Toward purple-to-green-to-transmissive-to-black color switching in polymeric electrochrome

Article abstract of DOI:10.1021/ma501975m

A novel 3,4-ethylenedioxythiophene (EDOT)-based donor-acceptor electroactive monomer (HCQE) bearing carbazole subunit is synthesized, and then the corresponding polymer, poly(HCQE), is directly deposited onto ITO/glass surface via electrochemical process. Spectroelectrochemical studies demonstrate that poly(HCQE) is capable of showing both n- and p-doping processes. Hence, this low band polymer serves the multielectrochromic feature at three separate states (-0.4 to -2.0 V, -0.4 to 1.2 V, and 1.2 to 2.0 V) with a highly unique near colorless and colored states, fast switching times, redox switching stability, and exceptional transmittance change. To the best of our knowledge, this is the first report of a neutral state green multielectrochromic polymer exhibiting successive switching of colored-to-bleached-to-colored in three distinct regimes.

Full text of DOI:10.1021/ma501975m

Article
pubs.acs.org/Macromolecules
Toward Purple-to-Green-to-Transmissive-to-Black Color Switching in
Polymeric Electrochrome
†
†
†
§
†
̈
Elif Karabiyik, Emre Sefer, Fatma Baycan Koyuncu, Murat Tonga, Eyup Ozdemir,
̈
,‡
and Sermet Koyuncu*
†
‡
Department of Chemistry, Faculty of Sciences and Arts, and Department of Chemical Engineering, Faculty of Engineering,
anakkale Onsekiz Mart University, 17020 Canakkale, Turkey
Department of Chemistry, University of MassachusettsAmherst, Amherst, Massachusetts 01003, United States
C
§
̧
̧
*
S Supporting Information
ABSTRACT: A novel 3,4-ethylenedioxythiophene (EDOT)-based
donor−acceptor electroactive monomer (HCQE) bearing carbazole
subunit is synthesized, and then the corresponding polymer, poly(HCQE),
is directly deposited onto ITO/glass surface via electrochemical process.
Spectroelectrochemical studies demonstrate that poly(HCQE) is capable
of showing both n- and p-doping processes. Hence, this low band polymer
serves the multielectrochromic feature at three separate states (−0.4 to
−
2.0 V, −0.4 to 1.2 V, and 1.2 to 2.0 V) with a highly unique near colorless
and colored states, fast switching times, redox switching stability, and
exceptional transmittance change. To the best of our knowledge, this is the
first report of a neutral state green multielectrochromic polymer exhibiting
successive switching of colored-to-bleached-to-colored in three distinct
regimes.
INTRODUCTION
color. It requires the absorption bands in the red and blue parts
■
of the visible spectrum at the neutral state and also depletion in
the oxidized state with the same applied potential. However, a
dual control of both chromophores with the same applied
potential is rather challenging. For that reason, a limited
number of studies referring neutral state green polymer have
come out. In 2004, Wudl et al. reported the first example of
neutral state green polymer based upon a synthetic design
relying on the insertion of two separate chromophores
generating a dual absorbance in the visible spectrum.
Nevertheless, the polymer showed inadequate transmissivity
A major breakthrough with the discovery of polyacetylene in
1
970 has imparted a great effect on the development of
1
−5
conductive polymers.
Since then, the area has grown
enormously and also brought about many new and astonishing
applications due to unique features of polymers such as easy
processing, flexibility, light weight, tunability of band gaps, and
redox behavior as well as low cost. During the past three
decades, conjugated polymers are widely used for industrial
6
7
,8
applications such as organic light-emitting diodes (OLEDs),
9
,10
organic solar cells (OSCs),
organic field effect transistors
1
6,19
1
1,12
13,14
in the oxidized state.
Later, the Reynolds and Toppare
(
ces.
OFETs),
smart windows,
and electrochromic devi-
1
5,16
groups overcame this by synthesizing and studying donor−
acceptor polymers based on benzothiadiazole and quinoxaline
moieties along the polymer backbone. These neutral-state
green polymers having a single chromophore demonstrated
It is observed that a large number of device
applications based upon electrochromism rely on the
reproducibility of switching characteristics and the stability of
conducting polymers under operating conditions.
