Stimulus-Mediated Ultrastable Radical Formation

Full text of DOI:10.1016/j.chempr.2020.05.005

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Article
Stimulus-Mediated Ultrastable Radical
Formation
Jade A. McCune, Moritz F.
Kuehnel, Erwin Reisner, Oren A.
Scherman
oas23@cam.ac.uk
HIGHLIGHTS
Reversible switching behavior of
organic radicals in various DESs
Radical lifetime in air was >105
times that in water and >6 months
that in air
Stabilization through
supramolecular engineering of
the solvent environment
Model electrochromic device
constructed in the stable radical
state
We describe an approach to taming air-sensitive chemical moieties important for
solar energy conversation, electrochromic devices, photocatalysis, and
biomimetic systems through supramolecular engineering of the solvent
environment. Typically, transient radical species persist for over 6 months in air and
have a lifetime in air more than 105 times that observed in aqueous environments.
These findings will overcome many limitations for the use of these chemical
species in a variety of energy applications.
McCune et al., Chem 6, 1–12
July 9, 2020 ª 2020 Published by Elsevier Inc.
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Article
Stimulus-Mediated Ultrastable Radical Formation
Jade A. McCune,1 Moritz F. Kuehnel,1,3 Erwin Reisner,2 and Oren A. Scherman1,4,
*
SUMMARY
The Bigger Picture
Organic radicals are widely used
chemical species that form the
basis of solar energy
Organic radicals are reactive and often short-lived species typically
formed through either the addition of a chemical agent or photo-
chemical means. On account of their open-shell electronic structure,
they have attracted attention based upon their magnetic properties
and desirable spectroscopic behavior. Redox-sensitive molecules,
such as viologen (V), undergo one-electron reductions to form
radical species. These species hold significant potential in myriad
applications but are limited because they are rapidly quenched by
oxygen in air. Using methyl viologen (MV) as an example, we show
that the MV radical (MV+,) can be formed through electrochemical,
chemical, and photochemical stimuli as well as a novel thermal stim-
ulus in various deep eutectic solvents (DESs) and was found to be
exceptionally stable. The conductive properties of DESs allowed
for the fabrication of an aerobic electrochromic device through a
straightforward, economical approach. Our report represents a
unique approach to extending reactive radical lifetimes in air
without altering the parent structure.
conversation, electrochromic
devices, and photocatalysis and
biomimetic systems.
Their lifetimes, however, are short
lived under aerobic conditions on
account of their rapid reaction
with oxygen. Innovative ways to
stabilize such radical species
include alteration of their
chemical and electronic structure,
incorporation within mechanically
interlocked architectures, or
encapsulation within various
receptors. Herein, we describe an
approach to taming these
INTRODUCTION
sensitive chemical moieties
through supramolecular
The use of a stimulus to alter the structure and properties of redox-active molecules
has been exploited across a wide range of chemistries. Redox-responsive systems
are readily switched with electrochemical, chemical, or in some cases, photochem-
ical stimuli. Redox systems are desirable because, unlike other responsive systems,
changes occur in both the charge and spin states of the molecule.1 These systems
have been utilized for many applications, including drug delivery,2 displays,3 elec-
tronic memory,4 batteries,5 and tunable materials.1 Typical redox-active molecules
include metallocenes (such as ferrocene) and aromatic derivatives (such as tetrathia-
fulvalene, naphthalene diimides, and viologen derivatives).1 Isolation and character-
ization of organic radicals are often impeded by their instability and transient nature.
To overcome this limitation, encapsulation within receptors has been a common way
to stabilize and prolong the lifetime of such species. This has been demonstrated
elegantly for chemically reactive species,6–8 radical cations,9 and anions.10–12 Of in-
terest are dicationic viologen derivatives, which can undergo a one-electron reduc-
tion to form intensely colored radical cations with chemical and electrochemical ap-
plications as electron mediators. These species are stable in the absence of any
oxidizing agent because the unpaired electron is delocalized across the p-system.13
However, in the presence of molecular O2, their quenching is extremely rapid.14 The
electrochromic behavior of viologen derivatives, particularly the high optical
contrast between redox states, has led to their application in electrochromic devices
and displays. Companies have produced electrochromic devices based on viologen;
for example, Genetex has produced a best-selling electrochromic mirror currently
used in cars.13 Nevertheless, the poor dication solubility and short radical lifetime
of methyl viologen (MV) in air have limited its applications within current commercial
engineering of the solvent
environment. Exploring the
reversible redox-switching
behavior of methyl viologen as a
model organic radical in various
deep eutectic solvents (DESs), we
report the generation of radical
species in air upon application of
external stimuli with lifetimes 105
times greater that than in aqueous
environments, far exceeding
previous reports.
