690

Chemistry Letters Vol.38, No.7 (2009)

Single-Molecule Imaging of a Micro-Brownian Motion of a Chiral Helical -Conjugated
Polymer as a Molecular Spring Driven by Thermal Fluctuations
Ken-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
2
Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192
3SENTAN, Japan Science and Technology Cooperation (JST)1(Received April 20, 2009; CL-090394; E-mail: shinoken@jaist.ac.jp)
We observed a micro-Brownian motion of single moleculesof a chiral helical -conjugated polymer as a molecular springon mica substrate under n-octylbenzene at room temperature using high-speed (rapid scan) AFM. The average velocity at thechain end of the polymer was 67 nm s1 at the observation point.
The analytical result shows that the mean square displacement ina single polymer chain is proportional to the time and, hence,complies with Einstein’s law of Brownian motion. The diﬀusion
constant (D) is measured to be 8:2  1015 m2 s1 at the solid/
liquid interface.

Polymers are ubiquitous materials that display many excellent properties, and they have become indispensable in maintaining and developing our current way of life. In particular, -conjugated polymers are recognised as being part of the next generation of functional polymers for photonic and electronic applications.1–3 Nevertheless, it is diﬃcult to discuss the correlation
between their structures and properties at a molecular level,
since these are diverse, dynamic, and, in general, very complex.
If the structure and properties of a polymer could be directly observed, with minimal inferences or hypotheses, the relationshipbetween polymer structures and functions could be clariﬁed.
Consequently, molecular devices of a polymer might be createdbased on new design concepts and new working principles, may
be the soft nanomachines. Recently, we have achieved the direct
observation of single molecules in a -conjugated polymer
at room temperature using a scanning tunneling microscope(STM)4 and an atomic force microscope (AFM)5 to measure
the chiral helical structure. In addition, we succeeded in usinga total internal reﬂection ﬂuorescence microscope (TIRFM) toimage single-molecule dynamic ﬂuorescence6,7 and a combination of an AFM and an objective-type TIRFM to simultaneous
image the molecular structure and a novel ﬂuorescent phenomenon.8
In this study, we observed micro-Brownian motion of asingle molecule of a chiral helical -conjugated polymer(Figure 1),4,5,9 which has a structure of a molecular machine,namely a molecular spring, using a high-speed (rapid-scan)
AFM at a solid/liquid interface. A high-speed AFM oﬀers outstanding performance for observing the movement of single
molecules in aqueous solution at room temperature.10–12 However, we modiﬁed the speciﬁcations of this high-speed AFM in order to use even in the organic solvent, allowing successful imaging of the movement of a single molecule of a chiral helical conjugated polymer at room temperature. In addition, we measured the ﬂexibility of a single polymer chain as a fundamental
property of a molecular spring.

Figure 1. Chemical structure of ()-poly(MtOCAPA). The appearance is a yellow solid. Mw ¼ 1:05  106 , Mw =Mn ¼ 3:30
(GPC), cis % >90 (1 H NMR), ½25(c 0.0183,D ¼ 499
14CHCl3 ), (see Supporting Information ).
We cast tetrahydrofuran (THF, 50 mL  3) on freshly cleaved
mica for removing water and cast n-octylbenzene (5 mL) immediately. This allowed us to cast a dilute tetrahydrofuran (THF) solution (106 mol L1 , 5 mL) of a chiral helical -conjugated substituted phenylacetylene polymer [()-poly(MtOCAPA)] (Figure 1)
on a mica substrate for single-molecule imaging. Afterward, weconducted high-speed AFM observations in n-octylbenzene atroom temperature13 and measured the ﬂexible motion of astring-like substance 2 nm in height and 200 nm long (Figures 2aand 2b). The average velocity at the chain end of the polymer was
67 nm s1 at the observation point. Because the size of the stringtype substance corresponds to the value of the molecular model
calculation (Figure 2c), we concluded that the observed substanceis a single molecule of the chiral helical -conjugated polymer.
In particular, the zigzag structure is conﬁrmed in the 48th framein Figure 2a with a structural period, measured to be 24:0 
4:0 nm, based on the coarse and dense (compressional) helicalstructure of a polymer chain.5 When several molecules are simultaneously observed, their motion is not synchronous but rather
random (Figure 2d). We directly observed the reversible motionof elongation, curvatures, and folding of a single polymer chain.
It was found that single molecules of ()-poly(MtOCAPA) havesuitable ﬂexibility of their -conjugated main chain to serve as amolecular spring. In addition, this polymer chain was diﬃcult toform a molecular globule, because it has -conjugated mainchain. Furthermore, most of the polymer chains were not detachedfrom the substrate surface which would otherwise lead to diﬀusionin the solvent but rather were rolled onto the substrate surface(Figures 2a and 2d). This dynamic behavior can be explained by
the energy state of a single polymer chain adsorbed to a mica
surface in n-octylbenzene at room temperature.
We hypothesize that this behavior is due to thermal motion,
which reﬂects a compensation of energy of solvation and adsorption onto the substrate surface. To validate this conclusion, weconducted a detailed motional analysis. We measured the dimensional displacements of a single polymer chain in the X-direction
and ﬁtted it to a Gaussian curve (Figure 3a). The standard devi-Copyright  2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.7 (2009)

