Lyotropic supramolecular helical columnar phases formed by C 3-symmetric and unsymmetric rigid molecules

Article abstract of DOI:10.1002/chem.201202944

Unlike thermotropic liquid-crystalline C₃-symmetric molecules with flexible chains, the herein-designed fully rigid three-armed molecules (C₃-symmetric and unsymmetric) create a fancy architecture for the formation of lyotropic liqui

Full text of DOI:10.1002/chem.201202944

FULL PAPER
DOI: 10.1002/chem.201202944
Lyotropic Supramolecular Helical Columnar Phases Formed by C3-
Symmetric and Unsymmetric Rigid Molecules
Daoliang Wang, Youju Huang, Junjun Li, Lu Xu, Mingming Chen, Jiaojiao Tao, and
Liangbin Li*[a]
Abstract: Unlike thermotropic liquid-
crystalline C3-symmetric molecules
with flexible chains, the herein-de-
signed fully rigid three-armed mole-
cules (C3-symmetric and unsymmetric)
create a fancy architecture for the for-
mation of lyotropic liquid crystals in
water. First, hollow columns with
triple-stranded helices, analogous to
helical rosette nanotubes, are spontane-
ously constructed by self-organization
of the rigid three-armed molecules.
Then, the helical nanotubes arrange
into
phases, which show macroscopic chi-
AHCTUNGTRENGNrUN ality as a result of supramolecular
chiral symmetry breaking. Interestingly,
the helical nanotubes constructed by
the fully rigid molecules are robust and
stable over a wide concentration range
hexagonal
liquid-crystalline
in water. They are hardly affected by
ionic defects at the molecular periph-
ery, that is, further decoration of func-
tional groups on the molecular arms
can presumably be realized without
changing the helical conformation. In
addition, the formed columnar phases
can be aligned macroscopically by
simple shear and show anisotropic
ionic conductivity, which suggests
promising applications for low-dimen-
sional ion-conductive materials.
Keywords: chiral symmetry break-
ing · conducting materials · helical
structures · liquid crystals · supra-
molecular chemistry
Introduction
shapes[5] have been synthesized for the formation of supra-
molecular liquid crystals under thermotropic conditions.
Three-armed molecules (C3-symmetric or unsymmetric)
have received increasing attention in recent years due to
their diverse functional arms and peculiar assembled struc-
tures.[6] Meijer and co-workers prepared C3-symmetrical
compounds for amplification of chirality in their supra-
Owing to molecular recognition, self-assembly and supra-
ACHTUNGTRENNUNGmolecular function, supramolecular chemistry has been ex-
tensively studied in recent years and employed in various
applications.[1] Particularly supramolecular liquid crystals,
one of the most important branches of supramolecular
chemistry, have gained increasing attention due to their
complex superstructures and multiple functionalities.[2] By
combining the hierarchical structures of supramolecular self-
assembly and the unique properties of liquid crystals (e.g.,
molecular order and mobility), one can design various func-
AHCTUNGTRENNUNG
molecular aggregates.[7] Molecules with C3 symmetry are
also used to construct metal–organic frameworks with wide
applications like hydrogen storage and gas separation.[8]
With the aid of flexible aliphatic chains, three-armed mole-
cules are capable of programming supramolecular thermo-
tropic liquid crystals.[9] However, to the best of our knowl-
edge, there is no report on employing fully rigid three-
armed molecules to establish supramolecular liquid crystals
yet. With ionic groups at their periphery, fully rigid dis-
coid[10] or plank-shaped (chromonic)[11] molecules can show
special liquid-crystalline order in aqueous solution. In con-
trast to thermotropic liquid-crystalline C3-symmetric mole-
cules with flexible chains or chromonic liquid-crystalline
molecules with plank shape, self-assembly of fully rigid
three-armed molecules with ionic groups at their periphery
faces the difficulty of filling the large void space among the
individual arms. To minimize the contact between water
molecules and the hydrophobic arms, the rigid three-armed
molecules have to create a fancy architecture for the forma-
tion of lyotropic liquid crystal.
