Redox-induced ring shuttling and evidence for folded structures in long and flexible two-station rotaxanes

Article abstract of DOI:10.1135/cccc20031488

Two dumbbell-shaped components with tetraarylmethane-type stoppers - one hydrophobic and one hydrophilic - and a rod-like section containing a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety as electron-donating units, and their [2]r

Full text of DOI:10.1135/cccc20031488

1488
Yamamoto et al.:
REDOX-INDUCED RING SHUTTLING AND EVIDENCE FOR FOLDED
STRUCTURES IN LONG AND FLEXIBLE TWO-STATION ROTAXANES
Tohru YAMAMOTOa1, Hsian-Rong TSENGa2, J. Fraser STODDARTa3,*,
b3
Vincenzo BALZANIb1, Alberto CREDIb2, Filippo MARCHIONI and
b4,
Margherita VENTURI
*
a
Department of Chemistry and Biochemistry, University of California,
Los Angeles 405 Hilgard Avenue, Los Angeles, CA 90095-1569, U.S.A.;
e-mail: tohru_yamamoto@sdk.co.jp, tseng@chem.ucla.edu, stoddart@chem.ucla.edu
Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 40126 Bologna,
Italy; e-mail: vbalzani@ciam.unibo.it, acredi@ciam.unibo.it, marchioni@ciam.unibo.it,
1
2
3
b
1
2
3
4
mventuri@ciam.unibo.it
Received Jun e 27, 2003
Accepted July 9, 2003
Dedicated to Professor Sergio Roffia on the occasion of his retirement.
Two dum bbell-sh aped com pon en ts with tetraarylm eth an e-type stoppers – on e h ydroph obic
an d on e h ydroph ilic – an d a rod-like section con tain in g a tetrath iafulvalen e (TTF) un it an d
a 1,5-dioxyn aph th alen e (DNP) m oiety as electron -don atin g un its, an d th eir [2]rotaxan es, in -
corporatin g th e cyclobis(paraquat-p-ph en ylen e) (CBPQT4+) cycloph an e as th eir electron -
acceptin g rin g com pon en t, h ave been syn th esized, th e latter usin g tem plate-directed proto-
cols. Th e two am ph iph ilic [2]rotaxan es, wh ich differ from each oth er on ly in th e len gth s of
th e polyeth er ch ain s associated with th eir h ydroph ilic stoppers, were design ed in order (i) to
h ave th em exh ibit en h an ced am ph iph ilicities an d, by alterin g th e len gth s of polyeth er
ch ain s, (ii) to im prove th e qualities of th eir Lan gm uir–Blodgett film s, an d by rem ovin g th e
ph en olic residues, (iii) to in crease th e oxidative stabilities of th ese switch able m olecules, an d
so exten d th e lifetim es of electron ic devices fabricated from am ph iph ilic h ysteretic m olecu-
lar switch es of th is type. UV-VIS absorption an d 1H NMR spectra, as well as electroch em ical
m easurem en ts, sh ow th at both [2]rotaxan es exist to all in ten ts an d purposes in solution as
th e tran slation al isom er in wh ich th e CBPQT4+ cycloph an e surroun ds th e TTF un it. Eviden ce
h as also been obtain ed for th e presen ce in solution of folded con form ation s of th ese
[2]rotaxan es. Wh ile ox/red stim ulation of th e TTF un it causes sh uttlin g of th e CBPQT4+
cycloph an e between th e TTF an d DNP station s, reduction of CBPQT4+ causes un foldin g of
th e [2]rotaxan e m olecules.
Keyw ord s: Molecular sh uttles; Rotaxan es; Tetrath iafulvalen e; Cycloph an es; Ch arge-tran sfer
in teraction s; Bipyridin ium ion s; Ph otoch em istry; Electroch em istry.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)
doi:10.1135/cccc20031488

Folded Structures in Long and Flexible Two-Station Rotaxanes
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Two-station [2]rotaxan es are curren tly th e subject of exten sive in vestiga-
tion s sin ce con trollable sh uttlin g of th e rin g between th e two station s can
be exploited for m ech an ical an d switch in g purposes1,2 in con n ection with
th e con struction of m olecular-level m ach in ery an d in form ation proces-
sors3. In con tin uan ce of our studies in th is field4,5, we h ave syn th esized two
n ew [2]rotaxan es, con sistin g of th e electron -acceptin g cyclobis(paraquat-
p-ph en ylen e) (CBPQT4+) rin g com pon en t an d two n ovel dum bbell-sh aped
com pon en ts. Th e precursor dum bbell com poun ds, wh ich act as efficien t
tem plates for th e form ation of th e en circlin g CBPQT4+ rin g, com prise two
tetraarylm eth an e-type stoppers (on e h ydroph obic an d th e oth er h ydro-
ph ilic) an d a rod-like section con tain in g tetrath iafulvalen e (TTF) an d 1,5-di-
oxyn aph th alen e (DNP) derivatives as electron -don atin g un its wh ich act as
recogn ition sites (station s) for th e electron -acceptin g rin g. Th e two dum b-
bells differ from each oth er as a result of th e len gth s of th e polyeth er ch ain s
associated with th eir h ydroph ilic stoppers. Th ey are also an approxim ately
on e-to-on e m ixture of cis an d trans isom ers as a result of rotation (fast on
th e laboratory tim escale an d slow on th e NMR tim escale) aroun d th e form -
al carbon –carbon double bon d between th e two five-m em bered rin gs in
th eir TTF un its5a,5c. We will discover th at th is con figuration al isom erism is
also expressed in th e two [2]rotaxan es. Th e structural form ulas for th e
rotaxan es 1·4PF6 an d 2·4PF6 an d of th eir respective dum bbell com poun ds 3
an d 4 are sh own in Fig. 1.
Presen tly, our design of am ph iph ilic, bistable rotaxan es is bein g driven
(Fig. 2) by a desire to use com poun ds wh ich are well ch aracterized in solu-
tion , both with respect to th eir am ph iph ilic properties an d th eir redox
switch in g beh aviors, an d are an ticipated to be robust an d to act coh eren tly
in con den sed ph ases an d also in solid-state devices. To th is en d, we decided
to rem ove th e oxidatively sen sitive ph en olic residues from th e h ydroph ilic
stoppers from earlier gen eration s7 of m olecular switch es an d so im part im -
proved stabilities upon electron ic devices based on am ph iph ilic, bistable
rotaxan es by givin g th em m uch lon ger lifetim es durin g th eir voltage-driven
redox switch in g. Sin ce it is of param oun t im portan ce to in corporate th ese
m olecules in an orien ted an d coh eren t fash ion in such devices, th ey n eed
to be self-organ ized, for exam ple, in a two-dim en sion al fash ion at th e
air–water in terface as Lan gm uir–Blodgett (LB) film s. With th is objective in
m in d, we h ave varied th e len gth s of polyeth er ch ain s associated with th e
h ydroph ilic stoppers. In con trast with previously in vestigated7 com poun ds
of th is fam ily, th e h ydroph ilic stoppers presen t in 14+ an d 24+ sh ould ex-
h ibit m uch weaker electron -don or properties on accoun t of th e fact th at
th e four arom atic rin gs in th e tetrasubstituted tetraarylm eth an e core of th e
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Yamamoto et al.:
FIG. 1
Structural form ulas an d graph ical represen tation s of [2]rotaxan es 1·4PF6 an d 2·4PF6 an d th eir
dum bbell-sh aped com pon en ts 3 an d 4, both of wh ich are an approxim ately on e-to-on e m ix-
ture of cis an d trans isom ers
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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Yamamoto et al.:
h ydroph ilic stoppers are (i) n ot directly lin ked to oxygen atom s an d (ii) are
sterically in accessible for efficien t π–π stackin g. Th e presen t in vestigation
h as been un dertaken to establish wh eth er th is structural m odification in flu-
en ces th e redox-con trolled sh uttlin g an d con form ation al foldin g observed
in MeCN solution s of th ese two [2]rotaxan es. Earlier studies5b,5c,6 in solu-
tion h ave sh own th at, for related lon g an d flexible pseudorotaxan es an d
rotaxan es, th e foldin g-un foldin g processes can be ren dered con siderably
m ore com plex sin ce alon gside in teraction s of th e CBPQT4+ rin g with th e
DNP un it are probably supplem en ted by π–π stackin g in teraction s in volvin g
readily accessible electron -rich arom atic rin gs in th eir h ydroph ilic stoppers.
