Polyoxometalate-Based Hybrid Supramolecular Polymer via Orthogonal Metal Coordination and Reversible Photo-Cross-Linking

Article abstract of DOI:10.1021/acs.macromol.9b01825

To enrich the building blocks and functions of supramolecular polymer, functioned inorganic polyoxometalate clusters were introduced through a novel and efficient orthogonal self-assembly strategy. The design principle is based on the organic modification

Full text of DOI:10.1021/acs.macromol.9b01825

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Polyoxometalate-Based Hybrid Supramolecular Polymer via
Orthogonal Metal Coordination and Reversible Photo-Cross-Linking
Jing Yan, Huiya Huang, Zhiliang Miao, Qiuyu Zhang,* and Yi Yan*
Department of Applied Chemistry, School of Science, MOE Key Laboratory of Material Physics and Chemistry under Extraordinary
Condition, Ministry of Education, Northwestern Polytechnical University, Xi’an 710072, China
S
* Supporting Information
ABSTRACT: To enrich the building blocks and functions of
supramolecular polymer, functioned inorganic polyoxometalate
clusters were introduced through a novel and efficient
orthogonal self-assembly strategy. The design principle is
based on the organic modification of polyoxometalates
(POMs) with coordination groups and electrostatic interaction
of POMs with surfactants capped by photo-cross-linkable
groups. Because of the relatively independent nature of these
two interactions, it is possible to prepare POMs-based
supramolecular cross-linked network through coordination−
photo-cross-linking and vice versa, leading to the formation of main-chain-type POMs-based supramolecular polymer networks
and gels. The responsiveness of these two interactions also endows the resulted supramolecular polymer with different
responsiveness to light and coordination. Therefore, orthogonal self-assembly supramolecular polymerization of POMs provides
a facile strategy to improve the processability of inorganic clusters, paving a new avenue toward cluster-based materials and
electronic devices.
energy storage, photo/electrochromism, and magnetism to
industrial catalysis.18−21 Therefore, POMs are potential
candidates for functioned supramolecular polymers.22−24
Taking advantage of their negatively charged characteristic
and organic modification, POMs have been introduced to
supramolecular polymer systems through controlled polymer-
ization and ionic self-assembly.25−27 By using the strategy of
controlled polymerization, one can prepare only side-chain and
POMs-labeled polymers. New polymerization methods,
especially combination of different interactions, should be
explored to prepare different topological polymers. Orthogonal
self-assembly allows the integration of two and more separated
interactions in one system,28,29 which are broadly used to
prepare diverse supramolecular polymers.
Herein, we designed a rigid POMs-based monomer by
integrating the organic modification of POMs with terpyridine
ligands and ionic graft of photo-cross-linkable coumarin
groups. By use of the orthogonal interactions of coordination
and photo-cross-linking, POMs-containing supramolecular
polymers are prepared and well characterized. Because of the
orthogonality and responsiveness of these two interactions, the
prepared supramolecular polymer displays multiple responsive-
ness. Such a kind of strategy can be also extended to other
cluster systems and be potentially applied in different areas
such as chromism, smart catalyst, and so on.
1. INTRODUCTION
Supramolecular polymers are usually constructed from differ-
ent monomers via intermolecular interactions.1−3 Different
interactions such as hydrogen bonding,4 coordination,5
electrostatic interactions,6 host−guest interaction,7 and so on
have been used in supramolecular polymerization. The
flexibility and responsiveness nature of supramolecular
interactions enable supramolecular polymers with applications
in different areas, such as self-healing materials,8 smart
catalysts,9 drug and gene delivery vehicle,10 and so on.
Generally, there are still several problems obstructing the
development of supramolecular polymer systems.11 One is
about the molecular structure and molecular weight. Because
of the possible side reactions of intermolecular/intramolecular
cyclization, linear supramolecular polymers with higher
molecular weight are very difficult to prepare. The other
problem is about the functionalization of supramolecular
polymers. One of the possible solutions to these problems is to
use functioned rigid monomers. To enrich the composition
and functions of supramolecular polymers, functional building
blocks such as metal complexes,12 π-conjugated systems,13
functioned clusters (polyhedral oligomeric silsesquioxane
(POSS),14 polyoxometalates (POMs),15,16 boron cluster,17
etc.), and so on have been introduced into supramolecular
polymers.
