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A Novel Approach toward Polyfulvene: Cationic Polymerization of Enediynes Shudan Chen, Qiaoping Li, Shiyuan Sun, Yun Ding,* and Aiguo Hu* Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: Enediyne compounds have found limited applications in polymer science and material chemistry due to the poor regioselectivity and/or the step-growth nature in their radical polymerizations. However, the cationic cyclization of enediynes exhibits a high regioselective 5-exo-dig mechanism, providing a new strategy for the synthesis of polyfulvene derivatives. The expected polymers were successfully produced by cationic polymerization of enediynes induced by CF3SO3H, and a well-defined conjugated structure was confirmed by NMR, IR, and UV−vis spectroscopy. GPC analysis shows a relatively narrow molecular weight distribution, and the molecular weight reaches up to 62.9 kDa. On the other hand, the structural features of the obtained polymers and the mechanism of the cationic polymerization were investigated through kinetic study and MALDI-TOF MS analysis, which revealed a second-order consumption of enediyne monomer and the polymerization being probably terminated through intramolecular abstraction of proton from the neighboring group.

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Recently, poly(1,4-naphthalene),15,16 poly-p-phenylene (PPP),17,18 and poly(bis-o-diynylarene) (BODA polymers)19,20 prepared using the Bergman cyclization polymerization of enediynes have been reported. Although on-surface fabrication of the poly(1,6-di-2-naphthylhex-3-ene-1,5-diyne) (DNHD polymers) through Bergman cyclization showed perfect onedimensional polyphenylene chains in the STM image,18 other cases demonstrated an ill-defined structure of the main chain, which contains the fulvene structural defects caused by unwanted five-membered cyclization.15,21,22 It is supposed that the exclusive production of a benzenic ring during these polymerizations requires either a step-growth mechanism of the 1,4-diradical intermediates or a highly regioselective chain growth polymerization,21 which seems to be unattainable under radical polymerization condition, due to the ambiguous selectivity between Bergman cyclization, five-membered cyclization, and the dimerization of enediynes.23−25 This issue inspires our attempts to reevaluate the synthetic potency of

INTRODUCTION

Enediynes (EDY) find ever-increasing use in synthetic organic and material chemistry, since a group of natural antibiotics containing a (Z)-hex-3-ene-1,5-diyne fragment were discovered in the 1980s. The strong antitumor and antibacterial activity of the enediyne “warheads” has initiated a flurry of activities on the synthesis of biosimilar enediyne antibiotics.1−6 The biological activity of these enediyne compounds stems directly from the Bergman cyclization7,8 which generates highly reactive benzene-1,4-diradicals (Scheme 1), capable of cleaving DNA and rupturing the protein structure.9−11 On the other hand, the role of the diradical intermediate could be classified as either an initiator for other vinyl monomers12 or a monomer for selfpolymerization, which was therefore adopted as a method to generate conjugated aromatic polymers.13,14 Scheme 1. Bergman Cyclization of Enediyne

Received: October 25, 2016 Revised: December 31, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.macromol.6b02321 Macromolecules XXXX, XXX, XXX−XXX

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MeOH was added as an external nucleophile to intercept the incipient vinyl cation, which has been described by Overman and Sharp as a “nucleophile-promoted electrophilic cyclization” (NPEC).47,48 Remarkably, when treated with catalytic amounts of CF3SO3H in dichloromethane, only a little amount of 9c was yielded with rapidly disappearance of EDY 9. It is proposed that the poor productive rate suggested the extensive formation of oligomers.34 In this article, we performed a detailed research on the cationic polymerization of enediynes initiated by CF3SO3H. Delightedly, a series of expected polyfulvene derivatives were generated, which showed a clearly conjugated structure and narrow molecular weight distributions. According to MALDITOF MS and kinetic analysis, the cationic polymerizations start rapidly, following a second-order kinetics. In general, an intramolecular electrophilic cyclization is proposed to happen to terminate the polymerization. Regarding to this, introduction of electron-deficient group (such as F atom) probably is a good strategy to generate polyfulvene with high molecular weight.

