Photohealable ion gels based on the reversible dimerisation of anthracene

Article abstract of DOI:10.1039/c8cc07775d

We report a photohealable ion gel based on the photodimerisation of anthracene as a dynamic covalent bond. A tetra-arm poly(ethylene glycol) terminally functionalised with anthracene was synthesised and combined with an ionic liquid to form an ion gel. Th

Full text of DOI:10.1039/c8cc07775d

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Photohealable ion gels based on the reversible
dimerisation of anthracene†
Cite this: Chem. Commun., 2018,
54, 13371
Aya Saruwatari,‡ Ryota Tamate, *‡ Hisashi Kokubo
Masayoshi Watanabe
and
Received 28th September 2018,
Accepted 5th November 2018
*
DOI: 10.1039/c8cc07775d
rsc.li/chemcomm
We report a photohealable ion gel based on the photodimerisation of networks swollen with ionic liquids (ILs), can overcome this
anthracene as a dynamic covalent bond. A tetra-arm poly(ethylene defect due to the intrinsic nonvolatility of ILs. In this context,
glycol) terminally functionalised with anthracene was synthesised and we recently developed a new class of photoresponsive ion gels
combined with an ionic liquid to form an ion gel. The photodimerisa- by introducing azobenzene moieties into thermoresponsive
tion reaction was utilised to realise photohealing of the ion gels.
block copolymers18–20 and ionic liquids.21 In these systems,
the polarity change induced by the photoisomerisation of
One of the unique functions of living systems is their respon- azobenzene modulates the interaction strength between the
siveness to the surrounding environment.1 More specifically, polymers and the ILs. Consequently, differences were observed
living matter can change its shape and mechanical properties in the sol–gel transition temperatures (Tgel) of block copolymers
in response to external stimuli. Thus, inspired by living in ILs between the cis (cis-Tgel) and trans (trans-Tgel) forms of
systems, synthetic stimuli-responsive soft materials that can azobenzene. This difference could be exploited to achieve
respond to external stimuli such as temperature, light, pH, photohealing of the ion gel by a light-induced sol–gel transition
and chemicals have been intensively investigated in recent of the damaged segment.19 However, the nature of the physical
decades.2–5 Such responsive materials have versatile applica- cross-linking of the ion gel resulted in the gel failing to endure
tions including drug delivery systems,6 injectable gels,7 cell large strains, due to the chain pullout from the physical
culturing materials,8 artificial muscles,9 and self-healing association.22 In this regard, the photodimerisation reaction
materials.10,11 Among the various stimuli-responsive systems can introduce chemical cross-linking into molecular frame-
reported to date, light-responsive materials have attracted works, which can then be reversibly dimerised and dissociated
significant attention because of their beneficial properties, such by light irradiation.14,23,24 The dynamic covalent bonds formed
as high spatial and time resolutions, contactless manipulation, by photodimerisation have been previously utilised for the
and non-invasiveness.12–14 In this context, light responsiveness can photohealing of dry polymers25,26 and hydrogels;27 however,
be imparted to soft materials by the introduction of photochromic to date, the photohealing of ion gels through dimerisation
molecules into their molecular frameworks. For example, azo- reactions has yet to be reported.
benzene is a particularly well-known photochromic molecule
Thus, we herein describe the first report into photohealable
that exhibits light-induced cis–trans isomerisation. The polarity ion gels via the photodimerisation of anthracene (Fig. 1). In this
and molecular shape of azobenzene are modulated through system, anthracene undergoes a photo-induced reversible [4+4]
this cis–trans isomerisation, and so azobenzene-containing cycloaddition reaction, since anthracene dimerises upon irra-
materials exhibit unique photoinduced structural transitions diation at wavelengths 4350 nm, whereas the dimer is dissociated
in both dry and wet systems.15–17
by heating or by irradiation at wavelengths o300 nm.14 In this study,
One obvious drawback of wet systems such as hydrogels is the end group of tetra-arm poly(ethylene glycol) (tetraPEG) was
the solvent volatility, which renders their use challenging in functionalized with anthracene through the reaction between iso-
an open atmosphere. However, ion gels, which are polymer cyanate and hydroxyl groups.28 In the initial step of this reaction, the
amino group of 2-aminoanthracene was converted into an isocya-
Department of Chemistry and Biotechnology, Yokohama National University,
nate group using triphosgene at ambient temperature (Scheme S1a,
ESI†). In the second step, the hydroxyl-terminated tetraPEG
(Mn = 40 kDa) was reacted with an excess of the anthracene isocyanate
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
E-mail: tamate-ryota-tm@ynu.ac.jp, mwatanab@ynu.ac.jp
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
in the presence of a dibutyltin dilaurate catalyst to yield the
c8cc07775d
‡ These authors contributed equally to this work.
anthracene-terminated tetraPEG (tetraPEG–Ant) (Scheme S1b, ESI†).
