Planar, Twisted, and Curved π-Extended Pyrrolo[3,2-b]Pyrroles: Recent Advances in Synthesis, Properties, and Applications

The structure of 1,4-dihydropyrrolo[3,2-b]pyrrole is among the most widely used heteropentalene scaffolds due to its significant capability to maintain desirable π-conjugation and favorable electronic properties through various post-synthesis reactions. As a distinguished feature, pyrrolo[3,2-b]pyrroles have the most electron density among this class of aromatic heterocycles due to the structure consisting of two fused pyrrole rings. Because of its highly electron-rich nature, this bicyclic fused heterocyclic core has been used as an electron donor moiety in the structure of many functional dyes. Also, the derivatives of this compound have significant applications in various fields, such as semiconductors and optoelectronics. Since 2013, when a facile and efficient three-component reaction was introduced to facilitate access to this bicyclic heterocycle, the synthesis of this compound has attracted considerable research interest. Given the substantial increase in research reports in this field since then, we decided to examine, in this review article, the studies that have extended the amount of π-expansion around the central core of 1,4-dihydropyrrolo[3,2-b]pyrroles through post-transformation reactions.