Hierarchical Gelation of a Pd12L24 Metal-Organic Cage Regulated by Cholesteryl Groups

Article abstract of DOI:10.1021/acs.inorgchem.9b01171

The creation of supramolecular assemblies by assembly of structurally simple components via supramolecular interactions provides an opportunity to develop functional materials with hierarchical complexibility. Herein, we report an assembly approach to supramolecular gels based on metal-organic cages with tunable hierarchical structures and properties. A Pd₁₂L₂₄ cage (L is cholesteryl-functionalized 3,5-bis(4-pyridyl)benzene) bearing 24 cholesteryl groups is used as a supramolecular building unit and molecular platform for functionalization with tunable functional behaviors. The Pd₁₂L₂₄ cage motifs spontaneously self-assemble into gels where orthogonal metal-organic coordination and cholesteryl-cholesteryl interactions are involved in the gelation. The Pd₁₂L₂₄ cage exhibits a reversible transition between solution and aggregated gel states in response to temperature. The gelation and the mechanical property are rarely regulated by deuterated solvents and tetramethylsilane. The mechanical property of the gel materials is tunable by varying the content of cholesteryl groups of Pd₁₂L₂₄. Functional moieties (e.g., luminescent TPE group) can be introduced on the cage, and the luminescent property changes while the structure is maintained. The Pd₁₂L₂₄ gel shows visible anion-responsive behaviors arising from the hierarchical structure.

Full text of DOI:10.1021/acs.inorgchem.9b01171

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hierarchical Gelation of a Pd12L24 Metal−Organic Cage Regulated by
Cholesteryl Groups
Lihua Zeng, Yali Xiao, Jingxing Jiang, Haobin Fang, Zhuofeng Ke, Liuping Chen,
and Jianyong Zhang*
Sun Yat-Sen University, MOE Laboratory of Polymeric Composite and Functional Materials, School of Materials Science and
Engineering, Guangzhou 510275, China
S
* Supporting Information
ABSTRACT: The creation of supramolecular assemblies by assembly of
structurally simple components via supramolecular interactions provides an
opportunity to develop functional materials with hierarchical complexibility.
Herein, we report an assembly approach to supramolecular gels based on
metal−organic cages with tunable hierarchical structures and properties. A
Pd12L24 cage (L is cholesteryl-functionalized 3,5-bis(4-pyridyl)benzene) bearing
24 cholesteryl groups is used as a supramolecular building unit and molecular
platform for functionalization with tunable functional behaviors. The Pd12L24
cage motifs spontaneously self-assemble into gels where orthogonal metal−
organic coordination and cholesteryl−cholesteryl interactions are involved in
the gelation. The Pd12L24 cage exhibits a reversible transition between solution
and aggregated gel states in response to temperature. The gelation and the
mechanical property are rarely regulated by deuterated solvents and
tetramethylsilane. The mechanical property of the gel materials is tunable by
varying the content of cholesteryl groups of Pd12L24. Functional moieties (e.g., luminescent TPE group) can be introduced on
the cage, and the luminescent property changes while the structure is maintained. The Pd12L24 gel shows visible anion-
responsive behaviors arising from the hierarchical structure.
spaces.23−28 However, for the reported examples of cage-based
INTRODUCTION
■
polymer gels (particularly Johnson’s and Kato’s based on
In living organisms, small biomolecules, e.g., lipids, interact and
self-organize to form well-defined functional structures.
Inspired by nature’s elegant self-assembly processes and
structural amplification from small molecular components
into giant supramolecular nanostructures, chemists devote
themselves to the creation of complicated supramolecular
assemblies by assembly of structurally simple molecules via
Fujita-type Pd12L24 metal−organic cage14−17), covalent link-
ages are relatively less adaptable and less stimuli-responsive.
