Control of chirality and electrochromism in copolymer-type chiral PEDOT derivatives by means of electrochemical oxidation and reduction

Article abstract of DOI:10.1021/ma102861t

Copolymer-type chiral poly(ethylenedioxythiophene) [PEDOT] derivatives were synthesized by two kinds of electrochemical polymerizations of (i) chiral EDOT-based monomers using a cyclic voltammetric method in acetonitrile and (ii) achiral EDOT-based monomers using a potentiostatic method in chiral nematic liquid crystal (N*-LC) employed as an asymmetric reaction field. The electrochromic and morphological changes of the chiral PEDOT derivatives upon electrochemical doping and dedoping were investigated. Drastic and reversible changes in not only electrochromism but also circular dichroism between neutral and oxidized states of the chiral PEDOT derivatives were observed. This indicates that the helicity of the present polymers can be controlled by the electrochemical doping and dedoping procedures. It is found that the helically twisted chain and/or helically π-stacked structure formed in the neutral PEDOT derivative are suppressed in the oxidized state, where the main chain becomes planar due to a formation of quinoid structure and the interchain distances are lengthened due to intercalation of dopants into the chains. Spiral morphologies with right- and left-handed screw directions were observed for the polymers synthesized in the N*-LC and well correlated with spiral textures of the N*-LCs.

Full text of DOI:10.1021/ma102861t

ARTICLE
pubs.acs.org/Macromolecules
Control of Chirality and Electrochromism in Copolymer-Type Chiral
PEDOT Derivatives by Means of Electrochemical Oxidation and
Reduction
Yong Soo Jeong and Kazuo Akagi*
Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
S Supporting Information
b
ABSTRACT: Copolymer-type chiral poly(ethylenedioxythiophene)
[PEDOT] derivatives were synthesized by two kinds of electrochemi-
cal polymerizations of (i) chiral EDOT-based monomers using a cyclic
voltammetric method in acetonitrile and (ii) achiral EDOT-based
monomers using a potentiostatic method in chiral nematic liquid
crystal (N*-LC) employed as an asymmetric reaction field. The
electrochromic and morphological changes of the chiral PEDOT
derivatives upon electrochemical doping and dedoping were investi-
gated. Drastic and reversible changes in not only electrochromism but
also circular dichroism between neutral and oxidized states of the chiral
PEDOT derivatives were observed. This indicates that the helicity of
the present polymers can be controlled by the electrochemical doping
and dedoping procedures. It is found that the helically twisted chain and/or helically π-stacked structure formed in the neutral
PEDOT derivative are suppressed in the oxidized state, where the main chain becomes planar due to a formation of quinoid
structure and the interchain distances are lengthened due to intercalation of dopants into the chains. Spiral morphologies with right-
and left-handed screw directions were observed for the polymers synthesized in the N*-LC and well correlated with spiral textures of
the N*-LCs.
conjugated polymer.11ꢀ16 The second one is a polymerization
1. INTRODUCTION
of achiral monomer in an asymmetric reaction field consisting of
Conjugated polymers have many interesting electrical and
chiral nematic liquid crystal.17ꢀ19 The third one is a usage of both
optical properties and profound applicabilities for inherently
liquid crystalline (LC) conjugated polymer and chiral dopant.
transporting materials, light-emitting displays, rechargeable bat-
That is, when the chiral dopant is added to the nematic LC
teries, and electrochemical devices and sensors.1ꢀ6 Electroche-
conjugated polymer, the chiral nematic LC phase is induced in
mical polymerization for the conjugated polymer is useful
the polymer.20,21 The chiral conjugated polymers thus prepared
because it can be carried out at room temperature and homo-
are expected to exhibit circular dichroism in absorption and even
geneous polymer films can be formed directly at the electrode
surface by which the film thickness is well controlled. One of the
most attractive properties of the conjugated polymers is a so-
in fluorescence, yielding circularly polarized absorption and
luminescence.
