Chirality in dynamic supramolecular nanotubes induced by a chiral solvent

Article abstract of DOI:10.1002/chem.200902399

Amplification of chirality has been reported in polymeric systems. It has also been shown that related effects can occur in polymer-like dynamic supramolecular aggregates, if a subtle balance between noncovalent interactions allows the coupling between a chiral information and a cooperative aggregation process. In this context, we report a strong majority-rules effect in the formation of chiral dynamic nanotubes from chiral bisurea monomers. Furthermore, similar helical nanotubes (with the same circular dichroism signature) can be obtained from racemic monomers in a chiral solvent. Competition experiments reveal the relative strength of the helical bias induced by the chiral monomer or by the chiral solvent. The nanotube handedness is imposed by the monomer chirality, whatever the solvent chirality. However, the chirality of the solvent has a significant effect on the degree of chiral induction.

Full text of DOI:10.1002/chem.200902399

FULL PAPER
DOI: 10.1002/chem.200902399
Chirality in Dynamic Supramolecular Nanotubes Induced by a Chiral Solvent
Benjamin Isare,[a, b] Mathieu Linares,[c, d] Loussinꢀ Zargarian,[e] Serge Fermandjian,[e]
Motofumi Miura,[f] Shigeyasu Motohashi,[f] Nicolas Vanthuyne,[g]
Roberto Lazzaroni,[c] and Laurent Bouteiller*[a, b]
Abstract: Amplification of chirality has
been reported in polymeric systems. It
has also been shown that related ef-
fects can occur in polymer-like dynamic
supramolecular aggregates, if a subtle
balance between noncovalent interac-
tions allows the coupling between a
chiral information and a cooperative
aggregation process. In this context, we
report a strong majority-rules effect in
the formation of chiral dynamic nano-
tubes from chiral bisurea monomers.
Furthermore, similar helical nanotubes
(with the same circular dichroism sig-
nature) can be obtained from racemic
monomers in a chiral solvent. Competi-
tion experiments reveal the relative
strength of the helical bias induced by
the chiral monomer or by the chiral
solvent. The nanotube handedness is
imposed by the monomer chirality,
whatever the solvent chirality. Howev-
er, the chirality of the solvent has a sig-
nificant effect on the degree of chiral
induction.
Keywords: chirality
· chiral sol-
vents · nanotubes · self-assembly ·
supramolecular chemistry
Introduction
[a] Dr. B. Isare, Dr. L. Bouteiller
UPMC Univ Paris 06, UMR 7610
Chimie des Polymꢀres, 75005, Paris (France)
Fax : (+33)144277089
The control of chirality amplification is a prominent objec-
tive, because understanding how a slight enantiomeric
excess in a racemate can be amplified, may help unravel the
mystery of the origin of chirality in biological systems. Am-
plification of chirality is also important for chemistry and
materials science, in applications such as enantioselective
catalysis and chiral separation or sensing.[1] The amplifica-
tion of chirality was first studied in polymeric systems and is
now a well known phenomenon.[2] More recently, it has
been shown that related effects can also occur in polymer-
like dynamic supramolecular aggregates,[3] if a subtle bal-
ance between noncovalent interactions allows the coupling
between a chiral information and a cooperative aggregation
process. So far, most studies have dealt with non-linear ef-
fects occurring when a chiral monomer is mixed with a non-
chiral monomer (sergeants-and-soldiers principle) or with its
racemate (majority-rules principle).[3] In contrast, only few
studies concern the effect of chiral additives,[4] and even
fewer report on the use of chiral solvents.[5–7] However, from
a practical point of view, the introduction of chiral informa-
tion in a dynamic supramolecular aggregate, via a simple
chiral solvent instead of an elaborate chiral monomer, is cer-
tainly very attractive.
E-mail: laurent.bouteiller@upmc.fr
Homepage: http://www.lcp.upmc.fr/
[b] Dr. B. Isare, Dr. L. Bouteiller
CNRS, UMR 7610
Chimie des Polymꢀres, 75005, Paris (France)
[c] Dr. M. Linares, Dr. R. Lazzaroni
Service de Chimie des Matꢁriaux Nouveaux
Universitꢁ de Mons/Materia Nova
Place du Parc, 20, 7000 Mons (Belgium)
[d] Dr. M. Linares
Current address: Department of Physics, Chemistry and Biology
Linkçping University, 58183 Linkçping (Sweden)
[e] Dr. L. Zargarian, Dr. S. Fermandjian
LBPA - UMR 8113, Ecole Nationale Supꢁrieure de Cachan
Bꢂtiment de l’Institut d’Alembert (IDA)
61 avenue du Prꢁsident Wilson, 94235 Cachan Cedex (France)
[f] M. Miura, Prof. S. Motohashi
College of Pharmacy
Nihon university 7-1
Narashinodai 7-chome Funabashi-shi, Chiba 274-8555 (Japan)
[g] Dr. N. Vanthuyne
UMR 6180 - Universitꢁ Paul Cꢁzanne
Avenue Escadrille Normandie Niꢁmen
13397 Marseille cedex 20 (France)
We have recently shown that racemic bisureas EHUT-rac
self-assemble in non polar solvents to form extremely long
nanotubes, through a strongly cooperative process.[8] These
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902399.
Chem. Eur. J. 2010, 16, 173 – 177
ꢃ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
173

