A new class of dendritic metallogels with multiple stimuli- Responsiveness and as templates for the in situ synthesis of silver nanoparticles

Article abstract of DOI:10.1002/chem.201302780

A new class of poly(aryl ether) dendritic ligands containing a pyridine functionality at the focal point and the corresponding AgI complexes through metal-ligand coordination were designed, synthesized, and fully characterized. Compared with the dendritic ligands, the corresponding dendritic complexes exhibited much better gelation ability for various organic solvents at very low critical gelation concentrations. The gel-sol phase transition temperatures and morphologies could be finely tuned by binding silver ion to the ligand. A preliminary study revealed that multiple noncovalent interactions, such as AgI-pyridine coordination, solvophobic interaction, and π-π stacking, synergistically enable the formation of stable metallogels. Interestingly, these metallogels could intelligently respond to multiple external stimuli including temperature, chemicals, and shear stress, leading to gel-sol phase transitions. In addition, these dendritic metallogels were successfully applied as templates for the in situ formation and stabilization of silver nanoparticles without the use of any chemical reducing/stabilizing agents.

Full text of DOI:10.1002/chem.201302780

DOI: 10.1002/chem.201302780
Full Paper
&
Gels
A New Class of Dendritic Metallogels with Multiple Stimuli-
Responsiveness and as Templates for the In Situ Synthesis of
Silver Nanoparticles
Zhi-Xiong Liu,[a] Yu Feng,*[a] Zhi-Yong Zhao,[a] Zhi-Chao Yan,[b] Yan-Mei He,[a] Xu-Jun Luo,[a]
Chen-Yang Liu,[b] and Qing-Hua Fan*[a]
Abstract: A new class of poly(aryl ether) dendritic ligands
containing a pyridine functionality at the focal point and the
corresponding AgI complexes through metal–ligand coordi-
nation were designed, synthesized, and fully characterized.
Compared with the dendritic ligands, the corresponding
dendritic complexes exhibited much better gelation ability
for various organic solvents at very low critical gelation con-
centrations. The gel–sol phase transition temperatures and
morphologies could be finely tuned by binding silver ion to
the ligand. A preliminary study revealed that multiple non-
covalent interactions, such as AgI–pyridine coordination, sol-
vophobic interaction, and p–p stacking, synergistically
enable the formation of stable metallogels. Interestingly,
these metallogels could intelligently respond to multiple ex-
ternal stimuli including temperature, chemicals, and shear
stress, leading to gel–sol phase transitions. In addition, these
dendritic metallogels were successfully applied as templates
for the in situ formation and stabilization of silver nanoparti-
cles without the use of any chemical reducing/stabilizing
agents.
Introduction
metal–ligand interactions but also endow with the outstanding
redox, catalytic, and optical properties associated with the
metal centers, which are not accessible to low molecular
weight organogels.[2,5] So far, a variety of metallogels formed
from various transition-metal complexes, including silver,
copper, palladium, gold, platinum, and iron complexes have
been reported.[2–6] Some of them were found to be responsive
to different external stimuli such as light, redox, chemicals, and
mechanical stress.[4c–e,5a,6] In addition, metallogels have recently
been utilized for the in situ-synthesis of metal nanoparticles.[7]
Despite the great success achieved in developing metallogels,
there are few reports of functional metallogels with multiple
stimuli-responsive properties.[8]
Recently, supramolecular gels have attracted considerable at-
tention due to their potential applications in tissue engineer-
ing, sensors, optoelectronic devices, nanoporous materials, cat-
alysis, template synthesis, and so on.[1] Metallogels are a rela-
tively new kind of supramolecular gel in which metallic ele-
ments actively participate in gel formation.[2] Generally, two dif-
ferent strategies have been adopted to fabricate the
metallogels. One is the utilization of discrete small-molecular
metal complexes as the gelators to construct metallogels
through H-bonding, p–p stacking, unconventional metal–
metal interactions, and so on.[3] The other is coordination poly-
mers as metallogels, which are achieved by metal–ligand coor-
dination upon the addition of metal ions.[4]
Dendrimers and dendrons are highly branched macromole-
cules with well-defined molecular structures and have been
widely used as building blocks in the self-assembly of supra-
molecular gel-phase materials.[9] Their unique architectures also
render them capable of being incorporated with metal ions
into the different positions, including the dendritic core, the
tether termini, the dendritic branches, or the branching points.
To date, a number of organometallic dendrimers with metal
centers at different positions have been reported and exhibit-
ed unusual properties (e.g., catalytic and redox activity, con-
ductivity, luminescence, and magnetism).[10] Surprisingly, there
have been very limited examples of hierarchical self-assembly
of metallodendrimers.[11] The group of Aida reported an ele-
gant example in which trinuclear dendritic pyrazolate com-
plexes self-assembled into micrometer-scale, luminescent su-
perhelical fibers.[11a] However, to the best of our knowledge,
there is no report on multi-stimuli-responsive dendritic metal-
logels obtained from metallodendrimers.
