Synthesis and characterization of triphenylamine-based polymers and their application towards solid-state electrochromic cells

Article abstract of DOI:10.1039/c6ra12112h

Four novel triphenylamine-based polymers, PJK1, PJK2, PJK3 and PJK4 were successfully synthesized and fully characterized by ¹H NMR, UV-VIS spectroscopy, cyclic voltammetry (CV), GPC and spectroelectrochemistry. These polymers are easily soluble in many common organic solvents, which make them appropriate for film deposition via spray-coating. We fabricated electrochromic cells comprising ITO-coated glass/polymer/gel electrolyte/ITO-coated glass and patterned the color change by applying direct current with different voltages. We report herein color changes, from the neutral to oxidized form as follows: for PJK1, orange to dark green; for PJK2, light yellow to reddish brown; for PJK3, light blue to grey; and for PJK4, green to bluish green. The majority of the copolymers exhibited very good thermal stabilities, as evidenced by less than a 5% weight loss in temperatures exceeding 400 °C. To further characterize, we simulated the electrochemical and optical properties, which are good agreement with the experimental data.

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Synthesis and characterization of triphenylamine-based polymers
and their application towards solid-state electrochromic cells
Jaemyeng Jeong‡, Rangaraju Satish Kumar‡, Mergu Naveen and Young-A Son*
Received 00th January 20xx,
Accepted 00th January 20xx
Four novel triphenylamine-based polymers, PJK1, PJK2, PJK3 and PJK4 were successfully synthesized and fully chara-
DOI: 10.1039/x0xx00000x
1
cterized by H NMR, UV-VIS spectroscopy, cyclic voltammetry (CV), GPC and spectroelectrochemistry. These polymers are
www.rsc.org/
easily soluble in many common organic solvents, which make them appropriate for film deposition via spray-coating. We
made fabricated electrochromic cells comprising ITO-coated glass/polymer/gel electrolyte/ITO-coated glass and patterned
the color change by applying direct current with different voltages. We report herein color changes, from the neutral to
oxidized form as follows: for PJK1, orange to dark green; for PJK2, light yellow to reddish brown; for PJK3, light blue to
grey; and for PJK4, green to bluish green. The majority of the copolymers exhibited very good thermal stabilities, as
evidenced by less than a 5% weight loss in temperatures exceeding 400 °C. To further characterize, we simulated the
electrochemical and optical properties, which are good agreement with the experimental data.
nitrogen atom is connected to three electron-rich phenyl
propeller-like geometry.19 Electron-rich
1. Introduction
groups in
a
triphenylamine derivatives are easily oxidized to form stable
polarons, an oxidation process that is always associated with a
clear change in coloration of the molecule. Additionally,
different thiophene-based derivatives have gained much
attention during the past years, owing to the variation of the
optical and electronic properties by simple structural
modifications.20,21 Fluorene is a biphenyl structure with rigid
plane and huge π-conjugated molecule, whose derivatives
have been widely used in organic electro-luminescence
materials, photochromic materials two-photon absorbing
materials, because of their very high photoluminescence,
thermal stability, electroluminescence quantum efficiency,
good solubility.22-24
Electrochromism is generally known as a reversible optical
change in a material induced by an external voltage; many
organic and inorganic compounds exhibit electrochromism
throughout the electromagnetic spectrum.1 The idea of an
electrochromic display was first suggested in 1961.2 This
property led to the expansion of many technological
applications such as smart windows,3 automatic anti-glazing
mirrors,4 chameleon materials,5 and electrochromic displays.6,7
In recent years, WO34 and viologen-based8 electrochromic
devices were commercialized. In the series of electrochromic
compounds, conjugated polymers can display high coloration
over a wide color range at low operating voltage with fast
switching capabilities, high optical contrasts, rapid response
times and the ability to modify their structure to create
multicolor electrochromes.9 Conjugated electrochromic
polymers, such as poly(thiophene) (PTh), poly(aniline) (PANI)
and poly(pyrrole) (PPy), have been extensively studied.10
Previously, many electrochromic organic polymers were
Here in this research we designed a series of conjugated
polymers as EC cells, wherein, triphenylamine (TPA) and
thiophene or fluorene were chosen to build the backbone of
our target molecule with the principle of different color
properties and these copolymers are highly conjugated and
giving a small band gap should therefore be more conducting
nature. We reported the synthesis and full characterization of
obtained
by
electropolymerization
or
oxidative
polymerization,11,12 but chemical polymerization has more
advantages than these polymerization methods. Because in
electropolymerization or oxidative polymerization methods we
always get the problems, such as the lack of film uniformity
over large surface, low material recovery, inability to form
large amounts of processable polymer and irregular linkages
within the polymer backbone.13,14
four triphenylamine-based polymers: PJK1, PJK2, PJK3 and
Triphenylamine (TPA) derivatives, which are electron-
donating in nature, show excellent electrochemical stability,
optoelectronic and electrochromic behavior.15-18 The
characteristic structural feature of triphenylamine is the
nitrogen center, which also serves as its electroactive site. The
Fig. 1 Polymer structures and color transitions.
