Comparison of liquid-crystalline properties of symmetric and nonsymmetric liquid crystal trimers

Article abstract of DOI:10.1080/15421400701617749

A series of nonsymmetric liquid crystal trimers, -(4-cyanobiphenyl-4'- yloxy)alkyl 4'-co-(4-cyanobiphenyl-4'-yloxy)alkoxy]-4-biphenylcarboxylates, were prepared. The compounds exhibited enantiotropic nematic behavior. The melting points, the nematic-isotr

Full text of DOI:10.1080/15421400701617749

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Comparison of Liquid-
Crystalline Properties of
Symmetric and Nonsymmetric
Liquid Crystal Trimers
Toshio Itahara a & Hisashi Tamura a
a Faculty of Engineering, Kagoshima University ,
Korimoto, Kagoshima, Japan
Published online: 22 Sep 2010.
To cite this article: Toshio Itahara & Hisashi Tamura (2007) Comparison
of Liquid-Crystalline Properties of Symmetric and Nonsymmetric Liquid
Crystal Trimers, Molecular Crystals and Liquid Crystals, 474:1, 17-27, DOI:
10.1080/15421400701617749
To link to this article: http://dx.doi.org/10.1080/15421400701617749
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Mol. Cryst. Liq. Cryst., Vol. 474, pp. 17–27, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701617749
Comparison of Liquid-Crystalline Properties of
Symmetric and Nonsymmetric Liquid Crystal Trimers
Toshio Itahara
Hisashi Tamura
Faculty of Engineering, Kagoshima University, Korimoto,
Kagoshima, Japan
A series of nonsymmetric liquid crystal trimers, x-(4-cyanobiphenyl-40-yloxy)alkyl
40-co-(4-cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxylates, were prepared.
The compounds exhibited enantiotropic nematic behavior. The melting points,
the nematic–isotropic transition temperatures, and the entropy changes associated
with the nematic–isotropic transition showed remarkable odd–even effects as the
length of the spacers was varied, in which the even members exhibited higher
values. The liquid-crystalline properties were compared with those of analogous
symmetric trimers. The nematic–isotropic transition temperatures and the
associated entropy changes of the nonsymmetric trimers exhibited lower values
in comparison with those of the symmetric trimers.
Keywords: liquid crystals; melting point; nematic–isotropic transition; nonsymmetric
trimers; odd–even effects
INTRODUCTION
Liquid crystal trimers are defined as molecules containing three
mesogenic units joined by two flexible spacers [1] and have been exten-
sively studied as model compounds for liquid-crystalline polymers
[1–20]. A comparison of the liquid-crystalline properties of nonsym-
metric and symmetric liquid crystal oligomers [4] containing trimers
is of interest in connection with the effect of the symmetry of the mole-
cules on the molecular aggregation and liquid-crystalline behavior.
However, the investigations have dealt almost exclusively with
nonsymmetric and symmetric liquid crystal dimers [4,13,17]. In the
present article, data is presented that compare the liquid-crystalline
Address correspondence to Toshio Itahara, Faculty of Engineering, Kagoshima
University, Korimoto, Kagoshima, 890-0065, Japan. E-mail: itahara@be.kagoshima-u.ac.jp
17

18
T. Itahara and H. Tamura
SCHEME 1 Structures of BCBOn, TCBOn, and compounds (1).
properties of nonsymmetric liquid crystal trimers with those of
symmetric liquid crystal trimers.
4-Cyano-40-alkoxybiphenyl derivatives are typical liquid-crystalline
molecules. Furthermore, it is known that liquid crystal dimers containing
two 4-cyanobiphenyl groups, such as BCBOn, exhibit unusual transition
behavior [21,22]. In this connection, symmetric liquid crystal trimers such
as TCBOn, which are derivatives of BCBOn and contain three biphenyl
groups as the mesogenic units, have been prepared [1–3,6,14]. We report
the synthesis and characterization of new nonsymmetric liquid crystal
trimers (1) (n ¼ mþ 1, m ¼ 3–11) containing three biphenyl groups as
the mesogenic units (Scheme 1). The nonsymmetry of 1 was achieved by
the introduction of one ester group into TCBOn. The thermodynamic data
of 1 were compared with those of TCBOn, for which data are available [1].
EXPERIMENTAL
Elemental analyses were performed in the Analytical Center of Kyoto
University. The 1H NMR spectra (400 MHz) were obtained with a Jeol
GSX 400 spectrometer. The chemical shifts (d values) were measured
in parts per million (ppm) downfield from tetramethylsilane as inter-
nal reference. The IR spectra were recorded with a Jasco FT = IR-420
spectrometer. The measurements in CDCI3 were made at room
temperature with a 0.1-mm KBr cell. Differential scanning calorimetry
(DSC) measurements were carried out with a Shimadzu DSC-60.
Microscopy observations were performed under a Nikon Eclipse
E600 POL equipped with a hot stage (Linkam LK-600PH).

