Yellow electrochromic polymer materials with fine tuning electrofluorescences by adjusting steric hindrance of side chains

Article abstract of DOI:10.1039/c7ra03097e

A series of novel conducting conjugated yellow-to-transmissive electrochromic (EC) polymers were designed and synthesized to research their structure-property relationships, achieving electrofluorescent (EF) switching with applied external potential. These bifunctional materials are based on repeat units of bi-phenyl-1,3,4-oxadiazole and thiophene derivatives with different side chains. The polymers present a similar yellow color and emit various fluorescences with applied reduction potential, and they can turn to nearly transparent and non-fluorescent with oxidization potential. The optical contrast, electrochemical, electrochromic and electrofluorescent switching properties were also characterized in detail. A device containing these polymers was also fabricated that can achieve electrochromic and electrofluorescent switching simultaneously, making these polymers promising candidates for electro-bifunctional materials.

Full text of DOI:10.1039/c7ra03097e

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Yellow electrochromic polymer materials with fine
tuning electrofluorescences by adjusting steric
hindrance of side chains†
Cite this: RSC Adv., 2017, 7, 25444
*
Jian Liu,
Yuchen Shi, Jingchuan Wu, Mei Li, Jianming Zheng and Chunye Xu
A series of novel conducting conjugated yellow-to-transmissive electrochromic (EC) polymers were designed
and synthesized to research their structure–property relationships, achieving electrofluorescent (EF)
switching with applied external potential. These bifunctional materials are based on repeat units of bi-
phenyl-1,3,4-oxadiazole and thiophene derivatives with different side chains. The polymers present
a similar yellow color and emit various fluorescences with applied reduction potential, and they can turn to
nearly transparent and non-fluorescent with oxidization potential. The optical contrast, electrochemical,
electrochromic and electrofluorescent switching properties were also characterized in detail. A device
containing these polymers was also fabricated that can achieve electrochromic and electrofluorescent
switching simultaneously, making these polymers promising candidates for electro-bifunctional materials.
Received 16th March 2017
Accepted 21st April 2017
DOI: 10.1039/c7ra03097e
rsc.li/rsc-advances
designed a series of p-conjugated polymer capable of electro-
uorochromic switching and a device containing electro-
Introduction
chromic polymer and uorescent polymer to modulate color
and uorescence respectively.10,11 Above all, these reported
materials are little to achieve the electrouorescent and elec-
trochromic effect synchronous by one material.
Stimuli-responsive materials with optical change have gained
much popularity in recent decades as they are promoted
candidates for sensors, logical gates and military camouage.1
Wherein, material for electrical stimuli attracted considerable
research interest for display application, as the electrical stimuli
can be applied rapidly and reversibly. Electrochromism and
electrouorescence are the most important parts in electrical
stimuli material eld.2–4 The material can change color or
emission uorescence quickly and reversibly in response to
external potential so that they are widely used in smart
windows, LED displays and mirrors.5–7
From the aspect of electrochromic material, yellow-to-
transmissive attaches importance to fulll both of the
subtractive primary color sets: cyan–magenta–yellow (CMY) and
red–yellow–blue (RYB). A series of highly stable anodic yellow
electrochromic polyamides with high contrast of optical trans-
mittance change have synthesized by Guey-Sheng Liou's group.8
John R. Reynolds and his co-workers have also designed many
yellow electrochromic polymers, which have excellent optical
contrast and switching stability.9 These materials really
provided strategies to explore variety of electrochromic mate-
rials, but presented either multicolored change or only simple
electrochromic effect. In addition, Eunkyoung Kim's group have
Thiophene derivatives with long side chains have been
widely used in soluble electrochromic polymer materials, as
they exhibited unique advantages such as tunable colors, facial
processing procedure, high coloration efficiency and short
response time.12–16 However, these thiophene derivatives poly-
mers have strongly aggregation-caused quenching (ACQ)
effect,17 which limits the uorescent application in a state of
aggregation, such as particles or lm. In order to design the
above mentioned bifunctional material, oxadiazole derivatives,
excellent uorescent functional molecule,18,19 was introduced in
polymer backbone to act as electrouorescent functional group.
