Synthesis and physico-chemical properties for 2-oxochromen-6-yl 4-(4-alkoxyphenylazo)benzoates

Article abstract of DOI:10.1080/15421400903060557

This paper describes the preparation and thermal properties for homologous series of 2-oxochromen-6-yl 4-(4-alkoxyphenylazo)benzoates (compounds 1-n). Compounds 1-n show nematic phase exclusively, where the nematic-isotropic phase transition temperatures are between 293° C for 1-4 and 256° C for 1-8. Interestingly, compound 1-8 exhibits smectic A and re-entrant nematic phases in addition to the nematic one. The smectic A phase are characterized using a small angle X-ray diffraction study, indicating the smectic A phase has a partially bilayer molecular arrangement. Copyright Taylor & Francis Group, LLC.

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Synthesis and Physico-
Chemical Properties for
2-Oxochromen-6-yl 4-(4-
Alkoxyphenylazo)benzoates
Yuki Morita a , Hiroshi Uemura a , Kentaro Shimoi a
Kazuo Kasatani a & Hiroaki Okamoto a
,
a Department of Advanced Materials Science and
Engineering , Faculty of Engineering, Yamaguchi
University , Ube, Yamaguchi, Japan
Published online: 05 Oct 2009.
To cite this article: Yuki Morita , Hiroshi Uemura , Kentaro Shimoi , Kazuo Kasatani
& Hiroaki Okamoto (2009) Synthesis and Physico-Chemical Properties for 2-
Oxochromen-6-yl 4-(4-Alkoxyphenylazo)benzoates, Molecular Crystals and Liquid
Crystals, 508:1, 163/[525]-172/[534], DOI: 10.1080/15421400903060557
To link to this article: http://dx.doi.org/10.1080/15421400903060557
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Mol. Cryst. Liq. Cryst., Vol. 508, pp. 163=[525]–172=[534], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903060557
Synthesis and Physico-Chemical Properties for
2-Oxochromen-6-yl 4-(4-Alkoxyphenylazo)benzoates
Yuki Morita, Hiroshi Uemura, Kentaro Shimoi,
Kazuo Kasatani, and Hiroaki Okamoto
Department of Advanced Materials Science and Engineering, Faculty
of Engineering, Yamaguchi University, Ube, Yamaguchi, Japan
This paper describes the preparation and thermal properties for homologous
series of 2-oxochromen-6-yl 4-(4-alkoxyphenylazo)benzoates (compounds 1-n).
Compounds 1-n show nematic phase exclusively, where the nematic–isotropic
phase transition temperatures are between 293ꢀC for 1–4 and 256ꢀC for 1–8.
Interestingly, compound 1–8 exhibits smectic A and re-entrant nematic phases
in addition to the nematic one. The smectic A phase are characterized using a
small angle X-ray diffraction study, indicating the smectic A phase has a partially
bilayer molecular arrangement.
Keywords: re-entrant nematic phase; smectic A phase; synthesis; X-ray diffraction
study
INTRODUCTION
It is well known that liquid crystal materials incorporating strongly
polar terminal groups such as cyano and=or nitro group exhibit
interesting mesomorphic behavior, including a re-entrant nematic
and several smectic phases, where the large dipole moment along
to the longitudinal direction of the molecule is supposed to play an
important role [1].
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,
Culture, Sports, Science and Technology of Japan.
Address correspondence to Hiroaki Okamoto, Department of Advanced Materials
Science and Engineering, Faculty of Eng., Yamaguchi University, Tokiwadai 2557,
Ube, Yamaguchi 755-8611, Japan. E-mail: oka-moto@po.cc.yamaguchi-u.ac.jp
163=[525]