Liquid Crystal Trimers
19
TABLE 1 Yields and IR Spectral Data of x-(4-Cyanobiphenyl-40-yloxy)alkyl
40-Hydroxy-4-biophenylcarboxylate (3)
n
Yield (%)
cmꢀ1 (CDCl3)
3
4
5
6
7
8
9
10
11
53
55
57
55
50
55
52
50
48
3610, 2933, 2858, 2227, 1714, 1606, 1495, 1279, 1180
3602, 2935, 2856, 2227, 1712, 1605, 1495, 1279, 1180
3604, 2929, 2856, 2227, 1713, 1605, 1495, 1281, 1180
3604, 2936, 2860, 2227, 1713, 1605, 1495, 1279, 1180
3591, 2935, 2858, 2227, 1714, 1606, 1497, 1279, 1180
3593, 2933, 2858, 2227, 1714, 1606, 1497, 1280, 1180
3593, 2933, 2858, 2227, 1713, 1606, 1497, 1281, 1180
3596, 2931, 2856, 2227, 1711, 1606, 1495, 1281, 1180
3593, 2931, 2856, 2227, 1711, 1606, 1497, 1281, 1180
x-(4-Cyanobiphenyl-40-yloxy)alkyl 40-Hydroxy-
4-biophenylcarboxylate (3)
A mixture of a-bromo-x-(4-cyanobiphenyl-40yloxy)alkane (2) [23,24]
(5.0 mmol) and 40-hydroxy-4-biphenylcarboxylic acid (6.0 mmol) in
N,N-dimethylformamide (200 ml) in the presence of K2CO3 (5.0 mmol)
was stirred for 24 h at room temperature. The reaction mixture was
evaporated to give a residue, which was chromatographed over silica
gel. Elution of chloroform resulted in the recovery of 2. Further elution
of a mixture of chloroform and methanol (9:1) gave 3. The yields of 3
(n ¼ 3–12) are shown in Table 1.
x-(4-Cyanobiphenyl-40-yloxy)alkyl 40-[x-(4-Cyanobiphenyl-
40-yloxy)alkoxy]-4-biphenylcarboxylate (1)
A mixture of 2 (1.0 mmol) and 3 (1.0 mmol) in N,N-dimethylformamide
(40 ml) in the presence of K2CO3 (1.0 mmol) was stirred for 24 h at
room temperature. The reaction mixture was evaporated to give a
residue, which was chromatographed over silica gel. Elution of chloro-
form gave the liquid crystal trimers (1), and small amounts of 2 and 3
were also recovered. The yields of 1 (n ¼ m þ 1, m ¼ 3–11) are shown
in Table 2.
RESULTS AND DISCUSSION
Liquid crystal trimers contain two flexible spacers. To study
the dependence of the flexible spacers of liquid crystal trimers on
liquid-crystalline properties, a series of compounds (1) (n ¼ m þ 1) in
which the number of atoms of the two flexible spacers is equal was prepared.