enediynes and extend the radical polymerization to other reactive species. The conjugated structures of enediynes leads to a narrowed HOMO/LUMO gap, rendering such modified alkyne systems better donors and better acceptors than the parent acetylene. As a result, enediyne cyclizations are readily to proceed under the action of various reagents: lithium metal,26−29 radicals,30−33 electrophiles,26,34−36 transition metal complexes,37−39 nucleophiles,40−42 etc. Among all the reactions, cyclizations under the action of electrophiles show a high selectivity of the 5-exo-dig pathway, mainly because of the extremely low stability of the phenyl cation which would otherwise be generated through the 6-endo-dig pathway. In general, interaction of the first triple bond of such an enediyne with an electrophile (E+) leads to the formation of a carbenium ion or π-complex, which then attacks the second triple bond to form a five-membered ring. Subsequent nucleophilic attack on the intermediate generates two isomers of benzofulvene derivatives, which are also regarded as valuable precursors for conjugated architectures.35,43−45 Indeed, cationic cycloaromatization reactions of enediynes were known long before the Bergman cyclization. In the 1960s, Whitlock et al.26,46 found that 1,2-bis(phenylethynyl)benzene (EDY 9) could be transformed into benzofulvene derivatives 9a and 9b (Scheme 2) under the action of bromine or iodine in

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RESULTS AND DISCUSSION All the enediyne compounds (EDY 1−12) were synthesized through Sonogashira coupling reaction between acetylene derivatives and 1,2-diiodocyclohexene (1) or 1,2-diiodobenzene (2) as shown in Scheme 3. Starting from commercial available cyclohexanone, 1,2-dibromocyclohexene was obtained within a few steps according to a literature procedure, which was then converted to 1,2-diiodocyclohexene (1).49 These diiodides enable the cross-coupling reaction taking place smoothly to give the desired enediyne compounds in high yields. Terminal alkynes with various substituents were easily available, unless deprotection of the TMS group of some substituted ethynyltrimethylsilane (3a−c) was failed due to the volatile nature or side reactions. To solve this problem, tetrabutylammonium fluoride trihydrate (TBAF·3H2O) was added to the Sonogashira coupling reaction system to achieve the “one-pot” method.50 When induced by CF3SO3H, all the enediyne compounds undergo a fast cationic polymerization at room temperature (except for EDY 12). The 1H and 13C NMR spectra of the enediyne compounds EDY 1−12 and the corresponding polymer P1−12 are provided in the Supporting Information. For alkyl-substituted polymers, the peaks attributed to the alkyl protons at about 0.5−2.5 ppm (P1,5−8 1H NMR) and the alkyl carbon at about 14, 20, and 30 ppm (P1,5−8 13C NMR)

Scheme 2. Electrophilic Cyclization of 1,2Bis(phenylethynyl)benzene

chloroform solutions. When induced by a strong acid, such as H2SO4 and CF3SO3H, the precursor 9a or 9b would further hydrolyze to generate ketone 9c.34 In these systems, AcOH or

