Thermal analysis of supramolecular hydrogen-bonded liquid crystals formed by nonyloxy and alkyl benzoic acids

Article abstract of DOI:10.1080/15421406.2012.762500

A novel series of supramolecular hydrogen-bonded complexes (supramolecular hydrogen-bonded liquid crystal, SMHBLC) has been isolated from equimolar ratios of p-n-nonyloxy benzoic acid (9BAO) and seven p-n-alkyl benzoic acids (nBA) whose carbon number varied from ethyl to octyl. Complementary hydrogen bonds are formed between them. They have been investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and further these data are utilized for the construction of phase diagram. Characteristic textures of the orthogonal and tilted phases have been recorded and compared. A new smectic ordering referred to as smectic X is characterized by various techniques. Odd-even effects are evinced in transition temperatures. The order of the transitions is distinguished by thermal analysis. The magnitude of experimental optical tilt angle in smectic C of all the homologs is fitted to a power law and the value of exponent is found to concur with the mean field theory predicted value.

Full text of DOI:10.1080/15421406.2012.762500

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Thermal Analysis of Supramolecular
Hydrogen-Bonded Liquid Crystals Formed
by Nonyloxy and Alkyl Benzoic Acids
C. Kavitha a , N. Pongali Sathya Prabu a & M. L. N. Madhu Mohan a
a Liquid Crystal Research Laboratory (LCRL), Bannari Amman
Institute of Technology, Sathyamangalam, India
Published online: 02 Apr 2013.
To cite this article: C. Kavitha , N. Pongali Sathya Prabu & M. L. N. Madhu Mohan (2013) Thermal
Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals Formed by Nonyloxy and Alkyl Benzoic
Acids, Molecular Crystals and Liquid Crystals, 574:1, 96-113, DOI: 10.1080/15421406.2012.762500
To link to this article: http://dx.doi.org/10.1080/15421406.2012.762500
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Mol. Cryst. Liq. Cryst., Vol. 574: pp. 96–113, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.762500
Thermal Analysis of Supramolecular
Hydrogen-Bonded Liquid Crystals Formed
by Nonyloxy and Alkyl Benzoic Acids
C. KAVITHA, N. PONGALI SATHYA PRABU,
AND M. L. N. MADHU MOHAN∗
Liquid Crystal Research Laboratory (LCRL), Bannari Amman Institute
of Technology, Sathyamangalam, India
A novel series of supramolecular hydrogen-bonded complexes (supramolecular
hydrogen-bonded liquid crystal, SMHBLC) has been isolated from equimolar ratios
of p-n-nonyloxy benzoic acid (9BAO) and seven p-n-alkyl benzoic acids (nBA) whose
carbon number varied from ethyl to octyl. Complementary hydrogen bonds are formed
between them. They have been investigated by polarizing optical microscopy (POM),
differential scanning calorimetry (DSC) and further these data are utilized for the con-
struction of phase diagram. Characteristic textures of the orthogonal and tilted phases
have been recorded and compared. A new smectic ordering referred to as smectic X is
characterized by various techniques. Odd–even effects are evinced in transition temper-
atures. The order of the transitions is distinguished by thermal analysis. The magnitude
of experimental optical tilt angle in smectic C of all the homologs is fitted to a power
law and the value of exponent is found to concur with the mean field theory predicted
value.
Keywords Hydrogen-bonded liquid crystals; odd–even effect; optical tilt angle; smec-
tic X; thermal analysis
1. Introduction
In liquid crystals, hydrogen bond is an important driving force for engineering directional
elements into supramolecular assemblies of noncovalently connected molecules [1,2]. The
earliest example of the unique geometrical ramification of hydrogen bonding in supramolec-
ular organization is the work of Bradfield and Jones [3], who, started some 70 years ago,
showed that certain hydrogen-bonded dimmers of organic acids exhibit liquid crystallinity
in their melts. Aromatic acid derivatives with alkoxy and alkyl terminal groups are known
to show mesomorphism due to the dimerization of their carboxylic acid groups through
hydrogen bonding [3] and this mesomorphism results from the proper combination of
molecular interactions and the shape of molecules. Interest and curiosity in the relation-
ship between hydrogen bond formation and mesomorphism have been exploited in the last
decade by Paleos [4], Kato and Frechet [5], and Kato et al. [6]. An archetypal example is
∗Address correspondence to M. L. N. Madhu Mohan, Liquid Crystal Research Laboratory
(LCRL), Bannari Amman Institute of Technology, Sathyamangalam 638401, India. Tel.: +91-
9442437480; Fax: +91-4295-223-775. E-mail: mln.madhu@gmail.com
96

