Helical carbon and graphite films prepared from helical poly(3,4-ethylenedioxythiophene) films synthesized by electrochemical polymerization in chiral nematic liquid crystals

Article abstract of DOI:10.1002/anie.201308462

Helical carbon and graphite films from helical poly(3,4- ethylenedioxythiophene) (H-PEDOT) films synthesized through electrochemical polymerization in a chiral nematic liquid-crystal (N-LC) field are prepared. The microscope investigations showed that the

Full text of DOI:10.1002/anie.201308462

Angewandte
Chemie
DOI: 10.1002/anie.201308462
Carbon Materials
Helical Carbon and Graphite Films Prepared from Helical Poly(3,4-
ethylenedioxythiophene) Films Synthesized by Electrochemical Poly-
merization in Chiral Nematic Liquid Crystals**
Satoshi Matsushita, Bairu Yan, Shinsuke Yamamoto, Yong Soo Jeong, and Kazuo Akagi*
Abstract: Helical carbon and graphite films from helical
poly(3,4-ethylenedioxythiophene) (H-PEDOT) films synthe-
sized through electrochemical polymerization in a chiral
nematic liquid-crystal (N*-LC) field are prepared. The micro-
scope investigations showed that the H-PEDOT film synthe-
sized in the N*-LC has large domains of one-handed spiral
morphology consisting of fibril bundles. The H-PEDOT films
exhibited distinct Cotton effects in circular dichroism spectra.
The highly twisted N*-LC with a helical pitch of smaller than
ogy can be regarded as a promising approach for the
fabrication of the carbon films with hierarchical morphology
if the precursor morphology is preserved during the carbon-
ization, even at high temperature. Recently, chiral nematic
(N*) mesoporous carbon films with a high specific surface
area that have been prepared from nanocrystalline cellulose-
silica composite films serving as carbonization precursors
[1]
have been reported.
Currently, the development of the “morphology-retaining
carbonization method” has demonstrated that iodine-doped
helical polyacetylene (H-PA) films are promising precursors
to produce helical graphite films with unique spiral morphol-
1
mm produced the H-PEDOT film with a highly ordered
morphology. The spiral morphologies with left- and right-
handed screws were observed for the carbon films prepared
from the H-PEDOT films at 8008C and were well correlated
with the textures and helical pitches of the N*-LCs. The spiral
morphologies of the precursors were also retained even in the
graphite films prepared from the helical carbon films at
[2]
ogies consisting of helical graphite fibers. The H-PA films
are synthesized through asymmetric interfacial polymeri-
[3]
zation in N* liquid-crystal (N*-LC) fields. However, further
improvements in efficiency and effectiveness for the prepa-
ration of helical graphite films are desired. To this end, the
following two possible approaches are considered appropriate
for improving the carbonization method. One is 1) to use the
electrochemical polymerization for the synthesis of the
carbonization precursors which is easier and safer than the
interfacial acetylene polymerization and can operate under
2
6008C.
H
elical carbon and graphite films with tunable helical sense
and degree of helicity are potentially applicable materials in
carbon-based electronics and nanotechnology because of
their electrical conductivity, high chemical and thermal
stability, and chirality derived from the helical structure.
However, it is intrinsically difficult to fabricate a carbon film
with a unique morphology, such as a hierarchical helical
structure. Although vapor-phase carbonization can produce
newly discovered allotropes of carbon such as fullerenes,
carbon nanotubes, and graphenes, there are some limitations
in producing carbon films with hierarchical morphology using
the carbonization method. In contrast, solid-state carbon-
ization of an organic polymer film with controlled morphol-
[
4]
ambient conditions. The other is 2) to use the helical
aromatic p-conjugated polymers as stable precursors instead
of the helical aliphatic ones with poor stability. The inherently
rigid backbone of the aromatic p-conjugated polymers will
suppress thermal degradation and will provide high thermal
durability during the carbonization process.
