Quantifying the working stroke of tetrathiafulvalene-based electrochemically-driven linear motor-molecules

Article abstract of DOI:10.1039/b511575b

A highly constrained [2]rotaxane, constructed in such a way that the tetracationic cyclobis(paraquat-p-phenylene) ring is restricted to reside on a monopyrrolotetrathiafulvalene unit, has been synthesised and characterised. This design allows the deslipping free energy barrier for the tetracationic ring in all three redox states of the rotaxane to be determined. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b511575b

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Quantifying the working stroke of tetrathiafulvalene-based
electrochemically-driven linear motor-molecules{
Sune Nygaard,ab Bo W. Laursen,bc Amar H. Flood,{b Camilla N. Hansen,a Jan O. Jeppesen*a and
J. Fraser Stoddart*b
Received (in Cambridge, UK) 16th August 2005, Accepted 7th September 2005
First published as an Advance Article on the web 10th October 2005
DOI: 10.1039/b511575b
monopyrrolotetrathiafulvalene (MPTTF) unit by the close proxi-
mity of the two stoppers attached to each end of the MPTTF unit.
A conventional large di-t-butylbenzylic stopper9 and a smaller
more unconventional one, in the form of a combination of a
thioethyl (SEt) group and diethyleneglycol substituent,5b were
chosen, thereby forcing any deslipping of the CBPQT4+ ring to
take place in the direction of the latter and smaller of the two
stoppers. During oxidation, the MPTTF unit acquires one and
subsequently two positive charges in a stepwise fashion, effectively
destabilizing the mechanically interlocked molecule and hence
lowering the effective barrier to deslipping of the CBPQT4+ ring.
The design of highly constrained 54+ allows us to determine the
deslipping free energy barrier for the tetracationic ring in all three
redox states of the rotaxane, i.e., when the MPTTF unit is (i)
neutral, (ii) singly and (iii) doubly oxidised. The thermodynamic
parameters associated with the deslipping barriers allow the
magnitude of the electrostatically-driven working stroke produced
in MPTTF/CBPQT4+-based molecular machines to be determined.
The [2]rotaxane 5?4PF6 was synthesised as outlined in Scheme 1.
(Methoxyethoxy)ethyl iodide was coupled with MPTTF building
block 1, following its in situ deprotection with CsOH?H2O, to give
2 in 87% yield. The tosyl protecting group on the MPTTF unit was
removed in good yield (93%) using NaOMe in a THF–MeOH
mixture. The resultant pyrrole nitrogen in 3 was alkylated (NaH/
DMF) with 3,5-di-t-butyl-bromomethylbenzene, affording the
dumbbell compound 4 in 93% yield. Self-assembly of 5?4PF6
A highly constrained [2]rotaxane, constructed in such a way
that the tetracationic cyclobis(paraquat-p-phenylene) ring is
restricted to reside on a monopyrrolotetrathiafulvalene unit,
has been synthesised and characterised. This design allows the
deslipping free energy barrier for the tetracationic ring in all
three redox states of the rotaxane to be determined.
Of all the p-electron rich guests that form 1 : 1 complexes with
the p-electron poor cyclobis(paraquat-p-phenylene) (CBPQT4+)
host,1 tetrathiafulvalene (TTF) is one of the most efficient.§"
Together with the fact that TTF can be oxidized reversibly2—at
+
?
+321 mV (MeCN, vs. SCE) to the TTF radical cation, and
then subsequently at +714 mV (MeCN, vs. SCE) to its TTF2+
dication—rendering its complexation with CBPQT4+ null and
void, its propensity to be encircled by a CBPQT4+ ring has made
it a prime candidate for the construction of molecular switches
in the form of bistable [2]catenanes3,4 and [2]rotaxanes.4,5
Furthermore, in bistable [2]rotaxanes,6 and in a doubly bistable
palindromic [3]rotaxane,7 the ability to induce mechanical
movement(s) of the CBPQT4+ ring component(s) along the
rod sections of the rotaxanes’ dumbbell components to another
much less p-electron rich site—for example, a 1,5-dioxy-
naphthalene one, by changing the redox states of the TTF unit
either chemically or electrochemically—has become the basis
for the construction of artificial linear motor-molecules.8 The
linear movements in such bistable, mechanically interlocked
molecules, which are undoubtedly activated by the electrostatic
repulsion between the oxidized and positively-charged TTF unit
and the tetracationic CBPQT4+ ring, become the basis for what
can be likened to a working stroke in a linear motor.
In this communication, we report on the synthesis and
thermodynamic
evaluation
of
a
highly
constrained
[2]rotaxane, 54+, constructed (Scheme 1) in such a way that the
location of the CBPQT4+ ring is restricted to reside on a
aDepartment of Chemistry, University of Southern Denmark, Odense
University, Campusvej 55, DK-5230, Odense M, Denmark.
