Electrochromic material containing unsymmetrical substituted N,N,N0,N0-Tetraaryl-1,4-phenylenediamine: Synthesis and their optical, electrochemical, and electrochromic properties

Article abstract of DOI:10.1002/pola.23880

A novel dibromo compound containing unsymmetrical substituted bi-triarylamine was synthesized. A conjugated polymer was prepared via the Suzuki coupling from the newly prepared dibromo compound and 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate). The gl

Full text of DOI:10.1002/pola.23880

Electrochromic Material Containing Unsymmetrical Substituted
N,N,N 0,N 0-Tetraaryl-1,4-phenylenediamine: Synthesis and Their Optical,
Electrochemical, and Electrochromic Properties
HAN-YU WU,1 KUN-LI WANG,2 DER-JANG LIAW,1 KUEIR-RARN LEE,3 JUIN-YIH LAI3
1Department of Chemical Engineering, National Taiwan University of Science and Technology, 10607, Taipei, Taiwan
2Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 10608, Taipei, Taiwan
3R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, 32023, Chung-Li, Taiwan
Received 15 October 2009; accepted 13 December 2009
DOI: 10.1002/pola.23880
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A novel dibromo compound containing unsymmetri-
cal substituted bi-triarylamine was synthesized. A conjugated
polymer was prepared via the Suzuki coupling from the newly
prepared dibromo compound and 9,9-dioctylfluorene-2,7-bis(tri-
methyleneboronate). The glass transition temperature (Tg) of the
conjugated polymer was 140 ꢀC, 10% weight-loss temperatures
(Td10) in nitrogen was 458 ꢀC, and char yield at 800 ꢀC in nitrogen
higher than 64%. Cyclic voltammogram of the polymer film cast
onto an indium-tin oxide (ITO)-coated glass substrate exhibited
two reversible oxidation redox couples at 0.70 and 1.10 V versus
Ag/Agþ in acetonitrile solution. The polymer films revealed excellent
stability of electrochromic characteristics, with a color change from
yellow green of the neutral form to the dark green and blue of oxi-
dized forms at applied potentials ranging from 0 to 1.3 V. The color
switching time and bleaching time were 4.25 and 7.22 s for 860 nm
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and 5.51 s and 6.48 s for 560 nm.
2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 1469–1476, 2010
KEYWORDS: conjugated polymers; electrochemistry; electro-
chromic; naphthalene; synthesis
INTRODUCTION In the last two decades, p-conjugated poly-
mers have attracted considerable interests because of their
potential applications in electrochromics,1–4 light emitting
diodes,5–9 organic thin film transistors,10–14 photovoltaics,15–18
and polymer memories.19–20 Fluorene and its analogous
derivatives have drawn much attention of optoelectronics
because they generally have good solubility, high luminescent
efficiency, and very good charge-transfer mobility in both
neutral and doped states.21–24 However, it is also known
that they have drawbacks such as unsatisfied thermal
stability and excimers formation in the solid state.25,26 The
applications for electrochromic conjugated polymers are
quite diverse due to several favorable properties of these
materials, like stable oxidation state, fast switching times,
and excellent switching reproducibility.27 This excellent prop-
erties led to the development of many technological applica-
tions such as self-darkening rear-view mirrors, adjustably
darkening windows, large-scale electrochromic screens, and
chameleon materials.28–30 Electron-rich triarylamines are
known to be easily oxidized to form stable polarons, and the
oxidation process is always associated with a noticeable
change of the coloration. Furthermore, triarylamine-based
polymers are not only widely used as the hole-transport
layer in electroluminescent diodes but also show interesting
electrochromic behavior.31–33
In this article, we, therefore, synthesized a new dibromo
monomer containing unsymmetrical N,N,N0,N0-tetraaryl-sub-
stituted 1,4-phenylenediamine moiety. The unsymmetric
structure with naphthalene in the dibromo compound was
designed to enhance the thermal stability and solubility. A
conjugated polymer derived from the dibromide via Suzuki
coupling reaction with a fluorene derivative was prepared,
and its general properties such as thermal and optical prop-
erties as well as electrochemical and electrochromic property
were investigated and discussed in detail in this study.
