Non-symmetric liquid crystal dimers: High thermal stability in nematic phase enhanced by thiophene-2-carboxylate moiety

Article abstract of DOI:10.1080/15421400902987248

Series of non-symmetric liquid crystal dimers, α-(4- benzylidenechloroaniline-4′-oxy) ω- [4-(thiophene-2-carboxyl) benzylideneaniline-4′-oxy]alkanes incorporating a thiophene-based moiety in one of the two mesogenic units have been synthesized and charact

Full text of DOI:10.1080/15421400902987248

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Non-Symmetric Liquid Crystal
Dimers: High Thermal Stability
in Nematic Phase Enhanced
by Thiophene-2-Carboxylate
Moiety
Guan-Yeow Yeap a , Tiang-Chuan Hng a , Daisuke
Takeuchi b , Kohtaro Osakada b , Wan Ahmad Kamil
Mahmood a & Masato M. Ito c
a Liquid Crystal Research Laboratory, School of
Chemical Sciences , Universiti Sains Malaysia ,
Penang, Malaysia
b Chemical Resources Laboratory , Tokyo Institute of
Technology , Midori-ku, Yokohama, Japan
c Department of Environmental Engineering for
Symbiosis, Faculty of Engineering , Soka University ,
Hachioji, Tokyo, Japan
Published online: 03 Aug 2009.
To cite this article: Guan-Yeow Yeap , Tiang-Chuan Hng , Daisuke Takeuchi , Kohtaro
Osakada , Wan Ahmad Kamil Mahmood & Masato M. Ito (2009) Non-Symmetric Liquid
Crystal Dimers: High Thermal Stability in Nematic Phase Enhanced by Thiophene-2-
Carboxylate Moiety, Molecular Crystals and Liquid Crystals, 506:1, 134-149, DOI:
10.1080/15421400902987248
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Mol. Cryst. Liq. Cryst., Vol. 506, pp. 134–149, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902987248
Non-Symmetric Liquid Crystal Dimers: High Thermal
Stability in Nematic Phase Enhanced
by Thiophene-2-Carboxylate Moiety
Guan-Yeow Yeap1, Tiang-Chuan Hng1, Daisuke Takeuchi2,
Kohtaro Osakada2, Wan Ahmad Kamil Mahmood1, and
Masato M. Ito3
1Liquid Crystal Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, Penang, Malaysia
2Chemical Resources Laboratory, Tokyo Institute of Technology,
Midori-ku, Yokohama, Japan
3Department of Environmental Engineering for Symbiosis,
Faculty of Engineering, Soka University, Hachioji, Tokyo, Japan
Series of non-symmetric liquid crystal dimers, a-(4-benzylidenechloroaniline-40-
oxy)-x-[4-(thiophene-2-carboxyl)benzylideneaniline-40-oxy]alkanes incorporating
a thiophene-based moiety in one of the two mesogenic units have been synthesized
and characterized. The nematogenic properties of the dimers were studied wherein
=
the flexible spacers made up by n methylene units (–CH2-) ranging from n 5 to
=
n 12. These dimers exhibit enantiotropic N phase with high thermal stability
(>187.0ꢀC). The N phase temperature range for the present homologues is found
to be strongly dependent on the length and parity of the methylene spacer which
connects the 4-benzylidenechloroaniline and 4-(thiophene-2-carboxyl)- benzylide-
neaniline mesogenic cores.
Keywords: enantiotropic N phase; high thermal stability; non-symmetric liquid crystal
dimers; thiophene
1. INTRODUCTION
The liquid crystals derived from oligomers have attracted much
attention for several reasons. Part of the growing interest resulted
from the fact that the oligomers exhibit considerable differences in
Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory,
School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang,
Malaysia. E-mail: gyyeap@usm.my or gyyeap_liqcryst@yahoo.com
134

