Metal-organic frameworks as hosts for photochromic guest molecules

Article abstract of DOI:10.1021/ic302856b

Several metal-organic framework compounds (MOF-5, MIL-68(Ga), MIL-68(In), MIL-53(Al)) were loaded with azobenzene (AZB), as confirmed by XRPD measurements and elemental analysis. By IR spectroscopy, it was shown that the light-induced trans/cis isomerization of AZB in these hybrid host-guest compounds is improved compared to that of solid AZB. A population of the excited cis state up to 30% has been obtained for AZB_0.66@MIL-68(In). However, no light-induced trans/cis isomerization was observed for AZB_0.5@MIL-53(Al). Structural models obtained from high-resolution synchrotron powder diffraction data show that AZB molecules are densely packed within the channels of MIL-53(Al) so that no trans/cis isomerization can occur. A different situation was observed for AZB in the larger channels of MIL-68(Ga). Thus, this investigation shows the influence of the host material on the switching behavior of the embedded AZB molecules.

Full text of DOI:10.1021/ic302856b

Article
pubs.acs.org/IC
Metal−Organic Frameworks as Hosts for Photochromic Guest
Molecules
D. Hermann,† H. Emerich,‡ R. Lepski,§ D. Schaniel,∥,⊥ and U. Ruschewitz*,†
†Department of Chemistry, University of Cologne, Greinstr. 6, 50939 Cologne, Germany
‡SBNL at the European Synchrotron Radiation Facility, BP 220, 6 rue Horowitz, F-38043 Grenoble, France
§Physics I, University of Cologne, Zulpicher Str. 77, 50939 Cologne, Germany
̈
∥Universite
́
de Lorraine, CRM2, UMR 7036, Vandoeuvre-les-Nancy, F-54506, France
⊥CNRS, CRM2, UMR 7036, Vandoeuvre-les-Nancy, F-54506, France
S
* Supporting Information
ABSTRACT: Several metal−organic framework compounds
(MOF-5, MIL-68(Ga), MIL-68(In), MIL-53(Al)) were loaded
with azobenzene (AZB), as confirmed by XRPD measure-
ments and elemental analysis. By IR spectroscopy, it was
shown that the light-induced trans/cis isomerization of AZB in
these hybrid host−guest compounds is improved compared to
that of solid AZB. A population of the excited cis state up to
30% has been obtained for AZB0.66@MIL-68(In). However, no light-induced trans/cis isomerization was observed for AZB0.5@
MIL-53(Al). Structural models obtained from high-resolution synchrotron powder diffraction data show that AZB molecules are
densely packed within the channels of MIL-53(Al) so that no trans/cis isomerization can occur. A different situation was observed
for AZB in the larger channels of MIL-68(Ga). Thus, this investigation shows the influence of the host material on the switching
behavior of the embedded AZB molecules.
achieved up to now.21,22 Azobenzene molecules weakly bound
via van der Waals interactions to the host have already been
incorporated in zeolites (ZSM-5) and aluminophosphates,
INTRODUCTION
■
Metal−organic frameworks (MOFs) are a class of inorganic−
organic hybrid materials that combine a crystalline nature with
where the lifetime of the metastable cis isomer was found to
an ordered pore structure.1 This field is widely explored with
depend on the chosen host matrix,23 in zinc saccharate, a 3D
respect to gas adsorption,2,3 the separation of gases4,5 and
coordination network,24 and in the flexible porous coordination
liquids,5−7 and heterogeneous catalysis.8 To explain the
selectivities in the adsorption of ethylbenzene, ortho-, meta-,
and para-xylenes, the crystal structures of loaded MIL-47(V)
were solved from X-ray powder diffraction data, indicating that
polymer [Zn2(terephthalate)2(triethylenediamine)]n.25 Most
remarkably, in the latter example, it was shown that switching
of AZB leads to a reversible structural transformation of the
flexible coordination polymer and thus to drastic changes in the
a different packing of the isomers is responsible for their
gas adsorption properties. We have already briefly reported our
differing adsorption behavior.5 However, the embedment of
results on the successful trans/cis isomerization of AZB
larger functional molecules is less investigated up to now.