Previous in vestigation s7 h ave sh own th at, in pseudorotaxan es, rotaxan es,
an d caten an es con tain in g TTF- an d DNP-type station s, on e predom in an t
tran slation al isom er8 is presen t – n am ely, th e on e in wh ich th e CBPQT4+
rin g en circles th e TTF un it sin ce it is th e m uch stron ger electron don or of
th e two. Here, we report our in vestigation on (i) th e ph otoph ysical proper-
ties an d (ii) th e electroch em ical beh avior of th e [2]rotaxan es 14+ an d 24+
,
an d of th e related dum bbell com poun ds 3 an d 4 in MeCN solution at room
tem perature. Model com poun ds for th e h ydroph obic an d h ydroph ilic stop-
pers – n am ely, 5 an d 6, respectively – are portrayed in Fig. 3, alon g with th e
structural form ulas for CBPQT·4PF6, TTF, an d 1,5-dim eth oxyn aph th alen e
(DMN). Th ese com poun ds will be used as referen ces in th e ph otoph ysical
an d electroch em ical in vestigation s for th e ph oto- an d electroactive un its
FIG. 3
Structural form ulas of m odel com poun ds 5, 6, TTF, CBPQT·4PF6, an d DMN
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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presen t in th e rotaxan es an d th e free dum bbells. Alth ough th ese in vestiga-
tion s did brin g som e un derstan din g to th e issue of con form ation al
isom erism between folded an d un folded con form ation s of 14+ an d 24+, th ey
did n ot allow us to probe th e con figuration al isom erism in volvin g cis an d
1
trans isom ers of 14+, 24+, 3, an d 4. An d so we record h ere (iii) h ow H NMR
spectroscopy h as been used to probe th e con figuration al isom erism presen t
in all th ese com poun ds in solution . Fin ally, we describe (iv) th e ch em ical
ox/red stim ulation con dition s th at prom ote “clean ” m ech an ical switch in g
between th e differen t redox states of th e two [2]rotaxan es.
RESULTS AND DISCUSSION
Synthesis
Th e procedures used in th e syn th esis of th e am ph iph ilic [2]rotaxan es
1·4PF6 an d 2·4PF6 are described in Sch em es 1–3. Th e two h ydroph ilic stop-
pers 15 an d 16 were obtain ed (Sch em e 1) in 99 an d 96% yields, respec-
tively, by treatin g th e alcoh ols 13 an d 14 with SOCl2 in CH2Cl2. Th e
preparation of th e alcoh ols 13 an d 14 begin s with ch lorin ation of th e
tri-p-tolylm eth an ol9 (7) in acetyl ch loride, followed by a Friedel–Craft reac-
tion usin g an excess am oun t of ph en ol at 110 °C to afford com poun d 8 in
41% yield. Subsequen t triflation an d Pd-catalyzed CO in sertion con verts
com poun d 8 to th e ester 9 in 77% yield. Th e h igh ly specific brom in ation s
of th ree aryl m eth yl groups in th e ester 9 with NBS were carried out in CCl4
usin g AIBN as catalyst. With out purification , th e resultin g tribrom ide 10
was reacted with dieth ylen eglycol m on om eth yleth er (11) in th e presen ce of
NaH an d with NaI an d [15]crown -5 (15C5) as catalysts. Th e resultin g in ter-
m ediate was th en reduced with LiAlH4 in THF to give th e alcoh ol 13 in
18% yield overall for th e th ree steps. Alcoh ol 14 was prepared in a sim ilar
yield (14%) by treatin g th e tribrom ide 10 with tetraeth ylen eglycol m on o-
m eth yleth er (12) un der th e sam e reaction con dition s em ployed in th e pre-
paration of com poun d 13.
Th e sequen ce of reaction s leadin g up to th e tem plate-directed syn th esis10
of th e rotaxan e 1·4PF6 from its precursor dum bbell com poun d 2 are sum -
m arized in Sch em e 2. Alkylation of th e 1-acetoxy-5-h ydroxyn aph th alen e11
(17) with 2-[2-(2-iodoeth oxy)eth oxy]tetrah ydropyran 12 (18) in th e presen ce
of K2CO3 gave com poun d 19 (63%), wh ich was subsequen tly reacted with
KOH in a THF/H2O m ixture to obtain th e deprotected alcoh ol 20 in 89%
yield. Th e alcoh ol was reacted with com poun d5c 21 un der alkylation con di-
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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tion s (K2CO3/NaI/18C6/THF) an d th e resultin g in term ediate was depro-
tected in th e presen ce of HCl in CH2Cl2 to afford com poun d 22 in a yield
of 96% overall. In th e presen ce of NaH, th e h alf-dum bbell com poun d 22
was reacted with th e ben zyl ch loride 15 usin g NaI an d 15C5 as catalysts to
produce th e dum bbell com poun d 3 in 37% yield. Th e [2]rotaxan e 1·4PF6
was obtain ed (Sch em e 2) by a tem plate-directed syn th esis10, usin g th e
dum bbell com poun d 3 as th e tem plate, to form a CBPQT4+ rin g from its
dication ic precursor13 23·2PF6 an d 1,4-bis(brom om eth yl)ben zen e (24). Th e
[2]rotaxan e 1·4PF6 was isolated in 45% yield as an an alytically pure green
solid after ch rom atograph y on silica gel usin g a 1% NH4PF6 solution in
Me2CO as th e eluen t. Th e tem plate-directed syn th esis10 of th e rotaxan e
2·4PF6 from its dum bbell-sh aped precursor 4 is outlin ed in Sch em e 3. Th e
ben zyl ch loride 16 an d th e h alf-dum bbell 22 were reacted un der alkylation
con dition s (NaH/NaI/15C5/THF) to afford th e dum bbell com poun d 4 in
23% yield. Th e tem plate-directed syn th esis10 of th e [2]rotaxan e 2·4PF6 pro-
ceeded in 61% yield, wh en th e reaction of th e dum bbell com poun d 4, th e
dication ic salt13 23·2PF6 an d th e dibrom ide 24, usin g exactly th e sam e reac-
tion an d isolation con dition s as th ose em ployed in th e preparation of
1·4PF6, was carried out.
Photophysical Properties
Th e ph otoph ysical properties of th e com poun ds sh own in Figs. 1 an d 3
h ave been studied in air-equilibrated MeCN solution s at room tem perature.
Each dum bbell, i.e., com poun ds 3 an d 4, con sists of four distin ct ch rom o-
ph oric un its – n am ely, a TTF un it an d a DNP m oiety, in addition to h ydro-
ph obic an d h ydroph ilic stoppers. Both [2]rotaxan es (14+ an d 24+) con tain
th e CBPQT4+ cycloph an e wh ich con sists of two equivalen t an d n on -
in teractin g bipyridin ium un its. For com parison purposes, we h ave also in -
vestigated th e properties of th e m odel com poun ds 5 an d 6 as well as TTF,
CBPQT4+ an d DMN sh own in Fig. 3. Com poun d 6 is con sidered to be a
good m odel for th e h ydroph ilic stoppers presen t in both 3 an d 4.
