A conjugated thiophene-based rotaxane: synthesis, spectroscopy, and modeling

Article abstract of DOI:10.1002/chem.200903353

A dithiophene rotaxane 1?βCD and its shape-persistent corresponding dumbbell 1 were synthesized and fully characterized. 2D NOESY experiments, supported by molecular dynamics calculations, revealed a very mobile macrocycle (β-CD). Steadystate and time-resolved photoluminescence experiments in solution were employed to elucidate the excited-state dynamics for both systems and to explore the effect of cyclodextrin encapsulation. The photoluminescence (PL) spectrum of 1?β-CD was found to be blueshifted with respect to the dumb- bell 1 (2.81 and 2.78 eV, respectively). Additionally, in contrast to previous observations, neither PL spectra nor the decay kinetics of both threaded and unthreaded systems showed changes upon increasing the concentration or changing the polarity of the solutions, thereby providing evidence for a lack of tendency toward aggregation of the unthreaded backbone.

Full text of DOI:10.1002/chem.200903353

FULL PAPER
DOI: 10.1002/chem.200903353
A Conjugated Thiophene-Based Rotaxane:
Synthesis, Spectroscopy, and Modeling
Leszek Zalewski,*[a] Michael Wykes,[b] Sergio Brovelli,[c] Massimo Bonini,[a]
Thomas Breiner,[a] Marcel Kastler,[a] Florian Dçtz,[d] David Beljonne,*[b]
Harry L. Anderson,*[e] Franco Cacialli,*[c] and Paolo Samorꢀ*[f, g]
Abstract: A dithiophene rotaxane 1ꢀb-
CD and its shape-persistent corre-
sponding dumbbell 1 were synthesized
and fully characterized. 2D NOESY ex-
periments, supported by molecular dy-
namics calculations, revealed a very
mobile macrocycle (b-CD). Steady-
state and time-resolved photolumines-
cence experiments in solution were em-
ployed to elucidate the excited-state
dynamics for both systems and to ex-
plore the effect of cyclodextrin encap-
sulation. The photoluminescence (PL)
spectrum of 1ꢀb-CD was found to be
blueshifted with respect to the dumb-
bell 1 (2.81 and 2.78 eV, respectively).
Additionally, in contrast to previous
observations, neither PL spectra nor
the decay kinetics of both threaded
and unthreaded systems showed
changes upon increasing the concentra-
tion or changing the polarity of the sol-
utions, thereby providing evidence for
a lack of tendency toward aggregation
of the unthreaded backbone.
Keywords: conjugation · oligothio-
phenes · optoelectronics · rotax-
anes · supramolecular chemistry
Introduction
ing its optical and electronic properties. Among various non-
covalent engineering strategies, the encapsulation of conju-
gated molecules by nonconjugated macrocycles to form ro-
taxanes was shown to represent a powerful method to ach-
ieve a high degree of control over several properties of mo-
lecular-based systems, as it can allow for simultaneous
1) improvement of molecular rigidity, 2) prevention of ag-
gregation, 3) enhanced solubility and chemical compatibility,
as well as 4) protection of the functional unit. The latter is
of importance when working with electron-rich chromo-
phores such as oligothiophenes,[5] since the shielding effect
of cyclodextrins can hinder radical or electrophilic attack of
Organic p-conjugated molecules incorporating electron-rich
thiophene moieties are an important and widely investigated
class of semiconductors.[1] In view of their capability of
transporting charges, oligothiophenes are attractive building
blocks for organic electronic applications including field-
effect transistors (FET),[2] light-emitting diodes (LED),[3]
and solar cells.[4]
Achieving a full control over the intermolecular interac-
tions in p-conjugated materials makes it possible to optimize
several physicochemical properties of the ensemble includ-
[a] Dr. L. Zalewski, Dr. M. Bonini, Dr. T. Breiner, Dr. M. Kastler
BASF SE, 67056 Ludwigshafen (Germany)
[e] Prof. H. L. Anderson
University of Oxford, Department of Chemistry
Chemistry Research Laboratory
E-mail: leszekzalewski@gmail.com
Mansfield Road, Oxford OX1 3TA (UK)
E-mail: harry.anderson@chem.ox.ac.uk
[b] M. Wykes, Dr. D. Beljonne
Laboratory for Chemistry of Novel Materials
Universitꢀ de Mons, 20, Place du Parc, 7000 Mons (Belgium)
E-mail: david@averell.umh.ac.be
[f] Prof. P. Samorꢁ
Nanochemistry Laboratory, ISIS-CNRS 7006
Universitꢀ de Strasbourg
8 allꢀe Gaspard Monge, 67000 Strasbourg (France)
E-mail: samorꢁ@isis-ulp.org
[c] Dr. S. Brovelli, Prof. F. Cacialli
Department of Physics and Astronomy (CMMP Group)
and London Centre for Nanotechnology
University College London, Gower Street, London (UK)
E-mail: f.cacialli@ucl.ac.uk
[g] Prof. P. Samorꢁ
ISOF-CNR, via Gobetti 101, 40129 Bologna (Italy)
[d] Dr. F. Dçtz
BASF SE, Global Research Center Singapore/Organic Electronics
61 Science Park Road, 117525 (Singapore)
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the conjugated backbone. In addition, the electronic proper-
ties of noncovalently interlocked chromophores inside the
cavity were found to be only mildly affected by the environ-
ment.
