Controllable donor-acceptor neutral [2]rotaxanes

FULL PAPER

Controllable Donor–Acceptor Neutral [2]Rotaxanes

[a]

Takahiro Iijima,^[a]Scott A.Vignon, Hsian-Rong Tseng,^[a]Thibaut Jarrosson,^[b]

[b]

Jeremy K.M.Sanders,*

Filippo Marchioni,^[c]Margherita Venturi,*^[c]

Emanuela Apostoli,^[c]Vincenzo Balzani,^[c]and J.Fraser Stoddart* ^[a]

Abstract: In pursuit of a neutral bi-

stable [2]rotaxane made up of two tetra-

arylmethane stoppers—both carrying

one isopropyl and two tert-butyl groups

located at the para positions on each of

three of the four aryl rings—known to

permit the slippage of the p-electron-

1/5DNP38C10 by slippage. Dynamic

¹H NMR spectroscopy in CD₂Cl₂

revealed that shuttling of the

1/5DNP38C10 ring occurs in NpNpR

and PmPmR, with activation barriers

of 277 K of 14.0 and 10.9 kcalmol^À1, re-

spectively, reflecting a much more pro-

nounced donor–acceptor stabilizing in-

teraction involving the NpI units over

the PmI ones. The photophysical and

electrochemical properties of the three

neutral [2]rotaxanes and their dumb-

bell-shaped precursors have also been

investigated in CH₂Cl₂. Interactions be-

tween 1/5DNP38C10 and PmI and NpI

units located within the rod section of

the dumbbell components of the [2]ro-

taxane give rise to the appearance of

charge-transfer bands, the energies of

which correlate with the electron-ac-

cepting properties of the two diimide

moieties. Comparison between the po-

sitions of the visible absorption bands

in the three [2]rotaxanes shows that,

in NpPmR, the major translatio-

nal isomer is the one in which

1/5DNP38C10 encircles the NpI unit.

Correlations of the reduction potentials

for all the compounds studied confirm

that, in this non-degenerate [2]ro-

taxane, one of the translational isomers

predominates. Furthermore, after deac-

tivation of the NpI unit by one-electron

reduction, the 1/5DNP38C10 macrocy-

cle moves to the PmI unit. Li⁺ions

have been found to strengthen the in-

teraction between the electron-donat-

ing crown ether and the electron-ac-

cepting diimide units, particularly the

PmI one. Titration experiments show

that two Li⁺ions are involved in the

strengthening of the donor–acceptor in-

teraction. Addition of Li⁺ions to

NpPmR induces the 1/5DNP38C10

macrocycle to move from the NpI to

the PmI unit. The Li⁺-ion-promoted

switching of NpPmR in a 4:1 mixture

of CD₂Cl₂and CD₃COCD₃has also

donating

1,5-dinaphtho[38]crown-10

(1/5DNP38C10) at the thermodynamic

instigation of p-electron-accepting rec-

ognition sites, in this case, pyromellitic

diimide (PmI) and 1,4,5,8-naphthalene-

tetracarboxylate diimide (NpI) units

separated from each other along the

rod section of the rotaxaneꢀs dumbbell

component, and from the para posi-

tions of the fourth aryl group of the

two stoppers by pentamethylene

chains, a modular approach was em-

ployed in the synthesis of the dumb-

bell-shaped compound NpPmD, as well

as of its two degenerate counterparts,

one (PmPmD) which contains two PmI

units and the other (NpNpD) which

contains two NpI units. The bistable

[2]rotaxane NpPmR, as well as its two

degenerate analogues PmPmR and

NpNpR, were obtained from the corre-

sponding dumbbell-shaped compounds

NpPmD, PmPmD, and NpNpD and

1

been shown by H NMR spectroscopy

to involve the mechanical movement of

the 1/5DNP38C10 macrocycle from the

NpI to the PmI unit, a process that can

be reversed by adding an excess of

[12]crown-4 to sequester the Li⁺ions.

Keywords:

molecular devices

self-assembly · template synthesis

bistable switches

·

rotaxanes

Lensfield Road, Cambridge CB2 1EW (UK)

Fax : (+44)122-333-6362

E-mail: jkms@cam.ac.uk

[a] T. Iijima, S. A. Vignon, Dr. H.-R. Tseng, Prof. J. F. Stoddart

California NanoSystems Institute and

Department of Chemistry and Biochemistry

University of California, Los Angeles

405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)

Fax : (+1)310-206-1843

[c] F. Marchioni, Prof. M. Venturi, E. Apostoli, Prof. V. Balzani

Dipartimento di Chimica “G. Ciamician”

Università di Bologna

E-mail: stoddart@chem.ucla.edu

Via Selmi 2, 40126 Bologna (Italy)

Fax : (+39)051-209-9456

E-mail: margherita.venturi@unibo.it

[b] T. Jarrosson, Prof. J. K. M. Sanders

Cambridge Center for Molecular Recognition

University Chemical Laboratory

Chem. Eur. J. 2004, 10, 6375 – 6392

DOI: 10.1002/chem.200400651

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FULL PAPER

Introduction

consistent with a model in which, as the hydrophilicities of

the rotaxanesꢀ tails or the surface pressure of the Langmuir

films are increased, the equilibria lie closer to the unfolded

conformations.

During the past decade, supramolecular chemistry^[1]has pro-

vided synthetic chemists with a powerful methodology in

the form of template-directed protocols^[2]to construct mo-

lecular-scale machines.^[3,4]The opportunity to incorporate

wholly synthetic motor molecules, as the active components

during the engineering of nanoelectromechanical systems^[5]

and in the fabrication of nanoelectronic devices,^[6]makes it

feasible to consider a bottom-up approach for the miniaturi-

zation of devices. Some of us at UCLA have helped to real-

ize molecular-switch tunnel junctions (MSTJs)^[6c]based on a

series of amphiphilic, bistable [2]rotaxanes,^[7]in which a tet-

racationic ring component, cyclobis(paraquat-p-phenylene)

(CBPQT⁴⁺), is located around a neutral dumbbell compo-

nent, containing two recognition sites—a tetrathiafulvalene

Figure 1. Schematic representations of a) the equilibrium between the

folded/unfolded conformations of a bistable [2]rotaxane in solution and

b) the equilibrium between possible folding/unfolding conformations of

the amphiphilic, bistable [2]rotaxane in solution. The p-electronic donat-

ing unit associated with hydrophilic stopper folds back to form an along-

side p–p stacking interaction with the CPBQT⁴⁺ring.

(TTF) unit and

a 1,5-dioxynaphthalene (DNP) ring

system—for the cyclophane situated some way apart from

each other within the central polyether section of the dumb-

bell, which is terminated by a hydrophobic stopper at one

end and by a hydrophilic stopper at the other. Although the

amphiphilic character of these bistable [2]rotaxanes allows

them to be self-organized^[6c,8]at the air–water interface in a

Langmuir trough, before being transferred as uniform mon-

olayers into the MSTJs of functioning devices, there is some

evidence^[7]from electrochemical experiments carried out by

Notwithstanding all the potential problems associated

with the self-organization of these positively charged, am-

phiphilic [2]rotaxanes at the air–water interface and their

transfer employing the LB technique, MSTJs have been fab-

ricated^[6c,8]by sandwiching self-organized monolayers of the

amphiphilic, bistable [2]rotaxanes between bottom polysili-

con electrodes and top electrodes of titanium covered with

aluminum. The devices have been shown^[6c]to exhibit hyste-

retic current–voltage responses on account of marked differ-

ences in conductance^[10]between the ground (low) state,

when the CBPQT⁴⁺ring encircles the TTF unit, and the

metastable (high) state, when the CBPQT⁴⁺ring encircles

the DNP ring systems, of a range of amphiphilic, bistable

[2]rotaxanes.^[7]However, the tetracationic nature of the

CBPQT⁴⁺ring component may introduce complications

during the LB stage of the device fabrication (Figure 2) or

the supporting counterions, which are supplied as four hexa-

1

some of us in Bologna, supported by H NMR spectroscopic

data obtained at UCLA, that some of these (amphiphilic)

bistable [2]rotaxanes adopt folded conformations in solution

(Figure 1). Also, since the thicknesses of the Langmuir–

Blodgett (LB) monolayers are always considerably less than

might be predicted on the basis of essentially linear amphi-

philes, it is not unlikely that some of the solution-state,

structural features of the molecules will be present, even in

compressed films at the air–water interface. Indeed, there is

some circumstantial evidence^[9]from in-depth studies of am-

phiphilic, bistable [2]rotaxanes with a variety of constitu-

tionally different (more and less) hydrophilic tails, that 1)

the thinner than expected LB films, 2) the observed foot-

prints in the first liquid-condensed and second highly con-

densed phases, and 3) their area differential, all compared

as a function of the constitution of the hydrophilic tails, are

consistent with the presence in the Langmuir films of an

equilibrium (Figure 2) between folded and unfolded confor-

mations. Moreover, the pressure–area isotherm data are

À

fluorophosphate anions (PF₆), may exercise some profound

influence during the electromechanical switching process-

es^[11]that involve the mechanical movement of the

CBPQT⁴⁺ring between the ground and the metastable

states, that is, the OFF and ONstates of the switch, respec-

Figure 2. Some of the possible conformational equilibria and self-organization behaviors displayed by an amphiphilic, bistable [2]rotaxane in solution

(left), in low-pressure condensed phases (middle), and in high-pressure condensed phases (right).

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Chem. Eur. J. 2004, 10, 6375 – 6392

Controllable [2]Rotaxanes

6375 – 6392

À

tively. The evident question arises: what do the ions, PF₆or

otherwise, do in the molecular monolayer of the solid-state

device (Figure 3) during the electromechanical movement of

the CBPQT⁴⁺ring? Do they follow it, thus conferring drag

upon the movement of the ring, for example? The answer

to this question is that we do not know and so would like to

be better informed on these fundamental issues pertaining

to molecular electronic devices containing positively charg-

ed bistable [2]rotaxane molecules.

