Light-Controllable Ionic Conductivity in a Polymeric Ionic Liquid

Article abstract of DOI:10.1002/anie.201912921

Polymeric ionic liquids (PILs) have attracted considerable attention as electrolytes with high stability and mechanical durability. Light-responsive materials are enabling for a variety of future technologies owing to their remote and noninvasive manipula

Full text of DOI:10.1002/anie.201912921

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: Light-controllable Ionic Conductivity in a Polymeric Ionic Liquid
Authors: Hui Nie, Nicole S. Schauser, Neil D. Dolinski, Jerry Hu, Craig
J. Hawker, Rachel A. Segalman, and Javier Read de Alaniz
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912921
Angew. Chem. 10.1002/ange.201912921
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10.1002/anie.201912921
Angewandte Chemie International Edition
RESEARCH ARTICLE
Light-controllable Ionic Conductivity in a Polymeric Ionic Liquid
Hui Nie,[a]† Nicole S. Schauser,[b]† Neil D. Dolinski,[b] Jerry Hu,[b] Craig J. Hawker,[a,b] Rachel A. Segal-
man,*[b,c] Javier Read de Alaniz*[a]
Abstract: Polymeric ionic liquids (PILs) have attracted considerable
attention as electrolytes with high stability and mechanical durability.
Light responsive materials are enabling for a variety of future
technologies due to their remote and noninvasive manipulation,
spatiotemporal control and low environmental impact. To address this
potential, responsive PIL materials based on diarylethene units have
been designed to undergo light-mediated conductivity changes. Key
to this modulation is tuning of the cationic character of the imidazolium
bridging unit upon photoswitching. Irradiation of these materials with
UV light triggers a ~70% drop in conductivity in the solid state that can
be recovered upon subsequent irradiation with visible light.
Significantly, this light responsive ionic conductivity enables
spatiotemporal and reversible patterning of PIL films using light. This
modulation of ionic conductivity allows for the development of light
controlled electrical circuits and wearable photodetectors.
of photo-responsive molecules in ionic liquid motifs has been
previously reported,[19–24] incorporating these structures into PILs
has yet to be demonstrated. The substantial challenge in
identifying a light-controllable PIL centers on translating photonic
energy into a property change that can control ion transport
through
a
polymer matrix. Key considerations include
straightforward synthesis, reversibility, operation in the solid state
and rapid response time. We speculate that photochromic
diarylethenes (DAEs) containing an imidazolium ring as the
bridging unit will serve as an especially appealing target in this
context. Photochromic DAEs can switch between ring open (DAE-
O) and ring closed (DAE-C) isomers both in solution and solid
state.[25–28] Moreover, the photoinduced rearrangement of the π
bond in the bridging imidazolium ring can be used to control the
electronic properties and chemistry of the imidazolium ring.
Photochromic diarylethenes bearing an imidazolium ring have
found applications in different areas such as photoswitchable
organocatalysts,[29–31] chemical sensors,[32] and as a new type of
Lewis acid.[33] For example, the Bielawski group used a DAE-
based organocatalyst with photoswitchable electron density at the
C2 position of the imidazole ring in N-heterocyclic carbene
adducts to regulate the rate of transesterification reactions.[29]
Additionally, Kawai showed that the open and closed isomers of
4,5-dithiazolylimidazolium derivatives possess different reactivity
towards strong nucleophiles.[25]
Introduction
Ionic liquids, such as imidazolium salts, have generated
significant interest as electrolytes across a wide range of dis-
ciplines due to their high ionic conductivity and minimal to no
volatility.[1] The introduction of ionic liquid motifs along polymer
backbones, forming polymeric ionic liquids (PILs), further
enhances this utility with improved stability, mechanical durability
and integration with other electronic components.[2,3] Light
responsive materials have similarly attracted considerable
attention in a wide range of applications based on their
advantages of remote and noninvasive manipulation, inherent
spatiotemporal control and minimization of the generation of
additional waste.[4–6] Surprisingly, combining the advantageous
features of both materials classes through the development of
“smart” PILs with light controllable conductivity has lagged
significantly behind other classes of electronic/ionic conducting
materials.[7–12] Advances in tunable PILs would therefore enable
cutting-edge applications based on photoconductive properties
such as light-controlled electronic circuits[13–15] and wearable
photodetectors.[16–18]
To enable the development of stimuli-responsive PILs, we sought
to design an ionic liquid based photoswitchable motif that could
be incorporated into a polymeric structure. While the incorporation
[a]
Dr. H. Nie, Prof. C. J. Hawker, Prof. J. Read de Alaniz
Department of Chemistry and Biochemistry, University of California-
Santa Barbara, Santa Barbara, California 93106, United States
E-mail: javier@chem.ucsb.edu
Figure 1. (a) Photochromism of imidazolium bearing DAE; (b) Scheme of PIL
with photoresponsive cation-anion interaction; (c) solid-state PIL film with
tunable, photoresponsive ionic conductivity.
