Mol. Cryst. Liq. Cryst., Vol. 381, pp. 21–32
Copyright # 2002 Taylor & Francis
1058-725X/02 $12.00+.00
DOI: 10.1080/10587250290102071

NOVEL CHIRAL FLUORO-SUBSTITUTEDBENZOYLOXYBENZOATES EXHIBITING AN
ANTIFERROELECTRIC MESOPHASE{
S. Shubashree, B. K. Sadashiva* and Surajit Dhara
Raman Research Institute, C. V. Raman Avenue,
Sadashivanagar, Bangalore 560 080, India

The synthesis and properties of a new series of chiral fluoro-substituted benzoyloxybenzoates with nonchiral chains ranging from heptyloxy to octadecyloxy
are reported. The mesomorphic properties of all of the compounds have been
examined by a combination of optical polarized light microscopy, differential
scanning calorimetry, and miscibility studies. They show rich polymorphism,and seven compounds exhibit the antiferroelectric (SmC*A) phase. The spontaneous polarization in the ferroelectric (SmC*) phase has been measured as afunction of temperature for selected compounds.
Keywords: ferroelectric; antiferroelectric; liquid crystals; spontaneous polarization; miscibility

INTRODUCTION
Since the discovery of antiferroelectricity in mesogenic materials [1], afairly large number of compounds exhibiting this phenomenon have beensynthesized [278]. The standard material, 4-(1-methylheptyloxycarbonyl)phenyl-40 -octyloxybiphenyl-4-carboxylate [MHPOBC] has been investigated in detail as it exhibits a number of phases, and the structure of theantiferroelectric phase was established using this material [9]. Lateralsubstituents have been used in liquid crystal chemistry to not only modify
the transition temperatures but also to alter the dipolar effects to advantage. A fluoro substitutent in terms of its van der Waal’s radius is notvery much different from that of hydrogen and combined with its large
Received 27 October 2001; accepted 26 February 2002.
The authors thank Mr. K. Subramanya for technical help.
{
Part of this work was presented at the International Union of Materials Research Societies
International Conference in Asia, Bangalore, India, October 13716, 1998, Abstract No. I, Pos
07, p. 296.
*Corresponding author. E-mail: sadashiv@rri.res.in
21

22

S. Shubashree et al.

electronegativity has been used widely to obtain useful physical propertiesof liquid crystalline materials. The influence of a lateral fluoro substitutent
in different positions of the core of many systems exhibiting antiferroelectric properties has also been examined [2,8,10]. It is known that a fluoro
substitutent ortho to the n-alkoxy chain in three ring triesters reduces thelongitudinal dipole moment but the mesophase sequence is retained [8]. It
has been reported by Demus et al. [11] that compounds with a fluoro
substituent ortho to the carboxylate group containing the chiral moiety inthree ring diesters show a wide temperature range of the antiferroelectricphase. The aim of the present study was to combine these two aspects, byintroducting one fluoro substitutent in each of the terminal phenyl rings ina three ring system and examine the mesomorphic properties with emphasis on the antiferroelectric properties. We have also compared themesomorphic properties of this homologous series of compounds withthose of the unsubstituted analogues.

EXPERIMENTAL
2-Fluoro-4-benzyloxybenzoic acid was prepared by first reacting ethyl 2fluoro-4-hydroxybenzoate with benzyl chloride in the presence of anhydrous potassium carbonate in butan-2-one, followed by hydrolysis of
the resulting ester with aqueous alkali. Its esterification with [S]-[þ]-octan2-ol in the presence of 1,3-dicyclohexylcarbodiimide (DCC) and 4-N,Ndimethylaminopyridine (DMAP) as catalyst gave compound 3. The
deprotection of the phenolic group was carried out through a palladiumcatalyzed hydrogenolysis reaction. The phenol 4 was condensed with theacid 5, which was prepared by treating ethyl 4-hydroxybenzoate withbenzyl chloride in the presence of anhydrous potassium carbonate in butan-2-one and hydrolyzing the resulting ester with aqueous alkali, using thesame esterification procedure in order to lengthen the molecule by one
phenyl ring. The final compounds were obtained by a classical esterification
reaction between the phenol 7 and an appropriate 3-fluoro-4-n-alkoxybenzoic acid. The 3-fluoro-4-n-alkoxybenzoic acids were prepared following
a procedure described previously [12]. The purification of final compounds
was carried out by column chromatography on silica gel using chloroform:petroleum ether (b.p. 60780 C) mixture as eluent followedby repeated crystallizations using suitable solvents. The purity of all theintermediates and final compounds were checked by thin layer chromatography (Merck Kieselgel 60F254 precoated plates) and by normalphase high-performance liquid chromatography using m porasil column(3.9 mm 6 300 mm, Waters Associates Inc.) and 1% acetone in dichloromethane as an eluent for the latter. The chemical structures of all ofBenzoyloxybenzoates Exhibiting an SmCA Phase23