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PJK4. The resulting copolymers were readily soluble in most h. The reaction mixture was allowed to cool to room
industrial solvents such as chloroform, dichloromethane, DMF, temperature, at which point 500 mL water was added, and the
and tetrahydrofuran. To evaluate the electrochemical mixture was extracted with ethyl acetate (2×250 mL). The
properties, all-solid-state electrochemical cells were prepared combined organic layers were dried over sodium sulfate,
by spray-coating chloroform solutions of the copolymers onto concentrated, and purified by silica gel column
ITO-coated glass and used a polymer gel as the electrolyte. chromatography (10% ethyl acetate in hexane) to yield 14.02 g
1
Neutral-to-oxidized color changes for the copolymers all (83%) of the product as a yellow and viscous oil. H NMR (300
differed, exhibiting transitions from orange to dark green, light MHz, CDCl3): δ 7.17-7.08 (m, 4H), 7.01-6.83 (m, 10H), 2.48 (t, J
yellow to reddish brown, light blue to grey, and green to bluish = 7.7 Hz, 2H), 1.57-1.44 (m, 2H), 1.35-1.21 (m, 2H), 0.85 (t, J =
green (Figure 1).
7.3 Hz, 3H) ppm; 13C NMR (75 MHz, CDCl3): δ 148.1, 145.3,
137.7, 129.2, 129.1, 124.7, 123.7, 122.1, 35.0, 33.6, 22.4, 13.9
ppm; IR νmax in cm-1(in CHCl3): 2930, 1589, 1508, 1491, 1276,
1075, 903, 833, 723, 649, 621; ESI-MS: 302 [M+H]+.
2. Experimental Section
2.1. Materials and Characterization
2.2.2. 2,3-diphenylthieno[3,4-b]pyrazine (7): Compound 6
(0.45 g, 3.95 mmol) was added to benzil (0.91 g, 4.34 mmol) in
ethanol (85 mL) and refluxed overnight. The solution was
evaporated and purified by column chromatography (10%
ethyl acetate in hexane) to afford a yellow solid (0.81 g, 71%).
1H NMR (600 MHz, CDCl3): δ 8.05 (s, 2H), 7.46-7.29 (m, 10H)
ppm; 13C NMR (150 MHz, CDCl3): δ 153.4, 141.6, 139.1, 129.6,
128.8, 128.1, 117.5 ppm; IR νmax in cm-1(KBr): 3084, 3054,
1597, 1576, 1231, 1074, 839, 816, 723; ESI-MS: 289 [M+H]+.
All solvents and reagents (analytical and spectroscopic grades)
were commercially obtained and used as received unless
otherwise noted. Fourier 300, 7.05 Tesla, 300 MHz, an AVANCE
III 600 spectrometer (Akishima, Japan) was operated at 600
MHz and 150 MHz for 1H and 13C NMR spectroscopy,
respectively. CDCl3 was used as the solvent, and Alice 4.0
software was used for analysis. The chemical shifts (δ values)
are reported in ppm downfield from an internal standard
(Me4Si). Mass spectra were recorded on a 4000 Q TRAP mass
spectrometer. HRMS spectra were obtained using a micrOTOF-
QII mass spectrometer. FT-IR spectra were recorded on an
ALPHA-P spectrometer. UV-visible absorption spectra were
recorded using an Agilent 8453 spectrophotometer. The
electrochemical measurements, i.e., cyclic voltammetry (CV)
experiments, were conducted using a Versa STAT 3 instrument
using polymer films sprayed on platinum button, the
electrolyte used was 0.1 M TBABF4 in propylene carbonate,
Ag/Ag+ electrode as the reference electrode and platinum
wire as the counter electrode.
2.2.3. 5,7-dibromo-2,3-diphenylthieno[3,4-b]pyrazine (8):
NBS (0.97 g, 5.4 mmol) was added to compound 7 (0.75 g, 2.6
mmol) in a mixture of chloroform/trifluoroacetic acid (16 mL;
1:1 v/v), and the mixture was stirred overnight in the dark. The
mixture was extracted with ethyl acetate and dried over
sodium sulfate, evaporated, and purified by column
chromatography (50% dichloromethane in hexane) to afford a
1
greenish-yellow solid (0.86 g, 74%). H NMR (300 MHz, CDCl3):
δ 7.46-7.27(m, 10H) ppm; 13C NMR (150 MHz, CDCl3): δ 154.7,
139.3, 138.2, 129.8, 129.3, 128.1, 104.9 ppm; IR νmax in cm-
1(KBr): 3051, 1596,1523, 1253, 1025, 811, 763, 739; HR-MS
(ESI-MS) m/z calcd. For C18H11Br2N2S [M+H]+: 446.8984, found
446.8989.
