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In Situ Monitoring of Unique Switching Transitions in the PressureAmplifying Flexible Framework Material DUT-49 by High-Pressure 129 Xe NMR Spectroscopy Jana Schaber,†,‡,§ Simon Krause,§,∥ Silvia Paasch,† Irena Senkovska,∥ Volodymyr Bon,∥ Daniel M. Többens,⊥ Dirk Wallacher,# Stefan Kaskel,∥ and Eike Brunner*,† †

Bioanalytische Chemie, Fachrichtung Chemie und Lebensmittelchemie, and ∥Anorganische Chemie I, Fachrichtung Chemie und Lebensmittelchemie, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany ⊥ Department of Structure and Dynamics of Energy Materials and #Department of Sample Environment, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany S Supporting Information *

ABSTRACT: The pronounced flexibility of special metal− organic frameworks (MOFs), so-called soft porous crystals, is attracting increasing research interest. Studies of host−guest interactions in such materials are especially powerful if the measurements are performed in situ. 129Xe NMR spectroscopy is favorable because it provides characteristic, structure-sensitive parameters such as chemical shifts. The combination of highpressure xenon adsorption with 129Xe NMR spectroscopy was used to elucidate the adsorption-induced phase transitions in the recently discovered pressure-amplifying framework material DUT49, showing a unique negative gas adsorption (NGA) transition. In the open-pore state, DUT-49op exhibits a hierarchical pore system involving both micro- and mesopores. After reaching a critical relative pressure of ca. 0.15, adsorbed xenon induces mesopore contraction, resulting in a purely microporous contracted-pore phase. This contraction is accompanied by release of xenon from the mesopores. Further increase of the pressure initiates the recovery of the mesopores without any indication of a structural intermediate in the NMR spectra. According to the NMR data, the structural transition induced by xenon is a collective, stepwise phenomenon rather than a continuous process. This is the first time that NGA has been studied by directly monitoring the guest and its interaction with the host framework. MOFs for natural gas storage applications.23 Therefore, flexible MOFs are considered to be very promising novel materials for applications in selective adsorption and catalysis as well. NMR spectroscopy is an efficient tool for monitoring the local structure, dynamics, and flexibility of both the host framework and adsorbed guests such as Xe and CO2.24−35 It thus provides information that aids the development of a fundamental understanding of the processes governing structural transformations in soft porous crystals. Studies of host−guest interactions are particularly powerful if the measurements are performed in situ.

1. INTRODUCTION The pronounced flexibility of special metal−organic frameworks (MOFs), so-called soft porous crystals,1−18 is attracting increasing research interest. Switching between a narrow-pore state and a large-pore state (as observed for the MIL-53 series,4,9−11 where MIL represents Matériaux de l′Institut Lavoisier) or transitioning from a closed-pore state to an openpore state beyond a certain, gas-dependent gate-opening pressure (for example in DUT-8(Ni),12−15 where DUT represents Dresden University of Technology) is usually accompanied by a dramatic increase in the unit cell volume, sometimes exceeding 100%. Apart from the change in pore volume, other changes in physical properties are triggered (optical, magnetic, or dielectric), rendering these materials promising for advanced sensing applications. Moreover, several recent publications19−22 reported the highly selective adsorption of components from gas mixtures by these flexible compounds. Examples are the selective adsorption of p-xylene from mixtures with other isomers19 and the low-pressure selectivity of special MOFs for nitric oxide.20 Another important property is the enhanced working capacity of flexible © XXXX American Chemical Society

2. EXPERIMENTAL METHODS 2.1. Synthesis of DUT-49. Cu(NO3)2·3H2O (SigmaAldrich, 99.5%), N-methyl-2-pyrrolidone (NMP) (AppliChem, 99%), acetic acid (Sigma-Aldrich, 99%), and ethanol (anhydrous) (VWR Prolabo) were used for the synthesis and Received: February 7, 2017 Revised: February 14, 2017 Published: February 15, 2017 A