Non-Symmetric Liquid Crystal Dimers
135
properties in comparison to the conventional low molecular mass
liquid crystals [1]. Also, previous findings on the dimers and trimers
have suggested that the oligomeric members can act as model com-
pounds for the semiflexible main chain liquid crystalline polymers
[2]. A striking similarity between the oligomeric and polymeric
derivatives which has received great attention from researchers is
the odd–even effect which refers to the spacer parity-dependent meso-
morphic properties [3]. Because of higher degree of structure linearity,
the members with even spacers possess higher phase transition
temperatures in comparison to the odd members which are less linear
[2,3]. Spacers with even parity possess more conformers which allow
parallel arrangement of mesogenic cores. On the other hand, odd
spacers possess more conformers which result in arrangement of cores
with bent configuration. Extensive investigation on this interesting
odd–even alternation of transition temperatures has been carried
out by Imrie and co-workers on various systems of dimers, trimers, and
tetramers such as the 4,40-bis[x-(4-cyanobiphenyl-40-yloxy)alkoxy]-
biphenyls [4].
Although many novel liquid crystal dimers have been reported so
far but the number of dimers with non-symmetric structure has still
been relatively small. Among these non-symmetric dimers, the
five-membered heterocyclic rings such as furan and thiophene in the
molecular structures are rare. Most of the known liquid crystals
incorporating one or more five-membered rings are not liquid crystal
oligomers and many of these compounds are smectogenic and nemato-
genic [5,6]. Previous studies have shown that heteroatoms in the
heterocyclic rings may cause changes in a variety of parameters such
as dielectric anisotropy [7]. Since the report on non-symmetric dimers
incorporating five-membered heterocyclic rings and the potential
influence of these rings on certain physical characteristics has
hitherto found to be rare, the structure-property relationships of these
dimers are indeed worth an exploration. In the mesophase, the
presence of a heterocyclic ring in either one of the two mesogenic
groups within the dimers can induce a different kind of intermolecular
interaction, and hence it will be interesting if further investigation can
focus on this subject.
The unconventional properties of non-symmetric dimers and our
interests to examine the influence of a heterocyclic ring on the liquid
crystalline properties have prompted us to prepare a novel series of
thiophene-based non-symmetric dimers. This series of non-symmetric
dimers contains eight homologous a-(4-benzylidenechloroaniline-40-
oxy)-x-[4-(thiophene-2-carboxyl)benzylideneaniline-40-oxy]alkanes with
the general molecular structure as shown below.

136
G.-Y. Yeap et al.
The dimers differ from one another in terms of the methylene
spacer, (CH2)n with odd and even n ranging from 5 to 12. The liquid
crystalline properties of the hexyl and octyl homologues in this series
are also compared with the previously reported analogous DL-
a-(4-benzylidenechloroaniline-40-oxy)-x-[2-methylbutyl-40-(400-phenyl)-
benzoateoxy]alkanes.
2. EXPERIMENTAL
2.1. Synthesis
All intermediates and title compounds in this series were synthesized
based on the routes given in Scheme 1. Compound 1 is the product of
esterification upon employment of Schotten–Baumann technique with
pyridine serving as the base. The condensation between compound 1
and 4-aminophenol in an ethanolic solution led to the formation of
the compound 2. Compounds 3a–3h were prepared via two-step synth-
eses starting from condensation reaction and then Williamson ether
synthesis in the presence of potassium carbonate anhydrous as
SCHEME 1

Non-Symmetric Liquid Crystal Dimers
137
reported elsewhere [8]. The title compounds 4a–4h were synthesized
by reacting compound 2 with compounds 3a–3h, respectively, with
potassium carbonate anhydrous and catalytic amount of potassium
iodide. The labels referring to the intermediary compounds 3a–3h
and title compounds 4a–4h with respective spacers (C5H10 to
C12H24) are shown in Table 1.
2.1.1. Synthesis of Compound 1
4-Hydroxybenzaldehyde (2.44 g, 20.0 mmole) was dissolved in 8 mL
pyridine in a round-bottom flask prior to the addition of 50 mL
benzene. 2-Thiophenecarbonyl chloride (2.93 g, 20.0 mmole) was then
added very slowly to the stirring solution. The mixture was refluxed
for 2 h. The undesired precipitate was immediately filtered off and
the resulting solution was left for evaporation. The product was
washed with absolute ethanol and subsequently recrystallized from
the mixture of chloroform and hexane. Yield 71%. Elemental analysis
(%): found, C 62.07, H 3.49; calculated (C12H8O3S), C 62.06, H 3.47. IR
(KBr) n=cmꢁ1, 1726 (C O ester), 1697 (C O aldehyde). 1H-NMR
(CDCl3) d=ppm, 7.23 (dd, 1 H, Th), 7.46 (d, 2 H, Ar), 7.73 (dd, 1 H,
Th), 7.98 (d, 2 H, Ar), 8.03 (dd, 1 H, Th), 10.05 (s, 1 H, CHO).
=
=
2.1.2. Synthesis of Compound 2
Compound 1 (2.79 g, 12.0 mmole) was dissolved in 40 mL absolute
ethanol in a round-bottom flask. As the solution was heated up, an
ethanolic solution of 4-aminophenol (1.31 g, 12.0 mmole) was added
dropwise. The mixture was refluxed for 3 h. The resulting hot solution
was subsequently cooled to 5ꢀC for 1=2 h to precipitate out the solids.
Upon filtration, the crude products were then recrystallized from the
mixture of chloroform and hexane. Yield 52%. Elemental analysis
(%): found, C 66.88, H 4.06, N 4.34; calculated (C18H13NO3S),
TABLE 1 Intermediary Compounds 3a–3h and Title
Compounds 4a–4h with Respective Spacers, (CH2)n
Intermediate
Compound
Spacer (CH2)n
3a
3b
3c
3d
3e
3f
4a
4b
4c
4d
4e
4f
C5H10
C6H12
C7H14
C8H16
C9H18
C10H20
C11H22
C12H24
3g
3h
4g
4h

