N,N,N′,N′- tetraphenyl-1,4-phenylenediamine-fluorene alternating conjugated polymer: Synthesis, characterization, and electrochromic application

Article abstract of DOI:10.1021/ma902138g

A novel N,N,N′N′-tetraphenyl-l,4-phenylenediamine (TPPA)-containing aromatic dibromide (3), N,N,-bis(4-bromophenyl)-N′, N′-bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)-1,4-phe.n.ylenediamine, was successfully synthesized. The novel, conjugated polymer (4) ha

Full text of DOI:10.1021/ma902138g

2236 Macromolecules 2010, 43, 2236–2243
DOI: 10.1021/ma902138g
N,N,N0,N0- Tetraphenyl-1,4-phenylenediamine-Fluorene Alternating
Conjugated Polymer: Synthesis, Characterization, and Electrochromic
Application
Wen-Hsiang Chen,† Kun-Li Wang,*,‡ Der-Jang Liaw,*,† Kueir-Rarn Lee,§ and Juin-Yih Lai§
†Department of Chemical Engineering, National Taiwan University of Science and Technology, 10607 Taipei,
Taiwan, ‡Department of Chemical Engineering and Biotechnology, National Taipei University of Technology,
§
10608 Taipei, Taiwan, and R&D Center for Membrane Technology, Department of Chemical Engineering,
Chung Yuan University, 32023 Chung-Li, Taiwan
Received September 26, 2009; Revised Manuscript Received January 7, 2010
ABSTRACT: A novel N,N,N0,N0-tetraphenyl-1,4-phenylenediamine (TPPA)-containing aromatic dibro-
mide (3), N,N-bis(4-bromophenyl)-N0,N0-bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)-1,4-phenylenediamine,
was successfully synthesized. The novel conjugated polymer (4) having number-average molecular weight
of 2.05 ꢀ 104 was prepared via Suzuki coupling from the dibromide (3) with 9,9-dioctylfluorene-2,7-diboronic
acid bis(1,3-propanediol) ester. The conjugated polymer (4) exhibited excellent solubility in common organic
solvents such as N-methyl-2-pyrrolidinone (NMP), tetrahydrofuran, chloroform, and toluene at room
temperature. The conjugated polymer (4) had useful levels of thermal stability associated with their relatively
high glass transition temperature (Tg = 144 °C), 10% weight-loss temperatures (Td10) in nitrogen (Td10
=
445 °C), and char yields at 800 °C in nitrogen (59.0%). The hole-transporting and electrochromic properties
were examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the
conjugated polymer films cast onto indium-tin oxide (ITO)-coated glass substrates exhibited two reversible
oxidation redox couples at E1/2 value of 0.67 and 1.05 V vs Ag/Agþ in acetonitrile solution. The conjugated
polymer film (4) revealed better stability of electrochromic characteristics and more rapid switching time than
model polymers M2 which has no electron-donating bis(4-(2,4,4-trimethylpentan-2-yl) phenyl)amine group
and M3 without electron-donating 2,4,4-trimethylpentan-2-yl substituent. The conjugated polymer film (4)
also showed high contrast with color change from pale yellowish to green and then to blue at applied
potentials ranging from 0.00 to 1.18 V.
