Kinetic and Thermodynamic Control of Structure Transformations in a

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Kinetic and Thermodynamic Control of Structure Transformations in a Family of Cobalt(II)-Organic Frameworks Qiang Chen, Rui Feng, Jian Xu, Yan-Yuan Jia, Ting-Ting Wang, Ze Chang, and Xian-He Bu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12925 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

Kinetic and Thermodynamic Control of Structure Transformations in a Family of Cobalt(II)-Organic Frameworks Qiang Chen,†,‡,§ Rui Feng,†,§ Jian Xu,†,§ Yan-Yuan Jia,‡,§ Ting-Ting Wang,†,§Ze Chang,*,†,§and Xian-He Bu*,†,‡,§ †

School of Materials Science and Engineering, National Institute for Advanced Materials, and

TKL of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China ‡

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai

University, Tianjin 300071, China §

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai

University, Tianjin 300071, China KEYWORDS: dynamic materials, metal-organic frameworks, structure transformation, kinetic control, thermodynamic control, sensing

ACS Paragon Plus Environment

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ABSTRACT

Dynamic metal-organic frameworks (MOFs) that respond to external stimuli have recently attracted great attention. However, the subtle control of dynamical processes as well as the illustration of the underlying mechanism, which is crucial for the targeted construction and modulation propose, is extremely challenging. Herein, we report the achievement of simultaneous kinetic and thermodynamic modulation of the structure transformation processes of a family of cobalt(II)-organic frameworks, through the rational combination of co-ligand replacement, solvent molecule substitution and ligand-based solid solution strategies. Based on the systematic investigation of the structural transformation behaviors, the underlying response mechanism and principles for modulation were illustrated. It is expected that this work can provide valuable hints for the study and further development of dynamic materials.


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INTRODUCTION Dynamic materials1-4 that exhibit stumili-responsive behaviors are quite desired in switches,5-7 sensing,8-11 drug delivery,12-14 and other related applications,15-17 in which the distinct physical properties under external stimuli can be used as detecting signals. Thus, the precise tuning of the dynamical behavior of a material is very important but extremely challenging. Among various candidate dynamic materials, metal-organic frameworks18-21 (MOFs) have attracted much attention for their structural diversity and extended chemical functionalization.22-25 By taking the advantage of their tailorable and crystalline nature,26 which largely facilitates the characterization of their structures in terms of X-ray crystallography, the structure-related transformation mechanism of dynamic MOF materials can be readily illustrated. More importantly, once the convertion mechanism was well understood, their dynamical behavior can be tuned accurately and flexibly by varying their constituent components. However, until now, the progress in this regard is very limited. This might be due to the multifarious factors influencing the transformation processes of dynamic MOFs, which makes the systematic tuning very difficult to be achieved. Herein, we report the achievement of subtle kinetic and thermodynamic control of the dynamic structure transformation behavior of a family of cobalt(II)-organic frameworks. The single-crystal to single-crystal (SC-SC) transformations of the MOFs are sensitive to the coordination of water molecules and accompanied by the removal/recovery of co-ligands and obvious colors change. Both the responsive behavior and the readily distinguishable state of this material make it a good candidate for sensing applications. Furthermore, the SC-SC feature of the transformation greatly benefits the understanding/study of the dynamic process. By employing the combined strategies of co-ligands replacement,27-29 solvent molecule


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substitution30-31 and ligand/solvent-based solid solution32-33 (Figure 1), the transformation behaviors of the materials could be effectively modulated in the thermodynamic and kinetic aspects within a certain range. To our knowledge, this is the first example of the simultaneously subtle kinetic34-36 (the rates of conversion) and thermodynamic37-39 (the thresholds of conversion) modulation of the structure transformation of dynamic MOFs materials.

Figure 1. Schematic diagram of the dynamic frameworks of MOFs and the strategies for controlling their structure transformation behavior. EXPERIMENTAL SECTION The main ligand 1,3,5-tris(p-imidazolylphenyl)benzene (tipb) was prepared according to literature procedure.40 The abbreviation of the carboxylate ligands and the corresponding samples are shown in Figure 2. Synthesis of [Co1.5(tipb)(SO4)(amtpa)0.5]·(DMF)n (BAM⊃ ⊃DMF): A mixture of tipb (0.1 mmol), 2-aminoterephthalic acid (H2amtpa, 0.1 mmol), CoSO4·7H2O (0.15 mmol) and two drops of pyridine in 5 mL DMF was sealed in a 16 mL Teflon lined stainless steel container and


