Odd-even effect in liquid crystal tetramers

Article abstract of DOI:10.1080/15421400802697665

A new series of liquid crystal tetramers, which contained four biphenyl groups joined by three flexible spacers, was prepared. The tetramers showed nematic and/or smectic phases. The melting points, the nematic-isotropic transition temperatures, and the e

Full text of DOI:10.1080/15421400802697665

Mol. Cryst. Liq. Cryst., Vol. 501, pp. 94–103, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802697665
Odd–Even Effect in Liquid Crystal Tetramers
Toshio Itahara and Hisashi Tamura
Faculty of Engineering, Kagoshima University, Korimoto,
Kagoshima, Japan
A new series of liquid crystal tetramers, which contained four biphenyl groups
joined by three flexible spacers, was prepared. The tetramers showed nematic
and=or smectic phases. The melting points, the nematic-isotropic transition
temperatures, and the entropy changes associated with the nematic–isotropic
transition showed a remarkable odd–even effect as the length of the spacers was
varied, in which the even members exhibited higher values. The transition proper-
ties of the liquid crystal tetramers were compared with those of liquid crystal
dimers and trimers, which consist of two and three biphenyl groups.
Keywords: biphenyl; flexible spacer; liquid crystal tetramer; odd-even effect; transition
property
INTRODUCTION
Emsley et al. [1] reported the unusual transition properties of liquid
crystal dimers containing two 4-cyanobiphenyl groups, BCBOn.
Furthermore, the transition properties of many liquid crystal dimers
and trimers, which contain two or three mesogenic units, have been stu-
died by many groups of workers [2–5]. In addition, some kinds of liquid
crystal tetramers, which are defined as molecules containing four meso-
genic units joined by three flexible spacers, have been prepared [6–12].
These compounds exhibit a remarkable odd–even effect in their transi-
tion properties, depending on the length and parity of the flexible
spacers [1–12]. Liquid crystal dimers, trimers, and tetramers are also
of interest as models for main chain liquid crystal polymers [13].
Address correspondence to Toshio Itahara, Faculty of Engineering, Kagoshima
University, Korimoto, Kagoshima 890-0065, Japan. E-mail: itahara@be.kagoshima-u.
ac.jp
94

Odd–Even Effect in Liquid Crystal Tetramers
95
FIGURE 1 Structures of tetramer (1), trimer (2), and BCBOn.
Previously, we reported on the synthesis of liquid crystal trimers (2)
containing three biphenyl groups [14]. In this article, we report the
synthesis and characterization of new liquid crystal tetramers (1) con-
taining four biphenyl groups as the mesogenic units. The transition
properties of 1 were compared with those of related liquid crystal
dimers and trimers containing two and three biphenyl groups, such
as BCBOn and trimers (2) (Fig. 1).
EXPERIMENTAL
Elemental analyses were performed at the Analytical Center of Kyoto
1
University. H-NMR spectra (400 MHz) were obtained with a JEOL
GSX 400 spectrometer. The chemical shifts (d-values) were measured
in parts per million (ppm) down-field from tetramethylsilane as an
internal reference. IR spectra were recorded with a JASCO FT=IR-
420 spectrometer. Measurements in CDCl3 were made with a 0.1-mm
KBr cell. Differential scanning calorimetry (DSC) measurements
were carried out with a Shimadzu DSC-60. Polarizing microscopy
observations were performed under a Nikon Eclipse E600 POL
equipped with a hot stage (Linkam LK-600PH).
a,x-Bis[4-[x-(4-cyanobiphenyl-4’-
yloxy)alkyloxycarbonyl]biphenyl-4’-yloxy]alkane (1)
The preparation of x-(4-cyanobiphenyl-40-yloxy)alkyl 40-hydroxy-4-
biophenylcarboxylate (3) has been reported [14]. A mixture of 3 (1.0
mmol) and a,x-dibromoalkane (0.7 mmol) in N,N-dimethylformamide
(40 ml) in the presence of K2CO3 (2.0 mmol) was stirred for 24 h at
room temperature. The reaction mixture was evaporated to give a
residue, which was chromatographed over silica gel. An elution with

