Synthesis and two-photon absorption property of new π -conjugated donor-acceptor polymers carrying different heteroaromatics

Article abstract of DOI:10.1007/s12039-013-0366-1

In this communication, we report the synthesis of three newly designed fluorescent polymers P1-P3, starting from simple thiophene derivatives through precursor polyhydrazide route. The new polymers, carrying donor and acceptor heterocyclic moieties with different spacer groups were found to be thermally stable and good of nonlinear optical (NLO) materials with two photon absorption property. The structures of newly synthesized monomers and polymers were confirmed by FTIR, NMR spectral and elemental analyses. Further, polymers were characterized by GPC and TGA studies. Their linear optical and electrochemical properties were evaluated by UV-vis, fluorescence spectroscopic and cyclic voltammetric (CV) studies, respectively, whereas their NLO properties were studied by Z-scan technique using Nd: YAG laser at 532 nm with 7 ns pulse. The electrochemical band gap of P1-P3 was determined to be 1.98, 1.91 and 2.05 eV, respectively. The NLO results reveal that polymers P1-P3 show good optical limiting property with TPA coefficient values 2.9 × 10 ^{- 11} m/W, 8.0 × 10 ^{- 11} m/W and 1.4 × 10 ^{- 11} m/W, respectively.

Full text of DOI:10.1007/s12039-013-0366-1

c
J. Chem. Sci. Vol. 125, No. 1, January 2013, pp. 29–40. ꢀ Indian Academy of Sciences.
Synthesis and two-photon absorption property of new π-conjugated
donor–acceptor polymers carrying different heteroaromatics
M S SUNITHA, K A VISHNUMURTHY and A V ADHIKARI∗
Department of Chemistry, National Institute of Technology Karnataka, Surathkal 575 025, India
e-mail: avadhikari123@yahoo.co.in, avchem@nitk.ac.in
MS received 8 March 2012; revised 30 June 2012; accepted 5 July 2012
Abstract. In this communication, we report the synthesis of three newly designed fluorescent polymers
P1–P3, starting from simple thiophene derivatives through precursor polyhydrazide route. The new polymers,
carrying donor and acceptor heterocyclic moieties with different spacer groups were found to be thermally
stable and good of nonlinear optical (NLO) materials with two photon absorption property. The structures
of newly synthesized monomers and polymers were confirmed by FTIR, NMR spectral and elemental analy-
ses. Further, polymers were characterized by GPC and TGA studies. Their linear optical and electrochemical
properties were evaluated by UV-vis, fluorescence spectroscopic and cyclic voltammetric (CV) studies, respec-
tively, whereas their NLO properties were studied by Z-scan technique using Nd: YAG laser at 532 nm with
7 ns pulse. The electrochemical band gap of P1–P3 was determined to be 1.98, 1.91 and 2.05 eV, respectively.
The NLO results reveal that polymers P1–P3 show good optical limiting property with TPA coefficient values
2.9 × 10−11 m/W, 8.0 × 10−11 m/W and 1.4 × 10−11 m/W, respectively.
Keywords. Conjugated polymers; optical materials; NLO; D–A polymers.
1. Introduction
With regard to NLO properties, a strong delocaliza-
tion of π-electrons in the polymer backbone is highly
significant as it determines a very high molecular polari-
zability and thus giving rise to remarkable optical non-
linearity. Large molecular hyperpolarizabilities and low
optical losses within the spectral region of interest are
the basic requirements for NLO applications. A detailed
literature review reveals that a general approach for
obtaining materials with important NLO properties
consists in synthesizing polymer framework involv-
ing electron-donor and electron-acceptor groups linked
through a π-conjugated spacer.13–15 Such D–A sys-
tems exhibit a prominent intramolecular charge trans-
fer (ICT) along the π-conjugated bridges, which is
crucial in promoting large optical nonlinearities and
ultra-fast responses due to instantaneous electronic
polarization. Thus, an optimal combination of various
factors such as π-delocalization length, donor–acceptor
moieties, dimensionality, confirmation and orientation
of molecular structure result in a large hyperpolari-
zability in the polymeric systems and hence it leads to
achieve good nonlinearities in them.16–23 Moreover, theo-
retical calculations further suggest that the electronic
nature and location of heterocyclic rings in the system
play a subtle role in the development of NLO properties
of donor–acceptor compounds.24–26
In recent times, there is tremendous increase in the
usage of organic nonlinear optical (NLO) polymers
in various applications like frequency doublers, opti-
cal storage devices, and electro-optic (EO) switches
and modulators.1–6 Out of various optical materials,
organic polymers exhibit several advantages over inor-
ganic materials, like large NLO effects, mechanical
endurance, low driving voltage and ease of process-
ing.7–11 Amongst many organic polymers, conjugated
polymers with their extended delocalization of π-
electrons have attracted much attention in optoelec-
tronic studies. It has been established that conjugated
polymers with π-excessive nature have greater ten-
dency to transport holes than electrons, whereas poly-
mers containing π-deficient heterocycles like pyridine,
pyran, and oxadiazoles show greater tendency to trans-
port electrons than holes.12 Consequently, in order to
have conjugated polymer with both hole and elec-
tron transporting groups in a single chain, the donor–
acceptor strategy is being widely used to tailor their
electronic, mechanical and physical properties.
∗For correspondence
29

30
M S Sunitha et al.
