Tunable Properties from Structural Reversibility: Responsive Metal-Organic Frameworks

Gary Suseno Literature Seminar November 8, 2012

Metal Organic Frameworks (MOFs) are defined as an ordered array of repeating units consisting of metals and organic linkers that span either in two or three dimensions1. Interesting properties of many MOFs arise from the fact that they are porous, giving them capability to store other molecules via adsorption mechanism or host-guest chemistry2,3. This capability of storage has led to applications in gas storage and separation processes4.

Since the discovery of MOFs, major improvement has been made in the systematic design and gas storage capacity of MOFs5,6. In addition, a significant effort has also been focused on the structural flexibility that is associated with a group of MOFs7,8. In fact, many unique properties such as gated and selective adsorption are associated with this structural flexibility9,10. Those properties might be used to explore new application of MOFs as stimuli responsive materials with switchable properties8. Responsive materials with switchable properties are particularly interesting because their functionality can be activated on-demand. This seminar will highlight some works related to the development of MOFs as a responsive material with switchable properties.

In the first work, Yaghi and co-workers11 reported a structurally non-interpenetrated MOF-123 that could reversibly transforms into an interpenetrated MOF-246 upon ligand removal or addition (Figure 1). This transformation is exquisite, as previously known MOFs could only have reversible degree of interpenetration. Isothermal adsorption and Powder X-Ray Diffraction (PXRD) studies show intermediate structures in the transition going from MOF-123 to MOF-246, and this gradual structural transformation is also associated with changes in gas adsorption profiles, until finally MOF-246 shows no adsorption capability due to complete interpenetration. This structurally reversible MOF-123 is an exhibition of reversibility in porosity and ultimately adsorption properties.

Figure 1: a) The idea of interpenetration (left) and the three-dimensional picture (right). b) Reversible process from MOF-123 (left) to MOF-246 (right). Interpenetrated network is shown in green.

Metal–Organic FrameworksDOI: 10.1002/anie.201202925

Reversible Interpenetration in a Metal–Organic Framework Triggeredby Ligand Removal and Addition**Sang Beom Choi, Hiroyasu Furukawa, Hye Jin Nam, Duk-Young Jung, Young Ho Jhon,Allan Walton, David Book, Michael O!Keeffe, Omar M. Yaghi, and Jaheon Kim*

Interpenetration is known for the structures of many mineralsand ice;[1] most notably for ice, it exists in doubly inter-penetrating (VI, VII, and VIII) and non-interpenetrating (Ih)forms with the latter being porous and having nearly half ofthe density of the former.[2] In synthetic materials, specificallyin metal–organic frameworks (MOFs), interpenetration isgenerally considered undesirable because it reduces poros-ity.[3] However, on the contrary, many advantageous proper-ties also arise when MOFs are interpenetrated, such asselective guest capture,[3a] stepwise gas adsorption,[4e]

enhanced framework robustness,[5] photoluminescence con-trol,[6] and guest-responsive porosity.[7] Therefore, variousstrategies have been suggested to control interpenetrationduring synthesis.[4] However, once these extended networkmaterials are prepared as interpenetrating or non-interpene-trating structures, the degree of interpenetration generallyremains unchanged, because numerous chemical bonds mustbe broken and subsequently reformed in a very concerted way

during the process unlike some interlocked coordinationcompounds in solution (Figure 1a).[8]

Some extended structures, such as [(ZnX2)3(tris-(4-pyr-idyl)triazine)2]n (X = Cl, Br, I) are known to show frameworkinterpenetration by heat treatments via amorphous inter-mediate phases.[9] In contrast to this irreversible phasetransformation, [Ag6Cl(3-amino-1,2,4-triazolate)4]OH·6H2Oshows a reversible change in the degree of interpenetrationbetween five-fold and six-fold interpenetration.[10] However,it has not been observed so far that extended structuresbehave like discrete molecular systems exhibiting full andreversible catenation. Herein, we report that the removal ofDMF ligands protruding into the small channels of a three-dimensional metal–organic framework, [Zn7O2(NBD)5-(DMF)2] (MOF-123; NBD = 2-nitrobenzene-1,4-dicarboxy-late, DMF = N,N-dimethylformamide) triggers the transfor-mation of this MOF to a doubly interpenetrating form, MOF-246, [Zn7O2(NBD)5], whose crystal structure is identical tothe backbone of MOF-123. Moreover, addition of DMF toMOF-246 results in the reverse transformation to give MOF-123 (Figure 1b).

