Multicolor luminescent supramolecular hydrogels based on cucurbit[8]uril and OPV derivative

Article abstract of DOI:10.1039/c9sm02004g

Supramolecular hydrogels have received considerable attention because of their various fascinating applications. Herein, an alkyl chain-modified oligo(p-phenylenevinylene) (Py-OPV) derivative was synthesized. When assembled with cucurbit[7]uril, its fluor

Full text of DOI:10.1039/c9sm02004g

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Multicolor luminescent supramolecular hydrogels
based on cucurbit[8]uril and OPV derivative†
Cite this: Soft Matter, 2019,
15, 9881
Yong-Zhen Chang, Yong Chen and Yu Liu
*
Supramolecular hydrogels have received considerable attention because of their various fascinating
applications. Herein, an alkyl chain-modified oligo(p-phenylenevinylene) (Py-OPV) derivative was synthesized.
When assembled with cucurbit[7]uril, its fluorescence intensity was enhanced without any change in color.
However, the molecules underwent J-type dimerization when encased in the cavity of cucurbit[8]uril, which
possessed different emission properties based on the monomer. By simply changing the concentration
of cucurbit[8]uril, the fluorescence properties of the assemblies were easily altered. In addition,
luminescent supramolecular hydrogels were constructed with Py-OPV and cucurbit[8]uril based on the
photopolymerization of acrylamide. By embedding the assembly in the polymer to confine it, hydrogels
emitting various tones of blue light were easily constructed. The preparation method of such luminescent
hydrogels provides a new reference method for the construction of specific luminescent materials.
Received 8th October 2019,
Accepted 14th November 2019
DOI: 10.1039/c9sm02004g
rsc.li/soft-matter-journal
Based on the above research results, we tried to make a supra-
molecular hydrogel with various fluorescence properties. Herein,
Introduction
As a unique material between solid and liquid materials, the an alkyl chain-modified amphipathic oligo(p-phenylenevinylene)
hydrogel combines the elasticity of solids and the fluidity of (Py-OPV) derivative, 1-methyl-4-((E)-4-((E)-2-(1-octylpyridin-1-ium-
liquids, and plays an extremely important role in the field 4-yl)vinyl)styryl)pyridin-1-ium iodide, was synthesized. When
1
of soft materials research. Supramolecular assembly, based assembled with cucurbit[7]uril, its fluorescence intensity was
2
–7
on the host–guest interactions,
provides a new way for enhanced without any change in color. However, the molecules
constructing such hydrogels with the advantages of simple proceeded via J-type dimerization when encased in the cavity of
operation, convenient synthesis and applications in the water cucurbit[8]uril, which emitted at different fluorescence wave-
8
–10
phase.
Among them, a multiple emission fluorescent hydro- lengths from the monomer. By simply changing the concen-
gel is a hot topic in the research field. For example, Lin et al. tration of cucurbit[8]uril, the fluorescence properties of the
reported an easy-to-make strong white AIE supramolecular assemblies were easily altered. In addition, a luminescent supra-
1
1
polymer as a color tunable photoluminescent hydrogel. More- molecular hydrogel was constructed with Py-OPV and cucurbit[8]uril
over, Liu et al. reported highly swollen hydrogels with tunable based on the photopolymerization of acrylamide. By embedding the
1
2
white-light emission by supramolecular self-sorting. In addi- assembly in the polymer to constrain it, hydrogels emitting in
tion, Huang et al. reported multifluorescent supramolecular various tones of blue light were easily constructed (Scheme 1).
polymeric hydrogels for encoding, reading, and transforming The preparation method of such luminescent hydrogels provides
1
3
information.
a new reference method for the construction of specific lumi-
Moreover, some reports about achieving multiple luminescence nescent materials.
14–19
emission by supramolecular methods have been reported.
Tao
et al. reported a facile cucurbit[8]uril-based supramolecular
approach to fabricate tunable luminescence materials in
Results and discussion
20
aqueous solutions. However, there are few reports about using
a single fluorophore to achieve multiple fluorescence emissions The amphiphilic guest Py-OPV that has two positive charges
21
for hydrogels. Moreover, our group has reported some supra- and an octyl tail was synthesized in 67.7% yield (Scheme S1,
22,23
molecular hydrogels with a variety of properties and uses.
