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

Article abstract of DOI:10.1080/15421400490478326

We synthesized twin dimers (TDs) with different linking groups between a mesogenic group and a flexible spacer, different spacer length, and different terminal alkyl chain length to clarify the effect of these groups on the mesomorphic properties of the T

Full text of DOI:10.1080/15421400490478326

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Synthesis and Thermal
Properties of Twin Dimers
Containing Cyanostilbene
Derivatives as a Mesogenic
Group
T. Mihara a , M. Ito 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: 18 Oct 2010.
To cite this article: T. Mihara , M. Ito & N. Koide (2004) Synthesis and
Thermal Properties of Twin Dimers Containing Cyanostilbene Derivatives as a
Mesogenic Group, Molecular Crystals and Liquid Crystals, 419:1, 69-86, DOI:
10.1080/15421400490478326
To link to this article: http://dx.doi.org/10.1080/15421400490478326
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Mol. Cryst. Liq. Cryst., Vol. 419, pp. 69–86, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490478326
SYNTHESIS AND THERMAL PROPERTIES OF TWIN
DIMERS CONTAINING CYANOSTILBENE
DERIVATIVES AS A MESOGENIC GROUP
T. Mihara, M. Ito, and N. Koideꢀ
Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601, Japan
We synthesized twin dimers (TDs) with different linking groups between a
mesogenic group and a flexible spacer, different spacer length, and different
terminal alkyl chain length to clarify the effect of these groups on the meso-
morphic properties of the TDs. A remarkable odd–even effect on the phase tran-
sition temperatures and nematic–isotrophic phase transition entropy changes
was observed for the TDs with different spacer length. Liquid crystallinity of
TDs with ester linking group was superior to that of TDs with ether linking
group. Length of terminal group played an important role in exhibiting a
smectic phase for TDs with ester linking group.
Keywords: cyanostilbene; odd–even effect; liquid crystal dimer
INTRODUCTION
Main-chain–type liquid crystalline polymers (MCLCPs) with wholly
aromatic rings and semirigid moieties have been found primarily as high-
performance polymers for molding resins with good mechanical properties.
MCLCPs can be classified into three types, Xyder, Vectra, and X7G,
depending upon their heat distortion temperatures. The chemical constitu-
ents of semirigid MCLCPs like X7G have consisted of the mesogenic core
and flexible spacer. Consequently, semirigid MCLCPs had lower phase tran-
sition temperatures than wholly aromatic MCLCPs, and thus semirigid
MCLCPs can easily exhibit mesophases in the wide temperature range.
Therefore many academic researchers have synthesized semirigid MCLCPs
to clarify the relationship between their chemical structures and thermal
properties [1].
ꢀCorresponding author. E-mail: nkoide@ch.kagu.sut.ac.jp
69

70
T. Mihara et al.
Thermal properties of semirigid MCLCPs were dependent upon the
chemical structure of the mesogenic core, flexible spacer length, the linking
group between the mesogenic core and flexible spacer, etc. The odd–even
effect is well known for the MCLCPs with different flexible spacer length.
The odd–even effect is as follows: nematic–isotropic phase transition
temperature and entropy changes from a nematic phase to an isotropic
phase change alternately with the number of methylene units in the
flexible spacer. The odd–even effect would be explained by the different
orientational order of the mesogenic groups in a nematic phase,
depending upon the number of methylene units with all-trans conformation
in the flexible spacer.
The clear odd–even effect on the transition temperatures and entropy
changes for the nematic–isotropic phase transition was observed for the
semirigid MCLCPs, as shown in Figure 1. Thus, the odd–even effect had
thermodynamic properties very familiar to the semirigid MCLCPs. Low
molecular weight model compounds having structural units constituting
the semirigid MCLCP offer good information on the odd–even effect on
the phase transition temperatures, as well as entropy changes of the semi-
rigid MCLCPs by experimental and computer simulation techniques [2,3].
There are two types of model compounds. One is a core model that had
a single mesogenic core with two terminal chains. Another is a twin dimer
(TD) model in which two mesogenic cores are linked by a flexible
spacer.
TDs are recognized as an exact structural model compound of semirigid
MCLCPs [2]. Abe and Furuya have found that the different orientation of
each mesogenic group in the nematic phase played an important role for
the odd–even effect on the nematic–isotropic phase transition behavior
of the TD by the conformation analysis of the flexible spacer [3]. Moreover,
TDs are recognized as interesting materials themselves, as well as the
structural model of the semirigid MCLCPs [4–6]. The symmetric Schiff’s
base dimers were found to exhibit a smectic polymorphism [7]. Symmetric
and nonsymmetric dimers consisting of the azobenzene mesogenic
group were also investigated. The spacer length had a profound influence
on the clearing temperatures of these dimers. A remarkable odd–even
effect on the transition temperatures and entropy changes for the
FIGURE 1 Chemical structure of main-chain liquid crystalline polymer containing
cyanostilbene moieties.

