Design, synthesis and characterization of a linear hydrogen bonded homologous series

Article abstract of DOI:10.1016/j.physb.2011.12.103

A linear hydrogen bonded liquid crystalline homologous series has been synthesized and characterized. Hydrogen bond is formed between pn-dodecyloxy benzoic acid and various pn-alkyl benzoic acids whose alkyl chain vary from octyl to ethyl. Synthesized complexes are characterized by FTIR, ¹H NMR and ¹³C NMR studies for inferring the formation of hydrogen bonds. Polarizing Optical Microscopy (POM) and DSC studies reveal various mesophases and their corresponding transition temperatures along with respective enthalpy values. All the seven synthesized complexes exhibit rich liquid crystalline mesomorphism. A new phase namely smectic X has been observed in five of the complexes with a narrow thermal range. This phase has been characterized by optical textural, DSC, tilt angle and helicoidal pitch studies. Smectic X is sandwiched between traditional smectic C and re-entrant smectic C (designated as C_R) phases. Homeotropic transition in nematic phase is observed in all the mesogens and thus these materials can be used as thermally controlled optical shutters. Tilt angle in smectic C, smectic X and smectic C_R phases have been experimentally elucidated for all the mesogens.

Full text of DOI:10.1016/j.physb.2011.12.103

Physica B 407 (2012) 859–867
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Physica B
journal homepage: www.elsevier.com/locate/physb
Design, synthesis and characterization of a linear hydrogen bonded
homologous series
C. Kavitha, N. Pongali Sathya Prabu, M.L.N. Madhu Mohan n
Liquid Crystal Research Laboratory (LCRL), Bannari Amman Institute of Technology, Sathyamangalam 638401, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A linear hydrogen bonded liquid crystalline homologous series has been synthesized and characterized.
Hydrogen bond is formed between p–n-dodecyloxy benzoic acid and various p–n-alkyl benzoic acids
whose alkyl chain vary from octyl to ethyl. Synthesized complexes are characterized by FTIR, 1H NMR
and 13C NMR studies for inferring the formation of hydrogen bonds. Polarizing Optical Microscopy
(POM) and DSC studies reveal various mesophases and their corresponding transition temperatures
along with respective enthalpy values. All the seven synthesized complexes exhibit rich liquid
crystalline mesomorphism. A new phase namely smectic X has been observed in five of the complexes
with a narrow thermal range. This phase has been characterized by optical textural, DSC, tilt angle and
helicoidal pitch studies. Smectic X is sandwiched between traditional smectic C and re-entrant smectic
C (designated as CR) phases. Homeotropic transition in nematic phase is observed in all the mesogens
and thus these materials can be used as thermally controlled optical shutters. Tilt angle in smectic C,
smectic X and smectic CR phases have been experimentally elucidated for all the mesogens.
& 2011 Elsevier B.V. All rights reserved.
Received 7 October 2011
Received in revised form
3 December 2011
Accepted 16 December 2011
Available online 23 December 2011
Keywords:
Hydrogen bonded liquid crystals
Smectic X
Re-entrant smectic C
Homeotropic transition
Thermally controlled optical shutter
1. Introduction
phase behavior. Mesogenic properties of HBLC can be tuned easily
by changing H-bond donor/ acceptor or percentage of respective
In the recent years the design of Hydrogen Bonded Liquid
Crystals (HBLC) with non-covalent interactions took a new turn in
the history of science, which made many researchers to hub their
attention towards synthesis of these self assembly systems
formed by inter molecular hydrogen bonding. For the formation
of liquid crystalline materials through hydrogen bonding interac-
tions, complementarity of the interacting components coupled
with the directionability of hydrogen bonds are the main factors
contributing to the exhibition of liquid crystallinity. The appro-
priate shape and stability of the hydrogen bonded complexes are
also required. Thus, for non-covalent interactions hydrogen bond-
ing acts as a powerful tool in assembling molecules. The forma-
tion of thermotropic liquid crystals with varying characteristics
will be based on a categorization of the interacting components
according to their structural features (alkyl carbon chain length).
Therefore several liquid crystalline materials can be obtained by
the interplay of the nature, the number and the location of the
recognizable groups in various substrates. It is noticed [1–6] that
in hydrogen bonded liquid crystals (HBLC) lower bonding and
activation energies showed a profound influence on their thermal
properties, viz, clearing points, enthalpies and mesomorphic
molar composition or by changing the length of the alkyl chain.
Stable and dynamic molecular complexes can be prepared by
simple molecular self assembly processes using such hydrogen
bonding. A number of such HBLC have been investigated by Kato
et al, which indicates that the mesomorphism results from proper
combination of molecular interactions and shape of the mole-
cules. It is inferred [7] that hydrogen bonding has pronounced
influence on crystallization and phase behaviors of multi compo-
nent supra molecular complexes formed by benzoic acids.
