Synthesis and Electrochromic Properties of Star-Shaped Oligomers with Phenyl Cores

Article abstract of DOI:10.1002/asia.201700890

A series of star-shaped conjugated oligomers, 1,3,5-tri(2′-thienyl) benzene (3TB), 1,3,5-tri(3′,4′-ethylenedioxythienyl) benzene (3EB), 1,3,5-tri[5′,2“-(3”,4“-ethylenedioxy-thienyl)-2′-thienyl] benzene (3ETB), and 1,3,5-tri[5′,2”-(3“,4”-ethylenedioxy-thie

Full text of DOI:10.1002/asia.201700890

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Accepted Article
Title: Synthesis and electrochromic properties of star-shaped oligomers
with phenyl cores
Authors: Jinming Zeng, Xiaoyuan Zhang, Xiaoting Zhu, and Ping Liu
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Synthesis and electrochromic properties of star-shaped oligomers with
phenyl cores
Jinming Zeng[a], Xiaoyuan Zhang[a], Xiaoting Zhu[a], Ping Liu*[a]
Abstract: A series of star-shaped conjugated oligomers, 1,3,5-tri(2'-
zene (3EB), 1,3,5-tri[5',2"-(3",4"-ethylenedioxythineyl)-2'-thienyl]
thienyl) benzene (3TB), 1,3,5-tri(3',4'-ethylenedioxythienyl) benzene (3EB), benzene (3ETB), and 1,3,5-tri[5',2"-(3",4"-ethylenedioxythineyl)-2'-
1,3,5-tri[5',2"-(3",4"-ethylenedioxy-thienyl)-2'-thienyl] benzene (3ETB),
and 1,3,5-tri[5',2"-(3",4"-ethylenedioxy-thienyl)-2'-thienyl]-4-(3',4'-ethyle-
nedioxythienyl)benzene (3TB-4EDOT), were synthesized. The star-shaped
polymer, poly(1,3,5-tri[5',2"-(3",4"-ethylenedioxythineyl)-2'-thienyl]benze-
ne) (P3ETB), was also prepared. The electrochemical and electrochromic
properties of these conjugated oligomers and polymer were investigated.
These oligomer and polymer films showed reversible, clear color changes
upon electrochemical doping and dedoping. The color of the P3ETB film
reversibly changed from orange to blue under doping and dedoping. The
switching times for doping and dedoping were 1.2 and 0.9 s, respectively.
thienyl]-4-(3',4'-ethylenedioxythineyl)benzene (3TB-4EDOT) (Scheme
1) were synthesized, and their electrochromic properties were
investigated.
Scheme 1 Molecular structures of 3TB, 3EB, 3ETB, and 3TB-4EDOT.
The electronic absorption spectra and fluorescence emission
spectra are important parameters for conjugated compounds. Figure 1a
shows the electronic absorption spectra of 3TB, 3EB, 3ETB, and 3TB-
4EDOT in DCM. They all absorbed in the wavelength range of 235-
428 nm. The maximum absorption peaks of 3TB, 3EB, 3ETB, and
3TB-4EDOT were located at 293, 302, 369, and 367 nm, respectively.
The band edge wavelengths (λedge) of 3TB, 3EB, 3ETB, and 3TB-
4EDOT were located at 330, 345 426, and 426 nm, respectively. From
the formula E = 1240 / λedge, the optical band gaps (Egopt) of 3TB, 3EB,
3ETB, and 3TB-4EDOT were calculated to be 3.76, 3.54, 2.91, and
2.91 eV, respectively. Figure 1b shows the fluorescence emission
spectra of 3TB, 3EB, 3ETB, and 3TB-4EDOT in DCM. The maximum
emission peaks of 3TB, 3EB, 3ETB, and 3TB-4EDOT were located at
370, 378, 423, and 448 nm, respectively. The electronic absorption
spectra and fluorescence spectra indicated that these oligomers all
possessed semiconductor properties.
