Preparation and optoelectronic behaviours of novel electrochromic devices based on triphenylamine-containing ambipolar materials

Article abstract of DOI:10.1039/c7tc02953e

Two new triphenylamine-containing ambipolar electrochromic materials with ether linkages, 1-(2-(4-(bis(4-methoxyphenyl)amino)phenoxy)ethyl)-1′-ethyl-[4,4′-bipyridine]-1,1′-diium tetrafluoroborate (TPA-Vio) and 2-(4-(bis(4-methoxyphenyl)amino)phenoxy)anthracene-9,10-dione (TPA-OAQ), were successfully synthesized and fabricated into novel electrochromic devices. These devices demonstrated interesting and much higher performance than devices derived from TPA-3OMe with respect to driving voltage, switching time, and electrochromic stability.

Full text of DOI:10.1039/c7tc02953e

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ARTICLE
Preparation and Optoelectronic Behaviours of Novel
Electrochromic Devices Based on Triphenylamine-containing
Ambipolar Materials
De-Cheng Huang,‡ Jung-Tsu Wu,‡ Yang-Ze Fan, and Guey-Sheng Liou*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Two new triphenylamine-containing ambipolar electrochromic materials with ether linkage, 1-(2-(4-(bis(4-
methoxyphenyl)amino)phenoxy)ethyl)-1'-ethyl-[4,4'-bipyridine]-1,1'-diium tetraflouroborate (TPA-Vio) and 2-(4-(bis(4-
methoxyphenyl)amino)phenoxy)anthracene-9,10-dione (TPA-OAQ), were synthesized successfully and fabricated into
novel electrochromic devices. Such devices demonstrated interesting and much higher performance than the devices
derived from the TPA-3OMe such as driving voltage, switching time and electrochromic stability.
www.rsc.org/
group has reported a series of electro-active high-performance
polymers based on TPA derivatives in respective of synthesis and
characteristic evaluation5. In addition, we proposed how to reduce
Introduction
Basically, “chromism” refers to the colour change within the material
which was subjected to an external stimulus such as light,
temperature, electric field, and so on. Electrochromism can be
definitely recognized as the reversible coloration in UV-vis-NIR region
of materials being associated with electrochemically induced
oxidative and reductive states. Generally, common electrochromic
(EC) materials can be classified into four different groups including
inorganic coordination complexes, transition-metal oxides,
conjugated polymers, and organic molecules1, 2. The electrochromic
device (ECD) of active optical modules possesses lots of advantages
such as high contrast, lower applied potential and power
consumption, colour control and bistable characteristics. In recent
years, with the increasing awareness of energy saving and carbon
reduction, the EC technology is gradually drawing more attention
and could be applied to smart windows for building construction,
sunroof and anti-glare rear-view mirrors for vehicles, electronic
papers and displays. The EC glasses have been mainly based on
inorganic metal oxide for several years, but the cost of price is still
becoming the brunt of major problem to produce in large quantity.
Organic EC materials have gradually become promising candidate
due to various characteristics, including flexibility necessary for
flexible device fabrications, easy process, high coloration efficiency,
and multicolour property within the same material.
applied oxidation potential which could enhance the electrochemical
stability, and also investigated the EC colour-merging behaviours by
designing different polymer structures and fabricating them into ECD
construction. As contrasted with polymeric EC materials, small
organic molecules have the merits such as fast mobility within the
electrolyte, lower cost, and easy to tune the EC properties via facile
structural modifications. According to the related studies we have
published6, TPA could readily lose electron during electrochemical
oxidation process to emerge stable radical cation with a discernible
change in colour. By introducing electron-donating substituents to
the p-position of phenyl groups in the TPA unit, it could not only be
a protective group to prevent electrochemical oxidative coupling
reaction and to afford stable cationic radicals but also decreased
oxidation potential owing to their electron-donating ability, resulting
in notably improved redox and EC reversibility of the obtained TPA-
based materials.
