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Cite This: J. Phys. Chem. A 2018, 122, 1831−1837

Intermolecular Hydrogen Abstraction from Hydroxy Group and Alkyl by T1(ππ*) of 1‑Chloro-4-nitronaphthalene Di Zhang,† Peipei Jin,† Meng Yang,† Yong Du,‡ Xuming Zheng,† and Jiadan Xue*,† †

Department of Chemsitry, Zhejiang Sci-Tech University, Hangzhou 310018, China Center for THz Research, China Jiliang University, Hangzhou 310018, China

‡

S Supporting Information *

ABSTRACT: Nanosecond transient absorption and theoretical calculations have been used to investigate the intermolecular hydrogen abstractions from alcohols and 1-naphthol by the lowest excited triplet (T1) of 1-chloro-4-nitronaphthalene upon excitation of the compound in organic solvents. The hydrogen abstraction of T1 from hydroxy group of 1-naphthol takes place through an electron transfer followed by a proton transfer through hydrogen bonding interaction with rate constants of ∼109 M−1 s−1. Hydrogen-bonding is crucial in this process, indicated by the observation of a half reduction for T1 yield when increasing the concentration of 1-naphthol. The hydrogen abstraction in this way can be decelerated by increasing solvent polarity and hydrogen-bonding donor ability. The T1 of 1-chloro-4-nitronaphthalene can undergo one-step H atom abstraction from alkyl hydrogen in alcoholic solvents, with rate constants of ∼104 M−1 s−1, and produce radical intermediates with the absorption maximum at 368 nm. DFT calculation results indicate both oxygens of the nitro group are active sites for hydrogen abstraction, and the difference of activation barriers for formation of two radical isomers is only 1.0 kcal/mol.

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INTRODUCTION Nitropolycyclic aromatic hydrocarbons (NPAHs) are persistent organic pollutants and can be generated from incomplete combustion of fossil fuels as well as atmospheric reaction of polycyclic aromatic hydrocarbons (PAHs) with nitrogen oxides.1−3 Comprehensive studies showed that NPAHs are more mutagenic and carcinogenic to organisms than their corresponding unsubstituted PAHs,2 and photolysis is their major degradation pathways at ambient conditions. It has been revealed that irradiation of NPAHs in solution populates their first lowest excited singlet state S1 (ππ*), which then undergoes three deactivation pathways: intersystem crossing (ISC) to the lowest excited triplet state T1, production of aryloxy radicals (ArO·) and nitrogen oxide (NO·), and generation of slight fluorescence for some selected nitronaphthalenes,4−8 nitropyrenes,9−11 nitroanthracenes12 compounds. Fluorescence upconversion4 and femtosecond transient absorption5,6,13 in combination with theoretical calculation studies4,7 suggested that the presence of Tn(nπ*), a higher excited triplet state and close to S1 in energy, controls the ultrafast ISC process and consequently the very short (∼200 fs) lifetime of S1. The fast ISC resulting from nitro group increasing the spin− orbital coupling of the excited singlet states with the triplet manifold makes T1 generated in a good yield and to be the major intermediate after photolysis of NPAHs. For example the T1 yield is about 0.93 ± 0.15 for 2-nitronaphthalene, 0.64 ± 0.12 for 1-nitronaphthalene, and 0.33 ± 0.05 for 2-methyl-1nitrophthanlene.14 These triplets of NPAHs at ambient © 2018 American Chemical Society

conditions have microsecond lifetimes and are easy to undergo energy transfer, electron transfer, protonation, and hydrogen abstraction reactions with their surrounding reactants.15−18 Hydrogen abstraction by T1 is one of the major reactions in the photochemistry of NPAHs in the organic phase of atmospheric aerosols, especially in polluted condition as hydroxy PAHs present in the diesel exhaust.19−21 Intermolecular hydrogen abstraction is one of the thoroughly studied photochemistry reactions, especially in the cases of aromatic carbonyl triplets.22 It has been well accepted that nπ* triplets are easier to undergo H atom abstraction than ππ* triplets. In the system of nπ* triplets, it is spin-allowed that the H atom makes a bond with the half-occupied n orbital of oxygen in plane. In contrast, in ππ* triplet systems, since the n orbital is fully occupied and both π and π* orbital half-occupied are outside the CO plane, H atom has to approach the carbonyl oxygen perpendicular to the plane, and this is spinforbidden. On the other hand, ππ* triplets can proceed the hydrogen abstraction efficiently by the way of state equilibrium or electron coupled proton transfer. Because the energy of an nπ* triplet decreases with decreasing solvent polarity, some acetophenone and their derivatives, for instance,23,24 having mixed nπ* and ππ* configurations in their triplets, can undergo H atom abstraction by reversion from ππ* to nπ* triplet Received: November 13, 2017 Revised: January 4, 2018 Published: February 12, 2018 1831