[b]
[c]
N. S. Schauser, Dr. N. D. Dolinski, Dr. J. Hu, Prof. C. J. Hawker,
Prof. R. A. Segalman
Materials Department and Materials Research Laboratory,
University of California-Santa Barbara, Santa Barbara, California
93106, United States
Prof. R. A. Segalman
Department of Chemical Engineering, University of California-Santa
Barbara, Santa Barbara, California 93106, United States
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201912921
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RESEARCH ARTICLE
Scheme 1. (a) Synthesis of primary amine functionalized DAE-BF4; (b) Synthesis of PIL PEO-stat-PAGE-DAE-BF4. Reagents and conditions: (i) N-3-
bromopropylphthalimide, cesium carbonate, DMF, 60 oC, 4 h; (ii) iodomethane, CH3CN, 80 oC, 16 h; (iii) hydrazine hydrate, 1,4-dioxane/H2O, reflux, overnight; (iv)
silver tetrafluoroborate, THF/MeOH, room temperature, 15 min; (v) 3-Mercaptopropanyl-N-hydroxysuccinimide Ester, 2,2-dimethoxy-2-phenylacetophenone, DCM,
365 nm light, 0 oC, 2 h; (vi) DAE-BF4, triethylamine, DCM, room temperature, 36 h.
Here we describe the design of photoresponsive PILs using
imidazolium containing DAEs as ionic pendants covalently
attached to a poly(ethylene oxide) backbone. The ring-open
isomer (DAE-O) contains a delocalized cationic change due to the
aromatic imidazolium ring, while the ring-closed form (DAE-C)
possesses more localized positive charge. This allows the binding
between mobile anions and imidazolium containing DAE cations
to be controlled, and thus the ionic conductivity of the PIL solid
film reversibly modulated (Figure 1).
notable feature of this robust synthetic route is the ability to
prepare gram quantities of DAE-containing PIL. The versatile
synthetic route also enables control and modulation of backbone
molecular weight, backbone identity and percent DAE-
incorporation, which are directions for future studies.
The electron density of the central imidazolium ring in DAE-BF4
can be reversibly modulated using two different wavelengths of
light. When DAE-BF4 is present as the ring-open isomer (DAE-O),
the imidazolium ring is conjugated with overlapping π-orbitals
containing 4n+2 π electrons. It is therefore expected to have
significant aromatic character and delocalized positive charge.
Irradiation with UV light triggers cyclization to give the ring-closed
form with associated electronic rearrangement leading to a non-
aromatic imidazolinium ring containing a more localized positive
charge. Key to this strategy is the reversibility of this cyclization
reaction which allows for regeneration of the aromatic ring-
opened system upon irradiation with visible light.