Electrochromism is a phenomenon in which the optical characteristics
of a material can be tuned by applying voltage.[1] In recent decades,
electrochromic materials have attracted considerable attention because
of their potential application in smart windows, antiglare rearview
mirrors, and displays.[2]
Electrochromic materials include inorganic compounds and
conjugated organic compounds.[3] Compared with inorganic
electrochromic materials such as tungsten trioxide,[4] conjugated
organic compounds are perhaps more promising as electrochromic
materials because of the ability to fine-tune their molecular structure,
and their high optical contrast and fast switching times.[5]
Research on organic thin-film electrochromic materials has
largely focused on line-shaped conjugated polymers,[6] such as
polythiophene, polypyrrole, polyaniline, and poly(3,4-ethylenedioxy-
thiophene) (PEDOT). In our previous work,[7] a series of line-shaped
conjugated oligothiophene derivatives were synthesized. Solid films of
these line-shaped oligothiophene derivatives showed reversible, clear
color changes upon electrochemical doping and dedoping. We have
also synthesized star-shaped conjugated oligothiophene derivatives,[8]
and studied their electrochromic properties. In general, the color
stability of organic electrochromic materials depends on the stability of
the ion radical. Thus, the stability of the ion radical of organic
electrochromic materials is very important. Star-shaped oligomers can
offer advantages in terms of higher stability of the ion radical,
compared with line-shaped oligomers.
Figure 1 (a) Electronic absorption spectra and (b) fluorescence emission spectra of
3TB, 3EB, 3ETB, and 3TB-4EDOT.
Many star-shaped conjugated oligomers have recently been
synthesized, and their electrochemical and optical properties have been
well studied. However, their electrochromic properties have received
little attention.
Electrochemical cyclic voltammetry (CV) experiments were
performed to determine the energy levels of the highest occupied
molecular orbital and lowest unoccupied molecular orbital of the
conjugated compounds. The cyclic voltammograms of 3TB, 3EB,
3ETB, and 3TB-4EDOT were carried out in DCM solution containing
0.1 M TBAP. A saturated calomel electrode, platinum wire, and glassy
carbon were used as the reference, counter, and working electrodes,
respectively. The initial oxidation potentials (Eoxonset) of 3TB, 3EB,
3ETB, and 3TB-4EDOT were located at 1.50, 0.85, 0.65, and 0.66 V
verses SCE, respectively. The initial reduction potentials (Eredonset) of
3TB, 3EB, 3ETB, and 3TB-4EDOT were located at ‒0.85, ‒0.75,
‒0.80, and ‒0.75 V verses SCE, respectively. This indicated that
In this study, a series of star-shaped conjugated oligomers, 1,3,5-
tri(2'-thienyl)benzene (3TB), 1,3,5-tri(3',4'-ethylenedioxythineyl) ben-
[a]
J. M. Zeng, X. Y. Zhang, X. T. Zhu, Prof. Dr. P. Liu
State Key Laboratory of Luminescent Materials and Devices, Re- search
Institute of Materials Science,South China University of Technology
Guangzhou 510640, P. R. China
E-mail: mcpliu@scut.edu.cn (P. Liu)
Supporting information for this article is given via a link at the end of
the document.
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electrochromic devices containing these star-shaped oligomers as
active layers might have low driving voltages. Table 1 shows
photoelectric properties of 3TB, 3EB, 3ETB, and 3TB-4EDOT. The
CV results also indicated that these oligomers possessed semiconductor
properties.
is reversible. However the solid state films of 3TB, 3EB, 3ETB, and
3TB-4EDOT could not change back to their intrinsic colors (colorless,
colorless, light yellow, and colorless, respectively) upon
electrochemical dedoping. One possible reason for this was that they
electropolymerized to their corresponding polymers P3TB, P3EB,
P3ETB, and P(3TB-4EDOT)) upon electrochemical doping,
respectively.[9] The results indicated that P3TB, P3EB, P3ETB, and
P(3TB-4EDOT) had clear, reversible color changes upon
electrochemical doping and dedoping.
Table 1 Photoelectric properties data of 3TB, 3EB, 3ETB, and 3TB-4EDOT
opt [a]
onset
onset
electro [b]
Compound
λmax/nm
λedge
λmax/nm
Eg
Eox
Ered
Eg
(absorption)
/nm
(emission)
/eV
/Vvs.SCE
/Vvs.SCE
/eV
3TB
3EB
293
302
369
367
330
345
426
426
370
378
423
448
3.76
3.54
2.91
2.91
1.5
-0.85
-0.75
-0.80
-0.75
2.35
1.60
1.45
1.41
0.85
0.65
0.66
3ETB
3TB-
4EDOT
opt
opt
electro
[a] Eg (eV)= 1240/λedge
;
Eg
:
the optical band gap. [b] Eg
(eV): the
electrochemical band gap.