DOI: 10.1021/acs.jpca.7b11146 J. Phys. Chem. A 2018, 122, 1831−1837

Article

The Journal of Physical Chemistry A state.25 In this way, the rate constant of H atom abstraction is determined by the energy gap of ππ* and nπ* triplet states. Electron transfer coupled proton transfer is also an efficient way for some ππ* triplets to undergo hydrogen abstraction.22 This reaction is dependent on the reduction and oxidation potentials of H-withdrawing and H-donating species, usually occurs within exciplex through hydrogen bonding, and results in ion pair or radical ion pair.26,27 Most NAPHs possess a ππ* configuration in their T1 states, and for instance the T1 of parent 1-nitronaphthalene has a ππ* configuration associated with the electron transition from the naphthalene frontier π orbital (HOMO) to the π* orbital localized at both naphthalene ring and nitro group (LUMO). Thus, the hydrogen abstractions in alcoholic solvents are too slow to detect intermediates directly. Martins28 and Tanaka15 observed the quench effect of alcohol on T1 states of 1nitronaphthalene and 2-nitronaphthalene respectively with the rate constant of about 104 M−1 s−1. Contrastively, Arce and coworkers obtained the second order rate constants of 108−109 M−1 s−1 for the T1 of 1-nitropyrene toward various phenolic compounds.29 This huge distinction in rate constants of hydrogen abstraction for T1 of NAPHs from alcohols and phenolic compounds brings out the questions of whether these two reactions are controlled by two different mechanisms, as illustrated for aromatic carbonyl ππ* triples, and whether the T1 of NAPHs has a similar reactivity to aromatic ketones toward H atom donors and is still under assessment. In this paper, we selected 1-chloro-4-nitronaphthalene (ClNpNO2) as model and recorded and identified the absorption spectrum of intermediates formed through its hydrogen abstraction from alcoholic solvents. By comparison with its reaction in the presence of 1-naphthol, we wish to provide evidence of the hydrogen abstraction mechanisms by T1(ππ*) of ClNpNO2 from hydroxy and alkyl hydrogens donors.

method35,36 employing the 6-31G(d) basis set. Minimum energy structures were confirmed through examination of their harmonic frequencies. Twelve electrons in eleven orbitals were selected for CASSCF computation. The selected orbitals include π and π* orbitals in NO2, the oxygen nonbonding orbital, and π and π* orbitals in the aromatic ring. Single-point calculations at the CASSCF(12,11) optimized geometric structures using complete-active-space second-order perturbation theory CASPT237,38 (16,13) were performed to further handle the remaining excitations involving inactive orbitals. All of the quantum mechanical calculations were done using Molpro39−41 and Gaussian42 software.

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RESULTS AND DISCUSSION Figure 1 displays nanosecond transient absorption spectra at various time delays (Figure 1a) and kinetics at selected

