Soluble variable color carbazole-containing electrochromic polymers

Article abstract of DOI:10.1021/ma050805x

Three new carbazole-containing polymers were synthesized through Suzuki cross-coupling between N-octylcarbazole-3,6-bis(ethyleneboronate) (NOctCz(BO₂Et)₂) and several arylene dibromides. 2,5-Dibromothiophene (DBT), 2,2′-dibromo-5,5′-bithiophene (DEBT), and 4,7-dibromo-2,1,3-benzothiadiazole (DBBTD) gave the three polymers PCzTh, PCzBi, and PCzBTD, respectively. Cyclic voltammetry showed two separate redox processes for each polymer, PCzTh (E_1/2 = 0.33 V and E_1/2 = 0.65 V vs Fc/ Fc⁺), PCzBi (E_1/2 = 0.32 V and E _1/2 = 0.60 V vs Fc/Fc⁺), and PCzBTD (E_1/2 = 0.56 V and E_1/2 = 0.84 V vs Fc/Fc⁺). The two redox couples signify the presence of stable radical cation and dication oxidation states. In addition, the three materials were observed to display electrochromic properties. UV/vis/NIR absorbance spectroscopy revealed that each of the polymers displayed a unique absorbance maximum (345 nm for PCzTh, 375 nm for PCzBi, and 460 nm for PCzBTD) in their neutral states. Spectroelectrochemical analyses demonstrated that the π to π* transition was depleted upon application of increasingly positive potentials, giving rise to color changes. The different color states were quantified by colorimetry. PCzTh exhibits a yellow neutral state, a faint green radical cation state, and a pink dication state. PCzBi is yellow in its neutral state, light orange as a radical cation, and gray when fully oxidized. PCzBTD is orange when neutral, green-gray at intermediate potentials, and slate-gray as a dication.

Full text of DOI:10.1021/ma050805x

7636
Macromolecules 2005, 38, 7636-7644
Soluble Variable Color Carbazole-Containing Electrochromic Polymers
David Witker and John R. Reynolds*
The George and Josephine Butler Polymer Research Laboratories, Department of Chemistry, Center for
Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611
Received April 16, 2005; Revised Manuscript Received July 6, 2005
ABSTRACT: Three new carbazole-containing polymers were synthesized through Suzuki cross-coupling
between N-octylcarbazole-3,6-bis(ethyleneboronate) (NOctCz(BO2Et)2) and several arylene dibromides.
2,5-Dibromothiophene (DBT), 2,2′-dibromo-5,5′-bithiophene (DBBT), and 4,7-dibromo-2,1,3-benzothia-
diazole (DBBTD) gave the three polymers PCzTh, PCzBi, and PCzBTD, respectively. Cyclic voltammetry
showed two separate redox processes for each polymer, PCzTh (E1/2 ) 0.33 V and E1/2 ) 0.65 V vs Fc/
Fc+), PCzBi (E1/2 ) 0.32 V and E1/2 ) 0.60 V vs Fc/Fc+), and PCzBTD (E1/2 ) 0.56 V and E1/2 ) 0.84 V vs
Fc/Fc+). The two redox couples signify the presence of stable radical cation and dication oxidation states.
In addition, the three materials were observed to display electrochromic properties. UV/vis/NIR absorbance
spectroscopy revealed that each of the polymers displayed a unique absorbance maximum (345 nm for
PCzTh, 375 nm for PCzBi, and 460 nm for PCzBTD) in their neutral states. Spectroelectrochemical
analyses demonstrated that the π to π* transition was depleted upon application of increasingly positive
potentials, giving rise to color changes. The different color states were quantified by colorimetry. PCzTh
exhibits a yellow neutral state, a faint green radical cation state, and a pink dication state. PCzBi is
yellow in its neutral state, light orange as a radical cation, and gray when fully oxidized. PCzBTD is
orange when neutral, green-gray at intermediate potentials, and slate-gray as a dication.
Introduction
color and multicolor electrochromic polymers. However,
some difficulties exist with many of the known electro-
chromic polymer systems. Poly(3-alkyl thiophene)s8 and
the poly(2,2-dialkylpropylenedioxythiophene)9 group of
polymers are two of the few electrochromic conjugated
polymer families that have been obtained through
means other than electropolymerization or oxidative
chemical polymerization. These techniques, while often
useful and easily implemented, possess several inherent
drawbacks. Anodic polymerization onto an ITO-coated
substrate can give the thin films necessary for practical
devices. However, obtaining large films of high quality
and uniformity, such as those necessary for electrochro-
mic windows, is often quite difficult due to nonuniform
potentials across the electrode surface. In addition, this
technique provides only a relatively small amount of
material, making potential large-scale uses of this
technique difficult. Chemical polymerization with an
oxidant such as FeCl3 can be used to produce large
quantities of material rather easily, as demonstrated
by Sugimoto et al.10 However, this approach often leads
to irregular linkages that dramatically diminish the
desired properties of the material.11 Ideally, electro-
chromic conjugated polymers would be produced through
a nonoxidative chemical process in which the reactive
sites on the monomers are clearly defined. This ap-
proach would give large quantities of materials with
regular and consistent structures. In addition, solubility
in common organic solvents is necessary in order to
facilitate processing, such as spin- and spray-casting of
films or printing onto the appropriate substrates.