The self assembly systems formed between alkyloxy and alkyl
carboxylic acids exhibit rich phase polymorphism. It has been
earlier reported by us that these self organized systems induce
variety of new phenomena like, reentrant phase occurrence [8,9],
light modulation [10], optical shuttering action [11–13] and field
induced transitions [10,15,16]. The carboxylic acids with other
acids are reported [8–16] to form complementary single and
multiple hydrogen bonds.
In the present work, we have successfully synthesized a novel
series of hydrogen bonded liquid crystal where the inter-mole-
cular hydrogen bonding occurred between p–n-dodecyloxy
benzoic acid and p–n-alkyl benzoic acids resulting in the forma-
tion of seven new HBLCs. Due to the presence of carboxylic acid
groups, a stable hydrogen bonded network is formed, whose
presence induces the formation of supra molecular structures
that exhibit liquid crystalline behavior. Results of homeotropic
n Corresponding author. Tel.: þ91 9442437480; fax: þ91 4295 223 775.
E-mail address: mln.madhu@gmail.com (M.L.N.M. Mohan).
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2011.12.103

860
C. Kavitha et al. / Physica B 407 (2012) 859–867
transition in nematic phase, which can be used as thermally
controlled optical shutters along with tilt angle in smectic C,
smectic CR and smectic X phases have been experimentally
elucidated for all the mesogens.
literature [1,2,4]. Molecular structure of the present homologous
series of p–n-dodecyloxy benzoic acid (12BAO) with p–n-alkyl
benzoic acids (nBA, where n¼2–8) is depicted in the Fig. 1 where
n represents the alkyl carbon number.
2. Material and methods
3. Results and discussion
Optical textural observations were made with a Nikon polar-
izing microscope (POM) equipped with Nikon digital CCD camera
system with 5 mega pixels and 2560 ꢀ 1920 pixel resolutions. The
liquid crystalline textures were analyzed and stored with the aid
of ACT-2U imaging software system. The temperature control of
the liquid crystal cell was equipped by Instec HCS402-STC 200
temperature controller (Instec, USA) to a temperature resolution
of70.1 1C. This unit was interfaced to computer by IEEE-STC 200
to control and monitor the temperature. The liquid crystal sample
was filled by capillary action in its isotropic state into a commer-
All the hydrogen bonded complexes isolated under the present
investigation are white crystalline solids and are stable at room
temperature (30 1C). They are insoluble in water and sparingly
soluble in common organic solvents such as methanol, ethanol,
benzene and dichloro methane. However, they show a high
degree of solubility in coordinating solvents like dimethyl sulf-
oxide (DMSO), dimethyl formamide (DMF) and pyridine. All these
mesogens melt at specific temperatures below ꢁ82 1C (Table 1).
They show high thermal and chemical stability when subjected to
repeated thermal scans performed during Polarizing Optical
Microscopy (POM) and DSC studies.
cially available (Instec, USA) polyamide buffed cell with 4 mm
spacer. Optical extinction technique [16] was used for determina-
tion of tilt angle. Transition temperatures and corresponding
enthalpy values were obtained by DSC (Shimadzu DSC-60, Japan).
FTIR spectra were recorded (ABB FTIR MB3000) and analyzed with
the MB3000 software. The p–n-dodecyloxy benzoic acid (12BAO)
and p–n-alkyl benzoic acids (nBA, where n¼2–8) were supplied
by Sigma Aldrich, (Germany) and all the solvents were of
HPLC grade.
3.1. Phase identification
The observed phase variants, transition temperatures and
corresponding enthalpy values obtained by DSC in the cooling
and heating cycles for the 12BAOþnBA complexes are presented
in Table 1. These data are in concurrence with POM data.
3.2. 12BAOþnBA homologous series
2.1. Synthesis of HBLC
The mesogens of the p–n-dodecyloxy benzoic acid (12BAO)
with p–n-alkyl benzoic acids (nBA, where n¼2–8) designated as
12BAOþnBA homologous series are found to exhibit character-
istic textures [17], viz., nematic (N) (threaded texture, Plate 1),
homeotropic region of nematic (Nh), (Plate 2), smectic C (schlieren
texture, Plate 3) smectic X (worm like texture, Plate 4) and
reentrant smectic C designated as smectic CR (broken focal conic
texture, Plate 5). The general phase sequence of various homologs
of 12BAOþnBA series in cooling and heating run can be shown as
All the inter-hydrogen bonded complexes examined in the
present study are prepared by mixing in 1:1 molar ratio dodecy-
loxy benzoic acid with various alkyl benzoic acids in DMF and
reprecipitating after the evaporation as described in the reported
OH
O
O
C
CnH2n+1
C12H25
O
C
HO
Fig. 1. Molecular structure of 12BAOþnBA homologous series.