During the past few years, some of us in Cambridge (Eng-

land) have pioneered^[12]

a donor–acceptor recognition

system based on neutral p-electron-accepting sites, as well

as neutral p-electron-donating sites, for the efficient assem-

bly of neutral catenanes and rotaxanes using template-dir-

ected synthesis.^[2]With reference to [2]rotaxanes,^[12i,k]the

neutral p-electron-rich aromatic crown ether, namely 1,5-di-

naphtho[38]crown-10 (1/5DNP38C10), has been located

around the neutral p-electron-accepting 1,4,5,8-naphthalene-

tetracarboxylic diimide^[13](NpI) unit. With the ultimate ob-

jective of realizing neutral [2]rotaxanes of high bistabilities

(and moderate metastability) that can be switched by con-

trolling the redox properties of the molecules, we have iden-

tified pyromellitic diimide^[14](PmI) as a much weaker recog-

nition site to compete with an NpI site for encirclement by a

1/5DNP38C10 macrocycle around the dumbbell-shaped

component of a [2]rotaxane. We would anticipate that p–p

stacking and other weak noncovalent interactions involving

the 1/5DNP38C10 macrocycle encircling the stronger (NpI)

of the two p-electron-accepting recognition sites would

result in these [2]rotaxanes existing essentially as one trans-

lational isomer,^[15]with the weaker (PmI) site to all intents

and purposes unpopulated by the 1/5DNP38C10 macrocycle.

Another requirement of this particular design of a neutral

molecular switch is that it can be thrown electromechanical-

ly between two different states. Since the reduction potential

of a bare NpI unit is considerably lower^[16]than that for a

bare PmI unit, the opportunity does exist to control

(Figure 4) the switching of such a bistable [2]rotaxane with

redox chemistry.

Figure 4. A schematic representation of redox switching in a bistable

[2]rotaxane.

After satisfying this requirement, the next challenge be-

comes one of being able to synthesize the potentially bista-

ble [2]rotaxane NpPmR, shown in Figure 5a, along with its

degenerate counterparts NpNpR and PmPmR illustrated in

Figure 5b and c, respectively. In general, template-directed

protocols^[2]have been applied to the synthesis of two-station

[2]rotaxanes in which the reliance is placed upon either ki-

netic or thermodynamic control,^[17]with the latter able to in-

volve dynamic covalent chemistry,^[18]as well as supramolec-

ular (noncovalent) synthesis, for example, slippage.^[19]Al-

though a number of different thermodynamic synthetic ap-

proaches are being pursued actively at UCLA and in Cam-

bridge, the research reported in this paper employs slippage

as the final step in the syntheses of the three [2]rotaxanes

NpPmR, NpNpR, and PmPmR from their corresponding

Figure 3. Proposed electromechanical mechanism which accounts for the hysteretic current–voltage response by an amphiphilic, bistable [2]rotaxane

monolayer in an MSTJ device. The devices which are described in reference [6c] consist of rotaxane molecules with 1) an amphiphilic dumbbell compo-

nent containing two recognition sites (a TTF unit and a DNP ring system) and two stoppers (one (upper) hydrophobic and the other (lower) hydrophil-

ic); and 2) a macrocycle, the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), accompanied by four hexafluorophosphate anions (PF₆^À, É), See

reference [6c] for the structural formulas of rotaxanes that give hysteretic current–voltage responses in device settings. Note that the graphical represen-

tations of the amphiphilic bistable [2]rotaxane molecules are highly idealized.

Chem. Eur. J. 2004, 10, 6375 – 6392

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Figure 5. Structural formulas for the a) bistable [2]rotaxane NpPmR and the degenerate [2]rotaxanes b) NpNpR, and c) PmPmR.

dumbbell-shaped compounds NpPmD, NpNpD, and

PmPmD; hence, the choice of the tetraarylmethane stop-

pers^[20]carrying two tert-butyl groups and one isopropyl

group which are known^[21]to permit the passage of

1/5DNP38C10 macrocycles in solution at slightly elevated

temperatures with respect to those normally found in a re-

search laboratory.

tetraarylmethane stoppers^[11]suitable for the slippage of

1/5DNP38C10 and 2) the PmI and/or NpI recognition sites,

separated by pentamethylene chains. Moreover, the reac-

tions chosen (Schemes 1–3) for connecting each fragment

were nucleophilic substitution reactions carried out under

Mitsunobu reaction conditions.

In this paper, we describe 1) the synthesis of two degener-

ate model [2]rotaxanes PmPmR and NpNpR and a non-de-

generate, bistable [2]rotaxane NpPmR; 2) their characteri-

zation by mass spectrometry, by (dynamic) NMR and UV-

visible spectroscopy, and by electrochemistry; and 3) the

switching of NpPmR by chemical (Li⁺ions) as well as elec-

trochemical means.

Synthesis: The routes employed in the syntheses of two de-

generate neutral, two-station [2]rotaxanes are outlined in

Scheme 1 and 2. The preparation of the dumbbell-shaped

compound PmPmD (Scheme 1) began with the alkylation of

the tetraarylmethane stopper 1^[20]by 5-chloro-1-pentanol in

MeCNin the presence of LiBr and [18]crown-6 (18C6) as

catalysts, and K₂CO₃as the base. Following flash column

chromatography, the alcohol 2 was obtained in 83%. Using

Mitsunobu reaction conditions (PPh₃/diethylazodicarboxy-

late (DEAD)), pyromellitic diimide (3) was reacted with the

alcohol 2 and the mono-THP-protected 1,5-pentandiol 4.^[22]

Without purification, the product was converted into the

half-dumbbell compound 5 by HCl-catalyzed deprotection

in wet CH₂Cl₂with an overall yield of 37%. The synthesis

of the dumbbell-shaped compound PmPmD was accom-

plished in 33% yield, once again by using Mitsunobu reac-

tion conditions (PPh₃/DEAD) on a mixture of compound 5,

the alcohol 2, and pyromellitic diimide (3) in THF. The

[2]rotaxane PmPmR was obtained by a template-assisted

slippage protocol, wherein the dumbbell-shaped compound

PmPmD served as the template. Compound PmPmD was

treated with an excess of the macrocycle 1/5DNP38C10

Results and Discussion

Design and synthetic strategy: Retrosynthetic analyses of

the degenerate [2]rotaxanes PmPmR and NpNpR and the

nondegenerate [2]rotaxane NpPmR suggested to us a limit-

ed number of viable synthetic pathways. Common to all of

the pathways was the slippage procedure,^[19]which we decid-

ed to employ to incorporate 1/5DNP38C10 around the

dumbbell-shaped compounds, that is, it constitutes the last

step of all three syntheses summarized in Schemes 1–3. In

the syntheses of the dumbbell-shaped compounds PmPmD,

NpNpD, and NpPmD, a modular approach was adopted to

assemble progressively the four fragments, namely 1) two

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Controllable [2]Rotaxanes

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Scheme 1. Synthesis of the PmI-containing degenerate [2]rotaxane PmPmR and its dumbbell-shaped precursor PmPmD.

(6)^[23]in the presence of LiBr in a CHCl₃/MeOH (95:5) sol-

vent mixture at 608C for 14 days. After column chromatog-

raphy, the [2]rotaxane PmPmR was obtained in a yield of

44% as an analytically pure yellow solid.

The template-directed synthesis of the nondegenerate

[2]rotaxane NpPmR from its dumbbell-shaped precursor

NpPmR is summarized in Scheme 3. The synthesis of the

dumbbell NpPmD was accomplished in 30% yield by carry-

ing out a Mitsunobu reaction (PPh₃/DEAD) between the

half-dumbbell compound 5, the alcohol 2, and naphthalene

diimide (7) in THF. The template-directed synthesis of the

[2]rotaxane NpPmR proceeded in 52% yield when a mix-

ture of the dumbbell-shaped compound NpPmD, the macro-

cycle 1/5DNP38C10 (6)^[23]and LiBr were reacted in a

CHCl₃/MeOH solvent mixture, by using exactly the same re-

action and purification conditions as those described for the

preparation of [2]rotaxanes NpNpR and PmPmR.

The sequence of steps employed in the synthesis of dumb-

bell-shaped compound NpNpD and degenerate rotaxane

NpNpR is summarized in Scheme 2. The half-dumbbell

compound 8 was obtained in an overall yield of 33% by al-

kylating naphthalene diimide (7) with the alcohol 2 and the

mono-THP-protected 1,5-pentanediol 4^[22]by using Mitsuno-

bu reaction conditions (PPh₃/DEAD), followed by HCl-cat-

alyzed deprotection in CH₂Cl₂. The synthesis of the dumb-

bell-shaped compound NpNpD was accomplished in 40%

yield after another Mitsunobu reaction carried out on com-

pound 8, the alcohol 2, and naphthalene diimide (7)^[24]in

THF. The template-directed synthesis of the [2]rotaxane

NpNpR proceeded in 42% yield when a mixture of the

dumbbell-shaped compound NpNpD, the macrocycle

1/5DNP38C10 (6)^[22]and LiBr is in a CHCl₃/MeOH solvent

mixture, employing exactly the same reaction and purifica-

tion conditions as those described for the preparation of ro-

taxane NpNpR.

Mass spectrometric investigation: The mass spectrometric

data for the dumbbell-shaped compounds NpNpD, PmPmD,

and NpPmD, and for the [2]rotaxanes NpNpR, PmPmR,

and NpPmR are summarized in Table 1. All of these com-

pounds were characterized by matrix-assisted laser-desorp-

tion ionization time-of-flight (MALDI-TOF) mass spec-

trometry. The positively-charged ions [M+Na]⁺or [M+H]⁺

were observed in their mass spectra.