sized by means of a nickel-catalyzed Yamamoto coupling.[21]
Both these approaches suffered from a poor control over
the polymerization ratio, thus leading to highly polydisperse
systems. Additionally, only a maximum 60% of insulation-
threading ratio (i.e., 0.6b-cyclodextrin per bithiophene unit)
was affordable.
Novel types of oligothiophene rotaxanes were synthesized
by Harada and co-workers.[22] By applying aqueous Suzuki
coupling conditions, a series of dimethyl-b-cyclodextrin-
based rotaxanes that featured various threading ratios and
lengths of the conjugated backbone were synthesized by
using insulated b-cyclodextrin bulky stoppers.
Instead we opted to focus on ultrarigid oligomeric thio-
phene-phenyl-based scaffolds as backbone, thereby exposing
aromatic end groups terminated with carboxylic acid moiet-
ies. Thus this class of oligorotaxane combines features that
are relevant for both electronic function of the molecule
and preprogrammed self-assembly through the interaction
between distinct functional groups, namely hydrogen bond-
ing among COOH moieties that belong to adjacent mole-
cules.
Scheme 1 portrays the synthesis of dithiophene rotaxane
1ꢀb-CD. Inclusion complex 3 was formed in a threading re-
action between dithiophene diboronic ester 2 and a fourfold
molar excess of b-cyclodextrin (b-CD). In the final stage,
complex 3 was capped by bulky iodoterphenylenedicarbox-
ylic acid 4 by means of an aqueous Suzuki coupling to yield
the formation of a rotaxane. Dithiophene rotaxane 1ꢀb-CD
was isolated and purified by reversed-phase chromatography
(RPC) in 2% yield. Monomer 2 (5,5’-di(1,3,2-dioxaborolan-
2-yl)-2,2’-bithiophene) and m-terphenylene acid 4 capping
group (3,5-bis(4-carboxyphenyl)-1-iodobenzene) were syn-
thesized according to previously reported procedures.[13,22,23]
Insulation of chromophores by cyclodextrin (a, b, or g)
macrocycles can significantly reduce interchain electronic in-
teractions within thin films[6] to limit redshift and quenching
of the luminescence.[7–10] Hitherto insulated molecular wires
have been formed by threading a relatively rigid polymer
backbone (i.e., dumbbell) through various types of rings like
cyclophanes,[11] cyclodextrins,[9,12,13] or incorporated into
inert matrixes of porous materials (zeolites, silica),[14] or
even wrapped by polymers.[15] A wide range of host–guest
inclusion complexes of cyclodextrins with organic dyes and
chromophores in aqueous solutions have been reported. The
formation of the inclusion complex relies on the effect of
hydrophobic forces to promote penetration of a cyclic cavity
by organic molecules, thus leading to stable complexes.[16]
The best environment for such a threading reaction is there-
fore the aqueous media, thereby rendering Suzuki coupling
in water a suitable reaction for synthesizing cyclodextrin-en-
capsulated rotaxanes. Under these conditions, bulky stop-
pers are typically covalently attached in the a and w posi-
tion of the guest molecules to trap the macrocycle and pre-
vent it from unthreading.[9,17]
Despite the great effort devoted to polymer-based rotax-
anes, less attention has been paid to oligomeric (i.e., mono-
disperse) derivatives. Monodisperse rotaxanes offer greater
control over the formation of structurally defined architec-
tures featuring preprogrammed electronic and photonic
properties.[18] Additionally, monodispersity of p-conjugated
oligomers allows for 2D and 3D crystal engineering. The
knowledge gained on oligomeric systems is a first step
toward attaining a greater understanding of the properties
and behavior of the polymer analogues.
Here we describe the synthesis of a novel p-conjugated
2,2’-bithiophene with bulky m-terphenylene stoppers encap-
sulated by b-cyclodextrin. The structure of the derived [2]ro-
taxane was confirmed by mass spectrometry, 1D and 2D
NMR spectroscopy, and modeling. The photophysical prop-
erties of derived molecules in solution were investigated by
means of time-correlated single-photon counting technique
in solution and quantum-chemical calculations. For the sake
of comparison, all the properties of the rotaxane were com-
pared to those of the corresponding dumbbell.