This led us to seek the rational design of supramolecular gels
based on metal−organic cages with a multistimuli responsive
property.21
Herein, a new metal−organic cage-based supramolecular gel
is designed and constructed via classical cholesteryl−
noncovalent, weak, and reversible interactions such as metal−
organic coordination, hydrogen bonding, π−π stacking, and
cholesteryl interactions. We use a Pd12L24 cage as supra-
van der Waals interactions.1 In nature multiple supramolecular
molecular building units to assemble gel materials with
hierarchical structures and highly tunable properties. Specifi-
interactions are usually present in a system, thus much effort
cally, the Fujita-type well-established Pd12L24 cage motif29−34
has been made to use orthogonal supramolecular interactions
to develop sophisticated complex systems.2−12 Recently, a
was employed. Pd12L24 is a large spherical nanocage assembled
from with 12 Pd(II) ions and 24 pyridine-capped bidentate
novel type of soft materials has been developed by
ligands. It shows rich host−guest chemistry and can
incorporation of well-defined metal−organic cages in gel
accommodate functional molecules in its interior.32−35 And
importantly, its outer surface can be readily exofunctionalized.
On the other hand, cholesteryl is a classical molecular building
block in gel chemistry, the derivatives of which are known to
stack via van der Waals interactions to produce directional
aggregates.35−42 Cholesteryl molecular scaffolds increase
gelation ability and bring reversibility in self-assembled
materials.13−21 It is promising in development of soft matter
that not only has a confined cavity but also is amenable to
materials processing.22 Such materials have shown interesting
properties including stepwise guest release,13 tailored polymer-
ization, and viscoelasticity.14−16 These properties are related to
supramolecular metal−organic cage building units with well-
defined cavities, considerable sizes, and desirable shapes. These
metal−organic cage compounds are well-known to offer
cavities suitable for the inclusion of a range of guest molecules,
and uncommon chemistry has been explored in their confined
Received: April 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01171
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Figure 1. Schematic representation of (a) Pd12L24 and gelation, (b) Pd12L24 with defect L0, and (c) Pd12L24 with doping L′.
1
Figure 2. H NMR spectrum of Pd12L24 in CDCl3−CD3CN (v/v 2:1) (323 K) and L in CDCl3−CD3CN (v/v 2:1) (293 K).
B
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Figure 3. (a) Reversible gel−solution phase transition, (b) SEM image of Pd12L24 gel (CDCl3−CD3CN) (the scale bar represents 500 nm),
rheological tests (c) frequency sweep at a 1% strain amplitude within the linear regime from 0.1 to 100 Hz, and (d) strain sweep at 1.0 Hz
frequency ranging from 0.01%−100% for Pd12L24 (CHCl3−CH3CN) and Pd12L24 (CDCl3−CH3CN).
supramolecular structures so that various functional groups can
be incorporated, such as redox-active naphthalimide,43
tetrathiafulvalene,44 near-infrared active nickel bis-
(dithiolene),45 and macrocyclic calix[4]arene.46,47 Based on
the Pd12L24 cage motif, orthogonal supramolecular inter-
actions, including metal−organic coordination and cholester-
ol−cholesterol interactions, have been employed to hierarchi-
cally self-assemble functional gels with tunable properties.
coordination cage was subsequently prepared quantitatively by
simple mixing of L and Pd(NO3)2·2H2O in a 2:1
stoichiometric molar ratio in CHCl3-MeCN (v/v 2:1) at 50
°C for 2 h. The formation of spherical complex Pd12L24 was
1
−
characterized by H NMR (24 PF6 anions are omitted for
clarity, the same hereinafter, Figures 2, S7, and S8). Compared
to L, the 1H NMR spectrum of Pd12L24 showed only one set of
signals, in good agreement with its discrete, highly symmetric
spherical structure. Due to coordination interaction between L
and Pd(II), most of the peaks corresponding to the ligand shift
downfield. In particular, the α-pyridyl protons Ha (from 8.73 to
9.57 ppm) and the β-pyridyl protons Hb (from 7.55 to 7.94
ppm) show downfield chemical shifts. 2D diffusion-ordered 1H
NMR spectroscopy (DOSY) showed that the phenyl proton
signals displayed the diffusion coefficient (1.8 × 10−10 m2 s−1)
in CDCl3−CD3CN at 323 K, and the size of the rigid core of
Pd12L24 was calculated to be 4.1 nm according to the Stokes−
Einstein equation.49,50 These data are consistent with those of
Fujita’s Pd12L24 (L = 1,3-bis(4-pyridyl)benzene (L0) or 3,5-
bis(4-pyridyl)benzaldehyde, Figures S9−S13),51 supporting
that a discrete Pd12L24 cage structure was persistent in solution
(for ESI-TOF mass spectrometry, see below).
