Electrochemical and nonlinear optical studies of new D-A type π-conjugated polymers carrying 3,4-benzyloxythiophene, oxadiazole, and 3,4-alkoxythiophene systems

Article abstract of DOI:10.1246/cl.2012.234

We investigated the nonlinear optical (NLO) properties of two newly synthesized conjugated polymers Pl and P2 carrying 1,3,4-oxadiazole, 3,4-dibenzyloxythiophene, and 3,4-dialkoxythiophene moieties along the main chain, as potential NLO active materials. Their structures have been well characterized. The nonlinear measurements were performed by Z-scan using 532 nm, 7 ns laser pulses. Calculated values of figure of merit and β follow the criteria for good NLO materials. These results suggest that polymers are promising materials for applications in photonics.

Full text of DOI:10.1246/cl.2012.234

doi:10.1246/cl.2012.234
234
Published on the web February 25, 2012
Electrochemical and Nonlinear Optical Studies of New D-A Type ³-Conjugated Polymers
Carrying 3,4-Benzyloxythiophene, Oxadiazole, and 3,4-Alkoxythiophene Systems
M. S. Sunitha,1 Airody Vasudeva Adhikari,*1 K. A. Vishnumurthy,1 N. Smijesh,2 and Reji Philip2
1Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Mangalore-575 025, India
2Light and Matter Physics Group, Raman Research Institute,
C. V. Raman Avenue, Sadashiva Nagar, Bangalore-560 080, India
(Received November 1, 2011; CL-111067; E-mail: avchem@nitk.ac.in)
We investigated the nonlinear optical (NLO) properties of
two newly synthesized conjugated polymers P1 and P2 carrying
1,3,4-oxadiazole, 3,4-dibenzyloxythiophene, and 3,4-dialkoxy-
thiophene moieties along the main chain, as potential NLO
active materials. Their structures have been well characterized.
The nonlinear measurements were performed by Z-scan using
532 nm, 7 ns laser pulses. Calculated values of figure of merit
and ¢ follow the criteria for good NLO materials. These results
suggest that polymers are promising materials for applications in
photonics.
HO
EtOOC
OH
2-(chloromethyl)benzene
O
O
COOEt
DMF, K CO
2
3
S
1
COOEt
EtOOC
S
2
(i) 5% alc.NaOH
+
(ii) H /H
O
2
O
O
O
ClOC
O
SOCl
OR
2
COOH
HOOC
COCl
OR
S
S
3
4
In recent times, NLO materials are gaining attention of
many researchers because of their potential applications in
optical communications, optical storage, optical computing,
optical switching, optical limiting, etc.1,2 Polymeric systems
with delocalized ³-electrons have been shown to possess a very
high molecular polarizability and third-order optical nonlinear-
ities. Recently, D-A type conjugated polymers gathered much
importance as they carry enhanced polarizability and dipole
moment, due to which they are widely used in high speed
nonlinear susceptibility studies.3-5 Among various D-A type
conjugated polymers, thiophene-based polymers and oligomers
are of special interest mainly due to their high thermal stability,
readiness to accommodate functional groups, and solubility in
common organic solvents. It is observed that enhancement in
their optical nonlinearity can be achieved by introducing proper
electron-donating and electron-withdrawing groups as well as
by optimizing steric repulsion between these groups through
structural modification. This generates a highly polarizable
charge-transfer system with an asymmetric electron distribu-
tion.6-8
Though several reports are available on NLO properties of
conjugated polymers, the quest to design and synthesize new
organic D-A type polymers possessing high molecular hyper-
polarizability and dipole moment (®) is still on.9 One way to
achieve this is to fine tune the optoelectronic properties of these
polymers depending on the choice of the functional substitu-
ents.10-13
Keeping this in view, we have designed new polymers with
3,4-aryloxy-/3,4-alkoxy-substituted thiophene as electron donor
moieties and 1,3,4-oxadiazole as electron-withdrawing segment
along the polymer chain with the hope of improved NLO
activity. Besides, the NLO properties of these new polymers
have been studied by Z-scan technique and their results are
discussed. These polymers are believed to possess high mo-
lecular hyperpolarizability due to effective delocalization of ³-
electron cloud along the polymer main chain. Moreover, charge
LiCl, Pyridine
NMP
CONHNH
H
NHNOC
2
2
S
5a, b
O
S
O
O
O
O
OR
OR
O
POCl
3
O
NH
NH
n
N
H
n
S
S
NH
O
O
S
N
N
O
N
N
OR
OR
6a, b
P1 and P2
where 5a,6a, P1: R = n-C
5b,6b, P2: R = n-C
H
12 25
H
6
13
Scheme 1. Synthetic route for the preparation of polymers P1 and
P2.
transfer would take place over nonconjugated bonds between
benzyl and thiophene units owing to the presence of -OCH2
linkage. Further, this may also decrease the torsional angle
between the benzyl groups in the polymeric chain.
The synthetic route toward the preparation of new inter-
mediates, monomers and polymers is outlined in Scheme 1.
The required intermediate 3,4-bis(benzyloxy)thiophene-2,5-di-
carboxylic acid (3) was synthesized by alkali hydrolysis of
the diethyl 3,4-bis(benzyloxy)thiophene-2,5-dicarboxylate (2),
which was obtained by condensation of diethyl 3,4-dihydroxy-
thiophene-2,5-dicarboxylate (1) with benzyl chloride in presence
of potassium carbonate and DMF. The diacid 3 was refluxed
with excess thionyl chloride to obtain 3,4-bis(benzyloxy)thio-
phene-2,5-dicarbonyl dichloride (4), which on treatment with the
dihydrazides 5a and 5b in presence of lithium chloride and
pyridine underwent smooth polycondensation to give required
polyhydrazides 6a and 6b. The compounds 6a and 6b obtained
were converted into corresponding target polymers P1 and P2
through cyclo-dehydration reaction using phosphorus oxychlo-
ride as dehydrating agent. The molecular structures of newly
synthesized intermediates, monomers, precursors, and the final
polymers were confirmed by their FTIR, 1H NMR, and ele-
Chem. Lett. 2012, 41, 234-236
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

