Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.7b01908

1,3,3-Trimethylindolino-6′-nitrobenzopyrylospiran (SP-1) as an example of a photoswitchable spiropyran was loaded into the pores of different prototypical metal-organic frameworks, namely MOF-5, MIL-68(In), and MIL-68(Ga), by a vapor-phase process. The su

Full text of DOI:10.1021/acs.inorgchem.7b01908

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Solution-Like Behavior of Photoswitchable Spiropyrans Embedded in
Metal−Organic Frameworks
Heidi A. Schwartz,† Selina Olthof,‡ Dominik Schaniel,§ Klaus Meerholz,‡ and Uwe Ruschewitz*,†
†University of Cologne, Institute of Inorganic Chemistry, Greinstraße 6, D-50939 Cologne, Germany
‡University of Cologne, Institute of Physical Chemistry, Luxemburger Straße 116, D-50939 Cologne, Germany
́
§Universite de Lorraine, CNRS, CRM2, F-54506 Nancy, France
S
* Supporting Information
ABSTRACT: 1,3,3-Trimethylindolino-6′-nitrobenzopyrylospiran (SP-1)
as an example of a photoswitchable spiropyran was loaded into the pores
of different prototypical metal−organic frameworks, namely MOF-5,
MIL-68(In), and MIL-68(Ga), by a vapor-phase process. The successful
incorporation in the pores of the MOF was proven by X-ray powder
diffraction, and the amount of the embedded photoswitchable guest was
determined by X-ray photoelectron spectroscopy and elemental analysis.
In contrast to the sterically hindered crystalline state, SP-1 embedded in
solid MOF hosts shows photoswitching under irradiation with UV light
from the spiropyran to its merocyanine form with a nearly complete
photoisomerization. Switching can be reversed by heat treatment. These
switching properties were confirmed by means of UV/vis and IR
spectroscopy. Remarkably, the embedded guest molecules show photo-
switching and absorption properties similar to those in the dissolved state,
so that MOFs might be considered as “solid solvents” for photoswitchable spiropyrans. In contrast to that, embedment of SP-1 in
the smaller pores of MIL-53(Al) was not successful. SP-1 is mainly adsorbed on the surfaces of the MIL-53(Al) particles, which
also leads to photoswitching properties.
INTRODUCTION
■
Photochromism is defined as the reversible transformation of a
molecule between two configurations induced by irradiation
with light, which results in changes in its absorption spectra.1
Depending on the nature of the molecule, light-triggered
structural changes may cause E/Z isomerizations (e.g.,
azobenzenes) or ring-opening/-closing reactions (e.g., diary-
lethenes/spiropyrans); retransformation can be induced by
either light (P-type chromophores) and/or heat treatment (T-
type chromophores).2
Figure 1. Photoinduced isomerization of the spiropyran SP-1 to its
zwitterionic merocyanine form via cleavage of the CSpiro−O bond.
ylospiran (SP-1), which was used as an example of a
photoswitchable spiropyran in this work.
Among the different types of photoresponsive molecules,
spiropyrans have been intensely studied since their discovery by
Hirshberg and Fischer in 19523 and independently by Chaude
and Rumpf in 1953.4 These molecules consist of two π systems
linked by a tetrahedral spiro carbon. They are known to be in
equilibrium with their two metastable merocyanine forms.
Upon irradiation with UV light the stable, closed, and rather
nonpolar spiropyran form (SP) transforms to an open, highly
polar merocyanine form (MC), which may exist in charge-
separated zwitterionic and quinoidal forms.3,4 UV light
irradiation causes cleavage of the CSpiro−O bond and formation
of a larger π-conjugated system, whereas ring reclosure is
achieved by visible light or heat, which makes spiropyrans T-
type chromophores.3,4 In Figure 1 the ring-opening/-closing
reaction is shown for 1,3,3-trimethylindolino-6′-nitrobenzopyr-
Due to the substantial structural reorganization, the switch-
ing of spiropyrans is hindered by spatial confinement in pure
solids, such as crystals or crystalline powders. Therefore,
photochromic behavior of spiropyrans has mainly been studied
in solution.5−8 It is noteworthy that the polarity of the solvent
influences the absorption properties of the resulting merocya-
nine dye by changing the energy gap between the ground state
and excited state.2 This solvatochromism results in red-shifted
absorption bands (bathochromic shift) in nonpolar solvents,
leading to a bluish appearance (cf. Figure 6). Blue-shifted
absorption bands (hypsochromic shifts) appear in polar
solvents, leading to reddish colors. Furthermore, the highly
́
Received: July 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01908
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(H2bdc = C8H6O4, terephthalic acid). The phase purity of both
compounds was checked by XRPD.
polar merocyanine molecules have a strong tendency to
aggregate.9 Arrangement of the dipoles occurs in either an
antiparallel (side by side, H-aggregates) or parallel fashion
(head to tail, J-aggregates), leading to additional blue- or red-
shifted absorption peaks, respectively, in comparison to the
isolated merocyanine molecule. These two aspects, solvato-
chromism and aggregation of the merocyanine dyes, have to be
considered when the switching behavior of spiropyrans in
solution is investigated. Similar effects are expected in the solid
state for spiropyrans either incorporated in porous matrices and
polymers or deposited as thin films.10−21 Here, the necessary
steric degree of freedom is realized by separating the single
molecules from each other. Surprisingly, only one publication
up to now has dealt with the incorporation of a spiropyran into
a thin film of a metal−organic framework (MOF),22 even
though MOFs appear to be ideal and very versatile host
matrices, which has already been shown for other photo-
chromic molecules.23−28
MIL-53(Al) (AlOH(C8H4O4)). MIL-53(Al) was synthesized according
to the protocol known in the literature45 with variations in the amount
of the starting materials and the applied temperature programs. A 1.95
g portion of Al(NO3)3·9H2O (5.20 mmol) and 432.00 mg of
terephthalic acid (2.60 mmol) were mixed with 5.0 mL of deionized
water in a 23 mL Teflon lined autoclave. The mixture was heated to
180 °C at 10 °C/h, kept at this temperature for 72 h, and afterward
cooled to room temperature at 5 °C/h. The resulting colorless powder
was washed with deionized water several times and dried in air. To
remove the embedded terephthalic acid molecules, the residue was
heated in air at 350 °C for 7 days followed by 3 days of heating at 400
°C. Finally, the resulting powder was heated at 100 °C for 1 h under
reduced pressure and stored under an argon atmosphere. The phase
purity was confirmed by XRPD.
