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978-1-4244-779 - /10/$26.00 ©2010 IEEE

Investigation of Selected Polymers with Different Azobenzene

Moieties for NLO Application

Oksana Krupka1,*, Oksana Nadtoka

1, Vitaliy Smokal

1, Hasnaa El Ouzzani

2, and Bouchta Sahraoui

2

*,1Kyiv National Taras Shevchenko University, Macromolecular Chemistry Department

Volodymyrska Str., 60, 01033 Kyiv, Ukraine Tel: (8044) 2393367, Fax: (8044) 2393100, e-mail: oksana_krupka@yahoo.com

2MOLTECH Anjou - UMR CNRS 6200 MINOS, University of Angers

2 Boulevard Lavoisier, 49045 Angers, France

ABSTRACT

The polymers with different azobenzene moiety were synthesized. The results of photochemical and optical activities of the corresponding polymers are presented. The third order nonlinear optical susceptibilities of studied compounds have been investigated in solutions using degenerate four wave mixing (DFWM) method at 532 nm. Keywords: nonlinear optics; NLO susceptibility; azopolymers; DFWM.

1. INTRODUCTION

Azobenzene systems possess the advantages for high optical nonlinearities due to photoinduced trans-cis

isomerization, molecular reorientation and nonlinear absorption [1-4]. The trans-cis transformation is connected with its volume change, just change of its rotational mobility. For example there is a ca. 10% decrease of the volume occupied by molecule passing from trans to cis form (Fig. 1). As such conformational changes are

connected with the change of π electron conjugation length, as result the material index of refraction is changing too.

N

N9.0 A

o

trans-

λ = 360 nmN N

o

5.5 A

cis-

λ = 470 nm

Fig. 1. Azobenzene isomerization.

In this vein azobenzene polymers are interesting because they combine the properties of anisotropy with photoresponsive behavior and can give rise to application in such areas as LC displays, NLO materials, information storage devices, etc. [5-8].

The present work deals with a photochromic polymers based on methacrylic azoesters. The azobenzene polymers containing NLO chromophores in each side chain unit with different groups of acceptor nature and flexible alkyl spacer have been synthesized via free radical polymerization. Besides, the introduction of methacrylate chain from methacryloyl chloride improved the solubility of azobenzol compounds, which provided much potentiality in film forming.

On the other hand, it was studied the influence of the push–pull electronic effect on the third-order NLO property of polymers by changing the substituents which have different electronic effect on the azobenzol group.

2. EXPERIMENTAL

2.1 Instruments1H NMR (400 MHz) spectra were recorded on the "Mercury-400" spectrometer using CDCl3 as solvent. Chemical shifts are in ppm from the internal standard tetramethylsilane (TMS).

The phase transitions were studied by differential scanning calorimetry (DSC) using Perkin Elmer DSC-2 instrument equipped with a IFA GmbH processor at the scan rate of 20ºC/min.

UV-VIS measurements were performed at room temperature either in solutions in a quartz liquid cell, or as thin films deposited on glass substrates, with a Perkin-Elmer UV/VIS/NIR Lambda 19 spectrometer.

2.2 Chemicals

The target azomonomers (M1 and M2) were synthesized by general methods described elsewhere [9]. The methacrylic monomers were synthesized by reaction the azobenzene alcohols with methacryloyl chloride in the presence of triethylamine as nucleophilic catalyst and hydrochloric acid acceptor. The polymers were

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synthesized by radical polymerization using AIBN as radical initiator. The structures of obtained polymers were confirmed by 1H NMR spectra.

4-[(Z)-[4-[(2-methyl-1-oxo-2-propenyl)oxy]phenyl]azo]-benzoic acid butyl ester (M1): Brown crystals;

yield 69%; mp 63º (by DSC). 1H NMR (CDCl3), δ (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, =CH2), 5.91 (s, 1H, =CH2), 2.05 (s, 3H, -CH3), 1.72 (m, 9H, C4H9). UV-vis

(ethanol) λmax: 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 %.

4'-(methacryloxy)hexyl xy-4-nitroazobenzene (M2): Orange crystals; yield 50%; mp 72º (by DSC). 1H

NMR (CDCl3), δ (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, =CH2), 5.95 (s, 1H, =CH2), 2.20 (s, 3H, -CH3), 4.10 (m, 2H, ArOCH2), 3.9 (m, 2H, HOCH2) 1.72 (m, 4H, H2).

UV-vis (ethanol) λmax: 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 %.

CH2 C

CH3

C O

O

N

N

C

O OC4H9

n n

(CH2)6

CH2 C

CH3

C O

O

O

N

N

NO2

P1 P2

Fig. 2. Structures of azopolymers.

Polymerization. Homopolymers P1 and P2 were synthesized by free-radical polymerization in toluene. The polymerization was carried out in 10 wt.% toluene solution of monomer with AIBN as free radical initiator (1 wt.% of monomer) at 80ºC over more than 30 hours. Polymers were isolated from the reaction solution by precipitation into methanol followed by reprecipitation from toluene into methanol and then dried at 50ºC overnight [9].

The obtained polymers were dissolved in dimethylformamide. The solutions of polymers were filtered through a 0.2 µm nylon filter before using.

Table 1. Characteristics of polymeric azoesters.

