Two-and three-dimensional liquid-crystal phases from axial bundles of rodlike polyphiles: Segmented cylinders, crossed columns, and ribbons between sheets

Article abstract of DOI:10.1002/anie.201103303

Pleated ribbons and bow ties: Rodlike mesogens (see scheme) with swallow-tail side chains arrange axially in ribbonlike bundles. At high temperatures the ribbons rotate, resulting in a novel 3D hexagonal liquid-crystal phase (see picture, left). At lower temperature, rotation locks in giving a structure of crossed aromatic and fluorinated columns (right). On further cooling the ribbons fuse into aromatic sheets with fluorinated columns intercalated. Copyright

Full text of DOI:10.1002/anie.201103303

DOI: 10.1002/anie.201103303
Liquid Crystals
Two- and Three-Dimensional Liquid-Crystal Phases from Axial
Bundles of Rodlike Polyphiles: Segmented Cylinders, Crossed
Columns, and Ribbons between Sheets**
Feng Liu, Marko Prehm, Xiangbing Zeng, Goran Ungar,* and Carsten Tschierske*
The design of programmed molecules for assembly into
complex nanostructures has received considerable attention
in recent years.[1–3] One successful approach to complex soft
matter has been achieved with polyphilic star-shaped mole-
cules, both polymeric[4] and molecular.[3,5,6] T-shaped bolaam-
phiphiles in particular provide a series of novel honeycomb
liquid-crystal (LC) morphologies (Figure 1a,b). In these
honeycombs rodlike units form polygonal cylinder frames,
Figure 1. Sequence of LC phases formed by rodlike bolaamphiphiles
with incompatible lateral and terminal groups, upon increasing the
with the terminal hydrogen-bonding groups fusing the
cylinder walls and the flexible lateral chains filling the cell
size of the lateral chain(s): a,b) polygonal cylinder phases (rods
interior (Figure 1a,b).[3] By adding a second side chain[7]
normal to column);[3] c) lamellar phases (rods in plane of layer);[10] and
incompatible with the first, we recently obtained new
structures based on periodic multicolor tiling patterns. Some
are of unprecedented complexity, with tiles of several differ-
ent shapes and “colors”.[8,9] However, increasing the side-
chain volume leads to lamellar phases (Figure 1c)[10] and, by
introducing branches, to inversion of the honeycombs, with
the column core now inhabited by bundles of aromatic rods
coaxial with the columns (Figure 1d).[11,12]
d) axial-bundle phases (bundles of rods parallel to columns).[11,12]
Molecules displaying these phases are sketched above: gray aromatic
core, blue glycerol, green side chain.
branches, a semiperfluorinated (RF) and a carbosilane one
(RSi). For this compound, three new LC phases were
discovered with decreasing temperature, each with an intri-
guing structure: a hexagonal 3D phase with correlated
modulated columns, a 3D phase with crossed columns, and
a phase with columns between layers.
Herein we report a new type of bolaamphiphile (1,
Scheme 1) having a lateral chain with two incompatible
Compound 1 (see the Supporting Information for the
synthesis)[13] shows these three stable LC phases (Scheme 1)
separated by reversible transitions. On cooling from the
isotropic liquid, a spherulitic texture develops, typical of
columnar LC (Figure 2a). All mesophases have positive
birefringence as confirmed by polarizing microscopy with a
l-retarder. The colors of the fans (Figure 2c, middle) reveal
the orientation of the slow axis as tangential rather than
[*] Dr. F. Liu, Dr. X. Zeng, Prof. Dr. G. Ungar
Department of Materials Science and Engineering
University of Sheffield
Mappin Street, Sheffield S13JD (UK)
E-mail: g.ungar@sheffield.ac.uk
Dr. M. Prehm, Prof. Dr. C. Tschierske
Organic Chemistry, Institute of Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle (Germany)
E-mail: carsten.tschierske@chemie.uni-halle.de
Prof. Dr. G. Ungar
WCU program C2E2, School of Chemical and Biological Engineering
Seoul National University, Seoul (Korea)
[**] This work was supported by the EC 7th and 6th Framework
Programmes, under contracts 2007-2013 (NANOGOLD, grant
agreement No. 228455), and ERAS-CT-2003-989409. It was also
supported by the DFG and EPSRC as part of the ESF EUROCORES
Programme SONS. G.U. acknowledges support from the WCU
program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology (R31-
10013). M.P. acknowledges the support by the Cluster of Excellence
“Nanostructured Materials” and DFG (FG 1145). For help with the
synchrotron experiments we thank Drs. Nick Terrill, Jen Hiller
(beamline I22), Steve Collins, and Alessandro Bombardi (beamline
I16) at Diamond, and Drs. Simon Brown and Paul Thompson
(BM28-XMaS) at ESRF.
Scheme 1. Structure, transition temperatures T [8C], and enthalpies
DH [kJmolꢀ1] (bottom, italics) of compound 1 (for differential scan-
ning calorimetry, see Figure S1 in the Supporting Information).
Cr =crystalline phase, LamSm/p2mm=lamellar phase with in-plane
periodicity correlated between layers on a p2mm lattice, Cmmm =
orthorhombic crossed-column phase, P6/mmm=3D hexagonal axial
bundle phase (see Figure 3), Iso=isotropic liquid.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103303.
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10599

