Liquid Crystals, 2002, Vol. 29, No. 6, 789± 797

Banana-shaped mesogens: eVect of 2-methylresorcinol as thecentral unit on the mesomorphic properties of  ve-ring esters
S. SHUBASHREE, B. K. SADASHIVA* and SURAJIT DHARA
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar ,Bangalore 560 080, India(Received 21 January 2002; accepted 25 February 2002)
Three homologous series of banana-shaped  ve-ring esters and one series of seven-ring esters
derived from 2-methylresorcinol have been synthesized. This has been carried out to understand the eVect of chemical structure on the mesomorphic properties of such compounds. It
is shown that the 2-methyl substituent in the central phenyl ring destabilizes the mesophases
when they are compared with those of the analogous unsubstituted compounds. Replacementof a phenyl ring by a biphenyl moiety (seven-ring esters) in the two arms of the bent-coremolecule enhances the thermal range of the mesophases as well as the clearing temperatures.
The mesophases have been characterized using techniques such as polarizing optical microscopy,
diVerential scanning calorimetry, and X-ray diVraction, and by electro-optical investigations.
In the 32 compounds investigated, two mesophases, B1 and B2 , have been identi ed.

1. Introduction
Thermotropic mesomorphism has been observed incompounds with three diVerent basic molecular shapes.
These are compounds with rod-like molecules or disclike molecules or banana-shape d molecules. There has
been great interest in compounds with banana-shape dmolecules as some of these have been reported to exhibit
ferroelectric [1] and antiferroelectric properties [2],though the molecules themselves are achiral. As a matterof fact, it was Vorländer who discovered long ago thatcompounds with non-linear structures exhibit mesophases though the exact nature of these was reported
much later [3, 4]. In the last few years several hundred
compounds with a bent molecular shape and exhibiting
mesomorphic properties have been synthesized [5–15 ]with a view to understanding the relationship between
the molecular structure and the mesophases they exhibit.
There have been several studies in which the central
phenyl ring has been substituted by chloro [16–18],
bromo [15], cyano [19] and nitro [20 ] substituents. In
all these compounds, which are derivatives of resorcinol,
the exact position of the substituent has an in uenceon the nature of the mesophase obtained. The results
obtained with various substituents on the central phenyl
ring have been summarized in a recent publication [21].
The mesomorphic properties of the derivatives of
2-methylresorci nol have not been studied in detail. A few
of the SchiV ’s base esters reported earlier [22] showeda phase transition from B to B as the isotropic phases
25*Author for correspondence; e-mail: sadashiv@rri.res.in

were cooled. The interesting observation was that bothof these mesophases are electro-optically switchable. The
other unusual observation for these derivatives was thaton attaching a  uorine to the outer phenyl ring ortho
to the terminal alkoxy chain, three diVerent unidenti edmesophases were obtained [21 ]. Interestingly , the replacement of  uorine by chlorine appears not only to depress
the melting point but also to eliminate two of themesophases.
With a view to examining the eVect of 2-methylresorcinol as the central unit on the mesomorphic
properties of compounds which are not SchiV ’s basederivatives, we have synthesized four homologous seriesof compounds having the general structure A shown
below:
5

O

O 3

O
Y

4

O

X

6

O

1
2

O

C H3

O
X

O

Y
R’

R’

Str uctur e A

X = F, Y = H, R’ = OCnH2n+1

Series I

X = F, Y = H, R’ = CnH2n+1

Series II

X = H, Y = F, R’ = OCnH2n+1

Series III

X = F, Y = H, R’ = -C6H4-CnH2n+1

Series IV

L iquid Crystals ISSN 0267-829 2 print/ISSN 1366-585 5 online © 2002 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/02678290210138830

