Synthesis and physico-chemical properties of polar liquid crystal materials incorporating a coumarin skeleton at the terminal position

Article abstract of DOI:10.1080/15421400802430000

This paper described the synthesis and physico-chemical properties of homologous series of 2-oxo-2H-chromen-6-yl 6-alkoxy-naphthalene-2-carboxylate (compounds 1-n). Compounds 1-n show a monotropic nematic (N) phase, where the average of the N-isotropic (I) phase transition temperatures is ca. 140C. The mesomorphic properties were also examined by binary phase diagrams for the mixture of 1-n (n=4 or 8) and 4-(cyano-4'-octyloxybiphenyl (8 OCB), or 4-heptyloxyphenyl 4-nonyloxybenzoate. The physico-chemical properties for compounds 1-n are compared with the corresponding 4-alkoxybenzoate derivatives (compounds 2-n) and 4-alkoxybiphenyl-4'-carboxylate ones (compounds 3-n). These results are discussed in terms of molecular structures and the electrostatic natures of the molecules.

Full text of DOI:10.1080/15421400802430000

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Synthesis and Physico-Chemical
Properties of Polar Liquid
Crystal Materials Incorporating
a Coumarin Skeleton at the
Terminal Position
Yuki Morita a , Hiroyuki Ushijima a , Kyohei Era a ,
Kazuo Kasatani a & Hiroaki Okamoto a
a Faculty of Engineering , Yamaguchi University,
Ube , Yamaguchi, Japan
Published online: 05 Apr 2011.
To cite this article: Yuki Morita , Hiroyuki Ushijima , Kyohei Era , Kazuo
Kasatani & Hiroaki Okamoto (2008) Synthesis and Physico-Chemical Properties
of Polar Liquid Crystal Materials Incorporating a Coumarin Skeleton at the
Terminal Position, Molecular Crystals and Liquid Crystals, 494:1, 282-292, DOI:
10.1080/15421400802430000
To link to this article: http://dx.doi.org/10.1080/15421400802430000
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Mol. Cryst. Liq. Cryst., Vol. 494, pp. 282–292, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802430000
Synthesis and Physico-Chemical Properties of Polar
Liquid Crystal Materials Incorporating a Coumarin
Skeleton at the Terminal Position
Yuki Morita, Hiroyuki Ushijima, Kyohei Era, Kazuo
Kasatani, and Hiroaki Okamoto
Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi,
Japan
This paper described the synthesis and physico-chemical properties of homologous
series of 2-oxo-2H-chromen-6-yl 6-alkoxy-naphthalene-2-carboxylate (compounds
1-n). Compounds 1-n show a monotropic nematic (N) phase, where the average of
the N-isotropic (I) phase transition temperatures is ca. 140ꢀC. The mesomorphic
properties were also examined by binary phase diagrams for the mixture of 1-n
(n ¼ 4 or 8) and 4-(cyano-40-octyloxybiphenyl (8 OCB), or 4-heptyloxyphenyl
4-nonyloxybenzoate. The physico-chemical properties for compounds 1-n are com-
pared with the corresponding 4-alkoxybenzoate derivatives (compounds 2-n) and
4-alkoxybiphenyl-40-carboxylate ones (compounds 3-n). These results are discussed
in terms of molecular structures and the electrostatic natures of the molecules.
Keywords: binary phase diagram; molecular structure; nematic phase; polar liquid
crystal; synthesis
INTRODUCTION
It has been known that the mesomorphic and physical properties of
liquid crystal materials strongly depend on their molecular structures
[1]. It is also well known that the terminal polar groups such as cyano
or nitro group, modifying a dielectric anisotropy, have an inevitable
role for liquid crystal materials for twisted nematic (TN) mode display
devices [2].
This work is supported by Industrial Technology Research Grant Program in 2006
from New Energy and Industrial Technology Development Organization (NEDO) of
Japan and Grant-in-Aids for Young Scientists (B) from the Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan.
Address correspondence to Hiroaki Okamoto, Department of Advanced Materials
Science and Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai
2557, Ube, Yamaguchi, 755-8611, Japan. E-mail: oka-moto@po.cc.yamaguchi-u.ac.jp
282