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
97
their work on hydrogen bonding between aromatic acids and 4-substituted pyridyl group
to stabilize low molar mass mesophases in binary mixtures, mixtures with polymeric com-
ponents [5,6], and in chiral, mesomorphic networks comprising nonmesogenic precursors
[7]. In view of the fact that Kato et al. prepared new liquid crystals induced by inter-
molecular hydrogen bonding between pyridyl moieties and carboxylic acid groups that are
proton acceptors and proton donors, respectively [5,6], many new liquid crystals such as
molecular liquid crystals [8,9], polymer liquid crystals [10,11], ferroelectric liquid crystals
[12–14], room-temperature liquid crystals [15] have been prepared by using this method,
because of the stability of intermolecular hydrogen bonding [14,16] and the synthetic di-
rectionality [13,15]. Thus, hydrogen bonding is one of the key interactions for chemical
and biological processes in nature and also plays an important role in molecular aggre-
gates for the association of molecules. Dipole–dipole interactions have long been taken
into consideration in the design of liquid crystalline molecules [17,18]. In contrast, and
with few exceptions, intermolecular hydrogen bonds that are stronger than dipole–dipole
interactions had been considered to be deleterious for thermotropic liquid crystallinity in
the past, because in many cases they cause molecular associations that raise melting temper-
atures or destroy molecular order of the mesophases, but recently [8,12,19–25] a number
of supramolecular mesogenic materials have been obtained by molecular self-assembly
through intermolecular hydrogen bonding. To explore the potential use of the hydrogen-
bonded liquid crystals, mesogens consisting of only two aromatic rings have been designed
to induce room-temperature mesophases [26]. Most of the published work up to early 1995
is summarized in two review papers, one, treating the subject in a general manner [20] and
the other focusing specifically on carbohydrates [27]. Examples of more recent publications
are quoted in Refs. [15,19,28–31].
The central theme of the present work is to study the physical and chemical proper-
ties of supramolecular hydrogen-bonded liquid crystals (SMHBLCs) formed by the blend
of various combinations of alkyl and alkoxy carboxylic acids through intermolecular hy-
drogen bonds. Through our previous experience [32–42] in designing, synthesizing, and
characterizing liquid crystals and in persistence of our efforts to understand the effect of
hydrogen bonds, eight such homologous series designated as mOØn (mBAO + nBA, where
n varied from ethyl to octyl while m varied from pentyl to dodecyl) can be deduced with
changing the alkyl and alkoxy carbon number. A systematic study of 12OØn (12BAO +
nBA), 11OØn (11BAO + nBA), and 10OØn (10BAO + nBA) homologous series has been
carried out by the authors [43]. In the present work, design, synthesis, and characterization
of seven 9OØn (9BAO + nBA) homologs are discussed.
2. Experimental
Observations of optical textures are made with a Nikon polarizing microscope (Nikon,
Tokyo, Japan) equipped with Nikon digital CCD camera system with 5 mega pixels and
2560 × 1920 pixel resolutions. The liquid crystalline textures are analyzed and stored
with the assist of ACT-2U imaging software system. The temperature control of the liquid
crystal cell is equipped with HCS402-STC 200 temperature controller (Instec, Boulder,
CO) to a temperature resolution of 0.1◦C. This unit is interfaced to a computer by IEEE –
STC 200 to control and monitor the temperature. The liquid crystal sample, in its isotropic
state is filled by capillary action into a commercially available polyamide 4-μm buffed
cell (Instec). Optical extinction technique [38] is used for the determination of tilt angle in
smectic C phase. Transition temperatures and corresponding enthalpy values are obtained

98
C. Kavitha et al.
by differential scanning calorimetry (DSC; Shimadzu DSC-60, Japan). Fourier transform
infrared (FTIR) spectra are recorded (ABB FTIR MB3000) and analyzed with the MB3000
software. The p-n-nonyloxy benzoic acid (9BAO) and p-n-alkyl benzoic acids (nBA, where
n varied from ethyl to octyl) are supplied by Sigma-Aldrich (Munich, Germany), and all
the solvents used are of high performance liquid chromatography (HPLC) grade.
2.1 Synthesis of SMHBLC
SMHBLC complexes examined in the present study are obtained by mixing 1:1 molar
ratio of nonyloxy benzoic acid with various alkyl benzoic acids separately in DMF and
reprecipitating after the evaporation as described in the reported literature [5,6]. Synthesis
scheme of the present homologous series (9OØn) of nonyloxy benzoic acid (9BAO) with
p-n-alkyl benzoic acids (nBA, where n represents the alkyl carbon number and n varied
from ethyl to octyl) is depicted in scheme along with the proposed molecular structure of
9OØn in Fig. 1.
3. Results and Discussion
SMHBLCs isolated under the present investigation are white crystalline solids and stable at
room temperature (∼30◦C). They are insoluble in water and sparingly soluble in common
organic solvents such as methanol, ethanol, benzene, and dichloro methane. However, a
high degree of solubility is found in coordinating solvents such as dimethyl sulfoxide
(DMSO), dimethyl formamide (DMF), and pyridine. All these mesogens melt at specific
temperatures below 76.7◦C (Table 1). They show high thermal and chemical stability when
subjected to repeated thermal scans performed during polarizing optical microscopy (POM)
and DSC studies.
3.1 Phase Identification
The observed phase variance, transition temperatures, and corresponding enthalpy values
obtained by DSC in the cooling and heating cycles for the 9OØn complexes are presented
in Table 1. It is found that these data are in concurrence with POM data. The mesogens
of the nonyloxy benzoic acid (9BAO) with various p-n-alkyl benzoic acids (nBA, where
n varied from ethyl to octyl) designated as 9OØn homologous series are found to exhibit
characteristic phases [18], viz., nematic (N; schlieren brushes, Plate 1), smectic C (schlieren
texture, Plate 2), and smectic F (mosaic-schlieren, Plate 3, i.e., mosaic platelets separated
by very fine lines containing schlieren-like brushes, but does not show actual intersections
in the form of crosses). A new phase, smectic X (worm-like texture, Plate 4), has been
identified in this complex. As previously mentioned, a systematic study of 12OØn, 11OØn,
and 10OØn homologous series has been carried out [43] in which smectic X phase has
been identified and characterized. The textural morphology and the thermal characteristics
of the smectic X phase observed in the present study exactly resemble the reported [43]
data.
A brief description of schlieren brushes of nematic phase is detailed below; Plate 1
specifies nematic phase encompassing point singularity with four brushes. These brushes
meet at point singularities on the surface of the preparation or point singularities are the
origins of the brushes. The black bands or schlieren occurring throughout the texture are
the regions of extinction and are often referred to as schlieren brushes [18]. The point

