Nematic 4-cyanophenyl esters incorporating a lateral fluorine atom

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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�. �jpa-00210083�

1395

Nematic 4-cyanophenyl ^esters incorporating ^a lateral fluorine atom Hp. ^Schad and S. M. Kelly

Brown Boveri Research Center, ^CH-5405 Baden, Switzerland

(Reçu ^le 4 fevrier ^1985, accepte ^{le 28 mars} 1985)

Résumé.

Les propriétés diélectriques, diamagnétiques ^et élastiques ^{de divers esters de fluoro-2 et fluoro-3} p-

cyanophényle ^ont été étudiées. Les esters sont dérivés des acides benzoiques, cyclohexacarboxyliques ^ou bicyclo- octancarboxyliques. ^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} pré-

sence et de la position de l’atome de fluor. Un de ces esters présente ^une très forte anisotropie diélectrique.

Abstract.

We have investigated ^the dielectric, diamagnetic, and elastic properties ^{of various 2-fluoro- and 3-} fluoro-4-cyanophenyl ^{esters. The esters are} derivatives of benzoic, cyclohexane ^or bicyclooctane ^{acids. For} compari-

son 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.

J. Physique ⁴⁶ (1985) ^{1395-1404 Ao8T} 1985;

Classification Physics ^Abstracts 61. 30G

1. Introduction..

Compounds ^{with one} strong dipole (usually provided by ^{a nitrile} function) ^{in a terminal} position ^{are essen-}

tial components of nematic mixtures for liquid crystal displays ^{such as} the twisted nematic [1] ^{and the} guest- host 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

these parameters ^{of an} additional dipole ^{attached to}

the same ring ^{as the nitrile} group ^we have synthesized

ester derivatives of 2-fluoro- and 3-fluoro-4-hydroxy-

benzonitrile. The acid part ^{of the esters contains either}

a 1,4-disubstituted benzene, ^{or a} cyclohexane, ^{or a} bicyclooctane ring. ^The synthesis ^and liquid crystal

transition temperatures ^{of these esters are} reported

elsewhere [3, 4]. ^{In this} publication ^we report ^values for the dielectric and elastic constants and the dia- magnetic anisotropy ^of representative homologues ^of

these compounds.

The strong ^terminal dipole ^moment of the cyano

group generally gives ^{rise to} strong ^{molecular correla-} tion [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].

The terminal position of the nitrile dipole ^{leads to} antiparallel configurations ^{with the exact} form of the

overlap depending ^{on the} charge separation ^within

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 ^aniso-

tropy ^{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 fluoro- substituted 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

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 transi- tion temperatures (TNI), + ^0.1 for the dielectric constants, ± ^0.001 for the refractive index of the

isotropic phase (ni), ^{± 4} % ^{for the} diamagnetic ^ani- sotropy (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 ^inves- tigated 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 fluoro- substituted 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 dependence of the dielectric constants is shown for 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 temperatures ^{for the} 2-substituted esters (X,

^F;

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 tempera-

ture 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 experi- mentally. ^{The molecular} dipole moments ^{of the} free molecule can normally be calculated from tabu-

Fig. 1.

The dielectric constants ell, ^s, and 8; ^of compounds

1 to 3 as functions of reduced temperature. For every ^com-

pound E is represented by the upper ^curve and El by ^the

lower curve.

Fig. ^2.

^The dielectric constants of compounds ^{4 to 6 as}

functions of reduced temperature.

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

Fig. ^3.

^The dielectric constants of compounds ^{7 to 9 as} functions of reduced temperature.

Fig. ^4.

The dielectric constants of compounds ^{10 and 11}

as functions of reduced temperature.

ester group and the fluorine ^atom. As the rotation of the phenyl part of the molecule about the carbon- oxygen single bond of the ester group is sterically

hindered an average between the extreme cases of free rotation and antiparallel arrangement ^{of the}

Fig. ^5.

^The dielectric constants of compounds 2, 8, ^and 11 as 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 deter-} mining ^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 asso-

ciation 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-fluoro-}

substituted 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 associa-}

tion 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

compound [12].

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 nema-} tic 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).

Table II.

Calculated molecular dipole moment p of compounds ^{1 to 9 and} correlation coefficient g.

Table III.

The dielectric constants, the diamagnetic anisotropy AX, and the elastic constants of compounds ^{1 to}

11 as a function of temperature.

1401

TABLE III (continued) -

TABLE III (continued).

The temperature dependence ^{of the} association

(when present) may complicate ^the temperature ^beha- viour of the dielectric constants, ^{as was} found for the esters with substitution in the 2-position (see Fig. 6).

E; and 7

(E ^{11 + 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 ana-} lysis ^{is rather} difficult due to the anisotropic ^internal

fields.

3.2 DIAMAGNETIC ANISOTROPY AND ELASTIC CONS- TANTS.

The diamagnetic anisotropy ^{of the esters}

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 > kll > 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

largest ^{elastic constants and those} with the fluorine 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} bicyclo-

octane derivates [13, 14] ^the k33/ki l ^{values of the} bicyclooctane ^esters (10 ^and 11) ^are very large (> 2).

Fig. ^6.

The dielectric mean 8

(E ^{I, + 2 El)/3 and si}

of compounds ^{2, 8, and 11 as} functions of reduced tempe-

rature.

Fig. ^7.

^The diamagnetic anisotropy ^of compounds ^{1 to} 6, 10 and 11 as a function of reduced temperature.

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. ^8.

^{The elastic constant} k 11 ^of splay deformation for

compounds 1-6, ^{10 and 11 as a} function of reduced tempe-

rature.

Fig. ^9.

^{The elastic constant} k22 of the twist deformation for compounds ^1-6, 10 and 11 ^as a function of reduced temperature.

Fig. 10.

^{The elastic constant} k33 of the bend deformation for compounds 1-6, 10 and 11 as a function of reduced temperature.

Fig. ^11.

^{The ratio} k33/kll ^of compounds ^{1-6, 10 and 11}

as a function of reduced temperature.

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-cyano- phenyl ^{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 ben- zoates

presumably ^{due to} steric effects

and is

completely absent for one ester. In that case the combi- nation of a large dipole ^moment with the lack of molecular association leads to large dielectric cons- tants and a dielectric anisotropy ^of nearly ^50.

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