Article Cite This: Chem. Mater. 2018, 30, 5478−5484
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Structure Transformation of a Luminescent Pillared-Layer Metal− Organic Framework Caused by Point Defects Accumulation Yi Qi, Huoshu Xu, Xiaomin Li, Binbin Tu, Qingqing Pang, Xiao Lin, Erlong Ning, and Qiaowei Li* Department of Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China
Chem. Mater. 2018.30:5478-5484. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/12/18. For personal use only.
S Supporting Information *
ABSTRACT: Pillared-layer metal−organic frameworks (MOFs) are often encountered to “collapse” upon external stimuli due to weak interactions between the layers and the pillars. However, the detailed local structural change, especially the accumulation of defects due to intricately disordered bond dissociations, is not clear due to the complicated and dynamic nature of the collapse. We report a luminescent pillared-layer MOF structure, FDM-22, using zinc dicarboxylates as layers and dipyridyl ligands as pillars, in which three different transformed structures were captured along the increasing number of coordination bond dissociations between zinc metals and pyridine linkers. The transformation is triggered by these local point defect formations in the MOF, which further contribute to the modulation of its luminescence property, as well as prominent change in the morphology and pore distribution of the MOF. Evidenced by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), each of the pillar ligands has only one pyridyl group coordinated to a Zn(II) ion eventually, with the other uncoordinated pyridyl group pointing to the pore. With ∼10% of the coordination bonds breaking within the framework, FDM-22 provides a high concentration of active metal sites in the framework.
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INTRODUCTION Metal−organic frameworks (MOFs),1−5 in which extensive diversity of structure design can be achieved, have attracted intensive attention due to their applications in gas storage,6 separation,7,8 and catalysis.9 In typical pillared-layer MOF structures,10−16 strong coordinating groups such as carboxylates connect with metals to build layers, and ligands with weak coordinating N atoms act as pillars. This feature allows anisotropic functionalization of MOFs by systematic modulations on the two ligands17−20 as well as directional and flexible responses21−25 to external stimuli. On the other hand, MOFs with long pillars tend to deform or “collapse” during desolvation, as indicated by much lower surface areas compared to their theoretical values based on the single crystal structures.26 Initiation of the collapse was frequently described as the coordination bond dissociation between the metal and the pillar;27 however, in-depth studies on the detailed local structure change upon framework activation are scarce in the literature14 because these pillared-layer networks tend to lose single crystallinity and transform to unidentified phases. What is more, in a broader context of considering these breaking points as defects (or active sites) in crystals, these collapsed MOFs provide opportunities in comprehending defect chemistry of MOFs for better designing active sites and exploring these MOFs for catalysis. © 2018 American Chemical Society
Herein, an interpenetrated pillared-layer MOF, named FDM-22, was constructed with zinc dicarboxylates as layers and luminescent pyridine ligands as pillars. Structure transformation of this MOF could be achieved by regulating the weak pillars’ variable coordinations, and we applied three different desolvation methods to capture three different stages along the transformation (Figure 1). Upon mild solvent removal process, the pristine network (Figure 1a) starts to contract due to the tilting of the pillars (Figure 1b), and as the pillar tilting accumulates, noticeable structure transformation due to zinc−nitrogen bond dissociation was observed (Figure 1c). The dynamic transformation has red-shifted the luminescence of FDM-22 significantly. Interestingly, this transformation was accompanied by the crystal morphology transition from intact to distorted and creviced as well as the formation of more mesopores. The point defects in the crystals associated with the zinc−nitrogen bond breaking were clearly evident in specific vibrational peaks intensity increase in Raman spectra. In addition, when the severe activation method was applied, the directional arrangement of weakly coordinating pillars has produced single crystals with four wavy facets Received: June 13, 2018 Revised: July 5, 2018 Published: July 6, 2018 5478
DOI: 10.1021/acs.chemmater.8b02511 Chem. Mater. 2018, 30, 5478−5484
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Chemistry of Materials