Viologen derivatives are well known as organic EC materials with
distinct colouring performances due to the capability of reversible
optical colour change in the reduction redox cycles. By cathodic
scanning, viologen would undergo reversible reduction from a
dication to a monocation radical, advancing a high contrast variation
in transmittance over the visible-light region7-9. By modifying
pendent substituents of viologen structure, the colour can be tuned
by means of adjusting different molecular energy levels. Moreover,
anthraquinone derivatives also have drawn attention to be redox-
active and near-infrared (NIR) EC materials. Anthraquinone
possesses strong electron withdrawing characteristic, and was
usually used in bleaching pulp for papermaking and conjugated
polymers as an electron acceptor unit.
Triphenylamine (TPA) derivatives are famous for their marvellous
photo- and electro-active properties, and are representative
alternatives for optoelectronic applications including light-emitters,
hole-transporters, photovoltaic, EC and memory devices3, 4. Our
Functional Polymeric Materials Laboratory, Institute of Polymer Science and
Engineering, National Taiwan University, 1 Roosevelt Road, 4th Sec., Taipei 10617,
Taiwan.*Email - gsliou@ntu.edu.tw; Fax: +886-2-33665237; Tel: +886-2-33665315
† Electronic supplementary information (ESI) available: Measurement, synthesis of
1-4, NMR spectra of 5-7 and the electrochromic data of ECD based on TPA-3OMe.
See DOI: 10.1039/x0xx00000x
In addition, anthraquinone derivatives are more interesting to be
cathodic EC materials because of having highly electrochemical
stability during reduction redox procedure10. Generally, one single EC
material is not enough to achieve high contrast and full-band
‡These authors contributed equally to this work.
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Journal Name
absorption in visible-light region. Therefore, by collocating of anodic calcd for C22H22BrNO3: C 57.74, H 4.99, N 5.94; found: C 57.79, H 4.93,
View Article Online
DOI: 10.1039/C7TC02953E
and cathodic EC materials together should be a judicious approach N 5.72.
to obtain complementary in colours; moreover, it also could become
mutual charge-storage layers to enhance the response capability. In
the previous studies11-14, Hsiao and our group have reported the
1-(2-(4-(Bis(4-methoxyphenyl)amino)phenoxy)ethyl)-1'-ethyl-[4,4’-
bipyridine]-1,1'-diium- tetraflouroborate (TPA-Vio) (6):
Firstly, 1.07 g (2.5 mmol) of 5 was dissolved in 60 mL acetonitrile,
triarylamine-based ambipolar EC polymers (e.g., polyimide,
then 1.35 g (5.0 mmol) of 1 was added into the solution, followed by
polyamide), meaning one polymer chain could exhibit interesting EC
stirring at reflux temperature for 72 hours. After reaction and
behaviours with multicolour characteristics in both oxidized and
filtration, the filter was dissolved in DI water and added into
reduced states. The resulted polymers possessed some crucial
saturated NaBF4 solution dropwisely to give precipitate. It was
properties such as excellent thermal stability, high coloration
filtered off and dried over vacuum to obtain 6 as pale purple powder
efficiency (CE), notable quick switching time (response capability),
(0.920 g, 60%). M.p.: 195-200 °C measured by Melting Point System
and outstanding electro-active reversibility.
1
at 5 °C min-1. H NMR (400 MHz, DMSO-d6, δ): 1.60 (t, 4H), 3.70 (s,
In 2017, Zhou group has prepared solution type electrochromic
6H), 4.52 (t, 2H), 4.71-4.76 (m, 2H), 5.13 (t, 2H), 6.84 (m, 12 H), 8.79-
devices derived from ambipolar materials with electron-withdrawing
8.83 (m, 4H), 9.41-9.45 (m, 4H); 13C NMR (100 MHz, DMSO-d6, δ):
16.45, 55.39, 56.74, 60.34, 66.51, 114.93, 115.77, 123.91, 124.94,
126.59, 126.84, 141.22, 142.52, 145.69, 146.50, 148.71, 149.37,
152.57, 154.96; ESI-MS m/z: calcd for (C34H35N3O3)+, 533.27; found,
533.27. Anal. calcd for C34H35B2F8N3O3: C 57.74, H 4.99, N 5.94;
found: C 57.79, H 4.93, N 5.72.
