Thermally controlled optical shutter in an inter-molecular hydrogen bonded liquid crystal

Article abstract of DOI:10.1016/j.physb.2011.08.014

Novel homologs series of supra-molecular liquid crystals have been isolated. Hydrogen bond is formed between mandelic acid (MD) and various homologs of p-n-alkyloxy benzoic acids (nOBA) and has been confirmed by FTIR studies. Optical polarizing microscopic observations show that all these materials exhibit rich liquid crystallinity with various mesophases. Phase transition temperatures and enthalpy values are experimentally evaluated by DSC studies and the phase diagram of homologous series has been constructed. An interesting feature is the observation of thermally controlled reversible optical shuttering action in one of the homolog, wherein with the increment of temperature the homeotropic texture changes to homogenous texture of smectic F. Thus, this optical shuttering phenomenon is reversible. Optical tilt angle data of two homologs have been fitted to power law equation and it is found that the mean field theory prediction is valid. The light intensity profile in homeotropic region of smectic F in one complex has been experimentally analyzed and a steep sudden decrement of the intensity of light manifesting the distortion of the molecular alignment is experimentally found.

Full text of DOI:10.1016/j.physb.2011.08.014

Physica B 406 (2011) 4139–4144
Contents lists available at SciVerse ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Thermally controlled optical shutter in an inter-molecular hydrogen
bonded liquid crystal
V.N. Vijayakumar, 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:
Novel homologs series of supra-molecular liquid crystals have been isolated. Hydrogen bond is formed
between mandelic acid (MD) and various homologs of p-n-alkyloxy benzoic acids (nOBA) and has been
confirmed by FTIR studies. Optical polarizing microscopic observations show that all these materials
exhibit rich liquid crystallinity with various mesophases. Phase transition temperatures and enthalpy
values are experimentally evaluated by DSC studies and the phase diagram of homologous series has
been constructed. An interesting feature is the observation of thermally controlled reversible optical
shuttering action in one of the homolog, wherein with the increment of temperature the homeotropic
texture changes to homogenous texture of smectic F. Thus, this optical shuttering phenomenon is
reversible. Optical tilt angle data of two homologs have been fitted to power law equation and it is
found that the mean field theory prediction is valid. The light intensity profile in homeotropic region of
smectic F in one complex has been experimentally analyzed and a steep sudden decrement of the
intensity of light manifesting the distortion of the molecular alignment is experimentally found.
& 2011 Elsevier B.V. All rights reserved.
Received 8 September 2010
Received in revised form
3 August 2011
Accepted 4 August 2011
Available online 11 August 2011
Keywords:
Liquid crystals
Chemical synthesis
Phase transitions
Optical properties
1. Introduction
versatility for commercial applications. For a liquid crystalline mate-
rial to be exploited for commercial applications, it should possess a
Liquid crystals science is gaining importance in the last few
decades. Among the various types of liquid crystals thermotropic
liquid crystals form an exclusive class. Many research groups [1–4]
around the globe are involved in these materials, which are being
used in display device applications. For the formation of liquid
crystalline materials through hydrogen bonding interactions, com-
plementarity of the interacting components coupled with the direc-
tionality of hydrogen bonds is the main factor [5–8] contributing to
the exhibition of liquid crystallinity. The first ever hydrogen bonded
liquid crystalline material has resulted from the dimerization of
aromatic p-n alkyloxy carboxylic acids (nOBA) [9,10]. There are
reports in the literature [11,12] that to obtain a mesogenic hydrogen
bonded liquid crystal it is enough if one of the compounds exhibits
mesogenic properties. It is a great surprise to note that a mesogenic
hydrogen bonded liquid crystal (HBLC) can be formed through two
non-mesogenic compounds [13,14] by hydrogen bonding. Room
temperature liquid crystals are derived [15,16] from the interaction
of stilbazole derivatives with several mono/dicarboxylic acids. Kato
and his group [17–21] did extensive work on synthesis and char-
acterization of various types of HBLC. In the recent past, many groups
[16–19] worked on HBLC due to their ease of synthesis, abundant
availability of various acceptor and donor functional groups and
large magnitude of tilt angle, large thermal range of smectic C phase
and optimum response time. Another interesting application of these
materials is in the area of light modulation. Thermotropic liquid
crystalline materials are reported [22] in the literature to exhibit
optical shuttering action and light modulating applications.
