D-A conjugated polymers containing substituted thiophene, 1,3,4-oxadiazole and non-conjugation linkers: Synthesis and study of optical and electrochemical properties

Article abstract of DOI:10.1007/s12039-016-1145-6

In this communication, we report synthesis and characterization of new D-A conjugated polymers (P1-P3) consisting of electron-donating (D) 3,4-didodecyloxythiophene, electron-accepting (A) 1,3,4-oxadiazole unit and non-conjugation linkers. The conjugated

Full text of DOI:10.1007/s12039-016-1145-6

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1145-6
D-A conjugated polymers containing substituted thiophene,
1,3,4-oxadiazole and non-conjugation linkers: Synthesis and study
of optical and electrochemical properties
PRASHANTH KUMAR K Ra, UDAYAKUMAR Da,∗, SIJI NARENDRAN N Kb,
CHANDRASEKHARAN Kb and RITU SRIVASTAVAc
aDepartment of Chemistry, National Institute of Technology Karnataka, Surathkal,
P.O. Srinivasnagar 575 025 India
bDepartment of Physics, National Institute of Technology Calicut, Kerala, 673 601 India
cPhysics of Energy Harvesting, National Physical Laboratory, New Delhi 110 012 India
e-mail: udayaravi80@gmail.com; udayakumar@nitk.ac.in
MS received 6 September 2014; revised 9 November 2014; accepted 16 November 2015
Abstract. In this communication, we report synthesis and characterization of new D-A conjugated poly-
mers (P1-P3) consisting of electron-donating (D) 3,4-didodecyloxythiophene, electron-accepting (A) 1,3,4-
oxadiazole unit and non-conjugation linkers. The conjugated segment in P1-P3 contains only five aromatic rings
resulting in short conjugation length, but has an alternate D-A arrangement which significantly enhances the
intramolecular charge transfer (ICT) interaction within the segment. As a result, these polymers exhibited low
optical band gap in the range 2.51–2.76 eV. Fluorescence emission studies revealed that the polymer thin films
emit intense blue light with emission maxima in the wavelength rage 430–480 nm. All three polymers undergo
both oxidation and reduction processes under electrochemical conditions. Further, these polymers (P1–P3)
exhibit low-lying HOMO and LUMO levels as a result of D-A structure of the conjugated segment. Polymer
light-emitting devices were fabricated using these polymers as emissive layer with a device configuration of
ITO/MoO3/polymer/LiF/Al. The test device based on P2 emitted blue light with a low threshold voltage of 5
V. Z-scan studies reveal that the polymers exhibit a strong optical limiting behavior. The value of the nonlinear
absorption coefficient (β) of polymers is of the order 10−11m/W which indicates that these materials may be
accomplished for fabricating optical limiters.
Keywords. Conjugated polymers; conjugation confinement; blue light-emitting polymers; cyclic voltamme-
try; nonlinear optics.
1. Introduction
optoelectronic devices, particularly polymer light emit-
ting diodes (PLEDs) and polymer solar cells (PSCs).6
Furthermore, the increase in effective π-electron delo-
calization in D−A polymers results in large third-order
A great deal of research work is going on in the field of
conjugated polymers due to their fascinating optoelec-
tronic properties.1 3 Conjugated polymers containing nonlinear optical susceptibilities, and hence these poly-
mers are considered also as potential candidates in the
area of nonlinear optics.7,8 Currently, much attention is
devoted towards the development of blue light-emitting
polymers, because blue light can be converted to green
or red using proper dyes, which means a blue light-
emitting device alone is capable of generating all col-
ors. It is known that the optical absorption spectra and
the emission color of conjugated polymers are directly
connected with their band gap energy. In this regard,
D-A conjugated polymers with extended conjugation
and low band gap energy absorb light at longer wave-
lengths (in the UV-Vis region) and hence emit light gen-
erally in green to red region, whereas blue emission can
be expected only in conjugated molecules or oligomers
with shorter conjugation length. But these molecules/
carbon and heteroatom frameworks with alternate donor-
acceptor (D-A) architecture along the polymer chain
are of great importance for directing material proper-
ties towards the desired optoelectronic applications.4 In
D-A conjugated systems, interaction between a strong
electron donor (D) and strong electron acceptor (A)
units gives rise to a low band gap energy.5 In addition,
energy levels of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) in these systems could be adjusted as desired
by judicious selection of donor and acceptor units.
Because of this reason, D−A conjugated polymers are
emerging as promising materials for the fabrication of
∗For correspondence

Prashanth Kumar K R et al.
oligomers usually exhibit poor processability and long- structural units incorporated in a variety of conjugated
term resistance towards crystallization, phase separation polymers. The cyanovinylene linkage, when present in
and other morphological degradations. One possible conjugation (i.e., when it is part of the conjugated sys-
approach to overcome these problems is to develop tem), generally extends the absorption wavelength of
polymers containing short conjugated segments inter- these systems towards red region. Whereas in polymer
connected through nonconjugated linkers. Such poly- P3, cyanophenylenevinylene unit is not a part of the
mers are expected to retain the electronic properties of conjugated system but is linked through the noncon-
the conjugated segment and possess the desired pro- jugated linker. The length of the conjugated segment
cessability and morphological stability associated with in P1-P3 is markedly short which may be comparable
higher molecular weight polymers.
with the length of a single repeating unit of fully con-
In our previous work, we observed that D-A conju- jugated polymers (Figure 1). But it is important to note
gated polymers containing 3,4-dialkoxythiophene, 1,3, the alternate arrangement of donor and acceptor units
4-oxadiazole and other conjugated linkers emit bluish in the conjugated segment which significantly enhances
green or green light.9 In conjugated polymers, based the intramolecular charge transfer (ICT) interaction thus
on the experimental work with oligomeric models, it leading to an extended absorption or low band gap.