DOI: 10.1021/acs.jpcc.7b01204 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

least 15 min after pressure changes and at least 30 min after temperature changes. 129Xe NMR signals were monitored during this equilibration time to ensure that the signal remained constant after the equilibration period. The sample temperature was calibrated following the literature by using the 1H NMR chemical shifts of methanol.42−44 This method can exhibit systematic errors of a few degrees. The temperature was therefore calibrated by an independent second method. The condensation pressure of xenon, that is, the pressure where liquefaction starts inside the tube, was measured at the given temperature. The corresponding temperature was then determined from the phase diagram of xenon.45

activation of DUT-49 (C40H20N2O8Cu2). The ligand 9,9′([1,1′-biphenyl]-4,4′-diyl)bis(9H-carbazole-3,6-dicarboxylic acid) (H4BBCDC) was synthesized following a procedure previously reported by our group.36 In a solvothermal synthesis, H4BBCDC (200 mg, 0.302 mmol) was dissolved in a mixture of 34 mL of NMP and 2.5 mL of acetic acid at 80 °C, and 184 mg (0.756 mmol) of Cu(NO3)2·3H2O was added. The solution was heated at 80 °C for 36 h. The blue precipitate was filtered and washed five times with fresh NMP, and the solvent was exchanged with anhydrous ethanol 12 times over 3 days. The activation was performed using a protocol involving supercritical carbon dioxide.37 2.2. Volumetric Adsorption Experiments. Volumetric adsorption experiments were carried out on a BELSORP-max instrument for adsorption up to 100 kPa and on a BELSORPHP instrument for adsorption up to 500 kPa. Gases of high purity were used (Xe, 99.995%; He, 99.999%). To achieve the adsorption temperature of 165 K, a closed-cycle He cryostat was used.36 The desired adsorption temperatures were maintained using the following cooling-bath mixtures: 190 K, ethanol and dry ice; 195 K, acetone and dry ice; 223 K, mixture of 100 g of ice and 143 g of CaCl2·6H2O. The temperature was monitored during the measurements, and deviations smaller than 0.5 K were allowed. 2.3. In Situ PXRD under Xenon Adsorption. Combined adsorption/powder X-ray diffraction (PXRD) experiments were conducted at the KMC-2 Beamline at the BESSY II synchrotron light source of the Helmholtz-Zentrum Berlin für Materialien and Energie using a recently established experimental setup.38 Monochromatic radiation with an energy of 8048 eV (λ = 1.5406 Å) was used for all experiments. The diffraction images were measured in 2θ scan mode using the Vantec 2000 area detector system (Bruker). A synchrotron beam with dimensions of 0.5 × 0.5 mm was used for the experiments. Corundum powder with a crystallite size of 5 μm served as an external standard. The images were integrated using Datasqueeze 2.2.9 software39 and processed using the Fityk 0.9.8 program.40 Indexing of the PXRD patterns and Pawley refinement were performed using the Reflex module implemented in Materials Studio software.41 2.4. NMR Spectroscopy. In situ high-pressure 129Xe NMR experiments were performed on an Avance 300 spectrometer (Bruker, Karlsruhe, Germany) at 83.02 MHz using a 10-mm HR probe (6-μs pulse length). Experiments were carried out using a homemade apparatus allowing sample pressurization directly inside the NMR magnet. The apparatus uses a highpressure single-crystal sapphire tube with a home-designed gasand vacuum-tight titanium valve.12 This apparatus can be connected either to a vacuum pump or to an external gas reservoir. Chemical shifts were referenced by measuring the signal of xenon gas inside the tube at various pressures at room temperature and extrapolating the chemical shift to zero pressure. The activated sample was placed into the sapphire tube under an argon atmosphere. Afterward, the sample was evacuated under high vacuum at about 10−5 mbar. The sample tube was then mounted into the NMR spectrometer and connected to an outside xenon reservoir equipped with a pressure gauge using a Teflon hose. This allowed sample pressurization in situ within the magnet. We were thus able to measure 129Xe NMR spectra at controlled temperature and under the desired pressure. The pressure was measured using a Heise ST-2H pressure sensor with an HQSC-2 module with ±0.2 bar uncertainty. Samples were allowed to equilibrate for at

3. RESULTS AND DISCUSSION Recently, we discovered the pressure-amplifying metal−organic framework DUT-4936,37 with exceptionally high porosity, which exhibits spontaneous desorption of gas with increasing pressure in a defined pressure range (e.g., upon adsorption of methane at temperatures between ca. 90 and 120 K, Figure 1). This

