The synthesis, characterization and third-order nonlinear optical character of poly(2,5-dipropargyloxybenzoate) containing a polar aromatic diacetylene

Article abstract of DOI:10.1016/j.dyepig.2010.05.012

The glass transition temperature of the novel compound, poly(2,5-dipropargyloxybenzoate) containing a polar aromatic diacetylene was 105 °C and differential scanning calorimetry showed two exothermic peaks due to the opening of the hexa-2,4-diynylene-1,6-

Full text of DOI:10.1016/j.dyepig.2010.05.012

Dyes and Pigments 88 (2011) 129e134
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The synthesis, characterization and third-order nonlinear optical character of
poly(2,5-dipropargyloxybenzoate) containing a polar aromatic diacetylene
Sandra L. Castañon a, Miriam F. Beristain a, Alejandra Ortega a, Gustavo Gómez-Sosa a,
,
c
Eduardo Muñoz b, Ana Laura Perez-Martínez a, Takeshi Ogawa a , M. Faisal Halim ,
*
Francis Smith c, Ardie Walser c, Roger Dorsinville c
a Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, Ciudad Universitaria, México DF 04510, Mexico
b Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, Ciudad Universitaria, México DF 01000, Mexico
c Electrical Engineering Department, Grove School of Engineering at the City College of New York, 140th St. & Convent Avenue New York, NY 10031, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The glass transition temperature of the novel compound, poly(2,5-dipropargyloxybenzoate) containing
a polar aromatic diacetylene was 105 ꢀC and differential scanning calorimetry showed two exothermic
peaks due to the opening of the hexa-2,4-diynylene-1,6-dioxy group and the aromatic diacetylene group.
Two types of free radical were detected in the electron spin resonance spectra of the heated polymer, one
formed by cross-linking of the main chain hexa-2,4-diynylene-1,6-dioxy group and another arising from
Received 8 January 2010
Received in revised form
18 May 2010
Accepted 20 May 2010
Available online 1 June 2010
the aromatic chromophore diacetylene. The third-order nonlinear optical susceptibility,
c
(3), of the
polymer film determined using the Z-scan technique was ꢁ4.5 ꢂ 1 ꢃ 10ꢁ10 esu. When the films were
irradiated by UV light at 140 ꢀC, they formed micro-cracks due to intense cross-linking.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Polar aromatic diacetylene
Nonlinear optics
Cross-linking
Z-Scan
Free radicals
1. Introduction
diacetylene-containing polymers, the conjugated structures
obtained are not the same as those of their light-sensitive, semi-
Organic materials having nonlinear optical susceptibility have
been studied intensively in recent years and several monographs
on the subject are available [1]. In the case of third-order nonlinear
optical (3-NLO) materials, although highly conjugated polymers
such as polydiacetylenes have been considered to be important
materials [2], because of their highly crystalline nature it is not easy
to obtain films of satisfactory optical quality for photonic device
construction. In this context, diacetylene-containing polymers may
provide an option since they can readily be prepared as thin films
and the polydiacetylene network can be developed in the films
when heated or irradiated. The current authors have previously
synthesized a series of light-sensitive polyamides containing
aromatic diacetylenes that afforded purple films with 3-NLO
susceptibility of 10ꢁ10 esu; however, the films were not quite
transparent due to their polycrystalline nature [3,4]. In order to
obtain films with adequate optical quality for photonic applica-
tions, amorphous polymer are required. In the case of amorphous
crystalline film counterparts.
Other type of polymer that show promise for 3-NLO applications
are those that contain discrete conjugated groups, as they are
usually amorphous and provide films of excellent optical quality by
spin coating or casting. However, there is very little work in this
field. The current authors have reported previously the 3-NLO
susceptibility of polymers containing fluorescein [5], tolan deriva-
tives [6] as well as a polar azo dye [7]. The polymers were amor-
phous and furnished thin films of excellent optical quality by spin
coating, with 3-NLO susceptibilities of the order of 10ꢁ10 esu. In this
work a novel polymer containing an aromatic polar diacetylene was
synthesized and its thermal and chemical properties, as well as its
3-NLO property using the Z-scan technique, were studied.
2. Experimental
2.1. Synthesis
The synthetic route to poly(400-nitrophenylbutadiynyl-40-N-ethyl-
anilinoethyl-2,5-dipropargyloxybenzoate) is shown in Scheme 1.
The iodination of ethylanilinoethanol (Aldrich) was carried out by
* Corresponding author. Tel.: þ52 55 5622 4728; fax: þ52 55 5616 1201.
E-mail address: ogawa@servidor.unam.mx (T. Ogawa).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.05.012