Polyoxometalates (POMs) are a kind of functioned
inorganic cluster with multiple composition and well-defined
structure. The varied compositions endow POMs with
properties ranging from fundamental research such as catalysis,
Received: September 2, 2019
Revised: November 20, 2019
© XXXX American Chemical Society
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1.86−1.18 (m, 40H), 0.87 (t, 3H, J = 7.2 Hz). 13C NMR (CDCl3, 100
MHz): 162.44, 161.39, 155.86, 143.67, 128.81, 112.97, 112.77,
112.35, 101.32, 68.65, 63.92, 51.40, 31.88, 29.58, 29.38, 29.23, 28.90,
26.27, 25.89, 22.81, 22.67, 14.13.
2. EXPERIMENTAL SECTION
2.1. Materials. [N(C4H9)4]4[α-Mo8O26], Mn(OAc)3·2H2O, and
[N(C4H9)4]4EDTA were prepared according to the corresponding
literature.30−32 N,N-Dimethylacetamide (DMAc) and acetone were
dried with CaH2 overnight and distilled prior to use. Fe(OTf)2 (98%)
was purchased from Shanghai CIVI Chemical Technology Co., Ltd.,
and ethyl bromoacetate (98%) was purchased from Aladdin. Other
chemicals and solvents were purchased from MACKLIN and used as
received.
Synthesis of Terpyridine Symmetrically Organic-Modified
Polyoxometalate TBA3[TPY-MnMo6] (Monomer 6). Potassium
hydroxide (15.60 g, 0.28 mol, 4.67 equiv), 4-hydroxybenzaldehyde
(7.33 g, 0.06 mol, 1 equiv), and 2-acetylpyridine (13.44 mL, 0.12 mol,
2 equiv) were dissolved in ethanol (100 mL), and then ammonium
hydroxide (150 mL) was added and heated at 50 °C for 24 h.
Adjusting the pH to 7 by dropwise addition of acetic acid, we
collected the formed precipitate and washed with ethanol (20 mL)
three times. The grass green powder was recrystallized twice in a
mixture of chloroform and methanol (CHCl3:CH3OH (v/v) = 1:2),
yielding 8.48 g of 3 as a white crystal; yield 43%. FT-IR (KBr, cm−1):
1610 (νCC), 1583 (νCC), 1522 (νCN), 1394 (νC−C), 793 (γC−H).
1H NMR (DMSO-d6, 400 MHz): 9.85 (s, 1H), 8.75 (d, 2H, J = 4.8
Hz), 8.65 (s, 2H), 8.65 (d, 2H, J = 7.6 Hz), 8.02 (dd, 2H, J = 7.6, 1.2
Hz), 7.79 (d, 2H, J = 8.8 Hz), 7.51 (dd, 2H, J = 7.2, 4.8 Hz), 6,97 (d,
2H, J = 8.4 Hz). Anal. Calcd for C21H15N3O (325.36 g/mol): N,
12.91; C, 77.52; H, 4.65. Found: N, 13.24; C, 77.98; H, 4.34. 13C
NMR (DMSO-d6, 100 MHz): 159.43, 155.95, 155.59, 149.76, 137.86,
128.68, 128.37, 124.89, 121.34, 117.50, 116.68.
Compound 3 (3.00 g, 9.22 mmol, 1 equiv) and finely grinded
potassium carbonate (3.82 g, 27.66 mmol, 3 equiv) were dispersed in
15 mL of anhydrous DMAc and stirred for 10 min at room
temperature, and then ethyl bromoacetate was added dropwise. The
mixture was heated at 60 °C for 24 h. The reaction mixture was
filtered and precipitate in 70 mL of water. After drying at 60 °C in a
vacuum oven, 4 was obtained as a white solid (3.53 g, 93%).33 FT-IR
(KBr, cm−1): 1751 (νCO), 1585 (νCC), 1516 (νCN), 789 (γC−H).
1H NMR (DMSO-d6, 400 MHz): 8.76 (d, 2H, J = 4.8 Hz), 8.67 (s,
2H), 8.66 (d, 2H, J = 8.8 Hz), 8.03 (dd, 2H, J = 7.6, 1.2 Hz), 7.88 (d,
2H, J = 8.4 Hz), 7.52 (dd, 2H, J = 7.2, 4.8 Hz), 7.14 (d, 2H, J = 8.8
Hz), 4.89 (s, 2H), 4.20 (q, 2H, J = 7.2 Hz), 1.24 (t, 2H, J = 7.2 Hz).
13C NMR (DMSO-d6, 100 MHz): 169.07, 161.17, 159.25, 156.06,
155.49, 149.81, 149.37, 137.92, 130.80, 128.72, 128.17, 124.97,
121.39, 117.87, 117.61, 115.85, 65.19, 61.23, 14.55.
