Responsive Self-Assembly of Supramolecular Hydrogel Based on Zwitterionic Liquid Asymmetric Gemini Guest

Article abstract of DOI:10.1002/chem.201801321

A low-molecular-weight supramolecular hydrogel has been fabricated based on host–guest interactions between β-cyclodextrin (β-CD) and an asymmetric gemini zwitterionic liquid (ZIL) containing an azobenzene and bis(trifluoromethanesulfonyl)imide. Reversible sol–gel phase transitions were triggered by light and temperature. The binding stoichiometry, mainly noncovalent interactions in the hydrogel, photo- and thermoresponsive mechanisms, and mode of inclusion in the complex were studied in detail by NMR spectroscopy, UV/Vis spectroscopy, isothermal titration calorimetry, and control experiments. Interestingly, the β-CD in the complex can be photo-manipulated to shuttle along the guest molecule, accompanied by a change in the circular dichroism signals. Reversible switching of the conductivity accompanies the sol–gel phase transition, which demonstrates the stability of the supramolecular hydrogel and its potential application in multi-stimuli-responsive electric sensor materials.

Full text of DOI:10.1002/chem.201801321

A Journal of
Accepted Article
Title: Responsive Self-Assembly of Supramolecular Hydrogel Based on
Zwitterionic Liquid Asymmetric Gemini Guest
Authors: Aoli Wu, Panpan Sun, Na Sun, and Liqiang Zheng
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201801321
Link to VoR: http://dx.doi.org/10.1002/chem.201801321
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Responsive Self-Assembly of Supramolecular Hydrogel Based on
Zwitterionic Liquid Asymmetric Gemini Guest
Aoli Wu, Panpan Sun, Na Sun, and Liqiang Zheng*
Abstract: Low-molecular-weight supramolecular hydrogel was
fabricated based on host-guest interaction between β-cyclodextrin
(β-CD) and asymmetric Gemini zwitterionic liquid (ZIL) which
contains azobenzene (Azo) and bis(trifluoromethanesulfonyl)imide
(TFSI-). Reversible sol-gel phase transition could be triggered by
light and temperature. The binding stoichiometry, main non-covalent
interactions in hydrogel, photo and thermal responsive mechanisms,
and inclusion complex mode were studied deeply by NMR
spectroscopy, UV-vis spectroscopy, isothermal titration calorimetry
(ITC) and control experiments. Interestingly, the β-CD in guest
molecule can be photo-manipulated to shuttle accompanied with the
change of circular dichroism (CD) signals. The reversible switches of
conductivity behind the sol-gel phase transition indicating the
stability of supramolecular hydrogel and their potential applications
in multi-stimuli responsive electric sensor materials.
between β-CD and Azobenzene (Azo) has also been extensively
studied based on the mechanism that the isomerization of Azo
can be reversibly tuned by UV and visible light, which induces
the changes of assembly modes and material properties.[6]
Zwitterions with both cation and anion tethered have attracted
much attention not only because ionic compounds can be easily
incorporated through electrostatic interaction to obtain functional
zwitterionic liquids (ZILs), but also because the ZILs combine
the excellent physicochemical properties of ionic liquids (ILs)
such as nonflammability, chemical and thermal stabilities, and
high ionic conductivity.[7] Based on the designability of ILs,
responsive functional groups such as Azo, ferrocene (Fc),
carboxyl and acrylamide are easily incorporated into the
structure to fabricate multi-stimuli responsive supramolecular
hydrogels. Yan et al. prepared gel based on polymer host
containing β-CD and ZIL containing Fc and TFSI-.[8]
Temperature, anion-exchange, and redox reactions are able to
induce sol-gel phase transition. Most of the researchers mainly
focus on the sol-gel transition behavior, specific responsive
mechanism and other performance changes behind the phase
transition phenomena have been rarely studied in-depth. Instead,
supramolecular host-guest hydrogels formed by small molecules
are more conducive for us to study the non-covalent interactions
in the system.
Herein, supramolecular hydrogel with photo and thermal
responsiveness has been fabricated by using β-CD and
reasonable designed asymmetric Gemini type ZIL which
contains two kinds of functional guest groups Azo and TFSI-.
