Synthesis and nonlinear optical properties of a novel polyester containing tricyanovinylthienyl group

Article abstract of DOI:10.1080/15421400903584390

A novel T-type polyester (7) containing 1-(2,5-dioxyphenyl)-2-{5-(1,2,2- tricyanovinyl)-2-thienyl}ethene groups as NLO-chromophores was synthesized and characterized. Polymer 7 was soluble in common organic solvents such as DMF and DMSO. It showed thermal stability up to 300°C in TGA with T_g value obtained from DSC near 113°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at the 1560nm fundamental wavelength was around 4.42×10^-9esu. The dipole alignment exhibited a greater thermal stability even at 10C higher than T_g, and there was no SHG decay below 125°C due to the partial main-chain character of the polymer structure, which is acceptable for nonlinear optical device applications.

Full text of DOI:10.1080/15421400903584390

Mol. Cryst. Liq. Cryst., Vol. 520: pp. 186=[462]–192=[468], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903584390
Synthesis and Nonlinear Optical Properties
of a Novel Polyester Containing
Tricyanovinylthienyl Group
HYO JIN NO, GA-YOUNG LEE, YOU JIN CHO, AND
JU-YEON LEE
Institute of Basic Science, Department of Chemistry, Inje University,
Obang-dong, Gimhae, Korea
A
novel T-type polyester (7) containing 1-(2,5-dioxyphenyl)-2-{5-(1,2,2-
tricyanovinyl)-2-thienyl}ethene groups as NLO-chromophores was synthesized
and characterized. Polymer 7 was soluble in common organic solvents such as
DMF and DMSO. It showed thermal stability up to 300ꢀC in TGA with Tg value
obtained from DSC near 113ꢀC. The second harmonic generation (SHG) coef-
ficient (d33) of poled polymer film at the 1560 nm fundamental wavelength was
around 4.42 ꢁ 10ꢂ9 esu. The dipole alignment exhibited a greater thermal stability
even at 10ꢀC higher than Tg, and there was no SHG decay below 125ꢀC due to
the partial main-chain character of the polymer structure, which is acceptable for
nonlinear optical device applications.
Keywords Differential scanning calorimetry (DSC); NLO; polyester; relaxation
of dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
Introduction
Recently nonlinear optical (NLO) materials are extensively studied because of their
potential applications in the field of electro-optic devices, including high speed
optical modulators and ultrafast optical switches [1–6]. In the developments of
NLO polymers for electro-optic device applications, stabilization of electrically
induced dipole alignment is one of important criteria [7]; in this context, two
approaches to minimize the randomization have been proposed namely the use of
cross-linking method [8–10] and the utilizing high glass-transition temperature (Tg)
polymers such as polyimides [11–14]. In general, polymers with the NLO chromo-
phores in the main chain have good thermal stability of dipole alignments, but they
often do not dissolve in organic solvents, and their intractability make them unusa-
ble to fabricate stable noncentrosymmetric films. Side chain NLO polymers have the
advantages such as good solubility and homogeneity, but they often suffer from poor
stability of dipole alignments at high temperatures. Recently we reported novel NLO
Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of
Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea. Tel.: þ82-55-320-
3221; Fax: þ82-55-321-9718; E-mail: chemljy@inje.ac.kr
186=[462]

Synthesis and NLO Properties of a Novel Polyester
187=[463]
Figure 1. Side chain NLO polymers (a) and T-type NLO polymers (b).
polyesters containing dioxynitrostilbenyl group [15] or dioxybenzylidenemalononi-
trile [16], which showed enhanced thermal stability of dipole alignments. In this work
we prepared a novel T-type polyester containing 1-(2,5-dioxyphenyl)-2-{5-(1,2,2-
tricyanovinyl)-2-thienyl}ethene groups as NLO chromophores. We selected the
latter as a NLO chromophore because it has a large dipole moment. Furthermore,
1-(2,5-dioxyphenyl)-2-{5-(1,2,2-tricyanovinyl)-2-thienyl}ethene groups constitute
novel T-type NLO polyesters (Fig. 1b), and these T-type NLO polyesters have not
yet been reported in the literature. Thus, we synthesized a new type of NLO poly-
ester, in which the pendant NLO chromophores are parts of the polymer backbones.
This T-type NLO polymer is expected to have the advantages of both main-chain
and side-chain NLO polymers namely stable dipole alignment and good solubility.
After confirming the structure of the resulting polymer we investigated its properties
such as solubility, thermal stability, second harmonic generation (SHG) activity,
and relaxation of dipole alignment. We now report the results of the initial phase
of the work.
Experimental
Materials
Synthetic method of polymer 7 is summarized in Scheme 1. Polymer 7: ginh ¼ 0.28
1
dL=g (c, 0.5 g=dL in DMSO at 25ꢀC). H NMR (DMSO-d6) d (ppm): 4.26–4.45
=
(s, 4H, 2 -CH2-O-), 4.54–4.78 (d, 4H, 2 Ph-O-CH2-), 6.84–7.72 (m, 6H, -CH CH-,
aromatic), 7.85–8.23 (m, 5H, aromatic). IR (KBr disc) (cmꢂ1): 3085 (w, ¼C-H),
=
=
2954 (m, C-H), 2219 (s, CN), 1720 (vs, C O), 1627 (s, C C). Anal. Calcd for
(C29H19N3O6S)n: C, 64.80; H, 3.55; N, 7.81.; S, 5.97. Found: C, 64.88; H, 3.61; N,
7.76; S, 5.93.
Measurements
1
IR, H NMR, and UV-Vis spectra were taken on a Shimadzu FT IR-8201PC infra-
red spectrophotometer, Varian 300 MHz NMR spectrometer, and Shimadzu
UV-3100S spectrophotometer, respectively. Elemental analyses were performed

