New diphenylamine-based donor-acceptor-type conjugated polymers as potential photonic materials

Article abstract of DOI:10.1016/j.reactfunctpolym.2011.08.011

A new series of donor-acceptor-type conjugated polymers (P1 and P2) carrying a diphenyl amine moiety were synthesized via Wittig condensation technique. The polymers structures were well established by FT-IR, ¹H NMR, elemental analysis and gel

Full text of DOI:10.1016/j.reactfunctpolym.2011.08.011

Reactive & Functional Polymers 71 (2011) 1119–1128
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
New diphenylamine-based donor–acceptor-type conjugated polymers
as potential photonic materials
K.A. Vishnumurthy a, M.S. Sunitha a, Reji Philip b, Airody Vasudeva Adhikari a,
⇑
a Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore 575 025, India
b Light and Matter Physics Group, Raman Research Institute, C.V. Raman Avenue, Sadashiva Nagar, Bangalore 560 080, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2011
Received in revised form 29 August 2011
Accepted 30 August 2011
Available online 10 September 2011
A new series of donor–acceptor-type conjugated polymers (P1 and P2) carrying a diphenyl amine moiety
were synthesized via Wittig condensation technique. The polymers structures were well established by
FT-IR, 1H NMR, elemental analysis and gel permeation chromatographic techniques. They exhibited good
thermal stability with an onset decomposition temperature of approximately 325 °C under nitrogen
atmosphere. The optical and electrochemical properties of the polymers were studied by UV–vis, fluores-
cence emission spectroscopy and cyclic voltammetry. They exhibited good fluorescence in dilute solu-
tions and showed solvatochromic behavior in various polar solvents. The electrochemical studies
revealed that the polymers possess low-lying LUMO energy levels that ranging from ꢀ3.47 to ꢀ3.73 eV
and high-lying HOMO energy levels that ranging from ꢀ5.57 to ꢀ5.81 eV. The thirdorder nonlinear opti-
cal properties of the polymers were investigated using the Z-scan technique. The effective two-photon
absorption (TPA) coefficients (b) of the polymers were found to be 0.645 ꢁ 10ꢀ10 and 0.212 ꢁ 10ꢀ10 m/W.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Thiophene
Cyanopyridine
Diphenylamine
Fluorescence
1. Introduction
tron-donor units along the main polymer chain would significantly
increase the NLO properties, mainly due to an enhancement of the
In recent years, a great deal of interest has been focused on the
synthesis of novel -conjugated polymers because of their intrigu-
hyperpolarizability. The desired optical properties can be achieved
when polymer backbones are tailored with different heterocyclic
systems that allow the fine-tuning of important physical and/or
photophysical properties.
p
ing properties, such as electrical conductivity [1], electrolumines-
cence [2], photovoltaic [3] and chemical-sensing [4] properties.
Additionally, conjugated polymers have been extensively studied
for nonlinear optical (NLO) applications [5–7], such as electro-opti-
cal (EO) modulation, optical switches, optical power limiting and
frequency doubling, because of their large optical nonlinearity, fast
response time, and easy processability for integrated assembly [8–
10].
Many conjugated polymeric systems that carry aromatic het-
erocyclic rings are known to exhibit an increased hyperpolarizabil-
ity compared to those that carry benzenoid systems [11–13]. This
increase in the hyperpolarizability occurs because the delocaliza-
tion energy of hetero-aromatics is lower than that of benzenoid
systems. Active chromophores that contain aromatic heterocycles
such as thiophene [14–16], thiazole [17,18] benzothiazole [18,19]
or their derivatives are among the most studied systems. Recently,
a donor–acceptor (D–A) approach has been adopted to tune the
nonlinear properties of conjugated polymers [20]. According to this
concept, the incorporation of alternate electron-acceptor and elec-
An extensive literature survey reveals that the incorporation of a
3,4-dialkoxythiophene moiety along the backbone of a polymer en-
hances the polymer’s dopability and decreases its band gap. Fur-
thermore, the incorporation of a highly electron-withdrawing
oxadiazole ring along the conjugation path increases the charge-
carrying properties of the polymer [21,22], which may alter its
NLO properties. In addition, the introduction of vinylene linkages
increases the planarity of the polymer chain by reducing the tor-
sional interactions between the hetero-aromatic rings, which leads
to a decrease in the band gap [21]. A pyridine ring is also a highly
electron-withdrawing moiety that exhibits good electron-trans-
porting abilities and optical properties when it is introduced into
the polymer main chain. The presence of a nitrile (CN) substituent
on the pyridine ring further enhances the charge-carrying proper-
ties of the resulting polymer [23]. Because of the fluorescent nature
of cyanopyridine, its presence in a polymer chain would improve
the optical properties. Conjugated polymers carrying cyanopyridine
are also known to exhibit good optical-limiting properties [24].
In general, the NLO properties of conjugated polymers can be
further enhanced either by doping the polymer with a NLO chro-
mophore (guest–host) or covalently incorporating an NLO moiety
⇑
Corresponding author. Tel.: +91 8242474046; fax: +91 8242474033.
E-mail addresses: avadhikari123@yahoo.co.in, avchem@nitk.ac.in (A.V. Adhi-
kari).
1381-5148/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2011.08.011

1120
K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
onto the polymer backbone [22]. The guest–host method has some
fatal disadvantages, such as the low solubility of the chromophore
in the polymer host, the fast decay of NLO activity due to orienta-
tional relaxation and sublimation of the chromophore. However,
the method of covalent incorporation of an NLO chromophore
leads to an enhancement of the chromophore density without
phase separation and is therefore advantageous over the guest–
host method. Among the various classes of NLO chromophores
reported in the literature, triphenylamine and diphenylamine
derivatives are good candidates because of their multifunctional
properties, such as good solubility, two-photon absorption, and
hole-conducting properties [23,24]. Therefore, the diphenylamine
moiety can be conveniently connected with a strong acceptor sys-
tem through ethylene-conjugated bridges to produce large first-
molecular hyperpolarizability in the resulting molecules.
Against this background, we have designed two new D–A type
conjugated polymers, P1 and P2, by introducing the highly elec-
tron-donating diphenylamine moiety into the polymer network.
In the synthetic design of the new polymer P1, alkoxythiophene
and diphenylamine moieties were connected as electron donors,
and an oxadiazole ring was connected as an electron acceptor. In
polymer P2, a diphenylamine group was attached as the electron
donor, and a cyanopyridine ring was used as the electron acceptor.
In addition, phenylene vinylene units were incorporated as conju-
gated spacers in both P1 and P2 to enhance the conjugation path
length, which in turn enhances their optical properties. In the pres-
ent work, the required monomers have been prepared from simple
thiodiglycolic acid through multistep reactions. The structures of
the new P1 and P2 polymers have been established by spectral
techniques, and their electrochemical, linear and nonlinear optical
properties have been evaluated to investigate the influence of their
structure on the properties.
with a gel permeation chromatograph (GPC) against polystyrene
standards with THF as the eluent.