130
S.L. Castañon et al. / Dyes and Pigments 88 (2011) 129e134
b, 30°C, 72h, 89%
a, 0°C, 3h
57%
N
N
I
N
c, 40°C, 24h, 67%
HO
HO
HO
(II)
(I)
(III)
d, 30°C, 24h, 85%
e, 35°C, 2h, 83%
O2N
Br
O2N
(IV)
(V)
f, 30°C, 24h
NO2
O2N
(III)
+
(V)
(VI)
70%
N
O2N
(VII)
OH
N
N
(VIII)
HO
OH
O
O
O
g, 60°C, 14 days
90%
O
O
(VII)
+
O
Cl
O
Monomer
O2N
N
h, 60°C, 48h, 99%
Polymer
Scheme 1. Synthetic route of poly (400-nitrophenylbutadiynyl-40-N-ethylanilinoethyl-2,5-dipropargyloxybenzoate). Reagents: a) KI, NaOCl, CH3OH/H2O, b) TMSA, Pd(PPh3)2Cl2,
PPh3, CuI, THF/Triethylamine under N2 atmosphere, c) AgNO3, CH3OH/H2O, d) same as (b), e) NaOH, CH3OH, f) CuCl, TMEDA, O2, acetone, g) Triethylamine, CH3CH2Cl2, h) CuCl,
TMEDA, O2, N,N-dimethylformamide.
Fig. 1. 1H NMR spectrum of the monomer in CDCl3.

S.L. Castañon et al. / Dyes and Pigments 88 (2011) 129e134
131
Fig. 3. DSC and TMA of polymer.
Fig. 2. FT-IR spectrum of the monomer (below) and polymer (above).
a similar method to that of Edgar and Steinmetz [8], and the iodide
derivative was then reacted with trimethylsilylacetylene (TMSA)
using the Sonogashira reaction to obtain N-ethyl-N-(2-hydrox-
yethyl)-4-ethynylaniline (III). 4-Nitroethynylbenzene (V) was
prepared also using the Sonogashira reaction from 4-bromoni-
trobenzene and TMSA (III) and (V) (excess) were coupled by
oxidative coupling in acetone using copper (I) chloride and
N,N,N0,N0-tetramethylethylenediamine (TMEDA) (VI) was insol-
uble and separated from the reaction system; the major product
(VII) and the minor product (VIII) were separated by washing
using different solvents such as acetone, chloroform, ethanol,
etc.Recrystallization of (VII) from acetone gave orange crystals
melting at 173 ꢀC (DSC) with a yield of 70% (VII) was then reacted
with an excess of 2,5-dipropargyloxybenzoyl chloride in dry
dichloroethane using triethylamine as an acid acceptor. Finally the
monomer was purified by column chromatography using silica gel
column and hexane/ethyl acetate (90/10) as eluent. Yellow orange
powder melted at 130 ꢀC (DSC). Elemental analysis calculated for
C33H26N2O6 : C, 72.52%,; H, 4.79%; N, 5.13%. Found: C, 72.28%;
H, 4.35%; N, 5.34%.
Scientific spectrophotometer model Nicolet 6700. Elemental
analysis was performed at the Analytical Center of Faculty of
Chemistry of our university. The molecular weight of the polymer
was determined by GPC using a Waters Model 2695 ALLIANCE at
35 ꢀC with dimethylformamide (DMF) as eluent with poly-
methylmethacrylate standard. The inherent viscosity was deter-
mined in DMF at 25 ꢀC. For differential scanning calorimetry (DSC)
and thermo-mechanical analysis (TMA), TA Instruments DSC 2910
and TMA 2940, were used, respectively. ESR spectra were taken
using a JEOL ESR spectrometer Model RE3X. The sample was placed
in a quartz tube with an inner diameter of 3 mm, supplied by
Wilmad LabGlass. The tube was heated in the cavity and the
spectra were taken after maintaining for 20 min at the temperature
of each measurement. The number of spins (radicals) was calcu-
lated with the double numerical integration of the first derivative
of the resonance curve from equation [9];
HB
H
Z
Z
ꢀ
ꢁ
A ¼
dH
dH0S H0
HA
HA
Where HA and HB are the initial and final parts of the resonance
curve, and S(H0) is the value of the absorption at field H0. The
NaCl:Mnþþ crystal calibrated by atomic absorption spectroscopy,
2.2. Characterization
1HNMR were taken using Bruker Avance 400 MHz spectrom-
eter, and FT-IR spectra were taken on KBr disk using Thermo
O
O
O
*
n
O
O
O
O
N
O
N
O2N
O2N
Fig. 4. DSC of diphenylbutadiyne and poly(2,5-dipropargyloxybenzoate) containing
Scheme 2. Polymer structure.
Disperse Red 1.