Electrochromic materials are those that exhibit dif-
ferent colors depending upon their oxidation state. A
wide variety of electrochomic materials are presently
1
known, ranging from metal oxides such as WO3 and
mixed-valence metal complexes like Prussian Blue2 to
organic molecules. In the realm of organic electrochro-
mic systems, both small molecules like viologens3 and
conjugated polymers4 have been shown to display elec-
trochromic effects. Electrochromic materials exhibit at
least two distinct color states and may give multiple
colors, depending on the structure of the material.
The applications for electrochromic conjugated poly-
mers are quite diverse due to several favorable proper-
ties of these materials. Since the various oxidation
states of conjugated polymers are quite stable, the colors
corresponding to these states are long-lasting. In addi-
tion, conjugated polymer electrochromics usually show
fast switching times, allowing for rapid color changes
from the materials upon application of appropriate
potentials.5 Finally, electrochromic polymers are known
to have excellent switching reproducibility, meaning
that they can be switched between their various color
states many times without any noticeable decline in
performance.6 One of the more prominent uses for
electrochromic polymers is in window applications,
where a device can be cycled between transparent and
tinted states through the use of a polymer with trans-
missive and absorptive color states. Also, as the palette
of available colors in electrochromic polymers grows,
these materials should become more useful for display
technology.
One of the interesting multicolor systems is that
based upon the 3,6-linked carbazole moiety.12 This unit
in conjugated polymer chains serves as a conjugation
break, which in turn leads to some useful properties.
Upon initial oxidation of a typical conjugated polymer,
an electron is removed from the conjugated backbone,
and a radical cation is formed as shown in Scheme 1
for a 1,4-phenylene linked bicarbazole. In most cases,
There has been a considerable body of work reported
concerning electrochromic polymers.7 Through the use
of a wide variety of conjugated structures, an impressive
array of available colors has been attained in both two-
* Corresponding author. E-mail reynolds@chem.ufl.edu.
10.1021/ma050805x CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/09/2005

Macromolecules, Vol. 38, No. 18, 2005
Carbazole-Containing Electrochromic Polymers 7637
Scheme 1. A Segment of
Poly(carbazole-co-phenylene) Depicting (a) Neutral
State, (b) Radical Cation State, and (c) Dication
Statea
Scheme 2. Synthetic Approach for NOctCz(BO2Et)2
and Suzuki Cross-Coupling Polymerization To Give
PCzAr Family
In this work, we have synthesized three new carba-
zole-containing polymers through the use of Suzuki
cross-coupling. This approach is well-suited for the task
in that the reactive species, boronate esters and halides,
can both be isolated and their purities verified, ensuring
that the polymers produced have the desired structures
with minimal defects.14 The repeat units of these
materials include N-octylcarbazole-thiophene (PCzTh),
N-octylcarbazole-bithiophene (PCzBi), and N-octylcar-
bazole-benzothiadiazole (PCzBTD). By varying the
comonomer used along with the carbazole unit, we have
been able to control the electronic properties of the
materials and therefore ultimately tune the colors
exhibited by the polymers. Inclusion of appropriate
substituents allows the polymers produced to be dis-
solved in a variety of organic solvents, such as chloro-
form, methylene chloride, toluene, and THF, thus
facilitating processing. The new electrochromic carba-
zole-containing polymers reported here have been spray-
cast onto ITO/glass slides, giving uniform films that
exhibit electrochromism. This effort represents a foray
into a new family of electrochromic polymers that can
be easily synthesized and processed. In addition, the
presence of conjugation-breaking 3,6-linked carbazole
units allows for these materials to support three oxida-
tion states, meaning that three distinct colors are
available in each polymer.
a Conjugation breaks are represented by dashed lines.
these radicals are unstable and combine to give dica-
tions. However, because of the conjugation breaks
present due to the inclusion of 3,6-linked carbazole
units, the radical cations are separated from one
another and do not combine. Upon further oxidation at
higher potentials, another electron is removed, giving
dications. The ability of these materials to support two
distinct oxidation states leads to a three-color electro-
chromic effect, with separate colors present for the
neutral, radical cation, and dication states. It is impor-
tant to note that while several examples of polymers
containing 2,7-linked carbazole units13 have been re-
ported, these materials cannot support three stable
oxidation states, as the 2,7-linked carbazole unit does
not function as a conjugation break.