the compounds were confirmed by using a combination of 1H NMR spectroscopy (Bruker AMX 400 spectrometer with tetramethylsilane as an internal standard), infrared spectroscopy (Shimadzu IR 435 spectrometer)
and mass spectrometry (JEOL SX 102/DA-6000). Specific optical rotations
were measured using chloroform as a solvent (Optical Activity AA 1000
polarimeter).
The initial phase assignments and transition temperatures were determined by thermal polarized light microscopy using a polarizing microscope (Leitz Laborlux 12 POL) in conjunction with a hot stage andcontroller (Mettler FP52 and FP5 control unit). Differential scanning calorimetry (Perkin-Elmer, Pyris 1D differential scanning calorimeter) wasused to determine both the transition temperatures and the heats oftransitions. The instrument was calibrated against an indium standard andall enthalpies are quoted in kJ mol1. Spontaneous polarization measurement was carried out using samples sandwiched between thin glass plates
coated with indium tin oxide (ITO) and etched appropriately to create an
active area for accurate measurement. In order to obtain sample aligned in
the homogenous (planar) bookshelf geometry, the plates were treated withpolyimide and rubbed unidirectionally. Mylar spacers were used to define
the thickness of the cell (typically 2 to 7 mm). To perform the polarization
measurements we adopted the modified form of Tower-Sawyer [13] technique proposed by Diamant et al. [14]. The measurement as a function oftemperature was carried out by applying a low frequency triangular field(30740 Hz) having an amplitude of 4715 Vpp.
The general method of preparation of the various compounds is shown
schematically in Figure 1.
The detailed procedure for the synthesis and characterization of one of
the compounds under study is given below.

[S]-[+]-1-Methylheptyl 4-[4-(4-decyloxy-3-fluorobenzoyloxy)benzoyloxy]-2-fluorobenzoate(ld)A mixture of 4-decyloxy-3-fluorobenzoic acid (0.139 g, 0.47 mmol),[S]-[þ]-1-methylheptyl 2-fluoro-4-(4-hydroxybenzoyloxy)benzoate (0.2 g,
0.47 mmol), 4-N,N-dimethylaminopyridine (DMAP) (0.006 g, 0.047 mmol)and dry dichloromethane (5 ml) was stirred for 5 min. To this mixture wasadded 4-N,N’-dicyclohexylcarbodiimide (DCC) (0.106 g, 0.47 mmol) andstirring continued overnight at room temperature. The dicyclohexyl urea
formed was filtered off and the filtrate diluted with dichloromethane(20 ml). The resultant solution was washed with 5% aqueous acetic acid
(3630 ml), water (3630 ml) and dried over Na2SO4. The residue obtainedon removal of solvent was chromatographed on silica gel and eluted withchloroform:petroleum ether mixture to yield a white solid which was

24

S. Shubashree et al.

FIGURE 1 Synthetic scheme for preparation of [s]-[þ]-l-methylhepthyl 4-[4-(4-nalkoxy-3-flourobenzoyloxy) benzoyloxy]-2-fluorobenzoates.