benzodipyrrolidone moieties15. In this article, we herein propose a
facile “linkage” approach to merge ambipolar EC behaviours by using
only one single small molecule. Two novel ambipolar EC materials, 1-
(2-(4-(bis(4-methoxyphenyl)amino)phenoxy)ethyl)-1'-ethyl-[4,4'-
bipyridine]-1,1'-diium tetrafluoroborate (6) and 2-(4-(bis(4-
methoxyphenyl)amino)phenoxy)anthracene-9,10-dione (7), were
synthesized and characterized fully, then were fabricated into the EC
devices to evaluate their EC performance. With the “linkage”
2-(4-(Bis(4-methoxyphenyl)amino)phenoxy)anthracene-9,10-dione
(TPA-OAQ) (7):
approach, the incorporation of counter EC moieties as charge- 5.53 g (40 mmol) of K2CO3 was firstly dissolved in 50 mL DMSO, 5.34
trapping layer into TPA units can be expected to have two colours g (22 mmol) of 2-chloroanthraquinone and 10.55 g (20 mmol) of 4
combination, reduce driving potential and switching time, and even were added into the flask successively at room temperature. The
intensify EC reversibility.
mixture was heated at 100 °C with stir for 24 hours monitored by TLC,
and then slowly poured into methanol/water (1/1) after cooling. The
target product was washed with hot methanol to obtain 7 as
yellowish powders (9.33 g, 84%). M.p.: 144-145 °C measured by
Melting Point System at 5 °C min-1. 1H NMR (400 MHz, DMSO-d6, δ):
3.74 (s, 6H), 6.87 (d, 2H), 6.93 (d, 4H), 7.05-7.08 (m, 6H), 7.45-7.51
(m, 2H), 7.89-7.94 (m, 2H), 8.15-8.22 (m, 3H); 13C NMR (100 MHz,
DMSO-d6, δ): 55.22, 112.55, 115.01, 120.92, 121.48, 122.41, 126.53,
126.70, 127.53, 129.91, 132.98, 134.27, 134.66, 135.15, 140.14,
146.06, 147.03, 155.75, 163.11, 181.24, 182.08. ESI-MS m/z: calcd for
(C34H25NO5)+, 527.17; found, 527.18. Anal. calcd for C34H25NO5: C
77.41, H 4.78, N 2.65; found: C 77.48, H 4.60, N 2.70.
Experimental
General Considerations
TPA-3MeO was prepared by similar method in accordance with the
previous literature16. Tetrabutylammonium tetrafluoroborate
(TBABF4) was given as follow procedure: DI water containing 5.00 g
tetrabutylammonium bromide was dropped into saturated NaBF4
aqueous solution. After dropwise, the solution gave desired white
precipitate and then directly recrystallized from hot water. The
remaining reagents were received from commercial sources and
used without further purification.
Fabrication of electrochromic device
Two ITO glasses (~5 Ω/□) were laminated with thermosetting
adhesives by full-auto dispenser and heated at 150 °C for 2 hours to
obtain vacant devices with gap of 120 μm (controlled by beaded glass
dispersing in the adhesives), and the active areas of devices were
controlled to be 2 cm × 2 cm. Only one tiny hole was left in one side
of the devices for the later injection of electrochromic liquid
electrolyte. After injection of electrolyte which the total amount of
propylene carbonate is ~0.048 mL inside the device based on its
vacant volume, the cells were eventually sealed completely by UV
gel. A liquid-type electrolyte assembly was based on TPA-3OMe
(0.015 M), 6 (0.030 M), and 7 (0.030 M), respectively, with 0.10 M
TBABF4 dissolving in propylene carbonate (PC). Fig. 1 depicts the
entire process of ECDs fabrication.
