Columnar liquid crystalline π-conjugated oligothiophenes

Article abstract of DOI:10.1039/b606144c

Polycatenar oligothiophenes possessing three alkoxy chains at each terminal self-organise into columnar liquid crystalline phases, and one-dimensional columnar stacks can be oriented uniaxially by mechanical shearing in the mesophases. The Royal Society o

Full text of DOI:10.1039/b606144c

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Columnar liquid crystalline p-conjugated oligothiophenes{
Takuma Yasuda, Kenji Kishimoto and Takashi Kato*
Received (in Cambridge, UK) 2nd May 2006, Accepted 19th June 2006
First published as an Advance Article on the web 10th July 2006
DOI: 10.1039/b606144c
respectively (see ESI{). They were soluble in common organic
Polycatenar oligothiophenes possessing three alkoxy chains at
each terminal self-organise into columnar liquid crystalline
phases, and one-dimensional columnar stacks can be oriented
uniaxially by mechanical shearing in the mesophases.
solvents such as chloroform, dichloromethane, THF and toluene.
The mesomorphic properties of 1a–c and 2a–c were examined
by differential scanning calorimetry (DSC) and polarised optical
microscopy. As summarised in Table 1, compounds 1a,b and 2b,c
exhibit enantiotropic columnar (Col) phases, whereas 1c and 2a
show monotropic Col phases only on cooling from the isotropic
melt. As a typical example, 1a bearing six dodecyloxy substituents
forms a Col phase at 79 uC (DH 5 27 kJ mol21) that becomes an
isotropic state at 101 uC (DH 5 1.7 kJ mol21).{ A fan texture
characteristic of columnar phases is observed under crossed
polarisers, as shown in Fig. 2. In contrast, analogous terthiophene-
and quaterthiophene-based compounds having six terminal alkoxy
chains did not show any liquid crystalline properties.{ Therefore, it
should be pointed out that appropriate choice of the core length
leads to the polycatenar oligothiophenes that are capable of
forming Col phases. The phase behaviour of these new compounds
is markedly different from that of previously reported liquid
crystalline oligothiophenes,8–10 in which nematic and/or smectic
Liquid crystalline p-conjugated materials have recently attracted
much attention because they have applications as electronic
materials that easily form macroscopically ordered nanostruc-
tures.1–3 Among them, columnar liquid crystals, e.g., triphenylene,
hexabenzocoronene and perylene diimide derivatives, have been
prepared for one-dimensional transportation of charge carriers,
and give rise to high mobilities along the columns.2 In order to
attain electronic and anisotropic conduction properties, precise
control of the mesoscopic order is of great importance for such
materials.4,5
p-Conjugated oligothiophenes are promising organic semicon-
ductors in the application of field-effect transistors (FETs).6
Recent research efforts have been devoted to the exploitation of
oligothiophene derivatives that could construct well-ordered
molecular assemblies having unique electrical and optical proper-
ties.7 Several smectic and nematic liquid crystalline oligothiophenes
have been reported so far,8–10 and some have also been applied in
FETs.10 Herein we present the first example of thermotropic
columnar liquid crystals based on p-conjugated oligothiophene
(Fig. 1). Our molecular design features three alkoxy chains
tethered to each terminal of the central aromatic core,11,12 which
facilitates one-dimensional molecular stacks in the liquid crystalline
phase.
Table 1 Liquid crystalline behaviour of 1a–c and 2a–c
c
Lattice parameter (A)
Compound Phase transition behavioura
˚
1a
1b
1c
2a
2b
2c
a
Cr 79 Colh 101 Iso
Cr 62 Cr9 80 Colh 100 Iso
Cr 69 Cr9 109 (Col 90)b Iso
45.6
47.8
d
—
Cr 118 Cr9 131 (Colh 109)b Iso 49.5
Cr 91 M 109 Colh 116 Iso
Cr 85 Cr9 102 Colh 117 Iso
51.4
55.3
New polycatenar quinquethiophenes (1a–c) and sexithiophenes
(2a–c) were synthesised by using palladium-catalysed coupling
reactions, and obtained as orange and reddish purple solids,
Transition temperatures (uC) determined by DSC (second heating;
5
uC min21). Cr: crystalline; M: mesophase; Colh: hexagonal
columnar; Iso: isotropic. Observed only on cooling. Colh phase at
b
c
d
90 uC for 1a,b and 105 uC for 2a–c. Impossible to obtain the XRD
pattern of the mesophase, but the optical texture is consistent with a
Col phase.
Fig. 1 Structures of columnar liquid crystalline oligothiophenes.
Department of Chemistry and Biotechnology, School of Engineering,
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
E-mail: kato@chiral.t.u-tokyo.ac.jp; Fax: (+81) 3-5841-8661
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures and characterisation for 1a–c and 2a–c and DSC, XRD and
polarised IR data. See DOI: 10.1039/b606144c
Fig. 2 Polarised optical photomicrograph of 1a in the Colh phase at
90 uC.
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Fig. 4 Polarised optical photomicrographs of the uniaxially oriented 1a
in the Colh phase at 90 uC by means of mechanical shearing. Rotating the
sample by 45u produces periodic bright (left) and dark (right) images under
the crossed Nicols condition. Arrows indicate the directions of the
mechanical shear force and polariser (P) and analyser (A) axes.
Fig. 3 X-ray diffraction pattern of 1a at 90 uC.
The macroscopically uniform alignment of columns might lead to
anisotropic charge carrier transport along the molecular stacking.4
We next studied spectroscopic properties of the polycatenar
oligothiophenes in the solution and condensed states (Fig. 5). The
UV–vis absorption spectrum of 1a in chloroform displays the p–p*