the layers are separated by 22.9 Å each other by the slender BP4VA. It is not surprising that FDM-22 is doubly interpenetrated, with the SBUs from the second framework sit in the body center of the first framework. This twofold interpenetration has placed the positionally disordered anthracene moieties in BP4VA exactly within the layers of [Zn2(BPDC)4]∞. Despite the interpenetration, FDM-22 still contains ∼3285 Å3 per unit cell accessible void space (62.1% of the unit cell) with open channels of ∼7.1 Å × 5.1 Å along the a-axis. With a distance of 15.9 Å between the two N atoms, BP4VA presents one of the longest pillar ligands ever used in pillaredlayer MOF construction.29,30 In addition, the vinylene groups provide more flexibility in the conformations of BP4VA. We would like to emphasize that these factors have made the synthesis of single crystals with high quality a challenge. Even with better crystals obtained from sophisticated stepwise heating (see the Supporting Information), the SXRD showed severe diffraction spots overlapping indicating multidomains (Figure S1). Indeed, under high-resolution optical microscope, clear intersecting lines along the crystal surfaces were observed in all the pristine crystals examined, indicating grain boundaries between multiple domains inside FDM-22 crystals (Figure S2). Nevertheless, a definite crystal structure with satisfactory refinement was achieved. In order to systematically study the structure transformation of the pillared-layer MOF upon activation, we applied three different solvent removal methods on FDM-22 after they were solvent exchanged with acetone: supercritical CO2 drying (FDM-22S), acetone evaporation in ambient air (FDM-22A), and dynamic vacuum at 115 °C (FDM-22V) (see the Supporting Information). All three guest-free samples show different powder X-ray diffraction (PXRD) patterns compared to that of the as-synthesized FDM-22 (phase I), indicating structure change upon solvent removal (Figure 3a and Figure S3). Specifically, FDM-22S and FDM-22A have resulted in the same PXRD pattern, suggesting the same newly transformed structure (phase II). We used in situ XRD to understand how fast the structure transformation from FDM-22 to FDM-22A happens. As shown in Figure S4, two strong peaks corresponding to the (001) and (221̅) crystal planes of
Figure 1. Illustration of structure transformation of a pillared-layer MOF. The pristine network (a) contracts with the pillars tilting (b) and then point defects due to joint breakage can be observed (c). Complete structure transformation ends with maximum number of defects achieved (d).
and two relatively undisturbed facets (Figure 1d), corresponding to high concentration of point defects on the pillar−layer joints. The obtained porous MOF structure has coordination bond dissociations happening at either end for all the pillars, as indicated by the 1:1 ratio of 5-coordinated Zn(II) vs 4coordinated Zn(II), evidenced by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Overall, with ∼10% of the coordination bonds breaking in FDM-22, it provides a rational design of accessible active sites with high concentration.
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RESULTS AND DISCUSSION The solvothermal reaction of Zn(NO3)2·6H2O, BPDC, and BP4VA28 [BPDC = 1,1′-biphenyl-4,4′-dicarboxylic acid, BP4VA = 9,10-bis((E)-2-(pyridine-4-yl)vinyl)anthracene] affords FDM-22 [Zn2(BPDC)2(BP4VA)]. Single crystal X-ray diffraction (SXRD) reveals that FDM-22 crystallizes in the monoclinic C2/m space group, and Zn-based secondary building units (SBUs)1 in paddle-wheel geometry connect with BPDC to construct square grids. The grids are furtherly linked together using BP4VA as pillars, forming the pillaredlayer MOF structure (Figure 2 and Supporting Information Table S1). The square grids have an edge length of 15.2 Å, and
Figure 3. (a) PXRD of FDM-22, FDM-22S, FDM-22A, and FDM22V, along with the simulated pattern of FDM-22. (b) Luminescence spectra of BP4VA, FDM-22, FDM-22S, FDM-22A, and FDM-22V. (c−e) SEM images of FDM-22S, FDM-22A, and FDM-22V, respectively.
Figure 2. BPDC and BP4VA ligands and the single crystal structure of interpenetrated FDM-22, with two frameworks in ball-and-stick and space-filling drawings, respectively. In the ball-and-stick drawing: C, blue; N, orange; O, red; Zn, yellow. H atoms are omitted for clarity. 5479
DOI: 10.1021/acs.chemmater.8b02511 Chem. Mater. 2018, 30, 5478−5484
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Figure 4. (a) Pore size distributions and 77 K low-pressure N2 adsorption isotherms (inset) of FDM-22S and FDM-22V. (b) Raman spectra of BPDC, BP4VA, FDM-22, FDM-22S, FDM-22A, and FDM-22V. The peaks in blue bands correspond to the vibrations of the pyridyl and vinylene groups in the pillars. (c) XPS spectra of FDM-22S, FDM-22A, and FDM-22V. (d−f) Crystal structure of pristine FDM-22, and artistic illustrations of FDM-22 with limited Zn−N bond dissociations and FDM-22 with half Zn−N bond dissociations.
