A mixed-linker ZIF containing a photoswitchable phenylazo group

Article abstract of DOI:10.1002/ejic.201100789

We report the synthesis and characterization of the new switchable Zn-based zeolitic imidazolate framework (ZIF) [Zn(Im)(aIm)] (1). The high-throughput investigation of the mixed linker system Zn²⁺/imidazole (HIm)/2-phenylazoimidazole (HaIm)/DMF at 85 °C led to 1, which is isostructural to ZIF-8 and crystallizes in a sodalite (SOD)-type structure. The preparation was also studied with microwave-assisted heating and ultrasound-assisted synthesis. The crystal structure was determined from single-crystal X-ray diffraction data. Although Im^- and aIm ^- ions are present in a 1:1 molar ratio, no ordering of the 2-phenylazo group was observed. Incorporation of the Im^- and aIm ^- linkers as an integral part of the framework structure was confirmed by elemental analysis, ¹³C and ¹⁵N MAS NMR, IR and Raman spectroscopy. In addition, the permanent porosity of 1 was demonstrated by N₂ sorption experiments and a specific surface area of S_BET = 580 m²g^-1 is observed. The photoswitching properties were investigated by UV/Vis spectroscopy as the cis and trans isomers exhibit different UV absorption spectra. Switching can be achieved by irradiation with UV light (λ = 355 nm), and back-switching using visible light (λ = 525 nm). Although changes in the UV/Vis spectra are detected, the switching process is only partially reversible.

Full text of DOI:10.1002/ejic.201100789

FULL PAPER
DOI: 10.1002/ejic.201100789
[Zn(C3H3N2)(C3H2N2–N=N–C6H5)], a Mixed-Linker ZIF Containing a
Photoswitchable Phenylazo Group
Stephan Bernt,[a] Mark Feyand,[a] Antje Modrow,[a] Julia Wack,[b] Jürgen Senker,[b] and
Norbert Stock*[a]
Keywords: Metal-organic frameworks
/ Microporous materials / Structure elucidation / Optical switching /
High-throughput methods
We report the synthesis and characterization of the new
switchable Zn-based zeolitic imidazolate framework (ZIF)
[Zn(Im)(aIm)] (1). The high-throughput investigation of the
mixed linker system Zn2+/imidazole (HIm)/2-phenylazoimid-
azole (HaIm)/DMF at 85 °C led to 1, which is isostructural to
ZIF-8 and crystallizes in a sodalite (SOD)-type structure. The
preparation was also studied with microwave-assisted heat-
ing and ultrasound-assisted synthesis. The crystal structure
was determined from single-crystal X-ray diffraction data.
Although Im– and aIm– ions are present in a 1:1 molar ratio,
no ordering of the 2-phenylazo group was observed. Incorpo-
ration of the Im– and aIm– linkers as an integral part of the
framework structure was confirmed by elemental analysis,
13C and 15N MAS NMR, IR and Raman spectroscopy. In ad-
dition, the permanent porosity of 1 was demonstrated by N2
sorption experiments and a specific surface area of SBET
=
580 m2 g–1 is observed. The photoswitching properties were
investigated by UV/Vis spectroscopy as the cis and trans iso-
mers exhibit different UV absorption spectra. Switching can
be achieved by irradiation with UV light (λ = 355 nm), and
back-switching using visible light (λ = 525 nm). Although
changes in the UV/Vis spectra are detected, the switching
process is only partially reversible.
