Microwave-assisted solvothermal synthesis and optical properties of tagged MIL-140A metal-organic frameworks

Article abstract of DOI:10.1021/ic4024234

A series of tagged MIL-140A-R frameworks have been synthesized using a microwave-assisted solvothermal method. Compared with their UiO-66-R polymorphs, the absorption energies in the MIL-140A-R series (R = NH₂, NO ₂, Br, Cl, and F) are extended toward the visible region because of the spatial arrangement of the linkers.

Full text of DOI:10.1021/ic4024234

Communication
pubs.acs.org/IC
Microwave-Assisted Solvothermal Synthesis and Optical Properties
of Tagged MIL-140A Metal−Organic Frameworks
Weibin Liang,† Ravichandar Babarao,‡ and Deanna M. D’Alessandro*,†
†School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia
‡Division of Materials Science and Engineering, CSIRO, Private Bag 33, Clayton South MDC, Victoria 3169, Australia
S
* Supporting Information
linkers in MIL-140, which leads to short-range dispersive
interactions that enhance the stability of the framework.6
ABSTRACT: A series of tagged MIL-140A-R frameworks
have been synthesized using a microwave-assisted
solvothermal method. Compared with their UiO-66-R
polymorphs, the absorption energies in the MIL-140A-R
series (R = NH2, NO2, Br, Cl, and F) are extended toward
the visible region because of the spatial arrangement of the
linkers.
In a previous study, we developed a microwave-assisted
solvothermal synthesis method for MIL-140.7 Our present goal is
to assess how functionalization of 1,4-benzendicarboxylic acid
(H2bdc) contributes to and modifies the chemical and physical
properties of MIL-140A ([ZrO(O2CC6H4CO2)]). Five ana-
logues of H2bdc with different functional groups (H2bdc-R; R =
NH2, NO2, Br, Cl, and F) were employed to produce a new series
of tagged MIL-140A systems (see Scheme 1).
etal−organic frameworks (MOFs) are a class of hybrid
Mmaterials formed by the self-assembly of metal ions or
clusters and polydentate bridging ligands.1 Because of the
virtually limitless combinations of metals and ligands, the
physicochemical properties of MOFs can be judiciously tuned
for specific applications. As a result, these materials have shown
promise for a number of diverse applications in areas including
molecular separation, gas sorption, catalysis, and luminescene.2
Recently, significant progress has been made in the UiO
isoreticular MOF series.3 This class of frameworks is
characterized by octahedral Zr6 secondary building units
(SBUs) bound to 12 linear dicarboxylate ligands (1,4-
benzenedicarboxylate in UiO-66, [Zr6O4(OH)4(O2CC6-
H4CO2)12]) to afford a three-dimensional periodic structure in
which each centric octahedral cage is connected to eight corner
tetrahedral cages through triangular windows.3b UiO-66 exhibits
exceptional thermal, chemical, and mechanical stability;4
however, its upper analogues (that is, frameworks with the
same topology but longer organic linkers), such as UiO-67
([Zr6O4(OH)4(O2CC12H8CO2)12]), exhibit comparatively low
stability even under atmospheric conditions.4,5
Serre and co-workers recently reported another series of
zirconium-based MOFs, which are polymorphs of the UiO series,
denoted MIL-140, with the general formula [ZrO(L)] (L = linear
dicarboxylate ligand).6 In MIL-140, instead of Zr6O clusters,
zirconium oxide chains act as SBUs and are oriented along the c
axis. Each cluster is connected to six other chains through the
dicarboxylate linkers. This results in one-dimensional triangular
channels rather than three-dimensional connecting pores in
UiO.6
Scheme 1. Synthetic Routes to MIL-140A and MIL-140A-R (R
= NH2, NO2, Br, Cl, and F)
The reaction conditions employed for synthesis of the MIL-
140A-R series were analogous to those for MIL-140A reported
previously in the literature.7 The microwave synthesis protocol
was selected for its distinct advantages in terms of reduction in
the synthesis time and increase in the energy efficiency.8 In
addition, our previous study indicated that MIL-140A-NH2 was
unattainable using the conventional electric heating method.7
The powder X-ray diffraction (PXRD) patterns obtained for
as-synthesized MIL-140A-R are presented in Figure 1a. The
positions and relative intensities of the peaks for the MIL-140A-R
frameworks were consistent with the as-synthesized MIL-140A
parent material,6 which demonstrates that the tagged MIL-140A-
R frameworks are topologically equivalent to MIL-140A.