RESULTS AND DISCUSSION
■
Exofunctionalization of a Pd12L24 Cage. Fujita-type
Pd12L24 cages are readily exofunctionalized by incorporation of
multiple functional groups at the vertex of building blocks
(organic linkers). To achieve this, cholest-5-en-3-ol (3β)-3-
(3,5-bis(4-pyridyl)phenoxy)propyl) carbonate (L) was de-
signed herein. L has two pyridyl groups for metal coordination
and assembly of Pd12L24 cage moiety. One cholesteryl residue
was attached to L via a flexible linker. The cholesteryl group
does not interfere with the Pd−pyridine bond, so the structure
of dynamic Pd12L24 cage motifs is not expected to be disrupted
during the self-assembly process. The Pd12L24 cage has 24
cholesteryl substituents on the exterior surface, providing for
further aggregation (Figure 1). By virtue of cholesteryl−
cholesteryl interactions, the spherical complex Pd12L24 may
further assemble into a larger aggregation or infinite network
with more complexity.
Gelation Tests of Pd12L24. The gelation ability of Pd12L24
was tested as shown in Table S1. Among the solvents
examined, a yellow opaque gel was obtained in CHCl3−
CH3CN (v/v 2:1, the same hereinafter) after 48 h.
Unexpectedly, the gelation time is remarkably shortened in
deuterated solvent. The gelation in CDCl3−CD3CN (0.5 h) is
much faster than CDCl3−CH3CN (8 h). It is worth
Bispyridylligand L was synthesis from 3,5-di(pyridin-4-
yl)phenol35 and cholesteryl alkyl bromide48 by a nucleophilic
substitution reaction (Scheme S1, Figures S1−S6). A Pd12L24
C
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mentioning that the deuterated solvent contains 0.03%
tetramethylsilane (TMS), which is usually used as an internal
standard for calibrating the chemical shift for NMR spectros-
copy. Serendipitously, TMS may accelerate the gelation as well.
If 1.0% TMS was added in the solvent mixture, the gelation
was achieved in a shorter time (5 h) and the gel was stronger
than those with less (0.1%) or no TMS added. Moreover, the
gelation was fully thermoreversible in CDCl3−CD3CN
deuterated mixture (Figure 3). The gel transformed to a pale
yellow transparent solution when heated at 55 °C for 1 min
Pd12L24 xerogel exhibit two signals centered at 343.8 and 338.6
eV in the fitted Pd 3d core level spectra, which are attributed to
Pd 3d3/2 and Pd 3d5/2, respectively, characteristic of divalent
Pd(II) (Figure S23). The appearance of positive CD signals
(λmax = 364 and 272 nm) for Pd12L24 xerogel indicates the
formation of chiral supramolecular structures induced by the
cholesterol group (Figure S24).
Rheological Property and Tuning. Rheology analyses
were carried out to measure the influence of deuterated
solvents (CDCl3 vs CHCl3) on the gelation. Rheological
response was assessed by means of dynamic rheology
measurement for Pd12L24 gels in CHCl3−CH3CN and
CDCl3−CH3CN (Figure 3). Frequency sweep response of
the gels shows that the storage modulus (G′) is considerably
higher than the corresponding loss modulus (G″) within the
frequency range of 0.01−10 Hz at a constant strain of γ =
1.0%, which indicates that both of the gels have predominantly
elastic rather than viscous characteristics. Strain sweep ranging
from 0.01−100% at a constant frequency (1 Hz) determines
the linear viscoelastic region of the gels. Below the strain of
about 5% the average G′ parameters remain roughly 5000 and
800 Pa, respectively, and it can be observed that both gels have
a much larger G′ than the value of G″, indicating the dominant
elastic property. G′ of the CHCl3 gel decreased from 4500
500 to 750 100 Pa in the linear viscoelastic regime, while G′
of the CDCl3 gel decreased from 105 10 to 20 5 Pa. The
difference between deuterated CDCl3 and regular CHCl3
solvents is greater than one order of magnitude. This means
that the gelator in CDCl3 is much softer than in nondeuterated
CHCl3, demonstrating that the deuterated solvent has a
significant influence on the mechanical property of the
corresponding gel. As known, the solubility property of
CDCl3 is virtually identical to that of regular CHCl3 solvent,
while deuterated and regular solvents have slight differences in
solubility depending on the relative purity, which could be on
the order of a few percent. The present gelation difference
should be attributed to the different interaction between
cholesteryl moieties and deuterated solvent molecules. To the
best of our knowledge, no gel material has been reported about
the subtle difference between deuterated and regular solvents.