Benzoyloxybenzoates Exhibiting an SmCA Phase25

crystallized repeatedly from isopropyl alcohol (0.274 g, 80%), m.p. 101 C,½aD25 ¼ 20 (1 mg/ml in CHCl3); nmax (nujol): 2950, 1730, 1720, 1710, 1610,
1460, 1280, 1240, and 1050 cm1; d: 0.8670.93 (6H, t, 2XCH3), 1.2771.9(27H, m, 1XCH3, 12XCH2), 1.8371.9 (2H, m, CH(CH3)CH2),4.1174.14 (2H, t, ArOCH2), 5.1575.2 (1H, m, COOCH), 7.078.27(10H, m, ArH).

RESULTS AND DISCUSSION
The transition temperatures together with the associated enthalpies forcompounds of series I are summarized in Table 1. As can be seen, all thecompounds are enantiotropic mesomorphic. A preliminary examination of
the different mesophases was carried out as follows. Compounds Ib, Ic, Ie,
and Ig exhibit all the mesophases observed in the series, and an ex-

TABLE 1 Transition Temperatures ( C) and Enthalpies (kJ mol1) (in Italics)
for [S]-[þ]-1-Methylheptyl 4-[4-(4-n-alkoxy-3-fluorobenzoyloxy)benzoyloxy]-2-fluorobenzoates
Compoundnumber
n
Ia

7

Ib

8

Ic

9

Id

10

Ie

11

If

12

Ig

14

Ih

16

Ii

18

Cr
63.0
20.8
69.5
21.8
93.5
29.3
101
31.9
90.0
27.9
60.5
27.7
57.0
39.5
62.0
34.7
52.0
34.8

SmC*A

SmC*g

7

112.5 7
0.04
116.5
117.0
{
{
117.0
117.5
0.02
0.04
118.0 7
0.07
112.5
113.0
0.01
0.05
111.0 7
0.05
98.0
99.5
{
0.04
7

7

7

SmC*

SmC*a
113.0
{
117.5
{
120.5
{
122.5
{
122.0
{
121.0
{
119.5
{
115.5
{
112.0
{

SmA
114.0
0.03
118.5
{
121.0
{
123.0
0.33
122.5
{
122.0
{
120.0
{
116.5
0.42
112.5
{

I
139.0
5.32
135.5
5.23
132.0
5.02
130.5
4.68
129.0
5.05
127.5
4.91
124.5
4.92
121.5
4.89
118.0
4.62

{; The enthalpy could not be measured; ; total enthalpy including any other crystal-crystal
transition.
Key: Cr, Crystalline phase; SmC*A, Antiferroelectric phase; SmC*g, Ferrielectric phase;
SmC*, Ferroelectric phase; SmC*a , Chiral smectic C alpha phase; SmA, Smectic A phase; I,
Isotropic phase; , Phase exists; 7, Phase does not exist.

26

S. Shubashree et al.

amination of compound Id under a polarizing microscope showed the following features. On cooling the isotropic liquid, focal-conic texture typicalof the smectic A phase was seen (this phase could be easily aligned
homeotropically to yield dark regions). On lowering the temperature, finebands appeared on the focal-conics (banded focal-conics) which indicatedthe transformation to a chiral smectic C phase. On decreasing the temperature further the bands became broader and highly colored. Two otherphase transitions seen in differential scanning calorimetric thermograms
could not be seen clearly under these conditions.
In order to establish the identify of the mesophases, the samples were
examined on different cells coated for homogeneous and homeotropicalignments and viewed under a polarizing microscope. When the isotropic
liquid of compound Id was cooled in a cell treated for homogeneousalignment, uniform areas with some focal-conic defects typical of thesmectic A phase [15] were seen. As the temperature was gradually decreased, this existing texture became striated with bright bands, whichgrew parallel to the rubbing direction. This type of texture has been observed for smectic C*a phase [15]. On further cooling the sample, thestriations appeared to twist, resulting in a ropelike texture. This type of
texture is typically observed for SmC* phase [15]. On reducing the temperature further, stripes started growing in a direction perpendicular to therubbing direction on the existing ropes. This texture was identical to thatobserved for the antiferroelectric (SmC*A) phase [15]. A typical photomicrograph of this texture is shown in Figure 2.
On cooling the isotropic liquid of compound Ib in a cell treated for a
homeotropic alignment below the SmC* phase, a birefringent texture was

FIGURE 2 Photomicrograph of compound Id showing SmC*A phase (homogeneously aligned) at 111 C.