96
T. Itahara and H. Tamura
TABLE 1 Elemental Analyses and Yields of Tetramers (1)
a
n
%
Calculated
Found
4
5
6
7
8
9
10
11
60
58
58
55
55
55
52
50
C65H58N2O8: C, 78.45, H, 5.87; N, 2.81%
C68H64N2O8: C, 78.74, H, 6.22; N, 2.70%
C, 78.41, H, 5.88; N, 2.76%
C, 78.72, H, 6.12; N, 2.70%
C, 78.78, H, 6.48; N, 2.79%
C, 79.03, H, 6.83; N, 2.49%
C, 79.53, H, 7.12; N, 2.37%
C, 79.82, H, 7.20; N, 2.82%
C, 79.93, H, 7.59; N, 2.47%
C, 79.60, H, 7.77; N, 2.13%
C
C
71H70N2O8: C, 79.01, H, 6.54; N, 2.60%
74H76N2O8: C, 79.26, H, 6.83; N, 2.50%
C77H82N2O8: C, 79.49, H, 7.10; N, 2.41%
C80H88N2O8: C, 79.70, H, 7.36; N, 2.32%
C
C
83H94N2O8: C, 79.90, H, 7.59; N, 2.25%
86H100N2O8: C, 80.09, H, 7.81; N, 2.17%
aYield based on the amount of 3 used.
chloroform or a mixture of chloroform and methanol (9:1) gave liquid
crystal tetramers (1). The yields, based on the amount of 3 used, are
shown in Table 1.
RESULTS AND DISCUSSION
Liquid crystal tetramers (1) were prepared according to the procedure
shown in Fig. 2. All the structures of 1 were confirmed on the basis of
elemental analyses (Table 1), the IR spectral data (Table 2), and
1H-NMR spectral data (Table 3). The IR spectral data of 1 showed
the presence of a cyano group at 2227 cmꢀ1 and an ester group at
1
ca. 1711 cmꢀ1. The H-NMR spectral data of 1 were similar to those
of liquid crystal trimers (2) [14], but the integration ratio of the
absorptions at d 4.34–4.44 (CH2COO) to d 4.00–4.11 (CH2O) was 1:2.
The thermal mesomorphic phases of tetramers (1) were investi-
gated by differential scanning calorimetry (DSC) and polarizing micro-
scopy. Each of the tetramers, except 1 (n ¼ 4), exhibited enantiotropic
FIGURE 2 Synthesis of liquid crystal tetramers (1).

Odd–Even Effect in Liquid Crystal Tetramers
97
TABLE 2 IR Spectral Data of Tetramers (1)
cmꢀ1=CDCl3
n
4
5
6
7
8
9
10
11
2951, 2873, 2227, 1712, 1606, 1525, 1495, 1471, 1277, 1248, 1182, 1115
2943, 2862, 2227, 1712, 1606, 1523, 1495, 1471, 1279, 1248, 1182, 1115
2943, 2862, 2227, 1711, 1605, 1525, 1495, 1470, 1279, 1248, 1182, 1115
2937, 2860, 2227, 1711, 1606, 1523, 1495, 1471, 1279, 1248, 1180, 1115
2937, 2858, 2227, 1711, 1606, 1523, 1495, 1471, 1281, 1248, 1182, 1115
2933, 2858, 2227, 1710, 1606, 1523, 1495, 1471, 1279, 1248, 1182, 1115
2931, 2856, 2227, 1710, 1606, 1523, 1495, 1471, 1279, 1248, 1182, 1115
2931, 2856, 2227, 1712, 1605, 1525, 1495, 1470, 1279, 1248, 1182, 1115
TABLE 3 1H-NMR Spectral Data of Tetramers (1)
n
d=CDCl3
4
8.05 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 7.00 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.44 (broad, 4H), 4.11 (broad, 4H), 4.06 (t, 4H,
J ¼ 7 Hz), 2.01 (broad, 8H), 1.92 (quintet, 4H, J ¼ 7 Hz), 1.72 (quintet, 2H,
J ¼ 7 Hz).
5
6
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.39 (t, 4H, J ¼ 6 Hz), 4.05 (t, 4H, J ¼ 6 Hz), 4.04 (t, 4H,
J ¼ 6 Hz), 1.8–1.9 (m, 12H), 1.6–1.8 (m, 8H).
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.36 (t, 4H, J ¼ 6 Hz), 4.03 (t, 4H, J ¼ 6 Hz), 4.02 (t, 4H,
J ¼ 6 Hz), 1.8–1.9 (m, 12H), 1.4–1.6 (m, 14H).
7
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.35 (t, 4H, J ¼ 6 Hz), 4.01 (t, 8H, J ¼ 6 Hz), 1.75–1.85
(m, 12H), 1.4–1.6 (m, 20H).
8
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.34 (t, 4H, J ¼ 6 Hz), 4.00 (t, 8H, J ¼ 6 Hz), 1.75–1.85
(m, 12H), 1.35–1.6 (m, 26H).
9
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.33 (t, 4H, J ¼ 6 Hz), 4.00 (t, 8H, J ¼ 6 Hz), 1.75–1.85
(m, 12H), 1.35–1.6 (m, 32H).
10
11
8.07 (d, 4H, J ¼ 8 Hz), 7.68 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.59 (d, 4H,
J ¼ 8 Hz), 7.53 (d, 4H, J ¼ 8 Hz), 7.52 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.33 (t, 4H, J ¼ 6 Hz), 4.00 (t, 8H, J ¼ 6 Hz), 1.75–1.85
(m, 12H), 1.3–1.6 (m, 38H).
8.07 (d, 4H, J ¼ 8 Hz), 7.69 (d, 4H, J ¼ 8 Hz), 7.63 (d, 4H, J ¼ 8 Hz), 7.61 (d, 4H,
J ¼ 8 Hz), 7.55 (d, 4H, J ¼ 8 Hz), 7.51 (d, 4H, J ¼ 8 Hz), 6.99 (d, 4H, J ¼ 8 Hz),
6.98 (d, 4H, J ¼ 8 Hz), 4.33 (t, 4H, J ¼ 6 Hz), 4.00 (t, 8H, J ¼ 6 Hz), 1.75–1.85
(m, 12H), 1.3–1.6 (m, 44H).

