Trigonal columnar self-assembly of bent phasmid mesogens

Article abstract of DOI:10.1039/c7cc06714c

Three compounds with a bent rod-like aromatic core and with three alkoxy chains at each end were synthesised by click reaction. The compounds form a columnar liquid crystal phase with non-centrosymmetric trigonal p31m symmetry, the columns having a 3-arm star-like cross-section.

Full text of DOI:10.1039/c7cc06714c

ChemComm
View Article Online
View Journal
COMMUNICATION
Trigonal columnar self-assembly of bent phasmid
mesogens†
Cite this:DOI: 10.1039/c7cc06714c
a
b
b
a
Huifang Cheng,‡ Ya-xin Li,‡ Xiang-bing Zeng,
Hongfei Gao,
a
bc
Received 27th August 2017,
Accepted 21st September 2017
Xiaohong Cheng * and Goran Ungar
*
DOI: 10.1039/c7cc06714c
rsc.li/chemcomm
Three compounds with a bent rod-like aromatic core and with polycatenar compounds in the triple-network cubic phase,
three alkoxy chains at each end were synthesised by click reaction. where the networks are also made up of short column
1
0
The compounds form a columnar liquid crystal phase with non- segments. Columns made up of more circular strata, contain-
centrosymmetric trigonal p31m symmetry, the columns having a ing discotic or fan-shaped mesogens, are also understood to
1
1–14
3
-arm star-like cross-section.
feature helical twist.
The question we address here is whether a trigonal colum-
The term ‘‘phasmid’’ mesogen is used to describe rod-like nar LC phase can be formed, with three-fold rather than six-fold
molecules with three flexible chains, usually alkyl, at each symmetry and with non-cylindrical columns. In honeycomb-
end, and the name refers to the similarity with the six-legged like inverted columnar phases of side-chain-containing rod-like
1
insects known by the same name. Usually such mesogens form amphiphiles, in some cases the cross-section of the prismatic
the hexagonal columnar (Colhex) liquid crystal (LC) phase, channels with aromatic walls is hexagonal; thus although the
where typically three aromatic rods lie parallel to each other columns are not circular, naturally the symmetry is still hex-
8
,9
and perpendicular to the column axis, forming a stratum of agonal, plane group p6mm. However, in two cases amphi-
2
the column; the alkyls fill the space between the columns.
philes of that kind were reported to form honeycombs
1
5,16
Phasmids are a subclass of a wider class of rod-like molecules with only three-fold symmetry (see below).
Such non-
with more than one chain at each end, known as polycatenar centrosymmetric structures are rare in liquid crystals but are
mesogens. Columnar phases, particularly the most commonly of interest for their potential ferroelectric, pyroelectric or non-
observed hexagonal phase, are normally assembled from disk- linear optical properties. The question remains whether such
3
–7
like or fan-shaped molecules.
Alternatively a variety of three-fold symmetry is possible in the more conventional
‘
‘honeycomb’’ columnar phases are seen in rod-like amphiphiles columnar phases where the hard aromatic moieties occupy
8
,9
with flexible side-chains. One may ask why phasmidic col- the centre, rather than the periphery of the column. Here
umns, with elongated rods lying normal to the column axis, we show that columns with a three-arm star cross-section
pack on a lattice with hexagonal symmetry, when such packing and trigonal symmetry can form from bent-core, or banana-
is normally reserved for round cylinders or hexagonal prisms. shaped phasmids. While the crystallographic plane group
The answer may be in that the successive strata in a column are belongs to the hexagonal system, the phase symmetry is
rotated, forming a helix, making the averaged cross-section actually trigonal.
circular. Such helical twist of polycatenar strata has recently
been suggested by the observation of chirality in non-chiral have a bend of 1091 in the centre due to the sp -hybridized
The aromatic rod-like core in the compounds described here
3
methylene group – see Scheme 1. The compounds are labelled
3
IC /n, with n = 10, 12, 14 the number of carbon atoms in each
a
Key Laboratory of Medicinal Chemistry for Natural Resources,
of the six terminal alkyl chains. They were synthesized by a
Chemistry Department, Yunnan University, Yunnan, 650091, P. R. China.