Presently, thiophene and oxadiazole based polymers 2. Experimental
are of special interest among the D–A type conju-
gated systems mainly due to their high thermal stability,
readiness to accommodate functional groups and solu-
bility in common organic solvents which offer great
promise for practical device applications. Also, such
conjugated polymers with different spacer groups like
benzene, biphenyl, naphthalene, anthracene, pyridine,
etc. were reported to possess good molecular hyper-
polarizability.27 Further, it was shown that incorpora-
tion of different five-membered heterocyclic rings as
spacer groups of the polymeric system could lead to
enhanced molecular hyperpolarizability. For instance,
in certain polymers replacement of benzenoid moie-
ty with heteroaromatic rings such as thiophene, furan
effectively brought about increase in the electron delo-
calization,28,29 while in push–pull polyenes presence of
benzene rings caused saturation in their molecular non-
linearity.24 Against this background, several NLO chro-
mospheres containing polarizable five-membered hete-
roaromatics with lower aromatic stabilization energy
were developed as effective D–A type π-conjugative
systems. Further, inclusion of electron withdrawing
1,3,4-oxadiazole offers an improved π-electron delo-
calization across the D–A links and provides signifi-
cant electron transportation. Also, it was established
that inclusion of electron deficient pyridine moiety in
thiophene-based D–A type polymer increases the elec-
tron affinity that makes the polymer more resistant to
oxidation and produces improved electron-transporting
properties.
2.1 Materials
The required starting material diethyl 3,4-
dihydroxythiophene-2,5-dicarboxylate (1), was syn-
thesized according to the reported procedures.30,31
Tetrahydrofuran (THF) and acetonitrile (ACN), dried
over CaH2 were used. Thiodiglycolic acid, diethyloxa-
late and tetrabutylammoniumperchlorate (TBAPC)
were purchased from Lanchaster (UK). Vanillin,
sodium borohydride, phosporous tribromide, and
lithium chloride were purchased from Aldrich and
were used as received. All the solvents and reagents
were of analytical grade, purchased commercially and
used without further purification.
2.2 Instrumentation
Infrared spectra of all intermediate compounds,
monomers and polymers were recorded on a Nicolet
Avatar 5700 FTIR (Thermo Electron Corporation). The
UV-visible and fluorescence spectra were taken in GBC
Cintra 101 UV-visible and Perkin Elmer LS55 fluo-
1
rescence spectrophotometers, respectively. H NMR
spectra were obtained with 400 MHz on Bruker NMR
spectrometer using TMS/solvent signal as internal refe-
rence. Elemental analyses were performed on a Flash
EA1112 CHNS analyzer (Thermo Electron Corpora-
tion). Electrochemical studies of the polymers were car-
ried out using AUTOLAB PGSTAT30 electrochemical
analyzer. Cyclic voltammograms were recorded using
a three-electrode cell system, with glassy carbon but-
ton as working electrode, a platinum wire as counter
electrode and an Ag/AgCl electrode as the reference
electrode. Molecular weights of the polymers were
determined with WATER’s make Gel Permeation Chro-
matograph (GPC) against poly(styrene) standards with
teterhydrofuran (THF) as an eluent. The thermal sta-
bility of polymers was studied by SII-EXSTAR6000-
TG/DTA6300 thermogravimetric analyzer. Q-Switched
Nd:YAG laser was used for NLO studies.
Keeping all these in view, we designed three new
D–A type conjugated polymers (P1–P3) carrying 3,4-
bis(4-(decyloxy)-3-methoxybenzyloxy)thiophene unit
as strong electron density donor and 1,3,4-oxadiazole
as strong electron acceptor moieties. Additionally, dif-
ferent spacer groups, viz. phenyl (P1), thiophene (P2)
and pyridine (P3) moieties were introduced as π-
conjugation bridges in between donor and acceptor
groups in order to study their effect on electrochemical,
linear and nonlinear optical properties. Accordingly,
we synthesized the unknown polymers (P1–P3) start-
ing from simple thiophene derivative through precur-
sor polyhydrazide route. The structures of newly syn-
thesized monomers and polymers were confirmed by
1
FTIR, H NMR, 13C NMR spectral methods followed
2.3 Synthesis of monomers
by elemental analyses. Further, the molecular weight
and thermal stability of polymers were evaluated by
GPC and TGA studies, respectively. We have investi-
gated their linear optical and electrochemical proper-
ties and determined their band gaps. Further, their NLO
properties were studied by Z-scan technique using Nd:
YAG laser at 532 nm with 7 ns pulse.
The synthetic route towards the preparation of required
intermediates and monomers is outlined in scheme 1.
The alkylated vanillin (2), obtained from vanillin
was reduced to corresponding alcohol (3) using
sodium borohydride. The resulting alcohol was then
bromomethylated using phosphorous tribromide.

Synthetic route for monomers
31
CH2Br
CH2OH
CHO
CHO
(iii)
(ii)
(i)
OCH3
OCH3
OC10H21
OCH3
OC10H21
OCH3
OC10H21
OH
1
4
3
2
H3CO
H3CO
O
OC10H21
OC10H21
H21C10
H3CO
O
H
21C10O
H3CO
(iv)
HO
O
O
OH
COOEt
O
(v)
4
+
CONHNH2
COOEt
H2NHNOC
EtOOC
EtOOC
S
S
5
S
7
6
(i) C10H21Br, DMF, K2CO3, (ii) NaBH4, MeOH, (iii) PBr3, DEE, (iv) DMF, K2CO3, (v) C2H5OH, NH2NH2. 2H2O
Scheme 1. Synthesis of monomers.
