Effect of cyano substituents on electron affinity and electron-transporting properties of conjugated polymers

A series of cyano-containing distyrylbenzenes were synthesized as the model compounds to systematically study the effect of cyano substituents on the redox behaviors of conjugated molecules. By introducing the electron-withdrawing functional groups (cyano and dicyanovinyl) onto the phenylene ring, both electron affinity and electrochemical stability of the resulting distyrylbenzenes are greatly enhanced. The results enabled us to design and synthesize a new class of highly electron affinitive, fluorene-based copolymers with these cyano-containing chromophores as comonomers. The effects of acceptor strength and side chain on electron-transporting properties of these polymers were also investigated. By properly adjusting copolymer compositions, a combined high electron affinity and transport was achieved in a statistic copolymer, poly(fluorenebenzothiadiazole-cyanophenylenevinylene) (PFB-CNPV). An external quantum efficiency up to 0.88% and brightness as high as 4730 cd/m(2) were achieved in a double-layer light-emitting diode (LED) using PFB-CNPV as the emitting layer.