A skeletal double gyroid formed by single coaxial bundles of catechol based bolapolyphiles

Article abstract of DOI:10.1039/c8cc06956e

A bolapolyphile, derived from a linear p-terphenyl core with glycerol groups at each end and two octadecyl chains fixed to the same side of the π-conjugated rod, is synthesized and is found to form a new type of self-assembled cubic LC phase with the Ia3d lattice. Each unit cell involves 16 polar aggregates interconnected by 24 coaxial rod-bundles at the single-molecule length, forming two interwoven skeletal networks with three-way junctions, being separated by a continuum of the lateral alkyl chains located around the gyroid minimal surface.

Full text of DOI:10.1039/c8cc06956e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Skeletal double gyroid formed by single coaxial bundles of
catechol based bolapolyphiles
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
Silvio Poppe,† Changlong Chen,† Feng Liu* and Carsten Tschierske*
DOI: 10.1039/x0xx00000x
www.rsc.org/
11,12
A bolapolyphile, derived from a linear p-terphenyl core with blocks, such as electron-, hole- or ion-conducting units.
Gyroid
glycerol groups at each end and two octadecyl chains fixed to the phases involving different arrangements of π-conjugated rods are
same side of the π-conjugated rod, is synthesized and is found to known which are collated in Fig. 1b-d. In the most often observed
form a new type of self-assembled cubic LC phase with Ia3;¯d structure (b), formed by rod-like molecules, having alkyl chains or
1
3
lattice. Each unit cell involves 16 polar aggregates interconnected other bulky groups at the ends of the rods (polycatenar
1
4
by 24 coaxial rod-bundles at single-molecule length, forming two molecules ), the rods are arranged perpendicular to the networks
interwoven skeletal networks with three-way junctions, being and the clashing of the chains leads to a helical organization of the
1
5
separated by a continuum of the lateral alkyl chains located molecules along the networks.
around the gyroid minimal surface.
a)
b)
Two-dimensional (2D) and 3D crystal engineering has significantly
1,2
advanced in the recent decades, but the understanding of the
molecular self-assembly in soft matter systems, where the
individual molecules do not have fixed positions, is still at the
3
infancy. Though a number of fascinating superstructures was
observed for amphiphiles in aqueous systems and the reasons
4,5,6
forming those were explained by simple geometrical models,
a
c)
d)
fundamental knowledge of targeted molecular design towards
more complex soft matter structures is still missing. Cubic soft
matter phases represent examples of complex 3D structures which
7
have received significant attention. They can be formed by
bicontinuous networks with distinct valences of their branchings or
by micellar aggregates (bicontinuous and micellar phases,.
respectively) and the aggregates can either involve the lipophilic
chains or the polar groups and water, leading to normal and
8
inverted types, respectively. The double gyroid is probably the
Figure 1: a) The double gyroid cubic phase involving two networks (gray) separated by
the gyroid minimal surface (magenta). (b-d) The known arrangements of π-conjugated
rods (green) in the LC gyroid phase: b) helical organization of polycatenar molecules
most common cubic phase and is formed by two mutually
interwoven networks with three-way junctions and separated by
9
the gyroid minimal surface (Fig. 1a). Though this kind of (alkyl chains at both ends of the rods) perpendicular to the networks; c) organization of
bicontinuous cubic phases is well known for lyotropic systems and X-shaped molecules (chains in opposite lateral positions) with the π-conjugated rods on
1
0
the minimal surface and perpendicular to it, and d) coaxial rod-bundle phase formed by
T-shaped bolapolyphiles with a branched lateral chain, involving pairs of coaxial rod-
bundles forming the skeletal networks; adapted from Ref. 16.
as morphology in block copolymers, only recently it was used for
functional materials by inclusion of proper functional building
1
6,17
a.
More recently two new Ia3;¯d cubic phases
reported for rod-like molecules having bulky chains laterally
attached to the rod-like -conjugated core and hydrogen
were
Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-
Straße 2, 06120 Halle.
State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong
University, Xi’an 710049 (P.R. China)
These authors contributed equally to this work.
b.
π
†
bonding glycerol groups at the ends. As the lateral chains are
Electronic Supplementary Information (ESI) available: [Methods, syntheses,
analytical data, additional XRD and structural data]. See DOI: 10.1039/x0xx00000x
incompatible with the rods as well as the glycerols, these
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molecules involve more than only two incompatible building synthesis and the analytical data of
Journal Name
1
are described in the
blocks are considered as polyphiles, more precisely as Electronic Supporting Information (ESI).
bolapolyphiles, due to the position of the polar groups at both
3
,18
ends of the lipophilic core. Depending on the number and
position of the lateral lipophilic chains, T-shaped
a)
b)
1
bolapolyphiles, having only one linear or branched lateral
9
2
0
chain, and X-shaped bolapolyphiles, having two chains at
opposite sides, can be distinguished. The cubic phases with
Ia3;¯d symmetry formed by these molecules are shown in Fig.
