Optical activity of heteroaromatic conjugated polymer films prepared by asymmetric electrochemical polymerization in cholesteric liquid crystals: Structural function for chiral induction

Article abstract of DOI:10.1021/ma400302j

We electrochemically polymerized various achiral heteroaromatic monomers in left-handed helical cholesteric liquid crystal (CLC) media. Circular dichroism (CD) spectroscopy revealed that most of the resulting conjugated polymer films exhibited both the first negative and second positive Cotton effects near their absorption maxima. This indicates left-handed helical aggregation of the conjugated main chains, which is consistent with left-handed helical order of the CLC. This result suggests that the left-handed helical CLC environment induced left-handed helical aggregation of the polymers during the electrodeposition. However, CD intensity of the polymers depends on the structure of the parent monomers. Systematic investigation of the relationship between monomer structures and optical activity of the polymers indicates that linearity of the conjugated main chains and excluded volume interaction between the monomers and the CLC are important factors for producing optical activity of the polymers.

Full text of DOI:10.1021/ma400302j

Article
pubs.acs.org/Macromolecules
Optical Activity of Heteroaromatic Conjugated Polymer Films
Prepared by Asymmetric Electrochemical Polymerization in
Cholesteric Liquid Crystals: Structural Function for Chiral Induction
Kohsuke Kawabata,† Masaki Takeguchi,‡ and Hiromasa Goto†,*
†Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan
‡Microstructure Characterization Platform Promotion Office, National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047,
Japan
S
* Supporting Information
ABSTRACT: We electrochemically polymerized various
achiral heteroaromatic monomers in left-handed helical
cholesteric liquid crystal (CLC) media. Circular dichroism
(CD) spectroscopy revealed that most of the resulting
conjugated polymer films exhibited both the first negative
and second positive Cotton effects near their absorption
maxima. This indicates left-handed helical aggregation of the
conjugated main chains, which is consistent with left-handed
helical order of the CLC. This result suggests that the left-
handed helical CLC environment induced left-handed helical
aggregation of the polymers during the electrodeposition.
However, CD intensity of the polymers depends on the
structure of the parent monomers. Systematic investigation of the relationship between monomer structures and optical activity
of the polymers indicates that linearity of the conjugated main chains and excluded volume interaction between the monomers
and the CLC are important factors for producing optical activity of the polymers.
has been reported that electrochemical polymerization of
achiral monomers in a cholesteric liquid crystal (CLC)
medium, which has helical molecular order, produced optically
active conjugated polymer films.21, The polymers thus prepared
exhibited circular dichroism (CD) at the wavelengths where
absorption due to the π−π* transition was observed, despite
the absence of any chiral structure in the monomer repeating
unit. This indicates that during the electrodeposition the chiral
environment due to the CLC affects formation of 2-D
intramolecular or 3-D intermolecular chiral structures that
induce the optical activity of the resulting polymer. This
method is referred to as ‘‘CLC asymmetric electrochemical
polymerization’’. This is a unique method to prepare optically
active conjugated polymers even from achiral monomers.
Furthermore, thus-prepared conjugated polymer films show
optical textures characteristic of the CLC medium used,
indicating that conjugated polymer chains are mesoscopically
oriented similarly to the periodically helical molecular order in
the CLC. However, understanding details of the polymerization
mechanism and supermolecular structure in the film is still
insufficient and requires systematic studies.
INTRODUCTION
■
Chiral polymers have attracted much interest because chiral
molecular structures provide optical activity, such as circular
dichroism (CD) and optical rotatory dispersion, nonlinear
optical response, and chiral recognition and catalytic proper-
ties.1−6
Conjugated polymers are also of great interest because of
their optical absorption and emission properties over the
ultraviolet−visible and near-infrared regions, which can be
modulated by designing the chemical structure.7,8 For example,
in the neutral state, poly-p-phenylenes, polyfluorenes and
polycarbazoles absorb ultraviolet light and emit blue light,9−11
and polythiophenes absorb and emit visible light,12 while
poly(3,4-ethylenedioxythiophene) (PEDOT), polyisothianaph-
thene, and donor−acceptor-type conjugated polymers exhibit
absorption and emission bands that extend into the near-
infrared region.7,13,14 The absorption and emission properties
of the conjugated polymers can also be modulated by chemical
and electrochemical redox processes. Through the redox
process conjugated polymers deform their conjugated back-
bones between the benzenoid structure in the neutral state into
the quinonoid structure in the ionized state, accompanied by
change in their electronic states.
For the construction of helically aggregated polymer
structures, various chiral structural units have been introduced
both to side chains and main chains.15−20 However, recently, it
Received: February 11, 2013
Revised: February 25, 2013
© XXXX American Chemical Society
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Chart 1. Monomers Used for Asymmetric Electrochemical Polymerization
In this study, systematically designed monomers were used
for the CLC asymmetric electrochemical polymerization to
induce chiroptical activity in various conjugated polymers, in
order to understand the aggregated polymer structure and the
chiral induction mechanism in the CLC asymmetric electro-
chemical polymerization.
electrochemical polymerization of the monomers theoretically
provides α-position-linked conjugate polymers.
CLC ELECTROLYTE SOLUTIONS
■
Generally, CLC phase can be induced from nematic liquid
crystals (NLCs) by addition of a chiral compound, so-called
chiral inducer. However, chiral inducers require good
miscibility with the mother NLC. Otherwise, the addition of
chiral compounds into the LC matrix may break the entire LC
order like adding an impurity. In this study, cholesteryl oleyl
carbonate (COC), a kind of cholesterol derivative, was used as
a chiral inducer as reported in previous studies.24 The CLC
medium was prepared by addition of COC (136.2 mg, 0.20
mmol) as a chiral inducers to 4-cyano-4′-n-hexyl biphenyl
(6CB) (1.32 g, 5.0 mmol). To the CLC medium (150 mg) was
added tetrabutylammonium perchlorate (TBAP) (0.34 mg, 1.0
μmol) and monomers (1−8 μmol) to afford CLC electrolyte
solutions (Table 1).
MONOMERS
■
2,2′-Bithiophene (BT), 2,2′:5′,2″-terthiophene (ter-T), 3,3′-
dimethyl-2,2′-bithiophene (3DMBT), 4,4′-dimethil-2,2′-bithio-
phene (4DMBT), 3,4,3′,4′-tetramethyl-2,2′-bithiphene
(TMBT), 1,3-di(2-thienyl)isothianaphthene (T-ITN), 3,4-
ethylenedioxythiophene (EDOT), 2,2′-bis(3,4-ethylnenediox-
ythiophene) (bis-EDOT), thieno[3,2-b]thiophene (TT), 2,2′-
bithieno[3,2-b]thiophene (bis-TT), 2,7-di(2-thienyl)fluorene
(T-Fl), N-methyl-2,7-di(2-thienyl)carbazole (T-Cz1), 3,7-
di(2-thienyl)dibenzothiophene (T-Dbt), 4,4′-di(2-thienyl)-
biphenyl (T-Bp), 2,7-di(2-furyl)fluorene (F-Fl), N-methyl-
2,7-di(2-furyl)carbazole (F-Cz1), 3,7-di(2-furyl)-
dibenzothiophene (F-Dbt), 4,4′-di(2-furyl)biphenyl (F-Bp),
4,7-di(2-thienyl)-benzo[1,2,5]thiadiazole (T-Btdaz), 4,7-di(2-
furyl)-benzo[1,2,5]thiadiazole (F-Btdaz), 4,7-bis-thieno[3,2-b]-
thiophen-2-yl-benzo[1,2,5]thiadiazole (TT-Btdaz), and N-
methyl-3,6-di(2-thienyl)carbazole (3,6T-Cz1) were each used
as monomers for CLC asymmetric electrochemical polymer-
izations, which are listed in Chart 1. Synthetic routes to the
monomers are described in Scheme 1.
Liquid crystallinity of the CLC electrolyte solutions was
confirmed by polarizing optical microscopy (POM) and
differential scanning calorimetry (DSC). The electrolyte
solutions were confirmed to maintain CLC phase even after
the addition of TBAP and the monomers by POM observation
of fingerprint textures that are characteristic of the CLC phase
(Figure 1). In the POM observation for the CLC electrolyte
solutions no precipitated TBAP or monomers were observed,
indicating good homogeneity of the solution. DSC measure-
ments show the phase transition temperatures of the CLC
medium decreased slightly after addition of TBAP and BT
(Figure S2, Supporting Information). In the temperature range
from 16 to 27 °C, the CLC electrolyte solution exhibit a stable
CLC phase under both cooling and heating. The helical half-
pitch of the CLC medium at 22 °C was measured to be 1.14
μm by the Grandjean−Cano method.25 This helical half-pitch
length is long enough not to demonstrate any optical effects,
such as selective reflection or optical band gap, at the
absorption region of the resulting polymers (300−900 nm).
Under the assumption that the electrochemical polymer-
ization reaction proceeds via the radical coupling reaction as
described by Genies et al.,22 the reactive sites can be
determined by spin-density distribution in the monomers in
the radical cationic state.23 Density functional theory (DFT)
calculation revealed that in the potential reactive sites with
eliminable protons in the monomers, external α-carbons of the
heteroaromatic rings in the monomers have the highest spin
density (Figure S1, Supporting Information). Specifically, the
most reactive sites in the monomers in the radical cationic state
are the external α-positions of the heteroaromatic rings. Thus,
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Scheme 1. Synthesis of Monomers
The helical twisting power of COC is estimated to be 11.4
cell was first heated to form an isotropic phase of the CLC
electrolyte solution, and then cooled and kept at 22 °C to
maintain the CLC phase. In the polymerization cell helical axes
of the CLC electrolyte were parallel to the substrate electrodes
to form fingerprint textures. 4.0 V of direct current voltage was
applied between the ITO electrodes to start electrochemical
polymerization. After the reaction for 10−30 min, the CLC
electrolyte solution was rinsed off with hexane and acetone to
μm−1.
CLC ASYMMETRIC ELECTROCHEMICAL
POLYMERIZATION
■
The CLC electrolyte solutions were injected between two
indium−tin−oxide- (ITO-) coated glass electrodes, separated
by a Teflon spacer of 0.20 mm thickness. The polymerization
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Table 1. Constituents of Cholesteric Liquid Crystal Electrolytes
poly(ter-T) prepared in acetonitrile, indicating that the
polymerization reaction in this study proceeds in a similar
way to the conventional method using common organic
solvents (Figure S5, Supporting Information). Furthermore, the
absence of absorption bands at 2230 and 1740 cm−1 due to
CN stretching (νCN) of 6CB and CO stretching (νCO
)
of COC, respectively, indicates that the CLC solvent was
completely removed after polymerization.
Molecular weight of the polymers was evaluated by matrix-
assisted laser desorption ionization time-of-flight mass spec-
troscopy (MALDI-TOF-MS) using dithranol as a matrix. The
spectra of the polymers show periodic patterns that correspond
to molecular weights of the monomer repeating units (Figures
S6 and S7, Supporting Information). The spectra show low
average molecular weights; however, these spectra can only
detect low molecular weight fractions.
Figure 1. POM image of CLC electrolyte solution containing
bithiophene (BT) monomer at 22 °C under crossed Nicols.
OPTICAL PROPERTIES OF THE POLYMERS
■
CD and UV−vis spectra of polymer films prepared by the CLC
asymmetric electrochemical polymerization are shown in Figure
2. Linear dichroism of the polymers were evaluated to be
negligible intensity against CD observed as shown in Figure
S10, Supporting Information, and removed from the CD
spectra. Poly(BT) and poly(ter-T) synthesized in this study
exhibited optical properties consistent with those of the same
polymers previously synthesized by using similar methods to
this study.24,26 Poly(BT) and poly(ter-T) in this study show
different absorption maxima around 470 and 490 nm,
respectively, despite the same chemical structure of the
resulting polymers (Figure 2a). This different effective
conjugation length is caused by different reactivity of the
monomer in the radical cationic state. It has been previously
reported that in electrochemical polymerization, ter-T is less
reactive than BT due to delocalization of π-electrons over the
entire molecule. The low reactivity of ter-T results in lower
afford polymeric thin films on an anodic surface. The polymer
films were fully reduced by exposure with hydrazine vapor prior
to subsequent measurements.
The polymers prepared by the CLC asymmetric electro-
chemical polymerization were obtained as insoluble and
infusible films on ITO glass electrodes. According to the
DFT calculation results, polymerization reactions proceed via a
radical coupling reaction between the α-positions of the
heteroaromatic rings. In IR spectra of the polymers, absorption
bands due to α-position C−H out-of-plane bending vibration
(δC−H) of the heteroaromatic monomers disappeared after
electrochemical polymerization (Figures S3 and S4, Supporting
Information). This elucidates that the polymers grew via
coupling between the α-position of the monomers. IR spectra
of poly(ter-T) show a similar shape to the spectrum of
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Figure 2. CD (top) and UV−vis (bottom) spectra of the polymer in the neutral state.
molecular weight of polythiophene than that prepared from
BT.27 The difference in the molecular weights of the polymers
is also indicated by the difference in their IR spectra. The IR
spectra of poly(ter-T) showed more intense absorption (at 693
cm−1) due to out-of-plane C−H vibration (δC−H) at the α-
position of thiophene rings than did poly(BT), which indicates
that poly(ter-T) has more unreacted terminal α-positions than
poly(BT) (Figure S3a,b, Supporting Information). As a result,
poly(ter-T) shows several absorption shoulders due to the
well-defined low molecular weight polymers, while poly(BT)
shows the absorption maximum and onset at longer wave-
lengths than poly(ter-T). In CD spectra, poly(ter-T) and
poly(BT) show bisignate Cotton effect near their absorption
maximum wavelengths. These spectra are similar to those of
previously reported helically aggregated polythiophenes.28,29
This type of bisignate Cotton effect represents the chiral
exciton coupling via Davydov splitting,30 indicating chiral
aggregation of polythiophene backbones. Poly(ter-T) and
poly(BT) show first negative and second positive Cotton
effects, indicating that polythiophene backbones form left-
handed helical aggregation.30 This left-handed helical aggrega-
tion is consistent with the left-handed helical order of the CLC
medium consisting of 6CB and COC. On the contrary, poly(T-
ITN) shows first positive and second negative Cotton effect at
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around 550 nm of absorption maximum wavelength, indicating
right-handed helical aggregation of the conjugated backbone.
Poly(3DMBT) and poly(4DMBT) show different absorp-
tion maxima at around 430 and 410 nm, respectively, despite
the same chemical structure of head-to-head and tail-to-tail
poly(methylthiophene) backbone (Figure 2b). The difference
can be explained by two possible structural defects in the
poly(3DMBT) backbones. One is α−β linkage defects due to
potential reactivity at uncapped external β-positions of
3DMBT,31 while, in electrochemical polymerization of
4DMBT, methyl substituents at external β-positions prevent
formation of α−β linkage defects. The other possible structural
defect in poly(3DMBT) is syn-defects which can be caused
both in the monomer units and between the monomer units.
The syn-defects in the monomer units are spontaneously
formed because syn-conformation is energetically favorable to
anticonformation for 3DMBT in the neutral state, while
anticonformation is favorable for 4DMBT in the neutral state,
according to inter-ring rotation energy profiles calculated by the
DFT method (Figure 3). In the radical cationic states,
conjugated main chain. These three spectra are not zero-
crossing at absorption maxima, while unsubstituted poly(BT)
and poly(ter-T) show zero-crossing points at absorption
maxima in the CD spectra. This indicates that in the CD
spectra of poly(3DMBT), poly(4DMBT), and poly(TMBT), a
monosignate negative Cotton effect due to one-handed twisted
conjugated single chains is superimposed on the bisignate
Cotton effect due to helical aggregation between the
conjugated chains. The one-handed twisted conformation in
the methyl-substituted polythiophenes is caused by steric
hindered methyl groups, while unsubstituted polythiophene
have coplanar conjugated structures, which is supported by the
DFT calculation (Figure S8, Supporting Information).
Poly(TT) and poly(bis-TT) show almost identical absorp-
tion spectra, and yield first negative and second positive Cotton
effect at around their absorption maximum wavelengths.
However, the CD intensity of poly(TT) is smaller than that
of poly(bis-TT) (Figure 2c). To the contrary, poly(bis-
EDOT) and PEDOT show significant difference in CD spectra
despite the same chemical structure and UV−vis and IR
absorption spectra (Figure 2c and Gigure S3g,h, Supporting
Information). In the CD spectra, poly(bis-EDOT) shows
distinct first negative and second positive Cotton effect at
around its absorption maximum (550 nm), while PEDOT
shows no detectable Cotton effect. This significant difference in
optical activity, in spite of the same UV−vis and IR absorption,
suggests that the choice of parent monomers is quite important
for the construction of chiral polymer structure during CLC
asymmetric electrochemical polymerization.
Figure 2d shows CD and UV−vis absorption spectra of three
donor−acceptor (D-A)-type conjugated polymers, poly(T-
Btdaz), poly(F-Btdaz), and poly(TT-Btdaz). Poly(T-Btdaz)
and poly(F-Btdaz) show intermolecular charge transfer
absorption bands at around 560 nm, which are characteristic
of D−A conjugated polymers. At short wavelengths (300 nm
∼400 nm), poly(T-Btdaz) and poly(F-Btdaz) show one and
two absorption bands, respectively. Poly(TT-Btdaz) shows a
charge-transfer-type band at a little longer wavelength than
poly(T-Btdaz) and poly(F-Btdaz) (580 nm), indicating that
the bis(thienothiophene) unit functions as a stronger donor
than bithiophene or bifuran. In the CD spectra, three D−A
polymers show two or three bisignate Cotton effects near
absorption maxima. The bisignate Cotton effects at long
wavelengths are more intense than those at short wavelengths,
although the absorption intensities at both long and short
wavelengths are at the same level. Among the D−A polymers,
poly(TT-Btdaz) shows much stronger CD than poly(T-Btdaz)
and poly(F-Btdaz).
Parts e and f of Figure 2 show CD and UV−vis spectra of the
polymers prepared from thiophene- and furan-disubstituted
four-ring monomers, respectively. These polymers show similar
absorption bands near 300 nm -500 nm because of the similar
chemical structure. However the biphenyl-based polymers,
poly(T-Bp) and poly(F-Bp), show maximum absorption bands
at shorter wavelengths than the others. This is because the
unbridged biphenyl is less planar and has less effective
conjugation than bridged biphenyl systems such as fluorene,
carbazole, and dibenzothiophene. Additionally, poly(T-Cz1)
and poly(F-Cz1) have slightly longer absorption maximum
wavelengths than the other biphenyl systems.
Figure 3. Energy profiles for inter-ring rotation of 4DMBT in the
neutral (open circle) and radical cationic (solid circle) state and
3DMBT in the neutral (open square) and radical cationic (solid
square) state calculated by DFT method (B3LYP-6-31G*). Syn-
conformation: from 0° to 90°. Anticonformation: from 90° to 180°.
anticonformation is energetically favorable to syn-conformation
for both 3DMBT and 4DMBT; however, there is a high energy
barrier between syn- and anticonformation, more than 10 kcal
mol−1. Therefore, according to the Franck−Condon principle,
once syn-3DMBT in the neutral state is electrochemically
oxidized, 3DMBT rarely reforms its syn-conformation into
anticonformation. Another kind of syn-defect can be formed
between the monomer units during the radical coupling
reaction, which is more easily formed in 3DMBT than
4DMBT because of the steric hindered methyl substituents at
the external β-positions of 4DMBT. Thus, poly(3DMBT) has
more structural defects in the conjugated backbone than
poly(4DMBT), resulting in shortening its effective conjugation
length. Poly(TMBT) shows an absorption maximum at 346
nm, which is shorter than that of poly(3DMBT) and
poly(4DMBT). This is because poly(TMBT) has two steric
hindered methyl groups per one thiophene unit to increase
dihedral angles between thiophenes.
In the CD spectra, poly(3DMBT), poly(4DMBT), and
poly(TMBT) exhibit bisiganate Cotton effects with different
intensities, which have first negative and second positive
Cotton effects. The relatively small CD intensity of poly-
(3DMBT) is consistent with structural defects in the
In the CD spectra, these polymers, except for poly(T-Cz1)
and poly(3,6T-Cz1), show first negative and second positive
Cotton effects around their absorption bands. On the contrary,
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poly(T-Cz1) shows intense first positive and second negative
Cotton effect, while poly(3,6T-Cz1) shows no clear Cotton
effect.
interaction between the chiral environment and the dissolved
monomers and growing oligomers. One of the most important
anisotropic interactions between the CLC and the solvated
monomers and oligomers can be excluded volume interaction.
Generally, a large length/diameter (L/D) ratio for a molecule
dissolved in LC solvent causes strong excluded volume
interaction, while a low L/D ratio results in little anisotropic
interaction. The small CD in poly(TT), poly(T-Btdaz),
poly(F-Btdaz), and poly(3,6T-Cz1), and no CD in PEDOT,
can be explained by the small L/D ratios of the parent
monomers resulting in little anisotropic interaction during
polymerization.
Another important factor for enhancing CD due to the chiral
exciton coupling in the helical aggregation can be the angle of
the optical transition dipole moment relative to the polymer
chain axis director. For example, zigzag- shaped conjugated
polymers are unable to demonstrate one-handed chiral exciton
coupling at each local point even if the polymer chains form the
helical aggregation as a whole (Figure 6). This is because the
DISCUSSION
■
Most of the polymers in this study showed the first negative
and the second positive Cotton effects (negative couplet) in
their CD spectra around their maximum absorption wave-
lengths. These Cotton effects are characteristic of Davydov
splitting-type CD by chiral exciton coupling between left-
handed helically ordered chromophores. In the conjugated
polymers, conjugated backbones function as chromophores,
whose transition moments are along the conjugated backbones.
Therefore, the negative couplets in the CD indicate that the
conjugated polymers form left-handed helical order of their
rigid conjugated main chains in their aggregated state, as do the
CLC molecules (Figure 4).30
Figure 4. Illustration of the left-handed helical LC-order (a) and
aggregated polymer structure (b) in this study.
Figure 6. Illustration of the left-handed helically aggregated structure
of the zigzag-shaped polymers (a and b). The red and blue chains are
next to each other (b).
This helical order of the polymers is supported by the
periodic birefringence structure observed in POM images of the
polymer films as shown in Figure 5. The POM images for
zigzag- conjugated polymer chains are unable to form one-
handed helical conformation at each local point, resulting in
fluctuation of the angle between their optical transition dipole
moments, as shown in Figure 6b. Such fluctuation of the angle
between the chromophore may decrease the total CD intensity.
Thus, the small CD intensities of poly(T-Btdaz), poly(F-
Btdaz), and poly(3,6T-Cz1) compared to poly(TT-Btdaz)
and poly(T-Cz1) can be due to their zigzag-conjugated chains
as well as the small L/D ratio of the monomers (Figure 7).
To the contrary, poly(T-ITN) and poly(T-Cz1) show the
first positive and second negative Cotton effect (positive
couplet) in the CD spectra, indicating right-handed helical
aggregation. This result is inconsistent with the left-handed
helical sense of the CLC solvent. However, elongation of the
alkyl chain attached to the nitrogen atom of poly(T-Cz1)
results in the opposite sign of CD of the polymers (poly(T-
Czm) m = 2−8) as shown in Figure 8a. A significant spectral
difference is observable between poly(T-Cz1) and poly(T-
Cz2) despite only one-carbon difference between the
monomers. Interestingly, the CD intensity and absorption
bandwidths of the poly(T-Czm) decrease with the increase of
the number of carbon atoms in the alkyl chains. A series of
furan derivatives poly(F-Czm) (m = 1−6) also show such
spectral variation; however, they show only negative couplets
(Figure 8b). This spectral variation can be explained by two
mechanisms. One is that sterically hindered side alkyl chains
suppress the interchain exciton coupling. The other is that
lowering the L/D ratio of the monomers due to laterally
attached alkyl chains decreases the anisotropic interaction
Figure 5. POM images under crossed Nicols of poly(4DMBT) (a)
and poly(bis-EDOT) (b) films prepared by CLC asymmetric
electrochemical polymerization.
polymer films under crossed Nicol show fingerprint texture,
which is similar to that of the CLC solvent used. It is well-
known that CLCs form periodic helical molecular order that
shows fingerprint textures, in which the width of each stripe
corresponds to the distance required for the LC director to
rotate 180°. The distance is the “helical half-pitch”. Therefore,
the fingerprint texture of the polymer films similar to that of the
CLC strongly suggests periodic helical orientation of the
polymer chains. Besides, the helical half-pitch in the fingerprint
texture of the polymer films is in good agreement with that of
the CLC solvent.
For the construction of the helical aggregation during the
polymerization reaction, the chiral environment induced the
polymers to form chiral 3-D structures through anisotropic
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Figure 7. Optimized geometry of the model oligomers for poly(T-Btdaz) (a), poly(F-Btdaz) (b), poly(TT-Btdaz) (c), poly(3,6T-Cz1) (d), and
poly(T-Cz1) (e) calculated by DFT method (B3LYP-6-31G*).
To compare the aggregated structures of poly(T-Czm),
surface morphology of the polymers were observed by SEM
(Figure 9). In the SEM images, poly(T-Cz1) forms nanoplates
Figure 9. SEM images of poly(T-Cz1), poly(T-Cz2), poly(T-Cz3),
and poly(T-Cz4).
which align along the fingerprint texture. This structure
suggests two kinds of anisotropic growth of the poly(T-Cz1)
in the CLC. First, the fingerprint textures of the polymers
indicate the anisotropic growth of the polymers along the
mesoscopically periodic helical order of the CLC during the
polymerization. Second, the formation of nanoplates indicates
strong anisotropic interaction between the polymers and
oligomers through the deposition process. The nanoplates are
one-handed helically stacked to form the fingerprint texture.
Among the series of poly(T-Czm), such nanoplate formation
was observed only in poly(T-Cz1). The positive couplet CD
observed in poly(T-Cz1) can be derived from the right-handed
helically distorted nanoplates; whereas poly(T-Cz2), poly(T-
Figure 8. UV−vis (bottom) and CD (top) spectra of poly(T-Czm)
(a) and poly(F-Czm) (b) in the reduced state.
between the monomers and the CLC during the polymer-
ization reaction, resulting in disorder in the helical aggregation
polymer structure.
Cz3), and poly(T-Cz4) show globular surfaces forming
convex-concave structures along the fingerprint texture. Addi-
tionally, the convex-concave fingerprint texture disappears with
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the increase in the side alkyl chain length. Poly(T-Czm) with
alkyl chains longer than pentyl group show globular surface and
no fingerprint texture. The disappearance of the convex-
concave structure in the polymers with long alkyl chains
indicates that the lateral alkyl chain decreases anisotropic
interaction between the chiral environment and the monomers
and the growing oligomers due to the low L/D ratio of the
monomers.
COC/6CB were confirmed to exhibit continuous CLC phase
by POM observation (Figure S9, Supporting Information).
Therefore, CP/6CB and (S)-8BpB6/6CB CLC systems have
left-handed helical molecular order.
In both these CLC solvents, as well as in the COC/6CB
solvent, BT was electrochemically polymerized to form
polymeric films. Poly(BT) films prepared in CP/6CB and
(S)-8BpB6/6CB CLCs exhibit almost the same UV−vis
absorption and CD spectra (Figure 10). COC and CP have
USE OF DIFFERENT CHIRAL INDUCERS
■
Cholesterol derivatives are often used as chiral inducers because
their steroid skeletons function as a good mesogen with many
asymmetric carbons. Likewise, synthetic chiral inducers require
mesogens and some asymmetric carbons. As well as COC,
cholesteryl pelargonate (CP) and 4′-(4-hexyloxy-benzoyloxy)-
biphenyl-4-carboxylic acid-(S)-1-methyl-heptyl ester ((S)-
8BpB6) in Chart 2 also function as chiral inducers in 6CB;
Chart 2. Chiral Inducers
Figure 10. UV−vis (bottom) and CD (top) spectra of poly(BT)
prepared in the CLCs with different chiral inducers, cholesteryl oleyl
carbonate (red), cholesteryl pelargonate (green), and (S)-8BpB6
(blue) in the reduced state.
the same chiral steroid skeleton, while (S)-8BpB6 has one
asymmetric carbon next to an ester group. The similar CD
spectra of the three poly(BT)s strongly suggest that the
helically aggregated polymer structures were induced, not by
chiral structural units of the chiral inducers, but by the helical
molecular order of CLC phase.
however, they have different helical twisting powers (βM = 10.5
μm−1, 19.5 μm−1, respectively). (S)-8BpB6 was synthesized via
Mitsunobu reaction to introduce an symmetric carbon (Scheme
2). By using CP and (S)-8BpB6 as chiral inducers two CLC
electrolyte solutions were prepared as follows, to have the same
helical half-pitch as the CLC containing COC in this study (ρ =
1.14 μm): to 6CB (132 mg, 500 μmol) was added CP (11.5
mg, 21.8 μmol) or (S)-8BpB6 (6.1 mg, 11.5 μmol), TBAP
(0.34 mg, 1.0 μmol), and BT (0.67 mg, 4.0 μmol) to afford
CLC electrolyte solutions.
These two LC electrolyte solutions (CP/6CB and (S)-
8BpB6/6CB) were confirmed to form a CLC phase at room
temperature by POM observation. The helical senses of the two
CLC electrolytes were determined by the contact method with
COC/6CB as a left-handed helical standard CLC. The
boundary area between the two new CLC electrolytes and
CONCLUSIONS
■
A series of aromatic conjugated monomers were electro-
polymerized in a left-handed helical CLC medium consisting of
6CB and COC. Most of the resulting polymer films show
Davydov splitting-type negative couplet near their absorption
maximum wavelength in their CD spectra, which indicates left-
handed helical aggregation of conjugated main chains.
However, the CD intensities of the resulting polymers strongly
depend on the parent monomer structures. The electro-
chemical polymerization of systematically designed monomers
provided some guidelines for obtaining high optical activity of
the polymers: (1) excluded volume interactions between
Scheme 2. Synthesis of the Chiral Inducer
I
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2,5-Dibromo-3-methylthiophene (2). To a solution of 3-
methylthiophene (9.8 g, 100 mmol) in acetic acid (50 mL) and
chloroform (50 mL) was added N-bromosuccinimide (37.4 g, 210
mmol) slowly at room temperature. The mixture was refluxed at 80 °C
for 6 h. After the reaction, the mixture was extracted with chloroform
and NaOH(aq) and washed with water, and the organic layer was
dried over magnesium sulfate. The crude product was purified by
column chromatography (eluent: hexane) to afford a colorless liquid
(22.9 g, 90 mmol, yield = 90%). 1H NMR (400 MHz; CDCl3; TMS):
δ 2.15 (s, 3H), 6.76 (s, 1H). 13C NMR (100 MHz; CDCl3; TMS): δ
15.13, 108.36, 110.12, 131.86, 138.05.
monomers and LC molecules are important; (2) a linear
conjugated backbone of the resulting polymers is preferable;
(3) a long alkyl chain can prevent chiral exciton coupling
between the conjugated main chains. Furthermore, obtaining
the same optical properties using different chiral inducers with
the same helical half-pitch indicates that the driving force of the
helical induction to the polymers is not the chiral structural
units of the chiral inducers, but rather the helical molecular
order of the CLC continuum.
3,3′,5,5′-Tetrabromo-4,4′-dimethyl-2,2′-bithiophene (3). To
a solution of lithium diisopropylamide (freshly prepared from
diisopropylamine (14.3 mL, 102 mmol) and 1.6 M n-butyllithium in
hexane (58.4 mL, 94 mmol) in tetrahydrofuran (180 mL)) was added
2,5-dibromo-3-methylthiophene (2) (21.8 g, 85 mmol) dropwise over
10 min at −78 °C. After stirring for 2 h at −78 °C, CuCl2 (8.07 g, 60
mmol) was added to the mixture in one portion, and the mixture was
gradually warmed up to room temperature. After 6 h, the reaction was
quenched with 1 M HCl(aq) (100 mL), and the mixture was extracted
with chloroform, and the organic layer was dried over magnesium
sulfate. The crude product was purified by recrystallization from
chloroform to afford a white solid (18.4 g, 36 mmol, yield = 85%). 1H
NMR (400 MHz; CDCl3; TMS): δ 2.26 (s, 6H). 13C NMR (100
MHz; CDCl3; TMS): δ 16.29, 111.08, 114.92, 128.27, 137.27.
3,3′-Dibromo-4,4′-dimethyl-2,2′-bithiophene (4). A solution
solution of 3,3′,5,5′-Tetrabromo-4,4′-dimethyl-2,2′-bithiophene (3)
(16.3 g, 32 mmol) in ethanol (100 mL), 0.3 M HCl(aq) (20 mL), and
acetic acid (20 mL) was refluxed at 110 °C, and zinc dust (5.9 g, 90
mmol) was added portionwise to the mixture. After 3 h, the zinc dust
was removed by hot filtration, and the filtrate was cooled in a fridge.
The precipitate was recovered by filtration and purified by
recrystallization from ethanol to afford colorless crystal (10.1 g, 28
MATERIALS
■
2,2′-Bithiophene, 2,2′:5′,2″-terthiophene, tetrabutylammonium per-
chlorate, cholesteryl oleyl carbonate, and cholesteryl pelargonate were
purchased from Tokyo Chemical Industry (TCI). 3,4-Ethylenediox-
ythiophene was purchased from TCI and distilled prior to use. 4-
Cyano-4′-n-hexyl biphenyl was purchased from Merck. 2,2′-Bithieno-
[3,2-b]thiophene was purchased from Aldrich.
SYNTHESIS OF MONOMERS
■
T-Czm, F-Czm, F-Bp, and F-Fl monomers were previously
synthesized by our group.32−35
3,3′-Dibromo-2,2′-bithiophene (1). To a solution of 3-
bromothiophene (9.78 g, 60 mmol) in tetrahydrofuran (100 mL)
was added lithium diisopropylamide (freshly prepared from
diisopropylamine (8.4 mL, 60 mmol) and 1.6 M n-butyllithium in
hexane (38 mL, 61 mmol) in tetrahydrofuran (20 mL)) dropwise over
15 min at −78 °C. After stirring for 1 h at −78 °C, CuCl2 (8.07 g, 60
mmol) was added to the mixture in one portion, and the mixture was
gradually warmed up to room temperature. After 6 h, the reaction was
quenched with 1 M HCl(aq) (100 mL), and the mixture was extracted
with dichloromethane, and the organic layer was dried over
magnesium sulfate. The crude product was purified by column
chromatography (eluent: hexane), and recrystallized from hexane/
ethanol) to afford a pale yellow solid (6.36 g, 19.6 mmol, yield = 65%).
1H NMR (400 MHz; CDCl3; TMS): δ 7.08 (d, 2H, J = 5.4 Hz), 7.41
(d, 2H, J = 5.4 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ 112.65,
127.53, 128.89. 130.81.
3,3′-Dimethyl-2,2′-bithiophene (3DMBT). To a solution of
3,3′-dibromo-2,2′-bithiophene (1) (1.30 g, 4.0 mmol) and dichloro-
(1,3-bis(diphenylphosphino)propane)nickel (0.054 g, 0.10 mmol) in
ether (20 mL) was added methylmagnesium iodide (freshly prepared
from iodomethane (2.27 g, 16.0 mmol) and magnesium (0.41 g, 17.0
mmol) in ether (10 mL)) dropwise at 0 °C, and the reaction mixture
was gradually warmed up to room temperature. After 6 h, the reaction
was quenched with methanol (3 mL) carefully. The mixture was
extracted with ether and water, and the organic layer was dried over
magnesium sulfate. The crude product was purified by silica gel
column chromatography (eluent: hexane) to afford colorless liquid
(0.70 g, 3.6 mmol, yield = 90%). 1H NMR (400 MHz; CDCl3; TMS):
δ 2.17 (s, 6H), 6.92 (d, 2 H, J = 4.8 Hz), 7.26 (d, 2H, J = 4.8 Hz). 13C
NMR (100 MHz; CDCl3; TMS): δ 14.63, 124.94, 129.37, 129.98,
136.48.
4,4′-Dimethyl-2,2′-bithiophene (4DMBT). To a solution of 3-
methylthiophene (3.93 g, 40 mmol) in dried THF (100 mL) was
added n-butyllithium (1.6 M in hexane) (26.3 mL, 42 mmol) dropwise
over 10 min at −78 °C. After stirring at −78 °C for 1 h, CuCl2 (5.65 g,
42 mmol) was added in one portion, and the mixture was gradually
warmed up to room temperature. After stirring overnight, aqueous
HCl solution (2M, 80 mL) was added. The mixture was extracted with
ether and the organic layer was dried over magnesium sulfate. The
mixture was purified by silica gel column chromatography (eluent:
hexane) to afford a white solid (2.87 g, 69% purity by 1H NMR) which
contained a head-to-tail compound. Further purification by recrystal-
lization from methanol gave a colorless crystal (1.26 g, 6.5 mmol, yield
= 33%). 1H NMR (400 MHz; CDCl3; TMS): δ 2.24 (d, 6H, −CH3, J
= 1.0 Hz), 6.76 (s, 2H, 3,3′-H(thiophene)), 6.95 (d, 2H, 5,5′-
H(thiophene), J = 1.0 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ
15.73, 119.44, 125.77, 137.35, 138.33.
1
mmol, yield = 88%). H NMR (400 MHz; CDCl3; TMS): δ 2.28 (d,
6H, J = 1.1 Hz), 7.13 (quartet, 2H, J = 1.1 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 16.70, 115.85, 122.28, 129.39, 137.63.
3,3′,4,4′-Tetramethyl-2,2′-bithiophene (TMBT). To a solution
of 3,3′-dibromo-4,4′-dimethyl-2,2′-bithiophene (4) (1.41 g, 4.0 mmol)
and dichloro(1,3-bis(diphenylphosphino)propane)nickel (0.054 g,
0.10 mmol) in ether (18 mL) was added methylmagnesium iodide
(freshly prepared from iodomethane (2.27 g, 16.0 mmol) and
magnesium (0.41 g, 17.0 mmol) in ether (10 mL)) dropwise at 0
°C, and the reaction mixture was gradually warmed up to room
temperature. After 6 h, the reaction was quenched with methanol (3
mL) carefully. The mixture was extracted with ether and water, and the
organic layer was dried over magnesium sulfate. The crude product
was purified by silica gel column chromatography (eluent: hexane) to
1
afford a white solid (0.83 g, 3.7 mmol, yield = 93%). H NMR (400
MHz; CDCl3; TMS): δ 2.03 (s, 6H), 2.19 (d, 6H, J = 0.8 Hz), 6.94 (d,
2H, J = 0.8 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ 13.37, 15.40,
120.57, 130.18, 136.14, 137.84.
Benzene-1,2-dicarbothioic acid di-S-pyridin-2-yl Ester (5).
To a solution of pyridine-2-thiol (6.67 g, 60 mmol) and triethylamine
(10 mL, 72 mmol) in tetrahydrofuran (100 mL) was added a solution
of phthaloyl dichloride (6.09 g, 30 mmol) in tetrahydrofuran (90 mL)
all at once with vigorous stirring at 0 °C, and then the reaction was
immediately quenched with 1 wt % HCl(aq) (400 mL). The mixture
was extracted with chloroform, and washed with 10 wt % NaOH(aq),
and the organic layer was dried over magnesium sulfate. The crude
product was purified by recrystallization from chloroform/ethyl acetate
1
to afford pale yellow solid (6.79 g, 19 mmol, yield = 64%). H NMR
(400 MHz; CDCl3; TMS): δ 7.31 (ddd, 2H, J = 2.0, 5.2, 7.0 Hz), 7.65
(dd, 2H, J = 3.2, 5.8 Hz), 7.74−7.80 (m, 4H), 7.89 (dd, 2H, J = 3.2,
5.8 Hz), 8.64 (ddd, 2H, J = 0.8, 2.0, 4.6 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 123.74, 128.62, 130.45, 132.03, 136.82, 137.31,
150.41, 151.35, 190.23.
1,2-Di(2-thienoyl)benzene (6). To a solution of benzene-1,2-
dicarbothioic acid di-S-pyridin-2-yl ester (5) (3.52 g, 10 mmol) in dry
THF was added 2-thienyl magnesium bromide (20 mmol) in THF
solution dropwise at 0 °C over 10 min. After stirring at 0 °C for 30
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min, the reaction was quenched with 10% HCl aqueous solution (100
mL). The mixture was extracted ether and washed with 10% NaOH
aqueous solution and water. The organic layer was dried over
magnesium sulfate. The solvent was removed under vacuum to afford a
tetrahydrofuran (200 mL) and water (200 mL) was refluxed at 100
°C for 4 h. After the reaction, the solvent was evaporated, and
concentrated HCl was added to the residue. The precipitate was
extracted with chloroform and water. The organic layer was dried over
magnesium sulfate. Removing the solvent under reduced pressure gave
a white solid (18.03 g, 97.8 mmol, yield = 98%). 1H NMR (400 MHz;
CDCl3; TMS): δ 7.32 (d, 1H, J = 5.0 Hz), 7.65 (d, 1H, J = 5.0 Hz),
8.10 (s, 1H). 13C NMR (100 MHz; CDCl3; TMS): δ 119.83, 127.33,
132.66, 133.67, 138.93, 145.19, 167.35.
Thieno[3,2-b]thiophene (TT). A solution of thieno[3,2-b]-
thiophene-2-carboxylic acid (9) (9.21 g, 50.0 mmol) and copper
(2.00 g,) in quinoline (80 mL) was refluxed at 260 °C to generate
carbon dioxide gas. After 30 min of reaction, the solution was cooled
to room temperature and extracted with ether and washed thoroughly
with 1 M HCl(aq). The organic layer was dried over magnesium
sulfate. The crude product was purified by silica gel column
chromatography (eluent: hexane) to afford a white solid (5.75 g,
41.0 mmol, yield = 82%). 1H NMR (400 MHz; CDCl3; TMS): δ 7.26
(d, 1H, J = 5.0 Hz), 7.38 (d, 1H, J = 5.0 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 119.34, 127.32, 139.41.
1
orange solid (2.92 g, 9.8 mmol, yield = 98%). H NMR (400 MHz;
CDCl3; TMS): δ 7.07 (dd, 2H, 4,4′-H(thiophene), J = 3.6, 4.2 Hz),
7.47 (dd, 2H, 3,3′-H(thiophene), J = 1.1, 4.2, Hz), 7.64 (dd, 2H, 4,5-
H(benzene), J = 3.2, 5.8 Hz), 7.66 (dd, 2H, 5,5′-H(thiophene), J = 1.1,
5.0 Hz), 7.74 (dd, 2H, 3,6-H(benzene), J = 3.2, 5.8 Hz). 13C NMR
(100 MHz; CDCl3; TMS): δ 121.98, 129.18, 130.58, 134.92, 135.09,
139.31, 144.03, 188.25.
1,3-Di(2-thienyl)isothianaphthene (T-ITN). A solution of 1,2-
di(2-thienoyl)benzene (6) (0.60 g, 2.0 mmol) and Lawesson’s reagent
(1.09 g, 2.7 mmol) in toluene (70 mL) was refluxed at 100 °C for 2 h.
After the reaction, solvent was removed and the crude product was
purified by column chromatography (eluent: hexane/chloroform = 7/
3) followed by recrystallization from hexane/ethanol to afford orange
1
solid (0.45 g, 1.5 mmol, yield = 75%). H NMR (400 MHz; CDCl3;
TMS): δ 7.13−7.16 (m, 4H, 5,6-H(isothianaphthene), 4,4′-H(thio-
phene)), 7.35 (dd, 2H, 3,3′-H(thiophene), J = 1.6, 3.6 Hz), 7.37 (dd,
2H, 5,5′-H(thiophene), J = 1.6, 5.2 Hz), 7.94 (dd, 2H, 4,7-
H(isothianaphthene), J = 3.2, 6.8 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 121.51, 124.78, 125.54, 125.59, 126.49, 127.88,
135.26, 135.59.
5-Tributylstannylthieno[3,2-b]thiophene (10). To a solution
of thieno[3,2-b]thiophene (TT) (1.40 g, 10.0 mmol) in tetrahy-
drofuran (50 mL) was added 1.6 M n-butyllithium (6.25 mL, 10.0
mmol) dropwise at −78 °C. After stirring at −78 °C for 1 h,
tributylstannyl chloride (3.58 g, 11.0 mmol) was added to the mixture
at −78 °C, and the mixture was gradually warmed up to room
temperature. After stirring for 4 h, the reaction was quenched with
water (50 mL). The mixture was extracted with ether, and the organic
layer was dried over magnesium sulfate. Removing the solvent under
2,2′-Bis(3,4-ethylnenedioxythiophene) (bis-EDOT). To a
solution of 3,4-ethylenedioxythiophene (2.84 g, 20 mmol) in dried
THF (70 mL) was added n-butyllithium (1.6 M in hexane) (12.5 mL,
20 mmol) dropwise over 5 min at −78 °C. After stirring at −78 °C for
1 h, CuCl2 (2.69 g, 20 mmol) was added in one portion, and the
mixture was gradually warmed up to room temperature over 2 h.
Water (30 mL) and triethylamine (10 mL) was added. The mixture
was extracted with chloroform and the organic layer was dried over
magnesium sulfate. The crude compound was passed through a short
plug of silica (neutralized with triethylamine) by using chloroform as
an eluent. The chloroform solution (300 mL) was poured into hot
hexane (700 mL) and cooled in 0 °C. The precipitate was collected by
filtration to afford a light green yellow solid (1.53 g, 5.43 mmol, yield =
1
reduced pressure gave a tan oil (4.08 g, 9.5 mmol, yield = 95%). H
NMR (400 MHz; CDCl3; TMS): δ 0.90 (t, 9H, J = 7.2 Hz), 1.12−1.16
(m, 6H), 1.31−1.40 (m, 6H), 1.55−1.63 (m, 6H), 7.23 (d, 1H, J = 5.6
Hz), 7.26 (s, 1H), 7.34 (d, 1H, J = 5.6 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 10.91, 13.67, 27.28, 28.95, 118.99, 126.53, 126.98,
140.64, 141.57, 145.26.
4,7-Dibromobenzo[1,2,5]thiadiazole (11). To a solution of
benzo[1,2,5]thiadiazole (6.81 g, 50 mmol) in 47% hydrobromic
acid(aq) (100 mL) was added a solution of bromine (25 g, 156.4
mmol) in 47% hydrobromic acid (aqueous) (75 mL) through a
dropping funnel over 1 h at room temperature, then the mixture was
refluxed at 100 °C. After 6 h, the mixture was cooled to room
temperature, and sodium thiosulfate aqueous solution was added to
the mixture. The precipitate was recover by filtration, and purified by
silica gel column chromatography (eluent: dichloromethane) to afford
1
54%). H NMR (400 MHz; CDCl3; TMS): δ 4.23−4.34 (m, 8H,
−O−C2H4−O−), 6.27 (s, 2H, H(thiophene). 13C NMR (100 MHz;
CDCl3; TMS): δ 141.20, 136.99, 109.88, 97.51, 64.99, 64.59.
3-Bromothiophene-2-carbaldehyde (7). 3-Bromothiophene
(20.0 g, 122.7 mmol) was added dropwise to a solution of lithium
diisopropylamide (freshly prepared from diisopropylamine (17.2 mL,
122.7 mmol) and 1.6 M n-butyllithium (76.7 mL, 122.7 mmol) in
tetrahydrofuran (210 mL)) at 0 °C. After stirring the mixture for 1 h,
dimethylformamide (9.5 mL, 122.7 mmol) was added to the mixture
at 0 °C and the mixture was warmed up to room temperature. After
stirring fo r 4 h, water (100 mL) was added to the mixture and
extracted with ether three times. The organic layer was dried over
magnesium sulfate. Evaporating in vacuo to afford a pale tan liquid
1
a pale yellow solid (12.90 g, 43.9 mmol, yield = 88%). H NMR (400
MHz; CDCl3; TMS): δ 7.73 (s, 1H). 13C NMR (100 MHz; CDCl3;
TMS): δ 113.90, 132.35, 152.94.
4,7-Di(2-furyl)benzo[1,2,5]thiadiazole (F-Btdaz). A solution of
2-tributylstannylfuran (0.75 g, 2.1 mmol), 4,7-dibromo-benzo[1,2,5]-
thiadiazole (11) (0.29 g, 1.0 mmol), and Pd(PPh3)4 (0.023 g, 0.020
mmol) in toluene (4 mL) was refluxed at 90 °C for 24 h. After cooling
to room temperature, the mixture was purified silica gel column
chromatography (contain 10% v/v of potassium carbonate) (eluent:
chloroform/hexane = 1/1) and recrystallized from hexane to afford a
1
(21.7 g, 113.8 mmol, yield = 93%). H NMR (400 MHz; CDCl3;
TMS): δ 7.16 (d, 1H, J = 4.8 Hz), 7.72 (dd, 1H, J = 1.2, 5.2 Hz), 9.99
(d, 1H, J = 1.2 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ 120.37,
132.04, 134.84, 136.91, 183.01.
Ethyl Thieno[3,2-b]thiophene-2-carboxylate (8). 3-Bromo-
thiophene-2-carbaldehyde (7) (21.0 g, 110 mmol) was added to a
mixture of potassium carbonate (20.7 g, 150 mmol) and 2-sulfanyl
acetate (13.8 g, 115 mmol) in dimethylformamide (200 mL) at room
temperature. After stirring for 3 days at room temperature, the mixture
was poured into water (600 mL) and extracted with dichloromethane.
The organic layer was dried over magnesium sulfate. The crude
product was purified by silica gel column chromatography (eluent:
dichloromethane) to afford a yellow liquid (21.3 g, 100 mmol, yield =
91%). 1H NMR (400 MHz; CDCl3; TMS): δ 1.40 (t, 3H, J = 7.0 Hz),
4.39 (quartet, 2H, J = 7.0 Hz), 7.28 (d, 1H, J = 5.8 Hz), 7.58 (d, 1H, J
= 5.8 Hz), 7.99 (s, 1H). 13C NMR (100 MHz; CDCl3; TMS): δ 14.36,
61.38, 119.75, 125.57, 131.61,135.19, 138.72, 143.88, 162.69.
Thieno[3,2-b]thiophene-2-carboxylic Acid (9). A solution of
ethyl thieno[3,2-b]thiophene-2-carboxylate (8) (21.39 g, 100 mmol)
and lithium hydroxide monohydrate (8.39 g, 200 mmol) in
1
red solid (0.18 g, 0.68 mmol, yield = 68%). H NMR (400 MHz;
CDCl3; TMS): δ 6.63 (dd, 2H, J = 3.6, 4.8 Hz), 7.58 (dd, 2H, J = 0.8,
4.8 Hz), 7.67 (dd, 2H, J = 0.8, 3.6 Hz), 8.04 (s, 2H). 13C NMR (100
MHz; CDCl3; TMS): δ 112.14, 112.47, 121.76, 123.50, 142.79,
150.13, 151.30.
4,7-Di(2-thienyl)-benzo[1,2,5]thiadiazole (T-Btdaz). A solu-
tion of 2-tributylstannylthiophene (0.78 g, 2.1 mmol), 4,7-dibromo-
benzo[1,2,5]thiadiazole (11) (0.29 g, 1.0 mmol), and Pd(PPh3)4
(0.023 g, 0.020 mmol) in toluene (4 mL) was refluxed at 90 °C for
24 h. After cooling to room temperature, the mixture was purified
silica gel column chromatography (contain 10% v/v of potassium
carbonate) (eluent: chloroform/hexane = 1/2) and recrystallized from
hexane to afford an orange solid (0.19 g, 0.62 mmol, yield = 62%). 1H
NMR (400 MHz; CDCl3; TMS): δ 7.22 (dd, 2H, J = 3.7, 5.1 Hz), 7.46
(dd, 2H, J = 1.2, 5.1 Hz), 7.87 (s, 2H), 8.12 (dd, 2H, J = 1.2, 3.7 Hz).
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13C NMR (100 MHz; CDCl3; TMS): δ 125.79, 125.99, 126.81,
127.50, 128.01, 139.34, 152. 63.
Hz). 13C NMR (100 MHz; CDCl3; TMS): δ 121.60, 122.17, 130.39,
131.60, 133.90, 137.66.
3,7-Dibromodibenzothiophene 5,5-Dioxide (14). To a sol-
ution of dibenzothiophene 5,5-dioxide (13) (9.73 g, 45 mmol) in
concentrated H2SO4 (330 mL) was added dibromoisocyanuric acid
(14.9 g, 52 mmol) in one portion at room temperature. After stirring
for 20 h, the mixture was poured into ice water (1000 mL). The white
precipitate was filtered. The filter cake was washed with 5%
NaOH(aq)/chloroform, and the organic layer was dried over
magnesium sulfate. The crude product was recrystallized from
chloroform to afford white solid (6.95 g, 18.6 mmol, yield = 41%).
1H NMR (400 MHz; CDCl3; TMS): δ 7.64 (d, 2H, J = 8.1 Hz), 7.77
(dd, 2H, J = 1.7, 8.1 Hz), 7.94 (d, 2H, J = 1.7 Hz). 13C NMR (100
MHz; CDCl3; TMS): δ 122.92, 124.60, 125.58, 129.58, 137.15,
138.87.
3,7-Dibromodibenzothiophene (15). To a solution of 3,7-
Dibromo-dibenzothiophene 5,5-dioxide (14) (4.00 g, 10.7 mmol) in
ether (100 mL) was added lithium aluminum hydride (2.00 g, 52.7
mmol) slowly over 10 min at room temperature. After gently refluxing
for 4 h, the mixture was cooled to room temperature, and 3 mL of
water and concentrated HCl (10 mL) was carefully added to the
mixture in turn. The precipitate was filtered off and washed thoroughly
with dichloromethane. The crude product was purified by silica gel
column chromatography (eluent: ethyl acetate) followed by
recrystallization from chloroform/ethanol to afford a white solid
(1.90 g, 5.6 mmol, yield = 52%). 1H NMR (400 MHz; CDCl3; TMS):
δ 7.56 (d, 2H, J = 7.6 Hz), 7.93−7.96 (m, 4H). 13C NMR (100 MHz;
CDCl3; TMS): δ 120.82, 122.62, 125.49, 128.18, 133.75, 140.94.
3,7-Di(2-furyl)dibenzothiophene (F-Dbt). A solution of 2-
tributylstannylfuran (0.75 g, 2.1 mmol), 3,7-dibromodibenzothiophene
(0.34 g, 1.0 mmol), and Pd(PPh3)4 (0.023 g, 0.020 mmol) in toluene
(8 mL) was refluxed at 90 °C for 24 h. After cooling to room
temperature, the mixture was purified silica gel column chromatog-
raphy (contain 10% v/v of potassium carbonate) (eluent: chloroform)
and recrystallized from chloroform/hexane to afford a pale yellow solid
(0.17 g, 0.54 mmol, yield = 54%). 1H NMR (400 MHz; CDCl3;
TMS): δ 6.53 (dd, 2H, J = 1.6, 3.2 Hz), 6.76 (d, 2H, J = 3.2 Hz), 7.52
(d, 2H, J = 1.6 Hz), 7.76 (dd, 2H, J = 1.5, 8.2 Hz), 8.11 (d, 2H, J = 8.8
Hz), 8.15 (d, 2H, J = 1.5 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ
105.66, 111.91, 117.70, 120.73, 121.69, 129.46, 134.33, 140.36, 142.39,
153.67.
4,7-Bis[thieno[3,2-b]thiophen-2-yl]benzo[1,2,5]thiadiazole
(TT-Btdaz). A solution of 2-tributylstannyl-thieno[3,2-b]thiophene
(10) (0.90 g, 2.1 mmol), 4,7-dibromo-benzo[1,2,5]thiadiazole (11)
(0.29 g, 1.0 mmol), and Pd(PPh3)4 (0.023 g, 0.020 mmol) in toluene
(10 mL) was refluxed at 90 °C for 24 h. After cooling to room
temperature, the mixture was purified silica gel column chromatog-
raphy (contain 10% v/v of potassium carbonate) (eluent: tetrahy-
drofuran) and recrystallized from chloroform/hexane to afford a
1
purple solid (0.20 g, 0.49 mmol, yield = 49%). H NMR (400 MHz;
CDCl3; TMS): δ 7.31 (d, 2H, J = 5.0 Hz), 7.46 (d, 2H, J = 5.0 Hz),
7.88 (s, 2H), 8.50 (s, 1H). 13C NMR (100 MHz; CDCl3; TMS): δ
119.57, 120.62, 125.66, 126.51, 128.44, 139.65, 140.43, 141.21, 152.58.
2,7-Di(2-thienyl)fluorene (T-Fl). A solution of 2-tributylstan-
nylthiophene (1.40 g, 3.7 mmol), 2,7-dibromofluorene (0.59 g, 1.8
mmol), and Pd(PPh3)4 (0.040 g, 0.035 mmol) in toluene (4 mL) was
refluxed at 90 °C for 24 h. After cooling to room temperature, the
mixture was purified silica gel column chromatography (contain 10%
v/v of potassium carbonate) (eluent: dichloromethane) and recrystal-
lized from chloroform to afford a pale brown solid (0.35 g, 1.05 mmol,
1
yield = 58%). H NMR (400 MHz; CDCl3; TMS): δ 3.98 (s, 2H),
7.10 (dd, 2H, J = 3.6, 5.1 Hz), 7.29 (dd, 2H, J = 1.2, 5.1 Hz), 7.37 (dd,
2H, J = 1.2, 3.6 Hz), 7.65 (dd, 2H, J = 1.3, 8.0 Hz), 7.77 (d, 2H, J = 8.0
Hz), 7.80 (d, 2H, J = 1.3 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ
36.92, 120.25, 122.54, 122.93, 124.62, 124.96, 128.09, 133.07, 140.74,
144.16, 144.86.
4,4′-Di(2-thienyl)biphenyl (T-Bp). A solution of 2-tributylstan-
nylthiophene (0.78 g, 2.1 mmol), 4,4′-diromobiphenyl (0.31 g, 1.0
mmol), and Pd(PPh3)4 (0.023 g, 0.020 mmol) in toluene (8 mL) were
refluxed at 90 °C for 24 h. After cooling to room temperature, the
mixture was purified silica gel column chromatography (contain 10%
v/v of potassium carbonate) (eluent: chloroform) and recrystallized
from chloroform to afford a white solid (0.22 g, 0.69 mmol, yield =
69%). 1H NMR (400 MHz; CDCl3; TMS): δ 7.11 (dd, 2H, J = 3.4, 5.2
Hz), 7.31 (dd, 2H, J = 1.0, 5.2 Hz), 7.37 (dd, 2H, J = 1.0, 3.4 Hz), 7.64
(d, 2H, J = 8.6 Hz), 7.71 (d, 2H, J = 8.6 Hz). 13C NMR (100 MHz;
CDCl3; TMS): δ 123.19, 125.08, 126.44, 127.21, 128.01, 133.57,
139.46, 144.10.
3,6-Dibromo-N-methylcarbazole (12). A solution of 3,6-
dibromocarbazole (0.39 g, 1.2 mmol), iodomethane (0.28 g, 2.0
mmol), and potassium carbonate (0.28 g, 2.0 mmol) in acetone (3
mL) was stirred at room temperature. After 24 h, the mixture was
extracted with dichloromethane and water, and the organic layer was
dried over magnesium sulfate. The crude product was purified by silica
gel column chromatography (eluent: chloroform/hexane = 1/4) to
3,7-Di(2-thienyl)dibenzothiophene (T-Dbt). A solution of 2-
tributylstannylthiophene (0.78 g, 2.1 mmol), 3,7-dibromodibenzothio-
phene (0.34 g, 1.0 mmol), and Pd(PPh3)4 (0.023 g, 0.020 mmol) in
toluene (10 mL) was refluxed at 90 °C for 24 h. After cooling to room
temperature, the mixture was purified silica gel column chromatog-
raphy (contain 10% v/v of potassium carbonate) (eluent: chloroform)
and recrystallized from chloroform/hexane to afford a white solid
(0.22 g, 0.62 mmol, yield = 62%). 1H NMR (400 MHz; CDCl3;
TMS): δ 7.12 (dd, 2H, J = 3.8, 5.0 Hz), 7.32 (d, 2H, J = 4.8 Hz), 7.41
(d, 2H, J = 3.6 Hz), 7.71 (dd, 2H, J = 1.4, 8.2 Hz), 8.07 (d, 2H, J = 1.2
Hz), 8.10 (d, 2H, J = 8.4 Hz). 13C NMR (100 MHz; CDCl3; TMS): δ
114.67, 119.91, 121.87, 123.08, 123.64, 125.28, 128.21, 133.38, 140.65,
144.19.
1
afford a white solid (0.24 g, 0.69 mmol, yield = 58%). H NMR (400
MHz; CDCl3; TMS): δ 3.80 (s, 3H,), 7.25 (d, 2H, J = 8.6 Hz), 7.56
(dd, 2H, J = 1.8, 8.6 Hz), 8.12 (d, 2H, J = 1.8 Hz). 13C NMR (100
MHz; CDCl3; TMS): δ 29.32, 110.13, 112.05, 123.18, 123.33, 129.06,
139.82.
N-Methyl-3,6-di(2-thienyl)carbazole (3,6T-Cz1). A solution of
2-tributylstannylthiophene (0.51 g, 1.36 mmol), 3,6-dibromo-N-
methylcarbazole (12) (0.22 g, 0.65 mmol), and Pd(PPh3)4 (0.016 g,
0.014 mmol) in toluene (3 mL) was refluxed at 90 °C for 24 h. After
cooling to room temperature, the mixture was purified silica gel
column chromatography (contain 10% v/v of potassium carbonate)
(eluent: chloroform/hexane = 3/7) to afford a white solid (0.13 g, 0.36
mmol, yield = 56%). 1H NMR (400 MHz; CDCl3; TMS): δ 7.11 (dd,
2H, J = 3.6, 5.2 Hz), 7.27 (dd, 2H, J = 1.2, 5.2 Hz), 7.35−7.38 (m,
4H), 7.75 (dd, 2H, J = 2.0, 4.8 Hz), 8.33 (d, 2H, J = 1.6 Hz). 13C
NMR (100 MHz; CDCl3; TMS): δ 29.31, 108.92, 117.93, 122.12,
123.13, 123.73, 124.64, 126.00, 128.00, 140.95, 145.60.
Dibenzothiophene 5,5-Dioxide (13). A solution of dibenzo-
thiophene (9.21 g, 50.0 mmol) and 33% H2O2(aq) (25 mL) in acetic
acid (150 mL) was refluxed at 130 °C. After 4 h, the mixture was
cooled to room temperature and cooled in a fridge. The precipitate
was filtered to give a white solid (10.15 g, 46.9 mmol, yield = 94%). 1H
NMR (400 MHz; CDCl3; TMS): δ 7.53 (dt, 2H, J = 1.2, 7.6 Hz), 7.64
(dt, 2H, J = 1.2, 7.6 Hz), 7.79 (d, 2H, J = 7.6 Hz), 7.83 (d, 2H, J = 7.6
SYNTHESIS OF CHIRAL INDUCERS
■
Synthetic routes of chiral inducer were described in Scheme 2.
4′-Hydroxybiphenyl-4-carboxylic Acid (16). A suspen-
sion of 4-cyano-4′-hydroxybiphenyl (9.76 g, 50 mmol) and
sodium hydroxide (10.00 g, 250 mmol) in ethanol (50 mL) and
water (300 mL) were refluxed at 120 °C for 24 h. After the
reaction, the mixture was cooled to room temperature and
poured into 500 mL of water. Hydrochloric acid was added into
the mixture with vigorous stirring until pH was around 1 to
precipitate white solid. The resulting white precipitate was
recovered by vacuum filtration to afford white solid (10.42 g,
1
48.6 mmol, yield = 97%). H NMR (400 MHz; DMSO-d6;
TMS): δ 6.96 (d, 2H, J = 8.6 Hz), 7.66 (d, 2H, J = 8.6 Hz), 7.89
L
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(d, 2H, J = 8.2 Hz), 8.05 (d, 2H, J = 8.2 Hz), 9.81 (s, 1H). 13
NMR (100 MHz; CDCl3; TMS): δ 115.87, 125.83, 128.12,
128.52, 129.60, 129.91, 144.31, 157.90, 167.23.
C
Promotion of Science, Grant in-Aid for Scientific Research,
22550161. K.K. is a research fellow of the Japan Society for the
Promotion of Science.
4′-Hydroxybiphenyl-4-carboxylic Acid (S)-1-Methyl-
Heptyl Ester (17S). To a solution of 4′-hydroxy-biphenyl-4-
carboxylic acid (16) (3.21 g, 15.0 mmol), triphenylphosphine
(3.99 g, 15.2 mmol), and (R)-2-octanol (1.98 g, 15.2 mmol) in
THF (20 mL) was added diisopropyl azodicarboxylate (40% in
toluene) (3.07 g, 15.2 mmol) dropwise at 0 °C. After stirring
for 12 h, the solvent was evaporated. The crude product was
purified by silica gel column chromatography (eluent: chloro-
form/ethyl acetate = 9/1) to afford white solid (4.19 g, 12.8
ABBREVIATIONS
■
CLC, cholesteric liquid crystal; CD, circular dichroism; BT,
2,2′-bithiophene; ter-T, 2,2′:5′,2″-terthiophene; 3DMBT, 3,3′-
dimethyl-2,2′-bithiophene; 4DMBT, 4,4′-dimethil-2,2′-bithio-
phene; TMBT, 3,4,3′,4′-tetramethyl-2,2′-bithiphene; T-ITN,
1,3-di(2-thienyl)isothianaphthene; EDOT, 3,4-ethylenedioxy-
thiophene; bis-EDOT, 2,2′-bis(3,4-ethylnenedioxythiophene);
TT, thieno[3,2-b]thiophene; bis-TT, 2,2′-bithieno[3,2-b]-
thiophene; T-Fl, 2,7-di(2-thienyl)fluorene; T-Cz1, N-methyl-
2,7-di(2-thienyl)carbazole; T-Dbt, 3,7-di(2-thienyl)-
dibenzothiophene; T-Bp, 4,4′-di(2-thienyl)biphenyl; F-Fl, 2,7-
di(2-furyl)fluorene; F-Cz1, N-methyl-2,7-di(2-furyl)carbazole;
F-Dbt, 3,7-di(2-furyl)dibenzothiophene; F-Bp, 4,4′-di(2-furyl)-
biphenyl; T-Btdaz, 4,7-di(2-thienyl)-benzo[1,2,5]thiadiazole; F-
Btdaz, 4,7-di(2-furyl)-benzo[1,2,5]thiadiazole; TT-Btdaz, 4,7-
bis-thieno[3,2-b]thiophen-2-yl-benzo[1,2,5]thiadiazole; 3,6T-
Cz1, N-methyl-3,6-di(2-thienyl)carbazole; DFT, density func-
tional theory; NLC, nematic liquid crystal; COC, cholesteryl
oleyl carbonate; 6CB, 4-cyano-4′-n-hexyl biphenyl; TBAP,
tetrabutylammonium perchlorate; POM, polarizing optical
microscopy; DSC, differential scanning calorimetry; ITO,
indium−tin−oxide; MALDI-TOF-MS, matrix-assisted laser
desorption ionization time-of-flight mass spectroscopy; L/D,
length/diameter; CP, cholesteryl pelargonate
1
mmol, yield = 85.6%). H NMR (400 MHz; CDCl3; TMS): δ
0.86 (t, 3H, J = 6.6 Hz), 1.27- 1.43 (m, 11H), 1.58−1.79 (m,
2H), 5.18 (sextet, 1H, J = 5.8 Hz), 5.49 (s, 1H), 6.95 (d, 2H, J
= 8.4 Hz), 7.52 (d, 2H, J = 8.4 Hz), 7.60 (d, 2H, J = 8.4 Hz),
8.08 (d, 2H, J = 8.4 Hz). 13C NMR (100 MHz; CDCl3; TMS):
δ 14.05, 20.10, 22.58, 25.41, 29.16, 31.73, 36.07, 71.89, 115.84,
126.41, 128.57, 128.91, 130.04, 132.59, 145.08, 155.99, 166.44.
4′-(4-Hexyloxybenzoyloxy)biphenyl-4-carboxylic Acid
(S)-1-Methylheptyl Ester ((S)-8BpB6). To a solution of 4-
hexyloxybenzoic acid (0.91 g, 5.0 mmol) and N,N'-dicyclohex-
ylcarbodiimide (1.03 g, 5.0 mmol) in dichloromathane (15 mL)
was added a solution of 4′-hydroxy-biphenyl-4-carboxylic acid-
(S)-1-methyl-heptyl ester (17S) (1.31 g, 4.0 mmol) and N,N-
dimethyl-4-aminopyridin (0.61 g, 5.0 mmol) in dichloroma-
thane (15 mL) dropwise over 1 h. After stirring for 20 h at
room temperature, white precipitate was removed by filtration.
The crude product was purified by silica gel column
chromatography (eluent: chloroform) followed by recrystalliza-
tion from hexane/ethanol to afford white solid (1.81 g, 3.4
REFERENCES
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1
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AUTHOR INFORMATION
Corresponding Author
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