Thermogravimetric analyses (TGA) were performed on a
Mettler Toledo instrument using a heating rate of 20 °C/min
and nitrogen flow rate of 50 mL/min. Number-average (Mn)
and weight-average (Mw) molecular weights were determined
by gel permeation chromatography (GPC) using a Waters
model 2690 instrument with THF (HPLC grade, Aldrich) as the
eluting solvent. Spectroelectrochemical data of the polymers
films were recorded by using an Instek model GPS-3303
instrument as a DC power source. Electrochromic polymer
films were produced by spraying solutions of the polymers (5
mg/mL in chloroform) onto Indium tin oxide (ITO)-coated glass
slides (square : surface resistivity : 8-12 Ω/sq, 25 mm × 25 mm
× 1.1 mm, Sigma-Aldrich). The polymers were spray-coated
onto the active surface of an ITO plate using a mask.
2.2.4.
3,3''-dioctyl-[2,2':5',2''-terthiophene]-3',4'-diamine
(13): Compound 12 (3.03 g, 5.4 mmol) was stirred in a mixture
of HCl/ethanol (24 mL/35 mL) and cooled in an ice bath. Tin(II)
chloride dihydrate (40 g, 177 mmol) was added in small
portions to maintain the temperature at 25 °C. After the
reaction was stirred overnight, the pH was adjusted to 10 with
4 M KOH, and the solution was extracted with ethyl acetate.
The organic layer was dried with sodium sulfate and purified
by column chromatography (70% ethyl acetate in hexane) to
1
afford the desired compound as a brown oil (1.96 g, 72%). H
NMR (300 MHz, CDCl3): δ 7.29 (d, J =5.3 Hz, 2H), 6.99 (d, J =5.1
Hz, 2H), 3.35 (s, 4H), 2.62 (t, J =7.7 Hz, 4H), 1.65-1.52 (m, 4H),
1.37-1.15 (m, 20H), 0.87 (t, J =6.5, 6H) ppm; 13C NMR (150
MHz, CDCl3): δ 142.33, 134.57, 129.28, 127.88, 125.29, 109.01,
31.88, 30.81, 29.45, 29.39, 29.25, 28.94, 22.65, 14.08 ppm; IR
νmax in cm-1(in CHCl3): 3413, 3325, 2922, 2852, 1613, 1572,
1496, 1451, 904, 829, 725, 648; ESI-MS: 503 [M+H]+.
2.2. Synthesis of Monomer
2.2.1. 4-butyl-N,N-diphenylaniline (1): Bromobenzene (26.69
g, 170 mmol), 4-butylaniline (8.344 g, 56 mmol) and sodium
tert-butoxide (19.23 g, 200 mmol) were added to a solution of
bis[2-(diphenylphosphino) phenyl]ether (DPEPhos) (0.914 g,
1.7 mmol) and bis(dibenzylideneaceton)palladium (Pd(dba)2)
(0.789 g, 1.3 mmol) in toluene (160 mL) under an inert
atmosphere. The resulting solution was stirred at 90 °C for 24
2 | J. Name., 2012, 00, 1-3
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2.2.5. 5,7-bis(3-octylthiophen-2-yl)-2,3-diphenylthieno[3,4- mmol), 2,7-dibromo-9,9-dioctyl-9H-fluorene (0.2 g, 0.37
b]pyrazine (14): Compound 13 (1.85 g, 3.7 mmol) was added mmol), Pd(0)(PPh3)4 (0.02 g, 0.017 mmol), THF (15 mL) and 2
to a solution of benzil (0.85 g, 4.0 mmol) in ethanol (65 mL) M K2CO3 (5 mL). Yield (140 mg, 56%). 1H NMR (600 MHz,
and refluxed overnight. The solution was concentrated by CDCl3): δ 7.76-7.72 (m, 2H), 7.60-7.54 (m, 8H), 7.24-7.19 (m,
evaporation and purified by column chromatography (10% 4H), 7.15-7.11 (m, 4H), 2.64-2.59 (m, 2H), 2.60-1.92 (br, 4H),
ethyl acetate in hexane) to afford the desired compound as a 1.66-1.60(m, 2H), 1.43-1.37 (m, 2H), 1.31-1.02 (m, 16H), 0.99-
1
purple solid (2.32 g, 92%). H NMR (300 MHz, CDCl3): δ 7.55- 0.94(t, J = 7.34, 4H), 0.83-0.77(m, 9H) ppm.