Iso"N-Nh-N"Sm C-Sm X-Sm CR"Crystal (12BAOþ8BA)
Table 1
Transition temperatures and enthalpy values obtained by various techniques of 12BAOþnBA homologous series.
Complex
Phase Variance Techniques Melt
N
Homeotropic (Nh) Nh
C
X
CR
Crystal
n
n
@
n
n
n
n
12BAOþ8BA NNhNCXCR
12BAOþ7BA NNhNCXCR
12BAOþ6BA NNhNCXCR
12BAOþ5BA NNhCXCR
12BAOþ4BA NNhCXCR
12BAOþ3BA NNhC
DSC (h)
DSC (c)
POM
57.0 (20.89) 125.9 (4.62)
122.0 (4.53)
100.0 (1.35)
111.7 (0.13) 97.3 (1.26)
44.5 (16.75)
44.7
122.7
116.3
n
112.3
97.8
n
91.6
n
90.5
@
DSC (h)
DSC (c)
POM
62.3 (32.46)
129.3 (7.92)
127.3 (7.5)
91.8 (0.82)
n
n
n
139.9 (0.49)
140.6
91.7 (0.96) 47.9 (37.67)
129.0
127.8
n
119.3
n
93.2
92.0
n
48.1
DSC (h)
DSC (c)
POM
60.4 (17.46) 129.5 (2.87)
101.1 (0.09)
@
n
n
n
130.1 (0.19) 120.3 (2.86)
51.3 (15.51)
51.8
129.6
121.1
@
115.6
112.2
@
101.6
n
100.5
n
DSC (h)
DSC (c)
POM
64.2 (33.59) 129.3 (4.77)
n
n
129.8 (0.36) 125.9 (4.32)
109.1 (0.08)
56.9 (22.72)
57.4
130.5
126.7
@
109.7
n
99.7
n
99.0
n
DSC (h)
DSC (c)
POM
61.6 (16.56) 127.2 (3.47)
n
n
n
128.8 (0.34) 123.6 (3.3)
54.2 (11.07)
54.9
129.7
82.1 (13.26) 129.0 (1.45)
126.3 (2.45)
124.1
n
119.3
n
97.6
97.0
DSC (h)
DSC (c)
POM
n
n
69.9 (13.63)
70.2
126.9
106.2
n
92.1
n
12BAOþ2BA NNhC
DSC (h)
DSC (c)
POM
64.8 (27.88) 128.9 (3.89)
126.2 (3.82)
n
n
53.9 (25.4)
54.2
126.8
112.1
110.5
Temperatures in 1C while enthalpy in J/g is given in parenthesis.
n
not resolved.
@
monotropic transition.

C. Kavitha et al. / Physica B 407 (2012) 859–867
861
Plate 1. Nematic schlieren texture of 12BAOþ8BA complex.
Plate 4. Fully grown smectic X phase observed in 12BAOþ8BA complex.
Plate 2. Texture of homeotropic region (Nh) of nematic observed in 12BAOþ8BA
Plate 5. Broken focal conic texture of smectic CR in 12BAOþ8BA complex.
complex.
involving H-bond dimmer and the Fermi resonances in the pre-
cursor (Fig. 2a (i)) are observed at 2554 cmꢂ1 and 1913 cmꢂ1
,
respectively [18,19]. Once the complexation is complete the O–H
stretching band and its Fermi resonance appear at 2561 cmꢂ1 and
1906 cmꢂ1, respectively, and no peak due to dimerrization of
benzoic acid moieties is seen. Moreover, the carbonyl band
observed at 1690 cmꢂ1 of the pre-cursors shifts to 1682 cmꢂ1
after complexation due to formation of hydrogen bond between
carboxylic acids. These results support the hydrogen bonded
network structures have been obtained by the dominant forma-
tion of the thermodynamically favored inter-molecular hydrogen
bonds between the individual components [19]. These results
concur with the reported results on H-bond mesogens [20].
Plate 3. Schlieren texture of smectic C in 12BAOþ8BA complex.
3.3.2. 1H NMR studies
Iso-N-Nh"N-Sm C-Sm X"Sm CR"Crystal (12BAOþ7BA)
Iso"N-Nh-N-Sm C-Sm X-Sm CR"Crystal (12BAOþ6BA)
The proposed molecular structure is verified by the 1H NMR
studies. The 1H NMR spectra are recorded for all the synthesized
complexes under present investigation. As a representative case,
the spectrum of 12BAOþ4BA is discussed. 1H NMR spectrum of
the complex is recorded in CDCl3 with TMS (tetramethylsilane) as
the internal standard. The following chemical shifts were
observed in the spectrum:
Iso"N-Nh-Sm C-Sm X-Sm CR"Crystal (12BAOþ5 and
4 BA)
Iso"N-Nh-Sm C"Crystal (12BAOþ3 and 2 BA).
