A new three-way supramolecular switch based on redox-controlled

interconversion of hetero- and homo-guest-pair inclusion inside a host

moleculew

Ilha Hwang, Albina Y. Ziganshina, Young Ho Ko, Gyeongwon Yun and Kimoon Kim*

Received (in Cambridge, UK) 24th October 2008, Accepted 25th November 2008

First published as an Advance Article on the web 8th December 2008

DOI: 10.1039/b818889k

A novel three-way supramolecular switch based on the inter-

conversion of hetero-guest-pair (D–A) and homo-guest-pair

(D2 or A2) inclusion inside cucurbit[8]uril is reported, which

can be selectively controlled by chemical or electrochemical

stimuli.

Design and synthesis of artificial molecular machines and

switches has been a subject of intense study in recent years

because of their potential applications in the creation of

nanometre scale molecular devices.1,2 While most molecular

and supramolecular switches have been designed to perform a

two-way switching operation based on bistability, design and

synthesis of new molecular/supramolecular switches operating

in three ways3 or more would be useful for creating advanced

molecular devices and would further widen the scope of the

chemistry. Simultaneous inclusion of two identical or different

guest molecules in a molecular4,5 or supramolecular host,6,7

which made it possible to study new forms of stereochemistry

and isomerism as well as highly controlled chemical reactions

in a confined space, may also be useful in the construction of

new supramolecular switches and logic gates. Nevertheless,

such a possibility has rarely been explored, since it requires

precise control of selective inclusion/release or pairing/unpair-

ing of guest molecules in a host using external stimuli, which is

difficult to achieve.

Cucurbit[8]uril (CB[8]),8,9 a member of the cucurbit[n]uril

host family, exhibits remarkable host–guest behavior includ-

ing the encapsulation of two identical or different guest mole-

cules inside the cavity to form a stable ternary complex.8a,10

For example, it encapsulates 2,6-dihydroxynaphthalene (HN)

and methyl viologen (MV2+) inside the cavity to form the

stable 1 : 1 : 1 complex (MV2+�HN)CCB[8], which is driven

by the markedly enhanced charge-transfer (CT) interaction

between the p-electron donor and acceptor molecules inside

the hydrophobic cavity of CB[8].10 A wide variety of supra-

molecular assemblies have been synthesized and applications

have been explored based on this chemistry.11,12 We also

reported that one-electron reduction of MV2+ in the presence

of CB[8] results in the rapid generation of a 2 : 1 inclusion

complex (MV+�)2CCB[8] comprising a dimer of the cation

radical MV+� encapsulated in the cavity of CB[8].13 Similarly,

oxidation of a tetrathiafulvalene (TTF) derivative produces a

stable TTF cation radical dimer trapped inside CB[8],

(TTF+�)2CCB[8].14 In addition, the reduction of the donor–

acceptor complex (MV2+�HN)CCB[8] in the presence of

1 equiv. of free MV2+ results in near-quantitative formation

of (MV+�)2CCB[8] and free HN, which led us to design a

redox-driven molecular machine behaving as a molecular loop

lock that can be locked and unlocked with a key and a redox

stimulus.15

Having achieved this, we decided to explore new supra-

molecular switches by taking advantage of the remarkable

ability of CB[8] to encapsulate/release two guest molecules in a

controlled manner. Herein, we report a novel three-way

supramolecular switch composed of a water-soluble TTF deri-

vative (1), MV2+, and CB[8], based on the redox-controlled,

highly selective interconversion between hetero- and homo-

guest-pairs inside a host (Scheme 1). A key feature of this

system is the exclusive formation of homo-guest-pairs encap-

sulated in CB[8], 32+ and 42+, upon reduction and oxidation,

respectively, of the host-stabilized D–A complex 22+.