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devices, which instead use substituted or immobilized viologen derivatives,
requiring additional costly manufacturing steps. The use of receptors to stabilize
MV radical cations has been reported. Kim and co-workers demonstrated enhanced
stability upon encapsulation inside a macrocyclic host, cucurbit[n]uril.15,16 This
concept was shown to be extended to the formation of 2D supramolecular organic
frameworks.17 The formation of a crystalline supramolecular complex between MV2+
and a bambusuril macrocycle was shown by Sindelar and co-workers.18 The crystals
could undergo photoinduced electron transfer to form MV+, radicals within the crys-
tals, which were found to have a half-life of several hours in air. Recently, the encap-
sulation of MV2+ in a zeolite resulted in a photochromic material through electron
transfer between the MV2+ and the anionic zeolite framework. The dense packing
of MV2+ within the zeolite pores resulted in a short electron-transfer pathway and
thus photochromic behavior in addition to stability of the radical state given that
the framework prevented contact with oxygen.19 Stoddart and co-workers have
put significant research effort into constructing mechanically interlocked molecules
based on MV2+. The most exploited example is the formation of cyclobis(paraquat-
p-phenylene) (CBPQT4+), i.e., ‘‘blue box,’’ where two viologen moieties are joined
through a spacing bridge to form a box-like structure.20 Although not air stable, it
allows access to a more stable MV+, radical species, which can undergo molecular
recognition to form charge-transfer complexes and extended structures, such as ro-
taxanes21 and catenanes.22,23
Deep eutectic solvents (DESs) were first introduced by Abbott and co-workers in
2003 as a new class of ionic liquids (ILs) composed of two non-symmetric compo-
nents, which can undergo complexation.24,25 Typically, one component is a qua-
ternary ammonium salt (commonly choline chloride [ChCl]), and the other is a
metal salt or hydrogen-bond donor (e.g., amide, alcohol, or acid).26,27 The
hydrogen-bonding interactions that occur during complexation result in charge
delocalization, and thus the melting point for the complex is lower than the indi-
vidual melting points of the components. The precise organization of the compo-
nents upon complexation, and the resulting liquid structure, is not yet fully under-
stood for all eutectic mixtures given that the field is still in its infancy. Recent
studies based upon computational and neutron scattering data have probed the
precise hydrogen-bonding interactions and the bulk liquid structure for the most
common DES, ChCl-urea (1:2).28,29 Reports on the nanostructure (dynamic molec-
ular configuration) of DESs,28 particularly the effect of water on them,30 are also
emerging.
Although DESs are not composed of discrete anionic and cationic species and have
chemical properties very different from those of ILs, they do have similar physical
properties, such as low vapor pressure, non-flammability, and a wide temperature
range over which they remain liquid.26,27 Both DESs and ILs are ‘‘tunable’’ solvents
1Melville Laboratory for Polymer Synthesis,
Department of Chemistry, University of
Cambridge, Lensfield Road, Cambridge CB2
1EW, UK
in that their properties can be altered for specific applications according to the se-
lection of individual components, and over 106 different combinations have been
predicted.26 In contrast to that of ILs, the preparation of DESs is facile, non-toxic,
2Christian Doppler Laboratory for Sustainable
and inexpensive, and many components are produced annually on a multi-ton scale.