Th e absorption spectra of th e CBPQT4+ cycloph an e an d th e four m odel
com poun ds are sh own in Fig. 4. Th e in set in th is figure illustrates th e fluo-
rescen ce spectra of 5, 6, an d DMN (TTF an d CBPQT4+ do n ot exh ibit an y
em ission ). Th e absorption spectra of th e two dum bbells are practically th e
sum m ation of th e spectra of th e com pon en t un its. No em ission is observed
for th e two dum bbells 3 an d 4, m ost likely because th e fluorescen t excited
states of both th e stoppers, an d th e DNP un it are quen ch ed, as a result of
en ergy tran sfer to th e lower-lyin g, n on -em ittin g excited states in th e TTF
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Yamamoto et al.:
un it. On goin g from th e dum bbells to th e rotaxan es, broad an d relatively
weak absorption ban ds are expected to appear in th e visible region as a con -
sequen ce of ch arge-tran sfer (CT) in teraction s between th e electron acceptor
cycloph an e an d th e electron -don or un its of th e dum bbell. Previous in vesti-
gation s h ave sh own th at such a CT ban d h as a m axim um aroun d 850 n m
wh en CBPQT4+ surroun ds a TTF station 5c,14,15 an d aroun d 520 n m wh en it
en circles a DNP station 16. Th e lower en ergy of th e CT ban d in volvin g th e
TTF un it is a con sequen ce of th e stron ger electron -don or power of TTF
com pared with th at of a DNP un it. Th e absorption spectra of th e rotaxan es
14+ an d 24+ in th e UV region are sim ilar, but n ot iden tical to th e sum m a-
tion of th e spectra of th e dum bbell an d rin g com pon en ts. Th ey display
(Fig. 5) a broad, relatively weak absorption ban d with a m axim um aroun d
840 n m (ε = 2700 l m ol–1 cm –1 for 14+ an d 3200 l m ol–1 cm –1 for 24+) th at is
n ot presen t in th e com pon en t spectra. Th e presen ce of th is ban d an d th e
absen ce of a ban d aroun d 520 n m sh ow th at th e predom in an t tran slation al
isom er of both 14+ an d 24+ is by far th e on e in wh ich th e CBPQT4+ cyclo-
ph an e surroun ds th e TTF un it. Furth erm ore, we propose th at th e sligh tly
larger absorban ces of th e rotaxan es, com pared with th eir m olecular com po-
n en ts in th e 350–550 n m region , could arise from con tribution s from
alon gside CT in teraction s between th e CBPQT4+ rin g, surroun din g th e TTF
un it, an d eith er (i) th e DNP m oiety, or (ii) th e stoppers, or (iii) both . Th ese
con clusion s are in agreem en t with th e fact th at folded con form ation s are
FIG. 4
Absorption spectra of th e CBPQT4+ cycloph an e an d of th e m odel com poun ds for th e
ch rom oph oric un its in corporated in th e dum bbell-sh aped com poun ds 3 an d 4. Th e em ission
spectra of th e fluorescen t m odel com poun ds are sh own in th e in set (air-equilibrated MeCN so-
lution , room tem perature)
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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required in order to be able to in terpret th e electroch em ical results wh ich
are n ow about to be discussed.
Electrochemical Behavior
Th e electroch em ical experim en ts were carried out in argon -purged MeCN
solution at room tem perature. Th e poten tial values are referred to SCE.
Sin ce th e rotaxan es 14+ an d 24+ con tain several electroactive un its, th ey
sh ow rath er com plex electroch em ical beh avior. We h ave focused our in ves-
tigation m ain ly on th e oxidation processes of th e TTF an d DNP electron -
don atin g un its of th e dum bbell com pon en ts, an d on th e reduction of th e
electron -acceptin g bipyridin ium un its presen t in th e CBPQT4+ cycloph an e
com pon en t. For com parison purposes, th e electroch em ical properties of
free dum bbells 3 an d 4, an d of th e m odel com poun d 6 h ave also been in -
vestigated. Th e beh avior of th e free CBPQT4+ cycloph an e16–18 an d th e
5b,5c
m odel com poun ds TTF 15,19, DMN 5b,5c,20, an d 5
is already kn own .
Before exam in in g th e results obtain ed for th e rotaxan es 14+ an d 24+, let us
con sider th e an ticipated effect of m ech an ical in terlockin g on th e redox pro-
cesses of th e cycloph an e an d of th e dum bbell com pon en ts. Th e oxidation
of a TTF un it or a DNP m oiety is stron gly con dition ed by th e presen ce of a
surroun din g CBPQT4+ cycloph an e because of (i) th e occurren ce of don or-
acceptor in teraction s, an d (ii) th e stron g electrostatic repulsion , arisin g
upon oxidation between th e ch arged radical cation an d th e tetracation ic
cycloph an e. By con trast, it is expected th at reduction of th e CBPQT4+
FIG. 5
Absorption spectra of [2]rotaxan es 14+ an d 24+ an d sum of th e spectra of th eir m olecular com -
pon en ts (dash ed lin e). Th e en larged view of th e CT ban ds of 14+ an d 24+ is sh own in th e in set
(MeCN solution , room tem perature)
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Yamamoto et al.:
cycloph an e is weakly in fluen ced by th e presen ce of a th readed, un ch arged
electron don or sin ce n o electrostatic effect will arise upon reduction . Th e
reduction process sh ould, h owever, be n oticeably in fluen ced by th e pres-
en ce of folded con form ation s sin ce th e h igh ly ch arged cycloph an e is
stron gly exposed to extern al in teraction s. It follows th at th e oxidation pro-
cesses will reveal in form ation on th e n ature of th e tran slation al isom ers
presen t in solution an d also on th e occurren ce of an y sh uttlin g m ove-
m en ts, wh ereas th e reduction processes will be m uch m ore sen sitive for ex-
posin g th e existen ce of an y foldin g/un foldin g rearran gem en ts in solution .
Oxidation processes. Th e data relatin g to th e oxidation of th e m odel com -
poun d 6, th e dum bbells 3 an d 4, an d th e rotaxan es 14+ an d 24+ are gath ered
togeth er in Table I alon g with th e already available data for th e oth er com -
poun ds. Th e m ost im portan t results con cern th e oxidation processes associ-
ated with (Fig. 6) th e TTF un it. In th e dum bbell com pon en ts, th e TTF un it
un dergoes two well-separated, m on oelectron ic, an d reversible processes,
wh ereas, in th e rotaxan es, th e two processes m erge at a poten tial alm ost
equal to th at observed for th e secon d process in th e dum bbells. In oth er
words, in th e rotaxan es, th e first TTF oxidation process is sh ifted (410 m V)
toward m ore positive poten tial values, com pared with th e correspon din g
dum bbells, wh ereas th e secon d oxidation process is n ot displaced at all.
Such beh avior sh ows th at, in th e rotaxan es, th e TTF un it is surroun ded by
Bipyridinium Units
TTF Unit
4+
2
4
30 µA
–1.0
–0.5
0
+0.5
+1.0
+1.5
E, V vs SCE
FIG. 6
DPV pattern s of [2]rotaxan e 24+ an d its correspon din g dum bbell-sh aped com pon en t 4. Th e
curren t in ten sity h as been corrected to take in to accoun t differen ces in con cen tration s an d dif-
fusion coefficien ts (argon -purged MeCN solution , room tem perature, scan rate 20 m V s–1
pulse h eigh t 75 m V)
,
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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Yamamoto et al.:
(an d en gaged in CT in teraction s with ) th e CBPQT4+ cycloph an e, an d th at,
after th e first oxidation , th e cycloph an e m oves away from th e TTF• + un it.
Such a m ech an ical m ovem en t takes place on th e tim e-scale of m illisecon ds,
as eviden ced by th e depen den ces of cyclic voltam m ogram s on th e sweep
rates. Th is beh avior h as already been observed in TTF-con tain in g pseudo-
rotaxan es15.
Th e irreversible oxidation process of th e dum bbells at about +1.2 V, as-
sign ed to th e oxidation of th e DNP un it – by com parison with th e corre-
spon din g m odel com poun d (Table I) – is n ot observed (Fig. 6) in th e
rotaxan es, in keepin g with th e fact th at it h as been sh ifted to m ore positive
poten tials, th us m ergin g with th e oxidation processes (> +1.4 V) associated
with th e stoppers. Such a sh ift is con sisten t with th e displacem en t of th e
CBPQT4+ cycloph an e on th e DNP m oiety after oxidation of th e TTF un it. It
is in deed kn own 21 th at a DNP un it en circled by CBPQT4+ un dergoes oxida-
tion at +1.55 V.
Fin ally, it sh ould also be n oted th at th e beh avior of th e rotaxan es 14+ an d
24+ is quite sim ilar, an observation wh ich suggests th at th e differen t len gth s
of th e polyeth er ch ain s associated with th e h ydroph ilic stopper do n ot play
a sign ifican t role, at least as far as th e oxidation processes are con cern ed.
Reduction processes. Th e electroch em ical properties of th e CBPQT4+ cyclo-
ph an e are kn own to ch an ge wh en it is en gaged in electron don or-acceptor
in teraction . Th e first bielectron ic reduction process is expected to m ove
an d, in fact, (i) does m ove sligh tly toward m ore n egative poten tial values in
pseudorotaxan es15 an d rotaxan es6c, an d (ii) splits in to two distin ct pro-
cesses in caten an es, in wh ich th e two bipyridin ium un its are topologically
n on -equivalen t14,16,17. Splittin g, h owever, h as also been observed for som e
pseudorotaxan es14 an d rotaxan es2a,5c,22
.
