Experimental and theoretical investigation of a new rapid switching near-infrared electrochromic conjugated polymer

Article abstract of DOI:10.1002/pola.24104

A new rapid switching near-IR electrochromic conjugated propeller-shape polymer (PBTPAFL) with lower oxidation potential containing a di-triarylamine group was synthesized via Suzuki coupling approach. The observed UV-vis-NIR absorption changes in the PBT

Full text of DOI:10.1002/pola.24104

Experimental and Theoretical Investigation of a New Rapid Switching
Near-Infrared Electrochromic Conjugated Polymer
HAN-YU WU,1 KUN-LI WANG,2 JYH-CHIANG JIANG,1 DER-JANG LIAW,1 KUEIR-RARN LEE,3 JUIN-YIH LAI,3 CI-LIANG CHEN1
1Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
2Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan
3R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan
Received 19 March 2010; accepted 24 April 2010
DOI: 10.1002/pola.24104
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A new rapid switching near-IR electrochromic con-
molecular weight conjugated polymer having high thermal
stability with Td10 more than 450 ꢀC has excellent solubility in
common organic solvents such as NMP, THF, chloroform, tol-
uene, xylene, and benzene at room temperature (25 ꢀC) due to
the propeller-shape structure and long alkyl chain on fluorene.
Herein, from the combination of the experimental and compu-
tational study, we proposed a mechanism on the basis of the
jugated propeller-shape polymer (PBTPAFL) with lower oxida-
tion potential containing
a
di-triarylamine group was
synthesized via Suzuki coupling approach. The observed UV-
vis-NIR absorption changes in the PBTPAFL film at various
potentials are fully reversible and associated with strong color
changes from the original light green to dark green and then
to a Prussian blue. Excellent continuous cyclic stability of the
molecular orbital theory to explain the electrochromic oxida-
C
V
electrochromic characteristics with
a
rapid color switching
tion behavior. 2010 Wiley Periodicals, Inc. J Polym Sci Part
time 2.58 s and bleaching time 1.76 s was found as well.
Compared with P1 and P2, the introduction of more electron-
donating propyl phenyl group in the para position of
PBTPAFL lowered the oxidative potential and prevented cou-
pling reaction during the electrochromic procedure. The high
A: Polym Chem 48: 3913–3923, 2010
KEYWORDS: computer modeling; conjugated polymers; electro-
chemistry; electrochromic; molecular orbital theory; near-IR;
rapid switching; theoretical
INTRODUCTION Organic conjugated monomers and polymers
have attracted a great deal of attention over the last three
decades since the discovery that conductivity could be
achieved in p-doped polyacetylene was made.1 These materi-
als have found use in many applications, such as photovol-
taics,2–5 light emitting diodes,6–10 field effect transistors,11–14
electrochromics,15–18 and polymer memories19–21 etc. Elec-
trochromic polymers, an important class of conducting mate-
rials, have received significant attention in the academy as
well as in industry due to their potential use in various
applications, such as in information display and storage,
rear-view mirrors (automotive industry), and smart win-
dows.22 Increasingly, near-infrared (NIR)- absorbing electro-
chromic materials are receiving more attention due to their
potential applications in biological sensing, optical communi-
cation, data storage, and thermal control and thermal emis-
sion detectors for spacecraft.23,24 Electrochromism is broadly
defined as the reversible and visible change in transmittance
and/or reflectance that is associated with an electrochemi-
cally induced oxidation or reduction. The color change is
commonly between a transparent state and a colored state
or between two colored states. For display devices, a poly-
mer that can switch between opaque and transparent states
is desirable as a polymer with a transparent conducting state
is applicable in antistatic coating technology.22
Triarylamine and its derivatives are well-known for photo
and electroactive properties that were found in optoelec-
tronic applications, such as photoconductors, hole transport-
ers, and light emitters.25–30 It also can be easily oxidized to
form radical cations, and the oxidation process is always
associated with a noticeable change of coloration. Thus,
many triarylamine-based electrochromic polymers have been
reported in the literature.31–36 However, the electro-gener-
ated cation radical of triarylamine is not stable and could
dimerize to form tetraphenylbenzidine by coupling reaction.
When the phenyl groups are substituted at the para position,
the coupling reactions are greatly prevented. Specifically,
incorporation of electron-donating substituents at the para
position of triarylamines afforded stable radical cations and
lowered the oxidation potentials.37–39
In this article, we report the new fluorene-based conjugated
polymer (PBTPAFL) containing propeller-shape di-triaryl-
amine with an electron-donating pendent propyl phenyl
groups. The introduction of electron-donating propyl phenyl
substituent is expected to prevent the coupling reaction and
Additional Supporting Information may be found in the online version of this article. Correspondence to: D.-J. Liaw (E-mail: liawdj@mail.ntust.edu.tw)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3913–3923 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
lower the oxidation potentials. In addition, the 2,2-diphenyl
propane structure in the polymer chain could enhance the
thermal stability and chemical resistance.40–42 The thermal
stability, electrochromic characteristics as well as theoretical
analysis of oxidation pathway of PBTPAFL in molecular orbi-
tals (MOs) view are investigated.