DES applications to date have been based upon their excellent solubilizing proper-
ties31 and mainly focused on areas such as electrodeposition32 but have also been
used for the synthesis of nanoparticles,33 for gas adsorption,34 and as a solvation
medium for traditional organic transformations.35,36
SynGas Chemistry, Department of Chemistry,
University of Cambridge, Lensfield Road,
Cambridge CB2 1EW, UK
3Present address: Department of Chemistry,
Swansea University, Singleton Park, Swansea SA2
8PP, UK
4Lead Contact
*Correspondence: oas23@cam.ac.uk
https://doi.org/10.1016/j.chempr.2020.05.005
Here, we utilize the nanostructure of DESs as a novel supramolecular confined envi-
ronment to explore the redox properties of MV as an exemplar organic radical
2
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species and show stimuli-responsive behavior toward electrochemical, chemical,
and photochemical stimuli, as well as a novel thermal stimulus. In all cases, we
observe the typically short-lived radical cation to be extremely persistent in DESs
even in oxygenated environments with drastically extended lifetimes approximately
105-fold longer than in water. Accessing a long-lived air-, light-, and heat-stable
redox-responsive radical under ambient aerobic conditions holds tremendous po-
tential in areas including switchable displays and smart windows. The persistence
of MV+, in DESs negates bleaching and renders current efforts to improve solubility
and write-erase efficiencies redundant. Moreover, in contrast to current technolo-
gies, our approach is economically and environmentally preferable as it uses
components already produced on a multi-tone scale annually and requires only
solution-based processing methods.
RESULTS AND DISCUSSION
Electrochemical Switching
MV is a redox-active molecule (Figure 1A) that can undergo a one-electron reduction
to form an intensely colored blue solution (Figure 1B). To determine how the solvent
nanostructure affects the redox behavior of MV, we explored the electrochemical
response of MV2+ in different DESs. DESs themselves are known to act as electro-
lytes,37 and therefore, all measurements were undertaken with solutions of MV2+
in neat DES with all redox potentials given in reference to the hydroxymethylferroce-
ninium/hydroxymethylferrocene redox couple (FcMeOH), serving as an internal
reference. Cyclic voltammetry in three typical DESs—ChCl-urea, ChCl-ethylene gly-
col, and ChCl-glycerol—showed that MV2+ could undergo two consecutive one-
electron reductions (Figures 1C and S1), and both processes were fully reversible.
We performed consecutive cycling between MV2+, MV+, and MV0 to demonstrate
cycling stability, a key requirement for electrochromic and battery applications.
Remarkably, all DES-MV solutions were found to be stable over 100 cycles with no
change in the electrochemical response and no sign of MV degradation. This finding
was particularly impressive given that the measurements were performed in air
without any degassing of the solutions. Such stability in air is unusual for radical cat-
ions such as MV+, which are highly reactive toward oxygen, and suggests stabiliza-
tion of MV+ in DES environments.
Chemical Switching
Switching between MV2+ and MV+, by using a chemical stimulus was investigated
through monitoring the optical properties upon the addition of different reducing
agents (Figure S2). A visible color change from colorless to an intense blue was
observed, indicating the formation of the MV+, radical cation with a characteristic
UV-visible (UV-vis) spectrum (lmax = 399 and 609 nm). To re-oxidize MV+, to MV2+
,
we bubbled solutions with air to introduce O2 into the system and quench the
radical. When purged with air, the solution slowly returned to colorless, which
was associated with the loss of all absorption between 300 and 800 nm
(Figures 1D and S3).
Photochemical Switching
MV2+ has shown photochromic behavior in solution,38 within films,39 and in the crys-
talline state,18,39 leading to applications in photonic devices, erasable data-storage
systems, and liquid crystals. The photochromic behavior is reported to occur
through photoinduced electron transfer between the counter anion and MV2+
cation.39 Typically, this requires irradiation with middle UV light; however, using nat-
ural light as a stimulus is highly desirable for applications such as self-tinting
windows.
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Figure 1. Summary of Electrochemical, Chemical, and Photochemical Redox Behavior of MV in an Example DES: ChCl-Glycerol
(A) Structural representation of the redox behavior of MV and changes in magnetic behavior upon oxidation and reduction.
(B) Images of vials showing MV in ChCl-glycerol DES (0.38 mM) in air before and after reduction.
(C) Cyclic voltammetry in air of MV in ChCl-glycerol DES (5 mM) shows the second cycle (red) and the 102nd cycle (black) with FcMeOH as an internal
standard. The asterisk denotes the FcMeOH+/FcMeOH couple (glassy carbon working electrode, v = 100 mV sÀ1, 40ꢀC).
(D) UV-vis spectra of MV2+ in ChCl-glycerol DES (0.39 mM) upon the addition of a reducing agent (monoethanolamine) to form MV+, (blue) and after
purging with air (red).
(E) Photographs depicting the growth of MV+, species (blue) at the interface between the solution and the incident light with time (see Video S1).