EXPERIMENTAL
Materials
N-Phenyl-1-naphthylamine was purchased from Ouchi Shinko
Chemical Industrial, 1-bromo-4-iodobenzene, iodobenzene, bis
(dibenzylideneacetone) palladium [Pd(dba)2], 1,10-bis(diphenyl-
phosphino)-ferrocene (DPPF), sodium tert-butoxide and tetrakis
(triphenylphosphine) palladium(0) [Pd(PPh3)4] were purchased
from Acros, 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate)
was purchased from Aldrich and recrystallized from ethyl acetate
and n-hexane before used. Tetrabutylammonium perchlorate
(TBAP) was purchased 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
Additional Supporting Information may be found in the online version of this article. Correspondence to: K.-L. Wang (E-mail: klwang@ntut.edu.tw)
or D.-J. Liaw (E-mail: liawdj@mail.ntust.edu.tw)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1469–1476 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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.
129.65 (C2), 130.46 (C17), 135.14 (C12), 139.64 (C4), 141.15
(C18), 145.50 (C8), 153.70 (C5)
ELEM ANAL. Calcd. for C22H16N2O2 : C, 76.63% : H, 4.74%; N,
8.23%. Found: C, 76.16%; H, 4.66%; N, 8.35%.
Measurements
Infrared (IR) spectra were recorded in the range 400–4000 cmꢁ1
for the synthesized monomers and polymer in a KBr disk (Bio-Rad
Digilab FTS-3500). Elemental analysis was made on a Perkin-
Elmer 2400 instrument. Nuclear magnetic resonance (NMR) spec-
tra were recorded using a BRUKER AVANCE 500 NMR. Weight-av-
erage molecular weights (Mw) and number-average molecular
weights (Mn) were determined by gel permeation chromatography
(GPC). Four Waters (Ultrastyragel) columns (300 ꢂ 7.7 mm2,
guard, 105, 104, 103, and 500 Å in series) were used for GPC analy-
sis with tetrahydrofuran (THF; 1 mL minꢁ1) as the eluent. The elu-
ents were monitored with a UV detector (JMST Systems, VUV-24)
at 254 nm. Polystyrene was used as the standard. Thermogravi-
metric data were obtained on a TA instrument Dynamic thermog-
ravimetric analysis (TGA) 2950 under nitrogen flowing condition
at a rate of 20 cm3 minꢁ1 and a heating rate of 10 ꢀC minꢁ1. Differ-
ential scanning calorimetric analysis was performed on a differen-
tial scanning calorimeter (DSC; TA instrument 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 solu-
tions were recorded on a JASCO V-550 spectrophotometer at room
temperature in air. The fluorescence spectra were recorded by a
Shimadzu RF-5031 spectrophotometer. Cyclic voltammetry (CV)
was performed with CHI model 619A with indium-tin oxide (ITO)
was used as a working electrode and a platinum wire as an auxil-
iary electrode at a scan rate of 50 mV sꢁ1 against a Ag/Agþ refer-
ence electrode in a solution of 0.1 M tetrabutylammonium perchlo-
rate (TBAP)/acetonitrile (CH3CN). The spectroelectrochemical cell
was composed of a 1-cm cuvette, ITO as a working electrode, a
platinum wire as an auxiliary electrode, and a Ag/Agþ reference
electrode. Photoluminescence spectra were measured with a Jasco
FP-6300 spectrofluorometer.
Synthesis of N-(4-Aminophenyl)-N-phenyl-1-
naphthalylamine (NAPA)
In a 500-mL three-neck round-bottom flask equipped with a
stirrer bar under nitrogen atmosphere, nitro compound
(NAPN; 8 g, 23 mmol), Pd/C (0.2g), ethanol 200 mL were
added. After 7 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, the solution was filtered
to remove the catalyst, and the crude product was recrystal-
lized from ethanol to afford green solid with a 75% yield.
