Photoactive liquid crystalline polyesters based on bisbenzylidene and pyridine moieties

Article abstract of DOI:10.1016/j.molstruc.2009.10.038

Photoactive main chain liquid crystalline polyesters containing bis(benzylidene)acetone and aromatic heterocyclic pyridine were synthesized by solution polycondensation. Chemical structure of monomers and polymers were confirmed by FT-IR, ¹H and ¹³C NMR spectroscopy. Thermal property was investigated by TGA and DSC. Optical property of polyesters was studied in chloroform solution and in thin film under UV irradiation and exhibited cis-trans-photoisomerization. This behavior was witnessed by solubility, carbonyl group absorption band in FT-IR, optical microscopy as well as DSC analysis through existence of liquid crystallinity after irradiation of polymers.

Full text of DOI:10.1016/j.molstruc.2009.10.038

Journal of Molecular Structure 963 (2010) 219–227
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Photoactive liquid crystalline polyesters based on bisbenzylidene
and pyridine moieties
*
G. Deepa, R. Balamurugan, P. Kannan
Department of Chemistry, Anna University, Chennai-600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 August 2009
Received in revised form 22 October 2009
Accepted 22 October 2009
Available online 29 October 2009
Photoactive main chain liquid crystalline polyesters containing bis(benzylidene)acetone and aromatic
heterocyclic pyridine were synthesized by solution polycondensation. Chemical structure of monomers
and polymers were confirmed by FT-IR, 1H and 13C NMR spectroscopy. Thermal property was investigated
by TGA and DSC. Optical property of polyesters was studied in chloroform solution and in thin film under
UV irradiation and exhibited cis–trans-photoisomerization. This behavior was witnessed by solubility,
carbonyl group absorption band in FT-IR, optical microscopy as well as DSC analysis through existence
of liquid crystallinity after irradiation of polymers.
Keywords:
Liquid crystalline polymer
Bisbenzylidene chromophore
Optical property
Ó 2009 Elsevier B.V. All rights reserved.
Photoisomerization
1. Introduction
Thus, LCPs has been extensively studied; several review articles
described progress on this domain [26–28]. Conjugated organic
Liquid–crystalline polymers (LCPs) are of interest ascribable
their ability to combine the properties of simple molecular liquid
crystals with those of polymers [1,2]. LC moieties can be
designed to exhibit a conformational change on the molecular le-
vel in response to thermal [3], pH [4], electrical [5,6] or optical
[7–10] stimulation. The increasing attention in synthesis, charac-
terization and application of photoactive liquid crystalline
polymer systems provide fascinating advantage in molecular,
supramolecular design and in photo controlled changes of optical
property.
The thermotropic main chain liquid crystalline polymers ex-
tend opportunity for development of high performance materials
with improved processability, thermal stability and mechanical
property over the past decades [11–18]. Furthermore, aromatic
polyesters composed of entirely rigid, linear aromatic ester units
possess high melting temperature [19–21]. They are not only
demonstrating thermotropic liquid crystalline (TLC) mesophases
but also good physical properties. Incorporation of methylene
spacer plays a significant role in determining the relationship be-
tween thermo-mechanical history, structural and morphological
organization of polymers [22–25]. Nature of heterocyclic unit
has high potential in TLC formation owing to permanent dipole
moment arising from polarity of hetero atoms in the molecules.
dye molecules are promising materials for electronic and pho-
tonic application suitable for technological innovation [29]. His-
torically, azo dye containing polymers were studied with
interest for their specific photo induced effects [30,31]. When
illuminated with a polarized light of appropriate wavelength,
azobenzene moiety undergoes a reversible trans–cis–trans isom-
erization process. The present investigation dealt with a similar
photo induced effect noticed with bis(benzylidene)acetone moi-
ety is hitherto unreported in the literature which comprises
synthesis of photoactive main chain liquid crystalline polymers
containing bis(benzylidene)acetone with aromatic pyridine het-
erocyclic units. They exhibited photoisomerization behavior un-
der UV irradiation. This phenomenon was demonstrated by
various supporting evidences and discussed.
