Synthesis and nonlinear optical properties of novel polyester containing tricyanovinylthiophene with enhanced thermal stability of second harmonic generation

Article abstract of DOI:10.1080/15421400902950170

A novel Y-type polyester (7) containing 1-(2,4-dioxyethoxy)phenyl-2-{5-(1, 2,2- tricyanovinyl)-2-thiophenyl)}ethenyl 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 134°C. The second harmonic generation (SHG) coefficient (d₃₃) of poled polymer film at the 1,560nm fundamental wavelength was around 6.74×10^-9 esu. The dipole alignment exhibited high thermal stability and there was no SHG decay below T_g due to the partial main-chain character of polymer structure, which was acceptable for NLO device applications. Copyright Taylor & Francis Group, LLC.

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Synthesis and Nonlinear
Optical Properties of
Novel Polyester Containing
Tricyanovinylthiophene with
Enhanced Thermal Stability of
Second Harmonic Generation
Ga-Young Lee a , Hyo Jin No a & Ju-Yeon Lee a
a Institute of Basic Science, Department of
Chemistry , Inje University , Obang-dong, Gimhae,
Korea
Published online: 25 Jun 2009.
To cite this article: Ga-Young Lee , Hyo Jin No & Ju-Yeon Lee (2009) Synthesis and
Nonlinear Optical Properties of Novel Polyester Containing Tricyanovinylthiophene
with Enhanced Thermal Stability of Second Harmonic Generation, Molecular Crystals
and Liquid Crystals, 504:1, 196-203, DOI: 10.1080/15421400902950170
To link to this article: http://dx.doi.org/10.1080/15421400902950170
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Mol. Cryst. Liq. Cryst., Vol. 504, pp. 196–203, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902950170
Synthesis and Nonlinear Optical Properties of
Novel Polyester Containing Tricyanovinylthiophene
with Enhanced Thermal Stability of Second
Harmonic Generation
Ga-Young Lee, Hyo Jin No, and Ju-Yeon Lee
Institute of Basic Science, Department of Chemistry, Inje University,
Obang-dong, Gimhae, Korea
A
novel Y-type polyester (7) containing 1-(2,4-dioxyethoxy)phenyl-2-{5-(1,2,2-
tricyanovinyl)-2-thiophenyl)}ethenyl groups as NLO-chromophores was synthe-
sized 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 134ꢀC. The second harmonic generation (SHG) coefficient
(d33) of poled polymer film at the 1,560 nm fundamental wavelength was around
6.74 ꢁ 10ꢂ9 esu. The dipole alignment exhibited high thermal stability and there
was no SHG decay below Tg due to the partial main-chain character of polymer
structure, which was acceptable for NLO device applications.
Keywords: differential scanning calorimetry (DSC); NLO; polyester; relaxation of
dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
INTRODUCTION
Recently functional polymers with nonlinear optical (NLO) activity
are extensively studied because of their potential applications in the
field of electro-optic devices [1–5]. In the developments of NLO poly-
mers for electro-optic device applications, stabilization of electrically
induced dipole alignment is one of important criteria [6]; in this
context, two approaches to minimize the randomization have been
This work was supported by the Korea Research Foundation Grant funded by the
Korean Government (#2009-0070572).
Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of
Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea. E-mail: chemljy@
inje.ac.kr
196

Synthesis and NLO Properties of Novel Polyester
197
proposed namely the use of cross-linking method [7–8] and the utiliz-
ing polymers with high glass-transition temperature (Tg) such as
polyimides [9–11]. Polymers with the NLO chromophores 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
impossible to fabricate stable noncentrosymmetric films. Side chain
NLO polymers have the advantages such as good solubility and homo-
geneity, but they often suffer from poor stability of dipole alignments
at high temperatures. Recently we reported novel NLO polyesters con-
taining dioxynitrostilbenyl group [12] and dioxybenzylidenemalononi-
trile [13], which showed enhanced thermal stability of dipole
alignments. In this work we prepared a novel Y-type polyester con-
taining
1-(2,4-dioxyethoxy)phenyl)-2-{5-(1,2,2-tricyanovinyl)-2-thio-
phenyl)}ethenyl group as a NLO-chromophore. We selected the
latter as a NLO chromophore because it has a large dipole moment.
Furthermore, 1-(2,4-dioxyethoxy)phenyl)-2-{5-(1,2,2-tricyanovinyl)-2-
thiophenyl)}ethenyl groups constitute novel Y-type NLO polyesters
(Fig. 1b), and these Y-type NLO polyesters have not yet been reported
in the literature. Thus, we synthesized a new type of NLO polyester, in
which the pendant NLO chromophores are components of the polymer
backbones. This mid-type NLO polymer is expected to have the advan-
tages 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.
FIGURE 1 Side chain NLO polymers (a) and Y-type NLO polymers (b).