17
20−24
excellent transmissivity in the oxidized state.
Furthermore,
Electrochromism is a phenomenon in which a material shows
a reversible color change upon applied electric field. New
absorption bands at different parts of the visible and NIR
regions can be generated by switching between different redox
states. The resulting color changes of the material are examined
by the spectroelectrochemical measurement by the utilization
of both electrochemical and spectroscopic methods. This
measurement offers a new route to study and clearly analyze
electroactive species or products of redox reactions. However, it
is a main prerequisite to produce the entire color spectrum for a
possible utilization of electrochromic materials in display
in 2008, the first neutral-state black polymeric electrochrome
was reported on the use of the donor−acceptor approach by
25
Reynolds et al.
Among the electrochromic polymers, poly(3,4-ethylenediox-
ythiophene) (PEDOT) and its derivatives have taken the lead
in electrochromic applications due to easy processability, easy
switching between two redox states, low band gap, and high
chemical stability given by an electron-donating ethylenedioxy
Received: September 24, 2014
Revised: November 25, 2014
1
8
technologies. So far, most of the electrochromic materials
have exhibited mainly blue and red colors and lacked a green
©
XXXX American Chemical Society
A
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2
6,27
group.
PEDOT polymers containing carbazole moieties
energy levels, oxidation and reduction potential onsets of HCQE and
poly(HCQE) were used, and the onset potentials of oxidation and
reduction were determined from intersection of the tangents between
the baseline and the signal current. The potentials were calibrated to
inserted in the main chain as building blocks and also in the
side chain as pendant groups have been used as electroactive
and photoactive materials in various electrochromic applica-
+
1
5,28,29
the ferrocene redox couple E°(Fc/Fc ) = 0.38 V (vs Ag wire) in
tions.
The multielectrochromism in the systems of
37
CH Cl using the equation E
= −e(E − E ) + (−4.8 eV).
2
2
HOMO
ox
Fc
carbazole moiety incorporated into the polymer backbone at
the 3,6-position is due to the effective conjugation break. The
radical cations formed upon initial oxidation and dications upon
further oxidation at higher potentials are well separated from
one another and do not combine. Thus, two distinct oxidation
states provide a three-color electrochromism resulting in the
separate colors found at three states.
Liu have shown the multielectrochromism studies with distinct
colors present for different states in a design of carbazole
moiety introduced into main chain as a pendant group.
Herein, we report the synthesis of a new monomer named as
-(5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-3-(9-
hexyl-9H-carbazole-3-yl)quinoxalin-2-yl)-9-hexyl-9H-carbazole
HCQE). Poly(HCQE) has an ambipolar character (n- and p-
UV−vis absorption spectra were measured using an Analytic Jena
Speedcord S-600 diode-array spectrophotometer. The optical band
gaps (E ) of products were calculated from their absorption edges
g
3
8
(λonset) using the equation E = 1241/λonset. Photoluminescence
g
spectra were recorded on a PTI QM1 fluorescence spectrophotometer.
Spectroelectrochemical measurements were performed using
39
3
0,31
absorption spectra of this polymer film upon applied potentials.
The spectroelectrochemical cell includes a quartz cuvette, an Ag wire
RE), a Pt wire counter electrode (CE), and ITO/glass as transparent
In addition, Hsiao and
(
working electrode (WE). These measurements were carried out in 0.1
M TBAPF as supporting electrolyte in CH CN.
