Solvatochromic rotaxane molecular shuttles

Article abstract of DOI:10.1039/c0cc05755j

A strongly fluorescent bistable rotaxane is described in which the relative position of the macrocyclic ring with respect to a solvatochromic fluorophore gives a strong response in the spectral domain.

Full text of DOI:10.1039/c0cc05755j

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COMMUNICATION
Solvatochromic rotaxane molecular shuttlesw
¨
Duygu Deniz Gunba-s, Leszek Zalewskiz and Albert M. Brouwer*
Received 23rd December 2010, Accepted 22nd February 2011
DOI: 10.1039/c0cc05755j
A strongly fluorescent bistable rotaxane is described in which the
relative position of the macrocyclic ring with respect to a
solvatochromic fluorophore gives a strong response in the
spectral domain.
Mechanically interlocked architectures, such as catenanes and
rotaxanes, offer an attractive framework for the study of
relatively weak intermolecular interactions, without the
complications of dissociation of the assemblies in solution.1
Manipulation of the intercomponent interactions can give rise
to molecular switches and motors.2 One of the challenges in
this field is not only to control the co-conformational isomerism
and the dynamics of the shuttling motion in which the
macrocyclic ring moves back and forth between stations, but
also to be able to detect the state of the system with high time
resolution and high sensitivity. Fluorescence spectroscopy is
potentially ideal for this, because it can be used to detect
processes down to the picosecond time scale, and its sensitivity
even allows the observation of the properties of individual
molecules.3 A number of rotaxanes containing fluorescent
groups have been described in the literature,4,5 but with few
exceptions6,7 the fluorophores do not have sufficient brightness
and photostability to be suitable for single molecule observation.
In order to make the properties of the fluorophore sensitive to
interaction with the moveable ring,6 in the present work we
have incorporated a bright and solvatochromic chromophore8
into a hydrogen-bonded rotaxane.
Scheme 1 Two translational co-conformers of rotaxane 1.
hindrance results from the second CQO group of the imide.
The yield of the ring closure reaction forming the rotaxane is
relatively low, 16% based on the amount of thread subjected
to the reaction.
The 1H NMR spectra of rotaxane 1 and thread 2 in CD2Cl2
(Fig. 1) indicate that the CH2 groups between the imide and
the glycine carbonyl groups in the rotaxane (Hd and Hk) are
substantially shielded due to the aromatic ring current effect of
Molecular switch 1 (Scheme 1) was assembled using a
protocol developed by Leigh and co-workers.9,10 In order to
place the macrocyclic ring directly in contact with an imide
unit, a new template was introduced in this work, consisting of
a simple amide, linked to the imide by a CH2 group. It is well
known that an imide is not a good binding site for the
tetra-amide macrocycle.11 The macrocyclic ring can be formed
using a simple amide as a template, albeit in low yield.12
The combination of amide and imide provides a template
that resembles the ideal trans-1,4-dicarbonyl unit,9 but steric
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam,
P.O. Box 94157, 1090 GD Amsterdam, The Netherlands.
E-mail: a.m.brouwer@uva.nl; Fax: +31 205255680;
Tel: +31 205255491
w Electronic supplementary information (ESI) available: Synthesis
and characterization, experimental procedures, additional details of
photophysical results. See DOI: 10.1039/c0cc05755j
z Present address: Reckitt Benckiser Produktions GmbH, Ludwig-
Bertram-Str. 8–10, 67059 Ludwigshafen, Germany.
Fig. 1 1H NMR spectra of rotaxane 1 and thread 2 (400 MHz,
CD2Cl2, 298 K). Resonances are labelled as shown in Scheme 1.
c
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Chem. Commun., 2011, 47, 4977–4979 4977

