60⊂COPY1-Pd and 5.5~5.8 Å in C70⊂COPY1-Pd (Figure 3, Figure S79, SI), which are obviously smaller than the diameter of fullerenes, hinting at that the release of the entrapped fullerenes is very difficult. The large variations on the dihedral angles and the sizes of the crevices

We observed that the encapsulations of COPY1-Pd for C60/70 occurred readily at elevated temperatures but very slow at ambient temperature, which is obviously different from most reported fullerene hosts that readily form fullerene complex at room temperature. We set about investigating temperature-driven encapsulations by 1H NMR at the range of temperatures from 303 to 343K. The simplified second-order rate constants of complexation of COPY1-Pd for C60/70 were calculated (Figure S35–45, SI) and summarized in Table 1. In both cases, the rate constants were improved significantly with the increase of temperature while an inversed trend in half-life was observed (Figure S35-44, SI). The data also showed that the encapsulation process for C60 is faster than that for C70 at each given temperature. ⧧

⧧

The Erying plots showed that the ΔH and ΔS values are 94.4 kJ.mol-1 and 20.0 J.mol-1.K-1 for associating C60, and 99.3 kJ.mol-1 and 29.4 J.mol-1.K-1 for associating C70, respectively. The large ⧧

⧧

ΔH values and small ΔS values revealed that encapsulations of fullerenes were dominated mainly by enthalpy. Given the values of ⧧

⧧

ΔH and ΔS are independent of temperatures, the activated free ⧧

energies ΔG for C70 are larger at least 2 kJ·mol-1 than those for C60 at the range of temperatures from 303 to 403 K (Table S2, SI), indicating the preferential encapsulation for C60 over C70. Interestingly, the formation of C60⊂COPY1-Pd or C70⊂COPY1Pd was observed directly from a mixture of COPY1 and PdCl2 when heating with C60/70 at 90 °C within 5 min (Figure S46-52, SI), which is much shorter than the encapsulation from the cage COPY1-Pd, suggesting that the encapsulation process is highly cooperative for a mixture of COPY1, PdCl2 and fullerenes. The observations in combination with the DFT calculations supported our speculation on the function of the crevices of the cage that allow fullerenes to enter the cage but prevent their escape from the cavity.

Table 1. Summary of Kinetic and Thermodynamic Parameters of the Formation of C60  COPY1-Pd and C70  COPY1-Pd Derived from NMR Measurements.a

T (K)

kb (10-2M-1s-1) C60

C70

303

0.39±0.01

0.17±0.01

313

1.90±0.05

0.69±0.02

323

5.6±0.2

2.4±0.1

333

14±0.4

7.2±0.2

343

39.0±1.4

19.0±0.6


⧧c

ΔH (kJ·mol-1)

ΔS (J·mol-1·K1)

ΔG d (kJ·mol-1)

C60

C70

C60

C70

C60

C70

94.4

99.3

22.0

29.4

87.9

90.5

⧧c

⧧

4

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a Kinetic experiments were carried out in (0.5 mM) TCE-d using 2 an equimolar mixture of the host and fullerenes. b The association rate constant (k) was calculated using the simplified second-order rate, ⧧

⧧

1/c=kt+1/c0. c The values of ΔH and ΔS were derived from Erying ⧧

Plots based on 1H NMR experiments. d The value of ∆G (kJ mol-1) at ‡ ‡ ‡ 298 K was calculated using the relationship ∆𝑮 = ∆𝑯 ―𝑻∆𝑺 .

Temperature-dependent selective encapsulating preference. The binding preference of COPY1-Pd was first tested by displacement experiments. The addition of 1 equiv C60 to C70⊂COPY1-Pd in TCE-d2 resulted in about 50% conversion of C70⊂COPY1-Pd to C60⊂COPY1-Pd at 90°C within 5 days. The complete conversion was observed at 130 °C within 20h (Figure S53-55, SI). In contrast, no displacement of C60 was detected when excess C70 (10 equiv) were added to C60⊂COPY1-Pd in TCE-d2 at 130 °C within several days (Figure S56-57, SI), presumably due to the smaller activated free energy for C60 complex than that for C70 complex (Table S2, SI).