Material synthesis
4-(2-Bromoethoxy)-N,N-bis(4-methoxyphenyl)aniline
(5):
(TPA-OBr)
A mixture of 4-(bis(4-methoxyphenyl)amino)phenol (1.4 g, 5 mmol)
4 was dissolved in 1,2-dibromoethane (20 mL, 227 mmol), then
dropped potassium hydroxide solution (1.1 g, 20 mmol) with
tetrabutylammonium bromide (0.026 g, 0.08 mmol) stirring at 90 °C
for 24 h. After cooling to room temperature, water (100 mL) was
added, and the crude product was extracted with CH2Cl2 then
condensed and dried, giving the 1.71 g (86% in yield) of brown
viscous liquid. Further purification by using flash column with eluent
(dichloromethane: hexane = 1:2) to obtain the yellow liquid (78% in
total yield). 1H NMR (400 MHz, DMSO-d6, δ): 3.71 (s, 6H), 3.77 (t, 2H),
4.26 (t, 2H), 6.86 (m, 12H); 13C NMR (100 MHz, DMSO-d6, δ): 31.76,
55.53, 68.42, 115.06, 115.93, 124.38, 125.06, 141.53, 153.40, 155.11;
ESI-MS m/z: calcd for (C22H22BrNO3)+, 427.08 ;found, 427.09. Anal.
Results and discussion
Materials synthesis
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Electrochemistry
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DOI: 10.1039/C7TC02953E
Electrochemical and EC behaviours of TPA-3OMe, 6 and 7 were
investigated by cyclic voltammetry (CV) measurements using a
customized OTTLE cell with 10-3 M of these EC materials,
containing anhydrous propylene carbonate (PC) as solvent and
0.1 M of TBABF4 as supporting electrolyte. Their CV diagrams
are shown in Fig. 2, and the results of the respective half-wave
redox potentials are summarized in Table 1. Comparing to TPA-
3OMe, the oxidation voltages of
6 and 7 are higher due to the
replacement of one less electron-donating substituent,
respectively. With the incorporation of the soft segment of
ether linkage, the cathodic EC moieties within 6 and 7 could be
isolated and behaved independently. Furthermore, the redox
potentials of anodic (TPA) and cathodic (viologen and
anthraquinone units) EC functionalities seem to be matched
with each other, and could be further confirmed in the ECDs by
their switching behaviours.
Fig. 1 Schematic diagram of fabrication process for ECDs based on
liquid-type EC electrolyte.
TPA-3OMe (m.p.: 92-93 °C by melting point system at 5 °C min-1;
lit.17: 90–92 °C) was obtained by copper-catalyzed reaction of p-
anisidine with 4-iodoanisole, and the result is in agreement with the
previous study. TPA-Vio (6) was synthesized by nucleophilic
substitution reaction of 5 with 1 to give yield of 60%. TPA-OAQ (7)
was prepared by potassium carbonate-mediated reaction of 4 with
2-chloroanthraquinone to give yield of 84%. NMR spectroscopy and
element analysis were used to characterize structures of the
intermediate 5 and final compounds, TPA-Vio (6) and TPA-OAQ (7).
Scheme 1 depicts the whole synthetic procedures, and the structures
of the obtained intermediates and two targeted compounds (5-7)
were confirmed well with the presented molecular structures by
NMR measurements as summarized in Fig. S1-9, respectively.
Spectroelectrochemistry
The spectroelectrochemical measurements were also
conducted by using the same condition mentioned above using
an OTTLE cell built from a commercial UV-vis cuvette to
demonstrate the EC behaviours of these materials, and the
results at different redox applied potentials are shown in Fig. 3
.
Scheme 1 Synthesis of ambipolar TPA derivatives 6 and 7.
Fig. 2 CV diagrams for 0.001 M of (a) TPA-3OMe, (b) 6 and (c) 7
in 0.1 M TBABF4 PC solution at the scan rate of 50 mV/s and
platinum net as the working electrode.
Table 1 Redox data of TPA-3OMe, 6 and 7.
Oxidation
Reduction
(vs. Ag/AgCl)
(vs. Ag/AgCl)
Material
Eonset (V)a)
E1/2 (V)b)
0.61
Eonset (V)a)
E1/2 (V)b)
-
TPA-3OMe
0.53
0.67
0.65
-
6
7
0.76
0.73
-0.26
-0.71
-0.35
-0.84c)
a) Onset, half-wave potentials were obtained from CV.
b) Half-wave potentials from cyclic voltammograms.
c) Reduction potentials from cyclic voltammograms.
