highly stable tetrathiafulvalene radical dimers in [3...

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Highly stable tetrathiafulvalene radical dimers in[3]catenanesJason M. Spruell1†‡, Ali Coskun1‡, Douglas C. Friedman1, Ross S. Forgan1, Amy A. Sarjeant1,

Ali Trabolsi1, Albert C. Fahrenbach1, Gokhan Barin1, Walter F. Paxton1, Sanjeev K. Dey1,

Mark A. Olson1, Diego Benıtez2, Ekaterina Tkatchouk2, Michael T. Colvin3, Raanan Carmielli3,

Stuart T. Caldwell4, Georgina M. Rosair5, Shanika Gunatilaka Hewage4, Florence Duclairoir6,

Jennifer L. Seymour1, Alexandra M. Z. Slawin7, William A. Goddard III2, Michael R. Wasielewski3,

Graeme Cooke4* and J. Fraser Stoddart1*

Two [3]catenane ‘molecular flasks’ have been designed to create stabilized, redox-controlled tetrathiafulvalene (TTF)dimers, enabling their spectrophotometric and structural properties to be probed in detail. The mechanically interlockedframework of the [3]catenanes creates the ideal arrangement and ultrahigh local concentration for the encircled TTF unitsto form stable dimers associated with their discrete oxidation states. These dimerization events represent an affinityumpolung, wherein the inversion in electronic affinity replaces the traditional TTF-bipyridinium interaction, which is over-ridden by stabilizing mixed-valence (TTF)2

†1 and radical-cation (TTF†1)2 states inside the ‘molecular flasks.’ Theexperimental data, collected in the solid state as well as in solution under ambient conditions, together with supportingquantum mechanical calculations, are consistent with the formation of stabilized paramagnetic mixed-valence dimers, andthen diamagnetic radical-cation dimers following subsequent one-electron oxidations of the [3]catenanes.

The observation of p–p radical-cation dimers on the redoxstimulation of electron rich or poor heterocycles is longprecedented1. However, investigations on such systems have

been hindered by the inherent instability of even the simplestradical-cation dimers, such that they have only been observed atlow temperatures or in the solid state2–6, an attribute that hascaused them to be considered as mere novelties, rather than thefundamental cornerstone of functional devices7. Specifically,methyl viologen (MV2þ), the predominant member of a class oforganic salts8 valued in previous times for their herbicidal activitiesand endemic to many disciplines and industries9, was observed10–15

long ago to form diamagnetic radical-cation dimers following theone-electron reduction of the dication. However, these dimerswere only stable at low temperatures and high concentrations instrictly air-free environments. Similar p–p radical-cation andmixed-valence dimers of tetrathiafulvalene (TTF)16,17 are knownto form upon the one-electron oxidation of the neutral compoundto its stable radical-cation. As the simple parent compound,however, the dimeric complex is once again only stable at lowtemperatures and in the solid state4,7.

In recent years, a renaissance18–30 in the radical-cationdimer chemistry of both MV2þ and TTF has been spurred on bythe discovery that inclusion complexes of each species exhibitexceptionally high stabilities in solution at room temperature.Kim and co-workers18,19 observed that cucurbit[8]uril31 enhances

the stability of both (MV†þ)2 and (TTF†þ)2 radical-cationdimers by encapsulating the desired species, such that theycould be studied in solution at room temperature. Subsequently,Fujita et al.20 observed mixed-valence dimers of (TTF)2

†þ

stabilized within self-assembled cages. When held in place cova-lently as members of crown ethers21, molecular clips22 or appendedto calixarenes23,28,29, TTF units also form stable radical-cation dimers.

Of particular interest for radical-dimerized species is the factthat the redox activation used in their production represents anaffinity umpolung—a concept borrowed from classical32 reactivityumpolung—whereby an inversion in supramolecular affinity33,34,rather than polarity, is brought about by redox events. Thisaffinity reversal has been used to drive switching in carefullydesigned molecular machinery24–27,30, whereby the ground statesof both MV2þ and TTF form stable complexes with electron-rich or electron-poor species, respectively, and the high affinitiesfor their dimerized states upon redox stimulation forces the adop-tion of a switched molecular (co)conformation26,30. Here we eluci-date further the concept of affinity umpolung through theexploration of a series of tetrathiafulvalene dimer pairs formedand stabilized at two different oxidation states under ambient con-ditions, held together by virtue of mechanical bonds in two[3]catenane molecular frameworks, demonstrating electronic andtranslational switching.

1Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA, 2Materials and Process Simulation Center,California Institute of Technology, Pasadena, California 91125, USA, 3Department of Chemistry, Argonne-Northwestern Solar Energy Research (ANSER)Center, and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, USA, 4Glasgow Centre for Physical OrganicChemistry, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, G12 8QQ, UK, 5Department of Chemistry, School of Engineering andPhysical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK, 6Laboratory of Ionic Recognition and Coordination Chemistry (UMR_E 3 CEA-UJF),17 rue des Martyrs, 38054, Grenoble Cedex 9, France, 7School of Chemistry and Centre for Biomolecular Sciences, University of St Andrews, North Haugh,St Andrews, KY16 9ST, UK; †Present address: California NanoSystems Institute and Materials Research Laboratory, University of California, Santa Barbara,California 93106, USA. ‡These authors contributed equally to the manuscript. *e-mail: [email protected]; [email protected]

ARTICLESPUBLISHED ONLINE: 25 JULY 2010 | DOI: 10.1038/NCHEM.749

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Results and discussionThe use of donor–acceptor interactions for the preparation of switch-able mechanically interlocked molecules has been borne out in morethan two decades worth of intensive research. One of the mostcommon and useful recognition motifs present in rotaxanes and cate-nanes of this type is that incorporating as one of its ring components,the p-electron-poor cyclophane35, cyclobis(paraquat-p-phenylene)(CBPQT4þ), which encircles single p-electron-rich units with highaffinities. The use of redox-active p-electron-rich units, such astetrathiafulvalene (TTF) in association with CBPQT4þ, enableselectrochemically induced molecular switching, which has beenused in diverse applications, such as information storage36, micro-scale mechanical actuation37, and molecular storage and release38.