tional matACHTUNGTRENNUNG
erials with desired performances.[2b,3] For example,
low-dimensional ion-conductive materials were fabricated
by using supramolecular liquid crystals.[4] Up to now, a great
number of molecular building blocks with diverse ge
ACHTUNGTRENNUNGrical shapes such as rod, fan, disk, cone and other exotic
ACTHUNGERTNNUNGoHCATUNGTRNEmNUNG et-
[a] D. Wang,+ Dr. Y. Huang,+ J. Li, L. Xu, M. Chen, J. Tao,
Prof. Dr. L. Li
National Synchrotron Radiation Lab and College of Nuclear Science
and Technology
CAS Key Laboratory of Soft Matter Chemistry
Department of Polymer Science and Engineering
University of Science and Technology of China
No. 96, JinZhai Road, Hefei, Anhui, 230026 (P. R. China)
Fax : (+86)551-5141078
Herein, we designed and synthesized three fully rigid mol-
ecules to investigate the effects of topology and hydrophilic-
ity on supramolecular assembly. These molecules with three-
way rigid rod junctions (Scheme 1) have different numbers
E-mail: lbli@ustc.edu.cn
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202944.
Chem. Eur. J. 2013, 19, 685 – 690
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685

Figure 1. Phase behaviors of various compositions in water at room tem-
perature.
Scheme 1. Chemical structures of P73, P72, and P71.
Figure 2. Polarizing optical micrographs of P73 in water at concentrations
of a) 7 and b) 13 wt%.
of sulfo groups at their periphery, which lead to their corre-
sponding C3-symmetric and unsymmetric molecular struc-
tures. In the designations P73, P72 and P71, 7 represents the
number of phenylene rings, indicating equally long arms in
the molecules, and 1, 2 and 3 the number of ionic groups at
the molecular periphery. Unexpectedly, P73, P72 and mix-
ture P73/P71 spontaneously assemble into cylinders with
triple-stranded helices in aqueous solution, which further ar-
range into hexagonal liquid-crystalline phases at high con-
centrations. The supramolecular helical cylinders can be
aligned macroscopically by simple shear, which brings about
anisotropic ionic conductivity in the liquid-crystalline
phases.
are present in the solution states at low concentrations for
1
these molecules, as verified by H NMR measurements (see
Supporting Information S-Figure 2).
The P72 system shows liquid-crystalline and gel states at
lower concentrations than the P73 system, that is, the
number of hydrophilic groups at the molecular periphery af-
fects self-assembly. Compound P71 with one sulfo group
cannot form a LC in water due to its low solubility
AHCTUNGTREG(NNNU !1 wt%). With the same average number but different dis-
tribution of sulfo groups on the arms, the blended P73/71
system shows LC and gel states with concentrations (2 and
7 wt%) comparable to those of the P72 system (2 and
8 wt%). This indicates that the distribution of hydrophilic
group on the arms barely affects the self-assembly behavior
provided the average ionic density ensures sufficient solubil-
ity. The P73-Py system, which has pyridinium instead of
proton as counterion, exhibits similar liquid-crystalline be-
havior with slightly lower concentrations for the LC and gel.
Results and Discussion
Phase behavior of the liquid crystals: To explore how the
ionic distributions and counterions affect the liquid-crystal-
line structures, three pure compounds (P73, P72 and P71), a
complex of P73 and pyridine (1/2 molar ratio), denoted P73-
Py, and a mixture of P73 and P71 (1/1 molar ratio), denoted
P73/71, were studied. The liquid-crystalline behavior of
these systems in water was studied by polarized optical mi-
croscopy (POM, Figure 1). With increasing concentration,
solution, liquid-crystal (LC) and gel-like states were ob-
served. Typical PO micrographs of the P73/water system are
shown in Figure 2a (7 wt%, LC) and b (13 wt%, gel). At
high concentration, the gel displays a stringlike texture and
can be aligned by simple shear (Figure 2b). At low concen-
tration, a transformation from gel to fluid liquid crystal
takes place, which exhibits “tiger-skin” texture (Figure 2a).