Additionally, color and emission uorescence of conjugated
polymers can be precisely tuned by controlling molecular
structure. Some literatures have reported for tuning colors and
emission uorescences of polymers, such as M. Sassi et al. have
reviewed the side chains functionalization approach to tune
electrochromic colors and Atilla Cihaner successfully controlled
the electrochromic colors by adjusting polymer backbone struc-
ture.20,21 Besides, Seogjae Seo' group have reported different
electrouorescent polymers with various uorophores.22 It is
observed that reports about ne tuning of color and uorescence
are really rare. As for solution-processable conducting polymer,
the long side alkyl chains could bring obvious steric hindrance,
and the resulted distortion of polymer backbone may cause the
reduction of average conjugated length, further resulting change
of color and uorescence of functional material.23 Therefore,
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of
So Matter Chemistry, Department of Polymer Science and Engineering, University of
Science and Technology of China, Hefei 230026, P. R. China. E-mail: chunye@ustc.
edu.cn; Tel: +86 551 63603459
† Electronic supplementary information (ESI) available: 1H NMR spectra of target
polymer and structural schematic diagram of device. See DOI: 10.1039/c7ra03097e
25444 | RSC Adv., 2017, 7, 25444–25449
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adjusting the steric hindrance by varying side chains is an
effective way to control color and uorescence of polymers.
Hence similar compounds with different side chains were
designed and synthesized to research for functional material
with various colors and uorescences.
In this article, combining uorescent oxadiazole derivatives
and electrochromic thiophene derivatives, a series of new con-
ducting conjugated polymers with different side chains which can
achieve bifunctions of electrochromic and electrouorochromic
switching were designed and synthesized, yielding insight for the
future development of functional materials. The optical absorp-
tion, electrochemical, electrochromic and electrouorescent
properties of synthesized bifunctional polymer materials were
characterized in detail as presented below. In addition, the device
comprising these polymers was fabricated, and its electrochromic
and electrouorescent switching can be achieved synchronously
by an applied external voltage.
Scheme 1 The synthesis route of compounds.
Experimental section
Materials
and dried to get white solid under vacuum condition. The crude
product was directly used for next step without further
purication.
All chemicals used in this paper were commercial products and
used as received without further purication unless otherwise
noted. Acetonitrile (ACN) and lithium perchlorate (LiClO4) were
purchased from Sigma-Aldrich Chemical. HPLC tetrahydro-
furan (THF) and N-methyl-2-pyrrolidone (NMP) were dried with
4A molecular sieves and stored in argon atmosphere for further
use.
2,5-Bis(4-bromophenyl)-1,3,4-oxadiazole
The mixture of 4-bromo-N0-(4-bromobenzoyl)benzohydrazide
and phosphorus oxychloride was reuxed for 6 h. Aer cooling
to room temperature, the reaction mixture was poured slowly on
ice. The resulted precipitate was ltered, washed with water,
dried. Then the needle crystals were collected through recrys-
1
Instruments
tallization from ethanol. H NMR (400 MHz, CDCl3): d (ppm)
7.90 (d, 4H), 7.60 (d, 4H).
NMR spectra were measured by Bruker Avance AV400. The
absorption spectra of lms were measured with a UV-vis-NIR
spectrophotometer (V-670; Jasco). Cyclic voltammetric (CV)
measurements were carried out using CHI 660D electrochemical
workstation in a three-electrode cell with working electrode (lm
coated on tin indium oxide (ITO) glasses, lms area were
approximately 0.7 cm ꢀ 3.0 cm), pseudo-reference electrode (Ag/
Ag+), and counter electrode (platinum wire) in electrolyte of 0.1 M
LiClO4/ACN. Spectroelectrochemical analysis was performed
with electrolytic cells using 1 cm UV-cuvette, lms (0.7 cm ꢀ 3.0
cm) on ITO glasses as working electrode, platinum wire as
counter electrode, and silver wire as reference electrode. The
uorescent spectra and electrouorescent behaviour of polymer
lm were characterized by OBIN YVON Fluorolog-3-TAV uores-
cence spectrophotometer. The lms were prepared by spraying
chloroform solution of corresponding materials onto ITO glass.
Photographs of the polymer lms were collected with a Canon
(IXUS 125 HS) digital camera.