FIG. 7
Sch em atic represen tation of th e foldin g/un foldin g con form ation al rearran gem en ts associated
with th e first reduction process of th e two bipyridin ium un its in rotaxan es 14+ an d 24+. Th e lo-
cation of th e doubly reduced rin g alon g th e dum bbell com pon en t can n ot be clearly estab-
lish ed from th e voltam m etric experim en ts
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Folded Structures in Long and Flexible Two-Station Rotaxanes
1503
In th e case of th e rotaxan es 14+ an d 24+, th e first bielectron ic reduction
process of th e CBPQT4+ com pon en t splits in to two distin ct processes
(Table I), sh owin g th at th e equivalen ce of th e two bipyridin ium un its is
lost. Th is result can be accoun ted for23 by assum in g th at CBPQT4+, wh ile
en circlin g th e TTF un it, is in volved in n on sym m etric alon gside in teraction s
with th e oth er don or un its. Such beh avior im plies th e presen ce of folded
con form ation s – possibly of a “pseudocaten an e” type (Fig. 7) – a suggestion
wh ich h as been advan ced previously to explain th e results obtain ed in th e
case of oth er lon g an d flexible pseudorotaxan es14 an d rotaxan es5,6. Th e
som ewh at sim ilar beh aviors exh ibited by both 14+ an d 24+ sh ow th at, in th e
case of th e reduction processes as well, th e differen t len gth s of th e poly-
eth er ch ain s associated with th e h ydroph ilic stoppers apparen tly do n ot
play a sign ifican t role, at least in an electroch em ical con text.
Th e secon d reduction of th e two bipyridin ium un its of th e cycloph an e
takes place (Table I) sim ultan eously at a poten tial value m ore n egative th an
th at observed for th e free cycloph an e. Th is result sh ows th at, after th e first
reduction process, th e two bipyridin ium un its of th e cycloph an e (i) still in -
teract with electron -don atin g un its, an d, m ore in terestin gly, (ii) th ey be-
com e equivalen t. Th e latter observation stron gly suggests th at th e first
reduction of th e two bipyridin ium un its causes un foldin g (Fig. 7).
1H NMR Spectroscopic Investigations
Th e 1H NMR spectra of th e [2]rotaxan es 1·4PF6 an d 2·4PF6, recorded in
CD3CN solution s, display alm ost iden tical arrays of sign als, with th e excep-
tion of th e tigh t package of reson an ces com pressed in to th e region from δ
3.30 to 4.70 ppm for th e O-m eth ylen e proton s presen t in four differen t por-
tion s of th e dum bbell com pon en t of th e rotaxan es. Clearly, th e rotaxan e
24+ con tain s 12 m ore O-m eth ylen e portion s th an does its lower h om olog
14+. Th e 1H NMR spectrum of th e [2]rotaxan e 1·4PF6 is sh own in Fig. 8.
Wh en all said an d don e, h owever, th e differen ce in th e len gth s of th e
polyeth er ch ain s, associated with th e h ydroph ilic stoppers in th e two com -
poun ds, h as little or n o in fluen ce on h ow th e rem ain ders of th e m olecules
beh ave in solution . It would n ot be un reason able to assum e th at th ey exist
in folded con form ation s – as in dicated by both ph otoph ysical an d electro-
ch em ical in vestigation s – in volvin g alon gside π–π stackin g in teraction s be-
tween th e DNP un it an d th e CBPQT4+ rin g. Such an in teraction h as already
been detected5c by 1H NMR NMR spectroscopy in bistable rotaxan es with
closely related structures, particularly with respect to th e rod portion s of
th eir dum bbells.
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1504
Yamamoto et al.:
In agreem en t with th e results of th e electroch em ical in vestigation s, on ly
one tran slation al isom er – th e on e in wh ich th e CBPQT4+ rin g en circles th e
TTF un it – is detected by 1H NMR spectroscopy to be presen t in th e in -
ten sely green solution s of each of th e two rotaxan es 1·4PF6 an d 2·4PF6. Th e
1H NMR spectra of both of th ese com poun ds in CD3CN at 500 MHz exh ibit
sim ilar spectra – i.e., in each case, two pairs of sin glets of un equal in ten si-
ties, reson atin g at δ 5.98 an d 6.06 ppm , an d 6.17 an d 6.27 ppm for 1·4PF6
(Fig. 8) an d at δ 5.96 an d 6.06 ppm , an d 6.16 an d 6.22 ppm for 2·4PF6 – for
th e con stution ally h eterotopic m eth in e proton s on th e TTF un its en circled
by CBPQT4+ rin gs. Th e fact th at th ere is also an equilibratin g m ixture of cis
an d trans isom ers as a result of slow rotation on th e 1H NMR tim escale
aroun d th e form al carbon –carbon double bon d between th e two substi-
tuted five-m em bered rin gs of th e TTF un its accoun ts24 for th e sign al pat-
FIG. 8
1H NMR spectrum of th e rotaxan e 1·4PF6 recorded in CD3CN at 500 MHz
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Folded Structures in Long and Flexible Two-Station Rotaxanes
1505
tern s of th eir two m eth in e proton s associated with th ese un its wh ich are
extern ally diastereotopic, in addition to bein g in tern ally con stitution ally
h eterotopic. So far, we h ave been un able to establish wh ich of th e isom ers,
th e cis or th e trans, is th e m ajor on e in CD3CN solution at room tem pera-
ture. It is in terestin g th at, in th e case of th e 14+ an d 24+, h eavily skewed
tran slation al isom erism is accom pan ied by fin ely balan ced con figuration al
isom erism . It is, h owever, always possible th at, wh en th e progression
(Fig. 2) from th e solution ph ase to th e con den sed ph ase on in to solid-state
devices is m ade, th e supram olecular forces associated with th e self-
organ ization of th e am ph iph ilic, bistable rotaxan es will sh ift th e equilib-
rium in favor of on e of th e two con figuration al isom ers. Or th e equilibrium
proportion s of th e cis an d trans isom ers could persist, leadin g to th e form a-
tion of dom ain s rich in on e or oth er of th e two isom ers. On th e basis of
wh at we kn ow at th is stage about th e in ter-relation sh ips between m olecular
an d supram olecular structures as we cross ph ase boun daries, an yth in g is
possible, it h as to be said.
Mechanical Switching by Chemical Ox/Red Stimulation
It is kn own th at TTF an d its derivatives can be m on o- an d di-oxidized by
stoich iom etric am oun ts of Fe(III) perch lorate. Such oxidation processes can
be followed by ch an ges25 in absorption spectra (TTF• +: λ = 434 n m , ε =
15 000 l m ol–1 cm –1; λ = 577 n m , ε = 4000 l m ol–1 cm –1; TTF2+: λ = 348 n m ,
FIG. 9
Absorption spectra of [2]rotaxan e 14+ an d its m on o-oxidized form . Th e en larged view of th e
CT ban d of 14+ an d of th e differen ce between th e spectra of 15+ an d 3+ (dotted an d dash ed
lin e) is sh own in th e in set (for details, see th e text)
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1506
Yamamoto et al.:
ε = 9000 l m ol–1 cm –1). We h ave foun d th at th is observation is also born e
out for th e TTF un it in th e dum bbells 3 an d 4. In both rotaxan es, th e ab-
sorption ban ds with m axim a aroun d 840 n m – related to th e CT in terac-
tion s between TTF an d CBPQT4+ – disappear (Fig. 9) upon addition of on e
equivalen t of Fe(III) perch lorate. Wh en th e spectrum of th e m on o-oxidized
dum bbell com pon en t is subtracted from th e spectrum of th e correspon din g
m on o-oxidized rotaxan e, a weak absorption ban d with a m axim um of
aroun d 520 n m appears (in set in Fig. 9), con firm in g th at th e CBPQT4+
cycloph an e m oves from th e TTF to th e DNP station upon m on o-electron
oxidation . Successive reduction of TTF• +, perform ed by addin g a stoich io-
m etric am oun t of th e 1,1′-dim eth yl-4,4′-bipyridin ium dication in its m on o-
reduced form , m oves th e rin g back to th e TTF station , th ereby restorin g th e
origin al startin g state of th e ch em ically ox/red system .
CONCLUSIONS
Th e absorption spectra an d th e results obtain ed from electroch em ical oxi-
dation sh ow th at th e [2]rotaxan es 14+ an d 24+ exist to all in ten ts an d pur-
poses as th e tran slation al isom er in wh ich th e CBPQT4+ cycloph an e
surroun ds th e TTF un it. Th e electroch em ical reduction studies in dicate th at
th e two bipyridin ium un its of th e cycloph an e are n on -equivalen t, im plyin g
th e presen ce of asym m etric alon gside in teraction s. It can th us be in ferred
th at th e rotaxan e structure is n ot stretch ed out, but folded up in som e m an -
n er (Fig. 1) in solution . Upon ox/red (electroch em ical or ch em ical) TTF-
localized stim ulation , th e CBPQT4+ cycloph an e sh uttles between th e TTF
an d DNP station s. Upon red/ox (electroch em ical) bipyridin ium -localized
stim ulation , som e kin d of un foldin g/foldin g con form ation al ch an ge takes
place in solution . Th e differen t len gth s of th e polyeth er ch ain s appen ded to
th e h ydroph ilic stopper in 14+ an d 24+ apparen tly do n ot play a sign ifican t
role in con trollin g th is con form ation al ch an ge in solution . Un til n ow, m ost
of th e in terest in rotaxan es h as been focused on con trolled rin g-sh uttlin g.