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Fig. 1 The chemical structure of tetraPEG–Ant and a conceptual illustra-
tion of photohealing of the tetraPEG–Ant ion gel.
Fig. 2 Variation in the UV-vis spectra for a 1 wt% tetraPEG–Ant solution in
[C2mim][NTf2] with time (a) under UV light (l = 365 nm) at room temperature,
and (b) upon heating at 150 1C following UV irradiation.
The anthracene modification efficiency was then calculated by
1H NMR, and the efficiency reached 99.5% (Fig. S1, ESI†). Almost
the same gel permeation chromatography (GPC) traces of tetraPEG
and tetraPEG–Ant indicate that no degradation of polymer chains shows changes in the degree of dimerisation calculated from the
and dimerisation reaction of anthracene occurred during the reaction absorption peak at 365 nm under UV irradiation/heating cycles.
(Fig. S2, ESI†).
Although the slight decrease in the degree of dimerisation in later
Due to the good affinity of PEG toward various ILs, tetra- cycles also implies the presence of irreversible reactions, a
PEG–Ant can form solutions in many common ILs. In this moderate reversibility was confirmed for the dimerisation/
study, a well-known imidazolium-based IL, 1-ethyl-3-methylimid- dissociation of anthracene in ILs.
azolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2])
At polymer concentrations above the chain overlap concen-
was used as a hydrophobic solvent for tetraPEG–Ant. The tration, photo-induced gelation of tetraPEG–Ant in ILs is expected
photo/thermo-responsivities of tetraPEG–Ant in [C2mim][NTf2] through the photodimerisation of anthracene at the polymer
was investigated via UV-vis spectrophotometry. Thus, the UV-vis terminal, which acts as a chemical cross-linking point. Thus,
spectra for the 1 wt% solution of tetraPEG–Ant in [C2mim][NTf2] Fig. S5 (ESI†) shows photographic images before and after UV
upon UV irradiation at 365 nm are shown in Fig. 2a. As irradiation of the tetraPEG–Ant solutions in [C2mim][NTf2] at
indicated, the absorption peaks at B365 nm decreased gradually different polymer concentrations. Indeed, photoinduced gelation
in intensity during UV irradiation, which indicates that in the IL, was confirmed at polymer concentrations 47 wt%. The required
a [4+4] cycloaddition reaction of the anthracene moieties took concentration for gelation was higher than those required for
place at the polymer terminal. We found that the dimerisation of tetraPEG hydrogels and ion gels formed through A–B type cross-
anthracene was slower than in aqueous systems,29 which could coupling reactions.32,33 This could be accounted for by considering
be attributed to the higher viscosities of ILs compared to water. intramolecular dimerisation and the lower reaction rate of dimer-
Similar observations were also made for the [2+2] cycloaddition isation (B60%). Nevertheless, the prepared tetraPEG–Ant formed
reaction of coumarin in an IL.30 In addition, we note that the mechanically stable ion gels at lower concentrations than pre-
photodimer of anthracene can be thermally dissociated to viously reported photohealable ion gels that had been prepared
regenerate the monomer form.31 Upon heating at 150 1C, the through the physical association of triblock copolymers.19 Subse-
absorption peaks increased in intensity over time, suggesting quently, the rheological changes caused by the photodimerisation
that the anthracene monomer was regenerated at the polymer and thermal dissociation of the dimers during UV irradiation and
terminal (Fig. 2b). However, a small increase in the spectral heating were investigated by oscillatory shear measurements.
baseline was also observed during heating. As UV-vis spectra for Fig. 3 shows the variation in the storage (G0) and loss (G00) moduli
pure [C2mim][NTf2] under UV irradiation and upon heating did with time upon sequential cycles of UV irradiation and heating at
not show any significant change (Fig. S3, ESI†), it is suggested 150 1C. Initially, a 10 wt% solution of tetraPEG–Ant in [C2mim][NTf2]
that some irreversible cleavage of anthracene or degradation of the was irradiated by UV light at 365 nm, which resulted in sharp
PEG polymer chains took place at high temperatures. Fig. S4 (ESI†) increases in G0 and G00. In addition, the crossover of G0 and G00,
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Fig. 3 Variation in G0 and G00 for a 10 wt% solution of tetraPEG–Ant in
[C2mim][NTf2] as a function of time under alternating UV irradiation at
365 nm (2 h) and heating at 150 1C (3 h) at a frequency = 1 Hz and strain
amplitude = 1%.