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Fig. 5 CV diagrams of ECDs containing 0.03 M of (a) 6 and (b) 7 in 0.1
M of TBABF4/PC solution at the scan rate of 50 mV/s. One of the ITO
glass served as the working electrode and the other ITO glass
simultaneously served as counter electrode and reference electrode.
Fig. 3 Electrochromic behaviour and the corresponding photographs
for (a) TPA-3OMe, (b) 6 and (c) 7 at the applied potentials from 0 to
(a) 1.0, (b) 1.0, -0.4 and (c) 1.0, -1.2 V (vs. Ag/AgCl) based on 0.001 M
of material in PC containing 0.1 M of TBABF4.
Electrochromic devices and properties
Moreover, ECDs derived from ambipolar EC materials
revealed the similar results as TPA-3OMe when revealed the same redox driving voltage, respectively, to
1.0 V was applied to the systems; 360 and 727 nm for while achieve coloured stage regardless of the different phases of
366 and 734 nm for , respectively, corresponding to the similar applied voltage (Fig. 5a), and the working potential for (1.2 V)
6 or 7
Both 6 and 7
6
7
6
colour change (from transparent to cyan during oxidation was much lower than the device derived from TPA-3OMe (2.2
procedures). For the cathodic EC behaviour, two new V; Fig. S10). In addition, the incorporation of counter EC
absorption peaks at 399 and 606 nm for
potential increased from 0 to -0.4 V owing to the formation of the absorption of new peaks in visible-light region at 606 nm
viologen cation radicals, resulting in colour change from (viologen unit) and 724 nm (TPA unit) for gradually increased
colourless at the original dication form to blue colour in the simultaneously and could be merged together to reach high
reduced state. For , the absorption of two new peaks at 402 ΔT% of 87% at 606 nm and 96% at 724 nm as shown in Fig. 6a
6 appeared when the moieties not only have the effect to reduce the driving voltage,
6
7
and 543 nm could be observed and increased obviously from 0 and
to -1.2 V. The formation of monoanion radical in the
anthraquinone moiety switches the colour from almost
colourless to pink. Furthermore, in order to get more insight to
the EC behaviours of the ambipolar materials, two slices of ITO
coated glass were used as working and auxiliary electrodes,
respectively, to simulate the coloration mechanism in the
b.
device system. By using
6 as example, it exhibited reversibly
colouring and bleaching changes on both electrodes in the same
time at the applied potential of 0.8 V as depicted and proposed
in the Fig. 4
.
Fig. 6 (a) Absorbance spectrum and (b) transmittance spectrum of
ECD based on 6 at the applied potential from 0 to 1.2 V and its
corresponding photograph. (c) Absorbance spectrum and (d)
transmittance spectrum of ECD based on 7 at the applied potential
from 0 to 1.8 V and its corresponding photograph.
Fig. 4 The schematic diagram of colour change for the ambipolar
materials using 6 as model with quartz glass square at the applied
potential 0.8 V (vs. Ag/AgCl).
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Thus, two complementary colours (cyan and blue) could be
merged together, resulting in much deeper blue colour and
higher ΔL* (L*, 91.38-40.09; a*, 0.25-13.09; b*, 0.28-52.54).
Similarly, the absorption intensity at 535 nm (anthraquinone
unit) and 734 nm (TPA unit) could also increase simultaneously
at higher applied voltage of 1.8 V for the other device based on
View Article Online
DOI: 10.1039/C7TC02953E
ambipolar material 7, and could combine two complementary
colours (cyan and pink) with ΔT% of 68% at 535 nm and 91% at
734 nm to produce a deeper violet colour (L*, 68.51; a*, 2.50;
b*, -11.36) of the device as shown in Fig. 6c and d. As the
Fig. 8 Chronoamperometry curves and the corresponding in situ
absorbance at the respective wavelengths of ECDs with (a) 6
between 0 and 1.2 V; (b) 7 between 0 and 1.8 V.
potential voltage turned off to 0 V, both of the devices could
easily bleach back to their original states. This outcome could
be partly responsible for the viologen/anthraquinone moieties
possessing the capability to induce the electrons losing from
TPA moiety during oxidative process, and then the resulted
monocation radical viologen and monoanion radical
anthraquinone also could facilitate electrons back to
monocation TPA during reductive process.
