Formation of a Cubic Liquid Crystalline Nanostructure with π-Conjugated Fluorinated Rods on the Gyroid Minimal Surface

Article abstract of DOI:10.1002/chem.201700905

Bicontinuous cubic phases are of significant importance for numerous applications, for example, for the crystallization of membrane proteins, as photonic materials and as templates for porous silica. A new variant of bicontinuous cubic liquid crystalline

Full text of DOI:10.1002/chem.201700905

A Journal of
Accepted Article
Title: Formation of a cubic liquid crystalline nanostructure with π-
conjugated fluorinated rods on the gyroid minimal surface
Authors: Marko Poppe, Changlon Chen, Feng Liu, Silvio Poppe, and
Carsten Tschierske
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Formation of a cubic liquid crystalline nanostructure with π-conjugated fluorinated
rods on the gyroid minimal surface
Marco Poppe,[a, ‡] Changlong Chen,[b, ‡] Feng Liu,[b,*] Silvio Poppe[a] & Carsten Tschierske[a,*]
[a] M. Poppe, S. Poppe, Prof. Dr. C. Tschierske
Department of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Str.
2, 06120 Halle, Germany
E-mail: carsten.tschierske@chemie.uni-halle.de
[b] C. Chen, Prof. Dr. F. Liu
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, P. R. China
E-mail: feng.liu@xjtu.edu.cn
‡ These authors contributed equally to this work.
Abstract: Bicontinuous cubic phases are of significant importance for numerous applications,
for example, for the crystallization of membrane proteins, as photonic materials and as
templates for porous silica. A new variant of bicontinuous cubic liquid crystalline phase with
Ia3¯d lattice is reported here for X-shaped bolapolyphiles. It is shown that in this class of
compounds cubic phase induction can be achieved by proper aromatic core fluorination; in
addition, the first cubic phases having π-conjugated oligo(phenylene ethynylene) rods on the
gyroid minimal surface were obtained. These new structures composed of 3D folded layers of
parallel arranged π-conjugated rods should allow charge transport in all three dimensions,
which is of interest for organic semiconductor applications.
Among nanoscale morphologies the gyroid type bicontinuous cubic phase with Ia3¯d space
group, occurring at the transition from lamellar to columnar organization, is one of the most
intriguing modes of amphiphilic self-assembly observed on different length scales in
numerous systems ranging from thermotropic and lyotropic liquid crystals to block copolymer
morphologies.[1,2] As shown in Figure 1c there are two interwoven networks separated by the
1
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gyroid infinite minimal surface.[1-3] Typically the molecules forming these cubic phases are
binary amphiphiles where the interfaces developing by nano-segregation of the incompatible
segments assume saddle splay curvature.[4,5] Increased complexity arises if rigid linear
segments are incorporated,[4] namely in rod-like molecules having bulky chains, like
oligo(ethylene oxide), oligo(propylene oxide) (rod-coil molecules),[5a] siloxanes,[5b]
fluorinated chains[5c-f] and long,[2a] branched[5d] or multiple alkyl chains at the ends
(polycatenar molecules).[6,7] In the cubic phases of these molecules the rod-like units are
organized in two interwoven networks and the flexible chains fill the spaces between them. In
the networks the rods are arranged almost perpendicular to the networks with opposite helical
twist between them (Figure 1a).[8]
Another kind of Ia¯3d phase was recently reported for rod-like T-shaped molecules
with terminal glycerol groups and branched semi-perfluorinated lateral chains. In these cubic
phases the rod-like units form bundles of molecules (Figure 1b) aligned with their long axes
parallel to the networks.[9-11] Whatsoever in all these previously reported Ia3¯d phases of rod-
like molecules, the gyroid minimal surface, separating the two networks, is only formed by
disordered alkyl chains.
