Synthesis and thermal properties of twin dimers containing cyanostilbene groups as a mesogenic group

Article abstract of DOI:10.1080/15421400490425847

Twin dimers (TDs) with a different linking group (ester or ether) between the mesogenic group (cycmostilbene moiety) and the flexible spacer were synthesized to clarify the effect of the linking group on the mesomorphic properties. The sequence of the est

Full text of DOI:10.1080/15421400490425847

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SYNTHESIS AND THERMAL
PROPERTIES OF TWIN DIMERS
CONTAINING CYANOSTILBENE
GROUPS AS A MESOGENIC
GROUP
H. Aoki a , T. Mihara a & N. Koide a
a Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601 Japan
Published online: 15 Jul 2010.
To cite this article: H. Aoki , T. Mihara & N. Koide (2004) SYNTHESIS AND
THERMAL PROPERTIES OF TWIN DIMERS CONTAINING CYANOSTILBENE GROUPS AS
A MESOGENIC GROUP, Molecular Crystals and Liquid Crystals, 408:1, 53-70, DOI:
10.1080/15421400490425847
To link to this article: http://dx.doi.org/10.1080/15421400490425847
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Mol. Cryst. Liq. Cryst., Vol. 408, pp. 53–70, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400490425847
SYNTHESIS AND THERMAL PROPERTIES OF TWIN
DIMERS CONTAINING CYANOSTILBENE GROUPS
AS A MESOGENIC GROUP
H. Aoki, T. Mihara, and N. Koide*
Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601 Japan
Twin dimers (TDs) with a different linking group (ester or ether) between the
mesogenic group (cyanostilbene moiety) and the flexible spacer were synthesized
to clarify the effect of the linking group on the mesomorphic properties. The
sequence of the ester linking group and the position of the cyano group in
the mesogenic core made an important contribution to the thermal properties of
the twin dimers. A remarkable odd–even effect in the phase transition
temperatures and entropy changes from a nematic phase to an isotropic fluid
was observed for all the synthesized twin dimers.
Keywords: cyanostilbene; liquid crystal dimer; odd–even effect
INTRODUCTION
Aromatic main chain liquid crystalline polymers (MCLCPs) with wholly
aromatic rings or semiflexible moieties have been found primarily as high-
performance polymers for molding resins with good mechanical properties.
MCLCPs can be classified into three types such as Xyder (Amoco,
Augusta), Vectra (Hoechst-Celanese, New Jersey), and X7G (Eastman-
Kodak, Tennessee), depending upon their heat distortion temperatures.
Especially the chemical constituents of semiflexible MCLCPs like X7G
(polyethylene terephthalate modified with rigid p-hydroxybenzoic acid)
are the mesogenic rigid core and a flexible spacer. Consequently, semi-
flexible MCLCPs had lower phase transition temperatures and high solu-
bility. Therefore, semiflexible MCLCPs are suitable candidates for the
assessment of reliable correlations between their chemical structural
*Corresponding author. E-mail: nkoide@ch.kagu.sut.ac.jp
53

54
H. Aoki et al.
FIGURE 1 Possible chemical structure of ester group linked to phenyl group.
characteristics and physical properties, including thermal properties and
formation of mesophases [1].
Thermal properties of semiflexible MCLCPs are dependent upon the
chemical structure of rigid core, flexible spacer length, the linking group
between the rigid core and a flexible spacer, etc. There are two types of
ester linking groups as shown in Figure 1. Asrar et al. reported that the
mesomorphic phase structure of the semiflexible MCLCPs is deeply influ-
enced by the sequence of the ester linking bond between the rigid core and
the flexible spacer [1]. A nematic phase was exhibited for the semiflexible
MCLCPs with the ester linking group, as shown in Figure 1(a), while a
smectic phase was displayed for the semiflexible MCLCPs with the ester
linking group, as shown in Figure 1(b). Roviello et al. and Blumstein et al.
reported that the 4,40-dihydroxy-a-methylstilbene-aliphatic acid polymers
and the 4,40-dihydroxy-2,20-dimethylazoxybenzene-alkanedioic acid poly-
mers exhibited a nematic phase [2,3].
We have already reported the synthesis and thermal properties of the
4,40-dihydroxycyanostilbene-alkanedioic acid polymers shown in Figure 2
[4,5]. These semiflexible MCLCPs exhibited an enantiotropic nematic phase.
Thus a nematic phase was easily displayed for the semiflexible MCLCPs
with the ester linking group as shown in Figure 1(a). On the other hand,
Watanabe et al. reported that a smectic phase was shown for the semi-
flexible MCLCPs with biphenyl groups as a mesogenic group and the ester
linking group as shown in Figure 1(b) [8]; however, the semiflexible MCLCP
with the ester linking group as shown in Figure 1(b) is little investigated.
The effect of the flexible polymethylene spacer on the thermal proper-
ties of MCLCPs is well assessed in a number of publications. The odd–even
effect of melting points (Tm ) and clearing points (Ti ) is well-known
behavior for the MCLCPs with different flexible spacer lengths. Both
FIGURE 2 Chemical structure of main chain type liquid crystalline polyester
containing cyanostilbene moieties.

