Liquid crystalline PEDOT derivatives exhibiting reversible anisotropic electrochromism and linearly and circularly polarized dichroism

Article abstract of DOI:10.1039/c0jm04183a

The multifunctional poly(3,4-ethylenedioxythiophene) [PEDOT] derivatives that exhibit liquid crystallinity, reversible anisotropic electrochromism, and linearly and circularly polarized dichroism are developed. A series of copolymer-type liquid-crystalline PEDOT (LC PEDOT) derivatives are synthesized and evaluated in terms of optical and electrochromic properties. Two of the polymers are monosubstituted LC PEDOT derivatives with phenylcyclohexyl (PCH) or cyanobiphenyl (CB) mesogenic side chain moieties linked to the main chain via an ester bond. Four are disubstituted polymers bearing PCH or CB side chain moieties linked to the main chain via an ester or an ether bond. All of the polymers showed thermotropically nematic LC (N-LC) phases of an enantiotropic nature. Macroscopically aligned LC PEDOT derivatives were prepared by rubbing the polymer films in their N-LC phases, yielding linearly polarized fluorescence and anisotropic electrochromism. The morphologies of the polymer films reversibly changed between the neutral and oxidized states through electrochemical dedoping and doping procedures. The electrochromism between the neutral and oxidized states occurred in the parallel and perpendicular directions of the aligned polymer film, affording a novel anisotropic electrochromism. The chiral nematic LC (N*-LC) phases were also induced in the polymers by adding chiral binaphthyl derivatives as chiral inducer into the LC PEDOT derivatives. The polymer films with the induced N*-LC phase exhibited circular dichroism, implying the formation of an interchain helically π-stacked structure or intrachain helically twisted structure.

Full text of DOI:10.1039/c0jm04183a

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PAPER
Liquid crystalline PEDOT derivatives exhibiting reversible anisotropic
electrochromism and linearly and circularly polarized dichroism†
*
Yong Soo Jeong and Kazuo Akagi
Received 1st December 2010, Accepted 26th April 2011
DOI: 10.1039/c0jm04183a
The multifunctional poly(3,4-ethylenedioxythiophene) [PEDOT] derivatives that exhibit liquid
crystallinity, reversible anisotropic electrochromism, and linearly and circularly polarized dichroism
are developed. A series of copolymer-type liquid-crystalline PEDOT (LC PEDOT) derivatives are
synthesized and evaluated in terms of optical and electrochromic properties. Two of the polymers are
monosubstituted LC PEDOT derivatives with phenylcyclohexyl (PCH) or cyanobiphenyl (CB)
mesogenic side chain moieties linked to the main chain via an ester bond. Four are disubstituted
polymers bearing PCH or CB side chain moieties linked to the main chain via an ester or an ether bond.
All of the polymers showed thermotropically nematic LC (N-LC) phases of an enantiotropic nature.
Macroscopically aligned LC PEDOT derivatives were prepared by rubbing the polymer films in their
N-LC phases, yielding linearly polarized fluorescence and anisotropic electrochromism. The
morphologies of the polymer films reversibly changed between the neutral and oxidized states through
electrochemical dedoping and doping procedures. The electrochromism between the neutral and
oxidized states occurred in the parallel and perpendicular directions of the aligned polymer film,
affording a novel anisotropic electrochromism. The chiral nematic LC (N*-LC) phases were also
induced in the polymers by adding chiral binaphthyl derivatives as chiral inducer into the LC PEDOT
derivatives. The polymer films with the induced N*-LC phase exhibited circular dichroism, implying
the formation of an interchain helically p-stacked structure or intrachain helically twisted structure.
(3,4-ethylenedioxythiophene) (PEDOT) is
a relatively new
1. Introduction
member in the family of conducting polymers.14–18 PEDOT has
many interesting properties, including a low half-wave potential
and a small band gap.19,20 This polymer is more electrochemically
stable during cyclic voltammetry and has greater air and thermal
stability in terms of electrical properties, compared with other
kinds of polythiophene derivatives.21,22
The optical and electrochemical properties of p-conjugated
conducting polymers are intriguing. In organic electronics, con-
ducting polymers have a number of advantages over small
molecules, in particular, their stability, mechanical properties
and ease of processing. Electrically conjugated polymers have
been regarded as excellent materials to be applied to electronic
devices, electrolytic capacitors, actuators, sensors, artificial
muscles and light-emitting diodes (LEDs).1–13 Among them,
conjugated polymers with small band gaps have been drawing
much attention because they potentially produce materials with
high conductivities and desirable optical properties. The
synthesis of p-conjugated polymers with small band gaps is one
of the major focuses in the field of organic conductors.
