Complex liquid-crystalline superstructure of a π-conjugated oligothiophene

Article abstract of DOI:10.1002/anie.200701585

(Chemical Equation Presented) A good connection: A polyphilic oligothiophene derivative (see formula) forms a complex liquid-crystalline phase, in which the π-conjugated rods are organized in a honeycomb-like network of square cylinders (see picture). Thi

Full text of DOI:10.1002/anie.200701585

Communications
DOI: 10.1002/anie.200701585
Liquid Crystals
Complex Liquid-Crystalline Superstructure of a p-Conjugated
Oligothiophene**
Marko Prehm, Günther Götz, Peter Bäuerle, Feng Liu, Xiangbing Zeng, Goran Ungar, and
Carsten Tschierske*
Amphiphiles consisting of a short rod-like aromatic core and
terminally and laterally attached chains incompatible with the
core and with each other have been shown to form a wide
range of novel liquid-crystal (LC) phases.[1,2] Most prominent
among them are honeycomb phases, an inverted version of
columnar phases in which the hard aromatic rods separate the
soft channels.[1,2] Bolaamphiphiles are one type of such LC-
forming compounds, having a biphenyl core, a polar group at
each end, and a nonpolar chain attached laterally.[1] The
purpose of the present work is twofold: 1) to investigate the
possibility of introducing significantly longer rod-like moi-
eties (in the oligomer range), thus scaling up the honeycomb
nanostructures and widening the scope of potential applica-
tions, and 2) to incorporate a building block with proven
electronic conductivity[3,4] to demonstrate that self-assembled
p-conjugated nanostructures of high complexity can be
produced with potentially superior electrical properties and
ease of alignment.
rod-like oligothiophene core, two polar hydrogen-bonding
groups at the ends to provide a bolaamphiphilic structure, and
a number of lipophilic alkyl chains laterally attached to the
rod-like core.[8,9] The length of the p-conjugated rod is about
three times that of the previous biphenyl-based bolaamphi-
philes, resulting in a substantial structural scale-up.
These new materials were synthesized by palladium-
catalyzed cross-coupling reactions[10] of a,a’-dibrominated
quaterthiophene 1 and glycerol-functionalized benzene bor-
onic acid 2,[1a] with subsequent deprotection of the diol groups
(Scheme 1). The final glycerol-functionalized oligothiophenes
4a–4c were purified by centrifugal TLC and subsequent
Herein, we present the first example of a honeycomb-like
structure involving p-conjugated oligothiophenes. Several
examples of thermotropic nematic, smectic,[5] and recently
also columnar organization of oligothiophene-based p-con-
jugated systems[6] have been reported. However, the self-
organized structures obtained in this way were still relatively
simple, restricted to layers and columns.[5–7] The structure
described herein is composed of cylinders with a large square
cross section, separated by p-conjugated walls. The com-
pound forming this arrangement was designed by combining a
Scheme 1. Synthesis of compounds 4a–c (a R=H, b R=CH3,
c R=C6H13).
[*] Dr. M. Prehm, Prof. Dr. C. Tschierske
Institute of Chemistry, Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Str. 2, 06120 Halle (Germany)
Fax: (+49)345-5525-346
repeated crystallization (see the Supporting Information for
synthetic procedures and analytical data). Compounds 4a–c
were investigated by polarizing microscopy, differential
scanning calorimetry (DSC), and X-ray diffraction. As
summarized in Table 1, compound 4a with two dodecyl
chains shows an enantiotropic liquid-crystalline phase,
whereas compound 4b with two additional methyl groups in
the periphery shows only a monotropic (metastable) meso-
phase on cooling from the isotropic melt. Compound 4c, in
which the methyl groups are replaced by longer hexyl groups,
is not liquid-crystalline. This finding indicates that these
peripheral chains inhibit LC phase formation, probably by
disturbing hydrogen bonding between the polar groups.
The following discussion is focused on the liquid-crystal-
line compound 4a. Upon cooling this compound from the
isotropic liquid state, a transition to a birefringent fluid phase
can be observed. The melting point is 1308C and the
E-mail: Carsten.tschierske@chemie.uni-halle.de
Dr. G. Götz, Prof. Dr. P. Bäuerle
Institut für Organische Chemie II und Neue Materialien
Universität Ulm
Albert Einstein Allee 11, 89081 Ulm (Germany)
F. Liu, Dr. X. Zeng, Prof. G. Ungar
Department of Engineering Materials
University of Sheffield
Mappin Street, Sheffield S1 3JD (UK)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(GRK 894), the government of Sachsen-Anhalt through the Cluster
of excellence “Nano-structured Materials” and the Fonds der
Chemischen Industrie. We thank Dr. C. Martin for helping to set up
the experiment at Daresbury Synchrotron and CCLRC for granting
the beam time.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Liquid-Crystalline Behavior of Compounds 4a–c.
Compd
R
Phase transitions[a]
4a
4b
4c
HCr 130 (31.5) Col squ/p4mm 151 (3.4) Iso
CH3
C6H13
Cr 122 (46.8) (M 109 (0.5))[b] Iso
Cr 126 (35.4) Iso
[a] Transition temperatures (T [8C]) and transition enthalpy values
(DH [kJmolÀ1], in parentheses) as determined by DSC (peak temper-
