Preparation of furan-based monomers and asymmetric electrochemical polymerization in cholesteric liquid crystals: Optical activity and selective reflection

Article abstract of DOI:10.1080/15421400902987636

1,4-Di(2-furyl)-2,5-substituted phenylenes were synthesized by a Stille cross-coupling reaction. Subsequently, these monomers were electrochemically polymerized in a cholesteric liquid crystal (CLC) electrolyte solution. The polymers thus prepared showed

Full text of DOI:10.1080/15421400902987636

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Preparation of Furan-Based
Monomers and Asymmetric
Electrochemical Polymerization
in Cholesteric Liquid Crystals:
Optical Activity and Selective
Reflection
Kohsuke Kawabata a , Hiroyuki Yoneyama a &
Hiromasa Goto a
a Graduate School of Pure and Applied Sciences,
Institute of Materials Science , University of
Tsukuba , Tsukuba, Ibaraki, Japan
Published online: 17 Dec 2009.
To cite this article: Kohsuke Kawabata , Hiroyuki Yoneyama & Hiromasa Goto
(2009) Preparation of Furan-Based Monomers and Asymmetric Electrochemical
Polymerization in Cholesteric Liquid Crystals: Optical Activity and Selective
Reflection, Molecular Crystals and Liquid Crystals, 515:1, 3-15, DOI:
10.1080/15421400902987636
To link to this article: http://dx.doi.org/10.1080/15421400902987636
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Mol. Cryst. Liq. Cryst., Vol. 515, pp. 3–15, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902987636
Preparation of Furan-Based Monomers and Asymmetric
Electrochemical Polymerization in Cholesteric Liquid
Crystals: Optical Activity and Selective Reflection
Kohsuke Kawabata, Hiroyuki Yoneyama, and
Hiromasa Goto
Graduate School of Pure and Applied Sciences, Institute of Materials
Science, University of Tsukuba, Tsukuba, Ibaraki, Japan
1,4-Di(2-furyl)-2,5-substituted phenylenes were synthesized by a Stille cross-
coupling reaction. Subsequently, these monomers were electrochemically polymer-
ized in a cholesteric liquid crystal (CLC) electrolyte solution. The polymers thus
prepared showed circular dichroism, birefringence, and a fingerprint texture
similar to that of the CLC electrolyte. Furthermore, one of the polymers displayed
selective reflection under illumination by a white light emitting diode.
Keywords: cholesteric liquid crystals; diffraction; electrochemical polymerization;
electrochromism
INTRODUCTION
Conjugated polymers such as polythiophene [1,2], polypyrrole [3,4],
polyaniline [5–7], and polyfuran [8] have been studied because of their
electrochemical and optical properties, as well as their electrical con-
ductivity. The synthesis of various substituted conjugated polymers
or conjugated polymers containing heteroatoms has been studied for
various applications such as electrochromic devices [9], photovoltaics
[10], organic electro-luminescent devices [11], actuators [12], and
biosensors [13]. Although polyethylenedioxythiophene has received
significant recent attention, little research has been reported on poly-
mers containing oxygen, with the exception of the electrochemical
synthesis of nonchiral polyfurans by Reynolds et al. [14,15].
Address correspondence to H. Goto, Graduate School of Pure and Applied Sciences,
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573,
Japan. Fax: 81-29-853-4490. E-mail: gotoh@ims.tsukuba.ac.jp
3

