Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light

Article abstract of DOI:10.1002/chem.201805391

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H′-octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB), into the pores of the well-known metal-organic frameworks MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF-AZB_0.34@MIL-53(Al), a structural model based on high-resolution synchrotron powder diffraction data was developed and the host-guest and guest-guest interactions were elucidated from this model. These interactions of O?H???F and π???π type were confirmed by significant shifts of the O?H frequencies in loaded and unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably, all of the synthesized F-AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo-isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.

Full text of DOI:10.1002/chem.201805391

A Journal of
Accepted Article
Title: Metal-Organic Frameworks as Hosts for Fluorinated
Azobenzenes: A Path Towards Quantitative Photoswitching with
Visible Light
Authors: Daniela Hermann, Heidi A. Schwartz, Melanie Werker,
Dominik Schaniel, and Uwe Ruschewitz
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To be cited as: Chem. Eur. J. 10.1002/chem.201805391
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Metal-Organic Frameworks as Hosts for Fluorinated
Azobenzenes: A Path Towards Quantitative Photoswitching with
Visible Light
Daniela Hermann,[a] Heidi A. Schwartz,[a] Melanie Werker,[a] Dominik Schaniel,[b] Uwe Ruschewitz*[a]
Dedicated to Prof. Dr. Thomas F. Fässler on the Occasion of his 60th Birthday
Abstract: Fifteen new photochromic hybrid materials were
Accordingly, for potential applications the photochromic
molecular entities are incorporated in polymers[9-11], crystalline[12]
synthesized by gas phase loading of fluorinated azobenzenes,
namely
ortho-tetrafluoroazobenzene
(tF-AZB),
4H,4H’-
and amorphous thin films[13-15], porous matrices like zeolites[16-18]
or molecular cages.[19-20]
,
octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-
AZB), into the pores of the well-known metal-organic frameworks
MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their
composition was analysed by elemental (CHNS) and DSC/TGA
analysis. For pF-AZB0.34@MIL-53(Al) a structural model based on
high-resolution synchrotron powder diffraction data was developed
and the host-guest and guest-guest interactions were elucidated
from this model. These interactions of O-H…F and … type were
confirmed by significant shifts of the O-H frequencies in loaded and
unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably,
all of the synthesized F-AZB@MOF systems can be switched with
visible light and some of them show almost quantitative (> 95 %)
photo-isomerization between its E and Z forms with no significant
fatigue after repeated switching cycles.
Recently, metal-organic frameworks (MOFs) were established
as a new class of host materials for photochromic guests.[21-23]
MOFs are hybrid materials consisting of inorganic knots, chains
or layers (mainly metal-oxo units), which are bridged by rigid
multifunctional organic molecules or anions (named “linker”) to
form crystalline porous materials with a very high structural
flexibility.[24,25] To incorporate the photochromic functionality into
these MOFs three general approaches were described:
1) The photochromic entity is part of the linker backbone of
the respective MOF.[26-27] But it is a severe drawback of
this approach that the large structural changes upon
irradiation often lead to a degradation of the framework.
2) The photochromic functionality is added as a substituent
to the linker.[28-37] This leads to the necessary sterical
freedom for successful photoisomerisation, but like for 1)
typically elaborate synthetic efforts are needed to obtain
these linker molecules.
Introduction
3) The most simple way is to directly embed the
photochromic molecules into the empty pores of a MOF
by gas phase loading or by solution based processes.[38]
Not only the low synthetic efforts, but also the high
flexibility with respect to the embedded guests are
advantages of this approach.
Photochromic materials have found wide-spread applications e.g.
in colour changing sunglasses[1] or as data storage devices.[2]
According to Hirshberg photochromism is the reversible
structural transformation of a molecular entity between two
configurations induced by irradiation with electromagnetic waves
leading to a change of the resulting absorption properties.[3]
These structural changes are for example E/Z isomerisations
(e.g. azobenzenes, stilbenes) or ring-opening/-closing reactions
(e.g. spiropyrans, diarylethenes).[1] For all of them the structural
transformation typically leads to significant changes of the size,
shape and properties like dipole moments of these molecules so
that their photochromic behaviour is largely influenced by the
surrounding medium. Thus the photoswitching behaviour is well
established in solution,[4-8] but mainly suppressed in crystalline
solids of the respective molecules due to spatial hindrance.
In the following we will focus on approach 3), which has
successfully been used to embed azobenzene dyes,[39,40]
spiropyrans/merocyanines[41,42]
and
diarylethenes[43-45]
in
different MOF hosts. For all of them successful photoswitching
of the embedded guests was proven.
[a]
Dr. D. Hermann, Dr. H. A. Schwartz, M. Werker, Prof. Dr. U.
Ruschewitz
Scheme 1. E/Z isomerization of azobenzene (AZB).
