Dipole architecture of molecules and mesomorphic behavior of liquid crystals with rigid T-shaped mesogenic fragment

Article abstract of DOI:10.1134/S107036320808015X

A number of liquid crystalline polyesters having a rigid T-shaped mesogenic fragment and differing in dipole architecture were synthesized and examined by polarizing optical microscopy, differential scanning calorimetry, and IR and ¹H NMR spectroscopy. The thermal stability of the mesophase in the given series of compounds was shown to increase with extension of arms in the T-shaped mesogenic fragment, as well as on replacement of the terminal ester groups (COOAlk) therein by ether moieties (OAlk). Such replacement also enhances smectogenic properties.

Full text of DOI:10.1134/S107036320808015X

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 8, pp. 1559–1564. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.V. Zuev, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 8, pp. 1324–1329.
Dipole Architecture of Molecules and Mesomorphic Behavior
of Liquid Crystals with Rigid T-Shaped Mesogenic Fragment
V. V. Zuev
Institute of High-Molecular Compounds, Russian Academy of Sciences,
Bol’shoi pr. 31, St. Petersburg, 199004 Russia
e-mail: zuev@hq.macro.ru
Received January 31, 2008
Abstract—A number of liquid crystalline polyesters having a rigid T-shaped mesogenic fragment and differing
in dipole architecture were synthesized and examined by polarizing optical microscopy, differential scanning
1
calorimetry, and IR and H NMR spectroscopy. The thermal stability of the mesophase in the given series of
compounds was shown to increase with extension of arms in the T-shaped mesogenic fragment, as well as on
replacement of the terminal ester groups (COOAlk) therein by ether moieties (OAlk). Such replacement also
enhances smectogenic properties.
DOI: 10.1134/S107036320808015X
Development of synthetic methods initiates a
discovery of new liquid crystalline compounds whose
molecular structure is untypical of common liquid
crystals. The liquid crystalline order is controlled by
fine balance between the molecular and dipole
architecture of molecules. I previously synthesized a
series of compounds having a rigid T-shaped
mesogenic fragment [1]. The goal of the present work
was to synthesize aromatic oligoesters with T-shaped
mesogenic fragment and to study their phase behavior
and its dependence on the molecular and dipole
architecture. The results of calculations in terms of the
extended Flory–Huggins lattice theory showed that
appearance of liquid crystalline order in T-shaped
mesogenic systems was possible only when each arm
of the mesogenic fragment itself was capable of acting
as mesogen in common (calamitic) liquid crystals [2].
Therefore, one step of the study was to elucidate the
effect of the size of the arm in the mesogenic fragment
on the mesomorphic behavior of such compounds.
An appropriate method for building up complex
monomeric structures having a potential mesogenic
fragment was described in [3]. This approach makes it
possible to simplify synthetic routines related to
variation of molecular structure in the series of
compounds under study. Following this approach,
trimellitic (benzene-1,2,4-tricarboxylic) acid was
Scheme 1.
COCl
COOH
ClOC
COCl + 3
HO
COO
COOH
NaOH
OOC
HOOC
COO
COOH
I
1559

1560
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Scheme 1. (Contd.)
COO
COCl
COO
PCl5
R
+ 3 HO
OOC
ClOC
COO
COCl
COO
II
COO
R
N(Et)3
OOC
OOC
R
COO
R
III−V
O(CH2)nCH3, n = 4–7; IV, R = O(CH2)nCH3, n = 4–7; V, R = COO(CH2)nCH3, n = 4–9.
III, R
Scheme 2.
selected as a base fragment. Esterification of the
corresponding acid chloride with p-hydroxybenzoic
acid gave compound I which was treated with
phosphorus pentachloride to obtain the desired
monomer II. The latter was brought into reactions with
various phenols, and a series of liquid crystalline
compounds III–V with different molecular and dipole
architectures was thus synthesized (see Scheme 1).
COO
COO
O
O
ClOC
OOC
O
VI
The above procedure for the synthesis of
compounds III–V is specific due to the use of
phosphorus pentachloride instead of more convenient
thionyl chloride. The reason is that treatment of II (as
well as of trimellitic acid [4]) with thionyl chloride
gives anhydride VI rather than the desired tricarbonyl
trichloride (Scheme 2).
The properties of compounds III–V as liquid
crystals were examined by polarizing optical
microscopy and differential scanning calorimetry (see
table). Liquid crystalline 1,2,4-tris(4'-alkoxybiphenyl-
4-yloxycarbonyl)benzenes of the general formula VII
were studied previously [1] (Scheme(3).
Scheme 3.
OCnH2n+1
COO
COO
OOC
OCnH2 +1
CnH2n+1
n
VII, n = 5–8.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 8 2008

