Rigid Rod Conjugated Polymers for Nonlinear Optics. 3. Intramolecular H Bond Effects on Poly(phenyleneethynylene) Chains

Abstract

This paper describes the synthesis and the X-ray and the optical characterization of conjugated soluble polymers in which the planarity of the backbone has been improved by intramolecular H bonds. The solubility of the polymers was enhanced by increasing the number of alkyl chains on the aryl group. The electron density on the phenyl ring was also modified by the amino and ester groups, introducing electron-donor or electron-acceptor groups, which also increase the polarizability. The polymers have been synthesized by polycondensation using a palladium-catalyzed coupling reaction, between a bromoaryl and an ethynylaryl unit, with HBr elimination. This method allows the insertion of a triple bond between two phenyl groups. It was initially used in our laboratory to obtain oligomers or high molecular weight polymers, having important nonlinear susceptibilities. The new polymers have been characterized by UV−visible and Raman spectroscopies, as well as nonlinear optical measurements of the third-order susceptibilities (χ(3)). These measurements allowed us to show the influence of the H bonds between the aryl groups on the absorption wavelength. In addition, the nonlinear optical measurements show that the χ(3) value of the pPY (6.8 × 10-10 esu) is close to the highest values obtained in acetylenic polymers such as polydiacetylenes.