ESI†), and its CB[8]-mediated supramolecular assembly and
luminescence behaviors were investigated. With the increase
in the concentration, the optical transmittance of Py-OPV at
College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin, 300071, P. R. China. E-mail: yuliu@nankai.edu.cn
480 nm gradually decreased, and the critical aggregation concen-
ꢀ
1
tration (CAC) of Py-OPV was measured as 0.06 mmol L (Fig. S6
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9sm02004g
and S7, ESI†). In addition, the fluorescence of free Py-OPV had an
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Fig. 1 (a) Fluorescence spectra of Py-OPV (10.0 mM) upon the addition of
increasing concentrations of CB[7] in aqueous solutions at 25 1C (lex
=
4
30 nm). (b) Fluorescence spectra of Py-OPV (10.0 mM) upon the addition
of increasing concentrations of CB[8] in aqueous solutions at 25 1C (lex
=
3
91 nm). (c) Photographs of the solutions of Py-OPV (10.0 mM) upon the
addition of increasing concentrations of CB[8] host (0–1.0 equiv.) in water
under UV light at 365 nm.
Scheme 1 (a) Fabrication of smart luminescent materials via host–guest molecules were encapsulated into the hydrophobic cavities of CB[7]
interactions between CB[8] and Py-OPV, which form different supra-
molecular assemblies in water, and the synthesis of the hydrogels.
in the form of monomeric molecules with enhanced fluorescence.
The maximum fluorescence emission peak of the system was still at
(
b) Photographs of the solutions of Py-OPV (10.0 mM) upon the addition
471 nm. When CB[7] reached twice the amount of Py-OPV, the
of increasing concentrations of CB[8] (Py-OPV : CB[8] = 1 : 0, 0.2, 0.5, 1) in
water under UV light at 365 nm.
increase in the fluorescence intensity tended to be gentle, further
demonstrating that CB[7] and Py-OPV were bound in a 2 : 1 mode
(Fig. S12, ESI†). However, only the fluorescence intensity was
emission maximum at 471 nm (Fig. S8, ESI†), and the emission enhanced in this process. There was no change in the emission
intensity decreased with the increase in the concentration, prob- wavelength, indicating that the fluorescence color of the system
ably due to the aggregation-caused quenching (ACQ) effect trig- remained unchanged. Therefore, we considered switching to CB[8]
gered by the pꢁꢁ ꢁp stacking of the Py-OPV skeleton. The host–guest as the host to try adjusting the emission color of the system.
binding and supramolecular assembly of Py-OPV with macrocyclic
Since the larger cavity of CB[8] may accommodate two
compounds were expected to reduce the ACQ effect and thus Py-OPV molecules, the binding behavior of CB[8] with Py-OPV
enhance the fluorescence intensity of Py-OPV. Herein, CB[7] and was also studied. First, a UV-vis absorption experiment was
CB[8] were selected as the host macrocyclic compounds to explore performed with the concentration of Py-OPV in water fixed at
ꢀ
1
their supramolecular assembly and fluorescence enhancement 10.0 mmol L and the proportion of CB[8] gradually increased.
behaviors towards Py-OPV. A phenomenon similar to that of CB[7] was observed, indicat-
First, the concentration of the aqueous solution of Py-OPV ing that CB[8] could also bind Py-OPV via the simple host–guest
ꢀ1
was fixed at 10.0 mmol L , and different ratios of the aqueous interactions (Fig. S15, ESI†). In addition, the Job’s plots gave
1
solution of CB[7] were gradually added to the Py-OPV solution. the stoichiometry of 1 : 1 (Fig. S16, ESI†). In the H NMR
With the addition of CB[7] (0–3 equiv.), the absorption intensity spectral analysis (Fig. 2), when 0.5 equivalent of CB[8] was
of Py-OPV at 392 nm gradually weakened with a red shift. added, the aromatic region of Py-OPV had a significant shifts,
Moreover, the absorption at 440 nm was enhanced (Fig. S9, while the aliphatic region had no obvious change. When
ESI†). This may indicate that Py-OPV molecules were entrapped 1 equivalent of CB[8] was added, the aliphatic region also under-
in the hydrophobic cavities of CB[7], which limited the rotation went significant shift and the aromatic region saw the appearance
of Py-OPV and weakened the pꢁ ꢁ ꢁp stacking. When CB[7] was of new peaks. A possible reason might be that for the CB[7]/Py-
twice the amount of Py-OPV, the enhancement in the absorp- OPV system, CB[7] and Py-OPV were bound in a 2 : 1 ratio, whereas
tion intensity at 440 nm tended to be steady, indicating that two CB[7] molecules were respectively bound to the aromatic
CB[7] and Py-OPV might be bound in a stoichiometric 2 : 1 ratio group and the alkyl chain to form the pseudorotaxane. However,
(
(
Fig. S10, ESI†), which was further verified by the Job’s plots for the CB[8]/Py-OPV system, CB[8] and Py-OPV were bound in the
Fig. S13, ESI†). In the H NMR spectral analysis, it also was n:n mode, where one CB[8] molecule encapsulated the aromatic
1
observed that the binding sites of CB[7] with Py-OPV were not groups of the two Py-OPV molecules into its cavity, and the other
only in the aromatic regions, but also in the aliphatic regions, encapsulated the long alkyl chain to form the supramolecular
leading to the formation of pseudorotaxanes (Fig. S14, ESI†).