4
K. Kawabata et al.
Unfortunately, due to its high oxidation potential, furan monomer
requires a relatively high voltage for polymerization compared to other
monomers which can form conducting polymers. Polyfurans are
expected to exhibit advantageous ionic conductivity due to the high
electronegativity of their oxygen atoms [16].
The synthesis of chiral conjugated polymers has been of interest in
both chiral chemistry and conjugated polymer research [17,18]; how-
ever, few furan-based chiral polymers have been reported. To obtain
chiroptical conjugated polymers, a chiral substituent was introduced
into the main chain [19]. Recently, a new approach to obtaining
chiroptical electroactive conducting polymers, chiral electrochemical
polymerization from achiral monomers in a cholesteric liquid crystal
(CLC) [20], has been developed. We refer to this method as ‘‘cholesteric
liquid crystal asymmetric electrochemical polymerization.’’
We began the present research with the synthesis of 1,4-di(2-furyl)-
2,5-substituted phenylene monomers by a Stille cross-coupling reac-
tion with a Pd(0) complex catalyst [21]. From these achiral monomers,
optically active conjugated polymers were prepared by electrochemical
polymerization in a CLC electrolyte solution. Finally, we examined the
spectroelectrochemistry of the polymers prepared by cholesteric liquid
crystal asymmetric electrochemical polymerization, and observed
their surface using polarizing optical microscopy.
EXPERIMENTAL
Materials
Tetrabutylammonium perchlorate (TBAP), and cholesteryl oleyl
carbonate were obtained from Tokyo Kasei Japan. 4-cyano-40-n-hexyl-
biphenyl (6CB) was purchased from Merck. The indium tin oxide (ITO)
glass consists of a 0.2–0.3 mm-thick ITO layer on glass (9 X=cm2) was
purchased from Furuuchi Chemical Corporation.
Technique
Differential scanning calorimetry (DSC) analysis was carried out
with DSC6200 (Seiko Instruments) at scan rate of 5ꢀC minꢁ1. Fourier
transform infrared (FT-IR) absorption spectroscopy measurement
was carried out with FT-IR-300 spectrometer (JASCO). Optical
absorption spectra of the polymers were obtained with a HITACHI
U-2000. Circular dichroism (CD) measurements were performed with
a JASCO J-720. Electrochemical measurements of polymers were per-
formed using an electrochemical analyzer PGSTAT 12 (AUTOLAB,

Preparation of Furan-Based Monomers
5
the Netherlands). Optical texture observations were carried out using
a Nikon ECLIPS LV 100 high resolution polarizing microscope with a
Nikon LU Plan Fluor lens and a Nikon CFIUW lens.
Synthesis of Monomer
Four different 1,4-di(2-furyl)-2,5-substituted phenylenes (3a–3d) were
synthesized. Mono-substituted furan with tributyltin (1) was coupled
with dibromophenylenes (2a–2d) in the presence of [Pd(PPh3)4] in
toluene solution to afford three-ring type furan based monomers
(Scheme 1).
These monomers were electrochemically polymerized in the CLC
solvent, which had a 3-D helical structure. It is generally known that
the CLC phase has a helical structure and that the addition of a small
amount of chiral compound into a nematic liquid crystal (NLC) can
induce the formation of a CLC phase [22]. In order to construct a chiral
reaction field, we prepared a CLC electrolyte solution, consisting of
6CB as an NLC solvent, cholesteryl oleyl carbonate as a CLC inducer,
TBAP as a supporting salt, and the monomers. The molecular formu-
las and compositions of the constituents of the CLC electrolyte solution
are summarized in Table 1.
Synthesis of 1,4-Di(2-furyl)phenylene (3a)
1,4-Dibromophenylene 2a (1.65 g, 7 mmol) and 2-(tributylstannyl)-
furan 1 (5.00 g, 14 mmol) were mixed into toluene (13 mL) under
nitrogen. After refluxing the mixture for 30 min, tetrakis(triphenyl-
phosphine)palladium(0) [Pd[PPh3]4] (0.10 g, 0.087 mmol) was added.
After a reaction of 24 h at 80ꢀC, the toluene in the mixture was
SCHEME 1 1) [Pd(PPh3)4], toluene. 2) Cholesteric liquid crystal, electrochemi-
cal polymerization.

6
K. Kawabata et al.
TABLE 1 Constituents of the Cholesteric Liquid Crystal (CLC) Electrolyte
Solution Containing the Monomers
Molecular structure
Mass ratio
Quantity
2.5 mg
Monomer
1.8 wt%
NLC solvent
CLC inducer
89.8 wt%
8.0 wt%
0.125 g
11.25 mg
Supporting salt
0.4 wt%
0.5 mg
evaporated. The purification by chromatography on silica gel (eluent:
dichloromethane) and recrystallization from hexane followed by eva-
poration afforded 3a (1.34 g, 6.4 mmol) as a white solid (Yield: 90%).
1H NMR (270 MHz, d from TMS (ppm), CDCl3): d 6.48 (m, 2 H), 6,66
(d, 2 H, J ¼ 3.5 Hz), 7.47 (m, 2 H), 7.68 (d, 4 H, J ¼ 0.8 Hz).
Synthesis of 2,5-Di(2-furyl)toluene (3b)
2,5-dibromotoluene 2b (0.55 g, 2.2 mmol) and 2-(tributylstannyl)-
furan 1 (1.57 g, 4.4 mmol) were mixed into toluene (4 mL) under nitro-
gen. After refluxing the mixture for 30 min, [Pd[PPh3]4] (0.052 g,
0.045 mmol) was added. After a reaction of 24 h at 90ꢀC, the toluene
in the mixture was evaporated. The purification by chromatography
on silica gel (eluent: chloroform) and recrystallization from hexane