Synthesis and Thermal Properties of Twin Dimers
71
mesomorphic–isotropic transition was observed for the dimer series. The
thermal stability of a smectic A phase exhibited by the dimer series
increased with increasing terminal chain length but decreased with increas-
ing spacer length [8].
Hogan et al. and Attard et al. reported thermal properties of nonsym-
metric dimers with different mesogenic groups such as cyanobiphenyl
and alkylanilinebenzylidene [9,10]. The reports proposed that novel phases
involving intercalated smectic phases stabilized by the mixed mesogenic
group interaction. The magnitude of the odd–even effect was a sensitive
function of the geometry of the linking group between the flexible alkyl
chain and the mesogenic groups [4,5,11].
Watanabe et al. reported that the smectic layered structures of a, --
bis(4,40-butoxybiphenyl-carbonyloxy) alkanes was dependent upon the
number of carbon atoms in the flexible spacer [12]. The authors found that
the dimers with an even number of methylene units in the alkyl spacer
formed a smectic A phase and that the dimers with an odd number of
methylene units in the alkyl spacer formed a bilayer smectic C2 phase.
The orientation of the mesogenic groups was controlled by the confor-
mation of the flexible spacer. The authors also reported that the achiral
TD formed antiferroelectric smectic liquid crystal [13]. The exhibition of
the antiferroelectric smectic phase would be controlled by the alkyl spacer
conformation and the linking group.
FIGURE 2 Chemical structures of twin dimers containing cyanostilbene derivatives.

72
T. Mihara et al.
We reported synthesis and thermal properties of TD having cyanostil-
bene moieties as a mesogenic group [14]. In the report, we discussed the
relationship between the mesomorphic properties and the linking group
between the flexible spacer and the mesogenic group, and found that the
linking group played an important role for the exhibition of mesophase.
In this report, we synthesized TDs having the cyanostilbene moieties
with different terminal chains or different flexible spacer lengths as shown
in Figure 2, and investigated thermal properties of the dimers with differ-
ent linking group or different terminal group.
EXPERIMENTAL
Materials
4-Methoxyphenyl acetonitrile was purchased from Midorikagaku corpor-
ation (Tokyo). TDs were prepared according to Schemes 1 and 2, respect-
ively. Typical synthetic procedures were described below.
4-Formylphenymethoxymethylether(1)
Diisopropylamine (10.7 g, 147.0 mmol) was added to a tetrahydrofuran
solution (10 ml) of p-hydroxybenzaldehyde (6.2 g, 49.0 mmol). After the
SCHEME 1 Synthesis of twin dimer I, III, and IV series.