Nematic LC Materials having a Coumarin Skeleton
283
In our earlier papers [3–7], liquid crystal materials having a
terminal carbonyl group, such as cyclohexanone, 1,3-dioxa-2-one,
benzofuran-2-one, and so on, exhibit a smectic A (SmA) phase with a
polar nature. However, these liquid crystal materials are difficult to
exhibit a thermally stable nematic (N) phase and are not suitable for
applying the TN mode display devices. On the other hand, liquid crys-
tal materials having a 2H-pyran-2-one or 2-oxo-2H-chromen-6-yl
group at the terminal position show a nematic phase [8].
This paper describes that homologous series of 2-oxo-2H-chromen-6-
yl 6-alkoxy-naphthalene-2-carboxylate (compounds 1-n) were synthe-
sized and their physico-chemical properties were compared with the
corresponding 4-alkoxybenzoate and 4-alkoxybiphenyl-40-carboxylate
derivatives (compounds 2-n and 3-n).
Interestingly, compounds 2-n and 3-n form organo-gels in various
organic solvents [6,9], while compounds 1-n show no gelation ability,
so that mesomorphic properties for compounds 1-n are reported in
this paper (Figure 1).
EXPERIMENTAL
Materials
Compounds 1-n were synthesized in a moderate yields according to
Scheme 1.
1
The chemical structures were confirmed by H NMR and IR spec-
troscopy, and the purity (99%) was checked by HPLC. The spectra
data for compounds 1-n are shown in appendix.
Method
The transition temperatures and latent heats were determined using a
Seiko SSC-5200 DSC, where indium (99.9%) was used as a calibration
SCHEME 1 Synthetic scheme for compounds 1-n. Reagents and conditions: i)
H2SO4, EtOH, reflux for 4 h, 85%; ii) CnH2nþ1Br, K2CO3, 3-pentanone, reflux
for overnight, 52–79%; iii) 6 M NaOH, DMSO, 3 h, then poured into 1 M HCl,
71–93%; iv) SOCl2, 70ꢀC for 1 h; v) Pyridine, toluene, 80ꢀC, overnight, 60–70%.

284
Y. Morita et al.
FIGURE 1 Chemical structures for compounds 1-n–3-n
standard (mp ¼ 156.6ꢀC, 28.4 J=g). The DSC thermogram was operated
at a heating or cooling rate of 5ꢀC minꢁ1. The mesophases were charac-
terized using a Nikon POH polarizing microscope fitted with a Mettler
thermo-control system (FP-900). 1H NMR spectra were measured
using
a JEOL EX-270 spectrometer, where tetramethylsilane
was used as an internal standard. IR spectra were recorded with a
Shimadzu Prestige-21 infrared spectrometer. The purity of the
materials was checked by HPLC.
TABLE 1 Transition Temperatures (ꢀC) and Latent Heats (kJ molꢁ1) for
Compounds 1-n
Transition temperatures (ꢀC)
N
Latent heats (kJ molꢁ1
)
Compounds 1-n
n
C
I
mp
N-I
4
5
6
7
8
.
.
.
.
.
168
177
167
159
165
(.
(.
(.
(.
(.
152)
140)
139)
135)
137)
.
.
.
.
.
37.3
41.0
38.6
36.9
41.4
0.2
0.2
0.4
0.3
0.4
C, N, and I indicate crystal, nematic, and isotropic phases, respectively.
Parentheses indicate a monotropic transition.

Nematic LC Materials having a Coumarin Skeleton
285
FIGURE 2 Polarized micrograph for compound 1–5 at 140ꢀC on cooling
process.
FIGURE 3 Plots of transition temperatures (ꢀC) vs. the carbon number (n)
for compounds 1-n: Diamonds and circles show melting points and nematic–
isotropic liquid phase transitions, respectively. Dashed line denotes a mono-
tropic transition.

286
Y. Morita et al.
RESULTS AND DISCUSSION
Thermal properties for compounds 1-n determined by a differential
scanning calorimeter, are summarized in Table 1.
The thermal data for the melting phenomenon were obtained by a
first heating process of the crystals obtained by recrystallization from
ethanol. The monotropic N-isotropic (I) phase transition data were
obtained from a cooling process.
Compound 1–5, for example, shows an endotherm at 177ꢀC corre-
sponding to melting phenomenon on the first heating process. The
molten materials showed a small exotherm at 140ꢀC due to a phase
transition on the cooling process.
The formed texture is shown in Figure 2. The mesophase shows a
typical schlieren texture at 140ꢀC on cooling process under polarized
microscope observation. Therefore, the mesophase for compound 1–5
is assigned to be a monotropic N phase.
All the homologous series shows a monotropic N phase as shown
in Table 1, and the small latent heats for the N-I phase transitions
FIGURE 4 Binary phase diagram for the mixture of 4-cyano-40-octyloxybi-
phenyl (8OCB) (left) and compound 1-4 (right): Diamonds, triangles and cir-
cles show melting points, smectic A (SmA)–nematic (N) and N–isotropic
liquid (I) phase transitions, respectively. Dashed lines denote monotropic
transitions.

