Synthesis and characteristics of novel TPA-containing electrochromic poly(ether sulfone)s with dimethylamino substituents

Article abstract of DOI:10.1016/j.electacta.2020.137552

Newly designed dimethylamine-substituted triphenylamine (TPA) derivatives, N,N'-(1,4-phenylene)bis(N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N'',N''-dimethylbenzene-1,4-diamine) (NTPPA-2Si) and N,N'-((1,1′-biphenyl)-4,4′-diyl)bis(N-(4-((tert-butyldimethy

Full text of DOI:10.1016/j.electacta.2020.137552

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Synthesis and Characteristics of Novel TPA-Containing
Electrochromic Poly(ether sulfone)s with Dimethylamino Substituents
Chiao-Ling Huang , Yu-Ruei Kung , Yu-Jen Shao ,
Guey-Sheng Liou
PII:
S0013-4686(20)31945-9
DOI:
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https://doi.org/10.1016/j.electacta.2020.137552
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Chiao-Ling Huang ,
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Poly(ether sulfone)s with Dimethylamino Substituents, Electrochimica Acta (2020), doi:
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Synthesis and Characteristics of Novel TPA-Containing
Electrochromic Poly(ether sulfone)s with Dimethylamino Substituents
a
b
a
a,c
Chiao-Ling Huang, Yu-Ruei Kung, Yu-Jen Shao, Guey-Sheng Liou*
a
Institute of Polymer Science and Engineering, National Taiwan University, No.1, Sec. 4,
Roosevelt Rd., Taipei 10617, Taiwan.
xb
Department of Chemical Engineering and Biotechnology, Tatung University, No.40, Sec. 3,
Zhongshan N. Rd., Taipei 10452, Taiwan.
c
Advanced Research Center for Green Materials Science and Technology, National Taiwan
University, Taipei 10617, Taiwan.
Corresponding author: gsliou@ntu.edu.tw (G. S. Liou)
Keywords: electrochromism, triphenylamine, dimethylamino group, poly(ether sulfone)s,
silyl method
Abstract
Newly designed dimethylamine-substituted triphenylamine (TPA) derivatives, N,N'-(1,4-
phenylene)bis(N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N'',N''-dimethylbenzene-1,4-
diamine)
(NTPPA-2Si)
and
N,N'-((1,1'-biphenyl)-4,4'-diyl)bis(N-(4-((tert-
butyldimethylsilyl)oxy)phenyl)-N'',N''-dimethylbenzene-1,4-diamine) (NTPB-2Si), with silyl
ether protecting groups were readily synthesized. Subsequently, novel electroactive aromatic
poly(ether sulfone)s (PES), NTPPA-PES and NTPB-PES, could be obtained from silyl
polycondensation. The PESs were readily soluble in commonly used laboratory organic
solvents and could be solution-cast into tough and amorphous films with moderate levels of
o
glass-transition temperature around 220 C and thermal stability without significant weight
o
loss up to 400 C under nitrogen or air atmosphere. The Nernst equation method was used to
explore the number of electrons transferred at each oxidation step of the targeted two
monomers. Furthermore, these two anodic electrochromic PESs were introduced into
1

electrochromic devices accompanied with cathodic heptyl viologen (HV), and the resulted
devices demonstrated a high coloration contrast and excellent electrochemical stability.
1
. Introduction
Electrochromics are generally categorized as materials that undergo reversible color
change under an applied electric field. Electrochromic (EC) materials are comprised of redox-
active moieties that exhibit significant color changes and the transparency upon
electrochemical reduction or oxidation process. Since Platt discovered the EC phenomenon in
[
1]
1
961, then Deb published the first sandwich-type electrochromic device (ECD) fabricated
[2]
by using amorphous WO thin film in 1969, EC technology is still prospering from then
3
[
3-5]
on.
oxides,
Nowadays, a flood of EC materials were reported and applied widely in our daily life, such as
There is a large number of chemical species exhibiting EC properties, such as metal
[
6-8]
[9,10]
[11-13]
[14,15]
coordination complexes,
viologens,
and conducting polymers.
[
16-19]
color changing eyewear, smart windows, display and adaptive camouflages etc.
Over the past few decades, arylamine-based EC materials have attracted significant
[
20-22]
interest owing to the multicolored electrochromism as well as electrochemical stability.
It
is worth mentioning that polymers containing triphenylamine (TPA) units not only possess
dramatic color changes during oxidation but also show excellent reversibility and outstanding
stability, which make them suitable candidates as electrochromic materials for practical
[23-29]
utilization.
[
30]
The related studies of TPA and its derivatives have been conducted extensively, and
by introducing methoxy group into TPA system at the para position of phenyl ring can
effectively prevent the coupling reaction to form TPA dimer, thus effectively enhance the
[
31-33]
redox stability.
In addition, dimethylamino group can not only reveals stronger electron
donating ability than methoxy group but also provide more oxidation site when attached into
anodic TPA-based EC materials. The simplest and typical material with dimethylamino
2

