2D and 3D ordered columnar liquid crystal phases by bundles of bolaamphiphiles with swallow-tail side chains

Article abstract of DOI:10.1021/ja805742t

Two new mesophases, a longitudinally correlated and a noncorrelated (columnar) hexagonal phase, were obtained for a polyphilic bolaamphiphile with laterally attached swallow-tail chains; the unique feature of these mesophases, which makes them distinct from previously reported LC phases, is the organization of rod-like units in bundles parallel to the column axis. Copyright

Full text of DOI:10.1021/ja805742t

Published on Web 10/21/2008
2D and 3D Ordered Columnar Liquid Crystal Phases by Bundles of
Bolaamphiphiles with Swallow-Tail Side Chains
Marko Prehm,† Feng Liu,‡ Xiangbing Zeng,‡ Goran Ungar,*,‡ and Carsten Tschierske*,†
Martin-Luther-UniVersity Halle-Wittenberg, Institute of Chemistry, Organic Chemistry, Kurt-Mothes Str. 2,
D-06120 Halle, Germany, and Department of Engineering Materials, UniVersity of Sheffield, Mappin Street,
Sheffield S1 3JD, U.K.
Received July 23, 2008; E-mail: g.ungar@sheffield.ac.uk; carsten.tschierske@chemie.unihalle.de
Liquid crystalline (LC) self-assembled superstructures combine
order and mobility, the features indispensable for their technological
applications, as well as being at the basis of life.1 The discovery
that disk-like molecules can organize into LC phases in which the
molecules stack up in columns2 initiated intense activity in the field
of columnar LC phases.3 Especially the overlapping π-systems of
aromatic disk-like molecules within the self-assembled columns lead
to interesting charge carrier properties which stimulated much
research in this field.4 Like disk-like molecules, rod-like (calamitic)
molecules can also form columnar LC phases if two or three alkyl
chains are attached to each end of a rod-like unit.5,6 In the columnar
LC phases of these polycatenar compounds5 and related dumbbell
molecules,7 the layers of smectic phases are disrupted into stripes
which adopt a periodic 2D organization. In these columns the rod-
like cores are on average perpendicular or highly tilted to the column
long axis (Figure 1b-d). Hence, in none of the known low-molar-
mass columnar LC phases is the mesogen long axis parallel to the
column axis (however for polymers, see below). MC/MD simula-
tions by Frenkel et al. using hard spherocylinders are ambiguous
about the existence of such a phase, the large system favoring a
3D “crystal”.8
(see below). The observed texture is characteristic for columnar
phases (Figure 2b). In the whole temperature range the X-ray
diffraction pattern (Figure 2d) is characterized by diffuse wide angle
scattering (maximum at d ) 0.50 nm), confirming the LC nature,
i.e. lack of preferred molecular position, in both mesophases. The
small angle diffraction pattern of a surface aligned sample (Figure
2d) indicates a 2D lattice with hexagonal symmetry. This is
confirmed by the 1:1/ꢀ3:1/2 ratio of d-values of small angle powder
reflections (see Figure 3a, ahex ) 3.11 nm).
Figure 2. Compound 1: (a) Molecular structure; phase transitions (second
DSC heating scan, 10 K min-1): Cr 81 °C [21.3 kJ ·mol-1], 3D 130 °C [-],
Colhex154 °C [7.8 kJ ·mol-1], Iso; Cr ) crystalline solid, Iso ) isotropic
liquid, Colhex ) hexagonal columnar phase, 3D ) correlated Colhex phase.
(b) Texture between crossed polarizers at 140 °C. (c) Texture with a λ-plate
retarder, the indicatrix orientation in the compensator, and in the two types
of fans is shown in the inset. (d) X-ray diffraction pattern of a surface aligned
sample at T ) 129 °C (X-ray beam parallel to the surface).
Figure 1. Columnar LC phases formed by molecules incorporating
anisometric units: (a) disk-like molecules and (b-d) polycatenar rod-like
molecules; in (c) and (d) the view is along the columns.
The electron density map reconstructed from the small angle
diffraction pattern in the Colhex phase at 140 °C is shown in Figure
3c (see Supporting Information for details). Each column has a
central circular density minimum (red) and six maxima at the
periphery (purple). Due to dynamic averaging the minimum contains
the aromatic cores, glycerol units, and much of the alkyl parts of
the side chains, while the peripheral maxima are dominated by the
electron-rich perfluorinated segments. From these data it is con-