DIPOLE ARCHITECTURE OF MOLECULES AND MESOMORPHIC
Properties of compounds III–V
1561
The arm of the mesogenic fragment in molecules
III is longer than that in VII by a benzoate moiety. The
melting points of compounds III increase by 35–40°C
as compared to VII, and the temperature range of their
liquid crystalline phase sharply extends, so that no
transition to the isotropic state is observed up to
decomposition temperature of III, i.e., above 300°C.
Thus the extension of the arm in the mesogenic
fragment by a benzoate moiety increases the thermal
stability of the mesophase by more than 150°C.
Analogous enhancement of the thermal stability of
mesophase with rise in the number of para-linked
benzene rings in the mesogenic fragment is typical of
calamitic liquid crystals [5]. All compounds III show
smectic mesomorphism, as follows from the confocal
textures of the liquid crystalline phase observed using
polarizing optical microscope. This means that
extension of arms in T-shaped mesogenic fragment
either leads to loss of nematic mesomorphism or the
transition temperature to the nematic phase falls
beyond the thermal stability range of compounds III.
Comp.
no.
ΔHm,
ΔHi,a
mp, °C
Ti,a °C
n
J g–1
J g–1
4
5
6
7
4
5
6
7
3
4
5
6
7
8
9
173.9
175.2
174.1
172.0
127.0
125.0
121.3
118.0
91.2
5.0
6.7
–
–
–
III
IV
V
–
5.6
–
–
5.8
–
–
27.5
28.6
26.4
26.2
29.4
28.1
20.1
15.6
18.3
5.2
190.7b
188.0c
187.0
188.9
124.0
138.0
144.0
72.5
152.0
–
2.6
2.8
2.1
3.0
1.8
1.7
1.2
1.0
2.6
–
86.3
74.2
17.0
68.0
In the next step of the study, the effect of dipole
architecture of T-shaped mesogenic fragments on their
mesomorphic behavior was examined. For this
purpose, the structure of compounds VII was modified
via introduction of an ester group between the benzene
rings in the arms (compounds IV). In going from VII
to IV, the length of the arm changes relatively slightly,
but the latter contain a group possessing a dipole
moment directed at an angle of ~55° with respect to
the principal molecular axis of the arm. The result is
that the melting points of IV are lower by 15–20°C
than those of analogous compounds VII (see table),
and the temperature of transition into the isotropic
phase increases by about 50°C. In other words,
introduction of additional dipolar components into the
T-shaped mesogenic fragment (as well as increase of
its polarizability) enhances the thermal stability of the
mesophase, and reduction of the melting point (due to
lower molecular symmetry) leads to considerable
extension of the temperature range of liquid crystalline
state. Increased polarizability and dipole moment of
the mesogenic fragment in IV strongly destabilize
nematic mesophase of these compounds as compared
to structures VII. As follows from the data of
polarizing microscopy and differential scanning
calorimetry, nematic phase is observed only in a
temperature range of about 1°C before transition to
isotropic phase for n-pentyl and n-hexyl derivatives
IV. These compounds displayed schlieren textures
13.0
16.0
4.3
92.0
0.5
a
Ti and ΔHi stand, respectively for the temperature and enthalpy
of transition into isotropic state. Smectic–nematic phase transi-
tion temperature 189.6°C. Smectic–nematic phase transition
temperature 186.8°C.
b
c
with singularities of 1/2 and 1, which is typical of
nematics [6] (see figure, a), while confocal textures
typical of smectics [6] were observed in the other
ranges of liquid crystalline state (see figure, b). Thus
increase in the polarizability and dipole constituent of
the arms in the T-shaped mesogenic fragment enhances
the smectogenity and thermal stability of the
mesophase.
The phase behavior and thermal stability of
calamitic liquid crystals are largely determined by the
nature of the terminal groups [5]. With a view to reveal
analogous relations in the series of T-shaped
mesogens, the terminal alkoxy groups in IV were
replaced by ester moieties (compounds V). It turned
out that such replacement leads to lowering of both
melting points and temperatures of transition into
isotropic state (see table). All compounds V showed
only smectic mesophase. An analogous pattern is
observed for calamitic liquid crystals: terminal ester
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 8 2008

