Amphiphilic design of a discotic liquid-crystalline molecule for dipole manipulation: Hierarchical columnar assemblies with a 2D superlattice structure

Article abstract of DOI:10.1002/anie.201207708

A liquid-crystalline dibenzophenazine that was forced to adopt a head-to-head stacking arrangement to generate a large dipole moment adapted to this stacking by hierarchical assembly into a 2D hexagonal superlattice (see picture). Thus, the net dipole and the contact area between immiscible side chains were minimized. Homeotropic alignment of the assembly on glass gave rise to directional charge transport. Copyright

Full text of DOI:10.1002/anie.201207708

Angewandte
Chemie
DOI: 10.1002/anie.201207708
Liquid Crystals
Amphiphilic Design of a Discotic Liquid-Crystalline Molecule for
Dipole Manipulation: Hierarchical Columnar Assemblies with a 2D
Superlattice Structure**
Ming-Che Yeh, Yu-Lou Su, Mei-Chun Tzeng, Chi Wi Ong,* Takashi Kajitani,* Hideo Enozawa,
Masaki Takata, Yoshiko Koizumi, Akinori Saeki, Shu Seki, and Takanori Fukushima
Columnar liquid-crystalline (LC) materials composed of disk-
shaped aromatic molecules arranged in one-dimensional (1D)
columns have attracted increasing attention owing to their
potential utility for solution-processable organic electronic[1]
and ionic devices.[2] Because carrier transport relies on these
1D columns, a long-range intracolumnar molecular order is
particularly important. Charge-transfer complexation has
been reported to be effective in reinforcing the intracolumnar
1D order of discotic columnar LC assemblies.[3] Williams and
co-workers demonstrated that the incorporation of electron-
withdrawing substituents into aromatic mesogens results in
the enhancement of p stacking to stabilize the LC state.[4] This
molecular-design strategy based on so-called “p polariza-
tion”[4,5] probably gives rise to a dipole in the aromatic
mesogens. As a result, LC molecules tend to p stack in a head-
to-tail manner, and the dipole is canceled out within
individual columns (Figure 1b). In this context, one may
wonder how LC molecules composed of p-polarized meso-
gens assemble when they bear a particular functionality that
would hamper head-to-tail stacking, and in turn, how the
entire LC assembly would cope with the large intracolumnar
dipole generated upon head-to-head stacking (Figure 1c).
Herein we report the interesting finding that such a molecular
Figure 1. a) Molecular structures of dibenzo[a,c]phenazine derivatives
C10, 1TEG, and 1EG and the benzo[b]triphenylene derivative 2TEG
b,c) Schematic illustrations of 1D columnar assemblies with head-to-
tail (b) and head-to-head arrangements (c) of the aromatic mesogen.
Yellow arrows indicate the dipole moment of the dibenzo[a,c]phenazine
core.
1
.
design, uncomfortable for the molecule in terms of dipole
interactions, leads to the formation of a 2D superlattice
structure that is unprecedented in columnar LC assemblies
composed of disk-shaped aromatic molecules. We noticed
that this hierarchical structure not only has the advantage that
the dipoles are canceled out intercolumnarly but also that
a homeotropic alignment of the LC columns is adopted.
Dibenzo[a,c]phenazine, which possesses a dipole moment
along the longer molecular axis,[6] is known to serve as
a mesogenic core for columnar LC assemblies (see Figure S1a
and Table S2 in the Supporting Information).[4a–d] We
previously reported that the derivative with six decyloxy
side chains, 1C10 (Figure 1a), exhibits a hexagonal columnar
(Colh) mesophase over a wide temperature range.[7] In study
described herein, we designed an amphiphilic derivative, 1TEG
(Figure 1a), with two triethylene glycol (TEG) chains on the
phenazine ring and four decyloxy chains on the fused benzene
rings. We anticipated that this amphiphilic derivative could
adopt a head-to-head arrangement upon p stacking if micro-
phase separation between immiscible TEG and paraffinic side
chains occurred in preference to cancellation of the dipole
(Figure 1c). Compound 1TEG was synthesized by a procedure
similar to that reported previously (see Scheme S1 in the
Supporting Information). As reference molecules, we also
prepared 1EG (Figure 1a), with shorter oxyethylene side
chains, and 2TEG, which was obtained by a ring-closing
reaction of the corresponding 2,3-bisphenylnaphthalene
derivative (see Scheme S2 in the Supporting Information).
