A self-assembled helical anthracene nanofibre whose P- and M-isomers show unequal linear dichroism in a vortex

Article abstract of DOI:10.1039/c1cc14241k

An anthracene derivative, having benzamide substituents bearing optically active alkyl chains, in hexane forms one-handedly biased helical supramolecular nanofibres, which show unequal linear dichroism (LD) in right- and left-handed vortex flows.

Full text of DOI:10.1039/c1cc14241k

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COMMUNICATION
A self-assembled helical anthracene nanofibre whose P- and M-isomers
show unequal linear dichroism in a vortexw
Yasunari Ando, Tomoki Sugihara, Kenjiro Kimura and Akihiko Tsuda*
Received 14th July 2011, Accepted 15th September 2011
DOI: 10.1039/c1cc14241k
8
An anthracene derivative, having benzamide substituents bearing
optically active alkyl chains, in hexane forms one-handedly
biased helical supramolecular nanofibres, which show unequal
linear dichroism (LD) in right- and left-handed vortex flows.
responsible. In fact, the formation of helical ribbons in such a
stirred solution has recently been confirmed. However, actual
hydrodynamic behaviors of the aggregates in the fluids have
not sufficiently been clarified experimentally. The present
study reports a novel phenomenon that one-handedly biased
helical supramolecular polymers showed unequal LD
responses in the solutions stirred in clockwise (CW) and
counterclockwise (CCW) directions.
Small molecules cannot orient along the macroscopic flowing
direction of the solvent molecules because Brownian motions are
dominant over hydrodynamic interactions at the molecular level.
However, when molecules or molecular assemblies are long
enough to average local effects of the Brownian motions of the
solvent molecules, they might sense fluidic flows. Actually, the
certain nanoscale molecules and molecular assemblies, having
anisotropic structures, have been known to align in sheared
The chiral AN molecule was designed by reference to the
molecules that form helical supramolecular polymers,
6
,7
reported previously.
AN was synthesized starting from
anthracene, which was successfully converted to 9,10-
bis(aminomethyl)anthracene, coupled with an (R)- or
1
(
S)-enantiomer of 3,4,5-[tris-3,7-dimethyloctyloxy]benzoic acid
fluids. With this background, we recently succeeded in a spectro-
and characterized by means of NMR, IR and electronic
scopic visualization of a macroscopic helical alignment of a
supramolecular polymer of J-aggregated zinc porphyrin in
torsional flows of the vortex by mechanical rotary stirring of a
absorption spectroscopy, together with mass spectrometry
9
(
see ESIw). In the absorption spectroscopy, CHCl
3
solution
À5
2,3
of AN at 25 1C with the concentration of 8.3 Â 10
M
fluid in an optical cell. Then, with an expectation that helical
4,5
displayed a characteristic spectral pattern of anthracene with
fine structure (Fig. S1, ESIw). However, in hexane AN showed
broaden red-shifted absorption bands at lmax of 407, 385 and
nano objects can sense torsional directions of the vortexes, in
the present study, we have synthesized helical supramolecular
nanofibres, composed of anthracene derivatives [(R)- and
3
66 nm, together with a blue-shifted one at 248 nm [Fig. 2
(
S)-AN] having benzamide substituents bearing optically active
6,7
(top), green curves]. Since those lower and higher energy bands
1
essentially originate from L , L transitions and B transition,
a b b
alkyl chains (Fig. 1), and investigated their flowing behaviors
1
in the vortex using LD spectroscopy.
1
1
being polarized along the shorter and longer in-plane molecular
Ribo et al. have previously reported that electrostatic
´
10
axis, respectively, AN molecules most likely form a J-aggregate
J-aggregates of (4-sulfophenyl)porphyrin derivatives, pre-
pared in aqueous media upon rotary stirring, become optically
active, and suggested that hydrodynamic selection of either the
