Diindolo[2,3‐b:2′,3′‐f]pyrrolo[3,2‐b]pyrroles as Electron‐Rich, Ladder‐Type Fluorophores: Synthesis and Optical Properties

Abstract Previously unknown ladder‐type heteroacenes were synthesized in just two steps from aromatic amines, 2‐nitrobenzaldehydes, and biacetyl. A recently discovered multicomponent process affording 1,4‐dihydropyrrolo[3,2‐ b ]pyrroles was followed by the Cadogan reaction, which resulted in simultaneous closure of two additional pyrrole rings. Rod‐like compounds synthesized in this way were stable in the absence of light, and they were transformed into bis( tert ‐butoxycarbonyl) derivatives. The rigid herringbone‐like structure resulted in a strong absorption in the λ =350–410 nm region and very strong violet and blue emission, which was bathochromically shifted relative to the emission of the parent 1,4‐dihydropyrrolo[3,2‐ b ]pyrroles. The optical properties of these compounds strongly suggest that the conjugation spanned the entire π system. The HOMO energy level was located at −4.6 to −5.1 eV.