Preparation of Furan-Based Monomers
7
followed by evaporation afforded 3b (0.42 g, 1.9 mmol) as a white solid
1
(Yield: 83%). H NMR (270 MHz, d from TMS (ppm), CDCl3): d 2.53
(s, 3 H), 6.47 (m, 1 H), 6.50 (m, 1 H), 6.56 (dd, 1 H, J ¼ 3.5, 0.7 Hz),
6.66 (dd, 1 H, J ¼ 3.5, 0.7 Hz), 7.47 (dd, 1 H, J ¼ 1.8, 0.7 Hz), 7.50
(dd, 1 H, J ¼ 1.8, 0.7 Hz), 7.54 (m, 2 H), 7.72 (m, 1 H).
Synthesis of 2,5-Di(2-furyl)-p-xylene (3c)
This compound was prepared by the same procedure for synthesis
of 3b. Quantity used: 2,5-dibromo-p-xylene 2c (0.55 g, 2.1 mmol), 2-
(tributylstannyl)furan 1 (1.49 g, 4.2 mmol), toluene (4 mL), [Pd[PPh3]4]
(0.048 g, 0.042 mmol) (0.45 g, 1.9 mmol, light yellow solid, Yield ¼ 89%).
1H NMR (270 MHz, d from TMS (ppm), CDCl3): d 2.51 (s, 6 H), 6.50
(m, 2 H), 6.57 (d, 2 H, J ¼ 3.3 Hz), 7.51 (m, 2 H), 7.59 (s, 2 H).
Synthesis of 1,4-Diethyl-2,5-di(2-furyl)benzene (3d)
This compound was prepared by the same procedure for synthesis
of 3b. Quantity used: 1,4-dibromo-2,5-diethylbenzene 2d (0.50 g,
1.7 mmol), 2-(tributylstannyl)furan
1 (1.21 g, 3.4 mmol), toluene
(4 mL), [Pd[PPh3]4] (0.041 g, 0.036 mmol). (0.37 g, 1.4 mmol, light yel-
low solid, Yield ¼ 81%). 1H NMR (270 MHz, d from TMS (ppm), CDCl3):
d 1.26 (t, 6 H, J ¼ 7.5 Hz), 2.85 (q, 4 H, J ¼ 7.4 Hz), 6.51 (m, 4 H), 7.52
(m, 4 H, C(fu5,50)H).
Polymerization in a Cholesteric Liquid Crystal
The CLC electrolyte solution was injected between two indium
tin oxide (ITO)-coated glass electrodes sandwiching a Teflon sheet
(ꢂ0.2 mm) spacer. Polymerization was performed with a constant
4.0 V direct-current voltage applied between the electrodes. During
polymerization, the temperature was held at 22.0ꢀC in order to main-
tain the CLC phase. After 20 min, a thin polymer film was deposited
on the anodic ITO glass electrode. The residual CLC solution was
washed off with hexane to afford a steady green film. GPC measure-
ments for the polymers could not be performed because the polymers
are insoluble and infusible.
RESULTS AND DISCUSSION
Differential Scanning Calorimetry Analysis
Figure 1 shows DSC curves of 6CB with CLC inducer, and the CLC
electrolyte solution containing monomer, supporting salt, and CLC
inducer. The CLC electrolyte solutions exhibited stable CLC phase
in the temperature range of 7–23ꢀC. In each CLC solution, the

