Photo and electrically switchable behavior of azobenzene containing pendant bent-core liquid crystalline polymers

Article abstract of DOI:10.1002/pola.26455

New class of photo and electrically switchable azobenzene containing pendant bent-core liquid crystalline monomers (AZBM 1, 2, and 3) and their polymers (AZBP 1, 2, and 3) are reported. The synthesized precursors, monomers, and polymers were characterized by FT-IR, ¹H, and ¹³C NMR spectroscopy. Thermal stability of polymers was examined by thermogravimetric analysis and revealed stable up to 260 °C. The mesophase transition of monomers and polymers are observed through polarized optical microscopy (POM) and further confirmed by differential scanning calorimetry (DSC). The electrically switching property of monomers and their polymers were studied by electro-optical method. Among the three monomers AZBM 1, 2, and 3, AZBM 1 and 2 exhibit antiferroelectric (AF) switching and AZBM 3 exhibits ferroelectric (F) switching behavior. On the other hand, low molecular weight polymers (AZMP 1, 2, and 3) show weak AF and F switching behavior. The photo-switching properties of bent-core azo polymers are investigated using UV-vis spectroscopy, trans to cis isomerization occurs around 25 s for AZBP-1 and 30 s for AZBP-2 and 3 in chloroform, whereas reverse processes take place around 80 and 90 s.

Full text of DOI:10.1002/pola.26455

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Photo and Electrically Switchable Behavior of Azobenzene Containing
Pendant Bent-Core Liquid Crystalline Polymers
M. Vijay Srinivasan,1 P. Kannan,1 A. Roy2
1Department of Chemistry, Anna University, Chennai, India
2Soft Condensed Matter, Raman Research Institute, Bangaluru, India
Correspondence to: P. Kannan (E-mail: pakannan@annauniv.edu)
Received 25 September 2012; accepted 25 October 2012; published online 27 November 2012
DOI: 10.1002/pola.26455
ABSTRACT: New class of photo and electrically switchable azo-
benzene containing pendant bent-core liquid crystalline
monomers (AZBM 1, 2, and 3) and their polymers (AZBP 1, 2,
and 3) are reported. The synthesized precursors, monomers,
and polymers were characterized by FT-IR, 1H, and 13C NMR
spectroscopy. Thermal stability of polymers was examined by
thermogravimetric analysis and revealed stable up to 260 ꢀC.
The mesophase transition of monomers and polymers are
observed through polarized optical microscopy (POM) and
further confirmed by differential scanning calorimetry (DSC).
The electrically switching property of monomers and their
polymers were studied by electro-optical method. Among the
three monomers AZBM 1, 2, and 3, AZBM 1 and 2 exhibit
antiferroelectric (AF) switching and AZBM 3 exhibits ferroelec-
tric (F) switching behavior. On the other hand, low molecular
weight polymers (AZMP 1, 2, and 3) show weak AF and F
switching behavior. The photo-switching properties of bent-
core azo polymers are investigated using UV-vis spectros-
copy, trans to cis isomerization occurs around 25 s for AZBP-
1 and 30 s for AZBP-2 and 3 in chloroform, whereas reverse
C
V
processes take place around 80 and 90 s.
2012 Wiley Peri-
odicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51,
936–946
KEYWORDS: azo polymers; ferroelectricity; liquid-crystalline
polymers (LCP)
INTRODUCTION Ferroelectric and antiferroelectric liquid crys-
tals are interesting materials due to their unusual electric
properties useful for modern optoelectronics, piezoelectric,
pyroelectric, optical devices, and display technology.1–4 Ferro-
electric and antiferroelectric behavior is identified from bent-
core (BC) molecules.5,6 The switching property arises due to
the smectic phase of BC molecule and the switchable meso-
phase attributed to polar character and supramolecular chiral-
ity of the BC molecule. In addition to the polar order, if the
long axes of molecules are tilted with respect to layer normal,
the phase is designated as SmCP phase (where P stands for
polar order). If the polar axes are the same in adjacent layers,
the SmCP phase behaves to be ferroelectric (F) while the po-
lar axes alternate from layer to layer in the ground state,
SmCP is antiferroelectric (AF). These BC molecules have
attracted much attention due to their interesting new type of
liquid crystalline phases which have properties somewhat dif-
ferent from those exhibited by conventional and non-conven-
tional liquid crystals.7–10 Bent-core molecule exhibits variety
of phases (B1–B8), these B phases is called as banana phases,
which depends on linking group, number of aromatic ring, lat-
eral substitution, and terminal alkyl chain.11,12 Among them
B2, B5, and B7 phases are electrically switchable. In general,
shorter terminal alkyl chain containing BC molecules exhibit
B1 phase (Colr) and are non-switchable phase, whereas longer
alkyl chain BC molecule exhibit SmCP phase (B2, B7). In the
B2 phase, polar switching can be observed as AF/F switching
based on polarization of the BC molecule. B2 phase exhibits,
four types of arrangements based on orientation of tilt and
polarization in successive layers, results in SmCSPA, SmCAPA,
SmCSPF, and SmCAPF (where the subscripts S, A of C denote
synclinic, anticlinic).13,14
Liquid crystalline polymers (LCPs) are the hottest research
topic in the field of liquid crystals. Due to their unusual
properties (anisotropic, optical, electrical, and mechanical),
LCPs have been used widespread in commercial applications,
such as high-strength fibers, thermoplastics, displays, optical
storage devices, medical application, etc.15–18 Among LCPs,
the bent-core liquid crystalline polymers (BCLCPs) possess
macroscopic polar order with variety of useful properties,
such as dielectric, piezo, and pyroelectricity.12,19,20 Hence,
the developments of BCLC material in to polymeric mole-
cules are the interesting research topic utilitarian in poten-
tial application. In general, most of the bent-shaped polymers
are reported as main-chain21,22 as well as side-chain poly-
mers.23–25 The side-chain bent-core polymers are derived
Additional Supporting Information may be found in the online version of this article.