138
G.-Y. Yeap et al.
C 66.86, H 4.05, N 4.33. IR (KBr) n=cmꢁ1, 3448 (OH), 1690 (C O ester),
=
1
=
1627–1600 (C N). H-NMR (CDCl3) d=ppm, 4.70 (s, 1 H, OH), 6.89 (d,
2 H, Ar), 7.21 (2d, 3 H, Ar and Th), 7.38 (d, 2 H, Ar), 7.69 (dd, 1 H, Th),
=
7.96 (d, 2 H, Ar), 8.02 (dd, 1 H, Th), 8.50 (s, 1 H, CH N).
2.1.3. Syntheses of Compounds 3a-3 h
Compounds 3a–3h were synthesized according to the previously rep-
orted method [8]. The carbon, hydrogen, and nitrogen (CHN) microana-
lytical results of these intermediates conform to the calculated values.
2.1.4. Synthesis of Compound 4a
In a round-bottom flask containing potassium carbonate anhydrous
(0.14 g, 1.0 mmole), compound 2 (0.16 g, 0.5 mmole) was dissolved in
40 mL acetone and heated up. Subsequently, compound 3a (0.19 g,
0.5 mmole) in another 40 mL acetone was added dropwise to the solu-
tion. The mixture was refluxed for 16 h. The acetone solution was then
left at room temperature to evaporate off. The potassium carbonate
anhydrous was removed in 50 mL water. The desired precipitate was
filtered off and recrystallized twice, initially from ethyl acetate and,
subsequently, from the mixture of chloroform and hexane. Yield
52%. Elemental analysis (%): found, C 69.41, H 5.04, N 4.51; calculated
(C36H31N2O4SCl), C 69.39, H 5.01, N 4.50. IR (KBr) n=cmꢁ1
,
=
=
2943–2871 (C-H alkyl), 1729 (C O ester), 1622–1605 (C N), 1252
1
(O-CH2). H-NMR (CDCl3) d=ppm, 1.60–1.99 (m, 6 H, CH2), 4.10 (t,
4 H, OCH2), 7.02 (d, 4 H, Ar), 7.17 (d, 2 H, Ar), 7.22 (t, 1 H, Th), 7.27
(m, 4 H, Ar), 7.35 (d, 2 H, Ar), 7.70 (d, 1 H, Th), 7.87 (2d, 4 H, Ar),
=
=
8.02 (d, 1 H, Th), 8.38 (s, 1 H, CH N), 8.43 (s, 1 H, CH N).
Whilst compounds 4b, 4d, 4f, and 4h were recrystallized from the
mixture of ethyl acetate and chloroform, compounds 4c, 4e, and 4g
were purified by the same solvent system applied for compound 4a.
2.1.5. Synthesis of Compound 4b
Yield 73%. Elemental analysis (%): found, C 69.75, H 5.24, N 4.42;
calculated (C37H33N2O4SCl), C 69.74, H 5.22, N 4.40. IR (KBr)
n=cmꢁ1, 2940–2863 (C-H alkyl), 1738 (C O ester), 1620–1605 (C N),
1248 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.62–1.91 (m, 8 H, CH2),
4.10 (t, 4 H, OCH2), 6.99 (d, 4 H, Ar), 7.15 (d, 2 H, Ar), 7.20 (t, 1 H,
Th), 7.26 (m, 4 H, Ar), 7.34 (d, 2 H, Ar), 7.67 (d, 1 H, Th), 7.86 (t, 4 H,
=
=
=
=
Ar), 8.02 (d, 1 H, Th), 8.37 (s, 1 H, CH N), 8.42 (s, 1 H, CH N).
2.1.6. Synthesis of Compound 4c
Yield 51%. Elemental analysis (%): found, C 70.10, H 5.44, N 4.31;
calculated (C38H35N2O4SCl), C 70.08, H 5.42, N 4.30. IR (KBr)