between two π-systems changes the π−π interaction and thus shifts the luminescence of the solids. Upon activation, it is very possible for the two interpenetrated nets to move relatively and thus to shorten the distance between BP4VA from one net and BPDC from the other net. More importantly, the pillared-layer nature of FDM-22 allows the lateral slide of the layers and vertical tilt of the pillars,21,33 which cause the PXRD change and impact the interaction between BP4VA and BPDC more significantly. However, the FDM-22S and FDM-22A with the same XRD pattern (phase II) show different luminescence, suggesting that an additional contribution besides identical phase transition must be taken into account. Under certain severe solvent removal process, the moderate metal−pyridine coordination bonds are more inclined to be affected.34,35 Therefore, beyond the pre-existing grain boundaries, more point defects may be created in FDM-22A as a result of the Zn−N coordination dissociations between the pillars and layers compared to FDM22S, which allow more dangling pillars to move with some degree of freedom and to form a closer π−π interaction with surrounding ligands; thus, a luminescence red-shift was observed. In the case of FDM-22V, following the more severe dynamic vacuum with heating, more dissociation reactions result in a higher concentration of local point defects, which surpasses the critical concentration of point defects that triggered the transition from phase II to III. In other words, the state of FDM-22A may be the snapshot before the second phase transition threshold is met, in which a significant number of point defects due to bond dissociations have changed the luminescence remarkably, but still not enough to make the long-range structure change happen in the crystals. PXRD and luminescence spectroscopy support our proposal on the macroscopic structure transformation caused by microscopic point defects due to bond dissociations in FDM-22. In addition, scanning electronic microscopy (SEM)
FDM-22 disappear within the first 20 min, while two new prominent peaks from FDM-22A rise gradually along the exposure time. Interestingly, the phase transformation also accompanies the crystal color change from yellow to orange (Figure S8), which we will discuss later. Moreover, dynamic vacuum with heating has given rise to another completely new structure FDM-22V (phase III). For all three activated samples, their phases remain unchanged after dry heating for 2 days at 100 °C (Figures S5−S7), while in the presence of N,N-dimethylacetamide (DMA) solvent (with or without BP4VA), phase III could transform back to phase II after heating. Unfortunately, the qualities of all the three guest-free samples were not satisfactory for SXRD analysis due to the peak broadening. This commonly observed feature26,29−31 makes our attempts to index unit cell parameters using PXRD data not successful. Benefiting from the luminescent BP4VA, tracking the MOF’s luminescence change during structure transformation could be an effective and sensitive probe to look into the local environment around the pillars. BP4VA shows an emission peak at 516 nm under excitation of 365 nm light,28 and when it is employed as the pillars in FDM-22, the light yellow crystals show an emission peak at 544 nm (Figure 3b). This red-shift of 28 nm could be explained by the formation of a larger conjugated π-electron system with extended delocalization environment after pyridyl groups in BP4VA coordinating with Zn.32 After activated with supercritical CO2, FDM-22S has the emission peak at 557 nm, a 13 nm wavelength red-shift compared to FDM-22. What is more, the emission peaks of orange FDM-22A and FDM-22V crystals shift further to larger wavelength, reaching 585 and 597 nm, respectively. In addition, a similar red-shift trend can also be observed for the samples suspended in acetone (Figure S9). The luminescence shift indicates the electronic structure change of the BP4VA pillars. For example, adjusting the distance 5480
DOI: 10.1021/acs.chemmater.8b02511 Chem. Mater. 2018, 30, 5478−5484
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Chemistry of Materials
increase in the vibrations of the pyridyl and vinylene groups in BP4VA following the two steps of phase transitions (the band assignments are listed in Table S2). Obviously, this phenomenon is not due to the desolvation or any distortion and bending in the SBUs and linkers. We use a Zn−bipyridyl MOF structure, MOF-508a,33 as the model compound in the Raman spectroscopy study. Confirmed by single-crystal X-ray diffraction, MOF-508a shows phase transformation to MOF508b with only subtle distortion in the metal coordination geometry upon activation, but without any bond dissociation. No specific peak intensity enhancement or new peak emerging in Raman was observed in MOF-508b (Figure S14). Furthermore, a similar increase in the Raman peak intensities was also reported when specific motifs in biomolecules gain more degrees of freedom after bond dissociation.37 In other words, by activation of FDM-22 (Figure 4d), the Zn−N coordination bond dissociations leave the BP4VA ligands dangling in the framework (Figure 4e), and the dangling ligands provide closer interactions with the framework. Moreover, 1H NMR spectra of the DCl-digested FDM-22S, FDM-22A, and FDM-22V show that the ratios of BPDC and BP4VA remain almost the same compared to the pristine FDM-22, ruling out the possibility of notable amounts of vacancy defects caused by ligands being washed out during the solvent exchange process (Figures S20−S23). For the first time, the pillars’ role which changes from supporting the layers to dangling between the layers was unfolded by continuous monitoring in Raman spectroscopy. The coordination bond