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 o ther functions in order to increase solubility, in order that this mig ht lead to higher molecular weights and longer conjugated backbones. W e chose also to act on the electron density of the conjugated chain by means of an electroactive functional group or an electron donor or el ectron acceptor attached to the phenyl moiety. We first synthesized so me polymers with alkoxy groups and long hydrocarbon tails of 10 and 12 carbon atoms. We compared these model polymers with other polymers, h omopolymers based on phenyl groups bearing thioether substituents or a lkyl esters as electron accepters, and also alternated copolymers base d on the former and other electron donors, anthryl or thienyl groups, or accepters, phenyl substituted by ester or nitro groups. The charact erization by GPC of the poly(phenyleneethynylene) substituted with two dodecyl ether chains on the phenyl group shows the highest average mo lecular weight ever synthesized on this type of homopolymer, finding a value as high as 10(5) g/mol. The new homopolymers and copolymers bas ed on the phenyl alkyl ether were similarly characterized. The spectro scopic linear properties in solution and in solid thin films confirm a large shift of the main absorption peak of the pi-pi transition in t he visible range according to the donor or acceptor character of the s ubstituent or comonomer group. Studies by Raman spectroscopy of the al koxy phenyl homopolymer and the corresponding trimer show a shift of t he triple bond stretching band and thus a greater delocalization for t he polymer. Highly birefringent liquid crystalline phases are induced by rubbing or shearing and are confirmed by an observation of the opti cal dichroism along the shearing direction.