All new compounds were characterized unambiguously by 1H
[*] M.-C. Yeh, Y.-L. Su, M.-C. Tzeng, Prof. Dr. C. W. Ong
Department of Chemistry, National Sun Yat Sen University
Kaohsiung 804 (Taiwan)
E-mail: cong@mail.nsysu.edu.tw
M.-C. Yeh, Dr. T. Kajitani, Dr. H. Enozawa, Dr. Y. Koizumi,
Prof. Dr. T. Fukushima
RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
E-mail: kajitani@riken.jp
Prof. Dr. T. Fukushima
Chemical Resources Laboratory, Tokyo Institute of Technology
R1-1 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
Prof. Dr. M. Takata
RIKEN SPring-8 Center
1-1-1 Kouto, Sayo, Hyogo 679-5198 (Japan)
Dr. Y. Koizumi, Dr. A. Saeki, Prof. Dr. S. Seki
Department of Applied Chemistry, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
[**] This research was supported by KAKENHI (21350108 (T.F.) and
23750170 (T.K.)). The synchrotron X-ray diffraction experiments
were performed at BL45XU in the SPring-8 Center with approval
(proposal 20110065).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207708.
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and 13C NMR spectroscopy and MALDI-TOF mass spec-
trometry.[6]
In differential scanning calorimetry (DSC) studies, 1TEG
displayed an LC mesophase between 69 and 808C (on
heating) and between 76 and 578C (on cooling; Figure 2;
corresponds to the lateral core-to-core distance between 1D
columns of 1TEG. The diffraction peak at q = 17.8 nmꢀ1 can be
assigned to the face-to-face distance (0.35 nm) of the p-
stacked 1TEG molecules along the columnar axis. Two
diffraction peaks with d spacings of 4.11 and 2.40 nm were
detected in a smaller-angle region (Figure 3a). By considering
pffiffiffi
the reciprocal spacing ratio (1: 3), we attributed these peaks
to diffractions from the (100) and (110) planes of an addi-
tional hexagonal lattice with a lattice parameter (a’) of
4.75 nm (Figure 3b, green hexagon). These observations
clearly indicate that LC 1TEG forms a 2D hexagonal super-
lattice structure.
As reported previously,[7] compound 1C10, which bears
decyloxy side chains (Figure 1a), forms a Colh structure in the
LC mesophase with a lattice parameter identical to that of the
smaller hexagonal structure observed for LC 1TEG (2.55 nm).
However, even in a detailed XRD study of 1C10 with
a synchrotron radiation source, we were unable to detect
diffraction peaks indicative of the presence of a 2D super-
lattice (see Figure S5).[6] We also investigated the phase
behaviors of 1EG and 2TEG (Figure 1a) by means of DSC and
temperature-dependent XRD and found that neither com-
pound exhibited a LC mesophase but rather that both
underwent a crystal-to-melt or glass-to-melt transition (see
Figures S2, S4, and S6, and Table S1).[6] There is a clear
difference between the structures of 1TEG and 1C10, which only
bears paraffinic side chains. Compound 1EG has oxyethylene
side chains, but they are too short to endow the molecule with
a distinct amphiphilic character. Although 2TEG is similar in
structure to 1TEG, its benzo[b]triphenylene core is devoid of
nitrogen atoms, and the existence of a dipole along the longer
molecular axis can hardly be expected (see Figure S1b and
Table S3).[6]
Figure 2. DSC trace for a second heating/cooling cycle (scan rate: 58C
minꢀ1) of 1TEG. Cr: crystal, Colh: hexagonal columnar LC phase, Iso:
isotropic melt.
see also Table S1 in the Supporting Information). The powder
X-ray diffraction (XRD) pattern of the LC mesophase of 1TEG
in a glass capillary showed four peaks with d spacings of 2.21,
1.27, 1.10, and 0.84 nm (Figure 3a; see also Figure S3 in the
Supporting Information), which were indexed as diffractions
from the (100), (110), (200), and (210) planes, respectively.
Thus, LC 1TEG forms a Colh structure with a lattice parameter
(a) of 2.55 nm (Figure 3b, orange hexagon). This value
All of the above observations allow us to rationalize the
formation of the 2D superlattice structure in LC 1TEG as
shown in Figure 4. We assume that side-chain miscibility
prevails over the dipole–dipole interaction and results in
a head-to-head p-stacking arrangement of 1TEG (Figure 4b).
To minimize both the net dipole of the LC system and the
contact area of the immiscible side chains, p-stacked columns
of 1TEG assemble triangularly into a cylindrical architecture
(Figure 4c), with the TEG side chains localized in the center.
It is likely that the resultant cylinders, each of which consists
of three columns of p-stacked 1TEG, arrange laterally to form
an extended hexagonal lattice (Figure 4d). Indeed, the small
(a = 2.55 nm) and large lattice parameters (a’ = 4.75 nm)
determined by XRD are consistent with the geometry of
the proposed hierarchical structure.
