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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Magnetic Metal−Organic Framework Exhibiting Quick and Selective Solvatochromic Behavior along with Reversible Crystal-toAmorphous-to-Crystal Transformation Peng Hu,†,⊥ Lei Yin,‡,⊥ Angelo Kirchon,§,⊥ Jiangli Li,† Bao Li,*,† Zhenxing Wang,*,‡ Zhongwen Ouyang,‡ Tianle Zhang,† and Hong-cai Zhou*,§ †

Key laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ Wuhan National High Magnetic Field Centre & School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China § Department of Chemistry, Department of Materials Science and Engineering, Texas A&M Energy Institute, Texas A&M University, College Station, Texas 77843-3255, United States S Supporting Information *

ABSTRACT: By utilizing a flexible tetrapyridinate ligand, tetrakis(4-pyridyloxymethylene) methane(L), a novel multifunctional soft porous framework, [Co(NCS)2(L)]·2H2O·CH3OH (1), was constructed. This framework exhibits quick and selective solvatochromic and vapochromic behavior during a reversible crystal-toamorphous-to-crystal (CAC) transformation. Importantly, the rapid CAC transition can selectively be triggered by methanol molecules, even at low concentrations of liquid or gaseous methanol. This reproducible transition can be monitored by single-crystal and power X-ray analysis, IR, and UV−vis, which all powerfully illustrate the selectivity and sensitivity of this system. In addition, the typical magnetic behavior of single ion magnets (SIMs) has been successfully introduced into this 3D framework, and the modified dynamic relaxations have been investigated via experimental and theoretical analysis. The consistent observations of both experimental and theoretical results support that the distortions of the metal coordination environments should be responsible for the finely tuned SIM behavior.

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INTRODUCTION Metal−organic frameworks (MOFs), as one class of highly porous materials, are ever-increasing because of their structural tunability and diversity and wide range of potential applications.1,2 The unique properties of MOFs have been utilized in versatile applications, including gas storage and separations, heterogeneous catalysis, luminescence, and so on.3−5 Nowadays, it can be stated that there are overwhelming demands for sensor based materials that display high sensitivity and selectivity in a broad area of applicable fields. These fields are closely related with economic developments and human life which include but are not limited to the management of industrial processes, medical diagnostics, the detection of a chemical threat, food quality control, and environmental monitoring.6 Taking into account the excellent structure− activity relationship within MOFs, a small number of researchers have begun to explore the potential of MOFs as chemical sensors.7,8 Up to now, most of the reported MOF based sensors have been formulated based on luminescence sensing due to the fact that the organic linkers within MOFs contain aromatic subunits which could be readily luminescent when responding to the external excitation.9,10 Compared to luminescent MOF sensors, solvochromic or vaporchromic © XXXX American Chemical Society

MOFs have received much less attention even though they can display some of the simplest and most powerful sensing routines.11 The color of these novel solvochromic or vaporchromic MOF based sensors can be visibly changed in accordance with the sensing signal in order to identify specific molecules and more importantly can be observed directly by the naked eye. The common mechanisms of solvochromic effects in these chemosensors are related with solvent polarity, the interaction between specific solvent-chromophore, and/or the variance of the coordination environments of the metal centers within the MOF.11,12 Although yet to be systematically investigated, the special structural−activity relationships of MOFs could furnish a great advantage over other classes of chemosensory materials. In the future, more and more MOFs with rapid sensitivity and selectivity to specific guest molecules could be constructed and investigated in order to uncover the mystery of the veil. In the field of materials science, how to construct phase change materials (PCMs) has become an emerging and hot topic in recent years because of their potentials as molecular Received: March 15, 2018

