Synthesis and Properties of Ladder-Type BN-Heteroacenes and Diazabenzoindoles Built on a Pyrrolopyrrole Scaffold.

A simple, three-step synthesis of BN-heteroacenes and diazabenzoindoles based on the pyrrole[3,2-b]pyrrole scaffold has been developed. The incorporation of BN units has proven to be effective in modulating the electronic properties and molecular geometries of the π-conjugated backbone, creating a new type of heteroarenes. The unique planar structure and high rigidity of BN-compounds result in very high absorption coefficients and high fluorescence quantum yields, and, at the same time, very small Stokes shifts. A striking difference has been observed for a second type of derivatives: diazabenzoindoles, which remain virtually nonfluorescent, despite having a similar, rigid structure. The former class of heterocycles is characterized by a strong absorption around 400 nm and intense fluorescence observed in the 395-426 nm region, which results in very small Stokes shifts of less than 900 cm(-1).