20
T. Itahara and H. Tamura
TABLE 2 Elemental Analyses and Yields of x-(4-cyanobiphenyl-40-yloxy)alkyl
40-[x-(4-Cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxylate (1)
Yield
m
n
(%)
Calcd.
Found
3
4
64
62
63
57
60
59
C46H38N2O5: C, 79.06; H, 5.48; N, 4.01% C, 78.83; H, 5.33; N, 3.39%
C48H42N2O5: C, 79.32; H, 5.82; N, 3.85% C, 79.07; H, 5.83; N, 3.85%
4
5
6
7
8
5
6
7
8
9
C
50H46N2O5: C, 79.55; H, 6.14; N, 3.71% C, 79.39; H, 6.08; N, 3.65%
C52H50N2O5: C, 79.77; H, 6.44; N, 3.58% C, 80.02; H, 6.44; N, 3.58%
C
C
54H54N2O5: C, 79.77; H, 6.71; N, 3.45% C, 79.99; H, 6.67; N, 3.48%
56H58N2O5 1=2 H2O:
C, 79.44; H, 6.91; N, 3.31%
C, 79.31; H, 7.01; N, 3.30%
9
10
59
56
C58H62N2O5: C, 80.34; H, 7.21; N, 3.23% C, 80.44; H, 7.20; N, 3.19%
10 11
C60H66N2O5ꢁ1=2H2O:
C, 79.65; H, 7.44; N, 3.07%
C, 79.70; H, 7.47; N, 3.10%
11 12
57
C
62H70N2O5ꢁ1=2H2O:
C, 79.98; H, 7.66; N, 3.03%
C, 79.88; H, 7.68; N, 3.00%
The new liquid crystal trimers were synthesized according to the pro-
cedure shown in Scheme 2. The reaction of 4-cyano-40-hydroxybiphe-
nyl with a,x-dibromoalkanes gave a-bromo-x-(4-cyanobiphenyl-40-
yloxy)alkanes (2) [23,24]. The treatment of 2 with 40-hydroxy-4-biphe-
nylcarboxylic acid in N,N-dimethylformamide in the presence of
SCHEME 2 Synthesis of nonsymmetric liquid crystal trimers (1).

21

22
T. Itahara and H. Tamura
K2CO3 gave compounds 3. The IR spectral data of 3 showed the pres-
ence of a phenolic OH group at ca. 3600 cmꢀ1 and an ester group at
1
1711–1715 cmꢀ1 (Table 1). The H NMR spectral data of 3 are sum-
marized in Table 3. The absorptions of 3 at d 4.84–5.03 (s, 1H, D2O
exchangeable) were assigned to the phenolic OH. These IR and
1H NMR spectral data suggest that the structures of 3 were x-(4-cya-
nobiphenyl-40-yloxy)alkyl 40-hydroxy-4-biophenylcarboxylates, not 40-
[x-(4-cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxylic acids. The
reaction of 3 with 2 in N,N-dimethylformamide in the presence of
K2CO3 gave a new liquid crystal, trimer (1). The yields and the
elemental analyses of 1 (n ¼ m þ 1, m ¼ 3–11) are shown in Table 2.
The structures of 1 were further confirmed on the basis of IR spectral
1
data (Table 4) and H NMR spectral data (Table 5).
The thermal mesomorphic phases of 1 (n ¼ m þ 1, n ¼ 3–11) were
analyzed by means of differential scanning calorimetry (DSC) and
polarizing microscopy. All the compounds exhibited enantiotropic
nematic behaviors. The nematic phases had a Schlieren optical
texture. The transition properties of 1 are listed in Table 6.
The thermodynamic data of 1 were compared with those of TCBOn [1].
Figure 1 shows the dependence of the transition temperatures on
the number (k) of atoms of the flexible spacers for 1 and TCBOn. In
Table 1, k is n þ 2 and m þ 3 for 1 and k is n þ 2 for TCBOn. The melt-
ing points and nematic–isotropic transition temperatures of both 1
and TCBOn exhibited an odd–even effect as the number (k) of atoms
of the flexible spacers was varied, in which the even numbers exhib-
ited higher values. The odd–even effects of TCBOn are explained on
the basis of the all-trans conformation [1]. The odd–even effects of 1
and TCBOn were similar to each other, but the nematic–isotropic
TABLE 4 IR Spectral Data of x-(4-cyanobiphenyl-40-yloxy)alkyl 40-
[x-(4-Cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxylate (1)
m
n
cmꢀ1 (CDCl3)
3
4
5
6
7
8
9
10
11
4
5
6
7
8
2940, 2875, 2228, 1714, 1605, 1495, 1396, 1279, 1248, 1180, 1115
2940, 2873, 2227, 1714, 1606, 1495, 1396, 1277, 1248, 1180, 1115
2941, 2875, 2227, 1714, 1605, 1495, 1396, 1279, 1248, 1180, 1115
2941, 2862, 2227, 1711, 1606, 1495, 1396, 1279, 1248, 1180, 1115
2937, 2860, 2227, 1713, 1606, 1495, 1394, 1279, 1248, 1180, 1115
2935, 2858, 2227, 1714, 1606, 1495, 1394, 1280, 1248, 1180, 1115
2933, 2858, 2227, 1711, 1605, 1495, 1394, 1279, 1248, 1180, 1115
2931, 2856, 2227, 1711, 1606, 1495, 1392, 1281, 1250, 1180, 1115
2929, 2856, 2227, 1712, 1606, 1495, 1396, 1281, 1250, 1180, 1115
9
10
11
12

