Counterion-induced translational isomerism in a bistable [2]rotaxane

Article abstract of DOI:10.1021/ol048518l

(Chemical Equation Presented) Translational isomerization can be induced by changing the anions associated with a bistable rotaxane in which the tetracationic cyclophane (blue box), cyclobis(paraquat-p-phenylene), encircles a dumbbell component containing

Full text of DOI:10.1021/ol048518l

ORGANIC
LETTERS
2004
Vol. 6, No. 23
4167-4170
Counterion-Induced Translational
Isomerism in a Bistable [2]Rotaxane
Bo W. Laursen,† Sune Nygaard,†,‡ Jan O. Jeppesen,*,‡ and J. Fraser Stoddart*,†
California NanoSystems Institute and Department of Chemistry and Biochemistry,
UniVersity of California, Los Angeles, 405 Hilgard AVenue,
Los Angeles, California 90095-1569, and Department of Chemistry, Odense UniVersity
(UniVersity of Southern Denmark), CampusVej 55, DK-5230, Odense M, Denmark
joj@chem.sdu.dk
Received July 28, 2004
ABSTRACT
Translational isomerization can be induced by changing the anions associated with a bistable rotaxane in which the tetracationic cyclophane
(blue box), cyclobis(paraquat-p-phenylene), encircles a dumbbell component containing bispyrrolotetrathiafulvalene (green) and a dioxynaph-
thalene (red) recognition sites. The rotaxane was isolated as both its hexafluorophosphate and tris(tetrachlorobenzenediolato)phosphate(v)
-
(TRISPHAT ) salts. Photophysical measurements and NMR spectroscopy carried out in acetone (CD3COCD3) and acetonitrile (CD3CN) solutions
-
reveal that the much larger TRISPHAT anion favors predominantly the encirclement of the green site by the blue box.
[2]Rotaxanes, comprised of a dumbbell component contain-
ing 1,5-dioxynaphthalene (DNP) and tetrathiafulvalene (TTF)
recognition sites for the ring component, cyclobis(paraquat-
p-phenylene) (CBPQT4+), have been shown1 to display
bistability that is electrochemically controllable insofar as
the occupation of the two sites by the CBPQT4+ ring can be
dictated by the oxidation state of the TTF site, i.e., when it
is neutral, it is encircled by the ring, and when it is oxidized,
the ring moves to the DNP site. In solution, comprehensive
photophysical, electrochemical, and 1H NMR spectroscopic
studies have provided2 a detailed picture of this redox-
activated shuttling process in terms of its kinetics and
thermodynamics. In addition, recent electrochemical inves-
tigations on a bistable rotaxane, self-assembled to a gold
electrode, have established3 that this switching process is
conserved in molecular monolayers in a “half-device”. A
similar electrochemical mechanism is believed4 to be re-
sponsible for the bistability observed in crossbar memory
devices in which Langmuir-Blodgett monolayers of am-
† University of California, Los Angeles.
‡ Odense University (University of Southern Denmark).
(1) (a) Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo,
F. M.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org.
Chem. 2000, 65, 1924-1936. (b) Jeppesen, J. O.; Perkins, J.; Becher, J.;
Stoddart, J. F. Org. Lett. 2000, 2, 3547-3550. (c) Jeppesen, J. O.; Perkins,
J.; Becher, J.; Stoddart, J. F. Angew. Chem., Int. Ed. 2001, 40, 1216-1221.
(d) Tseng, H.-R.; Vignon, S. A.; Stoddart, J. F. Angew. Chem., Int. Ed.
2003, 42, 1491-1495. (e) Jeppesen, J. O.; Nielsen, K. A.; Perkins, J.;
Vignon, S. A.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi, M.;
Balzani, V.; Becher, J.; Stoddart, J. F. Chem.sEur. J. 2003, 9, 2982-
3007. (f) Yamamoto, T.; Tseng, H.-R.; Stoddart, J. F.; Balzani, V.; Credi,
A.; Marchioni, F.; Venturi, M. Collect. Czech. Chem. C. 2003, 68, 1488-
1514. (g) Tseng, H.-R.; Vignon, S. A.; Celestre, P. C.; Perkins, J.; Jeppesen,
J. O.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi, M.; Balzani,
V.; Stoddart, J. F. Chem.sEur. J. 2004, 10, 155-172.
(2) (a) Jeppesen, J. O.; Becher, J.; Stoddart, J. F. Org. Lett. 2002, 4,
557-560. (b) Jeppesen, J. O.; Vignon, S. A.; Stoddart, J. F. Chem.sEur.
