Creating coordination-based cavities in a multiresponsive supramolecular gel

Article abstract of DOI:10.1002/chem.201406517

Abstract Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal-organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O-Pd₂L₄ cages is driven by coordination between Pd²⁺ and a photochromic dithienylethene bispyridine ligand (O-PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O-Pd₂L₄ cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three-dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light-induced phase and structural transformations readily occur owing to the reversible photochromic open-ring/closed-ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd₂L₄ cage-based gels show multiple reversible gel-solution transitions when thermal-, photo-, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids. Metal-organic cages were utilized as building units for the first time to generate supramolecular gels, providing a synthetic approach to cavity-containing gels that display multi-responsiveness towards environmental physical and chemical stimuli (see figure).

Full text of DOI:10.1002/chem.201406517

DOI: 10.1002/chem.201406517
Full Paper
&
Cage Compounds
Creating Coordination-Based Cavities in a Multiresponsive
Supramolecular Gel
[
a]
[a]
[a]
[a]
[a]
Shi-Chao Wei, Mei Pan, Yuan-Zhong Fan, Haoliang Liu, Jianyong Zhang,* and
[a, b]
Cheng-Yong Su*
Chem. Eur. J. 2015, 21, 1 – 11
1
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

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Abstract: Creating cavities in varying levels, from molecular
containers to macroscopic materials of porosity, have long
been motivated for biomimetic or practical applications.
Herein, we report an assembly approach to multiresponsive
supramolecular gels by integrating photochromic metal–or-
ganic cages as predefined building units into the supra-
molecular gel skeleton, providing a new approach to create
cavities in gels. Formation of discrete O-Pd L cages is driven
ther interconnected through supramolecular interactions to
form a three-dimensional (3D) gel matrix to trap a large
amount of solvent molecules. Light-induced phase and
structural transformations readily occur owing to the reversi-
ble photochromic open-ring/closed-ring isomeric conversion
of the cage units upon UV/visible light radiation. Further-
more, such Pd L cage-based gels show multiple reversible
2
4
gel–solution transitions when thermal-, photo-, or mechani-
cal stimuli are applied. Such supramolecular gels consisting
of porous molecules may be developed as a new type of
porous materials with different features from porous solids.
2
4
2
+
by coordination between Pd and a photochromic dithie-
nylethene bispyridine ligand (O-PyFDTE). In the presence of
suitable solvents (DMSO or MeCN/DMSO), the O-Pd L cage
2
4
molecules aggregate to form nanoparticles, which are fur-
Introduction
cyclodextrin, curcurbituril, calixarene and macrocycles, and co-
[7–9]
ordination complexes of metallocyclic and grid structures.
Supramolecular gels are a kind of stimulus-responsive soft ma-
terial that can undergo smart changes in response to external
On the other hand, by incorporating active moieties into mo-
lecular gelators, functional supramolecular gels can be fabricat-
[
1]
[10]
stimuli, thereby showing potential applications in materials
ed with modulated properties.
For example, light-induced
[
2]
[3]
science, drug delivery, and construction of supramolecular
reversible gelation can be achieved by introducing photoac-
[
4]
[11]
nanostructures. They are usually assembled from low-molecu-
lar-mass organogelators based on a “bottom-up” approach
through synergistic effect of various noncovalent interactions,
such as hydrogen bonding, p–p stacking, metal–organic coor-
tive/photochromic moieties like azobenzene or dithienyle-
[5]
dination, hydrophobic interactions, and van der Waals forces.
These weak and therefore reversible interactions make supra-
molecular gels responsive and readily tunable by mechanical,
thermal, and chemical stimuli, such as temperature, light, pH,
and ultrasound. Nevertheless, some uncertainty of the gel
structure is present. For example, the solid-like gel skeleton is
known to be amorphous in nature and the network porosity is
[
6]
hard to control and define.
As a way to tune the gel structure and porosity more defi-
nitely, endowing supramolecular gelators with predefined cavi-
ties is expected to be able to open up a new application gate
to gel materials, for example, incorporating host–guest chemis-
[
7]
try of cavitands into the gel systems. However, this promising
field combining interface of supramolecular gel and host–
guest chemistry is still in its infancy, and endeavors devoted to
creation of such gel materials are only limited to a few cavity-
containing gelators, for example, covalent host molecules like
2 4
Figure 1. Schematic representation of gelation process from O-Pd L cage
molecules to hierarchically porous supramolecular gels showing reversible
gel-to-solution transformations under external stimuli.
[
a] S.-C. Wei, Dr. M. Pan, Y.-Z. Fan, H. Liu, Prof. J. Zhang, Prof. C.-Y. Su
MOE Laboratory of Bioinorganic and Synthetic Chemistry
State Key Laboratory of Optoelectronic Materials and
Technologies, Lehn Institute of Functional Materials
School of Chemistry and Chemical Engineering
Sun Yat-Sen University, Guangzhou 510275 (China)
E-mail: zhjyong@mail.sysu.edu.cn
[12]
[13]
thene chromophores into gelators. Considering the above
background, we herein report a new synthetic approach to
supramolecular gels by using photoactive metal–organic cages
(
MOCs) as gelators. The resulting supramolecular gels contain
cesscy@mail.sysu.edu.cn
Pd L coordination cages with predefined internal cavities, of-
2
4
[
b] Prof. C.-Y. Su
fering hierarchical porosity of mesopores and nanocages, as
well as multiresponsive properties as shown in Figure 1.
