Recycling by-products in new small molecular electrochromic materials with ultra bistability

Article abstract of DOI:10.1016/j.dyepig.2018.10.078

The potential of three arylamine based molecules for electrochromic applications is investigated herein. The latter, by-product of a Buchwald-Hartwig reaction were fully characterized, instead of being thrown away, due to their structural analogy with well-known electrochromic analogues. It turns out that impressive and stable color changes were recorded when embedded in electrochromic devices. Finally, a flexible patterned electrochromic demonstrator was also fabricated, preliminary steps toward a potential and more environmental friendly signposting mass application.

Full text of DOI:10.1016/j.dyepig.2018.10.078

Accepted Manuscript
Recycling by-products in new small molecular electrochromic materials with ultra
bistability
Yue Jiang, Zhike Xian, Yuying Meng, Guofu Zhou, Clément Cabanetos, Jean Roncali,
Jun-ming Liu, Jinwei Gao
PII:
S0143-7208(18)32052-7
https://doi.org/10.1016/j.dyepig.2018.10.078
DYPI 7144
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 September 2018
Revised Date: 31 October 2018
Accepted Date: 31 October 2018
Please cite this article as: Jiang Y, Xian Z, Meng Y, Zhou G, Cabanetos Clé, Roncali J, Liu J-m, Gao J,
Recycling by-products in new small molecular electrochromic materials with ultra bistability, Dyes and
Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.10.078.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Recycling By-Products in New Small Molecular Electrochromic
Materials with Ultra Bistability
a
a
b
c, f
d
d
Yue Jiang , Zhike Xian , Yuying Meng , Guofu Zhou , Clément Cabanetos *, Jean Roncali *,
e
a
Jun-ming Liu , and Jinwei Gao *
a
Institute for Advanced Materials, Academy of Advanced Optoelectronics, and Guangdong
Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China
Normal University, Guangzhou 510006, P. R. China.
b
Sun Yat-Sen University, School of Chemistry, Guangzhou, 510275, PR China
c
Guangdong Provincial Key Laboratory of Optical Information Materials and Technology &
Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics,
South China Normal University, Guangzhou 510006, P. R. China
d
Group Linear Conjugated Systems, Moltech-Anjou UMR CNRS 6200, University of Angers,
2
Bd Lavoisier, 49045 Angers, France
e
Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China
f
Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, P. R. China
Abstract
The potential of three arylamine based molecules for electrochromic applications is
investigated herein. The latter, by-product of a Buchwald-Hartwig reaction were fully
characterized, instead of being thrown away, due to their structural analogy with well-known
electrochromic analogues. It turns out that impressive and stable color changes were recorded
when embedded in electrochromic devices. Finally, a flexible patterned electrochromic
demonstrator was also fabricated, preliminary steps toward a potential and more
environmental friendly signposting mass application.
Keywords
Electrochromism, organic small molecules, bistability

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1
. Introduction
Electrochromism (EC) refers to the phenomenon of tuning the light absorption or
transmittance of a material by applying an external electric stimulation through the
electrochemical oxidation or reduction process [1-2]. By switching the color from the visible
to the near-infrared region, electrochromic devices are usually used to modulate the in-door
temperature in buildings [3-4], as reflecting devices [5-6], such as auto rearview mirror [7], or
as signpost due to low starting voltage [8].
In a field dominated by metal oxide (WO [9], NiO [10]) and organometalic complexes
3
[
11] as active materials, organic EC materials have attracted considerable research attention
due to their potential compatibility with flexible, printable and low-cost processing
technologies [12-14]. Liu’s group has developed a lot of organic small molecules with linear
and star-shape structures for EC devices, which presenting plenty of colors after
polymerization. However, the application of non-polymerized small molecules on EC devices
is seldomly reported [15-18].
Arylamine based building blocks have already demonstrated their interest for
near-infrared electrochemical purpose such as the N, N, N’, N’-Tetraphenylphenylenediamine
(
TPPA), a typical mixed-valance (MV) moiety [19]. Widely studied [20], it is noteworthy that
most of the reported studies on such block focus on increasing the cycling stability and
decreasing the responding time [21-23]. However, and although barely discussed, it turns out
that improving the stability of doped species is of great importance to save energy by
maintaining the off-power state as long as possible [24].
In this context, we report herein the characterization and use of three arylamine based
molecules (Scheme 1). Fully characterized by UV-visible spectroscopy and cyclic
voltammetry, conventional electrochromic devices were also fabricated revealing impressive
bistability. In addition, the realization of a flexible Indium-doped tin oxide (ITO)-free
solid-state EC device was also investigated.