The stoichiometric ratio of inclusion complex and the main non-
covalent interactions present in the system were deeply
investigated. It is noteworthy that the complex pattern between
β-CD and Azo can be manipulated by UV light irradiation,
accompanied with the changes of Cotton effect. Due to the
intrinsically conductive characteristic of ZIL, the supramolecular
hydrogel paves a convenient way on responsive electrical
sensing materials.
Introduction
Supramolecular gels which fabricated based on non-covalent
interactions such as hydrogen bonding, host-guest, π-π stacking,
and metal-coordination have attracted increasing attention due
to their potential applications in smart materials, biochemistry,
electrochemistry, sensors, and drug delivery.[1] Such non-
covalent interactions enable molecules cross-link dynamically
which is beneficial to fabricate intelligent gels that respond to
light, temperature, pH, magnetic field and other stimulus
signals.[2] Due to remotely controlled without pollution to the
system, light and temperature are more competitive as external
stimuli to modulate supramolecular gels reshaped and
reprocessed, giving rise to non-destructive responsive
materials.[3]
It is noteworthy that, highly selective molecular recognition
endows host-guest gels valuable properties that subtle structural
changes on the building blocks can significantly influence the
macroscopic performances of the system. Cyclodextrins,
pillararenes, cucurbiturils, and crown ethers are common
macrocyclic host molecules. Whereas, cyclodextrins are the
most widely used in responsive supramolecular gels.[4] It has
been reported that bis(trifluoromethanesulfonyl)imide (TFSI-) can
form inclusion complexation with β-cyclodextrin (β-CD)
accompanied by temperature responsiveness due to relatively
high combine energy.[5] In addition, host-guest interaction
Results and Discussion
Molecular structure selection and gel preparation
To fabricate low-molecular-weight supramolecular host-guest
hydrogel, asymmetric ZIL denoted as AzoIPS-LiTFSI containing
two different guest molecules has been masterly designed and
synthesized (Scheme 2). AzoIPS-LiTFSI and β-CD were added
in water at designed compositions. Then the mixture was heated
to a homogeneous solution. After cooling down to room
temperature, the orange hydrogel was formed instantaneously
and confirmed by the inverted test tube method (Figure 1a).
Interestingly, sol-gel reversible phase transition can be achieved
by light irradiation and thermal treatment (Figure 1b and 1c). In
order to prove the importance of this structure design
[a]
A. Wu, P. Sun, N. Sun, Prof. Dr. L. Zheng
Key Laboratory of Colloid and Interface Chemistry
Shandong University, Ministry of Education
Jinan, 250100, P. R. China
E-mail: lqzheng@sdu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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(asymmetric ZIL), comparative tests were fabricated. As shown
in Figure 1d and 1e, only suspension can be observed in the
absence of AzoIPS or LiTFSI. It can be concluded that the
coexistence of two kinds of guest molecules is necessary for the
preparation of supramolecular hydrogel. The microstructure of
the xerogel was detected by SEM (Figure S1), in which loose
porous structures were constructed by stacked blocks.
The viscoelasticity of prepared hydrogel was detected by
stress sweep mode and oscillatory sweep mode. As shown in
Figure 2a, storage modulus (G’) is bigger than the loss modulus
(G’’) before the critical stress value, and both moduli are
independent of the applied stress. Further increase of shear
stress induces G’ and G’’ decrease rapidly. The oscillatory data
(Figure 2b) also show that G’ is thoroughly bigger than G’’. In
addition, G’ and G’’ are independent of frequency. The
experimental results indicated that the system actually formed a
cross-linked hydrogel.[9]
The signals of Hd and Hc (7.7-7.8 ppm) show splitting and upfield
shifts, while the signals of other aromatic protons (7.4-7.5 ppm)
show splitting and downfield shifts, which indicating the host-
guest interaction between β-CD and Azo.[10] The overall
chemical shifts for the protons of AzoIPS-LiTFSI reach the
maximum at a molar ratio of 3:1, indicating 3:1 complexation of
β-CD and AzoIPS-LiTFSI. In addition, the chemical shifts of the
fluorine signals in the TFSI- anion have also gradually moved
upfield with the molar ratio of β-CD/AzoIPS-LiTFSI increased
(Figure 3b, 19F NMR spectra). It can be determined that the
inclusion complexation between β-CD and TFSI- induces
changes in the chemical environment of fluorine atoms (TFSI- in
D2O and TFSI- in β-CD). Compared to D2O, a less polarized
environment of β-CD cavity increases the shielding effect and
causes the fluorine signal shift upfield. Similarly, it was only
when the molar ratio was 3:1 that the fluorine of TFSI- showed
maximum upfield shift, which is consistent with the phenomenon
1
of H NMR. It is reasonably assumed that three β-CDs can be
connected by one AzoIPS-LiTSFI to form inclusion complexes.
