Multi-responsive self-healing metallo-supramolecular gels based on "click" ligand

Article abstract of DOI:10.1039/c2jm31347b

Metallo-supramolecular gels were prepared using a ligand macromolecule containing the tridentate 2,6-bis(1,2,3-trizol-4-yl)pyridine (BTP) ligand unit synthesized via CuAAC "click" chemistry in the main chain, together with transition metal ions and/or lanthanide ions. The gelation and gel properties, e.g. swelling, emission, rheological properties, thermo- and chemo-responsive properties, can be tuned by the careful selection of metal ions and their combinations, solvents, concentration, etc. Most interestingly, the gels exhibited repeatable autonomic healing ability. Moreover, the repeatable self-healing ability of the gels found practical application in the repairing of metal coatings.

Full text of DOI:10.1039/c2jm31347b

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ISSN 0959-9428
PAPER
M. Vallet-Regí et al.
Supramolecular mechanisms in the synthesis of mesoporous magnetic
nanospheres for hyperthermia
0959-9428(2012)22:1;1-2
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ARTICLE TYPE
Multi-responsive self-healing metallo-supramolecular gels based on “click” ligand
b
a
a
a
a
a
Jinchun Yuan, Xiuli Fang, Lingxing Zhang, Guangning Hong, Yangju Lin, Qifeng Zheng, Yuanze
a
a
a
a
b
Xu, Yonghong Ruan, Wengui Weng,* Haiping Xia, and Guohua Chen
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Metallo-supramolecular gels were prepared using a ligand macromolecule containing tridentate 2,6-
bis(1,2,3-trizol-4-yl)pyridine (BTP) ligand unit synthesized via CuAAC “click” chemistry in the main
chain together with transition metal ions and/or lanthanide ions. The gelation and gel properties, e.g.
swelling, emission, rheological properties, thermo- and chemo-responsive properties, can be tuned by
careful selection of metal ions and their combinations, solvent, concentration, and so on. Most
interestingly, the gels exhibited repeatable autonomic healing ability. Moreover, the repeatable self-
healing ability of gels found practical application in repairing of metal coatings.
1
0
been used to produce both bulk materials and gels with self-
Introduction
healing properties. Compared to dynamic covalent chemistry,
non-covalent interactions are generally more susceptible to
external environment. In this sense, supramolecular systems are
smarter than the dynamic covalent entities. Indeed, self-healing
systems based on non-covalent interactions usually exhibited
extraordinary healing and multi-responsive characteristics.
1
2
2
3
3
4
4
5
0
5
0
5
0
5
Self-healing polymeric materials are an increasingly active
research area of particular scientific and commercial interests.
Inspired by biological systems, such materials can undergo either
autonomic healing or induced healing triggered by specific
mechanisms to repair damage, thereby restoring the strength and
function. The first mending approach was based on the
1
5
5
6
6
7
7
0
5
0
5
0
5
Among the various non-covalent interactions, of most interest to
us are the metal-ligand interactions that are not only
2
microencapsulation of reactive species, which are usually able
thermodynamically stable but also kinetically labile. There are a
to polymerize or react with the matrix when released upon crack
formation. This reaction at the damage site was irreversible and
the supply of healing agents was depleted locally, therefore, the
repair could not be repeated. Later, specially designed 3D
microvascular networks were adopted to attain multiple healing
cycles of a single crack. However, some limitations still remain.
Some other approaches included solvent microencapsulation,
nanoparticle
22,23
vast range of accessible metal-ligand complexes.
And their
characteristics, e.g., motif structure, thermodynamic stability and
dynamics, etc., can be easily tuned depending on the combination
of metal ion and ligand used. Furthermore, utilization of metal-
ligand interactions may afford materials that have attractive
functional properties associated with the metal ion. In particular,
Weng and coworkers have reported the construction of fast room
temperature healable colloidal gels via the self-assembly of
3
4
5
6
segregation,
coaxial
electrospinning,
7
thermoset/thermoplastic blending, magnetic and resistance
ditopic
ligand,
consisting
of
a
2,6-bis(1-
8
9
heating, UV crosslinking, and so on. Nevertheless, repetitive
healing was very rare in such systems.
methylbenzimidazolyl)pyridine (BIP) moiety attached to either
end of an oligo-PEG core, in the presence of transition
1
0,11
Dynamic covalent chemistry and non-covalent interactions,
22a-c
metal/lanthanide ions.
