A multi-responsive organogel and colloid based on the self-assembly of a Ag(i)-azopyridine coordination polymer

Article abstract of DOI:10.1039/d1sm00013f

In this work, through the coordination ofC₃symmetric azopyridine ligands and Ag(i), coordination polymers with azo groups on the main chain were prepared. Thetranscoordination polymer formed an organogel with a network of nanofibers at low crit

Full text of DOI:10.1039/d1sm00013f

Soft Matter
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A multi-responsive organogel and colloid based
on the self-assembly of a Ag(I)-azopyridine
coordination polymer†
Cite this: Soft Matter, 2021,
17, 3654
a
b
Botian Li, *a Da Xiao,a Xiaodong Gai,a Bo Yan,a Haimu Ye,
Liming Tang
a
and Qiong Zhou
*
In this work, through the coordination of C3 symmetric azopyridine ligands and Ag(I), coordination
polymers with azo groups on the main chain were prepared. The trans coordination polymer formed an
organogel with a network of nanofibers at low critical gelation concentrations, and it exhibited the
abilities of self-healing and multi-stimuli response to heating, light, mechanical shearing, and chemicals
due to the presence of dynamic coordinating bonds. On the other hand, the cis coordination polymer
was found to assemble into nanoparticles to give a responsive colloid, which can produce fibrous
precipitation in several days upon visible light irradiation due to the isomerization of the azo groups. This
work provides a novel example for the design of a multi-responsive organogel and colloid based on the
structural transformation of coordination polymers.
Received 4th January 2021,
Accepted 8th February 2021
DOI: 10.1039/d1sm00013f
rsc.li/soft-matter-journal
Gels are a unique category of responsive materials with their
entire three-dimensional network entrapping the solvent
1 Introduction
Stimuli-responsive materials showing dramatic property changes in inside.9,10 If the network is built by the nano/micro assembly
response to external environmental stimuli have been attracting of coordination polymers, a so-called coordination polymer gel
sustained attention due to the wide range of applications in soft (CPG) forms.11–13 Compared with other metallogels afforded
robots,1 tissue engineering,2 biosensors,3 display,4 etc. This great from discrete metal complexes (metallogelators), CPGs benefit
interest has led to the development of numerous responsive from the higher metal content and faster response of the
materials wherein reversible transformations are achieved by intro- coordination polymer, and thus they can be designed as a
ducing supramolecular interactions, such as hydrogen bonds, novel responsive material.14,15 In particular, CPGs in response
metal–ligand coordination, hydrophobic interactions, and p–p to complex and multiple stimuli have become a focus of
stacking.5 Among these supramolecular systems, coordination innovation in the design and fabrication of smart materials,
polymers have been of increasing interest in recent years, since as a single-responsive performance sometimes cannot ensure
they are mainly constructed from metal–ligand coordination that is accurate and remarkable feedback in a complicated environ-
not only thermodynamically stable but also kinetically labile, and ment. To date, some studies have been reported to prepare
this feature endows fast and precise stimuli-responsiveness to the CPGs that respond to many external inputs, including mechanical
materials.6,7 Generally, coordination polymers can be facilely stress,16 change of temperature,17 sonication18 and chemicals.19,20
synthesized through the coordination reaction of bridging ligands Recently, Mauro et al. introduced azobenzene groups into coordi-
and metal ions, and their characteristics, e.g., the assembly struc- nation polymers and fabricated
a soft gel actuator with
ture, stability, and functionality, can be tuned by the metal ions and photosensitivity.21,22 However, to date, the incorporation of
ligands utilized,8 which make them a promising prospect for multiple responses to light, heating, mechanical force and
potential applications in various fields.
chemicals into one CPG has not been reported.