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EXPERIMENTAL AND CALCULATION METHODS 1-Chloro-4-nitronaphthalene (98%), 1-naphthol (99%), sodium iodide, ferrocene, and perchloric acid were purchased from J&K scientific without further purification. Spectroscopic grade of acetonitrile, cyclohexane, acetonitrile, methanol, 2propanol, tetrahydrofuran were used to prepare sample solutions. The nanosecond transient absorption (ns-TA) measurements were performed on a LP-920 laser flash photolysis setup (Edinburgh Instruments, U.K.). The 355 nm pump laser pulse was obtained from the third harmonic output of a Nd:YAG Qswitched laser, and the probe light was provided by a 450 W Xe arc lamp. These two light beams were focused onto a 1 cm quartz cell. Signals analyzed by asymmetrical Czerny−Turner monochromator were detected by a Hamamatsu R928 photomultiplier, and the signal was processed via an interfaced computer and analytical software. Vertical excitation energies and oscillator strengths were obtained using time-dependent density functional theory (TDDFT),30−32 after the geometry optimization and vibrational frequency computation using the B3LYP33,34/6-311G(d,p) level of theory. The transition states for the H atom abstraction from methanol to ClNpNO2 were determined by TS method using UB3LYP/6-311++G(d,p) level of theory and confirmed through intrinsic reaction coordinate (IRC) scans. Stationary structures of 1-nitronaphthalene (1NN) in the three lowest triplet electronic states (T1, T2, T3) were optimized by means of the complete active space self-consistent field (CASSCF)

Figure 1. (a) Nanosecond transient absorption spectra recorded in 2propanol under argon saturated condition at various time delays after 355 nm excitation of ClNpNO2, and kinetics (black) monitored at (b) 585 nm and (c) 368 nm with exponential fit curves (red).

Scheme 1. Two Isomers of Radical ClNpNO2H·, Formed through H-Atom Abstraction by T1 State

wavelengths (Figure 1b), obtained after 355 nm laser excitation of ClNpNO2 in 2-propanol under argon saturated condition. As shown in Figure 1a, two broad bands centered at 404 and 587 nm were detectable immediately upon the laser pulse excitation, and accompanying their decay, a sharp absorption with maximum at 368 nm could be observed. The 404 and 587 nm broad bands are assigned to T1 of ClNpNO2 mainly based 1832

DOI: 10.1021/acs.jpca.7b11146 J. Phys. Chem. A 2018, 122, 1831−1837

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The Journal of Physical Chemistry A Table 1. DFT Calculation Predicted Vertical Transition Energy (in nm) and Oscillator Strength for the Two Isomers of ClNpNO2H· isomer 1

isomer 2

wavelength (nm)

oscillator strength

wavelength (nm)

oscillator strength

527.1 394.5 366.8 351.9 347.7 340.1 309.7

0.0009 0.0011 0.1756 0.0000 0.0100 0.0414 0.0282

529.03 381.42 373.23 361.27 352.60 330.13 301.03

0.000 0.1359 0.0130 0.0013 0.0173 0.0222 0.0055

Figure 4. Plots of the pseudo first order reaction rate of T1 monitored at 585 nm against concentrations of 1-naphthol in cyclohexane (CHX), acetonitrile (ACN), and 2,2,2-trifluoroethanol (TFE).

420 nm. The 587 nm absorption of T1 in 2-propanol moves to 570 nm in cyclohexane and 624 nm in aqueous solution respectively, exhibiting obvious red shift when increasing solvent polarity. This indicates that T1 has ππ* configuration in all solvents examined here, which also agrees very well with prior reports for other NPAHs.8,9,12,43 As presented in Figure 1, the decay of T1 was fitted well by a single exponential function with the time constant of 3110 ns recorded at 585 nm, and the same for 405 nm (not shown). While the T1 is diminishing, the 368 nm sharp absorption band (blue in Figure 1a) appears more and more dominant, especially at the late delays (10 μs spectrum for instance). The species with absorption at 368 nm is unstable, and the decay can be fitted well using a double exponential function with time constants of 156 μs and 1.8 ms, respectively (Figure 1c). The former one is assigned to contribution from the 1chloro-4-naphthaloxy radical (ClNpO·), generated through dissociation from the excited state of ClNpNO2, on the basis of, first, literature about the absorption and the lifetimes of aryloxy radicals,44,45 second, results that the lifetime of this species was not sensitive to oxygen quenching and the yield of the species was not affected by the quenching of T1 with ferrocene, and third, observations of processes with similar decay constant formed in the solvents of acetonitrile and cyclohexane. However, the 1.8 ms long-lived absorption band with maximum at 368 nm was only detected in 2-propanol, methanol, and tetrahydrofuran (Figure 1S in Supporting Information) but not in acetonitrile or cyclohexane. Thus, it was assigned to the radicals formed through H atom abstraction by nitro oxygens of ClNpNO2 T1 state from alcohols (hereafter ClNpNO2H·), analogous to H atom abstraction of aromatic carbonyl compounds.26,46,47 This assignment is further confirmed by TD-DFT calculation result, which predicts ClNpNO2H·, two isomers (Scheme 1) having the strongest transitions at 367 and 381 nm respectively as shown in Table 1, consistent very well with the experimental observation. Both isomers of ClNpNO2H· can be generated because the energy difference of their activation barrier is only 1 kcal/mol; vide infra. The formation of the radical ClNpNO2H· was measured to have a second order rate constant of (2.2 ± 0.1) × 104 M−1 s−1 as shown in Figure 2, by monitoring initial growth at 365 nm in order to avoid the interference of triplet−triplet