1
7
Cu(I)-catalyzed click reaction. All three compounds display a
liquid crystal phase, with the texture recorded by polarized
E-mail: xhcheng@ynu.edu.cn
b
c
Department of Materials Science and Engineering, University of Sheffield,
Sheffield, S1 3JD, UK. E-mail: g.ungar@sheffield.ac.uk
Department of Physics, Zhejiang Sci-Tech University, Xiasha College Park,
Zhejiang, 310018, China
optical microscopy (POM) featuring clear developable domains
18
(
‘‘spherulites’’), typical of a columnar LC phase – see Fig. 1.
The transition temperatures and heats of transition on 1st
heating, cooling and 2nd heating, determined by DSC in
combination with X-ray diffraction and nonlinear optical
†
Electronic supplementary information (ESI) available: Synthesis, analytical data,
molecular simulation video. See DOI: 10.1039/c7cc06714c
Both authors contributed equally to this work.
‡
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
of the diazonium salt and subsequent substitution with sodium
azide.
4
was esterified with the appropriate 3,4,5-trialkoxybenzoic acids
3
3
20
2
/n to 4-azidophenyl-3,4,5-trialkoxybenzoates 5 /n. Finally click
reaction between bisacetylene 8 and aromatic azides 5 /n produced
the target compounds IC /n. All of the target compounds were
3
3
purified by column chromatography. For more details, see ESI.†
X-ray scattering at small and wide angles (SAXS and WAXS)
experiments were performed to characterize the LC phases.
3
Scheme 1 Synthesis of compounds IC /n. Reagents and conditions:
i) KOH, CH CH OH, reflux, overnight; (ii) (a) NaNO /HCl, 0 1C, 1 h;
b) NaN
CH Cl , 0–5 1C, 18 h; (iv) LiAlH
bromide, acetone, reflux; (vi) tert-butanol, THF, H
CuSO O, 25 1C, 20 h.
ꢁ5H
(
3
2
2
(
3
, 0–5 1C, 5 h; (iii) dicyclohexylcarbodiimide, 4-dimethylaminopyridine, Grazing incidence SAXS (GISAXS) on well oriented thin films on
2
2
4 3
, AlCl , THF, 65 1C, 48 h; (v) KOH, propargyl
silicon facilitated indexing of X-ray reflections.
2
O, sodium ascorbate,
Furthermore, for any potential future application of such materials
in electronic or optical devices, orientation in thin film needs to be
understood. Fig. 2 shows a transmission powder SAXS and a GISAXS
4
2
3
pattern of the main liquid crystal phase, on the example of IC /12.
3
3
Equivalent patterns for compounds IC /10 and IC /14 are shown in
ESI,† Fig. S4–S6. The measured and calculated d-spacings for all three
ꢀ
2
compounds are listed in Tables S1–S3 in ESI.† The d values of the
three peaks in the SAXS pattern are in the ratio 1 : 3 : 4, which is typical
of a two-dimensional hexagonal lattice, with the respective Miller
indices (10), (11) and (20). The GISAXS patterns confirm this indexing,
as seen in Fig. 2b and Fig. S4a, S5a (ESI†). The fact that the reflections
are not confined to the equatorial plane means that the columns are
oriented parallel to the substrate surface (planar anchoring), rather
than perpendicular to it. This is consistent with the appearance of the
optical micrographs in Fig. 1 and Fig. S3 (ESI†). As the real space unit
cell is related to the reciprocal cell by 901 rotation, the fact that
reflections (10) and (20) are on the meridian means that the real
3
Fig. 1 Polarized optical microscopy textures of IC /14 at 70 1C. (b) is
recorded with a full-wave (l) plate. For explanation of colours see ESI.†
3
a
Table 1 The phase transition temperatures of compounds IC /n
ꢀ1
Compd
n
T/1C [DH/kJ mol
]
3
IC /10
10
mCr 69.6 [17.3] Cr
1
2
107.0 [33.3] Iso k 87.4 [1.2] p31m hexagonal cell lies on one of its sides on the substrate. That implies