Electrochromic devices were fabricated to investigate the
electrochromic properties of the star-shaped oligomers. The device
structure was an indium tin oxide (ITO) glass slide (or ITO PET
slide)/electrochromic active layer/gel electrolyte/ITO glass slide (or
ITO PET slide). Figure 2 shows a schematic of the structure of the
electrochromic device.
Figure 3 Electronic absorption spectra and digital photographs of the (a) 3TB, (b)
3EB, (c) 3ETB, and (d) 3TB-4EDOT, upon electrochemical doping and dedoping.
Table 2 Colors of 3TB, 3EB, 3ETB, and 3TB-4EDOT films upon electrochemical
doping and dedoping.
Figure 2 Schematic of the electrochromic device.
Eletrochromic
active layer
intrinsic
color
Doping
state
Dedoping
state
Doping
Dedoping
voltage/V
3TB, 3EB, 3ETB, and 3TB-4EDOT were deposited on ITO glass
slides by vacuum deposition. In the devices, ITO glass slides were used
as the electrode, and the conducting gel was used as the semi-solid
state electrolyte. The gel electrolyte consisted of PMMA, TBAP, ACN,
and PC (7:3:70:20, mass ratio). Electrochemical doping and dedoping
were performed using the electrochromic cycling tester.
voltage/V
3TB
colorless
light
green
yellow-
brown
+2.8
+2.2
+1.5
+1.5
-2.0
-1.2
-0.8
-1.5
3EB
colorless
red
blue
blue
yellow
orange
orange
3ETB
light
yellow
Figure
3 shows electronic absorption spectra and digital
photographs of devices containing 3TB, 3EB, 3ETB, and 3TB-4EDOT,
upon electrochemical doping and dedoping. Table 2 shows the colors
of 3TB, 3EB, 3ETB, and 3TB-4EDOT solid state films under neutral
conditions, and their colors upon electrochemical doping and dedoping.
The color of the 3TB solid state film changed from colorless to light
green upon applying +2.8 V, and then from light green to yellow-
brown upon applying ‒2.0 V. The change in color from light green to
yellow-brown is reversible. The color of the 3EB solid state film
changed from colorless to red upon applying +2.2 V, and then from red
to yellow upon applying ‒1.2 V. The change in color from red to
yellow is reversible. The color of the 3ETB solid state film changed
from light yellow to blue upon applying +1.5 V, and then from blue to
orange upon applying ‒0.8 V. The change in color from blue to orange
is reversible. The color of the 3TB-4EDOT solid state film changed
from colorless to blue upon applying +2.0 V, and then from blue to
orange upon applying ‒1.5 V. The change in color from blue to orange
3TB-4EDOT
colorless
In order to confirm whether the oligomers have been polymerized
upon electrochemical doping, the polymer based on the 3ETB was
prepared by electropolymerization. The electropolymerization of 3ETB
was carried out in ACN solution containing 0.1 M TBAP. ITO glass,
platinum wire, and a saturated calomel electrode were used as the
working, counter, and reference electrodes, respectively. The
electrolyte was purged with N2 for 10 min to eliminate oxygen.
Figure
4
shows cyclic voltammograms of the electro-
polymerization of 3ETB. 3ETB was electropolymerized on the ITO
glass (or pET) substrates, when the potential was scanned between 0 V
and +1.0 V versus SCE at a scan rate of 50 mV/s. In the first cycle, the
initial oxidation potential of the 3ETB monomer was located at 0.46 V
versus SCE. As cycling progressed, the initial oxidation potential
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decreased and the current density increased. This indicated that P3ETB
was deposited on the ITO glass (or ITO PET) substrate electrode.[10]
After a great many cycles, no appreciable change was observed in the
redox response of P3ETB, which indicated that it was highly stable.
substrate. One reason for this was the lower conductivity of ITO PET
compared to ITO glass.
Figure 4 Electropolymerization of 3ETB in ACN solution containing 0.1 M TBAP on
(a) ITO glass and (b) ITO PET substrates.
Figure 5 shows spectroelectrochemical properties of the P3ETB
film on the ITO glass substrate at different potentials. The orange
P3ETB film gradually transformed to gray, when the potential changed
from 0 V to +1.0 V versus SCE, and then transformed to blue when the
potential was further increased to +1.5 V versus SCE. The electronic
absorption spectra showed that the intensity of the absorption peak at
480 nm decreased, and that a new absorption peak gradually appeared
Figure 6 Voltage-time and current-time curves of P3ETB films on (a) ITO glass and
(b) ITO PET substrates. Magnified view of a single current-time curve on (c) ITO
glass and (d) ITO PET substrates.
at 550‒800 nm, upon electrochemical doping. This was attributed to
[11]
the formation of polarons.