Several publications are concerned with the inclusion of
incorporated in MOF-5.14 In this work, we will present our
investigations on further AZB@MOF hybrid materials and
precursor molecules in MOFs to obtain metal nanoparticles
additionally the crystal structures of MOFs with embedded
inside the MOF framework through decomposition of the guest
photochromic guest molecules, an essential knowledge for an
molecules.9,10 Fer
́
ey et al. investigated MOFs as hosts for drug
understanding of the different optical properties obtained for
the investigated materials.
molecule encapsulation.11,12 In a different approach, we have
started a while ago to incorporate photoswitchable complexes13
and azo dyes14 into different MOFs to obtain new materials
with interesting optical properties. Because of its well-known
photochromic behavior, that is, well-separated absorption bands
in the UV/vis spectrum for the trans and cis isomers,15
azobenzene (AZB) can act as a molecular switch, when
illuminated with light. This process is well-studied in solution.16
For porous coordination polymers (MOFs), it was found that
switching of azo groups, covalently bound to linker molecules
as a side chain, is possible,17−20 whereas when incorporated in
the MOF backbone, switching of azo groups could not be
EXPERIMENTAL SECTION
■
X-ray Powder Diffraction. Laboratory measurements were
carried out on a STOE Stadi P diffractometer (Ge monochromator,
PSD detector) with Cu−Kα1 radiation, a step size of 0.01°, and a
measurement time of 5 s/step. For each diffraction pattern, four
measurements were added.
Received: December 28, 2012
Published: February 14, 2013
© 2013 American Chemical Society
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Synchrotron Powder Diffraction. High-resolution synchrotron
measurements of AZB5@MOF-5 (1) were performed with a NaI
scintillation detector at the beamline BL9 of DELTA, TU Dortmund,
at 298 K with a wavelength of 0.815765 Å and a step size of 0.004°.
The measurement time was 4 s/step.
metal salt and H2bdc were mixed with 5.0 mL of DMF in a 23 mL
Teflon autoclave and heated to 100 °C at 20 °C/h. After 48 h, the
mixture was cooled to room temperature at 5 °C/h. The colorless
powders were washed with DMF and dried in air. The removal of
embedded DMF molecules was achieved by heating in air to 200 °C
for 12 h. The activated samples were stored in a glovebox to prevent
them from rehydration.
The syntheses described above were carried out with 207.4 mg of
Ga(NO3)3·xH2O and 100.0 mg of H2bdc/408.2 mg of
In(NO3)3·5H2O and 200.0 mg of H2bdc.
MIL-53(Al) (AlOH(C8O4H4)). The synthesis of MIL-53(Al) was
carried out based on the literature procedure33 with variations in the
amount of the starting materials and the applied temperature
programs.
Al(NO3)3·9H2O (1.95 g) and H2bdc (432.0 mg) were mixed with
5.0 mL of H2O in a 23 mL Teflon autoclave. The mixture was heated
to 180 °C at 10 °C/h, and the temperature was held for 72 h and then
cooled down to room temperature at 5 °C/h. MIL-53(Al)as was
obtained as a colorless powder, washed with H2O and DMSO, and
dried in air. For removal of unreacted and embedded H2bdc, the
substance was heated in air for 7 days at 350 °C, followed by 3 days at
400 °C. After cooling to room temperature, MIL-53(Al)lt (
AlOH(C8O4H4)·H2O) was obtained.
Guest@MOF Systems. To load the respective MOF with
azobenzene via a gas phase reaction, the host material was placed in
a Schlenk tube and activated for 1 h at 70 °C in a dynamic vacuum.
Azobenzene was added, and the inhomogeneous mixture was heated at
50−60 °C in vacuum, until the colorless MOF turned homogeneously
orange. The excess of azobenzene resublimated at the top of the glass
vessel. To prevent the loaded powders from the additional adsorption
of water or, in the case of MOF-5, decomposition by contact with air,
all compounds were stored in a glovebox under argon atmosphere. For
AZB0.5@MIL-53(Al) (4), it was shown that AZB can be removed from
the host by suspending 4 in acetone. The acetone solution turned
yellow and after repeating the procedure three times and finally
washing the residue with water, MIL-53(Al)lt (AlOH(C8O4H4)-
·H2O) was obtained. Similar results were obtained for AZB5@MOF-5
(1).
AZB5@MOF-5 (1). The reaction was carried out following the
general synthesis procedure as described above with 200.0 mg of
MOF-5 (0.26 mmol) and 308.0 mg of azobenzene (1.69 mmol) in the
ratio 1:6.5.
AZB0.66@MIL-68(In) (2). The reaction was carried out with 50.0
mg of solvent-free MIL-68(In) (0.17 mmol) and 46.5 mg of
azobenzene (0.26 mmol, ratio 1:1.5).