ꢀ
MP ¼ 129 C measured by DSC at 10 ꢀC/min; IR (KBr)
3356, 3445 cmꢁ1 (NH2 stretch).
1H NMR.(CDCl3): d (ppm) ¼ 3.54 (s, 2H, Hm); 6.60–6.63 (m,
2H, He,); 6.83–6.87 (m, 3H, Ha andHc); 7.02–7.04 (d, 2H, Hd, J
¼ 9.96 Hz); 7.14–7.17 (m, 2H, Hb); 7.32–7.34 (d, 1H, Hf, J ¼
9.96 Hz); 7.37–7.39 (m, 1H, Hj); 7.45–7.48 (t, 2H, Hg andHk, J
¼ 7.47 Hz); 7.73–7.75 (d, 1H, Hl, J ¼ 9.96 Hz); 7.88–7.90 (d,
1H, Hh, J ¼ 9.96 Hz); 8.02–8.04 (d, 1H, Hi, J ¼ 9.96 Hz).
Synthesis of N-(4-Nitrophenyl)-N-phenyl-1-
naphthalylamine (NAPN)
13C NMR (CDCl3): d (ppm) ¼ 116.01 (C7), 119.31 (C3),
119.71 (C1), 124.46 (C14), 125.75 (C17), 125.83 (C6), 125.92
(C11), 126.04 (C5), 126.27 (C16), 126.45 (C10), 128.26 (C12),
128.80 (C2), 131.06 (C13), 135.18 (C9), 139.82 (C15), 142.25
(C8), 144.01 (C18), 149.58 (C4).
In a 500-mL three-neck round-bottom flask was placed N-phenyl-
1-naphthylamine (10 g, 45.6 mmol), 4-fluoro-nitrobenzene (8 g,
45.6 mmol), sodium hydride (2 g, 83 mmol), and dimethyl sulfoxide
(DMSO, 120 mL). The mixture was heated with stirring at 120 ꢀC
for 24 h. The reaction mixture was cooled and then poured into 1 L
of saturated sodium chloride solution. The crude product was puri-
fied by silica gel column chromatography (n-hexane: toluene ¼
4:1) to afford pure product with MP 163 ꢀC measured by DSC at 10
ꢀC/min. IR (KBr): 2924 (ArAH), 1598 (ANO2 assymetric), 1313
(ANO2 symetric), 810 (CAN strech).
ELEM ANAL. Calcd. for C22H18N2: C, 85.13% : H, 5.85%; N,
9.02%. Found: C, 85.27%; H, 5.83%; N, 9.03%.
1H NMR (CDCl3): d (ppm) ¼ 6.82–6.84 (d, 2H, Hd, J ¼ 10.41
Hz); 7.18–7.20 (m, 1H, Ha,); 7.31–7.37 (m, 4H, Hb and Hc);
7.44–7.48 (m, 2H, Hg and Hj); 7.52–7.56 (m, 2H, Hh and Hk);
7.90–7.93 (t, 2H, Hf and Hi, J ¼ 7.50 Hz); 7.96–7.98 (d, 1H,
Hl, J ¼ 10.41 Hz); 8.02–8.04 (d, 2H, He, J ¼ 10.41 Hz).