The compound (4) was then condensed with 3,4- under reduced pressure; to this 20 mL of cold water was
dihydroxythiophene-2,5-dicarboxylate (5) to yield 3,4- added and then solvent extracted using 15 mL of diethyl
bis(4-(decyloxy)-3-methoxybenzyloxy)thiophene-2,5-
ether twice. The organic layer was dried over anhydrous
dicarboxylate (6), which was then converted to cor- sodium sulphate and then distilled under pressure to get
responding bishydrazide (7) using hydrazine hydrate. white solid product, which was recrystalized using ethyl
The procedures followed for their synthesis are given acetate.
below.
Mp: 53–55◦C. FTIR (cm−1): 3349, 2915, 2850, 1512,
1458, 1236, 1133, 1024. 1H NMR (DMSO-d6, δ ppm):
6.87–6.73 (m, 3H, Ar), 4.99–4.96 (t, 1H, -OH), 4.37–
4.35 (d, 2H, Ar-CH2), 3.7 (s, 3H, OCH3), 1.67–1.21
(m, 16H, aliphatic), 0.80–0.83 (t, 3H, -CH3). Element.
Anal. Calcd. for C18H30O3: C, 73.43%; H, 10.27%;
Found: C, 73.38%; H, 10.12%.
2.3a Synthesis of 4-decyloxy-3-methoxybenzaldehyde
(2): 1-Bromodecane (1.3 g, 1 mmol) was added to
a mixture of 4-hydroxy-3-methoxybenzaldehyde (1 g,
6 mmol) and potassium carbonate (0.9 g, 1 mmol) in
25 mL of DMF. The reaction mixture was stirred at
60◦C. The reaction mixture was then cooled and poured
into water. The product was then extracted twice using
20 ml of diethyl ether. The organic layer was dried
over sodium sulphate and solvent was removed under
reduced pressure to yield the white semi-solid crys-
talline product.
2.3c Synthesis
methoxybenzene (4): The compound
of
4-(bromomethyl)-1-ethoxy-2-
(1 g,
3
3.40 mmol) was dissolved in diethyl ether; to this
0.3 mL of phosphorous tribromide (1 mmol) was added
drop-wise and kept for stirring at room temperature
for 5 h. After completion of the reaction, the reaction
mixture was poured into water and the organic layer
was separated, dried over sodium sulphate and solvent
was evaporated under reduced pressure to obtain white
puffy solid. The product obtained was recrystalized
using ethyl acetate. FTIR (cm−1): 2915, 2850, 1511,
1458, 1254, 1087.
Mp: 60–61 C, FTIR (cm−1) 2916, 2849, 1679, 1583,
◦
1
1458, 1128. H NMR (DMSO-d6, δ ppm): 9.79 (s,
2H, -CHO), 7.50–7.11 (m, 3H, Ar), 4.03–4.01 (t, 2H,
-OCH2-), 3.79 (s, 3H, -OCH3), 1.73–1.21 (m, 16H,
aliphatic), 0.83–0.80 (t, 3H, -CH3). Element. Anal.
Calcd. for C18H28O3: C, 73.93%; H, 9.65%; Found: C,
73.90%; H, 9.59%.
2.3b Synthesis of (4-decyloxy-3-methoxyphenyl)methanol
(3): To a solution of 3.42 mmol (1 g) of 4-decyloxy-
3-methoxybenzaldehyde (2) in 15 mL methanol sodium
borohydride (0.03 g, 0.85 mmol) was added pinch-wise
during 15 min with stirring. After the complete addition
of sodium borohydride, the reaction mixture was stirred
at room temperature for 2 h. The solvent was removed
2.3d Synthesis of diethyl 3,4-bis(4-(decyloxy)-3-
methoxybenzyloxy)thiophene-2,5-dicarboxylate
(6):
To a mixture of compound 5 (1 g, 3.84 mmol) and
potassium carbonate (1.16 g, 8.46 mmol) in DMF,
solution of compound 4 in DMF was added drop-wise
with stirring. The reaction mixture was refluxed for
8 h at 60◦C, then cooled and poured into water to get

32
M S Sunitha et al.
creamish white solid. The product was recrystalized -NH-), 4.30–4.24 (t, 3H, 3.93–3.90), 3.68 (s, 3H,
using ethyl acetate to get white crystalline solid.
-OCH3), 1.69–1.24 (m, 32H, aliphatic), 0.82–0.86 (t,
Mp: 70–73◦C. FTIR (cm−1): 2914, 2848, 1711, 1467, 3H, -CH3). Element. Anal. Calcd. for C42H64N4O8S:
1
1234, 1143, 1040. H NMR (DMSO-d6, δ ppm): C, 64.26%; H, 8.22%;N, 7.14%; S, 4.08%; Found: C,
6.97–6.83 (m, 6H, Ar), 5.06 (s, 2H, -OCH2), 4.30–4.25 64.12%; H, 8.02%; N, 6.98%, S, 3.89%.
(q, 2H, -CH2), 3.92–3.89 (t, 3H, -CH3), 3.65 (s, 3H,
-OCH3), 1.70–1.23 (m, 32H, aliphatic), 0.86–0.82 (t,
3H, -CH3). Element. Anal. Calcd. for C46H68O10S: C,
2.3f Synthesis of diacid chlorides (9a–c): Excess of
67.95%; H, 8.43%; S, 3.94%; Found: C, 67.91%; H,
thionyl chloride (5 mL) was added to a flask containing
8.33%; S, 3.78%.
corresponding diacid (8a–c) (0.3 g) and then a drop of
DMF was added. The reaction mixture was refluxed for
5 h. The excess thionyl chloride was removed by distil-
lation under reduced pressure. The residue was washed
with methylene dichloride to remove trace amount of
thionyl chloride.