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Scheme 2. Synthesis of the NpI-containing degenerate [2]rotaxane NpNpR and its dumbbell-shaped precursor NpNpD.

Scheme 3. Synthesis of the bistable [2]rotaxane NpPmR containing both NpI and PmI units and its dumbbell-shaped precursor NpPmD.

(Dynamic) ¹H NMR spectroscopic study: We have em-

ployed variable-temperature ¹H NMR spectroscopy to

probe the kinetics and thermodynamics of the shuttling

processes^[26](Figure 6) in the two degenerate rotaxanes,

PmPmR and NpNpR. At low temperatures, the shuttling

processes are slow on the ¹H NMR timescale and the signals,

shown in Table 2, separate out into two equal intensity sets

of signals. As the temperature is increased, the shuttling

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Controllable [2]Rotaxanes

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Table 1. MALDI-TOF MS data for the dumbbell-shaped compounds

NpNpD, PmPmD and NpPmD, and for the [2]rotaxanes NpNpR,

PmPmR and NpPmR.

NpNpD PmPmD NpPmD

ion [M+Na]⁺[M+H]⁺[M+Na]⁺[M+Na]⁺[M+Na]⁺[M+H]⁺

m/z 1640 1718 1690 2277 2377 2304

NpNpR

PmPmR NpPmR

1

process becomes fast on the H NMR timescale and aver-

aged signals are observed. Table 2 gives low- and high-tem-

perature chemical shifts in CD₂Cl₂for a few sample probe

protons for each degenerate rotaxane. In order to study the

dynamics of shuttling, one set of probe protons for each ro-

taxane was chosen. The aromatic protons on the hydropho-

bic stopper, which give rise to the signals centered on d=

6.75 and 6.83 ppm were chosen as the probes for NpNpR,

while the resonances for the protons of the PmI unit appear-

ing at d=6.96 and 8.32 ppm were used for PmPmR. These d

values were obtained from spectra recorded in CD₂Cl₂at

209 K. Rate constants were then determined at different

temperatures for these shuttling processes by simulating the

partial spectra (see Figure 7 for the simulations of NpNpR)

by using Spinworks 2.1^[27]and matching them as closely as

possible with the experimentally observed spectra. The ki-

netic and derived thermodynamic data are listed in Tables 3

and 4. From a comparison of DG^°values recorded in these

two tables, it is evident that NpNpR [for which DG^°ranges

from 13.9 (at 305 K) to 14.1 kcalmol^À1(at 266 K)], contain-

ing the stronger electron-accepting NpI units, presents a

considerably larger barrier (3.1 kcalmol^À1at 277 K) for the

shuttling of the 1/5DNP38C10 macrocycle relative to that

Figure 7. Simulated (top) and experimental (bottom) partial ¹H N MR

spectra (600 MHz) recorded for NpNpR in CD₂Cl₂at a) 294 K, b) 277 K,

c) 266 K, and d) 226 K. The displayed region shows some of the resonan-

ces for the aromatic protons of the hydrophobic stopper (H_St, see

Figure 5). The rate constants (k_ex) used to simulate the spectra are shown

beside the spectra.

Table 3. Kinetic and thermodynamic data^[a]for shuttling in PmPmR in

CD₂Cl₂.

[c]

[d]

T [K]^[b]

k_ex[s^À1

]

DG^°[kcalmol^À1

]

226

239

248

277

294

45

300

1050

14500

50000

11.4

11.2

11.0

10.9

[a] These data were obtained by simulating the signals for the protons of

the PmI unit (H_Pm) at d=8.32 and 6.96 ppm (209 K), which coalesce into

a single peak at d=7.63 ppm (266 K). [b] Calibrated using neat MeOH

sample. [c] Measured using line-shape analysis method. [d] Æ

0.2 kcalmol^À1

.

Figure 6. A graphical representation of the shuttling process in a degener-

ate [2]rotaxane with two identical stations.

Table 2. ¹H NMR chemical shift values for select protons of the degener-

ate shuttles at high and low temperature in CD₂Cl₂at 600 MHz.

Table 4. Kinetic and thermodynamic data^[a]for shuttling in NpNpR in

CD₂Cl₂.

[c]

[d]

Assign- Limiting d values

T

Limiting d values

T

T [K]^[b]

k_ex[s^À1

]

DG^°[kcalmol^À1

]

ment^[a](low temperature) [K] (high temperature) [K]

266

277

294

305

16

55

250

650

14.1

14.0

13.9

PmPmR H_Pm

6.96 (br), 8.32 (br) 209 7.67 (s)

319

248

H_St

6.75 (d), 6.81 (d)

209 6.78 (d)

NpNpR H_Np

8.21 (br), 8.80 (m)

6.75 (d), 6.83 (d)

2.85 (hp), 2.86 (hp) 209 2.87 (h)

1.26 (s), 1.27 (s) 209 1.29 (s)

248 8.52 (br)

209 6.79 (d)

332

294

277

[a] These data were obtained by simulating the signals for the aromatic

protons of the hydrophobic stopper (H_St) at d=6.83 and 6.75 ppm

(226 K), which coalesce into a single peak at d=6.79 ppm (294 K).

[b] Calibrated using neat MeOH sample. [c] Measured using line shape

H_St

H_iPr

H_tBu

[a] Structural assignments for these protons can be found in Figure 5.

analysis method. [d] Æ0.2 kcalmol^À1

.

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FULL PAPER

observed in PmPmR [for which

DG^°ranges from 10.9 (at

294 K) to 11.4 kcalmol^À1(at

226 K)], with its weaker elec-

tron-accepting PmI units.

In the case of the bistable

[2]rotaxane, NpPmR, no shut-

tling is observed by dynamic

¹H NMR spectroscopy. The

1/5DNP38C10 macrocycle re-

sides, to all intents and purpos-

es, on the NpI unit, at least to

within the limits of detection

provided by variable-tempera-

ture ¹H NMR spectroscopy.

This heavily biased situation is

evident from comparing the

spectra presented in Figure 8.

Signals for the protons (H_Np) on

the NpI unit encircled by the

macrocycle appear at d=8.13

and 8.23 ppm (Figure 8c), while

no signal is observed at

d~8.8 ppm where a resonance

for the protons (H_Np) of the

free NpI unit would be expect-

ed to appear (Figure 8a). Simi-

larly, a signal for the protons

(H_Pm) on the free PmI unit is

observed at d=8.31 ppm in the

spectrum

(Figure 8b)

of

NpPmR. No signal correspond-

ing to an encircled PmI unit is

observed in the region of d=

6.9–7.0 ppm. From the differ-

ence (>3 kcalmol^À1) in the

DG^°values (compare data in

Tables 3 and 4) for shuttling in

PmPmR and NpNpR, a rough

Figure 8. ¹H NMR Spectra (600 MHz) at room temperature in CD₂Cl₂for a) NpNpR, b) NpPmR, and

c) PmPmR. The insets show the partial structures for the three rotaxanes. Comparison of the spectrum for

NpPmR with the two for the degenerate rotaxanes allows the observed signals to be assigned to the transla-

tional isomer where the ring resides on the NpI unit. The structural assignments for H_iPrand H_Stcan be found

in Figure 5.

estimate of the difference in binding energies of the NpI

and PmI recognition sites would be ~3 kcalmol^À1. This pre-

diction assumes that the difference in the free-energy barrier

for shuttling in the two degenerate, two-station rotaxanes is

dictated by ground-state interactions rather than transition-

state ones. Whatever the rationalization, a free-energy dif-

ference between the two translational isomers of NpPmR of

~3 kcalmol^À1would mean that less than 1% of NpPmR re-

sides in the translational isomer (not shown in Figure 5) in

which the 1/5DNP38C10 macrocycle encircles the weaker

electron-accepting PmI recognition site.

Yet another dynamic process can be observed in the two

rotaxanes, NpNpR and NpPmR, which contain NpI units.

As the temperature of the CD₂Cl₂of these rotaxanes is low-

ered to 209 K, the signals corresponding to the protons of

the NpI units, encircled by 1/5DNP38C10 macrocycles, sepa-

rate (Figure 9) into multiple peaks. This behavior reflects a

decrease in the local symmetry of the NpI unit, such that

protons that were previously equivalent, on account of a

particular site-exchange mechanism, are no longer equiva-

1

Figure 9. Partial H NMR spectra (600 MHz) of NpNpR and NpPmR, re-

corded in CD₂Cl₂, showing the signals corresponding to the protons of

the NpI unit, encircled by the 1/5DNP38C10 macrocycle, at a) 248 K,

b) 239 K, c) 226 K, and d) 209 K.

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Chem. Eur. J. 2004, 10, 6375 – 6392

Controllable [2]Rotaxanes

6375 – 6392

lent on the ¹H NMR timescale. A diagram explaining the

origin of this asymmetry is shown in Figure 10. In the ab-

sence of the macrocycle, the protons labeled H_aand H_c, as

well as H_band H_d, would be equivalent as a result of sym-

metry, both axial and planar symmetry relating them. How-

ever, the macrocycle does not share the same plane (and

also axis most likely) of symmetry, possessing instead a C₂

axis, which is perpendicular to the plane of the NpI unit, re-

sulting in all four protons being nonequivalent. A number of

possible co-conformational changes could be proposed that

would explain the site exchanges observed (Figure 10) for

the two degenerate forms, but no clear evidence is available

(Figure 11). The model compound 9 for the PmI recognition

site shows a structured absorption band with l_max=320 nm

(e_max=2600m^À1cm^À1) and no emission (Figure 12). The

model compound 10 for the NpI recognition site exhibits

Figure 10. A schematic representation of the NpI unit encircled by a

1/5DNP38C10 macrocycle. If the indicated exchange process is slow on

the ¹H NMR timescale, then all four protons would give rise to different

signals in the spectrum, because the macrocycle does not share the plane

of symmetry that would make H_aand H_c, as well as H_band H_d, equiva-

lent in the absence of the macrocycle. The squares and triangles repre-

sent the constitutionally distinct ends of the NpI unit in the rotaxane. H_a

and H_cundergo exchange between sites U and V. H_band H_dundergo ex-

change between sites X and Y.