can be assumed that, the effective conjugation length of Hence, we anticipated that this D-A structure with short
the polymer is not larger than 7–10 units.10 Based on conjugated length would facilitate blue emission in
this fact and from the results obtained in our previous these polymers (P1-P3).
work, we thought to interrupt the π-conjugation in D-A
conjugated polymers containing 3,4-dialkoxythiophene
2. Experimental
and 1,3,4-oxadiazole units by introduction of noncon-
jugation linkers expecting that the confinement of the
π-conjugation would shift their emission wavelength to
blue region. Though a few reports are available in the
literature on conjugated polymers containing alternat-
ing π-conjugated chromophore and saturated aliphatic
soft block,11 16 reports on D-A conjugated polymers
containing nonconjugated linker are limited.17,18 In
view of these facts, three polymers (P1-P3) with well-
defined short conjugated length were synthesized and
characterized in the present study. The conjugated seg-
ment in P1-P3 consists of 3,4-didodecyloxythiophene
unit as the electron donor (D) moiety and 1,3,4-
oxadiazole as the electron acceptor (A) unit. The length
of the conjugated segment is the same in polymers P1
and P3 while it is slightly longer in polymer P2 as it
contains additional phenylenevinylene units. The con-
jugated segments are connected through different non-
conjugated linkers and the chemical structure of the
repeating unit in P1-P3 is as shown in Figure 1. In P3,
the repeating unit contains also cyanophenylenevinylene
units. Cyanophenylenevinylene is one of the important
2.1 Materials and methods
All solvents and other reagents were purchased com-
mercially and used without further purification. 2,2ꢁ-
(3,4-Didodecyloxythiophene-2,5-diyl)bis[5-((4-triphe-
nylphosphonion methyl) phenyl)-1,3,4-oxadiazole]di-
bromide (2) was synthesized according to the reported
procedure9 starting from thiodiglycolic acid. 4,4ꢁ-
[Hexane-1,6-diylbis(oxy)]dibenzaldehyde (4) was syn-
thesized following the procedure reported by Yuan et al.13
2.2 Instrumentation
A Stuart SMP3 melting point apparatus was used for the
determination of melting point of the synthesized com-
pounds. 1H NMR spectra were obtained with an AMX
400 MHz NMR spectrometer using TMS as internal ref-
erence. Infrared spectra of all intermediate compounds,
monomers and polymers were recorded on a Nico-
let Avatar 330 FTIR (Thermo Electron Corporation).
Donor
H
C
O
OC
N
H
25 12
12 25
Polymer with
N
N
N
Nonconjugated
linker
shorter conjugation
length for blue
emission
S
O
O
n
Acceptor
Conjugated
Segment
Figure 1. Schematic representation of the arrangement of D and A units in conjuga-
tion and the non-conjugated linker in polymers P1–P3.

D-A conjugated polymers containing substituted thiophene
Molecular weight of the polymers was determined defined per substrate and were used to assess the repro-
using an Agilent 1200 gel permeation chromatography ducibility of the device performance. The fabricated
system. Thermo gravimetric analysis was carried out devices were finally annealed at 100◦C in vacuum for
using an EXSTAR TG/DTA 7000 (SII Nanotechnology 5 min before being characterized. Characterization of
Inc.) thermal analyzer. Differential scanning calorime- the light-emitting device was carried out at room tem-
try studies were performed using a SHIMADZU DSC- perature under ambient conditions without protective
60 thermal analyser. Absorption spectra of polymers encapsulation. I-V characteristics of these devices were
were recorded using a CINTRA - 101 (GBC scientific measured using a Keithley 2400 source meter.
equipment) UV–Vis spectrophotometer. Fluorescence
spectra were recorded using a JASCO F P2200 spec-
trofluorimeter. The electrochemical studies of the poly-
2.4 Z-scan measurements
The Z-scan technique19 was developed to simultane-
ously measure the magnitude of both the nonlinear
refraction (NLR) and the nonlinear absorption (NLA).
In the “open aperture” Z-scan, which provides informa-
tion about the nonlinear absorption coefficient, a laser
beam is used for molecular excitation and its propaga-
tion direction is taken as the z-axis. The beam is focused
using a convex lens and the focal point is taken as z =
0. The beam has maximum energy density at the focus,
and the energy density drops symmetrically on either
side of the focus (i.e., for positive and negative values of
z). In the experiment, the sample is placed in the beam
at different positions with respect to the focus (different
values of z), and the corresponding transmission is mea-
sured. The sample sees different laser intensity at each
position and therefore the position-dependent transmis-
sion will give information on its intensity-dependent
transmission as well. The nonlinear absorption coeffi-
cient can be calculated from this nonlinear transmission
information. The sample was placed in a 1 mm cuvette.
The transmission of the sample at each point was mea-
sured using two pyroelectric energy probes (Rj7620,
Laser Probe Inc). One energy probe monitored the input
energy while the other monitored the energy transmit-
ted through the sample. The second harmonic output
(532 nm) of a Q-switched Nd:YAG laser (Quanta Ray,
Spectra Physics) was used to excite the molecules. The
temporal width (FWHM) of the laser pulses was 7 ns.
Laser pulse energy of 25 μJ was used for the experi-
ments. The pulses were fired in the ‘single shot’ mode,
allowing sufficient time between successive pulses to
avoid accumulative thermal effects in the sample.
mers were carried out using an AUTOLAB PGSTAT 30
electrochemical analyzer. The cyclic voltammograms
were recorded using a three-electrode cell system, with
a glassy carbon disk as working electrode, a platinum
wire as counter electrode and Ag/AgCl electrode as the
reference electrode. The polymer solution in chloro-
form was placed on the glassy carbon electrode and the
solvent was evaporated to get the polymer coated work-
ing electrode. The electrolyte used in the experiment is
tetraethylammoniumperchlorate (0.1 M solution in ace-
tonitrile) and measurements were made at a scan rate
of 25 mV s−1. All measurements were calibrated using
ferrocene/ferrocenium (Fc/Fc+) standard (EFC = 0.53
vs. Ag/AgCl).