1
2
1
2.2. Characterization. D (1H, 13C) and D (1H−1H COSY, H
DOSY) spectra were recorded on a Bruker AVANCE 400 MHz
spectrometer in CDCl3 or DMSO-d6 with tetramethylsilane (TMS) as
internal reference. Diffusion-ordered spectroscopy (DOSY) experi-
ments were taken with “ledbpgp2s1d” and “ledbpgp2s” pulprog.
Electrospray ionization (ESI) mass spectra were acquired with a
Bruker micrOTOF-Q II electrospray instrument. FT-IR spectra were
performed on a Bruker Tensor 27 FT-IR spectrometer equipped with
a DTGS detector (32 scans) with resolution of 4 cm−1 on a KBr
pellet. UV−vis spectra were collected on a Unico UV-2600A
spectrophotometer scanning from 190 to 700 nm. For the UV−vis
titration experiments, each sample was stirred for at least 3 min to
ensure saturated coordination. For the Job’s plot experiment, the total
concentration of monomer 6 or 7 and Fe(OTf)2 was controlled at 0.1
CFe(II)
i
y
j
z
z
z
j
mM, while XFe(II)jXFe(II)
=
was varied from 0.0 to 1.0
CLigand + CFe(II)
k
{
with intervals of 0.1. Each set was stirred for at least 60 min before
UV−vis measurement. For the UV irradiation-induced photo-cross-
linking, three UVSP8 V1209 LED spot light sources (365 nm, 2200
mW/cm2) were applied in different directions of the cuvette with
distance of 0.3 cm (Figure S8). Typically, a 10 μM DMAc solution of
7 was used in the photo-cross-linking experiment. Organic elemental
analysis (C, H, and N) was performed on a Vario EL III elemental
analyzer. Scanning electron microscopy (SEM) images were acquired
on a VEGA 3 LMH TESCANVEGA3LMH electron microscope and
an FEI Helios G4 CX microscope. Transmission electron microscopy
(TEM) images were acquired on an FEI TalosF200X electron
microscope with accelerated voltage of 200 kV without staining.
Supramolecular polymerization before and after UV irradiation can be
monitored by rheological measurements. In this test, frequency sweep
was performed by using a stress-controlled rheometer (AR-G2, TA
Company) with a fixed oscillation stress of 0.5 Pa at 20 °C. The
frequency range is from 0.01 to 25 Hz. The geometry is 20 mm
rough-surface parallel plate with a gap of 200 μm.
Compound 4 (1.50 g, 3.64 mmol, 1 equiv), finely grinded
potassium carbonate (1.00 g, 7.28 mmol, 2 equiv), and tris(hydroxy-
methyl)methylaminomethane (Tris) (0.44 g, 3.64 mmol, 1 equiv)
were suspended in 15 mL of anhydrous DMAc and heated at 50 °C
for 48 h. After reaction, the mixture was concentrated and precipitated
in 60 mL of water. The white precipitate was collected and dried at 60
°C in a vacuum oven to give 5 (1.37 g, 77%). FT-IR (KBr, cm−1):
2.3. Synthesis and Preparation. Synthesis of Coumarin-
Terminated Quanternized Ammonium Surfactant (2). 7-Hydrox-
ycoumarin (1.62 g, 0.010 mol, 1 equiv), 1,12-dibromododecane (5.6
g, 0.017 mol, 1.7 equiv), and potassium carbonate (4.14 g, 0.030 mol,
3 equiv) were suspended in 50 mL of anhydrous acetone. The mixture
was refluxed at 56 °C overnight. After reaction, the mixture was
filtered and washed with dichloromethane. The solvent was removed
by a Rotavap to give the crude product. The crude was purified on
silica gel with petroleum ether and petroleum ether:dichloromethane
(v/v = 2:1) as eluent to give 1 as a white solid (2.23 g, 55%). FT-IR
(KBr, cm−1): 1722 (νCO), 1622 (νCC), 571 (νC−Br), 496 (δC−Br).
1H NMR (CDCl3, 400 MHz): 7.63 (d, 1H, J = 9.6 Hz), 7.36 (d, 1H, J
= 8.4 Hz), 6.83 (d, 1H, J = 8.4 Hz), 6.80 (s, 1H), 6.24 (d, 1H, J = 9.6
Hz), 4.01 (t, 2H, J = 6.8 Hz), 3.40 (t, 2H, J = 6.8 Hz), 1.91−1.23 (m,
20H). 13C NMR (CDCl3, 100 MHz): 162.44, 161.36, 155.93, 143.52,
128.72, 113.03, 112.91, 112.37, 101.31, 68.67, 34.14, 32.84, 29.52,
29.44, 29.33, 28.98, 28.78, 28.18, 25.96.