dissociations in the FDM-22 could also be demonstrated by probing the coordination environment of Zn with the sensitive XPS. As shown in Figure 4c, deconvolution of the Zn 2p3/2 regions reveals the presence of two distinct Zn 2p3/2 peaks for all three samples. Specifically, the peaks at 1021.8 eV on average correspond to the 5coordinated Zn, binding to one pyridyl N from BP4VA as well as four carboxylate O from BPDC, in the same range as other MOFs with 5-coordinated Zn(II) (Figure S15). On the other hand, the peaks at 1022.5 eV on average point to the 4coordinated Zn, as a result of the bond dissociations between the pyridyl N and Zn. The 0.7 eV shift of Zn 2p3/2 peaks is in accordance with that in similar 5-coordinated Zn-based complexes when the axial Zn−N bond dissociates, such as the shift from 1021.8 to 1022.1 eV when (ammine)(tetraphenylporphyrinato)Zn(II) eliminates the NH3 ligand.38 However, the integration of the two deconvoluted peaks reveals different ratios of Zn with two geometries. The percentages of 4-coordinated Zn are 31%, 43%, and 49% in FDM-22S, FDM-22A, and FDM-22V, respectively, suggesting more and more dangling BP4VA ligands from phase I to phase III. It is suggested that these defects could be partially healed by heating crystals of FDM-22V in DMA solvent along with the phase transformation back to phase II checked by PXRD (Figure S7).17 It is worth mentioning that the ratio between the two geometries is ∼1:1 in FDM-22V (Figure 4f), indicating that statistically half of the Zn−N bonds were dissociated. By looking at this magic number, it is quite tempting to suggest that for each of the BP4VA pillars one end is released from the coordination with Zn and the other end remains bonded with another Zn. With a high concentration of bond dissociations, FDM-22V allows maximum utilization of local dynamic sites (∼19 point defects in every 100 nm3 space throughout the whole crystal) in a definite extended structure.
could provide more direct information on the crystal morphology change upon activation. The SEM images of FDM-22S show the intact cubic crystals with only a few straight lines on the surface after the supercritical CO2 drying (Figure 3c). Notably, the lines may be attributed to the preexisting grain boundaries in FDM-22 crystals. Interestingly, many parallel crevices with 0.3−1.0 μm width were observed on four facets of each FDM-22A cubic crystal (Figure 3d), while the other two parallel facets of the crystals remain intact and smooth. Considering the robustness difference between the layers and pillars in the pillared-layer MOF, it is natural to assign the intact facets as the ab-plane of the crystals, where the [Zn2(BPDC)4]∞ layers base, and the straight crevices are perpendicular to the direction of the BP4VA pillars. This observation unveils that specific MOF structure change on the layer−pillar joints is obvious from SEM, despite the same PXRD for FDM-22S and FDM-22A. Furthermore, for the FDM-22V crystals, on top of the crevices similar to that in FDM-22A, we observed significant distortion and shrinkage of the crystals (Figure 3e). Specifically, the four facets with curved crevices show wavy textures, while the two facets corresponding to ab-planes are less distorted (Figure S10). This previously unseen phenomenon further suggests that the structure change is mainly due to the pyridyl-containing pillar ligands rather than the carboxylate-based ligands. Overall, the degree of crystal morphology change increases along with the activation condition upon switching from mild supercritical CO2 drying to air drying to harsh dynamic vacuum with heating, in the same way as with the luminescence red-shift. To evaluate the porosity of FDM-22 after structure transformation, N2 adsorption isotherms at 77 K were measured on FDM-22S and FDM-22V. Because of the randomly distributed defects introduced upon activation, we found the surface areas were variable among more than five parallel samples for both MOFs (Figures S11 and S12). Nevertheless, distinct isotherm characteristics could be observed for these two sets of samples. Two representative adsorption isotherms are shown in Figure 4a, and the Brunauer−Emmett−Teller (BET) surface areas of FDM-22S and FDM-22V are calculated to be 135 and 112 m2 g−1, respectively. Despite similar profiles at the P/P0 < 0.45 region for both samples, FDM-22S shows a more significant N2 uptake with hysteresis afterward, suggesting more mesopores. The surface area values are far below the theoretical value based on pristine FDM-22 (3201 m2 g−1, see the Supporting Information), indicating evident structure transformation upon solvent removal. The pore size distribution of FDM-22S shows both micropores at ∼1.2 nm and a wide range of mesopores at 3.0−10.0 nm (Figure S13). Local structure collapse due to the Zn−N coordination dissociations was accounted for the mesopore formation. On the other hand, FDM-22V shows a 42% mesopore volume loss compared to FDM-22S (Figure 4a) in the 3.0−10.0 nm range, suggesting mesopores closing induced by the unbearable amount of defects and extensive collapse in the dilapidated frameworks. To further verify our proposal on the point defects accumulation by Zn−N bond dissociations, Raman spectroscopy was used to observe the BP4VA molecule vibrational modes change upon different activation methods.36 As shown in Figure 4b, the activated samples FDM-22S, FDM-22A, and FDM-22V display a significant peak intensity increase at 476, 969, 991, 1203, 1243, 1312, 1341, 1559, and 1636 cm−1 compared to the pristine FDM-22, which corresponds to the 5481
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Figure 5. (a) Structure of FDM-22V. (b) Structure of MOF-508b.33 (c) XANES of FDM-22V and MOF-508b. (d) EXAFS of FDM-22V and MOF-508b.