In this work, the chiral PEDOT derivatives were synthesized
using two kinds of electrochemical polymerizations, as shown in
called electrochromism, where the polymer changes in color
upon an electrochemical reductionꢀoxidation (redox).7,8 Elec-
Table 1. The PEDOT derivatives with chiral side chains were
trochromic polymers give switchable functions and controllable
synthesized through the cyclic voltammetric method by electro-
optical properties.9 Poly(3,4-ethylenedioxythiophene) [PEDOT] is
chemically polymerizing EDOTs derivatives bearing chiral
a representative electrochromic polymer with versatile functional
groups at the phenylene moieties. Subsequently, the induced
properties such as high electrical conductivity, low band gap,
chiral PEDOT derivatives with achiral side chains were synthe-
redox activity, thermal stability, and excellent transparency in the
sized through the potentiostatic method by polymerizing EDOT
doped state.10 It is now desirable to cultivate novel polymeriza-
derivatives bearing achiral groups at phenylene moieties in an
tion methods to afford multifunctional and high-performance
PEDOT derivatives useful for plastic electronics.
Chiral conjugated polymers are expected to show circularly
asymmetric reaction field consisting of chiral nematic LC
(N*-LC). The copolymer-type chiral PEDOT derivatives thus
polarized luminescence when they have chiral centers and/or
they form helical structures. There are several methods to induce
a chirality on the conjugated polymer. One of them is an
introduction of chiral moiety into the side chain of the
Received: December 16, 2010
Revised: March 6, 2011
Published: March 18, 2011
r
2011 American Chemical Society
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Table 1. Two Kinds of Electrochemical Polymerizations Used for Synthesis of Chiral PEDOT Derivatives
synthesized were investigated in terms of electrochromic proper-
ties, chirality, and morphology upon electrochemical doping and
dedoping. We report that the chirality and electrochromism of
the copolymer-type chiral PEDOT derivatives are controlled by
means of electrochemical oxidation and reduction procedures.
wire was used as a counter electrode and Ag/Agþ was used as a reference
electrode. ITO glass (1 cm ꢁ 5 cm, 1 mm in thickness, 10 Ω/cm2) was
used as a working electrode. SEM observations were carried out a JEOL
7500F electron microscope. Optical texture observation was performed
using a Carl Zeiss polarizing optical microscope equipped with a Linkam
THMS 600 heating and cooling stage.
2.3. Electrochemical Polymerization. We used two methods of
the electrochemical polymerizations for syntheses of copolymer-type
chiral PEDOT derivatives. First, it may be helpful to characterize these
methods. The cyclic voltammetric method enables us to control the film
thickness by changing the scan number for oxidation and reduction
procedures and hence to obtain the polymer thin films with uniform
thickness. However, this method is not useful for the electrochemical
polymerization using the N*-LC because the helical twisting structure of
the N*-LC is easily distorted and even destroyed by the repetition of the
oxidation and reduction procedures. Thus, this method is only available
for the electrochemical polymerization using an isotropic solvent, and in
fact it is used for polymerizations of the chiral monomers [(R)- and (S)-
mono-1] in acetonitrile (see Table 1). On the other hand, the po-
tentiostatic method allowed us to electrochemically polymerize achiral
and chiral monomers, although the thickness of the polymer film cannot
be so precisely controlled as the cyclic voltammetric method. Never-
theless, one can retrieve the liquid crystals and the chiral dopants for the
subsequent polymerizations. Thus, the potentiostatic method is used for
the polymerizations of the achiral monomers [(R)- and (S)-mono-2] in
the N*-LC (see Table 1).
2. METHOD
2.1. Chemicals. 2,5-Dibromobenzoic acid and tributyltin chloride
were purchased from Aldrich and used without further purification. N,N,
N0,N0-Tetramethylethylenediamine (TMEDA), azodicarboxylic acid
diethyl ester (40% in toluene) (DEAD), and (R)- and (S)-2-octanol
were purchased from Tokyo Chemical Industry without further pur-
ification. Tetrahydrofuran (THF), acetonitrile (ACN), and triethyla-
mine were purified by the ordinary method, dried, and distilled
immediately before use. 3,4-Ethylenedioxythiophene (EDOT) was
vacuum-distilled before use. The chemical compounds (R)- and (S)-
2,20-dihydroxy-1,10-binaphthyl (optical purity, 0.99) were purchased
from commercially available sources. The mesogenic compound 4-
(trans-4-n-pentylcyclohexyl)phenol [PCH500] was purchased from
Kanto Chemical Ltd.