Clearly, the incorporation of metal ions into the supramolec-
ular gels could not only subtly tune the gelation ability by
[a] Z.-X. Liu, Dr. Y. Feng, Dr. Z.-Y. Zhao, Y.-M. He, X.-J. Luo, Prof. Dr. Q.-H. Fan
Beijing National Laboratory for Molecular Sciences
and CAS Key Laboratory of Molecular Recognition
and Function, Institute of Chemistry and Graduate School
Chinese Academy of Sciences (CAS)
Beijing 100190 (P.R. China)
E-mail: fengyu211@iccas.ac.cn
fanqh@iccas.ac.cn
[b] Z.-C. Yan, Prof. Dr. C.-Y. Liu
Beijing National Laboratory for Molecular Sciences
and CAS Key Laboratory of Engineering Plastics
Institute of Chemistry, CAS
Beijing 100190 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302780.
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Recently, we found that the peripherally dimethyl isophtha-
late (DMIP)-functionalized poly(benzyl ether) dendrons were
a very unique class of dendritic gelators that could form stable
gels in a wide range of organic solvents.[12a] Later, we incorpo-
rated the azobenzene groups into these dendron backbones
to fabricate photoresponsive dendritic organogels.[12d] In this
work, as a part of our continuing efforts,[12] we report the syn-
thesis of a new class of pyridine-functionalized poly(aryl ether)
dendritic ligands Pyr-Gn (n=0–2) and their corresponding
silver ion complexes Pyr-Gn/Ag (Scheme 1). The resulting met-
Figure 1. 1H NMR spectra of A) dendritic ligand Pyr-G2 and B) dendritic com-
plex Pyr-G2/Ag in CDCl3.
shifted downfield upon coordination to the silver ion. The syn-
thetic details and characterization data are provided in the Ex-
perimental Section. Their purity and chemical structures were
Scheme 1. Synthetic route to the dendritic ligands Pyr-Gn (n=0–2) and their
corresponding AgI-coordinated complexes Pyr-Gn/Ag (n=0–2).
1
confirmed by H/13C NMR spectroscopy, HR-ESI mass spectrom-
etry, as well as elemental analysis.
allic complexes exhibited excellent gelation abilities and rever-
sible gel–sol phase transition in response to multiple external
stimuli including temperature, chemicals, and shear stress. In
addition, these metallogels could act as templates for the in
situ formation and stabilization of silver nanoparticles without
the use of any chemical reducing/stabilizing agents.
Gelation behavior and sol–gel phase transition temperature
With dendritic silver complexes Pyr-Gn/Ag (n=0–2) in hand,
their gelation abilities were then evaluated in various organic
solvents and mixed solvents. It was found that stable metallo-
gels were formed in most organic solvents through a heating–
cooling process, and the critical gelation concentrations (CGCs)
were collected in Table 1. In general, the dendritic complexes
Pyr-Gn/Ag (n=0–2) showed much better gelation abilities with
more wide-ranging gelling solvents and much lower CGCs
than the parent dendritic ligands Pyr-Gn (n=0–2). It was found
that the Pyr-G0/Ag complex could form metallogels in some
solvents such as toluene, ethyl acetate, and methanol. In sharp
contrast, the parent Pyr-G0 ligand could not form organogels
under otherwise the identical conditions because of its good
solubility. It was noted that the first-generation complex Pyr-
G1/Ag formed stable and translucent metallogels in almost all
the solvents tested. The lowest CGC in ethanol was close to
1.7 mgmLÀ1, suggesting one dendritic molecule could immobi-
lize approximate 1.1ꢁ104 solvent molecules upon coordination
to silver ions. The highest generation complex Pyr-G2/Ag also
showed excellent gelation abilities. Interestingly, it was found
that Pyr-G2/Ag could form stable gels in ionic liquid
Results and Discussion
Synthesis and characterization of the dendritic ligands and
their silver complexes
According to our previous method,[13] the pyridine-functional-
ized poly(aryl ether) dendritic ligands Pyr-Gn (n=0–2) were
prepared by simple nucleophilic substitution of the corre-
sponding dendrons HO-GnCOOMe (n=0–2) with 4-(chlorome-
thyl)pyridine hydrochloride in the presence of potassium car-
bonate. Their silver complexes Pyr-Gn/Ag (n=0–2) were then
generated in situ by adding silver trifluoromethanesulfonate
(AgOTf; 0.5 equiv) in methanol to a solution of the dendritic li-
gands in chloroform. The coordination of sliver ions to the
1
dendritic ligands could be confirmed by H NMR. As shown in
Figure 1, the chemical shifts of the aromatic protons at a-(Hf)
and b-positions (He) of the pyridine moiety of Pyr-G2 were
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stability than the metallogels
from Pyr-G1/Ag, suggesting an
interesting positive “dendritic
effect”.