2. Experimental
2.1. Materials and methods
p-Methylacetophenone, benzaldehyde, ammonium acetate,
thionyl chloride, and p-hydroxybenzaldehyde were used as re-
ceived. 2-chloroethanol, 4-bromobutanol (Merck) 1-bromohexa-
nol, 1-bromooctanol, 1-bromodecanol and anhydrous potassium
carbonate were used as received. Solvents such as acetone, glacial
acetic acid, pyridine, ethanol, methanol, benzene, tetrahydrofuran,
triethylamine (Spectrochem, India) were purified and dried using
reported procedures [32,33].
* Corresponding author. Tel.: +91 44 2220 3155; fax: +91 44 2220 0889.
E-mail address: pakannan@annauniv.edu (P. Kannan).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.10.038

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G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
2.2. Synthesis of precursors
2.3.2. Synthesis of 1,3-bis[4-(m-hydroxyalkyloxy)benzylidene)]
acetone (m = 2, 4, 6, 8 and 10) (i–v)
2.2.1. Synthesis of (4-phenyl)-2,6-bis(4-methylphenyl)pyridine
(PBMPP)
The monomers (i–v) were prepared by Williamson aryl–alkyl-
ether synthesis. A typical procedure followed for the synthesis of
1,3-bis[4-(2-hydroxyethyloxy) benzylidene)]acetone (i) is as follows:
1,3-bis(4-hydroxybenzylidene) acetone (3 g; 0.011 mol) was dis-
solved in dry dimethylformamide (20 ml). Potassium carbonate
(7.8 g; 0.056 mol) was then added to it; instantly change in color
from yellow to red was observed as an indication for formation
of anion. The temperature was kept at 90 °C for 1 h. This was fol-
lowed by addition of 2-chloroethanol (2.3 ml; 0.033 mol) drop-
wise to reaction mixture with constant stirring for 24 h at 90 °C.
At the end of reaction, the mixture was cooled to room tempera-
ture and poured over crushed ice to precipitate product. The
product was filtered, washed with excess of distilled water and
dried in vacuum. The crude precipitate thus obtained was recrys-
tallized from ethanol–water mixture (50:50; yield 81%; m. p.
149–151 °C) [38].
Mixture of p-methylacetophenone (10.51 ml, 0.078 mol), benz-
aldehyde (4 ml, 0.039 mol), ammonium acetate (30 g; 0.0389
mol) and glacial acetic acid (75 ml, 0.131 mol) were taken and re-
fluxed for 5 h with constant stirring. Then the reaction mixture
cooled to ambient, an oily layer thrown out and crystallized slowly.
The crystals thus obtained was filtered, washed with acetic acid
(50%), cold ethanol and dried at 60 °C under vacuum (yield 63%;
m. p. 154–156 °C) [34].
IR(KBr): Absence of peak at 1680 cmÀ1 for carbonyl group con-
firms the formation of product. 1H NMR (CDCl3)–d: 2.42(s, 6H,
CH3), 7.2–8.2 (m, 13H, aromatic protons). 13C NMR (CDCl3)–d:
21.32(ArACH3), 127–129.41(aromatic carbons), 157.41(C@N in
heterocyclic ring), 116.57(b-Carbon in pyridine ring), 150.09(c-C
in pyridine ring).
IR (KBr): 1682 cmÀ1
(m
C@O), 3359 cmÀ1 (spacer-OH) and
1564 cmÀ1 mC@C exocyclic); 1H NMR-d: 9.2(s, 2H, OH), 7.7 (s, 2H,
(
2.2.2. Synthesis of (4-phenyl)-2,6-bis(4-carboxyphenyl)pyridine
(PBCPP)
CH@), 6.8–7.4(m, 6H, aromatic-H), 2.1–2.5(m, 8H, CH2 spacer),
3.8(t, 8H, AOCH2). 13C NMR (CDCl3) d: 26–28(ArAOACH2ACH2A),
64–67 (ArAOACH2A), 125–128 (aromatic carbons).
The remaining monomers (ii–v) were synthesized by an analo-
gous procedure using respective bromoalkanols instead of 2-chlo-
roethanol, with 1,3-bis(4-hydroxbenzylidene)acetone respectively.
PBMPP (4.5 g, 0.013 mol) was dissolved in pyridine-water mix-
ture (each 150 ml) and heated to 50 °C. KMnO4 (5 equivalents;
10.61 g; 0.0671 mol) was added in one portion with vigorous stir-
ring. Then the temperature was elevated to 100 °C and stirred for
2 h. An additional portion of KMnO4 (10.61 g, 0.067 mol) was
added and stirring continued for another 1 h. Excess KMnO4 was
quenched with ethanol; solid filtered and washed with hot water.