High-resolution synchrotron powder diffraction data of AZB0.66
@
MIL-68(In) (2), AZB0.66@MIL-68(Ga) (3), and AZB0.5@MIL-53(Al)
(4) were collected at the Swiss Norwegian BeamLine (SNBL BM01B)
at ESRF. The wavelength was calibrated with a Si standard NIST 640c
to 0.50195 Å. The diffractometer is equipped with six counting
channels, delivering six complete patterns collected simultaneously
with a small 1.1° offset in 2θ. A Si(111) analyzer crystal is mounted in
front of each NaI scintillator/photomultiplier detector. Data were
collected at 298 and 120 K (cryostreamer with cold N2) between 0.5
and 30.0° in 2θ with steps of 0.002° and 300−500 ms integration time
per data point. For each sample, several patterns were collected and
summed up for the final pattern. The data collection time for each
sample was 2−3 h (298 K) and 6−13 h (120 K).
All synchrotron powder experiments were carried out in sealed glass
capillaries of 0.7−1.0 mm in diameter (Hilgenberg).
Structure Solution and Refinement. The obtained diffraction
patterns were indexed with the programs ITO26 and FOX.27 Le Bail
fits in Jana200628 were used to refine the unit cells, zero shifts, and
profile parameters. For the structure solution, the atomic positions of
MIL-53(Al)·0.7H2bdc29/MIL-68(Ga)30 were taken as starting param-
eters of the host together with the refined unit cell parameters.
Furthermore, the molecular structure of azobenzene31 was used on the
starting position 0 0 0 in FOX. The global optimization algorithm was
run in the parallel tempering mode and converged after approximately
80 000 trials for AZB0.5@MIL-53(Al) (R values: Rp = 0.1608 and wRp
= 0.2057). The occupancy of AZB was refined in the structure solution
process. For the structure solution of AZB0.66@MIL-68(Ga), two
symmetry-independent AZB molecules were placed in the unit cell,
and the occupancy was set to 0.25 for both and fixed in the structure
solution process. Convergence was achieved after approximately
300 000 trials with R values of Rp = 0.1719 and wRp = 0.1750.
The Rietveld refinement of the obtained structure models was
performed with GSAS.32 To get a stable refinement, the azobenzene
molecules, as well as the benzenedicarboxylate anions, were fixed with
soft constraints (bond lengths, angles, and planes according to the
literature values). The isotropic displacement parameters of the guest
molecule atoms and the host atoms were coupled with constraints and
refined afterward, if possible. The final stable refinement of AZB0.5@
MIL-53(Al) had a total of 135 parameters. For AZB0.66@MIL-68(Ga),
additional soft constraints for the Ga−O distance were necessary. The
MOF network atom positions, unit cell, zero shift, and profile
parameters were stably refined with a total of 70 parameters. The
isotropic displacement parameters were fixed. The atom positions of
both symmetry-independent azobenzene molecules had to be refined
separately with the other parameters being fixed.
AZB0.66@MIL-68(Ga) (3). The reaction was carried out with 50.0
mg of solvent-free MIL-68(Ga) (0.20 mmol) and 54.7 mg of
azobenzene (0.30 mmol, ratio 1:1.5).
AZB0.5@MIL-53(Al) (4). The reaction was carried out with 60.0 mg
of MIL-53(Al)lt (0.27 mmol) and 39.4 mg of azobenzene (0.22 mmol,
ratio 1:0.8).
IR Spectroscopy. Extinction IR measurements were performed as
KBr pellets on a Nicolet 5700 spectrometer. After recording the IR
spectra of the ground state, the compounds were illuminated with 325
nm laser light to induce the trans → cis isomerization. To investigate
the reconversion to trans-AZB, the compounds were illuminated with
442 nm laser light. The successive irradiation times for all compounds
as well as the wavelengths are given in Table S1 (Supporting
Information). In Figure 2 and Figures S2 and S3 of the Supporting
Information, the ground state before irradiation is shown together with
the measurements underlined in Table S1.
UV/vis Spectroscopy. The UV/vis spectrum of the solid
compound AZB0.5@MIL-53(Al) was measured in a transparent KBr
pellet on a Varian CARY 05E spectrometer.
Elemental Analysis. Elemental analysis of carbon, hydrogen, and
nitrogen was performed with a HEKAtech GmbH EuroEA 3000
Analyzer. In a glovebox, approximately 2 mg of each compound was
filled into a tin cartridge.