132
S.L. Castañon et al. / Dyes and Pigments 88 (2011) 129e134
wavelength used in these measurements the linear absorption of
3
2
1
0
the thin films was weak and thermal effects were negligible. The
known Z-scan response of a quartz plate and a 1 mm solution of CS2
were used as references to calibrate the experimental setup. After
calibration, the third-order nonlinear coefficient values of the
sample could be calculated from the difference between the
normalized peak and valley transmittances, following the standard
Z-scan method. The difference between the normalized peak and
valley transmittance DTpꢁv was measured for the reference and the
sample and the third-order nonlinear coefficient was calculated
ð3Þ
using:
c
¼
cðr3efÞðð
D
TpꢁvnoÞSampleLSample=ð TpꢁvnoÞref Lref Þ.
D
Sample
Where Lsample/ref is the optical length of the sample/reference.
3. Results and discussion
Fig. 1 shows 1HNMR spectra of the monomer. The peak at
2.54 ppm corresponding to the terminal acetylenic proton dis-
appeared when polymerized indicating that the polymerization
took place. The FT-IR spectra shown in Fig. 2 supports the NMR
spectra, in which the peaks due to the terminal acetylene eC^CH
at 3284 cmꢁ1 and the propargyl acetylene C^C at 2120 cmꢁ1 dis-
appeared. The newly formed diacetylene (hexa-2,4-diynylene)
groups in the polymer appeared in the shoulder of the peak at
2197 cmꢁ1 due to the chromophore diacetylene. The structure of
the polymer is shown in Scheme 2. The inherent viscosity at 25 ꢀC
in DMF was 0.17 dl/g, and the molecular weight was found to be
8110 (Mn) and 12,560 (Mw).
300
400
500
600
700
Wavelength (nm)
Fig. 5. UV/Vis absorption spectra of films before (solid line) and after (dotted line)
heating at 120 ꢀC with UV irradiation.
was employed for calculation of spins taking both spectra under the
same conditions.
2.3. Film preparation
Fig. 3 shows the DSC diagram of the polymer. Its Tg was found to
be around 105 ꢀC, which was also observed by thermo-mechanical
analysis (TMA) shown in Fig. 3. The exothermic peak which started
at around 140 ꢀC is due to the reaction of diacetylene groups of the
polymer main chain (hexa-2,4-diynylene). This is commonly
observed in most of the polymers whose main chains consist of the
hexa-2,4-diynylene-1,6-dioxy group, and a highly cross-linked
structure is formed when heated in the solid state at this temper-
ature [10,11]. An example of such polymers is shown in Fig. 4,
where the poly(2,5-dipropargyloxybenzoate) containing Disperse
Red 1, initiates its exothermic reaction of cross-linking at around
140 ꢀC, while an aromatic diacetylene, diphenylbutadiyne, initiates
its reaction at around 180 ꢀC. The overlapping of these two
exothermal peaks gives the DSC of the polymer (Fig. 3).
Fig. 5 shows the absorption spectra of the film before (solid line)
and after (dotted line) heating at 140 ꢀC in argon with simultaneous
UV irradiation. The peak of the diacetylene chromophore dis-
appeared, and the spectrum became similar to that of oligomeric
diphenylbutadiynes [12] indicating that various conjugated
systems were formed as shown in Schemes 3 and 4. The film
became completely insoluble in solvents.
Polymer films were prepared by spin coating of dime-
thylformamide solution on a glass substrate heating at 60 ꢀC. They
were dried in a vacuum oven at 80 ꢀC for 120 min. Films thickness
was determined by a Sloan Dektak III profilometer. Films with
thickness around 1 mm were prepared. UV/Vis spectra were taken
using a Varian spectrophotometer Model Cary 400 Cone. The film
was also heated at 140 ꢀC in argon and irradiated by a medium
pressure mercury lamp (400 W) of Aceglass Co. The refractive index
and extinction coefficient of polymer were determined using
a
Jobin-Yvon Model Uvisel spectroscopic ellipsometer. Three
separate measurements were done near the Brewster angle, i.e.,
from 50 to 70ꢀ, with an increment of 10ꢀ in each step. The wave-
length range of these measurements was 260e830 nm, with an
increment of 10 nm per step, controlled by an automatic mono-
chromator. DeltaPsi2 software was used in order to fit ellipsometric
experimental data to optical properties. A refractive index of 1.6
was determined for the film, obtained by extrapolation to 1064 nm.
2.4. The Z-scan measurement
The 3-NLO properties of the polymer films were investigated
with 25 ps laser pulses, with a repetition rate of 20 Hz, from a Nd:YAG
The ESR spectra of the polymer are shown in Fig. 6, and the
changes in the number of spins with temperature rise are shown in
Fig. 7. The material contained radicals even at room temperature
before heating. The spectra were taken several days after the
polymer was prepared, and therefore the radicals were thought to
laser at wavelength 1064 nm at 0.1e1
of focal length 20 cm, and the beam radius was 35
power at focus was about (I0 w 0.2e2.5 GW/cm2). At the excitation
m
J. The Z-scan setup had a lens
mm. The peak
Scheme 3. Cross-linking of hexa-2,4-butadiynylene-1,6-dioxy groups and the stabilization of the resulted radicals with the chromophore diacetylene.