7.42 (m, 4H), 7.41-7.29 (m, 8H), 7.03 (d, J =5.1 Hz, 2H), 2.97 (t,
J =7.7 Hz, 4H), 1.81-1.68 (m, 4H), 1.43-1.14 (m, 20H), 0.86 (t, J 2.3.3. Poly(4-butyl-N-(4-(2,3-diphenylthieno[3,4-b]pyrazin-5-
=6.5 Hz, 6H) ppm; 13C NMR (150 MHz, CDCl3): δ 152.3, 141.0, yl)phenyl)-N-phenylaniline (PJK3): The procedure used to
139.1, 137.7, 134.8, 130.0, 129.5, 128.8, 127.9, 126.8, 125.3, synthesize PJK1 was followed, using 4-butyl-N,N-bis(4-(4,4,5,5-
31.9, 30.4, 30.3, 29.7, 29.5, 29.3, 22.7, 14.1 ppm; IR νmax in cm- tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (0.2 g, 0.37
1(KBr): 3083, 3060, 2951,1577, 1511, 1246, 1209, 713, 694, mmol), 5,7-dibromo-2,3-diphenylthieno[3,4-b]pyrazine (0.17 g,
640; ESI-MS: 678 [M+H]+.
0.37 mmol), Pd(0)(PPh3)4 (0.02 g, 0.017 mmol), THF (15 mL)
and 2 M K2CO3 (5 mL). Yield (108 mg, 51%). 1H NMR (600 MHz,
2.2.6.
5,7-bis(5-bromo-3-octylthiophen-2-yl)-2,3-diphenyl CDCl3): δ 8.23-8.17 (m, 4H), 7.55-7.50 (m, 4H), 7.36-7.27 (m,
thieno[3,4-b]pyrazine (15): NBS (1.14 g, 6.4 mmol) was added 6H), 7.25-7.21 (m, 2H), 7.18-7.07 (m, 6H), 2.64-2.57 (m, 2H),
to a solution of compound 14 (1.97 g, 2.9 mmol) in a mixture 1.67-1.60 (m, 2H), 1.43-1.37 (m, 2H), 0.99 (t, J = 7.53, 3H) ppm.
of chloroform/trifluoro acetic acid (14 mL; 1:1 v/v), and the
mixture was stirred overnight in dark. The reaction mixture 2.3.4. Poly(4-butyl-N-(4-(4-octyl-5-(7-(3-octylthiophen-2-yl)-
was extracted with ethyl acetate, dried over sodium sulfate 2,3-diphenylthieno[3,4-b]pyrazin-5-yl)thiophen-3-yl)phenyl)-
and
purified
by
column
chromatography
(50% N-phenylaniline (PJK4): The procedure used to synthesize
dichloromethane in hexane) to afford the desired compound PJK1 was followed, using 4-butyl-N,N-bis(4-(4,4,5,5-
as a greenish, dark purple solid (2.39 g, 98%). 1H NMR (300 tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (0.2 g, 0.37
MHz, CDCl3): δ 7.55 (d, J =6.2 Hz, 4H), 7.39-7.30 (m, 6H), 6.98 mmol),
5,7-bis(5-bromo-3-octylthiophen-2-yl)-2,3-
(s, 2H), 2.91 (t, J =7.7 Hz, 4H), 1.78-1.66 (m, 4H), 1.45-1.18 (m, diphenylthieno[3,4-b]pyrazine (0.31 g, 0,37 mmol),
20H), 0.87 (t, J =6.7 Hz, 6H) ppm; 13C NMR (150 MHz, CDCl3): δ Pd(0)(PPh3)4 (0.02 g, 0.017 mmol), THF (15 mL) and 2 M K2CO3
152.79, 141.14, 138.80, 137.65, 132.14, 130.07, 129.62, (5 mL). Yield (184 mg, 52%). 1H NMR (600 MHz, CDCl3): δ 7.63-
129.06, 128.10, 124.24, 114.44, 31.87, 30.53, 30.06, 30.08, 7.53 (m, 8H), 7.37-7.29 (m. 6H), 7.21-7.09 (m, 10H), 3.03-2.97
29.73, 29.49, 29.29, 22.66, 14.08 ppm; IR νmax in cm-1(KBr): (m, 4H), 2.64-2.58 (m, 2H), 1.84-1.16 (m, 28H), 0.99-0.79 (m,
3019, 1215, 905, 753, 726; HR-MS (ESI-MS) m/z calcd. For 9H) ppm.
C42H46Br2N2S3Na [M+Na]+: 857.1064, found 857.1037.
3. Results and Discussion
2.3. Copolymer synthesis
3.1. Monomer Synthesis
2.3.1. Poly(4-butyl-N-(4-(3,4-dinitrothiophen-2-yl)phenyl)-N-
phenylaniline (PJK1): 4-butyl-N,N-bis(4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)phenyl)aniline (0.2 g, 0.37 mmol), 2,5-
The synthetic routes for all of the monomers are shown in
Schemes 1-3. In Scheme 1, compound 1 was prepared by using
dibromo-3,4-dinitrothiophene (0.12 g,
0.37 mmol),
the bis(dibenzylideneaceton)- palladium,
ligand (bis-[2-
Pd(0)(PPh3)4 (0.02 g, 0.017 mmol), and aqueous 2 M K2CO3 (5
mL) were added to dry THF (15 mL). The mixture was
vigorously stirred at 85-90 °C for 48-72 h. After the solution
cooled to room temperature, the reaction mixture was poured
into a cold mixture of methanol/deionized water (10:1 v/v).