Liquid crystals are easily aligned by external forces, such as
sheer stress and electric and magnetic fields. A p-conjugated
polymer is insoluble and infusible because of the integrity of the
main chain backbone. If a conjugated polymer is substituted with
a liquid crystalline (LC) group, the polymer is soluble in organic
solvents and it is aligned by the spontaneous orientation of the
LC group. Besides, it can be macroscopically aligned by external
perturbations, in a way similar to LC molecules. With the
combination of liquid crystallinity and electrochromic func-
tionality, we expect to prepare linearly dichroic electrochromic
materials.23–26
Blocking the 3- and 4-positions of thiophene with an ethyl-
enedioxy group yields a highly electron-rich fused heterocycle
3,4-ethylenedioxythiophene (EDOT), in which the ethylenedioxy
bridge provides minimal steric repulsion between the rings. Poly
Meanwhile, it is well known that when a small amount of
chiral compound is added, as a chiral inducer, to a nematic
LC (N-LC), a chiral nematic LC (N*-LC) phase is induced by
virtue of chiral induction27 that is also called sergeant-soldiers.28
Although the chiral induction has been applied mainly to LC
molecules, the application to LC polymers has been seldom
Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto,
615-8510, Japan. E-mail: akagi@fps.polym.kyoto-u.ac.jp
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0jm04183a
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achieved. If the even conjugate polymers bear an N-LC nature,
they might exhibit an N*-LC phase by using the chiral inducer,
giving rise to chirality and/or helicity.29–37 The LC PEDOT
derivative with the induced chirality is expected to have circularly
dichroic absorption and emission.
(11.96 g, 68 mmol) in anhydrous diethyl ether (70 ml) under argon
atmosphere at room temperature. The mixture was stirred at
room temperature for 10 min, and then refluxed for 30 min. The
mixture was cooled to ꢁ78 ꢀC, and tributylstannyl chloride (11.56
g, 58 mmol) was added slowly over 1 h. The reaction mixture was
warmed up to room temperature and stirred for 3 h. The resulting
solution was extracted three times with 300 ml of diethyl ether.
The organic layer was then washed with an aqueous solution of
saturated sodium chloride. After drying over Na2SO4 and filtra-
tion, diethyl ether was removed using a rotary evaporator. The
residue was purified by column chromatography with hexane on
pretreated silica gel which was washed with neat triethylamine,
then hexane. The solvent was removed in vacuum and the residue
was further purified through vacuum distillation to give 12.9 g of
colourless liquid (yield: 73%). Anal. calcd for C18H32O2SSn: C,
50.13; H, 7.48; O, 7.42; S, 7.44; Sn, 27.53. Found: C, 49.83; H, 7.53;
In this work, to cultivate multifunctional and high perfor-
mance conjugated polymers useful for plastic electronics, we
developed a series of copolymer-type LC PEDOT derivatives by
chemical polymerizations. Macroscopically aligned polymer
films were prepared by rubbing the films in the N-LC phase. The
anisotropic electrochromism was observed in the aligned films.
Subsequently, helical polymer films were prepared through
a formation of the N*-LC phase constructed by adding the chiral
inducer to the polymer in the N-LC phase. Circular dichroism
was observed in absorption spectra of the LC PEDOT films with
interchain helically p-stacked structure or intrachain helically
twisted one.
1
S, 6.24; Sn, 27.73. H NMR (270 MHz, CDCl3, d, from TMS):
d 6.57 (s, 1H), 4.19 (m, 4H, –OCH2), 0.24–0.44 (t, 9H, –CH3) ppm.
13C NMR (270 MHz, CDCl3, d, from TMS): d 141.8, 141.5, 107.1,
99.6, 77.5, 77.0, 3.1, 3.0, 2.8 ppm.
2. Experimental
Techniques
2,5-Dibromohydroquinone
All 1H NMR and 13C NMR spectra were measured with an EX-
270 spectrometer. CDCl3 was used as the deuterated solvent, and
trimethyl silane (TMS) was used as an internal standard. Infrared
spectroscopic measurements were carried out with a Jasco FT-IR
550 spectrometer, using the KBr method. The phase transition
temperatures were determined using a Perkin-Elmer differential
scaꢀnning calorimeter (DSC) at a constant heating/cooling rate of
10 C minꢁ1, and the first cooling and second heating processes
were recorded. Optical texture observations were performed
using a Carl Zeiss polarizing optical microscope (POM), equip-
ped with a Linkam THMS 600 heating and cooling stage. The
molecular weights of the polymers were determined by gel
permeation chromatography (GPC) with a Shodex A-80M
column. X-Ray diffraction (XRD) measurements were per-
formed with a Rigaku D-3F diffractometer in which the X-ray
power and scanning rate were set to 1200 mW and 5 degrees per
minute, respectively. Molecular mechanics (MM) calculations
for the polymers were carried out with a Silicon Graphics Cerius2
system.
A solution of 1,4-dibromo-2,5-dimethoxybenzene (14.2 g, 48
mmol) in dichloromethane (60 ml) was stirred for 30 min at room
temperature. Then boron tribromide (25.0 g, 100 mmol) was
slowly added to the solution and was refluxed for 24 h at 40 ꢀC.
ꢀ
The solution was allowed to cool to 0 C. The precipitate was
added to a large amount of water and stirred for 3 h at 0 ꢀC. The
solution was filtered to give 11.3 g as a white solid (yield: 89%).
