Transition between tangential and co-axial liquid crystalline honeycombs in the self-assembly of Y-shaped bolapolyphiles

Article abstract of DOI:10.1039/C8CC06281A

p-Terphenyl based Y-shaped bolapolyphiles self-assemble into liquid crystalline honeycombs with co-axial organization of the π-conjugated rods around columns filled by alkyl sidechains. In this new hexagonal honeycomb with P6/mmm symmetry the orientation of the π-conjugated rods is perpendicular to the tangential honeycombs formed at lower temperature or with longer chain length.

Full text of DOI:10.1039/C8CC06281A

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ARTICLE TYPE
Transition between tangential and co-axial liquid crystalline
honeycombs in the self-assembly of Y-shaped bolapolyphiles
Anne Lehmann,a Marko Prehm,a Changlong Chen,b Feng Liu,b,∗ Xiangbing Zeng,c Goran Ungar,c,∗
Carsten Tschierskea,∗
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
be observed for all compounds with n ≥ 8. For compound A11, as
50 a representative example, optical micrographs taken between
crossed polarizers are shown in Fig. 2. The lamellar LC phase
(SmA+) has a typical birefringent fanꢀlike texture (Fig. 2a). On
cooling the birefringence increases (green to second order
orange) due to an increasing orientational order parameter of the
55 rods, and at the next transition a highly viscous mosaicꢀlike
texture develops (Fig. 2b), indicative of layer stiffening in the
phase designated as P6/mmm. At the subsequent phase transition
to the low temperature (LT) phase (p4mm) the mosaics become
broken and fluid again, accompanied by a significant decrease in
60 birefringence (color change from 2nd order orange via 1st order
green to blue, see Fig. 2c), meaning that either the degree of order
or the orientation mode of the πꢀconjugated rods had changed.
p-terphenyl based Y-shaped bolapolyphiles self-assemble into
liquid crystalline honeycombs with co-axial organization of
the π-conjugated rods around columns filled by alkyl
10 sidechains. In this new hexagonal honeycomb with P6/mmm
symmetry the orientation of the π-conjugated rods is
perpendicular to the tangential honeycombs formed at lower
temperature or with longer chain length.
Liquid crystals (LCs) are nowadays indispensible for displayꢀ and
15 information technology.1 The architecture of the industrially used
LCs is relatively simple, but in the recent decades the complexity
of LC molecules and their selfꢀassembly has been significantly
increased.1,2 Columnar LC phases, for example, were found for a
wide range of molecular shapes from discꢀlike, via taperꢀshaped,
20 and starꢀshaped to rodꢀlike.1,3 The usual mode of organization of
the mesogens in these columnar LCs is by stacking in columns,
arranged on a 2d lattice, in most cases with hexagonal symmetry
and embedded in the fluid continuum formed by the attached
flexible chains. The orientation of the molecules is perpendicular
25 to the columns or slightly tilted to the normal of the column long
axis, as shown in Fig. 1a for the multiꢀchain (polycatenar4) rodꢀ
like molecules. In the recent two decades Tꢀshaped and Xꢀshaped
polyphiles have paved the way to new modes of columnar selfꢀ
assembly, namely the LC honeycombs, having an inverted
30 structure with the flexible chains inside the columns and the
nanoꢀsegregated rodꢀlike mesogens forming the framework
around them (Fig. 1c). In these liquid honeycombs the ratio of the
volume of the alkyl chains to the length of the rods determines
the number of rods arranged in the polygonal circumference of
35 each alkylꢀfilled cell.5 , 6 , 7 The orientation of the mesogens is
perpendicular to the column axis z6 or tilted.8 Only recently have
the coꢀaxial rodꢀbundle phases, with the molecules parallel to z,
been reported (Fig. 1b).9 Herein we report a structure (Fig. 1d)
with πꢀconjugated rods oriented coꢀaxially in honeycombs, thus
40 being inverted to structures (b) and (c) in Fig. 1 either with
respect to the position or orientation of the rods, respectively.
65
70
75
Figure 1. Hexagonal columnar organizations in LC phases. (a) formed by
polycatenar molecules4 and (bꢀd) formed by bolapolyphiles with lateral
80 chains; (b) coꢀaxial rodꢀbundle phase;9 (c) tangential hexagonal
honeycomb,6, and (d) the coꢀaxial honeycomb (P6/mmm) reported herein.
