Effects of nonmesomorphic substituted benzonitriles on the dielectric properties of cyanobiphenyl liquid crystals

Article abstract of DOI:10.1134/S0036024408070285

The effects of the cyano-substituted aromatic nonmesomorphic additives N-(3,4-dicyanophenyl)-4'-nonyloxybenzamide (I), 3,4-dicyanophenyl-4'- nonyloxybenzoate (II), 3,4-dicyanophenyl-3',4',5'-tris-(4 - undecyloxyphenylbenzoate)benzoate (III), and 4-(6-hydroxyhexyloxy)benzonitrile (IV) on the dielectric properties of 4-hexyloxy-4 -cyanobiphenyl and a liquid-crystal mixture E3 were studied. Dielectric anisotropy, birefringence, and orientational ordering were studied to show that monocyano derivative IV most strongly increased the dielectric anisotropy of mesomorphic compositions because of its effective incorporation into the liquid crystal matrix.

Full text of DOI:10.1134/S0036024408070285

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 7, pp. 1211–1214. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © E.V. Aleksandriiskaya, S.A. Kuvshinova, I.V. Novikov, V.V. Aleksandriiskii, T.V. Tararykina, V.E. Maizlish, V.A. Burmistrov, 2008, published in Zhurnal
Fizicheskoi Khimii, 2008, Vol. 82, No. 7, pp. 1364–1367.
OTHER PROBLEMS
OF PHYSICAL CHEMISTRY
Effects of Nonmesomorphic Substituted Benzonitriles
on the Dielectric Properties of Cyanobiphenyl Liquid Crystals
E. V. Aleksandriiskayaa, S. A. Kuvshinovaa, I. V. Novikova, V. V. Aleksandriiskiia, b,
T. V. Tararykinaa, V. E. Maizlisha, and V. A. Burmistrova, b
a Ivanovo State University of Chemical Technology, Ivanovo, Russia
b Institute of the Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
e-mail: NMR@isuct.ru
Received February 8, 2007
Abstract—The effects of the cyano-substituted aromatic nonmesomorphic additives N-(3,4-dicyanophe-
nyl)-4'-nonyloxybenzamide (I), 3,4-dicyanophenyl-4'-nonyloxybenzoate (II), 3,4-dicyanophenyl-3',4',5'-
tris-(4''-undecyloxyphenylbenzoate)benzoate (III), and 4-(6-hydroxyhexyloxy)benzonitrile (IV) on the
dielectric properties of 4-hexyloxy-4'-cyanobiphenyl and a liquid-crystal mixture E3 were studied. Dielec-
tric anisotropy, birefringence, and orientational ordering were studied to show that monocyano derivative
IV most strongly increased the dielectric anisotropy of mesomorphic compositions because of its effective
incorporation into the liquid crystal matrix.
DOI: 10.1134/S0036024408070285
INTRODUCTION
,
CN
R
Because of their unique properties (fluidity com-
bined with anisotropy of physical properties), liquid
crystals (LCs) have found wide use in various fields of
engineering, research, and technology. LCs hold spe-
cial position as materials used in optoelectronic devices
for mapping information. The problem of reducing
control voltages by decreasing the threshold of LC
reorientation is a challenge. It can be solved by
increasing dielectric anisotropy with maintaining
other service characteristics at an acceptable level [1].
These problems cannot be solved by methods of
molecular design alone because the set of potential
synthons (fragments) for the design of rodlike
mesogens (rings, bridging groups, and lateral and ter-
minal substituents) is limited [2]. Profound modifica-
tion of mesomorphic and physical properties can also
be achieved via selective doping with special addi-
tives, both liquid-crystalline and nonmesomorphic
[3]. Thus, it was shown [4] that the anisotropy of
dielectric permittivity could be significantly increased
without decreasing viscosity and elasticity character-
istics by introducing small amounts of high-polarity
nonmesogenic dinitriles. However, the effects of polar
nonmesogens on the physical characteristics of LC
compositions have not been investigated.
where R = C6H13O (4-hexyloxy-4'-cyanobiphenyl,
6OCB) (Cr 54°C 76°C I) and R =
C5H11 (55%), C5H11O (15%), C7H15O (13%), C8H17O
(17%) in a liquid-crystal mixture based on cyanobi-
phenyls E3 that forms the nematic phase over a wide
N
temperature range [5] (Cr
–2°C
N
56°C I). N-(3,4-dicyanophenyl)-4'-nonyloxyben-
zamide (I), 3,4-dicyanophenyl-4'-nonyloxybenzoate (II),
3,4-dicyanophenyl-3',4',5'-tris-(4''-undecyloxyphenyl-
benzoate)benzoate (III), and 4-(6-hydroxyhexyl-
oxy)benzonitrile (IV) were used as modifiers:
CN
C9H19O
CONH
(I)
CN
CN
CN
C9H19O
COO
(II)
R'
CN
The aim of the present work was to study the effects
of small amounts of cyano-substituted aromatic non-
mesomorphic admixtures on the dielectric properties of
the nematic LC
R'
COO
CN
(III)
R'
1211