Results and Discussion
Previous work has demonstrated that photochromism in polymer
systems is highly dependent on the Tg of the polymer matrix,
where switching is favored in lower Tg polymers.[34] Here, a
statistical copolymer of ethylene oxide and allyl glycidyl ether
(PEO-stat-PAGE) (~10 mol% AGE) with a Tg of around -60 oCwas
chosen as the PIL precursor.[35,36] First, a scalable and robust
synthetic route was developed for DAEs bearing reactive primary
amines for attachment to the polymer backbone. From the
reported DAE 1, N-alkylation of the imidazole unit with N-3-
Modulating the electron density of the imidazolium ring through
light irradiation is expected to results in a stronger association of
the BF4- anions with the closed DAE form than the open DAE form
(Figure 2a). To demonstrate the aromaticity change and thus the
ability to modulate electron density on the imidazolium unit and
the anion interactions between the two photoisomers, initial
bromopropylphthalimide,
followed
by
methylation
with
iodomethane afforded DAE 3. DAE-BF4 was produced by
deprotection of the phthalimide group and subsequent anion
metathesis. A N-hydroxysuccinimidyl (NHS) ester strategy was
used to attach the DAE-BF4 to the PEO-stat-PAGE copolymer.
The NHS ester group was first introduced onto PEO-stat-PAGE
segments via thiol-ene click reaction, which can then selectively
couple with DAE-BF4 under mild conditions to produce the desired
DAE-containing PIL, PEO-stat-PAGE-DAE-BF4 (Scheme 1). A
1
studies examined changes in the H NMR spectra for the DAE
units
in
PEO-stat-PAGE-DAE-BF4
before
and
after
photoswitching. Significantly, the C2 proton undergoes an upfield
shift from 9.3 ppm to 8.9 ppm in CD2Cl2 on UV light irradiation
(Figure 2b and Figure S2). This shift is fully consistent with the
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10.1002/anie.201912921
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RESEARCH ARTICLE
corresponding small molecule analog where an upfield shift from
8.7 ppm to 8.2 ppm in CD3CN occurs on ring closure (Figure S1).
Considering the imidazolium C2 proton sense a larger effective
magnetic field in the aromatic ring-open isomer (DAE-O), an
upfield shift is expected upon cyclization to the ring-closed isomer
(DAE-O) despite the decrease in electron density of the ring-
closed imidazolinium photoproduct. This decrease in electronic
density upon photocyclization is supported by the 13C NMR where
the C2 carbon of the imidazolium shifts from 136.6 ppm to 150.6
ppm (Figure S3). A similar effect of decreasing the electron
density and breaking aromaticity upon photocyclization has been
reported by Kawai[25], Bielawski[29] and Branda.[33]
Additional solid-state NMR cross-polarization experiments before
-
photoswitching revealed the ion carrier BF4 interacts strongly with
the DAE cation units in PEO-stat-PAGE-DAE-BF4 rather than with
other segments within the polymer. As shown in Figure 2c, proton
peaks at 8.1 ppm (protons of a), 3.9 ppm (protons of b and b’) and
a shoulder around 2.4 ppm (protons of c) in the 19F−1H cross
polarization magic angle spinning (CPMAS) proton NMR
-
spectrum were observed, revealing the BF4 cations are
structurally close to the C2 position of the imidazolium ring.[37,38]
Details of peak assignments and analysis of distance between
BF4- and imidazolium cation before and after photoswitching from
analysis of the corresponding CPMAS proton NMR spectra are
shown in Figure S3, S4, S5 and Table S1. Here, analogous to
similar small molecule photochromic systems,[25] it is deduced that
the non-aromatic closed form (imidazolinium) has stronger
interaction with the anion than aromatic open form (imidazolium)
in our polymer system. However, we cannot rule out other factors
that may affect anion mobility, such as reorientation of polymer
chains and/or differential packing effects of the diarylethene units
upon photocyclization.
