Molecular loop lock: A redox-driven molecular machine based on a host-stabilized charge-transfer complex

Full text of DOI:10.1002/anie.200461806

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Host–Guest Chemistry
Molecular Loop Lock: A Redox-Driven
Molecular Machine Based on a Host-Stabilized
Charge-Transfer Complex**
Woo Sung Jeon, Eunju Kim, Young Ho Ko, Ilha Hwang,
Jae Wook Lee, Soo-Young Kim, Hee-Joon Kim, and
Kimoon Kim*
Dedicated to Professor Dong H. Kim
Artificial molecular machines have received much attention
in recent years because of their potential application in the
creation of nanometer-scale molecular devices.[1,2] A wide
variety of molecular machines such as shuttles,[3a] rotors,[3b]
muscles,[3c] ratchets,[3d] pistons and cylinders,[3e] scissors,[3f] and
elevators[3g] have been reported. Nevertheless, the design and
synthesis of new molecular machines that are reminiscent of
macroscopic machines would further widen the scope of this
area of chemistry.
Cucurbit[8]uril (CB[8]),[4] a member of the host family
cucurbit[n]uril, which has a cavity that is similar to that of g-
cyclodextrin, exhibits remarkable host–guest properties
including the encapsulation of a hetero-guest-pair inside the
cavity.[5] For example, it encapsulates methyl viologen (MV2+)
and 2,6-dihydroxynaphthalene (Np(OH)2) inside the cavity to
form the stable 1:1:1 complex 12+. Formation of the complex is
driven by the markedly enhanced charge-transfer (CT)
interaction between the electron-deficient and electron-rich
guest molecules inside the hydrophobic cavity of CB[8].[5a]
This discovery led us to build several novel supramolecular
assemblies such as supramolecular amphiphiles that led to
[*] Dr. W. S. Jeon, E. Kim, Dr. Y. H. Ko, I. Hwang, Dr. J. W. Lee,[+]
Dr. S.-Y. Kim, Dr. H.-J. Kim,[++] Prof. Dr. K. Kim
National Creative Research Initiative Center
for Smart Supramolecules and
Department of Chemistry
Division of Molecular and Life Sciences
Pohang University of Science and Technology
San 31 Hyojadong, Pohang 790-784 (Republic of Korea)
Fax: (+82)54-279-8129
E-mail: kkim@postech.ac.kr
[+] Current address: Department of Chemistry
Dong-A University
840 Hadan-dong, Busan 604-714 (Republic of Korea)
[
++] Current address: Department of Applied Chemistry
Kumoh National Institute of Technology
188 Shinpyung-dong, Gumi, Gyeongbuk 730-701 (Republic of
Korea)
[**] We gratefully acknowledge the Creative Research Initiative Program
and the International R&D Cooperation Program of the Korean
Ministry of Science and Technology for support of this work, and we
thank the Korean Ministry of Education (BK 21 Program) for
graduate studentships to W.S.J., E.K., and I.H. This paper is
dedicated to Professor Dong H. Kim, a founding father of Pohang
University of Science and Technology, on his retirement.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 87 –91
DOI: 10.1002/anie.200461806
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vesicles,[5b] molecular loops,[5c] supramolecular polymers on
surfaces,[5d] and molecular necklaces[5e] by exploiting the host-
stabilized CT interactions. Despite the progress in the
construction of self-assembly systems, the redox properties
of the host-stabilized CT complexes, in particular, the inter-
play between the redox process and the exchange of the guest
within the host-stabilized CT complexes have not been
reported. These properties may provide a novel operating
principle of molecular machines or stimuli-responsive mate-
rials. Herein we report the redox-coupled guest-exchange
properties of CB[8]-stabilized CT complexes which demon-
strate the interconversion of hetero- and homo-guest-pair
inclusion in a molecular host triggered by an external stimulus
(Scheme 1). We also report a molecular loop lock, a novel
redox-driven molecular machine based on this phenomenon.
Treatment of a mixture of methyl viologen (MV2+) and
the ternary complex 12+ (1:1) with a reducing agent such as
sodium dithionite (Na2S2O4) results in a drastic change in the
UV/Vis spectrum. The appearance of new absorption bands
at l = 365, 540, and 884 nm (see Supporting Information)
supports the near-quantitative formation of the 2:1 inclusion
complex (MV+C)2ꢀCB[8] (22+)[6] and free Np(OH)2
(Scheme 1). Introduction of O2 into the solution regenerates
12+ and MV2+. This behavior is confirmed by UV/Vis and
NMR spectroscopy, and this result demonstrates the rever-
sible conversion of hetero- and homo-guest-pair inclusion
inside CB[8] triggered by a redox stimulus.