2.3 Fabrication of polymer light-emitting devices
The polymer light emitting diodes were fabricated with
device configuration as ITO/MoO3 (5 nm)/Polymer
(100 nm)/LiF(1nm)/Al. ITO(Indium Tin Oxide) coated
glass substrates (procured from Vin Karola, USA) hav-
ing sheet resistance ∼20 ꢀ/sq were patterned 2-mm-
wide ITO lines using laser ablation. Sequentially the
patterned ITO substrates were cleaned by ultasonica-
tion for 15 min each in acetone, trichloroethylene and
iso-propanol solvents and finally dried in vacuum oven.
The cleaned ITO substrates were exposed to oxygen
plasma for 5 min to enhance the work function. There-
after, a thin film (thickness ∼5 nm) of MoO3 was
deposited by thermal deposition onto the ITO sub-
strate. This was followed by spin casting of the emit-
ting layer (P1-P3) (with a thickness of about 100 nm)
above the MoO3 layer at a speed of 2000 rpm from
chlorobenzene solution (10 mg/mL) filtered through a
0.45 μm Teflon filter, followed by vacuum annealing at
120oC for ∼2 h in order to remove the organic fraction.
Finally, the coated ITO glass plate was transferred to
a deposition chamber, where LiF and Al cathode lay-
ers (with thickness of about 50 and 150 nm, respec-
tively) were vacuum deposited on the polymer layer
2.5 Synthesis
2.5a Synthesis of monomer M and compounds 1 and
2: These compounds were prepared following the
reported procedure.9
by thermal evaporation at a pressure of 1 × 10−6 Torr. 5,5ꢁ-(3,4-bis(dodecyloxy)thiophene-2,5-diyl)bis(2-p-
Four pixels, each of active area of 4 × 4 mm2 are tolyl-1,3,4-oxadiazole) (M): 1H NMR (400 MHz,

Prashanth Kumar K R et al.
CDCl3) δ: 8.01 (d, 4H, Ar), 7.34 (d, 4H, Ar), 4.31 (t, 1231, 1055, 951, 808, 714. Element. Anal. Calcd. For
4H, -OCH2), 2.45 (s, 6H, Ar-CH3), 1.91-1.25 (m, 40H, C6H72N4O8S (%): Calcd. C 71.41, H 7.19, N 5.55, O
-(CH2)10-), 0.87 (t, 6H). FTIR (cm−1): 2914 and 2848
(-C-H), 1556 (C=N), 1481, 1464, 1350, 1051, 958.
Element. Anal. Calcd. (%) For C46H64N4O4S: Calcd. C
71.83, H 8.39, N 7.29, S 4.16; Found C 71.85, H 8.36,
N 7.30, S 4.18.
12.68, S 3.17. Found: C 71.45, H 7.14, N 5.50, O 12.62,
S 3.11.
2.6 Synthesis of polymers
2.6a Synthesis of polymer P1: To a solution of
compound 1 (1 g, 1.1 mmol) in DMF (10 mL), 4-
hydroxyphenol (0.13 g, 1.1 mmol) and K2CO3 (0.76
g, 6 mmol) were added and the resulting mixture was
heated at 90◦C for about 9 h under inert atmosphere.
Completion of the reaction was confirmed by TLC, then
the reaction mass was poured into a beaker containing
distilled water. A few drops of concentrated HCl were
added under stirring to get a precipitate. The obtained
precipitate was filtered, washed with excess of water
and then dried under vacuum to get polymer P1. Yield:
2,2ꢁ-(3,4-didodecyloxythiophene-2,5-diyl)bis[5-(4-bro-
momethylphenyl)-1,3,4-oxadiazole] (1): 1H NMR
(400 MHz, CDCl3), δ (ppm): 8.15 (d, 4H, Ar), 7.54
(d, 4H, Ar), 4.59 (s, 4H, Ar–CH2–Br), 4.28 (t, 4H,
–OCH2–), 1.88 (m, 4H, –O–C–CH2–) 1.56-1.25 (m,
36H, (–CH2)9–), 0.87 (t, 6H, –CH3). FTIR (cm−1):
2925, 2841(-C-H), 1589 (>C=N), 1512, 1452, 1359,
1215, 1055, 959, 811, 729. Element. Anal. Calcd. (%)
For C46H62Br2N4O4S: Calcd. C 59.59, H 6.75, N 6.05,
S 3.45; Found C 59.56, H 6.74, N 6.10, S 3.42.
1
80%. Orange powder. M.p.: >300◦C, H NMR (400
2,2ꢁ-(3,4-didodecyloxythiophene-2,5-diyl)bis[5-((4-tri-
phenylphosphonion methyl)phenyl)-1,3,4-oxadiazole]
dibromide (2): 1H NMR (400 MHz, CDCl3), δ (ppm):
7.89–7.23 (m, 38H, Ar), 6.1 (s, 4H, Ar–CH2–), 4.31-
4.28 (t, 4H, –OCH2–), 1.88-1.25 (m, 40H, (–CH2)10–),
0.88 (t, 6H, –CH3). FTIR (cm−1): 2920, 2845,
1585 (>C=N), 1486, 1429, 1366, 1101, 1056,
855, 712, 689, 509. Element. Anal. Calcd. (%) For
C82H92Br2N4O4P2S: Calcd. C 67.84, H 6.39, N 3.86, S
2.20; Found C 67.