2.3e Synthesis of 3,4-bis(4-(decyloxy)-3-methoxybenzyloxy)
thiophene-2,5-dicarbohydrazide
(7): One
gram
(1.23 mmol) of bisester (6) was added to a solution of
0.30 mL (6.15 mmol) of hydrazine hydrate in 25 mL
of ethanol. The reaction mixture was refluxed for 5 h.
Upon cooling, white precipitate was obtained. The
product was then filtered, washed with alcohol and
dried to get white solid. The product was recrystalized
using chloroform.
2.4 Synthesis of polymers
The synthetic route towards the synthesis of poly-
mers P1–P3 from corresponding monomers is shown
in scheme 2. The diacids were refluxed with excess
thionyl chloride to obtain diacid chlorides, which on
treatment with the dihydrazide 7, in the presence of
Mp: 133–135◦C. FTIR (cm−1): 3386, 3325, 2919,
1
2857, 1664, 1513, 1466, 1306, 1247, 1137, 1025. H
NMR (DMSO-d6, δ ppm): 8.80 (s, 2H, -NH2), 7.01–
6.77 (m, 6H, Ar), 5.14 (s, 4H, -OCH2), 4.54 (s, 1H,
HOOC
R
COOH
ClOC
R
COCl
where R=
S
N
9a-c
8a-c
PH3, P3
PH1, P1
PH2, P2
OC10H21
OCH3
OCH3
H21C10
O
OCH3
H21C10
O
OC10H21
OCH3
O
ClOC
COCl
O
O
O
NH
O
O
O
9a
(i)
NH
S
NH
NH
(ii)
O
n
O
O
n
PH1
S
N
N
N
N
P1
OC10H21
OCH3
OCH3
OCH3
H21C10
O
H21C10
O
OC10H21
OCH3
O
O
ClOC
COCl
O
S
O
O
O
9b
NH
S
NH
O
(ii)
NH
S
O
O
(i)
NH
n
7
+
S
S
n
O
N
N
N
N
PH2
P2
OC10H21
OCH3
OCH3
OCH3
H21C10
O
OC10H21
H21C10
O
OCH3
n
O
O
O
O
N
ClOC
N
COCl
O
O
O
O
(ii)
9c
(i)
NH
S
S
NH
NH
NH
N
n
N
N
N N
O
P3
PH3
O
(i) NMP, LiCl, Pyridine, (ii) POCl3
Scheme 2. Synthesis of polymers.

Synthetic route for monomers
33
lithium chloride and pyridine underwent polycondensa- 34H, aliphatic), 0.84–0.82 (t, 6H, -CH3). Weight aver-
tion to give required polyhydrazides PH1–PH2 in good age molecular weight (Mw): 8121 g/mol, Number aver-
yield. The polyhydrazides on cyclodehydration with age molecular weight (Mn):3812 g/mol, Polydispersity
phosphorous oxychloride yielded target D–A type poly- index (PDI): 2.13.
mers. The experimental procedures for the synthesis of P2: FTIR (cm−1): 2924, 2854, 1597, 1469, 1248, 1021.
new polymers are as follows.
1H NMR (DMSO-d6,δ ppm): 8.15–7.69 (m, 6H, Ar),
7.22–6.98 (m, 2H, Th), 5.72 (s, 4H, -OCH2-Ar), 1.66–
1.22 (m, 34H, aliphatic), 0.85–0.81(t, 6H, -CH3). Mw:
8198 g/mol, Mn: 4738 g/mol, PDI: 1.73.
2.4a General procedures for the synthesis of polyhy-
drazides (PH1–PH3): To a stirred solution of 0.5 g of
monomer (7) in 20 mL N-methylpyrrolidinone (NMP)
containing LiCl (1 g) and 1–2 drops of pyridine, another
monomer diacid chloride (8a–c) was added drop-wise.
The reaction mixture was then heated to 80◦C and
stirred for 6 h. After cooling to room temperature, it
was plunged into cold water and the separated precipi-
tate was filtered and washed with ethanol to give cor-
responding polyhydrazide (PH1–PH3). Further, poly-
mers were purified by soxhlet extraction technique
using ethyl acetate and finally it was dried in vacuum
oven at 40◦C.
P3: FTIR (cm−1): 2916, 2852, 1580, 1449, 1264,
1023. 1H NMR (DMSO-d6,δ ppm): 1H NMR (DMSO-
d6,δ ppm): 8.21–7.78 (m, 3H, pyridine), 8.13–7.67(m,
6H, Ar), 5.74 (s, 4H, -OCH2-Ar), 4.05–4.03 (t, 4H, -
OCH2-alkyl), 3.79 (s, 6H, -OCH3), 1.68–1.26 (m, 34H,
aliphatic), 0.87–0.84 (t, 6H, -CH3). Mw: 6560 g/mol,
Mn: 3329 g/mol, PDI: 1.97.
