Nematic 4-cyanophenyl esters incorporating a lateral
fluorine atom
Hp. Schad, S.M. Kelly
To cite this version:
Hp. Schad, S.M. Kelly. Nematic 4-cyanophenyl esters incorporating a lateral fluorine atom.
Journal de Physique, 1985, 46 (8), pp.1395-1404. <10.1051/jphys:019850046080139500>.
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J.
Physique 46 (1985) 1395-1404
Ao8T 1985;
1395
Classification
Physics
Abstracts
61. 30G
Nematic
4-cyanophenyl
Hp. Schad and S.
M.
Brown Boveri Research
esters
incorporating
a
lateral fluorine atom
Kelly
Center, CH-5405 Baden, Switzerland
(Reçu le 4 fevrier 1985, accepte le 28 mars 1985)
Les propriétés diélectriques, diamagnétiques et élastiques de divers esters de fluoro-2 et fluoro-3 pcyanophényle ont été étudiées. Les esters sont dérivés des acides benzoiques, cyclohexacarboxyliques ou bicyclooctancarboxyliques. Pour permettre la comparaison, les propriétés des esters non fluorés correspondants ont été
mesurées. Nous discutons du changement des propriétés des esters, soit qu’ils diffèrent par la composition du noyau
de la partie acide ou par la position de l’atome de fluor. Les propriétés physiques dépendent largement de la présence et de la position de l’atome de fluor. Un de ces esters présente une très forte anisotropie diélectrique.
Résumé.
2014
Abstract.
We have investigated the dielectric, diamagnetic, and elastic properties of various 2-fluoro- and 3fluoro-4-cyanophenyl esters. The esters are derivatives of benzoic, cyclohexane or bicyclooctane acids. For comparison we also measured the same properties of the corresponding non fluorinated esters. The change in the properties
of the esters with either variation of the ring in the acid part of the ester with fixed position of the fluorine atom or
with variation of the position of the fluorine atom with fixed ring structure is discussed. The physical properties of
the esters have been found to be strongly dependent upon the presence and position of the fluorine atom. A very
large value for the dielectric anisotropy of one of the esters has been determined.
2014
1. Introduction..
Compounds with one strong dipole (usually provided
by a nitrile function) in a terminal position are essential components of nematic mixtures for liquid crystal
displays such as the twisted nematic [1] and the guesthost displays [2]. These compounds exhibit large
dielectric anisotropies which are compatible with
the low threshold voltage values required for these
electrooptic effects. The values and the temperature
dependence of the dielectric and elastic constants of
the nematic phases of these compounds are very
important for the optimum performance of the above
mentioned displays. In order to study the effect on
parameters of an additional dipole attached to
these
the same ring as the nitrile group we have synthesized
ester derivatives of 2-fluoro- and 3-fluoro-4-hydroxybenzonitrile. The acid part of the esters contains either
cyclohexane, or a
bicyclooctane ring.
synthesis and liquid crystal
transition temperatures of these esters are reported
elsewhere [3, 4]. In this publication we report values
a
1,4-disubstituted benzene,
or a
The
for the dielectric and elastic constants and the diamagnetic anisotropy of representative homologues of
these compounds.
The strong terminal dipole moment of the cyano
group generally gives rise to strong molecular correlation [5]. In a simple model such cyano compounds
can be described as a binary mixture consisting of
virtually uncorrelated molecules (monomers) and
pairwise associated molecules (dimers) which are in
dynamic, temperature dependent equilibrium [6].
position of the nitrile dipole leads to
antiparallel configurations with the exact form of the
overlap depending on the charge separation within
The terminal
the molecules and on steric factors. A consequence of
the antiparallel arrangement of the molecules is
that the dipole moment of the dimer is zero and the
effective dielectric constants are smaller than without
association. This is because only the dipole of the
monomer contributes to the orientational part of the
polarization. A second consequence is that the
length/breath ratio of the dimer is greater than that of
the monomer (depending on the geometrical anisotropy of the dimer). Thus the nematic to isotropic
phase transition temperature (TNI) is higher than
without association [4]. The degree of the molecular
correlation and the form of the dimers of the fluorosubstituted esters and their influence on the physical
properties (in particular on the dielectric constants
and nematic-isotropic transition temperatures) are
of marked interest.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019850046080139500
1396
2.
ExperimentaL
The experimental methods for the determination of
the liquid crystal transition temperatures, density,
dielectric, diamagnetic, and elastic constants have
been described and discussed elsewhere [5].
The accuracy of the data is ± 0.2 OC for the transition temperatures (TNI), + 0.1 for the dielectric
constants, ± 0.001 for the refractive index of the
isotropic phase (ni), ± 4 % for the diamagnetic anisotropy (Ax), and ± 4 % for the elastic constants
(k11’ k22, k33). The derived correlation factor (g) is
accurate within ± 5 %. The particle number density N
is calculated from the relationship p/M. L, where p is
the density ( ± 5 x 104), M is the molecular mass,
and L is Avogadro’s number.
