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.