Synthesis and Thermal Properties of Twin Dimers
73
SCHEME 2 Synthesis of twin dimer II series.
mixture was stirred for 1 h, chloromethylmethylether (9.0 g, 112.0 mmol)
was added dropwise to the mixture under an ice bath. The reaction mixture
was stirred for 13 h at room temperature. After the reaction mixture was
poured into water, the aqueous mixture was extracted with ethyl acetate.
The ethyl acetate solution was dried over magnesium sulfate. The ethyl
acetate was evaporated under reduced pressure. The yellow liquid was
obtained in an 89.9% yield (6.7 g) and was used without further purification.
1H-NMR (CDCI3) d ppm: 3.5 (s, 3H, CH3), 5.5 (s, 2H, CH2), 7.2 (d, 2H,
Ar-H). 7.8 (d, 2H, Ar-H), 9.9 (s, 1H, CHO).
4-hexyloxybenzylcyanide (2)
1-Bromohaxane (3.7 g, 23.0 mmol) and potassium carbonate (0.2 g, 68.0
mmol) was added to a dimethylformamide solution (5 ml) of p-hydroxyben-
zyl cyanide (3.0 g, 23.0 mmol). The reaction mixture was heated at 80ꢁC for
4 h. The mixture was poured into water. The aqueous solution was
extracted with chloroform. The chloroform solution was dried over mag-
nesium sulfate. The chloroform was evaporated under reduced pressure.
The brownish liquid was obtained in a 95.5% yield (14.3 g) and was used
without further purification.
1H-NMR (CDCI3) d ppm: 0.8 (s, 3H, CH3), 1.3 (m, 4H, CH2), 3.7 (s, 2H,
CH2–CN), 4.0 (m, 2H, OCH2), 6.8 (d, 2H, Ar-H), 7.2 (d, 2H, Ar-H).
40-(2-methyl-1-butoxy)benzylcyanide (2)
This compound was synthesized according to the synthetic method of
the compound (2) by use of 2-methylbutanol tosylate. The crude product
was obtained. This compound was used without further purification.

74
T. Mihara et al.
4-Hexyloxy-40methoxymethyloxycyanostilbene (3)
Potassium hydroxide(1.2 g, 20.0 ) was dissolved in ethanol (50 ml). The
compound (1) (1.23 g, 8.0 mmol) was added to the ethanol solution. After
the compound (2) was added to the ethanol solution with stirring, the
reaction mixture was stirred for 4 h at room temperature. The obtained
precipitate was washed with an HCl aqueous solution and then was washed
with water until a neutral aqueous solution was obtained. The product was
purified by recrystallization from methanol and was obtained in a 64.0%
yield (1.9 g).
1H-NMR (CDCl3) d ppm: 0.8 (t, 3H, CH3), 1.3–1.9 (m, 6H, CH2), 3.5 (s, 3H,
OCH3), 4.0 (m, 2H, OCH2), 5.5 (s, 2H, OCH2), 6.8 (d, 2H, Ar-H), 7.1 (d, 2H,
Ar-H), 7.6 (d, 2H, Ar-H), 7.8 (d, 2H, Ar-H).
4-Hexyloxy-40-hydroxycyanostilbene (4)
The compound (3) was added to the mixture of methanol and 2N HCI
aqueous solution. Then tetrahydrofuran was added to the mixture to dis-
solve the compound (3). The reaction mixture was stirred for 12 h at
50ꢁC. An aqueous solution of sodium hydrogen carbonate was added to
the reaction mixture, and then the aqueous solution was extracted with
diethyl ether. The diethyl ether was evaporated to dryness. The yellow
solid was purified by recrystallization from methanol and was obtained in
an 88.0% yield (1.1 g).
1H-NMR (CDCl)3 d ppm: 0.8 (t, 3H, CH3), 1.3–1.9 (m, 6H, CH2), 4.0 (m,
2H, OCH2), 6.8 (m, 4H, Ar-H), 7.4 (s, 1H, CH=), 7.6 (d, 2H, Ar-H), 7.8 (d,
2H, Ar-H). IR (nujol) m cmꢂ1: 3297 (OH, phenol), 2229 (CN), 1605 and
1508 (aromatic group).
4-(2-methylbutoxy)-40-hydroxycyanostilbene (4)
This compound was synthesized according to the same procedure as
synthetic method of 4-hexyloxy-40-hydroxycyanostilbene.
1H-NMR (CDCl3) d ppm: 1.0 (m, 6H, CH3), 1.2–1.3 (m, 1H, CH), 1.5–1.6
(m, 1H, CH2), 1.8–1.9 (m, 1H, CH2), 3.7–3.8 (m, 2H, OCH2), 6.0 (s, 1H,
OH), 6.9 (d, 4H, Ar-H), 7.4 (s, 1H, CH=), 7.6 (d, 2H, Ar-H), 7.8 (d, 2H,
Ar-H).
Twin Dimer I (n = 6): TD-I-6
The compound (4) (1.1 g, 1.0 mmol) was dissolved in tetrahydrofuran
(30 ml). Triethylamine (0.9 g, 8.6 mmol) was added to the tetrahydrofuran
solution. A tetrahydrofuran solution (30 ml) of suberoyl chloride (0.6 g, 2.9
mmol) was added dropwise to the tetrahydrofuran solution of the com-
pound (4) and triethylamine. The reaction mixture was stirred for 24 h at
room temperature. The tetrahydrofuran was evaporated to dryness. The

