Columnar mesophases constructed by hierarchical self-organization of rod-like diacetylene molecules

Article abstract of DOI:10.1039/c1jm10817d

We report a new class of liquid crystalline system exhibiting columnar mesophases, which are constructed by the hierarchical supramolecular self-organization of a rod-like diacetylene derivative as the primary building block. A series of 1,3-alkadiynyl-4-benzoic acids (mDA) with different alkyl chain lengths (m = 8-16) were synthesized and their mesophase properties were investigated by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction under temperature control. The mDA exhibited three types of mesophases, i.e., a discotic lamellar phase, D_L, and two columnar rectangular phases, Col_rd and Col_ro, with disordered and ordered stacking structures of the mesogens in a column, respectively, depending on the length of the alkyl substituent and temperature. We have revealed that the columnar mesophase structures are constructed with the rod-like mDA molecules as the starting building blocks by hierarchical self-organization. The hydrogen-bonded dimers of the carboxylic acid with a rigid framework are assembled to provide a disc-like mesogen unit, which consists of a π-stacked core-part and peripheral alkyl-chains. The discotic supramolecular mesogens further stack in ordered and disordered columnar structures, producing the Col_ro and Col_rd mesophases, or in the layer D_L mesophase. This is a rare example for the discotic mesophases using the rod-like diacetylenes with a non-branched molecular structure.

Full text of DOI:10.1039/c1jm10817d

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PAPER
Columnar mesophases constructed by hierarchical self-organization
of rod-like diacetylene molecules†
a
Tomoyo Shimogaki, Satoshi Dei, Kazuchika Ohta and Akikazu Matsumoto
a
b
a
*
Received 24th February 2011, Accepted 17th May 2011
DOI: 10.1039/c1jm10817d
We report a new class of liquid crystalline system exhibiting columnar mesophases, which are
constructed by the hierarchical supramolecular self-organization of a rod-like diacetylene derivative as
the primary building block. A series of 1,3-alkadiynyl-4-benzoic acids (mDA) with different alkyl chain
lengths (m ¼ 8–16) were synthesized and their mesophase properties were investigated by differential
scanning calorimetry, polarized optical microscopy, and X-ray diffraction under temperature control.
The mDA exhibited three types of mesophases, i.e., a discotic lamellar phase, DL, and two columnar
rectangular phases, Colrd and Colro, with disordered and ordered stacking structures of the mesogens in
a column, respectively, depending on the length of the alkyl substituent and temperature. We have
revealed that the columnar mesophase structures are constructed with the rod-like mDA molecules as
the starting building blocks by hierarchical self-organization. The hydrogen-bonded dimers of the
carboxylic acid with a rigid framework are assembled to provide a disc-like mesogen unit, which
consists of a p-stacked core-part and peripheral alkyl-chains. The discotic supramolecular mesogens
further stack in ordered and disordered columnar structures, producing the Colro and Colrd
mesophases, or in the layer DL mesophase. This is a rare example for the discotic mesophases using the
rod-like diacetylenes with a non-branched molecular structure.
