Callimatic and T-shaped liquid crystalline compounds with fluorinated terminal groups

Article abstract of DOI:10.1134/S1070363208090119

A series of liquid crystalline polyesters with a rigid T-shaped or a stick-like mesogenic fragment and fluorinated terminal groups of various structures was synthesized and studied by means of polarization optical microscopy, differential scanning calorim

Full text of DOI:10.1134/S1070363208090119

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 9, pp. 1707–1710. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.V. Zuev, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 9, pp. 1475–1478.
Callimatic and T-Shaped Liquid Crystalline Compounds
with Fluorinated Terminal Groups
V. V. Zuev
Institute of Macromolecular Compounds, Russian Academy of Sciences,
Bolshoi pr. 31, St. Petersburg, 199004 Russia
e-mail: zuev@hq.macro.ru
Received March 31, 2008
Abstract―A series of liquid crystalline polyesters with a rigid T-shaped or a stick-like mesogenic fragment
and fluorinated terminal groups of various structures was synthesized and studied by means of polarization
1
optical microscopy, differential scanning calorimetry, and IR and H NMR spectroscopy. It was shown that the
effect of the fluorinated group on the thermostability of the mesophase depends on the shape of the mesogenic
fragment. The thermostability of the mesophase in callimatic liquid crystalline compounds with perfluorinated
terminal groups is higher and in those with difluoromethyl terminal groups containing one hydrogen atom is
lower compared with their alkyl analogs. This is connected with the possibility of weak hydrogen bonding that
destroys the liquid crystalline order. The thermostability of the mesophase in compounds with a T-shaped
mesogenic fragment and any fluorinated terminal groups is always higher compared with their alkyl analogs.
DOI: 10.1134/S1070363208090119
The possibility of formation of the liquid crystalline
order in organic compounds is primarily controlled by
shape effects [1]. In this connection it seems attractive
to introduce such substituent as fluorine in liquid
crystalline compounds. The fluorine atom is the
smallest of all possible substituents, except for
hydrogen, and thus produces weak steric perturbations
in the parent hydrocarbon molecules. At the same time,
the C–F bond is very strong, and, therefore, fluorinated
compounds much more thermally stable than their
unsubstituted analogs. Fluorinated alkanes feature a
high conformational rigidity, what should stabilization
the mesophase in liquid crystalline compounds. The
low polarizability of fluorine reveals itself in
extremely low surface energies of perfluorinated
compounds, due to which they have found a broad
practical application [2]. Fluorinated substituents are
introduced in liquid crystals to modify and optimize
their optical, dielectric, and viscoelastic properties.
Such materials are used in displays and optoelectronic
devices [3]. The aim of the present work was to
synthesize and study the phase behavior of aromatic
oligoesters of a stick or T-shaped mesogenic fragment,
and assess the effect of fluorination of terminal alkyl
groups.
One of the promising methods for preparing low-
molecular and polymeric liquid crystalline compounds
involves constructing complex monomers containing a
potentially mesogenic fragment in its structure [4].
Such approach allows one to simplify synthetic
procedures associated with varying the molecular
structure in a series of compounds under investigation.
Terephthalic acid was chosen as a basis for con-
structing a stick-like mesogenic fragment, and trimellic
(
benzene-1,2,4-tricarboxylic) acid was used for
constructing a T-shaped one. Esterification of cor-
responding acid chlorides with p-hydroxybenzoic acid
yielded polyacids which were converted to acid
chlorides I and II. Monomer I was reacted with
fluorinated alcohols to synthesize a series of liquid
crystalline compounds III. Compound II was reacted
with fluorinated esters of p-hydroxybenzoic acid to
obtain a series of liquid crystalline compounds IV. For
preparing completely substituted esters, a double
excess of corresponding alcohols or phenols should be
used. The reaction proceeds very fast (5–20 min)
compared to alkyl analogs. It should be noted that if
this the reaction time is exceeded at least 2 times, the
complex monomer is almost completely destroyed.
1707

1
708
ZUEV
ClOC
COCl + 2 HO
COOH
COO
COOH
NaOH
HOOC
OOC
COO
COCl + 2 HOCH (CF CF ) R
2 2
2 n
SOCl2
ClOC
OOC
I
COO
COOCH (CF CF ) R
2 2
2 n
N(Et)3
R(CF CF ) CH OOC
OOC
2
2 n
2
III
COCl
COCl + 3 HO
ClOC
COOH
COO
COOH
NaOH
HOOC
OOC
COO
COOH
COO
COCl
PCl5
ClOC
OOC
+ 3 HO
COOCH (CF CF ) R
2 2
2 n
COO
COCl
N(Et)3
II
COO
COO
COOCH (CF CF ) R
2 2
2 n
R(CF CF ) CH OOC
OOC
OOC
2
2 n
2
COO
IV
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 9 2008
COO
COOCH (CF CF ) R
2 2 2 n