Zn2+ or Co2+ ions. By employing imidazolate derivatives or
mixtures thereof, various zeolitic topologies with numerous
functional groups have been obtained.[17,18,19]
Introduction
Metal-organic frameworks (MOFs) have gained in-
creased attention in recent years due to their specific chemi-
cal and physical properties, such as pore size distribution,
surface properties and chemical functionality.[1–4] They con-
stitute a class of porous compounds that bridge the gap
between microporous zeolites and ordered mesoporous sil-
ica-based materials.[5] MOFs are constructed from inor-
ganic building units that are connected by organic linkers.[6]
The choice of the linker molecule can vary the pore size,
chemical functionality and physical properties such as sorp-
tion.[7,8] Functionality can be introduced directly by using
functionalized linkers such as aminoterephthalic acid,[9,10]
by coordination of guest molecules to unsaturated metal
sites[11] or by postsynthetic covalent modification.[12,13]
Tian et al. have reported a new class of MOFs[14,15] called
zeolitic imidazolate frameworks (ZIFs).[16] These com-
pounds contain Zn2+ or Co2+ ions that are tetrahedrally
surrounded by imidazolate linkers, which each bridge two
One goal in our current studies on MOFs is the introduc-
tion of functionality that can be modified by external stim-
uli. For example, sorption properties can be changed by ion
exchange,[20] porosity can be switched by guest exchange[21]
and the opening of pores can be triggered by gas adsorp-
tion.[22] In addition, guest-induced colour change[23] and
temperature-induced cooperative spin-crossover behaviour
in a 3D coordination polymer have been observed.[24] We
are interested in the use of photoswitchable organic linker
molecules for the synthesis of MOFs and have recently
demonstrated the reversible switching of the mixed-linker
MOF CAU-5, which contains azophenyl-4,4Ј-bipyridine
and 2,6-naphthalenedicarboxylate ions.[25]
Aromatic molecules that contain azo groups, such as
azobenzene or arylazoimidazole derivatives, are textbook
examples for photoisomerization reactions.[26,27] In general
the trans isomer of azobenzene is the thermodynamically
more stable form[28,29] and exhibits two distinct absorption
maxima. One with a lower intensity at λmax = 444 nm (n
Ǟ π* transition) and another at λmax = 316 nm (π Ǟ π*
transition). The cis isomer exhibits an absorption maximum
at λmax = 437 nm (n Ǟ π* transition) and the π Ǟ π* transi-
tion is shifted to λmax = 270 nm. The trans isomer can be
switched to the cis isomer by irradiation with UV light (ca.
365 nm). The switching process is schematically shown in
[a] Institut für Anorganische Chemie, Christian-Albrechts-
Universität zu Kiel,
Max-Eyth-Straße 2, 24118 Kiel, Germany
Fax: +49-431-8801774
E-mail: stock@ac.uni-kiel.de
[b] Anorganische Chemie III, Universität Bayreuth,
Universitätsstr. 30, 95447 Bayreuth, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100789.
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Photoswitchable Metal-Organic Framework
Figure 1. This is clearly visible in the UV/Vis spectra as the
intensity of the band at λmax = 316 nm decreases and the
intensity of the band at λmax = 444 nm increases. Therefore,
both isomers have distinct UV/Vis spectra and can easily be
distinguished. The reversible back-switching can be
achieved by heating or irradiation (ca. 440 nm).
Figure 3. Discovery library of the high-throughput investigation.
Empty circles denote clear solutions. The amounts used in each
reactor are given in Table S2.
Figure 1. Schematic representation of the switching process.
MOFs containing azo groups are known in the litera-
ture.[25,30,31,32] In most of these structures the azo groups
are an integral part of the linker molecules. Thus, switching
is strongly hindered and has not been demonstrated to date.
In contrast, reversible switching should be feasible for linker
molecules with azo groups that protrude into the pores. The
structure of a 1,1Ј-bis[(2-phenylazo)imidazol-1-yl]methane-
based MOF that contains two phenylazo groups was re-
cently published but no switching properties were shown.[33]
Here, we present the synthesis and detailed characteriza-
tion of a switchable mixed-linker ZIF, [Zn(Im)(aIm)] (1),
which contains imidazolate (Im–) and 2-phenylazoimidazol-
ate (aIm–) ions.
When HaIm was solely employed as the organic linker,
reactions in methanol led exclusively to clear solutions,
whereas reactions in DMF yielded zinc formate (CCDC-
266350),[36] which is due to the partial hydrolysis of DMF.