J. 2003, 9, 4611-4625. (c) Kang, S.; Vignon, S. A.; Tseng, H.-R.; Stoddart,
J. F. Chem.sEur. J. 2004, 10, 2555-2564.
(3) Tseng, H.-R.; Wu, D.; Fang, N. X.; Zhang, X.; Stoddart, J. F.
ChemPhysChem 2004, 5, 111-116.
(4) (a) Luo, Y.; Collier, C. P.; Jeppesen, J. O.; Nielsen, K. A.; DeIonno,
E.; Ho, G.; Perkins, J.; Tseng, H.-R.; Yamamoto, T.; Stoddart, J. F.; Heath,
J. R. ChemPhysChem 2002, 3, 519-525. (b) Diehe, M. R.; Steuerman, D.
W.; Tseng, H.-R.; Vignon, S. A.; Star, A.; Celestre, P.C.; Stoddart, J. F.;
Heath, J. R. ChemPhysChem 2003, 4, 1335-1339.
10.1021/ol048518l CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/20/2004

Figure 1. Structural formulas of the green translational isomer of the bistable [2]rotaxane 14+ with the anions PF6 and TRISPHAT-
-
displayed on either side.
phiphilic bistable rotaxanes are sandwiched between two
electrodes, one Si or C and the other Ti/Al, in “full devices”.
In all these rotaxanes, the four positive charges associated
with the CBPQT4+ ring component are balanced by four
sors in having a symmetric bispyrrolo-TTF (BPTTF) rec-
ognition site instead of the previously used monopyrrolo-
TTF1c,e,4 or simple TTF1d,g,3 sites.
1‚4PF6 was synthesized by conventional methods, and its
TRISPHAT- salt was produced by mixing 1‚4PF6 with an
excess of racemic morpholinium‚TRISPHAT6a in CHCl3. The
-
hexaflourophosphate (PF6 ) counterions. Recent studies5
carried out on Langmuir layers of these rotaxanes, as their
PF6- salts, suggest that the structures of the monolayers are
strongly influenced by interactions between the CBPQT4+
ring and its counterions with the water surface. Almost no
attention, however, has been paid to the role of the coun-
terions, even although it is very likely that they play an
important role, not only in influencing the packing structure
of the Langmuir monolayers but also because they may
perturb the electron-accepting properties and, hence, shuttling
dynamics of the CBPQT4+ ring component, particularly in
apolar solutions and in solid-state devices where electrostatic
attraction and repulsion between the charged moieties are
more significant because they are much less shielded by
solvent molecules.
We have prepared the tris(tetrachlorobenzenediolato)-
phosphate(v) (TRISPHAT-) salt of the amphiphilic bistable
[2]rotaxane 14+ and compared (Figure 1) its solution-state
properties with those of its corresponding PF6- salt. Lacour’s
TRISPHAT- anion6 was chosen on account of its consider-
able size,7 low polarity,8 and redox stability.9 The tetraca-
tionic 14+ resembles many of the redox-switchable bistable
[2]rotaxanes previously investigated both in solution and in
crossbar devices.1-5 However, 14+ differs from its predeces-
-
more water-soluble PF6 anions and morpholinium cations
were removed by washing the organic phase with H2O.
Excess of morpholinium‚TRISPHAT was removed subse-
quently by reprecipitating the product from Me2CO/Et2O to
afford 1‚4TRISPHAT in 72% yield. See Supporting Infor-
mation for more details on the syntheses of this rotaxane
and its spectrometric and spectroscopic characterization. The
electrospray mass spectrum (ES-MS) of the [2]rotaxane
1‚4TRISPHAT shows peaks at m/z 1992, 1072, and 612
corresponding, respectively, to the doubly positively charged
[M - 2TRISPHAT]2+, the triply positively charged [M -
3TRISPHAT]3+, and quadruply positively charged [M -
4TRISPHAT]4+ ions. A comparison of the 31P NMR spectra,
both recorded in CD3COCD3, of 1‚4PF6 and 1‚4TRISPHAT
revealed the complete disappearance of the resonance
-
centered on -144.2 ppm associated with the PF6 anions
and the appearance of a signal resonating at -80.7 ppm,
which is characteristic of the TRISPHAT- anion.6a The 19
F
NMR spectrum of 1‚4PF6 and 1‚4TRISPHAT, also recorded
in CD3COCD3, showed one intense doublet for the PF6- ion
resonating at -72.5 ppm, whereas no resonances were
observed in the 19F NMR spectrum of 1‚4TRISPHAT. These
-
observations are clear indications that all four PF6 anions
have been exchanged by four TRISPHAT- anions.