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from 1,3-phenylene unit with five aromatic rings, symmet-
ric24 as well as asymmetric,25 wherein the polymerizable
functional groups are generally used as 1,3-diene,26 metha-
crylates or acrylates group,23 and siloxane group.27 Interest-
ingly, side-chain bent-core liquid crystalline polymer pre-
dominantly demonstrate rich mesomorphism compare with
main-chain28 as well as side-chain29 and dendronized liquid
crystalline polymers30–36 and which are classified as follows
(i) side-chain bent-core liquid crystalline polymer
(SBCLCPs),15 (ii) bent-core mesogen-jacketed liquid crystal-
line polymers (BMJLCPs),37 (iii) central linked bent-core liq-
uid crystalline polymer (CBCLCPs),38 and (iv) hydrogen-
bonded side chain bent-cores liquid crystalline polymers
(HBCLCPs)26 were reported based on the polymeric back
bone attached with position of bent-core mesogen pendant.
Though BCLCs have been extensively investigated, bent-core
side-chain polymers were scarcely achieved with barely
detectable polar switching properties.23,26
eter in CDCl3 with TMS as an internal standard.ꢀDSC meas-
urements were performed at a heating rate of 5 C/min, the
monomers and polymers taken in an aluminum pan with a
pierced lid, in dry nitrogen atmosphere with an empty pan
as reference on TA instrument. Gel permeation chromato-
graphic (GPC) measurements were performed with Shimadzu
GPC and THF used as the eluant at a flow rate of 1.0 mL/
min. All GPC data were calibrated with polystyrene stand-
ards. The samples were dissolved in THF (10 mg/mL) and
filtered through a 0.45 lm PTFE filter before the measure-
ments. The LC texture of monomers and polymers are
observed using a Euromex polarizing optical microscope
(POM) equipped with a Linkem HFS91 heating stage and a
TP-93 temperature programmer. Samples were placed in
between two thin glass cover slips and melted with heating
and cooling at the rate of 2 ꢀC/min. The photographs were
taken from cannon 1000 D camera, the spontaneous polar-
ization (Ps) in the SmCP phase measured by a triangular
wave method in a planner-aligned cell using a digital oscillo-
scope (Agilent, model MSO6012A) and an arbitrary wave-
form generator (Agilent, model 33220A) coupled with a
power amplifier (Trek, model 601). The absorbance spectra
of polymers were measured on a Shimadzu UV-1650 spectro-
photometer using chloroform as solvent. The photoisomeri-
zation of azobenzene between the E and Z-form was carried
out using a 6W high-pressure Hg lamp (Sankyo, Japan) for
UV irradiation (365 nm), (Hitachi, Japan) for visible light
(560 nm).
This work illustrates the novel BC monomers and their BCLC
polymers, which are constructed with different linking
groups such as azo, azomethine, and ester with higher alkyl
chain (dodecyl spacer) on both terminal chain as well as
spacer. However, none of the polymers consisting of the
above linkage have so far been reported to exhibit dual
switching behavior namely photo and electrically switchable
liquid crystalline polymers. Bent-shaped compounds were
reported to exhibit variety of mesophase like B1, B2, B4, BX,
and Nu with azo and azomethine linkages.39–43 In this article,
we have focused bent-core liquid crystalline polymers
(BCLCPs) and these bent-core molecules behave with respect
to polymeric back-bone. The BCLCPs combined with azo and
imino functional groups is an important development in the
switchable material. As expected, this material is found to
form banana mesophases. An imino linkages introduced in
this molecule is more conducive to exhibit mesomorphism.
The azobenzene is considered to be a prototype molecular
switch thus the BCLCPs behave as dual switch with light as
well as electric field.
RESULTS AND DISCUSSION
Synthesis
Asymmetric BCLCPs can be synthesized from asymmetric
bent-core mesogens and connected with polymerizable func-
tional groups. Asymmetric BCLCs have fascinated in recent
years, because they exhibit a new phase behavior achieved by
selective cleavage of protected groups at different chemical
conditions. Recently, asymmetric BCLCs were synthesized with
high yield can be readily achieved with selective functional
groups. In this work, three different monomers and respective
of polymers were achieved to explore the structure–property
relationship effect on mesophase. The three different BCLC
monomers are constructed based on number and nature of ar-
omatic ring; asymmetry is introduced in the bent-core mole-
cules using different linking groups such as azo, azomethine,
and ester linkage. Here, asymmetric bent-core mesogens is
connected with polymeric back-bone through dodecyl spacer
as well as terminal chain. The synthesis of three different
monomers and respective polymers is shown in the Schemes
1, 2, and 3. The detailed synthetic procedure of precursors,
monomers, and polymers are given in Supporting Information.