11
investigated by Kato and Frechet, Lehn et al.,12 and other
groups.13–15 Columnar liquid crystalline properties for hetero-
meric hydrogen bonded systems using supramolecular mesogens
with a pseudo disc-like shape have been revealed.16
ꢀ
Introduction
Discotic liquid crystals with a columnar stacking structure have
attracted growing interest because of their significant potential
for fundamental physics and chemistry as well as applications to
organic electronic devices, such as displays, light-emitting diodes,
photovoltaic diodes, field effect transistors, and memories,1–5
since the first identification of the liquid crystal composed of disc-
shaped molecules in 1977 by Chandrasekhar et al.6 The molec-
ular structure of mesogens used for the discotic liquid crystals
usually implies a rigid aromatic core and flexible alkyl chains
surrounding the core. A large number of the discotic mesogens,
such as substituted benzenes,6,7 triphenylenes,8 porphyrins,9 and
phthalocyanines,10 have been synthesized in order to investigate
their mesophase structure as well as the physical properties. Since
the late 1980s, supramolecular liquid crystals using a well-defined
mesogenic complex constructed by complementary intermolec-
ular hydrogen bonding between different molecules have been
Other types of mesogens having a non-disc structure are also
used as the building blocks for the discotic and columnar liquid
crystals. Phasmids and biforked molecules with a rod-like rigid
core with two half-disc moieties at the ends, called polycatenar
mesogens, based on their molecular structures, exhibit
a columnar mesophase.17 An increase in the number of flexible
alkyl chains bound to the inner rigid core of polycatenars leads to
breaking of the nematic or lamellar (smectic) mesophase struc-
ture and forming a ribbon-like aggregate, being further orga-
nized to form cubic and columnar mesophase structures.
Another rod-like compound exhibiting a columnar phase is the
polygonal cylinder molecule, in which an increase in the volume
of the flexible chain in the mesogen induces the formation of non-
lamellar mesophases, i.e., columnar, cubic, and the other 3D
ordered one.18 The rod–coil and coil–rod–coil molecules con-
sisting of an aromatic rigid part and poly(ethylene oxide) coils
also prefer cubic and hexagonal columnar structures.19
aDepartment of Applied Chemistry and Bioengineering, Graduate School of
Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka,
558-8585, Japan. E-mail: matsumoto@a-chem.eng.osaka-cu.ac.jp; Fax:
+81-6-6605-2981
bSmart Materials Science and Technology, Department of Bioscience and
Textile Technology, Interdisciplinary Graduate School of Science and
Technology, Shinshu University, Ueda, Nagano, 386-8567, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10817d
An early example of mesophase formation by hydrogen
bonding is that of the alkoxy- and alkyl-substituted benzoic acids
and related compounds, which form
a hydrogen-bonded
dimer.20–23 Hydrogen bonding between carboxylic acids gener-
ates a long lath-like structure consisting of successive conjugating
units as the rigid core, exhibiting a nematic or smectic phase.
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A rigid diacetylene moiety is also used as the core part of the
mesogen for the construction of nematic, lamellar, and
columnar liquid crystals;24 for example, a micellar triclinic
mesophase is obtained by the supramolecular assembly of
phasmid-type diacetylene molecules.25 Recently, we found that
1,3-icosadiynyl-4-benzoic acid, which has a simple and linear
molecular structure consisting of rigid aromatic and diacetylene
units and a flexible alkyl chain, exhibits a highly ordered liquid
crystalline state in the temperature range of 100–180 ꢀC.26 In the
present study, we have synthesized 1,3-alkadiynyl-4-benzoic
acids (mDA) with different alkyl chain lengths (m ¼ 8–16 in
Scheme 1), and investigated their liquid crystalline properties
using differential scanning calorimetry (DSC), polarized optical
microscopy (POM), and X-ray diffraction (XRD) under
temperature control. We now report a new class of discotic
liquid crystal system, which is constructed by the hierarchical
supramolecular self-organization of the disc-shape mesogen
consisting of a rod-like diacetylene derivative as the primary
building block.
When the mDAs were cooled from the IL state, they trans-
formed into the DL phase for all the compounds except 8DA. For
9DA, the DL phase was directly crystallized to give K2, but all the
other compounds 10DA–16DA changed once into the Colrd
phase. During the further cooling process, another phase, Colro,
was observed for 10DA–13DA before crystallization, while 14DA
and 16DA were directly crystallized from the Colrd phases,
resulting in the K3 phase. Here, Colrd and Colro are the columnar
rectangular phases with disordered and ordered stacking struc-
tures of the mesogens in a column, respectively. The crystal phase
(K2 or K3) produced by the crystallization from the liquid crys-
talline states upon cooling was stable enough to remain at a lower
temperature without any further crystal-to-crystal transition into
another crystalline phase (K1 or K2). These reversible and irre-
versible steps for the phase transitions are confirmed by the DSC
traces and the POM observation (Fig. 2).
