Optically active liquid-crystalline fullerodendrimers from enantiomerically pure fulleropyrrolidines

Article abstract of DOI:10.1039/c0cc02709j

A synthetic methodology based on the 1,3-dipolar cycloaddition reaction was developed to design enantiomerically pure liquid-crystalline fullerodendrimers.

Full text of DOI:10.1039/c0cc02709j

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Optically active liquid-crystalline fullerodendrimers from
enantiomerically pure fulleropyrrolidineswz
a
a
b
c
´
´
Frederic Lincker, Philippe Bourgun, Helen Stoeckli-Evans, Isabel M. Saez,*
John W. Goodby*c and Robert Deschenaux*a
Received 21st July 2010, Accepted 20th August 2010
DOI: 10.1039/c0cc02709j
A synthetic methodology based on the 1,3-dipolar cycloaddition
reaction was developed to design enantiomerically pure liquid-
crystalline fullerodendrimers.
CH2Cl2/heptane mixture as eluent (Fig. S1), with diastereo-
isomer (R,S)-4 eluting first. Deprotection of (R,S)-4 and
(R,R)-4 with trifluoroacetic acid led to the corresponding
carboxylic acid derivatives which were condensed with
phenol-based second generation dendron 59 to give enantio-
merically pure fullerodendrimers (S)-6 and (R)-6.
Chiral nanotechnology1 has recently attracted growing interest
in the scientific community for optically active building blocks,
self-assembling and self-organizing nanomaterials,2 and as
unique components in materials science for the development
of innovative devices such as molecular switches3 and molecular
motors.4 However, despite their unique photophysical and
electrochemical properties, only a few optically active fullerene
derivatives have been reported to be prepared via regio- and
stereo-selective 1,3-dipolar cycloaddition reactions.5–8
Slow diffusion of heptane in a CH2Cl2 solution containing the
compound that eluted first on HPLC for the synthesis of (R,S)-4/
(R,R)-4 gave dark red crystals which were analyzed by X-ray
diffraction (Fig. 1). The (R,S)-4 derivative crystallized in the
chiral orthorhombic space group P212121 as a CH2Cl2 solvate.
The circular dichroism spectra of diastereoisomers
4
(Fig. S2) and enantiomers 6 (Fig. 2) are in agreement with
their absolute spatial configuration. Indeed, from literature
data,5,8 a negative Cotton effect at 430 nm corresponds to the
S-configuration of the stereogenic center in the pyrrolidine
ring, as found for (R,S)-4 and (S)-6, whereas a positive Cotton
effect at 430 nm corresponds to the R-configuration, as found
for (R,R)-4 and (R)-6. Compound (R,S)-4 for which the
stereochemistry was established by means of X-ray diffraction
fully supports the CD data.
Recently, we described the synthesis, characterization and
liquid-crystalline properties of four diastereoisomerically pure
fulleropyrrolidines, all of which exhibited chiral nematic (N*)
phases.9 Based on this approach, we have now developed a
synthetic methodology for the design of optically active liquid-
crystalline fullerodendrimers from enantiomerically pure
fulleropyrrolidines, which have allowed us to explore the
relationship between molecular structure, intermolecular
interactions and mesoscopic organization in relation to
chirality transfer. Our conceptual design is based on a 1,3-
dipolar cycloaddition reaction which generates a rotationally
fixed stereogenic center in a pyrrolidine ring adjacent to a
[60]fullerene (C60) moiety.10
Both fullerodendrimers display an enantiotropic mesophase
between the glass transition at 70 1C and the isotropization at
110 1C (Table 1, Fig S3). The enthalpy of isotropization is
identical for both enantiomers. Comparison of isotropization
temperatures and enthalpy of the transitions between dendron
5 and fulleropyrrolidines (R)-6 and (S)-6 indicates that C60
destabilizes the mesophase.
Esterification of bromoacetic acid with commercially
available (R)-(+)-a-methyl-2-naphthalenemethanol (R)-1 led
to ester derivative (R)-2 (Scheme 1). The latter compound was
converted to the amine (R)-3. Subsequent condensation of
(R)-3 with formaldehyde and C60 under standard 1,3-dipolar
cycloaddition conditions10 gave diastereoisomers (R,S)-4 and
(R,R)-4 which were easily and efficiently separated by
HPLC using a non-chiral mPorasil stationary phase and
The defect textures of the compounds were examined by
polarized optical microscopy. The samples were heated to the
isotropic state and cooled (0.1 1C minꢀ1) to 105 1C. Upon
annealing at this temperature, samples of (R)-6 and (S)-6
displayed the Grandjean-plane texture with oily streak defects
(floating edge dislocations; Fig. 3, Fig. S4). The presence of
this texture unequivocally identifies the phase as the N* phase.
Rotation of the analyzer showed that the enantiomers have
opposite twist directions, the helical macrostructure of the
mesophase being right-handed for (R)-6 and left-handed
for (S)-6.
a
´
ˆ
Institut de Chimie, Universite de Neuchatel, Avenue de Bellevaux 51,
Case postale 158, 2009 Neuchaˆtel, Switzerland.
E-mail: robert.deschenaux@unine.ch
b
´
ˆ
Institut de Physique, Universite de Neuchatel, Rue Emile-Argand 11,
2009 Neuchaˆtel, Switzerland
c Department of Chemistry, The University of York, Heslington, York,
UK YO10 5DD. E-mail: jwg500@york.ac.uk, iss500@york.ac.uk
w This article is part of the ‘‘Carbon Nanostructures’’ web-theme issue
for ChemComm.
On prolonged annealing, the Grandjean texture gradually
became populated by fingerprint defects arising from
homeotropic boundary conditions (Fig. S5). (R)-6 developed
the characteristic textures with more ease than (S)-6. The pitch
(r) of the chiral nematic phase was determined from the
fingerprint texture, since the distance between adjacent,
equidistant dark (or bright) lines corresponds to half of the
pitch length.12 The pitch values at room temperature are
r = 3.4 ꢁ 0.1 mm for (R)-6 and r = 4.4 ꢁ 0.1 mm for (S)-6.
z Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data of new compounds, HPLC chromato-
grams, UV-vis and CD spectra of (R,S)-4 and (R,R)-4, crystallo-
graphic cif file for (R,S)-4, DSC traces of (S)-6 and (R)-6, and
photomicrographs of the Grandjean texture of (S)-6 and fingerprint
texture of (R)-6. CCDC 785100. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0cc02709j
c
7522 Chem. Commun., 2010, 46, 7522–7524
This journal is The Royal Society of Chemistry 2010