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heated at 160 °C for 3 days. The container was then cooled to room temperature at a rate of 3 °C/h. The blue crystalline product obtained was separated by filtration and washed with DMF. Yield: 75% (based on tipb). Synthesis of BP⊃ ⊃DMF, BDH⊃ ⊃DMF, BNA⊃ ⊃DMF, BAN⊃ ⊃DMF: The synthesis procedure is similar to that of BAM⊃DMF except that co-ligand H2amtpa was replaced by terephthalic acid (H2pta), 2,5-dihydroxyterephthalic acid (H2dhtpa), 1,4-naphthalenedicarboxylic acid (H2nada) and 9,10-anthracenedicarboxylic acid (H2anda). Synthesis of ligand-based solid solution [Co1.5(tipb)(SO4)(pta)0.375(nada)0.125]·(DMF)n (BPNA31⊃ ⊃DMF): A mixture of tipb (0.1 mmol), H2pta (0.075 mmol), H2nada (0.025 mmol), CoSO4·7H2O (0.15 mmol) and two drops of pyridine in 5 mL DMF was sealed in a 16 mL Teflon lined stainless steel container and heated at 160 °C for 3 days. The container was then cooled to room temperature at a rate of 3 °C/h. The blue crystalline product obtained was separated by filtration and washed with DMF. Yield: 75% (based on tipb). Synthesis of ligand-based solid solutions of BPNA11⊃ ⊃DMF, BPNA13⊃ ⊃DMF, BPAN41⊃ ⊃DMF, BPAN32⊃ ⊃DMF, BPAN11⊃ ⊃DMF, BPAN23⊃ ⊃DMF, and BPAN14⊃ ⊃DMF: The synthesis procedure is similar to that of BPNA31⊃DMF except that H2pta (0.075 mmol), H2nada (0.025 mmol) were replaced by mixed co-ligands (BPNA31⊃DMF: H2pta 0.075 mmol and H2nada 0.025 mmol; BPNA11⊃DMF: H2pta 0.05 mmol and H2nada 0.05 mmol; BPNA13⊃DMF: H2pta 0.025 mmol and H2nada 0.075 mmol; BPAN41⊃DMF: H2pta 0.08 mmol and H2anda 0.02 mmol; BPAN32⊃DMF: H2pta 0.06 mmol and H2anda 0.04 mmol; BPAN11⊃DMF: H2pta 0.05 mmol and H2anda 0.05 mmol; BPAN23⊃DMF: H2pta 0.04 mmol and H2anda 0.06 mmol; BPAN14⊃DMF: H2pta 0.02 mmol and H2anda 0.08 mmol).


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Synthesis of BP⊃ ⊃CH3OH, BP⊃ ⊃EtOH, BP⊃ ⊃DMF, BP⊃ ⊃DMAC, and BP⊃ ⊃NMP: A mixture of tipb (0.1 mmol), H2pta (0.1 mmol), CoSO4·7H2O (0.15 mmol) and two drops of pyridine in 5 mL solvent (CH3OH, EtOH, DMF, DMAC, or NMP) was sealed in a 16 mL Teflon lined stainless steel container and heated at 160 °C for 3 days. The container was then cooled to room temperature at a rate of 3 °C/h. The blue crystalline product obtained was separated by filtration and washed with corresponding solvent used.

Figure 2. The main ligand tipb and dicarboxylic co-ligands used for the construction of isoreticular MOFs. The Characterizations of the Ligand-Based Solid Solutions: To determine the ratio of the co-ligands in the ligand-based solid solutions, the crystals of BPNA31, BPNA11, BPNA13, BPAN41, BPAN32, BPAN11, BPAN23, and BPAN14 were destroyed by diluted hydrochloric acid and the co-ligands (dicarboxylate ligands) were separated out for NMR tests. The results of NMR tests reveal that the ratios of nada2- and pta2- ligands in BPNA31, BPNA11, and BPNA13 are about 2.91:1, 0.92:1 and 0.32:1 (Figure S34). The ratios of anda2- and pta2- ligands in