Thermogravimetric analysis revealed that all frameworks,
except MIL-140A-NO2, exhibited thermal stabilities up to ca. 450
°C. In addition, the theoretical and experimental weight loss
profiles for MIL-140A and MIL-140A-R (calculated from their
chemical formulas) were in good agreement, except for MIL-
140A-NO2. The relatively low thermal stability (ca. 300 °C) and
disparity in the weight loss profile for MIL-140A-NO2 (54% in
the experiment compared with 61% in theory) are likely to result
from the low crystallinity of the material (broad and low-intensity
The chemical and mechanical stabilities of the MIL-140
frameworks exceed those of the UiO series, especially in the case
of the analogues with longer linkers.5,6 These improvements can
be attributed to (i) the reduced flexibility of the infinite
zirconium oxide chains in MIL-140 compared to the Zr6O
clusters in UiO and (ii) the presence of π-stacking of the aromatic
Received: September 24, 2013
Published: October 29, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
appeared yellow, in contrast to the white color of their
corresponding ligands. The color changes were quantified by
diffuse-reflectance UV−vis analysis (Figure 2). For comparison,
the UiO-66-R analogues were also synthesized according to the
reported methods.9
The UV−vis spectrum of MIL-140A exhibits absorption bands
below 350 nm, similar to the ligand H2bdc itself and its UiO-66
analogue, whereas absorption bands are observed in the spectra
of MIL-140A-NH2 and UiO-66-NH2 up to ∼500 nm (Figure 2).
Garcia et al. reported that electron transfer takes place from the
photoexcited organic linker to the metal oxo cluster within MOF-
5, a phenomenon termed linker-to-cluster charge transfer.10
Herein, the visible-light absorption for MIL-140A-NH2 is likely
to be attributed to the electronic transition from the amine-
containing chromophore to the Zr centers.11
Figure 1. PXRD spectra (a) and SEM images (b) of as-synthesized MIL-
140A (black) and MIL-140A-R [R = NH2 (red), NO2 (blue), Br
(purple), Cl (green), and F (yellow)].
While each of the MIL-140A-R analogues displayed an
absorption band edge at ca. 475 nm, which could be fitted with
a Gaussian peak with a maximum at ∼400 nm, the UiO-66-R
frameworks functionalized with electron-withdrawing groups
(NO2, Br, Cl, and F) and their corresponding dicarboxylate
ligands are transparent between 400 and 600 nm. Considering
the similarity in the chemical compositions of UiO-66-R and
MIL-140A-R (ZrO clusters linked by H2bdc with pendent
functional groups),3b,6 such changes in the optical properties
between these two polymorphs may be rationalized on the basis
of differences in their ligand arrangements. It is well-known that
the optical properties of materials are not only related to the
identities of the component metal centers and ligands but that
they are also dependent on the structural features of the
multidimensional system, which may give rise to intermolecular
energy transfer.2a
In UiO-66-R, the dicarboxylate ligands are spatially separated
in the three-dimensional coordination space. However, in MIL-
140A-R, 50% of the aromatic linkers are aligned by π stacking,6
which results in short-range interactions between the ligands.
Indeed, previous density functional theory (DFT) calculations
have indicated that the stacking interaction in MIL-140A is
comparable to that present in H2bdc crystals.6 In an independent
peaks as evidenced by the PXRD spectra; Figure 1a). A
hydrothermal stability test confirmed that this class of materials
is stable in liquid water at room temperature.
Prior to examining the porosities of the materials, a Soxhlet
washing procedure was performed (N,N′-dimethyformamide
exchange with methanol), followed by pore activation at
moderate temperature (120 °C) in vacuo. All materials were
found to retain moderate porosity (Brunauer−Emmett−Teller
surface areas ranged from 150 to 335 m2·g−1) despite the
presence of different functional groups on the bridging ligands.