Although many complex architectures have been achieved usingCBPQT4þ as the primary electron-poor ring, the interactions dominat-ing the workings of such mechanically interlocked molecules are thosebetween the CBPQT4þ ring and only onep-electron-rich occupant. Byexpanding the p-electron-poor ring system by one phenylene unit toyield cyclobis(paraquat-4,4′-biphenylene), 14þ—a cyclophane39

known to allow co-occupancy of two p-electron-rich guests—thestudy of guest–guest interactions occurring within the ring as well asthose occurring between the host and the guest has become possible.Herein is an account (Fig. 1) of how we have begun our studies ofTTF–TTF interactions between the two heterocycles housed withinthe cyclophane 14þ, either as the unfunctionalized units, as in the2:1 complex (TTF2 , 1).4PF6, or as part of simultaneously catenatedmacrocycles in the [3]catenanes 2.4PF6 and 3.4PF6.

Although ternary complexes of dioxynaphthalene units and 1 4þ

are well known, investigations of the nature of similar TTF com-plexes have never been conducted to our knowledge. The slow evap-oration of an emerald green MeCN solution containing an excess ofTTF, in the presence of 1.4PF6, afforded green crystals of highquality suitable for X-ray crystallographic analysis. The solid-statesuperstructure revealed (Fig. 1a) the inclusion of two TTF unitshoused comfortably within the cavity of 1 4þ with a TTF...TTFaverage interplanar distance of 3.77 Å. Electrochemical investi-gations (see Supplementary Information) performed on this[3]pseudorotaxane indicate that, because the TTF units are free to

2·4PF6 3·4PF6(TTF2 1)·4PF6

a b c

4 PF6–

N

N

N

N

SS

SS

O

OO

OO

O

OO

S S

S S

O

OO

OO

O

OO

++

++

N

N

N

N

S S

S SO

O

O OO

O

O

O OO

SS

SSO

O

OOO

O

O

OOO

++

++N

N

N

N

SS

SS

S S

S S

++

++

4 PF6–

4 PF6–

Figure 1 | Species of interest. a–c, The structural formulae (top), graphical representations (middle), and solid-state (super)structures (bottom) of the

[3]pseudorotaxane (TTF2 , 1).4PF6 (a), the [3]catenane 2.4PF6 (b) and the [3]catenane 3.4PF6 (c). The crystal (super)structures are displayed as

perspective views in ball-and-stick representations. The inclusion of the TTF units within 14þ constitutes a valuable supramolecular synthon, as well as

providing an ideal arrangement to form stable TTF dimers in the oxidized states of both [3]catenanes 2.4PF6 and 3.4PF6. Disordered PF62 counterions,

hydrogen atoms and MeCN solvent molecules have been omitted for clarity. The cyclobis(paraquat-4,4′-biphenylene) ring 14þ is illustrated in blue, the TTF

units in green, the dioxynaphthalene units in red, the butadiyne fragments in purple, the oxygens in pink, and the ethylene glycol carbons in grey.

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come and go from the complex, they behave independently; twoone-electron oxidation events are observed that correspondexactly with those for the parent TTF compound alone. Thissuggests that radical dimerization is not encouraged within thecavity of 1 4þ, but rather is repulsed by the Coulombic forces thatdevelop between the oxidized TTF units and the encircling tetraca-tionic cyclophane. This 2:1 complex has proved to be useful,however, as a valuable supramolecular synthon in the developmentof two related but further elaborated [3]catenanes. The introductionof the mechanical bond into the doubly threaded TTF architecturein the form of [3]catenanes has been used to endow stability that isnot observed within the [3]pseudorotaxane to TTF-dimerized stateswithin these mechanically interlocked molecules. This has enabled acomprehensive investigation of their properties.

In common with the template-directed syntheses of other high-order catenanes incorporating the cyclophane 14þ, the [3]catenane2.4PF6 (Fig. 1b) was prepared using the ‘clipping’ method, in whichthe formation of the tetracationic cyclophane was templated by thep-electron-rich TTF units present in the two separate macrocycles.Specifically, the reaction (Fig. 2a) of the partially formed cyclophane5.2PF6 and 6 with two equivalents of the TTF-containing macrocycle4 over 14 days at room temperature in DMF resulted in the formationof the [3]catenane 2.4PF6 in 33% yield; the related [2]catenane wasalso isolated in 5% yield. In addition to bearing the doubly threadedTTF motif, this [3]catenane also boasts the presence of secondaryp-electron-rich 1,5-dioxynaphthalene (DNP) units; these are com-petitive binding stations designed to take the place of the TTF unitswithin the tetracationic cyclophane following their ejection during(electro)chemical switching of the molecule.

