SYNTHESIS OF PLANAR POLY(P-PHENYLENE) DERIVATIVES FOR MAXIMIZATION OFEXTENDED PI-CONJUGATION

Described is the synthesis of a ladder polymer 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 force the consecutive units into planarity. The brid ging units are sp2 hybridized thus allowing for greater pi-electron fl ow between the consecutive phenyl units by lowering the band gap betwe en the hydroquinoidal and the quinoidal forms of the phenylene backbon e. When the bridges are n-dodecyl substituted, the fully planar struct ures (with Mn < 5 000) are soluble in hot chlorobenzene from which fle xible free standing films can be cast. The n-butyl substituted polymer s and the higher molecular weight n-dodecyl substituted polymers are s oluble in CH2Cl2/trifluoroacetic acid mixtures. The optical spectra of the planar systems are compared to that of the parent nonplanarized p olymers, some analogous planar oligomers, and oligo(p-phenylenes).