Chiral smectic *C mesogens having ester and amide central linkages

Article abstract of DOI:10.1080/15421400801915027

Ten members of a chiral homologous series 4-(4'-n-alkoxy benzoyloxy benzoyl)4''- S- (+) 2-methyl butoxy anilides were synthesized in multiple step synthesis. Series exhibits chiral smectic *C and cholesteric mesophases. In this series, n-propyl to n-dodecyl derivatives exhibit smectic *C and cholesteric mesophases. n-Tetradecyl and n-hexadecyl derivatives exhibit only smectic *C phases. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits odd-even effect for cholestric-isotropic transition temperatures. The effect of amide linkage and chirality is discussed in detail. High-temperature cholestric phases exhibit vivid colors. Two compounds were analyzed by calorimetric study (C12 and C14) to confirm the transition temperatures of which was observed under polarizing microscope. The mesogenic behavior of the series compared with other series where amide central linkage differs in its position. The mesogenic thermal stability is compared with other related homologous series. The result indicates that amide linkage enhances smectic as well as cholesteric thermal stability but enhancement in smectic phase is much higher. Compounds are characterized by elemental analysis, IR spectral data, Mass spectral data, DSC, NMR, and microscopic study.

Full text of DOI:10.1080/15421400801915027

Mol. Cryst. Liq. Cryst., Vol. 482, pp. 103–116, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400801915027
ꢀ
Central Linkages
Chiral Smectic C Mesogens Having Ester and Amide
R. C. Tandel1 and R. A. Vora2
1Chemistry Department, Faculty of Science, M. S. University of
Baroda, Vadodara, Gujarat, India
2Applied Chemistry Department, Faculty of Technology and
Engineering, M. S. University of Baroda, Vadodara, Gujarat, India
Ten members of a chiral homologous series 4-(40-n-alkoxy benzoyloxy benzoyl)400-
S- (þ) 2-methyl butoxy anilides were synthesized in multiple step synthesis. Series
exhibits chiral smectic ꢀC and cholesteric mesophases. In this series, n-propyl to
n-dodecyl derivatives exhibit smectic ꢀC and cholesteric mesophases. n-Tetradecyl
and n-hexadecyl derivatives exhibit only smectic ꢀC phases. The plot of transition
temperatures versus number of carbon atoms in alkoxy chain exhibits odd–even
effect for cholestric-isotropic transition temperatures. The effect of amide linkage
and chirality is discussed in detail. High-temperature cholestric phases exhibit
vivid colors. Two compounds were analyzed by calorimetric study (C12 and C14
)
to confirm the transition temperatures of which was observed under polarizing
microscope. The mesogenic behavior of the series compared with other series where
amide central linkage differs in its position. The mesogenic thermal stability is
compared with other related homologous series. The result indicates that amide
linkage enhances smectic as well as cholesteric thermal stability but enhancement
in smectic phase is much higher. Compounds are characterized by elemental
analysis, IR spectral data, Mass spectral data, DSC, NMR, and microscopic study.
Keywords: chiralsmecticꢀC; ester and amidecentrallinkages;mesogenic; odd–even effect
INTRODUCTION
It is well established that optically active nematogens exhibit choles-
teric mesophases. The term ‘‘Chiral Nematic’’ was coined by Dolphin
[1] to describe this general class of liquid crystals. Number of chiral
nematic liquid crystals have been synthesized based on schiff bases
Received May 10, 2007; accepted September 3, 2007.
Address correspondence to Dr. Ramnik Tandel, Chemistry Department, Faculty of
TechnologyandEngineering,Vadodara,Gujarat390002,India. E-mail:tandelcy@yahoo.com
103

