Click synthesis and reversible electrochromic behaviors of novel polystyrenes bearing aromatic amine units

Article abstract of DOI:10.1002/pola.25990

Novel electrochromic polymers were prepared by the click postfunctionalization of poly(4-azidomethylstyrene) with alkyne-containing aromatic amine units in the presence of Cu(I) catalysts. Two kinds of aromatic amine units, tris(4-alkoxyphenyl)amine and N,N,N',N'-tetraphenyl-p- phenylenediamine, were introduced into polystyrene side chains, which were completely characterized by gel permeation chromatography-multiangle light scattering, nuclear magnetic resonance, and infrared spectroscopies, and elemental analysis. Thermal analyses demonstrated the high stability with the decomposition temperatures exceeding 300 °C even after postfunctionalization. The UV-vis absorption spectra of the polymer thin films revealed negligible absorption in the visible region, as reasonably confirmed by visual observation. The polymer thin films were prepared by spray-coating on an indium tin oxide-coated glass plate. Cyclic voltammograms of these films exhibited anodic peaks ascribed to the oxidation of the side-chain aromatic amine moieties. The tris(4-alkoxyphenyl)amine unit displayed one-step oxidation at 0.287 V (vs. Ag/AgCl), while the N,N,N',N'-tetraphenyl-p-phenylenediamine unit showed two-step oxidations at 0.297 and 0.641 V. These oxidation processes produced new colors of the polymer films. The former triarylamine-based chromophore provided a blue color after the oxidation, while the latter phenylenediamine-based chromophore showed a potentially controlled green and dark blue colors. The reversibility and switching behaviors of these color changes were also comprehensively investigated. 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 Aromatic amine-based active electrochromophores were quantitatively introduced into a polystyrene side chain by using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction. The reversible electrochromic behaviors of the functionalized polystyrenes were investigated by spectroelectrochemistry. Copyright

Full text of DOI:10.1002/pola.25990

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Click Synthesis and Reversible Electrochromic Behaviors of Novel
Polystyrenes Bearing Aromatic Amine Units
Yongrong Li,1 Tsuyoshi Michinobu2,3
1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552,
Japan
2Global Edge Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
3PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan
Correspondence to: T. Michinobu (E-mail: michinobu.t.aa@m.titech.ac.jp)
Received 27 November 2011; accepted 30 January 2012; published online 23 February 2012
DOI: 10.1002/pola.25990
ABSTRACT: Novel electrochromic polymers were prepared by
the click postfunctionalization of poly(4-azidomethylstyrene)
with alkyne-containing aromatic amine units in the presence of
Cu(I) catalysts. Two kinds of aromatic amine units, tris(4-alkox-
yphenyl)amine and N,N,N0,N0-tetraphenyl-p-phenylenediamine,
were introduced into polystyrene side chains, which were
completely characterized by gel permeation chromatography–
multiangle light scattering, nuclear magnetic resonance, and
infrared spectroscopies, and elemental analysis. Thermal analy-
ses demonstrated the high stability with the decomposition
temperatures exceeding 300 ꢀC even after postfunctionaliza-
tion. The UV–vis absorption spectra of the polymer thin films
revealed negligible absorption in the visible region, as reason-
ably confirmed by visual observation. The polymer thin films
were prepared by spray-coating on an indium tin oxide-coated
glass plate. Cyclic voltammograms of these films exhibited an-
odic peaks ascribed to the oxidation of the side-chain aromatic
amine moieties. The tris(4-alkoxyphenyl)amine unit displayed
one-step oxidation at 0.287
V (vs. Ag/AgCl), while the
N,N,N0,N0-tetraphenyl-p-phenylenediamine unit showed two-
step oxidations at 0.297 and 0.641 V. These oxidation proc-
esses produced new colors of the polymer films. The former
triarylamine-based chromophore provided a blue color after
the oxidation, while the latter phenylenediamine-based chro-
mophore showed a potentially controlled green and dark blue
colors. The reversibility and switching behaviors of these color
C
V
changes were also comprehensively investigated. 2012 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2111–
2120, 2012
KEYWORDS: electrochemistry; functionalization of polymers;
polyamines; redox polymers
INTRODUCTION Solution-processable electrochromic poly-
mers are an important class of materials for the applications
to electrochromic displays and mirrors, smart windows, and
information storage.1 Promising building blocks are mainly
classified into metallo-supramolecular polymers and organic
aromatic polymers.2 In contrast to the metal-containing
materials, purely organic polymers that display perfectly re-
versible redox activities are limited. For example, Reynolds
et al.3 reported a series of conjugated polymers composed of
3,4-dioxythiophene- and 3,4-dioxypyrrole-based main chains
capable of undergoing reversible discoloration upon oxida-
tion and coloration upon reduction. The colors in the neutral
(reduced) state are controlled by designing the polymer
structures because of the ease of structural tuning of the or-
ganic materials. Donor–acceptor alternating structures are
such an example of the spectral/band gap engineering.4
triphenylamine moieties readily dimerize upon a one-elec-
tron oxidation forming N,N,N0,N0-tetraphenylbenzidine units.6
Therefore, the appropriate molecular design is required to
enhance the chemical stability of the oxidized states. Intro-
duction of electron-donating substituents into the p-positions
of the triphenylamine moieties effectively improves the life-
time of the oxidized states. Further improvement is usually
achieved when p-phenylenediamine structures with lower
oxidation potentials are adopted. This molecular design of
aromatic amines has indeed been used in the development
of hole-transporting materials and organic ferromagnetic
polymers.7 Also, Liou et al.8 successfully applied the redox
characteristics of the aromatic amines to designing novel
electrochromic polyamines. They reported a series of polya-
mides and polyimides incorporating the aromatic amine moi-
eties into the main chain. Because of the excellent engineer-
ing plastic features, the polymers displayed a high thermal
stability, good film formation capability, and reversible
electrochromism.