5
2.8 [12.0] M m 56.5 [12.3] p3 + Cr 65.4 [2.4] p31m 90.3
1
that the (10) plane is parallel to the substrate, suggesting in turn that it
[
1.2] Iso + Cr
2
108.6 [3.3] Iso
2
1
3
is the most densely packed plane (see Fig. S9, ESI†). In compounds
IC /12
12
14
m Cr
5
1
86 [21.1] Cr
2
110.2 [41.5] Iso k 88.6 [0.7] p31m
110.3 [35.7] Iso IC /12 and IC /14 another metastable mesophase (M-phase) has been
3
3
8.3 [11.5] M m62.1 [11.6] p31m 89 Cr
mCr 38.7 [30.5] Cr 73 [23.1] p31m 82.7 [0.8] Iso k76.2
0.3] p31m 58.5 [14.6] M m 62.5 [14.7] p31m + Cr 68.2
1.0] p31m 83.9 [0.6] Iso
2
3
IC /14
1
2
observed in rapidly cooled samples. The GISAXS pattern is presented
in Fig. S7 (ESI†). The structure of this complex phase is not clear at
present, and is the subject of further investigations.
[
[
2
a
Peak DSC transition temperatures [and enthalpies] on 1st heating (m),
followed by cooling (k), followed by 2nd heating (m), all at 5 K min
ꢀ1
.
Cr , Cr = crystal, p31m = hexagonal columnar phase with three-fold
1
2
symmetry, Iso = isotropic melt, and M is a complex metastable
unidentified LC phase.
studies, are listed in Table 1. The DSC traces are shown in
Fig. S1 and S2 of ESI.†
3
All of the target compounds IC /n were synthesized by click
reaction between the bisacetylene 8 and the appropriate substituted
3
aromatic azides 5 /n (Scheme 1). Reduction of commercially avail-
0
19
3
able 4,4 -dihydroxybenzophenone 6 with LiAlH
,4’-dihydroxydiphenylmethane 7, which was etherified with pro-
pargylbromide to yield bisacetylene 8. 4-Azidophenol 4 was synthe-
4
/AlCl
yielded
4
3
Fig. 2 (a) Powder SAXS curve of IC /12 recorded at 80 1C at station I22 at
3
Diamond. (b) GISAXS diffraction pattern of IC /12 recorded at 80 1C on
sized from commercially available 4-aminophenol 3 by formation beamline B28 at ESRF.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017

View Article Online
ChemComm
Communication
Table 2 Comparison of the values of lattice parameter (in nm) of
the hexagonal columnar phase in the three compounds at different
temperatures
Compound
60 1C
70 1C
80 1C
90 1C
3
IC /10
5.03
5.23
5.41
4.98
5.18
5.35
4.94
5.13
5.28
4.89
3
IC /12
3
IC /14
3
3
3
The lattice parameters of IC /10, IC /12 and IC /14 at different
temperatures are listed in Table 2. As expected, the value of lattice
parameter a is seen to increase with increasing end-chain length. The
continuous trend suggests that the basic phase structure is the same
in all three compounds. It is also seen in Table 2 that a decreases with
increasing temperature, a common trend observed in columnar
12
phases, particularly those containing a large aliphatic fraction.
In order to determine the molecular arrangement in the colum-
nar phase, we first need to establish the orientation of the molecular
cores relative to the column axis and then determine the number of
molecules in the unit cell. Using the l retardation plate (Fig. 1b and erated 20 mm above the substrate surface in IC /12 and IC /14; (b)
Fig. S3b, d, f, ESI†) we confirm that the slow axis, hence the long
Fig. 3 The Col/p31m phase: (a) temperature dependent intensity of the
second harmonic (400 nm) vs. background for 800 nm excitation, gen-
3
3
electron density map, with schematic molecules overlaid; (c) snapshot of
molecular dynamic simulation: purple = aromatic, grey and white =
axis of the aromatic core, is perpendicular to the column axis (see
ESI†). For the calculation of the number of molecules refer to Table
aliphatic chains (see Video in ESI†); (d) schematic of the Col/p31m phase.