The redox-coupled guest-exchange process was further
investigated by cyclic voltammetry (Figure 1). Methyl viol-
ogen (MV2+) shows two reversible waves that correspond to
the redox couples MV2+/MV+C and MV+C/MV0 (Figure 1a).
Compared to MV2+, 12+ exhibits a moderate negative shift of
the first reduction peak and a large negative shift of the
second reduction peak (Figure 1b). Furthermore, the oxida-
tion process that corresponds to the first reduction process of
12+ shows two peaks at À0.71 V and À0.50 V (vs SCE—
saturated calomel electrode), the latter of which is almost the
same as that for the oxidation of 22+.[6] With increasing scan
rates the oxidation peak at À0.71 V increases, whereas the
peak at À0.50 V decreases (see Supporting Information), as
often seen in processes that involve electron transfer followed
by a chemical reaction. The addition of MV2+ (1 equiv) results
in a small positive shift of the first reduction wave, disappear-
ance of the oxidation peak at À0.71 V, and concomitant
Figure 1. Cyclic voltammograms of a) MV2+ (0.5 mm) and b) 12+
(0.5 mm) in the absence of free MV2+, and c) 12+ (0.5 mm) in the pres-
ence of free MV2+ (1 equiv) in phosphate buffer solution (0.1m,
pH 7.0). Scan rate=100 mVsÀ1; – – – different scan rate.
increase of the peak at À0.50 V (Figure 1c).[7] A spectroelec-
trochemical study shows that the absorption spectrum of the
species generated by the electrolysis of 12+ (applied potential,
À0.85 V vs SCE, in the presence or absence of MV2+
(1 equiv)) is essentially identical to that of 22+ (see Supporting
Information). Taken together, these results suggest that the
reduction of 12+ initially generates the one-
electron-reduced species 1+C, which contains
MV+C and Np(OH)2 encapsulated in CB[8],
and then reacts with free MV+C to undergo the
rapid guest exchange that leads to 22+ and free
Np(OH)2 (see Supporting Information). Note
that the regeneration of 12+ and MV2+ by the
oxidation of 22+ and Np(OH)2 probably does
not follow the reverse pathway because the 1:1
mixture of 22+ and Np(OH)2 is thermodynami-
cally far more stable than the 1:1 mixture of 1+C
and MV+C. Instead, it is more likely to occur
through another pathway that involves the
initial generation of the 1:1 complex
Scheme 1. Interconversion of hetero- and homo-guest-pair inclusion in CB[8] triggered by a redox
stimulus. MV=methyl viologen, Np(OH)2 =2,6-dihydroxynaphthalene, CB[8]=cucurbit[8]uril.
MV2+ꢀCB[8] by the oxidation of 22+, as we
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demonstrated previously,[6] which then reacts with free
Np(OH)2 to produce 12+ (see Supporting Information).
The discovery of the redox-coupled guest-exchange
process prompted us to design and synthesize a redox-
driven molecular machine that behaves as a molecular loop
lock, which can be switched on and off (locked and unlocked)
by means of a key and a redox stimulus (Schemes 2 and 3).
The guest molecule 33+, which contains a naphthalen-2-yloxy
(Np) unit and a viologen unit linked to each other by a flexible
Figure 2. Absorption spectra of 43+ (0.25 mm) before (a and inset)
and after (c) reduction with Na2S2O4 in the presence of MV2+
(1 equiv) in carbonate buffer (pH 10.0). Optical path=1 mm;
g zero absorption.
Scheme 2. Formation of the molecular loop lock 43+ (folded “locked”
state) through the formation of an intramolecular CT complex inside
CB[8], and the redox-induced formation of ternary complex 53+ (open
“unlocked” state).
tether as well as a bulky cationic unit at the terminal,[8] was
synthesized in four steps (see Supporting Information).