polymer. In addition, 2D NMR was also performed to investigate
The fluorescence properties of the system were also studied. With the binding mode between Py-OPV and CB[8]. In the ROESY
the addition of CB[7], the fluorescence intensity of Py-OPV increased spectrum (Fig. S19, ESI†), the protons of CB[8] showed clear
and was eventually steady (Fig. 1a). This was because more Py-OPV NOE correlations with the protons of both the aromatic region
9
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Fig. 3 SEM images of (a) Py-OPV and (b) CB[8]/Py-OPV assembly.
intensity of Py-OPV slightly decreased with the addition of
a-cyclodextrin or b-cyclodextrin but significantly quenched with
the addition of sulfonated calix[4]arene. These phenomena demon-
strated that only CB[8] could be used to regulate the fluorescence
1
Fig. 2 H NMR titration of the CB[8]/Py-OPV system in D
2
O at 298 K.
(
a) Resonance signals for the free Py-OPV (1.0 mM). (b and c) Changes in the behaviors of Py-OPV and to achieve the multicolor fluorescence
resonance signals of Py-OPV in the presence of 0.5 and 1.0 equiv. of CB[8]. emission.
Supramolecular hydrogels have many applications in the
scientific fields of research and have attracted considerable
2
2–32
(
peaks H
indicating that the aromatic and aliphatic components of Py-OPV form a transparent and elastic supramolecular hydrogel based
were both included in the cavity of CB[8]. on the photopolymerization of acrylamide. By simply irradiating a
As shown in Fig. 1b, the fluorescence performance of mixture of the CB[8]/Py-OPV assembly, acrylamide, N,N-methylene-
2
, H
8
) and the aliphatic region (peaks H10, H11) of Py-OPV, attention.
Fortunately, the CB[8]/Py-OPV assembly could
0
0
the CB[8]/Py-OPV system was quite different from that of bis(acrylamide), ammonium persulfate and N,N,N ,N -tetramethyl-
CB[7]/Py-OPV. When CB[8] was gradually added, the fluores- ethylenediamine under sunlight for about five minutes, the mixture
cence intensity of Py-OPV at 471 nm decreased, and a new solution quickly became a transparent hydrogel. In addition,
broad peak appearing at 550–650 nm with the fluorescence the resultant hydrogel showed good viscoelastic properties and
intensity gradually increasing. This change implied that the mechanical properties. Fig. S26a (ESI†) shows the plots of
0
00
fluorescent color of the solution should have a large change storage moduli (G ) and loss moduli (G ) of the hydrogel versus
with the gradual addition of CB[8]. In order to clearly explore the applied range of external strain at a fixed angular frequency
ꢀ
1
0
00
00
the emission color changes during this process, eleven aqueous of 10 rad s . Under the strain from 0.1% to 110%, G and G
0
solutions with the Py-OPV/CB[8] ratios ranging from 1 : 0 to 1 : 1 remained nearly unaltered. Besides, G was larger than G ,
were prepared. As the proportion of CB[8] increased, the demonstrating that the hydrogel network was very stable and
fluorescence color of the system changed gradually in the order the physical structure remained undamaged. When the strain
0
00
of blue, white, light yellow, and light green (Fig. 1c). According kept increasing, G began to decrease while G increased.
