An Electrochemically Controllable Nanomechanical Molecular System Utilizing Edge-to-Face and Face-to-Face Aromatic Interactions

Article abstract of DOI:10.1021/ol0268541

(Matrix Presented) A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic π-system (hydroquinone) upon reduction, the nanom

Full text of DOI:10.1021/ol0268541

ORGANIC
LETTERS
2002
Vol. 4, No. 22
3971-3974
An Electrochemically Controllable
Nanomechanical Molecular System
Utilizing Edge-to-Face and Face-to-Face
Aromatic Interactions
Heon Gon Kim,† Chi-Wan Lee,† Sunggoo Yun,† Byung Hee Hong,†
Young-Ok Kim,‡ Dongwook Kim,† Hyejae Ihm,† Jung Woo Lee,† Eun Cheol Lee,†
,†
P. Tarakeshwar,† Su-Moon Park,‡ and Kwang S. Kim*
National CreatiVe Research InitiatiVe Center for Superfunctional Materials and Center
for Integrated Molecular Systems, Department of Chemistry, Pohang UniVersity of
Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea
kim@postech.ac.kr
Received September 5, 2002
ABSTRACT
A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone
moiety can readily be converted into an aromatic π-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system
exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction
and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-
to-face aromatic interactions and vice versa, respectively.
A great deal of interest has recently been evinced in the
development of molecular scale analogues of mechanical
devices (molecular machines) such as rotors, shuttles,
turnstiles, ratchets, tweezers, switches, and memory devices,
in anticipation of their potential applications as mechanical
and electronic nanodevices.1 In particular, various switching
modes changing molecular shapes have been investigated.2
It would be particularly useful if the switching mode could
be controlled. Thus, we have investigated a novel nanome-
chanical molecular flipper-like system wherein the confor-
mational change can be triggered electrochemically.
In the recent past, the design and development of novel
molecular entities (ionophores, nanodevices) using π-electron
containing systems has been an active research topic.3-6 One
of the interesting insights obtained therein was that subtle
changes in the π-electron densities could significantly
influence both the nature and the magnitude of the resulting
intermolecular interactions. Therefore, a way to control the
π-electron density would yield novel molecular systems with
† Center for Superfunctional Materials.
‡ Center for Integrated Molecular Systems.
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U.S.A. 2001, 98, 5481. (c) Kelly, T. R. Acc. Chem. Res. 2001, 34, 514. (d)
Koumura, N.; Geertsema, E. M.; Meetsma, A.; Feringa, B. L. J. Am. Chem.
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J. F. Angew. Chem., Int. Ed. 2000, 39, 3348. (f) Pease, A. R.; Jeppesen, J.
O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res.
2001, 34, 433. (g) Cardona, C. M.; Mendoza, S.; Kaifer, A. E. Chem. Soc.
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10.1021/ol0268541 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/09/2002