32−36
6
3
Colorimetry measurements were performed using an Analytic Jena
Specord S600 UV−vis spectrophotometer which consists of a
chromometer module (standard illuminator D65, CIE system; the
color is made up of three attributes: luminance (L), hue (a), and
3
(
4
0
saturation (b)). Platinum cobalt DIN ISO 621, iodine DIN EN 1557,
and Gardner DIN ISO 6430 are the references of colorimetric
measurement. These parameters were applied for the neutral and
oxidized states of the polymer deposited onto ITO/glass surface.
dopable) which implies that the same polymer can behave as
both the cathodically and anodically coloring material in an
electrochromic device. It exhibits a green color in the neutral
state, near colorless in the first oxidized state, and then cyan,
blue, and black colors in the second oxidized state of the p-
doping. On the other hand, the green color of the film turns
into red, violet, and purple on n-doping. Since the electroactive
carbazole pendant groups have a separate oxidation potential
and their oxidation has no color interference due to near
colorless first oxidation state, the resulting polymer film exhibits
a highly unique near colorless and colored oxidized states, fast
switching times, redox switching stability, and exceptional
transmittance change with a multielectrochromism behavior at
three distinct states of −0.4 to −2.0 V, −0.4 to 1.2 V, and 1.2 to
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The HCQE monomer
has been synthesized in four steps (Scheme 1). The initial
compound, 9-hexyl-9H-carbazole, was prepared according to
41
the previously reported work. 1,2-Bis(9-hexyl-9H-carbazole-3-
yl)ethane-1,2-dione was obtained via standard Friedel−Crafts
reaction between 9-hexyl-9H-carbazole and oxalyl chloride in
the presence of AlCl in DCM. The yield of the compound 1 is
3
very low (35%) because of the formed polymeric side products.
Then, the dibromo compound bearing carbazole subunits,
compound 2, was synthesized from the condensation reaction
between carbazole-dione compound 1 and 3,6-dibromoben-
zene-1,2-diamine in the presence of PTSA as a catalyst in
toluene. Finally, the standard Stille reaction was applied to
obtain target donor−acceptor compound of HCQE. After
completion of the synthetic works, the chemical structures of
the initial compounds and the target product were elucidated
2
.0 V (Figure 1). For the first time, we have demonstrated by
1
by using FT-IR and H NMR spectroscopy (Figures S1−S5).
Poly-HCQE was synthesized in a reaction medium
containing 2.0 × 10− M HCQE and 0.1 M TBAPF6/
CH Cl :CH CN (3:1 v/v) via repetitive cycling at a scan rate
3
2
2
3
Figure 1. Color changes of poly(HCQE) at different potentials.
−
1
of 100 mV s . The electroactive polymer was directly
deposited onto the working electrode (platinum disk or
ITO/glass surface). Potentiodynamic electrochemical polymer-
ization of HCQE was studied by repetitive cycling between 0
and 1.10 V exhibiting a new redox couple at lower potentials
(Figure 2). The increase in current density after each successive
cycle clearly indicated the deposition of poly(HCQE) onto the
surface of working electrode (WE). The polymer film deposited
onto ITO/glass surface was electrochemically dedoped in
monomer-free electrolyte solution and then partly dissolved in
chloroform by sonicating for UV−vis absorption and
fluorescence measurements.
this work a neutral state green multielectrochromic polymer
possessing the consecutive transitions of colored-to-bleached-
to-colored oxidation states, thereby allowing this material as a
possible electrochrome for display applications.
EXPERIMENTAL SECTION
■
All synthetic steps are given in the Supporting Information. FT-IR
spectra were recorded by a PerkinElmer FT-IR Spectrum One using
−1
1
an ATR system (4000−650 cm ). H NMR (Bruker Avance DPX-
4
00) data were recorded at 25 °C using CHCl -d solvent and TMS as
3
internal standard.
Optical and Electrochemical Properties. The UV−vis
absorption and fluorescence spectra of HCQE and poly-
Electrochemical measurements were carried out using a Biologic
SP-50 electrochemical workstation. The electrochemical cell consists
of a Ag wire as reference electrode (RE), a Pt wire as counter electrode
(
HCQE) were recorded in CHCl solution (Figure 3). In the
3
UV−vis absorption spectrum of HCQE, the absorption peak at
316 nm is attributed to the π−π* transition; the peak at the 429
nm charge transfer band is associated with electronic transition
(
CE), and platinum as a working electrode (WE) immersed in 0.1 M
TBAHF as the supporting electrolyte. All measurements were carried
6
out under an argon atmosphere. To calculate the HOMO−LUMO
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a
Scheme 1. Synthetic Route for HCQE Monomer
a
Reagents and conditions: (a) 1-hexyl bromide, KOH, toluene, 110 °C, 24 h; (b) oxalyl chloride, AlCl , DCM, rt, 18 h; (c) 3,6-dibromobenzene-1,2-
3
diamine, PTSA, toluene, 24 h, 110 °C; (d) Pd(PPh ) , 3 M KOH, toluene, 120 °C, 24 h.