Results and Discussion
Synthesis and solution characterization: In aqueous solu-
tions, oligothiophenes form strong and stable complexes
with native b-cyclodextrin and its derivatives (such as hy-
droxypropyl-b-cyclodextrin and dimethyl-b-cyclodextrin).[19]
Thiophene-based polypseudorotaxanes were readily synthe-
sized by electropolymerization and chemical oxidation of
2,2’-bithiophenes with FeCl3.[20] The first polythiophene-de-
rived polyrotaxane, with anthracene stoppers, was synthe-
Scheme 1. Synthesis of dithiophene rotaxane 1ꢀb-CD. Reagents and con-
ditions: i) water, b-cyclodextrin, room temperature, 4 h, ii) water, lithium
carbonate, palladium(II) acetate, 808C, 16 h. Dumbbell 1 was synthesized
in a separate reaction under the same conditions, except without the
AHCTUNGTREGUNcNN yclodextrin.
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Thiophene-Based Rotaxane
FULL PAPER
Dumbbell 1 was synthesized in a separate reaction under
the same coupling conditions and purified by reversed-phase
chromatography with a 73% yield.
The chemical structure of the dithiophene rotaxane was
confirmed by ESI mass spectrometry and 2D NMR spec-
1
troscopy. Figure 1 shows part of the H NMR spectra of the
dumbbell 1 (Figure 1a) and the rotaxane 1ꢀb-CD (Fig-
ure 1b). Dithiophene protons of dumbbell 1 exhibit a simple
pattern (two doublets at d=7.52 (f+g) and 7.88 ppm (e+
f)), thus reflecting the symmetric structure. The presence
and the influence of the asymmetric b-cyclodextrin in the
NMR spectrum of the rotaxane 1ꢀb-CD in Figure 1b are
evidenced by the splitting and shifting of dithiophene dou-
blets. Furthermore, an upfield shift of two aromatic protons
(C) is observed. The proximity of the macrocycle to the di-
thiophene unit in 1ꢀb-CD was examined by a 2D NOESY
experiment (Figure 2). The pattern of NOEs revealed strong
through-space contacts between H5 and H6 of the narrow
5,6-rim of b-cyclodextrin with aromatic hydrogen atoms (la-
beled C) of the stopper, whereas hydrogen atom H3 of the
wide 2,3-rim showed intense NOEs with two aromatic pro-
tons (C’). Dithiophene hydrogen atoms of 1ꢀb-CD (labeled
e and f) showed intense interactions with H6 of the narrow
5,6-rim of b-cyclodextrin, whereas hydrogen atoms h and g
exhibited intense NOEs with H3 of the wide rim of the mac-
rocycle. Slightly less intense NOEs are also observed be-
tween f and H3, g and H5, and C’ and narrow 5,6-rim pro-
tons H5, H6, and H6’. To gain information on the position
of b-CD with respect to dumbbell 1, one must consider that
the total intensity for a given NOE interaction is a result of
an ensemble average over all possible interproton distances.
The contribution to the total NOE intensity at each possible
interproton distance is the product of the intensity of the
NOE interaction (proportional to the inverse sixth power of
the interproton distance) and a Boltzmann weight (the
value of the probability distri-
Figure 1. 1H NMR of a) dumbbell
[D6]DMSO at room temperature, 400 MHz. Please note that C’ protons
are lying in the range d=8.0 and 8.2 ppm.
1
and b) rotaxane 1ꢀb-CD in
Molecular dynamics simulations: Computational studies can
assist the interpretation of both optical and NMR spectra by
elucidating the relationship between structure and spectro-
scopic response. The combination of molecular modeling
and spectroscopies in the characterization of cyclodextrin
host–guest complexes has been reviewed by Lipkowitz.[24]
Here we used molecular dynamics (MD) simulations to
sample the distribution of b-CD positions and orientations
with respect to the dumbbell 1 guest at 300 K. We then cal-
culated relative NOE intensities of proton–proton interac-
bution function (PDF) at that
distance). In addition, if multi-
ple protons have the same
chemical shift as the equivalent
protons in the seven units of b-
CD, the intensities for all pro-
tons will be summed. Molecular
dynamics simulations were em-
ployed to gain insight into this
complex picture by elucidating
the average relative NOE in-
tensity as a function of b-CD
position and predicting the
PDF. Conclusions concerning
the interpretation of the
NOESY spectrum and the posi-
tion of b-CD with respect to
dumbbell 1 are presented in the
molecular dynamics section
Figure 2. Part of the 1H NMR NOESY spectrum of [2]rotaxane 1ꢀb-CD in [D6]DMSO, at room temperature,
400 MHz. Please note that C’ protons are lying in the range d=8.0 and 8.2 ppm.
below.
