Chirality Induction through Nano-Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self-Assembly of X-Shaped Bolapolyphiles

Article abstract of DOI:10.1002/anie.201911245

The single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks, hold a significant place in ordered nanoscale morphologies for their potential applications as photonic crystals, metamaterials and templates for porous ceramics and metals. Here, we report the first alternating network cubic liquid crystals. They form through self-assembly of X-shaped polyphiles, where glycerol-capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic side-chains fill their respective separate channel networks. This new self-assembly mode can be considered as a two-color symmetry-broken double gyroid morphology, providing a tailored way to fabricate novel chiral structures with sub-10 nm periodicities using achiral compounds.

Full text of DOI:10.1002/anie.201911245

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Chirality Induction through Nano-phase Separation: Alternating
Network Gyroid (I4132) Phase by Thermotropic Self-Assembly of
X-Shaped Bolapolyphiles
Authors: Changlong Chen, Robert Kieffer, Helgard Ebert, Marko
Prehm, Rui-bin Zhang, Xiangbing Zeng, Feng Liu, Goran
Ungar, and Carsten Tschierske
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201911245
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http://dx.doi.org/10.1002/ange.201911245

10.1002/anie.201911245
Angewandte Chemie International Edition
Chirality Induction through Nano-phase Separation: Alternating Network Gyroid
(I4132) Phase by Thermotropic Self-Assembly of X-Shaped Bolapolyphiles
Changlong Chen, Robert Kieffer, Helgard Ebert, Marko Prehm, Rui-bin Zhang, Xiangbing
Zeng, Feng Liu*, Goran Ungar*, Carsten Tschierske*
C. Chen, Prof. Dr. F. Liu, Prof. Dr. G. Ungar
State Key Laboratory for Mechanical Behaviour of Materials, Shaanxi International Research
Center for Soft Matter, Xi’an Jiaotong University, Xi’an 710049 (P. R. China)
E-mail: feng.liu@xjtu.edu.cn
Dr. R. Kieffer, Dr. H. Ebert, Dr. M. Prehm, Prof. Dr. C. Tschierske
Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Straße 2,
06120 Halle (Germany)
E-mail: carsten.tschierske@chemie.uni-halle.de
Dr. R. Zhang, Dr. X. Zeng, Prof. Dr. G. Ungar
Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD
(UK)
E-mail: g.ungar@sheffield.ac.uk
Dr. R. Zhang
Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018 (P. R. China)
Keywords: single gyroid; cubic liquid crystal; spontaneous chirality induction; columnar
liquid crystal; chiral structures
Abstract: The single gyroid phase as well as the alternating double network gyroid,
composed of two alternating single gyroid networks, hold a significant place in ordered
nanoscale morphologies for their potential applications as photonic crystals, metamaterials
and templates for porous ceramics and metals. Here we report the first alternating network
cubic liquid crystals. They form through self-assembly of X-shaped polyphiles, where the
1
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10.1002/anie.201911245
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glycerol-capped terphenyl rods lie on the gyroid surface while the semiperfluorinated and
aliphatic side-chains fill their respective separate channel networks. This new self-assembly
mode can be considered as a two-color symmetry-broken double gyroid morphology,
providing a tailored way to fabricate novel chiral structures with sub-10 nm periodicities
using achiral compounds.
New routes to chirality from initially achiral systems are of particular contemporary interest
for obtaining chiral templates in asymmetric synthesis and catalysis,[1] for use in different
fields of material- and nano-science[2] as well as for the understanding of the fundamental
principles of the emergence of biological homochirality.[3] Creating chirality in liquids and
liquid crystals (LCs), having no fixed positions of individual molecules, is especially
challenging.[4,5] Nevertheless, it was recently achieved by mirror symmetry breaking through
synchronization and locking-in of transient chiral conformations and configuration.[6] Here we
report a new approach to spontaneous generation of chirality, in this case based on nano-phase
segregation. In the reported case breaking the inherent mirror symmetry of the double gyroid
cubic phase (Ia3d, Q230), known from lyotropic[7] and thermotropic liquid crystals (LCs)[8]
(Figure 1a) is achieved by self-assembly of X-shaped polyphilic molecules with two different
chains at opposite sides of a rod-like molecular core.[8b] The cores organize along the gyroid
minimal surface, thus forming a wall that separates the two enantiomeric infinite networks
involving these chains. Nano-phase separation of the two poorly compatible side-chains,
semiperfluorinated and aliphatic, into their own networks (blue and red in Figure 1b), gives
rise to a gyroid cubic phase with two chemically non-equal networks (the ’single gyroid’
I4132, Q214). This structure has broken mirror symmetry and represents the first alternating
network gyroid cubic LC, and the first LC with chirality solely based on phase separation.
