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Aug 7, 2017 - The host−guest symmetries, inclusion orientation, structural phase transition, and molecular dynamics of perovskite-like azido coordin...
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Matching of Host−Guest Symmetry/Orientation and Molecular Dynamics in Two Double Perovskite-Like Azido Coordination Polymers Ying Zeng,†,‡ Rui-Kang Huang,‡ Zi-Yi Du,*,† Chun-Ting He,*,‡ Wei-Xiong Zhang,*,‡ and Xiao-Ming Chen‡ †

Key Laboratory of Jiangxi University for Functional Materials Chemistry, College of Chemistry and Chemical Engineering, Gannan Normal University, Ganzhou 341000, China ‡ MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Two new double perovskite-like azido coordination polymers with trimethylammonium as the guest cation, namely, (Me3NH)2[CrNa(N3)6] and (Me3NH)2[CrK(N3)6], have been prepared. The molecular dynamics for both compounds are investigated and are clearly uncovered by the first-principles molecular dynamics simulation and the significant dielectric relaxation. Structural analyses of these compounds in combination with their analogue (Me3NH)[Mn(N3)3] reveal that the guest trimethylammonium has flexible structural adaptability, especially with a variety of disordered distributions, to match the different symmetries of varied metal-azido frameworks. Interestingly, the replacement of the divalent metal ion by mixed monovalent/ trivalent metal ions can change the symmetry, shape, and charge distribution of the host cage unit; thus it may influence and regulate the arrangement of the guest inclusion and its molecular dynamics as well as the structural phase transition.

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INTRODUCTION For a long time, inorganic perovskite oxides have played a central role in condensed-matter physics and solid-state chemistry, and they have been utilized in a diverse array of functional materials such as ferroelectrics, piezoelectrics, superconductors, sensors, memory devices, catalyst electrodes, and photovoltaics.1,2 In recent years, hybrid organic−inorganic coordination polymers (CPs) mimicking the inorganic ABO3type perovskite structure have also aroused great interest, from both the theoretical and application points of view.3−13 The perovskite-like CPs are usually assembled by the inclusion of a guest species into a well-matched host cage unit, with a general formula of ABX3 (A: monovalent noncoordinated cationic guest; B: bivalent metal ion possessing an octahedral coordination geometry; X: monovalent anion acting as a bridging ligand). Sometimes double perovskite-like CPs with a modified formula of A2(B′·B″)X6 can also be obtained by replacing the divalent metal ion with alternately arranged monovalent/trivalent metal ions.5a,e,6,7,11b Thus, the abundant variations of metal species, bridging ligands, and organic cation © 2017 American Chemical Society

components offer enormous chemical possibilities for creating perovskite-like CPs. The ideal inorganic perovskite structure adopts the cubic space group Pm3̅m. By lowering the symmetry from this aristotype architecture, many perovskite structures with distortions can be afforded. For hybrid perovskite-like CPs, the possible disordered distributions for their A or X components make the resultant crystal space groups more diversified, meaning they can even crystallize in a space group with higher symmetry than the prototype Pm3̅m, such as Fm3̅m.11b From an order−disorder perspective, the combination of a host cage unit and one guest cation within the perovskite-like CPs can be categorized into four basic types: (I) ordered host cage and ordered guest cation; (II) ordered host cage and disordered guest cation; (III) disordered host cage and ordered guest cation; (IV) disordered host cage and disordered guest cation. Among them, types I and II are the Received: June 9, 2017 Published: August 7, 2017 9946

DOI: 10.1021/acs.inorgchem.7b01478 Inorg. Chem. 2017, 56, 9946−9953

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Scheme 1. Symmetry Matching of the Host Cage Unit and the Guest Cation in the Three Phases of (Me3NH)[Mn(N3)3]a

a

The images are drawn based on the supplementary crystallographic data of reference 10a. H atoms have been omitted for clarity.