198
G.-Y. Lee et al.
EXPERIMENTAL
Materials
Synthetic method of polymer 7 is summarized in Scheme 1. Polymer 7:
ginh ¼ 0.31 dL=g (c, 0.5 g=dL in DMSO at 25ꢀC). 1H NMR (DMSO-d6) d
4.26–4.52 (m, 4H, 2 -CH2-O-), 4.52–4.78 (m, 4H, 2 Ph-O-CH2-),
6.53–7.72 (m, 11H, -CH ¼ CH-, aromatic), 7.90–8.13 (m, 5H, aromatic).
IR (KBr) 3086 (w, ¼C-H), 2958 (m, C-H), 2219 (s, CN), 1721 (vs, C ¼ O),
1596 (s, C ¼ C) cmꢂ1. Anal. Calcd for (C29H19N3O6S)n: C, 64.80; H,
3.55; N, 7.81.; S, 5.97. Found: C, 64.88; H, 3.62; N, 7.74; S, 5.91.
Measurements
IR, 1H NMR, and UV-Vis spectra were taken on a Shimadzu FT
IR-8201PC infrared spectrophotometer, Varian 300 MHz NMR spec-
trometer, and Shimadzu UV-3100S spectrophotometer, respectively.
Elemental analyses were performed using a Perkin-Elmer 2400
CHN elemental analyzer. Tg values were measured on a TA 2920 dif-
ferential scanning calorimeter (DSC) in a nitrogen atmosphere. ꢀTA
Q50 thermogravimetric analyzer (TGA) with a heating rate of 10 C=
ꢀ
min up to 800 C 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
[14]. Second harmonic generation (SHG) measurements were made
using a Maker fringe technique [15].
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer 7
The synthetic route for polymer 7 is presented in Scheme 1. Monomer
5 was condensed with terephthaloyl chloride in a dry pyridine to yield
polyester 6, and it was reacted with tetracyanoethylene in anhydrous
DMF [16] to yield polyester 7 containing the NLO-chromophore
1-(2,4-dioxyethoxy)phenyl-2-{5-(1,2,2-tricyanovinyl)-2-thiophenyl)}ethenyl
group. The resulting polymer was purified by Soxhlet extraction for 2
days with methanol as a solvent. The chemical structure of the poly-
1
mer was confirmed by H NMR, IR spectra, and elemental analysis.

Synthesis and NLO Properties of Novel Polyester
199
SCHEME 1 Synthetic scheme and structure of polymer 7.
1
Elemental analysis results fit the polymer structure. H NMR spec-
trum of the polymer showed a signal broadening due to polymeriza-
tion, but the chemical shifts are consistent with the proposed
polymer structure. The IR spectrum of polymer 7 showed strong carbo-
nyl peak near 1,722 cmꢂ1, indicating the presence of ester bond. The
IR spectrum of the same polymer sample showed strong absorption
peak near 2,219 cmꢂ1, indicating the presence of nitrile group. These
results are consistent with the proposed structure, indicating that
the tricyanovinyl groups are attached well to thiophene ring. The
number average molecular weight (Mn) of the polymer 7, determined
by GPC, was 17,800 (Mw=Mn ¼ 1.98). The structural feature of this
polymer is that it has pendant NLO chromophores, which are parts
of the polymer main chains. 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 0.31 dL=g. Polymer
7 showed strong absorption near 550 nm by the NLO-chromophore
1-(2,4-dioxyethoxy)phenyl)-2-{5-(1,2,2-tricyanovinyl)-2-thiophenyl)}ethenyl
group. Having well defined polyester (7), we investigated its
properties.

