Communication pubs.acs.org/IC
Molecular Scissoring: Facile 3D to 2D Conversion of Lanthanide Metal Organic Frameworks Via Solvent Exfoliation Hong Sheng Quah, Li Ting Ng, Bruno Donnadieu, Geok Kheng Tan, and Jagadese J. Vittal* Department of Chemistry, National University of Singapore, 117543, Singapore S Supporting Information *
[Ln(ADC)1.5(DMA)3] occurs during the solvent exchange of dimethylformamide (DMF) by dimethylacetamide (DMA), and this is accompanied by the formation of nanoplatelets as verified by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). Interestingly, the 2D CPs were not converted back to the 3D structure, despite several attempts. Finally, upconversion fluorescence by exciting the MOFs at near-infrared wavelengths (800 nm) was demonstrated. Five lanthanide MOFs, [Ln2(ADC)3(DMF)4]·DMF (1−5), have been synthesized by solvothermal reactions of the Ln(NO3)3 and H2ADC in a ratio of 1:6 in a mixture of DMF and ethanol at 120 °C for 2 days. The solid state structures of 1−5 are isostructural and crystallized in the triclinic space group P1̅ with Z = 1. A crystallographic inversion center is present in the dimer. Each Ln(III) in the dimer is coordinated to two DMF ligands and chelated by one ADC ligand. Further the Ln(III) atoms in the dimer are bridged by four ADC ligands. Of these, two are bridging in a μ2-fashion and two in a μ2-κ2:κ1 fashion (Figure 1a). In the Ln2(ADC)3(DMF)4 repeating unit, each Ln(III) has a LnO9 core. The connectivity of the six-connected octahedral nodes with the ADC spacer ligand generates pcu topology (Figure 1b,c). PLATON calculations33 show that the total potential solvent accessible void volume is in the range 41.0−41.7% for the structure without DMF. The structures have a solvent accessible void of 9.8−10.4% when only the lattice solvent was removed. The lanthanide CPs [Ln(ADC)1.5(DMA)3] (Ln = Nd (6), Gd (7), Pr (8), Ce (9) and Sm (10)) were obtained when the crystallization solvent was replaced with DMA. These isostructural compounds crystallized in the trigonal space group R3̅ with Z = 6. The asymmetric unit contains one-third of the formula unit. The nine-coordinate Ln(III) is on the crystallographic three-fold rotation axis with three ADC ligands chelating the Ln(III) and three coordinated DMF molecules in a cofacial manner (Figure 1d). The ADC ligand has a crystallographic center of inversion. Connectivity with the second carboxylate group in the adc produced the 2D honeycomb structure with hcb topology (Figures 1e,f). The DMF ligands in the alternate Ln(III) atoms are facing one side of the layer. These facial DMA ligands are facing each other in the alternate sheets around the crystallographic −3 center. In other words, the void created by the corrugated hexagonal nets are filled completely by the ABAB stacking. In fact, the total potential solvent accessible volume is only ∼1.2%. Hence, it is
ABSTRACT: Five lanthanide MOFs with pcu topology have been exfoliated into nanoplatelets of two-dimensional structures via sonication in the dimethylacetamide solvent. These nanosheets are fluorescent under two-photon excitation dominated by the ligand, indicating energy upconversion ability.