Some of the Colro and Colrd phases were not observed during
the heating process because of a superheating occurrence.28 This
can be rationally explained by the Gibbs energy–temperature
(G–T) diagram. The results for 10DA and 13DA are shown in
Fig. 3 as typical examples. When the phase transition from K to
Colro is obstructed by the superheating, the remaining K portion
undergoes a direct phase transition from K to Colrd. This case is
applied to 12DA and 13DA. The phase transition from K to
Colro at 123 ꢀC is not observed, and the transition to Colrd occurs
at 135 ꢀC for 13DA as shown in Fig. 3(a). Moreover, if the phase
transition from not only K to Colro, but also Colro to Colrd does
not occur by superheating, the remaining K can undergo the
phase transition to DL, as seen for 10DA and 11DA [Fig. 3(b)].
When the phase transition temperatures are close to each other,
the superheating tends to readily occur. If the heating is carried
out at an infinitely low scanning rate, the Colro and Colrd phases
Results and discussion
Mesophase behaviors
All the mDAs were prepared by the Cadiot–Chodkiewicz
coupling reaction of the corresponding 1-iodo-1-alkynes with
4-ethylbenzoic acid27 (see ESI†). The synthetic route of the mDAs
is shown in Scheme 1.
The phase transition temperatures and their enthalpy changes
for the mDAs are listed in Table 1. Fig. 1 shows the phase
diagrams versus the alkyl chain lengths (m) of the mDAs during
the heating and cooling processes. Each compound showed
a crystalline phase (K1) at room temperature and underwent
a crystal-to-crystal phase transition before reaching a mesophase
upon heating. The 8DA melted at 186.6 ꢀC and provided the
isotropic liquid (IL) state without showing any mesophase after
the three-step crystal-to-crystal phase transitions. When the
mDAs with m ¼ 9–11 were heated, the crystalline K2 or K3 phase
transformed into a mesophase at 157.7, 145.9, and 148.6 ꢀC, for
9DA, 10DA and 11DA, respectively. Upon further heating, the
mesophase cleared into the IL state at 184.6, 183.5, and 176.3 ꢀC,
respectively. For 12DA–16DA with larger alkyl substituents, the
crystalline K3 or K4 state first changed into a mesophase at
may appear, although no additional mesophase was observed at
the heating rate of 2–10 C minꢁ1
ꢀ
.
Fig. 4 shows the typical optical textures observed for the
mesophases of the 10DA, which were observed during the cool-
ing process from the IL state by POM. We can clearly see
a dendritic texture, which is characteristic of a discotic meso-
ꢀ
phase, at 182 C. A focal-conic texture and a terrace structure,
which are characteristics of a lamella structure, are also seen in
this mesophase. During further cooling, the dendritic aggregates
and the focal-conic fan textures transformed into a mosaic
texture, as observed in the photograph for the Colrd phase. A
terrace structure was more clearly observed for the Colrd phase
than for the DL phase. The optical textures of the two Colr
phases were close to each other, but a striated texture was
additionally observed in the Colro phase [Fig. 4(c)].
ꢀ
117.3–139.1 C, then transformed into the other type of meso-
morphic structure at 152.0–156.8 ꢀC, and finally the phase
cleared into the IL state at 165.2–172.6 ꢀC. The mesophase
structures observed during a heating process are assigned as
columnar rectangular phases (Colr) and a discotic lamellar phase
(DL), as described later.
X-Ray diffractions
We carried out the XRD experiments in order to accurately
determine the mesophase structures. Fig. 5 shows the XRD
profiles for 13DA recorded at 166, 152, and 135 ꢀC, which
correspond to the DL, Colrd, and Colro phases, respectively.