Introduction
modification of the conjugated backbone and to get multichro-
mism from the same material.12
Electrochromism is known as the reversible electromagnetic
absorbance/transmittance and color change resulting from the
oxidation or the reduction of the material in response to an
externally applied potential by electrochemical means.1 This
interesting property led to the development of many techno-
logical applications such as automatic antiglazing mirror,2 smart
windows,3 electrochromic displays,4 and chameleon materials.5
Hitherto, a wide variety of electrochromic materials showing high
coloration efficiencies, low operating voltage, and fast switching
capabilities6 have been developed, which can be classified into
several distinct categories such as metal oxides [tungsten trioxide
(WO3) or iridium dioxide (IrO2)],7 mixed-valence metal com-
plexes (Prussian blue),8 small organic molecules (viologens,
bipyridinium, and phthalocyanines),9 and conjugated poly-
mers.10 Conjugated organic polymers11 have received significant
attention throughout the course of the past two decades, stemm-
ing not only from their high conductivities in the doped state
but also from a variety of optoelectronic and redox properties
for advanced technological applications such as light-emitting
diodes, photovoltaic cells, and electrochromic cells. Moreover,
the most important features of conjugated polymers are the
possibilities to fine-tune the color through chemical structure
Recently, triphenylamine (TPA) derivative materials, includ-
ing small molecules and macromolecules, have extensively been
investigated for hole transporters,13 light emitters,14 and polymer
electronic memories.15 For instance, Kang et al.13k reported that
triphenylamine-fluorene alternating conjugated copolymers
with pendant acceptor groups (aldehyde, monocyano, and di-
cyano group) were applied for solar cell field. On the other hand,
Chen et al.14d reported that copolyfluorenes containing bipo-
lar groups (hole-transporting triphenylamine and electron-
transporting 1,2,4-triazole group) were useful for enhancing
electroluminescence of MEH-PPV {poly[2-methoxy-5-(20-ethyl-
hexyloxy)-1,4-phenylenevinylene]}. Thus, different functional
substituents on triphenylamine had diverse properties and appli-
cations. TPA can also be considered as a good electrochromic
material as it easily oxidizes to form radical cation with a
noticeable change of coloration.16 Although TPA has many
advantages, Adams and co-workers17 reported that TPA could
be easily dimerized to form tetraphenylbenzidine during the
anodic oxidation pathway. For electrochromic materials, this
dimerization could be considered as an undesired side reaction,
which might cause irreversible defect after several redox switches.
To avoid this behavior, incorporating electron-donating substit-
uents such as alkyl or alkoxy group at the para-position of
the TPA group could prevent the coupling reactions and
afford stable cationic radicals with lower potential.18 The redox
*To whom all correspondences should be addressed: Ph 886-2-
27712171 ext 2558, Fax 886-2-27317117; e-mail klwang@ntut.edu.tw
(K.-L.W.); Ph 886-2-27376638 or 886-2-27335050, Fax 886-2-23781441
or 886-2-27376644; e-mail liawdj@mail.ntust.edu.tw (D.-J.L.).
pubs.acs.org/Macromolecules
Published on Web 02/04/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 5, 2010 2237
Scheme 1. Synthesis Route of N,N-Bis(4-bromophenyl)-N0,N0-bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)-1,4-phenylenediamine (3)
property, electron-transfer process, multicoloring electrochro-
mism, and photoelectrochemical behavior of N,N,N0,N0-tetra-
substituted-1,4-phenylenediamines (TPPA) are important for
technological application.19
In this report, we prepared TPPA-containing dibromo mono-
mer with electron-donating group (2,4,4-trimethylpentan-2-yl
substituted), and their corresponding conjugated polymers were
obtained by Suzuki coupling. The basic properties of the con-
jugated polymer such as solubility as well as thermal and optical
properties are described. The electrochemical and electrochromic
characteristics of the conjugated polymer are also investigated
herein. For comparing the stability and switching time of
electrochromic characteristics with synthesized conjugated poly-
mer, the model polymers M2 and M3 were synthesized as well.
Monomer and Polymer Synthesis. Synthesis of 4-Nitro-N,N-
bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)aniline (1) (Scheme 1).
In a 500 mL three-neck round-bottom flask was placed bis-
(4-(2,4,4-trimethylpentan-2-yl)phenyl)amine (30.00 g, 76.20 mmol),
4-fluoronitrobenzene (12.90 g, 91.45 mmol), sodium hydride
(2.74 g, 114.31 mmol), and dimethyl sulfoxide (DMSO, 120
mL). The mixture was heated with stirring at 120 °C for 48 h.
The reaction mixture was cooled and then poured into 1 L
of methanol. The yellow precipitate was collected by filtration
and dried under vacuum. The product was purified by silica
gel column chromatography (dichloromethane:n-hexane = 1:3)
and recrystallized from ethyl acetate to afford yellow crys-
tals, 4-nitro-N,N-bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)ani-
line (1), in a 60% yield; mp 167 °C (by DSC at a scan rate of
10 °C min-1).
Synthesis of 4-Amino-N,N-bis(4-(2,4,4-trimethylpentan-2-yl)-
phenyl)aniline (2) (Scheme 1). In a 500 mL three-neck round-
bottom flask equipped with a stirrer bar under nitrogen atmo-
sphere, 7.00 g (13.60 mmol) of nitro compound (1) and 0.22 g of
10% Pd/C were suspended in 150 mL of ethanol. The suspension
solution was heated to reflux, and 7 mL of hydrazine mono-
hydrate was added slowly to the mixture from a dropping funnel
over a period of 1 h under dry nitrogen. After a further 12 h of
reflux, the mixture solution was filtered to remove Pd/C, and the
filtrate was cooled to precipitate. The crude product was col-
lected by filtration and recrystallization from ethanol to afford
white crystals (80% yield); mp 95 °C (by DSC at a scan rate of
10 °C min-1).