General procedure of 10–30
3,4-Dimethoxythiophene (1 eq.), alkyl alcohol (3 eq.), p-toluene-
sulfonic acid (0.1 eq.), and toluene were added in a three-necked
ask equipped with Soxhlet extractor with 4A molecular sieves
in cellulose thimble. The reaction mixture was reuxed for 24 h,
aer cooling to room temperature, the reaction solution was
washed with deionized water and dried under vacuum. The
resulted crude product was puried with column chromatog-
raphy on silica gel with hexanes/methylene chloride (10 : 1).
1
Compound 10. H NMR (400 MHz, CDCl3): d (ppm) 6.24 (s,
2H), 3.85–3.88 (t, 4H), 1.74–1.80 (m, 2H), 1.22–1.48 (m, 8H),
0.86–0.94 (m, 12H);
1
Compound 20. H NMR (400 MHz, CDCl3): d (ppm) 6.24 (s,
2H), 3.85–3.88 (t, 4H), 1.78–1.82 (m, 2H), 1.25–1.50 (m, 16H),
0.86–0.94 (m, 12H);
1
Compound 30. H NMR (400 MHz, CDCl3): d (ppm) 6.24 (s,
The synthesis of compound is as follows shown in Scheme 1:
2H), 3.84–3.92 (t, 4H), 1.76–1.80 (m, 4H), 1.20–1.50 (m, 20H),
0.86–0.94 (m, 6H).
Bromo-N0-(4-bromobenzoyl) benzohydrazide
3,4-Dimethoxythiophene, dibromoneopentyl glycol, p-
4-Bromobenzoyl chloride and hydrazine monohydrate were toluene-sulfonic acid, and toluene were added in three-necked
mixed in THF at ice bath for 1 h, and then the reaction mixture ask equipped with Soxhlet extractor with 4A molecular sieves
was transferred to room temperature for another 3 h. The in cellulose thimble. The mixture was reuxed overnight, then it
resulted precipitate was ltered, washed with saturated was cooled, and respectively extracted with saturated brine and
aqueous NaHCO3 solution and deionized water for three times, anhydrous ether. The extracted solution was dried under
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Polymer 4. 1H-NMR (400 MHz, CDCl3): d (ppm) 7.50–8.20 (s,
8H), 4.0 (s, 4H), 3.2–3.60 (s, 8H), 0.56–1.48 (m, 32H).
vacuum, and crude product was puried by silica gel column
chromatography, with petroleum ether/chloroform (10 : 1) as
1
eluent, white powder was obtained as compound 400. H-NMR
(400 MHz, CDCl3): d (ppm) 6.49 (s, 2H), 4.12 (s, 4H), 3.65 (s, 4H).
A dried round-bottom ask lled with 100 mL of DMF,
12 mmol of 2-ethyl hexanol (4 eq.), and 18 mmol of NaH (6 eq.)
was heated at 90 ꢁC for 3 h. Then 3 mmol of ꢁcompound 400 was
added and continued to react for 24 h at 95 C. Aer reaction,
the reaction mixture was cooled and extracted with saturated
brine and anhydrous ether twice. The organic phase was dried
over magnesium sulfate, and the solvent was removed by rotary
evaporation under reduced pressure. The obtained crude
product was puried by column chromatography with petro-
Fabrication of device
A device (3.45 ꢀ 3.45 cm2) containing various polymers was
fabricated, and its structural schematic diagram is shown in
Fig. S11.† The polymer lm sprayed onto the ITO glass was used
as working electrode, NiO counter electrode was introduced in
device, and electrolyte of LiClO4/ACN solution was bubbled with
argon for 4 h and then injected into the device. The device was
fabricated in a glove box under argon atmosphere in order to
avoid any moisture and oxygen contamination. Aer gelation,
the edges of device were sealed by UV-curable sealant.
1
leum to give colorless oil as compound 40. H-NMR (400 MHz,
CDCl3): d (ppm) 6.45 (s, 2H), 4.0 (s, 4H), 3.47 (s, 4H), 3.28 (d, 4H),
1.2–1.6 (m, 20H), 0.8–1.0 (m, 12H).