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406517.
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[
23,24]
controlled release.
Specifically, many analogous Pd L coor-
Results and Discussion
2 4
dination cages have already been reported to show interesting
[23]
guest-binding ability within its cavity
or light-triggered
Design approach
[24]
guest encapsulation.
The design of supramolecular gels consisting of porous mole-
[
14,15]
cules
may provide a new type of porous material with dif-
Gelation studies
ferent features from known porous solids. For the crystal engi-
neering of porous solids, the design and assembly of zeolite-
like porous metal–organic frameworks (MOFs) by using dis-
crete MOCs or coordination polyhedra as supramolecular
It is well known that the dithienylethene chromophore can un-
dergo open-ring to closed-ring isomerization by irradiation
with UV light, and the reversal by visible-light irradiation. Addi-
tionally, diarylethenes represent most promising switch units
because of their unique fatigue resistance and thermal irrever-
[16]
building blocks (SBBs) has been well-established (Figure 2a).
[25]
sibility. Therefore, a bipyridine-type ligand with diarylethe-
ne(dithienylcyclopentene) moiety as spacer, PyFDTE, was syn-
thesized (Scheme 1, see the Supporting Information for experi-
Scheme 1. Chemical structures of O-PyFDTE and O-PyDTE.
mental details) for the gelation test. The open-ring isomer O-
PyFDTE (O-Form; light-brown color) can be transformed to the
closed-ring isomer C-PyFDTE (C-Form; deep-blue color) upon
UV irradiation, and its reverse process occurs photochemically
upon visible light, readily identifiable by chemical shifts in the
Figure 2. Schematic representations of a) porous solids and b) cavity-con-
taining gels.
In contrast, supramolecular gels undergo readily gel-to-solu-
tion phase transitions when external stimuli are applied. Addi-
tionally, such gels are in a unique phase that, by weight and
volume, they are mostly liquid, yet they behave like a solid
due to the existence of 3D cross-linked networks within the
1
H NMR spectra and photochromism in the UV/Vis spectra
(Figure 3 and the Supporting Information, Figures S1 and S2).
Both the isomers, O-PyFDTE and C-PyFDTE, were tested for
their gelation behaviors with various metal salts, including
[Pd(NO ) ], [Ni(NO ) ]·6H O, [Cu(NO ) ]·3H O, [Zn(NO ) ]·6H O,
[
17]
liquid (Figure 2b).
The formation of a gel matrix often demands collaborative
3
2
3 2
2
3 2
2
3 2
2
[Cd(NO ) ]·4H O and [Co(NO ) ]·6H O. Only [Pd(NO ) ] was
3
2
2
3 2
2
3 2
[
6,17]
interplaying of multiple intermolecular interactions,
which
found to be able to induce gelation with O-PyFDTE, which can
is in contrast to cage-containing MOFs that are usually assem-
bled from periodic growth of cage SBBs in a well-ordered fash-
[18]
ion through directional metal–ligand coordination.
It is
known that the aggregation of cage SBBs in solution can
[19]
create fascinating superstructures like vesicles and micelles;
however, incorporation of cage SBBs into gel networks still
waits for exploration. As exemplified in Figure 1, the first step
is to assemble the Pd L coordination cages with an internal
2
4
cavity as supramolecular gelators. Secondly, these cage precur-
sors aggregate in the presence of suitable solvents (here
DMSO or MeCN/DMSO) to form nanoparticles. Finally, once the
nanoparticles further aggregate in a cross-linked and disor-
dered fashion rather than periodic crystallization, formation of
gel matrix is possible when sufficient and appropriate intermo-
[6]
lecular interactions are available for gelation (Figure 1). Such
a supramolecular gel may bring in versatile host–guest chemis-
try of nanoscaled coordination cages, since they have been
proven to be able to selectively encapsulate guest molecules
Figure 3. Gelation routes of CCG-1 from photochemically reversible O- and
C-forms of PyFDTE, and its versatile responsiveness towards light-, thermo-,
mechano-, and anion-stimuli, showing reversible or irreversible gel-to-solu-
tion transitions as well as color changes.
[20]
or ions and modify the reactivity of guest species,
thus
and
[
21]
[22]
showing potential applications in catalysis, sensors,
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be simply checked by the inversion test. Meanwhile, gelation
was found to occur in DMSO among various pure organic sol-
Figure S4). Energy dispersive X-ray spectroscopy disclosed the
presence of N, F, S and Pd elements in the gel (the Supporting
Information, Figure S5). Fourier transform infrared spectra of
the xerogel unveiled the presence of PyFDTE ligand in the
metal coordination sphere (the Supporting Information, Fig-
vents tested (DMSO, DMF, DMA, CH CN, 1,4-dioxane, THF, see
3
the Supporting Information, Table S1). Gels could also be ob-
tained in MeCN/DMSO solvent mixtures (v/vꢀ1:1). It is notice-
able that no gelation was observed for the closed-ring C-
PyFDTE with any tested metal salts and solvents (C-PyFDTE so-
lution was obtained by exposing an O-PyFDTE solution to UV
light with transformation of O-Form to C-Form>95%). Trans-
parent gels could be obtained in DMSO when the Pd/O-
PyFDTE ratio varied from 1:2 to 1:1 (the Supporting Informa-
tion, Table S2). Additionally, gelation could be observed under
À1
ure S6). The strong absorption band at 1383 cm is assignable
À
to the NO3 vibration. The surface analysis by X-ray photoelec-
tron spectroscopy showed a signal at 338.2 eV in the Pd 3d
5
/2
II
region, confirming the coordination of Pd to PyFDTE (the Sup-
porting Information, Figure S7).
