The first porous MOF with photoswitchable linker molecules

Article abstract of DOI:10.1039/c0dt01629b

We synthesized a porous twofold interpenetrated MOF [Zn₂(NDC) ₂(1)] (coined CAU-5) using 3-azo-phenyl-4,4′-bipyridine (1), 2,6-naphthalenedicarboxylic acid, and Zn(NO₃)₂· 6H₂O. The azo-functionality protrudes into the pores, and can be switched, by irradiation with UV light (365 nm), from the thermodynamically stable trans-isomer to the cis-isomer. Back-switching was achieved thermally and with an irradiation wavelength of λ_max = 440 nm.

Full text of DOI:10.1039/c0dt01629b

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PAPER
The first porous MOF with photoswitchable linker molecules†‡
Antje Modrow,a Dordaneh Zargarani,b Rainer Hergesb and Norbert Stock*a
Received 24th November 2010, Accepted 31st January 2011
DOI: 10.1039/c0dt01629b
We synthesized a porous twofold interpenetrated MOF [Zn2(NDC)2(1)] (coined CAU-5) using
3-azo-phenyl-4,4¢-bipyridine (1), 2,6-naphthalenedicarboxylic acid, and Zn(NO3)2·6H2O. The
azo-functionality protrudes into the pores, and can be switched, by irradiation with UV light (365 nm),
from the thermodynamically stable trans-isomer to the cis-isomer. Back-switching was achieved
thermally and with an irradiation wavelength of lmax = 440 nm.
as aminoterephthalic acid or other terephthalic acid derivatives.5
The latter route permits further modification by simple organic
reactions. This is known as the post-synthetic modification of
MOFs.16,17,18
Linker molecules containing an azo-functionality have also been
incorporated into the backbone of MOF structures.19,20,21,22,23 How-
ever, photochemically induced switching has not been achieved so
far, since trans-cis-isomerization is hindered in the rigid framework
(Fig. 1A). In cases where the azo-group is covalently attached
to the inner pore wall, however, switching should be possible
(Fig. 1B).
Introduction
Porous materials are excellent candidates for many applications,
for example in catalysis, drug delivery, gas storage or gas
separation.1,2,3,4 Crystalline compounds exhibiting defined pores
and cages in the microporous and mesoporous range are especially
promising. While in the past microporous zeolites were the focus
of interest, MOFs have been studied over the last decade because
of their structural diversity.5,6,7,8,9 The structural and chemical
variability is generated by a modular assembly of the MOFs that
provides the possibility to adjust pore sizes and to fine-tune the
shape and functionality of the pores.10,11 In general MOFs are built
of inorganic bricks connected by organic linkers, which are often
dicarboxylic acids and diamines.12
The first studies on MOFs focused on the synthesis of new
crystalline materials exhibiting permanent porosity. Materials
with large pores and cages extending to the mesoporous range
were of special interest.6,7,8 In these studies compounds such as
MIL-53 and MIL-88 were obtained, which contain highly flexible
frameworks.13,14,15 Recently, studies have been conducted focusing
on the chemical functionality of the pores. Functionality can
be introduced by synthesizing structures with open coordination
sites or by using functionalized organic linker molecules, such
aInstitut fu¨r Anorganische Chemie, Christian-Albrechts-Universita¨t, Max-
Eyth-Straße 2, D-24118 Kiel, Germany. E-mail: stock@ac.uni-kiel.de;
Fax: (+49) 0431-880-1775
Fig. 1 Two ways to introduce azo-functionalized molecules into MOFs.
(A) Azo-linker molecules as part of the backbone of the MOF, (B)
azo-group covalently attached to the inner pore wall and protruding into
the pore.
bOtto-Diels-Institut fu¨r Organische Chemie, Christian-Albrechts-
Universita¨t, Otto-Hahn-Platz 4, D-24098 Kiel, Germany.