2.3. Synthetic plan
Schemes 1 and 2 show the synthetic routes for the preparation
of new monomers and their polymerization to the target polymers.
In Scheme 1, compound 1 was alkylated using tetradecyl bromide
in the presence of sodium hydride to produce alkylated diphenyl-
amine 2, which was later converted into the corresponding dialde-
hyde 3 via the Vilsmeyer–Haak reaction. In Scheme 2, the required
chalcone 4 was prepared from tolualdehyde and 4-methylaceto-
phenone using the Claisen–Schmidt reaction. The product was
then cyclized to cyanopyridine 5 by reaction with malononitrile
in the presence of sodium methoxide. Further, two methyl groups
of compound 5 were brominated via the Wholzigler method using
NBS and BPO. The resulting dibromo derivative 6 was conveniently
converted to phosphonium Wittig salt 7, which, upon treatment
with dicarboxaldehyde 3 in ethanol–chloroform medium, yielded
polymer P2 in good yield.
As shown in Scheme 2, the diesters of 3,4-ditetradecyloxy
thiophene were readily converted to 3,4-ditetradecyloxythioph-
ene-2,5-carboxydihydrazides 9 by the action of hydrazine hy-
drate in alcoholic medium. This dihydrazide was tolylated to
yield
3,4-bis(tetradecyloxy)-N02,N05-bis(4-methylbenzoyl)thio-
phene-2,5-dicarbohydrazide 10, which, upon treatment with
phosphorus oxychloride gave 5,50-(3,4-bis(tetradecyloxy) thio-
phene-2,5-diyl)bis(2-p-tolyl-1,3,4-oxadiazole) 11 [25] in good
yield. This bisoxadiazole compound 11 was then Wohl–Ziegler
brominated using N-bromo succinimide (NBS) in carbon tetra-
chloride, and the resulting 5,50-(3,4-bis(tetradecyloxy)thio-
phene-2,5-diyl)bis(2-(4-(bromomethyl)phenyl)-1,3,4-oxadiazole)
12 was further converted to 5,50-(3,4-bis(tetradecyloxy)thio-
phene-2,5-diyl)bis(2-(4-triphenylphosphonionmethyl)phenyl)-1,3
,4 oxadiazole) 13 upon treatment with triphenylphosphine in
the presence of DMF. The reaction of compound 13 with diphe-
2. Experimental
2.1. Materials and methods
nylaminedicarboxaldehyde
3 in ethanol–chloroform medium
yielded polymer P1.
3,4-Ditetradecyloxythiophene-2,5-dicarboxylate (structure 8)
was synthesized according to the reported procedure [21]. All
chemicals used in the present work were procured from Sigma–Al-
drich and Lancaster (UK). All solvents were of analytical grade;
they were purchased and used without further purification.
2.4. Syntheses of intermediates, monomers and polymers
2.4.1. Synthesis of N-tetradecyl diphenylamine 2
Sodium hydride (0.7185 g, 29.5 mmol) was added to the solu-
tion of diphenylamine 1 (5 g, 29.5 mmol dissolved in 50 ml of
DMF) and the resulting mixture was stirred for approximately
30 min. 1-Bromo tetradecane (8.29 g, 29.5 mmol) was then slowly
added to the reaction mixture, and the mixture was stirred for 5 h
at room temperature. After completion of the reaction, the resulting
mixture was extracted with ethyl acetate/brine and then dried with
Na2SO4. The solvent was removed by evaporation. The resulting
crude product was purified by column chromatography using hex-
ane and ethyl acetate. Yield: 8.8 g (82%). 1H NMR (400 MHz, CDCl3),
d (ppm): 7.36–7.04 (m, 10H, aromatic protons), 3.83 (t, 2H, ANCH2),
1.74 (m, 2H, ANCH2CH2A), 1.34–1.24 (m, 24H, ACH2CH2), 0.87 (t,
3H, CH3). FTIR (cmꢀ1): 2920, 2852, 1590, 1494, 1309, 891, 743. Anal.
Calcd for C26H39N: C, 85.42; H, 10.75; N, 3.83. Found: C, 85.45; H,
10.78; N, 3.84.
2.2. Instrumentation
Infrared spectra of all intermediate compounds and polymers
were recorded on a Nicolet Avatar 5700 FTIR (Thermo). The UV–
visible and fluorescence spectra were recorded on a GBC Cintra
101 and a JASCO FP-6200 spectrofluorometer, respectively. 1H
NMR spectra were obtained on a Bruker 400 MHz FT-NMR spec-
trometer using the TMS/solvent signal as an internal reference. Ele-
mental analyses were performed on a Flash EA1112 CHNS analyzer
(Thermo Electron Corporation). Mass spectra were recorded on a
Jeol SX-102 (FAB) mass spectrometer. Electrochemical studies
were performed using an AUTOLAB PGSTAT30 electrochemical
analyzer. Molecular weights of the polymers were determined
Scheme 1. Synthesis of N-substituted diphenylamine dialdehyde.

K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
1121
Scheme 2. Synthesis scheme for monomers and polymers.
2.4.2. Synthesis of N-tetradecyl-diphenylamine-4,40-dicarbaldehyde 3
Freshly distilled phosphorous oxychloride (12.6 ml, 82.6 mmol)
was added drop-wise to 5.5 ml of anhydrous DMF at 0 °C over a
period of 30 min. Later, alkylated diphenylamine 2 (5 g, 13.6 mmol
in 20 ml of 1,2-dichloroethane) was added to the above solution
and stirred at 90 °C for 48 h. This solution was cooled to room
temperature, poured into ice water, and neutralized to pH 6–7 by
the drop-wise addition of saturated sodium hydroxide solution.
The dialdehyde 3 was extracted with ethyl acetate. The organic
layer was dried with anhydrous Na2SO4, and the solvent was sub-
sequently removed under reduced pressure. The crude product
was purified by column chromatography. A light-orange solid

1122
K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
was obtained. Yield: 4.1 g (71%). 1H NMR (400 MHz, CDCl3), d
(ppm): 9.88 (s, 2H, aldehydic), 7.81 (d, 4H, aromatic), 7.14 (d, 4H,
aromatic protons), 3.83 (t, 2H, NCH2), 1.74 (m, 2H, ANCH2CH2A),
1.34–1.24 (m, 24H, ACH2CH2), 0.87 (t, 3H, CH3). FTIR (cmꢀ1):
2915, 2848, 1685, 1578, 1502, 1359, 1157, 820, 719. Element. Anal.
Calcd. for C28H39NO2: C, 79.76; H, 9.32; N, 3.32. Found: C, 76.79; H,
9.28; N, 3.35 (Scheme 1).