Commercially available N,N’-dimethylformamide (Acros Organics),
Ga(NO3)3·xH2O (ABCR), In(NO3)3·xH2O (ABCR), Al(NO3)3·
9H2O (ABCR), and terephthalic acid (Alfa Aesar) were used without
further purification.
Since the first presentation of MOF-5 by Yaghi and co-
workers in 1999,29 metal−organic frameworks have become a
promising field of research. MOFs are hybrid materials,
consisting of an inorganic node and an organic linker molecule,
forming structures with potential voids.30 MOFs offer various
potential applications such as gas adsorption,31,32 separation of
gases33,34 and liquids,34−36 heterogeneous catalysis,37 and drug
molecule encapsulation.38,39 The variable design and possible
functionalization of the linker molecules lead to MOFs with a
wide variety of properties.40 The specific design of MOF pores
results in different chemical and physical environments; hence,
various forms of host−guest interactions may be expected for
photoswitches embedded in different MOF host materials.
Therefore, it is the aim of this work to synthesize and
characterize different SP-1@MOF systems to examine the
influence of the MOF hosts on the optical and photoswitching
properties of the embedded SP-1 molecules and to compare the
resulting properties with those obtained in solution. It should
be noted that our approach is different from the concept, where
the photoactive functionality is part of the backbone26,41 or a
substituent25,42,43 of the linker forming the MOF framework.
This necessarily leads to a high immobilization of the
photoswitchable unit, whereas in our guest@host systems a
higher mobility of the embedded guest molecules is expected.
Preparation of SP-1@MOF Systems. A mixture of the respective
activated MOF and SP-1 was ground under an argon atmosphere. The
resulting homogeneous powder was placed into a small glass vessel
inside a Schlenk tube and heated at 145−150 °C and a reduced
pressure of ∼5 × 10−2 mbar for several hours. The excess of SP-1
resublimated at the top of the glass tube. To prevent the absorption of
water and decomposition upon contact with air and moisture, all
compounds were stored in a glovebox under an argon atmosphere.
SP-1x@MOF-5 (1). The synthesis was carried out as described above
with 60.00 mg (0.08 mmol) of MOF-5 and 126.00 mg (0.4 mmol) of
SP-1, yielding a pink powder.
SP-1x@MIL-68(In) (2). The synthesis was carried out as described
above with 60.00 mg (0.20 mmol) of MIL-68(In) and 64.00 mg (0.20
mmol) of SP-1, yielding a pastel pink powder.
SP-1x@MIL-68(Ga) (3). The synthesis was carried out as described
above with 60.00 mg (0.24 mmol) of MIL-68(Ga) and 77.00 mg (0.24
mmol) of SP-1, yielding a pastel pink powder.
The purities of the resulting guest@MOF materials were checked
by XRPD. The XRPD patterns of 1−3 (loaded and unloaded) are
shown in Figure 2 and Figures S1 and S2 in the Supporting
Information. The resulting unit cell volumes are given in Table 1. The
composition, i.e. the amount of loading, of 1−3 was obtained from
elemental analysis and XPS measurements, which are summarized in
Table 2. Details are given in Figures S3−S5 and Tables S1−S4 in the
Supporting Information.
EXPERIMENTAL SECTION
■
1,3,3-Trimethylindolino-6′-nitrobenzopyrylospiran (SP-1,
C19H18N2O3). SP-1 was used as purchased (TCI) without further
purification. The purity of SP-1 was confirmed by H and 13C NMR
1
spectroscopy.
MOF Synthesis. MOF-5 (Zn4O(C8H4O4)3) was provided by
BASF SE and used without further purification. The phase purity was
checked by XRPD (X-ray powder diffraction).
MIL-68(Ga) (GaOH(C8H4O4)·0.9DMF·zH2O) and MIL-68(In) (InOH-
(C8H4O4)·1.0DMF·zH2O). MIL-68(Ga) and MIL-68(In) were synthe-
sized by following mainly the protocol given in the literature.44 The
corresponding metal salt and terephthalic acid were mixed with 5.0 mL
of DMF in a 23 mL Teflon lined autoclave. The mixture was heated to
100 °C at 20 °C/h, kept at this temperature for 48 h, and afterward
cooled to room temperature at 5 °C/h. The resulting colorless powder
was washed several times with DMF and dried in air. To remove the
embedded DMF molecules, the residue was heated in air at 200 °C for
12 h and then at 100 °C for 1 h under reduced pressure and was stored
afterward under an argon atmosphere. In a typical synthesis 207.40 mg
of Ga(NO3)3·xH2O (0.81 mmol calculated for x = 0) and 100.00 mg
of H2bdc (0.60 mmol) or 408.20 mg of In(NO3)3·xH2O (1.04 mmol
calculated for x = 0) and 200.00 mg of H2bdc (1.20 mmol) were used
Figure 2. XRPD patterns of SP-1@MOF-5 (1) (red) and unloaded
MOF-5 (black), measured at 298 K (BM01B/ESRF: λ = 0.50561 Å
(1) and λ = 0.504477 Å (unloaded MOF-5)). As both patterns were
recorded with different wavelengths 1/d was chosen as the x axis and a
small offset along y (+ 4%) was applied.