C70 at room temperature as evidenced by the disappearance of the 1H NMR signals of C 60/70⊂COPY1-Pd with the concomitant appearance of a new set of signals corresponding to COPY1 (Figure S65, SI). When the above mixtures were heated again at 90 °C for 3h, we were delighted to find the complete regeneration of C60/70⊂COPY1-Pd as demonstrated by 1H NMR spectra. Moreover, the above dissociation/association process could be reversibly operated at least twice (Figure S66-67, SI). Thus, these finding indicated that the processes for release of the encapsulated fullerenes and regeneration of the occupied cage based on dissociation/association are highly efficient and robust. Encouraged by the unprecedented selective binding of cage COPY1-Pd for C60 over C70 and highly efficient and revisable association/dissociation process, we decided to use COPY1-Pd for direct separation and purification of high-purity C60 and C70 from fullerene soot. The procedure includes three main steps (Figure 5): a) Extracting C60 and C70 from fullerene soot through the formation of C60/70⊂COPY1-Pd. b) Isolating C70 via the selective displacement with C60. c) Releasing C60 and recovering ligand COPY1 by dissociation. In a typical experiment (Figure 5), 400mg of COPY1 was applied to purify C60 and C70 from fullerene soot (816 mg). We were delighted to find that the isolations of high-purity (≥99.8%) C60 and C70 are nearly quantitative and the recovering mass of the ligand is 388 mg with a recovering yield of 97%.

Figure 4. Temperature-tunable selective encapsulations of COPY1-Pd for C60/C70. (A) The molar fraction of C70⊂COPY1-Pd vs C60⊂COPY1-Pd in various temperatures and times. The data were extracted from 1H NMR experiments. (B) Illustration of temperaturetunable binding preference COPY1-Pd for C60 over C70.

To obtain detailed information about selective binding preference of COPY1-Pd for C60 over C70, competition experiments were carried out. Equimolar amounts of COPY1-Pd, C60 and C70 in TCE-d2 were heated at a given temperature from 70 to 130°C and monitored by 1H NMR at room temperature at different intervals (Figure 4A). Two independent sets of signals corresponding to C60⊂COPY1-Pd and C70⊂COPY1-Pd were observed, but no unoccupied COPY1-Pd was detected in all cases, indicating that all COPY1-Pd were occupied by either C60 or C70. The molar ratios of C70⊂COPY1-Pd vs C60⊂COPY1-Pd were decreased slightly at 70 ~ 80°C but significantly at 90 ~ 110°C within 72h (Figure S58-62, SI). Interestingly, COPY1-Pd exclusively extracted C60 from the equimolar mixtures of C60 and C70 in TCE-d2 beyond 12h at 130°C (Figure 4B). Moreover, this exclusive extraction was also performed excellently in the mixture of C60 and 10 equiv. C70 (Figure S63–64, SI). These observations definitely revealed that COPY1-Pd has a higher affinity for C60 than C70. The preferential binding for C60 over C70 is attributed to the better fit of C60 in the cavity of COPY1-Pd, which results in the perfect electronic and structural complementarity between C60 and corannulene units as demonstrated by the DFT computation (vide supra).

Separation and purification of C60/C70 from fullerene soot. Before fullerene purifications, we turned our attentions to releasing encapsulated fullerenes and regenerating COPY1-Pd or recovering COPY1. 4-Dimethylaminopyridine (DMAP) was chosen to dissociate the cage so as to release fullerene and ligand. Upon additions of 60 equivalents DMAP to the solution of C60⊂COPY1-Pd or C70⊂COPY1-Pd in TCE-d2 lead to easy dissociations of the complexes and to complete release of C60 and