unusual characteristics. In this connection, derivatives of the
quinones, whose electronic characteristics can be electro-
chemically or photochemically controlled, have recently been
used to fabricate nanostructures.7
Given the current interest in fabricating novel nano-
mechanical systems, we have designed a new molecular
system, 2,11-dithio[4,4]metametaquinocyclophane 1 contain-
ing a quinone moiety (Figure 1). The lower part of the
the reversible process of conformational characteristics of
the quinocyclophane system can yield a controllable nano-
mechanical device.
In an effort to investigate the possible utility of confor-
mational characteristics, we have studied the interaction
energies between p-benzoquinone and benzene using ab initio
calculations.8 In the neutral state of the benzoquinone-
benzene pair (simulating 1), the stacked conformer is 5 kcal/
mol more stable than the T-shaped one. On the other hand,
in the dianionic state of the p-benzoquinone-benzene pair
(simulating 2), the T-shaped conformer is 6 kcal/mol more
stable than the stacked one, because of (i) the enhanced
charge-charge electrostatic interaction between the posi-
tively charged H atom of the benzene and the negatively
charged quinone dianion moiety in the T-shaped conformer
and (ii) the strong π-H interaction between the large
π-electron density of the dianionic quinone moiety and the
edge H atom of the benzene. These interaction energies are
indeed reflected in the conformational energy study of
cyclophanes 1 and 2. The stacked conformer of 1 is 7 kcal/
mol more stable than the T-shaped one, whereas the T-shaped
conformer of 2 is 9 kcal/mol more stable than the stacked
one, indicating that the conformational energy change caused
by the side linkers between benzene and quinone/hydro-
quinone in the cyclophane system is not large. The predicted
structures of 1 and 2 (Figure 2) are shown in comparison
Figure 1. Schematic view of the conformational change of the
upper benzene ring in the normal state 1, the dianionic state 2, and
the reduced state 3. The reaction between 1 and 2 that is composed
of two single electron redox steps (through 1-•) by electrochemical
process is fast, whereas that between 2 and 3 is very slow.
quinocyclophane 1 has a p-benzoquinone moiety, which is
an electrochemically active species and has large electron
affinity (∼40 kcal/mol) from highly enhanced aromatic
π-conjugation upon accepting two electrons. Thus, 1 can
readily change to the dianionic state 2 as an extremely fast
process involving electron transfer. In the presence of
solvents containing H+ sources, the dianionic species changes
into hydroquinone, and thus 2 can be converted into 2,11-
dithio[4,4]metametahydroquinocyclophane 3, as a slow
process involving proton exchange. All these reactions are
reversible or pseudoreversible.
An interesting point of this system is that the structural
changes of edge-to-face (T-shaped) and face-to-face (stacked)
orientations can be electrochemically controlled, and hence
Figure 2. X-ray crystal structure of 1 (left) and ab initio (MP2/
6-31G*) structures of 1 (middle) and 2 (right).
with the X-ray structure of 1 (to be discussed below). After
a time, in the case of the interaction energy between
p-hydroquinone and benzene (simulating 3), both T-shaped
and stacked conformations are similar in energy. However,
the T-shaped conformation is slightly more favored (by ∼1
kcal/mol) at a more accurate ab initio level.9 Therefore, the
(3) (a) Hunter, C. A. Chem. Soc. ReV. 1994, 101. (b) Adams, H.; Carver,
F. J.; Hunter, C. A.; Morales, J. C.; Seward, E. M. Angew. Chem., Int. Ed.
1996, 35, 1542. (c) Kim, E.; Paliwal, S.; Wilcox, C. S. J. Am. Chem. Soc.
1998, 120, 11192. (d) Ren, T. et al. J. Biomol. Struct. Dyn. 1997, 15, 401.
(e) Hong, B. H.; Lee, J. Y.; Cho, S. J.; Yun, S.; Kim, K. S. J. Org. Chem.
1999, 64, 5661. (f) Tarakeshwar, P. et al. J. Am. Chem. Soc. 2001, 123,
3323. (g) Kim, K. S.; Tarakeshwar, P.; Lee, J. Y. Chem. ReV. 2000, 100,
4145. (h) Hobza et al. J. Am. Chem. Soc. 1994, 116, 3500. (i) Tsuzuki, S.
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(4) Schladetzky, K. D.; Haque, T. S.; Gellman, S. H. J. Org. Chem. 1995,
60, 4108.
(8) MP2/6-31+G* calculations for the p-benzoquinone-benzene and the
p-benzoquinone dianion-benzene pairs, and MP2/6-31G* calculations for
the cyclophane systems using the Gaussian 98 suite of programs (Frisch.
M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Zarkzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K.
N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J.
V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian Inc.: Pittsburgh, PA,
1998).
(5) (a) Choi, H. S. et al. J. Org. Chem. 2002, 67, 1848. (b) Oh, K. S. et
al. Org. Lett. 2000, 2, 2679. (c) Choi, H. S.; Kim, K. S. Angew. Chem., Int.
Ed. 1999, 38, 2256; Angew. Chem. 1999, 111, 2400. (d) Choi, H. S.; Suh,
S. B.; Cho, S. J.; Kim, K. S. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 12094.
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Bioorg. Med. Chem. Lett. 1993, 3, 263. (c) Lee, J. Y. Theoretical Studies
on the Electronic Structure of the Benzene Dimer and the Aromatic-
Aromatic Interactions; Application to the Cyclophanes. B.S. Dissertation,
Pohang University of Science and Technology, Pohang, Korea, 1992. (d)
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348.
3972
Org. Lett., Vol. 4, No. 22, 2002