Similar textures are observed for the liquid-crystalline
phases of other molecular systems (see Supporting Informa-
tion S-Figure 1). Note that aggregates rather than monomers
Structural characterization by SAXS and WAXS: To deter-
mine the supramolecular structures of all samples, synchro-
tron-based small- and wide-angle X-ray scattering (SAXS/
WAXS) studies were performed at room temperature. All
systems show scattering signals of hexagonal phases at suffi-
ciently high concentrations, especially in their gel states. Fig-
ure 3a–c show representative two-dimensional SAXS and
WAXS patterns of a hexagonal phase in a highly aligned
sample, whereby the orientation direction is indicated by
double-headed arrow. The corresponding one-dimensional
diffraction curves for all systems at their gel concentrations
are shown in Figure 3d (SAXS) and e (WAXS), in which
the abscissa corresponds to the modulus of the scattering
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Lyotropic Supramolecular Helical Columnar Phases
FULL PAPER
metry of hexagonal lattice, one
can expect a linear dependence
between the spacing between
adjacent columns a and the re-
ciprocal square root of the
mass concentration cꢀ1/2. Our
experimental results indeed co-
incide with this expectation
(Figure 3 f). All systems show
good linear relations between
a and cꢀ1/2 in the concentration
range of the hexagonal phase.
Furthermore, the molecular
number in a single layer of col-
umns can be derived from the
linear slope, which is about
three for all systems (see Sup-
porting Information Part 3.1).
Considering that even at very
high concentration the spacing
a (ꢁ10 nm) is far larger than
the diameter of a single mole-
Figure 3. X-ray scattering patterns of aligned samples (arrow indicates shear direction). a) Small-angle region,
b) wide-angle region and c) same as b), but medium-angle region. (d) 1D SAXS and (e) WAXS curves for dif-
ferent samples (P73: 13 wt%, P73-Py: 10 wt%, P72: 12 wt%, P73/71: 10 wt%). f) Relationship between con-
centration and spacing of adjacent columns in each sample system.
vector q. The three distinct diffraction peaks with a ratio of
q values of 1:31/2:2 and, moreover, an inconspicuous peak at
71/2 lead to the conclusion that the liquid-crystalline phases
of all investigated systems adopt hexagonal symmetry, and
the three peaks can be indexed as (1,0), (1,1) and (2,0) (see
Figure 3a and d). Orthogonal to the three major SAXS
peaks, a sharp peak is observed for each system at a q value
of 18.6 nmꢀ1 in the wide-angle region (Figure 3b and e),
which corresponds to a layer spacing of about 0.34 nm.
These scattering data indicate long-range order for both in-
plane and interlayer arrangements. In addition, four scat-
cule (ꢁ3.5 nm, trans form of P73 molecule), it is reasonable
for three molecules in a single layer to construct a column
through layer-by-layer stacking, due to the coplanar configu-
ration of P73 molecules. The trans form of P73 dominates in
our system and the cis form is negligible, as was verified by
1
UV/Vis spectra and H NMR data (see Supporting Informa-
tion S-Figure 5).
Symmetry breaking of supramolecular chirality: The X-ray
data in Figure 3c suggest a helical conformation of columns.
The CD spectra shown in Figure 4a further confirm helical
structures in all systems. A bisignate signal with two Cotton
effects around 330 and 375 nm and a crossover around
345 nm appears in each system, whereby the sign order re-
veals information about the chirality of supramolecular
structure.[12] However, positive and negative (designated by
the sign of the first long-wavelength band) bisignate CD sig-
nals both have a chance to show up in each system. For the
sake of simplicity, Figure 4a shows selected CD curves for
each system, while Figure 4b shows, for the example of the
P73/water system at 5 wt% (or 8 wt%), that the CD curves
can display positive (373/330 nm, +/ꢀ) or negative (373/
330 nm, ꢀ/+) bisignate signals for different test batches. In
view of the achiral P73 molecules, the CD signals mean that
chiral symmetry breaking occurs in our LC systems, but in a
spontaneous and unpredictable fashion.[13] Similar symmetry
breaking of chirality has also been reported in other supra-
molecular aggregates of molecules involving ionic groups.[14]
In addition, there are no obvious Cotton effects when the
concentration (2 wt%) is far below the liquid-crystalline
ACHTUNGTRENNUNGtering points are spread around the vertical direction with a
q value of 3.02 nmꢀ1, a phenomenon commonly regarded as
fingerprint of helical structure, are observed in all systems
(Figure 3c and e).
