Liquid Crystals, 2001, Vol. 28, No. 3, 483± 487

Preliminary communicationEŒect of lateral substituents on the mesophases formed by someachiral banana-shaped molecules
B. K. SADASHIVA*, H. N. SHREENIVASA MURTHY and SURAJIT DHARA
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar ,Bangalore 560 080, India(Received 24 August 2000; in  nal form 2 October 2000; accepted 10 October 2000)
Several compounds composed of banana-shaped molecules and exhibiting mesophases arereported. The eŒect of diŒerent lateral substituents on the type of mesophase formed isexamined and a comparison made with unsubstituted compounds. The mesophases have beencharacterized using techniques such as polarizing optical microscopy, diŒerential scanningcalorimetry, and X-ray diŒraction. The B2 phase shows antiferroelectric switching behaviour.

Following the observation of a ferroelectric phase inachiral compounds composed of banana-shape d moleculesby Niori et al. [ 1 ] , there has been increased interest insynthesizing new materials and studying their physicalproperties. Recently, a number of such compounds havebeen synthesized [ 2–12 ] and so far,  ve diŒerent mesophases have been clearly identi ed. Amongst these,two mesophases, viz. B and B , have been well charac1
2terized. The B1 phase is a two-dimensional phase with arectangular lattice similar to those observed in rectangularcolumnar mesophases of disc-like molecules. The B2 phase
is a  uid lamellar phase with antiferroelectric switchable
characteristics [ 13 ]. Although these two mesophases
have been observed in a relatively large number of compounds, the relationship between molecular structure andtheir liquid crystalline properties is not well understood.
In this preliminary communication, we examine theeŒects of a number of lateral substituents on the mesophases formed by compound s composed of achiral bananashaped molecules. To our knowledge such a study has
not been undertaken before, and such studies should
throw more light on our understanding of structure–property relationships in these bent-core compounds.
All the compounds are esters and are thermally stable;
they are derived from resorcinol and contain seven arylrings. All the compounds were prepared following thegeneral synthetic pathway shown in  gure 1; the detailed
procedure for the preparation of compound 10 has been
described previously [ 11 ] . The compounds were puri edby chromatographi c techniques. The structure of eachcompound was con rmed by spectroscopic methods.
*Author for correspondence; e-mail: sadashiv@rri.res.in

A preliminary identi cation of the liquid crystalline
phases were made by polarizing optical microscopy(POM) using a Leitz Laborlux 12 POL equipped witha Mettler 52 heating stage and FP5 controller. The
phase transition temperatures and associated enthalpies
were determined by diŒerential scanning calorimetry(DSC) using a Perkin-Elmer Pyris I D. The calorimeter
was calibrated using pure indium as a standard. The
phase structures were determined by X-ray diŒraction(XRD) experiments using CuK radiation from a rotating
aanode generator, Rigaku Ultrax 18, with a  at graphite
crystal monochromator . The diŒraction patterns werecollected on an image plate (Marresearch) . Unoriented
samples were contained in Lindemann capillaries andthe temperature was controlled to better than Ô 0.1ß C.
Electro-optical investigations were carried out with 4 mmthick polyimide coated ITO cells by applying a triangularvoltage using a function generator, Wavetek Model 39,
and an ampli er, Trek Model 601-2.
The mesophases exhibited by all the compounds could
be easily identi ed from their characteristic textures. All
the compounds display only one banana phase. The
transition temperatures together with the associatedenthalpies for the unsubstituted parent compounds(series I) and the various substituted derivatives (series II)
are summarized in tables 1 and 2, respectively. Compounds
1 to 4 are enantiotropi c and exhibit the same type ofmesophase. This shows a mosaic texture; a typicaltexture obtained on slow cooling of the isotropic liquidof compound 3 is shown in  gure 2. The clearing
enthalpy is fairly high at about 23 kJ molÕ 1 . This mesophase shows the characteristic behaviour of the B1 phase.
Compounds 5 and 6 show a diŒerent type of mesophase:

L iquid Crystals ISSN 0267-829 2 print/ISSN 1366-585 5 online © 2001 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/02678290010018015