1562
ZUEV
(a)
(b)
50 μm
Textures of compound IV (n = 5) at (a) 190 and (b) 180°C.
groups give rise to pronounced smectogenity as
compared to alkoxy groups and reduce themal stability
of the mesophase. It is interesting that the
isotropization temperature of compounds V increases
with extension of the alkyl substituent, reaching the
maximum value at n = 7. Here, unlike compounds III
and IV, a strong odd–even effect of n on the phase
transition temperature is observed. Some compounds
V (n = 6, 9) form liquid crystalline state even at room
temperature.
polarizing microscope, as well as by differential
scanning calorimetry on a Shimadzu 60 microcalori-
meter at a heating rate of 2–5 deg min–1.
4-n-Alkoxyphenols and 4'-n-alkoxybiphenyl-4-ols
were synthesized by the Williamson reaction from the
corresponding alkyl bromides and hydroquinone or
biphenyl-4,4'-diol in the presence of potassium
carbonate [7]. n-Alkyl 4-hydroxybenzoates were
obtained by esterification of 4-hydroxybenzoic acid
with the corresponding alcohols in benzene in the
presence of sulfuric acid with simultaneous removal of
the liberated water as azeotrope. Benzene-1,2,4-
tricarbonyl trichloride was prepared according to the
procedure described in [4].
Thus the results of the present study showed that
the phase behavior of T-shaped liquid crystals
generally follows the relations known for calamitic
systems: extension of the arms in the T-shaped
mesogenic fragment leads to enhanced smectogenity
and increases the thermal stability of the mesophase.
The smectogenity also increases with rise in the
polarizability and dipole constituent of the arms.
Increase in the number of ester groups in the
mesogenic fragment (and hence increase in the lateral
constituent of the dipole moment) makes the
mesophase less thermally stable.
1,2,4-Tris(4-carboxyphenyloxycarbonyl)benzene
(I). A solution of 5 g of benzene-1,2,4-tricarbonyl
trichloride in 30 ml of anhydrous carbon tetrachloride
was added dropwise over a period of 15 min under
vigorous stirring to a solution of 12 g of 4-
hydroxybenzoic acid in 150 ml of 0.1 N aqueous
sodium hydroxide. The mixture was stirred for 3 h and
adjusted to pH 8 by adding 0.1 N hydrochloric acid.
The precipitate was filtered off, washed with water, 0.1
N hydrochloric acid, and water again, and dried under
reduced pressure. Yield 7.5 g (66%), white powder
which did not melt up to decomposition point (300°C).
1H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 1,2,4-
C6H3: 8.79 s (1H), 8.56 d (1H, J = 7.85), 8.31 d (1H, J
= 7.85); OC6H4COOH: 7.41–7.45 m (4H, m-H), 7.50 d
(2H, m-H), 8.02–8.07 m (6H, o-H), 10.29 s (OH). IR
EXPERIMENTAL
1
The H NMR spectra were recorded on a Bruker
MSL 400 spectrometer at 400 MHz. The IR spectra
were measured on a Bruker Vertex instrument. The
phase transition temperatures were determined on a
melting point apparatus coupled with a Boëtius
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 8 2008