To further characterize the photoswitching properties of PEO-stat-
PAGE-DAE-BF4, a series of pump probe UV-Vis experiments
were carried out and coupled with kinetic modeling to understand
the impact of the surrounding environment on switching. For both
the small molecule, DAE-BF4, and polymer supported system,
PEO-stat-PAGE-DAE-BF4, irradiation with 300 nm light triggers
photocyclization to generate the ring-closed isomer. This
isomerization/switching reaction is characterized by a significant
change in absorption profile which is visually observed through a
colorless to blue transition and is due to the formation of extended
π-conjugation in the ring-closed isomer. The corresponding UV-
Vis absorption spectra show trends typically observed for ring-
closing reactions of DAE derivatives. The high-energy bands
(~300 nm), which are assigned to the n →π* and π→π*
transitions of the N-heterocycle and thiophene systems, become
less intense as broad bands centered at ~ 390 nm and 670 nm
appear (Figure S6, S8).[26,29] By tracking this change in peak
intensity (~300 nm and ~670 nm) the equilibrium ratio of the
closed form can be estimated (full details can be found in the
Supporting Information, Table S2, Table S3) and then fit with
kinetic models. Significantly, the UV triggered isomerization
reaction is well described by a first order dynamic equilibrium
model (Equation 1) that has been previously used to characterize
equilibrium processes for a variety of photochromic molecules.
Additionally, the kinetics of the reverse visible-light-driven ring
opening reaction are well described by a first order decay.[39,40]
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Figure 2. (a) Illustration of cation-anion interaction change of PEO-stat-PAGE-
DAE-BF4 with light irradiation. (b) 1H NMR overlay of photochromism of PEO-
stat-PAGE-DAE-BF4 (from open form to closed form) in CD2Cl2 with UV light
irradiation. (c) 19F−1H CPMAS proton NMR spectrum of PEO-stat-PAGE-DAE-
BF4 in solid state at -18 °C under MAS at 25 kHz.
As expected, the equilibrium amount of closed form for the small
molecule, DAE-BF4, increases as the solvent is changed from
DCM (~45%), to THF (~65%), and MeCN (~70%), respectively.
This change in isomer percentage with solvent polarity clearly
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10.1002/anie.201912921
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RESEARCH ARTICLE
illustrates the increased stabilization of the more localized positive
charge for the closed form (Figure S7). This ring-closed isomer
could also be reversibly switched to the open form with high
efficiency by irradiating a solution of DAE-BF4-C with visible light.
In contrast, this relationship between solvent polarity and isomer
percentage is not observed for the polymer supported DAE
derivative, PEO-stat-PAGE-DAE-BF4. For this material, all
solvents equilibrate to roughly the same percentage of closed
form DAE (~50%-55%) (Figure S9). It is proposed that the PEO-
stat-PAGE polymer backbone provides a consistent molecular
environment for the DAE side chains regardless of solvent, with
efficient photoswitching of the DAE units in a variety of
environments.
The ability to photoswitch PEO-stat-PAGE-DAE-BF4 PIL films
then raises the exciting possibility of tuning ionic conductivity by
controlling electron density of the imidazolium cation through
reversible photocyclizations (Figure 4a). The bulk ionic
conductivity of PEO-stat-PAGE-DAE-BF4 is 2 x 10-9 S/cm at room
temperature with ionic conductivity increasing to 4.5 x 10-7 S/cm
upon addition of 50 wt% propylene carbonate (PC) as plasticizer
(Figure S12).[41] Temperature-dependent ionic conductivity of
PEO-stat-PAGE-DAE-BF4 follows Arrhenius-like behavior with a
calculated activation energy of 1.07 eV, which is in a common
range seen for other polymer electrolytes (Figure S13). For
measurements of in-situ ionic conductivity change during light
irradiation, through-plane analysis of spin-coated PEO-stat-
PAGE-DAE-BF4 PIL thin films on pre-cleaned ITO with
evaporated Au electrodes for top contacts was conducted (Figure
S14). Due to the decreased photoswiching efficiency with
increasing temperature, all the ionic conductivity measurements
were conducted at room temperature (Figure S15).
the solvent polarity having only
photostationary states.
a minor effect on the
Figure 3. UV−Vis spectral changes of PEO-stat-PAGE-DAE-BF4 solid film upon
UV irradiation under ambient conditions. The photostationary state (PSS) is
reached after UV irradiation for 22 min.