(F) Photochromic cycling behavior between MV2+ and MV+, in ChCl-glycerol DES. Cycles consisted of photoirridation with solar light simulator for 5 min
following by purging with air for 5 min repeated over multiple cycles.
(G) UV-vis spectra showing the formation of MV+, from photoirridation of MV2+ (3.8 mM) in ChCl-glycerol DES with time.
(H) UV-vis spectra showing the oxidation of MV+, formed in (G) to MV2+ through purging with air (30 mL minÀ1).
The photochromism of MV in three different DESs was probed with simulated solar
light (AM 1.5G, 100 mW cmÀ2). Upon irradiation, the colorless MV2+ solution turned
blue, indicative of MV+, generation, which was confirmed by its characteristic absor-
bance in the UV-vis spectrum (Figure S4). The radical was observed to originate at
the interface of the solution as the incident light formed a blue layer that gradually
grew into the solution over time (Video S1; Figure 1E). Visualization of this process in
real time highlights the unique way in which the MV+, species is held within the nano-
structure of the DES. Remarkably, no difference was observed whether photoreduc-
tion experiments were performed under aerobic or anaerobic conditions. Moreover,
the resulting MV+, solutions were stable in air; measurements were taken over
several hours open to air without further irradiation, showing no detectable loss of
absorbance at 609 nm in the UV-vis spectrum. This suggested that the MV+ was
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not being quenched by atmospheric O2, indicating again that the DESs have a sta-
bilizing effect.
It was found that a MV solution could undergo multiple cycles of photoirradiation
and purging without any impact on performance (Figure 1F). The rate of photo-
chromic coloration could be further controlled by varying the nature of the DESs
(Figures 1G and S4). For alcohol-based DESs (ChCl-ethylene glycol and ChCl-
glyerol), the formation of MV+, was faster than that of amide-based ChCl-urea
DESs under the same conditions. We postulate that this was a result of the photore-
duction mechanism given that alcohols are known to rapidly quench the MV2+
excited state with the formation of MV+, 40 The rate of MV+, formation could be
.
further controlled by varying the MV2+ concentration (Figure S5). Decoloration of
the MV+, solution was achieved by purging the solution with air (Figures 1H and
S6) with the rate of decoloration dependent on gas flow into the solutions (Figure S7);
all solutions reverted to colorless within 150 s.
Thermal Switching
In order to employ heat as a stimulus, we made use of a thermal phenomenon
unique to ChCl-urea DES. We studied thermochromism by monitoring the UV-vis
spectrum of a solution of MV2+ in ChCl-urea DES as a function of temperature (Fig-
ure S8). Heating to 130ꢀC for 30 min in air resulted in a blue solution with charac-
teristic absorbances at l = 399 and 609 nm, consistent with the formation of MV+,
(Figure 2A). The presence of the paramagnetic MV+, radical was further corrobo-
rated by electron paramagnetic resonance (EPR) spectroscopy. At 80ꢀC, the solu-
tion was EPR silent (Figure S9A); however, upon heating to 130ꢀC, a strong EPR
signal appeared (Figure 2B; g = 2.0023), indicative of the paramagnetic MV+,
radical cation.18,19 Control spectra of ChCl-urea DES heated to 130ꢀC in the
absence of MV2+ (Figure S9B) were found to be EPR silent, showing that the para-
magnetic species is not a result of solvent degradation. Moreover, the EPR spectra
of the thermally reduced MV2+ was analogous to that of a chemically reduced spe-
cies (Figure S9C). Radical formation was similarly observed in other DESs when a
catalytic amount of urea was added.
We carried out detailed mass spectrometry (MS) and gas-phase infrared (IR) spec-
troscopy experiments to elucidate the mechanism. In both the MS and IR spectra,
the formation of NH3 (z2.6 mM) was observed when ChCl-urea DES was heated
to 100ꢀC (Figures 2C, S10, and S11). Upon further heating to 130ꢀC, NH3 was still
observed (z30 mM); however, in both the IR and MS spectra, (Figures 2D and
S12) additional peaks appeared, which were identified as trimethylamine (TMA)
(z2 mM) (Figures 2D and S13). This suggested that NH3 was involved in the forma-
tion of TMA.