Table 1. Critical gelation concentration (CGC) values and gelation ability of the dendritic ligands Pyr-Gn (n=0–
2) and the corresponding complex Pyr-Gn/Ag (n=0–2) in various organic solvents and mixed solvents.[a]
Solvent
Pyr-G0/Ag
Pyr-G0
Pyr-G1/Ag
Pyr-G1
Pyr-G2/Ag
Pyr-G2
toluene
anisole
benzyl alcohol
ethyl acetate
acetone
OG (35.2)
PG
I
OG (38.5)
PG
I
I
I
I
I
I
OG (33.3)
PG
PG
I
I
I
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
TG (6.0)
TG (5.0)
TG (3.8)
TG (5.9)
TG (3.4)
TG (3.0)
CG (37.3)
CG (6.6)
S
OG (22.4)
PG
PG
OG (19.0)
OG (30.0)
P
S
S
S
S
S
I
I
P
P
P
CG (5.0)
TG (3.5)
TG (5.7)
TG (4.4)
TG (3.0)
TG (2.6)
CG (2.7)
CG (14.0)
CG (19.1)
CG (3.0)
CG (7.3)
TG (5.1)
TG (5.6)
TG (2.0)
TG (3.8)
TG (7.2)
CG (9.4)
CG (3.2)
OG (17.8)
OG (25.6)
PG
P
Morphologies of organogels
P
The metal–ligand coordination
not only effectively tuned the
gelation abilities but also had
great influence on the morphol-
ogies of the gel systems. The
morphology of the gels of Pyr-
Gn/Ag (n=0–2) and the corre-
sponding ligands in different
solvents was investigated by
SEM and TEM (Figure 3 and the
Supporting Information, Figur-
es S1–S3). The xerogel of the
Pyr-G0/Ag complex from tetra-
hydrofuran consisted of rigid
ribbons with widths of 400 nm
to 2 mm and lengths of several
micrometers (Figure 3A and B).
2-hexanone
OG (11.9)
S
S
S
S
S
I
I
OG (11.0)
OG (19.7)
OG (11.2)
S
tetrahydrofuran
dichloromethane
chloroform
1,2-dichloroethane
1,1,2,2-tetrachloroethane
methanol
CG (5.4)
S
TG (1.9)
TG (1.7)
TG (7.5)
TG (3.6)
TG (4.1)
TG (6.4)
S
ethanol
2-methoxyethanol
THF/MeOH (3:1)
THF/H2O (3:1)
CH2Cl2/MeOH (3:1)
[Bmim]·PF6
OG(36.0)
OG(19.0)
OG (2.8)
[a] The value given in parentheses is the CGC in mgmLÀ1 to achieve gelation at 258C. For solvent mixtures, the
volume ratio is also given in parentheses. CG: clear transparent gel; PG: partial gelation; TG: translucent gel;
OG: opaque gel; S: soluble (>60 mgmLÀ1); I: insoluble upon heating; P: precipitation.
[Bmim]·PF6, (Bmim=1-butyl-3-methylimidazolium hexafluoro-
However, the higher generation complexes of Pyr-G1/Ag and
Pyr-G2/Ag exhibited entangled networks of thin solid fibers
(Figure 3C–F). The network of the Pyr-G1/Ag system was
found to be made up of poorly connected unbranched bun-
dles of straight fibers with diameters of 500–900 nm (Figure 3C
and D). The metallogel Pyr-G2/Ag showed an even higher den-
sity fibrillar network was observed, consisting of heavily
phosphate) which represents the first example of dendrimer
able to gelate ionic liquids. Similarly, the corresponding parent
dendritic ligands Pyr-G1 and Pyr-G2 displayed poor gelation
ability. They could gelate only few organic solvents with much
higher CGCs.
To further investigate the thermal stability of these metallo-
gel phase materials, the gel–sol phase transition temperatures
(Tgel) of the gels formed from the Pyr-Gn/Ag (n=1, 2) com-
plexes in acetone were measured at different concentrations
of the gelators by the tube-inversion method (Figure 2). Ex-
pectedly, the Tgel increased with increasing in the concentra-
tions, indicating higher stability of the gel networks at a higher
concentration. Notably, the metallogels formed from the
second-generation complex Pyr-G2/Ag showed higher thermal
Figure 3. TEM and SEM images of the corresponding xerogels of dendritic
metallogels from tetrahydrofuran: A, B) Pyr-G0/Ag, C, D) Pyr-G1/Ag, and E,
F) Pyr-G2/Ag.
Figure 2. Plots of Tgel against the concentration of Pyr-G1/Ag and Pyr-G2/Ag
complexes in acetone with the heating rate of 28CminÀ1 in a water bath.
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branched interconnecting thin fibers with diameters of 50–
200 nm (Figure 3E and F). On the contrary, the parent dendritic
ligand Pyr-G0 exhibited a random, disordered aggregate mor-
phology. The higher generation ligands Pyr-G1 and Pyr-G2 ag-
gregated mainly into straighter, more rigid solid fibers with di-
ameters of 200–800 nm (the Supporting Information, Fig-
ure S1). The morphology changes were also confirmed by TEM
images.