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the p-xylylene units of the macrocyclic ring.9,13 The two
co-conformers 1-pery and 1-ni are both present, and in rapid
equilibrium on the NMR time scale. Remarkably, Hk is much
more shielded (ꢀ0.520 ppm) than Hd (ꢀ0.221 ppm), indicating
a distinct preference for co-conformer 1-pery over 1-ni. If we
make the reasonable assumption that the inherent shielding of
the CH2 hydrogen atoms in both co-conformers is the same,
we arrive at a population ratio of 1-pery to 1-ni of about 7 : 3.
This finding can be explained by the higher electron density on
the carbonyl groups of the electron donor substituted perylene
imide, which strengthens the hydrogen bonds to the pery
station.6
The solvatochromic shifts of 1 are similar to those of a closely
related pyrrolidine substituted perylene monoimide which
lacks the bay substituents.8,14 The Stokes shifts for thread 2
do not show a systematic dependence on solvent polarity. It is
smallest in the most polar solvents used. For rotaxane 1,
however, Stokes shifts are more similar in all the solvents
and the values are slightly smaller than those found for thread
2. Positive solvatochromic shifts in absorption and emission as
observed here are normally interpreted in terms of an increase
in the molecular dipole moment upon excitation. The remarkable
lack of solvent dependence of the Stokes shift can be attributed to
a solvent induced change in the electronic structure, leading
to a larger ground state dipole moment in more polar
solvents.8,14 Upon excitation of the naphthalimide chromophore
at 345 nm the red fluorescence of the perylene unit was
observed, indicating an efficient excited state energy transfer
from the naphthalimide to the perylene chromophore
(90–97%) (Fig. S11 and Table S3, ESIw). No systematic
difference between thread 2 and rotaxane 1 was observed.
A linear regression of the frequency of the solvatochromic
bands of 1 and 2 with solvent properties was carried out to
evaluate the influence of hydrogen-bond accepting or donating
ability of the solvents on absorption and fluorescence spectra
according to Kamlet–Taft solvatochromic relationships:15
The absorption and fluorescence spectra of rotaxane 1 and
thread 2 show solvatochromic shifts, both absorption and
emission appearing at longer wavelengths in more polar
solvents. Fig. 2 depicts the absorption and fluorescence spectra
of rotaxane 1 in toluene and DMSO as compared with thread
2. Optical spectra in other solvents are given in the ESIw
(Fig. S7), and the data for all solvents used are summarized in
Table 1. In DMSO, the absorption and emission spectra of
thread and rotaxane are identical. This is not unexpected,
because this solvent is known to disrupt hydrogen bonds.4 In
toluene, on the other hand, the absorption and emission
maxima of the rotaxane are substantially red-shifted relative
to those of the thread.
n~ = n0 + aa + bb + sp*
(1)
In solvents of intermediate polarity, the bands gradually
shift to longer wavelengths with increasing solvent polarity.
where ~n is the property correlated (absorption or emission
maxima of 1 or 2 in the different solvents), no is a constant, and
a, b, p* are measures of the solvent’s hydrogen-bond donating
ability, hydrogen-bond accepting ability and ‘‘dipolarity–
polarizability’’, respectively. The a, b and s values give a
measure of the sensitivity of n~ to these factors. The observed
absorption and emission maxima in wavenumbers together
with the parameters for the solvents used (a, b, p*) are given in
Table S2 in the ESI.w Fitting the data and the parameters with
eqn (1) gives the coefficients listed in Table 2.
For both rotaxane 1 and thread 2, absolute values of a are
larger than those of b, which indicates that the hydrogen-bond
donating solvents interact more strongly with the chromophore.
The coefficient a is negative, implying that the hydrogen-bond
donating solvents stabilize the excited state more than the
ground state. This effect is stronger for thread 2 than for
rotaxane 1 because in the latter the interaction between the
chromophore and the macrocycle competes with the inter-
action between the chromophore and the solvent. For rotaxane 1,
a small positive b value is observed for the absorption spectra,
Fig. 2 Absorption and fluorescence spectra of 1 (red) and 2 (blue) in
toluene (line) and DMSO (dotted). All spectra are scaled to the same
maximum.
Table 1 Absorption and emission maxima (nm) and Stokes shifts
(cmꢀ1) of thread 1 and rotaxane 2 in different solventsa
1
2
Table 2 Coefficients (cmꢀ1) and standard deviations for the Kamlet–
Taft expression obtained by fitting the absorption and emission
energies (Table S1, ESIw) to eqn (1)a
Stokes shift
(ꢁ10ꢀ3
Stokes shift
(ꢁ10ꢀ3
b
b
labs lem
)
labs lem
)
Solvent
Toluene
Dibutylether 582 700
608 717
2.74
3.14
3.11
3.18
2.67
3.13
3.05
3.14
2.90
2.84
575 698
556 681
576 715
580 716
610 737
602 739
613 751
617 764
620 754
621 761
3.21
3.32
3.43
3.39
2.86
3.16
3.05
3.10
2.85
2.84
1
2
EtOAc
THF
CH2Cl2
Acetone
DMF
MeOH
Acetonitrile
DMSO
597 733
597 758
621 748
606 747
620 756
620 763
618 755
621 761
Abs (ꢁ10ꢀ3
)
Em (ꢁ10ꢀ3
14.6 ꢂ 0.1
)
Abs (ꢁ10ꢀ3
18.7 ꢂ 0.4
)
Em (ꢁ10ꢀ3
15.3 ꢂ 0.2
ꢀ0.8 ꢂ 0.1
ꢀ0.6 ꢂ 0.2
ꢀ1.8 ꢂ 0.2
)
Coefficient
no
a
b
17.7 ꢂ 0.2
ꢀ0.6 ꢂ 0.1
0.1 ꢂ 0.2
ꢀ0.5 ꢂ 0.1 ꢀ1.0 ꢂ 0.3
ꢀ0.5 ꢂ 0.2 ꢀ0.9 ꢂ 0.4
ꢀ1.3 ꢂ 0.2 ꢀ1.8 ꢂ 0.5
s
ꢀ1.7 ꢂ 0.2
a
The plots of observed absorption and fluorescence maxima versus the
maxima predicted by eqn (1) are illustrated in Fig. S9 in the ESI.w
a
b
The spectra are shown in Fig. S1 in the ESI.w lexc = labs (max)
.
c
4978 Chem. Commun., 2011, 47, 4977–4979
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whereas the fluorescence spectra give a negative b value. The
negligible magnitude of b implies that the frequency of the
solvatochromic band is insensitive to the solvent’s hydrogen-
bond accepting ability. In contrast, negative b values obtained
for the fluorescence spectra mean that as basicity of the
solvents increased, the frequency of the fluorescence band of
rotaxane 1 decreased. For thread 2 both absorption and
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larger in absolute magnitude for thread 2. The negative values
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Rotaxane
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oxidation, or by protonation of the amino group. In these
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This work was financially supported by the European
Union (Marie Curie Training Site ‘‘Molecular Photonic
Materials’’, contract number HPMT-CT-2001-00311), and
by NanoNed, a national nanotechnology program coordinated
by the Dutch Ministry of Economic Affairs.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4977–4979 4979