99

100
C. Kavitha et al.
Step 1
Rigid core
OH
O
OH
O
O
DMF
OC9H19
C
C9H19
O
C
C9H19O
C
HO
9BAO monomer
9BAO dimer
Rigid core
Step 2
OH
O
OH
O
DMF
CnH2n+1
C
H2 +1Cn
C
H2n+1Cn
C
n
O HO
nBA monomer
nBA dimer
Step 3
OH
OH
O
Excess of DMF
C9H19O
C
+
H2 +1Cn
C
n
O
Stirring for 6-12 h
1 mol
:
1 mol
OH
O
O
CnH2
C
n
+1
C9H19O
C
HO
9OØn
Figure 1. Synthesis scheme representing the formation of 9OØn hydrogen-bonded series and the
molecular structure of 9OØn homologous series.
singularities are characterized by
|S| = number of brushes/4
= 4/4 = 1
With a positive sign when the brushes rotate in the same direction as that in which the
polarizer and analyzer are simultaneously rotated in the crossed position and with a negative
sign they turn in the opposite sense. In general, singularities with S = +1/2, −1/2, +1, −1
are known for nematics, but for the smectic C phase, all the point singularities are of the
S = 1 type. Therefore, S = +1 value calculated for the present texture revealed that the
given Plate 1 is identical with that of a nematic phase with a texture enclosing schlieren
four brushes.

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
101
Plates 1–4. (1)Schlieren brush texture of nematic phase. (2)Schlieren texture of smectic C phase.
(3) Mosaic-schlieren texture of smectic F phase. (4)Fully grown finger print worm-like texture of
smectic X phase
The general phase sequence of various homologs of 9OØn series in cooling and heating
run of POM and DSC can be shown as
Iso. → N ꢀ Sm C ꢀ Sm X ꢀ Sm F ꢀ Crystal
Iso. ꢀ N ꢀ Sm C ꢀ Sm F ꢀ Crystal
Iso. ꢀ N ꢀ Sm C → Sm X → Sm F ꢀ Crystal
Iso. ꢀ N ꢀ Sm C ꢀ Crystal
(9OØ8)
(9OØ7)
(9OØ6)
(9OØ5, 9OØ4, 9OØ3)
(9OØ2)
Iso. ꢀ N → Sm C ꢀ Crystal
Monotropic and enantiotropic transitions are depicted as single and double arrows, respec-
tively. The 9OØn series exhibits new phase sequence compared to their precursors (9BAO
and nBA) and is clearly evident from Table 2.

102

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
103
Figure 2. FTIR spectra of 9OØ6 complex
3.2 Spectroscopic Studies
3.2.1 Fourier Transform Infrared Spectroscopy (FTIR). At room temperature, FTIR spec-
tra of all the SMHBLCs are recorded in the solid state (KBr). As a representative case,
the FTIR spectrum of 9OØ6 is illustrated in Fig. 2. In the alkyloxy benzoic acids, car-
boxylic acid exists in monomeric form and the stretching vibration of C=O is observed
at 1674 cm−1. Further it is reported [44] that when a hydrogen bond is formed between
carboxylic acids, it results in lowering of the carbonyl frequency, which has been detected
in the present hydrogen-bonded complexes. A noteworthy feature in the spectrum of the
9OØ6 is the appearance of a sharp peak at 1682 cm−1, which clearly suggests the dimmer
formation, in particular the carbonyl group vibration [44–47]. A carboxylic acid existing
in a monomeric form in dilute solution absorbs at about 1760 cm−1 because of the elec-
tron withdrawing effect. However, acids in concentrated solution or in solid state tend to
dimerize through hydrogen bonding. It is reported [24] that this dimerization weakens the
C=O bond and lowers the stretching force constant K, resulting in lowering of the carbonyl
frequency of saturated acids to ∼1682 cm−1 (Table 3). Hence in the present complexes, the
formation of hydrogen bonding is established by FTIR peak assignments. A similar trend
of result is followed in all the other homologs.
Table 3. FTIR stretching frequencies of the 9OØn complexes and their precursors
Complex (9OØn)
Precursor (nBA)
Precursor (9BAO)
9OØn
(CO)acid
(OH)acid
nBA
(CO)acid
(OH)acid
(CO)acid
1674
(OH)acid
2924
9OØ8
9OØ7
9OØ6
9OØ5
9OØ4
9OØ3
9OØ2
1682
1682
1682
1682
1682
1690
1682
2924
2924
2924
2924
2924
2924
2924
8BA
7BA
6BA
5BA
4BA
3BA
2BA
1682
1692
1687
1686
1686
1681
1687
2924
2927
2927
2927
2927
2959
2962