Compound 1 (1.90 g, 4.64 mmol, 1 equiv) and N,N-dimethyl-
dodecylamine (2.28 g, 10.68 mmol, 2.3 equiv) were dissolved in 50
mL of ethanol and refluxed at 80 °C overnight. The reaction mixture
was concentrated to ca. 5 mL and then precipitated in 100 mL of
diethyl ether to give target surfactant 2 as a white powder (2.89 g,
99%). FT-IR (KBr, cm−1): 1728 (νCO), 1614 (νCC), 1350 (νC−N).
1H NMR (CDCl3, 400 MHz): 7.63 (d, 1H, J = 9.6 Hz), 7.36 (d, 1H, J
= 8.4 Hz), 6.82 (d, 1H, J = 8.8 Hz), 6.79 (s, 1H), 6.23 (d, 1H, J = 9.6
Hz), 4.01 (t, 2H, J = 6.4 Hz), 3.50 (q, 4H, J = 8 Hz), 3.39 (s, 6H),
1
1651 (νCO), 1585 (νCC), 1518 (νCN), 791 (γC−H). H NMR
(DMSO-d6, 400 MHz): 8.76 (d, 2H, J = 4.8 Hz), 8.68 (s, 2H), 8.66
(d, 2H, J = 8.4 Hz), 8.03 (t, 2H, J = 7.6 Hz), 7.91 (d, 2H, J = 8.4 Hz),
7.53 (dd, 2H, J = 7.2 Hz), 7.28 (s, 1H), 7.18 (d, 2H, J = 8.4 Hz), 4.81
(t, 3H, J = 5.6 Hz), 4.59 (s, 2H), 3.62 (d, 6H, J = 5.6 Hz). 13C NMR
(DMSO-d6, 100 MHz): 168.21, 159.07, 156.01, 155.47, 149.76,
149.24, 137.86, 130.84, 128.72, 124.93, 121.36, 117.80, 116.07, 67.57,
62.43, 60.44.
Compound 5 (0.500 g, 1.028 mmol, 3.5 equiv), [N(C4H9)4]4[α-
Mo8O26] (0.633 g, 0.294 mmol, 1 equiv), and Mn(CH3COO)3·2H2O
(0.118 g, 0.440 mmol, 1.5 equiv) were dissolved in 15 mL of
anhydrous DMAc. The mixture was heated at 80 °C for 24 h. The
reaction mixture was filtered while hot to remove a black solid, and
the orange filtrate was exposed to diethyl ether vapor. After 24 h,
monomer 6 was obtained as an orange cubic crystal (0.769 g, 84%).34
FT-IR (KBr, cm−1): 1695 (νCO), 941, 922, 905 (νMoO), 667
1
(νMo−O−Mo). H NMR (DMSO-d6, 400 MHz): 64.16 (b, 12H), 8.78
(d, 4H, J = 3.6 Hz), 8.70 (s, 4H), 8.67 (d, 4H, J = 8 Hz), 8.03 (t, 4H, J
= 7.6 Hz), 7.91 (d, 4H, J = 4.4 Hz), 7.52 (dd, 4H, J = 6.8, 4.8 Hz),
7.17 (s, 4H), 4.92 (s, 4H), 3.15 (t, 24H, J = 7.6 Hz), 1.55 (quint,
24H), 1.30 (sext, 24H, J = 7.2 Hz), 0.92 (t, 36H, J = 7.2 Hz).
Preparation of Orthogonal Self-Assembly Monomer 7. A
solution of monomer 6 (0.356 g in 15 mL of MeCN) was added
dropwise to the solution of excess amount of surfactant 2 (0.844 g,
1.350 mmol, 10 equiv) in a mixture of CHCl3 and MeCN (1:3 (v/v),
B
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Scheme 1. Schematic Illustration for the Orthogonal Self-Assembly Strategy To Prepare POMs-Containing Supramolecular
Polymer via Combination of Coordination and Photo-Cross-Linking
60 mL). A light orange precipitate formed immediately. The mixture
was stirred for 4 h, filtered, and dried under vacuum to give monomer
7 as a light orange powder (0.19 g, 40%).35,36 FT-IR (KBr, cm−1):
1730 (νCO), 1614 (νCC), 943, 918, 901 (νMoO), 671 (νMo−O−Mo).