More recently, Rowan and coworkers
due to their reversible nature, offer intriguing opportunities of
constructing self-healing materials capable of repetitive repairing.
Self-healing systems based on dynamic covalent chemistry
developed so far involved the uses of Diels-Alder (DA)
reported an optically healable metallo-supramolecular elastomer
based on a macromonomer comprising of a poly(ethylene-co-
22d
butylene) core with BIP at the two termini.
In recent years, copper(I) catalyzed azide-alkyne 1,3-dipolar
cycloaddition (CuAAC) has attracted considerable attention due
to its “click” characteristics and has been widely used as a
powerful synthetic strategy in almost all areas of modern
1
2
13
14
reactions,
[2+2] cycloaddition,
acylhydrazone bonds,
disulfide
1
5
16
trithiocarbonate
units,
moiety
and
1
7
diarylbibenzofuranone. Their impressive repetitive healing
characteristics were a consequence of the responsiveness of the
functional groups to external stimuli such as temperature,
chemicals and photo. Non-covalent interactions, including
hydrogen bonding, hydrophobic association, π-π stacking,
ionic interactions and metal-ligand interactions, have also
chemistry ranging from macromolecular engineering to drug
24
discovery.
Interestingly, the 1,2,3-triazoles produced by
CuAAC “click” reaction have been found to be excellent novel
ligands and their coordinating properties of have also been well
1
8
19
20
2
1
22
25
studied. Recently, a number of metal chelating systems based
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HO
H
O
O
n
H
N
NCO
O
NCO
OCN
OCN
O
N
n
H
O
N
HO
N
N
OH
5
N
N
N
N
4
O
O
H
N
H
N
O
N
O
N
N
O
N
H
O
N
H
n
N
N
N
N
O
O
m
5
1
0
Scheme 1 Synthesis of ligand macromolecule 5 carrying BTP in the backbone using a polyurethane reaction.
on 1,2,3-triazoles and various 1,2,3-triazole containing ligands
functional materials. In particular, the tunable stability and
dynamics can be utilized for the construction of multi-responsive
supramolecular gels with repeatable self-healing ability.
The BTP containing ligand macromolecule 5 was effectively
synthesized via bis-isocyanate functionalized prepolymer
formation followed by chain extension using bis-hydroxyl
functionalized BTP ligand 4 as chain extender (Scheme 1). The
incorporation of BTP moieties was proven by 1H NMR, GPC and
25-27
synthesized by the “click” chemistry have been reported.
Due
to their facile synthesis and easy functionalization, 1,2,3-triazoles
and 1,2,3-triazole containing ligands have emerged as a novel
class of ligands in building new supramolecular architectures. In
particular, the tridentate “click” ligands 2,6-bis(1,2,3-triazol-4-
yl)pyridines (BTP), which are versatile alternatives to BIP and
terpyridine (TPY) ligands, have shown great potential for the
1
2
2
3
5
0
5
0
55
development of new metallo-supramolecular functional materials. 60 FTIR (Figures S5-S7 in Supporting Information). The number
However, so far only one publication by Hecht and coworkers
reported the preparation of metallo-supramolecular gels using
rigid polymers bearing BTP subunits and their properties
associated with the dynamic metal-ligand interactions,
averaged molecular weight, M , and polydispersity index, PDI, of
n
the ligand macromolecule were typically about 30,000g/mol and
1.5, respectively, based on GPC measurements using THF as
eluant and polystyrene standards. Since the poly(tetrahydrofuran),
2
6e
particularly self-healing behaviors, were not explored. Herein, 65 PTHF (or poly(tetramethylene glycol), PTMG), linker used has a
we show that ligand macromolecule bearing built-in tridentate
click” ligand 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) moiety
M of 2,000, the average number of BTP moieties incorporated in
a single polymer backbone is estimated to be about 12.
n
“
produced by “click” chemistry in the polymer backbone could be
obtained through a facile synthetic pathway and report for the
first time the construction of multi-responsive self-healing bulk
Supramolecular gelation
metallo-supramolecular gels by using the ligand macromolecule 70 To probe the gelation of 5 in the presence of Zn(II) and/or Eu(III),
together with transition metal ions and/or lanthanide ions.
solutions of Zn(OTf) or Eu(OTf) in various solvents were added
2 3
into solutions of 5 in chloroform of two different concentrations.