Department of Chemistry
University of Cologne
Greinstraße 6, D-50939 Köln, Germany
E-mail: uwe.ruschewitz@uni-koeln.de
Prof. Dr. D. Schaniel
The E/Z isomerisation of azobenzene (abbreviated to AZB in the
following), which is shown in Scheme 1, can be regarded as the
prototype of photochromism. The successful embedment of AZB
into the MOFs MOF-5,[40] MIL-68(In),[40] MIL-68(Ga),[40] MIL-
53(Al),[40] and DMOF-1[39] was already shown, and for the latter
the remote-controlled uptake and release of CO2 was proven.[39]
It is however a severe drawback of AZB that UV light that might
[b]
CNRS, CRM2
Université de Lorraine
F-54506 Nancy, France
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201805391
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interfere with the host matrix, is needed to induce the E/Z
isomerisation. Even more, the overlapping absorption bands of
the E and Z isomers result in photostationary states, i.e. a
complete photoswitching of pristine AZB from E to Z and vice
versa is not possible in most cases. For example, in zeolite NaY
> 90 % Z-AZB were achieved, but here the formation of a Na+-
AZB complex was postulated.[46] For AZB0.66@MIL-68(In) we
only reached a maximum of ca. 30 % of the excited Z state
under illumination with UV light ( = 325 nm),[40] whereas Yanai
et al. found 38 % of the Z isomer in AZB@DMOF-1 after UV light
irradiation using an ultrahigh-pressure Hg lamp.[39] However, it
was shown that modifying AZB with suitable substituents will
separate the absorption bands of the E and Z isomers to
overcome these limitations.[47] A very elegant way is the
introduction of fluorine atoms in the phenyl rings of AZB. For
ortho-tetrafluoroazobenzene (tF-AZB) photostationary states
with 91 % Z and 86 % E states were reached in acetonitrile
solutions.[48] Furthermore, wavelengths of the visible region of
the electromagnetic spectrum can be used to trigger the photo-
isomerization so that no longer UV light has to be applied.[48] It
was successfully shown that incorporating tF-AZO side groups
attached to suitable linker molecules into MOF structures leads
to solid materials with almost quantitative E/Z isomerization,
which can be switched with green and violet light,
respectively.[35,36] In the following we will show that also by direct
embedment of fluorinated derivatives of AZB in different MOF
hosts following approach 3) (s. above) photochromic
guest@MOF systems with an almost quantitative E/Z
photoswitching with visible light can be obtained, even though
the photoswitching unit is not covalently attached to the MOF
scaffold. Furthermore, we examined the underlying host-guest
interactions occurring in these guest@MOF systems and show
how they influence the photoswitching properties of these
composite materials. The fluorinated AZB derivatives used in
this work are given in Scheme 2. Some of their optical properties
are summarized in Table 1. The superior photoswitching
properties of tF-AZB are obvious from this compilation. The
crystal structures of E- and Z-tF-AZB were reported quite
recently and compared with those of the other azobenzenes
depicted in Scheme 1 and 2.[49] For thin (SURMOF) films of type
HKUST-1 the successful incorporation of tF-AZB was already
proven.[50]
Synthesis, Composition & Stability
A few years ago we started to establish MOFs as suitable hosts
for the embedment of photochromic guests like azobenzenes[40]
and spiropyrans[42]. To embed the photochromic guests we
developed
a
gas phase procedure (see Experimental
Section).[40,42] Gas phase loading and loading of the molten
guest[39,43] are preferable to solution based processes, as with
the former any solvents can be excluded from all further
considerations. Using this gas phase approach we obtained
highly crystalline materials, which were accessible to
characterisation by XRPD (X-ray powder diffraction) methods. In
Figure 1 the XRPD patterns of activated MIL-68(Ga) and MIL-
68(Ga) loaded with different photochromic azobenzene guests
(pristine azobenzene, tF-AZB, oF-AZB, pF-AZB) are shown. As
the 2 values of the reflections are mainly unchanged after
loading, it can be concluded that the MOF framework is still
intact. But as the reflection intensities are drastically modulated
depending on the scattering power of the embedded guest, i.e.
with increasing fluorine substitution, XRPD can be used as a
simple and fast method to confirm the successful embedment of
the guest. Note: the intensity loss of the first reflection just below
2 = 5° upon loading with oF-AZB and pF-AZB does not indicate
a decreasing crystallinity along the respective crystallographic
direction, as other reflections with these indices are not affected
in the same way (e.g. reflection just below 2 = 20°). If the guest
is not completely embedded in the MOF, additional reflections of
the guest are visible in the respective XRPD patterns (not
shown). These residues of the guest can be removed by further
heating in vacuum. Even more, for spiropyran, which was
attempted to be embedded in MIL-53(Al), we showed that
unchanged intensities after potential gas phase loading are a
clear indicator that (almost) no embedment of the guest within
the pores of the MOF has taken place.[42] In Figure S21
(Supporting Information) XRPD patterns of MOF-5 loaded with
different starting ratios of pristine azobenzene are shown. These
patterns show that a rough estimate of the quantitative amount
of the embedded guest can be obtained from these patterns and
vice versa that the amount of the embedded guests molecules
can be controlled by the ratio guest:host in the starting mixture.
Scheme 2.
E isomers of ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H’-
octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB) (f.l.t.r.).
<Table 1>
Figure 1. Sections of the XRPD patterns of activated MIL-68(Ga)ht (red),
AZB@MIL-68(Ga) (yellow), tF-AZB@MIL-68(Ga) (green), oF-AZB@MIL-
Results and Discussion
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68(Ga) (turquois), and pF-AZB@MIL-68(Ga) (gray) (STOE Stadi P; capillaries,
CuK1 radiation).
Similar results were obtained for pF-AZB embedded in MOF-5
(Figure S22, Supporting Information). The mass loss of 61.5 %
leads to the composition pF-AZB3.40@MOF-5, which is again in
very good agreement with the results of the elemental analysis:
pF-AZB3.57@MOF-5 (Table S4, Supporting Information). Here,
the decomposition of the MOF framework already starts at
400 °C, as described for MOF-5 in the literature.[54]
Compared with attempts to incorporate the photoactive
functionality as a part of the backbone or a substituent of the
linker used to construct the respective MOF the approach to
embed the photochromic molecule as a guest in the MOF
framework has the advantage of an easy synthetic accessibility
as well as a very high versatility with respect to the MOF and the
guest. One might object that the guest molecules are only
weakly bonded to the MOF framework so that the thermal
stability of the resulting guest@MOF systems is not high, but for
azobenzene embedded in thin films of type HKUST-1 it was
shown that when stored at room temperature the guest leaves
the MOF very slowly with a depletion (or desorption) time
constant   360 days.[50] However, when stored at 60 °C 
decreases significantly to 14.5 days.[50] For two pF-AZB
containing systems of this investigation the resulting DSC/TGA
curves are shown in Figure 2 and Figure S22 (Supporting
Information). They show that pF-AZB is released from MIL-
53(Al) above 150 °C (Figure 2), whereas from MOF-5 the
release already starts at 100 °C (Figure S22). So the influence
of the respective MOF host on the thermal stability of the
resulting guest@MOF system is obvious. Nonetheless, both
systems are stable at room temperature and the loss of pF-AZB
at this temperature can be neglected. From the TGA curve of
pF-AZB0.34@MIL-53(Al)1 (Figure 2) a mass loss of 34.6 % is
obtained leading to the composition pF-AZB0.30@MIL-53(Al),
which is in very good agreement with the results of the Rietveld
refinement and the elemental analysis (Table S4, Supporting
Information). The smooth DSC curve confirms that neither a
decomposition of the guest or the host occurs up to 500 °C. The
decomposition of the MOF starts above 500 °C (not shown),
which is in good agreement with its decomposition temperature
given in the literature (Tdec. = 500 °C).[53] A small peak in the
TGA curve at ca. 250 °C (Figure 2) is an artefact of the balance
in this measurement.
