Nonlinear optical effects in polymeric azoesters

Article abstract of DOI:10.1080/15421400802451915

The methacrylic monomers containing azobenzene moiety with flexible alkyl spacers of different lengths and groups of different nature are synthesized, and their physicochemical properties were determined by electronic absorption, 1H NMR spectroscopy, and element analysis. In the present work, the homopolymers built with the use of the free radical polymerization of methacrylic monomers incorporating an azobenzene side-group have been synthesized and structurally characterized. The results of photochemical and optical activities of the corresponding polymers are presented. Third-order nonlinear optical (NLO) properties of the azopolymer solutions are studied by the degenerate four wave mixing (DFWM) method.

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Nonlinear Optical Effects In
Polymeric Azoesters
Oksana Nadtoka a , Oksana Krupka a , Jerome Luk b
, Volodymyr Syromyatnikov a , Bouchta Sahraoui b &
Vitaliy Smokal a
a Kyiv National Taras Shevchenko University,
Macromolecular Chemistry Department, Kyiv,
Ukraine
b Laboratory POMA, University of Angers, Boulevard
Lavoisier, Angers, France
Version of record first published: 06 Jul 2012.
To cite this article: Oksana Nadtoka , Oksana Krupka , Jerome Luk , Volodymyr
Syromyatnikov , Bouchta Sahraoui & Vitaliy Smokal (2008): Nonlinear Optical Effects
In Polymeric Azoesters, Molecular Crystals and Liquid Crystals, 496:1, 302-309
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Mol. Cryst. Liq. Cryst., Vol. 496, pp. 302–309, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802451915
Nonlinear Optical Effects In Polymeric Azoesters
Oksana Nadtoka1, Oksana Krupka1, Jerome Luk2,
Volodymyr Syromyatnikov1, Bouchta Sahraoui2,
and Vitaliy Smokal1
1Kyiv National Taras Shevchenko University, Macromolecular
Chemistry Department, Kyiv, Ukraine
2Laboratory POMA, University of Angers, Boulevard Lavoisier,
Angers, France
The methacrylic monomers containing azobenzene moiety with flexible alkyl
spacers of different lengths and groups of different nature are synthesized, and
their physicochemical properties were determined by electronic absorption, 1H
NMR spectroscopy, and element analysis. In the present work, the homopolymers
built with the use of the free radical polymerization of methacrylic monomers
incorporating an azobenzene side-group have been synthesized and structurally
characterized. The results of photochemical and optical activities of the corre-
sponding polymers are presented. Third-order nonlinear optical (NLO) properties
of the azopolymer solutions are studied by the degenerate four wave mixing
(DFWM) method.
Keywords: aryl(meth)acrylates; azopolymers; radical polymerization; third-order NLO
properties
1. INTRODUCTION
Photoanisotropic materials are widely investigated for optical storage
and processing applications. The most promising of them are azo-
dye-polymer systems because of their large value of photoinduced
birefringence. Azobenzene systems possess the advantages for high
optical nonlinearities due to photoinduced trans-cis isomerization,
molecular reorientation, and nonlinear absorption [1]. Holograms with
very high diffraction efficiency and exceptional stability can be written
(and erased) by laser illumination in these materials. Azobenzene
Address correspondence to Oksana Nadtoka, Kyiv National Taras Shevchenko
University, Volodymyr’ka St., 64, Kyiv 01033, Ukraine. E-mail: oksananadtoka@ukr.net
302