MHz,CDCl3), δ (ppm): 6.9–8.2 (m, 12H, Ar), 5.11(s,
4H, -Ar-CH2-O-), 4.31 (t, 4H, -CH2-O-), 1.24–1.88 (m,
40H, (-CH2)10), 0.86 (t, 6H, -CH3). FTIR (cm−1): 2924,
2852 (–C–H), 2372, 1585 (–C=N–), 1497, 1377, 1278,
1230, 1049, 960, 825, 731.
2.6b Synthesis of polymer P2: In a round bottom
flask, a mixture of compound 2 (1.4 g, 1 mmol), com-
pound 4 (1.1 g, 1 mmol), ethanol (15 mL) and chlo-
roform (5 mL) was stirred under nitrogen atmosphere.
To the resultant clear solution, sodium ethoxide (0.33 g,
4.8 mmol) in ethanol (10 mL) was added dropwise at
room temperature. After stirring for 10 h, ethanol (10
mL) was added to the resulting solution and stirred for
1 h. The obtained polymer P2 was filtered and washed
well with ethanol followed by ethyl acetate and dried
under vacuum. Yield: 55%. Bright yellow solid. M.p.:
2.3.2
4,4ꢁ-[Hexane-1,6-diylbis(oxy)]dibenzaldehyde
1
(4)11: White solid. H NMR (400 MHz, CDCl3), δ
(ppm): 9.9 (s, 2H, –CHO), 7.82 (d, 4H, Ar), 7.00 (d,
4H, Ar), 4.12 (t, 4H, –O–CH2– ), 1.8–1.6 (m, 8H,
(–CH2)4–). FTIR (cm−1): 1686 (>C=O), 2945, 2847
(alkyl C-H), 2847, 1007, 800, 638, 513.
1
>3000C, H NMR (400 MHz,CDCl3), δ (ppm): 7.8–
8.1 (m, 8H, Ar), 7.6 (d, 4H, -Ar-CH=CH-Ar-), 6.6-7.5
(m, 8H, Ar), 4.31 (t, 4H, -CH2-O-), 4.0 (t, 4H, Ph-O-
CH2-), 1.24-1.86 (m, 48H, (-CH2)10 and (-CH2)4), 0.86
(t, 6H, -CH3). FTIR (cm−1): 2922, 2851 (–C–H), 1691
(–C=N–), 1490, 1379, 1250, 1175, 1038, 959, 833,
675.
2.5b Synthesis of monomer 3: A mixture of com-
pound 1 (2 g, 2 mmol), 4-hydroxy benzaldehyde (0.5
g, 4.7 mmol), K2CO3 (2 g, 1.4 mmol) and tetra-n-butyl
ammonium bromide (0.2 g, 0.62 mmol) in DMF (20
mL) was heated at 90◦C for 8 h and progress of the reac-
tion was monitored by TLC. After the completion of the
reaction, the mixture was poured into water under stir-
ring to obtain a precipitate. The precipitate was filtered, 2.6c Synthesis of polymer P3: In a round bottom
washed with methanol and dried. Yield: 80%, Pale yel- flask, a mixture of compound 3 (1.5 g, 1.5 mmol)
1
low solid. M.p.: 156◦C. H NMR (400 MHz,CDCl3), and 1,4-phenylenediacetonitrile (0.25 g, 1.6 mmol) was
δ (ppm): 9.9 (S, Ar–CHO), 8.2 (d, 4H, Ar), 7.9 (d, taken in a solvent mixture of ethanol (15 mL) and chlo-
4H, Ar), 7.6 (d, 4H, Ar), 7.1 (d, 4H, Ar), 5.25 (s, roform (5 mL). To this clear solution, sodium ethox-
4H, -O-CH2-Ar), 4.32 (t, 4H, -O-CH2), 1.25 - 1.9 (m, ide (0.16 g, 7 mmol) in ethanol (10 mL) was added
40H, (-CH2)10), 0.88 (t, 6H, -CH3). FTIR (cm−1): 1693 drop wise with stirring at room temperature under nitro-
(C=O), 2922, 2852, 1583 (>C=N), 1500, 1451, 1340, gen atmosphere and the reaction mixture was stirred for

D-A conjugated polymers containing substituted thiophene
20 h. Then, ethanol (30 mL) was added to the reaction cyloxythiophene-2,5-diyl)bis[5-((4-triphenylphospho-
mass, the solid separated was filtered and washed with nionmethyl)phenyl)-1,3,4-oxadiazole]dibromide (2). Al-
ethyl acetate. The solid compound thus obtained was kylation reaction was carried out between compound
taken in a RB flask, ethyl acetate (10 mL) was added 1 and 4-hydroxy benzaldehyde to yield monomer 3.
and the contents were stirred for about 20 h followed Polymerization reaction was carried out between com
by sonication for about 30 min. The reaction mass was pound 1 and 4-hydroxyphenol to yield polymer P1.
then filtered and washed well with ethyl acetate. The Wittig reaction was performed between compound 2
solid compound was dried to get the polymer as yellow and 4,4ꢁ-[hexane-1,6-diylbis(oxy)]dibenzaldehyde (4)
solid. Yield: 85%, M.p.: >300◦C, 1H NMR (400 MHz, to obtain polymer P2 whereas polymer P3 was syn-
CDCl3), δ (ppm): 7.1-8.2 (m, 22H, Ar and -C=CH-), thesized by a reaction between dialdehyde 3 with 1,4-
5.25 (s, 4H, -Ar-CH2-O-), 4.24 (t, 4H, –OCH2–), 1.2– phenylenediacetonitrile in presence of sodium ethoxide
1.92 (m, 40H, (–CH2)10–), 0.88 - 0.91 (t, 6H, –CH3). using a Knoevenagel condensation methodology.
FTIR (cm−1): 2924, 2853 (–C–H), 1655 (–C=N–),
2212 (–CN), 1597, 1423, 1379, 1255, 1178, 1018, 958,
829, 611.