0
00
to the above experimental phenomena and the literature However, G was still larger than G even at the strain of
2
0
reports, the mechanism of the multicolor emission may be 2000%, indicating that the hydrogel was very stable. Then,
that when two Py-OPV molecules were encapsulated in the the viscoelastic region was analyzed. 1% strain was chosen to
cavity of CB[8], they underwent a J-type dimerization, resulting investigate the colloidal sensitivity to the oscillation frequency. As
in the bathochromic shift from the blue light of a single Py-OPV shown in Fig. S26b (ESI†), G and G did not undergo any obvious
molecule to the yellow light of the dimer. With the gradual changes within the range of angular frequency (0.1–100 rad s ).
addition of CB[8], a certain proportion of Py-OPV converted to In addition, G stayed larger than G , indicating a good stability of
dimers, and the others remained as monomers. The Py-OPV hydrogel towards the oscillation. Moreover, the hydrogel was strong
monomers emitted blue light, but the dimers emitted yellow enough that it was not easy to destroy, even with scissors.
0
00
ꢀ1
0
00
light. As the Py-OPV : CB[8] ratio reached 1 : 0.2, the mixed
Moreover, the hydrogels remained nearly unchanged even
emission color became white because yellow light and blue after a month in the dark (Fig. S27, ESI†). Besides, the hydro-
light could complement each other. When CB[8] was further gels were stable in the presence of water (Fig. S28, ESI†). First,
added, the emission color turned green. In addition, the the hydrogels were lyophilized to form white solids. Then, they
morphology of the assembly was investigated by SEM. The were soaked in water overnight, leading to the recovery of the
CB[8]/Py-OPV assembly exhibited a regular three-dimensional hydrogels with the initial fluorescence emission. These hydro-
diamond-shaped structure with a side length of about 200 nm gels were also very stable to temperature. In order to test their
(
(
Fig. 3b). Moreover, the free Py-OPV formed a linear structure thermal stability, they were soaked in hot water at 80 1C for up
Fig. 3a), which was quite different from that of CB[8]/Py-OPV. to 10 minutes. To our surprise, their shape and fluorescence
In order to compare the effects of different macrocycles on properties did not exhibit any differences (Fig. S29, ESI†).
the emission color changes, a-cyclodextrin, b-cyclodextrin and
Significantly, the CB[8]/Py-OPV hydrogel emitted various
sulfonated calix[4]arene were used as control compounds, and tones of blue light by changing the ratio between CB[8] and
their effects on the fluorescence properties of Py-OPV were Py-OPV (Fig. 4a–c). Furthermore, the SEM images (Fig. 4d) of
studied (Fig. S20–S22, ESI†). It could be seen that the fluorescence the lyophilized hydrogels provided morphological information
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Scheme 2 Synthetic route of Py-OPV.
dichloromethane (50 mL), and then the reaction mixture was
stirred at 80 1C for 17 h. After cooling, the mixture was filtered,
and the solid was washed with dichloromethane, chloroform
and distilled water. A yellow solid (0.44 g) was obtained in
1
4
6.6% yield after drying under vacuum. H NMR (400 MHz,
CD
3
OD) d: 8.73 (d, J = 6.9 Hz, 2H), 8.52 (d, J = 6.4 Hz, 2H), 8.19
(d, J = 6.9 Hz, 2H), 7.94 (d, J = 16.3 Hz, 1H), 7.80 (d, J = 8.4 Hz,
2H), 7.76 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 6.4 Hz, 2H), 7.59 (d, J =
1
6.5 Hz, 1H), 7.51 (d, J = 16.2 Hz, 1H), 7.34 (d, J = 16.4 Hz, 1H),
+
Fig. 4 (a) Fluorescence spectra of the hydrogels upon the addition of
4.32 (s, 3H) ppm. ESI-MS (m/z): calculated for C H N : 299.4,
2
1
19 2
0, 0.1, 0.2, 0.3, 0.5, and 1.0 equiv. of CB[8] at 298 K (lex = 391 nm). (b) The
found: 299.2.
1931 CIE chromaticity coordinate changes from deep blue to light blue with
Synthesis of Py-OPV. 1-Iodooctane (1 mL, 5.5 mmol) was
the addition of 0, 0.1, 0.2, 0.3, 0.5, and 1.0 equiv. of CB[8] (lex = 391 nm). The
coordinates were (0.1690, 0.2584), (0.1897, 0.2869), (0.2034, 0.2981),
added using a syringe to a solution of A2 (0.20 g, 0.48 mmol) in
(0.2127, 0.3162), (0.2099, 0.3085) and (0.2123, 0.3084). (c) Photographs of N,N-dimethylacetamide (10 mL), and then stirred at 150 1C
the hydrogels upon the addition of 0, 0.1, 0.2, 0.3, 0.5, and 1.0 equiv. of CB[8]
for 2 h. After cooling, the mixture was filtered, and the solid
was washed using N,N-dimethylacetamide and diethyl ether.
under UV light at 365 nm. (d) SEM image of the hydrogel made without CB[8].