Table 2 summarizes the lattice constants for the possible meso-
phase structures and the observed and calculated d-spacing
values for 13DA as well as the assignment of the Miller indices
for each diffraction line (see ESI† for the XRD data for the all
mDAs).
Scheme 1
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Table 1 Phase transition temperatures (T/ꢀC) and enthalpy change (DH/kJ molꢁ1 in brackets) for the mDAsa
a
Phase nomenclature: K1, K2, K3, and K4 ¼ crystals, Colro and Colrd ¼ columnar rectangular phases, DL ¼ discotic lamellar phase, and IL ¼ isotropic
liquid.
Fig. 6 is a schematic representation for the possible packing
observed at larger angles (2q ¼ 7–15ꢀ) were also assigned based
on the comparison of the observed and calculated d-spacing
values, as shown in Table 2. The extinction rule for rectangular
lattices suggests a Colr phase with P21/a symmetry for the both
structures of discotic mesogens in the columnar mesophases. In
a columnar mesophase, discotic mesogens assemble themselves
to form a column perpendicular to a 2D lattice plane. In general,
the columnar mesophases are classified based on the 2D lattice
symmetry of the column packing as follows: (a) columnar
hexagonal phase (Colh), (b) columnar tetragonal phase (Coltet),
(c) columnar oblique phase (Colob), and (d) columnar rectan-
gular phase (Colr). The Colr phases further divide into four types
by the 2D lattice symmetry. Each phase implies the ordered and
disordered stacking of the mesogens in a column represented by
the subscripts o and d, respectively.
ꢀ
structures observed at 152 and 135 C.
In this study, if mDA prefers any smectic mesophase, such as
the smectic B and E phases, a sharp and strong diffraction due to
the alkyl chain stacking should appear at the position corre-
ꢁ
sponding to the d-value of ca. 4 A. In this study, such diffraction
was not observed [Fig. 5(a)]. Instead, other weak diffraction lines
were observed, which are assigned as diffractions due to the
columnar stacking of the discotic mesogens (hA and hB). Similar
characteristic lines were observed in the XRD profiles of 10DA–
12DA, independent of the length of the alkyl chain (see ESI†).
In Fig. 7, each lattice constant for the mesophase structures is
plotted as a function of the carbon number of the alkyl chain.
The a-axis values linearly increased with an increase in the alkyl
chain length, while the b-axis values were almost constant. This
suggests that the direction of the expansion of the alkyl domains
is always along the a-axis direction. The plots of the lattice length
of the a- and b-axes of the 16DA mesophases deviated from the
linear relationship for the DL and Colrd phases. This suggests
enhanced interaction between the long-alkyl chains. For 14DA
In Fig. 5(a) and (b), the DL phase of 13DA showed a series of
diffractions due to the 00l planes in a small angle region (2q ¼ 2–
8ꢀ), which are characteristics to a lamellar structure. In addition,
a weak diffraction was observed at a larger angle (2q ¼ 18.04ꢀ,
d ¼ 4.91 A) as well as broad diffractions at 10–30 .9b,29 The Colr
phases observed at 152 and 135 ꢀC showed another series of
diffraction lines in a small angle region. The lines observed
around 2–6ꢀ are assigned to the diffractions due to the (110),
ꢀ
ꢁ
ꢁ
(020), (220), and (130) planes for the lattice with a ¼ 54.63 A and
ꢁ
ꢁ
ꢁ
b ¼ 47.99 A for the Colrd phase and a ¼ 57.68 A and b ¼ 46.73 A
for the Colro phase [Fig. 5(c) and (d)]. The weak diffractions
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Fig. 1 Phase transition temperatures versus the alkyl-chain length (m) of
the mDAs determined during (a) the heating process and (b) the cooling
process by DSC. The crystalline samples were obtained by recrystalli-
zation from a mixture of tetrahydrofuran and methanol and used for the
DSC measurements.
