A new building block with intramolecular D-A character for conjugated polymers: ladder structure based on B←N unit

The general strategy to construct D-A type conjugated polymers is alternating copolymerization of electron-donating (D) monomer and electron-accepting (A) monomer. In this article, we report a new strategy to develop D-A type conjugated polymers, i.e. first fuse the D and A units into a polycyclic structure to produce a building block and then polymerize the building block with another unit. We develop a new building block with ladder structure based on B <- N unit, B <- N bridged dipyridylbenzene (BNDPB). In the skeleton of BNDPB, one diamine-substituted phenylene ring (D unit) and two B <- N-linked pyridyl rings (A unit) are fused together to produce the polycyclic structure. Owning to the presence of intramolecular D-A character, the building block itself exhibits narrow bandgap of 1.74 eV. The conjugated polymers based on BNDPB show unique electronic structures, i.e. localized HOMOs and delocalized LUMOs, which are rarely observed for conventional D-A conjugated polymers. The polymers exhibit smaller bandgap than that of the building block BNDPB and display near-infrared (NIR) light absorption (lambda(abs)=ca. 700 nm). This study thus provides not only a new strategy to design D-A conjugated polymers but also a new kind of building block with narrow bandgap.