It was found that gelation was dependent on the concentration of
Results and discussion
5
and the solvent used to dissolve the metal ions (Figure S40 in
Supporting Information). The first solution of had
concentration of 100 mg/mL (100 mg 5 dissolved in 1 mL
CHCl ). In the cases of acetonitrile and THF, upon progressive
“Click” ligand and ligand macromolecule
7
5
5
a
3
4
4
5
5
0
5
0
The tridentate BTP ligand has been reported to coordinate well to
transitional metal ions such as Zn(II), Ni(II), Fe(II), Ru(II) and
lanthanide metal ions such as Eu(III).
3
2
5,26
Coordination typically
addition of solutions of metal salts, gradual increase in viscosity
was firstly observed, then at some points (about 50% and 35% of
led to stable 1:2 (metal:ligand) complexes in the case of
transitional metal ions and 1:3 (metal:ligand) complexes in the 80 the stoichiometric amounts of required metal ions for Zn(II) and
case of lanthanide ions. Our UV-Vis titrations of 4 with Zn(OTf)2
and Eu(OTf)3 in acetonitrile also confirmed these findings
Eu(III), respectively) gels were formed. The difference in the
amounts of Zn(II) and Eu(III) required to reach the gel points is
consistent with their different binding characteristics, i.e. Zn(II)
binds to BTP in a 1:2 ratio and Eu(III) binds in a 1:3 ratio. When
(Figures S8-S11 in Supporting Information). Interestingly, it was
possible to adjust the complex stability and complexation
dynamics by not only chemicals but also the solvents, which can 85 methanol was used as the solvent to dissolve the metal salts,
be demonstrated by UV-Vis titrations of chemicals and solvents
with Zn:4 and Eu:4 complexes in acetonitrile (Figures S12-S39
in Supporting Information). There are many other transition metal
and lanthanide ions available. Therefore, it is possible to offer a
however, no gels can be formed, despite that a slight increase in
viscosity was observed. This difference in gelation should be due
to the chelating ability of methanol which prevented the
formation of stable or long-lived metal:BTP complexes. When a
number of different types of metal:BTP interactions of various 90 second solution of 5 having a concentration of 25 mg/mL was
properties. Utilization of the easily accessible BTP ligands thus
will create a new avenue to build metallo-supramolecular
used, however, no gelation was observed whichever solvent was
used to prepare the solutions of metal salts. Since the second
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5
4
3
2
1
5
10
10
10
10
10
10
4
5
5
10
10
10
10
3
2
1
5
G'-500%
G''-500%
G'-1%
G'-0/100
G''-0/100
G'-100/0
G''-100/0
G''-1%
60
-
0
1
0
10
1
10
2
10
0
5
10
15
20
1
Time (Min)
Frequency (Hz)
1
0
Fig. 1 Breakdown (solid symbols, measured with a strain of 500% at 0.15
Hz) and reformation (open symbols, measured with a strain of 1% at 1 Hz)
behaviours of a 0/100 gel (45 mg/mL) measured by a time sweep
experiment.
Fig. 2 Storage modulus G′ and loss modulus G″ as a function of
frequency for 100/0 (triangle symbols) and 0/100 (square symbols) gels
65 (45 mg/mL).
1
2
2
3
5
0
5
0
mL solvent), 16.7 mg/mL and 14.3 mg/mL for the three systems,
respectively, with the 100/0 one having the highest critical
concentration (the lowest swelling ratio) and the 0/100 one
having the lowest critical concentration (Figure S41 in
Supporting Information). Further addition of chloroform
eventually led to complete dissolution of the gels. Similarly, all
gels could be recovered by drying and reswelling. It is generally
believed that higher crosslinking leads to lower swelling ratio.
However, the Eu(III) containing systems, which are supposed to
be more crosslinked, exhibited lower critical gel concentrations
solution of 5 had a concentration only 1/4 of the first one, it is
assumed that at this dilute condition the tendency to form
intrachain loops would be higher than at concentrated conditions.
In addition, acetonitrile, THF and methanol are all coordinating
by nature, their presence will influence the lifetimes of metal-
ligand complexes. Nevertheless, upon condensation, gels could
be eventually obtained for the systems in which acetonitrile and
THF were used to prepare metal ion solutions. It is also
noteworthy that after solvent evaporation the resulting materials
from all the above mentioned different approaches could be re-
swollen by appropriate solvents to form gels. These observations
confirmed the dynamic nature of the Zn:BTP and Eu:BTP
complexes, which is critical to the multi-responsive and self-
healing properties.
