Guest-to-host transmission of structural changes for stimuli-responsive adsorption property

Article abstract of DOI:10.1021/ja2115713

We show that structural changes of a guest molecule can trigger structural transformations of a crystalline host framework. Azobenzene was introduced into a flexible porous coordination polymer (PCP), and cis/trans isomerizations of the guest azobenzene b

Full text of DOI:10.1021/ja2115713

Communication
pubs.acs.org/JACS
Guest-to-Host Transmission of Structural Changes for Stimuli-
Responsive Adsorption Property
Nobuhiro Yanai,†,∥ Takashi Uemura,* Masafumi Inoue, Ryotaro Matsuda, Tomohiro Fukushima,†
,†
†
‡,§
§
§
,†,‡,§
Masahiko Tsujimoto, Seiji Isoda, and Susumu Kitagawa*
†Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
‡
ERATO Kitagawa Integrated Pores Project, Kyoto Research Park Building #3, Shimogyo-ku, Kyoto 600-8815, Japan
§
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
*
S Supporting Information
ABSTRACT: We show that structural changes of a guest
molecule can trigger structural transformations of a
crystalline host framework. Azobenzene was introduced
into a flexible porous coordination polymer (PCP), and
cis/trans isomerizations of the guest azobenzene by light
or heat successfully induced structural transformations of
the host PCP in a reversible fashion. This guest-to-host
structural transmission resulted in drastic changes in the
gas adsorption property of the host−guest composite,
displaying a new strategy for creating stimuli-responsive
porous materials.
Figure 1. Schematic illustration of the concept of guest-to-host
structural transmission. Red and orange objects represent trans-AB and
cis-AB, respectively. The conformational change in the guest molecule
by external stimuli triggers a structural transformation of the crystalline
host framework. This structural transmission results in the efficient
switching of porous functions of the host−guest composite.
n intriguing challenge in host−guest chemistry is how to
A
achieve a transmission of structural changes from the
1
guest to the host and vice versa. Chemists are inspired by
sophisticated biological systems, such as in the perception of
light, where a photoinduced configurational change of the guest
chromophore retinal induces a structural change of the host
opsin protein, converting it from an inactive state to an
In this work, we constructed a guest-to-host structural
transmission system by using flexible PCPs/MOFs for the first
time. Azobenzene (AB) was included in a flexible host
compound, [Zn (terephthalate) (triethylenediamine)] (1;
2
activated signaling state. Implementation of such guest-to-host
2
2
n
or host-to-guest structural transmission in artificial systems
would lead to a variety of advanced stimuli-responsive
pore size = 7.5 Å × 7.5 Å), in which two-dimensional square
grids are bridged by triethylenediamine (see the Supporting
1
,3−5
properties, but the successful examples are rather limited.
17
Information). The pore structure of host 1 has been reported
To attain this structural transmission accompanied by
responsive physical or chemical properties, we utilized porous
coordination polymers (PCPs) or metal−organic frameworks
to be deformed by inclusion of particular aromatic guest
4,17−19
molecules.
The cis/trans isomerizations of AB by light
and heat efficiently triggered structural transformations of 1,
resulting in a drastic switching of the adsorption property of the
host−guest composite.
(
MOFs) composed of metal ions and organic ligands. PCPs/
MOFs have highly regular nanopores that can be utilized for
6
−16
storage, separation, and catalysis.
One of the most
trans-AB was fully introduced into the nanochannels of 1 at
1
20 °C, after which excess AB external to the host crystals was
advantageous features of PCPs/MOFs is that their flexible
frameworks are responsive to guest molecules while retaining
high regularity, which does not occur in conventional
microporous materials such as zeolites and activated
removed under reduced pressure. The obtained composite is
denoted as 1⊃AB. Scanning electron microscopy measure-
ments showed that the size of the composite crystals was ∼10
μm. X-ray powder diffraction (XRPD) measurements showed
that the host structure was changed by the inclusion of trans-
AB, clearly indicating the formation of the host−guest adduct
8
,9,14,15
14
carbons.
They are called soft porous crystals. If the
conformation of the guest is changed by external stimuli, the
host structure would be simultaneously transformed according
to the different guest shapes (Figure 1). This artificial guest-to-
host structural transmission has great potential to produce a
dynamic switching of functions of the host−guest composites,
such as porous, optic, electric, and magnetic properties.
(
Figure 2a,b). The number of AB molecules per unit cell of 1
was 1.0, as evidenced by elemental and thermogravimetric
Received: December 11, 2011
Published: February 28, 2012
©
2012 American Chemical Society
4501
dx.doi.org/10.1021/ja2115713 | J. Am. Chem. Soc. 2012, 134, 4501−4504

Journal of the American Chemical Society
Communication
1
Figure 3. H NMR spectra of CD Cl solutions of AB isolated from
2
2
(
1
a) 1⊃AB, (b) 1⊃AB(UV), and (c) the solid obtained after heating
⊃AB(UV) at 120 °C.
Figure 2. XRPD patterns of (a) 1, (b) 1⊃AB, (c) 1⊃AB(UV), (d) the
solid obtained after heating 1⊃AB(UV) at 120 °C, and (e) the adduct
of 1 with cis-stilbene. Schematic representations of the host structures
for each sample are also shown.
unsymmetrical cis isomer in 1⊃AB(UV) would lead to a
partial expansion of the host framework from the orthorhombic
to the tetragonal form. The nonordered arrangement of the
mixture of cis-AB and trans-AB in the pores is another possible
origin of the formation of the tetragonal structure.
analyses of 1⊃AB. The XRPD pattern of 1⊃AB is similar to
that of the compound obtained by introducing benzene into 1,
in which the overall framework connectivity remains similar to
that of 1 but the square grid structure is distorted to form a
We examined the dependence of the host−guest synchro-
nous structural change on the UV irradiation time. The ratio of
cis isomer increased with longer irradiation time and reached
saturation in ∼6 h. As the fraction of the cis isomer increased,
the relative intensity of the tetragonal host structure became
higher in the XRPD pattern. To check the stability of the
formed cis isomer, 1⊃AB(UV) was kept in the dark at 25 °C.
Interestingly, more than 80% of the cis isomer remained after 1
month. The half-life period of the thermal cis-to-trans
conversion of AB in solution is a few days, and thus, the
activation energy for the cis-to-trans conversion in the pores of
1
7
rhombic net. A Le Bail fitting analysis of the XRPD data of
1
⊃AB showed that crystal system was changed from the
original tetragonal to orthorhombic, and the angle between
sides of the rhombic net is 67°. The volume per dinuclear Zn2
3
unit for 1⊃AB is smaller (1042.0 Å ) than that for 1 (1169.3
3
Å ). Therefore, the accommodation of trans-AB resulted in the
induced fit and resulting shrinkage of the host framework.
The guest trans-AB of 1⊃AB exhibited photoisomerization
upon UV irradiation. The material obtained after UV irradiation
is described as 1⊃AB(UV). The trans-to-cis photoisomerization
of AB in the pores was confirmed by IR and NMR
26
1
should be higher than that in solution. The electron beam
diffractions in a transmission electron microscope (TEM) were
measured for 1⊃AB and 1⊃AB(UV). The diffraction pattern
from the particles of 1⊃AB is attributable to the orthorhombic
form of host 1 (Figure 4a). Some particles of 1⊃AB(UV)
measurements. The IR spectrum of 1⊃AB showed a peak at
6
90 cm− due to the trans isomer. After UV irradiation, a new
1
−1
peak emerged at 697 cm , which is attributable to the
formation of the cis isomer. The AB molecules were isolated
from the composites by dissolution of the host framework in
20
tetrasodium ethylenediaminetetraacetate (Na EDTA) solution.
4
1
In the H NMR spectrum of AB isolated from 1⊃AB(UV),
signals could be assigned to the aromatic protons of trans-AB
and cis-AB and gave a cis/trans isomer ratio of 38:62, which is
close to that of the photostationary state of AB in solution
2
1,22
(
Figure 3b).
There have been some reports of the
isomerization of AB that was immobilized as a part of rigid
23−25
MOF frameworks.
However, the isomerization yield in our
system is higher than that in the previous ones, probably
because the weaker steric constraints around the AB moieties
Figure 4. Typical electron-beam diffraction patterns in a TEM for
single particles of (a) 1⊃AB and (b) 1⊃AB(UV).
25
allow a more efficient isomerization reaction. Remarkably, the
photoisomerization of AB triggered a structural transformation
of the host framework, showing that photoinduced host−guest
structural transmission was successfully achieved. After UV
irradiation, the XRPD pattern changed to a superposition of the
two patterns for the orthorhombic and original tetragonal
structures (Figure 2c). As a control experiment, cis-stilbene was
introduced into host 1, as it is difficult to prepare the composite
showed the pattern for the tetragonal form (Figure 4b) and
others remained in the orthorhombic form, suggesting that the
host structural transformation occurred in the whole particle,
not in the crystalline domain of the particle.
To elucidate the reversibility of the host and guest structure
changes, 1⊃AB(UV) was treated with heat. After 1⊃AB(UV)
was heated at 120 °C for 1 h, all of the cis-AB was converted to
trans-AB (Figure 3c). The host structure was also completely
transformed to the orthorhombic form, as evidenced by the
26
of 1 with pure cis-AB. The host framework remained as the
tetragonal structure after the inclusion of cis-stilbene within 1
(
Figure 2e). From these results, the formation of the
4
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Journal of the American Chemical Society
Communication
XRPD measurements (Figure 2d). The reversible switching of
the host structure was attained concomitantly with the
isomerizations of guest AB.
To explore a function responsive to the guest-to-host
structural transmission, nitrogen adsorption measurements
were performed for the composite materials. The composite
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, crystal structure, thermog-
ravimetric analysis, Le Bail fitting analysis, IR spectra, UV
irradiation time dependence, and repeated changes in the
■
*
S
XRPD pattern and N adsorption. This material is available free
2
1
⊃AB did not adsorb N , which might be the consequence of
2
of charge via the Internet at http://pubs.acs.org.
pore blockage by the close contact of the host framework with
the guest AB (Figure 5a). Significantly, the adsorption amount
AUTHOR INFORMATION
■
Corresponding Author
uemura@sbchem.kyoto-u.ac.jp; kitagawa@icems.kyoto-u.ac.jp
Present Address
∥
Department of Materials Science and Engineering and
Department of Chemistry, University of Illinois, Urbana,
Illinois 61801, United States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Young
Scientists (A) and a Grant-in-Aid for Scientific Research on
Innovative Area “Emergence in Chemistry” from MEXT. N.Y.
acknowledges a JSPS Postdoctoral Fellowship for Research
Abroad.
Figure 5. Adsorption isotherms of N at 77 K for 1⊃AB (red) and
2
1⊃AB(UV) (blue).
dramatically increased after the UV irradiation. The isotherm of
⊃AB(UV) showed type-I behavior with a saturated adsorption
amount of 45 mL g , which suggests the microporosity of the
material. The expansion of the host framework to the tetragonal
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