The asymmetric polymerization method using the N*-LC
as a reaction field has profound versatility for the synthesis of
helical p-conjugated polymers that do not even have chiral
[
5]
substituents on side chains. In particular, the helical
poly(3,4-ethylenedioxythiophene) (H-PEDOT) film electro-
chemically synthesized in N*-LC shows clear spiral morphol-
[*] Dr. S. Matsushita, B. Yan, S. Yamamoto, Dr. Y. S. Jeong,
Prof. K. Akagi
Department of Polymer Chemistry, Kyoto University
Katsura, Kyoto 615-8510 (Japan)
[6]
ogy with electroactive properties. One of the feasible ways
of preparing helical graphite films is to use the helical p-
conjugated polymer films with spiral morphology based on
both of the above-described two approaches. In this study, we
prepared helical carbon and graphite films from the H-
PEDOT films as potential carbonization precursors. The H-
PEDOT films were synthesized through asymmetric electro-
chemical polymerization of bis-EDOT in N*-LC containing
an electrolyte (Scheme 1). Here we present a new aspect of
aromatic p-conjugated polymers by focusing on their avail-
ability as precursors in preparing helical carbon and graphite
films.
E-mail: akagi@fps.polym.kyoto-u.ac.jp
[
**] We would like to thank Dr. A. Kaito and Dr. M. Shimomura (AIST,
Japan) for the measurements of Raman spectra, Dr. S. Ohshima and
Dr. T. Saito (AIST, Japan) for the use of the graphitizing apparatus.
The authors are grateful to Dr. M. Kyotani (University of Tsukuba,
Japan) for his helpful cooperation and support. The authors are also
grateful to laboratory colleague Mr. C. Tao (Kyoto University, Japan)
for his experimental support. This work was supported by the
Ministry of Education, Culture, Sports, Science and Technology
(
Japan) through a grant-in-aid for scientific research (S, grant
number 20225007 and A, grant number 13370214), and a grant-in-
aid for young scientists (B, grant number 24750219).
Scheme 1 depicts the molecular structures of the di- and
tetra-substituted axially chiral dopants, D1 and D2, respec-
tively. The N*-LC of systems 1–3 were prepared by adding D1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308462.
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Figure 2 shows polarizing optical microscope (POM) and
scanning electron microscope (SEM) images of the neutral H-
PEDOT films synthesized in N*-LCs of system 3 with a helical
pitch of 341 nm (see Table S1 in the Supporting Information).
The SEM images show the formation of a large domain of
spiral morphology that is composed of a fibrous structure. The
screw direction of the spiral is opposite to the helical sense of
[
9]
the N*-LC (Table S2). The H-PEDOT film synthesized
using mono-EDOT as a monomer has no fibril morphology
[
6a]
but only shows a fingerprint texture. The use of bis-EDOT
with a rigid, rod-like molecular shape and a lower oxidation
potential causes a directional polymer growth and the
production of polymers with a high conversion rate, resulting
in a fibril structure. In fact, the oxidation potential (E ) of the
pa
EDOT dimer (0.9 V) is much lower than that of the EDOT
[
10]
monomer (1.5 V). It is considered that the high planarity of
EDOT itself also facilitates positive effects for the formation
of the PEDOT fibrils. There are several investigations about
the chemical synthesis of p-conjugated polymers with fiber
[11]
structures such as polyanilines, polypyrroles, and PEDOTs.
From the viewpoint of evaluating one-dimensionality in
physical, chiroptical, and electromagnetic properties, fiber-
structured materials are favorable. The distance between the
fibril bundles (150 nm) is very close to the half-helical pitch of
Scheme 1. Preparation of a helical graphite film using the carbon-
ization of a H-PEDOT film synthesized in an N*-LC field and molecular
structures of the chiral dopants.