2.4.7. Synthesis of 3,4-ditetradecyloxythiophene-2,5-
carboxydihydrazide 9
Diethyl-3,4-dialkoxythiophene-2,5-dicarboxylate (0.5 g) was
added to a solution of 5 ml of hydrazine monohydrate in 40 ml of
ethanol. The reaction mixture was refluxed for 3 h. After the solu-
tion was cooled to room temperature, a white precipitate was ob-
tained. The precipitate was filtered, washed with petroleum ether,
dried under vacuum and finally recrystallized from ethanol to pro-
duce a white crystalline solid. Yield: 92%. FTIR (cmꢀ1): 3412
(ANH2), 3341 (ANHA), 2915, 2848, 1650 (>C@O), 1501, 1302,
1043, 956, 720.
2.4.3. Synthesis of 1,3-bis(4-methylphenyl)prop-2-en-1-one 4
A mixture of p-tolualdehyde (5 g, 41.6 mmol) and 4-methylace-
tophenone (5.5 g, 41.6 mmol) was dissolved in 50 ml of ethanol
and stirred in the presence of potassium hydroxide solution
(2.3 g in 5 ml water) at room temperature. After 10 h, the obtained
solid was filtered; it was recrystallized from a chloroform–metha-
nol system to produce a yellow needle-shaped solid. Yield: 8.9 g
(90%). FTIR (cmꢀ1): 1644, 1591, 1169, 987, 805, 726. Element. Anal.
Calcd. for C17H16O: C, 86.40; H, 6.82. Found: C, 86.41; H, 6.84.
2.4.8. Synthesis of N2,N5-di-(4-methylbenzoyl)-3,4-ditetradecyloxy
thiophene-2,5-dicarbo hydrazide 10
To a clear mixture of dihydrazide 9 (5 g, 8.79 mmol) and 2 ml
of pyridine in 50 ml of NMP, two equivalents (2.71 g, 17.58 mmol)
of 4-methylbenzoyl chloride was added slowly at room tempera-
ture while stirring. The stirring was continued at room tempera-
ture for 1 h. The resulting solution was stirred at 80 °C for 5 h.
After cooling to room temperature, the reaction mixture was
poured into an excess of water to produce a precipitate. The pre-
cipitate was collected by filtration, washed with an excess of
water, dried in oven and recrystallized from an ethanol/chloro-
form mixture to produce the desired product. Yield: 82%. 1H
NMR (400 MHz, CDCl3), d (ppm): 10.22 (s, 2H, ANHA), 9.71 (s,
2H, ANHA), 7.75 (d, 4H, Ar, J = 8.4 Hz), 7.15 (d, 4H, Ar,
J = 8.0 Hz), 4.24 (t, 4H, AOCH2A, J = 7.0 Hz), 2.38 (s, 6H, ArACH3),
1.12–1.84 (m, 40H, A(CH2)12A) 0.80 (t, 6H, ACH2ACH3, J = 7.4 Hz).
FTIR (cmꢀ1): 3245 (ACOANHA), 2917, 2849 (aromatic), 1666,
1621, 1447, 1274, 1046, 835, 732. Element. Anal. Calcd. for
2.4.4. Synthesis of 2-methoxy-4,6-bis(4-methylphenyl)pyridine-3-
carbonitrile 5
Compound 4 (5 g, 21.1 mmol) was added slowly to a freshly
prepared sodium alkoxide solution (223.3 mmol of sodium in
100 ml of methanol) with stirring. Malononitrile (1.39 g,
21.1 mmol) was then added with continuous stirring at room tem-
perature until the reaction was complete. The separated solid was
collected by filtration and recrystallized from hot ethanol and chlo-
roform. Yield: 4.5 g (67%). M.p.: 142–144 °C. 1H NMR (400 MHz,
CDCl3) d (ppm): 8.00 (m, 2H, ArAH), 7.55 (m, 2H, ArAH), 7.44 (s,
1H, ArAH (pyridine)), 7.34–7.28 (m, 4H, ArAH), 4.19 (s, 3H,
AOACH3), 2.43 (s, 6H, ArACH3). FTIR (cmꢀ1): 2907, 2217, 1658,
1554, 1366, 1139, 818. Element. Anal. Calcd. for C21H18N2O: C,
80.23; H, 5.77; N, 8.91. Found: C, 80.25; H, 5.75; N, 8.93.
C50H76N4O6S: C, 69.73; H, 8.89; N, 6.51; S, 3.72. Found: C,
69.72; H, 8.84; N, 6.53; S, 3.71.
2.4.9. Synthesis of 2,20-(3,4-ditetradecyloxythiophene-2,5-diyl)bis[5-
(4-methylphenyl)-1,3,4-oxadiazole] 11
2.4.5. Synthesis of 4,6-bis[4-(bromomethyl)phenyl]-2-methoxypyri
dine-3-carbonitrile 6
A mixture of compound 10 (5 g, 6.2 mmol) and 50 ml of phos-
phorus oxychloride was stirred at 80 °C for 6 h. The reaction mix-
ture was then cooled to room temperature and poured into an
excess of ice-cold water. The resulting precipitate was collected
by filtration, washed with water and dried in an oven. Further puri-
fication was performed by recrystallization of the obtained solid
from an ethanol/chloroform mixture. Yield: 85%. M.p.: 98–100 °C.
1H NMR (400 MHz, CDCl3) d (ppm): 7.94 (d, 4H, Ar, J = 8.4 Hz),
7.29 (d, 4H, Ar, J = 8.0 Hz), 4.25 (t, 4H, AOACH2A, J = 6.6 Hz), 2.38
(s, 6H, ArACH3), 1.13–1.83 (m, 48H, A(CH2)12A), 0.82 (t, 6H,
ACH2ACH3, J = 7.0 Hz). FTIR (cmꢀ1): 2915, 2848, 1587 (AC@NA),
1556, 1481, 14650, 1279, 1054, 823, 726. Element. Anal. Calcd. for
A mixture of compound 5 (3 g, 9.5 mmol), N-bromosuccinimide
(1.38 g, 19.1 mmol), and 5 mg of benzoyl peroxide in 30 ml of car-
bon tetrachloride was refluxed for 8 h. After the solvent was re-
moved, 20 ml of water was added with stirring for 1 h. The
resulting crude product was recrystallized from an ethyl acetate/
chloroform mixture to produce pure white-colored solid. Yield:
3.5 g (77%). M.p.: 202–205 °C. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.00 (m, 2H, ArAH), 7.99–7.55 (m, 4H, ArAH), 7.5–7.3 (m,
2H, ArAH), 7.29 (s, 1H, ArAH (pyridine)), 4.81 (s, 6H, ArACH3),
4.13 (3H, AOACH3). FTIR (cmꢀ1): 2993, 2219, 1580, 1546, 1359,
1139, 1007, 821, 600. Element. Anal. Calcd. for C21H16Br2N2O: C,
53.42; H, 3.42; N, 5.93. Found: C, 53.46; H, 3.45; N, 5.94.