<Figure 1>
Here we report a new variant of bicontinuous Ia3¯d cubic structure, having π-
conjugated rods organized on the gyroid minimal surface. It is obtained by fluorination of the
oligo(phenylene-ethynylene) core of the X-shaped polyphile H (Figure 2a, all X = H) forming
a triangular honeycomb liquid crystalline (LC) phase (Colhex phase) in a small temperature
range (Figure S11, Table S9).[12a,b] In this LC phase quasi-infinite triangular prismatic cells
are formed by the packing of the aromatic cores which are fused to a triangular honeycomb by
the hydrogen bonding networks of the glycerol groups at both ends, running along the edges;
the lateral chains fill the interior of the honeycomb cells (Figure S12).[12-14] It is shown herein
that core-fluorination[15] completely changes the mode of self-assembly, leading to
bicontinuous cubic phases. In the compounds Fn fluorine substituents were added to the two
outer benzene rings of compound H and the substitution pattern is identical at both ends. In
the compounds names the subscripts indicate the positions of the fluorine substituents being
either peripheral (directed to the glycerol units, positions 3 and 5) or directed to the core of
the molecule (positions 2 and 6).
2
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<Scheme 1, 2>
Compounds bearing one (F2, F3) or two fluorine atoms (F23, F26 and F35) at each end
have been synthesized as shown in Scheme 1, where in a sequence of Sonogashira couplings
the core unit is built up first[16] and the glycerol units are attached in one of the final steps by
etherification with 1,2-isopropylidene-3-p-toluenesulfonyl-rac-glycerol. For compound F2356
involving two perfluorinated benzenes the synthesis starts with a nucleophilic aromatic
substitution of pentafluoroiodobenzene with 1,2-isopropylideneglycerol,[17] followed by
stepwise elongation of the aromatic core by successive Sonogashira couplings (Scheme 2). In
both routes the deprotection of the terminal glycerol units is the final step (see supporting
information).
Figure 2a shows the phase transitions on cooling for all fluorinated compounds Fn in
comparison with the non-fluorinated compound H.[12a,b] Core fluorination reduces the melting
and crystallization temperatures and widens the LC ranges (Table S1). With exception of H
and F26 all compounds show optically isotropic mesophases with high viscosity, as typical for
cubic phases. The structure of the cubic phases was determined by XRD with synchrotron
radiation. In the WAXS region there is only diffuse scattering (Figure S10), indicating the LC
nature of these cubic phases, i.e. there is no long range positional order for individual
molecules. In the SAXS patterns there are series of sharp reflections with reciprocal spacings
½
½
½
½
½
½
in a ratio of 1/d-values 6 : 8 : 20 : 22 : 24 : 26 (Figures 3a-c and S9) which were
indexed to a centred cubic lattice with space group Ia3¯d. The cubic lattice parameters are
around 9.1 nm for all compounds (Tables S2-S5). Based on the parameter the number of
molecules organized in each unit cell, calculated by dividing the unit cell volume acub3 by the
molecular volume (calculated with Immirzi’s crystal volume increments[18]) is almost the
same, about ca 450 molecules (Table S12).
<Figure 2>
The distance between the junctions of the networks is 3.2 nm which is much shorter
than the molecular length Lmol = 4.4 nm, measured between the ends of the glycerol groups in
the most stretched conformation (Figure 2b). Even an organization with fully intercalated
glycerol units would lead to a distance of 3.8 nm [(Lmol+Lcore)/2], still significantly exceeding
the distance between the junctions. Thus, an organization in rod-bundles along the networks
(Figure 1b, but having only one rod-bundle between each junction) can be excluded. The
3
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lateral distance between the two interwoven networks is acub  31/2/4 ~ 3.9 nm (Table S12).
