Reversible Structural Flexibility and Sensing Properties of a Zn (II

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Reversible Structural Flexibility and Sensing Properties of a Zn(II) Metal−Organic Framework: Phase Transformation between Interpenetrating 3D Net and 2D Sheet Yeonga Kim, Jeong Hwa Song, Woo Ram Lee, Won Ju Phang, Kwang Soo Lim, and Chang Seop Hong* Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-713, Korea S Supporting Information *

ABSTRACT: A three-dimensional Zn(II) framework, [Zn4O(L)3(DMF)2]·0.5DMF·H2O (1; H2L = 3,3′-dimethoxybiphenyl-4,4′-dicarboxylic acid) was prepared under a solvothermal reaction in DMF. The structure reveals that the 3-fold interpenetration is stabilized in the framework with a distinct secondary building unit of the formula [Zn 4 O(RCO2)6(DMF)2], slightly different from that of MOF-5. Phase transformations in 1 occur reversibly via two pathways of solvent exchange/resolvation and activation/resolvation, which is indicative of the presence of extensive structural flexibility. Nitrobenzene among tested solvents is selectively detected by 1, and the sensing event was operating repeatedly. The threedimensional framework of 1 with 3-fold interpenetration is uniquely converted to the two-dimensional Cu phase with no interpenetration, reflecting a drastic dimensionality variation.

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occur between the same dimensionalities.17−22 In contrast, a variation in framework dimensionalities upon solvent removal has been rarely demonstrated in MOFs,23−25 and thereby, it is a challenging task to seek such a system in the pursuit of flexible MOFs responsive to external stimuli. Herein we report the synthesis, crystal structure, and structural transformations of a three-dimensional (3D) framework of [Zn4O(L)3(DMF)2]·0.5DMF·H2O (1; H2L = 3,3′dimethoxybiphenyl-4,4′-dicarboxylic acid). The 3-fold interpenetrating net undergoes extensive reversible phase transformations through two pathways of solvent exchange/ resolvation and activation/resolvation, indicating significant structural flexibility. Moreover, this material exhibits selective sensing of NB with recyclability. In addition, transformations between the 3D Zn phase (1) with 3-fold interpenetration and the 2D Cu phase with no interpenetration take place, and this is a unique demonstration of a dramatic change of dimensionality in MOFs.

INTRODUCTION Metal−organic frameworks (MOFs) are porous crystalline solids that possess high surface areas and tunability of pore surface properties. Recent advances have enabled various potential applications of MOFs in gas storage and separation, catalysis, proton conductivity, and drug delivery.1−6 Rapid sensing of explosives and other hazardous chemicals is important in the view of safety and environmental issues and has been carried out using luminescent MOFs.7,8 In particular, nitrobenzene (NB) was selectively detected via the fluorescence quenching mechanism of MOFs associated with interactions between NB and the frameworks.9,10 When long organic spacers are incorporated in the construction of coordination networks, an interpenetrated structure can result from the accommodation of more than two nets simultaneously.11,12 This fascinating topology is obtained because of its stabilization by the filling of a large space of a single net with another net through noncovalent interactions.11 Despite the tight entanglement of the nets, the interpenetrated architectures can become dynamic upon solvent exchange. Hence, it is essential to further explore the structural reversibility and flexibility to gain insight into the intrinsic nature of interpenetrated MOFs. The structures of robust MOFs tend to be retained even during the solvation−desolvation process. Unlike rigid solids, soft coordination frameworks may undergo substantial changes with respect to ligand conformations and coordination geometries around metal centers when guest molecules are present or absent in the framework.13−16 The solvent-induced dynamic structural transformations in dynamic MOFs primarily © 2014 American Chemical Society

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EXPERIMENTAL SECTION

Reagents. 3,3′-Dimethoxybiphenyl-4,4′-dicarboxylic acid (H2L) was prepared according to the literature procedure.26 All the other chemicals and solvents in the synthesis were of reagent grade and used as received. All manipulations were performed under aerobic conditions. Received: January 9, 2014 Revised: February 20, 2014 Published: February 21, 2014 1933

