A single-layer approach to electrochromic materials

Article abstract of DOI:10.1002/pola.24433

This study is focused on the development of electrochromic (EC) materials that could be incorporated into electrically-driven switchable devices such as electrochromogenic glasses. The ultimate goal of this research is to depart from the complexity of the EC device construction which is in use today. Such construction consists of three layers each of them incorporating a specific functionality: the electrochromophore, the electrolyte and the ion storage, assembled between two transparent or reflective electrodes. In most of these conventional devices the electrolyte layer is a liquid or a gel. Since solid-state EC devices are of high commercial interest, we are exploring various avenues to reduce the number of layers to one layer that is all-solid and electrochromically/electrolytically and ionically functional. The design strategy is based on the use of polymers such as poly(epichlorohydrin-co- ethylene oxide), poly(vinyl butyral) and poly(ethylene-co-methacrylic acid) ionomer, to which EC properties were introduced by grafting reactions with specifically synthesized carbazole derivatives. A combination of analytical techniques was used to characterize the monomers and the carbazole-grafted polymers. A proof of concept was demonstrated for a single-layer, all-solid-state EC device consisting of a film of poly(ECH-co-EO) containing pendent carbazole groups, assembled between two transparent electrodes, Sn-doped In₂O₃ oxide-coated glasses.

Full text of DOI:10.1002/pola.24433

A Single-Layer Approach to Electrochromic Materials
SIMONA PERCEC, SUSAN TILFORD
DuPont Central Research and Development, Experimental Station, Wilmington, Delaware 19880
Received 27 September 2010; accepted 30 September 2010
DOI: 10.1002/pola.24433
Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This study is focused on the development of elec-
trochromic (EC) materials that could be incorporated into elec-
trically-driven switchable devices such as electrochromogenic
glasses. The ultimate goal of this research is to depart from
the complexity of the EC device construction which is in use
today. Such construction consists of three layers each of them
incorporating a specific functionality: the electrochromophore,
the electrolyte and the ion storage, assembled between two
transparent or reflective electrodes. In most of these conven-
tional devices the electrolyte layer is a liquid or a gel. Since
solid-state EC devices are of high commercial interest, we are
exploring various avenues to reduce the number of layers to
one layer that is all-solid and electrochromically/electrolytically
and ionically functional. The design strategy is based on the
use of polymers such as poly(epichlorohydrin-co-ethylene
oxide), poly(vinyl butyral) and poly(ethylene-co-methacrylic
acid) ionomer, to which EC properties were introduced by
grafting reactions with specifically synthesized carbazole deriv-
atives. A combination of analytical techniques was used to
characterize the monomers and the carbazole-grafted poly-
mers. A proof of concept was demonstrated for a single-layer,
all-solid-state EC device consisting of a film of poly(ECH-co-EO)
containing pendent carbazole groups, assembled between
two transparent electrodes, Sn-doped In2O3 oxide-coated
C
V
glasses.
2010 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 361–368, 2011
KEYWORDS: electrochromic device; functionalization of polymers;
phase transfer catalysis; polyelectrolytes; redox polymers
INTRODUCTION Our research is directed toward the con-
struction of simpler and all solid-state electrochromic (EC)
devices. This is a very challenging task as most of the exist-
ing technologies are based on liquid or gel electrolytes. From
a commercial standpoint, EC solid-state devices are more
desirable, because they eliminate the problems associated
with solvent evaporation and leakage. EC systems based on
solid polymeric electrolytes have been known for some
time.1 The most successful application of the polymer elec-
trolyte principle2 relies on polymer solvents such as poly
(ethylene oxide) and lithium salts. However, there are some
disadvantages associated with these polymer electrolytes.
