Direct measurement of the chiral quaternary structure in a π-conjugated polymer at room temperature

Full text of DOI:10.1021/ja003023m

J. Am. Chem. Soc. 2001, 123, 3619-3620
Direct Measurement of the Chiral Quaternary
3619
Structure in a π-Conjugated Polymer at Room
Temperature
,†,‡
§
‡
Ken-ichi Shinohara,* Satoshi Yasuda, Gen Kato,
§
,§,|,⊥
Machiko Fujita, and Hidemi Shigekawa*
Center for Interdisciplinary Research and
Department of Metallurgy
Graduate School of Engineering
Tohoku UniVersity, Aoba-yama 02
Sendai 980-8579, Japan
Institute of Applied Physics and CREST-JST
The UniVersity of Tsukuba
Tsukuba 305-8573, Japan
ReceiVed August 14, 2000
π-Conjugated polymers have been developed as advanced
materials for photonic or electronic applications. If the π-conju-
gated polymer chain can be controlled in the higher order struc-
ture, novel functions at the molecular level will become available
due to the unique π-electron system. Many studies confirming
the fact that a π-conjugated polymer has a helical structure have
already been completed.1-6 Most of these studies have provided
us with data on molecular aggregates or data on the average of
many molecules. Although we now understand that the main chain
of the polymer takes the form of a helix, does one chain have
both right- and left-handed helices? What is the ratio of the right-
handed helices to the left-handed ones? What about the regions
where the helix is reversed, and how does it dynamically change?
The answers already provided to all these fundamental questions
have been based only on conjecture. Therefore, it is necessary to
establish a technique that can determine the structure at the single-
molecule level in order to achieve the above-mentioned objective.
Figure 1. (a) Chemical structure of (-)-poly(MtOCAPA). (b) Circular
dichromism and (c) UV-vis spectra of (-)-poly(MtOCAPA) in tetrahy-
drofuran solution at 15 (blue line), 30 (black line), and 45 °C (red line),
respectively.
Here we show that a scanning tunneling microscope (STM)7
,8
also in the main chain, which had expanded π-conjugation. It
was found from these observations that the optically active men-
thyl in the side group induced chirality in the π-conjugated system
of the main chain. This CD spectrum shows that the main chain
of (-)-poly(MtOCAPA) has a secondary structure having an
excess of the one-sense helix structure, which is stabilized by
the bulky substituent. Because the intensity of this CD signal
increased in reverse proportion to the temperature of the polymer
solution, it is presumed that this main chain has a flexible helix
structure.
Scanning probe microscope imaging was used to directly
observe the structure of a single polymer chain. We initially tried
to use an atomic force microscope (AFM) image of a polymer
placed on a mica substrate under ambient conditions. Each single
polymer molecule that formed a chain could be distinguished,
and intertwined polymer chains could also be observed. An overall
image of the polymer molecules could be directly observed using
the AFM, but the π-conjugated system in the main chain could
not be observed by AFM since this polymer had a bulky menthyl
group and the main chain was hidden behind the side groups on
the front. To observe the main chain without being obstructed
by the bulky menthyl group, STM was used, since this allows us
to see molecular orbitals having the low-energy gap of the
allows us to see the π-electron orbital of a chiral quaternary
structure, where it was directly observed and its size was even
measured with high resolution.
We synthesized a π-conjugated polymer, an optically active
polyphenylacetylene bearing menthoxycarbonylamino groups
[
(-)-Poly(MtOCAPA), Figure 1a]. It was found that (-)-poly-
(MtOCAPA) has a high molecular weight, that is, its molecular
6
weight is of an order of 1 × 10 Da, that the cis content of the
polymer is 90 mol %, and the main chain has a cis-transoidal
high stereo-regularity.6,9 The primary structure of this polymer
was examined using a nuclear magnetic resonance spectrometer
1
(
H NMR). As is apparent from the spectrum of this polymer
observed using a circular dichroism spectrometer [CD, Figure 1b]
and an ultraviolet-visible absorption spectrometer [UV-vis,
Figure 1c], chirality was detected not only in the side group but
*
Authors for correspondence. E-mail: shinoken@material.tohoku.ac.jp and
hidemi@ims.tsukuba.ac.jp.
†
CIR, Tohoku University.
‡
Graduate School of Engineering, Tohoku University.
§
The University of Tsukuba.
|
The University of Tokyo.
⊥
CREST-JST.
(
1) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson,
1
0-12
S. Science 1995, 268, 1860-1866.
HOMO-LUMO on which a tunneling current flows.
a shows a typical low magnification image of the low current
STM of (-)-poly(MtOCAPA) placed on a highly oriented pyro-
Figure
(
2) Akagi, K.; Piao, G.; Kaneto, S.; Sakamaki, K.; Shirakawa, H.; Kyotani,
2
M. Science 1998, 282, 683-686.
(
3) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449-451.
13
(
4) Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett. 1993,
lytic graphite (HOPG) substrate under ambient conditions. Dur-
ing our observation, the sample bias voltage and the tunneling
current were maintained at +20.0 mV and 30.5 pA, respectively,
and an STM probe using a Pt/Ir tip was operated at a 3.05 Hz
2
009-2012.
(
5) Aoki, T.; Shinohara, K.; Kaneko, T.; Oikawa, E. Macromolecules 1996,
2
9, 4192-4198.
(
6) Pu, L. Acta Polym. 1997, 48, 116-141.
(
7) Driscoll, R.; Youngquist, M.; Baldeshwieler, J. Nature 1990, 346, 294-
14
scanning rate. In Figure 2a, it can be observed that the two
2
96.
π-conjugated polymer chains are intertwined to form a right-
(8) Shigekawa, H.; Miyake, K.; Sumaoka, J.; Harada, A.; Komiyama, M.
J. Am. Chem. Soc. 2000, 122, 5411-5412.
9) Lee, D.-H.; Lee, D.-H.; Soga, K. Makromol. Chem., Rapid Commun.
990, 11, 559-563.
(
(10) Porath, D.; Bezryadln, A.; de Vries, S.; Dekker: C. Nature 2000, 403,
635-638.
1
1
0.1021/ja003023m CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/21/2001