104
C. Kavitha et al.
3.3 DSC Studies
DSC thermograms of all the seven SMHBLCs are recorded in heating and cooling cycle.
The accurately weighed individual sample is crimped in aluminum pan and loaded into the
heating chamber. The instrument is programed to heat the sample with a predetermined
scan rate (5◦C·min−1 and 10◦C·min−1) and held at its respective isotropic temperature for
2 min so as to attain thermal stability. Nitrogen gas is continuously purged inside the heating
chamber. Cooling run is programed with identical scan rate (5◦C·min−1 and 10◦C·min−1).
The respective resultant equilibrium transition temperatures and corresponding enthalpy
values of the mesogens of the homologous series are listed separately in Table 1. POM
studies also concur with the DSC transition temperatures. As a representative case, DSC
thermogram in both the heating and cooling cycles of 9OØ7 is depicted in Fig. 3(a), which
Iso
C
N
F
Crystal
4
2
Cooling run
0
Heating run
-2
9OØ7
140
20
40
60
80
100
120
º
Temperature ( C)
0.6
9OØ8
0.4
0.2
X
C
0.0
N
-0.2
-0.4
80
90
100
110
º
Temperature ( C)
Figure 3. (a) DSC thermogram of 9OØ7 complex. (b) DSC thermogram of 9OØ8 complex depicting
smectic X phase in cooling run.

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
105
Table 4. Sum of enthalpy values obtained in heating and cooling run
DSC cycles
Hydrogen-
bonded
complexes
ꢀ Enthalpy values of
ꢀ Enthalpy values of
all transitions in
all transitions in
heating cycle (J·g−1
)
cooling cycle (J·g−1
)
9OØ8
9OØ7
9OØ6
9OØ5
9OØ4
9OØ3
9OØ2
28.16
29.97
20.19
27.27
32.88
45.94
36.23
27.59
28.65
18.54
26.45
31.23
44.09
35.41
shows various mesogenic transitions. In addition, the well-resolved phase transitions of
sample 9OØ8, corresponding to isotropic to nematic, nematic to smectic C, and smectic C
to smectic X in the cooling cycle are also depicted in Fig. 3(b).
3.3.1 Thermal Equilibrium Exhibited by the Complexes. According to principles of ther-
modynamics, the energy involved in the endothermic (heating) process and exothermic
(cooling) process should be exactly equal. In other words, for a thermally stable mesogen,
the total heat absorbed in the endothermic process should be equal to the sum of heats
evolved during the exothermic process. From the thorough observation of the DSC ther-
mograms of the Entire 9OØn series, it is not surprising to note that the sum of enthalpy
values obtained by the complexes both in the heating and cooling runs, for the various
phase transitions, i.e., from isotropic to nematic, nematic to smectic C, smectic C to smec-
tic X, smectic X to smectic F, and smectic F to crystal, are found to be almost identical.
Table 4 confirms the above statement and verifies the principles of thermodynamics. The
total enthalpy value obtained in both heating and cooling run is found to follow a typical
order, which is clearly portrayed in Fig. 4. The magnitudes of total enthalpy values for both
the heating and the cooling cycles are closely following one another with the former having
slightly higher value than that of the later one. This is an interesting observation that is
detected in the present SMHBLC series.
3.3.2 Thermal Analysis. Landau theory and mean field theory [48] are the theories that
discuss the phase transitions in liquid crystalline substance. Two theories on the whole
can be summarized as physical systems in which phase transition can occur is usually
characterized by one or more long range order parameters. A phase transition can be
accompanied either by a continuous change or by a discontinuous change in the equilibrium
value of the order parameter when the system transforms from one phase to the other. It is
said to be the first-order transition when it is discontinuous, and if the state is continuous,
it is assigned to be second-order transition. Thus, the theoretical description of a phase
transition is equivalent to the determination of the free-energy density as a function of
the order parameter, its spatial derivatives, and the temperature. Determination of phase
transition order by experimental technique carried out by Navard and Cox [49] supplements
the above-mentioned arguments.