nanotubes are in dynamic exchange with the monomer and
with simple hydrogen-bonded filaments.[8a] Moreover they
can form only if the solvent molecules are small enough to
be accommodated within the tubes, thereby contributing to
their stabilization.[8b,e] We therefore considered that this
host–guest system was well suited to study the possibility of
chirality transfer from the solvent to the supramolecular
nanotube. Herein, we report the formation of chiral dynam-
ic nanotubes i) from chiral monomers,[6a,9] or ii) from racemic
monomers in a chiral solvent, and iii) the competition be-
tween the two processes.
Circular dichroism (CD) spectroscopy was then used to
investigate further the effect of the chiral side chains. No
CD signal is observed for EHUT-SS in ethanol (data not
shown), probably because no supramolecular structure is
formed in this strongly interacting solvent, and the chiral
centers are too far from the urea and aromatic chromo-
phores. However, in cyclohexane at room temperature, a
strong bisignated Cotton effect is observed with a negative
and positive maxima at l=236 and 224 nm, respectively
(Figure 1). A weak positive band also appears at l=290 nm.
Results and Discussion
Enantio-enriched monomers EHUT-SS and EHUT-RR
were synthesized from toluene diisocyanate and the corre-
sponding optically pure[10] 2-ethylhexylamines. The optical
purity of the monomers was characterized by chiral HPLC:
their enantiomeric excess is 78% and 70% respectively (see
the Supporting Information). The reference compound
EHUT-rac was obtained by reacting toluene diisocyanate
with racemic 2-ethylhexylamine, and therefore contains the
four isomers EHUT-SS, -RR, -RS and -SR in equimolar
amounts (Scheme 1). The first step is to probe the supra-
Figure 1. CD spectra of 1 mm solutions in cyclohexane of: a) EHUT-SS at
208C (bold black line); b) EHUT-RR at 208C (bold grey line);
c) EHUT-SS at 608C (black line); d) EHUT-rac at 208C (dashed black
line).
The comparison of this CD signal to the UV/Vis absorption
spectrum (Figure S4) suggests that the bisignated Cotton
effect may result from an exciton coupling within the self-as-
sembled nanotube. In any case, the opposite CD signal ob-
tained with the mirror image isomer EHUT-RR, and the
disappearance of the CD signal at temperatures above the
tube-filament transition (518C) are clear evidence that there
is a transfer of chirality from the alkyl side chains of the mo-
nomer to the urea and aromatic chromophores within the
self-assembled nanotubes.
Mixing enantiomers is a simple method to obtain structur-
al information on a self-assembled system.[3a] Therefore,
stock solutions of EHUT-SS or EHUT-RR in cyclohexane
were combined in different proportions and stirred at room
temperature. Figure 2 shows the result of the CD measure-
ments at equilibrium.[12] The highly non-linear variation of
the ellipticity is remarkable and shows that an enantiomeric
excess of only 10% is sufficient to impose the chiral twist of
the majority enantiomer. It also shows that the nanotubes
are indeed in dynamic exchange at room temperature, be-
cause this chiral amplification can only occur if the assem-
blies of the pure enantiomers reorganize after mixing. More-
over, this strong majority-rules effect is also observed if sol-
utions of EHUT-SS and EHUT-rac are combined in differ-
ent proportions (Figure 2). This means that the dilution by
the other isomers (EHUT-SR and EHUT-RS) does not de-
stroy the cooperativity of the process. Van Gestel et al. have
recently reported a theoretical model, which quantitatively
Scheme 1. Structure of bisurea monomers.
molecular structure formed by an enantiopure monomer in
comparison to the racemic mixture, because in some cases,
chirality has been shown to strongly affect the assembly pat-
tern.[3f,11] Figure S1 and S2 show that the small angle neutron
scattering (SANS) curves and the FTIR spectra of EHUT-
SS and EHUT-rac are identical, proving that the enantio-en-
riched monomer forms the same supramolecular architec-
ture as the racemic mixture. Moreover, the high cooperativi-
ty of the transition between the nanotubes and the filaments
affords a sensitive measure of the relative stability of the
two structures.[8] This transition occurs at the same tempera-
ture for EHUT-SS and EHUT-rac (Figure S3), showing that
the tube and filament structures are not significantly affect-
ed by the chirality of the monomer.
174
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Chem. Eur. J. 2010, 16, 173 – 177

