Molecular Scissoring: Facile 3D to 2D Conversion of Lanthanide Metal Organic Frameworks Via Solvent Exfoliation

Article abstract of DOI:10.1021/acs.inorgchem.6b02222

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.

Full text of DOI:10.1021/acs.inorgchem.6b02222

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) (DMA) ] occurs during the solvent exchange of
1.5 3
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.
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 demon-
strated.
wo-dimensional (2D) layered nanosheets have been
Five lanthanide MOFs, [Ln (ADC) (DMF) ]·DMF (1−5),
2
3
4
T
largely studied due to their distinct electronic, mechanical,
and thermal properties and increased surface areas as compared
have been synthesized by solvothermal reactions of the
Ln(NO ) and H ADC in a ratio of 1:6 in a mixture of DMF
3
3
2
1
−4
to the bulk.
Among these, graphene, MoS , and BN have
and ethanol at 120 °C for 2 days. The solid state structures of
2
been widely studied for various applications, including energy
1−5 are isostructural and crystallized in the triclinic space
5
−9
storage, optoelectronics, and sensors.
Of late, highly
1
̅
group P with Z = 1. A crystallographic inversion center is
crystalline covalent organic frameworks (COFs) have generated
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
10
interest as they are robust and tailorable by organic syntheses.
As the metal−organic hybrid material, metal−organic frame-
works (MOFs), also known as porous coordination polymers
ligands. Of these, two are bridging in a μ
-fashion and two in a
-κ :κ fashion (Figure 1a). In the Ln (ADC) (DMF)
repeating unit, each Ln(III) has a LnO core. The connectivity
2
2
1
(PCPs), can form superstructures containing voids, it can be
μ
2
2
3
4
exploited for gas storage and separation, catalysis, sensing, ionic
9
11
conductivity, and drug delivery. Crystalline MOFs consisting
of a single or multiple layers (nanoMOFs) are also sought after,
similar to 2D materials, due to their unusual and differing
of the six-connected octahedral nodes with the ADC spacer
ligand generates pcu topology (Figure 1b,c). PLATON
33
calculations 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.
12−18
properties when compared to their bulk.
This may make it
easier to peel off the individual CP layers of the non-
interpenetrated 2D CPs when the adjacent layers weakly
interact with each other. Nonetheless, not much success has
The lanthanide CPs [Ln(ADC)1.5(DMA) ] (Ln = Nd (6),
3
19−21
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 R3 with Z = 6. The asymmetric unit contains one-third
̅
been achieved so far.
On the contrary, mechanically
exfoliating the materials and downsizing MOFs by microwave
and sonication methods provide a facile way to obtain
22−25
processable 2D MOFs.
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
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
2
6−28
of external stimuli.
Additionally, Farha et al. have made a
wide range of MOFs using the building block replacement
2
9,30
(BBR) approach.
Usually higher dimensional CPs have
been obtained in this way, and structural transformation 3D to
26−30
2D CPs are not commonly encountered.
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
3
1,32
magnetic studies.
Structural transformation of the 3D
32
32
MOFs [Ln (ADC) (DMF) ]·DMF (Ln = Nd (1), Gd (2),
Pr (3), Ce (4), and Sm (5)) with pcu topology to 2D CPs
Received: September 13, 2016
2
3
4
3
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02222
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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
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.
of the 2D CPs 6−10.
not surprising that there is no lattice solvent present in these
compounds.
Fortunately, the SEM image of a single crystal of 3 soaked in
DMA for 5 days, although broken, remained together (Figure
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.
2
f). 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
2
D CPs as depicted in Scheme 1. Potentially, the replaced
Scheme 1. Proposed Solvent Mediated Structural
Conversion of 3D MOFs to 2D Sheets with Nanoplatelets
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
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 H NMR spectroscopy and electrospray
ionization mass spectroscopy (see Figures S13−S22). The
1
2g).
B
DOI: 10.1021/acs.inorgchem.6b02222
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NMR spectrum did not indicate the presence of an ADC
ligand, while the mass spectrum indicated a minute
−6
concentration of 10 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 trans-
formation can be described as a solid−liquid interfacial
transformation. It should be noted that 3D to 2D structural
26−28
transformations are not very common,
and here this is
accompanied by downsizing of the crystals also.
The single photon emission profiles of 1−10 are analogous
to that of H ADC when excited at 350 nm, suggesting that the
2
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,
34
which provides a vibrational decay pathway. The profile of the
free acid H ADC, in chloroform solution, exhibits vibrational
2
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
35−37
molecules are packed in high concentration.
and CPs 6−10 exhibit luminescent emissions at approximately
= 436 nm and λ = 426 nm, respectively, in the solid
MOFs 1−5
λ
max
max
state. By spacing the ligands apart when locking them into
ordered structures, aggregation effects such as excimer
38
formation are reduced. Hence, the emission maxima of the
MOFs shared similarity with the solution photoluminescence of
H ADC.
2
Figure 3. Crystals 1−10 showing photoluminescence under two
photon excitation at wavelength 800 nm.
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
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02222.
■
*
S
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
(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
1
−10.
2
D CPs were obtained when DMA was used. By soaking,
AUTHOR INFORMATION
Corresponding Author
E-mail: chmjjv@nus.edu.sg.
■
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.
*
Notes
The authors declare no competing financial interest.
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
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(
23) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.;
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DOI: 10.1021/acs.inorgchem.6b02222
Inorg. Chem. XXXX, XXX, XXX−XXX