This value is close to the layer distance in the SmA phase of compound F26 (d = 3.7 nm) and
agrees well with the molecular length in a structure with intercalated glycerol groups,
meaning that the organization of the aromatic cores is perpendicular to the gyroid minimal
surface. However there are two different possibilities, either the polar glycerol groups form
the networks or the minimal surface. To distinguish these possibilities electron density (ED)
reconstruction was performed for the Ia3¯d phases of compounds F3, F35 and F2356 having
different degree of fluorination. There are clear differences in the relative intensities of the
reflections at larger q-values compared with the intensity of the strongest (211) scattering
(Figures 3a-c and S9), indicating a changing electron density distribution. Independent on the
chosen level of electron density (ED = 120, 200 respectively; note: ED is assigned from 0 to
255 after normalization for ED reconstruction, the latter one is the highest), the diameter of
the high ED networks increases from F2 (blue) via F35 (green) to F2356 (red) meaning that
compound F2356 with four fluorine at each peripheral benzene ring always has higher ED
networks with larger diameter compared with its analogues compounds with lower degree of
fluorination of the terminal benzene rings (Figure 3d-f). This confirms that the glycerol
groups form the networks and the aromatic cores are positioned between them on the minimal
surface. (Figure 1d). The area of the gyroid surface per unit cell is calculated to be about 203
nm2 [3b] available for the ~450 molecules and meaning that the cross sectional area of each
,
molecule is 0.4 nm2 (see Table S12), being in line with the proposed organization of the
molecules perpendicular to the minimal surfaces where the alkyl chains and aromatic cores
have a similar cross sectional area of ~0.2 nm2, agreeing well with the cross-sectional areas of
these units in the LC state.[5f,19a,20] More details of ED reconstruction, phase selection and
structure determination can be found in section 2.5 of the SI.
<Figure 3>
The diffuse wide angle scatterings in the cubic phases of all compounds Fn have a
non-symmetric shape with a shoulder which can be fitted to a distance d = 0.32-0.35 nm
(Figures 4a and S10), indicating some contribution of face-to-face π-stacking interactions
between the aromatics. The shoulder is completely missing in the Colhex phase of F2 (see
Figures 4a and S13b). Thus, it appears that π-stacking interaction develops in the Ia3¯d cubic
phase due to a restricted rotation and denser packing of the electron deficit fluorinated rings.
This scattering is relatively weak and diffuse, which might be due to the limited correlation
4
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length and the inherent saddle-splay curvature in the cubic phase. Nevertheless, it indicates
that (at least local) segregation of aromatic cores and alkyl chains indeed exists. This
segregation is expected to lead to triply-segregated network structures [21] or local domains in
the continuum. There is first indication for such domains in the ED maps of F3 (see Figure
S7d), but this requires further studies.
<Figure 4>
Analysis of the data in Figure 2a reveals that proper fluorine substitution can induce
and stabilize bicontinuous cubic phases. This is remarkable, as for rod-like 4-alkoxy-4’-
stilbazole complexes of silver dodecylsulfates the opposite effect was found;[6b] cubic phases
of the non-fluorinated complexes were removed by peripheral fluorination and the cubic
phase range was narrowed by inside directed fluorination. In contrast, all compounds Fn
having fluorine atoms in both peripheral positions 3 and 5 at each end (F35 and F2356) form
exclusively the cubic phases. Compounds bearing only one fluorine in the peripheral positions
(F3 and F23) form cubic phases, too, but on cooling a small range of a birefringent Colhex is
observed before the cubic phase is formed (Figure S2). However, on heating there is a direct
transition Cub-Iso (Table S1), meaning that the cubic phase is the thermodynamically stable
phase and the Colhex phase is only metastable (therefore shown in brackets in Figure 2a).
Especially remarkable is the broad cubic phase range of F2356 with two tetrafluorinated
benzene rings, because for other rod-like[17] and polycatenar mesogens[6e] with this
substitution pattern mesophase stability is known to be reduced and nematic phases were
usually preferred.