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Scheme 1. Illustration of the three molecular structure studied herein
2
2
. Experimental Section
.1. Materials and apparatus
All reagents and chemicals from commercial sources were used without further purification.
Reactions were carried out under nitrogen atmosphere unless otherwise stated. Solvents were
1
13
dried and purified using standard techniques. H NMR and C NMR spectra were performed on a
1
13
Bruker AVANCE III 300 ( H, 300 MHz and C, 75 MHz). High resolution mass spectra (HRMS)
and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF)
were performed on Bruker Biflex � . UV-Vis spectra were recorded with a PerkinElmer LAMBDA
9
50 UV/Vis/NIR Spectrophotometer. Cyclic voltammetry was performed in 0.10 M
Bu NPF /CH Cl (HPLC grade). Solutions were degassed by nitrogen bubbling prior to each
4
6
2
2
experiment. Experiments were carried out in a one-compartment cell equipped with platinum
electrodes and a saturated calomel reference electrode (SCE) using a Biologic SP-150 potentiostat
with positive feedback compensation. TGA and DSC experiments were recorded on a METTLER
TOLED instrument with scanning rate of 10 K/min. The film thickness was tested on the
profilometer (KLA-TENCOR, p-6).
2
.2. Synthesis and related compounds
General procedure for the preparation of Me2-D, F2-D and F4-D and
bis(4-fluorophenyl)amine by Buchwald-Hartwig reaction: The amino compound N-methylaniline
or 4-fluoro-N-phenylaniline or bis(4-fluorophenyl)amine or 4-fluoroaniline (1.2mmol),
1
-Bromo-4-iodobenzene
(1.8mmol),
Sodium
tert-butoxide
(346mg,
3.6mmol),

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Tris(dibenzylideneacetone)dipalladium(0) (45mg, 0.048mmol) were dried overnight under
vaccum at 95 °C before being transferred into a dry 100mL flask under argon. Then, 25mL of dry
and degassed toluene were added to the mixture followed by the Tri-tert-butylphosphine (0.07mL,
o
0
.29mmol). After another 10min degas by argon bubbling, the mixture was refluxed at 110 C
overnight. Once cooled down to room temperature, the media was quenched by distilled water and
extracted by diethyl ether
Me2-D: The crude product was purified by silica gel chromatography using petroleum:
1
dichlorometane:triethylamine (v:v, 2:1) as eluent to afford a light pale solid (3.1g, 49%). H NMR
1
3
(
300 MHz, CD Cl) δ 3.30(3H, s) 6.88(1H, t) 6.95(2H, d, J=7.63) 7.02(2H, s) 7.25(2H, t).
C
3
NMR (76 MHz, CD Cl) δ 40.40 118.22 119.85 123.36 129.09 143.83 149.33. HRMS-FAB+
3
(cal.288.1626, found 288.1623).
F2-D: The crude product was purified by silica gel chromatography using petroleum:
1
dichlorometane:triethylamine (v:v:v, 8:1:0.02) as eluent to afford a white solid (3.9g, 44%). H
1
3
NMR (300 MHz, DMSO) δ 6.95(2H, s) 6.97-7.17(3H+2H+2H, m) 7.29(2H, t). C NMR (76
MHz, DMSO) δ 116.66 116.95 122.81 123.02 125.42 126.45 126.56 129.98 142.82 147.89.
HRMS-FAB+ (cal.448.1751, found 448.1747).
F4-D: The crude product was purified by silica gel chromatography using petroleum:
1
dichlorometane: triethylamine (v:v:v, 4:1:0.02) as eluent to afford a white solid (4.6g, 40%). H
1
3
NMR (300 MHz, CD CN) δ 6.92(1H, s) 7.00(2H, d, J=7.58) 7.05(2H, d, J=4.66). C NMR (76
3
MHz, CD Cl) δ 115.94 116.23 124.26 125.32 125.42 142.77 143.93 143.97 157.00 160.21.
3
HRMS-FAB+ (cal.484.1563, found 484.1565).
2
.3. Fabrication of electrochromic cell
ITO was thoroughly cleaned by acetone, ethanol and deionized water in supersonic
successively, each for 20 min. Dried by N , plasma treatment was conducted to further clean ITO.
2
The active materials were dissolved in DMAc at a concentration of 10 mg/mL. The solution was
o
spun coated on ITO glass (500 rpm), which was placed on a hot plate at a temperature of 60 C to
remove solvent. PMMA (1 g) and LiClO (0.1 g) were dissolved in dry acetonitrile (5 g). Then,
4
propylene carbonate (1.5 g) was added. The mixture was acted as a supporting electrolyte. The
electrolyte was spread onto the active materials coated ITO glass. Finally, the other ITO glass was