Th e eviden ce foun d from redox-in duced un foldin g/foldin g con form a-
tion s26 in lon g an d flexible rotaxan es is yet an oth er reason for m akin g ev-
ery effort to exam in e th e structural arran gem en ts of am ph iph ilic, bistable
rotaxan es in differen t ph ases an d settin gs. So too is th e observation of th e
con figuration al isom erism with in th e dum bbell com pon en ts of th e two
rotaxan es. It is quite ch allen gin g to ach ieve all th e con trol you m igh t wish
to h ave in fun ction in g exotic m olecules. It is also h um blin g to n ote th at,
wh ile con form ation al isom erism in 14+ an d 24+ is exposed by ph otoph ysical
an d electroch em ical studies, it is h ard to detect by 1H NMR spectroscopy,
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Folded Structures in Long and Flexible Two-Station Rotaxanes
1507
wh ereas con figuration al isom erism , wh ich is so eviden t from a cursory in -
1
spection of th e H NMR spectra of 1·4PF6 an d 2·4PF6, is n ot at all tran spar-
en t in th e in form ation provided by ph otoph ysical an d electroch em ical
m easurem en ts. Th e lesson to be learn t is a fam iliar on e – keep all eyes open ,
for, wh at on e set of eyes fin ds difficult to see an d even m isses altogeth er,
an oth er set of eyes sees very clearly in deed.
EXPERIMENTAL
Syn th esis
General methods. Ch em icals were purch ased from Aldrich an d used as received. Th e com -
poun ds tri-p-tolylm eth an ol9 (7), 1-acetoxy-5-h ydroxyn aph th alen e11 (17), 2-[2-(2-iodoeth oxy)-
eth oxy]tetrah ydropyran 12 (18), com poun d5c 21 an d 1,1′-(1,4-ph en ylen edim eth ylen e)di-
(4,4´-bipyridin -1-ium ) bis(h exafluoroph osph ate)13 (23·2PF6) were all prepared accordin g to
literature procedures. Solven ts were dried followin g m eth ods described in th e literature27
.
All reaction s were carried out un der an an h ydrous argon atm osph ere. Th in layer ch rom atog-
raph y (TLC) was perform ed on alum in um sh eets, coated with silica gel 60F (Merck 5554).
Th e plates were in spected by UV ligh t an d, if required, developed in I2 vapor. Colum n ch ro-
m atograph y was carried out by usin g silica gel 60 (Merck 9385, 230–400 m esh ). Meltin g
poin ts were determ in ed on an Electroth erm al 9100 m eltin g poin t apparatus an d are un cor-
rected. All 1H an d 13C NMR spectra were recorded on eith er (i) a Bruker ARX400 (400 an d
100 MHz, respectively), (ii) a Bruker ARX500 (500 an d 125 MHz, respectively) or (iii) a
Bruker Avan ce500 (500 an d 125 MHz, respectively), usin g residual solven t as th e in tern al
stan dard. Sam ples were prepared usin g CDCl3, CD3COCD3 or CD3CN purch ased from Cam -
bridge Isotope Laboratories. All ch em ical sh ifts (in ppm ) are quoted usin g th e δ-scale, an d all
couplin g con stan ts (J) are expressed in Hz. Fast atom bom bardm en t (FAB) m ass spectra were
obtain ed usin g a ZAB-SE m ass spectrom eter, equipped with a krypton prim ary atom beam
utilizin g a m-n itroben zyl alcoh ol m atrix. Cesium iodide or poly(eth ylen e glycol) were em -
ployed as referen ce com poun ds. Electrospray m ass spectra (ESMS) were m easured on a VG
ProSpec triple focusin g m ass spectrom eter with MeCN as th e m obile ph ase. Microan alyses
were perform ed by Quan titative Tech n ologies, In c.
Compound 8. A m ixture of tri-p-tolylm eth an ol (7) (20 g, 66 m m ol) an d AcCl (103 g,
1.32 m ol) was h eated un der reflux for 24 h . After coolin g down to room tem perature, acetyl
ch loride was rem oved in vacuo to give a yellow oil, wh ich was th en m ixed with ph en ol
(80 g, 0.85 m ol) to obtain a reddish -black m ixture. Th e reaction m ixture was th en stirred at
100 °C for 18 h . Ph Me (200 m l) was added an d th e m ixture was wash ed with an aqueous so-
lution of NaOH (1.0 m ol l–1, 7 × 150 m l) an d dried (MgSO4). After rem oval of th e solven t,
th e residue was purified by colum n ch rom atograph y (SiO2: CH2Cl2) to give com poun d 8
(10.2 g, 41%) as an off-wh ite solid. 1H NMR (400 MHz, CDCl3): 2.34 (s, 9 H), 4.71 (s, 1 H),
6.70 (d, J = 8.5, 2 H), 7.06–7.12 (m , 14 H). 13C NMR (100 MHz, CDCl3): 21.0, 63.3, 114.2,
128.1, 131.0, 132.3, 135.2, 139.8, 144.4, 153.3. MS (FAB), m/z (%): 378 (83) [M]+. For
C
28H26O calculated: 88.85% C, 6.92% H; foun d: 88.51% C, 6.97% H.
Compound 9. Trifluorom eth an esulfon ic an h ydride (12.5 g, 44 m m ol) was added dropwise
to a solution of com poun d 8 (10 g, 26 m m ol), 2,6-dim eth ylpyridin e (5.3 m l, 6.0 m m ol) an d
DMAP (0.72 g, 45 m m ol) in an h ydrous CH2Cl2 (100 m l) at –30 °C. Th e reaction m ixture was
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1508
Yamamoto et al.:
th en stirred at –30 °C for 30 m in an d at room tem perature for 1 h . H2O (100 m l) was added
an d th e m ixture was extracted with CH2Cl2 (3 × 100 m l) an d dried (MgSO4). Th e com bin ed
organ ic layers were con cen trated an d th e residue was th en purified in a sh ort-path colum n
(SiO2: CH2Cl2/h exan e, 1:2) to give a yellow oil, wh ich was dissolved in a m ixture of an h y-
drous DMF (100 m l), an h ydrous MeOH (40 m l) an d Et3N (6.4 g, 63 m m ol). Pd(OAc)2 acetate
(0.65 g, 3.0 m m ol) an d 1,3-bis(diph en ylph osph in o)propan e (1.2 g, 3.0 m m ol) were added
an d th e reaction m ixture was stirred at 70 °C un der an atm osph ere of CO for 7 h . After
coolin g down to room tem perature, H2O (200 m l) was added an d th e m ixture was extracted
with CH2Cl2 (3 × 50 m l) an d dried (MgSO4). Th e com bin ed organ ic layers were con cen trated
an d th e residue was subjected to colum n ch rom atograph y (SiO2: CH2Cl2/h exan e, 1:1) to
yield com poun d 9 (14.2 g, 77%) as a pale yellow solid. 1H NMR (400 MHz, CDCl3): 2.32 (s,
9 H), 3.90 (s, 3 H), 7.05–7.11 (m , 12 H), 7.34 (d, J = 8.5, 2 H), 7.91 (d, J = 8.5, 2 H).
13C NMR (100 MHz, CDCl3): 20.9, 52.0, 64.2, 127.6, 128.4, 128.7, 130.9, 131.1, 135.6,
143.6, 152.8, 167.1. MS (FAB), m/z (%): 421 (95) [M]+. For C30H28O2 calculated: 85.68% C,
6.71% H; foun d: 85.50% C, 6.68% H.
Compound 13. A m ixture of com poun d 9 (6.26 g, 15 m m ol), NBS (8.21 g, 46 m m ol) an d
AIBN (50 m g, catalytic am oun t) in an h ydrous CCl4 (100 m l) was h eated un der reflux for 6 h .
