A new class of "electro-acid/base"-induced reversible methyl ketone colour switches

Article abstract of DOI:10.1039/c3tc30769g

Methyl ketone has been designed as a switching unit for electrically addressable molecular colour switches. A newly proposed mechanism of "electro-acid/base" (radical ions)-induced intermolecular proton transfer for the colour switch is proven clearly by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR) and in situ UV-Vis spectroscopy. A dramatic spectral absorption shift (about 291 nm) is observed during the switching, and blue, yellow and green colours are obtained by adjusting the substituents on the methyl ketone-bridged unit. The in situ "electro-acid/base" is far more convenient than the conventional chemical stimulus of acids or bases for the manipulation of the molecular switching properties. This new switching method and molecular structure manipulation will inspire and accelerate the further development of broad switching materials and applications in ultrathin flexible displays, etc.

Full text of DOI:10.1039/c3tc30769g

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Materials Chemistry C
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A new class of “electro-acid/base”-induced reversible
methyl ketone colour switches†
Cite this: J. Mater. Chem. C, 2013, 1,
5309
a
a
Yu-Mo Zhang, Minjie Li, Wen Li,a Zhiyuan Huang,a Shaoyin Zhu,a Bing Yang,a
*
b
a
*
Xiao-Chun Wang and Sean Xiao-An Zhang
Methyl ketone has been designed as a switching unit for electrically addressable molecular colour switches.
A newly proposed mechanism of “electro-acid/base” (radical ions)-induced intermolecular proton transfer
for the colour switch is proven clearly by cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS),
infrared spectroscopy (IR) and in situ UV-Vis spectroscopy. A dramatic spectral absorption shift (about
291 nm) is observed during the switching, and blue, yellow and green colours are obtained by adjusting
the substituents on the methyl ketone-bridged unit. The in situ “electro-acid/base” is far more
convenient than the conventional chemical stimulus of acids or bases for the manipulation of the
molecular switching properties. This new switching method and molecular structure manipulation will
inspire and accelerate the further development of broad switching materials and applications in
ultrathin flexible displays, etc.
Received 24th April 2013
Accepted 25th June 2013
DOI: 10.1039/c3tc30769g
www.rsc.org/MaterialsC
reversible enol–ketone tautomerization on pH changes. The
transformation between the keto and enol forms would func-
Introduction
Electro-switchable organic colour materials have been inten-
sively investigated over the past decades due to their broad
applications in displays,1 data recording,2 sensing,3 optical
communication,4 etc. Most of the colour switching is based on a
mechanism featuring the direct redox state change of the
materials, and they can be divided into two main categories.
The rst class involves conjugated oligomers and polymers,5
such as polythiophenes,6 polyaniline7 and polyselenophene.8
These materials have the advantage of easy processing, and the
disadvantage of slow response times that limits practical
applications. The second category includes small organic
molecules, such as 2,20-bipyridine derivatives,9 5-substituted
isophthalate derivatives10 and organometallic compounds.11
These materials have better colour intensity and a faster
response time, but usually suffer from instability and poor
reversibility and processability. Thus, there has been the
constant need for developing better electro-switchable organic
colour materials.
tion as a switchable bridge for alternative extended conjugation.
Additionally, both the colour tuning and processability of this
class of molecules could be easily improved by varying the
substituents on the aromatic rings. Recent research has indi-
cated that the keto–enol tautomerization can be manipulated by
photo-acid induced intramolecular proton transfer.12 This
inspires us to investigate the possibility of using electrically
generated radical ions as a trigger to induce the keto–enol
tautomerization. However, no reports have demonstrated that
the equilibrium of the keto–enol tautomerization can be
manipulated by an electric eld, which is because the keto–enol
tautomerization usually needs to be induced by a stronger acid/
base, and multiple electrochemical/electrophysical factors
make the elucidation of the reaction mechanism extremely
intricate.
Herein we report the rst example of using radical ions as
convenient acids or bases in situ for the reversible manipulation
of the structure and properties of methyl ketone molecules. Our
electrochromic materials have several appealing properties,
such as excellent colour intensity and contrasts, easy process-
ability, a fast switching speed, and good durability and
reversibility.