Another class of promising organic polymers is aromatic
polyamines that display coloration upon oxidation and dis-
coloration upon reduction.5 It is known that unsubstituted
C
V
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Recently, we reported a new concept of click postfunctionali-
zation using Cu(I)-catalyzed azide–alkyne cycloaddition,9 ac-
cessible from readily available azide-functionalized polymers
as a precursor, and various alkyne-containing small mole-
cules as a functional reactant.10 Using this method, the
desired functional groups can be introduced into the poly-
mer side chains in quantitative yields without the production
of any byproducts. The significant advantage of using this
method is the straightforward derivatization from a single
precursor polymer, which allows the systematic investigation
of its various properties in a short time period. In other
words, the repetition of tedious polymerization and purifica-
tion processes is not required in order to prepare multiple
polymer samples. We now report the postfunctional quanti-
tative introduction of aromatic amine-based electrochromo-
phores into a polystyrene precursor.11 Two different electro-
chromophores were designed and their electrochromic
behaviors comprehensively compared.
4-{[tert-Butyl(dimethyl)silyl]oxy}-N,N-bis(4-methoxyphe-
nyl)aniline (2)
Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3; 93.5
mg, 0.102 mmol), and tri-tert-butylphosphine (P(tBu)3; 114
mg, 0.610 mmol) were dissolved in dryꢀ toluene (80 mL)
under N2. After stirring for 10 min at 20 C, 1 (2.27 g, 10.2
mmol), 4-bromoanisole (5.70 g, 30.5 mmol), and sodium
tert-butoxide (NaOtBu; 2.34 g, 24.4 mmol) were added. The
solution was stirred at 100 ꢀC for 15 h. After cooling to 20
ꢀC, the reaction mixture was diluted with toluene (100 mL)
and washed with brine (100 mL ꢂ 2). The toluene layer was
dried over Na2SO4. Evaporation and column chromatography
(SiO2, hexane/ethyl acetate ¼ 4:1) afforded the desired prod-
uct (2.2 g, 51%) as a yellow solid.
1H NMR (300 MHz, C6D6): d 0.14 (s, 6 H), 1.01 (s, 9 H), 3.31
(s, 6 H), 6.72 (m, 4 H), 6.79 (m, 2 H), and 7.07 (m, 6 H). 13C
NMR (75 MHz, C6D6): d-4.39, 23.19, 25.85, 54.99, 115.00,
120.86, 124.66, 125.64, 140.75, 142. 44, 151.31, and 156.86.
Infrared (IR; neat): 2953, 2830, 1498, 1456, 1436, 1238,
1039, 906, 843, 818, 804, and 778 cmꢁ1. MALDI–TOF MS
EXPERIMENTAL
(dithranol): m/z: calcd for C26H33NO3Siþ: 435.22 g molꢁ1
found: 434.2 g molꢁ1 [MAH]þ.
;
Materials
All reagents were purchased form Kanto, Tokyo Kasei, Wako,
and Aldrich and used as received. 4-{[tert-Butyl(dimethyl)
silyl]oxy}aniline (1),12 (4-{[4-(diphenylamino)phenyl](phenyl)
amino}phenyl)methanol (5),12 and poly(4-azidomethylstyrene)
(7)13 were synthesized according to the reported method.