LC Cyanostilbene Dimers
55
nematic–isotropic phase transition temperatures and entropy changes from
a nematic phase to an isotropic melt decrease with increasing length of the
flexible spacer. The odd–even effect would be explained by the different
order of the mesogenic groups in the nematic phase, depending upon the
number of methylene units with all trans conformation in the flexible
spacer. Roviello et al. and Blumstein et al. reported the odd–even effect on
the transition temperatures, enthalpy changes, and entropy changes from a
nematic phase to an isotropic melt of thermotropic liquid crystalline
polyesters [2,3]. The clear odd–even effect on the transition temperatures
and entropy changes from a nematic phase to an isotropic melt were also
observed for the semiflexible MCLCPs, as shown in Figure 2. Thus the odd–
even effect was much like the very familiar thermodynamic properties of
the semiflexible MCLCPs. However, it is very difficult to understand the
odd–even effect on the transition temperatures and entropy changes from a
nematic phase to an isotropic melt, because the semiflexible MCLCPs have
structural complexity and a polymeric nature, ie., high molecular weight,
molecular weight distribution, etc.
Low molecular weight model compounds having fundamental structural
units for the semiflexible MCLCP are used to investigate the odd–even effect
on the phase transition temperatures and entropy changes of the semiflexible
MCLCPs by experimental and computer-simulation techniques [6,7]. There
are two types of model compounds. One is a core model that has a single
mesogenic core with two terminal chains. Another is a twin dimeric model
(twin dimer) in which two mesogenic cores are linked by a flexible spacer.
Twin dimers are recognized as an exact structural model compound of
semiflexible MCLCPs [6]. Abe et al. have found that the different order of
the mesogenic group in the nematic phase played an important role in the
odd–even effect on the nematic–isotropic phase transition behavior of the
twin dimer by the conformation analysis of the spacer [7].
In this report, we synthesized twin dimers having the cyanostilbene
moieties as a mesogenic group, as shown in Figure 3, and investigated the
thermal properties of the dimers with different linking groups. We also
discussed the effect of the position of the cyano group in the mesogen on
the thermal properties of the twin dimers.
EXPERIMENTAL
Materials
4-MethoxyphenylacetonitrilewaspurchasedfromMidorikagakuCorporation
(Tokyo).
Twin dimers (TDs) I-V were prepared according to Schemes 1, 2, and 3.
Typical synthetic procedures of the twin dimers were described below.

56
H. Aoki et al.
FIGURE 3 Chemical structures of synthesized twin dimers containing cyanos-
tilbene moieties.
4-Methyl-40-methoxy-a-cyanostilbene (I)
Potassium hydroxide (19.4 g, 0.35 mol) was dissolved in ethanol
(450 ml). p-Tolualdehyde (23 ml, 0.19 mol) was dissolved in the ethanol
solution. 4-Methoxyphenyl acetonitrile (25 ml, 0.17 mol) was added
dropwise to the ethanol solution. After the reaction mixture was stirred for

LC Cyanostilbene Dimers
57
SCHEME 1 Synthesis of twin dimer I and II series.