The mixed-linker system 1 was obtained with DMF and
orange, air-stable single crystals (Figure S3) suitable for
crystal structure determination were isolated from the mix-
ture containing Zn(NO3)2·6H2O/HIm/HaIm/DMF in the
molar ratio 1:3:1:97.
Compound 1 was also obtained using conventional heat-
ing (CH), microwave-assisted (MW) heating or ultrasound
(US). The last two methods led to a substantially reduced
reaction time (5 min). The three different synthetic methods
resulted in phase-pure products. The CH synthesis led to
large single crystals, whereas MW and US reactions yielded
microcrystalline powders.
Results and Discussion
The HaIm linker was synthesized from aniline and
HIm[34] and purified by column chromatography (Figure 2).
It was subsequently employed in the high-throughput inves-
tigation of the Zn(NO3)2·6H2O/HIm/HaIm/N,N-dimethyl-
formamide (DMF) system. High-throughput (HT) methods
allow the simultaneous investigation of different reaction
parameters in solvothermal syntheses (Figure S1, Support-
ing Information) and are useful in the discovery of new
phases and the subsequent synthesis optimization.[35] The
discovery library was set up varying the solvent (DMF and
methanol) and employing the molar ratios Zn2+/HaIm/HIm
= 1–3:1–4:0–3 (Figure 3 and Table S2). The reaction prod-
ucts were characterized by X-ray powder diffraction
(XRPD) measurements and the results are shown in Fig-
ure 3.
The orange, air stable compound was activated at 200 °C
in vacuo for characterization by XRPD, thermogravimetric
analysis (TGA), elemental analysis and IR, Raman, UV/Vis
and solid-state NMR spectroscopy.
Compound 1 is isostructural to ZIF-8, which crystallizes
in a sodalite (SOD)-type framework. The SOD structure
has been observed in compounds that contain Zn2+ or Co2+
with 2-methyl-, 2-nitroimidazole, imidazole-2-carbalde-
hyde or benzimidazole (ZIF-7, -8, -9, -65, -67, -90, -91,
-92).[19,37,38] Two linkers are incorporated in 1: Im– and
aIm–. Based on the single crystal data, no ordering of the
phenylazo groups takes place. 1 is isostructural to ZIF-8
and crystallizes in the space group I23. Thus, we were able
to establish the SOD framework as well as the position of
the azo group during structure refinement (see experimental
section for crystallographic data, Figures 4 and S4).
Indexing the XRPD pattern (Figures 5 and S5) unequiv-
ocally demonstrated the presence of only one crystalline
phase. The lattice parameter [a = 17.009(6) Å] compares
well with results from the single-crystal X-ray diffraction
experiment [a = 17.023(2) Å].
The composition of activated 1 was established by ele-
mental analysis and TGA. The observed and calculated C,
H and N values compare well, and the TGA curve shows a
Figure 2. Synthesis of HaIm.
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N. Stock et al.
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Figure 4. Asymmetric unit of 1.
Figure 7. 15N CP MAS NMR spectrum of 1. The spinning side-
band is marked with an asterisk.
Figure 5. XRPD patterns of 1 compared to the simulated pattern,
which is based on single crystal data.
weight loss of 73.14% for the CH and MW products and
66% for the US product between 340 and 700 °C (calcd.
73.54%) with ZnO as the final decomposition product
(Figure S6). The incorporation of the aIm linker as part
of the framework was confirmed by solid-state NMR and
Raman spectroscopy (Figures 6, 7 and 8). The characteris-
tic –N=N– asymmetric vibration band is located at
1441 cm–1 in the Raman spectrum, and characteristic aro-
matic =C–H stretching vibrations are observed between
3142 and 3047 cm–1.
Figure 8. Raman spectra of 1 (bottom) and HaIm (top). The trans –
N=N– vibration of 1 (1441 cm–1) and the aromatic =C–H stretch-
ing vibrations (3142–3047 cm–1) of the phenyl ring and the imid-
azolate ions are marked with asterisks.
which are shifted downfield. Two new signals (h and f, g)
for Im– are present.