98
T. Itahara and H. Tamura
liquid crystalline behavior. The tetramers exhibited nematic and=or
smectic liquid crystalline properties. Tetramers (1) (n ¼ 4 and 5)
showed only the nematic phase (Schlieren texture) and 1 (n ¼ 11)
displayed only the smectic A phase (focal-conic texture). Tetramers
(1) (n ¼ 6–10) manifested both the smectic and nematic phases.
Figure 3 shows the polarizing microscopic images of 1 (n ¼ 8, 9, and
10). Figures 3A, B shows the optical textures of the smectic phase at
146ꢁC and the nematic phase at 165ꢁC upon the second heating for
1 (n ¼ 8), respectively. The smectic phase had a focal-conic texture,
which was characteristic of smectic A. Tetramer (1) (n ¼ 9) also
FIGURE 3 Representative polarizing microscopic images of tetramers (1),
obtained upon the second heating. (A): Tetramer (1) (n ¼ 8), SmA phase at
146ꢁC; (B): Tetramer (1) (n ¼ 8), N phase at 165ꢁC; (C): Tetramer (1) (n ¼ 9),
SmC phase at 170ꢁC; (D): Tetramer (1) (n ¼ 9), N phase at 180ꢁC; (E):
Tetramer (1) (n ¼ 10), SmA phase at 140ꢁC; (F): Tetramer (1) (n ¼ 10), N phase
at 155ꢁC.

Odd–Even Effect in Liquid Crystal Tetramers
99
manifested both the smectic and nematic phases, but the smectic
phase was smectic C (Schlieren texture), not smectic A. Figures 3C,
D shows the Schlieren textures of the tilted smectic phase at 170ꢁC
and the nematic phase at 180ꢁC upon the second heating, respectively.
Figures 3E, F shows the optical textures of the smectic A phase (focal-
conic texture) at 140ꢁC and the nematic phase at 155ꢁC upon the sec-
ond heating for 1 (n ¼ 10), respectively. The transition properties of
tetramers (1) are summarized in Table 4.
Tetramers (1) contain two kinds of flexible spacers, such as the
O(CH2)nþ1O and COO(CH2)nO groups, but the number (k) of atoms
in the two flexible spacers is equivalent, i.e., k ¼ n þ 3. Figure 4 shows
the occurrence of the liquid crystal phases of tetramers (1). As we
varied the number (k) of atoms in the flexible spacers, the nematic–
isotropic transition temperature exhibited an odd–even effect, in
which the even numbers exhibited higher values. Figure 4 also indi-
cates an odd–even behavior of the melting point and smectic-nematic
transition temperature.
The transition properties of tetramers (1) were compared with
those of trimers (2) and dimers, BCBOn. Figure 5 indicates the
presence of an odd–even effect of the melting point and the nematic–
isotropic transition temperature on the number (k) of atoms in the
flexible spacers for 1, 2, and BCBOn, but the transition temperature
of 1 (k ¼ 11ꢀ14) seems to deviate somewhat from the expected values.
This deviation may come from the formation of the smectic phase of 1,
because 2 and BCBOn exhibit only the nematic phase but no smectic
TABLE 4 Transition Properties of Tetramers (1)
n
T
M=ꢁC
T
SN=ꢁC
T
SI=ꢁC
T
NI=ꢁC
DSM=R
DSSN=R
DSSI=R
DSNI=R
4
5
6
7
8
9
10
11
193
242
160
214
143
151
138
178
(174)
251
182
224
175
188
167
14.2
13.7
13.5
12.8
12.3
14.2
17.2
16.9
(0.44)
3.57
0.57
3.65
0.95
3.79
1.72
(153)
(194)
160
173a
150
(1.60)
(0.40)
1.78
0.82a
1.02
190
6.37
TM: melting point.
TSN: smectic A-nematic transition temperature except for n ¼ 9.
SI: smectic A-isotropic transition temperature.
T
TNI: nematic–isotropic transition temperature.
( ): upon cooling.
aTSN: smectic C-nematic transition temperature.