The most notable are related to their glass transition (Tg)
temperature, which is not low enough to allow ionic conduc-
tivity at room temperature. Alternative methods of improving
the conductivity are, therefore, desirable. Finding combina-
tions of EC materials with polymer electrolytes exhibiting
higher conductivity at room temperature could have a signif-
icant impact on the applications of EC materials.3–6
ples include poly(epichlorohydrin-co-ethylene oxide) [poly
(ECH-co-EO)], poly(vinyl butyral) (PVB), and Na neutral-
ized poly(ethylene-co-methacrylic acid)7 [poly(E-co-MA)] or
R
SurlynV, respectively. All three types of polymers are chemi-
cally modified to incorporate the EC moieties via alkyl or
alkyl oxide linkers. In the case of poly(ECH-co-EO), the chlo-
rine side groups are used as grafting sites for derivatives of
carbazole. The carbazole-modified poly(ECH-co-EO) retains
elastomer functionality capable of dissolving inorganic salts
and permitting their transport in a dissociated state. In the
case of PVB and poly(E-co-MA), the AOH and ACOONa
groups, respectively, serve as grafting sites for carbazole
derivatives. In both cases, the alkyl spacers act as internal
plasticizers, which compensate for the conformational stiff-
ness of the polymer backbone. In addition, their Tg can be
further controlled by the addition of plasticizers. The
attached pendant electroactive species exhibit new optical
absorption bands (e.g., show a new color) in conjunction
with an electron transfer or redox reaction in which they ei-
ther gain or lose electrons. The new absorption can arise
from photoexcitation of an electron from a lower energy
level (or ground state) to a higher one, either in the same
molecule, which is an intramolecular excitation, or within a
neighboring moiety, which involves an optical charge trans-
fer. The grafting design offers the possibility of achieving
This article describes how polymer EC compositions
obtained by the modification of different types of polymers
show potential to be utilized in solid-state EC devices. The
EC compositions consist of polymer matrices selected from
(a) low Tg elastomers, (b) hydrophilic polymers that can
tolerate a high level of plasticizers, and (c) ionomers. Exam-
Correspondence to: S. Percec (E-mail: simona.percec@usa.dupont.com)
C
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 361–368 (2011) 2010 Wiley Periodicals, Inc.
A SINGLE-LAYER APPROACH TO EC MATERIALS, PERCEC AND TILFORD
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
control of EC properties by placing appropriate redox-active
chromophores in selective positions along the polymer back-
bone. This approach is quite versatile, allowing the attach-
ment of various EC groups to different polymer backbones.
Alternatively, different electroactive moieties can be used to
substitute different positions of the aromatic and heteroaro-
matic electrochromophores to vary the conduction properties
and/or achieve a controlled response with voltage fluctua-
tions. The ends of the electrochromophore molecules can be
individually functionalized with groups such as ASH, pyridyl,
ACN, and ASCN to promote the absorption and self-assem-
bly (SA) on metal surfaces, including Au, Cu, Pd, Pt, Ni, and
Al. Other functional groups such as ACOOH and AP(O)(OH)2
can also be introduced with the specific aim of providing
binding or SA to Sn-doped In2O3 oxide (ITO) surfaces.
Another possibility is to attach two or more types of chro-
mogenic groups, with similar or different substituents (e.g.,
one electron-donor and the other electron-acceptor to the
metal). Self-doped chromophores can be also obtained by
incorporating ionic groups (e.g., Naþ, Kþ, or Liþ as cationic
groups, and SO3ꢀ or ClO4ꢀ as anions) on the electrochromo-
phore. This report will be limited to the synthesis of electro-
chromophores and their reactions with poly(ECHO-co-EO),
PVB, and poly(E-co-MA).
carbazole with acrylonitrile in the presence of benzyltri-
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methylammonium hydroxide (TritonV B) as catalyst using a
procedure adapted from the literature.8 Carbazole (1.7 g,
0.0055 mol) and acrylonitrile (20 g, 0.369 mol) were intro-
duced into a 100-mL round-bottomed glass flask equipped
with magnetic stirrer. The flask was placed in an ice-bath
R
and allowed to cool for 15 min. Then, TritonV B (40% in
MeOH) was added dropwise (four drops) via a 50-lL syringe
for a total of 40 lL. The paste changed color from white to
yellow. The reaction mixture was allowed to warm-up slowly,
but there was no sign of an exothermic reaction. After 2 h,
heat was supplied via a heating mantle. Once the tempera-
ture reached 65 ꢁC, the paste became an orange solution.
The flask was heated for 1 h at 70 ꢁC. The product precipi-
tated in the flask after it had cooled. The precipitate was
then filtered and recrystallized from acetonitrile.