Fig. 3 Schematic G–T (Gibbs energy–temperature) diagrams for (a)
13DA and (b) 10DA. Symbols indicate the transitions observed during
(B) both heating and cooling processes, (,) only heating process and
(C) only cooling process.
and 16DA, the Colro (P21/a) phase with an ordered columnar
stacking was not detected. This is due to the fact that long alkyl-
chains prefer a robust lamellar packing structure with a fully
expanded (trans zigzag) conformation in the crystalline state
than a highly ordered alignment of the discs in the mesophases.
The enthalpy changes determined by DSC (Table 1) also
support the assignment of these mesophase structures. High DH
values (11.1–15.6 kJ molꢁ1) were observed for the transition from
the IL state to the DL phase, and the relatively small one for the
DL to Colrd transition (DH ¼ 0.17–0.29 kJ molꢁ1). A slight
change in the local structure during the disordered-to-ordered
transition of the Colr phase has resulted in the lowest DH values
(DH ¼ 0.03–0.14 kJ molꢁ1).
Molecular stacking
In the diffraction profile of 13DA at the Colro phase [Fig. 5(d)],
two weak diffractions were detected at 2q ¼ 21.59 and 23.32ꢀ
ꢁ
(d ¼ 4.11 and 3.81 A, respectively) in addition to the line
Fig. 4 Optical textures observed for the mesophases of 10DA during the
cooling process from the IL state. (a) DL phase at 182 ꢀC, (b) Colrd phase
at 159 ꢀC and (c) Colro phase at 150 ꢀC.
Fig. 2 DSC traces of (a) 10DA and (b) 12DA. The heating and cooling
rates are 10 ꢀC minꢁ1
.
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Fig. 5 XRD profiles of 13DA for (a, b) DL phase at 166 ꢀC, (c) Colrd
phase at 152 ꢀC and (d) Colro phase at 135 ꢀC.
ꢁ
observed at 4.82 A, while the Colrd phase gave only the single
ꢁ
diffraction of 4.86 A [Fig. 5(c)]. The two additional diffractions
Fig. 6 Representation for the possible packing structures of discotic
mesogens in the columnar mesophases. (a) Columnar hexagonal (Colh),
(b) columnar tetragonal (Coltet), (c) columnar oblique (Colob), and (d)
columnar rectangular (Colr) phases.
observed in a wider angle region are due to the alignment of the
disc-shaped mesogens in a column. The line corresponding to d ¼
ꢁ
4.1 A is assigned as the diffraction due to the core parts (hA),
ꢁ
while that observed at 3.8 A is due to the alkyl-chain stacking
(hB), as illustrated in Fig. 8. The hA value observed for the Colro
ꢁ
phase was constant as 4.1 A, being higher than that of the
ꢁ
expected values (3.5 A) for the p–p stacking with the face-to-face
interaction of the aromatic cores and independent of the m value.
For the Colro phase, the Z values are estimated as 15–19 with
the assumption that the density (r) is equal to unity. In this case,
two discs are included in each unit cell, as shown in Fig. 6(d).
Accordingly, 7.5–9.5 mDA molecules, which correspond to 4–5
dimers, are included in each disc. It is noted that the Z value
increased with an increase in the alkyl chain length of the
substituent of the mDAs (see Table S1 in ESI†) because of an
increase in the molecular stacking force. Fig. 9 shows a schematic
illustration for the construction of mesophase structures by the
hierarchical self-organization of the rod-like diacetylene
molecules.
Thus is due to the edge-to-edge type stacking of the mesogens in
ꢁ
a column. The hB value slightly increased from 3.71 to 3.81 A
with an increase in the m value from 10 to 13 (see Table S1 in
ESI†) due to the steric repulsion of the fluctuating alkyl chains in
the mesophases.