4-[Bis(4-methoxyphenyl)amino]phenol (3)14
To a solution of 2 (1.50 g, 3.44 mmol) in THF (30 mL), tetra-
butylammonium fluoride (1 M in THF, 7 mL) was added
under air. The mixture was stirred at 20 C for 20 min. Col-
Measurements
ꢀ
1H- and 13C-NMR spectra were measured on a JEOL model
AL300 spectrometer at 20 ꢀC. Chemical shifts are reported
in parts per million downfield from SiMe4, using the sol-
vent’s residual signal as an internal reference. The resonance
multiplicity is described as s (singlet), br s (broad singlet), d
(doublet), and m (multiplet). Attenuated total reflectance
Fourier transform infrared (ATR–FTIR) spectra were
recorded on a JASCO FT/IR-4200 spectrometer. Gel permea-
tion chromatography (GPC) was measured on a JASCO sys-
tem equipped with polystyrene gel columns using THF as an
eluent at the flow rate of 1.0 mL minꢁ1. Absolute molecular
weights were determined by using the detector miniDAWN
Tristar. Elemental analyses were conducted on a PerkinElmer
2400-SeriesII CHNS/O Analyzer. Thermogravimetric analysis
(TGA) was carried out on a Seiko SII TG 6220 under nitro-
gen flow at the heating rate of 10 ꢀC minꢁ1. Absorption
spectra of spectroelectrochemistry experiments were meas-
ured on an Agilent 8453 UV–vis spectrophotometer. Other
UV–vis-near infrared spectra in solutions were recorded on a
JASCO V-670 spectrophotometer. Potential applications were
performed using a cyclic voltammetry setup at 20 ꢀC in
dehydrated CH3CN-containing 0.1 M (nC4H9)4NClO4 in three-
electrode cell. The working, reference, and auxiliary electro-
des were an indium tin oxide (ITO), Ag/AgCl/CH3CN/
(nC4H9)4NPF6, and a platinum wire, respectively. All poten-
tials are referenced to the Ag/AgCl (Ag/Agþ) couple unless
otherwise stated. Polymer thin films were prepared on an
ITO electrode (10 X, about 0.8 ꢂ 2.5 cm2) by spray-coating
of polymer solutions (2.0 g Lꢁ1 in THF), followed by drying
umn chromatography (SiO2, hexane/ethyl acetate ¼ 2:1)
afforded the desired product (1.0 g, 90%) as yellow oil.
1H NMR (300 MHz, C6D6): d 3.31 (s, 6 H), 3.80 (br s, 1 H),
6.46 (m, 2 H), 6.72 (m, 4 H), and 7.04 (m, 6 H). IR (KBr):
3387, 1497, 1462, 1441, 1233, 1178, 1104, 1033, and 824
cmꢁ1. MALDI–TOF MS (dithranol): m/z: calcd for C20H19NOþ3 :
321.14 g molꢁ1; found: 320.15 g molꢁ1 [MAH]þ.
4-Methoxy-N-(4-methoxyphenyl)-N-[4-(prop-2-yn-1-yloxy)
phenyl]aniline (4)
A solution of 3 (500 mg, 1.56 mmol), K2CO3 (500 mg, 4.68
mmol), and KI (51.0 mg, 0.312 mmol) in dry DMF (30 mL)
was heated to 80 ꢀC under N2 for 1 h. Propargyl bromide
(222 mg, 1.87 mmol) was slowly added over 0.5 h. The reac-
ꢀ
tion mixture was stirred at 80 C for 2 days. After cooling to
20 ꢀC, water (100 mL) was added, and the crude product
was extracted with CH2Cl2. Column chromatography (SiO2,
hexane/ethyl acetate ¼ 4:1) afforded the desired product
(460 mg, 85%) as brown viscous liquid.
1H NMR (300 MHz, C6D6): d 1.98 (s, 1 H), 3.31 (s, 6 H), 4.18
(s, 2 H), 6.70 (m, 4 H), 6.79 (m, 2H), and 7.03 (m, 6 H). 13C
NMR (75 MHz, C6D6): d 55.02, 56.03, 75.41, 79.27, 115.02,
116.14, 124.60, 125.59, 142.36, 143.43, 153.31, and 155.79.
IR (KBr): 3281, 2119, 1497, 1462, 1441, 1234, 1212, 1177,
1105, 1030, and 824 cmꢁ1. MALDI–TOF MS (dithranol): m/z:
calcd for C23H21NO3þ: 359.15 g molꢁ1; found: 358.15 g molꢁ1
[MAH]þ.
ꢀ
in vacuo at 50 C.
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SCHEME 1 Synthesis of aromatic amine moieties: (a) 4-bromoanisole, NaOtBu, Pd2(dba)3, P(tBu)3, and toluene, 100 ꢀC, 24 h; (b)
(nC4H9)4NF, THF, 20 ꢀC, 2 h; (c) BrCH2CBCH, K2CO3, KI, DMF, N2, 80 ꢀC, 48 h; (d) DCC, DMAP, HCBC(CH2)3COOH, CH2Cl2, 20 ꢀC, 16 h.
4-{[4-(Diphenylamino)phenyl](phenyl)amino}benzyl
hex-5-ynoate (6)
1237, 1178, 1106, 1033, and 825 cmꢁ1. Elemental analysis:
calcd for (C46H41N5O2)n: C 74.11, H 5.83, N 10.80, O 9.26;
To a solution of 5-hꢀexynoic acid (204 mg, 1.35 mmol) in found: C 74.83, H 5.74, N 11.13%.
CH2Cl2 (13 mL) at 0 C, N,N0-dicyclohexylcarbodiimide (DCC;
306 mg, 1.48 mmol), 4-dimethylaminopyridine (DMAP; 16.5
P2
To a solution of 7 (146 mg, 0.92 mmol per repeat unit) in
mg, 0.135 mmol), and 5 (1.2 g, 2.7 mmol) were added. The
reaction mixture was allowed to warm to 20 ꢀC and stirred
for 16 h. After the filtration of white precipitates, the solvent
was evaporated and column chromatography (SiO2, CHCl3/
hexane ¼ 4/1) afforded the desired product (530 mg, 74%)
as a brown solid.