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covered under atmosphere condition.
3
3
. Results and discussion
.1. Synthesis
The “undesired” synthesis of what will become the target compounds is depicted in
Scheme 2. Indeed, in an attempt to synthesize the bromo-arylamine derivatives 4, 5 and 6,
precursors of push-pull molecules for organic photovoltaics, a non-negligible quantity of
by-products, corresponding to the double Buchwald-Hartwig reaction of the N-methylaniline,
the 4-fluoro-N-phenylaniline and the bis(4-fluorophenyl)amine on the1-bromo-4-iodobenzene
was isolated. The structure and purity of these three by-products, namely Me2-D, F2-D and
F4-D, were characterized by nuclear magnetic resonance (NMR) spectroscopy and high
resolution mass spectrometry (see Supporting Information). Hence, instead of throwing these
products away, we have decided to explore their properties and, at the end, exploit an
undesired reaction.
Scheme 2. Synthetic route of 4, 5, 6, Me2-D, F2-D and F4-D.
3
.2. Stability and thermal properties
Their stability and thermal properties have been thus investigated by differential
scanning calorimetry (DSC) and thermal gravimetry (TGA), as shown in Fig. 1. All these
o
o
three compounds show good stability until 280 C for Me2-D and around 330 C for F2-D and
F4-D. Replacement of phenyl ring by methyl reduces the molecular size and prominently
lowers the melting temperature (T ) of ca 50°C compared to the unsubstituted TPPA
m
reference compound [25]. As expected, introduction of fluorine atoms also led to a significant
decrease of T for both F2-D and F4-D [25]. Interestingly, the crystallization behavior of
m

ACCEPTED MANUSCRIPT
o
F2-D turns out to be a cold crystallization accompanied with a glass transition at ca 37 C,
which is probably due to limited free molecular movement induced by the fluorine atoms. The
two F atoms in F2-D molecule deteriorate the intermolecular interaction due to the steric
repulsion of electronically “hard” C-F bond, while four F atoms in F4-D enhance the
intermolecular interaction with the further increased electron withdrawing capability.
Therefore, the DSC of F2-D is more like a polymer with “soft and rigid” parts whereas F4-D
shows strong tendency to crystalize.
Fig. 1. TGA (A, B, C) and DSC (D, E, F) traces recorded for Me2-D (A, D), F2-D (B, E) and
F4-D (C, F) under N atmosphere, rate: 10K/min.
2
3
.3. UV-vis-NIR Absorption Spectra
Fig. 2 shows the UV-visible-NIR absorption spectra recorded in THF solution and as thin
film spun cast on quartz sheets. Data are gathered in Table 1. With π−π* transitions recorded
at ca 250 and 300 nm, it turns out that the direct substitution of a phenyl by a methyl group or
the insertion of a fluorine atom has a limited impact on the molecular scale (∆λmax ≈ 5 nm).
In the solid state, broader absorption bands and red-shifted absorption edges with onsets at ca