Table 2 Assignment of XRD data for the 13DA mesophase
First, a pair of hydrogen-bonded carboxylic acids (i.e.,
a carboxylic acid dimer) is formed. The mDA dimer has a rigid
framework consisting of the successive diacetylene, p-phenylene,
and carboxylic acid dimer moieties. The dimers are self-assem-
bled and form a disc-like structure, which consists of a p-stacked
core part and peripheral alkyl-chains. The flexible alkyl-chains
fluctuate in the mesophases. The dimer stacking distance was
Spacing
Miller
indices
ꢁ
dobs/A
ꢁ
dcalcd/A
Mesophase structure
DL at 166 ꢀC, a ¼ 35.01 A
35.01
17.35
11.58
4.91
36.06
24.00
17.83
15.50
11.07
6.56
35.01
17.50
11.67
(001)
(002)
(003)
ꢁ
ꢁ
ꢁ
4.78–4.88 A for the Colr phases and 4.91–5.04 A for the DL
phases. This suggests that the p stacking interaction in the core
of the DL phase is smaller than that in the Colr mesophases [see
also Fig. 7(d)]. The disc-like mesogens produced by the dimer
stacking are further assembled to form columnar and lamellar
mesophases, Colr and DL, depending on the temperature and the
m values.
Colrd (P21/a) at 152 ꢀC,
36.06
24.00
18.03
15.35
10.99
6.65
(110)
(020)
(220)
(130)
(240)
(270)
ꢁ
ꢁ
a ¼ 54.63 A, b ¼ 47.99 A
4.86
Colro (P21/a) at 135 ꢀC,
36.31
23.36
18.00
15.52
11.10
6.51
4.82
4.11
3.81
36.31
23.36
18.16
15.04
10.83
6.50
(110)
(020)
(220)
(130)
(240)
(270)
ꢁ
ꢁ
a ¼ 57.68 A, b ¼ 46.73 A,
In the IR spectra of the mDAs, the characteristic peaks due to
the C]O stretching were observed at 1741 and 1700 cmꢁ1 at
180 ꢀC [Fig. 10(a)], indicating that some carboxyl groups are
connected to each other to form a dimer by hydrogen bonding,
while the others are free in the isotropic state. The intensity of the
peak due to the free carboxylic acid at 1741 cmꢁ1 decreased and
that of the peak due to the dimer formation at 1700 cmꢁ1
Z ¼ 18.87 for r ¼ 1.0 g cmꢁ3
hA
hB
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Fig. 7 Lattice constants versus alkyl-chain length, m, of mDAs. (B) a-
axis and (C) b-axis values in the (a) DL, (b) Colrd, and (c) Colro phases.
(d) Other molecular stacking distance in the (,) DL, (:) Colrd, and (>)
Colro phases.
increased during the cooling process from the IL state to the
mesophases. No free carboxyl group was included in the crys-
talline state at 100 ꢀC. Thus the IR spectra support the formation
of a robust hydrogen-bonded dimer of the diacetylene molecules
in the mesophases. The fact that similar fluorescence spectra
[Fig. 10(b)] as well as UV-vis absorption spectra (lmax ¼ 299 nm)
were observed for mDA independent of the mesophase structures
suggests that each mesophase implies a similar packing structure
of the phenylene groups. We further investigated the possibility
Fig.
9
Schematic illustration showing construction of mesophase
structures by hierarchical self-organization of mDA molecules.
of the mesophase formation using the methyl ester of mDA in
order to check the role of a carboxylic acid moiety in the dimer
formation. As a result, the metꢀhyl ester of 12DA did not show
any transition peak below 200 C except for a melting point at
32.0 ꢀC. This result supports the fact that the dimer formation is
indispensable for the appearance of the mesophase characteris-
tics in the mDA series.
Fig. 8 (a) Possible stacking structure of discotic mesogens with a rigid
core and flexible alkyl chains in a column and (b) molecular stacking
appropriate for the polymerization. The hA and hB are stacking distances
of the core and alkyl chain, respectively. The dimers are required to stack
Fig. 10 (a) Temperature-dependent IR spectrum of 12DA during the
cooling process from 200 to 80 ꢀC at the cooling rate of 2.5 ꢀC minꢁ1
.