Non-Symmetric Liquid Crystal Dimers
139
n=cmꢁ1, 2938–2862 (C-H alkyl), 1723 (C O ester), 1621–1606 (C N),
1247 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.51–1.88 (m, 10 H, CH2),
4.06 (t, 4 H, OCH2), 7.01 (d, 4 H, Ar), 7.16 (d, 2 H, Ar), 7.20 (t, 1 H,
Th), 7.28 (m, 4 H, Ar), 7.34 (d, 2 H, Ar), 7.69 (d, 1 H, Th), 7.86 (2d,
=
=
=
=
4 H, Ar), 8.01 (d, 1 H, Th), 8.37 (s, 1 H, CH N), 8.41 (s, 1 H, CH N).
2.1.7. Synthesis of Compound 4d
Yield 63%. Elemental analysis (%): found, C 70.42, H 5.63, N 4.20;
calculated (C39H37N2O4SCl), C 70.41, H 5.61, N 4.21. IR (KBr)
n=cmꢁ1, 2937–2856 (C-H alkyl), 1738 (C O ester), 1620–1605 (C N),
1247 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.44–1.88 (m, 12 H, CH2),
4.07 (t, 4 H, OCH2), 7.00 (2d, 4 H, Ar), 7.15 (d, 2 H, Ar), 7.21 (dd,
1 H, Th), 7.25 (m, 4 H, Ar), 7.33 (d, 2 H, Ar), 7.66 (dd, 1 H, Th), 7.84
=
=
=
(2d, 4 H, Ar), 8.00 (dd, 1 H, Th), 8.37 (s, 1 H, CH N), 8.41 (s, 1 H,
=
CH N).
2.1.8. Synthesis of Compound 4e
Yield 52%. Elemental analysis (%): found, C 70.75, H 5.82, N 4.13;
calculated (C40H39N2O4SCl), C 70.73, H 5.79, N 4.12. IR (KBr)
n=cmꢁ1, 2932–2853 (C-H alkyl), 1731 (C O ester), 1622–1605 (C N),
1250 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.41–1.84 (m, 14 H, CH2),
4.05 (t, 4 H, OCH2), 6.99 (2d, 4 H, Ar), 7.16 (d, 2 H, Ar), 7.21 (dd,
1 H, Th), 7.26 (m, 4 H, Ar), 7.35 (d, 2 H, Ar), 7.68 (dd, 1 H, Th), 7.85
=
=
=
=
(t, 4 H, Ar), 8.01 (dd, 1 H, Th), 8.37 (s, 1 H, CH N), 8.41 (s, 1 H, CH N).
2.1.9. Synthesis of Compound 4f
Yield 70%. Elemental analysis (%): found, C 71.04, H 5.99, N 4.05;
calculated (C41H41N2O4SCl), C 71.03, H 5.96, N 4.04. IR (KBr) n=cmꢁ
1
=
=
, 2940–2863 (C-H alkyl), 1739 (C O ester), 1620–1606 (C N), 1249
1
(O-CH2). H-NMR (CDCl3) d=ppm, 1.39–1.87 (m, 16H, CH2), 4.06 (t,
4 H, OCH2), 7.00 (2d, 4 H, Ar), 7.15 (d, 2 H, Ar), 7.19 (dd, 1 H, Th),
7.25 (m, 4 H, Ar), 7.33 (d, 2 H, Ar), 7.68 (dd, 1 H, Th), 7.82 (2d, 4 H,
=
=
Ar), 7.99 (dd, 1 H, Th), 8.36 (s, 1 H, CH N), 8.41 (s, 1 H, CH N).
2.1.10. Synthesis of Compound 4g
Yield 54%. Elemental analysis (%): found, C 71.34, H 6.15, N 3.98;
calculated (C42H43N2O4SCl), C 71.32, H 6.13, N 3.96. IR (KBr)
n=cmꢁ1, 2936–2852 (C-H alkyl), 1731 (C O ester), 1622–1605 (C N),
1249 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.35–1.85 (m, 18 H, CH2),
4.05 (t, 4 H, OCH2), 7.00 (2d, 4 H, Ar), 7.16 (d, 2 H, Ar), 7.21 (dd,
1 H, Th), 7.26 (m, 4 H, Ar), 7.35 (d, 2 H, Ar), 7.70 (dd, 1 H, Th), 7.85
=
=
=
(2d, 4 H, Ar), 8.02 (dd, 1 H, Th), 8.37 (s, 1 H, CH N), 8.41 (s, 1 H,
=
CH N).