Similar to the low-molecular-weight organogels based on
the silver-coordinated complex,[4d,e,8c] the dendritic metallogels
showed a quick response to some anions such as the chloride
ion, bromonium ion, iodine ion, sulfate ion, and undergo gel–
sol transformation (Figure 4A and the Supporting Informtion,
Driving force analysis
The excellent gelation properties of these dendritic complexes
promoted us to study the driving forces toward gelation. Ac-
cording to our previous study,[12a] solvent titration experiment,
temperature-dependent (TD) 1H NMR spectroscopy and
powder X-ray diffraction (PXRD) were applied to investigate
the interactions between the individual components (the Sup-
porting Information, Figures S4–7). It had been reported that
the solvophobic interactions could effectively enhance supra-
molecular organizations involving p–p stacking structures.[14]
The formation of p–p stacking interactions of these dendritic
complexes of Pyr-Gn/Ag (n=1, 2) in solution and in the gel
state was first studied by using a solvent titration experiment.
In this study, the Pyr-G1/Ag complex was chosen for the sol-
vent titration experiment to explore the aggregation behavior.
As shown in Figure S5 in the Supporting Information, increas-
ing the volume ratio of CCl4 resulted in obvious upfield shifts
of the resonance signals for the aromatic protons on pyridine
moiety, whereas only slight upfield shifts of the peripherally di-
methyl isophthalate (DMIP) and the internal aromatic rings
were observed. This result indicated that strong solvophobic
interactions could enhance aromatic p–p stacking interactions
of the polar pyridine complex. In addition, the TD-1H NMR ex-
periments and PXRD study confirmed the p–p stacking interac-
tions of DMIP. In the TD-1H NMR experiments, the resonance
signals for the aromatic protons on the peripheral DMIP rings
and the internal benzyl rings of Pyr-G2/Ag complex were
found to be slightly shifted upfield upon lowering the temper-
ature from 328 to 278 K (the Supporting Information, Fig-
ure S7), suggesting the p–p stacking interactions.[12a] This p–p
interactions were also evidenced by PXRD study. As shown in
Figure S8 in the Supporting Information, a prominent reflec-
tion characteristic of a typical p–p stacking distance was ob-
served in the wide-angle region at approximate 3.5 ꢂ of the
PXRD patterns. On the basis of the above results, multiple non-
covalent interactions such as AgI-coordination, solvophobic
and p–p stacking interactions synergistically enable the forma-
tion of stable metallogels.
Figure 4. A) Photographs of the Pyr-G2/Ag complex metallogel in acetone
(5.6 mgmLÀ1) before and after the addition of potassium iodide, SEM and
TEM images of the xerogel of B, D) Pyr-G2/Ag complex and C, E) the aggre-
gates from sol state after the addition of iodide ions.
Figure S9). As an example, when potassium iodide (1.0 equiv
relative to the silver ion) was carefully added onto the metallo-
gel of Pyr-G2/Ag complex in acetone (1.0 mL, 5.6 mgmLÀ1), the
metallogel collapsed gradually and transformed into a clear
light-yellow solution with a small amount of pale precipitate. If
excess silver trifluoromethanesulfonate (AgOTf) was added,
a light-yellow stable metallogel could be reformed (Figure 4A).
The change could be clearly confirmed by SEM and TEM meas-
urements (Figure 4B–E). Compared with the dense fibrillar net-
work of highly interwined thin fibers with diameters of 50–
200 nm, the SEM and TEM images, after being dried from the
appropriate solution, exhibited straight, rigid, thick solid fibers
with diameters of 200–800 nm. It was noted that the reformed
gel showed very similar fibrillar morphology compared with
that of the original one.
Additionally, the above-described gel system could also re-
spond to other chemicals except anions (the Supporting Infor-
mation, Figures S9 and S10). A drop of pyridine was carefully
added onto the surface of the metallogel, the gel quickly
became broken and finally tuned into a clear solution (the
Supporting Information, Figure S10). Similarly, the addition of
a solution of ammonia in water or 1,10-phenanthroline in etha-
nol could also completely destroy the metallogel.
Sol–gel transformations of dendritic metallogels by chemical
stimulus
With introduction of silver ions, the metal–ligand interaction
could not only finely tune gel properties, but also expand the
scope of the functionalities, especially with potential in smart
or stimuli-responsive materials.[6,8]
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Sol–gel transformations of metallogels by mechanical stimu-
li, a rheological study
of magnitude greater than G’’ (ꢀ3.2ꢁ103 Pa), indicating elastic
characters of this sample. Both moduli G’ and G’’ remained
roughly constant below a critical strain value of 0.7%, known
as the upper limit of the linear viscoelastic regime. Above this
strain value, a sharp decrease of G’ and G’’ was observed and
the value of G’’ became larger than that of G’, representing
a partial breakup of the dendritic metallogel that begins to
flow.