According to the previous reports, β-CD tends to form inclusion
complexes with TFSI- or Azo with a 1:1 stoichiometric ratio.[11]
Interestingly, the chemical shifts of protons in AzoIPS almost
unchanged after the molar ratio of β-CD:AzoIPS increasing to
2:1 (Figure S2), which can be inferred that one AzoIPS was
combined by two β-CDs.
Figure 1. Preparation of supramolecular hydrogel (a) β-CD/AzoIPS-LiTFSI
(3:1) and their disassembly induced by (b) UV light and (c) temperature.
Comparative tests (d) β-CD/LiTFSI (1:1); (e) β-CD/AzoIPS (2:1); (f) β-
CD/C10IPS-LiTFSI (3:1); (g) β-CD/AzoIPS-LiCl (3:1). The concentration of β-
CD was fixed at 200 mM.
Figure 3. (a) Partial 1H NMR spectra (molecular structure marked with red) for
different molar ratios of β-CD to AzoIPS-LiTFSI (from 0:1 to 4:1). (b) 19F NMR
spectra (using trifluoroacetic acid as an internal reference) for various molar
ratios of β-CD to AzoIPS-LiTFSI (from 0:1 to 4:1). All the samples were
detected in D2O at 25.0 oC. The concentration of AzoIPS-LiTFSI was fixed at 2
mM.
Isothermal titration calorimetry (ITC) was used to further
investigate the binding stoichiometry of building blocks (Azo and
TFSI-) with β-CD and the thermodynamic mechanism behind the
supramolecular self-assemble. As can be seen from Figure 4a,
two binding sites were observed when β-CD was added
dropwise into the aqueous AzoIPS-LiTFSI solution. The first
titration jump appeared at a molar ratio of 1:1, demonstrating a
strong 1:1 inclusion complexation between β-CD and AzoIPS-
LiTFSI. With continued addition of β-CD, a second titration jump
occurred at a molar ratio of 3:1, which demonstrated a moderate
2:1 inclusion complexation between β-CD and AzoIPS-LiTFSI.
The results can be interpreted that there are two different
Figure 2. The G’ and G’’ of supramolecular hydrogel (β-CD/AzoIPS-LiTFSI 3:1
250 mM) (a) as a function of shear stress at a constant frequency of 1 Hz at
25.0 oC; (b) as a function of frequency at 25.0 oC.
Gel formation mechanism
In order to acquire detailed insight into the mechanism of gel
1
formation, H NMR and 19F NMR spectra were used to analyze
the host-guest interaction. As shown in Figure 3a (1H NMR
spectra), significant impact of β-CD on the protons of AzoIPS-
LiTFSI was observed with the increase of β-CD concentration.
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binding affinities of β-CD’s complexation with TFSI- and Azo
moieties.[12] As appeared in Figure 4b, the binding constant of
complexation between β-CD and AzoIPS was 2.29 × 103 ± 127
M-1 and the stoichiometric ratio was 1.77, which was smaller
than 2. It can be interpreted that there are free AzoIPS and β-CD
molecules without host-guest interactions presented in the
equilibrium status.
the chemical shifts from 25.0 oC to 50.0 oC are different for pure
AzoIPS-LiTFSI (0.263 ppm) and in the presence of β-CD (0.219
ppm). The possible explanation is that partial TFSI- anions
escape from the cavity of β-CDs, inducing the change of
polarizable environment. The above results further confirmed the
host-guest interaction between β-CD and TFSI- and revealed the
mechanism of the thermo-responsive sol-gel transition.