Monotropic and enantiotropic transitions are depicted as
single and double arrows, respectively.
(a) Broad resonance signals are observed approximately in the
range of 0.5–2.8 ppm for the methylene group. In the
12BAOþ4BA complex these signals are observed between
2.685 and 0.878 ppm, which has been attributed to the
existence of the backbone methylene.
3.3. Spectroscopic studies
3.3.1. Fourier Transform Infrared Spectroscopy (FTIR)
(b) Sets of multiplets between 7.963–7.933 ppm, 7.675 ppm and
7.255–7.229 ppm and 6.916–6.887 are assigned to aromatic
protons.
(c) The existence of the methoxy proton unit resonance showed a
signal at 4.009 ppm.
IR spectra of all the HBLC complexes are recorded in the solid
state (KBr) at room temperature. As a representative case, Fig. 2a
illustrates the FTIR spectra of 12BAOþ8BA in solid state at room
temperature. It can be noticed that the O–H stretching band

862
C. Kavitha et al. / Physica B 407 (2012) 859–867
Fig. 2. (a) FTIR spectra of 12BAOþ8BA complex. (a(i)) FTIR spectra of pure 8BA.
The non appearance of a peak at 11.0–12.0 ppm for the
The sharp signal obtained at 156.410 ppm is attributed to the
(e)
(d)
carboxylic acid hydrogen in the 1H NMR spectra also stands
a proof for the involvement of that hydrogen in inter-
molecular hydrogen bonding.
oxy-carboxylic acid carbon.
(f) The sharp signal obtained at 161.689 ppm is attributed to the
carboxylic acid carbon.
The shifts obtained through 1H NMR study are found in
accordance to the proposed molecular structure.
Thus the proposed structure of the organic molecule is further
confirmed with 13C NMR studies.
3.3.3. 13C NMR studies
The projected organic molecular structure of the complex is
verified through 13C NMR study. The 13C NMR spectra are
recorded for all the synthesized complexes under present inves-
tigation. As a representative case, the spectrum 12BAOþ4BA is
discussed. This study infers the information regarding the number
of non equivalent carbons and also to identify the types of carbon
atoms (methyl, methylene, aromatic, carbonyl and so on). Hence
the carbon skeleton of the organic molecule can be deduced
completely with the aid of the 13C NMR study.
13C NMR spectrum of the complex was recorded in CDCl3 with
TMS (tetramethylsilane) as the internal standard. AS a represen-
tative case, the spectrum of 12BAOþ4BA is elaborately discussed.
The recorded spectrum includes the following chemical shifts:
3.4. DSC studies
DSC thermograms are recorded in heating and cooling cycle.
The sample is heated with a scan rate of 10 1C/min and held
at its isotropic temperature for 2 min so as to attain thermal
stability. The cooling run is performed with the same scan rate of
10 1C/min. The respective 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 represen-
tative case the DSC thermogram including both the heating and
cooling cycles are depicted in Fig. 3. A graph designated as Fig. 3
depicts the endothermic and exothermic DSC thermogram of
12BAOþ8BA complex. It is plotted for the DSC heating and
cooling thermograms by assigning the temperature variation
(a) Broad resonance magnetic signals are observed approxi-
mately in the range of 34.493–15.882 ppm, which contributes
to the methylene carbon.
(b) A multiplet shift achieved at 7.825–7.607 ppm is assigned to
the methyl carbon.
along X-axis and the corresponding enthalpy value (
Y-axis.
DH) along
(c) The existence of the alkyloxy carbon in the proposed mole-
cule is confirmed by the shift observed at 72.095–
61.812 ppm.
3.5. Phase diagram of pure p–n-alkyloxy benzoic acid
(d) Multiple resonance magnetic signals obtained at 125.332–
107.678 ppm are due to the benzene carbon.
The phase diagram of pure 12BAO is reported [21,22] to
compose of two phases namely, nematic and smectic C.

C. Kavitha et al. / Physica B 407 (2012) 859–867
863
smectic CR to crystal interfaces. Moreover there is no appreci-
able change in isotropic temperatures.
(g) The occurrence of smectic X phase is attributed to the l/d ratio
of the homologs series. The lower homologs failed to have a
threshold value of l/d ratio, which in turn reflects in the
absence of smectic X phase.
2
0
Nh
K
N
Iso.
C
Cooling run
(h) A gradual decrease in the crystallization temperatures is
observed from pentyl benzoic acid to ethyl benzoic acid.
4. Optical tilt angle measurement
Heating run
The optical tilt angle has been experimentally measured by
optical extinction method [16] in smectic C and smectic CR phases
of all the members of the present 12BAOþnBA homologous
series. Figs. 5–10 depict such variation of optical tilt angle with
temperature for 12BAOþnBA (where n¼2–8) series. In Figs. 5–10
the theoretical fit obtained from the Mean field theory is denoted
by the solid line. It is observed from the Figs. 5–10, that the tilt
-2
12BAO+8BA
50
100
Temperature / °C
angle increases with decreasing temperature and attains
a
Fig. 3. DSC thermograms obtained in cooling cycle of 12BAOþnBA complexes.