Chirality in Supramolecular Nanotubes
FULL PAPER
(Figure 2 inset and Figure S6). This improved model is now
compatible both with previous experimental data[8c] and
with the present CD results.
These results on chiral monomers encouraged us to inves-
tigate the effect of a chiral solvent. Limonene was selected
because of i) its size which is compatible with the inner
space of the nanotubes,[8b] ii) the absence of polar groups
which would interfere with self-assembly by hydrogen bond-
ing, and iii) its commercial availability. FTIR and DSC ex-
periments (Figures S7 and S8) confirm that the nanotubes
are formed in limonene, with a tube-to-filament transition
temperature of 388C. Figure 3a shows the symmetrical CD
Figure 2. Net helicity ((De236ÀDe224)/(De236ÀDe224
)max) versus enantiomeric
excess (ee) of 1 mm bisurea solutions in cyclohexane at 208C, for the fol-
*
*
lowing mixtures: EHUT-SS/EHUT-RR ( ); EHUT-SS/EHUT-rac ( );
*
EHUT-RR/EHUT-rac ( ).The curve is a fit of the EHUT-SS/EHUT-RR
data.[13c] The ee is calculated as ([S]À[R])/([R]+[S]), for which [R] and
[S] are the ethylhexyl concentrations, as deduced from the measured op-
tical purity of the monomers. The insets show optimized geometries for
an EHUT-RR (left) or EHUT-SS (right) nanotube, determined by molec-
ular mechanics and dynamics (see Figure S6 for details).
describes chiral amplification effects in one-dimensional self
assemblies.[13] They showed that the two relevant parameters
for such a system are the energetic penalty for a helix rever-
sal (ereversal) and the energetic penalty for the presence of a
“wrong” enantiomer in an assembly of given helicity
(emismatch). Following the fitting procedure of van Gestel
et al.,[13c,d] it is possible to obtain an excellent agreement
with our experimental data (Figure 2) if values of the pa-
rameters are ereversal ꢀ17 kJmolÀ1 (=7 kBT) and emismatch
ꢁ
0.1 kJmolÀ1 (=0.05 kBT). The very low value for the mis-
match penalty is not really surprising and reflects the small
energetic destabilization of a (R)-ethylhexyl group within a
predominantly (S)-ethylhexyl domain. Notably, the huge
length of the tubes[8d] means that the overall mismatch
energy for a given tube can still be significant. Moreover,
the large value for a helix reversal penalty is very informa-
tive, because it implies that helix reversals are not likely to
occur along a nanotube. This in turn means that in race-
mates, the bisureas tend to self-assemble into a mixture of
right-handed or left-handed nanotubes (in adequate propor-
tions), rather than in a kind of multiblock copolymer.[14] Fur-
thermore, the unfavorable interaction between right-handed
and left-handed nanotubes indicates a major influence of
the chiral side chains on either the hydrogen-bonding pat-
tern or the steric environment of the nanotubes. This last
conclusion contradicts the previously reported molecular
model for the nanotube for which no twist was visible.[8c]
However, the absence of twist was a consequence of the mo-
lecular simulation procedure adopted; periodic boundary
conditions were imposed to avoid end effects owing to the
absence of hydrogen bonding.[15] In the present work, molec-
ular dynamics simulations were performed on long tubes
with free extremities, to lift the geometrical constraint im-
posed by the periodic boundary conditions. With this new
procedure, some twisting of the nanotube clearly appears
Figure 3. a) CD spectra at 208C of bisurea solutions: EHUT-rac in (S)-li-
monene (black bold line) or (R)-limonene (grey bold line) (10 mm);
EHUT-SS (black line) or EHUT-RR (grey line) in cyclohexane
(1 mm).[16] b) Helicity measured at l=238 nm versus the enantiomeric
excess of EHUT-rac in (S)-limonene/(R)-limonene mixtures.[16]
spectra measured for EHUT-rac solutions in (S)- or (R)-li-
monene. The Cotton effects obtained for EHUT-rac in (S)-
limonene and EHUT-SS in cyclohexane have nearly identi-
cal shapes: it indicates that the nanotubes formed by the
racemic monomer mixture in the chiral solvent and the
chiral monomer in the achiral solvent have very similar
structures. Moreover, the handedness of the helical assembly
is the same whether it is induced by the (S)-limonene sol-
vent or by the EHUT-SS monomer. Now, this raises the
question of whether the chiral induction by the solvent is
quantitative (like the chiral induction by the monomer) or
only partial. The fact that the signal intensities in the CD
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www.chemeurj.org
175