Results and Discussion
In addition to the presence of three colors, another
advantage of electrochromic polymers containing 3,6-
linked carbazole units is the elimination of color depen-
dence on molecular weight. In typical conjugated poly-
mers, electronic properties of oligomers change as the
length of the chains increases. This effect continues until
a maximum conjugation length is reached. Above this
critical length, the physical properties of the material
may change, but the conjugation length remains con-
stant. However, with a material containing conjugation
breaks in each repeat unit, the conjugation length of
the polymer is set by the structure of the chromophore,
meaning that both oligomers and high molecular weight
materials will be the same color. Since shorter chains
are often easier to synthesize and can show higher
solubility, leading to easier processing, conjugated
polymers whose electrochromic properties are indepen-
dent of molecular weight can be quite useful.
Monomer Syntheses. N-Octylcarbazole-3,6-bis(eth-
yleneboronate) (NOctCz(BO2Et)2) was synthesized from
3,6-dibromo-N-octylcarbazole according to an estab-
lished procedure as outlined in Scheme 2.15 The crude
material was recrystallized from hexanes and then from
ethyl acetate immediately prior to use. After drying in
vacuo at 100 °C, the 1H NMR spectrum of the material
displayed all the expected resonances with no discern-
ible peaks corresponding to impurities. 2,5-Dibro-
mothiophene and 2,2′-dibromo-5,5′-bithiophene were
synthesized using NBS in DMF along with thiophene
and 2,2′-bithiophene, respectively. 2,5-Dibromothiophene
(DBT) was distilled, and 2,2′-dibromo-5,5′-bithiophene
(DBBT) was recrystallized from acetone/water shortly
before use. 4,7-Dibromobenzo-2,1,3-thiadiazole (DBBTD)
was produced through the reaction of benzo-2,1,3-
thiadiazole and bromine in aqueous HBr. This monomer
was recrystallized from acetone/water immediately prior

7638 Witker and Reynolds
Macromolecules, Vol. 38, No. 18, 2005
Table 1. Summary of GPC Data (Polystyrene Standard)
for PCzAr Polymers
polymer
Mn (g/mol)
Mw (g/mol)
PDI
Xn
PCzTh
PCzBi
PCzBTD
2600
3700
2800
4100
5800
3700
1.57
1.57
1.32
7.2
8.4
6.8
contain both carbazole units and the appropriate arylene.
Since each material is treated with 6 M HCl to remove
any terminal boronates after completion of the polym-
erization, it is expected that the polymers obtained
contain N-octylcarbazole or the appropriate bromoarylene
as end groups. The 1H NMR spectrum of N-octylcarba-
zole shows that the protons at the 3- and 6-positions
overlap those at the 2- and 7-positions, giving a complex
multiplet at 7.66 ppm. For each polymer sample, the
corresponding peak integrates to roughly two hydrogens
per repeat unit, indicating that very few protons in the
3- and 6-positions of the carbazole unit are present and
that the material is indeed a polymer. However, since
the crucial signals are not isolated from the peaks
corresponding to the protons at the 2- and 7-positions
of the carbazole unit, molecular weight estimation by
1H NMR is not feasible.
Figure 1. 1H NMR spectra of (a) distal N-octyl protons, (b)
R-methylene protons, (c) carbazole protons, and (d) comonomer
protons.
to use. After distillation (DBT) and drying in vacuo at
100 °C (DBBT, DBBTD), the identities of these materi-
1
als were verified by H NMR. Tetrakis(triphenylphos-
phine)palladium was synthesized by addition of hydra-
zine to a solution of palladium(II) chloride and triphenyl-
phosphine in DMSO at 120 °C. The solid was recrystal-
lized from benzene/methanol, dried in vacuo, and used
within a few hours after preparation.
GPC analysis was used to estimate the molecular
weight of each sample. The molecular weight data are
summarized in Table 1. It was determined that the
polymers were all of rather low molecular weights. All
three polymers gave number-average molecular weights
of about 3000 g/mol. This value corresponds to a degree
of polymerization of seven to eight, meaning that the
polymers synthesized here have an average of about 30
aromatic rings per chain. The identity of the arylene
dibromide used in the polymerizations had no signifi-
cant effect on the molecular weights of the materials.
This result is unexpected in that electron-poor DBBTD
would be expected to facilitate the coupling reaction by
more readily participating in the oxidative addition step
of the catalytic cycle. It is important to note that the
molecular weights of these polymers do not affect their
optical or redox properties, owing to the conjugation-
breaking nature of the carbazole units. As noted below,
the molecular weights obtained were adequate for
processing and film formation, and thus, further opti-
mization of the polymerization was not attempted.