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Figure 1. (a) Structure of the secondary building unit of [Zn4O(L)6(DMF)2] for 1. (b) View of 3-fold interpenetrating nets in the ac plane. (c) Schematic representation of the simplified nets. (d) Side view of the nets in the bc plane. [Zn4O(L)3(DMF)2]·0.5DMF·H2O (1). Zn(NO3)2·6H2O (10 mg, 0.034 mmol) and H2L (4 mg, 0.013 mmol) were put in a glass tube and dissolved in DMF (0.5 mL). The sealed tube was placed in a preheated oven (100 °C) and reacted for 2 days. Colorless plate crystals were formed, which were washed with DMF and dried in air. Yield: 64%. Anal. Calcd for C55.5H55.5N2.5O22.5Zn4: C, 48.33; H, 4.06; N, 2.54. Found: C, 48.04; H, 4.18; N 2.97. Reversible Phase Transformation between 1 and 1a upon MeOH Exchange and DMF Resolvation. Complex 1 was immersed in MeOH at room temperature for 3 days, and the resulting solid was washed with MeOH and air-dried to give the MeOH exchanged sample (1a). Complex 1a was soaked in DMF at room temperature for 3 d, and the resulting solid was washed with DMF and air-dried to produce the phase of 1. These processes were checked by PXRD, IR, and TG. Anal. Calcd for C48.1H49.8O25.8Zn4 (1a; [Zn4O(L)3(DMF)2]· 0.1MeOH·6.7H2O): C, 44.36; H, 3.85. Found: C, 44.00; H, 3.76. Phase Transformation from 1a to 1b upon Activation. Complex 1a was desolvated in vacuum at 200 °C for 2 h to afford the activated sample (1b). This process was checked by PXRD, IR, and TG. Anal. Calcd for C48H36O19Zn4 (1b; [Zn4O(L)3]): C, 48.93; H, 3.08. Found: C, 48.84; H, 3.04. Reversible Phase Transformation between 1b and 1 upon DMF Solvation and Activation. Complex 1 was desolvated in vacuum at 200 °C for 2 h to afford the activated sample (1b). Complex 1b was immersed in DMF at room temperature for 3 d to yield the DMF solvated sample (1). These processes were checked by PXRD, IR, and TG. [Cu2L2(DMF)2]·6H2O (Cu Phase). This compound was synthesized under solvothermal conditions in DMF/H2O (7:1, v/v) according to the literature.26 Phase Transformation from Zn Phase (1) to Cu Phase. Complex 1 (40 mg, 0.029 mmol) was immersed in a 20 mL of DMF/ H2O (1:7, v/v) solution of Cu(NO3)2·3H2O (193 mg, 0.8 mmol) and sonicated for 1 min. The resulting solution was placed in a preheated oven at 80 °C and reacted under solvothermal conditions for 3 h, 12 h, 1 d, and 3 d. The solids were washed with DMF/H2O (7:1) several times. The soaking process was repeated 3 times for 3 d to obtain the samples. The color and PXRD pattern of each sample were checked to identify the phase of the solid, and the Cu phase started to appear at 12 h.

Phase Transformation from Cu Phase to Zn Phase. The Cu phase (40 mg, 0.081 mmol) was immersed in a 20 mL of DMF solution of Zn(NO3)2·6H2O (238 mg, 0.8 mmol) and sonicated for 1 min. The solution was put in a preheated oven at 100 °C and reacted under solvothermal conditions for 3 h, 1 d, 3 d, and 6 d. The solid was washed with DMF several times. The soaking process was repeated 3 times for 3 d to give the solids. The color and PXRD pattern of each sample were checked to identify the phase of the solid, and the Zn phase started to appear at 3 d. Physical Measurements. Elemental analyses for C, H, and N were performed at the Elemental Analysis Service Center of Sogang University. Infrared spectra were obtained from KBr pellets with a Bomen MB-104 spectrometer. Thermogravimetric analyses were carried out at a ramp rate of 10 °C/min in a N2 flow using a Scinco TGA N-1000 instrument. PXRD data were recorded using Cu Kα (λ = 1.5406 Å) on a Rigaku Ultima III diffractometer with a scan speed of 2°/min and a step size of 0.01°. Photoluminescence was measured with a Hitachi F-7000 FL spectrophotometer. The ICP data were collected on ICP-AES (ICP-OES; JY Ultima2C; Jobin Yvon, France) spectrometer in Korea Basic Science and Institute. Crystallographic Structure Determination. X-ray data for 1 were collected on a Bruker SMART APEXII diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Preliminary orientation matrix and cell parameters were determined from three sets of ω/ϕ scans at different starting angles. Data frames were obtained at scan intervals of 0.5° with an exposure time of 30 s per frame. The reflection data were corrected for Lorentz and polarization factors. Absorption corrections were carried out using SADABS. The structure of 1 was solved by direct methods and refined by full-matrix least-squares analysis using anisotropic thermal parameters for non-hydrogen atoms with the SHELXTL program. Some carbon atoms (C46, C47, and C53) were disordered over two sites with occupancy of 0.5 and were isotropically refined. All hydrogen atoms except for hydrogens bound to the disordered atoms were calculated at idealized positions and refined with the riding models. Crystal data of 1: empirical formula = C54H50N2O21Zn4, Mr = 1324.44, monoclinic, space group Cc, a = 17.0836(11) Å, b = 16.9192(11) Å, c = 21.0915(14) Å, β = 110.954(4)°, V = 5693.1(6) Å3, Z = 4, Dcalc = 1.545 g cm−3, μ = 1.743 mm−1, 23187 reflections collected, 12003 unique (Rint = 0.0439), R1 = 0.0543, wR2 = 0.0912 [I > 2σ(I)]. 1934