CALLIMATIC AND T-SHAPED LIQUID CRYSTALLINE COMPOUNDS
Properties of compounds III and IV
1709
–
1
a
–1 а
Compound
n
R
mp, °С
m
ΔH , J g
Т
i
, °С
i
ΔH , J g
b
b
b
b
b
c
b
b
IIIa
IIIb
IIIc
IIId
IVa
IVb
IVc
IVd
1
2
2
3
1
2
2
3
H
H
F
163 (130)
5.3 (65.2)
4.2 (72.0)
49.2 (72.0)
6.7 (74.5)
– (221)
– (1.1)
b
b
b
161 (150.5)
– (191.6)
– (10.5)
b
b
b
b
162.5 (150.5)
158.5 (148.5)
191.8 (191.6)
14.4 (10.5)
1.2 (11.3)
–
b
b
H
H
H
F
174 (175)
b
b
b
154 (–)
5.5 (–)
>300 (–)
b
b
b
b
142 (86.3)
7.4 (28.1)
203 (138)
1.8 (1.7)
b
b
b
b
144 (86.3)
13.5 (28.1)
12.1 (15.6)
217 (138)
2.1 (1.7)
b
b
b
b
H
118 (17)
192 (72.5)
1.2 (1.0)
a
b
T
i
and ΔH
i
are the temperature and enthalpy of isotropic transition. Parenthesized values relate to the corresponding alkyl analogs;
c
data for compounds III are taken from [8] and for compounds IV, from [7]. Monotropic smectic A at 160–153°C.
The liquid crystalline properties of compounds III–
III, observed in the polarization microscope, reveal a
IV were studied by means of polarization optical
microscopy and differential scanning calorimetry (see
the table). The isotropization temperature of callimatic
liquid crystalline compound IIIc containing a
perfluorinated alkyl substituent scarcely differs from
that of its alkyl analog. The mesophase type, too,
remains the same (smectic A, as judjed from the focal
conic texture observed in the polarization microscope).
The melting point and the isotropization enthalpy most
commonly increase in going from compounds with
alkyl terminal groups to their perfluorinated analogs
smectic-type mesophase. With compounds with a
perfluorinated terminal group, the transition tempera-
tures occur to be higher, but, in whole, the effect dif-
fers from that observed with calimatic-type com-
pounds. This observation can be explained by the fact
that the presence of weak H-bonds in compounds with
a 3D T-shaped mesogenic fragment does not prevent
packing in the liquid crystalline order, but this order
differs from that observed in callimatic liquid crystals
(3D rather than planar).
Hence, perfluorinated terminal groups in liquid
crystalline compounds always stabilize the mesophase,
while the effect of partially fluorinated groups may
vary, depending on the structure of the mesomorphic
compound and on the type of the liquid crystalline
order.
[
3]. The melting points increase with all the other
compounds III. At the same time, the isotropization
temperature of compound IIId with a long (heptyl)
fluorinated substituent is practically the same as that of
the alkyl analog, compound IIIb exhibits no liquid
crystalline properties, and compound IIIa has only a
monotropic mesophase in the temperature range 160–
EXPERIMENTAL
1
53°C. Hence, the appearance of a single hydrogen
atom in the perfluorinated methyl group e destabilizes
the mesophase. An analogous effect was observed
previously by Cowling et al. [5]. A possible explana-
tion is as follows. As known, the C–H bond incur-
porating the carbon atom that bears simultaneously
fluorine atoms is strongly polarized and thus becomes
capable of forming weak H-bonds with neighboring
fluorinated substituents [6]. Such bonding evidently
prevents formation of the liquid crystalline order by
disturbing the parallel location of stick-like molecules.
1
The H NMR spectra were taken on a Bruker
Avance 400 (400 MHz) NMR spectrometer. The IR
spectra were recorded on a Bruker Vertex device. The
phase-transition temperatures were measured on a
polarizing microscope equipped with a Boetius hot
stage and on a Shimadzu 60 differential scanning
microcalorimeter at the heating rate 2–5°C/min.
4
-n-Fluoroalkyl p-hydroxybenzoates were obtained
by azeotropic esterification from corresponding al-
cohols, using sulfuric acid as catalyst and carbon
tetrachloride as solvent. Acid chloride I was obtained
by the procedure in [4], and compound II was
synthesized according to [7].
A different picture is observed with compounds IV
with a T-shaped mesogenic fragment. The isotro-
pization temperature is always increased, i.e. the
mesophase is stabilized. The textures of compounds
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 9 2008