In homeotropically aligning cells smectic layers form parallel
to the surfaces. In this case the lamellar phase (SmA+), appearing
at the highest temperature, is completely dark between crossed
This mode of honeycomb selfꢀassembly with coꢀaxial rods was 85 polarizers. This indicates that, on average, the terphenyls are
observed for the pꢀterphenyl based compounds An with two polar
glycerol end groups and a peripheral lateral chain (see formula in
45 Table 1). These compounds were synthesized as described in the
ESI (Scheme S1†) and their data are collated in Table 1.
perpendicular to the layers. Under these conditions the phase
transition to the next LC phase (P6/mmm) is difficult to
recognize; the texture remains dark and only the viscosity
increases (Fig. S2a, b†). This is the first hint that another
Lamellar LC phases (SmA+) are dominant in most An 90 optically uniaxial mesophase has formed A low birefringent
compounds (n = 5ꢀ13) and additional nonꢀlamellar LC phases can
(gray) “spherulitic” texture, typical of columnar phases, with
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Table 1. Phase transitions and lattice parameters of the LC phases of compounds An. a
An Phase transitions T/°C [ꢁH/kJmolꢀ1]
SmA+: d1, d2 (nm) P6/mmm: ahex, c (nm) p4mm: asqu (nm)
A5 Iso 203 [6.7] SmA+ 114 [18.2] Cr
A7 Iso 191 [6.6] SmA+ 105 [16.2] Cr
2.16, 1.83
n.d.
ꢀ
ꢀ
ꢀ
ꢀ
A8 Iso 187 [6.0] SmA+ 116 [0.8] P6/mmm 101 [18.1] Cr
A9 Iso 182 [5.2] SmA+ 130 [0.8] P6/mmm 95 [ꢀ] M 65 [2.4] Cr
A10 Iso 182 [4.7] SmA+ 157 [1.0] P6/mmm 115 [1.8] Colsqu/p4mm 78 [11.3] Cr
A11 Iso 179 [4.0] SmA+ 164 [1.2] P6/mmm 136 [2.4] Colsqu/p4mm 84 [13.4] Cr
2.20, 2.2
2.19, 2.32
2.13, 2.51
2.16, 2.66
2.44, 2.20
2.64, 2.23
2.84, 2.17
3.05, 2.22
3.27, 2.21
3.28, 2.20
ꢀ
ꢀ
ꢀ
2.81
2.86
2.86
A12 Iso 178 [3.5] SmA+ 169 [1.4] P6/mmmb 131 [2.3] Colsqu/p4mm 98 [17.1] Cr 2.14, 2.83
A13 Iso 177 [3.0] SmA+ 172 [1.7] P6/mmmb 141 [3.2] Colsqu/p4mm 107 [18.4] Cr 2.12, 2.84
2.83
A14 Iso 179 [6.2] Colrec/p2gg 129 [26.0] Cr
ꢀ
7.00, 6.58 (p2gg)c
a
Values determined on cooling with rate 10 K minꢀ1 by DSC (peak temperatures); abbreviations: Cr = crystalline solid, Iso = isotropic liquid, SmA+ =
orthogonal lamellar phase with diffuse small angle scattering; P6/mmm = 3D hexagonal LC phase with coꢀaxial rod orientation in a hexagonal
honeycombs; Colsqu/p4mm = tangential square honeycomb LC phase; Colrec/p2gg = pentagonal honeycomb LC phase; M = unknown mesophase; n.d. =
not determined; for transitions on heating, more details and calculations, see Tables S1 and S8†, for DSC traces, see Fig. S1† and for XRD data see Figs
S7ꢀS15 and Tables S2ꢀS7†; b phase coexists with Colsqu/p4mm; c parameters a and b of the Colrec/p2gg phase.
An unusual feature of this SmA phase is an additional diffuse
20 scattering with relatively high intensity in the SAXS pattern
(SmA+). In a homeotropically aligned sample with the Xꢀray
beam parallel to the layers (Figs. 3a and S8b†) the maximum of
this diffuse scattering (d2) is on the equator, i.e. perpendicular to
the layer reflection. It indicates additional strong short range inꢀ
25 plane density fluctuations with a mean distance between 1.8 and
2.8 nm. This distance increases with alkyl chain length (Table 1).