B
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multichamber UHV system at a pressure of 5 × 10−10 mbar using a
Phoibos 100 hemispherical analyzer (Specs). As the excitation source a
Mg Kα anode was used (hν = 1252.6 eV, probing depth ∼10 nm).
Due to charging effects during measurements caused by the low
conductivity of the powder samples, the binding energy scale as
measured by XPS was shifted by a few electronvolts for the different
samples. To account for this, the binding energies were corrected such
that adventitious carbon is positioned at 284.8 eV. Integrated peak
areas of characteristic core level excitations were used to calculate the
embedded amount of SP-1 inside the MOF matrices. For this, the peak
areas of N, Zn, In, Ga, and Al were evaluated and corrected by their
relative sensitivity factors (RSF).47 Details are given in Figures S3−S5
and S7 and Table S1 in the Supporting Information.
Table 1. Results of Le Bail Fits of High-Resolution
Synchrotron Powder Diffraction Data of Compounds 1−3 in
Comparison with the Unit Cell Volumes of the Respective
Unloaded MOF
1
2
3
T/K
298
298
298
GOF
0.80
0.97
0.91
Rp
0.186
0.167
0.205
Rwp
0.291
0.248
0.325
V/Å3
17111.8(3)
17156.0(4)
5911.5(2)
5901.1(2)
5217(1)
5197.0(7)
V/Å3 (unloaded MOF)
Elemental Analysis. Elemental analysis of carbon, hydrogen, and
nitrogen was carried out with a HEKAtech GmbH EuroEA 3000
Analyzer. Approximately 2 mg of each compound was filled into a tin
cartridge under an argon atmosphere. For each sample three
measurements were carried out, from which a mean value was
calculated (Tables S2−S5 in the Supporting Information). To estimate
the SP-1 to MOF ratio the following procedure was applied: as only
the SP-1 guest contains nitrogen, x in SP-1x@MOF was optimized in a
way that calculated and found nitrogen contents agree as well as
possible. For SP-1x@MOF-5 this leads to very good agreements
between the calculated and found carbon and hydrogen values as well.
However, significant discrepancies are found for the systems based on
the MIL family as hosts. This is discussed in more detail below.
Reflection Spectroscopy. Measurements were carried out with a
PerkinElmer Lambda 1050 spectrometer. Samples were measured on
an adhesive film in the range 250−750 nm. Data were recorded before
and after irradiation with UV light (λ = 365 nm, 1 min).
SP-1x@MIL-53(Al) (4). The synthesis was carried out as described
above with 60 mg (0.28 mmol) of MIL-53(Al) and 92 mg (0.28
mmol) of SP-1, yielding a pastel pink powder. The purity of the
resulting guest@MOF material was checked by XRPD. No obvious
changes in intensities in comparison to pristine MIL-53(Al) were
found (Figure S6 in the Supporting Information); therefore, it was
concluded that obviously only a very small amount of SP-1 was
embedded in the pores of the MOF. As no additional peaks for SP-1
were found, we concluded that SP-1 was adsorbed as an amorphous
thin film on the surfaces of MIL-53(Al) particles. Nonetheless, the
composition of this SP-1@MIL-53(Al) system was determined from
elemental analysis and XPS measurements: The results are also
presented in Table 2. Details of these investigations can be found in
Figure S7 and Tables S1 and S5 in the Supporting Information.
Synchrotron Powder Diffraction. High-resolution synchrotron
powder diffraction data of pristine and loaded MOFs were recorded at
the Swiss Norwegian BeamLine (SNBL, BM01B) at the European
Synchrotron (ESRF, Grenoble/France). The wavelength was
calibrated with a Si standard NIST 640c to 0.50561 and 0.504477
Å, respectively. The diffractometer is equipped with five counting
channels, delivering five complete patterns collected with a small 1.1°
offset in 2θ. A Si(111) analyzer crystal is mounted in front of each NaI
scintillator/photomultiplier detector.
For all experiments the substances were filled in glass capillaries (1.0
mm i.d., Hilgenberg) and sealed under an argon atmosphere. The
capillaries were mounted on a spinning goniometer. Data were
collected at 298 K between 1.1 and 25° in 2θ with steps of 0.002° and
100 ms of integration time per data point. Typical recording times
were ∼20 min per scan. For each sample five such scans were added.
Data from all detectors and scans were averaged and added to one
pattern with local software. To determine the unit cell parameters
precisely, Le Bail fits in Jana200646 were performed by refining the
lattice parameters, zero shifts, background (manual background
function with 30 points), and profile parameters (TCH profile
function with GW and LY as refined parameters). In all Le Bail fits the
same set of parameters was used. These Le Bail fits are given in Figures
S8−S10 in the Supporting Information.
UV/vis Spectroscopy. UV/vis spectra of the SP-1@MOF systems
were recorded using transparent KBr pellets with a Varian CARY 4000
spectrometer.
Transparent pellets were prepared as follows: half a spatula of the
substance was carefully ground with six spatulas of dried KBr. The
mixture was pressed for 30 min at a pressure of ∼530 bar, yielding a
thin, transparent, slightly colored pellet. The KBr pellet was placed
into the sample holder, and the sample chamber was evacuated to 10−5
mbar. Spectra were recorded before and during irradiation with UV
light (λ = 365 nm). For thermal relaxation, the pellet was placed in a
furnace; details of the irradiation and relaxation times as well as applied
temperatures are given in Table S6 in the Supporting Information.