atures in the second heating scan) at a scanning rate of 10 KminÀ1
;
abbreviations: Cr=crystalline solid, Colsqu/p4mm=square columnar
liquid-crystalline phase with plane group p4mm, M=unknown meso-
phase, Iso=isotropic liquid; [b] monotropic mesophase, observed only
on cooling.
Figure 2. X-ray powder diffraction pattern of compound 4a at 1358C.
The inset shows the high-resolution powder pattern obtained with a
synchrotron source.
transition to the isotropic liquid takes place at 1518C (for
DSC traces, see Figure S1c in the Supporting Information). In
the temperature range of the liquid-crystalline phase, a
spherulitic texture typical for columnar phases can be
observed between crossed polarizers (Figure 1a). In addition
to the birefringent texture, large optically isotropic regions
can also be observed (Figure 1b). These findings indicate an
optically uniaxial LC phase, which could be either a
hexagonal or a square columnar phase.
on the conformation. Taking this value into account, it is
reasonable to assume that the rigid rod-like oligothiophene
moieties self-assemble into a honeycomb-like network with a
square cross section of the cells. As shown in Figure 3a, the
oligothiophene cores form the cylinder walls, which are fused
together by the hydrogen-bonding networks at the termini,
thus forming distinct columns running parallel to the edges.
The hydrogen-bonding networks within these columns give
rise to the formation of the square net structure, but they also
provide the attractive forces with a component parallel to the
columns, thus leading to the cylinder structure. The interior of
the square cells is filled with the lateral dodecyl chains. The
number of molecules in the unit cell, assuming a thickness of
0.48 nm (corresponding to the maximum of the diffuse wide-
angle scattering) was calculated as n = 4 on average. Accord-
ingly, the thickness of the partitioning walls amounts to
approximately two aromatic cores.
This model is in complete agreement with the recon-
structed electron density map shown in Figure 3b, which was
obtained from the intensities of the three small-angle
reflections measured from the high-resolution powder pattern
obtained with a synchrotron source (see inset in Figure 2).
The intensities and a description of the electron-density
reconstruction procedure are given in the Supporting Infor-
mation. The map clearly shows the square cross section of the
regions with lowest electron density (red), representing the
central alkyl columns, and a higher electron density for the
square frames around these columns, formed by the aromatic
cores and the diol groups. Consistent with the model, the
electron density is the highest (purple) in the middle of the
cylinder walls where the electron-rich sulfur atoms of the
thiophene rings are located. The hydrogen-bonding networks
of the diol groups provide an intermediate electron density
(blue) at the edges.
Figure 1. Polarized optical photomicrographs of the Colsqu phase of
compound 4a at 1408C as seen between crossed polarizers (a color
version is shown in Figures S1a and b in the Supporting Information);
dark areas, especially visible in (b), represent homeotropically aligned
regions in which the orientation of the long cylinder axes is uniform
and perpendicular to the substrate surfaces.
The X-ray powder diffraction pattern is shown in Figure 2
(see also Figure S2 in the Supporting Information). The wide-
angle scattering is diffuse and has its maximum at d = 0.48 nm,
which confirms the liquid-crystalline nature of this phase. In
the small-angle region, two intense peaks corresponding to
distances d = 3.52 and 2.49 nm and an additional weak peak
corresponding to d = 1.77 nm were observed. These peaks can
be indexed to the (10), (11), and (20) reflections of a square
lattice with p4mm plane group symmetry and a lattice
parameter asqu = 3.5 nm. This lattice parameter corresponds
to the length L of the molecule 4a measured between the two
diol groups, which is between L = 3.3 and 3.7 nm, depending
The above results confirm that it is possible to construct
complex self-organized superstructures by appropriate
molecular design of extended p-conjugated rods. Though
liquid-crystalline cylinder phases were recently reported for
bolaamphiphilic biphenyl derivatives[1] and facial amphiphilic
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Communications
focus on exploring the electronic properties of the LC phases
of these p-conjugated LC materials and the crystalline phases
developing below the LC transitions, which is of interest for
their potential use in self-organized organic electronic
devices.
Received: April 11, 2007
Revised: August 9, 2007
Published online: September 4, 2007
Keywords: liquid crystals · molecular electronics ·
.
oligothiophenes · self-assembly · X-ray diffraction
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Figure 3. a) Organization of compound 4a in the Colsqu/p4mm phase;
white=cylinders of the alkyl chains; blue=hydrogen-bonding net-
works; gray=aromatic cores. b) Reconstructed electron density map;
four unit cells are shown. Density regions: high (pink)=thiophenes;
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