Crystal Structure & Host-Guest Interactions
To clarify the arrangement of the fluorinated azobenzene guest
embedded in a MOF host and to understand the underlying
host-guest interactions as well as to confirm the reliability of the
compositions obtained from elemental and DSC/TGA analysis
we performed a comprehensive structural analysis with a
subsequent Rietveld refinement on a selected compound. For
this analysis we have chosen pF-AZB embedded in MIL-53(Al)
for the following reasons: according to Table 3 the initial state of
pF-AZB consists to 100 % of the E isomer, so that in the
refinements only this isomer has to be taken into account.
Furthermore, the host MIL-53(Al) shows orthorhombic symmetry,
which minimizes possible disorder compared to a MOF with a
higher, i.e. cubic symmetry like MOF-5. It is remarkable that
compared to already published AZB0.50@MIL-53(Al) (Pnma, no.
62)[40] we found a symmetry change for pF-AZBx@MIL-53(Al)
(Imma, no. 74). Details of the structure solution and refinement
are given in the Experimental Section, in Figure 3 views of the
resulting crystal structure are given.
A
space-filling
representation was chosen to show that the guest molecules fit
perfectly into the pores of MIL-53(Al) taking the respective van
der Waals radii into account. H…F distances between the BDC2-
linkers of the MOF and pF-AZB guests start at 295 pm. This
value is as expected larger than the sum of the van der Waals
radii of H and F (256 pm).[55] The occupancy factor of the
position of the pF-AZB molecule refined to 8.6 % resulting in the
composition pF-AZB0.34@MIL-53(Al). This composition, which
will be used in the following to name this compound, is in
reasonable agreement with the results of the elemental (pF-
AZB0.27@MIL-53(Al)) and DSC/TGA analysis (pF-AZB0.30@MIL-
53(Al), Figure 2 and Table S4, Supporting Information).
Figure 3. Space-filling presentation of the crystal structure of pF-
AZB0.34@MIL-53(Al) with a view along a channel ([100] direction, left) and a
side-view with an “opened” channel (right); Al: large white, C: grey, O: red, H:
small white, F: green, N: blue spheres.
Figure 2. DSC/TGA curve of pF-AZB0.34@MIL-53(Al).
1 Here, the composition as derived from the Rietveld refinement (see
next section) is used to name this compound.
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In our former work on pristine azobenzene (AZB) embedded in
different MOFs we presented the crystal structures of
AZB0.50@MIL-53(Al) and AZB0.66@MIL-68(Ga).[40] As in all these
investigations (Rietveld, elemental and DSC/TGA analysis) a
good agreement for the amount of the embedded guest
molecules was found, we conclude that all methods are suitable
to quantify the loading. In Figure 4 the dominating host-guest
interactions in AZB0.50@MIL-53(Al)[40] and pF-AZB0.34@MIL-
53(Al) are compared, as assigned after close inspection of both
crystal structures. In AZB0.50@MIL-53(Al) O-H… interactions
between the hydroxyl group of the MOF and both phenyl rings of
AZB (O- H…C = 265 and 276 pm) are found. These interactions
are depicted in Figure 4 (left) as dashed lines. Furthermore a
parallel orientation of the phenyl rings of the BDC2- linkers and
the AZB guests suggests … interactions. The planes of two
AZB guests are separated by 349 pm and the planes of a guest
and a BDC2- linker by 382 pm. So for the latter only weak
interactions must be assumed. In both cases the centres of the
rings are shifted by approx. 300 pm pointing to a parallel
displaced stacking (offset stacked). Some host-guest
interactions found in pF- AZB0.34@MIL-53(Al) are depicted in
To analyse these host-guest and guest-guest interactions in
more detail IR spectra of the activated (“empty”) and loaded
MOF were recorded. Especially the MOFs of type MIL-53 and
MIL-68 are well-suited for such investigations, as the OH groups
in these MOFs are sensitive probes for such interactions. All
samples were prepared under inert conditions (glovebox) and
the spectrometer itself was also housed in a glovebox to provide
completely inert conditions and to exclude any water impurities.
In Table 2 the shifts of the resulting O-H frequencies are
summarized as  ̃ values, i.e. referenced to ̃ (OH) of the
activated (“empty”) MOF.
Table 2. Shifts of IR frequencies of O-H stretching vibrations in loaded
MOFs of the MIL-53 and MIL-68 series referenced to the activated MOF.
E isomer
̃ / cm-1
Z isomer
̃ / cm-1
AZB0.50@MIL-53(Al)
tF-AZB0.19@MIL-53(Al)
pF-AZB0.34@MIL-53(Al)
-39
-39
-28
-10
-11 / -17[a]
-10
AZB0.66@MIL-68(Ga)
tF-AZB0.58@MIL-68(Ga)
pF-AZB0.63@MIL-68(Ga)
-39
-39
Figure
4
(right). Contrary to AZB0.5@MIL-53(Al) O-H…F
-14
-24
-3 / -10[a]
-3 / -10[a]
interactions (shortest O-H…F distance: 213 pm) seem to
dominate and only weak O-H… interactions are observed
(shortest O-H…C distance: 281 pm compared to 265 pm in
AZB0.50@MIL-53(Al)). But this finding is not surprising, as the
high degree of fluorination of the pF-AZB guest leads to an
inversion of its quadrupole moment, i.e. the fluorine substituents
bear the negative charge and the aromatic ring can no longer
act as an electron donor.[56] The orientation of the phenyl rings of
two guest molecules is not in agreement with attractive …
interactions. 320 pm is found as the shortest C-C distance
between a pF-AZB guest and the BDC2- linker. The rings are
arranged almost directly one above each other with only a very
small shift, which is in-between a stacked and an offset stacked
arrangement.[57] This can also be explained by the inverted
quadrupole moment of the perfluorinated ring leading to an
interaction with the unfluorinated phenyl ring of the BDC2- linker
of a different type. These interactions result in a slight shift of the
frequency of ̃ (C=C) from 1506 cm-1 in the IR spectrum of
unloaded MIL-53(Al) to 1513 cm- 1 in the IR spectrum of pF-
AZB0.34@MIL-53(Al).
