Twofold 6π-Electrocyclization as a Universal Route Toward Multi-heteroatom-doped Nanographenes Build on 1,4-Dihydropyrrolo[3,2-b]pyrrole Core

Here we present a highly versatile synthetic strategy based on twofold 6π-electrocyclization accompanied with HBr elimination as a novel approach towards centrosymmetric multi-heteroatom-doped nanographenes build around an electron-rich 1,4-dihydropyrrolo[3,2-b]pyrrole core. A straightforward synthesis from readily available substrates offers a unique possibility of fusing the 1,4-dihydropyrrolo[3,2-b]pyrrole subunit not only with carbocyclic building blocks, such as electron-deficient phenanthrenes, chrysenes, or [4]helicenes, but also with heterocyclic systems, such as benzo[b]furan and 5-thiatruxene. The clear advantage of this strategy is that there is no requirement to assemble complex scaffolds possessing bromoaryl units since bromine atom is introduced by bromination of 1,4-dihydropyrrolo[3,2-b]pyrrole core which, because of its exceptionally electron-rich character, is universal and straightforward reaction. The obtained χ-shaped and S-shaped nanographenes containing 10 or more fused rings, exhibit visible-range emissions characterized by fluorescence quantum yields reaching 48%. Computational studies of the reaction mechanism revealed that the 6π-electrocyclization is kinetically favourable over photo-induced direct arylation. Steady-state UV/Visible spectroscopy reveals that upon photoexcitation, the prepared S-shaped N-doped nanographenes undergo mostly radiative relaxation leading to large fluorescence quantum yields. We anticipate that this chemistry will empower the creation of new materials with various functionalities.