S.L. Castañon et al. / Dyes and Pigments 88 (2011) 129e134
133
8
O
7
O
N
O
6
N
O
O
N
N
5
·
N
O
4
N
3
2
O2N
NO2
·
O2N
1
NO2
NO2
20
40
60
80
100
120
140
160
NO2
Temperature (ºC)
Scheme 4. Oligomerization of aromatic diacetylene chromophore in the polymer.
Fig. 7. Decrease in the number of spins with temperature increase observed in the ESR
spectra. * based on the molecular weight of monomer.
have been formed during this period. The broad signals starting at
room temperature are considered to be due to the radicals formed
by the reaction of the main chain hexa-2,4-diynylene groups at
room temperature. The room light might have been the cause of the
reaction. It is known that amorphous polymers containing aliphatic
diacetylenes and hexa-2,4-diynylene-1,6-dioxy groups undergo
cross-linking sometimes even at room temperature [13] as shown
in Scheme 3. However, such radicals are usually not observed by ESR
at room temperature because they are not stable unless they are
trapped and immobilized in solid. Therefore, these radicals might be
trapped in the solid polymer or stabilized by the aromatic diac-
etylene groups in the system, and thus they can be observed by ESR
spectra. The stabilization of transient radicals by aromatic diac-
etylenes is known for the free radical polymerization of vinyl
monomers in the presence of aromatic diacetylenes. The stabilized
propagating radicals show their ESR spectra at polymerization
temperatures [14,15]. This means that the radicals in Scheme 3 can
be stabilized by the aromatic diacetylene and detected by ESR. Note
that the broad signals are those from main chain diacetylenes, and
the sharp signal that appears above 100 ꢀC is that of stable oligo-
meric diradicals of the aromatic diacetylene (Scheme 4). The
decrease in the number of radicals with the increase in temperature
is probably due to the radical recombination as can be seen in Fig. 7.
These disappearing radicals are mainly the aliphatic hexadiynylene
groups. The new sharp peak that appears above 100 ꢀC is due to the
radicals formed by the reaction between the aromatic diacetylenes
shown in Scheme 4, and g value of 2.0038 is that of highly conju-
gated stable carbon radicals. The ESR signal above 140 ꢀC, is a typical
signal of stable diradicals of oligomeric aromatic diacetylenes
[12,16,17]. The typical behavior of Z-scan data is shown in Fig. 8. The
c
(3) value of untreated film determined with 1064 nm was found to
be ꢁ4.5 ꢂ1 ꢃ10ꢁ10 esu. That of the film irradiated with UV lamp for
30 min at 120 ꢀC, gave a value of ꢁ6.7 ꢂ 1 ꢃ10ꢁ10 esu. This nonlinear
response was, as expected in a transparent sample, mainly real at
the wavelength and intensities investigated, and they are consid-
ered to be quite high for common organic conjugated systems
compared with those of highly conjugated polymers such as poly-
diacetylenes and polythiophenes, whose c(3) are of the order of
10ꢁ12 esu [18]. The (3) value of this polymer arises from the discrete
c
conjugated group of the polymer (chromophore diacetylene),
whose concentration is high at 58% w/w. The polymer also contains
substantial amounts of radicals, and this might increase c
(3) value.
There are some discussions on the effects of open-shell and spins-
containing polymers on the 3-NLO properties, and it is said that
they enhance 3-NLO properties [19e21]. However, various poly-
mers containing discrete conjugated groups are known to possess
Fig. 6. Effect of heating on the ESR signals of the polymer. Magnetic central field:
320 mT, microwave power: 4 mW, microwave frequency: 9.1054 GHz, modulation
amplitude: 0.2 mT, modulation frequency: 100 KHz, time constant: 0.03 s, and scan
range: ꢂ40 mT, gain: 10.0. At each temperature the sample was kept for 20 min before
measuring.
c
(3) values of an order of 10ꢁ10 esu [5e7]. It was expected that the
films irradiated with UV light at 140 ꢀC would have much higher
3-NLO coefficients, because of expansion of conjugation due to the