We believe it is possible to develop good colour-switchable
molecules by constructing multiple aromatic units with quick
alternative connection units, and methyl ketone seems a good
candidate for such a switchable bridge, as it undergoes a
aState Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, Jilin 130012, China. E-mail: liminjie@jlu.
edu.cn; seanzhang@jlu.edu.cn; Fax: +86-431-85163812; Tel: +86-431-85163812
bInstitute of Atomic and Molecular Physics, Jilin University, Changchun, Jilin 130012,
China
Results and discussion
Design and synthesis
The methyl ketone-bridged derivatives used in this study were
prepared via the Friedel–Cras acylation of aromatic rings with
acid chlorides. In order to improve the acidity of the a-H and
† Electronic supplementary information (ESI) available: experimental procedures
and synthesis. See DOI: 10.1039/c3tc30769g
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facilitate the tautomerization, Ar1 was designed to include
electron donating groups and Ar2 to include electron with-
drawing groups. All of the compounds used in this paper were
puried and fully characterized by conventional analytical
methods, such as 1H-NMR, 13C-NMR, UV-Vis, MS etc. (Scheme 2
and ESI†).
Colour switching mechanism
Verication of the switch feasibility. As an example, the
electro-switchable property of M1 (Scheme 2) was rst studied
by cyclic voltammetry (CV) in acetonitrile (Fig. 1). A brilliant
blue colour appeared when M1 began to be reduced at ꢀ1.03 V,
and disappeared at 0.2 V. The intensity of absorption was
dependent on the redox state of M1, conrming that the colour
switch can be triggered alternately by the electric eld. What's
more, the colour change was very obvious from colourless
(lmax ¼ 273 nm) to bright blue (lmax ¼ 564 nm), with a
maximum absorption wavelength red shi of 291 nm (Fig. 2).
Proof of the keto–enol tautomerization of M1. UV-Vis spec-
Scheme 2 Synthetic scheme for M1–M11. Reagents: (a) SOCl2, CH2Cl2; (b) AlCl3,
CH2Cl2; (c) the same as (a) and (b) with different ST1; (d) TBAPF6, CH3I, CH3CN,
ꢀ1.2 V; (e) t-BuOK, CH3I, THF, N2; (f) ethylene glycol, p-toluene sulphonic acid,
toluene.
M1 (ꢀ1.17 V) is assigned to the one-electron reduction of the
nitro group rather than the carbonyl,13 by comparison with
those of nitrobenzene (ꢀ1.25 V) and M11 (ꢀ2.0 V), a nitro-
eliminated derivative of M1. The reduction of the nitro group is
further conrmed by X-ray photoelectron spectroscopy (XPS)
and infrared spectroscopy (IR). For M1, a single N 1s binding
energy peak at 405.6 eV (Fig. 3A) for the nitro group is observed.
However, when the M1 solution is electrically reduced at ꢀ1.2 V,
a new peak at 401.8 eV appears, which suggests that the nitro
group is reduced under these conditions. In addition, a
1
troscopy, IR, H-NMR and GC (Gas Chromatography) spectro-
scopic analyses of both the electrically reduced and non-
reduced M1 solutions indicates the formation of the enolate
isomer of M1 during the electrical process. This conclusion is
based on the fact that the absorption peak of the M1 solution
(564 nm, Fig. 2) aer being electrically reduced is nearly iden-
tical to that of the enolate of M1 (M1-A, 568 nm) generated by a
strong chemical base (7 equivalents of t-BuOK in CH3CN solu-
tion, Fig. 2). Additionally, its enolate structure is proved by 1H-
NMR spectroscopy (Fig. S2†). Furthermore, the IR data indicates
clearly that the methyl ketone group is converted to the enolate
(1548 cmꢀ1). Comparing the IR spectrum of the M1 solution
with its reduced solution at ꢀ1.2 V, a new peak representing C]
C stretching absorption at 1548 cmꢀ1 is observed, along with
the decrease of the stretching vibration absorption intensities
of both the carbonyl at 1687 cmꢀ1 and the C–H of methylene
decrease in the nitro stretching absorptions (1517 cmꢀ1
,
1352 cmꢀ1) of the blue reduced solution are observed, further
indicating that the colour change is associated with the reduc-
tion of the nitro group (Fig. 3B).