2.2. Measurements. NMR spectra of the monomers were re-
corded on a JEOL-NMR spectrometer (EX-400) using CDCl3 as a
solvent. Electrochemical experiments were carried out using a potentio-
stat (Autolab PGSTAT12). UVꢀvis. absorption spectroscopy (Hitachi
U-3500) and circular dichroism (CD) spectroscopy (JASCO J-720)
were used to characterize the polymer films. The surface morphologies
of the polymer films were investigated using a JEOL JSM-6400 scanning
electron microscope. In the electrochemical characterization, a platinum
Electrochemical polymerizations were performed in a standard three-
electrode cell containing an ITO-coated glass slide as a working
electrode, a platinum foil as a counter electrode, and a Ag/Agþ as a
reference electrode. As shown in Table 1, (R)- and (S)-poly-1 were
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Scheme 1. Synthetic Routes for Monomers [(R)- and (S)-mono-1] and Polymers [(R)- and (S)-poly-1]
synthesized through the electrochemical cyclic voltammetric method
from solutions containing 0.01 M of monomers and 0.1 M tetra-n-
butylammonium perchlorate (TBAP) in anhydrous acetonitrile. (R)-
and (S)-poly-2 were synthesized by the potentiostatic method for
electrochemical polymerization in the N*-LC field.
3. RESULTS AND DISCUSSION
3.1. Synthesis of Monomers. Chiral compounds of (R)- and
(S)-1,4-bis(2-(3,4-ethylenedioxy)thienyl)-2-benzoic acid 1-methyl
heptyl ester [(R)- and (S)-mono-1] were prepared and used as
chiral monomers for the electrochemical polymerizations. The
Figure 1. Electrochemical polymerization using cyclic voltammetric
method for synthesis of (S)-poly-1 in acetonitrile including 0.1 M
TBAP. Potential scanning of at 20 mV/s was repeated to increase the
film thickness according to the cyclic voltammetric method.
synthetic route for mono-1 is described in Scheme 1. The
dibromo compounds (R)-1 and (S)-1 were prepared by Mitsu-
nobu esterification reaction by between (R)- and (S)-octanol and
2,5-dibromobenzoic acid with diethyl azodicarboxylate (DEAD)
and triphenylphosphine (TPP) in tetrahydrofuran (THF) solu-
tion. The reaction proceeded through an SN2-type Walden
inversion at the chiral center without racemization, resulting in
the formation of the desired compound. 3-Tributylstannyl-3,4-
ethylenedioxythiophene was prepared by a direct lithiation of an
equal molar ratio of 3,4-ethylenedioxythiophene, followed by a
reaction with tributyltin chloride. (R)- and (S)-mono-1 were
synthesized by Stille coupling using Pd(PPh3)2Cl2 as a catalyst
between 3-tributylstannyl-3,4-ethylenedioxythiophene and 2,5-
(R)- and (S)-dibromobenzoic acid 1-methyl heptyl ester.
3.2. Polymerization. The oxidative electrochemical polymer-
izations for the copolymers [(R)- and (S)-poly-1] were synthe-
sized in acetonitrile with 0.1 M tetrabutylammonium perchrolate
as electrolyte and 0.01 M monomer on ITO glass electrodes.
Figure 1 shows the repeated potential scans for the electroche-
mical polymerization of the monomer with chiral substituent of
(S)-configuration. The results for the electrochemical polymer-
izations of (R)- and (S)-mono-1 are similar to each other (see
Figure S1). During the oxidative scan, two oxidation peaks
appeared at ꢀ0.12 and þ0.58 V for (R)-mono-1 and two
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Figure 2. Cyclic voltammetry of monomer-free (S)-poly-1 in 0.1 M
TBAP/acetonitrile solution: (a) 10, (b) 20, (c) 40, (d) 60, (e) 80, and (f)
100 mV sꢀ1
.
Figure 4. CD spectra of thin films of neutral and oxidized (R)- and (S)-
poly-1 on ITO glasses.