Synthesis and Thermal Properties of Twin Dimers
75
residue was washed with an HCl aqueous solution and then was washed
with water until a neutral aqueous solution was obtained. The product
was purified by recrystallization twice from a mixed solvent (methanol=
chloroform = 1=1). The white solid was obtained in a 58.5% (0.79 g).
1H-NMR (CDCl3) d ppm: 0.9 (t, 6H, CH3), 1.3–1.9 (m, 24H, CH2), 2.6 (t,
4H, OOCCH2), 4.0 (t, 4H, OCH2), 6.8 (d, 4H, Ar-H), 7.1 (d, 4H, Ar-H), 7.4
(s, 2H, CH=), 7.6 (d, 4H, Ar-H), 7.8 (d, 4H, Ar-H).
Twin Dimer II (n = 6): TD-II-6
The twin dimer was obtained using the compound (1) in Scheme 2
according to our literature [14]. The yellow product was obtained in a
36.4% yield. NMR and Fourier transform infrared (FTIR) measurements
supported the finding that synthesized twin dimers had desirable chemical
structures.
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 cm ꢂ 1 resolution. DSC mea-
surements 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
microscopy and DSC measurements. X-ray diffraction patterns were
recorded with a RIGAKU RINT2500 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). Mesomorphic
structure of twin dimers was investigated by X-ray measurements.
UV-vis-NIR spectroscopy measurements were carried out with a HITACHI
U-3410 spectrophotometer.
RESULTS AND DISCUSSION
In our previous work, we found that the linking group between the meso-
genic group and the flexible spacer, as well as the position of the cyano
group in the mesogenic group, played an important role for the exhibition
of mesophases for twin dimers (TDs) containing cyanostilbene moieties
[14]. In this study, to clarify the influence of the terminal group of TDs con-
taining cyanostilbene moieties, we synthesized TDs containing cyanostil-
bene moieties with different terminal chain lengths, different flexible
spacer lengths, or both, as shown in Figure 2. Thermal properties of the