In the last few decades, many kinds of stimuli-responsive
colloids have been developed for drug delivery,23 controlled
release,24,25 etc. Traditionally, the response of these colloids is
achieved by introducing some catalysts or agents to change the
chemical structure of the matrix.26–28 However, this method
leads to a complex procedure with additional reagents, which
may contaminate the system. Therefore, the photo-responsive
colloids based on a simple, clean, and efficient photochemical
a Department of Materials Science and Engineering, China University of Petroleum,
Beijing, 102249, P. R. China
b Key Laboratory of Advanced Materials of Ministry of Education of China,
Department of Chemical Engineering, Tsinghua University, Beijing, 100084,
P. R. China. E-mail: botian.li@cup.edu.cn, zhouqiong_cn@163.com
† Electronic supplementary information (ESI) available: Supplementary figures,
tables, movies, synthesis routes. See DOI: 10.1039/d1sm00013f
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reaction have been proposed to address this problem.29 So far, 2.2 Synthesis of 4-(3-pyridylazo)phenol
most of these research studies applying UV irradiation to
3-Aminopyridine (1 g) was dissolved in 20 mL of deionized
water. Then, 4 mL of hydrochloric acid (30%) was added
dropwise into the solution. The solution was kept in an ice-
water bath, and 0.89 g of sodium nitrite dissolved in 10 mL of
deionized water was added within 10 min under stirring.
Subsequently, the reaction mixture was added to a 60 mL
aqueous solution containing 3 g of phenol within 10 min. After
continuing the reaction at 0 1C for 10 min, 1.6 g of NaOH was
introduced and the resulting precipitate was filtered. The crude
product was washed three times with deionized water and dried
in vacuum for 12 h to give a yellow powder (1.73 g). Yield: 82%.
1H NMR (600 MHz, 298 K, DMSO-d6): d = 10.45 (s, 1H, OH),
9.03 (s, 1H, pyridine 2-H), 8.68 (d, J = 4.4 Hz, 1H, pyridine 6-H),
8.11 (d, J = 8.4 Hz, 1H, pyridine 4-H), 7.84 (d, J = 8.8 Hz, 2H,
phenyl 3-H), 7.58 (d ꢁ d, J = 4.8, 8.0 Hz, 1H, pyridine 5-H),
6.98 ppm (d, J = 8.8 Hz, 2H, phenyl 2-H) (Fig. S1, ESI†). FTIR
(KBr, cmꢀ1): 3020, 3072, 1588, 1501, 1482, 1415, 1282, 1140.
ESI-MS m/z: [M + H]+ calcd 200.07; found 200.1446.
manipulate the well-designed colloids have shown the feasibility
of photo-responsiveness;30,31 nevertheless, the coordination
polymer colloid in response to visible light under mild condi-
tions remains a challenge.
To demonstrate the strategy of preparing
a stimuli-
responsive gel and colloid by the assembly of a coordination
polymer, herein, we report the fabrication of a multi-responsive
organogel and colloid by employing two azo isomers of the Ag(I)
coordination polymer. The azo groups were introduced into the
main chain of the coordination polymer through the coordina-
tion of Ag(I) and azopyridine ligand, thereby affording the trans
and cis conformers of the coordination polymer. The coordina-
tion polymer with trans configuration can produce a CPG
constituted of the nanofiber networks, and its gel–sol phase
transition can be triggered in response to multi-stimuli includ-
ing light, heating, mechanical shearing, and chemicals. It was
found that the photoisomerization of azo moieties on the
coordination polymer was retarded significantly compared with
that of azo ligands, showing the macromolecular hindrance
effect. Meanwhile, the isotropic assembly of the coordination
polymer with cis configuration produced a colloidal dispersion
composed of nanoparticles, and both visible light irradiation
and heating could change the nanoparticles into fiber aggre-
gates due to the isomerization of the coordination polymer, and
caused precipitation and demulsification. This research pre-
sents a novel example of a responsive colloid derived from the
assembly transformation of coordination polymers.
2.3 Synthesis of tris(4-(3-pyridylazo)phenyl)benzene-1,3,5-
tricarboxylate (trans-D)
First, 1 g of 4-(3-pyridylazo)phenol was dissolved in 50 mL of
THF, and then 0.44 g of 1,3,5-benzenetricarbonyl trichloride
dissolved in 10 mL of THF was added dropwise. Subsequently,
0.71 g of DMAP was added into the flask, and the solution was
refluxed for 1.5 h. Afterwards, the solvent was rotary evaporated
and 60 mL ethanol was introduced. The resulting precipitate
was filtered and purified by column chromatography to give a
yellow powder of ligand trans-D (0.37 g). Yield: 30%.