After coolin g down to room tem perature, th e reaction m ixture was filtered an d th e solid was
wash ed with CCl4 (20 m l). Th e com bin ed organ ic filtrate was con cen trated an d purified by
colum n ch rom atograph y (SiO2: CH2Cl2) to give com poun d 10 as an off-wh ite solid. Com -
poun d 10 was dissolved in THF (20 m l) an d added to a solution of dieth ylen eglycol
m on om eth yleth er (11) (18 g, 150 m m ol), 15-crown -5 (10 m g, catalytic am oun t), NaI (10 m g,
catalytic am oun t) an d NaH (3.0 g, 120 m m ol) in THF (200 m l). Th e reaction m ixture was
h eated un der reflux for 20 h . After coolin g down to room tem perature, H2O (200 m l) was
added an d th e m ixture was extracted by CH2Cl2 (3 × 150 m l) an d dried (MgSO4). Th e com -
bin ed organ ic layers were con cen trated to give a brown oil, wh ich was th en dissolved in
THF (200 m l). LiAlH4 (0.5 g, 13 m m ol) was added in sm all portion s over a period of 15 m in
an d th e reaction m ixture was stirred at room tem perature for 24 h . H2O (10 m l) was slowly
added an d th e m ixture was extracted by CH2Cl2 (3 × 100 m l) an d dried (MgSO4). After re-
m oval of th e solven t, th e residue was subjected to colum n ch rom atograph y (SiO2:
EtOAc/MeOH, 95:5) to give com poun d 13 (2.05 g, 18%) as a pale-yellow oil. 1H NMR
(500 MHz, CDCl3): 3.35 (s, 9 H), 3.52–3.54 (m , 6 H), 3.63–3.68 (m , 18 H), 4.51 (s, 6 H), 4.62
(s, 2 H), 7.16–7.23 (m , 16 H). 13C NMR (125 MHz, CDCl3): 58.9, 64.1, 64.7, 69.6, 70.4, 70.5,
71.8, 72.9, 126.1, 126.9, 130.8, 130.9, 131.0, 135.7, 138.4, 146.0. MS (FAB), m/z (%): 747
(100) [M]+. For C44H58O10 calculated: 70.75% C, 7.83% H; foun d: 70.58% C, 7.72% H.
Compound 14. A m ixture of com poun d 9 (2.5 g, 5.9 m m ol), NBS (3.40 g, 19 m m ol) an d
AIBN (15 m g, catalytic am oun t) in an h ydrous CCl4 (30 m l) was h eated un der reflux for 7 h .
After coolin g down to room tem perature, th e reaction m ixture was filtered an d th e solid was
wash ed with CCl4 (20 m l). Th e com bin ed organ ic filtrate was con cen trated an d purified by a
sh ort-path colum n (SiO2: CH2Cl2) to give com poun d 10 as a off-wh ite solid. Com poun d 10
was dissolved in THF (10 m l) th en added dropwise to a solution of tetraeth ylen eglycol
m on om eth yleth er (12) (8.0 g, 38 m m ol), 15-crown -5 (10 m g, catalytic am oun t), NaI (10 m g,
catalytic am oun t) an d NaH (1.26 g, 50 m m ol) in THF (100 m l). Th e reaction m ixture was
h eated un der reflux for 48 h . After coolin g down to room tem perature, th e m ixture was fil-
tered an d th e solid was wash ed with THF (50 m l). Th e com bin ed filtrate was con cen trated to
obtain a brown oil, wh ich was th en dissolved in THF (100 m l). LiAlH4 (0.5 g, 13 m m ol) was
added in sm all portion s over a period of 15 m in an d th e reaction m ixture was stirred at
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Folded Structures in Long and Flexible Two-Station Rotaxanes
1509
room tem perature for 48 h . H2O (10 m l) was added an d th e m ixture was extracted by
CH2Cl2 (100 m l) an d dried (MgSO4). After rem oval of th e solven t, th e residue was subjected
to colum n ch rom atograph y (SiO2: EtOAc/MeOH, 90:10) to give com poun d 14 (0.85 g, 14%)
as a colorless oil. 1H NMR (500 MHz, CDCl3): 3.14 (s, 9 H), 3.32–3.45 (m , 48 H), 4.33 (s, 6 H),
4.38 (s, 2 H), 6.99–7.03 (m , 16 H). 13C NMR (125 MHz, CDCl3): 58.6, 60.0, 61.2, 63.9, 64.0,
69.4, 70.2, 70.3, 70.4, 71.0, 71.6, 72.6, 125.9, 126.6, 130.7, 130.9, 135.6, 139.0, 145.4, 145.9.
MS (MALDI), m/z: 1033.5455 [M + Na]+. For C56H82O16 calculated: 66.51% C, 8.17% H;
foun d: 66.66% C, 8.26% H.
Compound 15. Th ion yl ch loride (1.8 g, 15 m m ol) was added dropwise to th e solution of
com poun d 13 (0.75 g, 1.5 m m ol) in an h ydrous CH2Cl2 (10 m l). After stirrin g at room tem -
perature for 2 h , th e reaction m ixture was con cen trated an d th e residue was purified by col-
um n ch rom atograph y (SiO2: CH2Cl2) to give com poun d 15 (0.76 g, 99%) as a colorless oil.
1H NMR (500 MHz, CDCl3): 3.37 (s, 9 H), 3.54–3.56 (m , 6 H), 3.64–3.68 (m , 18 H), 4.53 (s,
6 H), 4.56 (s, 2 H), 7.07–7.24 (m , 16 H). 13C NMR (125 MHz, CDCl3): 45.9, 60.2, 64.2, 69.8,
70.6, 70.7, 72.1, 73.0, 126.9, 127.6, 130.9, 131.2, 134.9, 135.9, 145.8, 147.0. MS (FAB), m/z
(%): 765.32 (40) [M + H]+.
Compound 16. Th ion yl ch loride (290 g, 1.9 m m ol) was added dropwise to a solution of
com poun d 14 (190 m g, 0.19 m m ol) in an h ydrous CH2Cl2 (10 m l). After stirrin g at room
tem perature for 2 h , th e reaction m ixture was con cen trated an d th e residue was purified by
colum n ch rom atograph y (SiO2: CH2Cl2) to give com poun d 15 (186 m g, 96%) as a colorless
oil. With out furth er ch aracterization , com poun d 15 was used directly to alkylate com poun d
22 an d so form th e dum bbell com poun d 4.
Compound 19. A solution of 1-acetoxy-5-h ydroxyn aph th alen e (17) (1.58 g, 7.88 m m ol),
2-[2-(2-iodoeth oxy)eth oxy]tetrah ydropyran (18) (3.55 g, 12 m m ol), K2CO3 (3.0 g, 22 m m ol),
an d 18-crown -6 (10 m g, catalytic am oun t) in an h ydrous MeCN (50 m l) was stirred un der re-
flux for 48 h . After coolin g down to room tem perature, th e reaction m ixture was filtered
an d th e solid was wash ed with MeCN (50 m l). Th e com bin ed organ ic filtrate was con cen -
trated an d th e residue was subjected to colum n ch rom atograph y (SiO2: CH2Cl2/MeOH, 95:5)
to give com poun d 19 (1.84 g, 63%) as a pale-yellow oil. 1H NMR (500 MHz, CD3CN):
1.48–1.90 (m , 6 H), 2.56 (s, 3 H), 3.50–4.41 (m , 10 H), 4.72 (t, J = 3.2, 1 H), 7.09 (d, J = 8.5,
1 H), 7.42 (d, J = 8.5, 1 H), 7.53–7. 65 (m , 3 H), 8.31 (d, J = 8.5, 1 H). 13C NMR (125 MHz,
CD3CN): 20.3, 21.1, 26.2, 31.4, 62.7, 67.4, 69.1, 70.2, 71.4, 99.7, 106.8, 114.3, 118.2, 120.0,
120.8, 125.8, 127.8, 129.0, 147.7, 155.5, 170.6. MS (FAB), m/z (%): 374 (100) [M]+. For
C
21H26O6 calculated: 67.36% C, 7.00% H; foun d: 67.40% C, 7.02% H.