Figure 1. Schematic representation of the xenon adsorption process on DUT-49 derived from adsorption studies (cf. Supporting Information): (a) without a structural transition at higher temperatures (237 K, cf. Figure 2), (b) with a structural transition and NGA at lower temperatures (200 K, cf. Figure 3). Color code: Xe, green; C, dark gray; Cu, turquoise; O, red; H, light gray; N, blue.

pressure amplification originates from a collapsing metastable state traversed during mesopore filling of the hierarchical pore structure (micropore diameters, 1.0 and 1.7 nm; mesopore diameter, 2.4 nm). Structural transformation, namely, mesopore contraction, leads to a purely microporous state and is accompanied by partial gas release (negative gas adsorption, NGA). Interestingly, a similar NGA transition could be observed at 75 kPa during the adsorption of xenon at 195 K (see Supporting Information, Figures S1−S3). This allows the NGA to be studied by in situ 129Xe NMR spectroscopy. Previous investigations focused on elucidating the structural transitions responsible for the unique behavior without an in-depth analysis of guest interactions with the flexible host framework. Xenon is particularly well-suited to the investigation of host− guest interactions by NMR spectroscopy because it provides characteristic, structure-sensitive parameters such as the B


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it is remarkable that the chemical shift at higher relative pressures significantly exceeds the chemical shift of bulk liquid xenon. This can be explained by the influence of xenon−wall interactions in addition to xenon−xenon interactions. In summary, the observations made at 237 K in the open-pore (op) form of DUT-49 do not provide indications for pronounced framework flexibility at this temperature. In contrast, a markedly different situation in terms of the pressure dependence of the chemical shift of xenon adsorbed in DUT-49 was observed when adsorption was carried out at 200 K (Figure 3, selected spectra are shown at the top). After an initial

chemical shift, chemical shift anisotropy, signal intensities, and relaxation times.26−31 Our home-built special apparatus12 allows in situ high-pressure NMR spectroscopic studies by applying variable gas pressures up to ca. 30 bar at variable temperatures down to ca. 200 K inside the NMR spectrometer. Gases such as 129Xe, 13CO2, and 13CH4 used in our investigations enable adsorption/desorption processes to be followed by observing the signals of adsorbed gases and correlating these NMR-derived parameters with volumetrically measured adsorption/desorption isotherms. In previous investigations, we showed that no pronounced structural transformation or NGA occurs at higher adsorption temperatures (e.g., at 298 K for methane).36,37 Similarly to these findings, no significant hysteresis could be observed for Xe adsorption at 223 and 237 K (Figure 2 and Supporting

Figure 2. Chemical shifts of adsorbed xenon in DUT-49 measured during adsorption (solid circles) and desorption (open circles) at 237 K in comparison to the adsorption isotherm at 223 K (red squares) measured volumetrically. The chemical shift of bulk liquid xenon at 237 K is indicated by the dotted horizontal line.

Information, Figures S1 and S2). The isotherm of the nonflexible DUT-49 was recorded (Figure 2). It exhibits the expected shape for a material with micro- and small mesopores. As can be seen in this figure, the 129Xe NMR chemical shift of adsorbed xenon in DUT-49 as a function of relative pressure (p/p0) at 237 K exhibits a reversible behavior without pronounced hysteresis. The adsorbed xenon moves rapidly within the pore system (i.e., exchanges between the different pores), thus giving rise to one single signal at an averaged chemical shift. The chemical shift of adsorbed xenon27 can be written as δ = δ W + δ Xe − Xe(ρXe )

Figure 3. Top: 129Xe NMR signals of adsorbed xenon on DUT-49 measured at 200 K for various relative pressures during the adsorption experiment (op, open-pore phase; cp, contracted-pore phase). Bottom: Chemical shifts of adsorbed xenon in DUT-49 at 200 K (blue circles) measured during the adsorption experiment (solid symbols denote signals of DUT-49op, open symbols denote those of DUT-49cp). The chemical shift of bulk liquid xenon at 200 K is given as a horizontal dotted line. The volumetric Xe adsorption isotherm measured at 195 K is also shown for comparison (red squares).