1212
ALEKSANDRIISKAYA et al.
Table 1. Dipole moments of LCs and nonmesogenic sub-
stances
ch. d. a. (pure for analysis) grade were used without
additional purification.
The dipole moments (µ) of the mesogen molecules
were calculated using the Guggenheim–Smith equation
[7] and the experimental permittivities and refraction
indices of dilute solutions of LCs in benzene at 25°C.
The error in dipole moments did not exceed 0.01D. Per-
mittivities were measured at a frequency of 10 kHz
using an LCR-817 (INSTEK) instrument in a tempera-
ture-controlled ( 0.01 K) plane parallel cell with a gap
of 0.2 mm in a magnetic field of 0.2 T. The error in ε did
not exceed 0.02.
Compound
µ, D
Compound
µ, D
6OCB
5.4
5.1 [5]
9.8
II
8.3
9.9
5.3
E3
I
III
IV
R' =
C11H23O
COO
The order parameter was determined as described
in [8] from the 1H NMR spectra obtained on a Bruker
AC-200 spectrometer equipped with a BVT3000 tem-
perature accessory. Birefringence ∆n = n|| – n⊥ was
measured by refractometry at a wavelength of 589 nm
to an accuracy of 0.02. The refractive indexes of the
ordinary beam (no = n⊥) in mesomorphic and isotropic
liquid states (nis) were directly measured on an Abbe
temperature-controlled refractometer at 589 nm
( 0.0005). The prism surface of the refractometer was
preliminarily polished for sample orientation with an
LC surface. The refractive index of the extraordinary
beam (ne = n||) was determined from the equation for
HO(CH2)6O
CN
(IV)
EXPERIMENTAL
Compounds I–III were prepared and purified by a
procedure similar to that described in [6]; 4-(6-hydrox-
yhexyloxy)benzonitrile (IV) was synthesized by boil-
ing 4-hydroxybenzonitrile and 6-chloro-1-hexanol in
DMF in the presence of potassium carbonate. The prod-
uct was purified by recrystallization from a mixture of
ethanol and water. 4-n-Alkoxy-4'-cyanobiphenyls of
the mean value, n2 = (ne2 + 2n2o )/3, evaluated by
extrapolating nis to the nematic phase region taking
into account the temperature dependence of the den-
sity of LC.
ε
RESULTS AND DISCUSSION
Since the dielectric properties of LCs depend
strongly on the polarity of nonmesomorphic modifiers,
we used the second Debye method [7] to measure
the dipole moments of compounds I–IV presented in
Table 1. An analysis of these data shows that the intro-
duction of the second nitrile substituent into the aro-
matic ring is accompanied by an increase in the polarity
of the nonmesogen. Figure 1 shows the temperature
dependences of the components of the permittivity of
the LC mixture E3 and compositions based on it con-
taining 3% admixtures I–IV in the nematic and isotro-
pic phases. Since the introduction of nonmesomorphic
admixtures changes phase transition temperatures, we
used the scale of reduced temperatures (íred = í – íNI),
which represent deviations from the nematic–isotropic
phase transition point.
ε||
20
15
10
5
1
2
3
4
5
εI
ε⊥
An analysis of the data presented in Fig. 1 shows
that admixtures have a pronounced effect on the permit-
tivity of LC. In the isotropic phase, phenyl benzoate II
was the only compound that produced an appreciable
effect, while in the nematic phase, all modifiers
increased both permittivity components, ε⊥ and ε|| (the
latter increased to a greater extent). This led to an
increase in the dielectric anisotropy ∆ε = ε|| – ε⊥ (Fig. 2)
in the series IV > II, III > I. A comparison of these
–40
–20
0
20
Tred
Fig. 1. Dependences of the components of dielectric permit-
tivity on reduced temperature for (1) the E3 mixture and its
modifications (E3 + 3% modifier: (2) I, (3) II, (4) III, and
(5) IV) on reduced temperature.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 7 2008

