Synthesis and phase transition behavior of novel liquid crystal tetramers

Article abstract of DOI:10.1080/15421400903065739

We synthesized novel liquid crystal tetramers possessing cyanobiphenyl and phenylpyrimidine moieties, and investigated their phase transition behavior. The liquid crystal tetramers exhibited nematic (N) and smectic A (SmA) phases. X-ray diffraction measurements suggest that the SmA phase has an intercalated structure. We discuss the transition behavior of the newly designed tetramers in terms of their molecular shape. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421400903065739

Mol. Cryst. Liq. Cryst., Vol. 509, pp. 263=[1005]–273=[1015], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903065739
Synthesis and Phase Transition Behavior
of Novel Liquid Crystal Tetramers
Tsuyoshi Narumi1, Atsushi Yoshizawa1,
Jun Yamamoto2, and Yoichi Takanishi2
1Department of Frontier Materials Chemistry, Graduate School
of Science and Technology, Hirosaki University, Hirosaki, Japan
2Department of Physics, Graduate School of Science, Kyoto University,
Oiwake-cho, Kitashirakawa, Skyo-ku, Kyoto, Japan
We synthesized novel liquid crystal tetramers possessing cyanobiphenyl and
phenylpyrimidine moieties, and investigated their phase transition behavior.
The liquid crystal tetramers exhibited nematic (N) and smectic A (SmA) phases.
X-ray diffraction measurements suggest that the SmA phase has an intercalated
structure. We discuss the transition behavior of the newly designed tetramers in
terms of their molecular shape.
Keywords: liquid crystals; phase transition; tetramers
1. INTRODUCTION
Supramolecular assemblies composed of oligomeric liquid crystals
are of current interest in the design of new liquid crystalline mate-
rials [1]. Dimeric liquid crystals are attractive because they exhibit
different properties from the corresponding low molar mass meso-
gens [2–5]. For instance, transition properties of dimeric liquid crys-
tals are known to depend on the length and parity of the flexible
spacer. Furthermore, liquid crystal trimers [6–15] and tetramers
[16,17] have been reported. Pronounced odd-even effects have
been observed in the transition properties of linear liquid crystal
This work was partly supported by a Grant-in-Aid for Scientific Research from the
Japan Society for the Promotion of Science (No. 19550175).
Address correspondence to Atsushi Yoshizawa, Department of Frontier Materials
Chemistry, Graduate School of Science and Technology, Hirosaki University,
bunkyo-cho, Hirosaki 036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
3
263=[1005]

264=[1006]
T. Narumi et al.
FIGURE 1 Liquid crystal oligomers possessing cyanobiphenyl and phenyl-
pyrimidine units.
oligomers on varying the spacer length. The transition behavior is
interpreted in terms of how the spacers control the average molecu-
lar shape [12,16].

Liquid Crystal Tetramers
265/[1007]
We have reported liquid crystal oligomers in which cyanobiphenyl
and phenylpyrimidine moieties are connected via a flexible spacer
[18–20], 1,3-dihydroxybenzene [21], 3,4-dihydroxybenzoic acid [22–24]
or 3,5-dihydroxybenzoic acid [25], as shown in Figure 1. Intermolecular
interactions between such mesogenic moieties were found to have sig-
nificant effects on the molecular assemblies in the liquid-crystalline
phases: (i) antiparallel interactions between cyanobiphenyl moieties
promote the formation of N and SmA phases, (ii) interactions between
phenylpyrimidine moieties were found to stabilize smectic phase [26],
and (iii) interactions between cyanobiphenyl and phenylpyrimidine
moieties are known to induce an SmA phase [27]. Linear compound
[18] and the V-shaped compound [21] exhibited a phase sequence of
I – N – SmA –SmCanti. The k-shaped compound [22] was found to
show a stable incommensurate SmA phase, however, the Y-shaped
compound [25] exhibited a direct transition from isotropic liquid to
SmCanti phase. The K-shaped compound [24] showed a monotropic
nematic phase.
For the present study, we prepared novel liquid crystal tetramers
possessing cyanobiphenyl and phenylpyrimidine moieties, and investi-
gated their phase transition behavior.
2. EXPERIMENTAL
2.1. Spectroscopic Analysis
The structures of the final products were elucidated by infrared
(IR) spectroscopy (BIO RAD FTS-30) and proton nuclear magnetic
resonance (1H NMR) spectroscopy (JEOL JNM-ECA500).
2.2. Preparation of Materials
2.2.1. 1,6-Bis{4-[4-(4-cyanophenyl)phenyloxycarbonyl]-
2-{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}
phenyloxy}hexane, I-6
To
a
solution of 5-octyl-2-(4-hydoxyphenyl)pyrimidine (2.84 g,
10.0 mmol) and 1,6-dibromohexane (3.42 g, 14.0 mmol) in cyclo-
hexanone (40 ml) was added potassium carbonate (1.38 g, 14.0 mmol).
The resulting mixture was stirred at 80ꢀC for 9 h. After filtration
of the precipitate, the solvent was removed by evaporation. The
residue was purified by column chromatography on silica gel with a
toluene=ethyl acetate (15=1) mixture as the eluent. Recrystallization
from ethanol gave 2-[4-(6-bromohexyloxy)phenyl]-5-octylpyrimidine;
yield 2.16 g (4.8 mmol, 48%).