It is noteworthy that, the thermodynamics of the interaction
between β-CD and LiTFSI in water is relatively complicated
(Figure 4c). The overall binding process is a coexistence of
endothermic (positive signals) and exothermic (negative signals)
steps, leading to significantly different titration curves. Therefore,
it is unsuitable to obtain the thermodynamic information by fitting
the titration curve. However, according to our previous report,[13]
the binding stoichiometry of β-CD and TFSI- was 0.859. In
general, the real stoichiometric ratio between AzoIPS-LiTFSI
and β-CD was smaller than 3:1 due to the coexistence of free
guest molecules and inclusion complex in the supramolecular
system. It is well known that, the driving force for inclusion
complexation is usually the exothermic host-guest interaction.
The presence of competitive interaction can shed light on the
complicated binding process.[14] It should be noted that, there
exists strong hydrogen bonds between TFSI- anions and water
molecules, due to the heavily fluorinated of selected TFSI- anion.
During the titration of β-CD, TFSI- released from water and
entered into the cavity of β-CD to form hydrogen bonding with
the hydroxyl group in β-CD. Therefore, positive signals in the
titration curve may be caused by the destruction of hydrogen
bonds between TFSI- anions and water molecules.
Figure 5. Temperature-dependent 19F NMR spectra of β-CD/AzoIPS-LiTFSI
(3:1 6 mM) in D2O. The sample was heated from 25.0 to 50.0 °C and then
cooled down to 25.0 °C (from bottom to top).
It is well known that hydrophobic interaction and reasonable
size-compatible fit between the cavity of β-CD and guest
molecule are the main driving forces for the β-CD inclusion
complexes. The selected hydrophobic TFSI- anion and Azo
group are suitable size-matched host-guest pairs with β-CD,
which promotes the binding. Obviously, oversized guest
molecule cannot be entrapped in the β-CD cavity, and oversized
β-CD cavity cannot ensure van der Waals force (which is strictly
dependent on the distance between two molecules) to stabilize
the host-guest interaction.[15] As can be seen from Figure 1f,
unstable hydrogel was obtained after the Azo was replaced by
decyl due to relatively long distance between alkyl chain and the
cavity of β-CD. When the TFSI- was replaced by hydrophilic and
small sized Cl-, supramolecular hydrogel could not form (Figure
1g). It can also be concluded that host-guest interaction is the
necessary prerequisite for the formation of supramolecular
hydrogel.
As ion dissociator, zwitterion AzoIPS can dissolve inorganic
salt LiTFSI. As can be seen from Figure 6, the absorption band
of AzoIPS located at 1159 cm-1 belonging to C-H in-plane
vibration of imidazole. The bands at 1186 and 1043 cm-1
ascribed to the S=O stretching vibrations.[16] For LiTFSI, the
peaks at 1140, 1198 and 1061 cm-1 attributed to the C-SO2-N
bond and CF3 symmetric stretching vibration respectively.[13] For
AzoIPS-LiTFSI, these absorption peaks significantly shift to
1193, 1137, and 1051 cm-1, indicating the electrostatic
interaction between AzoIPS and LiTFSI.
Figure 4. Titration data of β-CD (15 mM) titrated into (a) AzoIPS-LiTFSI (0.5
mM) (b) AzoIPS (0.75 mM) (c) LiTFSI (1.5 mM) recorded at 25.0 °C.
Temperature-dependent 19F NMR spectra were carried out to
prove the presence of hydrogen bonding. As shown in Figure S3,
the fluorine signal of AzoIPS-LiTFSI solution is at about -77.519
ppm at 25.0 °C. Significant upfield shift can be observed with the
increasing of temperature (-77.782 ppm at 50.0 oC). Upon
cooling, the original fluorine signal of TFSI- anion was observed
again. Therefore, it can be concluded that there are hydrogen
bonding between TFSI- anion and water molecules. As a control
experiment (Figure 5), the same experimental phenomenon was
o
obtained (from -77.908 ppm at 25.0 C to -78.127 ppm at 50.0
oC) for β-CD/AzoIPS-LiTFSI complex (molar ratio 3:1) due to the
presence of hydrogen bonds between β-CD and TFSI-. However,
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1
CD/AzoIPS-LiTFSI from 0:1 to 3:1). As a control experiment, H
NMR spectra without addition of β-CD was detected and shown
in Figure S4. There is no significant chemical shift was observed
1
after UV irradiation. The results of H NMR spectra demonstrate
that the photoisomerization of Azo changes the inclusion mode
of β-CD and guest molecules.