58
H. Aoki et al.
SCHEME 2 Synthesis of twin dimer III series.
SCHEME 3 Synthesis of twin dimer IV and V series.

LC Cyanostilbene Dimers
59
4 at room temperature, the precipitate was washed with aqueous HCl and
then with water until a neutral aqueous solution was obtained. The product
was purified by recrystallization from
a mixed solvent (acetone/
methanol ¼ 1/2) and was obtained in a 72.9% yield (39.2 g). 1H-NMR
(CDCl3) d ppm: 2.4(s, 3H, CH3), 3.8(s, 3H, OCH3), 7.5(s, 1H, CH ¼), 7.0–
7.9(m, 8H, Ar-H).
4-Hydroxy-40-methyl-cyanostilbene (II)
The intermediate (I) (20 g, 80.6 mmol) was dissolved in acetic acid
(80 ml). A hydrobromic acid aqueous solution (47%, 80 ml) was added to
the acetic acid solution. The reaction mixture was heated at 120ꢀC for 12 h
and then was poured into water. The precipitate was washed with water
until a neutral aqueous solution was obtained. The product was obtained in
1
a 62.1% yield (11.8 g) and was used without further purification. H-NMR
(CDCl3) d ppm: 2.4(s, 3H, CH3), 3.9(s, 1H, OH), 7.5(s, 1H, CH¼),
7.0–7.9(m, 8H, Ar-H).
Twin Dimer I (n ¼ 6): TD I-6
The intermediate (II) (1.1 g, 4.3 mmol) was dissolved in tetrahydrofuran
(THF) (40 ml). Triethylamine (1.8 ml, 12.8 mmol) was added to the THF
solution. A THF solution (10 ml) of suberoyl chloride (0.45 ml, 2.1 mmol)
was added dropwise to the THF solution. The reaction mixture was stirred
at room temperature for 24 th. The precipitate was removed by filtration.
THF was evaporated under reduced pressure. The residue was washed
with aqueous HCl and with water until a neutral aqueous solution was
obtained. After the product was washed with methanol, the product
was purified by silica gel column chromatography (eluent chloroform) and
1
was obtained in a 55.5% yield (0.72 g). H-NMR (CDCl3) d ppm: 1.3–1.8
(m, 8H, CH2), 2.4(s, 6H, CH3), 2.8(t, 4H, OOCCH2), 7.5(s, 2H, CH¼),
7.1–7.8(m, 16H, Ar-H).
4-Methoxy-40 -methyl-a-cyanostilbene (III)
The intermediate (III) was synthesized from p-anisaldehyde and
p-xylylcyanide according to the similar synthetic procedure of the
intermediate (I). The product was purified by recrystallization from a
mixed solvent (acetone/methanol ¼ 1/2). The product was obtained in a
74.8% yield (40.2 g).1H-NMR (CDCl3) d ppm: 2.4(s, 3H, CH3), 3.8(s, 3H,
CH3), 7.3(s, 1H, CH¼), 6.9–7.8(m, 8H, Ar-H).
Intermediate (IV)
The intermediate (IV) was prepared according to the similar synthetic
procedure of the intermediate (II). The product was used without further
1
purification. The product was obtained in a 34.8% yield (6.8 g). H-NMR