C50H72N4O4S: C, 72.77; H, 8.79; N, 6.79; S, 3.89. Found: C, 72.72;
H, 8.72; N, 6.81; S, 3.9.
2.4.6. Synthesis of [4,6-bis[44-triphenyl phosphonion methyl) phenyl]-
2-methoxy, 3-cyano pyridine]dibromide 7
2.4.10. Synthesis of 2,20-(3,4-ditetradecyloxythiophene-2,5-diyl)bis[5-
(4-bromomethylphenyl)-1,3,4-oxadiazole] 12
A solution of dibromide compound 6 (1 g, 6.3 mmol) and tri-
phenyl phosphine (3.34 g, 12.7 mmol) in 5 ml of DMF was refluxed
with stirring for 8 h. The reaction mixture was cooled to room tem-
perature and poured into 50 ml of ethyl acetate. The resulting mix-
ture was sonicated for approximately 30 min to induce
precipitation. The obtained white-colored amorphous solid was fil-
tered off, washed with an excess of ethyl acetate and dried at 40 °C
for 10 h. Yield: 82%. M.p.: above 300 °C. 1H NMR (400 MHz, CDCl3)
d (ppm): 8.14–8.12 (m, 2H, ArAH), 7.99–7.50 (m, 33H, ArAH),
7.16–7.12 (m, 4H, ArAH), 5.26 (s, 4H, ArACH2), 4.13 (s, 3H,
AOACH3). FTIR (cmꢀ1): 3365, 3051, 2853, 2211, 1658, 1430,
1103, 728, 682, 495. Element. Anal. Calcd. for C57H46Br2N2OP2: C,
68.68; H, 4.65; N, 2.81. Found: C, 68.66; H, 4.68; N, 2.84.
A mixture of compound 12 (3 g, 3.9 mmol), N-bromosuccini-
mide (1.38 g, 7.8 mmol) and 5 mg of benzoyl peroxide in 30 ml of
benzene was refluxed for 5 h. After the solvent was removed,
20 ml of water was added with stirring for 1 h. The resulting crude
product was recrystallized from a methyl acetate/chloroform mix-
ture. Yield: 65%. 1H NMR (400 MHz, CDCl3) d (ppm): 7.92 (d, 4H, Ar,
J = 8.4 Hz), 7.27 (d, 4H, Ar, J = 8.0 Hz), 4.5 (s, 4H, ArACH2ABr), 4.24
(t, 4H, AOACH2A, J = 6.6 Hz), 1.13–1.83 (m, 40H, A(CH2)12A), 0.80
(t, 6H, ACH2ACH3, J = 7.0 Hz). FTIR (cmꢀ1): 2915, 2849, 1557
(AC@NA), 1482, 1465, 1279, 1054, 952, 822, 726. Element. Anal.
Calcd. for C50H70Br2N4O4S: C, 61.09; H, 7.18; N, 5.70; S, 3.26.
Found: C, 61.05; H, 7.15; N, 5.71; S, 3.28.

K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
1123
2.4.11. Synthesis of 2,20-(3,4-ditetradecyloxythiophene-2,5-diyl)bis[5-
((4-triphenyl phosphonium methyl)phenyl)-1,3,4-oxadiazole]
dibromide 13
those in compound 5. The structure of monomer 7 was also evi-
denced by its 1H NMR data; the methyl protons of its phosphonium
salt resonated at d 5.26 and were substantially deshielded com-
pared to those of compound 6.
A solution of dibromide compound 12b (1 g, 1.07 mmol) and
triphenyl phosphine (0.565 g, 2.157 mmol) in 5 ml of DMF was re-
fluxed with stirring for 10 h. The reaction mixture was cooled to
room temperature and poured into 50 ml of ethyl acetate. The
resulting precipitate was collected by filtration, washed with an
excess of ethyl acetate and dried at 40 °C for 10 h. (13b) Yield:
72%. M.p.: above 300 °C. 1H NMR (400 MHz, CDCl3), d (ppm):
7.21–7.90 (m, 38H, ArAH), 5.95 (s, 4H, ArACH2APA), 4.25 (t, 4H,
AOCH2A, J = 6.9 Hz), 1.12–1.83 (m, 48H, A(CH2)12A), 0.79 (t, 6H,
ACH2ACH3, J = 7.0 Hz). FTIR (cmꢀ1): 2922, 2855, 1583, 1459,
1352, 1048, 956, 727. Element. Anal. Calcd. for C86H100Br2N4O4P2S:
C, 68.52; H, 6.69; N, 3.72; S, 2.13. Found: C, 68.58; H, 6.66; N, 3.38;
S, 2.23.
Similarly, the structure of 3,4-ditetradecyloxythiophene-2,5-
carboxydihydrazide (9) was established by its FTIR and 1H NMR
spectra. Its FTIR spectrum showed sharp peaks at 3412 and
1650 cmꢀ1, which indicate the presence of ANH2 and >C@O
groups, respectively. Its 1H NMR spectrum displayed sharp singlets
at d 8.3 and d 4.9 for the ANHA and ANH2 protons, respectively.
The conversion of bishydrazide 9 to biscarbohydrazide 10 was con-
firmed by the FTIR and elemental analysis studies. The biscarbo-
hydrazide exhibited sharp peaks at 3245 and 1666 cmꢀ1, which
indicate the presence of ANHA and >C@O groups, respectively.
Furthermore, its 1H NMR spectrum showed peaks at d 2.38, d
10.22 and d 9.71, which indicate the presence of the tolyl methyl
protons and amidic protons, respectively. The formation of com-
pound 10 from 11 was established by FTIR, 1H NMR and mass spec-
tral analyses. The disappearance of bands in the regions at 3245
and 1666 cmꢀ1 and the appearance of a new band at 1587 cmꢀ1
indicate the formation of an oxadiazole ring in the molecule. The
formation of the ring was further confirmed by the 1H NMR spec-
trum, which showed no characteristic peaks due to amidic protons
and deshielding of the aromatic protons. The conversion of com-
pound 11 to compound 12 was confirmed by 1H NMR studies
and elemental analysis. In the 1H NMR spectrum of 12, the disap-
pearance of the tolyl methyl protons and the appearance of a
new peak at d 4.5 confirms the bromination of the methyl groups
attached to the aromatic rings. The formation of compound 13
was further confirmed by 1H NMR studies and elemental analysis.
The 1H NMR spectrum showed a peak at d 5.95, which indicates the
presence of a ACH2A group attached to the aromatic ring and a
phosphonium group on either side. The structures of both poly-
mers were established by 1H NMR, FTIR and gel permeation chro-
matographic techniques.