Design, synthesis and characterization of supra-molecular
materials that can be used for applicational purposes is a
challenging task. These hydrogen bonded materials can be used
for applications only if they possess a phase that can be tuned by
an external stimulus like an electric or thermal field. The design of
the molecular structure helps in a proper molecular alignment
of the phase, which plays a pivotal role in ascertaining the extent
of utility of the mesogen. With our previous experience [23–30] in
synthesis and characterizing of various types of liquid crystals
and with the above aims, novel series of hydrogen bonded
mesogens are synthesized. Light modulation and optical shutter-
ing action can be realized with proper design of the mesogens. In
the present work thermally controlled optical shuttering action
has been examined in one of the mesogens, which can be
exploited for commercial applications.
2. Materials and methods
n Corresponding author. Tel.: þ91 9442437480; fax: þ91 4295 223 775.
E-mail address: mln.madhu@gmail.com (M.L.N. Madhu Mohan).
The p-n-alkyloxy benzoic acids (nOBA) and mandelic acid are
supplied by Sigma Aldrich, Germany, and all the solvents used
0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2011.08.014

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V.N. Vijayakumar, M.L.N. Madhu Mohan / Physica B 406 (2011) 4139–4144
were of HPLC grade. Inter-molecular hydrogen bonded mesogens
are synthesized by the addition of one mole of p-n-alkyloxy
benzoic acids (nOBA) with 1 mole of mandelic acid to N,N-
dimethyl formamide (DMF). Further, they are subjected to con-
stant stirring for 12 h at ambient temperature (30 1C) till a white
precipitate in a dense solution is formed. The white crystalline
crude complexes so obtained by removing excess DMF are then
recrystallized with dimethyl sulfoxide (DMSO) and the yield
varied from 75% to 90%. Yield of higher homologs complexes are
observed to be more (90%) compared to their lower counterparts
(75%). The molecular structure of the present homologous series
of p-n-alkyloxy benzoic acids with mandelic acid is depicted in
Fig. 1, where n represents the alkyloxy carbon number. The
temperature control of the liquid crystal cell is equipped by
Instec HCS402-STC 200 temperature controller (Instec, USA) to a
temperature resolution of þ0.1 1C. This unit is interfaced to a
Pentium computer by IEEE-STC 200 to control and monitor the
temperature. Optical textural observations are made with a Nikon
polarizing microscope equipped with Nikon digital CCD camera
system with 5 mega pixels and 2560 ꢀ 1920 pixel resolutions. The
liquid crystalline textures are processed, analyzed and stored
with the aid of ACT-2U imaging software system. The liquid
crystal sample is filled by capillary action in its isotropic state
into a commercially available (Instec, USA) polyamide buffed cell
The optical extinction method is used for calculating optical tilt
angle. The light intensity profile has been experimentally ana-
lyzed by controlling the temperature and the intensity of the light
from the liquid crystal sample is measured by a photodiode (TSL
252) connected to a 6½ digit digital multimeter (Keithley 2100).
The transition temperatures and corresponding enthalpy values
are experimentally obtained by differential scanning calorimetry
(DSC; Shimadzu DSC-60).
3. Results and discussion
All the mesogens isolated under the present investigation are
white crystalline solids and are stable at room temperature. They
are insoluble in water and sparingly soluble in common organic
solvents such as methanol, ethanol, and benzene and dichloro
methane. However, they show a high degree of solubility in
coordinating solvents like dimethyl sulfoxide (DMSO) and
pyridine. All these mesogens melt at specific temperatures below
115 1C (Table 1). They show high thermal and chemical stabilities
when subjected to repeated thermal scans performed during
polarizing optical microscope (POM) and DSC studies.