groups is N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD), which has already been
[
34-36]
investigated as display applications for a long period of time.
EC films based on 4-
(dimethylamino)triphenylamine-functionalized polyamides with high contrast ratio and rapid
[37]
switching behavior have been successfully reported in 2008. Recently, two small molecular
TPA derivatives with dimethylamino substituents, NTPPA and NTPB, have also successfully
[38]
synthesized in our group for the application of electrofluorochromic devices.
[
39-41]
Apart from high-performance polymers with TPA units like polyamides,
[
42,43]
[44]
[45,46]
polyimides,
polybenzoxazines
and epoxies,
poly(ether sulfone)s (PESs), a
category of thermoplastic polymers with good thermal stability, strong corrosion resistance
and excellent mechanical properties, are commonly used in many realms, such as aviation,
[
47,48]
aerospace, microelectronics, nanotechnology, filtration and so on.
In addition, amorphous
PES films are also colorless with high transparency which is an important merit for the further
[
49]
optical applications.
Thus, a series of multi-colored EC arylamine-based aromatic
polyethers with methoxy protecting groups have been prepared by our group that could reveal
the multi-colored (with 1 to 3 oxidation stages) behavior by increasing the electroactive
[50]
sites. However, it is still difficult to obtain PES arylamine-based PESs with four or more
electroactive sites due to the complex synthesis route. Therefore, in this work, we introduce
the dimethylamino unit into PES to increase the electroactive site via the facile synthesis
route,
Herein, by integrating two dimethylamino groups and silyl ether protecting groups into
N,N,N',N'-tetraphenyl-p-phenylenediamine (TPPA) and N,N,N',N'-tetraphenylbenzidine (TPB)
moieties, two novel EC materials with four electroactive nitrogen centers, NTPPA-2Si and
NTPB-2Si, have been prepared in this study. The Nernst equation has been applied here to
investigate the electrochemical behaviors of monomers during the oxidation process. The
obtained TPA-based monomers with disilyl ether groups can further react with bis(4-
fluorophenyl)sulfone to obtain two novel electroactive poly(ether sulfone)s, NTPPA-PES and
3

NTPB-PES. These PESs are expected to have four electroactive sites that increases two more
additional oxidation stages than the corresponding PESs without dimethylamino groups. In
addition, the incorporation of ether linkage is also effective to enhance their solubility for
preparing colorless polymer films with high transparency and the corresponding EC behaviors
are described hereby.
2
2
. Experimental Section
.1 Materials
Commercially available chemical reagents, including 4-bromophenol (Alfa), tert-
butyldimethylsily
chloride
(ACROS),
imidazole
(ACROS),
N,N-dimethyl-p-
phenylenediamine (Alfa), tris(dibenzylideneacetone)dipalladium(0) (Pd (dba) ) (ACROS),
2
3
tri-tert-butylphosphine (P(tBu) ) (Alfa), sodium tert-butoxide (NaOtBu) (ACROS), 1,4-
3
bromobenzene (ACROS), 4,4'-dibromo-1,1'-biphenyl (ACROS), palladium (II) acetate
(
Pd(OAc) ) (Lancaster), bis(4-fluorophenyl) sulfone (ACROS), and cesium fluoride
2
(
ACROS) were used as received. Tetra-n-butylammonium tetrafluoroborate (TBABF ) was
4
acquired by ion exchanged of saturated sodium tetrafluoroborate aqueous solution with
saturated tetra-n-butylammonium bromide aqueous solution, and then recrystallized by ethyl
acetate.
2
.2 Monomer and Polymer Synthesis
4-Bromophenoxy)-tert-butyldimethylsilane (Br-Si)
-Bromophenol (30 g, 0.17 mol) was added into a 500 mL three-necked round-bottom
(
4
flask and dissolved in 200 mL of dimethylformamide (DMF) under nitrogen atmosphere.
Then, imidazole (14 g, 0.21 mol) and tert-butylchlorodimethylsilane (Cl-TBDMS, 30 g, 0.20
mol) were added under 0 ℃ ice bath. The reaction was stirred for 20 h and was slowly
returned to room temperature. After that, the mixture was poured into cold water and
4

extracted with hexane. The organic phase was washed with saturate NaHCO3(aq) two times
and dried over anhydrous sodium sulfate (MgSO . The solvent was removed under reduced
4
)
pressure on a rotary evaporator and then a vacuum pump to obtain 48 g of colorless liquid
1
(
96% yield). mp < room temperature. H NMR (400 MHz, DMSO-d , δ, ppm): 7.38-7.36 (d, 2
6
H, H ), 6.79-6.77 (d, 2 H, H ), 0.92 (s, 9 H, H ), 0.15 (s, 6 H, H ).
a
b
d
c
N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N',N'-dimethylbenzene-1,4-diamine (NDPA-
Si)
Tris(dibenzylideneacetone)dipalladium(0) (Pd (dba) , 0.7361 g, 0.80 mmol) and tri-tert-
2
3
butylphosphine (P(tBu) , 0.4 mL, 1.60 mmol) were added into a 250 mL round-bottom flask
3
containing 100 mL anhydrous toluene in the glove box preferentially. After stirring at room
temperature under nitrogen atmosphere for 20 minutes to undergo the ligand exchange, N,N-
dimethyl-p-phenylenediamine (13.61 g, 0.10 mol) was added to the flask sequentially and
stirred at 50 ℃ till the N,N-dimethyl-p-phenylenediamine dissolved completely. Br-Si (22.90
g, 0.08 mol) and sodium tert-butoxide (NaOtBu, 10.00 g, 0.10 mol) were then added into the
solution. The mixture was stirred at 90 ℃ for 17 hours and then extracted with EA and water
till the water layer was cleared. The organic layer was dried over MgSO and rotary
4
evaporator. The residual was purified by flash column chromatography and recrystallized
1
from hexane/EA to obtain 20.1 g of pale orange crystal (73% yield), mp: 93-95 ℃. H NMR
(
400 MHz, DMSO-d , δ, ppm): 7.34 (s, 1 H, H ), 6.91-6.89 (d, 2 H, H ), 6.79-6.77 (d, 2 H,
6 d e
1
3
H ), 6.70-6.65 (m, 4 H, H ), 2.80 (s, 6 H, H ), 0.94 (s, 9 H, H ), 0.14 (s, 6 H, H ). C NMR
c
b+f
a
h
g
(
125 MHz, DMSO-d , δ, ppm): 147.2 (C ), 145.4 (C ), 139.8 (C ), 134.3 (C ), 120.2 (C ),
6 9 2 6 5 3
1
19.7 (C ), 116.3 (C ) 114.1 (C ), 41.0 (C ), 25.6 (C ), 17.9 (C ).
7 4 8 1 12 11
N,N'-(1,4-phenylene)bis(N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N'',N''-
dimethylbenzene-1,4-diamine) (NTPPA-2Si)
5