60
H. Aoki et al.
(CDCl3) d ppm: 2.4(s, 3H, CH3), 3.8(s, 1H, OH), 7.5(s, 1H, CH¼),
7.0–7.8(m, 8H, Ar-H).
Twin Dimer II (n ¼ 6): TD II-6
The intermediate (IV) (1.1 g, 4.3 mmol) was dissolved in THF (40 ml).
Triethylamine (1.8 ml, 12.8 mmol) was added to the THF solution. A THF
solution (10 ml) of suberoyl chloride (0.45 ml, 2.1 mmol) was added
dropwise to the THF solution. The reaction mixture was stirred at room
temperature for 24 h. The precipitate was removed by filtration. THF was
evaporated under reduced pressure. The residue was washed with aqueous
HCl and with water until a neutral aqueous solution was obtained. After the
product was washed with methanol, the product was purified by silica gel
column chromatography (eluent chloroform) and was obtained in a 62.5%
1
yield (0.81 g). H-NMR (CDCl3) d ppm: 1.5–1.8(m, 8H, CH2), 2.4(s, 6H,
CH3), 2.6(t, 4H, OOCCH2), 7.5(s, 2H, CH¼), 7.0–7.9(m, 16H, Ar-H).
Intermediate (V)
Potassium hydroxide (19.4 g, 0.35 mol) was dissolved in ethanol
(450 ml). p-Terephthalaldehydic acid (25 g, 0.17 mol) was dissolved in
the ethanol solution. p-Xylylcyanide (25 ml, 0.19 mol) was added drop-
wise to the ethanol solution. After the reaction mixture was stirred for
12 h at room temperature, the precipitate was washed with aqueous HCl
and then with water until a neutral aqueous solution was obtained. The
product was purified by recrystallization from acetone and was obtained
in a 76.1% yield (38.2 g). 1H-NMR (CDCl3) d ppm: 2.4(s, 3H, CH3),
7.2–8.1(m, 9H, Ar-H and ¼ CH), 13.2(s, 1H, COOH).
Twin Dimer III (n ¼ 6): TD III-6
The intermediate (V) (1.4 g, 5.3 mmol), 1,6-hexanediol (0.3 ml,
2.5 mmol) were dissolved in THF (80 ml). 1-[3-(Dimethylamino)propyl]-
3-ethylcarbodiimide hydrochloride (2.0 g, 10.4 mmol) and a small amount
of 4-dimethylaminopyridine as a catalyst were added to the THF solution.
The reaction mixture was stirred at room temperature for 48 h. The pre-
cipitate (urea derivative as a by-product) was removed by filtration. THF
was evaporated under reduced pressure. The residue was washed with
aqueous HCl to remove the remained urea derivative as a by-product of the
esterification reaction and with water until a neutral aqueous solution was
obtained. After the product was washed with methanol, the product was
purified by recrystallization from a mixed solvent (benzene/methanol ¼
1
5/1) and was obtained in a 40.7% yield (0.60 g). H-NMR (CDCl3) d ppm:
1.6–1.9(m, 8H, CH2), 2.4(s, 6H, CH3), 4.4(t, 4H, COOCH2), 7.5(s, 2H,
CH¼), 7.2–8.1(m, 16H, Ar-H).

LC Cyanostilbene Dimers
61
Intermediate (VI)
p-Hydroxybenzylaldehyde (2.4 g, 21.2 mmol) and 1,6-dibromohexane
(1.5 ml, 10.0 mmol) were dissolved in dimethylformamide (7.5 ml). Potas-
sium carbonate (3.4 g, 25.2 mmol) was added to the solution. After the
reaction mixture was heated at 110ꢀC for 4 h, it was poured into water
(300 ml). The white precipitate was washed with water and was obtained in
a 55.1% yield (1.8 g). The white precipitate was used without further
purification. 1H-NMR (CDCl3) d ppm: 1.5–2.0(m, 8H, CH2), 4.2(t, 4H,
OCH2), 6.0(d, 4H, Ar-H), 7.8(d, 4H, Ar-H), 9.9(s, 2H, CHO).
Twin Dimer IV (n ¼ 6): TD IV-6
Potassium hydroxide (8.0 g, 0.12 mol) was dissolved in ethanol (300 ml).
p-Xylylcyanide (1.7 ml, 13.3 mmol) and the intermediate (VI) (1.8 g,
5.5 mmol) were added to the ethanol solution. After the reaction mixture was
stirred at 60ꢀC for 2 h, the light yellowish precipitate obtained was washed
with aqueous HCl and then with water until a neutral aqueous solution was
obtained. The product was purified by recrystallization from a mixed solvent
(methanol/chloroform ¼ 1/3). The yellow product was obtained in a 72.2%
yield (2.2 g).1H-NMR (CDCl3) d ppm: 1.5–2.0(m, 8H, CH2), 2.4(s, 6H, CH3),
4.1(t, 4H, OCH2), 7.5(s, 2H, CH¼), 6.8–7.8(m, 16H, Ar-H).
Intermediate (VII)
p-Hydroxybenzylcyanide (2.8 g, 21.2 mmol) and 1,6-dibromohexane
(1.5 ml) were dissolved in dimethylformamide (5 ml). Potassium carbonate
(3.4 g, 25.2 mmol) was added to the mixture solution. The reaction mixture
was heated at 110ꢀC for 4 h and then was poured into water (300 ml). The
precipitate obtained was washed with water. The white precipitate was
purified by silica gel column chromatography (eluent chloroform). The
product was obtained in a 34.5% yield (1.2 g). 1H-NMR (CDCl3) d ppm: 1.5–
1.7(m, 8H, CH2), 3.6(s, 4H, CNCH2), 4.0(t, 4H, OCH2), 6.8(d, 4H, Ar-H),
7.3(d, 4H, Ar-H).
Twin Dimer V (n ¼ 6): TD V-6
Potassium hydroxide (2.0 g, 30.5 mmol) was dissolved in ethanol (150 ml).
p-Tolualdehyde (0.4 ml, 3.4 mmol) and the intermediate (VII) (0.51 g,
1.44 mmol) were added to the ethanol solution. After the reaction mixture was
stirred at 60ꢀC for 2 h, the light yellowish precipitate obtained was washed
with aqueous HCl and then with water until a neutral aqueous solution was
obtained. The product was purified by recrystallization from a mixed solvent
(methanol/chloroform¼ 1/3). The yellow product was obtained in a 62.9%
yield (0.62 g). 1H-NMR (CDCl3) d ppm: 1.5–2.0(m, 8H, CH2), 2.4(s, 6H, CH3),
4.1(t, 4H, OCH2), 7.5(s, 2H, CH ¼), 6.8–7.8(m, 16H, Ar-H).