S. Shubashree et al.

790

We have also compared the mesomorphic properties ofthese compounds with those of the unsubstituted parent
compounds and report here our results.
2. Synthesis
The banana-shape d compounds belonging to the threeseries I, II and IV were synthesized following thegeneral scheme shown in  gure 1 and those belonging
to series III were prepared following the general pathway
shown in  gure 2. The commercially obtained 2-methylresorcinol was puri ed before use. The 4-n-alkyl and
4-n-alkoxybenzoic acids were prepared following procedures described in the literature [23, 24]. 4-Benzyloxybenzoic acid was prepared by  rst reacting ethyl
4-hydroxybenz oate with benzyl chloride in the presence ofanhydrous potassium carbonate in butan-2-on e followedby hydrolysis of the resulting ester with aqueous alkali.
2-Fluoro 4-hydroxybenzoi c acid and 3- uoro-4-hydrox ybenzoic acid were synthesized as described earlier[25]. 2-Fluoro-4-benzyloxybenzoi c acid and 3- uoro-4benzyloxybenzoic acid were prepared following a procedure described previously [26 ]. 4-n-Alkylbiphenyl4¾ -carboxylic acids were prepared following a proceduredescribed by us [27]. A typical and standard procedure
for the synthesis and characterization of one of thecompounds under investigation is given below.

O

Figure 1.

Series I I

Series I V

Synthetic pathway for the preparation of thecompounds of series I, II and IV.

HO

P h C H 2O

OH

( v iii )OH

CH3

DC C , DMAPC H C l3O

O
OO

( ix )CH3

P hC H 2O

O C H 2P h

H 2, 5% P d - C
1,4 - Dioxane

O

O
O

O

(x )

CH3

HO

OH

F

DC C , DMAPC HC l3O( xi )

H 2n+1 C nO
OH

O
O
F
H 2n+1 C n O

O
O

O

O
C H3

O
O

Series I I IFO C n H 2n+1

n = 7,8,9,10,11,12,14,16,18.
Figure 2.

Series I+(i)

Synthetic pathway for the preparation of thecompounds of series III.

2.1. 2-Methyl-1,3-phenylenebis[2- uoro 4-benzyloxybenzoate ] (iii )A mixture of 2- uoro-4-benzyloxybenzoi c acid (3 g,
12 mmol ), 2-methylresorcinol (0.76 g, 6.11 mmol), N,Ndimethylaminopyridin e (DMAP, 0.15 g, 1.2 mmol) anddry dichloromethane (50 ml ) was stirred for 5 min. To
this mixture was added N,N-dicyclohexylcarbodiimide(DCC, 27.77 g, 12.1 mmol ) and stirring was continued
overnight at room temperature. The N,N-dicyclohexylurea formed was  ltered oV and the  ltrate diluted withdichloromethane (50 ml ). The resultant solution waswashed with 5% aqueous acetic acid (2 Ö 20 ml ) andwater (3 Ö 30 ml ) and then dried over anhydrous sodium
sulphate. The solvent was removed from the  lteredsolution to yield a residue which was puri ed by column
chromatograph y on silica gel using chloroform as eluent,
to yield a white material which was crystallized from a
mixture of chloroform and petroleum ether (b.p. 60–80ß C).
Yield 2.74 g (78%); m.p. 149–149.5ß C; nmax (nujol ): 2950,
1740, 1720, 1620, 1580, 1470, 1340, 1260, 1120 and
1070 cmÕ 1 ; 1 H NMR (CDCl3 ) d: 2.12 (s, 3H, ArCH3 ),5.14 (s, 4H, ArOCH2 )2 , 6.76–8.1 (m, 19H, ArH ).