7
7
8
0
5
0
(
higher swelling ratios). A plausible explanation for this
observation is that at similar concentrations the systems
containing higher ratios of Zn(II) were weaker in mechanical
strength and therefore tended to flow at higher critical
concentrations, although crosslinked networks still remained at
this point.
Swelling properties of metallo-supramolecular gels
Rheological properties of metallo-supramolecular gels
The dynamic properties of the complexes were further
investigated by swelling study of the gels. Three systems were
studied containing different stoichiometric ratios of Zn(II) to
Eu(III) ranging from 100% Zn(II) and no Eu(III) (100/0), to 50%
Zn(II) and 50% Eu(III) (50/50), and to no Zn(II) and 100% Eu(III)
8
9
9
5
0
5
To provide more insight into the mechanical properties of the gels,
rheological characterization was carried out. In this case, toluene
instead of chloroform and 1,1,2,2-tetrachloroethane was used to
prepare the gels due to its higher boiling point and lower toxicity.
After sample loading, a time sweep experiment under oscillatory
shear was first conducted to study the breakup and reformation of
the gel network. Fig. 1 shows the storage modulus G′ and loss
modulus G″ from the time sweep experiment of a 0/100 sample
with a concentration of 45 mg/mL. The results show that when
sheared with a strain of 500% at a frequency of 0.15 Hz, the gel is
effectively broken into a sol, as revealed by the fact that storage
modulus G′ is lower than loss modulus G″. After the large strain
shear is ceased, an almost instantaneous crossover of storage
modulus and loss modulus (G′ > G″) is observed, indicating the
reconstitution of a gel network. Interestingly, the equilibrium
3
4
4
5
5
0
5
0
(0/100). In each case, a piece of dried gel of 50 mg was used.
Swelling study was performed by means of stepwise addition of
chloroform and checking the stability of the gels by inverting the
vials after equilibration at each step. The equilibration time,
defined as the time required for the absorption of solvent and the
following homogenization of the gels, was different for different
systems. It took about 30 min for the 100/0 system and about 3
min for the 0/100 one, while the time required for the 50/50 one
lied in between 3-30 min. The observation that longer
equilibration times were required for the Zn(II) containing
systems can be explained by the fact that Zn(II) forms stronger
and less dynamic complex with BTP than does Eu(III). It should
be pointed out that if different amounts of dried gels were used,
the equilibration times might be different. Based on the inverting
vial method, the critical concentrations for the gels to stay stable
1
00 storage modulus value characteristic of the fully developed is also
almost reached instantaneously. Replacing Eu(III) with Zn(II) or
changing gel concentration gave similar recovery behaviors
(
Figure S42, Figure S44 and Figure S45 in Supporting
(
no flowing) were about 20 mg/mL (20 mg dried gel swollen by 1
Information). This recovery (healing) is much more effective than
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5
4
3
2
1
0
10
10
10
10
10
10
5
5
5
G'-0/100
G''-0/100
G'-100/0
G''-100/0
60
-
0
1
0
10
1
10
2
10
3
10
1
Strain (%)
1
0
Fig. 3 Storage modulus G′ and loss modulus G″ as a function of strain
amplitude for 100/0 (star symbols) and 0/100 (diamond symbols) gels (45
mg/mL).
Fig. 4 Picture of gels (20 mg/mL in chloroform) with varying amounts of
Zn(II) and Eu(III) (from left to right Zn/Eu: 100/0, 75/25, 50/50, 25/75,
65 0/100) under 365 nm UV light demonstrating the effect of metal
combination on the photoluminescence colour.
1
2
2
3
3
4
4
5
0
5
0
5
0
5
that of the colloidal metallo-supramolecular gels based on BIP
22a-c
ligands reported by Weng and coworkers.
Such an
extraordinary recovery is consistent with the thermodynamically
stable but kinetically labile nature of the metal:BTP complexation
and is of great importance to applications where a fast response is
required.