Figure 3. UVꢀvis and near-IR absorption spectra of (S)-poly-1 film
coated on ITO electrode, measured at different potentials ranging from
ꢀ0.7 to þ0.7 V in acetonitrile with 0.1 M TBAP vs Ag/Agþ.
oxidation peaks at ꢀ0.11 and þ0.58 V for (S)-mono-1 in the first
scan. On the return scan, the polymer is dedoped. A growing
polymer through the redox process was observed, for instance, by
examining changes in absorption intensity at 510 nm of (S)-poly-
1 during the cyclic voltammetric polymerization on ITO-coated
glass (see Figure S2). All peaks exhibit an increased current
response, indicating the formation of an electroactive polymer
film accompanied by an increase of film thickness. After the
polymerization, the polymer thin films were washed with an
electrolyte solution and acetonitrile.
3.3. Characterization. Figure 2 shows the cyclic voltammo-
gram of (S)-poly-1 film at scan rates of 10, 20, 40, 60, 80, and 100
mV sꢀ1 in 0.1 M TBAP/acetonitrile. The half-wave oxidation
potential of the polymer (E1/2) was ꢀ0.32 V vs Ag/Agþ
(Eox/p = ꢀ0.10 V and Ered/p = ꢀ0.54 V at 100 mV sꢀ1), similar
to the values for poly[1,4-bis(2-(30,40-ethylenedioxy)thienyl)-2-
methoxy-5,200-ethylhexyloxybenzene] [poly(BEDOT-MEHB)].22
The polymer exhibited a well-defined and quasi-reversible redox
process. It is worthy noting that (R)- and (S)-poly-1 display very
similar electrochemical behavior, probably because the chirality of
the polymers does not affect the redox behavior.
3.4. Spectroelectrochemistry of Polymer Films. It is desired
at this stage to gain a deeper insight into the relationship between
polymer structure and electrochemical properties. The spectro-
electrochemistry is expected to give key properties of conjugated
polymers such as a band gap (Eg) and the intergap states that
appear upon doping. Therefore, the spectroelectrochemical
measurements were performed in the UVꢀvis and near-infrared
(NIR) wavelength range using a quartz cell. The quartz cell
includes three electrodes (working, counter, and reference) and
the electrolytic solution (0.1 M TBAP). Before the measurement
Figure 5. SEM images of (S)-poly-1 prepared by electrochemical
polymerization on ITO glass: (a) neutral state and (b) oxidized state.
of the polymer a spectroscopic blank has been evaluated on an
ITO glass without the polymer under the same conditions. The
absorbance of the ITO glass is automatically subtracted from the
spectroscopic data points by the potentiostat. During the spec-
troelectrochemical measurement the working electrode potential
(ITO) is swept from ꢀ0.7 to þ0.7 V by the potentiostat.
Figure 3 shows the spectroelectrochemical analysis for the (S)-
poly-1 at different voltages varying between ꢀ0.7 and þ0.7 V.
The polymer film in the neutral state (ꢀ0.7 V) gives an
absorption band at 500 nm. This is assigned to πꢀπ* transition
of conjugated main chain and responsible for purple color of the
neutral film. In the initial stage of the oxidation (ꢀ0.2 V), the
band at 500 nm gradually decreases in intensity and new bands at
740 and 1450 nm emerge. This is accompanied by a color change
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Figure 6. Electrochemical doping and dedoping processes of chiral PEDOT derivative, (S)-poly-1 in TBAP electrolyte system. Changes in (a)
electronic structures between neutral and oxidized states and electrochromic color, (b) interchain helical π-stacking structure, and (c) morphology.
in the polymer from purple to green. As the oxidation proceeds
further (from ꢀ0.2 to þ0.1 V), the two new bands increase in
intensity. During the oxidation, the band at 740 nm shows a slight
red shift from 740 nm (ꢀ0.2 V) to 770 nm (þ0.1 V), and the
band at 1450 nm shows a large blue shift from 1450 nm (ꢀ0.2 V)
to 1000 nm (þ0.1 V). Finally, these two bands merge into an
intense and broad band around 860 nm (þ0.5 to þ0.7 V);
meanwhile, the band at 500 nm disappears. The isosbestic point
at 610 nm is observed in the oxidation process, indicating an
existence of equilibrium between the neutral and oxidized states.