Figure 5. Flow chart for the direct separation and purification of C60 and C70 from fullerene soot. Methods: a) A mixture of COPY1-Pd, generated by heating a mixture of 400 mg (0.2553 mmol) of COPY1 and 166 mg of Pd(CH3CN)2Cl2 in TCE (255 mL), and 813 mg of fullerene soot (determined by NMR and HPLC, Figure S68-69, SI) containing 77.3 mg of C60 and 17.0 mg of C70 was heated at 90°C for 2 h to completely extract fullerenes C60 and C70 in the form of C60/70⊂COPY1-Pd; the organic phase was collected by filtration; b) To the above filtrate, C60 (14.6 mg, 0.02 mmol) was added to quantitatively displace C70 from C70⊂COPY1-Pd with concomitant formation of equimolar C60⊂COPY1-Pd at 130°C within 48 h; the solvent was evaporated under reduced pressure. The residue was suspended in CS2 (10mL). The solid C60⊂COPY1-Pd was filtered off, and the filtrate was collected and evaporated to provide C70 (17 mg, quantitative) with high-purity (≥99.5% determined by HPLC, Figure S70, SI); c) To the solution of the above C60⊂COPY1-Pd in DCM (200 mL) was added DMAP (934 mg). After stirred at room temperature for 10 mins, the solvent was removed and methanol (200 mL) was added to precipitate C60, which were collected by filtered and washed by additional methanol (50 mL×3) to quantitatively yield 91.5 mg of C60 with high-purity (≥99.5% determined by HPLC


5


analysis, Figure S70, SI). The ligand COPY1 was purified by a short column chromatography on Al2O3.

Finally, this procedure was applied for consecutive purification of fullerene soot up to 5 cycles. At the end of each cycle, the crude COPY1 was used to generate the cage in situ without chromatographic purification (Figure S71-76, SI). 500mg of COPY1 were used to separate and purify 5.08g of fullerene soot (1016mg per cycle). The purification data were summarized in Table 2. After 5 cycles, we totally isolated 560 mg (Table 2 notes a and b) of high-purity (≥99.8 %) C60 with a mass loss of 2.5%. As for C70, the recovering yield and purity of C70 for the first three cycles were about 99 % and 99.8 %, respectively. But the isolated C70 was found to be slightly contaminated by C60 (0.6~1.1 %, determinated by the HPLC analysis) in the last two cycles, presumably resulting from the mass loss of COPY1 during the experimental setups. Fortunately, this C60 impurity could be readily removed by heating the impurity with equimolar unoccupied cage COPY1-Pd within 48h at 130°C (Figure S77, SI). Thus, the total amount of isolated high-purity (≥99.8 %) C70 is 102 mg with a mass loss of 4 %. The recovery mass of COPY1 was 470 mg with a recovering yield of 94 % after 5 cycles. This means the average mass loss of COPY1 is as low as 1.5 % per cycle. Both mass loss of fullerenes and host were unprecedentedly lower than most reported examples.

Table 2. Summary of the separation and purification of C60/C70 from fullerene soot using 500 mg COPY1 a

C60

C70

Run 1

Run 2

Run 3

Run 4

Run 5

Mass (mg)b

113.2

112.4

112.2

111.2

111.1

Recovery (%)

98.5

97.9

97.7

96.9

96.8

Purity (%)c

≥99.5

≥99.5

≥99.5

≥99.5

≥99.5

Mass (mg)

21.0

20.9

20.9

20.6

20.3

Recovery (%)

99.1

98.7

98.7

97.2

95.7

Purity (%)c

≥99.5

≥99.5

≥99.5

99.4 e

98.9 e

Mass (mg)d

crude

crude

crude

crude

470

Recovery (%)

n.d.

n.d.

n.d.

n.d.

94

COPY1

aThe fullerene soot was purchased from Beijing HWRK Chem Co., LTD and the content of C60 and C70 were 9.51 % and 2.09 %, respectively, determined by 1H NMR (Figure S68, SI) and HPLC (Figure S69, SI). 5.08 g fullerene soot contains 483.1 mg C60 and 106.2 mg C70. bThe total theoretical mass of C60 per turn is 114.8 mg, including two parts: 96.6 mg C60 in fullerene soot, and 18.2 mg C60 that was added to displaced encapsulated C70. After 5 cycles, the total theoretical mass of C60 is 574.1 mg. cThe purity of C60/70 was determined by HPLC analysis. dThe crude COPY1 was washed three times by 0.1N HCl solution and used for next cycle without further chromatographic purification. eHigh-purity (≥99.8%) could be achieved after further purification.

