Synthesis and mesomorphic properties of fluorinated Schiff's base esters containing alkyl and alkoxy end groups

Article abstract of DOI:10.1080/15421400802198268

Two series of fluorinated Schiff's base esters have been synthesized and characterized. Their chemical structures were identified by FTIR, 1H NMR, and elemental analysis (EA). Their mesomorphic properties were studied by polarizing optical, microscopy (PO

Full text of DOI:10.1080/15421400802198268

This article was downloaded by: [University of California, San Diego]
On: 07 October 2014, At: 19:47
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis and Mesomorphic
Properties of Fluorinated
Schiff's Base Esters Containing
Alkyl and Alkoxy End Groups
a
a
a
Qiang Wei , Huai Yang & Yanbin Wang
a
School of Materials Science and Engineering,
University of Science and Technology Beijing ,
Beijing , China
Published online: 31 Aug 2012.
To cite this article: Qiang Wei , Huai Yang & Yanbin Wang (2008) Synthesis and
Mesomorphic Properties of Fluorinated Schiff's Base Esters Containing Alkyl and
Alkoxy End Groups, Molecular Crystals and Liquid Crystals, 487:1, 31-38, DOI:
10.1080/15421400802198268
To link to this article: http://dx.doi.org/10.1080/15421400802198268
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or

indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 487, pp. 31–38, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802198268
Synthesis and Mesomorphic Properties
of Fluorinated Schiff’s Base Esters Containing
Alkyl and Alkoxy End Groups
Qiang Wei, Huai Yang, and Yanbin Wang
School of Materials Science and Engineering, University of Science and
Technology Beijing, Beijing, China
Two series of fluorinated Schiff’s base esters have been synthesized and character-
1
ized. Their chemical structures were identified by FTIR, H NMR, and elemental
analysis (EA). Their mesomorphic properties were studied by polarizing optical,
microscopy (POM) and differential scanning calorimeter (DSC). It was found that
most of the compounds with alkyl end group exhibited enantiotropic smectic A and
nematic phase; while that with alkoxyl behaved differently, some exhibited enan-
tiotropic smectic A and nematic phase; others exhibited monotropic smectic A
and enantiotropic nematic phase or unique enantiotropic nematic phase. And for
both series, their melting point and clearing point temperature decreased and
thermal ranges of mesophase increased with the length of their end groups (n).
Furthermore, it was also found that the increase of n was propitious to the stability
of smectic phase but went against for that of nematic.
Keywords: fluorinated liquid crystal; mesomorphic property; Schiff’s base; synthesis
INTRODUCTION
The mesomorphic behaviors of liquid crystals are basically dependent on
their molecular structures in which a slight change in their molecular
geometry may bring about considerable change in their mesomorphic
properties. Detailed studies by liquid crystal researchers have led to
empirical rules which included the effect of chemical constitution on
the nematic and smectic mesophases [1]. Most of these studies have
been focused on Schiff’s bases ever since the discovery of 4-methoxyben-
0
zylidene-4 -butylaniline (MBBA) which exhibits a room temperature
Address correspondence to Huai Yang, School of Materials Science and Engineering,
University of Science and Technology Beijing, Beijing 100083, China. E-mail: yanghuai@
mater.ustb.edu.cn
31

3
2
Q. Wei et al.
nematic phase [2]. Over the past few decades, many Schiff’s base ester
liquid crystals have been synthesized and investigated [1,3–8]. It is
proved that the terminal or lateral groups, such as -F, -Cl, -CN, -CH3,
and -OCH , had significant influence on the liquid crystalline proper-
3
ties of the Schiff’s base esters as well as in other liquid crystals. The
introduction of terminal or lateral polar groups into the molecular
system would lead to a change in molecular polarizability as well as
to a change in the physical properties such as clearing point [9]. Thus,
by introduction of different groups into the molecular system, new
Schiff’s base liquid crystals of diverse mesomorphic properties can be
obtained. Previous studies [10] indicated that the replacement of one
or several hydrogen atoms by fluorine confers to the resulting materials
unusual and peculiar properties such as optical and chemical stability,
wide mesomorphic temperature range, low melting point, low viscosity,
and low conductivity. So the studies of fluorinated liquid crystals are of
great importance, and many researchers [10–13] have devoted them-
selves into this field. And some studies have been made on fluorinated
Schiff’s base ester liquid crystals [3,13,14].
The aim of the present study was to synthesize and characterize two
series of new fluorinated Schiff’s base esters whose structure was
indicated in Fig. 1, and to study their mesomorphic properties by
using differential scanning calorimeter (DSC) and polarizing optical
microscopy (POM). The effects of the ‘‘soft tail’’ with different length
of alkyl or alkoxy carbon chains on the mesomorphic properties of
the fluorinated Schiff’s base esters were also to be examined.
EXPERIMENT
All solvents were AR quality. All starting materials were commercially
available reagents and used without further purification. The
FIGURE 1 Structure of the aim compounds.