76
T. Mihara et al.
TDs were investigated by polarizing optical microscopy and differential
scanning calorimetry (DSC) measurements. Mesomorphic phase structures
of TDs were investigated by X-ray measurements.
Figure 3 shows DSC curves of TD-I-6 with ester linking group between
the mesogenic group and the flexible spacer. Three peaks are detected on
both heating and cooling scans of TD-I series. The peaks at 93ꢁC and at
167ꢁC on heating scan are assigned to the melting and clearing tempera-
tures, respectively. This assignment is supported by observation of the
optical texture of TD-I-6. A schlieren texture was observed in the
temperature range between melting and clearing points. TD-I-6 displays
two mesophases in the temperature range between melting and clearing
points.
Figure 4 shows X-ray patterns of TD-I-6. A sharp peak in the small-angle
region and a broad peak in the wide-angle region are observed in the X-ray
pattern of TD-I-6 at 105ꢁC. A d-spacing based upon the sharp peak in the
˚
small-angle region is 43 A, while the calculated molecular length of the
˚
TD is 49 A. The structure of the mesophase in the lower mesomorphic tem-
perature range would be a smectic C phase due to observation of a schlie-
ren texture and the X-ray pattern for the twin dimer, while in the higher
mesomorphic temperature range a broad peak in the wide-angle region is
detected in the X-ray pattern of TD-I-6. The phase structure of TD-I-6 in
FIGURE 3 DSC curves of TD-I-6.

Synthesis and Thermal Properties of Twin Dimers
77
FIGURE 4 X-ray patterns of TD-I-6.
the higher mesomorphic temperature range is a nematic phase based upon
observation of the schlieren texture and this X-ray pattern.
The X-ray pattern of TD-I-3 at 115ꢁC on heating scan was similar to that
of TD-I-6 at 105ꢁC as shown in Figure 4. The d-spacing of TD-I-3 due to the
˚
sharp peak in the small-angle region is 39 A, while the calculated molecular
˚
length of TD-I-3 with all-trans conformation is 41 A. The layer spacing is
slightly shorter than that of the calculated molecular length of TD-I-3. In
the lower mesomorphic temperature range (109ꢁC–126ꢁC on heating scan)
at TD-I-3, initially the optical texture similar to a focal conic texture was
observed at 115ꢁC on heating scan; however, annealing of the sample in
the lower mesomorphic temperature range led to the appearance of a
schlieren texture. Therefore, we determined that the phase structure of
TD-I-3 in the lower mesomorphic temperature range was a smectic C
phase, although at the present time we cannot clarify the reason why the
change in the optical texture of TD-I-3 was induced by annealing in the
lower mesomorphic temperature range.
In the X-ray pattern of TD-I series with relatively shorter spacer in
higher mesomorphic temperature range, a peak in both the small-angle
and the wide-angle regions of TD-I-3 and TD-I-4 was detected. This
X-ray pattern of TD-I series with shorter spacer was similar to that
observed in the lower mesomorphic temperature range of TD-I-6, although
the peak in the small-angle region was very small. The layer structure of

78
T. Mihara et al.
the smectic phase would become loose because the intensity of the peak
in the small-angle region was weak compared with that detected in the
smectic C phase. In the higher mesomorphic temperature range of TD-I
series with relatively shorter spacer, the phase structure would be a
cybotactic nematic phase due to the observation of a schlieren texture
and the X-ray pattern similar to the smectic phase [15–18].
Thermal properties of TD-I series are summarized in Table 1. Smectic C
and nematic phases are observed for TD-I-3, TD-I-4, and TD-I-6. On the
other hand, an optical texture characteristic of typical liquid crystals could
not be observed in the lower mesomorphic temperature range between
87ꢀC and 50ꢀC on cooling scan of TD-I-5, although birefringence was shown
in this temperature range. DSC curves ꢀof TD-I-5 are shown in Figure 5. The
phase transition enthalpy change at 87 C on cooling scan was 0.7 Jg ꢁ1. The
value of the enthalpy change would be too small for crystallization. We
detected the exothermal peak at 50ꢀC in the DSC curve of TD-I-5 on cool-
ing scan. The peak was attributed to the crystallization of TD-I-5. The DSC
curve of TD-I-5 on cooling scan supported that the phase structure in the
temperature range between 87ꢀC and 50ꢀC would not be crystal but a
mesophase: however, X-ray patterns of TD-I-5 in the lower mesomorphic
temperature range were similar to that of a crystal. Therefore, at the
present time we cannot clarify the phase structure in the lower
mesomorphic temperature range of TD-I-5 on cooling scan. TD-I series
TABLE 1 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to the Isotropic Melt of Twin Dimer I Series
n
3
Phase transition temperatures=ꢀC
DSNI(Jmol-1K-1)
3.8 (4.0)
4
5
6
9.5 (10.8)
3.1 (3.0)
11.1 (13.2)
Cr, Cr0 crystal; SmC, smectic C phase; N, nematic phase; M, mesophase; I, isotropic phase;
DSNI, entropy change from a mesophase to the isotropic melt on cooling scan.
Parentheses ( ) denote DSNI on heating scan.