1H NMR (DMSO-d6, 400 MHz): 64.18 (b, 12H), 8.77 (d, 4H, J = 3.6
Hz), 8.70 (s, 4H), 8.66 (d, 4H, J = 7.6 Hz), 8.02 (t, 4H, J = 7.2 Hz),
7.98 (d, 3H, J = 9.6 Hz), 7.91 (s, 4H), 7.61 (d, 3H, J = 8.4 Hz), 7.51
(t, 4H, J = 6.4 Hz), 7.16 (s, 4H), 6.94 (s, 3H), 6.92 (d, 3H, J = 8.8
Hz), 6.27 (d, 3H, J = 9.2 Hz), 4.92 (s, 4H), 4.04 (t, 6H, J = 6.0 Hz),
3.18 (t, 12H), 2.96 (s, 18H), 1.77−1.10 (m, 120H), 0.83 (t, 9H, J =
6.0 Hz). Anal. Calcd for C159H226N11MnMo6O37·6H2O (3514.13 g/
mol): N, 4.25; C, 52.72; H, 6.62. Found: N, 3.84; C, 53.14; H, 6.51.
ESI-MS (negative mode, m/z): 628.2, 1437.0, 2370.2.
The monomer structure and the corresponding synthetic
approach are shown in Figure 1. Generally, the easily
organically modified Anderson-type cluster [MnMo6O18]3−
was used, which was synthesized from the esterification
reaction between tris(hydroxymethyl)-modified TPY (5),
TBA4[Mo8O26], and Mn(OAc)3.34,40 Then, coumarin-termi-
nated quaternized ammonium (2) was electrostatically linked
to the as synthesized TPY-MnMo6 (monomer 6) to prepare
the target monomer 7 with the following characteristics: (i)
Rigidity. Based on the single-crystal structure of the Anderson-
type [MnMo6O18]3− cluster and TPY, the diameter of the
disklike POMs (0.86 nm)35 is close to the width of TPY (ca.
0.94 nm),34,41 which makes this cluster a rigid rod. The two
TPY groups symmetrically attached to the Anderson cluster
through covalent bond behave like a ditopic ligand,42 which
can stoichiometrically form 1:2 coordination complexes with
divalent metal ions, such as Fe2+, Zn2+, and so on. (ii)
Flexibility. The three coumarin-terminated surfactants were
electrostatically interacted with the negatively charged
Anderson cluster, allowing their free movement around the
POM cluster and endowing the monomer and resulted
supramolecular polymer with good flexibility and solubility.35
(iii) Orthogonality and reversibility. The coordination between
TPY and metal ions is independent of the photo-cross-linking
property of coumarin groups. Meanwhile, the reversibility of
coordination and photo-cross-linking also guaranteed the
resulted supramolecular polymer a fully reversible and
stimulus-responsive system.
3. RESULTS AND DISCUSSION
3.1. Monomer Design and Synthesis. Free radical
polymerization37,38 and ring-opening metathesis polymer-
ization (ROMP)27 have been used to prepare POMs-
containing polymers. However, POMs were found to be
potential radical quenchers,39 which gave the resulted polymer
with Đ as large as 1.8 even in the strategy of controlled radical
polymerization such as conventional atom transfer radical
polymerization (ATRP).39 To avoid the possible radical
quenching as well as the use of the precious Grubbs catalyst,
a combined strategy of orthogonal coordination and photo-
cross-linking was proposed. Therefore, we designed a
polyoxometalate-based supramolecular monomer bearing
coordination terpyridine (TPY) symmetrically modified on
the equatorial position and photo-cross-linkable coumarin
groups electrostatically attracted on the axial positions, as
shown in Scheme 1. There are several characteristics of this
monomer: (i) the introduction of rigid POMs cluster can avoid
the cyclization in conventional organic supramolecular
polymer and enable supramolecular polymer with higher
molecular weight; (ii) the functioned POMs can also enrich
the functionalization of supramolecular polymer.
The successful preparation of target monomer 7 was
confirmed by 1H NMR, FT-IR, ESI-MS, and organic elemental
analysis. As shown in Figure 2B, both the complete
disappearance of signals from tetrabutylammoniums (TBAs)
and the preservation of signals from TPYs as well as the
1
integration of corresponding H NMR peaks confirmed that
three coumarin-terminated surfactants were attached to the
C
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Figure 1. Synthetic approach for monomer 7.