DIPOLE ARCHITECTURE OF MOLECULES AND MESOMORPHIC
1563
spectrum (KBr), ν, cm–1: 3400–3100, 2869, 2860, 1740
(C=O), 1608, 1496, 1455, 1398, 1286, 1049, 1001,
937, 806, 742. Found, %: C 63.45; H 3.50. C30H18O12.
Calculated, %: C 63.16; H 3.18.
H, J = 8.08), 6.98 d (4H, m-H, J = 8.08), 6.93 d (2H,
m-H, J = 8.08); AlkOC6H4O: 7.11 d (6H, o-H, J =
8.08), 7.44 d (6H, m-H, J = 8.08), 3.98 t (6H, OCH2),
1.79–1.80 m (6H), 1.60–1.67 m (6H), 1.39–1.47 m
(6H), 0.95 t (9H). IR spectrum (KBr), ν, cm–1: 3039,
2927, 2854, 1740 (C=O), 1604, 1498, 1441, 1269,
1251, 1197, 1160, 1072, 1001, 917, 802, 756. Found,
%: C 71.87; H 5.90. C63H60O15. Calculated, %: C
71.58; H 5.72.
1,2,4-Tris(4-chloroformylphenyloxycarbonyl)ben-
zene (II). A mixture of 5 g of compound I and 10 g of
phosphorus pentachloride was heated for 5 h at 140°C.
Phosphoryl chloride was distilled off, and the residue
was recrystallized from chloroform. Yield 4.85 g
(88%), yellow crystals, mp 126°C. 1H NMR spectrum
(CDCl3), δ, ppm (J, Hz): 1,2,4-C6H3: 8.86 s (1H), 8.56
d (1H, J = 8.14), 8.15 d (1H, J = 8.14); OC6H4COOH:
8.28–8.35 m (4H, o-H, J = 8.7), 8.11–8.17 (2H, o-H, J =
8.7), 7.43 d (4H, m-H, J = 8.8), 7.46 d (2H, m-H, J =
8.8). IR spectrum (KBr), ν, cm–1: 2869, 2860, 2782,
1750 (C=O), 1608, 1496, 1455, 1398, 1286, 1251,
1226, 1201, 1049, 1001, 937, 806, 742. Found, %: C
57.40; H 2.70; Cl 17.23. C30H15O9Cl3. Calculated, %:
C 57.58; H 2.42; Cl 17.00.
The other compounds of series IV were synthesized
in a similar way; yield 85–90%.
1,2,4-Tris[4-(4-n-pentyloxycarbonylphenyloxycar-
bonyl)phenyloxycarbonyl]benzene (V, n = 4) was
synthesized as described above for compound III (n =
1
4). Yield 88%, white crystals. H NMR spectrum
(CDCl3), δ, ppm (J, Hz): 1,2,4-C6H3: 8.90 d (1H, J =
1.64), 8.50 d.d (1H, J = 8.08, 1.64), 8.17 d.d (1H, J =
8.08, 1.64); OC6H4CO: 8.30 d (4H, o-H, J = 8.08),
8.35 d (2H, o-H, J = 8.08), 7.45 d (4H, m-H, J = 8.08),
7.47 d (2H, m-H, J = 8.08); AlkOC6H4CO: 7.32 d (6H,
1,2,4-Tris[4-(4′-n-pentyloxybiphenyl-4-yloxycar-
bonyl)phenyloxycarbonyl]benzene (III, n = 4). A o-H, J = 8.08), 8.13 d (6H, m-H, J = 8.08), 4.33 t (6H,
solution of 210 mg of trichloride II, 50 mg of 4'-
pentyloxybiphenyl-4-ol, and 0.2 ml of triethylamine in
10 ml of anhydrous dichloroethane was stirred for 5 h
at room temperature using a magnetic stirrer. The
solution was poured into 50 ml of ethanol, and the
precipitate was filtered off, dried, and reprecipitated
once again from chloroform into ethanol. Yield 350 mg
OCH2), 1.73–1.80 m (6H), 1.60–1.67 m (6H), 1.39–
1.47 m (6H), 0.95 t (9H). IR spectrum (KBr), ν, cm–1:
3076, 2960, 2933, 2873, 1742, 1717(C=O), 1602,
1506, 1466, 1414, 1385, 1260, 1227, 1197, 1160,
1072, 1001, 917, 802, 756. Found, %: C 69.80; H 5.10.
C66H60O18. Calculated, %: C 69.46; H 5.30.