Addition of 1 equiv. of CB[8] into an aqueous solution

containing a 1 : 1 mixture of 1 and MV2+ in an inert atmos-

phere produced the ternary complex (MV2+�1)CCB[8] (22+)

which has been characterized by NMR, UV/Vis, and mass

spectrometry (see ESIw). First, the 1H NMR signals for the

pyridinium protons of MV2+ and the TTF protons of 1 were

Scheme 1

National Creative Research Initiative Center for SmartSupramolecules (CSS) and Department of Chemistry, PohangUniversity of Science and Technology (POSTECH), Pohang, 790-784,Korea. E-mail: kkim@postech.ac.kr; Fax: +82 54-279-8129;Tel: +82 54-279-2113w Electronic supplementary information (ESI) available: Experimentaldetails and spectral characterization data. See DOI: 10.1039/b818889k

416 | Chem. Commun., 2009, 416–418 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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upfield-shifted relative to those in the free guests, which

supports the proposal that MV2+ forms a D–A complex with

the TTF unit of 1 inside the cavity of CB[8] (Fig. S1, ESIw).The ternary complex 22+ was also characterized by various 2D

NMR methods including ROESY and DOSY (diffusion-

ordered NMR spectroscopy). Intermolecular NOE cross-

peaks were also observed between the pyridinium protons of

MV2+ and the TTF protons of 1 in the ROESY spectrum

(Fig. S3, ESIw). Furthermore, the signals for MV2+, 1, and

CB[8] in the DOSY spectrum (Fig. S4, ESIw) are all lined up,

indicating the formation of the stable ternary complex. The

hydrodynamic volume estimated from the diffusion coefficient

(2.37� 10�10 m2 s�1) of 22+ is 2480 A3, almost 1.7 times larger

than that of CB[8] itself (1500 A3). Second, the UV/Vis

spectrum of 22+ reveals a characteristic CT absorption band

centered at 925 nm as shown in Fig. 1 (black solid line).

Finally, the parent ion peak at 984.3 in the ESI mass spectrum

of 22+ also confirmed the 1 : 1 : 1 complex formation between

1, MV2+, and CB[8] (Fig. S2, ESIw).Treatment of the ternary complex 2

2+ with Na2S2O4

resulted in a drastic change in the UV/Vis spectrum. The

appearance of new absorption bands at l = 366, 539, and

894 nm (Fig. 1, blue solid line) supported the near-quantitative

formation of the 2 : 1 inclusion complex (MV+�)2CCB[8]

(32+) (Scheme 1).13 Introduction of O2 into the solution

regenerated 22+. Furthermore, treatment of the ternary com-

plex 22+ with Fe(ClO4)3 caused the appearance of new

absorption bands at l = 402, 538, and 756 nm (Fig. 1, red

solid line) supporting the near-quantitative formation of the

2 : 1 inclusion complex (1+�)2CCB[8] (42+) (Scheme 1).14

Addition of sodium metabisulfite (Na2S2O5) or ascorbic acid

into the solution regenerated 22+. Taken together, these

results demonstrated the reversible formation of the radical

dimers inside CB[8] 32+ and 42+ triggered by reduction and

oxidation, respectively, of 22+.

The redox-controlled, reversible conversion of hetero- and

homo-guest-pair inclusion in CB[8] was further investigated by

cyclic voltammetry. Fig. 2 compares the cyclic voltammograms

of 22+ and a 1 : 1 mixture of 1 and MV2+.16 First, a reduction

peak of 22+ was observed at �0.80 V (vs. SCE) while the

corresponding oxidation peaks were observed at �0.70 V

(shoulder) and �0.49 V. As a similar behavior has been

observed previously in (MV2+�HN)CCB[8],15 essentially the

same interpretation can be given here. The reduction of

22+ initially generates the one-electron-reduced species

2+�, which contains MV+� and 1 encapsulated in CB[8]