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CONCLUSIONS In conclusion, a new class of corannulene based molecular cage was designed and prepared through metal directed self-assembly, which quantitatively encapsulates fullerenes C60 and C70 with unprecedentedly thermodynamic stability due to the small crevice sizes, multiple favorable CH- interactions and concave-convex aromatic interaction between fullerenes and cage, reminiscent of carcerands that permanently entrap small guests like solvent molecules during their synthesis, which was firstly reported by Cram.64,65 Although hemicarceplexes of C60 and C70 have been reported,21,68 carcerands that can selectively encapsulate large aromatic molecules like fullerenes are still very rare. Unlike the previously reported carceplexes in which guest release can only occur by the rupture of covalent bonds of the carcerands, in the present self-assembled system, the entrapped fullerenes can be easily released through dissociation, and the carcerand-like cage can also be readily regenerated through self-assembly. More importantly, the cage displays a temperature-dependent selective binding preference for C60 over C70. These unique properties enable us to develop an easy and efficient procedure for separation and purification of C60 and C70 in high purity from fullerene soot, without the help of recrystallization or HPLC. It appears that the procedure based on our host is an attractive methodology for separating and purifying fullerenes C60 and C70 in comparison with the traditional chromatographic technique, which is a tedious, energy- and time-consuming process. These findings encourage us to develop such new supramolecular architecture with enlarging the cavity by elongating the bridges for selective encapsulating higher fullerenes.

EXPERIMENTAL SECTION General Information. All starting chemicals were obtained from commercial sources and used without further purification, unless indicated otherwise. Corannulene was provided by Prof. Jay S. Siegel at School of Pharmaceutical Science and Technology, Tianjin University, Tianjin, China. Analytical TLC was carried out using tapered silica plates with a preadsorbent zone. Crude compounds were purified by flash column chromatography, using flash grade silica gel and 0 – 20 psig pressure, performed at ambient pressure. Per-deuterated solvents for NMR spectroscopy were obtained from Cambridge Isotope Laboratories and used as received. NMR spectra were obtained with a Bruker spectrometer (1H, 400, 500, and 600 MHz) or a JEOL Delta spectrometer (1H, 400 and 600 MHz) using chloroform-d (CDCl3), acetonitrile-d3 or the mixture of them as solvent. The chemical shift references were as follows: (1H) chloroform, 7.26 ppm (chloroform-d), (13C) chloroform-d, 77.16 ppm (chloroform-d); (1H) 1,1,2,2-tetrachloroethane-d2, 6.00 ppm (1,1,2,2-tetrachloroethane-d2), (13C) 1,1,2,2tetrachloroethane-d2, 73.8 ppm (1,1,2,2-tetrachloroethane-d2). (1H) tetramethylsilane (TMS), 0.00 ppm (the mixture of chloroform-d and acetonitrile-d3). Typical 1D FID was subjected to exponential multiplication with a line broadening exponent (LB) of 0.3 Hz (for 1H) and 1.0 Hz (for 13C). 2D NOESY was recorded with the mixing time of 300 ms. UV-vis spectra were obtained on Shimadzu (UV-2401PC) spectrophotometer, using 10-mm or 1mm path-length quartz cells. IR spectra were recorded on FTIR Spectrometer (IR Affinity-1) with thin KBr disk. 128 scans were acquired, with a resolution of 2 cm−1. UV-vis spectra were obtained on Shimadzu (UV-2401PC) spectrophotometer, using 10 mm or 1 mm path-length quartz cells. The Lambert-Beer plots were obtained by recording the spectra of the sample solutions at six concentrations, covering the 0.1 – 2.0 range in absorbance (A). Mass spectra (ESI, MALDI) were acquired on GCT and FT-ICR