Unexpectedly, it was found that these dendritic metallogels are
sensitive to shear stress. For instance, the metallogel Pyr-G2/
Ag in tetrahydrofuran was changed into a viscous sol by vigo-
rous shaking. Upon resting, the gel could be regenerated
spontaneously (Figure 5A). Although the thixotropic metallo-
gels have been known for quite a while,[4c,6e,8b,d] there is no
report on thixotropic dendritic metallogels. Thus, the thixo-
tropic properties have been investigated in detail by rheologi-
cal techniques.
The thixotropic property of this dendritic metallogel was fur-
ther investigated. By monitoring the time evolution of G’ and
G’’, Figure 5D revealed an extremely rapid recovery of the gel
state. Just when the external stress was canceled, the sample
recovered its prominent elastic property immediately and be-
haved as a gel-like material (G’>G’’). Moreover, G’ reached its
initial value of the gel state (ꢀ2.4ꢁ104 Pa before shearing).
Even if the gel was sheared at a high strain (100%) enough to
destroy the metallogel for 60 seconds, the sample was com-
pletely transformed into the sol state (G’<G’’). However, it
showed elastic character (G’>G’’) just when the high stress
was removed (the Supporting Information, Figure S11). Clearly,
the dendritic metallogel exhibited a smart thixotropic property.
To further verify thixotropic property and the reproducibility
of this dendritic metallogel, another more serious experiment
was conducted, and the result is depicted in Figure 5E. Once
To detect the tolerance performance of Pyr-G2/Ag metallo-
gel in tetrahydrofuran (20 mgmLÀ1), a frequency sweep was
conducted. As seen from Figure 5B, the value of G’, associated
with energy storage, was larger than G’’, associated with loss
of energy. This result indicated dominant elastic characters of
the sample. Furthermore, the G’ and G’’ exhibited slight de-
pendence as the frequency decreased in range of 0.1–
100 radsÀ1, demonstrating that the gel has a good tolerance
to external forces.
The linear regime of the Pyr-G2/Ag metallogel system was
then determined. As shown in Figure 5C, at low strain values,
the value of G’ (about ~2.4ꢁ104 Pa) was just about one order
Figure 5. A) Reversible sol–gel phase transition of Pyr-G2/Ag complex metallogel in tetrahydrofuran triggered by shear-stress; and Storage modulus G’ and
loss modulus G’’ values of a Pyr-G2/Ag metallogel (THF, 20 mgmLÀ1) on rheological experiments of B) frequency sweep; C) strain sweep; D) time sweep;
E) three cycles of deformation and recovery processes.
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a high strain was applied (100%), the gel was completely
broken and transformed into sol state, suggested by the larger
value of G’’ than that of G’. The elasticity immediately recov-
ered (G’>G’’) when the high strain was canceled and the
sample could even successfully recover to its initial strength,
as indicated by the similar storage modulus G’ as its original
value. Importantly, this thixotropic process could be repeated
several times. The thixotropic properties of the metallogel in
other solvents such as acetone had also been investigated by
rheological experiments (the Supporting Information, Fig-
ure S12).
Ag metallogel in chloroform exhibited a characteristic surface
plasmon band at 452 nm, indicating the formation of silver
nanoparticles (Figure 6B). The TEM images of the xerogel of
Pyr-G2/Ag metallogel from acetone clearly showed the forma-
tion of spherical silver nanoparticles, which are mostly ar-
ranged along the fibers (Figure 6C and the Supporting Infor-
mation, Figure S14). On the basis of an analysis of the HR-TEM
images, their sizes varied from 10 to 20 nm (Figure 6D). In ad-
dition, the related selected area electron diffraction (SAED) ex-
periment confirmed the crystalline nature of the particles (the
Supporting Information, Figure S15). An energy dispersive X-
ray (EDX) spectrum of the same area proved the presence of
silver in the particles (the Supporting Information, Figure S16).
Alternatively, silver nanoparticles could be also produced when
the metallogels were exposed to natural light for days. Their
sizes were found to vary from 3 to 6 nm, which are smaller
than those obtained from UV-light irradiation (the Supporting
Information, Figure S17). Consequently, the dendritic metallo-
gels could be served as a directing medium for the synthesis
of silver nanoparticles, which were exclusively attached on the
fibers, providing a long, ordered assembly of gel-nanocompo-
sites.