Interestingly, the LC columns of hierarchically assembled
1TEG spontaneously align homeotropically on a glass sub-
strate. Under a polarized optical microscope (POM), a film of
1TEG sandwiched between glass plates at a mesophase temper-
ature (758C) showed a dark field over a large area (Figure 5a,
inset). On the other hand, optical microscopy (OM) showed
dendritic textures (Figure 5a). These observations are typical
for homeotropically aligned Colh assemblies.[8] In contrast,
a POM image of an LC film of paraffinic 1C10 showed both
bright birefringent texture and a dark field as a result of LC
domains of horizontally and homeotropically aligned col-
Figure 3. a) Synchrotron radiation XRD pattern of 1TEG at 758C on
heating in a glass capillary (f=1.5 mm) and a magnification of the
pattern (inset: ꢀ20, scattering vector q=1–9 nmꢀ1). Values in paren-
theses are Miller indices. b) Schematic illustration of the small
(orange) and large (green) hexagonal lattices observed by XRD for LC
1TEG
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Figure 4. Hierarchical self-assembly of 1TEG: a) 1TEG molecule; b) p-stacked column of 1TEG with a head-to-head arrangement; c) triangular
assembly of p-stacked columns; d) hexagonal superlattice with structural parameters indicated. The yellow arrows indicate the dipole moment of
the dibenzo[a,c]phenazine core.
Sm are the photocarrier-generation yield and the sum of the
mobilities of generated charge carriers, respectively (Fig-
ure 5d). From the FP-TRMC profile, the maximum transient
conductivities (fSmmax) along the perpendicular (orange) and
parallel (green) directions to the substrate surface were
evaluated as 1.0 ꢀ 10ꢀ4 and 2.7 ꢀ 10ꢀ5 cm2 Vꢀ1 sꢀ1, respectively.
Accordingly, the degree of anisotropy (fSm /fSm ) in the LC
?
k
film of 1TEG was calculated to be 3.7. Although an LC film of
1C10 also displayed a FP-TRMC signal (Figure 5e), the value
of fSm (1.7 ꢀ 10ꢀ4 cm2 Vꢀ1 sꢀ1) was lower than that of fSm
?
k
(2.6 ꢀ 10ꢀ4 cm2 Vꢀ1 sꢀ1), and the degree of anisotropy was as
small as 0.7.
It has been suggested that the homeotropic columnar
orientation of LC materials is possible when nucleation
occurs from a glass/LC interface rather than from the bulk.[10]
A recent report also showed that long-range 2D lattice order
can be a dominant factor for such a columnar orientation.[11]
In light of these notions, the strong preference of LC 1TEG for
a homeotropic orientation most likely results from the
hierarchical superlattice structure, which could enhance the
2D order of columnarly assembled 1TEG to a certain upper
length scale. Furthermore, the TEG side chains of 1TEG
localize at each lattice point of the superlattice (Figure 4d),
and this arrangement gives rise to hydrophilic domains that
can potentially interact with hydroxy groups on a SiO2
substrate. Presumably, such a surface event also plays a role
in the development of homeotropic alignment.
Figure 5. a) OM micrograph (inset: POM micrograph) of 1TEG at 758C
and b) POM micrograph of 1C10 at 1008C, on cooling from the
isotropic melt (scale bar: 50 mm). c) Schematic illustration of an LC
film of 1TEG sandwiched between quartz plates for an FP-TRMC
experiment. d,e) FP-TRMC profiles at 708C of LC films of 1TEG (d) and
1C10 (e) as measured perpendicular (orange) and parallel (green) to
the microwave electric-field vector.
umns, respectively (Figure 5b). Thus, LC 1C10 does not adopt
a homeotropic orientation predominantly.
The contrasting orientational behavior of LC 1TEG and 1C10
was clearly demonstrated by the measurement of flash-
photolysis time-resolved microwave conductivity (FP-
TRMC). The FP-TRMC technique enables the evaluation,
without electrodes, of the intrinsic carrier-transport proper-
ties of materials.[9] For this experiment, an LC film of 1TEG
sandwiched between two quartz plates was placed in a micro-
wave resonant cavity in such a way that the microwave
electric-field (E-field) vector could be polarized either
perpendicular or parallel to the substrate surface (Figure 5c).
Upon exposure of the LC film to laser light, the transient
conductivity of the LC film displayed a rise and decay profile,
whereby the transient conductivity is given by fSm, and f and
In conclusion, we demonstrated that the LC dibenzo-
[a,c]phenazine 1TEG, which contains decyloxy and TEG side
chains, self-assembles to form a 2D hexagonal superlattice.
This type of hierarchical structure has not previously been
reported for columnar LC assemblies composed of disk-
shaped aromatic molecules. In contrast to most reported
examples,[12] we ventured to adopt a molecular design that
could hamper a head-to-tail packing arrangement, which is
favorable for the canceling out of dipoles between adjacent
molecules. As a consequence, 1TEG chooses an unprecedented
way to cancel out the dipoles in the LC material: 1D
triangular assemblies consisting of three p-stacked columns of
1TEG arrange laterally to form a large hexagonal lattice
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Received: September 24, 2012
Published online: November 26, 2012
Keywords: amphiphiles · dipole–dipole interactions ·
.
liquid crystals · organic semiconductors · self-assembly
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