Treatment of 33+ with CB[8] (1 equiv) in water resulted in
the exclusive formation of the stable 1:1 complex 43+ through
the formation of the intramolecular CT complex between the
Np and viologen units inside CB[8], as confirmed by ESI-MS
and UV/Vis and NMR spectroscopy analyses (see Supporting
Information). In particular, the upfield-shifted signals for the
protons of the Np and viologen units and the downfield-
shifted signals for the protons of the linker and cationic
terminal in the 1H NMR spectrum of 43+ (see Supporting
Information) are consistent with the formation of a molecular
loop[5c] with a “closed” or “locked” conformation as illus-
trated in Scheme 2.
Figure 3. 1H NMR spectra obtained after reduction of 43+ with Na2S2O4
in carbonate buffer (pH 10.0) a) in the absence of MV2+, b) in the pres-
ence of MV2+ (0.5 equiv), and c) in the presence of MV2+ (1.0 equiv).
The signals labeled with correspond to the CB[8] host.
*
8 ppm) for the protons of 53+ are close to those for free 33+
which indicates that the Np unit is now located outside CB[8]
and that 53+ has an open or “unlocked” conformation as
schematically shown in Scheme 2. Introduction of O2 into the
solution of 53+ regenerates 43+ and MV2+ as confirmed by UV/
Vis and NMR spectroscopy. Thus, 43+ with a “closed”
conformation is converted into 53+ with an “open” conforma-
tion upon reduction in the presence of MV2+, and the process
can be reversed by oxidation.[11] This system thus behaves as a
molecular loop lock that can be locked and unlocked with a
key and a redox stimulus (Scheme 3): The two species 43+ and
53+ represent the locked and unlocked states, respectively, and
MV2+, which is activated by reduction, plays the role of the
key. It may be regarded as a “safeguarded” lock that requires
not only a key but also an activation process to open.[10,12,13]
In summary, we have demonstrated the redox-coupled
guest-exchange of CB[8]-stabilized CT complexes which
illustrates unprecendented interconversion, triggered by an
The 1H NMR spectrum of 43+ is not affected by the
addition of MV2+ (1 equiv) which indicates that the 1:1 host–
guest complex formed by the intramolecular CT interaction is
much more stable than the ternary complex formed by the
intermolecular CT interaction between the Np unit of 43+ and
MV2+ inside CB[8]. However, treatment of a solution
containing 43+ and MV2+ (1 equiv) with Na2S2O4 results in
the formation of the ternary complex 53+ (Scheme 2) in which
the one-electron-reduced viologen unit of 32+C interacts with
MV+C inside CB[8]. The formation of this complex was
confirmed by the appearance of new absorption bands at l =
368, 550, and 890 nm, which are characteristic of a CB[8]-
stabilized viologen radical-cation dimer (Figure 2).[9,10] Owing
to the paramagnetic nature of 53+, the signals for the Np unit
are broad, but are clearly observed by NMR spectroscopy
(Figure 3).[10] Furthermore, the chemical shift values (d ꢁ 7–
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f) T. Muraoka, K. Kinbara, Y. Kobayashi, T. Aida, J. Am. Chem.
´
Soc. 2003, 125, 5612; g) J. D. Badjic, V. Balzani, A. Credi, S. Silvi,
J. F. Stoddart, Science 2004, 303, 1845.
[4] a) J. Kim, I.-S. Jung, S.-Y. Kim, E. Lee, J.-K. Kang, S. Sakamoto,
K. Yamaguchi, K. Kim, J. Am. Chem. Soc. 2000, 122, 540; b) J. W.
Lee, S. Samal, N. Selvapalam, H.-J. Kim, K. Kim, Acc. Chem.
Res. 2003, 36, 621.
Scheme 3. Illustration of the working mode of a molecular loop lock
with a key.
[5] a) H.-J. Kim, J. Heo, W. S. Jeon, E. Lee, J. Kim, S. Sakamoto, K.
Yamaguchi, K. Kim, Angew. Chem. 2001, 113, 1574; Angew.
Chem. Int. Ed. 2001, 40, 1526; b) Y. J. Jeon, P. K. Bharadwaj,
S. W. Choi, J. W. Lee, K. Kim, Angew. Chem. 2002, 114, 1574;
Angew. Chem. Int. Ed. 2002, 41, 4474; c) J. W. Lee, K. Kim, S. W.
Choi, Y. H. Ko, S. Sakamoto, K. Yamaguchi, K. Kim, Chem.