3. Results and discussion
3.1 Characterization of the new monomers
and polymers
PH1: FTIR (cm−1): 3298, 2918, 2851, 1621, 1453,
1
1274, 1021. H NMR (DMSO-d6, δ ppm): 10.94 (s,
1H, -NH), 9.16 (s, 1H, -NH), 7.22–6.98 (m, 6H, Aro-
matic), 5.71 (s, 4H, -OCH2-Ar), 3.91 (s, 6H, -OCH3),
3.37–3.35 (t, 6H, -OCH2,-alkyl), 1.66–1.22 (m, 34H,
aliphatic), 0.83–0.81 (t, 6H, -CH3).
Structures of newly synthesized intermediates,
monomers and final polymers were confirmed by their
1
FTIR, H NMR and 13C NMR spectra, followed by
elemental analysis. Formation of diester (2) from 4-
hydroxy-3-methoxybenzaldehyde (1) was confirmed
PH2: FTIR (cm−1): 3225, 2921, 2852, 1622, 1481,
1
1261, 1024. H NMR (DMSO-d6, δ ppm): 10.65 (s,
1
by its FTIR, H NMR spectral data and elemental
1H, -NH), 9.17 (s, 1H, -NH), 8.59–7.66 (m, 6H, Ar),
7.22–6.83 (m, 2H, Th), 5.72 (s, 4H, -OCH2-Ar), 3.93–
3.90 (t, 4H, -OCH2-alkyl), 3.75 (s, 6H, -OCH3), 1.68–
1.23 (m, 34H, aliphatic), 0.85–0.82 (t, 6H, -CH3).
PH3: FTIR (cm−1): 3199, 2917, 2850, 1612, 1452,
analysis. Its FTIR spectrum showed sharp peaks at
2916 cm−1 and 2849 cm−1 indicating the presence of
alkyl chain, and another sharp peak at 1679 cm−1 that
corresponds to aldehydic carbonyl group. Further, its
1H NMR spectrum displayed a singlet at 9.79 ppm due
to aldehyde proton, multiplet at 7.50–7.11 ppm for aro-
matic protons, and a triplet at 4.03–4.01 ppm for alkoxy
-OCH2. Furthermore, a singlet appeared at 3.79 ppm
corresponds to methoxy group attached to benzene
ring and peaks at 1.73–1.21 as multiplet and 0.83–0.80
as triplet correspond to alkyl chains and –CH3 group.
Structure of (4-decyloxy-3-methoxyphenyl)methanol
1
1250, 1016. H NMR (DMSO-d6, δ ppm): 10.78 (s,
1H, -NH), 9.29 (s, 1H, -NH), 8.10–8.33 (m, 3H, pyri-
dine), 8.01–7.79 (m, 6H, Ar), 5.76 (s, 4H, -OCH2-Ar),
4.04–4.01 (t, 4H, -OCH2-alkyl), 3.78(s, 6H, -OCH3),
1.69–1.26 (m, 34H, aliphatic), 0.86–0.83 (t, 6H, -CH3).
1
2.4b General procedure for the synthesis of polymers
(P1–P3): Polyhydrazide (PH1–PH3, 0.3 g) was dis-
persed in 20 mL of phosphorus oxychloride. The reac-
tion mixture was refluxed for 12 h. After cooling to
room temperature, the reaction mixture was poured into
ice cold water. The precipitate was collected by filtra-
tion and was washed with water, ethanol, followed by
ethyl acetate and finally dried under vacuum at 40◦C.
P1: FTIR (cm−1): 2917, 2850, 1581, 1450, 1271, 1031.
1H NMR (DMSO-d6,δ ppm): 7.49–7.31 (m, 6H, Aro-
matic), 5.70 (s, 4H, -OCH2-Ar), 3.96–3.93 (t, 6H, -
OCH2,-alkyl), 3.86 (s, 6H, -OCH3), 1.66–1.21 (m,
(3) was confirmed by its FTIR, H NMR spectral and
elemental analyses. Its FTIR spectrum showed a broad
peak due to OH group at 3349 cm−1, sharp peaks at
2915 and 2850 cm−1 corresponding to -CH- stretching.
1
The H NMR spectrum showed multiplet for aromatic
protons at 6.87–6.73 ppm, and triplet at 4.99–4.96 ppm
that corresponds to –OH group. Further, it displayed
peaks at 4.37–4.35 ppm appeared as doublet for –CH2
of benzyl group, a singlet at 3.7 ppm for methoxy
attached at meta position, multiplet at 1.67–1.21 ppm
for alkyl protons and triplet at 0.80–0.83 ppm that
corresponds to methyl group at the end of alkyl chain.

34
M S Sunitha et al.
Formation of compound 4-(bromomethyl)-1-ethoxy-2- sence of aliphatic protons of benzyl group attached to
methoxybenzene (4) was confirmed by its FTIR spec- 3,4 positions of the thiophene. The cyclization of poly-
trum, which showed the disappearance of –OH peak hydrazide PH1 to target polymer P1 was established
1
at 3349 cm−1. Further, structure of 6 was confirmed by by FTIR and H NMR spectral data. The disappea-
1
its IR, H NMR, 13C NMR spectral data and elemen- rance of peaks due to amide and carbonyl stretching fre-
tal analysis. Its FTIR spectrum showed sharp peaks quencies and appearance of a new peak at 1581 cm−1
at 2914 and 2848 cm−1 due to -CH- stretching and a due to >C=N group in FTIR spectrum of P1 clearly
sharp peak at 1711 cm−1 which corresponds to car- indicated the formation of 1,3,4-oxadiazole ring. Fur-
1
bonyl group of ester. Its H NMR spectrum displayed ther, 1H NMR spectrum of P1 showed no peaks due to
a multiplet for aromatic protons at 6.97–6.83 ppm, a amide group in the region of 9–10 ppm confirming the
singlet at 5.06 for -OCH2 of substituted benzyl group, cyclization.