The high branch functionality Pd12L24 cage subunits14,15
makes it possible to finely tune the rheological property by
changing the cholesterol number of the cage moiety (Figure
1). The cholesterol number of the cage moiety was readily
tuned through defect engineering by varying the ratio of L and
L0 (without cholesteryl group) in the Pd12L24 cage. Various
ratios of L:L0 were employed to assemble a series of Pd12L24
metal−organic cages with mixed ligands for gelation. Gels were
successfully achieved under similar conditions when the molar
ratio of L:L0 was greater than 3.5:1, and the gel was only
thermoreversible in fully deuterated solvents like Pd12L24. In
contrast, no gel was obtained when the ratio of L0 increased to
more than 20% (Table S2). This further confirms that
cholesteryl moieties decide the gelation. Pd12(4L-L0)24 gel
(L:L0 = 3:1, CHCl3−CH3CN) with doping of 20% L0 was
investigated herein in detail. The morphology of Pd12(4L-L0)24
gel was characterized by scanning electron microscopy (SEM),
showing that the gel kept the same porous networked structure
consisting of interconnected nanoparticles (30−80 nm) as
Pd12L24 (Figure S25).
1
and returned to a gel upon cooling to room temperature. H
NMR showed weak and broad resonance signals in the gel
state at 293 K, attributed to the fact that most of the cage
molecules are fixed as part of the gel network. As shown above,
well-resolved signals were observed at 323 K, a temperature at
which a gel-to-solution transition occurred for the gel.
Reversible sol−gel transition was achieved only in fully
deuterated solvents. Instead, turbid fluids were obtained
when normal solvents were used and the phase transition
was not reversible. This is the first observation that deuterated
solvent and TMS affects the gelation. Deuterated solvents have
higher viscosity, and it may change the gelation reversibility
significantly and accelerate the gelation. TMS has a
pseudospherical structure with methyl groups, and the
increasing TMS content may enhance the intermolecular van
der Waals interactions and accelerate the gelation.52,53 This
may help develop responsive systems for specific analytes with
deliberately weak supramolecular interactions.
To reveal the cholesteryl−cholesteryl interactions on the
spherical molecule Pd12L24, a model complex PdL2L00 with
2
two cholesteryl groups (L00 represents 1-methoxy-3,5-di-
(pyridin-4-yl)benzene) was chosen for computational optimiz-
ing with the terminal N atoms frozen, which do not coordinate
to Pd in the model according to the published single crystal
structure of Pd12L24.29 The theoretical calculations show that
two neighboring cholesteryl moieties are twisted significantly
to become closer via van der Waal’s interaction, and the
shortest C−C distance of cholesteryl moieties are 3.46 Å
(Figures S14 and S15). Therefore, two cholesteryl moieties are
attracted to each other in this model complex, and it also
suggests that the gel matrix is stabilized mainly by van der
Waal’s interactions between cholesteryl moieties.