Compound g, º λmax, nm n Mw Mw/ n

P1 123 331 3500 4410 1.26

P2 112 343 5000 6900 1.38

2.3 Degenerate four wave mixing experiment

Nonlinear optical properties on solutions of P1 and P2 (C =12 g/L) were investigated using DFWM method. Various experimental techniques can be used for estimating the third order nonlinearity of given materials, even though each technique addresses only a particular facet of the nonlinearity due to the frequency dispersion of

3χ < > . The DFWM measurements were performed using Nd:YAG laser (Quantel Model YG472) working at 532

nm with 30 ps pulses duration and 1 Hz repetition rate. In this experiment, three optical pulses (two pump beams I1 and I2 and a probe beam I3) were obtained by using two beam splitters [10].

Carbon disulfide (CS2) was used as reference material to calibrate the DFWM measurements (2

3

CSχ < > =1.11×10-12

esu). The P1 and P2 were contained in 2 mm thick quartz cuves. The geometry of the DFWM experimental setup is presented in Figure 3. As a source of excitation we used

532 nm from a Q-switched mode locked Quantel Nd:YAG laser of 30 ps duration, and a repetition frequency of 1 Hz. First beam-splitter divide the beam to synchronize the acquisition from a fast photodiode. A glan prism (G) and a half-plate ( /2) allow to modify the intensity of the laser beam. Second beam-splitter reflects about 6% of intensity (probe beam I3). Non-reflected part of beam goes to next (third) beam splitter, which divides it in two

pump beams I1 and I2. Their intensities satisfy the relation 1 2( 0) ( )I z I z L= = = . The probe wave I3 is a weak

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probe beam 2

3 1( 10 )I I−= which makes an angle of 12° with the direction of the two pump waves. The signal

wave I4 is emitted in the opposite direction to the probe beam.

) θI1

I2

I3I4

z

NL

Fig. 3. Standard backward degenerate four-wave mixing (DFWM) geometry

(I1 and I2 - pump waves, I3- probe wave, I4 - fourth wave).

3. RESULTS AND DISCUSSION

The DFWM reflectivity (R) was calculated from the propagation equation of the four beams in interaction and took into account linear and nonlinear absorption and can be expressed as follows [11, 12]:

( )( ) ( )

24

2

3

0

0coth

2

I KR

Iq qL

φ= =

−, (1)

with

2

2 2

2q K

φ= − , ( )1 22 I Iφ α β= − − + ,

and ( ) ( )2

32 2

2 3 3

1 22

48' "K I I

n c

π χ χλ

< > < >= + , (2)

where α, β, L, λ and n are the linear absorption coefficient, the nonlinear absorption coefficient, the sample thickness, the laser wavelength and the linear refractive index, respectively.

We used the nonlinear transmission measurement to determine the two-photon absorption coefficient (β) that corresponds to the imaginary parts of the third order nonlinear optical susceptibility at 532 nm in the picosecond regime. We should notice that the investigated compounds exhibited only linear absorption at the excitation wavelength.

Therefore, the only parameter that remains to be determined is third order nonlinear optical susceptibility 3χ < > .

Taking into account linear and nonlinear absorption, the variation of the intensity of the pump waves along the z-axis may be expressed:

( )dI

I Idz

α β= − + . (3)

The transmission can be calculated from the following equation:

( )( )

(0) (0)(1 )

L

L

I L eT

I I e

α

αα

α β

−

−= =+ −

. (4)

The best fit of Eq. (4) to experimental data allows us to deduce α and β coefficients. Thus knowing the analytic formula of the DFWM reflectivity R and measuring the I4 intensity, one can determine the absolute value of

3χ < > .

The calculated values of linear absorption coefficient ( ), nonlinear absorption coefficient ( ) and third-order

nonlinear optical susceptibility ( 3χ < > ) are collected in Table 2.

Table 2. The values of the linear absorption coefficient ( ), nonlinear absorption coefficient ( ), and the absolute

values of the third-order nonlinear optical susceptibility ( 3χ < > ) of P1 and P2 dissolved in DMF.

Sampleα

1[ ]cm−

β[ ]cm GW

3

DFWMχ < >

-13[10 esu]

DMF 0.19 0.01 1.38

P1 1.83 0.01 2.26

P2 4.76 0.02 3.71

As shown in Table 2, all methacrylic azopolymers have high the third-order nonlinear optical susceptibility. After polymerization, the obtained polymers have repeated azobenzene side chains as NLO chromophore in each chain unit. Although the degree of polymerization is not very high, this is influenced by the steric effect of

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azobenzene chromophore in polymer side chain. We observed that the third-order nonlinear optical susceptibility

( 3χ < > ) of studied polymer P2 is higher than for P1 due to the strongest electron-donating and electron-

withdrawing groups. When the electron withdrawing group such as nitro group lies on the other end of azobenzene, the strong push-pull electronic structure was formed. The fluidity of electrons in the molecular chain is better and the intra-molecular charge transfer can be performed well, which is beneficial to enhancing

the NLO property. The value of the third-order nonlinear optical susceptibility ( 3χ < > ) for P1 and P2 is higher

then for LC [13]. In this aspect 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 materials have gained a wide popularity in various fields because plastics have a low specific gravity, are chemically inert.

4. CONCLUSIONS

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

ACKNOWLEDGEMENTS

The authors would like acknowledge the support of COST MP0702 action.

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