Carbazole-based azo group-containing single component polymer exhibiting photorefractive performance

Abstract

Photorefractive (PR) polymers have been investigated widely mostly with host–guest polymer composites [1, 2] and fully functionalized polymers [3, 4]. Host– guest polymer composites exhibit poor stability due to phase separation, while fully functionalized polymers are difficult to be synthesized in most cases. Therefore, much effort has been paid in constructing bifunctional polymers with both photoconductivity and electro-optic (EO) effect for photorefractive purpose in recent years. Such bi-functional photorefractive polymers may have good stability against phase separation and be much easier to synthesize; the minor composition of photo-sensitizer may be incorporated in a composite manner. In constructing the bi-functional photorefractive polymers, carbazole-based polymers have attracted considerable attention, which is because of the fact that polyvinylcarbazole (PVK) based host–guest polymers have been realized to exhibit high EO response and good photorefractive performance. But the high glass transition temperature (Tg) of PVK made it difficult to pole at room temperature to achieve orientational birefringence. Lowering the Tg by adding plasticizer normally resulted in the deduction of photorefractive performance due to the relative lowering of chromophore concentration. On the other hand, the direct polymerization of a functionalized carbazole monomer normally gives a polymer with low molecular weight owing to both the bulkiness of the monomer and the side effect of the functional group, which is not beneficial to the film forming property of the polymer. In our effort to synthesize carbazole-based azo group-containing bi-functional photorefractive polymer with low Tg through a post-azo-coupling reaction, we surprisingly found that the single component polymer exhibits good PR performance at room temperature without adding any sensitizer and plasticizer. The polymer synthesis is shown in Scheme 1; a precursor polymer bearing carbazole group with a long alkylene chain was prepared through conventional free radical polymerization; then the precursor polymer was allowed to react with diazonium salt in nitrobenzene with the help