E-mail: joj@chem.sdu.dk
bCalifornia NanoSystems Institute and Department of Chemistry and
Biochemistry, University of California, Los Angeles, 405 Hilgard
Avenue, Los Angeles, CA, 90095-1569, USA.
E-mail: stoddart@chem.ucla.edu
cNano-Science Center, University of Copenhagen, Universitetsparken 5,
DK-2100, København Ø, Denmark
{ Electronic Supplementary Information (ESI) available: Experimental
details on the syntheses of 5?4PF6, spectroscopic details and deslipping
data. See http://dx.doi.org/10.1039/b511575b
{ Present address: Chemistry Department, Indiana University, 800 East
Kirkwood Avenue, Bloomington, IN 47405, USA.
Scheme 1 Synthesis of the highly constrained [2]rotaxane 5?4PF6.
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144 | Chem. Commun., 2006, 144–146

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was carried out from its precursor dumbbell 4 by employing a
standard high pressure clipping procedure, in which 4 acts as a
template for the reaction between 1,10-[1,4-phenylenebis(methyle-
ne)]bis(4,49-bipyridin-1-ium) bis(hexafluorophosphate)1 and 1,4-
bis(bromomethyl) benzene, followed by counterion exchange,
yielding the product in 20% yield overall. It was characterised{
fully by mass spectrometry and NMR spectroscopy.
The deslipping of 54+ was carried out in PhCN at sufficiently
high temperatures to promote the CBPQT4+ ring to overcome the
steric barrier imposed by the SEt stopper. Deslipping was
monitored by absorption spectroscopy at room temperature using
the decrease in the intensity of the characteristic charge transfer
(CT) band at 844 nm (PhCN), originating from the CT interaction
between the MPTTF unit and the CBPQT4+ ring, as a probe for
the extent of deslipping. The rate constants for deslipping were
evaluated{ at 405, 409, 413, 429 and 447 K by first order kinetics,
giving deslipping barriers (DG{) of 32.3, 32.4, 32.5, 33.1 and
Scheme 2 A cartoon representation illustrating the effect of adding
1.0 equiv. of Fe(ClO4)3 to the rotaxane 54+. Firstly, the mono-oxidised
rotaxane 55+ is formed. Secondly, thermally-activated deslipping of 55+
generates the free ring component and the dumbbell as a radical cation 4+,
which then is further oxidised by a second equiv. of 55+ to generate the
1
33.5 kcal mol21, respectively, in PhCN. A H NMR spectrum{
recorded of the reaction mixture remaining after a similar
deslipping process in CD3SOCD3 revealed that it contained free
dumbbell 4 and the free CBPQT4+ ring, with only minor amounts
of decomposition products being observed.
non-oxidised rotaxane 54+ and the doubly-oxidised dumbbell 42+
.
following the disappearance of the characteristic 750 nm band,
originating from the doubly-oxidised MPTTF2+ unit located inside
the CBPQT4+ ring’s cavity. Assuming the operation of first order
kinetics, DG{ values of 19.8, 20.0, 20.0, 20.1 and 20.2 kcal mol21 in
MeCN at 276, 280, 283, 288 and 291 K, respectively, were
obtained.{
Addition of 1.0 equiv. of Fe(ClO4)3 to 54+ in MeCN resulted in
the mono-oxidation of the MPTTF unit, giving 55+. Deslipping of
the CBPQT4+ ring from 55+ was monitored by absorption
spectroscopy, utilising the distinctive 465 nm absorption band
(Fig. 1) originating from the mono-oxidised MPTTF+ unit
formally located inside the cavity of the CBPQT4+ ring as a
probe. The intensity of this band diminishes as a function of time,
giving rise to a broad absorption band centred at 625 nm. This
absorption is characteristic{ of the uncomplexed doubly-oxidised
MPTTF2+ unit of the free dumbbell 42+. These spectroscopic
observations suggest that a dismutation (Scheme 2) follows the
initial deslipping. The oxidation potentialsI for 42+/+ and 55+/4+
also agree well with this model. The spectroscopic deslipping data
were obtained at 313, 318, 323, 329 and 335 K and follow first
order kinetics. An analysis{ based on the dismutation model gave
deslipping barriers (DG{) of 24.6, 24.6, 24.8, 25.1 and
25.1 kcal mol21, respectively, in MeCN.
Since the DG{ values have been determined at different
temperatures, the enthalpic (DH{) and entropic (DS{) contribu-
tions to the deslipping processes can be calculated from plots{ of
DG{ against T. The kinetic parameters obtained from these plots
are summarised in Table 1, together with the extrapolated DG{
values at 300 K.