Scheme 3. Synthesis of Enediyne Compounds and Polymerization

B


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temperature to 100 °C, the EDY 12 were conclusively converted to the desired polyfulvene product (P12). The occurrence of cycloaromatization reaction was confirmed by NMR spectroscopy analysis. To correctly clarify the conformation of M12, we predicted 1H NMR chemical shifts of two possible stereoisomers by quantum chemical calculations. Simulated with WP0451/6-311++G(d,p) computational level and Gauge-Including Atomic Orbital (GIAO)52 method, as shown in Figure S1, the NMR spectrum of the E-isomer clearly resembles the experimental one (bottom panel). Since the Zisomer could not be isolated, there is no wonder that the Eisomer predominated or was the sole reaction product. Figure 2 shows a comparison of the 1H and 13C NMR spectra of EDY 12, M12, and O12 (oligomers precipitated from methanol). After the formation of fulvene moiety, the symmetry of the molecule was lowered. One proton on the benzo ring of M12 shifts greatly up to 8.20 ppm due to the electron-withdrawing effect of the neighboring trifluoromethylsulfonate group; the other three protons merged together showing a multiplet at 7.36 ppm. Two difluorophenyl rings are no longer symmetrical anymore; the aromatic protons of these group split into three sets of multiplets with an integration ratio of 1:1:4. Most importantly, a sharp peak corresponding to the proton on the fulvene ring shows up at 6.94 ppm, confirming the 5-exo-dig cycloaromatization. The structure of M12 was also proved by HR-MS analysis; a peak at m/z 500.0317 correlated well with the theoretical molecular weight of M12 (500.0316). Comparison of 13C NMR spectra of EDY 12, M12, and O12 comes up with the same conclusion. The acetylenic carbon signals at 80.4 and 97.2 ppm in EDY 12 disappeared after cycloaromatization, indicating that the reaction takes place on both triple bonds. An additional quartet signals (J = 319 Hz) at 118.1 ppm is attributed to the trapping of trifluoromethylsulfonate group in M12. The 1H and 13C NMR of O12 are similar to those of M12, indicating that the structure of the repeating unit in O12 is similar to M12. All the peaks in O12 are broadened because of the restricted motion of the conjugated main chain. Some tiny peaks also show up in aromatic region probably due to the complicated shielding and deshielding effects on internal aromatic protons and carbons in this conjugated system. These appealing results provided a direct evidence of the 5-exo-dig mechanism during the cationic polymerization. Formation of the conjugated backbones was further confirmed via UV−vis and fluorescence spectroscopy. The UV−vis spectra of enediyne (EDY 5) and polymer (P5) show that after polymerization strong adsorption tailing up to 500 nm (Figure S2), indicating the formation of long conjugated system. Figures 3a and 3b show the emission spectra of the enediyne compounds and their polymers. It is noteworthy that all the polyfulvenes show a clearly red-shift in comparison to the corresponding monomers, demonstrating a higher π-

were all broadened, and new broad peaks at 6.0−8.5 ppm (P1− 12 1H NMR) and 110−160 ppm (P1−12 13C NMR) emerged for all the synthesized polymers as well, indicative of the polymerization of enediyne compounds and the formation conjugated polymers. The 13C NMR analysis showed that the signals of acetylenic carbons at about 80−95 ppm have completely disappeared after the polymerization, corroborating the polymerization of the enediyne compounds. The cationic polymerization of enediyne compounds was further supported by IR analysis. Figure 1 shows a comparison of FT-IR spectra of

Figure 1. IR spectra of EDY 2 and P2.

an enediyne precursor (EDY 2) and the corresponding polymer (P2). Four characteristic bands (1439, 1488, 2854, and 2926 cm−1) are observed in both monomer and polymer. In particular, the bands at 1439, 2854, and 2926 cm−1 correspond to the bending and stretching vibrations of the saturated C−H bonds. While the one at 1488 cm−1 is characteristic of the CC double bond. Those at 1595, 1746, 1802, 1876, and 1948 cm−1 corresponded to the stretching vibrations of the unsaturated bonds of the enediyne compound, moved to 1739 cm−1 after polymerization. The characteristic band of νC−H of the unsaturated structure (3073 cm−1) moved to 3130 cm−1 as well, indicative of the preparation of conjugated polymers. The stretching vibrations band of CC at 2200 cm−1 in EDY 2 totally disappeared in P2, which is direct evidence for the reaction of the alkynyl groups after induced by CF3SO3H. Taking advantage of the steric hindrance and electrondeficient effect of the 2,6-difluorophenyl groups in EDY 12, we have successfully trapped the monomeric cyclization product of the cationic polymerization, in which only an addition product of CF3SO3H (M12) was generated when the reaction was conducted at 60 °C, as shown in Scheme 4. By raising the Scheme 4. CF3SO3H-Induced Reaction of EDY 12

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Figure 2. 1H and 13C NMR spectra of EDY 12 (a, b), M12 (c, d), and O12 (e, f).

Figure 3. Emission spectra of enediynes and polymers obtained in ethyl acetate solution (c = 10−3 mg/mL). (a) Excitation wavelengths: 256 nm. (b) Excitation wavelengths: 310 nm. (c) Emission spectra of the polymerization system of EDY 5 at room temperature (excitation wavelengths: 256 nm; recorded from 0 to 24 h).