1H NMR (600 MHz, 298 K, DMSO-d6): d = 9.16 (s, 3H,
benzene H J = Hz), 9.12 (s, 3H, pyridine 2-H), 8.78 (d, J =
4.8 Hz, 3H, pyridine 6-H), 8.22 (d, J = 8.8 Hz, 3H, pyridine 4-H),
8.10 (d, J = 8.8 Hz, 6H, phenyl 3-H), 7.70 (d, J = 8.8 Hz, 6H,
phenyl 2-H), 7.66 ppm (d ꢁ d, J = 8.8, 4.8 Hz, 3H, pyridine 5-H)
(Fig. S2, ESI†). FTIR (KBr, cmꢀ1): 1752, 1590, 1494, 1420, 1214.
ESI-MS m/z: [M + H]+ calcd 754.21; found 754.4189.
2 Experimental
2.1 Materials and equipment
1,3,5-Benzenetricarbonyl trichloride, phenol, 3-aminopyridine, 4-
aminopyridine, sodium nitrite, sodium hydroxide, and 4-
dimethylaminopyridine (DMAP) were purchased from Shanghai
Aladdin Co. Silver nitrate was supplied from Sinopharm Chemical
Reagent Co., Ltd. Hydrochloric acid, N,N-dimethylformamide
(DMF), N,N-dimethylacetamide (DMAc), tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dichloromethane,
2.4 Synthesis of 4-(4-pyridylazo)phenol
cyclohexane and ethanol were purchased from Beijing Chemical NaOH (0.318 g) and phenol (0.747 g) were dissolved in 6 mL of
Works. All other chemicals were of reagent grade or better. The deionized water (vial A), and 0.5 g of 4-aminopyridine was
1
synthesized compounds were characterized using H NMR (Japan dissolved in 1.2 mL of hydrochloric acid (30%) (vial B). Subse-
JEOL JNM-ECA600) and FT-IR (Bruker TENSOR II). The transmit- quently, 0.383 g of sodium nitrite dissolved in 1 mL of deio-
tance, particle size, and zeta potential were measured by dynamic nized water was introduced into vial B at 0 1C within 1 min, and
light scattering (DLS) with a particle size analyzer (Litesizer then the solution was carefully poured into vial A. After 5 min,
500 Anton-Paar). The gels were characterized using TEM (Japan 0.211 g of NaOH dissolved in 1 mL of deionized water was
JEOL JEM 2100, 200 kV), SEM (Japan Hitachi SU8010), X-ray introduced and the resulting precipitate was filtered. The crude
diffraction (XRD) (Bruker AXS D8), and elemental analysis (Elemen- product was washed with deionized water three times and
tar Vario EL III). The rheological measurement was performed using dried in vacuum for 12 h to give an orange powder (0.42 g).
a rheometer (Anton Paar Physica MCR-300). UV-vis absorption Yield: 40%.
spectra were recorded with a UV-vis absorption spectrometer (Japan
1H NMR (600 MHz, 298 K, DMSO-d6): d = 10.56 (s, 1H, OH),
Shimadzu UV-3600). The light response was determined with a 8.73 (d, J = 6.0 Hz, 2H, pyridine 2-H), 8.34 (d, J = 8.8 Hz, 2H,
365 nm UV lamp (Japan Iwata UV-201 SA, 200 mW cmꢀ2) and an phenyl 3-H), 7.64 (d, J = 6.0 Hz, 2H, pyridine 3-H), 6.94 ppm
LED lamp (4500 nm, 30 mW cmꢀ2).
(d, J = 8.8 Hz, 2H, phenyl 2-H) (Fig. S3, ESI†). FTIR (KBr, cmꢀ1):
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3043, 1580, 1469, 1403, 1295, 1135. ESI-MS m/z: [M + H]+ calcd was added. The yellow colloidal dispersion was immediately
200.07; found 200.1446.
formed after shaking by hand.