It is well known that there is chiral center in β-CD. Inclusion
complexation between achiral guest chromophore and β-CD
could induce circular dichroism (CD) signal in which information
about complexation mode could be obtained. As shown in
Figure 7d, no CD signal for AzoIPS-LiTFSI or β-CD aqueous
solutions could be detected. While, for β-CD/AzoIPS-LiTFSI
complex, CD signal was observed (blue curve) at the
wavelengths similar to the UV-vis absorption spectra of trans-
Azo. It can be clearly observed that, two strong positive Cotton
signals ascribing to π-π* transition of Azo emerge at 264 nm and
320 nm and a weak negative Cotton signal corresponding to n-
π* transition appears at about 465 nm with a splitting band at
about 360 nm, indicating strong host-guest interaction between
chiral β-CD and achiral Azo. It is the chiral β-CD that leading to
the chirality of the complexation. The results suggest that the π-
π* transition dipole of Azo is parallel to the principal axis of β-CD,
while n-π* transition dipole of Azo is perpendicular to the axis of
β-CD.[18] After UV irradiation, significant CD signal transition was
observed, which is corresponding to the UV-vis absorption
spectra of cis-Azo. That is, the decreased positive Cotton effect
at about 300 nm and increased intensity of Cotton effect at
about 400 nm. This spectrum (magenta curve) shows the
positive and negative CD signals corresponding to π-π* and n-
π* transitions of the cis-Azo respectively, demonstrating that cis-
Azo moiety entrapped in the β-CD cavity and the both transition
moments are tilted to the principal axis of β-CD, while the trans-
Azo moiety excluded from the β-CD.[10,19]
Figure 6. FTIR spectra of AzoIPS, LiTFSI and AzoIPS-LiTFSI.
Photo-responsive mechanism
As can be seen from Figure 1b, UV irradiation of the hydrogel
produced a fluid sol state. It has already been known that trans-
Azo can be entrapped into the cavity of β-CD due to its size
matching and hydrophobic nature. Trans-cis isomerization
caused by UV light illumination will induce size changes and
hydrophilicity enhancement of the Azo. Therefore, Azo moiety
may exclude from the cavity of β-CD. To study the mechanism
1
of the photo-responsive sol-gel transition, UV-vis and H NMR
spectroscopy were used to confirm trans-cis photoisomerization.
Figure 7a shows the UV-vis absorbance spectra of β-
CD/AzoIPS-LiTFSI (3:1 0.12 mM) aqueous solution with different
UV irradiation times. Without UV light treatment, a strong
absorption peak located at about 320 nm is belong to the π-π*
transition of Azo. After UV irradiation for 2 min, significantly
intensity decrease and blue shift (300 nm) of this peak was
observed. Meanwhile, the intensity of absorption band located at
425 nm which ascribed to the n-π* transition increased
slightly.[17] Obviously, longer irradiation time cannot bring
significant change in the UV-vis spectra due to maximum trans-
cis photoisomerization has been finished. After visible light
irradiation for 20 min, trans-AzoIPS-LiTFSI was obtained again.
In addition, the reversible trans-cis transition of β-CD/AzoIPS-
LiTFSI complex can be repeated more than five times without
decomposition of the components (Figure 7b).