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) show that Pd12L24 gels prepared
from deuterated or regular solvents have sponge-like porous
networked structures consisting of interconnected nano-
particles, the sizes of which are in the range of 20−50 nm
(Figures 3 and S16−S19). The nanoparticle may be aggregated
from several or dozens of cage molecules. Elemental
composition analysis by energy dispersive X-ray spectroscopy
(EDX) reveals the presence of the corresponding Pd metal
atoms (Figure S20). The wet gel shows broad powder X-ray
diffraction patterns, revealing its amorphous nature (Figure
S21). Fourier transform infrared (FT-IR) for the Pd12L24 wet
gel shows the stretching vibrations of CO and COC
groups at 1743 and 1200 cm−1, respectively (Figure S22). A
stretching vibration at 1665 cm−1 is attributed to the presence
of the CN bond, characteristic of the pyridine moiety. It
shifts to lower frequency in comparison with L (1670 cm−1),
which indicates the coordination of L to a palladium atom
through the nitrogen atom. The strong absorption band at
The effect of doping of 20% L0 on the viscoelastic property
of Pd12L24 gel was studied by rheological measurements
(Figure 4). Frequency sweep experiments show that decreasing
−
approximately 1384 cm−1 is observed for NO3 . X-ray
photoelectron spectroscopy (XPS) elemental survey scans of
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Figure 4. Rheological tests. (a) Frequency sweep at a 1% strain amplitude within the linear regime from 0.1 to 100 Hz, and (b) strain sweep at 1.0
Hz frequency ranging from 0.01−100% for Pd12L24 (CHCl3−CH3CN) and Pd12(4L-L0)24 (CHCl3−CH3CN).
̅
Figure 5. (a) ESI-TOF-MS spectrum of Pd12(2L-22L′)24 (PF6 as counterion), where a series of signals of Pd12L′24 (M′), Pd12LL′23 (M1), and
Pd12L2L′22 (M2) are shown, and (b) measured (top) and calculated (bottom) isotope patterns for various charge states observed from Pd12LL′23
(M1).
the cholesteryl number facilitated depression in the elastic
modulus. The elastic modulus decreases with doping of L0 and
decreased cholesteryl groups. G′ was about 3600 Pa for
Pd12L24 (CHCl3−CH3CN), while the lowest G′ was about 450
Pa for Pd12(4L-L0)24 (CHCl3−CH3CN). For the latter, less
cholesteryl groups take part in the cholesteryl−cholesteryl
interactions, leading to weaker elastic moduli. Therefore,
increasing cholesteryl content in the Pd12L24 cage results in
formation of a greater number of cross-links, and hence, the
gelation may be induced.
E
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Figure 6. Excitation and emission spectra of (a) Pd12L24 solution in CHCl3 (c = 1.0 × 10−5 mol L−1, λex = 316 nm, λem = 369 nm), (b) Pd12(23L-
L′)24 solution in CHCl3 (c = 1.0 × 10−5 mol L−1, λem = 437 nm, λex = 316 or 375 nm), and (c) Pd12(23L-L′)24 gel (λem = 529 nm, λex = 371 nm).
Luminescence Property Tuning of Pd12L24 Gels. As a
molecular platform for functionalization, the luminescence
property of Pd12L24 is readily tunable by one-step self-
assembly. A heteroleptic Pd12(23L-L′)24 metal−organic cage
was designed and synthesized via 4% doping of L′ with
tetraphenylethylene (TPE) moiety (Figures 5 and S26−
S29).54,55 A pair of bis-pyridyl ligands (L and L′) appended
tetraphenylethene groups and cholesterol groups, respectively,
are combined with Pd2+ cations in a 1:23:12 ratio to obtain the
Pd12(23L-L′)24 cage. The quantitative formation of Pd12(23L-
L′)24 is characterized by 1H NMR (Figures S30−S32). In good
SEM and TEM show that Pd12(23L-L′)24 gel keeps the
characteristic sponge-like porous networked structure consist-
ing of interconnected nanoparticles ranging from 30 to 80 nm.
