Study on the Relations of Structure and the Ultrafast Optical Kerr Effect of Polynitriles
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2 WANG , Y. H. ZOU
'Peking Univ., College of Chemistry and Molecular Engineering, Beijing 100871, P. R. China Peking Univ., Dept. of Physics, Mesoscopic Physics Lab, Beijing 100871, P. R. China
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ABSTRACT A series of polybenzonitrile derivatives, prepared by plasma glow discharge processes or chemical polymerization, were characterized by FT-IR, UV-Vis, GPC and VPO etc. The secondorder nonlinear optical hyperpolarizability y of various benzonitrile monomers and their, polymers were measured by the femtosecond time-resolved optical Kerr effect technique, and the structureproperty relationships were discussed. The power law dependence of 3y on the averagepolymerization-degree of several polybenzonitrile derivatives were also reported. INTRODUCTION Organic conjugated polymers have attracted much attention because of their optical nonlinearity, fast response, relatively low cost, ease of fabrication and integration into devices[l2]. Many contributions concerned on the third-order polarizability and structure correlation study of some typical conjugated polymers, such as polyacetylene, polythiophene, polyaniline etc.[3-6], manifested that the molecular geometry, electronic state, the donor or acceptor substitutents and conjugated chain length affected the third-order nonlinear susceptibility of the materials. These polymers are all of carbon-carbon or heteroaromatic conjugation system. We have reported a novel conjugated system, polynitriles with unique third order nonlinear optical properties in the previous papers [7, 8]. In this paper we report the further study on the synthesis of various polybenzonitrile
derivatives, and the relationship between molecular structure and optical nonlinearity of polybenzonitriles. EXPERIMENTS Benzonitrile(BN, >99%) and p-tolunitrile(TN, >98%) were purified by vacuum distillation just before use. The p-aminobenzonitrile(ABN, >98%) and p-nitrobenzonitrile(NBN, >99%) monomers were recrystallized from ethanol, and p-hydroxybenzonitrile(HBN, >97%) recrystallized from hot water. 3,4-Dimethoxybenzonitrle (DMBN)was synthesized according to literature[9]. Plasma polymerization of Benzonitriles (BN, ABN, NBN, NBN and TN) was carried out by radio-frequency plasma glow discharge. The process was described in literature[8]. PTN was dissolved in chloroform and reprecipitated by anhydrous ether. PHBN was dissolved in tetrahydrofuran and precipitated by anhydrous ether. Both PTN and PHBN were extracted with anhydrous ether to remove unreacted monomers. Poly(3,4-dimethoxybenzonitrile) (PDMBN) were prepared by chemical polymerization. The monomer and boron-trifluoride-anisole complex catalyst were sealed in a glass tube under vacuum, polymerized at 100°C for 24 hours. The polymers prepared by plasma method had a large molecular weight distribution from 2 to 5.1, so the polymers were fractionated by Gel Permeation Chromatography(WATERS 208 GPC analyzer) with dimethylformamide or tetrahydrofuran. The average number molecular weight of selected 271 Mat. Res. Soc. Symp. Proc. Vol. 488 01998 Materials R
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