1H NMR spectra were detected to further prove the trans-cis
isomerization and calculate the content of trans-isomers before
and after UV illumination. By calculating the proton signals
shown in Figure 7c, about 80% trans-content present in the
complex before UV light treatment. After UV irradiation over 30
min, shielding effect and magnetically anisotropic effect caused
by the change of polarity induce corresponding proton signals of
cis-Azo appear in upfield position. Meantime, the content of
trans-isomer was reduced to 15%. The results can be
determined that UV and visible light treatment cannot achieve
complete conversion. It is noteworthy that, UV illumination
translocates the original protons of trans-isomer moves to an
opposite direction compared with Figure 3a (the molar ratio of β-
Figure 7. UV-vis absorption spectra of β-CD/AzoIPS-LiTFSI aqueous solution
(3:1 0.12 mM) (a) with different UV irradiation times; (b) the maximum
absorption intensity after exposed to alternate UV and visible light. (c) 1H NMR
spectra of β-CD/AzoIPS-LiTFSI (3:1 6 mM) in D2O before and after UV
illumination. (d) CD spectra of AzoIPS-LiTFSI, and β-CD aqueous solution as
well as β-CD/AzoIPS-LiTFSI complex before and after UV irradiation.
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2D ROESY 1H NMR technique was used to detect the position
of AzoIPS moiety correlated with β-CD. As shown in Figure S5
(β-CD/AzoIPS-LiTFSI 3:1 15 mM), only protons of Azo group
To understand the correlation mode of Azo and β-CD better,
density functional theory (DFT) calculations were used to
optimize the structures of trans- and cis-AzoIPS-LiTFSI
display correlation peaks with the inner protons of host molecule. (Scheme 1a and 1b). The needed lengths of groups were
However, no significant correlation signals between the alkyl
protons of AzoIPS-LiTFSI and β-CD protons were observed,
demonstrating that other parts of AzoIPS-LiTFSI apart from Azo
group and TFSI- anion do not have host-guest interaction with β-
CD. Combine with above results, β-CD only includes trans-Azo
moiety and TFSI- anion.
marked in the picture. According to above experimental results
and the structure parameters of β-CD in the literature,[20] the
possible inclusion mode between β-CD and trans- or cis-
AzoIPS-LiTFSI can be described in Scheme 1c. Before UV light
treatment, one Azo group included in two β-CDs, which is
benefit for the hydroxyl in β-CD to form hydrogen bonding with
other adjacent β-CD. Thus, tightly linked host molecules induce
the formation of stable supramolecular hydrogel. After UV
irradiation, one β-CD escaped from Azo. Therefore, β-CDs are
not able to connect with each other. The loss of cross-linking
points disrupts the stability of hydrogel. For thermal
responsiveness, the destruction of hydrogen bonds at elevated
temperature also induces sol-gel phase transition. Therefore, it
can be concluded that host-guest interaction between β-CD and
two kinds of guests, electrostatic interaction between imidazole
cation and TFSI- anion, and hydrogen bonds between β-CDs as
well as β-CD and TFSI- promote the formation of supramolecular
hydrogel. (Scheme 1d)
In order to get detailed insight into the effect of UV light
irradiation on the inclusion mode of β-CD and AzoIPS moiety,
1
partial 2D ROESY H NMR spectra for the mixture of β-CD and
AzoIPS-LiTFSI before and after UV illumination was exhibited
(Figure 8). The hydrogen atoms on various carbons are labeled.
Before UV irradiation (Figure 8a), these spectra display the
correlation peaks between the protons of C3, C6 and C5 in β-CD
and the protons (Ha, Hb, Hc, Hd, and He) in the trans-Azo moiety
indicating that only Azo group included in the β-CD. Figure 8b
1
shows the 2D ROESY H NMR spectrum of complex after UV
irradiation. The correlation signals between β-CD protons and
trans-Azo protons were obviously weakened instead of the
partial protons for cis-Azo. Interestingly, new correlation peaks
between C3 protons and Hf were observed, which demonstrate
the sliding of β-CD on the position of guest molecule.
Scheme 1. Optimized structures of (a) trans-AzoIPS-LiTFSI, (b) cis-AzoIPS-
LiTFSI by DFT calculations. Color code for atoms: dark gray, carbon; light gray,
hydrogen; dark blue, nitrogen; light blue, fluorine; red, oxygen; yellow, sulfur;
purple, lithium. (c) Inclusion mode between β-CD and AzoIPS-LiTFSI before
and after UV irradiation. (d) Diagram of gel formation mechanism.
