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 mode l molecules of poly(phenyleneethynylene) derivatives by two different routes. This paper describes the synthesis and the X-ray and optical c haracterization of conjugated soluble oligomers, as well as model trim ers and pentamers of the same structure in which the solubility was im proved by fixing flexible alkyl chains of 10 or 12 carbons to the back bone. The electron density on the phenyl ring was also enhanced or red uced 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 un it, with HBr elimination. This method allows the insertion of a triple bond between two phenyl groups. It was initially used in our laborato ry in order to obtain high molecular weight polymers. It has been adap ted to favor the production of lower molecular weight compounds, by sh ortening the reaction time or by an important increase of the amount o f catalyst. The trimers and pentamers were synthesized step by step by using a route which involves a selective protection-deprotection meth od, followed by the palladium coupling reaction. Model compounds and o ligomers were characterized by UV-visible and Raman spectroscopies, as well as nonlinear optical measurements of the third order susceptibil ities (chi((3))) These measurements allowed us to show the influence o f the donor/acceptor groups and the chain lengths on the absorption wa velength. In addition, the nonlinear optical measurements show that th e chi((3)) value of the pentamer (1.6 x 10(-10) esu) is close to the v alue of the polymer (4.6 x 10(-10) esu).