However, no colour change is observed during the electrical
reduction of the nitro group of M9 (a two-substituted derivative
on the methylene group of M1) and M10 (a carbonyl-protected
derivative of M1) (Fig. S1†). Those results clearly prove that the
colour observed from the CV experiment of M1 is closely related
at 1325 cmꢀ1
.
In addition, the existence of the enolate M1-A in the elec-
troreduction process is further proved by reaction with iodo-
methane. The blue colour of the reduced solution fades
gradually aer the addition of iodomethane, and M8 is the only
identied product obtained from the reaction. M8 is conrmed
1
to be a mono-methylated derivative of M1 by H-NMR and GC
(Fig. S3 and S4†). This is further strong evidence proving the
formation of M1-A during this electrochromic process.
The reduction process. To fully understand the electro-
chemical colour switching mechanism, detailed CV analyses of
M1 and its derivatives were performed. The reduction peak of
Fig. 1 (a) Changes in the absorption at 564 nm (top) and the cyclic voltam-
mogram (bottom) of M1 (1.0 ꢁ 10ꢀ3 M) in acetonitrile with 0.1 M TBAPF6 using a
Scheme 1 Illustration of the design of the colour switches by bridging two
aromatic motifs with a methyl ketone group.
glassy carbon electrode (d ¼ 3 mm). Scan rate: 50 mV sꢀ1
.
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from the direct one-electron reduction of M1. Thus, we
proposed that a radical anion (M1-RA, an “effective electro-
base”) is rst generated from the one-electron reduction of the
nitro group of M1. Then this “electro-base” produces the col-
oured M1-A, and turns itself into a radical (M1-R) by extracting a
proton from nearby M1 molecules. When M1-R loses a electron
during the reverse process, a protonated cation (M1-C, an
“effective electro-acid”) is generated and reacts with a nearby
M1-A molecule. Consequently, both species become the initial
colourless ketone form.
The mechanism of the intermolecular proton transfer is
supported by both theoretical and experimental evidence. The
reversible colour change is observed when a mixture of nitro-
benzene and the nitro-absent M6 (or M7, Fig. S15 and S16†) is
stimulated by an electric eld. The observed colour is identical
to the colour from M6 (or M7) being treated with a chemical
base. Meanwhile, any one of the species alone shows no colour
change under the same treatment (Fig. S6 and 7†). In other
words, even though M6 and M7 have no nitro group, their
methyl ketone units can still be switched electrically in the
presence of nitrobenzene under the reduction voltage of the
nitro group via an intermolecular proton transfer between M6
(or M7) and a radical anion of nitrobenzene.
Fig. 2 Absorption spectra of M1 (1.0 ꢁ 10ꢀ5 M) (black curve) in acetonitrile,
treated with potassium tert-butoxide (red curve), and then neutralized with
CH3COOH (blue curve), and absorption spectra of M1 (1.0 ꢁ 10ꢀ3 M) (green
curve) in acetonitrile with 0.1 M TBAPF6, stimulated by a ꢀ1.2 V voltage (pink
curve), then by a 0.8 V voltage (yellow).
The transformation from M1 to M1-RA is exergonic by 6.4
kcal molꢀ1 (Fig. S17†), calculated with B3LYP/6-31+G(d,p),14
which suggests that the reaction involving the intermolecular
proton transfer is thermodynamically more favorable.
Thus, we could further conclude that the mechanism consists
of the following processes. (i) M1 is rst converted to M1-RA
through a one-electron reduction. Then the M1-RA (“electro-
base”) grabs a proton from the methylene group of a nearby M1
molecule to generate M1-R15 and M1-A (the colourant). (ii) When
M1-R is transformed to the acidic M1-C via a one-electron
oxidation, the M1-C gives the proton back to the M1-A. Then the
protonated M1-A transforms back to M1 again by the keto–enol
tautomerization, and this process results in a colour reversal.
Performance of the colour switch
Response speed. The intermolecular proton transfer reaction
induced by the radical anion (“electro-base”) is very fast. There
is still a change in colour even at a scan rate of 10 V sꢀ1 in the CV
experiment. Furthermore, a voltage impulse at 3.0 V for 50 ms
can also result in a detectable colour change of the ITO devices
(Fig. S10†). In addition, the colour intensity is found to increase
gradually with the elevation of the voltage.