2.4.12. General procedure for the synthesis of polymers P1 and P2
Monomer
3 (0.5 g, 0.11 mmol) and monomer 13 (1.7 g,
0.11 mmol) were dissolved in a mixture of 5 ml of chloroform
and 15 ml of ethanol. Sodium ethoxide (0.020 g, 1 mmol in 10 ml
of ethanol) was added to the reaction mass at room temperature
under a nitrogen atmosphere. Then the reaction mixture turned
yellow in color. It was stirred for 12 h at room temperature. After
the completion of the reaction, solvent was removed under re-
duced pressure. The crude reaction mass was then poured into
an excess of methanol and stirred for approximately 30 min. The
obtained polymer was filtered and then washed thoroughly with
acetone, re-dissolved in chloroform and poured into excess of
methanol to remove the oligomers. The resulting precipitate was
collected by filtration and dried under vacuum for 24 h to give a
greenish-yellow-colored powder. Polymer P2 was prepared by a
similar method (Scheme 2). The characterization data of P1and
P2 are given below.
P1: 1H NMR, (400 MHz, CDCl3), d (ppm): 8.1–7.1 (m, 12H,
ArAH), 7.1–6.9 (m, 2H, ACH@CHA), 4.31 (m, 4H, AOACH2A),
3.73–3.71 (m, 2H, ANCH2A), 2.2–1.1 (m, 72H, aliphatic), 0.87 (t,
9H, ACH2CH3). FTIR (cmꢀ1): 2919, 2850, 1585, 1499, 1366, 1175,
827, 726. Weight-average molecular weight (Mw): 12,900.
P2: 1H NMR, (400 MHz, CDCl3), d (ppm): 8.2–7.0 (m, 16H,
ArAH), 7.0–6.8 (m, 2H, ACH@CHA), 4.20 (m, 4H, AOACH3), 3.72–
3.69 (m, 2H, ANCH2A), 2.2–1.1 (m, 24H, aliphatic), 0.87 (t, 3H,
ACH2CH3). FTIR (cmꢀ1): 2919, 2849, 2217, 1579, 1503, 1356,
1237, 1009, 818. Weight-average molecular weight (Mw): 8400.
The weight-average molecular weights of the polymers P1 and
P2 were 12,900 and 8400, respectively. The polydispersity of these
polymers were 2.2 and 1.90, respectively. The thermal stability of
these polymers was studied by thermogravimetric analysis (TGA)
in a nitrogen atmosphere using a 3 °C minꢀ1 temperature ramp
from 50 to 700 °C. The thermogram (Fig. 1) showed that the poly-
mers exhibit a high thermal stability. The loss of less than 6–8%
weight at temperatures less than 300 °C may be attributed to the
degradation of the alkyl and alkoxy side chains in the polymers.
The gradual weight loss above 300 °C may be attributed to the deg-
radation of the polymer backbone, which led to the residue. Similar
3. Results and discussion
3.1. Characterization
The structures of the intermediates and the target polymers
were established by elemental analysis and spectroscopic tech-
niques. The structure of the dicarboxaldehyde monomer 3 was
confirmed by 1H NMR and FTIR spectroscopic measurements. The
1H NMR spectra of the monomer 3 showed peaks at d 9.88 that cor-
respond to aldehydic protons, and its FTIR spectrum showed a
strong peak at 1685 cmꢀ1, which confirms the presence of aldehy-
dic protons. The spectral characteristics of chalcone 4 matched the
reported data. The cyclization of the chalcone to cyanopyridine 5
was confirmed by the 1H NMR spectrum of 5, which showed a sig-
nal at d 4.19 that corresponds to AOCH3 protons of the pyridine
ring. The FTIR spectrum of
5 displayed a sharp peak at
2217 cmꢀ1, which indicates the presence of cyano groups. The for-
mation of compound 6 was confirmed by its 1H NMR spectrum, in
which it showed a signal at d 4.81 that corresponds to tolyl methyl
protons. These protons are deshielded to a greater extent than
Fig. 1. Thermogravimetric traces of P1 and P2.

1124
K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
mers appeared at ꢀ0.93 and ꢀ0.67 eV for P1 and P2, respectively.
These reduction potentials are even lower than that of 2-(4-biphe-
nylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, PBD) the most
widely used electron-transporting material in polymer light emit-
ting diodes PLEDs. The onset oxidation and reduction potentials
were used to estimate the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy
h
i
oxd
levels of the polymers. The equations, EHOMO ¼ ꢀ E
þ 4:4 eV
onset
h
i
red
oxd
red
and ELUMO ¼ ꢀ E
ꢀ 4:4 eV , where E
and E
are the onset
onset
onset
oxd
potentials vs. a standard calomel electrode (SCE) for the oxidation
and reduction of the referred material, were used for the calcula-
tion. The electrochemical potentials and energy levels of the poly-
mer are tabulated in Table 1. The HOMO energy levels of polymers
P1 and P2 are estimated to be 5.57 and 5.81 eV, respectively. These
values are almost the same as that of poly(cyanoterphthalylidene)
(CNPPV), which indicates that the polymers have a hole-injection
ability similar to that of CNPPV when they are used in PLEDs.
The LUMO energy levels of polymers P1 and P3 are estimated to
be 3.47 and 3.73 eV, respectively. These values are greater than
those of CN-PPV (3.02 eV), which indicates that these polymers ex-
hibit better electron-injection ability when they are used in PLEDs
[21,30]. The electrochemical band gaps of the polymers were cal-
culated from the difference between HOMO and LUMO energy lev-
els and were 2.1 and 2.0 eV for polymers P1 and P2, respectively.
The electrochemical behavior of these polymers can be ex-
plained on the basis of their structure–activity relationship. In gen-
eral, when electron-withdrawing substituents are attached to
conjugated molecules, the electron density in the p-system of the
conjugated molecule will be decreased. Consequently, the mole-
cule will be stabilized and its oxidation potential will be increased.
This results in a shift of the HOMO energy level to a lower energy.
Similarly, the presence of tetradecyloxythiophene and N-tetra-
decyldiphenylamine units in P1 and P2 influences the HOMO en-
ergy level because of their strong electron-donating ability.
Usually, the band gap of any conjugated polymer can be influenced
by conjugation length, solid-state intermolecular ordering and the
presence of electron-withdrawing or electron-donating moieties.
The variation of these properties allows the optical and electro-
chemical behavior of such polymers to be tuned. The effective con-
jugation length, which is dependent upon the torsional angle
between the repeating units along the polymer backbone, can be
controlled by the introduction of sterically hindered bulky alkoxy
side chains to twist the units out of plane [31]. In this context,
diphenylamine-carrying bulky alkyl substituents have a major role
in the enhancement of oxidation potentials [32].