188=[464]
H. J. No et al.
Scheme 1. Synthetic scheme and structure of polymer 7.
using a Perkin-Elmer 2400 CHN elemental analyzer. Tg values were measured on a
TA 2920 differential scanning calorimeter (DSC) in a nitrogen atmosphere. TA Q50
thermogravimetric analyzer (TGA) with a heating rate of 10ꢀC=min up to 800ꢀC was
used for the thermal degradation of polymers under nitrogen. The number average
molecular weight (Mn) and weight average molecular weight (Mw) of the polymer
were estimated by gel permeation chromatography (GPC) (columns styragel
HR5E4E; solvent THF). The alignment of the NLO-chromophore of the polymer
was carried out by corona poling method. The refractive index of the polymer
sample was measured by the optical transmission technique [17]. Second harmonic
generation (SHG) measurements were made using a Maker fringe technique [18].
Results and Discussion
Synthesis and Characterization of Polymer 7
The synthetic route for polymer 7 is presented in Scheme 1. Diol 5 was condensed
with terephthaloyl chloride in a dry pyridine to yield polyester 6, and it was reacted
with tetracyanoethylene in anhydrous DMF [19] to yield polyester 7 containing the
NLO chromophore 1-(2,5-dioxyphenyl)-2-{5-(1,2,2-tricyanovinyl)-2-thienyl}ethene
group. The resulting polymer was purified by Soxhlet extraction for 2 days with
methanol as a solvent. The chemical structure of the polymer was confirmed by
1H NMR, IR spectra, and elemental analysis. Elemental analysis results fit the poly-
mer structure. 1H NMR spectrum of the polymer showed a signal broadening due to
polymerization, but the chemical shifts are consistent with the proposed polymer
structure. The IR spectrum of polymer 7 showed strong carbonyl peak near
1720 cmꢂ1, indicating the presence of ester bond. The IR spectrum of the same poly-
mer sample showed strong absorption peak near 2219 cmꢂ1, indicating the presence
of nitrile group. These results are consistent with the proposed structure, indicating

Synthesis and NLO Properties of a Novel Polyester
189=[465]
that the tricyanovinyl groups are attached well to thiophene ring. The number
average molecular weight (Mn) of the polymer 7, determined by GPC, was 17200
(Mw=Mn ¼ 1.94). The structural feature of this polymer is that it has pendant
NLO chromophores, which are parts of the polymer main chain. Thus the resulting
polymer 7 is mid type of side chain- and main chain NLO polymers, and is expected
to have both of their merits. The polymer 7 was soluble in common solvents such
as acetone, DMF, and DMSO, but was not soluble in methanol and diethyl ether.
The inherent viscosity value was 0.28 dL=g. Polymer 7 showed strong absorption
near 527 nm by the NLO chromophore 1-(2,5-dioxyethoxy)phenyl)-2-{5-(1,2,2-
tricyanovinyl)-2-thienyl)}ethene group. Having well defined polyester 7, we investi-
gated its properties.
Thermal Properties of Polymer
The thermal behavior of the polymer was investigated by TGA and DSC to deter-
mine the thermal degradation pattern and glass transition temperature. Polymer 7
showed a thermal stability up to 300ꢀC from its TGA thermogram. Tg value of
the polymer 7 measured by DSC was around 113ꢀC. This Tg value is higher than
those of the polyesters containing dioxynitrostilbene, which are in the range
90–95ꢀC [15] or dioxybenzylidenemalononitrile, which is near 83ꢀC [16]. The TGA
and DSC studies show that the decomposition temperature of the polyester 7 is
higher than the corresponding Tg. This indicates that high-temperature poling for
a short term is feasible without damaging the NLO chromophore.
Nonlinear Optical Properties of Polymer
The NLO properties of polymer were studied by the SHG method. To induce non-
centrosymmetric polar order, the spin-coated polymer film was corona-poled. As the
temperature was raised to 120ꢀC, 6.5 kV of corona voltage was applied and this tem-
perature was maintained for 30 min. The poling was confirmed by UV-Vis spectrum.
After electric poling, the dipole moments of the NLO chromophores were aligned
and the UV-Vis spectra of polymer 7 exhibited a slight blue shift and a decrease
in absorption due to birefringence. From the absorbance change, the order para-
meter of the poled film could be estimated, which is related to the poling efficiency.
The estimated order parameter value U was equal to 0.12 for polymer 7 (U ¼ 1ꢂA1=A0,
where A0 ¼ 1.9642 and A1 ¼ 1.7354 are the absorbances of the polymer film before
and after poling, respectively). The refractive index of the sample was measured
by the optical transmission technique [17]. SHG measurements were performed
at a fundamental wavelength of 1560 nm using a mode locked Nd-YAG laser.
Nonlinear optical properties of polymer 7 are summarized in Table 1. In order to
determine the microscopic second-order susceptibility of the polymer, the angular
SHG dependence was recorded. Figure 2 shows the angular dependence of SHG
signal in a poled polymer 7. The SHG values were compared with those obtained
from a Y-cut quartz plate. SHG coefficients (d33, d31) were derived from the analysis
of measured Maker-fringes with the Pascal fitting program according to the litera-
ture procedure [18]. The measured values of d33 and d31 for polymer 7 were
4.42 ꢁ 10ꢂ9 esu and 1.51 ꢁ 10ꢂ9 esu, respectively. These values are higher than those
of the polyesters containing dioxynitrostilbene [15], and are similar with those of the
polyesters containing dioxybenzylidenemalononitrile [16]. If we increase the length

