Design and synthesis of 9,9-dioctyl-9H-fluorene based electrochromic polymers

Article abstract of DOI:10.1002/pola.27935

Two novel heterocycle-fluorene-heterocycle monomers, 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene (Th-F-Th) and 5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(2,3-dihydrothieno[3,4-b][1,4]dioxine) (EDOT-F-EDOT), were synthesized via Stille coupling reaction and electropolymerized to form corresponding polymers P(Th-F-Th) and P(EDOT-F-EDOT). Furthermore, the optoelectronic properties of the obtained monomers and polymers were explored using cyclic voltammetry (CV), UV-vis, and emission spectra and in situ spectroelectrochemical techniques. The band gap values of monomers calculated by DFT were 3.75 eV for EDOT-F-EDOT and 4.03 eV for Th-F-Th, while that of P(EDOT-F-EDOT) and P(Th-F-Th) were brought down to 1.70 and 2.10 eV, respectively. Both polymers exhibited excellent redox activity and electrochromic performance. P(EDOT-F-EDOT) exhibited a maximum optical contrast of 25.8% at 500 nm in visible region with a response time of 1.2 s. In addition, the coloration efficiency of P(EDOT-F-EDOT) was calculated to be 220 cm² C^-1.

Full text of DOI:10.1002/pola.27935

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Design and Synthesis of 9,9-Dioctyl-9H-Fluorene Based Electrochromic
Polymers
Qinglin Jiang, Shijie Zhen, Daize Mo, Kaiwen Lin, Shouli Ming, Zhipeng Wang, Congcong Liu,
Jingkun Xu, Yuanyuan Yao, Xuemin Duan, Danhua Zhu, Hui Shi
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, 330013, People’s
Republic of China
Correspondence to: J. Xu (E-mail: xujingkun@tsinghua.org.cn)
Received 3 July 2015; accepted 22 September 2015; published online 22 October 2015
DOI: 10.1002/pola.27935
ABSTRACT: Two novel heterocycle-fluorene-heterocycle mono-
mers, 2,20-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene (Th-F-
Th) and 5,50-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(2,3-dihydro-
thieno[3,4-b][1,4]dioxine) (EDOT-F-EDOT), were synthesized via
Stille coupling reaction and electropolymerized to form corre-
sponding polymers P(Th-F-Th) and P(EDOT-F-EDOT). Further-
more, the optoelectronic properties of the obtained monomers
and polymers were explored using cyclic voltammetry (CV),
UV–vis, and emission spectra and in situ spectroelectrochemi-
cal techniques. The band gap values of monomers calculated
by DFT were 3.75 eV for EDOT-F-EDOT and 4.03 eV for Th-F-
Th, while that of P(EDOT-F-EDOT) and P(Th-F-Th) were brought
down to 1.70 and 2.10 eV, respectively. Both polymers exhib-
ited excellent redox activity and electrochromic performance.
P(EDOT-F-EDOT) exhibited
a maximum optical contrast of
25.8% at 500 nm in visible region with a response time of
1.2 s. In addition, the coloration efficiency of P(EDOT-F-EDOT)
was calculated to be 220 cm2 C21
.
2015 Wiley Periodicals,
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Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 325–334
KEYWORDS: conducting polymers; copolymerization; electro-
chemical polymerization; electrochemistry; electrochromics;
P(EDOT-F-EDOT); spectroelectrochemistry
electron-rich polymeric backbone possessing high oxygen
INTRODUCTION Electrochromism is defined as a reversible
change in absorbance/transmittance in response to an exter-
nally applied potential on a material.1 Electrochromic (EC)
materials have extensive applied prospects as intelligent
materials, such as smart windows,2 optical displays,3 and
electrochromic mirrors.4 Conducting polymers (CPs) as elec-
trochromic materials, since they were discovered, have
attracted large number of attention due to outstanding colo-
ration efficiencies, ease of color control through structure
modification and fast switching ability.5
content and inhibiting a, b- and b, b- cross-links.8
Polyfluorenes (PFs), due to excellent chemical and thermal
stabilities, easy functionalization at the 9 positions and good
fluorescence properties, have been used as electroactive mate-
rials.9 Based on the advantages of PF and PEDOT, Nie et al.9
have reported a kind of electrochromic polymer poly(2,7-
bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)29H-fluorene)
(P(EDOT-FE)), a typical “poly(heterocycle-fluorene-hetero-
cycle)” material, which showed a good electrochromic per-
formance including good optical contrast (36% at 625 nm),
high CE, fast response time (0.5 s at 625 nm), satisfactory opti-
Among the numerous candidate CPs that have been studied,
polythiophenes are a class of important EC materials because
of high electrical conductivity, good redox behavior, and excel-
lent electrochromic performance including fast response time
and high contrast in the visible and NIR regions.6 Also, poly-
thiophenes exhibited facile bandgap tunability through struc-
tural modification, which has been proved by the reported
novel materials in past few years.7 In polythiophenes family,
3,4-ethylenedioxythiophene (EDOT) is the most common pre-
cursor used for electrochemical polymerization due to
€
cal memory and long-term stability. Furthermore, Onala and
coworkers synthesized three new monomers based on furan
and fluorene and investigated the optical properties of their cor-
responding polymers, which showed electrochromic properties
and smaller band gap than PF (i.e., 3.1 eV).10 Subsequently doz-
ens of fluorene analogs based electrochromic polymers have
been achieved (their structures are shown in Scheme 1), some
of which displayed exceptional performance.