Anal. calcd for C8H8Br2O2: C, 26.90; H, 1.50; Br, 59.65; O, 11.95.
Found: C, 26.7; H, 1.6; Br, 59.7. 1H NMR (270 MHz, CDCl3, d,
from TMS): d 6.6 (d, 2H), 5.2 (d, 2H, –OH) ppm. 13C NMR (270
MHz, CDCl3, d, from TMS): d 110.7, 122.5, 155.4, 110.7, 122.5,
155.4 ppm.
Pentylcyclohexylphenoxy-1-hexyl alcohol (PCH506OH)
Potassium carbonate (11.6 g, 84 mmol), potassium iodide (4.65 g,
28 mmol) and p-(trans-4-pentylcyclohexyl)phenol (6.8 g, 28
mmol) were added to 100 ml butanone in a three-neck round-
bottom flask equipped with a magnetic stirrer and argon line at
room temperature. Then 6-bromo-1-hexanol (5.07 g, 28 mmol)
was very slowly added and the mixture was refluxed for 48 h at
Materials
ꢀ
N,N-Dimethylformamide (DMF), tetrahydrofuran (THF),
chloroform and toluene were distilled prior to use. EDOT was
obtained from Aldrich Chemical. For purification, EDOT was
distilled under vacuum (85 ꢀC at 0.65 Torr). Diethyl azodi-
carboxylate (40 wt% in toluene; DEAD) and triphenylphosphine
(TPP) were used as received from Tokyo Kasei (TCI). (R)- and
(S)-2,20-Dihydroxy-1,10-binaphthyls (0.99 optical purity) were
purchased from commercially available sources. The mesogenic
compound 4-(trans-4-n-pentylcyclohexyl)phenol [PCH500] was
purchased from Kanto Chemical Ltd.
60 C. After evaporation, the solvent was extracted three times
with 300 ml of diethyl ether. The organic layer was then washed
with an aqueous solution of saturated sodium chloride. After
drying over Na2SO4 and filtration, diethyl ether was removed
using an evaporator. The crude product was passed through
a column chromatograph (chloroform) and recrystallized from
ethanol to give 8.11 g as a white solid (yield: 84%). 1H NMR (270
MHz, CDCl3, d, from TMS): d 7.4 (s, 2H), 6.9 (s, 2H), 4.2 (s, 1H),
3.9 (t, 2H), 3.7 (t, 2H), 1.8–1.0 (m, 29H) ppm. 13C NMR (270
MHz, CDCl3, d, from TMS): d 156.6, 138.6, 127.7, 114.9, 68.8,
62.0, 43.1, 37.9, 33.1, 31.4, 29.5, 25.3, 14.1 ppm.
2,5-Bis(tributylstannyl)-3,4-ethylenedioxythiophene
6-(4-Cyano-40-biphenyl)-oxy-1-hexyl alcohol (CB06OH)
n-Butyl lithium (22.7 ml, 58 mol, 2.6 M in hexane) was slowly
added to a stirred solution of 3,4-ethylenedioxythiophene (4.14 g,
29 mmol) and N,N,N0,N0-tetramethylethylenediamine (TMEDA)
Potassium carbonate (4.84 g, 35 mmol), potassium iodide (2.91 g,
18 mmol) and p-(trans-4-pentylcyclohexyl)phenol (3.60 g, 18
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mmol) were added to 100 ml butanone in three-neck round-
bottom flask equipped with a magnetic stirrer and argon line at
room temperature. Then 6-bromo-1-hexanol (4.15 g, 18 mmol)
was slowly added and the mixture was refluxed for 48 h at 80 ꢀC.
After evaporating, the solvent was extracted three times with
300 ml of diethyl ether. The organic layer was then washed with
an aqueous solution of saturated sodium chloride. After drying
over MgSO4 and filtration, diethyl ether was removed using an
evaporator. The crude product was passed through a column
chromatograph (chloroform) and recrystallized from ethanol to
7.7 (s, 1H), 7.4 (s, 2H), 7.0 (s, 2H), 4.7 (t, 4H), 4.2 (t, 2H), 4.0 (t,
2H), 2.3 (t, 2H), 1.84–0.85 (m, 29H) ppm. 13C NMR (270 MHz,
CDCl3, d, from TMS): d 167.9, 156.6, 150.5, 147.6, 138.7, 138.2,
133.4, 133.1, 130.6, 130.1, 128.8, 128.2, 127.1, 114.1, 111.3, 111.0,
68.7, 65.3, 65.0, 43.7, 37.7, 29.6, 29.3, 27.1, 26.8, 22.7, 21.8, 14.1,
10.8 ppm.
P2 to P6 were polymerized using the methods similar to that
described for P1. Details of the polymerization are given in the
ESI†.
1
give 6.37 g as a white solid (yield: 91%). H NMR (270 MHz,
3. Results and discussion
CDCl3, d, from TMS): d 7.7 (s, 2H), 7.6 (s, 2H), 7.5 (s, 2H), 6.9 (s,
2H), 3.9 (t, 2H), 3.7 (t, 2H), 1.8–1.3 (m, 8H) ppm. 13C NMR
(CDCl3, 270 MHz): d 159.5, 145.2, 131.7, 127.9, 126.8, 119.0,
68.1, 62.9, 33.1, 29.4, 25.3 ppm.