Colorless filtrate thus collected was concentrated and acidified
with HCl. White precipitate thus obtained was collected by filtra-
tion, washed with water, and dried in vacuum (yield 75%; m. p.
320 °C) [35].
2.3.3. Synthesis of 1,3-bis[4-(4-hydroxybutyloxy)benzylidene]acetone
(ii)
Yield 77%; m. p. 145–147 °C IR (KBr): 1680 cmÀ1
exocyclic); 1H
(mC@O),
3346 cmÀ1 (spacer-OH) and 1571 cmÀ1
NMR-d: 8.8 (s, 2H, OH), 7.4(s, 2H, CH@), 6.9–7.3(m, 6H, aromatic-
(mC@
C
IR (KBr): 1680 cmÀ1 C@O), and 3449 cmÀ1(spacer-OH) 1H
(m
H), 3.2(s, 4H, bCH2), 1.8 (s, 2H, cCH2), 2.1–2.5(m, 8H, CH2 spacer),
NMR–d: 7.3–8.0 (m, aromatic-H), 11–12 (for COOH).13C NMR
(CDCl3) d: 127–130(aromatic carbons), 156.2 (C @NA).169.4
(COOH).
3.8(t, 8H, AOCH2). 13C NMR (CDCl3) d: 25–28(ArAOACH2ACH2A),
64–67 (ArAOACH2A), 126–129(aromatic carbons).
2.3.4. 1,3-bis[4-(6-hydroxyhexyloxy) benzylidene]acetone (iii)
Yield 79%; m. p. 140–142 °C. IR (KBr): 1684 cmÀ1
(
m
C@O),
mC@C exocyclic); 1H NMR-
d: 9.0(s, 2H, OH), 7.5(s, 2H, CH@), 6.9–7.3(m, 6H, aromatic-H),
2.2.3. 1,3-Bis(4- hydroxybenzylidene) acetone (BHBA)
3376 cmÀ1 (spacer-OH) and 1569 cmÀ1
(
p-Hydroxybenzaldehyde
(12.2 g, 0.1 mol)
and
acetone
(3.7 ml, 0.05 mol) were dissolved in methanol (160 ml), then dry
HCl gas purged and stirred at room temperature until content of
flask turned to dark pink color. Precipitate thus obtained was fil-
tered and dried in vacuum at 30 °C. It was recrystallized from
methanol (yield 90%, m. p. 222–224 °C) [36].
3.0(s, 4H, bCH2), 1.6(s, 2H,
cCH2), 2.2–2.6(m, 16H, CH2 spacer),
3.7(t, 8H, AOCH2). 13C NMR (CDCl3) d: 25–29(ArAOACH2ACH2A),
63–68 (ArAOACH2A), 126–130(aromatic carbons). Elem. Anal.
Calcd. for C29H38O5 (466.2): C, 74.63; H, 8.26. Found: C, 74.42; H,
8.12.
IR (KBr): 1682 cmÀ1 C@O), and 1564 cmÀ1
(m (mC@C exocyclic) for
aromatic OH; 1H NMR–d: 9.2(s, 2H, OH), 7.7 (s, 2H, CH@),
6.8–7.4(m, 6H, aromatic-H), 2.1–2.5(m, 8H, CH2 spacer). 13C NMR
(CDCl3) d: 125–128(aromatic carbons).
2.3.5. 1,3-bis[4-(8-hydroxyoctyloxy) benzylidene]acetone (iv)
Yield 78%; m. p. 135–137 °C. IR (KBr): 1679 cmÀ1
(m
C@O),
C@C exocyclic). 1H
NMR–d: 8.5(s, 2H, OH), 7.8 (s, 2H, CH@), 7.0–7.4(m, 6H, aro-
3396 cmÀ1(spacer-OH) and 1590 cmÀ1
(m
2.3. Synthesis of monomers
matic-H), 2.6(s, 4H, bCH2),1.9(s, 2H,
cCH2), 2.4–2.7(m, 24H, CH2
2.3.1. Synthesis of (4-phenyl)-2, 6-bis(4-chlorocarbonylphenyl)
pyridine(a)
spacer), 3.9(t, 4H, AOCH2). 13C NMR (CDCl3)d: 26–
29(ArAOACH2ACH2A), 69 (ArAOACH2A), 130–137 (aromatic car-
bons). Elem. Anal. Calcd. for C33H46O5 (522.26): C, 75.71; H, 8.68.