UV/vis spectra of SP-1 dissolved in different solvents were
measured on a Varian Cary50 Scan photospectrometer (Figure S11
in the Supporting Information).
IR Spectroscopy. IR spectra of solid SP-1@MOF compounds
were measured with a Nicolet7500 FT-IR spectrometer using
transparent KBr pellets. Transparent pellets were prepared as
described for the UV/vis experiments. The KBr pellet was placed
into the sample holder, and the sample chamber was evacuated to 10−5
mbar. Scans were recorded in the range 360−4000 cm−1 with a
resolution of 2 cm−1. Ninety scans for each sample were recorded and
added. Irradiation times varied. They are given in the figures of the
respective spectra.
X-ray Powder Diffraction. To check the purity of the crystalline
samples, 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 80.70° in 2θ
with steps of 0.01° and a measurement time of 5 s/step. For each
sample seven such scans were added.
IR spectra of pristine MIL-68(In) and MIL-68(Ga) were measured
as pure solids with a PerkinElmer Spectrum 400 FT-IR/FT-FIR
spectrometer. Scans were recorded in the range 400−4000 cm−1 with a
resolution of 4 cm−1. One scan for each sample was recorded. Selected
XPS. For XPS measurements, SP-1@MOF powders were placed on
an adhesive copper foil. Measurements were performed on a
Table 2. Ratio of SP-1 per Formula Unit of the Respective MOF for 1−4 Calculated from XPS Measurements and Elemental
Analysis
XPS
elemental analysis
SP-1x@MOF-5 (1)
SP-1:(Zn4O(bdc)3)
SP-1:(In(OH)(bdc))
SP-1:(Ga(OH)(bdc))
SP-1:(Al(OH)(bdc))
2.26:1
0.30:1
0.24:1
0.10:1
2.4:1
SP-1x@MIL-68(In) (2)
SP-1x@MIL-68(Ga) (3)
SP-1x@MIL-53(Al) (4)
0.45:1
0.25:1
0.06:1
C
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IR spectra are shown in Figure 8 and in Figures S12−S16 in the
Supporting Information, focusing on the ν(OH) region.
volumes of the MIL-68 family show a slightly increased unit cell
volume upon loading, which might indicate repulsive
interactions between guest and host, whereas the decrease of
the unit cell volume of MOF-5 upon loading might point to
attractive interactions between guest and host. However, as
these changes are very small (approximately 0.2−0.4%), it will
be essential to confirm this finding by complementary methods:
e.g., quantum chemical calculations.
In contrast, for MIL-53(Al) no changes in reflection
positions and intensities were observed after loading, although
especially the reflection positions of such flexible (“breathing”)
MOFs should be very sensitive to guest incorporation. As in the
diffraction pattern of loaded MIL-53(Al) (Figure S6 in the
Supporting Information) no reflections in addition to the
expected MOF structure were found, though a color change
occurred during the loading process (see above), we concluded
that only a very little amount of SP-1 was embedded in the
pores of MIL-53(Al) and/or merely adsorption on the MOF’s
surface as an amorphous thin film took place. Nonetheless, SP-
1@MIL-53(Al) (4) was further investigated by XPS, IR, and
UV/vis spectroscopy in order to clarify the interaction between
MIL-53(Al) and SP-1. However, these investigations cannot
unambiguously prove whether SP-1 is adsorbed in the pores of
MIL-53(Al) or on its surface.
The successful embedment of SP-1 into the pores of MIL-
68(In) and MIL-68(Ga) can also be concluded from the shifts
of the OH frequencies in the IR spectra before and after loading
(Table S7 in the Supporting Information). These OH groups of
the MIL-68 frameworks are a sensitive probe for guest uptake.50
Shifts of −14 cm−1 (MIL-68(In)) and −6 cm−1 (MIL-68(Ga))
confirm the successful embedment of SP-1, whereas for MIL-
53(Al) a shift of +4 cm−1 indicates a different behavior for this
system, as already concluded from the XRPD measurements.
Composition (Degree of Loading). To understand the
switching behavior of the synthesized SP-1@MOF systems and
to correctly interpret possible host−guest and guest−guest
interactions, it is essential to know the composition of these
systems: i.e., to which fraction the MOF’s pores are filled with
guests. The ratio of guest molecule per formula unit of the
respective MOF was determined by means of XPS and
elemental analysis. In our previous work on azobenzene
(AZB) embedded in different MOF hosts, we only used
elemental analysis to determine the composition of the guest@
MOF systems.51 These results were in good agreement with the
results of Rietveld refinements on AZB0.5@MIL-53(Al) and
AZB0.66@MIL-68(Ga).51 However, the molecular structure of
SP-1 is much more complex than that of AZB so that we have
failed up to now to obtain a reliable structure model for SP-1@
MOF from powder diffraction data. Therefore, we used XPS
data in this work to corroborate the results of the elemental
analysis by a second method. As nitrogen is only found in the
guest, not in the MOF framework, we compared the nitrogen
intensities of typical XPS signals with the intensities of XPS
signals of the respective metal cations of the MOF host. The
results of the XPS measurements are shown in Table 2 and
compared with the results of the elemental analysis. More
detailed information is given in Figures S3−S5 and S7 and
Tables S1−S5 in the Supporting Information. The results of the
elemental analysis and the XPS measurements are in the same
range. However, larger discrepancies are found for the MIL-
68(In) and MIL-53(Al) samples. But, as the calculated and
measured CHN values already show some significant deviations
(Tables S2−S5), we conclude that XPS analysis seems to be the
RESULTS AND DISCUSSION
■
In this work we present our results on MOFs as suitable hosts
for photoswitchable spiropyrans and, in a second step, how the
MOF matrix influences the switching properties of the
embedded dye. In this first systematic investigation we have
concentrated on the influence of the metal node so that only
MOFs with bdc2− as a linker ligand (H2bdc = C8H6O4, 1,4-
benzenedicarboxylic acid = terephthalic acid) were selected.