Solvothermal reactions between Zn(NO3)2·6 H2O andH2NBD in a mixed solution of DMF/methanol producedcrystals of MOF-123, the framework of which is formulated as[Zn7O2(NBD)5(DMF)2]. The crystal structure shows that inthe heptanuclear inorganic secondary building unit (SBU,[Zn7O2(CO2)10(DMF)2]), which shares a structural similaritywith zinc benzoate complexes,[11] [Zn7O2(2,6-difluorobenzoa-

Figure 1. a) Catenation and decatenation processes in molecules (left)are observed in three-dimensional MOFs (right), which can be termedinterpenetration and deinterpenetration. b) MOF-123 is converted intothe interpenetrating crystals of MOF-246 by the removal of coordinatedDMF (large pink spheres). C black, N blue, O red, Zn blue polyhedra.The interpenetrated framework is shown in green.[*] S. B. Choi, Dr. Y. H. Jhon, Prof. J. Kim

Department of Chemistry, Soongsil UniversitySeoul 156-743 (Korea)E-mail: jaheon@ssu.ac.kr

Dr. H. Furukawa, Prof. O. M. YaghiDepartment of Chemistry, University of California, andThe Molecular Foundry, Lawrence Berkeley National LaboratoryBerkeley, CA 94720 (USA)

Dr. H. J. Nam, Prof. D.-Y. JungDepartment of Chemistry, Institute of Basic Sciences and Sung-kyunkwan Advanced Institute of NanotechnologySungkyunkwan University, Suwon 440-746 (Korea)

Dr. A. Walton, Dr. D. BookSchool of Metallurgy and Materials, University of BirminghamBirmingham B152TT (UK)

Prof. M. O’KeeffeDepartment of Chemistry and BiochemistryArizona State University, Tempe, AZ 85287 (USA)

Prof. O. M. YaghiCenter for Global Mentoring, Graduate School of EEWS (WCU),Korea Advanced Institute of Science and Technology (KAIST)Daejeon 305-701 (Korea)

[**] This work is supported by the Hydrogen Energy R&D Center, one ofthe 21st Century Frontier R&D Programs (the Ministry of Education,Science and Technology of Korea to J.K.). We thank Jihye Yoon andYoon Jeong Kim for their experimental support, and Dr. Seung-HoonChoi (Insilicotech Co. Ltd.), Prof. Kimoon Kim (POSTECH), Dr.Hexiang Deng, Kyle Cordova (UC Berkeley), and Dr. Carolyn B.Knobler (UCLA) for their valuable comments.

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.201202925.

AngewandteChemie

8791Angew. Chem. Int. Ed. 2012, 51, 8791 –8795 ! 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The second work was reported by Kitagawa and co-workers12; they introduced a system where structural transformation can be triggered by exposure to UV light (Figure 2). Their approach is to introduce trans-azo-benzene molecule as a guest molecule inside [Zn2(terephtalic acid)2(triethylenediamine)] MOF. Inclusion of the azo-benzene forced the MOF to adopt an orthorhombic structure and the composite does not have nitrogen gas adsorption capacity. However, UV irradiation of the azobenzene inside the MOF will induce isomerization of the guest from trans-azobenzene to cis-azobenzene. The cis-azo-benzene would force the MOF to adopt a tetragonal structure, and the composite showed improvement in the gas adsorption capacity. The tetragonal structure can be reverted back to the orthorhombic structure by heating the system. This system is showing an example of switchable gas adsorption capacity in MOFs by using UV trigger.

Figure 2: Illustration of tunable adsorption capacity that is induced by structural conformation of the guest molecules. The red and orange objects are the different isomers of the guest molecules.