The decreases in the surface area (223, 120, 150, 213, and 283
m2·g−1 for R = NH2, NO2, Br, Cl, and F versus 335 m2·g−1 for
MIL-140A) are largely attributed to the reduced free space
available and the increased molecular weights of the function-
alized MOFs.3c
Scanning electron microscopy (SEM) images show that the
functionalized materials appear as platelike crystallites (Figure
1b), consistent with previous results for MIL-140A and MIL-
140A-NH2.7 Interestingly, the resulting crystalline powders
exhibit different colors depending on the functionalization of
the linker (Figure 2). MIL-140A and MIL-140A-NH2 exhibited
the same color as their constituent dicarboxylic ligand (white for
H2bdc and MIL-140A and yellow for H2bdc-NH2 and MIL-
140A-NH2). However, MIL-140A-R (R = NO2, Br, Cl, and F)
́
study, Bredas et al. reported that strong face-to-face interactions
are predicted when the distance between two stilbene dimers is
less than ∼5 Å, resulting in modification of the highest occupied
(HOMO) and lowest unoccupied (LUMO) molecular orbital
levels of the dimer and a corresponding red shift in absorption.12
Considering that the minimum C−C distance of a bdc π-dimer in
MIL-140A is ∼3.8 Å, there is a strong probability for the
occurrence of such a phenomenon in this system.
Photoluminescence (PL) analysis was performed to gain
insight into the origin of the absorption bands in MIL-140A-R.
Several types of PL in MOFs have been classified, including (i)
ligand-based luminescence (particularly from highly conjugated
ligands), (ii) metal-centered emission (widely observed in
lanthanide MOFs), and (iii) charge-transfer luminescence.
Under 405 nm laser excitation at room temperature (at the
maximum of the lowest-energy absorption band in Figure 2), the
MIL-140A-R frameworks (R = NH2, NO2, Br, Cl, and F) exhibit
similar PL spectra (see the SI), which are characterized by
emission peaks at 470 and 503 nm. In general, ligand-based
luminescence is dependent on the chemical nature of the
linkers.13 Given the variety of funtionalized ligands examined in
this study, the fluorescence observed in the MIL-140A-R series is
more likely attributed to linker-to-cluster charge transfer rather
than ligand-based luminescence.
Figure 2. Diffuse-reflectance UV−vis spectra and digital images (insets)
of MIL-140A-R and UiO-66-R with their respective tagged ligands
H2bdc-R (R = H, NH2, NO2, Br, Cl, and F).
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Inorganic Chemistry
Communication
Le Bars, G.; Van de Voorde, B.; Vandichel, M.; Houthoofd, K.; Vimont,
A.; Daturi, M.; Waroquier, M.; Van Speybroeck, V.; Kirschhock, C.; De
Vos, D. E. J. Am. Chem. Soc. 2013, 135 (31), 11465−11468. (f) Wu, H.;
Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.; Yildirim, T.; Zhou,
W. J. Am. Chem. Soc. 2013, 135 (28), 10525−10532.
Considering the differences in the structures of the UiO-66-R
and MIL-140A-R polymorphs provides some insight into the
origins of their differing optical properties. In UiO-66-R, the bdc-
R linkers are spatially isolated, and the LUMO of the bdc-R
ligands (R = NO2, Br, Cl, and F) lies relatively lower in energy
than the Zr6O cluster SBUs. Thus, the energy-transfer pathway
from the ligand to the zirconium-based cluster (either n → π* or
π → π*) is relatively inefficient. Because of π-stacking
interactions between the aromatic linkers in MIL-140A-R, the
HOMO and LUMO levels of the bdc-R ligands are likely to be
modified such that the efficiency of the energy migration from
the lowest excited singlet state of the π-stacked bdc-R linkers to
the zirconium oxide chains is enhanced.
In summary, spectral analysis of a series of tagged MIL-140A-R
frameworks, which have been synthesized using a microwave-
assisted solvothermal method, reveals that the optical response of
MIL-140A-R (R = NO2, Br, Cl, and F) can be tailored toward
absorption in the visible region. In view of the extensive interest
in the photochemical properties of UiO-66-R (R = H, NH2) and
related MIL frameworks, which have been shown to act as
photocatalysts for hydrogen generation and the selective
oxidation of alcohols,14 it is of interest to compare the optical
properties of the less widely studied MIL-140A-R polymorph
with a view toward its potential applications. This study
demonstrates that it is possible to engineer the physical and
chemical properties of frameworks by varying the spatial
arrangement of the ligands. High-level DFT calculations are
currently underway to elucidate the origin of variations in the
optical band gaps of these materials.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional data. 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: deanna@chem.usyd.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Science & Industry Endowment
Fund and the Australian Research Council. The authors are
grateful to Dr. Ellie Kable (Australian Center for Microscopy &
Microanalysis, University of Sydney) for assistance with
fluorescence confocal microscopy.
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