Figure 2. Plot of the pseudo first order reaction rate of T1 monitored at initial time at 365 nm against concentrations of methanol (MeOH) in acetonitrile, fitted linearly, with the slope (k) indicated close to the curve.

Figure 3. Comparison of transient absorption spectra recorded at 500 ns after photolysis of ClNpNO2 in cyclohexane in the presence of 0.225 mM 1-naphthol and at 10 μs in 2-propanol without 1-naphthol. The two ∼366 nm bands agree very well.

on their similarity with those of 1-nitronaphthalene and 2methyl-1-nitronaphthalene triplets in previous works.8 Meanwhile, the result that both 404 and 587 nm bands are sensitive to oxygen and can be quenched by ferrocene with the second order reaction rate constant of kFe = (6.0 ± 0.07) × 109 M−1 s−1, close to diffusion rate, further supports the T1 assignment. Here the 404 nm band intensity cannot represent the real oscillation strength ( f) of T1, since the ground state bleaching of ClNpNO2 also contributes in this region before 1833

DOI: 10.1021/acs.jpca.7b11146 J. Phys. Chem. A 2018, 122, 1831−1837

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The Journal of Physical Chemistry A Scheme 2

annihilation at typical wavelengths attributed to T1 in the acetonitrile−methanol mixed solvent. In the presence of 1-naphthol (NpOH), the radical ClNpNO2H· can also be detectable in cyclohexane. The transient absorption spectrum recorded at 500 ns after photolysis of ClNpNO2 in cyclohexane in the presence of 0.225 mM 1-naphthol is displayed in Figure 3. It possesses sharp absorptions at 366 and 391 nm, and the latter one has been identified as 1-naphthoxyl radical due to loss of H atom for 1-naphthol, since this species has maximum absorption around 400 nm in acetronitrile.44 The 366 nm band in Figure 3 overlaps very well with the one formed in 2-propanol with maximum at 368 nm, except a slight shift due to solvent polarity. In the presence of 1-naphthol, the decay of T1 obeys second order reaction rate law, and the rate constants were measured as kNpOH = (8.4 ± 0.2) × 109 M−1 s−1 in cyclohexane, kNpOH = (6.7 ± 0.2) × 109 M−1 s−1 in acetonitrile, and kNpOH = (2.9 ± 0.1) × 109 M−1 s−1 in 2,2,2-trifluoroethanol, respectively. These second order reaction rate constants were obtained by plotting the pseudo first order reaction rates of T1 monitored at 585 nm against concentrations of 1-naphthol in three solvents as shown in Figure 4. The T1 of parent 1-nitronaphthalene has a ππ* configuration associated with the electron transition from the naphthalene frontier π orbital (HOMO) to the π* orbital localized at both naphthalene ring and nitro group (LUMO).4,43 Compared to 1nitronaphthalene, the para-chloride substitution just slightly decreases the electron density on the nitro group, as transient absorption spectra reveal that the T1 of ClNpNO2 still has a ππ* character in nonpolar and polar solvents. Thus, in the case of reaction of T1 with 1-naphthol, the so fast reaction rate constant at a magnitude of ∼109 M−1 s−1 could be explained as the hydrogen abstraction taking place through an electron transfer followed by a proton transfer through hydrogenbonding interaction, as illustrated in Scheme 2. (It cannot

Figure 5. Plots of the normalized yield (with error bar) of T1 of ClNpNO2, obtained by normalization of the absorption intensities at 585 nm at time zero in acetonitrile against concentrations of 1naphthol.