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BPAN41, BPAN32, BPAN11, BPAN23, and BPAN14 are about 4.68:1, 1.55:1, 0.98:1, 0.61:1 and 0.24:1 (Figure S35). The ratios in the complexes are basically consistent with the feed ratio employed in the solvothermal reaction. The ratio of anda2- and pta2- ligands in BPAN41 is a little higher. This might be attributed to weighing error for the amount of the co-ligands in synthetic raw materials is too small. RESULTS AND DISCUSSION The as synthesized Co-MOFs are blue blocky crystals with the formula [Co1.5(tipb)(SO4)(coligand)0.5]·(DMF)n. X-ray crystallographic analysis revealed that they have similar structures. There are two crystallographically independent CoII centers involved in the asymmetric unit and both of them are four-coordinate with distortional tetrahedral coordination geometry (Figure S1a, S3a, S5a, S6a, S8a and S9). The tipb ligands and sulfate anions linking CoII ions formed a three dimensional network. Rhombic channels are formed by two-fold interpenetration of the networks along c directions. The dicarboxyalate co-lignads are located in the rhombic channels and link the interpenetrating nets, which formed self-penetrating frameworks (Figure S1b, S3b, S5b, S6b, and S8b). The blue crystals (except for BAN) are sensitive to water moleculars. When they were exposed to moisture or immerged in water, the blue crystals will turn to red. X-ray crystallographic analysis of the red samples revealed that water molecules replace the dicarboxyalate co-lignads coordinating with CoII ions (Figure S2a, S4a and S7a). The dicarboxyalate co-lignads show partly disordered coordination state (Figure S7) or totally dissociated from the framework and stay in the channel in highly disorded state (Figure S2b, S4b and S7b). Along with the SC-SC transformations, the coordination number of the CoII ions


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changed from 4 to 6, which induced the color change of the sample.41-42 Different from other samples, the color of BAN remained unchanged even after immerged in water for one week, indicating the absence of coordination enviroment change in this condition. Byong the change of color, the changes of coordination of CoII ions also bring deformations of the frameworks. The channels defined by the two-fold interpenetrated networks (linked by tipb ligands, sulfate anions and CoII ions) changed from rhombus to oval (Figure S1, S2, S3, S4, S6 and S7). Additionally, the transformation changes the framework from neutral to cationic and the dissociated dicarboxyalate co-ligand anions distributed in the pores to balance the charge of the whole MOF. It should be noted that in the case of RNA, although the sample tuned to red and the coordination number of CoII ions changed to six, indicated by single-crystal X-ray diffraction analyses, only about 40% of the nada2- ligands were dissociated from the framework, and the coordination environments of the retained nada2- ligands exhibited a disordered geometry coordinated with CoII ions (Figure S7). However, the PXRD pattern of RNA is exactly similar to those of RP, RAM, and RDH (Figure S21). Presumably, the residual amount of dicarboxylic acids on the frameworks of RP and RAM are too little to be identified by single-crystal X-ray diffraction analyses (Figure S2 and S4). During the investigation of transformations of this system, an interesting phenomenon has been noticed. For the as synthesized blue crystals based on different co-ligands, the conversion rates vary greatly at the same condition. Since the SC-SC transformations are accompanied by the removal/recovery of dicarboxylic co-ligands, these phonmena suggest that the dicarboxylic co-ligands may have tremendous impacts on the dynamic of the transformation process.


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To explore the details of the dynamic properties of the MOFs, the structure transformation behaviors as well as the sensing performances of the as-synthesized isoreticular MOFs toward H2O were studied systematically. The as-synthesized samples were immerged in H2O at room temperature for visual observation, and the time-dependent color change of the samples were recorded for comparision. As shown in Figure 3a, BP⊃DMF shows the color change from blue to red (RP), while BAM⊃DMF and BDH⊃DMF from blue to reddish-brown (RAM and RDH). Besides, no obvious change was further observed after about 8 minutes. In contrast, the color change process of BNA⊃DMF is rather slower, which lasted about one week and no obvious color change was detected within the initial 8 minutes. Remarkably, different from the four samples mentioned above, the color of BAN remained unchanged even after one week Obviously, the prominent difference in the conversion rates of BP⊃DMF, BAM⊃DMF, BDH⊃DMF, and BNA⊃DMF suggests that co-ligands have an important effect on the kinetics of the structure transformation process.27, 43-44

Figure 3. (a) The samples of the isoreticular MOFs immerged in water for different periods of time. (b) The kinetic transformation behaviors of H2pta/H2nada-based solid solutions. (c) The time for complete conversion increases exponentially with the increase of the ratio of anda2- of the ligand-based solid solutions.