Figure 11. Absorption and emission spectra of the stopper precursor 2

and the crown ether 6 in air-equilibrated CH₂Cl₂at room temperature.

to allow us to be precise at this time about the details of this

mechanism. A rough estimate of the DG^°value for the ex-

change process can be made, based on the coalescence be-

haviors (Figure 9) of the signals, and gives a value of

11.5 kcalmol^À1at 239 K for both rotaxanes.^[28]Similar proc-

esses have been observed previously in catenanes^[29]and ro-

taxanes^[30]containing 1,5-dioxynaphthalene units.

a

strongly structured band (l_max=380 nm, e_max=

26000m^À1cm^À1), characteristic of its naphthalene core and a

fluorescence band (highest energy feature at l_max=390 nm,

F=1.210^À3, t<0.5 ns), which is the mirror image of the

absorption band (Figure 12). The data obtained for 9 and 10

are in agreement with previously reported results for com-

pounds in the same family.^[12a,32]

Photophysical investigations: The photophysical properties

of all the compounds investigated were studied in air-equili-

brated CH₂Cl₂at room temperature. The [2]rotaxanes

PmPmR, NpNpR, and NpPmR (Figure 5 and Schemes 1–3)

contain four different types of chromophoric units, namely,

the 1,5-dioxynaphthalene ring systems present in the

1/5DNP38C10 macrocycle and the tetraarylmethane stop-

pers as well as the PmI and/or NpI recognition sites present

in the dumbbell components. We have also investigated

model compounds related to the units contained in the

dumbbell components, namely, the tetraarylmethane deriva-

tive 2 (Scheme 1) and the diimides 9 and 10.

In CH₂Cl₂, 1/5DNP38C10 exhibits an absorption band

with l_max=295 nm, (e_max=16000m^À1cm^À1) and a strong

emission band (highest energy feature at 330 nm, F=0.10,

t=7.6 ns),^[31]while the stopper precursor 2 shows absorption

bands below 300 nm and a moderately intense (F=0.04)

and short-lived (t=1.6 ns) emission band with l_max=310 nm

Figure 12. Absorption and emission spectra of model compounds 9 and

10 in air-equilibrated CH₂Cl₂at room temperature.

Chem. Eur. J. 2004, 10, 6375 – 6392

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FULL PAPER

The absorption spectra of the dumbbell compounds

PmPmD, NpNpD, and NpPmD correspond to those expect-

ed from perusal of the spectra of the component units. As

for the emission, however, the high-energy fluorescence of

the stoppers is quenched, because of energy- and/or elec-

tron-transfer processes involving the diimide units, and the

structured band of the NpI unit in NpNpD and NpPmD is

partially quenched—presumably by a photoinduced elec-

tron-transfer process involving one of the stoppers—to

about 50% of the intensity observed for model com-

pound 10.

The absorption spectra of the [2]rotaxanes PmPmR,

NpNpR, and NpPmR differ considerably from what would

have been expected on the basis of the spectra of their com-

ponent units. It should be noted that there are strong differ-

ences in the spectral regions of both the ring component

and diimide units. Furthermore, the strong decrease in the

intensity of the UV region is accompanied (Figure 13) by

the appearance of weak and broad absorption bands in the

visible region. The presence of such low-energy excited

states, a feature which is a generally observed for strong

charge-transfer (CT) interactions, accounts for the lack of

emission of the three [2]rotaxanes.^[3d,4d,33]

We note that the maximum of the weak CT band in the

visible region moves (Table 5) from 435 nm for PmPmR to

485 nm for NpNpR, a result that is consistent with the

higher electron-accepting power of the NpI unit relative to

that of the PmI one (see the section on electrochemical

Table 5. Charge-transfer bands of the investigated compounds and com-

plexes.^[a]

l_max[nm]

e [m^À1cm^À1

]

PmPmR

NpNpR

435

485

490

460

550

460

560

460

1500

700

850

~350

~400

500

NpPmR

[6·9·Li₂]²⁺

[6·10·Li₂]²⁺

[PmPmR·Li₂]²⁺

[NpNpR·Li₂]²⁺

[NpPmR·Li₂]²⁺

900

550

[a] In air-equilibrated CH₂Cl₂at room temperature.

properties) and in agreement with the data previously ob-

tained for related systems.^[12g]We also note that the visible

band for NpPmR is practically the same as that for NpNpR,

an observation expected for the translational isomer of

NpPmR in which the macrocycle encircles the NpI unit.

Electrochemical investigations and redox-induced switching

of the bistable rotaxane: Electrochemical experiments—

namely, cyclic voltammetry (CV) and differential pulse vol-

tammetry (DPV)—were carried out in argon-purged CH₂Cl₂

solutions at room temperature. All the potential values are

referred to SCE. Since the [2]rotaxanes PmPmR, NpNpR,

and NpPmR all contain several electro-active units, they dis-

play a rather complex electrochemical behavior. We have

focused our attention on the electrochemical behavior of

the PmI and NpI recognition sites in order to understand, in

the case of NpPmR, which site, if any, is surrounded prefer-

entially by the macrocyclic ring and whether it is possible to

induce ring shuttling by electrochemical stimulation.

Table 6 summarizes the results obtained. Both the model

compounds 9 and 10 exhibit two monoelectronic and rever-

Table 6. Reduction processes of the PmI and NpI recognition sites con-

tained in the PmPmD, NpNpD, NpPmD dumbbells, and in the PmPmR,

NpNpR, NpPmR [2]rotaxanes, and in the model compounds 9 and 10.^[a]

II

NpI

II

PmI

E^I_NpI[V]^[b]

E^I_PmI[V]^[b]

E

[V]^[b]

E

[V]^[b]

9

10

PmPmD

NpNpD

NpPmD

PmPmR

À0.89(1)

À1.42(1)

À1.42(2)

À0.65(1)

À1.05(1)

À0.88(2)

À0.64(2)

À0.65(1)

À1.05(2)

À1.05(1)

À0.90(1)

À0.87(1)

À1.07(1)^[c]

À1.45(1)

À1.43(2)

NpNpR

NpPmR

À0.65(1)

À1.05(2)

À0.93(1)^[c]

À0.74(1)^[c]

À1.06(1)^[c,d]

À1.06(1)^[d]

À1.47(1)

[a] Room temperature argon-purged CH₂Cl₂solution; tetrabutylammoni-

um hexafluorophosphate as the supporting electrolyte, glassy carbon as

working electrode. [b] Potential value vs SCE; reversible process unless

otherwise noted; number of the exchanged electrons reported in paren-

theses. [c] Process affected by a relatively slow heterogeneous electron-

transfer rate. [d] For details, see the text.

Figure 13. Absorption spectra in air-equilibrated CH₂Cl₂at room temper-

ature of the [2]rotaxanes PmPmR, NpNpR, and NpPmR (full lines) com-

pared with the sums of the spectra of the component units (dashed lines).

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Controllable [2]Rotaxanes

6375 – 6392

sible reduction processes.^[12g,34]The dumbbells PmPmD and

NpNpD, which contain two PmI and NpI units, respectively,

show bielectronic and reversible reduction processes at ex-

actly the same potentials as observed for the corresponding

model compounds (Figure 14), indicating that the two chem-

and the correlation diagram of the reduction potentials, ob-

served in the dumbbell and rotaxane species, is presented in

Figure 16. The results obtained indicate that, in the case of

PmPmR and NpNpR, the two recognition sites are not

equivalent. One unit is reduced at the same potential as in

the corresponding dumbbell, whereas the second one is re-

duced at a more negative potential. This behavior demon-

strates that, on the timescale of the heterogeneous electron-

transfer process, the 1/5DNP38C10 macrocycle is localized

on one of the two units. We also note that, in both [2]ro-

taxanes, the second reduction process of the two electron-

accepting units takes place simultaneously and approximate-

ly at the same potential as the second reduction process ob-

served in the corresponding dumbbell. This behavior estab-

lishes that there is no appreciable interaction between the

macrocycle and the monoreduced PmI^Àand NpI^Àunits.

In the case of the NpPmR rotaxane, which contains two

different recognition sites, the first reduction process

(À0.74 V) occurs (Table 6) at a potential more negative than

that expected for a disengaged NpI unit—see the first reduc-

tion of the NpNpD and NpPmD dumbbells, and of the

NpNpR rotaxane which occur at approximately À0.65 V.

This result indicates clearly that the 1/5DNP38C10 macrocy-

cle encircles the NpI recognition site. It is also evident that

the second reduction wave of NpPmR is a two-electron

process that occurs at a potential (À1.06 V) more negative

than that of a disengaged PmI unit—see the first reduction

of the PmPmD dumbbell and PmPmR rotaxane and the

second reduction of the NpPmD dumbbell which both occur

at about À0.90 V. This result demonstrates that, in NpPmR,

the reduction of the NpI recognition site causes the dis-

placement of the 1/5DNP38C10 macrocycle to the PmI unit.

The shape of the CV curves as a function of sweep rate indi-

cates that such a displacement is fast on the electrochemical

timescale. As a consequence of the switching process, the

monoreduced NpI^Àunit is disengaged and is therefore ex-

pected to undergo second reduction at the same potential

(À1.05 V) as in the dumbbell species. This sequence of

events is consistent with the bielectronic nature of the

second reduction wave at À1.06 V of NpPmR that can

therefore be assigned to first reduction of engaged PmI and

second reduction of disengaged NpI. The switching of the

macrocycle from the one-electron reduced NpI^Àto the PmI

recognition site is confirmed by the fact that the first reduc-

tion of PmI occurs at the same potential (À1.07 V) as that

of the second reduction process of PmPmR. The third re-

duction wave (À1.47 V) for NpPmR can be assigned quite

straightforwardly to the reduction of a substantially disen-

gaged PmI^Àunit. When both the recognition sites are mono-

reduced, there is apparently no appreciable interaction with

the macrocycle, just as in the cases of the degenerate

NpNpR and PmPmR [2]rotaxanes.