Furthermore, suitable MOFs for this investigation were chosen
to meet the following requirements:
(1) The guest SP-1 has to fit into the pores of the MOF, and
photoswitching inside these pores has to be possible;
therefore, an adequate pore size is required.
(2) The absorption maxima of guest and host should not
overlap so that the switching process can be followed by
spectroscopic methods. Basically, colorless MOFs are
preferable.
Therefore, we have chosen MOF-5 (Zn4O(bdc)3),29 MIL-
68(In) (In(OH)(bdc)),44 MIL-68(Ga) (Ga(OH)(bdc)),44 and
MIL-53(Al) (Al(OH)(bdc))45 as suitable MOFs for this study,
while 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (de-
noted SP-1 in the following) was selected as the photoactive
guest molecule. From the crystal structure of SP-148 the size of
one molecule including van der Waals radii was estimated (∼15
Å × 8 Å × 8 Å), which should fit into the pores of the MOFs
we have selected for this investigation. The crystal structure of
the merocyanine form of SP-1 is not known, but from a similar
merocyanine49 its molecular size can be estimated to be ∼16.5
Å × 7 Å × 9.5 Å. Thus, during and after switching it should also
fit into the pores of the selected MOFs. Note: for the MOFs of
the MIL series an embedment of the guest molecules along the
porous channels is assumed.
The MOFs were loaded via a gas-phase reaction to exclude
any influence of solvent molecules in these investigations.
During the loading process, the mixture of the previously
colorless MOF and yellow SP-1 forms a slightly colored
powder, giving a first impression of the successful combination
of MOF and guest.
Embedment. The successful embedment of SP-1 into the
MOF’s pores was confirmed by X-ray powder diffraction
(XRPD). After the MOF’s pores were loaded, the integrity of
the MOF framework remained intact, as indicated by the
almost unaltered reflection positions. However, significant
changes in the reflection intensities were observed for SP-1@
MOF-5 (1), SP-1@MIL-68(In) (2), and SP-1@MIL-68(Ga)
(3). The changes in the reflection intensities indicate that the
electron density within the MOF’s pores must have changed, as
expected after embedment of a guest. No additional reflections
pointing to free dye molecules were observed. In Figure 2 the
diffraction patterns of the unloaded MOF-5 as well as 1 are
shown as examples (for diffraction patterns of unloaded/loaded
MIL-68(In) and MIL-68(Ga), see Figures S1 and S2 in the
Supporting Information).
It is remarkable that the Bragg peaks of 1 are shifted to
higher angles in comparison to unloaded MOF-5, indicating a
contraction of the MOF-5 framework upon incorporation of
SP-1. Le Bail fits using the software Jana200646 confirm the
observed decrease of the unit cell volume. In Table 1 the unit
cell volumes of 1−3 are given. It is noteworthy that the unit cell
D
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Figure 3. Reflection spectra (298 K) of 1−4 (respectively (a)−(d)) before (blue) and after (red) irradiation with UV light (λ = 365 nm, 1 min) with
photographs of 1−4 before and after irradiation with UV light.
much more accurate method to determine the degree of
loading in such guest@MOF systems.
To compare the results of the switching properties of SP-1
embedded in different MOF hosts, UV/vis spectra of all
compounds 1−4 were recorded before and after irradiation
with UV light (λ = 365 nm, 1 min). In Figure 3 the resulting
reflection spectra of 1−4 are shown and compared. It should be
noted that additional adsorption of SP-1 as an amorphous film
on the surface of the respective metal−organic framework is
expected to lead to a shoulder in the reflection spectrum, as the
different host−guest interactions within a pore and on a surface
will affect the optical properties. However, even for MIL-
53(Al), for which such a scenario has been assumed, no
shoulder is visible in the spectra shown in Figure 3. For the
latter this leads to the assumption that only a very small amount
of SP-1 is embedded in the pores of MIL-53(Al) and the
reflection spectrum is dominated by the species adsorbed on
the surfaces of the MOF’s particles.
Immediately after loading, a deep pink color is observed for 1
without UV light irradiation (Figure 3a)). The local minimum
in the reflection spectrum of nonirradiated 1 at ∼520 nm can
be assigned to the open merocyanine form of SP-1. On
irradiation (λ = 365 nm, 1 min), the reflection further decreases
in this range, indicating a proceeding isomerization of SP-1 to
its merocyanine configuration (MC-1) inside the pores of
MOF-5. This process can be reversed by heating the sample,
but a complete isomerization back to the SP-1 form was not
achieved. A similar finding was reported by Zhang et al., stating
On comparison of the XPS peaks of nitrogen in Figures S3−
S5 and S7 in the Supporting Information, the peaks in
compounds 1−3 appear somewhat broader than the nitrogen
peak for compound 4. As an interaction of the guest with the
MOF framework might lead to such a broadening, this can be
interpreted as another confirmation that SP-1 was successfully
incorporated in 1−3 but not in 4.
Photoswitching. In solution, spiropyrans show trans-
formation to their merocyanine form upon irradiation with
UV light. The planar structure and extended π conjugation
between the chromene and indoline parts of the merocyanine
leads to a reduction of the band gap, where the absorption
maximum is located between 500 and 600 nm, being strongly
dependent upon the polarity of the surrounding environ-
ment.52,53 Recovery of the closed spiropyran form is achieved
by irradiation with visible light or heat treatment. It is known
that the acidity of the environment (saltlike structures are
formed in the presence of acids)54−59 and metal complex-
ation60−68 also influence the stability and the absorption
properties of the merocyanine dye. However, these two aspects
do not have to be taken into consideration in our investigation,
as the MOFs we have used do not contain any coordinatively
unsaturated metal sites (CUM) for complexation and its ligand
(bdc2−) is not acidic enough to protonate the merocyanine.
