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SYNTHESIS AND PROPERTIES OF NEMATIC
LIQUID CRYSTALS EXHIBITING A POSITIVE
DIELECTRIC ANISOTROPY
V. Titov, E. Kovshev, A. Pavluchenko, V. Lazareva, M. Grebenkin
To cite this version:
JOURNAL DE PHYSIQUE Colloque C1, suppliment au no 3, Tome 36, Murs 1975, page Cl-387
Classification Physics Abstracts
7.130
SYNTHESIS AND PROPERTIES OF NEMATIC LIQUID CRYSTALS
EXHIBITING A POSITIVE DIELECTRIC ANISOTROPY
V. V. TITOV, E. I. KOVSHEV, A. I. PAVLUCHENKO, V. T. LAZAREVA and M. F. GREBENKlN Institute for Organic Intermediaries & Dyes, Moscow, U. S. S. R.
Rksumb. - Des series de cristaux liquides comportant des groupements benzonitrile et cinna- monitrile en position para ont et6 preparkes. Leurs temperatures de transition et leurs constantes ditlectriques ont Cte dCterminCes. L'introduction dans des cristaux liquides d'un groupe nitrile en position para ne change pas de mani6re significative leurs stabilites thermiques, mais augmente considerablement leurs anisotropies dielectriques. L'adjonction d'une double liaison dans des mesomorphes accroit leurs stabilites thermiques mais joue tres peu sur leurs anisotropies dielec- triques.
Abstract.
-
A series of liquid crystalline materials comprising 4-substituted benzonitriles and cinnamonitriles were prepared, and their transition temperatures and dielectric constants were investigated. The introduction of a 4-cyano group in a liquid crystal (LC) molecule is not accompa- nied by significant changes in mesophase thermal stabilities but it leads to materials with a consi- derable positive dielectric anisotropy. Introduction of an additional double bond in LC molecules increases mesophase thermal stabilities without significant changes in their dielectric anisotropies.I . Introduction. - There is interest in nematic liquid crystals exhibiting a positive dielectric aniso- tropy (PDA), due to their use in various electro- optical devices which utilise field orientational effects. These devices possess some advantages over those which utilise dynamic scattering [I].
Many published papers have discussed the relations between the constitution of LC molecules with a negative dielectric anisotropy (NDA) and their pro- perties [2]. LC with a PDA, in contrast to those with a NDA, have a strong permanent dipole moment along the molecular axis. Interactions between permanent dipoles is a factor which influences the thermal proper- ties of LC. Thus it is impossible to predict a priori the properties of LC with a PDA taking into consideration the known properties of LC with a NDA.
We have synthesized and investigated a large number of liquid crystalline compounds in which the PDA was provided by introduction of a cyano group, although other functions also could provide PDA. The only PDA materials whose synthesis has been previously reported are the N-(4-a1koxybenzylidene)- 4'-aminobenzonitriles [3].
In addition to the transition temperatures of the synthesized compounds their dielectric properties, which determine their electrooptic behaviour, were investigated.
2. Experimental.
-
Transition temperatures were determined using a Mettler FP-5 apparatus having micro-furnaces FP-51 and FP-52, and a Projectinapolarizing microscope. Dielectric properties of LC were investigated for layers of 80-100 mp thickness. A
magnetic field of 7 kG was used t o align the LC. The
cell capacitance was measured by an AC bridge at
1 000 Hz : the accuracy was estimated to be
+
1%.
Satisfactory elemental analyses were obtained for all compounds.Intermediate p-hydroxy- [4] and p-aminobenzo- nitrile [3], p-hydroxy- [5] and p-aminocinnamo- nitrile [7] were obtained by known methods. p-alkyl- benzaldehydes were synthesized by reaction of n-alkyl- benzenes with chlormethyl ether and purified via bisulphite derivatives. p-alkylcinnamic acids were obtained by condensation of p-alkylbenzaldehydes with malonic acid.
Compounds I-V and XI were obtained by reaction of the appropriate acid chlorides with p-hydroxy- benzonitrile (I-V) or p-hydroxycinnamonitrile (XI) in pyridine at 200 for 20 hours.