right- or left-handed helical conformation of the aggregates is
with head-to-tail arrangement along the anthracene short axis
1
1
(
vide infra). While heating the sample solution to 60 1C, the
absorption spectrum changed to that as observed in CHCl with
3
recovery of the fine structure (lmax = 394, 374, 355 nm and 259 nm)
due probably to the dissociation [Fig. 2 (top), black curve]. Scanning
electron microscopy (SEM) performed on a silicon substrate with a
À5
solution of self-assembled AN in hexane (8.3 Â 10 M), after being
air-dried, revealed the formation of linear fibres (Fig. S2, ESIw).
Here, AN monomers in nonpolar solvents may self-assemble via the
multi hydrogen bonding interactions of its benzamide functionalities
and p–p interaction of anthracene components to form J-aggregated
supramolecular nanofibres. Photodimerization of AN, which
Fig. 1 Chemical structures of (R)- and (S)-AN.
12
deforms the anthracene p-plane to a butterfly shaped structure,
thus, leads to dissociation of the assemblies (Fig. S3, ESIw).
In circular dichroism (CD) spectroscopy, hexane solutions of
Department of Chemistry, Graduate School of Science, Kobe
University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan.
E-mail: tsuda@harbor.kobe-u.ac.jp; Fax: +81-78-803-5671;
Tel: +81-78-803-5671
w Electronic supplementary information (ESI) available: Synthesis,
SEM, LD spectroscopy of AN nanofibres. See DOI: 10.1039/
c1cc14241k
(
R)- and (S)-enantiomers of AN provide Cotton effects in the range
of absorption bands of the anthracene unit, and their spectral
patterns become mirror image in each other [Fig. 2 (bottom)].
1
1748 Chem. Commun., 2011, 47, 11748–11750
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Fig. 2 Absorption (top) and CD (bottom) spectra without stirring of
hexane solutions of (R)-AN ( ) and (S)-AN ( ) at 25 1C, and
(
R)-AN (TT) at 60 1C in quartz optical cuvettes having light path length
of (a) 1 mm and (b) 10 mm for measurement range of 220–320 and
20–450 nm, respectively. [AN] = 8.3 Â 10À5 M.
Fig. 4 (a) Constant frequency-shift topography and (b) its height profile
(Positions A to B) of the (R)-AN fibres (on HOPG surface) immersed in
3
the 200 mM KCl solution. Interaction force (frequency-shift): Df=150 Hz,
Nomal-coated silicon cantilevers with a nominal spring constant of
As expected, heating the sample solution leads to a gradual
decrease of the Cotton effects and a complete loss at 60 1C. These
results indicate that the supramolecular polymer composed of
À1
(R)
2 Nm (Nanosensors, PointProbe Plus NCH-10T). Scan speed is
4
5
0 s per frame. (c) Constant frequency-shift topography of the (R)-AN
fibres immersed in the purified water. Df=100 Hz, gold-coated silicon
cantilevers with a nominal spring constant of 42 Nm . Scan speed is
(
R)- or (S)-AN has a dominant handedness of the P- or M-helix.
À1
1
In such chiral multi anthracene arrays, B transition of anthracene
b
1
20 s per frame. (d) Feedback error distribution of the (R)-AN fibres in
around 250 nm has been known to give strong exciton-coupled
(
c). Scan speed is 60 per frame.
CD, whose Cotton effects allow empirical estimation of their
10
absolute configurations. Since (R)- and (S)-AN assemblies
actually show positive to negative and negative to positive Cotton
effects, respectively, at this absorption region [Fig. 2a (bottom)],
they may dominantly adopt CW and CCW twist orientations,
which allow formation of P- and M-helical assembly, respectively
observed in a 2D crystalline phase (Fig. 4a and b). Since the
molecular size of AN is calculated as B3 nm, helical
J-aggregation of AN molecules as illustrated in Fig. 3 (left)
may allow formation of the discrete nanofibre with the
relatively larger diameter (centre). Although the suggested
helical structure of the single nanofibres could not be clarified
in our technique at this stage, we have succeeded in visualizing
the twisted structures of larger nanofibres (Fig. 4c and d). The