stacked conformer of 1 and the T-shaped conformer of 2
are highly stable, while in the case of 3, the T-shaped
conformer is slightly favored.
To verify these predictions, we have synthesized 1 and 3
(Scheme 1). The X-ray crystal structure of 1 confirms the
Scheme 1
Figure 3. 1H NMR spectra of 1 (top) and 3 (bottom) in CDCl3.
by the anisotropic ring current of the hydroquinone ring. This
is in agreement with ab initio calculations for which the
T-shaped conformation for the benzene-p-hydroquinone pair
is slightly favored over the stacked one as previously
mentioned. It was not possible to study the conformation of
2 by direct experimental measurement,14 because 2 is a very
short-lived transient species. However, it is unambiguous that
2 should have the T-shaped structure not only because 3
favors the T-shaped conformation but also because the
T-shaped conformer of 2 shows very high stability in ab initio
calculations.15
The cyclic voltammogram of 1 shows two clear reversible
redox reactions between 1 and 2 (Figure 4). It has been
known that two reduction peaks for the quinone represent
two single-electron reductions, resulting in the formation of
quinone dianion. Typically, in aprotic media, quinones show
two reduced states separated by about 0.7 V, which cor-
respond to the formation of a radical anion species and a
dianion species of quinones, respectively.16 This is in
agreement with the reduction charcteristics of 1. Two well-
separated reduced states of 1 (1-• and 2) are formed in the
aprotic solvent of acetonitrile upon reduction.17 Therefore,
the electronic states of 1 and 2 (through 1-•) can be easily
transformed into each other by simple electrochemical control
of the redox reaction, which results in large conformational
flapping motions due to a preference for the stable confor-
mation caused by the change in the electronic state of the
quinone moiety.
enhanced stability for the stacked conformer in agreement
with the calculated structure (Figure 2).10 In the case of 3, it
is slowly transformed into 1 in a day, and so it was not
possible to obtain single crystals of 3 (even in the absence
of oxygen gas to hinder oxidation).11 However, since
Gelmann and co-workers4 showed that the X-ray structure
of the 2,11-dithio[4,4]metametacyclophane is the T-shaped
conformer, 3 is likely to have the T-shaped conformation in
consideration of the similarity in aromaticity between
benzene and hydroquinone.12,13
Corroborative evidence of such conformational change
between 1 and 3 is further provided by NMR spectra (Figure
3). The chemical shifts for Ha of 1 and 3 are 7.09 and 6.57
ppm, respectively. The high upfield shift for Ha of 3 reveals
that 3 has the T-shaped conformation, in which Ha is shielded
(9) MP2/6-311++G** calculations with MP2/6-31G* zero-point energy
correction for the benzene-hydroquinone pair.
(10) Crystal data for the structure of 1 have been deposited in the
Cambridge Crystallographic Data Center as supplementary publication no.
CCDC-177472. Crystal data of 1: monoclinic, space group P21/c with a )
16.7998(8) Å, b ) 7.3924(4) Å, c ) 14.2021(7) Å, and R ) γ ) 90°, â )
91.3370(10)° for empirical formula C20H22O2S2; V ) 1763.29(15) Å3, Z )
4, T ) 223(2) K. Fcalcd ) 1.350 g cm-3, 2θmax ) 48.34°. Final full-matrix
least-squares refinement of F2 with all 2796 independent reflections and
217 variables converged to R1 (I > 2σ(I)) ) 0.0389, wR2 (all data) )
0.1041, and GOF ) 1.604.
(14) We tried to analyze the conformation of 2 in basic condition
containing an aqueous NaOD solution by using the NMR spectroscopy,
but it was not possible due to the instability of 2 in this condition.
(15) In comparison with 3, 2 has additional electrostatic energy between
the quinone dianion and the positively charged H atom in the benzene in
favor of the T-shaped conformer. In addition, the T-shaped energy is 9
kcal/mol lower than the stacked one in ab initio calculations (the difference
of 9 kcal/mol is much larger than the possible error tolerance of a few
kcal/mol).
(16) Chambers, J. Q. In The Chemistry of the Quinonoid Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Vol. 2, Chapter
12, pp 719-757; 1974; Vol. 1, Chapter 14, pp 737-791.
(17) We tried to obtain a protonated species, 3, in the presence of the
acidic additive, upon electrochemical reduction of 1. However, it resulted
in the breakage of thioether linkages.
(11) The transformation of 3 into 1 with the lapse of time was confirmed
by the UV spectra as well as the NMR spectra (see Supporting Information).
(12) The aromatic-aromatic interactions between the stacked benzene-
benzene pair and the stacked benzene-hydroquinone pair are almost the
same (∼2 kcal/mol) according to ab initio calculations as well as chemistry
insights.
(13) To confirm the conformation of 3, we synthesized 2,11-dithio[4,4]-
metametahydroquinocylophane dimethyl ether 8. The X-ray structure of
compound 8 showed that it has a T-shaped conformation (see the Supporting
Information).
Org. Lett., Vol. 4, No. 22, 2002
3973