Since the monomers have poor solubility in methanol, normal
precipitation process is not sufficient to remove unreacted
monomers. Further purification was achieved by using the col-
umn chromatography technique and dried in vacuum over
P2O5 at 70 ꢀC for 24 h. Chemical structures of these mono-
mers and polymers were confirmed with 1H and 13C NMR
analysis, the characteristic resonance of the vinyl group around
EXPERIMENTAL
Materials
Solvents ethanol, methanol, dichloromethane, ethyl acetate,
chloroform, and acetone were purified by the reported pro-
cedure.44 4-Aminophenylacetamide (Sigma-Aldrich, India),
phenol, sodium nitrate, potassium hydroxide, 4’-hydroxybi-
phenyl-4-carboxylic acid, sodium hydroxide, potassium
carbonate, hydrochloric acid (Merck, India), 4-hydroxybenzal-
dehyde, 3-hydroxybenzaldehyde, resorcinol, potassium
dichromate, potassium iodide, N,N-dicyclohexylcarbodiimide
(DCC), benzyl chloride, palladium carbon (10%), 12-bromo-
1-dodecanol, 12-bromo-1-dodecane, methacrylic acid, and
4-(dimethylamino)pyridine (DMAP) (Aldrich) were used as
received.
Measurements
The infrared spectra of BCLCPs were obtained on a Perkin
Elmer FT-IR spectrometer using KBr pellets. 1H and 13C
NMR spectra were recorded on a Bruker 400 MHz spectrom-
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SCHEME 1 Synthetic route of AZBM-1 and AZBP-1.
6.09 and 5.53 ppm can be clearly seen is assigned to meth-
acryl protons of each monomer. BCLCPs were successfully syn-
thesized using free radical polymerization method from its
monomer. The vinyl group proton is vanished after polymer-
ization broad resonance signal around 1.69 ppm assigned to
resonance signal of side-chain methylene protons, whereas the
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SCHEME 2 Synthetic route of AZBM-2 and AZBP-2.
signal at 2.10 ppm in all monomers corresponds to methyl
protons, shifted up field to 0.88 ppm in the polymers confirm-
ing the completeness of polymerization reaction. All the syn-
thesized monomers and polymers are soluble in DMF, DCM,
CHCl3, partially soluble in acetone and insoluble in ethanol,
methanol, and benzene.
(AZBM-2) exhibit SmCP with fluid like mesophase on heating
but the filamentary growth pattern with strips and ribbons
with fringe pattern (SmCP) was observed during cooling from
isotropic23,45 and revealed that AZBM-2 is enantiotropic in
nature. The AZBM-2 indicates somewhat lower mesophase
transition temperature than AZBM-3. Though AZBM-2 and 3
monomers are five-ring system, the presence of biphenyl moi-
ety in the AZBM-3 is responsible for higher transition temper-
ature when compared with AZBM-2.
Mesomorphic Properties of the Monomers and Their
Polymers
The transition temperature of BCLC monomers and polymers
are observed by DSC and phase transition confirmed by
POM. On cooling from the isotropic phase, the monomers
(AZBM-1, 2, and 3) exhibit SmCP textures with fluid likes
mesophases. Figure 1 depicts the mesomorpic properties of
monomers (AZBM 1-3). The AZBM-1 evidences filamentary
pattern with banana leaf-like texture which is highly fluid
mesophases with mesophase transition higher than the other
monomers22 and indicating that AZBM-1 was constructed
with six-aromatic ring and other monomers (AZBM 2 and 3)
were constructed by five-aromatic ring. The monomer
During heating AZBM-3 demonstrated a banana leaf-like tex-
ture. While cooling, filamentary like pattern was developed
from droplet in isotropic phase, filamentary texture was
transformed into broken fan like texture (SmC)22 and further
cooling no phase transition, except crystallization which is
depicted in Figure 1(a,b). All three monomers exhibit SmCP
mesophase with different transition temperature depends on
number and nature of aromatic ring in the bent-core mole-
cules and terminal alkyl chain lengths.12,23,46,47 From POM
study, all the polymers (AZBP-1, 2, and 3) exhibited grainy
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SCHEME 3 Synthetic route of AZBM-3 and AZBP-3.
like texture while cooling from the isotropic phase at a rate
of 5 C/min. as shown in Figure 2.
more number of aromatic ring (six ring) where as other
polymer has five-ring system.22,23 The monomers and poly-
mers display different mesophase and transition temperature
and these depends on structure–property of the molecules
might be influenced by nature of central-core, number of
aromatic ring and the chain length.