Banana mesogens from 2-methylresorcinol
2.2. 2-Methyl-1,3-phenylen ebis(2- uoro-4-hydroxybenzoate ) (iv)
A mixture of 2-methyl-1,3-phenylen e bis(2- uoro4-benzyloxybenzoate ) (2 g, 3.4 mmol ), 5% Pd-C catalyst(0.5 g) and 1,4-dioxane (50 ml ) was stirred at 55ß C in an
atmosphere of hydrogen until the calculated quantity ofhydrogen was absorbed. The reaction mixture was then ltered and removal of the solvent gave a residue which
was crystallized from ethyl acetate. Yield 1.3 g (94%);
m.p. 258–260ß C (d); n(nujol ): 3400, 2950, 1710, 1620,
max
1460, 1260, 1110, 1080 and 1060 cmÕ 1 ; 1H NMR (CDCl3 )d: 2.18 (s, 3H, ArCH3 ), 3.4 [s, 2H, (ArOH )2 ], 6.87–8.2(m, 9H, ArH).
2.3. 2-Methyl-1,3-phenylen e bis[4-(4-n-octa decyloxybenzoylox y)-2- uorobenzoate ] (8)This was synthesized following the procedure used
for the preparation of compound iii; the quantities
used were: 2-methyl-1,3-p henylene bis(2- uoro-4-hydroxy benzoate) (0.1 g, 0.25 mmol), 4-n-octadecyloxybenzoi cacid (0.195 g, 0.5 mmol), DCC (0.114 g, 0.55 mmol),
DMAP (0.006 g, 0.05 mmol ) and anhydrous chloroform(5 ml ). The product was crystallized from a butan-2one-acetonitrile mixture. Yield 77%; m.p 110ß C; nmax
(nujol): 2900, 1740, 1720, 1600, 1470, 1240, 1120,
1160 and 1060 cmÕ 1 ; 1 H NMR (CDCl ) d: 0.86–0.89
3(t, 6H, 2 Ö CH ), 1.2–1.57 (m, 64H, 32 Ö CH ), 2.17
3
2(s, 3H, ArCH3 ), 4.04–4.07 [t, 4H, (ArOCH2 )2 ], 6.98–8.2(m, ArH, 17H ).
All the compounds were puri ed by chromatographi ctechniques and by repeated crystallization using suitable
solvents. The chemical structures of all the compounds
were con rmed by 1H NMR spectroscopy (Bruker AMX
400 spectrometer) using tetramethylsilane as internal
standard, by infrared spectroscopy (Shimadzu FTIR-8400
spectrophoto meter) and by elemental analysis (Carlo-Erba
1106 analyser) .
3. Experimental
The thermal behaviour of the homologous series ofcompounds was examined by polarizing optical microscopy (POM) (Leitz Laborlux 12 POL) using a heatingstage and controller (Mettler FP52 and FP5, respectively) .
DiVerential scanning calorimetry (DSC) (Perkin-Elmer,
Pyris ID) was employed to determine the transition
temperatures, as well as the enthalpies of transition. This
instrument was calibrated using indium as standard. The
thermograms were generally recorded at a scanning rateof 5ß C minÕ 1 and the enthalpies are quoted in kJ molÕ 1.
Phase identi cation was made by observation of the
optical textures. This was con rmed by X-ray diVraction(XRD) studies using CuK radiation from a rotatingaanode generator with a  at graphite crystal monochromator. These diVraction studies were performed on

791

powder samples in Lindemann capillaries and diVraction
patterns were collected on an image plate (Marresearch) .
The electro-optical studies were carried out employing
a standard experimental set-up wherein the sample wasmounted between ITO coated glass plates and viewed
by POM. The d.c.  eld was generated using a suitable
power supply. The switching current response wasobtained using the triangular voltage method.