AZB0.66@MIL-68(In)
tF-AZB0.57@MIL-68(In)
pF-AZB0.32@MIL-68(In)
-55
-49
-16
-55
-12
-16
[a] split signals
The most significant shifts were obtained for AZB as guest. This
points to the fact that the O-H… and … interactions discussed
for AZB0.50@MIL-53(Al) are obviously stronger than the O-H…F
and … interactions found in pF-AZB0.34@MIL-53(Al). The
guest-guest and host-guest interactions with tF-AZB seem to be
somewhere in-between: in MIL-68(In) a strong shift was found,
whereas smaller shifts are observed for tF-AZB embedded in
MIL-53(Al) and MIL-68(Ga). Possibly, tF-AZB is able to form
different types of interactions (O-H…F, C-H… etc.), which are
dependent on the specific arrangement of the guest within the
MOF pores. This is supported by the significant shifts upon E 
Z isomerization, which are specific for tF-AZB as guest. This will
be discussed in more detail in the next chapter, where also a
correlation between the photochromic properties and the host-
guest and guest-guest interactions, as indicated by the results
documented in Table 2, will be attempted.
Photochromic Properties
In our previous work we showed that IR spectroscopy is well-
suited to characterize the photochromic behaviour of
azobenzene[40] and spiropyran[42] embedded in different MOF
hosts, as the IR bands of the guest hardly overlap with those of
the MOF host (Figure 5). Therefore it allows for a direct
quantitative analysis of the switching process via the
determination of the changes of the relevant IR bands. This
procedure seems to be preferable to the elution of the guest and
analysis of the E/Z ratio by e.g. ultraperformance liquid
chromatography (UPLC).[52]
Figure 4. Details of the crystal structures of AZB0.5@MIL-53(Al)[40] (left) and
pF-AZB0.34@MIL-53(Al)[this work] (right) emphasizing some of the occurring host-
guest interactions; colour coding as in Figure 3.
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(maximum of the respective photo isomer) were obtained by
irradiation with  = 532 nm (EZ) and  = 405 nm (ZE).
Notably, fluorination of azobenzene allows photoswitching with
visible light similar to solutions,[48,52] whereas for pristine
azobenzene UV light ( = 325 nm) has to be used for the EZ
isomerization.[40]
Figure 5. IR spectra of pF-AZB@MOF-5 after light induced isomerization with
 = 532 nm (EZ) and  = 405 nm (ZE). The IR spectra of the as-
synthesized state and pristine MOF-5 are given as a dark grey and a red curve,
respectively. Arrows mark bands assigned to the E (blue) and Z (green)
isomer.
We therefore used this approach to characterize the
photochromic behaviour of the novel guest@MOF systems. In
Figure 5-7 and Figures S23-S25 (Supporting Information) the IR
spectra, we obtained before and after illumination with light of
different wavelengths, are given for pF-AZB@MOF-5, tF-
AZB@MIL-68(In), and pF-AZB@MIL-68(In). After testing several
wavelengths for photoswitching we observed that the best
results were obtained with  = 532 nm for EZ and  = 405 nm
for ZE switching. In Table 3 the results of the quantitative
analyses of the IR spectra are summarized. For each of the
investigated systems the maximum amounts of E and Z isomers
are given, which were obtained by the illumination experiments.
Additionally, the composition of the initial state (before
illumination) is listed. More details are given in the Experimental
Section.
E-pF-AZB shows absorption maxima at  = 310 nm and
 = 453 nm in solution.[52] We have chosen pF-AZB3.57@MOF-5
for first illumination experiments with different wavelengths. The
changes in the obtained IR spectra (Figure 5) after irradiation
were compared with the spectra given in the literature and
specific bands were assigned to the two isomers. For pF-
AZB3.57@MOF- 5 the highest amount of Z-pF-AZB was obtained
by illumination with green light ( = 532 nm) in good agreement
with the results obtained for solutions (cp. Table 1).[52] The back
transformation ZE was achieved by illumination with violet light
( = 405 nm). Similarly, the isomerization of tF-AZB in solution
was described with  > 450 nm (EZ, 91 %) and  = 410 nm
(ZE, 86 %).[48,52] For tF-AZB embedded in MIL-68(In) (Figure
6) we achieved the highest amount of EZ isomerization with
 = 532 nm and ZE isomerization with  = 405 nm, similar to
pF-AZB. We also used  = 476 nm to trigger the EZ
isomerization in tF-AZB@MIL-68(In). As can be seen in Figure 6
an EZ isomerization was achieved, but the amount of the Z
isomer was significantly smaller than the amount obtained with
 = 532 nm. With all other MOF hosts or with oF-AZB as third
photochromic guest used in this investigation the best results
Figure 6. IR spectra of tF-AZB@MIL-68(In) after light induced isomerization
with  = 476 nm / 532 nm (EZ) and  = 405 nm (ZE). The as-synthesized
state is given as a dark grey curve. Arrows mark bands assigned to the E
(blue) and Z (green) isomer.
Remarkably, in some spectra the bands of one isomer are
almost completely extinguished after irradiation. In pF-
AZB@MIL-68(In) (Figure 7) the band at 1110 cm-1, assigned to
Z-pF-AZB, is no longer observed after irradiation with violet light
( = 405 nm), i.e. the Z isomer is completely transformed to the
respective E isomer by irradiation. Similarly, the intensity of the
band at approx. 770 cm-1 in tF-AZB@MIL-68(In) (Figure 6)
decreases strongly upon irradiation with violet light, again
pointing to an almost complete ZE isomerization. Upon
irradiation with green light ( = 532 nm) the intensities of these
bands increase indicating a successful EZ isomerization.
Figure 7. IR spectra of pF-AZB@MIL-68(In) after light induced isomerization
with  = 532 nm (EZ) and  = 405 nm (ZE). The as-synthesized state is
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given as a dark grey curve. Arrows mark bands assigned to the E (blue) and Z
(green) isomer.
As a control experiment pure crystalline tF-AZB and pF-AZB
were investigated under the same conditions. The initial state
(before illumination) of pF-AZB consists of a mixture of both
isomers and photochemical conversion of one isomer into the
other was not possible in the crystalline material. In contrast, tF-
AZB shows photo-isomerization in the solid state as well (Figure
S23a), Supporting Information). By illumination of the initial state
containing also a mixture of both isomers with violet light
( = 405 nm) a complete isomerization to 100 % E-tF-AZB was
achieved. Illumination with green light ( = 532 nm) led to 37 %
of the Z isomer.