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Since the kinetic transformation process of the MOFs are found to be largely depending on the backbone of the dicarboxylate ligands, the method of ligand-based solid solution was adopted aiming at a more accurate control of the conversion rates.45-48 It is expected that the combination of different dicarboxylate ligands in the MOFs could result in transformation behaviors between the MOFs based on single dicarboxylate ligands. Considering the pronounced difference in the conversion rates between BP⊃DMF and BNA⊃DMF as well as the similar colors of the transformed RP and RNA (Figure 3a), H2pta and H2nada were selected as the mixed co-ligands to regulate the kinetic of the transformation process. By adjusting the ratio of H2pta and H2nada in the reaction conditions, BPNA31⊃DMF, BPNA11⊃DMF, and BPNA13⊃DMF were successfully synthesized. 1H NMR experiments revealed that the ratios of co-ligands pta2- and nada2- in BPNA31⊃DMF, BPNA11⊃DMF, and BPNA13⊃DMF are about 3:1, 1:1, and 1:3, which consistent well with the feed ratio employed (Figure S34). X-ray crystallographic analysis of BPNA11⊃DMF, as an typical example, shows that pta2- and nada2- ligands are substitutionally disorder in the framework (Figure S9a). As shown in Figure 3b, when the samples of H2pta/H2nada-based solid solutions MOFs were immerged in water at room temperature, it was observed that the rate of the color change follows the sequence of BP⊃DMF > BPNA31⊃DMF > BPNA11⊃DMF > BPNA13⊃DMF > BNA⊃DMF, indicating that the sample with higher content of pta2- exhabits faster conversion rate. The color change process from blue to red takes about 8 min, 22 min, 7.5 h, 20 h, and 5 days for the samples, respectively. With the increasing molar ratios of anda2-/pta2-, the time required for the complete conversion increases exponentially (Figure 3c). Therefore, in the presence of solvent molecules (here is DMF), the strategy of co-ligand-based solid solutions


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could be utilized to enable the elaborate control of the kinetic of the transformation process successfully. On the other hand, since water molecules displaces the co-ligands and coordinates with CoII ions to trigger the structure transformation process, it plays an critical role. For better understanding of the transformation behaviors of the isoreticular MOFs, water vapor sorption experiments were performed. Prior to adsorption measurement, the as-synthesized samples were activated by solvent-exchange with methanol and subsequently evacuat at 105 oC to obtain the desolvated samples. As shown in Figure 4a, water sorption isotherms of BP, BAM, BDH, and BNA show similar profile involving a sharp increase of water uptake around 0.58-0.76 mmHg. The color of the samples are determined to be red at the end point of the sharp increase (Figure S16b), indicating the completeness of the structure transformation. The abruptly increased water uptakes should be attributed to coordination of H2O that triggered the structure transformations as well as the altered pore characters (formation of cationic framework and presence of anionic carboxylate ligands guest) that also benifit the adsorption of water molecules. Noted that the abruptly increased water uptake was not observed in the case of BAN due to the absence of the structural transformation, which agree well with the results observed in kinetic investigations (Figure 3a). Since the water sorption isotherms were obtained based on thermodynamically equilibrated uptakes under certain pressure points, their features can reveal the thermodynamics properties of the transformation process. For BP, BNA, BAM, and BDH, the similar transformation points around 0.65 mmHg suggest their similarities in the transformation process from the thermodynamic perspective. Considering the similar transformation mechanism determined by the crystal structures anslysis, the similarity in transformation point suggests the nature of the


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transformation procedures: In spite of the distinct functional groups and the backbones of the dicarboxylate ligands, the transformation process should be mainly affected by the breaking of carboxylate-CoII coordination bonds and the formation of H2O-CoII coordination bonds, which should be quite similar in different samples. This principle could also be applied for the understanding of the the isotherms of BPNA31, BPNA11, and BPNA13 (Figure 4a), as well as the further modulation of the dynamic behavior of the MOFs.

Figure 4. (a) Water adsorption isotherms of BP, BNA, BAN, BAM, BDH, PNA31, PNA11, and PNA13 at 273 K. The ROA data of vapor adsorptions of BPNA31 (b), BPNA11 (c), BPNA13 (d). On the other hand, in order to investigate the kinetic of the transformation behavior of the desolvated samples, the response rate of BPNA31, BPNA11, and BPNA13 toward H2O