Table 1. Summary of Crystal Data and Structural Refinements for 2 and 3 at Two Different Temperatures

a

compound

2

2

3

3

T (K) empirical formula formula weight space group a (Ǻ ) b (Ǻ ) c (Ǻ ) V (Ǻ 3) Z Dcalcd (g cm−3) μ (mm−1) GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)a

123(2) C6H8N20Cr1Na1 447.41 R3̅c 9.5642(4) 9.5642(4) 36.058(2) 2856.5(2) 6 1.561 0.666 1.025 0.0278, 0.0676 0.0365, 0.0718

296(2) C6H8N20Cr1Na1 447.41 R3̅c 9.6318(2) 9.6318(2) 36.629(2) 2942.9(2) 6 1.515 0.646 1.085 0.0375, 0.0775 0.0746, 0.0937

123(2) C6H20N20Cr1K1 463.52 R3̅c 9.9541(6) 9.9541(6) 36.115(4) 3099.0(4) 6 1.490 0.794 1.028 0.0456, 0.1151 0.0584, 0.1204

296(2) C6H20N20Cr1K1 463.52 R3̅m 10.0280(2) 10.0280(2) 18.4561(3) 1607.31(3) 3 1.437 0.766 1.019 0.0312, 0.1000 0.0335, 0.1040

R1 = ∑||F0| - |Fc|/|∑|F0|, wR2 = {∑w[(F0)2 - (Fc)2]2/∑w[(F0)2]2}1/2

investigate the symmetry matching of this guest cation with other types of noncentrosymmetric host cages. Hence, as an expansion of our previous work, herein we report two new double perovskite-like azido CPs with trimethylammonium acting as the guest cation, namely, (Me3NH)2[CrNa(N3)6] (2) and (Me3NH)2[CrK(N3)6] (3). Their syntheses, crystal structures, host−guest symmetries, guest orientation, dielectric properties, and the structural phase transition of 3 are described, as well as the inherent guest molecular dynamics uncovered by the first-principles molecular dynamics (MD) simulation and the dielectric relaxation spectroscopy.

two most common ones, while type III is less observed. Notably, the perovskite-like CPs are apt to undergo structural phase transitions upon external stimuli (such as temperature or pressure), accompanied with a transform from type I to II/III and even to IV. In addition, it has been found that their structural phase transitions and the related physical properties primarily depend on the symmetry breaking of their fine crystal structures and the inherent molecular dynamics of these compounds. Therefore, understanding the symmetry matching between the host cage unit and guest cation within these perovskite-like CPs, as well as the molecular dynamics of the host−guest components are critical for the design and practical application of such compounds. Recently, we11 and other researchers9,10 have synthesized and studied a series of perovskite-like CPs on the basis of a rod-like azido ligand, mainly focusing on their structural phase transitions and the related properties. Although the topological structures of these perovskite-like azido compounds are the same, the crystallographic symmetries of their host cage units and the cationic guests can varied in a wide range, especially taking into account the possible disordered distribution for the guest cation and the N 3 − ligand. For example, the trimethylammonium cation of (Me3NH)[Mn(N3)3]10a (1) itself shows no symmetries in the α phase (Scheme 1). However, to match the 2-fold symmetry or an inversion center of the host cage unit in the β/γ phase, it adopts two types of disordered distribution over two/four sites, which is also related by 2-fold or 3̅ symmetry, respectively. Such a flexible structural adaptability for the guest cation prompts us to further

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EXPERIMENTAL SECTION

Materials and Instrumentation. All chemicals were obtained from commercial sources and used without further purification. Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer using KBr pellets from 4000 to 400 cm−1. Powder X-ray diffraction (PXRD) patterns (Cu Kα) were collected on a Bruker Advance D8 θ−2θ diffractometer. The heat capacity (Cp) measurement was carried out on the Physical Property Measurement System (PPMS). The complex permittivities were measured under a nitrogen atmosphere, using a Tonghui TH2828A LCR meter in a Mercury iTC cryogenic environment controller of Oxford Instrument, and the samples were ground and pressed into tablets under a pressure of ca. 4 MPa. Syntheses. NaN3 (3.0 mmol) was dissolved in an aqueous solution of (Me3NH)(NO3) (2.0 mmol, 4 mL), and then Cr(NO3)3·9H2O (0.3 mmol) was added into the above solution, and the resultant turbid liquid was filtered to afford a dark green solution, which was allowed to stand at room temperature. A few days later, green block-shaped crystals of 2 were deposited from the filtrates, in a ca. 65% yield based 9947