Qinglin Jiang, Shijie Zhen, and Daize Mo contributed equally to this manuscript.
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use. 9,9-Dioctyl-2,7-dibromofluorene (98%; J&K chemical), n-
butyllithium (2.5 mol L21 in hexanes; Energy Chemical),
chlorotributyltin (98%; Energy Chemical) and tetrakis (tri-
phenylphosphine) palladium (0) (99%; Energy Chemical)
were stored at 4 8C and used directly without further purifi-
cation. N,N-Dimethylformamide (DMF, analytical grade; Bei-
jing East Longshun Chemical Plant) and dichloromethane
(analytical grade; Beijing East Longshun Chemical Plant)
were purified by distillation with calcium hydride under a
nitrogen atmosphere before use. Tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6, 99%; Energy Chemical) was
dried under vacuum at 60 8C for 24 h before use.
Monomer Synthesis
Two novel monomers, which comprise 9,9-dioctyl-2,7-dibro-
mofluorene core symmetrically linked to Th and 3,4-ethoxy-
lenedioxythiophene, namely 2,20-(9,9-dioctyl-9H-fluorene-2,7-
diyl)dithiophene (Th-F-Th) and 5,50-(9,9-dioctyl- 9H-fluorene-
SCHEME 1 Structures of reported poly(heterocycle-fluorenes-het-
erocycle) electrochromic materials. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
2,7-diyl)bis(2,3-dihydrothieno[3,4-b][1,4]dioxine)
(EDOT-F-
EDOT), were prepared in satisfactory yield by the synthetic
route in Scheme 2. In addition, the two monomers were syn-
thesized and their properties were investigated for the first
time as we known.
Although fluorene analogs based polymers exhibited good elec-
trochromic properties, however, the poor solubility in common
organic solvents limit their applications. One of the most prom-
ising ways to overcome these problems is to introduce alkyl
chain structures at the bridge position of fluorene (C-9) posi-
tion, which has been proved in previous work.11 Substituents
not only change the solubility of the system, but also affect the
oxidation potential of monomer, the band gap and optical prop-
erties of the polymers. In addition, it was predicted that the
alkyl substituted electrochromic polymer showed better con-
trast ratios and switching speeds than the parent polymer.12–14
Tributyl(thiophen-2-Yl)stannane and Tributyl(2,3-
dihydrothieno[3,4-B][1,4] dioxin-5-Yl)stannane
A solution of Th (or EDOT) in dry THF was cooled to 278
8C, and blanked by under an argon atmosphere.15,16 n-BuLi
(2.5 mol L21 in hexane) was slowly added dropwise. The
mixture was stirred for 1–2 h at 278 8C. Then a solution of
chlorotributyltin in dry THF was added slowly at 240 8C,
and the mixture was stirred for 2 h at the same temperature.
After that, the reaction mixture was warmed to room tem-
perature, and then was stirred for another 10 h under an
inert atmosphere (Ar). The organic layer was dried with
anhydrous MgSO4 and filtered with funnel, and then the sol-
vent was removed under reduced pressure by rotary evapo-
ration. The stannic derivative was used without other
purification for the Stille coupling reactions.
According to these consideration, two novel heterocycle-
fluorene-heterocycle monomers, Th-F-Th and EDOT-F-EDOT,
were synthesized via Stille coupling reaction and electropoly-
merized to form corresponding polymers P(Th-F-Th) and
P(EDOT-F-EDOT). In addition, electrochemical behavior, and
electrochromic properties of the obtained polymers were
systematically investigated by cyclic voltammetry (CV) and
in situ spectroelectrochemical techniques. To elucidate the
effect of the terminal group, the obtained results of EDOT-F-
EDOT and corresponding polymer are compared with its close
thiophene (Th) based analogues Th-F-Th and P(Th-F-Th).