Nematic LC Materials having a Coumarin Skeleton
287
also agree with the assignment of the N phase. The transition
temperatures for compounds 1-n are plotted against the carbon
number (n) as shown in Figure 3.
Compounds 1-n exhibit an even-odd effect in the N-I transition tem-
peratures and the latent heats. Compounds 2-n and 3-n, on the other
hand, show the N phase without any even-odd effect, where the aver-
age of the N-I transition temperatures are ca. 80ꢀC and 240ꢀC, respect-
ively as described in our earlier papers [8].
Binary phase diagrams for the mixtures of compounds 1 -n (n ¼ 4 or
8) and polar liquid crystal material are examined in order to reveal the
mesomorphic property. Figures 4 and 5 show the binary phase dia-
grams for the mixture of 4-cyano-40-octyloxybiphenyl (8OCB), a typical
polar liquid crystal material, and compounds 1–4, and 1–8, respect-
ively. The N phases for compounds 1–4 and 1–8 are well miscible with
that of 8OCB all over the phase diagrams as shown in Figures 4 and 5.
The N-I transition temperatures increase with increasing the mol
concentration of compound 1–4, while the SmA-N transition tempera-
tures decrease considerably and the SmA phase disappears at 50ꢀC
around 10 mol% of compound 1–4 as shown in Figure 4.
FIGURE 5 Binary phase diagram for the mixture of 8OCB (left) and com-
pound 1–8 (right): Diamonds, triangles and circles show melting points,
SmA-N and N-I phase transitions, respectively. Dashed lines denote mono-
tropic transitions.

288
Y. Morita et al.
Figure 5 shows the binary phase diagram for the mixture of 8OCB
and compound 1–8. The thermal property of the N-I transitions is
similar to that in Figure 4, while the SmA-N transition temperatures
increase a little with increasing the mol concentration of compound
1–8. The SmA phase could not observed at ca. 70ꢀC around 60 mol%
of 1–8, because of recrystallization of the mixture. These results
indicate the thermal stability of the extrapolated SmA phase is
around 100ꢀC, which is extremely lower than that of compound 3–8
(TSmA-N ¼ 208ꢀC).
Figure 6 shows the binary phase diagram for the mixture of
4-heptyloxyphenyl 4-nonyloxybenzoate and compound 1–4.
The transition temperatures of the smectic phase considerably
increase with increasing the mol concentration of compound 1–4, in
spite of showing no smectic property. The SmA-I transition
temperatures show a notable upward convexity around 40 mol% of
compound 1–4, and then the SmA phase disappears around 70 mol%.
A similar tendency is also observed in the phase diagram for
the mixture of 4-heptyloxyphenyl 4-nonyloxybenzoate and compounds
FIGURE 6 Binary phase diagram for the mixture of 4-heptyloxyphenyl
4-nonyloxybenzoate (left) and compound 1–4 (right): Diamonds, triangle (filled)
triangles (open) and circles show melting points, SmC-N, SmA-N and N-I phase
transitions, respectively. Dashed lines denote monotropic transitions.

Nematic LC Materials having a Coumarin Skeleton
289
1–8 as shown in Figure 7, where the maximum SmA-N transition
temperature is 120ꢀC around 70 mol% of 1–8.
It is well known that a lot of liquid crystal mixtures consisting of
polar and non-polar liquid crystals show a notable enhancement
and=or an induction of the SmA phase. In these phenomena, dipole–
dipole and=or electron donor–acceptor interactions are considered to
play important roles [10]. In Figures 6 and 7, the polar and non-polar
components correspond to compounds 1-n and 4-heptyloxyphenyl
4-nonyloxybenzoate, respectively.
These results indicate that the terminal coumarin skeleton for
compounds 1-n gives a polarity in the liquid crystal core.
The molecular structures and dipole moments for compounds 1–8,
2–8 and 3–8 are calculated by a semi-empirical molecular orbital
calculation (MOPAC, AM1) [11], and the molecular dimensions and
dipole moments are shown in Figure 8.
The calculated dipole moments for compounds 1–8, 2–8 and 3–8
are similar (8 D), while the molecular lengths are different due to
the core structures. Interestingly, the molecular breadths are almost
the same and independent of the core structures. Therefore, the order
FIGURE 7 Binary phase diagram for the mixture of 4-heptyloxyphenyl
4-nonyloxybenzoate (left) and compound 1–8 (right): Diamonds, triangle (filled)
triangles (open) and circles show melting points, SmC-N, SmA-N and N-I phase
transitions, respectively. Dashed lines denote monotropic transitions.