ꢀ
The polymers AZBP-1, 2, and 3 depict grainy or thread like
texture can also be viewed in small birefringence domains
when compared with their monomer texture and the domain
sizes in polymers are much lesser due to the high viscosity
of the polymeric materials. The AZBP-1 grainy/threaded is
fluid like mesophase when compared with other polymers,
the high viscosity of the polymeric materials is attributed
Figure 3 indicates the DSC thermograms of AZBM-1, 2, and 3
ꢀ
measured at the rate of 5 C/min for first heating and cooling.
All the monomers indicate two transition peaks during
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FIGURE 1 POM microphotograph SmCP phase is observed for monomers (a) AZBM-1 at 101 ꢀC, (b) AZBM-2 at 95 ꢀC, (c) AZBM-3
at 100 ꢀC, and (d) SmC phase with broken fan like texture of AZBM-3 at 100 ꢀC.
heating and cooling cycle, which is suggesting that all the
monomers are enantiotropic in nature. Here, the lower tem-
perature and higher temperature peaks correspond to crystal-
liquid crystal transition (Tm) and liquid crystal-isotropic tran-
sition (Tc), respectively. LC texture of SmCP phase of these
monomers confirmed by electro-optical experiment and phase
transition temperatures and enthalpies value are summarized
in Table 1. The polymers also show two transition peaks Tm
and Tc. Apart from this, a weak glass transition could be
observed at 60–70 ꢀC which is shown in Figure 4 and DSC
analysis reveals that polymers are enantiotropic in nature.
The phase transition temperatures of the polymers are sum-
marized in Table 2. The mesophase transition temperature of
polymer (AZBP-1) has shown slight difference between the
other polymers of AZBP-2 and 3. The three polymers follow
similar mesophase transition temperature trend as that of
their monomers (AZBM-1, 2, and 3).
lowed ‘‘three-stage’’ decomposition; the decomposition might be
started with cleavage of azomethine, then cleavage of azo group
ascribed from evolution of nitrogen gas and final stage decom-
position ascribed to the pyrolytic cleavage of ester linkage of the
aromatic backbone.48 The decomposition of the polymers was
almost completed at 700 ꢀC and no further weight loss was
observed. The thermal stability of the polymers increases in the
order of AZBP-1> AZBP-3>AZBP-2.The presence of number of
aromatic ring and biphenyl were expected to introduce rigidity
into the polymer which consequently bring up the thermal sta-
bility. If the number of ring increases in AZBP-1 rigidity
increases and is thermally stable when compared with other
five-aromatic ring polymers AZBP-3>AZBP-2, among this
AZBP-3 shows somewhat higher stability which is due to the
presence of biphenyl moiety present in the polymer. The char
yield was calculated by measuring the amount of residual sub-
stance present at 700 ꢀC during the TGA analysis. The results of
TGA data revealed that azobenzene-based polymers exhibited
high thermal stability and char yield (Table 1).
Thermal Stability of Polymers
The thermal stability of the polymers (AZBP-1, 2, and 3) were
evaluated using TGA at a heating rate of 20 ꢀC/min and depicted
in the Figure 5. The corresponding data of polymer AZBP-1, 2,
and 3 are listed in Table 2. The azobenzene polymers indicate
10% weight loss around at 290–360 ꢀC and 50% weight loss at
Electro-Optical Investigations of Monomer and Polymer
The investigation on electrical-switching behavior of mono-
mers AZBM-1, 2, and 3 were subjected to electrical field
using electro-optical method. The AZBM-1 is filled with
transparent sandwich type ITO cell (indium-tin oxide -coated
glass plates) with thickness 8 lm and sample filled by
ꢀ
390–435 C indicating their good thermal stability. The results
ꢀ
disclosed that the polymers were stable up to 260 C and fol-
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FIGURE 3 DSC thermograms of monomers (AZBM-1, 2, and 3).
intensity increases and the field induced transition are
reversible indicating the antiferroelectric switching (SmCPA)
as exhibited in Figure 6(a). The similar electro-switching
responses found in AZBM-2 [Fig. 6(b)], all these observa-
tions are quite similar to those obtained for the
12,23,49
SmCPA.
Similarly, the AZBM-3 was filled in an ITO cell with a thick-
ness of 7.5 lm is then subjected to electric field. The current
response of AZBM-3 was measured under a triangular wave
voltage of 40 V and frequency of 75 Hz which results the
single polarization current peaks per half cycle were
observed even at low frequency. This indicates the ferroelec-
tric-switching behavior of mesophase.50,51 The switching
current response trace obtained for AZBM-3 is displayed in
Figure 6(c). It is noteworthy to mention that while compar-
ing the polarization current profiles, compounds AZBM-1, 2
demonstrates antiferroelectric-switching behavior; on the
other hand, AZBM-3 establish ferroelectric switching behav-
ior. The monomers evidence different switching behavior
depends on structure–property of the molecules influenced
by nature of central-core, number of aromatic ring and the
FIGURE 2 POM microphotograph of grainy/ threaded like tex-
ture is observed for polymers (a) AZBP-1 at 120 ꢀC, (b) AZBP-2
at 105 ꢀC and (c) AZBP-3 at 110 ꢀC.