164=[526]
Y. Morita et al.
FIGURE 1 Chemical structure for compounds 1-n.
Recently, several liquid crystal materials having a terminal
carbonyl group such as lactone and cyclic carbonate groups were pre-
pared and it has been clarified that the terminal carbonyl groups affect
the mesomorphic and dielectric properties differently from the
terminal cyano and nitro groups. In earlier papers [2–6], for example,
we have reported that a terminal 1,3-dioxan-2-one and the related
skeletons affect their mesomorphic, physical, and dielectric properties.
Furthermore, we have also investigated some liquid crystal materials
having a 2-oxo-chromen (coumarin) skeleton at the terminal position of
the molecule and their benzoate system shows not only mesomorphic
properties but also gelation one [7,8].
This paper describes the preparation and the thermal properties
of homologous series of 2-oxochromen-6-yl 4-(4-alkoxyphenylazo)
benzoates (compounds 1-n) as shown in Figure 1. The experimental
results are discussed in terms of the molecular structure.
EXPERIMENTAL
Materials
Compounds 1-n were synthesized according to Scheme 1, where
compounds 1-n and synthetic intermediates were characterized by
1
means of IR and H NMR spectra and the results are as follows.
Physical data for compounds 1-n are as follows.
IR and 1H NMR spectra for compound 1-4: IR (KBr disc); n ¼ 1242.2,
1597.1, 1734.0, 1743.7, and 2960.7 cmꢀ1
.
1H NMR (CDCl3); d ¼ 1.01
(3H, t, J ¼ 6.5 Hz), 1.56 (2H, six., J ¼ 6.5 Hz), 1.83 (2H, qui.,
J ¼ 6.5 Hz), 4.08 (2H, t, J ¼ 6.5 Hz), 6.50 (1H, d, J ¼ 9.6 Hz), 7.03
(2H, d, J ¼ 8.9 Hz), 7.42 (3H, s), 7.71 (1H, d, J ¼ 9.6 Hz), 7.97
(2H, d, J ¼ 8.9 Hz), 7.99 (2H, d, J ¼ 8.9 Hz), and 8.33 (2H, d,
J ¼ 8.6 Hz) ppm.
IR and 1H NMR spectra for compound 1–5: IR (KBr disc);
n ¼ 1138.0, 1259.5, 1735.9, and 2956.9 cmꢀ1
.
1H NMR (CDCl3);
d ¼ 0.96 (3H, t, J ¼ 6.9 Hz), 1.38–1.54 (4H, m), 1.85 (2H, qui.,
J ¼ 6.9 Hz), 4.07 (2H, t, J ¼ 6.9 Hz), 6.50 (1H, d, J ¼ 9.6 Hz), 7.03

Re-Entrant Phenomena of Coumarin Mesogen
165/[527]
SCHEME 1 Synthetic scheme for compounds 1-n; Reagents and conditions; a)
NaNO2, 1 M HCl, b) Phenol in 6 M NaOH, then poured into 2 M HCl, c) H2SO4,
EtOH, reflux, d) CnH2nþ1Br, K2CO3, 3-pentanone, reflux, e) 6 M NaOH, then
poured into 2 M HCl, f) SOCl2 70ꢁC, g) 6-hydroxycoumarine, pyridine, 70ꢁC.
(2H, d, J ¼ 8.9 Hz), 7.42 (3H, s), 7.72 (1H, d, J ¼ 9.6 Hz), 7.97 (2H, d,
J ¼ 8.9 Hz), 7.99 (2H, d, J ¼ 8.9 Hz), and 8.32 (2H, d, J ¼ 8.6 Hz) ppm.
IR and 1H NMR spectra for compound 1–6: IR (KBr disc);
n ¼ 1139.9, 1238.3, 1720.5, and 2941.4 cmꢀ1
.
1H NMR (CDCl3);
d ¼ 0.92 (3H, t, J ¼ 6.5 Hz), 1.36–1.57 (6H. m), 1.84 (2H, qui.,
J ¼ 6.5 Hz), 4.07 (2H, t, J ¼ 6.5 Hz), 6.50 (1H, d, J ¼ 9.2 Hz), 7.02 (2H,
d, J ¼ 8.9 Hz), 7.42 (3H, s), 7.71 (1H, d, J ¼ 9.6 Hz), 7.97 (2H, d,
J ¼ 8.9 Hz), 7.99 (2H, d, J ¼ 8.9 Hz), and 8.33 (2H, d, J ¼ 8.9 Hz) ppm.
IR and 1H NMR spectra for compound 1–7: IR (KBr disc);
n ¼ 1143.8, 1290.2, 1728.2, and 2939.5 cmꢀ1
.
1H NMR (CDCl3);
d ¼ 0.91 (3H, t, J ¼ 6.5 Hz), 1.24–1.53 (8H, m), 1.84 (2H, qui.,
J ¼ 6.5 Hz), 4.07 (2H, t, J ¼ 6.5 Hz), 6.50 (1H, d, J ¼ 9.6 Hz), 7.03 (2H,
d, J ¼ 8.9 Hz), 7.42 (3H, s), 7.71 (1H, d, J ¼ 9.4 Hz), 7.97 (2H, d,
J ¼ 8.9 Hz), 7.99 (2H, d, J ¼ 8.9 Hz), and 8.32 (2H, d, J ¼ 8.9 Hz) ppm.
IR and 1H NMR spectra for compound 1–8: IR (KBr disc);
n ¼ 1139.9, 1244.1, 1734.0, 1745.6, and 2924.1 cmꢀ1. 1H NMR (CDCl3);
d ¼ 0.90 (3H, t, J ¼ 6.5 Hz), 1.21–1.57 (10H, m), 1.84 (2H, qui.,
J ¼ 6.5 Hz), 4.05 (2H, t, J ¼ 6.6 Hz), 6.50 (1H, d, J ¼ 9.6 Hz), 7.03 (2H,
d, J ¼ 8.9 Hz), 7.42 (3H, s), 7.72 (1H, d, J ¼ 9.6 Hz), 7.97 (2H, d,
J ¼ 8.6 Hz), 7.99 (2H, d, J ¼ 8.6 Hz), and 8.33 (2H, d, J ¼ 8.6 Hz) ppm.
METHOD
The transition temperatures and latent heats were determined using a
Seiko SSC-5200 DSC, where indium (99.9%) was used as a calibration
standard (mp ¼ 156.6ꢁC, 28.4 J=g). The DSC thermogram was operated
at a heating or cooling rate of 5ꢁC minꢀ1. The mesophases were char-
acterized using a Nikon POH polarizing microscope fitted with a
Mettler thermo-control system (FP-900). The homogeneous and