UV/vis absorbance spectra of the neutral polymers in
solution (methylene chloride for PCzTh and PCzBi,
toluene for PCzBTD) are given in Figure 2a. These data
show that the colors of the polymers produced through
this method can be tuned by varying the identity of the
aryl dibromide used as a comonomer. PCzTh absorbs
at a fairly short wavelength, giving solutions of this
material a bright yellow color with a λmax of 345 nm.
Addition of another thiophene ring to the repeat unit,
as in PCzBi, shifts the absorbance to slightly longer
wavelengths, giving a λmax of 375 nm. Finally, as
expected on account of the donor-acceptor interaction
between the electron-poor benzothiadiazole unit and the
relatively electron-rich carbazole, PCzBTD shows the
greatest λmax at 460 nm, leading to an orange color. As
a point of comparison, Figure 2b shows UV/vis spectra
and photography of neutral polymer films produced by
spray-coating on ITO/glass. The spectra presented here
are somewhat broadened and red-shifted as compared
to their solution counterparts. This effect may be a
result of some light scattering by the polymer films but
is most likely due a wider range of environments for
the chromophore in the solid state as opposed to solution
Polymer Syntheses. The polymers described here
were all produced using Suzuki cross-coupling tech-
niques given in Scheme 2. A two-phase mixture of the
monomers, Pd(PPh3)4, and Aliquat 336 in toluene and
2 M aqueous potassium carbonate served as the polym-
erization medium. The carbazole unit, (NOctCz(BO2-
Et)2), carries the boronate esters and is present to act
as a conjugation break and also as a carrier of the
solublizing octyl chain. The aryl dibromides serve as
electronic tuning units, influencing the electronic prop-
erties of the polymers and ultimately allowing for some
degree of control over the various color states of the
materials. NOctCz(BO2Et)2 allows for a large number
of carbazole-containing polymers to be synthesized by
the procedure described here. This monomer can be
combined with any of the numerous commercially
available and easily synthesized aryl dihalides under
the reaction conditions described here to give a wide
variety of colors in soluble, easily processed electrochro-
mic polymers.
Structural Characterization. Analysis of the poly-
mers by 1H NMR spectroscopy in 1,1,2,2-tetrachloroet-
hane-d2 at 120 °C is given in Figure 1. Each spectrum
shows signals corresponding to the protons (a) bonded
to carbons 2-8 of the aliphatic N-octyl group (2.0-0.8
ppm) as well as the methylene protons (b) adjacent to
the carbazole nitrogen (4.2 ppm). While the chemical
shifts of the more distal protons are not affected by the
structure of the polymer backbone, in PCzBTD, the
signal corresponding to the N-bonded methylene unit
is shifted slightly downfield as compared to PCzTh and
PCzBi. This change can be attributed to the strong
electron-withdrawing character of the BTD units in the
polymer backbone. In addition, peaks representing the
aromatic protons (c) of the carbazole unit (8.5-7.0 ppm)
were also observed. Finally, resonances (d) indicating
the presence of the arylene groups (7.2 ppm for PCzTh,
7.2-7.0 ppm for PCzBi, and 8.8 ppm for PCzBTD)
unique to each of the individual materials were also
present. The differences in the aromatic region among
the spectra indicate that the polymers synthesized

Macromolecules, Vol. 38, No. 18, 2005
Carbazole-Containing Electrochromic Polymers 7639
Figure 3. Photographs of a sample of PCzBTD in methylene
chloride with gradual oxidation by SbCl5.
Figure 2. UV/vis spectra of PCzAr family (a) in solution and
(b) as films. Photographs above each peak show solution and
film colors.
and not a change in the optoelectronic nature of the
chromophore.
Solution Doping. Solutions of the polymers in
methylene chloride (2 mL, ∼0.01 mg/mL) were treated
with 20 µL portions of a 100 µM solution of antimony
pentachloride in methylene chloride. The addition of
SbCl5 was continued until no further spectral changes
were observed. For each polymer, the absorbance spec-
tra were unaffected by additions of SbCl5 above roughly
1 equiv SbCl5 per 2 repeat units. Since SbCl5 is a much
stronger oxidant than the oxidized polymer and the
hexachloroantimonate ion generated by the doping
process does not contribute any color to the solutions,
this doping method is effective for gauging the color
changes of polymer solutions at varying degrees of
oxidation as shown in Figure 3 for PCzBTD. In this
instance, the distinct orange (neutral) to transmissive
gray-blue (fully oxidized) states are evident as extremes,
while the solution passes through less saturated inter-
mediate colors upon oxidation. As the oxidant is added
to the polymer solution, the π to π* transition is
gradually depleted, leading in each case to broad ab-
sorbances in the near-infrared region of the spectrum,
as shown in Figure 4. Each spectrum shows isosbestic
points between the π to π* transition given by the
neutral polymer and the absorbances that occur with
addition of the oxidant. The well-defined isosbestic
Figure 4. UV/vis/NIR spectra of (a) PCzTh, (b) PCzBi, and
(c) PCzBTD as a function of added SbCl5. Dashed lines indicate
the visible region of the spectrum. Arrows signify the spectral
changes with increasing oxidant. 20 µL aliquots of a 100 µM
SbCl5 solution were added to 2 mL of a ∼0.01 mg/mL polymer
solution until no further spectral changes could be observed.