Synthesis of Bis[4-(2-phenyl-2-propyl)phenyl]-
4-nitrophenylamine (NTPA)
In a 500 mL three neck round-bottom flask, bis[4-(2-phenyl-2-
propyl)phenyl]amine (20.00 g, 49 mmol), 4-fluoro–nitroben-
zene (5.23 g, 49 mmol), sodium hydrate (1.17 g, 49 mmol), and
dimethyl sulfoxide (DMSO, 120 mL) were added. The mixture
ꢀ
was heated with stirring at 120 C for 48 h. The reaction mix-
EXPERIMENTAL
ture was cooled and then poured into 1 L methanol. The yellow
precipitate was collected by filtration and dried under vacuum.
The product was purified by silica gel column chromatography
(n-hexane : dichloromethane ¼ 2:1) to afford bis[4-(2-phenyl-
2-propyl)phenyl] nitrophenyl in a 50% yield (12.99 g); mp ¼
151 ꢀC by DSC, IR (KBr): 1493, 1296 cm-1 (NO2 stretch)
Materials
Bis[4-(2-phenyl-2-propyl)phenyl]amine was purchased from
Ouchi Shinko Chemical Industrial, 1-bromo-4-iodobenzene,
iodobenzne, bis(dibenzylideneacetone) palladium [Pd(dba)2],
1,10-bis(diphenylphosphino)- ferrocene (DPPF), sodium tert-
butoxide, and tetrakis (triphenylphosphine)palladium (0)
(Pd(PPh3)4) were purchased from Acros, 9,9-dioctylfluorene-
2,7-bis(trimethylene boronate) was purchased from Aldrich
and recrystalized from ethyl acetate and n-hexane before
used. Tetrabutylammonium perchlorate (TBAP) was pur-
chased from Acros and recrystallized twice by ethyl acetate
under nitrogen atmosphere and then dried in vacuo prior to
use. DMSO was dried and distilled over calcium hydride
under an inert atmosphere. Toluene was dried and distilled
over sodium under an inert argon atmosphere. All other
reagents were used as received from commercial sources
1H NMR. (CDCl3): d (ppm) ¼ 1.77 (s, 12H, Hd), 6.93–6.95 (d,
4H, Hf, J ¼ 9.4 Hz); 7.12–7.14 (d, 4H, He, J ¼ 10.05 Hz),
7.24–7.27 (m, 2H, Ha), 7.28–7.30 (d, 2H, Hh, J ¼10.05 Hz),
7.35–7.36 (m, 4H, Hb), 7.37–7.38 (d, 4H, Hc, J ¼ 5.02 Hz),
8.05–8.08 (d, 2H, Hg, J ¼ 9.3 Hz)
13C NMR (CDCl3): d (ppm) ¼ 30.65 (C6), 42.62 (C5), 117.42
(C12), 125.31 (C13), 125.71 (C1), 125.88 (C9), 126.62 (C2),
128.03 (C3), 128.13 (C8), 139.62 (C14), 142.80 (C10), 148.26
(C7), 150.01 (C4) 153.46 (C11).
Elem Anal. Calcd for C36H34N2O2: C, 82.10%; H, 6.51%; N,
5.32%. Found: C, 81.87%; H, 6.39%; N, 5.21%.