Palladium(II) acetate (Pd(OAc) , 130 mg, 0.58 mmol) and tri-tert-butylphosphine
2
(
P(tBu) , 0.13 mL, 0.54 mmol) were added into a 100 mL round-bottom flask containing 65
3
mL anhydrous toluene in the glove box preferentially. After stirring at room temperature
under nitrogen atmosphere for 20 minutes to undergo the ligand exchange, 1,4-bromobenzene
(3.42 g, 14.5 mmol), NDPA-Si (10.27 g, 30.0 mmol) and sodium tert-butoxide (NaOtBu, 4.50
g, 46.8 mmol) were added into the container sequentially and then the solution was refluxed
for 4 hours. After cooling to room temperature, the mixture was extracted with water and
dichloromethane till the water phase was cleared. The organic layer was dried over MgSO4
and rotary evaporator. The residual was purified by column chromatography and then
recrystallized by EA/MeOH to obtain 7.4 g of brown needle crystal (67% yield), mp: 165-
1
1
6
67 ℃. H NMR (500 MHz, THF-d , δ, ppm): 6.94-6.92 (d, 4 H, H ), 6.88-6.86 (d, 4 H, H ),
8
c
e
.77 (s, 4 H, H ), 6.68-6.65 (t, 8 H, H ), 2.88 (s, 12 H, H ), 0.98 (s, 18 H, H ), 0.18 (s, 12 H,
d
b+f
a
h
1
3
H ). C NMR (125 MHz, THF-d , δ, ppm): 151.3 (C ), 148.5 (C ), 143.8 (C ), 143.8 (C ),
g
8
6
11
2
8
1
1
39.1 (C ), 127.3 (C ), 125.0 (C ) 123.8 (C ), 121.1 (C ), 114.6 (C ), 41.2 (C ), 26.3 (C ),
5
4
9
7
10
3
1
14
9.0 (C ).
1
3
N,N'-((1,1'-biphenyl)-4,4'-diyl)bis(N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N'',N''-
dimethylbenzene-1,4-diamine) (NTPB-2Si)
6

Palladium(II) acetate (Pd(OAc) , 130 mg, 0.58 mmol) and tri-tert-butylphosphine
2
(
P(tBu) , 0.13 mL, 0.54 mmol) were added into a 100 mL round-bottom flask containing 65
3
mL anhydrous toluene in the glove box preferentially. After stirring at room temperature
under nitrogen atmosphere for 20 minutes to undergo the ligand exchange, 4,4'-dibromo-1,1'-
biphenyl (4.52 g, 14.5 mmol), NDPA-Si (10.27 g, 30.0 mmol) and sodium tert-butoxide
(NaOtBu, 4.50 g, 46.8 mmol) were added into the container sequentially and then the solution
was refluxed for 4 hours. After cooling to room temperature, the mixture was extracted with
water and dichloromethane till the water phase was cleared. The organic layer was dried over
MgSO and rotary evaporator. The residual was purified by column chromatography and then
4
recrystallized by EA/DCM to obtain 9.5 g of yellow granular crystal (78% yield), mp: 178-
1
1
81 ℃. H NMR (500 MHz, THF-d , δ, ppm): 7.33-7.31 (d, 4 H, H ), 6.98-6.95 (m, 8 H,
8
e
H ), 6.90-6.89 (d, 4 H, H ), 6.74-6.68 (m, 8 H, H ), 2.90 (s, 12 H, H ), 1.00 (s, 18 H, H ),
c+f
d
b+g
a
i
1
3
0
1
.20 (s, 12 H, H ). C NMR (125 MHz, THF-d , δ, ppm): 152.1 (C ), 148.9 (C ), 148.8 (C ),
h 8 6 5 13
43.1 (C ), 138.4 (C ), 133.8 (C ), 128.0 (C ) 127.5 (C ), 126.4 (C ), 121.6 (C ), 121.3
2
10
9
4
8
11
7
(
C ), 114.6 (C ), 41.1 (C ), 26.3 (C ), 19.0 (C ).
12 3 1 16 15
Preparation of NTPPA-PES and NTPB-PES poly(ether sulfone)s
CsF (0.35 g, 2.3 mmol) was added into a 50 mL two-necked round-bottom flask and
dried at 150 ℃ for 3 hours under the vacuum system. After cooling to the room temperature,
NTPPA-2Si (0.76 g, 1.0 mmol) and 6.4 mL of anhydrous NMP were added into the flask and
stirred at 120 ℃ for 3 hours. After bis(4-fluorophenyl)sulfone (0.25 g, 1.0 mmol) was added
into the reaction, the temperature was raised to 180 ℃ and stirred for 12 hours. While the
temperature returned to room temperature, the obtained polymer solution was slowly poured
into methanol. The filtered was further purified by using Soxhlet extractor for 24 hours. After
that, the obtained NTPPA-PES was dried under vacuum system at 120 ℃ (Yield: 96%).
NTPB-PES was also prepared by the same reaction condition.
7