hindrance of the long or branched alkyl chains. Although the enediynes with aromatic substituents emit at longer wavelength (λmax 360−390 nm) due to the higher conjugated degree in these compounds, the obtained polymers from all the enediyne compounds show quite similar broad emission peaks at around 430 nm, implying identical backbone structure of these polyfulvenes. Shown as Figure 3c, the process of the cationic polymerization was monitored by the evolution of the emission spectra over 24 h (using EDY 5 as representative). The spectra display a decrease of emission peak at 324 nm (corresponded to enediyne moiety), and a peak (398 nm) ascribed to the

conjugated degree of the polymers. The emission maximum (λmax) varies with different substituents, which could roughly divide into two parts: the enediyne compounds with alkyl substituents (EDY 5−8) and aromatic substituents (EDY 2, 3, 9, and 10). In particular, the enediyne compounds with a homologous series of alkyl chains exhibit similar emission maximum appearing at 324 nm, while the emission spectra of their corresponding polyfulvenes show a tendency of blue-shift with the increasing of n-alkyl chain length (443, 427, and 415 nm). The λmax of the isopentyl-based polyfulvene (P7) is even shorter, recorded as 397 nm. This hypsochromic change of the emission maximum is probably due to the stronger steric D


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Figure 4. Conversion−time (left) and kinetic (right) curve for the polymerization of EDY 5 (Ct = 1 − [M]/[M]0, [M]0 = 33.6 mmol/L, [TfOH]0 = 0.07 mmol/mL, −78 °C, in dichloromethane).

Table 1. Molecular Weights and Polydispersity Indexes of the Polyfulvene Derivatives Mw (kDa) PDI

P1

P2

P3

P4

P5

P6

P7

P8

P9

P10

P11

P12

62.9 2.22

10.6 1.09

5.02 1.08

7.34 1.12

24.5 1.12

23.9 1.78

4.71 1.08

24.7 1.31

2.83 1.14

1.63 2.05

3.10 1.10

40.3 1.25

Scheme 5. Possible Mechanism of the Cationic Polymerization of EDY

oligomer intermediates emerged during the first 20 min, which further red-shifts to about 430 nm due to the formation of polybenzofulvene conjugated backbone. Kinetic analysis of the cationic polymerization of EDY 5 was also conducted at −78 °C by monitoring the consumption of EDY 5 via gas chromatography (GC). As shown in Figure 4, the conversion of monomers boomed to 60% within the first hour, but the rate slowed down quickly. Interestingly, the polymerization progress

corresponds well with a second-order kinetics equation (Figure 4b), implying that both the initiation and propagation steps are concentration dependent, which is quite different from other ionic polymerization. The addition of proton to enediyne to initiate the cycloaromatization polymerization is probably the rate-limiting step, followed by much faster chain propagation due to the higher reactivity of the vinyl cation terminal group toward another enediyne monomer. Regarding to this E


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Figure 5. MALDI-TOF mass spectra of P1, P2, P5, and P6.

demonstrated by Whitlock et al. Actually, the dominant series of signals correspond to integral multiples of the m/z value of a monomer unit, indicating an intramolecular abstraction of proton must be happened to quench the cationic polymerization, which was also revealed by GPC analysis. On the other hand, according to the MS spectra of P1, P5, and P6, there is another peak series differs from the dominant one mentioned above. The difference between these two series is about 57 or 71, which matches well with the calculated m/z of butyl and pentyl groups, respectively, probably due to an elimination of the alkyl groups at the chain end during laser dissociation.53 In consideration of all the structural characterization and end-group analysis, a complete reaction scheme of the cationic polymerization of enediyne was proposed as shown in Scheme 5. In the initiation step, the proton from CF3SO3H first attacks one of the triple bonds in the enediyne to form a carbenium ion or π-complex, which then attacks the second triple bond to form a five-membered ring. The newly generated vinyl cation further reacts with other enediyne molecules to extend the polymer chain. The chain propagation would be terminated through two different routes differs with substituents. For instance, without a strong electron-withdrawing group (F atom), the aromatic rings are good nucleophiles which can easily be attacked by the vinyl cation. Therefore, electrophilic substitution takes place between the vinyl cation and the aromatic ring to form indeno[2,1-a]indene structure (route a). On the other hand, since vinyl cations are strongly (>20 kcal/ mol) disfavored relative to alkyl cations,23 the termination reactions of alkyl-substituted polymers start with a hydride transfer. The newborn alkyl cation subsequently electrophilic attack the double bond to generate a cyclization product (route b).

