The chemistry of 1,4-dihydropyrrolo[3,2-b]pyrroles

1,4-Dihydropyrrolo[3,2-b]pyrrole (DHPP) skeleton is comprised of only two fused pyrrole rings and yet, in non-fused form, it had to wait for discovery until 1972. The [3,2-b]-mode of fusion leads to several important consequences such as exceptionally high electron-density, ease of functionalization and high symmetry. In this Chapter we provide a comprehensive overview of multiple synthetic methods leading to this aromatic, heterocyclic skeleton. Although older methods comprised of some truly spectacular transformations, it was the development of multicomponent reaction of aromatic aldehydes, aromatic primary amines and butane-2,3-dione leading directly to 1,2,4,5-tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles, which triggered the intense exploration of chemical space. We concentrate on presenting the newest and the most spectacular synthetic approaches leading to heretofore unknown, ladder-type heterocyclic skeletons. The increased scientific activity has resulted in the preparation of planar, helical and bowl-shaped π-expanded 1,4-dihydropyrrolo[3,2-b]pyrroles and numerous research applications in, for example, optoelectronics. We provide a critical overview of photophysical properties of DHPP and especially of its π-expanded analogs. 1,4-Dihydropyrrolo[3,2-b]pyrroles made a contribution into deepening of our understanding of excited-state symmetry breaking and fluorescence of nitroaromatics. The synthesis, reactivity, and photophysical properties are exhaustively presented.