This results was consistent with
The stabilities of P3ETB films on ITO glass and ITO PET
substrates were measured by cyclic voltammetry, in ACN solution
containing 0.05 M TBAP (Figure 7). The potential was scanned
between 0 V and +1.3 V versus SCE, at a scan rate of 50 mV/s. After
500 cycles, the electric charge loss of the P3ETB film on the ITO glass
substrate was 19.2%. The corresponding electric charge loss of the
P3ETB film on the ITO PET substrate was 19.8%. This result indicated
that the P3ETB film had good electrochemical and electrochromic
stability.
electrochromic properties of device based on 3ETB, which confirmed
that the oligomers have been polymerized in electrochromic devices
upon electrochemical doping.
Figure 5 Spectroelectrochemistry of the P3ETB film on an ITO glass substrate in
Figure 7 1st and 500th cyclic voltammograms of P3ETB films on (a) ITO glass and (b)
ACN solution containing 0.1 M TBAP.
ITO PET substrates, in ACN containing 0.05 M TBAP.
The response time is an important parameter for evaluating
electrochromic materials. The response time of the P3ETB film was
measured by chronoamperometry, in a typical three-electrode system.
Figure 6a and 6b show voltage-time and current-time curves of the
P3ETB film on ITO glass and ITO PET substrates, respectively. Figure
6c and 6d show magnified views of single cycles on ITO glass and ITO
PET substrates, respectively. Figure 6c shows that the doping and
dedoping times of the P3ETB film on the ITO glass substrate were 1.2
and 0.9 s, respectively. Figure 6d shows that the doping and dedoping
times of the P3ETB film on the ITO PET substrate were 16 and 7.5 s,
respectively. The doping and dedoping times of the P3ETB film coated
on the ITO PET substrate were longer than those on the ITO glass
Figure 8 Digital photographs of changes in the colors of devices based on P3ETB
films on (a) ITO glass and (b) ITO PET substrates, upon electrochemical doping and
dedoping.
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The structure of the electrochromic devices was ITO glass (or
ITO PET)/P3ETB film/electrolyte/ITO glass (or ITO PET). The
conducting gel was used as the semi-solid state electrolyte. The gel
electrolyte consisted of PMMA, TBAP, ACN, and PC (7:3:70:20, mass
ratio).The color changes of the device upon electrochemical doping
and dedoping are shown in Figure 8. The device based on the ITO
glass substrate exhibited reversible color changes from orange to blue,
upon electrochemical doping and dedoping. The doping and dedoping
response times of the device based on the P3ETB film on the ITO glass
substrate were 1.9 and 1.3 s, respectively. The device based on the ITO
PET substrate exhibited reversible color changes from orange to blue,
upon electrochemical doping and dedoping. The doping and dedoping
times of the device based on the P3ETB film on the ITO PET substrate
were 18 and 9.2 s, respectively. Therefore, the rigid and flexible
electrochromic devices based on P3ETB both showed clear and
reversible color changes. This results was consistent with
electrochromic properties of device based on 3ETB, which further
confirmed that the oligomers have been polymerized in electrochromic
devices upon electrochemical doping.
[1] a) J. R. Platt, J. Chem. Phys. 1961, 34, 862; b) Y. M. Zhang, F. L. Xie, W. Li, Y.
Y. Wang, W. R. Zhang, X. J. Wang, M. J. Li, S. X. -A. Zhang, J. Mater. Chem.
C 2016, 4, 4662; c) H. C. Lu, S. Y. Kao, T. H. Chang, C. W. Kung, K. C. Ho,
Sol. Energy Mater. Sol. Cells 2016, 147, 75.
[2] a) L. M. Huang, C. W. Hu, C. Y. Peng, C. H. Su, K. C. Ho, Sol. Energy Mater.
Sol. Cells 2016, 145, 69; b) P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky,
Electrochromism and Electrochromic Devices, Cambridge University Press,
Cambridge, 2007.
[3] Roger J. Mortimer, Chem. Soc. Rev. 1997, 26, 147.
[4] a) C. Y. Yan, W. B. Kang, J. X. Wang, M. Q. Cui, X. Wang, C. Y. Foo, K. J. J.