[12]
the N*-LC (about 170 nm). The distance can be controlled
by choosing the chirality and concentration of the chiral
or D2 into an N-LC, 4-cyano-4’-n-pentylbiphenyl
(
5CB; see the Experimental Section). Figure 1
shows UV/Vis, circular dichroism (CD), and gabs
spectra of the oxidized and neutral H-PEDOT
films synthesized in N*-LC of system 3 including
D2. The as-grown PEDOT film including the
À
perchlorate ion (ClO ) as a dopant species is
4
called the “oxidized PEDOT film” hereafter. The
electrochemically produced aromatic p-conju-
gated polymers are usually obtained in a doped
(
oxidized) state. The oxidized PEDOT film can
be reduced to produce a “neutral PEDOT film”
through an electrochemical dedoping process,
although the reduction is not completely ach-
ieved (for details see the Supporting Informa-
tion). The neutral H-PEDOT films showed
bisignate Cotton effects in the CD spectra,
which indicates the formation of a polymer
assembly with an interchain helically p-stacked
[
7]
structure. According to the exciton coupling
[
8]
theory, the (R)- and (S)-PEDOT films have
right-handed (P-helicity) and left-handed (M-
helicity) helical senses, respectively. The degree
of circular polarization in absorption was eval-
uated using the dissymmetry factor, g . This
abs
factor is defined by the equation gabs
=
Figure 1. UV/Vis, CD, and gabs spectra of the a) oxidized (“as-polymerized”) and
b) neutral H-PEDOT films synthesized in N*-LCs of system 3 [polymerization;
5CB:(R)- or (S)-D2:bis-EDOT:TBAP=100:1:2:0.2 (mole ratio), ITO glass (anode), ITO
glass (cathode), 4 V, 5 minutes, 208C] [dedoping; TBAP: 0.05m, in MeCN, Pt wire
2
(e Àe )/(e +e ) = De/e. The H-PEDOT films
L
R
L
R
À2
had values of g on the order of 10 in the
abs
absorption region. From these results, the H-
PEDOT films are anticipated to yield chiral
carbonization precursors bearing interchain one-
handed helical p-stacked structures.
(
anode), ITO glass (cathode), 4 V, 5 minutes, room temperature]. (R)- and (S)-PEDOT
denote the H-PEDOT films synthesized in N*-LCs including the chiral dopants of (R)-
and (S)-configurations, respectively. UV/Vis spectra of the neutral H-PEDOT films
have a lmax of 493–495 nm.
1
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À1
of the graphite film is on the order of 10 Scm . The Raman
À1
spectrum of the carbon film shows a broad band at 1358 cm
that is attributed to the disordered structure (D-band) and
À1
a strong peak at 1589 cm corresponding to the graphitic
2
structure of the sp hexagonal carbon network (G-band). The
À1
two Raman bands at 1358 and 1589 cm become sharp, and
at the same time, the former and the latter decreases and
increases in intensity, respectively, after the graphitization
(Figure 3c and d). If the oxidized H-PEDOT films are used as
carbonization and graphitization precursors, the preparation
of helical graphite films with a high crystallinity can be
expected. Therefore, the oxidized H-PEDOT films were
employed as precursors.
The X-ray photoelectron spectroscopy (XPS) measure-
ment of the PEDOT-800 films was achieved to investigate the
quantitative sulfur content and bonding states of sulfur in the
carbon film. The XPS survey spectra showed O 1s, C 1s, S 2s,
and S 2p peaks. The C 1s peak increased considerably and the
O 1s and S 2p peaks decreased in intensity, respectively, after
carbonization at 8008C. The PEDOT-800 films have no Cl 2p
peak on the surface that is in agreement with the result of the
elemental analysis. Figure 3e and f shows high-resolution C 1s
and S 2p spectra of the oxidized PEDOT-800 film, respec-
Figure 2. POM images of the neutral H-PEDOT films synthesized in
a) (R)- and b) (S)-N*-LCs of system 3, respectively [5CB:(R)- or (S)-
D2:bis-EDOT:TBAP=100:1:2:0.2]. Arrows indicate the directions of
the polarizer and analyzer. SEM images of c) left- and d) right-handed
H-PEDOT films with spiral morphologies synthesized using c) (R)- and
d) (S)-D2, respectively.