Although bistability has traditionally been achieved within two-station systems, we recently prepared40 a single-station switchable[2]catenane in which the elimination of the secondary DNP unitresulted in simplified switching behaviour. Moreover, the

template-directed synthesis of this [2]catenane was facilitated bythe highly efficient ‘threading-followed-by-clipping’ protocol41,42.First, a bispropargyl-functionalized TTF derivative was threadedthrough the CBPQT4þ ring, after which a mild Eglinton Cu2þ-based oxidative alkyne homocoupling43,44—a reaction well-estab-lished for the efficient synthesis of catenanes42,45–49—induced macro-cyclization to give the product [2]catenane in high yield. We appliedthe same synthetic protocol to form the switchable [3]catenane3.4PF6 (Fig. 1c), incorporating the expanded cyclobis (paraquat-4,4′-biphenylene) 14þ as the templating p-electron-poor macrocycle.After mixing the p-electron-rich bispropargylated TTF derivative 7with the cyclophane 1.4PF6 in a 5:1 molar ratio in MeCN,Cu(OAc)2

.H2O (1.0 equivalent per terminal alkyne) was added tobring about the desired oxidative homocoupling, affording the[3]catenane 1.4PF6 (Fig. 2b) after 3 days as a green solid in a 50%overall yield. Remarkably, in this case, no [2]catenane or ring-in-ring complex by-products were isolated—a finding that highlightsthe fundamental differences between the ‘clipping’ and ‘threading-followed-by-clipping’ protocols used in the production of 2.4PF6and 3.4PF6, respectively. In addition to providing the [3]catenanein higher yield in a shorter reaction time, the resulting doublythreaded TTF motif of 3.4PF6 was framed by structural butadiyneunits instead of secondary binding stations as in 2.4PF6. Thisfeature has subsequently been found to have a profound influenceon the stability of the various oxidation states of the catenane.

The mechanically interlocked molecular structures of 2.4PF6 and3.4PF6 were confirmed by X-ray crystallographic analyses per-formed on the green single crystals of each, obtained by slowvapour diffusion of iPr2O into MeCN solutions of the catenanesat 0 8C. Perspective views of both [3]catenanes are displayed inFig. 1 as ball-and-stick representations. In each case, both TTFunits (shown in green) exist in their trans configurations, arehoused within the cyclobis(paraquat-4,4′-biphenylene) cavities

N

N

N

N

SS

SS

O

OO

OO

O

OO

S S

S S

O

OO

OO

O

OO

N

N

N

N

S S

S SO

O

O OO

O

O

O OO

SS

SSO

O

OOO

O

O

OOO

SS

SS

O

OO

O

OO

OO

+

7

1·4PF6

N

N

N

N

SS

SSO

O

OOO

O

O

OOO

N

N

N

N

Br Br

+DMF/23°C/14d

MeCN/23°C/72 h

Cu(OAc)2 · H2O

50%

33%

3·4PF6

2·4PF64

6

5·2PF6

a

b

4 PF6–

4 PF6–

4 PF6–

++

++

+

+

+

+

2 PF6–

++

++

++

Figure 2 | [3]Catenane synthesis. Template-directed syntheses of the [3]catenanes 2.4PF6 and 3.4PF6 in which two TTF units are threaded through a

cyclobis(paraquat-4,4′-biphenylene) ring and mechanically interlocked. a, The ‘clipping’ reaction of compound 5.2PF6 with 6 around two TTF-containing

macrocycles 4 results in the formation of the [3]catenane 2.4PF6 in 33% yield. b, An alternative ‘threading-followed-by-clipping’ protocol affords the related

[3]catenane 3.4PF6 in 50% yield, following the Cu2þ-catalysed Eglinton oxidative coupling of the terminal alkyne fragments extending from each threaded

TTF derivative 7 around the templating cyclophane 1.4PF6. DMF, N,N-dimethylformamide; MeCN, acetonitrile.

ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.749
















(shown in blue) and are stabilized by a combination of weak inter-actions. These stabilizing elements consist of p–p stacking contactswith close distances (averaging 3.58 Å) between the bipyridiniumfaces and the included TTF units, as well as [C–H...O] close con-tacts50, observed between some of the crown ether oxygens andthe a-bipyridinium hydrogens. As expected, the DNP units in2.4PF6 (shown in red) participate in alongside short interplanarcontacts (3.49 Å) with the outside faces of the bipyridiniumunits. In keeping with previously reported Eglinton-derivedcatenanes40,42, the butadiyne fragments of 3.4PF6 (shown inpurple) align themselves approximately parallel to the externalbipyridinium faces with interplanar distances of 3.64 Å. Of con-siderable interest within the mechanically interlocked frameworksof 2.4PF6 and 3.4PF6, as well as the parent supramolecularsynthon (TTF2 , 1).4PF6, is the relationship within the ‘molecularflask’ between the two side-by-side TTF units. When both TTF unitsare neutral—as in 2 4þ and 3 4þ—they are expected to experiencevery little electronic interaction with each other. The crystal struc-tures of 2 4þ and 3 4þ support this notion as the TTF units experi-ence a slipped-stacked arrangement within the tetracationiccyclophane, their average planes being separated by 3.68 Å ineach case. It is this close internal contact and the resulting highlocal concentrations of TTF units that provide 2.4PF6 and 3.4PF6with the possibility of forming strongly coupled TTF dimersacross their various oxidation states.