The solid polymer was collected by filtration and washed with
methanol. The recovered polymer was then washed for 24 h in
a Soxhlet apparatus using acetone to remove oligomers and
catalyst residues. Finally, the polymer was redissolved in
chloroform, precipitated from cold methanol and dried (112
mg, 61%). 1H NMR (600 MHz, CDCl3): δ 7.42-7.38 (m, 4H), 7.22-
7.09 (m, 8H), 2.63 (t, J = 7.5, 2H), 1.66-1.60 (m, 2H), 1.43-
1.35(m, 2H), 0.96(t, J = 7.24, 3H) ppm.
(diphenylphosphino)phenyl]ether); compounds 2-6 were
synthesized according to reported procedures.25-30 Compound
7 was successfully prepared by condensation with benzil in
ethanol. In IR spectrum disappearance of the N-H stretching
band of the amino functionality at 3355 cm-1 was indicated
successful arylation to compound 7. Compound
8 was
synthesized in good yield using NBS in a mixture of TFA and
chloroform at room temperature (Scheme 2). The bromination
was confirmed by disappearance of the peak at δ 8.05 in the
1H-NMR spectrum and characterization data for compound 8
fully matched with the reported data (Scheme 2).31 Compund
9-12 prepared according to reported procedure.32-38 To
synthesize compound 15 (Scheme 3), we modified a reported
route.38 First, compound 12 was reduced using a mixture of
HCl/ethanol and tin(II) chloride to compound 13. The
previously reported yield for this reaction is 52%, which we
were able to increase to 72% here. Subsequently, compound
13 was condensed with the benzil to give 14 in 92% yield, and
2.3.2. Poly(4-butyl-N-(4-(9,9-dioctyl-9H-fluoren-2-yl)phenyl)-
N-phenylaniline (PJK2): The procedure used to synthesize
PJK1 was followed, using 4-butyl-N,N-bis(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (0.2 g, 0.37
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then brominated with NBS in TFA and chloroform to afford Suzuki coupling reaction of 4-butyl-N,N-bis(4-(4,4,5,5-
compound 15 in 98% yield. Using this method, we obtained a tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (3) with
higher overall yield for these 3 steps compared with that compounds 5, 8, 15 and 16, respectively. The reactions were
obtained using the previous method. The structures of all the performed in the presence of Pd(PPh3)4 and potassium
1
synthesized compounds were confirmed by H-NMR, 13C-NMR, carbonate in THF solution (Scheme 4).39 The 1H NMR spectrum
IR and mass spectral analyses. Compound 16 was obtained of PJK1 in CDCl3 showed peaks indicative of the 12 protons
from a commercial source.
assignable to the phenyl rings of triphenylamine at δ 7.30-7.36
ppm and 6.98-7.16 ppm, while signals of the Ar-CH2 alkyl side
chain of the phenyl units appeared at δ 2.56 ppm as a triplet,
and the terminal -CH3 appeared as a triplet at δ 0.89. Similarly,
the 1H NMR spectrum of PJK2 in CDCl3 exhibited peaks
attributable to the 18 protons of the fluorene and phenyl
groups in the aromatic region. The Ar-CH2 alkyl side chain on
the phenyl units appeared at δ 2.56 ppm as a triplet, while the
-CH3 protons appeared at δ 0.89 as a triplet. The 1H NMR
spectrum of PJK3 in CDCl3 revealed peaks in the aromatic
region for the phenyl groups, and the peaks in the aliphatic
region was also confirmed the structure. For the spectrum of
PJK4, the two protons of the thiophene-CH2 alkyl side chain
appeared at δ 2.93 ppm as a broad signal, and protons
attributable to the Ar-CH2 alkyl side chain on the phenyl units
3.2. Polymer Synthesis
Polymers PJK1, PJK2, PJK3, and PJK4 were synthesized by the
1
appeared at δ 2.53 ppm as a triplet. Detailed H-NMR data of
the polymers can be found in the experimental section, while
the 1H NMR spectra can be found in the Supporting
Information.
Scheme 1 Synthesis of 4-Butyltriphenylamine derivative.