In (b and c) the view is down the columns. Note that according to the
S4 in ESI.† To estimate molecular volume in the LC phase the
GISAXS pattern in Fig. 2b and Fig. S4a, S5a (ESI†) the column orientation in
molecule is split in two parts. The volume of the rigid aromatic part this figure is as in a film on a horizontal substrate viewed along the
2
2
substrate surface (see Fig. S9 in ESI†).
is estimated using the methods of crystalline volume increments.
ꢀ
3
For the aliphatic part the density of 0.8 g cm is assumed, being in
between the values for liquid n-alkanes and amorphous polyethy-
lene. From the measured area of the unit cell and an assumed in Fig. 3b–d. This arrangement is favoured for steric reasons, as the
intermolecular spacing along the column axis c in the usual range bunch of six alkyls emanating from the end of a star arm faces the
between 0.4 and 0.45 nm, we obtain close to 3 as the number empty concave ‘‘armpit’’ of the adjacent column. To test the viability
of molecules per unit cell. As we don’t know the actual value of c of the two arrangements and their variants, molecular models of
(no sharp feature is observed in the 0.3–0.5 nm WAXS range), in crystals were built with the experimental unit cell parameter a and
Table S4 (ESI†) we give the calculated value of c assuming that the with c from Table S4 (ESI†). These were then subjected to molecular
number of molecules, which must be an integer, is exactly 3. As can dynamics annealing. Space could not be filled uniformly with
be seen, the value of c is in the range between 0.407 and 0.433, model A, while a very satisfactory density distribution was achieved
increasing slightly with the length of the terminal chains.
with model B – see snapshot in Fig. 3c. A short video of the
With the above in mind, we consider two alternative models of dynamics of model B is available from the ESI.†
molecular packing. In both models three molecules cluster together
The columnar structure in Fig. 3 has 3-fold rather than 6-fold
in one stratum, with their aromatic cores back-to-back, forming a symmetry, and the plane group is p31m rather than the usual p6mm
three-arm star. The models differ in the arrangement of the star- symmetry of the Colhex phase. While the p6mm plane group has a
profiled columns. In model A the centres of the stars are situated at centre of inversion, p31m has not. To distinguish between them, we
apices of hexagons, producing a hexagonal honeycomb with aro- performed second harmonic generation (SHG) experiments. Fig. 3a
matic walls where the central methylene groups occupy alternative shows a large increase in SHG signal above the background level on
corners of the hexagon. This packing model resembles that found in cooling from Iso to Colhex phase, showing clearly that the latter phase
anchor-shaped mesogens consisting of a bent-rod aromatic core lacks centre of symmetry and is thus a trigonal phase (see also ESI†).
15
with a flexible chain attached to the inside of the bend point.
The lack of centre of symmetry causes a problem when recon-
However, while in the case of anchor mesogens the ends of the structing the electron density (ED) map from X-ray diffraction
aromatic arms were linked via hydrogen-bonding terminal groups, intensities. That is, the phase f of the structure factor of one of
the present compounds do not. In fact the bulky end-chains on the three reflections, the (11), is not limited to 01 or 1801, a condition
adjacent molecules would face each other and clash, having to bend that applies to all reflections in centrosymmetric space groups.
sharply in order to fill the interior of the hexagonal channel. Even A value of f different from 01 or 1801 breaks the hexagonal
rotating the star-like columns by 601 about their axis does not solve symmetry. Choosing an arbitrary angle f = 1201 we obtain the ED
3
the packing problem as tested by molecular simulation.