dopant. The H-PEDOT films synthesized in N*-LCs of
systems 1 and 2 had the distance of 1.5–1.6 mm and 270–
2
80 nm, respectively (Figures S1–S3). The highly
twisted H-PEDOT film synthesized in N*-LC includ-
ing D2 shows an intense CD signal (Figure 1) with
crystal-like ordered morphology (Figure 2a,b, and
Figures S4–S6). The chiral dopant of D2 induces
highly twisted N*-LC with a helical pitch of 341–
[
13]
6
21 nm.
This is approximately five to nine times
shorter than the helical pitch of N*-LC induced by the
chiral dopant of D1 (3.1 mm). The addition of a small
amount of D2 (0.5–1 mol%) into an N-LC enabled us
to prepare a highly twisted N*-LC; the molar concen-
tration of the chiral dopant in systems 2 and 3 is one-
half of or equal to that of D1 in system 1 (Table S1).
The gabs of the neutral H-PEDOT film synthesized in
system 3 was higher than that of the H-PEDOT film
synthesized in system 1 (Figure 1 and Figure S7).
The X-ray diffraction (XRD) analyses for the
graphite films prepared from the oxidized and neutral
PEDOT films at 8008C and subsequent heating at
2
2
6008C (abbreviated as oxidized and neutral PEDOT-
600 films) indicated that the former has a higher
crystallinity than the latter (Figure 3a,b and Fig-
[
14]
ure S8).
a sharp (002) plane of a graphitic crystal at 3.46 ꢀ
25.78 in 2q). This is attributed to the difference in
crystallinity between the neutral and oxidized PEDOT
The oxidized PEDOT-2600 film shows
(
[
15]
films as carbonization precursors. The closely packed
polymer chains stabilized by the doped ClO4 ions in
À
the oxidized PEDOT film will crosslink effectively
during the heating process. As a result, the graphitized
PEDOT film with high crystallinity is prepared from
Figure 3. XRD patterns of a) oxidized and b) neutral PEDOT, carbon, and
graphite films. c) Raman scattering spectra of oxidized PEDOT, carbon, and
graphite films, and d) their magnified spectra. High-resolution e) C 1s and
the oxidized PEDOT film. The electrical conductivity f) S 2p XPS spectra of the oxidized PEDOT-800 film.
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[
2]
tively. The main peak of the C 1s spectrum at 284.6 eV is
morphology-retaining carbonization. This is due to the
infusibility of the PEDOT films at high temperature, which is
characteristic of aromatic p-conjugated polymers with rigid
backbone. Figure 4d shows a SEM image of the oxidized H-
PEDOT-2600 film that was prepared using the N*-LC of
system 3 including the chiral dopant of (R)-D2. It should be
emphasized that the graphite film prepared by the heat
treatment at 26008C has almost the same surface morphology
as that of the original H-PEDOT film and that of the helical
carbon film prepared at 8008C, except for a thermal shrink-
age.
2
assigned to sp carbon atoms, suggesting that the networks of
2
[16]
sp carbon bonds are formed to some extent. The subpeaks
at higher binding energy indicate the existence of carbon
atoms bonding to sulfur and oxygen atoms. The S 2p spectrum
can be deconvoluted into two clear peaks at 163.8 and
1
65.0 eV corresponding to S 2p3/2 and S 2p1/2 of thiophenic
sulfur, respectively. Thus, it can be inferred that the sulfur is
mainly found in thiophene-like structures in the carbon
material. This result is reasonable by taking account of the
[
17]
molecular structure of the precursor. The atomic percent-
age of oxygen, carbon, and sulfur of the oxidized PEDOT-800
film is 2.06, 95.22, and 2.72%, respectively. A clear difference
in the quantitative sulfur content and bonding states of sulfur
between the neutral and oxidized PEDOT-800 films was not
observed.
