Rational Designs of Multifunctional Polymers-Conjugated Photorefractive Polymers
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ABSTRACT A novel type of photorefractive polymers, containing a conjugated backbone and a second order NLO chromophore, has been developed. A new polymerization method based on the Stille coupling reaction was explored to synthesize these materials. It was expected that the conjugated backbone would absorb photons in the visible region and play the triple role of charge generator, charge transporter and backbone. Several physical measurements demonstrated this expectation. Two beam coupling experiments clearly indicated an asymmetric optical energy exchange between two beams without applying an external electrical field. This is a conclusive demonstration for photorefractivity. Also, large optical gain of 5.7 cm- 1 was observed under zero-field condition, which is comparable to most inorganic photorefractive materials. The versatility of the reaction makes possible the advancement of new polymer structures. INTRODUCTION Photorefractive (PR) effect was discovered almost thirty year ago in an inorganic single crystal, LiNbO 3 .1 Since then, research on photorefractive effects was focused exclusively on inorganic materials, such as ferroelectric crystals (LiNbO3, LiTaO3, BaTiO3), semiconductors (GaAs, InP, CdF2), and sillenites (Bil2SiO 20 ). 1-4 However, since 1990, photorefractive studies have been extended into organic materials, both doped organic crystal and polymeric materials. 5 -8 Organic photorefractive materials exhibit reasonably large electro-optic response and a lower dielectric constant than inorganic materials; they are versatile in structural design and have good processibility for device fabrications. In a photorefractive material, four functional species exist simultaneously: photocharge generator, charge transporter, charge trap, and second order nonlinear optical moiety. A general approach in designing photorefractive polymers is to prepare composite polymers doped with different small molecules that play different roles (6). While these composite systems enjoy the ease of preparation, problems such as phase separations and the instability of electrooptical activity make it desirable to synthesize photorefractive polymers in which all of the species are covalently attached. We have succeeded in developing a new approach to synthesizing novel photorefractive polymers containing an NLO chromophore, a charge generator and a transporting compound, all covalently linked to the polymer backbone. 7 ,8 Two beam coupling experiments revealed that the refractive index grating, caused by the space charge field with a phase shift of 900, is the major contribution to the optical gain, a demonstration of the photorefractive effect. More recently, we developed a new concept in designing photorefractive polymers, i.e. synthesis of conjugated photorefractive polymers that contain a conjugated backbone, a second order NLO chromophore, and a small amount of charge trapper (see Scheme 1).9 The rationale behind designing these new materials is that the conjugated backbone absorbs photons in the visible region and plays
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