Previous attempts to produce single gyroid structure were based on replication from butterfly
2
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10.1002/anie.201911245
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wings,[9] lithography[10] and templating.[11] The alternating double network gyroid was found
in narrow composition ranges of multiblock copolymer blends.[11,12] in all cases leading to
structures in the >100 nm range. The new concept reported herein provides a tailored way to
fabricate chiral structures with much smaller sub-10 nm periodicities, which are of great
potential in nano-templating and as enantiospecific membranes for use in enantiomer
separation.
a)
b)
Ia3d (achiral)
I4132 (chiral)
Figure 1. a) The double-network gyroid Ia3d cubic phase and b) the alternating double
gyroid cubic phase involving two networks comprising different components separated by the
G minimal surface (this work).
To this end a series of X-shaped molecules has been specifically designed and synthesized.
These molecules are based on a p-terphenyl core terminated by two hydrogen-bonded polar
glycerol groups, and bearing two laterally attached incompatible chains, i.e. an aliphatic
hydrocarbon chain (RH) and a semiperfluorinated chain (RF) containing a short -(CH2)n-
spacer and a long perfluorinated end segment -CmF2m+1 (see Table 1). For details of synthesis
and experimental procedures of investigation, see Supporting Information (SI). Most
compounds form enantiotropic LC phases except for compound 1c which only exhibits a
monotropic soft crystal phase (see Table 1). Of main focus here are compounds 1b, 2 and 3
which all form a novel alternating double network gyroid phase with lattice parameters below
10 nm upon both heating and cooling.
3
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Table 1. Structure information, transition temperatures (T/°C), associated enthalpy changes
(H/kJ mol-1, lower lines, in italics) and lattice parameters of X-shaped molecules 1-3.
Comp.
RH
n
RF
Phase transitions[a]
Lattice
parameter
/nm[b]
Volume fraction
of side chains[c]
(%)
1a
1b
1c
2
-C20H41
-C20H41
-C20H41
-C20H41
-C22H45
6
6
6
4
4
-C8F17
-C10F21
-C12F25
-C12F25
-C10F21
Cr1 18 Cr2 55 Colhex/p6mm 81 iso
6.89
9.19
6.32
8.70
8.88
62.7
64.5
66.2
64.9
64.0
1.1* 4.2*
2.5
G 24 Cub/I4132 88 iso
2.0
-
Cr 106 (CrLam 103) iso
2.0*
11.5
Cr 104 Cub/I4132 110 iso
11.4 1.9
3
Cr 35 Cub/I4132 102 iso
16.0 2.4
[a] Recorded from first DSC heating at 10 K·min-1 (see Figure S1) and POM; brackets mean
[b]
metastable phase (only observed upon heating); Determined by synchrotron powder small
[c]
angle X-ray scattering;
Volume fraction of side chains measured using Material Studio.
Abbreviations: Cr, Cr1, Cr2 = crystalline solid; G = glassy solid; Col/p6mm = Hexagonal
columnar phase with p6mm symmetry; Cub/I4132 = alternating double network gyroid cubic
*
phase with I4132 symmetry; CrLam = lamellar soft crystal phase; Iso = isotropic liquid.
Partial crystallization.
As the length of the perfluorinated side chain is increased, compounds 1a-c exhibit a phase
sequence Colhex/p6mm – Cub/I4132 – CrLam (Table 1 and Figure 2a-c). Compound 1a, bearing
the shortest semiperfluorinated chain, is found to form a hexagonal columnar phase showing a
birefringent fan-like texture interspersed with black homeotropic regions (Figure S2a). The
sharp Bragg peaks in SAXS are indexable on a 2D hexagonal lattice with unit cell parameter
ahex = 6.89 nm, and showing only diffuse scattering in the wide angle region, characteristic of
a LC (Figures 2a, S3 and Tables 1, S1). The lattice parameter corresponds to about three times
the molecular length (Lmol = 2.3 – 2.6 nm measured between the two terminal polar groups).
The electron density map reconstructed based on p6mm (Figure S10) shows a partly
segregated two-color tiling composed of a lower-density (alkyl) column and two higher
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density (mixed) columns.[13] The analysis of the two-color Colhex phase is described in
Section 5 of SI. The aromatic cores make up the walls between the columns with glycerol
groups forming the hydrogen bonding networks at cell edges (Figure 4g).