In addition, measurements of storage modulus G′ and loss
modulus G″ were performed as a function of the oscillatory
frequency. The results of the 100/0 and 0/100 samples with a
concentration of 45 mg/mL are depicted in Fig. 2. It is seen that
G′ of the 100/0 sample is dependent on the frequency and is
slightly larger than G″ at low frequencies, whereas G′ of the
70
7
5
0
/100 sample is almost constant and about one order of
magnitude higher than G″ over the entire frequency range.
Similar behaviors were observed for the two systems at a
concentration of 60 mg/mL (Figure S43 and Figure S46 in
Supporting Information). These differences in rheological
behaviors from frequency sweeps imply that at similar conditions
Zn(II) containing gels are less elastic and weaker than the gels
containing only Eu(III), consistent with the swelling results.
Finally, the viscoelasticity of the gels were investigated by
strain sweep tests. Fig. 3 shows the moduli as a function of strain
amplitude of the 100/0 and 0/100 samples with a concentration of
80
45 mg/mL. Both samples show large linear viscoelastic regions,
where logG′ and logG″ remain essentially constant up to a strain
of about 30%, beyond which logG′ starts to decrease abruptly.
The crossovers of G′ and G″, an indicative of the onset of viscous 85 25/75 gel (c) at RT and (d) after being heated at 120 °C, 0/100 gel (e) at
flow, both occur at a strain of around 100%. These large linear
regimes and crossover strains imply that the gels are stretchy and
Fig. 5 Pictures of 100/0 gel (a) at RT and (b) after being heated at 140 °C,
RT and (f) after being heated at 110 °C under 365 nm UV light. Gels
were swollen with chloroform with a concentration of 50 mg/mL.
can withstand relatively large deformation. This is very different
from the behaviors of the colloidal metallo-supramolecular gels
i.e. 100% Zn(II) (100/0), 25% Zn(II) and 75% Eu(III) (25/75),
22a-c
previously reported by Weng and coworkers.
9
0
and 100% Eu(III) (0/100), were selected. Upon heating, the 100/0
gel turned into a free flowing sol until at a temperature up to as
high as about 140 °C. However, the gel-sol transition
temperatures were much lower, ca. 120 °C and 110 °C for the
gels containing 75% and 100% Eu(III), respectively. This is
consistent with the weaker complexation of Eu(III) with BTP.
While little change in the emission of the gel containing only
Zn(II) could be observed when heated, the emissions from
Thermo- and chemo-responsive properties of metallo-
supramolecular gels
5
0
Having demonstrated that the emission of the gels can be
manipulated, the gel-sol transition and optical response upon
exposure to heating were studied. In this case, only three systems,
9
5
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5
0
5
0
5
0
5
5
Fig. 7 Optical microscopic images (magnification 150X) showing a cut (a)
and its healing (b) of a coating prepared with a gel containing no Zn(II)
and 70% Eu(III) (0/70) on copper surface.
1
1
2
6
0
showed sensitive response to the nerve gas agent mimic triethyl
phosphate, however, neither of them could be destroyed by
triphenylphosphine. It is thus anticipated that careful selection of
metal ions, including other metal ions not shown here, and their
combinations would allow construction of gel systems with
tunable sensory response to chemicals.
6
5
Self-healing properties of metallo-supramolecular gels
The observations illustrated above clearly show that the Zn:BTP
and Eu:BTP complexes are thermodynamically stable and
kinetically labile, suggesting the potential for self-healing of the
gels. Thus, to test this hypothesis, we first prepared a gel
containing no Zn(II) and 100% Eu(III) (0/100) swollen by
1,1,2,2-tetrachloroethane. 1,1,2,2-tetrachloroethane was selected
due to its high boiling point and good ability to form stable gels
in this study. Interestingly, the gel exhibited impressive self-
healing properties. As depicted in Fig. 6, four deliberately cut
blocks of gels, two of which were dyed with a small amount of
rhodamine B, could form a stable self-supporting bar by simply
joining together the fresh surfaces and keeping contact for about
7
7
8
8
0
5
0
5
Fig. 6 Photographs of a gel (100 mg/mL in 1,1,2,2-tetra-chloroethane)
containing no Zn(II) and 100% Eu(III) (0/100) showing the self-healing
behaviour and properties of the healed gel. (a) four blocks of cut gels, two
red ones were dyed with rhodamine B; (b) jointed blocks by simply
joining together the fresh surfaces; (c) stable self-supporting bar formed
after healing for 6 h at RT without any external intervention; (d)
squeezing, (e) bending and (f) stretching of the healed gel bar. (g)
Simplified schematic illustration of the self-healing mechanism: I, cutting
caused the break of the polymer linker between two BTP ligands; II,
completely separated blocks after cutting; III, healing as a consequence of
exchange of ligand macromolecules between two adjacent blocks.