† Electronic supplementary information (ESI) available: procedure for
the synthesis of 3-azo-phenyl-4,4¢-bipyridine, results of the UV/vis and
Raman spectroscopic measurements, CO2 and Ar sorption isotherms,
XRD patterns. CCDC reference number 801833. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01629b
The most prominent member of the switchable azo dyes is
azobenzene.24,25,26 In general the trans-isomer of azobenzene is the
thermodynamically more stable form, which exhibits two distinct
absorption maxima; one with a lower intensity at lmax = 444 nm
(n → p* transition) and another maximum at lmax = 316 nm (p →
p* transition). The cis-isomer exhibits an absorption maximum
at lmax = 437 nm (n → p* transition) and the p → p* transition
is shifted to lmax = 270 nm. The trans-isomer can be switched to
the cis-isomer by irradiation with UV light (l ~ 365 nm). This is
clearly visible in the UV/vis spectra, since the intensity of the band
‡ Crystal data for CAU-5 (STOE IPDS, Mo-Ka radiation, 170 K):
H24C40N4O8Zn2, M = 819.46 g mol-1, monoclinic, space group C2/c, a =
3
˚
˚
˚
˚
19.060(4) A, b = 17.898(4) A, c = 13.932(3) A, V = 4417.1(15) A , Z = 8,
15528 reflections measured, 4258 independent reflections (Rint = 0.0422),
The final R1 and wR(F2) values (I > 2s(I)) are 0.0418 and 0.1202. The
final R1 and wR(F2) values (all data) are 0.0545 and 0.1288. The goodness
of fit on F2 was 0.967. CCDC number 801833 contains the crystallographic
data.
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Fig. 2 Final step of the synthesis of 3-azo-phenyl-4,4¢-bipyridine (1).
at lmax = 316 nm decreases and the intensity of the band at lmax
=
temperature and 150 mL ethyl acetate was added. The organic
phase was separated and washed twice with 50 mL water and
dried using magnesium sulfate. The solvent was removed under
vacuo and the product was chromatographically separated with
444 nm increases. Therefore, both isomers have distinct UV/vis
spectra and can easily be distinguished. The reversible back-
switching can be achieved thermally by heating or by irradiation
(l ~ 440 nm) (Fig. 1).
1
ethyl acetate over silica gel. H-NMR (220 MHz, CDCl3, 300 K,
Since many MOFs are based on mixtures of dicarboxylic acid
and diamine linker molecules,27,30 we used azo-functionalized
4,4¢-bipyridine in combination with a rigid dicarboxylic acid to
synthesize new MOFs. Depending on the size of the dicarboxylic
acid it should be possible to fine tune the pore size of the MOF
in order to allow the incorporation of the azo-functionalized 4,4¢-
bipyridine linker molecule.30
Here we present the synthesis of the switchable azo-linker
molecule 3-azo-phenyl-4,4¢-bipyridine (1), its use in the synthesis
of the new porous MOF [Zn2(NDC)2(1)] named CAU-5 (CAU =
Christian-Albrechts-University) and the switching properties of
both.
TMS, numbering according to Fig. 2): d = 8.93 (d, 4J = 0.54 Hz,
1H, H-1), 8.77 (d, 3J = 5.07 Hz, 1H, H-2), 8.72 (dd, 3J = 4.54 Hz,
4J = 1.63 Hz, 2H, H-5), 7.78 (dd, 3J = 6.79 Hz, 4J = 3.03 Hz, 2H,
H-4), 7.65 (dd, 3J = 8.28 Hz, 4J = 1.39 Hz, 1H, H-3), 7.50–7.48 (m,
5H, H-6,7,8) ppm. MS (EI, 70 eV): m/z (%) = 260 (100) [M]+. MS
(CI, isobutane): m/z (%) = 261 (100) [M+H]+.