3.1. Synthesis and structures of polymers
Fig. 1 shows the six kinds of LC PEDOT derivatives synthesized.
Two of the polymers are monosubstituted LC PEDOT derivatives
bearing phenylcyclohexyl (PCH) or cyanobiphenyl (CB) mesogenic
side chain moieties that are linked to the main chain via an ester
bond. Four of them are disubstituted LC PEDOT derivatives with
PCH or CB mesogenic side chain moieties linked to the main chain
via an ester or an ether bond. The liquid crystallinity of the polymers
was examined in terms of the number of substitutions (one or two)
and the linkage between the main and side chains (ester or ether).
The LC substitutions, PCH506OH and CB06OH, were
synthesized through the Williamson etherification reaction, and
the synthesis pathways are shown in Scheme 1. Hydroquinone and
6-(4-(4-Pentylcyclohexyl)phenoxy)hexyl 2,5-dibromobenzoate
(M1)
A solution of triphenylphosphine (2.62 g, 10 mmol) and diethyl
azodicarboxylate (DEAD) (4.31 g, 40 wt% in toluene, 10 mmol)
in THF (10 ml) was stirred for 1 h at room temperature. Then,
2,5-dibromobenzoic acid (2.68 g, 10 mmol) and PCH506OH
(3.46 g, 10 mmol) in THF (80 ml) were slowly added via pressure
equalized dropping funnel. The reaction mixture was stirred at
room temperature overnight under an argon atmosphere. TLC
indicated completion of the reaction. The solution was extracted
with dichloromethane, washed with water thoroughly, and dried
over anhydrous sodium sulfate. The dichloromethane layer was
removed by evaporation and was purified by column chroma-
tography (silica gel, dichloromethane). The solvent was removed
in vacuum and the residue was further purified through vacuum
2,5-bis(tributylstannyl)-3,4-ethylene
dioxythiophene
were
prepared by a directed lithiation reaction, using twice the relative
mole ratio of EDOT and followed by a reaction with tributyl-
stannyl chloride. The LC-substituted dibromophenylene deriva-
tives M1–M6) were prepared by the Mitsunobu reaction between
the LC substituents (PCH506OH and CB06OH) and 2,5-dibro-
mobenzoic acid for M1 and M2, 2,5-dibromoterephtalic acid for
M3–M4, and 1,4-dibromohydroquinone for M5–M6. The poly-
merizations were carried out by Stille coupling reactions between
2,5-bis(tributylstannyl)-3,4-ethylenedioxythiophene (2) and the
LC-substituted dibromophenylene derivatives M1–M6), as
shown in Scheme 2.
1
distillation to give 2.2 g as a white solid (yield: 57%). H NMR
(270 MHz, CDCl3, d, from TMS): d 7.8 (s, 1H), 7.5 (s, 1H), 7.3 (s,
1H), 7.1 (d, 2H) 6.9 (d, 2H), 4.2 (t, 2H), 3.9 (t, 2H), 2.4 (m, 4H),
2.0–0.8 (m, 29H) ppm. 13C NMR (270 MHz, CDCl3, d, from
TMS): d 166.3, 157.2, 140.2, 137.5, 137.1, 136.5, 128.2, 121.1,
118.8, 68.0, 43.7, 38.6, 35.5, 34.9, 32.6, 29.3, 29.0, 26.7, 25.3, 24.0,
15.5 ppm.
P1 and P2 are monosubstituted PEDOT derivatives in which
the PCH and CB mesogens, combined with hexamethylene
spacers, are linked to the main chains via ester bonds,
M2 to M6 were prepared using the methods similar to that
described for M1. Details of the monomer synthesis are given in
the ESI†.
Poly[6-(4-(4-pentylcyclohexyl)phenoxy)hexyl-3,4-
ethylenedioxythiophene benzoate] (P1)
P1 was synthesized by Stille reaction between 2,5-bis(tributyl-
stannyl-3,4-ethylenedioxythiophene) and M1. A solution of M1
(596 mg, 1.0 mmol), 2,5-bis(tributylstannyl-3,4-ethyl-
enedioxythiophene) (720 mg, 1.0 mmol), dichlorobis(triphenyl-
phosphine)palladium(II) (Pd(PPh3)Cl2) (15 mg, 0.02 mmol) in
5 ml of toluene was stirred under argon at 100 ꢀC for 72 h. The
reaction mixture was poured into a large amount of methanol
(300 ml) containing hydrochloric acid (HCl) and vigorously
stirred for 3 h. The resulting precipitate was collected by filtration
and dissolved in the minimum amount of THF (5 ml) and stirred
in methanol (300 ml) for 24 h. After filtration, the product was
dried under vacuum to give 0.63 g as a yellow powder (yield:
48%). 1H NMR (270 MHz, CDCl3, d, from TMS): d 7.8 (s, 1H),
Fig.