Table 1. Parameters Refined by Fitting the EXAFS Spectra of FDM-22V andMOF-508ba sample
shell
CN
r/Å
σ2/Å2
ΔE0/eV
R-factor
FDM-22V MOF-508b
Zn−O/N Zn−O/N
4.7 ± 0.3 5
2.00 ± 0.01 2.02 ± 0.01
0.009 ± 0.001 0.008 ± 0.001
4.0 ± 0.7 4.3 ± 0.9
0.0013 0.0015
CN, coordination number; r, distance between absorber and backscatter atoms; σ2, Debye−Waller factor to account for both thermal and structural disorders; ΔE0, inner potential correction; R-factor indicates the goodness of the fit. In the FDM-22V fitting, S02 was fixed to 1.20 as determined from MOF-508b fitting. a
within pillared-layer MOF structure allows us to deliberately design defects in specificity. With tremendous progress occurring in the area of defect engineering of MOFs, thrilling research opportunities will present themselves in terms of using porous pillared-layer MOFs as a platform for incorporating defect sites and applying these active sites into catalysis.
To get direct insights and more evident proof of the local structural change in the MOFs with point defects, FDM-22V (Figure 5a) was analyzed with Zn K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Despite limited accuracy in determining the coordination numbers of Zn(II) by XANES due to its absence of any pre-edge structure, the careful monitoring of relative intensity of the white line can still provide reliable information on the metal coordination environment when a model compound with the same coordination was chosen.39 Structurally robust MOF-508b33 (Figure 5b), in the same coordination fashion as pristine FDM22, was used as the model compound. As shown in Figure 5c, the relative peak height of MOF-508b at the absorption maximum is 1.74, compared to 1.56 for FDM-22V. Considering the coordination number of Zn(II) in MOF508b being 5, which is supported by the single-crystal X-ray diffraction, an average coordination number of less than 5 for Zn(II) in FDM-22V seems reasonable.40 In addition, in the EXAFS in R space (Figure 5d), FDM-22V exhibits a prominent peak at 1.52 Å from Zn−N/O shell with a coordination number of 4.7 ± 0.3 (Figure S16 and Table 1), compared to MOF-508b with a coordination number of 5 (Figure S17), in agreement with the results from XPS. With each radial distribution function fitting well with their respective models (Table 1), the results confirm that partial Zn−N bond dissociations happen after solvent removal in FDM-22, resulting in FDM-22V with high concentration of local defects.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02511. Synthesis and characterizations, SXRD, PXRD, optical and electronic microscopy, adsorption isotherms, Raman spectroscopy, TGA, XAS, and 1H NMR (PDF) Crystallographic data for FDM-22 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (Q.L.). ORCID
Qiaowei Li: 0000-0002-5987-9465 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21571037 and 21733003) and the Science & Technology Commission of Shanghai Municipality (15QA1400400, 16520710100, and 17JC1400100). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.
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CONCLUSION In conclusion, we illustrate the point defects formation caused by coordination bond dissociations in pillared-layer FDM-22 and study how the point defects concentration guides the structure transformation. The point defects accumulation in FDM-22 results in the luminescence shift and pore size distribution change. Without breaking the layers in this MOF, all the pillars could transform from connecting two neighboring layers to hanging to one layer. The anisotropicity
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DOI: 10.1021/acs.chemmater.8b02511 Chem. Mater. 2018, 30, 5478−5484
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Chemistry of Materials Bishydrazone Ligands on the Structure of Zinc(II) Complexes: A Comparative XANES Study. Inorg. Chem. 1999, 38, 38−43.
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DOI: 10.1021/acs.chemmater.8b02511 Chem. Mater. 2018, 30, 5478−5484