Figure 14. Cyclic voltammograms (argon-purged CH₂Cl₂; scan rate=

50 mVs^À1) of the dumbbells PmPmD, NpNpD, and NpPmD and of 9 and

10, which serve as model compounds for the recognition sites. The CV

waves have been corrected for different concentrations and diffusion co-

efficients.

ically and topologically equivalent units in each dumbbell

behave independently. As expected, NpPmD, which con-

tains both PmI and NpI recognition sites, shows four dis-

tinct, monoelectronic and reversible reduction processes for

noninteracting units (Figure 14).

The CV patterns for the reduction of the [2]rotaxanes

PmPmR, NpNpR, and NpPmR are illustrated in Figure 15

It is also interesting to note that the reduction potential

of the engaged NpI unit is more negative in NpNpR

(À0.93 V) than in NpPmR (À0.74 V). This result can be ac-

counted for by considering that, in the NpPmR rotaxane, re-

duction of the NpI unit is followed by a fast shuttling to the

other (PmI) recognition site, whereas in the NpNpR rotax-

ane, reduction of the engaged NpI cannot be followed by

Figure 15. Cyclic voltammograms (argon-purged CH₂Cl₂; scan rate=

50 mVs^À1) of the [2]rotaxanes PmPmR, NpNpR, and NpPmR. The CV

waves have been corrected for different concentrations and diffusion co-

efficients.

Chem. Eur. J. 2004, 10, 6375 – 6392

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FULL PAPER

Figure 16. Correlations of the reduction potentials of the [2]rotaxanes PmPmR, NpNpR, and NpPmR with the dumbbells PmPmD, NpNpD, and

NpPmD.

shuttling because the other recognition site has already been

reduced as well.

of the visible band from 550 to 460 nm, demonstrating that

the association constant of [6·9·Li₂]²⁺is larger than that of

Simulations of the CV patterns on the basis of the above

discussed mechanism confirmed that shuttling is a fast proc-

ess (k>105 s^À1) and showed that in all three [2]rotaxanes

the reduction waves of the encircled units are not fully re-

versible, because of a slow heterogeneous electron transfer

(k<0.02 cms^À1). Such a behavior has already been observed

for electroactive guests encapsulated in cage-like hosts, such

as cyclodextrins, hemicarcerands, and dendrimers.^[35]

[6·10·Li₂]²⁺

.

Chemical switching of the bistable rotaxane: Previously, the

interaction of Li⁺ions with PmI and NpI units and

1/5DNP38C10 (6) has been investigated.^[36]Here, we report

in more detail on the nature of these interactions. Addition

of LiClO₄(dissolved in MeCN) to a solution of 6 in CH₂Cl₂

causes small spectral changes in the absorption and emission

bands of the macrocycle. No spectral change was observed

when LiClO₄was added to solutions of 9 or 10 in CH₂Cl₂. In

CH₂Cl₂, association of either 9 or 10 with 1/5DNP38C10 (6)

does not take place. When a solution containing equimolar

amounts (8.010^À5m) of 6 and 9 or 10 was titrated by a so-

lution of LiClO₄in MeCN, strong spectral changes were ob-

served—see, for example, Figure 17. Noticeably, a broad

band with a maximum at 460 and 550 nm, respectively, was

formed (Table 5), indicating that the presence of Li⁺ions

promotes formation of a CT complex (presumably with a

pseudorotaxane-like superstructure) between 1/5DNP38C10

and 9 or 10. Titration plots (see the inset in Figure 17, for

example) indicate that such complexes contain two Li⁺ions

and exhibit high association constants. Some fuzziness in the

isobestic points suggests that a 1:1 species is first formed,

followed by formation of a 1:2 species. Titration of the

[6·10·Li₂]²⁺complex with 9 causes (Table 5) a displacement

Figure 17. Spectral changes (optical path=1 cm) observed upon titration

of 9 and 6 (CH₂Cl₂, 8.010^À5m) with LiClO₄(MeCN). The inset shows

absorbance changes at 460 nm.

Titration of the degenerate PmPmR and NpNpR [2]ro-

taxanes with a solution of LiClO₄in MeCNcaused strong

spectral changes to occur. As illustrated in Figure 18 for

PmPmR, the CT band of the rotaxane (l_max=435 nm,

Figure 14, Table 5) decreases in intensity and moves to the

red, giving a band with l_max=460 nm, quite similar to that of

the [6·9·Li₂]²⁺complex and reaching its maximum for addi-

tion of two equivalents of LiClO₄. In the same manner, ad-

dition of two equivalents of LiClO₄to NpNpR causes the

displacement of the CT band from 485 to 560 nm (Table 5).

These results show clearly that the [2]rotaxanes are able to

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Chem. Eur. J. 2004, 10, 6375 – 6392

Controllable [2]Rotaxanes

6375 – 6392

Figure 20. Schematic representation of the lithium-induced switching

process of the bistable NpPmR. Addition of two equivalents of a lithium

cation forms a complex between the crown ether, the PmI unit and two

lithium cations that is stronger than the complex formed between the

NpI unit and crown ether alone. Removal of the lithium cations disrupts

this complex and causes the macrocycle to return to the stronger elec-

tron-accepting NpI unit.

Figure 18. Spectral changes (optical path=1 cm) observed upon titration

of the degenerate PmPmR rotaxane (CH₂Cl₂, 8.010^À5m) with LiClO₄

(MeCN).

complex two Li⁺ions in order to form the [PmPmR·Li₂]²⁺

and [NpNpR·Li₂]²⁺species. Since the presence of Li⁺af-

fects the CT bands, we can also conclude that the metal ions

are located close to the units involved in the donor–acceptor

interactions.

We have also investigated the lithium-ion-induced switch-

ing process (Figure 20) in the bistable [2]rotaxane using

¹H NMR spectroscopy as the probe.^[37]The spectrum of the

neutral NpPmR at room temperature in CD₂Cl₂is displayed

in Figure 21a. This bistable [2]rotaxane exists predominately

as the translational isomer (Figure 8) in which the

1/5DNP38C10 macrocycle resides on the NpI unit. In order

to induce the rotaxane to switch, an excess of LiClO₄

dissolved in ~200 mL of CD₃COCD₃was added to the

sample, which was then shaken vigorously to ensure com-

plete mixing of the two solutions in the NMR sample tube.

1

After addition of the LiClO₄, the H NMR spectrum was re-

corded (Figure 21b) again at room temperature. Comparing

this spectrum (Figure 21b) with that (Figure 21a) for the

rotaxane, prior to addition of the lithium salt, reveals

some distinct changes in chemical shifts. The signal at

d=8.19 ppm for the protons (H_Np) of the encircled NpI unit

is absent and signals for the “uncomplexed” NpI unit

(H_Np) appear at d=8.62 and 8.66 ppm. Also, the signal cor-

responding to the “uncomplexed” PmI unit (H_Pm

) at

d=8.24 ppm is no longer present in the spectrum. Unfortu-

nately, the signal corresponding to the encircled PmI unit

(H_Pm) is obscured by the other aromatic signals.

Figure 19. Spectral changes (optical path=5 cm) observed upon titration

of the nondegenerate NpPmR rotaxane (CH₂Cl₂, 8.010^À5m) with

LiClO₄(MeCN).

Further support for the presence of essentially only a

single translational isomer before and after addition of

LiClO₄can be obtained by examining the signals corre-

sponding to the methyl groups of the hydrophobic stopper

(see the expansion in Figure 21). Both of the major transla-

tional isomers possess two constitutionally distinct stoppers

as a result of the two different recognition sites within the

dumbbell. A consequence is that all of the signals for the

protons of the stoppers should appear in the form of two

equal-intensity signals, assuming the ring is located solely on

one of the two recognition sites. These signals can be seen

clearly in the expansions, in which the methyl groups of the

tert-butyl group give rise to two singlets, while the methyl

groups of the isopropyl group give rise to two doublets, both

before and after addition of the lithium salt.

All the foregoing spectroscopic and electrochemical re-

sults indicate that, in the nondegenerate NpPmR rotaxane,

the 1/5DNP38C10 macrocycle is located around the NpI sta-

tion. However, the experiments performed on the pseudo-

rotaxane-like species show that the presence of Li⁺ions

favors formation of the [6·9·Li₂]²⁺complex in preference to

the [6·10·Li₂]²⁺one. Taken together, these results suggest

that addition of Li⁺ions to the NpPmR rotaxane should

switch the 1/5DNP38C10 macrocycle from the NpI to the

PmI recognition site. Such a switching is indeed observed, as

indicated by the spectral changes displayed in Figure 19.

They show that, upon addition of Li⁺ions, the CT band cor-

responding to the 1/5DNP38C10–NpI interaction is replaced

by a CT band characteristic of the “Li⁺-ion-assisted”

1/5DNP38C10–PmI interaction (Table 5).

Another more subtle indication of the nature of the lithi-

um complex comes from the change of the NpI signals.

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FULL PAPER

Before addition of lithium salt,

the NpI unit gives rise to one

peak at d=8.19 ppm. After ad-

dition of LiClO₄, however, two

doublets are observed at d=

8.62 and 8.66 ppm. This differ-

ence in chemical shift arises

from the fact that the two ends

of the NpI unit are not con-

stitutionally the same—see

Figure 10, in which the squares

and triangles represent different

structural entities attached to

the NpI unit. Although this sep-

aration exists both before and

after addition of the lithium

salt, the presence of the Li⁺

ions accentuates the constitu-

tional heterotopicity of the two

ends, causing the Dd value to

be larger and thus making it

easier to observe. Before addi-

tion of the lithium salt, the Dd

value is so small that they

appear as only a single reso-

nance.