points are to be expected for oxidation of conjugated
polymers having discrete chromophores in solution, as
the chromophores are necessarily well-separated from

7640 Witker and Reynolds
Macromolecules, Vol. 38, No. 18, 2005
one another under the relatively dilute conditions. This
separation eliminates the range of chromophore oxida-
tion potentials commonly observed in the solid state.
The difference between solution and solid state spectra
of the PCzAr materials upon oxidation will be presented
in a following section. It is interesting to note that the
polymers do show some deviance from the isosbestic
points after the addition of a relatively large amount of
SbCl5. This effect may be due to some degree of
overoxidation, meaning that the polymer is oxidized to
such a great extent that structural changes may occur,
making reneutralization impossible.
Film Formation. Films were produced by spraying
solutions of the polymers (methylene chloride for PCzTh
and PCzBi, toluene for PCzBTD) onto ITO-coated glass
slides. As electroactive polymers become more common
and find more uses in practical devices, it becomes
necessary to easily process these materials. While
electropolymerization of a desired monomer gives the
films necessary for device construction, it is often
difficult to obtain even films of any considerable size
due to potential gradients across the electrode surface.
On the other hand, spray-casting allows for films to be
produced in which size is not a consideration. Simply
by applying a uniform coating of a polymer solution,
smooth, even films can be obtained. This attribute of
the technique is especially important for applications
such as electrochromic windows which are necessarily
larger than a few square centimeters. In addition to the
elimination of size considerations, spray-casting is more
material-economic than the more traditional spin-cast-
ing technique, in that only a small fraction of the active
material applied to the substrate is not incorporated into
the film. The films produced by spray-casting with
solutions of the PCzAr materials were insoluble in both
acetonitrile and propylene carbonate, allowing for elec-
trochemistry to proceed.
Polymer Film Electrochemistry. Cyclic voltam-
mograms of the three polymers described here were
obtained by casting films of the polymers onto a
platinum button electrode (area ) 0.02 cm2). After the
scans were completed, a small quantity of ferrocene was
introduced to the cell, and a final scan was performed
in order to calibrate the results. As depicted in Figure
5, two redox processes were observed for each polymer,
indicative of the radical cation and dication states. As
expected, the materials with more electron-rich comono-
mers, PCzTh (E1/2 ) 0.33 V vs Fc/Fc+ for the radical
cation, E1/2 ) 0.65 V vs Fc/Fc+ for the dication) and
PCzBi (E1/2 ) 0.32 V vs Fc/Fc+ for the radical cation,
E1/2 ) 0.60 V vs Fc/Fc+ for the dication), showed less
positive oxidation potentials than the more electron-
deficient PCzBTD (E1/2 ) 0.56 V vs Fc/Fc+ for the radical
cation, E1/2 ) 0.84 V vs Fc/Fc+ for the dication).
As expected, a slight decrease in the current recorded
was observed after a few scans. The current density of
the peaks corresponding to oxidation to the radical
cation state decresased by about 5% and the that of the
transition from radical cation to dication by about 15%.
This effect is common among electroactive polymers.
However, the current response stabilized after 5-6
scans, indicating that these materials possess good
cycling stability and can be switched between color
states many times without degradation.
Figure 5. Cyclic voltammograms of (a) PCzTh, (b) PCzBi, and
(c) PCzBTD at 20 mV/s in 0.1 M TBAPF6/propylene carbonate.
E1/2 values are indicated by dashed lines.
ments in 0.1 M TBAPF6/acetonitrile. Prior to recording
absorption spectra, the films were conditioned by ap-
plying potentials between -500 and 1200 mV vs a silver
wire through five cycles. This treatment produced a
constant current response and allowed for reproducible
cycling. The spectroelectrochemical series of each mate-
rial is given in Figure 6. Initial scans of the films were
recorded with the working electrode potential set at
-500 mV vs Ag/Ag+ in order to ensure that the films
were completely neutral. The π to π* transitions in the
neutral films are somewhat red-shifted from those
observed for the solution spectra as shown in Figure 2.