[Zn2(NDC)2(1)] (CAU-5). CAU-5 was formed in a solvother-
mal reaction in 0.2–0.5 ml glass vials (Biotage). The starting ma-
terials Zn(NO3)2·6H2O (11.3 mg, 0.037 mmol), H2NDC (8.1 mg,
0.037 mmol) and 1 (4.8 mg, 0.019 mmol) were stirred in a mixture
of 0.375 ml DMF and 0.375 ml CH3OH for one day at room
temperature. The reaction mixture was heated at 120 ◦C in an
isothermal oven for two days. The crystalline orange product was
filtered and washed three times with 1 ml DMF. The product
was dried at room tempe◦rature in air followed by activation in
an isothermal oven at 80 C. Elemental analysis of the activated
sample [Zn2(NDC)2(1)] (M = 819 g mol-1) calc. (%): C 58.63, H
2.95, N 6.84; found (%): 59.45, H 2.65, N 6.7. TG/DTA analysis
Experimental
Materials
The chemicals 3-aminopyridine (Sigma Aldrich), pyvaloyl chlo-
ride (Merck), tetramethylethylenediamine (TMEDA, Merck),
nitrosobenzene (Fluka), n-BuLi, 1.6 M (Acros) and Pd(PPh3)4
(ABCR) were obtained commercially and used as received.
3-Azo-phenyl-4,4¢-bipyridine (1) was synthesized from 3-
aminopyridine and pivalic acid chloride in a five step syn-
thesis (Fig. S1 and S2†). The chemicals Zn(NO3)2·6H2O,
methanol and dimethylformamide were purchased from Merck
and BASF, respectively, in reagent grade. Zn(NO3)2·6H2O and
methanol were used as obtained. Dimethylformamide was
dried over CaH2 and subsequently distilled in vacuo. 2,6-
Naphthalenedicarboxylic acid was synthesized by hydrolysis of
dimethyl-2,6-naphthalenedicarboxylate (Sigma Aldrich).
◦
of CAU-5: no weight loss was detected up to 390 C (Fig. S2†).
Between 390 and 510 ◦C a total weight loss of 80.6% (weight
loss calc. 80.1%) was observed in two steps which are due to the
oxidative decomposition. The final product was identified as ZnO
by X-ray powder diffraction (XRPD).
Characterization
Elemental analyses were carried out using a Eurovektor Eu-
roEA Elemental Analyzer. Raman spectra were recorded using a
Bruker FRA 106 Raman spectrometer. UV-measurements were
performed with a Cary 5000 (Varian). Switching experiments
of 1 in solution and CAU-5/BaSO4 mixtures were performed
using a 150 W Xenon-lamp and an Orieal Apex monochromator
(Newport Corporation). Thermogravimetric (TG) analyses were
Syntheses
The detailed procedure of the five step synthesis of 1 is given in
the ESI.† Only the last reaction step leading to 1 is presented here
in detail (Fig. 2).
◦
carried out in air (25 ml min-1, 25–900 ◦C, 4 C min-1) on a
NETZSCH-STA-449 C thermal analyzer. Sorption measurements
were realized with a BELSORP Max from Bel Japan. INC; for
proper activation the sample was heated in vacuum for 5 h at
150 ◦C. XRPD measurements were carried out in transmission
mode using a STOE high-throughput powder diffractometer
equipped with an image-plate position-sensitive detector (IPPSD).
Switching experiments of pure microcrystaline powder of CAU-
5 were performed using two LED light sources from NICHIA
(l = 365 nm: NC4U133E and l = 455 nm: NS6C083A). The
3-Azo-phenyl-4,4¢-bipyridine (1).
A solution of 4-iodo-3-
(phenylazo)pyridine (1.23 g, 3.9 mmol) in 35 ml toluene and
12 ml EtOH was mixed with 2-(4-pyridyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (0.897 g, 4.3 mmol). K2CO3 (1.99 g, 12.87
mmol) and tetrakis-(triphenylphosphine)-palladium(0) (0.075 g,
0.065 mmol) were added under N2-atmosphere and stirred under
reflux for 78 h. The reaction progress was checked by thin
layer chromatography. The reaction mixture was cooled to room
4218 | Dalton Trans., 2011, 40, 4217–4222
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Table 1 Crystal data for CAU-5 [Zn2(NDC)2(1)], 1 = 3-azo-phenyl-4,4¢-
bipyridine
achieved with visible light (l = 455 nm) and a XRPD pattern was
also recorded. This experiment was performed three times.