Commun. 2002, 2692; d) K. Kim, D. Kim, J. W. Lee, Y. H. Ko, K.
Kim, Chem. Commun. 2004, 848; e) Y. H. Ko, K. Kim, J.-K.
Kang, H. Chun, J. W. Lee, S. Sakamoto, K. Yamaguchi, J. C.
Fettinger, K. Kim, J. Am. Chem. Soc. 2004, 126, 1932.
external stimulus, of hetero- and homo-guest-pair inclusion in
a molecular host. Furthermore, we have synthesized a redox-
driven molecular machine based on this phenomenon that
behaves as a molecular loop lock, which requires both a key
and an activation process to open. Further studies on the
novel molecular machine and its applications are in progress.
[6] We previously demonstrated that the 1:1 inclusion complex
MV2+ꢀCB[8] undergoes disproportionation to form an equal
mixture of the 2:1 complex 22+ ((MV+C)2ꢀCB[8]) and free CB[8]
upon chemical or electrochemical reduction and that the
reaction is reversible: W. S. Jeon, H.-J. Kim, C. Lee, K. Kim,
Chem. Commun. 2002, 1828.
[7] The small, reversible wave at À1.1 V in Figure 1c, which
increases with increasing scan rates, corresponds to the MV+C/
MV0 redox couple of free MV+C. The subsequent spectroelec-
trochemical study revealed no absorption bands that correspond
to free monomeric MV+C after electrolysis of 12+ in the presence
of MV2+ (1 equiv, see Supporting Information) which implies
that the reaction between 1+C and MV+C that leads to 22+ is
relatively slow on the timescale of cyclic voltammetry experi-
ments. The second reduction wave of 12+ in the absence
(Figure 1b) or presence (Figure 1c) of MV2+ is apparently
related to the reduction of the 2:1 complex 22+ generated in the
first reduction step, but we do not clearly understand this process
at the moment.
Experimental Section
3·3Br: The tribromide salt of N-(3-(naphthalen-2-yloxy)propyl)-N’-
(3-(trimethylamino)propyl-4,4’-bipyridinium) (34+·3BrÀ) was pre-
pared according to a reported procedure[5c] with a minor modification.
Detailed procedures are described in Supporting Information.
4·3Br: CB[8]·H2SO4·16H2O (20.0 mg, 11.6 mmol) was added to a
solution of 3·3Br (7.0 mg, 9.7 mmol) in D2O (4 mL), and the resulting
mixture was sonicated for 1 min. Undissolved solid was filtered off,
and the filtrate was slowly evaporated under reduced pressure to yield
the title product (18.2 mg, 92%). 1H NMR (500 MHz, D2O, 25 8C,
TMS): d = 8.84 (d, J(H,H) = 6.3 Hz, 1H; Py), 8.79 (d, J(H,H) =
6.6 Hz, 2H; Py), 8.76 (d, J(H,H) = 6.3 Hz, 1H; Py), 6.85–6.78 (m,
2H; Py, Np), 6.74 (d, J(H,H) = 6.5 Hz, 3H; Py, Np), 6.65 (d, J(H,H) =
8.9 Hz, 1H; Np), 6.62 (d, J(H,H) = 4.8 Hz; Np), 6.56 (d, J(H,H) =
8.1 Hz, 1H; Np), 6.51 (d, J(H,H) = 3.4 Hz, 2H; Np), 6.06 (d, J(H,H) =
1.9 Hz, 1H; Np), 5.72 (dd, J(H,H) = 9.2, 15.3 Hz, 16H; CB[8]), 5.47 (s,
16H; CB[8]), 5.08–5.02 (m, 2H; Py-CH2), 4.91–4.88 (m, 2H; Py-CH2),
4.62–4.57 (m, 2H; OCH2), 4.21–4.16 (dd, J(H,H) = 9.2, 15.3 Hz, 16H;
CB[8]), 3.66 (t, J(H,H) = 9.1 Hz, 2H; NCH2), 3.57–3.56 (m, 6H;
NCH2), 2.99–2.66 (m, 4H; CH2), 1.46 ppm (t, J(H,H) = 7.11 Hz, 9H;
[8] The bulky cationic unit was introduced at the terminal to prevent
the formation of intermolecular CT complexes and intermolec-
ular viologen radical-cation dimers inside CB[8] upon reduction.