a quatert at 4.30–4.25 ppm and triplet at 3.92–3.89
The newly synthesized D–A type polymers are
that corresponds to -CH2- and –CH3 of ester. A singlet soluble in common organic solvents such as chloro-
appeared at 3.65 ppm is attributed to methoxy group form, toluene, and chlorobenzene at room tempera-
attached to benzene. The peaks at 1.70–1.23 as mul- ture. The weight average molecular weight of the poly-
tiplet and 0.86–0.82 as triplet correspond to aliphatic mer was determined by gel permeation chromatogra-
and end methyl group protons. Further, its 13C NMR phy (GPC) against polystyrene standards in THF. The
showed a series of peaks at 159.85, 152.63, 148.73, weight average molecular weight (Mw) and PDI of
148.26, 128.62, 121.05, 119.20, 112.65, 112.53, 75.84, polymers P1–P3 in THF solution was determined to be
68.17, 61.28, 55.33, 31.22, 30.61, 28.93, 28.88, 28.68, 8121 g/mol, 8198 g/mol, 6560 g/mol and 2.13, 1.73 and
28.61, 25.44, 22.01, 13.94, and 13.86 ppm confirming 1.97, respectively.
the presence of carbons of carbonyl, thiophene, benzyl,
The thermogravimetric traces of the polymers P1–
alkoxy and ester groups. The structure of monomer 7 P3 are as shown in figure 1. It revealed that the onset
1
was confirmed by its FTIR, H NMR and elemental decomposition temperature of the polymer under nitro-
analyses. The FTIR spectrum showed sharp peaks gen was 250–300◦C. The initial decrease in the mass of
at 3386 and 3325 cm−1 that correspond to –NH and polymers continuously was attributed to the loss of the
–NH2, and peaks at 2919 and 2857 cm−1 due to alkyl alkoxy side chain and the amount of this weight loss
-CH- stretching. A strong peak appeared at 1664 cm−1 was found to increase with pendant chain length fur-
implied the presence of carbonyl group of carbonyl ther. The second weight loss step took place that corres-
1
hydrazide. Furthermore, H NMR spectrum confirms ponds to the degradation of polymer backbone leaving
the formation of compound 7 from compound 6, its behind a residue. Polymer P1 underwent degradation
spectrum showed broad singlet at 8.80 ppm for -NH2 faster than polymers P2 and P3 due to the presence of
and multiplet at 7.01–6.77 ppm for aromatic protons. vinylene linkage in the polymer back bone.
Oxymethylene protons of benzyloxy group appeared
at 5.14 ppm as singlet and another -NH- appeared as
singlet at 4.54 ppm. Alkoxy –OCH2 appeared as triplet
at 4.30–4.24 ppm and a singlet appeared at 4.54 ppm
for mehoxy group attached to benzyl group. The peaks
at 1.69–1.24 as multiplet and 0.82–0.86 as triplet for
aliphatic and methyl protons.
The precursor polymer, i.e., polyhydrazide PH1,
showed a peak at 3298 cm−1 that corresponds to -NH
of amide and also a sharp peak appeared at 1621 cm−1
that shows the presence of carbonyl group. Further, 1H
NMR of precursor polyhydrazide PH1 displayed peaks
of amide protons at 10.94 ppm and 9.16 ppm and aro-
matic protons resonated as multiplets at 7.49–7.31 ppm.
It also showed a singlet at 5.70 ppm due to the pre-
sence of –OCH2 between attached to thiophene moiety.
A singlet at 3.86 ppm appeared for –OCH3 attached to
bezyl ring, and a triplet at 3.96–3.93 ppm for –OCH2
of alkyl chain appeared. Further, multiplet at 1.66–
1.21 ppm and triplet at 0.84–0.82 ppm showed the pre-
Figure 1. Thermogravimetric traces of P1–P3.

Synthetic route for monomers
35
Table 1. Electrochemical potentials, energy levels and electrochemical band gap of P1–P3.
Polymer Eoxd
Ered
Eoxd (onset) Ered (onset) EHOMO (eV) ELUMO (eV) Eag (eV)
P1
P2
P3
1.48 −0.95
1.37 −0.80
1.50 −0.94
1.35
1.27
1.22
−0.63
−0.64
−0.83
−5.75
−5.67
−5.62
−3.77
−3.76
−3.57
1.98
1.91
2.05
aElectrochemical band gap
3.2 Electrochemical studies
molecular orbital (LUMO). The equations ELUMO
=
ꢀ
ꢁ
ꢀ
ꢁ
Red
Oxd
− E
− 4.4eV and EHOMO = − E
+ 4.4eV
Onset
Onset
Red
Oxd
The electrochemical properties of the polymers were
studied by cyclic voltammetry carried out in 0.1 M
tetrabutylammonium perchlorate (TBAP). The cyclic
voltammogram of polymers coated on a glassy car-
bon electrode was measured on AUTOLAB PGSTAT
30 electrochemical analyzer, using a Pt counter elec-
trode and Ag/AgCl reference electrode at a scan rate
of 25 mV/s. The electrochemically determined LUMO,
HOMO energy levels and calculated band gaps are
given in table 1.