Measurements
IR spectra were recorded in the range 400–4000 cmꢁ1 for the
synthesized monomers and polymer on a KBr disk (Bio-Rad
Digilab FTS-3500). Elemental analysis was made on a Perkin-
Elmer 2400 instrument. NMR spectra were recorded using a
Bruker Avance 500 NMR. Weight-average (Mw) and number-av-
erage (Mn) molecular weights were determined by gel permea-
tion chromatography (GPC). Four Waters (Ultrastyragel) col-
umns (300 mm ꢂ 7.7 mm, guard, 105, 104, 103, and 500 Å in
series) were used for GPC analysis with tetrahydrofuran (THF, 1
mL minꢁ1) as the eluent. The eluents were monitored with a UV
detector (JMST Systems, VUV-24, USA) at 254 nm. Polystyrene
was used as the standard. Thermogravimetric data were
obtained on a TA instrument Dynamic TGA 2950 under nitrogen
flowinꢀg condition at a rate of 20 cm3 minꢁ1 and a heating rate
of 10 C minꢁ1. Differential scanning calorimetric analysis was
performed on a differential scanning calorimeter (TA instru-
ment TA 910) under nitrogen flowing condition at a rate of 50
cm3 minꢁ1 and a heating rate of 10 ꢀC minꢁ1. UV-vis spectra of
the polymer films or solutions were recorded on a JASCO V-550
spectrophotometer at room temperature in air. The fluorescence
spectra were recorded by a Shimadzu RF-5031 spectrophotom-
eter. Cyclic voltammetry was performed with CHI model 619A
with ITO was used as a working electrode and a platinum wire
as an auxiliary electrode at a scan rate of 50 mV/s against a Ag/
Agþ reference electrode in a solution of 0.1 M tetrabutylammo-
nium perchlorate (TBAP)/acetonitrile (CH3CN). The spectroe-
lectrochemical cell composed of a 1 cm cuvette, an ITO as a
working electrode, a platinum wire as an auxiliary electrode,
and a Ag/Agþ reference electrode. Absorption spectra in spec-
troelectrochemical analysis were measured with a JASCO V-670
spectrophotometer. Photoluminescence spectra were measured
with a Jasco FP-6300 spectrofluorometer.
Synthesis of Bis[4-(2-phenyl-2-propyl)phenyl]-
4-aminophenylamine (ATPA)
In a 500 mL three neck round-bottom flask equipped with a
stirrer bar under nitrogen atmosphere, nitro compound
(NTPA) (20 g, 38 mmol ), Pd/C (0.4 g), ethanol 200 mL were
added. After 14 mL hydrazine monohydrate was slowly added
to the mixture from a dropping funnel, the solution was
stirred at reflux temperature for 12 h. Then the solution was
cooled down to room temperature, filtered to remove the cat-
alyst, and the crude product was recrystallized from ethanol
ꢀ
to afford 12.5 g (yield: 66%) of white solid, mp 110–112 C,
IR (KBr) 3432, 3356 cmꢁ1 (NH2 stretch).
1H NMR (DMSO-d6): d (ppm) ¼ 1.57 (s, 12H, Hd), 5.02
(s, 2H, NH2), 6.56–6.58 (d, 2H, Hg, J ¼ 10 Hz), 6.77 (s, 4H,
Hf), 6.79 (s, 4H, He), 6.98–7.00 (d, 2H, Hh, J ¼ 10 Hz), 7.09–
7.12 (m, 2H, Ha), 7.19–7.20 (d, 4H, Hb, J ¼ 5 Hz), 7.20–7.21
(d, 4H, Hc, J ¼ 5 Hz).
13C NMR (DMSO-d6): d (ppm) ¼ 30.3 (C6), 41.7 (C5), 114.9
(C13), 120.73 (C9), 125.3 (C1), 126.2 (C3), 126.9 (C8), 127.8
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(C2), 127.9 (C12), 135.2 (C14), 142.5 (C10), 145.4 (C7), 145.9
(C11), 150.3 (C4).
propyl)phenyl] dibromo compound (BBTPA) (200 mg, 0.25
mmol) were added. The flask equipped with a condenser,
which was then evacuated and filled with nitrogen several
times to remove traces of air, then the degassed toluene 5
mL was added. Once the two monomers were dissolved, tet-
rakis (triphenylphosphine)palladium(0) (Pd(PPh3)4, 8.6 mg,
0.007 mmol) and 3M aqueous sodium carbonate solution
(2 mL) were added, and the mixture was stirred for 48 h at
105 ꢀC under nitrogen. The reaction mixture was cooled to
room temperature, and the organic layer were separated,
washed with water, and precipitated into methanol. The
green polymer sample was filtered, washed with excess
Elem Anal. Calcd for C36H34N2: C, 87.05%: H, 7.31%, N,
5.64%. Found: C, 86.9%; H, 7.13%, N, 5.61%
methanol, dried, and purified by
a Soxhlet extraction
with methanol for 2 days. IR (KBr) 1269 cmꢁ1 (ArAN ),
3029 cmꢁ1 (ArAH stretch).
Synthesis of N,N-bis(4-bromophenyl)-N0,N0-di(4-
isopropylphenyl)-1,4-phenylenediamine (BBTPA)
1H NMR.(CDCl3): d (ppm) ¼ 0.75 (br, 4H, Ho), 0.79–0.82 (m,
6H, Hu), 1.08–1.22 (m, 20H, Hp, Hq, Hr, Hs, and Ht), 1.68 (t,
12H, Hd), 2.05 (br, 4H, Hn), 7.03–7.06 (br, 6H, He and Hh),
7.11 (br, 6H, Hf and Hg), 7.17–7.20 (m, 4H, Hi), 7.25 (br, 2H,
Ha), 7.28–7.30 (m, 8H, Hc and Hj), 7.57–7.60 (m, 8H, Hb, Hm,
and Hk), 7.75–7.77 (m, 2H, Hl).