1 Copolymer-type LC PEDOT derivatives with mono- and
disubstituted side chains: PCH-LC polymers (P1, P3 and P5) and CB-LC
polymers (P2, P4 and P6).
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Scheme 1 Synthetic pathways of the liquid crystalline (LC) moieties.
respectively. P3 and P4 are disubstituted PEDOT derivatives in
which two PCH and CB mesogens, combined with hexam-
ethylene spacers, are linked to the main chains via ester bonds,
respectively. The structures of P5 and P6 are the same as those of
P3 and P4, respectively, except that ether linkages replace the
ester bonds.
polystyrene (PS). The number-average molecular weights (Mn) of
the polymers varied between 3300 and 6700. The dispersion
ratios (Mw/Mn, Mw ¼ weight-average molecular weights) of the
polymers are 1.3–2.2, and the degrees of polymerizations (DPs)
are 4–11, which correspond to 8–22 aromatic rings. The relatively
low DPs of the polymers are due to the steric hindrance of the LC
side chains during the polymerization. The effect of the steric
hindrance is increased in the disubstituted PEDOT derivatives
with LC mesogens on both side chains, especially in P5 and P6. It
was possible to prepare cast films to form the polymers for
evaluating the chemical and optical properties of the films.
3.2. Molecular weights and dispersion ratios
Table 1 shows the molecular weights and dispersion ratios of the
polymers, evaluated by GPC measurements using standard
Scheme 2 Synthetic pathways of the copolymer-type LC PEDOT derivatives.
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Table 1 The molecular weights, dispersion ratios and degrees of poly-
merization of the polymers, P1–P6, evaluated by GPC with a polystyrene
standard
correspond to distances between main chains or LC side chains
(see Fig. S5†).
Table 2 summarizes the phase transition temperatures deter-
mined from the DSC measurements. The conjugated polymers
are highly viscous even in the isotropic phase because of their
rigidity of main chains. In addition, the viscosity of the polymer
increases as the temperature becomes close to the phase transi-
tion temperature from the isotropic to LC phase. This is because
the polymers tend to be further associated with each other, which
serves for a preparation of spontaneous alignment for the LC
phase. These situations should give a supercooling of the
isotropic phase. On the other hand, the primary and higher-order
structures of the polymer formed in the N-LC phase remain
unchanged in the glassy state. The polymers also exhibit the
supercooling in the N-LC phase, where the phase transition
temperature from the N-LC to the glassy state shifts to a lower
temperature.
Polymer Mn
Mw
Mw/Mn DPa Number of aromatic ring
P1
P2
P3
P4
P5
P6
3300 5500 1.7
6700 14 900 2.2
6
11
6
6
4
12
22
12
12
8
6300
5100
4100
4100
9900 1.6
7800 1.5
5200 1.3
5200 1.3
5
10
a
Degree of polymerization.
3.3. Thermal properties
The thermal properties of the polymers were evaluated with the
POM and DSC measurements. All the polymers showed ther-
motropic liquid crystallinity with an enantiotropic nature. Fig. 2
shows typical polarizing optical micrographs (POMs) of P1 and
P2. Schlieren textures were observed and are characteristic of
nematic LC phases. The assignment of nematic LC phase is
supported by X-ray diffraction (XRD) results; for instance, the
polymers have only a broad peak in the wide angle region aꢀt
It is apparent from Table 2 that the polymers with CB side
chains (P2 and P4) have higher and wider LC temperature
regions than those with PCH side chains (P1 and P3). The polar
cyanide (CN) group of the CB side chain enables interchain
electrostatic interactions between the terminal sites of the side
chains, which should stabilize the LC phase to yield a wider LC
temperature region. The disubstituted polymers (P3 and P4)
have higher and wider LC temperature regions than the corre-
sponding monosubstituted polymers (P1 and P2). A notable
difference in the LC temperature region can be seen between P4
and P6 in which the former and the latter have ester and ether
bonds, respectively, to link the CB moieties with the main chains;
P4 has a wider LC temperature region than P6. This implies that
the ester bond is more rigid than the ether bond and can suppress
the internal rotation of the side chains, leading to a more stable
LC phase in P4.
24.8ꢀ in 2q (d ¼ 3.6 A) and a weaker overtone peak at 12.5
ꢀ
ꢀ
ꢀ
ꢀ
(d ¼ 7.1 A) for P2, a broad peak at 19.0 (d ¼ 4.7 A) for P3, and
ꢀ
ꢀ
ꢀ
a broad peak at 19.9 (d ¼ 4.5 A) for P5. The peaks of 3.6–4.7 A
Here it is of interest to note that the LC PEDOT derivatives
have more stable LC phases, compared with LC poly(p-phenyl-
ene) [PPP] and other kinds of LC aromatic conjugated poly-
mers.41 The high degree of stability of the LC phase can be
related to the high planarity of the main chains of the PEDOT
derivatives. When the p-conjugated polymer forms a LC phase,
Table 2 The phase transition temperatures and LC temperature regions
of the polymers, determined from DSC measurements
Fig. 2 Polarizing optical micrographs (POMs) of polymers. (a) Schlieren
texture of P1 at 87 ꢀC in the cooling process and (b) Schlieren texture of
P2 at 162 ꢀC in the cooling process.