NLO Effects in Polymeric Azoesters
303
polymers are interesting because they combine the properties of
anisotropy with photoresponsive behavior and can give rise to applica-
tions in such areas as LC displays, NLO materials, information sto-
rage devices, etc. [2–7].
Among basic requirements to the materials exhibiting the NLO
activity are the polarizability (electrons need to be greatly perturbed
from their equlibrium positions), asymmetric charge distribution
(incorporation of D-A molecules), and acentric crystal packing.
On the molecular level, a molecule must possess an excited state
with energy close to that of the ground state, the large oscillator
strength for electronic transitions from the ground to the excited
state, and a large difference between the dipole moments in the
ground and excited states. Such requirements are implemented due
to the charge transfer between electron donating and electron
withdrawing groups as a dipolar highly polarizable donor-p-acceptor
(D-p-A) system.
Such factors as the increased length, increased planarity, overall
increased conjugation, and the strength of donors and acceptors
play a significant role in the NLO activity. In this paper, we
designed a new series of photochromic polymers based on meth-
acrylic azoesters (poly-AzoMA). The azobenzene polymers contain-
ing NLO chromophores in each side chain unit with different
groups of the acceptor nature and a flexible alkyl spacer have been
synthesized via free radical polymerization. On the other hand, the
introduction of methacrylate chains ensures a good solubility of
azobenzene compounds in different organic solvents that provides
a wide potentiality for the fabrication of optical and optoelectronic
devices.
In this article, we turn attention to the investigation of the influ-
ence of the push-pull electronic effect on the third-order NLO proper-
ties of polymers. For this purpose, the poly-AzoMA should have
different substituents and a flexible alkyl spacer.
2. EXPERIMENTAL
2.1. Materials
The general structures and main characteristics of poly-AzoMA are
shown in Figure 1 and Table 1. All the chemicals were purchased from
commercial suppliers and used without further purification unless
otherwise noted. The structures of all the precursors and final pro-
1
1
ducts were confirmed by H NMR spectroscopy. The H NMR spectra
were done on samples dissolved in CDCl3.

304
O. Nadtoka et al.
FIGURE 1 Structures of azopolymers.
Monomer Synthesis. The synthetic route for the target azomo-
nomers (M1 and M2) is shown in Scheme 1. The corresponding
monomers were synthesized by general methods. The azocompound
(0.06 mole) and triethylamine (9.0 ml) were dissolved in THF
(200 ml). The solution was kept in an ice bath for 10 min. A solution
of distilled methacryloyl chloride (6.0 ml, 0.06 mole) was added drop-
wise to the above mixture. The additional methacryloyl chloride was
added, and the resulting mixture was stirred at room temperature
overnight. Then the solution was poured into distilled water (1 l)
and the obtained residue was filtered and air-dried. Recrystallization
of monomers was carried out from ethanol.
4-[(Z)-[4-[(2-methyl-1-oxo-2-propenyl)oxy]phenyl]azo]-benzoic
acid butyl ester (M1) Brown crystals; yield 69%; mp 63ꢀC (by DSC). 1H
NMR (CDCl3), d(ppm): 7.89 (d, 2H, Ar), 7.73 (d, 2H, Ar), 7.93 (d, 2H, Ar),
6.94 (d, 2H, Ar), 6.34 (s, 1H, ¼ CH2), 5.91 (s, 1H, ¼ CH2), 2.05 (s, 3H,
ꢁCH3), 1.72 (m, 9H, C4H9). UV-vis (ethanol) kmax: 350, 485 nm. Elem.
Anal. Calcd for C21H22O4N2: C, 38.25%; H, 6.01%; N, 7.65%. Found:
C, 38.05%; H, 5.88%; N, 7.61%.
40-(methacryloxy)hexyloxy-4-nitroazobenzene (M2) Orange
crystals; yield 50%; mp 72ꢀC (by DSC). 1H NMR (CDCl3), d(ppm):
8.02 (d, 2H, Ar), 8.34 (d, 2H, Ar), 7.68 (d, 2H, Ar), 7.10 (d, 2H, Ar),
6.30 (s, 1H, ¼CH2), 5.95 (s, 1H, ¼ CH2), 2.20 (s, 3H,ꢁCH3), 4.10 (m,
2H, ArOCH2), 3.9 (m, 2H, HOCH2) 1.72 (m, 4H, H2). UV-vis (ethanol)
k
max: 365, 490 nm. Elem. Anal. Calcd for C21H25O5N3: C, 61.74%; H,
4.18%; N, 13.50%. Found: C, 61.70%; H, 4.16%; N, 13.52%.
TABLE 1 Characteristics of Polymeric Azoesters
ꢀ
Compound
Tg, C
kmax, nm
Mn
Mw
Mw=Mn
P1
P2
123
112
331
343
3500
5000
4410
6900
1.26
1.38