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assuming that the chemical shift is referenced relative to the gas-phase chemical shift extrapolated to zero pressure. The term δW summarizes all of the interactions of xenon and the internal surface. The second term describes the influence of xenon−xenon interactions and depends on the xenon density ρXe inside the pores. For low pressures, one can write δXe−Xe = ΔXe−XeρXe. The parameter ΔXe−Xe is temperature-dependent. The pressure dependence of the chemical shift should therefore resemble that of the conventional volumetric adsorption/ desorption isotherms because the chemical shift is correlated with ρXe. This is indeed observed (see Figure 2). Furthermore,

continuous increase in chemical shift in the low-p/p0 region, a sudden jump from 135 ppm at p/p0 = 0.13 to 235 ppm at p/p0 = 0.18 was observed. In a rigid MOF, such a slight pressure increase would result in only a slight change in chemical shift, as can be seen in the isotherm recorded at 237 K (see Figure 2). Furthermore, the signal intensity decreased at p/p0 = 0.18 compared to that at p/p0 = 0.13. This means that increasing xenon−xenon interactions cannot be responsible for this huge change in chemical shift. Therefore, we conclude that the C


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The Journal of Physical Chemistry C pronounced jump in the chemical shift indicates a structural transition resulting in much stronger xenon−wall interactions (cf. eq 1). The average pore size must decrease considerably to achieve strongly increasing xenon−wall interactions at almost constant xenon loading. It is known for other materials that decreasing average pore diameters result in increasing chemical shifts for adsorbed xenon.28,29 In situ adsorption studies performed on DUT-49 with methane at 111 K and with Xe at temperatures of 195 K and below (see Supporting Information, Figures S1 and S2, Table S1) indeed revealed that the mesopores in DUT-49op contract through a conformational change of the linkers and the material becomes purely microporous (DUT-49cp). This results in “negative” gas adsorption, that is, desorption of previously adsorbed methane or xenon from the MOF (cf. Figure 3). After the explained jump, the chemical shift then increases only slightly up to a relative xenon pressure of 0.55, as reflected by a corresponding plateau in the adsorption isotherm. At p/p0 = 0.61, a second signal of adsorbed xenon appears at ca. 240 ppm. Its intensity increases continuously with rising p/p0, whereas the signal at higher chemical shift decreases continuously until it disappears at p/p0 = 0.88. This indicates a second structural transition from the contracted-pore (cp) phase to the open-pore (op) phase taking place in the relative pressure range between 0.55 and 0.88. The presence of two separate signals corresponding to the op and cp states reveals that there is no fast exchange of xenon between these two states, indicating that the transition is a collective phenomenon taking place in larger domains/ crystallites. The coexistence of the two states over a certain relative pressure range shows that the transformation does not take place at the same pressure for all domains/crystallites. This results in a gradual slope of the adsorption isotherm. We did not observe any indication for an intermediate state; that is, the phase transition in the adsorption experiments seems to be a stepwise switching rather than a continuous process, in agreement with in situ PXRD studies.37 Note that parameters such as crystal sizes and lattice defects can strongly influence the switching behavior of flexible MOFs. Their distribution can cause different crystallites to switch at different pressures. The cp−op structure transition is accompanied by a significant increase in signal intensity, whereas the chemical shift changes only slightly. Obviously, the mesopores are restored and immediately filled with xenon at these relatively high pressures (cf. Figure 4). This explains the high chemical shift that changes only slightly with p/p0. Stepwise pressure release (desorption) reveals a pronounced hysteresis compared with the adsorption process (compare Figures 3 and 4). Only one signal is present down to a relative pressure of 0.23. A second peak at higher chemical shifts is observed for p/p0 < 0.23, indicating the op−cp structural transformation to DUT-49cp, in agreement with in situ PXRD results (see section 2 of the Supporting Information). The material finally adopts the cp state after the op−cp phase transition below p/p0 = 0.25. This explains the high chemical shift at low pressure caused by strong xenon−wall interactions.

