A rigid donor-acceptor daisy chain dimer

Article abstract of DOI:10.1039/c2cc32499g

A functionalised cyclobis(paraquat-p-phenylene) attached by a rigid linker to a tetrathiafulvalene unit, which is incapable of self-complexation, forms preferentially a [c2]daisy chain which undergoes rapid disassociation and reassociation on the ¹H NMR time-scale above room temperature. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc32499g

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A rigid donor–acceptor daisy chain dimerwz
Dennis Cao,ab Cheng Wang,a Marc A. Giesener,a Zhichang Liua and J. Fraser Stoddart*ab
Received 6th April 2012, Accepted 30th April 2012
DOI: 10.1039/c2cc32499g
A functionalised cyclobis(paraquat-p-phenylene) attached by a
rigid linker to a tetrathiafulvalene unit, which is incapable of
self-complexation, forms preferentially a [c2]daisy chain which
rigid spacer of an appropriate length between the donor and
the CBPQT4+ ring might eliminate the possibility for intra-
molecular interactions altogether, thus favouring the for-
mation of daisy chains. Herein, we report the synthesis and
characterisation of a daisy chain-forming compound 14+
which consists (Fig. 1) of a tetrathiafulvalene (TTF) unit
joined by rigid aromatic linkers to a CBPQT4+ ring for the
all but exclusive construction of a donor–acceptor [c2]daisy
chain. Several important considerations were taken into
account in designing 14+, including (i) the rigidity of the linker
which was enforced by a phenylacetylene-containing spacer
whose length (11.6 A) is greater than the length (9.9 A) of
the CBPQT4+ cavity, ensuring that self-complexation cannot
occur, (ii) the choice of TTF as the electron-rich donor since
previously we have noted14 that 1-ethynyl-5-hydroxynaphthal-
ene derivatives have a low binding affinity for the CBPQT4+
ring, and (iii) the use of a phthalimide linker in place of one
of the xylylene units in the CBPQT4+ ring because sub-
stituents attached to the imide nitrogen are oriented at right
angles to the mean plane of the CBPQT4+ ring, a situation
which maintains a plane of symmetry in a perpendicular
direction, thus avoiding the generation of isomeric [c2]daisy
chains.
1
undergoes rapid disassociation and reassociation on the H NMR
time-scale above room temperature.
Artificial molecular muscles and actuators have become a
source of interest in recent years because of their incorporation
into nanoelectromechanical systems (NEMS)1 in order to
transduce nanoscale molecular motions into macroscopic
movements.2 Elastomers,3 conducting polymers,4 and carbon
nanotubes,5 capable of actuation, have all been a part of this
interest. Simultaneously, mechanically interlocked molecules2
(MIMs) which mimic protein filaments in biological systems,
have generated attention since the discrete relative movements
of their components can be controlled by a number of different
stimuli. To this end, in addition to electrochemically-
stimulated doubly bistable [3]rotaxanes,6 both acid–base7
and chemically8b actuated bistable [c2]daisy chains have been
introduced. The latter have been designed and synthesised
around metal–ligand coordination,8 hydrogen bonding,7,9
hydrophobic,10 and p-donor/acceptor interactions,11 as the
sources of their mutual recognition units. More often than
not, however, daisy chain monomers self-assemble in solution to
give10c–d,11 a mixture of linear and cyclic oligomers, depending on
the concentration of the monomer.
The synthesis (Scheme 1) of 1ꢀ4PF6 begins15 with a Diels–
Alder reaction between 2,5-dimethylfuran and maleic anhydride.
Subsequent elimination of H2O from the adduct under strongly
acidic conditions yields the anhydride 2 which, when condensed
with 4-iodoaniline, yields 3. Dibromide 4, generated by the
NBS bromination of 3, was hydrolysed to afford the diol 5 in
order to ‘‘protect’’ the benzylic bromides during the sub-
sequent Sonogashira coupling. The TTF derivative 616 was
then coupled to 5 to yield the intermediate 7. Since standard
PBr3 bromination to regenerate the dibromide proved to be
too harsh, mesylation of the diol followed by chloride sub-
stitution was used to generate the dichloride 8 which was
In the case of donor–acceptor-based daisy chains, there
have been numerous attempts to attach flexible donating units
to the electron-deficient cyclobis(paraquat-p-phenylene)12
(CBPQT4+) ring. These monomers, however, have shown13
a strong tendency to self-complex, rather than form higher
order superstructures, no doubt as a consequence of the
considerable entropic penalty associated with the generation
of supramolecular polymers, be they cyclic or acyclic. In order
to circumvent self-complexation, we envisage that the use of a
a Center for the Chemistry of Integrated Systems, Department of
Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
IL 60208, USA. E-mail: stoddart@northwestern.edu;
Fax: (+1)-847-491-1009; Tel: (+1)-847-491-3793
b NanoCentury KAIST Institute and Graduate School of EEWS
(WCU), Korea Advanced Institute of Science and Technology
(KAIST), 373-1 Guseong Dong, Yuseong Gu, Daejeon 305-701,
Republic of Korea
w This article is part of the ChemComm ‘Aromaticity’ web themed
issue.
Fig. 1 The structural formula of 14+ which contains a TTF unit
linked to a CBPQT4+ ring by means of a phenylacetylene spacer.
z Electronic Supplementary Information (ESI) available: Synthesis
and characterization. See DOI: 10.1039/c2cc32499g
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6791–6793 6791