Preparation of the Potassium Salt of 2-(N-Carbazolyl)-
propionitrile. The potassium salt of 2-(N-carbazolyl)pro-
pionitrile was obtained by reacting 2-(N-carbazolyl)propio-
nitrile with potassium hydroxide in ethanol at reflux. KOH
(1.7 g) was dissolved in water (10 g), and then added to a
mixture of EtOH (15.8 g) and N-propionitrile carbazole
(0.293 g, 0.00132 mol) in a 100-mL round-bottomed glass
flask equipped with magnetic stirrer. After the addition of
the KOH, a condenser was attached, and the reaction mixture
was heated to reflux for 3 h. The flask was then attached to
a rotary evaporator, and the EtOH and water were removed
until a precipitate formed. The precipitate was filtered and
recrystallized from EtOH. The recrystallized material was
dried overnight at room temperature under nitrogen.
EXPERIMENTAL
Materials
All reagents were obtained from Sigma–Aldrich unless other-
wise indicated. Poly(E-co-MA) sodium salt ionomer was
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obtained as SurlynV from E. I. du Pont de Nemours (DuPont),
Wilmington, DE. PVB was also obtained from DuPont. Sn-
doped-ITO was obtained from Southwall Technologies, Palo
Alto, CA.
Preparation of Bromohexyl Carbazole. Tetrabutylammonium
bromide (TBAB) (2.622 g, 0.008 mol) and NaOH (50%, 3
mL) were placed in a 50-mL round-bottomed flask. 1,6-
Dibromohexane (4.392 g, 0.018 mol), benzene (3 mL), and
carbazole (1 g, 0.00598 mol) were mixed, forming a suspen-
sion. This suspension was then poured into the reaction flask
while stirring. The flask was then stoppered and allowed to
stir at room temperature. The reaction was allowed to con-
tinue at room temperature for 1.5 h and then overnight
(16 h). The reaction product was extracted three times with
25-mL methylene chloride followed by a final washing with
water (pH ¼ 7). A 1H NMR spectrum taken of the resulting
compound indicated that further purification was needed to
remove the unreacted dibromohexane. To purify the product,
a silica column was loaded with the crude product. Then, the
column was flushed with 500 mL of hexane to remove the
unreacted dibromohexane compound. Subsequently, the elu-
ent was changed to a 50/50 hexane/methylene chloride mix-
ture to collect the final product. An oily product was isolated
and diluted with EtOH. The precipitate that formed on cool-
ing was filtered off and dried under vacuum. The 1H NMR
spectrum of this compound is consistent with the expected
product. Bromobutyl carbazole was obtained in a similar
manner by the reaction of carbazole with dibromobutane.
Methods
Infrared Spectroscopy
Infrared spectroscopy (IR) analyses were carried out with a
Nicolet instrument, model Magna-IR 760 E.S.P.
1H NMR
1
The H NMR characterization of the monomers was obtained
at 400 or 500 MHz using chloroform-d1 as solvent and tetra-
methylsilane as internal standard.
Cyclic Voltammetry
Cyclic voltammetry (CV) experiments of the chromogenic
compounds and devices were obtained with a voltammetric
analyzer, CV-50W from Bioanalytical Systems.
Synthesis of Monomers
All reactions were carried out under an inert atmosphere.
All reagents were used without further purification unless
otherwise indicated. Thin-layer chromatography (TLC) analy-
sis was performed using EM Science silica gel 60 (F254)
plates (0.25 mm). Chromatographic purifications were per-
formed by flash chromatography on EM Science silica gel
(230–400 mesh).