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molecules was evaluated based on the sorption kinetic data obtained from the sorption experiments. It is expected that significant differences should be observed in different samples based on the results of transformation experiments in water condition (Figure 3). However, when analyzing the rate of adsorption (ROA) data of the samples, it was found that with the increasing percentage of nada2-, the rate of the vapor adsorption of the ligand-based solid solutions just slightly increased (Figure 4b, c and d). Besides, as observed in the water adsorption isotherms of BP, BPNA31, BPNA11 and BPNA13, and BNA (Figure 4a, S14), the conversion pressure point indicated by the cusp increase of water uptake also decreased marginally with the increase of nada2-. These phenomena indicate that, with the absence of solvent molecules, it is easier for BNA, which is composed of co-ligand with larger backbone, to transform than BP under the same condition. In addition, in the H2pta/H2nada-based solid solution MOFs, the occourance of structural transformations turned to be much easier along with the increase of the nada2percentage. These results are diametrically opposed to the conversion rates observed previously (Figure 3) with the DMF included MOFs, indicating that the solvent molecules occupying the channels of the frameworks might also play an important role in the transformation process. To verify the function of solvent molecules in the transformation process, the transformation behaviors of the desolvated samples were investigated, and the results show that the transformation process could be completed immediately upon the addition of water (Figure 5a and Transformation of desolvated samples.avi). Compared with the samples containing solvent molecules (Figure 3b), the distinguished transformation behaviors of the desolvated samples confirmed that solvent molecule is another key factor influencing the kinetic of the transformations.


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For better understanding of the effect of solvent molecules on the transformation process, a family of BP frameworks accommodating different solvent molecules, that is, BP⊃CH3OH, BP⊃EtOH, BP⊃DMF, BP⊃DMAC, and BP⊃DEF, were synthesized and investigated about their transformation rates in water.49-51 In spite of the identical framework, the conversion rate of the samples shows a decreasing trend accompany with the increased volume of the solvent molecules (Figure 5b), thus suggesting that the kinetic of the transformations are actually dominated by the synergy of co-ligands and solvent molecules in the frameworks. This principle implies that the materials with target transformation rate could be fabricated by modulating the co-ligands and/or solvent molecules.

Figure 5. (a) The desolvated samples of H2pta/H2nada-based solid solutions MOFs exposed in water within 1min. (b) The transformation behaviors of BP with different solvent molecules. The synergy of co-ligands and solvent molecules can be illustrated by the conversion mechanism. As established before, the essence of structure transformations is that water molecules replace the dicarboxylic acid ligands and coordinate with CoII ions, accompanied by the conversion of CoII coordination geometries. The backbones of the co-ligands and the solvent molecules occupying the channels of the frameworks have significant impacts on the


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transformation procedures. X-ray crystallographic analysis of BP⊃DMF, BP⊃DMAC and BP⊃NMP shows that, the solvent molecules were located in confined chambers defined by coligands (Figure 6a). Then the interactions between the co-ligands and the solvent molecules should be proportional to the volume of their backbones, which will influence/resist the diffusive rate of water molecules in the channels during the dynamic process. Thus, the conversion rates should decrease with the increasing volume of the solvent molecules and the backbone of the coligands.

Figure 6. (a) The relative positions of pta2- ligands (red) and solvent molecules (green) in BP⊃DMF, BP⊃DMAC and BP⊃NMP. (b) The interactions between anda2- ligands (red) and the inner wall atoms of the rhombic channels (cyan) in BAN. Solvent molecules omitted for clarity.


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In contrast, when the solvent molecules were removed from the frameworks, as demonstrated in the adsorption experiments, there was enough space left in the channels to accommodate the co-ligands dissociated from the frameworks. With the increasing dimension of the co-ligands, the crowdedness between co-ligands and the framework will increase (Figure 6), which could promote the dissociation/disorder of co-ligands. It means that, without solvent molecules, the samples constructed by co-ligands having larger backbone are easier to transform when encountering water (Figure 5a, S15). That accounts for the opposite trends observed with samples with solvent molecules, i.e. in water adsorptions, with the increasing ratio of nada2- in the H2pta/H2nada-based solid solutions MOFs, the conversion pressure point decreased (thermodynamics), while the rate of the vapor adsorption increased (kinetics) slightly. While, if the backbone of co-ligands increased to a certain extent, the interaction between co-ligand and the framework will play a dominant role, and the behavior of the MOF will be totally different. By comparing the crystal structures of BAN with that of the other MOFs, it was found that the bulky backbone of anda2- ligand further confined the space in the channels, resulting in a relatively stable state so that the dissociation of the anda2- from the framework in the presence of H2O was inhibited (Figure 6b). The water sorption behavior of BAN is entirely different from that of other transformable samples. The results indicate that the interaction between dicarboxylate ligands and the framework could thermodynamically influence the structure transformation behavior, which could be further utilized to modulate the dynamic behavior of these materials. Since the dicarboxylate ligands could influence the transformation procedures thermodynamically, the deliberate manipulation of the thermodynamic conversion processes was performed coresspondingly. Since anda2- has stronger interaction with the framework,