As applications for these novel PIL materials typically involves
operation in the solid state, our focus was then directed to
investigating the photochemical behavior of PEO-stat-PAGE-
DAE-BF4 spin-coated thin films. Irradiation of PEO-stat-PAGE-
DAE-BF4 films with UV light again resulted in a distinct color
change from near colorless to blue due to photoswitching of the
DAE-BF4 units. In analogy with solution behavior, the UV−Vis
spectra of PEO-stat-PAGE-DAE-BF4 films exhibited an
absorption at ~300 nm (Figure 3, Figure S10) with rapid
switching being observed upon UV irradiation leading to the
appearance of new bands at 390 nm and 672 nm. Significantly,
subsequent irradiation with visible light reversed these spectral
changes, resulting in color loss. For photoswitching of the PIL thin
films, kinetic tracking of the absorption maximum at 672 nm over
time is shown in Figure S11 and Table S4. The above data
clearly demonstrates the reversible photoswitching of polymer-
supported DAE-BF4 units in a solid film with isomerization ratio of
~ 38-44%. It should also be noted that under the same
measurement conditions, the irradiation time required to reach the
photostationary state is only marginally longer for both UV and Vis
light irradiation when compared to solution, again consistent with
Figure 4. (a) Scheme illustrating the in situ conductivity measurement and ionic
conductivity change of PEO-stat-PAGE-DAE-BF4 films with light irradiation. (b)
Ionic conductivity change of above solid films with light irradiation. (c) Open to
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10.1002/anie.201912921
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RESEARCH ARTICLE
closed form isomerization by monitoring UV-Vis absorption intensity at 672 nm
of above solid films with light irradiation.
As shown in Figure 4b, the ionic conductivity of PEO-stat-PAGE-
DAE-BF4 rapidly decreased by ~70% upon UV irradiation and
subsequently could be switched back to its original value upon
exposure to visible light. Compared with solution-based, light
responsive ionic liquids, this new type of solid PIL material not
only has the advantage of high ionic conductivity modulation but
also simplified processing/operation for practical applications.[20–
Figure 5. (a) Schematic illustration of light patterning ionic conductivity of PEO-
stat-PAGE-DAE-BF4 with
a through-plane configuration; (b) Local ionic
conductivity with electrode 1 and 2 at different states.
24]
Significantly, the kinetics and time scale of ionic conductivity
change parallels the change of isomerization of DAE-BF4 units
tracked by UV-Vis absorption spectroscopy, supporting that the
photoswitch controls the property change (Figure 4c). This ionic
conductivity change with light irradiation was reproducible for
multiple cycles (Figure S16), albeit with a slight variation of ionic
conductivity likely due to degradation of the DAE units as seen in
UV-Vis studies.[42] DSC measurements show that the Tg of PEO-
stat-PAGE-DAE-BF4-O and PEO-stat-PAGE-DAE-BF4-C, stays
Conclusion
In summary, we have developed a robust synthetic platform and
demonstrated reversible photoswitching of DAE-based PILs.
