Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 3888–3891
Part 1, No. 6B, June 1999c°1999
Publication Board, Japanese Journal of Applied Physics

Effect of the Dipole-Dipole Interaction on the Self-Assemblyof Cyclodextrin Inclusion Complexes
Satoshi YASUDA1 , Koji M IYAKE1 , Jun S UMAOKA2 , Makoto KOMIYAMA2, ∗ and Hidemi S HIGEKAWA1,2,∗
1 Institute of Applied Physics, and CREST, Japan Science and Technology Corporation, University of Tsukuba, Tsukuba 305-8573, Japan
2 Department of Chemistry and Biotechnology, Graduate School Engineering, University of Tokyo, Tokyo 113-8656, Japan(Received February 15, 1999; accepted for publication April 2, 1999)

We performed scanning tunneling microscopy (STM) observations on self-assembled α-cyclodextrin (CyD) inclusion complexes on HOPG for the guest compounds of (1) H2 O, (2) MeOH (Methanol) and (3) p-NP ( p-nitrophenol). The observed
structures of α-CyD-H2 O and α-MeOH complexes were channel-like with twofold symmetry, however, α-CyD- p-NP showeda structure different from that of the others, namely, a threefold symmetric arrangement. On the other hand, the dipole momentscalculated for α-CyD-H2 O and α-MeOH complexes were similar and directed from the secondary to the primary hydroxyl
groups of the α-CyD. However, the dipole moment on the α-CyD- p-NP complex was oblique to the axis for the other twocomplexes. Taking into account this difference in the dipole moment and the hydrogen bonding between the CyDs, the structural differences observed by STM could be explained comprehensively. The obtained results indicate the existence of a highpotential for the structural control of the self-assembled α-CyD complexes by changing the guest compounds.
KEYWORDS: STM, Cyclodextrin, self-assemble, inclusion complex, dipole moment

1. Introduction
Recently, self-assembly of molecular arrangements has
been attracting considerable attention because of its high potential as a nano-science technology for the realization of desired materials. Since the ordering of a self-assembled molecular arrangement depends on the intermolecular forces, microscopic analysis is important for understanding the selfassembly mechanism.
Scanning tunneling microscopy (STM) is a very powerfultool for studying nanostructures on an atomic scale and has
been widely used for the direct observation of self-assembled
molecules. Many STM observations have been conducted,
such as for linear n-alkanethiol,1) planar liquid crystal2, 3) andspherical Fullerene molecules.4) However, the intermolecularinteractions in these materials are mainly due to van der Waals
forces, and studies on molecules with anisotropic interactions
are still insufficient.
Cyclodextrins (CyDs) are cyclic oligosaccharides and consist of 6–12 glucopyranose units in a chair conformation.
Schematic model of α-CyD, which consists of six glucopyranose units, is shown in Fig. 1.5, 6) Hydroxyl groups are locatedon both ends of the torus-shaped structure, and are termed theprimary and secondary hydroxyl groups. Due to the presenceof these hydroxyl groups, the external wall of a CyD is hydrophilic and there is a strong anisotropic interaction between
CyDs. On the other hand, many methyl groups exist on theinner wall of the cavity, and the nonbonding electron pairs of
the glucosidic oxygen bridges are directed towards the cavityinterior. Therefore, the cavity has a hydrophobic characteristic, and CyDs include various hydrophobic molecules therein.
From these characteristics, CyDs are expected to play an important role in the field of artificial enzymes and biosensor
devices. Therefore, the study of CyD self-assembly is very
important from both the fundamental and practical points ofview.
Recent STM observations revealed that α-CyD inclusion
complexes were self-assembly-ordered on a HOPG substrate,

and their arrangements were different from those of the threedimensionally grown crystals.7, 8) The arrangements of theα-CyD inclusion complexes differed, and the included guest
molecules were considered to strongly influence the interactions between the CyDs. However, since the effect of theguest-dependent conformation is not sufficient to explain thedifferences in the arrangements, it is necessary to provide
some other mechanism to understand the phenomenon.
A rather large dipole moment (10–20 D) is known to exist
on CyDs, and its effect on the formation of inclusion complexes was reported in a previous paper.9) Until now, no attention has been paid to the relation between the dipole moment
and CyD molecular arrangement. In this paper, we analyze
the effect of the dipole moment on the arrangement of theself-assembly-ordered α-CyD inclusion complexes.
2. Experimental and Calculation
Three types of inclusion complexes, (1) α-CyD-H2 O, (2)α-CyD-MeOH, and (3) α-CyD- p-NP, were prepared as samples by dropping a 1.0 mM solution of each onto a freshly
cleaved HOPG surface, and then drying naturally. As isschematically shown in Fig. 2, each α-CyD cavity includes(a) two H2 O molecules, (b) a single MeOH molecule and (c)a single p-NP molecule.10–12) STM observations were per-∗ Corresponding

authors. E-mail address: hidemi@ims.tsukuba.ac.jp,mkomi@chembio.t.u-tokyo.ac.jp WWW: http//dora.ims.tsukuba.ac.jp/3888

Fig. 1. Schematic structure of α-cyclodextrin (α-CyD).