290
Y. Morita et al.
FIGURE 8 Molecular structures, dimensions and dipole moments for
compounds 1–8, 2–8, and 3–8, calculated by a semi-empirical molecular orbital
calculation (MOPAC, AM1) [11].
of the length-to-breadth ration is as follows; 3-8 (5.07) > 1–8
(4.79) > 2–8 (4.36), and similar to the order of the N-I phase transition
temperatures.
It is well known that the length-to-breadth ratio also causes the
even-odd alternation on the N-I phase transition temperatures [12].
Only compounds 1-n show the even-odd alternation as described
before, indicating that the core structure also affects the even-odd
effect on the N-I phase transition temperatures.
CONCLUSION
We have prepared a homologous series of compounds 1-n, which show
the monotropic N phase with the even-odd effect on the N-I phase
transition temperatures and latent heats. The N-I transition tempera-
ture for compound 1–8 is ca. 140ꢀC. The terminal coumarin skeleton
gives a polarity in the liquid crystal core.

Nematic LC Materials having a Coumarin Skeleton
291
REFERENCES
[1] Gray, G. W. (1972). Advances in Liquid Crystals, Academic Press Inc.: New York
and London, Vol. 2.
[2] Pohl, L. & Finkenzeller, U. (1990). Physical properties of liquid crystals. In: Liquid
Crystals, Application and Uses, Bahadur B., (Ed.), World Scientific: Singapore, Ch.
4, Vol. 1, p. 140.
[3] Okamoto, H., Okamoto, T., & Takenaka, S. Chem. Lett., 2000, 1040.
[4] Okamoto, H., Okamoto, T., Petrov, V. F., & Takenaka, S. (2001). Mol. Cryst. Liq.
Cryst., 364, 719.
[5] Morita, Y., Tasaka, T., Habu, K., Okamoto, H., Kasatani, K., & Takenaka, S. (2004).
Trans. Mater. Res. Soc. Jpn., 29, 831.
[6] Zhang, F., Morita, Y., Kawabe, K., Tasaka, T., Okamoto, H., & Takenaka, S. (2005).
Chem. Lett., 34, 1156.
[7] Morita, Y., Zhang, F., Tasaka, T., Yamaguchi, R., Okamoto, H., & Takenaka, S.
(2006). Bull. Chem. Soc. Jpn., 79, 163.
[8] Morita, Y., Tasaka, T., Yamaguchi, R., Okamoto, H., & Takenaka, S. (2005). Mol.
Cryst. Liq. Cryst., 439, 2075.
[9] Morita, Y., Kawabe, K., Zhang, F., Okamoto, H., Takenaka, S., & Kita, H. (2005).
Chem. Lett., 34, 1650.
[10] Kelker, H. & Hatz, R. (1980). Handbook of Liquid Crystals, Verlag Chemie:
Weinheim, 340.
[11] Dewar, M. J. S., Zoebisch, E. G., Healy, E. F., & Steward, J. J. P. (1985). J. Am.
Chem. Soc., 107, 3902.
[12] Demus, D. (1998). Chemical structure and mesogenic properties. In: Handbook of
Liquid Crystals, Demus, D., Gooby, J., Gray, G. W., & Spiess, H.-W., (Eds.),
Wiley-VCH: Weinheim, Chapter 6, Vol. 1.
APPENDIX
Spectra data for compounds 1-n are as follows:
Compound 1–4; IR (KBr disc) n ¼ 1190.0, 1718.5, 1741.7, and
1
2941.4 cmꢀ1, H NMR (CDCl3) d ¼ 1.02 (3H, t, J ¼ 7.3 Hz), 1.57 (2H,
six., J ¼ 7.6 Hz), 1.86 (2H, qui., J ¼ 6.7 Hz), 4.13 (2H, t, J ¼ 6.6 Hz),
6.48 (1H, d, J ¼ 9.6 Hz), 7.19 (1H, d, J ¼ 2.3 Hz), 7.23 (1H, dd, J ¼ 8.9,
2.3 Hz), 7.38–7.48 (3H, m), 7.70 (1H, d, J ¼ 9.8 Hz), 7.81 (1H, d,
J ¼ 8.6 Hz), 7.88 (1H, d, J ¼ 8.9 Hz), 8.13 (1H, dd, J ¼ 8.6, 1.7 Hz),
8.69 (1H, d, J ¼ 1.3 Hz) ppm.
Compound 1–5; IR (KBr disc) n ¼ 1190.0, 1718.5, 1741.7, and
1
2926.0 cmꢀ1, H NMR (CDCl3) d ¼ 0.96 (3H, t, J ¼ 7.0 Hz), 1.38–1.56
(4H, m), 1.87 (2H, qui., J ¼ 6.6 Hz), 4,12 (2H, t, J ¼ 6.3 Hz), 6.48 (1H,
d, J ¼ 9.6 Hz), 7.18 (1H, d, J ¼ 2.4 Hz), 7.22 (1H, dd, J ¼ 8.9, 2.2 Hz),
7.38–7.44 (3H, m), 7.70 (1H, d, J ¼ 9.6 Hz), 7.81 (1H, d, J ¼ 8.9 Hz),
7.88 (1H, d, J ¼ 8.9 Hz), 8.13 (1H, dd, J ¼ 9.0, 1.9 Hz), 8.69 (1H, d,
J ¼ 1.4 Hz) ppm.
Compound 1–6; IR (KBr disc) n ¼ 1190.1, 1718.5, 1741.7, and
1
2926.0 cmꢀ1, H NMR (CDCl3) d ¼ 0.93 (3H, t, J ¼ 7.4 Hz), 1.37–1.57