Sm CR
Sm C
11
10
9
140
120
100
80
12BAO+nBA
Isotropic
N
8
N
C
Nh
Sm X
7
Cr
X
6
5
12BAO+8BA
94
60
Crystal
90
92
96
40
Temperature / °C
2
4
6
8
Alkyl carbon number
Fig. 5. Optical tilt angle measurement in smectic C,
12BAOþ8BA complex.
X
and CR phases of
Fig. 4. Phase diagram of 12BAOþnBA homologous series.
SmCR
14
3.5.1. Phase diagram of 12BAOþnBA
Sm C
Phase diagram of p–n-dodecyloxy benzoic acid and p–n-alkyl
benzoic acids (12BAOþnBA) homologs series are depicted in
Fig. 4. The following points can be elucidated from Fig. 4:
12
10
(a) The 12BAOþnBA hydrogen bonded homologous series exhi-
bits nematic as orthogonal phase while smectic C, smectic X
and smectic CR as tilted phases.
(b) The nematic phase along with homeotropic region of nematic
and smectic C are prevailing throughout the homologs of the
series.
Sm X
(c) The smectic X phase is induced only from butyl to octyl
benzoic acids quenching the thermal range of smectic C.
Further smectic X is sandwiched between smectic C and re-
entrant smectic C phases.
(d) The thermal span of nematic and re-entrant C phase are
observed to be invariant with increasing alkyl chain length.
(e) The mesogenic thermal span is found to increase with
increasing alkyl chain length.
8
12BAO+7BA
105
6
90
95
100
110
115
Temperature / °C
(f) An interesting observation in the present homologs series is
the absence of odd–even effect at isotropic to nematic and
Fig. 6. Optical tilt angle measurement in smectic C,
12BAOþ7BA complex.
X and CR phases of

864
C. Kavitha et al. / Physica B 407 (2012) 859–867
15
Sm C
Sm X
Sm CR
Sm CR
Sm C
14
12
10
8
10
5
Sm X
12BAO+6BA
12BAO+4BA
110
6
100
105
110
100
Temperature / °C
Temperature / °C
Fig. 7. Optical tilt angle measurement in smectic C,
12BAOþ6BA complex.
X
and CR phases of
Fig. 9. Optical tilt angle measurement in smectic C,
12BAOþ4BA complex.
X
and CR phases of
Smectic C
12BAO+3BA
12BAO+2BA
Sm CR
Sm X
Sm C
12
10
5
10
8
6
12BAO+5BA
0
4
0
1
2
3
98
100
102
104
106
108
(T - TC) / °C
Temperature / °C
Fig. 10. Optical tilt angle measurement in smectic
complexes (where n¼3 and 2).
C
phase of 12BAOþnBA
Fig. 8. Optical tilt angle measurement in smectic C,
12BAOþ5BA complex.
X and CR phases of
4.1. Optical tilt angle measurements in smectic C, X and CR phases
saturation value. These large magnitudes of the tilt angle are
attributed to the direction of the soft covalent hydrogen bond
interaction, which spreads along molecular long axis with finite
inclination [23].
4.1.1. Tilt angle measurement in 12BAOþ8BA complex
The onset of the smectic C phase is observed at 97.3 1C. As the
temperature is decreased, the optical tilt angle value increases
from 71 to 10.41 and attains a saturation value at 95 1C as can be
noticed from Fig. 5. The optical tilt angle has been fitted to power
law and the solid line indicates the fitted data. Further the
Tilt angle is a primary order parameter [24] and the tempera-
ture variation is estimated by fitting the observed data of
the relation
y
(T) to
magnitude of
Mean field theory predicted value.
b (0.5) is observed to be in concurrence with the
y
ðTÞa
ðTCꢂTÞb
ð1Þ
On further decrease of temperature the smectic C phase of
12BAOþ8BA paves way for a new phase namely smectic X at
91.6 1C whose thermal range is observed to be very narrow
(ꢁ1.1 1C). The magnitude of tilt angle increases with decreasing
temperature and attains a saturated value. The tilt angle magni-
tude varies from 91 to 10.51 in this smectic X phase and as the
phase is fully grown the magnitude of the tilt angle saturates at
10.51.