2.3 Measurement
Melting point was mainly detected by OptiMelt-Automated Melting Point System at
1
the scan rate of 5 ℃/min and DSC at the scan rate of 5 ℃/min if mentioned. H NMR spectra
13
were recorded on Bruker DPX-400NMR (400 MHz) and C NMR was recorded on Bruker
AVIII-500MHz FT-NMR (125 MHz). The definition of splitting pattern is carried out as
follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Fourier transform infrared
(FT-IR) spectra were obtain by PerkinElmer Spectrum 100 Model FT-IR spectrometer.
Electrochemistry was executed with CH Instrument 611B Electrochemical Analyzer or CH
Instrument 612C Electrochemical Analyzer. Ultraviolet–visible (UV-Vis) spectra were carried
out by Agilent 8453 UV-visible Spectroscopy System. CIELAB data were collected from
JASCO V-650 UV-VIS spectrophotometer. Cyclic voltammetry was conducted by using the
optically transparent thin layer electrochemical (OTTLE) cell (platinum gauze, 80 mesh, as
working electrode; platinum wire as an auxiliary electrode; Ag/AgCl, 0.3 M KCl as a
reference electrode) or two-electrode device with 20 mm×20 mm working area at the scan
rate of 50 mV/s. Differential pulse voltammetry was conducted with the same system at the
scan rate of 2 mV/s, pulse amplitude of 50 mV, pulse width of 25 ms and pulse period of 0.2
s. Spectroelectrochemistry was conducted with the same system at the range of 300 and 1100
nm. UV-vis spectra for the wavelength range of 300–1600 nm were recorded by Hitachi U-
4
100 UV-vis-NIR spectrophotometer. The inherent viscosity was specified with a Tamson
TV-2000 viscometer at 30 °C at the concentration 0.5 g/dL. A Waters chromatography unit
interfaced with a Waters 2410 refractive index detector was used to conduct gel permeation
chromatographic (GPC) analysis. Two Waters 5 μm Styragel HR-2 and HR-4 columns (7.8
mm I. D. × 300 mm) were connected with NMP in series as the eluent at a flow rate of 0.5
mL/min at 40 °C and were calibrated with polystyrene standards.
2
.4 Preparation of the films
8

4
00 µL solutions of PESs in o-dichlorobenzene (2 mg/1 c.c.) were casted onto the
2
ITO-coated glass substrates (3×2.5 cm ) and dried at room temperature for 6 hours, and then
1
60 ℃ for 3 hours under vacuum. Subsequently, the obtained ITO-coated glass was cut into
2
2
the size of 3×0.7 cm for further measurements or 2×2 cm for fabrication of EC devices.
2.5 Spectroelectrochemical measurement
Spectroelectrochemical measurements were applied to investigate the preliminary
optical behavior of the two EC materials, NTPPA-2Si and NTPB-2Si, respectively, carried
out by an optically transparent thin-layer electrode (OTTLE) coupled with UV-vis-NIR
spectroscopy. The OTTLE cell was placed in the optical path of light beam in the
spectrophotometer, which allowed us to acquire electronic absorption spectra during
electrochemical experiments in a 0.1 M TBABF /NMP solution with 1 mM of the targeted
4
EC materials. Besides, these new EC polymer films (NTPPA-PES and NTPB-PES) were
casted on the ITO-coated glass substrate as working electrode were investigated by UV-vis-
NIR spectroscopy in dry acetonitrile (CH CN) containing 0.1 M of TBABF as electrolyte
3
4
under nitrogen atmosphere
2.6 Device fabrication
First, prepare the polymer films on ITO substrates according to the previous method
2
and then erase the area over 2*2 cm with chloroform. Thermoset adhesive was dispensed on
the ITO glass by full-auto dispenser and a tiny hole would be retained. Another blank ITO
glass was then pasted onto it and baked at 150 ℃ for two hours. The gap was controlled by
dispersing glass ball with a grain size about 120 µm into the adhesive. The liquid-type
electrolyte (~0.05 mL) was then injected into the device by using a sample vacuum
encapsulating system and the tiny hole would be filled up by UV gel. Stick copper tape on
both side of the device for further measurement.
9

3. Results and Discussion
3.1 Characterization of Monomer
In order to obtain high molecular weight PESs with dimethylamino groups, the
protecting silane unit as functional groups of monomers are necessary. Starting material (4-
bromophenoxy)-tert-butyldimethylsilane (Br-Si), a high purity colorless liquid, was
synthesized from 4-bromophenol with an equivalent of tert-dimethylsilyl chloride under base
[
51]
condition by the reported procedures.
N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N',N'-
dimethylbenzene-1,4-diamine (NDPA-Si), an important intermediate product, was prepared
by Buchwald-Hartwig amination with a yield of 73%. Two novel final targeted monomers
with
silyl
protecting
groups,
N,N'-(1,4-phenylene)bis(N-(4-((tert-
butyldimethylsilyl)oxy)phenyl)-N'',N''-dimethylbenzene-1,4-diamine) (NTPPA-2Si) and
N,N'-((1,1'-biphenyl)-4,4'-diyl)bis(N-(4-((tert-butyldimethylsilyl)oxy)phenyl)-N'',N''-
dimethylbenzene-1,4-diamine) (NTPB-2Si), were synthesized from the similar procedure
with the yield of 67% and 78%, respectively, as shown in Scheme 1. The synthesis and
characterization of these compounds are illustrated in Supporting Information. Fourier
transform infrared (FT-IR) spectra of the monomers were illustrated in Figure S1, and the
formation of NTPPA-2Si and NTPB-2Si could also be confirmed. Relative to NDPA-Si, the
-1
medium N-H stretching of secondary amine in the region of 3310-3350 cm disappeared in
the spectra of NTPPA-2Si and NTPB-2Si, and all the compounds showed similar
-
1
characteristic bands such as the aliphatic C-H stretching at 2800-3000 cm , the C-N
-1
-1
stretching at 1252 cm and the Si-C stretching at 823 cm . The NMR spectra for the
intermediate and two monomers were also measured and depicted in Figure S2 to Figure
S13. In addition, all the peaks related to the molecular structures could be assigned with the
aid of two-dimensional (2D) COSY and HSQC NMR spectra. Thus, the results of all the
10