1
POMs cluster. The detailed H NMR assignment is shown in
was used due to the following merits: (i) its coordination with
terpyridine can be visualized as a purple color arising from the
characteristic metal-to-ligand charge transfer (MLCT) absorp-
tion at 578 nm;43 (ii) its relatively higher binding constant
with TPYs.44 As expected, UV−vis titration of monomers 6
and 7 with Fe(OTf)2 displayed gradually increasing of the
absorption at 578 nm, accompanied by the color change from
colorless to purple for both monomers 6 and 7 (as shown
insets in Figure 3A,B). It should mentioned that the addition
of cationic Fe(II) to the solution of negatively charged POMs
do not lead to any precipitation, which may imply no specific
Coulomb interaction between these two charged species. To
further identify the stoichiometric ratio between Fe(II) and the
ditopic monomer 6 or 7, Job’s plot experiments were carried
out. As shown in Figure 3C,D, both monomers 6 and 7
displayed a coordination stoichiometric ratio of 1.0 with
Fe(II),45,46 which confirmed that Fe(II) quantitatively
coordinates with terpyridine groups, without any other
charge−charge interaction or coordination interactions with
Figure 2B, which was in good agreement with the
corresponding 1H−1H COSY (Figure S2). 1H NMR
integrations from TPYs and the coumarin surfactants also
indicated the completely exchange of TBAs by coumarin-
terminated surfactants. The POMs cluster kept intact during
the cation exchange process as indicated by the characteristic
1
broad peak around 64 ppm in H NMR, attributing to the
three −CH2− groups attached to the [MnMo6O18]3− cluster
(Figure 2B, inset) as well as the characteristic triplet peaks
around 1000 cm−1 and the singlet at 671 cm−1 in the FT-IR
spectrum (Figure 2C). The ESI-MS signals at m/z of 2370.2
and 1064.5 (as shown in Figure 2D and Figure S3) matched
well with the molecular structure of monomer 7, which is also
in good agreement with the corresponding organic elemental
analysis result. As shown in Figure S4, the UV−vis spectrum of
monomer 7 displayed both absorption from coumarin-
terminated surfactant at 322 nm and absorption from the
terpyridine group in monomer 6 at 285 nm. The relatively
separated absorption peaks allowed monitoring the orthogonal
self-assembly process by UV−vis spectra.
1
other groups. H NMR titration experiments were used to
track the coordination process. As shown in Figure 3E, with
the gradual addition of 1.5 equiv of Fe(II) to monomer 7,
signals for the protons from terpyridine groups became broad
and gradually moved to upfield,47 especially for protons H16,
H17, and H18 (Figure 3F), which were due to the shielding
effect of the metal complex. After addition of 1.5 equiv of
3.2. Coordination Induced Supramolecular Polymer-
ization and Its Reversibility. The as-prepared TPY
symmetrically modified monomers 6 and 7 can be treated as
ditopic ligands, which can form 1-D coordination polymers
with different metal ions, such as Zn2+, Fe2+, and Ru2+. Fe2+
D
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Figure 2. (A) Chemical structure of monomer 7 and corresponding labeling of protons. (B) 1H NMR spectra of coumarin-terminated surfactant 2,
monomer 6, and monomer 7 in DMSO-d6; the inset is enlarged from 61 to 67 ppm for monomers 6 and 7. (C) FT-IR spectra of coumarin-
terminated surfactant 2, monomer, 6 and monomer 7. (D) ESI-MS spectrum of monomer 6 and the corresponding peak assignments.
1
Fe(II), the H NMR spectrum became constant. It should be
noted that protons from coumarin groups do not change
of Fe(II), almost no obvious supramolecular polymerization
was observed, since the D do not change too much (Figure
S5); in the case of 2.0 mM, the D decreased obviously
indicating the formation of supramolecular polymer; upon
further increase of the concentration to 10 mM, the formed
supramolecular polymer became a gel-like state, indicating the
molecular weight of the resulted supramolecular polymer is
high enough (Figure 4C). However, no D was obtained as it is
not suitable for the DOSY experiment. Therefore, we can
estimate that the critical polymerization concentration of
monomer 6 is around 2.0 mM.41,50
The formation of the 1-D coordination supramolecular
polymer can also be confirmed by microscopy. As shown in
Figure 4A, by drop-casting 1 mg/mL DMF solution of
monomer 7 on a silica wafer, spherical aggregates constructed
from nanofibers with ca. 40 nm width were observed. Addition
of 1.0 equiv of Fe(II) to solution of monomer 7 resulted in the
formation of fibers with lengths as long as 20 μm and widths of
150 nm, which further proved the formation of the 1-D
coordination polymer (Figure 4B). In the case of 5 mM
monomer 7, a supramolecular organogel was formed after
addition of 1.2 equiv of Fe(II). The SEM image (Figure 4C)
revealed that the gel network was composed of longer fibers
with length of several hundred micrometers.