O2N
Br
NO2
Br
Fuming HNO3
NBS
Fuming H2SO4
Conc. H2SO4
Br
Br
CHCl3, CH3COOH
S
S
S
5
3.3. GPC and TG analysis
4
The number-average molecular weight (Mn) and weight-
average molecular weight (Mw) of the copolymers ranged
from 2600 to 5400 and 4200 to 21,600, respectively, with
polydispersity index ranging from 1.6 to 4.0. These data are
summarized in Table 1. The thermal properties of all the
copolymers were investigated by TGA under a nitrogen
atmosphere and data are summarized in Table 1. Most
copolymers exhibited very good thermal stabilities, losing less
than 5% of their weight until well above 400 °C. The amount of
SnCl2
HCl, EtOH
O
O
H2N
NH2
Ph
Ph
N
N
NBS
N
N
CHCl3, TFA
EtOH
S
Br
Br
S
S
7
8
6
Scheme 2 Synthesis of 5,7-dibromo-2,3-diphenylthieno[3,4-b]pyrazine.
Scheme 3 Synthesis of 5,7-bis(5-bromo-3-octylthiophen-2-yl)-2,3-diphenylthieno[3,4-b]pyrazine.
4 | J. Name., 2012, 00, 1-3
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Ar
C8H17
C8H17
O2N
NO2
N
N
N
N
S
S
S
S
S
C8H17 C8H17
5
16
8
15
O
B
O
B
Ar
n
O
O
N
N
THF, K2CO3
Pd(0)(PPh 3
Br Ar Br
+
)
4
3
C4H9
C4H9
Scheme 4 Synthesis of copolymers.
carbonized residue (char yield) of these polymers in nitrogen
atmosphere was more than 61% at 600 °C.
Fig. 2 Cyclic voltammograms of (A) PJK1; (B) PJK2; (C) PJK3; (D) PJK4 on platinum
button with propylene carbonate in 0.1 M tetrabutylammonium tetrafluoro borate.
Scan rate: 50 mV s-1 at 18°C.
3.4. Cyclic voltammetry studies
freshly cast polymer film upon the repetitive insertion and
extraction of counter ion through the oxidation process.38
The cyclic voltammograms (CVs) of all the polymers can be
found in Figure 2 and the results are summarized in Table 1.
Cyclic voltammograms were obtained by casting films of
polymers onto a platinum button, propylene carbonate using
0.1 M TBABF4 as the supporting electrolyte, an Ag/Ag+
electrode as the reference electrode and a platinum wire as
the counter electrode, at a scan rate of 50 mV/s. The oxidation
voltages of PJK1-PJK4 films were 0.83 V, 0.67 V, 0.36 V and
0.46 V respectively. All new polymers are stable upon
oxidation with potential of oxidation ranging from 0.36 to 0.83
V vs SCE. Here almost we observed that oxidation peaks has
higher peak currents than reduction peaks and it shows these
polymers better electron donors than acceptors.41
Fig. 3 Schematic cross-section of electrochromic cells
PJK1
PJK2
PJK3
PJK4
Cycling stability upon oxidation of all polymers has been
conducted (see the Supporting Information). A thin film of
polymer was placed on a platinum button and cyclic
voltammetry was done at speed rate of 50mV/s. The cyclic
voltammograms of PJK1 between -0.5 and +1.5 V vs Ag/Ag+
shows a slight shift of the anodic peak (0.02 V) and a reduction
of the current density of 40% after 500 cycles. For PJK2, the
cyclic voltammograms at same conditions shows a shift of the
anodic peak (+0.02 V) and a decrease in the current density
after 500 cycles. For PJK3 current decreased around 30%.
Finally PJK4 shows a shift of the anodic peak (+0.02 V) and a
decrease of 30% in the current density. The minor shift of the
anodic peak and the reduction of the current densities upon
cycling can be attributed to an alteration in the film
morphology or activation of new electro active site from
Fig. 4 Polymers with before and after applied potentials.
3.5. Spectroelectrochemical studies
Spectroelectrochemical measurements were performed on
films of the polymers deposited onto ITO-coated glass
substrates following the spray-coating technique reported by
Reynolds et al.,40 which facilitated formation of uniform
polymeric layers. The gel electrolyte was spread on the
polymer-coated side of the electrode, another ITO glass was
sandwiched under atmospheric conditions, and glass spacers
were used to fix the cell. To prevent leakage, an epoxy resin
was applied to seal the device (see Figure 3). The gel
electrolyte prepared by using PMMA, LiClO4 and propylene
carbonate in dry acetonitrile.39 This electrolyte allow ion
transport and utilized to afford a single layer electrochromic
cell (Figure 4). DC voltage was fixed between 0.0 and 2.8 V for
Table 1 GPC, TGA and CV oxidation potential data of Polymer Derivatives.
Polymer
PJK1
Mn
Mw
PDI
2.3
3.1
1.6
4.0
Tdec in oC
288
Eox vs SCE
0.83
5400
4700
2600
5300
12400
15000
4200
PJK2
467
0.67
PJK3
502
0.36
PJK4
21600
460
0.46
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Fig. 5 Absorption spectra of PJK1 (a), PJK2 (b), PJK3 (c) and PJK4 (d) solid state EC using gel electrolyte (DC power supply).
