Synthesis and chemical properties of polyphenylenes cross-linked by electron-accepting viologen moiety

Abstract

Polyphenylene (PP) with NH2 side groups, namely, polymer-1, was synthesized by the Pd-complex-catalyzed reaction of 2,5-dibromoaniline with 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. The reaction of polymer-1 with 1,1′-bis(2,4-dinitrophenyl)-4,4′-bipyridinium dichloride (salt-1) in 1:0.5, 1:1, and 1:2 molar ratios eliminated 2,4-dinitroaniline to yield PPs cross-linked by a viologen (1,1′-disubstituted 4,4′-bipyridinium dications) moiety, polymer-2a, polymer-2b, and polymer-2c, respectively. The UV–vis spectrum of polymer-2c showed absorptions due to the viologen radical cation formed by electron transfer from the polymer backbone to the viologen moiety. The p-doping state of polymer-2c was stable in air. The ESR spectrum of polymer-2c confirmed the generation of the viologen radical. In contrast to photoluminescent polymer-1, the polymers cross-linked by a viologen moiety showed no photoluminescence because the viologen contained within them acted as a quencher. The electrical conductivity of the pellet molded from polymer-2c was higher than that of polymer-1 due to the self-doping in polymer-2c.