1378

Chemistry Letters Vol.36, No.11 (2007)

Single Molecular Imaging of a micro-Brownian Motion and a Bond Scissionof a Supramolecular Chiral -Conjugated Polymer as a Molecular Bearing
Driven by Thermal Fluctuations
1Ken-ichi Shinohara,1;3 Noriyuki Kodera,2 and Toshio Ando2;3
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahi-dai, Nomi 923-1292
2Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192
3SENTAN, Japan Science and Technology Cooperation (JST)(Received August 31, 2007; CL-070940; E-mail: shinoken@jaist.ac.jp)

The micro-Brownian motion of a single polymer chain was
directly measured by high-speed (rapid-scan) AFM on a mica
substrate under an aqueous solution at room temperature. The
diﬀusion constant was 1:3  1014 m2 /s, which is a basic property of a molecular bearing at a solid–liquid interface. The molecule was a micrometer-long supramolecular chain in which conjugated polyrotaxane was coupled by a – interaction.
Bond scission of this supramolecular chain was directly observed, and the dynamic folding behavior of a single polymer
chain was also imaged.

moves. While conducting an experiment, we envisioned a new
design concept for a molecular device that has the ﬂexibilityof a single molecule. We collected images for a total of
14.08 s and irradiated with an ultraviolet (UV) pulsed laser during the last 5.6 s as shown in Figure 1c. During the pulsed laser
irradiation, the cantilever was bent owing to the light pressure of
the laser beam. Thus, the probe pressed the observation surface
for an instant. The scanning line, which looks like a scratch,showed this state from 8.48 s to 14.08 s. Hence, we observed
the probe pressing onto a single polymer chain with a high probe

Polymers are very useful materials that display many excellent properties and have become indispensable in maintaining
and developing our current way of life. Especially, -conjugatedpolymers are recognized as part of the next-generation of functional polymers for photonic and electronic applications.1–3 Nevertheless, it is diﬃcult to discuss the correlation between theirstructures and properties at the molecular level because polymers are diverse, dynamic and can be very complex. If the structure and properties of a polymer could be directly observed, thenwith minimal inferences or hypotheses, the relationship between
polymer structures and functions could be clariﬁed. Consequently, molecular devices of a polymer might be created on the basisof new design concepts and new working principles, which maylead to soft nanomachines. Recently, we have directly observed
single molecules in a -conjugated polymer at room temperatureusing a scanning tunneling microscope (STM)4 and an atomic
force microscope (AFM)5 to measure a chiral helical structure
and a total internal reﬂection ﬂuorescence microscope (TIRFM)
to image single molecular dynamic ﬂuorescence.6,7
-Conjugated polymers may be ﬂexible because they are organic molecules. We have used high-speed (rapid-scan) AFM8
to observe their ﬂexibility and motion driven by thermal ﬂuctuations. The observed polymer is the same as those in previous
research where the molecular structure and functions were simultaneously observed using a combination of an AFM and an
objective-type TIRFM.9 Figures 1a and 1b show the chemicalstructure and optimized molecular model, respectively. This-conjugated polymer has a molecular bearing structure, which
is a kind of soft nanomachines. Figure 1c shows the ﬂexible motion of a single chain of the supramolecular -conjugated polymer on mica under an aqueous solution at room temperature. Because its height, which is ca. 1.5 nm, corresponds to the height of-cyclodextrin, the polymer chain is clearly a single molecule asshown in Figure 1d. Various studies have been trying to apply
rigid rod-like -conjugated polymers to a molecular wire formolecular electronics devices, but it is troublesome if the wire

Figure 1. (a) Chemical structure of a polyrotaxane: (+)-poly[AEPErotaxa-(-CyD)]. (b) A molecular mechanics calculation optimized
model of a supramolecular polymer. (c) High-speed AFM images ofa single molecule of a supramolecular chiral -conjugated polymer
on mica under aqueous solution at room temperature. (8.48–14.08 s)
UV pulsed laser of 355 nm irradiating 20 nJ at 20 Hz. (13.92–14.08 s) The dynamic folding behavior of the cut end in a single polymer chain. (8.64 s) The imaging of chain scission in a single molecule.
The point of a scission is indicated by a red arrow. XY range:
400 nm  400 nm. Z range: 5.0 nm. Scan rate: 160 ms per frame. Pixel:
100  100. Dynamic images of single molecular (+)-poly[AEPErotaxa-(-CyD)] as a Supporting Information.10 (d) A cross sectional
result of single polymer chain on the red line indicated in a high-speed
AFM image at 0 s as shown in Figure 1c.

Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.11 (2007)

1379Figure 2. (a) Analytical results of the displacement in a single polymer chain of (+)-poly[AEPE-rotaxa-(-CyD)] at scanning rates of 80, 120, 150,181.8, 200, 250, and 300 ms per frame in high-speed AFM imaging (without UV pulsed laser irradiation). Lines are the best ﬁts of the data to agaussian curve; the standard deviations are 5.81, 6.40, 5.76, 7.43, 6.49, 7.76, and 8.99 nm, respectively. (b) Mean-square displacement as a functionof time of tip-scanning cycle in the direction of the X axis. The mean-square displacements were obtained from the standard deviations in Figure 2a.
The plots were ﬁtted to equation, hx2 i ¼ 2Dt. A diﬀusion constant of a micro-Brownian motion is 1:3  1014 m2 /s.

tip pressure while irradiating, which subsequently cut the single
polymer molecule as shown in the image at 8.64 s. We also observed a folding phenomenon, i.e., the cut end folded dynamically (from 13.92 to 14.08 s). The fact that the probe tip pressure cut
the polymer chain supports our conclusion that the polymer is asupramolecular polymer coupled by molecular interaction because molecular interaction force is weaker than a covalentbond. Thus, information in terms of structural chemistry canbe obtained. Figure 1c shows the images under an aqueous solution at room temperature. We consider that this is a micro-Brownian motion, which is excited by the Brownian motion of watermolecules at room temperature. Unlike STM, AFM uses the AC(tapping) mode and the probe touches the surface. It is speculated that molecules move mainly owing to the touching probe tip,
although there may also be Brownian motion. Hence, we conducted a detailed analysis. We changed the probe scanning speed
and analyzed the one-dimensional displacement of a single polymer molecule in the probe scanning direction. In practice, wechanged the frame rate to 80, 120, 150, 181.8, 200, 250, and300 ms per frame, collected images under the seven diﬀerentconditions, and analyzed the displacement. We made a histogram of these analysis results with the displacement as the horizontal axis and the observation number as the vertical axis, andﬁtted the data to the Gaussian curve as shown in Figure 2a.
Finally, we plotted the square of the standard deviation of this
distribution (i.e., mean-square displacement, hx2 i) and the probe
scanning cycle time (t) as shown in Figure 2b. The result showsthat the mean-square displacement in a single supramolecularpolymer chain is proportional to the time and, hence, complies
with Einstein’s Law of Brownian motion.

hx2 i ¼ 2Dt

ð1Þ

Here, D is the diﬀusion coeﬃcient. In this result, the D value
is calculated as 1:3  1014 m2 /s in the one-dimensionalmotion. Hence, this motion is a Brownian motion at solid–liquidinterface, and the motion in a single polymer chain is a microBrownian motion, i.e., thermal motion.
We anticipate that our studies will be the basis for creating
innovative molecular devices such as molecular motors ormolecular electronics/photonics materials, which utilize thermal
stochastic energy as the driving source.
This research was ﬁnancially supported by the Grantin-Aids for Scientiﬁc Research for Young Scientists (A) from
the Ministry of the Education, Culture, Sports, Science andTechnology (MEXT), Japan (No. 15685007) (to K. S.).
References and Notes
1 R. E. Martin, F. Diederich, Angew. Chem., Int. Ed. 1999, 38, 1350.
2 H. Meier, Angew. Chem., Int. Ed. Engl. 1992, 31, 1399.
3 H. Bleier, Organic Materials for Photonics, Elsevier, Amsterdam, 1993,
p. 77.
4 K. Shinohara, S. Yasuda, G. Kato, M. Fujita, H. Shigekawa, J. Am. Chem.
Soc. 2001, 123, 3619; Editors’ Choice, Science 2001, 292, 15.
5 K. Shinohara, T. Kitami, K. Nakamae, J. Polym. Sci., Part A: Polym. Chem.
2004, 42, 3930.
6 K. Shinohara, S. Yamaguchi, H. Higuchi, Polym. J. 2000, 32, 977.
7 K. Shinohara, S. Yamaguchi, T. Wazawa, Polymer 2001, 42, 7915.
8 T. Ando, N. Kodera, E. Takai, D. Maruyama, K. Saito, A. Toda, Proc. Natl.
Acad. Sci. U.S.A. 2001, 98, 12468.
9 K. Shinohara, T. Suzuki, T. Kitami, S. Yamaguchi, J. Polym. Sci., Part A:Polym. Chem. 2006, 44, 801.
10 Supporting Information is also available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.

Published on the web (Advance View) October 20, 2007; doi:10.1246/cl.2007.1378