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spectrometer (Bruker Daltonics Inc. APEXII, BIFLEX III), respectively. High-resolution mass spectra were recorded on a SCIEX TripleTOF 5600+ System. 3-Bromo-5-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)pyridine (3). Compound 2 were synthesized according to previous reported methods.69 To a solution of 3-Bromo-5-hydroxypyridine (2 g, 11.5 mmol) and compound 2 (5.5 g, 17.2 mmol) in DMF (40 ml) was added K2CO3 (4.8 g, 34.5 mmol). The reaction was stirred at 70 oC for 16 hours. The reaction mixture was cooled to room temperature and quenched with water and subsequently extracted with EtOAc. The combined organic extracts were washed with brine, dried over Na2SO4, filtered and concentrated. The crude product was purified by column chromatography on silica gel eluted with a solvent gradient from DCM to DCM/MeOH (50/1). The solvent was evaporated to yield colourless oil (3 g, Yield: 81.7%). IR (KBr, cm–1): 3493, 2927, 2360, 2131, 1647, 1564, 1423, 1288, 1249, 1107, 871, 702. 1H NMR (400 MHz, CDCl3) δ 8.25 (d, J = 8.2 Hz, 2H), 7.38 (s, 1H), 4.15 (t, J = 4.7 Hz, 2H), 3.90 – 3.81 (m, 2H), 3.71 (dd, J = 6.3, 3.4 Hz, 2H), 3.68 – 3.60 (m, 4H), 3.53 (dd, J = 5.7, 3.6 Hz, 2H), 3.36 (s, 3H). 13C NMR (151 MHz, CDCl3) δ 155.5, 143.1, 136.7, 124.4, 120.3, 72.0, 71.0, 70.7, 70.6, 69.5, 68.4, 59.0. TOF-HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z): Calcd. for C12H19BrNO4 at [M+H]+: 320.0497(100%), 321.0531(13.0%), 322.0477(97.3%), 323.0511(12.6%); Found: 320.0497(100%, 0 ppm), 321.0533(12.8%, 1 ppm), 322.0470(96.9%, 2.1 ppm), 323.0507(12.1%, 1.2 ppm). 3-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)-5((trimethylsilyl)ethy-nyl)pyridine (4). A mixture of compound 3 (3 g, 9.4 mmol), trimethylsilylacetylene (4 ml, 28.3 mmol), CuI (178 mg, 0.94 mmol) and Pd(PPh3)2Cl2 (658 mg, 0.94 mmol) in TEA (20 mL) was stirred at 90 oC under Ar for 16 h. Then, water (30 mL) was added, and the resulting solution was extracted with EA (3×100 mL). The combined organic extracts were dried over Na2SO4 and evaporated under reduced pressure to give the crude product. The crude product was purified by column chromatography on silica gel eluted with a solvent gradient from PE/EA (5/1) to PE/EA (1/1). The solvent was evaporated to yield a brown oil (2.8 g, Yield: 88.9%). IR (KBr, cm–1): 3493, 3049, 2927, 2879, 2827, 2463, 2360, 2156, 1647, 1564, 1554, 1423, 1417, 1352, 1317, 1292, 1288, 1226, 1172, 1058, 945, 848, 761. 1H NMR (400 MHz, CDCl3) δ 8.31 (d, J = 8.8 Hz, 2H), 7.30 (s, 1H), 4.22 – 4.15 (m, 2H), 3.93 – 3.86 (m, 2H), 3.76 (d, J = 5.0 Hz, 2H), 3.73 – 3.65 (m, 4H), 3.61 – 3.55 (m, 2H), 3.41 (s, 3H), 0.29 (s, 9H). 13C NMR (151 MHz, CDCl3) δ 154.4, 145.3, 138.4, 123.5, 120.5, 101.5, 98.2, 72.1 , 71.1 , 70.9 , 70.8 , 69.7 , 68.2 , 59.2, 0.0. TOF-HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z): Calcd. for C17H28NO4Si at [M+H]+: 338.1788(100%), 339.1783(5.1%), 339.1821(18.4%); Found: 338.1789(100%, 0.3 ppm), 339.1786(6.1%, 0.9 ppm), 339.1816(17.9%, 1.5 ppm). 3-Ethynyl-5-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)pyridine (5). To a solution of compound 4 (2.8 g, 8.3 mmol) in THF (15 ml) was added Bu4NF (2.8 g, 8.3 mmol). The reaction was stirred at rt for 2 h. The reaction mixture was quenched with water and subsequently extracted with EtOAc. The combined organic extracts were washed with brine, dried over Na2SO4, filtered and concentrated. The crude product was purified by column chromatography on silica gel eluted with a solvent gradient from DCM to DCM/MeOH (100/1). The solvent was evaporated to yield brown oil (1.78 g, Yield: 81%). IR (KBr, cm–1): 3493, 2927, 2877, 2360, 2131, 1647, 1564, 1417, 1288, 1109, 871, 702. 1H NMR (400 MHz, CDCl3) δ 8.36 (s, 2H), 7.33 (s, 1H), 4.20 (dt, J