Experimental Section
Materials. The chemical bis(4-(2,4,4-trimethylpentan-2-yl)-
phenyl)amine was purchased from Ouchi Shinko Chemical
Industrial. 4-Fluoronitrobenzene (Acros, 99%), sodium
hydride (Aldrich), hydrazine monohydrate (Merck), 10% pal-
ladium on activated carbon (10% Pd/C) (Merck), 1-bromo-
4-iodobenzene (Acros, 98%), bis(dibenzylideneacetone)palla-
dium [Pd(dba)2] (Acros), 1,10-bis(diphenylphosphino)ferrocene
(DPPF) (Acros, 98%), anhydrous potassium carbonate
(Merck), and sodium tert-butoxide (Aldrich, 98%) were used
without further purification. Tetrabutylammonium perchlorate
(TBAP) was obtained from Fluka and recrystallized twice from
ethyl acetate and then dried in vacuum prior to use. The catalyst
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) was pur-
chased from Acros. 9,9-Dioctylfluorene-2,7-diboronic acid bis-
(1,3-propanediol) ester was purchased from Aldrich and
recrystallized from mixture of ethyl acetate and n-hexane 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.
Synthesis of N,N-Bis(4-bromophenyl)-N0,N0-bis(4-(2,4,4-tri-
methylpentan-2-yl)phenyl)-1,4-phenylenediamine (3) (Scheme 1)
by Buchwald-Hartwig Reaction20. A mixture of 1.00 g (2.06
mmol) of amino compound 2, 1.17 g (4.13 mmol) of 1-bromo-4-
iodobenzene, 23.72 mg (4.13 ꢀ 10-2 mmol) of Pd(dba)2, 45.71
mg (8.25 ꢀ 10-2 mmol) of DPPF, 0.59 g (6.19 mmol) of sodium
tert-butoxide, and 6 mL of dry toluene was charged in a three-
necked flask kept under a nitrogen atmosphere. The mixture

2238 Macromolecules, Vol. 43, No. 5, 2010
Scheme 2. Synthesis Route of Conjugated Polymer 4 via Suzuki Coupling
Chen et al.
was heated to reflux for 6 h. After the completion of the reaction,
the solvent was removed under reduced pressure, and the residue
was extracted with dichloromethane/water. The collected or-
ganic layer was dried over MgSO4 overnight and then filtered
to remove MgSO4. After removing the solvent of filtrate, the
residue was purified by silica gel column chromatography
(dichloromethane:n-hexane = 1:3) and recrystallized from hex-
ane to obtain white crystals (61% yield); mp 205 °C (by DSC at a
scan rate of 10 °C min-1).
mmol) of Pd(dba)2, 237.9 mg (4.30 ꢀ 10-1 mmol) of DPPF,
3.10 g (32.21 mmol) of sodium tert-butoxide, and 10 mL of dry
toluene was charged in a three-necked flask kept under a nitro-
gen atmosphere. The mixture was heated to reflux for 12 h. After
the completion of the reaction, the solvent was removed under
reduced pressure, and the residue was extracted with dichloro-
methane/water. The collected organic layer was dried over
MgSO4 overnight and then filtered to remove MgSO4. After
removing the solvent of filtrate, the residue was purified by silica
gel column chromatography (n-hexane) to obtain colorless
viscous liquid (bis(4-bromophenyl)phenylamine, 85% yield).
The model polymer M2 was prepared analogously as the
conjugated polymer 4. Yield: 88%.