Fig. 2. Cyclic voltammetric traces of P1 and P2.
behavior has been observed for some of the reported polythio-
phenes [26,27]. The newly synthesized polymer is soluble in com-
mon organic solvents such as chloroform, dichloromethane,
tetrahydrofuran and dimethylsulfoxide, and this polymer showed
good processability and film-forming ability. The solubility may
be attributed to the presence of bulky alkyl and alkoxy groups on
the aromatic rings in the polymers.
3.2. Electrochemical studies
Cyclic voltammetry (CV) was employed to determine redox
potentials of the new polymers and to estimate the HOMO and
LUMO levels, which is important in determining the band gap. Cyc-
lic voltammograms of the polymers coated on a glassy carbon elec-
trode were obtained on an AUTOLAB PGSTAT-30 electrochemical
analyzer using a Pt counter electrode and an Ag/AgCl reference
electrode immersed in an electrolyte [0.1 M (n-Bu)4NClO4 in aceto-
nitrile] at a scan rate of 25 mV/s [28].
All the measurements were calibrated using ferrocene as a stan-
dard [29]. As shown in Fig. 2, the newly synthesized polymers were
electroactive either in the cathodic region or in the anodic region.
The onset oxidation potentials (p-doping) were estimated to be
1.09 and 1.41 eV for P1 and P2, respectively. For the n-doping
(reduction) process, the onset reduction potentials for the poly-
3.3. Linear optical properties
The solution-phase UV–vis absorption and fluorescence emis-
sion spectra of the new polymers were recorded at room tempera-
ture in dilute solutions. All spectral data are summarized in Table
2. UV–vis absorption spectra were recorded in different polar sol-
vents (Figs. 3 and 4). The enhancement in the absorption maxi-
mum of polymer P2 is attributed to the strong electron-
withdrawing nature of the cyanopyridine along the polymer back-
bone. The absorption maximum of polymer P1 was blue-shifted
compared to that of P2. This shift is most probably due to the pres-
Table 1
Electrochemical characterization table.
Polymer
Eoxd
Ered
Eoxd (onset)
Ered (onset)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
P1
P2
1.17
1.69
ꢀ1.46
ꢀ1.20
1.09
1.41
ꢀ0.93
ꢀ0.67
ꢀ5.57
ꢀ5.81
ꢀ3.47
ꢀ3.73
2.1
2.0

K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
1125
Table 2
UV absorption and fluorescence emission spectra of P1 and P2 in different polar solvents.
Polymers
UV–visible absorption and fluorescence emissions in different solvents (nm) [(UV–visible) fluorescence emission]
Quantum yield
Chlorobenzene
Chloroform
Tetrahydrofuran
Dimethyl sulfoxide
In THF (%)
P1
P2
(443) 525
(458) 540
(446) 532
(465) 548
(458) 536
(472) 555
(460) 547
(484) 570
39
44
Fig. 3. UV absorption spectra of P1 in different polar solvents.
Fig. 5. Fluorescence emission spectra of P1 in different polar solvents.
Fig. 4. UV absorption spectra of P2 in different polar solvents.
Fig. 6. Fluorescence emission spectra of P2 in different polar solvents.
ence of the alkoxy groups on the thiophene ring of the polymer
backbone, which usually leads to an increase in the dihedral angle
of consecutive aromatic units. The increased dihedral angle results
in the reduction of the electronic conjugation and hence induces an
enhanced band gap [33].
Consequently, the energy gap E (S1, S2) is enlarged so that the cou-
pling of the S1 state directly to the ground state stays open, and the
inter-system crossing from the S1 to the T state is enhanced
[36,37].
Fluorescence studies of P1 and P2 were performed in different
polar solvents (Figs. 5 and 6). The polymers displayed very good
fluorescence behavior in different solvents when irradiated with
UV light (Figs. 7 and 8). The fluorescence emission spectra were ob-
tained by irradiative excitation at the wavelength of the absorption
maximum. The polymers exhibited a strong solvatochromic effect
[34,35]. Based on the results of these studies, the emission maxima
of the polymers were red-shifted as the polarity of the solvent was
increased. In particular, maximum solvatochromism was observed
in P2 because of the presence of the cyanopyridine ring in the
backbone. In a highly polar solvent such as THF or DMSO, the emis-
sive S1 state of the intra-molecular charge transfer (ICT) is strongly
solvated, and its energy is therefore dramatically decreased.
The presence of high-electron-releasing thiophene and diphe-
nyl amine moieties along the polymer chain was observed to in-
duce a red shift (lower energy) of their emission maxima. This
shift is mainly attributed to energy losses through dissipation of
vibrational energy during the decay. This is normally influenced
by interaction between the fluorophore and the solvent molecules
around the excited dipole, by hydrogen bonding and by the forma-
tion of charged complexes. In addition, the introduction of the
pyridinyl moiety in polymer P2 increases its electron affinity,
which makes the polymer more resistant to oxidation and also
gives the polymer better electron-transporting properties. Further-
more, it avoids fluorescence quenching due to the inter-system
crossing (ISC) effect [38]. The quantum yield for emission in solu-

1126
K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
Physics) was used to excite the molecules. The temporal width
(FWHM) of the laser pulses was 7 ns. The pulses were fired in
the ‘‘single shot’’ mode, which allowed sufficient time between
successive pulses to avoid accumulative thermal effects in the
sample.
Fig. 9 shows the open-aperture Z-scan curves of the polymers in
THF solutions. In each case, we observed two-photon absorption
(TPA). The nonlinear activity of the polymer can be explained by
considering pulse duration, pump intensity and wavelength. It
can be explained by the transitions: ground state S0 to higher ex-
cited singlet states Sn (two-photon or multi-photon excitation),
the first excited singlet state S1 to higher excited states Sn, or the
T1 to Tn states in the triplet manifold. The last two processes are
known as excited-state absorption (ESA), and if their cross-sections
are larger than that of the ground-state linear absorption, then
these processes are referred to as reverse saturable absorption
(RSA). The net effect is then known as an ‘‘effective’’ TPA process.
The nonlinear transmission behavior of the present samples can
therefore be modeled by defining an effective nonlinear absorption
Fig. 7. Fluorescence emission of P1 in different polar solvents under UV irradiation.
coefficient a(I), as given by:
a0
ꢀ ꢁ
aðIÞ ¼
þ bI
ð1Þ
I
1 þ
Is
where a0 is the unsaturated linear absorption coefficient at the
wavelength of excitation, Is is the saturation intensity (intensity at
which the linear absorption drops to half its original value), and b
is the effective TPA coefficient. To calculate the output laser inten-
sity for a given input intensity, we first numerically evaluated the
output intensity from the sample for each input intensity by solving
the propagation:
Fig. 8. Fluorescence emission of P2 in different polar solvents under UV irradiation.