In the previous work, the conformation of azo-benzene triggers the structural transformation of the MOF, which is correlated to the gas adsorption capacity. However, one could think the reversed process might also be possible. Kitagawa and co-workers13 introduced a composite system by using [Zn2(terephtalic acid)2(triethylenediamine)] as the MOF and distyryl benzene (DSB) as the guest (Figure 3). DSB is a dye molecule known to have fluorescence property that is dependent on its structural conformation, where a planar structure is known to have a higher quantum yield for the excitation-emission process. Introduction of gas molecules into the composite will force the MOF’ structure from orthorombic to tetragonal as well as planarization of the DSB. The planarization will produce higher quantum yield in the excitation-emission process, shown by higher emission intensity. This process is a showcase of controlled fluorescent switching by gas adsorption. Coupled with the fact that this composite is selective toward certain gases, these properties are useful for the development of gas sensors.

Figure 3: Illustration of reversible fluorescent response from [MOF + DSB] composite. This process is induced by CO2 gas inclusion.

Guest-to-Host Transmission of Structural Changes for Stimuli-Responsive Adsorption PropertyNobuhiro Yanai,†,∥ Takashi Uemura,*,† Masafumi Inoue,† Ryotaro Matsuda,‡,§ Tomohiro Fukushima,†

Masahiko Tsujimoto,§ Seiji Isoda,§ and Susumu Kitagawa*,†,‡,§

†Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,Nishikyo-ku, Kyoto 615-8510, Japan‡ERATO Kitagawa Integrated Pores Project, Kyoto Research Park Building #3, Shimogyo-ku, Kyoto 600-8815, Japan§Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

*S Supporting Information

ABSTRACT: We show that structural changes of a guestmolecule can trigger structural transformations of acrystalline host framework. Azobenzene was introducedinto a flexible porous coordination polymer (PCP), andcis/trans isomerizations of the guest azobenzene by lightor heat successfully induced structural transformations ofthe host PCP in a reversible fashion. This guest-to-hoststructural transmission resulted in drastic changes in thegas adsorption property of the host−guest composite,displaying a new strategy for creating stimuli-responsiveporous materials.

An intriguing challenge in host−guest chemistry is how toachieve a transmission of structural changes from the

guest to the host and vice versa.1 Chemists are inspired bysophisticated biological systems, such as in the perception oflight, where a photoinduced configurational change of the guestchromophore retinal induces a structural change of the hostopsin protein, converting it from an inactive state to anactivated signaling state.2 Implementation of such guest-to-hostor host-to-guest structural transmission in artificial systemswould lead to a variety of advanced stimuli-responsiveproperties, but the successful examples are rather limited.1,3−5

To attain this structural transmission accompanied byresponsive physical or chemical properties, we utilized porouscoordination polymers (PCPs) or metal−organic frameworks(MOFs) composed of metal ions and organic ligands. PCPs/MOFs have highly regular nanopores that can be utilized forstorage, separation, and catalysis.6−16 One of the mostadvantageous features of PCPs/MOFs is that their flexibleframeworks are responsive to guest molecules while retaininghigh regularity, which does not occur in conventionalmicroporous materials such as zeolites and activatedcarbons.8,9,14,15 They are called soft porous crystals.14 If theconformation of the guest is changed by external stimuli, thehost structure would be simultaneously transformed accordingto the different guest shapes (Figure 1). This artificial guest-to-host structural transmission has great potential to produce adynamic switching of functions of the host−guest composites,such as porous, optic, electric, and magnetic properties.

In this work, we constructed a guest-to-host structuraltransmission system by using flexible PCPs/MOFs for the firsttime. Azobenzene (AB) was included in a flexible hostcompound, [Zn2(terephthalate)2(triethylenediamine)]n (1;pore size = 7.5 Å ! 7.5 Å), in which two-dimensional squaregrids are bridged by triethylenediamine (see the SupportingInformation).17 The pore structure of host 1 has been reportedto be deformed by inclusion of particular aromatic guestmolecules.4,17−19 The cis/trans isomerizations of AB by lightand heat efficiently triggered structural transformations of 1,resulting in a drastic switching of the adsorption property of thehost−guest composite.trans-AB was fully introduced into the nanochannels of 1 at