3.1. Phase identification
with 4 mm spacer. Silver wires are drawn as leads from the cell.
The observed phase variants, transition temperatures and
corresponding enthalpy values obtained by DSC in cooling and
heating cycles for various homologous complexes are presented
in Table 1.
3.2. Infrared spectroscopy (FTIR)
Infrared spectra of free p-n-alkyloxy benzoic acid, mandelic
acid and their inter-molecular H-bonded complexes are recorded
in the solid state (KBr) at room temperature. As a representative
case, Fig. 2 illustrates the FTIR spectra of the hydrogen bonded
Fig. 1. Molecular structure of MDþnOBA inter-hydrogen bonded complex.
Table 1
Comparison of transition temperatures obtained by various techniques.
Carbon
Phase variance
Technique
Crystal to melt
N
C
F
G
Crystal
5
F
DSC (H)
DSC (C)
POM (C )
108.8 (115.68)
#
107.6 (37.54)
108.1
84.6 (48.65)
84.9
6
7
8
9
NF
DSC (H)
DSC (C)
POM (C )
95.9 (79.0)
93.5 (58.25)
75.1 (26.97)
94.1 (58.55)
#
102.0 (79.0)
93.6 (36.85)
93.9
109.4 (1.03)
109.8
79.1 (34.11)
79.4
NF
DSC (H)
DSC (C)
POM (C )
#
104.8 (merged)
87.5 (6.70)
89.9
108.7 (4.48)
109.2
72.4 (28.02)
72.7
NFG
NFG
DSC (H)
DSC (C)
POM (C )
#
106.9 (37.85)
94.5 (63.3)
94.9
100.7 (18.30)
90.3 (Merged)
90.7
109.2 (1.25)
109.6
50.3 (20.34)
50.6
DSC (H)
DSC (C)
POM (C )
106.8 (3.78)
103.0 (8.57)
103.4
#
105.1 (3.00)
105.5
89.3 (17.98)
89.8
81.8 (44.46)
82.1
10
11
NCF
DSC (H)
DSC (C)
POM (C)
DSC (H)
DSC (C)
POM (C )
86.30 (37.22)
97.4 (58.23)
#
#
95.8 (19.83)
90.6 (58.78)
90.9
108.8 (3.52)
109.2
100.0 (4.30)
100.3
72.9 (36.52)
73.2
NCFG
110.8 (merged)
113.9 (1.10)
114.2
#
#
#
101.9 (12.70)
102.3
87.0 (14.58)
87.5
72.4 (8.03)
72.9
62.1 (27.10)
62.5
12
NC
DSC (H)
DSC (C)
POM (C)
95.3 (83.74)
111.9 (merged)
108.7 (4.34)
109.1
#
86.9 (21.94)
87.3
70.7 (49.22)
71.1
#, monotropic transition; H, heating run; C, cooling run.

V.N. Vijayakumar, M.L.N. Madhu Mohan / Physica B 406 (2011) 4139–4144
4141
Fig. 2. FTIR spectra of MDþ9OBA inter-hydrogen bonded complex.
complex of MDþ9OBA in solid state at room temperature (31 1C).
The solid state spectra of free alkyloxy benzoic acid are reported
[31] to have two sharp bands at 1685 cmꢁ1and at 1695 cmꢁ1 due
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 confirm these DSC results along with the
results of monotropic transition.