The 15N CP MAS NMR spectrum shows three signals
that can be assigned to aIm and Im. The signal at 107 ppm
is due to the nitrogen atoms of the azo group, and those at
–172 and –175 ppm can be assigned to the nitrogen atoms
of the imidazolate ions (Figure 7). A 15N MAS NMR spec-
trum of the pure HaIm molecule cannot be recorded due
to its very slow spin relaxation.
Although the aIm linker protrudes into the SOD cages,
permanent porosity was demonstrated by N2 sorption ex-
periments at 77 K (Figure 9). The N2 sorption isotherm of
the activated sample (CH, 200 °C, 12 h, vacuum) shows a
rapid increase at low p/p0 values followed by a plateau,
which is typical of type I isotherms. Evaluating the data
with the Brunauer–Emmett–Teller (BET) equation resulted
in a specific surface area (SBET) of 580 m2 g–1 with a micro-
pore volume (Vp) of 0.26 cm3 g–1. The N2 sorption iso-
therms of the MW and US samples show similar behaviour
Figure 6. 13C MAS NMR spectrum of 1 (top) and 13C CP MAS
NMR spectrum of pure HaIm (bottom).
The 13C cross polarization (CP) magic angle spinning but with slightly lower specific surface areas (US: SBET
=
(MAS) NMR spectrum of the HaIm linker (Figure 6, bot- 544 m2 g–1, Vp = 0.25 cm3 g–1; MW: SBET = 507 m2 g–1, Vp
tom) shows five signals that can be clearly assigned. The = 0.26 cm3 g–1). The specific surface area of 1 is significantly
signals of 1 can be assigned to both imidazolate linkers lower than ZIF-8 [SBET = 1030 m2 g–1, Vp = 0.49 cm3 g–1
(Figures 6, top, and S8). Due to the deprotonation of HaIm (calculated with PLATON as 0.54 cm3 g–1)], in which a
and HIm, only two signals for a, b, f and g are observed, methyl group protrudes into the SOD cages.[12,32]
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Photoswitchable Metal-Organic Framework
(confinement effect). Repeating this procedure led to the
observation of the same behaviour. After three switching
cycles the initial curve cannot be reached.
Conclusions
We have synthesized the porous, air-stable ZIF [Zn-
(Im)(aIm)] (1), which contains photoswitchable azophenyl
groups. This compound was formed in a solvothermal reac-
tion using a mixed-linker system. X-ray diffraction experi-
ments demonstrated its structural relationship with ZIF-8,
and the incorporation of aIm– was proven by Raman and
solid-state NMR spectroscopy. Although the large phen-
ylazo group protrudes into the cage, 1 shows permanent
prosity. UV/Vis switching experiments demonstrated the
cis–trans isomerization and showed partial reversibility of
the switching process.
Figure 9. N2 sorption isotherms of 1 (triangles: CH product, penta-
gons: US product, stars: MW product).
The switching properties of 1 were investigated using
UV/Vis spectroscopy (Figures 10 and S9). Switching the
azo groups from trans to cis configuration was ac-
complished by UV irradiation (355 nm, 150 W xenon lamp,
1 h). Back-switching was achieved by irradiation with vis-
ible light (525 nm, 150 W xenon lamp, 1 h) but not ther-
mally (100 °C in air for 14 h). The switching of HaIm is
hard to observe as fast thermal back-switching (cis to trans)
takes place. In contrast, N-alkyl-substituted imidazolate de-
rivatives exhibit much lower rate constants.[24,25]
Experimental Section
General: Synthetic procedures for HaIm and 1 and selected spectro-
scopic data are described in this section. All chemicals were used
as obtained, unless stated otherwise.
2-Phenylazoimidazole (HaIm):[34] A mixture of aniline (10.6 mL,
116 mmol) and tetrafluoroboric acid (57.5 mL, 50%) was cooled to
0 °C. A solution of sodium nitrite in deionized water (18 mL) was
slowly added. The precipitate was separated and washed with
ethanol and diethyl ether to obtain benzenediazonium tetra-
fluoroborate (26.7 g).