1
6,17
1c,d.
In structure (c), found for X-shaped molecules, the
rods are organized in layers with saddle-splay curvature, thus
1
6
c)
d)
forming the minimal surface, whereas in structure (d), found
for T-shaped molecules with branched chains, the
conjugated rods are organized to form the skeletons of the
π
-
1
7
two infinite networks, i.e. orthogonal to case (b). Within this
cubic phase the three-way junctions of the two
interpenetrating networks are connected by pairs of end-to-
end connected bundles, each bundle being one molecule in
1
7
length and having ~12 molecules in the diameter.
O
I
I
+
Low
High
O
O
B(OH)2
C
18
H
37
O
OC18H
37
Figure 3: Characterisation of 1: a) continuous heating scan from crystal to Iso state via
Cub/Ia3;¯d phase, the inset shows the diffuse wide angle scattering of the Cub/Ia3;¯d
i
O
O
o
phase at 110 C; b) SAXS diffractogram of the Cub/Ia3;¯d phase at 100 °C; c)
O
O
O
O
O
O
reconstructed electron density map for the Cub/Ia3;¯d phase with added schematic
molecules, only high-electron-density regions (purple) are shown; d) surface-and-
contour plot of electron density distribution of [001] cross section, the continuous
colour scale is shown underneath.
C18H37
O
OC18H37
ii
HO
HO
OH
OH
Compound
1 was investigated by polarized optical
microscopy (POM), differential scanning calorimetry (DSC) and
powder X-ray diffraction (XRD) using a synchrotron source.
Phase transition temperatures and associated enthalpy values
are given in Fig. 2 (see Fig. S1 for DSC traces). The compound
shows an optically isotropic mesophase, appearing completely
dark between crossed polarizers (see Fig. S2) above the
melting point at T = 93 °C until the transition to the isotropic
C18H37
O
OC18H37
-1
-1
1
: Cr 93 °C [77.4 kJmol ] Cub/Ia3;¯d 128 °C [1.1 kJmol ] Iso
Figure 2: Structure, synthesis and phase transitions of the bolapolyphile
1.
Reagents and conditions: i) Pd(PPh , sat. NaHCO , H O, THF, reflux, 5h, 43%, ii)
2
3
)
4
3
HCl (10%), MeOH, THF, reflux, 6 h, 90%. Phase transitions were determined by liquid state at T = 128 °C. The transition from this isotropic
-
1
DSC on the first heating scan with 10 Kmin (peak temperatures); abbreviations: mesophase to the isotropic liquid is indicated by a relatively
-
1
Cr = crystalline solid; Cub/Ia3;¯d = double gyroid cubic phase with Ia3;¯
d
small transition enthalpy (1.1 kJmol ) and a sudden decrease
of viscosity, as typically observed for Cub-Iso transitions.
symmetry, Iso = isotropic liquid.
In the temperature range of the isotropic mesophase the
XRD wide-angle scattering is completely diffuse, like in the
liquid state, with a maximum at d = 0.46 nm (see the inset of
Fig. 3a), corresponding to the mean lateral distance between
the molecules. This indicates that there is no positional long
range order of individual molecules and this phase thus
represents an optically isotropic liquid crystalline (LC)
mesophase. In the small angle X-ray scattering (SAXS) pattern
Herein we report a new type of bolapolyphiles being
derived from a catechol based rod-like p-terphenyl core, which
can be considered as π-shaped. In the reported compound two
long identical alkyl chains are fixed at the same side of a p-
terphenyl rod and this compound exclusively forms a new type
of axial rod bundle cubic phase with Ia3;¯d lattice (see Fig. 3).
In this skeletal cubic phase only a single rod-bundle, instead of
1
7
two, as observed in all previously known cases, is involved.
Compound reported in this work was synthesized as shown
in Fig. 2 by a Suzuki cross coupling reaction of 1,4-diiodo-2,3-
(
Fig. 3a,b) there is a series of sharp Bragg peaks. The d-values
of the peaks show a characteristic ratio of 6: 8: 14: 16:
20: 22: 24: 26: 32, which is indexed to the (211), (220),
321), (400), 420), (332), (422), (431) and (440) reflections of a
cubic lattice with Ia3;¯d symmetry and a lattice parameter acub
7.76 nm (see Table S1). Based on this SAXS pattern the
1
√
√
√
√
2
1
√
√
√
√
√
2
2
dioctadecyloxybenzen with two equivalents 4-[(2,2-dimethyl-
(
2
,3-dioxolane-4-yl)methoxy] benzene boronic acid, followed
3
1
2
by deprotection of the glycerol groups. Details of the
4
=
electron density (ED) map, shown in Figs. 3c,d and S3, was
2
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1
7
reconstructed. The purple surfaces enclose the high ED regions structure. From compound
1
via the biphenyl
the chain volume to molecular length ratio
chain/Lmol increases (see Table S3). Accordingly, the LC phase
contain the alkyl chains (see Figs. 3d and S3). The ED histogram structure changes from the gyroid involving single rod-bundles
of the Ia 3;¯ lattice, supporting the chosen phase via the larger gyroid formed by pairs of rod-bundles to straight
2
to the
2
8a
involving the aromatic cores and the glycerol groups (see Fig. terphenyl
c,d), while the low-ED regions around the two networks
3
3
V
d
combination, is shown in Fig. S4. The two electron rich columns formed by quasi-infinite chains of rod-bundles.