It is generally difficult to prepare undoped and neutral
[
18]
PEDOT through electrochemical synthesis. This is because
the dedoping of the oxidized PEDOT film cannot be
completely achieved, as mentioned above. Here, it is desir-
able to compare the present neutral and oxidized PEDOT
films with the “entirely neutral PEDOTs”, which can be
prepared by another synthesis method, in terms of feature,
structure, and thermal behavior in carbonization. The entirely
neutral PEDOT was synthesized through dehalogenation
polycondensation in an isotropic solvent instead of the
Interestingly, the helical morphology of the oxidized H-
PEDOT film remained unchanged after the carbonization at
8
008C. A spiral morphology consisting of fibril bundles is
observed in a domain. The helical carbon films prepared using
the (R)- and (S)-PEDOT films exhibit spiral morphologies
with left- and right-hand screw directions, respectively (Fig-
ure 4a and b). Therefore, it can be argued that the helical
senses of the spirals consisting of carbon fibrils are control-
lable by choosing the chirality of the chiral dopant. The
distance between the fibril bundles was 130 nm. The mor-
phologies of the H-PEDOT films synthesized in N*-LCs of
systems 1 and 2 were also preserved even after the carbon-
ization (Figures S9 and S10). The morphology-retaining
carbonization is possible not only for the oxidized H-
PEDOT film but also for the neutral H-PEDOT film bearing
[
19]
electrochemical oxidative polymerization.
The polymer
was obtained in powder form and was found to be insoluble in
regular organic solvents. The carbon particles obtained from
the neutral PEDOT at 8008C showed slightly better crystal-
linity than that of the PEDOT-800 films with an amorphous
structure (Figure 3a,b, and Figure S11). The difference in the
degree of crystallinity between the carbonized materials may
be attributed to the difference in hydrogen content and in the
polymerization methods used to synthesize the corresponding
precursor polymers (for details see the Supporting Informa-
tion). Finally, although the entirely neutral PEDOTs could be
potentially useful carbonization precursors, especially in
terms of the degree of crystallinity, they are not able to
produce helical graphite or even carbon films. Therefore,
among the three types of PEDOTs (oxidized PEDOT film,
neutral PEDOT film, and entirely neutral PEDOT), the
oxidized PEDOT films are the most promising precursors for
the preparation of helical carbon and graphite films.
À
almost no ClO4 dopant (Figure 4c). This is quite different
from the carbonization of a PA film, where the use of an
oxidized film obtained from iodine doping is required for the
In summary, we found that the oxidized PEDOT films
consisting of closely packed polymer chains, which are
stabilized by the electrostatic interaction between the cationic
polymer chain and the anionic dopant, perform effective
carbonization, leading to the highly crystalline graphite films.
It was also demonstrated that the carbonization of a helical
aromatic p-conjugated polymer film with a tunable helical
sense and degree of helicity is a promising approach for the
preparation of hierarchically controlled helical carbon and
graphite films. The present helical carbon atoms might exhibit
unique electrochemical properties characteristic of the helical
structure. The induced solenoid magnetism, based on high
electrical conductivity and well-controlled helical structure,
might be one of the most anticipated and outstanding
properties of helical graphites.
Figure 4. SEM images of the oxidized H-PEDOT-800 films with spiral
morphologies that were prepared using the a) (R)- and b) (S)-N*-LCs
of system 3, respectively [5CB:(R)- or (S)-D2:bis-EDOT:T-
BAP=100:1:3:0.5]. c) SEM image of the neutral H-PEDOT-800 film
that was prepared using N*-LC of system 3 including (R)-D2. d) SEM
image of the oxidized H-PEDOT-2600 film that was prepared using the
Experimental Section
The molecular structures of the chiral dopants, the N-LC, the
supporting electrolyte (tetra-n-butylammonium perchlorate;
(R)-N*-LC of system 3.