Scheme 1 Reagents and conditions: (i) bromoacetic acid, DCC, DMAP, CH2Cl2, 0 1C to r.t., 97%; (ii) benzylamine, Et3N, THF, 0 1C to r.t., 88%;
(iii) C60, formaldehyde, toluene, reflux, 20 h, 25% for (R,S)-4 and 31% for (R,R)-4; (iv) TFA, CH2Cl2, r.t.; (v) 5, DCC, DPTS, CH2Cl2, 0 1C to r.t.,
63% for (R)-6 and 63% for (S)-6. For abbreviations, see ref. 11.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7522–7524 7523

This response of the cholesteric phase, i.e. similar magnitude
of the pitch (and hence of helical twisting power) and opposite
helical sense, mirrors the observation of their identical
magnitude but opposite sign Cotton effects in solution, as
expected for enantiomers.
The results described here regarding the relationship
between the molecular chirality, the handedness of the
resulting macroscopic helix and the induced helical twisting
power of the chiral nematic phase demonstrate unambiguously
that the mesophase chirality is controlled at the molecular
level by the stereogenic center of the fulleropyrrolidine ring.
In conclusion, we have reported an elegant concept for the
synthesis of enantiomerically pure fulleropyrrolidines. The latter
materials were used as chiral platforms for the design of optically
active liquid crystals. In these materials, the nature of the
mesomorphic dendron controls the type of mesophase exhibited,
whereas the stereochemical structure of the fulleropyrrolidine
dictates the handedness of the helical supramolecular structure of
the mesophase, and ultimately the transmission of molecular
chirality to the bulk self-organizing mesophase.
Fig. 1 View of the molecular structure of the compound (R,S)-4
(the solvent molecule has been omitted for clarity).
R.D. thanks the Swiss National Science Foundation
(Grant No. 200020-119648) for financial support. We also
acknowledge support from the European Science Foundation
(ESF) under the EUROCORES SONS II – LCNANOP project.
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dendron 5 and fullerodendrimers (S)-6 and (R)-6a
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Tg/1C
Transition
T/1C
DH/kJ molꢀ1
59
(S)-6
(R)-6
51
70
70
N - I
N* - I
N* - I
164
110
110
2.2
0.3
0.3
a
Tg, glass transition temperature; N, nematic phase; N*, chiral
nematic phase; I, isotropic liquid. Temperatures are given as the onset
of the peaks obtained during the second heating run; the Tg values
were determined during the first cooling run.
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Fig. 3 Grandjean-plane texture displayed by (R)-6 at 105 1C (ꢂ100).
7524 Chem. Commun., 2010, 46, 7522–7524
c
This journal is The Royal Society of Chemistry 2010