Metallogels as templates for the in situ-synthesis of silver
nanoparticles
Preparation of metallic nanoparticles within supramolecular
gels in the absence of chemical reducing/stabilizing agents
could increase their potential applications.[7] Among the nu-
merous metallic nanoparticles studied to date, silver nanoparti-
cles are of interest due to their optical and antibacterial prop-
erties.[15]
Considering the presence of silver ion in the gels, we tried
to prepare the silver nanoparticles by in situ-reduction of silver
ions in the absence of chemical reducing/stabilizing agents. In-
terestingly, the formation of silver nanoparticles was visually
predicated by the color change of the Pyr-G2/Ag metallogel
system from colorless to brown after exposure to UV light
(365 nm) for about ten minutes (Figure 6A). The silver nano-
particles were further characterized by using a UV/Vis spec-
trometer and TEM. It was found that the system of the Pyr-G2/
Conclusion
We have designed and synthesized a series of pyridine-func-
tionalized poly(aryl ether) dendritic ligands and their corre-
sponding complexes through silver ion–pyridine coordination.
Compared with the dendritic ligands, the dendritic silver com-
plexes exhibited better gelation ability for various organic sol-
vents with lower CGCs. It was found that the gel properties,
such as gel-sol phase transition temperatures and morpholo-
gies, could be finely tuned by biding silver ion to the dendritic
ligand. A preliminary study revealed that multiple noncovalent
interactions such as AgI–pyridine coordination, solvophobic in-
teractions, and p–p stacking synergistically promoted the for-
mation of stable metallogels. Interestingly, these metallogels
could intelligently respond to multiple external stimuli includ-
ing temperature, chemicals, and shear stress, leading to rever-
sible gel–sol phase transitions. In addition, these dendritic met-
allogel systems could be applied as templates for the in situ-
formation and stabilization of silver nanoparticles. Their further
applications in the fabrication of other metal nanoparticles and
in catalysis will be performed in our laboratory.
Experimental Section
General methods
All starting materials were obtained from commercial suppliers and
used as received. All solvents were distilled with suitable drying
agents. Moisture-sensitive reactions were performed under an at-
mosphere of dry argon. H NMR and 13C NMR spectra were record-
1
Figure 6. A) Photograph of Pyr-G2/Ag metallogel from chloroform after UV-
light irradiation (365 nm) for about ten minutes; B) UV/Vis absorption spec-
tra of the silver nanoparticles obtained from Pyr-G2/Ag (1 mm path length);
C) TEM images of the silver nanoparticles formed in situ from Pyr-G2/Ag in
acetone under UV-light irradiation; D) HR-TEM images of the silver nanopar-
ticles.
ed on Bruker AMX 300 Spectrometer (1H: 300 MHz; 13C: 75 MHz) or
Bruker AMX-600 spectrometers (1H: 600 MHz; 13C: 150 MHz) at
298 K. Chemical shifts were reported in parts per million (ppm) rel-
ative to the internal standards, partially deuterated solvents or tet-
ramethylsilane (TMS). HRMS-ESI were obtained on a Bruker APEX IV
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instrument. Elemental analyses were performed on a Carlo–Erba-
1106 instrument. Powder wide-angle X-ray diffraction (WAXD) pat-
terns of xerogels were obtained by using a Japan Rigaku D/MAX-
2500/PC diffractometer with CuKa radiation (l=1.54178 ꢂ). The UV/
Vis absorption spectra were obtained from a Lambda 950 UV/Vis-
NIR spectrophotometer.
Synthesis of Pyr-G2
Following the procedure for Pyr-G1: 4-(chloromethyl)pyridine hy-
drochloride (0.072 g, 0.439 mmol), potassium carbonate (0.135 g,
1.254 mmol), HO-G2COOMe (0.500 g, 0.418 mmol) and anhydrous
DMF (10 mL) yielded Pyr-G2 (0.520 g, 97%) as an off-white solid
1
after flash column chromatography. H NMR (600 MHz, CDCl3): d=
3.93 (s, COOCH3, 24H), 5.09 (s, ArCH2O, 4H), 5.13 (s, ArCH2O+
PyCH2O, 10H), 7.02 (s, ArH, 2H), 7.04 (s, ArH, 4H), 7.13 (s, ArH, 2H),
7.14 (s, ArH, 1H), 7.41 (d, J=4.8 Hz, PyH, 2H), 7.82 (d, J=1.2 Hz,
ArH, 8H), 8.29 (s, ArH, 4H), 8.62 ppm (d, J=4.2 Hz, PyH, 2H);
13C NMR (75 MHz, CDCl3): d=166.0, 159.2, 158.7, 158.6, 149.8,
146.1, 138.9, 138.2, 131.8, 123.3, 121.6, 120.0, 119.0, 118.8, 113.4,
113.2, 70.0, 69.7, 68.1, 52.4 ppm; HRMS-ESI: m/z: calcd for
C70H64NO23: 1286.38636 [M+H]+; found: 1286.38545; elemental
analysis calcd (%) for C70H63NO23: C 65.36, H 4.94, N 1.09; found: C
65.04, H 5.06, N 1.18.