Synthesis of Model Compound 4,40-Bis(2,4,4-trimethylpentan-
2-yl)triphenylamine (M1). A mixture of 3.00 g (7.62 mmol) of
bis(4-(2,4,4-trimethylpentan-2-yl)phenyl)amine, 1.55g(7.62mmol)
of 1-iodobenzene, 87.6 mg (1.52 ꢀ 10-1 mmol) of Pd(dba)2, 168.9
mg (3.05 ꢀ 10-1 mmol) of DPPF, 1.10 g (11.43 mmol) of sodium
tert-butoxide, and 10 mL of dry toluene was charged in a three-
necked flask kept under a nitrogen atmosphere. The mixture was
heated to reflux for 12 h. After the completion of the reaction,
the solvent was removed under reduced pressure, and the residue
was extracted with dichloromethane/water. The collected organic
layer was dried over MgSO4 overnight and then filtered to remove
MgSO4. After removing the solvent of filtrate, the residue was
purified by silica gel column chromatography (n-hexane) to obtain
colorless liquid (65% yield).
Synthesis of the Conjugated Polymers 4 via Suzuki Coupling
(Scheme 2). To a three-necked flask was added 0.1757 g (0.3146
mmol) of 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-pro-
panediol) ester and 0.25 g (0.3146 mmol) of dibromo compound
3. The flask equipped with a condenser was then evacuated
and filled with nitrogen several times to remove traces of air,
and then 5 mL of the degassed toluene was added. Once the
two monomers were dissolved, 10.9 mg (9.4369 ꢀ 10-3 mmol)
of tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] and
5 mL of 3 M aqueous sodium carbonate solution were added,
and the mixture was stirred for 48 h at 105 °C under a nitrogen
atmosphere. After completing the reaction, the reaction mixture
was cooled to room temperature, and the organic layer was
separated, washed with water, and precipitated into methanol.
The light yellow-green fibrous polymer sample was filtered,
washed with excess methanol, dried, and purified by a Soxhlet
extraction with acetone for 2 days. Yield: 92%.
Measurements. FT-IR spectra were recorded in the range
4000-400 cm-1 on a Bio-Rad FTS-3500 spectrometer. Elemen-
tal analyses were performed on Perkin-Elmer 2400 C, H, and N
1
analyzer. The H and 13C NMR spectra were recorded on a
Bruker DRX-500 instrument operating at 500 MHz for proton
and 125 MHz for carbon. Weight-average (Mw) and number-
average (Mn) molecular weight were determined by gel permea-
tion chromatography (GPC). Five Waters (Ultrastyragel)
3
4
5
˚
columns 300 ꢀ 7.7 mm (guard, 500, 10 , 10 , 10 A in a series)
were used for GPC analysis with tetrahydrofuran (THF) (1 mL
min-1) as an eluent. The eluents were monitored with a refrac-
tive index detector (RI 2000). Polystyrene was used as a stan-
dard. The melting temperature (Tm) and glass transition
temperature (Tg) were measured on a DuPont 9000 differential
scanning calorimeter (TA Instruments TA 910) at a heating
rate of 10 °C min-1 from 50 to 250 °C under a steady flow of
nitrogen. The glass transition temperature (Tg) was recorded on
second run heating. The recorded temperatures were calibrated
using indium as standard. Thermogravimetric data were ob-
tained on a Perkin-Elmer TG/DTA (Diamond TG/DTA).
Experiments were carried out on approximately 3-5 mg sam-
ples at a heating rate of 10 °C min-1 from 50 to 800 °C under
nitrogen or air flowing conditions (20 cm3 min-1). UV-vis
spectra of the polymer films or solutions were recorded on a
JASCO V-550 spectrophotometer at room temperature in air.
Photoluminescence spectra were measured with a HORIBA
Jobin Yvon FluoroMax-3 spectrofluorometer. Fluorescence
quantum yield (ΦF) values of the samples in THF were mea-
sured by using 9,10-diphenylanthracence in THF as a reference
standard (ΦF = 0.9) with excitation at 346 nm. All corrected
Synthesis of Bis(4-bromophenyl)phenylamine and Model Poly-
mer M2. A mixture of 1.00 g (10.74 mmol) of aniline, 6.08 g
(21.48 mmol) of 1-bromo-4-iodobenzene, 123.5 mg (2.15 ꢀ 10-1

Article
Macromolecules, Vol. 43, No. 5, 2010 2239
Table 1. Molecular Weight and Thermal Properties of the Conjugated Polymers
Td10d (°C)
polymer
yielda (%)
Mn ꢀ 10-4 b
Mw ꢀ 10-4 b
PDIb
Tgc (°C)
N2
air
char yielde (%)
4
M2
92
88
2.05
1.75
3.52
3.16
1.72
1.80
144
133
445
438
412
417
59.0
63.9
a Yield was determined gravimetrically. b Determined by gel permeation chromatography (GPC) in THF using polystyrene as a standard. c Tg was
determined by DSC at a heating rate of 10 °C min-1
.
d The decomposition temperature at 10% weight loss (Td10) was determined by TG at a heating rate
of 10 °C min-1 e Residual weight percentage at 800 °C in nitrogen.