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Figure 4. (a) Absorption spectra of 2 after irradiation with UV light (λ = 365 nm) and heat treatment. (b) Photographs of the switching process of a
pressed powder of 2 (for details see text).
that 1-(2-hydroxyethyl)-3,3-dimethylindolino-6′-nitrobenzo-
pyrylospiran is also stabilized in its open MC form within the
pores of JUC-120, an analogue of MIL-100.22 We think that
these results clearly show that the pores of MOF-5 and JUC-
120 establish a chemical environment, which is preferred by
molecules with a high dipole moment such as spiropyrans in
their open MC form.
For 2 and 3 a somewhat different behavior of the SP-1
molecule in the pores of MIL-68(In) and MIL-68(Ga) is
observed. When they are embedded in the respective MOF,
nonirradiated 2 and 3 show only slight coloration (Figure 3b,c).
After irradiation with UV light the color changes to deep purple
and the reflection spectra of 2 and 3 show minima at ∼560 nm
for 2 and at ∼566 nm for 3 (Figures 3b,c), which can be
assigned to the open MC-1 form. The reflection minima of 2
and 3 are shifted to slightly longer wavelengths in comparison
to 1, leading to a more bluish color. Similar to the results shown
above for 1, the isomerization between SP-1 and MC-1 is not
light (λ = 365 nm, 1 min).
Figure 5. Reflection spectra (298 K) of 1−4 after irradiation with UV
sterically hindered inside the pores of MIL-68(In) and MIL-
68(Ga). Switching of 2 and 3 is fully reversible by heating the
sample. This is shown for 2 as an example in Figure 4. Figure 4a
shows the absorption spectra of 2 irradiated with UV light
(solid line) and after heating (broken line). The MC-1 band
decreases significantly after heating. In Figure 4b a pressed
powder of nonirradiated whitish 2 is irradiated with UV light (λ
= 365 nm) using a shadow mask. After irradiation, a powder
with a deep purple color is obtained. The nonirradiated sections
of the pressed powder show the pastel pink color of
nonirradiated 2 after removal of the shadow mask. After
heating pastel pink 2 is recovered in the whole powder.
In Figure 3d the reflection spectra of 4 are shown. Even
Figure 6. Photographs of (left) MC-1 dissolved in solvents with
different polarities after irradiation with UV light (λ = 365 nm, 1 min)
and (right) 1−4 (from left to right) after irradiation with UV light (λ =
365 nm, 1 min).
though a successful embedment of the dye molecule in the
pores of MIL-53(Al) could not be confirmed, photochromic
behavior of the combination of this MOF with SP-1 was
observed. Comparable to the case for 2 and 3, 4 shows
photochromic behavior upon irradiation with UV light (see
Figure 3). Interestingly, the light-induced transformation of SP-
1 to its MC-1 form causes a deep blue color with a reflection
minimum at ∼599 nm, which is significantly red shifted in
comparison to 2 and 3.
It is obvious from Figure 3 that the colors of irradiated 1−4,
i.e. MC-1 embedded in different MOFs, vary significantly: a
more reddish color is found for MC-1 embedded in MOF-5,
whereas a shift to bluish colors is found for MC-1 embedded in
different MOFs of the MIL series. These results are
summarized in Figures 5 and 6 (right), which show the
reflection spectra of 1−4 after irradiation with UV light (λ =
365 nm) and the colors of the resulting powders. As mentioned
above, the energy gap between the ground and excited states of
MC-1 is influenced by the polarity of the solvent in solution.
Therefore, we conclude that the different colors and reflection
spectra found for irradiated 1−4 can be ascribed to variations in
polarity inside the different MOF pores. Thus, the pores within
MOF-5 should be rather polar, leading to a blue (hypsochro-
mic) shifted absorption band and a reddish color. For MIL-
68(In) and MIL-68(Ga) the difference between the ground
state and excited state of the merocyanine form in 2 and 3 is
not as high as in 1, as a bathochromic shift of the absorption
band is found leading to a more bluish color. Therefore, it can
be concluded that the pores within MIL-68(In) and MIL-
68(Ga) establish an environment which is less polar than that
of MOF-5. As the same bdc2− linker is used in these MOFs, this
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difference must be related to their knots, which are [Zn4O]6+
(MOF-5) and [Ga(OH)]2+/[In(OH)]2+ units (MIL-68). Just
focusing on the charges of these knots, one indeed expects a
higher polarity for pores in MOF-5, as has been experimentally
observed. The difference between Ga(OH)2+ and In(OH)2+
due to the increasing charge/radius ratio from In3+ to Ga3+ is
obviously too small to lead to a significant change in the
recorded spectra.
For 4 the situation is not as clear as for 1−3. As already
mentioned, apparently only a small amount of SP-1/MC-1 is
embedded in the pores of MIL-53(Al), which are the smallest
of all MOF pores of this investigation. Additionally, SP-1/MC-
1 is assumed to be adsorbed on the surfaces of the MOF
particles. For the [Al(OH)]2+ knots in MIL-53(Al) a polarity
between those of MOF-5 and MIL-68 is expected, whereas an
adsorption on the surface should be less polar. From the
reflection measurements we conclude that SP-1 must be mainly
adsorbed on the surfaces of MIL-53(Al) particles, resulting in a
red-shifted reflection minimum in comparison to 1−3 (Figure
5). This red-shifted minimum leads to a bluish color of 4
(Figure 6, right).