Scheme 1 Synthesis of 1ꢀ4PF6.
reacted in MeCN with an excess of 4,40-bipyridine to yield
separated from each other, while the peaks for the methylene
protons closest to the phthalimide unit separate into an AX
system. We hypothesize that this AX system is a result of
the stable nature of the [c2]daisy chain at low temperatures
where the imide functionality sits exclusively on one side of the
CBPQT4+ face, imposing diastereotopism upon the HMTD
protons on opposite sides of the CBPQT4+ ring. As a con-
sequence of the sidedness of the CBPQT4+ ring in the dimeric
[c2]daisy chain, the resonances corresponding to the bipyridi-
nium protons, HaD1, HaD2, HbD1, and HbD2, as well as to the
phenylene protons, HPBD, also divide up into four and two sets
of resonances, respectively. Upon increasing the temperature
of a CD3CN solution of 1ꢀ4PF6 from 233 to 323 K, changes
occur (Fig. 4) in the 1H NMR spectra. The resonances
corresponding to HaD1, HaD2, HbD1, HbD2, and HPBD coalesce
between 248 and 263 K, indicating that the rotations of the
pyridinium and phenylene rings become fast on the NMR
time-scale. The peaks for the protons employed to probe the
energy barriers for rotation, using the coalescence method,
results17 in very similar energies of activation DGz, namely
13.8–14.5 kcal molꢁ1 from the bipyridinium and phenylene
protons, indicating that we are looking at a situation involving
numerous probes and realizing they reflect the same
mechanism—removal of TTF units from inside CBPQT4+
rings followed by pyridinium and phenylene ring rotations. At
around 293 K, whereas the HTTF2D and HTTF3D resonances
essentially spread out into the baseline, between 293 and
323 K, some of the peaks begin to shift while the HTTF2D
and HTTF3D resonances reappear at around 6.3 ppm, similar
to the TTF resonances in the free model compound S1
9ꢀ2PF6 after counterion exchange. Finally, 1,4-bis(bromomethyl)-
benzene was reacted with 9ꢀ2PF6 in the presence of template 10
to yield 1ꢀ4PF6 after counterion exchange and purification by
high-performance liquid chromatography. High resolution
electrospray ionization mass spectrometry of a 3 mM solution
of 1ꢀ4PF6 revealed peaks at m/z = 1326.0789 and 2797.1162 Da,
ꢁ
corresponding to the loss of one PF6 counterion from the
monomer and dimer of 14+, respectively.
UV-Vis spectrophotometric investigations on a model TTF
compound (S1z) and CBPQT4+ (S24+z) confirm that the TTF
units functionalized with a rigid linker bind to the CBPQT4+
ring. Indeed, when handling 1ꢀ4PF6, it was noticeable imme-
diately that the colour of the solution is dependent on
the concentration (Fig. 2) and the temperature. At higher
concentrations, the green colour resulting from the charge
transfer (CT) between TTF and CBPQT4+ persists while, at
lower concentrations, the yellow colour corresponding to a
characteristic absorption band appears to dominate. A serial
dilution of a solution of 1ꢀ4PF6 in MeCN from 2.7 mM down
to 0.1 mM reveals (see SIz) a CT band at 780 nm for the
interaction between TTF and the CBPQT4+ ring. The inten-
sity of the band decreases as the concentration is lowered.
Applying the Benesi–Hildebrand method (see SIz) reveals a non-
linear relationship between the inverse concentration and the
inverse change in absorption intensity, an observation which
suggests that the formation of the CT complex is most likely
the result of dimerisation rather than as a consequence of self-
complexation or oligomerisation.
The equilibrium (Fig. 3a) between the monomer and dimeric
[c2]daisy chain can be followed by variable temperature (VT)
1H NMR spectroscopy. At low temperatures, relatively sharp
resonances corresponding to the cyclic dimer can be observed.
With the assistance of 1H-1H-g-DQF-COSY and 1H-1H
ROESY NMR (see SIz), the resonances for all the protons
in 14+ can be assigned (Fig. 3b) at 233 K. The resonances
corresponding to the TTF protons are shifted upfield and well
Fig. 3 (a) The proposed equilibrium between the monomer and
dimer of 14+. (b) 1H NMR spectrum (600 MHz, 2 mM, 233 K,
CD3CN) of 1ꢀ4PF6 and assignment of the resonances.
Fig. 2 Solutions of 14+ in MeCN demonstrating the change in colour
as a function of concentration.
c
6792 Chem. Commun., 2012, 48, 6791–6793
This journal is The Royal Society of Chemistry 2012