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L. Zalewski, D. Beljonne, H. L. Anderson, F. Cacialli, P. Samorꢁ et al.
tions based on the MD trajectory and generated plots of
NOE intensity as a function of b-CD position with respect
to dumbbell 1, thereby providing insight into which struc-
tures contribute strongly to each peak in the NOESY spec-
trum. Details of the MD simulations can be found in the Ex-
perimental Section.
In quantifying the position of the b-CD along the dithio-
phene axis, a metric d was defined as the distance between
the center of mass (COM) of b-CD and the COM of the
first benzene ring of the stopper on the wide-rimmed side of
the b-CD. The average distance between the first benzene
ring of the stopper on the wide-rimmed side of the b-CD
and the first benzene ring of the stopper on the narrow-
rimmed side of the b-CD amounts to 1.2 nm, so d may lie in
the range 0 to 1.2 nm with a distance of 0.6 that corresponds
to b-CD being perfectly centered along the dithiophene
backbone. The metric d was measured every 1 ps over a
total sampling time of 1.05 ms. If the MD sampling is statisti-
cally converged, then a normalized histogram of d values
will represent the real probability distribution function
(PDF), and a potential of mean force (PMF) or the free
energy as a function of d can be obtained by Boltzmann in-
version. Details of how statistical uncertainties were estimat-
ed can be found in the Experimental Section. The PDF and
PMF of d (Figure 3) feature relatively small undulations on
the order of KT at 300 K over the range 0.2 to 0.9 nm, indi-
cating that at 300 K BCD can diffuse over this range of the
reaction coordinate without encountering significant free
energy barriers.
Figure 4. b-CD tilt angle q defined as the average angle between vectors
that connect opposite oxygen linkers and the dumbbell 1 axis vector be-
tween backbone terminal carbon atoms. As b-CD contains seven oxygen
linkers, the average is over four angles, one of the oxygen atoms being
used twice. An angle of 908 corresponds to zero tilt (q=08). Hydrogen
atoms have been omitted for clarity.
cated at (d,q) values of (0.47,75), which corresponds to the
macrocycle being slightly off center along the dithiophene
backbone and slightly tilted (see inset of Figure 4). Smaller
peaks and significant nonzero probability over the range
0.2<d<0.9 and 608<q<908 indicate that the free-energy
barriers for b-CD to leave the largest probability peak are
small and hence that the macrocycle is very mobile at
300 K.
To check for consistency between the MD results and the
NOESY spectrum (Figure 2), we investigated the relation-
ship between intensities of individual NOE peaks and the
geometry of the system. NOE intensity of a single-proton
pair varies with the inverse sixth power of the distance be-
tween the protons. The intensity of protons with identical
chemical shifts are summed in the experimental spectrum,
so here we sum the inverse sixth powers of interproton dis-
tances between all equivalent proton pairs for each frame in
the MD trajectory. The dependence of the relative NOE in-
tensity on b-CD position d was investigated by binning tra-
jectory frames according to d and calculating the average in-
tensity in each histogram bin. The total intensity observed in
the spectrum is an ensemble average integral over all possi-
ble geometries of the system, which, for the sake of this
analysis, we represent as an integral over all d of the product
of I(d), the average relative NOE intensity as a function of
d and PDF(d), the probability as a function of d. How much
information can be gleaned from the integrated total inten-
sity and I(d) depends on how sensitive I(d) is to a change in
d; if the intensity is constant over all d we can learn nothing
about the PD; conversely, if the intensity is strong over only
a small range of d, a weak or strong experimental intensity
tells us that the PDF for that d range are small or large, re-
spectively. Figure 6 presents a selection of the most intense
and d-dependent NOE interactions.
Figure 3. Probability distribution function (PDF) and potential of mean
force (PMF) as a function of b-CD position d, thus indicating that the
macrocycle is very mobile at 300 K. Points are joined by cubic spline in-
terpolations.
To asses the tilt of the b-CD with respect to the dithio-
phene axis, a second metric q was defined as the average
angle between the dumbbell 1 backbone axis (defined by
the backbone terminal carbon atoms) and four vectors that
cross the average plane of the b-CD from opposite oxygen
linkers (see Figure 4).
The metrics d and q were combined into a two-dimension-
al histogram shown in Figure 5. This indicates the b-CD can
be strongly tilted and that the main probability peak is lo-
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Thiophene-Based Rotaxane
FULL PAPER
function of time (not shown)
displayed linear behavior be-
tween 0.5 and 1 ns. Inserting
the slope into the 1D diffusion
2
equation
<jd(t)ꢁd(t0)j >=
2Dt yielded the diffusion coeffi-
cient. Performing this analysis
on twenty-one independent
MD trajectories yielded an
average
D
of (4.1ꢂ1.8)ꢃ
10ꢁ6 nm2 ps. Using the expres-
sion t=d2/2D, the diffusion
model predicts that b-CD re-
quires (44ꢂ20) ns to diffuse
along the backbone from d=
0.25 to 0.85 nm.