((MV+��1)CCB[8]); the small oxidation peak at –0.70 V

corresponds to the oxidation of this species. Then, it reacts

with another 2+� to undergo a rapid guest exchange leading to

32+ ((MV+�)2CCB[8]), free 1, and CB[8]; the oxidation peak

at �0.49 V corresponds to the oxidation of 32+. Second, the

oxidation peak of 22+ was observed at 0.18 V while the

corresponding reduction peaks were observed at 0.11 V

(shoulder) and �0.12 V. This process can be explained in

a similar manner. Upon oxidation, one-electron-oxidized

species 23+� ((MV2+�1+�)CCB[8]) is generated, which under-

goes a rapid guest exchange leading to 42+ ((1+�)2CCB[8]),

free MV2+, and CB[8] (or 42+ and MV2+CCB[8]); the

reduction peaks at 0.11 V and �0.12 V correspond to the

reduction of 23+� and 42+, respectively. With decreasing scan

rate, the small peaks corresponding to 2+� and 23+�, observed

at �0.70 V and 0.11 V, respectively, decreased, whereas the

peaks at �0.52 V and �0.10 V increased (Fig. S5, ESIw), asoften seen in processes that involve electron transfer followed

by a chemical reaction.

Fig. 1 Absorption spectra of 22+ (0.1 mM, black solid line) in H2O

and after addition of Na2S2O4 (for reduction, blue solid line) or

Fe(ClO4)3 (for oxidation, red solid line), respectively. The absorption

spectrum of a 1 : 1 mixture of 1 andMV2+ is also shown (black dashed

line).

Fig. 2 Cyclic voltammograms of a 1 : 1 mixture of 1 and MV2+

(0.5 mM each, dashed line), and 22+ (0.5 mM, solid line) in phosphate

buffer solution (0.1 M, pH 7.0). Scan rate = 100 mV s�1.

Fig. 3 Absorption spectra of 22+ (0.5 mM) before (dashed line) and

after (solid line) applying (a) �0.8 V or (b) +0.3 V, respectively.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 416–418 | 417

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Finally, the formation of 32+ and 42+ was also confirmed

by spectroelectrochemistry (Fig. 3). When �0.8 V was applied

to a solution of 22+, its UV/Vis spectrum changed as shown in

Fig. 3a, which indicated the formation of 32+. On the other

hand, when +0.3 V was applied to the 22+ solution, the

spectrum changed as shown in Fig. 3b, indicating the forma-

tion of 42+. These results demonstrate that the formation of

methyl viologen radical dimer ((MV+�)2CCB[8]) (32+) or

TTF radical dimer ((1+�)2CCB[8]) (42+) can be controlled

by applying a proper voltage to 22+.

In summary, we have demonstrated a novel three-way

supramolecular switch based on the redox-coupled guest-

exchange process of a CB[8]-stabilized D–A complex. A key

feature of the system is the unprecedented, three-way inter-

conversion of hetero-guest-pair (D–A) and homo-guest-pair

(D2 or A2) inclusion inside CB[8], which can be selectively

controlled by chemical or electrochemical stimuli. This

principle can be extended not only to design artificial mole-

cular machines and logic gates, but also to create delicate

supramolecular organizations that reversibly assemble and

disassemble upon applying redox stimuli. Work is in progress

in our laboratory along this line.

We gratefully acknowledge the Creative Research Initiative

and the Brain Korea 21 Program of the Korean Ministry of

Education, Science and Technology for support of this work.

Notes and references

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13 W. S. Jeon, H.-J. Kim, C. Lee and K. Kim, Chem. Commun., 2002,1828–1829.

14 A. Y. Ziganshina, Y. H. Ko, W. S. Jeon and K. Kim, Chem.Commun., 2004, 806–807.

15 W. S. Jeon, E. Kim, Y. H. Ko, I. Hwang, J. W. Lee, S.-Y. Kim,H.-J. Kim and K. Kim, Angew. Chem., Int. Ed., 2005, 44, 87–91.

16 Free guest molecules 1 and MV2+undergo two, reversible, one-electron oxidations and reductions, respectively in aqueous solu-tion containing supporting electrolytes such as LiClO4, NaNO3,and KCl. However, 22+slowly precipitated in these supportingelectrolyte solutions. Therefore, we decided to use phosphatebuffer as a supporting electrolyte, in which the second reductionand second oxidation waves of 2

2+were irreversible and poorlyobserved. Therefore, this study was limited to investigation of thefirst reduction and first oxidation waves of the complex.

418 | Chem. Commun., 2009, 416–418 This journal is �c The Royal Society of Chemistry 2009

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