DMF (12 mL), 6 (490 mg, 0.92 mmol), sodium ascorbate
(18.2 mg, 0.092 mmol), and copper(II) sulfate pentahydrate
(11.4 mg, 0.046 mmol) were added under argon. The mix-
ꢀ
ture was stirred at 20 C for 24 h. The solution was poured
into MeOH/CH2Cl2 (10/1). The precipitate was filtered,
washed with MeOH, and dried under vacuum, yielding the
pale yellow solid (530 mg, 84%).
1H NMR (300 MHz, C6D6): d 1.69 (m, 2 H), 1.77 (m, H), 2.00
(m, 2 H), 2.25 (m, 2 H), 5.01 (s, 2 H), 6.91 (m, 3 H), 7.02 (m,
4 H), and 7.20 (m, 14 H). 13C NMR (75 MHz, C6D6): d 17.83,
23.85, 32.83, 65.91, 69.50, 83.24, 122.85, 123.05, 123.56,
124.30, 124.43, 125.70, 126.03, 129.57, 129.60, 130.06,
130.39, 143.01, 143.68, 148.13, 148.32, 148.35, and 172.24.
IR (neat): 3287, 3034, 2934, 2118, 1731, 1587, 1502, 1487,
1310, 1265, 1150, 829, 752, 694, and 626 cmꢁ1. MALDI–TꢁO1F
1H NMR (300 MHz, C6D6): d 0.64 (br s, 3n H), 2.02 (br s, 2n
H), 2.31 (br s, 2n H), 2.72 (br s, 2n H), 4.89 (br s, 4n H),
6.83 (m, 4n H), 6.95 (m, 8n H), and 7.07 (m, 12 nH). IR
(neat): 3034, 2923, 2097, 1731, 1588, 1503, 1489, 1264,
753, and 694 cmꢁ1
.
Elemental analysis: calcd for
(C46H41N5O2)n: C 79.40, H 5.94, N 10.06, O 4.60; found: C
79.50, H 5.49, N 10.41%.
MS (dithranol): m/z: calcd for C37H32N2Oþ2 : 536.25 g mol
found: 535.4 g molꢁ1 [MAH]þ.
;
RESULTS AND DISCUSSION
P1
Synthesis of Aromatic Amine Moieties
To a solution of 7 (146 mg, 0.92 mmol per repeat unit) in
DMF (12 mL), 4 (330 mg, 0.92 mmol), sodium ascorbate
(18.2 mg, 0.092 mmol), and copper(II) sulfate pentahydrate
(11.4 mg, 0.046 mmol) were added under argon. The mix-
Triarylamine derivatives have traditionally been synthesized
by Ullmann chemistry, while the recent progress in the Pd-
catalyzed cross-coupling reactions enabled the high yielding
and large-scale production of various aromatic amine mole-
cules.15 Therefore, we prepared the redox-active aromatic
amine moieties, tri(4-alkoxyphenyl)amine and N,N,N0,N0-tetra-
phenyl-p-phenylenediamine units, by the Pd-catalyzed reac-
tions. Starting from 4-{[tert-butyl(dimethyl)silyl]oxy}aniline
ꢀ
ture was stirred at 20 C for 24 h. The solution was poured
into MeOH/CH2Cl2 (10/1). The precipitate was filtered,
washed with MeOH, and dried under vacuum, yielding the
pale yellow solid (395 mg, 83%).
1H NMR (300 MHz, C6D6): d 0.65 (br s, 5n H), 3.37 (s, 6n H), 1, the Pd-catalyzed amination with the twofold 4-bromoani-
5.04 (br s, 3n H), 6.74 (br s, 6n H), 6.88 (br s, n H), and sole constructed the triarylamine framework 2 in 51% (71%
7.03 (br s, 9n H). IR (neat): 3034, 2932, 1501, 1503, 1462, yield for each amination reaction; Scheme 1). Deprotection
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new Cu catalyst was developed for the homogeneous reac-
tion in organic solvent media,16 the initial combination of
CuSO4ꢃ5H2O and sodium ascorbate effectively promoted the
cycloaddition between the azide moieties of 7 and the
terminal alkyne of the aromatic amine units. The GPC–MALS
measurements revealed
a reasonable molecular weight
increase. It should be noted that relative molecular weights
determined by the comparison to standard polystyrenes
were not reliable in this case probably due to the strong ad-
hesive feature of the functionalized polystyrenes P1 and P2.
The Mn and Mw/Mn values were 77,500 and 1.44 for P1 and
98,100 and 1.29 for P2, respectively (Table 1). The absence
of the terminal alkynes and azide groups in P1 and P2 was
confirmed by the 1H NMR and IR spectra. Because P2
showed a gradual color change in chloroform, the NMR spec-
tra were taken in deuterized benzene or dimethyl sulfoxide.