1-Bromo-4-iodobenzene (3.4 g, 12 mmol), ATPA (2.9 g, 6
mmol), Pd(dba)2 (0.07 g, 0.12 mmol), DPPF (0.13 g, 0.24
mmol), sodium tert-butoxide (2.6 g, 27 mmol) and dry tolu-
ene (10 mL) were charged in a three-necked flask kept
under nitrogen. The mixture was heated to reflux for 10 h.
After the completion of the reaction, the solvent was
removed under vacuum, and the residue was extracted with
dichloromethane–water. The organic layer was dried over
MgSO4 and filtered. After removing the solvent, the residue
was purified by silica gel column chromatography (toluene:
n-hexane ¼ 1:4). Then the product was recrystallized from
ethyl acetate, and the white solid was obtained in a 65ꢁ%1
yield (3.06 g), mp 225 ꢀC by DSC. IR (KBr) 1070 cm
(ArABr), 1286 cmꢁ1 (ArAN), 3032 cmꢁ1 (ArAH stretch)
13C-NMR (CDCl3): d (ppm) ¼ 14.06, 22.59, 23.82, 25.61,
29.21, 29.22, 30.05, 30.77, 31.78, 40.50, 42.47, 55.21, 67.96,
119.88, 120.90, 123.15, 123.69, 125.22, 125.49, 125.55,
125.90, 126.77, 127.49, 127.78, 127.97, 135.48, 139.36,
139.74, 144.70, 145.29, 146.93, 150.75, 151.63.
1H NMR (CDCl3): d (ppm) ¼ 1.71 (s, 12H, Hd), 6.92–6.97 (m,
6H, Hh, and Hi); 6.99–7.01 (m, 6H, He and Hg), 7.12–7.14 (d,
4H, Hf, J ¼ 8.6 Hz), 7.18–7.22 (m, 2H, Ha), 7.30–7.31 (d, 8H,
Hb and Hc, J ¼ 4.3 Hz), 7.34–7.36 (d, 4H, Hj, J ¼ 8.8 Hz)
13C NMR (CDCl3): d (ppm) ¼ 30.7(C6), 42.5 (C5), 114.9 (C18),
123.4 (C8), 124.7 (C12), 124.8 (C16), 125.6 (C1), 125.9 (C13),
126.73 (C2), 127.5 (C9), 128.0 (C3), 132.2 (C17), 140.9 (C14),
144.1 (C11), 144.9 (C10), 145.0 (C7), 146.6 (C15), 150.6 (C4)
Synthesis of the Model Compound, (M1)
Iodobenzene (1.4 g, 7 mmol), bis[4-(2-phenyl-2-propyl)-
phenyl]amine(2.4 g,
6 mmol), Pd(dba)2 (0.07 g, 0.12
Elem Anal. Calcd for C48H42Br2N2: C, 71.47%: H, 5.25%, N,
3.47%. Found: C, 72.00%; H, 5.30%; N, 3.09%
mmol), DPPF (0.13g, 0.24 mmol), sodium tert-butoxide
(2.6 g 27 mmol) and dry toluene (10 mL) were charged
in a three-necked flask kept under nitrogen. The mixture
was heated to reflux for 8 h. After the completion of the
reaction, the solvent was removed under vacuum and the
residue was extracted with dichloromethane–water. The
organic layer was dried over MgSO4 and filtered. After
removing the solvent, the residue was purified by silica
gel column chromatography (toluene: n-hexane ¼ 1: 6).
Then the viscous product was obtained in a 55% yield
(1.57 g).