PJK1. At 0.0 V, polymer PJK1 is in a neutral state, and the color increasing the DC voltage, this absorption peak decreased, and
is orange. The change in absorption of the film of PJK1 at a new absorption peak appeared in the region of 680 to 900
various applied potentials is shown in Figure 5a. In the neutral nm, resulting in a grey color (Figure 5c). Finally, PJK4 exhibited
form, at 0 V, PJK1 exhibited a strong absorption at 438 nm. a green color in its neutral state at 0.0 V, corresponding to an
Upon application of the voltage, this absorption band absorption maximum at 655 nm. Upon increasing the voltage,
decreased while a new absorption peak starting at 580 nm and a new peak appeared starting at 700 nm and extending above
extending to above 900 nm in the NIR region grew in, while 900 nm in the NIR region, resulting in a color change to bluish
the color changed to green. The new absorption band green (Figure 5d). Moreover, these polymers displayed high
exhibited a higher extinction coefficient value at 2 V, and then molar extinction coefficient values in solution state, which was
decreased after that voltage. The color change of the confirmed to present a good coloration efficiency (see
fabricated electrochromic cell was uniformed across the film.
Figure 5b displays the spectroelectrochemical spectrum of
supporting information, Figure 49).42
PJK2 at various DC voltages up to 2.8 V. When a voltage of 0.0 3.6. Electrochromic switching
V was applied, the polymer was in the neutral state with a light
Electrochromic switching studies for the all polymers were
yellow color. When the applied voltage was increased to a
maximum of 2.8 V, the color changed from light yellow to
reddish brown with a uniform color change across the entire
film. For potentials up to 1.7 V, PJK2 exhibited an absorption
band at 399 nm; when the voltage was increased, this
absorption band slowly diminished while a new absorption
peak appeared between 430 and 580 nm, corresponding to a
reddish brown color. PJK3, which was light blue in color,
exhibited absorption maxima at 598 nm up to 1.5 V. By
done to monitor the % transmittance (T) as a function of time
at their absorption maximum and to regulate the response
time by stepping potential repeatedly between the neutral and
oxidized states.43 The active area of the polymer film on ITO-
glass is around 1 cm2 in TBABF4, in propylene carbonate. Fig. 6
represents the %T and current density changes of PJK1, PJK2,
PJK3 and PJK4 as a function of the time at their longer
wavelength absorption maximum, 731 nm for PJK1 with 0 to
2.5V potential, 480 nm for PJK2, 776 nm for PJK3 and 1033 nm
6 | J. Name., 2012, 00, 1-3
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Fig. 6 Electrochromic switching under an applied square voltage signal for (A) PJK1 at 731 nm, (B) PJK2 at 480 nm, (C) PJK3 at 776 nm and (D) PJK4 at 1033 nm respectively, as a
function of time.
for PJK4 with 0 to 2.3V potential by applying square-wave electronic transitions, oscillator strength and absorption
potential steps of 12 s (complete cycle time is 24 s) with 20 wavelength were generated.48
cycles. The optical contrast of the PJK1 was found to be 59% at
731 nm with a switching time of 3.5 seconds for coloring wavelength and oscillator strength) and transition properties
process and 2.9 seconds for bleaching process.44,45 PJK2
PJK3 in the neutral and oxidation states were used for investigation
As shown in Table 2 and Fig 8, optical (absorbance
,
and PJK4 exhibit electrochromic performance with contrast to better understand the optical transition of PJK materials.
value (∆T) 68%, 37%, 64% with switching time of 3.5s/3s, For the neutral state of PJK1, HOMO→LUMO+2 transiꢀon
3.5s/3.1s and 3.6s/3.0s respectively. From the data, it can be mainly dominates S0→S7 transiꢀon with the absorpꢀon
clearly found that PJK1, PJK2 and PJK4 were have the better wavelength of 328 nm, which is contributed by charge transfer
stability and higher percent transmittance contrast than PJK3 from the whole to the triarylamine part (Fig. 8). The transitions
after regular switching for 500s (Figure 6). Mainly PJK1 and HOMO→LUMO, HOMO-2→LUMO and HOMO-1→LUMO+1
PJK4 have the good optical contrasts in the NIR region, which correspond to PJK2
is a very important property for various NIR applications, such
as camouflage devices and polymer based field-effect
transistors.
, PJK3 and PJK4 in the natural state mainly
3.7. Theoretical investigation
For this study, all the calculations were done for PJK polymer
series using commercial package Gaussian 03.46 As shown in
Fig. 7, the optimized geometries of monomers at neutral and
oxidation states were obtained using Hartree-Fock (HF)
combined with 3-21G* bases set.47 By using the semi-empirical
Fig. 7 Optimized geometries of monomers of conjugated polymers.
quantum chemical method ZINDO including 25 lowest-energy
7 | J. Name., 2012, 00, 1-3
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Table 2 The calculated absorption wavelength (λ), oscillator strength (ƒ) and the corresponding MO transitions of monomers in neutral and oxidation states. The electronic
transitions corresponding to the first excited state and the largest oscillator strength are shown.