2,20-(9,9-Dioctyl-9H-fluorene-2,7-diyl)dithiophene
(Th-F-Th)
9,9-Dioctyl-2,7-dibromofluorene (0.5 g, 0.91 mmol) and trib-
utyl(thiophen-2-yl)stannane (1.02 g, 2.73 mmol) were dis-
solved in dry DMF (40 ml) and tetrakis(triphenylphosphine)
palladium (0) (100 mg, 0.091 mmol) was added under an
inert atmosphere (Ar). The mixture was refluxed for 12 h at
100 8C under argon atmosphere (Ar). Then, the mixture was
cooled to room temperature, with an equal amount of water
poured into the flask, extracted with CH2Cl2, and washed
EXPERIMENTAL
Chemicals
Thiophene (Th, 99%; TCI, Shanghai) and EDOT (98%; Sigma-
Aldrich) were distilled under N2 atmosphere once before
SCHEME 2 Synthetic route of monomers: (1) DMF, Pd(PPh3)4, 100 8C; (2) DMF, Pd(PPh3)4, 100 8C.
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with saturated NaHCO3 solution and water three times suc-
cessively. The combined organic layer was dried over anhy-
drous MgSO4 and concentrated under reduced pressure to
leave a crude residue. Purification by chromatography on
silica gel (EA:PE 5 1:10) afforded 0.30 g Th-F-Th as a faint
yellow liquid (60%). 1H NMR (400 MHz, CDCl3): d 7.72 (d,
J 5 8 Hz, 1H), 7.64 (t, J 5 24 Hz, 2H), 7.43 (d, J 5 4 Hz, 1H),
7.33 (d, J 5 8 Hz, 1H), 7.15 (t, J 5 8 Hz, 1H), 2.06 (m, 2H),
1.11 (m, 10H), 0.83 (t, J 5 12 Hz, 3H), 0.72 (s, 2H). Calcd for
C37H46S2: C, 80.14; H, 8.30; S, 11.55. Found: C, 80.05; H,
8.37; S, 11.58.
5,50-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(2,3-
Dihydrothieno[3,4-B][1,4] dioxine) (EDOT-F-EDOT)
Synthesis of EDOT-F-EDOT was carried out in a similar man-
ner as Th-F-Th under similar conditions. The reagents were
9,9-dioctyl-2,7-dibromofluorene (0.51 g, 0.91 mmol), tribu-
tyl(2,3-dihydrothieno[3, 4-b][1,4]dioxin-5-yl)stannane (1.22 g,
2.73 mmol) and tetrakis(triphenylphosphine) palladium(0)
(0.1 g, 0.091 mmol). Yield: 64%; faint yellow solid (0.59 g). 1H
NMR (400 MHz, CDCl3): d 7.72 (d, J 5 8 Hz, 1H), 7.64
(d, J 5 12 Hz, 1H), 6.31 (s, 1H), 4.28 (d, J 5 28 Hz, 4H), 1.98
(t, J 5 16 Hz, 2H), 1.29 (t, J 5 28 Hz, 5H), 1.09 (d, J 5 16 Hz,
6H), 0.81 (d, J 5 4 Hz, 3H), 0.78 (s, 2H). Calcd for C41H50S2O4:
C, 73.43; H, 7.46; S, 9.55; O, 9.55. Found: C, 73.29; H, 7.57; S,
9.47; O, 9.67.
FIGURE 1 Anodic polarization curves of 0.01 mol L21 Th-F-Th
(A) and EDOT-F-EDOT (B) in CH2Cl2-Bu4NPF6 (0.1 mol L21).
Potential scan rates: 50 mV s21. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
Characterization
1H NMR and 13C NMR spectra were recorded on a Bruker
AV 400 NMR spectrometer with CDCl3 or DMSO-d6 as the
solvent and tetramethylsilane as an internal standard (TMS,
singlet, chemical shift: 0.0 ppm). UV–vis spectra of the mono-
mers and polymers were taken by using Cary 5000 UV–vis-
NIR spectrophotometer. With an F-4500 fluorescence spec-
trophotometer (Hitachi), the fluorescence spectra of the
monomers were determined. Infrared spectra were deter-
mined with a Bruker Vertex 70 Fourier transform infrared
(FTIR) spectrometer with samples in KBr pellets. Electro-
chemical tests and polymerization of the monomers were
performed in a one-compartment cell with the use of Model
263A potentiostat-galvanostat (EG&G Princeton Applied
Research) under computer control. The surface morphology
was captured in tapping mode by atomic force microscopy
(AFM, Veeco Multimode). Spectroelectrochemical and kinetic
studies were recorded on a Specord 200 Plus (Analytikjena)
spectrophotometer at a scan rate of 2000 nm min21 and the
potentials were controlled using Versa Stat 3 (Princeton
Applied Research) electrochemical workstation.