Synthesis of dendritic ligand
The synthesis and characterization of the dendritic synthons of
HO-GnCOOMe (n=0–2) has been prepared previously according to
our previous method.[12a]
Synthesis of Pyr-G0
General procedures for the synthesis of the Pyr-Gn/Ag
(n=0–2) complexes
A 100 mL round-bottom flask was charged with 4-(chloromethyl)-
pyridine hydrochloride (0.820 g, 5.00 mmol), dimethyl 5-hydroxyi-
sophthalate (1.155 g, 5.50 mmol), potassium carbonate (2.070 g,
15.00 mmol), potassium iodide (0.831 g, 5.00 mmol), and acetone
(60 mL). The mixture was heated to reflux with stirring for 6 h
under a nitrogen atmosphere. The solvent was evaporated under
reduced pressure. Water (40 mL) and dichloromethane (60 mL)
were then added to the residue. The organic phase was separated
and the water phase was extracted with dichloromethane (50 mLꢁ
3). The combined organic phase was washed with brine (50 mL).
The organic phase was dried over sodium sulfate, and then con-
centrated. The residue was purified by flash column chromatogra-
phy on silica gel to afford Pyr-G0 (1.323 g, 88%) as an off-white
Silver trifluoromethanesulfonate (AgOTf, 0.5 mol equiv) dissolved
in methanol (1.0 mL) was slowly added into a solution of the ac-
cording dendritic ligand of Pyr-Gn (n=0–2) in chloroform (10 mL).
The mixture was continuously stirred for 5 h in darkness. Then, the
solvent was evaporated under reduced pressure to afford an off-
white solid.
1
Synthesis of Pyr-G0/Ag: Yield: 99%. H NMR (300 MHz, DMSO): d=
3.90 (s, COOCH3, 12H), 5.36 (s, PyCH2O, 4H), 7.53 (d, J=6.0 Hz, PyH,
4H), 7.80 (d, J=1.5 Hz, ArH, 4H), 8.11 (s, ArH, 2H), 8.60 ppm (d, J=
6.0 Hz, PyH, 4H); 13C NMR (75 MHz, DMSO): d=165.0, 158.1, 150.4,
146.7, 131.6, 122.3, 122.1, 119.6, 67.9, 52.6 ppm; HRMS-ESI: m/z:
calcd for C32H30AgN2O10: 709.09459 [MÀOTf]+; found: 709.09406;
elemental analysis calcd (%) for C33H30AgF3N2O13S: C 46.11, H 3.52,
N 3.26; found: C 45.99, H 3.64, N 3.35.
1
solid. H NMR (300 MHz, CDCl3): d=3.95 (s, COOCH3, 6H), 5.18 (s,
PyCH2O, 2H), 7.39 (d, J=5.4 Hz, PyH, 2H), 7.83 (s, ArH, 2H), 8.33 (s,
ArH, 1H), 8.65 ppm (d, J=5.4 Hz, PyH, 2H). 13C NMR (75 MHz,
CDCl3): d=165.9, 158.2, 149.8, 145.6, 132.0, 123.7, 121.6, 120.0,
68.5, 52.5 ppm; HRMS-ESI: m/z: calcd for C16H16NO5: 302.10230
[M+H]+; found: 302.10230; elemental analysis (%) calcd for
C16H15NO5: C 63.78; H 5.02; N 4.65. found: C 63.28; H 5.00; N 4.49.
Synthesis of Pyr-G1/Ag: Yield: 100%. 1H NMR (300 MHz, CDCl3):
d=3.93 (s, COOCH3, 24H), 5.09 (s, ArCH2O, 8H), 5.14 (s, PyCH2O,
4H), 7.00 (s, ArH, 4H), 7.15 (s, ArH, 2H), 7.52 (d, J=6.0 Hz, PyH,
4H), 7.79 (d, J=0.9 Hz, ArH, 8H), 8.26 (s, ArH, 4H), 8.75 ppm (d, J=
6.3 Hz, PyH, 4H); 13C NMR (75 MHz, CDCl3): d=166.0, 158.4, 158.3,
152.0, 149.2, 138.6, 131.8, 123.4, 122.4, 120.0, 119.3, 113.1, 69.7,
67.5, 52.5 ppm; HRMS-ESI: m/z: calcd for C68H62AgN2O22:
1365.28397 [MÀOTf]+; found: 1365.28074.
Synthesis of Pyr-G1
Synthesis of the Pyr-G2/Ag: Yield: 100%. 1H NMR (300 MHz,
DMSO): d=3.85 (s, COOCH3, 48H), 5.11 (s, ArCH2O, 8H), 5.18 (s,
ArCH2O+PyCH2O, 20H), 7.07 (s, ArH, 12H), 7.13 (s, ArH, 6H), 7.41
(d, J=4.2 Hz, PyH, 4H), 7.69 (s, ArH, 16H), 8.02 (s, ArH, 8H),
8.54 ppm (d, J=4.2 Hz, PyH, 4H); 13C NMR (75 MHz, CDCl3): d=
165.9, 159.3, 158.7, 158.5, 150.8, 150.0, 139.3, 138.4, 132.0, 123.4,
122.7, 120.2, 119.3, 118.9, 113.6, 113.2, 70.1, 69.8, 67.8, 52.3 ppm;
HRMS-ESI: m/z: calcd for C140H126AgN2O46: 2677.66272 [MÀOTf]+;
found: 2677.66311.
