Synthesis and nonlinear optical properties of novel Y-type polyester containing nitrophenylazoresorcinoxy group

Article abstract of DOI:10.1080/15421406.2011.566074

A new Y-type polyester (3) containing nitrophenylazoresorcinoxy groups as NLO chromophores, which are components of the polymer backbone was prepared. Polyester 3 is soluble in common organic solvents such as N,N-dimethylformamide and acetone. It shows a

Full text of DOI:10.1080/15421406.2011.566074

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Synthesis and Nonlinear Optical
Properties of Novel Y-type Polyester
Containing Nitrophenylazoresorcinoxy
Group
a
a
a
You Jin Cho , Mi Sung Kim & Ju-Yeon Lee
a
Department of Chemistry , Institute of Basic Science, Inje
University , Obang-dong, Gimhae, Korea
Published online: 16 May 2011.
To cite this article: You Jin Cho , Mi Sung Kim & Ju-Yeon Lee (2011) Synthesis and Nonlinear Optical
Properties of Novel Y-type Polyester Containing Nitrophenylazoresorcinoxy Group, Molecular Crystals
and Liquid Crystals, 539:1, 115/[455]-120/[460], DOI: 10.1080/15421406.2011.566074
To link to this article: http://dx.doi.org/10.1080/15421406.2011.566074
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Mol. Cryst. Liq. Cryst., Vol. 539: pp. 115=[455]–120=[460], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.566074
Synthesis and Nonlinear Optical Properties of
Novel Y-type Polyester Containing
Nitrophenylazoresorcinoxy Group
YOU JIN CHO, MI SUNG KIM, AND JU-YEON LEE
Department of Chemistry, Institute of Basic Science, Inje University,
Obang-dong, Gimhae, Korea
A new Y-type polyester (3) containing nitrophenylazoresorcinoxy groups as NLO
chromophores, which are components of the polymer backbone was prepared. Poly-
ester 3 is soluble in common organic solvents such as N,N-dimethylformamide and
ꢀ
acetone. It shows a thermal stability up to 240 C from thermogravimetric analysis
with glass-transition temperature (T ) obtained from differential scanning calor-
g
ꢀ
imetry near 116 C. The second harmonic generation (SHG) coefficient (d33) of
poled polymer films at the 1064 nm fundamental wavelength is around 4.63 ꢁ
ꢂ9
ꢀ
10
esu. The dipole alignment exhibits a thermal stability even at 4 C higher than
ꢀ
T , and no SHG decay was observed below 120 C due to the partial main-chain
g
character of polymer structure, which is acceptable for NLO device applications.
Keywords Differential scanning calorimetry (DSC); dipole alignment; NLO;
polyester; 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 [1–3]. In the developments of
NLO polymers for electro-optic device applications, stabilization of electrically
induced dipole alignment is one of important criteria [4]; in this context, two
approaches to minimize the randomization have been proposed namely the use of
cross-linking method [5] and the utilizing high glass-transition temperature (Tg)
polymers such as polyimides [6]. Polyesters with amino-sulfone azobenzene chromo-
phores in the main chain generate strong and stable reversible birefringence [7]. NLO
polyesters containing azobenzene mesogens in the main chain exhibit high thermal
and temporal stabilities [8]. Polyesters containing cyanophenylazoaniline moiety in
the side chain show good temporal stability of second-order nonlinearity [9]. In gen-
eral, 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,
Address correspondence to Ju-Yeon Lee, Department of Chemistry, Institute of Basic
Science, 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
115=[455]