106
C. Kavitha et al.
9OØ8
40
30
20
DSC heating cycle
DSC cooling cycle
2
4
6
8
Alkyl carbon number
Figure 4. Total enthalpy values obtained in heating and cooling run of DSC thermograms of entire
9OØn complexes.
Navard and Cox [49] utilized thermal analysis for the differentiation of first- and
second-order transitions of liquid crystal complexes based on DSC thermograms. The
DSC scanning rate or the weight of the sample measured can be varied for this purpose,
but for the accurate results and convenience, the former is preferred in the present study.
Transition peaks obtained by DSC studies at two scan rates, one being twice the other, are
measured and their ratio (Cox parameter, NC) is employed for this purpose. The first- and the
second-order transitions can be classified based on the ratio (NC) of the measured heights,
√
1< NC ≥ 2 for an isothermal first-order transition and NC = 2 for a second-order transition.
For all the homologs, DSC thermograms at 5◦C·min−1 and 10◦C·min−1 are obtained
and as a representative case, the thermal analysis of 9OØ8 complex is discussed. The DSC
thermograms of the sample 9OØ8 at 5◦C·min−1 and 10◦C·min−1 scan rates are represented
in Fig. 5. The magnitude of the corresponding peaks, the phase segregations along with
the cooling run at 5◦C·min−1 (a) and 10◦C·min−1 (b) scan rates are depicted in Fig. 5. The
values of NC for the isotropic to nematic, nematic to smectic C, and smectic C to smectic
X transitions for 9OØ8 complex are presented in Table 5. From this qualitative analysis,
the values of NC are calculated and found to be 1.70 for isotropic to nematic transition,
Table 5. Comparison of DSC peak heights along with NC values across various transitions
in 9OØ8 complex
Peak height
Complex
9OØ8
DSC
Nematic
Sm C
Sm X
5◦C·min−1
10◦C·min−1
0.34 mW
0.58 mW
1.70
0.04 mW
0.07 mW
1.75
0.21 mW
0.37 mW
1.76
Ratio (NC)
(10◦C·min−1)/(5◦C·min−1
)
Note. Sm, smectic.

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
107
2.0
(0.34)
9OØ8
(0.21)
(0.04)
1.5
1.0
0.5
a
Iso.
Sm X
Sm C
N
(0.58)
(0.37)
(0.07)
b
80
90
100
110 120 130 140
º
Temperature ( C)
Figure 5. DSC thermograms of 9OØ8 complex performed at two different scan rates. Thermogram
(a) and (b) represents scan rates of 5◦C·min−1 and 10◦C·min−1, respectively
1.75 for nematic to smectic C transition, and 1.76 for smectic C to smectic X transition.
Thus, NC is found to be below 2 indicating that all these transitions are of first order.
Quantitatively, the higher value of enthalpy obtained in DSC studies (Table 1) also serves
as an additional evidence for its first-order transition. Further, from the enthalpy values
based on the above-mentioned procedures, we can classify smectic C to smectic X as weak
first-order transition.
3.4 Phase Diagram of Precursors
Phase diagrams of precursors 9BAO and nBAs are composed of nematic, smectic C, and
nematic, smectic G, respectively (Table 2).
3.4.1 Phase Diagram of 9OØn. Phase diagram of nonyloxy benzoic acid and p-n-alkyl
benzoic acids homologous series (9OØn) are depicted in Fig. 6. Following points can be
elucidated from Fig. 6:
(a) The 9OØn hydrogen-bonded homologous series exhibits nematic as orthogonal
phase and smectic C, smectic X, and smectic F as tilted phases.
(b) Compared to the precursors, in the present SMHBLCs two smectic phases namely
smectic X and smectic F is induced while smectic G phase is completely annihi-
lated.
(c) Smectic X phase is induced only in octyl benzoic acid and hexyl benzoic acid,
quenching the thermal range of smectic C. Further smectic X is sandwiched be-
tween smectic C and smectic F phases.
(d) The thermal span of nematic phase is observed to be invariant with increasing alkyl
chain length.
(e) An interesting observation is the presence of odd–even effects at isotropic to
nematic and nematic to smectic C transition temperatures.

108
C. Kavitha et al.
9OØn
Isotropic
N
140
120
100
80
C
F
X
X
6
60
Crystal
2
4
8
Alkyl carbon number
Figure 6. Phase diagram of 9OØn homologous series.
(f) A gradual decrease in the crystallization temperatures is observed from octyl
benzoic acid to butyl benzoic acid.
(g) The occurrence of smectic X phase is attributed to the l/d ratio of the homologous
series. The lower homologs failed to posses the threshold value of l/d ratio, which
in turn reflects in the absence of smectic X phase.
3.4.2 Odd–Even Effect. The origin of the odd–even effect [48,50,51] can be understood
from the consideration of the molecular structure of 9OØn. In the even numbers of the
series, the disposition of the end group is to enhance the molecular anisotropy and hence
molecular order, whereas in the odd number it has the opposite effect. As the chain length
increase, their flexibility increases and the odd–even effect observed in the present series
is in accordance with the quantitative calculations proposed by Marcelja [50].
From Fig. 7(a) and (b), it can be observed that the magnitudes of the isotropic to nematic
and nematic to smectic C transition temperatures corresponding to the homologous with
even carbon number (9OØ2, 9OØ4, 9OØ6, and 9OØ8) exhibit one type of behavior, while
the odd counter parts (9OØ3, 9OØ5, and 9OØ7) show a different increment and is known
as odd–even effect.
Considering flexibility due to end chains, Mercelja [50] incorporated the configu-
rational statistics of the end chains in the theory of the nematic phase in a Flory-type
calculation [52]. In addition to conformational energy, each C–C bond in the end chain is
subjected to a mean field, which depends both on the orientational order of the rigid central
part and that of the end chain. A high probability of occurrence of the all trans-configuration
for short chains is considered for the strong odd–even effect for lower homologous. The
increase in conformational flexibility for the higher homologs is seen as an extension of
this effect.
Luckhurst [53] refined the calculations made by Pink model, which shows that while
the odd–even effect in transition temperature can be understood as a consequence of
the odd–even effect in the chain-order parameter with respect to the molecular axis, the
odd–even effect in the rigid segment-order parameter is the consequence of at least such