Results and discussion
Optical properties
General procedure of polymer 10–40
The polymer 10–40 were polymerized by oxidative polymeriza-
tion, and color and absorbance spectra could be seen in
Fig. 1(a), the polymer lms present orange, yellow, red and
purple respectively, correspondingly, the absorbance peak in
spectra locates at the range of 450–600 nm. The diversity of
absorbance spectra is what we based on to design the materials
with ne tuning uorescences (Fig. 1(b)).
The photos and spectra of polymer 1–4 lms were displayed
in Fig. 1(c) and (d). With introduction of oxadiazol derivative, all
the lms show a similar yellow color with the corresponding
wavelength around 440 nm, as the higher electron delocaliza-
tion energy of backbone benzene and oxadiazole than thio-
phene could cause a blue shi to yellow color region.24,25 As for
PL emission spectra, polymer lms show a slight change from
yellow to orange bright uorescence due to introducing excel-
lent uorescent group of oxadiazole derivative. The variety of
uorescences may be ascribed to steric hindrance of different
side chains. As steric hindrance of branch chains is bigger than
The compound was dissolved in a small amount of chloroform,
then nitromethane solution dissolved with ferric chloride (5 eq.)
was added. With the addition, the reaction solution gradually
turned to green, the reaction was continued for 6 h, poured into
large amount of methyl alcohol, ltered, and the ltrate was
washed with Soxhlet extraction via methanol, hexane and
chloroform respectively. The solvent was removed by rotary
evaporation under reduced pressure to obtain the solid.
General procedure of polymer 1–4
2,5-Bis(4-bromophenyl)-1,3,4-oxadiazole (1 eq.), thiophene
derivative (1 eq.), potassium carbonate (2.6 eq.), palladium
acetate (0.04 eq.), trimethylacetic acid (0.3 eq.) were added into
dried two-necked ask in glove box. Then dried N-methyl-2-
pyrrolidone (NMP) was injected to dissolve the mixture and
ꢁ
the reaction mixture was heated to 120 C for 1 h under agita-
tion with N2 atmosphere. Upon cooling to the room tempera-
ture, the mixture was poured into methanol solution with
vigorous stirring. The resulted precipitate was ltered through
cellulose thimble and puried via Soxhlet extraction for 24 h
with methanol and hexane respectively. The resulted solid was
dissolved in chlorobenzene and heated to 60 ꢁC. Diethyl
dithiocarbamic acid dieth-ylammonium salt (Pd-scavenger, 4
eq. of Pd content) and 18-crown-6 were added and the solution
was stirred for 4 h. The reaction mixture was cooled to room
temperature, and precipitated into methanol. The precipitate
was ltered, washed via Soxhlet extraction for 24 h with meth-
anol and hexane respectively again and dried under vacuum
overnight.
1
Polymer 1. H NMR (400 MHz, CDCl3): d (ppm) 8.11 (s, 4H),
7.87 (s, 4H), 3.85–3.88 (t, 4H), 1.24–1.54 (m, 10H), 0.86–0.94 (m,
12H);
1
Polymer 2. H NMR (400 MHz, CDCl3): d (ppm) 8.10 (s, 4H),
7.88 (s, 4H), 3.88–4.06 (t, 4H), 1.14–1.50 (m, 18H), 0.86–0.94 (m,
12H);
Fig. 1 (a) The photos and absorbance spectra of the polymer 10–40
film. (b) CIE 1931 coordinates the fluorescence emission of different
polymers. (c) The photos and absorbance spectra of polymers; (d) the
photos and PL emission of polymers.
1
Polymer 3. H NMR (400 MHz, CDCl3): d (ppm) 8.07 (s, 4H),
7.86 (s, 4H), 3.90–4.08 (t, 4H), 1.76–1.80 (m, 4H), 1.20–1.50 (m,
20H), 0.86–0.94 (m, 6H);
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Fig. 3 Spectroelectrochemistry of P1 (a), P2 (b), P3 (c) and P4 (d) films
coated on ITO substrate from chloroform (2 mg mLꢂ1) solution at
different applied potentials between ꢂ1.6 V and 1.8 V, the potential
increases as the direction of arrows.