Azo-compounds VI and X were synthesized by coupling of diazotated p-aminobenzonitrile (VI) or p-aminocinnamonitrile (X) and phenol with a follow- ing acylation by aliphatic acid chlorides.
Azoxy-compounds VII (mixtures of isomers) were obtained by oxydation of Vl with hydrogen peroxide in acetic acid.
Schiff bases VIII and IX were obtained by conden- sation of p-alkyl- or p-alkoxybenzaldehydes with p- aminocinnamonitrile in benzene or toluene in the presence of catalytic amounts of p-toluenesulphonic acid or piperidine, with azeotropic distillation of the water formed.
C1-388 V. V. TITOV, E. I. KOVSHEV, A. I. PAVLUCHENKO, V. T. LAZAREVA AND M. F. GREBENKIN Compound index 1
-
Ia b C d e f g 4-Substituted benzonitriles (*) Transition temperatures PC) 4 average-
10.7 average+
6.4 average IVa C2Hs -CH=CHCOO- 106.0-1 17.0 b C3H7 -CH= CHCOO- 98.7-124.6 c C4H9 -CH=CHCOO- 83.9-1 13.6 d C 5 H ~ I -CH=CHCOO- 74.5-118.5 e C 6 H ~ 3 -CH=CHCOO- 73.0-107.4 f C7H15 -CH=CHCOO- 57.8-109.8 g (AH, 7 -CH=CHCOO- 61.1-111.1 -CH=CHCOO- -CH=CHCOO- -CH=CHCOO- -CH=CHCOO- -CH ==CHCOO- -CH= CHCOO- -CH=CHCOO- -CH=CHCOO- -- -- - average+
35.3+
6.5+ 29.3
+ 35.6
+
53.0+
49.3+
60.9+
56.5+
44.5 average VIa C,H5CO0-N=N-
128.0-1 35.0 b C3H7CO0-N=N-
1 15.6- 124.0 c C4H9CO0-N=N-
102.9-109.5 d C5Hl ,COO-N=N-
107.7-1 10.5 e C,H13CO0-N=N-
98.2-104.7 f C7H1 ,COO-N=N-
109.0-104.7 7.3 C,Hl ,COO-N=N-
102.0-107.5 average+
4.6SYNTHESIS AND PROPERTIES OF NEMATIC LIQUID CRYSTALS 61-389 Compound index 1
-
Transition tem~eratures VIIa average+
54.1 3. Results and discussion.-
Most of the investi-gated compounds have enantiotropic nematic phases. Only the compounds Ia-d are monotropic.
Some higher homologues of compounds VIII and IX have both nematic and smectic mesophases. In the case of compounds Xe-g the smectic phase is even formed by the lower homologues. Compounds VIII and I X g h form a smectic mesophase with a schlieren texture. After cooling of the nematic phase a metastable fan texture forms, which passes into schlieren texture after slight sliding of the cover glass.
Azo-compounds X form several smectic textures, which were not investigated in detail.
Some conclusions can be draw form the listed results (tables I, 11, Fig. 1-3) on the influence of terminal substituents and central linkages on transition tempe- ratures. The central linkages, in order of increasing thermal stability of the mesophase are ,:
Alkyl carbon chain length n
FIG. 1. -Influence of terminal substituents on LC clearing
points.
for the terminal substituents,
alkyl
<
alkoxy--
acyloxy.Mesophase temperature ranges have approximately the same order with the exception of azo-compounds, benzonitrile derivatives of which have the lowest meso- morphic ranges (Table I).
SYNTHESIS AND PROPERTIES OF NEMATIC LIQUID CRYSTALS
Alkyl carbon chain length n
FIG. 2. -Influence of central bonds on LC clearing points.
those of compounds with NDA having a similar struc- ture. Thus the presence of a strong permanent dipole moment. along the molecular axis of a LC does not affect mesophase thermal stability. In this connection, note should be taken of the clearing temperature increase in the series of p,p'-dialkoxyderivatives on
passing from azo- to azoxy-compounds ; in the latter there is a dipole moment perpendicular to the mole- cular axis. We have also observed an analogous increase in our azoxy-compounds (Table I). One can suppose that the direction but not magnitude of the permanent dipole moment influences the mesophase thermal stabi- lity, which is increased to a greater degree by a perpen- dicular moment than by parallel one.