RESULTS AND DISCUSSION
■
We selected colorless MOFs for our experiments to minimize
the influence of the host lattice in optical investigations.
Furthermore, it was checked by simple geometric consid-
erations that AZB should fit into the pores or channels of the
MOF structures. Finally, three MOFs of the MIL family (MIL:
́
Materiaux de l’Institut Lavoisier) were chosen for our
experiments, as they crystallize in crystal structures of lower
symmetry (orthorhombic) so that symmetry-caused disorder is
minimized, helping in the structural elucidation of AZB
embedded in the MOF channels (see below). After activation
of the chosen MOF hosts, AZB was embedded in MOF-534
(Zn4O(bdc)3, bdc2− = 1,4-benzenedicarboxylate), MIL-68-
(In)30 (In(OH)(bdc)), MIL-68(Ga)30 (Ga(OH)(bdc)), and
MIL-53(Al)33 (Al(OH)(bdc)) via a gas phase process to
exclude any solvent molecules from all further considerations.
Synthesis: MIL-68(Ga) (GaOH(C8O4H4)·0.9DMF·zH2O) and
MIL-68(In) (InOH(C8O4H4)·1.0DMF·zH2O). MIL-68(Ga) and MIL-
68(In) were synthesized according to the literature.30 The respective
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The successful inclusion of the guest molecules was confirmed
by XRPD measurements, elemental analysis, and UV/vis
measurements. After embedment of AZB in MOF-5, MIL-
68(In), and MIL-68(Ga), significant changes of the reflection
intensities were observed caused by the electron density of the
guest molecules inside the MOF pores. However, the
diffraction angles mainly remained unchanged, indicating that
the MOF lattice is still intact. For loaded AZB5@MOF-5 (1)
and unloaded MOF-5, this is shown in Figure 1. (For
Figure 1. XRPD pattern of AZB5@MOF-5 (blue) and unloaded
MOF-5 (red) at 298 K (STOE Stadi P, Cu−Kα1 radiation).
diffraction patterns of loaded/unloaded MIL-68(In) and MIL-
68(Ga), see Figures S4 and S5 in the Supporting Information.)
On the contrary, for MIL-53(Al), a flexible porous coordination
polymer, a change of the reflection intensities and positions
occurs (Figure S6 in the Supporting Information). Through the
inclusion of AZB molecules, the framework “breathes”,
resulting in a deformation of the porous channels and thus
the unit cell. From elemental analysis (Supporting Information,
Table S2), the ratio of guest molecules per formula unit of the
host material was determined: AZB5@MOF-5 (1), AZB0.66
@
MIL-68(In) (2), AZB0.66@MIL-68(Ga) (3), and AZB0.5@MIL-
53(Al) (4). The UV/vis measurement of (1) showed that only
the thermodynamically more stable trans isomer15 was
embedded in MOF-5 (Supporting Information, Figure S1), as
expected from the synthesis procedure at elevated temperatures
(60−80 °C).
To investigate the isomerization process of the embedded
AZB molecules, the polycrystalline powders were prepared as
transparent KBr pellets and illuminated with UV light (λ = 325
nm) to induce the trans → cis conversion. For AZB0.66@MIL-
68(In) (2) and AZB5@MOF-5 (1), the reverse process was
examined by irradiation with blue light (λ = 442 nm). Before
and after each irradiation, IR spectra were collected (Figure 2;
for IR spectra of pure AZB and AZB0.5@MIL-53(Al) (4), see
the Supporting Information, Figures S2 and S3).
Figure 2. IR spectra of azobenzene (AZB) loaded MOFs before (dark
blue) and after (red) irradiation with UV light (λ = 325 nm). For
AZB0.66@MIL-68(In) (2) (top), the IR spectrum after irradiation with
λ = 442 nm (light blue) is additionally shown.
azobenzene of 12%. The obtained fraction of cis-azobenzene
embedded in MOFs depends slightly on the host material at a
given irradiation time with a maximum for MIL-68(In). The
quantitative analysis of the band areas results in a ratio of 30%/
27%/25% cis-azobenzene for AZB0.66@MIL-68(In) (2),
AZB0.66@MIL-68(Ga) (3), and AZB5@MOF-5 (1), respec-
tively.37 When exposed to light with longer wavelengths (442
nm), the reverse process is induced, resulting in the recovery of
almost 100% trans-azobenzene, as shown exemplarily for
AZB0.66@MIL-68(In) (2) in Figure 2 (top). Because of a
progressing opacity of the KBr pellet, a longer irradiation to
gain 100% of the cis-isomer was not possible. However, for
MIL-53(Al) as host material, no conversion of the embedded
trans-azobenzene could be achieved using even longer
irradiation times or different wavelengths (Supporting
Information, Table S1).