1H NMR (CDCl3): d (ppm) ¼ 1.67 (s, 12H, Hd), 6.96–6.98 (m,
6H, Hg and Hf), 7.06–7.09 (m, 5H, He and Hi), 7.15–7.24 (m,
4H, Ha and Hh), 7.27–7.30 (d, 8H, Hb and Hc, J ¼ 10 Hz)
Synthesis of the Conjugated Polymer, (PBTPAFL)
To a three-necked flask 9,9-dioctylfluorene-2,7-bis(trimethy-
leneboronate) (138.4 mg, 0.25 mmol) and bis[4-(2-phenyl-2-
13C NMR (CDCl3): d (ppm) ¼ 30.7 (C6), 42.45 (C5), 122.18
(C12), 123.5 (C9), 123.74 (C14), 125.54 (C1), 126.74 (C3),
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of the
dibromo monomer and conju-
gated polymer
127.46 (C8), 127.95 (C2), 129.03 (C13), 144.83 (C11), 145.25
(C10), 147.92 (C7), 150.71 (C4)
ysis, FTIR, and NMR spectra. The characteristic band for the
aryl bromide group of BBTPA monomer was observed at
1070 cmꢁ1 in the FTIR spectrum. Figure 1 shows the 1H
NMR [Fig. 1(a)] and 13C NMR [Fig. 1(b)] spectra as well as
two dimensional spectra including COSY [Fig. 1(c)] and
HMQC [Fig. 1(d)] of the dibromo monomer. The assignments
of the proton and the carbon peaks are assisted by the
integral of proton peaks and the two-dimensional COSY
[Fig. 1(c)] and HMQC [Fig. 1(d)] techniques. The basic COSY
and HMQC procedures give two dimensional spectra from
which almost all of the H-1H and H-13C connectivity can be
determined. All of the aromatic protons of BBTPA are in the
range of 6.9–7.4 ppm, only the methyl protons appear at
1.71 ppm. Elemental analysis, IR, and NMR spectra clearly
confirm that the intermediate compounds and BBTPA syn-
thesized herein were consistent with the proposed structure.
The conjugated polymer PBTPAFL was synthesized by
Suzuki polycondensation of an equimolecular mixture of the
dibromide (BBTPA) and diboronate. The polymerization was
carried out for 48 h using 3 mol % Pd(PPh3)4 as the catalyst
in a two-phase mixture of degassed toluene and aqueous
K2CO3 (3M). The chemical structure of the polymer was veri-
fied by FTIR and NMR spectra as described in experimental.
The 1H NMR spectrum and peak assignments of the pre-
pared conjugated polymer can be find in the supporting
information.
1
1
Synthesis of the Model Polymer (M2)
The model polymer (M2) was prepared according to the pro-
cedures described in the published article.43
RESULTS AND DISCUSSION
Synthesis and Characterization
The number-average molecular weight of the prepared poly-
mer was as high as about 20,400 and the polydispersity
index was 1.79. The polymer appears good solubility in com-
mon organic solvents, such as NMP, THF, dichloromethane,
chloroform, toluene, xylene, and benzene at room
temperature.
The general synthetic route toward the monomers and poly-
mer was outlined in Scheme 1. The ATPA was prepared from
the condensation of bis[4-(2-phenyl-2-propyl)phenyl]amine
with 4-fluoronitrobenzene followed by palladium-catalyzed
reduction. The dibromo compound, BBTPA, containing
N,N,N0,N0-tetraphenyl-1,4-diphenylenediamine group, was
synthesized via facile Buchwald-Hartwig reaction from ATPA
and 1-bromo-4-iodobenzene. The structure of intermediate
compounds and BBTPA was all confirmed by elemental anal-
Thermal Properties
The thermal stability of PBTPAFL under nitrogen was deter-
mined by thermogravimetry analysis (TGA), and their phase
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FIGURE 1 (a) 1H NMR spectra of monomer BBTPA (b) 13C NMR spectra of monomer BBTPA (c) COSY spectra of monomer BBTPA
(d) HMQC spectra of monomer BBTPA. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
transition behavior was evaluated with differential scanning
calorimetry (DSC). The TGA traces in nitrogen and air atmos-
phere are sketched in Figure 2. The PBTPAFL showed excel-
lent thermal stability with decomposition temperature at
435 and 453 ꢀC in air and nitrogen, respectively. The glass
transition temperature of PBTPAFL was 137 oC, which is
much higher than that of poly(9,9-dioctylfluorene) (POF, Tg
¼ 66 ꢀC).43 The conjugated polymer PBTPAFL could obtain
the higher glass transition temperature than poly(9,9-dioctyl-
fluorene) due to introduction of rigid N,N,N0,N0-tetrasub-
stitued- 1,4-diphenylenediamine moieties44,45 into the back-
bone of poly(9,9-dioctylfluorene).
their photophysical properties are summarized in Table 1.