4. Results and discussion
The transition temperatures together with the associatedenthalpies for the homologous compounds of series I,
are summarized in table 1. As can be seen, compounds
1, 2, and 3 are non-mesomorphic , while the remaining
compounds exhibit a monotropi c mesophase. The optical
microscopic textures of these compounds were examinedas thin  lms obtained by sandwiching the samples
between a glass slide and a cover slip and viewed by
POM. All the compounds show similar textures, whichlook like leaves; sometimes a schlieren texture is also
obtained. A typical photomicrograp h of the texture
exhibited by compound 8 is shown in  gure 3. This issimilar to that observed for the antiferroelectric B2 phase
and, as we shall see later, this mesophase has been con rmed as a B phase. The clearing transition enthalpies
2
for compounds 7 and 8 are of the order of 24 kJ molÕ 1 .
A plot of the transition temperatures as a function of
Table 1. Transition temperatures (ß C) and enthalpies (kJ molÕ 1 ),in italics, for compounds of series I. Key: Cr 5 crystalline
phase; B2 5 antiferroelectric banana phase 2; I 5 isotropic
phase; B1 5 two dimensional banana phase 1;
phaseexists; — phase does not exist. () indicates a monotropictransition.
Compound

n

1

10

2

11

3

12

4

13

5

14

6

15

7

16

8

18

Cr

B2
109.5
94.9 a
117.0
55.7
119.5
60.8
118.0
65.9
115.0
116.7 a115.5
118.9 a114.0
137.0 a110.0
147.2 a

I

—

—

—

—

—

—

(

103.0)(

105.0)(

107.0)(

108.0)
23.3
109.5)
24.8

b
b
b

(

a Total enthalpy including any other crystal–crystal transition.
b The enthalpy could not be measured as the sample crystallizes

immediately.

S. Shubashree et al.

792

Table 2. Transition temperatures (ß C) and enthalpies (kJ molÕ 1 ),in italics, for compounds of the following series [ 28 ]. For
key see table 1.
O
O

O
O

O

O

F

O
F

O

H 2n+1C nO

Figure 3.

Photomicrograph of the antiferroelectric B2 phaseof compound 8 at 107ß C.

the number of carbon atoms in the alkyl chain for thisseries is shown in  gure 4. The curve for the clearing
temperatures is typical of that obtained for the B2 phase–
isotropic transition points [11 ]. In order to examine theeVect of the 2-methyl substituent on the mesomorphic
properties we compare below the transition temperatures(table 2) for the analogous compounds without the
methyl substituent [28]. It is clear that in the compounds with the 2-methyl substituent, the melting points
are increased and the clearing temperatures are reduced
although the same mesophase is retained. Also, the mesophases in the parent compounds (table 2) are enantiotropic, while three of the methyl substituted compounds
are non-mesomorphic and the remaining  ve give monotropic phases. In eVect, the 2-methyl substituent suppresses the thermal range of the mesophase in thesecompounds.

Io

T emperature / C

116

112
Cr
108

B2

104

100
10

11

12

13

14

Compound

n

9

10

10

11

11

12

12

13

13

14

14

15

Cr

B2
96.5
44.9
95.0
49.4
93.0
48.1
91.0
44.4
94.5
63.2
99.5
87.9

I
98.0
19.8
101.0
20.8
105.0
22.8
109.0
21.9
110.5
24.4
112.5
25.9

The transition temperatures and associated enthalpies
for the homologous compounds of series II are summarized in table 3. Compounds 15 and 16 are nonmesomorphic, while compounds 17, 18 and 19 givemonotropic mesophases; compounds 20 and 21 areenantiotropic mesomorphic with narrow thermal rangesof the mesophases. From the optical textures as well asother physical measurements, the mesophase exhibitedby the compounds of this series has been characterizedas B2 . A plot of the transition temperatures as a functionof the number of carbon atoms in the alkyl chain isshown in  gure 5. The clearing temperatures fall on a
Table 3. Transition temperatures (ß C) and enthalpies (kJ molÕ 1 ),in italics, for compounds of series II. For key see table 1.

120

9

O C nH 2n+1

15

16

17

18

19

Number of carbon atoms in the n-alkoxy chain

Figure 4. Plot of transition temperatures versus the numberof carbon atoms in the n-alkoxy chain for compounds ofseries I.

Compound

n

15

9

16

10

17

11

18

12

19

14

20

16

21

18

a,b See table 1.