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

formed in air at room temperature using a Pt/Ir tip. All STM
images were obtained in the constant height mode.
In order to analyze the molecular interactions, the dipole
moment was calculated for each case. For the atomic coordinates of the guest compounds, the parameters determined forthe crystal structures by the X-ray diffraction method wereused.10–12)

Fig. 2. Schematic structures of (a) α-CyD-H2 O, (b) α-CyD-MeOH, and(c) α-CyD- p-NP inclusion complexes.

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3. Results and Discussion
3.1 α-CyD-self-assemblies observed by STM
Figure 3 shows STM images of (1) α-CyD-H2 O, (2)α-CyD-MeOH, and (3) α-CyD- p-NP. As shown in Figs. 3(a)and 3(b), α-CyD-H2 O and α-CyD-MeOH complexes showchannel-like structures with twofold symmetry, which isschematically shown in Fig. 3(d). On the other hand, theα-CyD- p-NP complex (Fig. 3(c)) has a threefold symmetric arrangement as schematically shown Fig. 3(e). Since allmolecules are completely included in the α-CyD cavity as
shown in Fig. 2, they could not have a direct influence on thearrangement. Therefore, some secondary effect on the interaction is expected. In the following sections, we analyze theeffect of the dipole moment on the self-assembly arrangementof CyD inclusion complexes.
3.2 Dipole moment induced on α-CyD inclusion complexes
The dipole moments calculated for (1) α-CyD-H2 O, (2)α-CyD-MeOH, and (3) α-CyD- p-NP inclusion complexes
are shown in Fig. 4. The dipole moments induced on
α-CyD-H2 O and α-CyD-MeOH complexes are directed fromthe secondary to the primary hydroxyl groups, as shownin Figs. 4(a) and 4(b), respectively. However, on the
α-CyD- p-NP complex, the dipole has an oblique orientation
relative to the other cases. (Fig. 4(c)).
Since the dipole moment of the p-NP molecule itself is directed from the primary hydroxyl groups to the secondary hydroxyl groups, the component from the CyD molecule must
contribute to the dipole structure. How does this occur? When
α-CyD includes small molecules, such as H2 O and MeOH,
two glucose residues of the CyD molecule are rotated inward,
so that the CyD molecule has a distorted structure.11, 12) However, when α-CyD includes a larger molecule such as p-NP,
the rotation of the glucose residue is suppressed, and little
distortion is induced on the CyD.10) Therefore, the difference

Fig. 3. STM images of (a) α-CyD-H2 O, (b) α-CyD-MeOH, and (c) α-CyD- p-NP inclusion complexes and their structural models with(d) twofold and (e) threefold symmetric arrangement.

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

Fig. 4.

Dipole moments induced on (a) α-CyD-H2 O, (b) α-CyD-MeOH, and (c) α-CyD- p-NP inclusion complexes.

Fig. 6. Structural models with (a) twofold and (b) threefold symmetric arrangement. Arrows indicate the dipole moments induced on CyD inclusion
complexes.

Fig. 5. Dipole moments induced on (a) distorted and (b) non-distorted
α-CyD molecules.

in the distortion may influence the electronic structure of theinclusion complexes. In order to clarify this point, we calculated the dipole moments of distorted and undistorted α-CyD
molecules without any guest compounds.
Figure 5(a) shows the dipole moment for a distorted αCyD. Rotated glucose residues are indicated by dotted circles.
The dipole is directed from the secondary to the primary hydroxyl groups. On the other hand, for the undistorted α-CyD,
the dipole is induced differently from the case involving distortion, as shown Fig. 5(b). The result indicates that the distortion of one glucose residue strongly influences the dipole
moment induced on the α-CyD molecule as predicted.
3.3 Structural analysis of α-CyD self-assemblies
Next, taking into account the effect of the dipole moment,
we consider the difference in the arrangement observed bySTM. For the α-CyD-H2 O and α-CyD-MeOH complexes,
their dipole moments are directed from the secondary to theprimary hydroxyl groups as shown in Figs. 4(a) and 4(b).

Therefore, the dipole-dipole interaction works together withthe hydrogen bond interaction in the same direction, and possibly results in the arrangement of the channel-like structure with twofold symmetry as shown in Fig. 6(a). On theother hand, the dipole moment of the α-CyD- p-NP complex is obliquely induced as shown in Fig. 4(c), and thestrongest orientation of the dipole-dipole interaction is different from that of the hydrogen bonds. Therefore the channellike arrangement shown in Fig. 6(a) may be suppressed forthe α-CyD- p-NP complex. A possible structure for theα-CyD- p-NP complex is shown in Fig. 6(b). These models
show good agreement with the observed structures.
4. Conclusion
We performed STM observations of the self-assembled
CyD inclusion complexes by changing the guest compounds.
The structure of the α-CyD- p-NP complex was different fromthose of α-CyD-H2 O and α-CyD-MeOH complexes, which
could not be explained solely by the conformational difference related to the guest molecules. Taking into consideration
the effect of the induced dipole moment on the molecules, wecould comprehensively explain the structural arrangement observed by STM. In addition, the change in the distortion of theα-CyD molecules was found to strongly influence the direc-

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6Btion of the dipole moment. These results strongly indicate thehigh possibility of the structural control of the self-assemblyof α-CyD by changing the guest compounds.
Acknowledgements
We thank Dr. Ishida and Dr. Hata of Univ. of Tsukuba for
the fruitful discussion. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. One of the authors (S. Y.)
was financially supported by the Japan Society for the Promotion of Science (JSPS) Fellowships for Japanese Junior Scientists.

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