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Inorganic Chemistry on Cr. The PXRD on bulky crystals indicated that the experiment pattern matches well with the simulated one (Figure S1a, see Supporting Information). IR (KBr, cm−1): 3455(m), 3381(m), 3106(m), 2046(s), 1468(s), 1382(m), 1350(s), 1294(m), 1227(m), 1046(m), 974(m), 655(m), 606(m). KN3 (10.0 mmol) was dissolved in an aqueous solution of (Me3NH)(NO3) (8.0 mmol, 18 mL), and then Cr(NO3)3·9H2O (0.5 mmol) was added into the above solution, which was allowed to stand at room temperature. A few days later, green block-shaped crystals of 3 and colorless block-shaped unknown crystals were deposited from the filtrates. The green crystals were picked out manually, in a ca. 60% yield based on Cr. The PXRD on bulky crystals indicated that the experiment pattern matches well with the simulated one (Figure S1b). IR (KBr, cm−1): 3380(m), 3093(m), 3022(m), 2061(s), 1467(s), 1383(m), 1349(s), 1298(m), 1249(m), 1046(m), 975(m), 659(m), 607(m). Caution! Although our sample never exploded during handing, azide metal complexes are potentially explosive. Only a small amount of material should be prepared, and it should be handled with caution. Single-Crystal X-ray Crystallography. The single-crystal X-ray diffraction intensities for 2 and 3 were collected on a Rigaku XtaLAB P300DS single-crystal diffractometer at 123(2) K and on a Smart ApexII CCD diffractometer at 296(2) K. Both diffractometers are equipped with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were applied by using multiscan program REQAB or SADABS.14 The structures were solved by direct methods and refined using full-matrix least-squares technique with the SHELX program package.15 All hydrogen atoms were generated geometrically. All non-hydrogen atoms were refined with anisotropic thermal parameters, whereas all hydrogen atoms were refined isotropically. Crystallographic data and structural refinements for 2 and 3 at two different temperatures are summarized in Table 1, and the selected bond lengths are listed in Table S1. More details about the crystallographic data have been deposited as Supporting Information. MD Simulations. The first-principles MD simulations of 2 and 3 were performed with the Materials Studio 5.5 package,16 based on the spin-polarization density functional theory (DFT). To accelerate the DFT calculations, the reduced unit-cells for both compounds were adopted, and the lattice parameters were fixed during the simulations. Before the MD simulations, the geometries of all structures were optimized with the Dmol3 module. The widely used generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional and the double numerical plus d-functions (DND) basis set were used for the nonmetal atoms, while the effective core potentials (ECP) were employed for the metal atoms. Also, thermal smearing for the orbital occupancy was used to accelerate convergence. Then, the simulations were performed in a constant-volume and constant-temperature (NVT) ensemble with the temperature set at 373 K (in order to speed up the molecular movement and reduce the simulation time). The simple Nosé−Hoover thermostat and random initial velocities method were used. The total simulation time was 3 ps with a time step of 1.0 fs. The first half was used as equilibrium, and the following half was adopted for statistical analysis.

Figure 1. Cage unit and packing diagrams of 2. The disordered (Me3NH)+ cations are shaded transparently. The 3-fold axis is shaded in purple.