In the 1H NMR spectra, the terminal alkyne proton peaks
detected at 1.98 and 1.74 ppm for 4 and 6, respectively,
FIGURE 1 1H NMR spectra of (a) 4 and P1 and (b) 6 and P2 in
C6D6 at 20 ꢀC. The residual C6H6 and water contamination are
marked as * and #, respectively.
of the tert-butyl(dimethyl)silyl group followed by the Wil-
liamson reaction with propargyl bromide yielded the alkyne-
functionalized tris(4-alkoxyphenyl)amine unit 4. The total
yield of the three steps was 40%. The hydroxyl group-func-
tionalized N,N,N0,N0-tetraphenyl-p-phenylenediamine 5 was
prepared from 4-bromotriphenylamine via the two-step
Pd-catalyzed amination reactions according to a literature
method.12 Similar to the synthesis of 4, the Williamson reac-
tion was attempted. However, this reaction was not success-
ful in this case. Therefore, the hydroxyl group of 5 was
esterified under standard conditions using DCC and DMAP,
for conversion into the alkyne functionality. The desired
compound 6 was obtained in a moderate yield of 74%. The
total yield to produce this molecule from commercially avail-
able compounds was 46%. The chemical structures of the
two aromatic amine chromophores were fully characterized
by 1H and 13C NMR, IR, and MALDI–TOF mass
measurements.
Polymer Synthesis
The azide-functionalized polystyrene derivative
7 was
selected as a readily available precursor polymer. The abso-
lute molecular weight (Mn) and the polydispersity (Mw/Mn)
determined by gel permeation chromatography–multiangle
light scattering (GPC–MALS; multiangle light scattering) were
15,400 and 1.71, respectively. The presence of the azide
group was confirmed by the strong vibration peak at 2095
cmꢁ1 of the IR spectrum (Fig. 1), and the azide content in
the repeat unit was determined to be 96% from the elemen-
tal analysis.
The new alkyne-functionalized aromatic amines 4 and 6
were subjected to the click postfunctionalization of 7 in the
presence of a Cu(I) catalyst in DMF, yielding new polystyrene
derivatives P1 and P2, respectively (Scheme 2). Although a
SCHEME 2 Synthesis of P1 and P2 by click postfunctionaliza-
tion: (a) CuSO4ꢃ5H2O, sodium ascorbate, DMF, 24 h.
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TABLE 1 Summary of Molecular Weights and Thermal
Properties of the Polymers
T5% (ꢀC)b
Tg (ꢀC)c
a
a
Polymer
Mn
Mw/Mn
7
15,400
77,500
98,100
1.71
1.44
1.29
–
–
P1
P2
a
308
331
113
97
Molecular weights determined by GPC-MALS (multiangle light
scattering).
b
The 5% weightꢁl1oss temperatures determined by TGA at the heating
rate of 10 ꢀC min
c
.
Glass transꢁit1ion temperatures determined by DSC at the scanning rate
of 10 ꢀC min
.
completely disappeared after the postfunctionalization, de-
spite the occurrence of partial broadening (Fig. 1). Moreover,
in the IR spectra, both the strong azide vibrational peak at
2095 cmꢁ1 of 7 and the alkyne CAH and CBC vibrational
peaks detected at 3281 and 2119 cmꢁ1 for 4 and at 3287
and 2118 cmꢁ1 for 6 apparently disappeared after the post-
functionalization. As an example, the IR spectra of 4, 7, and
P2 are depicted in Figure 2. More importantly, elemental
analyses revealed the content of the pendent aromatic amine
units to be 98.5% for P1 and 98.0% for P2 from the C/N ra-
tio. All these results strongly support the occurrence of the
quantitative click postfunctionalization.
FIGURE 3 TGA curves of P1 and P2 measured at the heating
rate of 10 ꢀC minꢁ1 under flowing nitrogen.
ꢀC were 57.2 and 65.3%, respectively. The high-char yields
of these polymers could be explained by their high aromatic
contents. The glass transition temperatures (Tg) of P1 and
ꢀ
P2 were 97 and 113 C, respectively. These values are com-
parable to the reported Tg of polystyrene, ensuring that this
postfunctionalization does not lower the thermal stabilities
of the precursor polymers.
Optical and Electrochemical Properties
Thermal Properties
Both P1 and P2 are soluble in the common organic solvents,
such as toluene, benzene, chlorobenzene, o-dichlorobenzene,
THF, and dichloromethane at room temperature. However,
they are insoluble in some polar solvents, such as acetoni-
trile and dimethyl sulfoxide, at room temperature. The pres-
ence of the clear solubility boundary made it possible to pre-
pare thin films of these polymers by a spray-coating method
on an ITO substrate and undergo electrochemical measure-
ments in the thin-film states.
Thermal stability is very important when practical applica-
tions are considered. To determine the thermal properties of
the postfunctionalized polymers P1 and P2, TGA and differ-
ential scanning calorimetry measurements were performed
at the scanning rate of 10 ꢀC minꢁ1 under flowing nitrogen.