104
R. C. Tandel and R. A. Vora
[1–4], aromatic esters [5–8], azo-esters [9–11], and biphenyls [12–15].
Chiral nematics, ferroelectric smectic C phases, and chiral smectic A
mesogens are of great interest due to their applications in recent times
[16,17]. Interest in the synthesis of chiral smectic ÃC liquid crystals has
increased considerably since the advent of a fast switching bistable,
electro optical device based on their ferroelectric properties [18–20].
In the early literature, there were few references for mesogenic com-
pounds having amide linkages and terminal chiral linkage. However,
to the best of our knowledge , the first homologous series containing
(ÀNHÀCOÀ) amide group as one of the central linkages was reported
by Vora and Gupta [21,22]. Soon after that, Kalyvas and McIntyre
[23] reported a symmetrical homologous series having an ester and
amide central linkages. Subsequently, Brown [24] has also mentioned
that compounds with ÀCONHÀ may exhibit mesomorphism. It was
thought of interest to reverse the amide linkage and to study its effect
on the mesomorphic properties of such homologous series and the effect
of chirality on a similar no chiral mesogenic series; with this in view it
was planned to synthesize chiral mesogenic homologous series with
reverse amide linkage compared to earlier studies and explore tÃhe
possibility of obtaining chiral nematic (cholesteric), chiral smectic C
phases, and classical smectic and nematic phases.
In the last few years, several mesogens and model compounds are
reported having amide and ester linkages [25–30].
EXPERIMENTAL
The route adopted for the synthesis of chiral mesogens is shown in
Figure 1. The purity of the final compounds was checked by the
thin-layer chromatography (TLC) method.
Microanalysis of compounds was recorded on Coleman Instrument.
IR spectra and Fourier Transform Infrared Spectrometer (FTIR) spec-
tra were recorded on Perkin–Elmer spectrophotometer and NICOLET
Impact-400 FTIR, respectively. NMR spectras were recorded on Hitachi
NMR R-1200 Instrument. Liquid crystalline properties were investi-
gated on Leitz-Labourlux 12Pol (Germany) polarizing microscope
provided with kofler heating stage. Differential Scanning Calorimetry
(DSC) were investigated on Mettler TA 4000 system. Electrospray
Ionization Mass Spectrometry (ESI-MS) were recorded on a LC-MS
Shimadzo-2010 Instrument.
Procedure
4-n Alkoxy benzoic acids and 4-n alkoxy benzoyloxy benzoic acids
9–11 were synthesized by the method described in the literature.

Chiral Smectic Mesogens
105
FIGURE 1 Synthetic route to homologous series I. Here, R ¼ n-C m H 2m þ 1,
m ¼ 3 to 8, 10, 12, 14, and 16.
S(þ)-1-bromo-2-methyl butane was synthesized from S(þ) 2-methyl
butanol (12) and S-(þ)-4-(b-methyl butoxy) anilines were synthesized
by the method described in the literature [31].
Preparation of 4-(40-n-Alkoxy Benzoyloxy Benzoyl)400-S (þ)-b-
methyl Butoxy Anilides
Respective 4-n-alkoxy benzoyloxy benzoyl chloride (0.01 mole) was
dissolved in about 5 ml of dry pyridine. The solution of S-( þ )-4 (b-
methyl butoxy) aniline (0.01 mole ) in 10 ml of dry pyridine was added
slowly to cold solution of acid chloride of respective 4-n alkoxy benzoy-
loxy benzoyl chloride. The mixture was heated on water bath for about
half an hour and was allowed to stand overnight. It was acidified with
cold 1:1 hydrochloric acid. Precipitates were filtered and washed by
water followed with sodium hydroxide and water. The resulting
ester-amides were recrystallized several times from Dimethyl Forma-
mide (DMF) until constant transition temperatures were obtained