Found: C, 75.79; H, 8.93.
PBCPP (4 g; 0.01 mol) was dissolved in dry benzene (25 ml).
Thionyl chloride (2.2 ml; 0.03 mol) was added drop-wise to the
mixture with vigorous stirring. At the end of addition, one drop
of DMF was added and bath temperature slowly brought up to
70 °C. As the reaction proceeds, dispersed diacid was slowly dis-
solved in benzene and then refluxed for 4 h. At the end of reaction,
benzene and excess of thionyl chloride were removed by vacuum
distillation; a yellow colored powder was obtained and used
in situ for subsequent reaction (yield 81%; m. p. 160–162 °C) [37].
2.3.6. 1,3-bis[4-(10-hydroxydecyloxy)benzylidene]acetone (v)
Yield 82%; m. p. 131–133 °C. IR (KBr): 1683 cmÀ1
(m
C@O),
mC@C exocyclic) 1H NMR-
d: 9.0(s, 2H, OH), 7.4 (s, 2H, CH@), 6.8–7.4(m, 6H, aromatic-H),
3395 cmÀ1 (spacer-OH) and 1592 cmÀ1
(
3.0(s, 4H, bCH2), 1.8(s, 2H,
cCH2), 1.8–2.5(m, 32H, CH2 spacer),
IR (KBr): 2920 cmÀ1
(m
CH3) and 1600 cmÀ1 (C@C and C@N). 1H
3.6 (t, 8H, AOCH2). 13C NMR (CDCl3)d: 26–30(ArAOACH2ACH2A),
68 (ArAOACH2A), 129 (aromatic carbons). Elem. Anal. Calcd. for
C37H54O5 (578.76): C, 76.71; H, 9.38. Found: C, 76.83; H, 9.48.
NMR–d: 7.3–8.0 (m, aromatic-H), 2.4(s, 6H, CH3).13C NMR (CDCl3)
d: 25–28(CH3), 127–130(aromatic carbons), 156.2 (C @NA).

G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
221
2.4. Polymerization
using KBr pellets. High-resolution 1H and 13C NMR spectra were re-
corded on a Bruker spectrometer at 500 MHz in CDCl3. Tetrameth-
ylsilane (TMS) was used as an internal standard. TGA thermograms
were recorded on a NETZSCH-Gerâtebau Gmbh thermal analysis
system. DSC measurements were performed at a heating rate of
10 °C/min up to 500 °C on sample taken in an aluminium pan with
a pierced lid, in dry nitrogen atmosphere with an empty pan as ref-
erence. Polarized optical microscopy (POM) study was performed
on a Euromex polarizing microscope equipped with a Linkem HFS
91 heating stage and a TP-93 temperature programmer. Small
quantity of sample was placed between two thin glass cover slips,
heated and cooled at the rate of 5 °C minÀ1 and mesophases ob-
served. Photographs were taken on a Nikon FM10 camera and ex-
posed on Kodak 200 film. Elemental analyses were performed
using Perkin Elmer 2400 elemental analyzer. Photoisomerization
behavior was investigated by observing absorptions between 320
and 420 nm on a Shimadzu UV-160A UV–visible recording spectro-
photometer in thin film as well as chloroform solution (10À2 M).
Typical procedure adopted is as follows: Polymer film was formed
on a quartz plate and subjected to UV irradiation emanating from
a 125 W medium pressure mercury lamp kept at a distance of
10 cm from sample at various intervals of time followed by UV
absorption measured on the spectrophotometer respectively. This
procedure was repeated until reduction in absorption was
completed.