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wo-dimensional (2D) layered nanosheets have been largely studied due to their distinct electronic, mechanical, and thermal properties and increased surface areas as compared to the bulk.1−4 Among these, graphene, MoS2, and BN have been widely studied for various applications, including energy storage, optoelectronics, and sensors.5−9 Of late, highly crystalline covalent organic frameworks (COFs) have generated interest as they are robust and tailorable by organic syntheses.10 As the metal−organic hybrid material, metal−organic frameworks (MOFs), also known as porous coordination polymers (PCPs), can form superstructures containing voids, it can be exploited for gas storage and separation, catalysis, sensing, ionic conductivity, and drug delivery.11 Crystalline MOFs consisting of a single or multiple layers (nanoMOFs) are also sought after, similar to 2D materials, due to their unusual and differing properties when compared to their bulk.12−18 This may make it easier to peel off the individual CP layers of the noninterpenetrated 2D CPs when the adjacent layers weakly interact with each other. Nonetheless, not much success has been achieved so far.19−21 On the contrary, mechanically exfoliating the materials and downsizing MOFs by microwave and sonication methods provide a facile way to obtain processable 2D MOFs.22−25 Apart from the usual methods to synthesize CPs/MOFs with desired structure and functionality, they can be accessed through the structural conversions from other CPs, MOFs, or metal complexes in either solution or solid state with the help of external stimuli.26−28 Additionally, Farha et al. have made a wide range of MOFs using the building block replacement (BBR) approach.29,30 Usually higher dimensional CPs have been obtained in this way, and structural transformation 3D to 2D CPs are not commonly encountered.26−30 In this work, five 2D CPs and five 3D MOFs with lanthanide metal ions have been synthesized with anthracene dicarboxylate ligand (ADC) as a spacer ligand. Previously, this ligand was used to construct several lanthanide coordination polymers for magnetic studies.31,32 Structural transformation of the 3D MOFs [Ln2(ADC)3(DMF)4]·DMF (Ln = Nd (1),32 Gd (2),32 Pr (3),32 Ce (4), and Sm (5)) with pcu topology to 2D CPs © XXXX American Chemical Society
Received: September 13, 2016
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DOI: 10.1021/acs.inorgchem.6b02222 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Crystallographic packing of the lanthanides MOFs and CPs. (a) Repeating unit, (b) packing, and (c) topology of the 3D Ln-MOFs 1−5. (d) Building unit, (e) packing, and (f) topological representation of the 2D CPs 6−10.
Figure 2. SEM images of CP 6−10 crystals obtained from 1−5 by microwave synthesis. (a) 6, (b) 7, (c) 8, (d) 9, and (e) 10. (f) SEM image of 3 after soaking the crystal in DMA for 5 days. (g) Exfoliated CPs 6−10 (left to right) dispersed in DMA after a week of standing.
not surprising that there is no lattice solvent present in these compounds. Since by adjusting the solvent during the reactions controls the dimensionality of the MOF that forms, MOF 1−5 were subjected to sonication in DMA. Their PXRD patterns (Figure S5) indicated a structural transformation of the 3D to 2D framework. In addition, MOF 1−5 compounds can be synthesized in 5 min as microcrystals using microwave synthesis with the addition of octanol in the above synthesis. Although the crystallites do not have uniform shape and size, their sizes are in the range 1−10 μm. The PXRD patterns of the microcrystals were matched with the simulated patterns from the single crystal data to confirm their formation and purity. Despite changing from ethanol to octanol, it has no effect on the dimensionality of the MOF that forms. Presumably, the more sterically bulky DMA allowed the formation of a lower dimensionality as compared to DMF. The microcrystals of the MOFs 1−5 were used for solvent assisted exfoliation by soaking, microwave, sonication, and solvothermal methods (see SI for procedure). The PXRD patterns of the products obtained from these experiments (see Figure S6) indicate that they have been transformed to the 2D MOFs. The PXRD patterns did not have any traces of the peaks corresponding to the 3D MOFs 1−5, suggesting complete conversion. The SEM images in Figure 2 show the nanoplatelets of the crystals. The crystals were observed to be cuboidal before exfoliation (see Figure S11). After exfoliation, the crystals have been broken down from micro- to submicron size, and their morphologies were altered as well. The disc-like platelets are approximately 40−100 nm in width. Their thicknesses were estimated to be under 100 nm from some particles vertically arranged in the SEM images, and they are able to remain dispersed in DMA solution after standing for a week (Figure 2g).