235
Table 1. Electrochemical potentials, energy levels, and electro-
chemical band gap of P1 and P2
a
b
Eoxd
(onset)
Ered
(onset)
EHOMO ELUMO
/eV /eV
Eg
/eV
Eg
/eV
P1
P2
1.37
1.01
¹1.05
¹1.19
¹5.77 ¹3.35 2.42
¹5.41 ¹3.21 2.20
2.45
2.38
b
aElectrochemical band gap. Optical band gap.
(a)
Figure 2. UV-visible absorption spectra of P1 in solution and film
states.
(b)
Figure 3. UV-visible absorption spectra of P2 in solution and film
states.
Figure 1. Cyclic voltammetric waves of P1 (a) and P2 (b).
mental analyses. Thermogravimetric analysis showed that
polymer is thermally stable up to 300 °C. The detailed character-
ization data are given in Supporting Information.21
Cyclic voltammetry (CV) was employed to determine redox
potentials of new polymers and then to estimate the HOMO,
LUMO energy and band gap of the polymers using a reported
equation.14 Electrochemical data of P1 and P2 are summarized
in Table 1 and CV waves are as shown in Figure 1. These
reduction potentials are lower than that of 2-(4-tert-butylphen-
yl)-1,3,4-oxadiazole (PBD), one of the most widely used
electron-transporting materials.14-16 It has been observed that
P1 exhibited lower band gap compared to P2. The variation in
band gap of P1 and P2 may be attributed to difference in the
alkyl chain length of alkoxy substituents at positions 3 and 4
of thiophene ring. The length of alkyl substituent affects the
electronic energy levels significantly. In general, the introduc-
tion of sterically hindered bulkier alkoxy side chain twists the
monomer units out of plane leading to loss of planarity. Hence,
increase in band gap has been observed in P1.
The absorption measurements were taken for dilute solu-
tions (10¹5 M) and for film state having thickness of 1.83 ¯m for
P1 and 1.64 ¯m for P2. The optical absorption maxima for
polymers P1 appeared at 388 and 395 nm (Figure 2) and for P2
appeared at 372 and 379 nm (Figure 3) for solution and film
state, respectively. The absorption maxima of the polymers in
film state showed bathochromic shift of ca. 7 nm compared to
that obtained in solution state, indicates the presence of
interchain interactions in the solid state of polymers.
Figure 4. Photoluminescence spectra of P1 and P2.
The fluorescence emission spectra of polymers P1 and P2 in
THF showed emission peaks at 502 and 480 nm, respectively
(excitation wavelength 380 nm) as shown in Figure 4. Their
optical band gaps (Eg) calculated from the absorption edge of the
spectrum were found to be 2.45 and 2.38 eV (Table 1) for P1
and P2, respectively.
Figures 5 and 6 show the open aperture Z-scans and fluence
curves of P1 and P2, respectively. Numerically, a TPA type
process was found to give the best fit to the measured Z-scan
data. Both samples have a linear absorption of about 50% at the
excitation wavelength when taken in a 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 for a two-photon
absorption process.17
Z
þ1
TðzÞ ¼ ½1=³1=2qðzÞꢀ
ln½1 þ qðzÞ expðꢁ¸2Þꢀd¸
ð1Þ
ꢁ1
Chem. Lett. 2012, 41, 234-236
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

