Rigid rod conjugated polymers for nonlinear optics .2. Synthesis and characterization of phenylene-ethynylene oligomers

In order to make a comparison between the properties of low molecular weight poly(phenyleneethynylene) type compounds and the corresponding high molecular weight polymers, we have synthesized oligomers and model molecules of poly(phenyleneethynylene) derivatives by two different routes. This paper describes the synthesis and the X-ray and optical characterization of conjugated soluble oligomers, as well as model trimers and pentamers of the same structure in which the solubility was improved by fixing flexible alkyl chains of 10 or 12 carbons to the backbone. The electron density on the phenyl ring was also enhanced or reduced by introducing either electron-donor or electron-acceptor groups. The oligomers were 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 in order to obtain high molecular weight polymers. It has been adapted to favor the production of lower molecular weight compounds, by shortening the reaction time or by an important increase of the amount of catalyst. The trimers and pentamers were synthesized step by step by using a route which involves a selective protection-deprotection method, followed by the palladium coupling reaction. Model compounds and oligomers were characterized by UV-visible and Raman spectroscopies, as well as nonlinear optical measurements of the third order susceptibilities (chi((3))) These measurements allowed us to show the influence of the donor/acceptor groups and the chain lengths on the absorption wavelength. In addition, the nonlinear optical measurements show that the chi((3)) value of the pentamer (1.6 x 10(-10) esu) is close to the value of the polymer (4.6 x 10(-10) esu).