.
fluorescence excitation spectra were found to be equivalent to
their respective absorption spectra. Cyclic voltammetry (CV;
CHI model 619A) was conducted with the use of a three-
electrode cell in which 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
tetrabutylammonium perchlorate (TBAP)/acetonitrile (CH3CN).
Voltammograms are presented with the positive potential point-
ing to the left and with increasing anodic currents pointing
downward. 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. Absorp-
tion spectra in spectroelectrochemical analysis were measured
with a JASCO V-550 spectrophotometer.
Results and Discussion
Synthesis. Novel N,N,N0,N0-tetraphenyl-1,4-phenylene-
diamine (TPPA)-containing dibromo compound 3 and the
conjugated polymer 4 containing TPPA and fluorene groups
were synthesized according to the synthetic route outlined in
Schemes 1 and 2, respectively. The identification techniques
such as elemental analysis, IR, and NMR spectra includ-
ing 1H NMR, 13C NMR, HMQC, and COSY were used to
identify structures of the intermediate compounds, the di-
bromo monomer 3, and the conjugated polymer 4. The
confirmation data are shown in the Supporting Information.
The results of the elemental analyses for the intermediate
compounds and the target dibromo monomer 3 are in good
agreement with the proposed structures. The NMR spectra
also agree well with the proposed molecular structures of
compound 3 and the conjugated polymer 4.
Basic Characterization. The solubility behavior of conju-
gated polymer 4 was tested qualitatively. The conjugated
polymer 4 was highly soluble in comment organic solvent
such as N-methyl-2-pyrrolidinone (NMP), tetrahydrofuran
(THF), dichloromethane, chloroform, toluene, xylene, and
benzene at room temperature, and the enhanced solubility
could be attributed to the introduction of the bulky pendent
N,N,N0,N0-tetraphenyl-1,4-phenylenediamine (TPPA) moi-
ety and alkyl chain including fluorene group and 2,4,4-
trimethylpentan-2-yl substituted into the repeating unit.
Thus, the excellent solubility makes the conjugated polymer
4 potential candidates for practical applications by spin-
coating, dip-coating, or inject-printing processes to obtain
thin films for optoelectronic devices.
The molecular weights of the conjugated polymers were
determined by gel permeation chromatography (GPC) using
the polystyrene as the standard and are summarized in
Table 1. The number-average molecular weight of the pre-
pared conjugated polymers 4 and M2 was as high as 2.05 ꢀ
104 and 1.75 ꢀ 104 with polydispersity index of 1.72 and 1.80,
respectively.
The thermal properties of the conjugated polymers were
investigated by DSC and TGA techniques, and the results
are summarized in Table 1. All the polymers indicated no
clear melting endotherms up to the decomposition tempera-
tures on the DSC thermograms (Figure 1). This result
Figure 1. TGA and DSC curve of the conjugated polymers 4 and M2
with a heating rate of 10 °C min-1
.
supports the amorphous nature of these triphenylamine-
containing polymers.21 In Figure 1, the glass transition
temperature of the conjugated polymer 4 were observed at
144 °C, which is much higher than that of poly(9,9-di-
octylfluorene) (POF) (POF; Tg = 75 °C)22,23 and 4,40-bis-
[N,N-(m-tolyl)phenylamino]biphenyl (TPD; Tg = 60 °C),24
indicating that the use of these conjugated polymers may
greatly improve the device durability, which related to the
Tg’s of the materials as reported by Tokito et al.24 The
relatively high glass transition temperature has the thermal
advantage for the application of electronic materials.25 It
also observed that the glass transition temperature of the
model polymer M2 (Tg = 133 °C) was slightly lower than
that of conjugated polymer 4 (Tg = 144 °C) due to incor-
poration of bulky N,N,N0,N0-tetraphenyl-1,4-phenylenedi-
amine (TPPA) moiety into the conjugated polymer. Tokito
et al.24 investigated that the oligomerization of triphenyl-
amine brought about increase of the Tg. Typical TGA
spectra of conjugated polymers 4 and M2 in a nitrogen
atmosphere are shown in Figure 1. The conjugated polymers
4 and M2 exhibited a similar TGA pattern with no significant
weight loss below 400 °C in a nitrogen atmosphere. The
decomposition temperatures at 10% weight loss (Td10) and
char yield of the conjugated polymers 4 and M2, examined
by TG analysis, were 445 and 438 °C in nitrogen, 412 and
417 °C in air, and 59.0% and 63.9% in nitrogen, respectively.