1
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TBAP), and the monomer are depicted in Scheme S1. Di- and
tetrasubstituted axially chiral binaphthyl derivatives were synthesized
d) S. Matsushita, Y. S. Jeong, K. Akagi, Chem. Commun. 2013,
49, 1883 – 1890.
[
13,20]
as described in previous reports.
The former is (R)- or (S)-2,2’-
[7] CD measurements were performed through the rotation of the
sample to exclude both the artifact and linear polarization in the
Cotton effect.
[8] “Exciton Chirality Method: Principles and Applications”: N.
Berova, K. Nakanishi in Circular Dichroism: Principles and
Applications, 2nd ed. (Eds.: N. Berova, K. Nakanishi, R.
Woody), Wiley, New York, 2000; chap. 12, pp. 337 – 382.
[9] T. Mori, M. Kyotani, K. Akagi, Macromolecules 2010, 43, 8363 –
8372.
PCH506-1,1’-binaphthyl [abbreviated as (R)- or (S)-D1]; the latter is
(
(
R)- or (S)-2,2’-PCH5012-6,6’-PCH5-1,1’-binaphthyl [abbreviated as
R)- or (S)-D2].
The helical twisting powers of the chiral dopants, the concen-
tration ratios, and the helical pitches of the N*-LCs are summarized in
Table S1. 5CB was used as a parent LC for the N*-LC. The N*-LC of
system 1 was prepared by adding 1 mol% of the chiral dopant, D1,
into 5CB. The N*-LCs of systems 2 and 3 were prepared by adding 0.5
and 1 mol% of D2 into the N-LC, respectively.
The electrochemical polymerizations in asymmetric reaction
fields were performed using the N*-LCs for the production of H-
PEDOT films. The N*-LC solutions were prepared by adding 0.5–
[10] P. Leriche, P. Blanchard, P. Frꢁre, E. Levillain, G. Mabon, J.
Roncali, Chem. Commun. 2006, 275 – 277.
[11] a) J. Huang, S. Virji, B. H. Weiller, R. B. Kaner, J. Am. Chem.
Soc. 2003, 125, 314 – 315; b) X. Zhang, S. K. Manohar, J. Am.
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[14] Since the H-PEDOT films were too thin to obtain a sufficient
amount for reliable characterizations, freestanding PEDOT
films were electrochemically synthesized in MeCN as an
isotropic solvent instead of a N*-LC solution under similar
conditions. The obtained PEDOT films were then carbonized
and subsequently graphitized, yielding the freestanding
PEDOT-800 and PEDOT-2600 films, respectively. The neutral
and oxidized PEDOT, PEDOT-800, and PEDOT-2600 films
were characterized in terms of thermal, structural, and electrical
properties by elemental analysis, SEM, IR spectroscopy, ther-
mogravimetric analysis, XRD, XPS, Raman spectroscopy, and
electrical conductivity. Details can be found in the Supporting
Information.
1
mol% of D1 or D2, 3 mol% of bis-EDOT, and 0.5 mol% of TBAP
into 5CB. The full experimental details of the polymerization methods
are presented in the Supporting Information. The H-PEDOT films
for the measurements of UV/Vis and CD spectra were synthesized in
the N*-LC solutions that were prepared by adding 0.5–2 mol% of D1
or D2, 2 mol% of bis-EDOT, and 0.2 mol% of TBAP into 5CB. The
dedoping process was performed by applying 4 V to the H-PEDOT
films in an acetonitrile (MeCN) solution of TBAP (0.05 m) for
five minutes at room temperature.
The PEDOT film was carbonized at 8008C using the electric
À1
furnace for 1 h with a heating rate of 108C min . The carbonized film
was furthermore heated at 26008C with a graphitizing apparatus for
3
0 minutes in flowing argon gas.
Received: September 28, 2013
Revised: November 14, 2013
Published online: January 22, 2014
Keywords: chirality · electrochemistry · helical structures ·
.
liquid crystals · polymerization
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