Chemical formula
Zn2(NDC)2(1)
Results and discussion
Crystal system
Space group
Monoclinic
C2/c
3-Azo-phenyl-4,4¢-bipyridine (1) was obtained as a pure phase
product and employed in the synthesis of CAU-5. Starting from the
synthesis procedure of [Zn2(NDC)2(4,4¢-bipy)], reported by Kim
et al.,30 the partial and full replacement of 4,4¢-azobipyridine by 1
was investigated. While partial replacement led only to the same
threefold interpenetrated, non porous structure, full replacement
of 4,4¢-bipyridine by 1 gave rise to the porous title compound
CAU-5 (Fig. S4†). The framework structure of CAU-5 crystallizes
in the monoclinic space group C2/c with one Zn-atom, one NDC2-
ion and one-half azobipyridine-molecule in the asymmetric unit.
Zn-containing paddle wheel units are observed which are bridged
by naphthalenedicarboxylate ions in the a,b-plane to form a 2D
square-grid (Fig. 3). The axial sites of these paddle wheel units
are occupied by the azobipyridine linker molecules 1, and the
azo-phenyl groups are disordered. In contrast to [Zn2(NDC)2(4,4¢-
bipy)] which has a threefold interpenetrated structure, in CAU-5
only twofold interpenetration is observed. This is due to the steric
demand of 1. The purity of CAU-5 was demonstrated by XRPD
measurements, TG/DTA (Fig. S2†) and elemental analysis as
well as Raman spectroscopy. The calculated and measured XRPD
patterns compare well and only one additional reflection of low
intensity was observed at 15◦ (2 theta), which is due to the presence
of guest molecules in the pores (Fig. 4).
The incorporation of the linker molecule 1 was also confirmed
by Raman spectroscopy (Fig. S5, Table S1†). The characteristic
band for the symmetric N N vibration of the trans-isomer is
located at 1443 cm-1 and 1444 cm-1 in the spectra of the pure linker
molecule 1 and CAU-5, respectively. The Raman spectrum of
CAU-5 contains the characteristic bands of both linker molecules,
the naphthalenedicarboxylic acid as well as 1.
The permanent porosity of CAU-5 was demonstrated by N2,
CO2 and Ar sorption measurements (Fig. 5, Fig. S6 and S7,†
respectively). The N2 sorption curve of the activated sample
(150 ◦C, 5 h in vacuum) shows a sharp increase at low p/p0
values and a well defined plateau, which is typical for a type
I isotherm. Fitting the data with the Brunauer–Emmett–Teller
(BET) equation, a specific surface area of 554 m2 g-1 is calculated.
Using the t-plot method, a micropore volume of 0.23 cm3 g-1
is obtained. This value compares well with the pore volume
(0.225 cm3 g-1) calculated using PLATON.31
˚
a/A
19.060(4)
17.898(4)
13.932(3)
111.66(1)
4417.1 (15)
8
819.46
15528
4258 /3350
0.0422
0.0418
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Formula weight/g mol-1
Total number of data collected
Unique/obsd data/I > 2s(I)
R(int
)
R1/I > 2s(I)
R1, wR2 (all data)
0.0545, 0.1288
samples were analysed using a PANalytical X’Pert diffractometer
equipped with a PixCell semiconductor detector system using Cu-
Ka irradiation. All measurements were performed in reflection
geometry.
Structure determination and refinement
CAU-5 [Zn2(NDC)2(1)] crystallizes to form air-stable orange rods
(0.07 ¥ 0.07 ¥ 0.12 mm3). The single-crystal XRD measurement
was performed on a STOE IPDS diffractometer. The instrument
is equipped with a fine focus sealed tube (Mo-Ka radiation,
l = 71.073 pm). For data reduction and absorption correction
the programs XRED and XSHAPE were used.28 The crystal
structure was solved by direct methods and refined using the
program package SHELXTL.29 Since for the single-crystal XRD
measurement a non activated crystal was used, disordered guest
molecules were present in the pores. Therefore, for the final
structure refinement the SQUEEZE procedure was applied as
implemented in the PLATON software.31 Results of the structure
determination are summarized in Table 1.