The critical exponent
y (T) to the above Eq. (1) is found to be 0.50 to agree with the
b value estimated by fitting the data
of
Mean field prediction [25,26]. The solid lines in the Figs. 5–10
depict the fitted data for various mesogens. Further, the agree-
ment of magnitude of
infers the long-range interaction of transverse dipole moment for
the stabilization of tilted smectic C and CR phases.
b (0.5) with the Mean field value (0.5)

C. Kavitha et al. / Physica B 407 (2012) 859–867
865
On further decrease of temperature to 90.5 1C the re-entrant
critical exponent
b
value estimated by fitting the data of
y
(T) is
smectic CR is observed. The magnitude of the tilt angle is 8.71 at
90.5 1C and increases to 10.51 before attaining a saturated value at
89.5 1C.
In all these tilted phases viz., smectic X and smectic CR phase,
the magnitude of the saturated optical tilt angle measured is
approximately equal to the saturated tilt angle value observed in
smectic C phase.
observed to follow the Mean field theory predicted value.
On decreasing the temperature in small steps of 0.1 1C at
99.7 1C a new phase designated as smectic X with very narrow
thermal range (ꢁ0.7 1C) is observed. The saturated magnitude of
the tilt angle is observed to be 10.61 at 99.6 1C.
On further decreasing the temperature to 99.0 1C a new phase
designated as reentrant smectic CR phase is observed. The
saturated magnitude of the tilt angle is observed to be 11.61 at
98.1 1C.
4.1.2. Tilt angle measurement in 12BAOþ7BA complex
In all the three tilted phases viz. smectic C, smectic X and
smectic CR phases, the saturated magnitude of the optical tilt
angle is observed to be almost invariant as can be seen from Fig. 8.
In this hydrogen bonded complex the smectic C phase origi-
nates at 119.3 1C. The variation of tilt angle with temperature is
depicted in Fig. 6. In smectic C phase with decreasing temperature
the optical tilt angle value increases from 6.61 and attains a
saturation value of 141 at a temperature of 111 1C. The fitted tilt
value obtained from Eq. (1) is shown as a solid line in Fig. 6. The
4.1.5. Tilt angle measurement in 12BAOþ4BA complex
The onset of smectic C is observed at 119.3 1C in 12BAOþ4BA
complex. Fig. 9 illustrates the variation of optical tilt angle
value with temperature. From Fig. 9 it can be observed that the
magnitude of the tilt angle increases with decrement in tempera-
ture. The magnitude of the tilt angle saturates to 12.51 at 115.5 1C.
The theoretical tilt value obtained from Eq. (1) is shown as a solid
critical exponent
b value estimated by fitting the data of y (T) is
observed to follow the Mean field theory predicted value.
At 93.2 1C a new phase designated as smectic X with a narrow
temperature range (ꢁ1.5 1C) is observed. As the temperature
decreased in small steps the magnitude of the tilt angle varied
from 111 to 141 in this phase before attaining a saturated value
of 141.
line in Fig. 9. The critical exponent
b value estimated by fitting the
data of (T) is observed to follow the Mean field theory
y
predicted value.
On further decrease of temperature re-entrant smectic phase
designated as CR is observed at 91.7 1C. The magnitude of the tilt
angle is observed to increase with the decrement of temperature.
As the phase is fully grown the magnitude of the tilt angle
saturates to a value of 14.11 at 89.8 1C.
In the three tilted phases viz. smectic C, smectic X and smectic
CR phases, the magnitude of the optical tilt angle is almost
the same.
As the temperature is further decreased, at 97.6 1C a new
phase designated as smectic X with a very narrow thermal range
(ꢁ0.6 1C) is observed. In this phase, as the temperature is
decreased in small steps of 0.1 1C, the magnitude of the tilt angle
increases and gets saturated as shown in Fig. 9. The magnitude of
the tilt angle saturates to 12.51 at 97.3 1C.
On further decrement of temperature to 97.0 1C a re-entrant
smectic phase designated as CR is observed. The magnitude of the
tilt angle in this phase varies between 11.31 and 13.51 with
temperature. The saturated value of 13.51 is observed at 95.8 1C.
In all the three tilted phases viz. smectic C, smectic X and
smectic CR phases, the saturated magnitude of the optical tilt
angle is observed to be almost invariant as can be seen from Fig. 9.
4.1.3. Tilt angle measurement in 12BAOþ6BA complex
The onset of smectic C phase in 12BAOþ6BA is observed at
112.2 1C. As the temperature is decreased the magnitude of the
tilt angle increases and attains a saturated value. The variation of
temperature dependence of tilt angle is depicted in Fig. 7. The
magnitude of the optical tilt angle value varied between 7.11 and
12.71 with decreasing temperature and attains a saturation value
of 12.71 at a temperature of 107.7 1C. The theoretical tilt value
obtained from Eq. (1) is shown as a solid line in Fig. 7. The critical
4.1.6. Tilt angle measurement in 12BAOþ3BA complex
In this hydrogen bonded complex the smectic C phase origi-
nates at 92.1 1C. The variation of tilt angle with temperature is
depicted in Fig. 10. In smectic C phase, with decrement in
temperature the optical tilt angle value increases from 4.41 and
attains a saturation value of 10.91 at a temperature of 94.6 1C. The
fitted tilt value obtained from Eq. (1) is shown as a solid line in
exponent
b value estimated by fitting the data of y (T) is observed
to follow the Mean field theory predicted value.