liner nanofibres of (R)-AN clearly show right-handed P-helical
structures, whose direction is in agreement with that expected
from the above CD spectral study, and the observed diameters
of B50 nm indicate multi bundling of the single nanofibres in
their structure (Fig. 3, right).
(Fig. 3, left). This assignment is also supported by a previously
reported chiral anthracenophane, in which two anthracene
1
3
components are connected by optically active spacers. The
anthracenophane adopts a CCW orientation in X-ray crystallo-
graphy, and gives the CD spectrum in solution, whose pattern is
essentially the same as that of (S)-AN assemblies.
Frequency-modulation
atomic
force
microscopy
(
FM-AFM) performed on a HOPG substrate with a solution
À5
of self-assembled (R)-AN in hexane (8.3 Â 10 M), after dried
With these structural features of AN assemblies in mind, we
have investigated hydrodynamic behaviors of such twisted
2
supramolecular nanofibres in torsional vortex flows. Orientation
over hexane vapor for 3 h, allowed us to obtain high-
1
4,15
resolution images of the linear fibres (Fig. 4).
The parallel
aligned single nanofibres with a 4–5 nm diameter, that also
agrees to the height profile in the cross section image, are
of the nanofibres of AN in solution can be characterized by using
LD spectroscopy. The spectrometers were equipped with a
1
0 Â 10 Â 40 mm quartz optical cuvette, in which sample
solutions (3 mL) were stirred mechanically using
a
+
2.0 Â 5.0 mm Teflon-coated magnetic stir bar at the bottom
of the cuvette, 13 mm below the centre of a +8.0 mm wide
polarized light pass (Fig. 5a). While a hexane solution of AN
À5
(
8.3 Â 10 M) without stirring was LD silent, it became LD
active upon mechanical rotary stirring (Fig. 5b and c). For
example, when stirred at 1350 rpm in CW direction, the hexane
solution containing nanofibres composed of (R)-AN displayed
intense LD bands at the absorption bands of anthracene
component [401 (Dod. = À0.048), 381 (À0.032), and 362 nm
(
À0.016)] (Fig. 5b, blue curve). The LD intensity was dramatically
Fig. 3 Schematic illustrations of a twisted J-aggregation of (R)-AN
molecules (left and centre) and a helical bundle of the nanofibres
decreased when the measurement was demonstrated in octane
(right). OR: tris-(R)-3,7-dimethyloctyloxy group.
having higher viscosity than hexane (Fig. S4, ESIw). Since the
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Chem. Commun., 2011, 47, 11748–11750 11749

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the solution, respectively. The results presented herein become a
new knowledge for the macroscopic hydrodynamic behavior of a
helical supramolecular assembly, but still remaining a contro-
versy for the origins of the observed LD differences. Extension of
this phenomenon to artificial conjugated polymers is a fascinating
next project that is actively in progress in our laboratory.
The present work was sponsored by a Grant-in-Aid for
Scientific Research (B) (No. 22350061) from the Ministry of
Education, Science, Sports and Culture, Japan, by TEPCO
Research Foundation, and the Toray Science and Technology
Grant. The AFM observations in this work were supported by a
Grant-in-Aid for Scientific Research on Priority Areas (No. 477)
‘‘Molecular Science for Supra Functional Systems’’.
Notes and references
Fig. 5 (a) Schematic illustration of a sample solution with rotary CW
stirring in a 10 Â 10 Â 40 mm quartz optical cuvette containing a magnetic
stir bar. For LD spectroscopy, an +8.0 mm wide linear polarized light is
allowed to pass through the sample solution at a position 14 mm above the
bottom of the inner surface of the optical cuvette. LD spectra of nanofibres
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(
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1
2
1
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1750 Chem. Commun., 2011, 47, 11748–11750
This journal is c The Royal Society of Chemistry 2011