3.2 Structural characterization
The structure of synthesized compounds and the poly-
1
mers was confirmed by their spectral analysis. The H
3. Results and Discussion
NMR spectrum of compound 3 showed singlets at δ
4.95 and δ 9.9 ppm due to methylenoxy (Ar-CH2-O-
Ar) and aldehyde protons respectively. In addition, four
3.1 Synthesis of monomers and polymers
Synthetic routes of monomers and polymers are out- distinct doublets appeared in the aromatic region. The
lined in Figure 2. 2,2ꢁ-(3,4-didodecyloxythiophene-2,5- peaks at δ 8.2 and 7.9 ppm correspond to ortho and
diyl)bis[5-(4-bromomethylphenyl)-1,3,4-oxadiazole] (1) meta protons (ortho and meta positions with respect
was treated with triphenyl phosphine in the presence of to -CHO group), respectively, of the terminal phenyl
DMF to get the required compound, 2,2ꢁ-(3,4-didode- ring. Whereas, the doublets at δ 7.6 and 7.1 ppm are
OC
N
H
H
C
O
H
C
O
OC
N
H
12 25
25 12
25 12
12 25
PPh /DMF
3
N
N
N
N
N
N
-
+
+
-
S
S
BrPh
P
O
O
Br
105oC
O
O
PPh Br
3
Br
3
1
2
H
C
O
OC
N
H
H
O
25 12
12 25
N
N
N
TBAB
K2CO3, DMF, 90oC
OHC
O
O
CHO
1
+
S
O
O
3
O
Δ
O
K2CO3, DMF, 90oC
O
O
O
H
O
4
O
1,6-Dibromohexane
OC
H
H
C
O
12 25
N N
25 12
K2CO3, DMF, 90oC
N
N
H
H
O
O
1
+
S
O
O
O
O
P1
n
OC
N
H
H
C
O
12 25
N
25 12
N
N
2 C H OH/C H ONa
2
5
2
5
4
+
S
O
O
CHCl , RT
3
O
O
P2
S
n
CN
H
C
O
OC
N
H
25 12
12 25
N
CH2CN
N
N
NC
C H OH/C H ONa
2
5
2 5
O
O
O
O
3
+
n
CHCl , RT
3
P3
CH CN
2
Figure 2. Synthesis of intermediate compounds and polymers (P1, P2 and P3).

Prashanth Kumar K R et al.
due to ortho and meta protons of the phenyl ring con- eluent against polystyrene standards. The number aver-
nected to 1,3,4-oxadiazole unit. Further, FTIR spectrum age molecular weight (Mn) of the polymers was found
of compound 3 showed a peak at 1693 cm−1 which to be 6717 for P1, 21199 for P2 and 24877 for P3 with
corresponds to the -C= O stretching of aldehyde group. polydispersity (PD) of 1.62, 1.03 and 1.05 for P1, P2
The successful reaction between Wittig salt (2) and 4- and P3, respectively.
hydroxy benzaldehyde with the formation of polymer
P1 was evident by the absence of a singlet δ 4.54 ppm
3.3 Thermogravimetric analysis
(due to CH2-PPh3 group) and appearance of a singlet
at δ 5.11 ppm (due to -Ar-CH2-O-Ar-) in the 1H NMR
Thermogravimetric analysis (TGA) curves of polymers
P1, P2 and P3 are as shown in Figure 3. The analysis
spectrum of P1. The 1H NMR spectrum of polymer P2
displayed a peak at δ 7.6 ppm due to protons of alkene
was carried out under nitrogen atmosphere at a heat-
ing rate of 5◦C/min. These polymers exhibit good ther-
mal stability with onset decomposition temperature of
324◦C, 204◦C and 330◦C. respectively for P1, P2 and
P3. Among three polymers, P2 showed lower onset
decomposition temperature because of the presence
of alkoxy (hexyloxy) group along the polymer chain
which decomposes at a lower temperature as compared
to the methyleneoxy phenyl group present in other two
polymers. The initial weight loss to about 5% below
110◦C in case of polymer P1 could be due to the
loss of trapped moisture or solvent or trace amount of
monomer impurity. Differential scanning calorimetry
(DSC) studies were performed to observe glass tran-
sition temperature (Tg) of the polymers. The samples
were heated up to 320◦C under nitrogen atmosphere at
a heating rate of 5◦C/min. No Tg or melting point was
observed suggesting that the polymers are either having
very high Tg or are highly crystalline in nature and
decompose before melting.
group thus confirming the successful Wittig conden-
sation between compounds 2 and 4. Similarly, the H
1
NMR spectrum of polymer P3 showed a peak at δ 7.9
ppm due to the cyano substituted alkene group along
with other characteristic peaks. The spectral characteri-
zation data of the intermediate compound and polymers
are given in the experimental part. All three polymers
(P1, P2 and P3) are soluble in common organic solvents
like chloroform, methylene dichloride, tetrahydrofuran,
chlorobenzene, DMF, DMSO, etc., at room tempera-
ture, which could be due to the presence of long dode-
cyloxy groups at 3- and 4- positions of the thiophene
ring. Molecular weight of the polymers was deter-
mined by gel permeation chromatography using THF as
100
P1
P2
P3
80
3.4 Linear optical properties
60
40
20
0
The UV–Vis absorption and fluorescence emission
spectral studies were utilized to investigate photophys-
ical properties of P1, P2 and P3 in dilute CHCl3 solu-
tion. Table 1 lists out the optical absorption and emis-
sion features for the polymers. This set of data allows
for establishment of clear structure-property relation-
ships which typically depend on conjugated π-systems
present in thse polymers. The absorption spectra of
polymers in solution (Figure 4a) show maxima at 360,
430 and 380 nm for P1, P2 and P3. respectively, which
0
200
400
600
800
Temperature (oC)
Figure 3. TGA graphs of P1, P2 and P3.