<Table 3>
As described in our former work for AZB embedded in different
MOF hosts only a maximum of 30 % Z-AZB was reached upon
irradiation.[40] For potential applications this is certainly
a
drawback as well as the necessity of UV light to induce the
photo-isomerization. As highlighted in Table 3 the situation is
different for fluorinated azobenzenes as photochromic guests.
All compounds with pF-AZB as guest molecule can be
transferred to 100 % of the E isomer by illumination with violet
light ( = 405 nm). However, with green light only a maximum of
79 % Z isomer is reached within a MIL-68(In) host. For potential
applications, hybrid materials with embedded tF-AZB are
preferable, as in almost all MOF hosts E and Z ratios above
90 % were reached (Table 3). One might speculate that the size
of the MOF pores influences the photo-isomerization, as for pF-
AZB as photochromic guest the largest Z ratio is observed within
the large pores of MIL-68(In). However, within the pores of
MOF-5 with a similar size only 50 % of Z-pF-AZB were reached
and no such size-dependence is observed for different MOFs
with tF-AZB as guest at all. In Table 2 the shifts of OH
frequencies of the MIL-53/MIL-68 frameworks upon guest
loading are given. It is quite remarkable that upon irradiation and
photo-isomerization these signals show the most significant
shifts for systems with tF-AZB guests. Two exemplary IR spectra
are given in Figure 8. Upon irradiation and photo-isomerization
the host-guest and guest-guest interactions change in these tF-
AZB@MOF systems. Consequently, the ratio of E and Z isomers
seems to be less influenced by the pore size of the respective
MOF host, but more significantly by the underlying host-guest
and guest-guest interactions, which were discussed above for
AZB0.50@MIL-53(Al) and pF-AZB0.34@MIL-53(Al) (Figure 4). For
a rational design of an optimized guest@MOF system it seems
to be indispensable to understand these interactions in more
detail.
Figure 8. Sections (region of OH frequencies) of the IR spectra of tF-
AZB@MIL- 68(In) (above) and tF-AZB@MIL-53(Al) (below) after light induced
isomerization with  = 532 nm (EZ) and  = 405 nm (ZE). The as-
synthesized state is given as a dark grey curve.
It was a remarkable finding of our work on AZB embedded in
different MOFs that for AZB0.50@MIL-53(Al) no photo-
isomerization was achieved even after very long irradiation
times.[40] From the crystal structure it was concluded that the
AZB guests are densely packed within the pores of MIL-53(Al)
so that photo-isomerization is hindered for sterical reasons.[40]
As can be seen in Table 3, tF-AZB (100 % Z), oF-AZB (56 % Z),
and pF-AZB (“little Z”) embedded in MIL-53(Al) allow photo-
isomerization, at least to some extent. But from chemical
analysis (Table S4, Supporting Information) it was found that the
loading in these systems is different: guestx@MIL-53(Al) with x =
0.50 for AZB, x = 0.19 for tF-AZB, x = 0.18 for oF-AZB, and x =
0.30 (or 0.34 depending on the analytical method) for pF-AZB.
Even when considering the different volumes of the guests
(AZB: V/Z = 248.33(5) Å3; tF-AZB: V/Z = 272.90(2) Å3; oF-AZB:
V/Z = 289.90(5) Å3; pF-AZB: V/Z = 284.65(5) Å3)[49] the packing
of AZB (x = 0.50) within the pores of MIL-53(Al) is much denser
than that of tF-AZB (x = 0.19). Hence, varying Z ratios for the
different guest@MOF systems are not surprising.
Kitagawa and co-workers embedded AZB in flexible DMOF-1
and found a reversible structural transformation of the MOF
framework upon photo-isomerization of the AZB guest molecule,
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which was used to remote-control the uptake and release of
CO2.[39] This structural transformation was reached, although
photo-isomerization only led to a Z/E ratio of 38:62.[39] As MIL-
53(Al) and its Ga analogue are also known to show flexible
(“breathing”) behaviour upon guest inclusion,[53,58] we tested our
guest@MIL-53 systems for such behaviour. In Figures S26 and
S27 (Supporting Information) the synchrotron powder diffraction
patterns of tF-AZB0.19@MIL-53(Al) (capillary,  = 0.3 mm) after
irradiation with green ( = 532 nm; several hours) and violet light
( = 405 nm; several hours) are compared with those of non-
irradiated tF-AZB0.19@MIL-53(Al). As the reflection positions are
unchanged no structural transformation has taken place under
these conditions. Slight changes of the reflection intensities
indicate a different arrangement of the tF-AZB guests within the
pores of MIL-53(Al) upon irradiation. In Figure S28 (Supporting
Information) smaller sections of the synchrotron powder
diffraction patterns are compared. The peaks obtained upon
violet light irradiation ( = 405 nm, ZE isomerization) appear
slightly broadened. It should be noted that the low penetration
depth of UV/vis light into solid matter might be the reason that
no structural changes of tF-AZB0.19@MIL-53(Al) were observed
upon irradiation with green or violet light. Notably, differing
results were published for structural transformations of flexible
MOF hosts induced by photo-isomerization of embedded guests.
Kitagawa and co-workers reported such an effect for AZB
embedded in flexible DMOF-1,[39] whereas photo-isomerization
of related PAP (2-phenylazopyridine)[59] or DTE (1,2-bis(2,5-
dimethyl-thien-3-yl)-perfluorocyclopentene)[43] did not lead to a
structural transformation of the same host upon irradiation.
almost quantitative (> 95 %) switching between E and Z isomers
was observed in a solid state material. This core message of our
work is summarized in Figure 9. It is also notable that this
switching can be reached with green and blue light only, i.e. no
UV light is needed to trigger the E/Z isomerisation.
Figure 9. Almost quantitative E/Z isomerisation in tF-AZB@MIL-68(In) with
green and blue light, respectively.