Two bands at 776 and 690 cm−1 can be assigned to trans-
azobenzene.35 After exposure to UV light, a decrease of their
intensities combined with the appearance of two new bands at
759 and 703 cm−1 was detected, deriving from a partial
conversion of trans- to cis-azobenzene. Such a successful
isomerization process was found in AZB5@MOF-5 (1),
AZB0.66@MIL-68(In) (2), and AZB0.66@MIL-68(Ga) (3). In
a lower ratio and only after longer irradiation times, it was also
induced in pure solid azobenzene, in contrast to reports in the
literature.36 From our experiments, we estimated a ratio of cis-
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To understand the different switching behavior of AZB0.5@
MIL-53(Al) (4) and AZB0.66@MIL-68(Ga) (3) and to
characterize the underlying host−guest interactions, the
knowledge of their crystal structures and thus the arrangement
of the guest molecules within the respective host materials is
essential. As all substances are only available as polycrystalline
powders after the gas phase loading, the structure solutions had
to be carried out from X-ray powder diffraction data. To get the
best data possible, high-resolution synchrotron powder
diffraction data were recorded at beamline BM01B at the
ESRF/Grenoble at low temperatures (120 K) to diminish the
motion of the guest molecules within the MOF pores.
The diffraction patterns obtained at room temperature and
120 K were indexed with the programs ITO26 and FOX.27 Le
Bail fits in Jana200628 (Supporting Information, Figures S7, S8,
S10−S12) were used to refine the unit cells, zero shifts, and
profile parameters. The structures of AZB0.66@MIL-68(Ga) (3)
and AZB0.5@MIL-53(Al) (4) were finally solved with FOX.27
The atomic positions of the known crystal structures of MIL-
68(Ga)30 and MIL-53(Al)29 with the unit cell parameters
obtained from the Le Bail fits were used as starting parameters
together with the known molecular structure of trans-
azobenzene.38 The positions of azobenzene were determined
by a global optimization algorithm (parallel tempering) in
direct space and refined with the program GSAS.32 More details
are given in the Experimental Section. The crystallographic data
are summarized in Table 1.
The crystal structure of AZB0.5@MIL-53(Al) (4) is shown in
Figure 3. As the structure of MIL-53(Al) is well-known,29,33 the
structure description will focus on the arrangement of the guest
molecules inside the MIL-53(Al) pores. Compound 4
crystallizes in the orthorhombic space group Pnma (No. 62)
with four formula units per unit cell, as already found for MIL-
53(Al)as.33 AZB occupies the general site 8d, but due to steric
reasons, a simultaneous occupation of all symmetry-generated
positions is not possible. The refinement of the occupancy of all
positions of the AZB molecule converged at a value of 0.24, in
good agreement with the results of the elemental analysis. As
described for other systems,6 due to dynamic or static disorder
over all these positions, no reduction of symmetry is observed.
AZB is oriented almost parallel to the one-dimensional
channels of the host. The shortest C−C distance between
AZB and the host is 3.49 Å (C7−C3x) and the shortest C−N
distance is 3.47 Å (N2−C3x). These distances point to weak
van der Waals interactions, as also obvious from the space-
filling representation in Figure 3 (bottom). Furthermore, the
aromatic rings of the AZB guest and the bdc linker of the host
are aligned parallel in such a way that π−π interactions are
likely to occur. Assuming that only every fourth AZB position
within a channel is occupied, the C−C distances between two
AZB molecules start at 3.34 Å (C1−C4) and the shortest C−N
distances at 3.48 Å (N2−C6), again in the range of van der
Waals interactions. Compared to the room-temperature crystal
structure of trans-azobenzene,38 these distances are slightly
shorter (C−C: 3.540(2) Å and C−N: 3.622(1) Å). From
Figure 3 (bottom), it is obvious that the light-induced
isomerization to cis-azobenzene is obviously sterically hindered
within the channels of MIL-53(Al).