Electrosynthesis and Electrochemical Tests
All the electrochemical experiments and polymerization of
the monomers were performed in a one-compartment cell
with the use of Model 263A potentiostat-galvanostat (EG&G
Princeton Applied Research) under computer control. For
electrochemical tests, the working and counter electrodes
were both Pt wires with a diameter of 0.1 mm, respectively.
To obtain a sufficient amount of the polymer films for char-
acterization, Pt sheet, or ITO-coated glass were employed as
the working electrode and another Pt sheet was used as the
counter electrode. They were placed 5 mm apart during the
examinations. Prior to each experiment, these electrodes
mentioned above were carefully polished with 1500 mesh
abrasive paper (for ITO: immersed in ethanol for 6 h and
then cleaned by ultrasonically wave for 15 min), cleaned suc-
cessively with water and acetone, and then dried in air. An
Ag/AgCl electrode directly immersed in the solution served
as the reference electrode, with its electrode potential cali-
brated against a saturated calomel electrode, and it revealed
sufficient stability during the experiments. All the solutions
were deaerated by a dry nitrogen stream and maintained
under a slight overpressure through all the experiments to
avoid the effect of oxygen.
Electrosynthesis and Electrochemical Tests
Spectroelectrochemistry was recorded on a Specord 200
Plus (Analytikjena) spectrophotometer at a scan rate of
2000 nm min21. A three-electrode quartz cell was utilized
consisting of an Ag/AgCl electrode, a Pt wire counter elec-
trode, and an ITO coated glass working electrode. All meas-
urements were carried out in CH2Cl2-Bu4NPF6 (0.1 mol L21).
It should be noted here that the quartz cell filled with
CH2Cl2-Bu4NPF6 (0.1 mol L21) and ITO-coated glass without
deposited film were used as the background for spectroelec-
trochemical measurements.
The polymer films were grown potentiostatically at the opti-
mized potential in CH2Cl2 containing 0.1 mol L21 Bu4NPF6
as the supporting electrolyte. After polymerization, the poly-
mer films were washed repeatedly with anhydrous CH2Cl2 to
remove the electrolyte, monomer, and oligomer.
The optical density (᭝OD) at the specific wavelength (kmax) of
the sample was determined by %T values of electrochemically
oxidized and reduced films, using the following equation17
:
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CE5DOD=Qd
(2)
The doping charge Qd is defined as the relation between the
injected/ejected charge during the redox process (᭝Q) and
working electrode area (S) at the specific dominant wave-
length (kmax) of the sample as illustrated by the following
equation17,18
:
Qd5DQ=S
(3)
᭝Q can be directly obtained from the charge-time (C-t)
curves, which conversed from the potential-time (E-t) curves
directly recorded during the electrochromic experiments. S
can be calculated by the area of the polymer films deposited
onto the ITO glass.
Details of Computations
All calculations were carried out using the Gaussian 03 pro-
gram. The structure of the oligomers were optimized without
symmetry constraints using a hybrid density functional19,20
and Becke’s three-parameter exchange functional combined
with the LYP correlation functional (B3LYP)21,22 using the 6-
31G(d,p) basis set (B3LYP/6-31G(d,p)). Vibrational frequen-
cies were evaluated at the same level to identify the real
minimum energy structures.
RESULTS AND DISCUSSION
Anodic Polarization Curves
Figure 1 shows the anodic polarization curves of 0.01
mol L21 monomers (EDOT-F-EDOT and Th-F-Th) in CH2Cl2-
Bu4NPF6 (0.1 mol L21). The oxidation of Th-F-Th and EDOT-
F-EDOT were initiated at 0.87 and 0.71 V vs. Ag/AgCl,
respectively. The difference between the onset oxidation
potentials of two monomers was mainly due to electron-rich
polymeric backbone possessing high oxygen content and
inhibiting a, b- and b, b- cross-links.23 However, these values
are much lower than fluorene (1.05 V)24, EDOT (1.2 V),25
and Th (2.0 V),26 which could be attributed to the increase
of p-conjugated systems.27 The method employing EDOT as
the terminal groups presents an effective way to reduce oxi-
dation potential of monomer.
FIGURE 2 CV of 0.01 mol L21 EDOT-F-EDOT (A) and Th-F-Th
(B) in CH2Cl2-Bu4NPF6 (0.1 mol L21). Potential scan rates: 100
mV s21
.