100
T. Itahara and H. Tamura
FIGURE 4 Dependence of liquid crystal phase on the number (k) of atoms
in the flexible spacers for tetramers (1); k ¼ n þ 3; C: crystal; N: nematic;
S: smectic; I: isotropic liquid.
FIGURE 5 Comparison of transition temperatures of tetramer (1), trimer (2),
and BCBOn; (A) Melting point; (B) Nematic–isotropic or smectic–isotropic
transition temperature; k ¼ n þ 3 for tetramer (1) and trimer (2); k ¼ n þ 2
for BCBOn; &: Tetramer (1); ~: Trimer (2); : BCBOn.
.

Odd–Even Effect in Liquid Crystal Tetramers
101
phase. Furthermore, Fig. 5B shows that the nematic–isotropic
transition temperature is significantly dependent on the number (k)
of atoms in the flexible spacers for even-membered tetramers, trimers,
and dimers, compared with odd-membered tetramers, trimers, and
dimers.
Although the entropy changes associated with the melting points of
tetramers (1), trimers (2), and BCBOn did not result in an odd–even
effect on the number of atoms in the flexible spacers (Fig. 6A), these
compounds evinced an odd–even effect with the entropy changes
associated with the nematic–isotropic transition (Fig. 6B). The entropy
changes associated with the nematic–isotropic transition were depen-
dent on the number (k) of atoms in the flexible spacers for odd-
membered tetramers, trimers, and dimers. The values of the entropy
change of the odd-membered tetramers resembled those of the corre-
sponding odd-membered trimers and dimers. On the other hand, as
for the even-membered tetramers, trimers, and dimers, the values of
the entropy changes were only slightly affected by the number (k) of
atoms in the flexible spacers. It can be seen from Fig. 6B that the
average value of the even-membered tetramers (k ¼ 8, 10, and 12) is
FIGURE 6 Comparison of entropy change associated with phase transition of
tetramer (1), trimer (2), and BCBOn; (A) Melting point; (B) Nematic–isotropic
or smectic–isotropic transition; k ¼ n þ 3 for tetramer 1 and 2; k ¼ n þ 2 for
BCBOn; &: Tetramer (1); ~: Trimer (2); : BCBOn.
.

102
T. Itahara and H. Tamura
ca. 3.7, the average value of the even-membered trimers (k ¼ 8, 10, and
12) is ca. 2.85, and the average value of even-membered dimers (k ¼ 8,
10, and 12) is ca. 2.0. Imrie et al. [15–17] have already reported similar
thermodynamic behavior of liquid crystal dimers and trimers,
which was explained on the basis of the molecular shapes of the all-
trans-conformation of the compounds containing odd- and even-
membered flexible spacers. The present results substantiate the
regular array seen in the thermodynamic behavior of dimers, trimers,
and tetramers, which consist of two, three, and four biphenyl groups
as the mesogenic units.
CONCLUSION
A new series of liquid crystal tetramers, which contained four biphenyl
groups joined by three flexible spacers, was prepared. The tetramers
exhibited nematic and=or smectic phases. The melting points, the
nematic–isotropic transition temperatures, and the entropy changes
associated with the nematic–isotropic transition showed an odd–even
effect as the number of atoms in the spacers was varied, in which the
even members exhibited higher values. The transition properties of
the tetramers were compared with those of related liquid crystal
dimers and trimers containing two and three biphenyl groups. The
entropy changes associated with the nematic–isotropic transition were
only slightly affected by the number of atoms in the flexible spacers for
even-membered tetramers, trimers, and dimers. The values of the
entropy changes of the even-membered tetramers, the even-membered
trimers, and the even-membered dimers regularly decreased, in that
order.
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