= 5.3, 2.5 Hz, 2H), 3.95 – 3.88 (m, 2H), 3.80 – 3.74 (m, 2H), 3.74 – 3.65 (m, 4H), 3.58 (dt, J = 5.7, 2.5 Hz, 2H), 3.41 (s, 3H), 3.23 (s, 1H). 13C NMR (151 MHz, CDCl3) δ 154.4, 145.2, 138.5, 132.1, 128.6, 123.8, 119.4, 72.0, 71.0, 70.7, 70.6, 69.5, 68.1, 59.0. TOF-HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z): Calcd. for C14H20NO4 at [M+H]+: 266.1392(100%), 267.1426(15.2%); Found, 266.1392(100%, 0 ppm), 267.1421(15.9%, 1.9 ppm). 3-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)-5((tributylstannyl)eth-ynyl)pyridine (6). To a stirred solution of compound 5 (265 mg, 1 mmol) in THF (3 mL) was added n-BuLi (0.75 mL, 1.2 mmol; 1.6M in hexane) at -78°C under Ar. After being stirred at -78°C for 30 min, W-Bu3SnCl (0.4 mL, 1.5 mmol) was added. The reaction mixture was allowed to warm to room temperature and stirred for 16 h. The reaction mixture was quenched with saturated NH4Cl solution, extracted with EtOAc (2×200 mL). The combined organic extracts were washed with water (150 mL), brine (150 mL), dried over anhydrous Na2SO4 and concentrated. The crude product was purified by column chromatography on silica gel eluted with a solvent gradient from DCM to DCM/MeOH (50/1). The solvent was evaporated to yield brown oil (345 mg, Yield: 81%). IR (KBr, cm–1): 3493, 2928, 2920, 2360, 2131, 1647, 1564, 1521, 1423, 1417, 1404, 1288, 1249, 871, 702, 602. 1H NMR (400 MHz, CDCl3) δ 8.27 (s, 1H), 8.21 (d,