236
Table 2. Figures of merit calculated at 532 nm
¹1
¹1
n2/cm W
W
T
I/J ms
¹10
¹11
P1
P2
1.142 © 10
7.31 © 10
5.0
4.8
¹1
¹1
3.25 © 109
4.12 © 109
backbone. Moreover, the alkoxy and benzyloxy groups act
as solubilising groups thereby facilitating the solubility of the
polymers. All these factors collectively favored the nonlinear
absorption of the polymers as a whole. The enhancement is well
perceived as the resulting third-order nonlinear susceptibility has
increased 10-fold when compared to the results of some of the
D-A type polymers already reported.12,13
Figure 5. Input intensity versus the normalized transmittance for
the polymer P1. Inset shows the Z-scan curve.
It can be concluded that two new conjugated polymers P1
and P2 were designed and synthesized successfully using the
precursor polyhydrazide route. They exhibited good thermal
stability up to 300 °C. Their UV-visible absorption spectra red
shift of ca. 7 nm was observed in thin films. The electrochemical
band gaps were determined to be 2.42 and 2.20 eV for P1
and P2, respectively. The polymers showed strong absorptive
nonlinearity due to an effective TPA process. The values of
effective TPA absorption coefficients and figure of merits
indicate a high optical nonlinearity in polymers. Hence they
can find useful applications in photonics.
References and Notes
1
2
3
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Figure 6. Input intensity versus the normalized transmittance for
the polymer P2. Inset shows the Z-scan curve.
where T(z) is the sample transmission at position z, q(z) = ¢I0L/
[1 + (z/z0)2], where I0 is the peak intensity at the focal point,
L = [1 ¹ exp(¹¡l)]/¡, where l is the sample length and ¡ is the
4
5
6
7
8
linear absorption coefficient, and z0 = ³½ 2/- is the Rayleigh
0
range, where ½ is the beam waist radius at focus and - is
0
the light wavelength, ¢ is the effective TPA coefficient. The
numerically calculated values of the effective TPA coefficient
¹1
for polymers from the experiment are 5.3 © 10¹11 mW for
P1 and 1.4 © 10¹11 mW¹1 for P2. For comparison, under similar
excitation conditions, NLO materials like Cu nanocomposite
9
¹10
glasses had given effective TPA coefficient values of 10
to
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5.3 © 10
and 1.9 © 10¹9 mW¹1, respectively.18,19
The figures of merit such as W and T were calculated using
¢, intensity of light and equation reported earlier,20 which are
important parameters to determine significant material properties
required to achieve efficient optical switching. The values of
n2(-), W, and T have been calculated for polymers P1 and P2
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explained based on their structure. In the polymers, alkoxy
and benzyloxy pendants substituted at 3- and 4-positions of
thiophene ring behave as donor moieties whereas 1,3,4-oxadi-
azole behaves as strong acceptor moiety. This alternate D-A
arrangement gives rise to high ³-electron density along the
polymeric chain and are easily polarizable which in turn results
in enhanced delocalization of the electrons in the polymer
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Chem. Lett. 2012, 41, 234-236
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/