(5) (a) Lee, I. C.; Curtis, F. W.; Yamamoto, T.; Tseng, H.-R.; Flood, A.
H.; Stoddart, J. F.; Jeppesen, J. O. Langmuir 2004, 20, 5809-5828. (b)
Nørgaard, K.; Jeppesen, J. O.; Laursen, B. W.; Simonsen, J. B.; Weygand,
M. J.; Kjaer, K.; Stoddart, J. F.; Bjørnholm T. In preparation.
In Me2CO solution, 1‚4PF6 exists as an approximately 1:1
mixture of the two possible translation isomers10 where the
CBPQT4+ ring is located around the DNP recognition site
in one isomer and around the BPTTF site in the other one.
These two translation isomers give raise to characteristic
charge transfer (CT) absorption bands centered on 550 nm
(red) and 825 nm (green), respectively, in the visible spec-
trum. Similar equimolar proportions of translational isomers
have been observed1c,e previously in bistable [2]rotaxanes
containing monopyrrolo-TTF units, while bistable [2]rotax-
anes containing simpler TTF units usually exist1d,f,g almost
exclusively as the green isomer. The apparently less favorable
(6) (a) Lacour, J.; Ginglinger, C.; Grivet, C.; Bernardinelli, G. Angew.
Chem., Int. Ed. Engl. 1997, 36, 608-610. (b) Lacour, J.; Ginglinger, C.;
Favarger, F.; Torche-Haldimann, S. Chem. Commun. 1997, 2285-2286.
-
(7) Molecular modeling of the PF6 and TRISPHAT- anions indicates
that they have diameters of ca. 4 and 14 Å, respectively, at least when the
ions are approximated to spheres. For crystal structures of TRISPHAT-
salts, see ref 6a and: Lacour, J.; Bernardinelli, G.; Russell, V.; Dance, I.
CrystEngCommun. 2002, 4, 165-170.
(8) Lacour, J.; Barche´chath, S.; Jodry, J. J.; Ginglinger, C. Tetrahedron
Lett. 1998, 39, 567-570.
(9) If 1‚4TRISPHAT is going to be used in a redox-switchable context,
as in crossbar devices, for example, it is important that the TRISPHAT-
anions do not undergo redox chemistry within the potential range where
the bistable [2]rotaxane 14+ switches. We therefore investigated the redox
chemistry of Me4N‚TRISPHAT in MeCN solution by cyclic voltammetry
and found that this salt is not redox active in the range from -2 to +1.7 V
vs SCE using ferrocene/ferrocenium as the internal standard.
(10) Schill, G.; Rissler, K.; Fritz, H.; Vetter, W. Angew. Chem., Int. Ed.
Engl. 1981, 20, 187-189.
4168
Org. Lett., Vol. 6, No. 23, 2004

binding between the monopyrrolo-TTF units1e and the
CBPQT4+ ring has been ascribed to less efficient [C-H‚‚‚O]
interactions between the ethyleneglycol linkers in the dumb-
bells and the CBPQT4+ ring, caused by the extended nature
of the TTF derivative, rather than by its π-electron donor
strength, which is almost similar to that of the simpler TTF
unit. In agreement with this interpretation, the green isomer
of 1‚4PF6 becomes favored at higher temperatures in polar
solvents, where intramolecular [C-H‚‚‚O] interactions sta-
bilizing the red isomer become less important.
Exchanging the counterions of 14+ from PF6 to
-
TRISPHAT- in a separating funnel (Figure 2) changes the
1
Figure 3. Partial H NMR spectra (400 MHz) of 1‚4PF6 (a) and
1‚4TRISPHAT (b) recorded in CD3COCD3 at 295 K, showing the
signals for the benzylic methylene protons as two singlets in the
ratio of 2:1 for each of the two translational isomers.
-
isomer, was observed on replacing PF6 with TRISPHAT-
counterions.