L. Bouteiller et al.
spectra of Figure 3a are very similar does not necessarily
mean that full chiral induction is achieved by chiral limo-
nene, because the CD signal intensity is known to also
depend on the solvent polarizability.[17]
Gestel et al. was formally written for the case of a mixture
of chiral monomers, it is equally valid for the present case
of a one-dimensional assembly for which helicity is con-
trolled by a chiral solvent. The lack of cooperativity of the
experimental curve (Figure 3b) does not enable to deter-
mine both parameters ereversal and emismatch independently.
However, we can assume that the helix reversal penalty de-
pends mainly on the hydrogen bonding pattern of the nano-
tube and therefore not much on the solvent. If the previous-
ly determined minimum value of ereversal is used
(17 kJmolÀ1), then a value emismatch =1.5 JmolÀ1 (=0.0006 kT)
affords a good fit of the data (Figure S9). Although this
value is quite low, it is not unrealistic. Even lower values
have been found in the case of chiral solvation for helical
polymers.[2]
To address this question, we performed the following ex-
periment. Stock solutions of EHUT-rac in (S)- or (R)-limo-
nene were combined in different proportions and stirred at
room temperature. Figure 3b shows the result of the CD
measurements at equilibrium; the ellipticity is a linear func-
tion of the solvent enantiomeric excess. This points to a very
different behavior (and energetics) when the helicity of the
nanotubes is biased through a chiral solvent or through a
chiral monomer. However, we still do not know if full chiral
induction is induced by the solvent, because no saturation of
the helicity is reached.[13c]
Finally, the respective magnitude of the nanotube helicity
control by a chiral monomer or by a chiral solvent was
probed through the following competition experiments.
Figure 4 shows the results of the four possible combinations
Conclusions
In conclusion, we have shown for the first time that chiral
dynamic self-assembled nanotubes can be obtained from a
racemic monomer in a chiral solvent. The helical bias in-
duced by the particular solvent tested (limonene) is not as
strong as the helical bias induced by a chiral monomer, but
is still very significant (about 33%). These results do not
allow us to determine if the solvent control on the nanotube
helicity is affected by the limonene molecules present within
the nanotube or outside or both. However, it is tempting to
assume that a host–guest type of interaction is operative, be-
cause it should be stronger than simple solvation. Finally, it
is therefore not unreasonable to expect that a better chiral
guest can be found, which will induce a full helical bias of
the nanotubes.
Figure 4. CD spectra at 208C of 10 mm bisurea solutions: EHUT-SS in
*
(S)-limonene (c c); EHUT-SS in (R)-limonene (cꢆc);
EHUT-RR in (R)-limonene (cꢆc); EHUT-RR in (S)-limonene
Acknowledgements
*
(c c).
We thank FranÅois Bouꢁ (LLB, Saclay) for assistance with SANS experi-
ments.
between the two chiral monomers and the two chiral sol-
vents. Clearly, the handedness of the nanotube is imposed
by the monomer because the Cotton effect is of the same
sign for EHUT-RR in both (S)- and (R)-limonene. Howev-
er, the intensity difference between these two spectra con-
firms the significant effect of the solvent. It is reasonable to
assume that full chiral induction is reached for EHUT-RR
in (R)-limonene, because this is already the case in cyclo-
hexane (Figure 2). Therefore, the intensity of the Cotton
effect for this system (De=24.5 LmolÀ1 cmÀ1, Figure 4)[18]
can be used to normalize the value obtained in the case of
EHUT-rac in (R)-limonene (De=8.3 LmolÀ1 cmÀ1, Fig-
ure 3a): this yields a value of 33% for the chiral induction
by the chiral solvent. Although full chirality induction by
the solvent alone is not reached, this value is quite signifi-
cant.
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Chirality in Supramolecular Nanotubes
FULL PAPER
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[18] This value is the average between EHUT-RR and EHUT-SS data.
The difference between both data is due to the difficulty to precisely
control the pathlength of the very thin cell (0.001 cm).
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Received: August 31, 2009
Published online: November 11, 2009
Chem. Eur. J. 2010, 16, 173 – 177
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