Figure 2. a-c) Synchrotron SAXS diffractograms of the mesophases of compounds 1a-c at the
given temperatures. d-g) Reconstructed electron density maps in 2×2×2 unit cell. The
boundaries are estimated by the volume fractions of each segment: d) network formed by
semiperfluorinated chains; e) network composed of the aliphatic chains; f) gyroid minimal
surface formed by molecular cores; g) overall view of the electron density map (purple =
perfluorinated chains, cyan = rod-like cores of aromatic and polar segments, red = alkyl
chains). h, i) Views along a right-handed (h) and a left-handed (i) 41 screw axis of the
Cubbi/I4132 phase, illustrating the opposite chirality of the two networks.
As the side chain length is increased, the honeycomb is replaced by new mesophases. The
mesophase in compound 1b, having a slightly longer semiperfluorinated chain, grows with a
completely dark texture under crossed polarizers (Figure S2b). This isotropic mesophase has
high viscosity, which is typical of a cubic phase. The reciprocal d-spacings from the SAXS
reflections are in the ratio 21/2 : 61/2 : 81/2 : 141/2 : 221/2 : 261/2 : 301/2 : 341/2: 361/2 : 381/2 : 421/2
and could be indexed on a body-centered cubic lattice (for Miller indices see Figure 2b).
Alternative indexing on a primitive Bravais lattice, with the first reflection as (100) instead of
(110), would have had the a/d of the 4th reflection equal to 71/2 instead of 141/2, a (h2+k2+l2)1/2
value unobtainable for any combination of Miller indices. Thus a primitive lattice is excluded.
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Furthermore, due to the clear absence of the (200) reflection (a/d = 41/2), only one space group,
I4132, satisfies the observed extinction conditions, which include the 41 screw axis condition
00l: l = 4n. Note that the 9th peak can be indexed as either (600) or (442), hence its presence
in powder SAXS does not violate the screw-axis condition. Just to safeguard ourselves against
the remote possibility that the absence of (200) is coincidental, we constructed the electron
density map using Im3m symmetry. However in this case we obtain a body centered micellar
structure with the fluorinated chains forming the micelles (Figure S11a); the high curvature of
the micelles is unlikely considering the molecular structure, volume fractions and dimensions.
Accordingly, the mesophase in compound 1b is assigned as a bicontinuous cubic with
symmetry I4132 (Figures 2d-g and S11b). Thus we have the first documented case of a I4132
cubic phase in liquid crystals, thermo- or lyotropic.
Since the intensities of all remaining reflections are smaller than 1% of the strongest (110)
reflection (Tables S2, S4 and S5), the electron density map is dominated by that reflection,
whose phase is either +π/2 or –π/2 rad. The map constructed using one of these phases is
simply the reverse of that obtained using the other, meaning that the maps are identical except
for a change of origin. Different representations of the map are shown in Figure 2d-g, where
purple color indicates the regions of high, and red the regions of low electron density. The
cyan intermediate density region follows the familiar gyroid surface of minimum curvature.
The map is closely related to those of the double gyroid phase, except that in the latter case
the color of the regions on both sides of the minimum surface would be the same, as both
networks in the Ia3d phase have either lower or higher density than the minimal surface,
depending on the compound. Thus we can conclude that, as in the double gyroid, in our I4132
phase there are two infinite networks with 3-way branched channels separated by the gyroid
surface. Only here one network contains the high-density RF and the other the low-density RH
chains (Figure 2d, e, respectively). In fact the high-density network is of a core-shell type,
with the perfluorinated chain ends in the center of the channels surrounded by the short
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aliphatic spacers. The gyroid surface is composed of the glycerol-terminated cores lying
within it (Figure 2f, g).
Figure 3. AFM phase images of the I4132 phase of compound 3 recorded at 40C after
cooling in situ from 110C (Iso) to 90C at 0.5 K/min and then at 3 K/min to 40C. (a) a (110)
plane, (c) a (111) plane (Fourier filtered). (b) and (d) show respectively (110) and (111) cuts
through an electron density map (the 2×2×2 = 8 unit cell box). See also frontal view of (b)
with marked dimensions in Figures S12. (e) Model of the surface layer of unit cells showing
network segments as rods. The white domes are the suggested blobs of aggregated RF chains
seen as dark dots in (c).