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
109
9OØn
130
125
120
Isotropic - Nematic transition temp.
2
4
6
8
Alkyl carbon number
9OØn
100
80
Nematic - Smectic C transition temp.
2
4
6
8
Alkyl carbon number
Figure 7. (a) Odd–even effect across isotropic to nematic transition temperatures in 9OØn series.
(b) Odd–even effect across nematic to smectic C transition temperatures in 9OØn series.
an effect in the mean-square deviation and used to make successful calculations for com-
pounds that have two rigid cyano biphenyl moieties linked by flexible spacers, in which
the odd–even alteration in transition temperature is very strong. In compounds with alkyl
and alkoxy end chains, it is already known [43] that the isotropic to nematic transition
temperatures show odd–even effects. In addition to this odd–even effect, at the interface of
nematic to smectic C transition temperature is also noticed. For short alkyl chain lengths,
this alteration effect is more pronounced, whereas it becomes less marked with longer alkyl
chain lengths. Odd carbon atom alkyl chain (3BA, 5BA, and 7BA) having a terminal CH3,
extends along the molecular axis increasing the transition temperature, whereas in an even
number carbon chain (2BA, 4BA, 6BA, and 8BA), the terminal chain lies off the molecular
axis decreasing the transition temperature, which may be the appropriate explanation for
the occurrence of this effect. The entropy of melting increases with flexible chains, which
serves to lower the melting point and to augment the liquid crystalline range.

110
C. Kavitha et al.
4. Occurrence of Smectic C and Measurement of Optical Tilt Angle
Smectic C phase is the first classified phase in which the molecular long axes of the
constituent molecules were found to be tilted with respect to the normal of the layer planes.
A large number of experimental studies are there concerning the effect of small changes
in the molecular structure on the incidence and temperature dependence of the smectic C
phase. The occurrence and abundance of high thermal span of smectic C in the present
SMHBLC can be justified by the molecular structure that is supplemented by the following
arguments:
a. Molecules having two terminal alky chains especially alkyl/alkoxy groups exhibit
smectic C phase predominantly.
b. Approximately, symmetrical molecular structure of the mesogens usually favors
smectic C phase.
c. The molecular structure of the constituent molecules, particularly the length of the
chains of the terminal alkyl or alkoxy groups, influences the occurrence of smectic
C phase.
The variation of tilt angle θ(T) with temperature, reflecting the growth of order param-
eter in smectic C phase is measured by optical extinction method [38] in all the homologs of
9OØn series. The tilt angle is found to increase with decreasing temperature. It is observed
from the Fig. 8 that θ(T) attains a characteristic maximum value. It can be observed from
Fig. 8 that the magnitude of tilt angle increases with the increment in the alkyl chain.
The large magnitudes of the tilt angle in higher homologs are attributed to the enhanced
orientational disorder introduced by the lengthy flexible part of the molecule. Hence, higher
homologs with extended flexible part in SMHBLCs are argued to contribute positively to
the inclined [23] soft covalent hydrogen bonding interaction for the realization of tilted
phases of applicational interest.
The observed temperature variation of θ(T) appears to follow a power law relationship
given by
θ(T )α(TC − T )β,
(1)
9OØn
- Smectic C
15
10
5
nBA =
8BA
7BA
6BA
5BA
4BA
3BA
2BA
0.0
0.5
1.0
1.5
2.0
2.5
º
T-T / C
C
Figure 8. Temperature variation of tilt angle in smectic C phase of 9OØn complexes (n varied from
ethyl to octyl carbon number). Solid lines indicate the fit deduced from mean field theory.