Fluorinated Schiff’s Base Esters
33
FIGURE 2 Synthetic routes of the aim compounds.
synthetic route to the aim structures was summarized in Fig. 2 and
procedures were summarized below.
Infrared (IR) spectra were recorded on a Nicolet 510P instrument
1
and H NMR spectra on Bruker AV400 instrument in CDCl . EA
3
measurements were carried out on a Thermo Finnigan Flash EA1112
elemental analyzer. The mesomorphic properties of the compounds
and complexes were studied by an Olympus BX-51 polarizing micro-
scope equipped wilh a Linkam Scientific LTS 350 heating=freezing
stage. For DSC, a Perkin-Elmer DSC-6 instrument was used; the
ꢀ
scanning rate was 10 C=min, the samples were sealed in aluminum
capsules, and the holder atmosphere was dry nitrogen. The smectic
subphase was identified by POM observation by using homeotropic
oriented liquid crystal cells.
Synthesis of 4-Hexyloxybenzoic Acid
Literature method [15] was used to prepare 4-hexyloxybenzoic acid. IR,
ꢁ1
n=cm : 2552–3075 (acidic -OH), 1682 (C=O), 1605, 1580, 1510 (aro-
1
matic C=C), 1251 (C-O-Ar). H NMR (CDC1 , d ppm): 0.90–1.84 (m,
3
1
6
1H, alkyl protons), 4.01–4.04 (m, 2H, J ¼ 6.48 Hz, -CH -O), 6.92–
2
.94 (d, 2H, J ¼ 7.48 Hz, Ar-H), 8.04–8.06 (d, 2H, J ¼ 7.64 Hz, Ar-H).
Anal. Calcd. For C H O : C, 70.24; H, 8.16. Found C, 70.32; H, 8.15.
1
3
18
3
Synthesis of 4-Hexyloxybenzoic Acid 4-Formylphenyl Ester
Dicyclohexylcarbodiimide 10.3 g (50 mmol) and N,N-dimethylamino-
pyridine 20 mg were added to a solution of 4-hexyloxybenzoic acid