Cyclic voltammetry performed at room temperature was used toinvestigate the electrochemical switching mechanisms of 2.4PF6 and

3.4PF6 (Fig. 3c,e). The removal of the four electrons associated withthe two TTF units (labelled I to IV in Fig. 3) occurs in each case aseither a single-electron or concerted two-electron process.Examination of the full range cyclic voltammetry spectrum of2.4PF6 in Fig. 3c reveals two one-electron oxidations occurringwith peak potentials at 400 and 600 mV (labeled I and II), followedby a two-electron oxidation at 810 mV (labelled III and IV) versusAg/AgCl. By comparison, the complete potential scans for the con-stitutionally different [3]catenane 3.4PF6 reveal four separate one-electron oxidation events (labelled I to IV in Fig. 3e), occurringwith peak potentials at 405, 655, 1,320, and 1,470 mV versusAg/AgCl. In both [3]catenanes, the first two oxidation processes(I and II) result in the formation of stabilized mixed-valence,(TTF)2

†þ, and radical-cation, (TTF†þ)2, dimers. A large potentialgap exists between the first two (I and II) and final two (III andIV) oxidation events of 3.4PF6, amounting to shifts in excess of530 mV for the second oxidation of the TTF units compared withthe 4,4′(5′)-bis[(2-(2-(2-hydroxyethoxy)ethoxy)ethoxy)-methanol]tetrathiafulvalene control compound S1 (Fig. 3d)40. This indicates thata stabilized species is formed during the second oxidation event (II).

In order to describe these oxidation events in both catenanes, wepropose two similar switching mechanisms shown in Fig. 4,whereby the TTF units of 2.4PF6 and 3.4PF6 are oxidized individu-ally in three and four separate steps, respectively. This leads to twostable dimers of tetrathiafulvalene—as the mixed-valence state(TTF)2

†þ, labelled 2 5þ and 3 5þ, and the radical-cation dimer(TTF†þ)2, labelled 2 6þ and 3 6þ—as well as a novel radical-cation/butadiyne dimer, labeled 3 7þ. The secondary bindingstations within 2.4PF6 act in combination to destabilize theradical-cation dimer state found in 2 6þ, such that a concertedtwo-electron oxidation, occurring at 810 mV vs Ag/AgCl, causesthe circumrotation of the TTF2þ dicationic units from inside the tet-racationic cyclophane so that the DNP units can take up occupancywithin the cyclophane in 2 8þ. Owing to a lack of secondary bindingstations to vie for co-occupancy within the cyclophane, the oxidizedstates of 3, especially 3 6þ, enjoy considerable stability, as evidencedby the much larger potentials needed to oxidize the TTF units fully.This stability is maintained despite the inherent destabilizing inter-actions associated with the inclusion of positively charged elementsconstituting the (TTF†þ)2 radical-cation dimer within a tetracatio-nic capsule. The formation of 3 7þ occurs after the stepwise ejectionof the TTF2þ dication from inside the tetracationic cyclophane—anact that causes the circumrotation of one of the crown ethers suchthat its butadiyne unit occupies the cavity of the cyclophane—leaving a TTF†þ radical-cation associated with the other crownether and the newly presented butadiyne to establish a stabilizinginteraction inside the cyclophane. The final oxidation of the3 7þ species results in the ejection of the second TTF2þ dicationfrom inside the tetracationic cyclophane in 3 8þ, forming the trans-lational state where both dications occupy alongside positions withrespect to the bipyridinium units on the cyclophane.

Further evidence of the translational changes involved in the for-mation of 2 8þ and 3 8þ is brought into focus once again when weconsider the re-reduction of these highly charged species. Thefirst re-reductions occur as two-electron processes (labelled V),reforming the stabilized radical-cation (TTF†þ)2 dimer in 2 6þ

and 3 6þ. These re-reductions occur very near the potential for thefirst re-reductions of the TTF control compounds (Fig. 3b,d). Thisindicates that the TTF2þ dications, after being removed from thecavity of the tetracationic cyclophane, are re-reduced to radical-cations while residing outside the cyclophane. It is worth notingthat the reformation of the (TTF†þ)2 dimer in 2 6þ and 3 6þ stronglyfavours the insertion of these dicationic (TTF†þ)2 radical-cationdimers inside the tetracationic cyclophane, despite the chargerepulsion they must experience in that position. Further reductionsof the molecules occur as two distinct one-electron events with the

50 μA

a

b

c

d

e

I

IIIII

IV

VVI

–1.5 –1.0 –0.5 0.0 0.5

E (V vs Ag/AgCl)

1.0 1.5 2.0

I

IIIII, IV

VVIVII

VII

Figure 3 | Cyclic voltammetry. a–e, Full range voltammagrams (1 mM in

MeCN, 100 mM Bu4NPF6, 200 mV s21, performed at room temperature) of

1.4PF6 (a), the control TTF macrocycle 4 (b), the [3]catenane 2.4PF6 (c),

the control triethylene glycol TTF compound S1 (d), and the [3]catenane

3.4PF6 (e). Two one-electron and one two-electron oxidation processes exist

for the [3]catenane 2.4PF6 occurring with peak potentials at 400 (I),

600 (II), and 810 (III,IV) mV versus Ag/AgCl. Four distinct one-electron

oxidation processes are observed for the [3]catenane 3.4PF6 at 405 mV (I),

655 mV (II), 1.32 V (III), and 1.47 V (IV). Each of these processes occurs at

a potential significantly shifted from the analogous redox process for control

compounds 4 (b) and S1 (d). These perturbed oxidation potentials indicate

the formation of stabilized mixed-valence (TTF)2†þ and radical-cation

(TTF†þ)2 dimer states for both [3]catenanes, as well as a TTF†þ-butadiyne

interaction in 3.4PF6.