Figure 7. Plot of the absorption maxima (in nm) of MC-1 dissolved in
different solvents (blue dots) against the elution power ε0 on alumina
taken from Snyder.69 These values were linearly fitted (blue dotted
line). The reflection minima of the different SP-1@MOF systems
(green diamonds) were added to this curve classifying the polarity of
the respective MOFs.
It is also remarkable that the colors of the nonirradiated
guest@MOF systems 1−4 differ significantly, which can be
directly correlated to the amount of MC-1 embedded in the
pores of the respective MOF (in addition to SP-1, see
absorption spectra in Figure 3). With increasing polarity of the
pores the amount of the open polar MC-1 form increases. As
already mentioned, a similar finding was reported for thin films
of JUC-120, an analogue of MIL-100,22 where after embedment
of a spiropyran larger amounts of the open merocyanine form
were found, which was also attributed to the polarity of the
pores in JUC-120.22
As all these observations (switching properties, dependence
of the MC-1 color on the MOF’s polarity) resemble very much
the behavior of spiropyrans dissolved in solvents with different
polarities,2 we conclude that MOFs might be considered as
“solid solvents” for this class of photochromic molecules. In
Figure 6 the correspondence between MC-1 dissolved in
different MOFs and solvents is shown. From methanol to
toluene the polarity of the solvent decreases (Figure 6, left) and
the absorption of MC-1 is increasingly red shifted, leading to a
more bluish color. This correlates nicely with the colors of MC-
1 embedded in different MOFs (Figure 6, right).
To classify the polarities of the different MOFs used in this
investigation, the absorption maxima of the merocyanine (MC-
1) dissolved in solvents of different polarities (Figure S11 in the
Supporting Information) were plotted against the elution
power ε0 of the respective solvent on alumina, which was taken
from Snyder.69 In Figure 7 the resulting values (blue dots) are
shown, which give an almost linear fit (blue dotted line).
Adding the reflection minima of the different SP-1@MOF
systems (green diamonds), one can estimate the polarity of the
respective MOF host compared to these solvents.
According to the plot in Figure 7, MOF-5 is even more polar
than methanol and both MOFs of the MIL-68 series show
polarities between those of acetonitrile and acetone, whereas
the polarity of MIL-53(Al) (better: it’s surface) almost equals
that of toluene. However, note that for SP-1@MIL-53(Al) our
results indicate that SP-1 is merely adsorbed on the surfaces of
the MOF particles. These results nicely show the similarity
between SP-1 dissolved in different solvents and embedded in
different MOFs so that our conclusion to understand MOFs as
“solid solvents” for such solvatochromes seems to be reasonable.
Of course, more examples with other solvatochromes
embedded in different MOFs are needed to support this
hypothesis. This is part of our current investigations in this
field.
As shown above, the successful isomerization of SP-1 to its
MC-1 form was confirmed by means of UV/vis spectroscopy
(Figure 3). In addition, IR spectroscopy was employed as a
powerful tool to gain more detailed information about the
amount of switching from SP-1 to MC-1 in 1−4, as has
previously been shown for some azobenzene@MOF systems.51
As the formation of the merocyanine moiety leads to a cleavage
of the CSpiro−O bond in SP-1 (Figure 1), the decrease of this
band at ∼950 cm−1 can be used to follow the formation of MC-
1.70 Therefore, IR spectra on polycrystalline powders of 1−4
pressed to thin transparent KBr pellets were recorded before
and during irradiation with UV light (λ = 365 nm). These
spectra are shown in Figure 8.
Formation of the merocyanine moiety is proven for all
compounds 1−4, as the intensity of the band at ∼950 cm−1
decreases upon irradiation with UV light (λ = 365 nm).
Depending on the MOF host, a complete isomerization took
up to ∼17 h. Long irradiation times are required, as MC-1
formed at the surface of the pellets strongly absorbs the
incoming UV light, such that SP-1 molecules in the bulk receive
a significantly lower UV dose, which hinders their switching.
This assumption is validated by a significant decrease in the
Cspiro−O absorption band after turning the KBr pellets to their
opposite sides (cf. Figure 8).
CONCLUSION
■
In conclusion, we synthesized four new switching systems
consisting of a photoswitchable spiropyran (SP-1) embedded in
four different metal−organic frameworks: namely, MOF-5,
MIL-68(In), MIL-68(Ga), and MIL-53(Al). The resulting SP-
1@MOF systems were thoroughly investigated by means of
XRPD, XPS, and elemental analysis to confirm the successful
loading and the degree of loading. It was found that the pores
of MOF-5, MIL-68(In), and MIL-68(Ga) were successfully
filled with SP-1, whereas the smaller pores of MIL-53(Al) were
only filled with a minor amount of SP-1 and the major amount
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Figure 8. IR spectra of 1−4 (respectively (a)−(d)) during irradiation with UV light (λ = 365 nm) focusing on the decrease (the direction of the
decrease is marked with an arrow) of the band of the Cspiro−O bond at approximately 950 cm−1. Measurements labeled with an asterisk mark the
spectra which were recorded after turning the KBr pellets of 1−4 to their opposite sides.
was adsorbed on the surfaces of the MIL-53(Al) particles.
Nonetheless, photochromic behavior was confirmed for all
compounds by UV/vis and IR spectroscopy.
However, these authors did not ascribe their finding to the
different polarities within the MOF pores.
As mentioned in the Introduction, merocyanines in their
zwitterionic forms are known to have a strong tendency to form
aggregates. However, as in the spectra shown in this
contribution no additional blue (H-aggregates)- or red-shifted
(J-aggregates) absorption bands were found, no such
aggregation can be concluded from our spectra. However,
after very long irradiation times with UV light (>12 h) we
observed some hints that aggregation might take place. This is
currently being investigated by our group.