3
4
difference in redox behavior of poly(HCQE) and the initial
compound HCQE can be deduced from the CV depicted in
Figure 4. During CV scan in the anodic regime, HCQE
ox
m,a
exhibited an oxidation peak at E = 0.80 V vs Ag wire at
anodic scan, and this oxidation potential is higher than that of
ox
m,a
ox
m,c
ox
poly(HCQE) (E = 0.55 V and E = 0.45 V; E = 0.50 V
m,1/2
vs Ag wire) (Figure 4a,b). Because of the extended conjugation
along the polymer backbone, an oxidation potential was
observed at lower potentials compared to the corresponding
initial compound (HCQE). It is clearly seen that the
electroactive pendant carbazole moiety in both monomer and
polymer structures can be easily oxidized at 1.5 V upon applied
higher potentials (Figure S6).
On the other hand, during the cathodic regime, the cyclic
voltammogram of HCQE exhibits a characteristic reduction
red
m,1/2
peak at −1.35 to −1.41 V (E
= −1.38 V) attributed to the
reversible reduction process of the quinoxaline moiety (Figure
4a). Besides, a more negative shift was observed upon the
polymerization since the donor character of the main chain
rises with increasing π−π conjugation on the polymer
Figure 2. Repeated potential scans of HCQE in 0.1 M TBAHF in
6
−1
CH CN/CH Cl (1/3; v/v) with a scan rate of 100 mV s .
3
2
2
from the donor moieties to the electron-withdrawing quinoxa-
line unit (Figure 3). Because of increased effective conjugation
on the polymer backbone, the charge transfer band was
broadened and bathochromically shifted (about 230 nm)
relative to corresponding monomer, suggesting π−π interaction
between polymer chains. Moreover, two specific absorption
bands which are crucial for neutral state green conducting
polymers were determined. The emission spectrum of poly-
backbone. Finally, a reduction peak was observed with a half-
red
m,1/2
wave potential (E
poly(HCQE).
= −1.49 V) of −1.40 to −1.58 V for
The scan rate dependence of the polymer film was studied by
cyclic voltammetry. The linear relationship between peak
current and scan rate demonstrates that the polymer film firmly
attaches onto ITO electrode, and the electrochemical process is
consistent with non-diffusion-controlled behavior (Figure
(
(
(
HCQE) showed that emission maxima shifted to NIR region
λmax: 778 nm) relative to the corresponding monomer HCQE
λmax: 538 nm) due to a red-shift in absorbance of the charge
4
2,43
5
).
The linear behavior indicates that the redox-active
species pursue the characteristic of the surface confined nature
of the electrochemical process. Thus, it is seen that the polymer
on the ITO/glass surface does not diffuse into the electrolyte
solution even in the high scan rates. On the other hand, the
positive or negative charges can diffuse through the polymer
transfer band upon polymerization, and the intensity was quite
quenched relative to the corresponding monomer HCQE.
The electrochemical properties of HCQE and poly(HCQE)
were investigated by cyclic voltammetry (CV). An explicit
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Figure 3. UV−vis absorbance and fluorescence spectra of HCQE (a) and poly(HCQE) (b). Insets: the corresponding photos of compounds with
right) and without (left) UV light (366 nm).
(
Figure 4. Cyclic voltammograms of HCQE (a) and poly(HCQE) (b) in 0.1 M TBAPF /DCM with a scan rate of 100 mV s−1 vs Ag wire.
6
Figure 5. Scan rate dependence of the poly(HCQE) film deposited onto the ITO electrode in 0.1 M TBAPF /CH CN at different scan rates from
6
3
−1
2
5 to 1000 mV s vs Ag wire.