Quantitative switching in solids is still rare, but not
unprecedented.[36,60] From the results of the photophysical
investigations summarized in Table 3 tF-AZB is the most
interesting guest molecule, which already showed very
remarkable photochromic properties in solution (Table 1) or as a
side group in other MOFs.[35,36] Concerning the host material
MIL- 68(In) seems to be preferable due to its higher chemical
stability compared to e.g. MOF-5 and the high photo-
isomerization rates that were achieved within this host (Table 3).
Therefore, we have chosen tF-AZB0.57@MIL-68(In) to investigate
the photochemical stability of this system. In Figure 10 the ratios
of E-tF-AZB after three switching cycles with  = 532 nm and  =
405 nm, resp. are shown, which reveal no significant fatigue in
these measurements. For possible applications it should be
noted that a successful synthesis of thin films of MIL-68(In) has
already been described.[61]
Conclusions
In summary, we have synthesized and characterized 15 new
photochromic materials by embedment of fluorinated
azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB),
4H,4H’-octafluoroazobenzene
(oF-AZB),
and
perfluoroazobenzene (pF-AZB), into the pores of the five well-
known MOFs, i.e. MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga),
and MIL-68(In). The successful embedment was confirmed by
X-ray powder diffraction and the amount of loading determined
from elemental (CHNS), DSC/TGA, and – for one example –
Rietveld analysis. The latter was performed on pF-AZB0.34@MIL-
53(Al). From the resulting crystal structure host-guest and guest-
guest interactions were elucidated revealing O-H…F and …
type interactions, which were compared with the interactions
found in the already described crystal structure of AZB0.5@MIL-
53(Al). In compounds with MOFs of the MIL-type series shifts of
the O-H frequencies of the respective hosts were used to
quantitatively assign these interactions. Notably, the largest
shifts and thus strongest interactions were found for
unfluorinated azobenzene as guest. Similarly, Bléger and co-
workers used the O-H stretching frequencies of an UiO-66 type
MOF with tF-AZB side groups to follow the photo-isomerization
in this system.[35]
Figure 10. Ratio of E-tF-AZB in tF-AZB0.57@MIL-68(In) after three switching
cycles and irradiation with  = 532 nm and  = 405 nm, resp.. For the standard
deviation a value of ± 2 % was estimated.
However, the most remarkable finding of our investigations is
that for some of the synthesized F-AZB@MOF systems an
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with the azobenzene via
a
gas phase procedure. Under reduced
To improve the photochromic properties of the presented F-
AZB@MOF systems it is essential to understand the underlying
host-guest and guest-guest interactions in more detail. In our
investigation we give first insights how these interactions can be
determined. It is very remarkable that for tF-AZB as guest the IR
frequencies show the most significant shifts upon photo-
isomerization (Table 2). This indicates that the host-guest and
guest-guest interactions change upon E/Z isomerization. Hence,
influencing these interactions is a powerful tool to modulate the
photochromic properties of these materials. Additionally, we
found that the photochromic properties are affected by the
size/shape of the MOF’s pore, by the degree of guest loading,
i.e. the packing of the guest molecules within the MOF’s pores,
and/or by the chemical environment given by the MOF pores.
For spiropyrans embedded in MOFs we used the solvatochromic
behaviour of the excited merocyanine form to investigate the
influence of the MOF’s polarity on the photostationary state.[42]
But, as the excited Z form of azobenzene is not solvatochromic,
this approach cannot be used here and any further interpretation
remains speculative at the moment. Nonetheless, to improve the
photophysical properties of these guest@MOF systems for
possible applications it is indispensable to understand the
underlying interactions in more detail, which is the focus of our
current research in this field.
pressure (∼5 × 10−2 mbar) the respective AZB guest was sublimed on to
the activated MOF, which was heated from 50 °C to 70 °C (tF-AZB) or
75 °C (oF-AZB, pF-AZB) for 2-3 hours. The end of the loading process
was recognized, when no longer red crystals of the azobenzene are
visible on the MOF material. The excess of the respective azobenzene
resublimated at the top of the glass tube. To prevent the absorption of
water or the decomposition of the MOF upon contact with air or moisture,
all compounds were stored in a glovebox under a dry argon atmosphere
(H2O < 1 ppm, O2 < 1 ppm), where all further handling was carried out. In
Table S1 (Supporting Information) the detailed weighted samples of all
experiments are listed. A photograph of some of the resulting powders is
given as Figure S5 (Supporting Information). The purity and composition
of the resulting composites guest@MOF were analyzed by XRPD (Le
Bails fits: Table S2, Figures S6-S19 in the Supporting Information) and
elemental analysis (Table S3, Supporting Information). The resulting
compositions are summarized in Table S4 (Supporting Information).
Details of the methods are given below.
X-ray Powder Diffraction. Laboratory measurements were carried out
on a STOE Stadi P diffractometer (Ge monochromator, PSD detector)
with Cu Kα1 radiation. Data were collected at 298 K between 4 and
80.59° in 2θ with steps of 0.01° and a measurement time of 5 s/step. For
each sample four such scans were added. Alternatively, data were
recorded on a Huber G670 diffractometer (Ge monochromator, image
plate detector, room temperature, Cu Kα1 radiation), ca. 120-800 min per
scan. Samples were sealed in glass capillaries ( = 0.3 – 0.5 mm) under
inert conditions (Argon filled glovebox) prior to all measurements.
Synchrotron Powder Diffraction. High-resolution synchrotron powder
diffraction data were recorded at the Swiss Norwegian BeamLine (SNBL,
BM01B)[65] at the European Synchrotron (ESRF, Grenoble/France). The
wavelength was calibrated with a Si standard NIST 640c to 0.50195 Å
and 0.50544 Å, respectively. The diffractometer is equipped with six
counting channels, delivering six complete patterns collected with a small
1.1° offset in 2θ. A Si(111) analyser crystal is mounted in front of each
NaI scintillator/photomultiplier detector. Data were collected at 298 K and
120 K (cryostreamer) with steps of 0.002° (2) and 100-500 ms
integration time per data point. Typical recording times were 20-60 min
per scan. For each sample 5-8 such scans were added. Data from all
detectors and scans were averaged and added to one pattern with local
software.
Experimental Section
MOF synthesis. The colourless metal-organic frameworks MOF-5,[54]
MIL-53(Al),[53] MIL-53(Ga),[58] MIL-68(Ga),[62] and MIL-68(In)[62] were
selected for this investigation. Details of their syntheses are given in the
Supporting Information. MOF-5 was provided by BASF and used without
any further purification.