The effect of b-CD on the
torsional degrees of freedom of
dumbbell 1 was also assessed
by comparing the distributions
of rotaxinated and nonrotaxi-
nated backbones. Angles for
Figure 5. Histogram of b-CD orientation and position and structures from each probability peak. The bin spac-
ing is 0.1 nm for d and 18 for the tilt angle q. From left to right the structures shown above have (d,q) values
of (0.24,67), (0.46,73), (0.6,85), and (0.79,72), respectively. Atoms in the dithiophene dumbbell are represented
as van der Waals spheres, whereas the b-CD is represented by bonds and van der Waals surface mesh.
the rotaxinated system were measured over the 1.05 ms tra-
jectory of the solvated system previously described. Angles
for the nonrotaxinated system were taken from a 0.6 ms sim-
ulation in the gas phase using a stochastic dynamics integra-
tor to mimic the random forces a solvated molecule would
be subject to. The distributions were symmetrized by folding
thiophene–thiophene (TT) and thiophene–phenyl (TP) tor-
sion angles ꢁ1808<w<1808 into the range 08<w<1808
and 08<w<908, respectively. To quantify the average tor-
sion angle (<w90 >) with respect to planarity, whether cis or
trans, the TT distribution was also folded into the range 08<
w<908. Figure 7 shows the distribution of TT and TP tor-
sion angles and corresponding <w90 > values inset. Both
distributions indicate a subtle planarization of the dumbbell
1 upon b-CD rotaxination. The average reduction in <w90 >
is 6.3, 4.6, and 1.28 for TP at b-CD narrow rim, TP at b-CD
wide rim, and TT, respectively.
Figure 6. Average proton–proton distances to the inverse sixth power es-
timating relative NOE intensities as a function of b-CD position. The
NOE interactions C’-5 and C-5 are the most sensitive to b-CD position
and have been plotted with a thicker line.
Optical properties: We focused our attention on the optical
properties of dumbbell 1 and 1ꢀb-CD rotaxane with a view
to unravel the kinetics of excited states and their depend-
ence on cyclodextrin encapsulation. Recently, comparison of
threaded and unthreaded polymeric systems has allowed sig-
nificant insight into the consequences of threading on the
optical properties of conjugated polymers and on the forma-
tion and decay of interchain and intrachain excitonic spe-
cies.[7,25] With this in mind, we performed steady-state and
time-resolved photoluminescence experiments on diluted
solutions of 1 and 1ꢀb-CD in dimethyl sulfoxide (DMSO).
Both systems are easily soluble in DMSO due to the polari-
ty of the carboxylic units of the bulky end groups.
Many of the interactions are strong over a wide range of
d, thus making it impossible to extract information on the b-
CD position from the NOESY spectrum with a useful level
of resolution. The NOE interactions of e and h are problem-
atic as they are not particularly well resolved in the spec-
trum, thereby making it hard to evaluate the total intensity
of each interaction separately. C’-5 and C-5 interactions are,
however, sensitive to d, the former being intense over the
range d<0.5, the latter being intense over the range d>0.5.
Since both interactions feature intense peaks in the spec-
trum, we can conclude that both regions d>0.5 and d<0.5
are sampled at 300 K, in agreement with PDF predicted by
molecular dynamics simulations.
We started our analysis by considering the optical absorp-
tion and time-integrated PL spectra of solutions of 1ꢀb-CD
and dumbbell 1 (5ꢃ10ꢁ2 mgmLꢁ1 of the conjugated portion)
in DMSO, reported in Figure 8. The absorption spectra
Our MD simulations also provide information on the
timescale of b-CD motion along the dumbbell backbone.
The mean-squared displacement of b-CD position d as a
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3937

L. Zalewski, D. Beljonne, H. L. Anderson, F. Cacialli, P. Samorꢁ et al.
molecules present a well-defined vibronic structure with a
0–0 fluorescence band at 2.81 and 2.78 eV, respectively. As
observed for other encapsulated systems,[8,26] also in this
case the PL spectrum of the rotaxane is blueshifted with re-
spect to the unthreaded backbone. Possible causes of this
blueshift include a reduction of intermolecular interactions,
the modified solvation environment or a sterically driven in-
crease in twist angle between adjacent thiophene units on
the backbone. MD simulations indicate, however, that a
sterically driven decrease in twist angle between adjacent
thiophene units occurs, thereby dismissing the latter poten-
tial explanation of the observed blueshift.