The cyclic voltammograms of polymers P1–P3 dis-
played distinct oxidation and reduction processes as
shown in figures 2, 3 and 4. They showed reduc-
tion peaks at −0.95, −0.80 and −0.94 eV, respec-
tively. These reduction potentials are lower than that
of 2-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), one
of the most widely used electron transporting materi-
als.12,32 In the anodic sweep, polymers P1–P3 displayed
small oxidation peaks at 1.48, 1.37 and 1.50 eV, respec-
tively. The onset oxidation and reduction potentials
were used to estimate energy levels of highest occupied
molecular orbital (HOMO) and the lowest unoccupied
were used for the calculations. Here E
and E
are
Onset
Onset
the onset potentials versus SCE for the oxidation and
reduction processes.33
Generally, band gap of any conjugated polymer is
influenced by the extent of its conjugation, solid-state
ordering, the presence of electron-withdrawing and
electron-donating moieties in the polymer main chain.
Also, many other factors such as the nature of (sol-
ubilizing) side-chains, the conformation of the poly-
mer backbone and its chemical constituents normally
influence their band gap. Consequently, one can tune
the electrochemical behaviour of any conjugated poly-
mer by varying the above properties. As a rule, the
presence of electron-donating and withdrawing groups
causes a partial charge separation along the polymer
back bone and hence it lowers the band gap. It is evi-
dent that the effective conjugation length in a poly-
mer can be easily controlled by incorporating proper
electron-releasing and electron-withdrawing heteroaro-
matic/aromatic moieties while the torsion angle among
the repeating units can be monitored by introducing
the bulky alkoxy side chains which would bring about
twisting the units out of plane.
Figure 2. Cyclic voltammetric waves of P1.
Figure 3. Cyclic voltammetric waves of P2.

36
M S Sunitha et al.
Figure 4. Cyclic voltammetric waves of P3.
Figure 6. UV-vis absorbance spectra of P1–P3.
Figure 5 shows the energy level band diagram of the
HOMO and LUMO levels of polymers P1–P3, PPV,
ITO anode and Al cathode. The HOMO and LUMO
energy levels of polymers P1–P3 were determined to be
−5.75, −5.67, −5.62 eV and −3.77, −3.76, −3.57 eV,
respectively. These results indicate that the HOMO and
LUMO energy levels of polymers and the work func-
tions of ITO (−4.80 eV) and Al (−4.28 eV) with simi-
lar energy barrier (about 0.5 to 0.9 eV) match well.
Thus, the obtained results of electrochemical studies
suggest that polymers P1–P3 are promising candidates
for applications in electroluminescent devices.
P3 appeared at 360, 377 and 356 nm, respectively. The
absorption maxima of P2 has been red shifted compared
to P1 and P3. This is originated due to the presence of
thiophene ring in the polymer P2 which has enhanced
the electron delocalization in the polymer. Further, the
appearance of shoulder peak in polymer P3 at 475 nm
has been attributed to the intramolecular charge-transfer
(ICT) transition between the thiophene and pyridine
moieties,34,35 and its absorption maximum at 350 nm
is due to the π–π* transition. The fluorescence emis-
sion spectra of polymers P1, P2 and P3 in THF showed
emission peaks at 554, 566, and 552 nm, respectively
as shown in figure 7. These data indicate that the poly-
mers emit intense green light when photoexcited. The
3.3 Linear optical properties
The UV-vis absorption spectra of polymers P1–P3 in
dilute THF solution were recorded and their spectra are
shown in figure 6. The absorption measurements were
taken for dilute solutions (10−5 M) of polymers. It has
been observed that absorption maxima of P1, P2 and
Figure 5. Energy level band diagram of P1–P3.
Figure 7. Fluorescence emission spectra of P1–P3.

Synthetic route for monomers
37
Table 2. Absorption maxima, emission maxima, quantum yield of P1–P3.
Polymer
Absorption maxima
Emission maxima
Quantum yield (%)
P1
P2
P3
360
377
356
554
566
552
34
31
29
quantum yield of P1–P3 is evaluated using 0.1 M qui- the open aperture z-scan data, the nonlinear absorption
nine sulphate in H2SO4 as standard. The spectral data coefficient of the material can be calculated.
are summarized in table 2.
In Z-scan set-up, we used a stepper-motor controlled
linear translation stage to move the sample through the
beam in precise steps. The samples were taken in 1 mm
cuvettes. The transmission of the sample at each point
was measured by means of two pyroelectric energy
probes (Rj7620, Laser Probe Inc.). One energy probe
monitors the input energy, while the other monitors the
transmitted energy through the sample. The second har-
monic output (532 nm) of a Q-switched Nd:YAG laser
(Quanta Ray, Spectra Physics) was used for exciting
the molecules. The laser pulse width is 7 nanoseconds.
Laser pulse energy of approximately 190 microjoules
was used for the experiments. The pulses were fired in
the ‘single shot’ mode, allowing sufficient time between
successive pulses to avoid accumulative thermal effects
in the sample.