3
4
Q. Wei et al.
1
1
1.1 g (50 mmol) and 4-hydroxylbenzaldehydc 6.1 g (50 mmol) in
20 mL of dichloromethane (DCM). The mixture was stirred for 12 h.
The dicyclohexylurea was filtered off and the solvent from the filtrate
was removed under reduced pressure. The crude product was purified
by column chromatography (silica gel, DCM) and then evaporated at
ꢁ1
reduced pressure gave the white crystals at a yield of 76%. IR, n=cm :
960, 2872, 2932, 2850 (aliphatic C-H), 1735 (C=O of ester), 1695 (C=O
2
of aromatic aldehyde), 1605, 1579, 1510 (aromatic C=C), 1265 (C-O-
1
Ar). H NMR (CDC1 , d ppm): 0.92–1.85 (m, 11H, alkyl protons),
3
4
.03–4.08 (m, 2H, J ¼ 6.30 Hz, -CH -O), 6.97–7.00 (d, 2H, J ¼ 8.31 Hz,
2
Hz, Ar-H), 7.39–7.42 (d, 2H, J ¼ 7.98 Hz, Ar-H), 7.96–7.98 (d, 2H,
J ¼ 7.92 Hz, Ar-H), 8.13–8.16 (d, 2H, J ¼ 8.34 Hz Ar-H), 10.03 (s, H,
Ar-CHO). Anal. Calcd. For C H O : C, 73.60; H, 6.79. Found C,
2
0
22
4
7
3.46; H, 6.81.
Synthesis of Aim Compound 6OAPE
Several drops of acetic acid, 4-hexyloxybenzoic acid 4-formylphenyl
ester 3.26 g (10 mmol) and 2,3,4-trifluorophenylaimne 1.47 g (10 mmol)
was dissolved in 100 mL of absolute ethanol. The mixture was stirred
under refluxing for 7 h, then cooled and left unstirred overnight. The
crude product was collected and recrystallized from ethanol to give
white crystals of the aim compound 6OAPE at an 82% yield. IR,
ꢁ
1
n=cm : 2960, 2936, 2863 (CH -, -CH -), 1731 (C=O), 1628 (C=N),
3
2
1
1
605, 1502 (aromatic C=C), 1264 (C-O-Ar). H NMR (CDC1 , d ppm):
3
0
.94–1.90 (m, 11H, alkyl protons), 4.07–4.10 (d, 2H, J ¼ 6.52 Hz,
-
CH -O), 6.96–6.99 (d, H, J ¼ 10.36 Hz, Ar-H), 7.01–7.03 (d, 3H,
2
J ¼ 8.76 Hz, Ar-H), 7.38–7.40 (d, 2H, J ¼ 8.36 Hz, Ar-H), 8.02–8.04
(
d, 2H, J ¼ 8.28 Hz, Ar-H), 8.20–8.17 (d, 2H, J ¼ 8.72 Hz, Ar-H), 8.55
(
s, H, CH=N). Anal. Calcd. For C H F NO : C, 68.56; H, 5.31; N,
2
6
24
3
3
3
.08. Found C, 68.67; H, 5.33, N, 3.09.
Synthesis of 4-Heptylbenzoic Acid 4-Formylphenyl Ester
Same synthetic procedure to 4-hexyloxybenzoic acid 4-formyl-phenyl
ester was used to prepare 4-heptylbenzoic acid 4-formylphenyl ester
1
that gave the white crystals at a yield of 82%. IR, n=cm : 2957,
2
922, 2850 (CH -, -CH -), 1738 (C=O of ester), 1696 (C=O of aromatic
3 2
1
aldehyde), 1600, 1500 (aromatic C=C), 1262 (C-O-Ar). H NMR
CDCl , d ppm): 0.87–1.68 (m, 13H, alkyl protons), 2.69–2.73 (m, 2H,
(
3
J ¼ 7.84 Hz, Ar-CH ), 7.32–7.34 (d, 2H, J ¼ 7.72 Hz, Ar-H), 7.40–
2
7
.42 (d, 2H, J ¼ 8.12 Hz, Ar-H), 7.96–7.98 (d, 2H, J ¼ 7.36 Hz, Ar-H),