Table 1. Electrochemically Determined HOMO and LUMO Energy Levels, Electrochemical Band Gaps (E′), and
g
Spectroscopically Estimated Optical Band Gap (E ) Values of HCQE and Poly(HCQE)
g
molecules
HCQE
reduction potential (V) oxidation potential (V) HOMO (eV) LUMO (eV) E′g, electrochem band gap (eV) Eg, optical band gap (eV)
red
ox
E
E
E
E
E
E
= −1.41
= −1.35
= −1.27
= −1.40
= −1.58
= −1.36
E
E
= 0.80
= 0.68
−5.10
−3.15
1.95
1.34
2.27
m,c
red
m,a
red
m,on
red
p,c
red
p,a
m,a
ox
m,on
ox
poly(HCQE)
E
E
E
= 0.55
= 0.45
= −0.02
−4.40
−3.06
1.63
p,a
ox
p,c
red
p,on
ox
p,on
film under applied potentials referring to as having diffusive
characteristics at the redox states.
In addition, the HOMO−LUMO energy levels and the
electrochemical band gap (E′) of the compounds were
g
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estimated from the onset oxidation and reduction potentials
the polymer film nearly colorless in a stepwise fashion with a
residual absorption tail starting at 500 nm. Apparently, a high
level of transparency can be seen by the naked eye upon
oxidation. Finally, the green color (L*:57; a:-46; b:15) of the
polymer film was converted to nearly colorless (L*:72; a:-3; b:-
2) after application of positive potential.
As the applied potential was gradually increased (1.2−2.0 V),
a new broad absorption peak at about 650 nm appeared and the
peak intensity increased drastically (Figure 8). The pendant
(
Table 1). The electrochemical band gaps of HCQE and
poly(HCQE) were found as 1.95 and 1.34 eV, respectively.
The frontier molecular orbitals of the monomer HCQE were
investigated by using density functional theory (DFT) with the
Spartan10 program at the parameters of B3LYP and 6-31+G*
basis sets. The charge distribution in the frontier molecular
orbitals is shown in Figure 6. All hexyl chains were replaced
Figure 6. Molecular orbital diagrams of the HOMO and LUMO levels
of HCQE.
with an ethyl chain for simplicity. As shown in Figure 6, the
LUMO orbital is more localized on the electron-accepting
quinoxaline moiety, while HOMO orbitals (HOMO and
HOMO−1) are delocalized on the electron-donating thiophene
and pendant carbazole moiety, respectively. Hence, the results
of DFT calculation also confimed that the structure shows an
increase in stability against over oxidation and also explain the
formation of two distinct state electrochromism in the anodic
regime.
Figure 8. Spectroelectronic absorption spectra and the color changes
of poly(HCQE) deposited onto ITO/glass surface with the applied
potential of 1.2−2.0 V in 0.1 M TBAPF /CH CN on p-doping.
6
3
Spectroelectrochemical Properties. Oxidation and re-
duction of the polymer film are accompanied by impressive
color changes based upon the chemical structure of the
polymer. To study these color changes in detail, the electro-
optical properties of the poly(HCQE) film were investigated by
monitoring absorption spectra upon applied different potentials
to the film. Poly(HCQE) has absorption bands at two distinct
wavelengths (400 and 700 nm) which are necessary to produce
a green color at the neutral state (Figure 7). Upon applied
positive potential of −0.4 to +1.2 V to the poly(HCQE) film,
the intensity of the valence band−conduction band (π−π*
transitions) at about 400 nm and charge transfer band at about
carbazole units may oxidize and dimerize at higher applied
voltages leading to a biscarbazole structure, which may be a
result of NIR absorption due to the intervalence charge transfer
(
IVCT) effect. The IVCT effect is known as one electron
transfer from a neutral to a positively charged nitrogen atom on
carbazoles.
