Second-Order Nonlinear Optical Polymers with Different Chromophore Arrangements
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INTRODUCTION Organic polymeric materials have long been considered promising candidates for the fabrication of electro-optical devices. 1 Design and syntheses of nonlinear optical (NLO) materials can be accomplished through several strategies. One attractive approach is incorporation of NLO chromophore covalently onto polymer backbones. According to the arrangements of dipolar chromophores, NLO polymers can be divided into side-chain and main-chain polymers characterized by different configurations shown in figure 1. For convenience, we adopt the abbreviations SD, RD, HT, and HH to represent the side-chain, random, head-to-tail, and headto-head polymers. Although most of the NLO polymers synthesized to date are side-chain 37 polymers, 2 -3 a few main-chain polymers have been reported. -
~yyy
RD: HT: r
HH:
Side-chain
Main-chain
Figure 1. Different arrangements of dipolar chromophores in polymers For practical electro-optic devices, the materials must exhibit large and stable nonlinearities. Crosslinking of polymers has proven to be an effective approach to stabilizing poling-induced NLO activity (e. g., SHG). 8 The magnitude of optical nonlinearity depends upon chromophore 529 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
number density and poling efficiency, and therefore depends upon the nature of chromophore incorporation. Unfortunately, a systematic comparison can not be made based on the previously reported NLO polymers due to uncontrolled structural variation. In this paper, we report on design and syntheses of a series of NLO polyesters based on the same chromophore (functionalized amino-nitro azobenzene) but with the different chromophore arrangements mentioned previously. Poling behaviors of these polymers were investigated by an in-situ poling and SHG experiment. EXPERIMENTAL 1. Syntheses o f Monomers The syntheses of dye-bearing monomers are depicted in scheme 1. Compound 1 was synthesized according to a modified literature procedure. 9 2,2'-[4-[(4-Nitrophenyl)azo] iminobisethanol (Disperse Red 19 ) was prepared following the literature method. 10 Diazo coupling reactions between N-substituted anilines and diazonium salts produced monomer 1, 2 and 3. All the monomer structures were confirmed by IH NMR and elemental analyses.
Scheme 1 CI 2OH
H2N
NO2
., l
C H3 -
CH 2OH +N2
/H
b
H3CN-
N0 2
CH 2OH
R-Br/K 2C03 CH3 ,,, M(onomer 1: R= -CH 2CH OH M(onomer 2: R= -(CH2)4COOEt
R/IJ-•j
I
Br-(CH 2)6-Br K2CO3
CH3 _O (CH2)6
CH2 OH
/N
CH3
ý NO 2
ý (0H2 )6
•cH
/
N-"{ •)' N N
-
2 OH NO
CH2
Monomer 3 2. Syntheses of Polymers Scheme 2 shows the syntheses and structures of the polymers. The SD, RD, and HH polymers were prepared by conventional condensation polymerization and purified by reprecipitation. The HT polymer was synthesized by melt-phase polytransesterification catalyzed by dibutyltin diacetate.
530
Scheme 2 HO
OH •
0---O
N
CIOC-(CH2)4-COCI
N
•O
SR N
N02
Monomer I
C1OC-(CH 2)4-C ICi
Monomer 2
Py/NMP
C-(CH2)4-CO+
N
Py/NMP
N
--
NO2 OH 3 "
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