166/[528]
Y. Morita et al.
homeotropic alignments between glass surfaces were achieved by
treatment of glass plates with polyimide (Tore SP-810) and cetyl-
trimethyl ammonium bromide, respectively. 1H NMR spectra were
measured using a JEOL EX-270 spectrometer, where tetramethylsi-
lane 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 and DSC thermograms using a
‘DSCPURITY’ program (SII instrument) instead of elemental analysis.
X-ray diffraction experiments for the SmA phase were performed
˚
using a Rigaku RINT 2200 diffractometer, where CuKa (k ¼ 1.541 A)
was used as an X-ray source. The reflection angle was calibrated by
a comparison of both right and left angles. The temperature was con-
trolled using a Rigaku PTC-20A controller. The samples were heated
to the re-entarant nematic phase and filled into the quartz capillary
(1.0 mm/). The sample was oriented by a constant magnetic field
(480 G) and the measurements were carried out during the heating
process.
The molecular lengths were calculated by a semi-empirical mole-
cular orbital calculation, where minimization of the total energy was
achieved by an AM1 method [9].
RESULTS AND DISCUSSION
Mesomorphic properties for compounds 1-n were determined by
means of DSC measurements and polarized microscope observations.
Compound 1–7 shows endotherms at 124ꢁC and 263ꢁC correspond-
ing to the melting and mesophase–isotropic (I) phase transition
phenomena on heating process. The molten material showed a small
exotherm at 261ꢁC due to the phase transition during a cooling
process. The formed texture is shown in Figure 2. The mesophase
shows a typical Schlieren texture under polarized microscope observa-
tion (see Fig. 2). Therefore, the mesophase is assigned to be a nematic
(N) phase.
For compound 1–8, a typical focal conic fan texture is also observed
at 216ꢁC in addition to the Schlieren one on cooling process as shown
in Figure 3(a) and (b). We assume that the mesophases have an
orthogonal alignment of the molecule, since the mesophases show a
typical homeotropic texture (dark under a cross-polarized microscope)
under a homeotropic alignment. Therefore, the mesophases are
assigned to be the N and smectic A (SmA) phases, whereas analogues
of compounds 1-n, 4-methyl-7-coumaryl 4-(4-alkoxyphenylazo) benzo-
ates, show a smectic C phase as well as the N phase [10]. Interestingly,
compound 1–8 exhibits another Schlieren texture under a homogeneous

Re-Entrant Phenomena of Coumarin Mesogen
167/[529]
FIGURE 2 Polarized micrograph for compound 1–7 at 255ꢁC.
FIGURE 3 Polarized micrographs for compound 1–8: (a) nematic (N) phase
(254ꢁC), (b) smectic A (SmA) phase (202ꢁC), (c) re-entrant nematic (Nre) phase
(132ꢁC).