the ionic Na−N bond is somewhat weak in 2, with their bond lengths being 2.071(2) and 2.458(3) Å at 296 K, respectively. The symmetries of the host cage unit and the guest cation in 2 are different compared to all of the three phases of 1. For 2, the Cr3+ ion is located at an inversion center and a 3-fold axis, whereas the Na+ ion lies on one 3-fold and three 2-fold axes simultaneously. Alternatively, the N atom of the [Me3NH]+ guest lies on a 3-fold axis. The crystallographically independent N3− ligand situates at a general position. It is noted that the [Me3NH]+ guest is 2-fold disordered while the host cage unit is ordered, corresponding to type II mentioned above. In addition, by means of differential scanning calorimeter (DSC) and variable-temperature single-crystal X-ray diffractions (Table 1), it was found that 2 shows no structural phase transitions in a wide temperature range, which is different than most of the perovskite-like azido CPs reported previously.9−11 When the Na+ ion in 2 was replaced by K+ ion, compound 3 can be obtained. The variable-temperature single-crystal X-ray diffractions (Table 1) and Cp measurement (Figure S2) reveal that it underwent a structural phase transition at around 220 K. For convenience, we label the phase below/above the phase transition temperature as 3α/3β phase, respectively. The structure of 3α phase is isomorphous to 2, which crystallizes in the same space group R3̅c but with a larger unit-cell volume at 123 K (ca. 8.5%). Interestingly, it is worthy to point out that the polar trimethylammonium guest in 2 shows two opposite orientations, with its N−H bond running across the diagonal 3fold axis within the noncentrosymmetric cage unit (Figure 1), whereas that in 3α related by a similar 3-fold axis is unidirectional, with the N-bound H atom orienting toward the Cr3+ ion rather than the K+ ion (Figure 2). This phenomenon may be attributed to a much longer K−N bond length in comparison with that of the Cr−N bond (2.711(2) vs 2.044(1) Å at 123 K), which makes the N3− anions much closer to the Cr3+ ion than the K+ ion. As a result, the [Me3NH]+ cation in 3α tends to be close to the N3− anions within the anionic cage unit, owing to the electrostatic interaction between them. Thus, only one orientation for the [Me3NH]+ cation was observed in 3α. For 2, the difference of the Na−N and Cr−N bond lengths is not large enough; hence, within the tolerance range, two opposite orientations can be achieved in such a cage unit. After phase transition, the space group changes to a different trigonal space group R3̅m (Table 1). The unit cell parameter c in 3β is roughly halved in comparison with that of 3α. The

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RESULTS AND DISCUSSION Structural Analyses and Comparisons. Compound 2 crystallizes in the trigonal space group R3c̅ . Its crystal structure can be roughly described as a distorted double perovskite-like structure. Each Cr3+ or Na+ ion in 2 is surrounded by six N atoms from six N3− ions, all of which act as end-to-end bridging ligands between the heterometallic Cr3+ and Na+ ions, thus leading to a three-dimensional cage-like framework (Figure 1). The common structural feature of it is the anionic [Cr0.5Na0.5(N3)3]− cage unit enclosed by 12 Cr−N−N−N− Na fragments, which encapsulate a guest [Me3NH]+ cation. The Cr3+ and Na+ cations are arranged orderly and alternately over two types of metal sublattices, in a chess-board manner. It is noted that the covalent Cr−N bond is relatively strong while 9948

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are 2-fold or 4-fold disordered, which are related by 2-fold or 3̅ symmetry, respectively. As for the [Me3NH]+ cation in 2, although it also shows a 2-fold disorder, there is no symmetry between pairs of the disordered [Me3NH]+ cations. However, such a pair of disordered [Me3NH]+ cations are confined in two opposite orientations, and each is related by a C3 symmetry. First-Principles MD Simulations. As it is well-known, MD simulation is a powerful tool to probe into the microscopic molecular-motion process (e.g., vibration, translation, or rotation) of a crystalline system.17 To get a direct insight into the MD of 2 and 3β, we performed the first-principles MD simulations (for simulation details, see the Experimental Section). Derived from the MD simulation, the MD snapshots of 2 (Figures 4,S4) show that the N−H bond for the Figure 2. Cage unit and packing diagrams of 3α. The 3-fold axis is shaded in purple.

volume of a cage unit in 3β is slightly larger than that in 2 (267.9 vs 245.2 Å3 at room temperature). For 3β, both of the Cr3+ and K+ ions are located at an inversion center, one 3-fold axis, three 2-fold axes, and three mirrors simultaneously (Figure 3). Alternatively, the [Me3NH]+ cation remains unidirectional Figure 4. Overlapped conformations about the Cr(N3)6 and (Me3NH)+ fragments from seven snapshots derived from the MD simulation for 2 at 373 K (see Figure S4), showing their dynamic changes over the simulation time. The N-bound H atoms are shaded in pink while the C-bound H atoms have been omitted for clarity.