140
G.-Y. Yeap et al.
2.1.11. Synthesis of Compound 4h
Yield 67%. Elemental analysis (%): found, C 71.62, H 6.32, N 3.89;
calculated (C43H45N2O4SCl), C 71.60, H 6.29, N 3.88. IR (KBr)
n=cmꢁ1, 2936–2850 (C-H alkyl), 1738 (C O ester), 1620–1606 (C N),
1250 (O-CH2). 1H-NMR (CDCl3) d=ppm, 1.33–1.85 (m, 20 H, CH2),
4.04 (t, 4 H, OCH2), 7.00 (d, 4 H, Ar), 7.16 (d, 2 H, Ar), 7.21 (t, 1 H,
Th), 7.26 (m, 4 H, Ar), 7.34 (d, 2 H, Ar), 7.70 (d, 1 H, Th), 7.85 (2d,
=
=
=
=
4 H, Ar), 8.01 (d, 1 H, Th), 8.37 (s, 1 H, CH N), 8.41 (s, 1 H, CH N).
2.2. Characterization
The molecular structures of all intermediary and title compounds were
elucidated via spectroscopic methods. The FT-IR technique was carried
out on a Perkin Elmer 2000 FTIR spectrometer. The samples were
prepared in the form of KBr pellets and analyzed within frequency range
1
of 4000–400cmꢁ1. The investigation by H-NMR was performed on a
Bruker 400 MHz UltrashieldTM spectrometer. The compounds were
dissolved in deuterated chloroform (CDCl3) with 0.03% (v=v) TMS as
internal standard. CHN microanalyses were performed using a Perkin
Elmer 2400 LS Series CHNS=O analyzer. Texture observation was
performed on a Carl Zeiss Axioskop 40 polarizing microscope linked to
a Linkam TMS94 temperature controller and LTS350 hot-stage. In order
to determine the phase transition temperatures, associated enthalpies
and entropy changes for the nematic-isotropic transition, differential
scanning calorimetry was carried out using a Seiko DSC6200R
calorimeter with the heating and cooling rate of ꢂ 5ꢀC minꢁ1 at Tokyo
Institute of Technology, Japan. The nature of the mesophase was
identified conclusively using a Bruker D8 powder X-ray diffractometer
for representative compound 4f at Warsaw University, Poland.
3. RESULTS AND DISCUSSION
3.1. Liquid Crystalline Properties
3.1.1. The Odd–Even Effect on Phase Transition Temperatures
All non-symmetric dimers (4a–4h) in this series are enantiotropic nema-
togens. The phase transition temperatures, respective enthalpies, and
entropy changes for the nematic-isotropic transition are shown in
Table 2. The dependence of all transition temperatures and N phase
temperature ranges upon the number of methylene units, n in the
spacers during heating run is shown in Fig. 1a. From the graph, it is
apparent that the Cr-N and N-I transition temperatures are strongly
correlated with the parity of the spacer. The alternation of the melting