As the temperature is further decreased at 101.6 1C a new
phase designated as smectic X with a very narrow (ꢁ1.1 1C)
thermal range have been observed. In this phase as the tempera-
ture is decreased in small steps, the magnitude of the tilt angle
increases from 111 to 12.71 and saturates as shown in Fig. 6. The
saturated magnitude of the tilt angle 12.71 is observed at 100.6 1C.
On further decrement of temperature to 100.5 1C a re-entrant
smectic phase designated as CR is observed. The magnitude of the
tilt angle in this phase varies between 91 and 12.81 with
temperature. The saturated value of 12.81 is observed at 98.5 1C.
In all the three tilted phases viz. smectic C, smectic X and
smectic CR phases, the saturated magnitude of the optical tilt
angle is observed to be almost invariant as can be seen from Fig. 7.
Fig. 6. The critical exponent
b value estimated by fitting the data
of (T) is observed to follow the Mean field theory predicted
y
value. In this lower homolog smectic X and smectic CR phases
have been quenched.
4.1.7. Tilt angle measurement in 12BAOþ2BA complex
The onset of the smectic C phase is observed at 110.5 1C. As the
temperature is decreased, the optical tilt angle value increases
from 6.61 to 10.51 and attains a saturation value. The optical tilt
angle has been fitted to power law and the solid line indicates the
fitted data. Further the magnitude of
b (0.5) is observed to be in
concurrence with the Mean field theory predicted value. Like
12BAOþ3BA complex in this lower homolog also the smectic X
and smectic CR phases have been quenched. The variation of tilt
angle with temperature is depicted in Fig. 10.
4.1.4. Tilt angle measurement in 12BAOþ5BA complex
In the 12BAOþ5BA complex the onset of smectic C is observed
at 109.1 1C. Fig. 8 illustrates the variation of optical tilt angle
value with temperature. From the Fig. 8 it can be observed that as
temperature is decreased the magnitude of the tilt angle increases
initially and attains a saturated value. The magnitude of the tilt
angle saturates to 10.61 at 106 1C. The theoretical tilt values
obtained from Eq. (1) are shown as a solid line in Fig. 8. The
4.2. Possible reasons for the occurrence of re-entrant smectic CR
The appearance of finger print texture with decrease of
temperature (in n¼4–8 as sandwiched between SmC and SmCR)

866
C. Kavitha et al. / Physica B 407 (2012) 859–867
can be explained on the premise of a frustration (i.e., deformation
or disruption) of the long range tilt (grown in the higher
temperature SmC phase) ordering. With the decrease of tempera-
ture, the molecules of SmC phase come closer. As such, the
separation of dipolar pair decreases to increase the repulsion,
which leads to the loss of stability of the high temperature
C-phase. However, if one molecule in nth smectic layer re-adjusts
to slide onto the adjacent layer, the repulsive interaction can be
reduced. Thus, the spoilage of long range layered tilt order
(especially in homologs, viz., for n¼4–8) leads to the protrusion
of end chains into the adjacent layers. Further, the steric factors
(as manifested by the protruding end chains) become prevalent
with decreasing temperature. A broken tilt order with decreasing
temperature leads to the formation of domains with features of
SmC. They are familiarized with cybotactic nematic domains.
However, further decrease of temperature naturally leads to the
growth of some sort of correlation among these domains. Thus,
the growth of correlation among tilted smectic domains leads for
their coalescing to result in the growth of smectic CR phase.
measured. As a representative case the temperature variation of
12BAOþ8BA helical structure is discussed.
The deformation of the helicoidal structure has been experi-
mentally analyzed by varying the temperature. As the tempera-
ture is decremented in small steps the variation of the helix is
noted at each step. The variation of normalized helix with
temperature is depicted as Fig. 11. The unwinding of the helix
with temperature in smectic X phase indicates that this phase is
not only tilted but also possesses helicoidal structure. Liquid
crystal in the smectic X phase acts as a grating and a diffraction
pattern is thus observed on the screen. The first order diffraction
pattern observed is recorded and depicted as Plate 6.
5.4. Ordering in smectic X
Smectic X phase is sandwiched between traditional smectic C
and re-entrant smectic C referred as smectic CR. The molecular
orientation in various phases is depicted in Fig. 12. From Fig. 12 it
can be seen that the flipping of the molecules is taking place. 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 re-orientation. Thermo dynamical conditions favor
this smectic X phase with narrow thermal span where the flipping
of the molecules occur.
