Redox-Active Doubly Boron-Doped Indenofluorenes: Isomer-Directed Access to Open- and Closed-Shell Electronic States...

Indenofluorenes have become an important class of polycyclic aromatic hydrocarbons that are of interest for their nonalternant structures and intriguing properties. In this report, we unveil the reversible redox behavior of doubly boron-doped indenofluorenes (DBIFs). The diboraindeno[2,1-b]fluorene isomer is disclosed for the first time, where reduction of 18 π-electron DBIFs yield new 19 π-electron radical anions and 20 π-electron dianions. Notably, the DB[2,1-b]IF radical anion is the first isolated and structurally characterized 19 π-electron indenofluorene derivative, displaying ultra low-energy absorption in the near-infrared (NIR)-IIb region (1594 nm, tailing to 1850 nm). In addition, the synthesis of three 20 π-electron DBIFs highlight isomer-dependent electronic structure modulation. While the para-quinodimethane (QDM) DB[1,2-b]IF is NMR-active and has a singlet ground state (NIR emissive, 900 nm), the meta-QDM DB[2,1-b]IF compounds are EPR-active and have a triplet ground state (NIR absorption out to 1350 nm), both of which are isoelectronic with their respective pure-hydrocarbon indenofluorene analogs. Initial reactivity studies of the reduced compounds were conducted, which reveal that the two boron centers react cooperatively to afford unusual macrocyclic products. These results highlight reduced, aryl-substituted DBIFs as true electronic mimics for carbonaceous indenofluorenes, with expanded redox capabilities, optical properties, and chemical reactivities that result from boron-doping.