The other compounds of series V were synthesized
in a similar way; yield 85–90%.
1
(90%), white crystals. H NMR spectrum (CDCl3), δ,
ppm (J, Hz): 1,2,4-C6H3: 8.88 s (1H), 8.56 d (1H, J =
8.08), 8.31 d (1H, J = 8.08); OC6H4CO: 8.28–8.36 m
(4H, o-H), 8.11–8.17 d (2H, o-H), 7.38 d (6H, m-H, J
8.08); AlkOC6H4C6H4: 6.86–7.01 m (12H, o-H), 7.30–
7.35 m (6H, m-H), 7.40–7.65m (6H, m-H), 4.0 t (6H,
OCH2), 1.79–1.80 m (6H), 1.60–1.67 m (6H), 1.39–
1.47 m (6H), 0.95 t (9H). IR spectrum (KBr), ν, cm–1:
3039, 2927, 2854, 1740 (C=O), 1604, 1498, 1441,
1269, 1251, 1197, 1160, 1072, 1001, 917, 802, 756.
Found, %: C 75.40; H 5.70. C81H72O15. Calculated, %:
C 75.68; H 5.65.
4-Chloroformylphenyl 3,5,8,10-tetraoxo-2,6,9-
trioxa-1,7(1,4),4(1,2)-tribenzenacyclodecaphane-44-
carboxylate (VI). A mixture of 5 g of compound I and
50 ml of thionyl chloride was heated for 5 h under
reflux. Excess thionyl chloride was distilled off, and
the residue was recrystallized from chloroform. Yield
1
4.60 g (83%), yellow crystals, mp 147°C. H NMR
spectrum (DMF-d7), δ, ppm (J, Hz): 1,2,4-C6H3: 8.91 s
(1H); 8.65 d (1H, J = 7.85); 8.41 d (1H, J = 7.85);
ClCOC6H4O: 8.04 d (2H, o-H, J = 8.08), 6.97 d (2H,
m-H, J = 8.08); C6H4COOCOC6H4: 8.15 d (4H, o-H, J =
The other compounds of series III were synthesized
in a similar way; yield 85–90%.
13
8.08), 7.56 d (4H, m-H, J = 8.08). C NMR spectrum
(DMF-d7), δC, ppm: 167.74 (COCl); 167.10, 167.04
(CO, anhydride); 165.31, 164.69, 164.40 (C=O);
154.73, 154.52, 154.47; 134.47, 133.09, 132.77,
132.34, 132.03, 131.69, 131.62, 131.53, 131.43,
129.89, 129.73, 129.03, 122.65, 122,57, 122.43,
122.33, 116.24, 115.66. IR spectrum (KBr), ν, cm–1:
2869, 2860, 1760 (C=O), 1603, 1504, 1455, 1398,
1286, 1049, 1001, 937, 744. Found, %: C 63.40;
1,2,4-Tris[4-(4-n-pentyloxyphenyloxycarbonyl)
phenyloxycarbonyl]benzene (IV, n = 4) was
synthesized as described above for compound III (n = 4).
Yield 87%, white crystals. 1H NMR spectrum (CDCl3),
δ, ppm (J, Hz): 1,2,4-C6H3: 8.89 d (1H, J = 1.64), 8.58
d.d (1H, J = 8.08, 1.64), 8.16 d.d (1H, J = 8.08, 1.64);
OC6H4CO: 8.28 d (4H, o-H, J = 8.08), 8.33 d (2H, o-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 8 2008

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ZUEV
Makromol. Chem., Rapid Commun., 1991, vol. 12, no. 3,
p. 151.
N2.70; Cl 6.23. C30H15O10Cl. Calculated, %: C 63.12;
H 2.65; Cl 6.21.
3. Zuev, V.V. and Skorokhodov, S.S., Vysokomol. Soedin.,
Ser. B, 1987, vol. 29, no. 6, p. 440.
ACKNOWLEDGMENTS
4. Drechsler, G. and Heidenreich, S., J. Prakt. Chem., 1965,
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 06-08-00195a).
vol. 27, no. 1, p. 163.
5. Handbook of Liquid Crystals, Demus, D., Goodby, J.,
Gray, G.W., Spiess, H.-W., and Vill, V., Eds., Weinheim:
Wiley, 1998.
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