8
K. Kawabata et al.
CLC-crystal phase transition temperature in the first cooling was
lower than that in the second heating, because of supercooling of
the CLC phase. Furthermore, the addition of the monomers and
supporting salt into the CLC solutions decrease the phase transition
temperatures. However, in the case of the CLC solution containing
1,4-diethyl-2,5-di(2-furyl)benzene, the CLC-crystal phase transition
in the first cooling increased. Isotropic-CLC phase transition tempera-
tures decrease with increase of alkyl side chain length of the mono-
mers in the CLC electrolyte solutions.
Infrared Absorption Spectroscopy
FT-IR absorption spectra of 6CB, cholesteryl oleyl carbonate, and
poly(1,4-di(2-furyl)phenylene) are shown in Fig. 2. 6CB shows two
absorption bands at 2930 and 2845 cmꢁ1 assignable to nCH2 of alkyl
chain, and an absorption band at 2226 cmꢁ1 due to nCN of CN end
group. Cholesteryl oleyl carbonate shows two absorption bands due
to nCH2 of alkyl chain similar to that of 6CB, and an absorption
band at 1738 cmꢁ1 due to nC=O of the ester moiety. Contrary, poly
FIGURE 1 Differential scanning calorimetry curves of a cholesteric liquid crys-
tal (CLC) solution without supporting salt and monomers (a). CLC electrolyte
solutions containing supporting salt and monomers (1,4-di(2-furyl)phenylene
(b), 2,5-di(2-furyl)toluene (c), 2,5-di(2-furyl)-p-xylene (d), 1,4-diethyl-2,5-di
(2-furyl)benzene (e).

Preparation of Furan-Based Monomers
9
(1,4-di(2-furyl)phenylene) prepared in the CLC shows no absorption
bands at 2226 cmꢁ1 due to 6CB and at 1738 cmꢁ1 due to cholesteryl
oleyl carbonate. These results suggest that the polymer contains
neither 6CB nor cholesteryl oleyl carbonate.
Optical Absorption and CD
Figure 3 shows the CD spectra and optical absorption spectra of
the poly(1,4-di(2-furyl)phenylene) at various applied potentials vs.
Ag=Agþ reference electrode during oxidation. This polymer at 0 V, in
reduced state, exhibited CD signal of ꢁ111 mdeg at 472 nm. During
the oxidation process, the CD intensity of the polymer decreased to
ꢁ26 mdeg accompanied by the red shift from 472 nm to 483 nm. At
the same time, the absorption band due to p ꢁ pꢃ transition of conju-
gated main chain increased and shifted to long wavelengths. Further-
more, an absorption band due to polarons at around 700–800 nm
appeared. These results indicate that the change in electronic state
of the polymer provides change in color and chiroptical property.
The electrochemical oxidation process produces polarons and bipolar-
ons on the main chain accompanied by structural change from benze-
noid structure to planar quinonoid structure, resulting in increases of
coplanarity and rigidity of the main chain. The signal shift in the CD
FIGURE 2 Fourier transform infrared (FT-IR) absorption spectra for 6CB,
cholesteryl oleyl carbonate, and poly(1,4-di(2-furyl)phenylene).

10
K. Kawabata et al.
FIGURE 3 CD (top) and optical absorption (bottom) spectra of the poly(1,4-di
(2-furyl)phenylene) prepared by cholesteric liquid crystal asymmetric electro-
chemical polymerization at various applied potentials vs. Ag=Agþ reference
electrode during oxidation process.
of the polymer by the redox process can be explained by the structural
change in the main chain.
The poly3b–3d in a reduced state also exhibited the negative Cotton
effect at 473 nm (poly3b), 473 nm (poly3c), and 440 nm (poly3d) as
shown in Fig. 4. The CD signals of the polymers are at nearly identical
wavelengths, however, the CD intensities of the polymers are different.
An increase of alkyl side chain length of the polymers decreased in the
CD intensities. This result suggests that introduction of the alky side
chains disturbs the formation of the helical structure, resulting in
deformation of helical order structure of the polymers.
Therefore, the present cholesteric liquid crystal asymmetric electro-
chemical polymerization of furan-based monomer was successfully
performed. At the same time, we observed electrochromism in these
polymers. The color of the polymers changed from light green to black
after the electrochemical oxidation. Furthermore, the intensity of the
CD was tuned by application of voltage. This confirmed that the poly-
mer shows optical activity and that this optical activity was changed
during the oxidation process by application of voltage to the polymer.