Preparation of 2-(N-Carbazolyl)propionitrile. 2-(N-Carba-
Synthesis of 2-(2-Carbazol-9-yl-ethoxy)ethanol. The title
zolyl)propionitrile was synthesized via cyanoethylation of
compound was produced in two steps. In the first step, 2[2-
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ARTICLE
(chloroethoxy)ethoxyl]tetrahydropyran was prepared by the
reaction of 2-(2-chloropethoxy)ethanol and 3,4-dihydro-2H-
pyran in the presence of concentrated hydrochloric acid. In a
dry box, 2-(2-chloroethoxy)ethanol (50.0 g, 0.401 mol) and
135-mL chloroform were added to a 500-mL three-necked
flask equipped with a stirring bar, addition funnel and sep-
tum. 3,4-Dihydo-2H-pyran (39.7 g, 0.472 mol) in 35-mL chlo-
roform was added dropwise over 30 min using an addition
funnel. At the end of the addition, the flask was warm. The
addition funnel was removed from the flask and replaced
with a stopper, and the flask was transferred to the hood
and placed under an atmosphere of nitrogen. Twelve drops
of concentrated HCl were added dropwise via a syringe. The
septum was then replaced with a water condenser. The reac-
tion was heated at 40ꢁ for 1 h, and then allowed to cool to
room temperature. The flask was opened to the air, K2CO3
(9 g) was added to the reaction mixture, and the mixture
was stirred for 2 min before filtering. The solvent was
removed via rotary evaporation, and the product was dried
under vacuum for 8 h at room temperature to give 84 g
(0.406 mol) of 2[2-(2-chloroethoxy)ethoxyl]tetrahydropyran.
In the second step, 2-(2-carbazol-9-yl-ethoxy)-ethanol was
prepared in the following manner. Carbazole (20 g, 0.12
mol), 2[2-(chloroethoxy)ethoxyl]tetrahydropyran (38 g, 0.12
mol), benzyltriethylammonium chloride (TEBA) (4.2 g,
0.025 mol), NaI (1.0 g, 0.0055 mol), 100 mL of 50% NaOH
were performed using 100-mL THF. The THF extracts were
subjected to column chromatography to give a purified
carboxylic acid derivative.
Reaction of Poly(E-co-MA) with Bromoalkyl Carbazole
Bromoalkyl carbazole prepared as shown before was reacted
with poly(E-co-MA) sodium salt ionomer according to the
following procedure. Poly(E-co-MA) (0.5 g), DMAc (1.4 g),
and 1,2-dichlorobenzene (8.1 g) were added to a 25-mL
round-bottomed glass flask that was attached to a condenser.
This mixture was heated at 110ꢁ for 1 h until poly(E-co-MA)
was completely dissolved. TBAB (0.0281 g) and bromo-
butane carbazole (0.0263 g) were added with stirring. The
reaction was heated for 4 h and allowed to stir at room
temperature under N2 overnight. Approximately 0.5 mL of
the reaction mixture was added to 20 mL of a 1:9 methylene
chloride:hexane solution. The precipitated polymer was
filtered, washed with CH2Cl2 and hexane, and dried at room
temperature under vacuum.
The Reaction of PVB with Carbazole Propionic Acid
The reaction of PVB with carbazole propionic acid (prepared
in multiple steps as previously indicated) was performed in
the following manner. PVB (0.185 g) was placed in a 100-mL
round-bottomed flask with a stirring bar. THF (10 mL)
was added, and the PVB went into solution after 15 min. 9-
Propionic acid carbazole (0.239 g, prepared as previously
described), 4-(dimethylamino)-pyridine (0.0092 g), and 1,3-
dicyclohexylcarbodimide (0.206 g) were added, in that order.
The solution was homogeneous and colorless. An additional
15 mL of THF were added after 20 min. The reaction was
stirred at room temperature under N2 purge for 2 days. The
reaction mixture was then poured in 150-mL distilled water.
The resulting precipitated compound was washed twice with
100-mL distilled water, separated by filtration, and dried
under vacuum at 85ꢁ overnight.
solution, and 100-mL benzene were added to
a flask
equipped with a stir bar, condenser, and a nitrogen inlet. The
mixture was refluxed under nitrogen for 6 h. The reaction
mixture was diluted with 100 mL of benzene and the
organic phase was separated, washed with water, dried over
MgSO4 and filtered through acidic Al2O3. The solvent was
removed via rotary evaporation. The product was dissolved
in 400-mL MeOH. Concentrated HBr (2 mL) was added to
the flask, and the reaction was stirred overnight at room
temperature. The reaction was neutralized with 10% aque-
ous NaOH. The MeOH was removed by rotary evaporation to
give a dark orange oil. The final product was further purified
by recrystallization twice from methanol.