4
4,45
As a consequence of oxidation of carbazole
moiety on the polymer subunit, the poly(HCQE) film switched
from a nearly colorless oxidized state to a fully oxidized state by
revealing colors of (L*:82; a:-3; b:-2) cyan (L*:55; a:-31; b:-
1
0), dark blue (L*:31; a:2; b:-19), and black (L*:22 ; a:0; b:-6).
Further, an n-type feature can be also confirmed by an
7
00 nm decreased, and a new intense absorption band centered
at about 950 nm emerged due to low-energy charge carriers.
This depletion in absorption bands of the visible region makes
absorption change in the NIR region beside a redox couple at
negative potentials revealed in cyclic voltammetry. Anion
radicals formed on the quinoxaline moiety of the poly(HCQE)
structure upon applied negative potentials. As the negative
potentials are applied to the film from −0.4 to −2.0 V (Figure
9
), a decrease in the absorption bands at about 400 and 700 nm
and the formation of new absorption bands at 510 and 950 nm
were observed by accompanying the color change from green
(
(
L*:57; a:-46; b:15) to violet (L*:50; a:56; b:-33) and purple
L*:50; a:76; b:-66).
The double step chronoamperometry technique was used to
monitor the transmittance changes between the neutral and the
oxidized states upon electrochromic switching of poly(HCQE).
Electrochromic parameters of the polymer film were analyzed
with the changes in transmittance (increments or decrements
of the absorption band with respect to time) while switching
the potential step wisely between the neutral and oxidized
states with a residence time of 5 s. In the oxidation, the change
in maximum percentage transmittance (%ΔT) of poly(HCQE)
between the neutral (−0.4 V) and oxidized state (1.2 V) was
found 37% (Figure 10). Besides, the polymer switches very
rapidly between the neutral and oxidized state, and response
Figure 7. Electronic absorption spectra and the color changes of
poly(HCQE) deposited onto ITO/glass surface with the applied
potential of −0.4 to 1.2 V in 0.1 M TBAPF /CH CN on p-doping.
6
3
E
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dual n- and p-type dopable behavior and having another
electroactive carbazole moiety as a subunit, poly(HCQE) as an
electrochromic material has multiple separate colors at various
oxidation and reduction states. Optical and spectroelectro-
chemical analysis of the green conjugated polymer brought out
a nearly colorless and colored oxidized states, excellent
transmittance change, fast switching times, and long-term
stability. To our knowledge, this is the first report of a neutral
state green polymer demonstrating the consecutive switching of
colored-to-bleached-to-colored oxidation states, which might
open up a new horizon in electrochromic display applications.
ASSOCIATED CONTENT
■
*
S
Supporting Information
1
All experimental details; FT-IR and H NMR spectra of initial
compounds and HCQE monomer. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 9. Electronic absorption spectra and the color change of
poly(HCQE) deposited onto ITO/glass surface with the applied
potential of −0.4 to −2.0 V in 0.1 M TBAPF /CH CN on n-doping.
6
3
AUTHOR INFORMATION
■
times in this regime were measured as 0.45 and 0.3 s.
Poly(HCQE) film achieved an excellent optical activity of 98%
after 1000 cycles (Figure 10).
Corresponding Author
*
Fax +90-286-2180541; Tel +90-286-2180540; e-mail
skoyuncu@comu.edu.tr (S.K.).
Another important criterion for identifying the electro-
chemical performance of the materials is the coloration
efficiency (CE). The CE is defined as the change in optical
density per unit of inserted charge. The ideal material would
demonstrate a large transmittance change with a small amount
of charge, which eventually results in large CEs. CE is
calculated using the formulas CE = OD/Qd and OD =
Notes
The authors declare no competing financial interest.
̈
E.O.: deceased March 25, 2014.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Scientific and Technical
Research Council of Turkey (TUBITAK, project #:
110T830) for financial support.
log(Tcolored/Tbleached), where Q is the injected/ejected charge
d
between the neutral and oxidized states and Tcolored and Tbleached
are the transmittance in the oxidized and neutral states,
respectively. Using these equations, CE of poly(HCQE) was
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−1
2
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■
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dx.doi.org/10.1021/ma501975m | Macromolecules XXXX, XXX, XXX−XXX