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Y. Morita et al.
(6H, m), 1.87 (2H, qui., J ¼ 6.6 Hz), 4, 12 (2H, t, J ¼ 6.3 Hz), 6.48 (1H, d,
J ¼ 9.5 Hz), 7.18 (1H, d, J ¼ 2.3 Hz), 7.23 (1H, dd, J ¼ 8.3, 2.3 Hz), 7.38–
7.46 (3H, m), 7.70 (1H, d, J ¼ 9.6 Hz), 7.81 (1H, d, J ¼ 8.6 Hz), 7.88 (1H,
d, J ¼ 8.9 Hz), 8.13 (1H, dd, J ¼ 8.6, 1.6 Hz), 8.69 (1H, d, J ¼ 1.3 Hz)
ppm.
Compound 1–7; IR (KBr disc) n ¼ 1192.0, 1712.7, 1741.7, and
1
2922.1 cmꢁ1, H NMR (CDCl3) d ¼ 0.91 (3H, t, J ¼ 7.1 Hz), 1.26–1.57
(8H, m), 1.88 (2H, qui., J ¼ 6.6 Hz), 4,12 (2H, t, J ¼ 6.6 Hz), 6.48 (1H,
d, J ¼ 9.5 Hz), 7.18 (1H, d, J ¼ 2.0 Hz), 7.24 (1H, dd, J ¼ 8.9, 2.4 Hz),
7.36–7.45 (3H, m), 7.70 (1H, d, J ¼ 9.6 Hz), 7.81 (1H, d, J ¼ 8.6 Hz),
7.88 (1H, d, J ¼ 8.9 Hz), 8.13 (1H, dd, J ¼ 8.6, 1.7 Hz), 8.69 (1H, d,
J ¼ 1.3 Hz) ppm.
Compound 1–8; IR (KBr disc) n ¼ 1192.0, 1716.6, 1741.7, and
1
2922.1 cmꢁ1, H NMR (CDCl3) d ¼ 0.90 (3H, t, J ¼ 6.3 Hz), 1.24–1.59
(10H, m), 1.87 (2H, qui., J ¼ 6.6 Hz), 4,12 (2H, t, J ¼ 6.6 Hz), 6.48
(1H, d, J ¼ 9.6 Hz), 7.18 (1H,d, J ¼ 2.3 Hz), 7.24 (1H, dd, J ¼ 8.9,
2.3 Hz), 7.38–7.45 (3H, m), 7.70 (1H, d, J ¼ 9.5 Hz), 7.81 (1H, d,
J ¼ 8.6 Hz), 7.88 (1H, d, J ¼ 9.2 Hz), 8.13 (1H, dd, J ¼ 8.6, 2.0 Hz),
8.69 (1H, d, J ¼ 1.3 Hz) ppm.