Cr

B2
111.0
76.4 a107.0
75.0 a109.0
97.6 a112.0
98.7 a104.5
96.2 a
104.5
33.6
107.0
91.9 a

I

—

—

—

—

(

99.0)

(

101.0)
23.6
104.0)
24.5
106.0
24.5
108.0
26.1

b

(

Banana mesogens from 2-methylresorcinol
112
I
110

o

Temperature / C

108
Cr

106

B2
104
B2

102
100
98
8

10

12

14

16

18

Number of carbon atoms in the n-alkyl chain

Figure 5. Plot of transition temperatures versus the numberof carbon atoms in the n-alkyl chain for compounds ofseries II.

smooth curve with a rising trend on ascending the series.
As mentioned earlier, this is the typical trend observed forB2 phase–isotropic liquid transition points. Replacementof the terminal n-alkoxy chain in series I by the n-alkyl
chain has a marginal eVect on the mesomorphism. For
comparison purposes, the transition temperatures andthe corresponding enthalpies for the parent compounds[28] are shown in table 4. The parent compounds 22
and 23 are non-mesomorphi c as also are the corresponding 2-methyl substituted compounds 15 and 16.
However, compounds 24 to 27 give enantiotropic phases
Table 4. Transition temperatures (ß C) and enthalpies ( kJ molÕ 1 ),in italics, for compounds of the following series [28 ]. For
key see table 1.
O
O

O
O

O

F

O

O
F

C nH 2n+1

n

22

9

23

10

24

11

25

12

26

14

27

16

Cr

B2
100.5
53.3
102.5
49.2
101.5
28.5
99.5
31.22
97.5
55.1
96.5
42.5

while the analogous mesophases for 17, 18 and 19 aremonotropic. It can also be seen that in the 2-methyl
substituted compounds there is a reduction in thethermal range of the enantiotropic B phase. For
2example, compound 27 has a thermal range of 14ß C
while the corresponding analogous compound 20 has arange of only 1.5ß C. Again, this clearly indicates thatthe 2-methyl substituent suppresses the thermal rangeof the mesophase.
The compounds containing a  uoro substituent ortho
to a terminal n-alkoxy chain (homologues of series III )show a marked diVerence. All the compounds of thisseries are non-mesomorphi c and the melting points, alongwith the associated enthalpies, are collected in table 5.
In contrast, the parent compounds show interesting
phase behaviour [28]. Therefore the 2-methyl substituent totally suppresses the mesophase in these compounds. To sum up, the 2-methyl substituent destabilizes
the mesophases in all these series of compounds.
We have reported [12] earlier that by employing abiphenyl moiety on the side arms of the bent-coremolecules, the thermal range of the mesophase can beenhanced considerably. Since the thermal range of themesophase of the  ve-ring esters is some what small, we
decided to examine the in uence of the 2-methyl substituent on such compounds containing a biphenyl
moiety. The transition temperatures and the associatedenthalpies for these compounds (series IV) are summarized in table 6. All the compounds have fairly wide
thermal ranges of the mesophase. Compounds 37, 38
and 39 exhibit only one mesophase. When a sampleof these as a thin  lm was examined by POM highly
Table 5. Transition temperatures (ß C) and enthalpies (kJ molÕ 1 ),in italics, for compounds of series III. For key see table 1.
Compound

n

28

7

29

8

30

9

31

10

32

11

33

12

34

14

35

16

36

18

O

H2n+1C n

Compound

793

I

—

—

—

—
104.0
22.2
105.5
23.9
108.5
25.9
110.5
26.6

a See table 1.

Cr

I
154.0
85.2 a148.0
84.3 a144.5
53.9 a143.0
60.1 a142.0
51.2 a141.0
59.2 a140.0
71.2 a139.0
87.4 a138.5
89.8 aS. Shubashree et al.