Figure 8. Partial 2D ROESY 1H NMR spectra for the mixture of β-CD and
AzoIPS-LiTFSI with the concentration of β-CD 15 mM (molar ratio 3:1
D2O/DMSO 9/1, v/v).
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Nitrosobenzene (98%), p-toluidine (99%), N-bromosuccinimide (99%)
and benzoyl peroxide (98%), imidazole (99%), 1,3-propanesultone (99%),
lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99%), lithium chloride
(LiCl) (99%), 1-bromodecane (98%) were purchased from J&K Scientific
Co., Ltd. Trifluoroacetic acid (99%), β-CD (96%) was purchased from
Aladdin Chemical Reagent Co., Ltd. Organic solvents were purchased
from Shanghai Chemical Co., Ltd. All the above materials were used
without further purification. Ultra-pure water was used throughout the
experiment. The purity of the products was confirmed by 1H NMR on a
Bruker Avance 400 MHz NMR.
Conductivity of hydrogel
Based on the charged character of AzoIPS-LiTFSI and thermal-
responsive sol-gel transition of the supramolecular hydrogel,
alternating current impedance was used to study the conductive
performance of the prepared supramolecular hydrogel (Figure 9).
The ionic conductivity of sample at 25.0 C is 1.84×10-4 S·cm-1,
o
which increases to 2.93×10-4 S·cm-1 at 40.0 C (sol solution at
o
this temperature). Interestingly, significant reversible switching of
conductivity was achieved accompanying with thermal-
responsive sol-gel phase transition. Moreover, the cyclic
conductivity conversion could be repeated many times without
addition of any supporting electrolytes. Such performance is
beneficial greatly for the potential application on temperature
sensitive soft materials.
Synthesis
AzoBr (1) AzoBr was synthesized according to the previous report.[21]
Azomim (2) Imidazole (0.68 g), NaOH (0.44 g) and ultra-pure water
(0.44 mL) were added to a neck round bottom flask. AzoBr (2.5 g) was
dissolved in THF and added dropwise into the above mixture at 60 oC
under N2 atmosphere. After refluxed for 3 days, THF was removed by
rotary evaporation. The crude product was extracted three times with
CH2Cl2/H2O. The organic layer was concentrated and dried under
vacuum. 1H NMR (CDCl3, δ): 5.21 (s, 2H), 6.94 (s, 1H), 7.13 (s, 1H),
7.26-7.30 (m, 2H), 7.48-7.61 (m, 4H), 7.89-7.93 (m, 4H).
AzoIPS (3) Azomin (1.8 g, 6.9 mmol) was dissolved in acetone. An
equimolar amount of 1,3-propanesultone (0.84 g, 6.9 mmol) dissolved in
acetone was added dropwise to the solution in ice bath under N2
atmosphere. The mixture was then stirred for 5 days at room temperature.
After that, the solution was filtered and the solid was purified with
acetone for three times. The orange powder AzoIPS was dried under a
vacuum at room temperature. 1H NMR (DMSO, δ): 2.07-2.17 (m, 2H),
2.39-2.45 (t, 2H), 4.31-4.37 (t, 2H), 5.54 (s, 2H), 7.57-7.64 (m, 5H), 7.82-
7.95 (m, 6H), 9.34 (s, 1H).
AzoIPS-LiTFSI (4) AzoIPS-LiTFSI was obtained by dissolving equimolar
AzoIPS and LiTFSI in MeOH/H2O mixture solvent. After stirring overnight,
the solvent was removed by rotary evaporation. The product was dried
under vacuum at room temperature.
Figure 9. Conductivities of the supramolecular hydrogel (β-CD/AzoIPS-LiTFSI
3:1 200 mM) at gel phase (25.0 oC) and sol solution (40.0 oC) induced by
alternating temperature changes at 25.0 and 40.0 °C, respectively.
AzoIPS-LiCl The synthesis of AzoIPS-LiCl (Figure S6a) is similar to the
synthesis AzoIPS-LiTFSI.
Conclusions
C10IPS The synthesis of C10IPS (Figure S6b) is similar to the previous
report.[22] 1H NMR (D2O, δ): 0.66-0.71 (t, 3H), 1.08-1.12 (m, 14H), 1.68-
1.73 (m, 2H), 2.13-2.21 (m, 2H), 2.72-2.77 (t, 2H), 4.00-4.06 (t, 2H), 4.18-
4.23 (t, 2H), 7.34-7.39 (m, 2H).