For PBTPAFL in the toluene solution, a major absorption at
around 387 nm and a minor absorption at 313 nm were
observed. Because of the maximum absorption spectra of
BBTPA (319 nm) in toluene is near 313 nm, we ascribe the
minor absorption at 313 nm to the p-p* transition of triphe-
nylamine groups. As the maximum absorption peak for
poly(9,9-dioctylfluorene) is 387 nm,43 we ascribed the major
absorption to p-p* transition derived from the conjugated
polymer backbone. The absorption spectra in solid state are
similar to that in toluene solution. The absorption peaks of
the solid film were red-shifted to 319 and 388 nm due to
the interchain interaction of the polymer. Upon excitation at
the maximum absorption wavelength, the PBTPAFL and
BBTPA exhibit blue emission color in toluene solution with
emission peaks at 451 and 409 nm. The emission peak of
Optical Properties
The optical absorption and emission spectra of PBTPAFL and
BBTPA in toluene and solid films are shown in Figure 3, and
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Photophysical Properties of Conjugated Polymers and
the Monomer
Solution (Toluene)
PL
Film
Polymer
Code
Abs
UPL
(%)a
Abs
PL
(nm)
(nm)
(nm)
(nm)
PBTPAFL
POF43
BBTPA
a
313, 387
387
451
414
409
87
85
—
319, 388
389
466
435
429
319
326
Quantum yield (within an error margin of 63%) in toluene solution
was calculated in an integrating sphere with 9,10-diphenylanthracene as
the standard (UF ¼ 0.9).
coated glass substrate as the working electrode in dry aceto-
nitrile (CH3CN) containing 0.1 M of TBAP as an electrolyte
under nitrogen atmosphere. Oxidative and reductive cycle of
the polymer is shown in Figure 4(a). The PBTPAFL showed
two reversible oxidation redox couples at E1/2 values of 0.72
(Eonset ¼ 0.75 V) and 1.05 V in the oxidative scan. Com-
paring with previous result,39 the electron-donating propyl
phenyl substituted PBTPAFL showed lower oxidative poten-
tial than the P1(E1/2¼ 0.87 V), which is 2,4,4-trimethyl pen-
tan-2-yl substituted and P2 (E1/2¼ 0.93 V) which is without
FIGURE 2 TGA and DSC results of PBTPAFL. (heating rate of
20 ꢀC/min). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
BBTPA was partially overlapped by the absorption of the poly-
mer that revealed a Fo¨rster energy transfer phenomenon.46
The fluorescent quantum yield for PBTPAFL was 87% in tolu-
ene solution. The solid state PL spectra were red-shifted by 15
and 20 nm from the corresponding solution spectra. This phe-
nomenon is probably attributable to intermolecular interac-
tions that planarize segments in the monomers/polymers lead-
ing to better conjugation in the solid state.47,48 The emissions
of PBTPAFL were red-shifted both in solution and solid state
as compared to BBTPA because of the enhanced conjugated
length by Suzuki coupling reaction.
Electrochemical Properties
The electrochemical behavior of PBTPAFL was investigated
by cyclic voltammetry conducted by film cast on an ITO-
FIGURE 4 Cyclic voltammograms of (a) PBTPAFL, P1, and P2 in
film on an indium-tin oxide (ITO)- coated glass substrate in CH3CN
containing 0.1 M TBAP. The scanning rate is 0.1 V/s. (b) Cyclic vol-
tammograms of PBTPAFL, M1, and M2 in film on an indium-tin ox-
ide (ITO)-coated glass substrate in CH3CN containing 0.1 M TBAP.
The scanning rate is 0.1 V/s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 3 Normalized UV and PL spectra of PBTPAFL and
BBTPA. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
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TABLE 2 Electrochemical Properties of the PBTPAFL and model compounds
a
E1/2 (V)
HOMOd (eV)
LUMOd (eV)
b
c
First
Second
Eonset (V)
Eg (eV)
PBTPAFL
0.72
1.09
1.16
1.05
—
0.75
1
2.81
3.49
2.88
ꢁ4.93
ꢁ5.18
ꢁ5.19
ꢁ2.12
ꢁ1.69
ꢁ2.37
M1
M2
a
—
1.03
d
E
1/2: average potential of the redox couple peaks
Calculated from the equation: HOMO ¼ ꢁ(Eox
ꢁ EFconset) ꢁ4.8,
onset
Eox/onset (Fc/Fc vs. Ag/Agþ) ¼ 0.62 V.
LUMO ¼ HOMO þ band gap.
b
c
The data were calculated from polymer films by the equation Eg
¼
1240/konset
.
the substituent [Fig. 4(a)], indicating that the more electron-
donating group introduced, the more oxidative potential low-
ered. From the oxidation potential relative to ferrocene/fer-
rocenenium, corresponding to ꢁ4.8 eV for ferrocene below
the vacuum level,49 we can approximately calculate the
HOMO energy level of the conjugated polymer. The LUMO
level of the polymer was calculated according to the equa-
tion: LUMO ¼ HOMO þ Eg.39 The electrochemical properties
of PBTPAFL and model compounds (M1 and M2) are sum-
marized in Table 2. The onset oxidation potentials are situ-
ated from 0.75 to 1.03 V, from which their HOMO levels are
estimated to be ꢁ4.93 to ꢁ5.21 eV [Fig. 4(b)]. From the op-
tical bandgaps, the LUMO levels are ꢁ1.69 to ꢁ2.37 eV.