Providing that the deslipping transition state is not affected to
any significant degree by the oxidation state of the MPTTF unit, a
potential energy diagram for the deslipping of the highly
constrained [2]rotaxane 54+ in its three oxidation states can be
constructed (Fig. 2). Thus, based on the thermodynamic data
obtained (Table 1) for the different deslipping processes occurring
in 54+, 55+ and 56+, the potential energy that this linear motor
acquires as it is driven into the first and second oxidation state can
be quantified as 4.9 and 8.7 kcal mol21, respectively. It is evident
that the oxidation of the MPTTF unit reduces the energy barrier
that the CBPQT4+ ring has to overcome in order for deslipping to
occur by raising the energy levels of the potential wells in the 55+
and 56+ oxidation states compared to the ground state 54+ such
that they become net repulsive to the dicationic MPTTF2+ unit,
i.e., DG300 . 0 in Fig. 2. Progressive lowering of the energy barrier
Formation of the doubly-oxidised [2]rotaxane 56+ was accom-
plished by adding an excess of Fe(ClO4)3 in MeCN. The
thermodynamics of the deslipping of 56+ were established by
Table 1 Kinetic parametersa for the deslipping of the CBPQT4+ ring
in 54+, 55+ and 56+
DH{
DS{
DG{ (300 K)
Solvent
(kcal mol21
)
(cal mol21 K21
)
(kcal mol21
)
54+
55+
56+
a
PhCN
MeCN
MeCN
19.9
15.5
12.6
230.6
228.8
226.2
29.1
24.2
20.4
The DH{ and DS{ values were obtained from the intercept and gra-
dient of the straight line plots of DG{ against T using the relation-
ship DG{ 5 DH{ 2 TDS{, where T is the absolute temperature; the
estimated errors are ¡5% for DH{ and ¡10% for DS{ and DG{.
Fig. 1 Time lapse absorption spectra of 5?4PF6 recorded in MeCN at
323 K after addition of 1.0 equiv. of Fe(ClO4)3.
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Chem. Commun., 2006, 144–146 | 145

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than in Me2CO. Ka values for the complexation of CBPQT4+ with
MPTTF derivatives have been reported5d in the range 1300–4000 M21 in
Me2CO at room temperature. Consequently Ka values at room
temperature in the range 5000–16000 M21, corresponding to DGu values
in the range of 25–26 kcal mol21, in MeCN/PhCN are expected.
1 J. F. Stoddart and H.-R. Tseng, Proc. Natl. Acad. Sci. U. S. A., 2002,
99, 4797.
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G. Schukat and E. Fangha¨nel, Sulfur Rep., 2003, 24, 1; (c) J. O. Jeppesen,
M. B. Nielsen and J. Becher, Chem. Rev., 2004, 104, 5115.
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Chem., 2000, 65, 1924; (b) C. P. Collier, G. Mattersteig, E. W. Wong,
Y. Luo, K. Beverly, J. Sampaio, F. M. Raymo, J. F. Stoddart and
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H.-R. Tseng, S. Kang, J. R. Heath and J. F. Stoddart, Chem.-Eur. J.,
2004, 10, 6558; (b) A. H. Flood, J. F. Stoddart, D. W. Steuerman and
J. R. Heath, Science, 2004, 306, 2055; (c) P. M. Mendes, A. H. Flood
and J. F. Stoddart, Appl. Phys. A: Solid Surf., 2005, 80, 1197.
5 (a) Y. Luo, C. P. Collier, J. O. Jeppesen, K. A. Nielsen, E. DeIonno,
G. Ho, J. Perkins, H.-R. Tseng, T. Yamamoto, J. F. Stoddart and
J. R. Heath, ChemPhysChem, 2002, 3, 519; (b) J. O. Jeppesen,
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M. T. Gandolfi, M. Venturi, V. Balzani, J. Becher and J. F. Stoddart,
Chem.-Eur. J., 2003, 9, 2982; (c) S. Kang, S. A. Vignon, H.-R. Tseng
and J. F. Stoddart, Chem.-Eur. J., 2004, 10, 2555; (d) J. O. Jeppesen,
S. Nygaard, S. A. Vignon and J. F. Stoddart, Eur. J. Org. Chem., 2005,
196; For recent examples of bistable rotaxanes based on metal–ligand
bonding, see; (e) J.-P. Sauvage, Chem. Commun., 2005, 1507; and on
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D. Martel, F. Paolucci, A. M. Z. Slawin and J. K. Y. Wong, J. Am.
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Fig. 2 Scaled schematic representation of the potential energy surface of
54+, in its three oxidation states 54+, 55+ and 56+ as a function of the
reaction coordinate. The bottom of the well for 54+ is approximated{{ by
the binding energy 26 kcal mol21 between free MPTTF and the
CBPQT4+ ring.
is mainly a result of the electrostatic repulsion** between MPTTF+
or MPTTF2+ units and the tetracationic CBPQT4+ ring. Thus, a
linear motor built from the same components has upwards of ca.