Steric, polar and electronic effects of core fluorination are considered as the
preliminary origins of the transition from triangular honeycombs to the cubic phases. For rod-
like molecules peripheral core substitution favours smectic phases and inside directed
substitution distorts layer formation, thus leading to nematic phases.[6b,19] Because the
molecules Fn are arranged parallel to each other in the saddle-splay deformed layers of the
Ia3¯d phases any stabilization of smectic phases, as for example provided by core-core
interactions involving the electron deficit fluorinated aromatics and electrostatic interactions,
should stabilize these cubic phases, too.[15,22] However, for compounds Fn these core-core
interaction could, in principle, also stabilize the organization of the aromatics along the
honeycomb walls. Therefore, the situation becomes more complex and especially steric
effects, depending on the position of fluorination[20] and destabilizing the honeycombs, need
5
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to be considered. In the case of peripheral fluorination (F3, F35, F23 and F2356) the narrow
vertices in the triangular honeycombs are obviously unfavourable for this kind of self-
assembly due to the clashing of the fluorine substituents (Figure 4b). Hence, fluorine
substitution destabilizes the Colhex phases and supports an intercalated packing of the glycerol
units (Figure 4c), which reduces the interface curvature between polar and nonpolar nano-
segregated regions, thus favouring the formation of the saddle-splay lamella in the Ia3¯d
phases (Figure 4c). Figure 5 shows that replacing the peripheral fluorine of F3 by the larger
chlorine completely removes the honeycomb, leaving only the cubic phase. Nevertheless, the
larger size of Cl reduces the mesophase stability and the even larger (and non-polar) methyl
group completely removes the columnar as well the cubic phase, probably by distorting the
hydrogen bondings between the glycerols.
On the other hand, fluorine substituents directed towards the core region of the
molecules (F2, and F26) distort the lateral core-core packing, as known for rod-like molcules
where nematic phases are formed.[19] For compounds Fn this substitution pattern leads to the
lowest LC phase stabilities and depression of the cubic phases (Figures 2, S13 and S14).
Simultaneously, the disturbance of the glycerol packing in the triangular prismatic cells is
reduced for F2, which retains the Colhex phase over a broader temperature range. For F26 the
strong distortion of the core-packing by in total four of these core-directed substituents
disrupts the triangular honeycomb, leading to a lamellar (SmA) phase with the lowest phase
stability and fully mixed organization of alkyl chains and aromatic cores in layers (diffuse
WAXS without any shoulder, see Figure S14b) separated by the polar glycerol layers. For
compounds F23 and F2356, combining peripheral with core-directed fluorination, the negative
effect of core directed fluorination on cubic phase formation and mesophase stability is
obviously compensated by the cubic phase stabilizing effects of the additional peripheral
fluorines.
<Figure 5>
In summary, a new variant of bicontinuous cubic self-assembly with Ia3¯d lattice is
found, which is different from, but complimentary to the organization of polycatenar
mesogens by swapping the aromatic cores from the networks to the continuum. The parallel
arranged π-systems on the infinite gyroid minimal surface should allow charge carrier
mobility in all three dimensions of space. Though this requires further engineering of the core
structure, this new type of cubic phases is of potential interest for organic semiconductor
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applications, as the quasi-infinite layers of π-conjugated cores should be less sensitive to
defects compared to the narrow and helical twisted networks[8] in the cubic phases of
polycatenars.[23] Moreover, this work provides a blueprint for the design of 3D continuous
assemblies with tethered nano-rods.[24]
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft (Ts 39/21-2) and the
National Natural Science Foundation of China (No. 21374086). We thank beamline BL16B1
at SSRF (Shanghai Synchrotron Radiation Facility, China) for providing the beamtimes.
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Scheme and Figure captions
Scheme 1. Synthesis of F2, F3, F23, F26 and F35.a
a Reagents and conditions: i) TiPS-Cl, Imidazole, DCM, rt, 90 – 95%, ii) [Pd(PPh3)4], CuI,
NEt3, reflux, 75 – 90%, iii) K2CO3, DCM/MeOH (2:1), rt, 35 – 65%, iv) Bu4NF, THF, 20 °C,
80 – 90%, v) K2CO3, DMF, 120 °C, 24 h, 60 - 75%, vi) PPTS, THF/MeOH (1:1), 55 °C, 60 –
70%.
a
Scheme 2. Synthesis of F2356
.
a Reagents and conditions: i) K2CO3, DMF, 40 °C, 3d, 59%, ii) NEt3, [Pd(PPh3)4], CuI, 40 °C,
94%, iii) K2CO3, DCM/MeOH (2:1), 65%, iv) PPTS, MeOH/THF (1:1), 40 °C, 55%.