Because of the stability of the film and the good adhesion
between the polymer and ITO substrate, the PBTPAFL exhib-
ited excellent reversibility of electrochromic characteristics
in 900 sec continuous cyclic scans between 0.0 and 1.14 V,
changing color from light green to deep green and then
Prussian blue. The electrochromic characteristics are dis-
cussed below.
and M2 are very different with those of PBTPAFL, indicating
that the oxidations do not result only from the electron re-
moval of nitrogen atoms. Many previous literatures proposed
the mechanism of di–triarylamine polymer oxidation mecha-
nism.37,39,49–52 They supposed the first electron was
removed from the nitrogen atom with larger electron density
whereas the second electron was removed from the other
nitrogen atom, which is close to atomic orbital opinion. How-
ever, from the electrochemical study of model compounds
and the PBTPAFL as abovementioned, it is not suitable to
explain them by atomic orbital opinion. If the oxidations
occurred only in the nitrogen atoms, the oxidation peaks
should have appeared in the same bias potentials. Herein,
we try to use a theoretical approach to give a more reasona-
ble interpretation for the oxidation behavior.
All theoretical calculations in this study were carried out
using quantum mechanical package Gaussian 03.53 Equilib-
rium structure for each fluorene-based polymer was deter-
mined using DFT with the B3LYP functional and the 6-31G*
basis set. It has been shown that B3LYP/6-31G* gives decent
ground state structures of conjugated polymers.54 The sketch
map of the studied basic structure and optimized structure
by B3LYP/6-31G* are plotted in Figure 5. The electronic
Theoretical Study of PBTPAFL for Oxidation Mechanism
Cyclic voltammograms of M1, M2, and PBTPAFL are shown
in Figure 4(b). The position of the oxidation peaks of M1
FIGURE 5 Sketch map of the studied structure (M3) and optimized structure by DFT/B3LYP/6-31G*. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
RAPID SWITCHING ELECTROCHROMIC CONJUGATED POLYMER, WU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Selected Atomic Charge Distribution of Part Atoms in Ground State, Oxidation State 1, Oxidation State 2, and Charge
Difference of DQ1 and DQ2
G. Sa
Ox1b
Ox2c
DQ1d
0.041142
DQ2e
0.014702
1 N
ꢁ0.659187
0.292118
0.273066
0.290841
ꢁ0.661311
0.285703
0.286134
ꢁ0.062142
0.101624
ꢁ0.192804
ꢁ0.618045
0.260064
0.306211
0.250610
ꢁ0.625383
0.246415
0.249998
ꢁ0.065516
0.103355
ꢁ0.187125
ꢁ0.603343
0.265325
0.289432
0.256052
ꢁ0.609480
0.242756
0.243338
ꢁ0.101586
0.121779
ꢁ0.172132
2 C
ꢁ0.03205
0.033145
ꢁ0.04023
0.035928
ꢁ0.03929
ꢁ0.03614
ꢁ0.00337
0.001731
0.005679
0.005261
ꢁ0.01678
0.00544
12 C
22 C
32 N
33 C
43 C
130 C
132 C
133 C
0.015903
ꢁ0.00366
ꢁ0.00666
ꢁ0.03607
0.018424
0.014993
a
d
e
G. S: ground state.
DQ1 ¼ atomic charge difference of Ox1 and G. S.
DQ2 ¼ atomic charge difference of Ox2 and Ox1.
b
Ox1: losing one electron.
Ox2: losing two electrons.
c
states of neutral structure and oxidized structures of
PBTPAFL were simulated and the main results were sum-
marized in Table 3. (see detailed distribution in the support-
ing information) The main atomic charge differences were
located on 1N, 2C, 12C, 22C 32N, 33C, 43C atoms. For the
first oxidation (losing first electron), the 1N, 12C, and 32N
atoms contribute 4.1, 3.3, and 3.6% electron, whereas the
2C, 22C, 33C, and 43C contributed 3.2, 4, 3.6, and 3.9% elec-
tron, respectively. For the second oxidation, the 1N, 32N,
132C, and 133C atoms contribute 1.5, 1.6, 1.8, and 1.5% of
an electron, whereas the 12C and 130C get 1.7 and 3.6% of
an electron, respectively. The electron density contour of
ground state and oxidation states were plotted by Gauss
View as shown in Figure 6, which illustrates the electron
density distribution of HOMO state of basic unit (ground
state) was mainly located on the N,N,N0,N0-tetraphenyl phen-
ylene diamine moiety, and extended to the fluorene moiety
for the SOMO electronic state of the first oxidation state.