The other device based on 7 as shown in Fig. 6d and e
required 5.5 s for coloration at 535 nm and 5.3 s for
decolouration (anthraquinone moiety); 3.2 s for coloration at
734 nm at 1.8 V and 5.2 s for decolouration (TPA moiety)
without applied voltage.
The results summarized in Table S1 are noteworthy that the
colouration of ECDs with ambipolar materials proceeded during
the colouring process faster than the corresponding ECD based
on TPA-3OMe EC material only with anodic EC unit owing to the
According to the switching investigation of ECDs based on the
individual ambipolar EC materials, the results are summarized
in Fig. 7. As depicted in Fig. 7a and
b, the switching time of ECD
derived from exhibited 1.7 s and 2.7 s at 1.2 V for coloring
6
fact that the connecting charge trapping moieties in the
could effectively induce the electron gaining and losing. The
amount of injected charge (Qin) obtained from Fig. 7c and were
14.012 and 14.134 mC/cm2 for colouring process of devices with
and , respectively. According to the results described above,
such devices demonstrated enhanced performance comparing
to the device with TPA-3OMe Fig. S11a).
As shown in Fig. 8, the electrochemical stability of the ECDs
based on and was determined by evaluating the
electrochromic CE (η= OD/Q) and injected charge (electro-
6 and 7
process at 724 nm (TPA moiety) and 606 nm (viologen moiety),
while 9.7 s and 9.5 s, respectively, for bleaching process without
applied voltage.
f
6
7
(
6
7

activity) at various switching cycles, and the results are
summarized in Table S2 and S3, respectively. After 1000
scanning cycles, both ECDs derived from
6 and 7 ambipolar EC
materials still could maintain high reversibility and electro-
activity without significant decay, while the ECD derived from
TPA-3OMe exhibited much worse electrochemical stability
even only for 20 cycles (Fig. S11c), probably owing to the much
higher switching potential and without complementary
cathodic EC moieties. The CE of ECD fabricated from
6 could
reach to 112 and 177 cm2/C at 606 and 724 nm, and remain
their reversibility with decay of 7 and 6%, respectively, after
1000 cycles between 0 and 1.2 V. On the other hand, the CE of
ECD prepared from
7
showed CE 72 and 162 cm2/C at 535 and
734 nm, respectively, with decay of 10 and 5% after 1000 cycles.
These outcomes are also in agreement with the assumption
mentioned above that the incorporation of counter EC moieties
into TPA unit could effectively improve the switching response
time and enhance the electrochemical stability.
Fig. 7 Transmittance changes at (a) 724 nm and (b) 606 nm, and (c)
chronoamperometry curve of device containing 6 between 0 and 1.2
V. Transmittance changes at (d) 535 nm and (e) 734 nm, and (f)
chronoamperometry curve of device containing 7 between 0 and 1.8
V.
Conclusions
The high-performance ECDs with ambipolar EC materials TPA-
Vio (6) and TPA-OAQ (7) have been fabricated successfully and
investigated on the basis of spectroelectrochemical and EC
switching studies, respectively. By the incorporation of cathodic
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EC moieties into TPA as charge trapping layer, the resulted
ambipolar ECDs not only could reduce the driving voltage and
switching time over the EC procedure when compared to the
devices only with TPA, but also reveal the capability to merge
the colours contributed from both anodic TPA and cathodic EC
moieties. Therefore, the electrochemical stability and EC
performance of the obtained ECDs containing ambipolar EC
materials could be further enhanced than ever. These results
demonstrate and manifest conclusively that the incorporation
of suitable counter EC moieties into TPA derivatives should be a
facile and feasible approach to fabricate novel ECDs.
View Article Online
DOI: 10.1039/C7TC02953E
Acknowledgements
The authors gratefully acknowledge the Ministry of Science and
Technology of Taiwan for the financial support.
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC02953E
Table of content
By combining anodic and cathodic moieties into an ambipolar material via the ether
linkage, the electrochromic performance could be enhanced.
ITO
1.2 V
electrolyte
ITO