capillary action in isotropic phase and cooled slowly from
the isotropic phase (1 ꢀC per min.). The current responses
were measured across a 1 kX resistance under a triangular
wave voltage of 70 V and frequency of 75 Hz. The filamen-
tary texture patterns of SmCP phase is transformed to a bro-
ken-fan texture while applying A.C. electric field and this fan
texture remains even after removal of electric field, which
clearly indicates the presence of spontaneous polarization in
the smectic layers and the polarization tends to align along
the applied electric field in the SmCP phase. Two polariza-
tion current peaks per half cycle were observed at a very
low threshold voltage, if the voltage is increased, the peak
FIGURE 4 DSC thermograms of the polymers (AZBP 1, 2, and 3).
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TABLE I Phase Transition Temperature (T in 8C) of Monomers
(AZBM-1–3) by DSC
DSC (ꢀC)
Monomer Code Cr
SmCP
Iso
AZBM-1
AZBM-2
AZBM-3
*
*
*
97.9 [15.2]
76.9 [15.9]
91.8 [36.8]
*
*
*
112.8 [15.9]
101.4 [17.4]
105.9 [09.4]
*
*
*
Cr, crystalline phases; SmCP, polar smectic
phase; ‘‘*’’ phase exists.
C phase; Iso, Isotropic
chain length. The switching current response trace obtained
for AZBM-1 and 2 is testified in Figure 6(a,b). The spontane-
ous polarization (Ps) of AZBM-1 and 2 are measured from
its switching current of their SmCP phase and found to be
90, 109 nC/cm2. It was observed that all the monomers
(AZBM-1, 2, and 3) are prone to polymerize under the A.C.
electric field and, hence, all the data were measured using
freshly prepared cells, and the measurement time was lim-
ited to be <5 min.23
FIGURE 5 The thermal stability of the polymers (AZBP-1, 2,
and 3) were evaluated using TGA at a heating rate of 20 ꢀC/
min.
Photo-Switching Studies of Bent-Core Monomer
and Its Polymer
The photo-switching property of AZBP-1 was performed in
chloroform using UV-vis spectroscopy with absence and
presence of UV light illumination. The absorption spectra of
azobenzene containing bent-core polymers (AZBP-1, 2, and
3) are pictured in Figure 8. The absorption spectra of AZBP-
1, 2, and 3 shows three absorbance maxima at 256–261 nm,
366–367 nm, and 446–447 nm, respectively. The strong
absorbance in the UV region at 366–367 nm corresponds to
p-p* transition of the E isomer and a weak absorbance in
the visible region at 446–447 nm represent to n-p* transi-
tion of Z isomer.28,33,53–56 The absorption maximum at 264
nm is not important in photo-switching, remaining two
absorption peaks were considered for photoisomerization
processes. The AZBP-1 is illuminated using UV light with
365 nm at different time intervals and the absorption spec-
tra were immediately recorded. The absorption maximum at
366 nm slowly diminishes whereas the absorption maximum
around 446 nm gradually increases, which indicates that E
isomer transformed to Z isomer. After 30 s illumination,
there is no remarkable change in absorption spectrum which
confirms the saturation of E!Z isomerization processes. The
existence of photoisomerization is further confirmed by two
isobestic points existed at 327 and 435 nm.28,38,55,56 Figure
8 shows the E/Z photoisomerization of AZMP-1- 3 upon ex-
posure of UV light. The photoisomerization of AZMP-1 takes
Electro-optical investigations of polymers were also carried
out in similar ITO cells as previously discussed. The AZBP-1
is filled in ITO cell there is no change in LC texture when
observed using POM during applying electric fields at low
voltage, if increasing electric-field above 110 V, a very weak
switching behavior is observed. The polarization current of
AZBP-1 in response to an applied triangular-wave field
shows two small humps per half wave function [Fig. 7(a)],
indicating perhaps an antiferroelectric switching behavior.
Most of the LCPs did not switch upon applying electric
field while weak switching behavior was observed in low
molecular weight polymers. AZBP-2 also shows electrical
switching responses with weak antiferroelectric behavior
[Fig. 7(b)].23 Similarly, electrical switching experiment was
carried out on AZBP-3 which shows ferroelectric switching
behavior [Fig. 7(c)]. The ferroelectric behavior of AZBP-3 is
found from presence of one small hump per half cycle
when applied electrical field. All these observations were
quite similar to those obtained for the SmCP mesophase
exhibited by the polymers.26,29,50–52 Whereas in the low
intensity of the switching humps indicates that not all the
molecular dipoles were switched during the electro-optic
measurement because of high viscosity of the polymer
samples.23
TABLE 2 Thermal Property, Molecular Weight, and Absorption Maxima of Polymers (AZBP-1–3)
Char yield
at 700 ꢀC (%)
k max (nm)
k max (nm)
ꢀ
Mn
ꢀ ꢀ
Mw=Mn
Polymers
Yield (%)
Tg
Tm
Tc
Td
in chloroform
in film
AZBP-1
AZBP-2
AZBP-3
74
75
70
77
70
74
107.7
94.5
129.1
113.6
122.2
273
260
265
19
11
15
11, 143
11, 742
9,979
1. 31
1.57
366, 446
366, 446
367, 447
366
366
367
100.7
1. 26
ꢀ
Tg, glass transition temperature; Tm, melting temperature; Tc, clearing temperature; Td, decomposition temperature; Mw, weight-average molecular
ꢀ
ꢀ
ꢀ
weight; Mn, number-average molecular weight; Mw=Mn, molecular weight distribution.