9 kcal mol21 to use from the powered movement of the CBPQT4+
ring following two-electron oxidation of the MPTTF unit. On
account of the electrostatic nature of this working stroke, it should
be noted that higher as well as lower values are to be expected in
other environments, depending on the local dielectric and specific
ion pairing.
6 (a) H.-R. Tseng, D. Wu, N. X. Fang, X. Zhang and J. F. Stoddart,
ChemPhysChem, 2004, 5, 111; (b) D. W. Steuerman, H.-R. Tseng,
A. J. Peters, A. H. Flood, J. O. Jeppesen, K. A. Nielsen, J. F. Stoddartand
J. R. Heath, Angew. Chem., Int. Ed., 2004, 43, 6486; (c) T. D. Nguyen,
H.-R. Tseng, P. C. Celestre, A. H. Flood, Y. Liu, J. F. Stoddart and
J. I. Zink, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10029.
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H.-R. Tseng, J. F. Stoddart, M. Baller and S. Magonov, Appl. Phys.
Lett., 2004, 85, 5391.
We thank the University of Southern Denmark and the
Danish Natural Science Research Council (SNF, grants #21-
03-0014 and #21-03-0317), the Oticon and MODECS founda-
tions, the Center for NanoScale Innovation for Defense
(CNID), and the Defense Advanced Research Projects
Agency (DARPA) for financial support.
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A Journey into the Nanoworld, Wiley-VCH, Weinheim, 2003; (b) Towards
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Notes and references
§ One of us, and later Bryce, investigated the green 1 : 1 complex formed
between CBPQT4+ and TTF. Originally, in a communication,10 a Ka value
of 51 M21 for the 1 : 1 complex in MeCN at 300 K was reported.
Subsequently, it wasfound that this Ka valuewas in error. After numerous
experimentshadbeencarriedoutbyboththeStoddartgroup11andBryce,12
a consistent conclusion was reached—that is, that the Ka value is ca.
10000M21 inMeCNat 298K,whileinMe2COat298Kitisca. 2600M21
.
Later,theabilityofCBPQT4+tohostpolyether-linkedTTFderivativeswas
investigated,3a,13 and it was found that the ethyleneoxy substituents are of
paramount importance in assisting the complexation process by virtue of
…
theirenteringinto[C–H O]interactionswithsomeofthea-CH hydrogen
atoms in the bipyridinium units of CBPQT4+. Finally, the 1 : 1 complex
formation between CBPQT4+ and structurally modified TTF derivatives
hasbeeninvestigated,14 andithasbeenconcluded15 thatthestrengthofthe
binding between TTF derivatives and CBPQT4+ is strongly dependent on
the p-electron donating ability of the TTF derivatives.
12 W. Devonport, M. A. Blower, M. R. Bryce and L. M. Goldenberg,
J. Org. Chem., 1997, 62, 885.
13 M. Asakawa, P. R. Ashton, V. Balzani, A. Credi, G. Mattersteig,
O. Matthews, N. Montalti, N. Spencer, J. F. Stoddart and M. Venturi,
Chem.-Eur. J., 1997, 3, 1992.
" Beside CBPQT4+ being able to host TTF and its derivatives, it has very
recently been demonstrated16 that TTF can be used as an efficient template
for the synthesis of CBPQT4+
.
I Based on the half-wave potentials, there is a 16 mV driving force for this
14 (a) J. Lau, M. B. Nielsen, N. Thorup, M. P. Cava and J. Becher, Eur.
J. Org. Chem., 1999, 3335; (b)M.R.Bryce,G.Cooke,F.M.A.Duclairoir
and V. M. Rotello, Tetrahedron Lett., 2001, 42, 1143.
15 M. B. Nielsen, J. O. Jeppesen, J. Lau, C. Lomholt, D. Damgaard,
J. P. Jacobsen, J. Becher and J. F. Stoddart, J. Org. Chem., 2001, 66,
3559.
reaction.{
** Thenatureofthepotentialenergysurfaceinthevicinityoftheinteraction
oftheCBQPT4+ringwiththeMPTTF+radicalcationorMPTTF2+dication
is expected to be transition state-like in character. Consequently, we
associate the lowest energy states with highly distorted co-conformations.
{{ Several studies5b,11–16 have shown that the complexation of CBPQT4+
with different TTF derivatives is around four times stronger in MeCN
16 G. Doddi, G. Ercolani, P. Mencarelli and A. Piermattei, J. Org. Chem.,
2005, 70, 3761.
146 | Chem. Commun., 2006, 144–146
This journal is ß The Royal Society of Chemistry 2006