106
R. C. Tandel and R. A. Vora
TABLE 1 (40n-Alkoxy benzoyloxy benzoyl)400-S-(þ)-b-methyl butoxy anilides
ꢁ
Transition temperatures C
Sr. No.
R ¼ n-alkyl group
Smectic
Cholesteric
Isotropic
1
2
3
4
5
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
175
202
214
218
214
273
264
260
272
269
264
236
300 (d)
316 (d)
308 (d)
315 (d)
301 (d)
306 (d)
294
6
7
207
194
8
9
10
Dodecyl
Tetradecyl
Hexadecyl
174 (168.3)
153 (151.1)
139
193.5 (190.9)
256 (254.7)
213 (209.5)
193
–
–
(d) Decomposition, () indicates DSC data.
(Table 1). The yield of purified ester-amides varies between 65–70%. The
synthesized compounds give satisfactory elemental analysis (Table 2).
Spectral Analysis of 4-(40-n- Alkoxy Benzoyloxy Benzoyl)400-S
(þ)-b-Methyl Butoxy Anilides
IR(KBr) Spectrum (n-Prpyloxy Derivative)
3325 cmꢀ1 and 1645 cmꢀ1 ꢀNH stretching and bending of secondary
ꢀ1
amide, 1600 cmꢀ1 stretching vibration of C O of amide, 1510 cm
=
VCꢀH bending of alkene (ꢀCH2ꢀ), 750 cmꢀ1 VCꢀH bending of aromatic
ring, 1730 cmꢀ1 carbonyl stretching of aromatic ring of ester, 2900 cmꢀ1
and 1415 cmꢀ1 VCꢀH stretching, and bending vibrations of ꢀCH3,
1600 cmꢀ1 and 1210 cmꢀ1 of aromatic ring. Other signals are observed
at 1165, 820 cmꢀ1, etc.
ꢁ
TABLE 2 Average Thermal Stabilities C
Series
(I) R0 ¼ ꢂC5H11
307.0 (C3–C8)
238.0
(A) R0 ¼ ꢂC5H11
200.17
(B) R0 ¼ C4H9
Cholesteric=Nematic to
Isotropic (C1 to C8)
Smectic-Chol.=or to
Nematic=Isotropic (C6–C16
Commencement
256.75
199.83 (C12, C14, C16
C12
)
–
–
)
C3
of Smectic mesophase

Chiral Smectic Mesogens
107
IR(KBr) Spectrum (n-Octyloxy Derivative) (Fig. 2)
3325 cmꢀ1 and 1644 cmꢀ1 ꢀNH stretching and bending of secondary
ꢀ1
amide, 1600 cmꢀ1 stretching vibration oꢀf1C O of amide, 1515 cm
=
VCꢀH bending of alkene (ꢀCH2ꢀ), 750 cm VCꢀH bending of aromatic
ring, 1730 cmꢀ1 carbonyl stretching of aromatic ring of ester, 2900 cmꢀ1
and 1410 cmꢀ1 VCꢀH stretching, and bending vibrations of ꢀCH3,
1600 cmꢀ1 and 1210 cmꢀ1 of aromatic ring. Other signals are observed
at 1165, 810 cmꢀ1, etc.
FTIR(KBr) Spectrum (n-Decyloxy Derivative) (Fig. 3)
3346 cmꢀ1 and 1648 cmꢀ1 ꢀNH stretching and bending of secondary
ꢀ1
ꢀ1
=
amide, 1604 cm . Stretching vibration of C O of amide, 1514 cm
VCꢀH bending of alkene (ꢀCH2ꢀ), 759 cmꢀ1 VCꢀH bending of aromatic
ring, 1735 cmꢀ1 carbonyl stretching of aromatic ring of ester, 2924 cmꢀ1
and 1414 cmꢀ1, VCꢀH stretching, and bending vibrations of ꢀCH3,
1604 cmꢀ1 and 1206 cmꢀ1 of aromatic ring. Other signals are observed
at 1165, 827, 690 cmꢀ1, etc.
ESI-MS were recorded on a LC-MS Shimadzo-2010 instrument and
mass values are recorded as:
Mass spectrum (n-pentyloxy derivative):
[Mþ þ NH4þ , relative intensity (%)]
144.2(24), 160(78), 202(100), 219(41), 247(17), 438(17.5), 495(15).
FIGURE 2 Representative IR spectrum of 4-(40n-alkoxy benzoyloxy benzoyl)
400-S (þ) -b-methyl butoxy anilides for compounds R ¼ n-C3 H7 and n-C8 H17
.