2.5 Synthesis of tris(4-(4-pyridylazo)phenyl)benzene-1,3,
5-tricarboxylate (trans-Z)
3 Results and discussion
First, 0.3 g of 4-(4-pyridylazo)phenol and 0.159 g of triethyl-
amine were dissolved in 15 mL of THF, and then 0.132 g of 1,3,
5-benzenetricarbonyl trichloride dissolved in 1 mL of THF was
added dropwise. After 10 min, the solvent was rotary evaporated
and the resulting solid was purified by column chromatography
to give a yellow powder of ligand trans-Z (0.19 g). Yield: 51%.
1H NMR (600 MHz, 298 K, DMSO-d6): d = 9.09 (s, 3H, benzene
H), 8.82 (d, J = 6.0 Hz, 6H, pyridine 2-H), 8.10 (d, J = 8.8 Hz,
6H, phenyl 3-H), 7.75 (d, J = 6.0 Hz, 6H, pyridine 3-H), 7.69 ppm
(d, J = 8.8 Hz, 6H, phenyl 2-H) (Fig. S4, ESI†). FTIR (KBr, cmꢀ1): 1749,
1589, 1495, 1204. ESI-MS m/z: [M + H]+ calcd 754.21; found 754.4459.
3.1 Formation of gel via self-assembly
In previous work, we reported several CPGs with various
morphologies fabricated through coordination of metal ions
and symmetrical ligands with pyridyl groups.32–34 To develop a
CPG with multi-stimuli response, herein, azo-pyridyl groups
were introduced into the C3-symmetric molecule, and the
resulting ligands with meta-pyridine and para-pyridine were
designed, namely ligand trans-D and trans-Z, respectively. The
molecular structures of the synthesized compounds were char-
acterized by 1H NMR (Fig. S1–S4, ESI†), and their two-step
synthetic routes are shown in Fig. S5 and S6 (ESI†). After the
addition of the Ag(I) solution to the ligand DMF solution at
room temperature, the corresponding coordination polymers
trans-AgD and trans-AgZ immediately assembled and aggre-
gated from the solution due to their poor solubility in the
solvent. It is worth noting that only trans-AgD formed a stable
gel an orange-yellow color (Fig. 1a and Movie S1, ESI†), while
trans-AgZ produced a suspension under the same conditions
(Fig. S7, ESI†). According to the observation of SEM and TEM
(Fig. 1b and c), trans-AgD assembled into long fibers with a
diameter in the range of 50–200 nm, and built the gel network
by entanglement and interlacing with each other. Compara-
tively, the microstructure of trans-AgZ was composed of short
2.6 Preparation of coordination polymer gel
For a typical procedure, silver nitrate (2.1 mg, 0.0124 mmol)
was added to vial A and dissolved in 0.2 g of DMF. Ligand trans-
D (6.1 mg, 0.0083 mmol) was added to vial B and dissolved in
0.8 g of DMF by heating. Rapid mixing of A and B produced a
trans-AgD organogel (0.8 wt%, Ag(I)/trans-D feeding ratio = 3 : 2).
The gel was confirmed by the vial inversion test. Following a
similar procedure, trans-AgZ precipitation was obtained by
employing the ligand trans-Z.
2.7 Preparation of coordination polymer colloid
For a typical procedure, ligand trans-D (6.1 mg, 0.0083 mmol) thick fibers, which could not form a network (Fig. S7, ESI†).
was added to vial A and dissolved in 0.8 g of DMF by heating. Although the crystal architecture analysis from a single-crystal
The solution was irradiated by 365 nm UV light for 2 min to XRD was not successful, their different morphologies might be
produce cis-D solution. Afterwards, silver nitrate (2.1 mg, deduced by the reported study on similar Ag(I) coordination
0.0124 mmol) dissolved in 0.2 g of DMF was added to vial A polymers, as the meta-pyridine coordination polymer usually
to generate cis-AgD solution, and then 5 mL of deionized water had a closer packing in topology than the para-pyridine
Fig. 1 Gel morphology of trans-AgD. (a) Photograph, (b) TEM image and (c) SEM image of trans-AgD gel. (d) The possible assembly structure of the
trans-AgD coordination polymer.