2.4.1 Synthesis of poly{1,3-bis[4-(m-alkyloxy)benzylideneacetone]-(4-
phenyl)-2,6-bis(4-phenyl) pyridine dicarboxylate}s (m = 2, 4, 6, 8 and
10) (I–V)
The polymers were prepared by solution polycondensation
technique at room temperature using triethylamine (TEA) as an
acid acceptor. Typical procedure for synthesis of poly{1,3-bis
[4-(2-ethyloxy)benzylideneacetone]-(4-phenyl)-2,6-bis(4-phenyl)
pyridine dicarboxylate} (I) is as follows: 1,3-bis[4-(2-hydroxyethyl-
oxy)benzylidene)]acetone (i) (1 g; 0.0028 mol) was dissolved in
THF(20 ml). To this TEA (0.39 ml, 0.0028 mol) was added with con-
stant stirring. Then (4-phenyl)-2,6-bis(4-chlorocarbonylphe-
nyl)pyridine (1.217 g, 0.0028 mol) dissolved in THF (20 ml) was
added drop-wise to reaction mixture at ambient temperature and
reaction continued for 24 h. Then, it was refluxed, cooled and
poured into excess of methanol. A yellow colored precipitate thus
formed was purified by re-precipitation using THF-methanol, fil-
tered and dried in vacuum (yield 82%, m. p. 164–166 °C) [39].
2.5. Characterization
The intrinsic viscosity of polymers was determined in an Ubbe-
lohde viscometer using chloroform at 30 °C. Infrared spectra were
obtained on a Thermomattson satellite model FT-IR spectrometer
Scheme 1. Synthesis of monomers and polymers.

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G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
3.2. Mesomorphic studies
3. Results and discussion
3.1. Synthesis and characterization
DSC measurements and POM observations were performed to
confirm the existence of mesomorphic behavior of monomers
and polymers. Table 1 shows variation in transition temperatures
with spacer length (m). Fig. 4 displays representative DSC thermo-
grams of monomers and polymers and indicated three endother-
mic transitions correspond to glass transition temperature (Tg),
crystalline–liquid crystalline (Tm), and liquid crystalline–isotropic
transitions (Ti), respectively. The DSC analysis revealed Tm and Ti
of polymers I–V were decreased with an increase in flexible meth-
Pyridine based diacid chloride monomer was prepared by mod-
ified Chichibabin reaction. 1,3-Bis-(4-hydroxybenzylidene)acetone
was prepared by reacting acetone and p-hydroxybenzaldehyde in
the presence of dry HCl gas. It was subsequently converted to
1,3-bis[4-(m-hydroxyalkyloxy)benzylidene)]acetone (m = 2, 4, 6, 8
and 10) by Williamson aryl–alkyl-ether synthesis. Solution poly-
condensation method was carried out at 60 °C in THF solvent in
the presence of triethylamine as an acid acceptor to produce poly-
esters with good yield (77–84%; Scheme 1). The solubility behavior
of polyesters was examined in different solvents and soluble in
CHCl3, CH2Cl2, DMF and THF and insoluble in methanol, ethanol,
2-propanol, benzene and toluene attributed to flexible methylene
chain and polarity of ester linkage. Intrinsic viscosity of polymers
obtained in chloroform solution as a measure of molecular weight,
values are in the range of 0.73–0.99 dl gÀ1 and increases with
increasing length of aliphatic chains. The solution polycondensa-
tion provided moderate molecular weight polymers.
ylene chain [40]. The transition temperatures (Tm and
DT) of repre-
sentative monomers and polymers are shown in Fig. 5. Tm and Ti of
polymers were in the range of 94–154 °C and 123–166 °C, respec-
tively. The identification of mesophases and transition tempera-
tures were conducted by examining textures of polymer melt
sandwiched between glass substrate in polarized optical micro-
scope. The POM microphotographs of mesomorphic textures of
monomers and polymers are shown in Fig. 6 and data listed in Ta-
ble 1. The pyridine based monomer and v displayed liquid crystal-
line state at 160 °C, and 127 °C, and become isotropic at 165 °C
and133 °C respectively. The optical texture suggests the liquid
crystalline phase is nematic [41,42]. Polymer I exhibited a transi-
tion at 166 °C on DSC heating scan, ascribed to isotropic transition
temperature (Ti). The lower methylene chain containing polymer
did not show mesophase transitions in DSC and thermal decompo-
sition of polymer occurs after melting instead of isotropic transi-
tion temperature attributed to high rigidity of the polymer
system owing to the presence of shorter spacers caused broaden-
ing of molecular geometry and prevented liquid crystallinity.