Preliminary communication

484

Figure 1. Synthetic pathway used
to obtain the banana-shapedmesogens.
Table 1. Transition temperatures (ß C) and enthalpy values( kJ molÕ 1 ) (in italics) for compounds of series I. Cr 5crystal, I 5 isotropic; the nomenclature of the mesophases
B1 and B2 are those suggested at the Workshop on‘Banana-shaped liquid crystals: Chirality by Achiral
Molecules’, December 1997, Berlin.
CompoundnCr

1

7E

28
E

3

9E

410E

511E

612E

163.5 a
50.56
153.5
36.14
141.0
50.98
159.0
42.87
151.0 a61.42
121.0 a53.14

B2B1—E—E—E—
EEE

209.0
25.45
209.0
26.99

I
235.0
23.46
226.0
23.81
219.0
24.24
211.0
23.76
EEE

—
E—
EE

a crystal–crystal transition; enthalpy denoted is the sum ofall transitions.

on cooling the isotropic liquid of these compounds, anunspeci ed  nger-print texture is obtained and a typical
texture for compound 6 is shown in  gure 3. This
mesophase is less viscous than those of the lower homologues. The mesophase exhibited by compounds 5 and6 has been identi ed as the B2 phase. The clearing
enthalpies of all six compounds in table 1 are comparable .
In order to con rm the identity of these mesophases
XRD measurements have been carried out. The diŒraction
pattern of an unoriented sample of the mesophase ofcompound 3 shows a diŒuse peak in the wide angle
Ã , indicating a liquid-like in-plane
region with d ~ 4.7 A
order. In the small angle region two re ections at d1 5
Ã and d 5 24.8 AÃ were obtained which could be32.7 A2
indexed as the (1 1) and (0 2) re ections from a rectangularÃ and b 5 49.7 A
Ã . A typical diŒraction
lattice with a 5 43.5 A
pattern obtained for this compound is shown in  gure 4.
This is similar to the patterns observed for the standard
material [ 2, 3 ] as well as our own observations [ 11, 12 ] .
Based on these we have identi ed this mesophase to be
B in nature. The XRD pattern of an unoriented sample
1Preliminary communication
Table 2.

485

Transition temperatures (ß C) and enthalpy values ( kJ molÕ 1 ) (in italics) for compounds of series II. (See table 1 for
explanation of symbols.)

CompoundnX

710FE

811FE

912FE

1010ClE

1111ClE

1212ClE

1310CH3E

1411CH3E

1512CH3E

1610OCH3E

1711OCH3E

1812OCH3ECr
141.5
27.96
142.0
35.86
146.0
40.04
132.0
20.2
128.0
15.91
131.0
15.15
139.0
68.23
138.5
74.35
114.0
56.38
124.0
46.32
124.0
52.0
130.5
62.12

B2

B1—
E—
E—
E—
E—
E—
E

—

(E(EE124.0) a
15.42
128.5
16.27

—
(E(E197.0
22.70
191.0
22.13
185.5
22.07
156.5
16.05
153.0
15.36
148.0
14.34127.5) a
13.87
EEEEEEE—EE—(E123.5) a17.6127.5) a18.5I119.5) a15.0

—
E—E

E

a Monotropic transition

Figure 2. Optical photomicrograph of the B1 phase developingfrom the isotropic liquid of compound 3 at 217 ß C.

Figure 3. Optical photomicrograph of the B2 phase developingfrom the isotropic liquid of compound 6 at 208 ß C.

of compound 6 also displayed a diŒuse peak in the wide
Ã , indicative of a liquid-likeangle region, with d ~ 4.6 A
in-plane order. In addition, four orders of lamellarre ections (in the ratio 1 : 2 : 3 : 4) could be seen. The
angular intensity pro le of this mesophase is shown in gure 5. From these data and the textural characteristics,
we have identi ed this mesophase to be a B2 phase.
The switching behaviour of the B2 phase wasobserved using electro-optical measurements as well as
by simultaneously viewing the sample under a polarizing

microscope. A cell of 4 mm thickness was prepared forhomogeneous alignment of the sample, and was  lledin the isotropic phase and cooled slowly. After thetransition to the mesophase, the applied  eld was
gradually increased to get better alignment and saturated
current peaks. By applying a su ciently high (200 Vpp )triangular voltage, a current response peak per half
period could be seen on the oscilloscope screen. A typicalswitching current response obtained for compound 6 isshown in  gure 6. The current was measured across aPreliminary communication