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FIGURE 6 Switching current response of monomers. (a) AZBM-1 observed at 105 ꢀC, 70 V, 75 Hz, (b) AZBM-2 observed at 95 ꢀC,
40 V, 75 Hz, the occurrence of two peaks in each half period of the applied triangular wave is an indication of an antiferroelectric
switching, and (c) AZBM-3 observed at 95 ꢀC, 40 V, 75 Hz, the presence of one peak is indication of an ferroelectric switching.
place at 30 s whereas other polymers AZMP-2 and 3 take
place at 35 s. Here, the AZMP-1 has six-member ring and
other polymers has five-member ring. Generally, six-member
ring system is rigid in nature which can undergo easy photo-
isomerization when compare with five-ring system. Whereas
AZMP-2 and 3 shows similar photoisomerization attributed
that both are five-ring system with similar structure. The
photoisomerization studies reveal that the AZMP-1 shows
somewhat faster photoisomerization than other polymers.
The time taken for photoisomerization depends on the
nature of linking group (electron withdrawing group/donat-
ing group) and size of the substituent in the azobenzene
moiety.38,43,48,53
upon illumination with 560 nm visible light are 80 s for
AZBP-1 and 90 s for AZBP-2 and 3. The photoisomerization
studies reveal that BCLCPs show trans to cis photoisomeriza-
tion take place at 30, 35, and 35 s whereas reversible proc-
esses take place at 80, 90 and 90 s. The bent-core monomers
and polymers exhibit very strong photoisomerization phe-
nomena with UV as well as visible light illumination and also
thermally reversible.
Recently, the researchers focus their attention on solid film
to show the photo-switching property for commercial exploi-
tation. Faster photoisomerization and thermal stability is a
crucial parameter for the creation of storage devices in solid
samples. Hence, the UV-vis spectroscopic properties of the
synthesized polymers were investigated in thin film for poly-
mer AZBP-1–3. Figure 8(d) shows the absorption spectra of
polymer (AZBP-1) in thin film. The thin film of polymer
shows an absorption band at 366 nm nearly same as in solu-
tion phase at 365 nm. The polymer film irradiated with 365
nm UV light, the decrease in absorption at 366 nm was
observed ascribed to E to Z isomerization at 80 s. After
reaching photo-stationary state, the film was irradiated with
560 nm visible light and absorbance increase at 366 nm,
there is no remarkable change after 110 s at 450 nm and
confirms the photoisomerization processes. The existence of
photoisomerization is further confirmed by two isobestic
points existed at 328 and 433 nm.28,38,55,56 Similarly, other
The reversible photoisomerization behaviors of BCLCPs are
observed using visible light illumination or keeping the solu-
tion in dark. The resultant solution (Z isomer) of all the
BCLCPs is placed in the dark environment; the absorption
band in the range of 366–367 nm was regained slowly
attributed to the slow thermal cis to trans back isomeriza-
tion of azobenzene unit at ambient temperature in dark, this
well-known processes of the thermal back reaction confirms
the BCLCPs are thermally reversible behavior. Similarly, the Z
isomer of AZBP-1 sample were irradiated with 560 nm visi-
ble light which causes the cis to trans back isomerization
very rapidly compared with that at ambient temperature in
dark.28,38,56 The time required to recover the original state
FIGURE 7 Switching current response of polymers (a) AZBP-1 observed at 105 ꢀC, 110 V, 75 Hz, (b) AZBP-2 observed at 95 ꢀC, 110
V, 75 Hz, presence of two small hump is an indication of an antiferroelectric switching, and (c) AZBP-3 observed at 95 ꢀC, 110 V,
75 Hz, the presence of one small hump or peak is indication of an ferroelectric switching.
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FIGURE 8 The absorption spectra of azobenzene containing bent-core polymers (a) AZBP-1, (b) AZBP-2, (c) AZBP-3 polymers in so-
lution, and (d) AZBP-1 in thin film were recorded with different exposure time of UV light. 0 s corresponds to absence of UV light
illumination
polymers indicate trans to cis photoisomerization at 90 s
whereas reverse processes take place at 120 s. The time
taken to attain the photo-stationary state in thin film is high
when compared with that in solution phase indicating that
steric effects play a vital role in thin film.28,29,57
carried out in solution as well as film. AZBP-1–3 indicate
trans to cis photoisomerization take place at 30, 35, and 35 s
whereas reversible processes occurred at 80, 90, and 90 s in
chloroform. Our investigations revealed that the thermal
properties as well as the photochromic properties of these
azo-substituted polymers vary as the number and nature ar-
omatic ring present in the polymers. The photo- and electro-
optical switching behavior of these BCLCPs might be
exploited in the field of optical data storage device, molecu-
lar switches, and pyroelectric materials.