DOD5 logðTox=Tref
Þ
(1)
The CE is defined as the relation between the injected/
ejected charge as a function of electrode area (Qd) and the
change in optical density (᭝OD) at the specific dominant
wavelength (kmax) of the sample as illustrated by the follow-
ing equation17
:
FIGURE 3 UV–vis spectra (a) and emission spectra (b) of Th-F-Th (A) and EDOT-F-EDOT (B) in CH2Cl2. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 1 Optical Data of EDOT-F-EDOT and Th-F-Th
absorption spectra were generally observed in hybrid conju-
gated samples containing long side chain.14,28,30,31
Samples
k (nm)
e (L mol21 cm21
)
As shown in Table 1, it was noted that the absorption peak
of EDOT–F–EDOT (kmax 5 368 nm, e 5 67,000 L mol21 cm21
)
EDOT-F-EDOT
368
380
358
367
67,000
54,000
124,000
97,000
shifted to longer wavelength with decreasing absorption
coefficient compared with that of Th–F–Th (kmax 5 358 nm,
e 5 124,000 L mol21 cm21), which could be attributed to
electron-donating ethylenedioxy group of EDOT. In addition,
the optical band gap of monomer was calculated on the basis
of the formula Eg 5 1240/konset, where konset is the onset of
p-p* transition. The values of the optical bandgap for EDOT–
F–EDOT and Th–F–Th were 3.08 and 3.19 eV, respectively
(Table 2). The results are again consistent with the more
electron-donating ability of EDOT in conjugated block.
Th-F-Th
e is the absorption coefficient.
Cyclic Voltammetry
The electrochemical performances of two monomers were
examined by CV in CH2Cl2-Bu4NPF6 (0.1 mol L21), as shown
in Figure 2. The formation of this loop [Fig. 2(A,B)] can be
explained as characteristic of the nucleation process, and only
appeared on the first cycle. As the CVs scan continued, the
peak current of the redox system increased regularly, indicat-
ing that electroactive polymer was formed. Meanwhile, visual
inspection during electrochemical experiments revealed the
formation of compact and homogeneous polymers on the elec-
trode surface. From the CVs results, both P(EDOT-F-EDOT) and
P(Th-F-Th) exhibited broad redox waves. The phenomenon
usually appears in electrochemical behavior of Th/EDOT-based
compounds,28,29 and could be ascribed to the wide distribu-
tion of the conjugated polymer chain length and the mutual
transition of multiple conductive species during the electropo-
lymerization. In addition, EDOT-F-EDOT [Fig. 2(B)] exhibited
better electropolymerization performances, which might be
attributed to the electron-donating ethylenedioxy group and
less structural defects.18,23 It was noted that the alkyl chain
substituents led P(EDOT-F-EDOT) film to exhibit sharper redox
peaks than the parent P(EDOT-FE) film, which could be
ascribed to the separation between polymer chains due to
existence of alkyl chain.
The fluorescence emission spectra of monomers in CH2Cl2
were carefully determined, as shown in Figure 3(B). In agree-
ment with the UV–vis spectral results, the emission spectra dis-
played significant red shift when the terminal unit change from
Th (kem 5 412 nm) to EDOT (kem 5 422 nm). The difference
was mainly attributed to the electron-donating ethylenedioxy
group as in the case of the shift in UV–vis spectra.18,22
Structural Characterization
FTIR spectra of the oligomers and the hybrid polymers
(Fig. 4) were recorded to elucidate their structure and inter-
pret the polymerization mechanism. In the functional group
region, the peak at 3070 cm21 of EDOT-F-EDOT, and the
peak at 3109 cm21 of Th-F-Th, is attributed to the @CAH
vibration of Th rings. However, these peaks disappeared in
the spectra of P(EDOT-F-EDOT) and P(Th-F-Th), indicating
that EDOT-F-EDOT and Th-F-Th could be electropolymerized
at a-position of EDOT and Th, respectively. In the fingerprint
region, the peaks at 1208 cm21 [Fig. 4(A)] and 1173 cm21
[Fig. 4(B)] should be assigned to the @CAH in-plane defor-
mation vibration of Th rings, and 883 cm21 [Fig. 4(A)] and
908 cm21 [Fig. 4(B)] results from out-of-plane deformation
vibration. However, all these peaks disappear or weaken in
FT-IR spectrum of the polymer, which further demonstrated
that Th-F-Th and EDOT-F-EDOT were electropolymerized
through a, a0-coupling of Th rings.