to the frequency
n(CQO mode). The doubling feature of this
stretching mode confirms the dimeric nature of alkyloxy benzoic
acid at room temperature [31]. Further, in the present MDþ9OBA
hydrogen bonded complex a band appearing at 2924 cmꢁ1is
As a representative case, the phase transition temperatures and
enthalpy values of nonyloxy benzoic acid and mandelic acid mesogen
(MDþ9OBA) are discussed. Fig. 3 illustrates the thermogram of
MDþ9OBA hydrogen bonded complex recorded at a scan rate of
10 1C/min for the heating and cooling runs. The cooling run of DSC
thermogram shows four distinct transitions, namely isotropic to
nematic, nematic to smectic F, smectic F to smectic G and smectic
G to crystal with transition temperatures 105.1, 103.0, 89.3 and
81.8 1C and corresponding enthalpy values 3.00 J/g, 8.57 J/g, 17.98 J/g
and 44.46 J/g, respectively, while in the heating cycle two distant
transitions, namely crystal to melt and melt to smectic F are
obtained at 94.1 1C and 106.8 1C with corresponding enthalpy values
of 58.55 J/g and 3.78 J/g, respectively. Nematic and smectic G are
observed to be monotropic transitions in the heating run.
assigned to
n
(O–H mode) of the carboxylic acid group.
(CQO mode) may be attributed to the
The doubling nature of
n
dimeric nature of the acid group at room temperature [31]. The
corresponding spectrum of solution state (chloroform) shows a strong
intense band, suggesting the existence of monomeric form of benzoic
acid. A noteworthy feature in the spectra of MDþ9OBA complex is
the appearance of a broad band at 1682 cmꢁ1 and non-appearance of
the doubling nature of n(CQO mode) of benzoic acid moiety. This
clearly suggests that the dimeric nature of the benzoic acid dissoci-
ates and prefers to exist in a monomeric form upon complexation.
3.3. MDþnOBA homologous series
All these transition temperatures of the present homologous
series concur with those of optical polarizing microscopic studies.
The mesogens of the mandelic acid and alkyloxy benzoic acid
homologous series are found to exhibit characteristic textures
3.5. Phase diagram of MDþnOBA homologous series
[32], viz., nematic (Schlieren threaded), smectic
C (marble
threaded), smectic F (checkered board texture) and smectic G
(multi-colored mosaic texture). The general phase sequence of the
mandelic acid and alkyloxy benzoic acids in the cooling run can
be shown as
The phase diagram of pure p-n-alkyloxy benzoic acid is
reported [23] to compose of three phases, namely nematic,
smectic C and smectic G. The phase diagram of mandelic acid
with p-n-alkyloxy benzoic acid complexes is depicted in Fig. 4.
A careful observation of Fig. 4 reveals the following points:
Isotropic-Sm F-Crystal (MDþ5OBA)
Isotropic-N-Sm F-Crystal (MDþ6OBA, MDþ7OBA)
Isotropic-N-Sm F-Sm G-Crystal (MDþ8OBA, MDþ9OBA)
Isotropic-N-Sm C-Sm F-Crystal (MDþ10OBA)
Isotropic-N-Sm C-Sm F-Sm G-Crystal (MDþ11OBA)
Isotropic-N-Sm C-Crystal (MDþ12OBA)
a) The phase diagram is composed of four phases namely
nematic, smectic C, smectic F and smectic G.
b) Nematic phase is present in all homologs while the higher
ordered smectic F phase is observed in all homologs except
MDAþ12OBA.
c) It is interesting to note that MDþ11OBA exhibits all the four
phases (NCFG), while MDþ5OBA has only one phase (F).
d) Smectic F phase of MDþ11OBA exhibits both homogenous and
homeotropic alignments.
3.4. DSC studies
e) In general, MDþ5OBA has the smallest mesogenic thermal range
while MDþ8OBA has the largest mesogenic thermal range.
The lower homologs (MDþnOBA, n¼5–7) exhibit mono/bi
phase variance with the phase being either nematic or smectic F
or both.
DSC thermograms are obtained in heating and cooling cycles. The
sample is heated at a scan rate of 10 1C/min and held at its isotropic
temperature for 1 min so as to attain thermal stability. The cooling

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V.N. Vijayakumar, M.L.N. Madhu Mohan / Physica B 406 (2011) 4139–4144
Fig. 3. DSC thermogram of MDþ9OBA inter-hydrogen bonded complex.