120 °C, after which excess AB external to the host crystals wasremoved under reduced pressure. The obtained composite isdenoted as 1⊃AB. Scanning electron microscopy measure-ments showed that the size of the composite crystals was ∼10μm. X-ray powder diffraction (XRPD) measurements showedthat the host structure was changed by the inclusion of trans-AB, clearly indicating the formation of the host−guest adduct(Figure 2a,b). The number of AB molecules per unit cell of 1was 1.0, as evidenced by elemental and thermogravimetric

Received: December 11, 2011Published: February 28, 2012

Figure 1. Schematic illustration of the concept of guest-to-hoststructural transmission. Red and orange objects represent trans-AB andcis-AB, respectively. The conformational change in the guest moleculeby external stimuli triggers a structural transformation of the crystallinehost framework. This structural transmission results in the efficientswitching of porous functions of the host−guest composite.

Communication

pubs.acs.org/JACS

© 2012 American Chemical Society 4501 dx.doi.org/10.1021/ja2115713 | J. Am. Chem. Soc. 2012, 134, 4501−4504

NATUREMATERIALS DOI: 10.1038/NMAT3104ARTICLES

DSB

1

75°90°

ZnZn

OO

N

N

OO

OO

OO

OO

CO2

CO2

!"1 DSB "1 [DSB + CO2]

1!

Figure 2 | Introduction of reporter molecule DSB into flexible host 1, and structural and fluorescence changes of the composite by co-adsorption of CO2.The original and deformed host structures are denoted as 1 and 10, respectively. The adsorption of CO2 on 10 � DSB induces the coupled changes of the PCPstructure and DSB conformations, resulting in the critical fluorescence response towards CO2. Pictures of the composite materials were taken at 195 Kunder ultraviolet irradiation (excitation at 366 nm). The removal of CO2 from 1� [DSB+CO2] by vacuuming at 195 K regenerates the original composite10 � DSB.

a clear blue fluorescence was observed from the composite onintroducing CO2 gas at 195K. The fluorescence colour changestarted within a few seconds, and was complete within a fewminutes. Figure 3d shows the fluorescence spectra of 1

0 � DSBunder different CO2 pressures. The fluorescence spectrum of1

0 � DSB under vacuum was broad, which may be explained byhindrance of the relaxation process from the twisted state to theplanar excited state30,40. The spectra changedmarkedly and featuredvibronic structures above a specific threshold pressure (P =30 kPa)that corresponds to the step of the adsorption isotherm. Theappearance of vibronic spectra is attributable to the planarizationof the DSB molecules30,40. Although conformational planarizationusually extends the effective ⇡-conjugation length of molecules,the excitation and fluorescence spectra showed a slight blue shiftafter CO2 uptake, which is probably because of the alterationof the aggregation structure of DSB (Fig. 3d and SupplementaryFig. S8)29,30,35. The original guest fluorescence of 10 � DSB wasregenerated by removingCO2 from the composite 1� [DSB+CO2],confirming the easy reuse of the detection system. Note that theshape and position of the fluorescence spectrum of 10 �DSB wasnot affected by N2, O2, and Ar (Supplementary Fig. S9); thus, thecomposite selectively sensed CO2 among the atmospheric gases.

Furthermore, the particular threshold pressure was detected by aclear change in the solid-state fluorescence. This type of detectionis important for quality control to ensure the proper functionsof CO2-containing gases in industrial processes, and to safeguardagainst their waste streams into the atmosphere8. Notably, theDSB fluorescence in rigid PCPs was not sensitive to CO2, whichhighlights the importance of the framework transformation for thefluorescence response (Supplementary Fig. S10).