The thermal analysis data are summarized in Table 1, and
the TGA curves are depicted in Figure 3. Both postfunctional-
ized polymers are thermally stable and the 5% decomposi-
ꢀ
tion temperatures (T5%) of P1 and P2 were 308 and 331 C,
respectively. The residual soot amounts of P1 and P2 at 500
The UV–vis absorption and fluorescence spectra of P1 and
P2 were measured both in THF and in the thin-film state,
and the data are summarized in Table 2. These optical prop-
erties are ascribed to the side-chain aromatic amine units,
because the precursor polystyrene did not provide these
spectra. The absorption spectrum of P1 in THF showed a
well-defined main peak (longest wavelength absorption max-
imum: kmax) at 298.5 nm accompanied by a weak long wave-
length shoulder peak, characteristic of the pꢁp* and n–p*
transitions of the tris(4-alkoxyphenyl)amine units (Fig. 4).17
TABLE 2 Summary of the optical properties of the
functionalized polystyrenes
In THF
Film
kmax kem (nm)
Polymer (nm) [kex (nm)]
Stokes kmax kem (nm)
shift (cmꢁ1
) (nm) [kex (nm)]
P1
P2
298.5 398.5 [298.5] 8406
314.0 401.0 [314.0] 6910
302.5 400.0 [302.5]
322.5 401.5 [322.5]
FIGURE 2 IR spectra of 4, 7, and P1.
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FIGURE 4 Normalized UV–vis absorption [(a) in THF; (c) in film] and fluorescence spectra [(b) in THF; (d) in film] of P1 and P2.
The kmax of P2 was detected at 314.0 nm in THF. This value
bathochromically shifted compared to that of P1 due to the
extended aromatic moiety. However, the emission maxima
(kem) of P1 and P2 in THF appeared at similar positions of
398.5 and 401.0 nm, respectively. Accordingly, the Stokes
shift of P1, calculated from the kmax and kem values, was
8406 cmꢁ1, which was greater than that of P2 (6910 cmꢁ1).
The optical band gaps of P1 and P2, determined from the
end absorptions in THF, were 3.22 (385 nm) and 3.26 eV
(380 nm), respectively. The polymer thin films, prepared by
casting the solutions onto a quartz plate, displayed a slight
bathochromic shift in both kmax and kem compared to the so-
lution spectra. The most significant shift was observed for
the kmax value of P2 (8.5 nm), suggesting the strong aggrega-
tion propensity of the larger aromatic chromophores in the
side chains. Although a broadening of the end absorptions
also occurred, the main peaks remained in the UV region,
thereby maintaining its visual transparency.
eventually determined to be 1 mV sꢁ1. The exfoliation of the
polymer films was probably due to the partial solubilization
of the formed polycations in CH3CN, although it was unclear
by visual observation.
The redox activities of these polymers originate from the
side-chain aromatic amine moieties. The P1 film displayed a
single well-defined oxidation peak at 0.289 V (vs. Ag/Agþ),
ascribed to one-electron removal from the tris(4-alkoxyphe-
nyl)amine moiety [Fig. 5(a)]. Although the anodic current in-
tensity of the oxidation process was greater than the ca-
thodic one of the corresponding reduction process, no new
oxidation peaks appeared even after the repeated scans. The
repeated scans led to a gradual decrease in the anodic cur-
rent intensity and the current intensities of both oxidation
and reduction finally became almost the same, again sup-
porting the partial solubilization of the polymer films when
they were oxidized. This result basically suggested no side
Cyclic voltammograms (CVs) of these polymer films were
measured in
a poor solvent, dry CH3CN, with 0.1 M
TABLE 3 Summary of the redox potentials and energy levels of
the functionalized polystyrenes
(nC4H9)4NClO4 as the supporting electrolyte. The redox
potentials and the calculated energy levels are listed in Table
3. The polymer films were prepared by the spray coating of
the THF solutions onto an ITO-coated glass plate. Because
they were almost insoluble in CH3CN, the redox behavior in
the film states was directly evaluated using the ITO as a
working electrode. Solution CVs of the aromatic polyamines
Eox,1
(V)a
Eox,2
(V)a
Opt. BG
(eV)b
HOMO
(eV)c
LUMO
(eV)d
Polymer
P1
P2
a
0.287
0.297
–
3.22
3.26
ꢁ4.98
ꢁ5.08
ꢁ1.76
ꢁ1.82
0.641
Potentials versus Ag/Agþ in CH3CN with 0.1 M (nC4H9)4NClO4.
are usually measured at the scanning rate of 50–100 mV sꢁ1
,
b
c
Optical band gap.
but it was found that the polymer films peel off the ITO sur-
face when scanned at this rate. Decreasing the scanning rate
solved this problem. The most suitable scanning rate was
Calculated from the Eox,1 values on the basis of Fc/Fcþ ¼ ꢁ4.8 eV. Fc/
Fcþ ¼ 0.1 V (vs. Ag/Agþ).
d
Calculated from the HOMO levels and the optical band gaps.
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FIGURE 5 Cyclic voltammograms (CVs) of the thin films of (a) P1 and (b) P2 on an ITO-coated glass plate in CH3CN containing 0.1
M (nC4H9)4NClO4 at the scanning rate of 1 mV sꢁ1. Potentials versus Ag/Agþ.
reactions under the measurement conditions, which was con-
firmed by the reversible electrochromic behavior (vide infra).