Fig. 5. Temperature variation of tilt angle in smectic C phase of MDþ10OBA and
MDþ11OBA. Dotted and solid lines denote the fit.
120
110
100
90
Isotropic
Tilt angle is a primary order parameter [33,35] in tilted phases,
viz., smectic C. The temperature variation is estimated by fitting
the observed data of y(T) to the relation
N
y
ðTÞ ¼ AðTꢁTCÞb
ð1Þ
F
C
The critical exponent
b
value estimated by fitting the data of
F
G
y(T) to the above Eq. (1) is found to be 0.50 in both the complexes
80
F
and agrees with the mean field [34] prediction. The value of the
constant A is found to be 9.5 and 12 for MDþ10OBA and
MDþ11OBA, respectively. The dotted and solid lines in Fig. 5
70
G
depict the fitted data. Further, the agreement of
b with mean field
60
value shows the long-range interaction of transverse dipole
moment for the stabilization of tilted smectic C phase.
50
Crystal
9
MDL+nOBA
6
40
4. Thermally controlled optical shuttering action
5
7
8
10
11
12
Alkyloxy carbon number
Optical shutters are usually realized by the application of
external electrical stimulus [26,36] to the liquid crystalline
sample. The orientation of the molecules with respect to the
increasing thermal field gives rise to gradual optical extinction,
which is referred to as optical shuttering action.
In the present compound (MDþ11OBA) the optical shuttering
action is observed with variation of thermal field. In the sense
that when the liquid crystal sample is cooled from isotropic to
smectic F, in the smectic F phase the optical light intensity is
observed to fall gradually in small steps with the decrease of
temperature.
It is interesting to note a homeotropic like alignment in
smectic F phase of MDþ11OBA complex which has been estab-
lished by various techniques like DSC thermogram, textural
observations, optical tilt angle and light intensity studies. Thus,
this mesogen can be used in optical shutter applications. The
following sections describe in detail about the results obtained.
Fig. 4. Phase diagram of MDþnOBA homologous series.
f) In contrast, the higher homologs (MDþnOBA, n¼7–11) exhibit
three/four phase variance with the phases being nematic,
smectic C, smectic F and smectic G.
g) New phases are induced at regular increment of alkyloxy
carbon number. As an example, in MDþ6OBA nematic phase
is induced and in MDþ8OBA smectic G phase is induced while
in MDþ10OBA smectic C phase is induced
3.6. Optical tilt angle studies
For the hydrogen bonded MDþ10OBA and MDþ11OBA com-
plexes, optical tilt angles have been experimentally measured by
the optical extinction method [33] in corresponding smectic C
phase. From Fig. 5 it is observed that the tilt angle increases with
decreasing temperature and attains a saturation value, which is in
accordance with the mean field theory [34]. The saturated values
of the tilt angle in MDþ10OBA and MDþ11OBA are observed to
be around 201. These large magnitudes of the tilt angle are
attributed [16] to the direction of the soft covalent hydrogen
bond interaction, which spreads along molecular long axis with
finite inclination.
4.1. DSC thermogram of MDþ11OBA
The DSC thermogram of MDþ11OBA is illustrated in Fig. 6.
From the cooling run of the DSC thermogram all the phase
transitions exhibited by the compound, viz., isotropic to nematic,
nematic to smectic C, smectic C to smectic F (homogeneous),
smectic F (homogenous) to smectic F (homeotropic), smectic F
(homogenous) to smectic G and smectic G to crystal with transition

V.N. Vijayakumar, M.L.N. Madhu Mohan / Physica B 406 (2011) 4139–4144
4143
Plate 1
Plate 1. Homeotropic texture of smectic F phase of MDþ11OBA. (All the textures
are digitally recorded with a magnification of 10 ꢀ . )
Fig. 6. DSC thermogram of MDþ11OBA inter-hydrogen bonded complex.
temperatures 113.9, 101.9, 87, 85.1, 72.4 and 62.1 1C with corre-
sponding enthalpy values 3.11 J/g, 12.70 J/g, 14.58 J/g, merged,
8.03 J/g and 27.10 J/g, respectively, are clearly identified as peaks
at various temperatures. An interesting observation is that the
transition from homeotropic to homogenous alignment smectic F
is clearly identified as a prominent peak in the DSC thermogram.