To determine if this ion-in-

duced switching process was re-

versible, [12]crown-4 is added

to the sample after recording

the spectrum of the switched

rotaxane. This crown ether acts

as a sequestering agent^[38]for

Li⁺ions and competes with the

rotaxane for binding to the Li⁺

ions. An excess of [12]crown-4

was added to drive the equili-

brium completely towards the

Li⁺·[12]crown-4 complex, re-

turning the rotaxane to its neu-

1

tral ground state. The H N MR

spectrum (Figure 21c) of the so-

lution containing the rotaxane,

LiClO₄, and excess [12]crown-4

was then recorded . The signal

at d=8.1 ppm arises from an

impurity present in the com-

mercial sample of [12]crown-4.

Other than a slight upfield shift

for some of the signals, presum-

ably caused by a change in the

polarity of the solvent, the ob-

servable signals are the same as

those for the neutral ground-

state rotaxane, indicating that

the switching process has been

Figure 21. ¹H NMR spectra (600 MHz) of NpPmR at room temperature in CD₂Cl₂a) before and b) after addi-

tion of excess of LiClO₄dissolved in 200 mL of CD₃COCD₃. The insets show the partial structures for the ro-

taxane before and after addition of the lithium. The expansion shows the signals corresponding to the methyl

groups on the hydrophobic stopper.

reversed.

6388

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 6375 – 6392

Controllable [2]Rotaxanes

6375 – 6392

nyl)methyl]phenol^[20](1) , 5-[(tetrahydropyan-2-yl)oxy]pentan-1-ol^[22](4),

1,5-dinaphtho[38]crown-10,^[23](6) and 1,4,5,8-naphthalenecarboxylic di-

imide^[24](7) were all prepared according to literature procedures. Solvents

were dried following methods described in the literature.^[25]All reactions

were carried out under an anhydrous argon atmosphere. Thin-layer chro-

matography (TLC) was performed on aluminum sheets, coated with

silica-gel 60F (Merck 5554). The plates were inspected by UV light and,

if required, developed in I₂vapor. Column chromatography was carried

out by using silica-gel 60 (Merck 9385, 230–400 mesh). Melting points

were determined on an Electrothermal 9100 melting point apparatus and

Conclusion

Important and successful steps have been taken towards the

design, synthesis, and characterization of redox-controlled

donor–acceptor, neutral [2]rotaxanes that could fulfill the

role of switches in a future generation of molecular elec-

tronic devices. NMR spectroscopic and photophysical inves-

tigations have demonstrated quite independently and

beyond any shadow of a doubt, that, in the nondegenerate

[2]rotaxane NpPmR with both PmI and NpI recognition

sites located along the rod section of the dumbbell compo-

nent, by far the most stable translational isomer is the one

in which the 1/5DNP38C10 macrocycle encircles the NpI

recognition site, which is itself a much better electron ac-

ceptor than the PmI one. Furthermore, a detailed electro-

chemical investigation of the redox-controllable, donor–ac-

ceptor, bistable [2]rotaxane reveals that, upon one-electron

reduction, its NpI recognition site is deactivated and the

1/5DNP38C10 macrocycle moves to the PmI one. Also, it is

worthy of note that when this second recognition site is de-

activated by electrochemical reduction, the 1/5DNP38C10

macrocycle apparently is no longer involved in any donor–

acceptor interactions. This electrochemical behavior is just

what is wanted in a nanoelectromechanical switch, that is,

there exists the potential to destabilize the metastable state

of a bistable switch (Figure 3). All that is needed now is to

introduce amphiphilicity^[7]into this neutral bistable [2]ro-

taxane so that it can be self-organized as a Langmuir film^[8,9]

and transferred as a molecular monolayer by means of the

Langmuir–Blodgett technique into a two-terminal crossbar

device.^[6]This research goal is being pursued presently with

some vigor as we seek to broaden the scope of bistable ro-

taxanes in molecular electronic devices from charged tetra-

cationic systems^[6–11]to neutral molecules.

Research rarely happens without throwing up a few sur-

prises and presenting the odd bonus to the researchers. This

collaborative venture has been no exception in so far as it

has uncovered a practical way by which the neutral bistable

[2]rotaxane can also be switched chemically using lithium

cations. The ability of two Li⁺ions to form a 2:1 co-complex

in solution with pseudorotaxanes,^[36]self-assembled between

substrates that contain electron-accepting diimide (PmI or

NpI) units and the electron-donating 1/5DNP38C10 macro-

cycle acting as a receptor, can be used to switch NpPmR.

Since the pole-dipole interactions involving the Li⁺ions and

the oxygen atoms in the polyether loops of the

1/5DNP38C10 macrocycle are stronger in the case of a PmI-

encircled unit, addition of Li⁺ions to the neutral bistable

[2]rotaxane induces the 1/5DNP38C10 macrocycle to move

from the NpI to the PmI recognition site. This observation

is a significant one in the context of the future design and

operation of artificial (supra)molecular machines^[4,39]that will

be driven by some kind of chemomechanical mechanism.^[40]

1

are uncorrected. All H and ¹³C NMR spectra were recorded on either 1)

a Bruker ARX400 (400 MHz and 100 MHz, respectively), 2) a Bruker

ARX500 (500 MHz and 125 MHz, respectively), or 3)

a Bruker

Avance500 (500 MHz and 125 MHz, respectively), with residual solvent

as the internal standard. Samples were prepared in CDCl₃and CD₃OD

purchased from Cambridge Isotope Labs. All chemical shifts are quoted

using the d scale, and all coupling constants (J) are expressed in Hertz

(Hz). Fast atom bombardment (FAB) mass spectra were obtained using a

ZAB-SE mass spectrometer, equipped with a krypton primary atom

beam utilizing a m-nitrobenzyl alcohol matrix. Cesium iodide or poly-

(ethylene glycol) were employed as reference compounds. Matrix-assist-

ed laser-desorption ionization time-of-flight (MALDI-TOF) mass spectra

were measured on an IonSpec Fourier transform mass spectrometer.

Photophysical experiments: All measurements were performed at room

temperature in air-equilibrated CH₂Cl₂(Merck Uvasol^TM) solutions (2

10^À5–110^À4m). UV-visible absorption spectra were recorded with

a

Perkin–Elmer Lambda 40 spectrophotometer. Uncorrected fluorescence

spectra were obtained with a Perkin–Elmer LS-50 spectrofluorimeter,

equipped with a Hamamatsu R928 phototube. Naphthalene in air-equili-

brated cyclohexane (F=0.036)^[41]was used as the standard for evaluating

the luminescence quantum yields. The estimated experimental errors are:

2 nm on band maxima, Æ10% on the molar absorption coefficients and

fluorescence intensity.

Electrochemical experiments: Cyclic voltammetric (CV) and differential

pulse voltammetric (DPV) experiments were carried out in argon-purged

CH₂Cl₂(Romil Hi-Dry^TM) at room temperature with an Autolab 30 mul-

tipurpose instrument interfaced to a personal computer. The working

electrode was a glassy carbon electrode (0.08 cm², Amel); its surface was

routinely polished with 0.3 mm alumina–water slurry on a felt surface, im-

mediately prior to use. In all cases, the counterelectrode was a Pt spiral,

separated from the bulk solution with a fine glass frit, and an Ag wire

was used as a quasi-reference electrode. Ferrocene (E_1/2=À0.46 V vs

SCE in CH₂Cl₂)^[42]was present as the internal standard. In all the electro-

chemical experiments the concentration of the compounds was in the

range 510^À4–110^À3m and tetrabutylammonium hexafluorophosphate

solution of a 100 times higher concentration was added as supporting

electrolyte. Cyclic voltammograms were obtained with sweep rates in the

range 0.02–1.0 Vs^À1; the IR compensation implemented within the Auto-

lab 30 was used, and every effort was made throughout the experiments

in order to minimize the resistance of the solution. In any instance, the

full reversibility of the voltammetric wave of ferrocene was taken as an

indicator of the absence of uncompensated resistance effects. The DPV

experiments were performed with a scan rate of 20 or 4 mVs^À1(pulse

height 75 and 10 mV, respectively) and a duration of 40 ms. The reversi-

bility of the observed processes was established by using the criteria of 1)

separation of 60 mV between cathodic and anodic peaks, 2) the close to

unity ratio of the intensities of the cathodic and anodic currents, and 3)

the constancy of the peak potential on changing sweep rate in the cyclic

voltammograms. The same halfwave potential values were obtained from

the DPV peaks and from an average of the cathodic and anodic CV

peaks, as expected for reversible processes. The number of exchanged

electrons in the redox processes of the investigated dumbbells and [2]ro-

taxanes was measured by comparing the current intensity of the CV

waves and the area of the DPV peaks with those found for the two rever-

sible and monoelectronic reduction processes of the model compounds 9

and 10, after correction for differences in the diffusion coefficients and

concentrations.^[43]The experimental errors associated with the potential

values were estimated to be Æ10 mV. Digital simulation of the experi-

mental CVs were obtained by using the software package DigiSim

3.05.^[44]

Experimental Section

General methods: Chemicals were purchased from Aldrich or TCI and

used as received. The compounds 4-[4-isopropylphenylbis(4-tert-butylphe-

Chem. Eur. J. 2004, 10, 6375 – 6392

www.chemeurj.org

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

6389

J. F. Stoddart, J. K. M. Sanders, M. Venturi et al.

FULL PAPER

Compound 2: A solution of 5-chloro-1-pentanol (999 mg, 8.15 mmol), 1

(2.00 g, 4.08 mmol), K₂CO₃(1.69 g, 12.23 mmol), LiBr (10 mg, cat.