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it is necessary for the polymer to be aligned spontaneously in
a domain. The spontaneous alignment of the conjugated polymer
owes to a self-assembly of the polymers and also to an excluded
volume effect. It is likely that a planar conjugated polymer is
more preferable for the formation of the self-assembly because of
its ability to have p-overlap interaction and/or van der Waals
interaction between the polymer chains, than a less planar
conjugated polymer such as poly-p-phenylene (PPP) derivatives.
Actually, the dihedral angle between phenylene rings in PPP is
estimated to be about 30ꢀ because of steric repulsions between
neighboring hydrogen atoms belonging to the different phenyl-
ene rings. Thus, the planar conjugated polymer can form a stable
self-assembly feasible for the spontaneous alignment, which leads
to a stable LC phase.
The casting of the polymers gave rise to a broadening and a red
shift of the absorption bands for the polymer main chain. As
shown in Table 3, the polymers with ester bonds linking the side
chains (P1–P4) exhibited small red shifts of 8–10 nm, and the
polymers with the ether bonds linking the side chains (P5 and P6)
showed large red shifts of 33–34 nm. This means that the
planarity of the conjugated main chain with the ether linkages is
enhanced when the polymer is fabricated into the cast films.
Hence, the flexible ether bond is more favorable to the formation
of interchain p-stacking or interchain associations, leading to an
increase in the effective conjugation length.
Fig. 3 shows the fluorescence spectra of the polymers in
chloroform and as cast films. Polymers (P1 and P2) in chloro-
form showed blue fluorescence when they were excited by UV
light at 365 nm from a 16 W lamp. The spectroscopic measure-
ments indicate that the fluorescence bands of the polymers (P1–
P6), attributed to the p*–p transition of the main chains, are
located at the wavelength from 495 to 514 nm, which corre-
sponds to blue emission. The Stokes shifts for the polymers in
chloroform ranged from 73 to 90 nm. The quantum yields of the
polymer fluorescence were also evaluated by dissolving quinine
sulfate in 1 N sulfuric acid as the standard. As summarized in
Table 3, the quantum yields ranged from 12 to 21%, depending
on the number of the LC substitutions. The monosubstituted
polymers (P1 and P2) appear to have higher quantum yields than
the disubstituted polymers (P4–P6).
3.4. Optical properties
Fig. 3 shows the UV-Vis and fluorescence spectra of P1 and P2 in
chloroform and as cast films. The optical spectra of P3–P6 are
given in the ESI†. The optical properties of the polymers are
summarized in Table 3. The polymers with PCH side chains
dissolved in chloroform showed three absorption bands: < 250,
318 (shoulder) and 410 nm for P1; 280, 320 and 415 nm for P3;
and 280, 350 (shoulder) and 426 nm for P5. The two bands at the
shorter wavelength (250–280 nm) and the middle wavelength
(318–350 nm) were assigned to the mesogenic core of the side
chain. The band at the longer wavelength (410–426 nm) was
assigned to the p–p* transition of the conjugated main chain.
The polymers with CB side chains in chloroform showed two
absorption bands: 297 and 427 nm for P2; 296 and 412 nm for
P4; and 296 and 415 nm for P6. The bands at the shorter (296–
297 nm) and longer (412–427 nm) wavelengths were assigned to
the mesogenic core of the side chains and the conjugated main
chains, respectively. It should be noted that the effective conju-
gation lengths of the polymers are not remarkably affected by the
structural differences of the side chains (PCH and CB moieties),
the number of substitution (mono- and di-substitutes) and the
linkages between the main and side chains (ester and ether
linkages) so far as the polymers in solution are concerned.
The fluorescence of the polymers in cast films was investigated.
The films showed a green fluorescence at 515–534 nm for P1–P4
and a greenish yellow one at 554–558 nm for P5 and P6. The P5
and P6 films form more p-stacked structures between the poly-
mer chains than the P1–P4 films. This agrees with the results
from the absorption spectra. The Stokes shifts for the films are in
the range 92–106 nm, which is substantially larger than the range
observed in the chloroform solution (73–90 nm).
3.5. Reversible electrochromism
It is useful to explain briefly the process of the electrochemical
dedoping and doping. The polymer film cast on an ITO
glass is immersed in an acetonitrile solution including 0.1 M
Table 3 The optical properties of the polymers in chloroform and cast
films
UV-vis
(in
CHCl3)
UV-vis
(cast film) CHCl3)
Fluorescence (in
Fluorescence
(cast film)
lmax
/
lmax
nm
/
Da/ Em
Dc/ Fd Ex
/
Df/ Ex
nm nm nm (%) nm nm nm nm
/
b
/
e
/
b
Em
e
Polymer nm
P1
P2
P3
P4
P5
P6
410
427
415
412
426
415
418
435
423
8
8
8
495 85 21 363 524 106 376
99 381
92 375
500 73 21 362 534
502 87 12 361 515
422 10 502 90 12 371 528 106 397
459 33 514 88 13 376 558 99 395
449 34 502 87 14 418 554 105 371
a
Difference in wavelength of absorption band between solution and film.
b
c
d
Fig. 3 UV-Vis absorption (upper) and fluorescence (lower) spectra of
the polymers in chloroform and as cast films: (a) P1 and (b) P2. The insets
contain photographs of the fluorescent colors of the polymers in chlo-
roform and cast films.