Imidazole (6.8 g, 100 mmol) was added to a solution of sodium
hydrogen carbonate (4.5 g, 53.6 mmol) in deionized water (45 mL).
After homogenization of the solution, benzenediazonium tetrafluo-
roborate (19.2 g, 100 mmol) in deionized water (100 mL) was
added. A brown precipitate formed immediately and the mixture
was stirred for 30 min and allowed to stand for another 30 min.
The precipitate was separated and washed with deionized water.
The product was purified by column chromatography on basic alu-
minium oxide with ethyl acetate (+1% triethylamine) to give HaIm
(9.8 g, 57%) as orange needles. C9H8N4 (172.07): calcd. C 62.78,
H 4.68, N 32.54; found C 62.54, H 4.64, N 32.61. 1H NMR
(200 MHz, [D6]DMSO, 300 K, numbering according to Figure 2):
δ = 7.38 [s, 2 H, 1,2-H], 7.6 [m, 3 H, 5,6,7-H], 7.85 [m, 2 H, 4,8-
H], 13.2 [br. s, 1 H, 3-H] ppm. MS-EI: m/z (%) = 172.0 [M]+ (77%),
144 (100), 117 (57), 105 (10); (CI) 173 [M + H]+ (65%), 144 (100),
117 (48), 105 (11).
Figure 10. UV/Vis spectra of 1 before irradiation (black line), after
irradiation at 355 nm for 1 h (red line) and after irradiation at
525 nm for 1 h (green line). Only one cycle is presented for clarity
and more cycles are shown in Figure S9. The reversibility of the
switching process (based on the πǞπ* absorption band at 392 nm)
is shown in the inset. Every whole number represents a cycle of
switching to the cis product and back-switching to the trans prod-
uct.
Based on results reported for HaIm,[28,29] the bands can
be assigned to the πǞπ* (392 nm) and nǞπ* transitions
(450 nm). Upon irradiation with UV light, changes in the
intensities of these bands are observed; the intensity of the
πǞπ* band decreases, which is accompanied by an increase
in the intensity of the nǞπ* band. Back-switching with vis-
ible light leads to an increase in the intensity of the πǞπ*
band and a decrease in the intensity of the nǞπ* band.
Although back-switching is not fully reversible, repeated
switching and back-switching led to the corresponding
changes in the UV/Vis spectra.
[Zn(Im)(aIm)] (1): Single crystals of 1 were formed from a solvo-
thermal reaction in 2 mL Teflon® autoclaves in a high-throughput
reactor (see Supporting Information). Solutions of Zn(NO3)2·6H2O
(193 μL, 0.3 m), HaIm (193 μL, 0.3 m) and imidazole (580 μL,
0.3 m) in DMF were mixed and additional DMF (433 μL) was
added. The reaction mixture was heated at 85 °C in an isothermal
oven for 96 h. The crystalline orange product was collected by fil-
tration and washed with DMF (2 mL) and acetone (5 mL). The
product was dried at room temperature in air for five days followed
by 12 h at 200 °C in vacuo. C12H10N6Zn (302.03): calcd. C 47.46,
H 3.32, N 27.68; found C 46.85, H 3.64, N 27.04.
Microwave-Assisted Synthesis of 1: In a typical reaction, solutions
of Zn(NO3)2·6H2O (193 μL, 0.3 m), HaIm (193 μL, 0.3 m) and
The partial reversibility could be due to a photobleach-
ing effect or the steric hinderance of the switching process imidazole (580 μL, 0.3 m) in DMF with additional DMF (433 μL)
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N. Stock et al.
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were mixed in a 2 mL glass vial sealed with a Teflon®-coated cap.
The reaction mixture was stirred and exposed to microwave irradia-
tion for 5 min at 100 °C (Biotage Initiator Eight EXP). The orange
solid was collected by centrifugation and redispersed in DMF
(2 mL). The redispersing and centrifugation steps were repeated
twice more with acetone. The product was dried at room tempera-
ture in air for five days followed by 12 h at 200 °C in vacuo.