1
16=[456]
Y. J. Cho, M. S. Kim, and J.-Y. Lee
Figure 1. Main chain NLO polymers (a), Side chain NLO polymers (b), and Y-type NLO
polymers (c).
and their intractability make them unusable 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 polyesters containing dioxynitrostil-
benyl group [10] or dioxybenzylidenemalononitrile [11], which showed enhanced
thermal stability of dipole alignments. In this work reported here, we prepared
new Y-type polyester containing nitrophenylazoresorcinoxy groups as NLO chro-
mophores. We selected the latter as NLO chromophores because they are expected
to have high optical nonlinearities. Furthermore, these nitrophenylazoresorcinoxy
groups constitute novel Y-type NLO polyesters (Fig. 1c), 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 parts of the
polymer backbone. This Y-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 thermal and nonlinear optical properties.
Experimental
Materials
Synthetic method of polymer 3 is summarized in Scheme 1. The polymer was
further purified by extraction in a Soxhlet extractor with methanol and dried under
vacuum, yielding 4.29 g (90% yield) of polymer 3. Inherent viscosity (g ) ¼ 0.30
inh
ꢀ
1
dL=g (c, 0.5 g=dL in DMSO at 25 C). H NMR (DMSO-d ) d (ppm): d 4.45–4.87
6
(
7
m, 8H, 2 -O-CH -CH -O-), 6.68–6.82 (s, 1H, aromatic), 6.85–7.10 (s, 1H, aromatic),
ꢂ1
2 2
.58–8.33 (s, 9H, aromatic). IR (KBr disc) (cm ): 3085 (w, ¼C-H), 2926 (m, C-H),
1
721 (vs, C=O), 1600 (s, N=N), 1521, 1342 (vs, N=O), 1180 (m, N=N). Anal. Calcd
for (C H N O ) : C, 60.38; H, 4.01; N, 8.80. Found: C, 60.48; H, 4.09; N, 8.72.
2
4
19
3
8 n

Synthesis and NLO Properties of Novel Polyester
117=[457]
Scheme 1. Synthetic scheme and structure of polymer 3.
Measurements
Infrared (IR) spectra were obtained with a Shimadzu FT IR-8201PC infrared
1
spectrophotometer. H NMR spectra were obtained with a Varian 300 MHz NMR
spectrometer. UV-visible absorption spectra were obtained with a Shimadzu
UV-3100S spectrophotometer. T values were measured using a TA 2920 differential
g
scanning calorimeter DSC in a nitrogen atmosphere. A TA Q50 thermogravimetric
ꢂ1
ꢀ
ꢀ
analyzer 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
(
M ) and weight-average molecular weight (M ) of the polymers were estimated
n w
using gel permeation chromatography (GPC; styragel HR5E4E columns; tetrahy-
drofuran (THF) solvent). The refractive index of the sample was measured using
the optical transmission technique [12]. Second harmonic generation (SHG) mea-
surements were made using a Maker fringe technique [13]. SHG coefficients (d )
3
3
were derived from the analysis of measured Maker-fringes.
Results and Discussion
Synthesis and Characterization of Polymer 3
The synthetic route for polymer 3 is presented in Scheme 1. Polyester 3 was synthe-
sized through the reaction of monomer 2 containing NLO chromophore with stoi-
chiometric amounts of terephthaloyl chloride in pyridine. The chemical structure
1
of the resulting polymer was confirmed by H NMR, IR spectra, and elemental
1
analysis. The chemical shifts in H NMR spectrum of the polymer are consistent
with the proposed polymer structure. The signal at 4.45–4.87 is assigned to the eth-
ylene protons attached to oxygen. The aromatic protons are shown in the range
6
.68–8.33 ppm. The IR spectrum of polymer 3 shows strong carbonyl peaks near
ꢂ1
1
absorption peaks near 1521 and 1342 cm
721 cm indicating the presence of ester bond. The spectrum also shows strong
ꢂ1
due to nitro group indicating the
presence of nitroazobenzene unit. These results are consistent with the proposed
structure, indicating that the NLO chromophores remained intact during the
polymerization. The number average molecular weight (M ) of the polymer 3,
n
determined by GPC, was 18800 (M =M ¼ 1.97). The structural feature of this
w
n

