RIGID-ROD CONJUGATED POLYMERS FOR NONLINEAR OPTICS .1. CHARACTERIZATION AND LINEAR OPTICAL-PROPERTIES OF POLY(ARYLENEETHYNYLENE) DERIVATIVES

New rigid rod and conjugated polymers based on poly(aryleneethynylene) have been synthesized by a palladium-catalyzed polycondensation. Long alkyl side chains were bound to the aryl groups by ester, ether, or other functions in order to increase solubility, in order that this might lead to higher molecular weights and longer conjugated backbones. We chose also to act on the electron density of the conjugated chain by means of an electroactive functional group or an electron donor or electron acceptor attached to the phenyl moiety. We first synthesized some polymers with alkoxy groups and long hydrocarbon tails of 10 and 12 carbon atoms. We compared these model polymers with other polymers, homopolymers based on phenyl groups bearing thioether substituents or alkyl esters as electron accepters, and also alternated copolymers based on the former and other electron donors, anthryl or thienyl groups, or accepters, phenyl substituted by ester or nitro groups. The characterization by GPC of the poly(phenyleneethynylene) substituted with two dodecyl ether chains on the phenyl group shows the highest average molecular weight ever synthesized on this type of homopolymer, finding a value as high as 10(5) g/mol. The new homopolymers and copolymers based on the phenyl alkyl ether were similarly characterized. The spectroscopic linear properties in solution and in solid thin films confirm a large shift of the main absorption peak of the pi-pi* transition in the visible range according to the donor or acceptor character of the substituent or comonomer group. Studies by Raman spectroscopy of the alkoxy phenyl homopolymer and the corresponding trimer show a shift of the triple bond stretching band and thus a greater delocalization for the polymer. Highly birefringent liquid crystalline phases are induced by rubbing or shearing and are confirmed by an observation of the optical dichroism along the shearing direction.