The luminescence property of Pd12L24 was studied. The
luminescence spectrum of L in CHCl3 solution (1.0 × 10−5 M)
displays a blue emission with a maximum at 377 nm upon
excitation at 258 or 310 nm, while solid L displays a greenish
yellow emission (λmax = 504 nm) upon excitation at 449 nm
(Figure S37). After cage formation, the CHCl3 solution of
Pd12L24 (1.0 × 10−5 mol L−1) displays an emission at λmax
=
369 nm upon excitation at 316 nm, which may be attributed to
the luminescence of 3,5-di(pyridin-4-yl)benzene moiety
(Figure 6). However, Pd12L24 gel did not generate any
luminescence, probably due to the quenching effect imposed
by Pd2+ and nitrate after gelation.56,57
1
agreement with its cage structure, the H NMR spectrum
1
shows one set of signals, and the 2D H DOSY spectrum
shows that the phenyl proton signals displayed the diffusion
coefficient of 2.23 × 10−10 m2 s−1. The diameter of the rigid
core of Pd12(23L-L′)24 was calculated to be 33.3 Å according
to the Stokes−Einstein equation. Mass spectrometry could not
be performed for Pd12L24 or Pd12(23L-L′)24 due to the easy
gelation and solubility restriction probably arising from
stronger intermolecular cholesteryl−cholesteryl interactions
and the resulting aggregation. Instead, ESI-TOF mass
spectrometry was performed successfully with the DMSO
solution of Pd12(2L-22L′)24 (L:L′ = 2:22) after exchange of
NO3− with PF6− anions. A distribution of various coordination
cages with 22−24 L′ ligands incorporated was observed, i.e.,
Pd12L′24 (M′), Pd12LL′23 (M1), and Pd12L2L′22 (M2), with a
After 4% doping of L′, the luminescence spectrum of
Pd12(23L-L′)24 solution in CHCl3 (1.0 × 10−5 mol L−1)
changes significantly, giving multiple peaks with a maximum at
437 nm under 375 nm excitation and another weak emission at
369 nm (λex = 316 nm). In comparison, the luminescence
spectrum of L′ in CHCl3 solution (1.0 × 10−5 mol L−1)
displays multiple emissions, with an emission at λmax = 387 nm
upon excitation at 265 or 305 nm and multiple emissions at
λmax = 433 nm upon excitation at 372 nm (Figure S38).
Therefore, for the Pd12(23L-L′)24 solution the main emission
at 437 nm is assigned to the TPE unit of L′, while the weak
emission at 369 nm is attributed to the 3,5-di(pyridin-4-
yl)benzene moiety of L. The Pd12(23L-L′)24 gel phase shows
the emission maximum at 529 nm (λex = 371 nm), which is
compared with L′ in the solid state (λem = 530 nm).
Fluorescence lifetime data show that Pd12(23L-L′)24 gel has
series of [M′-(PF6 )n]n+, [M1-(PF6 )n]n+, and [M2-(PF6 )n]n+
−
−
−
(n = 6−12) recognizable signals (Figures 5 and S33−S35),
24+·24PF6 related
−
confirming the existence of [Pd12LnL′24−n
]
structures. Pd12(23L-L′)24 gel was obtained when the hot
solution was cooled and kept standing overnight (Figure S36).
F
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a higher weighted mean lifetime (0.25 ns) than Pd12L24 gel
(0.18 ns) (Table S3). Additionally, Pd12L24 wet gel exhibits a
low fluorescence quantum yield (Φ = 0.4%) because Pd2+ and
nitrate is a fluorescence quencher,56,57 while Pd12(23L-L′)24
wet gel shows a slight increase in the Φ value (0.7%). These
results imply that the Pd12L24 cage may restrict the
intermolecular rotation and turn on the fluorescence of TPE
unit.58,59 Briefly, both the solution and gel states of Pd12(23L-
L′)24 show the characteristic emission of TPE, showing that
the luminescence of Pd12L24 may be tunable while maintaining
their structure. However, it should be noticed that the
luminescence is severely quenched by Pd2+ and nitrate in the
present system.
visually sensitive and selective toward CN−, while F− and I−
can also be detected.