Table 1. Linear optical characterization data of monomer M and polymers P1–P3.
Absorption maxima Emission maxima
Absorption edge
film (nm)
Optical band
gap Eogpt (eV)
Polymer
solution (nm) film (nm)
solution (nm)
film (nm)
P1
P2
P3
M
360
430
380
355
365
437
387
–
425
460
454
422
434
479
465
–
454
494
449
–
2.73
2.51
2.76
–

D-A conjugated polymers containing substituted thiophene
signify π → π∗ transition in the polymers. The corres- P2 which increases the extent of π-conjugation. Also,
ponding spectra of polymer films (Figure 4b) did not these vinylene linkages serve to planarize the polymer
show appreciable shift in the absorption maxima which backbone by overcoming torsional interactions between
implies that there is no much difference in the confor- rings which further extends the π-conjugation in the
mations of the polymer in the two states. Fully conjuga- polymer. The polymers showed little absorption in the
ted systems generally show a red shift in the absorption longer wavelength region (500–800 nm) which may be
maxima in the film state when compared with that of the due to the formation of polymer aggregates.20 The opti-
solution state due to increased extent of the π-stacking cal band gap of these polymers was found to be in the
in the film state. Such an effect is limited in P1-P3 range 2.51–2.76 eV. The band gap of these polymers is
because the interruption of the conjugation along the slightly higher when compared with those of fully con-
polymer chain by non-conjugate linkers decreases the in- jugated polymers which could be attributed to the inter-
terchain interactions in these polymers. Further, the ruption of conjugation by non-conjugated linkers. The
long alkoxy pendants on the thiophene ring may also fluorescence emission spectra of polymers P1, P2 and
push apart any two adjacent polymer chains thus P3 in dilute CHCl3 solution and in film form are shown
decreasing the extent of the π-stacking. A red shift in in Figure 5, and the corresponding emission maxima
the absorption maximum was observed for P2 when com- are tabulated in Table 1. These polymers (P1, P2 and
pared with those of P1 and P3. This could be due to the P3) emit intense bluish color. The blue shift of the spec-
presence of additional conjugated units (phenylvinyl) in tra is a result of the conjugation confinement by the
1.0
500
P1
P2
P3
P1
P2
P3
0.8
0.6
0.4
0.2
0.0
400
300
200
100
0
300
400
500
600
700
800
400
450
500
550
600
650
700
Wavelength (nm)
(a)
Wavelength (nm)
(a)
500
400
300
200
100
0
1.0
0.8
0.6
0.4
0.2
0.0
P1
P2
P3
P1
P2
P3
200
300
400
500
600
700
800
300
400
500
600
700
800
(b)
Wavelength (nm)
Wavelength (nm)
(b)
Figure 4. UV-vis absorption spectra of (a) polymer solu- Figure 5. Fluorescence emission spectra of (a) polymer
tions in dilute CHCl3 solution and; (b) polymer films. solutions in dilute CHCl3 solution and; (b) polymer films.

Prashanth Kumar K R et al.
introduction of non-conjugated linkers. The confine- molecular orbital (HOMO) and lowest unoccupied mol-
ment of the effective conjugation does not allow the ecular orbital (LUMO) energy levels of the newly syn-
charge carriers to diffuse along the polymer chain, thus thesized polymers. Cyclic voltammograms of P1–P3
limiting their transport to quenching sites.21,22 A notable shown in Figure 8 reveal that all three polymers are both
variation is observed in the absorption and emission p- and n-dopable which could be attributed to the pres-
behavior between P1 and P3 though the conjugation ence of both electron donor and electron acceptor units
unit remains the same in these polymers. This may with an alternate D-A arrangement in the conjugated
be due to the presence of additional conjugated unit segment of the polymers. The onset oxidation and onset
(cyanophenylene units) in P3 as compared to P1. The reduction potentials were used to estimate the HOMO
conjugation length of this unit (with three phenyl rings and the LUMO energy levels of polymers according to
and two cyanovinylene units) is quite significant to alter the following equations,23 25
oxd
red
the absorption and emission behavior of the polymer.
EHOMO = −[E
+ 4.4eV] and ELUMO = −[E
+
onset
oxd
onset
red
The optical properties of the polymers were com- 4.4eV], where E
and E
are the onset potentials
onset
onset
pared with those of a conjugated monomer (M) versus standard calomel electrode (SCE) for the oxida-
which contains oxadiazole-thiophene-oxadiazole seg- tion and reduction processes of a polymer, respectively.
ment (Figure 6). The conjugation length of M is almost All the polymers showed an oxidation peak at around
the same as that of the conjugated segment in P1. The 1.8 V with a reduction peak at around -1.7 V. The poly-
monomer showed absorption maximum in the range mers exhibit similar redox potentials and their HOMO/
355–360 nm and an emission maximum at 422 nm LUMO energy levels also are comparable (Table 2) as
(Figure 7), the values match with the corresponding the structure of the conjugated segment in these poly-
absorption and emission maxima respectively of P1. mers is almost the same. However, an additional oxi-
This observation further confirms the confinement of dation peak was observed at a lower potential value
conjugation length in the polymers by the introduction (0.84 V) for P2, which could be due to the oxidation
of nonconjugated linkers.
of the alkoxy linkers. Similarly, polymer P1 showed
a reduction peak at −2.05 V due to the reduction of
phenoxy linker whereas facile reduction of the cyano-
containing linker in P3 is evident by the appearance of
a small reduction peak at −1.06 eV. All three polymers
(P1–P3) exhibit low-lying HOMO and LUMO levels
because of the D-A structure of the conjugated segment.