It must be noticed that this supposition is not in correspondence with a molecular-statistics theory of the nematic phase [8], which neglects interactions of permanent dipoles. But interactions between induced and permanent dipole moments are possible.
A considerable increase of a mesophase thermal
stability and mesomorphic range occur if a supple- mentary vinyl-group is introduced in the LC molecule
Z
4
6
8 10Alkyl carbon chain length n
FIG. 3.
-
Influence of an additional double bond on LC clearing points.(Fig. 3) ; a double bond near the end of the molecule stabilizes a mesophase to a greater degree than in the centre (compounds XI and V). The observed meso- phase thermal stability increase is analogous to the clearing point elevation, which was observed during transition from LC with two phenylene rings to compounds with three phenylene rings [91. It is appa- rently associated with an increased polarizability along
RG. 4.
-
Temperature dependence of dielectric anisotropy of Ic (1), Ie (2), If (3) and f Ic+
3 If mixture (4). The measuringC1-392 V. V. TITOV, E. I. KOVSHEV, A. I. PAVLUCHENKO, V. T. LAZAREYA AND M. F. GREBENKIN the molecular axis, caused in our case by addition of
double-bond n-electrons to a common conjugation. Great magnitudes of positive dielectric anisotropies are observed for all LC investigated, i. e. ell is conside-
rably larger than s, (s,,
-
3 E,). In most cases E, is.about 6.5 to 8 near the melting point, though for Ie and If, sl N 10. Some decrease of is observed
with an increase of alkyl group length (Ie, f and IVc, f), which can be explained by a molecular density decrease. Introduction of a supplementary double bond in the LC molecule does not change the dielectric constants much. It is possible that LC polarity and polarizability
increases are in this case counteracted by a molecular density decrease.
We have also found for a mixture
3 Ic
+
+
If, As =+
22 at 25 0C while earlier a typical valueAE =
+
14 was indicated [I].Acknowledgment.
-
We wish to thank Dr. L. M. Blinov for many helpful discussions during the prepa- ration of this paper, Dr. V. P. Grigoryan for quantita- tive elemental analyses, and Mrs. I. F. Agafonova, Mrs. T. V. Koroleva and Miss N. I. Yegorova for technical assistance in the preparation of many of the compounds listed.FIG. 5. -Temperature dependence of dielectric anisotropy of Vd (I), IVc (2), XIb (3), IXd (4), VIIIc (V) and IVf (V).
LC dielectric properties Compound index
-
Ic Ie If VIIIc IXc IVc IVf Vd XIb References[I ] BOLLER, H., SCHEPPER, H. and SCHADT, M., Proc. ZEEE 60 [6] DOLTER, R. G. and CARRAN, C., J. Ant. Chem. Soc. 82 (1960)
(1972) 1002. 4153.
[21 DE J E ~ ,
6'-
H- and VAN DER VEEN, G., Phi[@s Res. Rep- 27 [7] GRAY, G . W. and JONES, B., J. Chem. Soc. (1954) 1476.(1972) 172. [8] M ~ R , W. and SAUPE, A., 2. Naturforsch. 14a (1959) 882.
[3] CASTELLANO, J. A., GOLDMACHER, J. E., BARTON, L. A. and
QNE, J. s., J . Q , . ~ . (-hem. 33 (1968) 3501. RCA carp., [91 DEWAR, M. G- S- and GOLDBERG, R. S., J. Org, Chem. 35
Brit. 1228606 (1971). (1 970) 2771.
[4] BAKER, W. and CARRUTHERS, G. N., J. Chem. Soc. (1937) 1101 GOLDMACHER, J. E. and HEILME~ER, G . H., U. S. 3,499,702
479. (1970) ;