3. Results and discussion.
The molecular structures of the compounds investigated and values for some of their physical properties
are shown in tables la-d. The data for the isotropic
phases (indicated by the index i) were measured just
above the nematic to isotropic transition and those
for the itematic phases at t,,(= TITNI)
0.95. The
complete data of the temperature dependences of the
dielectric, diamagnetic, and elastic. properties are
given in table III. The data for two of the non fluorosubstituted compounds (4 and 7) have been already
published [5, 7] but are recorded in the tables for
purposes of comparison.
3.1 DIELECTRIC CONSTANTS.
The temperature
of
the
dielectric
constants
is shown for
dependence
the benzoates (compounds 1-3) in figure 1, for the
heptyl homologue of the cyclohexane compounds (4-6)
in figure 2, for the pentyl homologue of the cyclohexane
compounds (7-9) in figure 3, and for the bicyclooctane
compounds (10 and 11) in figure 4. The « saturation »
in the dielectric anisotropy As = E - B1. at lower
F;
temperatures for the 2-substituted esters (X,
X2 H) is most unusual (see also Fig. 5) and will be
discussed below (E is the dielectric constant with the
electric field parallel to the optical axis and B 1. that
perpendicular). Associating liquids are well described
by the formalism given by Kirkwood and Froehlich [8]
who introduced a factor g to represent the degree of
the correlation of the molecular dipoles ( g
1 means
no correlation and g
0 means complete correlation).
The factor g can be regarded (using the simple model
of unassociated molecules (monomers) and associated
molecular pairs (dimers)) as the proportion of the
free molecules in the binary mixture [6]. The temperature dependence of g is a consequence of the
temperature dependent equilibrium. Values for g
can be calculated from the Kirkwood-Froehlich
equation [9] provided the parameters referred to
below are known. The density, the refractive index and
the dielectric constants can be determined experimentally. The molecular dipole moments of the
free molecule can normally be calculated from tabu=
Fig. 1.
as
pound
lower
The dielectric constants ell, s, and 8; of compounds
functions of reduced temperature. For every comEis represented by the upper curve and El by the
-
1 to 3
curve.
-
=
=
=
=
The dielectric constants of compounds 4 to 6
Fig. 2.
functions of reduced temperature.
-
as
lated group and bond moments [10,11]. In the case of
the fluorine substituted esters an approximation must
be made, since the resulting dipole moment depends
on the orientational correlation of the dipoles of the
1397
The dielectric constants of
Fig. 3.
functions of reduced temperature.
-
compounds
7 to 9
as
Fig.
11
5.
as
The dielectric constants of compounds 2, 8, and
functions of reduced temperature.
-
ketone function and the fluorine atom is assumed. The
experimental data for the refractive indices are listed
in table I and the derived dipole moments and the
calculated correlation factors are collated in table II
for compounds 1-9. The values of the dipole moments
of the molecules with the fluorine atom in 3-position
(X I H ; X2 F) are larger while those with the
fluorine atom in 2-position (Xl
F; X2 H) are
smaller than those of the unsubstituted compounds
(XI X2 H). The same sequence is shown for the
isotropic dielectric constant E; and the dielectric
anisotropies (see Table Ia-d), because the differences in
the squares of the dipole moments which are determining factors [8] are larger than the differences in the
correlation factors of the esters. It is clear from the
g-values in table II that the degree of molecular association of the fluoro-substituted esters is lower than
that of the non fluoro-substituted esters. In the
case of the cyclohexane esters (4-9) the 2-fluorosubstituted compounds (5 and 8) show the lowest
degree of association. In the case of the benzoates
(1-3) the 3-fluoro-substituted ester (3) exhibits no
association whatsoever (g
1). The lack of association combined with the large dipole moment of 6.1 D
leads to the very high positive dielectric anisotropy
(As --- 50) found for this compound. The isotropic
dielectric constant si of compound 3 shows the 1/T
dependence typical for non associating polar liquids.
The large dielectric anisotropy (As) of the ester (3) and
the small values for its elastic constants (see Tables I
and III) give rise to a very low threshold voltage
(0.3 V) for a twisted nematic display containing this
=
=
=
=
=
=
4.
The dielectric constants of compounds 10 and 11
functions of reduced temperature.
Fig.
as
-
ester group and the fluorine atom. As
the rotation of
the phenyl part of the molecule about the carbonoxygen single bond of the ester group is sterically
hindered an average between the extreme cases of
free rotation and antiparallel arrangement of the
compound [12].
=
1398
Table I.