Synthesis of fluorinated azobenzenes. For the synthesis of tF-AZB
and oF-AZB we used the protocol that was described earlier in detail.[49]
tF-AZB: C12H6F4N2 (254.19): Calcd  C, 56.70%, H, 2.38%, N, 11.02%;
Found  C, 56.92%, H, 2.89%, N, 10.35%; oF-AZB: C12H2F8N2 (326.15):
Calcd  C, 44.19%, H, 0.62%, N, 8.59%; Found  C, 44.64%, H, 0.85%,
N, 7.96%.
An additional synchrotron powder diffraction pattern was collected at
beamline BL9 of the DELTA synchrotron radiation facility, Dortmund.[66]
The measurement was performed at 295 K with a wavelength of
 = 0.81577 Å using a NaI scintillation detector.
pF-AZB was synthesized mainly following the procedures given in the
literature.[63,64] 0.5 g pentafluoroaniline (2.73 mmol) were dissolved in 30
mL toluene and 3.03 g Lead(IV)-acetate (6.83 mmol) were added,
whereon the suspension turned red. After stirring at room temperature
overnight the brown precipitate was filtered off. The solution was washed
with acetic acid (50%) and the organic phase was dried with MgSO4.
After removing the solvent at reduced pressure the residue was purified
by column chromatography (CH2Cl2:cyclo-hexane = 3:7). 128.8 mg
(yield: 37%) perfluoroazobenzene were obtained as a red solid. NMR
signals were assigned according to the literature:[64] 19F-NMR (282MHz,
CDCl3):  /ppm = -161.2 (m, 4F, trans-2F, 2’F, 6F, 6’F), -158,6 (m, 4F,
cis-2F, 2’F, 6F, 6’F), -150,4 (m, 2F, cis-4F, 4’F), -148,3 (m, 6F, trans-F3,
F3’, F4, F4’, F5, F5’), -146,6 (m, 4F, cis-F3, F3’, F5, F5’). C12F10N2
(362.13): Calcd  C, 39.80%, H, 0%, N, 7.74%; Found  C, 39.65%, H,
0.40%, N, 6.77%. Small amounts of hydrogen might result from minor
amounts of moisture or solvents (CH2Cl2 or cyclo-hexane).
For all experiments the substances were filled in glass capillaries
( = 0.7 – 1.0 mm) and sealed under an argon atmosphere. The
capillaries were mounted on spinning goniometers.
Analysis of powder diffraction data. The WinXPow software
package[67] was used for raw data handling and visual inspection of the
data. To determine the unit cell parameters of the resulting guest@MOF
systems precisely, Le Bail fits in Jana2006[68] were performed by refining
the lattice parameters, zero shift, background, and profile parameters
(pseudo-Voigt) for each pattern. Depending on the instrument also
parameters for reflection asymmetry and anisotropy were included in the
refinements. The results of these Le Bails fits are summarized in Table
S2 and the plots of these fits are shown in Figures S6-S19 (all
Supporting Information). They were visualized using Gnuplot 4.6.[69]
Preparation of guest@MOF systems (guest: tF-AZB, oF-AZB, pF-
AZB). In general a mixture of the respective activated MOF was loaded
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Structure Solution and Refinement. The high quality diffraction pattern
of pF-AZB0.34@MIL-53(Al) obtained at the ESRF at 120 K was used to
elucidate the arrangement of the guest within the pores of this MOF. For
the structure solution, the atomic positions of MIL-53(Al)·0.7H2bdc[70]
were used as starting parameters of the host together with the unit cell
parameters obtained from the Le Bail fits. Furthermore, the known
molecular structure of E-perfluoroazobenzene[71] was introduced on the
starting position 000 in FOX.[72] The global optimization algorithm was run
in the parallel tempering mode and converged after approximately
130000 trials for pF-AZB@MIL-53(Al) (Rp = 0.2452 and wRp = 0.1699).
The occupancy of pF-AZB was fixed at 0.125 in the solution process. The
resulting structural model was used as a starting model for a Rietveld
for the different fluorinated AZB guests: pF-AZB (texp = 30-60 min; E/A =
24-95 J/cm2), oF-AZB (texp = 15-58 min; E/A = 12-95 J/cm2), and tF-AZB
(texp = 23-126 min; E/A = 23-278 J/cm2). For pF-AZB (texp = 780 min) and
tF-AZB (texp = 840 min) also long exposures overnight were tested.
For a quantitative analysis the extinctions of the bands of the E and Z
isomers were determined (integration of the areas under the respective
IR peaks after subtraction of the base line) and the maximum amount of
the respective isomer was calculated based on the linear relationship
between the concentration of a species and the extinction in the
measured IR spectrum (Lambert-Beer law). In those systems, where a
complete isomerization of one isomer was observed after irradiation, the
amount of this isomer in the other spectra was directly calculated by
relating it to this 100 % value. However, in most of the investigated
systems no complete isomerization was obtained so that the extinction
for 100% E or Z isomer had to be estimated using the following
procedure: for each isomer one characteristic band with a strong intensity
change after irradiation was chosen. From the change of the peak
intensities for the highest and lowest amount of both isomers and the
remaining intensities that were not extinguished by irradiation completely,
the theoretical extinctions for 100 % E and 100 % Z isomer were
estimated. Whenever possible, several such peaks were examined and a
mean value was calculated. These mean values were used to determine
the relative amounts of both isomers. A worked example is given on page
3 of the Supporting Information to illustrate this procedure.
refinement with GSAS.[73] To get
a stable refinement, the E-
perfluoroazobenzene molecules, as well as the benzenedicarboxylate
anions of the MOF, were fixed with soft constraints (bond lengths, angles,
and planes according to the literature values).[70,71] The isotropic
displacement parameters of the atoms of the pF-AZB guest and the host
were coupled with constraints and refined afterward, if possible. In the
final refinements also the occupancy of the guest was refined resulting in
pF- AZB0.34@MIL-53(Al). This occupancy is in good agreement with the
result of the elemental analysis: pF-AZB0.3@MIL-53(Al) (Tables S3 and
S4, Supporting Information). The final refinement of pF-AZB0.34@MIL-
53(Al) converged slowly with a total of 119 parameters (Rp = 0.0858 and
wRp = 0.1082). The resulting fit is given as Figure S20 and selected
details of the Rietveld refinement are summarized in Table S5, both
Supporting Information. The reliability of the resulting structural model is
corroborated by the good fit (Figure S20), the almost coincident
compositions obtained from Rietveld refinement (x = 0.34) and elemental
analysis (x = 0.30) as well as the reasonable resulting distances between
the pF-AZB guest and the MOF host, which have already been discussed
in the “Results and Discussion” paragraph. Diamond[74] was used for the
visualization of the crystal structure of pF-AZB0.34@MIL-53(Al).