It is a remarkable finding that the photoswitching properties
of embedded SP-1 strongly depend on the MOF host. Before
irradiation with UV light already a larger amount of SP-1
switched to its merocyanine form (MC-1) after embedment in
MOF-5, whereas in MIL-68(In) and MIL-68(Ga) only minor
amounts of MC-1 were found before UV irradiation. After
irradiation with UV light (λ = 365 nm) increasing amounts of
MC-1 were found, depending on the time of the irradiation
(also for SP-1@MIL-53(Al)). However, the color of the MC-1
form strongly depends on the MOF host, as a more reddish
color was found in MOF-5 and more bluish colors were found
in the MOFs of the MIL series. We were able to show that this
is due to the larger polarity within the pores of MOF-5 in
comparison to MIL-68. As this behavior resembles very much
the behavior of SP-1 dissolved in solvents with different
polarities, MOFs might be considered as “solid solvents” for such
photoswitchable molecules. During the preparation of this
paper a similar finding was reported by Shustova and co-
workers, who found that after embedment of the chromophore
MeO-oHBI in different MOF types the emission wavelength
varies by 100 nm depending on the respective MOF.71
Recently Gascon and co-workers72 gave an interesting
overview on photoswitch@MOF hybrid materials. Most of
the presented approaches contain the photoswitchable moiety
as a part of the MOF linker, which typically requires complex
syntheses. Our approach to embed readily available photo-
switchable molecules in the pores of well-known MOFs seems
to be an easy alternative, which is not restricted to bulk
materials, but can also be extended to thin films.73 In their
review Gascon and co-workers state that “... SP/MC-based
MOFs still remain unexplored.”72 With this contribution we
present the very first results on these SP/MC-based MOFs,
which seem to offer new and remarkable photophysical
properties.
H
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Photomerocyanines in Bilayers-Clay Matrices. J. Chem. Soc., Chem.
Commun. 1991, 7, 532−533.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01908.
■
S
(12) Casades, I.; Constantine, S.; Cardin, D.; García, H.; Gilbert, A.;
́
Marquez, F. “Ship-in-a-Bottle” Synthesis and Photochromism of
Spiropyrans Encapsulated within Zeolite Y Supercages. Tetrahedron
2000, 56, 6951−6956.
(13) Schomburg, C.; Wark, M.; Rohlfing, Y.; Schulz-Ekloff, G.;
Details of XPS measurements/calculations and elemental
analyses, IR and UV/vis spectra, XRPD patterns of
loaded and unloaded MOFs, and XRPD patterns with Le
Bail fits (PDF)
Wohrle, D. Photochromism of Spiropyran in Molecular Sieve Voids:
̈
Effects of Host−guest Interaction on Isomer Status, Switching Stability
and Reversibility. J. Mater. Chem. 2001, 11, 2014−2021.
(14) Casades, I.; Alvaro, M.; García, H.; Pillai, M. N. Modified
Mesoporous MCM-41 as Hosts for Photochromic Spirobenzopyrans.
Photochem. Photobiol. Sci. 2002, 1, 219−223.
AUTHOR INFORMATION
Corresponding Author
*E-mail for U.R.: Uwe.Ruschewitz@uni-koeln.de.
ORCID
Selina Olthof: 0000-0002-8871-1549
Uwe Ruschewitz: 0000-0002-6511-6894
Author Contributions
H.A.S. performed the syntheses and all experiments, which
were coplanned by H.A.S. and U.R.. S.O., D.S., and K.M.
provided the different spectroscopic instruments as well as their
expertise in these methods. All authors discussed the results and
the final version of the manuscript, which was written by H.A.S.
and U.R.
■
(15) Kinashi, K.; Harada, Y.; Ueda, Y. Thermal Stability of
Merocyanine Form in Spiropyran/silica Composite Film. Thin Solid
Films 2008, 516, 2532−2536.
(16) Dattilo, D.; Armelao, L.; Fois, G.; Mistura, G.; Maggini, M.
Wetting Properties of Flat and Porous Silicon Surfaces Coated with a
Spiropyran. Langmuir 2007, 23, 12945−12950.
́
(17) Renkecz, T.; Mistlberger, G.; Pawlak, M.; Horvath, V.; Bakker,
E. Molecularly Imprinted Polymer Microspheres Containing Photo-
switchable Spiropyran-Based Binding Sites. ACS Appl. Mater. Interfaces
2013, 5, 8537−8545.
(18) Kundu, P. K.; Olsen, G. L.; Kiss, V.; Klajn, R. Nanoporous
Frameworks Exhibiting Multiple Stimuli Responsiveness. Nat.
Commun. 2014, 5, 3588.
(19) Li, Z.; Wan, S.; Shi, W.; Wei, M.; Yin, M.; Yang, W.; Evans, D.
G.; Duan, X. A Light-Triggered Switch Based on Spiropyran/Layered
Double Hydroxide Ultrathin Films. J. Phys. Chem. C 2015, 119, 7428−
7435.
Notes
The authors declare no competing financial interest.
(20) Lee, C.-Y.; Hu, C.-H.; Cheng, S.-L.; Chu, C.-C.; Hsiao, V. K. S.
Reversible Photoluminescence in Spiropyran-Modified Porous Silicon.
J. Lumin. 2015, 159, 246−250.
(21) Lukyanov, B. S.; Metelitsa, A. V.; Voloshin, N. A.; Alexeenko, Y.
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L. Solid State Photochromism of Spiropyrans. Int. J. Photoenergy 2005,
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge H. Emerich for his
technical support and expertise at the Swiss Norwegian
BeamLine (SNBL BM01B) at the ESRF (Grenoble), S. Kremer
for the elemental analyses, and BASF SE for providing MOF-5.
(22) Zhang, F.; Zou, X.; Feng, W.; Zhao, X.; Jing, X.; Sun, F.; Ren,
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