To probe the mechanism of anion responsiveness for the gel,
1H NMR experiments were carried out for metal−organic cage
Pd12L024 as a model (Figures S39−S41). First, 2 equiv of TBA
salts of various anions, including F−, Cl−, CN−, H2PO42−, BF4 ,
−
PF6 , ClO4 , OTf−, Br−, I−, and OAc−, were each mixed with
−
−
the (CD3)2SO solution of Pd12L024. H NMR spectra of BF4 ,
1
−
−
−
PF6 , ClO4 , and OTf− show negligible variation. For CN−,
Cl−, F−, and H2PO42−, the Pd12L024 cage collapses to give free
ligand L0. For OAc−, Br−, and I−, a series of stray peaks show
that complex mixtures were obtained. Accordingly, the
mechanism of anion responsiveness is proposed. The Pd12L24
cage unit may be disrupted when the gel was treated with CN−,
Cl−, F−, Br−, I−, OAc−, or H2PO42−. In comparison with Cl−,
F−, Br−, I−, OAc−, and H2PO42−, the stronger binding of CN−
with Pd2+ may efficiently destroy the gel matrix and liberate the
ligand L, causing the turn-on luminescence. The liberation of
ligand L was further confirmed by ESI mass spectrometry
experiments showing isotopic peak patterns at m/z 719.4785,
corresponding to L[M + H]+. The selective sensibility thus
arises from both the coordination binding and the gelation
ability.
Anion Responsiveness of Pd12L24 Gel. The anion
responsiveness of Pd12L24 gel was investigated considering
that anion responsive behaviors are well-known for gels60 and
Pd-cages.61−64 Tetrabutylammonium (TBA) salts with various
counteranions, ClO4 , OTf−, CN−, H2PO4 , OAc−, BF4 , Br−,
−
−
−
F−, Cl−, I−, and PF6 , in 100 μL of CH3CN (molar ratio of
−
anion/L = 10:1), were layered on top of the vials containing
the gel (0.2 mL) (Figure 7). After the vials were left standing at
CONCLUSIONS
■
To summarize, we have developed a protocol to obtain
supramolecular gels based on metal−organic cage motifs via
orthogonal metal−organic coordination and cholesteryl−
cholesteryl supramolecular interactions. The Pd12L24 cage
appended with 24 cholesteryl groups shows aggregation
behaviors by virtue of cholesteryl−cholesteryl interaction
which interconnects the Pd12L24 cage motifs to achieve
gelation. Pd12L24 gel is readily tunable and shows a rich
responsive property. A reversible solution−gel transition is
achieved in fully deuterated CDCl3−CD3CN solvents other
than in regular solvents. Deuterated solvents affect the
rheological property remarkably. The rheological property
can be regulated by changing the content of cholesteryl moiety
via defect engineering with different proportions of un-
functionalized ligand L0. Higher stiffness was related to higher
content of the cholesteryl groups and cross-links. The
tetraphenylethylene group is employed to regulate the
luminescence property of Pd12L24 gel. Moreover, the gel is
visually sensitive and selective toward CN− and other anions
arising from the hierarchical structure.
In the present study, the Pd12L24 gel has a hierarchical
structure with the Pd12L24 cage subunits interconnected via
supramolecular interactions, and rich functionality may be
introduced (e.g., the luminescent tetraphenylethylene). The
functional diversity may be further increased through
modification of the organic ligands or metal ions of metal−
organic cage subunits. It provides a new opportunity to prepare
supramolecular soft materials through combination of multiple
metal−organic coordination and other supramolecular inter-
actions. The approach opens up the possibility of generating
gel phases with hierarchical complexibility. Future work to
study the host−guest chemistry of these cage molecules will
fully exploit the function possibilities of this type of gel
material.
Figure 7. Anion responsive tests of Pd12L24 wet gel when a solution of
−
−
−
NBu4X (ClO4 , OTf−, CN−, H2PO4 , OAc−, BF4 , Br−, F−, Cl−, I−,
−
and PF6 ) was added (a) under day light and (b) under 365 nm UV
radiation, and (c) the proposed sensing mechanism toward CN−. In a
typical test, a solution of 10 equiv of NBu4X in 100 μL of CH3CN was
added on the top of the gel (0.2 mL, cL = 0.013 mol L−1).
room temperature for 12 h, a unique response was observed for
CN−. The gel collapsed and a yellow precipitate emerged upon
treatment with CN−, while the resulting mixture showed bright
luminescence upon 365 nm UV radiation. It could be noticed
that the gel generally kept the shape along with faded color for
ClO4 , OTf−, BF4 , PF6 , H2PO4 , and OAc−. The gel
partially collapsed for halide ions F−, Br−, and I−, and the gel
color changed (F− green, Br− yellow, I− brown). In contrast to
CN−, all the other anions did not induce visible fluorescence
change. These observations indicate that the Pd12L24 wet gel is
−
−
−
−
G
DOI: 10.1021/acs.inorgchem.9b01171
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Article
amphiphilic coordination cage-based emulsifier. J. Am. Chem. Soc.