Polymer II appears as grainy in the temperature range of
163–156 °C during cooling cycle. The observation from POM for
III and IV indicates enantiotropic nature of mesophases, where
polymers showed similar feature and assigned nematic phase of
Schlieren texture, IV at 121 °C and V also exhibits nematic phase
from its isotropic state at 123 °C. Mesophase is stable in a temper-
The polymers were characterized by FT-IR, 1H and 13C NMR
spectroscopy. The spectral values are in accordance with assigned
structures. The FT-IR spectrum of IV is shown in Fig. 1. The stretch-
ing vibration at 1599 cmÀ1 is due to CH@CH and C@O stretching
vibration of ester appeared around 1719 cmÀ1. Aryl alkyl ethers
give rise to strong bands an asymmetric CAOAC stretching near
1257 cmÀ1 and a symmetric stretching near 1043 cmÀ1. The 1H
NMR spectrum of III in CDCl3 is illustrated in Fig. 2. The benzyli-
dene aromatic protons are resonated around 6.79–7.32 ppm, alkyl
group connected with ether linkage resonated at 3.85 ppm and ole-
finic protons appeared as a singlet around 7.02 ppm. The 13C NMR
spectrum of IV in CDCl3 is shown in Fig. 3, the C@O signal of ester
group resonated at 165.94 ppm, together with carbon signals for
aromatic rings resonated around 128.76–130.62 ppm. The carbon
in aliphatic chains (CH2)mÀ2 resonated around 22–25 ppm and
OCH2A next to ester group appeared at 68.91 ppm. The other poly-
mers displayed similar 13C NMR spectra.
ature range (
DT) over 30 °C and finally solidifies at 90C. Conse-
quently Ti and Tm were decreased monotonically at the length of
Fig. 1. The FT-IR spectrum of polymer IV, (a) before and (b) after irradiation.

G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
223
Fig. 2. 1H NMR spectrum of polymer III.
Fig. 3. 13C NMR spectrum of polymer IV.

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G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
Table 1
DSC and POM data of polymer I–V.
Monomer/
polymer
DSC (°C)
POM (°C)
Tm Ti
Tg
Tm
Ti
D
T
D
T
Mesophase
a
v
I
II
III
IV
160 165
127 133
5
6
–
6
161 167
126 133
6
7
–
7
Nematic
Nematic
No mesophase
Grainy
–
–
-
–
166
–
–
154 160
156 163
71 143 158 15 144 160 16 Nematic
66 104 129 25 107 131 24 Schlieren
Nematic
V
-
94
123 29 95
125 30 Smectic
increased spacers. This is ascribed to decrease in polarity and rigid-
ity of the system as increased with spacer lengths.
Except I, the remaining polymers showed enantiotropic nematic
phases. The ester linkages bridging the spacer and rod-like core
had an essential effect on stability of mesophases. It is well known
that characteristic liquid crystal behavior has an essential relation-
ship with molecular interaction between mesogenic moieties of
main chain liquid crystalline polymers.
Fig. 5. Transition temperatures of representative monomers and polymers.
is attributed to the presence of ether linkage in the spacers were
expected to introduce significant flexibility and consequently
brought down thermal stability. Degradation occurred in two-step
manner; first step between 435 and 460 °C corresponds to cleavage
of ester linkage and second step between 580 and 640 °C may be
the outcome of cleavage of aryl–alkyl–ether linkage present in
the polymer chain. Char yield was calculated by measuring the
amount of residual substance at 800 °C. The data revealed that char
yield increased with decreasing spacer lengths as follows:
Dimethylene > tetramethylene > hexamethylene > octamethy-
lene > decamethylene.
3.3. Thermal property
The thermal behavior of polymers was evaluated by TGA in
nitrogen atmosphere at a heating rate of 10 °C/min, their traces
shown in Fig. 7, and data presented in Table 2. Thermal stability
was evaluated by 5% and 50% weight loss at minimum tempera-
ture. The results disclosed that they are stable up to 326 °C and
start degrading thereafter in nitrogen. The thermal stability in-
creases with decreasing spacer lengths [I > II > III > IV > V] [43]. It
Fig. 4. Representative DSC thermograms of monomers and polymers.