Reaction of PVB with Carboxylic Acid of
2-(2-Carbazol-9-yl-ethoxy)ethanol
PVB (1 g) was placed in a 100-mL round-bottomed flask
with a stir bar. DMSO (10 mL) was added, and PVB
was solubilized with stirring. 1,3-Dicyclohexylcarbodiimide
(0.206 g) was dissolved in 1.5-mL DMSO. 4-(Dimethyl-
amino)-pyridine (0.0092 g) was also dissolved in 1.5 mL of
DMSO. Once these solutions were completely homogeneous,
they were added to the solution of PVB in DMSO under stir-
ring and nitrogen. Then, the carboxylic acid of 2-(2-carbazol-
9-yl-ethoxy)-ethanol (0.62 g, 0.00198 mol) prepared as
shown in the previous section, was added to this reaction
mixture. The reaction was stirred and kept under a N2
blanket for 2 days. Then, the reaction mixture was poured
into 150 mL of distilled water. The water was decanted off,
and the polymer was washed two more times with 100 mL
of water. The water was decanted off, and the polymer was
Preparation of the Carboxylic Acid of 2-(2-Carbazol-9-yl-
ethoxy)ethanol. The 2-(2-carbazol-9-yl-ethoxy)ethanol was
transformed into its carboxylic acid in two steps. In the first
step, 2-(2-carbazol-9-yl-ethoxy)ethanol (0.584 g, 0.00227
mol) and 12.5-mL THF were introduced into a 100-mL
round-bottomed glass flask equipped with stirring in a dry
box. Separately, sodium hydride (0.0546 g, 0.00227 mol) was
mixed with 12.5-mL THF. The slurry obtained was added
very slowly to the flask. After complete addition, the reaction
mixture was allowed to stir under nitrogen at room temper-
ature overnight (16 h). Then, to this mixture, methyl chloroa-
cetate (0.326 g in 25 mL THF) was added dropwise under
nitrogen at room temperature. This mixture was allowed to
stir at room temperature for 4 h. Then, the flask was taken
out from the dry box and 30 mL of distilled water were
added, followed by the addition of 75 mL of brine solution.
The resulting solution was transferred into a separatory fun-
nel. The THF was then separated. Two additional extractions
ꢁ
dried in the vacuum oven at 85 C overnight.
Reaction of Poly(EPI-co-EO) with Potassium Salt
of 2-(N-Carbazolyl)propionitrile
A solution of poly(ECH-co-EO) was made from 0.46 g (0.005
mol) of polymer in 30 mL of anhydrous THF. Complete
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of 2-(N-carbazolyl)propionitrile and
ACOOK derivative (a) and bromoalkyl carbazole (b).
dissolution was achieved by stirring it overnight at room
temperature. This solution was heated with the potassium
carboxylic salt (0.293 g, 0.001 mol) of 2-(N-carbazolyl)pro-
pionitrile prepared as previously shown (in a ratio CH2Cl/
KOOC ¼ 1:1) in the presence of a stoichiometric amount
(0.322 g, 0.001 mol) of tetrabutylammonium bromide. The
SCHEME 2 Synthesis of 2-(2-carbazol-9-yl-ethoxy)-ethanol.
dependent on the type of amine.13 Aromatic amines such as
carbazole do not react without a catalyst. We adopted a pro-
cedure from the literature14 using benzyltrimethylammonium
R
hydroxide (TritonV B) as a catalyst, in the presence of which
ꢁ
reaction mixture was heated at 60 C for 96 h. The resulting
the reaction proceeds vigorously. The potassium salt was
obtained with KOH at reflux in ethanol. These reactions
are outlined in Scheme 1(a). The N-alkylation reaction of car-
bazole with dibromides is shown in Scheme 1(b). In this
case, the bromoalkyl carbazoles were prepared under mild
conditions using phase-transfer catalysis. The reactions were
carried out in benzene in the presence of TBAB and aqueous
50% sodium hydroxide (NaOH).
polymer was precipitated in water, filtered, and dried in a
vacuum oven. A 22-mg sample was dissolved in ꢂ0.7 mL of
deuterated THF. The sample was heated to 60 ꢁC until dis-
1
solved, and was analyzed by H NMR.