Fluorinated Schiff’s Base Esters
35
8
.10–8.12 (d, 2H, J ¼ 7.76 Hz, Ar-H), 10.03 (s, H, Ar-CHO). Anal.
Calcd. For C H O : C, 77.75; H, 7.46. Found C, 77.69; H, 7.48.
2
1
24
3
Synthesis of 6APE
The synthetic procedure was same to 6OAPE and the white crystals
ꢁ
1
were got at a yield of 86%. IR, n=cm : 2960, 2932, 2854 (CH -,
3
1
-
CH -), 1735 (C=O), 1627 (C=N), 1601, 1502 (aromatic C=C).
H
2
NMR (CDCl , d ppm): 0.87–1.66 (m, 13H, alkyl protons), 2.69–2.74
3
(
4
8
m, 2H, J ¼ 7.40 Hz, Ar-CH), 6.90–7.03 (m, 2H, Ar-H), 7.32–7.37 (m,
H, J ¼ 7.85 Hz, Ar-H), 7.98–8.01 (d, 2H, J ¼ 8.16 Hz, Ar-H), 8.11–
.14 (d, 2H, J ¼ 7.74 Hz, Ar-H), 8.52 (s, H, CH=N). Anal. Calcd.
For C H F NO : C, 71.51; H, 5.78; N, 3.09. Found C, 71.59; H,
2
7
26
3
2
5
.79, N, 3.09.
TABLE 1 Phase Transition Temperature and Associated
Enthalpy Changes of nAPE and nOAPE
ꢀ
Phase transition temperature ( C) and enthalpy
ꢂ
Compound
changes (kJ=mol, in parentheses)
8
1ð52:0Þ
165ð1:9Þ
173ð0:3Þ
6
5
4
3
2
6
5
4
3
2
APE
APE
APE
APE
APE
OAPE
OAPE
OAPE
OAPE
OAPE
ꢂ
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
* SA
* N
* I
)4
)163
)172
5
7
5ð33:6Þ
159ð1:8Þ
178ð0:4Þ
* SA
* N
* I
)3
)156
)176
9
8
2ð28:6Þ
155ð1:1Þ
182ð0:5Þ
* SA
* N
* I
)4
)151
)180
6
1
01ð25:8Þ
145ð1:3Þ
180ð0:4Þ
* SA
* N
* I
)4
)142
)178
6
1
14ð22:7Þ
126ð0:5Þ
189ð0:5Þ
* SA
* N
* I
)6
)124
)186
2
7
7ð24:8Þ
169ð0:5Þ
198ð0:4Þ
* SA
* N
* I
)4
)167
)197
3
9
1ð29:8Þ
161ð0:4Þ
203ð0:5Þ
* SA
* N
* I
)5
)159
)201
1
1
02ð28:9Þ
142ð0:4Þ
212ð0:8Þ
* SA
* N
* I
)5
)140
)209
1
1
20ð33:0Þ
218ð0:7Þ
* SA
* N
* I
)8
)
)216
0
120ð0:2Þ
1
34ð30:8Þ
235ð0:5Þ
* N
02
* I
)1
)233
A
Cr: crystal; S : smectic A; N: nematic; I: isotropic.

3
6
Q. Wei et al.
RESULTS AND DISCUSSION
The liquid crystalline properties of the two series of aim compounds
were studied by DSC and POM. Their optical textures were shown
in Fig. 3. Their phase variants, transition temperatures, and associa-
ted enthalpy were listed in Table 1 and their phase transition data in
heating scan were represented graphically in Fig. 4.
All the members of the two series were mesomorphic. Series nAPE
(
n ¼ 2–6) exhibited similarly mesomorphic properties, they all showed
enantiotropic smectic A and nematic phase with no respect to the
length change of their alkyl chains, which indicated that the phase
variant of this series were mainly depended on their core structure.
As to series nOAPE, 6OAPE, 5OAPE, and 4OAPE showed enantiotro-
pic smectic A and nematic phase, but 3OAPE exhibited a monotropic
FIGURE 3 Optical textures of the aim compounds on cooling cycle. (a) Fan-
ꢀ
shaped. texture 6APE. 162 C, ꢃ 200: (b) Schlieren texture. 6APE.
ꢀ
ꢀ
1
69 C. ꢃ 200: (c) Black field. 6APE. 160 C homeotropic oriented. ꢃ 200: (d)
ꢀ
Focal conic texture 6APE. 149 C. homeotropic oriented. ꢃ 100: (e) Fan-shaped
ꢀ
ꢀ
texture. 6OAPE. 163 C ꢃ 100: (f) Schlieren texture. 6OAPE. 192 C ꢃ 100: (g)
ꢀ
Black field. 6OAPE. 154 C. homeotropic oriented ꢃ 100.

Fluorinated Schiff’s Base Esters
37
FIGURE 4 Effect of carbon atoms in the end groups on the phase transition
properties.
smectic A and enantiotropic nematic phase, as for 2OAPE, only an
enantiotropic nematic phase can be observed, which indicated that
the length of alkoxy ‘‘tail’’ had more effect on the liquid crystal proper-
ties than of alkyl ‘‘tail’’ in these studied compounds. The typical optical
textures of the two series were listed in Fig. 3.
The results shown in Fig. 4 revealed that series nAPE showed little
and series nOAPE almost showed no even–odd effects. And on the
whole, the melting point and clearing point temperature of both series
decreased with the number of carbon atoms (n) in their end groups.
These results also testified that the core structure of these studied
compounds predominated in shaping their liquid crystal properties
as compared to their alkyl or alkoxy chains which is considered the
main reason of the even–odd effects. Furthermore, with the increase
of n, the thermal range of smectic phase of both series broadened;
and at the same time, their thermal range of nematic phase narrowed.
When compared nAPE with nOAPE, it was found that the melting
point and clearing point temperature of nAPE always lower than their
corresponding compounds of nOAPE.
CONCLUSION
We have successfully synthesized and characterized two series of
fluorinated Schiff’s base ester liquid crystals. It was found that most
of the compounds with alkyl end group exhibited enantiotropic smectic
A and nematic phase; while that with alkoxyl behaved differently,
some exhibited enantiotropic smectic A and nematic phase; others
exhibited monotropic smectic A and enantiotropic nematic phase or
unique enantiotropic nematic phase. And for both series their melting