Synthesis and Thermal Properties of Twin Dimers
79
FIGURE 5 DSC curves of TD-I-5.
displayed a remarkable odd–even effect on phase transition temperatures
and entropy changes from a nematic phase to the isotropic melt (DSNI).
We also synthesized TD-II series with the ether linking group between
the mesogenic group and the flexible spacer to clarify the effect of the link-
ing group on the mesomorphic properties of TDs. A schlieren texture was
observed for TD-II-4. DSC curves and X-ray patterns of TD-II-4 are shown
in Figure 6 and 7, respectively. TD-II-4 displayed two mesophases in the
temperature range between 149 and 179ꢁC on heating scan. The d-spacing
˚
of TD-II-4 wasꢁ42 A based on the peak in the small-angle region of the X-ray
˚
pattern at 158 C, while the calculated molecular length of TD-II-4 was 45 A.
Therefore, the phase structure in the lower mesomorphic temperature
range of TD-II-4 would be a smectic C phase. In the higher mesomorphic
temperature range, a cybotactic nematic phase would be shown due to ob-
servation of the schlieren texture and a weak peak in the small-angle region
of the X-ray pattern, as shown in Figure 7. A cybotactic nematic phase was
also observed in the higher mesomorphic temperature range of TD-II-5.
Thermal behavior of TD-II series with ether linking group is summarized
in Table 2. It would be difficult to exhibit a smectic phase for TD-II series
compared to TD-I series with ester linking group.
TD-II series displayed a remarkable odd–even effect on phase transition
temperatures and DSNI. TD-I series easily exhibited the liquid crystalline
phases compared to TD-II series. Therefore, we synthesized TD-III series

80
T. Mihara et al.
FIGURE 6 DSC curves of TD-II-4.
FIGURE 7 X-ray patterns of TD-II-4.

Synthesis and Thermal Properties of Twin Dimers
81
TABLE 2 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to the Isotropic Melt of Twin Dimer II Series
n
3
Phase transition temperatures=ꢁC
DSNI(Jmol-1K-1)
3.8
4
5
6
12.4 (12.3)
4.1 (4.1)
8.6 (9.0)
Cr, Cr0, crystal; Smc, smectic C phase; N, nematic phase; M, mesophase; I, isotropic phase;
DSNI, entropy change from a mesophase to the isotropic melt on cooling scan.
Parentheses ( ) denote DSNI on heating scan.
with different terminal chain length to investigate the effect of the terminal
chain length on the thermal behavior of TDs with ester linking group. Here
we fixed the length of methylene spacer, because TD-I-6 displayed liquid
crystalline phases and simple thermal behavior without a recrystallization.
Phase transition temperatures and DSNI for TD-III series are summarized
in Table 3. TD-III, with a relatively short terminal chain, showed an enan-
tiotropic nematic phase, while TD-III-6 and TD-III-7, with longer terminal
chains, displayed an enantiotropic smectic phase. This is conventional ther-
mal behavior for low molar mass liquid crystals. The temperature range of
the smectic phase for TD-III-7 was wider than that of TD-III-6. Terminal
chain length played an important role in the exhibition of a smectic phase
and the stability of the smectic phase for TDs. In the higher mesomorphic
temperature range of TD-III-7, a cybotactic nematic phase was observed.
The possible packing models for the smectic C and cybotactic nematic
phase of TD series are shown in Figure 8. The models were proposed
assuming that TDs form all-trans conformation. Figure 8(a) displays the
packing model for the smectic C phase of TD series with even-numbered
flexible spacer such as TD-I-4, TD-I-6, TD-II-4, TD-III-6, and TD-III-7.
The mesogenic groups of the even-numbered TD series can be parallel to
each other. On the other hand, Figure 8(b) shows the packing models
for the smectic C phase of TD-I-3 that have odd-numbered flexible spacer.
TD-I-3 must have a bent conformation assuming that it forms all-trans con-
formation. The mesogenic groups of TD-I-3 can be tilted to each other