The electron affinity (LUMO level) of the poly-
mers were higher than those of 2-(4-biphenyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole (PBD),26 aluminum
tris(8-hydroxyquinolinate) (Alq3)27 and cyano-poly(p-
phenylenevinylene) (CN-PPV),28 which are some of
the widely used electron transporting materials. Hence,
polymers P1–P3 may be considered as promising active
materials in the fabrication of PLEDs. The electrochem-
ical band gap (Eegl) of the polymers was calculated based
on the formula Eegl = ELUMO− EHOMO and the values are
tabulated in Table 2. Among three three polymers, sim-
ilar to the results obtained in the optical studies, poly-
mer P2 with higher extent of π-conjugation showed the
lowest electrochemical band gap (Eegl).
3.5 Electrochemical properties
Cyclic voltammetric (CV) studies were used to determine
redox potentials and to estimate the highest occupied
H
C
O
OC
H
25 12
12 25
N N
N
N
CH
H C
3
3
S
O
O
M
Figure 6. Structure of monomer M.
0.8
800
0.6
0.4
0.2
0.0
600
400
200
0
3.6 Electroluminescent properties
To carry out preliminary studies on electrolumines-
cence properties of polymers, polymer light-emitting
devices were fabricated with a device configuration of
ITO/MoO3/polymer/LiF/Al. P1 and P2-based devices
emitted blue light whereas P3-based device was not
stable under testing (ambient) conditions and did not
300
400
500
600
700
Wavelength (nm)
Figure 7. UV-Vis absorption and fluorescence emission
spectra of monomer (M) in solution.

D-A conjugated polymers containing substituted thiophene
emit light at all. The presence of cyanovinylene group, 3.7 Nonlinear optical properties
which is susceptible to undergo oxidative degradation
under ambient conditions may be one of the reasons The open aperture Z-scan data obtained from samples
for the failure of P3-based device. The current density- P1, P2 and P3 dissolved in chloroform are shown in
voltage characteristics of polymer P2 is shown in Figure 10. Samples P1, P2 and P3 show linear trans-
Figure 9. The current density increases exponentially mittance of 82, 52 and 70% respectively, at the excita-
with the increasing forward bias voltage, which is a tion wavelength when taken in the 1 mm cuvette. The
typical diode characteristic. The threshold voltage of transmission of the samples get decreased as the input
P1 and P2 is 22.5 and 5.2 V, respectively, which sug- light intensity increases (intensity is maximum at z = 0)
gests that P2 could be a promising material for the which signifies the strong optical limiting behavior of
fabrication of blue-emitting diodes. However, further the polymers. Therefore, strong two-step excited state
studies on optimization of device configuration, device absorption will happen along with weak genuine two-
efficiency and life time are essential to consider the photon absorption (2PA) resulting in an optical limit-
practical utility of these polymers.
ing phenomenon. The net effect is then known as an
8
6
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
4
2
0
-2
-4
-6
-8
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
(a)
(b)
Potential (V)
Potential (V)
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
Potential (V)
(c)
Figure 8. Cyclic voltammogram of (a) P1, (b) P2 and (c) P3 at 25 mV/s scan rate in ACN/TEAPC (0.1 M) solution.
Table 2. Electrochemical potentials and energy levels of polymers P1, P2 and P3.
oxd
onset
red
onset
Polymer
E
E
Eoxd
Ered
HOMO (eV)
LUMO (eV)
Eegl (eV)
P1
P2
P3
+1.58
+1.42
+1.60
−1.49
−1.31
−1.28
+1.84
+0.85/ + 1.89
+1.87
−1.77
−1.72
−5.98
−5.82
−6.0
−2.91
−3.09
−3.12
3.07
2.73
2.88
−1.72/−1.14

Prashanth Kumar K R et al.
“effective” 2PA process. It was noticed that a 2PA type by dividing the output intensity with the input inten-
process gives the best fit to the obtained experimental sity and normalizing it with the linear transmittance.
data. The Z-scan data obtained were therefore numeri- As seen from Figure 10, there is superior concurrence
cally fitted to the nonlinear transmission equation for a between the experimental data and numerical simula-
2PA process, given by equation,
αo
tion. The numerically estimated value of β for P1,
P2 and P3 are 34 × 10−11, 65 × 10−11 and 0.75 ×
10−11 m/W respectively. Interestingly, these values are
comparable with the β values obtained for some fully
conjugated D-A polymers containing thiophene and
α(I) =
+ βI
(1)
I
1 +
Is
where, α (m−1) is the unsaturated linear absorption
coefficient at the wavelength of excitation, and Is
(W/m2) is the saturation intensity (intensity at which
the linear absorption drops to half its original value).
β (m/W) = σN is the excited state absorption (ESA)
coefficient, where σ is the ESA cross section and N(I)
is the intensity-dependent excited state population den-
sity, β is the effective 2PA coefficient (Table 3). For
calculating the output laser intensity for a given input
intensity, first we numerically evaluate the output inten-
sity from the sample for each input intensity by solving
the propagation equation,
1.0
0.8
0.6
P1
ꢀꢁ
ꢂ
ꢃ
dI
dZꢁ
αo
-12000 -8000 -4000
0
4000
8000 12000
= −
+ βI I
(2)
I
z (microns)
1 +
Is
using the fourth order Runge–Kutta method. Here Z’
indicates the propagation distance within the sample.
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 irradiation area.
The normalized transmittance (Tnorm) is then calculated
P2
1.0
0.8
0.6
0.4
0.4
0.3
0.2
0.1
0.0
-12000 -8000 -4000
0
4000 8000 12000 16000
z (microns)
P3
1.0
0.9
0.8
0
2
4
6
8
Voltage (V)
Figure 9. Current density-voltage characteristics of device
ITO/MoO3/P2/LiF/Al.
-12000 -8000 -4000
0
4000 8000 12000 16000
Table 3. Numerically estimated β value of the polymers.
z (microns)
Polymer
P1
P2
P3
Figure 10. Open aperture Z-scan curve of polymers P1, P2
and P3. Hollow circles are data points while the solid curves
are the numerical fit obtained using equation (2).