13C NMR (CDCl3): d (ppm) ¼ 116.47 (C6), 123.20 (C9),
124.94 (C3), 125.25(C1), 125.93 (C7), 126.28 (C11), 126.49
(C15), 127.06 (C14), 127.38 (C10), 127.98 (C13), 128.64 (C16),
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Synthesis of N,N-Bis(4-bromophenyl)-N0-phenyl-N0-
(naphthalen-1-yl)-1,4-phenylenediamine (NAPHDIBr)
1-Bromo-4-iodobenzene (13 g, 48 mmol), NAPA (7.5 g, 24
mmol), Pd(dba)2 (0.4 g, 0.72 mmol), DPPF (0.39 g, 0.72
mmol), sodium tert-butoxide (7 g, 72 mmol) and dry toluene
(100 mL) were charged in a three-necked flask kept under
nitrogen. The mixture was heated to reflux for 6 h. After the
completion of the reaction, the solvent was removed under
vacuum and the residue was extracted with dichlorome-
thane–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 n-hexane and
ethyl acetate (V/V ¼ 4/1), and the light yellow solid was
obtained. MP ¼ 166 ꢀC measured by DSC at 10 ꢀC/min; IR
(KBr) 1070 cmꢁ1 (ArABr); 1282 cmꢁ1 (ArAN); 3034 cmꢁ1
(ArAH stretch).
105 ꢀC under nitrogen. The reaction mixture was cooled to
room temperature, and the organic layer was separated,
washed with water, and precipitated into methanol. The
green polymer sample was filtered, washed with excess
methanol, dried, and purified by a Soxhlet extraction with
methanol for 2 days. IR (KBr) 1261 cmꢁ1 (ArAN); 3031
cmꢁ1 (ArAH stretch).
1H NMR (CDCl3): d (ppm) ¼ 0.75 (s, 4H, Hs); 0.85–0.88 (m,
6H, Hy); 1.08–1.28 (m, 20H, Ht, Hu, Hv, Hw, and Hx); 2.00–
2.05 (br, 4H, Hr); 6.93–6.96 (t, 1H, Ha, J ¼ 7.5 Hz); 7.05–7.11
(br, 6H, Hc, He, and Hg); 7.21–7.29 (m, 4H, Hb, and Hd); 7.38–
7.45 (m, 2H, Hh, and Hl); 7.47–7.55 (m, 3H, Hi, and Hp);
7.58–7.62 (m, 8H, Hg, Hm, Hn, and Ho); 7.75–7.77 (m, 4H,
Hf); 7.91–7.93 (d, 1H, Hj, J ¼ 10 Hz); 8.03–8.05 (d, 1H, Hk,
J
¼ 10 Hz).
13C NMR (CDCl3): d (ppm) ¼ 14.09, 22.64, 23.84, 25.60,
29.26, 29.28, 29.57, 30.07, 31.86, 40.47, 40.53, 55.20, 67.95,
119.86, 120.89, 121.61,122.83, 123.39, 123.59, 124.15,
124.28, 125.48, 126.13, 126.35, 127.03, 127.17, 127.25,
127.77, 128.31, 128.43, 128.75, 129.13, 135.32, 139.55,
139.91, 141.51, 143.51, 144.24, 146.95, 148.58, 151.61.
1H NMR (CDCl3): d (ppm) ¼ 6.89–6.98 (m, 9H, Ha,Hd, He and
Hf); 7.02–7.04 (d, 2H, Hc, J ¼ 10.05 Hz); 7.19–7.22 (t, 2H,
Hb, J ¼ 7.54 Hz); 7.32–7.34 (d, 4H, Hg, J ¼ 10.05 Hz); 7.36–
7.41 (m, 2H, Hh and Hl); 7.47–7.52 (m, 2H, Hi and Hm);
7.78–7.80 (d, 1H, Hn, J ¼ 10.05 Hz); 7.89–7.91 (d, 1H, Hj, J ¼
10 Hz); 7.96–7.98 (d, 1H, Hk, J ¼ 10.15 Hz).
13C NMR (CDCl3): d (ppm) ¼ 114.88 (C12), 122.41 (C1),
121.57 (C3), 122.98 (C17), 124.76 (C7), 126.06 (C19), 126.16
(C6), 126.32 (C14), 126.38 (C13), 126.59 (C20), 127.20 (C18),
128.44 (C15), 129.09 (C2), 131.26 (C21), 132.20 (C11), 135.30
(C22), 140.52 (C5), 143.25 (C16), 144.77 (C8), 146.55 (C9),
148.22 (C4).