Decreasing the concentration of the three-armed mole-
cules weakens the long-range order or decreases the correla-
tion length of the hexagonal symmetry, which is manifested
in broadening of the first-order peaks (1,0) and (1,1) as well
as weakening of the second-order peak (2,0) in SAXS pat-
terns (see Supporting Information S-Figure 3). Nevertheless,
the hexagonal symmetries persist even at relatively low con-
centrations in all systems. Similar smooth transition of hex-
agonal phase was also observed in the P phases (hexagonal
array), which were formed via the columns with effectively
circular cross section in chromonic systems.[11b–d]
The q value of the (1,0) peak in all systems declines
smoothly with decreasing concentration, whereas the peaks
corresponding to the layer spacing at a q value of 18.6 nmꢀ1
and helical structure at a q value of 3.02 nmꢀ1 remain un-
changed in position. In other words, increasing water con-
tent expands the distance among adjacent columns, but this
does not affect the internal organization of the columns.
This indicates that the columns are stable and robust, and
cannot be easily broken by adding water. Based on the sym-
1
concentration (5 wt%). H NMR studies show that P73 mol-
ecules exist as aggregates rather than monomers even at
1.5 wt% concentration (see Supporting Information S-
Figure 2). Thus, the macroscopic chirality has a close rela-
tion with the ordered liquid-crystalline phase, because the
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687

L. Li et al.
Figure 5. a) Electron-density profile of the 2D unit cell for P73/water
system at a concentration of 13 wt%. Schematic representation of the su-
pramolecular helical column in P73 system in b) top and c) side views.
Figure 4. a) CD spectra of different samples at selected concentrations
(P73: 5 wt%, P73-Py: 8 wt%, P72: 6 wt%, P73/71: 3 wt%). b) CD spec-
tra of the P73/water systems at different concentrations; (1) and (2) rep-
resent different test batches.
From the top view, a water channel of about 3 nm and a col-
umnar diameter of about 8.5 nm are present in a helical ro-
sette nanotube. The fact that the diameter of the nanotube
is close to the spacing a between adjacent columns at very
high concentration suggests that the model is reasonable.
From the side view, a triple-stranded helix is built up
through layer-by-layer stacking, which gives the spiral and
twining distribution of ions. Due to the periodic distribution
of sulfo groups, anisotropic ionic conductivity is expected in
the aligned hexagonal phase.
independent aggregates are insufficient to induce symmetry
breaking of chirality due to racemization.
Assembly model of the helical column: How do these rigid
molecules assemble into a helical column? As shown in Fig-
ure 3c, the four scattering points have a 208 angle referring
to the SAXS peaks of the in-plane hexagonal order (or 708
referring to the WAXS peak). The projection along the col-
umnar axis gives q=3.02cos708=1.03 nmꢀ1 in reciprocal
space, which corresponds to a helical period of 6.10 nm
along the columnar axis. As the columns have a layered
structure with spacing of 0.34 nmꢀ1, a helical period would
contain 6.10/0.34=18 layers. That is, the first layer in a heli-
cal column would overlap geometrically with the 19th layer.
Both the helical pitch and the layer spacing remain un-
changed with varying molecular concentration, while the in-
tercolumnar distance increases with decreasing concentra-
tion (see Supporting Information S-Figure 4). This suggests
that the helical signal originates from intracolumn order,
which is not affected by the interaction among columns.
To obtain a clear picture of the observed hexagonal
phase, we reconstructed the electron density profile from
the small-angle diffraction densities of the powder pattern
(see Supporting Information Part 3.2). All systems show
similar electron-density profiles for their hexagonal phases.
Figure 5a shows the profile for P73/water system at 13 wt%
concentration. The continuous regions of low electron densi-
ty (0.33 eꢁ3) correspond to water solvent, while the annuli
of high density and central holes of intermediate density
correspond to the helical columns and water channels, re-
spectively. Based on this electron-density profile and X-ray
and CD data, a schematic model for supramolecular helical
column is given in Figure 5b (top view) and c (side view).