Neutral state
Oxidation state
Statea
S0→S1
S0→S7
S0→S1
S0→S19
S0→S1
S0→S3
S0→S1
S0→S5
λ (nm)
532
328
318
222
482
340
544
397
ƒ (a.u.)
Transition character
State
λ (nm)
818
614
750
528
652
429
858
572
ƒ (a.u.)
0.84
0.37
1.22
0.97
0.42
0.44
0.68
0.59
Transition character
0.02
0.54
1.15
1.02
0.36
0.74
0.44
0.81
HOMO→LUMO+1 (11%)
HOMO→LUMO+2 (49%)
HOMO→LUMO (49%)
HOMO-3→LUMO (27%)
HOMO→LUMO (78%)
HOMO-2→LUMO (26%)
HOMO→LUMO (42%)
HOMO-1→LUMO+1 (63%)
S0→S1
S0→S2
S0→S1
S0→S2
S0→S1
S0→S7
S0→S1
S0→S3
HOMO→LUMO (91%)
HOMO-1→LUMO (79%)
HOMO→LUMO (92%)
HOMO-1→LUMO (89%)
HOMO-1→LUMO (70%)
HOMO-5→LUMO (41%)
HOMO→LUMO (65%)
HOMO-2→LUMO (37%)
PJK1
PJK2
PJK3
PJK4
Fig. 8 Electronic transitions of PJK1, PJK2, PJK 3 and PJK4 in ground and excited states.
dominate S0→S1, S0→S3 and S0→S5 electronic transiꢀons the conjugation length in conjugated polymers PJK1, PJK2,
having large oscillator strength with absorption at 318, 340 PJK3 and PJK4 in the film state can be explained from this
and 397 nm respectively. Which are contributed by charge theoretical study. Thus, the NIR electrochromic phenomena of
transfer from triarylamine to the rest of the molecule. For the reported conjugated polymers can be successfully proved by
oxidation state of PJK1, PJK3 and PJK4, HOMO→LUMO, theoretical investigation.
HOMO-1→LUMO and HOMO→LUMO transiꢀons respecꢀvely,
mainly controls S0→S1 transiꢀon with maximum absorpꢀon at
818, 652 and 858 nm, respectively. While, for PJK2 in the
oxidation state, HOMO-1→LUMO transiꢀon related to 528 nm
mainly dominates S0→S2 transiꢀon. The absorpꢀon
wavelengths for the conjugated polymers are close to the
experimental values (Fig. 5). Upon oxidation, the changes in
4. Conclusions
We
have
prepared
four
soluble,
electrochromic
triphenylamine-based copolymer derivatives and reported
their fabrication into electrochromic cells using ITO-glass. The
electrochromic devices were shown to exhibit good color
8 | J. Name., 2012, 00, 1-3
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22 K. L. Wang, M. K. Leung, L. G. Hsieh, C. C. Chang, K. R. Lee, C.
L. Wu, J. C. Jiang, C. Y. Tseng and H. T. Wang, Org. Electron.,
2011, 12, 1048-1062.
23 X. H. Ouyang and Y. P. Huo, Appl. Phys. A, 2011, 105, 891-
895.
24 S. Marco and J. Andres, J. Am. Chem. Soc., 2010, 132, 8372-
8377.
25 H. Hong, M. Y. Jo, Y. E. Ha and J. H. Kim, Macromol. Res.,
2013, 21, 321-326.
changes when modulated between their neutral and oxidized
states and color changes for PJK1, the color changed from
orange to dark green; for PJK2, from light yellow to reddish
brown; for PJK3, from light blue to grey; and for PJK4, from
green to bluish green. These polymers revealed well switching
times, is an important candidate for electrochromic display
applications. These polymer electrochromic cells may be useful
for conductive textiles, whose interesting features could find
applications in adaptive camouflage.
26 Y-L. Chu, C-C. Cheng, Y-C. Yen and F-C. Chang, Adv. Mater.,
2012, 24, 1894-1898.
27 R. M. Kellogag, A. P. Schaap, E. T. Harper and N. D. H.
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Acknowledgements
This study was supported by the National Research Foundation
of Korea (NRF) funded by the Ministry of Science, ICT and
Future Planning (Grant no. 2015063131).
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Graphical Abstract
A series of triphenyl amine based new molecules, coded PJK1, PJK2, PJK3 and PJK4
synthesized for electrochromic cells. Here we achieved color change for PJK1, orange to dark
green; for PJK2, light yellow to reddish brown; for PJK3, light blue to grey; and for PJK4, green
to bluish green.