266/[1008]
T. Narumi et al.
To a solution of 2-[4-(6-bromohexyloxy)phenyl]-5-octylpyrimidine
(2.13 g, 4.8 mmol) and ethyl 3,4-dihydroxybenzoate (3.46 g, 19.0 mmol)
in cyclohexanone (20 ml) was added potassium carbonate (1.97 g,
14.3 mmol). The resulting mixture was stirred at 70ꢀC for 8 h. After
filtration of the precipitate, the solvent was removed by evaporation.
The residue was purified by column chromatography on silica gel with
a toluene=ethyl acetate (19=1) mixture as the eluent. Recrystallization
from ethanol gave ethyl 4-hydroxy-3-{6-[4-(5-octylpyrimidin-2-yl)
phenyloxy]hexyloxy}benzoate; yield 1.15 g (2.1 mmol, 44%).
To a solution of ethyl 4-hydroxy-3-{6-[4-(5-octylpyrimidin-2-yl)
phenyloxy]hexyloxy}benzoate (1.10 g, 2.0 mmol) and 1,6-dibromohex-
ane (0.24 g, 1.0 mmol) in cyclohexanone (10 ml) was added potassium
carbonate (0.28 g, 2.0 mmol). The resulting mixture was stirred at
130ꢀC for 5 h. After filtration of the precipitate, the solvent was
removed by evaporation. The residue was purified by column
chromatography on silica gel with a toluene=ethyl acetate (9=1)
mixture as the eluent. Recrystallization from ethanol gave 1,6-bis
{4-ethyloxycarbonyl-2-{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]hexyloxy}
phenyloxy}hexane; yield 1.03 g (0.9 mmol, 88%).
1,6-Bis{4-ethyloxycarbonyl-2-{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]
hexyloxy}phenyloxy}hexane was added to a solution potassium hydro-
xide (0.90 g, 16 mmol) in ethanol (95%, 20ml). The resulting mixture
was stirred under reflux for 2 h. The solution was acidified with hydro-
chloric acid. Water (20 ml) was added to the mixture and the aqueous
phase was extracted with dichloromethane (3ꢁ 20 ml). The organic
layers were combined, dried over magnesium sulfate, filtered and
evaporated. 1,6-Bis{4-carboxy-2-{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]
hexyloxy}phenyloxy}hexane was obtained; yield 0.64 g (0.6 mmol, 71%).
To a solution of 1,6-bis{4-carboxy-2-{6-[4-(5-octylpyrimidin-2-yl)phe-
nyloxy]hexyloxy}phenyloxy}hexane (0.56 g, 0.5 mmol) in dichloro-
methane (30 ml), 4-cyano-40-hydroxybiphenyl (0.23 g, 1.2 mmol), N,N0-
dicyclohexylcarbodiimide (0.25 g, 1.2 mmol) and 4-(N,N-dimethylamino)
pyridine (0.25 g, 1.2 mmol) were added. The resulting solution was
stirred at room temperature for 25 h. Precipitated materials were
removed by filtration. After removal of the solvent by evaporation,
the residue was purified by column chromatography on silica gel with
a toluene=ethyl acetate (4=1) mixture as the eluent. Recrystallization
from ethanol gave the desired product; Yield 84 mg (0.1 mmol, 11%).
dH (500 MHz, CDCl3, TMS): 8.55 (s, 4H, Ar-H), 8.33 (d, 4H, Ar-H,
J¼ 9.1Hz), 7.83 (dd, 2H, Ar-H, J¼ 8.3 Hz, 2.0 Hz), 7.72 (d, 4H, Ar-H, J¼
8.6 Hz), 7.67 (d, 4H, Ar-H, J¼ 8.6 Hz), 7.67 (d, 2H, Ar-H, J¼ 2.3 Hz), 7.62
(d, 4H, Ar-H, J ¼ 8.6 Hz), 7.31 (d, 4H, Ar-H, J¼ 8.6 Hz), 6.95 (d, 4H, Ar-H,
J¼ 9.0 Hz), 6.93 (d, 2H, Ar-H, J¼ 8.2 Hz), 4.09 (t, 4H, -OCH2-, J¼ 6.6 Hz),