J = 2.7 Hz, 1H), 7.22 (s, 1H), 4.14 (t, J = 4.8 Hz, 2H), 3.85 (t, J = 4.7 Hz, 2H), 3.76 – 3.70 (m, 2H), 3.69 – 3.59 (m, 4H), 3.54 (dd, J = 5.7, 3.6 Hz, 2H), 3.37 (s, 3H), 1.59 (q, J = 8.1 Hz, 6H), 1.36 (q, J = 7.3 Hz, 7H), 1.06 (dd, J = 9.1, 7.0 Hz, 6H), 0.91 (t, J = 7.3 Hz, 9H). 13C NMR (151 MHz, CDCl3) δ 154.3, 145.3, 137.5, 123.4, 121.2, 106.2, 97.8, 72.0, 71.0, 70.7, 70.6, 69.6, 68.0, 59.1, 28.9, 27.0, 13.7, 11.3. TOF-HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z): Calcd. for C26H46NO4Sn at [M+H]+: 556.2449(100%), 557.2482 (28.1%), 558.2461(14.2%), 560.2480(17.8%); Found, 556.2450(100%, 0.2 ppm), 557.2479(27.8%, 0.5 ppm), 558.4256(13.9%, 0.9 ppm), 560.2462(16.9%, 3.2 ppm). 1,3,5,7,9-Pentachlorocorannulene (4). The procedure was slightly modified from the one previously reported in the literature.70 Iodine monochloride (15 mL, 15 mmol, 1 M in dichloromethane) was cooled to -78 oC and corannulene (250 mg, 1 mmol) was added. The mixture was allowed to warm to room temperature over 10 h and stirred an additional 3 days at ambient temperature. The solution was poured into 50 mL of chloroform and washed 2 times with sodium sulfite aqueous (10 mL) and 2 times with water (20 mL). The resulting yellow suspension was evaporated to dryness and triturated 2 times with hexanes (10 mL) and 3 times with dichloromethane (10 mL). The resulting yellowish solid was recrystallized from 1,1,2,2-tetrachloroethane to yield a pale yellow solid (240 mg, Yield: 57%). IR (KBr, cm–1): 3450, 1610, 1420, 1300, 1175, 960, 875, 810. 1H NMR (400 MHz, TCE-d2): δ 7.96 (s, 5H). 13C NMR (151 MHz, TCE-d2, 100°C): not obtained due to its poor solubility under the measured condition. HRMS-MALDI-TOF: Calcd. for C20H6Cl5 at [M+H]+: 422.8883 (100%), 424.8853 (63.9%), 420.8912 (62.5%), 423.8916 (21.6%), 425.8887 (13.8%), 421.8946 (13.5%); Found: 422.8873 (100%, 2.4 ppm), 424.8842 (63.5%, 2.6 ppm), 420.8892 (60.3%, 4.7 ppm), 423.8906 (22.1%, 2.4 ppm), 425.8890 (14.1%, 0.7 ppm), 421.8935 (13.3%, 2.6 ppm). 1,3,5,7,9-Penta-((5-(2-(2-(2methoxyethoxy)ethoxy)ethoxy)pyridi-ne-3-yl)ethynyl) corannulene (COPY1). To a mixture of compound 7 (76 mg, 0.18 mmol), Pd(OAc)2 (21 mg, 0.09 mmol), t-BuOK (21 mg, 0.18 mmol) and IPr·HCl (77 mg, 0.18 mmol) under nitrogen was added dry THF (20 mL), and this solution stirred at 50 °C for 30