To further probe the underlying mechanism, we prepared four deuterated DES var-
iants. These had no deuteration (HH-DES), deuteration of ChCl (DH-DES), deutera-
tion of urea (HD-DES), or deuteration of both components (DD-DES). When these se-
ries of deuterated DES mixtures were heated, the NH3 produced was shown to
originate from the urea component given that no difference was observed between
the MS spectra of HH-DES and DH-DES, whereas HD-DES and DD-DES showed an
additional peak at m/z = 20 corresponding to NH3 (Figure S12). TMA was shown to
arise from the ChCl component, and the predominant peak at m/z = 58 (N(CH3)3)
was shifted to m/z = 67 in both DH-DES and DD-DES, indicating the formation of
N(CD3)3 (Figure S12). It is likely that smaller quantities of dimethylamine and methyl-
amine were also formed; however, they were masked by the TMA fragmentation.
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Figure 2. Thermal Switching of MV in ChCl-Urea DES
(A) UV-vis spectra of MV+, in ChCl-urea DES after heating of the MV2+ solution to 130ꢀC. The spectrum shows characteristic lmax = 399 and 609 nm (inset:
photo of the MV+, solution).
(B) EPR spectrum of MV+, in ChCl-urea DES (1 mM) formed through thermal stimulus.
(C) Schematic depicting the formation of DES and structural changes after thermal treatment.
(D) Gas-phase IR spectra of ChCl-urea DES heated to 130ꢀC (blue) and a reference spectrum of NH3 (black). Zoomed inset shows 2,500–3,000 and 2,000–
1,000 cmÀ1 regions, heated DES (blue), and the NMe3 reference (red).
(E) Proposed mechanism for MV2+ reduction induced within ChCl-urea DES upon the introduction of a thermal stimulus.
The proposed cycle (Figure 2E) was initiated by the generation of NH3 at 100ꢀC from
condensation of urea to form biuret.41 When the temperature was increased to
130ꢀC, NH3 reacted with ChCl to form TMA and monoethanolamine (MEA) (which
was not observed in the gas phase because of its low vapor pressure), which is a
known reducing agent.42,43 UV-vis studies confirmed that adding MEA to MV2+ re-
sulted in the formation of MV+, (Figure S14), whereas adding NH3 and TMA did
not (Figures S15 and S16). Serendipitously, the reaction of MEA with MV2+ formed
MV+, regenerating NH3.42,43 The cycle (Figure 2E) shows how both ChCl and urea
DES components play key roles. When a deuterated MV isotopolog, d6-MV2+, was
used, no change in the MS was observed, and thus any potential role of MV2+ itself
in the reduction pathway was ruled out (Figure S17).
Thermal-mediated reduction had not been previously reported; therefore, this new
stimulus was tested for another class of radical species, perylene bis(diimides) (PDI)
(Figure S18). Heating a solution of PDI in ChCl-urea DES to 130ꢀC resulted in thermal
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Figure 3. Stability of MV+, Species in DES under Aerobic Conditions
(A) Long-term UV-vis measurement of MV+, in ChCl-urea DES (0.39 mM). Spectra were recorded over a 100 h period with no change in absorbance.
(B) Images showing a solution of MV+, in ChCl-urea DES being poured into air-saturated water highlight how the DES structure is protecting the radical
from O2. The solution remained blue until complete mixing had occurred and the DES nanostructure was destroyed.
(C) Overview of the stability of MV+, in three different DES and comparison to water (pH 13). Solutions of MV+, were prepared by chemical reduction
(MEA, 90.6 mL), and reoxidation as a function of time was measured through continuous acquisition of UV-vis spectra during purging with air (30 mL
minÀ1 for DES; 0.1 mL minÀ1 for water [pH 13]).
reduction analogous to that observed for MV2+, forming a radical species as evi-
denced by UV-vis and EPR spectroscopies (Figure S19) The EPR spectrum was anal-
ogous to that of a PDI radical formed from chemical reduction (Figure S20).
Long-Lived Nature of the Radical Species
Quenching of MV+, occurs through reaction with oxygen and typically happens
readily in the liquid and solid state upon exposure to air. However, throughout all
experiments, we observed that the radical species was remarkably persistent under
aerobic conditions. To probe the lifetime of the radical in air, we exposed a solution
of MV+, in ChCl-urea DES to air and recorded UV-vis spectra over a 100 h period
(Figure 3A). The solution was not degassed, nor was any attempt to minimize oxygen
made; however, the solution remarkably showed no change in absorbance at 609 nm
during this time period. Solutions prepared and stored in air were found to remain
blue for >6 months, and thus we forecast a lifetime of >>6 months for MV+, in
DES. The persistence of the radical in the DES environment despite exposure to
air suggested that oxygen diffusivity was minute. A time-lapse video of MV+, in
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DES (Video S2) showed the solution turning colorless at the interface, whereas the
bulk remained blue for over a month. This indicated that oxygen was able to
permeate the solution and quench the radical but on a much slower timescale
than observed in any other solvent (weeks versus seconds).