1
H NMR and ESI time-of-flight mass spectroscopic studies
evidently verified that the gel particles are composed of
À1
1
an O-PyFDTE concentration as low as 0.025 molL (the Sup-
[Pd (O-PyFDTE) ] cage molecules. H NMR titration was per-
2
4
porting Information, Table S3). As a representative, the gel of
formed in [D ]MeCN/[D ]DMSO solvent mixture (v/v=2:1) in
3 6
À1
Pd/O-PyFDTE=1:2 system (c =0.025 molL ) was investigated
which no gelation occurred (Figure 5). Upon proportional addi-
L
in detail (denoted as coordination cage-based gel-1, CCG-1). In
general, CCG-1 was obtained through two routes shown in
2
+
Figure 3: 1) Assembly of Pd and O-PyFDTE in 1:2 ratio gives
rise to homogeneous solution of O-Pd L cages, which under-
2
4
goes gelation for about 10 h at room temperature to lead to
2
+
CCG-1; 2) Assembly of Pd and C-PyFDTE in 1:2 ratio in the
dark gives rise to deep-blue solution of C-Pd L cages, which
2
4
cannot undergo gelation in the dark, but undergoes gelation
readily when exposed in visible light for about 10 h at room
temperature, offering CCG-1 as in route A. For comparison,
ligand PyDTE cannot gelate any tested metal salts under simi-
lar conditions (see below).
The gel CCG-1 is thermo-reversible (Figure 3). Upon heating,
a gel-to-solution transition was observed, and the solution re-
verted to the gel by cooling to room temperature. The gel-to-
1
solution transition temperature (T ) elevated considerably
Figure 5. H NMR titration of O-PyFDTE with [Pd(NO ) ] in [D ]MeCN/
gel
3
2
3
with increasing gelator concentration (the Supporting Informa-
[D
shown.
6
]DMSO solvent mixture (v/v=2:1). Only aromatic proton signals are
tion, Figure S3). Specifically, Tgel increased from 70 to 908C as
À1
the gelator concentration increased from 0.025 to 0.10 molL .
2
+
Additionally, the solution mixture with lower concentration
took a much longer time for gelation. For example, the gela-
tion of Pd into the solution of O-PyFDTE, the spectral profile
of the pure ligand showed drastic changes. New proton signals
appeared along with relatively decrease of the ligand signals.
When the metal-to-ligand ratio reached to 1:2, the NMR spec-
trum turned into one set of new signals, corresponding to res-
onance patterns of pure ligand with all chemical shifts of the
ligand protons shifted remarkably downfield, indicating that
À1
tion of a 0.025 molL solution took about 10 h, whereas a so-
À1
lution of 0.10 molL only took 2 min. When the concentration
À1
was above 0.06 molL , the gelation occurred fast (ꢀ10 min)
(the Supporting Information, Figure S3).
The morphology of gel CCG-1 was studied by scanning elec-
2
+
tron microscopy (SEM) and transmission electron microscopy
TEM) (Figure 4). The 3D gel network was aggregated from
complexation had occurred between O-PyFDTE and Pd to
[26]
(
lead to the assembly of [Pd (O-PyFDTE) ] cages. The protons
2 4
nanoparticles of about 20–50 nm size. The gel skeleton com-
posed of nanoparticles is amorphous in the wet gel and xero-
gel states, evidenced by the broad and featureless diffraction
peaks of powder X-ray diffraction (the Supporting Information,
H and H on the pyridine ring in the neighboring positions of
b e
N donor showed much more significant downfield shifts in
comparison with other protons, clearly confirming the coordi-
nation through PdÀN bonding due to the electron-withdraw-
2
+
ing effect by the Pd ion. No further resonance changes hap-
pened when Pd/PyFDTE ratios were increased from 1:2 up to
2
:2, suggesting that the Pd/L=1:2 cages are the most thermo-
dynamically preferred solution species. Furthermore, high-reso-
lution ESI-MS spectra of a dilute dispersion of the gel in MeCN
showed salient peaks at m/z 788.0231 and 1213.0285 (the Sup-
porting Information, Figure S8), exactly corresponding to the
3
+
2+
species [Pd L +NO ] and [Pd L +2NO ] , respectively, veri-
2
4
3
2
4
3
fied by closely matched isotopic distribution simulations. No
other obvious coordination species of higher molecular mass
Figure 4. a) SEM and b) TEM images of gel CCG-1 (the bars represent 1.0
and 0.1 mm from left to right, respectively).
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were observed. These spectroscopic results and solution coor-
dination behaviors may support the formation of Pd L cage
2
4
molecules and the integration of the cage molecules into the
gel skeleton.