ELEM ANAL. Calcd. for C34H24Br2N2: C, 65.83% : H, 3.90%; N,
4.52%. Found: C, 65.99%; H, 4.07%; N, 4.50%.
Synthesis of the Model Compound, M1
Iodobenzene (1.4 g, 7 mmol), N-phenyl-1-naphthylamine
(1.3 g, 6 mmol), Pd(dba)2 (0.2 g, 0.36 mmol), DPPF (0.72
mmol), sodium tert-butoxide (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 dichloro-
methane and 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 1:2). Then the light green solid product was
obtained in a 71% yield.
Synthesis of the Conjugated Polymer, NAPHFL
To a three-necked flask was added 9,9-dioctylfluorene-2,7-
bis (trimethyleneboronate) (138.4 mg, 0.25 mmol), NAPH-
DIBr (200 mg, 0.25 mmol). The flask equipped with a con-
denser 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(triphenyl-phosphine)palladium (0) [Pd(PPh3)4], 8.6 mg,
(0.007 mmol) and 3 M aqueous sodium carbonate solution
(2 mL) was added, and the mixture was stirred for 48 h at
1H NMR (CDCl3): d (ppm) ¼ 6.95–6.98 (t, 2H, Ha, J ¼ 7.5
Hz); 7.07–7.09 (d, 4H, Hc, J ¼ 10 Hz); 7.21–7.24 (t, 4H, Hb, J
¼ 7.5 Hz); 7.36–7.40 (m, 2H, Hh and Hd); 7.47–7.52 (t, 2H,
He and Hi, J ¼ 7.5 Hz); 7.80–7.81 (d, 1H, Hj, J ¼ 5 Hz); 7.91–
7.93 (d, 1H, Hf, J ¼ 8 Hz); 7.99–8.00 (d, 1H, Hg, J ¼ 5 Hz).
13C NMR (CDCl3): d (ppm) ¼ 121.62 (C1), 121.83 (C3),
124.26 (C10), 126.08(C12), 126.32 (C7), 126.34 (C6), 126.39
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
(C13), 127.21 (C11), 128.34 (C8), 129.05 (C2), 131.27 (C5),
135.27 (C9), 143.57 (C14), 148.44 (C4).
Synthesis of the Model Compound, M2
The model compound was prepared according to the proce-
dures published in the paper.34
RESULTS AND DISCUSSION
Synthesis and Characterization
The synthesis procedures of dibromo monomer (NAPHDIBr)
containing unsymmetrical bi-triarylamine structure were out-
lined in Scheme 1. The precursor, N-(4-aminophenyl)-N-
phenyl-naphthalen-1-amine (NAPA), was obtained with
hydrazine monohydrate under a nitrogen atmosphere by Pd/
C-catalytic reduction of compound N-(4-nitrophenyl)-N-phe-
nylnaphthalen-1-amine (NAPN), which was prepared from N-
phenyl-1-naphthylamine and 4-fluoro-nitrobenzene in the
presence of sodium hydride. The new aromatic dibromo
compound, NAPHDIBr, having ditriarylamine and naphtha-
lene groups was successfully synthesized by the Buchwald-
Hartwig reaction with 1-bromo-4-iodobenzene. Elemental
SCHEME 1 Synthesis of the dibromo monomer and conjugated
polymer.
NAPHDIBr were consistent with the proposed structures.
The conjugated polymer, NAPHFL, was synthesized by
Suzuki coupling polymerization of an equimolecular mixture
of the dibromide (NAPHDIBr) and 9,9-dioctylfluorene-2,7-
bis(trimethyleneboronate) as shown in Scheme 1. The poly-
merization 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 (3 M). The chemical structure of
the polymer was verified by FTIR and NMR spectra. Figure 1
shows the aromatic part 1H NMR spectra of NAPHFL in
CDCl3, where all the peaks are consistent with the expected
structure. The aliphatic hydrogens attached on the fluorene
appear at the range of 0.5–2.2 ppm and the integrations of
the peaks are consistent with the structure. The Mn of the
prepared polymer was as high as about17,200 and the poly-
dispersity index was 2.99.