Fortunately, the SEM image of a single crystal of 3 soaked in DMA for 5 days, although broken, remained together (Figure 2f). The fault lines were clearly seen formed in parallel in the crystal. It is proposed that physical exfoliation could have occurred directionally via the insertion of the DMA molecules uniformly into the channels of the 3D framework via concentration gradient-based diffusion. The DMA molecules then proceed to chemically displace DMF solvent, which is relatively loosely coordinating to the metal center. This replacement produces a scissoring effect accompanied by a change in coordination sphere resulting in the formation of the 2D CPs as depicted in Scheme 1. Potentially, the replaced Scheme 1. Proposed Solvent Mediated Structural Conversion of 3D MOFs to 2D Sheets with Nanoplatelets
DMF molecules remaining in the solution can cause the transformation to be reversible. Despite sonicating CPs 6−10 for 10 h in pure DMF solvent, no conversion was observed as followed from the PXRD patterns (see Figure S7). The DMA used for exfoliation of the compounds was checked after the reaction using 1H NMR spectroscopy and electrospray ionization mass spectroscopy (see Figures S13−S22). The B
DOI: 10.1021/acs.inorgchem.6b02222 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry NMR spectrum did not indicate the presence of an ADC ligand, while the mass spectrum indicated a minute concentration of 10−6 M of the ligand present. These results suggested the possibility of ligand leaching during the transformation which could not entirely be attributed to a solution state transformation. More appropriately, the transformation can be described as a solid−liquid interfacial transformation. It should be noted that 3D to 2D structural transformations are not very common,26−28 and here this is accompanied by downsizing of the crystals also. The single photon emission profiles of 1−10 are analogous to that of H2ADC when excited at 350 nm, suggesting that the emission is derived from similar excitation states. No obvious lanthanide emission is seen, suggesting the fastest and lowest energy of the decay path is ligand-centered and lanthanide emission is quenched. Although the ligand could act as an antenna to sensitize lanthanide emission, the presence of a coordinating solvent molecule provided a possible decay pathway for de-excitation to occur. This is mainly due to the proximity of C−H and C−O bonds near the lanthanide center, which provides a vibrational decay pathway.34 The profile of the free acid H2ADC, in chloroform solution, exhibits vibrational features with λmax at 401, 421, and 444 nm (Figure S23). Its solid state emission is distinctly different and shows a featureless broad emission peaked at 518 nm. The red-shifted spectrum is due to the excimer emission formed when molecules are packed in high concentration.35−37 MOFs 1−5 and CPs 6−10 exhibit luminescent emissions at approximately λmax = 436 nm and λmax = 426 nm, respectively, in the solid state. By spacing the ligands apart when locking them into ordered structures, aggregation effects such as excimer formation are reduced.38 Hence, the emission maxima of the MOFs shared similarity with the solution photoluminescence of H2ADC. The as prepared crystals 1−10 are two photon active under the excitation of a pulsed laser at 800 nm. The colors of the confocal images are arbitrarily represented, and the optical images are displayed in Figure 3. Although the MOFs and CPs are fluorescing under two photon excitation, the quantum efficiency of a single crystal is too low for the detector to be quantitatively accurate with the emission spectra via the confocal setup. An attempt to increase the laser power to get better spectrum resulted in the decomposition of the solids. The emission spectra shown resemble the single photon luminescence, which is supplied in the Supporting Information (see Figure S10). In summary, a total of 10 Ln-MOFs and Ln-CPs have been synthesized with ADC as the organic linker. 3D MOFs were obtained when DMF solvent was used for crystallization, while 2D CPs were obtained when DMA was used. By soaking, sonicating, or heating under solvothermal or microwave conditions with DMA, the 3D MOFs were structurally transformed to 2D CPs of nanoplatelets with the thickness estimated to be under 100 nm. Though weak, these Ln-MOFs are potential upconverting solid state materials that will be relatively resistant to dissolution and photobleaching as compared to solution dyes. Their facile exfoliation into dispersible solution enhanced the proccessibility of these MOF materials.
Figure 3. Crystals 1−10 showing photoluminescence under two photon excitation at wavelength 800 nm.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02222. General characterization procedure, synthesis procedure, Tables S1−S3 and Figures S1−S33 (PDF) Accession Codes
Crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif, CCDC 1487983−1487992 correspond to 1−10.
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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 We thank the Ministry of Education, Singapore, for financial support through NUS FRC Grant No. R-143-000-604-112. H.S.Q. sincerely acknowledges his NGS scholarship. C
DOI: 10.1021/acs.inorgchem.6b02222 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b02222 Inorg. Chem. XXXX, XXX, XXX−XXX