2
3
5
0
6
h at room temperature without any external intervention.
Although the healed scars were still visible, the joints between
the different blocks in the repaired were strong enough to sustain
squeezing, bending and stretching. Surprisingly, the repaired gel
might not break at the scar location upon stretching (Movie in
Supporting Information). When the healed gel was cut at the
joints into separated blocks and brought into contact together as
before, the self-healing process occurred once again. This cutting
and healing cycle can be repeated several times. Moreover, if the
Eu:BTP in the Eu(III) containing gels were found readily
quenched by heating (Fig. 5). As the temperature was returned to
3
4
4
5
0
5
ambient, both gels were reformed and the emission from the 90 blocks were not cut to obtain fresh surfaces, the fusion was also
Eu:BTP could be regained. In addition, the gel-sol transition and
emission quenching-recovery could be repeated many times.
Therefore, the gels, especially the Eu(III) containing gels, may
find potential uses as temperature sensor with reversible
characteristics.
To study the response to chemical stimuli, a spectrum of
chemicals having different polarity, pH and chelating properties
were tested. It was found that most of the chelating chemicals
selected were able to destroy the gels and at the same time caused
changes in photoluminescence (Figures S53-S63 and Table S1 in
possible and took similar time. When a portion or all of Eu(III)
was replaced with Zn(II), the gels healed equally well, except that
higher gel concentrations were generally required to obtain
mechanically stable blocks and bars (Figure S65 and Figure S66
95 in Supporting Information).
Self-healing properties of bulk coating
Self healing of the metallo-supramolecular gels may find many
potential practical uses. One interesting application is the healing
Supporting Information). Whereas bipyridine readily caused the 100 of metal coatings made of 5 and various types of metal ions,
deconstruction of the gel containing only Zn(II), it had no
effecton the gel containing only Eu(III). In addition, both gels
which can be applied to a number of different metal surfaces.
While detailed studies are in progress, preliminary results showed
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that supramolecular Eu:5 crosslinking system formed strong and
spectrometer at 25 °C using residual protonated solvent signals as
tough solid coating layer on the surface of copper and cuts on the
coating could be easily healed by applying a small amount of
suitable solvent, e.g. toluene, onto the damage area, followed by
internal standard. Gel permeation chromatography (GPC) data
were calibrated against polystyrene standards and collected on a
Waters 1515 equipped with a Waters 2414 Refractive Index
5
0
5
subsequent drying (Fig. 7). Solvent enabled healing has been 60 Detector using THF as eluant. FTIR spectra were collected on a
2
1b
adopted to the healing of hydrogel thin films
and solid
Nicolet Avatar 330 (Thermo Electron Corporation, USA). UV-vis
spectra were obtained on Thermo Scientific Evolution 300.
Photoluminescence spectra were acquired on Hitachi F7000
fluorescence spectrometer. Rheological measurements were
2
1c
coatings assembled via LBL technique on substrates base on
elelctrostatic interactions. However, to the best of our knowledge,
this is the first time self-healing of a metallo-supramolecularly
1
crosslinked coating enabled by solvent treatment is demonstrated. 65 conducted on an ARES G2. Optical microscopy was performed
Due to excellent healing properties and the fact that trazole
using VHX-600 digital microscope (Keyence, UK) in refractive
mode.
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8
derivative ligands can effectively adhere to metal surfaces, we
are expecting to develop a series of all new supramolecular
crosslinking coatings with self-healing abilities.