In a 100 mL round-bottom flask, a mixture of 4-(chloromethyl)pyri-
dine hydrochloride (0.239 g, 1.457 mmol), potassium carbonate
(0.450 g, 4.163 mmol), and HO-G1COOMe (0.708 g, 1.388 mmol) in
anhydrous N,N-dimethylformamide (10 mL) was stirred under nitro-
gen for 6 h at 608C. The reaction mixture was then poured into
water (250 mL) and stirred. The resulting precipitate was filtered,
washed with water, and dissolved in dichloromethane (300 mL).
The organic layer was washed with brine (50 mL), dried over
sodium sulfate, and then concentrated under reduced pressure.
The residue was purified by flash column chromatography on silica
gel to afford Pyr-G1 (0.585 g, 67%) as an off-white solid. 1H NMR
(300 MHz, CDCl3): d=3.94 (s, COOCH3, 12H), 5.14 (s, ArCH2O, 4H),
5.16 (s, PyCH2O, 2H), 7.04 (s, ArH, 2H), 7.15 (s, ArH, 1H), 7.45 (d, J=
5.1 Hz, PyH, 2H), 7.83 (s, ArH, 4H), 8.30 (s, ArH, 2H), 8.64 ppm (d,
J=5.1 Hz, PyH, 2H); 13C NMR (75 MHz, CDCl3): d=166.0, 158.8,
158.5, 149.8, 146.2, 138.4, 131.9, 123.4, 121.6, 120.1, 119.2, 113.4,
69.9, 68.2, 52.5 ppm; HRMS-ESI: m/z: calcd for C34H32NO11:
630.19699 [M+H]+; found: 630.19817; elemental analysis calcd (%)
for C34H31NO11: C 64.86, H 4.96, N, 2.22; found: C 64.45, H 4.97, N
2.23.
Gelation test
A weighed sample of a dendritic metallogelator was mixed with
a solvent (0.5 mL) in a septum-capped vial and heated in an oil
bath until the solid was dissolved. Then the sample vial was cooled
to room temperature. The aggregation state was then assessed. If
no flow was observed when inverting the vial, a stable gel was
formed and noted as gelation (G). If part of the mixture formed
a gel but flow was still observed, the phenomenon was recorded
Chem. Eur. J. 2014, 20, 533 – 541
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as partial gelation (PG). If precipitation occurred, P was noted, and
if the clear solution (>60 mgmLÀ1) was retained, it was marked as
soluble (S). If the compound was unable to dissolve, it was noted
as insoluble (I). Repeated heating and cooling confirmed the
thermo-reversibility of the gelation process. The critical gelation
concentration (CGC) of the gelators was determined by measuring
the minimum amount of gelator required for the formation of
a stable gel at room temperature.
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Rheological measurements
Rheological measurements were carried out with a stress-con-
trolled rheometer (TA Instruments, AR-G2) equipped with steel par-
allel-plate geometry (40 mm diameter). The gap distance was fixed
at 750 mm. A solvent-trapping device was placed above the plate
to avoid evaporation. All measurements were made at 108C. Strain
sweep at a constant frequency (6.28 radsÀ1) was performed in the
0.05–200% range to determine the linear viscoelastic region (LVER)
of the gel sample. The frequency sweep was obtained from 0.1 to
100 radsÀ1 at a constant strain of 0.1%, well within the linear
regime determined by the strain sweep. A thixotropic study was
conducted to examine the recovery behavior of dendritic metallo-
gels after the strain sweep. The recovery of the storage modulus of
the destroyed gel is monitored at
a constant frequency
(6.28 radsÀ1) under a low strain (0.1%) just after the strain sweep
progress. The storage modulus G’ and the loss modulus G’’ are re-
corded as functions of time in the recovery process.
The cycle of deformation and recovery include two steps: 1) Defor-
mation: A constant oscillatory strain (100%) that is enough to de-
stroy the gel is applied to the fresh gel in the sample holder for
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;
2) Modulus recovery in a time sweep: the recovery of the storage
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ˇ
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In a typical preparation, the solid Pyr-G2/Ag complex (12.0 mg)
was dissolved in the respective solvents (1.0 mL) by gentle heating,
and then cooled to room temperature to form stable metallogels.
The resulting metallogels were exposed to UV light (365 nm) for
minutes or natural light for days. The hybrid materials were charac-
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (No. 21074140 and 91027046) and the National Basic Re-
search Program of China (973 program, No. 2013CB932800) for
financial support.
Keywords: dendrimers · gels · ligands · nanoparticles · silver ·
supramolecular chemistry
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Received: July 16, 2013
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Published online on December 11, 2013
Chem. Eur. J. 2014, 20, 533 – 541
www.chemeurj.org
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim