Highly twisted helical polyacetylene with morphology free from the bundle of fibrils synthesized in chiral nematic liquid crystal reaction field

Article abstract of DOI:10.1021/ja070701x

We synthesized novel axially chiral binaphthyl derivatives with highly twisting powers by substituting phenylcyclohexyl (PCH) mesogenic moieties into 2,2′ positions or 2,2′,6,6′ positions of binaphthyl rings. The di- and tetrasubstituted binaphthyl deriva

Full text of DOI:10.1021/ja070701x

Published on Web 06/19/2007
Highly Twisted Helical Polyacetylene with Morphology Free
From the Bundle of Fibrils Synthesized in Chiral Nematic
Liquid Crystal Reaction Field
Munju Goh,† Mutsumasa Kyotani,‡ and Kazuo Akagi*,†
Contribution from the Department of Polymer Chemistry, Kyoto UniVersity, Katsura, Kyoto
615-8510, Japan, and Tsukuba Research Center for Interdisciplinary Materials Science (TIMS),
UniVersity of Tsukuba, Ibaraki 305-8573, Japan.
Received January 31, 2007; E-mail: akagi@star.polym.kyoto-u.ac.jp
Abstract: We synthesized novel axially chiral binaphthyl derivatives with highly twisting powers by
substituting phenylcyclohexyl (PCH) mesogenic moieties into 2,2′ positions or 2,2′,6,6′ positions of binaphthyl
rings. The di- and tetrasubstituted binaphthyl derivatives, abbreviated as D-1 and D-2, respectively, were
adopted as chiral dopants to induce chiral nematic liquid crystals (N*-LCs) available for synthesis of helical
polyacetylene. The helical twisting power (âM) of D-2 was 449 µm-1, which was ca. 2.6 times larger than
that of D-1 (171 µm-1). We prepared two kinds of induced N*-LCs with 5 µm and 270 nm in helical pitch
by adding the chiral dopants D-1 and D-2 into the host N-LCs, respectively. The helical polyacetylene
synthesized in the N*-LC containing D-2 exhibited highly screwed fibrils, but not a bundle of fibrils. This
result is in quite contrast to the usual fibril morphology, where the screwed fibrils are gathered to form the
bundle of fibrils, as observed in the helical polyacetylene synthesized in the N*-LC containing a chiral
dopant with moderate helical twisting power, such as D-1. It is of keen interest that the helical pitch (270
nm) of the N*-LC including D-2 is much smaller than the diameter (ca. 1 µm) of the bundle of fibrils, which
should depress the formation of the bundle of fibrils. The morphology free from the bundle of fibrils might
enable us to evaluate more precisely intrinsic electromagnetic properties of a single screwed fibril of helical
polyacetylene.
field to produce chiral conducting polymers.5 It has been found
that the chirality of the helical polyacetylene is controllable by
Introduction
There has been increasing interest in chiral conducting
polymers, because of their potential application for nanowires,1
electrodevices,2 and biological technologies.3 Since helical
polyacetylene characterized by hierarchical spiral morphology
was synthesized in chiral nematic liquid crystal (N*-LC),4 the
N*-LC has been currently focused as an asymmetric reaction
selecting the chirality of the chiral dopant to be added to the
nematic LC (N-LC). The helical polyacetylene is anticipated
to be a prototype of a molecular solenoid owing to its helical
structure and high electrical conductivity.1,4 It is therefore
desirable to synthesize a highly screwed helical polyacetylene
for inducing the solenoidal magnetism. For this aim, it is
essentially important to construct a more highly twisted LC
reaction field.
† Kyoto University.
‡ University of Tsukuba.
(1) (a) Lee, H. J.; Jin, Z. X.; Aleshin, A. N.; Lee, J. Y.; Goh, M. J.; Akagi, K.;
Kim, Y. S.; Kim, D. W.; Park, Y. W. J. Am. Chem. Soc. 2004, 126, 16722.
(b) Aleshin, A. N.; Lee, H. J.; Park, Y. W.; Akagi, K. Phys. ReV. Lett.
2004, 93, 196601. (c) Aleshin, A. N.; Lee, H. J.; Akagi, K.; Park, Y. W.
Microelectron. Eng. 2005, 81, 420. (d) Aleshin, A. N.; Lee, H. J.; Jhang,
S. H.; Kim, H. S.; Akagi, K.; Park, Y. W. Phys. ReV. B 2005, 72, 153202.
(e) Lee, S. W.; Kim, B.; Lee, D. S.; Lee, H. J.; Park, J. G.; Ahn, S. J.;
Campbell, E. E. B.; Park, Y. W. Nanotechnology 2006, 17, 992. (f) Ofuji,
M.; Takano, Y.; Houkawa, Y.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.;
Mori, T.; Goh, M.; Guo, S.; Akagi, K. Jpn. J. Appl. Phys. 2006, 45, 1710.
(2) (a) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Christiaans, M. P. T.;
Meskers, S. C. J.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc.
1996, 118, 4908. (b) Peeters, E.; Christiaans, M. P. T.; Janssen, R. A. J.;
Schoo, H. F. M.; Dekkers, H. P. J. M.; Meijer, E. W. J. Am. Chem. Soc.
1997, 119, 9909.
(3) (a) Bross, P. A.; Schoberl, U.; Daub, J. AdV. Mater. 1991, 3, 198. (b) Li,
W.; Wang, H. L. J. Am. Chem. Soc. 2004, 126, 2278. (c) Iwaura, R.;
Hoeben, F. J. M.; Masuda, M.; Schenning, A. P. H. J.; Meijer, E. W.;
Shimizu, T. J. Am. Chem. Soc. 2006, 128, 13298.
(4) (a) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.; Shirakawa, H.; Kyotani,
M. Science 1998, 282, 1683. (b) Akagi, K.; Piao, G.; Kaneko, S.; Higuchi,
I.; Shirakawa, H.; Kyotani, M. Synth. Met. 1999, 102, 1406. (c) Akagi, K.;
Guo, S.; Mori, T.; Goh, M.; Piao, G.; Kyotani, M. J. Am. Chem. Soc. 2005,
127, 14647. (d) Goh, M. J.; Kyotani, M.; Akagi, K. Curr. Appl. Phys. 2006,
6, 948.
On the other hand, from a viewpoint of nanoscience, it is
very intriguing to elucidate the physical properties of the single
helical fibril. The helical polyacetylene chains are self-assembled
owing to van der Waals interaction to form a helical fibril, which
is a bundle of polymer chains. Besides, the helical fibrils are
gathered to form a bundle of fibrils. The bundles of fibrils give
a spiral morphology in a domain, resulting in a multidomain of
helical structure.4 However, the insolubility and infusibility of
polyacetylene make it difficult to separate the bundle of fibrils
into single fibrils. This situation is similar to the case of carbon
(5) (a) Kang, S. W.; Jin, S. H.; Chien, L. C.; Sprunt, S. AdV. Funct. Mater.
2004, 14, 329. (b) Goto, H.; Akagi, K. Angew. Chem., Int. Ed. 2005, 44,
4322. (c) Goto, H.; Akagi, K. Macromolecules 2005, 38, 1091. (d) Goto,
H.; Nomura, N.; Akagi, K. J. Polym. Sci., Part A: Polym. Chem. 2005,
43, 4298. (e) Goto, H.; Jeong, Y. S.; Akagi, K. Macromol. Rapid Commun.
2005, 26, 164. (f) Goto, H.; Akagi, K. Chem. Mater. 2006, 18, 255. (g)
Goto, H.; Akagi, K. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1042.
9
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J. AM. CHEM. SOC. 2007, 129, 8519-8527
8519

A R T I C L E S
Goh et al.
Table 1. Molecular Structures of Nematic Liquid Crystals and Chiral Dopants
nanotubes; due to substantial van der Waals attractive interac-
tions, carbon nanotubes tend to form aggregates which can be
hardly separated into single tubes.6
containing chiral compounds.4,9,10 However, even when the
binaphthyl derivative with the highly twisting power was used,
the helical pitch of the induced N*-LC was not in nano-order,
but nearly equal to or larger than 1 µm. Thus, to affect the axial
chirality more effectively to the environmental LC molecules,
the high miscibility reinforced by liquid crystallinity of the chiral
dopant itself as well as the high helical twisting power were
required.
In this work, we synthesized novel di- and tetrasubstituted
binaphthyl derivatives, where the phenylcyclohexyl (PCH)
moieties are substituted at the 2,2′ positions or the 2,2′, 6,6′
positions of the binaphthyl rings4 (see Table 1). Among them,
the tetrasubstituted binaphthyl derivative (D-2), having the direct
linkage between the PCH moieties and the binaphthyl rings at
the 6 and 6′ positions, gave helical pitches of nano-order (270
nm) when added as a chiral dopant into N-LC. Thus, the
prepared N*-LC enabled us to synthesize a highly twisted nano-
single-fibril of helical polyacetylene.
The only way for getting the single fibril, that has been
recently put forward, is to soak a piece of polyacetylene film
in N,N-dimethylformamide solution including a surfactant such
as hexaethylene glycol monododecyl ether, and then to ultra-
sonicate it for dispersion.1a-d However, this procedure might
lead to cleavages and/or damage of the fibrils during the
ultrasonication. This would prevent us to evaluate intrinsic
electromagnetic properties of the single fibril of helical poly-
acetylene. Here we have found that the helical polyacetylene
film, which consists of only single fibrils but not the bundle of
fibrils, can be synthesized by using a highly twisted N*-LC
reaction field.
The helical pitch of the N*-LC can be adjusted by changing
the concentration of the chiral dopant or by changing the helical
twisting power of the chiral dopant itself.7 However, the
mesophase temperature region of the N*-LC is also affected
by the concentration of the chiral dopant. Namely, it becomes
narrow as the concentration of the chiral dopant increases, and
finally the mesophase will be destroyed when the concentration
is close to a critical value.8 Therefore, the latter approach
utilizing the chiral dopants with various helical twisting powers
might be preferable. It has been elucidated so far that the
binaphthyl derivatives have not only a good miscibility toward
N-LC by virtue of their LC moieties but also a higher helical
twisting power due to axial chirality than an asymmetric center
Methods
All experiments were performed under argon atmosphere. Tetrahy-
drofuran (THF) and dichloromethane (CH2Cl2) were distilled prior to
use. Williamson etherification and Suzuki coupling reactions were used
to obtain the chiral dopant. The chemical compounds, (R)- and (S)-
2,2-dihydroxy-1,1-binaphthyl (optical purity, 0.99), were purchased
from commercially available sources. The mesogenic compounds,
4-(trans-4-n-pentylcyclohexyl)phenol [PCH500] and 4-(trans-4-n-pen-
tylcyclohexyl)bromobenzene [PCH5Br], were purchased from Kanto
Chemical Ltd.
Nematic LCs of phenylcyclohexane derivatives, 4-(trans-4-n-pro-
pylcyclohexyl)ethoxybenzene (PCH302) and 4-(trans-4-n-propylcy-
clohexyl)butoxybenzene (PCH304), and disubstituted binaphthyl de-
(6) (a) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P.
L.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias,
F.; Shon, Y.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280,
1253. (b) Ajayan, P. M. Chem. ReV. 1999, 99, 1787.
(9) (a) Gottarelli, G.; Hibert, M.; Samori, B.; Solladie, G.; Spada, P.;
Zimmermann, R. J. Am. Chem. Soc. 1983, 105, 7318. (b) Solladie, G.;
Zimmermann, R. Angew. Chem., Int. Ed. Engl. 1984, 23, 348. (c) Gottarelli,
G.; Spada, G. P.; Bartsch, R.; Solladie, G.; Zimmermann, R. J. Org. Chem.
1986, 51, 589. (d) Bhatt, J. C.; Keast, S. S.; Neubert, M. E.; Petschek, R.
C. Liq. Cryst. 1995, 18, 367. (e) Proni, G.; Spada, G. P.; Lustenberger, P.;
Welti, R.; Diederich, F. J. Org. Chem. 2000, 65, 5522.
(10) (a) Heppke, G.; Lo¨tzsch, D.; Oestreicher, F. Z. Naturforsch., A: J. Phys.
Sci. 1986, 41, 1214. (b) Kanazawa, K.; Higuchi, I.; Akagi, K. Mol. Cryst.
Liq. Cryst. 2001, 364, 825. (c) Rokunohe, J.; Yoshizawa, A. J. Mater. Chem.
2005, 15, 275.
(7) Gottarelli, G.; Mariani, P.; Spada, G. P.; Samori, B.; Forni, A.; Solladie,
G.; Hibert, M. Tetrahedron 1983, 39, 1337.
(8) (a) Lee, H.; Labes, M. M. Mol. Cryst. Liq. Cryst.1982, 84, 137. (b) Hatoh,
H. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1994, 250, 1. (c)
Semenkova, G. P.; Kutulya, L. A.; Shkol’nikova, N. I.; Khandrimailova,
T. V. Kristallografiya 2001, 46, 128. (d) Guan, L.; Zhao, Y. J. Mater.
Chem. 2001, 11, 1339.
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Helical Polyacetylene Nanofiber
A R T I C L E S
Scheme 1. Synthetic Routes for Tetrasubstituted Axially Chiral Binaphthyl Derivative D-2
rivatives [(R)-, (S)-2,2′-PCH506-1,1′-binaphthyl, abbreviated as (R)-,
(S)-D-1] were synthesized according to the previous reports.4 The
tetrasubstituted binaphthyl derivatives [(R)-, (S)-2,2′-PCH5012-6,6′-
PCH5-1,1′-binaphthyl, abbreviated as (R)-, (S)-D-2] were synthesized
by substituting the PCH moieties without methylene spacers into the
6,6′ positions of the binaphthyl ring. Synthetic routes for the tetrasub-
stituted binaphthyl derivatives, D-2, are shown in Scheme 1. As
reference compounds to the D-2, we synthesized [(R)-2,2′,6,6′-PCH506-
1,1′-binaphthyl, abbreviated as RC-1] and [(R)-2,2′-PCH506-6,6′-
PCH5-1,1′-binaphthyl, abbreviated as RC-2], by means of the previ-
ously reported procedure10b and the synthetic method similar to that
used for D-2, respectively (see the Supporting Information).
in H2O (20 mL) and THF (20 mL) overnight, and then it was washed
several times with H2O and CHCl3. The organic layer was subjected
to chromatography on silica using n-hexane/CH2Cl2 ) 1 as an eluent
to give a colorless glassy material, D-2 (1.2 g, 96%).
Proton (1H) and carbon (13C) NMR spectra were measured in CDCl3
using a JEOL 270 MHz NMR spectrometer. Chemical shifts are
represented in parts per million downfield from tetramethylsilane as
an internal standard. The chemical properties of the compounds
synthesized are given in the last section. The microscope observation
was carried out under crossed nicols by using a Nikon ECLIPSE E
400 POL polarizing optical microscope, equipped with a Nikon
COOLPIX 950 digital camera and a Linkam TH-600PM and L-600
heating and cooling stage with temperature control. The phase transition
temperatures of the chiral dopants and the N*-LC were determined
using a Perkin-Elmer differential scanning calorimeter (DSC 7) and a
TA Instrument Q100 DSC apparatus at a constant heating and cooling
rate of 10 °C/min, where the results in the first cooling and the second
heating processes were recorded. XRD measurements were performed
with a Rigaku D-3F diffractometer, in which X-ray power was set at
12 kW.
PCH500 (5 g, 20.3 mmol) and K2CO3 (3 g, 22 mmol) were dissolved
in acetone (100 mL) and stirred at 60 °C. 1,12-Dibromododenane (6.6
g, 20.3 mmol) was added dropwise to the solution at 60 °C. The mixture
was stirred at 60 °C. After 12 h, the reaction mixture was quenched
with H2O and extracted with CHCl3. The organic layer was dried with
Na2SO4. The solvent was removed in vacuo after filtration. The organic
layer was subjected to chromatography on silica using n-hexane/CH2-
Cl2 ) 1 as an eluent to give a white powder, 1 (4.6 g, 46%). PCH5Br
(5 g, 16.2 mmol) and Mg (1.2 g, 48.5 mmol) were dissolved in dried
THF (20 mL) and stirred for 2 h at room temperature. Trimethylborate
(1.4 g, 24.3 mmol) was added dropwise to the reaction mixture at
-78 °C. The reaction mixture was stirred for 5 h at room temperature.
The mixture was washed several times with HCl, H2O, and CHCl3 and
recrystallized from n-hexane to give a white powder 2 (2.2 g, 52%).
The (R)-, (S)-6,6′-dibromo-2,2′-dihydroxy-1,1′-binaphthyl (3) was
synthesized using the bromination reaction.11 The hydroxy groups of
3 (1 g, 2.25 mmol) were substituted by Williamson etherification with
1 (2.45 g, 4.95 mmol) in acetone (20 mL) using K2CO3 as a base at
60 °C, giving the compound (R)-, (S)-6,6′-dibromo-2,2′-PCH5012-1,1′-
binaphthyl (4) (2.63 g, 92%). Compounds of 2 (0.55 g, 2 mmol),
4 (1 g, 0.79 mmol), 20 wt % Na2CO3(aq), and a catalytic amount of
Pd(PPh3)4 were added to the reaction mixture. The mixture was refluxed
Results and Discussion
Mesomorphic Properties of Chiral Dopants. Mesomorphic
properties of the binaphthyl derivatives were investigated with
differential scanning calorimeter (DSC), polarizing optical
microscope (POM), and X-ray diffraction (XRD) measurements.
Although the disubstituted binaphthyl derivative (D-1) and
tetrasubstituted one, RC-2, showed no mesophase, the tetra-
substituted derivatives (D-2 and RC-1) showed liquid crystal-
linity. The phase transition temperatures of D-2 were as
follows: G f 79 °C f Sx f 99 °C f I and G r 52 °C r Sx
r 63 °C r I in the heating and cooling processes, respectively.
Note that G, S, and I denote glassy, smectic, and isotropic
phases, respectively. The POM of D-2 showed a focal conic
texture characteristic of smectic (Sx) phase. The XRD of D-2
(11) (a) Pu, L. Chem. ReV. 1998, 98, 2405. (b) Cui, Y.; Evans, O. R.; Ngo, H.
L.; White, P. S.; Lin, W. Angew. Chem., Int. Ed. 2002, 41, 1159.
9
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A R T I C L E S
Goh et al.
measured at 90 °C showed the sharp diffraction peak at the
small angle region, 2θ ) 2.4°, corresponding to 36.8 Å, which
is ascribed to the interlayer distance. The diffraction peak in
the wide angle region (2θ ) 18° to 22°) was assigned to the
intermolecular distance of LCs (from 4.9 to 4.07 Å). From the
POM and XRD results, the LC phase of D-2 was assigned to
the smectic phase. (The results of DSC, POM, and XRD are
available in the Supporting Information.)
Characterization of Chiral Nematic LC (N*-LC). The N*-
LC was prepared by adding a small amount of chiral dopant
into an equimolar mixture of the N-LCs of PCH302 and
PCH304, as shown Table 1. It should be noted that although
each component (PCH302 or PCH304) shows an LC phase, the
LC temperature region is very narrow, i.e., less than 1-2 °C.
This is not suitable for acetylene polymerization irrespective
of the N-LC or N*-LC reaction field, because the exothermal
heat evoked during the acetylene polymerization would raise
the temperature inside a Schlenk flask and easily destroy the
LC phase into an isotropic one. Hence, we prepared an LC
mixture by mixing the equimolar two nematic LCs. In the N-LC
mixture, the nematic to isotropic temperature, TN-I, and the
crystalline to nematic temperature, TC-N, might be raised and
lowered, respectively. In fact, the mixture exhibited the LC
phase in the region from 20 to 35 °C. The change of TN-I upon
an addition of Ti(O-n-Bu)4-AlEt3 catalyst was examined
through DSC measurement. Taking account of the effect of
supercooling of LCs, the catalyst solution consisting of the LC
mixture and the chiral dopant was found available for room-
temperature polymerization ranging from 5 to 25 °C. This
sufficiently wide temperature region enabled us to perform the
acetylene polymerization in the N*-LC phase.
Figure 1. Plots of inverse helical pitch vs mole fraction of chiral dopant.
Note that the horizontal axis is expressed by mole fraction, not mole
percentage, in order to make it easier to evaluate the helical twisting power
(âM) that is defined as âM ) [(1/p)/c]cf0, where c is the mole fraction of
chiral dopant in the N*-LC.
Evaluation of Helical Twisting Power (âM). Subsequently,
the helical twisting powers (âM) of the chiral dopant, i.e., an
ability to convert N-LC into N*-LC, was evaluated using the
equation
âM ) [(1/p)/c]cf0
where p is the helical pitch in micrometers and c is the mole
fraction of the chiral dopant in the N*-LC.9b,14 We plotted the
inverse pitch (1/p) as a function of the mole fraction of the chiral
dopant and obtained a linear relationship between 1/p and c in
the low-concentration regime, as shown in Figure 1. The helical
twisting powers of D-1 and D-2 were 171 µm-1 and 449 µm-1
,
respectively (see Table 1). It is clear that the helical twisting
power of the chiral dopant D-2 is ca. 2.6 times larger than that
of D-1. This may be rationalized with a difference in the number
of substituents. Namely, the axially twisting torque of D-2 is
more effectively transferred to environmental nematic LC
molecules, by virtue of intermolecular interactions between the
four PCH substituents of D-2 and the PCH moieties of LC
molecules, rather than in the case of D-1 bearing two PCH
substituents. The equilibrium geometries of the chiral dopants
are available in the Supporting Information. Note that although
the chiral dopant of the (S)-configuration should have the same
helical twisting power as that of the (R)-configuration, the former
gave a smaller value in the helical twisting power than the latter,
irrespective of the di- or tetrasubstituted binaphthyl derivatives,
as shown in Table 1. This is due to the fact that (S)-binaphthol
used as a starting compound has a lesser optical purity than
(R)-binaphthol, although both binaphthols are commercially
available.
Contribution of Helical Twisting Power and Miscibility
of Chiral Dopant in Helical Pitch of N*-LC. Since the helical
pitch of the N*-LC might be essential to control the morphology
of helical polyacetylene, it is worthwhile to examine in more
detail how the helical twisting power (âM) and the miscibility
of the chiral dopant affect the helical pitch of the N*-LC. Table
2 shows some properties of the tetrasubstituted binaphthyl
derivatives.
The molecular fragments in both the nematic liquid crystals
and the chiral dopants (binaphthyl derivatives) are composed
of only phenyl, cyclohexyl, and alky moieties as well as ether-
type oxygen. These molecular fragments are chemically stable
toward not only extremely reactive Ziegler-Natta catalyst such
as triethylaluminum and tetrabutoxytitanium but also other kinds
of transition metal (Ni, Pd, Pt) compounds. This means that
the present chiral nematic liquid crystal is available for
asymmetric reaction solvent for other kinds of polymerizations
and even for chemical reactions of chiral compounds. The
potential versatility as an asymmetric reaction solvent in the
present chiral nematic liquid crystal is worthy to be emphasized
from the viewpoint of further application.
The helical pitch of N*-LCs were precisely evaluated by
measuring the distance between Cano lines appearing on the
surface of a wedge-type cell under the POM microscope.4c,10b,12
Meanwhile, when the helical pitch was smaller than 1 µm, it
was evaluated with the selective light reflection method. The
helical pitch was evaluated according to the equation
p ) λmax/n
where λmax is the center wavelength for the maximally reflected
light and n is the mean refraction index of N*-LC (n = 1.5).4d,13
The structural difference between RC-1 and RC-2 is that the
PCH moieties are indirectly and directly linked with the 6,6′
positions of the binaphthyl rings, respectively. That is, the
absence of the hexamethylene spacer, [-(CH2)6-] leads to a
(12) (a) Grandjean, F. C. R. Acad. Sci. 1921, 172. (b) Cano, R. Bull. Soc. Fr.
Mineral. 1968, 91, 20. (c) Heppke, G.; Oestreicher, F. Mol. Cryst. Liq.
Cryst. Lett. 1978, 41, 245. (d) Gottarelli, G.; Samori, B.; Stremmenos, C.;
Torre, G. Tetrahedron 1981, 37, 395.
(13) (a) Friedel, G. Ann. Phys. (Paris) 1922, 18, 273. (b) Yoshida, J.; Sato, H.;
Yamagishi, A.; Hoshino, N. J. Am. Chem. Soc. 2005, 127, 8453.
(14) Kuball, H. G.; Ho¨fer, T. In Chirality in Liquid Crystals; Kitzerow, H. S.,
Bahr, C., Eds.; Springer Press: New York, 2000; Vol. 1, Chapter 3, p 67.
9
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Helical Polyacetylene Nanofiber
A R T I C L E S
Table 2. Comparison between Several Types of Tetrasubstituted
Axially Chiral Binaphthyl Derivatives ((R)-Configuration)
RC-1
RC-2
D-2
rigiditya
low
high
high
helical twisting power (âM)
liquid crystallinityb
maximum mol %c
helical pitch of induced
N*-LCd
200 µm-1
yes
510 µm-1
no
449 µm-1
yes
1.5
940 nm
0.25
1.3 µm
1.5
270 nm
a Rigidity in linkages between 6 and 6′ positions of the binaphthyl rings
and the PCH moieties. b Liquid crystallinity of chiral dopant itself.
c Maximum mole percentage of chiral dopant, over which N*-LC changes
into solid state through phase separation. Mole percentage ) mole of chiral
dopant/(mole of PCH302 + mole of PCH304) × 100. d Helical pitch of
N*-LC induced by the chiral dopant of maximum mole percentage.
Figure 2. Change of helical pitch in N*-LC as a function of mole
percentage of dopant. The helical pitches longer and shorter than 1 µm
were evaluated by Cano’s wedge method and the selective light reflection
method, respectively.
allowed us to prepare highly twisted N*-LC whose helical pitch
is nano-order such as 270 nm. It should be emphasized here
that although the liquid crystallinity of the chiral dopant has no
direct influence on the helical twisting power of the N*-LC, it
plays a role to increase the miscibility of the chiral dopant,
leading to an increase of the upper limit concentration usable
for the chiral dopant to the parent N-LC.
Asymmetric Reaction Field Constructed with N*-LC.
Figure 2 shows the changes of helical pitches in N*-LCs as a
function of mole percentage of chiral dopants D-1 and D-2.
The PCH moieties substituted in the binaphthyl derivatives
enhance a miscibility of the chiral dopants D-1 and D-2 in the
nematic mixture of PCH302 and PCH304, even at relatively
high mole percentage (1.5 mol %) of the binaphthyl derivatives.
The helical pitches of N*-LCs varied from 9.4 to 3.2 µm with
change of the mole percentage of D-1 (from 0.25 to 1.5 mol
%) and from 5.24 µm to 270 nm with D-2 (from 0.05 to 1.5
mol %). This result shows that the chiral dopant D-2 has a highly
helical twisting power enough to give the nano-order helical
pitch in N*-LC. We also confirmed that the N*-LCs induced
by D-2 showed selective light reflections in the visible region,
as shown in Figure 3. The color depends on the helical pitch of
the N*-LC. The N*-LCs with blue, green, and red colors in the
selective light reflection were prepared by using the chiral
dopant D-2 of 0.75, 1, and 1.5 mol %, respectively.
rigidity in the linkage between the PCH moieties and the
binaphthyl rings, resulting in high helical twisting power of
510 µm-1 in RC-2, which is about 2.5 times higher than that
(200 µm-1) of RC-1. However, RC-2 has no liquid crystallinity
owing to its rigidity, which depresses the miscibility of the chiral
dopant to the parent N-LC. In fact, the maximum mole
percentage (0.25 mol %) usable as the dopant was one-sixth
that of RC-1 (1.5 mol %) having the liquid crystallinity. As a
result, the N*-LC induced by the chiral dopant of RC-2 has a
longer helical pitch (1.3 µm) than that (910 nm) by the dopant
The phase transition temperature (clearing temperature) from
chiral nematic to isotropic phase depends on the molecular
structure and the mole percentage of the chiral dopant, as well
as the intermolecular interactions between the chiral dopant and
the N-LCs.15 Figure 4 shows the N*-LC-isotropic phase
transition temperatures of the mixed systems as a function of
mole percentage of the chiral dopant. Since the chiral dopant
of D-2 has liquid crystallinity itself, the intermolecular interac-
tions between D-2 and the nematic molecules should be larger
than that those in the case of D-1. Thus, the increase in mole
percentage of D-2 raises the clearing point.16 This suggests a
thermal stability of the N*-LC phase including D-2. However,
the N*-LC including D-1 showed a reverse trend, i.e., a lowering
of the clearing temperature with increasing the mole percentage
of D-1.
of RC-1, in spite of the larger helical twisting power (510 µm-1
of RC-2.
)
Meanwhile, D-2 shows the rigidity in linkage between the
6,6′ positions of the binaphthyl rings and the PCH moieties,
similar to that of RC-2. Thus, it has a higher helical twisting
power (449 µm-1), although the value is slightly smaller than
that (510 µm-1) of RC-2. The difference is due to an increase
of the length of the methylene spacer between the 2,2′ positions
of the binaphthyl rings and the PCH moieties; the dodecam-
ethylene spacer [-(CH2)12-] in D-2 makes the PCH fragments
at the 2,2′ positions of the binaphthyl rings more flexible than
in the case of RC-2. This results in a slightly smaller twisting
power than RC-2. At the same time, however, owing to the
increase of the flexibility, D-2 has liquid crystallinity itself,
similar to the case of RC-1. Actually, the liquid crystallinity
increased the miscibility of D-2 to the parent N-LC, yielding a
maximum concentration of 1.5 mol %. As a consequence, both
the high helical twisting power and the high miscibility of D-2
(15) (a) Bauman, D. Mol. Cryst. Liq. Cryst. 1988, 159, 197. (b) Bauman, D.;
Martynski, M. T.; Mykowska, E. Liq. Cryst. 1995, 18, 607. (c) Deussen,
H. J.; Shivaev, P. V.; Vinokur, R.; Bjornholm, T.; Schaumburg, K.;
Bechgaard, K.; Shivaev, V. P. Liq. Cryst. 1996, 21, 327.
(16) Van Hecke, G. R. J. Phys. Chem. 1985, 89, 2058.
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J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007 8523

A R T I C L E S
Goh et al.
nm, respectively. Figure 5 shows POMs of the N*-LCs,
System-1 and System-2. System-1 gave a fingerprint texture
with striae (Figure 5a). The distance between the striae
(2.5 µm) corresponds to a half of the helical pitch in N*-LC.
Meanwhile, the POM of System-2 gave a fan-shaped texture,
but no striae was observed (Figure. 5b). This is due to the fact
that the distance between the striae formed in System-2 is too
small to be detected in the POM microscope with a resolution
limit of ca. 1 µm.
The helical senses of the N*-LCs were examined through a
miscibility test4c,17 and a selective light reflection in circular
dichroism (CD) spectra.13a,18 Whereas the miscibility test can
be used regardless of length in helical pitch, the selective light
reflection method can be used only when the helical pitch is
between the UV and near-infrared region (from 250 nm to
1 µm). This is the reason why we used both methods to evaluate
the helical sense for System-1 (helical pitch of 5 µm) and
System-2 (helical pitch of 270 nm). However, we will hereafter
focus on only the results of the selective light reflection method.
The results of the miscibility test are given in the Supporting
Information. It is known that the cholesteryl oleyl carbonate is
a left-handed cholesteric LC showing a selective light reflection
in the visible region. Therefore, cholesteryl oleyl carbonate was
used as a reference for determining the handedness of the N*-
LC. As shown in Figure 6, the N*-LC inducing (R)-D-2 showed
a negative sign, while the N*-LC including (S)-D-2 and
cholesteryl oleyl carbonate showed peaks of positive sign in
the CD spectra. These results indicate that the handed senses
of the (R)- and (S)-N*-LCs are right- and left-handed, respec-
tively.
Figure 3. Selective light reflection spectra of N*-LCs. The N*-LCs of
curves a, b, c, and d contain 1.5, 1.0, 0.75, and 0.5 mol % of chiral dopant
(R)-D-2, respectively. Photographs of the selective light reflections,
wavelengths of the selective light reflection, and helical pitches of N*-LCs
are shown below.
Synthesis of Helical Polyacetylene. As seen in Figure 5,
the N*-LCs of System-1 and System-2 can be regarded as
weakly and highly twisted asymmetric reaction fields, respec-
tively. Thus, two kinds of N*-LCs were prepared as solvents
using Ziegler-Natta catalyst consisting of Ti(O-n-Bu)4 and Et3-
Al. The concentration of Ti(O-n-Bu)4 was 50 mM, and the mole
ratio of the cocatalyst to catalyst, [Et3Al]/[Ti(O-n-Bu)4], was
4.0. The catalyst solution was aged for 30 min at room
temperature. The polymerization temperature was kept constant
at 20 to 21 °C to maintain the N*-LC phase. The initial pressure
of acetylene gas was 140 to 150 torr, and the polymerization
time was 3 to 5 min. After polymerization, polyacetylene film
was carefully stripped off from the container and washed with
toluene several times and subsequently with a methanol solution
of hydrochloric acid (1 N) under argon gas at room temperature.
The film was dried with vacuum pumping on a Teflon sheet
and was stored in a freezer at -20 °C.
Figure 4. N*-LC-isotropic phase transition temperatures of the mixed
systems (PCH302, PCH304, and dopant) as a function of mole percentage
of chiral dopant (D-1 or D-2). The transition temperatures of N*-LC were
measured with DSC in the cooling process.
Figures 7 and 8 show scanning electron microscope (SEM)
photographs of the helical polyacetylene films synthesized in
right-handed N*-LC with a helical pitch of 5 µm [(R)-System-
1] and left-handed N*-LC with a helical pitch of 270 nm [(S)-
System-2], respectively. The hierarchical helical structures are
observed in both helical polyacetylene films. It is confirmed
that the screw directions of the fibril and even the bundle of
fibrils are opposite to the helical sense of the N*-LC. For
instance, in the right-handed N*-LC of (R)-System-1, the fibrils
are screwed left to form the bundle of fibrils (Figure 7).
From a viewpoint of reaction field, the stable and highly
twisted N*-LC is preferred. The N*-LC including D-2 of 1.5
mol % showed thermal stability and a small helical pitch of
270 nm. Meanwhile, the N*-LC including D-1 of 1.5 mol %
showed a relatively larger helical pitch of 3.2 µm and lower
thermal stability than in the case of D-2 (see Figure 4). Hence,
the N*-LCs including D-1 of 1 mol % {PCH302/PCH304/D-1
) 100:100:2 (mole ratio)} and D-2 of 1.5 mol % {PCH302/
PCH304/D-2 ) 100:100:3 (mole ratio)}, abbreviated as Sys-
tem-1 and System-2, respectively, were prepared as the
asymmetric reaction field for acetylene polymerization. The
helical pitches of System-1 and System-2 were 5 µm and 270
(17) Finkelmann, H.; Stegemeger, H. B. B. Phys. Chem. 1978, 82, 1302.
(18) (a) Fergason, J. L. Mol. Cryst. Liq. Cryst. 1966, 1, 293. (b) Saeva, F. D.;
Wysocki, J. J. J. Am. Chem. Soc. 1971, 93, 5928.
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8524 J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007

Helical Polyacetylene Nanofiber
A R T I C L E S
Figure 5. Polarizing optical micrographs of N*-LCs at 27 °C. The insets show the magnified photographs. (a) (S)-System-1 containing 1.0 mol % of chiral
dopant, (S)-D-1. (b) (R)-System-2 containing 1.5 mol % of chiral dopant, (R)-D-2.
Similarly, in the left-handed N*-LC of (S)-System-2, the fibrils
are screwed right, although no bundle is formed. The relationship
between the screw direction of the fibril and the helical sense
of the N*-LC has been partly rationalized in the previous work4c
and will be discussed in more detail elsewhere.19 It is of
particular interest that the highly twisted N*-LC (System-2)
gave the fibrils but not the bundle of fibrils (Figure 8c). This is
in quite contrast to the morphology of helical polyacetylene
synthesized in the moderately twisted N*-LC (System-1). It is
evident from Figures 7c and 8c that the polyacetylene fibrils
synthesized in (S)-Sysmtem-2 are more highly twisted than
those in (R)-System-1.
changing the mole percentage of the chiral dopant of D-2. Figure
9 shows a series of SEM photographs of helical polyacetylene
films synthesized in the above-mentioned N*-LCs. The N*-
LC with helical pitch of 2.3 µm gave the bundle of fibrils in
morphology of helical polyacetylene (Figure 9, parts a and b).
In the case of the N*-LC with a helical pitch of 850 nm, the
bundle of consisting of several fibrils were observed. The
interdistance between the bundles was less than 550 nm (Figure
9, parts c and d). When the N*-LC of helical pitch of 470 nm
was used, almost single fibrils were observed, but a part of the
fibrils were overlapped (Figure 9, parts e and f). These results
indicate that the morphology of helical polyacetylene is
dominated by the degree of helical pitch of the N*-LC but not
by the species of the chiral dopant. Figure 10 shows a schematic
representation of the relationship between the twisting degree
of the N*-LC and the hierarchical morphology of helical
polyacetylene. In the weakly twisted N*-LC, polyacetylene
In order to elucidate the relationship between the degree of
helical pitch of the N*-LC and the morphology of helical
polyacetylene, we prepared various N*-LCs with helical pitches
(2.3 µm, 850 nm, and 470 nm) between 5 µm and 270 nm, by
(19) (a) Akagi, K. Polym. Int., in press. (b) Akagi, K. Unpublished result, 2002.
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A R T I C L E S
Goh et al.
Figure 8. SEM micrographs of helical polyacetylene film synthesized in
the left-handed (S)-System-2 {PCH302/PCH304/(S)-D-2 ) 100:100:3 (mole
ratio)} with a helical pitch of 270 nm. The photographs of (b) and (c) show
the magnified one of (a).
Figure 6. Reflection CD spectra of the cholesteryl oleyl carbonate and
the N*-LCs induced by (R)-D-2 and (S)-D-2. (R)-N*-LC; PCH302/PCH304/
(R)-D-2 ) 100:100:2 (mole ratio), (S)-N*-LC; PCH302/PCH304/(S)-D-2
) 100:100:2 (mole ratio).
Figure 7. SEM micrographs of helical polyacetylene film synthesized in
the right-handed (R)-System-1 {PCH302/PCH304/(R)-D-1 ) 100:100:2
(mole ratio)} with a helical pitch of 5 µm. The photographs of (b) and (c)
show the magnified one of (a).
fibrils (whose diameters are from 70 to 120 nm) are gathered
to form the bundle of fibrils (whose diameters are up to 1 µm).
Interestingly, the interdistance between the bundles of fibrils is
close to half of the helical pitch. The bundle of fibrils can be
formed in the N*-LC whose helical pitches are up to 1 µm.
However, in the highly twisted N*-LC (whose helical pitch is
narrower than 1 µm), the polyacetylene has highly screwed
fibrils but not the bundle of fibrils. Therein, the diameters of
the fibrils are in the range from 70 to 120 nm. This may be due
to the fact that the helical pitch of 270 nm in the strongly twisted
N*-LC is smaller than the diameter (ca. 1 µm) of the bundle of
fibrils. This situation might prevent the formation of the bundle
of fibrils. The morphology free from the bundle of fibrils should
make it much easier to evaluate electromagnetic properties of
the screwed fibril.
Figure 9. SEM micrographs of helical polyacetylene films synthesized in
left-handed (S)-N*-LCs. (a and b) (S)-N*-LC containing 0.125 mol % of
(S)-D-2; helical pitch, 2.3 µm. (c and d) (S)-N*-LC containing 0.5 mol %
of (S)-D-2; helical pitch, 850 nm. (e and f) (S)-N*-LC containing 0.75 mol
% of (S)-D-2; helical pitch, 470 nm.
favorable product because of the so-called cis opening mech-
anism of acetylene polymerization, while the trans form is a
thermodynamically stable product.20 The cis form is actually
transformed to the trans form, depending on the degree of the
exothermal heat during the acetylene polymerization. Since the
exothermal heat is evolved in the polymerization at 20 °C, the
cis-trans isomerization is enhanced to give high trans content
in the present helical polyacetylene. Helical polyacetylene
Properties of Helical Polyacetylene. The helical polyacety-
lene films with morphology free from the bundle of fibrils have
high trans contents of 90%. This is mainly due to the
polymerization temperature such as 20-21 °C. It is known that
the cis form of the polyacetylene segment is a kinetically
(20) Chien, J. C. W., Ed. Polyacetylene; Academic Press: London, 1984; Chapter
2, p 24.
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8526 J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007

Helical Polyacetylene Nanofiber
A R T I C L E S
Figure 10. Relationship between the twisting degree of N*-LC and the morphology of helical polyacetylene.
showed high electrical conductivities such as ∼1.8 × 103 to
2.0 × 103 S/cm at room temperature after iodine doping.
Meanwhile, the polyacetylene films synthesized in Systems-1
and 2 have bulk densities of ca. 0.5 and 0.96 g/cm2, respectively.
This indicates that the single fibrils give a more closed packing
in morphology than the bundle of fibril structure.
(m, 8H, Ph-O-CH2-CH2-, Ar-O-CH2-CH2-), 6.80-8.03 (m,
26H, Ar-H). 13C NMR (CDCl3): δ ) 14.24, 22.82, 25.76, 26.19, 26.76,
29.26, 29.49, 29.56, 29.65, 32.32, 33.47, 33.76, 34.44, 34.68, 37.41,
37.48, 43.79, 44.35, 67.97, 69.83, 114.16, 116.13, 120.50, 125.29,
125.67, 125.91, 126.93, 127.12, 127.47, 129.21, 129.41, 133.19, 135.90,
138.66, 139.78, 139.90, 146.54, 154.44, 157.10. Specific rotation:
[R]25 ) -43.5 deg·dm-1·g-1·cm3.
589
Conclusions
(S)-2,2′-PCH5012-6,6′-PCH5-binaphthyl.Anal.Calcdfor(C112H158O4)n
(1568.45)n: C, 85.77; H, 10.15. Found: C, 85.09; H, 9.89. H NMR
1
The N*-LC induced by the novel tetrasubstituted binaphthyl
derivative (D-2) used as a chiral dopant gave rise to helical
pitches of 270-850 nm, depending on the concentration of the
chiral dopant. By virtue of the high helical twisting power and
liquid crystallinity of D-2, we obtained the N*-LC with helical
pitch of nano-order by adding high mole percentage of the chiral
dopant to the nematic LC without destruction of the liquid
crystalline phase. The helical pitches (270 to 850 nm) are smaller
than the radius (1 µm) of the bundle of fibrils of polyacetylene.
Especially, the highly screwed chiral nematic liquid crystal with
helical pitch of 270 nm depresses the formation of the bundle
of fibrils, resulting in a bundle-free fibril morphology consisting
of single fibrils. Thus, we found that the degree of screwing in
the chiral nematic reaction field is a key factor to control the
bundle formation and/or depression in fibril morphology of
polyacetylene.
(CDCl3): δ ) 0.87-1.53 (m, 100H, CH, CH2, CH3), 1.69-1.77 (m,
4H, CH2), 1.83-1.95 (m, 16H, cyclohexane, Ph-CH-CH2-), 2.35-
2.55 (m, 4H, cyclohexane, Ph-CH-CH2-), 3.88-4.00 (m, 8H, Ph-
O-CH2-CH2-, Ar-O-CH2-CH2-), 6.80-8.03 (m, 26H, Ar-H).
13C NMR (CDCl3): δ ) 14.10, 22.70, 25.63, 26.06, 26.63, 29.13, 29.38,
29.42, 29.52, 32.18, 33.59, 33.63, 34.31, 34.55, 37.28, 37.35, 43.67,
44.22, 67.84, 69.71, 114.02, 116.00, 120.38, 125.17, 125.55, 125.78,
126.70, 127.34, 128.61, 129.10, 129.28, 133.07, 135.78, 138.54, 139.66,
141.38, 146.42, 154.31, 156.95. Specific rotation: [R]25 ) +30.2
589
deg·dm-1·g-1·cm3.
(R)-2,2′-PCH506-6,6′-PCH5-binaphthyl (RC-2). Anal. Calcd for
(C100H134O4)n (1400.13)n: C, 85.78; H, 9.65. Found: C, 86.09; H, 9.67.
1H NMR (CDCl3): δ ) 0.87-1.52 (m, 80H, CH, CH2, CH3), 1.83-
1.95 (m, 16H, cyclohexane, Ph-CH-CH2-), 2.34-2.50 (m, 4H,
cyclohexane, Ph-CH-CH2-), 3.63-3.68 (t, 4H, J ) 6.26 Hz, Ph-
O-CH2-CH2-), 3.92-3.98 (m, 4H, Ar-O-CH2-CH2-), 6.69-8.01
(m, 26H, Ar-H). 13C NMR (CDCl3): δ ) 14.23, 22.82, 25.53, 26.77,
29.21, 29.40, 32.32, 33.71, 33.76, 34.44, 34.69, 37.43, 37.49, 43.78,
44.34, 67.64, 69.78, 114.12, 116.28, 120.63, 125.29, 125.72, 125.91,
126.93, 127.16, 127.42, 129.29, 129.47, 133.17, 136.0, 138.54, 139.68,
146.57,154.43,157.0.Specificrotation: [R]25589 )-50.1deg·dm-1·g-1·cm3.
It is expected that the highly twisted helical polyacetylene
without the bundle of fibrils might be feasible for evaluation
of unprecedented electromagnetic properties of a single fibril
of conducting polymer.
Experimental Section
Acknowledgment. This work was supported by a Grant-in-
Aid for Science Research in a Priority Area “Super-Hierarchical
Structures” (No. 446) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and partly by Iketani
Science and Technology Foundation.
(R)-2,2′-PCH506-binaphthyl (D-1). Anal. Calcd for (C66H86O4)n
(943.39)n: C, 84.03; H, 9.19. Found: C, 84.08; H, 9.21. 1H NMR
(CDCl3): δ ) 0.87-1.53 (m, 48H, CH, CH2, CH3), 1.84-1.87 (m,
8H, cyclohexane, Ph-CH-CH2-), 2.36-2.45 (m, 2H, cyclohexane,
Ph-CH-CH2-), 3.72 (t, 4H, J ) 6.42 Hz, Ph-O-CH2-CH2-), 3.86-
3.97 (m, 4H, Ar-O-CH2-CH2-), 6.76-7.96 (m, 20H, Ar-H). 13C
NMR (CDCl3): δ ) 14.13, 22.72, 25.41, 25.47, 26.67, 29.10, 29.33,
32.22, 33.68, 34.61, 37.32, 37.41, 43.74, 67.69, 69.69, 114.25, 115.94,
120.81, 123.43, 125.48, 126.02, 127.55, 127.78, 129.06, 129.29, 134.20,
139.91,154.50,157.18.Specificrotation: [R]25589 )+22.5deg·dm-1·g-1·cm3.
(R)-2,2′-PCH5012-6,6′-PCH5-binaphthyl (D-2). Anal. Calcd for
(C112H158O4)n (1568.45)n: C, 85.77; H, 10.15. Found: C, 85.90; H,
10.22. 1H NMR (CDCl3): δ ) 0.87-1.55 (m, 100H, CH, CH2, CH3),
1.69-1.77 (m, 4H, CH2), 1.83-1.95 (m, 16H, cyclohexane, Ph-CH-
CH2-), 2.35-2.55 (m, 4H, cyclohexane, Ph-CH-CH2-), 3.88-4.00
Supporting Information Available: Differential scanning
calorimeter (DSC), polarizing optical microscope (POM), and
X-ray diffraction (XRD) measurements of chiral dopant, D-2,
the equilibrium geometries of chiral dopants (D-1 and D-2),
the synthetic routes for RC-2, and the results of the miscibility
test. This material is available free of charge via the Internet at
http://pubs.acs.org.
JA070701X
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J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007 8527