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Indeed, when the NMR tube was removed from the spectro-
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green upon cooling to room temperature. Going higher in
temperature than 323 K results in the decomposition of the
compound.
We have demonstrated the preferential formation of a
[c2]daisy chain as a consequence of the rigidity in the monomer
unit which rules out intramolecular interactions leading to self-
complexation. The supramolecular complex undergoes rapid
disassociation and reassociation on the 1H NMR timescale above
RT as indicated by the fact that certain protons on the CBPQT4+
ring undergo fast exchange. The next challenge is to design and
synthesize rigid donor–acceptor daisy chains with bistability.
This research is supported by the National Science Foundation
(NSF) under grant CHE-0924620. D. C. and J. F. S. were
supported by the WCU Program (NRF R-31-2008-000-10055-0)
funded by the Ministry of Education, Science and Technology,
Korea. D. C. acknowledges support from an NSF Graduate
Research Fellowship.
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17 The rate constants, kc, at the coalescence temperatures, Tc, were
determined using the approximate expression, kc = p(Dn)/(2)12, in
which Dn is the limiting chemical shift (in Hz) between the
exchanging proton resonances. The Eyring equation, DGz
=
c
ꢁRTln(kcꢀh/kBTc) was used to calculate the DGzc value at the lower
limit of the Tc value for the (i) HaD1 (Dn = 49.8 Hz, kc = 110.6,
Tc = 263 K, DGz = 14.0 ꢂ 0.1 kcal molꢁ1), (ii) HaD2 (Dn =
c
14.6 Hz, kc = 32.5, Tc = 248 K, DGz = 14.5 ꢂ 0.4 kcal molꢁ1),
c
(iii) HbD (Dn = 48.6 Hz, kc = 108.0, Tc = 263 K, DGzc = 14.0 ꢂ
0.1 kcal molꢁ1), and (iv) HPBD (Dn = 47.5 Hz, kc = 105.4, Tc =
258 K, DGz
I. O. Sutherland, Annu. Rep. NMR Spectrosc., 1972, 4, 71–235.
=
13.8
ꢂ
0.1 kcal molꢁ1
) resonances. See:
c
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6791–6793 6793