CONCLUSION
We have demonstrated the synthesis, mesomorphic, photo-
and electro-optical properties of asymmetry BCLCMs and its
BCLCPs. All the monomers and polymers evidence liquid
crystalline phases; the AZBM-1 and 2 show SmCP phase
with antiferroelectric switching (AF) behavior whereas
AZBM 3 exhibit SmCP phase with ferroelectric switching (F)
behavior. Similarly, the polymers exhibit grainy like texture
and display weak AF and F switching behavior compared
with respective of monomers. The monomers and polymers
proves thermal properties in the following order of AZBM-
1> AZBM-3>AZBM-2 and AZBP-1> AZBP-3>AZBP-2,
respectively. The photo-switching properties polymers were
ACKNOWLEDGMENTS
M. V. Srinivasan and P. Kannan sincerely acknowledge to Coun-
cil of Scientific and Industrial Research (CSIR), New Delhi, India
[CSIR scheme no. 01(2095)/07/EMR-II] for the financial sup-
port. M. V. Srinivasan honestly thanks to University Grants Com-
mission, New Delhi, Government of India [UGC Meritorious
Research Fellowship-Fl4-112006(BSR0/7-7I2007(BSR), dt.
04.07.2011]. Instrumentation facility provided under FIST-DST
WWW.MATERIALSVIEWS.COM
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28 C. Saravanan, S. Senthil, P. Kannan, J. Polym. Sci. Part A:
Polym. Chem. 2008, 46, 7843–7860.
and DRS-UGC to Department of Chemistry, Anna University,
Chennai are gratefully acknowledged.
29 C. Saravanan, P. Kannan, J. Polym. Sci. Part A: Polym.
Chem. 2010, 48, 1565–1578.
30 J. G. Rudick, V. Percec, Acc. Chem. Res. 2008, 41,
REFERENCES AND NOTES
1641–1652.
1 N. A. Clark, S. T. Lagerwall, Appl. Phys. Lett. 1980, 36,
899–901.
31 J. G. Rudick, V. Percec, New J. Chem. 2007, 31, 1083–1096.
32 B. M. Rosen, C. J. Wilson, D. A. Wilson, M. Peterca, M. R.
Imam, V. Percec, Chem. Rev. 2009, 109, 6275–6540.
2 M. Koden, Ferroelectrics 1996, 179, 121–129.
3 V. Lapanik, V. Bezborodov, S. Timofeev, W. Haase, Appl.
Phys. Lett. 2010, 97, 2519–2513.
33 S. A. Prokhorova, S. S. Sheiko, C. H. Ahn, V. Percec, M.
Moeller, Macromolecules 1999, 32, 2653–2660.
4 W. Martienssen, H. Warlimont, Springer Handbook of Con-
densed Matter and Materials Data, Springer Berlin Heidelberg,
2005; Vol. XVII, p 1–1025. (ISBN:978-8-540-44376-6).
34 V. Percec, C. H. Ahn, G. Ungar, D. J. P. Yeardley, M. Moller,
S. S. Sheiko, Nature 1998, 391, 161–164.
35 V. S. K. Balagurusamy, G. Ungar, V. Percec, G. Johansson,
J. Am. Chem. Soc. 1997, 119, 1539–1555.
5 T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe,
J. Mater. Chem. 1996, 6, 1231–1233.
36 V. Percec, D. Schlueter, Y. K. Kwon, J. Blackwell, M. Moller,
P. J. Slangen, Macromolecules 1995, 28, 8807–8818.
6 R. Amaranatha Reddy, C. Tschierske, J. Mater. Chem. 2006,
16, 907–961.
37 X. F. Chen, K. K. Tenneti, C. Y. Li, Y. Bai, R. Zhou, X. Wan,
X. Fan, Q. F. Zhou, Macromolecule 2006, 39, 517–524.
7 G. Ungar, V. Percec, M. N. Holerca, G. Johansson, J. A. Heck,
Chem. Eur. J. 2000, 6, 1258–1266.
38 M. Vijay Srinivasan, P. Kannan, J. Mater. Sci. 2011, 46,
5029–5043.
8 V. Percec, D. Tomazos, J. Heck, H. Blackwell, G. Ungar, J.
Chem. Soc. Perkin. Trans. 1994, 2, 31–44.
39 V. Prasad, D. S. Shankar Rao, S. Krishna Prasad, Liq. Cryst.
2001, 28, 643–646.
9 V. Percec, M. Glodde, T. K. Bera, Y. Miura1, I. Shiyanovskaya,
K. D. Singer, V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A.
Rapp, H.-W. Spiess, S. D. Hudson, H. Duan, Nature 2002, 419,
384–387.
40 V. Prasad, A. Jakli, Liq. Cryst. 2004, 31, 473–479.
41 S. W. Kang, K. A. Suresh, L. Joshi, Q. Wang, S. Kumar,
J. Am. Chem. Soc. 2005, 127, 17224–17227.
10 G. Johansson, V. Percec, G. Ungar, K. Smith, Chem. Mater.
1997, 9, 164–175.