The reverse changes in spectra are observed in the reduction
process. At the present time, the assignments of the new bands
(740 and 1450 nm) and the intense and broad band around 860
observed in the final stage of the oxidation are not clearly
elucidated, remaining them undissolved. However, one may
argue that this sort of electrochromism is reversible and can be
controllable by the electrochemical doping and dedoping
procedures.
spectra were observed for the neutral (R)- and (S)-poly-1.
However, when the polymer was oxidized through a doping,
the CD band was markedly weakened in intensity. The applica-
tion of voltage of ꢀ0.7 V for the dedoping restored the CD bands
to the original intensities of the neutral state. It is suggested that
the changes of CD bands in the neutral and oxidized states might
be related to a chirality change in the polymer. In the neutral
state, the polymer takes a helically twisted structure because the
polymer main chain has a benzonoid structure and a relatively
high degree of freedom around the internal rotation of the main
chain. Meanwhile, in the oxidized state, the polymer main chain
takes a quinoid structure, and the planarity of the main chain is
enhanced. The chirality of the polymer weakens as the twisting
degree decreases. At the same time, the distance between tꢀhe
polymer main chains is expanded because the dopants (ClO4
)
are intercalated into the main chains (see Figure 6). The increase
of interchain distance weakens the helical π-stacking, causing a
decrease in CD intensity.
Thus, the CD band observed in the neutral polymer is
attributed to the two structural factors: (i) an intrachain twisting
(helical conformation) and (ii) an interchain helical π-stacking
(chiral stacking). The loss of CD intensity in the oxidized state is
due to (iii) an intrachain planarization (planar conformation) of
the quinoid structure and (iv) an increase of interchain distance
3.5. CD Spectra of Polymer Films. Figure 4 shows circular
dichroism (CD) spectra of (R)- and (S)-poly-1 films. Cotton
effects were observed for absorption region at 450 nm for πꢀπ*
transition of the polymer main chain in the neutral state. (R)-
poly-1 showed a bisignate Cotton effect, i.e., a positive CD band
at 452 nm and a negative one at 556 nm. Mirror images in CD
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Figure 7. POM images of (R)- and (S)-N*-LCs systems containing a monomer, an electrolyte, and a chiral dopant. Images recorded for N*-LC phase at
23 °C in the cooling process.
(intercalation of dopant into main chains), leading to a vanishing
of the helically π-stacked structure. It is not straightforward to
discriminate between the two possibilities of intrachain (i and iii)
and interchain (ii and iv) structural factors. Nevertheless, by
taking account of bisignate shapes of the CD spectra for neutral
(R)- and (S)-poly-1 shown in Figure 4, the interchain structural
factor (ii and iv) is the most likely for the explanation of the
CD band.
3.6. Morphologies of Polymer Films. Figure 5 shows scan-
ning electron microscope (SEM) images of (S)-poly-1 films
prepared by 40 scans on ITO glass. The polymer film in the
neutral state has a twisted and shrunk morphology (Figure 5a).
Meanwhile, the film in the oxidized state shows a swollen
morphology (Figure 5b), since the dopants are contained
between the polymer main chains. In fact, the film thickness of
the polymer increased by about 20% due to the swelling after the
oxidization. The swelling of the oxidized film is attributed to the
intercalation of dopants such as ClO4ꢀ into the interchains and
the porous sites of the film in accompanying with the electro-
chemical doping. It is of keen interest that the morphological
change between the neutral and oxidized states is reversible and
hence controllable by the electrochemical doping and dedoping
procedures.