spectroscopic analyses suggest the successful preparation of the targeted disilyl ether
monomers.
3.2 Characterization of Polymers
Two new types of PESs were prepared from the reaction of disilyl ether protecting
groups of the multi-arylamine based monomers and bis(4-fluorophenyl)sulfone as shown in
Scheme 2. During the polymerization process, the silyl protecting groups were deprotected by
CsF to form phenoxide salts and further reacted with difluoride via nucleophilic substitution
to produce PESs. This kind of method could avoid extra steps, such as pre-deprotection and
[
52]
purification of the resulted diphenols. The results of all the spectroscopic analyses of FT-IR
1
and H-NMR spectra for the PESs shown in Figure S14 and Figure S15 suggest the
successful preparation of the target polymers.
The inherent viscosity and molecular weight of the obtained PESs are summarized in
Table S1. The solubility behavior of the polymers listed in Table S2 reveals high solubility in
most organic solvents, such as NMP, DMAc, THF, CHCl and o-DCB (o-dichlorobenzene)
3
due to the bulky, packing-disruptive and three-dimensional propeller-like multi-arylamine
units in the polymer backbone, indicating that these highly processable PESs should be
suitable for solution-casting, spin-coating or inkjet-printing to afford high-performance thin
films as potential candidates for optoelectronic devices applications.
The thermal properties of PESs were investigated by thermogravimetric analysis (TGA)
(Figure S16) and differential scanning calorimetry (DSC) (Figure S17) and the results are
summarized in Table S3. The prepared PESs with wholly aromatic structure exhibited high
o
thermal stability without significant weight loss up to 400 C under both nitrogen and air
o
atmosphere, and the carbonized residues (char yields) in nitrogen atmosphere at 800 C could
reach to 45%. Furthermore, these PESs also possessed high glass-transition temperatures (Tg)
of 220 and 226 °C for NTPPA-PES and NTPB-PES, respectively.
11

3.3 Electrochromic Properties of the Monomers
3.3.1 Electrochemical properties
The electrochemical properties of these two newly synthesized TPA-based EC
monomers, NTPPA-2Si and NTPB-2Si, were investigated by differential pulse voltammetry
DPV). The measurements were conducted under an optically transparent thin-layer
(
electrochemical (OTTLE) cell in anhydrous N-methyl-2-pyrrolidone (NMP) containing 1 mM
of EC monomers and using 0.1 M of tetrabutylammonium tetrafluoroborate (TBABF ) as the
4
supporting electrolyte under nitrogen atmosphere. The resulted DPV diagrams for NTPPA-
2
Si and NTPB-2Si are depicted in Figure 1a and Figure 2a. Since the aliphatic
dimethylamine behaves stronger electron donating nature than aromatic amine, therefore, not
only the oxidation potentials of the NTPPA-2Si and NTPB-2Si compounds at the first and
second stages could be effectively reduced but also should exhibit four oxidation stages.
However, oxidation potentials may be very close or even overlap as the result of symmetric
and similar dimethylamino-containing TPA electroactive surroundings. Thence, the DPV
diagram of NTPPA-2Si exhibited merely three major oxidation peaks at 0.46, 0.67 and 1.08 V
together with EC coloring images from colorless at neutral state to green and then deep green
at the semi-oxidation and fully oxidation stages, respectively, as shown in Figure 1. In order
to evaluate the electrochemical redox stability behaviors of the newly synthesized two
monomers, optical recovery test from neutral to different oxidative stages of the monomers
was conducted to confirm their electrochemical reversibility. Regardless of optical recovery
in first and second oxidation stages, NTPPA-2Si could maintain excellent recovery up to
9
9.8% and 98.8%, respectively as depicted in Figure 1b and Figure 1c, respectively. On the
contrary, NTPB-2Si with four theoretical electroactive sites revealed only two broader
oxidation peaks at 0.60 V and 1.00 V maybe could be ascribed to the symmetric nitrogen
centers of bis(diphenylamine)s linked via more distanced biphenyl than phenyl units as the
12