mechanism feature, the control over this cationic polymerization of enediyne would be more difficult than other ionic polymerization. The calculated half-life of the EDY 5 is about 0.4 h at −78 °C, corroborating the high rate of the polymerization. Since these polymers are structurally much more compact in comparison with general polystyrene standards, the molecular weights would be severely underestimated when determined by gel permeation chromatography (GPC). Therefore, a dualdetector GPC equipped with a refractive index detector and a multiangle static laser light scattering (MALLS) detector was used for the determination of molecular weight and polydispersity of the polyfulvenes. The weight-average molecular weights (Mw) and polydispersion index (PDI) of these polymers are listed in Table 1. It should be noted that the presence of aryl groups at the alkynyl termini of enediyne monomer typically results in low Mw of the polymer, which is probably due to an analogous Friedel−Crafts reaction between the vinyl cation terminal group and the neighboring aromatic ring could easily take place to quench the cationic polymerization (Scheme 5a). Interestingly, by blocking all the sites on the aromatic substituents that would otherwise undergo the Friedel−Crafts reaction with electron-deficient F atoms, the chain termination reaction was successfully inhibited to give polymer with much higher molecular weight (P12). On the other hand, enediynes with alkyl groups at the alkynyl termini typically give polymers with Mw over 20 kDa. When an isopentyl group was deliberately introduced at the alkynyl termini (EDY7), the Mw of P7 was drastically dropped to 4.7 kDa. The abstraction of a hydrogen atom from the tertiary carbon in the isopentyl group is much easier than from other secondary or primary carbon, which resulted in a much faster chain termination in the polymerization of EDY 7. This effect of substituent groups on the Mw of the polymer draws a clear clue on the chain termination mechanism which will be discussed in detail below. MALDI-TOF mass spectrometry is another powerful tool to analyze the structure of the polyfulvene derivatives. Though only a few oligomers were detected, as shown in Figure 5, lots of information could be obtained from these spectra. One or two series of signals, which have an equal interval consistent with the calculated m/z value of a monomer unit, are observed in all the spectra. However, none of them can be ascribed to the expected chain-end structures terminated by hydroxyl group as

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CONCLUSIONS A variety of polyfulvene derivatives were successfully produced by cationic polymerization of enediynes induced by CF3SO3H, which has never reported before. According to NMR, IR, and UV−vis spectra, the polymers showed an identical well-defined conjugated structure despite the different substituents. The polymerization process were corresponded well with secondorder kinetics equation revealed by GC detection. After quenched by saturated NaHCO3 after 24 h, the cationic polymerization led to a series of narrow-distribution and highmolecular-weight (up to 62.4 kDa) polyfulvene derivatives. In F