Preparation of an EC Device
The poly(E-co-EO) modified with carbazole pendent groups
and the salt, lithium perchlorate (LiClO4), were mixed in
anhydrous THF. Separately, two ITO-coated glasses were
rinsed with isopropanol and dried under nitrogen. Then, using
the THF solution made from the components previously men-
tioned, an EC film was coated on one piece of clean ITO. The
resulting EC film on ITO was sandwiched using another piece
of clean ITO. Wire leads were connected to each side of the
two ITO slides. An appropriate voltage (depending on the
electrochromophore) was applied, and the change in color
was observed. Then, the potential applied was reversed and
the return to the initial color was detected.
2-(2-Carbazol-9-yl-ethoxy)-ethanol was produced according
to a literature procedure15 via protection, deprotection
chemistries in two steps, as shown in Scheme 2. Accordingly,
2[2-(chloroethoxy)ethoxyl]tetrahydropyran was synthesized
first by the reaction of 2-(2-chloroethoxy)ethanol with 3,4-
dihydro-2H-pyran. In the second step, this compound was
reacted with carbazole in benzene and aqueous NaOH using
RESULTS AND DISCUSSION
Different types of new electrochromophores have been
reported for the synthesis of functionalized polymers.8
Organic–inorganic hybrids consisting of polyether phases,
which act as a solid solvent for lithium ions are also
known.9–12 Examples of electrochromophores that can be
used for the assembly of solid-state, single-layer devices
have not been described in the literature. To be functional,
such a layer needs to combine EC, electrolytic, and ionic
properties. The main goal of this study was to explore a
combination of technical approaches, including grafting and
phase-transfer catalysis, for the design and synthesis of novel
electrochromophores. Their use in the assembly of a solid-
state, single-layer EC device was demonstrated in the case of
poly(ECH-co-EO) grafted with carbazole.
Synthesis of Carbazole Derivatives
2-(N-Carbazolyl)propionitrile was synthesized via the cyano-
ethylation of carbazole with acrylonitrile. It is known that
the ease with which acrylonitrile reacts with amines is
FIGURE 1 CV of ferrocene in a 0.1 M solution of tetrabutylam-
monium hexafluorophosphate in acetonitrile using Ag wire as
a reference electrode.
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ARTICLE
SCHEME 4 Reaction of PVB with the carboxylic acid of 2-(2-car-
bazol-9-yl-ethoxy)-ethanol.
where: A ¼ 2070 nm/1705 nm (OH/methylene), B ¼ 1925
nm/1705 nm (water/methylene).
FIGURE 2 CV of 2-(2-carbazol-9-yl-ethoxy)-ethanol.
The results indicating the OH% of the PVB starting material
and of two samples of PVB after their modification with a
low and a high level of carboxylic acid of 2-(2-carbazol-9-yl-
ethoxy)-ethanol can be seen in Table 1.
TEBA phase-transfer catalyst to obtain 2-(2-carbazol-9-yl-
ethoxy)-ethanol.
CV experiments of 2-(2-carbazol-9-yl-ethoxy)-ethanol were
carried out using ferrocene as a reference (Fig. 1). The
voltammogram of 2-(2-carbazol-9-yl-ethoxy)-ethanol displays
oxidation and reduction peaks (Fig. 2) appearing near two
positive voltages, 98 and 140 mV, respectively. The peaks are
not symmetrical, probably resulting from the kinetic limita-
tions in the redox reaction.
R
Reaction of SurlynV with Bromo Alkyl Carbazole
Bromo-alkyl carbazole, prepared as shown in the Experimen-
tal section, was reacted with poly(E-co-MA) sodium salt
ionomer according to Scheme 5.