Flexible PCPs can also differentiate gases with similar properties,which allows fluorescence detection of these gases. We employedCO2 and C2H2 as target gases because it is important to distinguishC2H2 from CO2 in the production and use of C2H2; however, thisdifferentiation is difficult because of their similar physicochemicalproperties (Supplementary Table S2)41–43. The adsorption isothermof C2H2 for 1

0 � DSB shows a steep rise in the low-pressureregion and reaches saturation, which is very different from thestepwise isotherm of CO2 (Fig. 4a). The observed difference mightbe due to the stronger interaction of C2H2 with the pore wallsthan that of CO2. The pore walls are mainly composed of aromaticand aliphatic moieties, but there are negatively polarized oxygenatoms of carboxylate ligands that can form attractive electrostaticinteractions with the positively polarized hydrogen atoms of

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© 2011 Macmillan Publishers Limited. All rights reserved

In conclusion, MOFs are a versatile area of research owing to its unique porous and flexible structure. These properties have been exploited not only to develop MOFs for gas storage, but also stimuli responsive MOFs with switchable properties. Significant progress has been accomplished over the years and more research will continue in this promising area in order to produce MOFs whose properties can be activated on-demand.

References

1. Batten, S. R.; Champness N. R.; Chen, X. M.; Garcia-Martinez, J.; Kitagawa, S.; Öhrström L.; O’Keeffe, M.; Suh, M. P.; Reedijk, J. Coordination Polymers, Metal-Organic Frameworks and the Need for Terminology Guidelines. CrystEngComm. 2012, 12,3001-3004.

2. Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective Gas Adsorption and Separation in Metal-Organic Frameworks Chem. Soc. Rev. 2009, 38,1477-1504.

3. Dybstev, D. N.; Chun, H.; Kim, K. Rigid and Flexible: A Highly Porous Metal-Organic Framework with Unusual Guest-Dependent Dynamic Behavior. Angew. Chem. Int. Ed. 2004, 43, 5033-5036.

4. Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre, J. Metal Organic Frameworks- Prospective Industrial Applications. J. Mat. Chem. 2006, 16, 626-636.

5. Rowsell, J. L. C.; Yaghi, O. M. Strategies for Hydrogen Storage in Metal-Organic Framework. Angew. Chem. Intl. Ed. 2005, 44,4670-4679.

6. Furukawa, H.; Kim, J.; Ockwig, N. W.; O’Keeffe, M.; Yaghi, O. M. Control of Vertex Geometry, Structure Dimensionality, Functionality, and Pore Metrics in the Reticular Synthesis of Crystalline Metal-Organic Frameworks and Polyhedra. J. Am. Chem. Soc. 2008, 130, 11650-11661.

7. Férey G.; Serre, C. Large Beathing Effects in Three-Dimensional Porous Hybrid Matter: Facts, Analyses, Rules, and Consequences. Chem. Soc. Rev. 2009, 38, 1380-1399.

8. Horike S.; Shimomura S.; Kitagawa S. Soft Porous Crystal. Nature Chemistry 2009, 1, 695-704.

9. Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa S. Porous Coordination-Polymer Crystals with Gated Channels Specific for Supercritical Gases. Angew. Chem. Int. Ed. 2003, 42, 428-431.

10. Tanaka, D.; Nakagawa, K.; Higuchi, M.; Kubota, Y.; Kobayashi, T. C.; Takata, M.; Kitagawa, S. Kinetic Gate-Opening Process in Flexible Porous Coordination Polymer. Angew. Chem. 2008, 120, 3978-3982.

11. Choi, S. B.; Furukawa, H.; Nam, H. J.; Jung, D.; Jhon, Y. H.; Walton, A.; Book, D.; O’Keeffe, M.; Yaghi, O. M.; Kim, J. Reversible Interpenetration in a Metal-Organic Framework Triggered by Ligand Removal and Addition. Angew. Chem. Int. Ed. 2012, 51, 8791-8795.

12. Yanai, N.; Uemura, T.; Inoue, M.; Matsuda, R.; Fukushima, T.; Tsujimoto, M.; Isoda, S.; Kitagawa, S. Guest-to-Host Transmission of Structural Changes for Stimuli-Responsive Adsorption Property. J. Am. Chem. Soc. 2012, 134, 4501-4504.

13. Yanai, N.; Kitayama, K.; Hijikata, Y.; Sato, H.; Matsuda, R.; Kubota, Y.; Takata, M.; Mizuno, M.; Uemura, T.; Kitagawa, S. Gas Detection by Structural Variatins of Fluorescent Guest Molecules in a Flexible Porous Coordination Polymer. Nature Materials 2011, 10, 787-793.