The P2 film also exhibited strong oxidation peaks at 0.297
and 0.641 V. The first oxidation potential (Eox,1) was attrib-
uted to the one-electron removal from the p-phenylenedi-
amine unit forming an aminium cationic radical. A further
electron removal occurred at the second oxidation potential
(Eox,2) yielding a bication. Similar to P1, the repeated scan-
ning of the CV curves did not provide a new peak, thus
showing the high potential of P2 as a solution-processable
electrochromophore.
aminium cationic radical of the N,N,N0,N0-tetraphenyl-p-phen-
ylenediamine unit.18 The intensity of these bands increased
as more positive potentials were applied up to 0.45 V, lead-
ing to a green film. The significant persistency of the ami-
nium cationic radical realized the high stability of the green
color. However, the further increase in the applied potentials
led to a decrease in the peak intensities at 405 and 820 nm
and the appearance of another new peak at 620 nm, result-
ing in a dark blue film. The second color change in the P2
film is attributed to the oxidation from the aminium cationic
radical to the bication. The presence of the isosbestic points
The electronic energy levels, highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO), of P1 and P2 were estimated from the combination
of the redox potentials and optical band gaps. The HOMO
levels of P1 and P2 were determined to be ꢁ4.98 and
ꢁ5.08 eV, respectively, on the basis of ferrocene/ferrocinium
(Fc/Fcþ) ¼ ꢁ4.8 eV. The LUMO levels of P1 and P2, calcu-
lated from the determined HOMO levels and the optical band
gaps, were ꢁ1.76 and ꢁ1.82 eV, respectively. These values
are comparable to those of reported aromatic polyamines.5,8
Electrochromic Properties
Based on the optical and electrochemical studies, the electro-
chromic behavior of the polymer thin films was evaluated.
Spectroelectrochemistry was used to monitor the spectral
changes in the electrochromic films of P1 and P2. The spray-
coated thin films on the ITO-coated glass plate together with
the reference and counter electrodes were placed in a 1-cm
cuvette. The UV–vis-near infrared spectra were measured in
CH3CN-containing 0.1 M (nC4H9)4NClO4 at 20 ꢀC. When
potentials more positive than 0.20 V (vs. Ag/Agþ) were
applied to the thin film of P1, new absorption bands cen-
tered at 390 and 720 nm appeared, resulting in the film’s
color change from almost transparent to blue (Fig. 6).17 Con-
sidering the CV curve, this color change is ascribed to the
generation of the aminium cationic radical, and this radical
showed a sufficient stability in the solid state. In the case of
the P2 film, two-step color changes upon electrochemical ox-
idation were observed (Fig. 7). The application of a 0.15 V
potential produced new bands centered at 405 and 820 nm.
The lower-energy band at 820 nm is characteristic of the
FIGURE 6 Electrochromic behavior of the P1 film on the ITO-
coated glass plate in CH3CN-containing 0.1 M (nC4H9)4NClO4 at
applied potentials from 0 to 0.40 V (vs. Ag/AgCl).
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FIGURE 7 Electrochromic behavior of the P2 film on the ITO-coated glass plate in CH3CN-containing 0.1 M (nC4H9)4NClO4 at
applied potentials (a) from 0 to 0.45 V and (b) from 0.45 to 0.9 V (vs. Ag/AgCl).
basically suggests no side reactions during these oxidation
steps under the stated measurement conditions. The stability
of the electrochemically generated poly(cationic radical)s
was estimated from the time-dependent UV–vis-near infrared
spectral changes of the thin films on the ITO plate. Both poly
(cationic radical)s were almost persistent under the inert
atmosphere, while the half-lives of the poly(cationic radical)s
of P1 and P2 under air were 69 and 231 h, respectively.
neutral polyamines to the aminium cationic radicals were
fully reversible when slow scanning rates were adopted. Fast
scanning of the potentials induced peeling of the films from
the ITO electrode, as indicated in the CV measurements. To
gain insight into the film stability and coloring mechanism in
detail, the response time and switching processes were
investigated. The absorbance ascribed to the aminium cati-
onic radicals was monitored as a function of time by UV–vis-
near infrared spectroscopy, and the absorbance profiles of
the P1 and P2 films are shown in Figures 8 and 9, respec-
tively. The pulse time for the P1 film at 720 nm and for the
P2 film at 820 nm was determined to be 20 and 50 s,
Reversible color production by the desired potential applica-
tions is essential for the fabrication of practical electrochro-
mic devices. For both the P1 and P2 films, the UV–vis-near
infrared spectral changes in the oxidation process from the
FIGURE 8 Current consumption (upper) and absorbance
change (lower) at 720 nm upon eletrochromic switching
between 0 and 0.27 V of the P1 film (ꢄ1 lm thickness) on the
ITO-coated glass plate (coated area: 2.5 cm ꢂ 0.8 cm).
