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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02511 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018
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Chemistry of Materials
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, Qiaowei Li* Department of Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and Shang‐ hai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China 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 complicate 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 num‐ ber 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.
INTRODUCTION Metal–organic frameworks (MOFs)1‐5, in which exten‐ sive diversity of structure design can be achieved, have attracted intensive attention due to their applications in gas storage6, separation7,8, and catalysis9. In typical pil‐ lared‐layer MOF structures10‐16, 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 ligands 17‐ 20 , as well as directional and flexible responses21‐25 to ex‐ ternal stimuli. On the other hand, MOFs with long pillars tend to deform or “collapse” during desolvation, as indi‐ cated by much lower surface areas compared to their the‐ oretical values based on the single crystal structures26. Initiation of the collapse was frequently described as the coordination bond dissociation between the metal and the pillar27; however, in‐depth studies on the detailed lo‐ cal structure change upon framework activation are scarce in the literatures14, 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 opportu‐ nities in comprehending defect chemistry of MOFs for better designing active sites and exploring these MOFs for catalysis. Herein, an interpenetrated pillared‐layer MOF, named FDM‐22, was constructed with zinc dicarboxylates as
Figure 1. Illustration of structure transformation of a pillared‐ layer MOF. The pristine network (a) contracts with the pillars tilting (b), then point defects due to joint breakage can be observed (c). The complete structure transformation ends with maximum number of defects achieved (d). 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
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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.
Figure 3. (a) PXRD of FDM‐22, FDM‐22S, FDM‐22A, and FDM‐22V, 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.
FDM‐22 significantly. Interestingly, this transformation was accompanied by the crystal morphology transition from intact to distorted and creviced, as well as the for‐ mation of more mesopores. The point defects in the crys‐ tals associated with the zinc–nitrogen bond breaking were clearly evident in specific vibrational peaks intensity in‐ crease in Raman spectra. In addition, when severe activa‐ tion method was applied, the directional arrangement of weakly coordinating pillars has produced single crystals with four wavy facets and two relatively undisturbed fac‐ ets (Figure 1d), corresponding to high concentration of point defects on the pillar–layer joints. The obtained po‐ rous 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. 4‐coordinated 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 concen‐ tration.
RESULTS AND DISCUSSION
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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 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 fur‐ therly linked together using BP4VA as pillars, forming the pillared‐layer MOF structure (Figure 2 and Table S1, See the Supporting Information (SI)). The square grids have an edge length of 15.2 Å, and the layers are separated by 22.9 Å each other by the slender BP4VA. It is not surpris‐ ing that FDM‐22 is doubly interpenetrated, with the SBUs from the second framework sit in the body center of the first framework. This two‐fold 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 × 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 pillared‐layer MOF construction29,30. In addition, the vinylene groups provide more flexibility in the confor‐ mations 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 SI), the SXRD showed severe diffraction spots overlapping indicating multi‐domains (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 transfor‐ mation of the pillared‐layer MOF upon activation, we applied three different solvent removal methods on FDM‐ 22 after they were solvent exchanged with acetone: Super‐ critical CO2 drying (FDM‐22S), acetone evaporation in the Ambient air (FDM‐22A), and dynamic Vacuum at 115 °C (FDM‐22V) (See the SI). All three guest‐free samples show different powder X‐ray diffraction (PXRD) patterns com‐ pared to that of the as‐synthesized FDM‐22 (Phase I), indicating structure change upon solvent removal (Fig‐ ures 3a and S3). Specifically, FDM‐22S and FDM‐22A have resulted 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 (22ī) crystal planes of FDM‐22 disappear within the first 20 minutes, while two new prominent peaks from FDM‐22A rise gradually along the exposure time. Interestingly, the phase transformation also accompanies with the crystal color change from yellow to orange (Figure S8), which we will discuss later. Moreover, dynamic vacuum with
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Chemistry of Materials
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.
heating, has given rise to another completely new struc‐ ture FDM‐22V (Phase III). For all three activated samples, their phases remain unchanged after dry heating for 2days at 100 °C (Figure 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 param‐ eters using PXRD data not successful. Benefiting from the luminescent BP4VA, tracking the MOF’s luminescence change during structure transfor‐ mation 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 light28, 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 Zn32. 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, simi‐ lar 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 between two π‐systems changes the π–π interaction, and thus shifts the
luminescence of the solids. Upon activation, it is very pos‐ sible 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 laterally slide of the layers and vertically tilt of the pillars21,33, which cause the PXRD change and impact the interaction be‐ tween BP4VA and BPDC more significantly. However, the FDM‐22S and FDM‐22A with the same XRD pattern (Phase II) show different luminescence, sug‐ gesting that additional contribution besides identical phase transition must be taken into account. Under cer‐ tain severe solvent removal process, the moderate metal‐ pyridine coordination bonds are more inclined to be af‐ fected34,35. 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 FDM‐22S, which allow more dangling pillars to move with some degree of freedom and to form closer π–π interaction with surrounding ligands, thus luminescence red shift was ob‐ served. In the case of FDM‐22V, following the more severe dynamic vacuum with heating, more dissociation reac‐ tions result in 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 significant amount of point defects due to bond dissocia‐ tions have changed the luminescence remarkably, but still not enough to make the long‐range structure change happen in the crystals.
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Figure 5. (a) Structure of FDM‐22V. (b) Structure of MOF‐508b33. (c) XANES of FDM‐22V and MOF‐508b. (d) EXAFS of FDM‐ 22V and MOF‐508b.
PXRD and luminescence spectroscopy support our pro‐ posal on the macroscopic structure transformation caused by microscopic point defects due to bond dissociations in FDM‐22. In addition, scanning electronic microscopy (SEM) could provide more direct information on the crys‐ tal 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 pre‐existing grain boundaries in FDM‐22 crystals. In‐ terestingly, many parallel crevices with 0.3–1.0 μm in width were observed on four facets of each FDM‐22A cu‐ bic crystals (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 ab‐plane of the crystals, where the [Zn2(BPDC)4]∞ lay‐ ers base; and the straight crevices are perpendicular to the direction of the BP4VA pillars. This observation un‐ veils 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 with that in FDM‐22A, we observed significant distortion and shrink‐ age 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 (Fig‐ ure S10). This previously unseen phenomenon further suggests that the structure change is mainly due to the pyridyl containing pillar ligands rather than the carbox‐ ylate‐based ligands. Overall, the degree of crystal mor‐ phology change increases along with the activation condi‐ tion switched from mild supercritical CO2 drying to air drying to harsh dynamic vacuum with heating, in the same trend 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. Due to the ran‐ domly 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 representa‐ tive 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, re‐ spectively. Despite similar profiles at P/P0