Fig. 4 (a) Photographic images of the pristine (left), cut (centre), and
healed (right) 10 wt% tetraPEG–Ant ion gels. The cut ion gel pieces were
re-contacted and healed by heating at 150 1C for 10 h and by UV irradiation
overnight under an argon atmosphere. (b) Stress–strain curves for the
pristine ion gel (green solid line), the healed ion gel by heating and UV
irradiation (blue dashed line) and the healed ion gel only by UV irradiation
(black dotted line).
which is indicative of the sol–gel transition, was observed within 1 h.
After 2 h irradiation, G0 was 42 kPa, which was comparable to that
of our previously reported physically crosslinked photo-healable ion
gel with a polymer concentration of 20 wt%,19 indicating that a anthracene units at the surface. In addition, we note that thermal
mechanically stable ion gel had been formed through photodimer- annealing at high temperatures has been previously employed to
isation of the terminal anthracene groups as chemical cross-linking dissociate dimerised terminal anthracene moieties.36,37 In this
points. After subsequent heating of the sample to 150 1C in the case, when the ion gel segments were brought into contact and
absence of light, G0 and G00 gradually decreased over time. Thermo- thermally treated at 150 1C, healing of the cut surfaces was indeed
gravimetric analysis (TGA) indicated that the ion gel was thermally observed by UV light irradiation (Fig. 4a).
stable at 150 1C for 410 h (Fig. S6, ESI†), and so the decreases in G0
The mechanical properties of the pristine and healed tetra-
and G00 upon heating were likely due to a reduction in the number of PEG–Ant ion gel sheets were then investigated by tensile tests
chemical cross-linking points due to the dissociation of anthracene (Fig. 4b). As indicated, the pristine tetraPEG–Ant ion gel
dimers at high temperatures. The observed variations in G0 and G00 exhibited excellent stretchability, and the elongation at break
upon UV irradiation and heating were confirmed over four cycles; reached 950%. Although the A–A type coupling reaction inevi-
however, the gradual decrease in G0 as the number of cycles tably includes self-reaction, which does not contribute to the
increased was again indicative of the occurrence of irreversible network elasticity, the high stretchability of the tetraPEG–Ant
reactions during cycling.
ion gel indicates that the terminal chemical cross-linking of
Finally, the photohealing ability of the tetraPEG–Ant ion gel was branched polymer chains via photodimerisation can withstand
investigated. For this purpose, an ion gel sheet was prepared by significantly higher strains than ion gels formed by physical
irradiating the 10 wt% tetraPEG–Ant/[C2mim][NTf2] solution with associations (e.g., triblock copolymer-based gels).19 We also
UV light in a rectangular mould. The resulting tetraPEG–Ant ion found that the ion gel sheet healed by thermal treatment and
gel sheet was then cut into two pieces using a sharp blade. As the UV irradiation could be elongated over 700%, thereby indicat-
mechanochemical cycloreversion of anthracene photodimers has ing that it recovered good mechanical strength. Upon compar-
been previously reported at crack surfaces through fluorescent ison of the fracture energies of the pristine and healed ion gels,
sensing,34,35 the weak bonds present in the anthracene photo- the healing efficiency was calculated to be 66%. This implies
dimer were expected to be cleaved preferentially in the ion gel that a sufficient quantity of anthracene photodimers acting as
network. However, when the cut surfaces of the two ion gels were chemical cross-linking points were reformed through the
brought back into contact with one another and irradiated with UV photohealing process. Furthermore, the stress–strain curve
light, healing of the cut segments was very incomplete. This shapes were similar for the pristine and healed ion gels,
implies that a sufficient amount of anthracene monomer is suggesting that the bulk polymer network was not altered
required to promote the bimolecular reaction of the chain end significantly by heating or UV irradiation. On the other hand,
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This work was financially supported by Grants-in-Aid for
Scientific Research (15H05758 to M. W. and 18K14280 to R. T.)
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