β (m/W)
3.4 × 10−11
6.5 × 10−11
0.75 × 10−11

D-A conjugated polymers containing substituted thiophene
1,3,4-oxadiazole units9 which suggests that the confine-
ment of π-conjugation does not affect significantly the
NLO properties of these D-A systems. Among P1–P3,
polymer P2 with higher extent of π-conjugation exhib-
ited the highest value for β. Hence these polymers are
promising candidates for optical limiting applications.
References
1. Skotheim T A and Reynolds J R 2007 In Conjugated
Polymers -Theory, Synthesis, Properties and Character-
ization 3rd edn. (Boca Raton: CRC Press)
2. Park S H, Cho S, Lee J K, Lee K and Heeger A J 2009
Org. Electron. 10 426
3. Guo X, Baumgarten M and Müllen K 2013 Prog. Polym.
Sci 38 1832
4. Bian L Y, Zhu E W, Tang J, Tang W H and Zhang F J
2012 Prog. Polym. Sci. 37 1292
5. Balan A, Gunbas G, Durmus A and Toppare L 2008
Chem. Mater. 20 7510
4. Conclusions
Three D-A conjugated polymers (P1, P2 and P3) con-
taining nonconjugated linkers and 3,4-didodecyloxythio-
phene unit as electron-rich moiety were synthesized
through a multistep synthetic route. Thermogravimet-
ric analysis revealed that polymer P2 containing alkoxy
linker exhibit lower onset decomposition temperature
when compared with that of polymers P1 and P2 which
contain methyleneoxy phenyl linkers. Interruption of
the π-conjugation by nonconjugated linker affects the
linear optical properties of the polymers. As a result,
these polymers showed slightly higher optical band gap
than that of fully conjugated polymers and the emission
maxima of these polymers shifted to the blue region.
It is evident from the cyclic voltammetry studies that
all three polymers (P1–P3) exhibit low-lying HOMO
and LUMO levels because of the D-A structure of the
conjugated segment. The test light-emitting device
based on P2 with a device configuration of ITO/MoO3/
polymer/LiF/Al, emitted blue light with a low thresh-
old voltage of 5.2 V. Z-scan studies revealed the strong
optical limiting behavior of these polymers. Though the
confinement of pi conjugation affect the linear optical
properties of the polymers, it did not show any sig-
nificant effect on their NLO properties and their 2PA
coefficient (β) value is comparable with that of fully
conjugated D-A polymers. These results reveal that the
synthesized polymers are the promising materials for
optoelectronic device applications.
6. Zhang Z-G and Wang J 2012 J. Mater. Chem. 22 4178
7. Albota M, Beljonne D, Bredas J L, Ehrlich J E, Fu J Y,
Heikal A A, Hess S E, Kogej T, Levin M D, Marder S
R, McCord-Moughon D, Perry J W, Rockel H, Rumi M,
Subramaniam G, Webb W W, Wu X L and Xu C 1998
Science 281 1653
8. Huang C, Sartin M M, Cozzuol M, Siegel N, Barlow S,
Perry J W and Marder S R 2012 J. Phys. Chem. A 116
4305
9. Murali M G, Dalimba U and Sridharan K 2012 J. Mater.
Sci. 47 8022
10. Tian B, Zerbi G, Schenk R and Mullen K 1991 J. Chem.
Phys. 95 3191
11. Wang J and Leung L M 2013 Dyes Pigm. 99 105
12. Gudeika D, Grazulevicius J V, Sini G, Bucinskas A,
Jankauskas V, Miasojedovas A and Jursenas S 2014
Dyes Pigm. 106 58
13. Yuan Z, Yu Z and Tan S 2006 J. Mater. Sci. 41 6103
14. Xia A, Yuan Z, Tan S, Zou Y, Tan Z, Yang Y and Li Y
2007 Eur. Polym. J. 43 1394
15. Sim J H, Kim S J, Yamada K, Yokokura S, Natori I,
Natori S and Sato H 2009 Synth. Met. 159 85
16. Hay M and Klavetter F L 1995 J. Am. Chem. Soc. 117
7112
17. Xu X, Zhang H, Liu X, Zhuang Q and Han Z 2010 Eur.
Polym. J. 46 528
18. Zhao M, Hashimoto K and Tajima K 2013 Synth. Met.
175 9
19. Sheik-Bahae M, Said A A, Wei T H, Hagan D J and Van
Stryland E W 1990 IEEE J. Quant. Electron. 26 760
20. Nguyen T-Q, Doan V and Schwartz B J 1999 J. Chem.
Phys. 110 4068
21. Sun R G, Wang Y Z, Wang D K, Zheng Q B, Kyllo
E M, Gustafson T L, Wang F and Epstein A J 2000
Synth. Met. 111 595
22. Kyllo E M, Gustafson T L, Wang D K, Sun R G and
Epstein A J 2001 Synth. Met. 116 189
23. Yang C J and Jenekhe S A 1995 Macromolecules 28
1180
Supplementary Information
1H NMR spectra of monomer 3 and polymers P1–P3
are available at www.ias.ac.in/chemsci.
24.
Agrawal A K and Jenekhe S A 1996 Chem. Mater. 8 579
25. Leeuw D M D, Simenon M M J, Brown A R and
Einerhand R E F 1997 Synth. Met. 87 53
26. Pommerehne J, Vestweber H, Guss W, Mahrt R F,
Bässler H, Porsch M and Daub J 1995 Adv. Mater. 7 551
27. Destruel P, Jolinat P, Clergereaux R and Farenc J 1999
J. Appl. Phys. 85 397
Acknowledgements
UKD thanks SERB, DST India for the financial support
(Ref. No.SB/FT/CS-145/2012).
28. Bradley D D C 1993 Synth. Met. 54 401