Benzoyloxybenzoates Exhibiting an SmCA Phase27

obtained. The characteristic feature of this phase was that the texture
appeared to be in constant motion even though the temperature was held
constant. This type of texture was seen for ferrielectric phase [16], and wehave identified the same (SmC*g) phase for the above compound. In addition, compounds Ic, Ie, and Ig also exhibit this phase. Therefore, fromthese textural observations we have been able to identify that compounds
discussed above exhibit SmA, SmC*a, SmC*, SmC*g, and SmC*A phases. In
order to conclusively establish the identity of the different smectic phases,
a miscibility study of the nonyloxy homologue (compound Ic) was carriedout with the well-known standard material 4-(1-methylheptyloxycarbonyl)phenyl-40 -octyloxybiphenyl-4-carboxylate (MHPOBC) which exhibits theabove mesophases. An isobaric binary phase diagram constructed is shownin Figure 3. The mixtures were prepared as weight/weight ratio and mixed
well in their isotropic states. In this diagram a complete miscibility of all thephases over the entire composition range can be seen.
A plot of the transition temperatures as a function of the number of carbon
atoms in the n-alkoxy chain is shown in Figure 4. The plot shows fairly smooth
curve relationships for like transitions. These transitions could also be seenclearly in thermograms obtained on a differential scanning calorimeter.
In order to understand the effect of fluoro substituents in the present
series of compounds, a comparison of the mesophases and the transition
temperatures of the unsubstituted parent compounds (series II) as well ascompounds of series III synthesized by Faye et al. [8] have been made. The
three series of compounds are detailed below.

X ¼ F; Y ¼ F

Series I

n ¼ 7; 8; 9; 10; 11; 12; 14; 16; 18:

X¼Y¼H
Series II ½8 n ¼ 7; 8; 9; 10; 11; 12:
X ¼ F; Y ¼ H Series III ½8 n ¼ 8; 9; 10; 11; 12:
When a fluoro substituent is introduced ortho to the n-alkoxy chain as inseries III [8], the following marginal changes are observed. The homologues
with n ¼ 9 to 12 exhibit SmC*A, SmC*F12, SmC*, and SmA phases, while
homologues with n ¼ 8 to 10 exhibit SmC*a phase in addition to the abovephases. Also, none of the homologues of this series showed SmC*F12 phase.
The SmC*A phase is retained as compared to the unsubstituted compounds

28

S. Shubashree et al.

FIGURE 3 Miscibility phase diagram of binary mixtures of compound Ic and thestandard material MHPOBC.

FIGURE 4 A plot of transition temperatures versus the number of carbon atoms in
the n-alkoxy chain.