G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
225
Fig. 6. Representative HOPM photographs of monomers and polymers at 20Â magnification.
were monitored using UV spectrophotometer. Fig. 8 shows the
change of UV–visible spectra of III both in chloroform solution
and in thin film as a typical example of photochemical behavior
upon irradiation at k > 300 nm. As shown in Fig. 8, during irradia-
tion, a continuous decrease in absorption at 367 nm, ascribable
*
to
p–p transition of bisbenzylidene chromophore and increase
of absorption band at 285 nm were noted with an isobestic point
at 302 nm. The isobestic point indicates photoisomerization pro-
cess takes place upon irradiation (Fig. 9). The similar trend was de-
Table 2
Thermal decomposition data of polymer I–V.
Polymer
TGA
5%
Fig. 7. TGA thermograms of polymers I–V.
50%
% Char yield at 800 °C
I
403
380
363
340
326
534
503
477
473
469
19.45
18.14
14.53
12.09
11.33
3.4. Optical property
II
III
IV
V
The optical property was studied in chloroform solution and in
thin film; by irradiating under UV light, the structural changes

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G. Deepa et al. / Journal of Molecular Structure 963 (2010) 219–227
tected for remaining polymers. It was observed in the initial 15 s
irradiation that an increase in intensity as well as small blue shift
at 367 nm for poly[bis(benzylidene)]acetone esters. This increase
in intensity reverses on further irradiation and decreases regularly
thereafter may be assigned to disorganization caused by randomly
oriented bisbenzylidene-acetone chromophore as polymer changes
to isotropic state. Such disordered structure could be introduced by
trans-to-cis photoisomerization of bisbenzylidene chromophore on
irradiation. This kind of isomerization is reported in similar low
molecular weight compounds [44] which disrupt parallel stacking
of chromophore in aggregate attributed to nonlinear conformation
of cis-isomer (Fig. 9). Similar results were observed and reported by
Gangadhara and Kishore [45].
To support this photoisomerization phenomenon, polymer
samples were subjected to irradiation under UV light for long
time and then examined by solubility test, FT-IR, DSC and
POM. The photo-irradiated polymers were found to be soluble
in organic solvents and confirmed photoisomerization process.
The peak at 1719 cmÀ1 assigned to C@O carbonyl group conju-
gated with peak at 1635 cmÀ1 assigned to C@C double bond of
chalcone moiety was unaltered to higher wave number
(Fig. 1b), in FT-IR spectroscopy, indicates the unaffected nature
of conjugated system in chalcone.
The liquid crystalline texture unchanged for III, when irradi-
ated for more than 4 h and testify photoisomerization process.
There is an identical mesophase region between Tm and Ti no-
ticed before and after irradiated samples in DSC analysis. During
initial stages of photo-irradiation of nematic LC sample contain-
ing trans-form, dark spots were unobserved in the POM. It sug-
gests the isotropic domains were unorganized at irradiated
sites where sufficient amount of cis-form produced upon photo-
irradiation and nematic-isotropic phase transition did not take
place in local domain. This is attributed to trans-and-cis isomers
show a rod-like molecular structure, and trans–cis isomerization
produces little change in molecular shape [46]. All these
evidences clearly confirmed the cis–trans-photoisomerization
phenomenon prevailed in the molecular system as that of azo-
benzene polymers.
Fig. 8. Changes in UV spectral characteristics during photolysis of polymer III in
both solution and thin film at various time intervals.
Fig. 9. Photochemical reactions of bis(benzylidene)acetone.

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227
4. Conclusion
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New class of thermotropic main chain liquid crystalline polyes-
ters containing bisbenzylidene and pyridine units has been synthe-
sized, where the effect of mesogenic chain extender, with various
spacer lengths, their properties investigated and reported. The
presence of heterocyclic ring and bulky pendant group in polyester
backbone are resistant to high temperature and modify the proper-
ties of polymers. The intrinsic viscosity data disclosed the polymers
are of moderate molecular weights. The bisbenzylidene containing
photoreactive functionality undergo photoisomerization under the
influence of UV irradiation and further supported by FT-IR, POM
and DSC analysis. The performance of irradiation sensitive poly-
mers depends not only on inherent photo reactivity of their func-
tional groups but also on spatial arrangements. The thermal
stability displayed good thermal stability characteristics. Liquid
crystalline property of monomers and polymers demonstrated
development of mesophase depends on length of flexible spacers.
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The author R.B. gratefully acknowledges the Council of Scien-
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of Senior Research Fellowship.
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