A different picture is found for AZB0.66@MIL-68(Ga) (3), as
shown in Figure 4. AZB molecules are only found within the
larger six-membered channels of the host. Two symmetry-
independent sites are occupied by AZB molecules, both located
on the general position 16h. The occupancies of both sites
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Figure 3. (top) Unit cell of AZB0.5@MIL-53(Al) (4) with a view along
[010]. (bottom) Space-filling representation of 4. Note: not all
symmetry-related AZB positions are shown; each position is occupied
with 24%.
Figure 4. (top) View of the crystal structure of AZB0.66@MIL-68(Ga)
(3) along [001]. (bottom) Space-filling representation of 3. Note: not
all symmetry-related AZB positions are shown; each position is
occupied with ∼21%.
refined to very similar values (∼0.21), again in reasonable
agreement with the results of the elemental analysis. Only one
AZB is oriented parallel to the one-dimensional channels along
[001]; the other AZB is almost perpendicular to the former in
the (001) plane. The shortest C−C distance between AZB and
the host is 2.99 Å (C20−C9), and the shortest C−N distance is
3.79 Å (N1−C8). These distances are slightly different to those
obtained for AZB0.5@MIL-53(Al) (4), but still in the same
range, as expected for van der Waals interactions (see space-
filling representation in Figure 4 (bottom)). It cannot be
excluded that, for AZB0.66@MIL-68(Ga) (3) with two
symmetry-independent sites for the AZB guests, the
uncertainties in determining the guests positionsand such
the intermolecular distancesare higher than those obtained
for 4. The standard deviations obtained from the Rietveld
analysis surely underestimate these uncertainties. Furthermore,
due to the fact that two symmetry-independent positions are
only partly occupied, a reasonable assignment of intermolecular
AZB distances is not possible. Figure 4 shows unambiguously
that, within the larger channels of MIL-68(Ga), the light-
induced isomerization to cis-azobenzene can occur without any
sterical hindrance.
ization was found. Crystal structures reveal that the size and the
shape of the host’s channels, as well as the orientation of
azobenzene within these channels, are responsible for this
different behavior. Fujita et al. reported porous coordination
networks consisting of ZnI2 and tris(4-pyridyl)triazine, which
were loaded with (Z)-stilbene via a solution process.39 The
successful inclusion was confirmed by an X-ray single-crystal
structure analysis. Irradiation (λex = 400−500 nm) of this
inclusion compound suspended in a solution resulted in a 98%
conversion to (E)-stilbene. This inclusion compound can even
act as a catalyst for the Z → E photoisomerization, but the
authors did not report any dependence of the photo-
isomerization upon sterical hindrance within the pores of the
porous coordination network. This aspect of our work seems to
be quite new, so we are planning to broaden our studies to
further MOFs with different shapes and sizes of the pores as
well as to other guests, such as fluorinated and perfluorinated
azobenzene.40 Also, the influence of the concentration of the
guest within the host’s pores on the optical properties shall be
investigated. First results show that MOF-5 can be loaded with
lower ratios of guest molecules. Finally, the investigation of
XRPD patterns of different AZB@MOF systems under
irradiation seems to be worthwhile, as already shown by
Kitagawa et al.25
CONCLUSION
■
In conclusion, we have embedded photochromic azobenzene in
different MOFs and showed that the trans/cis isomerization can
also occur within the confined channels or pores of these hosts
and is even improved compared to that of pure solid
azobenzene. However, for MIL-53(Al), no trans/cis isomer-
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(22) Schaate, A.; Duhnen, S.; Platz, G.; Lilienthal, S.; Schneider, A.
̈
ASSOCIATED CONTENT
* Supporting Information
Details of the irradiation experiments, elemental analyses, UV/
vis and additional IR spectra, XRPD patterns of loaded and
unloaded MOFs, and XRPD patterns with Le Bail and Rietveld
fits. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
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Chem., Int. Ed. 2003, 42, 1848.
(25) Yanai, N.; Uemura, T.; Inoue, M.; Matsuda, R.; Fukushima, T.;
Tsujimoto, M.; Isoda, S.; Kitagawa, S. J. Am. Chem. Soc. 2012, 134,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: uwe.ruschewitz@uni-koeln.de .
Notes
■
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
D.H. acknowledges the Fonds der Chemischen Industrie for a
doctoral stipend. We thank D. Hertel and K. Meerholz for
measurements at their IR spectrometer under laser irradiation.
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dx.doi.org/10.1021/ic302856b | Inorg. Chem. 2013, 52, 2744−2749