1
analysis, Fourier transform IR (FTIR), H NMR, and 13C NMR
spectra as well as two dimensional NMR techniques were
used to identify structures of the intermediates NAPN,
NAPA, and the desired dibromo compound, NAPHDIBr. The
FTIR spectrum of NAPN showed nitro characteristic bands
at 1313 and 1598 cmꢁ1. After reduction, the characteristic
band of the NAPA showed the typical NH2 stretch at 3356
and 3445 cmꢁ1, and the nitro characteristic bands disap-
peared. The characteristic band for the aryl bromide group
of NAPHDIBr monomer was observed at 1070 cmꢁ1 in the
Basic Properties
1
FTIR spectrum. The H NMR and 13C NMR spectra as well as
The polymer has good solubility in common organic solvents,
such as NMP, THF, dichloromethane, chloroform, toluene, xy-
lene, and benzene at room temperature. The excellent solu-
bility may result from the aliphatic segment and the propel-
ler structure of the noncoplanar bi-triarylamine moiety. The
TPA-based compounds are noncoplanar and propeller shape,
two dimensional spectra including COSY and HMQC of the
dibromo monomer were showed in the Supporting Informa-
tion. All aromatic protons of the NAPHDIBr are in the range
of 6.89–7.98 ppm. Elemental analysis, IR, and NMR spectra
clearly confirm that the compounds, NAPN, NAPA, and
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ARTICLE
polymer (NAPHFL) and the monomer (NAPHDIBr) in solid
state were red-shifted by 43 and 5 nm for from the corre-
sponding solution spectra. This phenomenon is probably at-
tributable to intermolecular interactions that planarize seg-
ments in the monomers/polymers leading to better
conjugation in the solid state38,39 The emissions of NAPHFL
was red-shifted both in solution and solid state as compared
to NAPHDIBr because of the enhanced conjugated length by
Suzuki coupling reaction.
Electrochemical Properties
The electrochemical behavior of NAPHFL was investigated
by CV conducted by film cast on an ITO-coated glass sub-
strate as the working electrode in dry CH3CN containing 0.1
M of TBAP as electrolyte under nitrogen atmosphere. Oxida-
tive and reductive cycles of the polymer and model com-
pounds are shown in Figure 3. The NAPHFL show two re-
versible oxidation redox couples at E1/2 values of 0.72 (Eonset
¼ 0.68) and 1.11 V in the oxidative scan. For understanding
the oxidation order of the two nitrogen atoms in the poly-
mer, model compounds (M1 and M2) were prepared and
their electrochemical behaviors are also shown in Figure 3.
The M1 and M2 have reversible oxidation peak at 1.27 V
(Eonset ¼ 1.01 V) and 1.46 V (Eonset ¼ 1.03 V), respectively.
Comparing the electhochemical data, we could find that M1
was more easily oxidized than M2, and prove the oxidation
order of nitrogen atoms for NAPHFL according to the elec-
tron density among the two nitrogen atoms on different tri-
phenylamine structures in each repeating unit (Table 2).
From these electrochemical data, the first electron removal
for NAPHFL could be assumed to occur at the nitrogen atom
on the pendent N,N-diphenyl-naphthalene-1-amine unit,
which was more electron-rich than the nitrogen atom on the
main-chain triphenylamine units (Scheme 2). From the oxi-
dation potential relative to ferrocene/ferrocenenium, which
can correspond to ꢁ4.8 eV for ferrocene below the vacuum
level,40 we can approximately calculate the HOMO energy
level of the conjugated polymer. The LUMO level of the poly-
mer was calculated according to the equation: LUMO ¼
HOMO þ Eg. The electrochemical properties of NAPHFL and
FIGURE
1
1H NMR spectrum of the conjugated polymer,
NAPHFL. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
so that they have more space than coplanar structures to let
solvent penetrate (or swelling). On the other hand, the rigid
structure of triphenylamine can enhance the Tg and thermal
stability as studied by many researches.35–37 The thermal
stability of NAPHFL under nitrogen was determined by TGA,
and their phase transition behavior was evaluated with DSC.