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
111
where TC represents the transition temperature, β the critical exponent component value,
and T is the temperature at which θ(T) is experimentally determined.
The data of θ(T) for the members of 9OØn in smectic C phase are fitted (Fig. 8) to
Eq. (1) to study the growth of order parameter in smectic C phase. The critical exponent β
value estimated by fitting the data of θ(T) to Eq. (1) is found to be 0.50 to agree with the
mean field predicted value [48,54] to infer the long-range interaction of transverse dipole
moment for the stabilization of the tilted smectic C phase.
5. Characterization of Smectic X Phase
Smectic X phase, which has been identified in two complexes namely 9OØ8 and 9OØ6, is
characterized by textural studies. Furthermore, the observed smectic X phase in the present
study exactly resembles the smectic X reported in our previous study [43].
In both the above complexes, smectic X is sandwiched between conventional smectic
C and smectic F phases. This serves as one of the strongest evidence to identify this new
phase, smectic X as smectic ordering.
5.1 Textural Study of Smectic X Phase
The hydrogen-bonded complexes 9OØ8 and 9OØ6 on cooling from isotropic phase forms
nematic droplets with schlieren four brushes (Plate 1), on further cooling the mesogen
nematic transforms into smectic C phase with schlieren texture (Plate 2). On additional
cooling, the schlieren texture is transformed into a worm-like texture (Plate 4), which is
designated as smectic X. The thermal width of this phase is observed to be very narrow
(<1◦C) and the fully grown phase is shown in Plate 4. The striations in the worm-like
texture are manifestation of the presence of helicoidal structure. With further decrement in
temperature, this phase paves way for mosaic-schlieren texture of smectic F phase as shown
in Plate 3. Interestingly, this smectic X phase is observed to be a monotropic transition.
5.2 DSC Studies of Smectic X Phase
Interestingly, the smectic X phase exhibits monotropic transition in the performed thermal
scans and hence it is observed only in the cooling run of the 9OØ8 complex (Fig. 3(b)).
5.3 Ordering in Smectic X
Smectic X phase is sandwiched between conventional smectic C and smectic F phases. It is
reported by us [43] that the molecular orientation in this phase is due to the flipping of the
molecules. The high magnitude of the helix in smectic X phase, compared to its preceding
and succeeding phases, stands as a token of evidence for the molecular reorientation.
Thermodynamical conditions favor this smectic X phase with narrow thermal span, where
the flipping of the molecules occur. POM, optical tilt angle, and helicoidal structural studies
enable this phase to be a smectic ordering [43]. This argument is valid with evidence where,
smectic X is sandwiched between two smectic orderings.
The ratio of length of the molecule (l) to the interlamellar distance (d) plays a pivotal
role in the formation of various phases. If the layer spacing is equal to the molecular
length, smectic phases are observed. However, if the layer spacing is smaller than the
molecular length, the molecules undergo rapid restricted molecular motion forming higher
order smectic phases. In very few cases, the molecules protrude into other smectic layers,

112
C. Kavitha et al.
which are referred as interdigitated phases. Based on the preliminary textural and thermal
data of 9OØ8 and 9OØ6, when the liquid crystal is cooled from smectic C, it is believed
that interdigitations takes place leading to the formation of smectic X phase. On further
cooling, the interdigitations ceases and higher order smectic F phase is formed.
6. Conclusions
Design and synthesis of liquid crystals with alkyl-nonyloxy terminal groups is carried out,
which paved way for synthesis of many more similar hydrogen-bonded liquid crystals by
varying the alkyl/alkyloxy terminal group. It is found that these liquid crystals exhibit rich
phase polymorphism with orthogonal and tilted phases. The abundance of thermal width
of smectic C is of interest along with the occurrence of new smectic ordering.
Acknowledgments
The divine blessings of almighty Bannari Amman, the infrastructural support rendered by
Bannari Amman Institute of Technology, and the financial support rendered by the Board
of Research in Nuclear Sciences (BRNS) of Department of Atomic Energy (DAE), India
(sanction no. 2012/34/35/BRNS) are gratefully acknowledged by the authors.
References
[1] (a) Lehn, J. M. (1995). Supramolecular Chemistry: Concepts and Perspectives, A Personal
Account Built Upon the George Fisher Baker Lectures in Chemistry at Cornell University:
Rome, New York; (b) Wuest, J. D. (1995). Mesomolecules: from Molecules to Materials,
Chapman and Hall: New York.
[2] Kumar, S. (2011). Chemistry of Discotic Liquid Crystals, CRC Press: Boca Raton, FL.
[3] Bradfield, A. E., & Jones, B. (1929). J. Chem. Soc., 2660.
[4] Paleos, C. M., & Siourvas, D. T. (2001). Liq. Cryst., 28, 1127.
[5] Kato, T., & Frechet, J. M. J. (1989). J. Am. Chem. Soc., 111, 8533.
[6] Kato, T., Frechet, J. M. J., Wilson, P.G., Saito, T., Uryu, T., Fujishima, A., Jin, C., & Kaneuchi,
F. (1993). Chem. Mater., 5, 1094.
[7] Kihara, H., Kato, T., Uryu, T., & Frechet, J. M. J. (1998). Liq. Cryst., 24, 413.
[8] Kato, T., Fujishima, A., & Frechet, J. M. J. (1990). Chem. Lett., 919.
[9] Kato, T., Wilson, P. G., Fujishima, A., & Frechet, J. M. J. (1990). Chem. Lett., 19, 2003.
[10] Kato, T., & Frechet, J. M. J. (1989). Macromolecules, 22, 3818.
[11] Kumar, U., Kato, T., & Frechet, J. M. J. (1992). J. Am. Chem., 114, 6630.
[12] Kumar, U., Frechet, J. M. J., Kato, T., Ujiie, S., & Iimura, K. (1992). Angew. Chem. Int. Ed.
Engl., 31, 1531.
[13] Yu, L. J. (1993). Liq. Cryst., 14, 1303.
[14] Tian, Y. Q., Su, F. Y., Shao, Y. Y., Lu, X. Y., Tang, X. Y., Zhao, X. G., & Zhou, E. L. (1995).
Liq. Cryst., 19, 743.
[15] Kato, T., Fukumasa, M., & Frechet, J. M. J. (1995). Chem. Mater., 7, 368.
[16] Kato, T., Uryu, T., Kanelechi, F., Jin, C., & Frechet, J. M. J. (1993). Liq. Cryst., 14, 1311.
[17] Gray, G. W. (1987). Thermotropic Liquid Crystals, Wiley Ed: Chichester.
[18] Gray, G. W., & Goodby, J. W. (1984). Smectic Liquid Crystals, Leonard Hill: Glasgow.
[19] Kato, T., & Frechet, J. M. J. (1995). Macromol. Symp., 95, 311.
[20] Paleos, C. M., & Tsiourvas, D. (1995). Angew. Chem. Int. Ed. Engl., 34, 1696.
[21] Lehn, J. M. (1993). Makromol. Chem. Macromol. Symp., 69, 1.
[22] Bruce, D. W., & Price, D. J. (1994). Adv. Mater. Opt. Electron., 4, 273.
[23] Sato, A., Kato, T., & Uryu, T. (1996). J. Polym. Sci., Part A: Polym. Chem., 34, 503.