Anisotropic ionic conductivity: The ionic conductivities of
the hexagonal phases were systematically studied and dis-
cussed here for the P73-Py system. Illustrations of cells for
the conductivity measurements and POM images of oriented
columns after simple shear are shown in S-Scheme 2 of the
Supporting Information. As shown in Figure 6, the ionic
conductivities in the direction parallel to the columnar axis
(s , Figure 6P) are significantly higher than those in the di-
k
rection perpendicular to the columnar axis (s , Figure 6V),
?
while the unaligned samples (Figure 6S) have ionic conduc-
tivities in between. At low concentrations where no liquid
Figure 6. Ionic conductivities of P73-Py/water system at different concen-
trations parallel (P) and perpendicular (V) to the column axis and for an
unaligned sample (S).
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Lyotropic Supramolecular Helical Columnar Phases
FULL PAPER
crystal phase forms, no anisotropic ionic conductivity is ob-
served in the sheared samples, which further confirms the
existence of aligned ionic nanochannels in the columnar
phases. The columns in hexagonal phases are robust and
stable over a wide concentration range, and thus promising
for use as low-dimensional functional materials as have
been successfully achieved for thermotropic liquid crys-
tals.[3c,4,15]
robust. As indicated by X-ray scattering data, intracolumn
order is not sensitive to variations in intercolumnar interac-
tion. In contrast to aggregation of flexible amphiphilic mole-
cules, which is driven by entropic changes, aggregation of
the rigid three-armed molecules is mainly driven by enthal-
py.[11] The strong interactions among adjacent arms (e.g., p–
p interactions, hydrogen bonds) support the robustness of
the microtubules. The robustness of the microtubules is also
manifested in the same self-assembly behavior of the C3-
symmetric P73 and the unsymmetric P72 molecules. The C3
Discussion: The helical rosette conformation is a new as-
sembly mode for three-armed molecules, which often self-
organize into single-stranded helical stacks according to pre-
vious literature.[7,21,6c] This unique triple-helical assembly of
fully rigid three-armed molecules seems novel but is rather
natural. It is well known that amphiphilic surfactants with a
hydrophilic head and hydrophobic tails can self-organize
into various well-defined nanoscale geometries in water, for
example, micelles, vesicles and microtubules.[16] The triple-
helical columns illustrated in Figure 5 are essentially micro-
tubules. However, unlike flexible amphiphilic surfactants,
symmetry of the P73 molecule allows a perfectly ho
ACHTUNGTRENNUNGmoACHTUNGTRENNUNGge-
AHCTUNGTREGnNNNU eous distribution of sulfo groups at the surface of the mi-
crotubules. With only two sulfo groups, the unsymmetric
configuration of the P72 molecule inherently introduces
“ion defects” on the surface if we refer to P73 as the “per-
fect distribution”. Clearly, the microtubules with strong in-
teractions among the arms have high tolerance to ionic de-
fects amounting to as much as one-third of the total ions,
which is further confirmed by the same phase behavior of
the P73/P71 blend. The high tolerance to ionic defects not
only demonstrates the robustness of the microtubules but
also suggests their versatility for further decoration on the
arms without sulfo groups, which may permit us to endow
the microtubules with diverse functions.