Communications
radial, that is, coincident with the columnar director. Since the
slow axis is known to be parallel to the biphenyl axis, it follows
that the biphenyls are coaxial with the columns.
That all three phases under discussion are LC is confirmed
by the lack of any sharp X-ray diffraction at wide angles (see
Figure 2d,e). The high-temperature phase has hexagonal
symmetry (Figure 2d,f, ahex = 3.73 nm). In both diffraction
patterns the sixfold axis is randomly oriented within the plane
of the film (horizontal in Figure 2d–f). Notably, in addition to
the hk0, there is also a weak but distinct Bragg reflection on
the equator at 1.8 nm; it is indexed as (001) (Figure 2 f). This
reflection appears at the isotropization temperature and
persists almost unchanged down to the crystal phase (Fig-
ure 2g). The periodicity c = 1.8 nm along the columns corre-
sponds to the length of the bolaamphiphile (l = 1.8–2.1 nm).
The correlation length along [001] is 150 nm, indicating long-
range order. We note that, although hkl cross-reflections are
not observed, this phase has true 3D long-range order.
Otherwise, the vertical width of the (001) GISAXS reflection
(Figure 2 f) would have exceeded that of the hk0 reflections.
The suggested space group of the 3D hexagonal phase is P6/
mmm. As shown above (Figure 2c), in this phase the
biphenyls are parallel to the column axis, surrounded by the
molten swallow-tailed side chains. From the volume of the
unit cell we calculate the number of molecules per cell to be
14 (for calculation, see Table S2 in the Supporting Informa-
tion). The 14 molecules sit side by side in a bundle, and each
bundle is centered at a vertex of a simple 3D hexagonal unit
cell (see Figure 3a). The segmentation of the columns is due
to segregation of the aromatic and the terminal glycerol
groups.
ꢀ
A 3D phase with R3m symmetry, in which the bundles are
longitudinally shifted by ꢁ c/3 between adjacent columns, was
reported for related terphenyl-based bolaamphiphiles with
two swallow tail side chains.[12] A possible reason for there
being no longitudinal shift in 1 is that it has only one swallow
tail attached to one end of the biphenyl, hence molecules on
Figure 2. Mesophases of 1. a) Polarized optical texture at T=908C
(P6/mmm phase) and b) at T=658C (LamSm/p2mm phase); c) single
spherulite of the P6/mmm phase between crossed polarizers (left),
with l-retarder plate (middle), and indicatrix orientation of the retarder
(right); d) XRD pattern (surface-aligned sample) at T=1258C
(P6/mmm) and e) at T=598C (LamSm/p2mm phase); f) grazing-
incidence small-angle X-ray scattering (GISAXS) pattern at T=808C
(P6/mmm); g) powder SAXS on continuous cooling at 5 Kminꢀ1
covering the three LC phases and the crystal; two regions of interest
are shown; the “001” peak changes from 001P6/mmm via 001Cmmm to
,
Figure 3. Electron-density maps obtained from synchrotron powder
patterns of compound 1 at a) T=808C; b) T=608C, and c) T=528C.
Red=low density (bolaamphiphilic moieties); blue=high density
(richest in RF chains); molecules are sketched in (a) and (c).
01Lam
.
Sm
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neighboring columns could fit well together if mutually
rotated by 1808 in an antiparallel configuration. In contrast,
the two side groups at each end of the related terphenyl
compounds (Figure 1d) would clash, hence the longitudinal
front plane in Figure 3a). Moreover, in Figure S4 in the
Supporting Information we added together three electron-
density maps of Figure 3b with three in-plane orientations:
ꢀ608, 08 and + 608. The combined map is almost indistin-
guishable from that of the hexagonal phase in Figure 3a.
Thus, we can conclude that the bundle columns are actually
noncircular ribbons even in the P6/mmm phase, and XRD
gives the time–space average. The reason for the noncircular
shape is thought to be the need for the side chains on all
biphenyls to access the surface and the peripheral corona, a
situation unachievable with 14 biphenyls densely packed in a
circular cylinder. Incidentally, there is a continuous slow
increase in the number of molecules in a bundle as temper-
ature decreases (see d spacing vs. temperature plot in Fig-
ure S3 and Table S2 in the Supporting Information). This
widening of the ribbons helps stop their rotation and locks