Because of lower aromatic content, the conjugated polymer 4
was lower char yield than that of conjugated polymer M2.
Optical and Electrochemical Properties. The optical proper-
ties of the conjugated polymers were investigated by UV-vis
and photoluminescence spectroscopy in solid state and
toluene (ca. 1 ꢀ 10-5 M solution). The results are sum-
marized in Table 2.
The UV-vis absorption of the conjugated polymer 4
exhibited a strong absorption around 387 nm and a
minor absorption around 314 nm in toluene solution. By

2240 Macromolecules, Vol. 43, No. 5, 2010
Table 2. Optical and Electrochemical Properties of the Conjugated Polymers
Chen et al.
solution λ (nm)a
film λ (nm)
Φ
PL (%)c abs max abs onset PL maxb
oxidation (V; vs Ag/Agþ in CH3CN)
Egopt (eV)d Eox/onset (V)e HOMO (eV)f LUMO (eV)f
polymer abs max
PL maxb
4
M2
314, 387 452
299, 386 423, 444 (s)g
84.3
69.5
318, 381
303, 379
440
430
462
429, 454 (s)g
2.82
2.88
0.57
0.89
-4.95
-5.28
-2.13
-2.40
a Polymer solution in toluene (ca. 1 ꢀ 10-5 M solution). b Excited at the absmax for both the solution and solid states. c Fluorescence quantum yields
(ΦF) values of the samples in dilute THF solution were measured in an integrating sphere using 9,10-diphenylanthracence in THF as a reference standard
(ΦF = 0.9) with excitation at 346 nm. d Calculated from UV absorption spectrum of polymer films by the equation: band gap = 1240/λ
.
e The
abs
onset
oxidation potential versus Ag/Agþ calculated from CV spectrum using ferrocene as internal standard. f Calculated from the equation HOMO =
) - 4.8, LUMO = HOMO þ band gap. g “s” means shoulder.
Fc
onset
ox
onset
-(E
- E
Figure 2. UV spectrum of the conjugated polymer 4 and the dibromo
compound 3 and PL spectrum of the conjugated polymer 4 in solution
and in film state.
comparing the absorption spectra of monomer 3 (Figure 2),
the minor absorption around 314 nm was ascribed to a π-π*
transition resulting from the conjugation between the aro-
matic rings and nitrogen atoms (TPPA unit). Since the
maximum absorption peak for poly(9,9-dioctylfluorene)
(POF) is 391 nm,22 the strong absorption around 387 nm
was assigned to π-π* transition derived from the conjugated
fluorene backbone. The model polymer M2 also exhibited
the similar results. The UV absorption peaks of the solid film
were observed at 318 and 381 nm for 4 and 303 and 379 nm
for M2. As shown in Figure 2, it is clearly observed that the
absorption wavelength is given a simultaneous bathochro-
mic shift of the high-energy transition (314 nm) and hypso-
chromic shift of the low-energy transition (387 nm) from
a dilute solution to a solid state thin film in UV spectrum. For
hypsochromic shift of the low-energy transition, it may be
attributed to the more disordered morphology of the back-
bone, resulting from conformational fluctuations of the
highly twisted and bulky TPPA units in the polymer back-
bone.26 For bathochromic shift of the high-energy transi-
tion, it may be due to the intrachain and/or interchain
mobility of the excitons. In addition, the dibromo monomer
3 also exhibited the similar results (Figure 2).
The fluorescence emission of the conjugated polymers 4
and M2 exhibited a maximum emission peak at 452 and
423 nm in toluene solutions, respectively. The fluorescence
quantum yield (ΦF) of the conjugated polymers 4 and M2 in
THF was estimated by comparing with the standard of 9,10-
diphenylanthracene (ca. 1 ꢀ 10-5 M solution). The fluore-
scent quantum yield for the conjugated polymers 4 and M2
was 84.3% and 69.5% in THF solution, respectively. The PL
emission peaks of the solid film were observed at 462 nm for 4
and 429 nm for M2. The solid-state PL spectrum of the
conjugated copolymers 4 and M2 is red-shifted by about
Figure 3. Cyclic voltammograms of the conjugated polymers 4, M2,
and M3 films and model compound M1 on an ITO-coated glass
substrate in CH3CN solutions containing 0.1 M TBAP. Scan rate =
50 mV/s. Oxidation redoxs of (a) ferrocene, (b) the conjugated polymer
M2 film and model compound M1, and (c) the conjugated polymers 4
and M3 films.