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S.L. Castañon et al. / Dyes and Pigments 88 (2011) 129e134
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4. Conclusions
A novel polymer containing polar aromatic diacetylene chro-
mophore was synthesized in order to study its 3-NLO properties.
The c(3) value of around ꢁ4e7 ꢃ 10ꢁ10 esu is considered to be
reasonably high for a polymer containing only small conjugated
molecules. The polymer cross-linked even at room temperature
through the hexa-2,4-diynylene-1,6-dioxy group of the main
chains. The ESR detected radicals formed by the reaction of main
chain diacetylenes, are stabilized by the chromophore aromatic
diacetylenes. The typical ESR signals of oligomeric diradicals of
aromatic diacetylene appeared at above 100 ꢀC.
The cross-linking of main chains via the hexa-2,4-diynylene-1,6-
dioxy groups is undesirable because the polymer films shrink on
heating above 140 ꢀC causing micro-cracks. Polymers with polar
diacetylene chromophores, without hexa-2,4-diynylene-1,6-dioxy
groups in the main chain, are being prepared, and their 2-NLO and
3-NLO properties will be reported in future works.
Acknowledgements
[20] Nakano M, Kishi R, Nakagawa N, Ohta S, Takahashi H, Furukawa S, et al.
This work was supported by CONACYT (Consejo Nacional de
Ciencia y Tecnologia) under the contract number of 49849-R.
Sandra Luz Castañon thanks for her postgraduate scholarship given
by CONACYT. Thanks are also due to Miguel Canseco, Salvador
Lopez, and Gerardo Cedillo, for their assistance in thermal analysis,
GPC and NMR spectroscopy, respectively.
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