Liquid Crystal Tetramers
267/[1009]
4.09 (t, 4H, -OCH2-, J¼ 6.6Hz), 4.01 (t, 4H, -OCH2-, J¼
6.3 Hz), 2.58 (t, 4H, Ar-CH2-, J¼ 7.7 Hz), 1.90–1.26 (m, 48H, aliphatic-H),
0.87 (t, 6H, -CH3, J¼ 6.9 Hz). n=cmꢂ1 (KBr): 2927,2855 (C-H), 2225
(CꢃN), 1729 (C¼O, -COO-), 1606,1584 (C¼C).
The other compounds presented in this paper were obtained by a
similar method to that for I-6. Analytical data for the other
compounds are listed below.
2.2.2. 1,7-Bis{4-[4-(4-cyanophenyl)phenyloxycarbonyl]-
2-{6-[4-(5-octylpyrimidin-2-yl)phenyloxy]
hexyloxy}phenyloxy}heptane, I-7
dH (500MHz, CDCl3, TMS): 8.56 (s, 4H, Ar-H), 8.33 (d, 4H, Ar-H,
J ¼ 8.6 Hz), 7.83 (dd, 2H, Ar-H, J ¼ 8.3 Hz, 2.0 Hz), 7.73 (d, 4H, Ar-H, J ¼
8.6 Hz), 7.67 (d, 4H, Ar-H, J ¼ 8.6 Hz), 7.66 (d, 2H, Ar-H, J ¼ 1.7 Hz), 7.62
(d, 4H, Ar-H, J ¼ 8.6 Hz), 7.31 (d, 4H, Ar-H, J ¼ 8.6 Hz), 6.95 (d, 4H,
Ar-H, J ¼ 9.2 Hz), 6.94 (d, 2H, Ar-H, J ¼ 9.2 Hz), 4.10 (t, 4H, -OCH2-,
J ¼ 6.6 Hz), 4.07 (t, 4H, -OCH2-, J ¼ 6.6 Hz), 4.02 (t, 4H, -OCH2-,
J ¼ 6.3 Hz), 2.58 (t, 4H, Ar-CH2-, J ¼ 7.8 Hz), 1.90–1.26 (m, 50H,
aliphatic-H), 0.87 (t, 6H, -CH3, J ¼ 6.9 Hz). n=cmꢂ1 (KBr): 2928,2855
(C-H), 2225 (CꢃN), 1728 (C¼O, -COO-), 1606,1584 (C¼C).
2.2.3. 1,8-Bis{4-[4-(4-cyanophenyl)phenyloxycarbonyl]-
2-{6-[4-(5-octylpyrimidin-
2-yl)phenyloxy]hexyloxy}phenyloxy}octane, I-8
dH (500 MHz, CDCl3, TMS): 8.56 (s, 4H, Ar-H), 8.34 (d, 4H, Ar-H,
J ¼ 9.2 Hz), 7.83 (dd, 2H, Ar-H, J ¼ 8.3 Hz, 2.1 Hz), 7.73 (d, 4H, Ar-H,
J ¼ 8.0 Hz), 7.68 (d, 4H, Ar-H, J ¼ 8.6 Hz), 7.67 (d, 2H, Ar-H, J ¼
2.3 Hz), 7.63 (d, 4H, Ar-H, J ¼ 8.6 Hz), 7.32 (d, 4H, Ar-H, J ¼ 8.6 Hz),
6.96 (d, 4H, Ar-H, J ¼ 9.2 Hz), 6.94 (d, 2H, Ar-H, J ¼ 8.6 Hz), 4.11 (t,
4H, -OCH2-, J ¼ 6.6 Hz), 4.06 (t, 4H, -OCH2-, J ¼ 6.6 Hz), 4.03 (t, 4H,
-OCH2-, J ¼ 6.6 Hz), 2.58 (t, 4H, Ar-CH2-, J ¼ 7.5 Hz), 1.91–1.26 (m,
52H, aliphatic-H), 0.88 (t, 6H, -CH3, J ¼ 7.2 Hz). n=cmꢂ1 (KBr):
2928,2855 (C-H), 2225 (CꢃN), 1727 (C¼O, -COO-), 1606,1584 (C¼C).
2.3. Physical Properties
The initial phase assignments and corresponding transition tempe-
ratures for the final products were determined by thermal optical
microscopy using a Nikon Optiphoto POL polarizing microscope
equipped with a Mettler FP82 hot stage and FP90 control processor.
The heating and cooling rates were 5ꢀC minꢂ1. Temperatures and