Another role of the cationic terminal unit is to improve the
solubility of the complex after reduction.
CH3); HRMS (ESI-MS): m/z: calcd for C80H90N35O17 [M-3Br]3+
604.2418; found: 604.2411.
:
[9] The bands at l = 368, 550, and 890 nm can also be attributed to
22+ ((MV+C)2ꢀCB[8]), which may be generated during the
reduction process. However, the intensity of the band at l =
368 nm increases linearly with increasing concentrations of
MV2+ (0–1.0 equiv) and is almost twice as high as that of 22+
generated by the reduction of a 1:1 mixture of MV2+ and CB[8]
at the same concentration (see Supporting Information). These
results thus indicate that the amount of 22+ generated during this
reduction process is insignificant. Besides the bands that are
assigned to 53+, shoulders at l = 395 and 610 nm are observed
(Figure 2) which can be attributed to the one-electron-reduced
species 42+C and MV+C that are in equilibrium with 53+. It also
suggests that the reduction of a 1:1 mixture of 43+ and MV2+ first
generates 42+C and MV+C, which further react to produce 53+ (see
Supporting Information).
Received: August 27, 2004
Revised: September 25, 2004
Keywords: charge transfer · electrochemistry · molecular
.
devices · self-assembly · supramolecular chemistry
[1] For reviews, see: a) V. Balzani, A. Credi, F. M. Raymo, J. F.
Stoddart, Angew. Chem. 2000, 112, 3484; Angew. Chem. Int. Ed.
2000, 39, 3348; b) “Molecular Machines Special Issue”: Acc.
Chem. Res. 2001, 34, 409 – 522.
[2] Electronic devices based on molecular machines have been
´
reported, see: a) C. P. Collier, E. W. Wong, M. Belohradsky,
F. M. Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams, J. R.
Heath, Science 1999, 285, 391; b) C. P. Collier, G. Mattersteig,
E. W. Wong, Y. Luo, K. Beverly, J. Sapaio, F. M. Raymo, J. F.
Stoddart, J. R. Heath, Science 2000, 289, 1172.
[10] Treatment of 43+ with Na2S2O4 in the absence of MV2+ produces
the one-electron-reduced species 42+C, whose UV/Vis spectrum is
similar to that of MV+C which indicates that the viologen unit of
the guest molecule exists in its radical-cation form. Owing to the
paramagnetic nature of the species, the signals for the protons
from the stopper unit (d ꢁ 1.4 and ꢁ 3.5 ppm) are broad.
However, the signals are clearly observed in the 1H NMR
spectrum of 42+C whereas those for the Np unit are not, which
indicates that the Np unit is still in close proximity to the
[3] For representative examples, see: a) P. L. Anelli, N. Spencer, J. F.
Stoddart, J. Am. Chem. Soc. 1991, 113, 5131; b) T. R. Kelly, H.
De Silva, R. A. Silva, Nature 1999, 401, 150; c) M. C. JimØnez,
C. O. Dietrich-Buchecker, J.-P. Sauvage, Angew. Chem. 2000,
112, 3484; Angew. Chem. Int. Ed. 2000, 39, 3284; d) L.
Mahedevan, P. Matsudaira, Science 2000, 288, 95; e) A. M.
Brouwer, C. Frochot, F. G. Gatti, D. A. Leigh, L. Mottier, F.
Paolucci, S. Roffia, G. W. H. Wurpel, Science 2001, 291, 2124;
viologen radical-cation unit. This result in turn suggests that 42+
C
maintains the same “locked” conformation with the one-
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electron-reduced viologen and Np units located inside CB[8].
Thus, the molecular loop lock does not open in the absence of
the key (MV2+) and the activation process (reduction).
[11] Preliminary electrochemical studies confirmed that this rever-
sible process can also be triggered by electrochemical stimuli.
Details of the electrochemical behavior of 43+ and related
compounds will be published elsewhere in due course.
[12] A “safeguarded” switch has been reported: J. W. Lee, K. Kim, K.
Kim, Chem. Commun. 2001, 1042.
[13] It may also be viewed as a reversibly operating molecular AND
logic gate: F. M. Raymo, S. Giordani in Encyclopedia of
Nanoscience and Nanotechnology, Vol. 5 (Ed.: H. S. Nalwa),
American Scientific, California, 2004, pp. 677 – 692.
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