bridge causing the more similar oxidation potentials to enhance the formation of one wave
two electron oxidation process. (Figure 2a). The EC color change images of NTPB-2Si
shown in Figure 2 from neutral colorless stage to green semi-oxidation, and then purple color
fully oxidation stages together with excellent redox reversibility confirmed by the highly
optical recovery ratio up to 98.5%.
3.3.2 Spectroelectrochemical properties
Both of the electroactive compounds were colorless in the neutral state (0 V). The
spectra of absorption changes for NTPPA-2Si at different working potential are shown in
Figure 1. When the electroactive amino groups within NTPPA-2Si began to oxidize, a new
shoulder peak at 430 nm and two other peaks centered at 744 and 1021 nm appeared
simultaneously. According the results from DPV diagram shown in Figure 1a, the first and
second oxidation potential peaks were very close and even overlapped, implying that each
electronic oxidation states may not be clearly determined. According to the observation of
oxidation stage from 0-0.8 V, the first three electrons (will discuss later) exhibited the same
tendency of absorption pattern as shown in Figure 1b. Notably, the broad absorption peak at
1
021 nm in the near-IR region within the range of 0-0.8 V could be classified as the
intervalence charge transfer (IVCT) due to the coupling interaction of cation radicals between
[
52]
two electroactive amine via phenylene bridge.
The first oxidation stage demonstrated
extremely high reversibility of 99.8 % based on the neutral form absorbance at 313 nm. As we
applied a higher potential from 0.9 V to 1.3 V, NTPPA-2Si could reach the fully and final
oxidation state of the fourth electron. The characteristic peaks ascribed to the first stage at 430
and 1021 nm would decrease while the new peak at 744 nm kept enhancing as shown in
Figure 1c. In the fourth and last oxidation state of NTPPA-2Si, the IVCT band in the near-IR
region started to decay since all the four oxidation sites in the structure of NTPPA-2Si were
completely oxidized, and the reversibility was 98.8% at this oxidation stage.
13

The DPV diagram of NTPB-2Si with biphenyl-linkage structure depicted in Figure 2a
exhibited only two broader peaks but not four or three peaks as the NTPPA-2Si with phenyl-
linkage structure that may be attributed to the more extended biphenyl bridge between two
diphenylamine units. Figure 2b displays the spectroelectrochemical behavior of NTPB-2Si in
the range of applied voltage 0-0.8 V, a shoulder peak at 455 and a broad absorption peak at
7
95 nm appeared simultaneously. At this applied voltage range, the first three oxidation
centers (will discuss later) were oxidized with a reversibility of 99.6%. As the applied
potential increased from 0.9 to 1.2 V, the remaining oxidation sites could then be oxidized as
depicted in Figure 2c, and the absorption peaks at 455 nm and 795 nm for the first oxidation
stage decrease their intensity while the new flat and broad absorption between 502 and 739
nm became obvious with a reversibility of 98.5% at this second oxidation stage.
3.3.3 Nernst equation: Exploring the number of electrons transferred at each oxidation stage
For the purpose of further exploring the actual situation of electronic oxidation at each
step of NTPPA-2Si and NTPB-2Si, spectroelectrochemical spectra and Nernst equation were
combined to investigate more detailed oxidation behaviors of the four redox-active sites
[
54-56]
within these two EC materials in this section.
, By utilizing the intensity change of
characteristic absorption peak in spectroelectrochemical spectra derived from OTTLE
measurement as the concentrations of electroactive compound at oxidation and neutral forms.
The ratio of oxidized [O] and neutral forms [N], [푂]/[푁], at each different applied potential
would be in accordance with the Nernst equation (1):
0
.059
[ꢁ]
0
퐸푎푝푝푙푖푒푑 = 퐸 +
ꢀ표푔 ( )
푛
[ꢂ]
(1)
0
where Eapplied and E represent the applied and standard potentials of redox reaction,
respectively; [O] and [N] are the oxidized and neutral species, and 0.059 represents the ratio
of the gas constant to the Faraday constant at room temperature. The [O]/[N] value could be
14

calculated from absorbance changes measured by recording spectra after achieving
equilibrium corresponding to each Eapplied. In the equation, the measurement was performed at
a gradually increasing applied potential; each applied potential was maintained until an
equilibrium value of [푂]/[푁] was established in the entire thin layer solution contiguous to
the platinum electrode by electrolysis and diffusion process. After obtaining the ratio of
[푂]/[푁], plotting 퐸푎푝푝푙푖푒푑 to logꢃ[푂]/[푁]ꢄ by linear regression. The resulting slope value
could be used to fit the coefficient of Nernst equation, and the resulted n value represented the
[57,58]
number of electrons oxidized in the step.
The absorption spectral change for NTPPA-2Si and the corresponding Nernst equation
plot depicted in Figure S18 and Figure S19, respectively, was divided into three different
voltage intervals (a, b) 0.4-0.48 V, (c, d) 0.5-0.6 V and (e, f) 0.6-0.7 V separately in Figure
S18. Since the oxidation potentials in this stage were close to form a broader peak, electrons
were not oxidized one by one independently. To gain the fitting lines, the [푂]/[푁] values
from different applied voltages at 750 nm was plotted with 퐸푎푝푝푙푖푒푑. Comparing the slope of
fitting line in Figure S18b (0.4-0.48 V) with the Nernstian value of 0.059 V, the obtained
value n = 1.58. By using the same approaches, n = 0.87 and n = 0.61 could be also
b
d
f
determined as shown in Figure S18d and Figure S18f. Through summing these n values (n +
b
n + n = 3.06), the total number of electrons transferred was confirmed approximately to be 3.
d
f
In other words, the applied voltage from 0.0 V to 0.8 V demonstrated three-electron transfer
manifested by two waves oxidation process. Furthermore, when applying higher voltage
interval to 0.96-1.06 V as shown in Figure S19, the slope of the fitting line revealed one-
electron transfer process (n = 1.13).
Similarly, for NTPB-2Si in Figure S20, the study of Nernst equation demonstrated that
NTPB-2Si undergoes the three-electrons oxidation process at applied potential from 0.5 to 0.7
V; and by using the same approach, n = 0.93 could be obtained at applied potential of 1.02-
b
15