1
during the addition of Fe(II). The H NMR results further
demonstrated that the only interaction between monomer 6 or
monomer 7 and Fe(II) is the coordination between terpyridine
and Fe(II); no interaction between polyoxometalate cluster or
coumarin groups and Fe(II) was observed.
The formation of 1-D supramolecular coordination polymer
can be proved by the increasing of the hydrodynamic radius
and decreasing of the diffusion coefficient. As shown in Figure
S5, for monomer 6 with concentration of 2 mM, the diffusion
coefficient (D) attributed to the POMs cluster decreased from
8.59 × 10−11 to 3.97 × 10−11 m2/s after the addition of 1.2
equiv of Fe(II),48 while signals for the TBAs counterions and
solvents (DMSO, H2O) do not change during the supra-
molecular polymerization process. All these data indicated that
the supramolecular polymerization happened between the
polyoxometalate clusters driven by coordination. As supra-
molecular polymerization is a concentration-dependent proc-
ess,12,49 a critical concentration of polymerization is estimated
by the DOSY experiment. As shown in Figure S6, different
concentrations of monomer 6 from 0.5 to 10 mM were used to
prepare the supramolecular polymer. For a lower concentration
of 0.5 mM, although the color changed to purple after addition
E
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Figure 3. UV−vis titration spectra for monomer 6 (A) and monomer 7 (B) with Fe(II) and the corresponding Job’s plot for monomer 6 (C) and
monomer 7 (D); insets are the corresponding images. (E) 1H NMR spectra of monomer 7 with different equivalent of Fe(II) in DMSO-d6. (F) 1H
NMR spectra for monomer 7 before (bottom) and after (top) the formation of 1-D coordination polymer with Fe(II).
Figure 4. SEM images of monomer 7 in DMF (1.0 mg/mL) (A), 1-D coordination polymer formed by monomer 7 (2 mM) and 1.0 equiv of
Fe(II) (B), and supramolecular organogel formed by monomer 7 (5 mM) and 1.2 equiv of Fe(II); the sample was freeze-dried before SEM (C).
Insets are the corresponding optical images.
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Figure 5. Optical images (A) and corresponding Tyndall scattering (B) of the supramolecular polymer solution during the addition of different
amount of TBA4(EDTA). (C) UV−vis spectra of the depolymerization process by adding different ratio of TBA4(EDTA); the inset is the
relationship between absorbance at 578 nm and the equivalent of TBA4(EDTA).
Figure 6. UV−vis spectra of (A) photoinduced dimerization of monomer 7; inset is the time-dependent kinetic process. (B) Photoinduced
depolymerization; inset is the corresponding kinetic plot.
One of the advantages of such a coordination system is the
reversibility. By adding competitive ligand TBA4(EDTA)32 to
the formed 1-D supramolecular polymer, the characteristic
purple color turned to colorless (Figure 5A), which implied the
dissociation of the coordination interaction between terpyr-
idine and Fe(II). By gradual addition of TBA4(EDTA), UV−
vis absorption at 294, 326, and 578 nm showed an opposite
trend toward the coordination process (Figure 5C), especially
the completely decrease of absorbance at 578 nm. Meanwhile,
the Tyndall scattering decreased dramatically after the addition
of TBA4(EDTA) (Figure 5B) also indicated the depolymeriza-
tion of the 1-D polymer. Such a coordination−dissociation
process can be repeated at least for four cycles (as shown in
Figure S7).
3.3. Photoinduced Polymerization and Orthogonal
Self-Assembly. Besides TPY as coordination site, the
terminal coumarin groups can also undergo photoinduced
dimerization to give cross-linked supramolecular polymer.51,52
As shown in Figure 6A, after the solution of monomer 7 was
irradiated with 365 nm UV light, the UV−vis absorbance at
287 and 314 nm gradually decreased with increasing
irradiation time, indicating the dimerization of coumarin
groups. The photoinduced dimerization process achieved a
photostationary state within 400 min with a dimerization
degree of 90%.52 During the photoinduced dimerization,
polymerization of monomer 7 can be expected, as indicated by
the increasing of Tyndall scattering (Figure S9).53 Meanwhile,
the photoinduced dimerization can also be depolymerized by
irradiating with 254 nm UV light. As shown in Figure 6B, the
depolymerization can be achieved within 30 min. Such a
photoinduced dimerization and depolymerization process is
reversible for at least four cycles, as shown in Figure S10. It
should be noted that during the UV irradiation process no
absorbance around 700 nm was observed, indicating that the
POMs cluster kept their characteristic structure and
composition.33
Because of the nature of orthogonal self-assembly, the
coordination and photoinduced dimerization could be
operated separately and can achieve corresponding reversibility
under a suitable stimulus, which can tune the reversible
formation and depolymerization of supramolecular polymers.