ꢀ
ꢁ
with a stacking distance (ds) of 4.8–5.0 A and a stacking angle of ca. 45 in
Each spectrum was recorded at 4 min intervals. (b) Temperature-
a disc for the polymerization to occur.
dependent fluorescence spectrum of 12DA.
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Table 3 Polymerization of 12DA in liquid crystalline and crystalline
statesa
peripheral flexible alkyl chains. The discotic mesogens further
stack in a columnar or layer structure. While the carboxylic acids
having a simple and calamitic molecular structure reported in the
literature exhibited only nematic and smectic mesophases,20–23
our present study revealed that the columnar and layer meso-
phases are thermodynamically favored even when linear and rod-
like molecules are used as the starting building blocks for the
supramolecular liquid crystalline system.
Phase
Temperature/ꢀC
Mn ꢂ 10ꢁ3
Mw ꢂ 10ꢁ3
Mw/Mn
DL
165
155
145
r.t.
1.38
1.84
1.76
2.26
1.27
1.23
Colrd
Colro
K
1.82
2.10
1.16
b
b
b
a
Polymerization conditions: UV irradiation for 5 min in the liquid
crystalline states or 400 kGy g-ray irradiation in the crystalline state.
The highly crystalline and high-molecular-weight polymers were
b
Acknowledgements
produced, being insoluble in tetrahydrofuran.
This work was partially supported by the Funding Program for
World-Leading Innovative R&D on Science and Technology
(FIRST Program) and Grant-in-Aids for Scientific Research
from the Japan Society for the Promotion of Science (JSPS).
The photopolymerization of 12DA at different temperatures
corresponding to the three kinds of liquid crystalline states
resulted in the formation of oligomers with the molecular weights
of 1.4–1.8 ꢂ 103, as shown in Table 3. This is in contrast to the
production of a highly crystalline and insoluble polymer during
the solid-state polymerization under g-ray radiation.30 The
molecular weight of the produced oligomers in the mesophases
corresponds to the polymerization degree of 4–6, agreeing well
with the aggregation number of the dimers in a disc. It is assumed
that the diacetylene dimers have a stacking structure similar to
that appropriate for the solid-state polymerization,31,32 that is,
References
1 Handbook of Liquid Crystals, ed. D. Demus, J. Goodby, G. W. Gray,
H.-W. Spiess and V. Vill, Wiley-VCH, Weinheim, 1998, vol. 2A and
2B.
2 P. S. Pershan, Structure of Liquid Crystal Phases, World Scientific,
Singapore, 1988.
3 S. Chandrasekhar, Liquid Crystals, Cambridge University Press,
Cambridge, 2nd edn, 1992.
4 P. J. Collings and M. Hird, Introduction to Liquid Crystals: Chemistry
and Physics, Taylor & Francis, London, 1997.
ꢀ
ꢁ
the stacking distance (ds) of 5 A and the stacking angle of 45 , as
shown in Fig. 8. The propagation occurs within a disc, resulting
in the formation of an oligomer, but not a high polymer. This
result indicates the formation of a compartmentalized disc
assembly, which is stable even in the discotic lamellar mesophase.
No transition to the non-discotic phase, such as a smectic or
nematic one, occurred due to the robust structures of the
assemblies. The stable discs in the mesophases resulted in the
oligomer formation with the noted molecular weights, which
correspond to the number of mesogens aggregated in a disc. The
liquid crystalline properties were revealed to vanish after the
complete polymerization (oligomerization) because of the ther-
modynamic properties of the liquid crystalline system sensitive to
the molecular structure of the mesogens. Previously, Busico
et al.33 reported a mesophase with a bidimensional rectangular
structure of lithium n-hexadecanoate, which is also one of linear-
shape molecules as the building blocks for higher-order meso-
phase appearance. More recently, we further observed the
mesomorphic properties of other related compounds including
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