5. Characterization of smectic X phase
Smectic X phase, which has been identified in five complexes
namely 12BAOþ8BA, 12BAOþ7BA, 12BAOþ6BA, 12BAOþ5BA
and 12BAOþ4BA is characterized by textural studies. In all the
above complexes, smectic X is sandwiched between traditional
smectic C and reentrant smectic CR phases. This serves as one of
the strongest evidence to identify this new phase smectic X as
smectic ordering.
POM, optical tilt angle and helicoidal structural studies enable
this phase to be a smectic ordering. This argument is valid with
1.0
12BAO+8BA
5.1. Textural study of smectic X phase
Smectic X
0.8
The hydrogen bonded complexes from 12BAOþ8BA to
12BAOþ4BA, on cooling from isotropic phase nematic droplets
are observed. On further cooling, a worm like texture is observed,
which is designated as smectic X and depicted as Plate 4. The
thermal width of this phase is very narrow (o1 1C) and the fully
grown phase is shown as Plate 4. The striations in the worm like
texture are manifestation of the presence of helicoidal structure.
This phase has been characterized in earlier hydrogen bonded
liquid crystals [16] by us. With further decrement of temperature
this phase paves way for broken focal conic texture of smectic CR
phase as shown in Plate 5. Interestingly, this smectic X phase is
observed to be monotropic. The nematic to smectic C transition is
observed to be second order and hence could not be resolved in
the DSC thermograms, similarly smectic X to smectic C is also
observed to be second order transition.
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
0.5
(T-Tc) / °C
5.2. DSC studies of smectic X phase
Fig. 11. Temperature variation of helicoidal structure of 12BAOþ8BA complex.
Interestingly the smectic X phase exhibits monotropic transi-
tion in the performed thermal scans hence it is observed only in
the heating run of the 12BAOþ6BA complex. In the other
complexes exhibiting this phase it is unresolved although the
thermal scan is performed at a scan rate of 5 1C/min. The enthalpy
value of this phase transition clearly indicates that it is a weak
second order transition.
5.3. Helical pitch measurement
In the present work, the helical pitch is measured by diffrac-
tion of He–Ne red laser light on liquid crystal sample filled in a
commercially available (Instec) buffed conducting cell. This
method can be used for measurement of the helical pitch of
short length. Helicoidal pitch of all the complexes has been
Plate 6. Diffraction pattern corresponding to helicoidal measurement of
12BAOþ8BA complex.

C. Kavitha et al. / Physica B 407 (2012) 859–867
867
nematic, the extinction of light intensity is noticed. The light
shuttering action in the entire thermal span of homeotropic
region of nematic phase can be clearly seen from Fig. 13. Thus
all these seven homologous in homeotropic region of nematic
phase can be thermally tuned for optically shuttering action,
which can be exploited for commercial applications.
Smectic C
Smectic X
Flipping
of dipoles
7. Conclusions
Smectic CR
(1) A novel series of HBLC is synthesized and characterized by
various techniques.
(2) Typical smectic phases like smectic X and smectic CR have
been observed in certain complexes.
Molecular alignment in
various phases
Fig. 12. Molecular orientation in various phases.
(3) All the phases observed are confirmed by the textural obser-
vation, optical tilt angle measurement and DSC studies.
(4) The observation of homeotropic region in nematic phases
observed in all the complexes enables the mesogens to be
used as a thermally tuned optical shutter.
1.00
0.75
0.50
0.25
0.00
12BAO+8BA
(5) Optical tilt angles for the various tilted phases like smectic C,
smectic X and smectic CR are measured and compared.
Homeotropic
Nematic
region of nematic
(N)
(Nh)
Acknowledgments
The divine blessings of almighty Bannari Amman and the
infrastructural support rendered by the Bannari Amman Institute
of Technology are gratefully acknowledged.
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All the seven hydrogen bonded complexes of 12BAOþnBA
exhibit nematic to homeotropic transition and the relevant
transition temperatures of various homologous are tabulated in
Table 1. In all the seven homologs studied, the nematic phase is
manifested by a threaded nematic texture as shown in Plate 1.
As a representative case, thermally tuned optical shuttering
action in 12BAOþ8BA complex is discussed. The onset of the
nematic phase is observed at 122.0 1C with threaded nematic
texture. The intensity of the plane polarized light pertaining to
the nematic texture under the crossed polarizers is measured
with a photo diode TSL 252 at various predetermined tempera-
tures. With the decrement of the temperature in small steps of
0.5 1C the onset of homeotropic region of nematic phase desig-
nated as Nh is noticed, which is shown as Plate 2. The intensity of
the plane polarized light in the nematic, and homeotropic
nematic phase are measured. Fig. 13 illustrates the variation of
normalized light intensity with the temperature. As the nematic
to homeotropic region of nematic occurs, the plane polarized light
intensity gradually decreases and in homeotropic region of
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