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34+ 35+ 36+ 37+

24+ 25+ 26+ 28+

38+

–1 e– –1 e– –1 e– –1 e–

+2 e–

+1 e–+1 e–

I II

III IV

V

VIVII

Mixed-valence Radical-cation

–1 e–

+1 e–

I

VII

–1 e–

+1 e–

II

VI

III, IV

V

–2 e–

+2 e–

Mixed-valence Radical-cationa

b

Disproportionation

Figure 4 | Stepwise oxidative dimerization mechanisms. a,b, The proposed oxidative switching mechanisms exhibited by the [3]catenane 2.4PF6 (a) and the

[3]catenane 3.4PF6 (b) each proceed in successive distinct oxidation events (labelled I to IV), forming stable intermediaries in the 5þ, 6þ, and 8þ oxidation

states. States 5þ and 6þ are composed of stabilized mixed-valence (TTF)2†þ and radical-cation (TTF†þ)2 dimers housed within the ‘molecular flask’ created

by each [3]catenane’s mechanically interlocked architecture. The analogous transient intermediate to 37þ is not observed in compound 2, presumably

because DNP co-occupancy within the cyclophane is more favourable following the higher oxidations (III, IV). Two molecules of the intermediate 37þ are

believed to disproportionate to give the more stable 36þ and 38þ combination. For each catenane, the re-reduction of the 8þ to the 4þ states proceeds in

three distinct steps labelled V to VII, reforming the stable intermediates 6þ and 5þ along the path to the ground state 4þ .

3.56 Å3.68 Å

34+ 35+ 36+

3.42 Å

Figure 5 | Solid-state structural analysis of the [3]catenane 3.4PF6, the TTF mixed-valence species 3.4PF6.ClO4 and the radical-cation dimer entity

3.2PF6.4ClO4. The structure of 34þ (left), 35þ (centre) and 36þ (right) displayed in two different side-on views, along with the average TTF interplanar

spacings. Oxidation of the TTF pair results in significant alteration of the relative arrangement, as well as the interplanar spacings of the two units. These

structural changes are related directly to the electronic bonding of the dimer structures housed within the ‘molecular flask’ across the 5þ and 6þ oxidation

states. Disordered counterions, hydrogen atoms and disordered solvent molecules are omitted for clarity. The cyclobis(paraquat-4,4′-biphenylene) ring is

shown in blue, the TTF units in green, the butadiyne fragments in purple, the oxygens in pink, and the ethylene glycol carbons in grey.






mixed-valence (TTF)2†þ dimers 2 5þ and 3 5þ, which form after the

first reduction (labelled VI) before returning to the ground states2 4þ and 3 4þ after the second (labelled VII). The application offurther reduction potentials results in two two-electron reversibleprocesses for the redox events of both bipyridinium moieties tothe radical-cation and neutral states. Both bipyridinium units areequivalent in the symmetrical [3]catenanes and hence the reductionand re-oxidation of these moieties occur simultaneously withrespect to each redox event. This behaviour is very similar to thatobserved (Fig. 3a) for the control tetracationic cyclophane 1.4PF6.

Because of the stability of the TTF-dimerized oxidized states of3.4PF6, we succeeded in growing single crystals of both 3 5þ and3 6þ. Each species was generated in the bulk solution by titratingeither one or two equivalents of Fe(ClO4)3 oxidant in MeCN intoa solution of 3.4PF6 in MeNO2. Single crystals of X-ray qualitywere grown after the slow vapour diffusion of Et2O into thestock solutions of 3 5þ at 0 8C and 3 6þ at –20 8C. The solid-statestructures of 3 4þ, 3 5þ and 3 6þ are rendered in two different side-on views in Fig. 5 in order to indicate the relationship establishedbetween the encircled TTF units in all three oxidation states. The

4.04.55.05.56.06.57.07.58.08.59.09.5

Hα HβHPh HTTF

HCH2 HCH2

HCCCH2O

δ (ppm)

0 equiv.oxidant

300

0.0

0.5

Abs

orpt

ion

1.0

1.5

2.0

2.5

600 900 1,200 1,500 1,800 2,100

λ (nm)

2 equiv.oxidant

4 equiv.oxidant

iv

iii

ii

iiiiviii

iiia

b

N

N

N

N

SS

SS

O

OO

OO

O

OO

S S

S S

O

OO

OO

O

OO

HCCCH2OH

Hβ

Hα

HCH2

HCH2

HTTF

HPh

N

N

N

N

SS

SS

O

OO

OO

O

OO

S S

S S

O

OO

OO

O

OO

N

N

N

N

SS

SS

O

OO

OO

O

OO

O

OO

OO

O

OO

SS

SS

++

++

HPh

++

++

++

++

0 equiv. — 34+ 2 equiv. — 36+ 4 equiv. — 38+

0 equiv.

