RIGID-ROD CONJUGATED POLYMERS FOR NONLINEAR OPTICS .3. INTRAMOLECULARH BOND EFFECTS ON POLY(PHENYLENEETHYNYLENE) CHAINS

This paper describes the synthesis and the X-ray and the optical chara cterization of conjugated soluble polymers in which the planarity of t he backbone has been improved by intramolecular H bonds. The solubilit y of the polymers was enhanced by increasing the number of alkyl chain s 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-c atalyzed coupling reaction, between a bromoaryl and an ethynylaryl uni t, with HBr elimination. This method allows the insertion of a triple bond between two phenyl groups. It was initially used in our laborator y to obtain oligomers or high molecular weight polymers, having import ant nonlinear susceptibilities. The new polymers have been characteriz ed by UV-visible and Raman spectroscopies, as well as nonlinear optica l measurements of the third-order susceptibilities (X((3))). These mea surements allowed us to show the influence of the H bonds between the aryl groups on the absorption wavelength. In addition, the nonlinear o ptical measurements show that the X((3)) value of the pPY (6.8 x 10(-1 0) esu) is close to the highest values obtained in acetylenic polymers such as polydiacetylenes.