Analysis of Supramolecular Hydrogen-Bonded Liquid Crystals
113
[24] Kato, T., Kihara, H., Uryu, T., Fujishima, A., & Frechet, J. M. J. (1992). Macromolecules, 25,
6836.
[25] Malik, S., Dhal, P. K., & Mashelkar, R. A. (1995). Macromolecules, 28, 2159.
[26] Fukumasa, M., Kato, T., Uryu, T., & Frechet, J. M. J. (1993). Chem. Lett., 22, 65.
[27] Prade, H., Miethchen, R., & Vill, V. (1995). J. Prakt. Chem., 337, 427.
[28] Treybig, A., Dorscheid, C., Weissflog, W., & Kresse, H. (1995). Mol. Cryst. Liq. Cryst., 260,
369.
[29] Araki, K., Kato, T., Kumar, U., & Frechet, J. M. J. (1995). Macromol. Rapid Commun., 16, 733.
[30] Miethchen, R., Holz, J., Prade, H., & Liptak, A. (1992). Tetrahedron, 48, 3061.
[31] Wilson, L. M. (1995). Liq. Cryst., 18, 381.
[32] Chitravel, T., Madhu Mohan, M. L. N., & Krishnakumar, V. (2008). Mol. Cryst. Liq. Cryst.,
493, 17.
[33] Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2010). Solid State Sci., 12, 482.
[34] Pongali Sathya Prabu, N., Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2011). Phys. B :
Condens. Matter, 406, 1106.
[35] Vijayakumar, V. N., Murugadass, K., & Madhu Mohan, M. L. N. (2009). Mol. Cryst. Liq. Cryst.,
515, 39.
[36] Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2009). Braz. J. Phys., 39, 601.
[37] Pongali Sathya Prabu, N., Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2011). J. Mol.
Struct., 994, 387.
[38] Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2009). Solid State Commun., 149, 2090.
[39] Madhu Mohan, M. L. N., Kumar, P. A., Goud, B. V. S., & Pisipati, V. G. K. M. (1999). Mater.
Res. Bull., 34, 2167.
[40] Pongali Sathya Prabu, N., Vijayakumar, V. N., & Madhu Mohan, M. L. N. (2011). Mol. Cryst.
Liq. Cryst., 548, 73.
[41] Kumar, P. A., Madhu Mohan, M. L. N., Potukuchi, D. M., & Pisipati, V. G. K. M. (1998). Mol.
Cryst. Liq. Cryst., 325, 127.
[42] Madhu Mohan, M. L. N., Kumar, P. A., & Pisipati, V. G. K. M. (2001). Mol. Cryst. Liq. Cryst.,
366, 431.
[43] (a) Kavitha, C., Pongali Sathya Prabu, N., & Madhu Mohan, M. L. N. (2012). Phys. B: Condens.
Matter, 407, 859; (b) Kavitha, C., Pongali Sathya Prabu, N., & Madhu Mohan, M. L. N. (2012).
Phase Trans., 85, 973.
[44] Pavia, D. L., Lampman, G. M., & Kriz, G. S. (2007). Introduction to Spectroscopy, Thomson,
Brooks/Cole: Belmont, CA.
[45] Nakamoto, K. (1978). Infrared and Raman Spectra of Inorganic and Co-ordination Compounds,
Interscience: New York.
[46] Xu, J. (2006). J. Mater. Chem., 16, 3540.
[47] Frechet, J. M. J., & Kato, T. (1992). Patent US No. 5139696.
[48] (a) Chandrasekhar, S. (1977). Liquid Crystals, Cambridge University Press: New York; (b)
Priestly, E. B., Wojtowicz, P. J., & Sheng, P. (1975). Introduction to Liquid Crystals, Plenum
Press: New York.
[49] Navard, P., & Cox, R. (1984). Mol. Cryst. Liq. Cryst., 102, 261.
[50] (a) Mercelja, S. (1973). Solid State Commun., 13, 759; (b) Mercelja, S. (1974). J. Chem. Phys.,
60, 3599.
[51] Senthil, S., Rameshbabu, K., & Wu, S. L. (2006). J. Mol. Struct., 783, 215.
[52] Flory, P. J. (1956). Proc. Roy. Soc. London, A234, 73.
[53] Luckhurst, G. R. (1986). Nematic Liquid Crystals formed from flexible molecules: a molecular
field theory. In: L. L. Chapoyed (Ed.), Recent Advances in Liquid Crystalline Polymers, Elsevier:
London, pp. 105–107.
[54] Stanley, H. E. (1971). Introduction to Phase Transition and Critical Phenomena, Clarendon
Press: New York.