The physical properties of compound 1 to 12 (TNI transition temperatures ; molecular weight M ; particle
number density Ni’ and dielectric constant 8¡ in the isotropic phase ; the birefringence An, dielectric anisotropy
A E and diamagnetic anisotropy A x, elastic constants of splay k11, twist k22, and bend k33 deformations in the nematic phase). The data for the isotropic phases were measured just above T NI and for the nematic phases at tr ( TI TNI) =
0.95.
-
=
Ia :
Compounds
1 to 3.
1399
TABLE I
(continued).
1400
Table II.
Table III.
11
as
-
-
Calculated molecular
dipole moment p of compounds
The dielectric constants, the diamagnetic anisotropy
a function of temperature.
1 to 9 and correlation
coefficient g.
AX, and the elastic constants of compounds 1
to
1401
TABLE III
(continued) -
1402
TABLE III
(continued).
The dielectric mean 8
Fig. 6.
(EI, + 2 El)/3 and si
compounds 2, 8, and 11 as functions of reduced tempe-
=
of
The temperature dependence of the association
(when present) may complicate the temperature behaviour of the dielectric constants, as was found for the
esters with substitution in the 2-position (see Fig. 6).
E; and 7
(E11 + 2 sl)/3 exhibit a curved temperature
dependence and for Ae a saturation is observed at a
low reduced temperature (see Fig. 5). Although the
association is presumed to be affected only weakly by
the long range order of the nematic phase, the analysis is rather difficult due to the anisotropic internal
fields.
rature.
=
3.2 DIAMAGNETIC ANISOTROPY AND ELASTIC CONSThe diamagnetic anisotropy of the esters
TANTS.
exhibits the normal temperature dependence (see
Fig. 7). The values for the benzoates are three times as
large as those of the cyclohexane and bicyclooctane
compounds due to the presence of two aromatic rings.
The bicyclooctanes have slightly lower values than the
cyclohexane compounds (presumably due to a smaller
particle number density).
The elastic constants of the esters containing
benzene (1-3), cyclohexane (4-6), and bicyclooctane
(10, 11) rings are shown in figures 8-10. For each
ester k33 &#x3E; kll &#x3E; k22 (except (5) where, close to
T, k33 kl l). The compounds with the fluorine
atom in the 2-position (Xl
F; X2 H) possess the
constants
elastic
and
those with the fluorine
largest
atom in the 3-position (Xi =’ H; X2
F) possess
the smallest elastic constants. The k33/ki i ratio,
which is important for multiplexed addressing of
the twisted nematic display (it should be
1), is
shown in figure 11. As is usually found for bicyclooctane derivates [13, 14] the k33/ki l values of the
bicyclooctane esters (10 and 11) are very large (&#x3E; 2).
-
=
=
=
The diamagnetic anisotropy of compounds 1 to
7.
10 and 11 as a function of reduced temperature.
Fig.
-
6,
The lowest value of k3-3/k11 is observed for the heptyl
homologue of the 2-fluoro-substituted cyclohexane
ester (5). The corresponding pentyl homologue exhibits
a much higher k33/kll ratio (see Tables Ic and III).
1403
Fig. 10. The elastic constant k33 of the bend deformation
compounds 1-6, 10 and 11 as a function of reduced
Fig. 8. The elastic constant k 11 of splay deformation for
compounds 1-6, 10 and 11 as a function of reduced tempe-
for
rature.
temperature.
r-
Fig.
for
The elastic constant k22 of the twist deformation
compounds 1-6, 10 and 11 as a function of reduced
9.
-
temperature.
-
Fig.
as a
The ratio k33/kll of compounds 1-6, 10 and 11
function of reduced temperature.
11.
-
1404
Thus the ratio of the elastic constants k3 3/k 11 depends
much more directly on the length of the alkyl chain
than on the position of the fluorine substituent
3 . 3 CONCLUSION. In this paper we have investigated
the variation in the dielectric, diamagnetic, and elastic
properties of 2-fluoro, 3-fluoro substituted 4-cyanophenyl esters and their unsubstituted analogues with
either variation of the ring of the acid part with fixed
position of the fluorine atom or with variation of the
position of the fluorine atom with fixed ring structure.
The diamagnetic anisotropy was found to depend
mainly on the ring structure and not on the position of
-
the fluorine substitution. The elastic constants of the
2-fluoro components are larger and those of the 3" j
fluoro components are smaller than those of the
unsubstituted analogues. The analysis of the dielectrics
constants showed that in addition to differences in the
dipole moment the degree of molecular association
also varied. The association is lowest for the benzoates
and is
presumably due to steric effects
completely absent for one ester. In that case the combination of a large dipole moment with the lack of
molecular association leads to large dielectric constants and a dielectric anisotropy of nearly 50.
-
-
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