This extra feature is likely to be due to the average distance
between domains arising from local segregation of the flexible
lipophilic alkyl chains from the polar and rigid bolaamphiphilic
30 cores. The volume fraction of the lateral chains increases from
0.20 to 0.39 between A5 to A13 (Table S8†), thus the chains
represent the minor component in all cases. This phase,
designated as SmA+,5,6 can thus be considered as a mesh
phase10,11 with the layers irregularly penetrated by alkyl chain
Figure 2. Textures of A11 as observed between crossed polarizers (white
arrows); aꢀc) in planar aligning cells (terphenyls parallel to the substrate
surface, 5 ꢀm PI coated cells) and d) in homeotropic alignment (terphenyls
35 domains. The relatively small meridional spread of the diffuse
SAXS maxima on the equator suggests that the alkyl domains are
columns perpendicular to the layers and cutting through several
of them, on average (see Fig. 4c).
perpendicular to the surfaces, between nonꢀtreated microscopy glass
plates) as observed on cooling. The insets show textures with additional
λ/2 plate at the same temperatures, the blue arrows indicate the direction
of the slow axis; for more details and explanations, see Figs. S2ꢀS5†.
At the next phase transition, observed in compounds A8-A13,
40 this diffuse scattering condenses into a sharp Bragg peak on the
equator (Figs. 3b and S8†) and then it can be indexed as the (100)
reflection of a 3D hexagonal lattice, spacegroup P6/mmm, with
ahex = 2.44 to 3.18 nm (Table 1). The positions of the (100) and
(200) layer reflections on the meridian are almost unchanged and
45 become the (001) and (002) reflections of periodicity c = 2.17ꢀ
2.22 nm). The GISAXS pattern in Fig. 3c confirms this
assignment. The positions of the diffuse equatorial WAXS
maxima do not change, indicating that the orientation of pꢀ
terphenyls and alkyl chains remains parallel to the column long
50 axis (Fig. 3b). The proposed structure, depicted in Fig. 4b, is
corroborated by the reconstructed electron density (ED) map in
Fig. 3d, where the alkyl chains forming infinite low ED columns
(red) are arranged on a regular hexagonal lattice and are
embedded in a high ED lamellar continuum (purple) with slightly
55 modulating ED due to the alternation of glycerols and terphenyls
in the penetrated layers. The ED difference between glycerols and
terphenyls is small, hence the weak (00l) reflections. The fact that
columns in the parallel to the surface, develops on further cooling
at the transition to the fluid LT phase (p4mm, Fig. 2d).
For all An compounds in the whole temperature range down to
crystallization, the WAXS pattern is diffuse with a maximum at d
= 0.45ꢀ0.47 nm, confirming true LC phases without fixed
positions of individual molecules (see Section S2.3 in ESI†). In
the high temperature phases of surfaceꢀaligned A5-A13 there is a
weak 1stꢀ and a stronger 2ndꢀorder layer reflection on the
meridian, confirming a homeotropic lamellar phase with a layer
10 thickness of d1 = 2.13ꢀ2.20 nm (Fig. 3a and Table 1). The dꢀvalue
is almost identical for all compounds and is close to the length of
the bolaamphiphilic core, measured between the secondary OH
groups in a compact conformation (Lmol,min = 2.3 nm, see Fig.
S17a†). This indicates a smectic phase with monolayer structure.
15 In contrast to the layer reflections, the diffuse WAXS maximum
is on the equator, confirming an on average nonꢀtilted parallel
organization of terphenyls and a significant part of the alkyl
chains perpendicular to the layers.
5
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(002) is stronger than (001) (Figs. 3c and S9b†) indicates a local
ED minimum in the middle of the layers, attributed to the fact
that the alkyls are tethered to the ends of the terphenyl cores (note
the weak modulation in diameter of alkyl columns in Fig. 3d).
chains the transition from P6/mmm to p4mm is not complete, i.e.
p4mm and P6/mmm coexist in the P6/mmm range (Fig. S13c†).