108
R. C. Tandel and R. A. Vora
FIGURE 3 Representative FTIR spectrum of 4-(40-n-decyloxy benzoyloxy
benzoyl)400-S (þ)-b -methyl butoxy anilide.
Mass spectrum (n-decyloxy derivative):
[Mþ þ Naþ , relative intensity (%)]
137(28), 165(80), 208(100), 213(8), 243(4.5), 341(5), 429(6), 560(20),
582(22).
NMR Spectra (n-Butoxy Derivative) (Fig. 4)
1H NMR (90 MHz, CHCl3)
1.0–1.79 (m, 16H of alkyl chain at C-4 and C-400);
4.2 (t, 4H of two methylene group in the alkoxy chain at C-4 and C-400)
7.11 (d, 4H at C-3, C-5, C-300 and C-500)
7.46 (d, 4H at C-30, C-50, C-200 and C-600)
8.17 (d, 2H at C-20 and C-60)
8.32 (d, 2H at C-2 and C-6)
8.9 (S, 1H of ꢀCOꢀNHꢀ of amide linkage).

Chiral Smectic Mesogens
109
1
FIGURE 4 H NMR spectrum of 4-(40-n-butoxy benzoyloxy benzoyl)400-S
(þ)-b-methyl butoxy anilides.
RESULTS AND DISCUSSIONS
Vora et al. [31,32] and Gupta [33] took up the study of number of
mesogenic series with ester and amide central linkages and also of
mesogenic series having two amide linkages. The observation of Vora
et al. made it clear that compared to an ester and azomethine linkage,
amide-central linkage enhances smectic and nematic phases, and
enhancement is more in case of smectic mesophase.
It has been observed by Griffin and others [16,17] that reversal of
the amide central linkage exhibits some remarkable differences in
liquid crystalline properties. It was thought proper to synthesize a
homologous series having reversal of amide linkage to compare with
other similar homologous series reported by our school. Moreover it
was thought proper that reversal of the amide linkage may induce
smectic C phase in the series which would be highly useful if it is
further converted to chiral smectic ꢂC by introducing chiral
substituent in the system.
With this in view, the following homologous Series I was synthe-
sized and their mesogenic properties were studied.
SERIES I

110
R. C. Tandel and R. A. Vora
Here, R ¼ (n-Cm H2m þ 1) and m ¼ 3 to 8, 10, 12, 14, 16.
For the synthesis of series I, intermediate 4-n-alkoxy benzoic acids
and 4-n-alkoxy benzoyloxy benzoic acids were synthesized, which are
mesogenic in nature. The transition temperatures of all the inter-
mediate acids compare well with the literature value with a few excep-
tions. The introduction of a chiral tail in the molecule will give chiral
smectic ꢂC phase which imparts ferroelectric properties to the molecules.
The series is synthesized as indicated in Figure 1. Homolog from
n-propyl derivatives onwards exhibit smectic ꢂC and cholesteric meso-
phase up to n-dodecyl derivative. n-Tetradecyl to n-hexadecyl deriva-
ꢂ
tives exhibit only smectic C phases.
The plot of transition temperatures against number of carbon atoms
in alkoxy chain for series I exhibits odd–even effect for cholesteric-
isotropic transition temperatures as well as for smectic-cholesteric
transition temperatures (Fig. 5).
The odd-even effect in the molecules, forming liquid crystals, gener-
ally possesses the basic skeleton of relatively rigid aromatic or alicyclic
rings with alkoxy or alkyl chains attached to one or both the ends.
Within the isotropic or nematic phases, different regions of each
molecules are continuously in contact with parts of neighbouring
FIGURE 5 Graph of transition temperatures versus number of carbon atoms
in alkoxy chain.