Figure 4. Top: 129Xe NMR signals of adsorbed xenon on DUT-49 measured at 200 K for various relative pressures during the desorption experiment. Bottom: Chemical shifts of adsorbed xenon in DUT-49 at 200 K (blue circles) measured during the desorption experiment (solid symbols denote signals of DUT-49op, open symbols denote those of DUT-49cp). The chemical shift of bulk liquid xenon at 200 K is given as a horizontal dotted line. The volumetric Xe adsorption/desorption isotherm measured at 195 K is also shown for comparison (adsorption, solid red squares; desorption, open red squares).

studies46 revealed that the adsorption enthalpy of the guest molecule is of major importance for this behavior. It provides the energy required to perform the endothermal structural contraction. At increasing temperature, however, both guest− host and guest−guest interactions decrease, thus making the structural transformation unfavorable. The op−cp transition, namely, mesopore-to-micropore contraction, is directly observed as an abrupt, large jump in the 129Xe NMR chemical shift. This reflects the stronger xenon−wall interactions in the cp state. At higher p/p0, reopening of the mesopores caused by the cp−op transition can also be directly monitored by observing the individual signals for the two phases. A pronounced hysteresis is observed in the desorption experiment, that is, at stepwise decreasing pressure. At low p/p0, DUT-49 undergoes mesopore contraction in a final op−cp transition. 129Xe NMR spectroscopy thus provides a unique view into the pore interior, revealing the collective character of NGA transitions and a two-phase coexistence over a certain relative pressure range.

4. CONCLUSIONS In summary, in situ high-pressure 129Xe NMR spectroscopy allows for the study of the unique phase transitions including NGA in DUT-49 by directly monitoring the adsorbed Xe atoms inducing these transitions. DUT-49 does not exhibit pronounced transitions for Xe adsorption at 237 K. In contrast, distinct structural transformations occur at 200 K. Theoretical D