UV–vis and Fluorescence Spectra of Monomers
UV–vis spectra and the absorption coefficient data of two
monomers in CH2Cl2 were carefully examined, as shown in
Figure 3(A). EDOT-F-EDOT and Th-F-Th showed similar
absorption in the region of 320–400 nm, which corre-
sponded to a distinctive p-p* transition.14,30,31 It was noted
that Th-F-Th exhibited scarcely discernible vibronic couplings
at 358 and 367 nm in its p-p* transition band. Like Th-F-Th,
the UV–vis spectra of EDOT-F-EDOT showed obvious vibronic
coupling at 368 and 382 nm in its absorption band. It was
suggested that vibronic splitting about conjugated monomers
correlate to the rigid structure.14,31 Furthermore, the splitted
Computational Results
To obtain a further interpretation of monomers, including
planar structure, HOMO2LUMO levels, and bandgap, density
functional theory calculations have been performed (Table 2
and Fig. 5). As it is seen from Table 1 the HOMO energy lev-
els of monomer increase from Th-F-Th (25.22 eV) to EDOT-
TABLE 2 The Calculated and Experimental Parameters of Th-F-Th and EDOT-F-EDOT
HOMO (eV)
LUMO (eV)
Eg,opt (eV)
Samples
kabs (nm)
kem (nm)
Eox,onset (V)
Exp
DFT
Exp
DFT
Exp
DFT
Th-F-Th
358
368
407
472
0.87
0.71
25.67
25.51
25.22
25.14
22.48
22.43
21.47
21.48
3.19
3.08
3.75
3.66
EDOT-F-EDOT
Exp: experimental parameters; DFT: the calculated parameters through density functional theory calculations.
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FIGURE 5 HOMO and LUMO orbitals of monomers. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 4 FTIR spectra of the oligomers and the hybrid poly-
mers: (A) EDOT-F-EDOT, (B) Th-F-Th, (C) P(EDOT-F-EDOT), and
(D) P(Th-F-Th). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
mula Eg 5 LUMO-HOMO, were 4.03 and 3.75 eV, respectively.
Furthermore, these values including HOMO, LUMO and band
gap were found to be higher than those from experimental
data, which could be explained by solvent effect.15
F-EDOT (25.14 eV), which is in agreement with electron
density of monomers. Compared with the change corre-
sponding to HOMO of monomers, the difference of LUMO
between Th-F-Th (21.47 eV) and EDOT-F-EDOT (21.48 eV)
was found to be insignificant. In addition, the band gap of
Th-F-Th and EDOT-F-EDOT, calculated on the basis of the for-
Electrochemistry of Polymer Films
To get a deeper insight into the electrochemical activity and
stability of obtained polymers, the electrochemical behaviors
FIGURE 6 Left column: CV of P(Th-F-Th) (A) and P(EDOT-F-EDOT) (B) films in monomer-free CH2Cl2-Bu4NPF6 (0.1 mol L21) at
potential scan rates of 25 mV s21, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, and 300 mV s21. Right column: plots of redox peak
current densities versus potential scan rates. jp is the peak current densities, and jp,a and jp,c denote the anodic and cathodic peak
current densities. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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films or interfacial charge transfer process. In agreement
with Figure 2, P(EDOT-F-EDOT) exhibited the sharper redox
peaks the parent P(EDOT-FE) film. The result further demon-
strated that the existence of alkyl chain facilitated better
redox performance.
It is well known that the good stability of CPs is very signifi-
cant for their applications in electronic devices.33 Therefore,
the long-term redox stability of two kinds of polymer films
was also investigated in CH2Cl2-Bu4NPF6 (0.1 mol L21), as
shown in Figure 7. It was noted that P(Th-F-Th) films exhib-
ited better redox stability than P(EDOT-F-EDOT), retaining
>89% of the electroactivity after 500 cycles, while P(EDOT-
F-EDOT) displayed inferior redox activity with the amount of
exchange charge remaining 79% after 500 cycles. In the lit-
erature EDOT based polymer generally exhibited superior
stability than Th based polymer.29,34236 The exceptional sta-
bility of P(Th-F-Th) might be ascribed to narrower potential
window compared with that of P(EDOT-F-EDOT) during
redox process, as shown in Figure 6 (P(Th-F-Th) between
20.42 and 0.94 V; P(EDOT-F-EDOT) between 21.1 and
0.5 V).
FIGURE 7 Long-term redox stability of P(Th-F-Th) (A) and
P(EDOT-F-EDOT) (B) films in monomer-free CH2Cl2-Bu4NPF6
(0.1 mol L21) at the potential scan rate of 150 mV s21. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
of polymers were determined carefully by CV in monomer-
free CH2Cl2-Bu4NPF6 (0.1 mol L21), as shown in Figure 6.