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Table 1 Gelation ability, CGC and Tgel of trans-AgD in various organic
solvents
whereas it featured obvious thermo-response with increasing
temperature. The process of heating and cooling could trigger
the reversible transition of the solution and gel states because
of the inherent property of noncovalent bonds, where the
temperature of gel–sol transition (Tgel) was recorded during
the heating process. Tgel in DMAc (76 1C) was lower than that in
DMF because the coordination polymer had better solubility in
DMAc (Table 1).
The kinetic growth of the fibers was characterized by DLS
measurements at a concentration below CGC (0.2 wt%). The
mean particle size increased up to 490 nm within 8 min along
with a widened size distribution (Fig. 2). Taking into account its
one-dimensional shape, the fiber length could be much longer.
The single peak series in Fig. 2a suggests almost no secondary
nucleation, inferring that the separation of nucleation and
growth might facilitate a relatively uniform assembly of nano
aggregates to nanofibers. With increasing time, the suspension
became opaque and viscous as the fibers thickened and over-
lapped with each other (Fig. 2b). If a concentration above CGC
was applied, the assembly process resulted in a gel.
Entrya
Solvent
Statusb
CGCc (wt%)
Tgel (1C)
d
1
2
3
4
5
THF
DMSO
DMF
DMAc
MeCN
P
S
G
G
P
—
—
0.33
0.45
—
—
—
93
76
—
a
b
Molar ratio of AgNO3/trans-D
=
3 : 2. P: precipitation; G: gel;
c
S: solution. Critical gelation concentration (CGC), trans-AgD mass
percentage. Gel–sol phase transition temperature, gel concentration
d
0.8 wt%.
coordination polymer, and that favored the one-dimensional
assembly to form longer nanofibers.35 In addition, under
different feeding ratios of Ag(I)/trans-D, the elemental analysis
of all organogel fibers showed a 1 : 1 ratio after the filtration of
solvent (Table S1, ESI†), and that result was also consistent with
the reported research.35 Based on the above-mentioned results
and reference, the possible assembly structure of the coordina-
tion polymer is proposed as shown in Fig. 1d.
All the nano aggregates displayed positive potential (9.3–11.2 mV)
via zeta potential determination since Ag(I) was distributed on the
fiber surface (Table S2, ESI†). Notably, the increase of Ag(I)/trans-D
ratio (from 1 : 1 to 2 : 1) triggered the acceleration of gelation
(Table S2, ESI†), while no gelation occurred at the ratio less than
1 : 1. This implied that the excessive Ag(I) was beneficial to the
nucleation and the fiber growth in the gelation process. From the
TEM observation (Fig. S8, ESI†), with the increase of Ag(I)/trans-D
ratio, the gel fiber became thinner and the amount of gel fiber
increased due to the increase of the nucleation concentration, which
promoted the network density and enhanced the stability of the
gel. To allow direct comparison, an Ag(I)/trans-D ratio of 3 : 2 was
used throughout the experiment.
The solvent also had great influence on the gelation process.