Switching experiment
The switching experiments of 3-azo-phenyl-4,4¢-bipyridine (1)
were performed in ethanol using a quartz cuvette. Successful
switching was demonstrated by recording the UV/vis spectra. The
stirred solution was irradiated with UV light (l = 365 nm) for 1 h
to switch the trans-isomer into the cis-isomer. The reversible back-
switching was achieved by irradiation with visible light (l = 440
nm) for 1 h. This switching experiment was repeated twice.
The switching experiments and UV/vis measurements for CAU-
5 were performed in a mixture of the microcrystalline powder
and BaSO4. The UV/vis spectra were measured in reflection
geometry. The data was transformed into adsorption spectra using
the Kubelka Munk equation. After 15 min irradiation time the
photostationary state was reached (Fig. S3†). The back-switching
from the cis- to trans-isomer could also be achieved thermally at
room temperature or by irradiation with visible light (l = 440 nm).
The switching was also investigated with XRPD measurements
using a ground sample of CAU-5. After recording a XRPD
pattern, the sample was irradiated with UV light (l = 365 nm)
using a LED light source. After an irradiation time of 15 min
a second XRPD pattern was recorded. The back switching was
The results of the switching experiments of 3-azo-phenyl-4,4¢-
bipyridine (1) in solution are shown in Fig. 6. The maximum at l =
321 nm can be assigned to the p → p* transition of the azo-group,
the band with a maximum at l = 460 nm to the n → p* transition.
Both maxima are well separated and can be easily distinguished.
In the trans-isomer, the p → p* transition has a higher intensity
than the n → p* transition. Irradiation for 1 h with UV light (l =
365 nm) leads to an increase of the band at l = 460 nm as well as
a decrease of the band at l = 321 nm (Fig. 6: blue, gray, and green
line). This is due to the trans-cis-isomerization. The reversible back
switching was achieved using visible light (l = 440 nm) as shown in
Fig. 6 (yellow, red, and purple line). The switching experiment was
repeated twice to demonstrate the reversible switching behaviour
of 3-azo-phenyl-4,4¢-bipyridine in ethanol.
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Fig. 3 Top: paddle wheel unit containing the 3-azo-phenyl-4,4¢-bipyridine and the naphthalenedicarboxylate ions as observed in the crystal structure of
CAU-5; bottom: ORTEP presentation of the asymmetric unit, C1B, C2B, C3B, C4B, C4B2, C5B and C5B2 constitute the 4,4¢-bipyridine molecule, N2,
N3, C1C, C2C, C3C, C3CA and C4 C the azo-phenyl moiety. Hydrogen atoms are omitted for clarity; thermal ellipsoids are shown at 85% probability.
Fig. 5 N2 adsorption- (filled squares) and desorption- (open squares)
isotherm of CAU-5.
Fig. 4 Calculated (bottom) and measured (top) XRPD patterns of
CAU-5. Allowed reflection positions are marked by ticks.
4220 | Dalton Trans., 2011, 40, 4217–4222
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(Fig. 7, Fig. S8†). The back switching was achieved employing
visible light (l = 440 nm). The intensity of the band at l = 462 nm
decreased again, due to higher trans-isomer concentration. The
results of repeated switching are shown in Fig. S9.†
Switching from the cis- to trans-isomer could also be achieved
thermally (Fig. S10†). After switching from the trans- to the cis-
isomer, the sample was left at room temperature in the dark for
24 h. The cis-isomer partially switched back to the trans-isomer.
To prove that the switching takes place in the crystalline net-
work, switching experiments using pure microcrystalline powder
of CAU-5 were performed. Trans-cis-isomerization should lead
to changes in the reflection intensities of XRPD patterns. As
expected, irradiation of the CAU-5 crystals with UV light (l =
365 nm) led to small changes in the reflection intensities in the
XRPD pattern (Fig. 8a). While no obvious shifts of the reflections
positions are detected, two small broad reflections at 10.1 and
15.3◦ (2 theta) are observed. After the irradiation with visible
light (l = 455 nm) the XRPD patterns showed nearly the same
intensities as before and the additional reflections were no longer
present (Fig. 8b). The experiment was performed three times and
showed the same results. Although a detailed crystallographic
explanation is not possible at this stage, the results demonstrate
unequivocally that the trans-cis-isomerization leads to changes
in the crystal structure and therefore to changes in the XRPD
patterns.