These PIL solid films feature continuously tunable ionic
conductivity levels with irradiation time, and a notable maximum
~ 70% drop in conductivity. Taking advantage of this unique light-
responsive ionic conductivity property of PIL thin films, in-situ
spatiotemporal patterning and reversible control over local
conductivity was successfully illustrated. This new light
responsive PEO-stat-PAGE-DAE-BF4 system is envisioned to
lead to new opportunities and research directions for PIL
materials, which hold great potential for smart electrical circuits
and wearable photodetectors. Moreover, the non-invasive nature
of the photomodulation step which avoids the use of any
additional chemical species for modulating ionic conductivity
provides a unique handle for future mechanistic studies. The
design of imidazolium bearing photoswitches with improved
fatigue resistance and thermal stability for next generation PILs
and the impact of fundamental ion-polymer interactions on ion
transport are under investigation.
the same (around 0 C) (Figure S17). The static DP 19F NMR
o
spectrum becomes slightly broader when the DAE-BF4 units in the
polymer are switched from an open to closed form with UV
irradiation, also suggesting a tighter binding and thus reduction in
-
the mobility of the BF4 anions. These observations are fully
consistent with a decrease in ionic conductivity (Figure S18). The
PEO-stat-PAGE-DAE-BF4 with PC sample shows similar light
responsive ionic conductivity behavior as described above
(Figure S19).
The unique light-responsive ionic conductivity property of PEO-
stat-PAGE-DAE-BF4 PIL thin films now enables in-situ
spatiotemporal patterning and reversible control over local
conductivity. As a demonstration (Figure 5), PEO-stat-PAGE-
DAE-BF4 thin films were patterned with Au electrodes (electrode
1 and 2). In state 1, the local ionic conductivity measured with
electrode 1 and 2 is approximately identical. However, when the
film is irradiated through a photomask with UV light, the DAE-BF4
units only undergo switching to their closed form under electrode
2. This results in a decrease in the local conductivity measured by
electrode 2 while electrode 1 remains unchanged (state 2). UV
light irradiation through the opposite photomask then leads to
switching of the DAE units under electrode 1 and a similar
decrease in ionic conductivity is observed (state 3). This
patterning and modulation of local conductivity can be completely
erased through Vis light irradiation, increasing the local
conductivity of both electrodes (state 1). This ability to pattern
ionic conductivity using simple masking and light exposure holds
significant potential for novel stimuli-responsive electrical devices.
Acknowledgements
H.N. and N.S.S. contributed equally. The research reported here
was primarily supported by the National Science Foundation
(NSF) through the Materials Research Science and Engineering
Center at UC Santa Barbara, DMR-1720256 (IRG-2). N.S.S.
acknowledges the Fannie and John Hertz Foundation and the
NSF Graduate Research Fellowship Program under Grant
1650114. Research (N.D.D.) was sponsored by the U.S. Army
Research Office and was accomplished under Contract Number
W911NF-19-D-0001 for the Institute for Collaborative
Biotechnologies. The views and conclusions contained in this
document are those of the authors and should not be interpreted
as representing the official policies, either expressed or implied,
of the U.S. Government. The U.S. Government is authorized to
reproduce and distribute reprints for Government purposes
notwithstanding any copyright notation herein.
Keywords: diarylethene • polymeric ionic liquid • photoswitch •
ionic conductivity • cation-anion interaction
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This article is protected by copyright. All rights reserved.

10.1002/anie.201912921
Angewandte Chemie International Edition
RESEARCH ARTICLE
RESEARCH ARTICLE
Light responsive polymeric ionic
liquids: Responsive PIL materials
based on diarylethene units have
been designed to undergo a light-
mediated change in conductivity. Key
to this modulation is a change in the
cationic character of the imidazolium
bridging unit upon photoswitching.
Irradiation of these materials with UV
light triggers a ~70% drop in
Hui Nie,[a]† Nicole S. Schauser,[b]† Neil
D. Dolinski,[b] Jerry Hu,[b] Craig J.
Hawker,[a,b] Rachel A. Segalman,*[b,c]
Javier Read de Alaniz*[a]
Page No. – Page No.
Light-controllable Ionic Conductivity
in a Polymeric Ionic Liquid
conductivity in the solid state that can
be recovered upon subsequent
irradiation with visible light.
This article is protected by copyright. All rights reserved.