Z. Chee, P. S. Lee, ACS Nano 2014, 8, 316; b) R. -T. Wen, Gunnar A.
Nikalasson, Claes G. Granqvist, ACS Appl. Mater. Interfaces 2015, 7, 28100.
[5] a) E.-C. Cho, C. W. C. Jian, Y. S. Hsiao, K. C. Lee, J. -H. Huang, Sol.Energy
Mater. Sol. Cells 2016, 150, 43; b) S. H. Hsiao, L. -C. Wu, Dyes and
Pigments 2016, 134, 51; c) J. A. Kerszulis, R. H. Bulloch, N. B. Teran, R. M.
W. Wolfe, J. R. Reynolds, Macromolecules 2016, 49, 6350; d) S. H. Hsiao, G.
S. Liou, Y. C. Kung, H. J. Yen, Macromolecules 2008, 41, 2800.
[6] a) A. F. Diaz, Juan I. Castillo, J. A. Logan, W. -Y. Lee, J. Electroanal. Chem.
1981, 129, 115; b) Bayer AG, Eur. Patent 339 340, 1988; c) G. Heywang, F.
Jonas, Adv. Mater. 1992, 4, 116; d) L. Zhang, Y. P. Wen, Y. Y. Yao, J. K. Xu,
X. M. Duan, G. Zhang, ElectrochimicaActa 2014, 116, 343; e) J. A. Kerszulis,
K. E. Johnson, M. Kuepfert, D. Khoshabo, A. L. Dyer and J. R. Reynolds, J.
Mater. Chem. C 2015, 3, 3211; f) P. M. Beaujuge, J. R. Reynolds, Chem. Rev.
2010, 110, 268; g) A. Durmus, G. E. Gunbas, P. Camurlu, L. Toppare, Chem.
Commun. 2007, 3246; h) W. Li, T. Michinobu, Poly. Chem. 2016, 7, 3165.
[7] a) B. B. Yin, C. Y. Jiang, Y. G. Wang, M. La, P. Liu, W. J. Deng, Synth. Met.
2010, 160, 432; b) M. La, M. M. Liu, P. Liu, W. J. Deng, Z. Tong, Chin. J.
Chem. 2008, 26, 1523.
In summary, a series of star-shaped oligomers, 3TB, 3EB,
3ETB, and 3TB-4EDOT, was synthesized. Their electrochromic
properties were investigated. These oligomer films showed reversible,
clear color changes upon electrochemical doping and dedoping.
However, these oligomer films could not change back to their intrinsic
colors upon electrochemical dedoping. The P3ETB film changed color
reversibly from orange to light green. The P3ETB film had a short
response time and good electrochemical stability. The doping and
dedoping times of the P3ETB film were 1.2 and 0.9 s, respectively.
[8] Y. P. Zhong, J. M. Gao, P. Liu, W. J. Deng, Mater. Res. Innov. 2015, 19, 51.
[9] Y. J. Ding, M. A. Invernale, D. M. D. Mamangun, A. Kumar and G. A. Sotzing,
J. Mater. Chem. 2011, 21, 11873.
Acknowledgements
This research was financially supported by the NSFC (Grant No.
20674022, 20774031, and 21074039), the Natural Science Foundation
of Guangdong (Grant No. 2006A10702003, 2009B090300025,
[10] K. W. Lin, S. L. Ming, S. J. Zhen, Y. Zhao, B. Y. Lu and J. K. Xu, Polym.
Chem. 2015, 6, 4575.
[11] W. S. Li, Y. T. Guo, J. J. Shi, H. T. Yu and H. Meng, Macromolecules 2016, 49,
7211.
2010A090100001,
2014A030313241,
2014B090901068,
and
2016A010103003), and the Ministry of Education of the People’s
Republic of China (Grant No. 20090172110011).
Keywords: star-shaped oligomers • thin films •
spectroelectrochemistry• electrochromic devices •
electrochromic properties
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Text for Table of Contents
Electrochromic materials: A series of star-shaped oligomers (3TB, 3EB, 3ETB, and 3TB-4EDOT)
with phenyl cores were synthesized, and their electrochromic properties were investigated. These
oligomer films show reversible and clear color changes upon electrochemical doping and dedoping
because of the star-shaped oligomers form stable ion radical upon electrochemical doping and dedoping.
J. M. Zeng, X. Y. Zhang, X. T. Zhu, P. Liu
Synthesis and electrochromic properties of star-shaped oligomers with phenyl cores

Corresponding author. E-mail address: mcpliu@scut.edu.cn (P. Liu)
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