To the best of our knowledge, CAU-5 is the first MOF with
a reversible switching azo-functionality inside the pores. In the
past, different metal–organic frameworks with azo-functionalized
linker molecules were described. For example MOF-9, which con-
sists of Tb2C6O12 clusters connected by azobenzenedicarboxylic
acid molecules.20 Another example was recently reported by
Kucha´r et al., who synthesized a compound based on Zn(II) atoms,
terephthalic acid and 4,4¢-azo(bis)pyridine molecules.22 In both
compounds, the azo-group is part of the rigid backbone of the
MOF and therefore switching is strongly hindered.
Fig.
6 Results of the UV/vis switching experiments of
3-azo-phenyl-4,4¢-bipyridine in ethanol. Top: UV/vis spectra recorded
from 250–550 nm; bottom: detailed view from 390–650 nm.
Although we were able to show the first example of a switchable
azo-functionality inside a metal–organic framework, the switching
effect has to be improved. This could possibly be achieved by
preventing the interpenetration of the network. Interpenetra-
tion can be prevented by using sterically demanding organic
linker molecules. This was previously demonstrated for the com-
pound Zn2BDC2(4,4¢-bipy) which has a twofold interpenetrated
structure.30 Replacing terephthalic acid by tetramethylterephthalic
acid, a non-interpenetrating, porous compound is obtained.30
Present results show a similar trend. The combination of naph-
thalenedicarboxylic acid and 4,4¢-bipyridine leads to a threefold
interpenetrated structure. By replacing 4,4¢-bipyridine by the steri-
cally more demanding linker molecule 1, a twofold interpenetrated
network structure is obtained, which shows permanent porosity.
Therefore, to improve the switching properties in our system, one
could imagine further increasing the steric demand of the linker
molecules and thus avoid interpenetration.
The analogous UV/vis switching experiment was performed
using well ground CAU-5 in a BaSO4 matrix (Fig. 7). Upon
irradiation with UV light (l = 365 nm) the intensity of the band
at l = 462 nm increased and the band at l = 365 nm decreased
Conclusions
The porous, air stable metal–organic framework compound,
[Zn2(NDC)2(1)] (CAU-5) with a free, switchable azo-group inside
the pores was obtained by solvothermal synthesis. Elemental and
thermal analysis confirmed purity. Although the CAU-5 structure
Fig. 7 Results of the UV/vis switching experiment of microcrystalline
CAU-5 in a BaSO4 matrix.
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Adam Wutkowski for TG/DTA measurements, Christina Rutz
for support in the preparation of 3-azo-phenyl-4,4¢-bipyridine
and Mark Feyand for additional help solving the single crystal
structure.
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Fig. 8 (a) Powder pattern of CAU-5 before (black) and after irradiation
with UV light (l = 365 nm) (red line). The reflection intensities change and
two new broad reflections occur at 10.1 and 15.3◦ (2 theta). (b) Powder
pattern of CAU-5 before (black) and after irradiation with visible light
(l = 455 nm) (green line). The recorded powder pattern of CAU-5 showed
nearly the same intensities as before and the additional reflections are no
longer present.
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ity that was demonstrated by sorption measurements. UV/vis
experiments unequivocally showed the possibility of repeated
reversible switching.
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Acknowledgements
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This work has been supported by the State of Schleswig-Holstein
and the Deutsche Forschungsgemeinschaft (DFG) via the SFB 677
Function by Switching. The authors thank Uschi Cornelissen and
Stephanie Pehlke for supporting all spectroscopic measurements,
Inke Jess and Christian Na¨ther for single crystal measurements,
29 G. M. Sheldrick, SHELXTL-PLUS Crystallographic System, Siemens,
Analytical X-Ray Instruments Inc. Madison, WI, 1992.
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4222 | Dalton Trans., 2011, 40, 4217–4222
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