Gelation study and preparation of gels
1
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8
0
5
0
5
To study the gelation behaviour, a solution of Zn(OTf)2 (0.05
mol/L) or Eu(OTf) (0.015 mol/L) in CH CN or THF or CH OH
Conclusion
3
3
3
was added dropwise into a stirred solution of 5 (100 mg or 25 mg)
in CHCl3 (1 mL). The total amount of metal added was adjusted
to provide for metal:BTP equivalent of 1.0 (accounting for 1:2
and 1:3 binding, respectively). During the addition of metal salt,
the viscosity change of the solution and gel formation was
monitored by visual observation. The resulting gel or sol was
allowed to dry under ambient conditions and further dried in
vacuo to produce solid supramolecular polymers Zn:5 and Eu:5,
which were kept for further uses. For systems containing both
Zn(II) and Eu(III), i.e. Zn/Eu:5, prescribed amounts of metal
solutions in CH CN (usually 0.05 mol/L for Zn(OTf) and 0.015
In summary, we have demonstrated that metallo-supramolecular
gels were prepared using a ligand macromolecule containing
tridentate 2,6-bis(1,2,3-trizol-4-yl)pyridine (BTP) ligand moiety
synthesized by CuAAC “click” chemistry in the backbone
together with transition metal ions and/or lanthanide ions. Due to
the supramolecular and dynamic nature of the metal-BTP
complexes, the gels exhibited multi-responsive properties and
repeatable autonomic healing ability. The gelation and the
properties can be tuned by careful selection of metal ions and
their combinations, solvent, concentration, and so on. The multi-
responsive properties make them attractive for developing
sensors and the repeatable self-healing ability may find many
practical applications including but are not limited to reshaping
and regeneration of gels and repairing of metal coatings. To the
best of our knowledge, this is the first report showing the
preparation of multi-responsive gels using macromolecule with
built-in “click” ligand unit in the backbone and their self-healing
properties originating from the dynamic nature the metal-ligand
interactions. The use of macromolecule carrying multiple ligand
moieties in the backbone can be advantageous over their
counterparts carrying ligand units only at the chain ends. Multi-
responsive materials, self-healing polymeric materials and “click”
chemistry are increasingly active research areas of particular
scientific and commercial interests. The discovery that tridentate
2
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3
3
4
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0
5
0
5
0
5
3
2
mol/L Eu(OTf) ) were added into solutions of 5 in CHCl
3
3
(usually 100 mg/mL) to form gels first. The gels were then dried
under ambient conditions and further dried in vacuo to produce
solid supramolecular polymers, which were kept for further uses.
In subsequent studies, metallo-supramolecular gels were
produced by reswelling dry supramoluecular polymers with
prescribed amounts of specific solvent.
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0
Rheological tests
Rheological measurements were performed using parallel plate
geometry. The plate used has a diameter of 25 mm and the gap is
set at 500 μm for all measurements. All measurements were
conducted at a temperature of 25±0.5 °C, and the samples were
covered with a thin layer of ethylene glycol to prevent
evaporation of the solvent or absorption of atmospheric water.
Gels used for theological measurements were swollen with
toluene instead of chloroform, due to the higher boiling point of
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5
“
click” BIP ligand was effective in building multi-responsive and
self-healing materials opens up new possibilities of designing
new applied functional materials via a combination of “click”
chemistry and supramolecular chemistry. Current efforts are
being made to evaluate the influence of polymerization procedure
and the design of ligand molecules on the properties of the gels
and solid films. In addition, studies are ongoing to develop the
applications of the multi-responsive and self-healing properties.
1
1
1
00 toluene. And the gels were made in two concentrations, 45
mg/mL and 60 mg/mL. After sample loading and gap setting, the
sample was first presheared with a large strain amplitude of 500%
at 0.15 Hz, then allowed to equilibrate for 20 min to complete the
gelation process, during which the sample was monitored by
05 oscillatory shear of small strain amplitude (1 %, in the linear
viscoelastic regime) at 1 Hz, in time steps of 30 s. After
equilibration, a frequency sweep was carried out to measure G′
and G″ as a function of deformation frequency from 0.01 Hz to
5
0
Experimental
General methods
All reactions requiring inert gas were performed under N2
atmosphere. Column chromatography was carried out with 200-
1
00 Hz at a strain amplitude of 1%. This frequency sweep was
10 then followed by a strain sweep in a strain range from 0.1-1000%
at 1 Hz.
400 mesh silica gel using the eluants specified. NMR spectra
5
5
were recorded on 400 MHz Brucker Avance II 400
a
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This work was supported by National Natural Science Foundation
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a
Department of Chemistry, College of Chemistry and Chemical
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Engineering, Xiamen University,Xiamen, Fujian 361005 (P. R. China).
Fax: +86-592-2186975; Tel: +86-592-2189790;
E-mail: wgweng@xmu.edu.cn
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Journal Name, [year], [vol], 00–00 | 7