In addition, the gelation dependence on the L/Pd ratios is
actually controlled by assembly of Pd L cage precursors as re-
2
4
vealed by the NMR titration study (Figure 5). When L/Pd ratio
was 2:1, quantitative Pd2L4 cages were obtained in MeCN/
2
+
DMSO solution. Excess Pd
ions (e.g., L/Pd=2:2) did not
affect the solution structure. However, when excess ligand was
present (e.g., L/Pd=2:0.67), Pd L cage molecules showed re-
2
4
markable dynamic behavior as evidenced by the broadened
and complicated signals. Such solution assembly behaviors of
Pd L cage showed profound influence of the gelation. Only
2
4
when all O-PyFDTE ligands were used up for formation of
Figure 7. Absorption spectra of the Pd/O-PyFDTE=1:2 solution
À4
À1
(
c
L
=1.0ꢁ10 molL in DMSO) under 254 nm UV light irradiation, showing
Pd L cages, that is, L/Pd=1:1 and 2:1, did the gelation occur
2
4
O-Pd to C-Pd structural conversion.
2
L
4
2 4
L
(the Supporting Information, Table S2). When the ligand was in
excess, that is, L/Pd=4:1, no gelation occurred. In addition, it
2
+
should be noted that the presence of too many excess Pd
Formation of the closed-ring C-Pd L cage structure in the
2 4
ions, for example, L/Pd=1:2, also prevented the gelation pro-
above process was evidenced by ESI-MS and NMR spectro-
scopic examination. The high-resolution ESI mass spectra of
the C-Pd L solution showed a clear sequence of peaks for
cess (the Supporting Information, Table S2).
2
4
4
+
À 3+
À 2+
[
Pd L ] , [Pd L +NO ] , and [Pd L +2NO ] species cor-
2 4 2 4 3 2 4
3
Light-triggered structural and gel-to-solution transitions
responding to sequential loss of nitrate anions from the intact
mother coordination cage [Pd ](NO (the Supporting Infor-
mation, Figure S9). This spectral observation closely resembles
the ESI-MS result of O-Pd cage mentioned above, because
the O-Pd and C-Pd isomers have exactly the same molec-
The Pd L solution underwent reversible photochromism in the
2
L
4
)
3 4
2
4
solution state upon irradiation with UV and visible light arising
from ring-closing/opening reactions as expected for dithienyle-
thene derivatives (Figure 6). Such structural interconversion be-
L
2 4
L
2
L
2 4
4
ular weight. However, the structural interconversion between
these two isomers can be easily identified by the in situ
1
H NMR monitoring (Figure 6). The initial and completely con-
verted Pd L cage species show well-recognized resonance pat-
2
4
terns corresponding to respective isomers (the Supporting In-
formation, Figure S10), whereas the incompletely converted in-
termediate shows a mixture of two isomers.
Light-induced structural conversion between the C-Pd2L4
and O-Pd L isomers readily leads to a gel-to-solution transition
2
4
(
Figure 1) because the C-Pd L isomer cannot form a gel at
2 4
room temperature but the O-Pd L isomer can. The transition
2
4
Figure 6. Interconversion between open-ring O-Pd
2
L
4
and closed-ring C-
from the brown gel state (CCG-1) to the blue solution state (C-
Pd
Pd
2
L
L
4
isomers in solution at 343 K and a) Pd/C-PyFDTE=1:2 solution (C-
Pd L ) was induced by 254 nm UV irradiation for 12 h. The re-
2
4
À1
2
4
) kept in the dark (c
and O-Pd mixture); c) under visible light for 30 min (O-Pd
Only aromatic proton signals are shown.
L
=0.025 molL ); b) under visible light for 1 min
sultant low viscosity solution was stable at room temperature
when kept in dark, but the blue solution faded upon exposure
to visible light for 20 min and turned to a gel again after 10 h.
The gel-to-solution transition was fully reversible and repeata-
ble for many times. Further investigations revealed that 15%
percentage conversion of the open-ring to closed-ring isomer
was enough to break down the gel. In other words, the critical
conversion percentage of C-Pd L cages is below 15% (deter-
(C-Pd
2
L
4
2
L
4
2 4
L
).
tween open-ring O-Pd L and closed-ring C-Pd L isomers can
2
4
2 4
be further monitored by in situ UV/Vis spectra. The major ab-
sorption band of the Pd/L=1:2 dilute solution in the open
form O-Pd L was found at about 290 nm, which is ascribed to
2
4
2 4
p–p* transition (Figure 7). Irradiation of the solution with
54 nm UV light led to appearance of new absorptions band
mined on the basis of the NMR spectral change).
2
at about 385 and 595 nm, which are attributed to the in-
Gelation mechanism
creased p-electron delocalization in the closed isomer C-Pd L .
2
4
The isosbestic points are centered at 257 and 320 nm, suggest-
ing that only two absorbing species, namely O-Pd L and C-
The above results reveal that formation of O-Pd L cage precur-
2
4
sors is essential in the gelation of CCG-1. The formation of the
2
4
Pd L isomers, are involved in light-triggered structural inter-
Pd L cage structure was further confirmed by two-dimensional
2
4
2 4
1
1
conversion.
diffusion-ordered spectroscopy (DOSY) H NMR. DOSY H NMR
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spectroscopy was utilized to measure the diffusion coefficients
of the solution species assembled from the Pd/O-PyFDTE=1:2
solution in [D ]MeCN/[D ]DMSO (v/v=2:1) (the Supporting In-
3
6
formation, Figure S11). The self-diffusion coefficient D for the
À10
2
À1
cage was determined to be 4.47ꢁ10 m s at 298 K. Accord-
ing to the Stokes–Einstein Equation, the approximate hydrody-
namic radius is calculated to be 9.21 ꢂ for the solution species.
The radius is in agreement with formation of the Pd L cage
2
4
structure. In addition, the Pd L cage complex precipitated by
2
4
diffusion of hexane into the MeCN/DMSO solution was found
to gelate DMSO again.
To understand the role of Pd L cage molecules in the gela-
2
4
1
tion, variable-temperature H NMR spectroscopy was investi-
gated within a temperature range 300–353 K for both O-Pd2L4
and C-Pd L isomers, which show distinguishable gel and solu-
1
Figure 8. H NMR spectra of O-PyFDTE/[Pd(NO ) ]=2:1 in [D ]MeCN/
3
2
3
2
4
[D ]DMSO solvent mixtures (v/v=1:19, 1:7, 1:3, 1:1, 5:3, 2:1)
=0.025 molL ) at room temperature. Only the signals of aromatic pro-
tons are shown. Prior to analysis, the reaction mixtures were allowed to
stand overnight. The v/v=1:19, 1:7, 1:3, 1:1 systems formed gels and the v/
v=5:3, 2:1 systems were solutions.
6
À1
(
c
L
tion states (the Supporting Information, Figure S12). At room
temperature, no salient resonance signals of O-Pd2L4 cages
were observed in the gel state, suggesting all the cage mole-
cules are fixed as part of the gel network. Upon heating the O-
Pd L gel system, a gradual improvement of the signal resolu-
the gel matrix as the temperature decreases or as the MeCN
content decreases. This suggests that the gel matrix is mainly
constituted by O-Pd L cage molecules and the solvent plays
2
4
tion was observed. Such increase of cage gelator signals upon
heating is due to the enhancement of molecular mobility and
2
4
[
11d,31]
dissociation of the gel network.
Above the gel-to-solution
an important role in the gelation process as well.
transition temperature (343 K), well-resolved signals of the
Pd L cage species were observed. The reverse cooling process
To reveal the role of counteranions, [Pd(CH CN) (BF ) ] in-
3
4
4 2
2
+
stead of [Pd(NO ) ] was used as the Pd source to treat with
2
4
3 2
À
afforded the opposite observations. This suggests that the O-
Pd L cage species aggregate gradually and finally form the gel
O-PyFDTE. Gelation occurred too when NO3 was replaced by
À
1
BF4 (the Supporting Information, Figure S15). H NMR spec-
troscopy revealed the formation of O-Pd L cage species in so-
2
4
as the temperature decreases. In contrast, the C-Pd L isomer
2
4
2 4
À
system persists in the solution state in the testing temperature
range. When measured in the dark at room temperature, the
spectrum showed broadened but salient proton signals, indi-
cating slowdown of the C-Pd L cage dynamics in solution but
lution. It suggests that both weakly coordinating NO and
3
À
BF4 anions are able to assist Pd L cage formation and there-
2
4
fore gelation. However, the coordinative anions exhibited
À
À
À
À
rather different behaviors, such as Cl , Br , I , and AcO ,
which significantly disturbed gelation process and exhibited
anion responsiveness.
2
4
no gelation. As expected, the proton resonances became well-
resolved with an increase in temperature.
To obtain more gel structural information, the O-Pd2L4
system was investigated in [D ]MeCN/[D ]DMSO solvent mix-
The anion responsiveness study was performed by the fol-
lowing method: Tetrabutylammonium (TBA) salts with different
3
6
1
tures by H NMR spectroscopy. As mentioned above, gelation
also occurs in MeCN/DMSO solvent mixtures (v/vꢀ1:1) but
sensitively depends on the MeCN/DMSO ratio. When the con-
tent of MeCN in MeCN/DMSO mixture increases, the system
transforms from gel (v/vꢀ1:1) to solution (v/v>1:1) (the Sup-
porting Information, Figure S13). Interestingly the signals of O-
Pd L cage were observed for the gels as seen for the solutions
counteranions, TBA-Cl, TBA-Br, TBA-I, TBA-BF , and TBA-OAc in
4
50 mL of DMSO (molar ratio of anion/L=2:1), were layered on
top of the vials containing the gel CCG-1 (0.2 mL). After the
vials were left standing at room temperature, the gels diffused
À
À
À
À
with Cl , Br , I , and AcO were all converted into solutions
À
within a few hours, whereas the gel with BF4 remained intact
1
(the Supporting Information, Figure S16). H NMR spectroscop-
2
4
(
Figure 8). The broadened signals turn sharp gradually as the
ic analysis of the resultant solution after diffusion with the co-
ordinating anions, showing the proton signals of free ligand
and confirming dissociation of Pd L cages. The results show
MeCN content increases, revealing that the O-Pd L cages inte-
2
4
grated in the gel network become loose and more dynamic
even if the overall gel state was retained. This observation is
comprehensible because intermolecular interactions are less fa-
vored in less polar MeCN than in DMSO, thus allowing faster
exchange between discrete cage molecules and those integrat-
2
4
À
À
À
À
that the coordinating anions (Cl , Br , I , and AcO ) may com-
pete with the PdÀN binding interactions within the O-Pd L
2
4
cage, thus causing dissociation of the cage building units
thereof destroying the gel matrix. In contrast, the non-coordi-
[
27]
À
ed in the gel network. In addition, when the gel (v/v=1:3)
nating anion (BF ) has little effect on the gelation state. There-
4
was heated, the resonances became well resolved (the Sup-
fore, the coordinating anions are able to trigger gel-to-solution
1
[28]
porting Information, Figure S14). These results from H NMR
transformation (Figure 1).
spectroscopy show that the O-Pd L cage molecules dissociate
To explore whether fluorine atoms on PyFDTE ligand play
a role in gelation, a structurally related ligand was prepared for
comparison, namely PyDTE (Scheme 1). PyDTE is an analogue
of PyFDTE without substituted fluorine atoms on the DTE
2
4
gradually from the gel matrix as the temperature increases or
as the MeCN content increases at room temperature; in other
words, O-Pd L cage molecules are gradually incorporated in
2
4
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&
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moiety. PyDTE undergoes similar open-ring to closed-ring iso-
meric transformation and assembles similar Pd L cages in so-
four OÀPyFDTE ligands in an unrestricted conformation, expos-
ing S atoms, F substituents, and aromatic organic moieties to
the outer sphere, facilitating formation of multiple intermolec-
ular interactions probably involving these heteroatoms, DMSO
2
4
lution. Unexpectedly, gelation did not occur for a Pd/PyDTE=
:2 mixture in DMSO under similar conditions. Instead, a viscous
1
À
solution was obtained. This observation implies that the fluo-
rine atoms may take part in the aggregation of Pd L precur-
solvent molecules, NO3 anions, as well as hydrophobic inter-
actions and p···p stacking interactions. The evidence of p···p
stacking interactions can be testified from UV/Vis spectra of
the O-Pd L cage solutions, which indicate that the adsorption
2
4
sors and subsequent gelation, presumably through fluorine-in-
[
29]
volved weak intermolecular interactions.
2
4
To understand why gelation only occurs to the open-ring O-
Pd L isomer rather than the closed-ring C-Pd L isomer, struc-
maximum shows significantly redshifted from 286 to 304 nm
À6
as the concentration increased from 1.0ꢁ10
to 1.0ꢁ
2
4
2 4
À4
À1
tural simulation has been carried out for both the isomers (see
the Supporting Information). The C-Pd L cage displays a much
10 molL (the Supporting Information, Figure S19). Forma-
tion of these intermolecular interactions may be able to stabi-
lize the initially aggregated nanoparticles, and to further
induce them to grow gradually. The nanoparticles continue to
grow to ꢁ150–170 nm after 6 h, and then fell beyond the test-
ing scope of DLS instrument (Figure 9). This is consistent with
the above SEM and TEM study. In comparison, different aggre-
gation was observed for the solids obtained from the C-Pd2L4
cage solution in DMSO (the Supporting Information, Fig-
ure S20) and the O-Pd L cage solution in MeCN/DMSO (the
2
4
more rigid conformation and regular shape, whereas the O-
Pd2L4 cage permits flexibility and geometric freedom to its
component ligands and substituting groups (the Supporting
Information, Figures S17 and S18). Moreover, the S atoms in
the dithienylethene moieties are oriented outwards in between
every two ligands in O-Pd L cage structure, but completely re-
2
4
strained inwards into the cage cavities in C-Pd L isomer. In-
2
4
stead, only hydrophobic methyl groups are positioned in the
vacant space between the ligands. These distinct structural ge-
ometry and conformation may impart different intermolecular
interacting environment to O-Pd L and C-Pd L isomers, thus
2
4
Supporting Information, Figure S21). During this period of
time, the aggregated nanoparticles may form an open network
to trap solvent molecules, finally yielding the gel CCG-1. There-
fore, the gelation process should start from assembly of the O-
Pd L cage molecules, proceeding through formation of nano-
2
4
2 4
causing different interactions between the cage molecules as
well as interactions with surrounding anions and solvent mole-
cules. O-Pd2L4 cage isomers may provide more chances to
form intermolecular interactions with adjacent species, facilitat-
ing gelation process, which indeed needs synergistic interplays
from multiple intermolecular weak forces.
2
4
particle dispersion, and subsequent aggregation into monolith-
[32]
ic networks (Figure 1).
However, it is worth mentioning that gelation is a complicat-
ed behavior subject to subtle influence of various intermolecu-
lar interactions including solvent–gelator interactions. The
exact gelation mechanism is hard to fully understand because
we cannot directly determine the molecular structure of the
gel matrix according to the present techniques. Therefore, the
gel structural information mainly comes from dissociated spe-
cies in the liquid phase. Since coordination oligomers/polymers
The aggregation process of O-Pd2L4 cage molecules into
larger nanoparticles was estimated by a dynamic light scatter-
ing (DLS) study (Figure 9). In the freshly prepared O-Pd L solu-
2
4
À1
tion (c =0.025 molL ), around 24 nm particles were recorded
L
to show the initial aggregation of O-Pd L molecules to nano-
2
4
particles. Electrostatic attraction mediated by counterions may
be one of important forces inducing the initial aggrega-
[6]
are well known to assist gelation, we cannot exclusively rule
out the presence of trace amounts of coordination oligomer/
polymer species in the O-Pd L gel; however, our experimental
[
19,30]
tion.
The Pd L cage unit carries four positive charges, thus
2 4
attracting counteranions to surround the cage species by elec-
trostatic forces. Sharing of the anionic sphere among the mul-
tivalent cage species may hold them together to form larger
2
4
results indicate that the gel skeleton is mainly based on O-
Pd L cage species and the contribution of possible polymeric
2
4
[
31]
nanoparticles. In the meantime, the O-Pd L cage contains
species is definitely beyond the NMR detection.
2
4
Thixotropic and rheological studies
The gel CCG-1 exhibits thixotropic response to external large
straining. It transformed into a free-flowing viscous liquid by
shaking the vial for 2–3 min, and the resulting free-flowing
liquid reverted back to the gel after resting for about 10 h
1
(
Figure 3). This process was monitored by H NMR spectrosco-
py (the Supporting Information, Figure S22). In the gel state no
salient proton signals were observed, however, some broad
signals of O-Pd L cages appeared after shaking. These signals
2
4
observed after shaking are similar to those before gelation.
This suggests that the cage molecules may escape from the
gel matrix and turn “free” to some degree upon shaking.
The thixotropism or mechano-responsive behavior of the gel
CCG-1 was investigated in detail by rheological techniques.
Figure 9. DLS particle size as function of reaction time for the Pd/O-
À1
PyFDTE=1:2 reaction system in DMSO (c
L
=0.025 molL ) at room tempera-
ture.
Chem. Eur. J. 2015, 21, 1 – 11
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7
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gels composed of cage molecules can afford similar thixotropic
property. Interestingly, the present gels have a different nano-
particle-based morphology. The mechano-responsive behavior
may be related to the electrostatic interactions and other weak
intermolecular interactions between O-Pd2L4 cage building
units, which are characteristic of non-directional and self-cor-
rective forces.
Conclusion
A multiple stimuli-responsive metal–organic gel (CCG-1) has
been obtained on the basis of Pd L coordination cage assem-
2
4
bly utilizing a dithienylethene-derivatized photochromic bis-
pyridine ligand. The gelation process involves firstly the forma-
2
+
tion of Pd L cage molecules from Pd salts and the open-
2
4
ring dithienylethenebispyridine isomer O-PyFDTE. Then the O-
Pd L cage molecules aggregate in DMSO to form nanoscaled
Figure 10. Rheological tests of gel CCG-1: a) strain sweep; b) frequency
sweep; c) time sweep; d) three cycles of deformation and recovery process-
es.
2
4
particles, which are further cross-linked to generate 3D gel
matrix with the aid of multiple supramolecular interactions.
The presence of O-Pd L cage building units was evidenced by
2
4
1
1
Firstly, the gel was monitored as functions of shear strain and
dynamic frequency at room temperature to determine the
linear viscoelastic regime (Figure 10a,b). At a low strain the
storage modulus G’ (ca., 69 Pa) was more than one order of
magnitude greater than the loss modulus G’’ (ca., 6 Pa), indi-
cating dominant elastic characters of the gel. The moduli are
H NMR and DOSY H NMR spectroscopies and ESI-MS. Control
experiments and structural simulation show that the open-ring
conformation and the fluorine substituents are important for
the intermolecular interactions and final gelation. The gel CCG-
1 is anion-responsive towards coordinative anions because the
addition of coordinative anions to the gel system dissociates
the Pd L cages, thus resulting in collapse of the gelatinous
[
33]
lower than those of normal low molecular-mass gels. Below
the strain of 5% as the upper limit of the linear viscoelastic
regime, the energy storage modulus and loss modulus kept
constant. With the strain increases gradually, the energy stor-
age modulus G’ fell sharply, whereas the loss modulus firstly in-
creased and then fell, indicating that the gel began to breakup
partially. At a critical value of 130% strain, the storage modulus
G’ was equal to the loss modulus G’’. Above the critical value
of 130% strain, the loss modulus G’’ became greater than the
storage modulus G’, which indicated that the network of the
gel was completely destroyed. It is clear that the values of G’
are always greater than those of the corresponding G’’ within
2
4
state to a solution. Remarkably, the viscoelastic gel CCG-1 ex-
hibits reversible multistimuli-responsive behaviors by exposure
to external temperature, light, and shear stress, showing vari-
ous stimulus-triggered gel-to-solution interconversions with
the overall Pd L cage topology well kept. Among these phase
2
4
interconversions, the gel-to-solution transition controlled by
UV/visible light irradiation is accompanied with the structural
transformations between the ring-open O-Pd L and the ring-
2
4
closed C-Pd L cage isomers. Nevertheless, the O-Pd L cage
2
4
2 4
molecules are kept intact in temperature- or shear-stress-regu-
lated gel-to-solution transitions. The present study suggests
that coordination cages of predefined shape/size and inner
cavity can be used intelligently as promising building blocks
for creation of responsive and smart soft materials. In this way,
versatile host–guest chemistry can be introduced into gel ma-
terials. It also suggests that supramolecular gels consisting of
porous molecules may be developed as a new type of porous
materials because they are mostly liquid, yet they behave like
a solid and they undergo readily gel-to-solution transitions.
À1
the frequency range 0.1–100 rads . Below the frequency limit
À1
of 100 rads , both moduli exhibited weak dependence on fre-
quency change, indicating typical viscoelastic behavior. Re-
markably, when the large strain was removed, the gel recov-
À1
ered immediately (G’>G’’) under 2% strain and 6.283 rads
by monitoring the time evolution of G’ and G’’ (Figure 10c).
This confirms that the gel features a thixotropic behavior,
which is a key property for real-life applications of gel-based
[
34]
materials.
To further confirm the gel thixotropic property, a typical thix-
otropy loop test was performed (Figure 10d). When a large
strain (500%) was applied for 10 s, the gel was completely de-
stroyed (G’’>G’). Once the high strain was canceled, the elas-
ticity recovered immediately after 20 s (G’>G’’). Such a thixo-
tropy test can be repeated three times at least. This recovery
time is comparable with that of low-molecular-mass organo-
Experimental Section
Materials and general methods
All starting materials and solvents were obtained from commercial
sources and used without further purification unless otherwise
[38]
indicated. 3-Bromo-2-methyl-5-(3-pyridyl)thiophene
and O-
[39]
PyDTE were synthesized according to the published methods.
The powder X-ray diffraction was recorded on a Bruker D8 Ad-
vance diffractometer at 40 kV, 40 mA with a Cu target tube and
a graphite monochromator. Infrared spectra were measured on
[
35]
[36]
gels and shorter than that of metallogels.
Thixotropic behavior has been previously studied for low-
[
35,37]
molecular-mass gels.
The present study unveils that the
&
&
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1
a Nicolet Avatrar 330 FT-IR spectrometer with KBr pellets. H NMR
within 20 s. G’ and G’’ were recorded as functions of time in both
spectra were recorded on AVANCE III 400 MHz Spectrometer. UV/
Vis absorption spectra were recorded using a Shimadzu/UV-250PC
spectrophotometer. The UV irradiation studies were carried out
the processes.
using standard hand-held lamps (254 and 365 nm) used for visual- Acknowledgements
izing TLC plates (WFH-12B), and the visible-light irradiation studies
were carried out by using a 150 W halogen source (Philips 13629
EKE) that was passed through a Y48(>450) cutoff filter to eliminate
higher energy light.
We acknowledge the 973 Program (2012CB821701), the NSFC
21103233, 21273007, 21350110212 and 91222201), the NSF of
(
Guangdong Province (S2013030013474), the Program for New
Century Excellent Talents in University (NCET-13-0615), and the
RFDP of Higher Education of China (20120171130006) for sup-
port.
Synthesis of O-PyFDTE
The synthetic route for PyFDTE is shown in Scheme S1 (the Sup-
À1
porting Information). A nBuLi/hexane solution (3.1 mL, 1.6 molL ,
5
a
5
mmol) was added dropwise at À788C under N atmosphere to
2
Keywords: coordination cages
palladium · photochromism · supramolecular gels
·
coordination modes
·
solution of 3-bromo-2-methyl-5-(3-pyridyl)thiophene (1.26 g,
mmol) in anhydrous THF (50 mL). After 30 min, octafluorocyclo-
penene (0.34 mL, 5 mmol) was slowly added and the reaction mix-
ture was stirred for 1 h at À788C. The reaction mixture was al-
lowed to slowly warm to the room temperature and quenched by
diluted hydrochloric acid. The mixture was extracted with diethyl
ether and the ethereal extract was dried with anhydrous MgSO4
and evaporated to dryness. The crude solid product was purified
by column chromatography on silica gel using petroleum ether/
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Published online on && &&, 2015
&
&
Chem. Eur. J. 2015, 21, 1 – 11
www.chemeurj.org
10
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Metal–organic cages were utilized as
building units for the first time to gen-
erate supramolecular gels, providing
a synthetic approach to cavity-contain-
ing gels that display multi-responsive-
ness towards environmental physical
and chemical stimuli (see figure).
&
Cage Compounds
S.-C. Wei, M. Pan, Y.-Z. Fan, H. Liu,
J. Zhang,* C.-Y. Su*
&
& – &&
Creating Coordination-Based Cavities
in a Multiresponsive Supramolecular
Gel
Porous Materials
Cavities on different molecular levels, from molecular
containers to macroscopic porous materials, have many
potential applications. In their Full Paper on page && ff., J.
Zhang, C.-Y. Su, and co-workers describe an assembly
approach to multiresponsive supramolecular gels by
integrating photochromic metal–organic cages as
predefined building units into the supramolecular gel
skeleton, providing a new approach to create cavities in
gels. Such supramolecular gels consisting of porous
molecules may be developed as a new type of porous
materials.
Chem. Eur. J. 2015, 21, 1 – 11
www.chemeurj.org
11
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