Non-Symmetric Liquid Crystal Dimers
141
TABLE 2 Phase Transition Temperatures (ꢀC), Associated Enthalpies
(kJ molꢁ1), and Nematic-Isotropic Entropy Changes of Compounds 4a–4h
(. ¼ Enantiotropic Phase; Cr ¼ Crystal, N ¼ Nematic, I ¼ Isotropic)
Compound
Cr
N
I
DSNI=R
4a
4b
4c
4d
4e
4f
. 126.3 (9.5)
. 166.1 (13.2)
. 135.4 (11.7)
. 149.1 (14.7)
. 131.9 (9.1)
. 149.6 (15.5)
. 134.6 (10.3)
. 150.2 (20.2)
. 203.1 (0.3)
. 245.8 (0.5)
. 202.9 (0.5)
. 224.2 (1.3)
. 196.7 (0.5)
. 207.6 (1.2)
. 187.9 (0.7)
. 193.4 (1.3)
.
.
.
.
.
.
.
.
0.06
0.12
0.11
0.32
0.13
0.31
0.18
0.33
4g
4h
temperatures into the N phase is strong although attenuation has not
been recorded. The members with even-parity spacers melt at higher
temperatures as compared to the odd counterparts. Similarly, the even
members also possess higher clearing temperatures (TNI) than the odd
members. However, as the series ascends towards the longer spacer,
the difference of TNI between neighbouring odd and even homologues
=
reduces substantially. As such, the TNI of compound 4h (n 12) is only
ꢀ
=
5.5 C higher than that in compound 4g (n 11). The strong attenuation
of TNI on increasing n has also been reported for other series of dimers
such as the cholesterol and cyanobiphenylyl-based liquid crystals [9].
This odd–even characteristic is part of the unique features of the dimers
and other oligomeric members which is not observed in the series of non-
oligomeric mesogens.
Upon increasing n, the suppression of the nematogenic properties of
the even members is more pronounced as compared to the odd mem-
=
bers (Fig. 1b). Compounds 4b and 4d (n 6 and 8, respectively) possess
larger N phase temperature range as compared to the odd compounds
=
4a and 4c (n 5 and 7, respectively). However, with the addition of
more methylene units, significant decreasing trend is observed in
the N phase temperature range for even members as compound 4h
ꢀ
=
=
(n 12) is 10.1 C smaller than that of compound 4g (n 11). Such char-
acteristic can be justified based on the influence of the spacers.
Regardless of the spacer length, the dimers with odd spacers presum-
ably possess less linear conformation and thus lower orientational
order compared to the even counterparts [3]. Every addition of two
methylene units from C5H10 to C11H22 does not result in significant
increase in the dilution of the cores if compared to the even members
which have higher shape anisotropy. As the result, the dilution effect
of the cores is more pronounced for the even members as the spacer

142
G.-Y. Yeap et al.
FIGURE 1 (a) The dependence of phase transition temperatures, T on the
number of methylene units, n in the flexible spacer and (b) The comparison
of N phase temperature range, T between odd and even members during
heating run.
length is increased. Therefore, with the increasing spacer length, the
nematogenic properties of the even dimers are reduced in greater mag-
nitude than the odd counterparts.
3.1.2. The Thermal and Optical Properties
The DSC results of compounds 4a–4h are consistent with the
results obtained via polarizing microscopy in terms of transition
temperatures for all heating and cooling cycles. The DSC traces of
representative compounds 4d and 4f are given in Figs. 2a,b, respec-
tively. Generally, the enthalpies of fusion for dimers with even-parity

Non-Symmetric Liquid Crystal Dimers
143
FIGUREꢁ21 The DSC traces of (a) compound 4d and (b) compound 4f with
ꢂ5ꢀCmin heating and cooling rates.
are higher than those detected for the odd dimers (Table 2). This
characteristic suggests that the intermolecular attractive forces for
the even dimers in the Cr lattice are stronger as compared to the
odd members owing to the more linear conformation of the former.

144
G.-Y. Yeap et al.
FIGURE 3 The dependence of the entropy change (DSNI=R) associated with
the nematic-isotropic transition on the number of methylene units, n in the
flexible spacer.
The nematic-isotropic (N-I) transitional entropy changes of the
dimers are expressed in dimensionless quantity, DSNI=R. The depen-
dence of the entropy change, DSNI=R associated with the N-I transi-
tion on the number of methylene units, n in the flexible spacer is
given in Fig. 3. The overall DSNI=R values of the even dimers 4b,
4d, 4f, and 4h fall in the range of 0.12–0.33. These values are
considerably lower than earlier reported dimers with even spacers
such as the dimers of the m-OnO-m and m-n-m series [10]. In order
to substantiate this point, a comparison in relation to the magnitude
of DSNI=R will be made later in the discussion. Despite the low
entropy changes of the even members, these values are still generally
higher than those detected for the odd counterparts. The DSNI=R
values of the even members are approximately 1.7–5.5 times larger
than those of the odd members except compound 4b. The DSNI=R
values for compounds 4a, 4c, 4e, and 4g are 0.06, 0.11, 0.13, and
0.18, respectively. The lower entropy changes of the odd dimers
are, however, not uncommon as the more biaxial odd dimers tend
to possess entropies of transition similar to the monomers [11]. Apart
from the odd dimers, it is interesting to note that the value for
compound 4b with hexyl spacer is far lower than its even counter-
parts. Whilst DSNI=R of compound 4d, 4f, and 4h are 0.32, 0.31,
and 0.33, respectively, the DSNI=R of compound 4b is only 0.12.
The unusually low DSNI=R value of 4b could be attributed to the
lower shape anisotropy of 4b as compared to the other three even
members with longer spacers [10]. The overall entropy changes