3620 J. Am. Chem. Soc., Vol. 123, No. 15, 2001
Communications to the Editor
Figure 3. (a) STM height image of (-)-poly(MtOCAPA), Bar: 5.0 nm,
V
s
) +20.0 mV and I ) 30.5 pA. (b) Molecular mechanics calculated
t
optimized model of 20-mer of cis-transoidal (-)-poly(MtOCAPA). Side-
view (upper). Top-view (bottom).
Figure 2. (a) Low current STM height image of intertwined two (-)-
poly(MtOCAPA) chains on HOPG at room temperature (V
s
) +20.0
) 30.5 pA). (b) String model of the intertwining polymer
mV and I
chains.
t
handed double helix structure, as in the case of the model shown
in Figure 2b, and the right-handed helix structure can also be
observed. The analytical result is shown in Figures 3 and 4. The
width of one right-handed helix chain was 0.9 nm [Figure 3a].
This width of 0.9 nm matched the width of the main chain back-
bone of polyphenylacetylene in the π-electronic system of the
6
cis-transoidal main chain structure 20-mer which was obtained
by optimization using the molecular mechanics calculation [Figure
3
b]. This corroborated the observations that the STM image
displays the π-electron orbital of the main chain of a polymer
and that the helix of the secondary structure is a super-helix
tertiary structure [Figure 4a]. As a result of the analyses [Figure
4
b], the pitch of this super-helix is 2 nm.15 It is shown to be a
super-helix with a close-packed structure, because the pitch agrees
with the width of the model in Figure 3b. In addition, it was shown
in Figure 4b that the super-helix width is 2 nm, that the helix
sense is right-handed, and that this precisely controlled super-
helix tertiary structure extends over a range of more than 10 nm.
When the polymer structure in Figure 2a was turned over, it could
be observed that the two π-conjugated polymer chains are
intertwined to form a right-handed double helix structure, as in
the case of the model shown in Figure 2b. This substantiated the
presence of a quaternary structure, which is much higher in the
chiral hierarchical structure. It was also found that the quaternary
structure is so soft that its form can be changed during probe
scanning. That is, we proved that the main chain of the (-)-poly-
Figure 4. (a) A close-up view of STM height image of (-)-poly-
(MtOCAPA) (V ) +20.0 mV and I ) 30.5 pA), (b) Analytical result
s
t
of super-helix pitch in the cross section of the STM image.
structure of the single π-conjugated polymer chains was confirmed
for the first time. However, in our current method, the molecule
itself is changed by the STM measurement, and the helical stru-
cture becomes loosened. Therefore, to determine the electronic
structures and the ratio of the two different helical-senses by the
effect of the chiral pendant groups, we need to take an approach
where we stabilize the STM measurements by fixing the mol-
ecules on the substrate, which is currently in progress. Finally,
this unique higher-order structure of the π-electronic system is
expected to provide novel photonic or electronic functions that
will lead us toward molecular devices.
Acknowledgment. We thank Professor Dr. Toshiki Aoki in Graduate
School of Science and Technology, Niigata University, for our using the
CD spectroscope.
Supporting Information Available: Experimental details of GPC,
CD, STM, molecular mechanics (MM) calculation, and synthesis of the
polymer (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(MtOCAPA) was flexible despite the π-conjugated system.
In the present study, it was shown that it was possible to
determine the structure of a single polymer chain using STM.
We observed that it has a higher-order structure, and the fine
(
11) Magonov, S. N.; Bar, G.; Cantow, H.-J.; Paradis, J.; Ren, J.; Whangbo,
M.-H.; Yagubskii, E. B. J. Phys. Chem. 1993, 97, 9170-9176.
12) Yoshimura, M.; Shigekawa, H.; Nejoh, H.; Saito, G.; Saito, Y.;
Kawazu, A. Phys. ReV. B 1991, 43, 13590-13593.
13) Although the single polymer chains were also observed in part, the
double helix structure shown in Figure 2 were observed mostly in many places.
14) Molecules on HOPG substrate were easy to move by the effect of
(
(
(
probe scanning. In this study, STM parameters were chosen as to obtain the
mostly stable STM imaging condition. Moving of the object under the effect
of the STM probing shows that it is not the well-known artifact image observed
on HOPG surface. Because the artifacts are the modulated structure of the
HOPG surface, they never move even if they are scanned repeatedly.
(
15) These values showed high-reproducibility in the stably obtained STM
images of the polymer chains.
JA003023M