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DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

method that two layers consist of Co(NCS)2 and L at room temperature. Single-crystal X-ray analyses (SCXRD) had been carried out at 173 K and show that 1 consisted of cobalt metal ion, isothiocyanate, and the tetrapyridyl ligand. The incorporation of the SCN groups could also be validated via the strong vibrational peak near 2050 cm−1 in the IR spectra (Figure S1 in the Supporting Information). The pink colored crystal samples are very unstable after leaving from the mother liquid and rapidly become blue (Figure 1). Powder X-ray diffraction

appliances, for example as data storage devices, sensors, and switches.13 Due to the existence of coordination bonds and flexible backbones, MOFs can also exhibit the phenomenon of phase transitions. Phase transitions within MOFs can be reversibly achieved via the external stimuli, and the resulting materials have been coined as third-generation soft porous materials.14 The transformation of single-crystal-to-singlecrystal (SCSC) within MOFs is a great example of these phase transitions, and SCSC transformations as well as several others can be utilized to synthesize new functional materials.15 In some rare cases, the crystallinity of MOFs can be destroyed on the loss of solvent molecules, resulting in an amorphous state; upon immersing into solvent again, the crystallinity could be recovered. This recovery indicates the presentation of crystal-to-amorphous-to-crystal (CAC) transitions.16,17 Strictly speaking, the amorphous state should be termed as a form with imperfect crystallinity, which lost the perfect crystallinity typical of MOFs. But without the polymer network completely collapsing, the original crystallinity can be recovered with the adsorption of guest molecules again.17 Compared to the transformation of SCSC, the CAC transformation is more attractive since it usually causes a rearrangement of the coordination sphere around the metallic centers and this rearrangement could possibly further affect the electronic structure, magnetic properties, or other physical functions (color, ferroelectric properties, or second-order nonlinear optical) of MOFs. Although several examples related with SCSC or CAC systems have been presented, most transition processes are still difficult to induce unless under extreme stimuli such as high temperature or specific pressure.18 At the intersection of chemosensors and phase transition materials, solvatochromic MOFs exhibit the process of CAC transformation and the responses to external stimuli desired to satisfy the needs of every aspect. With all the above considerations in mind, we selected cobalt (Co) ions, a flexible tetrapyridyl linker (tetrakis(4pyridyloxymethylene)methane (L)), and thiocyanate in order to build a flexible MOF system. Cobalt ions have a variety of coordination modes with corresponding color changes, as well as single cobalt complexes can display the performance of single ion magnets (SIMs).19 The flexible tetrapyridyl ligand exhibits a semirigid skeleton, which is able to reflect a respiratory effect during the loss or gain of guest molecules making it an ideal candidate to construct a MOF with a CAC transition. The introduction of thiocyanate ions is based on two considerations: First, it functions to guarantee the CoN6 coordination sphere, which is ideal for the typical magnetic behavior of single ion magnets (SIMs); Second, it functions to increase the interaction sites on the surface of MOF channels that are needed to maximize the interaction of the framework with guest molecules. In accordance with our assumptions, a flexible 2-fold interpenetrating MOF, [Co(NCS) 2 (L)]·2(H 2 O)· CH3OH (1), had been constructed, which exhibits quick and selective solvatochromic and vapochromic behavior during the reversible CAC transformation and this transformation can only be triggered by the existence of methanol. Herein, the detailed crystal structures, the interpenetration of the transition process, and the related properties observed during the transformation have been presented.

Figure 1. (a−d) View of the continuous color changes along (a) assynthesized sample, (b) first without methanol, (c) second with methanol, (d) second without methanol molecules; (e) Comparative view of the color change with methanol and without methanol (methanol is mixed with ethanol in a volume ratio of 1:20).

(PXRD) investigation revealed the blue state is amorphous as shown in Figure 2, and the presence of some weak diffraction peaks can be attributed to the solvent of the bottom sample on the glass plate being difficult to escape quickly during testing. Furthermore, the color and crystallinity would be recovered after immersing blue sample in a methanol solution or methanol rich atmosphere. PXRD investigations reveal the existence of minor changes in the recovered crystal structures, compared to the original MOF. The crystal data and structural parameters of two crystalline state are listed in Table S1.

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RESULTS AND DISCUSSION Synthesis of 1 and Noncrystalline State. The pink crystalline samples of 1 had been constructed via a diffusion B

DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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transformation with the purpose of detecting methanol molecules with great sensitivity and selectively. To explore the possibility of vapochromic phenomenon, the process of CAC transformation was monitored by UV−vis spectra under a methanol enriched atmosphere as shown in Figure 3. For the crystalline sample, the maximum absorption

Figure 2. PXRD patterns of solvated and desolvated samples.