The low oxygen diffusivity into the DES is a contributing factor to the long radical life-
time; however, it was unclear how stable the radical would be when introduced into
an oxygenated environment or when oxygen was forced into the DES environment.
As shown in Figure 3B, when a solution of MV+, was poured into air-saturated water,
the radical persisted in the DES solution. The unique nanostructure of the DES
shielded the radical species from interaction with oxygen in the water, and thus a
blue solution of MV+, in DES was present at the bottom of the flask. The blue color
remained until the two solutions were completely mixed. The persistence of the
radical was also observed when water was added to a DES MV+, solution (Video S3).
To demonstrate the stabilization of MV+, in DES versus water, we forced oxygen into
the system by purging aqueous and DES MV+, solutions with air (30 mL minÀ1) and
recording UV-vis spectra in parallel to monitor the quenching of MV+, (Figure 3C). It
was necessary to degas the aqueous MV+, solution and seal it under a N2 atmo-
sphere as, in air, the radical species did not persist long enough for any measure-
ments to be taken. In contrast, the DES solution was not degassed or sealed. Within
less than 4 min of purging with air (0.1 mL minÀ1), the aqueous MV+, solution was
completely quenched; however, in DES under more extreme purging conditions
(30 mL minÀ1), the radical species persisted for 2–15 h, an enhancement factor of
approximately 105. The prolonged radical lifetime was not found to be dependent
on the viscosity of the DESs. While the DES with the lowest viscosity, ChCl-ethylene
glycol (36 cP),26 did result in the shortest radical lifetime upon purging with air (Fig-
ure 3C), the MV+, was found to be significantly more persistent in a DES of medium
viscosity, ChCl-glycerol (376 cP),26 with a lifetime more than double (900 versus
400 min) that of the more viscous ChCl-urea DES (632 cP).26 These findings suggest
that stabilization of the radical species arises from a number of different factors,
notably not simply oxygen diffusivity. The non-linear relationship between viscosity
and lifetime indicates that the stabilization of the radical species arises from factors
other than the diffusivity of oxygen into the DES, which would be controlled by vis-
cosity. The stabilization most likely arises from a combination of different factors,
including poor solubility of oxygen in DESs (see Supplemental Information) and
the unique DES nanostructure and the interactions between the radical and the sol-
vent, which shield the species from oxygen present in the surrounding environment.
In this case, the nature of the hydrogen-bond-donor component (alcohol versus
amide) was found to play a role. To further probe the solvent-radical interactions
responsible for stabilization, we carried out extensive neutron scattering experi-
ments, which unfortuantely only revealed information on the bulk solvent
properties.44
Application toward Fabrication of a Smart Window
The redox behavior of MV has been exploited in the development of electrochromic
devices, such as displays, mirrors, and smart windows, by many industry leaders
because of the high optical contrast between the MV2+ and MV+, states. Typically,
this requires chemical modification, immobilization, encapsulation, or incorporation
into a nanomaterial to prevent bleaching of the MV+, state. In contrast, the MV+,
radical is ultrastable in DES without any costly modifications, and the electrochem-
istry of unmodified MV2+ was found to be completely reversible and stable over 100
cycles in air (vide supra). Moreover, the DES can act as an electrolyte itself, and all
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Figure 4. Fabrication and Performance of a ‘‘Smart Window’’ Based on MV in DES
(A) Illustration of a ‘‘smart window’’ shows color modulation with the application of bias.
(B) Schematic representation of the electrochromic device shows the components involved and the setup.
(C and D) Photographs of the ‘‘smart window’’ upon application of a (C) reductive bias (À3 V, 15 s) and (D) oxidative bias (À3 V, 90 s).
(E) Electrochemical switching of the ‘‘smart window’’ shows the alternating bias applied (top) and resultant change in absorbance at 609 nm monitored
by UV-vis spectroscopy (bottom) over multiple cycles.
components are cheap, non-toxic, and biodegradable, thus making this system
ideal for the fabrication of an electrochromic device (Figure 4). Unique to this system
are four different stimuli that can be used to modulate the color, whereas current
chromic devices typically use only one.