absorption maximum at 462 nm,§ which is shifted by about 40 nm
with respect to a,v-dialkyl-quinquethiophenes.15 This implies that
the carbonyl groups in 1a are effectively conjugated with the
central oligothiophene core. In the liquid crystalline and solid
states, the absorption peak is blue-shifted (ca. 10 nm) compared
with that in the solution, besides a new broad band arises around
540 nm. The photoluminescence (PL) spectrum of 1a in the Colh
phase exhibits a broad and red-shifted emission peak at 585 nm,
while the solution displays a structured emission with maxima at
544 and 575 nm (Fig. 5b). These spectral changes can be ascribed
to the formation of p-stacked aggregates with H-type parallel
stacking mode;16 these optical features support the aforementioned
self-organised p-stacked structure observed in the Colh phase.
In summary, we have demonstrated for the first time that
oligothiophenes exhibit columnar liquid crystalline phases. We
incorporated six flexible chains into the oligothiophenes to form
polycatenar mesogens. The intermolecular p–p interaction of
oligothiophene moieties and the nano-segregation are attributed
predominantly to the formation of such one-dimensional supra-
molecular assemblies. Uniaxial orientation of columnar structures
has been achieved in the mesophase. These columnar liquid
crystalline p-conjugated oligothiophenes are expected to be
phases are exclusively observed on account of their calamitic
molecular structures.
To confirm the columnar structures of the mesophases, X-ray
diffraction (XRD) measurements were performed for the
oligothiophenes. A representative XRD pattern of 1a is shown
˚
in Fig. 3. In the small-angle region, an intense peak at 39.6 A and
˚
two weak peaks at 22.8 and 19.7 A with a reciprocal d-spacing
ratio of 1 : !3 : 2 are observed. These peaks correspond to the (10),
(11) and (20) reflections, respectively, indicative of a two-
dimensional hexagonal arrangement of the columns with the
˚
intercolumnar distance of 45.6 A. It should be noted that the
intercolumnar distance of 1a is much smaller than the fully
˚
extended molecular length (ca. 57 A calculated by AM1). This
discrepancy may indicate that the oligothiophene molecules are
tilted with respect to the cross section of each column.3c The
intercolumnar distance increases with the elongation of the alkoxy
chain length or of the oligothiophene core length (Table 1).{
Taking these results into account, it is reasonable to assume that
the rigid-rod oligothiophene moieties self-assemble into a one-
dimensional domain and the outer part of the columns is filled
with the molten alkoxy segments, similar to the previously
reported polycatenar mesogens11,12 and rod–coil block mole-
cules.13 For these oligothiophenes, it can be estimated that
approximately 2.3–3.4 molecules are included inside a stratum of
each column on average.{ The intermolecular co-facial p–p
interaction between the oligothiophene moieties as well as nano-
segregation of the aromatic centre from the surrounding alkoxy
moieties will facilitate the organisation of these molecules into such
a columnar assembly in the mesophase.1b,14
Uniaxial orientation of the columnar structures is readily
attainable after the polydomain of the Colh phase of 1a is
mechanically sheared within the sandwiched glass plates.5
Polarised optical microscopy reveals that the columnar material
tends to align parallel to the shearing direction. As can be seen in
Fig. 4, the birefringence of the oriented sample of 1a alternately
changes from light to dark upon a 45u rotation under the crossed
Nicols condition.2e,5 In addition, polarised infrared spectra of the
oriented 1a in the Colh phase exhibited dichroism for the aromatic
CLC stretching band of the oligothiophene core at 1435 cm21.{
This observation can support the assumption that the p–p stacking
direction of the oligothiophenes is parallel to the shear direction.
Fig. 5 (a) UV–vis absorption and (b) PL spectra of 1a in chloroform
solution (red), Colh phase at 90 uC (green) and solid thin film (blue).
3400 | Chem. Commun., 2006, 3399–3401
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attractive candidates for charge-transporting materials in organic
optoelectronic devices.
Partial financial support by Grants-in-Aid for Creative Scientific
Research of ‘‘Invention of Conjugated Electronic Structures and
Novel Functions’’ (No. 16GS0209) (T. K.) from the Japan Society
for the Promotion of Science (JSPS), for Scientific Research B
(No. 17350065) (T. K.) and for The 21st Century COE Program of
‘‘The Frontiers of Fundamental Chemistry Focusing on Molecular
Dynamism’’ (T. K. and T. Y.) from the Ministry of Education,
Culture, Sports, Science and Technology is gratefully acknowl-
edged. We also thank Drs M. Yoshio and T. Hatano at The
University of Tokyo for helpful discussions. T. Y. is grateful for
financial support from the JSPS Research Fellowship for Young
Scientists.
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{ The average number of molecules in a column stratum (m) was estimated
according to the following equation: m 5 (!3 NA a2 h r)/2M, where NA is
Avogadro’s number, a is the lattice parameter (cf. Table 1), h is the layer
thickness (estimated as ca. 3.6–4.8 A based on the X-ray crystal structures
of oligothiophene derivatives),6i,15 r is the density (assumed to be 1 g cm23
and M is the molecular weight of the compound.
§ Compound 2a in chloroform showed the UV–vis and PL peaks at 470
and 548 nm, respectively.
˚
)
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