Non-Symmetric Liquid Crystal Dimers
145
exhibit a dramatic alternation as n is increased with the exception
=
involving n 6 as we mentioned earlier.
The textures of the title compounds 4a–4h under polarized light are
identified based on several characteristics. On thin samples, the N
phase appears as marble texture upon heating of the Cr phase. As
temperature is elevated, colour changes are observed in certain
domains due to the change in birefringence [12]. As it approaches
the isotropic phase, the N schlieren texture with both twofold and
fourfold brushes is then observed for compound 4g (Fig. 4). However,
upon heating, compound 4f exhibit the coexistence of homeotropic and
planar region. Our observation on this microscopic sample reveals the
presence of filamentary texture in the homeotropic region which
resembles that of SmA and SmC phases [13]. Upon cooling at
ꢁ5ꢀCminꢁ1, the filaments of compound 4f gradually transform to com-
pact domains as shown in Fig. 5. Such filamentary structure has yet to
be reported for N phase. However, upon mechanical shearing of the
sample, the occurrence of bright flashes suggests that the mesophase
is indeed nematogenic in nature [12]. The flash indicates reorientation
of the director and, therefore, also the optic axis of the N phase due to
the applied pressure upon shearing. In order to ascertain the meso-
phase conclusively, the powder X-ray diffraction analysis is conducted
for compound 4f. The absence of sharp reflection at low angle confirms
that layer periodicity is absent in the mesophase structure whilst the
broadly diffuse signal at wide angle region indicates the short range
order typical of the N phase [14].
FIGURE 4 The N schlieren texture (compound 4g) with both twofold and
fourfold brushes at 186.0ꢀC.

146
G.-Y. Yeap et al.
FIGURE 5 The filaments observed in the homeotropic region of N phase
(compound 4f) which initially form (left) upon cooling and gradually transform
to compact domain (right).
3.1.3. Comparison Studies Between Title Compounds
and DL-a-(4-Benzylidene- Chloroaniline-40-Oxy)-x-
[2-Methylbutyl-40-(400-Phenyl)benzoateoxy]alkanes
In order to observe the changes in mesomorphic properties as the
result of replacing one of the two mesogenic cores with a different core
whilst another remains the same, comparison is made between the
properties of the present dimers and those of the previously synthe-
sized racemic a-(4-benzylidenechloroaniline-40-oxy)-x-[2-methylbutyl-
40-(400-phenyl)benzoateoxy]alkanes [15]. These dimers are analogous
with the present thiophene-based dimers in terms of 4-benzylidene-
chloroaniline mesogenic group. To ensure that the comparison is rele-
vant, only compounds 4b and 4d from this series are chosen since the
=
=
dimers mentioned above only incorporate spacers with n 6 or n 8. The
general molecular structure of the reported dimers and respec-
tive phase transitions are shown below, where the dimers with the
spacers made up by hexyl and octyl groups are labelled as 5a and
5b, respectively.