A9 rapidly crystallizes and therefore its LT phase (M) could not
be investigated. No LT phase is observed in A8, while even the
5
The ahex parameter of the P6/mmm inꢀplane lattice increases 50 P6/mmm phase is absent in A7 and A5, leaving only the SmA+
significantly with increasing chain length, from 2.44 nm for A8 to
3.28 nm for A13. This is due to the expansion of the lipophilic
columns, amplified by a concomitant increase in the number of
molecules per unit cell from ncell = 14 for A8 to 22 for A13
phase (Table 1, Fig. 5a). Extension of the lateral chain of A13 by
just one more CH2 unit (A14) leads to a sudden and simultaneous
loss of the SmA+, P6/mmm and p4mm phases. Only a rectangular
columnar phase with p2gg lattice (a = 7.0 nm, b = 6.6 nm) is left
10 (Tables 1 and S8†), The number of molecules in the lateral cross 55 in A14, representing
a honeycomb composed of slightly
section of these honeycomb walls is nwall = 2.6ꢀ2.7 for all
compounds (for calculation, see section S2.5 in the ESI†). Thus
two to three (1.7ꢀ2.7) molecules are arranged sideꢀbyꢀside along
each of the six sides of a hexagonal cell around each alkyl
deformed pentagonal cells (Table 1, Fig. 5a, for details, see Figs.
S6, S15 and S20†).
15 column. This structure can be considered as
a coꢀaxial
honeycomb (Figs. 1d and 4b), having the rodꢀlike mesogens
arranged parallel to the column axis instead of perpendicular to it,
as in the previously reported tangential honeycombs (Fig. 1c).6
Due to this inverted organization the periodicity provided by the
20 bolaamphiphilic core structure leads to a periodicity along the
column axis (z) instead of fixing the honeycomb shape in the x-y
plane.
Figure 4. Schematic models of the three LC phases in compounds An: (a)
60 square tangential honeycomb p4mm; (b) hexagonal coaxial 3D phase
P6/mmm; (c) random mesh perforated layer phase SmA+. Grey =
terphenyl, blue = glycerol, white = alkyl.
SmA+, 165 oC
P6/mmm, 153 oC
(a)
(b)
In the series of isomeric compounds Bn (Fig. 5b), having the
lateral alkyl chain at the central benzene ring, the SmA+ phase is
65 formed over a much smaller chainꢀlength (n = 5,6) and
temperature range and the P6/mmm phase is completely absent.12
Instead, Bn compounds exhibit a series of more conventional
(d)
tangential honeycombs, as follows (Fig. 5b):
a centred
P6/mmm
(c)
rectangular (c2mm) phase consisting of rhombic cells in B8; a
70 highꢀtemperature square p4mm phase with square cells in B9,
B10 and B11, preceded in B9-B11 at lower temperature by a
rectangular p2mm phase. Finally, in B12 and B14 we see the
p2gg phase made up of pentagonal cells, as in A14.
Why has the transfer of attachment point from the central (Bn)
75 to the terminal benzene ring (An) caused such unusual phase
behaviour in An? As the sideꢀchain volume increases, either due
to increased temperature or to addition of extra CH2 groups, the
squares of the p4mm phase would be expected to turn into
pentagons or hexagons, as in the Bn and other Tꢀshaped
80 amphiphiles. However, in that case the chains in Bn compounds
emanating from the corners of such polygons would have a
problem reaching the centre of the cell. Even though
geometrically possible, stretching of alkyl chains to maintain high
density at the centre of pentagonal or even square cells at higher
85 temperatures would be entropically disfavoured. Hence an
alternative solution is adopted by the Yꢀshaped An molecules,
abandoning the tangential in favour of axial alignment of the
cores. There must, however, be a free energy penalty for this
inversion, as in most other cases the tangential structure prevails.
90 A possible penalty is the dilution of hydrogen bonding glycerol
groups in (perforated) 2D layers in P6/mmm and SmA+ phases,
in contrast to their concentration in well segregated 1D columns
in tangential honeycombs. Thus, as in many other examples, a
relative minor alteration in molecular design leads to frustration
95 resolved by the system adopting novel and potentially
advantageous mesostructures.
Figure 3. a,b) XRD patterns (obtained after subtraction of the scattering
25 pattern in the isotropic liquid state, for original patterns, see Fig. S10†) of
a surface aligned sample of compound A10 in (a) SmA+ and (b) P6/mmm
phases; c) GISAXS pattern of the P6/mmm phase of A13 at 165 °C on a
Si surface with either the (100) or the (001) axis perpendicular to the
substrate surface d) electron density (ED) map of the P6/mmm phase of
30 compound A9 (red = low ED, purple = high ED; see also Section S2.3 in
the ESI†).