42 N. G. Nagaveni, A. Roy, V. Prasad, J. Mater. Chem. 2012,
22, 8948–8959.
11 R. Dabrowski, Ferroelectrics 2000, 243, 1–18.
43 M. Vijay Srinivasan, P. Kannan, A. Roy, Liq. Cryst., in press.
DOI.10.1080/02678292.2012.721904.
12 S. Balamurugan, P. Kannan, M. T. Chuang, S. L. Wu, Ind.
Eng. Chem. Res. 2010, 49, 7121–7128.
44 D. D. Perin, W. L. F. Aramarego, Purification of Laboratory
Chemicals; Pergamon Press: New York, 1998.
13 H. Nadasi, W. Weissflog, A. Eremin, G. Pelzl, S. Diele, B.
Dasb, S. Grande, J. Mater. Chem. 2002, 12, 1316–1324.
45 G. Dantlgraber, D. Shen, S. Diele, C. Tschierske, Chem.
Mater. 2002, 14, 1149–1158.
14 S. Balamurugan, P. Kannan, K. Yadupati, A. Roy, Liq. Cryst.
2011, 38, 1199–1207.
46 A. Roy, N. V. Madhusudana, Eur. Phys. J. E 2005, 18,
253–258.
15 V. Percec, D. Tomazos, Adv. Mater. 1992, 4, 548–561.
16 V. Percec, J. Heck, G. Johansson, D. Tomazos, M. Kawa-
sumi, G. Ungar, J. Macromol. Sci. Pure Appl. Chem. A 1994,
31, 1031–1070.
47 I. Dierking, Textures of Liquid Crystals; Wiley-VCH Verlag
GmbH & Co. KGaA: Weinheim, 2003; pp 1–218.
48 S. R. P. Delphia, S. Senthil, P. Kannan, G. Vinitha, A. Rama-
lingam, A. Nooraldeen, J. Phys. Chem. Solids 2007, 68,
1812–1820.
17 V. Percec, Pure Appl. Chem. 1995, 67, 2031–2038.
18 V. Percec, M. Lee, Macromolecules 1991, 24, 2780–2788.
19 C. Keith, G. Dantlgraber, R. Amaranatha Reddy, U. Baumeis-
ter, C. Tschierske, Chem. Mater. 2007, 19, 694–710.
49 G. Dantlgraber, C. Keith, U. Baumeister, C. Tschierske, J.
Mater. Chem. 2007, 17, 3419–3426.
20 H. Takezoe, Y. Takanishi, Jpn. J. Appl. Phys. 2006, 45, 597–625.
50 V. Novotna, M. Kaspar, V. Hamplova, M. Glogarova, L.
Lejcek, J. Kroupa, D. Pociecha, J. Mater. Chem. 2006, 16,
2031–2038.
21 N. Gimeno, S. F. Antoni, N. Sebastian, R. Mezzenga, Blanca
Ros, M. Macromolecules 2011, 44, 9586–9594.
51 S. Umadevi, A. Jakli, B. K. Sadashiva, Soft Matter 2006, 2,
215–222.
22 E. J. Choi, E. C. Kim, C. W. Ohk, W. C. Zin, J. H. Lee, T. K.
Lim, Macromolecules 2010, 43, 2865–2872.
52 Y. Zhang, U. Baumeister, C. Tschierske, M. J. O. Callaghan,
C. Walker, Chem. Mater. 2010, 22, 2869–2884.
23 X. Chen, K. T. Kishore, C. Y. Li, Y. Bai, X. Wan, X. Fan, Q. F.
Zhou, L. Rong, B. S. Hsiao, Macromolecules 2007, 40, 840–848.
53 A. Natansohn, P. Rochon, Chem. Rev. 2002,102, 4139–4175.
24 L. Y. Wang, H. Y. Tsai, H. C. Lin, Macromolecules 2010, 43,
1277–1288.
54 L. Rahman, S. Kumar, C. Tschierske, G. Israel, D. Ster, G.
Hegde, Liq. Cryst. 2009, 36, 4, 397–407.
25 K. T. Kishore, X. Chen, C. Y. Li, Z. Shen, X. Wan, X. Fan, Q.
F. Zhou, L. Rong, B. S. Hsiao, Macromolecules 2009, 42,
3510–3517.
55 M. Yamada, M. Kondo, J. I. Mamiya, Y. Yu, M. Kinoshita, C.
J. Barrett, T. A. Ikeda, Angew. Chem. Int. Ed. 2008, 47,
4986–4988.
26 A. C. Sentman, D. L. Gin, Angew. Chem., Int. Ed. 2003, 42,
1815–1819.
56 J. H. Liu, P. C. Yang, Polymer 2006, 47, 4925–4935.
27 C. Keith, R. Amarnath Reddy, R.; C. Tschierske, Chem. Com-
mun 2005, 871–873.
57 S. H. Kim, C. H. Ahn, S. R. Keum, K. Koh, Dyes Pigments
2005, 65, 179–186.
946
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 936–946