them in a mutually parallel orientation, with the consequent
P6/mmm!Cmmm symmetry breaking.
ꢀ
shift in the R3m phase.
On decreasing the temperature, a transition to a lamellar
phase (see Figure 2b,e) takes place via an intermediate phase
(for powder SAXS of the complete series on cooling, see
Figure 2g). The transition exotherms are relatively broad, and
in the XRD patterns the intermediate phase can only be
found coexisting with one of the two adjacent phases (see also
the GISAXS pattern in Figure S2 and the temperature
dependence of d spacings in Figure S3 in the Supporting
Information). All but one of the diffraction peaks of the
intermediate phase index on a rectangular 2D lattice of c2mm
symmetry with lattice parameters a = 6.33 nm and b =
4.30 nm (see Table S1 in the Supporting Information). How-
ever, the (001) reflection of the P6/mmm phase persists in the
intermediate phase unaltered. Hence, the intermediate phase
is 3D orthorhombic, c = 1.80 nm, space group Cmmm. As can
be seen from the maps in Figure 3a,b, at the P6/mmm–Cmmm
transition the averaged column cross section turns from
circular to elliptical. Figure 3b also shows the presence of
undulating RF-rich columns lying perpendicular to the main
biphenyl columns. Thus, this crossed column mesophase
consists of two orthogonal interpenetrating sets of columns,
the modulated columns of the rod bundles and the undulating
RF-rich columns.[14]
Interestingly, the P6/mmm–Cmmm transition occurs by
continuous deformation of the hexagonal lattice (see the
splitting of the (100P6/mmm) reflection into (110Cmmm) and
(200Cmmm) in Figure 2g and Figure S3 in the Supporting
Information). This finding is indicative of a second-order
transition. Accordingly, it is expected that local domains of
orthorhombic order also exist in the hexagonal phase, with
three equally probable orientations in the xy plane; that is, the
ribbons undergo correlated rotation. The idea is illustrated in
Figure 4, where three orthorhombic cells (Figure 4a) are
superimposed (Figure 4b). Note the maxima in electron
density (purple) where three undulated ribbons overlap.
These density maxima (e.g. the two labeled 1 and 2) are
clearly seen in Figure 3a (the dotted line in Figure 4 is the
On further cooling, at the transition to the LamSm/p2mm
phase, the ribbons fuse to form infinite sheets. Indeed, the
layer spacing a = 3.24 nm is only slightly larger than a/2 =
3.16 nm of the Cmmm phase (Figure 3c). As typically
observed for T-shaped amphiphiles with large lateral sub-
stituents, in lamellar (Lam) phases the rodlike units are
parallel to the layer plane.[3,10] As shown by the persistence of
the (001) reflection through the P6/mmm–Cmmm–Lam
sequence (Figure 2g), this periodicity is retained, confirming
that the Lam phase has 2D long-range order (correlated layer
phase, LamSm/p2mm).[15] At the Cmmm–LamSm transition the
RF-rich columns are retained but change from undulated to
straight. Thus, the LamSm phase features fluorinated columns
between aromatic sheets.[16] This arrangement can be com-
pared to a similar morphology recently reported in a rod–coil
miktoarm star terpolymer.[17]
General comments on the phase sequence in laterally
substituted bolaamphiphiles can be made with reference to
Figure 1. The mesophases on the right (axial-bundle phases of
compounds with large branched side chains) and on the left
(polygonal honeycombs in compounds with smaller side
chains), are the inverse of each other. Loosely speaking, if
the aromatic and glycerol moiety is replaced by polar groups
and water, the situation is analogous to direct (left) and
inverse (right) lyotropic phases in polar and apolar amphi-
philes. Even the elliptical deformation of the column cross
section, which leads to rectangular phases when the Col–Lam
transition is approached, is similar to the behavior observed in
other self-organized amphiphilic systems. Current results
suggest that at least in some of those amphiphiles the
hexagonal phase may also consist of rotationally averaged
ribbons.
Received: May 13, 2011
Published online: September 20, 2011
Figure 4. a) Schematic top view of the crossed columns of the Cmmm
phase; b) superposition of three figures (a) after in-plane rotation by
ꢀ608, 08, and +608.
Keywords: bolaamphiphiles · liquid crystals · mesophases ·
X-ray diffraction
.
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