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H2pta/H2anda-based solid solutions MOFs, namely BPAN41, BPAN32, BPAN11, BPAN23, and BPAN14 were synthesized and investigated. 1H NMR tests reveal that the ratios of pta2- and anda2- in these samples are about 4:1, 3:2, 1:1, 2:3, and 1:4, respectively. X-ray crystallographic analysis of BPAN11⊃DMF shows that pta2- and anda2- ligands are substitutionally disordered in the framework.

Figure 7. Water adsorption isotherms of BP, BPAN41, BPAN32, BPAN11, BPAN23, BPAN14, and BAN. The conversion pressure point increased distinctly with the increased ratio of anda2-. Subsequently, water vapor adsorptions were performed to characterize the thermodynamic transformation process of these MOFs. As expected, in adsorption isotherms, the region with sharply increased uptake gradually shifted toward higher pressure along with the increased ratio of anda2- in the sample (Figure 7). These results indicated that the transformation process could be regulated thermodynamically by modifying the interactions between the framework and coligands.


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CONCLUSIONS In summary, the subtle kinetic and thermodynamic control of the transformation processes of a family of co-ligand-replaced dynamic cobalt(II)-organic frameworks were successfully performed. The conversion mechanisms of the transformations are illustrated in detail through systematic investigations, which provide the theoretical foundation for the subtle control of such dynamical processes. By means of the combined strategies of co-ligand replacement, solvent molecule substitution and ligand-based solid solution, the kinetic and thermodynamic processes of the transformations could be controlled accurately and flexibly. Notably, the strategy of ligand-based solid solution could be applied for the uninterrupted control of the kinetic (H2pta/H2nada-based solid solutions) and thermodynamic (H2pta/H2anda-based solid solutions) transformation processes. This work provides valuable methods and ideas for the study of similar solid dynamic materials and shed new light on the development of novel dynamic materials. ASSOCIATED CONTENT Supporting Information. This material is available free of charge on the ACS Publications website Expermental details, crystallographic data, supporting figures, adsorption data, PXRD, Solid UV-visible spectra, TGA, NMR spectra (PDF) Transformation of desolvated samples (avi) AUTHOR INFORMATION Corresponding Author


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(Z.C.) E-mail: [email protected],

*

(X.-H.B.) E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (21531005, 21421001, 91622111, and 21671112), Natural Science Fund of Tianjin, China (15JCZDJC38800), and the China Postdoctoral Science Foundation funded project (2015M571260). We thank Prof. Yabing He (Zhejiang Normal University) for the helpful discussions. REFERENCES (1) Cui, J.; Daniel, D.; Grinthal, A.; Lin, K.; Aizenberg, J., Dynamic Polymer Systems with Selfregulated Secretion for the Control of Surface Properties and Material Healing. Nat. Mater. 2015, 14, 790-795. (2) Grancha, T.; Ferrando-Soria, J.; Zhou, H.-C.; Gascon, J.; Seoane, B.; Pasan, J.; Fabelo, O.; Julve, M.; Pardo, E., Postsynthetic Improvement of the Physical Properties in a MetalOrganic Framework through a Single Crystal to Single Crystal Transmetallation. Angew. Chem. Int. Ed. 2015, 54, 6521-6525. (3) Jiang, S.; Jelfs, K. E.; Holden, D.; Hasell, T.; Chong, S. Y.; Haranczyk, M.; Trewin, A.; Cooper, A. I., Molecular Dynamics Simulations of Gas Selectivity in Amorphous Porous Molecular Solids. J. Am. Chem. Soc. 2013, 135, 17818-17830. (4) Kitagawa, S.; Uemura, K., Dynamic Porous Properties of Coordination Polymers Inspired by Hydrogen Bonds. Chem. Soc. Rev. 2005, 34, 109-119. (5) Du, Z.-Y.; Xu, T.-T.; Huang, B.; Su, Y.-J.; Xue, W.; He, C.-T.; Zhang, W.-X.; Chen, X.-M., Switchable Guest Molecular Dynamics in a Perovskite-Like Coordination Polymer Toward Sensitive Thermoresponsive Dielectric Materials. Angew. Chem. Int. Ed. 2015, 54, 914-918.


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