Reversible Transition between Crystalline and Noncrystalline States. When the resulting crystals of 1 were separated from the mother liquid, a rapid color change from pink to blue was observed using an electron microscope. After about 5 min, the pink samples completely turned blue. It was also observed that the shapes of the original and color-changed crystal samples were consistent and showed no evidence of any change or degradation. The mechanism of this color change could be ascribed to two reasons, one is that the release of methanol molecules may change the ligand field strength, and another is the distortion of the coordination sphere of cobalt ions caused by the structural transformation. The CAC transformation of 1 was also validated by the frequency of the asymmetric CN stretch of the pseudohalide ligands in IR spectra.20 Observed from the characteristic peaks of thiocyanate in two different states, the coordination field of the thiocyanate ions in the crystalline state is significantly stronger than in the amorphous state. The IR spectra of the crystalline sample exhibits a band at 2081 cm−1 which corresponds to the NCS group coordinated to Co(II) via the nitrogen atom, while the desolvated amorphous sample displays a band at 2065 cm−1. The band at higher energies can be explained by a stronger δ bonding interaction between nitrogen atoms of NCS and the cobalt center as well as by examining the electron withdrawing ability of the cobalt ions. The crystalline sample displays a stronger withdrawing affect than when it is in the amorphous state. This helps explain the larger wavenumber of the NCS group in the crystalline state.21 Furthermore, the quick and reversible color change from blue to pink was also observed when immersing the blue amorphous material in a methanol solution. IR spectra also reveal the recovery of the coordination sphere of cobalt ion in the crystalline state (Figure S1 in the Supporting Information). This recovery of the crystalline state signals a rapid and reversible crystal− amorphous−crystal transformation that is triggered by the loss or gain of methanol molecules. In order to examine the selectivity of the observed solvatochromic phenomenon, the blue amorphous sample was immersed in different solvent systems such as ethanol, isopropanol, acetone, DMF, etc.; however, no similar solvochromic phenomenon was observed, indicating the solvatochromic CAC transition is selective and specific to methanol molecules. In addition, as shown in Figure 1(e), the quick and reversible transformation could also be presented in the mixed solution of methanol and ethanol with volume ratio of 1:20, indicating the lower limitation of detection of methanol is very sensitive. Therefore, as-synthesized cobalt-based MOF exhibits a very quick and reversible solvochromic CAC

Figure 3. Color changes and UV spectra of fresh sample, sample with or without methanol atmosphere, and sample in the mixture atmosphere of methanol and ethanol (V/V, 1/20).

peak is around 490 nm, which is consistent with the presented pink color based on the color opponent process. After 5 min, a color change from pink to blue could be observed due to the loss of guest molecules. A broad peak from 550 to 650 nm with the maximum absorption peak at around 625 nm is presented in the spectra, which is in accordance with the blue color for amorphous sample. Continuously, the blue sample was set in the atmosphere of methanol, and a color change from blue to purple has been exhibited. Observed from UV spectra, two broad peaks with the highest absorption peaks at around 490 and 625 nm manifest the incomplete transformation from amorphous to crystal state. However, the transition from crystal to amorphous state is complete and verified by the UV spectra which display the characteristic broad peak from 550 to 650 nm. Finally, the amorphous sample was placed in the atmosphere of mixed solution containing methanol/ethanol with volume ratio of 1/20. The incomplete transition was also observed and was similar to the color in the pure methanol atmosphere. The maximum peaks in UV spectra qualitatively reflect the extent of crystal field splitting energy. For the sample with methanol molecules, larger splitting magnitude between t2g and eg orbitals existed due to the stronger interaction between cobalt ions and thiocyanato ions, in accordance with the results analyzed from IR spectra.22 All in all, in solution or atmosphere, the quick and reversible CAC transformation visualized by a color change, also called solvatochromic and vapochromic phenomenons, was triggered only by methanol molecules. It also exhibits specific selectivity and sensitivity, which could satisfy the necessary demands for novel chemosensor materials as described in the Introduction. C

DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Crystal Structure of 1 and 1 after Adsorbing Methanol. Similar crystal structures of fresh and revertible samples have been presented via the studies of SCXRD. There are hydrogen bonding interactions between the guest methanol molecules and thiocyanato groups coordinated on the central cobalt ions. Compound 1 crystallizes in the tetragonal crystal system I41/a, and the asymmetric unit consists of two SCN groups, two half cobalt ions, and one complete tetra-pyridinate ligand (Figure 4). The central cobalt center is coordinated by

results in a 2-fold interpenetrating structure (Figure 5). Evaluated by the program of PLATON, the volume of solvent

Figure 5. Partial view of packing mode of 2-fold interpenetrating structure of 1 with the view of the positions of methanol molecules in the final structure.

accessible should be 28.3%. The pores are filled with methanol molecules via hydrogen bonding interactions with thiocyanato ions (the distance and angle of H−O···S are 3.276(5) Å and 161.7(2)°). The larger than traditional hydrogen bonding distance can help explain the easy loss of methanol molecules from the fresh sample of 1. The framework after the loss of methanol molecules would shrink due to its naturally flexible skeleton which originated from the semirigid tetra-pyridinate ligand. This flexibility causes a lack of any long-range periodic order, validated by the broad “humps” in X-ray diffraction patterns. After readsorption of methanol molecules, the initial crystalline state could be recovered. A similar crystal structure of recovered sample was obtained and compared to 1. For example, the Co−Npyridyl and Co−NNCS bond lengths are 2.122(1) and 2.168(2) Å, respectively; the joints between cobalt ions and NCS− ions are bent with the C−N−Co angles ranging from 141.5(2) to 155.6(1)°; Σ values are 16.73 and 17.84°; the Npy···Ccentral···Npy angles in L range from 100.77(2)° to 114.55(2)°. Methanol molecules are housed in the irregular pores via weak hydrogen bonding interactions. The porous conformation and the specific interaction sites of the framework make it an easy way for the loss and gain of methanol molecules, which promotes the unexpected selectivity and sensitivity during the process of CAC transformation. Magnetic Properties and HF-EPR of Crystalline and Noncrystalline State. Variable temperature direct-current (dc) magnetic susceptibility measurements for a suspension of 1 in methanol as well as the dried amorphous state symbolized by a blue powder were collected under 1000 Oe at a temperature range of 1.8−300 K. The plot of χMT vs T is depicted in Figure 6. The values of χMT at 300 K are 2.43 and 2.62 cm3 K mol−1 for crystal and amorphous samples, respectively, which exceed the theoretical value of 1.875 cm3 K mol−1 for single Co(II) ions. This effect is caused by the existence of a strong spin−orbit coupling for the Co(II) ions. Along with the temperature cooling, χMT values show the obvious decrease below 100 and 150 K for both samples and reach the minimum values of 1.38 cm3 K mol−1 and 1.37 cm3 K mol−1 at 1.8 K due to the zero-field splitting and/or magnetic

Figure 4. (a) Perspective view of the asymmetric unit of 1 with methanol molecules; (b) and (c) view of the octahedral environment of two cobalt ions.

6N atoms belonging to four different pyridyl rings and two thiocyanato groups. The bond lengths of Co−Npyridine and Co− NNCS are 2.121(1) and 2.140(2) Å,23 respectively, which are both similar to the typical values of a high-spin Co2+ state. The joint between cobalt ions and NCS− ions is bent with the C− N−Co angles ranging from 144.5(2) to 153.6(1)°, which results in two asymmetric metal centers. Since centrosymmetry exists, the angles of N−Co−N in the octahedron are all near 90 and 180°, indicating a regular octahedron of cobalt centers (Figure 4b,c). For Co(1) and Co(2) atoms, the axial sites are taken up via two NCS− groups, and the four equatorial sites are occupied by four pyridyl groups. The octahedral distortion parameters (sum of the deviations of cis N−Fe−N angles from 90°), Σ values,24 are 17.1 and 21.22° for Co(1) and Co(2) ions, indicating the more regular CoN6 octahedron. Each cobalt(II) center links four tetrapyridinate ligands, and each L binds four cobalt(II) ions. The angles of N···Ccentral···N in the flexible ligand range from 101.18(1)° to 114.85(3)°, while the torsion angles of Ccentral−CH2−O−Cpyridyl fall in the range from 154.04(2) to 175.15(2)°, illustrating that the ligand diverges seriously from tetrahedral geometry and presents a unique relatively contracted conformation. As such, the ligands serve as quaternary bridges to connect four cobalt(II) centers to form 3D PtS porous structure with the point Schläfli symbol {42·84} calculated with TOPOS. Large pores had been impeded by mutual interpenetration of identical 3D frameworks, which D

DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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the two different states are close to the values in other Co(II) single-ion magnets (SIMs) (Table 1) .26 The signs of axial ZFS parameter D for both samples are comparable and negative, confirming the existence of easy-axis magnetic anisotropy in the six-coordinated octahedral environment of the cobalt ion. In addition, similar g factors of both samples were observed. However, the obvious difference of rhombic ZFC parameters for the two samples was observed (larger E value of crystalline sample was obtained compared to the value of noncrystalline one), which could be ascribed to the presence and absence of guest molecules. The ordered crystal structure tended to shrink along with the loss of guest molecules due to the original flexibility of the framework and causes the aperiodic arrangement of cobalt ions, which must be responsible for reducing the magnetic anisotropy of the amorphous sample, giving the smaller E value compared to the crystallographically ordered arrangement of crystalline sample. Furthermore, in order to validate the existence of a strong magnetic anistropy and the sign of the D values for the two different samples, high frequency/field electron paramagnetic resonance (HF-EPR) was measured on the suspensions in methanol of crystalline and amorphous samples at frequencies up to 260 GHz at 4.2 K. As the magnitude of the |D| values is out of the frequency limitation, only one signal peak was observed in the HF-EPR spectra of both samples (as shown in Figure 7). This fact can be attributed to a system (spin = 3/2) with large and negative D values.27 All signals result from the intra-Kramers transitions within the highest doublet mS = ±3/2 with ΔmS = ±3. Unambiguously, the observed sign of D values in HF-EPR experiments satisfies the results obtained from theoretical fitting for the variable temperature magnetic behavior, which further validates the phenomenon of strong magnetic anisotropy in both samples. The different simulated g factors must be ascribed to the ordered or disordered arrangement of coordination frameworks. The large calculated and observed magnetic anisotropy values for the octahedral cobalt(II) ions in the two different states indicate slow magnetic relaxation of SIMs, prompting us to detect the magnetic relaxation dynamics of the two states. First, the temperature/frequency dependence of the alternating current (ac) magnetic susceptibility measurements has been collected at different frequencies (1−999 Hz) in a 2.5 Oe ac field and different dc fields. Under a zero dc field, no slow magnetic relaxation behavior was presented (Figure S3 in the Supporting Information), suggesting the occurrence of the fast quantum tunnelling of magnetization (QTM) through the spinreversal barrier.28 When increasing the external dc field, the effect of QTM in the samples could be effectively prohibited, and the apparent phenomenon of slow magnetic relaxation is presented (Figure S4 in the Supporting Information). However, the tendency of temperature dependence of the ac susceptibility under high dc fields was not clear, indicating that the relaxation process could be suppressed by higher applied field. Then the measurement of temperature-dependent ac susceptibilities had been collected for both samples under the optimum 800 Oe dc field. The two types of the in-of-phase (χ′) and out-of-phase (χ″) ac signals clearly manifest the presence of a slow relaxation of magnetization (Figure S5 and S6 in the Supporting Information). The plots of Cole−Cole had been obtained from the ac magnetic susceptibilities for both samples (Figure 8) and fitted according to the generalized Debye model to inspect the distribution of the relaxation time.29 The fitting parameters

Figure 6. Variable-temperature dc susceptibility data of the suspensions in methanol of crystalline and amorphous samples. Solid lines indicate the best fits with the PHI program.

