Extension of Pyrrolopyrrole π‑System: Approach to Constructing Hexacyclic Nitrogen-Containing Aromatic Systems

A facile three-step
approach to synthesizing quinoline-fused pyrrolopyrroles
is reported. The crucial step in this synthesis is the condensation
of 2-aminophenyl substituted pyrrolopyrroles with aromatic aldehydes.
The resulting hexacyclic ladder-type dyes strongly absorb UV radiation
and exhibit fluorescence at 450–510 nm. The presence of pyridine-type
and pyrrole-type nitrogen atoms is important for the electronic properties
of this almost planar heterocycle. These heteroatoms, along with the
addition of moderate electron-withdrawing and electron-accepting substituents,
provide a means for fine-tuning of the emission characteristics of
the polycyclic conjugates.