In Table 1, several kinds of solvents are listed as candidates as
they could dissolve in both AgNO3 and ligand, but the trans-
AgD gel could only form in some polar solvents such as DMF
and DMAc. The gels could be obtained at very low concentra-
tions with CGCs as low as 0.33 wt% in DMF and 0.45 wt% in
DMAc, elevating trans-AgD to the category of supergelator.36
The organogel was stable for months at room temperature,
3.2 Driving forces of gelation
The prominent gelation behavior of trans-AgD encouraged us to
investigate the driving forces, and 1H NMR, UV-vis spectro-
scopy, FT-IR, and powder X-ray diffraction (PXRD) were utilized
in the characterization process. As shown in Fig. 3a, after the
addition of Ag(I), the 1H NMR signal of the pyridyl hydrogen
(Hd, He, Hf, Hg) shifted downfield, while the peaks of the phenyl
hydrogen (Ha, Hb, Hc) remained unchanged, proving the coordi-
nation of Ag(I) and the formation of trans-AgD. Comparing the
infrared spectra of the trans-AgD xerogel with those of the ligand
trans-D (Fig. 3b), the additional peak at 1662 cmꢀ1 was assigned to
the stretching vibration of the coordinated pyridyl ring, while the
ꢀ
peak at 1383 cmꢀ1 corresponded with the absorption of NO3
,
also the vibration peak of the pyridyl ring red-shifted from 1011 to
1027 cmꢀ1, reflecting the coordination of pyridine.
Besides coordination, the p–p interaction also played a
critical role in gel assembly. Fig. 3c shows the UV-vis absorption
spectra of trans-AgD in gel and sol states. A maximum absor-
bance peak at 325 nm was revealed in the DMAc solution,
corresponding to the p–p* transition of free azopyridine
Fig. 2 Dynamic measurements: (a) particle size distribution weighted by number in solution over time; (b) the plot of mean particle size and
transmittance against time (0.2 wt% solution concentration and 3 : 2 molar ratio).
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Fig. 3 Characterization of driving forces: (a) 1H NMR, (b) FT-IR, and (c) UV-vis spectra of trans-D and trans-AgD.
chromophores,37 whereas a hypsochromic shift of 27 nm was behavior can be repeated in several cycles, demonstrating that the
found in the gel state, demonstrating the formation of H aggregates network of the gel was self-healable and can reversibly recover after
by p–p stacking interaction.38 Meanwhile, the PXRD pattern of the removal of mechanical stimuli. Furthermore, as the temperature
trans-AgD exhibited a broad peak around 2y = 24.71 (d = 0.36 nm), raised above 90 1C, the G0 and G00 of the gel in DMF declined rapidly
implying the loose p–p stacking in the xerogel (Fig. S9, ESI†). All due to the destruction of the network, and this result was accorded
these results proved that both the p–p interaction and the coordi- with the Tgel measured in DMF, as shown in Table 1 (Fig. S10, ESI†).
nation were the key driving forces of gelation.
3.4 Light response
3.3 Mechanical response and self-healing property
The azo groups in trans-D were incorporated into the main
Thanks to their dynamic cross-linking point, many gels were chain of the coordination polymer through coordination, to
sensitive to mechanical stimuli and were able to ‘‘heal’’ themselves give an example of azopyridine CPG with a unique light
by the reconstruction of their 3D networks after mechanical destruc- response. Under UV irradiation, the photoisomerization of
tion, which enables the gels to resist external damage.7,12,34 In our the azopyridine of trans-D in the DMAc solution occurred as
work, it was found that the trans-AgD gel changed into a turbid sol evidenced through the spectral change (Fig. 5a and b) and
by vigorous shaking. After several minutes of resting, the gel could 1H NMR chemical shift (Fig. S11, ESI†). By prolonging the
be regenerated, and no fluid flowed down during the vial inversion irradiation time, the absorption intensity at 325 nm (p–p*
test (Fig. 6a). The mechano-responsive behavior of the gel was transition) rapidly decreased, and the intensity of the peak at
characterized in detail by the rheological measurements of the gel 435 nm concomitantly increased (n–p* transition). According to
(0.8 wt% in DMF). As shown in Fig. 4a, the gel displayed a soft and these results, we speculated that the ligand transformed from
elastic state under low strain (0.1%), as the storage modulus (G0) the trans-isomer to the cis-isomer (denoted as cis-D) with an
1
was larger than the loss modulus (G00) in the frequency range of equilibrium time of 80 s (Fig. 5a). From the H NMR analysis,
1–100 Hz. As the oscillation strain was higher than 10%, the gel the signal of the hydrogen on the ligand shifted to a higher field
underwent a gel–fluid transition (G0 o G00), showing a typical due to the isomerization, and 84% of the azopyridine turned
thixotropic behavior under mechanical shearing strain (Fig. 4b). into cis-isomer based on the integral calculation of 1H NMR
Dynamic step strain testing was used to characterize the recovery (Fig. S11, ESI†). Upon subsequent visible light irradiation, the
property of the gel. As can be seen from Fig. 4c, when high strain time-dependent spectra of UV-vis (Fig. 5b) and 1H NMR
was applied (90%), the gel was thoroughly disrupted as indicated by (Fig. S11, ESI†) showed an opposite tendency, since cis-D
the lower values of G0 than G00. Once low strain (0.1%) was reapplied, changed back to trans-D; nevertheless, it takes a longer time
the elasticity (G0 4 G00) recovered in a few seconds. This recovery to reach the photoreaction equilibrium (240 s).