The distance between the two 3D networks is acub×31/2/4 = 4.0 nm, which is the same as the
distance between the columns (the prismatic honeycomb cells) of the hexagonal phase of
compound 1a, which is ahex/31/2 = 4.0 nm (Figures S10 and S12). This equivalence is to be
expected, as in both cases we have an “inverse” thermotropic LC phase in which columns of
flexible chains are surrounded by “walls” of rigid aromatic-glycerol rods; in the columnar
case these are the honeycomb cell walls and in the cubic case the wall is the minimal surface
of the same constitution.
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To confirm our structure assignment in real space, compound 3 was imaged at temperatures of
the cubic phase by AFM. Images of two different crystallographic planes, (110) and (111), are
shown in Figure 3a, c. Phase contrast is due to the difference in shear modulus between the
stiffer RF chains (dark) and the softer RH chains (light). A comparison with the corresponding
cuts through the electron density map (Figure 3b, d) confirms the general correspondence in
geometry of the ED slices and the AFM images, the two methods agreeing in spacings and
angles within 3%. The measured distance between the rows of motifs in the (111) plane is
11.2 nm (Figure 3c) while the value calculated from SAXS is a6/2 = 10.9 nm, with the rows
inclined by exactly 60; the measured distance between the rows in the (110) plane of 7.0 nm
(Figure 3a) and the angle of 75 also compares well with the values of 6.8 nm and 73
measured from the (110) section through the map in Figures 3b and S14. However, while the
position of the dark RF spots on the triangular lattice in Figure 3c matches well with those of
the centers of the 3-arm stars in Figure 3d, the star-like feature is absent in the AFM image.
We attribute this discrepancy to surface reconstruction, as broken network segments at the
surface coalesce in blobs (white domes in Figure 3e) to minimize their surface energy.
Having established the structure of the new cubic LC, it is important to note that I4132 is a
chiral space group, and the phase is therefore chiral, even though the compounds forming it
are achiral. The chirality comes from the fact that the two enantiomorphous networks are
chemically different (see Figure 2h, i) which breaks the mirror symmetry. In contrast, the
achiral double gyroid Ia3d phase, having two identical but enantiomorphous networks,
possesses a number of glide planes. This is the first case of chirality induction in LCs being
exclusively mediated by nano-phase separation8b,14 of two chemically different molecular
segments (the alkyl and perfluoroalky chains) into distinct nano-scale domains (the two
networks).
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As in other cases of spontaneous mirror symmetry breaking, chirality is likely to be confined
to individual cubic domains and, in the absence of an external chiral bias, is likely to be a
conglomerate. However, as in the previous case of the chiral triple-network cubic phase of
achiral compounds, one chirality tends to win, eventually spreading over the entire sample in
a variant of Ostwald ripening.[6,15]
Figure 4. Phase sequence of polyphilic X-shaped molecules with increasing side chain
volume from left to right: (a, d, g) hexagonal columnar honeycomb, (b, e, h) alternating
network gyroid cubic, (c, f, i) lamellar phase. (a) schematic of two of the six sectors of a
hexagonal honeycomb cell; (b) average cross-section of a cubic network channel close to a
horizontal junction; (c) part of a (aliphatic) layer of the lamellar phase. (d, e, f) A column, a
network junction and a layer compared. (g, h, i) Perceived molecular arrangement in the three
phases: green = terphenyl cores; light green = glycerol groups; purple = semiperfluorinated
side chains; red = alkyl chains (side-chain regions are shown as continua).
In contrast to the previously reported cases of cubic and other optically isotropic LC phases
[refs. 5,6,15] optical activity or conglomerate formation cannot be observed by chiroptical
methods in the I4132 cubic phase reported here. The reason is that the chromophore is located
on or close to the gyroid minimal surface, which is achiral, whereas the two chiral networks
are filled with the alkyl chains and the fluorinated chains, respectively, whose absorption is
far away from the spectral range investigated either by polarizing microscopy or by CD.
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Hence, optical rotation and CD are negligible, leading to optical cryptochirality.16,17 However
optical activity and CD are only consequences of the lack of mirror symmetry and are not
inevitable, whereas XRD provides a direct proof of chirality, and works irrespective of
whether there is only one domain or a conglomerate. Because very strong peaks were
observed, that are forbidden in the achiral Ia3d lattice, there is no doubt that mirror
symmetry is broken.