The NAPHFL showed excellent thermal stability with decom-
position temperatures 448 and 458 ꢀC in air and nitrogen,
respectively. The glass transition temperature of NAPHFL
was 140 ꢀC which is much higher than that of poly(9,9-dio-
ctylfluorene) (POF; Tg ¼ 66 ꢀC).34 The high transition tem-
perature may result from the rigid N,N-diphenyl-N0-phenyl-
N0-naphthalen-1-yl-1,4-phenylene diamine structure.
Optical Properties
The optical absorption and emission spectra of NAPHFL in
toluene and NAPHDIBR in toluene and solid film state are
shown in Figure 2, and their photophysical properties were
summarized in Table 1. For NAPHFL in toluene solution, a
major absorption at around 381 nm and a minor absorption
at 307 nm were observed. Because of the maximum absorp-
tion (324 nm) of NAPHDIBr in toluene is near the minor
absorption (307 nm) of NAPHFL, we ascribe the peak at 307
nm to p–p* transition of N,N-bis(4-bromophenyl)-N-phenyl-
N-naphthalen1-yl-1,4-phenylene-diamine moieties group.
Since the maximum absorption peak for POF is 387 nm,34
we ascribe the major absorption to p-p* transition derived
from the conjugated polymer backbone. The absorption spec-
tra in solid state were similar to that in toluene solution.
The band edge of absorption peak of the polymer film exists
around 460 nm, which is ꢃ20 nm longer than that of the so-
lution. This fact indicates the existence of interchain interac-
tion in the solid-state polymer. On excitation at the maximum
absorption wavelength, the NAPHFL and NAPHDIBr exhibit
blue emission color in toluene solution with the emission
peaks at 473 and 454 nm. The fluorescent quantum yield for
NAPHFL is 79% in toluene solution. The PL spectra of the
FIGURE 2 UV–vis and PL spectra of NAPHDIBr and NAPHFL.
[Color figure can be viewed in the online issue, which is avail-
able at www.interscience.wiley.com.]
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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 Monomers
Solution (Toluene)
Film
Compound
Abs (nm)
PL (nm)
U
PL(%)a
Abs (nm)
PL (nm)
NAPHFL
NAPHDIBr
POF37
a
307, 381
324
473
454
414
79
–
310, 385
326, 363
389
516
459
435
387
85
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).
model compounds are summarized in Table 2. The onset oxi-
dation potentials are situated from 0.68 to 1.03 V, from
which their HOMO levels are estimated in the range of
ꢁ4.86 to ꢁ5.21 eV. From the optical bandgap, the LUMO lev-
els are calculated in the range of ꢁ2.08 to ꢁ2.39 eV. Because
of the stability of the film and the good adhesion between
the polymer and ITO substrate, the NAPHFL exhibited excel-
lent reversibility of electrochromic characteristics in 1200 s
continuous cyclic scans (measuring time limit of the instru-
ment) between 0.0 and 1.2 V, changing color from light
green to deep green and then blue. The electrochromic char-
acteristics will be discussed below.
to 344 nm, and then a new band grows up at 860 nm due
to first oxidation. When the potential was adjusted to a more
positive value of 1.2 V, corresponding to the second step oxi-
dation, the peak at 860 nm keep to raise and a new band at
560 nm slowly grow up. Meanwhile, the film changed from
original yellow green to dark green and then to a dark blue
oxidized form as shown in Figure 4. The color switching
times were estimated by applying a potential step, and the
absorbance profiles were followed in Figure 5. The switching
time was calculated at 90% of the full switch because it is
difficult to perceive any further color change with naked eye
beyond this point. Thin NAPHFL film requires 4.25 s at 1.2
V for switching and 7.22 s for bleaching at 860 nm, 5.51 s
for switching and 6.48 s for bleaching at 560 nm. Continuous
cyclic scans between 0.0 and 1.2 V, the polymer films still
exhibited good stability of electrochromic characteristics.