For compounds A9ꢀA13 there is an additional phase transition
from P6/mmm to a LT phase. Its SAXS and GISAXS patterns for
compounds A10-A13 can be indexed to a square lattice with
35 p4mm plane group (Figs. S11 and S12). The lattice parameters of
the p4mm phases of all four compounds are almost identical (asqu
= 2.81ꢀ2.86 nm, Table 1), being only slightly larger than the
molecular length in the most stretched conformation (Lmol,max
=
2.6 nm, see Fig. S17b†). This is typical of tangential honeycombs
40 with square cells. The phase structure is corroborated by ED
maps (see Figs. S11d and S14†) and is in line with the negative
birefringence (see inset in Fig. 2c and Fig. S2e,f†). This confirms
that the intramolecular πꢀconjugation pathway, i.e. the rod
orientation, is perpendicular to the columns. The structure is
45 depicted in Fig. 4a. For compounds A12 and A13 with longer
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This work is supported by DFG (392435074), NSFC
35 (21761132033, 21374086) and EPSRC (EPꢀP002250). We thank
Beamlines BL16B1 at SSRF, I22 at Diamond Light Source and
BM28 at ESRF for providing the beamtime.
An
There are no conflicts of interest to declare.
Notes and references
40 a Department of Chemistry, Martin Luther University Halle-Wittenberg,
Kurt-Mothes-Str. 2, 06120 Halle, Germany. E-mail:
carsten.tschierske@chemie.uni-halle.de
b State Key Laboratory for Mechanical Behavior of Materials, Xi’an
Jiaotong University, Xi’an 710049, P. R. China. E-mail:
45 feng.liu@xjtu.edu.cn
c Department of Materials Science and Engineering, Sheffield University,
Sheffield S1 3JD, UK. E-mail: g.ungar@sheffield.ac.uk
† Electronic Supplementary Information (ESI) available: [Synthesis,
analytical data, experimental techniques, DSC and XRD data, structural
50 models and additional discussions]. See DOI: 10.1039/b000000x/
Bn
1
J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. F. Gleeson,
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C. Tschierske, Angew. Chem. Int. Ed., 2013, 52, 8828; T. Kato, M.
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S. Kumar, Chemistry of Discotic Liquid Crystals, CRC Press, Boca
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4
5
Figure 5. Comparison of the LC phases on cooling (10 K minꢀ1) a) of the
Yꢀshaped (An) and b) the Tꢀshaped bolapolyphiles Bn12 depending on the
length of the lateral chain n; abbreviations: see Table 1 and 3DꢀHex =
P6/mmm; Colrec/c2mm = rhombic honeycomb; Colrec/p2mm = rectangular
honeycomb; Colsq = Colsqu in brackets were only observed on cooling.
5
6
7
a) C. Tschierske, Chem. Soc. Rev., 2007, 36, 1930; b) C. Tschierske,
C. Nürnberger, H. Ebert, B. Glettner, M. Prehm, F. Liu, X. B. Zeng
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A similar sequence SmA+ ꢀ P6/mmm has been observed for
facial amphiphiles with exchanged positions of alkyl chains and
10 polar groups at the pꢀterphenyl core.13 In this P6/mmm phase the
polar lateral groups form the columns on the hexagonal lattice
and the alkyl chains separate the layers of the coaxially aligned pꢀ
terphenyl rods (“channelled layer phase”, ChLhex).13 In another
related structure, the soꢀcalled “tubular nematic columnar phase”,
15 formed by hairyꢀrod polymers17 and some oligomesogens,14 the
mesogens are also coꢀaxial to lipophilic columns, but have only
orientational order. This indicates that coꢀaxial honeycombs
represent a general mode of LC selfꢀassembly.
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In summary, a new kind of LC honeycombs with coꢀaxial rod
20 orientation is reported, occurring at the transition from lamellar
phases to tangential honeycombs and competing with the
triangular and square honeycombs as well as with the
bicontinuous cubic phases15. This work shows that an inversion
of rod orientation in the honeycombs from tangential6 via tilted8
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to the orthogonal (SmA), tilted (SmC)1 and inꢀplane (Lam)
versions of lamellar phases.16 From an application point of view
the reported molecules represent simple generic model
compounds for the understanding and tailoring of the selfꢀ
30 assembly of larger πꢀconjugated molecules and lateral substituted
rodꢀlike macromolecules17 into wellꢀdefined superstructures, as
required for advanced organic electronic and light harvesting
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| Journal Name, [year], [vol], 00–00
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