23

24
T. Itahara and H. Tamura
TABLE 6 Transition Properties of x-(4-Cyanobiplienyl-40-yloxy)alkyl-
40-[x-(4-Cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxylate (1)
m
n
TCN (ꢂC)
TNI (ꢂC)
DSCN=R
DSNI=R
3
4
5
6
7
8
9
10
11
4
5
6
7
8
210
168
211
163
202
149
179
144
160
279
178
248
177
214
171
195
164
176
13.6
14.5
14.0
16.0
12.5
16.8
17.5
19.0
19.2
2.48
0.35
2.79
0.43
2.84
0.81
2.85
1.07
3.32
9
10
11
12
CN: melting point; NI: nematic–isotropic transition.
FIGURE 1 Dependence of the transition temperatures on number (k) of
atoms of flexible spacers for compounds (1) and TCBOn. k ¼ m þ 3 ¼ n þ 2
for compounds (1) and k ¼ n þ 2 for TCBOn [1]. .: melting points of TCBOn.
&: nematic–isotropic=transition temperatures of compounds (1). &:
nematic–isotropic transition temperatures of TCBOn.

Liquid Crystal Trimers
25
transition temperatures of 1 clearly exhibited lower values in compari-
son with those of TCBOn.
Figure 2 shows the dependence of the entropy change (DSCN=R)
associated with the melting transition on the number (k) of atoms of
the flexible spacers for 1 and TCBOn. In spite of the observation of
the odd–even effects of the melting points (Fig. 1), Fig. 2 did not show
similar odd–even effects. However, DSCN=R of 1 roughly exhibited
lower values in comparison with those of TCBOn. Figure 3 shows
the dependence of the entropy change (DSNI=R) associated with
the nematic–isotropic transition on the number (k) of atoms of the
flexible spacers for 1 and TCBOn. Figure 3 unquestionably shows
the odd–even effects of DSNI=R for 1 and TCBOn. The odd–even effects
of 1 closely resembled those of TCBOn. On the other hand, it can be
seen from Figs. 1 and 3 that the nematic–isotropic transition tempera-
tures and the values of DSNI=R of 1 were clearly lower than those
of TCBOn. It is thought that the differences between 1 and
TCBOn result from polarizability and shape of the liquid-crystalline
molecules.
FIGURE 2 Dependence of entropy change associated with melting point on
number (k) of atoms of flexible spacers for compounds (1) and TCBOn.
k ¼ m þ 3 ¼ n þ 2 for compounds (1) and k ¼ n þ 2 for TCBOn [1]. .:
Compounds (1). ꢂ: TCBOn.

26
T. Itahara and H. Tamura
FIGURE 3 Dependence of entropy change associated with nematic–isotropic
transition on number (k) of atoms of flexible spacers for compounds (1) and
TCBOn. k ¼ m þ 3 ¼ n þ 2 for compounds (1) and k ¼ n þ 2 for TCBOn [1].
&: compounds (1). &: TCBOn.
CONCLUSION
A series of nonsymmetric liquid crystal trimers, x-(4-cyanobiphenyl-40
yloxy)alkyl 40-[x-(4-cyanobiphenyl-40-yloxy)alkoxy]-4-biphenylcarboxy-
lates, were prepared from 4-cyano-40-hydroxybiphenyl, 40-hydroxy-4-
biphenylcarboxylic acid, and a,x-dibromoalkanes. Compounds (1)
(n ¼ m þ 1) exhibited enantiotropic nematic behavior. The transition
properties of 1 were compared with those of symmetric liquid crystal tri-
mers, such as TCBOn. The melting points, the nematic–isotropic tran-
sition temperatures, and the entropy change associated with the
nematic–isotropic transition (DSNI = R) of both 1 and TCBOn exhibited
an odd–even effect as the number (k) of atoms of the flexible spacers
was varied, in which the even numbers exhibited higher values, although
the entropy change associated with the melting transition (DSCN = R) did
not exhibit an odd–even effect. The odd–even effects of 1 and TCBOn
were similar to each other, but the nematic–isotropic transition tempera-
tures and DSNI=R of 1 clearly exhibited lower values in comparison with
those of TCBOn. Therefore, it may be concluded from the results of this
investigation that the introduction of nonsymmetry into the symmetric

Liquid Crystal Trimers
27
liquid crystal trimer will result in a lowering of the nematic–isotropic
transition temperatures and the associated entropy changes.
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