In summary, we have prepared photo and thermal responsive
supramolecular hydrogel based on β-CD and rational designed
asymmetric Gemini ZIL guest. The complex ratio between β-CD
and two kinds of guest molecules Azo and TFSI-, as well as
photo and thermal responsive mechanism has been meticulous
studied. The coexistence of two kinds of host-guest interactions
and hydrogen bonding are the requirements for the formation of
hydrogel. Interestingly, one trans-Azo group can be included in
two β-CDs. In addition, the β-CD in AzoIPS-LiTFSI can be
photo-manipulated to shuttle accompanied with the change of
CD signals. The conductive properties may highlight the
potential applications of stimuli-responsive hydrogel on electrical
sensing materials.
Experimental Section
Materials
Scheme 2. Synthetic routes of AzoIPS-LiTFSI.
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Sample preparation
DFT calculations were used to simulate the optimum configuration of
trans- and cis-AzoIPS-LiTFSI. Gaussian 09 package with the hybrid
B3LYP functional and the 6-31G (d, p) basis set were performed.
The hydrogels were prepared by weighing all components at designed
compositions in a screw-capped sample tube. The β-CD and AzoIPS-
LiTFSI were mixed at different molar ratios. The concentration indicated
in the article was the molar concentration of β-CD unless otherwise
specified. The mixtures were homogenized at 60 oC and equilibrated at
room temperature.
Conductivity measurements
Conductivities of the hydrogels at different conditions were determined by
an electrochemical impedance spectroscopy technique with 0.2
V
oscillating voltage in the frequency range from 1 Hz to 10 MHz using ITO
glasses as cell electrodes and teflon as a spacer. The conductivities (σ)
were calculated using the following equation:[23]
Photoisomeric experiments
To achieve trans-cis isomerization, the samples in quartz tube were
illuminated with a LUYOR-3109 UV light at room temperature. After
photoisomerization, all the samples were wrapped with aluminum foil to
prevent the transformation from cis- to tans-isomer.
where R is the resistance; and l and A are the thickness and area of the
sample, respectively.
NMR
1H NMR and 19F NMR spectra were recorded on a Bruker Avance 400
Acknowledgements
o
MHz NMR with D2O as solvent at 25.0 C. For 19F NMR measurements,
trifluoroacetic acid in D2O was used as an external reference (-79.45
ppm). 2D ROESY 1H NMR spectra were recorded by using D2O/DMSO
(9:1 v/v) mixed solvent at 25.0 oC.
The authors are grateful to the National Natural Science
Foundation of China (No.21573132, No.21773141).
Isothermal titration calorimetry (ITC)
Conflict of interest
ITC experiment was carried out with Microcal VP-ITC apparatus at 25.0
oC. The β-CD solution was titrated into the 1.4 mL AzoIPS-LiTFSI
solution via a 300 μL syringe with the total injection of 29 drops.
The authors declare no conflict of interest.
Keywords: supramolecular hydrogel • stimuli-responsive •
azobenzene • zwitterionic liquid • host-guest interaction
UV-vis and FTIR spectroscopy
The UV-vis spectroscopy measurements of 0.12 mM β-CD/AzoIPS-
LiTFSI (3:1) aqueous solutions before and after different UV irradiation
times were carried out on a UV-4100 spectrophotometer. The ultra-pure
water was utilized as a blank in the experiments. FTIR (PerkinElmer
Spectrum Two) was carried out at room temperature.
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Aoli Wu, Panpan Sun, Na Sun, Liqiang
Zheng*
Page No. – Page No.
Responsive Self-Assembly of
Supramolecular Hydrogel Based on
Zwitterionic Liquid Asymmetric
Gemini Guest
Reversible sol-gel phase transition of prepared supramolecular hydrogel could be
triggered by light and temperature. UV irradiation changes the inclusion complex
mode between β-CD and Azo, while thermal treat influences the hydrogen bonding
between β-CD and TFSI-.
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