Non-Symmetric Liquid Crystal Dimers
147
Comparison in terms of liquid crystalline properties between 5a and
5b and the present dimers with similar spacer length reveals that the
differences are pronounced. For instance, the DSNI=R values of 5a and
5b are 1.01 and 0.98, respectively. Whilst these values are within the
typical range for nematic-isotropic transition of the dimers, the
DSNI=R values of 4b and 4d, as mentioned earlier, are several times
lower than those of compounds 5a and 5b. Although the lower magni-
tude of DSNI=R is often associated with the decreased shape anisotropy
and the increased molecular biaxiality, it is premature to directly
attribute the lower DSNI=R of compounds 4b and 4d to the two possible
factors. This is, in part, due to the fact that the melting points and
mesophase thermal stability of compounds 4b and 4d are found to
be significantly higher than those of 5a and 5b. By replacing the
2-methylbutyl-40-(400-phenyl)benzoateoxy group with 4-(thiophene-2-
carboxyl)benzylideneaniline-40-oxy group, the clearing point for
compound 4b for instance, is 80.5ꢀC higher than that of compound 5a.
By substituting a mesogenic group with another one, it is important
to realize that a number of properties such as electronic effect, core
length, rigidity and fractional residue volume are also altered at the
same time. As a consequence, the differences in terms of transition
temperatures between 4b, 4d and 5a, 5b, as stated earlier, cannot
be directly justified based on the structure and chemical constitution
alone, without taking into account the changes in other properties.
Nevertheless, it is equally important to mention that the 4-(thio-
phene-2-carboxyl)benzylideneaniline group is larger in size and
surface area, as compared to the biphenyl core, which results in
stronger intermolecular attraction. At the same time, the presence of
a thiophene ring at one of the terminals of compounds 4b and 4d also
enhances the terminal attraction among molecules although it should
not be assumed that the lateral attraction is also strengthened. This
point is reflected appropriately as nematogenic properties rather than
the smectogenic properties, predominate compounds 4a–4h in this
series.
3.2. Spectroscopic Results
From the FT-IR spectra of compounds 4a–4h, the absorption band at
2943–2852 cmꢁ1 is assigned to the C-H stretching of the respective
methylene spacers. At lower frequency, the carbonyl group of the ester
linkage between the thiophene ring and the benzylideneaniline frag-
ment absorbs at 1739–1723 cmꢁ1. The azomethine groups give rise
to a band with medium intensity at 1622–1605 cmꢁ1. The vibrations
within these frequencies have also been reported for the liquid crystals

148
G.-Y. Yeap et al.
derived from phthalic acid [16]. The strongest band is observed in the
fingerprint region of 1252–1247 cmꢁ1 which can be attributed to the
presence of two ether (O-CH2) groups for every dimeric structure.
From the 1H-NMR spectra, the peaks, multiplicities and respective
chemical shifts are assignable to the protons of 4a–4h. The following
description is based on the 1H-NMR spectrum of compound 4d. At
high-field, the twelve methylene protons of the C8H16 spacer give rise
=
to two sets of multiplets at d 1.44–1.88 ppm.
=
A triplet at d 4.07 ppm assignable to four methoxy protons, on the
other hand, is similar to the absorption reported for 5-octylpyrimi-
dine-2-yl oligomers [17]. Although the overall structure is
non-symmetric due to two different mesogenic groups, the appearance
of only one triplet owing to the two pairs of methoxy protons indicates
the presence of symmetry in the flexible spacer. The appearance of six
=
doublets, three double of doublets and one multiplet at d 7.00–
8.00 ppm can be attributed to sixteen aromatic protons and three
=
thiophene protons. At low-field, two singlets observed at d 8.37 and
8.41 ppm can be assigned to the azomethine protons of 4-
benzylidenechloroaniline and 4-(thiophene-2-carboxyl)benzylidenea-
niline moieties, respectively.
4. CONCLUSIONS
Series of non-symmetric liquid crystal dimers, a-(4-benzylidenechlor-
oaniline-40-oxy)-x-[4-(thiophene-2-carboxyl)benzylideneaniline-40-oxy]-
alkanes has been synthesized and characterized. The title compounds
varying in terms of flexible methylene spacers (C5H10 to C12H24) exhi-
bit enantiotropic N phase with relatively high thermal stability
(>187.0ꢀC). The nematic-isotropic entropy changes of the eight homo-
logues are considerably lower than those of the previously reported
dimers. Under the polarized light, the N phase is identified based on
its characteristic marble and schlieren texture as well as the flashes
upon mechanical shearing. The nematogenic properties of the title
compounds have been attributed to the presence of a thiophene ring
at one of the terminals which induces predominant terminal intermo-
lecular attraction rather than lateral attraction.
ACKNOWLEDGMENTS
The main author (G.-Y. Yeap) acknowledges the Malaysian Ministry of
Science, Technology and Innovation (MOSTI) for the research grant
(eScience No.03–01-05-SF0366 or 305=PKIMIA=613328). The authors

Non-Symmetric Liquid Crystal Dimers
149
thank Professor Corrie T. Imrie (University of Aberdeen, Scotland)
and Professor Ewa Gorecka (Warsaw University, Poland) for some
helpful comments.
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