EFFECTS OF NONMESOMORPHIC SUBSTITUTED BENZONITRILES
1213
∆ε
results with the data from Table 1 shows that the
effect of the nonmesomorphic compounds on the
dielectric properties of LCs does not correlate with
their polarity. Moreover, monocyano derivative IV,
which had the lowest dipole moment, increased the
dielectric anisotropy of the LC mixture to the great-
est extent (Fig. 2).
15
5
4
3
2
Therefore, it is of interest to consider the behavior of
the mean permittivity ε = (ε|| + 2ε⊥)/3 of the mesophase
and the εextr values extrapolated from the isotropic liq-
1
10
uid state. As is known, ε < εextr [9] for the cyano deriv-
atives of LCs [9] because of the antiparallel dipole–
dipole association of polar molecules. An analysis of
Table 2 shows that the addition of nonmesogens I and
IV decreases δε = εextr – ε . This is evidence of a
decrease in the degree of association of cyanobiphenyls
in the mesophase. At the same time, for nonmesogens
II and III, δε changes sign.
5
–40
–30
–20
–10
0
Tred
The unexpectedly strong effect of low-polarity IV
on the dielectric properties of the LC mixture can be
attributed to an increase in orientational ordering of the
mixture caused by doping or to a change in the pre-
ferred orientation of dipoles. Measurements of the ori-
entation order parameters (S) of mixed LCs involves
serious difficulties [8], and we therefore obtained the
temperature dependences of S for pure LC 6OCB and
its mixture with nonmesogen IV (3%) and the temper-
ature dependences of birefringence (Fig. 3). The order
parameters were measured by NMR, and the refractive
indexes, by refractometry.
Fig. 2. Dependences of dielectric anisotropy on reduced
temperature for the system based on E3 ((1–5) see Fig. 1
for notation).
without destabilizing it, and, secondly, an increase in
orientational order cannot be the reason for a substan-
tial increase in dielectric anisotropy.
Modifier IV is the only nonmesogen that has a pro-
ton-donor hydroxyl group and can therefore in princi-
An analysis of these data shows that admixture ple participate in specific interactions with electron-
effects on the above-mentioned characteristics are very donor fragments, primarily with the cyano groups of
insignificant. This is evidence that, first, the nonme- LCs. In our opinion, the formation of the hydrogen-
sogen is effectively incorporated into the LC matrix bonded complexes
R
CN HO (CH2)6
O
CN
can lead to the following important effects. The for- accompanied by an increase in the net dipole moment
mation of a supermolecule of this kind should be along the optical axis of an LC and, accordingly, an
Table 2. Dielectric properties of LC compositions at Tred = –10 K
LC
ε
εextr
δε
ε||
ε⊥
∆ε
E3
10.15
10.62
10.68
11.1
11.00
10.76
10.21
10.99
11.52
10.77
10.83
0.85
0.14
15.98
16.90
17.59
17.6
7.24
7.50
7.78
7.85
8.01
7.56
7.47
8.75
9.5
E3 + I
E3 + II
–0.47
–0.11
0.12
9.81
9.75
10.16
8.38
9.32
E3 + III
E3 + IV
6OCB
11.40
10.35
10.58
18.17
15.94
16.79
0.42
6OCB + IV
0.25
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 7 2008

1214
ALEKSANDRIISKAYA et al.
S
0.6
∆n
0.18
0.13
0.08
0.5
1
2
0.4
0.3
–20
–10
0
Tred
–20
–10
0
Tred
Fig. 3. Dependences of the order parameter S and birefringence ∆n on reduced temperature for (1) 6OCB and (2) a 6OCB–IV mixture.
2. M. F. Grebenkin and A. V. Ivashchenko, Liquid Crystal-
increase in the parallel permittivity component (ε||).
line Materials (Khimiya, Moscow, 1989) [in Russian].
In addition, intermolecular H-bonding can be treated
as an interaction that competes with the antiparallel
association of mesogen molecules, which is known
[10] to balance dipoles and reduce the ∆ε effective
value. Note that the formation of H-bonded com-
plexes of this kind should be accompanied by the
effective incorporation of a nonmesogen into an LC
matrix. This is indirectly confirmed by the observa-
tion that the orientational order parameter and bire-
fringence of nonmesomorphic modifier IV remain
unchanged (Fig. 3).
3. S. M. Kelly and M. O’Neill, in Handbook of Advanced
Electronic and Photonic Materials and Devices, Vol. 7:
Liquid Crystals, Display, and Laser Materials, Ed. by
H. S. Nalwa (Academic, New York, 2000).
4. S. T. Wu, et al., Jpn. J. Appl. Phys. 37 (10B), L1254
(1998).
5. S. Sen, P. Brahma, K. Subir, and S. B. Roy, Acta Phys.
Pol., A 65 (1), 47 (1984).
6. T. V. Tararykina, V. E. Maizlish, G. P. Shaposhnikov,
et al., Zhidkie Kristally
i
Ikh Prakticheskoe
Ispol’zovanie, Nos. 1–2(15–16), 97 (2006).
ACKNOWLEDGMENTS
7. V. I. Minkin, O. A. Osipov, and Yu. A. Zhdanov, Dipole
Moments in Organic Chemistry (Khimiya, Leningrad,
1968) [in Russian].
This work was financially supported by the Russian
Foundation for Basic Research, project no. 05-03-
32571a, and the program of the Department of Chem-
istry and Materials Science of the Russian Academy of
Sciences, project no. 20Kh.
8. Yu. V. Molchanov, A. F. Privalov, P. P. Yakutseni, and
P. M. Borodin, Yadernyi Magnitnyi Rezonans, No. 6,
113 (1981).
9. W. H. de Jeu, Philos. Trans. R. Soc. London, Ser. A 309,
217 (1983).
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