200
G.-Y. Lee et al.
Thermal Properties of Polymer
The thermal behavior of the polymer was investigated by TGA and
DSC to determine the thermal degradation pattern and glass transi-
tion 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 134ꢀC. The TGA and DSC studies showed that the
decomposition temperature of the polyester 7 was 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 noncentrosymmetric polar order, the spin-coated polymer film
was corona-poled. As the temperature was raised to 140ꢀC, 6.5 kV of
corona voltage was applied and this temperature was maintained for
30 min. The poling was confirmed by UV-Vis spectrum. The UV-Vis
spectra of the polymer 7 before and after poling are presented
in Figure 2. 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 bire-
fringence. From the absorbance change, the order parameter of the
FIGURE 2 UV-Vis absorption spectra of a film of polymer 7 before and after
poling.

Synthesis and NLO Properties of Novel Polyester
201
TABLE 1 Linear and Nonlinear Optical Properties of Polymer 7
c
Polymer kmaxa(nm)
550
d33b(esu)
U
d31b(esu)
film thicknessd(mm)
n
7
6.74 ꢁ 10ꢂ9 0.18 1.92 ꢁ 10ꢂ9
0.52
1.52
aPolymer film after poling.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [15].
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 [14].
poled film could be estimated, which is related to the poling efficiency.
The estimated order parameter value U was equal to 0.18 for polymer
7 (U ¼ 1ꢂA1=A0, A1 ¼ 0.7307, A0 ¼ 0.8890, where A0 and A1 are the
absorbances of the polymer film before and after poling). The refrac-
tive index of the sample was measured by the optical transmission
technique [14]. 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 3 shows the angu-
lar dependence of SHG signal in a poled polymer 7. The SHG values
were compared with those obtained from a Y-cut quartz plate. SHG
FIGURE 3 Angular dependence of SHG signal in a poled film of polymer 7.

202
G.-Y. Lee et al.
coefficients (d33) were derived from the analysis of measured
Maker-fringes with the Pascal fitting program according to the litera-
ture procedure [15]. The preliminary measured values of d33 and d31
for polymer 7 were 6.74 ꢁ 10ꢂ9 esu and 1.92 ꢁ 10ꢂ9 esu, respectively.
These values are intermediate in the NLO polymers prepared so far.
If we increase the length 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 polye-
ster, there was not resonant contribution to this d33 value.
To evaluate the high-temperature stability of the polymer, we stu-
died the temporal stability of the SHG signal. In Figure 4, 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 3.5ꢀC=min from 25 to 200ꢀC. The
polymer film exhibited a thermal stability up to Tg and no significant
SHG decay was observed below 135ꢀC. This SHG thermal stability is
at the high rank in the known NLO polyesters. In general, side chain
NLO polymers lose the thermal stability of dipole alignment below Tg.
Stabilization of dipole alignment is a characteristic of main chain
FIGURE 4 Normalized SHG signal of polymer 7 as a function of temperature
at a heating rate of 3.5ꢀC=min.

Synthesis and NLO Properties of Novel Polyester
203
NLO polymers. The exceptional high thermal stability of second
harmonic generation of polymer 7 was due to the stabilization of dipole
alignment of NLO chromophore, which stemmed 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 Y-type NLO polyester 7 with pendant NLO chromophores as
parts of the polymer backbones was prepared and characterized. This
mid-type polymer 7 is soluble in common organic solvents and showed
a thermal stability up to 300ꢀC with Tg value around 134 ꢀC. The SHG
coefficient (d33) of corona-poled polymer film was 6.74 Â 10À9 esu. This
polymer exhibits SHG stability up to Tg and no significant SHG decay
was observed below 135ꢀC. This high thermal stability of optical
nonlinearity stemmed from the stabilization of dipole alignment of
the NLO-chromophore.
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