Table 2. CASPT2(16,13)/Aug-cc-PVDZ Calculated Excitation Energies and Configuration of Triplet States Minimums

a

The values were obtained by setting ground state (S0) as energy zero.

Figure 6. DFT calculation predicted transition states and energy levels of the reactants and products in two channels of H atom abstraction by T1 of ClNpNO2 from methyl hydrogen of methanol. R1, R2, TS1, TS1, P1, and P2 were all obtained in triplet manifold. 1834

DOI: 10.1021/acs.jpca.7b11146 J. Phys. Chem. A 2018, 122, 1831−1837

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The Journal of Physical Chemistry A

deficient as that in nπ* state, and 1-naphthol being more acidic relative to the alcohols enables the hydrogen bonding to be stronger. This was also indicated by our observations that the second order reaction rates of hydrogen abstraction were ∼109 M−1 s−1 for phenol and ∼104 M−1 s−1 for tetrahydrofuran (THF) respectively, although they have similar ionization potential (IP) values (8.8 eV for THF and 8.4 eV for phenol). On the other hand, alcohols examined here have higher IP (10.1 eV for 2-proponal and 10.9 eV for methanol) than 1naphthol (7.8 eV) also making the electron transfer harder to occur between alcohols and the T1 of ClNpNO2. The absence or very weak hydrogen bonding between alcohols and T1 of ClNpNO2 as well as high IP of alcohols suggests that the hydrogen abstraction could take place from cleavage of the C− H bond. In addition to electron coupled proton transfers, H atom abstraction also can proceed through equilibrium between ππ* and nπ* triplet states which are close in energy.25 However, our theoretical calculation results for unsubstituted 1-nitronaphthalene indicate this situation hardly occurs since both T1 and T2 states are ππ* configuration, and there is a big energy gap of 32 kcal/mol between T1(ππ*) and T3(nπ*) minimums, as shown in Table 2. After all the enthalpy difference between nπ* and ππ* triplets is only 0.7 kJ/mol for p-methylacetophenone,25 which has ππ* configuration in T1 and a moderate rate constant (∼105 M−1 s−1) toward alcoholic hydrogen. DFT calculation was used to determine the transition states and activation barriers of the H atom abstraction by T1 of ClNpNO2 from methyl hydrogen of methanol. The calculation results presented in Figure 6 show that both oxygens of the nitro group are potential candidates to abstract H atom, since activation energies were predicted to be 7.6 and 8.6 kcal/mol for formation of two isomers. It was proposed here that ISC could take place quickly by way of curve-crossing of the potential surface after H atom abstraction by T1 and generating the radicals 1 and 2, which have an energy difference of 2.2 kcal/mol. Thus, both isomers of ClNpNO2H· are possible to be formed during the H atom abstraction and their yields are dependent on the energy difference of activation barriers.