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(10) Basak, A.; Roy, S. K.; Roy, B.; Basak, A. Synthesis of highly strained enediynes and dienediynes. Curr. Top. Med. Chem. 2008, 8, 487. (11) Kar, M.; Basak, A. Design, Synthesis, and Biological Activity of Unnatural Enediynes and Related Analogues Equipped with pHDependent or Phototriggering Devices. Chem. Rev. 2007, 107, 2861. (12) Rule, J. D.; Wilson, S. R.; Moore, J. S. Radical polymerization initiated by Bergman cyclization. J. Am. Chem. Soc. 2003, 125, 12992. (13) Chen, S.; Hu, A. Recent advances of the Bergman cyclization in polymer science. Sci. China: Chem. 2015, 58, 1710. (14) Gerstel, P.; Barner-Kowollik, C. RAFT Mediated Polymerization of Methyl Methacrylate Initiated by Bergman Cyclization: Access to High Molecular Weight Narrow Polydispersity Polymers. Macromol. Rapid Commun. 2011, 32, 444. (15) Cheng, X.; Ma, J.; Zhi, J.; Yang, X.; Hu, A. Synthesis of Novel “Rod-Coil” Brush Polymers with Conjugated Backbones through Bergman Cyclization. Macromolecules 2010, 43, 909. (16) Miao, C.; Zhi, J.; Sun, S.; Yang, X.; Hu, A. Formation of Conjugated Polynaphthalene via Bergman Cyclization. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2187. (17) Tour, J. M. Soluble Oligo- and Polyphenylenes. Adv. Mater. 1994, 6, 190. (18) Sun, Q.; Zhang, C.; Li, Z.; Kong, H.; Tan, Q.; Hu, A.; Xu, W. On-Surface Formation of One-Dimensional Polyphenylene through Bergman Cyclization. J. Am. Chem. Soc. 2013, 135, 8448. (19) Smith, D. W.; Shah, H. V.; Perera, K. P. U.; Perpall, M. W.; Babb, D. A.; Martin, S. J. Polyarylene networks via Bergman cyclopolymerization of bis-ortho-diynyl arenes. Adv. Funct. Mater. 2007, 17, 1237. (20) Rettenbacher, A. S.; Perpall, M. W.; Echegoyen, L.; Hudson, J.; Smith, D. W. Radical Addition of a Conjugated Polymer to Multilayer Fullerenes (Carbon Nano-onions). Chem. Mater. 2007, 19, 1411. (21) Johnson, J. P.; Bringley, D. A.; Wilson, E. E.; Lewis, K. D.; Beck, L. W.; Matzger, A. J. Comparison of “Polynaphthalenes” Prepared by Two Mechanistically Distinct Routes. J. Am. Chem. Soc. 2003, 125, 14708. (22) Alabugin, I. V.; Gilmore, K.; Patil, S.; Manoharan, M.; Kovalenko, S. V.; Clark, R. J.; Ghiviriga, I. Radical Cascade Transformations of Tris(o-aryleneethynylenes) into Substituted Benzo[a]indeno[2,1-c]fluorenes. J. Am. Chem. Soc. 2008, 130, 11535. (23) Alabugin, I. V.; Gold, B. Two Functional Groups in One Package”: Using Both Alkyne π-Bonds in Cascade Transformations. J. Org. Chem. 2013, 78, 7777. (24) Peterson, P. W.; Mohamed, R. K.; Alabugin, I. V. How to Lose a Bond in Two Ways - The Diradical/Zwitterion Dichotomy in Cycloaromatization Reactions. Eur. J. Org. Chem. 2013, 2013, 2505. (25) Haberhauer, G.; Gleiter, R.; Fabig, S. Enediyne Dimerization vs Bergman Cyclization. Org. Lett. 2015, 17, 1425. (26) Whitlock, H. W.; Sandvick, P. E.; Overman, L. E.; Reichardt, P. B. Chemical behavior of o-bis(phenylethynyl)benzene toward some electrophilic and nucleophilic reagents. J. Org. Chem. 1969, 34, 879. (27) Bradshaw, J. D.; Solooki, D.; Tessier, C. A.; Youngs, W. J. Lithium-Induced Cyclization of Tetrabenzocyclyne. A Novel Zipper Reaction of Cyclic o-Ethynylbenzenes. J. Am. Chem. Soc. 1994, 116, 3177. (28) Peterson, P. W.; Shevchenko, N.; Alabugin, I. V. Stereoelectronic Umpolung”: Converting a p-Donor into a σ-Acceptor via Electron Injection and a Conformational Change. Org. Lett. 2013, 15, 2238. (29) Peterson, P. W.; Shevchenko, N.; Breiner, B.; Manoharan, M.; Lufti, F.; Delaune, J.; Kingsley, M.; Kovnir, K.; Alabugin, I. V. Orbital Crossings Activated through Electron Injection: Opening Communication between Orthogonal Orbitals in Anionic C1−C5 Cyclizations of Enediynes. J. Am. Chem. Soc. 2016, 138, 15617. (30) Kovalenko, S. V.; Peabody, S.; Manoharan, M.; Clark, R. J.; Alabugin, I. V. 5-Exo-dig radical cyclization of enediynes: The first synthesis of tin-substituted benzofulvenes. Org. Lett. 2004, 6, 2457.

addition, end-group analysis of MALDI-TOF MS indicated the occurrence of an intramolecular electrophilic cyclization during the chain termination. Further research may focus on structural optimization of the polyfulvene derivatives by the introduction of suitable electron-withdrawing groups and the exploration of their optical and electronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02321.

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Experimental details (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.H.). *E-mail [email protected] (Y.D.). ORCID

Aiguo Hu: 0000-0003-0456-7269 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474027, 91023008), Shanghai Shuguang Project (07SG33), New Century Excellent Talents in University, PhD Programs Foundation of Ministry of Education of China, Shanghai Leading Academic Discipline Project (B502), and the “Eastern Scholar Professorship” support from Shanghai Local Government.

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