From Figure 6, it is obvious that the electronic density con-
tours also show that the electron lone pair of the nitrogen
atoms have strong coupling with the p electrons. Therefore,
we proposed a new oxidation mechanism of PBTPAFL as
shown in Scheme 2. We supposed that the first electron was
removed from the HOMO of the molecule instead of the lone
pair electron of nitrogen atom, forming a single occupied
molecular orbital (SOMO). The second electron was removed
from SOMO to form second oxidation state. In conclusion, we
proposed a new oxidation mechanism on the basis of the
molecular orbital theory. That is, the electron removal (oxi-
dation) of the molecule was contributed by the all atoms in
the HOMO (first oxidation) or SOMO (second oxidation) of
the molecule and not only by nitrogen atom.
Electrochromic Characteristics
Electrochromism of the polymer thin film was examined by
an optically transparent thin-layer electrode (OTTLE)
coupled with UV-vis-NIR spectroscopy. The electrode prepa-
rations and solution conditions were identical to those used
in cyclic voltammetry. The typical electrochromic absorbance
spectrum of PBTPAFL is shown in Figure 7. Upon first oxida-
tion (increasing applied voltage from 0 to 1.05 V), the inten-
sity of the absorption peak at 387 nm gradually decreased,
FIGURE 6 Electronic density contours of
the frontier orbitals for optimized PBTPAFL
at ground state and first oxidation state.
[Color figure can be viewed in the online
issue,
which
is
available
at
wileyonlinelibrary.com.]
3920
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
SCHEME 2 Proposed mechanism of poly-
mer oxidation
whereas a new broadband having its maximum absorption
wavelength at 1050 nm in the NIR region gradually
increased in intensity. When the potential was adjusted to a
more positive value of 1.46 V, corresponding to the second
oxidation, the broadband with maximum absorption at 1050
nm decreased gradually, and a new broadband with two
peaks centered at around 610 and 865 nm rose. The
observed UV-vis-NIR absorption changes in the PBTPAFL
film at various potentials were fully reversible, which was
associated with strong color changes from original light
green to dark green and then to a Prussian blue of the oxi-
dized form as shown in Figure 7.
very stable and has good adhesion with the ITO substrate. In
Figure 6, the electronic density contours of the frontier orbi-
tals clearly show that the electron distribution of oxidized
PBTPAFL did not include the outside propylphenyl group,
indicating the coupling reaction is not easy to occur in this
polymer. This theoretical investigation is consistent with the
stable electrochromic experimental results.
CONCLUSIONS
A new rapid switching near-IR electrochromic conjugated
propeller-shape polymer with di–triarylamine group was
successfully synthesized via Suzuki coupling. Comparing with
previous result, excellent electrochromic performance was
observed from light green to dark green and then to a Prus-
sian blue with a quick switching time (2.58 s) and bleaching
time (1.76 s), lower oxidative potential (Eonset ¼ 0.75 V) and
continuous cyclic stability by the successful introduction of
the more electron-donating propylphenyl substituent. The
conjugated polymer (PBTPAFL) having highly thermal stabil-
ity with Td10 at 453 oC has excellent solubility in common
organic solvents at room temperature. Combining the experi-
ments and theoretical computation, we proposed a new oxi-
dation mechanism for PBTPAFL. The first electron was
removed from the highest occupied molecular orbital
(HOMO) instead of specific atoms like nitrogen atom.
The color switching times were estimated by applying a
potential step, and the absorbance profiles were followed
(Fig. 8). The switching time was calculated at 90% of the full
switch because it is difficult to perceive any further color
change with the naked eye beyond this point. Thin film of
PBTPAFL required only 2.58 s for switching time and 1.76 s
for bleaching time at 610 nm, which was much faster than
P1 (8.6s for switching time and 2.8 s for bleaching time) and
P2 (11.8 s for switching time and 3.4 s for bleaching time).39
After continuous cyclic scans between 0.0 V and 1.46 V in
900 s, the polymer films still exhibited excellent stability of
electrochromic characteristics, indicating that the film was
FIGURE 7 Electorchromic behavior of PBTPAFL thin film (in
CH3CN with 0.1 M TBAP As the supporting electrolyte) from 0
to 1.46 v (V vs Ag/Agþ).
FIGURE 8 Current consumption and potential step absorptom-
etry of PBTPAFL (in CH3CN with 0.1 M TBAP as the supporting
electrolyte) by applying a potential step (0.00– 1.42 V).
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
24 Qiao, W.; Zheng, J.; Wang, Y.; Zheng, Y.; Song, N.; Wan, X.;
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