R
Redox Properties of Carbazole Modified SurlynV
R
A film of carbazole-modified SurlynV, prepared as is indi-
As it is not possible to react 2-(2-carbazol-9-yl-ethoxy)-etha-
nol directly with PVB, it was transformed into its carboxylic
derivative in two steps. In the first step, it was reacted with
sodium hydride and, in the second step, the carboxylic acid
derivative was obtained from the ethoxy compound and
methyl chloroacetate as is indicated in Scheme 3.
cated in the Experimental section, was made on Pt foil in a
dry box by dissolving the dried polymer in aꢁmixture of 81%
1,2-dichlorobenzene and 14% DMAc at 110 C. After solvent
evaporation, the film was tested for redox properties in an
electrochemical cell. The working electrode was the Pt with
the film on top. The counter electrode was also Pt. The refer-
ence electrode was silver wire. The test solution was aceto-
nitrile with 0.1 M tetrabutylammonium hexafluorophosphate
Polymer Modification Reactions with
Carbazole Derivatives
R
as supporting electrolyte. The CV of SurlynV is shown in
Figure 3 and, as expected, indicates no redox activity. The CV
of the bromopropyl carbazole is shown in Figure 4. In con-
The reaction of PVB with the carboxylic acid of 2-(2-car-
bazol-9-yl-ethoxy)-ethanol was performed according to
Scheme 4.
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trast, SurlynV modified with butyl carbazole (Fig. 5) indicates
the presence of two redox peaks (175 and 4 mV), in good
agreement with the redox activity exhibited by the bromo-
butyl carbazole compound (Fig. 4).
The degree of modification of PVB with the carboxylic acid
of 2-(2-carbazol-9-yl-ethoxy)-ethanol was estimated using IR
spectroscopy. The OH level was calculated using the follow-
ing formula that takes into account the IR region from 2150
to 1600 nm.
TABLE 1 Estimated OH% of the PVB Starting Material and of
Two Samples of PVB after Modification with a Low and a High
Level of Carboxylic Acid of 2-(2-Carbazol-9-yl-ethoxy)-ethanol
OH% ¼ 21.9872[A – (0.1 ꢃ B)] þ 0.3841
No.
Samples
PVB
OH (%)
1
2
18.82
16.50
PVB grafted with a low level
of the carboxylic acid
of 2-(2-carbazol-9-yl-ethoxy)-
ethanol
3
PVB grafted with a high level
of the carboxylic acid
of 2-(2-carbazol-9-yl-ethoxy)-
ethanol
No measurable
OH peak (2070 nm)
SCHEME 3 Synthesis of the carboxylic acid of 2-(2-carbazol-9-
yl-ethoxy)-ethanol.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 5 Reaction of bromoalkyl carbazole with poly(E-co-
MA) sodium salt ionomer.
Poly(ECH-co-EO) with Pendant Carbazole Groups
Poly(ECH-co-EO) contains ꢂ66% epichlorohydrin. The intent
was to displace the chlorine from the epichlorohydrin by
reaction with the potassium salt of 2-(N-carbazolyl)
propionitrile.
FIGURE 4 CV of bromopropylcarbazole.
There are major drawbacks regarding the nucleophilic dis-
placement of chlorine from poly(ECH-co-EO). They include
dehydrochlorination side reactions and degradation result-
ing from the formation of the vinylether units.14 For these
considerations, to achieve the chemical modification of poly
(ECH-co-EO), we chose to use phase-transfer catalysis. The
synthetic scheme for this preparation is shown in Scheme 6.
The 1H NMR spectrum of the carbazole-modified resulting
polymer was consistent with the proposed structure and
shows 57% conversion. This spectrum and the assignments
are shown in Figure 6.
1H NMR 500 MHz, d8-THF: 8.053 (br, 2H), 7.516 (br, 2H),
7.404 (br, 2H), 7.157 (br, 2H), 4.672 (br, 2H), 4.1 (d, 3.5H),
3.687 (br), 2.858 (br, 2H).
The UV–vis spectra of the potassium salt of the carbazole
derivative and of poly(ECH-co-EO) modified with this deriva-
tive can be seen in Figure 7. It can be observed that the modi-
fied polymer exhibits the signature of the carbazole derivative.