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RESULTS AND DISCUSSION

Crystal Structure. A solvothermal reaction of Zn2+ and H2L in DMF at 100 °C produced colorless plate crystals of 1. The secondary building unit (SBU) consists of four Zn atoms with the formula [Zn4O(R-CO2)6(DMF)2], coordinated by one central oxygen atom, six carboxylate ligands from L2−, and two DMF molecules (Figure 1a and Figure S1, Supporting Information). The structure of SBU in 1 is similar to that in MOF-5 with the formula [Zn4O(R-CO2)6] where each Zn atom is tetrahedrally surrounded by six carboxylates of terephthalates.27 A distinct feature of the SBU in 1 is the two coordination environments around the central Zn atoms; three Zn centers adopt tetrahedral geometries, and the other Zn atom possesses an octahedral geometry. The Zn center with octahedral geometry is additionally bound by two DMF molecules in the cis mode. To assess the framework structure, the organic linker of L2− is represented by a stick and the Zn4O cluster is represented by a joint. One framework extracted from the entire structure corresponds to a six-connected uninodal bsn net with a point symbol {48.54.63} (Figure S2, Supporting Information).28 The nets are mutually entangled to generate the 3-fold interpenetration (full interpenetration vectors, Class Ia; Figures 1b,c). The side view of the nets shows that a parallelogram shaped ladder is present in the front face, while the other type of ladder is located in the back face (Figure 1d). These ladders are linked together via the organic linker to form the bsn topology (Figure S3, Supporting Information). This 3D structure can be compared with a Zn(II) complex bridged by L2− in which the bridging ligand acts as bis(bridging bidentate) syn−anti modes to construct a 2D coordination sheet with ZnO4 SBU, clearly different from that of 1.26 Other frameworks using L2− and its analogue were demonstrated, and they include Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and lanthanides as metal nodes.29−32 Structural Flexibility. In the thermogravimetric (TG) curve of 1, the weight loss of 13.9% in the temperature range 30−243 °C is consistent with the removal (14.0%) of 2.5DMF and 0.5H2O (Figure S4, Supporting Information). An abrupt loss is evident above 355 °C, indicating the thermal decomposition of the framework. To check the structural flexibility, the as-synthesized sample of 1 was soaked in MeOH for 3 days at room temperature. The powder X-ray diffraction (PXRD) pattern of the MeOH-exchanged phase (1a) is different from that of the as-synthesized phase, suggesting the formation of a new phase in which the CO stretching vibration at 1657 cm−1, characteristic of a DMF molecule, was absent (Figure 2 and Figure S5, Supporting Information). The original phase (1) was regenerated by immersing 1a in DMF for 3 days, which was confirmed by PXRD. The presence of DMF in the resolvated phase of 1 was identified by the advent of the CO peak in the Nujol infrared (IR) spectrum. Further, evacuation of 1a at 200 °C for 2 h afforded the activated sample of 1b. The TG curve of 1b corroborated the absence of DMF molecules, which was also supported by IR spectroscopy (Figures S5 and S6, Supporting Information). Meanwhile, the activated phase was obtained via another route; 1 was heated under vacuum at 200 °C for 2 h to produce 1b. Phase 1b was immersed in DMF for 3 days, and the resolvated phase (1) was eventually obtained, as confirmed by PXRD and IR spectroscopy. These observations indicate that the extensive phase transformations occurring among the as-synthesized, MeOHexchanged, and activated forms arise reversibly via two

Figure 2. (top) Schematic representation of the reversible structural flexibility among 1, MeOH exchanged, and activated samples. (bottom) PXRD patterns of the transformed samples.