168/[530]
Y. Morita et al.
alignment surface at 149ꢁC on cooling of the SmA phase as can be seen
in Figure 3(c). The mesophase below the SmA phase is assigned to be a
re-entrant N (Nre) phase as a result.
The transition temperatures for 1-n are summarized in Table 1 and
plotted against the carbon numbers (n) in the alkoxy chain (Fig. 4).
The N–I transition temperatures for 1-n decrease on ascending the
homologues, while the melting points are almost independent of the
carbon numbers (n). Compound 1–8 has the phase sequence of crystal
(C)–Nre–SmA–N–I, while compound 1–7 C–N–I as described before.
It is known that the re-entrant phenomenon in liquid crystals was
frequently found in cyano and nitro compounds [11], which is so-called
‘‘polar liquid crystals’’. To the best of our knowledge, the appearance of
the Nre phase of compound 1–8 is a first example for a single liquid
crystal material having a carbonyl group at the terminal position of
the molecules, suggesting the introduction of the coumarin skeleton
gives a polar nature in liquid crystal materials.
The SmA phase was also characterized by means of X-ray diffrac-
tion measurements, and the X-ray profile for compound 1–8 at
160ꢁC is shown in Figure 5.
The reflection peak arising from a smectic layer was observed at
ꢁ
˚
2h ¼ 2.39 (36.9 A), on the other hand, there was no apparent peak in
the wide-angle region. The results also support the characterization
of the SmA phase of compound 1–8.
The phase transition behavior and layer structures of the SmA
phase were further characterized using small angle X-ray diffraction
measurements in variable temperatures as shown in Figure 6.
The diffraction peak appeared obviously at 160ꢁC on heating
process, while there is no apparent peak at 150ꢁC. The intensity of
TABLE 1 Transition Temperatures for Compounds 1-n
Compounds 1-n
Transition temperature (ꢁC)
SmA
n
C
Nre
N
I
4
5
6
7
8
.
.
.
.
.
136
130
119
124
125
–
–
–
–
.
–
–
–
–
.
.
.
.
.
293
280
272
263
256
.
.
.
.
.
149
.
216
C, Nre, SmA, N and I indicate crystal, re-entrant nematic, smectic A, nematic and
isotropic phases, respectively.
ꢂLatent heats for Nre – SmA and SmA – N transitions could not be observed by DSC
measurements.

Re-Entrant Phenomena of Coumarin Mesogen
169/[531]
FIGURE 4 Plots of transition temperatures against the number of the carbon
atoms (n) for compounds 1-n; Diamonds, circle (filled), triangle and circles
(open) denote melting points, Nre–SmA, SmA–N and N–I transition tempera-
tures, respectively.
diffraction peaks becomes large and sharp with increasing the
temperatures, suggesting the order of the molecular arrangements
arise in the SmA phase. These results also support the existence of
the Nre phase below the SmA one.
FIGURE 5 X-ray profile for compound 1–8 at 160ꢁC.

170/[532]
Y. Morita et al.
FIGURE 6 X-ray profiles for compound 1–8 at (a) 160ꢁC, (b) 165ꢁC, (c) 170ꢁC.
The diffraction angles arising from the SmA layer are observed at
ꢁ
˚
2h ¼ 2.39 (d ¼ 36.9 A) and independent of the temperature between
160ꢁC and 170ꢁC.
In order to consider the molecular arrangements in the SmA phase,
the molecular structure and dipole moment of compound 1–8 were
estimated using a semi-empirical molecular calculation (AM1 method)
[9] as shown in Figure 7.
˚
The molecular length (l) of compound 1–8 is calculated to be 30.8 A,
˚
˚
which is 6.1 A shorter than the layer spacing (36.9 A) of the SmA
phase, so that the SmA phase is assigned to be so-called ‘‘SmAd’’ phase,
which is frequently observed in polar liquid crystal materials. The
characterization of the SmAd phase is also supported by the
semi-empirical molecular orbital calculation, in which compound 1–8
has a large dipole moment (8.5 D) mainly arising from the terminal
coumarin skeleton and should have a polar liquid crystal nature. On
the other hand, the length of the coumarin skeleton for compound
FIGURE 7 Molecular structure and dipole moment for compound 1–8, calcu-
lated by a semi-empirical molecular orbital calculation (AM1).

Re-Entrant Phenomena of Coumarin Mesogen
171/[533]
˚
1–8 is calculated to be 6.1 A, agreed with the difference of the layer
spacing (d) and molecular length (l).
These results suggest that there are two possibilities for the mole-
cular arrangements in the SmAd phase as shown in Figure 8.
In Figure 8(a), coumarin skeletons are gathered around the smectic
layer boundary and the parts of the compound 1–8 including the alkyl
chain are arranged in the center of the smectic layers. In Figure 8(b),
on the other hand, coumarin skeletons are arranged in the center of
the smectic layer and some parts of the terminal alkyl chain are gath-
ered around the smectic layer boundary, which is the conventional
molecular arrangement in the SmAd phase.
In our earlier papers, some liquid crystal materials have unconven-
tional molecular arrangements, because of the electrostatic and steric
effects of the terminal groups [12–19].
It seems to be difficult to clarify the molecular arrangements in the
SmAd phase since there are not enough experimental data at present.
FIGURE 8 Possible molecular arrangements in the SmA phase for compound
1–8: coumarin groups are arranged around (a) the smectic layer boundary and
(b) the center of the smectic layer.

172/[534]
Y. Morita et al.
However, the re-entrant phenomena for compound 1–8 and the
gelation properties for the corresponding benzoate systems [7,8] might
have some relationship in molecular arrangement.
Further examination is now underway, and the results will be
published elsewhere.
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