Benzoyloxybenzoates Exhibiting an SmCA Phase29of the series II. For example, the homologue with n ¼ 8 in series II is monotropic, but it is rendered enantiotropic with a narrow thermal range of 2 C
after the introduction of the fluoro substituent. These results indicate thatthe steric hindrance due to the presence of the fluoro substituent is minimal,
although the clearing temperatures are considerably lowered. In the case ofcompounds of series I where there are two fluoro substituents, one ortho tothe terminal n-alkoxy chain and the other to the carboxylate group close to
the chiral center, we have observed a decrease in both the melting as well as
the clearing temperatures. As far as the SmC*A phase is concerned, thehomologues with n ¼ 8 to 12 in all three series of compounds exhibit thisphase. It is interesting to note, however, that in the present series of compounds I, this phase is not only enantiotropic but the thermal range is quite
large. For example compound If has a thermal range of 50.5 C. In contrast,
the parent homologues (series II) with n ¼ 8 and 9 show metastable SmC*A
phase. Another significant point to note is that for sufficiently long chain
lengths (compound Ih and Ii), the SmC*A phase disappears and the SmC*
phase with a wide thermal range predominates. In addition, enantiotropic
SmC*a phase is observed in all the homologues of this series I.
Thus, these results show that the introduction of two fluoro substituents
ortho to the terminal n-alkoxy chain as well as the carboxylate group close
to the chiral moiety influences the stabilization of the SmC* and SmC*A
phases while reducing the melting and clearing temperatures.
The temperature dependence of spontaneous polarization (PS) was
determined, and plots of PS versus reduced temperature (T-T*) were obtained for five homologues (compounds Id, Ie, If, Ih, and Ii). A typical plot
obtained for compound If is shown in Figure 5.
All of the compounds of series I follow a general pattern, and PS was
found to decrease with increase in temperature. The value lies close to zeroat T* (T* is the SmC*a-SmC* transition temperature). One of the interesting features is that a decrease in the magnitude of the polarization values was observed with an increase in the chain length. A plot of PS at
T-T* ¼ 4 C as a function of chain length is shown in Figure 6. It can beseen that for n ¼ 10, the value for PS is about 61 nC/cm2, whereas forn ¼ 18, it is reduced to about 51.5 nC/cm2.
The effect of a polar substituent on the magnitude of PS can be explained in terms of (1) proximity between the lateral polar group and thechiral center and (2) the favorable orientation of the dipoles of the polargroup and the chiral moiety. It is believed that a polar group present close
to the chiral center increases PS due to the restricted internal rotation of
the polar group by steric hindrance with the chiral group [17]. This is truein systems having ether as the linking group between the chiral moiety andthe core. However, when the linking group is a carboxylate group, the increase or decrease in the magnitude of PS due to the presence of a lateral

30

S. Shubashree et al.

FIGURE 5 Plot of spontaneous polarization versus reduced temperature forcompound If.

FIGURE 6 Plot of the sponateous polarization at T-T* ¼ 4 C as a function of then-alkoxy chain length.

Benzoyloxybenzoates Exhibiting an SmCA Phase31

polar substituent depends on the direction of orientation of the dipoles
associated with the polar group and the chiral group. This is because in thelatter case, since the chiral tail is conformationally correlated with the coreless tightly than in the former case, the internal rotation of the polar group
becomes more feasible and hence the dipoles can orient appropriately. An
increase in the value of PS is observed when the dipoles add up and adecrease in the value is seen when they cancel each other. The favorable
orientation of the dipoles thereby either opposing each other or adding toeach other in turn depend on the stable conformation the molecule would
prefer to take so as to relieve the dipole-dipole interaction between thepolar substituent and the group linking the core to the chiral moiety.
The results obtained from the measurement of spontaneous polarization
for the decyloxy homologue of the difluoro (series I), parent (series II), andmonofluoro (series III) substituted compounds are given below:
Series I
Series II
Series III

PS ¼ 64 nC/cm2
PS ¼ 80 nC/cm2
PS ¼ 100 nC/cm2

It is seen that for compound that has a fluoro substituent ortho to then-alkoxy chain (serie III) the value of PS is about 100 nC/cm2, which ishigher as compared to the unsubstituted parent compound (series II).
However, when a second fluoro substituent is introduced close to the chiral
center (series I), the value of PS decreases to about 64 nC/cm2. This decrease in the value of spontaneous polarization may be explained as follows.
The 1-methylheptyl group is slightly away from the core and is separated
from it by a carboxylate group. The fluoro substituent, which is ortho to thiscarbonyl group, is free to orient itself so as to relieve the dipole-dipole interaction with the carbonyl group and adopt a conformation as shown below.

It is probable that in this stable conformation the dipole of the lateral
fluoro substituent is opposing that of the carbonyl group, as a result of
which there is a decrease in the value of PS.

32

S. Shubashree et al.

To summarize, a new homologous series of difluoro-substituted compounds have been synthesized. The fluoro substituents, while reducing themelting and clearing temperatures, have enhanced the thermal range of the
SmC* and SmC*A phases when compared with those of the unsubstituted
parent compounds. In addition, it has been found that the value of spontaneous polarization decreases as a result of introduction of outer edge
fluoro substituent close to the chiral moiety.

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