4.2. Optical polarizing microscopic textural observation
As discussed above the compound MDþ11OBA exhibits rich
phase polymorphism with phases like nematic, smectic C, smectic
F and smectic G. Checkered board texture is a token of evidence
for the smectic F phase.
When this sample is cooled from isotropic, various phase
transitions are observed. The Schlieren threaded texture of
nematic is transformed to marble threaded texture of smectic C.
On further cooling the marble threaded texture changes to a dark
region, which is designated as the homeotropic region of smectic
F (Plate 1). With further decrement of temperature this home-
otropic region of smectic F paves way for homogenous region of
smectic F with a texture of checkered board (Plate 2).
Plate 2
Plate 2. Homogenous texture of smectic F phase of MDþ11OBA. (All the textures
are digitally recorded with a magnification of 10 ꢀ .)
This serves as a strong textural evidence to suggest the
presence of homeotropic and homogenous textures in smectic F.
4.3. Intensity profile of the optical texture
The sample MDþ11OBA is filled in a commercially available
buffed cell (Instec) and silver leads are drawn for contact. The
intensity profile in homeotropic region of smectic F has been
experimentally analyzed by varying the thermal field and the
intensity of the light from the liquid crystal sample is measured by
a photo-diode (TSL 252). As temperature is decremented in small
steps, the variation of the intensity of the texture is noted at each step
and plotted as shown in Fig. 7. During the transition from homo-
genous region to homeotropic region of biaxial smectic F there is a
steep sudden decrement of the intensity of light, manifesting the
distortion of the molecular alignment. In the homeotropic region of
smectic F, which is referred to as the optical shutter, the optical
profile is completely vanished. As temperature is further decreased,
the homogenous region of smectic F reappears. Thus the intensity of
light again starts to increase as shown in Fig. 7. Thus the phase
variance in the cooling run can be given as
Fig. 7. Temperature variation of intensity in the smectic F phase of MDþ11OBA.
It has been recently reported [36] that in the biaxial phase the
Schlieren texture will not be completely extinct since the field
does not affect the distribution of second director normal to the
main director. The present result is in concurrence with the above
published observation. Thus the liquid crystal behaves as an
optical shutter (Plate 1). Hence this HBLC may be used as a light
modulator.
Sm F (homogenous)-Sm F (homeotropic)-Sm F (homogenous)

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V.N. Vijayakumar, M.L.N. Madhu Mohan / Physica B 406 (2011) 4139–4144
5. Conclusions
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The central idea of designing hydrogen bonded liquid crystals
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are found to be mesogenic. As expected increasing the chain
length favored occurrence of smectic C phase in higher homologs,
quenching the thermal span of smectic F. Extended chain length
of the present series influenced the steep fall of isotropic tem-
peratures compared to the pure benzoic acids. One of the
synthesized complexes, MDþ11OBA, exhibits thermally con-
trolled reversible optical shuttering action.
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Acknowledgments
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[26] V.N. Vijayakumar, M.L.N.Madhu Mohan, Solid State Commun. 149 (2009)
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[27] V.N. Vijayakumar, M.L.N.Madhu Mohan, Solid State Sci. 12 (4) (2010) 482.
[28] X. Han, X. Wei, X. Wang, R.A. Shanks, Adv. Mater. Res. (2011) 152 and 284.
[29] V.N. Vijayakumar, M.L.N.Madhu Mohan, Braz. J. Phys 39 (2009) 601.
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[31] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Co-ordination
Compound, Interscience, New York, 1978.
The authors acknowledge the financial support rendered by
Department of Science and Technology (DST), New Delhi, and
infrastructural support provided by Bannari Amman Institute of
Technology.
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