The stability and reversibility. To verify the stability and
reversibility of the colour switch, the absorbance changes at
564 nm of M1 in both a thin liquid lm and a poly(methyl
methacrylate) (PMMA) lm in an ITO cell were monitored by
UV-Vis spectroscopy (Fig. 5). The performance of 50 switching
Fig. 3 (A) The XPS spectra of M1 before (black curve) and after (red curve)
reduction at ꢀ1.2 V in acetonitrile with 0.1 M TBAPF6. (B) IR spectra of M1
before (black curve) and after (red curve) reduction at ꢀ1.2 V in acetonitrile with
0.1 M TBAPF6.
to the methyl ketone with an unsubstituted a-proton, which cycles for both the liquid and solid devices are shown in Fig. 5.
enables the molecules to undergo a keto–enol tautomerization. For the solid device, there is much room to improve its stability
Hypothesis of the mechanism. All of the results demonstrate and reversibility. These results conrm that this type of
that the electrically generated coloured intermediate is an switchable molecule could be very stable under an inert envi-
enolate of the methyl ketone-bridged molecule (M1-A, an anion ronment, and M1-A and M1-R could be easily reversed back to
from the loss of a proton from M1) instead of a radical anion the colourless M1 without any observable side products.
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Fig. 4 The switch mechanism of M1.
Fig. 6 (A) Absorption spectra of M1–M7 in acetonitrile after the voltage is
supplied. (B) A real object illustration of an electrically-driven display of “JLU”
based on M1. (C) The real colour of M2–M7 switched electrically in their ITO
devices, with the switch threshold (from CV) provided in brackets (the switch
thresholds of M6 and M7 are labelled as those of the mixture with nitrobenzene).
Conclusions
Fig. 5 (A) Absorbance variation (at 564 nm) of the thin liquid film of M1 in an
ITO device with the switch cycles generated by alternating ꢀ3.0 V (3 s) and 2.0 V
(5 s) voltages. (B) Absorbance variation (at 564 nm) of the PMMA film of M1 in an
ITO device with the switch cycles generated by alternating ꢀ3.5 V (5 s) and 2.0 V
(25 s) voltages.16
In conclusion, a new class of “electro-acid/base”-triggered
reversible keto–enolate colour switches was proposed and
demonstrated with methyl ketone-bridged molecules. The
switching mechanism of intermolecular proton transfer induced
by radical ions as an “electro-acid/base” has been clearly proved.
Colour tunability. Investigation into the colour tunability The new colour switches are speedy and fairly durable, with
and switchability of M1–M7 provides further insight into how several attractive properties such as large colour contrasts, colour
substituent effects alter the electrochromic properties. Blue, tunability and good reversibility. A dramatic spectral absorption
yellow and green colours have been obtained by changing the shi (of about 291 nm) is observed during the switching via keto–
conjugation length, as well as the electron-donating ability on enolate tautomerization. Blue, yellow and green colours are
the donor segments (Ar1) and electron-withdrawing ability on obtained by adjusting either side of the substituents on the
the acceptor segments (Ar2) (Fig. 6, Scheme 2). We nd that methyl ketone unit. The switching speed of the electrically-
increasing the conjugation length or adding a new donor group induced colour switching is less 50 ms. The colour switches are
on the Ar1 side does not have a signicant effect on the colour of used in several prototype ITO devices, and repeated switching in
the enolate isomers of M1–M7. This indicates that the oxygen the devices is achieved. This newly-demonstrated concept of
anion portion of the coloured intermediates is a much stronger “electro-acid/base” in situ for molecular switching is far more
electron donor than Ar1, and it plays the dominant role in the convenient than the conventional chemical stimulus of acids or
colour. However, changing the electron-withdrawing functional bases for practical applications in IT devices, not only because the
group on Ar2 has pronounced effects on both the switchable electric eld is the simplest and easiest method for imple-
colour and the switch threshold of the molecules. The ortho- mentation, but also it can control both the reaction progress and
positioned nitro group on Ar2 (M5) has a slightly higher direction quickly and precisely in a closed system based on the
switching potential value (from ꢀ1.03 V to ꢀ1.06 V) compared needs of the device. This important concept of using an “electro-
with its para-positioned counterpart in M1, but its maximum acid/base” for the reversible manipulation of the molecular
absorption is signicantly red shied from 564 nm to 601 nm. structure and properties will certainly inspire and accelerate the
The CVs of M1–M7 are shown in Fig. S11–S16.† The stronger the further development of broad switching materials and their
electron withdrawing nature of the acceptors, the lower the applicationsinultrathinexibledisplays, datarecording, sensing
reduction potentials that are needed, and vice versa.
and optical communication, etc.