To gain further insight into the fluorescence kinetics of
the investigated systems, in Figure 9 we report the PL decay
curves for both systems collected at the emission maximum
Figure 9. Time-dependent decays of 1ꢀb-CD (circles) and dumbbell 1
(triangles) in DMSO (5ꢃ10ꢁ2 mgmLꢁ1) excited at 3.3 eV. The PL decays
were fitted with a single exponential function, thereby yielding an experi-
*
Figure 7. Torsion angle distributions from simulations at 300 K, thus indi-
cating a subtle planarization of the backbone. The value <w90 > defines
the average angle with respect to planarity, whether cis or trans. a) Black:
dumbbell, <w90 >=37.13; red: rotaxane, <w90 >=35.92. b) Black:
dumbbell, <w90 >=46.84; red: b-CD narrow rim, <w90 >=40.50; green:
b-CD wide rim, <w90 >=42.22.7
mental lifetime t=505 ps. The fitting curve is reported as solid lines (
:
!
with b-CD, : without b-CD).
in DMSO. The fluorescence kinetics of both threaded and
unthreaded systems are single exponential with a lifetime
t=505 ps. This result is consistent with the observation of a
similar PL quantum yield for the two systems (FPL =0.12ꢂ
0.01) and confirms that the radiative lifetime of intrinsic ex-
citons, tRAD =t/FPL =4.2 ns, is independent of rotaxina-
tion.[26] Furthermore, both the spectral properties and decay
dynamics suggest that aggregation is strongly disfavored in
both investigated systems, possibly as a result of the pres-
ence of the carboxylic functionalities on the end groups, the
twofold biphenyl twist on each meta-terphenyl terminus, and
the relatively short length of the dithiophene backbone.
Excited-state calculations: Finally, we performed excited-
state calculations on the series of molecules from bithio-
phene (2T) to quaterthiophene (4T) as well as the dumbbell
1 to confirm the experimental observation that dumbbell 1
emits at the same energy as 4T, thus indicating that conjuga-
tion extends over the first ring of each stopper. Calculations
were performed with TURBOMOLE 5.9[27] using time-de-
pendent density functional theory (TD-DFT), the B3LYP
Figure 8. Optical absorption and continuous wave (cw) photolumines-
cence spectra excited at 3.34 eV for solutions (5ꢃ10ꢁ2 mgmLꢁ1 of the
conjugated portion) of 1ꢀb-CD and dumbbell 1 in DMSO (c: with b-
CD, a: without b-CD).
functional, the 6-31gACHTUNRGTNE(NUG d,p) basis set, the m4 numerical quad-
rature grid, and an SCF convergence criterion of 10ꢁ7. The
geometry of the lowest singlet electronic excited state of
each molecule was optimized in the gas phase, thereby
yielding emission vertical transition energies. Vertical transi-
show a first absorption band in the UV spectral region at
3.24 and 3.16 eV for the rotaxane and the dumbbell, respec-
tively. The PL spectra of both threaded and unthreaded
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Thiophene-Based Rotaxane
FULL PAPER
tion (VT) energies can also be extracted from the experi-
mental fluorescence spectra by calculating the intensity-
weighted average transition energy. Values for thiophene
oligomers were based on fluorescence spectra from ref. [23].
Figure 10 plots the predicted VT energy versus the experi-
mental one. This confirms that dumbbell 1 does indeed emit
at very similar energies to 4T. Differences between the ex-
perimental and predicted energies are likely due to the limi-
tations of the B3LYP functional and also the lack of a treat-
ment of solvent effects.
Experimental Section
Diboronic ester
2
(5,5’-di(1,3,2-dioxaborolan-2-yl)-2,2’-bithiophene)[22]
and aryl iodide stopper 4 (3,5-bis(4-carboxyphenyl)iodobenzene) were
synthesized according to previously reported procedures.[23]
Experimental procedures: 1H NMR spectra were recorded using
a
Bruker Avance 400 spectrometer at 400 MHz. The residual solvent peak
was used as reference for calibration ([D6]DMSO: d=2.50 ppm). Peaks
are described as singlet (s), doublet (d), triplet (t), doublet of doublet
(dd), and multiplet (m). Unless otherwise noted, spectra were recorded
at 258C. 13C NMR spectra were recorded using a Bruker Advance 400
spectrometer at 100 MHz. All spectra were measured in broadband de-
coupled conditions. Chemical shifts are given in ppm. The residual sol-
vent peak was taken as reference ([D6]DMSO: d=39.43 ppm). Unless
otherwise noted, spectra were recorded at 258C. 2D NMR (COSY,
NOESY) were recorded using a Bruker Avance 400 spectrometer. High-
resolution mass spectrometry (HRMS) analyses were performed using a
Bruker Micro TOF mass spectrometer at the Service de Spectromꢀtrie
de Masse, Universitꢀ de Strasbourg. The observed pattern was always
compared to the theoretical pattern.