1
18=[458]
Y. J. Cho, M. S. Kim, and J.-Y. Lee
polymer is that it has pendant NLO chromophores, which are parts of the polymer
main chain. Thus the resulting polymer 3 is mid type of side-chain and main-chain
NLO polymer, and is expected to have both of their merits. The polymer 3 is soluble
in common solvents such as acetone, DMF, and DMSO, but is not soluble in meth-
anol and diethyl ether. The inherent viscosity value is 0.30 dL=g. Polymer 3 shows
strong absorption near 398 nm due to the NLO chromophore nitrophenylazoresor-
cinoxy group.
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 3
ꢀ
shows a thermal stability up to 240 C from its TGA thermogram. The 10% weight
ꢀ
loss degradation temperature (T ) of polymer 3 is near 262 C. T value of the poly-
d
g
ꢀ
mer 3 measured by DSC is around 116 C. This T value is higher than those of the
g
ꢀ
polyesters containing dioxynitrostilbene, which are in the range 90–95 C [10] or
ꢀ
dioxybenzylidenemalononitrile, which is near 83 C [11].
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 120 C, a corona voltage of 6.5 kV was applied and this
temperature was maintained for 30 min. The poling was confirmed by UV-visible
spectrum. After electric poling, the dipole moments of the NLO chromophores were
aligned and the UV-visible absorption of polymer 3 exhibits a slight blue shift and a
decrease in absorption due to birefringence. SHG measurements were performed at a
fundamental wavelength of 1064 nm using a mode locked Nd-YAG laser. Nonlinear
optical properties of polymer 3 are summarized in Table 1. In order to determine the
microscopic second-order susceptibility of the polymer, the angular SHG depen-
dence was recorded. Figure 2 shows the angular dependence of SHG signal in a
poled polymer 3. The SHG values were compared with those obtained from a
Y-cut quartz plate. SHG coefficients (d , d ) were derived from the analysis of mea-
3
3
31
sured Maker-fringes with the Pascal fitting program according to the literature pro-
ꢂ9
cedure [13]. The measured values of d and d31 for polymer 3 are 4.63 ꢁ 10 esu and
3
3
ꢂ9
1.58 ꢁ 10 esu, respectively. Since the second harmonic wavelength is at 532 nm,
which is not in the absorptive region of the resulting polyester, there is not resonant
contribution to this d33 value.
Table 1. Nonlinear optical properties of polymer 3
a
b
d33 (esu)
b
d31 (esu)
c
Polymer kmax (nm)
Film thickness (mm)
n
3
398
4.63 ꢁ 10ꢂ9 1.58 ꢁ 10ꢂ9
0.48
1.73
a
b
c
Polymer film after poling.
SHG coefficients (d33) were derived from the analysis of measured Maker-fringes [13].
Film thickness was determined by the optical transmission technique [12].

Synthesis and NLO Properties of Novel Polyester
119=[459]
Figure 2. Angular dependence of SHG signal for a poled film of polymer 3.
To evaluate the high-temperature stability of the polymer, we studied the tem-
poral stability of the SHG signal. Figure 3 shows the dynamic thermal stability study
of the NLO activity of a film of polymer 3. 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 3.5 C=min from 25 to 200 C. The
ꢀ
polymer film exhibits a thermal stability even at 4 C higher than T and no signifi-
g
cant SHG decay is observed below 120 C. In general, main-chain NLO polymers
ꢀ
have good thermal stability of dipole alignments, but they often do not dissolve in
organic solvents. Side-chain NLO polymer systems have good solubility, but they
often suffer from poor stability of dipole alignments at high temperatures. The high
thermal stability of second harmonic generation of polymer 3 is due to the stabiliza-
tion of dipole alignment of NLO chromophore, which stems from the partial main
Figure 3. Normalized SHG signal of polymer 3 as a function of temperature at a heating rate
ꢀ
of 3.5 C=min.

1
20=[460]
Y. J. Cho, M. S. Kim, and J.-Y. Lee
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 3 with pendant NLO chromophores as parts of the
polymer backbone was prepared and characterized. This Y-type polymer 3 is sol-
ꢀ
uble in common organic solvents and shows a thermal stability up to 240 C with
ꢀ
T value around 116 C. The SHG coefficient (d ) of corona-poled polymer film
g
33
ꢂ9
ꢀ
is 4.63 ꢁ 10 esu. This polymer exhibits SHG stability even at 4 C higher than
ꢀ
T and no significant SHG decay is observed below 120 C. This high thermal stab-
g
ility of optical nonlinearity stems from the stabilization of dipole alignment of the
NLO chromophores, which constitute parts of the polymer main chain
Acknowledgment
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. 2010-0020951).
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