268/[1010]
T. Narumi et al.
enthalpies of transition were investigated by differential scanning
calorimetry (DSC) using a Seiko DSC 6200. The materials were
studied at a scanning rate of 5ꢀC minꢂ1 after being encapsulated in
aluminum pans. The XRD patterns of the sample on cooling process
were obtained using a real-time X-ray diffractometer (D8 Discover;
Bruker AXS GmbH) equipped with a hot stage and a temperature-
control processor. A sample was put on a convex lens, which was
placed in a custom-made temperature stabilized holder (stability
within ꢄ0.1ꢀC). The X-ray apparatus was equipped with a cross-
¨
coupled Gobel mirror on a platform system with a two-dimensional
position-sensitive proportional counter (PSPC) detector (HI-Star;
Bruker AXS GmbH). X-rays were generated at 40 kV and 40 mA; a
parallel Cu Ka X-ray beam was used to irradiate the sample. Each dif-
fraction pattern was obtained using the PSPC detector at a camera
distance of 150 mm for a short counting time of 30 s.
3. RESULTS AND DISCUSSION
3.1. Synthesis
We designed novel liquid crystal tetramers I-n (see Fig. 2). Compound
I-6 was prepared by the synthesis outlined in Scheme 1. Purification of
the final products was carried out using column chromatography over
silica gel with a toluene=ethyl acetate (4=1) mixture as the eluent, fol-
lowed by the recrystallization from ethanol. The structures were elu-
1
cidated using IR and H NMR measurements.
3.2. Phase Transition Behavior
Transition temperatures and transition enthalpies for compounds I-n
measured by optical polarized light microscopy and differential
scanning calorimetry (DSC) are listed in Table 1. Compound I-6
FIGURE 2 Molecular structure of compound I-n.

Liquid Crystal Tetramers
269/[1011]
SCHEME 1 The preparative route for the liquid crystal tetramer I-6.
exhibited an SmA phase, whereas compounds I-7 and I-8 exhibited N
and SmA phases. Weissflog et al. reported that a dimer possessing two
laterally attached substituents exhibits an SmA phase [28]. Imrie and

270/[1012]
T. Narumi et al.
TABLE 1 Phase Transition Temperatures (ꢀC) and Transition Enthalpies
(kJ molꢂ1) (in Square Brackets) for I-n
Compound
Cr
SmA
N
I
mp=ꢀC
I-6
I-7
I-8
. 172
. 97.8
. 130
. 209 [31.2]
. 202.6 [a]
. 191 [14.0]
.
.
.
208
191
178
. 202.8 [a]
. 198 [15.8]
[a] The I-N and N-SmA transitions occurred simultaneously. The total value of both
transition enthalpies was 30.4 kJ molꢂ1
.
Henderson discussed that packing constraints depress appearance of
an N phase of the dimer [2]. The appearance of the N phase for the
compounds I-7 and I-8 is thought to be attributed to antiparallel inter-
actions between the neighboring cyanobiphenyl moieties.
3.3. X-Ray Diffraction Studies
In order to understand a structure of the SmA phase, XRD measure-
ments were carried out. Figure 3 shows an X-ray diffraction pattern
in the small angle region of compound I-6 in the SmA phase. A sharp
reflection was observed at 2h ¼ 3.82ꢀ, corresponding to the spacing of
˚
23.1 A. Figure 4 shows the expected elongated molecular conformation
and length calculated using the MM2 method. Although stability of
the configuration from MM2 method is still questionable due to the
molecular flexibility, we consider the single configuration to discuss
FIGURE 3 X-ray diffraction pattern of compound I-6 in the SmA phase.

Liquid Crystal Tetramers
271/[1013]
FIGURE 4 Expected molecular configuration and length by the MM2 calcula-
tion for compound I - 6.
the phase structure at the first approximation. The molecular length
˚
was estimated as 41 A. The obtained layer spacing is shorter than
the molecular length. Therefore compound I - 6 is considered to form
an intercalated SmA phase. Figure 5 shows a possible model of mole-
cular organization in the SmA phase. The model suggests that: 1) the
close-packed molecular shape produces favorable molecular packing
and, 2) intermolecular interactions between cyanobiphenyl and phe-
nylpyrimidine moieties exist in each layer.
4. CONCLUSIONS
We prepared novel liquid crystal tetramers possessing cyanobiphenyl
and phenylpyrimidine moieties, and found that they exhibit N
and SmA phases. X-ray studies suggest that the SmA phase has an
intercalated layer structure composed of the tetramers with a
close-packed conformation.

272/[1014]
T. Narumi et al.
FIGURE 5 Molecular arrangements suggested for compound I - 6 in the SmA
phase.
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