1
.1 V for the one-electron oxidation process as shown in Figure S21. These results derived
from Nernst equation approach together with DPV diagrams are summarized in Table 1.
3. 3.4 Electrochemical stability
The electrochemical stability of NTPPA-2Si and NTPB-2Si was examined by repetitive
cyclic voltammetry (CV) test as shown in Figure 3 and Figure 4. Referring to DPV results,
the CV tests were divided into three voltage stages up to 0.55 V, 0.8 V and 1.4 V for NTPPA-
2
Si and two stages up to 0.75 V and 1.4 V for NTPB-2Si, respectively, for continuous 100
cycles at the scan rate of 25 mV/s. And demonstrated that the former three electrons transfer
oxidation at the oxidation stage with lower applied potentials exhibits extremely high
electrochemical stability while the last stage at higher applied potential reveals less stable
redox behaviors for both of the compounds. From the EC recovery tests of absorption spectra
for these two compounds mentioned above in Figure 1b, 1c and Figure 2b, 2c, respectively,
the results also demonstrated the same trend that these two EC compounds reveals excellent
reversibility (>99.6%) at the first oxidation stage while decreases to 98.5% at the last stage.
3.4 Electrochromic Properties of the Polymers
3.4.1 Electrochemical properties
The electrochemical properties of the two new poly(ether sulfone)s, NTPPA-PES and
NTPB-PES, were investigated by cyclic voltammetry with polymer thin films casted on the
ITO-coated glass substrate as working electrode in the dry acetonitrile (CH CN) containing
3
0
.1 M of TBABF as an electrolyte under nitrogen atmosphere for oxidation measurements,
4
and the results are summarized in Figure S22. The oxidation potentials of NTPPA-PES
showed four obvious oxidation peaks at 0.58, 0.68, 1.06 and 1.27 V, separately, for the four
electroactive nitrogen centers in Figure S22a. Although NTPB-PES also has four
electroactive nitrogen centers as NTPPA-PES, while the oxidation behavior was different as
depicted in Figure S22b that the first oxidation peak at 0.69 V should be two-electron process
16

contributed from subsequent dimethylamine groups, and the second oxidation peak at 1.22 V
also exhibited two-electron process derived from tetraphenyl-benzidine (TPB) unit.
3.4.2 Spectroelectrochemical properties
According to the four electroactive nitrogen centers from dimethylamine and arylamine
units, the EC process was divided into four steps. For NTPPA-PES, when the applied
potentials increased positively from 0 to 0.6 V, the characteristic absorption peak of neutral
state at 322 nm decreased gradually while a new wide range absorption band with three peaks
grew up at 600, 845 and 1049 nm during the first electron oxidation (Figure 5a). As the
potential was further increased to 0.7 V, all of the absorption peaks belonged to the first
oxidation state reduced their intensity, and the characteristic absorption around 400-500 nm
maintained its shoulder intensity, resulting in color change from blue to pale blue at the
second oxidation state (Figure 5b). Comparing to the oxidizing ability of aromatic amine
(
TPA) unit, the aliphatic amine (NMe ) moiety should possess stronger basicity which leads to
2
have lower oxidation voltage. Therefore, the first two oxidation steps could be attributed to
[37,59]
the dimethylamino groups.
The EC images and the CIELAB data of semi-oxidized state
NTPPA-PES film at the first two redox steps as depicted in Figure 5 show the color change
from highly transparent neutral state (0 V, L*: 97.45, a*: 1.60, b*: 0.07) to a blue (0.6 V, L*:
6
9.11, a*: -25.16, b*: 0.97), and then to pale blue (0.7 V, L*: 89.16, a*: -7.61, b*: 13.30). As
the applied potential increased further to the third oxidation step (0.8-1.0 V), the characteristic
absorption of the typical arylamine characteristic absorption peak at 415 nm and another
intense inter-valence charge transfer (IV-CT) broad band at near IR region centered at 942 nm
grew up accompanying with the color change to green (1.0 V, L*: 58.88, a*: -11.86, b*:
1
4.94) at the third oxidation step (Figure 5c). The broad absorption at the near-IR region
ascribed to the IV-CT excitation caused by the electron coupling between neutral nitrogen and
mono-cationic radical of TPA nitrogen centers in TPPA moiety via the phenyl bridge, which is
17

[
52]
consistent with the phenomenon classified by Robin and Day. For the fully oxidation state
at applied voltage (1.1-1.3 V), the two characteristic absorption peaks (415 nm, 942 nm) of
the third oxidation state decreased, and a new sharp absorption peak at 766 nm appeared with
a color change to dark blue (1.3 V, L*: 40.07, a*: -1.52, b*: -19.00) as shown in Figure 5d.
The reduced intensity of NIR absorption band demonstrated the further oxidation of mono-
cationic radical species to the formation of dication in the TPPA segments. Subsequently, as
the applied potential returned to 0 V, the film could recover to the original colorless neutral
state.
NTPB-PES also could exhibit four stages of anodic color change by applying different
related potentials. At the first stage from 0.0 to 0.6 V, the characteristic peak in the UV region
decreased gradually while a broad peak centered at 760 nm grew up. The image of the EC
film changed from colorless (0 V, L*: 99.90, a*: -0.07, b*: 3.34) to green (0.6 V, L*: 78.10,
a*: -28.90, b*: 13.94) at first oxidation state (Figure 6a). When the potential was set to 0.8 V,
the characteristic absorption broad peak at 760 nm of the first oxidation state disappeared and
the color would change back to almost colorless (0.8 V, L*: 98.47, a*: -4.61, b*: 17.07) at
second oxidation state as shown in Figure 6b after both electroactive sites of dimethylamino
groups were oxidized to cation radicals. This decolorization behavior (disappearance of bands
situated both in visible and NIR region) was similar to dimethylamine-substituted organic
[60]
dyes, such as methylene blue and Rodamine B under electrochemical oxidation process.
For the third oxidation state from 0.9 to 1.0 V, two new typical oxidation characteristic
absorption peaks at 485, 885, and a broad IV-CT band at 1380 nm od TPB moiety grew up
with the color change to orange (1.0 V, L*: 58.83, a*: 14.44, b*: 55.74). When the applied
voltage was adjusted to fully oxidation state (1.1-1.2 V), the two peaks at 485 and 1380 nm
decreased, while a sharp and intensive peak at 833 nm arose rapidly with a strong color
change from orange to dark blue (1.2 V, L*: 18.90, a*: -1.79, b*: -25.45) as shown in Figure
6d. This NTPB-PES film also could recover to the same original colorless state as NTPPA-
18