1
710
ZUEV
Bis[4-(1,1-dihydroperfluoropentyl)oxycarbonyl-
J 8.08); oxybenzoate fragment: 8.31 d (4H, ortho, J
8.08), 8.35 d (2H, ortho, J 8.08), 7.45 d (4H, meta, J
8.08), 7.47 d (2H, meta, J 8.08); terminal oxybenzoate
fragment: 7.32 d (6H, ortho, J 8.08), 8.13 d (6H, meta,
phenyl]benzene-1,4-dicarboxylate (IIIc). A mixture
of 0.5 g of acid chloride I, 0.8 g of 1,1-dihydro-
perfluoropentyl alcohol, 0.4 g of triethylamine, and 15 ml
of dry tetrachloroethane was vigorously stirred for
J 8.08), 4.85 t (6H, OCH , J 13.3). Found, %: C 56.80;
2
1
5 min on a magnetic stirrer. The resulting suspension
H 2.10; F 20.01. C H O F . Calculated, %: C 56.67;
6
6
33 18 15
poured into 150 ml of ethanol. The precipitate that
formed was filtered off, crystallized from ethanol, and
dried in a vacuum. Yield 0.55 g (76%), white powder.
H 2.38, F 20.37.
ACKNOWLEDGMENTS
–
1
IR spectrum (KBr), ν, cm : 3103, 3071, 3057, 2892,
1
1
736 (C=O), 1603, 1504, 1433, 1416, 1277, 1200,
165, 1141, 1119, 1077, 1019, 971, 880, 702. H NMR
The work was financially supported by the Russian
Foundation for Basic Research (project no. 06-08-
00195a).
1
spectrum (CDCl ), δ, ppm (J, Hz): 8.36 s (4H), 8.18 d
3
(
4H, J 8.8), 7.39 d (4H, J 8.8), 4.85 t (4H, J 13.3).
REFERENCES
Found, %: C 53.45; H 2.50; F 30.01. C H O F .
3
2
16
8 10
Calculated, %: C 53.50; H 2.24; F 29.44.
1
2
.
.
Handbook of Liquid Crystals, Demus, D., Goodby, J.,
Gray, G.-W., Spiess, H.-W., and Vill, V., Eds., Wein-
heim: Wiley–VCH, 1998.
The other compounds III and IV were synthesized
analogously in 45–75% yields.
Ishikawa, N. and Kobayashi, E., Fluorine Compounds,
Tokyo: Kodansha Scientific, 1982. Translated under the
title Soedineniya ftora. Sintez i primenenie, Moscow:
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Bis[4-(1,1,5-trihydroperfluoropentyl)oxycarbonyl-
phenyl]benzene-1,4-dicarboxylate (IIIb). IR spec-
–
1
trum (KBr), ν, cm : 3103, 3071, 3057, 2892, 1736
(C=O), 1603, 1504, 1433, 1416, 1277, 1200, 1165,
1
3.
Hird, M., Chem. Soc. Rev, 2007, vol. 36, no. 12, p. 2070.
1
141, 1119, 1077, 1019, 971, 880, 702. H NMR
spectrum (CDCl ), δ, ppm (J, Hz): 8.36 s (4H), 8.18 d
4. Zuev, V.V. and Skorokhodov S.S., Vysokomol. Soedin.,
Ser. B., 1987, vol. 29, no. 6, p. 440.
3
(
4H, J 8.8), 7.39 d (4H, J 8.8), 6.08 t.t (2H, J 51.9, J
1 2
5
2
2
.4), 4.85 t (4H, J 13.5). Found, %: C 58.95; H 2.70; F
5
6
7
.
.
.
Cowling, S.J., Hall, A.W., Coodby, J.W., Wang, Y., and
Gleeson, H.F., J. Mater. Chem., 2006, vol. 16, no. 11,
p. 2181.
2.81. C H O F . Calculated, %: C 58.02; H 2.74; F
32
18
8 8
2.94.
Tris[4-[4-(1,1-dihydroperfluoropentyl)oxycarbonyl-
phenoxycarbonyl]phenyl]benzene-1,2,4-tricarboxy-
Caminati, W., Melandri, S., Moreschini, P., and Fave-
ro, P.G., Angew. Chem., Int. Ed., 1999, vol. 38, no. 13,
p. 2924.
–
1
late (IVc). IR spectrum (KBr), ν, cm : 3103, 3080,
3
1
spectrum (CDCl ), δ, ppm (J, Hz): 1,2,4-benzene
fragment: 8.91 s (1H), 8.60 d (1H, J 8.08), 8.17 d (1H,
Zuev, V.V., Zh. Obshch. Khim., vol. 78, no. 8, p. 1324.
057, 2982, 1743 (C=O), 1603, 1508, 1415, 1359,
1
263, 1235, 1160, 1062, 1014, 919, 843, 756. H NMR
8. Leblanc, J.P., Tessier, M., Judas, D., Friedrich, C., Noel, C.,
and Mareshal, E., Macromolecules, 1993, vol. 26, no. 17,
p. 4391.
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 9 2008