2018, 140, 17384−17388.
(11) Sun, Y.; Li, S.; Zhou, Z.; Saha, M. L.; Datta, S.; Zhang, M.; Yan,
X.; Tian, D.; Wang, H.; Wang, L.; Li, X.; Liu, M.; Li, H.; Stang, P. J.
Alanine-based chiral metallogels via supramolecular coordination
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(12) Samanta, S. K.; Quigley, J.; Vinciguerra, B.; Briken, V.; Isaacs, L.
Cucurbit[7]urilenables multi-stimuli-responsive release from the self-
assembled hydrophobic phase of a metal organic polyhedron. J. Am.
Chem. Soc. 2017, 139, 9066−9074.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01171.
■
S
Experimental details regarding general methods and
syntheses, typical procedures, and computational details,
including schemes, tables, and figures (PDF)
(13) Foster, J. A.; Parker, R. M.; Belenguer, A. M.; Kishi, N.; Sutton,
S.; Abell, C.; Nitschke, J. R. Differentially addressable cavities within
metal-organic cage-cross-linked polymeric hydrogels. J. Am. Chem.
Soc. 2015, 137, 9722−9729.
(14) Zhukhovitskiy, A. V.; Zhong, M.; Keeler, E. G.; Michaelis, V.
K.; Sun, J. E. P.; Hore, M. J. A.; Pochan, D. J.; Griffin, R. G.; Willard,
A. P.; Johnson, J. A. Highly branched and loop-rich gels via formation
of metal-organic cages linked by polymers. Nat. Chem. 2016, 8, 33−
41.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zhjyong@mail.sysu.edu.cn.
ORCID
Zhuofeng Ke: 0000-0001-9064-8051
Jianyong Zhang: 0000-0002-3297-7524
Author Contributions
■
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(15) Wang, Y.; Gu, Y.; Keeler, E. G.; Park, J. V.; Griffin, R. G.;
Johnson, J. A. Star polyMOCs with diverse structures, dynamics, and
functions by three-component assembly. Angew. Chem. 2017, 129,
194−198.
Notes
(16) Gu, Y.; Alt, E. A.; Wang, H.; Li, X.; Willard, A. P.; Johnson, J. A.
Photoswitching topology in polymer networks with metal-organic
cages as crosslinks. Nature 2018, 560, 65−69.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(17) Uchida, J.; Yoshio, M.; Sato, S.; Yokoyama, H.; Fujita, M.;
Kato, T. Self-assembly of giant spherical liquid-crystalline complexes
and formation of nanostructured dynamic gels that exhibit self-healing
properties. Angew. Chem., Int. Ed. 2017, 56, 14085−14089.
(18) Wei, S.-C.; Pan, M.; Fan, Y.-Z.; Liu, H.; Zhang, J.; Su, C.-Y.
Creating coordination-based cavities in a multiresponsive supra-
molecular gel. Chem. - Eur. J. 2015, 21, 7418−7427.
(19) Feng, J.; Zeng, L.; Chen, K.; Fang, H.; Zhang, J.; Chi, Z.; Su,
C.-Y. Gelation of luminescent supramolecular cages and trans-
formation to crystals with trace-doped-enhancement luminescence.
Langmuir 2016, 32, 12184−12189.
(20) Ganta, S.; Chand, D. K. Multi-stimuli-responsive metallogel
molded from a Pd2L4-type coordination cage: Selective removal of
anionic dyes. Inorg. Chem. 2018, 57, 3634−3645.
■
We thank Prof. Z. Chi for helpful discussion regarding the
luminescent properties. We gratefully acknowledge the NSFC
(51573216) for financial support.
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