62
H. Aoki et al.
Characterization
1H-NMR was carried out with a JEOL JNM-LA 400 spectrometer using
CDCl3 as the solvent. Infrared spectra were recorded on a JEOL JIR 7000
spectrometer. Spectra were collected at 4 cmꢁ1 resolution. DSC measure-
ments were conducted with a Mettler DSC821e. Optical microscopy was
performed on a Nikon polarizing optical microscope, OPTIPHOTO-POL,
equipped with a Mettler FP80 controller and a FP82 hot stage. Thermal
properties of synthesized materials were investigated by optical micro-
scope and DSC measurements. X-ray diffraction patterns were recorded
with a RIGAKU RINT2500 series with Ni-filtered CuKa radiation. The
sample in quartz capillary (diameter 1 mm) was held in a temperature-
controlled cell (RIGAKU LC high-temperature controller). The meso-
morphic structure of twin dimers was investigated by X-ray measurements.
RESULTS AND DISCUSSION
We synthesized twin dimer (TD) series containing cyanostilbene groups.
Thermal properties of the TD series were investigated by polarizing optical
microscopy and DSC measurements. DSC curves of TD I-5 and I-6 were
shown in Figure 4. A peak assigned to the phase transition from the crystal
to the isotropic melt was shown on heating run of TD I-5. On the other
FIGURE 4 DSC curves of twin dimers I (TD I-6, broken line; TD I-5, solid line).