map in Fig. 3b, onto which the schematic molecules of an IC /n
In the alternative model (B) the self-assembled stars are located compound are superimposed. The high density regions (purple)
on a triangular lattice all with the same orientation – see the models represent the aromatic moieties, and the blur comes from the time
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
and space averaging in the liquid crystal. We note that choosing a
The authors acknowledge funding for this work from EPSRC
different value of f merely changes the extent of deviation from (EP-K034308, EP-P002250), NSFC (No. 21664015, 21364017,
circularity of the high-density maxima – see Fig. S8 (ESI†). 21602195) and YEDF (ZD2015001). YL thanks CSC for stipend
We consider that the efficient back-to-back packing of the and UoS for waiving tuition fee. For help with synchrotron
three bent cores in this structure is facilitated by a degree of experiments we thank Prof. N. Terrill and Dr O. Shebanova,
flexibility in the core, particularly around the oxymethylene beamline I22, Diamond Light Source, and Drs O. Bikondoa,
linkage. The long-range polarity, on the other hand, is believed S. Brown and P. Thompson of beamline BM28 at ESRF.
to be secured by the high barrier for uncorrelated rotation
around the column axis.
It is interesting to compare the present structure with that of
a bent-core phasmid compound also having six terminal C H
1
4
29 Conflicts of interest
chains but with longer arms and a pyridine ring as the bend
2
3
There are no conflicts to declare.
point. The model proposed for the hexagonal phase had four
rather than three molecules in a stratum resulting in a 4-arm
star. p6mm symmetry was assumed. Interestingly, at lower
temperatures a phase transition to a polar columnar phases
took place but unlike in the current case, the poling was
parallel rather than perpendicular to the column axis. Longitu-
dinal poling was explained by an out-of-plane distortion of
the stars.
References
1 H. T. Nguyen, C. Destrade and J. Malth ˆe te in Handbook of Liquid
Crystals, ed. D. Demus, J. Goodby, G. W. Gray, H. W. Spiess, V. Vill,
Wiley-VCH Verlag GmbH, 1998, p. 865.
2
D. Fazio, C. Mongin, B. Donnio, Y. Galerne, D. Guillon and
D. W. Bruce, J. Mater. Chem., 2001, 11, 2852.
3
T. Woehrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum,
J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann and
S. Laschat, Chem. Rev., 2016, 116, 1139.
It is also appropriate to compare the present structure with
3
those of a number of mesogens with 3-fold (C ) symmetry in
their chemical structure. In the columnar phase of the 3-arm
4 W. Pisula, M. Zorn, J. Y. Chang, K. M u¨ llen and R. Zentel, Macromol.
Rapid Commun., 2009, 30, 1179.
2
4
star molecules by Lehmann et al. the molecules are actually
thought to be distorted into an E-shaped conformation, making
them effectively fan-shaped and thus assembling as other
fan-shaped mesogens such as dendrons. In other cases the fact
5
6
M. O’Neill and S. M. Kelly, Adv. Mater., 2011, 23, 566.
M. Funahashi, J. Mater. Chem. C, 2014, 2, 7451.
7 T. Kato, M. Yoshio, T. Ichikawa, B. Soberats, H. Ohno and M. Funahashi,
Nat. Rev. Mater., 2017, 2, DOI: 10.1038/natrevmats.2017.1.
C. Tschierske, Angew. Chem., Int. Ed., 2013, 52, 1.
8
that the non-centrosymmetric 3-arm star molecules of C symmetry
3
9 G. Ungar, C. Tschierske, V. Abetz, R. Holyst, M. A. Bates, F. Liu,
M. Prehm, R. Kieffer, X. B. Zeng, M. Walker, B. Glettner and
A. Zywocinski, Adv. Funct. Mater., 2011, 21, 1296.
formed the Col phases was thought to be the result of rotational
hex
25
averaging. However, even though the diffraction pattern has
hexagonal symmetry, this does not preclude the real-space structure
10 C. Dressel, F. Liu, M. Prehm, X. B. Zeng, G. Ungar and C. Tschierske,
Angew. Chem., Int. Ed., 2014, 53, 13115.
having only trigonal symmetry. One has to remember that the Laue 11 E. Fontes, P. A. Heiney and W. H. Jeu, Phys. Rev. Lett., 1988, 61, 1202.