794

Table 6. Transition temperatures (ß C) and enthalpies ( kJ molÕ 1 ),in italics, for compounds of series IV. For key see table 1.
Compound

n

37

8

38

9

39

10

40

11

41

12

42
43
44

14
16
18

Cr

B2
168.0
28.5
150.0
29.1
147.8
38.4 a147.6
43.5 a136.0
43.3 a135.8
48.6 a132.0
56.8 a
127.5
44.0

B1

I

—

208.0 c—197.0 c—194.2 cbbb192.0 c—b193.0 c—b

194.5 c

—

b

194.0

—

b

192.5 c—ba,b See table 1.
c Beyond this temperature the compound decomposes.

colourful spherulitic growth could be seen as the isotropic liquid was cooled. As the temperature was lowered
further, the spherulites coalesced to form a mosaic-liketexture. A typical photomicrograp h of the texture exhibitedby compound 39 at 150ß C is shown in  gure 6. From
this, as well as XRD studies, this mesophase has beencharacterized as a B1 phase. The remaining compounds
40 to 44 of the series show a mesophase that has beencharacterized as B2 . A comparison of the mesomorphic
behaviour of these compounds with that of three ofthe parent compounds [12] reveals the following. While
all three parent compounds exhibit a B phase with2
fairly large thermal range, the analogous 2-methyl substituted compounds show a slightly diVerent behaviour.
Compound 39 with a n-decyl chain shows a B1 phase
while the corresponding parent compound exhibits a B2
phase. However, compounds 40 and 41 show a B phasesimilarly to the parent compounds. The main diVerenceis that the methyl substituted compounds have higher
melting points and tend to decompose at the clearing
points, while the parent compounds have lower melting
points and do not decompose. It is also interesting tonote that despite all this, the clearing temperatures areabout the same for these six compounds.
5. X-ray diVraction studies
The XRD studies were carried out on non-oriented
samples. The sample was generally  lled into a 0.5 mm
diameter Lindemann capillary tube in the isotropic
phase and the ends of the tube were  ame sealed. The
diVraction patterns were collected on an image platewith an eVective pixel size of 100 mm, procured from
Marresearch. The diVraction pattern was recorded on cooling the sample into the mesophase. A typical diVraction
pattern obtained for compound 8 at 107ß C is shown in gure 7. As can be seen this shows four re ections inthe small angle region with spacings 42.1, 21.4, 14.3 andÃ which can be indexed as (0 0 1), (0 0 2), (0 0 3)
10.7 Aand (0 0 4) re ections. The periodicities of the structure
spacing are in the ratio 1: 1/2: 1/3: 1/4; this clearly
indicates a layered structure. In addition to these re ections
Ã due
a broad diVuse wide angle re ection at about 4.74 A
to the alkoxy chain can also be seen. The X-ray angularintensity pro le depicted in  gure 7 is identical to thatobserved for the B2 phase [12]. Based on these results,
as well as on the microscopic textural observation, thismesophase has been identi ed as B2 . The lamellar spacing
Ã obtained for compound 8 corresponds to a tiltof 42.1 A
angle of 56.3ß which is calculated assuming the molecule
has the conformation shown in  gure 8 in which themethylene units of the chains are in a fully extended alltrans-conformation.

2

I ntensity / arb units

001

002

2

003

004

4

6

8

10

e /degFigure 6.

Photomicrograph of the two-dimensional B1 phaseof compound 39 at 150ß C.

Figure 7.

X-ray angular intensity pro le of the B2 phase ofcompound 8 at 107ß C.

Banana mesogens from 2-methylresorcinol

Figure 8. Molecular model of compound 8 in which thechains are in a fully extended all trans-conformation.