Also, the absorption peaks are broader, and the onset
of absorption is not as sharp with the solid samples as
compared to the solution spectra. These effects are most
likely due to dopant ions remaining in the solid from
the initial conditioning treatment. In addition, as
mentioned previously, the solid state gives rise to
different environments for the individual chromophores,
leading to a broader range of absorbances.
Spectroelectrochemistry. The films obtained by
spraying the polymer solutions onto ITO/glass slides
were used to perform spectroelectrochemistry experi-
The differences among the three samples correspond
quite well to the other data obtained here. The onset of

Macromolecules, Vol. 38, No. 18, 2005
Carbazole-Containing Electrochromic Polymers 7641
ences between polymers in solution and those cast as
films. The solid state gives rise to a wider range of
oxidation potentials as well as a wider range of absor-
bances. The differing oxidation potentials are expressed
in the absorption spectra of the polymers as an absence
of isosbestic points since there are certainly more than
two electrochemically different chromophores in the film
during the oxidation process. Thus, it is important to
note that the absence of isosbestic points is not neces-
sarily proof of the presence of stable radical cations as
it may be that this absence is simply an expression of
the different chromophore environments.
Colorimetry. To accurately convey the color of a
material, a more detailed description than that given
by absorbance spectroscopy is necessary. A convenient
way to approach this task is use colorimetric measure-
ments.16 In such experiments, three attributes of color
can be determined: hue, or dominant wavelength,
saturation, and luminance. These measurements are
usually accomplished with electrochromic polymers by
recording the Yxy color space coordinates of the material
at different potential steps. The data obtained can then
be plotted using CIE 1931 color coordinates to visually
depict the path of the material’s color change during
potential switching.
Figure 7 depicts the CIE color space plots for the three
polymers discussed here. The two materials with elec-
tron-rich comonomers, PCzTh and PCzBi, are both
yellow in their neutral states, while the donor-acceptor
polymer, PCzBTD, is orange. As the applied potential
to each material is increased, the color of each material
moves toward the white point in the center of the color
space plot. It is interesting to note that each material
changes color upon the application of relatively low
potentials, but then the color stabilizes at intermediate
potentials before changing again to a final color. This
phenomenon can be attributed to the formation of the
three oxidation states of each polymer, neutral, radical
cation, and dication.
The three color states can be best compared and
contrasted by plotting the linear distance between each
point on the color space plot and the initial point,
representing the completely neutral polymer. Since the
color space plot represents different colors by their
position on the plot, the distance from one point to
another can be interpreted as a measure of the color
difference between the two potentials represented by the
points. To determine the magnitude of the vector
between any two points on the color space plot, the
differences between the x and y coordinates of the point
in question and the initial point are squared and then
summed with the square root of the resulting sum being
representative of the linear distance between the two
points. Figure 8 shows the relationship between the
linear distance between each point on the CIE 1931
color space plot and the initial point as a function of
the applied electrode potential. In each case, two steps
are depicted, suggesting that three color states are
indeed present for each material. In addition, colori-
metrically determined color swatches for each color state
are included. These results are of special note when
studied in conjunction with the spectroelectrochemistry
results. While the spectral series shown in Figure 6
nicely details the evolution of the spectra, the nonlinear
change in color with potential is difficult to see. Close
examination of Figure 6b, for example, shows a number
of closely bunched points at relatively low potentials,
Figure 6. Spectroelectrochemical series of (a) PCzTh (400-
1350 mV vs Ag/Ag+), (b) PCzBi (300-1150 mV vs Ag/Ag+),
and (c) PCzBTD (650-1500 mV vs Ag/Ag+). Potential steps
are 50 mV. Dashed lines represent the visible region of the
spectrum. Arrows represent the changes in the spectral profiles
with increasing potential.
a color change was observed at 400, 500, and 900 mV
for PCzBi, PCzTh, and PCzBTD, respectively. The color
change continued as the applied potential was increased
in a stepwise fashion until it reached 1100, 1350, and
1500 mV for for PCzBi, PCzTh, and PCzBTD, respec-
tively. The various optical properties of each polymer
in its various oxidation states were studied extensively
by colorimetry and will be discussed below. At very
positive potentials, all of the samples were observed to
gradually become a transmissive yellow color with very
little difference between the various polymers. After
exposure to these high potentials, the films were
observed to become detached from the substrates, and
they could not be returned to their neutral states.
As compared to the solution doping results, an
important difference can be observed. While isosbestic
points are readily observed in the solution spectra, they
are absent in the film spectra. As stated earlier, the
absence of isosbestic points may be explained by differ-

7642 Witker and Reynolds
Macromolecules, Vol. 38, No. 18, 2005
Figure 8. Plots of distance between points in CIE color space
plots and the initial point for (a) PCzTh, (b) PCzBi, and (c)
PCzBTD. Steps represent different color states.