Fig. 2 The CV curves of the P1 (a), P2 (b), P3 (c) and P4 (d) films in
electrolyte of 0.1 M LiClO4/ACN.
linear chain, P3 has a red shi compared with other three
polymers. However, the emission peak of P4 owning branch
chain occurs a red shi compared to that of P1 and P2. This may
be caused by the ring attached to backbone structure, which can
reduce the winding between side chains, further enhancing the
conjugated degree of polymer backbone. Therefore, gradual red
shi of emission spectra was observed with reduction of steric
hindrance, resulting an obvious visible change from yellow to
orange uorescence under irradiation of 365 nm light. The
trend is as follows: P2 < P1 < P4 < P3. The Commission Inter-
nationale de L'Eclairage (CIE) coordinates of various polymers
are calculated to be (0.366, 0.599), (0.312, 0.632), (0.427, 0.556)
and (0.388, 0.575) as shown in Fig. 1(b).
yellow color in oxidization state and present approximate
transparent color in reduction state.
Electrochromic switching property
Transmittance change, a function of time at the maximum
absorption wavelength achieving the largest change between
reduction and oxidized state, and the switching time, the time
needed to achieve 95% of transmittance changes, are crucial
factors for electrochromic application. As shown in Fig. 4, P1–
P4 lms reveal transmittance change (DTmax) of 30%, 35%, 31%
and 32% at the maximum absorption wavelength respectively,
and all the polymer lms possess similar switching time of 2 to
3.2 s.
Electrochemical properties
To obtain a deep understanding of electrochemical behaviours
of these polymers, the electrochemistry of polymers in 0.1 M
LiClO4/ACN was explored by cyclic voltammetry (CV). The onset
oxidization voltages (Eonset) of P1–P4 lms were 1.08 V, 1.14 V,
1.04 V and 1.06 V respectively (Fig. 2), which were ascribed to
the inuence of different steric hindrances analyzed above.
Obviously, weaker steric hindrance could induce conjugated
length of polymers, further reducing oxidization voltage. This is
also consistent with the optical results of polymers.
Spectroelectrochemistry
The electrochromic properties of polymer lms were evaluated
by spectroelectrochemistry. The changes in color and optical
properties in details would be observed when different poten-
tials were applied to lms. Optical change, typical electro-
chromic absorbance spectra and photos of color change were
displayed in Fig. 3. With increased applied potential, the
absorption in wavelength of about 440 nm decreases indicating
the formation of radical cations (polarons) and further dica-
tions (bipolarons) at the expense p–p* transition in a higher
Fig. 4 Electrochromic switching optical transmittance change of
polymer films monitored at their lmax in electrolyte of 0.1 M LiClO4/
ACN. (a) P1 at 416 nm, (b) P2 at 412 nm, (c) P3 at 418 nm and (d) P4 at
doping state.26,27 Additionally, all the polymers show similar 425 nm.
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Table 1 Electrochromic properties of P1–P4 film
Polymer
lmax (nm)
DT%
treda (s)
toxb (s)
P1
P2
P3
P4
416
412
418
425
30
35
31
32
2.9
2.8
2.9
2.7
2.2
3
2.1
2.4
a
b
tred is the response time for reduction process. tox is the response
time for oxidization process.
Above all, typical electrochromic parameters such as
maximum absorption wavelength, transmittance change and
response time were displayed in Table 1.
Electrouorescent property
Fig. 6 Electrofluorescent switching PL intensity of polymer films
monitored at their lmax in electrolyte of 0.1 M LiClO4/ACN. (a) P1 at
547 nm, (b) P2 at 539 nm, (c) P3 at 561 nm and (d) P4 at 554 nm.
The polymer lms emit uorescent colors by UV excitation and
electrouorescent switching properties of polymer lms were
characterized by monitoring their photoluminescent properties
with different applied external potentials in a three-electrodes
cell with electrochemical working station and uorescence
spectrophotometer combinedly (Fig. 5 and 6). All the polymers
show similar electrouorescent switching properties with
different potentials. Taking P1 lm for example, upon applying
potential from ꢂ1.6 V to 1.8 V, the bright uorescence was
extinguished to nearly dark gradually. The uorescence could
reversibly return to its original state when applying potential of
ꢂ1.6 V. Additionally, there is no obvious shi of emission peak
(545 nm) of polymer lm in potential change process, indi-
cating that the quenching of uorescence of polymer lm
originated from the produced cation radical in polymer back-
bone during redox process, rather than the production of side
product.28,29 Furthermore, P1 lm reveals a quenching time of
2.4 s and a recovery time of 2.3 s, which matches with the
electrochromic response time. Other electrouorescent prop-
erties of polymer lms were depicted in Table 2.