(e) SEM image of the hydrogel made with 1.0 equiv. of CB[8].
A yellow solid (0.37 g) was obtained in 67.7% yield after drying
1
under vacuum. H NMR (400 MHz, DMSO-d ) d: 8.98 (d, J =
6
of the hydrogel made without CB[8] as lamellar nanosheets,
while the hydrogel made with 1.0 equiv. of CB[8] were thicker
and bigger lamellar nanosheets with numerous holes. This
illustrated that the addition of CB[8] changed the microstructure
of the hydrogel and further changed the fluorescence properties
of the hydrogel to some extent.
6
4
3
.8 Hz, 2H), 8.89 (d, J = 6.4 Hz, 2H), 8.26 (dd, J = 6.8, 10.0 Hz,
H), 8.08 (dd, J = 8.0, 15.8 Hz, 2H), 7.88 (s, 4H), 7.67 (d, J =
.6 Hz, 1H), 7.63 (d, J = 3.6 Hz, 1H), 4.53 (t, J = 7.6 Hz, 2H), 4.27
(s, 3H), 1.95–1.87 (m, 2H), 1.29–1.25 (m, 10H), 0.87 (t, J = 6.4 Hz,
1
3
3
1
1
6
H) ppm. C NMR (101 MHz, DMSO-d ) d: 152.5, 152.1, 145.2,
44.3, 139.8, 139.5, 136.9, 136.8, 128.8, 124.6, 124.5, 124.1,
23.7, 59.9, 47.1, 31.1, 30.5, 28.4, 28.3, 25.4, 22.1, 13.9 ppm.
2
+
ESI-MS (m/z): calculated for C H N : 206.3, found: 206.2.
Anal. calcd for C H I N ꢁH O (684.11): C, 50.89; H, 5.60;
2
9
36 2
Experimental
2
9
36 2
2
2
All the reagents and solvents were commercially available and used N, 4.09. Found: C, 50.69; H, 5.70; N, 4.00.
as received unless otherwise noted. All aqueous solutions were
prepared in distilled water. Column chromatography was performed
Preparation of hydrogels
on a 200–300 mesh silica gel. NMR spectra were recorded on The aqueous solution of Py-OPV (250 mL, 0.1 mM) was added to
a Bruker AV400 instrument. UV/vis spectra were recorded on a the aqueous solution of CB[8] (0, 25 mL, 50 mL, 75 mL, 125 mL,
Shimadzu UV-3600 spectrophotometer in a quartz cell at 298 K. 250 mL, and 0.1 mM), and then was added with water to 500 mL.
Steady state fluorescence emission spectra were recorded in a Moreover, acrylamide (0.9384 g, monomers), N,N-methylenebis-
conventional quartz cell at 298 K on a Varian Cary Eclipse (acrylamide) (0.564 mg, cross-linkers), ammonium persulfate
0
0
equipped with a Varian Cary single-cell Peltier accessory to control (1.598 mg, photo initiator) and N,N,N ,N -tetramethylethylene-
the temperature. The fluorescence lifetimes were measured by diamine (5 mL, cross-linking accelerator) were dissolved in 6 mL
3
5
time-correlated single photon counting on a FLS920 instrument water. After slightly stirring, each 1 mL of the mixture was
Edinburg Instruments Ltd, Livingstone, UK) with a H pulse added into the above assembly solution. Then, after 5 minutes
(
2
lamp. The SEM images were recorded on a JEOL JSM-7500F of exposure to sunlight, they became transparent and elastic
scanning electron microscope, operating at an accelerating hydrogels.
voltage of 30 keV.
Conclusions
Synthetic route of Py-OPV (Scheme 2)
3
3
Synthesis of A2 . Methyl iodide (0.4 g, 2.5 mmol) was added In summary, a luminescent supramolecular system was
3
4
using a syringe to a solution of A1 (1.80 g, 6.3 mmol) in successfully constructed through the host–guest interactions
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