3
8
Q. Wei et al.
point and clearing point temperature decreased and thermal ranges of
mesophase increased with the length of their end groups. Further-
more, it was also found that the increase of n was propitious to the
stability of smectic phase but went against for that of nematic.
Additionally, it was also found that the melting point and clearing
point temperature of nAPE always lower than their corresponding
compounds of nOAPE.
ACKNOWLEDGMENTS
This research was supported by the Scientific Program of Beijing
under Grant No. Y4050004040121 and the Major Project of Ministry
of Education of the People’s Republic of China under Grant No.
1
04187.
REFERENCES
[1] Ycap, G. Y., Ha, S. T., Lim, P. L., Boey, P. L., Ito, M. M., Sanehisa, S., & Youhei, Y.
(2006). Liq. Cryst., 33, 205.
[
[
2] Kelker, H. & Scheurle, B. (1969). Angew. Chem., Int. Ed. Engl., 8, 884.
3] Matsunaga, Y., Hikosaka, L., Hosono, K., Ikeda, N., Sakatani, T., Sekiba, K.,
Takachi, K., Takahashi, T., & Uemura, Y. (2001). Mol. Cryst. Liq. Cryst. Sci. Techn.,
Sect. A: Mol. Cryst. Liq. Cryst., 369, 103.
[
[
4] Prajapati, A. K. & Bonde, N. L. (2006). Mol. Cryst. Liq. Cryst., 461, 15.
5] Yeap, G. Y., Ha, S. T., Boey, P. L., Mahmood, W. A. K., Ito, M. M., & Youhei, Y.
(2006). Mol. Cryst. Liq. Cryst., 452, 73.
[
[
6] Hu, P., Zhao, K. Q., Zhang, L. F., Lin, Z. R., & Xu, H. B. (1999). Acta Chimica
Sinica, 57, 931.
7] Shin, S. T., Choi, H., Lee, C. K., Kwon, S. S., Kim, T. S., Choi, E. J., Kim, S. Y., Zin,
W. C., Kim, D. C., & Chien, L. C. (2004). Liq. Cryst., 31, 935.
8] Umadevi, S. & Sadashiva, B. K. (2005) Liq. Cryst., 32, 1233.
9] Yeap, G. Y., Ha, S. T., Lim, P. L., Boey, P. L., Mahmood, W. A. K., Ito, M. M., &
Sanehisa, S. (2004). Mol. Cryst. Liq. Cryst., 423, 73.
[
[
[10] Guittard, F., Taffin de Givenchy, E., Geribaldi, S., & Cambon, A. (1999). J. Fluorine
Chem., 100, 85.
[
[
[
11] Miethchen, R. & Hein, M. (2000). Carbohydr. Res., 327, 169.
12] William, R. & Doloier, J. (2005). J. Fluorine Chem., 126, 157.
13] Chudgar, N. K., Shah, S. N., Thakkar, I., Dhamalia, N., & Sharma, H. C. (1997).
Mol. Cryst. Liq. Cryst. Sci. Techn., Sect. C: Mol. Mater., 8, 301.
14] Matsunaga, Y., Miyake, T., Rikimaru, H., & Iwano, N. (1999). Mol. Cryst. Liq.
Cryst. Sci. Techn., Sect. A: Mol. Cryst. Liq. Cryst., 326, 155.
[
[
15] Xiao, J. M., He, W. L., Gao, J., & Yang, H. (2006). Journal of East China University
of Science and Technology (Natural Science Edition), Vol. 32, 160.