82
T. Mihara et al.
TABLE 3 Phase Transition Temperatures and Entropy Changes from a Nematic
Phase to the Isotropic Melt of Twin Dimer III Series
m
3
Phase transition temperatures=ꢁC
DSNI=(Jmol-1K-1)
11.7 (11.9)
4
12.6 (12.2)
5
6
7
11.6 (11.6)
11.1 (13.2)
14.2 (13.4)
Cr, crystal; SmC, smectic C phase; N, nematic phase; I, isotropic phase; DSNI, entropy
change from a mesonphase to the isotropic melt on cooling scan.
Parentheses ( ) denote DSNI on heating scan.
FIGURE 8 Possible packing models for smectic C and cybotactic nematic phases of
twin dimers: (a) smectic C phase for twin dimers with even-numbered flexible
spacer, (b) smectic C phase of TD-I-3, (c) cybotactic nematic phase.

Synthesis and Thermal Properties of Twin Dimers
83
[12,13]. Figure 8(c) indicates one of the models for the molecular array in
the cybotactic nematic phase. The cybotactic nematic phase has the mol-
ecular array with smectic C order (cybotactic group) in nematic molecular
array. The cybotactic group in the nematic structure is a cluster of liquid
crystal (LC) molecules that form the molecular array similar to the smectic
C phase. Both the conventional nematic array of LC molecules and the
cybotactic groups coexist in the cybotactic nematic phase [15–18].
Furthermore, to investigate the effect of an optical active moiety on the
thermal behavior of TDs, we synthesized TD-IV series, which had an optical
active terminal chain and different flexible spacer lengths. Phase transition
temperatures, entropy changes from mesophase to isotropic phase (DSMI),
and optical rotation of TD-IV series are summarized in Table 4. It does not
seem easy for TD-IV series to show enantiotropic mesophases. In DSC
curves of TD-IV-4, two peaks (at 125 and 155ꢁC) were observed on the
heating scan, while three peaks (at 155, 122, and 108ꢁC) were shown on
the cooling scan. The two peaks on the heating scan were assigned to
the melting and clearing temperatures, respectively, based upon the polar-
ized optical microscopy observation. While the three peaks on the cooling
scan were attributed to the clearing, mesophase–mesophase transition and
melting temperatures, respectively. An oily streak texture was observed in
the temperature range between 125 and 155ꢁC on the heating scan and
TABLE 4 Phase Transition Temperatures, Entropy Changes from a Mesophase to
the Isotropic Melt, and Optical Rotation of Twin Dimer IV Series
n
3
Phase transition temperatures=ꢁC
[a]25
DSMI=(Jmol-1K-1)
D
43.3
16.5
—
4
5
6
7
8.7 (8.1)
1.6
50.7
25.5
49.3
9.7
2.2
Cr, Cr0, crystal; Sm, smectic phase; Ch, cholesteric phase; M, mesophase; I, isotropic phase,
DSMI, entropy change from a mesophase to the isotropic melt on cooling scan.
Parentheses ( ) denote DSMI on heating scan.