3.4 Nonlinear optical properties
The Z-scan is a widely used technique developed by
Sheik Bahae et al. to measure the nonlinear absorption
coefficient and nonlinear refractive index of materials.36
The ‘open aperture’ Z -scan gives information about the
nonlinear absorption coefficient. Here, a Gaussian laser
beam is used for molecular excitation, and its propa-
gation direction is taken as the z-axis. The beam is
focused using a convex lens, and the focal point is taken
as z = 0. Obviously, the beam will have maximum
energy density at the focus, which will symmetrically
reduce towards either side of it, for the positive and
negative values of z. The experiment is done by plac-
ing the sample in the beam at different positions with
respect to the focus (different values of z), and mea-
suring the corresponding transmission. For a focused
The nonlinear transmission behaviour of the present
samples can therefore be modelled by defining an effec-
tive nonlinear absorption coefficient α(I), given by the
equation:
Gaussian beam, each z position corresponds to an input
√
laser energy density of F(z) = 4 ln 2Ein/π3/2ω(z)2,
and intensity of I(z) = F(z)/τ, where Ein is the input
laser pulse energy, ω(z) is the beam radius, and τ is
α0
ꢂ ꢃ ꢄ
α (I) =
+ βI,
(1)
1 + I Is
the laser pulse width. Thus the sample sees different where α0 is the unsaturated linear absorption coeffi-
laser intensity at each position, and hence, the mea- cient at the wavelength of excitation, I is the input
sured position-dependent transmission gives informa- laser intensity and is the saturation intensity (intensity
tion about its intensity-dependent transmission. From at which the linear absorption drops to half its original
Figure 8. Z-scan and fluence curves of P1.

38
M S Sunitha et al.
Figure 9. Z-scan and fluence curves of P2.
value). βI = σ N is the excited state absorption (ESA) where T (z) is the sample transmission at position z,
coefficient, where σ is the ESA cross section and N(I) where I0 is the peak intensity at the focal point, L =
ꢀ
ꢁꢃ
is the intensity-dependent excited state population den-
sity. For calculating the transmitted intensity for a given
input intensity, the propagation equation,
1 − exp (−αl) α, where l is the sample length and α
ꢃ
is the linear absorption coefficient, and z0 = πω02 λ is
the Rayleigh range, where ω0 is the beam waist radius
at focus and λ is the light wavelength, and β is the
effective TPA coefficient.
ꢀ
ꢁ
dI
dzꢁ
= − {α0/(1 + I/Is)} + βI I,
(2)
The numerically calculated values of the effective
TPA coefficient are found to be 2.9 × 10−11, 8.0 ×
10−11 and 1.4 × 10−11 m/W for P1–P3, respectively.
These observed values are comparable with those of
good NLO materials. Under similar excitation condi-
tions, Cu nanocomposite glasses were shown to possess
effective TPA coefficient values of the order 10−10 to
10−12 m/W,37 while bismuth nanorods and CdS quan-
tum dots were shown to have 5.3 × 10−11 m/W, 1.9 ×
10−9 m/W, respectively.38,39 From the results, it is evi-
dent that P1–P3 are potential candidates for optical
limiting devices.
The observed nonlinear behaviour of the polymers
can be explained based on their structure. The alter-
nate D–A arrangement in these polymers gives rise
to high π-electron density along the polymeric chain
and are easily polarizable which in turn results in
enhanced delocalization of the electrons in the polymer
was numerically solved. Here zꢁ indicates the propaga-
tion distance within the sample. Figures 8, 9 and 10
show fluence open aperture and Z-scan curves obtained
from the samples P1, P2 and P3, respectively. Numeri-
cally, a two-photon absorption (TPA) type process is
found to give the best fit to the measured Z-scan data.
Samples P1–P3 have a linear absorption of about 55,
68 and 50%, respectively at the excitation wavelength
when taken in the 1 mm cuvette. Therefore, strong two-
step excited state absorption would happen along with
genuine TPA in the present case. The net effect is then
known as an ‘effective’ TPA process. The data obtained
are fitted to the nonlinear transmission equation (3) for
a two-photon absorption process.
ꢅ ꢆ
ꢀ
ꢂ
ꢄꢁ
+∞
T (z) = 1 π1 2q(z) ∫−∞ ln 1 + q(z) exp −τ2 dτ,
/
(3)
Figure 10. Z-scan and fluence curves of P3.

Synthetic route for monomers
39
backbone. In the new polymers 3,4-bis(4-(decyloxy)- Acknowledgements
3-methoxybenzyloxy)thiophene acts as electron donor
Authors are grateful to Dr. Reji Philip, Raman Research
Institute (RRI), Bangalore for providing NLO analy-
sis and also thankful to the Indian Institute of Science
(IISc), Bangalore for extending NMR facility.
moiety, whereas 1,3,4-oxadiazole behaves as elec-
tron acceptor group. In polymer P1, the presence
of benzene ring which functions as spacer as well
as electron-rich group enhances the TPA coeffi-
cient when compared to P3. The higher TPA coef-
ficient value of P2 than that of P1 and P3 is
mainly ascribed to the presence of thiophene ring
in between electron-donating 3,4-bis(4-(decyloxy)-3-
methoxybenzyloxy)thiophene and electron-accepting
1,3,4-oxadiazole rings. As expected, the presence of
less aromatic thiophene ring as spacer group in poly-
mer P2 has enhanced the electron delocalization in
the main chain and also offers more effective π-
conjugation between donor and acceptor groups, in
turn resulting in larger nonlinearities. However, in
polymer P3 the TPA coefficient value is quite less,
which is mainly due to the presence of pyridine ring
next to the electron withdrawing 1,3,4-oxadiazole moi-
ety. In P1–P3, the presence of 3,4-bis(4-(decyloxy)-3-
methoxybenzyloxy)thiophene group facilitates the solu-
bility of the polymers due to the presence of long
alkyl chain, while 1,3,4-oxadiazole ring imparts rigid-
ity to the polymer chains. The spectra of intermediates,
monomers and polymers are given in the supplementary
information.
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