Compound 20. A solution of com poun d 19 (0.50 g, 1.34 m m ol) an d KOH (0.4 g,
6.7 m m ol), in THF (10 m l) an d H2O (10 m l) was h eated un der reflux for 3 h . After coolin g
down to room tem perature, H2O (20 m l) was added, an d th e m ixture was extracted with
CH2Cl2 (3 × 20 m l) an d dried (MgSO4). Th e com bin ed organ ic layers were con cen trated to
give a brown oil, wh ich was purified by colum n ch rom atograph y (SiO2: EtOAc/h exan e, 9:1)
to give com poun d 20 (0.40 g, 89%) as a reddish -brown oil. 1H NMR (400 MHz, CDCl3):
1.50–1.58 (m , 6 H), 3.48–3.50 (m , 1 H), 3.71–3.82 (m , 6 H), 4.00–4.03 (m , 2 H), 4.30–4.32
(m , 2 H), 4.66 (t, J = 3.2, 3 H), 6.84 (d, J = 8.5, 1 H), 7.25–7.36 (m , 2 H), 7.72–7.74 (m , 1 H),
7.86–7.88 (m , 1 H). 13C NMR (100 MHz, CDCl3): 19.4, 25.4, 30.6, 62.3, 66.7, 67.9, 69.8,
70.9, 99.0, 105.5, 109.3, 114.2, 114.4, 125.0, 125.2, 125.6, 127.1, 152.8, 154.5. MS (FAB),
m/z (%): 332.04 (100) [M]+.
Compound 22. A solution of com poun d 20 (0.27 g, 0.81 m m ol), com poun d 21 (0.87 g,
0.82 m m ol), K2CO3 (0.45 g, 3.25 m m ol), LiBr (20 m g, catalytic am oun t) an d 18-crown -6
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(20 m g, catalytic am oun t) in MeCN (50 m l) was h eated un der reflux for 36 h . After coolin g
down to room tem perature, th e reaction m ixture was filtered an d th e solid was wash ed with
MeCN (20 m l). Th e com bin ed filtrate was con cen trated an d th e residue was purified by col-
um n ch rom atograph y (SiO2: CH2Cl2/EtOH, 100:3) to give com poun d 22 (0.88 g, 96%) as an
oran ge oil. 1H NMR (500 MHz, CD3COCD3): 1.21 (t, J = 7.6, 3 H), 1.31 (s, 18 H), 2.62 (q, J =
7.6, 2 H), 2.70 (s, 1 H), 3.54–3.81 (m , 14 H), 3.97–3.99 (m , 4 H), 4.09–4.12 (m , 2 H),
4.30–4.33 (m , 8 H), 6.47–6.52 (4 × s, 2 H), 6.83–6.85 (m , 2 H), 6.96–6.99 (m , 2 H), 7.10–7.16
(m , 10 H), 7.30–7.32 (m , 4 H), 7.34–7.40 (m , 2 H), 7.85–7.88 (m , 2 H). 13C NMR (125 MHz,
CD3CN): 14.8, 25.1, 27.7, 30.5, 33.8, 60.9, 63.0, 67.3, 67.4, 69.1, 69.2, 69.8, 70.0, 70.1, 70.2,
71.4, 72.2, 106.8, 113.3, 114.3, 116.5, 116.6, 118.2, 120.0, 120.8, 124.3, 125.8, 126.7, 127.8,
129.0, 130.2, 130.4, 131.6, 134.5, 134.6, 139.5, 141.5,144.4, 144.7, 147.7, 148.3, 155.5,
156.6. MS (FAB), m/z (%): 1128 (55) [M]+.
Dumbbell compound 3. A solution of com poun d 15 (0.19 g, 0.25 m m ol) in THF (5 m l) was
added dropwise to a suspen sion of com poun d 22 (0.20 g, 0.177 m m ol), 15-crown -5 (10 m g,
catalytic am oun t), NaI (10 m g, catalytic am oun t), NaH (0.05 g, 2.0 m m ol) in THF (20 m l).
Th e reaction m ixture was h eated un der reflux for 24 h . After coolin g down to room tem per-
ature, th e m ixture was filtered an d th e solid was wash ed with THF (10 m l). Th e com bin ed
organ ic filtrate was con cen trated an d purified by colum n ch rom atograph y (SiO2: EtOAc) to
give dum bbell com poun d 3 (0.125 g, 37%) as a yellow oil. 1H NMR (500 MHz, CD3COCD3):
1.21 (t, J = 7.6, 3 H), 1.31 (s, 18 H), 2.62 (q, J = 7.6, 2 H), 3.30 (s, 9 H), 3.47–3.62 (m , 38 H),
3.94–3.96 (m , 4 H), 4.05–4.12 (m , 2 H), 4.25–4.26 (m , 8 H), 4.48–4.49 (m , 8 H), 6.26–6.29
(4 × s, 2 H), 6.78–6.80 (m , 2 H), 6.90–6.93 (m , 2 H), 7.05–7.28 (m , 32 H), 7.79 (d, J = 8.5,
2 H). 13C NMR (125 MHz, CD3CN): 14.8, 25.1, 27.7, 30.5, 33.8, 58.9, 60.9, 63.0, 64.1, 64.7,
67.3, 67.4, 69.1, 69.2, 69.6, 69.8, 70.0, 70.1, 70.2, 70.4, 70.5, 71.4, 71.8, 72.2, 72.9, 106.8,
113.3, 114.3, 116.5, 116.6, 118.2, 120.0, 120.8, 124.3, 125.8, 126.1, 126.7, 126.9, 127.8,
129.0, 130.2, 130.4, 130.8, 130.9, 131.0, 131.6, 134.5, 134.6, 135.7, 138.4, 139.5, 141.5,
144.4, 144.7, 146.0, 147.7, 148.3, 155.5, 156.6. MS (FAB), m/z (%): 1857.84 (100) [M + H]+.
For C109H132O18S4 calculated: 70.44% C, 7.16% H, 6.90% S; foun d: 70.42% C, 7.12% H,
6.86% S.
Dumbbell compound 4. A solution of com poun d 16 (186 m g, 0.181 m m ol) in THF (5 m l)
was added dropwise to a suspen sion of com poun d 22 (200 m g, 0.177 m m ol), 15-crown -5
(10 m g, catalytic am oun t), NaI (10 m g, catalytic am oun t), NaH (50 m g, 2.0 m m ol) in THF
(20 m l). Th e reaction m ixture was h eated un der reflux for 2 days. After coolin g down to
room tem perature, th e m ixture was filtered an d th e solid was wash ed with THF (10 m l). Th e
com bin ed organ ic filtrate was con cen trated an d purified by colum n ch rom atograph y (SiO2:
EtOAc/MeOH, 9:1) to give th e dum bbell com poun d 4 (85 m g, 23%) as a yellow oil. 1H NMR
(500 MHz, CD3COCD3): 1.21 (t, J = 7.6, 3 H), 1.31 (s, 18 H), 2.61 (q, J = 7.6, 2 H), 3.28 (s,
9 H), 3.45–3.79 (m , 64 H), 3.95–4.03 (m , 6 H), 4.29–4.31 (m , 6 H), 4.52–4.55 (m , 8 H),
6.45–6.50 (4 × s, 2 H), 6.81–6.82 (m , 2 H), 6.93–6.95 (m , 2 H), 7.15–7.35 (m , 32 H), 7.85 (d,
J = 8.5, 2 H). 13C NMR (125 MHz, CD3COCD3): 14.9, 25.2, 27.6, 30.6, 33.8, 59.0, 60.9, 63.1,
64.1, 64.7, 67.3, 67.4, 69.1, 69.2, 69.6, 69.8, 69.9, 70.0, 70.1, 70.2, 70.3, 70.4, 70.5, 71.4,
71.9, 72.4, 73.1, 105.9, 113.1, 114.2, 116.5, 116.6, 118.4, 120.1, 120.8, 124.3, 125.8, 126.1,
126.7, 126.8, 127.9, 129.1, 130.2, 130.3, 130.8, 130.9, 131.0, 131.6, 134.6, 134.6, 135.7,
138.4, 139.5, 141.5, 144.3, 144.7, 145.9, 147.7, 148.1, 154.3, 156.8. MS (FAB), m/z (%):
2122.00 (100) [M + H]+. For C121H156O24S4 calculated: 68.46% C, 7.41% H, 6.04% S; foun d:
68.19% C, 7.36% H, 6.06% S.