Emission wavelength. Stokes shift in solution. Quantum yield
(evaluated with quinine sulfate dissolved in sulfuric acid).
1
N
e
Wavelength of excitation light. f Stokes shift in film.
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Fig. 5 UV-Vis absorption spectra of the P2 film in neutral and oxidized
states. The neutral and oxidized states are reversibly generated through
electrochemical dedoping and doping procedures, respectively.
Fig. 4 (a) Optical photograph, (b) differential interference contrast
(DIC) micrograph and (c and d) SEM images of the P2 film in neutral and
oxidized states.
microscopic measurements of the aligned film showed that the
LC texture was maintained even in the glassy state, and that the
multidomain of the nematic LC phase was uniaxially aligned
along the rubbing direction.
tetra-n-butylammonium perchlorate used as a supporting elec-
trolyte. The voltages of ꢁ0.8 V and +0.8 V vs. Ag/Ag+ are applied
to the polymer film for the electrochemical dedoping and doping,
respectively. The electrochemical dedoping and doping of the
polymer produce the neutral and oxidized forms of the conju-
gated main chain, respectively. These forms are represented by
benzonoid and quinoid structures, respectively (for instance, see
Fig. 4c and d). In the doping state, the perchlorate anion is
located as a dopant near the oxidized polymer bearing positive
charges. Fig. 4 depicts an optical photograph, differential inter-
ference contrast (DIC) micrograph and scanning electron
microscope (SEM) images of the P2 film in neutral and oxidized
states. The P2 film exhibits electrochromism between the neutral
and oxidized states, which is generated by electrochemical
dedoping and doping. The yellow color of the P2 film in the
neutral state becomes black in the oxidized state (Fig. 4a). The
drastic change in color of the film is confirmed in the DIC
micrographs of the neutral and oxidized P2 films (Fig. 4b). The
SEM images show that the film surfaces in the neutral state are
smooth and flat, while the oxidized states have rough globular
morphologies. These morphologies reversibly change between
the neutral and oxidized states, which is similar to the electro-
chromic changes that will be shown below.
Fig. 6 shows the linearly polarized fluorescence (LPF) spectra
of the aligned P2 film on a quartz substrate in which the polarizer
was placed parallel or perpendicular to the rubbing direction.
The LPF intensity at 534 nm for unpolarized excitation light
(l ¼ 381 nm) in the parallel (I ) direction was larger than that in
k
the perpendicular one (It). The dichroic ratio (R) of the aligned
film was evaluated to be 2.6, where R is defined as R ¼ I /It.39–41
k
It is evident that the aligned P2 film has an anisotropy in fluo-
rescence, serving for a linearly polarized luminescent material.
Fig. 7 shows optical micrographs of the macroscopically
aligned P2 film in the neutral and oxidized states. Bright and
dark optical textures are observed when the polarizer is set
parallel and perpendicular to the aligned direction of the film,
respectively. Such linearly polarized dichroism can be seen in
both neutral (Fig. 7a and c) and oxidized states (Fig. 7b and d).
The reversible electrochromism between the neutral and oxidized
states occurs in the parallel and perpendicular directions of the
aligned P2 film. It can be remarked that this aligned film of the
PEDOT derivative has linearly polarized electrochromism.
Fig. 5 shows the UV-Vis absorption spectra of the P2 film in
the neutral and oxidized states. The neutral and oxidized states
are reversibly generated through electrochemical dedoping and
doping procedures, respectively. The absorption band at 420 nm
in the neutral state and the broad band located from 500 to 800
nm in the oxidized state are responsible for the yellow and black
colors in the dedoped and doped films, respectively. In other
words, the reversible electrochromic change occurs through the
electrochemical doping and dedoping.
3.6. Linearly polarized fluorescence (LPF)
We carried out the macroscopic alignment of the polymers by
a rubbing technique. The polymers were heated to the LC
temperature region on the quartz substrates and rubbed with
a glass rod. Then the polymers were cooled to room temperature
to yield macroscopically aligned films. Polarized optical
Fig. 6 Linearly polarized fluorescence (LPF) of aligned P2 films. The
dichroic ratio (I /It) was 2.6. The aligned P2 film on a quartz substrate
k
(size: 10 ꢂ 50 mm) shows LPF upon excitation with UV light (l ¼ 365
nm, 16 W handy lamp).