To elucidate the role of the counterions even further,
Me2CO/CD3COCD3 solutions of 1‚4PF6 and 1‚4TRISPHAT
were titrated with Bu4N‚PF6 and Me4N‚TRISPHAT, respec-
1
tively, and both the UV-vis and H NMR spectra were
-
monitored. In the case of 1‚4TRISPHAT, addition of PF6
ions very rapidly brings the isomer ratio back to that of the
pure PF6- salt, 1‚4PF6. Already, after addition of only 1 equiv
of Bu4N‚PF6, giving a 1:1 ratio of PF6- to TRISPHAT- ions
in solution, the isomer ratio reaches 80% of the value found
Figure 2. Photograph showing the color change accompanying
counterion exchange. In funnel a, the lower reddish CHCl3 phase
contains 14+, PF6-, morpholinium, and TRISPHAT- ions and is
topped with a carefully added H2O phase. In funnel b, we see the
result of shaking the separation funnel and allowing the phases to
-
for the pure PF6 salt. Titration of 1‚4PF6 with Me4N‚
TRISPHAT shows, on the other hand, that a much larger
excess of the TRISPHAT- ion is required to shift the isomer
distribution. The titration curves show (Figure 4) clearly that
the CBPQT4+ ring has a higher affinity for the PF6- ion than
for the larger TRISPHAT- ion.
-
separate: now the morpholinium and PF6 ions are in the H2O
phase, leaving 14+ with the TRISPHAT- ions in the now greenish
CHCl3 phase.
proportion of translational isomers in favor of the green
isomer, a phenomenon that can be observed directly by the
naked eye. The exact ratios of translation isomers can easily
be determined from the 1H NMR spectra recorded for 1‚4PF6
and 1‚4TRISPHAT, respectively, since several of the protons
in the dumbbell component give rise to two sets of signals,
one for each of the two translation isomers. From integration
of the resonances associated with the benzylic protons in
the hydrophilic stopper, the isomer distributions for 1‚4PF6
and 1‚4TRISPHAT were determined at 295 K in CD3COCD3
and CD3CN solutions. In CD3COCD3, 1‚4PF6 was found
(Figure 3) to contain 45% of the red translation isomer, while
1‚4TRISPHAT featured less than 5% of this isomer. These
observations can be accounted for by the fact that the
counterions must be closely associated with the tetracationic
rotaxane and must have a profound influence upon the
noncovalent bonding interactions that occur between the
different portions of the latter. In more polar solvents, where
the ions will be better solvated, such effects are expected to
be less significant. Hence in CD3CN, a much smaller change
in the isomer ratio, from 25 to 15% of the red translational
Figure 4. Translation isomer distribution in CD3COCD3 at 295 K
as a function of added salts determined by 1H NMR spectroscopy.
Dots (b) show the titration of 1‚4TRISPHAT by Bu4N‚PF6 with a
first-order exponential function fitted to the data points. Triangles
(2) show the titration of 1‚4PF6 with Me4N‚TRISPHAT with a
first-order exponential function fitted to the data points.
Org. Lett., Vol. 6, No. 23, 2004
4169

UV-vis titrations of more diluted Me2CO solutions (ca.
stration17 that the kinetics and the thermodynamics of the
switching by amphiphilic bistable rotaxanes vary in a
perfectly logical manner on traversing the environment from
the solution phase, through polymer matrixes and half-
devices into the full devices used in random access memory
circuits.5,16
1
10-4 M as compared with ca. 10-3 M for the H NMR
experiments) show similar trends on probing changes in the
characteristic CT absorption bands as functions of added
PF6- or TRISPHAT- salt. Several possible explanations may
be advanced to account for the counterion influence on the
translational isomer ratio in 14+. There are at least two effects
that favor the green isomer when the counterion is changed
Acknowledgment. We thank the Danish Natural Science
Research Council (SNF, Grants #21-03-0014 and #21-03-
0317), the Oticon and MODECS foundations, and the
Defense Advanced Research Projects Agency (DARPA) for
financial support.
-
from PF6 to TRISPHAT-: (1) One effect is an electronic
effect caused by the large size of the TRISPHAT- ion, which
increases the average distance between the tetracationic ring
and the negative charges of the ions in close ion-pairs,
causing the CBPQT4+ ring to become a stronger π-electron
acceptor and so favor binding to the stronger π-electron
donor, i.e., the BPTTF unit. (2) The other effect is a steric
effect whereby the larger TRISPHAT- ions shield the
CBPQT4+ ring in the close ion-pair, thus disfavoring the
[C-H‚‚‚O] interactions between the ring and the ethylene
glycol units when it resides on the DNP unit. The fact that
Supporting Information Available: Synthetic schemes
and experimental procedures for 1‚PF6 and 1‚TRISPHAT,
along with spectroscopic characterization. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL048518L
-
addition of a large excess of PF6 ions to 1‚4PF6 does not
lead to a pronounced increase in the proportion of the red
(12) The influence of solvation on the conformational isomerism of calix-
[4]arene and p-tert-butylcalix[4]arene has also been reported. See, for
example: (a) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713-1734. (b)
Cajan, M.; Lhotak, P.; Lang, J.; Dvorakova, H.; Stibor, I.; Koca, J. J. Chem.