1 equiv. Fe(ClO4)3

2 equiv. Fe(ClO4)3

3 equiv. Fe(ClO4)3

4 equiv. Fe(ClO4)3

Figure 6 | Spectroscopic evidence detailing the formation of TTF-based dimers on stepwise oxidation of the [3]catenane 3.4PF6. a, The UV–Vis–NIR

spectra of 341 (0.3 mM in MeCN, 1.0 cm pathlength, room temperature) and the oxidized states arising from the stepwise addition of 1.0–4.0 equivalents of

Fe(ClO4)3 (solution in MeCN) reveal absorbances indicative of (i) TTF-bipyridinium charge-transfer, (ii) the mixed-valence (TTF)2†þ dimer, (iii) the radical-

cation (TTF†þ)2 dimer, and (iv) the TTF2þ dication. b, Similarly, 1H NMR spectra (600 MHz, CD3CN, 293 K) of 341, 361 and 381 were recorded following the

addition of 0, 2.0, and 4.0 equivalents of tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Note that the resonances marked with red crosses in the

second spectrum arise from the reduced chemical oxidant. The presence of well-defined signals in the spectrum of 361 supports the conclusion that the

(TTF†þ)2 radical-cation dimer is a diamagnetic species.







negative counterions are not shown in Fig. 5 for clarity, but theirpresence—positionally locked although rotationally disordered—clearly indicates the oxidation state of each [3]catenane species.Specifically, four PF6

2 counterions accompany compound 3 4þ inthe solid state, whereas one ClO4

2 and four PF62 counterions

balance the charge on 3 5þ, and the charge on 3 6þ is cancelled byfour ClO4

2 and two PF62 counterions. Of greater interest are the

relative geometries and interplanar spacings adopted by the TTFunits within the cyclophane in each case. The slipped-stack arrange-ment observed between the TTF units in 3 4þ, separated by 3.68 Å,changes dramatically to a laterally aligned arrangement of the TTFunits, which are drawn much closer together—separated by only3.56 Å—in the (TTF)2

†þ mixed-valence state 3 5þ.An even more pronounced geometrical change occurs in 3 6þ,

wherein the TTF units overlap completely—separated by a mere3.42 Å—and form a (TTF†þ)2 radical-cation dimer within thetetracationic cyclophane. These distances and geometries are inagreement with those in previously reported structures of oxidizedTTF dimers in the solid-state2,51. Moreover, their presence assingly and doubly charged states within the tetracationic cyclophaneindicates a stabilizing interaction arising from their radical dimeri-zation, which overpowers the Coulombic repulsion experiencedbetween the charged species. It has been shown in the literature52

using high-level ab initio methods that the oxidative dimerizationof lone TTF units in solution is stabilized by solvent- and counter-ion-bridging; these types of counterion bridging interactions couldbe occurring within the solid-state structures of 3 5þ and 3 6þ whereshort [C-H...O] and [C-H...F] contacts are noted between the TTFunits and the surrounding ClO4

2 and PF62 counterions (see

Supplementary Information). The ability to grow and solve crystalstructures of excited states of donor-acceptor mechanically interlockedmolecules provides a powerful means of understanding their proper-ties across the complete switching cycle. We are currently applyingthese protocols to other oxidatively switchable molecular machines.In addition to the solid-state evidence, high-resolution mass spectraof 35þ and 36þ (see Supplementary Information) recorded from a sol-ution of 3.4PF6, following its oxidation with 1.0 and 2.0 equivalents ofFe(ClO4)3, provide further evidence for their stabilities within both thesolution and gas phases.

Further spectroscopic evidence for the occurrence of the oxi-dative TTF dimerization process, as well as the structural conse-quences of the oxidations, was provided by UV–Vis–NIR and 1HNMR spectroscopies. Titrations were carried out using Fe(ClO4)3for the acquisition of the UV–Vis–NIR spectra and tris(4-bromo-phenyl)ammoniumyl hexachloroantimonate (Magic Blue)53 torecord the 1H NMR spectra. These chemical titrations led to the oxi-dation of 3.4PF6 to each of its oxidation states (see SupplementaryInformation for similar studies performed on the [3]catenane2.4PF6). UV–Vis–NIR spectra (Fig. 6a), recorded following theaddition of 1.0–4.0 equivalents of the chemical oxidant, display (i)the characteristic40 absorbances for TTF-bipyridinium charge trans-fer in 3 4þ; (ii) a broad NIR signal at 2,040 nm, as well as anotherband at 613 nm, which was expected2,20,22,23 for (TTF)2

†þ mixed-valence dimers in 3 5þ; (iii) two strong bands centred at 806 and555 nm, which can be attributed2,19,22,23 to the (TTF†þ)2 radical-cation in 3 6þ; and (iv) a sharp absorbance at 375 nm, character-istic40 of the TTF2þ species present in 3 7þ and 3 8þ. The appearanceof these absorbances for species generated under equilibrationconditions in solution at ambient temperature and pressure in airsupports the proposed switching mechanism, which relies uponstable TTF mixed-valence and radical-cation dimer interactionsformed from the first and second oxidations, respectively, startingwith 3.4PF6.

Further structural data was gleaned from 1H NMR spectra(Fig. 6b) for all diamagnetic states of 3—namely, 3 4þ, 3 6þ and3 8þ—following oxidation. The ground-state spectrum of 3 4þ is

Disproportionation

1 equiv.

2 equiv.

3 equiv.