84
T. Mihara et al.
between 155 and 122ꢁC on cooling scan. This microscopy observation
supported that TD-IV-4 displayed an enantiotropic cholesteric phase.
Oily streak textures of cholesteric phases can be observed when the
helical axis of cholesteric helical structure is perpendicular to the glass
plate. We cannot estimate the helical pitch of TD-IV-4 from the optical tex-
ture. The selective reflection wavelength is related to the helical pitch of
cholesteric helical structures. Therefore we investigated the selective
reflection wavelength of cholesteric structures for TD-IV-4 and TD-IV-6
by UV-vis-NIR spectroscopy measurements. Transmittance spectra of TD-
IV-6 are shown in Figure 9. The selective reflection wavelengths of choles-
teric structures were about 880 nm at 140ꢁC for TD-IV-4 and about 900 nm
at 120ꢁC for TD-IV-6, respectively. The selective reflection peak for TD-IV-4
and TD-IV-6 was slightly shifted to a shorter wavelength with decreasing
temperature.
Figure 10 shows the X-ray patterns of TD-IV-4 at 115ꢁC on the cooling
scan. A sharp peak in the small-angle region and a broad peak in the
wide-angle region were observed in the X-ray pattern of TD-IV-4. The
˚
d-spacing of TD-IV-4 was 35 A due to the sharp peak in the small-angle
FIGURE 9 Transmittance spectra of planar-oriented samples of TD-IV-6 at
(a)110ꢁC, (b)115ꢁC, and (c)120ꢁC.

Synthesis and Thermal Properties of Twin Dimers
85
FIGURE 10 X-ray patterns of TD-IV-4.
region, while the calculated molecular length of TD-IV-4 with all-trans
˚
conformation was about 41 A. The d-spacing obtained was shorter than that
of the calculated molecular length. Therefore, we expected that the phase
structure in the lower mesomorphic temperature range would be a smectic
Cꢀ phase: however, a clear optical texture characteristic of smectic Cꢀ
phase was not observed, but birefringence was observed in the lower
mesomorphic temperature range of TD-IV-4.
A remarkable odd–even effect on DSMI on cooling scan was observed
for TD-IV series. TD-IV-even number series showed a cholesteric phase,
while TD-IV-odd number series did not exhibit a typical liquid crystalline
phase. This thermal behavior would originate from the difference in the
orientation of mesogenic groups based upon the flexible spacer length.
Birefringence was observed in the temperature range of TD-IV-odd num-
ber series; however, we detected the crystallization of TD-IV-odd number
series under annealing by optical microscopy measurements. We did not
observe a typical X-ray pattern for liquid crystalline phase, but an X-ray
pattern characteristic of a crystalline phase. In the X-ray pattern, many
sharp peaks were observed in the wide angle region. Crystallization would
occur during the X-ray measurements of TD-IV-odd number series. The
monotropic mesophase on cooling scan would be unstable thermodynami-
cally, and phase transition from mesophase to crystal would occur under
annealing.

86
T. Mihara et al.
CONCLUSIONS
We synthesized twin dimers (TDs) with different linking group between a
mesogenic group and a flexible spacer, different spacer length, and differ-
ent terminal alkyl chain length. Liquid crystallinity of TDs with ester linking
group was superior to that of TDs with ether linking group, although we
cannot clarify the origin of the difference in liquid crystallinity between
TDs with ester or ether linking group. A remarkable odd–even effect on
the phase transition temperatures and nematic–isotropic phase transition
entropy changes was observed for the TDs with different spacer length.
In contrast, length of terminal group played an important role in exhibiting
a smectic phase for TDs with an ester linking group. A clear odd–even ef-
fect on the phase transition temperatures and nematic–isotropic phase
transition entropy changes was not detected for the TDs with different
lengths of terminal group. The exhibition of cholesteric phase for TDs with
optically active terminal moiety was dependent upon the flexible spacer
length.
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