10 and 6 nm, respectively, from the corresponding solution
spectra (Figure 2). This phenomenon is probably attributa-
ble to intermolecular interactions, leading to aggregation
and possible excimer formation.13k,27
The electrochemical behavior of the conjugated polymers
was investigated by cyclic voltammetry conducted by film
cast on an ITO-coated glass substrate as the working elec-
trode in dry acetonitrile (CH3CN) containing 0.1 M of TBAP
as an electrolyte under a nitrogen atmosphere. The typical
cyclic voltammograms for the conjugated polymer 4 (with
substituted 2,4,4-trimethylpentan-2-yl group), M3 (without
substituted 2,4,4-trimethylpentan-2-yl group), M2, and
model compound M1 are shown in Figure 3 for comparison.
There are two reversible oxidation redox couples at E1/2
values of 0.67 and 1.05 V for the conjugated polymer 4 and
0.73 and 1.13 V for model polymer M3 and one reversible
oxidation redox couples at E1/2 = 0.93 for moldel compound
M1 and 1.03 for model polymer M2 in the oxidative scan.
From these electrochemical data between the conjugated
polymers 4 and M2 and model compound M1, the first
electron removal for the conjugated polymer 4 could be
assumed to occur at the N atom on the pendent 4,40-bis-
(2,4,4-trimethylpentan-2-yl)triphenylamine groups, which
was more electron-rich than the N atom on the main-chain
triphenylamine unit. The anodic oxidation pathway of the
conjugated polymer 4 was postulated as in Scheme 3. Com-
paring the electrochemical data in Figure 3c, we found that

Article
Macromolecules, Vol. 43, No. 5, 2010 2241
Scheme 3. Simplified Redox Process of Conjugate Polymer 4 from Its
Neutral State and Radical Cation State to Dication State
Figure 4. Spectral change of 4 thin film on the ITO-coated glass
substrate (in CH3CN with 0.1 M TBAP as the supporting electrolyte)
along with increasing of the applied voltage: 0-1.18 V vs Ag/Agþ
couple as reference. The inset shows the photographic images of the film
at indicated applied voltages.
The typical electrochromic absorption spectra of the con-
jugated polymer 4 film are presented in Figure 4 as a series of
UV-vis absorbance curves correlated to electrode poten-
tials. In the neutral form of conjugated polymer 4 (Figure 4),
at 0 V, the film exhibited strong absorption at wavelength
around 316 and 389 nm characteristic for TPPA and fluorene
group, respectively, but it was almost transparent in the
visible region. Upon oxidation of the conjugated polymer 4
film (increasing applied voltage from 0 to 0.95 V), the
intensity of the absorption peak at 316 and 389 nm gradually
decreased, while a new broad band at around 700-900 nm
gradually increased in intensity due to the first stage oxida-
tion. As the applied potential became more anodic to 1.18 V,
corresponding to the second step oxidation, the two new
peaks at 623 and 886 nm gradually increased in intensity.
The observed UV-vis absorption changes in the film of
conjugated polymer 4 at various potentials are fully rever-
sible and are associated with strong color changes; indeed,
they even can be seen readily by the naked eye. From the inset
shown in Figure 4, it can be seen that the film of conjugated
polymer 4 switches from a transmissive neutral state (pale
yellow) to a highly absorbing semioxidized state (green) and
a fully oxidized state (blue). The spectroelectrochemistry
characteristics of the model polymer M2 are also shown in
Figure 5. When the applied potential increased positively
from 0 to 1.04 V, the peak of characteristic absorbance at
385 nm for model polymer M2 decreased gradually, while a
new band grew up at 495 nm and a broad band rose at
around 700-900 nm. The new spectrum was assigned to the
formation of a cationic radical TPA moiety which appeared
the red color after oxidation. According to this result, it was
also proven that the second electron removal for the con-
jugated polymer 4 would be occurred at the N atom on the
main-chain triphenylamine unit. From the inset shown in
Figure 5, it can be seen that the film of M2 switches from a
transmissive neutral state (pale yellow) to a fully oxidized
state (red).
the conjugated polymer 4 (E1/2 = 0.67 V) was much more
easily oxidized than the model polymer M3 (E1/2 = 0.73 V).