[Me3NH]+ cation oscillates dynamically with an apparent amplitude, while the metal-azido framework vibrates slightly in the vicinity of their equilibrium positions. In contrast, the MD snapshots of 3β (Figures 5 and S5) reveal that not only the N−

Figure 3. Cage unit and packing diagrams of 3β. The disordered N3− ligands are shaded transparently. The 3-fold axis and mirror symmetry are shaded in purple and cyan, respectively.

in the cage unit and is related by a C3v symmetry, which contains a 3-fold axis and three mirrors simultaneously. It is noted that the [Me3NH]+ guest is ordered, while the N3− ligand is 2-fold disordered, corresponding to type III mentioned above. At 296 K, the Cr−N bond length in 3β [2.036(3) Å] is slightly shorter than that in 2, while the K−N bond length in 3β is much longer [2.742(3) Å] than the Na−N bond length in 2. By and large, the replacement of the B-site divalent metal cation by mixed monovalent/trivalent metal ions can change the symmetry, the shape, and the charge distribution of the host cage unit, thus influencing and regulating the structural phase transition, the arrangement of the guest inclusion as well as its molecular dynamics (see the below-mentioned discussion). The guest trimethylammonium in these crystalline perovskitelike azido CPs can synergistically adjust its molecular or disorder-distribution symmetries to match the different symmetries of various host cages (Figure S3). The [Me3NH]+ cations in 1α and 3α/3β are orderly arranged, with the former showing no symmetry while the latter featuring a C3v symmetry. Contrastively, the [Me3NH]+ cations in 1β/γ

Figure 5. Overlapped conformations about the Cr(N3)6 and (Me3NH)+ fragments from seven snapshots derived from the MD simulation for 3 at 373 K (see Figure S5), showing their dynamic changes over the simulation time. For display details, see the caption for Figure 4.

H bond of its [Me3NH]+ cation has a wider oscillation than that of 2, but also its metal-azido framework features a more intense vibration than that of 2. The notable orientational changes of the N3− anion in 3β coincides well with the 2-fold disorder of the N3− anion shown in its crystal structure. In addition, the MD simulation of 2 proved that the [Me3NH]+ cation could not flip between its two disordered sites, thus the disorder of the [Me3NH]+ cation is static rather than dynamic. Dielectric Properties and the Intrinsic Mechanisms. The MD of the host−guest components in 2 and 3 can be reflected by the variable-temperature dielectric spectra, which is generally associated with the motion of a dipole unit (especially during a structural phase transition).18 As shown in Figure 6, the complex dielectric permittivity (ε = ε′ − iε″, where ε′ and 9949

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Figure 6. Temperature-dependent ε′ (a) and ε″ (b) at various ac frequencies for powder-pressed pellet samples of 2. The inset plot in (b) is shown in Arrhenius representation, providing the linear fitting of ln(ω) versus 1/Tpeak.

Figure 7. Temperature-dependent ε′ (a) and ε″ (b) at various ac frequencies for powder-pressed pellet samples of 3. The inset plot in (b) is shown in Arrhenius representation, providing the linear fitting of ln(ω) versus 1/Tpeak.