chanical devices as a first step toward a propelling molecular
vessel or a molecular flipper18 that can be controlled by an
electrochemical process.
Acknowledgment. This work was supported by the
Creative Research Initiative Program of Korea Ministry of
Science and Technology and partly by the Korean Ministry
of Education (Brain Korea 21 program).
Supporting Information Available: Experimental pro-
cedures and characterizations of compound 1, 3-5, and 7;
X-ray structural information on 1; UV spectra of 1 and 3;
X-ray crystal structure of 8. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL0268541
(18) According to the molecular motional analysis based on our
calculations, the present quinocyclophane system can play the role of
molecular flipper in solvents by the high-frequency alternating current
applied to the cyclic voltammogram. That is, the system can be utilized as
a nanomechanical device in solvents. In the case of 1, no solvent molecules
exist in a reasonably large space between benzene and quinone rings because
of the presence of rich electron clouds which repel the electron clouds of
solvent molecules. On the other hand, in the case of 2, solvent molecules
surround the benzene ring moiety. Upon reduction, 1 changes to 2. During
this conformational change, the benzene ring moves closer to the quinone
ring without much disturbing solvent molecules. On the other hand, upon
oxidation, 2 changes to 1. For this conformational change, the solvent
molecules surrounding the benzene ring are squeezed out from the space
between the benzene and quinone rings as these rings stack to each other.
This results in a strong thrusting force toward the backward direction. This
would let the molecular system move backward by the external alternating
electric current. Though the moving distance would be limited to a short
distance for the present system, the system could be considered as a
preliminary version for the precursor of molecular flipper/vessel.
Figure 4. Cyclic voltammogram of 1 (1 mM) and TBAP (0.1 M)
in CH3CN at 25 °C (scan rate 100 mV/s).
In summary, we have designed a novel nanomechanical
molecular cyclophane system composed of quinone and
benzene rings that exhibits a flapping motion from the
electrochemical redox process. It shows the potential avail-
ability of utilizing the differences in nonbonding interaction
forces (devoid of bond formation/breaking) to yield a
nanomechanical device. The large flapping/flipping motion
from the T-shaped and stacked conformation and vice versa
could be applied to design molecular hinges, molecular
switches, and memory devices as well as mobile nanome-
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Org. Lett., Vol. 4, No. 22, 2002