486

Figure 6. Switching current response in the B2 phase ofcompound 6 obtained by applying a triangular voltage(200 Vpp): sample thickness 4 mm, temperature 150ß C,polarization ~ 760 nC cmÕ 2 .
Figure 4.

Figure 5.

X-ray intensity pro le of an unoriented sample ofcompound 3 at 155 ß C.

X-ray intensity pro le of an unoriented sample ofcompound 6 at 150 ß C.

10 kV resistance. The apparent ‘saturated polarization’PS was derived from the area under the peaks to beabout 760 nC cmÕ 2 and is nearly temperatur e independent
except close to the clearing point. The electro-opticalresponse of this sample was also observed under apolarizing microscope. On cooling the sample slowly
from the isotropic liquid under an a.c. electric  eld, astriped focal-conic texture appears when the transition
to the mesophase occurs. On increasing the voltage
further, the stripes become clearer and sharper. We haveobserved similar behaviour for the B phase of com2

pound 15 [ 11 ] . These observations clearly indicate thatthe mesophase is antiferroelectric in character.
We have examined the eŒects of some lateral substituentson the mesophases formed by these banana-shape dcompounds. The substituents used are  uoro, chloro,
methyl and methoxy groups. The same three homologues, viz. n 5 10, 11, 12 with each of these groups
in the same position, were studied. A comparison of
the mesophase types and transition temperatures of theunsubstituted parent compounds 4, 5 and 6 and thediŒerently substituted compounds 7 to 18 (see tables 1and 2) indicates the following. In all cases, from thesmall  uoro substituent to the large, bulky methoxy
group, there is a reduction of the clearing temperatures.
Except for compounds 9, 12 and 18, there is also a
general decrease in the melting points.
The introduction of a  uoro substituent increases themolecular lateral dimension marginally with a small
associated increase in the steric interactions. Since the uoro substituent is strongly electronegative, it alsoaŒects the polarity of the system, and hence the dipolarinteractions between neighbouring molecules will beconsiderable. Indeed, there will be a competition between
steric and dipolar interactions whenever a ring hydrogenis replaced by any substituent. Compound 4 shows a B1
phase while compounds 5 and 6 exhibit the B2 phase.
The corresponding  uoro-substitute d compounds 7, 8
and 9 show only the B1 phase, eliminating the B2 phasecompletely. Similar behaviour was observed for thechloro-substitute d compounds 10, 11 and 12. However,in the case of the methyl substituted derivatives 13, 14and 15, the phases present in the parent compounds are
retained although with considerable reductions in thetransition temperatures. In the case of the bulky methoxy

Preliminary communication
substituent, compounds 16, 17 and 18, the enantiotropi cmesophases present in the parent compounds arereplaced by monotropic behaviour but without aŒectingthe nature of the mesophases. As one proceeds from uoro to chloro, methyl and methoxy groups, their size
increases gradually. This results in the lowering of theclearing temperatures, in that order. Although the sizeof the chloro and methyl groups are about the same,
there is a diŒerence in the way they aŒect the mesophasesof the parent compounds. The main diŒerence between
these two substituents is that their dipoles are in opposingdirections. Also, both  uoro and chloro substituents
eliminate the B2 phase present in the parent compounds.
This suggests that dipolar eŒects are signi cant whencompared with steric factors in these compounds.
A number of other compounds with substituents indiŒerent positions have been prepared and the results ofstudies on these will be presented in a forthcoming
publication.
The authors thank Dr V. A. Raghunathan for help
with X-ray measurements, and Mr M. R. Subrahmanya mand Mr K. Subramanya for technical support.
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