4 equiv.

d

a

c

b

3,400 3,420 3,440

Magnetic field (gauss)

3,460

Figure 7 | Steady-state continuous wave EPR spectroscopy. Continuous

wave EPR spectra of the [3]catenane 3.4PF6 recorded during stepwise

titration with Fe(ClO4)3. a, From 0 to 1.0 equivalents. b, From 1.0 to 2.0

equivalents. c, From 2.0 to 3.0 equivalents. d, From 3.0 to 4.0 equivalents of

oxidant. The growth of a radical signal in response to the formation of 35þ,

as well as its attenuation on the formation of 36þ, indicate the paramagnetic

character of the former and diamagnetic of the latter. The lack of radical

character upon the addition of 3.0 equivalents of oxidant supports the

notion of the disproportionation of 37þ.






complicated by the presence of interconverting cis and trans config-urational isomers in the case of each TTF unit within the cavity—each conferring their respective local symmetries throughout the[3]catenane. Nonetheless, the resonances attributed to all of theimportant functions in the molecule were assigned using multidi-mensional techniques (see Supplementary Information) and areannotated on the spectra in Fig. 6b. After the addition of 2.0 equiva-lents of Magic Blue, both TTF resonances disappeared and the othersignals broadened. The presence of clearly defined signals for theoxidized species supports the fact that the (TTF†þ)2 radical-cationdimer is spin-coupled within 3 6þ. Similar behaviour has beenreported by Kim and co-workers19 who have also obtained an 1HNMR spectrum of the encapsulated (TTF†þ)2 radical cation-dimer—in their case as a supramolecular entity located insidecucurbit[8]uril. Finally, complete oxidation, following the additionof an excess of Magic Blue to form 3 8þ, resulted in the appearanceof signals for the TTF2þ protons, as well as the migration of the res-onances arising from the methylene groups neighbouring the buta-diyne units, indicating that circumrotation had occurred40 such thatthe butadiyne units moved inside the cavity of the tetracationiccyclophane, whereas the TTF2þ units were excluded from its cavity.

In order to probe the spin properties of 3.4PF6 in each of its oxi-dation states, we performed continuous wave electron paramagneticresonance (EPR) spectroscopy during the titration of the oxidantFe(ClO4)3 into a solution of the [3]catenane in MeCN under anambient atmosphere at 295 K. During the slow addition of theoxidant, a signal for the radical emerged, reaching a maximumintensity after the addition of 1.0 equivalent—producing the(TTF)2

†þ mixed-valence state of 3 5þ—as shown in Fig. 7a. Theradical signal of 3 5þ is broad and lacks hyperfine splitting as a con-sequence of slow tumbling of the molecule in solution. A broadsignal, also lacking hyperfine splitting, was observed (seeSupplementary Information) for the TTF model compound 7upon oxidation with 1.0 equivalent of Fe(ClO4)3. Continuedaddition of the oxidant resulted in the attenuation of the radicalsignal, following the addition of a total of 2.0 equivalents. Thisresult was surprising at first, considering that another radical

species is formed, at least formally, during the continued oxidationto 3 6þ. The attenuation in the signal can be attributed to the stronglypaired (singlet) nature of the two interacting formal radical speciesconstituting the (TTF†þ)2 radical-cation dimer. The presence of avery small radical signal can be attributed to a finite population ofTTF co-conformations in 3 6þ that are not encircled, wherein thefree TTF†þ units behave as noninteracting radical species. Duringthe addition of another equivalent of oxidant to give a total of 3.0equivalents, the expected emergence of the radical signal did notoccur. This result is best explained by the disproportionation ofthe transient 3 7þ species to form the stable 3 6þ and 3 8þ, both ofwhich are EPR silent. Finally, the addition of 4.0 total equivalentsof the oxidant causes the complete oxidation to 3 8þ, which bearsno radical character and is completely EPR silent.

The experimental data is commensurate with [3]catenanes exhi-biting stable TTF dimeric states as the mixed-valence (TTF)2

†þ andradical-cation (TTF†þ)2 forms in 5þ and 6þ states, respectively,while also being suggestive of a novel TTF†þ-butadiyne interactionin 3 7þ. In order to investigate the exact nature and stability of thesedimeric species found in the different oxidation states of each[3]catenane, we have used quantum-mechanical modelling.Density functional theory (DFT) using the M06 suite of func-tionals54,55—a combination that we have shown previously toprovide accurate structural and energetic descriptions of mechani-cally interlocked molecules40,56—shed light on the relative energeticlandscape of the [3]catenane across its five different oxidation states.We performed geometry optimizations on the both-in, one-in-one-out, and both-out co-conformations. This convention for namingdenotes the relative position of the TTF units—for each of the oxi-dation states, 3 4þ–3 8þ, at the M06-L/6-31G** level of theory in thegas phase. Comparison of the calculated structures with the exper-imental solid-state structures of 3 4þand 3 5þ revealed very similarvalues for both the interplanar distances and the relative orientationof the TTF units. However, the placement of the more flexible ethyl-ene glycol chains within the calculated and experimental structuresshows significant deviations; these differences are expected as thegeometrical minimizations were performed in the gas phase.Energies were calculated from the solved structures using theM06-2X functional, including a continuum solvation correctionfor MeCN.