NLO Effects in Polymeric Azoesters
305
SCHEME 1 Synthetic route for the azomonomers.
Polymerization. Homopolymers were synthesized by free-radical
polymerization in toluene. The polymerization was carried out in
10 wt % toluene solution of monomer with AIBN as a free radical
initiator (1 wt % of monomer) at 80ꢀC for more than 30 h. Polymers
were isolated from the reaction solution by precipitation into methanol
followed by reprecipitation from toluene into methanol and then dried
at 20ꢀC overnight. The synthetic procedure was described previously
[8] in detail.
The obtained polymers were dissolved in dimethylformamide. The
solutions of polymers were filtered through a 0.2 mm nylon filter before
using.
2.2. Instruments for Characterization
The synthesized azopolymers and organic intermediates were charac-
terized by 1H NMR spectroscopy. The 1H NMR (400 MHz) spectra were
recorded on a ‘‘Mercury-400’’ spectrometer using CDCl3 as a solvent.
Chemical shifts are in ppm from the internal standard tetramethylsi-
lane (TMS). The obtained results are in agreement with the proposed
structures.
The phase transitions were studied by differential scanning calori-
metry (DSC) using a Perkin Elmer DSC-2 instrument equipped with
a IFA GmbH processor at a scan rate of 20ꢀC=min. The calorimetry

306
O. Nadtoka et al.
calibration was carried out according to the well-known recent
recommendation [9–11] using sapphire and quartz as standard.
2.3. Degenerate Four-Wave Mixing Experiment
Nonlinear optical properties of the solutions of P1 and P2
(C ¼ 12 g=l) were investigated using the DFWM method. Various
experimental techniques can be used for estimating the third-order
nonlinearity of the given materials, even though each technique
addresses only a particular facet of the nonlinearity due to the fre-
quency dispersion of v<3>
. The DFWM measurements were
performed using a Nd:YAG laser (Quantel Model YG472) working
at 532 nm with the 30-ps duration of pulses and the 1-Hz repetition
rate [12]. In this experiment, three optical pulses (two pump beams
I1 and I2 and a probe beam I3) were obtained by using two beam
splitters (Fig. 2).
Carbon disulfide (CS2) was used as a reference material to calibrate
<3>
CS2
the DFWM measurements (v
¼ 1:11 ꢂ 10ꢁ12 esu). The P1 and P2
were contained in 2-mm-thick quartz cuvettes.
FIGURE 2 Experimental set up of DFWM: S – sample, RO – delay lines, G –
Glan prism, Phs and Phc – synchronization and control photodiodes, BS –
beam-splitters, PMT – photomultiplier tube, <1> and <2> – pump waves,
<3> – probe wave, <4> – fourth wave.

NLO Effects in Polymeric Azoesters
307
The DFWM reflectivity (R) was calculated from the propagation
equation of the four beams in interaction and took into account the lin-
ear and nonlinear absorption and can be expressed as follows [13–15]:
I4ð0Þ
I3ð0Þ
K2
q cothðqLÞ ꢁ 2
R ¼
¼
;
ð1Þ
h
i
2
/
with
and
ꢀ ꢁ
2ꢁ K2; / ¼ ꢁa ꢁ 2bðI1 þ I2Þ;
/
q2 ¼
2
ꢀ
ꢁ
2
h
i
48p3
n2ck
2
2
K2 ¼
ðv<3>0Þ þ ðv<3>00Þ I1I2;
ð2Þ
where a, b, L, k, and n are the linear absorption coefficient, nonlinear
absorption coefficient, sample thickness, laser wavelength, and linear
refractive index, respectively.
We used the nonlinear transmission measurement to determine the
two-photon absorption coefficient (b) that corresponds to the imagin-
ary parts of the third-order nonlinear optical susceptibility at
532 nm in the picosecond regime. We note that the investigated com-
pounds exhibited only the linear absorption at the excitation wave-
length.
Therefore, the only parameter that remains to be determined is the
third-order nonlinear optical susceptibility v<3>. Taking the linear and
nonlinear absorption into account, the variation of the intensity of
pump waves along the z-axis can be expressed as
dI
¼ ꢁða þ bIÞI
ð3Þ
dz
The transmission can be calculated from the equation
aeðꢁaLÞ
IðLÞ
T ¼
¼
:
ð4Þ
Ið0Þ a þ bIð0Þð1 ꢁ eꢁaL
Þ
The best fit of Equation (4) to experimental data allows us to evalu-
ate the coefficients a and b. Thus, knowing the analytic formula for
the DFWM reflectivity R and measuring the I4 intensity, we can
determine the absolute value of v<3>
.