Soc., Perkin Trans. 2 2002, 11, 1922-1929. (c) Alema´n, C.; den Otter, W.
K.; Tolpekina, T. V.; Briels, W. J. J. Org. Chem. 2004, 69, 951-958.
(13) It has also been shown that conformational isomerism in cyclohex-
anones such as 2-N,N-dimethylaminocyclohexanone is strongly influenced
by the polarity of the solvent. The conformational equilibrium between the
axial and equatorial conformation of 2-N,N-dimethylaminocyclohexanone
has been investigated, and it was found that the axial conformation was
dominant in CD3SOCD3 solution whereas the equatorial conformation was
the most dominant in more apolar solvents such as CDCl3. See: Freitas,
M. P.; Tormena, C. F.; Garcia, J. C.; Rittner, R.; Abraham, R. J.; Basso, E.
A.; Santos, F. P.; Cedran, J. C. J. Phys. Org. Chem. 2003, 16, 833-838.
isomer leads us to exclude mechanisms involving specific
-
interactions between 14+ and the PF6 ions favoring this
isomer.
The fact that the nature and size of the counterions have
such a profound influence upon the ratio of translational
isomers11-15 in bistable donor-acceptor rotaxanes where a
π-accepting tetracationic cyclophane shuttles between two
different π-donating recognition sites indicates that even
larger counterion effects will be operative in condensed-phase
systems, e.g., Langmuir and Langmuir-Blodgett mono-
layers,5 when they are employed in device settings.5,16 This
phenomenon is under investigations following the demon-
(11) Several other examples of translational isomerism, in both bistable
catananes and rotaxanes, induced by an environmental change such as
solvent and temperature have been reported. See: (a) Ashton, P. R.; Blower,
M.; Philp, D.; Spencer, N.; Stoddart, J. F.; Tolley, M. S.; Ballardini, R.;
Ciano, M.; Balzani, V.; Gandolfi, M. T.; Prodi, L.; Mclean, C. H. New. J.
Chem. 1993, 17, 689-695. (b) Ashton, P. R.; Ballardini, R.; Balzani, V.;
Credi, A.; Gandolfi, M. T.; Menzer, S.; Pe´rez-Garc´ıa, L.; Prodi, L.; Stoddart,
J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1995,
117, 11171-11197. (c) Leigh, D. A.; Moody, K.; Smart, J. P.; Watson, K.
J.; Slawin, A. M. Z. Angew. Chem., Int. Ed. Engl. 1996, 35, 306-310. (d)
Lane, A. S.; Leigh, D. A.; Murphy, A. J. Am. Chem. Soc. 1997, 119, 11092-
11093. (e) Leigh, D. A.; Murphy, A.; Smart, J. P.; Deleuze, M. S.; Zerbetto,
F. J. Am. Chem. Soc. 1998, 120, 6458-6467. (f) See ref 1c. (g) See ref 1e.
(h) Bottari, G.; Dehez, F.; Leigh, D. A.; Nash, P. J.; Pe´rez, E. M.; Wong,
J. K. Y.; Zerbetto, F. Angew. Chem., Int. Ed. 2003, 42, 5886-5889.
(14) Only one previous example of anion-induced translational isomerism
in a [2]rotaxane has been reported. See: Keaveney, C. M.; Leigh, D. Angew.
Chem., Int. Ed. 2004, 43, 1222-1224.
(15) Very recently, Li+ ions have been used to control the distribution
of translational isomers in a neutral bistable [2]rotaxane. See: Vignon, S.
A.; Jarrosson, T.; Iijima, T.; Tseng, H.-R.; Sanders, J. K. M.; Stoddart, J.
F. J. Am. Chem. Soc. 2004, 126,. 9884-9885.
(16) Flood, A. H.; Ramirez, R. J A.; Deng, W.-Q.; Muller, R. P.;
Goddard, W. A. III; Stoddart, J. F. Aust. J. Chem. 2004, 57, 301-322.
(17) (a) Tseng, H.-R.; Steuerman, D. W.; Peters, A. J.; Flood, A. H.;
Jeppesen J. O.; Nielsen, K. A.; Heath, J. R.; Stoddart, J. F. Angew. Chem.,
Int. Ed. In press. (b) Flood, A. H.; Peters, A. J.; Vignon, S. A.; Steuerman,
D. W.; Tseng, H.-R.; Heath, J. R.; Stoddart, J. F. Chem.-Eur. J. In press.
4170
Org. Lett., Vol. 6, No. 23, 2004