J. Am. Chem. Soc. 2001, 123, 3619-3620

3619

Direct Measurement of the Chiral Quaternary
Structure in a π-Conjugated Polymer at RoomTemperature
Ken-ichi Shinohara,*,†,‡ Satoshi Yasuda,§ Gen Kato,‡
Machiko Fujita,§ and Hidemi Shigekawa*,§,|,⊥
Center for Interdisciplinary Research andDepartment 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 advancedmaterials for photonic or electronic applications. If the π-conjugated polymer chain can be controlled in the higher order structure, 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 havealready been completed.1-6 Most of these studies have provided
us with data on molecular aggregates or data on the average ofmany molecules. Although we now understand that the main chainof the polymer takes the form of a helix, does one chain haveboth right- and left-handed helices? What is the ratio of the righthanded 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 toestablish a technique that can determine the structure at the singlemolecule level in order to achieve the above-mentioned objective.
Here we show that a scanning tunneling microscope (STM)7,8
allows us to see the π-electron orbital of a chiral quaternarystructure, where it was directly observed and its size was evenmeasured 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 molecularweight is of an order of 1 × 106 Da, that the cis content of thepolymer 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(1H 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 andhidemi@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,S. Science 1995, 268, 1860-1866.
(2) Akagi, K.; Piao, G.; Kaneto, S.; Sakamaki, K.; Shirakawa, H.; Kyotani,M. Science 1998, 282, 683-686.
(3) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449-451.
(4) Aoki, T.; Kokai, M.; Shinohara, K.; Oikawa, E. Chem. Lett. 1993,2009-2012.
(5) Aoki, T.; Shinohara, K.; Kaneko, T.; Oikawa, E. Macromolecules 1996,29, 4192-4198.
(6) Pu, L. Acta Polym. 1997, 48, 116-141.
(7) Driscoll, R.; Youngquist, M.; Baldeshwieler, J. Nature 1990, 346, 294296.
(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.
1990, 11, 559-563.

Figure 1. (a) Chemical structure of (-)-poly(MtOCAPA). (b) Circulardichromism and (c) UV-vis spectra of (-)-poly(MtOCAPA) in tetrahydrofuran solution at 15 (blue line), 30 (black line), and 45 °C (red line),
respectively.

also in the main chain, which had expanded π-conjugation. Itwas found from these observations that the optically active menthyl in the side group induced chirality in the π-conjugated systemof the main chain. This CD spectrum shows that the main chainof (-)-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 helixstructure.
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 menthylgroup and the main chain was hidden behind the side groups onthe front. To observe the main chain without being obstructedby the bulky menthyl group, STM was used, since this allows us
to see molecular orbitals having the low-energy gap of theHOMO-LUMO on which a tunneling current flows.10-12 Figure
2a shows a typical low magnification image of the low current
STM of (-)-poly(MtOCAPA) placed on a highly oriented pyrolytic graphite (HOPG) substrate under ambient conditions.13 During our observation, the sample bias voltage and the tunnelingcurrent 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
scanning rate.14 In Figure 2a, it can be observed that the two
π-conjugated polymer chains are intertwined to form a right(10) Porath, D.; Bezryadln, A.; de Vries, S.; Dekker: C. Nature 2000, 403,635-638.

10.1021/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,Vs ) +20.0 mV and It ) 30.5 pA. (b) Molecular mechanics calculated
optimized model of 20-mer of cis-transoidal (-)-poly(MtOCAPA). Sideview (upper). Top-view (bottom).
Figure 2. (a) Low current STM height image of intertwined two (-)poly(MtOCAPA) chains on HOPG at room temperature (Vs ) +20.0mV and It ) 30.5 pA). (b) String model of the intertwining polymer
chains.

handed double helix structure, as in the case of the model shownin Figure 2b, and the right-handed helix structure can also beobserved. 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 backbone of polyphenylacetylene in the π-electronic system of thecis-transoidal6 main chain structure 20-mer which was obtainedby optimization using the molecular mechanics calculation [Figure3b]. 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 [Figure4b], the pitch of this super-helix is 2 nm.15 It is shown to be asuper-helix with a close-packed structure, because the pitch agrees
with the width of the model in Figure 3b. In addition, it was shownin Figure 4b that the super-helix width is 2 nm, that the helix
sense is right-handed, and that this precisely controlled superhelix 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 areintertwined to form a right-handed double helix structure, as in
the case of the model shown in Figure 2b. This substantiated thepresence of a quaternary structure, which is much higher in thechiral hierarchical structure. It was also found that the quaternarystructure is so soft that its form can be changed during probe
scanning. That is, we proved that the main chain of the (-)-poly(MtOCAPA) was flexible despite the π-conjugated system.
In the present study, it was shown that it was possible todetermine 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, thedouble helix structure shown in Figure 2 were observed mostly in many places.
(14) Molecules on HOPG substrate were easy to move by the effect ofprobe scanning. In this study, STM parameters were chosen as to obtain themostly stable STM imaging condition. Moving of the object under the effectof the STM probing shows that it is not the well-known artifact image observedon HOPG surface. Because the artifacts are the modulated structure of theHOPG 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.

Figure 4. (a) A close-up view of STM height image of (-)-poly(MtOCAPA) (Vs ) +20.0 mV and It ) 30.5 pA), (b) Analytical resultof 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 structure becomes loosened. Therefore, to determine the electronicstructures and the ratio of the two different helical-senses by theeffect of the chiral pendant groups, we need to take an approach
where we stabilize the STM measurements by fixing the molecules on the substrate, which is currently in progress. Finally,
this unique higher-order structure of the π-electronic system isexpected to provide novel photonic or electronic functions thatwill lead us toward molecular devices.
Acknowledgment. We thank Professor Dr. Toshiki Aoki in Graduate
School of Science and Technology, Niigata University, for our using theCD spectroscope.
Supporting Information Available: Experimental details of GPC,CD, STM, molecular mechanics (MM) calculation, and synthesis of thepolymer (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.

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