Chiral Smectic Mesogens
111
molecules. Gray [36,37] has explained the odd-even effect based on the
geometry of alkoxy or alkyl chain.
The transition temperature curves fall steeply, whereas solid-meso-
morphic transition temperature curve exhibits rising tendency
initially, and then falls steeply (Fig. 5).
A typical behavior of transition temperature curves is observed in
the graph (Fig. 5). They fall steeply. In the same way, solid-mesomorphic
transition temperature curve exhibits rising tendency initially, and
subsequently falls steeply. Normally, mesomorphic homologous series
do not exhibit such behavior. The odd–even effect is observed for
cholestric-isotropic transition temperatures as well as for smectic–
cholesteric transition temperatures.
There is a close relationship between mesomorphism and molecular
constitution of organic compounds. Hence, thermal stability, a
measure of mesomorphism, can be correlated with the molecular con-
stitution of the compounds.
The average thermal stabilities are compared to different homolo-
gous series (Table 3). Average thermal stabilities of 4-(40-n-alkoxy
benzoyloxy benzoy)400-S(þ)-b-methyl butoxy anilides (series I) are com-
pared with those of.
1) S(þ)-4(40-b-methyl butoxy benzoyloxy)-400-n-alkoxybenzoyloxy
benzanilides. [A] [38]
2) 4-(40-n-alkoxybenzoyloxy)-400 -n-butoxy benzanilides. [B] [39]
The reference to the geometry of series I and series A (Fig. 6)
indicates they are essentially the same except that one of the central
TABLE 3 Elemental Analysis
Calculated (%)
Found (%)
Sr. No.
R n-alkyl group
C
H
N
C
H
N
1
2
3
4
5
6
7
8
9
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
72.89
73.26
73.62
73.96
74.28
74.58
75.13
75.64
76.10
76.52
6.72
6.95
7.16
7.36
7.54
7.72
8.05
8.35
8.62
8.87
3.04
2.95
2.86
2.78
2.71
2.64
2.51
2.39
2.28
2.18
72.60
72.84
73.51
73.65
74.17
74.38
75.34
74.79
75.70
76.35
6.78
7.10
7.06
7.14
7.46
7.08
8.13
7.79
8.36
8.90
3.13
2.87
2.70
2.69
2.66
2.86
2.61
2.58
2.31
2.09
10

112
R. C. Tandel and R. A. Vora
FIGURE 6 The molecular geometry of present series (I) and comparative
series (A and B).
linkage amide (ꢀCOꢀNHꢀ) is reversed in series A as ꢀNHꢀCOꢀ in
the molecules. In the present series, the nitrogen of amide linkage is
attached to terminal phenyl ring and can take part into group reason-
ating structures along with terminal 2-methyl butyl group which
contribute to polarizability of the molecules of series I. In the case of
series A, the nitrogen of amide linkage is attached to central phenyl
group which is attached to another phenyl ring via an ester linkage.
This would cause a difference in the polarizability of molecules of
series I and A. This is also reflected in the thermal stabilities of both
homologous series. The commencement of smectic mesophase is
delayed in series A which is at dodecyl derivative. The S(þ) b-methyl
butyl group at one end does induce smectic tendency in a system,
however reversal of amide linkage has made it more pronounced.
The molecules of series I and series B differ in reversal of amide link-
age and in fixed n-alkoxy group at one end which is n-butoxy in the
case of series B and S (þ) b-methyl butoxy group in the case of series
I. Series B does not exhibit smectic phase in the homologous series but
it commences from the n-propyl derivative in series I. Danuta et al.
[41] have synthesized a number of model compounds with ester-amide
linkage to understand mesogenic behavior of polyesteramides. Their
results indicate that reversal of amide linkage in the model com-
pounds where nitrogen of amide linkage is attached with terminal
phenyl ring exhibits higher thermal stability.
Two members (C12 and C14 of series I) were screened by DSC
method. Both members exhibit prominent endothermic peaks for
solid-mesogenic transitions, small peaks for smectic-cholesteric and
mesogenic-isotropic transitions. Both members are showing endo-
thermic peaks for phase transitions. Enthalpy and entropy values
are recorded in Table 4. Both members are showing endothermic
peaks as well as exothermic peaks on heating and cooling, respectively
(Figs. 7 and 8).