The steady-state CVs of polymers represented broad anodic
and cathodic peaks. The peak current densities, jp,a and jp,c
(Fig. 6), were all proportional to the scan rate, which indi-
cated that a reversible redox couple was fixed on the elec-
trode and the electrochemical processes were non-diffusion
controlled.21 Furthermore, there is
a stark difference
between the anodic and cathodic peak potentials in CV
curves, which due to hysteresis effect during redox process.
The main reasons are usually as follows32: slow heterogene-
ous electron transfer, local rearrangement effect of polymer
chains, slow mutual transformation of various electronic spe-
cies, and electronic charging of interfacial exchange corre-
sponding to the metal/polymer and polymer/solution
interfaces, etc. Significantly, the driving voltages of P(EDOT-
F-EDOT) was lower than that of P(Th-F-Th). The differences,
corresponding with the highest occupied states in the
valence band,23 were commonly observed in the electro-
chemistry of CPs and may be due to a number of factors
including the ease of diffusion of dopants in and out of the
FIGURE 8 AFM images of P(Th-F-Th) (A) and P(EDOT-F-EDOT)
(B) films deposited electrochemically on the ITO glass elec-
trode. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 9 Spectroelectrochemistry for P(Th-F-Th) (A) and P(EDOT-F-EDOT) (B) electropolymerized on ITO coated glass in
monomer-free CH2Cl2-Bu4NPF6 (0.1 mol L21). [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
Morphology
370 nm. Fortunately, once the applied potential reached 1.2
V, the absorption band at 700 nm began to increase, which
led to the color change from yellow (L* 5 88, a* 5 24,
b* 5 22) to dark green (L* 5 81, a* 5 212, b* 5 6).
The surface morphologies of CPs are closely related to their
properties, including electrical conductivity, redox activity,
and stability, etc.37 Therefore, the surface of the doped poly-
mer films deposited electrochemically on the ITO electrode
was observed by AFM images. As shown in Figure 8, the rms
roughnesses of P(Th-F-Th) and P(EDOT-F-EDOT) were 12.60
and 4.23 nm, respectively, indicating that the morphology of
P(EDOT-F-EDOT) was smoother than that of P(Th-F-Th).
In addition, the optical band gap (Eg) of these polymers was
calculated on the basis of the formula Eg 5 1240/konset
,
where konset is the onset of p-p* transition. The band gap of
the P(Th-F-Th) was 2.1 eV, and the value was higher than
that of P(EDOT-F-EDOT; 1.7 eV). Compared with P(EDOT-FE;
Eg 5 1.96 eV).
Electrochromic Properties
Spectroelectrochemical and optical switching studies of these
polymers were performed in monomer-free CH2Cl2-Bu4NPF6
(0.1 mol L21) to evaluate their electronic properties and
band gaps and, consequently, to elucidate the function of the
different terminal unit on the electronic properties of poly-
mer films. As can be seen in Figure 9, P(EDOT-F-EDOT)
revealed two well-defined absorption maxima at 364 and
500 nm in the neutral state; however, P(Th-F-Th) only shown
one absorption band at 370 nm. It showed that the electron
rich nature of EDOT resulted in a red shift in the spectrum.
Furthermore, the absorption band of P(EDOT-F-EDOT)
shifted to a longer wavelength of about 30 nm as compared
to the analog P(EDOT-FE).9 The result demonstrated that
alkyl chain was beneficial to the increase of the conjugated
block. Upon oxidation, P(EDOT-F-EDOT) showed the slightly
fading of absorbance at 500 nm and typical evolution of
peaks at more than 700 nm. Further oxidation, the absorp-
tion band at 775 nm began to decrease, whereas that in the
NIR regime (1080 nm) increased, indicating the transition
from polaron to bipolaron.19
P(EDOT-F-EDOT) also revealed lower band gap, which could
be ascribed to the longer conjugated block. However, the
combination of EDOT and 9,9-dioctyl-9H-fluorene units
under the same umbrella resulted in a hybrid compound,
which has the intermediate band gap between PF (2.95
eV)38 and PEDOT (1.61 eV).23 It can be concluded that the
band gap can be modified by incorporating Th/EDOT units
into PF backbone.
The electrochromic parameters, such as optical contrast ratio
(᭝T%) and response time are summarized in Table 4. The
potentials were switching alternatively between reduced and
oxidized states with residence time of 5 s.