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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[2]Rotaxane 1·4PF6. A solution of dum bbell com poun d 3 (0.125 g, 0.067 m m ol), 23·2PF6
(0.143 g, 0.205 m m ol) an d 1,4-bis(brom om eth yl)ben zen e (24) (0.053 g, 0.202 m m ol) in an -
h ydrous DMF (5 m l) was stirred at room tem perature for 10 days. Th e reaction m ixture was
directly subjected to colum n ch rom atograph y (SiO2) an d un reacted com poun d 3 was recov-
ered with Me2CO, wh ereupon th e eluen t was ch an ged to Me2CO/NH4PF6 (1.0 g NH4PF6 in
100 m l Me2CO) an d th e green ban d con tain in g [2]rotaxan e 1·4PF6 was collected. After re-
m oval of solven t, H2O (50 m l) was added an d th e resultin g precipitate was collected by fil-
tration to afford th e [2]rotaxan e 1·4PF6 (90 m g, 45%) as a green solid. 1H NMR (500 MHz,
CD3CN): 1.13–1.19 (m , 3 H), 1.23–1.26 (m , 18 H), 2.55–2.60 (m , 2 H), 3.25 (s, 9 H),
3.42–4.13 (m , 52 H), 4.44 (s, 6 H), 4.48 (s, 2 H), 5.38–5.71 (m , 8 H), 5.91, 5.99, 6.08, 6.19
(4 × s, 2 H), 6.47 (d, J = 8.5, 1 H), 6.56–6.65 (m , 3 H), 6.76 (d, J = 8.5, 1 H), 6.95–6.98 (m ,
1 H), 7.03–8.02 (m , 48 H), 8.38–8.97 (m , 8 H). MS (FAB), m/z (%): 2814 (8) [M – PF6]+, 2668
(13) [M – 2PF6]+, 2523 (8) [M – 3PF6]+. For C145H164F24N4O18P4S4 calculated: 58.86% C,
5.59% H, 1.89% N; foun d: 58.79% C, 5.54% H, 1.86% N.
[2]Rotaxane 2·4PF6. A solution of dum bbell com poun d 4 (81 m g, 0.038 m m ol), 23·2PF6
(81 m g, 0.114 m m ol) an d 1,4-bis(brom om eth yl)ben zen e (24) (30 m g, 0.114 m m ol) in DMF
(5 m l) was stirred at room tem perature for 10 days. Th e reaction m ixture was directly sub-
jected to colum n ch rom atograph y (SiO2) an d un reacted com poun d 4 was recovered with
Me2CO, wh ereupon th e eluen t was ch an ged to Me2CO/NH4PF6 (1.0 g NH4PF6 in 100 m l
Me2CO) an d th e green ban d con tain in g [2]rotaxan e 2·4PF6 was collected. After rem oval of
solven t, H2O (50 m l) was added, form in g a precipitate, wh ich was collected by filtration to
give th e [2]rotaxan e 2·4PF6 (0.074 m g, 61%) as a green solid. 1H NMR (500 MHz, CD3CN):
1.15–1.17 (m , 3 H), 1.91–1.93 (m , 18 H), 2.55–2.58 (m , 2 H), 3.25 (s, 9 H), 3.42–4.35 (m ,
76 H), 4.44–4.48 (m , 8 H), 5.43–5.66 (m , 8 H), 5.93, 6.00, 6.09, 6.19 (4 × s, 2 H), 6.46–6.80
(m , 2 H), 6.99–7.65 (m , 52 H), 8.59–8.97 (m , 8 H). MS (FAB), m/z (%): 3078.29 (57) [M –
PF6]+, 2934.05 (100) [M – 2PF6]+, 2788.29 (57) [M – 3PF6]+. For C157H188F24N4O24P4S4 calcu-
lated: 58.50% C, 5.88% H, 1.74% N; foun d: 58.20% C, 5.81% H, 1.69% N.
Ph otoph ysical Experim en ts
All th e m easurem en ts were perform ed at room tem perature in air-equilibrated MeCN solu-
tion s (2 × 10–5–1 × 10–4 m ol l–1). UV-Vis absorption spectra were recorded with a Perkin –
Elm er Lam bda 40 spectroph otom eter. Un corrected fluorescen ce spectra were obtain ed with a
Perkin –Elm er LS-50 spectrofluorim eter equipped with a Ham am atsu R928 ph ototube. Th e es-
tim ated experim en tal errors are: 2 n m on ban d m axim a, ±5% on th e m olar absorption coef-
ficien ts an d fluorescen ce in ten sity.
Electroch em ical Experim en ts
Cyclic voltam m etric (CV) an d differen tial pulse voltam m etric (DPV) experim en ts were car-
ried out in argon -purged MeCN solution at room tem perature with an Autolab 30 m ultipur-
pose in strum en t in terfaced to a person al com puter. Th e workin g electrode was a glassy
carbon electrode (0.08 cm 2, Am el); its surface was routin ely polish ed with 0.3 µm alum in a-
water slurry on a felt surface, im m ediately prior to use. In all cases, th e coun ter electrode
was a Pt spiral, separated from th e bulk solution with a fin e glass frit, an d an Ag wire was
used as a quasi-referen ce electrode. Decam eth ylferrocen e28 (E1/2 = –0.11 V vs SCE) was pres-
en t as in tern al stan dard. In all th e electroch em ical experim en ts th e com poun d con cen tra-
tion s were in th e ran ge 5 × 10–4–1 × 10–3 m ol l–1 an d tetrabutylam m on ium h exafluoro-
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

1512
Yamamoto et al.:
ph osph ate with 100 tim es h igh er con cen tration was added as supportin g electrolyte. Cyclic
voltam m ogram s were obtain ed with sweep rates in th e ran ge 0.02–1.0 V s–1; th e DPV experi-
m en ts were perform ed with a scan rate of 20 or 4 m V s–1 (pulse h eigh t 75 an d 10 m V, re-
spectively) an d a duration of 40 m s. Th e reversibility of th e observed processes was
establish ed by usin g th e criteria of (i) separation of 60 m V between cath odic an d an odic
peaks, (ii) th e close to un ity ratio of th e in ten sities of th e cath odic an d an odic curren ts, an d
(iii) th e con stan cy of th e peak poten tial on ch an gin g sweep rate in th e cyclic
voltam m ogram s. Th e sam e h alfwave poten tial values were obtain ed from th e DPV peaks an d
from an average of th e cath odic an d an odic CV peaks, as expected for reversible processes.
For irreversible processes th e poten tial values were estim ated from th e DPV peaks. Th e n um -
ber of exch an ged electron s in th e redox processes of th e in vestigated dum bbells an d
rotaxan es was m easured by com parin g th e curren t in ten sity of th e CV waves an d th e area of
th e DPV peaks with th ose foun d for th e two reversible an d bielectron ic reduction processes
of th e CBPQT4+ cycloph an e16–18, after correction for differen ces in th e diffusion coefficien ts
an d con cen tration s29. Th e experim en tal error on th e poten tial values was estim ated to be
±10 m V an d ±20 m V for reversible an d irreversible processes, respectively.
Ch em ical Ox/Red Experim en ts
A stan dardized Fe(ClO4)3 solution in MeCN at room tem perature was used to perform th e
m on o-oxidation of th e TTF un it con tain ed in th e dum bbells an d rotaxan es. Th e con cen tra-
tion s of th e dum bbell an d rotaxan e solution s were 2.0 × 10–5 m ol l–1. Th e reduction of th e
TTF• + species was perform ed by usin g a MeCN solution of th e 1,1′-dim eth yl-4,4′-bipyridin ium
radical cation , obtain ed by reducin g with zin c powder th e 1,1′-dim eth yl-4,4′-bipyridin ium
dication .
This research was supported by at the University of California at Los Angeles by the Defense
Advanced Research Project Agency (DARPA). T. Yamamoto thanks Showa Denko K K for a visiting
scholar fellowship. In Italy, the research was supported by the University of Bologna (Funds for Se-
lected Research Topics), MIUR (Supramolecular Devices Project) and EU (HPRN-CT-2000-00029).
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Folded Structures in Long and Flexible Two-Station Rotaxanes
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23. The splitting of the first reduction process of the two bipyridinium units in a rotaxane
composed of CBPQT4+ and a phenylenediamine-based dumbbell was attributed to an
increase in the electronic communication between such units through the interposed
electron-donor moiety (ref.22b). This explanation is unlikely in the present case, since
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encircled by CBPQT4+ (ref.15).
24. 1H NMR spectra recorded in CD3COCD3 (see Experimental) for the two dumbbell
compounds 3 and 4 also indicate that they are both slowly equilibrating mixtures of cis
and trans isomers, as shown in Fig. 1.
25. a) Hünig S., Kiesslich G., Quast H., Scheutzow D.: Liebigs Ann. Chem. 1973, 310;
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biopolymers, such as proteins: a) Pascher T., Chesick J. P., Winkler J. R., Gray H. B.:
Science 1996, 271, 1558; b) Chen E., Wittung-Stafshede P., Kliger D. S.: J. Am. Chem. Soc.
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Collect. Czech. Chem. Commun. (Vol. 68) (2003)