308
O. Nadtoka et al.
The geometry of the DFWM experimental setup is presented in
Figure 2. As a source of excitation, we used 532-nm pulses from a
Q-switched mode locked Quantel Nd:YAG laser of the 30-ps duration
with a repetition frequency of 1 Hz. The first beam-splitter divides
the beam to synchronize the acquisition from a fast photodiode. A Glan
prism (G) and a half-plate (k=2) allow us to modify the intensity of laser
beams. The second beam-splitter reflects about 6% of the intensity
(probe beam <3> ). The non-reflected part of a beam goes to the next
(third) beam splitter which divides it into two pump beams <1> and
<2>. Their intensities satisfy the relation I1ðz ¼ 0Þ ¼ I2ðz ¼ LÞ. The
probe wave <3> is a weak probe beam (I3 ¼ 10ꢁ2 I1) which makes
an angle of 12ꢀ with the direction of the two pump waves. The signal
wave <4> is emitted in the opposite direction to the probe beam.
3. RESULTS
The calculated values of linear absorption coefficient (a), nonlinear
absorption coefficient (b), and third-order nonlinear optical suscepti-
bility ðv<3>Þ are collected in Table 2.
We chose NLO active substances for comparison such as Liquid
Crystals (LC) and Cobalt Phthalocyanine (CoPc) which are known
for their strong nonlinear optical properties [17]. We observed that
the third-order nonlinear optical susceptibility ðv<3>Þ of the polymers
under study is one order of magnitude higher than that of LC. In
the case of CoPc, the value of the third-order nonlinear optical suscep-
tibility ðv<3>Þ is higher than that of P1 and P2. These polymers have a
number of advantages, first of all their homogenized film-forming and
further application in devices (microdevices such as micromachines
and microelectronic mechanical systems). In general, polymeric mate-
rials have gained a wide popularity in various fields, because plastics
have a low specific gravity and are chemically inert.
TABLE 2 Values of the Linear Absorption Coefficient (a) and the Absolute
Values of the Third-Order Nonlinear Optical Susceptibility ðv<3>Þ of P1 and P2
Dissolved in DMF; and those for Cobalt Phthalocyanine (CoPc) and Liquid
Crystal (LC) for Comparison
Sample
a, cmꢁ1
b, cm=GW
v<D3F>WM; 10ꢁ13 esu
DMF
P1
P2
CoPc
LC
0.19
1.83
4.76
5.11
9.92
0.01
0.01
0.02
0.32
ꢃ0
1.38
2.26
3.71
25.20 [16]
0.30 [16]

NLO Effects in Polymeric Azoesters
309
4. CONCLUSION
Two homopolymers containing azoesters in the side chain were
synthesized by the thermoinitiated free-radical polymerization process
and characterized. In this paper, we evaluated the linear and non-
linear absorption coefficients and the third-order NLO susceptibility
of various polymeric azoesters. These parameters have been measured
within the degenerate four-wave mixing method at a fundamental
wavelength of 532 nm in the picosecond regime. We also show that
the synthesized polymer P2 gives a larger effect on the NLO response.
We suggest that the value of the third-order nonlinear optical suscep-
tibility is determined by a steric delocalization of the active side chain
of azoester.
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