7


min. Then, compound 6 (1.0 g, 1.8 mmol) was transferred to the above solution and the reaction mixture was refluxed under Ar for 3 d. After removal of solvent, the residue was subjected to chromatography on silica gel. Elution with DCM/MeOH/NH3.H2O (from 100:1: 1 to 100:2:1) gave product as a yellow solid (152 mg, Yield: 54%). IR (KBr, cm–1): 3493, 2927, 2360, 2131, 1647, 1564, 1423, 1417, 1288, 1249, 1107, 871, 738, 702. 1H NMR (600 MHz, TCE-d2) δ 8.55 (s, 1H), 8.34 (d, J = 2.7 Hz, 1H), 8.31 (s, 1H), 7.52 (s, 1H), 4.27 – 4.19 (m, 2H), 3.90 – 3.85 (m, 2H), 3.71 (dd, J = 5.7, 3.6 Hz, 2H), 3.63 (dd, J = 5.7, 3.6 Hz, 2H), 3.60 (dd, J = 5.6, 3.6 Hz, 2H), 3.51 (dd, J = 5.6, 3.6 Hz, 2H), 3.32 (s, 3H). 13C NMR (126 MHz, TCE-d2) δ 153.5, 143.9, 137.4, 133.7, 130.3, 129.9, 122.6, 120.7, 119.0, 90.4, 88.8, 70.8, 69.8, 69.4, 69.3, 68.4, 67.2, 57.9. HRMS-ESI (1% HCOOH in MeCN, ion type, % RA for m/z): Calcd. for C90H96N5O20 at [M+H]+: 1566.6649(100%), 1567.6682(97.4%), 1568.6716(46.9%), 1569.6749(14.9%); Found, 1566.6639(94.5%, 0.3 ppm), 1567.6671(100%, 0.7 ppm), 1568.6710(48.9%, 0.4 ppm), 1569.6741(15.2%, 0.5 ppm). COPY1-Pd. COPY1 (0.5 μmol, 1 equiv), Pd(CH3CN)2Cl2 (2.5 μmol, 2.5 equiv), and TCE-d2(0.5 mL) were added to a glass test tube. When the mixture was stirred at at 90oC for 2 h, the 1H NMR spectrum revealed the formation of COPY1-Pd in quantitative yield. 1H NMR (600 MHz, TCE-d2 ) δ 8.58 (d, J = 3.1 Hz, 1H), 8.40 (dd, J = 7.3, 2.4 Hz, 1H), 8.22 (d, J = 4.2 Hz, 1H), 7.54 (s, 1H), 4.21 (s, 2H), 3.91 – 3.78 (m, 2H), 3.68 (dd, J = 5.8, 3.3 Hz, 2H), 3.64 – 3.60 (m, 4H), 3.53 – 3.51 (m, 2H), 3.33 (s, 3H). 13C NMR (126 MHz, TCE-d2) δ 154.9, 148.4, 141.5, 135.4, 132.4, 131.4, 126.0, 121.2, 121.1, 90.0, 88.2, 71.8, 70.9, 70.8, 70.3, 69.3, 69.0, 58.8. C60⊂COPY1-Pd. To a NMR tube was added C60 (5 mM in CS2, 50 μL, 1.0 equiv), the solvent was removed and added COPY1-Pd (0.5 mM in TCE-d2, 0.5 mL, 1.0 equiv). When the-tube was heated at 90oC for 12h, the 1H NMR spectrum revealed the formation of a C60⊂COPY1-Pd complex in quantitative yield. 1H NMR (600 MHz, TCE-d2) δ 8.54 (t, J = 2.8 Hz, 1H), 8.32 (s, 1H), 8.23 (d, J = 5.8 Hz, 1H), 7.54 (t, J = 3.0 Hz, 1H), 4.22 (s, 2H), 3.86 – 3.82 (m, 2H), 3.68 (m, 2H), 3.65 – 3.60 (m, 4H), 3.54 – 3.51 (m, 2H), 3.34 (s, 3H); 13C NMR (126 MHz, TCE-d2) δ 154.9, 148.2, 141.8, 141.6, 136.1, 133.7, 132.2, 126.4, 121.8, 121.5, 90.1, 88.8, 71.8, 70.8, 70.4, 70.3, 70.2, 69.0, 58.8. C70⊂COPY1-Pd.To a NMR tube was added C70 (5 mM in CS2, 50 μL 1.0 equiv), the solvent was removed and added COPY1-Pd (0.5 mM in TCE-d2, 0.5 mL, 1.0 equiv). When the-tube was heated at 90oC for 2 h, the 1H NMR spectrum revealed the formation of a C70⊂COPY1-Pd complex in quantitative yield. 1H NMR (600 MHz, TCE-d2 ) δ 8.66 – 8.60 (m, 1H), 8.58 (dd, J = 13.0, 2.4 Hz, 1H), 8.06 (d, J = 1.9 Hz, 1H), 7.58 – 7.54 (m, 1H), 4.24 (s, 2H), 3.89 – 3.83 (m, 2H), 3.71 – 3.68 (m, 2H), 3.67 – 3.60 (m, 4H), 3.56 – 3.50 (m, 2H), 3.34 (s, 3H). 13C NMR (126 MHz, TCE-d2) δ 155.0, 148.0, 147.2, 147.1, 146.8, 143.7, 135.9, 134.7, 131., 128.5, 126.9, 122.1, 121.6, 91.2, 89.6, 71.8, 70.9, 70.4, 70.3, 69.1, 68.9, 58.9.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental and computational details and characterization data, including spectra for NMR and HRMS (PDF)

Page 8 of 10

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge the financial supports from the 973 Program (2015CB856502) and the National Natural Science Foundation of China (21332008, 21572023 and 21672026).

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