ꢄꢂ
ꢂ
ꢃꢃ
ꢅ
dI
I
Is
¼ ꢀ
a
0= 1 þ
þ bI I
ð2Þ
dz0
tion was determined relative to quinine sulfate in 0.1 M H2SO4
according to the method described by John et al. [39] and Davey
et al. [40]. Polymers P1 and P2 were strongly emissive with high
quantum yields. These high quantum yields could be attributable
to the rigid structure of the polymer with the effect that relaxation
from the excited state through non-radiative (e.g., thermal) pro-
cesses would be reduced with consequently higher fluorescence
quantum yields [41].
using the fourth-order Runge–Kutta method. The input intensities
for the Gaussian laser beam for each sample position in the Z-scan
are calculated from the input energy, laser pulse width and irradia-
tion area. Here, ‘Z’ indicates the propagation distance within the
sample. The normalized transmittance was then calculated by
dividing the output intensity by the input intensity and normalizing
it with the linear transmittance. As seen from Fig. 8, there is good
agreement between the experimental data and the numerical sim-
ulation. The numerically estimated values of the effective TPA coef-
ficients are 0.64 ꢁ 10ꢀ10 and 0.21 ꢁ 10ꢀ10 m/W for polymers P1 and
P2, respectively.
3.4. Third-order nonlinear optical activity
3.4.1. Z-scan studies
A very convenient and fast experimental method to assess
materials for NLO (including optical limiting) is the Z-scan experi-
ment [42]. This method allows the measurement of the magnitude
of both the nonlinear refraction (NLR) and nonlinear absorption
(NLA) as a function of the incident laser intensity while the sample
is gradually moved through the focus of a lens (along the z-axis). In
the experiment, the sample is placed in the path of a high-intensity
laser beam at different positions with respect to the focus (differ-
ent values of z), and the corresponding transmission is measured.
The sample is then exposed to different laser intensity at each po-
sition; therefore, its position-dependent transmission will give
information on its intensity-dependent transmission as well. The
effective nonlinear absorption coefficients were calculated by fit-
ting theory. We used a stepper-motor-controlled linear translation
stage in our setup to move the sample through the beam in precise
steps. The sample was placed in a 1-mm cuvette. The transmission
of the sample at each point was measured using two pyroelectric
energy probes (Rj7620, Laser Probe, Utica, NY, USA). One energy
probe monitored the input energy while the other monitored the
energy transmitted through the sample. The second harmonic out-
put (532 nm) of a Q-switched Nd:YAG laser (Quanta Ray, Spectra
For comparison, under similar excitation conditions, good NLO
materials like copper (Cu) nanocomposite glasses have shown
effective TPA coefficient values of 10ꢀ10–10ꢀ12 m/W [43], function-
alized carbon nanotubes have shown 3 ꢁ 10ꢀ11 m/W [44], bismuth
(Bi) nanorods have shown 0.53 ꢁ 10ꢀ10 m/W [45] and cadmium
sulfide (CdS) quantum dots have exhibited 1.9 ꢁ 10ꢀ9 m/W [46].
Similarly, many
p-conjugated polymers have shown good two-
photon absorption coefficients [47] on the order of 10ꢀ10 m/W
with comparable optical limiting behavior. Recently, triphenyl-
amine-based conjugated polymers have shown a good two-photon
absorption coefficient of 1.1 ꢁ 10ꢀ10 m/W [48,10]. These values
demonstrate that the present samples exhibit an optical nonlinear-
ity comparable to those of good optical limiters reported in the lit-
erature, and are therefore potentially suitable for applications in
optical limiting devices.
An increase in the planarity and rigidity of a polymer chain is
known to enhance thirdorder susceptibilities because the in-
creased rigidity optimizes the overlap of
in enhanced electron delocalization. Also, the third-order optical
nonlinearity can be enhanced by an increase in the -delocalized
electron density and the carrier transport [49]. In the present
p-orbitals, which results
p

K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
1127
Fig. 9. Z-scan and fluence curves of P1 and P2.
study, both polymers showed good optical limiting behavior and
Acknowledgements
optical nonlinearity. This nonlinearity may be attributed to the
presence of extended conjugation and enhanced polarizability of
the molecule due to a D–A-type-arrangement in the polymer back-
bone. As previously discussed, the introduction of electron-defi-
cient cyanopyridine between a highly electron-donating moiety
such as alkyl-substituted diphenylamine has enhanced the D–A
nature of the resulting polymer P2 and, in turn, increased the ex-
tent of delocalization in the polymer strands. As a result, the opti-
cal nonlinearity of P2 has increased considerably.
The authors are grateful to the NITK, Surathkal; NMR Research
Centre, IISc Bangalore; and SICART-CVM, Gujarat, for providing
instrumental facilities.
References
[1] A.J. Heeger, Angew. Chem. Int. Ed. 40 (2001) 2591–2611.
[2] S. Kuwabara, N. Yamamoto, M. Prabhuodeyara, V. Sharma, K. Takamizu, M.
Fujiki, Y. Geerts, K. Nomura, Macromolecules 44 (2011) 3705–3711.
[3] M. Lanzi, L. Paganin, D. Caretti, L. Setti, F. Errani, React. Funct. Polym. 71 (2011)
745–775.
[4] C.S. Rhett, G.T. Andrew, M.H. Lim, S.J. Lippard, Org. Lett. 7 (2005) 3573–3575.
[5] S.R. Marder, B. Kippelen, A.K.Y. Jen, N. Peyghambarian, Nature 388 (1997) 845–
851.
4. Conclusion
[6] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in
Molecules and Polymers, John Wiley Inc., New York, 1991.
[7] J. Zyss, Molecular Nonlinear Optics Materials Physics and Devices, Orlando
Academic Press, 1994.
[8] F. Kajzar, K.S. Lee, A.K.Y. Jen, Adv. Polym. Sci. 161 (2003) 1–85.
[9] M. Behl, E. Hattemer, M. Brehmer, R. Zentel, Macromol. Chem. Phys. 203 (2002)
503–508.
[10] J.L. Hua, B. Li, F.S. Meng, F. Ding, S.X. Qian, H. Tian, Polymer 45 (2004) 7143–
7149.
[11] J. Hao, M.J. Han, K. Guo, Y. Zhao, L. Qiu, Y. Shen, X. Meng, Mater. Lett. 62 (2008)
973–976.
[12] D. Yanmaoa, J. Luc, Q. Xuc, F. Yanc, X. Xiac, L. Wangc, L. Huc, Synth. Met. 160
(2010) 409–412.
[13] R.M.F. Batista, S.P.G. Costa, M. Belsley, M.M. Raposo, Tetrahedron 63 (2007)
9842–9849.