which form microtubules with walls of liquid layers or
ACHTUNGERTNgNUNG a-
ACHTUNGTRENNUNGnized by flexible hydrophobic tails, the C3-symmetrical or
unsymmetrical molecules with rigid arms cannot pack freely
to shield their hydrophobic rods from water. The triple-heli-
cal conformation seems the best choice to minimize contact
between the arms and water, and also distributes the sulfo
groups evenly on the interface. In addition, the directionali-
ty of hydrogen bonds also contributes to the helical columns
according to previous literature.[16,12a,6b] There is no doubt
that the triple-stranded helical structure is governed by a
delicate balance among various interactions, such as disper-
sion forces, hydrophobic interactions, electrostatic forces
and so on.[17,18a,19]
Conclusion
Fully rigid three-armed molecules can self-organize into su-
pramolecular columnar liquid-crystalline phases with triple-
helical rosette nanotubes as the columns, which are robust
and stable over a wide concentration range in water. The
helical nanotubes are hardly affected by ionic defects at the
molecular periphery, and this would no doubt benefit fur-
ther functionalization of their liquid crystals. The formed
hexagonal phases exhibit two distinctive properties. First,
the supramolecular helical cylinders can be aligned macro-
scopically by simple shear, which leads to anisotropic ionic
conductivities in the hexagonal phases. Second, the ordered
liquid-crystalline structures of organized achiral molecules
showed macroscopic chirality, which indicates the occur-
rence of spontaneous chiral symmetry breaking in these sys-
tems. These interesting findings open the possibility to pre-
pare low-dimensional ion-conductive materials and some
other functional liquid-crystalline materials with single-
handed helices based on only achiral molecules.
An interesting feature of our systems is that the supra-
ACHTUNGTRENNUNGmolecular liquid crystals formed by achiral molecules show
macroscopic chirality. Intensive studies on amplification of
chirality in C3-symmetrical molecules suggested that chi-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
cessive rotation of the stacking molecules would produce a
chiral column; however, this does not result in macroscopic
chirality if the left- and right-handed columns are evenly dis-
tributed. The macroscopic chirality observed in our systems
may stem from the cooperative interactions in the supra-
ACHTUNGTRENNUNGmolecular liquid crystals, similar to the other symmetry-
breaking systems.[13,18] Due to the cooperative interactions,
the column formed at the very beginning will lead to the for-
mation of a single-handedness domain. This is analogous to
the autocatalytic pattern of primary seeding and secondary
Experimental Section
nucleation processes observed in sodium chlorate with chiACTHNUTRGNEUNGral
symmetry breaking.[18a,20] However, the handedness of the
initiating column is randomly dominated, because the unit
molecule is achiral. Hence we can obtain either levo- or
dextrorotatory enantiomers, as is evidenced by the mirror-
image CD curves (Figure 4b).
Design and synthesis of three-armed molecules: materials, Synthetic de-
tails and characterization of all new compounds are given in Part 1 of the
Supporting information.
Experimental instruments and methods: Olympus (Japan) BX-51-P mi-
croscope system was used to obtain polarizing optical micrographs. CD
spectra were recorded on a Jasco J-810 spectropolarimeter. Ionic conduc-
tivities were measured by the alternating-current impedance method ac-
cording to previous literature[15a] on a CHI660C electrochemistry work-
All of the experimental evidence suggests that the micro-
tubules formed by the fully rigid three-armed molecules are
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L. Li et al.
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ACHTUNGTRENNUNG
tron-radiation X-ray scattering station with a radiation wavelength of
0.154 nm and Mar 345 image plate as a detector at NSRL (China). Ori-
ented samples were obtained by fluid shear in a 1 mm Kapton-film tube
with a sample holder. As the area of the imaging plate cannot cover
SAXS and WAXS data at a single sample-to-detector distance, the
sample-to-detector distance was set at 400 and 200 mm to measure the
SAXS and WAXS data, respectively, during which the sample was fixed
on the sample holder without any movement. Some of the SAXS meas-
urements were carried out by using an in-house setup with a 30 W micro
X-ray source (Incoatec GmbH) and a multiwire proportional chamber
detector (Bruker Hi-star). The radiation wavelength was 0.154 nm. The
Fit2D software package was used to analyze the 2D SAXS/WAXS pat-
terns.
Acknowledgements
The authors would like to thank Prof. Goran Ungar (Sheffield), Prof.
Wim H. de Jeu (Amherst) and Prof. Lihua Yang (Hefei) for helpful dis-
cussions. This work is supported by the National Natural Science Founda-
tion of China (51033004, 50973103, 51120135002 and 20904050), the Fund
for One Hundred Talents of CAS, 973 program of MOST
(2010CB934504) and the Fundamental Research Funds for the Central
Universities. SAXS and WAXS measurements were done in NSRL and
SSRF.
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Received: August 17, 2012
Published online: November 14, 2012
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