ACCEPTED MANUSCRIPT
opt
3
80 nm, 373 nm and 367 nm corresponding to optical band gaps (Eg ) of 3.26, 3.32, 3.38 eV
of Me2-D, F2-D and F4-D respectively were recorded.
Fig. 2. Normalized UV-Vis absorption spectra of compounds Me2-D, F2-D, F4-D in THF
solution (A) and as thin films spin-cast on glass (B).
Table 1. Results of UV-Vis absorption spectroscopy for compounds Me2-D, F2-D, F4-D. S:
THF solutions, F: films spun-cast on glass from solutions
S
-1
-1
F
F
opt
Compd λmax (nm) ε (M .cm )
λ
max (nm)
λ
onset (nm) Eg (eV)
Me2-D 305,248
23000,
36000,
38000
317
309
308
380
373
367
3.26
3.32
3.38
F2-D
F4-D
303,239
238,300

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3
.4. Cyclic Voltammetry
Cyclic voltammetry (CV) was performed in THF solution in the presence of Bu NPF as the
4
6
supporting electrolyte (Fig. 3). All potentials were calibrated vs the ferrocene/ferrocenium
+
couple (Fc /Fc), used as internal standard. The electrochemical data are summarized in Table
2
. The cyclic voltamogramm of the three arylamine based compounds shows reversible
two-step one electron oxidation, assigned to the formation of stable radical cation and
dication respectively, and a non-reversible one-step reduction process [26]. The 0.2 V
1
negative shift of Epa observed for Me2-D confirms its stronger donor character. In addition,
comparison of the anodic patterns of F2-D and F4-D also reveals that the insertion of a
second fluorine atom on the remaining phenyl group constituting the triphenylamine moiety
has a limited impact on the electronics. More interestingly, no polymerization was observed
for all three molecules after hundred cycles of in the anodic region, thus highlighting a good
electrochemical stability (Supporting Information Fig. S1).
−
1
Fig. 3. CV curves (1 mM in 0.1 M Bu NPF /THF, scan rate 100 mV s , SCE reference
4
6
electrode, Pt working electrodes) of Me2-D, F2-D, F4-D.
Table 2. Electrochemical results of Me2-D, F2-D, F4-D.
1
2
Compd Epa (V) Epa (V) E (V) HOMO (eV) LUMO (eV)
pc
Me2-D
0.61
0.98
-1.39
-5.45
-2.19

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F2-D
F4-D
0.81
0.82
1.14
1.19
-1.36
-1.34
-5.67
-5.66
-2.35
-2.28
3
.5. Electrochromism
Conventional
quasi-solid
EC
cells
of
architecture:
glass/ITO/electrolyte/materials/ITO/glass, as schematically illustrated in Fig. 4 were
fabricated. In this configuration, successive depositions by spin coating were performed. First,
the active materials, dissolved in dimethylacetamide (DMAc) at a concentration of 10mg/mL,
then, the gel electrolyte, consisting of a mixture of PMMA, CH CN, LiClO and propylene
3
4
carbonate (PC). Finally, a second ITO substrate was used to complete the device. The
thickness of the electrochromic films was characterized with profilometer, and the correspond
data are ca. 70, 61 and 84 nm for Me2-D, F2-D and F4-D.
Fig. 4. Schematic diagram of electrochromic cell (A), initial state (B) and photos of displayed
colors after voltage supply on glass of Me2-D (C), F2-D (D), F4-D (E)
The transmittance and UV-visible-NIR absorption spectra were recorded in-situ under
different voltages ranging from 0 V to 2 V (Fig. 5). In neutral state, the devices are colorless
and almost transparent (Fig. 4B). No obvious change was monitored until the applied voltage
reaches 1V. Hence beyond this voltage, the color of the electrochromic cells turns from

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colorless to blue and green for Me2-D and both fluorinated derivatives respectively (Fig. 4C,
D, E), corresponding to optical signatures of their oxidized species. Greyscale obviously
shows that higher voltage increases the absorption bands and therefore decreases
transmittance. In addition, substituting one phenyl ring of the triphenylamine by methyl or the
introduction of two electron deficient fluorine atoms both induces a hypsochromic shift of the
intervalence charge-transfer (IV-CT) band from ca 830 nm of TPPA to 620 nm of Me2-D and
8
20 nm of F2-D respectively [10]. On the other hand, the insertion of four fluorine atoms
results in a bathochromic shifts of the derivatives IV-CT bands to 850 nm for F4-D. Finally,
although the three studied molecules exhibit similar optical properties in the neutral state, it is
noteworthy that their optical properties in the oxidized state significantly differs, which could
be correlated to the improved electron delocalization over the fluorinated molecular
backbones (F4-D).
Fig. 5. Transmittance (A, B, C) and UV-visible spectra (D, E, F) of displayed colors under