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Fig. 4 Storage modulus G0 and loss modulus G00 values from the rheological experiments of (a) frequency sweep (strain = 0.1%) and (b) strain sweep
(frequency = 1 Hz). (c) During the step strain measurement, the alternate step strain switched from low strain (0.1%) to high strain (90%) at a fixed
frequency (1 Hz).
Fig. 5 UV-vis spectra at different irradiation times in DMAc. (a) The trans-D solution under UV light (365 nm); (b) the cis-D solution under visible light;
(c) the trans-AgD gel (0.8 wt% in DMAc) dissolved in solution after UV light irradiation (365 nm); and (d) the cis-AgD solution under visible light.
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When adding Ag(I) to the DMAc solution of cis-D, the cis-AgD
coordination polymer was obtained, and the coordination
between Ag(I) and cis-D was confirmed by the lower-field
1
chemical shift of the pyridine hydrogen in H NMR (Fig. S12,
ESI†). On the contrary to the trans-AgD gel, cis-AgD was found to
be soluble in DMF or DMAc because the cis conformer of the
azo group possessed a larger dipole moment compared to its
symmetrical trans conformer, which favored solubility in polar
solvents.22 It is worth noting that, the photoisomerization rate
of cis-AgD in solution was largely slower than cis-D under the
same irradiation and concentration conditions with a remarkably
prolonged equilibrium time of 45 min (Fig. 5d), which was
also supported by the time-dependent spectra of 1H NMR
(Fig. S12, ESI†). This phenomenon coincided with the character-
istics of the main chain azo supramolecular polymers,39 as the
macromolecular steric hindrance retarded the configurational
transformation of the main chain azo group, thus the isomeriza-
tion of the azopyridine group on the coordination polymer was
‘‘locked’’ by the coordination of Ag(I). Furthermore, from Fig. 5c,
the dynamic isomerization in the trans-AgD gel was so signifi-
cantly hindered that the system was still far from the photo-
stationary state after 40 min of UV irradiation. Besides the
macromolecular effect, we assumed that the trans–cis transforma-
tion of the azopyridine group was further hampered in the nano
assemblies due to the close packing of the coordination polymer,
resulting in the decrease of the isomerization rate.
As a consequence of the trans-to-cis isomerization of the
coordination polymer, upon continuous UV irradiation, the gel
gradually changed to a solution from its surface (Movie S2,
ESI†). However, the corresponding solution was unavailable to
recover its gel state under visible light stimuli (Fig. 6a). Different
from the rapid gelation process in several seconds (Table S2,
ESI†), the cis-to-trans isomerization of cis-AgD proceeded at a very
slow rate to give great amounts of crystal nuclei to grow nano-
fibers, causing the larger aggregation of the coordination polymer
as particles in microscale (Fig. S13, ESI†). Taking advantage of
this feature, the trans-AgD gel could be designed as a smart
UV-patterning material with visible light stability (Fig. 6b). For
example, a layer of gel (0.8 wt%) was covered with a patterned
mask, and then irradiated with UV light for 3 min. After removing
the mask, the gel displayed the corresponding visual patterns.