pathways: solvent exchange/resolvation and activation/resolvation. It is worth noting that the framework 1 is truly flexible and reversible upon external inputs, which could be considered as a unique intrinsic feature of the interpenetrating nets with dynamic spaces.33 As shown in N2 isotherms at 77 K, 1b is nonporous (Figure S7, Supporting Information). Photoluminesence Properties. The photoluminescence spectra of the organic ligand H2L and 1 were acquired in DMF (Figure S8, Supporting Information). The broad band at 380 nm observed in the case of 1 can be attributed to the ligandbased emission, since the ligand shows a peak at 385 nm similar to the peak position of 1.34 To probe the sensing properties of 1, we recorded the emission data in different solvents such as acetonitrile, chloroform, ethyl acetate, ether, ethanol, dichloromethane, methanol, and nitrobenzene (NB).35−37 The emission bands were visible at similar positions in most solvents used, while in NB the luminescence signal completely disappeared (Figure 3). Although NB has been industrially used

Figure 3. Photoluminescence spectra of 1 in different solvents (excited at 280 nm). 1935

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the Zn phase was inspected by adding the Cu phase to excess Zn2+ ions under the same solvothermal conditions used in the synthesis of the Zn phase. The Zn phase emerged after 3 days together with the ZnO phase,43 suggesting that the 2D sheet phase transforms to the 3-fold interpenetrating 3D framework (Figure 4b). Notably, such concomitant conversion of dimensionality and interpenetration degree constitutes a unique example in MOFs, although the process occurs via dissolution and recrystallization under solvothermal conditions.

in the fabrication of plastics and pesticides, it is essential to detect NB because of its carcinogenic and toxic nature. The observation of the specific quenching effect of 1 solely in NB indicates that the compound serves as a potential luminescent sensor in detecting traces of the target species. The energy transfer from the MOF to the electron-deficient nitrobenzene through interspecies contacts occurs efficiently to quench the system.9,38,39 When NB was added to 1 in MeOH, the peak intensity around 380 nm was reduced with increasing concentration of NB (Figure S9, Supporting Information).10 The quenching efficiency, defined by (I0 − I)/I0 × 100%, where I0 and I are the luminescence intensities of 1 before and after the addition of NB, respectively, was estimated to be 20.1% for 10 ppm of NB, 91.1% for 50 ppm of NB, and 99.9% for 5000 ppm of NB. This efficiency is comparable to that of other MOFs in sensing NB.40,41 The detection ability of 1 could be restored by washing the sample with MeOH several times and by immersing it in MeOH for a few minutes (Figures S10 and S11, Supporting Information).42 The quenching−recovery cycles were tested 5 times without sizable intensity decay, indicating that this framework retains its stability and reversibility during the recycling process. Phase Transformation. The 2D sheet [Cu2L2(DMF)2]· 6H2O (Cu phase) was prepared by a solvothermal reaction of Cu(NO3)2·3H2O and H2L in DMF/H2O at 80 °C.26 This structure is quite different from 1 (Zn phase) with the 3-fold interpenetration. To examine the mutual transformation between Zn and Cu phases, the Zn phase was immersed in a 0.04 M Cu2+ solution and treated under reaction conditions identical to that used for the synthesis of the Cu phase. The color of the samples changed from pale yellow to green with increase in the reaction time. The PXRD profiles, which were acquired after 3 h, 12 h, 1 d, and 3 d, showed that the peaks for the Cu phase began to appear after 12 h (Figure 4a). An inductively coupled plasma (ICP) analysis was conducted to determine the relative metal contents of Zn and Cu during the phase transformation process (Figure S12, Supporting Information). Zn remained even after the solvothermal reaction for 3 days. The reverse transformation from the Cu phase to

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CONCLUSIONS In summary, we have prepared a 3D Zn(II) framework with 3fold interpenetration, which exhibits selective NB sensing with recyclability. The structure of the net is highly flexible because conversions among the three phases occur upon solvent exchange/resolvation and activation/resolvation. The 3D Zn phase with 3-fold interpenetration degree uniquely transformed to the 2D Cu phase without entanglements.

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic files in CIF format and additional structural data for the complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (The Ministry of Science, ICT & Future Planning (MSIP); NRF2013M1A8A1035849), by Basic Science Research Program (NRF-2012R1A1A2007141), and by the Priority Research Centers Program (NRF20100020209).

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REFERENCES

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Figure 4. (a) PXRD profiles of the as-synthesized Zn sample (1), samples after 3 h, 12 h, 1 d, and 3 d, and as-synthesized Cu sample. (b) PXRD profiles of the as-synthesized Cu sample, samples after 3 h, 1 d, 3 d, and 6 d, and the Zn sample. The asterisk denotes the ZnO phase. (c) Color changes observed during the transformations between Zn and Cu phases. 1936

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