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spectroelectrochemical cell (shown in Fig. S18†). In order to
measure the absorption spectra and the absorption lifetime of
the ITO device under the in situ voltage application, the direct
current mode was applied to the cell by a Bio-logic electro-
chemical work station.
Experimental
Synthesis
The general synthetic procedure for M1–M7 and M11 using M1 as
a template is as follows: SOCl2 (0.52 ml, 7.6 mmol, 1.2 eq.) was
added to a solution of 2-(4-nitrophenyl)acetic acid (1.14 g,
6.3 mmol, 1 eq.) in dichloromethane (10 mL) at 25 ꢂC and stirred
for 4 h, then benzene (6 mmol, 1 eq.) and AlCl3 (7.8 mmol, 1.3 eq.)
were added in sequence to the above solution in an ice bath under
a nitrogen atmosphere. Aer 4 h, the reaction mixture was poured
onto ice, and the aqueous layer was extracted with ethyl acetate
(3 ꢁ 10 mL). The combined organic layers were washed with brine
and dried over MgSO4. Aer being ltered and concentrated in
vacuum, the product was separated by column chromatography
using petroleum ether and ethyl acetate (v/v ¼ 10 : 1) as the eluent.
Preparation of sandwich-type ITO cells
ITO cells sandwiched with liquid lm. A two-electrode cell
was constructed with ITO glass electrodes. The mixture solution
containing methyl ketone-bridged molecules and TBAPF6 was
sandwiched between the ITO electrodes. Polydimethylsiloxane
(PDMS) lm was used as the spacer (shown in Fig. S19†).
ITO cells sandwiched with PMMA lm. A mixture of 30 wt%
of PMMA, 70 wt% of propylene carbonate, TBAPF6 and the
methyl ketone-bridged molecule was stirred for 24 h. A homo-
geneous phase with electrochromic properties was obtained.
The above mixture lm was laminated between two ITO
electrodes. The thickness was set to about 0.1 mm (shown
in Fig. S20†).
Characterizations
The UV-Vis absorption spectra were measured using a Shi-
madzu UV-2550 PC double-beam spectrophotometer.
The cyclic voltammograms were obtained from a Bio-logic
electrochemical work station. The experiments were performed
under the protection of argon in acetonitrile containing TBAPF6
(0.1 mol Lꢀ1) as the supporting electrolyte. The three-electrode
cell consisted of a glassy carbon working electrode (Shenhua,
China) with a surface area of 7.07 mm2, a Pt wire counter
electrode (Shenhua, China) and an AgNO3/Ag reference elec-
trode (Shenhua, China). All of the redox potentials were refer-
enced to internal ferrocene (E0 (Fc+/Fc0) ¼ 0.4 V vs. SCE), added
at the end of each CV experiment.
Acknowledgements
We thank Jilin University, State Key Lab of Supramolecular
Structure and Materials for start-up support. This work was
supported by the National Science Foundation of China (grant
no. 21072025). The authors also acknowledge Prof. Erkang
Wang, Dr Youxing Fang, Prof. Hansong Cheng, and Dr Shubin
Zhao for helpful discussions relating to this project.
The electron paramagnetic resonance (EPR) spectra were
recorded on a JEOL JES-FA 200 EPR spectrometer.
Notes and references
The GC (Gas Chromatography) analysis was performed on a
Shimadzu GCMS QP 2010 Plus instrument.
The ESI-HRMS analysis was performed on an Agilent 1290-
micrOTOF-Q II mass spectrometer. Accurate masses were
reported for the molecular ion [M + H]+ or [M]+.
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5314 | J. Mater. Chem. C, 2013, 1, 5309–5314
This journal is ª The Royal Society of Chemistry 2013