These patterns were stable under visible light for 2 days as the
solution in the pattern could not convert back to gel. It is well
known that heating can prompt the cis-to-trans isomerization of
the azo motif.40,41 By heating up to Tgel and then cooling, the gel
was regenerated and the patterns were erased. This process was
repeatable, imparting the CPG with the UV-heating rewritable
function.
Fig. 6 Behaviors of the trans-AgD organogel. (a) Photographs of the gel
in response to heating, shaking, UV/vis light, and chemicals. (b) Patterned
images generated by a repeatable write (UV light)-erase (heating–cooling)
process.
competing ligands, causing the disassembly of the nanofibers
and the destruction of the gel. As demonstrated in Fig. 6, after
introducing a small amount of ammonia (0.8 mg), the gel
(0.8 wt% in DMF) quickly changed into a clear solution in
several seconds, since ammonia could compete with ligand
+
trans-D by coordinating with Ag(I) to form the Ag(NH3)2
complex. The resulting solution was still reactive as a further
addition of Ag(I) could regenerate the gel. Another example of
the chemical response was derived from the reaction between
Ag+ and H2S. When engaging hydrogen sulfide gas, the gel
turned into a dark suspension, and the black particles were
ascribed to the precipitation of Ag2S. This process was irrever-
sible since the pyridyl ligands were consumed by the reaction
with H2S, thus the addition of Ag(I) in dark suspension did not
result in a gel (Fig. 6a). The chemical responsive behaviors
described above provided intuitive examples to demonstrate
the multi-sensitivity of the trans-AgD gel.
3.5 Chemical response
The CPGs contain great amounts of metal ions. These ions can
be used as reactive sites and render potential applications to
3.6 Coordination polymer colloid
the gels, such as catalysts,42 sensors,43 absorbents44 and so Besides organogel, the multi-responsive behavior of the coordina-
forth. The as-prepared trans-AgD gel was also chemically tion polymer was also discovered in its dispersing colloidal state.
responsive, as the Ag(I)–azopyridine coordination bonds were Different from the one-dimensional packing of trans-AgD, the
chemically dynamic, and can be destroyed by adding other assembly of cis-AgD favored the amorphous mode in phase
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Fig. 7 Responsiveness of the cis-AgD colloid. (a) Photographs of the preparation and the stimuli-response of the cis-AgD colloid. (b) SEM image of
cis-AgD before visible light irradiation. (c) SEM image of the precipitate after 3 days of visible light irradiation. (d) Illustration of the assembly structural
transformation of the coordination polymer.
separation due to the distorted stacking of the cis isomer,40 giving
Upon light irradiation by an LED lamp (0.1 W mꢀ2), the
dispersing particles in aqueous dispersion. When the solution dispersing colloid of cis-AgD slowly turned into precipitation in
of cis-AgD was poured into deionized water, a yellow colloidal 4 days at room temperature. It can be seen from Fig. 8 that the
dispersion was produced upon slight shaking (Fig. 7a and
Movie S3, ESI†). The cis-AgD colloid consisted of spherical nano-
particles with a diameter of 80–160 nm (Fig. 7b), and its self-
emulsification might come from the Ag(I) ions distributed on
the particle surface as inferred by the positive zeta potential
(+21.0 mV). It is worth noting that both the visible light irradiation
and heating (490 1C) triggered the irreversible coalescence and
precipitation (Fig. 7a). During this process, the coordination
polymer gradually transformed from nanoparticles to fiber-like
aggregates (Fig. 7c), implying that the demulsification was caused
by the change of the assembly structure of the coordination
polymer. The morphological transformation should be attributed
to the cis-to-trans isomerization expedited under visible light and
heating. The mechanism of the light/heating response is illu-
strated in Fig. 7d. It was a prominent example of the controlled
demulsification based on the morphological transformation,
where the isomerization of cis-AgD was greatly slower than ligand
cis-D because of the macromolecular ‘‘lock on’’ effect.
Fig. 8 Precipitate percentages in the cis-AgD colloidal dispersion for
different times with or without light exposure.
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Paper
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In summary, the coordination polymers bearing azo groups on
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This work provides a new strategy to design a multi-responsive
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