TABLE 3 Colorimetric Data for Synthesized Polymers
CIE color
Colors of
polymers
coordinates
Polymers
Neutral Oxidized Neutral Oxidized
P(Th-F-Th)
L* 88
81
As shown in Table 3, these changes of P(EDOT-F-EDOT) in
the absorption spectra were accompanied with color changes
from light gray (L* 5 79, a* 5 25, b* 5 214) to blue
(L* 5 70, a* 5 22, b* 5 0) during doping process. Compared
with P(EDOT-F-EDOT), P(Th-F-Th) showed unsatisfactory
electrochromic property. As the applied potential increased,
P(Th-F-Th) showed scarcely discernible absorption change at
a* 24
212
6
b* 22
P(EDOT-F-EDOT) L* 70
a* 22
79
25
214
b*
0
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TABLE 4 Electrochromic Parameters of P(EDOT-F-EDOT) and P(EDOT-FE)
Response time (s)
Oxidation Reduction
Polymers
k (nm)
Tred
Tox
DT
CE (cm2/C)
Eg (eV)
P(EDOT-F-EDOT)
500
775
1080
470
61.4%
75.8%
79.5%
16%
87.2%
68.8%
68.0%
41%
25.8%
7.0%
11.5%
25%
1.2
0.6
2.6
1.6
1.6
3.8
3.4
1.0
220
22
1.70
106
415
P(EDOT-FE)9
1.96
FIGURE 10 Electrochromic switching: (A) P(EDOT-F-EDOT) was switched between 20.5 and 1.0 V with a switch time of 10, 5, 3,
and 2 s at 500 nm; (B) optical absorbance change monitored for P(EDOT-F-EDOT) in monomer-free CH2Cl2-Bu4NPF6 (0.1 mol L21).
Switching time: 5 s. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
In the visible region (500 nm), P(EDOT-F-EDOT) showed 23.9%
transmittance change between the neutral and oxidized states
which was especially significant when considering the use in win-
dow- or display-type devices. The optical contrast was found to
be 11.5% at 1080 nm. Herein, P(EDOT-F-EDOT exhibited similar
electrochromic feature with P(EDOT-FE; Table 4), whereas supe-
rior than some analogs, such as poly(2,7-bis(2,3-dihydro-
thieno[3,4-b][1,4]dioxin-5-yl)250H-spiro[fluorene-9,40-pyrrolo
[1,2-a]quinoxaline])(PEQE).39 Unfortunately, the quality of the
polymer P(Th-F-Th) film coated on ITO was not good, so it did
not exhibit acceptable performance during polymer switching
between its redox states. The polymer film is not stable and
robust under ambient conditions and lost its electroactivity
after a few cycles (Fig. 10).
criterion for identifying the performance of electrochromic
material. CE of P(EDOT-F-EDOT) were calculated as
220 cm2 C21 (at 500 nm) and 106 cm2 C21 (at 1080 nm),
respectively (Table 4). The value was apparent inferior than
that of its analog P(EDOT-FE), whereas superior than that of
poly(heterocycle-fluorenes-heterocycle) electrochromic mate-
rials including PEQE and poly(2,7-di(thiophen-2-yl)250H-spi-
ro[fluorene-9,40-pyrrolo[1,2-a]quinoxaline]) (PTQT).39
In addition, the optical stability was controlled by using a
square wave potential step method together with measuring
its spectra. The optical activity obtained after 100 cycles of
Response time is an important parameter for a polymer
since it indicates the speed of counter ions moving into and
out of the polymer chains during the doping process.
P(EDOT-F-EDOT) exhibited fast response time between neu-
tral and oxidized state and achieved 95% of the optical con-
trasts within 0.6–3.8 s at all three wavelengths (Table 4).
Especially, the response time during oxidation process was
obvious faster than that of P(EDOT-FE) (1.6 s).9 It was noted
that the response time during oxidation was faster than the
reduction process, which could be ascribed to the difference
in charge transport rates between the two processes.
CE, calculated on the basis of the following formulas
DOD 5 log(Tox/Tred) and CE 5 DOD/Qd, is also a significant
FIGURE 11 Optical stability at 431 nm between 20.5 and 1.0 V.
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In summary, the synthesis and electrochemical polymeriza-
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F-Th and EDOT-F-EDOT) were comprehensively reported.
Structure-property relationships between P(Th-F-Th) and
P(EDOT-F-EDOT), including optical property, surface mor-
phology, electrochemical, and electrochromic performance,
were comparatively investigated. Based on electrochemical
result, the method employing Th/EDOT as the terminal
groups presents an effective way to reduce oxidation poten-
tial of monomer. Meanwhile, P(EDOT-F-EDOT) revealed good
electrochemical behavior and favorable electrochromic per-
formance, such as fast response time (0.6 s) and high CE
(220 cm2/C).
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ACKNOWLEDGMENTS
The authors are grateful to the National Natural Science Foun-
dation of China (grant number: 51303073, 51463008), Ganpo
Outstanding Talents 555 projects (2013), the Natural Science
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(YC2014-S441, YC2014-S431) for their financial support of this
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