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FIGURE 5 CV of SurlynV modified with bromopropylcarbazole.
are electrochemical cells where the EC electrode (transpar-
ent ITO modified with electrochromophore) is separated
from the counter electrode by an electrolyte. The assembly
of solid-state devices offers several advantages in applica-
tions requiring large surface area and flexibility and also
eliminates problems related to the lifetime of the device
such as solvent evaporation and leakage. Although the solid
state is commercially highly desirable, it is in a very early
stage of development with very few known examples. The
example we present is based on the fabrication of solid devi-
ces using two optically complementary conductive polymer
Characterization of EC Properties
An important aspect of the use of an EC material is how to
incorporate it into an EC device. The conventional devices
SCHEME 6 Reaction of poly(ECH-co-EO) with potassium salt of
2-(N-carbazolyl)propionitrile.
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FIGURE 3 CV of SurlynV.
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ARTICLE
FIGURE 6 1H NMR Spectrum of carbazole-modified poly(ECH-co-EO).
the use of liquid or gel electrolytes. For proof of concept, we
prepared a single-layer solid-state device, the schematic of
which is shown in Scheme 7.
First, a mixture was prepared from the 2-(N-carbazolyl)pro-
pionitrile-modified poly(EPI-co-EO) and LiClO4 in THF. Then,
an EC film was coated from this mixture and sandwiched
between two ITO-coated glasses, as described in the Experi-
mental section. An appropriate voltage (1 V) was applied for
20 s, and the change in color was observed. Then, the
applied potential was reversed, and the return to the initial
color was detected.
The EC mechanism associated with the presence of the car-
bazole moiety is supposed to involve an oxidation reaction
to form a delocalized radical cation in one of the positions
shown in Scheme 8.
FIGURE 7 UV–vis spectra of the potassium salt of the carbazole
derivative (1) and of poly(ECH-co-EO) modified with this deriva-
tive (2).
blends deposited on ITO-PET and a polymeric electrolyte.3 In
our case, the ultimate goal is to simplify the EC device con-
struction, to reduce the number of layers, and to eliminate
SCHEME 8 EC mechanism associated with the presence of
SCHEME 7 Simple schematic of a single-layer EC device.
carbazole.
A SINGLE-LAYER APPROACH TO EC MATERIALS, PERCEC AND TILFORD
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 8 Initial color of carbazole-electrolyte-elastomer film
(0.3175 mm) sandwiched between two ITO-coated glasses.
FIGURE 9 Change in color of carbazole-electrolyte-elastomer
film after applying 1 V for 20 s.
The EC response of the EC device made of carbazole-modi-
fied poly(EPI-co-EO) can be depicted by examining Figures 8
and 9. In Figure 8, the EC layer appears transparent and
uncolored, and the text placed under the device can be
clearly distinguished. By applying a low voltage (1 V), the
color changes to green and the transparency is reduced.
REFERENCES AND NOTES
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Paoli, M. A. Adv Mater 1998, 10, 1522–1525.
CONCLUSIONS
4 Imrie, C. T.; Ingram, M. D.; McHattie, G. S. Adv Mater 1999,
Several EC derivatives consisting of carbazole equipped with
alkyl or alkyl oxide linkers have been synthesized. 2-(N-Car-
bazolyl)propionitrile was synthesized via a cyanoethylation
reaction of carbazole with acrylonitrile in the presence of
benzyltrimethylammonium hydroxide as catalyst. Bromoalkyl
carbazoles were obtained from carbazole and dibromoal-
kanes under phase-transfer-catalyzed conditions. 2-(2-Carba-
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deprotection chemistry using TEBA phase-transfer catalyst.
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propionitrile under phase-transfer catalysis conditions to
avoid the dehydrochlorination side reactions and degradation
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5 Sotzing, G. A.; Reddinger, J. L.; Katritzky, A. R.; Soloducho,
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R
resulting from formation of the vinyl ether units. SurlynV
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was modified with bromobutyl carbazole in the presence of
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dichlorobenzene. PVB was EC functionalized with 9-propi-
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THF as a solvent. Various degrees of functionalization were
obtained, as determined by IR spectroscopy. The EC proper-
ties of these devices were investigated using bulk electrolysis
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SurlynV were also identified by cyclic voltammetry experi-
14 Percec, V. Polym Bull 1981, 5, 651–657.
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carbazole-modified p(ECH-co-EO) elastomer placed between
ITO electrodes that changed color when it was subjected to
a change in voltage.
15 Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanov-
skaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.;
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