PES when the applied potential was released to 0 V. According to the results shown in Figure
and Figure 6, these novel dimethylamino-substituted PESs demonstrated fascinating EC
behaviors with relatively low applied voltage and multiple oxidation stages.
5
3.5 Device Fabrication and EC Properties
The two EC polymers were further introduced to fabricate electrochromic device (ECD)
and the EC behaviors were systematically investigated. The procedure of preparing ECD is
illustrated in Figure S23 and the schematic diagram of the devices is depicted in Figure 7. In
addition, cathodic EC material, heptyl viologen (HV), was introduced into the device as
complementary EC material for building ambipolar system that not only could balance the
charge of electroactive species to reduce the driving redox voltage but also efficaciously
increase totally electrolyte concentration.
The spectroelectrochemical behaviors of ECDs based on NTPPA-PES and NTPB-PES
without/with HV are summarized in Figure S24-S27. The EC spectra changes of the prepared
ECDs at the four oxidation stages were in accordance with those of original polymer films but
higher applied voltage would be required in the device because of lower diffusion rate
between electrode and ions from the electrolyte. Taking NTPPA-PES based ECD as example,
the applied voltage required for the four oxidation stages were 2.3, 2.4, 2.6, 2.8 V (without
HV) and 1.0, 1.5, 1.8, 2.0 V (with HV), respectively. It is worth noting that the PES film
coupled with HV as ambipolar device system could effectively reduce driving voltage due to
the complementary effect.
The electrochemical, spectroelectrochemical and the corresponding EC coloring
behaviors of the ECDs derived from NTPPA-PES/HV and NTPB-PES/HV at the first
oxidation state are depicted in Figure 8. The oxidation potential peaks obtained by CV were
1
.01 and 0.96 V for NTPPA-PES/HV and NTPB-PBS/HV as shown in Figure 8a and 8b,
respectively. The EC behaviors and color changes of the devices at different working
19

potentials from 0 to 1.0 V are demonstrated in Figure 8c and 8d. For NTPPA-PES/HV
Figure 8c), three absorption peaks at 600, 845, and 1049 nm appeared with color change
(
from colorless (0 V, L*: 91.21, a*: -0.52, b*: -0.11) to navy blue (1.0 V, L*: 50.90, a*: -16.98,
b*: -28.32) at the first oxidation state. In the case of NTPB-PBS/HV (Figure 8d), two
absorption peaks emerged at 600 and 760 nm, and color changed from colorless (0 V, L*:
8
8.75, a*: -0.41, b*: 5.23) to blue (1.0 V, L*: 67.75, a*: -9.22, b*: -23.66) at the first
oxidation state. The absorption peak at 600 nm could be ascribed to the contribution from the
reduction of HV both in NTPPA-PES/HV and NTPB-PBS/HV ECDs. In addition, the long-
term electrochemical stability of these two ECDs were also confirmed by repetitive cyclic
voltammograms as shown in Figure 9. After 100 cycles of continuous repeated redox
scanning, both NTPPA-PES/HV and NTPB-PBS/HV devices still revealed excellent EC
behaviors and excellent cyclic electrochemical stability.
3. Conclusion
Two novel dimethylamine-containing TPA-based EC materials with silyl functional
groups, NTPPA-2Si and NTPB-2Si, were readily prepared by Buchwald-Hartwig amination
and then go through a silyl polycondensation to synthesize two poly(ether sulfone)s (PESs),
NTPPA-PES and NTPB-PES, respectively. The dimethylamino substituents not only provide
stronger electron-donating ability to reduce oxidation potentials but also reveal multiple
coloring stages comparing to their corresponding multi-arylamine structures without
dimethylamino substituents. According to the results from the spectroelectrochemical spectra
combining with Nernst equation, the numbers of multi-electrons transfer behaviors of the
redox-active monomeric compounds during oxidation procedure could be determined. The
obtained polymer films demonstrated fascinating four EC coloring-stage changes at relatively
lower voltages. The ECDs derived from these anodic materials accompanying with cathodic
HV exhibited high EC stability implying great potential for optoelectronic applications.
20

Authorship contribution statement
G.S.L and C.L.H conceived the study project. G.S.L guided and supervised the project. C.L.H
fabricated the devices and performed the measurements. C.L.H, Y.R.K and Y.J.S wrote the
first draft of the manuscript. G.S.L revised the manuscript.
Supporting Information: Supplementary material associated with this article can be found,
in the online version.
Acknowledgements
This work was financially supported by the “Advanced Research Center for Green
Materials Science and Technology” from The Featured Area Research Center Program within
the framework of the Higher Education Sprout Project by the Ministry of Education in
Taiwan (109L9006) and the Ministry of Science and Technology in Taiwan (MOST 109-
2
634-F-002-042, 107-2113-M-002-024-MY3, and 107-2221-E-002-066-MY3).
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Scheme 1. Synthetic route of NTPPA-2Si and NTPB-2Si.
29