amount), and [18]crown-6 (10 mg, cat. amount) in dry MeCN(80 mL)

was heated under reflux for 6 d. After cooling down to room tempera-

ture, the reaction mixture was filtered and the residue was washed with

MeCN(50 mL). The combined organic phase was concentrated in vacuo

and the crude product was purified by column chromatography (SiO₂:

CH₂Cl₂) to give 2 (1.95 g, 83%) as a white solid. M.p. 190–1928C;

¹H NMR (CDCl₃, 400 MHz): d=1.23 (d, J=6.9 Hz, 6H), 1.31 (s, 18H),

1.52–1.68 (m, 4H), 1.79–1.83 (m, 2H), 2.89 (septet, J=6.9 Hz, 1H), 3.66

(t, J=6.5 Hz, 2H), 3.95 (t, J=6.4 Hz, 2H), 6.76 (d, J=8.9 Hz, 2H), 7.08–

7.15 (m, 10H), 7.23 ppm (d, J=8.6 Hz, 4H); ¹³C NMR (CDCl₃,

400 MHz): d=22.4, 24.0, 29.1, 31.4, 32.5, 33.5, 34.3, 62.8, 63.2, 67.6, 113.0,

124.1, 125.2, 130.8, 131.0, 132.2, 139.5, 144.2, 144.7, 146.0, 148.3,

156.8 ppm; MS (FAB): m/z (%): 576 (55)[M]⁺, 457(45), 443(80), 397

(100); elemental analysis calcd (%) for C₄₁H₅₂O₂(576): C 85.37, H 9.09;

found: C 85.08, H 9.33.

148.1, 153.5, 156.6, 166.1, 166.2 ppm; MS (FAB): m/z (%): 576 (55) [M]⁺,

457(45), 443(80), 397 (100); MS (MALDI-TOF): m/z calcd for

C₁₄₃H₁₆₀N₄O₂₀: 2254; found: 2277 [M+Na]⁺; HRMS (MALDI-TOF): m/z

calcd for C₁₄₃H₁₆₀N₄O₂₀Na: 2277.1557; found: 2277.1570.

Compound 8: DEAD (724 mg, 4.16 mmol) was added dropwise at 08C

under an Ar atmosphere to

a solution of 2 (1.20 g, 2.08 mmol), 4

(392 mg, 2.08 mmol), (609 mg, 2.29 mmol), and PPh₃(1.09 g,

7

4.16 mmol) in THF (50 mL); the reaction mixture was stirred at room

temperature for 16 h. The mixture was filtered though silica gel and the

solution was evaporated in vacuo to obtain a crude product, which was

then purified by column chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:3).

The colorless band (R_f=0.6) was collected and dissolved in MeOH/

CH₂Cl₂(1:1, 100 mL). A concentrated HCl aqueous solution (0.5 mL)

was added and the reaction mixture was stirred at room temperature for

4 h. After removal of the solvent, the residue was purified by column

chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:5) to give 8 (0.63 g, 33%) as

1

a yellow solid. M.p. 257–2598C; H NMR (CDCl₃, 500 MHz): d=1.23 (d,

J=6.9 Hz, 6H), 1.30 (s, 18H), 1.50–1.70 (m, 6H), 1.76–1.88 (m, 6H), 2.86

(septet, J=6.9 Hz, 1H), 3.68 (t, J=6.4 Hz, 2H), 3.95 (t, J=6.3 Hz, 2H),

4.22 (q, J=7.1 Hz, 4H), 6.73 (d, J=8.9 Hz, 2H), 7.05–7.10 (m, 10H), 7.22

(d, J=8.6 Hz, 4H), 8.75 ppm (s, 4H); ¹³C NMR (CDCl₃, 400 MHz): d=

23.3, 23.7, 23.4, 27.8, 27.9, 29.1, 31.4, 32.3, 33.5, 34.3, 40.8, 62.7, 63.1, 67.4,

113.0, 124.0, 125.2, 126.6, 126.62, 126.7 130.7, 131.0, 132.2, 139.5, 144.2,

144.6, 146.0, 148.3, 156.8, 162.8, 162.9 ppm; MS (MALDI-TOF): m/z

calcd for C₆₀H₆₆N₂O₆Na: 933.5; found: 933.7; HRMS (MALDI-TOF): m/

z calcd for C₆₀H₆₆N₂O₆Na: 933.4813; found: 933.4803.

Compound 5: DEAD (700 mg, 4.02 mmol) was added dropwise at 08C

under an Ar atmosphere to

a solution of 2 (1.16 g, 2.01 mmol), 3

(478 mg, 2.21 mmol), (379 mg, 2.01 mmol), and PPh₃(1.06 g,

4

4.02 mmol) in THF (50 mL); the reaction mixture was stirred at room

temperature for 16 h. The mixture was filtered though silica gel and the

solution was evaporated in vacuo to obtain a crude product, which was

then purified by column chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:3).

The colorless band (R_f=0.5) was collected and dissolved in MeOH/

CH₂Cl₂(1:1, 100 mL). A concentrated HCl aqueous solution (0.5 mL)

was added and the reaction mixture was stirred at room temperature for

4 h. After removal of the solvent, the residue was purified by column

chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:5) to give 5 (0.65 g, 37%) as

Dumbbell NpNpD: DEAD (169 mg, 0.97 mmol) was added dropwise at

08C under an Ar atmosphere to a solution of 2 (280 mg, 0.49 mmol), 7

(142 mg, 0.53 mmol),

8 (442 mg, 0.49 mmol), and PPh₃(255 mg,

1

a yellow solid. M.p. 239–2418C; H NMR (CDCl₃, 400 MHz): d=1.23 (d,

0.97 mmol) in THF (20 mL); the reaction mixture was stirred at room

temperature for 2 d. The mixture was filtered though silica gel and the

solution was evaporated in vacuo to obtain the crude product which was

then purified by column chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:5)

J=6.9 Hz, 6H), 1.29 (s, 18H), 1.41–1.54 (m, 4H), 1.60–1.67 (m, 2H),

1.72–1.85(m, 6H), 2.86 (septet, J=6.9 Hz, 1H), 3.65 (t, J=6.5 Hz, 2H),

3.73–3.80 (m, 4H), 3.92(t, J=6.2 Hz, 2H), 6.73 (d, J=8.6 Hz, 2H), 7.05–

7.10 (m, 10H), 7.23 (d, J=8.6 Hz, 4H), 8.27 ppm (s, 2H); ¹³C N MR

(CDCl₃, 400 MHz): d=23.1, 23.6, 24.0, 28.2, 28.9, 31.4, 32.1, 33.5, 34.3,

38.6 41.7, 62.6, 63.1, 67.3, 112.9, 118.2, 124.1, 125.2, 129.1, 130.7, 131.0,

132.2, 137.2, 139.6, 144.2, 144.6, 146.0, 148.3, 156.7, 166.3, 166.3 ppm; MS

(MALDI-TOF): m/z calcd for C₅₆H₆₄N₂O₆Na: 883.5; found: 883.6; ele-

mental analysis calcd (%) for C₅₆H₆₄N₂O₆(860): C 78.11; H 7.49, N3.25;

found: C 77.89, H 7.59, N3.24.

to give NpNpD (0.35 g, 40%) as

a orange solid. M.p. 190–1928C;

¹H NMR (CDCl₃, 400 MHz): d=1.23 (d, J=6.9 Hz, 12H), 1.30 (s, 36H),

1.53–1.70 (m, 6H), 1.76–1.90 (m, 12H), 2.86 (septet, J=6.9 Hz, 2H), 3.96

(t, J=6.3 Hz, 4H), 4.22 (m, 8H), 6.74 (d, J=8.9 Hz, 4H), 7.05–7.10 (m,

20H), 7.23 (d, J=8.6 Hz, 8H), 8.73 ppm (AAꢀBBꢀ system, 8H);

¹³C NMR (CDCl₃, 400 MHz): d=23.7, 24.0, 27.7, 27.9, 31.4, 33.4, 34.3,

40.7, 63.1, 67.4, 112.9, 124.0, 125.2, 126.6, 126.7, 130.9, 131.0, 132.2, 139.5,

144.2, 144.6, 146.0, 148.3, 156.8, 162.8 ppm; MS (MALDI-TOF): m/z

calcd for C₁₁₅H₁₂₀N₄O₁₀: 1717; found: 1718 [M+H]⁺; HRMS (MALDI-

TOF): m/z calcd for C₁₁₅H₁₂₁N₄O₁₀: 1717.9083; found: 1717.9077 [M+H]⁺

.

Dumbbell PmPmD: DEAD (181 mg, 1.04 mmol) was added dropwise at

08C under an Ar atmosphere to a solution of 2 (300 mg, 0.52 mmol), 3

(124 mg, 0.57 mmol),

5 (448 mg, 0.52 mmol), and PPh₃(273 mg,

1.04 mmol) in THF (20 mL) and the reaction mixture was stirred at room

temperature for 2 d. The mixture was filtered though silica gel and the

solution was evaporated in vacuo to obtain a crude product, which was

then purified by column chromatography (SiO₂: CH₂Cl₂/Me₂CO, 100:5)

to give PmPmD (0.28 g, 33%) as a yellow solid. M.p. 213–2168C;

¹H NMR (CDCl₃, 400 MHz): d=1.24 (d, J=6.9 Hz, 12H), 1.30 (s, 36H),

1.50–1.60 (m, 6H), 1.73–1.90(m, 12H), 2.87 (septet, J=6.9 Hz, 2H), 3.70–

3.82 (m, 8H), 3.93 (t, J=6.3 Hz, 4H), 6.73 (d, J=8.9 Hz, 4H), 7.05–7.10

(m, 20H), 7.23 (d, J=8.6 Hz, 8H), 8.21 ppm (s, 4H); ¹³C NMR (CDCl₃,

400 MHz): d=23.6, 24.0, 27.8, 28.3, 28.9, 31.4, 34.3, 38.2, 38.6, 63.1, 67.3,

112.9, 118.2, 124.1, 125.2, 130.7, 131.0, 132.2, 137.2, 137.3, 139.6, 144.2,

144.6, 146.0, 148.3, 156.7, 166.2, 166.3 ppm; MS (MALDI-TOF): m/z

calcd for C₁₀₇H₁₁₆N₄O₁₀: 1617; found: 1640 [M+Na]⁺; HRMS (MALDI-

TOF): m/z calcd for C₁₀₇H₁₁₆N₄O₁₀Na: 1640.8622; found: 1640.8666 .