A small change can be observed upon conversion to the
radical cation state, with PCzTh taking on a faint green
color and PCzBi showing a bleached orange hue. Finally,
a greater difference can be seen in the dication states
of the two polymers. PCzTh is a faint pink color, while
PCzBi becomes a rather neutral gray. PCzBTD is orange
in its neutral state, green-gray as a radical cation, and
a neutral gray in its most oxidized state. The presence
of neutral grays in these materials is promising, as one
could imagine blending these materials with absorptive-
to-transmissive electrochromic polymers such as the
poly(ProDOT) family to give blends that would switch
Figure 7. CIE color space plots for (a) PCzTh, (b) PCzBi, and
(c) PCzBTD.
followed by a rapid absorption increase between 1.5 and
2.5 eV as the potential is stepped between 500 and 700
mV. These data correlate nicely with the results of
Figure 8b, where a distinct color change is evident at
ca. 600 mV.
The thiophene-containing polymers show very similar
colors in their neutral states, as both are bright yellow.

Macromolecules, Vol. 38, No. 18, 2005
Carbazole-Containing Electrochromic Polymers 7643
freshness and monomer purity, the conjugation-break-
ing nature of the 3,6-linked carbazole unit allows for
the color of the polymers to be unaffected by molecular
weight.
The three polymers described here show colors in
their various oxidation states that include yellow, green,
pink, and various grays as a result of the different
arylene dibromides used. Simply by using different
comonomers in the synthetic process, the colors of the
electrochromic materials can be tuned while still giving
an easily processed material. In addition to controlling
the colors of the polymers, the oxidation potential of the
polymers is strongly dependent on the electronic char-
acter of the arylene unit paired with carbazole in each
material. Solution doping trials and their companion
spectroelectrochemistry experiments demonstrated that
upon oxidation, whether with a chemical oxidant or
through electrochemical means, the absorbance corre-
sponding to the π to π* transition is depleted, and broad
absorbances appear in the near-infrared region of the
spectrum. As evidence of three stable color states
corresponding to three separate oxidation states of each
polymer, CV experiments show two distinct redox
processes: one corresponding to oxidation from the
neutral state to the radical cation and another repre-
senting further oxidation to the dication. In addition,
colorimetric measurements show three distinct color
states for each of the three polymers as various applied
potentials.
On the basis of the results observed here, it becomes
obvious that the next step for the polymers examined
to be used as the active components in practical devices
that call for electrochromic materials. The ability of
these polymers to be sprayed to form uniform films
means that there should be few, if any, limitations on
the size of the devices constructed. Also, to fully utilize
this family of materials, more members should be
synthesized in order to expand the palette of available
colors. The ease of synthesis should make such a task
relatively easily accomplished and will ultimately pro-
vide a very useful new circle of functional polymers.
Figure 9. Emission spectra of PCzAr solutions. Photographs
show emission of polymers when exposed to a standard
laboratory UV lamp.
from a colored state to a gray state. In such situations,
the specific color of the colored state and the intensity
of the gray state could be controlled by the identity of
the polymers used in the blend as well as the blending
ratio.
Photoluminescence. Fluorescence spectra were ob-
tained for each of the materials and are depicted in
Figure 9. The excitation wavelengths were 345, 375, and
460 nm for PCzTh, PCzBi, and PCzBTD, respectively.
The emission maxima recorded were 484 and 504 nm
for PCzTh, 528 and 555 nm for PCzBi, and 604 nm for
PCzBTD. The two thiophene-containing polymers, PC-
zTh and PCzBi, show two emissions, while PCzBTD
gives only a relatively broad emission. Although the
absorption, electrochemical, and colorimetric properties
of PCzTh and PCzBi have been observed to be quite
similar, the fluorescence spectra of these materials show
quite a difference. PCzTh emits an aqua-blue colored
light, while PCzBi emits a more intense green. Coupling
these results with the orange emissions from PCzBTD
gives a range of fluorescent colors that covers a great
portion of the visible spectrum. Although the photolu-
minescence efficiencies of these polymers were not
determined, a qualitative comparison suggests that they
may have efficiencies (ca. 0.1) comparable to or greater
than those of the carbazole-EDOT polymers reported
earlier.12a While an array of photoluminescent materials
was not a primary concern of this work, the preliminary
results given here suggest that the polymers described
may be useful for applications such as luminescent
sensors. To use these materials for such applications, a
more detailed study of their luminescent properties
would be necessary.