1
2 J. K. Kwon, S. N. Chvalun, J. Blackwell, V. Percec and J. A. Heck,
Macromolecules, 1995, 28, 1552.
symmetry (i.e. the symmetry of the diffraction pattern) is hexagonal
even if the crystal symmetry is trigonal. Indeed the columnar phase
of compounds described in ref. 26 showed strong second harmonic
generation (SHG), characteristic of non-centrosymmetric structures.
1
3 M. A. Shcherbina, X. B. Zeng, T. Tadjiev, G. Ungar, S. H. Eichhorn,
K. E. S. Phillips and T. J. Katz, Angew. Chem., Int. Ed., 2009, 48, 7837.
4 V. Percec, et al., J. Am. Chem. Soc., 2011, 133, 12197.
5 B. Glettner, F. Liu, X. B. Zeng, M. Prehm, U. Baumeister, G. Ungar
and C. Tschierske, Angew. Chem., Int. Ed., 2008, 47, 6080.
1
1
In other reported cases of C star-like mesogens only 6-fold sym-
3
26–28
metry was claimed for the columnar LC phase.
As hinted in the introduction, beside the already mentioned
16 X. B. Zeng, et al., Science, 2011, 331, 1302.
7 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed.,
001, 40, 2004; C. W. Tornøe, C. Christensen and M. Meldal, J. Org.
Chem., 2002, 67, 3057.
1
2
1
5
trigonal columnar phase in anchor-shaped compounds, there
has been another case of triangular honeycomb LC of straight 18 Y. Bouligand, J. Phys., 1980, 41, 1297.
1
2
9 A. K. Boal and V. M. Rotello, J. Am. Chem. Soc., 2002, 124, 5019.
0 X. Tan, L. Kong, H. Dai, X. Cheng, F. Liu and C. Tschierske,
Chem. – Eur. J., 2013, 19, 16303.
rod amphiphiles, where two incompatible side-chains were
attached at each side of the aromatic rod. While at higher
1
6
temperatures the two chain types were mixed in the triangular 21 G. Ungar, F. Liu, X. B. Zeng, B. Glettner, M. Prehm, R. Kieffer and
C. Tschierske, J. Phys.: Conf. Ser., 2010, 247, 012032.
2 A. Immirzi and B. Perini, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
Theor. Gen. Crystallogr., 1977, 33, 216.
channels, a second-order Curie-type transition on cooling
brought about a trigonal p31m phase in which the two side-
2
chain types were separated in triangles on two separate sub- 23 J. Matraszek, J. Mieczkowski, D. Pociecha, E. Gorecka, B. Donnio
and D. Guillon, Chem. – Eur. J., 2007, 13, 3377.
4 M. Lehmann and M. Jahr, Chem. Mater., 2008, 20, 5453.
5 G. Hennrich, A. Omenat, I. Asselberghs, S. Foerier, K. Clays,
T. Verbiest and J. L. Serrano, Angew. Chem., Int. Ed., 2006, 45, 4203.
6 E. Beltran, J. L. Serrano, T. Sierra and R. Gimenez, J. Mater. Chem.,
lattices. A recent Monte Carlo simulation helps understand
such p6mm–p31m transitions.
In summary, we have synthesised a series of bent-core
phasmid compounds and found that they form a trigonal
2
2
2
9
2
2
2
2
2012, 22, 7797.
columnar phase, where the aromatic columns adopt
a
7 S. K. Pathak, R. K. Gupta, S. Nath, D. S. S. Rao, S. K. Prasad and
A. S. Achalkumar, J. Mater. Chem. C, 2015, 3, 2940.
8 E. Westphal, M. Prehm, I. H. Bechtold, C. Tschierske and H. Gallardo,
J. Mater. Chem. C, 2013, 1, 8011.
9 S. George, C. Bentham, X. B. Zeng, G. Ungar and G. A. Gehring, Phys.
Rev. E, 2017, 95, 062126.
3-armed star-shaped cross-section. This finding indicates a
potential new path to creating non-centrosymmetric self-
assemblies that could, with suitable substituents, be used in
ferroelectric, pyroelectric or frequency-doubling optical devices.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017