[14] V.P. Rao, A.K.Y. Jen, K.Y. Wong, K.J. Drost, Tetrahedron Lett. 34 (1993) 1747–
1750.
[15] A. Abbotto, S. Bradamante, A. Facchetti, G.A. Pagani, J. Org. Chem. 62 (1997)
5755–5759.
Two new D–A-type conjugated polymers, P1 and P2, carrying
highly electron-deficient cyanopyridine/oxadiazole moieties and
electron-donating alkyl-substituted diphenylamine/dialkoxythi-
ophene systems were successfully synthesized and characterized
using various techniques. Linear optical studies indicated that
these polymers are excellent fluorescent materials. The results of
the electrochemical measurements revealed that the band gaps
of the polymers were significantly reduced by an increase in the
donor strength. A third-order nonlinear optical study using the Z-
scan technique showed reverse saturable absorption with good
TPA coefficients. The absorptive nonlinearity observed in these
polymers is of the optical-limiting type. As predicted, the new
polymers displayed good NLO properties, which are mainly attrib-
uted to the greater extent of delocalization and hence the increased
hyperpolarizability in the molecules. They are therefore considered
to be potential candidates for photonic applications.
[16] S. Yuquan, Z. Yuxia, L. Zao, W. Jianghong, Q. Ling, L. Shixiong, Z. Jianfeng, Z.
Jiayun, J. Chem. Soc., Perkin Trans. 1 (1999) 3691–3695.

1128
K.A. Vishnumurthy et al. / Reactive & Functional Polymers 71 (2011) 1119–1128
[17] C.W. Dirk, H.E. Katz, M.L. Schilling, L.A. King, Chem. Mater. 2 (1990) 700–705.
[18] S.M. Tambe, A.A. Kittur, S.R. Inamdar, G.R. Mitchell, M.Y. Kariduraganavar, Opt.
Mater. 31 (2009) 817–825.
[19] W.N. Leng, Y.M. Zhou, Q.H. Xu, J.Z. Liu, J. Polym. 42 (2001) 9253–9259.
[20] T. Cassano, R. Tommasi, F. Babudri, Opt. Lett. 27 (2002) 2176–2178.
[21] D. Udayakumar, A.V. Adhikari, Synth. Met. 156 (2006) 1168–1173.
[22] D. Udayakumar, A.V. Adhikari, Opt. Mater. 29 (2007) 1710–1718.
[23] S. Michelle, Liu, J. Xuezhong, S. Liu, H. Petra, A.K.Y. Jen, Macromolecules 35
(2002) 3532–3538.
[36] V.D. Gupta, V.S. Padalkar, K.R. Phatangare, V.S. Patil, P.G. Umape, N. Sekar, Dyes
Pigm. 88 (2011) 378–384.
[37] Z.J. Hu, P.P. Sun, L. Li, Y.P. Tian, J.X. Yang, J.Y. Wu, H.P. Zhou, L.M. Tao, C.K.
Wang, M. Li, G.H. Cheng, H.H. Tang, X.T. Tao, M.H. Jiang, Chem. Phys. 355
(2009) 91–98.
[38] N.J. Turro, Modern Molecular Photochemistry Benjamin Cummings, Publ. Co.,
New York, 1978. p. 48.
[39] A.M. John, A.D. Panagiotis, G.M. Vasilis, T. Lin-Ren, C. Yun, Eur. Polym. J. 45
(2009) 284–294.
[24] W. Hongli, L. Zhen, H. Bin, J. Zuoquan, L. Yanke, W. Hui, Q. Jingui, Y. Gui, L.
Yunqi, S. Yinglin, React. Funct. Polym. 66 (2006) 993–1002.
[25] T.Y. Wu, N.C. Lee, Y. Chen, Synth. Met. 139 (2003) 263–269.
[26] H. Xiao, X. Lingge, Polymer 41 (2000) 9147–9154.
[27] T.M. Swager, C.G. Gil, M.S. Wrighton, J. Phys. Chem. 99 (1995) 4886–4893.
[28] S. Qingjiang, W. Haiqiao, Y. Chunhe, L. Yongfang, J. Mater. Chem. 13 (2003)
800–806.
[29] J. Pommerehe, H. Vestweber, W. Guss, R.F. Mahrt, H. Bassler, M. Porsch, Daub,
J. Adv. Mater. 7 (1995) 551–554.
[30] M. Strukelji, F. Papadimitrakooulous, T.M. Miller, L.J. Rothberg, Science 267
(1995) 1969–1972.
[40] A.P. Davey, S.O. Elliott, W. Blau, J. Chem. Soc. Chem. Commun. (1995) 1433–
1434.
[41] J.K. Tai, Y.K. Jae, J.K. Kyung, L. Changjin, B.R. Suh, Synth. Met. 69 (1995) 377–
378.
[42] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. VanStryland, IEEE J. Quant.
Electron. 26 (1990) 760–769.
[43] B. Karthikeyan, M. Anija, C.S. Suchandsandeep, N.T.M. Muhammad, RejiPhilip,
Opt. Commun. 281 (2008) 2933–2937.
[44] H. Nan, C. Yu, B. Jinrui, W. Jun, J.B. Werner, Z. Jinhui, J. Phys. Chem. C. 113
(2009) 13029–13035.
[45] S. Sivaramakrishnan, V.S. Muthukumar, S.S. Sivasankara, K. Venkataramanaiah,
J. Reppert, A.M. Rao, M. Anija, R. Philip, N. Kuthirummal, Appl. Phys. Lett. 91
(2007) 093104–093107.
[31] E. Perzon, X.J. Wang, S. Admassie, O. Inganäs, M.R. Andersson, Polymer 47
(2006) 4261–4268.
[32] M. Vetrichelvan, R. Nagarajan, S. Valiyaveettil, Macromolecules 39 (2006)
8303–8310.
[46] P.A. Kurian, C. Vijayan, K. Sathiyamoorthy, C.S. Suchand Sandeep, P. Reji, Nano
Res. Lett. 2 (2007) 561–565.
[33] A.D. Schlüter, Polym. Sci. A: Polym. Chem. 39 (2001) 1533–1556.
[34] V.V. Jerca, F.A. Nicolescu, A. Baran, D.F. Anghel, D.S. Vasilescu, D.M. Vuluga,
React. Funct. Polym. 70 (2010) 827–835.
[47] M. Samoc, A. Samoc, B.L. Davies, Synth. Met. 109 (2000) 79–83.
[48] Y. Qian, K. Meng, C.G. Lu, B. Lin, W. Huan, Y. Cui, Dyes Pigm. 80 (2009) 174–
180.
[35] Y.P. Huang, S.H. Tsai, D.F. Huang, T.S. Tsai, T.J. Chow, React. Funct. Polym. 67
(2007) 986–998.
[49] L.P. Yu, L.R. Dalton, Synth. Met. 29 (1989) 463–470.