For example, in the sequence of coordination−photo-cross-
linking, monomer 7 was first coordinated with Fe(II) to give a
homogeneous 1-D coordination polymer solution (Figure 7A);
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Figure 7. UV−vis spectra of (A) coordination and (B) photoinduced dimerization after coordination. (C) FT-IR spectra of monomer 7, after
coordination with Fe(II), and further photoinduced dimerization, from top to bottom, respectively. (D) UV−vis spectra of the dedimerization
process. (E, F) SEM images of the orthogonal supramolecular polymer prepared in the sequence of coordination−photo-cross-linking.
further irradiation with 365 nm UV light led to the cross-
linking of the terminal coumarin groups, which resulted the
formation of orthogonal supramolecular networks (Figure 7B).
The coordination and photodimerization can be confirmed by
corresponding FT-IR spectra, especially the characteristic
coordination peaks at 1162−1266 cm−1 and corresponding
photodimerization signals around 1724 and 1701 cm−1 (Figure
7C).54 The peak at 1730 cm−1 shifted to 1724 cm−1 with a
relative decrease of intensity, and the peak at 1714 cm−1
shifted to 1701 cm−1 with relative increase of intensity, which
is in good agreement with the feature of coumarin
dimerization.55 Such kinds of supramolecular networks can
be disassembled by either 254 nm photoirradiation-induced
dedimerization or addition of TBA4(EDTA)-induced decoor-
dination, as indicating by corresponding UV−vis spectra
(Figure 7D). Interestingly, the mechanical property of the
monomer was improved during the orthogonal supramolecular
polymerization process. As shown in Figure S20, the storage
modulus (G′) is much larger than the loss modulus (G″) for
the coordination polymer, and both of them are independent
of the frequency, showing the formation of supramolecular gel.
After photo-cross-linking of the coumarin groups, both G′ and
G″ increased, which may indicate the formation of the cross-
linked network.
The nature of the orthogonal self-assembly ensures that such
supramolecular network can be also prepared in an alternate
sequence of coordination−photo-cross-linking. SEM images
revealed that the 1-D fiber-like structures were glued together
after photo-cross-linking (Figure 7E,F).
4. CONCLUSIONS
In summary, herein we demonstrate the successful application
of orthogonal self-assembly in the preparation of polyoxome-
talate-based hybrid supramolecular network through coordi-
nation and photodimerization. Such a strategy can be extended
to other types of organically modified polyoxometalates and
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clusters, which may be useful in the preparation of supra-
molecular materials, such as photochromism devices, smart
catalysts, and so on. For example, taking advantage of the metal
coordination as well as the reversible multielectron redox
process in POMs, the introduction of different metal ions can
enrich the colors of POMs-based chromism device as well as
new catalysis centers. Meanwhile, the orthogonal interaction
also endows the supramolecular polymer with suitable
processability, which provides a new avenue toward clusters-
based electronic devices.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.9b01825.
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2D NMR spectra, ESI-MS, UV−vis spectrum, and other
materials (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: yanyi@nwpu.edu.cn (Y.Y.).
*E-mail: qyzhang@nwpu.edu.cn (Q.Z.).
■
ORCID
Qiuyu Zhang: 0000-0002-4823-5031
Yi Yan: 0000-0003-4119-9047
Author Contributions
J.Y. and H.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the funding support from NSFC
(21975205, 21504068), Fundamental Research Funds for the
Central Universities (3102017jc01001, 3102018jcc040,
310201911cx031), Fundamental Research Program of Taicang
(TC2018JC01), Open Project of State Key Laboratory of
Supramolecular Structure and Materials (sklssm2019031), and
the Key Laboratory of Green Pesticide and Agricultural
Bioengineering (2017GDGP0101). The authors also thank
Prof. Liping Cao at the College of Chemistry and Materials
Science of Northwest University for his help with ESI-MS and
Prof. Kaiqiang Liu at the School of Chemistry and Chemical
Engineering of Shaanxi Normal University for his kind help on
rheological tests. The authors also thank the Analytical and
Testing Center of Northwestern Polytechnical University for
TEM and SEM testing.
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