Further extension of perfluorinated side chains removes the cubic phase and replaces it with a
monotropic soft crystal phase in compound 1c having the largest total side chain volume. This
phase appears on cooling and its SAXS Bragg peaks indicate a layer structure with a d = 6.32
nm layer thickness. A single sharp peak in the wide-angle region at d = 0.51 nm suggests a
hexagonal arrangement of the perfluorinated chains lying perpendicular to the layers. Thus,
compound 1c exhibits a soft lamellar crystal phase with ABCB stacking of four sublayers,
where A = disordered aliphatic layer, B = disordered aromatic-glycerol layer and C =
crystallized fluorinated layer (Figures 4i and S13). The fact that the alkyls are molten while
the fluoroalkyls are not can be understood by comparing the melting points of polyethylene
and Teflon, i.e. ca. 130C vs ca 300C, respectively. In fact between 30C and 300C Teflon
forms a hexagonal columnar phase[18] with hexagonally arranged chains with irregular helix
reversals.[19]
The phase sequence Colhex – Cubbi – Lam with increasing side-chain volume is consistent
with the established effect of molecular geometry, once it is accepted that the present
mesophases are of the “inverse” thermotropic kind. The present molecules could be regarded
as having a tapered shape, but with the chain ends located on average near the narrow end and
the rigid rod spanning the wide end of the wedge – see Figure 4a-c. This is the opposite of the
usual case of wedge-shaped mesogens where the aromatic is at the apex and the multiple
attached chains fan out at the wide end.[20] For such “normal” wedges it has been shown that
a key determinant of the adopted phase type is the dV/dr function, describing how volume
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10.1002/anie.201911245
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increases as one moves along r from the apex (r = 0) toward the wide end of the wedge.[21]
.
The same principle can be applied to the present inverse wedges. As a first approximation, the
cross-section area A of the wedge is A(r)  dV/dr  rq. Here q  1 (slice or triangle shape)
suites a columnar phase (Figure 4a), q  0 (rectangle, i.e. matching aromatic and aliphatic
cross-sections) suits a lamellar phase (Figure 4c), while 0 < q < 1 (shield shape) is most likely
to adopt a Cubbi phase (Figure 4b).[22] It is worth noting that a network of branched columns
in a Cubbi phase is intermediate between straight columns and layers, as depicted in Figure
4d-e. A more detailed schematic of the molecular arrangement in the three phase types in the
present compounds is given in Figure 4g-i.
In conclusion, we present the first I4132 alternating double network gyroid cubic LC.
The work demonstrates a novel way of inducing chirality, through phase separation in
chemically non-chiral systems. The network segments reported here are on the scale of only a
few nm between junctions, which is of great potential in nano-templating,[12d] for example as
enantiomer-specific membranes[8c] for use in enantiomer separation and production and
manipulation of circularly polarized light, chirality switching through thermally, chemically
or light-induced mesophase transitions.[ 23 ] Moreover, the present study shows how the
principle of self-assembled multicolor tiling can be extended from two dimensions (columnar)
to three dimensions (cubic),[13, 24 ] providing a new way to fabricate complex nano-
architectures.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The work is supported by the National Natural Science Foundation of China (Nos.
21761132033, 21374086, 21674099), the Deutsche Forschungsgemeinschaft (392435074)
and EPSRC (EP-K034308, EP-P002250). The authors are grateful to station I22, Diamond
Light Source and Beamline BL16B1 at SSRF (Shanghai Synchrotron Radiation Facility,
China) for providing the beamtime.
11
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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10.1002/anie.201911245
Angewandte Chemie International Edition
Self-sorting of the two poorly compatible side-chains of X-shaped polyphiles into their own
networks gives rise to a chiral gyroid cubic phase (the ’single gyroid’ I4132) with two
chemically non-equal networks. This is the first alternating network gyroid cubic liquid
crystal, and the first liquid crystal with chirality solely based on phase separation, allowing
the self-assembly of chiral structures with sub-10 nm periodicities.
Keywords: liquid crystals, single gyroid, chirality, mirror symmetry breaking, soft matter
Changlong Chen, Robert Kieffer, Helgard Ebert, Marko Prehm, Rui-bin Zhang, Xiangbing
Zeng, Feng Liu*, Goran Ungar*, Carsten Tschierske*
Chirality Induction through Nano-phase Separation: Alternating Network Gyroid (I4132)
Phase by Thermotropic Self-Assembly of X-Shaped Bolapolyphiles
14
This article is protected by copyright. All rights reserved.