Experimental Section
Chemicals. Thiophene, 2,2′-bithiophene, 1,2-phenylenedi-
amine, N-bromosuccinimide, carbazole, octyl bromide, Aliquat
336, antimony pentachloride, palladium(II) chloride, and tri-
methyl borate were purchased from Aldrich and used as
received. Thionyl chloride, HBr, bromine, ethylene glycol,
sodium hydride, toluene, DMF, methylene chloride, and THF
were purchased from Fisher and used as received. Tetrakis-
(triphenylphosphine)palladium was purchased from Strem and
used as received.
The syntheses of 2,5-dibromothiophene (DBT),17 2,2′-di-
bromo-5,5′-bithiophene (DBBT),18 benzothiadiazole,19 4,7-di-
bromobenzo-2,1,3-benzothiadiazole (DBBTD),20 and NOctCz-
15
(BO2Et)2 were accomplished using methods reported else-
where.
Instrumentation. Ambient temperature NMR spectra
were obtained using a Varian Mercury 300 MHz apparatus.
High-temperature NMR spectra were recorded with a Varian
Mercury broadband 300 MHz spectrometer. UV/vis/NIR ab-
sorbance spectra were recorded with a Cary 5E spectropho-
tometer. GPC data were collected using two 300 by 7.5 mm
Polymer Laboratories PL Gel 5µM mixed-C columns with a
2996 photodiode array detector. THF or chloroform was the
eluting solvent at a flow rate of 1 mL/min. Molecular weights
were referenced to a poly(styrene) standard. Colorimetric
measurements were accomplished using a Minolta CS-100
Chroma Meter with the CIE recommended 0/0 (normal/
normal) viewing geometry for transmittance measurements.
Conclusions
Through the use of an easily synthesized carbazole
diboronate and any number of readily available arylene
dibromides, Suzuki polycondensation can be used to give
electrochromic polymers which are soluble in common
organic solvents and can be cast into uniform films.
Although the molecular weight of the products is quite
sensitive to experimental conditions, such as catalyst

7644 Witker and Reynolds
Macromolecules, Vol. 38, No. 18, 2005
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Electrochemical measurements were carried out using an
EG&G Princeton Applied Research model 273 potentiostat. CV
data were collected using a platinum button working electrode
(area ) 0.02 cm2), a platinum flag counter electrode, and an
Ag/Ag+ reference. The electrolyte was 0.1 M TBAPF6 in
propylene carbonate. Spectroelectrochemistry was accom-
plished using films spray-cast from methylene chloride or
toluene onto ITO-coated glass slides (7 × 50 × 0.5 mm, 20
Ω/square, Delta Technologies). The films were dried in vacuo
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Representative Polymer Synthesis. N-Octylcarbazole-
3,6-bis(ethyleneboronate) (1 g, 2.39 mmol, 1 equiv), 2,5-
dibromothiophene (578 mg, 2.39 mmol, 1 equiv), and Aliquat
336 (300 mg, ∼20% of the monomer weight) were combined
in 10 mL of toluene. This solution was deoxygenated with
bubbling argon for 60 min. In the meantime, 10 mL of 2 M
aqueous potassium carbonate was refluxed while deoxygen-
ating with bubbling argon. After 60 min had elapsed, the
aqueous solution was cooled to room temperature and then
transferred via cannula to the monomer solution. Immediately
following the addition of the aqueous base, a solution of Pd-
(PPh3)4 (28 mg, 24 µmol, 0.01 equiv) in 2 mL of deoxygenated
toluene was transferred via cannula to the two-phase reaction
mixture. After addition of the catalyst, the reaction mixture
was heated to 90 °C and stirred for 3 days.
After this time, the reaction mixture was cooled to room
temperature and the organic phase was separated. This
solution was added to 500 mL of methanol, giving a precipitate.
The precipitate was collected by filtration and dissolved in 250
mL of methylene chloride. This solution was added to 250 mL
of 6 M HCl, and the two-phase mixture was stirred vigorously
for 60 min. Then, the organic phase was separated and washed
with water. The volume of the solution was reduced to ∼25
mL by rotary evaporation and added to 500 mL of methanol.
The resulting precipitate was collected by filtration and
washed with methanol, acetone, and hexanes in a Soxhlet
extractor. The polymer was redissolved in a minimal amount
of methylene chloride and added to 500 mL of methanol. The
resulting solid was collected by filtration and dried overnight
in vacuo.
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Acknowledgment. We gratefully acknowledge the
AFOSR (F49620-03-0091) for partial support of this
work. We also thank Mr. Bob Brookins for helpful
discussions concerning carbazole chemistry, Ms. Aubrey
Dyer for assistance with electrochemical measurements,
and Dr. Jeremiah Mwaura for assistance with photo-
luminescence experiments.
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