3.7. Mechanism of Electrochemical Redox Processes.
Upon the electrochemical oxidation, the polymer is oxidized to
give a polaron or bipolaron which strongly affects on the
electronic band structure.23 Figure 6 describes electrochemical
doping and dedoping processes and the changes in electrochro-
mic color of the chiral PEDOT derivative, (S)-poly-1. In the
initial stage of the oxidation, the neutral polymer changes into a
cationic polymer with a radical cation corresponding to the
polaron. The radical cation is delocalized over the conjugated
polymer chain. The color of the polymer film changes from a
deep purple to a light purple. In the further oxidation, the doping
proceeds and the dication corresponding to the bipolaron is
generated. As a result, the color of the film changes from a light
purple to an emerald green. The bathochromic shift of the
electrochromic color is attributed to an increase of the effective
conjugation length, which comes from a formation of a quinoid
structure in the oxidized polymer. In Figure 6 is also shown the
schematic representation of the changes in structure and mor-
phology of (S)-poly-1 upon the electrochemical doping and
dedoping procedures. Both the interchain helical π-stacking and
the intrachain twisting are weakened by the electrochemical
doping and strengthened by the electrochemical dedoping.
The weakening of the interchain helical π-stacking is accompa-
nied by a considerable expansion between the interchain dis-
tances. The expansion of the interchain distances should cause a
change in higher-order structure such as morphology. The
notable change in morphology between the neutral (shrunk
morphology) and oxidized states (swollen morphology), as
depicted in Figure 6, could be correlated with the change in
the interchain distances.
3.8. Preparation of Chiral Nematic Liquid Crystal (N*-LC).
Next, the electrochemical polymerizations in an asymmetric
reaction field were performed by using the N*-LC.24 Table 1
shows molecular structures of compounds used for the chiral
electrochemical polymerization. Synthetic details of the com-
pounds are given in the Supporting Information. 4-Cyano-40-n-
pentylbiphenyl (5CB) was used as a parent liquid crystal for the
N*-LC. The liquid crystallinity of 5CB was maintained even after
an addition of small amount of the chiral dopant. The N*-LC
system is composed of 0.8 M (R)- or (S)-binaphthyl-2,2-bis-
[p-(trans-4-pentylcyclohexyl)phenoxy-1-1-hexyl] ether [(R)- or
(S)-PCH506-binol] as the chiral dopant, 1.0 M 2,5-bis(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzoic acid 1-methyl-
heptyl ester [monomer], and 0.7 M tetrabutylammonium per-
chlorate [TBAP] in 5CB.
The N*-LC system was heated up to an isotropic temperature
to completely dissolve the monomer, the electrolyte, and the
chiral dopant in 5CB. POM images of the N*-LC systems showed
typical fingerprint textures, as seen in Figure 7. (R)- or (S)-N*-
LC systems showed thermotropic LC behavior. The phase
transition temperatures of the (R)- and (S)-N*-LC systems are
as follows: Cf 6 f N* f 28 f I, C r 1 r N* r 25 r I and
Cf 5 f N* f 27 f I, C r 1 r N* r 25 r I in heating and
cooling processes, respectively. Subsequently, the contact tests
were carried out to elucidate the screw directions of the N*-LC
systems with fingerprint textures. The cholesteryl oleyl carbonate
was used as the standard LC for the contact test. It was found that
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Figure 9. CD spectra of (R)- and (S)-poly-2 in neutral and oxidized
states.
Figure 8. UVꢀvis and near-IR absorption spectra of (S)-poly-2 film
coated on ITO electrode prepared at various electrode potentials
ranging from ꢀ0.7 to þ0.7 V in 0.1 M TBAP/acetonitrile vs Ag/Agþ.
the (R)- and (S)-N*-LC systems have left- and right-handed
screw directions, respectively.
3.9. Polymerization in Asymmetric Reaction Field. The N*-
LC system containing the monomer was injected into a cell
composed of sandwiched ITO glasses with a Teflon spacer (140
μm). The cell including the N*-LC system was heated to 50 °C
and then slowly cooled to room temperature. The N*-LC in the
cell showed a fingerprint texture at room temperature (Figure
S3). DC voltage of 3 V was applied to the cell for 1 h. As the
polymerization proceeded, the optical texture of the N*-LC
system became darker owing to the formation of polymer. As a
result of the polymerization, a polymer film of dark red color was
deposited on the inner glass of the cell. Once the polymerization
was complete, the polymer film was washed with methanol,
acetonitrile, and toluene, in that order.