Similar diVraction patterns were obtained for the mesophases of compounds 4–7 of this series, compounds
11–15 in series II and compounds 28–32 in series IV.
Hence, all these mesophases have been identi ed as B2 .
The XRD pattern of the mesophase exhibited bycompounds 37, 38 and 39 are identical. The angularintensity pro le of the mesophase of compound 39 at
150ß C is shown in  gure 9. This shows three re ections
Ã whichin the small angle region at 32.5, 24.78 and 14.1 A
could be indexed as (0 0 2), (1 0 1) and (2 0 0) re ections,
respectively, of a centred rectangular two-dimensionallattice as observed before for the B1 phase [5, 12]. The
Ã and b 5 65.0 A
Ã . Based
lattice parameters are a 5 26.8 A
on these data the mesophase has been identi ed as B1 .
The mesophases of compounds 37 and 38, which gave
similar results, have also been identi ed as B .

795

extinction brushes were formed. In the applied  eld, thebrushes were oriented at an angle of ~45ß with respect
to the orientation directions of the crossed polarizers.
On reversing the polarity of the applied  eld, the brushes
rotated in the same direction, i.e. the rotation of thebrushes in this case occurred in only one direction
independent of the sign of the applied  eld. On switching(a)1

6. Electrooptical studies
The eVect of a d.c. electric  eld on the B2 phase of compound 7 has been studied. The sample was held in a
5 mm thick cell made with conducting plates (ITO coatedglass plates), treated with polyimide and unidirectionallyrubbed. The sample was viewed between crossed polarizers in the polarizing microscope. As the sample wascooled at a slow rate of 0.1ß C minÕ 1 under a d.c.  eldof 4.2 V mmÕ 1 , circular domains with characteristic(b)

Intensity / arb units

002

101

200

2

4

6

8

10

e / deg

Figure 9.

X-ray angular intensity pro le of the B1 phase ofcompound 39 at 150ß C.

(c)Figure 10. Photomicrographs illustrating the eVect of a d.c.
electric  eld on compound 7; rubbed polyimide coatedplates at 105ß C; (a) 1 21.3 V, (b) 0 V, (c) Õ 21.3 V.

796

S. Shubashree et al.

oV the  eld, the orientation of the dark brushes remained
unaVected and the resultant texture had uniform periodic
stripes on the circular domain. This texture is characteristic of the racemic ground state [29] of the B phase.
2
Figures 10 (a, b, c) show the textures of compound 7 in
the presence and absence of the electric  eld.
Under low d.c.  elds the stripes become birefringent
and they begin to disappear as the voltage is increased.
For this compound the stripes remain up to 23 V after
which they begin to disappear. These observation s indicate
a racemic ground state in this compound which ischaracterized by synclinic interlayer correlation with an
antiferroelectric order from layer to layer.
In order to establish conclusively the antiferroelectric
nature of this phase, a switching current response pro le
was obtained under a triangular wave voltage for compound 20. The sample was  lled in its isotropic state
into a cell (ITO coated and etched to give a precise areaof 0.8 cm2 ) of 45 mm thickness and treated for homogenous alignment. When a triangular wave with a peakto-peak voltage of 173 V and a frequency of 65 Hz at
105.5ß C was applied, two fairly sharp peaks were recorded
during a half period indicating an antiferroelectri c switching behaviour . Figure 11 depicts a typical current responsepro le for this compound. The spontaneous polarization
determined by integrating the switching current peaks
amounts to about 410 nC cmÕ 2 .
7. Conclusions
Thirty-two compounds having banana-sha ped molecules
derived from 2-methylresorcino l have been synthesized
and their mesomorphic properties examined. The antiferroelectric B2 phase or the two-dimensional B1 phasehas been observed in many of the compounds. In one
series containing a  uorine atom on the outer phenyl

Figure 11. Switching current response in the B2 phase ofcompound 20 on applying a triangular voltage (173 VPP ,65 Hz, 105.5ß C).

ring, no mesophase could be obtained. The in uenceof 2-methylresorcino l as the central unit on the mesomorphic properties in the four homologous series has
been examined and it is concluded that this central unit
has a destabilizing eVect on the mesophases.
The authors wish to thank Dr V. A. Raghunathan forhelp in the XRD measurements and Mr K. Subrahmanya
and Ms K. N. Vasudha for technical support.
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