190=[466]
H. J. No et al.
Table 1. Linear and nonlinear optical properties of polymer 7
Polymer kmaxa(nm)
527
d33b(esu)
Uc
d31b(esu)
Film thicknessd (mm)
n
7
4.42 ꢁ 10ꢂ9 0.12 1.51 ꢁ 10ꢂ9
0.51
1.51
aPolymer film after poling.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [18].
cOrder parameter U ¼ 1ꢂA1=A0, where A0 and A1 are the absorbances of the polymer film
before and after corona poling, respectively.
dFilm thickness was determined by the optical transmission technique [17].
of the NLO chromophores, the SHG values would be improved. Since the second
harmonic wavelength was at 780 nm, which is not in the absorptive region of the
resulting polyester, there was not resonant contribution to this d33 value.
To evaluate the high-temperature stability of the polymer, we studied the
temporal stability of the SHG signal. In Figure 3, we present the dynamic thermal
stability study of the NLO activity of the film 7. To investigate the real time NLO
decay of the SHG signal of the poled polymer film as a function of temperature,
in situ SHG measurements were performed at a heating rate of 4ꢀC=min from 25
to 220ꢀC. The polymer film exhibited a greater thermal stability even at 10ꢀC higher
than Tg and no significant SHG decay was observed below 125ꢀC. This thermal stab-
ility is higher than those of the polyesters containing dioxynitrostilbene [15] or diox-
ybenzylidenemalononitrile [16], which exhibits SHG thermal stability up to 100ꢀC.
In general, main-chain NLO polymers have good thermal stability of dipole align-
ments, but they often do not dissolve in organic solvents, and their intractability
make them unusable to fabricate stable films. Side-chain NLO polymer systems have
the advantages such as good solubility and high loading level of NLO chromo-
phores, but they often suffer from poor stability of dipole alignments at high tem-
peratures. The exceptional high thermal stability of second harmonic generation
of polymer 7 is due to the stabilization of dipole alignment of NLO chromophore,
Figure 2. Angular dependence of SHG signal for a poled film of polymer 7.

Synthesis and NLO Properties of a Novel Polyester
191=[467]
Figure 3. Normalized SHG signal of polymer 7 as a function of temperature at a heating rate
of 4ꢀC=min.
which stems from the partial main chain character of the polymer structure. Thus,
we obtained a new type of NLO polyester having both the merits of main chain-
and side-chain NLO polymers namely stable dipole alignment and good solubility.
Conclusions
A novel T-type NLO polyester 7 with pendant NLO chromophores as parts of the
polymer backbones was prepared and characterized. This T-type polymer 7 is sol-
uble in common organic solvents and showed a thermal stability up to 300ꢀC with
Tg value around 113ꢀC. The SHG coefficient (d33) of corona-poled polymer film
was 4.42 ꢁ 10ꢂ9 esu. This polymer exhibits SHG stability even at 10ꢀC higher than
Tg and no significant SHG decay was observed below 125ꢀC. This high thermal stab-
ility of optical nonlinearity stemmed from the stabilization of dipole alignment of the
NLO chromophore, which constituted a part of the polymer main chain.
Acknowledgements
This research was supported by Basic Science Research Program Through
the National Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (No. 2009-0070572).
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