Synthesis of dumbbell 1: A solution of diboronic ester 2 (0.020 mmol,
6.8 mg), aryl iodide stopper 4 (0.045 mmol, 20 mg), lithium carbonate
(0.18 mmol, 13 mg), and palladium(II) acetate (4.5 mmol, 1.0 mg) in water
(degassed and nitrogen saturated, 5.0 mL) was heated to 808C for 18 h,
then allowed to cool. Hydrochloric acid (2.0m) was added to give a gelat-
inous precipitate that was collected by centrifugation and washed with
water (2ꢃ15 mL). The crude product was redissolved in DMSO, filtered,
and concentrated to 4 mL. Methanol was added to form a greenish pre-
cipitate, which was collected by using a centrifuge. The crude product
was purified on RP silica (gradient: H2O to H2O/CH3CN 20%) to give a
greenish solid (13 mg). Yield: 73%. 1H NMR (400 MHz, DMSO, RT):
d=7.52 (d, J=3.6 Hz, 2H), 7.88 (d, J=3.6 Hz, 2H), 8.03 (m, 14H),
8.09 ppm (m, 8H); 13C NMR (100 MHz, DMSO, RT): d=93.81, 123.97,
127.81, 130.39, 130.56, 135.28, 136.70, 141.32, 142.05, 144.00, 167.54 ppm;
MS (ESI): m/z: calcd for C48H29O8S2: 797.13 [MꢁH]ꢁ; found: 797.09.
Figure 10. Predicted- and measured-emission vertical-transition energies
that indicate that dumbbell 1 emits at similar energies to 4T and hence
&
that conjugation extends over the first ring of each stopper ( : thiophene
*
series, : dumbbell).
Synthesis of [2]rotaxane 1ꢀb-CD:
A solution of diboronic ester 2
Conclusion
(0.30 mmol, 96 mg) and b-cyclodextrin (1.2 mmol, 1.4 g) in water
(8.0 mL, degassed and nitrogen saturated) was stirred for 4 h at room
temperature, and then added to the reaction mixture containing aryl
In summary, we have described a joint synthetic, analytical,
optical, and computational investigation of a new rigid di-
thiophene b-CD rotaxane and its related dumbbell.
In DMSO, the presence of the threaded cyclodextrin has
only a subtle effect on the absorption and emission behavior
of the p system.
Molecular dynamics simulations provided direct evidence
for the spatial distribution and orientation of b-CDs with re-
spect to the dumbbell, whereas excited-state calculations
supported the optically detected extended conjugation for
both investigated molecules.
It was shown that b-CD is slightly tilted and centered
along the dithiophene backbone, with mobile ability over
the range 0.2 to 0.9 nm at 300 K. Due to reduced intermo-
lecular interaction, blueshifted absorption and photolumi-
nescence were observed. Our dithiophene b-CD rotaxane,
being based on a conformationally rigid and conjugated
backbone, represents a model system to explore the opto-
iodide stopper
4 (0.60 mmol, 280 mg), lithium carbonate (2.4 mmol,
180 mg), and palladium(II) acetate (36 mmol, 8 mg) in water (degassed
and nitrogen saturated, 60 mL). The reaction mixture was heated to 808C
overnight under nitrogen, then allowed to cool. Hydrochloric acid (2m)
was added to give a gelatinous precipitate that was collected by centrifu-
gation and washed with water (6ꢃ15 mL). Collected precipitate was dis-
solved in DMSO, filtered, and concentrated to 12 mL; chloroform was
added to form a precipitate that was collected by using a centrifuge.
Crude product (containing dumbbell and [2]rotaxane) was dissolved in
Li2CO3 solution (90 mmol) and purified on RP silica column (gradient:
H2O to H2O/CH3CN 10%), thereby eluting [2]rotaxane and dumbbell.
Both fractions were concentrated to a volume of 5 mL and acidified by
0.5m HCl to give precipitates that were collected by using a centrifuge,
washed with water, and dried. Yield: 2% of [2]rotaxane 1ꢀb-CD (12 mg,
yellowish solid) and 62% of dumbbell 1 (148 mg, greenish solid). For
[2]rotaxane (1ꢀb-CD): 1H NMR (400 MHz, DMSO, RT): d=3.29 (m,
7H), 3.41 (m, 21H), 3.57 (d, J=9.6 Hz, 7H), 3.65 (d, J=9.2 Hz, 7H),
3.71 (t, J=9.2 Hz, 7H), 4.80 (d, J=3.2 Hz, 7H), 7.20 (d, J=3.6 Hz, 1H),
7.36 (d, J=3.6 Hz, 1H), 7.57 (d, J=3.6 Hz, 1H), 7.64 (d, J=3.6 Hz, 1H),
7.91 (s, 2H), 8.00 (s, 2H), 8.02 (s, 2H), 8.06 (s, 4H), 8.09 (m, 2H),
8.13 ppm (m, 10H); 13C NMR (100 MHz, DMSO, RT): d=60.04, 72.40,
72.83, 73.51, 81.77, 102.42, 124.80, 127.70, 127.99, 130.56, 141.13, 141.44,
142.76, 144.03, 167.63, 167.77 ppm; MS (ESI): m/z: calcd for
C90H100O43S2: 1931.49 [MꢁH]ꢁ; found: 1931.41.