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hexamethylene moieties at the 2 and 20 positions of the
binaphthyl ring.33,34,38–40 The mole ratio of the polymer to chiral
inducer in the mixture was 100 : 5. The POMs show fingerprint
textures with stripes (striae) of about 1 mm that correspond to
a half helical pitch of the N*-LC. This means that the helical
pitches of P2 and P5 in N*-LC phases are 2 mm, because the
distance between the stripes (striae) of the fingerprint texture
corresponds to a half helical pitch of the N*-LC phase. It may be
useful to note here that the helical pitch of the N*-LC is defined
as the distance between the chains with twisting angles of 0ꢀ and
360ꢀ in one-handed helical p-stacked structure. In other words,
three stripes (striae) of the fingerprint texture observed in POM
correspond to the helical structures with twisting angles of 0ꢀ,
180ꢀ and 360ꢀ, at which the birefringences are the same values,
giving the stripes (striae) in the POM.
Fig. 9 shows the UV-Vis and CD spectra of the polymer films
with N*-LC phases. The polymer films were prepared by adding
the chiral compound, (R)- or (S)-CB06-BINOL to the PEDOT
derivatives at room temperature and then annealing the polymer
films at the N-LC temperature, followed by cooling them to
room temperature to attain the glass state. The N*-LC structure
and morphology of the LC conjugated polymer formed at high
temperature for LC phase are well retained even after cooling to
the temperature corresponding to the glass state.35,41,42 It is
apparent that the P2, P4 and P5 films show bisignate Cotton
effects in the absorption region of the p–p* transitions for the
main chain. These results imply that the polymers form inter-
chain helically p-stacked structures. In contrast, P1 and P3 give
monosignate Cotton effects, suggesting the formations of intra-
chain helically twisted structures. These results are also observed
in the case of the usage of PCH-substituted binaphthyl deriva-
tives as chiral inducers.
Fig. 7 The reversible electrochromic behavior of a macroscopically
aligned P2 film between the neutral and oxidized states. The polarizer is
set to (a and b) parallel and (c and d) perpendicular to the aligned
directions of the film in the neutral and oxidized states. (e) A photo of the
P2 film with the neutral and oxidized states.
Interestingly, the electrochromism of the film occurs upon
electrochemical doping and dedoping (Fig. 7e). The photo in this
figure was acquired without a polarizer. The reversible electro-
chromism is confirmed in terms of the parallel direction (Fig. 7a
and b) and the perpendicular one (Fig. 7c and d), although the
differences between the brightness and contrast are diminished
because of the polarizer in the measurement.
3.7. Chiral induction and circular dichroism (CD) spectra
P6 gives a much larger Cotton effect than the other polymers.
P6 shows a monosignate Cotton effect in the 400 to 550 nm
region, corresponding to the absorption of the polymer chain.
This implies that large chirality is induced in the polymer chain.
At the same time, a bisignate Cotton effect with a high intensity
Fig. 8 shows the POMs of P2 and P5 in the chiral nematic LC
(N*-LC) phase. The N*-LC phase is induced by adding a chiral
compound as a chiral inducer to the polymer in the N-LC phase.
The chiral compound (R)-CB06-BINOL is an axially chiral
binaphthyl derivative substituted with cyanobiphenyl and
Fig. 8 POMs of P2 and P5 in the N*-LC phases. The N*-LC phase is induced by adding a chiral inducer, (R)-CB06-BINOL, to the polymer in the N-
LC phase. The mole ratio of polymer to chiral induced was 100 : 5. The POMs show fingerprint textures with striae.
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thermotropically nematic LC phases of an enantiotropic nature.
Macroscopically aligned polymers, prepared by rubbing in their
N-LC phases, exhibited linear dichroism in the absorption and
fluorescence spectra. The morphologies of the polymer films are
found to reversibly change between the neutral and oxidized
states through electrochemical dedoping and doping procedures.
The reversible electrochromism between the neutral and oxidized
states also occurred in the parallel and perpendicular directions
of the aligned film, producing anisotropic electrochromism.
Chiral nematic LC (N*-LC) phases were induced in the polymers
by the addition of the chiral binaphthyl compound as a chiral
inducer into the LC PEDOT derivatives. The polymer films with
the induced N*-LC phase exhibited circular dichroism in
absorption, suggesting the formation of interchain helically p-
stacked structures or an intrachain helically twisted one,
depending on the structures of the polymers. The present liquid
crystalline PEDOT derivatives exhibiting reversible anisotropic
electrochromism and linearly and circularly polarized dichroism
could be used as multifunctional materials for advanced plastic
electronics.
Acknowledgements
The authors acknowledge Dr Hiromasa Goto, University of
Tsukuba for his helpful 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 Tech-
nology, Japan.
Fig. 9 UV-Vis and CD spectra of the polymer films on ITO glasses: (a)
P1 and (b) P2. The polymer films were prepared by adding a chiral
inducer, (R)- or (S)-CB06-BINOL, to the PEDOT derivatives at the N-
LC temperature and cooling the mixtures to room temperature.
was observed in the region from 250 to 350 nm, which corre-
sponds to the absorption of side chains with CB moieties. Note
that the scale of the vertical axis in the CD spectra for Fig. S3-c†
is nearly eight times larger than the others. It remains unclear
why such a large Cotton effect is observed in this region, and this
will require further investigation.
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