Synthesis of planar poly(p-phenylene) derivatives for maximization of extended .pi.-conjugation

Described is the synthesis of ladder polymers with a poly(p- phenylene) (PPP) backbone. The main PPP backbone was synthesized via palladium- catalyzed coupling of an arylbis(boronic ester) with an aryl dibromide. Imine bridges, formed by exposure of the polymer to trifluoroacetic acid, are used to forced the consecutive units into planarity. The bridging units are sup 2 hybridized thus allowing for greater pi-electron flow between the consecutive phenyl units by lowering the band gap between the hydroquinoidal and the quinoidal forms of the phenylene backbone. Upon planarization, bathochromic shifts of 210-240 nm occur for the n-dodecyl substituted polymer. The optical spectra of the planar systems are compared to that of the parent nonplanarized polymers, oligo(p-phenylenes), PPP and other near-planar PPP derivatives. When the bridges are n-dodecyl substituted, the fully planar structures can be made into flexible free standing films.