Chiral Smectic Mesogens
113
TABLE 4 4-(40n-Alkoxy benzoyloxy benzoyl)400-S-( þ )-b-methyl butoxy anilides
Calorimetric data
Peak temps.
R ¼ n-alkyl Weight Transition (microscopic
DH
J=G
DS
Total
Sr. No.
1
group
(mg)
state
reading)
J=g.ꢂK DS J=g.ꢂK
Dodecyl
12
K–Sc
168.3 (174.0) 11.70 0.0271
Sc–Chol 190.9 (193.5)
Chol–Iso 254.7 (256.0)
2.10 0.00463
3.73 0.00721
0.0389
0.0747
2
Tetradecyl
10
K–Sc
151.1 (153.0) 24.06 0.057
209.5 (213.0) 4.91 0.0177
Sc–Iso
In the present study of the type of smectic mesophase and choles-
teric mesophase are inferred from the textures observed under polar-
izing microscope ‘‘Leitz Laborlux Mciroscope’’ provided with heating
stage (Figs. 9 and 10).
CONCLUSION
A new mesogenic homologous series with chiral center have been
ꢁ
synthesized. n-Propyl to n-dodecyl derivatives exhibit smectic C and
FIGURE 7 Representative DSC thermogram of 4-(40-n’alkoxy benzoyloxy)
400-S(þ)-2-methyl butoxy anilides for compounds R ¼ n-C14 H29 and n-C12H25
on heating rate at 10ꢂC minꢀ1 in N2 gas.

114
R. C. Tandel and R. A. Vora
FIGURE 8 Representative DSC thermogram of 4-(40-n’alkoxy benzoyloxy) 400-
S(þ)-b-methyl butoxy anilides for compounds R ¼ n-C14 H29 and n-C12 H25 on
cooling rate at 10ꢁC minꢀ1 in N2 gas.
FIGURE 9 Optical polarized microphotograph of cholesteric liquid crystalline
phase of 4-(40-n-decyloxy benzoyloxy benzoyl) 400-S-(þ)-b-methyl butoxy
anilides at 270ꢁC on heating.

Chiral Smectic Mesogens
115
FIGURE 10 Optical polarized microphotograph of smectic and cholesteric
liquid crystalline phases of 4 - (40-n-decyloxy benzoyloxy benzoyl) 400-S-(þ)-b-
methyl butoxy anilides at 215ꢁC on cooling.
cholesteric mesophases. n-Tetradecyl and n-hexadecyl derivatives
ꢂ
exhibit only smectic C phases. The plot of transition temperatures
versus number of carbon atoms in alkoxy chain exhibits odd–even
effect for cholestric-isotropic transition temperatures as well as smectic–
cholesteric transition temperatures. Compounds exhibit smectic and
cholesteric phases due to chiral center in a molecule as well as ester
and amide central linkages.
ACKNOWLEDGEMENT
We are indebted to Professor S. G. Devi, Head, Chemistry Depart-
ment, Faculty of Science for taking interest in the work and providing
necessary facilities. We extend our sincere thanks to Professor N. D.
Jadav, Head of Applied Chemistry Department, Faculty of Technology
and Engineering, M. S. University of Baroda, Vadodara for providing
necessary facilities. We are thankful to Dr. Jigar Desai, Sr. Scientist,
Cadila Pharmaceutical Industries Limited, Ahmedabad for NMR and
Mass spectral analysis. We are also thankful to Dr. S. S. Madhave Rao
and Mr. S. L. Patel for various analyses.
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