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different voltage supply. A, D: Me2-D; B, E: F2-D; C, F:F4-D.
The reversibility of the process was then studied by applying square-wave of ±1.5V
reverse bias (Fig. S2). From then, coloration and bleaching response time (t and t ) as well as
c
b
on/off contrast ratio (∆T) were estimated for all EC devices and data are gathered in table 3.
Table 3. Electrochromic responding time and on/off contrast ratio of three compounds.
Compd
Me2-D
F2-D
tc (s)
158
200
39
tb (s)
219
207
237
∆T
42%(395nm) 55%(620nm)
70%(399nm) 65%(840nm)
67%(403nm) 63% (839nm)
F4-D
As mentioned above, it is of great importance to maintain the color of EC devices over
time for specific applications, such as signpost, at the off-power state. Hence, this decay was
evaluated for each molecule and is plotted in the Fig. 6. All devices were switched by
applying voltage of 2 V for 3 second before being left under ambient conditions in air. It turns
out that Me2-D based devices need up to 43 hours to switch back to a transmittance close to
the initial state, ie, measured at 0 V. Regarding the fluorinated derivatives, the EC devices of
F2-D and F4-D still showed some coloration after 4 days, indicating that a longer time period
of time is mandatory to fully recover the initial transmittance.
Fig. 6. Transmittance of off-power EC devices. A: Me2-D; B: F2-D; C: F4-D.
Finally, based on these promising results, a flexible patterned electrochromic
demonstrator was fabricated. The picture in Fig. 7 shows a uniform color of the letter “E”,
(
for Electrochromism) induced by applying voltage of 1 V to a metallic network transparent

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conductive electrode [27].
Fig. 7. Flexible EC device based on Me2-D after application of 1V voltage.
4
. Conclusion
In summary, three undesired by-products were isolated and fully characterized. Their
structural analogy to well-known electrochromic materials pushed us to investigated their
potential in this field. It results in the characterization of three new arylamine based molecules
with interesting optical, electrochemical and electrochromical properties. Indeed, impressive
color pallets were recorded and induced by a subtle change of the molecular backbone.
Finally, we also showed that i) the color decay of doped species require several days and ii)
the potential fabrication of flexible devices, preliminary steps toward a more environmental
friendly signposting mass application.
ASSOCIATED CONTENT
Supporting Information. The NMR spectra, cyclic voltammetry of 100 cycles and the
reduction process are attached in the Supporting Information which is available free of charge
on the ACS Publications website at DOI:
AUTHOR INFORMATION
Y. Jiang and Z. Xian contributed equally.

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Corresponding Author
*
*
*
E-mail for C. C.: clement.cabanetos@univ-angers.fr
E-mail for J. R.: jeanroncali@gmail.com
E-mail for J. G.: gaojw@scnu.edu.cn
Conflicts of interest
There are no conflicts to declare.
Acknowledgment
We thank the financial support from University Foundation (16KJ06), Guangdong Provincial
Foundation (2016KQNCX035), National Key R&D Program of China （ No.
2
016YFA0201002), NSFC grant (No. 51571094, 51431006, 51561135014, U1501244),
Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No.
IRT_17R40) and Guangdong Innovative Research Team Program (No. 2013C102). We also
thank the support from the Guangdong Provincial Engineering Technology Research Center
for Transparent Conductive Materials, National Center for International Research on Green
Optoelectronics (IrGO), MOE International Laboratory for Optical Information Technologies
and the 111 Project.
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Highlight:
. Recycle the by-products of Buchwald-Hartwig reaction and investigate their optical,
electrochemical, electrochromic properties;
1
2
. ultra long electrochromic bistability;
. flexible electrochromic demonstrator.
3