ACHTUNGTRENNUNGelectronic properties of insulated molecular wires.
Molecular modeling: In our MD simulations, 1 was modeled using a
modified version of the OPLSAA[28] forcefield (FF). Modifications in-
cluded deriving partial atomic charges fitted to the B3LYP/6-31g
MP2/6-31g
(d,p) electrostatic potential using restraints[29] to ensure equal
charges on equivalent atoms. The forcefield was also modified to repro-
ACHTUNGTRENNUNG(d,p)//
AHCTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 3933 – 3941
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3939

L. Zalewski, D. Beljonne, H. L. Anderson, F. Cacialli, P. Samorꢁ et al.
duce the accurate MP2/aug-cc-VTZ potentials of bithiophene,[30] biphen-
Acknowledgements
yl,[31] and thiophene phenyl (performed ourselves at the MP2/6-31g
ACTHNUTRGNEU(GN d,p)//
MP2/aug-cc-VTZ level using Gaussian 03[32]). b-CD was modeled using
the OPLSAA forcefield for carbohydrates,[33] whereas the solvent DMSO
was modeled using the united atom OPLS forcefield.[34]
Alan Grossfield is acknowledged for helpful discussions concerning the
statistical uncertainties in the PMF. This work was supported by the EC
Marie Curie projects THREADMILL (MRTN-CT-2006-036040) and SU-
PERIOR (PITN-CT-2009-238177), the EC FP7 ONE-P large-scale proj-
ect no. 212311, the NanoSci-E+ project SENSORS, the ESF-SONS2-SU-
PRAMATES project, and the International Center for Frontier Research
in Chemistry (FRC). The work in Mons is partly supported by the Inter-
MD simulations were performed on a system that comprised the 1ꢀb-
CD rotaxane solvated in a cubic box of 422DMSO molecules using
GROMACS 4.05.[35] The system was simulated in the NPT ensemble, the
temperature being regulated using the Nose–Hoover algorithm (300 K,
0.1 ps coupling time) and the pressure regulated using the Berendsen al-
gorithm (1 atm, 1 ps coupling time). During the simulations, the equa-
tions of motion were integrated using a time step of 2 fs and all bond
lengths were fixed at their equilibrium values. Nonbonded interactions
were calculated up to a cut-off distance of 1.5 nm. After an initial equili-
bration period, the b-CD was pulled from one end of the 1 to the other
by applying harmonic restraints to the distance (d) between the b-CD
center of mass (COM) and the COM of the first phenyl of the stopper
on the wide-rimmed side of the b-CD over a period of 10 ns. Twenty-one
geometries evenly spaced along the range of d=0.05–1.15 nm were taken
from the restrained trajectory and given new random velocities. Each
was then simulated for 50 ns without restraints, thereby resulting in a
total of 1.05 ms sampling time.
AHCTUNGERTGuNNUN niversity Attraction Pole IAP 6/27 of the Belgian Federal Governement,
and the Belgian National Fund for Scientific Research (FNRS/FRFC).
The calculations were carried out at the Interuniversity Scientific Calcu-
lation Facility (FUNDP, Namur, Belgium), for which the authors grate-
fully acknowledge the financial support of FNRS-FRFC. D.B. is a FNRS
Research Director.
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Assuming fully converged MD sampling, it is possible to determine the
PMF along d by Boltzmann-inverting the probability distribution. If the
sampling is not fully converged, there will be statistical uncertainties in
the probability distribution and corresponding uncertainties in the de-
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lower-lying regions and thus suffer the largest statistical uncertainties and
introduce the largest errors into the PMF. These statistical uncertainties
can be estimated by bootstrapping,[36] and here we perform the analysis
as implemented in the WHAM code.[37] The correlation time after which
points in the time series were assumed to be uncorrelated in the boot-
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ref. [38] for a review) to be 12 ns. Data points in the range 0.15 to
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Optical characterization: All data shown were taken for 5ꢃ10ꢁ2 mgmLꢁ1
solutions of dumbbell 1 and 1ꢀb-CD [2]rotaxane in DMSO. The solu-
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in the visible and near-ultraviolet (UV) region by means of a CCD-based
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diode laser at 375 nm (Edinburgh Instruments EPL-375) and a F-900
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curves in DMSO either at increasing molecular concentration (from
1 mgmLꢁ1 to 5ꢃ10ꢁ2 mgmLꢁ1 of the conjugated portion) or by titrating
the solutions with dichloromethane (from 10 to 99% in volume) using
the same diode laser and TCSPC unit described above. All experiments
were performed at room temperature and all measurements were cor-
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3940
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 3933 – 3941

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www.chemeurj.org
3941