FIGURE 9 Current consumption (upper) and absorbance
change (lower) at 820 nm upon eletrochromic switching
between 0 and 0.28 V of the P2 film (ꢄ1 lm thickness) on the
ITO-coated glass plate (coated area: 2.5 cm ꢂ 0.8 cm).
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2 (a) Han, F. S.; Higuchi, M.; Kurth, D. G. Adv. Mater. 2007, 19,
3928–3931; (b) Han, F. S.; Higuchi, M.; Kurth, D. G. J. Am.
Chem. Soc. 2008, 130, 2073–2081; (c) Nakamura, K.; Kanazawa,
K.; Kobayashi, N. Chem. Commun. 2011, 47, 10064–10066; (d)
Ikeda,T.; Higuchi, M. Langmuir 2011, 27, 4184–4189.
respectively, due to the starting point of the absorbance sat-
uration. The switching time was evaluated as 90% of the full
switch, because it is difficult to perceive any further color
change by the naked eye beyond this point. The results
revealed that the switching times of the coloring and bleach-
ing process for the P1 film at 720 nm (blue color) were 6.2
and 3.2 s, respectively. In the case of the P2 film at 820 nm
(green color), the slower coloring process of 11.1 s and
faster bleaching process of 1.8 s were observed. These values
are comparable to the reported values, supporting the clear
correlation between the response time and chemical struc-
tures.19 Importantly, these experiments revealed that the
color changes are reversibly achieved by applying cyclic
scans of potential pulses between the oxidation voltages and
the corresponding reduction voltages. The 100 cycle experi-
ments suggested that the decrease in the oxidized absorb-
ance was merely 5% for the P1 film and 12% for the P2
film, which were consistent with the repeated scans of the
CV curves. Unfortunately, the second color change of the P2
film from green to dark blue was not fully reversible upon
repeated pulse applications. The absorbance at 620 nm
became lower as more pulses were applied (not shown),
probably because of the undesired side reaction caused by
the unsubstituted para-positions of the triarylamine unit.
3 (a) Beaujuge, P. M.; Ellinger, S.; Reynolds, J. R. Nat. Mater.
2008, 7, 795–799; (b) Walczak, R. M.; Leonard, J. K.; Reynolds,
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M. R.; Babiarz, J. E.; Kiyak, K.; Reynolds, J. R. Macromolecules
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son, E. J.; Dyer, A. L.; Reynolds, J. R. Polym. Chem. 2011, 2,
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4 (a) Michinobu, T.; Okoshi, K.; Osako, H.; Kumazawa, H.; Shi-
gehara, K. Polymer 2008, 49, 192–199; (b) Michinobu, T.; Kuma-
zawa, H.; Noguchi, K.; Shigehara, K. Macromolecules 2009, 42,
5903–5905; (c) Michinobu, T.; Osako, H.; Seo, C.; Murata, K.;
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6 Sreenath, K.; Suneesh, C. V.; Kumar, V. K. R.; Gopidas, K. R.
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CONCLUSION
7 (a) Son, J. M.; Mori, T.; Ogino, K.; Sato, H.; Ito, Y. Macromo-
lecules 1999, 32, 4849–4854; (b) Michinobu, T.; Inui, J.; Nishide,
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We have developed a straightforward method for introducing
active electrochromophores into the reactive polystyrene
using the conventional click chemistry reaction. The total
synthetic yields of the aromatic amine-based electrochromo-
phore moieties amounted to 40–46%, suggesting the applic-
ability to large-scale synthesis. The tris(4-alkoxyphenyl)-
amine-based chromophore displayed a one-step color change
from transparent to blue, while the N,N,N0,N0-tetraphenyl-p-
phenylenediamine-based chromophore showed two-step
changes from transparent to green and finally to dark blue.
The production processes of the blue and green colors were
reversible due to the high chemical stability of the carefully
designed aminium cationic radicals. This useful postfunction-
alization methodology based on the combination of click
chemistry and electrochromism will be applied to a wide va-
riety of polymers and material surfaces.
8 (a) Liou,G.-S.;Lin,H.-Y.Macromolecules 2009, 42, 125–134; (b)
Yen, H.-J.; Liou, G.-S. Chem. Mater. 2009, 21, 4062–4070; (c)
Yen, H.-J.; Liou, G.-S. J. Polym. Sci., Part A: Polym. Chem.
2009, 47, 1584–1594; (d) Liou, G.-S.; Chang, H.-W.; Lin, K.-H.;
Su, Y. O. J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
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Sci., Part A: Polym. Chem. 2009, 47, 5378–5385; (f) Yen, H.-J.;
Liou, G.-S. J. Mater. Chem. 2010, 20, 9886–9894; (g) Huang,
L.-T.; Yen, H.-J.; Chang, C.-W.; Liou, G.-S. J. Polym. Sci., Part
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This work was supported, in part, by a Grant-in-Aid for Scien-
tific Research from MEXT, Japan. Y. L. acknowledges the finan-
cial support of the Sasakawa Scientific Research Grant from the
Japan Science Society. The authors thank Dr. Cha-Wen Chang for
his kind assistance in the electrochromic measurements.
9 Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. Int.
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