DSC/TGA. DSC/TGA measurements were performed with a Mettler
Toledo TGA/DSC 1 Stare (Al2O3 crucible; Ar stream with 30 mL/min;
heating rate 10 °C/min). Samples of approx. 2-6 mg were weighed out
and handled under inert conditions (glovebox). Diagrams are visualized
with Origin 8.5.0.[76]
Elemental Analysis. Elemental analysis of carbon, hydrogen, and
nitrogen was performed with a HEKAtech GmbH EuroEA 3000 Analyser.
In a glovebox, approximately 2 mg of each compound was filled into a tin
cartridge. Theoretical values were calculated for different ratios
guest:host and compared with the experimental values. The composition
with the best agreement in all three values (C, H, N content) was chosen
as the most likely one and used in the following (Tables S3 and S4 in the
Supporting Information). Compounds with MIL-53(Ga) as host were
excluded from these calculations, as weak extra reflections in the XRPD
patterns of the as-synthesized MOF indicated that it was not obtained as
a single-phase product.
Acknowledgements
The authors gratefully acknowledge H. Emerich and C.
Sternemann for their technical support and expertise at BM01B
(ESRF/Grenoble) and BL9 (DELTA/Dortmund), S. Kremer for
the elemental analyses, M. Prechtl for providing his IR
spectrometer housed in a glovebox, and BASF SE for donating
MOF-5. D.H. acknowledges the Fonds der Chemischen
Industrie for a doctoral stipend.
NMR spectroscopy. 1H and 19F NMR spectra were recorded on a
BRUKER Avance 300 MHz spectrometer in deuterated CDCl3 solutions
at room temperature (1H: 300.13 MHz; 19F: 282 MHz). The spectra were
analysed with the ACD/NMR Processor software.[75]
Keywords: azobenzene • fluorinated derivatives • metal-organic
frameworks • photochromism • solid state switching
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This article is protected by copyright. All rights reserved.

10.1002/chem.201805391
Chemistry - A European Journal
FULL PAPER
Table 1. Selected optical properties of azobenzene and some fluorinated derivatives.
absorption maxima
320 nm (,*)
450 nm (n,*)
250, 270 nm (,*)
450 nm (n,*)
305 nm (,*)
458 nm (n,*)
414 nm (n,*)
photoisomerization
46-53 % Z-AZB ( = 366 nm, cyclohexane)
thermal half-life of Z isomer
Reference(s)
[46,51,52]
E-AZB
Z-AZB
t1/2 = 2 days (room temperature)
t1/2 = 4 h (60 °C, acetonitrile)
E-tF-AZB
Z-tF-AZB
E-oF-AZB
Z-oF-AZB
E-pF-AZB
Z-pF-AZB
84 % Z-tF-AZB ( = 313 nm, acetonitrile)
91 % Z-tF-AZB ( > 450 nm, acetonitrile)
86 % E-tF-AZB ( = 410 nm, acetonitrile)
[48,52]
[52]
t1/2 = 700 days (25 °C)
t1/2 = 92 h (60 °C)
303 nm (,*)
456 nm (n,*)
~240 nm (,*)
413 nm (n,*)
310 nm (,*)
453 nm (n,*)
413 nm (n,*)
92 % Z-oF-AZB ( > 500 nm, acetonitrile)
92 % E-oF-AZB ( = 410 nm, acetonitrile)
91 % Z-pF-AZB ( > 500 nm, acetonitrile)
90 % E-pF-AZB ( = 410 nm, acetonitrile)
t1/2 = 27 h (60 °C)
[52]
not determined
Table 3. Maximum amounts of E and Z isomers for the investigated F-AZB@MOF systems, achieved by illumination with 405 and 532 nm, respectively.
AZB[40]
tF-AZB
oF-AZB
max. E / max. Z, [initial state]
n.d. [62–88 % E]
100 % / 56 % [99 % E]
n.d. [little Z]
pF-AZB
max. E / max. Z, [initial state]
100 % / 25 % [100 % E]
100 % / 0 % [100 % E]
100 % / n.d. [100 % E]
100 % / 27 % [100 % E]
100 % / 30 % [100 % E]
max. E / max. Z, [initial state]
93 % / 82 % [52 % E]
90 % / 100 % [37 % E]
90 % / 94 % [37 % E]
96 % / 93 % [45 % E]
95 % / 95 % [51 % E]
max. E / max. Z, [initial state]
100 % / 50 % [80 % E]
100 % / little Z [100 % E]
100 % / n.d. [100 % E]
100 % / 40 % [87 % E]
100 % / 79 % [55 % E]
MOF-5
MIL-53(Al)
MIL-53(Ga)
MIL-68(Ga)
MIL-68(In)
n.d. [62–88 % E]
88 % / 90 % [62 % E]
n.d.: not determined
This article is protected by copyright. All rights reserved.

10.1002/chem.201805391
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Azobenzenes with different degrees of
fluorination were embedded into the
pores of the metal-organic frameworks
MOF-5, MIL-53(Al), MIL-53(Ga), MIL-
68(Ga), MIL-68(In) and thoroughly
characterized thereafter. All
synthesized F-AZB@MOF systems
can be switched with visible light and
some of them show almost
Daniela Hermann, Heidi A. Schwartz,
Melanie Werker, Dominik Schaniel, Uwe
Ruschewitz*
Page No. – Page No.
Metal-Organic Frameworks as Hosts
for Fluorinated Azobenzenes: A Path
Towards Quantitative Photoswitching
with Visible Light
quantitative (> 95 %) photo-
isomerization between its E and Z
forms with no significant fatigue after
repeated switching cycles.
This article is protected by copyright. All rights reserved.