A detailed analysis (Fig. 8) of the calculated relative energiessuggests that for the 3 4þ, 3 5þ and 3 6þ oxidation states the both-in co-conformation is favoured by �20 kcal mol21. In fact, thisco-conformation is more strongly favoured in the higher oxidizedstates—by 6 kcal mol21 more in 3 5þ and 3 kcal mol21 more in3 6þ—despite the inclusion of additional charged species withinthe tetracationic cyclophane. We attribute this increased stabilityto the formation of (TTF)2

†þ mixed-valence and (TTF†þ)2radical-cation dimerized states in these oxidized species. It is impor-tant to state yet again that because the TTF-containing crown ethersin 3.4PF6 do not contain secondary p-electron-rich recognitionsites, bearing as they do only glycols and the butadiyne unit, nocompetition exists for occupancy within the cyclophane. This lackof competition between different recognition motifs allows evenweakly stabilized co-conformations to be observed and indeed occu-pied almost quantitatively in this single-station [3]catenane40. Anexample of such a weakly stabilized co-conformation is the novelTTF†þ-butadiyne interaction, which exists in the one-in-one-outco-conformation of 3 7þ. It was found by calculation to be slightlymore stable (�2 kcal mol21) than the both-out co-conformationof the same oxidation state. The observation of this stabilized inter-action is consistent with the cyclic voltammetry data in which athird sequential oxidation state exists (III in Fig. 3e), accountingfor the formation of 3 7þ. This interaction is also verified by themuch larger energetic penalty (25 kcal mol21) for converting theone-in-one-out co-conformation to the both-out co-conformation

Both-in60

40

20

–20

34+

35+

36+

37+

38+

–40

–60

Rel

ativ

e en

ergy

(kc

al m

ol–1

)

0

One-in-one-out Both-out

Figure 8 | Calculated energy landscape. DFT calculated relative energies for

the five oxidation states 34þ–38þ of the three co-conformations shown.

For each oxidation state the both-in co-conformation was arbitrarily set at

0 kcal mol21. Although the absolute energies between each oxidation state

are not directly comparable, the relative energetic differences between

co-conformations may be compared. The data indicate stable dimeric

structures (TTF)2†þ and (TTF†þ)2 in 35þ and 36þ, respectively, as well as a

stabilized TTF†þ-butadiyne interaction in 37þ.







of 3 6þ—a process that would break the stabilizing TTF†þ-buta-diyne interaction—as compared with the same process in 3 5þ.

ConclusionWe have demonstrated the efficient formation and enhanced stab-ility of (TTF)2

†þ mixed-valence and (TTF†þ)2 radical-cationdimers as well as an unprecedented TTF†þ-butadiyne dimerwithin ‘molecular flasks’ composed of mechanically interlocked[3]catenanes. The unique nature of the mechanical bond is respon-sible for the high stability of these electrochemically accessibledimeric states by providing both ideal geometric arrangementsand high apparent concentrations, while also enabling structuraland translational switching to occur. This affinity umpolungdrives the dimerization and stepwise circumrotation upon the suc-cessive oxidation of the TTF units such that finite structuralcontrol emerges through electronic stimulation. The ability to gen-erate and study in detail both the discrete mixed-valence andradical-cation tetrathiafulvalene dimers under ambient conditionswithin the mechanically interlocked framework of a [3]catenanemarks a departure from the current state-of-the-art and positionsthese and similar molecules as attractive candidates for electronicand spintronic device applications while also serving as valuablesynthetic models of natural analogues57.

Received 18 February 2010; accepted 14 June 2010;published online 25 July 2010

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AcknowledgementsThe authors acknowledge support from the Air Force Office of Scientific Research underthe Multidisciplinary Research Program of the University Research Initiative (awardnumber FA9550-07-1-0534, “Bioinspired Supramolecular Enzymatic Systems”) and theNational Science Foundation under CHE-0924620. M.R.W. was supported by the National

Science Foundation under Grant No. CHE-0718928. Proteomics and Informatic serviceswere provided by the CBC-UIC Research Resources Center Proteomics and InformaticsServices Facility, which was established by a grant from The Searle Funds at the ChicagoCommunity Trust to the Chicago Biomedical Consortium. Use of the Advanced PhotonSource was supported by the US Department of Energy, Office of Science, Office of BasicEnergy Sciences, under Contract No. DE-AC02-06CH11357. Use of the LS-CAT Sector 21was supported by the Michigan Economic Development Corporation and the MichiganTechnology Tri-Corridor (Grant 085P1000817). J.M.S. acknowledges the National ScienceFoundation for a Graduate Research Fellowship and Northwestern University for aPresidential Fellowship. M.T.C. thanks the Link Foundation for a fellowship. G.C. thanksthe EPSRC for support (GR/M32702, EP/E018211). G.C. thanks R. C. Hartley for his helpregarding preliminary EPR measurements and Patrice Woisel for advice regarding thepreliminary synthesis of cyclobis(paraquat-4,4′-biphenylene) and its complexationwith TTF.

Author contributionsJ.M.S., A.C., G.C., and J.F.S. conceived the project and prepared the manuscript. J.M.S.,A.C., G.B., and S.K.D. synthesized the different molecules studied in this work. R.S.F.,A.A.S., M.A.O., and A.M.Z.S. were responsible for growing single-crystals and/or solvingX-ray crystal structures. F.D., S.G.H., and S.T.C. were involved in the preliminaryinvestigations of the complexation behaviour of 1 and TTF. G.M.R. was responsible forsolving the X-ray structure of (TTF2 , 1). A.T. and A.C.F. were responsible forelectrochemical studies. M.T.C., R.C., and M.R.W. were responsible for the EPR studies.J.L.S. was responsible for the mass spectrometry. D.C.F. was responsible for NMRinvestigations. D.B., E.T., and W.A.G.III performed DFT calculations. W.F.P. providedinvaluable insights into the switching mechanisms.

Additional informationThe authors declare no competing financial interests. Supplementary information andchemical compound information accompany this paper at www.nature.com/naturechemistry. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressedto G.C. and J.F.S.





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