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(5) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Férey, G. A Rationale for the Large Breathing of the Porous Aluminum Terephthalate (MIL-53) Upon Hydration. Chem. - Eur. J. 2004, 10, 1373−1382. (6) Mellot-Draznieks, C.; Serre, C.; Surblé, S.; Audebrand, N.; Férey, G. Very Large Swelling in Hybrid Frameworks: A Combined Computational and Powder Diffraction Study. J. Am. Chem. Soc. 2005, 127, 16273−16278. (7) Serre, C.; Mellot-Draznieks, C.; Surblé, S.; Audebrand, N.; Filinchuk, Y.; Férey, G. Role of Solvent-Host Interactions That Lead to Very Large Swelling of Hybrid Frameworks. Science 2007, 315, 1828−1831. (8) Horike, S.; Shimomura, S.; Kitagawa, S. Soft Porous Crystals. Nat. Chem. 2009, 1, 695−704. (9) Boutin, A.; Coudert, F.-X.; Springuel-Huet, M.-A.; Neimark, A. V.; Férey, G.; Fuchs, A. H. The Behavior of Flexible MIL-53(Al) upon CH4 and CO2 Adsorption. J. Phys. Chem. C 2010, 114, 22237−22244. (10) Coudert, F.-X.; Mellot-Draznieks, C.; Fuchs, A.; Boutin, A. Double Structural Transition in Hybrid Material MIL-53 upon Hydrocarbon Adsorption: The Thermodynamics Behind the Scenes. J. Am. Chem. Soc. 2009, 131, 3442−3443. (11) Salles, F.; Ghoufi, A.; Maurin, G.; Bell, R. G.; Mellot-Draznieks, C.; Férey, G. Molecular Dynamics Simulations of Breathing MOFs: Structural Transformations of MIL-53(Cr) upon Thermal Activation and CO2 Adsorption. Angew. Chem., Int. Ed. 2008, 47, 8487−8491. (12) Hoffmann, H. C.; Assfour, B.; Epperlein, F.; Klein, N.; Paasch, S.; Senkovska, I.; Kaskel, S.; Seifert, G.; Brunner, E. High-Pressure in Situ 129Xe NMR Spectroscopy and Computer Simulations of Breathing Transitions in the Metal−Organic Framework Ni2(2,6-ndc)2(dabco) (DUT-8(Ni). J. Am. Chem. Soc. 2011, 133, 8681−8690. (13) Klein, N.; Hoffmann, H. C.; Cadiau, A.; Getzschmann, J.; Lohe, M. R.; Paasch, S.; Heydenreich, T.; Adil, K.; Senkovska, I.; Brunner, E.; Kaskel, S. Structural Flexibility and Intrinsic Dynamics in the M2(2,6ndc)2(dabco) (M = Ni, Cu, Co, Zn) Metal−Organic Frameworks. J. Mater. Chem. 2012, 22, 10303−10312. (14) Klein, N.; Herzog, C.; Sabo, M.; Senkovska, I.; Getzschmann, J.; Paasch, S.; Lohe, M. R.; Brunner, E.; Kaskel, S. Monitoring Adsorption-Induced Switching by 129Xe NMR Spectroscopy in a New Metal−Organic Framework Ni2(2,6-ndc)2(dabco). Phys. Chem. Chem. Phys. 2010, 12, 11778−11784. (15) Bon, V.; Klein, N.; Senkovska, I.; Heerwig, A.; Getzschmann, J.; Wallacher, D.; Zizak, I.; Brzhezinskaya, M.; Mueller, U.; Kaskel, S. Exceptional Adsorption-Induced Cluster and Network Deformation in the Flexible Metal−Organic Framework DUT-8(Ni) Observed by in Situ X-ray Diffraction and EXAFS. Phys. Chem. Chem. Phys. 2015, 17, 17471−17479. (16) Henke, S.; Schneemann, A.; Wütscher, A.; Fischer, R. A. Directing the Breathing Behavior of Pillared-Layered Metal−Organic Frameworks via a Systematic Library of Functionalized Linkers Bearing Flexible Substituents. J. Am. Chem. Soc. 2012, 134, 9464− 9474. (17) Henke, S.; Schneemann, A.; Fischer, R. A. Massive Anisotropic Thermal Expansion and Thermo-Responsive Breathing in Metal− Organic Frameworks Modulated by Linker Functionalization. Adv. Funct. Mater. 2013, 23, 5990−5996. (18) Murdock, C. R.; McNutt, N. W.; Keffer, D. J.; Jenkins, D. M. Rotating Phenyl Rings as a Guest-Dependent Switch in TwoDimensional Metal−Organic Frameworks. J. Am. Chem. Soc. 2014, 136, 671−678. (19) Mukherjee, S.; Joarder, B.; Manna, B.; Desai, A. V.; Chaudhari, A. K.; Ghosh, S. K. Framework-Flexibility Driven Selective Sorption of p-Xylene Over Other Isomers by a Dynamic Metal-Organic Framework. Sci. Rep. 2014, 4, 5761. (20) Xiao, B.; Byrne, P. J.; Wheatley, P. S.; Wragg, D. S.; Zhao, X.; Fletcher, A. J.; Thomas, K. M.; Peters, L.; Evans, J. S.; Warren, J. E.; Zhou, W.; Morris, R. E. Chemically Blockable Transformation and Ultraselective Low-Pressure Gas Adsorption in a Non-Porous Metal Organic Framework. Nat. Chem. 2009, 1, 289−294.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01204. Detailed data from volumetric Xe adsorption experiments (Figures S1−S3) and in situ PXRD (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Irena Senkovska: 0000-0001-7052-1029 Eike Brunner: 0000-0003-3511-9899 Present Address ‡

Leibniz-Institut für Polymerforschung (IPF) Dresden, Hohe Straße 6, 01069 Dresden, Germany. Author Contributions §

J.S. and S.K. contributed equally to the present work.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft (FOR 2433 “MOF switches”) and from the German Federal Ministry for education and research (BMBF, Project 05K13OD3) is gratefully acknowledged. Thanks are due to Helmholtz-Zentrum Berlin (HZB) for the allocation of the synchrotron radiation beamtime on KMC-2 beamline and travel grants.

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ABBREVIATIONS cp, contracted pore; DUT-49 or -8, Dresden University of Technology material number 49 or 8; H4BBCDC, 9,9′-([1,1′biphenyl]-4,4′-diyl)bis(9H-carbazole-3,6-dicarboxylic acid); MIL-53, Matériaux de l′Institut Lavoisier material number 53; MOF, metal−organic framework; NGA, negative gas adsorption; NMP, N-methyl-2-pyrrolidone; NMR, nuclear magnetic resonance; op, open pore; PXRD, powder X-ray diffraction

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REFERENCES

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