691

Figure 3. (a) Typical analytical results of the dimensional displacement in a single polymer chain of ()-poly(MtOCAPA)
in the X-direction at scanning rates of 82, 100, and 160 ms perframe in high-speed AFM imaging at a solid/liquid interface.
Lines are the best ﬁts of the data to a Gaussian curve; the standard deviations are 3.5, 4.0, and 4.8 nm, respectively. (b) Meansquare displacement as a function of time of tip-scanning cyclein the direction of the X axis. The mean-square displacements
were obtained from the standard deviation in (a). The plots wereﬁtted to the diﬀusion equation, hx2 i ¼ 2Dt.
This work was supported partly by Gtant-in-Aid for Young
Scientists (A) from MEXT, Japan (No. 1568007) (to K.S.), andalso by SENTAN, JST (to K.S. and T.A.).
Figure 2. High-speed AFM images in continuous scanningmode of a chiral helical -conjugated polymer [()-poly(MtOCAPA)] on mica in n-octylbenzene at room temperature.
(a) The observation area is 200  200 nm2 . The frame rate is252 ms per frame. Pixel: 100  100. (b) Analytical result of aline proﬁle in an AFM image of the 14th frame in (a). (c) An optimized molecular model of a polymer. (d) The area is 300  300
nm2 . The frame rate is 156 ms per frame. Pixel: 100  100.
Dynamic images of a single molecule of ()-poly(MtOCAPA)
in a Supporting Information.14
ation values were the time-average displacements at each observation time. The result shows that the mean-square displacement
in a single polymer chain is proportional to time and, hence,complies with Einstein’s law of Brownian motion (Figure 3b).
The diﬀusion constant (D) is measured to be 8:2  1015 m2 s1 .
Though the diﬀusion constant is low, this is the eﬀect of the adsorption of a molecule on the substrate. This result demonstrates
that the motion of a single polymer chain is micro-Brownian
motion at a mica/n-octylbenzene interface.
This polymer chain has a ﬂexible -conjugated chiral helix,
which serves as a structure of a molecular spring. A study ofhigh-speed AFM imaging of dynamical structural change ofthe coarse and dense (compressional) helix5 inside the single
chain is in progress. We anticipate that our studies will form
the basis for creating innovative molecular devices such as molecular motors or molecular electronics/photonics materials thatutilize thermal stochastic energy as one of the driving sources.

References and Notes
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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 K. Shinohara, T. Suzuki, T. Kitami, S. Yamaguchi, J. Polym. Sci.,
Part A: Polym. Chem. 2006, 44, 801.
9 V. Percec, J. Rudick, E. Aqad, Macromolecules 2005, 38, 7205.
10 T. Ando, N. Kodera, E. Takai, D. Maruyama, K. Saito, A. Toda,Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 12468.
11 T. Ando, T. Uchihashi, T. Fukuma, Prog. Surf. Sci. 2008, 83, 337.
12 K. Shinohara, N. Kodera, T. Ando, Chem. Lett. 2007, 36, 1378.
13 Amplitude of cantilever oscillation (Ap-p ) is 5 nm (free), and a set
point is 0.9. Although THF is a good solvent for this polymer,
n-octylbenzene has low miscibility.
14 Supporting Information is available electronically on the CSJJournal Web site, http://www.csj.jp/journals/chem-lett/index.
html.

Published on the web (Advance View) June 6, 2009; doi:10.1246/cl.2009.690