LC Cyanostilbene Dimers
63
hand, two peaks were detected on the cooling run of TD I-5. A schlieren
texture was observed in the temperature range between the two peaks.
The two peaks were assigned to the melting and clearing points, respec-
tively. The mesomorphic phase structure of the TD series was examined by
X-ray diffraction measurements. TD I-5 displayed a monotropic nematic
phase, while TD I-6 exhibited an enantiotropic nematic phase. Phase
transition temperatures and entropy changes from a nematic phase to an
isotropic melt (DSNI) for TD I-n are summarized in Table 1. On the heating
scan of the TD I series, a nematic phase was exhibited for TD I-(even
number) except TD I-2, while TD I-2 and TD I-(odd number) series did not
show any mesophases on the heating scan.
Phase transition temperatures and DSNI on the cooling scan of the TD I
series were summarized in Figures 5(a) and 5(b), respectively. Two peaks
were observed in DSC curves on the cooling run of the TD I series except
TD I-3, as shown in Figure 5(a). These two peaks were assigned to melting
and clearing temperatures, respectively. A schlieren texture was observed
in the temperature range between the two peaks detected by DSC
measurements of the TD I series. Only a broad peak was observed in the
wide-angle region of the X-ray pattern for the TD I series except TD I-3.
The reason why TD I-3 did not exhibit any mesophases was not
clarified. Clearing points of the TD I series showed a remarkable odd–even
effect. The clearing points of the TD I series decreased with increasing
flexible spacer length.
The DSNI of the TD I series also displayed a remarkable odd–even effect,
and the DSNI values increased with increasing flexible spacer length. This
TABLE 1 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to an Isotropic Melt of Twin Dimer I Series
Heating
Cooling
n
T
Cr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
TCr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
2
212
174
191
147
162
145
135
132
127
—
—
—
—
193
164
155
94
208
—
4.9
—
3
4
206
—
8.0
—
205
141
179
135
163
133
146
6.1
2.6
5
6
182
—
10.2
—
120
114
113
98
8.4
3.1
7
8
164
—
12.4
—
11.4
4.5
9
10
148
7.7
102
10.4
TCr, melting point; TNI, nematic-isotropic phase transition temperature; DSNI, entropy
changes based on the phase transition from a nematic phase to an isotropic melt.

64
H. Aoki et al.
FIGURE 5 (a) Phase transition temperatures and (b) entropy changes of twin
dimer I series. Open circles and solid squares represent melting and clearing points,
respectively. Closed circles indicate entropy changes based on the phase transition
from nematic to isotropic phase.
result indicates that the orientational order of the mesogenic groups in the
nematic phase would increase with increasing flexible spacer length.
The difference in the chemical structure between the TD I series and TD
II series was the position of the cyano group in the mesogenic core. TD II-4,

LC Cyanostilbene Dimers
65
TD II-6, and TD II-9 exhibited an enantiotropic nematic phase. On the other
hand, the TD II series, with the exception of the above mentioned three
TDs, displayed a monotropic nematic phase. Thermal properties of TD II
series are summarized in Table 2. Figure 6 shows phase transition tem-
peratures and DSNI values of the TD II series on the cooling run. The TD II
series have the same linking group between the rigid core and the flexible
spacer as the TD I series; however, the position of the cyano group in the
mesogenic core of the TD II series was different from that of the TD I
series. A schlieren texture was observed for the TD II series. The nematic
structure of the TD II series was supported by X-ray measurements.
Melting and clearing points for the TD II series displayed a remarkable odd–
even effect. The remarkable odd–even effect on melting points was not
observed for the TD I series with longer flexible methylene spacer (n > 6),
as shown in Figure 5(a). However in the case of the TD II series shown in
Figure 6(a), the clear odd–even effect on the melting points was displayed.
The position of the cyano group in the mesogenic core would play an
important role to exhibit the remarkable odd–even effect on the melting
point. Both melting and clearing points for the TD II series decreased with
increasing flexible spacer length. These clearing points for the TD II series
were nearly equal to those of the TD I series, while these melting points of
the TD II series were a little higher than those of the TD I series. As a
result, the mesomorphic temperature range of the TD I series was wider
than that of the TD II series.
The DSNI values of the TD II series displayed an odd–even effect, and the
DSNI values gradually increased with increasing flexible spacer length. Both
TD I and TD II series exhibited a nematic phase. However, the DSNI values
TABLE 2 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to an Isotropic Melt of Twin Dimer II Series
Heating
Cooling
n
T
Cr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
TCr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
2
216
171
192
146
165
131
156
115
146
—
—
—
—
171
156
182
116
147
114
142
95
202
157
200
135
170
133
154
126
146
4.2
2.1
4.9
2.3
5.4
2.4
8.0
3.7
12.6
3
4
202
—
4.5
—
5
6
175
—
5.2
—
7
8
—
—
9
10
148
—
3.4
—
134
TCr, melting point; TNI, nematic-isotropic phase transition temperature; DSNI, entropy
changes based on the phase transition from a nematic phase to an isotropic melt.