Preparation of Furan-Based Monomers
11
FIGURE 4 CD (top) and optical absorption (bottom) spectra of the polymers
at 0 V vs. Ag=Agþ reference electrode.
Surface Texture
The polarizing optical microscopic (POM) image of the CLC electrolyte
solution shown in Fig. 5a. The CLC electrolyte solution displays a
fingerprint texture typical of cholesteric phase. Additionally, the
polymers synthesized in the CLC show fingerprint texture under
cross-Nicol conditions, as shown in Fig. 5b–d, and Fig. 6b. The finger-
print textures of the polymers are similar to that of the CLC electro-
lyte solution. Although the CLC shows pearl-like multireflection
color, the polymers exhibit green. The fingerprint textures of the
polymers remained unchanged after annealing at 300ꢀC. These results
indicate that the polymers has no CLC property, and that molecular
imprinting during the electrochemical polymerization in the CLC
resulted in the fingerprint texture. The fingerprint texture of the
CLC arises from the helicity of the LC molecules. This suggests that
the polymers have a CLC like ordered structure, in which the linear
macromolecules are arranged in the 3-D helical structure of the CLC
molecules.
Helical pitch of CLC depends on the concentration of CLC inducers.
Figure 6 shows a series of POM images of poly3a prepared in CLC

12
K. Kawabata et al.
FIGURE 5 Polarizing optical microscopic images of cholesteric liquid crystal
(CLC) electrolyte solution containing the monomer (a), poly(2,5-di(2-furyl)
toluene) (b), poly(2,5-di(2-furyl)-p-xylene) (c), and poly(1,4-diethyl-2,5-di(2-furyl)
benzene) (d) under cross-Nicol conditions.
electrolyte solutions at different concentrations of the CLC inducer.
Poly3a films were prepared in these electrolytes with 4.0 wt%,
8.0 wt%, and 12.0 wt% of the CLC inducer in the electrolyte. The three
electrolytes show fingerprint textures with different pitch lengths of
ꢂ2.5 mm (Fig. 6a), ꢂ1.6 mm (Fig. 6b), and ꢂ0.8 mm (Fig. 6c), respec-
tively. These results suggest that the fingerprint texture of the poly-
mers thus obtained in the CLC electrolyte solution derives from
helical structure of the CLC. Therefore, the pitch length of the finger-
print texture of the polymers is controllable by concentration of the
CLC inducer.
Selective Reflection of Light
The surface of poly3a, originally green, shows jewel beetle-like irides-
cent reflection under illumination by white light emitting diode
(Fig. 7). This is the structural color derived from the periodic
convexo-concave stripes along the fingerprint texture. The polymer

Preparation of Furan-Based Monomers
13
FIGURE 6 Polarizing optical microscopic images of poly(1,4-di(2-furyl)
phenylene) prepared in CLC solutions at different concentrations of the CLC
inducer. (a; 4.0 wt%, b; 8.0 wt%, c; 12.0 wt%).
synthesized in this study is not CLC, but it can exhibit an interference
color reflection phenomenon due to the polymer has a CLC-like
periodic structure. In other words, helical periodic structure of the
present polymer produced by imprinting from CLC in the course of

14
K. Kawabata et al.
FIGURE 7 Iridescent reflection of white light from the poly(1,4-di
(2-furyl)phenylene) film prepared by cholesteric liquid crystal asymmetric
electrochemical polymerization.
electrochemical polymerization functions CLC type one-dimensional
photonic crystals.
CONCLUSION
1,4-Di(2-furyl)-2,5-substituted phenylene monomers were successfully
synthesized using a one-pot Stille cross-coupling reaction. Optically
active poly(1,4-di(2-furyl)-2,5-substituted phenylene)s were then pre-
pared by cholesteric liquid crystal asymmetric electrochemical poly-
merization of the achiral monomers in a CLC electrolyte solution.
The product polymers showed a fingerprint texture similar to that of
the CLC electrolyte solution and exhibited consistent CD. Further-
more, a change in CD intensity of the polymer was observed during
oxidation, where the polymers exhibit intense CD signals in the
reduced state, and weak signals in the oxidized state. The color of
the polymers also drastically changed, from light green to black,
during the redox process. The significant color change arises from
the furan structure in the main chain, which can provide high-
contrast electrochromic properties. These furan-based electro-active
polymers can be applied to chiroptical electrochromic displays.
Poly(1,4-di(2-furyl)phenylene) shows selective reflection of light, and

Preparation of Furan-Based Monomers
15
the periodic convexo-concave surface structure produces the structural
color.
ACKNOWLEDGMENTS
The author is grateful to the Chemical Analysis Center of University
of Tsukuba, for NMR spectra and elemental analysis data, and the
Engineering Workshop of University of Tsukuba, for glasswork.
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