3.10. Optical Properties of Polymer Films. The optical
textures of the polymers resemble to those of the N*-LCs. In
the spectroelectrochemical measurement, the working electrode
potential was controlled by the potentiostat and swept from þ0.7
to ꢀ0.7 V. Figure 8 shows the spectroelectrochemical analysis for
the (S)-poly-2 at voltages varying between þ0.7 and ꢀ0.7 V.
When the electrode potential was increased stepwise from ꢀ0.7
to 0 V, the absorptions in the near-IR region increased in
intensity, giving bands at 726 and 1250 nm. From 0 to þ0.3 V,
a different spectral change was observed; while the bands at 726
and 1250 nm increased, the band at 478 nm decreased. The
polymer in the neutral state showed an absorption band at
478 nm, which is due to πꢀπ* transition of the main chain
and responsible for the red color. From þ0.3 to þ0.7 V, the band
at 478 nm decreased, while those at 726 and 1250 nm increased.
This was accompanied by a change in color of the polymer from
red to green.
As shown in Figure 9, (R)- and (S)-poly-2 exhibit a strong
Cotton effect in the neutral state, but no Cotton effect was
observed in the oxidized state. (R)- and (S)-poly-2 gave com-
plementary mirror-image CD spectra. It is suggested that the
changes of CD signals in the neutral and oxidized states are
related to changes in chirality of the polymers. In the neutral
state, the polymer takes a twisted structure because the polymer
main chain has a benzonoid structure with a comparatively high
degree of freedom in terms of internal rotation of main chains.
Therefore, the strong CD intensity originates from the twisted
conjugated chain. Meanwhile, in the oxidized state, the polymer
takes a quinoid structure and the planarity of the main chain is
Figure 10. POM photographs of (R)-poly-2 (upper figure) and (S)-
poly-2 (lower figure) with left- and right-handed spiral morphologies,
respectively.
enhanced (see Figure 6). Thus, the chirality of the polymer is
weakened due to a decrease of twisting degree of the main chain.
Besides, in the oxidized state the interchain distance is expanded
because of existence of intercalating dopant (ClO4ꢀ) between
the polymer main chains.
Here, it is of interest to discuss the difference in absorption
spectrum and band gap between poly-1 and poly-2 (see Figures 3
and 8). When the band gap is defined as the onset of the
absorption band, the neutral poly-1 has a band gap of ∼1.7
eV, and the wavelength at maximum intensity of absorption
band, λmax, is 2.44 eV (508 nm). The neutral poly-2 has a band
gap of 1.8 eV, and its λmax is 2.59 eV (478 nm). It is evident that
the absorption band of the neutral poly-1 is located at longer
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poly-2 films are ca. 2 mm in thickness, which being 20 times
thicker than those (ca. 0.1 mm) of the poly-1 films, and they
(poly-2 films) are too thick to be affected in morphology by the
electrochemical doping and dedoping. Note that the difference in
film thickness between poly-1 and poly-2 is attributed to the
difference in the electrochemical polymerization techniques be-
tween the cyclic voltammetric method and the potentiostatic one.
Namely, in the latter (potentiostatic method), it is hard to control
the film thickness during the electrochemical polymerization. As a
result, the poly-2 prepared by the potentiostatic method is too thick
to enable the dopants to be dispersed homogeneously into the
whole of the film, yielding partly doped state.
Figure 11 depicts the optical micrographs of (R)-poly-2 film. The
micrographs are measured without a polarizer (Figure 11a) and
using a differential interference contrast technique (Figure 11b).
The oxidization was carried out by dipping a half of the film into the
electrochemical cell including the electrode. It is clear that the
neutral and oxidized areas of the film show reddish and bluish colors,
respectively (Figure 11a). Such an electrochromic behavior of poly-
2 is consistent with the results of UVꢀvis absorption spectra in
Figure 8. It is worthy noting that the fingerprint texture of the
neutral film of poly-2 remains unchanged after the oxidation
(Figure 11b). In other words, the morphology of the poly-2 does
not change by the electrochemical doping and dedoping proce-
dures, although the electrochromic change on the surface of the film
reversibly occurs between the neutral and oxidized states.