Figure 1. Bicontinuous cubic mesophases with Ia3¯d space group formed by molecules
involving rod-like units (green rods): (a) polycatenar molecules;[8] (b) rod-like bolapolyphiles
with branched lateral chains;[10] (c, d) the Ia3¯d phases of compounds Fn: (c) reconstructed
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electron density map of the cubic phase of compound F2356, with the two identical infinite
networks (gray) divided by the gyroid minimal surface (purple); (d) organization of the rod-
like cores of the molecules on the minimal surface.
Figure 2. a) LC phases and phase transitions depending on the substitution pattern as
recorded on cooling (rate 10 K min-1) compounds H[12a,b] and Fn; abbreviations: Cr =
crystalline solid, Colhex = hexagonal columnar LC phase (triangular honeycomb phase, for
model see Figure S12); Cub/Ia¯3d = cubic LC phase with space group Ia3¯d; SmA = lamellar
LC phase; values in brackets indicate metastable phases which can only be observed on
cooling; for transitions on heating, DSC traces and transition enthalpies, see Table S1 and
Figure S1; b) CPK model of compound F2356 with molecular dimensions.
Figure 3. a-c) SAXS diffractograms (see also Tables S2-5 and Figure S9); d, e) reconstructed
electron density maps of the Cub/Ia3¯d phases of compound F2356 (red), F23 (green) and F3
(blue) showing the isoelectron surfaces for different level of electron density (ED); f) showing
the cross-sectional profile of the networks in e) using grey scale (white = high electron
regions), red, green and blue represent isoelectron surfaces enclosing the high electron density
regions for compound F2356, F23 and F3 respectively, consisting of glycerol groups and
fluorinated benzene rings.
Figure 4. a) Comparison of the diffuse wide angle scatterings of compound F2 in the Colhex
phase at T = 150 °C and F23 in the Cub/Ia3¯d phase at T = 160 °C (see also Figure S10); b, c)
sketches showing c) the organization of the molecules in the triangular honeycombs and d)
the intercalated organization of the glycerols induced by the peripheral fluorines, leading to
reduced interfacial curvature and transition to Cub/Ia3¯d; the space between the rod-like cores
is filled by the lateral chains.
Figure 5. Effect of replacing fluorine (cv = 12.8 nm3)[17] by the larger chlorine (cv = 26.7
nm3) or a methyl group (cv = 31.7 nm3) on the LC phases, as observed on cooling with 10 K
min-1; for numerical data, see Table S1.
TOC text:
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10.1002/chem.201700905
Chemistry - A European Journal
The power of fluorination. Tailoring the fluorination pattern modifies the soft self-assembly
of oligo(phenylene ethynylene) based X-shaped polyphiles, leading to the first thermotropic
liquid crystalline cubic phase with the -conjugated rods forming the gyroid minimal surface.
The organization of the rods perpendicular to the minimal surface and parallel to each other
allows -stacking interactions in all three dimensions of space.
Keywords: liquid crystal • self-assembly • cubic phase • oligo(phenylene ethinylene) •
fluorine substitution
Si O
H
HO
Br
i-iii
Fn
Fn
ii, iii
Br
TMS
OC12H25
I
Si O
H
I
Fn
ii, iv
C12H25
O
OC12H25
HO
OH
C12H25
O
Fn
Fn
O
TosO
O
v,vi
OC12H25
HO
HO
O
O
OH
OH
C12H25
O
Fn
Fn
Scheme 1.
Scheme 2.
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10.1002/chem.201700905
Chemistry - A European Journal
a)
b)
c)
d)
Figure 1.
Figure 2.
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10.1002/chem.201700905
Chemistry - A European Journal
d)
a)
q [nm-1
]
ED=200
e)
b)
ED=120
q [nm-1
]
f)
c)
ED = 120
q [nm-1
]
Figure 3.
a)
I
[a.u.]
2 [°]
Figure 4.
T [°C]
Figure 5.
13
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10.1002/chem.201700905
Chemistry - A European Journal
TOC Figure
14
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