cluded that in this Colhex phase the bolaamphiphilic cores (terphenyls
+ glycerol groups) form the column cores which are surrounded
by the fluid continuum of the lateral chains. That the bolaam-
phiphilic cores are oriented parallel to the column axis is shown
by the color of the fans in the optical micrograph (Figure 2c) taken
with a λ-plate. The yellow and blue colors define the orientation
of the high-index axis as tangential rather than radial. Since the
columns are also tangential in the fans,10 and the high-index axis
is known to be parallel to the terphenyl long axis, it follows that
the terphenyls are coaxial with the columns.
Here we report two new modes of organization in columnar LC
phases where rod-like mesogenic units do lie parallel to the column
axis. This is achieved by lateral attachment of two branched
(swallow-tail) chains to a rigid p-terphenyl unit. Each of the
swallow-tail groups has a semiperfluorinated and an alkyl branch.
Glycerol groups are attached to both ends of the rod-like units,
providing end-to-end linking by hydrogen bonding (compound 1;
see Figure 2a). At reduced temperature long-range periodicity
gradually develops in addition to the 2D periodic spacing of
columns, giving rise to a 3D phase.
Compound 19 shows an enantiotropic (stable) liquid crystalline
phase between 81 and 154 °C with a phase transition at 130 °C
† Martin-Luther-University Halle-Wittenberg.
‡ University of Sheffield.
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14922 J. AM. CHEM. SOC. 2008, 130, 14922–14923
10.1021/ja805742t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
At ca. 130 °C an additional weak diffraction peak, corresponding
to d ) 2.03 nm, appears in the X-ray diffraction pattern (see Figure
3b), which upon further cooling continuously increases in intensity.
At the same time hk0 reflections show virtually no change. This
seems to indicate a second-order phase transition, and accordingly,
no peak can be found in the DSC traces (see Figure S1). Also the
optical texture does not change; only a continuous increase in
birefringence with decreasing temperature is seen. The width of
this additional peak is resolution limited, and its spacing corresponds
to the length of the bolaamphiphilic core (L ) 2.0-2.1 nm in a
conformation with compact glycerol groups). Therefore, it is
assigned to the establishment of long-range order (correlation length
at least 120 nm) along the column axis arising from the segregation
of the aromatic cores from the polar glycerol groups (Figure 3d).
In this mesophase adjacent columns must be in register, leading to
a mesophase with a 3D lattice based on a hexagonal arrangement
of columns. The intensification of the additional reflection suggests
that intercolumnar order increases with decreasing temperature.
in some respect related to columnar mesophases of hairy rod main
chain polymers,15 as the systems can be viewed as hydrogen-bonded
supramolecular polymers with the lateral chains as hairy side groups.
However, in contrast to these polymers the molecules reported here
are organized in bundles. Even some flexible and semiflexible
polymers without lateral chains exhibit a main-chain columnar LC
phase, but there again each chain forms a column of its own.16,17
The hairy rod polymers represent an important class of semicon-
ducting organic electronic materials for potential use in OLEDs,
organic transistors, and photovoltaic devices.18 Hence, investigation
of the low molecular weight molecules reported herein can provide
clues concerning the controlled organization of important functional
materials by directed design of molecular tectons.
Acknowledgment. This work, as part of the ESF EUROCORES
Programme SONS, was supported by funds from the DFG, EPSRC,
and the EC 6th Framework Programme, under Contract ERAS-
CT-2003-989409. We thank Dr. P. Boesecke for help with the
experiment on station ID02 and ESRF for the beamtime.
Supporting Information Available: DSC, X-ray data, synthesis and
analytical data. This material is available free of charge via Internet at
http://pubs.acs.org.
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Figure 3. Compound 1: Powder diffraction pattern (a) at T ) 140 °C
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