66
H. Aoki et al.
FIGURE 6 (a) Phase transition temperatures and (b) entropy changes of twin
dimer II series. Open circles and solid squares represent melting and clearing
points, respectively. Closed circles indicate entropy changes based on the phase
transition from nematic to isotropic phase.
of the TD I series, except for TD I-10, were larger than those of the TD II
series. Furthermore, the magnitude of the odd–even effect for the DSNI
values of the TD I series was larger than that of the TD II series.

LC Cyanostilbene Dimers
67
FIGURE 7 Phase transition temperatures of twin dimer III series. Open circles
represent melting points.
Figure 7 displayed melting points of the TD III series. The TD III series
has same mesogenic core as the TD I series; however, the sequence of the
ester linking group of the TD III series is opposite of that of the TD I series.
No mesophase was exhibited for the TD III series. A remarkable odd–even
effect was shown for the melting points of the TD III series. These results
TABLE 3 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to an Isotropic Melt of Twin Dimer IV Series
Heating
Cooling
n
T
Cr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
TCr/ꢀC
TNI/ꢀC
DSNI/(Jmolꢁ1Kꢁ1
)
2
190
156
206
125
149
141
155
159
147
192
—
4.0
—
182
141
200
105
133
77
192
—
5.2
—
3
4
—
—
—
—
—
—
5
109
167
117
149
120
139
1.6
11.5
3.7
10.8
6.0
9.6
6
167
—
11.4
—
7
8
—
—
—
—
109
51
9
10
—
—
132
TCr, melting point; TNI, nematic-isotropic phase transition temperature; DSNI, entropy
changes based on the phase transition from a nematic phase to an isotropic melt.

68
H. Aoki et al.
FIGURE 8 (a) Phase transition temperatures and (b) entropy changes of twin
dimer IV series. Open circles and solid squares represent melting and clearing
points, respectively. Closed circles indicate entropy changes based on the phase
transition from nematic to isotropic phase.
suggested that the sequence of the ester linking group for the TD series
played an important role to exhibit a mesophase. In the case of the twin
dimer series, which has the same sequence of the ester linking group as our

LC Cyanostilbene Dimers
69
TD III series, the orientation of the mesogenic groups in the smectic phase
was controlled by the conformation of the flexible spacer [8]. On the other
hand, our TD III series did not display any mesophases.
We prepared the TD IV and TD V series that had the ether linking group
between the mesogenic group and the flexible methylene spacer. TD IV-2
and IV-6 exhibited an enantiotropic nematic phase, while a monotropic
nematic phase was observed for the TD IV series with a longer methylene
spacer (n > 4). Thermal properties of TD IV series are summarized
in Table 3. Figure 8 displays phase transition temperatures and DSNI for the
TD IV series on the cooling run. A remarkable odd–even effect was
observed for the clearing points, melting points, and DSNI values. The TD IV
series exhibited a nematic phase, while, as shown in Figure 9, no meso-
phases were exhibited for the TD V series that had the cyano group at the
different position from TD IV series. The p-conjugated system from the
oxygen to the cyano group for the TD I and TD IV was longer than that
for the TD II and TD V. This result indicated that the position of cyano
group in the mesogenic core played an important role in exhibiting a
mesophase for the TD series with ether linking group.
With regard to exhibition of the mesophase, the TD I and TD IV series
are superior to another TD series. The TD I and TD IV series had the
sequence of o-phenyl group as a linking group and the cyano group near
the terminal group of the mesogenic core. The p-conjugated length from
the oxygen to the cyano group would influence the exhibition of the
FIGURE 9 Phase transition temperatures of twin dimer V series. Open circles
represent melting points.

70
H. Aoki et al.
mesophase for the TD series. In the case of the TD III series, the sequence
of the linking group was opposite to that of the TD I series. The donation of
the electron from the oxygen atom would not occur in the mesogenic group
of the TD III series because the TD III series had the sequence of phenyl–
carbonyl, as shown in Figure 1(b). The DSNI for TD IV series displayed a
remarkable odd–even effect. The DSNI values of the TD IV with odd-
membered series increased with increasing methylene spacer length, while
that of the TD IV with even-membered series decreased with increasing
methylene spacer length. As a result, the magnitude of the DSNI values for
the TD IV series became small. This result indicated that the conforma-
tional effect of the flexible spacer on the DSNI values decreased with
increasing flexible spacer length.
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