Electrochromic Characteristics
Electrochromism of the polymer thin film was examined by
an optically transparent thin-layer electrode coupled with
UV–vis spectroscopy. The electrode preparations and solution
conditions were identical to those used in CV. The typical
electrochromic absorbance spectra of NAPHFL are shown in
Figure 4. When the applied potentials increased positively
from 0 to 1.0 V, the characteristic peak of absorbance at 385
nm for neutral form NAPHFL decreased gradually and shifts
CONCLUSIONS
A novel conjugated polymer, NAPHFL, was successfully pre-
pared via Suzuki coupling reaction from a fluorene derivative
and a novel dibromo monomer containing unsymmetrical
N,N,N0,N0-tetraaryl-substituted 1,4-phenylenediamine. The
polymer exhibits excellent solubility in common organic sol-
ꢀ
vent, and has high thermal stability such as Td10 at 453 C in
ꢀ
nitrogen atmosphere and Tg at about 140 C. In electrochro-
mic behavior, it revealed excellent continuous cyclic stability
of electrochromic characteristic in 1200 s.
TABLE 2 Electrochemical Properties of NAPHFL and Model
Compounds
a
E1/2 (V)
b
Eg
HOMOc LUMOc
d
Compound First Second Eonset
(eV) (eV)
(eV)
NAPHFL
0.72 1.11
0.68
1.01
1.03
2.78 ꢁ4.86
3.05 ꢁ5.19
2.88 ꢁ5.21
ꢁ2.08
ꢁ2.14
ꢁ2.39
M1
M2
a
1.15
1.16
–
–
E1/2 (average potential of the redox couple peaks).
FIGURE 3 Cyclic voltammograms of NAPHFL, M1, and M2 in
film cast on an indium-tin oxide (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 avail-
able at www.interscience.wiley.com.]
b
The data were calculated from polymer films by the equation Eg
¼
1240/konset.
c
ox
onset
Calculated from the equation: HOMO ¼ ꢁ(E
ꢁ EFocnset) ꢁ 4.8, LUMO
¼ HOMO þ band gap.
d
Eox/onset (Fc/Fc vs. Ag/Agþ) ¼ 0.62 V.
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ARTICLE
SCHEME 2 Supposed redox process of
NAPHFL from its neutral state, radical cat-
ion state to dication.
FIGURE 4 Electrochromic behavior of NAPHFL thin film
(in CH3CN with 0.1 M TBAP as the supporting electro-
lyte) from 0 to 1.2 V.
FIGURE 5 Current consumption and
potential
step
absorptometry
of
NAPHFL (in CH3CN with 0.1 M TBAP as
the supporting electrolyte) by applying
a potential step (0.00–1.20 V).
ELECTROCHROMIC POLYMER, WU ET AL.
1475

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
19 Ling, Q. D.; Liaw, D. J.; Zhuc, C.; Chanc, D. S. H.; Kang, E.
T.; Neoh, K. G. Prog Polym Sci 2008, 33, 917–978.
The authors thank the National Science Council of the Republic
of China for the financial support of this work.
20 Ling, Q. D.; Liaw, D. J.; Teo, E. Y. H.; Zhu, C.; Chan, D. S. H.;
Kang, E. T.; Neoh, K. G. Polymer 2007, 48, 5182–5201.
21 Scherf, U.; List, E. J. W. Adv Mater 2002, 14, 477–487.
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