The introduction of electron-donating 2,4,4-trimethylpen-
tan-2-yl group not only greatly prevented the coupling
reaction but also lowered the oxidation potentials of the
electroactive conjugated polymer 4 as compared with the
corresponding model polymer M3 which has no electron-
donating 2,4,4-trimethylpentan-2-yl substituent.18
The energy of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
levels of the investigated the conjugated polymers 4 and M2
could be determined from the oxidation onset potentials and
the onset absorption wavelength of polymer films, and the
results are summarized in Table 2. For instance (Figure 3),
the oxidation onset potential for the conjugated polymer 4
was determined to be 0.57 V vs Ag/Agþ. The external
ferrocene/ferrocenium (Fc/Fcþ) redox standard Eonset(Fc/
Fcþ) was 0.42 V vs Ag/Agþ in CH3CN. Under the assump-
tion that the HOMO energy for the ferrocene standard was
-4.80 eV with respect to the zero vacuum level, the HOMO
energy for the conjugated polymer 4 was evaluated to be
-4.95 eV.
Spectroelectrochemical and Electrochromic Characteris-
tics. Spectroelectrochemistry experiments were conducted
to elucidate the optical properties of the electrochromic
films. For these investigations, the conjugate polymer film
was cast on an ITO-coated glass slide (a piece that fit in the
commercial UV-vis cuvette), and a homemade electroche-
mical cell was built from a commercial UV-vis cuvette. The
cell was placed in the optical path of the sample light beam in
a commercial diode array spectrophotometer. This proce-
dure allowed us to obtain electronic absorption spectra
under potential control in a 0.1 M TBAP/CH3CN solution.
For optical switching studies, polymer films were cast on
ITO-coated glass slides in the same manner as described
above, and each film was potential stepped between its
neutral (0 V) and oxidized (þ1.18 V) state. While the films
were switched, the absorbance at 389 and 886 nm was
monitored as a function of time with UV-vis spectroscopy.
Optical switching data for the cast film of the conjugated

2242 Macromolecules, Vol. 43, No. 5, 2010
Chen et al.
Therefore, the results of these experiments revealed that the
introduction of the electron-donating bis(4-(2,4,4-trimethyl-
pentan-2-yl) phenyl)amine group at the active sites of the
TPA or 2,4,4-trimethylpentan-2-yl substituent at the active
sites of the TPPA unit enhanced the redox and electrochro-
mic stability of the conjugated polymer 4, which could be a
good candidate as anodic electrochromic materials due to
their proper oxidation potentials, lower switching time, and
thin film formability.
Conclusions
A novel TPPA-containing conjugated polymer 4 was readily
prepared from the newly synthesized dibromo monomer 3 with
9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester
via Suzuki coupling. Introduction of extremely electron-donating
TPPA group and 2,4,4-trimethylpentan-2-yl substituent to the
polymer chain not only greatly prevented the coupling reaction
and lowered the oxidation potentials of the electroactive con-
jugated polymer but also led to good solubility and film-forming
properties of the conjugated polymer. In addition to high Tg and
good thermal stability, the conjugated polymer 4 also reveal
valuable electrochromic characteristics such as high contrast
(pale yellowish neutral form to oxidized blue forms) and good
electrochromic reversibility. Thus, these characteristics suggest
that the conjugated polymer have great potential for use in
optoelectronics applications.
Figure 5. Spectral change of model polymer M2 thin film on the ITO-
coated glass substrate (in CH3CN with 0.1 M TBAP as the supporting
electrolyte) along with increasing of the applied voltage: 0-1.04 V vs
Ag/Agþ couple as reference. The inset shows the photographic images
of the film at indicated applied voltages.
Acknowledgment. The authors thank the National Science
Council of the Republic of China for the financial support of this
work.
Supporting Information Available: Identification data such
as IR, elemental analysis and NMR spectroscopy for monomers
and conjugated polymers, and NMR spectrum such as 1H
NMR, 13C NMR, HMQC, and HMBC for dibromo compound
3 and conjugated polymer 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
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