anisotropy. The sample of reabsorbed methanol molecules exhibits similar magnetic behavior when compared to the fresh sample. The data of low-temperature magnetization data for these two samples was also recorded in the temperature range of 2−5 K and applied magnetic field from 2T to 7 T. At 2 K, the magnetization reaches 1.79 and 1.76 NμB at 7 T (Figure S2 in the Supporting Information), respectively, which is lower than 3 NμB for a Co(II) ion with S = 3/2. This nonsaturation behavior also reveals the possibility of appreciable magnetic anisotropy in these materials. With the consideration of the coordination environment of cobalt ions in the crystalline and noncrystalline states, the magnetic behavior in these two samples should be ascribed to the magnetic anisotropy of cobalt ion since the Co−Co interaction could be neglected. To gain insight into the magnetic anisotropy, both the experimental χMT versus T and M versus T curves had been fitted simultaneously via the PHI program25 by means of an anisotropic spin Hamiltonian (with gx = gy): ⎛ 2 S(S + 1) ⎞ 2 2 Ĥ = μB gS ·̂ Ĥ + D⎜Sẑ − ⎟ + E(Sx̂ − Sŷ ) ⎝ ⎠ 3

where μB, D, E, S, and H are the Bohr magnetron, the axial and rhombic ZFC parameters, the spin operator, and magnetic field vectors, respectively. After a variety of attempts during the fitting process, it was found that there was no good fit for this system with a positive D value but a good fit was obtained if D is negative. Finally, reasonable parameters for fitting magnetic data were obtained as shown in Table 1 and Figure 6 (fit line). The fitting lines match the experimental data well within the whole temperature region, and the large |D| values observed for Table 1. Fitting Results of the Susceptibility and Magnetization Data Using the PHI Program for Crystalline (a) and Noncrystalline Samples (b)

a b

gx = gy

gz

D(cm−1)

|E| (cm−1)

2.511 2.364

2.181 2.231

−86.911 −87.943

15.422 9.361 E

DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. HF-EPR spectra of crystalline (left) and amorphous sample (right) under variable frequencies at 4.2 K.

Figure 8. Cole−Cole polts of crystalline (a) and amorphous (b) sample.

have been gathered in Table S3 and S4. The parameter α (the deviation from the pure Debye model) for two samples varies in the range of 0.01−0.2 and 0.05−0.26, respectively, demonstrating a narrow distribution of relaxation times. Fitting of the relaxation time at a high temperature according to τ = τ0 exp(Ueff/kBT) gives an effective barrier Ueff equal to 7.05 K with a pre-exponential factor (τ0) of 1.09 × 10−5 s for crystalline sample, and 11.17 K with a pre-exponential factor (τ0) of 1.41 × 10−6 s for the amorphous sample, which are both in accordance with the empirical τ0 value 10−5−10−11 for single molecular magnets.30 The transitions between the crystalline and amorphous states are said to cause the distortions in the octahedral environments for the cobalt ions, which must be responsible for modifying the dynamic magnetic parameters.

transition was monitored by SCXRD and PXRD analysis and IR and UV−vis measurements, all powerfully illustrating the selectivity and sensitivity of this system. In addition, the typical magnetic behavior of SIMs has been successfully introduced into a novel 3D framework, and the modified dynamic relaxation was investigated via experimental and theoretical analysis. The consistent results of versatile experimental (spectra and magnetic measurements) and theoretical results support unambiguously that the distortions in the coordination environments of metal centers should be responsible for the fine-tuned SIM behavior. The reported novel framework displays remarkable versatility which makes this a promising chemosensory material, which can be an important contribution to the field of soft porous crystals.

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CONCLUSIONS In summary, we have reported a new multifunctional MOF displaying crystal-to-amorphous-to-crystal transformation along with a quick color change as well as fine-tuning of the dynamic magnetic relaxation exhibited by this MOF. Importantly, the quick CAC transformation can only be triggered by the release or absorption of methanol molecules, even at low concentrations of liquid or atmospheric gas. The reproducible

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00703. Additional experimental sections, characterization and physical measurements (PDF) F

DOI: 10.1021/acs.inorgchem.8b00703 Inorg. Chem. XXXX, XXX, XXX−XXX

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CCDC 1815174−1815175 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], 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]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bao Li: 0000-0003-1154-6423 Zhenxing Wang: 0000-0003-2199-4684 Hong-cai Zhou: 0000-0002-9029-3788 Author Contributions ⊥

P.H., L.Y., and K.A. contributed equally.

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 (21471062) and the Open Project of the State Key Laboratory of Physical Chemistry of the Solid Surface (Xiamen University) (201616). We gratefully acknowledge the Analytical and Testing Center, Huazhong University of Science and Technology, for analysis and spectral measurements. We also thank the staffs from BL17B beamline of National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility, for assistance during data collection.

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