Table 2 Electrofluorescent response time of P1–P4 film
Polymer
l (nm)
Quenching time (s)
Recovery time (s)
P1
P2
P3
P4
545
539
561
554
2.9
2.8
2.5
3.0
2.7
2.8
3.0
2.7
Bifunctional effect of fabricated devices
The photos of devices containing different polymers were dis-
played in Fig. 7. Applying an external potential, the device turns
to approximate transparent state, and the uorescence of device
was extinguished to nearly dark. The vanish of uorescence and
color is nearly synchronous, which may be caused by cation
radical produced by applied external potential. It can decrease
the absorption in visible region and also acts as strongly effec-
tive uorescence quencher.22,30 With reverse potential, the color
and uorescence can return to its original state. Aer dozen
cycles, the device couldn't work to achieve its bifunctional
effect, similar devices with long service time are in research in
our group. In addition, polymers show different uorescences
Fig. 5 Electrofluorescent behaviours of polymer film between ꢂ1.6 V
and 1.8 V and the insert is the photo of film in bleached state (left) and Fig. 7 (a) The images of fabricated device of different doping state
colored state (right). (a) P1; (b) P2; (c) P3; (d) P4. The potential increases under sunlight and UV irradiation. (b) Electrochromic and electro-
as the direction of arrows.
fluorescent switching arise from reversible redox reactions in device.
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in one device, which provides a new way to design material with 11 S. Seo, Y. Kim, J. You, B. D. Sarwade, P. P. Wadgaonkar,
various uorescences.
S. K. Menon, A. S. More and E. Kim, Macromol. Rapid
Commun., 2011, 32, 637–643.
¨
12 M. Akbayrak and A. M. Onal, Polym. Chem., 2016, 7, 6110–
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6119.
13 S. Govaerts, P. Verstappen, H. Penxten, M. Defour, B. Van
Mele, L. Lutsen, D. Vanderzande and W. Maes,
Macromolecules, 2016, 49, 6411–6419.
14 S. Ming, S. Zhen, X. Liu, K. Lin, H. Liu, Y. Zhao, B. Lu and
J. Xu, Polym. Chem., 2015, 6, 8248–8258.
15 Z. Xu, W. Wang, J. Wu, S. Mi, J. Zheng and C. Xu, New J.
Chem., 2016, 40, 5231–5237.
16 M. A. Invernale, J. G. Bokria, M. Ombaba, K.-R. Lee,
D. M. D. Mamangun and G. A. Sotzing, Polymer, 2010, 51,
378–382.
A series of novel conducting conjugated yellow-to-transmissive
electrochromic and electrouorescent polymers were designed
and synthesized to investigate the structure–property relation-
ship. With side chains of slight different steric hindrances, the
polymers present similar yellow color and emit ning tuned
uorescence with applied reduction potential, and they can
turn to nearly transparent state and non-uorescent state with
oxidization potential. The device containing various polymers
was also fabricated which can achieve electrochromic and
electrouorescent switching simultaneously, making these
polymers be promising candidate for electro-bifunctional
material.
17 G. He, W. Torres Delgado, D. J. Schatz, C. Merten,
A. Mohammadpour, L. Mayr, M. J. Ferguson, R. McDonald,
A. Brown, K. Shankar and E. Rivard, Angew. Chem., Int. Ed.,
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Acknowledgements
18 A. Baranczak, Y. Liu, S. Connelly, W. G. Du, E. R. Greiner,
J. C. Genereux, R. L. Wiseman, Y. S. Eisele, N. C. Bradbury,
J. Dong, L. Noodleman, K. B. Sharpless, I. A. Wilson,
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We gratefully acknowledge supports of this work by the
National Natural Science Foundation of China (51503193) and
the Chinese Academy of Sciences (Integrated system of high
efficiency building energy saving and its application, KFZD-SW-
403).
19 A. Paun, N. D. Hadade, C. C. Paraschivescu and M. Matache,
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