A simple device consisting of two pieces of fluorine-doped tin oxide (FTO) separated
by a membrane with ChCl-ethylene glycol DES on one side and the same DES with
MV2+ on the other was constructed (Figure 4A). Upon application of a negative bias
(À3 V) between the two electrodes, the device reversibly changed from colorless to
blue. This color was found to be stable even when the bias was removed (Figure 4B),
and only when a positive bias (+3 V) was applied did the device revert to the colorless
state (Figure 4C). Application of a negative bias (À3 V) led to an increase in the
absorbance at 609 nm, indicative of the formation of MV+,, whereas application of
a positive bias resulted in a decrease in absorbance at 609 nm, confirming reoxida-
tion to MV2+. Switching behavior of the device was rapid and robust, and the device
could undergo multiple switching cycles without bleaching, precipitation, or loss of
performance (Figure 4D). The use of DES thus allows for a highly desirable low-cost
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device construction using only inexpensive and widely available components,
namely ChCl (chicken feed), urea (produced on a multi-ton scale), and MV (common
herbicide). Its robust nature and excellent performance lead us to believe that this
technology holds significant potential in electrochromic applications.
Conclusions
The formation of MV+, in various DESs has been shown with electrochemical, chem-
ical, and photochemical stimuli and, for ChCl-urea DES, a unique thermal stimulus.
Typically, organic radicals, such as MV+,, are known to be unstable, rapidly
quenched upon exposure to oxygen present in air; however, in DES, we found the
radical to be extremely stable under aerobic conditions with a projected lifetime
of over 6 months. The stability is a specific feature of DESs and arises from the unique
chemical environment provided by the liquid nanostructure, which protects the
radical species from atmospheric oxygen. This supramolecular solvation shielding
phenomenon allowed the lifetime of MV+, in air to be extended without altering
the electronic or chemical structure of the molecule. Access to a persistent, air-sta-
ble, and redox-responsive radical in DES signifies a step change in the chemistry of
highly reactive chemical species unveiling a new methodology for taming these, and
undoubtedly other, moieties in air. The use of viologens in electrochromic devices,
including ‘‘smart windows,’’ has been impeded in the past by poor write-erase effi-
ciencies and rapid bleaching in air, requiring the development of derivatized mate-
rials. The air-stable MV+, in DESs was found to be applicable in the fabrication of a
smart window through cheap, solution-based methodology. The window fabricated
was inexpensive, robust, and responsive to multiple different stimuli, offering a new
approach to the design and construction of cost-effective switchable devices for
next-generation nanotechnology.
EXPERIMENTAL PROCEDURES
Resource Availability
Materials Availability
This study did not generate new unique reagents.
Lead Contact
Further information and requests for resources and reagents should be directed to
and will be fulfilled by the Lead Contact, Professor Oren A. Scherman (oas23@
cam.ac.uk).
Data and Code Availability
The published article includes all datasets generated or analyzed during this study.
This study did not generate any code.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.05.005.
ACKNOWLEDGMENTS
J.A.M. would like to acknowledge the EPSRC for a PhD studentship (EP/K503009/1).
O.A.S. acknowledges UK Engineering and Physical Sciences Research Council grant
EP/L027151/1. M.F.K. and E.R. acknowledge the Christian Doppler Research Asso-
ciation (the Austrian Federal Ministry of Science, Research, and Economy and the
National Foundation for Research, Technology, and Development), the OMV
Group, and the EPSRC (IAA Follow-on Fund). We thank Mr. Sam Schott for his
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assistance with EPR measurements and Dr. Kamil Sokołowski for useful discussions
throughout.
AUTHOR CONTRIBUTIONS
Conceptualization, J.A.M.; Methodology, J.A.M. and M.F.K.; Investigation, J.A.M.
and M.F.K.; Writing – Original Draft, J.A.M.; Writing – Review & Editing, J.A.M.,
M.F.K., E.R., and O.A.S.; Funding Acquisition, E.R. and O.A.S.; Resources, E.R.
and O.A.S.; Supervision, E.R. and O.A.S.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: November 15, 2019
Revised: February 28, 2020
Accepted: May 8, 2020
Published: June 12, 2020
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