ε″ are the real and imaginary parts, respectively) of 2 was measured over a wide frequency range (from 0.5 to 100 kHz) as a function of temperature (from 120 to 350 K). The ε′ and ε″ here both show notable frequency dispersion, suggesting the dynamical motion of polar components in 2. The frequencydependent dielectric response can be explicated in terms of dielectric relaxation of a rotatable or swingable dipole unit.19,20 According to the MD simulation results, the oscillation motion of the N-bound H atom for the trimethylammonium cation in 2 can be viewed as the main source of dielectric response, as it leads to the related motion between the host anionic framework and the guest cations and hence, it significantly changes the instantaneous dipole moments of the crystalline system. The relaxation process of 2 can be clearly demonstrated by the frequency dependence of ε″, in which the peak maxima were found at the temperatures (Tpeak) of 301.7, 306.4, 308.6, 317.7, 324.9, 329.9, 332.4, and 342.8 K for f = 0.5, 0.8, 1, 2.5, 5, 8, 10, and 25 kHz, respectively. Apparently, the Tpeak value moves toward higher frequencies during heating. On the basis of the Debye-type relaxation process, the temperature-dependent ε″ can be expressed as ε″(T) = ωτ(T)/[1 + ω2τ(T)2], where ω is the angular frequency of the test field and τ(T) is the relaxation time, which is a function of the temperature obeying the Arrhenius law: 1/τ = ω0exp[−Ea/(kBT)], where ω0, Ea, and kB are the pre-exponential factor, activation energy, and Boltzmann constant, respectively. The ε″ value reaches a maximum when ωτ(T) = 1. Thus, the test frequency can be used to estimate the value of τ at Tpeak. Accordingly, an ω0 value of 5.0 × 1017 s−1 and an Ea of 0.85 eV were obtained from the linear fitting of the plots of 1/Tpeak versus ln(ω) (Figure 6b, inset). The variable-temperature dielectric spectrum was also measured for 3 to detect its MD (Figure 7). An analogous relaxation process in 3 can also be uncovered by the frequency dependence of ε″, in which the peak maxima were found at the Tpeak of 254.6, 257.7, 259.6, 267.8, 274.3, 278.9, 281.0, 290.5, 298.3, 303.3, and 306.1 K for f = 0.5, 0.8, 1, 2.5, 5, 8, 10, 25, 50, 80, and 100 kHz, respectively. Thus, an ω0 value of 1.1 × 1017

s−1 and an Ea of 0.68 eV can be evaluated. The Ea value of 3 is somewhat smaller than that of 2, which can be ascribed to a larger cage space for the [Me3NH]+ guest in 3 (Figure S6). As a result, the oscillation amplitude for the [Me3NH]+ guest in 3 is somewhat wider than that in 2, as indicated by the larger anisotropic displacement ellipsoids of the [Me3NH]+ cation in the crystal structure of 3β. In fact, such a wide oscillation amplitude for the [Me3NH]+ guest in 3β can even lead to a synergistic swing of the N3− bridges, as demonstrated by the 2fold disorder distribution of the N3− anion in its crystal structure. In addition, the τ ratio for 3β and 2 at 296 K was calculated to be 4.5, showing that the dipole unit in 2 can recover to its equilibrium state in a shorter period under the AC electric field. Such a phenomenon can be explained by the kinetic energy curve derived from the MD simulations.21 As shown in Figure 8, the total kinetic energies of 2 and 3 are almost the same, and thus the molecular motion rate in 2 is much faster owing to its relatively smaller molecular weight.

Figure 8. Total kinetic energy of 2 and 3 derived from the MD simulations at 373 K. The profile and running average represent the trajectory and statistical averaging data, respectively. 9950

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CONCLUSIONS In summary, two new heterometallic perovskite-like azido CPs with trimethylammonium as the cation guest, i.e., 2 and 3, have been synthesized. The molecular dynamics for both compounds are investigated, which are clearly uncovered by the first-principles molecular dynamics simulation and the significant dielectric relaxation. The oscillation motion of the Nbound H atom for the trimethylammonium cation in 2 and 3 can be viewed as the main source of dielectric response. Structural analyses of 1, 2, and 3 reveal that the trimethylammonium cation guest has flexible structural adaptability, especially with rich forms of disordered distribution, to match the different symmetries of varied metal-azido frameworks. As a typical case study, the structural phase transition and the host−guest symmetry mechanism of the less observed III-type in 3 as well as the orientation pattern of a polar guest in the noncentrosymmetric host cage units of 2 and 3 are interpreted.

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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.inorgchem.7b01478. Details of crystal data and characterizations of 2 and 3 (PDF) Accession Codes

CCDC 1554295−1554296 and 1562986−1562987 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-Y.D.). *E-mail: [email protected] (C.-T.H.). *E-mail: [email protected] (W.-X.Z.). ORCID

Zi-Yi Du: 0000-0002-2794-7670 Wei-Xiong Zhang: 0000-0003-0797-3465 Xiao-Ming Chen: 0000-0002-3353-7918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21361002, 21661004, 21290173, 91422302, and 21671202). C.-T.H. is thankful to the National Postdoctoral Program for Innovative Talents (BX201600195). Z.-Y.D. is also thankful to the NSF of Jiangxi (20171BAB203001).

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REFERENCES

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