networks of the double gyroid structure are formed by Besides the volume to length ratio, the molecular topology and
column-like segments connected by three-way junctions. shape change in this series of compounds too. For compound
Based on the lattice parameter acub, which slightly decreases
3, having the bulky chains at both ends the formation of
with increasing the temperature (see Fig. S5), the distance hydrogen bonding networks between the glycerol groups is
between the junctions was always calculated to ~2.7 nm using limited and only a linear organization of molecules can be
the equation djunct = acub/(2∙√2). This value is close to the formed. In the case of compound 2, the lateral group is
molecular length Lmol = 2.5 nm, measured between the ends of attached to one end of the molecule, thus making the two
the glycerol groups in the most extended conformation. The ends different. The glycerols adjacent to the lateral chain have
distance of the two networks is about 3.4 nm, which is much a limited capability of hydrogen bonding, thus forming linear
larger than Lmol. Therefore, an organization of the rods on the junctions between pairs of rod bundles. The glycerol group at
minimal surface with the glycerols forming the networks, as the non-substituted end is less shielded and thus allows the
shown in Fig. 1c, can be excluded. Accordingly, we suggest that formation of junctions with higher valence. In this way the
each columnar network segment is formed by a single bundle double gyroid cubic phase of the biphenyl compounds
of coaxial molecules, aligned with the molecular long axes formed by two end-to-end connected longitudinal rod-
parallel to the network channels, as shown in Figs. 3c and 4. bundles. But in compound , the lateral chains are attached to
The number of molecules in each cubic unit cell (ncell) is the middle of the terphenyl core, the polar glycerol groups at
2 is
1
3
3
calculated from the unit cell volume (Vcell = acub = 467 nm ) both ends are equal and both are easily accessible and capable
3
and the molecular volume (Vmol = 1.44 nm ; calculated with of forming multivalent nodes. This leads to a structure formed
2
5
Immirzis crystal volume increments ), and assuming a packing by single longitudinal rod-bundles connecting each of the
density in the LC state corresponding to the mean value three way junctions.
2
6
between the solid (0.7) and the liquid (0.55), to be ncell = 290
molecules (ncell = Vcell/Vmol). As there are 24 rod-like network
segments, each bundle contains 12 molecules in cross section
on average (see Table S2). This value is equal to that found for
1
7
27
other cubic (Ia3;¯d, Pn3;¯m ) and the hexagonal columnar
rod bundle phases.
2
8
Figure 4: Model showing the two networks at single-molecule length between the
junctions, combining spheres formed by the aggregation of the glycerol groups at the
junctions (the rod bundles are shown with a time- and space averaged circular cross
section).
Figure 5: Development of the phase structure of coaxial rod-bundle phases depending
on the molecular structure of the bolapolyphile; data for 2 and 3 were taken from refs.
1
7,28b; for calculation of Vchain/Lmol and transition enthalpies of 2, 3, see Table S3.
It is noted that compound
1 is the first example of a non-
The main difference to the previously reported cubic cases
is that only a single coaxial rod-bundle is connecting each of
the junctions. The rod-bundles themselves represent ribbons
of six back-to-back packed pairs of terphenyls which are
rotationally disordered to give a circular cross section on time
and space average. Figure 5 shows the development of the
type of coaxial rod-bundle phase depending on the molecular
fluorinated bolapolyphile capable of forming a coaxial rod-
bundle phase, meaning that this is a general mode of soft
molecular self-assembly of properly designed bolapolyphiles,
not requiring semiperfluorinated segments in the lateral
chains. The effect of chain fluorination is therefore assumed to
be mainly of steric origin. The Ia3;¯
d cubic phase of the
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bolapolyphile belongs to a newly emerging type of cubic
Journal Name
1
phases being bicontinuous as it is formed by two networks.
However, as it contains 16 discrete nano-segregated
spheroidic aggregates at the nodes of the networks (each
involving ~36 glycerol groups) it can simultaneously be
considered as a micellar cubic phase (Fig. 4). This hybridization
of speroidic and network organization makes the family of
coaxial rod-bundle phases more complex and unique among
the cubic LC phases.
9
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