exclude the possibility that both oxygen are active with consequent generation of two isomers.) In our transient absorption experiments, while increasing the concentration of 1-naphthol from 0.075 mM to 0.45 mM, a ∼52% reduction for the triplet yield was observed, as displayed in Figure 5, in which the normalized yield of T1 was recorded by normalization of the absorption intensities at 585 nm at time zero in the presence of various concentrations of 1-naphthol in acetonitrile. This result demonstrates the presence of the interaction between ClNpNO2 T1 and 1-naphthol, viz. the T1 and 1naphthol exciplex formed. There would be two extremes if the hydrogen abstraction took place stepwise: proton transfer precedes electron transfer; electron transfer occurs first. Here although the T1 of ClNpNO2 has an increased electron density localized on the nitro group relative to the ground state, it is hard to be protonated due to several reasons. First, Martins and co-workers obtained pKa = −0.66 for protonated triplet 1nitronaphthalene (3NNH+) by acquiring the inflection point of second-order rate constants using the reaction of 1-nitronaphthalene T1 with chloride ion.13 This value indicates 3 NNH+ is easy to be deprotonated. Second, according our experimental results, the T1 of ClNpNO2 can be quenched by H+ with rate constant of (4.0 ± 0.2) × 106 M−1 s−1 (Figure 2S in Supporting Information), obtained in acetonitrile/water = 9:1 (v:v) mixed solvent with various concentrations of HClO4 present, much slower than kNpOH = (2.9−8.4) × 109 M−1 s−1 for T1 quenched by 1-naphthol. It suggests that hydrogen abstraction from 1-naphthol by T1 cannot proceed by way of proton transfer followed by electron transfer, although 1naphthol is easy to be deprotonated. On the other hand, a complete electron transfer preceding proton transfer would generate ClNpNO2 radical ion (ClNpNO2•−), which has a λmax ≈ 385 nm ever observed in aqueous solution,16 not identified in the reaction of T1 and 1-naphthol. This may be due to a partial electron transfer or the proceeding proton transfer occurs much faster within ultrafast time region so that it is too fast to be measured by the instrument used here. In the strong hydrogen bonding donor solvent 2,2,2trifluoroethanol, the second rate constant was observed with a 2- to 3-fold decrease relative to those in acetonitrile and cyclohexane as shown in Figure 4. This result may be accounted for in that the presence of hydrogen-bonding interaction between 2,2,2-trifluoroethanol and hydroxylic oxygen of 1-naphthol makes long pairs of the phenolic oxygen less available and results in less efficiency of the electron transfer from 1-naphthol to T1. It suggests that the retardation effect from hydrogen bonding between solvent and substrate is much more significant than that from increasing solvent polarity. In the hydrogen abstraction reaction of T1 of ClNpNO2 with 1-naphthol, the hydrogen-bonding interaction plays an important role during electron transfer, and it was expected that the hydrogen-bonding complex could decrease the reduction potential of the H-withdrawing species and oxidation potential of the H-donating species to an extent that electron transfer becomes favorable.26 The reduction of T1 yield when increasing the concentration of 1-naphthol demonstrates efficient formation of the hydrogen-bonding complex in the solutions. Contrastively, in the solvents containing various concentration of alcohols, no decreases for the T1 yield were observed although in the presence of 2,2,2-trifluoroethanol, a very strong hydrogen-bonding donor. This is not surprising as the ππ* character of T1 makes the oxygen not be as electron-

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CONCLUSION The T1 of ClNpNO2 can undergo one-step H atom abstraction from alkyl hydrogen in alcoholic solvents, for instance, methanol and 2-propanol, with rate constants of about ∼104 M−1 s−1, and produce intermediates ClNpNO2H· with the absorption maximum at 368 nm in 2-propanol. DFT calculation predicted that both oxygens of the nitro group are active sites for hydrogen abstraction, and the difference of activation barriers for formation of two radical isomers is only 1.0 kcal/ mol. The hydrogen abstraction of T1 from 1-naphthol takes place through an electron transfer followed by a proton transfer through hydrogen-bonding interaction with rate constants of ∼109 M−1 s−1. In this process, hydrogen bonding is crucial because with absence or very weak hydrogen bonding, the electron transfer could not proceed, although the hydrogen donor can easily provide an electron. The ππ* character of T1 of most NPAHs makes the nitro oxygen not to be as electrondeficient as that in nπ* state, and hydroxy hydrogen being more acidic relative to the alcoholic hydrogen enables the hydrogen bonding to be stronger. The hydrogen abstraction by way of electron coupled proton transfers of the T1 of ClNpNO2 can be decelerated by increasing solvent polarity and hydrogenbonding donor ability. 1835

DOI: 10.1021/acs.jpca.7b11146 J. Phys. Chem. A 2018, 122, 1831−1837

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The Journal of Physical Chemistry A

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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.jpca.7b11146. Nanosecond transient absorption spectra recorded in tetrahydrofuran and plot of pseudo first order reaction rate of T1 monitored at 585 nm against concentrations of acid mentioned in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiadan Xue: 0000-0002-3863-5947 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from Zhejiang Provincial Natural Science Foundation of China (Grant LY16B030008), National Natural Science Foundation of China (Grants 21202032 and 21473163), the National Basic Research Program of China (Grant 2013CB834604), and Science Foundation of Zhejiang Sci-Tech University (Grant 1206841Y).

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REFERENCES

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