C12H10N6Zn (302.03): calcd. C 47.46, H 3.32, N 27.68; found C
46.86, H 3.32, N 27.42.
ment parameters [Ueq(H) = –1.2·Ueq(C)] using a riding model with
dC–H = 0.93 Å. Details of the structure determination are given in
Table 1.
CCDC-836865 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/.
X-ray Powder Diffraction (XRPD): XRPD experiments were per-
formed using an XЈPert Pro PANalytical Reflection Powder Dif-
fraction System, with Cu-Kα radiation (λ = 154.0598 pm), equipped
with a PIXcel semiconductor detector from PANalytical. Products
of the HT investigations were characterized using a STOE HT X-
ray powder diffractometer (Cu-Kα radiation) equipped with an im-
age plate detector.
Ultrasound-Assisted Synthesis of 1: In a typical reaction, solutions
of Zn(NO3)2·6H2O (193 μL, 0.3 m), HaIm (193 μL, 0.3 m) and
imidazole (580 μL, 0.3 m) in DMF with additional DMF (433 μL)
were mixed in a 2 mL glass vial. The reaction mixture was soni-
cated using an ultrasonic generator with sonotrode (UP200S, Hi-
elscher-Ultrasound Technology, 200 W, 24 kHz) for 10 min. The
orange solid was collected by centrifugation and redispersed in
DMF (2 mL). The redispersing and centrifugation steps were re-
peated twice more with acetone. The product was dried at room
temperature in air for five days followed by 12 h at 200 °C in vacuo.
C12H10N6Zn (302.03): calcd. C 47.46, H 3.32, N 27.68; found C
46.94, H 3.46, N 26.63.
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic data, HT methodology, the experimental
data for the HT system where 1 was found and spectroscopic data.
Acknowledgments
Single-Crystal Structure Analysis: The crystal structure determi-
nation was performed with an imaging plate diffraction system
(IPDS-1) with Mo-Kα radiation from STOE & CIE. The structure
solution was carried out with direct methods using SHELXS-97
and structure refinements were performed against |F|2 using
SHELXL-97. The structure solution in the space group I43m (as
found for ZIF-8) did not lead to a reasonable structure model.
Choosing the subgroup I23 allowed the azophenyl rings to be as-
signed by a split model. The azophenyl rings were isotropically re-
fined and the Zn and imidazolate ions were refined anisotropically.
A numerical absorption correction was applied using X-Red (ver-
sion 1.31) and X-Shape (version 2.11) of the program package X-
Area. All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Refinement of the structure led to a Flack
parameter of 0.50(1). The model was therefore refined as a racemic
twin using the TWIN and BASF command implemented in
ShelXL. All aromatic C–H hydrogen atoms were positioned with
idealized geometries and were refined with fixed isotropic displace-
We acknowledge funding from the Deutsche Forschungsgemein-
schaft (DFG) (SFB 667, Function by Switching). We also thank
Ursula Cornelissen for undertaking the Raman and UV/Vis mea-
surements, Inke Jeß for the single crystal measurements, Adam
Wutkowksi and Jan Boeckmann for the DTA/TG measurements
and Dr. Frank Sönnichsen for recording solution 1H NMR spectra.
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Table 1. Selected crystal data and details of the structure determi-
nation of 1.
Formula
Zn2C16H5N10
468.08
cubic
M [gmol–1]
Crystal system
Space group
a [Å]
I23
17.023(2)
4933(1)
293
V [Å3]
T [K]
Z
6
Dcalcd. [gcm3]
0.945
1.472
μ [mm–1]
Θmax [°]
25.3
19788
1496
1320
0.100
Measured reflections
Unique reflections
Reflections [I0 Ͼ4σ(I0)]
Rint
R1 [all data]
R1 [I0 Ͼ4σ(I0)]
wR2 [all data]
wR2 [I0 Ͼ4σ(I0)]
Gof
0.1192
0.1109
0.2717
0.2706
1.25
Δρmax, Δρmin [eÅ–3]
0.51, –0.41
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Published Online: October 26, 2011
Eur. J. Inorg. Chem. 2011, 5378–5383
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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