4. CONCLUSION
The copolymer-type chiral poly(ethylenedioxythiophene)
[PEDOT] derivatives were synthesized by two kinds of electro-
chemical polymerizations of (i) the chiral EDOT-based mono-
mers using the cyclic voltammetric method in isotropic organic
solvent (poly-1) and (ii) the achiral EDOT-based monomers
using the potentiostatic method in the N*-LC used as an
asymmetric reaction field (poly-2).
Poly-1 exhibited drastic and reversible changes in electro-
chromism, circular dichroism, and even morphology between the
neutral and oxidized states. This implies that the electrochemical
doping and doping procedures are useful to control the chiroptical
and electrochemical properties of the chiral conjugated polymers.
Poly-2 showed spiral morphologies with right-and left-handed
screw directions which can be correlated to the optical textures of
the N*-LC reaction fields. The chirality and electrochromism of
poly-2 were also controlled by changing the polymer structures
between the neutral and oxidized states through the electro-
chemical doping and doping procedures.
The profiles of Cotton effects in CD spectra imply that the
neutral poly-1 has helically twisted main chain to form helically
π-stacked structure, whereas the neutral poly-2 has helically
twisted main chain. These helical structures formed in the
PEDOT derivatives are suppressed in the oxidized states because
the main chain becomes planar due to a formation of quinoid
structure and at the same time the interchain distances are
lengthened due to intercalation of dopants into the chains.
The present polymerization methods represent useful approaches
for the preparation of chiral conjugated polymers with multifunc-
tional properties such as electrochromism and circular dichroism.
Figure 11. Optical micrographs of (R)-poly-2: (a) optical micrograph
without polarizer; (b) differential interference contrast micrograph.
wavelength by 30 nm than that of the neutral poly-2. Namely, the
former has a longer effective conjugation length that the latter.
Such a difference can be related to the polymerization method.
As mentioned in section 2.3, the cyclic voltammetric (CV)
method enables us to control the film thickness by changing
the scan number for oxidation and reduction procedures and
hence to obtain the polymer thin film with uniform thickness. On
the other hand, the potentiostatic method is not capable for
controlling the film thickness, although it allows us to perform an
electrochemical polymerization even in the N*-LC. Actually,
poly-1 is synthesized by the CV method, yielding a thin film with
uniform thickness. The well-controlled uniform thickness of
poly-1 leads to polymer chains with long conjugation length
and even small amounts of polymer chains with further longer
conjugation length. The latter is responsible for the tailing of the
absorption band for poly-1, as found in Figure 3. On the other
hand, poly-2 is synthesized by the potentiostatic method, giving a
film with a nonuniform thickness. Hence, poly-2 consists of
polymer chains with relatively shorter conjugation length than
poly-1, and it gives no tailing in absorption band.
3.11. Morphologies and Micrographs of Polymer Films.
Figure 10 shows POM photographs of the polymer films in the
neutral state; (R)- and (S)-poly-2 have left- and right-handed
spiral morphologies, respectively. It appears that the polymer
replicates the optical texture of the N*-LC system. The helical
senses of the poly-2 films are quite in agreement with those of the
(R)- and (S)-N*-LC used as asymmetric reaction fields. How-
ever, these morphologies showed no change even after the
electrochemical doping and dedoping (see Figure 11). The result
is in contrast to the case of the poly-1 film. This is because the
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic details of compounds,
b
electrochemical polymerization using cyclic voltammetric method
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dx.doi.org/10.1021/ma102861t |Macromolecules 2011, 44, 2418–2426

Macromolecules
ARTICLE
for synthesis of (R)-poly-1 in acetonitrile including TBAP, changes
in absorption intensity at 510 nm of (S)-poly-1 during the cyclic
voltammetric polymerization on ITO-coated glass, electrochemical
polymerization cell for synthesis of PEDOT derivative using a
potentiostatic method. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Corresponding Author
*E-mail: akagi@fps.polym.kyoto-u.ac.jp.
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’ ACKNOWLEDGMENT
The authors acknowledge Dr. Hiromasa Goto, University of
Tsukuba, for his valuable helps and cooperation. This work was
supported by a Grant-in-Aid for Science Research (S) (No.
20225007) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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