Rotaxane NpNpR:

A solution of NpNpD (51.5 mg, 0.03 mmol), 6

(95.5 mg, 0.15 mmol), and LiBr (13.0 mg, 0.15 mmol) in CHCl₃/MeOH

(95:5, 1 mL) was heated at 608C for 14 d. After cooling down to room

temperature, the mixture was evaporated in vacuo to obtain a crude

product which was then purified by column chromatography (SiO₂:

CHCl₃/EtOAc, 80:20) to give NpNpR (30 mg, 42%) as a violet solid.

M.p. 212–2148C; ¹H NMR (CDCl₃, 500 MHz): d=1.23 (d, J=6.9 Hz,

12H), 1.29 (s, 36H), 1.59–1.79 (br, 6H), 1.81–2.07 (br, 12H), 2.87 (septet,

J=6.9 Hz, 2H), 3.77–4.33 (m, 44H), 6.07 (d, J=7.6 Hz, 4H), 6.60–6.65

(m, 4H), 6.76–6.81 (br, 4H), 6.83 (d, J=8.4 Hz, 4H), 7.05–7.10 (m, 20H),

7.22 (d, J=8.6 Hz, 8H), 8.00–9.00 ppm (br, 8H); ¹³C NMR (CDCl₃,

500 MHz): d=23.9, 24.8, 27.7, 27.8, 31.3, 33.3, 34.2, 40.4, 63.0, 67.2, 67.4,

69.7, 71.0, 71.2, 103.2, 112.8, 113.9, 123.4, 123.9, 124.6, 125.0, 130.6, 130.9,

132.1, 139.4, 144.1, 144.5, 145.8, 148.1, 152.9, 156.7, 162.8 ppm; MS

(FAB): m/z (%): 576 (55) [M]⁺, 457(45), 443(80), 397 (100); MS

Rotaxane PmPmR: A solution of PmPmD (48.5 mg, 0.03 mmol),

6

(95.5 mg, 0.15 mmol), and LiBr (13.0 mg, 0.15 mmol) in CHCl₃/MeOH

(95:5, 1 mL) was heated at 608C for 14 d. After cooling down to room

temperature, the mixture was evaporated in vacuo to obtain a crude

product which was then purified by column chromatography (SiO₂:

CHCl₃/EtOAc, 80:20) to give PmPmR (29.6 mg, 44%) as a yellow solid.

M.p. 196–1978C; ¹H NMR (CDCl₃, 500 MHz): d=1.23 (d, J=6.9 Hz,

12H), 1.29 (s, 36H), 1.53–1.65 (m, 6H), 1.78–1.94 (m, 12H), 2.87 (septet,

J=6.9 Hz, 2H), 3.63–3.73 (m, 8H), 3.90–4.10 (m, 36H), 6.28 (d, J=

7.6 Hz, 4H), 6.70–6.79 (m, 8H), 7.05–7.10 (m, 20H), 7.17 (d, J=8.4 Hz,

4H), 7.21 (d, J=8.6 Hz, 8H), 7.55–7.75 ppm (br, 4H); ¹³C NMR (CDCl₃,

500 MHz): d=23.7, 23.8, 24.3, 28.1, 28.3, 28.8, 29.6, 31.2, 33.3, 34.2, 38.0,

38.2, 63.0, 67.3, 67.8, 69.4, 71.1, 71.2, 104.4, 112.8, 113.9, 116.9, 123.9,

124.0, 125.0, 125.3, 130.6, 130.9, 132.1, 135.3, 139.5, 144.0, 144.5, 145.8,

(MALDI-TOF): m/z calcd for

C₁₅₁H₁₆₄N₄O₂₀: 2354; found: 2377

[M+Na]⁺; HRMS (MALDI-TOF): m/z calcd for C₁₅₁H₁₆₄N₄O₂₀Na:

2377.1870; found: 2377.2567.

Dumbbell NpPmD: DEAD (121 mg, 0.69 mmol) was added dropwise at

08C under an Ar atmosphere to a solution of 2 (200 mg, 0.35 mmol), 5

(299 mg, 0.35 mmol),

7 (102 mg, 0.38 mmol), and PPh₃(182 mg,

0.69 mmol) in THF (20 mL); the reaction mixture was stirred at room

temperature for 2 d. The mixture was filtered though silica gel and the

solution was evaporated in vacuo to obtain the crude product which was

then purified by column chromatography (SiO₂: CH₂Cl₂/EtOAc, 90:10)

to give NpPmD (0.17 g, 30%) as

a yellow solid. M.p. 168–1708C;

¹H NMR (CDCl₃, 500 MHz): d=1.23 (d, J=6.9 Hz, 12H), 1.30 (s, 36H),

6390

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 6375 – 6392

Controllable [2]Rotaxanes

6375 – 6392

1.46–1.67 (m, 6H), 1.75–1.91 (m, 12H), 2.86 (septet, J=7.0 Hz, 2H),

3.74–3.82 (m, 4H), 3.90–3.98 (m, 4H), 4.18–4.28 (m, 4H), 6.71–6.77 (m,

4H), 7.05–7.12 (m, 20H), 7.23 (d, J=8.4 Hz, 8H), 8.13 (s, 2H), 8.74 ppm

(AA’BB’ system, 4H); ¹³C NMR (CDCl₃, 500 MHz): d=23.5, 23.6, 23.9,

24.1, 27.3, 27.8, 27.9, 28.1, 28.8, 29.0, 31.3, 31.5, 33.3, 34.2, 38.2, 38.5, 40.4,

40.7, 63.0, 67.2, 67.3, 112.8, 112.9, 118.0, 123.9, 125.1, 126.4, 126.6, 130.6,

130.9, 132.1, 137.1, 139.4, 139.4, 144.0, 144.1, 144.5, 145.9, 148.1, 156.6,

156.7, 162.6, 162.7, 166.1 ppm; MS (MALDI-TOF): m/z calcd for

C₁₁₁H₁₁₈N₄O₁₀: 1667; found: 1690 [M+Na]⁺; HRMS (MALDI-TOF): m/z

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Rotaxane NpPmR: A solution of NpPmD (25.0 mg, 0.015 mmol), 6

(47.8 mg, 0.075 mmol), and LiBr (6.5 mg, 0.075 mmol) in CHCl₃/MeOH

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product which was then purified by column chromatography (SiO₂:

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M.p. 193–1968C; ¹H NMR (CDCl₃, 500 MHz): d=1.23 (dd, J=6.9,

1.5 Hz, 12H), 1.29 (d, J=2.5 Hz, 36H), 1.52–1.67 (m, 4H), 1.72–1.88 (m,

6H), 1.88–2.04 (m, 8H), 2.87 (m, 2H), 3.77 (t, J=7.3 Hz, 2H), 3.83–4.12

(m, 42H), 6.07 (d, J=7.6 Hz, 4H), 6.62 (t, J=8.0 Hz, 4H), 6.72 (d, J=

8.4 Hz, 2H), 6.82 (d, J=8.4 Hz, 4H), 7.05–7.10 (m, 20H), 7.18–7.27 (m,

8H), 8.20–8.30 ppm (br, 6H); ¹³C NMR (CDCl₃, 500 MHz): d=23.5, 23.8,

24.1, 24.5, 27.4, 27.8, 28.2, 28.8, 29.1, 31.3, 33.3, 34.2, 38.4, 38.5, 40.0, 40.2,

63.0, 67.2, 67.3, 67.5, 69.7, 71.1, 71.2, 103.3, 112.8, 112.9 113.9, 118.0,123.4,

123.9, 124.6, 124.9, 125.0, 125.1, 130.3, 130.4, 130.6, 130.9, 132.1, 132.2,

137.1, 139.4, 139.5, 144.1, 144.5, 145.8, 145.9, 148.1, 148.2, 152.9, 156.6,

156.7, 162.9, 163.0, 166.1, 166.2 ppm; MS (MALDI-TOF): m/z calcd for

C₁₄₇H₁₆₂N₄O₂₀: 2303; found: 2304 [M+H]⁺; HRMS (MALDI-TOF): m/z

calcd for C₁₅₁H₁₆₅N₄O₂₀: 2304.1860; found: 2304.1855.

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Acknowledgements

This research was supported at the University of California, Los Angeles

by the Defense Advanced Research Project Agency (DARPA) and in

part by the National Science Foundation under equipment grant CHE-

9974928. T.I. thanks the JSR Corporation for a visiting scholar fellowship.

This research was supported at the University of Bologna by the FIRB

(Manipolazione molecolare per macchine nanometriche) and the Euro-

pean Union through the Molecular-Level Devices and Machines Net-

work (HPRN-CT-2000–00029). The research was supported at the Uni-

versity of Cambridge by the Engineering and Physical Sciences Research

Council.

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Received: June 28, 2004

Published online: November 5, 2004

6392

ꢁ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

Chem. Eur. J. 2004, 10, 6375 – 6392

Article Doi

Controllable donor-acceptor neutral [2]rotaxanes

DOI: 10.1002/chem.200400651

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Article abstract of DOI:10.1002/chem.200400651

Full text of DOI:10.1002/chem.200400651

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